Group-Transfer Reactions of Ni(II)−Ni(II) Bridging Imido Complexes. Catalytic Formation of Carbodiimides and Isocyanates via Nitrene Transfer from Organoazides

The NHC-supported Ni(I) dimer {(IPr)Ni(μ-Cl)}2 (1; IPr = 1,3-di(2,6-di-isopropylphenyl)imidazolin-2-ylidene) yields the bridging d8−d8 imido complex {(IPr)NiCl}2(μ-NMes) (2; Mes = 2,4,6-Me3C6H2) upon combination with N3Mes. Tosylazide reacts with 1 to give the paramagnetic, dark red complex {(IPr)Ni}2(Cl)(μ-Cl)(μ-N,κ1-O:NSO2Tol) (3). Chloride abstraction from 2 with NaBArF4 affords the d8−d8 dimer [{(IPr)Ni}2(μ-Cl)(μ-NMes)]BArF4 (4; BArF4 = B[3,5-(CF3)C6H3)]4), and reduction of 2 with KC8 yields the neutral d8−d9 mixed-valence complex {(IPr)Ni}2(μ-Cl)(μ-NMes) (5). 2 undergoes stoichiometric nitrene group transfer to CO, PMe3, and CNR (R = CH2Ph, CMe3) to form MesNCO, MesNPMe3, and MesNCNR with regeneration of 1. Both 1 and 2 react with excess CNR to give monomeric, three-coordinate Ni(I) complexes (IPr)NiCl(CNR) (6a, R = CH2Ph; 6b R = CMe3). In the presence of excess CO, 1 undergoes disproportionation to give {(IPr)NiCl}2(μ-Cl)2 (7) and (IPr)Ni(CO)3. Similarly, reaction of 2 with PMe3 is accompanied by disproportionation of 1 to (IPr)NiCl2(PMe3) (8) and [(IPr)Ni]2. Catalytic carbodiimide (MesNCNR) formation is achieved with 10 mol % 1, N3Mes, and CNR. Catalytic mesityl isocyanate (MesNCO) formation is achieved with 10 mol % 4, N3Mes, and CO.