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(mu-Cl)}(2) (1; IPr = 1,3-di(2,6-di-isopropylphenyl)imidazolin-2-ylidene )yields the bridging d(8)-d(8) imido complex {(IPr)NiCl}(2)(mu-NMes) (2; Mes = 2,4,6-Me3C6H2) upon combination with N(3)Mes. Tosylazide reacts with 1 to give the paramagnetic, dark red complex 4 affords the {(IPr)Ni}(2)(Cl)(mu-Cl)(mu-N,kappa(1)-O:NSO(2)Tol) (3). Chloride abstraction from 2 with NaBAr4F affords the d(8)-d(8) dimer [{(IPr)Ni)(2)(mu-Cl)(mu-NMes)]BAr4F (4; BAr4F = B[3,5-(CF3)C6H3)](4)), and reduction of 2 with KC8 yields the neutral d(8)-d(9) mixed-valence complex {(IPr)Ni}(2)(mu-Cl)(mu-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)(mu-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, N(3)Mes, and CNR. Catalytic mesityl isocyanate (MesN=C=O) formation is achieved with 10 mol % 4, N(3)Mes, and CO.