Polyhedron 81 (2014) 457–464

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Polyhedron

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A multiprotic indole-based thiocarbohydrazone in the formationof mono-, di- and hexa-nuclear metal complexes

http://dx.doi.org/10.1016/j.poly.2014.06.0570277-5387/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +60 3 79674246; fax: +60 3 79674193.E-mail addresses: grdenia_77m@yahoo.com (A.A. Ibrahim), khaledi@um.edu.my,

hamid.khaledi@gmail.com (H. Khaledi), hapipah@um.edu.my (H.M. Ali).

Abeer A. Ibrahim a,b, Hamid Khaledi a,⇑, Hapipah Mohd Ali a

a Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysiab Presidency of Sulaimani Polytechnic University, Kurdistan Region, Iraq

a r t i c l e i n f o a b s t r a c t

Article history:Received 30 April 2014Accepted 24 June 2014Available online 9 July 2014

Keywords:Ditopic ligandBridging ligandSchiff baseCoinage metalPolynuclear complex

A new thiocarbohydrazone (LH4) derived from indole-7-carbaldehyde was synthesized and reacted withNiII, PdII, PtII, CuI and AgI salts to bind the metals in various coordination fashions, forming complexeswith different nuclearities. The reaction with CuCl in the presence of PPh3 produced a dinuclear CuI com-plex with the formula [Cu2Cl2(LH4)2(PPh3)2], in which LH4 binds the metals as a neutral l2-S-donorligand. The reaction of the thiosemicarbazone with AgI resulted in the formation of a hexanuclear com-plex featuring an Ag6S6 core. The ligand in this complex is monoanionic and uses the sulfur and a hydra-zinic nitrogen atom in binding to the metals. The reaction of LH4 with [MCl2(PPh3)2] (M = NiII, PdII and PtII)at room temperature yielded complexes of the type [M(LH2)(PPh3)]. The thiocarbohydrazone in thesecomplexes takes advantage of one indole N atom donor to act as a dianionic NNS tridentate ligand. Amodification of the conditions of the reaction with the PdII salt led to the formation of the dinuclear com-plex [Pd2Cl(LH)(PPh3)3]. In this molecule, the thiocarbohydrazone is triply deprotonated and chelates onemetal center by its dianionic NNS pocket, while coordinating the second metal ion through a hydrazinic Natom. The structure of the molecules was studied by X-ray crystallography and NMR spectroscopy.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

In the past decade, there has been growing interest in the coor-dination chemistry of thiocarbohydrazones. Depending on the sub-stituents and the reaction conditions, they have been shown toassume a variety of coordination modes to form mononuclear[1–8], dinuclear [9] or tetranuclear metal complexes [10–18]. Inthis context, particular attention has been devoted to the substitu-tion patterns which confer the thiocarbohydrazone ligand ditopiccharacter such that molecular rectangles are created [10–17].Moreover, a special example has shown the formation of an octa-nuclear-CuII complex from four fully deprotonated bis(salicylalde-hyde)thiocarbohydrazones, where the ligand used its maximaldonor capacity [19].

We have recently been exploring ligands derived from indole-7-carbaldehyde. The structure of indole-7-carbaldimines resemblesthose of the analogous salicylaldimines: both contain a potentialdonor atom linked to an iminic carbon via two intervening Catoms; however, the donor atom in indole-7-carbaldimines is the

softer and less electronegative N atom versus the O atom in salicyl-aldimines (Scheme 1).

In our previous articles we showed that indole-7-carbaldiminescan coordinate to metal ions in various modes to produce com-plexes incorporating the biologically important indole nucleus[20,21]. In continuation of our program to develop new indole-based complexes with biological significance, bis(indole-7-carbalde-hyde)thiocarbahydrazone was prepared and reacted with NiII, PdII,PtII, CuI and AgI ions. In order to prevent the formation of high orderaggregates and enhance the solubility of the products, which isessential for pharmacological applications, the reactions were con-ducted in the presence of triphenylphosphine. This article dis-cusses the coordination chemistry and the structures of theobtained complexes.

2. Experimental

2.1. Materials and measurements

Indole-7-carbaldehyde was purchased from the Sigma–AldrichCompany. Elemental microanalyses (C, H, N) were carried out ona Perkin-Elmer 2400 elemental analyzer. The NMR spectra wererecorded on a 400 MHz Bruker spectrometer. The IR spectra weretaken on a Perkin-Elmer Spectrum 400 ATR-FT-IR spectrometer.

NH

NN

OH

R R

Scheme 1. Indole-7-carbaldimines vs. salicylaldimines.

458 A.A. Ibrahim et al. / Polyhedron 81 (2014) 457–464

2.2. Synthesis of LH4

A mixture of indole-7-carbaldehyde (2.9 g, 20 mmol) and thio-carbohydrazide (1.06 g, 10 mmol) in methanol (40 mL) containinga few drops of HCl (37%) was stirred for 3 h. The solution was thenpartially evaporated, followed by addition of distilled water to pre-cipitate the yellowish product. It was filtered, washed with 40%aqueous methanol and dried over silica gel. Yield: 3.0 g, 83%. Anal.Calc. for C19H16N6S: C, 63.31; H, 4.47; N, 23.32. Found: C, 63.75; H,4.17; N, 23.28%. 1H NMR (DMSO-d6) d (ppm): 6.63 (2H, br.s, Ar-H);7.17 (2H, t, J = 7.4 Hz, Ar-H); 7.35 (1H, br.s, Ar-H); 7.43 (1H, br.s,Ar-H); 7.58 (2H, br.s, Ar-H); 7.75 (2H, d, J = 7.8 Hz, Ar-H); 8.52 (1H,s, HCNN); 8.83 (1H, s, HCNN); 11.29 (1H, s, indole-NH); 11.55 (1H,s, indole-NH); 12.11 (1H, s, NNH); 12.15 (1H, s, NNH). 13C NMR(DMSO-d6) d (ppm): 102.66, 119.70, 123.38, 124.15, 126.50, 126.99,128.56, 129.22 (Ar); 147.65, 147.94 (HCNN); 175.25 (CS).

2.3. Synthesis of [Cu2Cl2(LH4)2(PPh3)2]

A solution of PPh3 (0.262 g, 1 mmol) in methanol (5 ml) wasadded to a solution of CuCl (0.13 g, 1.3 mmol) in the same solvent(5 ml). The mixture was stirred for 15 min, followed by slow addi-tion of a solution of LH4 (0.36 g, 1 mmol) in 15 mL methanol–ace-tonitrile (50:50) at room temperature. The resulting brownishsolution was kept at 4 �C for five days, upon which olive crystalsof [Cu2Cl2(LH4)2(PPh3)2].6MeOH were formed. After determinationof the structure by X-ray crystallography, the crystals were groundand dried at 50 �C for other analytical purposes. Yield 0.38 g, 53%.Anal. Calc. for C74H62Cl2Cu2N12P2S2: C, 61.57; H, 4.33; N, 11.64.Found: C, 61.44; H, 4.08; N, 11.98%. 1H NMR (DMSO-d6) d (ppm):6.59 (2H, br.s, Ar-H); 6.65 (2H, br.s, Ar-H); 6.99 (2H, br.s, Ar-H);7.16 (2H, t, J = 6.8 Hz, Ar-H); 7.24–7.48 (34H, Ar-H); 7.58 (2H,br.s, Ar-H); 7.69 (4H, m, Ar-H); 7.77 (2H, d, J = 7.7 Hz, Ar-H); 8.55(2H, br.s, HCNN); 8.94 (2H, br.s, HCNN); 11.30 (2H, s, indole-NH);11.33 (2H, s, indole-NH); 12.32 (2H, s, NNH); 12.55 (2H, s, NNH).13C NMR (DMSO-d6) d (ppm): 102.86, 116.82, 117.92, 119.75,119.84, 123.96, 124.14, 124.70, 126.44, 126.96, 127.14, 128.75–129.37, 130.48, 131.57, 133.71–133.96 (Ar); 149.89, 150.29(HCNN); 172.43 (CS).

2.4. Synthesis of [Ag6(LH3)6]

To a solution of AgNO3 (0.17 g, 1 mmol) in methanol (5 ml) wasadded a solution of PPh3 (0.262 g, 1 mmol) in the same solvent(5 ml). The mixture was stirred for 15 min, followed by slow addi-tion of a solution of LH4 (0.36 g, 1 mmol) in 15 mL methanol–acetonitrile (50:50). The mixture was stirred at room temperaturefor 30 min and the resulting clear yellow solution was kept at 4 �Cfor a week to give a yellow precipitate. The solid was filtered,washed with methanol and dried over silica-gel. Yield: 0.23 g. Anal.Found: C, 48.94; H, 3.69; N, 17.32%. Recrystallization from DMF atroom temperature afforded X-ray quality crystals of [Ag6(LH3)6].6DMF. For elemental analysis and IR spectroscopy, the crystalswere ground and dried at 50 �C to remove the solvate molecules.Yield: 0.183 g, 39%. Anal. Calc. for C114H90Ag6N36S6: C, 48.83; H,3.24; N, 17.98. Found: C, 48.61; H, 3.09; N, 17.76%.

2.5. Synthesis of [Ni(LH2)(PPh3)]

A solution of [Ni(PPh3)2Cl2] (0.65 g, 1 mmol) in methanol(10 mL) was added to a solution of LH4 (0.36 g, 1 mmol) in 20 mLmethanol–acetonitrile (50:50), followed by the addition of a fewdrops of triethylamine. The mixture was stirred at room tempera-ture for 3 h and then kept at 4 �C for a week to give brownish crys-tals of [Ni(LH2)(PPh3)].2CH3CN. After determination of thestructure by X-ray crystallography, the crystals were ground anddried at 50 �C for other analytical purposes. Yield: 0.49 g, 72%. Anal.Calc. for C37H29N6NiPS: C, 65.41; H, 4.30; N, 12.37. Found: C, 65.38;H, 4.11; N, 12.24%.

2.6. Synthesis of [Pd(LH2)(PPh3)]

A solution of trans-[Pd(PPh3)2Cl2] (0.70 g, 1 mmol) in methanol(10 mL) was added to a solution of LH4 (0.36 g, 1 mmol) in 10 mLmethanol–acetonitrile (50:50), followed by the addition of a fewdrops of triethylamine. The mixture was stirred at room tempera-ture for 3 h to yield an orange precipitate. The solid was washedwith 50% aqueous methanol and dried at 50 �C. Yield: 0.57 g,79%. Anal. Calc. for C37H29N6PPdS: C, 61.12; H, 4.02; N, 11.56.Found: C, 61.47; H, 3.99; N, 11.33%. 1H NMR (DMSO-d6) d (ppm):6.10 (1H, br.s, Ar-H); 6.51 (2H, br.s, Ar-H); 7.00–7.07 (2H, m, Ar-H); 7.13 (1H, d, J = 6.8 Hz, Ar-H); 7.24 (1H, br.s, Ar-H); 7.41–7.62(11H, Ar-H); 7.67–7.84 (7H, Ar-H); 8.16 (1H, s, HCNN); 8.80 (1H,d, J = 11.8 Hz, HCNN); 10.04 (1H, br.s, indole-NH); 11.68 (1H, s,NNH). 13C NMR (DMSO-d6) d (ppm): 102.42, 102.59, 116.97,117.51, 118.61, 119.90, 122.34, 123.35, 126.09, 126.30, 127.64,128.35 (Ar); 129.21–129.45; 130.11, 130.52, 131.94, 132.43,134.17 (Ar); 135.25 (d, J [31P–13C] = 10.8 Hz, Ar), 139.70 (d, J[31P–13C] = 8.4 Hz, Ar); 142.71, 151.45 (HCNN); 168.80 (CS).

X-ray quality crystals of [Pd(LH2)(PPh3)]�DMSO were obtainedfrom a DMSO solution of the complex on standing for two daysat room temperature.

2.7. Synthesis of [Pt(LH2)(PPh3)]

A solution of cis-[Pt(PPh3)2Cl2] (0.79 g, 1 mmol) in methanol(10 mL) was added to a solution of LH4 (0.36 g, 1 mmol) in 20 mLmethanol–acetonitrile (50:50), followed by the addition of a fewdrops of triethylamine. The mixture was stirred at room tempera-ture for 3 h and then kept at 4 �C for a month to give red–orangecrystals of [Pt(LH2)(PPh3)].2CH3CN. After determination of thestructure by X-ray crystallography, the crystals were ground anddried at 50 �C for other analytical purposes. Yield: 0.73 g, 89%. Anal.Calc. for C37H29N6PPtS: C, 54.47; H, 3.58; N, 10.3. Found: C, 54.29;H, 3.44; N, 10.17%. 1H NMR (DMSO-d6) d (ppm): 6.21 (1H, br.s, Ar-H); 6.57 (1H, br.s, Ar-H); 6.83 (1H, br.s, Ar-H); 7.08 (1H, t, J = 7.6 Hz,Ar-H); 7.18 (2H, t, J = 7.6 Hz, Ar-H); 7.36 (1H, m, Ar-H); 7.47–7.62(11H, Ar-H); 7.73–7.90 (7H, Ar-H); 8.24 (1H, s, HCNN); 9.10 (1H,d, J = 10.7 Hz, HCNN); 10.00 (1H, br.s, indole-NH); 11.87 (1H, s,NNH). 13C NMR (DMSO-d6) d (ppm): 102.66, 103.23, 116.96,117.82, 118.58, 119.93, 122.36, 123.56, 126.31, 126.69, 128.22,128.36, 128.94–129.37, 130.05, 130.66, 132.17–132.63 (Ar);135.29 (d, J [31P–13C] = 11.44 Hz, Ar); 140.12 (d, J [31P–13C] = 6.7 Hz,Ar); 142.85, 149.92 (HCNN); 170.32 (CS).

2.8. Synthesis of [Pd2Cl(LH)(PPh3)3]

A solution of trans-[Pd(PPh3)2Cl2] (0.70 g, 1 mmol) in methanol(10 mL) was added to a hot solution of LH4 (0.36 g, 1 mmol) in10 mL methanol–acetonitrile (50:50), followed by the addition ofa few drops of triethylamine. The mixture was refluxed for 3 hand then evaporated to half of its original volume. The orange solidof the product was precipitated by addition of a small amount of

A.A. Ibrahim et al. / Polyhedron 81 (2014) 457–464 459

distilled water. The solid was washed with 50% aqueous methanoland dried at 50 �C. Yield: 0.40 g, 57%. Anal. Calc. for C73H58ClN6P3

Pd2S: C, 62.96; H, 4.20; N, 6.03. Found: C, 62.45; H, 3.99; N,6.21%. 1H NMR (DMSO-d6) d (ppm): 6.15 (1H, br.s, Ar-H); 6.56(2H, m, Ar-H); 7.05–7.80 (52 H, Ar-H); 8.24 (1H, s, HCNN); 8.86(1H, d, J = 11.90 Hz, HCNN); 10.05 (1H, br.s, indole-NH). 13C NMR(DMSO-d6) d (ppm): 102.47, 102.62, 117, 117.53, 118.64, 119.92,122.37, 123.54, 126.12, 126.32, 127.65, 128.37, 129.23–129.55,130.07, 130.59, 131.97, 132.42, 133.71–133.90 (Ar); 135.26 (d, J[31P–13C] = 11.44 Hz, Ar); 137.12 (d, J [31P–13C] = 11.44 Hz, Ar);142.72, 151.47 (HCNN); 168.89 (CS).

X-ray quality crystals of [Pd2Cl(LH)(PPh3)3].3DMSO wereobtained from a DMSO solution at room temperature.

2.9. Crystallography

Diffraction data were measured with a Bruker SMART Apex IICCD area-detector diffractometer (graphite-monochromatedMoKa radiation, k = 0.71073 Å). The orientation matrix, unit-cellrefinement and data reduction were all handled by the Apex2 soft-ware (SAINT integration, SADABS multi-scan absorption correction).The structures were solved using direct or Patterson methods(SHELXS) and were refined by the full matrix least-squares methodon F2 (SHELXL-2013). All the non-hydrogen atoms were refinedanisotropically. Drawings of the molecules were produced withXSEED [22]. Crystal data and refinement details are summarizedin Tables S1 and S2.

3. Results and discussion

3.1. Structure of LH4

The synthesis of LH4 was easily achieved through the condensa-tion of thiocarbohydrazide with two equivalents of indole-7-carb-aldehyde. The molecule may, in principle, adopt thiol or thionetautometic structures and they can be in so-called ‘‘syn’’ or ‘‘anti’’forms (Scheme 2). The syn geometry provides two NNS pocketsfor metal coordination, whereas the anti geometry affords oneNNN- and one NNS-binding pocket. The absence of the S–H bandin the IR spectrum (at ca. 2700 cm�1) and any signal attributableto the thiol proton in the 1H NMR spectrum indicate that LH4 existspredominantly as a thione tautomer in both the solid state andsolution. In the 1H NMR spectrum, the hydrazinic protons appearas two signals at d = 12.11 and 12.15 ppm. Similarly, each of the

Scheme 2. Isomeric s

indole-NH and also the azomethine protons give a distinct reso-nance, pointing out the inequivalency between the two sides ofthe molecule, compatible with the anti structural form.

LH4 was reacted with monovalent Cu and Ag and divalent Ni, Pdand Pt ions to give the corresponding metal complexes in moderateto high yield.

3.2. Structure of the copper(I) complex

The reaction of LH4 with an equimolar amount of CuCl and PPh3

afforded the Cu(I) complex [Cu2Cl2(LH4)2(PPh3)2] (Scheme 3). Thecrystal structure of the complex is shown in Fig 1. The thiocarbo-hydrazone ligand is almost planar (r.m.s. deviation = 0.151 Å) andadopts an anti geometry. Two neutral thiocarbohydrazones, actingas l2-S-donor ligands, doubly bridge pairs of Cu(I) atoms into acentrosymmetric dimer. The Cu centers within the Cu2(l2-S)2 coreare separated by 3.0681(6) Å, which is larger than the sum of theirvan der Waals radii (2.80 Å). One Cl atom and one PPh3 group com-plete a distorted tetrahedral geometry around each metal center,with coordination angles of ca. 96–119� (Table 1). The geometricparameters of the parallelogram and those pertaining to the metalcenters are compatible with the reported values for similar struc-tures [23–25]. The Cl atom is intramolecularly hydrogen bondedto N3, and intermolecularly H-bonded to a methanol solvate mol-ecule (Table S3). Theoretical calculations on similar dinuclear Cu(I)structures suggested that the hydrogen bonding between the hal-ogen ligands and the solvent molecules plays a crucial role in theformation of S-bridged dimers versus halogen-bridged or mono-meric structures [23].

The 1H and 13C NMR spectra of the molecule in DMSO-d6 are inagreement with the crystal structure, indicating the stability of thestructure in the solution. The 13C NMR spectrum shows an upfieldshift of the CS signal (�3 ppm) from that in the spectrum of LH4.

3.3. Structure of the silver(I) complex

The reaction of LH4 with AgNO3 in the presence of PPh3 affordedthe Ag(I) complex [Ag6(LH3)6] (Scheme 4). Although PPh3 is notincorporated in the structure of the final product, its presence inthe initial reaction was essential for a successful reaction. A similarobservation was made for the synthesis of an Au(I)-thiosemicarba-zone complex [26]. The crystal of the silver complex grew in therhombohedral space group R�3. The structure shows a hexanuclearAg(I) complex with a threefold roto-inversion axis passing through

tructures of LH4.

NHN

NH

HN

N

NH

S

CuCl

HN

N

HN

NH

S

NNH

Cu

NH

N

NH

HN

S

N HN

Cu

ClPh3P

ClPPh3

PPh32 2 2++

Scheme 3. Synthesis of the CuI complex.

460 A.A. Ibrahim et al. / Polyhedron 81 (2014) 457–464

the center (Fig. 2). Six monoanionic thiocarbohydrazone ligands,deprotonated at the hydrazinic N3 atoms, bind to the metal centersthrough their N3 and S atoms. Each Ag atom is coordinated by twobridging sulfur atoms and a nitrogen atom from three ligands in atrigonal plane, with the metal out-of-plane displacement being0.3299(10) Å. This coordination mode results in the creation ofan Ag6S6 core which features two alternate chair-like Ag3S3 rings(Fig. 3). Within the Ag6S6 core, each silver atom is in close proxim-ity to two other silver centers, with a separation of 2.9690(4) Å.This Ag. . .Ag distance is longer than that in metallic silver(2.88 Å), but shorter than the sum of the van der Waals radii oftwo silver atoms (3.44 Å), thus signifying the existence of weakmetal–metal interactions. Similar contacts and Ag6S6 assemblieshave been observed in a few other structures [27–30]. The thiocar-bohydrazone ligand deviates by up to 0.957(4) Å (for C5) from pla-narity and takes the anti form. The structure exhibitsintramolecular N–H. . .N and N–H. . .S hydrogen bonding as wellas intermolecular N–H. . .O and C–H. . .O interactions with DMF sol-vate molecules (Table S3). The low solubility of the Ag(I) complexin organic solvents hampered the study of the solution structure.

3.4. Structures of the mononuclear nickel(II), palladium(II) andplatinum(II) complexes

The reaction of [MCl2(PPh3)2], where M = Ni, Pd and Pt, with LH4

in the presence of triethylamine and at room temperature formed

Fig. 1. Molecular structure of [Cu2Cl2(LH4)2(PPh3)2] with thermal ellipsoids drawnat the 30% probability level. C-bound H atoms and methanol solvent molecules areomitted for clarity. Symmetry code: i = �x + 1, �y + 1, �z + 1.

complexes with the formula [M(LH2)(PPh3)] (Scheme 5). The X-raymolecular structures of the complexes resemble each other(Figs. 4–6), with the crystals of the Pt and Ni complexes being iso-morphous. The thiocarbohydrazone ligand is essentially planar(r.m.s. deviations = 0.171(4), 0.224(2) and 0.123(4) Å for the Ni,Pd and Pt complexes respectively) and adopts the syn configurationto provide two contiguous NNS-binding pockets; however, onlyone of them is occupied by a metal atom. The thiocarbahydrazoneis deprotonated at one indole nitrogen atom and one hydrazinicnitrogen atom and coordinates the metal center in a dianionicN,N’,S-tridentate fashion. One triphenylphosphine ligand com-pletes the square-planar geometry around the metal atom, whileblocking the fourth coordination site of the other NNS pocket.The metal-free NNS pocket forms an S(6) ring through anN4–H. . .N5 hydrogen bond. Crystal structures of PdII- and PtII- thio-carbohydrazones are unprecedented and the closest structures arethe analogous thiosemicarbazone complexes, which show compa-rable coordination bond lengths and angles to the present struc-tures [20]. The Ni complex has similar Ni–S and Ni–N1 distancesto those in the Ni complex of indole-7-carbaldehyde dithiocarbo-hydrazone [21], but Ni–N2 is relatively longer in the presentstructure, which could be due to the trans-influence of the PPh3

ligand.The IR spectra of the three complexes (after removal of the sol-

vate molecules) closely resemble each other (see supplementarymaterials). The 1H and 13C NMR spectroscopy of the Ni complexsuggested the decomposition of the compound in DMSO solution.In contrast, the 1D- and 2D-NMR spectra of the Pd and Pt complexes

Table 1Selected bond lengths [Å] and angles [�] for the Cu and Ag complexes.

[Cu2Cl2(LH4)2(PPh3)2].6MeOH [Ag6(LH3)6].6DMF

Bond lengthsCu–P 2.2376(7) Ag–N(3)#2 2.255(2)Cu–Cl 2.3430(7) Ag–S 2.4656(7)Cu–S#1 2.3559(7) Ag–S#3 2.5064(7)Cu–S 2.4413(7) Ag–Ag#4 2.9690(4)Cu–Cu#1 3.0681(6) Ag–Ag#2 2.9691(4)S–C(10) 1.723(2) S–C(10) 1.774(3)N(2)–N(3) 1.389(3) N(2)–N(3) 1.404(3)N(3)–C(10) 1.327(3) N(3)–C(10) 1.290(4)N(5)–N(6) 1.378(3) N(5)–N(6) 1.360(4)N(6)–C(10) 1.340(3) N(6)–C(10) 1.367(4)

Bond anglesP–Cu–Cl 114.77(3) N(3)#1-Ag–S 127.42(7)P–Cu–S#1 111.54(3) N(3)#1-Ag–S#3 117.56(7)Cl–Cu–S#1 113.14(2) S–Ag–S#3 109.34(3)P–Cu–S 118.81(2)Cl–Cu–S 96.62(2)S#1-Cu–S 100.51(2)

Symmetry transformations used to generate equivalent atoms: #1 �x + 1, �y + 1,�z + 1; #2 y, �x + y + 1, �z + 1; #3 �x + y + 1, �x + 2, z; #4 x � y + 1, x, �z + 1.

Ag

S

Ag

S

Ag

S

SAg

S

AgS

Ag

NN

NH

HN

N

NH

NN

NH

HN N HN

N NHN

NH

N

NH

NN

NH

HNN

HN

NNNH

HN

N

NH

NN

HNNH

NNH

NHN

NH

HNN

NH

S AgNO3

Scheme 4. Synthesis of the AgI complex.

Fig. 2. The hexagonal structure of [Ag6(LH3)6].6DMF. The hydrogen atoms of thethiocarbohydrazone ligands, except for those engaged in H-bonding, are omitted forclarity. Dashed lines represent the hydrogen bonds.

Fig. 3. Crystal structure of the Ag(I) complex showing the atom labelling scheme.Thermal ellipsoids are drawn at the 30% probability level. Symmetry codes:i = x � y + 1, x, �z + 1; ii = �y + 2, x � y + 1, z; iii = �x + y + 1, �x + 2, z; iv = y,�x + y + 1, �z + 1; v = �x + 2, �y + 2, �z + 1.

NHNNH

HN

N

NH

S [MCl2(PPh3)2] , Et3N

N

N

N

HN

S

N HN

M PPh3

M = NiII, PdII and PtII

Scheme 5. Synthesis of the mononuclear NiII, PdII and PtII complexes.

A.A. Ibrahim et al. / Polyhedron 81 (2014) 457–464 461

Fig. 4. Molecular structure of [Ni(LH2)(PPh3)] with thermal ellipsoids drawn at the50% probability level. Acetonitrile solvate molecules are not shown.

Fig. 5. Molecular structure of [Pd(LH2)(PPh3)] with thermal ellipsoids drawn at the50% probability level. DMSO solvate molecules are not shown.

Fig. 6. Molecular structure of [Pt(LH2)(PPh3)] with thermal ellipsoids drawn at the50% probability level. Acetonitrile solvate molecules are not shown.

Fig. 7. Molecular structure of [Pd2Cl(LH)(PPh3)3] with thermal ellipsoids drawn atthe 50% probability level. The C-bound H atoms are omitted for clarity. DMSOsolvate molecules are not shown.

462 A.A. Ibrahim et al. / Polyhedron 81 (2014) 457–464

(see supplementary materials) are consistent with the crystal struc-tures. In the 1H NMR spectra the HCNN resonance of the coordi-nated azomethine groups appears as a doublet due to couplingwith the phosphorus atom. The phosphorus atom is also coupled

NHNNH

HN

N

NH

S [PdCl2(PPh3)2] , E

Scheme 6. Synthesis of the

with some of the carbon atoms, splitting their 13C signals. The 13CNMR spectra show an upfield shift of the CS signal (5–7 ppm) anda downfield shift in the signal of the coordinated azomethine HCNN(2–3 ppm) from those in the spectrum of LH4.

t3N

N

N

N

N

S

N HN

Pd PPh3

PdCl

Ph3P

PPh3

dinuclear PdII complex.

Table 2Selected bond lengths [Å] and angles [�] for the Ni, Pd and Pt complexes.

[Ni(LH2)(PPh3)].2CH3CN [Pd(LH2)(PPh3)].DMSO [Pt(LH2)(PPh3)].2CH3CN [Pd2Cl(LH)(PPh3)3].3DMSO

Bond lengthsM(1)–N(1) 1.907(3) 2.0454(17) 2.035(3) 2.063(3)M(1)–N(2) 1.936(3) 2.0645(17) 2.061(3) 2.063(3)M(1)–S(1) 2.1718(11) 2.2654(5) 2.2800(11) 2.2635(14)M(1)–P(1) 2.2191(11) 2.2692(5) 2.2526(11) 2.2671(15)M(2)–N(6) 2.022(3)M(2)–P(3) 2.3184(13)M(2)–P(2) 2.3253(13)M(2)–Cl(1) 2.3299(14)S(1)–C(10) 1.748(4) 1.749(2) 1.747(5) 1.748(4)N(2)–N(3) 1.400(4) 1.394(2) 1.385(5) 1.394(4)N(3)–C(10) 1.285(5) 1.299(3) 1.302(5) 1.320(4)N(5)–N(6) 1.362(4) 1.367(3) 1.370(5) 1.347(4)N(6)–C(10) 1.364(5) 1.368(3) 1.368(6) 1.357(4)

Bond anglesN(1)–M(1)–N(2) 92.49(12) 91.46(7) 90.83(13) 91.66(12)N(1)–M(1)–S(1) 178.21(9) 173.68(5) 174.36(10) 175.38(9)N(2)–M(1)–S(1) 86.33(9) 84.11(5) 83.59(10) 84.22(9)N(1)–M(1)–P(1) 94.04(9) 93.81(5) 93.92(10) 94.32(9)N(2)–M(1)–P(1) 173.46(9) 173.07(5) 175.25(10) 174.00(9)S(1)–M(1)–P(1) 87.13(4) 90.969(19) 91.66(4) 89.81(4)N(6)–M(2)–P(3) 90.54(9)N(6)–M(2)–P(2) 89.82(9)P(3)–M(2)–P(2) 174.37(4)N(6)–M(2)–Cl(1) 174.18(9)P(3)–M(2)–Cl(1) 89.88(4)P(2)–M(2)–Cl(1) 90.32(4)

A.A. Ibrahim et al. / Polyhedron 81 (2014) 457–464 463

3.5. Structure of the dinuclear palladium(II) complex

When the reaction of LH4 with [PdCl2(PPh3)2] was carried out atreflux temperature, a dinuclear complex with the formula[Pd2Cl(LH)(PPh3)3] was obtained (Scheme 6). The structurerepresents an example of a complex with a triply deprotonatedthiocarbohydrazone ligand. Similar to the structure of the mono-nuclear Pd complex, the thiocarbohydrazone is in its syn formand accommodates a PdII ion in a dianionic NNS pocket to form abicyclic chelate ring (Fig. 7). A PPh3 ligand occupies the fourthcoordination site of the square planar geometry around the Pdcenter and blocks the fourth coordination site of the other NNSpocket. Therefore, the second PdII ion is coordinated externally tothe pocket via the deprotonated hydrazinic N6 atom. One chlorideand two PPh3 ligands complete the square planar geometry of thesecond Pd center. The N4 containing indole ring is N4–H. . .N5bonded to the azomethine N atom and is twisted with respect tothe bicyclic chelate ring by 32.86(7)�. Selected bond lengths andangles for the crystal structure are listed in Table 2. While the geo-metrical parameters associated with Pd1 are comparable to thosepertaining to the Pd center in the mononuclear structure, the Pd2coordination environment exhibits relatively longer Pd–P andshorter Pd–N distances, consistent with the trans-influence of thePPh3 ligands.

The NMR spectra of the complex in DMSO-d6 are in accordancewith the solid state structure and features similar P–C and P–Hcouplings as observed in the mononuclear Pd and Pt complexes.

4. Conclusions

LH4, a bis-Schiff base thiocarbohydrazone, is a potential poly-dentate ligand with four labile hydrogens which can undergo dif-ferent degrees of deprotonation on coordination to metal ions.This article presents examples of complexes with the thiocarbo-hydrazone acting as a neutral (CuI complex), monoanionic (AgI

complex), dianionic (NiII, PdII and PtII complexes) or trianionic (PdII

complex) ligand. Owing to the presence of two indolic rings in the

structure, the ligand offers two tridentate binding pockets, whichdepending on the geometry of the ligand could be of the NNN orNNS type. Metal binding by an NNS pocket occurred in the NiII, PdII

and PtII complexes, whereas occupation of the NNN pockets wasnot observed in this study. The coordination mode adopted bythe thiocarbohydrazone ligand is not only dependent on the natureof the metal, but also on the reaction conditions. The versatile coor-dination modes of the ligand led to the formation of complexeswith different nuclearities, i.e. mononuclear NiII, PdII and PtII, dinu-clear CuI and PdII and hexanuclear AgI complexes.

Acknowledgements

Financial support from the University of Malaya (HIR grantUM.C/625/1/HIR/151, Non-Ru grant CG033-2013 and PPP grantPV048/2012A) and the Center for Natural Products and DrugResearch, CENAR, (FL001-13BIO) is highly appreciated.

Appendix A. Supplementary data

CCDC 997103–997109 contain the supplementary crystallo-graphic data for this paper. These data can be obtained free ofcharge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, orfrom the Cambridge Crystallographic Data Centre, 12 Union Road,Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail:deposit@ccdc.cam.ac.uk. Supplementary data associated with thisarticle can be found, in the online version, at http://dx.doi.org/10.1016/j.poly.2014.06.057.

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