Synthesis, characterization and conducting properties of complex salts and heterobimetallic coordination polymers of the cyanodithioimidocarbonato ligand

Inorganic Chemistry Communications 9 (2006) 1058–1062 www.elsevier.com/locate/inoche

Synthesis, characterization and conducting properties of complex salts and heterobimetallic coordination polymers of the cyanodithioimidocarbonato ligand q Nanhai Singh *, Akhilesh Prasad, Rohit Kumar Sinha

1

Department of Chemistry, Banaras Hindu University, Varanasi 221 005, India Received 23 May 2006; accepted 3 July 2006 Available online 7 July 2006

Abstract The complex salts ðNPrn4 Þ2 ½M0 ðcdcÞ2  and the heterobimetallic coordination polymers [MM 0 (cdc)2] [M = Zn(II), Cd(II), Hg(II), 2Ag(I) or Pb(II); M 0 = Ni(II) or Cu(II); cdc2 = cyanodithioimidocarbonate] have been prepared and characterized by elemental analysis, solution and solid state conductivity, magnetic susceptibility, IR, 1H and 13C NMR, UV–Vis, EPR and FAB-mass spectrometry. All of the compounds exhibited semiconducting behaviour.  2006 Elsevier B.V. All rights reserved. Keywords: Complex salts; Heterobimetallic coordination polymers; Cyanodithioimidocarbonato; Semiconductivity;

Synthesis and study of properties of coordination polymers belong at present to one of the most fascinating fields in materials chemistry. The heterobimetallic complexes in general, including those of chalcogen rich ligands, are of great interest because of their structural diversities, cooperative bimetallic activity, molecular magnetism, electrical conductivity and as initiators for photooxidation reactions [1–19]. One of the important synthetic routes for the heterobimetallic coordination polymers is via the formation of metal complex building blocks called as metalloligands. A metalloligand is advantageous for designing of the heterometallic polymeric frameworks due to availability of free multidonor sites which could facilitate coordination to other metal ions with diverse electronic properties, coordination numbers and geometries. Complex anions of dithioq Presented at the 11th Symposium on Modern Trends in Inorganic Chemistry (MTIC-XI), December 8–10, 2005, Indian Institute of Technology Delhi, New Delhi, India. * Corresponding author. Tel.: +91 542 2307321x108; fax: +91 542 2368174. E-mail address: [email protected] (N. Singh). 1 Present address: Chemical Research Laboratory, Wockhardt Ltd., Aurangabad, India.

1387-7003/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2006.07.001

13

C NMR

ligands [M(dithio)2]n[M = transition metal ions; dithio = dinegative 1,1- and 1,2-dithioligands] are considered to be useful metalloligands for the construction of the heterometallic aggregates. Considerable attention has been paid to the synthesis, coordination chemistry and study of properties of heterobimetallic complexes [10,18–20] of 1,1- and 1,2-bifunctional unsaturated sulfur containing ligands such as isomaleonitriledithiolate (i-mnt2), 1,3-dithiole-2-thione-4,5-dithiolate (dmit2), maleonitriledithiolate (mnt2) and 1-ethoxycarbonyl-1-cyanoethylene-2,2-dithiolate (ecda2). Albeit complex salts [21–25] and a few homobimetallic complexes [21,24] such as [{Pt(cdc)(PMe2Ph)}2] and ½fRuðcdcÞðg6 -p-MeC6 Hi4 PrÞg2  of the ligand cyanodithioimidocar2 bonate ðC2 N2 S2 2 ; cdc Þ are reported, little attention has been paid to the synthesis of heterobimetallic complexes of this ligand, initially called ‘‘dithiocyanate’’ ion. To the best of our knowledge the only reported heterobimetallic [26] complex of this ligand is [Ag(PPh3)2]2[Ni(cdc)2]. The ligand cdc2 is akin (Fig. 1) to mnt2, i-mnt2 and ecda2. Nevertheless, significant points associated with this ligand are: (i) One of the carbon atom and its substituents are replaced by the N–C„N moiety and the terminal C–N

N. Singh et al. / Inorganic Chemistry Communications 9 (2006) 1058–1062

N

N C

SC

C N

C S

C

-

S

C

-

C2H5 O

S-

S-

C

C

N mnt2-

N

S-

C

C

1059

N

C S-

C

C S-

C N

O i-mnt 2-

ecda2-

cdc2-

Fig. 1. Structure of the ligands.

thereby reducing the planar symmetry of the ligand in monometallic complexes. (ii) This ligand has the potential to bridge two metal centers in the homo- and heterobimetallic complexes. (iii) Like the ligand i-mnt2, the ligand cdc2 may exhibit greater electron delocalization in its complexes through C–S, C@N, and N–C„N bonds which was suggested on the basis of surface enhanced Raman scattering [27] studies on i-mnt2 and (iv) Upon coordination, cdc2 forms four-membered chelate rings as compared to fivemembered rings in mnt2 complexes. This difference in behavior may result in varying degrees of strain thereby influencing the inter- and intraligand S–S distances which virtually affect the overall geometry [20,28] and molecular stacking of the complexes in the solid state. Staying with the above points herein we present the synthesis, characterization and properties of the complex salts ðNPrn4 Þ2 ½M0 ðcdcÞ2  and the heterobimetallic complexes [MM 0 (cdc)2] [M = Zn(II), Cd(II), Hg(II), 2Ag(I) or Pb(II); M 0 = Ni(II) or Cu(II)]. The ligand dipotassium cyanodithioimidocarbonate, K2C2N2S2 (K2cdc) was prepared according to literature procedure [29] and characterized by elemental analysis, IR and 13C NMR spectroscopies. Elemental analyses and experimental details pertaining pressed pellet conductivity, recording of IR, 1H and 13C NMR, UV–Vis and X-band EPR spectra were the same as described earlier [18,19]. The complex salts K2[M 0 (cdc)2] were prepared in situ according to reported methods [22,23] with slight modification. To a stirred methanol–water (60:40, v/v), 25 mL solution of K2cdc (0.39 g, 2 mmol), was added gradually 5 mL solution of Cu(NO3)2 Æ 3H2O(0.24 g, 1 mmol) in the same solvent mixture over a period of 15 min. The reddish– brown precipitate presumably of Cu(cdc) initially formed, is readily dissolved producing a wine-red solution of K2[Cu(cdc)2] (0.37 g, 1 mmol). This was additionally stirred for 30 min. K2[Cu(cdc)2] thus prepared was filtered to a 5 mL aqueous solution of NPrn4 I (0.63 g, 2 mmol) and stirred for 15 min. The microcrystalline orange–yellow complex salt ðNPrn4 Þ2 ½CuðcdcÞ2  formed immediately and was collected with the suction filtration and washed with methanol–water mixture, then methanol followed by diethyl ether and dried in vacuo over CaCl2. The dark green complex salt ðNPrn4 Þ2 ½NiðcdcÞ2  was prepared similarly using NiCl2 Æ 6H2O(0.24 g, 1 mmol), K2cdc (0.39 g, 2 mmol) and NPrn4 I (0.63 g, 2 mmol) in the above

solvent mixture. In this case the olive green precipitate of Ni(cdc) initially formed, is instantly dissolved resulting in a dark green solution of K2[Ni(cdc)2]. This 30 mL solution of K2[Ni(cdc)2] (0.37 g, 1 mmol) was filtered to an aqueous 5 mL solution of NPrn4 I (0.63 g, 2 mmol) and stirred for 15 min. The dark-green microcrystalline solid formed in a little while was filtered off washed with the methanol–water mixture, then methanol followed by diethyl ether and dried in vacuo over CaCl2. The heterobimetallic complexes have been prepared by utilizing the dipotassium salt of the metalloligand [M 0 (cdc)2]2 prepared in solution as described above. To a stirred 30 mL methanol–water solution of K2[M 0 (cdc)2] as outlined above, was added 5 mL of an aqueous solution of ZnSO4 Æ 7H2O (0.29 g, 1 mmol), CdCl2 Æ 2.5H2O (0.23 g, 1 mmol), Hg(NO3)2 Æ H2O (0.34 g, 1 mmol), AgNO3 (0.34 g, 2 mmol) or Pb(NO3)2 (0.33 g, 1 mmol). In each case the resulting mixture was stirred for 1 h. The solid, coloured compounds [30] formed in a little while were suction filtered, washed with methanol–water, methanol and finally with diethyl ether and dried in vacuo over CaCl2. Treatment of K2[M 0 (cdc)2] [M 0 = Ni(II) or Cu(II)] generated in in situ by reacting solutions of metal salts M 0 X2 and the ligand K2cdc in 1:2 molar ratio in a methanol– water mixture with two equivalents of a solution of NPrn4 I or one equivalent of metal salts MX2 solutions in the same solvent mixture yielded complex salt ðNPrn4 Þ2 ½M0 ðcdcÞ2  and the heterobimetallic complexes [MM 0 (cdc)2]. The ligand dipotassium cyanodithioimidocarbonate and the complexes were prepared according to the following reactions: 1. 2. 3. 4.

CS2 + H2NCN + 2KOH ! K2C2N2S2 (K2cdc) + 2H2O M 0 X2 + 2K2cdc ! K2[M 0 (cdc)2] + 2KX K2 ½M0 ðcdcÞ2  þ 2NPrn4 I ! ðNPrn4 Þ2 ½M0 ðcdcÞ2  þ 2KI K2[M 0 (cdc)2] + MX2 ! [MM 0 (cdc)2] + 2KX M = Zn(II), Cd(II), Hg(II), 2Ag(I) or Pb(II); M 0 = Ni(II) or Cu(II); X = Cl, NO or SO2 3 4 ; 2 2 cdc ¼ C2 N2 S2 (cyanodithioimidocarbonate)

These are air-stable solids and melt/decompose in the 102–>300 C temperature range. Micro-analytical data are consistent with the formulation of the complexes. The identity and purity of the compounds were confirmed by elemental analysis, IR, 1H and 13C NMR and positive

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ion FAB-mass spectrometry. The heterobimetallic complexes [MM 0 (cdc)2] are scarcely soluble in water, ethanol, methanol, acetone, dichloromethane, acetonitrile, benzene and dimethylformamide which limits the study of their solution properties such as solution conductivity measurement, NMR spectra and growing of single crystals. The salts ðNPrn4 Þ2 ½M0 ðcdcÞ2  [M 0 = Ni(II) or Cu(II)] are soluble in dimethylsulfoxide, KM  120 X1 cm2 mol1 is in agreement with their 2:1 ionic formulation [31]. Positive ion FAB-mass spectrum of ðNPrn4 Þ2 ½CuðcdcÞ2  does not show the molecular ion peak, however six prominent peaks at m/z, 525, 489, 295, 186, 142 and 114 with relative intensities 40, 15, 10, 100, 20 and 30 arise from the (C3H7)4N Æ C3H7-[Cu(C2N2S2)2]+, [{(C3H7)4N}2 Æ C2N2S2]+, ½CuðC2 N2 þ + + 2 þ S2 Þ2 2  , (C3H7)4N , (C3H7)3N and ½C2 N2 S2  fragments of this complex. In the IR spectra [21–23] of the ligand K2cdc, a strong band is observed for m(C„N) at 2151 cm1 and a strong absorption for m(C@N) at 1317 cm1.The corresponding stretching frequencies are located in the 2160–2185 cm1 and 1352–1427 cm1 ranges for the complex salts ðNPrn4 Þ2 ½M0 ðcdcÞ2  and the heterobimetallic complexes [MM 0 (cdc)2]. This observation clearly reveals the noninvolvement of the C„N group of the ligand in bonding. Considerable increase in the imido stretches m(C@N) in these complexes compared to the uncoordinated anionic ligand cdc2 indicate change in delocalization as a result of coordination of sulfur atoms of the ligand. It is noteworthy that m(C@N) for the heterometallic complexes lies in the 1352–1416 cm1 range as compared to complex salts at 1425 cm1. This shows even greater delocalization because of the additional (S, S) coordinating behaviour of the chelated cdc2 ligand in the complex salts to the added metal ions in a bridging fashion [21,26] in the heterobimetallic complexes. In the case of the complex salts ðNPrn4 Þ2 ½M0 ðcdcÞ2  the bands at 2900 cm1 due to C–H stretching absorptions of NPrn4 group are also observed. The features of the 1H and 13C NMR spectra of the ðNPrn4 Þ2 ½NiðcdcÞ2  complex differs significantly from that of ðNPrn4 Þ2 ½CuðcdcÞ2 . Sharp resonances at d 0.9 (triplet), d 1.6 doublet (broad) and d 3.15 (quartet) ppm for –CH3, –CH2 and –NCH2 protons of NPrn4 group in the diamagnetic salt ðNPrn4 Þ2 ½NiðcdcÞ2  are observed almost at the same positions as broad signals for the paramagnetic salt ðNPrn4 Þ2 ½CuðcdcÞ2 . It reveals that in dmso-D6 solution the diamagnetic nature of the nickel(II) complex is retained and that no perceptible interaction of solvent molecules with the metal ion occur. The resonance signal at d 3.35 ppm may be due to HOD. In the 13C NMR spectra, signals at d 227.46 and d 121.52 ppm due to the S2C@N and N–C„N moieties of the free ligand K2cdc are located at d 214.50 and d 114.56 ppm, respectively for the complex salt ðNPrn4 Þ2 ½NiðcdcÞ2 . Additionally, the resonances at d 59.30, d 14.82 and d 10.56 ppm represent the –NCH2, –CH2 and –CH3 carbons of the propyl group. The corresponding 13 C signals for the analogous paramagnetic copper(II)

complex salt observed at d 201.43, d 116.30–117.94 (triplet), d 59.51, d 14.96, and d 10.78 ppm, respectively, are relatively broad [32]. A perceptible downfield shift in the S2C@N carbon for both complex salts compared to the free ligand cdc2 shows the coordinated nature of the ligand through sulfur atoms. Conspicuous appearance of three resonances at d 116.30, 117.14 and 117.94 ppm for ðNPrn4 Þ2 ½CuðcdcÞ2  may be attributed to the presence of a trace of undetectable copper(I) impurities interacting with the nitrile group (–C„N) of the ligand and also to the interaction of the unpaired electron on Cu(II), 3d9 with the 13C nucleus in solution. Effective magnetic moment 1.75 BM for the complex salt ðNPrn4 Þ2 ½CuðcdcÞ2  is consistent with one unpaired electron on the copper(II) centre and rules out any possibility of copper–copper interaction in this complex. Its powder and DMSO solution X-band EPR spectra at liquid nitrogen temperature show well resolved parallel and perpendicular features into two sets of four lines. The g values for the solution spectrum gi = 2.0885 and g^ = 2.0152 together with Ai = 158 · 104 cm1 and A^ = 55 · 104 cm1 show square planar geometry [33–35] about copper(II). The value of gav = 2.0396 with the trend gi > g^ > ge is indicative of the presence of unpaired electron in the dx2 y 2 orbital of copper(II) in the ground state. The powder and solution (LNT) spectra of this salt are almost comparable suggesting that the proposed geometry about the copper(II) centre is retained in the solid as well as in frozen solution. The leff = 1.58–1.62 BM for copper containing neutral heterobimetallic complexes [MCu(cdc)2] is little bit lower than for one unpaired electron. This lowering in magnetic moment may be attributed to weaker inter-chain or interlayer exchange interaction between copper(II) ions in the polymeric array of these compounds in the solid state. In the powder EPR spectra of these complexes the electronic coupling between Cu(II) and M [M = Zn(II), Cd(II), Hg(II), 2Ag(I) and Pb(II) or Cu(II)–Cu(II) are not detected because of the diamagnetic nature of the metal ions M and weaker interactions between copper ions in the polymeric solid and show only an isotropic signal with g value  2.04 indicating presence of an unpaired electron in the d x2 y 2 orbital [34] of copper(II) in the ground state. The DMSO solution electronic spectrum of the salt ðNPrn4 Þ2 ½CuðcdcÞ2  shows weak bands at 620 nm (e = 2.5 · 102 M1 cm1) and 440 nm (e = 5 · 103 M1 cm1) which are assigned to d–d and M S (cdc2) charge transfer transitions, respectively, following square planar coordination about copper(II). Nujol mull electronic spectra of the bimetallic complexes [MCu(cdc)2] [M = Zn(II), Cd(II), Hg(II), 2Ag(I) or Pb(II)] closely resemble each other and show a broad structured band ranging from 350 to 640 nm. The lower energy end of this band i.e.  600 nm can be assigned to a d–d transition while the higher energy region i.e.  450 nm corresponds to a Cu(II) S (cdc2) charge transfer transition following square planar [22,23,31,36] environment about copper(II) in these complexes.

N. Singh et al. / Inorganic Chemistry Communications 9 (2006) 1058–1062

1061

2N

N

(NPr4n)2 +

C

C

C

S

S

C

M' S

N

S

N

(NPr4n )2 [M'(cdc)2 ] M' = Ni(II) or Cu(II) N

M S

S N

C

C

C

M' S

N

C N

S

Fig. 3. Temperature dependence of electrical conductivities of the heterobimetallic complexes.

M

[MM'(cdc)2] M' = Ni(II) or Cu(II) ; M = Zn(II), Cd(II), Hg(II) or Pb(II)

N

N

N C

C

C N

N

C

C Ag

S

S

S

M' S

N C S

M'

M' Ag

S C

S

S Ag S C

N

N C

N

S C

N C

S

Ag S

C N

N

[Ag2 M'(cdc)2] M' = Ni(II) or Cu(II) Fig. 2. Suggested structure of the complex salts and heterobimetallic complexes.

The diamagnetism of ðNPrn4 Þ2 ½NiðcdcÞ2  and [PbNi(cdc)2] and the weakly paramagnetic behaviour leff = 1.09 and 1.17 BM for the [CdNi(cdc)2] and [Ag2Ni(cdc)2] respectively suggest square planar coordination about nickel(II) in these species. The small parmagnetism may be arising as a result of weak axial intermolecular interaction of sulfur or nitrogen atoms of the ligand cdc2 with nickel(II) centers in the polymeric aggregate. The DMSO solution electronic absorption spectrum of ðNPrn4 Þ2 ½NiðcdcÞ2  encompasses bands at 620 nm (e = 4.5 · 102 M1 cm1) assignable to the dx2 y 2 ! dxy transition and the bands at 520 nm (e = 4.2 · 103 M1 cm1), 440 nm (e = 2.8 · 103 M1 cm1) and 420 nm (e = 9.8 · 103 M1 cm1) due to Ni(II) S (cdc2) charge transfer and intra-ligand charge transfer transitions following a square planar coordination [22,23,36] environment around nickel(II) in this complex. In Nujol mulls the feature of the electronic absorption bands in the 350–600 nm region are quite similar to those

of the diamagnetic [PbNi(cdc)2] and paramagnetic [CdNi(cdc)2] and [Ag2Ni(cdc)2] complexes. The lower energy region of the bands can be assigned to dx2 y 2 ! dxy transitions and the higher energy region to Ni(II) S (cdc2) charge transfer transitions for square planar geometry about nickel(II). In all the complexes the absorption bands below 400 nm correspond to intra-ligand chargetransfer transitions. Despite our best efforts we could not grow the single crystals of the complexes for X-ray crystallographic structural elucidation. However, based on the aforementioned discussion and by analogy with the known structures of dithiocomplexes [20,22,26], the ionic structures for the complex salt ðNPrn4 Þ2 ½M0 ðcdcÞ2  and the polymeric structure (Fig. 2) for the heterobimetallic complexes [MM 0 (cdc)2] where both copper(II) and nickel(II) possess square planar, silver(I) linear and remaining metal ions Zn(II), Cd(II), Hg(II) and Pb(II) tetrahedral geometry have been tentatively suggested. Pressed pellet electrical conductivities of the complexes were measured using two probe technique in the 303– 393 K temperature range. All the compounds exhibit room temperature conductivity rrt in the 109–108 S cm1 range. Some of the compounds exhibit semiconductivity in the given temperature range as their conductivity increases with the increase in temperature. The small values of rrt may be attributed to weaker intermolecular interaction through S  S or M  S bonding in the solid state. The plot (Fig. 3) of log r verses T1 is nearly linear and provides an activation energy of 0.20–1.9 eV in the considered temperature range. The conductivities of the diamagnetic and paramagnetic complexes are close indicating that the unpaired electron is not playing an important role in the conductivity mechanism. Acknowledgement Thanks are due to the Council of Scientific and Industrial Research (CSIR), New Delhi, for financial support to R.K Sinha, in the form of Senior Research fellowship.

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