Syntheses, structures and properties of homo- and heterobimetallic complexes of the type [Zn(tren)NCS]2[M(NCS)4] [tren = tris(2-aminoethyl)amine; M = Zn, Cu]

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Abstract

A 2:2:1:6 molar ratio of Zn(ClO4)2·6H2O, tris(2-aminoethyl)amine (tren), Zn(ClO4)2·6H2O/Cu(ClO4)2·6H2O and NH4NCS in methanol–water solution mixtures affords homo-/heterobimetallic compounds of the type [Zn(tren)NCS]2[M(NCS)4] (M = Zn, 1; M = Cu, 2) which have been characterized using microanalytical, spectroscopic, magnetic and other physicochemical results. The structures of the compounds are determined by X-ray diffraction measurements. Structural analyses reveal that 1 and 2 are isomorphous and consist of two discrete [Zn(tren)NCS]+ cations and a [M(NCS)4]2− (M = Zn/Cu) anion. Zinc(II) centers in the [Zn(tren)NCS]+ units adopt distorted trigonal bipyramidal geometry with ZnN5 chromophores coordinated through four N atoms of tren and one N atom of terminal thiocyanate. Each metal(II) center in [M(NCS)4]2− has a distorted tetrahedral coordination environment with an MN4 chromophore ligated by four N atoms of the terminal thiocyanates. In solid state, doubly N–H…S hydrogen bonded 1D chains of [Zn(tren)NCS]+ cations are interconnected by tetrahedral [Zn(NCS)4]2−/[Cu(NCS)4]2− anions through cooperative N–H…S and N–H…N (in 1) and N–H…S and C–H…S (in 2) hydrogen bonds resulting in 3D network structures. Establishment of such networks seems to be aiding the crystallization.

Introduction

The importance of hydrogen bonds [1], [2], [3] in the context of supramolecular chemistry [4], [5], [6], crystal engineering [7], [8] and molecular recognition [9], [10] has been well recognized in recent times. This secondary interaction has ramifications in systematic design of new functional materials [11], [12], [13], [14]. Bifunctional ligands that contain both metal ion binding groups and hydrogen bond donor and acceptor centers have pivot influence in the rational design of different metal–organic frameworks (MOF’s) with tunable properties. Pseudohalides like azide and thiocyanate are well known [15], [16], [17] for their versatile coordination motifs resulting in different 1D, 2D and 3D coordination polymers and polymer-based superstructures. Self-assembly [18] of the metal ions as geometry setters, organic ligands as spacers and pseudohalides as covalent and/or non-covalent bridges is an efficient approach to construct different metal–organic coordination frameworks [19], [20]. A popular approach in coordination chemistry is the use of cyanometallates [M(CN)n]Z− (M = VII, CrIII, MnIII and FeIII, n = 6, Z = 4 or 3; M = NiII, n = 4, Z = 2; M = MoV, n = 8, Z = 3) with multiple connectors to prepare multidimensional polymers and polymer-based superstructures in combination with suitable complementary units containing potential acceptor sites [21], [22], [23]. Recently, we are interested to extend this view with other hexapseudohalogenometallates specially with thiocyanate and reported the use of molecular ions [Cd(NCS)6]4− and [Mn(NCS)6]4− as multiple connectors [24], [25], [26] with suitable complementary units containing 3d/4d metal–ion centers ligated by tris(2-aminoethyl)amine (tren) and with potential acceptor sites. A range of molecular and crystalline architectures with different homo- and heteropolymers and bimetallic complexes is the result from such study. [Cd(NCS)6]4− forms 2D polymers of the type [Cd3(tren)2(NCS)6]n, [Ni2Cd(tren)2(NCS)6]n [24] and a trinuclear compound [Cu2Cd(tren)2(NCS)6] [25]; but [Mn(NCS)6]4− in combination with [Cu(tren)(NCS)]+ results in a bimetallic complex of the type [Cu(tren)(NCS)]4[Mn(NCS)6] [26] with hydrogen bonded network instead of covalently bonded polymers showing hydrogen bonding as the key factor in this complex leading to superstructure. Keeping in mind such difference in behaviour of cadmium(II) and manganese(II), we have extended this work with tetrapseudohalogenometallate such as [MII(NCS)4]2− (M = Zn and Cu). The two 3d ions copper(II) and zinc(II) find significant use in bioinorganic chemistry [27], [28], [29], [30]. A 2:2:1:6 molar ratio of Zn(ClO4)2·6H2O, tren, Zn(ClO4)2·6H2O/Cu(ClO4)2·6H2O and NH4NCS in aqueous methanol gives rise to bimetallic compounds of the type [Zn(tren)NCS]2[M(NCS)4] (M = Zn, 1; M = Cu, 2). Single crystal X-ray crystallographic studies show the presence of discrete [Zn(tren)NCS]+ and [Zn(NCS)4]2− in 1 and, [Zn(tren)NCS]+ and [Cu(NCS)4]2− in 2 which are hydrogen bonded to form 3D continuum. The details of syntheses, structures and other physicochemical properties are described below.

Section snippets

Materials

High purity tris(2-aminoethyl)amine (Fluka, Germany), ammonium thiocyanate (E. Merck, India), copper(II) carbonate (E. Merck, India), zinc(II) carbonate (E. Merck, India) and perchloric acid (E. Merck, India) were purchased from respective concerns and used as received. Copper(II) and zinc(II) perchlorate hexahydrates were prepared [31] by treatment of their respective carbonates with perchloric acid followed by slow evaporation on a steam-bath, filtration through a fine glass-frit, and

Synthesis and formulation

The homo- and heterobimetallic compounds were initially formed by reaction of [Zn(tren)(H2O)]2+ with [Zn(NCS)4]2− and [Cu(NCS)4]2−, respectively (Method A; Section 2.2.1) at room temperature with an aim to obtain dinuclear compounds of the composition [Zn(tren)(μ-NCS)Zn(NCS)3] and [Zn(tren)(μ-NCS)Cu(NCS)3]. However, microanalyses showed a 2:2:1:6 ratio of zinc(II), tren, zinc(II)/copper(II) and the pseudohalide, which is strongly suggestive of the composition [Zn(tren)NCS]2[Zn(NCS)4] (1

Conclusion

The use of molecular anion [M(NCS)4]2− in concert with [Zn(tren)NCS]+ containing bifunctional ligands with metal ion binding center as well as hydrogen bond donor acceptor sites leads to successful isolation of two new homo- and heterobimetallic compounds, viz. [Zn(tren)NCS]2[Zn(NCS)4] (1) and [Zn(tren)NCS]2[Cu(NCS)4] (2) which are X-ray crystallographically characterized. The failure to isolate compounds of the type [Cu(tren)NCS]2[Cu(NCS)4] and [Cu(tren)NCS]2[Zn(NCS)4] is indicative of special

Acknowledgements

B.K.G. thanks the DST, CSIR and UGC, New Delhi, India for financial support and S.C., K.B. and S.D. are grateful, respectively, to the CSIR and UGC, New Delhi, India for fellowships. The authors also acknowledge the Universiti Sains Malaysia for the Research University Golden Goose Grant No. 1001/PFIZIK/811012.

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