Electronically-coupled tungsten-tungsten quadruple bonds: Comparisons of electron delocalization in 3,6-dioxypyridazine and oxalate-bridged compounds

Malcolm H. Chisholm, Robin J.H. Clark, Judith Gallucci, Christopher M. Hadad, Nathan J. Patmore

Research output: Contribution to journalArticle

35 Citations (Scopus)

Abstract

The preparation of the 3,6-dioxypyridazine-bridged tungsten complex, [W2(O2CtBu)3]2(μ- H2C4N2O2), I, is described, along with its single-electron oxidized cation, I+, formed in the reaction between I and Ag+PF6-. Compound I has been structurally characterized as a PPh3 adduct, and I+PF 6- as a THF solvate, by single-crystal X-ray studies. The geometric parameters of these compounds compare well with those calculated for the model compounds [W2(O2CH)3] 2(μ -H2C4N2O2) and [W2(O2CH)3]2(μ-H 2C4N2O2)+ by density functional theory employing the Gaussian 98 and 03 suite of programs. The calculations indicate that the two W2 centers are strongly coupled by M2 δ-to-bridge π-bonding, and further coupled by direct M2⋯M2 bonding. Compound I is purple and shows an intense absorption in the visible region due to a metal-to-bridge charge transfer and, with excitation within this absorption, compound I exhibits pronounced resonance Raman bands associated with symmetric vibrations of the bridge and the M4 unit. The cyclic voltammogram of I in THF, the EPR spectrum of I+PF6 in 2-MeTHF and the electronic absorption spectrum of I+PF6- in THF are consistent with electron delocalization over both W2 units. These new data are compared with previous data for the molybdenum analogue, related oxalate-bridged compounds and closely related cyclic polyamidato-bridged Mo4- containing compounds. It is proposed that, while the electronic coupling occurs principally by an electron-hopping mechanism for oxalate-bridged compounds, hole-hopping contributes significantly in the cases of the amidate bridges and that this is more important for M = Mo than for M = W. Furthermore, for Class III fully delocalized mixed-valence compounds, the magnitude of Kc, determined from electrochemical methods, is not necessarily a measure of the extent of electron delocalization.

Original languageEnglish
Pages (from-to)8303-8313
Number of pages11
JournalJournal of the American Chemical Society
Volume126
Issue number26
Early online date15 Jun 2004
DOIs
Publication statusPublished - 7 Jul 2004
Externally publishedYes

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