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-ⁱPrC₆H₄ (1a), 2,4,6-Me₃C₆H₂ (1b), 2,6-ⁱPr₂C₆H₃ (1c); diimine = ArNC(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 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 (k₁) for MeI oxidative addition varied by a factor of >10³, depending on the steric bulk of the diimine ligand. For the bipy complex 1g, k₁ is the highest yet reported for a Rh(I) carbonyl complex (0.206 M^-1 s^-1, CH₂Cl₂, 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)I₃] (4f; Ar = 2,6-ⁱPr₂C₆H₃), the product of oxidative addition of I₂ to 1f, reveals distortion from ideal octahedral geometry caused by ortho ⁱPr groups on the diimine ligand.