Half-sandwich NHC-nickel(ii) complexes as pre-catalysts for the fast Suzuki coupling of aryl halides: a comparative study

Abstract

Cationic half-sandwich nickel complexes of general formula [Ni(NHC)(NCMe)(η⁵-C₅R₅)](PF₆) [NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) a, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) b; R = H, Me] were prepared from the reaction of their neutral homologues [Ni(NHC)Cl(η⁵-C₅R₅)] with 1 equiv. of KPF₆ in acetonitrile at room temperature. The new cationic complexes [Ni(IPr)(NCMe)(η⁵-C₅Me₅)](PF₆) 3a, [Ni(IMes)(NCMe)(η⁵-C₅Me₅)](PF₆) 3b and [Ni(IMes)(NCMe)(η⁵-C₅H₅)](PF₆) 4b were obtained in high yield and were fully characterized by ¹H and ¹³C NMR spectroscopy, IR spectroscopy, elemental analyses, and in the case of 3a by a single-crystal X-ray diffraction study. The neutral analogue of 3a, [Ni(IPr)Cl(η⁵-C₅Me₅)] 1a was also structurally characterized. Their geometries were compared and no significant structural differences were observed. Nevertheless solution NMR spectroscopy established that the acetonitrile ligand of the cationic species is labile in solution. This results in the absence of any rotational significant barrier about the nickel–carbene carbon bond at ambient temperature in solution in the sterically congested cationic complexes 3a and 3b, in contrast to their neutral analogues 1a and [Ni(IMes)Cl(η⁵-C₅Me₅)] 1b. The neutral and the cationic complexes catalyzed the cross-coupling of phenylboronic acid with aryl halides in the absence of co-catalysts or reductants. Surprisingly, the neutral or cationic nature of the complexes proved to have almost no influence on the reaction yields and rates. However, complexes bearing the bulky electron-rich pentamethylcyclopentadienyl ligand were much more active than those bearing the cyclopentadienyl ligand, and TOF of up to 190 h⁻¹, a high rate for nickel(II) complexes under similar conditions, were observed with these species.