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

The dimeric rhodium precursor [Rh(CO)(2)Cl](2) reacts with quinoline (a) and its three isomeric carboxaldehyde ligands [quinoline-2-carboxaldehyde (b), quinoline-3-carboxaldehyde (c), and quinoline-4-carboxaldehyde (d)] in 1: 2 mole ratio to afford complexes of the type cis-[Rh(CO)(2)Cl(L)] (1a-1d), where L = a-d. The complexes 1a-1d have been characterised by elemental analyses, mass spectrometry, IR and NMR (H-1, C-13) spectroscopy together with a single crystal X-ray structure determination of 1c. The X-ray crystal structure of 1c reveals square planar geometry with a weak intermolecular pseudo dimeric structure (Rh center dot center dot center dot Rh 3.573 angstrom). 1a-1d undergo oxidative addition (OA) with different electrophiles such as CH3I, C2H5I and I-2 to give Rh(III) complexes of the type [Rh(CO)(COR) Cl(L) I] {R = -CH3 (2a-2d), R = -C2H5 (3a-3d)} and [Rh(CO)Cl(L)I-2] (4a-4d) respectively. 1b exhibits facile reactivity with different electrophiles at room temperature (25 degrees C), while 1a, 1c and 1d show very slow reactivity under similar condition, however, significant reactivity was observed at a temperature similar to 40 degrees C. The complexes 1a-1d show higher catalytic activity for carbonylation of methanol to acetic acid and methyl acetate [Turn Over Frequency (TOF) 1551-1735 h(-1)] compared to that of the well known Monsanto's species [Rh(CO)(2)I-2](-) (TOF = 1000 h(-1)) under the reaction conditions: temperature 130 +/- 2 degrees C, pressure 33 +/- 2 bar, 450 rpm and time 1 h. The organometallic residue of 1a-1d was also isolated after the catalytic reaction and found to be active for further run without significant loss of activity. (C) 2010 Elsevier B.V. All rights reserved.