Dinitrogen coordination

Since the first step of all of these reactions is dinitrogen coordination to either the surface of the catalyst or transition metal center of the complex, let us briefly discuss the nature and importance of the M-N2 interaction, and the possible coordination modes ofN2 to transition metal centers. These issues were the subjects of many discussions in the literature [10, 11] and it is commonly agreed that the interaction of the N2 molecule with transition metal centers facilitates the activation of the N=N triple bond the stronger the M-N2 interaction, the easier to break the N=N triple bond. 

These examples would seem to indicate that the molybdenum atom, that for a long time was considered to be the specific site of dinitrogen coordination, is of little importance. It should be borne in mind, however, that the X-ray structure of the protein was obtained in the resting state. As noted, under such conditions, the Mo atom is assigned oxidation state IV and has a saturated coordination, hence not able to further coordinate nitrogen. EXAFS studies on the active protein indicate a Mo coordination different to that determined by X-ray diffraction. One hypothesis considers the dissociation of the homocitrate, induced by addition of electrons, that would leave vacant coordination sites which could then be saturated by the nitrogen molecule. 

This Re(I) complex, in dichloromethane-methanol (10 1) solution, displays a reversible one-electron oxidation (E° = +0.05 V vs. SCE) to the corresponding Re(II) complex [ReCl(N2)(PMe2Ph)4]+, 52 thus indicating the dinitrogen coordination is maintained upon the electron removal. 

Dinitrogen-coordinated complexes of Ti, Zr, Mo, and W have been studied as potent mediators in electrolysis media [491-495]. Preliminary results from the electroreduction of [Mo(N2RR )(S2C-NMe2)3]" reveal that the two-electron reduction of the cation leads to electroactivation of the... 

While it is not the purpose here to review all Cp-Ti compounds, there is obviously a very close relationship between CpTi moieties and their rearrangement product (CsH TiH via ring-to-titanium a-hydrogen shift, as has been alluded to above. The nature of the Ti-H species involved in reactions between low-valent titanium compounds and dinitrogen is not always clear. There seems little doubt that some form of bis(cyclopentadienyl) titanium is an active intermediate in dinitrogen coordination reactions (31, 37-39), but there is as yet no conclusive evidence that a Ti-H species is involved in the initial reaction (11), despite some claims to the contrary (40). The existence of different forms of Cp2Ti is referred to again below. 

The dinitrogen coordination has to be suitable for the overlap of both filled binding and empty antibonding dinitrogen orbitals with d-orbitals of iron atoms. 

At lower temperatures dinitrogen coordinates to [RhH(PCy3)2] forming the four-coordinate mononuclear complex [RhH(N2)(PCy3)2].46,47 49 55 56... 

Figure 6.10 Reduction and protonation of dinitrogen coordinated end-on terminally to iron centers with cis thiolate ligands.

Fig. 2 Dinitrogen coordination modes in transition metai compounds.

MacKay BA, Fryzuk MD. Dinitrogen coordination chemistry on the biomimetic... 

Crossland JL, Tyler DR. Iron-dinitrogen coordination chemistry dinitrogen activation and reactivity. Coord Chem Rev. 2010 254 1883-1894. 

Vidyarame I, Crewdson P, Lefebvre E, Gambarotta S. Dinitrogen coordination and cleavage promoted by a vanadium complex of a a, n, o-donor ligand. Inorg Chem. 2007 46 8836-8842. 

Yamamoto A, Miura Y, Ito T, et al. Preparation, x-ray molecular structure determination, and chemical properties of dinitrogen-coordinated cobalt complexes containing triphenylphosphine ligands and alkali metal or magnesium. Protonation of the coordinated dinitrogen to ammonia and hydrazine. Organometallics. 1983 2 1429-1436. 

---