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 = ArN C(Me)C(Me) NAr, Ar = 2- i PrC 6 H 4 ( 1a ), 2,4,6-Me 3 C 6 H 2 ( 1b ), 2,6- i Pr 2 C 6 H 3 ( 1c ); diimine = ArN C(Me)-2-py, Ar = C 6 H 5 ( 1d ), 2- i PrC 6 H 4 ( 1e ), 2,6- i Pr 2 C 6 H 3 ( 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)I 2 Me] ( 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)I 2 (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 alkyl groups in the ortho positions of N-aryl groups favoring migratory CO insertion. Rearrangement from octahedral to square-pyramidal geometry relieves steric strain between axial ligands and ortho alkyl substituents on the diimine. Kinetic studies showed that the second-order rate constant ( k 1 ) for MeI oxidative addition varied by a factor of >10 3, depending on the steric bulk of the diimine ligand. For the bipy complex 1g, k 1 is the highest yet reported for a Rh(I) carbonyl complex (0.206 M -1 s - 1, CH 2 Cl 2, 25 °C). Kinetic data are also reported for migratory CO insertion in 2b, f . An X-ray structure of [Rh(CO)(ArN C(Me)-2-py)I 3 ] ( 4f; Ar = 2,6- i Pr 2 C 6 H 3 ), the product of oxidative addition of I 2 to 1f, reveals distortion from ideal octahedral geometry caused by ortho i Pr groups on the diimine ligand.