Organochromium Complexes Bearing Noninnocent Diimine Ligands

Organochromium Complexes Bearing Noninnocent Diimine Ligands:

Abstract

We have prepared several anionic and neutral organochromium complexes featuring noninnocent α-diimine ligands. All neutral and anionic complexes refrain from insertion of the C=N bonds of the diimine ligand into the chromium–carbon bonds, presumably due to the reduced nature of the diimine ligand. However, insertion can be facilitated by one-electron oxidation. Thus, oxidation of [(^HL^iPr)CrR(THF)] [^HL^iPr = Ar–N=C(H)–(H)C=N–Ar in which Ar = 2,6-diisopropylphenyl and R = CH₂SiMe₃ or CH₃] with [(C₅H₅)₂Fe]⁺ presumably forms [(^HL^iPr)CrR(THF)]⁺, which contains a neutral diimine ligand coordinated to Cr^II. This cationic complex apparently undergoes immediate insertion of the C=N bond of the diimine ligand into the Cr–alkyl bond, followed by C–H activation of the isopropyl substituent of an N–aryl group on the diimine ligand. DFT calculations confirm that [(^HL^iPr)CrR(THF)]⁺ contains a neutral innocent diimine ligand that undergoes C=N bond insertion into the Cr–R bond.

A series of organochromium complexes bearing α-diimine ligands has been prepared. The compounds contain diimine ligands in the neutral, singly, and doubly reduced states. All the complexes resist alkyl migration to the diimine backbone, except for a cationic alkyl-Cr^II species that features a neutral diimine ligand. A C=N insertion mechanism followed by a hydride shift is proposed.