Four- and five-coordinate nickel(ii) complexes bearing new diphosphine–phosphonite and triphosphine–phosphite ligands: catalysts for N-alkylation of amines

Abstract

The reaction of Ph₂PCH₂OH with PhPCl₂ and PCl₃ in the presence of Et₃N afforded new phosphonite compounds PhP(OCH₂PPh₂)₂ 1 and P(OCH₂PPh₂)₃ 2, respectively. The reaction between 1 and [NiCl₂(DME)] in dichloromethane gave the five-coordinate complex [NiCl₂(1-κ³P,P,P)] 3. Conversely, 1 reacts with [NiCl₂(DME)] in the presence of NH₄PF₆ in dichloromethane to yield the four coordinate ionic complex [NiCl(1-κ³P,P,P)][PF₆] 4. The reactions between 1, [NiCl₂(DME)] and KPF₆ in the presence of RNC (R = Xylyl, ^tBu and ⁱPr) in dichloromethane yielded the five coordinate monocationic [NiCl(1-κ³P,P,P)(RNC)][PF₆] (R = Xylyl) and dicationic [Ni(1-κ³P,P,P)(RNC)₂][PF₆]₂ (R = ^tBu and ⁱPr) complexes, respectively. The analogous reaction of 2 with [NiCl₂(DME)] in the presence of KPF₆ gave complex [NiCl(2-κ⁴P,P,P,P)][PF₆], 8. The structures of all complexes were determined by single crystal X-ray diffraction studies and supported by spectroscopic methods. To demonstrate their catalytic application, N-alkylation reactions between primary aryl amines, benzyl and 4-methoxy benzyl alcohols were found to proceed smoothly in the presence of 2.5 mol% of complexes bearing ligand 1 and <0.5 mmol of KOBu^t in toluene at 140 °C. The C–N coupled products were formed in very good yields. Its substrate scope includes sterically encumbered, heterocyclic amines and aliphatic alcohol.