Does the Mode of Dioxygen Binding to Dinuclear Copper Complexes Depend on the Spectator Nitrogen-Containing Ligands? An ab Initio Theoretical Study.

Ab initio two-determinants GVB computations (required to get the appropriate representation of the lowest singlet states of such systems) have been carried out for (Cu(+)(NH(3))(n)())(2)-O(2) (n = 0-3) and [Cu(2)(&mgr;-pydz)(2)(cnge)(2)](2+)-O(2). Different dioxygen binding modes (ranging from perpendicular to parallel with respect to the Cu-Cu direction) on these complexes have been examined. The results obtained show unambiguously that the parallel arrangements are always the less stable ones. In the especially important case of (Cu(+)(NH(3))(3))(2)-O(2) complexes, both staggered and eclipsed conformations have been considered. They are found almost isoenergetic, and the optimized geometrical parameters are, for a perpendicular O(2) binding onto a staggered complex, in fine agreement with corresponding experimental data obtained from either oxyhemocyanin or its synthetic models. In the case of a (Cu(+)(NH(3))(4))(2)-O(2) complex, taken as a model for Karlin's [{(TMPA)Cu}(2)-O(2)](2+) complex, the computations tend to show that the experimental end-on (trans &mgr;-1,2) O(2) binding is due to the presence of four nitrogens in the copper's coordination shell. Regarding the complexes with [Cu(2)(&mgr;-pydz)(2)(cnge)(2)](2+), the results indicate that the dioxygen binding mode remains perpendicular even if the fixation of a third pyridazine is known to occur in a parallel manner on this complex.