SIMULTANEOUS BINDING OF BETAINE AND PSEUDOHALIDE TO CADMIUM - STRUCTURES OF 4 POLYMERIC MIXED-LIGAND CADMIUM(II) COMPLEXES

Four cadmium complexes containing betaine and pseudohalide ligands have been synthesised and structurally characterized by single-crystal X-ray analysis. The betaine-azide complex [Cd2(Me3NCH2CO2)2(N3)4] 1 and the betaine-isocyanate [Cd2(Me3NCH2CO2)2(NCO)4] 2 are isomorphous, crystallizing in the monoclinic space group C2/c with Z = 4. The two betaine-thiocyanate complexes [Cd3(Me3NCH2CO2)4(SCN)6(H2O)2] 3 and [Cd(Me3NCH2CH2CO2)(NCS)2] 4 are both triclinic, space group P1BAR with Z = 1 and 2, respectively. All four complexes are polymeric, the carboxy groups of the betaine ligands acting in three co-ordination modes and the pseudohalide ligands in two typical bridging modes. The structure of 1 or 2 consists of a virtually linear chain in which each pair of adjacent metal ions are triply bridged by one carboxylate plus two mu-1,1-pseudohalogenide ligands. The framework of 3 is a one-dimensional chain built from corner-sharing centrosymmetric 16-membered (-N-C-S-Cd)4 puckered rings containing mu-1,3-bridging thiocyanate groups in both anti and skew modes; both uni- and bi-dentate chelating modes of the betaine ligands coexist in this complex. The structure of 4 is constructed from an alternate arrangement of two kinds of rings which share corners to form a linear chain: one is an eight-membered chair-like (-N-C-S-Cd)2 ring bridged by a pair of mu-1,3-thiocyanate ligands and the other a four-membered (-O-Cd)2 ring bridged by two carboxylate ligands. The significance of these structural features and the co-ordination geometries at the cadmium centres are discussed.