Rhodium derivatives of lacunary heteropolytungstates illustrate metalloporphyrin analogies.: Reductive dimerization to Rh2-linked Keggin moieties

The heteropolyanion [PW11O39RhCl](5-) (1-Cl) was synthesized by hydrothermal reaction of [PW11O39](7-) and RhCl3 at 150 degrees C for 20 h. The tetramethylammonium salt of 1-Cl was characterized by elemental analysis, P-31 and W-183 NMR, and solution molecular weight determination by ultracentrifugation (found 2744 +/- 63, calcd 2816). The cyclic voltammogram of 1-Cl shows an irreversible reduction wave at -0.45 V vs Ag/AgCl, and controlled potential reduction at -0.5 V generates the dimeric metal-metal bonded species [(PW11O39Rh)(2)](10-) (2) The composition and structure of 2 have been confirmed by elemental analysis of the cesium salt, analytical ultracentrifugation (ionic weight 5821 +/- 186, calcd 5561), P and W NMR, and a limited structural analysis of the potassium salt, which revealed a Rh-Rh bond length of 2.52(2) Angstrom (K-10[(PW11O39Rh)(2)]. xH(2)O, triclinic, <P(1)over bar>; a = 12.703(6), b = 17.868(8), and c = 19.131(9) Angstrom; alpha = 96.56(2), beta = 97.12(2), and gamma = 91.318(33)degrees; Z = 2; V = 4278(3) Angstrom(3)). Aqueous solutions of 2 show an absorption band at 720 nm attributed to the pi*-->sigma* transition of the metal-metal bond. Photolysis (lambda > 670 nm) of acetone solutions of 2 with PhCH2Br yields [PW11O39RhBr](5-) (1-Br) and PhCH2CH2Ph. Oxidation of 2 by air, Br-2, and hypochlorite yields [PW11O39Rh(H2O)](4-) (1-aq), 1-Br, and 1-Cl, respectively. Photochemical, electrochemical, and ligand substitution routes to other [PW11O39RhL](n-) species, L = I-, CN-, CH3COO-, pyridine, or S-bonded dimethyl sulfoxide, are described. Each of these complexes has a characteristic P NMR resonance, and mixtures are separable by chromatography on Sephadex.