Synthesis and characterization of new rhodium and iridium complexes with trianisylphosphine, PAn3, and evaluation of their catalytic behavior in the homogeneous hydrogenation of cinnamaldehyde

A new family of rhodium and iridium compounds with the bulky tris(ortho-methoxyphenyl) phosphine (PAn3) was synthesized and characterized by NMR methods. The X-ray crystal structures of RhCl(PAn3)(COD) (1) and Ir[(PBz3)(PAn3)(COD)]PF6 (4) have been determined. A stabilizing agostic interaction has been crystallographically observed in both compounds, due to the steric hindrance of the ortho-substituted phosphine ligand and its presence has been associated to the fluxional behavior shown by the complexes on the NMR timescale in solution. Iridium complexes containing PBz3 and/or PAn3 have been evaluated as catalyst precursors for the hydrogenation of trans-cinnamaldehyde (CNA), and their activities have also been compared to those of other iridium complexes containing bulky phosphine ligands, such as PTol3 (tris-ortho-tolyl-phosphine). The catalytic experiments show that irrespectively of the phosphine combination, all of the evaluated catalysts prevalently hydrogenate the CC moiety. However, the product selectivity can be tuned by changing either the substrate/catalyst ratio or the phosphine ligand at the metal center. The catalyst with PAn3 proved to be more efficient and also gave higher yields of the enol product, indicating that stereoelectronic effects are responsible for the changes in selectivity.

Graphical abstractNew rhodium and iridium complexes with the bulky tris(ortho-methoxyphenyl)phosphine (PAn3) have been obtained and properly characterized, including X-ray diffraction structures for RhCl(PAn3)(COD) (1) and [Ir(PBz3)(PAn3)(COD)]PF6 (4). Among the new compounds, [Ir(PAn3)(py)(COD)]PF6 (6) and the analogues with PTol3 and PBz3 were tested and compared as precursors for the catalytic hydrogenation of trans-cinnamaldehyde, observing changes both in activity and selectivity. Figure optionsDownload full-size imageDownload as PowerPoint slide