Abstract
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–C C–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.
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