Synthesis, structure and reactivity of binuclear rhodium dicationic carbonyl complexes

Abstract

The reaction of Rh2(CO)3(dppm)2 (1) with an excess of triflic acid, HSO3CF3, yields the dicationic tricarbonyl complex Rh2(CO)3(dppm)2+2 (3). Complex 3 is fluxional and undergoes terminal and bridging CO exchange as well as coordination and dissociation of CF3SO3− in solution. The crystal structure of the η1-acetone complex Rh2(η1-OC(CH3)2)(μ- CO)(CO)(SO3CF3(dppm)2+ (4) has been determined. Complex 4 crystallizes in the triclinic space group P1 as its triflate salt with a benzene of crystallization in a unit cell of dimensions a = 12.087(4) Å, b = 15.268(4) Å, c = 20.602(4) Å, α= 87.06(2), β = 73.61(2), γ = 72.56(2)°, V = 3477.7 Å3 and Z = 2. Complex 4 possesses a binuclear structure in which the Rh centers are bridged by two dppm ligands and an asymmetrically bound CO ligand. The RhRh bond distance is 2.778(2) Å while the RhO(acetone) distance is 2.09(1) Å. The RhO(triflate) distance is 2.37(1) indicating a weak interaction. Complex 3 reacts readily and reversibly under a CO atmosphere to form the tetracarbonyl complex Rh2(CO)4(dppm)2+2 (5). Upon heating under vacuum, complex 3 undergoes facile CO loss to form the dicarbonyl species Rh2(μ-SO3CF3)(CO)2(dppm)2+2 (6), in situ. Complex 6 is thought to possess an A-frame structure with triflate coordinated in the bridgehead position. Complex 3 promotes H/D exchange with acetone-d6 and catalyzes the hydrogenation of acetone under 3 atm of H2 at 70°C at the rate of 1 turnover per day. The reaction of 3 with H2 leads to the generation of a moderately strong Bronsted acid whereas complex 5 reacts with H2 to produce the known cationic species Rh2(μ-H)(μ-CO)(CO)2(dppm)2+ (2).