Architectures in two crystalline homo and hetero two-dimensional polymers of the type [Cd 3(tren) 2(NCS) 6] n and [Ni 2Cd(tren) 2(NCS) 6] n [tren = Tris(2-aminoethyl)amine] through molecular ion bridges

Abstract

Cadmium(II) nitrate tetrahydrate – a metal ion template, tris(2-aminoethyl)amine (tren) – a tripodal amine and thiocyanate – a versatile intermediatary in a 3:2:6 mole ratio in water afford a homopolymer, [Cd 3(tren) 2(NCS) 6] n ( 1). Replacement of two equivalents of cadmium(II) salt by two equivalents of nickel(II) nitrate hexahydrate results in the heteropolymer, [Ni 2Cd(tren) 2(NCS) 6] n ( 2). Compound 1 crystallizes in the monoclinic C2/ c space group with a = 20.3480(17) Å, b = 10.8796(9) Å, c = 16.3495(10) Å, β = 110.009(7)° and Z = 4; 2 crystallizes in the same space group as in 1 with a = 19.584(5) Å, b = 11.034(3) Å, c = 15.760(4) Å, β = 100.712(4)° and Z = 4. 1 and 2 are new examples of homo- and hetero-polymers with the multiconnecting molecular ion bridge [Cd(NCS) 6] 4− in different geometric configurations. The single crystal X-ray structures of both neutral polymers consist of infinite two-dimensional sheets with two alternating environmentally different octahedral metal ion [M(II) = Ni(II)/Cd(II)] coordination spheres connected by four 1,3-thiocyanato bridging cis-/ trans-[Cd(NCS) 2(SCN) 4] 4−. The Cd(1) centres in 1 and 2 with the CdN 2S 4 chromophore are surrounded by four S atoms of the bridging thiocyanate and two terminal N atoms of the pseudohalide, while the CdN 6 chromophore of Cd(2) in 1 and NiN 6 chromophore of Ni(1) in 2 are completed by four tren N atoms and two N atoms of the bridging NCS − in cis alignment. In 1, the three tripodal amine N atoms and two bridging S atoms are engaged in strong intermolecular hydrogen bondings, which is responsible for the conformation and stability of the supramolecular network. The absence of such hydrogen bonding in 2 is due to a difference in the orientation of the tripodal amine from that in 1 in the polymeric net. The magnetic property of 2 has been investigated over the 2–300 K temperature range. The feature is characteristic of a Curie law for a typical magnetically isolated nickel(II) ion with very weak antiferromagnetic interactions ( J < 0.01 cm −1). The presence of a diamagnetic molecular ion with two 1,3-thiocyanate bridges may be one of the major reasons to hinder effective coupling of the paramagnetic centres.