LACUNARY POLYOXOMETALATE ANIONS ARE PI-ACCEPTOR LIGANDS - CHARACTERIZATION OF SOME TUNGSTORUTHENATE(II,III,IV,V) HETEROPOLYANIONS AND THEIR ATOM-TRANSFER REACTIVITY

Reaction of [Ru(H2O)6]2+ with [PW11O39]7-, followed by oxidation with O2 yields [PW11O39Ru(III)(H2O)]4- (1) isolated as the cesium salt. Cyclic voltammetry shows that 1 is reducible/oxidizable to the corresponding aquaruthenium(II) (2), oxoruthenium(IV), and oxoruthenium(V) derivatives. The pK(a) of 1 is 5.1, determined from a ca. 300 ppm shift of the P-31 NMR line between pH 3 and 6. At pH 3.0 and 23-degrees-C the rate of electron transfer between 1 and 2 was determined by P-31 NMR line-broadening to be 1.2 X 10(6) M-1 s-1. 2 reacts with pyridine, sulfoxides, dialkyl sulfides, and active alkenes (maleic, fumaric, crotonic acids, 1,4-dihydroxybut-2-ene) to form [PW11O39Ru(II)(L)]5- species, which are oxidizable to the Ru(III) stage only. At pH 3.0 and 20 +/- 1-degrees-C the half-life for substitution of DMSO for water on 2 is 3.5 h (k(obs) = 5.5 X 10(-5) s-1) and this rate is some 3 orders of magnitude slower than that for water exchange on [Ru(H2O)6]2+. The electronic spectra of the Ru(II) derivatives show, in addition to the expected d-d bands, broad intense charge-transfer absorption attributed to Ru(II) --> W(VI). Tungsten-183 NMR spectra of 2 and the dimethyl sulfoxide and maleic acid derivatives show the expected six-line (2:2:2:1:2:2) pattern but with resonances for the W atoms adjacent to Ru deshielded by as much as ca. 360 ppm. This effect is greatest for 2 (L = H2O) and least for L = maleic acid and is attributed to a partial delocalization of Ru pi-electron density onto the polytungstate ligand. The anomalous redox potential for Ru(III/II) in 2 (in comparison to other M(III/II) couples in [PW11O39M(H2O)]5-) is a further indicator of electron delocalization. In acidic solution, pH approximately 0, 1 is oxidized to the oxoruthenium(V) derivative in a single two-electron step, and this forms the basis of an electrocatalytic oxidation (40 turnovers) of dimethyl sulfoxide to the sulfone with > 90% current efficiency. The tetrabutylammonium salt of 1 in acetonitrile solution catalyzes the epoxidation of trans-stilbene by iodosylbenzene. Reduction of 2 to a heteropoly blue is not possible, due to catalytic hydrogen evolution, except in the presence of dimethyl sulfoxide which is catalytically reduced (30 turnovers) to dimethyl sulfide with ca. 50% current efficiency. Preliminary experiments show that the behavior of alpha-2-[P2W17O61Ru(III)(H2O)]7- parallels that of 1.