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

The heteropolyanion [PW 11 O 39 RhCl] 5- ( 1-Cl ) was synthesized by hydrothermal reaction of [PW 11 O 39 ] 7- and RhCl 3 at 150 °C for 20 h. The tetramethylammonium salt of 1-Cl was characterized by elemental analysis, 31 P and 183 W 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 [(PW 11 O 39 Rh) 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 (K 10 [(PW 11 O 39 Rh) 2 ]· x H 2 O, triclinic, P 1̄; 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) Å 3 ). 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 PhCH 2 Br yields [PW 11 O 39 RhBr] 5- ( 1-Br) and PhCH 2 CH 2 Ph. Oxidation of 2 by air, Br 2, and hypochlorite yields [PW 11 O 39 Rh(H 2 O)] 4- ( 1-aq ), 1-Br, and 1-Cl, respectively. Photochemical, electrochemical, and ligand substitution routes to other [PW 11 O 39 RhL] n - species, L = I -, CN -, CH 3 COO -, 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.