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

Abstract

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-4-carboxaldehyde ( d)] in 1:2 mole ratio to afford complexes of the type cis-[Rh(CO) 2Cl(L)] ( 1a– 1d), where L = a– d. The complexes 1a– 1d 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 X-ray crystal structure of 1c reveals square planar geometry with a weak intermolecular pseudo dimeric structure (Rh⋯Rh = 3.573 Å). 1a– 1d undergo oxidative addition ( OA) with different electrophiles such as CH 3I, C 2H 5I and I 2 to give Rh(III) complexes of the type [Rh(CO)(COR)Cl(L)I] {R = –CH 3 ( 2a– 2d), R = –C 2H 5 ( 3a– 3d)} and [Rh(CO)Cl(L)I 2] ( 4a– 4d) 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 ∼40 °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) 2I 2] − (TOF = 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 1a– 1d was also isolated after the catalytic reaction and found to be active for further run without significant loss of activity.