Evaluating the reducing power of 3-coordinate T-shaped NiI

The reactivity of (PNP)NiI, where PNP = (tBu2PCH2SiMe2)2N, with oxidants was evaluated. Towards the nitroxyl TEMPO, a 1:1 adduct is formed which was shown to have η2-TEMPO bound through both N and O, with the consequence that one P of the PNP ligand is displaced, leaving the pincer ligand bidentate to NiII. DFT calculations show that the bidentate character of TEMPO is due to steric clash between tBu and TEMPO ring methyl groups. Reaction of (PNP)Ni with I2, Br2, C2Cl6 and even CH2Cl2 all yield (PNP)NiIIX, but never (PNP)NiIIIX2. Excess Br2 instead oxidizes one phosphorus, yielding the zwitterion [(BrtBu2PCH2SiMe2)N(SiMe2CH2PtBu2)]NiBr2, whose structure is determined. DFT calculation of the species (PNP)NiIII(Br)2 yields reaction thermodynamics which show the reason for its absence, and also shows the low BDE of its Ni–Br bond. (PNP)Ni slowly catalyzes the polymerization of HCCR (R = H or Ph), but gives no detectable conversion to a new alkyne-derived nickel complex.