Dinitrogen complexes electron transfer

The nitration reagents (NO2 Y) for electrophilic aromatic nitration span a wide range and contain anions Y such as nitric acid (Y = OH-), acetyl nitrate (Y = OAc-), dinitrogen pentoxide (Y = NO3-), nitryl chloride (Y = Cl-), TV-nitropyridinium (Y = pyridine) and tetranitromethane [Y = C(N02)3-]. All reagents contain electron-deficient species which can serve as effective electron acceptors and form electron donor-acceptor (EDA) complexes with electron-rich donors including aromatic hydrocarbons107 (ArH, equation 86). Excitation of the EDA complexes by irradiation of the charge-transfer (CT) absorption band results in full electron transfer (equation 87) to form radical ion... 

Figure 1. The Nitrogenase Reaction. The electron transfer proteins ferredoxin (Fd) and flavodoxin (Fid) serve to couple the nitrogenase reaction to metabolically generated reducing equivalents. Ammonia synthesis requires 8 electrons 6 for the reduction of dinitrogen and 2 for the coupled, obligatory synthesis of H2. These reactions are catalyzed by the terminal component in the complex, the MoFe-protein. The electrons are transferred to the MoFe-protein from the Fe-protein in a process coupled to the hydrolysis of 2ATP/electron (Howard and Rees, 1994,1996).

In contrast, the kinetics of the alkylation of trans-[Mo(N2)(L)(dppe)2r (L = CN or SCN) with nBuI show a first-order dependence upon both the complex and the alkyl halide (88). These kinetics and the retention of the ligand, L, in the diazenido product are consistent with the mechanism shown in Scheme 4. The electron-rich complexes (A) undergo rate-limiting, outer-sphere electron transfer to yield the alkyl radical and fra/is-[M(N2)L(dppe)2] (B). Subsequent, rapid attack of the radical on the coordinated dinitrogen of (B) yields the diazenido product (C). 

On the basis of an EPR analysis, the reaction of M(acac) with NO2 (at room temperature it is present as N2O4) has been proposed to occur via a step-by-step oxidation of the metal complex with final formation of metal nitrates and the iminoxy free radical 111 (equation 79). The presence of a donor-acceptor complex between M(acac) and N2O4 has been postulated for the electron transfer to dinitrogen tetroxide. 

Peculiarities of the N2 molecule make it necessary to use special means of electron transfer to and inside the active center containing the substrate. The mechanism of the catalysis in protic surroundings, at least for dinitrogen reduction, presumably necessarily includes coupled one-electron transfer from an external electron donor and multi-electron transfer to the substrate coordinated in the polynuclear complex. The coupled electron transfer helps to activate and reduce the difficult substrate dinitrogen at ambient temperatures. 

Pre-steady-state stopped-flow and rapid quench techniques applied to Mo nitrogenase have provided powerful approaches to the study of this complex enzyme. These studies of Klebsiella pneumoniae Mo nitrogenase showed that a pre-steady-state burst in ATP hydrolysis accompanied electron transfer from the Fe protein to the MoFe protein, and that during the reduction of N2 an enzyme-bound dinitrogen hydride was formed, which under denaturing conditions could be trapped as hydrazine. A comprehensive model developed from a computer simulation of the kinetics of these reactions and the kinetics of the pre-steady-state rates of product formation (H2, NH3) led to the formulation of Scheme 1, the Thorneley and Lowe scheme (50) for nitrogenase function. 

Although dinitrogen complexes such as [M(N2)2(dppe)2] are not strictly organometallic, they closely resemble the carbonyls described in Section II,C,6. This analogy as well as the discovery of the syntheses of organo-nitrogen compounds via electron-transfer reactions have led us to include a brief description of the most recent developments. 

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