Electronic Effects in the Oxidative Addition of Iodomethane with Mixed-Aryl β-Diketiminate Chromium Complexes

The CrII complexes CpCr[DppNC(Me)CHC(Me)NC6H4Y] (1; Dpp = 2,6-(Me2CH)2C6H3) were used to prepare the corresponding CrIII iodo (2) and methyl (3) mixed-aryl β-diketiminate compounds with Y = OMe (a), CH3 (b), H (c), CF3 (d). Oxidation of the CrII precursors with iodine gave CpCr[DppNC(Me)CHC(Me)NC6H4Y](I) (2a−d). The CrIII methyl compounds 3a−d were prepared by reaction of the CrII complexes with iodomethane followed by MeMgI. The CrIII chloride complexes 4a−d were synthesized by salt metathesis of isolated CpCrCl2(THF) with the appropriate deprotonated mixed-aryl β-diketiminate. The structures of the paramagnetic complexes 1b,c, 2a,b, 3a−d, and 4a−d were determined by single-crystal X-ray diffraction. The rate constants for the single-electron oxidative addition of iodomethane with 1a−d were determined by monitoring the reaction by UV−vis spectroscopy. The rate constants correlated well with the electronic parameters of the Y substituents (ρ = −0.36). The rate constants for 1a−d were an order of magnitude greater than those previously obtained for the same reaction with CpCr[(DppNCMe)2CH] and related ortho-disubstituted symmetric β-diketiminate complexes.