Mixed-Valence Cu(I)-Cu(II) and Heterodimetallic Cu(I)-M(II) Bis(carboxylate-bridged) Complexes: Structural, Electrochemical, and Spectroscopic Investigations.

The synthesis, spectroscopic, electrochemical, and structural properties of a series of Cu(I)Cu(II) bis(carboxylate-bridged) complexes are described, together with related investigations of Cu(I)M(II) (M = Fe, Zn) analogues. Treatment of previously reported [Cu(2)(XDK)(MeCN)] (1) or [Cu(2)(PXDK)(MeCN)] (2), where H(2)XDK = m-xylylenediamine bis(Kemp's triacid imide) and H(2)PXDK = the propyl derivative of H(2)XDK, with 1 equiv of silver(I) triflate, trifluoroacetate, or tetrafluoroborate in THF afforded mixed-valence complexes [Cu(2)L(&mgr;-X)(THF)(2)], where X = triflate and L = XDK (4), PXDK (5); X = trifluoroacetate and L = XDK (6) and [Cu(2)L(THF)(4)]X, where X = tetrafluoroborate, L = XDK (7), PXDK (8). Compound 8 was also prepared from an equimolar mixture of (Et(4)N)[Cu(PXDK)] (3) and copper(II) triflate. Solid-state structural investigations of 4, 6, and 8 revealed symmetric, square pyramidal coordination environments about each copper atom and short Cu-Cu distances ranging from 2.3988(8) to 2.4246(12) Å. These features imply significant metal-metal bonding character, the nature of which was further interrogated. Comparative structural and ligand exchange studies with mixed-metal analogues [CuZn(PXDK)(OTf)(THF)(2)(H(2)O)] (9), CuFe(PXDK)(OTf)(NB)(MeCN) (10, NB = norbornene), and [CuZn(PXDK)(OTf)(NB)(H(2)O)] (11) revealed longer metal-metal distances ranging from 3.294(2) to 3.732(2) Å and monodentate, terminal triflate ligation. Variable-temperature and variable-field EPR studies showed that complexes 4-8 have fully delocalized electronic structures in the solid state and solution down to liquid helium temperatures. Molecular orbital calculations on simplified models of 4-8 revealed a Cu-Cu bonding interaction in the SHOMO and SOMO, comprising mainly sigma-type overlap between the d(x)()()2(-)(y)()()2 orbitals. In addition, cyclic voltammetric studies of compound 4 revealed a chemically reversible, electrochemically quasireversible one-electron reduction at a positive potential for a Cu(I)Cu(II) complex having a dianionic, oxygen-rich donor set. The relevance of these properties to the electronically similar Cu-Cu bonded system of the biological Cu(A) center is discussed.