Nickel alkynyl and allenylidene complexes: Synthesis and properties

摘要

Copper-catalyzed reaction of [Cp(PPh_3)NiCl] with the terminal alkynes H-C≡C-C(O)R (R = O-Menthyl, NMe_2, Ph) yields the alkynyl complexes [Cp(PPh_3)Ni-C≡C-C(O)R]. Subsequent O-methylation with either [Me_3O]BF_4 or MeSO _3CF_3 affords cationic allenylidene complexes, [Cp(PPh _3)Ni-C≡C-C(OMe)R]~+X? (X = BF_4, SO_3CF_3). N-Alkylation of Cp(PPh_3)Ni- pyridylethynyl complexes likewise gives cationic allenylidene complexes. [Cp(PPh_3)Ni-C≡C-C(CH)_4N] adds BF_3 at nitrogen. Modification of the ligand sphere in these nickel allenylidene complexes is possible by replacing PPh_3 by PMe_3 in the alkynyl complex precursors. The first allenylidene(carbene)nickel cation, [Cp(SIMes)N-C≡C-C(OMe)NMe_2]~+, is accessible by successive reaction of [Cp(SIMes)NiCl] with H-CC-C(O)NMe_2 and [Me_3O]BF_4. By the analogous sequence an allenylidene complex containing the chelating (diphenylphosphanyl)ethylcyclopentadienyl ligand can be prepared. DFT Calculations were carried out on the allenylidene complex cation [Cp(PPh_3)Ni-C≡C-C(OMe)NMe_2] ~+ and on its precursor, the alkynyl complex [Cp(PPh _3)Ni-C≡C-C(O)NMe_2]. Based on the spectroscopic data and a X-ray structure analysis the bonding in the new nickel allenylidene complexes is best represented by several resonance forms, an alkynyl resonance form considerably contributing to the overall bond.