Oxygen Atom Transfer to Cationic PCPNi(II) Complexes Using Amine-N-Oxides

Three PC_sp3P pincer ligands differing in the aryl group linking the phosphine arms with the anchoring carbon donor were used to support square planar Ni(II) bromide complexes 1-3_Br. Exchange of the coordinating bromide anion for the more weakly coordinating triflate (OTf) or hexafluoroantimonate (SbF₆) anions was accomplished by treatment with AgX or TlX salts to give compounds 1-3_X; compounds 1_OTf, 1_SbF, 2_Br, 2_OTf, 3_Br, and 3_SbF were all characterized by X-ray crystallography. The reactions of these Ni(II) compounds with the amine-N-oxide oxygen atom transfer agents ONMe₃ and ONMePh₂ were explored. For ONMe₃, reactions with 2 equiv gave products in which one arm of the pincer ligand was oxidized to a P═O unit, with the other amine-N-oxide ligated to the Ni(II) center, forming products 5-6_X; compounds 4_OTf, 5_OTf, and 6_SbF were characterized crystallographically. Transient amine-N-oxide adducts prior to ligand oxidation were observed in some reactions. For the more effective O atom donor ONMePh₂, reactions were very rapid and a second oxidation of the remaining phosphine arm was observed, producing a Ni(II) species with an OCO pincer ligand (7_SbF). All compounds were fully characterized. Experiments aimed at trapping transient Ni(IV) oxo intermediates (with cyclohexadiene, KH, and various Lewis acids) indicated that such species were not involved in the reaction. This was supported by density functional theory (DFT) computations at the B3PW91 level, which indicated that direct O atom insertion into the Ni-P bonds without the intermediacy of a Ni oxo species was the low-energy pathway.