Bis(diisopropylphosphinomethyl)amine Nickel(II) and Nickel(0) Complexes: Coordination Chemistry, Reactivity, and Catalytic Decarbonylative C–H Arylation of Benzoxazole

The facile one-step synthesis of five new bis­(diisopropylphosphinomethyl)­amine ligands RN­(CH2DIP)2 (DIP = diisopropylphosphine, R = Me–, iPr–, PhCH2–, 2-ThCH2–, and 2-FuCH2−) based on the use of the air-stable phosphonium salt [DIP­(CH2OH)2]Cl is presented. The phosphonium salt cleanly reacts with primary amines to afford amine-bridged bisphosphine ligands with variable backbone substitution in good yields. These DIP ligands are useful model systems for their chiral bisphospholane analogues. The coordination chemistry of neutral nickel­(II) complexes [(RDIP)­NiCl2], [(RDIP)­Ni­(CH3)2], [(iPrDIP)­Ni­(Cl)­CH2Si­(CH3)3], and [(RDIP)­Ni­(CH2Ph)2], as well as coordinatively unsaturated cationic nickel­(II) complexes [(RDIP)­Ni­(THF)­CH3]+BArF– and [(RDIP)­NiCH2Ph]+BArF–, has been studied by spectroscopic and X-ray diffraction methods. The cationic methyl complexes reacted cleanly with 2-butyne and 1,1-dimethylallene, yielding allylic complexes [(RDIP)­Ni­(pmcb)]+BArF– and [(RDIP)­Ni­(tma)]+BArF–, respectively (pmcb: η3-1,2,3,4,4-pentamethylcyclobutenyl; tma: η3-2,3,3-trimethylallyl). Nickel(0) complexes [(BzDIP)­Ni­(trans-stilbene)] and [(RDIP)­Ni­(C2H4)] were synthesized in one step from the corresponding dichloro complexes and have been fully characterized and analyzed by X-ray diffraction methods. [(MeDIP)­Ni­(C2H4)] reacted with phenyl-2-thiophenecarboxylate, yielding [(MeDIP)­Ni­(OPh)­(2-Th)] and [(MeDIP)­Ni­(CO)2] in a 2:1 ratio. Both complexes were also synthesized via alternative routes. The phenolato-thienyl complex represents an intermediate in the catalytic cycle of a recently reported decarbonylative arylation of azoles. Complexes [(RDIP)­Ni­(C2H4)] were shown to be active catalysts for this reaction.