N-N bond cleavage in diazoalkanes by a bis(imino)pyridine iron complex

The coordination chemistry of diazoalkanes with reduced iron complexes is an interesting area of study because diazoalkanes have been used as mimics of dinitrogen to get insight on N₂ coordination and functionalization. Further, iron serves as an intelligent alternative to expensive metal catalysts considering its occurrence in the catalytic center of nitrogenases. Chirik and coworkers have recently described an unprecedented hydrogenative N-N and N-C bond cleavage exhibited by bis(amino)pyridine iron diazoalkane complex, ( ^iPrPDI)FeN₂CHSiMe₃ (1-N₂CHSiMe ₃). In this account, they examined the scope of this chemistry and found a rare example of N-N bond cleavage in a family of monosubstituted diazoalkanes under mild and nonhydrogenative conditions. In particular, the addition of N₂CHPh to benzene-d₆ solution of bis(imino)pyridine iron dinitrogen complex, 1-(N₂)₂, rapidly and quantitatively afforded iron benzonitrile (1-NCPh) and iron benzaldimine (1-NHCHPh) as a result of N-N bond cleavage in diazoalkane (Fig. 1). The structures of these two new iron products were established through detailed NMR studies. Furthermore, 1-NHCHPh was independently prepared through hydrogenation of 1-NCPh, an extraordinary example of nitrile hydrogenation. 1-NHCHPh, in turn, delivered the iron amine complex 1-NH₂CH ₂Ph upon continued hydrogenation. The X-ray diffraction studies of 1-NHCHPh revealed a square planar Fe complex and chelate distortions indicative of two-electron reduction (Fig. 2). The scope of the N-N bond cleavage reaction was explored in a range of monosubstituted diazoalkanes (Fig. 1). In each case, 1-NCR and 1-NHCHR (Chemical Equation Presented) were obtained rapidly and quantitatively. It is noteworthy that with N₂CH^tBu, a third product, identified as 1-N₂CH^tBu, was obtained in 60% yield. However, experiments showed that this iron diazoalkane complex does not serve as intermediary in the formation of 1-NCR and 1-NHCHR. Figure 3 outlines the proposed mechanism for N-N bond cleavage in monosubstituted diazoalkanes using N₂CHPh as an illustrative example. The sequence starts with the nucleophilic attack of 1-(N₂)₂ on the diazoalkane carbon giving the putative complex 1-CHPh. This transient species undergoes [4π + 2π] cycloaddition with another diazoalkane to yield iron azine complex 2. The observed products were generated after 1,3-hydrogen shift followed by retrocyclization, nitrile and imine linkage isomerization, and capture by additional free iron complexes. (Chemical Equation Presented)