Tripodal PFe-N Complexes (X = B, Al, Ga): Effect of the Apical Atom on Bonding, Electronic Structure, and Catalytic N-to-NH Conversion.

Terminal dinitrogen complexes of iron ligated by tripodal, tetradentate P ligands (X = B, C, Si) have previously been shown to mediate catalytic N-to-NH conversion (NRR) with external proton and electron sources. From this set of compounds, the tris(phosphino)borane (P) system is most active under all conditions canvassed thus far. To further probe the effects of the apical Lewis acidic atom on structure, bonding, and NRR activity, Fe-N complexes supported by analogous group 13 tris(phosphino)alane (P) and tris(phosphino)gallane (P) ligands are synthesized. The series of PFe-N compounds (X = B, Al, Ga) possess similar electronic structures, degrees of N activation, and geometric flexibility as determined from spectroscopic, structural, electrochemical, and computational (DFT) studies. However, treatment of [Na(12-crown-4)][PFe-N] (X = Al, Ga) with excess acid/reductant in the form of HBAr/KC generates only 2.5 ± 0.1 and 2.7 ± 0.2 equiv of NH per Fe, respectively. Similarly, the use of [HNPh][OTf]/Cp*Co leads to the production of 4.1 ± 0.9 (X = Al) and 3.6 ± 0.3 (X = Ga) equiv of NH. Preliminary reactivity studies confirming PFe framework stability under pseudocatalytic conditions suggest that a greater selectivity for hydrogen evolution versus NRR may be responsible for the attenuated yields of NH observed for PFe and PFe relative to PFe.