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)

Abstract

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) Å, β=98.67(3)° and Z=2. The structure was solved for 4056 non-zero Mo Kα 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 Å) and two weak axial bonds (2.825(2) Å) to O(6) and O(6′) carbonyl oxygen atoms. This CuO(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)°) and the O(6)O(6′) axis is tilted 15.6° from the normal to the basal plane. The ESR and electronic spectra of the complex are consistent with the same structure being retained in CH 2Cl 2 solution. The ESR spectra of the complex in CH 2Cl 2 solution show nine lines of nitrogen superhyperfine splitting indicating clearly the coordination of four nitrogen atoms toward copper(II).