Rhodium carbonyl complexes containing pyridine carboxylic acid ligands: Reactivity towards various electrophiles and catalytic activity

The complex [Rh(CO)2Cl]2 reacts with two molar equivalent of pyridine carboxylic acids ligands Py-2-COOH(a), Py-3-COOH(b) and Py-4-COOH(c) to yield rhodium(I) dicarbonyl chelate complex [Rh(CO)2(L/)](1a) {L/ = η2-(N,O) coordinated Py-2-COO−(a/)} and non-chelate complexes [Rh(CO)2ClL//](1b,c) {L// = η1-(N) coordinated Py-3-COOH(b), Py-4-COOH(c)}. The complexes 1 undergo oxidative addition (OA) reactions with different electrophiles such as CH3I, C2H5I, C6H5CH2Cl and I2 to give penta coordinated Rh(III) complexes of the types [Rh(CO)(CORn)XL/], {n = 1,2,3; R1 = CH3(2a); R2 = C2H5(3a); X = I and R3 = CH2C6H5 (4a); X = Cl}, [Rh(CO)I2L/](5a), [Rh(CO)(CORn)ClXL//] {R1 = CH3(6b,c); R2 = C2H5(7b,c); X = I and R3 = CH2C6H5 (8b,c); X = Cl} and [Rh(CO)ClI2L//](9b,c). The complexes have been characterized by elemental analysis, IR and 1H NMR spectroscopy. Kinetic data for the reaction of 1a–b with CH3I indicate a first order reaction. The catalytic activity of 1a–c for the carbonylation of methanol to acetic acid and its ester is evaluated and a higher turn over number (TON = 810–1094) is obtained compared with that of the well-known commercial species [Rh(CO)2I2]− (TON = 653) at mild reaction conditions (temperature 130 ± 5 °C, pressure 35 ± 5 bar).

Graphical abstract[Rh(CO)2Cl]2 reacts with pyridine carboxylic acid ligands to yield rhodium(I) dicarbonyl chelate complex [Rh(CO)2(L/)] {L/ = η2-(N,O) coordinated Py-2-COO−} and non-chelate complexes [Rh(CO)2ClL//]{L// = η1-(N) coordinated Py-COOH}. The complexes undergo oxidative addition (OA) with different electrophiles such as CH3I, C2H5I, C6H5CH2Cl and I2. The complexes show higher efficiency as catalyst for carbonylation of methanol compared to the industrially used species [Rh(CO)2I2]−.Figure optionsDownload full-size imageDownload as PowerPoint slide