Spectroscopy and Bonding in Side-On and End-On Cu2(S2) Cores: Comparison to Peroxide Analogues

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Abstract
Spectroscopic methods combined with density functional calculations were used to study the disulfide−CuII bonding interactions in the side-on μ-η2:η2-bridged Cu2(S2) complex, [{CuII[HB(3,5-Pri2pz)3]}2(S2)], and the end-on trans-μ-1,2-bridged Cu2(S2) complex, [{CuII(TMPA)}2(S2)]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 3p orbitals relative to that of the O 2s/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 Cu2(S2)/Cu2(O2) complexes are more covalent than those of the end-on Cu2(S2)/Cu2(O2) complexes. This derives from the larger σ-donor interactions in the side-on μ-η2:η2 structure, which has four Cu−disulfide/peroxide bonds, relative to the end-on trans-μ-1,2 structure, which forms two bonds to the Cu. The larger disulfide/peroxide σ-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 Cu2(S2) 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 π*σ to Cu charge transfer excited states. A ligand σ* back-bonding interaction exists only in the side-on complexes, and there is more σ* mixing in the side-on Cu2(S2) complex than in the side-on Cu2(O2) complex. This σ* 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 νS-S/νO-O vibrational frequencies observed in the resonance Raman spectra of the side-on complexes.