Tripodal P3XFe-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 P-3(X) ligands (X = B, C, Si) have previously been shown to mediate catalytic N-2-to-NH3 conversion (N2RR) with external proton and electron sources. From this set of compounds, the tris(phosphino)borane (P-3(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-N-2 complexes supported by analogous group 13 tris(phosphino)alane (P-3(Al)) and tris(phosphino)gallane (P-3(Ga)) ligands are synthesized. The series of (P3Fe)-Fe-X-N-2([0/1-]) compounds (X = B, Al, Ga) possess similar electronic structures, degrees of N-2 activation, and geometric flexibility as determined from spectroscopic, structural, electrochemical, and computational (DFT) studies. However, treatment of [Na(12-crown-4)(2)][(P3Fe)-Fe-X-N-2] (X = Al, Ga) with excess acid/reductant in the form of HBAr4F/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*Co-2 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 (P3Fe)-Fe-X 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 (P3Fe)-Fe-Al and (P3Fe)-Fe-Ga relative to (P3Fe)-Fe-B.

Automated summary

Iron terminal dinitrogen complexes ligated by different tripodal phosphine ligands show similar electronic structures and N-2 activation, but the yield of ammonia production is affected when subjected to excess acid or reductant.