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7/29/2019 Review ReyaSynthesis, X-ray crystal structure analysis and properties of M(II)-N,N,N,N- tetraalkylpyridinedithioca

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Chapter-3

Synthesis, X-ray crystal structure analysis and properties of M(II)-N,N,N,N-

tetraalkylpyridinedithiocarboxamides M(II) = Co(II), Ni(II).

3.1 Introduction

Polydentate ligands, particularly with terdentate arrangements have been

extensively used in coordination chemistry in recent years. There are several reasons

for this interest. One of them is the possibility of obtaining low symmetry five or six-

coordinate complexes which are of interest from an electronic point of view especially

if significant steric ligand restrictions are present. The existence of unsymmetrical

metal environments in several biological systems (e.g., in copper enzymes) has been

related to their redox properties. The complexation of metal ions by pyridine-2,6-

dicarboxylic acid (dipicolinic acid, dipicH2), has been extensively studied. This stems

mainly from its ability to form stable chelates.1 It has been shown to act as a

bidentate2, meridional3 or as a bridging group.4 Other interesting properties are its

biological activity,5 its ability to stabilize transition metal ions in unusual oxidation

states,6 and its usefulness in analytical chemistry such as chemical analysis7 of iron at

concentrations down to 4 ppm and in corrosion inhibition8. Iron complexes of

dipicolinic acid are also involved in one of the greatest challenges, namely iron

induced activation of dioxygen and hydrogen peroxide. 9

N

OH OH

OON

NH HN

OO

(A) (B)

Among various ligands derived from pyridine-2,6-dicarboxylic acid (A) there

has been a growing interest in the development of the coordination chemistry of

pyridine dicarboxamide ligands, (B).10-22 The carboxamide [-C(O)NH-] group present

in the primary structure of proteins, is an important ligand construction unit for

coordination chemists. These ligands exhibit a range of coordination numbers,

geometries and nuclearities for transition metal and lanthanide ions. Upon

deprotonation of carboxamide nitrogen atoms, these centers and the pyridyl ringnitrogen of the anion chelate to the metal ions. Recently, a whole new class of ligands


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having fully substituted amide nitrogens, N,N,N,N-tetraalkylpyridine-2,6-

dicarboxamides (O-daap) and N,N,N,N-tetraalkylpyridine-2,6-dithiocarboxamides

(S-dapt) (Scheme 1) was synthesized and well characterized by Kapooret al.23-31 and

others.32 The symmetrical tertiary amide side arms at the 2- and 6- positions of the

central pyridine ring provide a variety of novel O~N~O and O~S~O tridentate

receptors for binding to metal ions.

(O-daap / S-dapt)

R = Me; O-dmap/S-dmpt

R = Et; O-deap/S-dept

R = iPr; O-dpap/S-dppt

R = iBu; O-dbap/S-dbpt

R = ph; O-dphap/ S-dphpt

NN

RR

N

O OR R

(S) (S)

Scheme 1

In the present work, molecular and crystal structure investigations of three

N,N,N,N-tetraalkylpyridine-2,6-dithiocarboxamide (S-dapt) ligands: N,N,N,N-

tetraethylpyridine-2,6-dithiocarboxamide (S-dept), N,N,N,N-tetraisobutylpyridine-

2,6-dithiocarboxamide (S-dbpt) and N,N,N,N-tetraphenylpyridine-2,6-

dithiocarboxamide (S-dphpt) has been carried out. Three new complexes of

N,N,N,N-tetraalkylpyridine-2,6-dithiocarboxamide (S-dapt) ligands withbiologically important metal ions such as Co(II) and Ni(II) have also been prepared

and well identified and characterized through melting points, elemental analysis, IR

and UV-Vis spectroscopy, conductivity measurements, thermal analysis (DT-TGA)

and single crystal X-ray crystallography. The relevant literature is discussed in the

preceding pages.

3.2 Literature survey

Investigations of the coordination chemistry of transition metals with non-

macrocyclic ligands containing amide functionality have received much attention in

recent years.10-44 The interest among such systems interest stems from their occurrence

in metalloproteins and metal complexes of glycopeptide antibiotics. The carboxamido

group, ubiquitous throughout nature in the primary structure of proteins, is an

important ligand construction unit for coordination chemists. Pyridine

dicarboxamides, a burgeoning class of multidentate ligands containing this linkage,

have been isolated using condensation reactions between pyridine dicarboxylic acid or


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the corresponding acid chloride and the appropriate amine. Upon deprotonation of the

carboxamide nitrogen atoms, these centers and the pyridyl nitrogen of the anion

chelate to metal ions. Pyridine dicarboxamide ligands have found use in asymmetric

catalyses, molecular receptors, dendrimer synthesis and platinum(II) complexes with

antitumour properties. The behaviour of pyridine carboxamides towards biologically

relevant d-block metals has been widely investigated. A number of groups are

engaged in the development of ligand systems that contain the pyridine-2,6-

dicarboxamide functionality, within larger ligand frames.

3.2.1.N,N-pyridine-2,6-dicarboxamides

Various polydentate N,N-pyridine-2,6-dicarboxamide ligands and their

complexes are are given in table 1. Mukherjee et al.15(a) have reported the structure of

[Et4N]2 [NiIIL2].H2O, [Et4N] [Ni

IIIL2].H2O and [NiIVL2].0.75H2O (H2L=2,6-bis(N-

phenylcarbamoyl)pyridine, L1). The X-ray structures of these compounds represent

the first crystallographically characterized NiN6 coordination sphere, with a common

pyridine bis-amide ligand. These authors have also described the synthesis, X-ray

structures and spectral, magnetic and redox properties of two novel six-coordinate

low-spin iron(III) and cobalt(III) complexes [Et4N] [FeL2]1.5H2O and [Et4N]

[CoL2]2H2O [L=L1]. These complexes are isomorphous and contain six-coordinated

metal ion bonded by four deprotonated amide nitrogens in equitorial plane and two

pyridyl nitrogens in the axial positions. Using L1 in its deprotonated form as H2L, a

mononuclear Ru(III) complex [Et4N][RuL2].H2O has been synthesized. Structural

analysis reveals that the RuN6 coordination comprises four deprotonated amide-N

species in the equatorial plane and two pyridine-N donors in the axial positions,

imparting a tetragonally compressed octahedron around Ru(III).22

Mascharak et al.10,12,17 and Mukherjee et al.15(b) have also studied the

complexation behaviour of a few pentadentate ligands (L2 and L3) along with

tridentate ligand (L1). These ligands have been successfully employed to prepare

highly stable low-spin iron(III) and cobalt(III) complexes. The deprotonated dianionic

ligands L1 and L3bind Cu(II), Co(II) and Co(III) in a pentadentate fashion with five

nitrogens. Quite in contrast, iron(III) gave the bis complex [Fe(Py3P)2]- in which two

Py3P2- ligands are coordinated to Fe(III) via the Namide-Npyridine-Namide portion of each

ligand. The authors failed to isolate a complex in which the nitrogen of the pendant


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Table 1: Few N,N-pyridinedicarboxamide pyridine dicarboxamide polydentate

ligands discussed in literature

Ligand Abbreviation

(ligand number)

Complexing

Metal

Reference(s)

N

NH HN

O O

H2L , (L1) Ni(II),

Ni(III),

Ni(IV),

Fe(II),

Co(III)

11,14,15

N

NH HNCH2

O O

H2C

H2C

CH2

N N

Py3PH2 , (L2) Fe(III),

Co(III),

Cu(II)

10,12b,13,

17, 18

N

NH HNCH2

O O

H2C

N N

MePy3PH2 , (L3) Fe(III) 12b

N

NH HNCH2

O O

CH2

H2C

CH2

N N

N N

PyPzPH2 or HAPH

, (L4)

Co(III),

Cu(II)

13,21,17


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N

NH HNCH2

O O

CH2

H2C

CH2

N NH2N

PMAH , (L5) Cu(II) 17

N

NH HN

O O

N NH3C CH3

H2LMe2 , (L6) Ru(II) 19

N

NH HN

O O

NN

(L7) Cu(II),

Ni(II),

Co(II),

Zn(II)

16,20

pyridine rings would coordinate to Fe(III). Complexation of Fe(III) with ligands

L2and L3 clearly suggested the exceptional stability provided by four carboxamido

nitrogens to the Fe(III) center which outweighed the chelate effect of the pentadentate

ligands.12(b) In a similar effort Mascharak et al.17 synthesized a designed dipeptide

ligand N,N-bis[2-(2-pyridyl)ethyl]pyridine-2,6-dicarboxamide (Py3PH2), L2 and

reported the synthesis, structure, and properties of [Cu(Py3P)], which was the first

example of a dipeptide complex of copper comprising a [CuN5] chromophore. These

authors also compared the structural and spectroscopic parameters of the distorted

square pyramidal complex [Cu(Py3P)] with those of two other Cu(II) complexes,

namely, [Cu(HAPH)][ClO4].1.6H2O, (HAPH, L4) and [Cu(PMA)]X, (X=ClO4-, BF4-)


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(PMAH, L5) that contains [CuN5] chromophore of similar geometry and one

deprotanated amido N donor in the first coordination sphere. The Cu(II) ion in

[Cu(Py3P)] is coordinated to five N atoms and the geometry around Cu is distorted

square pyramidal. Two deprotonated amido nitrogens and two nitrogens from the

pyridine rings reside in the basal plane while one pyridine nitrogen occupies the axial

position. Comparison of the structural features of [Cu(Py3P] with Cu(PMA) reveal

that the geometry around copper in [Cu(Py3P] is more distorted. Webb et al. have

investigated the reaction of N,N-bis(6-methyl-2-pyradyl)-2,6-pyridinedicarboxamide,

H2LMe2, L6with RuCl2(PPh3)2 which gave the complex [RuCl2(PPh3)(LMe2{H}2)].

Its X-ray structure showed that the Ru atom is coordinated to the nitrogen atoms of

the deprotonated amides and the central pyridine. The two pendant pyridines are both

protonated, and form H-bonds to the coordinated chlorine.19

Hiratani et al. synthesized pyridine derivatives containing two 8-

quinolylamino groups for heavy metal ion-chelation. It has been shown in solvent

extraction that 2,6-bis[N,N-(8-quinolyl)aminocarbonyl]pyridine, L7 can extract

only Cu(II) with excellent selectivity and efficiency from the aqueous phase (ph=

6.2) containing Cu(II), Ni(II), Co(II) and Zn(II) into chloroform phase.40 The

deprotonated amide groups coordinate readily to metal ions through the amide -N

atom, thus forming a stable delocalized electronic system. These authors have

concluded that coordination of a protonated amide group occurs almost universally

through the amide -O atom. The N-substituted picolinamides have been found to

coordinate to transition metal ions both as bidentate ligands (through ring-N and

amide-O) for 1: 2 complexes and as terdentate ligands in (1:1) complexes with

coordination through the deprotonated amide-N.20

Pyrazoles have also been used as N-donor ligands to bind metal ions. Their

use in bioinorganic chemistry is prompted by the fact that pyrazoles are potential

imidazole mimics and hence can serve in the development of ligand systems that

resemble active sites of metalloenzymes. During the past 25 years, numerous

complexes in which transition metal ions in different oxidation states are bonded to

2-pyrazole nitrogen have been isolated and structurally characterized. A close

scrutiny of the literature reveals that several Co(II) complexes with ligated 2-

pyrazoles have been reported and most of these complexes have been characterized

by X-ray crystallography. Quite in contrast, only a few cases are known in which


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coordination of a 2-pyrazole nitrogen to Co(III) centers has been implicated.

Mascharak et al.21 have synthesized three Co(III) complexes using a

potentially pentadentate ligand PyPz2PH2, L4 (PyPz2=N,N-bis[2-(1-

pyrazolyl)ethyl]pyridine-2,6-dicarboxamido) employs the pyridine nitrogen, twodeprotonated amido nitrogens, and two 2-pyradole nitrogens to bind cobalt in

[Co(PyPz2P)]. The sixth coordination site is either occupied by water molecule or

hydroxo-group.

3.2.2. N,N,N',N'-pyridine-2,6-dicarboxamides

Management of high level liquid waste has been the major concern of the

utilization of nuclear energy. One of the main problems of the treatment of such

waste concerns the selective separation by liquid-liquid extraction of trivalentactinide elements from highly acidic conditions with a huge amount of fission

products, such as trivalent lanthanide cations. Musikas was the first to rationalize

the approach of An(III)/Ln(III) separations using soft donor extracting agents.

Several tri-coordinated N-donor ligands were studied for the separation of actinide

from lanthanide trivalent cations. These studies also revealed differences of

separation factors inside the lanthanide(III) family, which are higher for the heavier

trivalent cations. Ligands possessing N and O donor atoms on their complexationsite, like the pyridinedicarboxamide (PDA) molecule, may also be good candidates

for such separation.

N

O O

RNN

R

R' R'

R=R'=H, H4PDA

R=R'= Me, Me4PDA

R=Me, R'=Ph, Ph2Me2PDA

R=R'=Ph, Ph4PDA

The wrapping of three tris-coordinated ligands offers to the complex a good

protection for the trivalent cations from solvation. Structures with

pyridinedicarboxylates as ligands have been found in the Cambridge

Crystallographic Structual Database. In most of these structures, the cation is

coordinated by three ligands in a tris-tridentate way. Structural and electronic

studies have been carried out on trivalent lanthanide cations (La III to LuIII) with

tetraethyl-PDA complex, and give relatively good stability constants in acetonitrile.


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In the way, the PDA molecule is promising in its use in such separation processes.

But in liquid-liquid extraction chemistry, the role of the substituent is very

important on the solubility and extraction factor of the ligan. In such a case, the

presence and the chemical properties of the substituent may also influence the

coordination of the cation by the ligand, in the structure and in the interaction

energy. In order to solve this question from a theoretical approach, taking in

account the electronic reorganization effects (charge transfer and polarization) upon

the ligand due to the high charge of the cation, the most appropriate methods to

describe the LnIII-PDA interaction come from quantum mechanics (QM)

simulations. Previous theoretical studies have been done on complexes with similar

methods and ligands. In particular, the influence on the LnIII-PDA interaction of

H4PDA, Me4PDA , Ph2Me2PDA, and Ph4PDA, substituents placed on the amidic

nitrogens of the PDA is discussed.33 In their study, Dobler et al.33 described

LnIII(PDA) complexes for the LaIII, EuIII, and LuIII cations placed, respectively at

the beginning, the middle and the end of the lanthanide family. The subject of their

study was the following: firstly, the ligands were compared in their isolated

geometry in vacuum; secondly, LaIII(H4PDA) complex was compared with

EuIII(H4PDA) and LuIII(H4PDA) complexes, and finally discussed the influence of

the R substituents on the cation-ligand interaction.

Quantum chemical simulations have been performed on LnIII (pyridine-

dicarboxamide) complexes in order to describe the interaction between the lanthanide

cations (LaIII, EuIII, and LuIII) and the ligand (R2R'2PDA). In particular, the influence

of substituents R (R2R'2 =H4, Me4, Ph2Me2, Ph4) on the interaction between the cation

and the R4PDA ligand has been discussed. The substitution of H 4 by Me4 stabilizes

the LnIII-ligand interaction of 33 kcal mol-1 (LaIII) to 38 kcal mol-1 (LuIII), and the Ph4

substitution by 69 kcal mol-1 (LaIII) and 75 kcal mol-1 (LuIII). Although the LnIII

(H4PDA)1 and LnIII (Ph4PDA)1 complexes have a C2v, symmetry, the four methyl

substituents reduce the symmetry of the complex to Cs, placing the cation out of the

central pyridyl plane. These results also emphasize that, for a given ligand, the

lutetium complexes are more stable than the lanthanum complexes, which can be

linked to the increasing experimental separation factors inside the lanthanide family.

Garcia-Lozano et al.32(a) have reported the crystalline and molecular structure and the

electronic properties of a copper(II) bis complex with N,N,N,N-


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tetraethylpyridinedine-2,6-dicarboxamide (O-deap), a potentially tridentate ligand

capable of bonding through two amide groups and the pyridine nitrogen. The

compound [Cu(O-deap)2][PF6]2 is built up of cationic [Cu(O-deap)2]2+ and PF6

groups. The geometry around copper is distorted octahedral. They have also reported

the synthesis and spectroscopic characterization of bis complexes of several other

divalent 3d cations with O-deap. In these systems, as well as in the tris complexes of

trivalent ions [Ln(O-deap)3](ClO4)3,32(a) the spectroscopic data suggest that the O-deap

molecules behave as tridentate ligands having ONO sets of donor atoms.

NC C

O

NN

O

H5C2

H5C2

C2H5

C2H5

O-deap

Diverse complexes of divalent Co, Ni and Cu and the ligands N,N,N,N-

tetramethylpyridine-2,6-dicarboxamide (tpda), N,N-dimethyl-N,N-diphenyl-

pyridine-2,6-dicarboxamide (dpda) and N,N,N,N-tetraisopropylpyridine-2,6-

dicarboxamide (ppda) have been investigated in order to illustrate the properties of

these ligands.32(b) The complexes [M(ppda)2](ClO4)2 (M= Co, Ni and Cu) and

[Co(ppda)2](ClO4)2(acet) (acet = acetone), had distorted octahedral chromophores.

The distortion is most pronounced in [Cu(ppda)2](ClO4)2. This severe distortion is not

present in octahedral [Ni(tpda)2](ClO4)2. The reaction of NiBr2 with dpda produced

two complexes; green NiBr2(dpda) was isolated at room temperature while red

NiBr2(dpda) was obtained from hot solution. Characterization data indicated

NiBr2(dpda) to have penta-coordinate chromophore. The Ni(II) ion has a distorted

square pyramidal enviorment with the donor atoms of dpda and a bromine atom

occupying the apical position. The coordination spheres of the crystalline complexes

NiBr2(dpda)(dmp) (dmp = 2,2-dimethoxypropane) and NiBr2(dpda)(acet) are

analogous to those ofNiBr2(dpda) andNiBr2(dpda), respectively.

Bis-complexes of Co(II), Ni(II) and Cu(II) with O-deap (N,N,N,N-

tetraethylpyridine-2,6-dicarboxamide) having the general formula [M(O-deap)2]X2

(X= ClO4, PF6) have been synthesized and characterized by i.r, electronic and e.p.r.

spectra.32(c) The spectroscopic results suggest that the O-deap acts in these compounds


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as a tridentate ligand through two oxygen atoms and a nitrogen (from pyridine ring)

atom. A distorted octahedral environment (MN2O4 chromophore) for the metal atoms

is inferred from the spectroscopic data.

Preez et al. have reported the synthesis of tpda and its complex with CuCl2.32(b)

In CuCl2.tpda, the coordination number around copper is five and its geometry is

square pyramidal. The carbonyl oxygen, pyridine nitrogen and one chlorine atom are

coplanar, and one chlorine atom is situated at the apex of the pyramid. The

dicarboxamido nitrogen of the fully substituted ligand N,N,N',N'-tetraethylpyridine-

2,6-dicarboxamide (O-deap), have been shown to be inactive as donors. In Cu(II)

complexes with N,N,N,N-tetraethyl pyridinedicarboxamide, [Cu-(O-deap)Cl2] I and

[Cu(O-deap)Cl(ClO4)] II, the ligand O-deap coordinates to the metal ion in tridentate

mode using the two carbonyl oxygens and pyridine nitrogen atom to form square

pyramidal geometries.23 Similar conclusion had been drawn by Soundarajan et al.32(d)

based on spectroscopic data on complexes of lanthanide perchlorate with O-deap.

Possibly, the concomitant hindered C-N bond rotation renders the amide nitrogens

almost inactive. Kapooret al. have shown that O-deap can act both as a tridentate as

well as a bidentate ligand.27,28

I II

The ligand N,N,N,N-tetraethylpyridine-2,6-dicarboxamide (O-deap) reacts

with CoX2 (X = PF6; BF4

; ClO4 and NO3

) salts to give the octahedral complexes,

[Co(O-deap)2(CH3CN)](PF6)2 and [Co(O-deap)2(H2O)2](X)2H2O. The composition

of the product is dependant upon the nature of the reaction medium. The complex

[Co(O-deap)2(CH3CN)](PF6)2 contains ON and ONO bonded O-deap ligands in

bidentate and tridentate coordinating modes, in which the coordination sphere of the

Co2+ ion is completed by attachment of an acetonitrile molecule. The Compounds

[Co(O-deap)2(H2O)2](X)2H2O with X = PF6 and ClO4

have identical cationic

species [Co(O-deap)2(H2O)]2+ in which Co2+ is bonded by O-deap ligands in a


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bidentate ON mode. As O-deap is coordinating in a bidentate mode, the two

coordinating water molecules are placed cis to each other in the octahedral

arrangement.28

The reactions of anhydrous copper(II) chloride with NaX (1:1 or 1:2) and AgX

(1:2) containing appropriate N,N,N,N-tetraalkylpyridine-2,6-dicarboxamides (O-

daap) in CH3CN yield monosubstituted 5-coordinated [Cu(O-deap)Cl(CF3SO3)] III,

[Cu(O-dpap)Cl(ClO4)] IV, [Cu(O-dbap)Cl(ClO4)] V, and 6-coordinated [Cu(O-

dpap)(CF3SO3)2]. H2O VI complexes, respectively ( X = -OClO3 and -OSO2CF3).29

III IV

V VI

The reactions of M(NO3)2xH2O [M = Co(II), Ni(II) and Cu(II)] with the appropriate

N,N,N,N-tetraalkylpyridine-2,6-dicarboxamide (O-daap) ligand in CH3CN yield

[Co(O-dmap)(NO3)2] VII, [Co(O-deap)(NO3)2] VIII, [Co(O-dpap)(NO3)2] , [Ni(O-

dmap)(H2O)3](NO3)2] IX, [Ni(O-deap)(H2O)2(NO3)](NO3)] X, [Cu(O-deap)(NO3)2],and [Cu(O-dpap)(NO3)2].

30 X-ray crystal structures of these complexes reveal that O-


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daap ligands coordinate to each metal ion in a tridentate coordination mode O~N~O

with nitrate group playing a vital role in molecular and crystal structures of all the

complexes. The coordination geometry in the two Co(II) complexes is approximately

pentagonal bipyramidal wherein the nitrate groups are bonded in a slightly

unsymmetrical bidentate chelating mode. The [Ni(O-dmap)(H2O)3](NO3)2 and

[Ni(O-deap)(H2O)2(NO3)](NO3) complexes exhibit octahedral geometry, the former

contains uncoordinated nitrate groups while the latter complex has one nitrate group

coordinated to metal ion in a unidentate fashion and the other nitrate group is outside

the coordination sphere. The Cu(II) in [Cu(O-dpap)(NO3)2] occupies a distorted

square pyramidal geometry and is linked to two nitrates in unidentate mode, although

one of the nitrate group is also involved in a weak interaction with the metal ion

through its other oxygen atom

VII VIII

IX X


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Reaction of anhydrous copper(II) chloride with N,N,N'N'-tetraisopropylpyridine-2,6-

dicarboxamide (O-dpap) leads to the synthesis of [Cu2Cl4L2]2CH3CN, XI.31 The

ligand coordinates to each Cu(II) ion in a bidentate fashion through pyridine nitrogen

and only one of the carbonyl oxygen atoms of the amide group rather than using

tridentate coordination mode O-N-O. The structure can be described as a dichloro

bridged centrosymmetric dimer where the two copper centers are pentacoordinated in

a square pyramidal fashion. The two pyramids are arranged sharing a basal edge with

their apical positions oriented opposite to each other.

XI

3.2.3. N,N,N',N'-pyridine-2,6-dithiocarboxamide

In order to investigate the effect of change in the donor set of atoms in the two

side arms on the coordination geometry adopted by the complexes, studies werecarried on N,N,N,N-tetraalkylpyridine-2,6-dithiocarboxamides (S-dapt) ligands

(Scheme 1).24-27 Crystal structures of Cu(II) complexes with S-dept and O-deap

ligands have shown important differences. Cu2Cl2(-S-dept)2][Cu2Cl4(-Cl)2], XIIis a

tetranuclear copper(II) complex, formed by a cationic [Cu2Cl2{-(S-dept)}2]2+ and an

anionic dinuclear complex [Cu2Cl4(-Cl)2]2-.25 The complex [Cu2(-Cl)2(S-

dept)2][CuCl3(EtOH)]2 is prepared in a similar manner as Cu2Cl2(-S-

dept)2][Cu2Cl4(-Cl)2] but using ethanol instead of acetonitrile. It consists of a

cationic binuclear moiety, [Cu2(-Cl)2(S-dept)2]2+, and two close anionic species with


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formula [CuCl3(EtOH)]- conforming a pseudodinuclear unit. In the cationic fragment,

each copper atom is pentacoordinated by the pyridilic N atom and the two S atoms of

the S-dept ligand, as well as by two chloro ligands acting in a bridging mode. On the

other hand, CuCl2(O-deap), II is a five coordinated trigonally distorted rectangular

pyramidal complex.23 The crystal and molecular structure of a 2:1 complex of CoCl2

with S-dept has been determined. The compound is built up from [Co(S-dept)(Cl)]

and [Co2Cl4(2-1)Cl)2]2 units, XIII.24

XII

XIII

The systematic investigation of electronic effects on the coordination geometry of

nickel(II) thiocyanate complexes with the tridentate N,N,N,N-tetraethylpyridine-2,6-

dithiocarboxamide (S-dept) and N,N,N,N-tetraethylpyridine-2,6-dicarboxamide (O-

deap) ligands shows a significant change in the geometry of the metallic site. Theircomplexes conform to composition [Ni2(-NCS)2(S-dept)2(NCS)2] and [Ni(NCS)2(O-


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deap)(CH3CN)].CH3CN, respectively.27 In the crystal lattice, complex [Ni2(-

NCS)2(S-dept)2(NCS)2] exists as a centrosymmetric dimer in which the dinuclear

core is bridged by two N-bonded thiocyanate groups. The near octahedral geometry of

the nickel atom is achieved through the two bridging N atoms of the thiocyanate

groups, three SNS donor atoms of the ligand S-dept and through the terminal nitrogen

atom of a non-bridging thiocyanate moiety. This complex presents the first example in

literature with the highest asymmetry in N-bridging thiocyanato ligands. In

[Ni(NCS)2(O-deap)(CH3CN)].CH3CN, the Ni ion is coordinated to ONO donor set of

atoms of O-deap and two N-bonded NCS terminal groups. The sixth coordination site

is completed by the N atom of an acetonitrile molecule. The five-coordinate Co(II)

complexes from reactions of CoX2 with S-dept (X = Br, XIV; I, XV; SCN, XVI) are

all distorted square pyramids, with an electronic configuration of high-spin. 26(a)

XIV XV

XVI

PRESENT WORK

Review of the literature on the preceding pages shows enough evidence that chemists

have been mainly drawn towards the ligational behaviour of N,N -pyridine-2,6-

dicarboxamides and N,N,N,N-pyridine-2,6-dicarboxamides. As we move towards


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corresponding thio derivatives (N,N,N,N-pyridine-2,6-dithiocarboxamides) the

number of complexes investigated become less. It is quite evident that ligands

containing thiocarboxamide groups provide an excellent opportunity to tune the

desired stereochemistry at the metal center without changing the number of donor

atoms. The ligand behavior of pyridine-2,6-dithiocarboxamides, appears to be

interesting because they contain both a soft sulphur donor site and a hard nitrogen

donor site and the presence of these disparate sites may lead to aggregation of soft and

hard metal centers. Therefore work done in chapter 3 of PART-II consists of

multianionic chelating ligands N,N,N,N-tetraalkylpyridine-2,6-dithiocarboxamides

and their complexes with transition metals such as Co(II) and Ni(II). The connection

of symmetrical tertiary thioamide side arms at the 2- and 6- positions of the central

pyridine ring provide a collection of versatile tridentate binding units. These ligands

have been designed to enhance steric interactions and enforce interesting distortions in

geometries or produce unexpected bonding modes on the metal center or constituent

amide groups. These ligands may prove instrumental in developing an understanding

of their influence on novel geometric and electronic properties imparted onto different

metal centers.

Therefore, in the present work six new N,N,N,N-tetraalkylpyridine-2,6-

dithiocarboxamide (S-dapt) ligands based crystal structures are reported (three ligands

(S-dept, S-dbpt, S-dphpt, Scheme 1) and three complexes [Co(S-dbpt)Br2] (1),

[Co(S-dppt)(SCN)2] (2) & [Ni(S-dept)2](ClO4)2H2O (3). All the ligands and

complexes have been well identified and characterized through melting points,

elemental analysis, IR and UV-Vis spectroscopy, conductivity measurements, thermal

analysis (DT-TGA) and single crystal X-ray crystallography.

3.3 Experimental

3.3.1. Materials

Solvents and other reagents were dried using standard techniques as described.45 All

reactions leading to the formation of ligands were carried out in anhydrous solvents

under dry N2 atmosphere.

3.3.2. Preparation of ligands

(a) N,N,N,N-tetraethylpyridine-2,6-dithiocarboxamide (S-dept) 25,26(a)

Step-I: Preparation of N,N,N,N-Tetraethylpyridine-2,6-dicarboxamide (O-deap):The ligand O-deap23,32(a),32(c) was prepared as per the method described.23 Pyridine-2,6-


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dicarboxylic acid was boiled under reflux with an excess of SOCl2 in order to obtain

the corresponding 2,6-dipiconyl chloride, which was further treated with four

equivalents of Et2NH in Et2O. The Et2NH2Cl was filtered off and O-deap was

obtained as a white crystalline solid on removal of the solvent under vacuum. O-

deap: Colour: White. M.p., 75 C. Anal. Calcd for C15H23N3O2: C, 64.98; H, 8.30 ; N,

14.88 %. Found: C, 64.61 ; H, 7.87 ; N, 14.88 %. 1H NMR (CDCl3): 1.16 (t, 6H,

CH3), 1.25 (t, 6H, CH3), 3.35 (q, 4H, CH2), 3.51 (q, 4H, CH2), 7.57 (q, 2H, py), 7.66

(t, 1H, py). IR (KBr pellet, cm-1): CO: 1640.

Step-II: Preparation of N,N,N,N-tetraethylpyridine-2,6-dithiocarboxamide(S-dept):

The ligand S-dept was pepared as per the method described.25,26(a) A mixture of

diethylpyridine-2,6-dicarboxamide (O-deap) (7.86 g, 0.028 mol) and P2S5 (3.93 g,

0.017 mol) was refluxed in benzene(40 mL) for 8 h. The reaction mixture was filtered

to removeunreacted P2S5. Crude S-dept was obtained by removal of thesolvent under

vacuum. Pure S-dept was obtained as shining yellow crystals on crystallization from

hot EtOH. S-dept: Colour: yellow. Yield: 6.80 g, (77%). M.p., 129-130 C. Anal.

Calcd for C15H23N3S2: C, 58.25; H, 7.44; N, 13.59; S, 20.71%. Found: C, 58.17; H,

7.51; N, 13.52; S, 20.18%. 1H NMR (CDCl3, TMS): 1.19 (t, 6H, CH3), 1.38 (t, 6H,

CH3), 3.42 (q, 4H, CH2), 4.07 (q, 4H, CH2), 7.42 (d, 2H, py), 7.74 (t, 1H, py). IR

(KBr pellet, cm-1): 1630 m, 1580 m, 1558 s, 1508 s, 1495 s, 1459 m, 1440 s, 1436 s,

1386 m, 1356 s, 1318 m, 1295 m, 1263 s, 1196 m, 1145 m,1117 m, 1096 m, 1076 s,

990 m, 818 m, 776 m, 723 m, 690 m, 626 m, 558 m, 508 m, 457 m, 418 m.

(b) N,N,N,N-tetraisopropylpyridine-2,6-dithiocarboxamide (S-dppt):26(b) The ligand

N,N,N,N-tetraisopropylpyridine-2,6-dithiocarboxamide (S-dppt) was prepared as per

the method given for S-dept except that N,N,N,N-tetraisopropylpyridine-2,6-

dicarboxamide(O-dpap)26(b),32(b),32(e) was used as the starting materials. The latter

compound was obtained as a white crystxalline solid by using a similar method as

reported for the preparation of O-deap and using the diisopropylamine. O-dpap:

Colour: White. M.p., 152 C. Anal. Calcd for C19H31N3O2: C, 68.40; H, 9.30; N,

12.01 %. Found: C, 68.30; H, 9.10; N, 11.88 %. 1H NMR (CDCl3): 1.05 (d, 12H,

CH3), 1.39 (d, 12H, CH3), 3.55 (hept, 2H, CH), 3.56 (hept, 2H, CH), 7.39 (d, 2H, py),

7.91 (t, 1H, py). IR (KBr pellet, cm-1

): CO:1638. S-dppt: Colour: Yellow. M.p., 142C. Anal. Calcd for C19H31N3S2: C, 62.49; H, 8.49; N, 11.50; 17.53%. Found: C,


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62.46; H, 7.84; N, 11.23 %. 1H NMR (CDCl3): 1.04 (d, 12H, CH3), 1.41 (d, 12H,

CH3), 3.55 (hept, 2H, CH), 3.56 (hept, 2H, CH), 7.39 (d, 2H, py), 7.91 (t, 1H, py). IR

(KBr pellet, cm-1): 1634 s, 1576 m, 1560 m, 1495 s, 1460 m, 1463 s, 1439 m, 1375 s,

1351s, 1340 m, 1312 w, 1231 w, 1188 m, 1147 s, 1117 m, 1082 m, 1060 w, 1038 m,

988 m, 973 m, 898 w, 833 w, 819 w, 799 m, 782 w, 759 m, 634 m, 610 m, 583m.

(c) N,N,N,N-tetraisobutylpyridine-2,6-dithiocarboxamide (S-dbpt):26(b) The ligand

N,N,N,N-tetraisobutylpyridine-2,6-dithiocarboxamide (S-dbpt) was prepared as per

the method given for S-dept except that N,N,N,N-tetraisobutylpyridine-2,6-

dicarboxamide(O-dbap) was used as the starting materials. The latter compound was

obtained as a white crystalline solid by using a similar method as reported for the

preparation of O-deap and using the diisobutylamine. O-dbap: Colour: White. M.p.,

61 oC. Anal. Calcd. for C23H39N3O2: C, 70.95; H, 10.02; N, 10.79. Found: C, 70.68;

H, 9.92; N, 10.60%. 1H NMR (CDCl3, TMS): 0.82 (d, 12H, CH3), 0.94 (d, 12H,

CH3), 1.67 (hept, 2H, CH), 2.04 (hept, 2H, CH), 3.20 (d, 4H, CH2), 3.26 (d, 4H, CH2),

7.49 (d, 2H, py), 7.77 (t, 1H, py). IR (KBr pellet, cm -1): C=O:1640 cm-1. S-dbpt:26(b)

Colour: Yellow. M.p. 87 oC. Anal. Calcd. for C23H39N3S2: C, 65.55; H, 9.26; N, 9.97;

S, 15.20%. Found: C, 66.13; H, 9.80; N, 9.47; 14.76%. 1H NMR (CDCl3, TMS):

0.72 (d, 12H, CH3), 0.94 (d, 12H, CH3), 1.69 (hept, 2H, CH), 2.04 (hept, 2H, CH),

3.20 (d, 4H, CH2),3.26 (d, 4H, CH2), 7.49 (d, 2H, py), 7.28(t, 1H, py). IR (KBr pellet,

cm-1): 1628 s, 1561m, 1465 s, 1341 m, 1279 m, 1246 m, 1192 w, 1150 m, 1116 m,

1078 m, 925 w, 814 m, 788 w, 722 w, 702 w, 639 w, 602 w.

(d) N,N,N,N-tetraphenylpyridine-2,6-dithiocarboxamide (S-dphpt): The ligand

N,N,N,N-tetraphenylpyridine-2,6-dithiocarboxamide (S-dphpt) was prepared as per

the method given for S-dept except that N,N,N,N-tetraphenylpyridine-2,6-

dicarboxamide (O-dphap)33 was used as the starting materials. The latter compound

was obtained as a white solid by using a similar method as reported for the

preparation of O-deap and using the diphenylamine. O-dphap: Colour: White. M.p.,

181 C. Anal. Calcd for C31H23N3O2: C, 79.30; H, 4.90; N, 8.88 %. Found: C, 79.31;

H, 4.90; N, 8.85%. 1H NMR (CDCl3, TMS): 6.96 (m, 12H, ph), 7.22 (m, 8H, ph),

7.29 (d, 2H, py)7.62 (t, 1H, py). IR (KBr pellet, cm -1): CO: 1644. S-dphpt: Colour:

Orange. M.p., 200 C. Anal. Calcd for C31H23N3S2: C, 74.25; H, 4.50; N, 8.38; S,12.77 %. Found: C, 73.68; H, 3.97; N, 7.98; S, 12.15 %. 1H NMR (CDCl3, TMS):


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6.98 (m, 12H, ph), 7.23 (m, 8H, ph), 7.28 (d, 2H, py), 7.63 (t, 1H, py). IR (KBr pellet,

cm-1): 1587 m, 1565 m, 1461 s, 1304s , 1280 m, 1226 m, 1157 m, 1093 m, 1073 m,

1051 m, 1023 m, 803 m, 771 m, 763 m, 744 s, 722 s, 696 s, 633 m, 594 s.

3.3.3. Preparation of complexes

(a) [Co(S-dbpt)Br2] (1): Anhydrous CoCl2 (4 mmol), dissolved in anhydrous ethanol

(20 mL), was added to a solution of KBr (8 mmol) in ethanol (20 mL). The mixture

was refluxed for about 4 h. The white precipitate of KCl was removed by filtration

and ligand S-dbpt (4 mmol), dissolved in ethanol (20 mL), was added to the solution.

After refluxing for about 5 h, complex 1 was obtained as dark green solid on keeping

at room temperature. The complex was crystallized from acetonitrile by slow

evaporation at room temperature. Colour: Dark Green. M.p., 235 C. Anal. Calcd. for

C23H39Br2CoN3S2: C, 43.12; H, 6.14; N, 6.56; S, 10.01. Found: C, 42.92; H, 5.94; N,

6.42; S, 9.88%. Molar conductance (-1 cm2 mol-1): 30 (CH3CN) (expected ranges for

1:1 and 1:2 electrolytes in CH3CN are 120-160 and 220-380, respectively). IR (KBr

pellet, cm-1): 1615 s, 1447 m, 1368 w, 1311w, 1248 w, 1164 w, 1081 w, 820 m, 643

w, 498 w, 468 w.

(b) [Co(S-dppt)(SCN)2] (2): Anhydrous CoCl2 (3 mmol), dissolved in anhydrous

ethanol (20 mL), was added to a solution of KSCN (6 mmol) in ethanol (20 mL). The

mixture was refluxed for about 4 h. The white precipitate of KCl was removed by

filtration and ligand S-dppt (4 mmol), dissolved in ethanol (20 mL), was added to the

solution. After refluxing for about 4 h complex 2 was obtained as green solid on

keeping at room temperature. The complex was crystallized from acetonitrile by slow

concentration at room temperature. Colour: Green. M.p., 215 C. Anal. Calcd. For

C21H31CoN5S4:C, 46.65; H, 5.78; N, 12.95; S, 23.72. Found C, 46.34; H, 5.66; N,

12.82; S, 23.68. Molar conductance (-1 cm2 mol-1): 32 (MeCN) and 28 (DMF). IR

(KBr pellet, cm-1): 2055 s, 1623 m, 1582 w, 1541 m, 1486 m, 1401 m, 1305 m, 1219

w, 1178 w, 1154 m, 1056 w, 1012 w, 989 w, 821 w, 662 w, 440 w.

(c) [Ni(S-dept)2](ClO4)2H2O (3): Anhydrous NiCl2 (4 mmol), dissolved in

anhydrous ethanol (20 mL), was added to a solution of NaClO 4 (8 mmol) in ethanol

(20 mL). The mixture was refluxed for about 4 h. The white precipitate of NaCl wasremoved by filtration and ligand S-dept (4 mmol), dissolved in ethanol (20 mL), was


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added to the solution. After refluxing for about 4 h complex 2 was obtained as reddish

brown solid on keeping at room temperature. The complex was crystallized from

acetonitrile by slow concentration at room temperature. Colour: Reddish brown. M.p.,

255 C. Anal. Calcd. For C30H48N6S4NiO9Cl2: C, 40.28; H, 5.41; N, 7.68; S, 14.34.

Found C, 40.44; H, 5.48; N, 7.48; S, 14.28. Molar conductance (-1 cm2 mol-1): 254

(CH3CN) (expected ranges for 1:1 and 1:2 electrolytes in CH3CN are 120-160 and

220-380, respectively). IR (KBr pellet, cm-1): 1581w, 1511 m, 1451 w, 1379 w, 1262

m, 1094 s, 1145 w, 1094 s, 915 w, 808 w, 670 w, 623 m; OH : 3425 br; ClO4-: 1094 s

and 623 m.


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Chemdraw figures of Ligands and complexes

* ipr = isopropyl; ibu = isobutyl; ph = phenyl

(a) N,N,N,N-tetraisobutylpyridine-2,6-dithiocarboxamide (S-dbpt)

N

SS

N(ipr)2(ipr)2N

(b)N,N,N,N-tetraisopropylpyridine-2,6-dithiocarboxamide (S-dppt)

N

SS

N(ibu)2(ibu)2N

(c)N,N,N,N-tetraphenylpyridine-2,6-dithiocarboxamide (S-dphpt)

N

SS

N(ph)2(ph)2N

ipr = isopropyl; ibu = isobutyl; ph = phenyl

(a) [Co(S-dbpt)Br2], (1) (b)[Co(S-dppt)(SCN)2] (2)

N

SS

N(ibu)2(ibu)2N

Co

Br Br

N

SS

N(ipr)2(ipr)2N

Co

SCN NCS


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(c)[Ni(S-dept)2](ClO4)2H2O (3)

N

S S

(ethyl)2N N(ethyl)2

N

SS

N(ethyl)2(ethyl)2N

Ni

2ClO4

H2O


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IR spectra of complexes 1-3


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3.4 Physical methods

Elemental analyses (C, H, N) were performed on a Perkin-Elmer model 2400 CHN

analyzer. IR spectra were recorded as KBr pellets on a Perkin- Elmer RX-1 FTIR

spectrophotometer. Thermal analyses were carried out on a Shimadzu-DTG 60

analyser. 1H NMR spectra of ligands were recorded on 300 MHz JEOL FT NMR

spectrometer with TMS as the reference compound. UV-Vis spectra were recorded on

Shimadzu Pharmaspec UV-1700 UV-vis spectrophotometer. Molar Conductance

values of millimolar solutions of the complexes were measured on a conductivity

bridge-Digital Conductivity Meter CC 601.

3.5 X-ray Crystallography

Crystallization of ligands (S-dept, S-dbpt, S-dphpt) and complexes (1-3) by

very slow evaporation of their saturated solutions in ethanol and acetonitrile,

respectively at room temperature yielded suitable single crystal for X-ray analysis.

The data for S-dept, S-dbpt, and complexes 1-3 were collected at 298 K on a Siemens

P4 Single crystal X-ray diffractometer using the XSCANS package.46(a) The data were

collected by the 2 scan mode with a variable scan speed up to a maximum of 2 =

60 using graphite monochromatised Mo-Kradiations ( = 0.71073 ). To monitor

the stability of the crystal 3 standard reflections were measured after every 97

reflections. The data for ligand S-dphpt was collected on a Bruker AXS KAPPA

APEX-II CCD diffractometer (Monochromatic Mo-K radiation) equipped with

Oxford cryosystem 700Plus. Unit cell refinement, data reduction and

integration were performed by SAINT V7.68A (Bruker AXS, 2009) and data

scaling was performed by SADABS V2008/1 (Bruker AXS).The data were corrected

for Lorentz and polarization effects. The structures were solved by direct methods

using SIR9746(b) and refined by Full-matrix least-squares refinement techniques onF2

using SHELXL-9746(c) in the WINGX package46(d) of programs. All non hydrogen

atoms were refined anisotropically. Hydrogen atoms of uncoordinated water molecule

in complex 3 were located through difference Fourier calculations. All other hydrogen

atoms were attached geometrically riding on their respective carrier atoms with U iso

being 1.5, 1.2 and 1.2 times the Uiso of their carrier methyl, methylene and aromatic

carbon atoms, respectively. Experimental details of the X-ray analyses of ligands aregiven in Table 2 and that of complexes 1-3 are provided in Table 3.


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Table 2: Crystal data and structure refinement for ligands S-dept, S-dbpt and S-dphpt.

Ligand S-dept S-dbpt S-dphpt

Empirical formula C15H23N3S2 C23H39N3S2 C31H23N3S2

Formula weight 309.48 421.69 501.64

T[K] 296(2) 296(2) 296(2)

Crystal system, Monoclinic Monoclinic Monoclinic

Space group P21/n P21/c P21/c

a[] 6.849(5) 11.163(5) 16.0429(12)

b[] 12.140(4) 17.901(4) 8.0781(6)

c[] 20.658(3) 13.490(3) 21.3781(17)

[] 90.460(5) 101.670(5) 109.000(4)

Volume [3] 1717.6(14) 2640.0(14) 2619.6(3)

Dcalcd. (Mg/m3) 1.197 1.061 1.272

Absorption coefficient [mm-1] 0.305 0.214 0.228

Reflections collected / unique 3488 / 3199 5157 / 4881 33318 / 5282

[R(int) = 0.0189] [R(int) = 0.0594] [R(int) = 0.0281]

Data / restraints / parameters 3199 / 0 / 181 4881 / 0 / 253 5282/ 0 / 325

Final R indices [I>2sigma(I)] R1 = 0.0449 R1 = 0.0990, R1 = 0.0539

wR2 = 0.1435 wR2 = 0.2214 wR2= 0.1645

R indices (all data) R1 = 0.0793 R1 = 0.2755, R1 = 0.0786

wR2 = 0.1803 wR2 = 0.3158 wR2= 0.1888

CCDC No. 810473 810472 810474


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Table 3: Crystal data and structure refinements for complexes 1, 2 and 3.

Complex 1 2 3

Empirical formula C23H39Br2CoN3S2 C21H31CoN5S4 C30H48Cl2N6NiO9S4

Formula weight 640.44 540.68 894.59

T[K] 296(2) 296(2) 295(2)

Crystal system, Monoclinic Monoclinic Triclinic

space group P21/n P21/c P-1

a[] 10.880(4) 11.612(2) 11.911(5)

b[] 18.808(5) 14.651(4) 12.288(3)

c[] 15.055(3) 16.524(3) 15.490(5)

[] 90 90 91.691(4)

[] 108.801(5) 105.130(10) 104.711(5)

[] 90 90 109.442(5)

Volume [3] 2916.3(14) 2713.7(10) 2051.6(12)

Dcalcd. (Mg/m3) 1.459 1.323 1.448

Absorption coefficient [mm-1] 3.487 0.958 0.862

Reflections collected / unique 5734 / 5437 5282 / 5024 8052 / 7637

[R(int) = 0.0656] [R(int) = 0.0308][R(int) = 0.0253]

Data / restraints / parameters 5437 / 0 / 280 5024 / 0 / 280 7637 / 0 / 469

Final R indices [I>2sigma(I)] R1 = 0.0544 R1 = 0.0390 R1 = 0.0657,wR2 = 0.1426 wR2 = 0.1148 wR2 = 0.1594

R indices (all data) R1 = 0.1037 R1 = 0.0504 R1 = 0.1048

wR2 = 0.1617 wR2 = 0.1233

wR2 = 0.1852

CCDC No 810475 810476 810477


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3.6. Results and discussion

3.6.1. IR and UV-Vis studies

The complexes 1-3 are obtained in good yield by mixing equimolar amounts ofS-dapt

ligands and the appropriate metal salts in anhydrous EtOH. The compounds are

readily soluble in common organic solvents such as such as chloroform, CH2Cl2, and

1,2-dichloroethane and are air and moisture stable. The relatively high conductivity

value for complex 3 as compared to the other two complexes clearly shows the

presence of ionic moieties in 3. The expected positive shifts in position and changes in

intensity of the principal IR bands of the pyridine ring are interpreted in favour of the

coordination of the ligands through the pyridine ring nitrogen atom47(a) in the

complexes. The presence of a band at 2055 cm1 in 2 is consistent with N-bonded

terminal thiocyanato groups.47(b)-(e)The corresponding (CS) modes are obscured by

ligand vibrations.47(f),(g) The presence of ionic perchlorate in complex 3 is indicated by

characteristic band at 1094 cm-1 and a weaker absorption at 623 cm-1 while a broad

band at 3425 cm-1 is attributed the the presence of uncoordinated water molecules.

The UV-Vis spectrum of complex 1 was investigated in methanol at room

temperature (Figure 1). In the visible region a band is observed with a maxima around

680 nm (14,705 cm-1; = 391 M-1 cm-1) along with a shoulder at 600 nm (16,666

cm-1; = 279 M-1 cm-1). This multiple structured feature is the result of overlapping

between two neighboring bands in this region and is characteristic of five coordinated

Co(II) square pyramidal complexes48(b) Similarly, In case of complex 2, a band around

690 nm (14,492 cm-1; = 350 M-1 cm-1) is observed with a shoulder around 670 nm

(14,925 cm-1; = 330 M-1 cm-1) in the visible region corresponding to the d-d

transitions (Figure 2).

Generally three spin allowed transitions from 3A2g3T2g,

3A2g3T1g(F) and

3A2g3T2g(P) falling within the range 800-1400 nm, 550-900 nm and 380-530 nm

respectively, are observed for Ni(II) octahedral complexes.48(b) The electronic

absorption spectrum of complex 3 (Ni(II) complex) were recorded in acetonitrile

solvent (Figure 3). In the visible range, two broad peaks were observed: one broad

band in the region 580-620 nm with a maxima at 600 nm (16,616 cm-1; = 96 M-1 cm-

1) and the other one as a shoulder at 420 nm (23,809 cm-1; = 380 M-1 cm-1) which

can be assigned to spin-allowed d-d transition bands of 3A2g 3T1g(F) and

3A2g


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3T1g(P), respectively. The third band corresponding to

3A2g

3T2g (800-1400 nm)

may have been shifted to the far infrared region beyond 1100 nm.

0

0.1

0.2

0.3

0.4

0.5

300 500 700 900 1100

nm

abs.

Figure 1: Electronic spectrum of complex 1 in acetonitrile showing d-dabsorption band inthe visible region.

Figure 2: Electronic spectrum of complex 2 in acetonitrile showing d-dabsorption band inthe visible region.

0

0.2

0.4

0.6

0.8

300 500 700 900 1100nm

abs.

Figure 3: Electronic spectrum of complex 3 in acetonitrile.


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3.6.2. Thermal studiesThe thermogravimetric analysis performed on complex 1 is shown in Figure 4.

The pyrolysis curve shows that the complex is thermally very stable as it stats losing

weight only above 200 C. Beyond this temperature it losses weight very rapidly and

up to 548 C the organic ligand S-dbpt is lost from the complex (Calc. wt. loss,

65.8; obs. wt. loss, 64.7 %). From 550 C onwards till 600 C, the complex loses the

two bromides entities as the total weight loss suffered by the present complex 1 from

beginning to 600 C corresponds to CoO being left as residue in the end (calc. wt.

loss, 24.8; obs. wt. loss, 23.5 %).

Figure 4: Thermogravimetric analysis (TGA) data of complex 1 as a function of temperaturebetween room temperature and 800 C.

The thermogravimetric analyses performed on complex 3 is shown in Figure

5. The percentage weight loss observed (% TGA) is plotted as a function of

temperature upto 600 C.The compound loses weight in two major steps. In the first

step between 130-138 C it undergoes weight loss corresponding to the two watermolecules present in the crystal lattice (obs. wt. loss, 2.2; calc. wt. loss, 2.0 %). The

compound is thermally very stable as even the lattice water is lost only above 130 oC.

This can be explained by having an insight into of the crystal structure of the complex

in which hydrogen bonded water dimers are encapsulated in a cage formed by

extensive intermolecular hydrogen bonding interactions between the complex

molecules and perchlorates. The anhydrous product formed after the removal of

lattice water is stable up to 270o

C. Beyond this temperature, in the second step itloses weight very rapidly up to 300 oC and finally the loss of organic ligand molecules


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along with the two perchlorate moieties gives NiS as the residue at the end at 400 oC

(obs. wt. loss, 88.9; calc. wt. loss, 88.2 %). Thereafter it does not suffer any weight

loss upto 600 oC.

0

20

40

60

80

100

0 100 200 300 400 500 600

Temperature (oC)

%Weight

Figure 5: Thermogravimetric analysis (TGA) data of complex 3 as a function of temperaturebetween room temperature and 800 C.

3.6.3. Molecular and crystal structures of N,N,N,N-tetraalkylpyridine-2,6-

dithiocarboxamide (S-dapt) ligands.

3.6.3(a).N,N,N,N-tetraethylpyridine-2,6-dithiocarboxamide (S-dept)

Figure 6: The ORTEP diagram and labelling scheme used for ligand S-dept. Ellipsoids are

drawn at 50% probability level.


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A search through Cambridge Crystal Structure Database revealed that although the

crystal structures of complexes with ligands S-dept and S-dbpt have been reported

but the molecular and crystal structure investigations of the neat ligands have never

been undertaken. There is no crystal structure report regarding S-dphpt either in any

complex or as an uncoordinated ligand.

Figure 6 shows the ORTEP representation and atom numbering scheme for the

ligand N,N,N,N-tetraethylpyridine-2,6-dithiocarboxamide (S-dept). It consists of a

pyridine ring with two amide side arms at 2 and 6 positions. The amide nitrogen atom

of each side arm is fully substituted with two ethyl groups. The C6-S1 and C11-S2

bond lengths of 1.664(3) and 1.653(3) respectively are in agreement with the

thioamide C=S bond lengths reported in similar ligands.49 Selected bond lengths and

angles are listed in Table 4. The torsional angles of 75.8(3) ( C2-C1-C11-S2) and -

60.5(3) (C4-C5-C6-S1) shows that the thiamide groups are much twisted out from

the plane of the pyridine ring. This may be attributed to the steric factors due to the

presence of ethyl groups on the fully substituted thiamide nitrogen atoms as in case of

pyridine-3-thiamide49(a)-(c) and pyridine-4-thiamide49(c) the corresponding torsional

angles are -35.69 and -36.96 respectively indicating that the thiamide moieties are

twisted out to a less degree from the pyridine ring plane. Furthermore, the C=S

groups are flipped out or in trans orientation with respect to the pyridine nitrogen

atom N1. Upon complexation to metal ion, these C=S groups flip inwards to

coordinate to the metal ion and become cis to pyridine nitrogen atom and thus the

ligand coordinate in a tridentate fashion through the two C=S groups and pyridine

nitrogen atom.

The orientations of the terminal carbon atoms on the fully substituted amide

nitrogen atoms is such that the atoms C8 and C15 are above their corresponding

amide planes while the atoms C10 and C13 are below so as to minimise steric strain

in the structure ( Figure 7).

The C15-H15BS2i (2.879(4) ) and C12-H12BS2i ( 2.850(4) , where i

= x+1,+y, +z) intermolecular hydrogen bonding interaction hold the ligand molecular

together in the crystal lattice to form a one dimensional chain running along a axis.

These parallel chains are in turn interlinked through C4-H4S2ii (2.829(3) , ii = -x-

1/2,+y+1/2,-z+1/2) hydrogen bonding interactions to give rise to a 2D corrugated

sheet parallel to ab plane (Figure 8)


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Figure 7: The terminal carbon atoms bonded to amide nitrogen atoms and their orientationswith respect to the corresponding thiamide planes in S-dept.

Figure 8: The 2D wavy (corrugated sheet) sheet formed through H-bonding interactions in

case of S-dept. The atoms involved in H-bonding are shown in ball and stick mode. The

hydrogen atoms other than involved in H-bonding interactions are omitted for clarity.


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Table 4: Important bond lengths and angles for S-dept, S-dbpt and S-dPhpt

S-dept

S2-C11 1.653(3) S1-C6 1.664(3)N2-C6 1.322(4) N2-C9 1.467(3)

N2-C7 1.477(3) C1)-N3 1.325(4)

C11-C1 1.500(4) N3-C12 1.472(4)

N3-C14 1.479(4) C1-N1 1.342(3)

C1-C2 1.383(4) N1-C5 1.338(3)

N2-C6-S1 124.8(2) C5-C6-S1 116.6(2)

N3-C11-S2 125.4(2) C1-C11-S2 117.3(2)

C6-N2-C9 120.9(2) C6-N2-C7 124.2(2)

C9-N2-C7 114.9(2) N3-C11-C1 117.3(2)

C11-N3-C12 123.6(2) C11)-N3-C14 120.8(2)

S-dbpt

C6-S1 1.660(7) C15-S2 1.654(8)

C1-N1 1.357(9) C5-N1 1.332(9)

C6-N2 1.339(9) C7-N2 1.470(9)

C11-N2 1.461(3) C15-N3 1.327(9)

N3-C15-S2 125.7(7) C1-C15-S2 115.4(6)

N1-C1-C2 121.9(8) N1-C1-C15 115.3(7)

N1-C5-C4 122.5(8) N1-C5-C6 116.3(7)

C4-C5-C6 121.2(8) N2-C6-C5 117.8(6)

N2-C6-S1 125.6(6) C5-C6-S1 116.6(6)

C9-C8-C7 107.4(8) N2-C11-C12 113.7(7)

S-dPhpt

C1-N1 1.333(3) C1-C2 1.380(3)

C5-N1 1.334(3) C5-C4 1.387(3)

C5-C6 1.505(3) C6-N2 1.343(3)


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C6-S1 1.646(2) C7-C12 1.375(4

C7-C8 1.388(3) C7-N2 1.447(3)

N1-C1-C2 123.6(2) N1-C1-C19 115.81(19)

C2-C1-C19 120.4(2) N1-C5-C4 123.5(2)

N1-C5-C6 114.77(19) C4-C5-C6 121.7(2)

N2-C6-C5 116.1(2) N2-C6-S1 125.88(17)

C5-C6-S1 118.06(17) C12-C7-C8 120.8(2)

C12-C7-N2 119.8(2) C8-C7-N2 119.4(2)

3.6.3(b)N,N,N,N-tetraisobutylpyridine-2,6-dithiocarboxamide (S-dbpt)

Figure 9: The ORTEP diagram and labelling scheme used in the structure analysis of ligandS-dbpt. Ellipsoids are drawn at 50% probability level

Figure 9 shows the ORTEP diagram and labelling scheme used in the structure

analysis of ligand N,N,N,N-tetraisobutylpyridine-2,6-dithiocarboxamide (S-dbpt).

The molecular structure ofS-dbpt is essentially the same as for ligand S-dept, only

difference being the nature of alkyl groups on the thiamide nitrogen atoms. In ligand

S-dbpt, there are bulky isobutyl groups on thiamide nitrogen atoms N1 and N2. The

C=S bond length values of are in agreement with the corresponding values in S-dept.

Important bond lengths and bond angles are summarized in Table 4. Slightly high

thermal parameters are observed for terminal carbon atoms attached to thiamide


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nitrogen atoms. Owing to steric factors the thiamide moieties are twisted away from

the pyridine ring plane as indicated by torsion angles of 61.3(3) [C4-C5-C6-S1] and -

65.2(4) [ C2-C2-C15-S2]. The isobutyl groups on the thiamide nitrogens on the side

arms of the pyridine ring are oriented in such a way so as to minimise the streric strain

in the structure. The carbon atoms C8, C9, C10, C21, C22 and C23 are oriented above

their respective thiamide planes while the atoms C12, C13 C14, C17, C18 and C19 lie

below (Figure 10).

The C8-H8S2i (2.896(2) , i = x-1, +y, +z) and C7-H7AS1ii (2.962(3) ,

ii = x, -y+1/2, +z+1/2) intermolecular hydrogen bonding interactions lead to the

formation of a two dimensional sheet structure parallel to ac plane (Figure 11)

Figure 10: The orientation of terminal carbon atoms of the isobutyl groups with respect totheir corresponding thiamide planes in S-dbpt.

Figure 11: The 2D sheet formed through intermolecular hydrogen bonding interactions in S-dbpt.


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3.6.3(c)N,N,N,N-tetraphenylpyridine-2,6-dithiocarboxamide (S-dphpt)

Figure 12: The ORTEP diagram and atom numbering scheme used for ligand S-dphpt.Ellipsods are drawn at 50% probability level.

The molecular structure of ligand N,N,N,N-tetraphenylpyridine-2,6-

dithiocarboxamide (S-dphpt) is similar to S-dept and S-dbpt ligands. Figure 12shows the ORTEP diagram along with numbering scheme used in the structure

analysis of S-dphpt. In this case two phenyl rings are attached to each of the two

nitrogen atoms of the thioamide side arms bonded to pyridine ring at 2 and 6

positions, instead of alkyl groups. The usual twisting of the amide moieties with

respect to the pyridine ring plane to accommodate phenyl rings is more pronounced in

this case as indicated by the torsional angles of -72.5(3) [N1-C1-C19-S2] and -

71.1(2) [N1-C5-C6-S1]. The orientation of the two twisted amide planes is such that

the C=S moieties are almost trans to each other with respect to the pyridine ring

(Figure 13(a)). Important bond lengths and bond angles are summarized in Table 4

To minimise the strain in the structure the two phenyl rings attached to the

thioamide nitrogen atom N2 orient themselves in such a way that the dihedral angle

between their respective planes is 68.86(3) (Plane 1 = C7-C8-C9C10-C11-C12 and

plane 2 = C13-C14-C15-C16-C17-C18). Similarly the dihedral angle between the

planes of the two phenyl groups attached to the thiamide nitrogen atom N1 is


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87.10(2) (Plane 1 = C20-C21-C22C23-C24-C25 and plane 2 = C26-C27-C28-C29-

C30-C31) (Figure 13(b)).

In the crystal lattice the ligand molecules are held together through C4-

H4S1i (2.943(3) , i = -x+1, -y+1, -z+2) and C9-H9S2ii 2.922(3) , ii = -x, -y+1,

-z+2) intermolecular hydrogen bonding interactions to form a 1D chain running

parallel to a axis. These parallel chains are in turn held together by C-H pi interactions

to form a two dimensional sheet structure parallel to ac plane (Figure 14).

Figure 13: (a) The trans orientation of the two C=S groups with respect to plane of thepyridine ring and (b) the dihedral angle between planes of the two phenyl rings attached tothioamide nitrogen atom in S-dphpt.

Figure 14: (a) H-bonded 1D chain and (b) Two dimensional sheet structure in case of S-dphpt. Hydrogen, green; Sulphur, purple; Carbon, black and Nitrogen, Blue.


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3.6.4. Molecular and crystal structures of M(II)-N,N,N,N-tetraalkylpyridine-2,6-

dithiocarboxamide complexes, M(II) = Co(II),Ni(II).

3.6.4(a). [Co(S-dbpt)Br2] (1)

Figure 15: ORTEP diagram and atom numbering scheme for complex 1. Ellipsoids are drawn

at 50% probability level.

The ORTEP diagram along with atom numbering scheme for complex 1 is

shown in Figures 15. The Co(II) ion in complex 1 has a five-coordinated

stereochemistry with CoNS2Br2 chromophore. The ligand S-dbpt (N,N,N,N-

tetraisobutylpyridine-2,6-thiocarboxamide) coordinates to the metal center in a

tridentate fashion using the pyridine nitrogen and the two thioamide sulphur atoms

while the remaining two positions are occupied by two Bromide groups. The Co1-N1,

Co-S1, Co-S2, Co-Br1 and Co-Br2 distance values are in agreement with the

corresponding values for a similar complex reported earlier.26(a) Important bond

lengths and angles and angles are listed in Table 5. As compared to the neat ligand S-

dbpt, both the coordinating C=S groups are flipped in and are in cis configuration to

pyridine nitrogen N1 with respect to C1-C15 and C5-C6 bonds, so as to be able to

coordinate to the metal ion in tridentate fashion. Furthermore, to maximize the

electron donation through coordination, both these C=S moieties of the tridentate

ligand S-dbpt are bending away from the pyridine ring plane (but remaining cis to


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pyridine nitrogen atom N1) as indicated by torsional angle values of -48 [N1-C1-

C15-S2] and 42 [N1-C5-C6-S1].

It is generally observed that most of the 5-coordinated complexes have neither

an ideal trigonal bipyramidal (tbp) nor

square pyramidal (sp) environment but

their geometry falls between sp and tbp

depending upon a parameter suggested

by Addison and co-workers.47(g) The

parameter is determined by the relation

= (-)/60 (where and are the larger

basal angles with > ) and its value may

vary from 0, representing ideal sp (square pyramidal) geometry to 1, denoting ideal

tbp (trigonal bipyramidal) geometry. The geometry of complexe 1 may be considered

as distorted square pyramidal, value being 0.37 [= N(1)Co(1)Br(2), 159.73(2)

and = S(2)Co(1)S(1), 137.34(3)].

In complex 1, the apical position in the square pyramid is occupied by the

bromine atom Br1 while sulfur atoms S1, S2, pyridine N1 and Br2 form the base of

the pyramid. A mean plane can be passed through S1, S2, N1 and Br2 atoms with -

0.065, -0.264, 0.6164, and 0.013 being deviations for these atoms from this plane,

respectively. The Co ion is 0.7313 above the mean plane defined by these four

atoms. The orientation of the terminal carbon atoms on the thioamide nitrogen atoms

N2 and N3 is such that the atoms C13, C14, C22 and C23 are above their respective

thioamide planes while the atoms C9, C10, C18 and C19 lie below. This orientation

minimises the steric strain in the structure due to bulky isobutyl groups on thioamide

nitrogen atoms. The increase in steric bulk on the amide side arms of the ligands has

no substantial effect on the geometry adopted by the corresponding complexes formed

as indicated by [CoBr2(S-dept)2]26(a)

and the present complex [CoBr2(S-dbpt)2], both

having distorted square pyramidal geometry with similar coordination environment.

The C3-H3S1i (2.897(2) ), C4-H4Br2i (2.931(2) ), C3-H3Br1i

(3.125(3) , where i = x+1/2, -y=1/2, +z+1/2) and C9-H9BBr2ii (3.046(2) , where

ii = -x-1/2, +y+1/2, -z+1/2+1) intermolecular hydrogen bonding interactions between

complex molecules leads to the formation of a 2D sheet parallel to ac plane in the

crystal lattice (Figure 16). An interesting feature of this two dimensional sheetstructure is the presence of two types of empty spaces/voids marked as A and B.


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Figure 16: Showing the 2D (two dimensional) sheet formed through intermolecular hydrogen

bonding among complex molecules along with two types of voids in it, market as A and B in

complex 1. The polyhedron (square pyramid) around each metal center is also shown.

Table 5: Important bond lengths [] and angles [ ] for complex [CoBr2(S-dbpt)], 1.

N1-Co1 2.140(4) S(1)-Co(1) 2.4454(17)

S2-Co1 2.4754(17) Co(1)-Br(1) 2.4115(13)

Co1-Br2 2.4286(13)

N1-Co1-Br1 92.88(13) N(1)-Co(1)-Br(2) 159.73(13)

Br1-Co1-Br2 107.38(5) N(1)-Co(1)-S(1) 79.27(13)

Br1-Co1-S1 108.93(5) Br(2)-Co(1)-S(1) 93.39(6)

N1-Co1-S2 79.66(12) Br(1)-Co(1)-S(2) 108.78(6)

Br2-Co1-S2 93.79(5) S(1)-Co(1)-S(2) 137.34(6)

C6-S1-Co1 94.5(2) C(15)-S(2)-Co(1) 92.07(19)


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3.6.4(b).[Co(S-dppt)(SCN)2] (2)

Figure 17: ORTEP diagram and atom numbering scheme for complex 2. Ellipsoids are drawnat 50% probability level.

The molecular structure of complex 2 is very similar to that of complex 1

except for the fact that there are two SCN groups in place of two Br groups and thecoordinating ligand (S-dppt) has isopropyl groups on the nitrogen atoms of thioamide

side arms. Figure 17 shows that ORTEP diagram and atom numbering schem used for

complex 2. The cobalt ion in the complex is pentacoordinated through a S-dppt ligand

molecule coordinating in a tridentate fashion while the other two vacancies are filled

by the two thiocyanates(-NCS) groups. The Co-S1, Co-S2, Co-N1, and Co-NSCN

distance values are in agreement with the values reported earlier in a similar

complex.26(a) Important bond lengths and angles and angles are listed in Table 6.

The torsion angles of -51.5 [N1-C5-C6-S1] and 36.0 [N1-C1-C13-S2]

indicate the degree by which both the coordinating C=S moieties are bending with

respect to the pyridine ring plane so as to maximise electron donation by coordination

to the metal ion. The geometry around the cobalt ion is distorted squarepyramidal

with value being 0.43 [angle = N1CoN5), 161.30 and angle = S2CoS1,

135.16]. The isothiocyanate nitrogen atom N4 occupies the apical position and

sulfur atoms S1 and S2, pyridine N1 and thiocyanate N5 form the base of the

pyramid. The mean plane through N1, N5, S1, S2 indicates that these atoms are 0.39,


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0.52, 0.05, 0.08 , respectively, above and below the mean plane. The Co ion is

0.74 above the mean plane defined by these four atoms. Such deviations are

energetically favourable and this phenomenon is observed in most square pyramidal

complexes. This has considerable significance since the four basal atoms effectively

restrict the approach of a ligand to the sixth coordination site.

An interesting aspect of the CoNCS linkages in 2 is

the bent observed with CoN4C20 and CoN5C21 angles of

168.3(2) and 164.7(2) in the present complex as well as in

other similarly coordinated complex.26(a) This linkage may be

either linear or angular, with examples of MNC angles

falling in the range 141174.50,51 The non-linearity of this

type may be attributed to steric factors.52 The bend is more

pronounced in the present complex [Co(SCN)2(S-dbpt)] as compared in [Co(SCN)2(S-

dept)]. It also suggests electron density localisation on the donor nitrogen atom such

that a canonical form I contributes to the structure.50,53,26(a)

Table 6: Important bond lengths [] and angles [ ] for complex [Co(SCN)2(S-dppt)],2.

C1-N1 1.341(3) N1-Co 2.152(2)

N4-Co 1.996(3) N5-Co 2.003(2)

S1-Co 2.4244(9) S2-Co 2.3561(9)

N4-Co-N1 94.37(9) N5-Co-N1 161.30(9)

N4-Co-S2 120.50(8) N5-Co-S2 89.99(8)

N1-Co-S2 79.83(6) N4-Co-S1 101.05(8)

N5-Co-S1 95.73(8) N1-Co-S1 81.02(6)

S2-Co-S1 135.16(3) C20-N4-Co 168.4(3)

C21-N5-Co 164.7(2) C6-S1-Co 90.26(9)

C13-S2-Co 99.44(9) N4-Co-N5 104.32(10)


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3.6.4(c) [Ni(S-dept)2](ClO4)2H2O (3)

Figure 18: ORTEP diagram and atom numbering scheme for complex 2. Ellipsoids are drawnat 50% probability level.

Figure 18 shows the molecular structure and labeling scheme used in the structural

analysis of complex 3, [Ni(S-dept)2](ClO4)2H2O. The complex consists of discrete

[Ni-(S-dept)2]2+ cations and perchlorate anions along with non-coordinating water

molecules and has the molecular composition of [Ni(S-dept)2](ClO4)2H2O. The

nickel atom Ni1 in the complex is hexacoordinated (NiN3S3 chromophore). Two

ligand (S-dept) molecules, each coordinate in a tridentate fashion uses the pyridine

nitrogen atom and both the thiocarboxamide sulphur atoms, thus bringing the metal-

ligand coordinating ratio to 1:2. The coordinating atoms N1,S1, S2 from one ligand

molecule and N2, S3, S4 atoms from the other coordinating ligand molecule

completes the distorted octahedral geometry around Ni1. A mean plane can be passed

through atoms N1, N5, S3 and S4 forming the basal plane around the metal ion with -

0.121 , -0.147 , +0.017 and +0.029 being the deviation of these atoms from

this plane respectively. The nickel atom Ni1 is displaced +0.015 from this plane.


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The distortion exhibited by the complex may be attributed to the steric hindrance

caused by the ethyl groups of the thioamide moiety. The atoms S1 and S2 occupy the

trans positions in the octahedra. The important bond lengths and bond angles are

listed in Table 5. In one ligand molecule a plane can be passed through C21, C26, N5,

S3 and S4 atoms, with -0.023 , +0.091 , -0.046 , -0.005 and +0.007 and

being deviation for theses atoms from this plane respectively. Similarly for the other

coordinating ligand molecule the atoms C6, C11, N1, S1 and S2 define a mean plane,

from which these atoms are deviated by +0.2 , -0.120 , -0.058 , -0.013 and

+0.015 . The torsional angles 91.9(2) (S1-Ni1-S3-C26), -87.3(2) (S1-Ni1-S4-

C21), 94.4(2) (S3-Ni1-S1-C6) and 107.3(2) (S3-Ni1-S2-C11) suggest that these

two planes containing the coordinating atoms are approximately at right angle to each

other so as to lower the steric strain and facilitate the coordination of both the ligand

molecules in the tridentate fashion to the metal ion. Furthermore in case of each

ligand molecule, the pyridine ring is bending out of the plane of the corresponding

coordinating atoms to minimize the steric strain and maximize stability of structure.

This is indicated by the torsion angles values 33.3(2) (N1-C1-C11-S2), -43.6(2)

(N1-C5-C6-S1), 36.4(3) (N5-C16-C26-S3) and -35.0 (N5-C20-C21-S4) with respect

to C1-C11, C5-C6, C16-C26 and C20-C21 bonds respectively. Important bond

lengths and angles and angles are listed in Table 7.

A comparision can be made with another transition metal complex [Co(O-

deap)2(H2O)2] (ClO4)2, involving a similar tridentate ligand O-deap coordinating to

the metal center in 1:2 metal-ligand ratio. But in that case one ligand molecule acts in

a tridentate fashion (using the pyridine nitrogen and both the carbonyl oxygen atoms)

while the other coordinates in bidentate mode using pyridine nitrogen atom and one of

the carbonyl oxygens. This is due to the fact that the oxygen atom of the

uncoordinated carbonyl group is strongly hydrogen bonded to the lattice water

molecule. In the present case of [Ni(S-dept)2](ClO4)2H2O, the sulphur atoms being

larger in size and lower in electronegativity value as compared to oxygen atom are

unable to get involved in any such strong hydrogen bonding interactions and thus both

the ligand molecules coordinate in tridentate fashion with the Ni(II) center.

The complex undergoes extensive intermolecular hydrogen bonding owing to

the presence of ionic perchlorates and water molecules in crystal lattice. The atoms

O1, O2, O3, O5, O6 and O7 are involved in hydrogen bonding interactionssummarized in table 8. A two dimensional sheet structure is formed owing to these


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hydrogen bonding interactions involving perchlorates (Figure 19). A close look at this

type of network shows that a water dimmer is trapped in a capsule made by hydrogen

bonding involving two complex molecules and two perchlorates (Figure 19).

Figure 19: Showing the extensive hydrogen bonding network involving ionic perchloratespresent in crystal lattice of complex 3. Hydrogen atoms other than involved in hydrogenbonding are removed for a clear view. Two water molecules (shown by ball and stick mode)encapsulated by cage formed by the hydrogen bonding between complex molecules and

perchlorates units (above). A magnified view of the cage trapping two water molecules(shown by space- filling model) (below). Carbon, black; Hydrogen, green; Nitrogen, blue;Sulphur, yellow; Nickel, cyan; Chlorine, purple and Oxygen, red).


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Table 7: Important bond lengths [] and angles [] for complex [Ni(S-

dept)2]ClO4H2O, 3.

N1-Ni1 2.064(4) N5-Ni1 2.048(4)

S1-Ni1 2.4401(16) S2-Ni1 2.3842(17)

S3-Ni1) 2.3982(16) S4-Ni1 2.4131(17)

O1-C2 1.425(5) O2-Cl2 1.390(6)

O3-Cl2 1.392(5) O4-Cl2 1.413(6)

O5-Cl1 1.372(6) O6-Cl1 1.406(6)

O7-Cl1 1.291(11) O8-Cl1 1.315(10)

S1-Ni1 2.4401(16) S2-Ni1 2.3842(17)S3-Ni1 2.3982(16) S4-Ni1 2.4131(17)

N5-Ni1-N1 168.99(16) N5-Ni1-S2 105.42(12)

N1-Ni1-S2 83.09(12) N5-Ni1-S3 84.17(12)

N1-Ni1-S3 102.93(11) S2-Ni1-S3 91.02(6)

N5-Ni1-S4 83.53(12) N1-Ni1-S4 89.44(11)

S2-Ni1-S4 90.94(7) S3-Ni1-S4 167.63(5)

N5-Ni1-S1 87.91(12) N1-Ni1-S1 83.78(12)S2-Ni1-S1 166.65(5) S3-Ni1-S1 89.59(6)

S4-Ni1-S1 91.31(6) O7-Cl1-O8 101.4(12)

O7-Cl1-O5 116.3(9) O4-Cl2-O1 108.1(4)

O8-Cl1-O5 107.0(5) O7-Cl1-O6 107.2(5)

O8-Cl1-O6 112.8(8) O5-Cl1-O6 111.8(4)

O2-Cl2-O3 111.2(5) O2-Cl2-O4 106.5(4)

O3-Cl2-O4 112.2(5) O2-Cl2-O1 109.3(4)

------------------------------------------------------------------


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Table 8. Important H-bonds and geometry forL

D-HA DA [] HA [] D-HA []

O2H19i 3.353(.010) 2.662(.008) 132

O1H18i 3.383(.009) 2.496(.006) 160

O5H7Bii 3.530(.009) 2.682(.006) 146

O7H12Bii 3.107(.023) 2.284(.023) 142

O1H7Aii 3.580(.008) 2.677(.005) 155

O1H17iii 3.201(.007) 2.440(.006) 139

O3H28Biii 3.478(.015) 2.569(.010) 158

O3H29Biii 3.448(.009) 2.560(.007) 152

O6.H27Biv 3.353(.008) 2.558(.006) 139

Symmetry transform to generate equivalent atoms: i = x, y, z; ii = x+1, +y, +z; iii = -

x, -y, -z+1; iv = -x, -y, -z.

3.7 Conclusions

In the present work six new crystal structures are reported (three N,N,N,N-

tetraalkylpyridine-2,6-dithiocarboxamide (S-dapt) ligands (S-dept, S-dbpt, S-dphpt,

Scheme 1) and three complexes [Co(S-dbpt)Br2] (1), [Co(S-dppt)(SCN)2] (2) &

[Ni(S-dept)2](ClO4)2H2O (3). The molecular structure investigations of the S-dapt

ligands reveal that the thiamide planes are twisted with respect to the pyridine ring

plane to accommodate the alkyl/phenyl groups attached to the thiamide nitrogen

atoms. The extent of twisting depends upon the steric bulk of the alkyl/phenyl groups.

The molecular structure investigations of M(II)-(S-dapt) complexes provide

ample evidence that cobalt has a greater tendency than nickel to give monomeric five-

coordinate complexes rather than octahedral complexes. Furthermore, the increase in

steric bulk on the amide side arms of the ligands has no substantial effect on the

geometry adopted by the corresponding complexes formed as indicated by: (a)

[CoBr2(S-dept)2]26(a)

and the present complex 1 [CoBr2(S-dbpt)2] and also (b) [Co(S-

dept)(SCN)2]26(a) and the present complex 2 [Co(S-dppt)(SCN)2]. In both the cases

(a) and (b), each of the complexes has a distorted square pyramidal geometry with

similar coordination environment although having different alkyl groups on thethiamide nitrogen atoms. At the same time, the change in the donor set of atoms in the


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S-dapt ligands as compared to in O-daap (S in S-dapt in place of O in O-daap) ensure

the tridentate coordination mode of S-dapt ligands as sulphur atom being larger in size

and less electronegtivity is unable to involve in any strong H-bonding interactions

(with medium or counterions) and thus is always free to coordinate to the metal ion.

Finally the crystal structure investigations show the formation of 2D sheets

through C-SH interactions (ligands) and C-HBr interactions (complex 1) and

through C-HO H-bonding interactions involving perchlorate ions in complex 3.


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review reyasynthesis, x-ray crystal structure analysis and properties of m(ii)-n,n,n′,n′- ...

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