Chelation and Bridging of Zinc(II) by Tetradentate N2S2 Donor Ligands: Similarities and Differences to Copper(I) and Copper(II) Systems

Reaction of zinc(II) triflate with neutral bis(benzimidazole-thioether) ligands yielded the complexes [Zn(bmbdh)(H2O)(eta(1)-SO3CF3)](SO3CF3) (1) with tetradentate chelating 1,6-bis(N-methyl-benzimidazol-2-yl)-2,5-dithiahexane (bmbdh), and dinuclear {[Zn(mu-bmdhp)(H2O)(eta(1)-SO3CF3)(2)}(SO3CF3)(2) (2) with bis-bidentate bridging 1,7-bis(N-methylbenzimidazol-2-yl)-2,6-dithiaheptane (bmdhp). In both cases, the oxygen donor ligands aqua and triflate adopt cis positions at the approximately octahedrally configured Zn2+ ions. The hexacoordination is facilitated by rather long Zn-S(thioether) distances in contrast to the typically shorter Zn-S(thiolate) bonds from classical zinc finger proteins or their models. With the shorter central (CH2)(n) chain (n = 2) the ligand bmbdh exhibits facial coordination of the two NSS donor sets in 1, similar as in [Cu(bmdhp)](+) ions with lower coordinate copper(I) but in contrast to the related {Cu-II(bmdhp)(H2O)](ClO4)(2) with the longer chain ligand bmdhp. In 2, the bis-bidentate coordination of bmdhp ligands to each Zn2+ ion occurs in an all-cis fashion for the pairs of N, S and O donor atoms, in contrast to related [Cu-I(mmb)(2)](BF4), mmb = 1-methyl-2-methylthiomethyl-1H-benzimidazole.