Mixed-valence Cu-I-Cu-II and heterodimetallic Cu-I-M-II bis(carboxylate-bridged) complexes: Structural, electrochemical, and spectroscopic investigations

The synthesist spectroscopic, electrochemical, and structural properties of a series of (CuCuII)-Cu-I bis(carboxylate-bridged) complexes are described , together with related investigations of (CuMII)-M-I (M = Fe, Zn) analogue s. Treatment of previously reported [Cu-2(XDK)(MeCN)] (1) or [Cu-2(PXDK)(Me CN)] (2), where H2XDK = m-xylylenediamine bis(Kemp's triacid imide) and H2P XDK = the propyl derivative of H2XDK, with 1 equiv of silver(I) triflate, t rifluoroacetate, or tetrafluoroborate in THF afforded mixed-valence complex es [Cu2L(mu-X)(THF)(2)], where X = triflate and L = XDK (4), PXDK (5); X = trifluoroacetate and L = XDK (6) and [Cu2L(THF)(4)]X, where X = tetrafluoro borate, L = XDK (7), PXDK (8). Compound 8 was also prepared from an equimol ar mixture of (Et4N)[Cu(PXDK)] (3) and copper(II) triflate. Solid-state str uctural 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) Angstrom. These features imply signifi cant metal-metal bonding character, the nature of which was further interro gated. Comparative structural and ligand exchange studies with mixed-metal analogues [CuZn(PXDK)(oTf)(THF)(2)(H2O)] (9), [CuFe(PXDK)(OTf)(NB)(MeCN)](2 ) (10, NB = norbornene), and [CuZn(PXDK)(OTf)(NB)(H2O)] (11) revealed longe r metal-metal distances ranging from 3.294(2) to 3.732(2) Angstrom and mono dentate, terminal triflate ligation. Variable-temperature and variable-fiel d EPR studies showed that complexes 4-8 have fully delocalized electronic s tructures in the solid state and solution down to liquid helium temperature s. 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(x2-y2) orbitals. In addition, cyclic voltammetric st udies of compound 4 revealed a chemically reversible, electrochemically qua sireversible one-electron reduction at a positive potential for a (CuCuII)- Cu-I complex having a dianionic, oxygen-rich donor set. The relevance of th ese properties to the electronically similar Cu-Cu bonded system of the bio logical Cu-A center is discussed.