Charge-Transfer Spectra of Structurally Characterized Mixed-Valence Thiolate-Bridged Cu(I)/Cu(II) Cluster Complexes

A series of Cu(II) and Cu(I)/Cu(II) complexes containing the cis-N(amine)2S(thiolate)2 copper complex rac-2 has
been synthesized to provide a basis for understanding the charge-transfer spectra of mixed-valence thiolate-bridged Cu(I)/Cu(II) complexes. In combination with Cu(Me2-13-N4ane), rac-2 yields a monobridged dinuclear
homovalent adduct, rac-5, while reaction with CuCl yields the mixed-valance pentanuclear complex rac-6. In the
presence of Cu(II)(acac)2, chiral R,R-1 reacts to form a mixed-valence pentanuclear cation R,R-7. rac-6 exhibits a
relatively short Cu(I)···Cu(II) contact [2.8231(9) Å] and associated structural features that suggest the presence of
a weak Cu(I)···Cu(II) interaction in a valence-trapped system. Additional structural features in rac-6 and R,R-7
include singly and doubly bridging thiolates, three- and four-coordinated Cu(I) ions, and varying Cu(I) ligand sets.
These features extend the types and complexities of electronic absorptions significantly. Spectra of rac-6 and
R,R-7 exhibit multiple overlapping absorptions over the entire visible and ultraviolet spectral regions studied, consonant
with these observations. Trends resulting from variations in structure type and oxidation state permit a first approach
toward developing a detailed assignment of the individual ligand Rydberg, LF, LMCT, MLCT, and possible MMCT
absorptions in these complexes.