Synthesis of a Rhodium Complex Featuring the Rh–H–B Linkage via a Hydride Migration from Rhodium to Borane: Study on the Electronic Deviation Induced by the Presence of the Boron Moiety

2012-11-12T00:00:00Z (GMT) by Hajime Kameo Hiroshi Nakazawa
The rhodium complex [{o-(Ph2P)­C6H4}2(Ph)­BH]­Rh­(CO)­(PPh3) (3-Rh) ,featuring a B–H–Rh interaction, was synthesized via phosphine ligand exchange between tris­(triphenylphosphine)­carbonylrhodium­(I) hydride, RhH­(CO)­(PPh3)3 (2-Rh), and the diphosphine–borane {o-(Ph2P)­C6H4}2BPh (5). In contrast, the reaction of 5 with the iridium analogue of 2-Rh, IrH­(CO)­(PPh3)3 (2-Ir), resulted in the formation of the iridium complex [{o-(Ph2P)­C6H4}3B]­Ir­(H)­(CO) (4-Ir), with a direct Ir→B interaction. These compounds 3-Rh and 4-Ir were fully characterized using 1H, 11B, and 31P NMR spectroscopy, X-ray diffraction analysis, and elemental analysis. Density functional theory calculations of 3-Rh revealed that the boron accepts a significant amount of electron density from the Rh center through a three-center–two-electron Rh–H–B interaction and that the presence of the boron moiety considerably weakens the Rh–CO bond. The electronic variation induced by the boron moiety can promote the facile CO elimination of 3-Rh. The “boron enhancement effect” observed in the catalytic transfer hydrogenation of ethyl phenyl ketone is also reported.