Zinc(II) vs cadmium(II) in organic chelate-free chemistry: Synthesis and characterization of 1-D [Zn2(N3)4(MeCN)3]n and 2-D [Cd3(N3)6(MeCN)2]n coordination polymers

The employment of a relatively unexplored reagent, namely the Me3SiN3, in the coordination chemistry of Zn(II) and Cd(II), in the absence of any external organic chelating/bridging ligand, has led to two new, azido-bridged coordination polymers with formulae [Zn2(N3)4(MeCN)3]n (1) and [Cd3(N3)6(MeCN)2]n (2), respectively. The reaction between Zn(ClO4)2·6H2O, Me3SiN3 and NEt3 in an 1:8:2 molar ratio afforded colorless needle-like crystals of the 1-D chain 1, whereas the exact same reaction but with Cd(ClO4)2·6H2O in place of the ZnII starting material resulted in the formation of colorless needles of the 2-D polymer 2. The repeating unit of 1 consists of a [Zn4(N3)8(MeCN)6] cluster with a defective dicubane core; the vertices of the latter are occupied by the four metal ions and the bridging ligation is provided by two μ3-1,1,1 (or 3.30) and six μ-1,1 (or 2.20) end-on N3− groups. In contrast, compound 2 comprises a central [Cd4(μ3-N3)4(MeCN)4]4+ cubane-like unit, unprecedented in the coordination chemistry of CdII, which is linked to [Cd(N3)4]2− tetrahedra through μ-1,1 end-on N3− groups, eventually forming the overall layered structure of 2. Topological analysis of 2 shows a trinodal (3,4,5) network with stoichiometry (3-c)2(4-c)(5-c)2 and a very rare topological type of 3,4,5L6. Terminal ligation in both 1 and 2 is solely provided by MeCN solvate molecules. Despite the optical inactivity of complex 1, the 2-D polymer 2 exhibits an intense violet-to-blue emission upon excitation at 225 or 277 nm in the solid-state, which is tentatively ascribed to an intra-ligand fluorescence from the coordinated azido groups within the robust 2-D network. Although it is strongly evidenced that the chemistry of ZnII and CdII follows the same general trends, and thus the majority of these coordination compounds are isomorphous, the present study demonstrates the formation of two chemically, structurally and optically different compounds of divalent Group 12 metal ions, under the exact same experimental conditions. This work may pave the way to further study the chemical, structural and physicochemical properties of organic chelate-free coordination polymers and clusters using different Me3SiX (X = OCN−, SCN−, CN− and RCO2−) reagents.