Mixed-Valence CuI−CuII and Heterodimetallic CuI−MII Bis(carboxylate-bridged) Complexes:  Structural, Electrochemical, and Spectroscopic Investigations

• Published in: Inorganic chemistry 1998-12, Vol.37 (26), p.6814-6826
• Main Authors: LeCloux, Daniel D, Davydov, Roman, Lippard, Stephen J
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic980763b

The synthesis, spectroscopic, electrochemical, and structural properties of a series of CuICuII bis(carboxylate-bridged) complexes are described, together with related investigations of CuIMII (M = Fe, Zn) analogues. Treatment of previously reported [Cu2(XDK)(MeCN)] (1) or [Cu2(PXDK)(MeCN)] (2), where H2XDK = m-xylylenediamine bis(Kemp's triacid imide) and H2PXDK = the propyl derivative of H2XDK, with 1 equiv of silver(I) triflate, trifluoroacetate, or tetrafluoroborate in THF afforded mixed-valence complexes [Cu2L(μ-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 = tetrafluoroborate, L = XDK (7), PXDK (8). Compound 8 was also prepared from an equimolar mixture of (Et4N)[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(H2O)] (9), [CuFe(PXDK)(OTf)(NB)(MeCN)]2 (10, NB = norbornene), and [CuZn(PXDK)(OTf)(NB)(H2O)] (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 σ-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 CuICuII complex having a dianionic, oxygen-rich donor set. The relevance of these properties to the electronically similar Cu−Cu bonded system of the biological CuA center is discussed.