title: Ligand redox effects in the synthesis, electronic structure, and reactivity of an alkyl-alkyl cross-coupling catalyst.
abstract: The ability of the terpyridine ligand to stabilize alkyl complexes of nickel has been central in obtaining a fundamental understanding of the key processes involved in alkyl-alkyl cross-coupling reactions. Here, mechanistic studies using isotopically labeled (TMEDA)NiMe(2) (TMEDA = N,N,N',N'-tetramethylethylenediamine) have shown that an important catalyst in alkyl-alkyl cross-coupling reactions, (tpy')NiMe (2b, tpy' = 4,4',4' '-tri-tert-butylterpyridine), is not produced via a mechanism that involves the formation of methyl radicals. Instead, it is proposed that (terpyridine)NiMe complexes arise via a comproportionation reaction between a Ni(II)-dimethyl species and a Ni(0) fragment in solution upon addition of a terpyridine ligand to (TMEDA)NiMe(2). EPR and DFT studies on the paramagnetic (terpyridine)NiMe (2a) both suggest that the unpaired electron resides heavily on the terpyridine ligand and that the proper electronic description of this nickel complex is a Ni(II)-methyl cation bound to a reduced terpyridine ligand. Thus, an important consequence of these results is that alkyl halide reduction by (terpyridine)NiR(alkyl) complexes appears to be substantially ligand based. A comprehensive survey investigating the catalytic reactivity of related ligand derivatives suggests that electronic factors only moderately influence reactivity in the terpyridine-based catalysis and that the most dramatic effects arise from steric and solubility factors.
authors.affiliation: Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, 72701, USA.
authors.forename: Gavin D
authors.initials: GD
authors.lastname: Jones
authors.forename: Jason L
authors.initials: JL
authors.lastname: Martin
authors.forename: Chris
authors.initials: C
authors.lastname: McFarland
authors.forename: Olivia R
authors.initials: OR
authors.lastname: Allen
authors.forename: Ryan E
authors.initials: RE
authors.lastname: Hall
authors.forename: Aireal D
authors.initials: AD
authors.lastname: Haley
authors.forename: R Jacob
authors.initials: RJ
authors.lastname: Brandon
authors.forename: Tatyana
authors.initials: T
authors.lastname: Konovalova
authors.forename: Patrick J
authors.initials: PJ
authors.lastname: Desrochers
authors.forename: Peter
authors.initials: P
authors.lastname: Pulay
authors.forename: David A
authors.initials: DA
authors.lastname: Vicic
mesh_terms: D000473:Alkanes / Q000138:chemical synthesis* / Q000737:chemistry; D002384:Catalysis; D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy; D006846:Hydrocarbons, Halogenated / Q000737:chemistry; D008024:Ligands; D008958:Models, Molecular; D009532:Nickel / Q000737:chemistry; D009942:Organometallic Compounds / Q000737:chemistry; D010084:Oxidation-Reduction; D011725:Pyridines / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet
chemical_list: D000473:Alkanes; D006846:Hydrocarbons, Halogenated; D008024:Ligands; D009942:Organometallic Compounds; D011725:Pyridines; D009532:Nickel