SYNTHESIS OF M2RH2 BIS(MU 3-CARBON DIOXIDE) COMPLEXES FROM THE REACTION BETWEEN RH(OH)(ETA 4-COD)2 AND CATIONIC METAL CARBONYLS

The M2Rh2 bis(μ3-CO2) complexes [Cp*(CO)(NO)Re(CO2)Rh(η4-cod)]2 (1) and [Cp*(CO)2MCO2Rh(η4-cod)]2, M = Fe (2) and Ru (3), were synthesized in moderate to high yields from treating [Rh(OH)(η4-cod)]2 (cod = 1,5-cyclooctadiene) with the carbonyl salts Cp*(L)(CO)2M+ (L = CO, M = Fe, Ru; L = NO, M = Re) and a base. [An alternative synthesis of 1 and its crystallographic structure determination has been reported.] Although details on using several bases in the synthesis of 2 are reported, the use of volatile EtMe2N in excess as the base is especially useful. IR and NMR spectral data are in accord with a M2Rh2 bis(μ3-CO2) core for 2 and 3:  they retain two μ3-[η1-C(M):η1-O(Rh):η1-O‘(Rh‘)] carboxylate ligands that resemble those that were found for 1 and the Rh(I) carboxylates [(RCO2)Rh(diene)]2. Complexes 2 and 3 with their “open-book” structures are not fluxional at room temperature. Complex 2, its norbornadiene analogue, [Cp*(CO)2FeCO2Rh(η4-nbd)]2 (4), and their 13C-labeled derivatives also are accessible from...