Reduction pathway of end-on terminally coordinated dinitrogen. V. N-N bond cleavage in Mo/W hydrazidium complexes with diphosphine coligands. Comparison with triamidoamine systems

N-N cleavage of the dialkylhydrazido complex [W(dppe)(2)(NNC5H10)] (B-W) upon treatment with acid, leading to the nitrido/imido complex and piperidine, is investigated experimentally and theoretically. In acetonitrile and at room temperature, B-W reacts orders of magnitude more rapidly with HNEt3BPh4 than its Mo analogue, [Mo(dppe)(2)(NNC5H10)] (B-Mo). A stopped-flow experiment performed for the reaction of B-W with HNEt3BPh4 in propionitrile at -70 degrees C indicates that protonation of B-W is completed within the dead time of the stopped-flow apparatus, leading to the primary prolonated intermediate (BH+)-H-W. Propionitrile coordination to this species proceeds with a rate constant k(obs)(1) of 1.5 +/- 0.4 s(-1), generating intermediate RCN-(BH+)-H-W (R = Et) that rapidly adds a further proton at N-beta and then mediates N-N bond splitting in a slower reaction (k(obs(2)) = 0.35 +/- 0.08 s(-1), 6 equiv of acid). k(obs(1)) and k(obs(2)) are found to be independent of the acid concentration. The experimentally observed reactivities of B-Mo or B-W with acids in nitrile solvents are reproduced by DFT calculations. In particular, geometry optimization of models of solvent-coordinated, N-beta-protonated intermediates is found to lead spontaneously to separation into the nitrido/imido complexes and piperidine/piperidinium, corresponding to activationless heterolytic N-N bond cleavage processes. Moreover, DFT indicates a spontaneous cleavage of nonsolvated B-W protonated at N-beta. In the second part of this article, a theoretical analysis of the N-N cleavage reaction in the Mo(III) triamidoamine complex [HIPTN3N]Mo(N-2) is presented (HIPTN3N = hexaisopropylterphenyltriamidoamine). To this end, DFT calculations of the (MoN2)-N-III triamidoamine complex and its protonated and reduced derivatives are performed. Calculated structural and spectroscopic parameters are compared to available experimental data. N-N cleavage most likely proceeds by one-electron reduction of the Mo(V) hydrazidium intermediate [HIPTN3N]Mo(NNH3)(+), which is predicted to have an extremely elongated N-N bond. From an electronic-structure point of view, this reaction is analogous to that of Mo/W hydrazidium complexes with diphos coligands. The general implications of these results with respect to synthetic N-2 fixation are discussed.