Studies Relevant to Catalytic Reduction of Dinitrogen to Ammonia by Molybdenum Triamidoamine Complexes (2005)

In this paper we explore several issues surrounding the catalytic reduction of dinitrogen by molybdenum compounds\nthat contain the [(HIPTNCH&lt;sub&gt;2&lt;/sub&gt;CH&lt;sub&gt;2&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;N]&lt;sup&gt;3-&lt;/sup&gt; ligand (where HIPT = 3,5-(2,4,6-i-Pr&lt;sub&gt;3&lt;/sub&gt;C&lt;sub&gt;6&lt;/sub&gt;H&lt;sub&gt;2&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;C&lt;sub&gt;6&lt;/sub&gt;H&lt;sub&gt;3&lt;/sub&gt;). Four additional plausible\nintermediates in the catalytic dinitrogen reduction have now been crystallographically characterized; they are &lt;b&gt;Mo&lt;/b&gt;NNH (&lt;b&gt;Mo&lt;/b&gt; = [(HIPTNCH&lt;sub&gt;2&lt;/sub&gt;CH&lt;sub&gt;2&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;N]Mo), [&lt;b&gt;Mo&lt;/b&gt;NNH&lt;sub&gt;2&lt;/sub&gt;][BAr‘&lt;sub&gt;4&lt;/sub&gt;] (Ar‘ = 3,5-(CF&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;C&lt;sub&gt;6&lt;/sub&gt;H&lt;sub&gt;3&lt;/sub&gt;), [&lt;b&gt;Mo&lt;/b&gt;NH][BAr‘&lt;sub&gt;4&lt;/sub&gt;], and &lt;b&gt;Mo&lt;/b&gt;(NH&lt;sub&gt;3&lt;/sub&gt;).\nWe also have crystallographically characterized a 2,6-lutidine complex, &lt;b&gt;Mo&lt;/b&gt;(2,6-Lut)&lt;sup&gt;+&lt;/sup&gt;, which is formed upon treatment\nof &lt;b&gt;Mo&lt;/b&gt;H with [2,6-LutH][B(C&lt;sub&gt;6&lt;/sub&gt;F&lt;sub&gt;5&lt;/sub&gt;)&lt;sub&gt;4&lt;/sub&gt;]. We focus on the synthesis of compounds that have not yet been isolated, which\ninclude &lt;b&gt;Mo&lt;/b&gt;NNH&lt;sub&gt;2&lt;/sub&gt;, &lt;b&gt;Mo&lt;/b&gt;NH, and &lt;b&gt;Mo&lt;/b&gt;(NH&lt;sub&gt;2&lt;/sub&gt;). &lt;b&gt;Mo&lt;/b&gt;NNH&lt;sub&gt;2&lt;/sub&gt;, formed by reduction of [&lt;b&gt;Mo&lt;/b&gt;NNH&lt;sub&gt;2&lt;/sub&gt;]&lt;sup&gt;+&lt;/sup&gt;, has not been observed.\nIt decomposes to give mixtures that contain two or more of the following: &lt;b&gt;Mo&lt;/b&gt;NNH, &lt;b&gt;Mo&lt;/b&gt;⋮N, &lt;b&gt;Mo&lt;/b&gt;(NH&lt;sub&gt;3&lt;/sub&gt;)&lt;sup&gt;+&lt;/sup&gt;, &lt;b&gt;Mo&lt;/b&gt;(NH&lt;sub&gt;3&lt;/sub&gt;),\nand ammonia. &lt;b&gt;Mo&lt;/b&gt;NH, which can be prepared by reduction of [&lt;b&gt;Mo&lt;/b&gt;NH]&lt;sup&gt;+&lt;/sup&gt;, is stable for long periods in the presence\nof a small amount of CrCp*&lt;sub&gt;2&lt;/sub&gt;, but in the absence of CrCp*&lt;sub&gt;2&lt;/sub&gt;, and in the presence of &lt;b&gt;Mo&lt;/b&gt;NH&lt;sup&gt;+&lt;/sup&gt; as a catalyst,\n&lt;b&gt;Mo&lt;/b&gt;NH is slowly converted into a mixture of &lt;b&gt;Mo&lt;/b&gt;⋮N and &lt;b&gt;Mo&lt;/b&gt;(NH&lt;sub&gt;2&lt;/sub&gt;). &lt;b&gt;Mo&lt;/b&gt;(NH&lt;sub&gt;2&lt;/sub&gt;) can be produced independently by\ndeprotonation of &lt;b&gt;Mo&lt;/b&gt;(NH&lt;sub&gt;3&lt;/sub&gt;)&lt;sup&gt;+&lt;/sup&gt; with LiN(SiMe&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt; in THF, but it decomposes to &lt;b&gt;Mo&lt;/b&gt;⋮N upon attempted isolation. Although\ncatalytic reduction of dinitrogen could involve up to 14 intermediates in a “linear” sequence that involves addition\nof “external” protons and/or electrons, it seems likely now that several of these intermediates, along with ammonia\nand/or dihydrogen, can be produced in several reactions between intermediates that themselves behave as proton\nand/or electron sources.