Rhodium(I) carbonyl complexes of quinoline carboxaldehyde ligands and their catalytic carbonylation reaction

The dimeric rhodium precursor [Rh(CO)2Cl]2 reacts with quinoline (a) and its three isomeric carboxaldehyde ligands [quinoline-2-carboxaldehyde (b), quinoline-3-carboxaldehyde (c), and quinoline-4carboxaldehyde (d)] in 1:2 mole ratio to afford complexes of the type cis-[Rh(CO)2Cl(L)] (1ae1d), where L 1⁄4 aed. The complexes 1ae1d have been characterised by elemental analyses, mass spectrometry, IR and NMR (1H, 13C) spectroscopy together with a single crystal X-ray structure determination of 1c. The Xray crystal structure of 1c reveals square planar geometry with a weak intermolecular pseudo dimeric structure (Rh/Rh 1⁄4 3.573 Å).1ae1d undergo oxidative addition (OA) with different electrophiles such as CH3I, C2H5I and I2 to give Rh(III) complexes of the type [Rh(CO)(COR)Cl(L)I] {R 1⁄4 eCH3 (2ae2d), R 1⁄4 eC2H5 (3ae3d)} and [Rh(CO)Cl(L)I2] (4ae4d) respectively. 1b exhibits facile reactivity with different electrophiles at room temperature (25 C), while 1a, 1c and 1d show very slow reactivity under similar condition, however, significant reactivity was observed at a temperature w40 C. The complexes 1ae1d show higher catalytic activity for carbonylation of methanol to acetic acid and methyl acetate [Turn Over Frequency (TOF) 1⁄4 1551e1735 h 1] compared to that of the well known Monsanto’s species [Rh(CO)2I2] (TOF 1⁄4 1000 h 1) under the reaction conditions: temperature 130 2 C, pressure 33 2 bar, 450 rpm and time 1 h. The organometallic residue of 1ae1d was also isolated after the catalytic reaction and found to be active for further run without significant loss of activity. 2010 Elsevier B.V. All rights reserved.