A new family of dinuclear rhodium complexes containing tertiary phosphanes in a semibridging or doubly bridging bonding mode

The reactions of [Rh2Cl(kappa(2)-acac)(mu-CPh2)(2)(mu-SbiPr(3))] (3) and [Rh-2(kappa(2)-acac)(2)(mu-CPh2)(2)(mu-SbiPr(3))] (4) with PMe3 lead to exchange of the bridging ligand and afford the novel PMe3-bridged counterparts 5 and 6, in which the phosphane occupies a semi-bridging (5) or a doubly bridging (6) position. In both cases, the bonding mode was confirmed crystallographically. Treatment of 6 with CO causes a shift of PMe3 from a bridging to a terminal position and gives the unsymmetrical complex [(kappa(2)-acac)Rh(mu-CPh2)(2)(mu-CO)Rh(PMe3)(kappa(2)-acac)] (7). Similarly to 5 and 6, the related compounds 10 and 11 with one or two acac-f(3) ligands were prepared. While both PEt3 and PnBu(3) react with 3 by exchange of the bridging stibane for phosphane to give compounds 12 and 13, the reactions of 4 with PMePh2 and PnBu(3) afford the mixed-valent (RhRhII)-Rh-0 complexes [(PR3)Rh(mu-CPh2)(2)Rh(kappa(2)-acac)(2)] (17, 18) in high yields. In contrast, treatment of 4 with PEt3 and PMe2Ph generates the phosphane-bridged compounds [Rh-2(kappa(2)-acac)(2)(mu-CPh2)(2)(mu-PR3)] (14, 15) exclusively. Stirring a solution of 14 (R = Et) in benzene for 15 h at room temperature leads to complete conversion to the mixed-valent isomer 16. The reaction of 6 with an equimolar amount of CR3CO2H (R = F, H) or phenol in the molar ratio of 1:10 results in substitution of one acac by one trifluoracetate, acetate, or phenolate ligand without disturbing the [Rh-2(mu-CPh2)(2)(mu-PR3)] core. From 6 and an excess of CR3CO2H, the symmetrical bis(trifluoracetato) and bis(acetate) derivatives [Rh-2(kappa(2)-O2CCR3)(2)(mu-CPh2)(2)(mu-PMe3)] (21, 22) were obtained.