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)](2) (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.
LeCloux
Daniel D.
DD
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and the Department of Chemistry, Northwestern University, Evanston, Illinois 60208.
Davydov
Roman
R
Lippard
Stephen J.
SJ