Structural, Electrochemical, and Spectroscopic Characterization of a Redox Pair of Sulfite-Based Polyoxotungstates: r-[W 18 O 54 (SO 3 ) 2 ] 4- and r-[W 18 O 54 (SO 3 ) 2 ] 5- Nigel Fay, ² Alan M. Bond,* ,² Carole Baffert, ² John F. Boas, ‡ John R. Pilbrow, ‡ De-Liang Long, § and Leroy Cronin* ,§ School of Chemistry, Monash UniVersity, Clayton, Victoria 3800, Australia, School of Physics, Monash UniVersity, Clayton, Victoria 3800, Australia, and WestCHEM, Department of Chemistry, The UniVersity of Glasgow, Glasgow, G128QQ, U.K. Received October 29, 2006 The synthesis, isolation, and structural characterization of the fully oxidized sulfite-based polyoxotungstate cluster (Pr 4 N) 4 {R-[W 18 O 54 (SO 3 ) 2 ]}‚2CH 3 CN and the one-electron reduced form (Pr 4 N) 5 {R-[W 18 O 54 (SO 3 ) 2 ]}‚2CH 3 CN has been achieved. R-[W 18 O 54 (SO 3 ) 2 ] 5- was obtained as a Pr 4 N + salt by reducing the “Trojan Horse” [W 18 O 56 (SO 3 ) 2 (H 2 O) 2 ] 8- cluster via a template orientation transformation. Acetonitrile solutions of pure R-[W 18 O 54 (SO 3 ) 2 ] 5- also were prepared electrochemically by one-electron bulk reductive electrolysis of R-[W 18 O 54 (SO 3 ) 2 ] 4- . Cyclic voltammetry of R-[W 18 O 54 - (SO 3 ) 2 ] 4- and R-[W 18 O 54 (SO 3 ) 2 ] 5- in CH 3 CN (0.1 M Hx 4 NClO 4 ) produces evidence for an extensive series of reversible one-electron redox processes, that are associated with the tungsten-oxo framework of the polyoxometalate cluster. Hydrodynamic voltammograms in CH 3 CN exhibit the expected sign and magnitude of the steady-state limiting current values for the R-[W 18 O 54 (SO 3 ) 2 ] 4-/5-/6- series and confirm the existence of a stable one-electron reduced species, R-[W 18 O 54 (SO 3 ) 2 ] 5- . Employment of the Randles-Sevcik (cyclic voltammetry) and Levich (rotating disk electrode) equations at a glassy carbon electrode (d ) 3 mm) enable diffusion coefficient values of 3.7 and 3.8 × 10 -6 cm 2 s -1 to be obtained for R-[W 18 O 54 (SO 3 ) 2 ] 4- and R-[W 18 O 54 (SO 3 ) 2 ] 5- , respectively. The tungsten polyoxometalates are highly photoactive, since measurable photocurrents and color changes are detected for both species upon irradiation with white light. EPR spectra obtained from both acetonitrile solution and solid samples, down to temperatures as low as 2.3 K, of the chemically and electrochemically prepared one-electron reduced species provided evidence that the unpaired electron in R-[W 18 O 54 (SO 3 ) 2 ] 5- is delocalized over a number of atoms in the polyoxometalate structure, even at very low temperatures. Introduction Since their discovery in the 19th century by Berzelius, 1 polyoxometalates have been extensively studied. Their wide- ranging structural, redox, and photochemical characteristics, high ionic charge, conductivity, and molecular weights are the primary reasons they are utilized in fields such as medicinal chemistry, 2-4 surface and solid-state chemistry, 5 materials science, 6-8 sensor technology, and catalysis. 9-12 Dawson or Wells-Dawson polyoxometalates, of general formula [M 18 O 54 (XO 4 ) 2 ] m- (where M ) Mo, W and X ) P, S), were first discovered in 1953 by Dawson, 13 having been previously postulated by Wells. 14 The Dawson-type poly- * To whom correspondence should be addressed. E-mail: alan.bond@ sci.monash.edu.au (A.M.B.); L.Cronin@chem.gla.ac.uk (L.C.). ² School of Chemistry, Monash University. ‡ School of Physics, Monash University. § The University of Glasgow. (1) Pope, M. T. Heteropoly and Isopoly Oxometalates; Springer: New York, 1983. (2) Rhule, J. T.; Hill, C. L.; Judd, D. A. Chem. ReV. 1998, 98, 327. (3) Aguey-Zinsou, K. F.; Bernhardt, P. V.; Kappler, U.; McEwan, A. G. J. Am. Chem. Soc. 2003, 125, 530. (4) Hasenkopf, B. Front. Biosci. 2005, 10, 275. (5) Klemperer, W. G.; Wall, C. G. Chem. ReV. 1998, 98, 297. (6) Ogliaro, F.; De Visser, S. P.; Cohen, S.; Sharma, P. K.; Shaik, S. J. Am. Chem. Soc. 2002, 124, 2806. (7) Yamase, T. Chem. ReV. 1998, 98, 307. (8) Pope, M. T.; Mueller, A. Angew. Chem., Int. Ed. 1991, 103, 56. (9) Mizuno, N.; Misono, M. Chem. ReV. 1998, 98, 199. (10) Hill, C. L. Angew. Chem., Int. Ed. 2004, 43, 402. (11) Hill, C. L. ComprehensiVe Coord. Chem. II 2004, 4, 679. (12) Sadakane, M.; Steckhan, E. Chem. ReV. 1998, 98, 219. (13) Dawson, B. Acta. Crystallogr. 1953, 6, 113. (14) Wells, A. F. Structural Inorg. Chem.; Oxford University Press; Oxford, 1945; p 344. Inorg. Chem. 2007, 46, 3502-3510 3502 Inorganic Chemistry, Vol. 46, No. 9, 2007 10.1021/ic062067e CCC: $37.00 © 2007 American Chemical Society Published on Web 03/29/2007