Sct

Ka

b

a

ARRA

KC1

MEAM

1

anctstctm

tatktaasi

1d

Spectrochimica Acta Part A 79 (2011) 1042– 1049

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

jou rn al hom epa ge: www.elsev ier .com/ locate /saa

pectral, structural elucidation and coordination abilities of Co(II) and Mn(II)oordination entities of 2,6,11,15-tetraoxa-9,17-diaza-1,7,10,16-(1,2)-etrabenzenacyclooctadecaphan-8,17-diene

umar Rajiva,∗, Johar Rajnib

Department of Chemistry, (SC) University of Delhi, New Delhi 110027, IndiaDepartment of Chemistry, G.G.S. I.P. University, New Delhi 110002, India

r t i c l e i n f o

rticle history:eceived 1 December 2010eceived in revised form 11 March 2011ccepted 13 April 2011

eywords:

a b s t r a c t

Designing tactics were tailored and followed by synthetic and formulation methodologies to pre-pare 2,6,11,15-tetraoxa-9,17-diaza-1,7,10,16-(1,2)-tetrabenzenacyclooctadecaphan-8,17-diene. Spec-tral techniques (MS, infrared, 1H NMR, 13C NMR, electronic and EPR), physiochemical measurements(elemental analysis, molar conductance and magnetic susceptibility), electrochemistry (cyclic voltam-metry) and classical mechanics (molecular modeling) were employed for structural elucidation of Co(II)

oordination entities (CEs)H and 13C NMRS

PRntimicrobialolecular modeling

and Mn(II) coordination entities having N2O4 chromophore. Comparative spectral analysis revealed legat-ing nature of N2O4 donor macrocycle and confirmed host/guest connectivity between ligand and metal(s).Mass spectrometry (MS) determined 1:1 stoichiometry in CEs. Further electrochemical study confirmedchange in oxidation and reduction patterns of CEs. Inhibiting potential (antifungal screened againstAspergillus flavus) showed enhanced antimicrobial properties of CEs as compared to ligand. Molecularmodeling was employed to find out different molecular features along with their stabilization energies.

. Introduction

Spectral techniques (MS, Infrared, 1H NMR, 13C NMR, electronicnd EPR) jointly provide a path to discover structural features ofatural or synthesized pure organic, inorganic and organometallicompounds. Modern spectral techniques certainly were some ofhe most interesting, important and powerful techniques used fortructural elucidation capable of providing a deeper look on struc-ural features of concerned compounds and materials [1]. Spectralonclusions explained theoretical and practical scenario of selec-ion and coordination abilities of donor atom(s) towards central

etal ion(s) during coordination [2].Macrocyclic ligands and their CEs, in general, were more stable

han analogous open chain ligands due to macrocyclic effect andzomethine linkage (>C N–), usually synthesized by condensa-ion of primary amines and carbonyl compounds. It is a commonlynown vital route and fruitful source for the synthesis of MCs. Struc-ural preorganization of ligand due to the presence of free donortoms leads in formation of CEs directly related to metal ion(s) size

nd its compatibility with ligand cavity. The high selectivity andtrong coordination ability of macrocycle towards transition metalon(s) have attracted considerable attention. Applicability of these

∗ Corresponding author. Tel.: +91 01234276530; fax: +91 01234276530.E-mail address: chemistry rajiv@hotmail.com (K. Rajiv).

386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2011.04.017

© 2011 Elsevier B.V. All rights reserved.

compounds are well known in catalytic and enzymatic reactions[3], magnetism, supramolecular structures and as electron carriersin redox reactions [4].

Macrocycles were used extensively as ligand because of thebinding abilities of donor atom(s) [O, S and N (nucleophile)]which tuned to force metal ion(s) to adopt different coor-dination geometries during coordination. Availability of suchhetero atom(s) also played a key role in the formation oforganometallic systems [5]. Coordination capabilities of donoratom(s) depend on donor potential of subunits linked throughaliphatic (–CH2–) or aromatic (–C6H5–) linkages. To understandconnectivity (acceptor/donor phenomenon) a key mechanismbetween metal/donor atoms, spectral techniques helped to eluci-date the structural features of compounds. In addition, theoreticalcalculations (Ligand Field Parameters) have been derived from UVspectra.

Herein, synthesis of CEs of Co(II) and Mn(II) having N2O4 chro-mophore macrocycle, i.e., 2,6,11,15-tetraoxa-9,17-diaza-1,7,10,16-(1,2)-tetrabenzenacyclooctadecaphan-8,17-diene, Fig. 1, werereported. An explanatory note was also presented having spectralfindings on selection and coordination potential of donor atomsof macrocycle with metal(s). Molecular mechanics, a very useful

computational tool, have been used to obtain minimisation energymodels for CEs [6]. Inhibiting potential (antifungal screened againstAspergillus flavus) was also reported and results obtained werearranged graphically.

K. Rajiv, J. Rajni / Spectrochimica Act

Fig. 1. Molecular model of macrocycle-MC; color code: C, light grey; N, blue; O,rr

2

2

S

2

wwmuPaoratCtStsit

prp

2

lrtaaa

ed. (For interpretation of the references to color in this figure legend, the reader iseferred to the web version of the article.)

. Experimental

.1. Materials and chemicals

Chemicals used were of AnalaR grade procured fromigma–Aldrich. Metal salts were purchased from E. Merck.

.2. Experimental protocols

Analytical data (C, H and N) of MC and CEs were obtainedith a Carlo-Ebra 1106 Elemental Analyzer. Molar conductanceas measured on Leeds Northrup 4995 Conductivity Bridge. Molaragnetic susceptibility was measured with powdered samples

sing Faraday method. Diamagnetic corrections were made usingascal’s constant and Hg[Co(SCN)4] as calibrant at room temper-ture (�g = 16.44 × 10−6 in c.g.s. units). IR spectra were recordedn Perkin Elmer 137 instrument as KBr pellets in wave numberegion 4000–400 cm−1. The 1H and 13C NMR spectra were recordedt ambient temperatures on Bruker AMX400 and DRX500 spec-rometers with TMS as internal reference and CDCl3 as solvent.hemical shifts (ı) were expressed in parts per million (ppm) rela-ive to (TMS) tetramethylsilane. UV–vis spectra were recorded on ahimadzu UV mini 1240 spectrometer. Electron impact mass spec-ra were recorded on JEOL, JMS, DX-303 mass spectrometer. EPRpectra of CEs were recorded as polycrystalline sample as well asn solution (at room temperature and LNT) on Varion E-4 EPR spec-rometer using diphenylpicrylhydrazyl (DPPH) as the g-marker.

Cyclic voltammetry measurements were made in a single com-artment cell with a platinum counter electrode and Ag/Ag+eference electrode at room temperature. Tetraethylammoniumerchlorate (TEAP) was used as a supporting electrolyte.

.3. Molecular modeling

Correct sequence of atoms was obtained to get reasonableow energy molecular models to determine their molecular rep-esentation in three dimensions. Complications of biochemicalransformations may be explored using output obtained. An

ttempt to gain a better insight on the molecular structure of the lig-nd and its complexes, geometric optimization and conformationalnalysis has been performed using MM+ [7] force field.

a Part A 79 (2011) 1042– 1049 1043

2.4. Pharmacology: in vitro antifungal assay

Antimicrobial activities (antifungal) of MC and CEs werescreened against A. flavus. Fresh stock solutions of ligand and CEswere prepared in DMSO according to required concentrations forexperiments. To ensure the effect of solvent on bacterial growth, acontrol test was performed with test medium supplemented withDMSO.

Growth of fungus was measured by reading diameter of fungalcolony. Screening for antifungal activities was carried out in vitroagainst A. flavus, following the procedure outlined [8] and their rel-ative inhibitory ratios (%) were determined using mycelium growthrate method. On complete mycelia growth, its diameters were mea-sured and inhibition rate was calculated according to formula:I = DI − Do/DI × 100 where I is inhibition rate, DI is average diam-eter of mycelia in the blank test and Do being average diameter ofmycelia in presence of CEs.

3. Synthesis of MC and CEs

3.1. Preparation of MC: 2,6,11,15-tetraoxa-9,17-diaza-1,7,10,16-(1,2)-tetrabenzenacyclooctadecaphan-8,17-diene

Condensation reaction of an amine and aldehyde generated pro-posed macrocycle shown in Scheme 1. An earlier reported syntheticroute [9] was followed, with slight modification, as required forthe preparation of 1,5-bis(o-aminophenoxy)propane. A detailedmethodology with formulated tactics for the synthesis of MC havingazomethine (–CH N–) and ether (–C6H4–O–CH2–) linkages was asfollows.

Hot ethanolic solution (90 cm3, absolute) of 1,5-bis(o-aminophenoxy)propane (2 mmol) was mixed with hot ethanolicsolution (90 cm3, absolute) of 1,5-bis(2-formylphenyl)-1,5-dioxapentane (2 mmol) in 100 mL round bottom flask in presenceof few drops of concentrated HCl. After complete addition, thesolution mixture was refluxed for 1–2 h at 80–82 ◦C. Dull whitepowdered compound was obtained, dried and dissolved in min-imum amount of CH2Cl2. Small amount of n-hexane was addedto precipitate out required compound and finally filtered as adull white crystalline solid. The mixture was cooled at roomtemperature and excess solvent removed under reduced pressureuntil a solid product was formed.

3.2. Synthesis of CEs of Co(II) and Mn(II)

CEs of CoCl2 or MnCl2 was prepared by mixing equimolaramounts of MC (0.5 mmol) and CoCl2 or MnCl2 (0.5 mmol) in min-imum quantity of C2H5OH (25 mL, absolute) for nearly 2–3 h at∼80–83 ◦C on a water bath until volume was reduced to ∼12 mL.A solid mass separated out on cooling at ∼5 ◦C was refrigeratedfor better crystallization. It was then filtered, washed with C2H5OHand dried over P2O5 under vacuum. The crystals were redissolvedfor recrystallization with warm ethanol, resulting in a clear solu-tion, formed small crystals when left undisturbed for weeks. CEswere soluble in DMSO, but insoluble in common organic solventsand H2O and were thermally stable up to ∼212–240 ◦C and thendecomposed.

4. Results and discussion

4.1. Characterization of MC

4.1.1. IR spectrum of MCIn IR spectrum of MC, several important peaks obtained

clearly depicted various structural features of MC. Absence

1044 K. Rajiv, J. Rajni / Spectrochimica Acta Part A 79 (2011) 1042– 1049

OH

NO2

Br Br

HO

O2NK2CO 3

DMF

O

NO2

O

O2N

DMF

5%, Pd, H 2 Atm

+ +

O

NH2

O

H2N

1,5-bis(o-nitrophenoxy)propane 1,5-bis(o-aminophenoxy)propane

Condensation with 1,5-bis(2-formylphenyl)-1,5-dioxopentane

O O

N N

O O

nd-L

he syn

o11bctva

r7c2qc

4

msaeedata

9.66–9.53 (s, 2H, –CH N–), 8.40–7.62 (m, 16H, –C6H4–), 3.91–3.87(t, 8H, –O–CH2–) and 2.10–2.00 (t, 4H, –CH2–) is illustrated inFig. 2.

Macrocycle (MC)-Liga

Scheme 1. Route for t

f strong absorption bands at 3355–3351 cm−1 and682–1680 cm−1 corresponds to �(–NH2) primary amine, i.e.,,5-bis(o-aminophenoxy)propane and �(>C O) carbonyl, i.e., 1,5-is(2-formylphenyl)-1,5-dioxapentane, respectively confirmingondensation between these groups, as a result macrocyclizationook place. Appearance of new medium to strong stretchingibration at 1613 cm−1 was assigned to �(–CH N–, m) known anzomethine linkage [10].

Strong to medium intense bands were observed in differentegions 3052–3049 cm−1, 1575–1557 cm−1, 1161–1154 cm−1 and68–763 cm−1, due to �(Ar–CH–) modes as expected. The pureharacteristic �(–CH–) mode of aliphatic groups were observed at955–2928 cm−1 and 2896 cm−1. Strong to medium stretching fre-uencies were found at 810–750 cm−1, in infrared spectrum of MC,orresponding to �(C–O) characteristic stretching modes [11].

.1.2. 1H NMR spectrum of MCDisappearance of signal at around 9.1 ppm corresponds to pri-

ary amine –NH2 confirmed complete condensation. A sharpinglet at 9.66–9.53 ppm corresponds to –HC N–, new link-ge formed during condensation, was observed dependent onlectronegative character of substituent benzyl ring [12]. Aslectron-affinity of substituent increases, azomethine proton shifts

ownfield due to increased deshielding effect. The shape, positionnd integration value of aromatic proton of benzyl ring connectedo side linkage appeared to be affected by more electronegativetoms, i.e., oxygen and nitrogen.

thesis MC-ligand-(L).

In addition, methylene chains were detected in form of tripletas a bridge between two oxygen atoms at 2.10–2.00 ppm. Othermultiplets resonated at 8.40–7.62 ppm (m, J = 8 Hz) due to presenceof –C6H4– configuration. Signals observed at 3.91–3.87 ppm con-firmed the presence of –O–CH2– protons in MC. The occurrence ofthis resonance in each of the above species as a ‘clean’ singlet wasdetected under the impact of intact (non-hydrolyzed) imine groups[13].

MC was soluble in common organic solvents. 1H NMR, (CDCl3) ı:

0123456789PPM

Fig. 2. 1H NMR spectrum of MC.

K. Rajiv, J. Rajni / Spectrochimica Acta Part A 79 (2011) 1042– 1049 1045

020406080100120140160180

4

ctt[amttdNe

2C(

4

dwiosesccf

Table 1Color, magnetic susceptibilities and molar conductivities of CEs.

Compound Color � (B.M.) aEM (�−1 cm2 mol−1)

[Co(L)]Cl2 Light pink 4.75 170[Co(L)](NCS)2 Dull pink 4.78 171[Co(L)](NO3)2 Dark pink 4.78 168[Co(L)](OSO3) Dull pink 4.75 167[Mn(L)]Cl2 Yellow 5.82 176[Mn(L)](NCS)2 Light yellow 5.85 172[Mn(L)](NO3)2 Dark yellow 5.83 165

TE

PPM

Fig. 3. 13C NMR spectrum of MC.

.1.3. 13C NMR spectrum of MCAppearance of sharp peaks in the range 123.30–121.14 ppm

orresponding to the aromatic carbon confirmed aromatic charac-eristic of MC. Downfield peaks at 163.39–162.71 ppm attributedo carbon atom of azomethine (–CH N–), confirmed condensation14]. A group of sharp peaks observed in the range 69.1–68.3 ppmnd 39.9–39.0 ppm corresponding to ether (–O–CH2–) and centralethylene (–CH2–) linkage. Another group of peaks observed in

he range 159.30–152.30 ppm 131.01–129.64 ppm correspondingo aromatic carbons linked to (–CH N–) and phenyl ring carbonsirectly attached to methylene (Ph–O–CH2–) linkage [15]. Thus, 13CMR spectrum, as illustrated in Fig. 3 was found in accordance witharlier findings discussed in IR and 1H NMR spectrum.

13C NMR (CDCl3) ı: C: 2,3,4,5,8,9,10,11,12,18,19,20,21,2,25,26,27,28 (123.30–121.14), C: 1,13,17,29 (131.01–129.64),: 7,23 (163.39–162.71) C: 6,24 (159.30–152.30), C: 14,16,30,3269.10–68.30) and C: 15,31 (31.20–31.00).

.1.4. Electron impact mass spectrum of MCMass spectral analysis of investigated MC gave more insight to

etect correct sequence of fragments. Peaks of different intensitiesere observed corresponding to various fragments directly reflect-

ng their stabilities. The base peak (100%) M+ indicated facile lossf hydrogen radical, a molecular ion peak M+ at m/z = 505, corre-ponds to molecular weight of MC. A general fragmentation patternvidenced primary cleavages at hetero-carbon bonds. Beside this

everal other peaks of different intensities were observed havingorresponding m/z values according to their molecular masses ofonstituent fragments [16]. Initiation in the cleavage in form of dif-erent pathways took place through loss of –O–CH2–CH2– fragment

able 2lemental analysis of MC and CEs.

Compound MC and CEs Calcd. (found)

Mol. wt & mass spectra (M+) M

(L) C32H30N2O4

Mol. wt.: 506.59; m/e: 506.22–

[Co(L)]Cl2 CoC32H30N2O4Cl2Mol. wt.: 636.43; m/e: 635.09

9.26 (9.02)

[Co(L)](NCS)2 CoC34H30N4O4S2

Mol. wt.: 681.69; m/e: 681.108.65 (8.34)

[Co(L)](NO3)2 CoC32H30N4O10

Mol. wt.: 689.53; m/e: 689.138.55 (8.34)

[Co(L)](OSO3) CoC32H30N2O8SMol. wt.: 661.59; m/e: 661.11

8.91 (8.34)

[Mn(L)]Cl2 MnC32H30N2O4Cl2Mol. wt.: 632.44; m/e: 631.10

8.69 (8.43)

[Mn(L)](NCS)2 MnC34H30N4O4S2

Mol. wt.: 677.69; m/e: 676.418.11 (8.09)

[Mn(L)](NO3)2 MnC32H30N4O10

Mol. wt.: 685.54; m/e: 685.138.01 (7.77)

[Mn(L)](OSO3) MnC32H30N2O8SMol. wt.: 657.59; m/e: 657.11

8.35 (8.33)

[Mn(L)](OSO3) Dull yellow 5.91 177

a Molar conductivity, 10−3 M DMF at 298 K.

from the parent molecule, followed by HCHO elimination as evidentin mass spectrum obtained.

4.1.5. Electronic spectrum for MCIn the electronic spectrum of ligand different intensities bands

were observed at 41,000 cm−1, 29,081 cm−1 and 21,007 cm−1. Bandobserved at 41,000 cm−1 was attributed to � → �* transition, indi-cated the presence of benzene moiety. The band observed at29,081 cm−1 and 21,007 cm−1 was because of the presence ofazomethine linkage.

4.2. Characterization of CEs of Co(II) and Mn(II)

Obtained coordination entities (CEs) were colored, non-hygroscopic solids, stable in air. Chemical compositions confirmedthe purity and stoichiometry of the neat and encapsulated CEs.Molar conductance measurements were found corresponding to1:2 electrolytes [17]. The color and molar conductance were repre-sented in Table 1.

Analytical data (C, H and N) had a good agreement with MS frag-mentation patterns. The findings obtained suggested monomericnature of CEs and were arranged in Table 2.

On the basis of elemental analysis, a general composition M(L)X2[where M = Co(II), and Mn(II), L = ligand and X = Cl−, NO3

−, NCS− and1/2SO4

2−] was proposed for CEs. Geometrical scenario of reportedCEs played important role in the formation of coordination envi-ronment of cyclic moiety along with used metal ion(s). Cavity of

MC had two different donor subunits, –CH2–O–CH2– and –CH N–,approached central metal ion(s) with different chromophore. Qual-itative reactions revealed the absence of anion(s) in [M(L)](X)2 ascounter ion(s).

%

C H N

75.87 (75.11) 5.97 (5.43) 5.53 (5.32)

60.39 (60.23) 4.75 (4.34) 4.40 (4.32)

59.90 (59.34) 4.44 (4.32) 8.22 (8.12)

55.74 (55.45) 4.39 (4.33) 8.13 (8.10)

58.09 (58.02) 4.57 (4.44) 4.23 (4.21)

60.77 (60.35) 4.78 (4.56) 4.43 (4.21)

60.26 (60.08) 4.46 (4.05) 8.27 (8.05)

56.06 (56.00) 4.41 (4.07) 8.17 (8.09)

58.45 (58.33) 4.60 (4.54) 4.26 (4.11)

1046 K. Rajiv, J. Rajni / Spectrochimica Acta Part A 79 (2011) 1042– 1049

Table 3Selected Infrared spectral bands (cm−1) with tentative assignments of MC and CEs.

Compounds �(CH N) �(C–H) �(M–N) �(M–O) �(NO3−) �(NCS−) �(SO4

2−)

(L) 1613s 2955w –[Co(L)]Cl2 1611s 2988w 540m 411m – – –[Co(L)](NCS)2 1609s 2977w 549m 402m – 2041s –[Co(L)](NO3)2 1611s 2976w 551m 404m 1383s – –[Co(L)](OSO3) 1610s 2976w 545m 409m – – 1139s, 1136s, 610m[Mn(L)]Cl2 1608s 2975w 537m 405m – – –

409m – 2042s –407m 1382s – –403m – – 1136s, 1135m, 610m

ts

4

lr1tpmefos4�

t[wd

4

addtfi

uBuMc

4M

(d(tdM

sa([a

CEs than free ion (786 cm−1), resulting in an increased distancebetween electrons, as a result affects size of the orbital.

[Mn(L)](NCS)2 1607s 2978w 549m

[Mn(L)](NO3)2 1608s 2973w 536m

[Mn(L)](OSO3) 1606s 2978w 554m

Beside analytical analysis and spectral presentation (mass, elec-ronic, infrared, 1H and 13C NMR), magnetic properties were alsoummarized and presented in Section 4.

.2.1. IR spectra of Co(II) and Mn(II) CEsIn the infrared spectra of CEs, a change was observed for ear-

ier value for the stretching vibration bands of �(–CH N–) in loweregions at 1611–1606 cm−1 than expected as evidenced in MC at613 cm−1. Shift in strong stretching vibration bands of azome-hine �(–CH N–) to lower region occurred due to the drift of loneair electron density of its donor atoms (nitrogen) towards centraletal ion(s) [18]. After this drift, new bond [�(M–N)] came into

xistence and was detected as medium to high intense bands inar infrared region at 554–536 cm−1, bathochromic shift, becausef charge transfer from donor atoms towards central metal whichtabilized CEs. Strong to medium stretching bands were found at11–402 cm−1, in infrared spectrum of CEs, confirmed new bonds(M–O) between metal and donor atoms.

Again it helped to identify the existence and formulation of CEshrough binding patterns between donor atoms and centre metals19], attributed to host/guest relationship. Infrared spectral dataas arranged sequentially in Table 3 so that conclusion can berawn by side by side comparison.

.2.2. Magnetic moments of CEs of Co(II) and Mn(II)Magnetic moments were recorded at room temperature (298 K)

nd calculated from susceptibility measurements correlating withiamagnetic contribution [20]. There was no temperature indepen-ent paramagnetic effect with no reduction of the moment belowhe spin only value due to spin orbit coupling with higher ligandeld terms.

Co-CEs show magnetic moments corresponding to threenpaired electrons [21] and their values lie in the range 4.78–4.75.M. Mn-CEs show magnetic moments corresponding to fivenpaired electrons and their values lie in the range 5.91–5.82 B.M.agnetic moment values of CEs were in tune with a high spin

onfiguration and has been presented in Table 1.

.2.3. Electronic spectra and ligand filed parameters of Co(II) andn(II)-CEs

Electronic spectra of CEs in DMSO exhibit three main features;i) One or two peaks at 342–300 nm and 340–310 nm were presentue to the intra-ligand charge transfer transition assigned as � → �*ii) intense peaks at 421–371 nm and 429–370 nm were present dueo ligand-to-metal charge transfer transition (iii) due to splitting of-orbital, band at 512 and 510 nm was present, for CEs of Co(II) andn(II), respectively.A sequencing of bands were observed in the electronic

pectra of Co-CEs, at 7633–6872 cm−1, 14,925–10,309 cm−1

nd 20,618–17,513 cm−1 assigned to 4T1g (4F) → 4T2g(4F), 4T1g4F) → 4A2g(4F) and 4T1g (4F) → 4T1g(4P) transitions, respectively22] characteristic of an octahedral geometry and related graphnnexed as Fig. 4. The ligand field parameters Dq (1018–944), B

Fig. 4. UV spectra of Co-CEs, 1-CoLCl2; 2-CoL(NO3)2; 3-CoLOSO3; 4-CoL(NCS)2.

(987–856), (0.94–0.86) and LFSE (96.4–45.4 kJ Mol−1) were cal-culated and their values (presented in brackets) were in goodagreement with earlier evidences. The lowering in B value for Co(II)from the free ion (1120 cm−1) suggested 76–88% covalent natureof CEs having octahedral geometry.

In the electronic spectra of Mn-CEs, four weak-intensityabsorption bands were observed at 17,111–18,231 cm−1, 23,121–28,987 cm−1, 28,476–28,987 cm−1 and 31,143–31,987 cm−1

assigned to the transitions: 6A1g → 4T1g(4G), 6A1g → 4Eg, 4A1g(4G)(10B + 5C), 6A1g → 4Eg(4D) (17B + 5C) and 6A1g → 4T1g(4P) (7B + 7C),respectively [23] as shown in Fig. 5.

Ligand field parameters, B and C of Mn-CEs were derived fromsecond and third transitions as they were free from the crystalfield splitting. These were linear combinations of certain coulombsand exchange integral, which were generally, treated as empiri-cal parameters obtained from the spectra of free ions. Orgel [24]calculated the values of Dq with the help of curve, transition ener-gies versus Dq, using the energy redistribution of the transition6A1g → 4T1g (4G). Slater Condon-shortly repulsion parameters F2and F4 were related to Racah parameters B and C as: B = F2 − 5F4and C = 35F4. The electron–electron repulsion was higher in Mn-

Fig. 5. UV spectra of Mn-CEs, 1-MnLCl2; 2-MnL(NO3)2; 3-MnLOSO3; 4-MnL(NCS)2.

ca Act

tt(ts

4

wbp2fwwta

asofwcM(qoCo

bm−[

4

arcbOrrbc

±tirwsTr

dp(pd0r

K. Rajiv, J. Rajni / Spectrochimi

On increasing delocalization, value of decreases upto lesshan one in Mn-CEs. The value of can be calculated fromhe Nephelauxetic parameter of ligand (hx), metal ion (km) as1 − ˇ) = hx × km. The value of hx calculated using co-valency con-ribution of Mn(II). The observed values for parameter and hxuggest that Mn-CEs have appreciable ionic character.

.2.4. EPR spectra of Co(II) and Mn(II)-CEsEPR spectra of CEs were recorded as polycrystalline sample as

ell as in DMSO solution at liquid nitrogen temperature (LNT)ecause of rapid relaxation of Co(II) broadened lines at higher tem-erature. Values of g||, giso and g⊥ lie in the range 3.97–3.61 and.60–2.40 and 1.99–1.84, respectively. Large deviation has beenound and g values obtained from the spin only value, i.e., 2.0023,as due to the large angular momentum contribution [25]. Co-CEsere of high spin type and had three unpaired electrons. Therefore,

he electronic configuration and splitting of the orbital could be t2gnd eg.

Combined action of electric field gradient and spin–spin inter-ction produced splitting of energy levels due to second orderpin orbital coupling between 6A1g ground state and lowest levelf 4A2g state. Further, a resonance was readily detected evenor large zero-field splitting since d5 is an odd-electron systemhose ground state is Kramer’s doublet and whose degeneracy

ould be completely removed by a magnetic field. Broad signals inn(II)-CEs was attributed to forbidden transitions where �m = ±M

M = electron spin quantum number) and �m /= 0 (m = nuclear spinuantum number). Thus broadening in powdered sample was anal-gous to that observed immobilized free-radicals, i.e., Mn-CEs ofoncanavalin Broadening results because of the rotational motionf Mn(II) which is highly restricted.

In solution, six lines arouse due to hyperfine interactionetween unpaired electrons of 55Mn nucleus (I = 5/2). The nuclearagnetic quantum number MI, corresponding to these lines were5/2, −3/2, −1/2, +1/2, +3/2 and +5/2, i.e., from low to high field

26].

.2.5. Cyclic voltammetry of Co(II) and Mn(II)-CEsVoltammetry measurements were carried out to ascertain

correlation between electrochemical properties and oxidationeduction states of central metal ion(s) within N2O4 chromophoreontaining CEs. The nature of one-electron couple was confirmedy comparing with current height of Fe(CN)6

3−/Fe(CN)64− system.

xidation of Co(II) to Co(III) was obtained from measurementseferred to Epc, the relative potential at maximum diffusion cur-ent, for Co(II)/Co(O) couple according to equation. Electrochemicalehaviour of Co(III)/Co(II) couple having positive potential indi-ated strong bond of metal to ligand in lower oxidation state.

Co-CEs exhibit metal and ligand-centered electro potential at1.71 V versus Ag/AgCl, Cl− electrode showing a quasi-reversible

wo step single electron transfer process. E0.5 values were recordedndependent of scan rate and �Ep increases with increasing scanate measured at 60–800 mVs−1. Ratio of cathodic peak currentith square root of the scan rate (Ipc/V0.5) was approximately con-

tant. Peak potential showed a small dependence with scan rate.he ratio (Ipa/Ipc) close to unity confirmed relation of redox coupleeaction to reversible one-electron transfer process.

Co-CEs exhibited one quasi-reversible oxidation process, evi-ent from peak to peak separation, �Ep > 100 mV, with redoxotentials at 1.09–1.21 V. The cathodic and anodic peak heightsIpc and Ipa) were same. The difference between potential of anodic

eak and cathodic peak remains constant. Co-CEs show two couplesue to one electron quasi-reversible reductions (�Ep > 100 mV) at.20 to −0.30 V and −0.40 to −0.60 V along with a two-electroneduction at −1.07 to −1.38 V [27] simultaneously.

a Part A 79 (2011) 1042– 1049 1047

Relative potential at maximum diffusion current for Co(II)/Co(O)couple was in accordance with the reaction and showed electroac-tive ability due to Co(II) centre and Epc values obtained confirmedmixed donor CEs. Redox behaviour of Mn-CEs displayed a chemi-cally reversible wave with scan rate of 100 mVs−1 and E0.5 valuesassumed had reversible potential because of Ep. It further showedthe influence of different anions present around the coordinationsphere on the geometrical features of the Mn-CEs.

Oxidation of Mn(II) to Mn(III) in two unresolved one electronprocesses [Mn(II) → Mn(II)Mn(III) → Mn(III)] and then to Mn(IV)is as follows: [Mn(III) → Mn(III)Mn(IV) → Mn(IV)]. Second signalswere relatively sharp comparatively to initial one, as evidenced bysmaller �Ep values of 70 and 75 mV, respectively [28].

Higher �Ep values than expected for a simultaneous two elec-tron process (�Ep = 57 mV/n), observed during oxidation of twoMn(II) centre to Mn(III) via two closely spaced one electron pro-cess, with a separation of 25 ± 10 mV. Average E0.5 values for thefirst unresolved two electron oxidation responses of SCN− > Cl−

decrease with increasing E0.5 = +0.56 to +0.47 V according to chainlength as for the second oxidation +1.30 to +1.20 V. Lengthening ofpolymethylene carbon chain showed an insignificant small shift inreduction potentials with varied oxidation potential show oxida-tion orbital to be ligand-based. Thus alkyl chains had slight effecton cyclic potentials of metal centre.

E0.5 values for Mn-CEs +0.40 V to +1.09 V reflected absence ofelectrostatic effects caused by a second metal centre in large ringcontaining macrocyclic ligand systems. The aromatic part of suchligand system reduces the binding ability of the bridgehead nitro-gen creating problems in oxidation process. As polymethylenecarbon chain of the Schiff base backbone lengthened, slight change(1–2 mV) in the reduction potential occurred suggesting reductionorbital to be metal-based.

4.2.6. FAB mass spectra of Co(II) and Mn(II)-CEsMass spectral study was applied to investigate complete frag-

mentation patterns of Co and Mn-CEs. In electron impact massspectra, peaks of different intensities corresponding to various frag-ments of Co and Mn-CEs were obtained along with their stabilities.A base peak M+, indicated the facile loss of hydrogen radical atm/z = 635 a.m.u. and m/z = 631 a.m.u. for CEs of Co(II) and Mn(II),respectively confirmed monomeric nature of CEs and proposedformula derived from chemical analysis. General fragmentationpattern revealed primary cleavages to occur at hetero-carbonbonds. Several peaks observed had various m/z values at differentintensities corresponding to molecular masses of the constituentfeatures. Their intensities gave an idea of their stabilities along withtheir geometrical configurations. The major cleavage pathwaystook place through the loss of –O–CH2–CH2– fragment, followed byelimination of HCHO. Main peak values for each compound werepresented in Table 2. In conclusion, presented generic model had1:1 stoichiometry and was same for rest of CEs. The experimentaland spectral analysis obtained from the analytical, vibrational, massand nuclear magnetic resonance studies were in best accordancewith the presented structure.

5. Pharmaceutical effectiveness of CEs

Pharmaceutical effectiveness of CEs depends on the connectivityof ligand to metal ion(s). Moreover, coordination reduces polarity ofused positively charged metal ion(s) and negatively charged donoratoms in MC. Antifungal activity of MC and CEs were performed by

agar plate technique against A. flavus and its results obtained weredisplayed graphically in Fig. 6.

CEs were directly mixed in a medium of different concentra-tions. The fungus was positioned in medium with the help of an

1048 K. Rajiv, J. Rajni / Spectrochimica Acta Part A 79 (2011) 1042– 1049

ic3i

naoeci

bpti

6

cii

ecbtfoar

b

FCr

Fig. 6. Comparative aspects of antifungal effectiveness.

noculum’s needle [29]. Petri dishes wrapped in polythene sheetsontaining some drops of C2H5OH were put in an incubator at0 ± 3 ◦C for 70–90 h. Enlargement of fungal colony was measured

n diameter.Results evidenced that comparative antifungal activity sig-

ificantly increased after complexation because of stabilizedzomethine bonds (–CH N–) with metal ion(s). It was alsobserved that (O) and (N) donor systems of ligand and CEs inhibitnzymatic activity. Enzymes require certain group for enhancingoncerned activity. It was thus susceptible to deactivation by metalon(s) on coordination.

CEs have potential to generate novel antimicrobial propertiesy displaying moderate to high affinities for most of the receptorsarticularly in the case of MnL(OSO3). In conclusion, it revealedhat presence of metal ion(s) in macrocycle play an important rolen enhancement of antimicrobial effectiveness.

. Molecular modeling of CEs

Molecular models were proposed as a standard to judge spe-ific interactions in topologies of the molecules of CEs. If deviationsn distances, angles or torsion were evidenced, specific electronicnteractions should perhaps be pursued.

In order to ascertain the structural features and related pref-rences to confirm spectral reports obtained; the coordinationapabilities of MC towards metal ion(s) effect bond lengths andond angles between used metal ion(s) and donor atoms (O or N),hereby, confirmed the formation of CEs. Taking in account theseacts, in this article, physical dimensions of the molecules helpedut to demonstrate molecular models for CEs with their topologicalssemblies. The process of determining energy minimization was

epeated several times to find global minimum energy [30].

The molecular structures of MC and CEs of Co(II) and Mn(II) haveeen shown in Figs. 1, 7 and 8. Data analysis for bond lengths and

ig. 7. Molecular model of Co-CEs; color code: C, light grey; N, blue; O, red; Cl, green;o, dark blue. (For interpretation of the references to color in this figure legend, theeader is referred to the web version of the article.)

Fig. 8. Molecular model of Mn-CEs; color code: C, light grey; N, blue; O, red; Cl,green; Mn, violet. (For interpretation of the references to color in this figure legend,the reader is referred to the web version of the article.)

angles of studied compounds reveal the following remarks con-cluded as follows.

The bond angles of the metal to donor atoms moi-ety were altered somewhat upon coordination; bonds anglesN(28)–Co(39)–N(10) and N(38)–Co(39)–O(27) were 179.427◦ and90.000◦, respectively as a consequence of bonding. All bond anglesin complexes were quite near to an octahedral geometry. All theactive groups taking part in coordination had bonds longer thanthat already existing in ligand (like –C–O–C– and –CH N–). Coor-dination significantly shortens for the N(38)–Co(39) that is 1.836as compared to O(27)–Co(39) which is 2.180. This is because bondlengths in the ligand between donor atoms probably gets affecteddue to the presence of hydrogen in azomethine (>CH N–). There isa large variation in N(37)–N(38) bond lengths on complexation. Itbecomes slightly longer as the coordination takes place via N atomof –CH N– group.

7. Conclusion

Condensation between carbonyl compounds and amines wereimplemented numerous times for the synthesis of macrocycles. Thesame methodology after slight modification (via reductive amina-tion, imine serves as an intermediate) was used for the synthesis ofmacrocycles having N2O4 chromophore. A sharp singlet appearedwithin 9.66–9.53 ppm in 1H NMR spectrum of macrocycle due tonewly formed bond, denoted as azomethine proton (–HC N–). Theposition of observed signal was found to be dependent on elec-tronegative character of benzyl ring.

As electron-affinity of substituent increased, azomethine protonshifted downfield due to increase in deshielding effect. The shape,position and integration values of this signal due to aromatic pro-ton in phenyl ring (C5) appeared to be affected by rate of exchange,relaxation time and concentration of the solution as well as sol-vent used. Natural atomic charge showed (O) and (N) atoms whichcarried negative charges, (C) atoms carried positive charges linkedwith (O) and (N) atoms and (C) atoms linked with (H) atoms carriednegative charges. It was easy to coordinate negatively charged (O)and (N) atoms with positively charged metal ion(s), which workedas Lewis acid and Base. Due to this, macrocycles have long beenemployed as selective host for a wide variety of guest moleculesand metal ion(s).

It was emphasized that level of preorganization of macrocy-cles, is much lower than commonly realized. Newly emerging types

of more highly preorganized ligands can be achieved accordingto desired selection and coordination abilities [31]. The bind-ing abilities of macrocycle highly depend on different fragmentsof constrained ligand detectable through spectral techniques as

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eported earlier [32,33]. Keeping such hurdles in mind, to detecttructural preorganization, spectral techniques (MS, infrared, 1HMR, 13C NMR, electronic and EPR), physiochemical analysis (ele-ental analysis, molar conductance and magnetic susceptibilityeasurements) electrochemistry (cyclic voltammetry) and classi-

al mechanics (molecular modeling) worked as a powerful tool andhat is why used jointly to get perfect results for complete structurallucidation of MC and CEs of Mn(II) and Co(II). Further theoreticalalculations such as binding and coordination abilities of –O–M–N–,O–M–O– and –N–M–N– linkages (M = metal, i.e., Mn(II) and Co(II)alts) confirmed chelation and strongly favoured spectral findings.emonstration of structural features is in best accordance withroposed molecular models for CEs.

cknowledgements

One of the authors (Rajiv) gratefully acknowledges his youngerrother Bitto for motivation. A Special thanks to the Universityrants Commission, New Delhi for financial assistance. Thanks to

.I.T. Bombay for recording EPR spectra. Thanks are also due to Solidtate Physics Laboratory, India for recording magnetic moments.

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