Synthesis, structure, and spectroscopic properties of copper(II) compounds containing nitrogen-sulphur donor ligands; the crystal and molecular structure of aqua[1,7-bis(N-methylbenzimidazol-2′;-yl)-2,6-dithiaheptane]copper(II) perchlorate

Abstract

The linear quadridentate N 2 S 2 donor ligand 1,7-bis(N-methylbenzimidazol-2′;-yl)-2,6-dithiaheptane (bmdhp) forms mono- and di-hydrate 1: 1 copper(II) complexes which are significantly more stable toward autoreduction than those of the non-methylated analogue. The deep green monohydrate of the perchlorate salt crystallises as the mononuclear aqua-complex, [Cu(bmdhp)(OH 2)][ClO 4 ] 2, in the monoclinic space group P2 1 /n, with Z = 4, a = 18.459(3), b = 10.362(2), c = 16.365(3) Å, and β = 117.14(1)°. The structure was solved and refined by standard Patterson, Fourier, and least-squares techniques to R = 0.047 and R′; = 0.075 for 3 343 independent reflections with l > 2σ(l). The compound consists of [Cu(bmdhp)(OH 2)] 2+ ions and ClO 4 - counter ions. The co-ordination around copper is intermediate between trigonal bipyramidal and square pyramidal, with Cu-N distances of 1.950(4) and 1.997(4) Å, Cu-O(water) 2.225(4) Å, and Cu-S 2.328(1) and 2.337(1) Å. In the solid state, the perchlorate dihydrate's co-ordination sphere may be a topoisomer of the monohydrate's. A new angular structural parameter, τ, is defined and proposed as an index of trigonality, as a general descriptor of five-co-ordinate centric molecules. By this criterion, the irregular co-ordination geometry of [Cu(bmdhp)(OH 2)] 2+ in the solid state is described as being 48% along the pathway of distortion from square pyramidal toward trigonal bipyramidal. In the electronic spectrum of the complex, assignment is made of the S(thioether) → Cu charge-transfer bands by comparison with those of the colourless complex Zn(bmdhp)(OH)(ClO 4). E.s.r. and ligand-field spectra show that the copper(II) compounds adopt a tetragonal structure in donor solvents.