A NOVEL EXAMPLE OF A DIRECT INTERACTION BETWEEN THE CARBONYL OXYGEN, O(6), OF A N(7)-BONDED 6-OXOPURINE AND COPPER(II) - PREPARATION, SPECTROSCOPIC STUDY AND CRYSTAL-STRUCTURE OF BIS(8-METHYLTHIOTHEOPHYLLINATO)BIS(PYRIDINE)COPPER(II)

The preparation, spectroscopic study and crystal structure of the complex bis(8-methylthiotheophyllinato)bis(pyridine)copper(II), [Cu(L)2(py)2], are reported. The complex crystallizes in the monoclinic system, space group P2(1)/c with a = 7.502(2), b = 20.352(4), c = 9.757(2) angstrom, beta = 98.67(3)-degrees and Z = 2. The structure was solved for 4056 non-zero Mo Ka reflections (R-0.040). The structure consists of discrete [Cu(L)2(py)2] molecules with the copper(II) atom located on a center of symmetry. Each molecule includes two trans N(7)/O(6) bidentate ligands and two trans pyridine molecules. The copper ligand environment can be described as a highly elongated octahedron with four strong Cu-N bonds forming an exact basal plane (Cu-N(7) = 1.999(2); Cu-N(15) = 2.032 angstrom) and two weak axial bonds (2.825(2) angstrom) to 0(6) and 0(6') carbonyl oxygen atoms. This Cu-0(6) distance is the shortest observed in a copper(II) complex with a 6-oxopurine. Owing to the N(7)/O(6) bite of the theophyllinato chelate, the Cu-N(7) bond does not lie as the expected lone-pair direction of N(7) (C(8)-N(7)-Cu = 141.8(2); Cu-N(7)-C(5) = 114.9(2)-degrees) and the 0(6)-O(6') axis is tilted 15.6-degrees from the normal to the basal plane. The ESR and electronic spectra of the complex are consistent with the same structure being retained in CH2Cl2 solution. The ESR spectra of the complex in CH2Cl, solution show nine lines of nitrogen superhyperfine splitting indicating clearly the coordination of four nitrogen atoms toward copper(II).