Phosphine-functionalized NHC Ni(ii) and Ni(0) complexes: synthesis, characterization and catalytic properties

Two families of nickel complexes bearing chelating diphenylphosphine-functionalized NHC ligands [Ni^II(ArNHCPPh₂)(allyl)]Cl 1a (Ar = Mes); 1b, (Ar = 2,6-iPr₂-C₆H₃) and [Ni⁰(ArNHCPPh₂)(alkene)] 2a (Ar = 2,6-iPr₂-C₆H₃, alkene = styrene); 2b (Ar = 2,6-iPr₂-C₆H₃, alkene = diethyl fumarate) have been prepared and fully characterized. VT-NMR experiments in solution reveal that the allyl derivatives 1a-b are stereochemically nonrigid. The solid-state structure of the Ni⁰ derivative 2b is also reported. These complexes display interesting catalytic properties in various cross-coupling reactions. The precatalyst [Ni⁰(ArNHCPPh₂)(styrene)] 2a was found to be the most active system. The bulkiness of the N-substituent on the imidazole ring and the low oxidation state of the metal center in 2a accounted for its enhanced catalytic performance. This system catalyzed effectively the coupling of (hetero)aryl chlorides with a range of nucleophiles including Grignard reagents, boronic acids, secondary amines and indoles.