Triphosphine-borane complexes

Figure 11 Triphosphine-borane complexes featuring M B interaction...

Figure 37 Optimized structures of the cage complexes deriving from the triphosphine-borane 46.

Figure 13 Anionic, neutral and cationic triphosphine-borane Rh complexes...

These reactivities encouraged the investigation of reduction into NH3 and spectacular results were obtained with triphosphine-borane Fe complexes. Various V-bound Fe complexes 65 relevant to the catalytic reduction of into NH were prepared. Most significant are the protonation of the [FeJ-NH complex into [Fe]-NHj and the reductive displacement of NH by N, which are the toal steps proposed to account for the fixation/reduction of at Fe (Scheme 25). This stoichiometric reaction sequence was adapted into a catalytic transformation. Treatment of the anionic complex 65d with excess and KCj at -78°C results in NHj production. Under these conditions, up to 7 NH3 molecules were generated per Fe complex and more than 40% of the furnished protons were delivered to Nj. The borane moiety plays a major role in this process, being capable of accommodating the various [FelN H, )] species involved in the catalytic cycle due to its versatile coordination properties. This is further supported by the inability of the related triphosphine-silyl complex to promote the reduction of Nj under similar conditions. 

Scheme 25 Catalytic reduction of into NH at a triphosphine-borane Fe complex...

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