Structure and spectra of dicopper(I) dioxygen species and their ammino complexes studied by ab initio calculations

Ab initio calculations were carried out on the species resulting from the c o-ordination of molecular oxygen to two Cu(I) centres. The calculations wer e performed at the HF and QCISD(T) levels with different sets of wave funct ions (a) STO-3G**; (b) 3-21G** basis; (c) 3-21G** for Cu and 6-31G*" for O. The highest symmetry structure of [CU2O2](2+) species in a closed shell si nglet state, namely the side-on mu-eta(2):eta(2)-[Cu2O2](2+) with D-2h symm etry, was found to be the lowest energy one (the global minimum) on the six -dimensional hypersurface. The open shell singlet of this species is at abo ut 65 cm(-1) and the triplet state at several eV higher. Some ammine comple xes of [Cu2O2](2+), studied by Ross and Solomon previously by the Xa-method , were also examined. Several features emerge: (a) the complex involving tr ans-[Cu2O2](2+) dissociates into 2[Cu(NH3)(2)](2+) + O-2; (b);' the bare ci s-[Cu2O2](2+), species, which is unstable with an uncompleted co-ordination sphere, is greatly stabilised by an endogenous bridge such as OH- and 2NH( 3) at each Cu centre; (c) the hydroxo bridged cis-[Cu2O2](2+) species shows two minima, one structure with short OO bond (1.2 Angstrom as in O-2) and another structure with long OO bond (1.4 Angstrom as in O-2(2-)); the two m inima are separated by a low (<300 cm(-1)) barrier, thus hopping between th em may occur. These findings cast light on the possible conversion of co-or dinated molecular oxygen to peroxide. (C) 2000 Elsevier Science B.V. All ri ghts reserved.