In 2003 we reported the first catalytic reduction of dinitrogen with protons and electrons. The process employs a molybdenum complex that contains a hexaisopropylterphenyl-substituted triamidoamine ligand ([HIPTN3N]Mo, Figure 1).Figure 1. The Mo(III) core of [HIPTN3N]Mo complexes.Dinitrogen is bound end-on in the trigonal coordination pocket and is reduced through a stepwise addition of protons ([2,6-lutidinium]BAr'4 (Ar' = 3,5-(CF3)2C6H3)) and electrons (decamethylchromocene). Dinitrogen is reduced in heptane to yield 7-8 equivalents of NH3 with the remaining electrons being used to make dihydrogen. Eight of the proposed intermediates in the Chatt-like reduction sequence have been isolated and characterized crystallographically and extensive calculations of the mechanism of reduction in the [HIPTN3N]Mo system support the proposed mechanism. A second example of the catalytic reduction of dinitrogen employs a Mo(0) complex, [Mo(L)(N2)2]2(�-N2)]2 (where L is a 'PNP pincer' ligand). Protons are added in the form of 2,6-lutidinium triflate, and electrons are added employing cobaltocene. No mechanistic details for this system have been elucidated. Both systems employ Mo catalysts, an acid that is readily reduced, and an organometallic reducing agent. Both are limited by loss of ligand from the metal. Both produce hydrogen as a byproduct (~1 equivalent or more). Recent synthesis of a TREN ligand that could lead to a more robust catalyst and higher turnovers will be described. A third dinitrogen reduction has been published recently that involves an iron-containing catalyst, H(ether)2[BArF] as the acid, and potassium graphite as the reducing agent. Approximately eight equivalents of ammonia are formed.

Reduction of dinitrogen catalytically to ammonia with protons and electrons

248th National Meeting (2014)

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