Divergent Carbonylation Reactivity Preferences of Nickel Complexes Containing Amido Pincer Ligands: Migratory Insertion versus Reductive Elimination

The reactivity of a series of nickel­(II) hydride, alkyl,
and anilide
complexes supported by amido diphosphine ligands, including symmetrical
[N­(o-C6H4PR2)2]− (R = Ph (1a), iPr (1c), Cy (1d)) and unsymmetrical
[N­(o-C6H4PPh2)­(o-C6H4PiPr2)]− (1b), with carbon
monoxide is described. Exposure of a benzene solution of [1a–d]­NiH to carbon monoxide under ambient conditions
leads to reductive elimination of diarylamine to give quantitatively
zerovalent nickel dicarbonyl complexes [H·1a–d]­Ni­(CO)2 (2). Migratory insertion
of CO into the Ni-R bonds of [1]­NiR in benzene solutions
affords Ni­(II)-acyl derivatives [1]­NiC­(O)­R (R = Me (3), Et (4), n-hexyl (5), 2-norbornyl (6)). Interestingly, further carbonylation
of acyl 3a and 4a generates [RC­(O)­N­(o-C6H4PPh2)2]­Ni­(CO)2 (R = Me (7a), Et (8a)) as a result of C–N bond-forming reductive elimination whereas
no reaction occurs for 3b–c or 4b–c under similar conditions. Carbonylation
of the anilide complexes [1]­NiNHPh produces carbamoyl
[1]­Ni­[C­(O)­NHPh] (9) as the final product
irrespective of the identity of the phosphorus substituents. The decisive
factors on the generation of these divergent carbonylation products
are discussed.