Studies Relevant to Catalytic Reduction of Dinitrogen to Ammonia by Molybdenum Triamidoamine Complexes

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Abstract
In this paper we explore several issues surrounding the catalytic reduction of dinitrogen by molybdenum compounds that contain the [(HIPTNCH2CH2)3N]3- ligand (where HIPT = 3,5-(2,4,6-i-Pr3C6H2)2C6H3). Four additional plausible intermediates in the catalytic dinitrogen reduction have now been crystallographically characterized; they are MoNNH (Mo = [(HIPTNCH2CH2)3N]Mo), [Mo NNH2][BAr‘4] (Ar‘ = 3,5-(CF3)2C6H3), [Mo NH][BAr‘4], and Mo(NH3). We also have crystallographically characterized a 2,6-lutidine complex, Mo(2,6-Lut)+, which is formed upon treatment of MoH with [2,6-LutH][B(C6F5)4]. We focus on the synthesis of compounds that have not yet been isolated, which include Mo NNH2, Mo NH, and Mo(NH2). Mo NNH2, formed by reduction of [Mo NNH2]+, has not been observed. It decomposes to give mixtures that contain two or more of the following: MoNNH, Mo⋮N, Mo(NH3)+, Mo(NH3), and ammonia. Mo NH, which can be prepared by reduction of [Mo NH]+, is stable for long periods in the presence of a small amount of CrCp*2, but in the absence of CrCp*2, and in the presence of Mo NH+ as a catalyst, Mo NH is slowly converted into a mixture of Mo⋮N and Mo(NH2). Mo(NH2) can be produced independently by deprotonation of Mo(NH3)+ with LiN(SiMe3)2 in THF, but it decomposes to Mo⋮N upon attempted isolation. Although catalytic reduction of dinitrogen could involve up to 14 intermediates in a “linear” sequence that involves addition of “external” protons and/or electrons, it seems likely now that several of these intermediates, along with ammonia and/or dihydrogen, can be produced in several reactions between intermediates that themselves behave as proton and/or electron sources.

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