Chatt Cycle Experimental Investigations

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 N-N and metal-N force constants resulting from normal coordinate analysis of the N2-, NNH-, NNH2-, and NNH3-complexes l ppe, 2 ppeF, 34ppeF, and 4 epeF as well as the nitrido and imido complexes 5, Ns and 6 pe C1 are graphically represented in Fig. 4 (15-17). Upon protonation of l pe to the NNH complex 2 peF, the N-N force constant decreases from 

In the meantime temperature-dependent stopped-flow measurements were conducted on the latter complex in order to determine the activation parameters of the N-N cleavage reaction (24). Plots of the absorption intensity at 418 nm vs. time at T — —35 to +15°C indicate biphasic kinetics with two rate constants 0bs(p and obs(2) in analogy to our measurements of the tungsten complex. This time, however, both rates depended upon the acid concentration. Interestingly much smaller rate constants 0bs(i) and 0bs(2) were found for all acid concentrations than given by Henderson et al. for his (single) rate constant kobs (up to 1 order of magnitude). Furthermore plots of 0bs(i) and kohs(2) vs. the acid concentration showed no saturation behavior but linear dependencies with slopes k and k and intercepts k und k, respectively (s — acid dependent and i — acid independent), Eq. (2)  

Electronic Structure and Reactivity of Molybdenum Nitrido and Imido Complexes 

After cleavage of the N-N bond the parent dinitrogen complex has been converted to a Mo(IV) or W(IV) nitrido or imido complex 

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In the preceding sections, experimental and theoretical studies on the mechanism and various modifications of the Chatt cycle have been presented. Our investigations have started with the detailed spectroscopic and theoretical characterization of the intermediates of the classic Chatt cycle. These studies are not yet... 

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