Quantifying Steric Effects of α-Diimine Ligands. Oxidative Addition of MeI to Rhodium(I) and Migratory Insertion in Rhodium(III) Complexes

The series of Rh(I) complexes [Rh(CO)(diimine)I] have been prepared (diimine = ArNC(Me)C(Me)NAr, Ar = 2-iPrC6H4 (1a), 2,4,6-Me3C6H2 (1b), 2,6-iPr2C6H3 (1c); diimine = ArNC(Me)-2-py, Ar = C6H5 (1d), 2-iPrC6H4 (1e), 2,6-iPr2C6H3 (1f); diimine = 2,2‘-bipy (1g)). An X-ray crystal structure showed 1f to have CO trans to the pyridyl nitrogen. Oxidative addition of MeI to 1a,d,e,g gave the stable methyl complexes [Rh(CO)(diimine)I2Me] (2a,d,e,g, respectively); an X-ray structure for 2a showed a pseudo-octahedral geometry with methyl trans to iodide. In contrast, the reactions of 1b,c with MeI gave the five-coordinate acetyl complexes [Rh(diimine)I2(COMe)] (3b,c, respectively); an X-ray structure for 3c displayed a distorted-square-pyramidal geometry. The methyl complex 2b was detected as a reactive intermediate during the reaction of 1b. The reaction of 1f with MeI attained an equilibrium with the methyl and acetyl products 2f and 3f. The different outcomes result from diimine ligand steric effects, with bulky a...