Chiral bimetallic assemblies and coordination polymers based on tetracyanonickelate: A striking reversible structural transformation

Four bimetallic assemblies involving chiral metal complexes and tetracyanonickelate, {[M(L)(m)][Ni(CN)(4)]center dot nH(2)O}(infinity) (M = Ni-II, m = 3, n = 4 (1), M = Cu-II, m = 2, n = 2 (2 and 4), M = Cu-II, m = 2, n = 0 (3), and where L = R,R=chxn = trans-cyclohexane-(1R,2R)-diamine for 1, 2 and 3; and S,S-chxn = tratis-cyclohexane-(1S,2S)-diamine for 4), have been synthesized and structurally characterized. The crystal structure analysis of compound 1 reveals that it consists of discrete [Ni(R,R-chxn)(3)](2+) cations and [Ni(CN)(4)](2-) anions. Compound 2 is a bimetallic assembly of a [Cu(R,R-chxn)(3)](2+) cation and a [Ni(CN)(4)](2-) anion. For compound 2 an interesting reversible phase transformation was observed. This bimetallic complex can be transformed into a one-dimensional polymer structure, 3, accompanied by a color change from violet to purple, simply by the removal of the water molecules of crystallization on heating. Rehydration of 3 gives 2, confirmed by powder X-ray diffraction. Compound 4 is also a one-dimensional zigzag polymer composed of alternating trans-[Cu(S,S-chxn)(2)](2+) cations and [Ni(CN)(4)](2-) anions. Circular dichroism (CD) measurements for 2 and 4, and immersion calorimetric studies on compound 3 have been carried out.