Phosphorus and olefin substituent effects on the insertion chemistry of nickel(II) hydride complexes containing amido diphosphine ligands

A series of nickel(II) hydride complexes supported by amido diphosphine ligands, including symmetrical [N(0-C6H4PR2)(2)](-) ([R-PNP](-) ; R = Ph, Pr-i, Cy) and unsymmetrical [N(0-C6H4PPh2)(0-(C6H4PPr2)-Pr-i)](-) ([Ph-PNP-Pr-i](-)), have been prepared for the investigation of olefin insertion chemistry. The unsymmetrical ligand precursor H[Ph-PNP-Pr-i] (1d) that features different substituents (phenyl and isopropyl) at the two phosphorus donors was prepared in 53% yield as colorless crystals. Treatment of Ni(COD)(2) (COD = cycloocta-1,5-diene) with H[R-PNP] (R = Ph (1a), Pr-i (1b), Cy (1c)) or 1d produced the corresponding our-coordinate nickel hydride complexes 2a-d. Attempts to isolate 2a led instead to the cyclooct-4-en-1-yl complex [Ph-PNP]Ni(eta(1)-C8H13) (3a) as a consequence of COD insertion into the Ni-H bond of 2a. The reactions of 2a,d with ethylene, I-hexene, and norbornene, respectively, generated cleanly the corresponding ethyl (4a,d), n-hexyl (5a,d), and 2-norbornyl (6a,d) complexes. The quantitative formation of 5a,d is indicative of exclusive 1,2-insertion of 1-hexene. In contrast, styrene inserts into the Ni-H bond of 2d in an exclusively 2,1-manner to afford [Ph-PNP-Pr-i]NiCH(Me)Ph (7d) quantitatively. The selective 2,1-insertion products [Ph-PNP]NiCH(Me)CO2Me (8a), [Pr-i-PNP]NiCH(Me)CO2Me (8b), [Cy-PNP]NiCH(Me)CO2Me (8c), and [Ph-PNP-Pr-i]NiCH(Me)CO2Me (8d) were also isolated from the reactions of methyl acrylate with the corresponding nickel hydride complexes 2a-d. The effects of the phosphorus and olefin substituents on the reactivity and regioselectivity of the olefin insertion reactions are discussed. In addition to solution NMR spectroscopic data for all new compounds, X-ray structures of 1d, 2b-d, 3a, and 5d-7d are reported.