Tripodal P₃^XFe–N₂ Complexes (X = B, Al, Ga): Effect of the Apical Atom on Bonding, Electronic Structure, and Catalytic N₂-to-NH₃ Conversion

Terminal dinitrogen complexes of iron ligated by tripodal, tetradentate P₃^X ligands (X = B, C, Si) have previously been shown to mediate catalytic N₂-to-NH₃ conversion (N₂RR) with external proton and electron sources. From this set of compounds, the tris(phosphino)borane (P₃^B) 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 N₂RR activity, Fe–N₂ complexes supported by analogous group 13 tris(phosphino)alane (P₃^(Al)) and tris(phosphino)gallane (P₃^(Ga)) ligands are synthesized. The series of P₃^XFe–N₂^([0/1−]) compounds (X = B, Al, Ga) possess similar electronic structures, degrees of N₂ activation, and geometric flexibility as determined from spectroscopic, structural, electrochemical, and computational (DFT) studies. However, treatment of [Na(12-crown-4)₂][P₃^XFe–N₂] (X = Al, Ga) with excess acid/reductant in the form of HBAr^F₄/KC₈ generates only 2.5 ± 0.1 and 2.7 ± 0.2 equiv of NH₃ per Fe, respectively. Similarly, the use of [H₂NPh₂][OTf]/Cp^*₂Co leads to the production of 4.1 ± 0.9 (X = Al) and 3.6 ± 0.3 (X = Ga) equiv of NH₃. Preliminary reactivity studies confirming P₃^XFe framework stability under pseudocatalytic conditions suggest that a greater selectivity for hydrogen evolution versus N₂RR may be responsible for the attenuated yields of NH₃ observed for P₃^(Al)Fe and P₃^(Ga)Fe relative to P₃^BFe