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, PyrPCP) were synthesized and their reactivity was studied. Reaction of RhI(PyrPCP)PR3 (2) (R = Et (a); Ph (b); Pyr (pyrrolyl, NC4H4) (c); Pyd (pyrrolydinyl, NC4H8) (d)) with MeI was strongly dependent on the sterics and nucleophilicity of PR3. Complex 2a (PEt3 cone angle, Θ°, 132°) reacted with MeI to give isomers of RhIII(PyrPCP)Me(I)PEt3, 3. Reaction of 2b (Θ°PR3 = 145°, R = Pyd (2d), Ph (2b), Pyr (2c)) with MeI was accompanied by release of PPh3 and is thought to proceed via the 14e intermediate RhI(PyrPCP). While the PPyd3 complex 2d reacted with MeI to give [RhIII(PyrPCP)Me(I)2][MePPyd3], 4a, the PPyr3 complex 2c did not react, owing to steric hindrance around RhI and the low nucleophilicity of PPyr3. The aptitude of complexes 2 toward activation of H2 was also examined. Our results support the involvement of 14e intermediates in the olefin hydrogenation process. The ancillary ligand substitution at the RhI center of 2 was found to proceed by an associative mechanism. ML5 d8 intermediates were clearly detected by 31P{1H} NMR at 213 K during equilibrium between 2a and 2c.