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,* ,† and Nathan J. Patmore † Contribution from the Department of Chemistry, The Ohio State UniVersity, 100 W. 18 th AVenue, Columbus, Ohio 43210-1185 and Christopher Ingold Laboratories, UniVersity College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom Received March 3, 2004; E-mail: Chisholm@chemistry.ohio-state.edu Abstract: The preparation of the 3,6-dioxypyridazine-bridged tungsten complex, [W2(O2C t Bu)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 + PF6 - 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-H2C4N2O2) + 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. Introduction Interest continues in the study of mixed-valence species, particularly for compounds that may be described as “almost delocalized” at the interface of Class II and III behavior on the Robin and Day Scheme, 1 where Class III represents fully delocalized and Class II, strongly coupled. A simple measure of the relative stability of the mixed-valence state is often gleaned from electrochemical studies, following the work of Taube and Richardson. 2 Other evaluations of the degree of electronic coupling have focused on the nature of electronic near-IR transitions whose spectral shape or form in particular can be informative about the nature of the potential energy surface of mixed-valence species. 3,4 As with the now classical Creutz-Taube Ru II -bridge-Ru III compounds 5 and other com- pounds bridged by conjugated π-systems, the electronic coupling falls off with distance as measured by electrochemistry. Recently, these types of study have been extended to dinuclear systems, wherein quadruple bonds have been linked by conju- gated dicarboxylate units, the simplest of which is oxalate 6-8 and more exotic examples include tamurate and texate [O 2 C(CHdCH) n CO 2 ] 2- , where n ) 3 and 4, respectively. 9,10 Recently, Cotton and co-workers extended their studies to cyclic polyamidato bridges between Mo 2 4+ centers. 11 These bridges are shown in Scheme 1 by the drawings A, B, and C. They noted that, while each could be viewed as corresponding stereochemically to the oxalate dianion, D, in terms of separation between the M 2 units, the electronic communication was, in all † Department of Chemistry, The Ohio State University. ‡ Christopher Ingold Laboratories, University College London. (1) Robin, M. B.; Day, P. AdV. Inorg. Radiochem. 1967, 10, 247. (2) Richardson, D. E.; Taube, H. Inorg. Chem. 1981, 20, 1278. (3) Nelson, S. F. Chem. Eur. J. 2001, 6, 581. (4) Brunschwig, B. S.; Creutz, C.; Sutin, N. Chem. Soc. ReV. 2002, 31, 168. (5) Creutz, C.; Taube, H. J. Am. Chem. Soc. 1969, 91, 3988. (6) Cayton, R. H.; Chisholm, M. H.; Huffman, J. C.; Lobkovsky, E. B. J. Am. Chem. Soc. 1991, 113, 8709. (7) Bursten, B. E.; Chisholm, M. H.; Clark, R. J. H.; Firth, S.; Hadad, C. M.; MacIntosh, A. M.; Wilson, P. J.; Woodward, P. M.; Zaleski, J. M. J. Am. Chem. Soc. 2002, 124, 3050. (8) Cotton, F. A.; Lin, C.; Murillo, C. A. J. Chem. Soc., Dalton Trans. 1998, 3151. (9) Cotton, F. A.; Donahue, J. P.; Murillo, C. A. J. Am. Chem. Soc. 2003, 125, 5436. (10) Cotton, F. A.; Donahue, J. P.; Murillo, C. A.; Perez, L. M. J. Am. Chem. Soc. 2003, 125, 5486. (11) Cotton, F. A.; Donahue, J. P.; Murillo, C. A.; Perez, L. M.; Yu, R. J. Am. Chem. Soc. 2003, 125, 8900. Published on Web 06/15/2004 10.1021/ja048768x CCC: $27.50 © 2004 American Chemical Society J. AM. CHEM. SOC. 2004, 126, 8303-8313 9 8303