Spectroscopy and bonding in side-on and end-on Cu2(S2) cores:: Comparison to peroxide analogues

Spectroscopic methods combined with density functional calculations were used to study the disulfide-Cull bonding interactions in the side-on mu-eta(2):eta(2)-bridged Cu-2(S-2) complex, [{Cu-II[HB(3,5-Pr(2)(i)pz)(3)]}(2)-(S-2)], and the end-on trans-mu-1,2-bridged Cu-2(S-2) complex, [{Cu-II(TMPA)}(2)(S-2)](2+), in correlation to their peroxide structural analogues. Resonance Raman shows weaker S-S bonds and stronger Cu-S bonds in the disulfide complexes relative to the O-O and Cu-O bonds in the peroxide analogues. The weaker S-S bonds come from the more limited interaction between the S(3)p orbitals relative to that of the O(2)s/p hybrid orbitals. The stronger Cu-S bonds result from the more covalent Cu-disulfide interactions relative to the Cu-peroxide interactions. This is consistent with the higher energy of the disulfide valence level relative to that of the peroxide. The ground states of the side-on Cu-2(S-2)/Cu-2(O-2) complexes are more covalent than those of the end-on Cu-2(S-2)/Cu-2(O-2) complexes. This derives from the larger sigma-donor interactions in the side-on mu-eta(2):eta(2) structure, which has four Cu-disulfide/peroxide bonds, relative to the end-on trans-mu-1,2 structure, which forms two bonds to the Cu. The larger disulfide/peroxide sigma-donor interactions in the side-on complexes are reflected in their more intense higher energy disulfide/peroxide to Cu charge transfer transitions in the absorption spectra. The large ground-state covalencies of the side-on complexes result in significant nuclear distortions in the ligand-to-metal charge transfer excited states, which give rise to the strong resonance Raman enhancements of the metal-ligand and intraligand vibrations. Particularly, the large covalency of the Cu-disulfide interaction in the side-on Cu-2(S-2) complex leads to a different rR enhancement profile, relative to the peroxide analogues, reflecting a S-S bond distortion in the opposite directions in the disulfide/peroxide pi*(sigma) to Cu charge transfer excited states. A ligand sigma* back-bonding interaction exists only in the side-on complexes, and there is more sigma* mixing in the side-on Cu-2(S-2) complex than in the side-on Cu-2(O-2) Complex. This sigma* back-bonding is shown to significantly weaken the S-S/O-O bond relative to that of the analogous end-on complex, leading to the low v(S-S)/v(O-O) vibrational frequencies observed in the resonance Raman spectra of the side-on complexes.