Structure and Reactivity of Rhodium(I) Complexes Based on Electron-Withdrawing Pyrrolyl-PCP-Pincer Ligands

Rhodium complexes based on the electron-withdrawing PCP-type pincer ligand dipyrrolylphoshinoxylene (DPyPX, (PCP)-P-Pyr) were synthesized and their reactivity was Studied. Reaction of Rh-1((PCP)-P-Pyr)PR3 (2) (R = Et (a); Ph (b); Pyr (pyrrolyl, NC4H4) (C); Pyd (pyrrolydinyl, NC4H8) (d)) with Mel was strongly dependent on the sterics and nucleophilicity of PR3. Complex 2a (PEt3 cone angle, Theta degrees, 132 degrees) reacted with Mel to give isomers of Rh-III((PCP)-P-Pyr)Me(I)PEt3, 3. Reaction of 2b (Theta degrees(PR3) = 145 degrees, R = Pyd (2d), Ph (2b), Pyr (2c)) with Mel was accompanied by release of PPh3 and is thought to proceed via the 14e intermediate Rh-I((PCP)-P-Pyr). While the PPyd(3) complex 2d reacted with Mel to give [Rh-III((PCP)-P-Pyr)Me(1)(2)][MePPyd(3)], 4a, the PPyr(3) complex 2c did not react, owing to steric hindrance around Rh-I and the low nucleophilicity of PPyr3. The aptitude of complexes 2 toward activation of H-2 was also examined. Our results support the involvement of 14e intermediates in the olefin hydrogenation process. The ancillary ligand substitution at the Rh-I center of 2 was found to proceed by an associative mechanism. ML5 d(8) intermediates were clearly detected by P-31{H-1) NMR at 213 K during equilibrium between 2a and 2c.