Mono- and dinuclear Ni(i) products formed upon bromide abstraction from the Ni(i) ring-expanded NHC complex [Ni(6-Mes)(PPh3)Br]

Abstract

Bromide abstraction from the three-coordinate Ni(I) ring-expanded N-heterocyclic carbene complex [Ni(6-Mes)(PPh₃)Br] (1; 6-Mes = 1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene) with TlPF₆ in THF yields the T-shaped cationic solvent complex, [Ni(6-Mes)(PPh₃)(THF)][PF₆] (2), whereas treatment with NaBAr^F₄ in Et₂O affords the dimeric Ni(I) product, [{Ni(6-Mes)(PPh₃)}₂(μ-Br)][BAr^F₄] (3). Both 2 and 3 act as latent sources of the cation [Ni(6-Mes)(PPh₃)]⁺, which can be trapped by CO to give [Ni(6-Mes)(PPh₃)(CO)]⁺ (5). Addition of [(Et₃Si)₂(μ-H)][B(C₆F₅)₄] to 1 followed by work up in toluene results in the elimination of phosphine as well as halide to afford a co-crystallised mixture of [Ni(6-Mes)(η²-C₆H₅Me)][B(C₆F₅)₄] (4), and [6MesH⋯C₆H₅Me][B(C₆F₅)₄]. Treatment of 1 with sodium salts of more strongly coordinating anions leads to substitution products. Thus, NaBH₄ yields the neutral, diamagnetic dimer [{Ni(6-Mes)}₂(BH₄)₂] (6), whereas NaBH₃(CN) gives the paramagnetic monomeric cyanotrihydroborate complex [Ni(6-Mes)(PPh₃)(NCBH₃)] (7). Treatment of 1 with NaO^tBu/NHPh₂ affords the three-coordinate Ni(I) amido species, [Ni(6-Mes)(PPh₃)(NPh₂)] (8). The electronic structures of 2, 5, 7 and 8 have been analysed in comparison to that of previously reported 1 using a combination of EPR spectroscopy and density functional theory.