Organochromium Complexes Bearing Noninnocent Diimine Ligands

• Published in: European Journal of Inorganic Chemistry 2012-01, Vol.2012 (3), p.520-529
• Main Authors: Kreisel, Kevin A., Yap, Glenn P. A., Theopold, Klaus H.
• Format: Article
• Language: English
• Subjects: Alkylation; Chromium complexes; Diimine ligands; Insertion; N ligands
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.201100803

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 [(HLiPr)CrR(THF)] [HLiPr = Ar–N=C(H)–(H)C=N–Ar in which Ar = 2,6‐diisopropylphenyl and R = CH2SiMe3 or CH3] with [(C5H5)2Fe]+ presumably forms [(HLiPr)CrR(THF)]+, which contains a neutral diimine ligand coordinated to CrII. 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 [(HLiPr)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‐CrII species that features a neutral diimine ligand. A C=N insertion mechanism followed by a hydride shift is proposed.