Dicarbonylrhodium(I) complexes of functionalized pyridine ligands and their catalytic activities

Reactions of dimeric complex [Rh(CO)2Cl]2 (1) with pyridine ester ligands methyl picolinate (a), methyl nicotinate (b), methyl isonicotinate (c), ethyl picolinate (d), ethyl nicotinate (e) and ethyl isonicotinate (f) in the 1:2 molar ratio afford the complexes of the type [Rh(CO)2ClL] (1a–f). The complexes 1a–f exhibit two equally intense ν(CO) bands in the range 1990–2091 cm−1 indicating cis-disposition of the two terminal carbonyl groups. The complexes 1a and 1d undergo partial decarbonylation reaction in solution to give the corresponding chelated monocarbonyl complexes [Rh(CO)Cl(methyl picolinate)] (1a′) and [Rh(CO)Cl(ethyl picolinate)] (1d′), respectively. The complexes 1a–f undergo oxidative addition reaction with different types of electrophiles like CH3I, C2H5I, C6H5CH2Cl and I2 to yield [Rh(CO)(COCH3)ClIL] (2a–f), [Rh(CO)(COC2H5)ClIL] (3a–f), [Rh(CO)(COCH2C6H5)Cl2L] (4a–f) and [Rh(CO)ClI2L] (5a–f) complexes, respectively. The complexes have been characterized by elemental analysis, IR and 1H NMR spectroscopy. The time taken by the different complexes 1a–f for the completion of oxidative addition reactions of CH3I are different and the complex 1f took the shortest time while the complex 1b required the longest time. The catalytic activity of the complexes [Rh(CO)2ClL] (1) in carbonylation of methanol is higher (TON = 844–1251) than the well known [Rh(CO)2I2]− species (TON = 653).

The complexes [Rh(CO)2ClL] (L = Py-2-COOR, Py-3-COOR and Py-4-COOR; R = CH3, C2H5) have been synthesized, which undergo partial decarbonylation to produce [Rh(CO)ClL′] (L′ = Py-2-COOR). The rate of oxidative addition reactions of electrophiles like CH3I, C2H5I, C6H5CH2Cl and I2 to the complexes depends on the ligand. The catalytic activity of the complexes in carbonylation of methanol is higher than that of the species [Rh(CO)2I2]−. Figure optionsDownload as PowerPoint slide