Controlling P−C/C−H bond cleavage in Nickel Bis(diphosphine) Complexes: Reactivity scope, mechanism, and computations

The synthesis of heteroleptic [Ni(P₂N₂)(diphosphine)][BF₄]₂ complexes and the cleavage of P−C and C−H bonds of the P₂N₂ ligand in those complexes are reported. The products are five-coordinate complexes in which Ni−C and P−H bonds have formed to give a cyclic moiety containing Ni−CH=NR₂. The reactivity of [Ni(P₂N₂)(diphosphine)][BF₄]₂ complexes is influenced by the rigidity of the diphosphine, the steric effect of the substituents, and length of the carbon linker of the diphosphine ligands. Diphosphine ligands bearing a rigid backbone (e.g., dmpbz, 1,2-bis(dimethylphosphino)benzene) or aromatic substituents (e.g., dppe, 1,2-bis(diphenylphosphino)ethane) react with [Ni(P^tBu₂N^Bn₂)(CH₃CN)₂][BF₄]₂ to give P−C/C−H bond cleavage products. Both [Ni(P^tBu₂N^Bn₂)(dmpe)(MeCN)][BF₄]₂ and [Ni(P^tBu₂N^Bn₂)(dmpm)(MeCN)][BF₄]₂ (dmpm = 1,2-bis-(dimethylylphosphino)methane) were prepared by the reaction of [Ni(P^tBu₂N^Bn₂)(CH₃CN)₂][BF₄]₂ with the corresponding diphosphine ligands. [Ni(P^tBu₂N^Bn₂)(dmpe)(MeCN)][BF₄]₂ readily undergoes P−C/C−H bond cleavage in nitromethane. In sharp contrast, [Ni(P^tBu₂N^Bn₂)(dmpm)][BF₄]₂ is stabilized by dmpm, a diphosphine with small bite angle, and does not show P−C/C−H bond cleavage reactivity. Computational results show that for complexes bearing less bulky diphosphine ligands, such as dmpm, the barriers for the rate-determining transition states are in some examples higher than 30 kcal/mol with the M06 functional, higher than those for complexes bearing more rigid or more bulky ligands, consistent with experimental studies. The calculated barriers for the first transition state correlated with increased values of the dihedral angle formed by the two NiP₂ planes.