Atypical Metal-Center Coordination in Two Cyanide-Based Coordination Polymers

Abstract

This contribution reports the crystal structure, solved and refined from single crystal XRD data, of two cyanide-based coordination polymers, with K[Cu2(CN)3] ⋅ H2O (1) and [Cd(H2O)]2Cd(H2O)2[Cu(CN)3Cu(CN)2]2 ⋅ 8H2O (2) as formula units. These coordination polymers are characterized by metal-metal bonds between copper atoms. In 1, the framework is formed by Cu−Cu(CN)3 pseudo tetrahedral blocks linked by planar Cu2(CN)3 units. In this material, a defined Cu−Cu bond was identified, which is congruent with the corresponding Raman spectrum. The K atom and water molecule occupy the framework cavities. In 2, the Cu−Cu bond is observed in Cu2(CN)6 blocks which remain linked to Cd(NC)2(H2O)4 and Cd(NC)4(H2O)2 units occupying their axial and equatorial coordination positions, respectively. The solid framework is completed by the −N=C−Cu−C=N− chains linked to Cd(NC)4(H2O)2 moieties. The crystal structure for 2 contains two different sites for the metal centers. The structural study was complemented by recording IR, Raman, and UV/Vis spectra. These two coordination polymers behave as semiconductor materials with a band gap of 2.79 and 3.43 eV, respectively, as derived from the Tauc plot of the corresponding UV/Vis spectrum. Band structure calculations properly supported these values. 1 is a material of an indirect band gap, while 2 has a direct band gap.