PYRAZOLYL-BRIDGED IRIDIUM DIMERS .15. SYNTHESIS OF BINUCLEAR CYCLOPENTADIENYL IR(III) COMPLEXES AND THE RHODIUM ANALOGS - REDUCTIVE ACCESS TO THE BIMETAL(II) STATE

Mononuclear M(III) complexes of formulae [MCpCl-(3-n)(R(2)pzH)(n)]((n -1)+) (M = Rh or Ir; n = 1,2) have been prepared through reactions of pyrazole (pzH, i.e. R = H) or substituted pyrazoles (R = Me or t-butyl ) with rhodium and iridium cyclopentadienyl or pentamethylcyclopentadi enyl precursors; for n = 1, the crystal and molecular structure has be en determined by using X-ray diffraction (11, M = Rh, R=H, FW = 377.96 g mol(-1) space group P2(1)/n, a = 7.1685(7), b = 15.740(3), c = 13.40 7(4)Angstrom, beta = 94.50(3)degrees), Dinuclear complexes of formulae [M(eta(5)-C(5)R(5))Cl(mu-pz)](2) (18, M = Rh, R = H; 19, M = Rh, R = H; 21, M = Ir, R = H; 20, M = Ir, R = Me; 21, M = Ir, R = H) containin g pyrazolyl bridges can be isolated through further reaction of the mo nonuclear compounds, or more directly from the chloro-bridged dimers [ M(eta(5)-C(5)R(5))Cl-2](2) by treatment with pyrazole in the presence of Et(3)N, although the dipyrazole iridium cation [IrCpCl(pzH)(2)](+) (16) does not undergo this type of dimerization. The dimeric complexe s, which possess a 'chair' geometry about the 6-membered bridging hete rometallocycle, have been shown to undergo a core conformational chang e ('chair':'boat') upon chemical reduction or halide abstraction. Chlo ride abstraction from 18 yields the binuclear product [{RhCp(mu-pz)}( 2)(mu-Cl)]BF4 (23) and reduction of either 18 or the C5H5 analog 19 gi ves access to the metal-metal bonded binuclear complexes [Rh(eta(5)-C( 5)R(5))(mu-pz)](2), 25 (R = Me) and 26 (R = H), which adopt a 'boat' c ore conformation. The reactivity of the metal-metal bonded products ha s been investigated: a triply-bridged single-fragment oxidative additi on product resulting from reaction with H+/MeOH ({[RhCp(mu-pz)](2)(mu- OMe)}O2CCF3, 29) has been structurally characterized (FW = 614.2 g mol (-1), space group P2(1)/n, a = 12.9647(3), b = 13.805(3), c = 11.593(3 ) Angstrom, beta = 90.44(2)degrees).