Dinitrogen complexes 17-electron intermediates

Anionic dinitrogen complexes of MeP and W of the type trans - [M(N2)X(Ph2PCH2CH2PPh2)2] (X = N3, CN, SCN) are moderately strong reducing agents with oxidation potentials in the vicinity of -1 V versus SCE. They undergo an outer sphere electron - transfer with alkyl halides to produce M dinitrogen 17 - electron intermediates and alkyl radicals. Scheme E. The fates of both... 

DFT calculations were performed on Mo dinitrogen, hydra-zido(2-) and hydrazidium complexes. The calculations are based on available X-ray crystal structures, simplifying the phosphine ligands by PH3 groups. Vibrational spectroscopic data were then evaluated with a quantum chemistry-assisted normal coordinate analysis (QCA-NCA) which involves calculation of the / matrix by DFT and subsequent fitting of important force constants to match selected experimentally observed frequencies, in particular v(NN), v(MN), and 8(MNN) (M = Mo, W). Furthermore time-dependent (TD-) DFT was employed to calculate electronic transitions, which were then compared to experimental UVATs absorption spectra (16). As a result, a close check of the quality of the quantum chemical calculations was obtained. This allowed us to employ these calculations as well as to understand the chemical reactivity of the intermediates of N2 fixation (cf. Section III). 

The suggestion for the catalytic cycle is presented in Scheme 4.18. Initially, an unsaturated iron complex is formed by expulsion of both dinitrogen molecules. Next, coordination of the alkene takes place, which is preferred since activation of hydrogen is also feasible. After alkene coordination, oxidative addition of hydrogen yields a formally 18-electron complex. Insertion of the alkene gave an alkyl complex, which recreated the starting complex via reductive elimination. Notably, the alkene complex also supports an isomerization of the double bond, hence an extension of possible intermediates is conceivable. 

Increasing the number of electrons reduces the activation of N2, because the electrons occupy the orbitals which are bonding with respect to the NN bond, and actually stabilize it. In agreement with this prediction dinitrogen is sufficiently activated to be reduced by protonation by dinuclear complexes of titanium(II), zirco-nium(Il), niobium(III), tantalum(III), molybdenum(IV), and tungsten(IV), whereas it is not reduced by protonation by certain d -d complexes, such as those of molybdenum(O), ruthenium(II), or rhodium(I). Apparently dinuclear complexes M-N=N-M in which M has the d electronic configuration can be intermediates in dinitrogen reduction in protic media, particularly if they represent part of polynuclear complexes (vide infra). 

It should be mentioned that consecutive two-electron reduction of dinitrogen is still usually considered likely in the literature, with diazene, N2H2, as an intermediate but stabilized by complexation with transition metals. At the same time there is no doubt that both 7r-bonds of N2 can be used simultaneously for four d-electrons (entering two degenerate 7ig -orbitals) from one, two, or several metal atoms forming a complex with dinitrogen, and separate cleavage of each 71-bond seems to be unnecessary, more so because it is thermodynamically unfavorable. Therefore, for-... 

---