Rhodium Derivatives of Lacunary Heteropolytungstates Illustrate Metalloporphyrin Analogies. Reductive Dimerization to Rh2-Linked Keggin Moieties

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The heteropolyanion [PW11O39RhCl]5- (1-Cl) was synthesized by hydrothermal reaction of [PW11O39]7- and RhCl3 at 150 °C for 20 h. The tetramethylammonium salt of 1-Cl was characterized by elemental analysis, 31P and 183W 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) Å (K10[(PW11O39Rh)2]·xH2O, triclinic, P1̄; a = 12.703(6), b = 17.868(8), and c = 19.131(9) Å; α = 96.56(2), β = 97.12(2), and γ = 91.318(33)°; Z = 2; V = 4278(3) Å3). Aqueous solutions of 2 show an absorption band at 720 nm attributed to the π*→σ* transition of the metal−metal bond. Photolysis (λ > 670 nm) of acetone solutions of 2 with PhCH2Br yields [PW11O39RhBr]5- (1-Br) and PhCH2CH2Ph. Oxidation of 2 by air, Br2, 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.