Mechanism of Catalytic Nitrene Transfer Using Iron(I)–Isocyanide Complexes

Low-spin iron­(I) complexes supported by bulky β-diketiminate ligands catalyze the formation of unsymmetric carbodiimides (RNCNR′, R ≠ R′) from isocyanides and organoazides. In situ EPR studies indicate that the catalytic resting state in these reactions is the tris­(isocyanide)­iron­(I) complex LFe­(CNR)3, and kinetic experiments show that the reaction is inverse second-order in isocyanide concentration, suggesting that two isocyanide molecules dissociate prior to the rate-determining step. An unusual iron–carbodiimide complex can be isolated independently, and facile displacement of the coordinated carbodiimide by both isocyanide and N2 demonstrate that product release is not kinetically limiting during catalysis. These experimental data fit a coherent mechanism for the catalytic C–N coupling reaction, and the implications for catalytic nitrene transfer are discussed.