The reduction of gaseous nitrogen (N2) is a challenge for industrial, biological and synthetic chemists, who want to understand the formation of ammonia (NH3) for agriculture and also want to form N-C and N-Si bonds for fine chemical synthesis. The iron-molybdenum active site of nitrogenase has inspired chemists to explore the ability of iron and molybdenum complexes to bring about transformations related to N2 reduction. This area of research has gained significant momentum, and the last two years have witnessed a number of significant advances in synthetic Fe-N2 and Mo-N2 chemistry. In addition, the identities of all atoms in the iron-molybdenum cofactor of nitrogenase have finally been elucidated, and the discovery of a carbide has generated new questions and targets for coordination chemists. This Perspective summarizes the recent work on iron and molydenum complexes, and highlights the opportunities for continued research.

This manuscript describes the formally iron(I) complexes L(Me)Fe(Py-R)(2) (L(Me) = bulky β-diketiminate; R = H, 4-tBu), in which the basal pyridine ligands preferentially accept significant unpaired spin density. Structural, spectroscopic, and computational studies on the complex with 4-tert-butylpyridine ((tBu)py) indicate that the S = 3/2 species is a resonance hybrid between descriptions as (a) high-spin iron(II) with antiferromagnetic coupling to a pyridine anion radical and (b) high-spin iron(I). When the pyridine lacks the protection of the tert-butyl group, it rapidly and reversibly undergoes radical coupling reactions that form new C-C bonds. In one reaction, the coordinated pyridine couples to triphenylmethyl radical, and in another, it dimerizes to give a pyridine-derived dianion that bridges two iron(II) ions. The rapid, reversible C-C bond formation in the dimer stores electrons from the formally reduced metal as a C-C bond in the ligands, as demonstrated by using the coupled diiron(II) complex to generate products that are known to come from iron(I) precursors.

dipp, 4) have been synthesized and used in polymerization experiments in addition to the previously known analogues with Ar = Ar = xyl, 1, or dipp, 5. The compounds were used as moderators for the polymerization of vinyl acetate (VAc) initiated by V-70, according to an OMRP mechanism. The alkylchromium(III) thermal initiator CpCr(nacnac xyl,xyl )(CH 2 CMe 3 ) (8) was synthesized from CpCr(nacnac xyl,xyl )(OTs) (7) and Mg(CH 2 CMe 3 ) 2 (dioxane), while 7 was obtained from CpCr(nacnac xyl,xyl )Cl (6) and AgOTs. The polymerizations carried out with (1-5)/V-70/VAc at elevated temperatures yielded rapid deactivation, suggestive of irreversible radical trapping.On the other hand, room temperature polymerizations carried out with 6/VAc proceeded, albeit slowly, to greater conversions. A labilizing effect of the Ar/Ar' steric bulk is suggested by QM/MM calculations of Cr III -C BDE for models of the OMRP dormant species with 1, 5, and the parent system where Ar = Ar' = Ph.Thermal deactivation of the nacnac xyl,xyl system has been evidenced, with formation of the acetate complex CpCr(nacnac xyl,xyl )(OAc), 9, as confirmed by an UV-vis study and by independent synthesis. This product is proposed to form by β-acetate transfer from the growing radical chain, triggered by a head-head monomer insertion.Compounds 2, 3, 6, 7, 8 and 9 have been structurally characterized by X-ray diffraction methods.

The pathway from N2 to NH3 at low-coordinate iron complexes is shown through crystallographic and spectroscopic characterization of intermediates, including bridging nitride, imide, and amides. Proton-coupled electron transfer plays a key role in the transformations.