Crucial role of Cu-S bonding for structural changes accompanying the reversible CuI/CuII transition in an unrestrained Cu(N[symbol: see text]S)2 coordination arrangement. An experimental and DFT study

The structure of reversibly oxidizable Cu(mmb)2](BF4) with 1-methyl-2-(methylthiomethyl)-1H-benzimidazole (mmb) as bidentate N,S-donor ligand has been determined and compared with that of the copper(II) species Cu(mmb)2(eta 1-ClO4)](ClO4). In the complex ions of the equilibrium CuI(mmb)2](+) + ClO4- reversible e- + CuII(mmb)2-(eta 1-ClO4)]+ the almost linear N-Cu-N backbone is invariant whereas the bonds to the thioether sulfur centers and especially the changing S-Cu-S angle (145.18(5) degrees for the CuII species, 109.33(3) degrees for the CuI form) reflect the metal oxidation state. In contrast to the perchlorate coordinating copper(II) species, CuI(mmb)2](BF4) contains a cation with a very large vacant site at the metal center, resulting in elliptical channels within the crystal. DFT calculations on CuI(mb)2]+, CuII(mb)2]2+, and CuII(mb)2(OClO3)]+ with mb = 2-methylthiomethyl-1H-benzimidazole confirm the essential role of the metal-sulfur bonds in responding to the reversible CuI/II electron transfer process, even in the absence of electronically stronger interacting thiolate sulfur centers or sophisticated oligodentate ligands.