Enhanced Ammonia Oxidation Catalysis by a Low-Spin Iron Complex Featuring Cis Coordination Sites

Abstract

The goal of using ammonia as a solar fuel motivates the development of selective ammonia oxidation (AO) catalysts for fuel cell applications. Herein, we describe Fe-mediated AO electrocatalysis with [(bpyPy 2 Me)Fe(MeCN) 2 ] 2+ , exhibiting the highest turnover number (TON) reported to date for a molecular system. To improve on our recent report of a related iron AO electrocatalyst, [(TPA)Fe(MeCN) 2 ] 2+ (TON of 16), the present [(bpyPy 2 Me)Fe(MeCN) 2 ] 2+ system (TON of 149) features a stronger-field, more rigid auxiliary ligand that maintains cis-labile sites and a dominant low-spin population at the Fe(II) state. The latter is posited to mitigate demetalation and hence catalyst degradation by the presence of a large excess of ammonia under the catalytic conditions. Additionally, the [(bpyPy 2 Me)Fe(MeCN) 2 ] 2+ system exhibits a substantially faster AO rate (ca. 50×) at significantly lower (∼250 mV) applied bias compared to [(TPA)Fe(MeCN) 2 ] 2+ . Electrochemical data are consistent with an initial E 1 net H-atom abstraction step that furnishes the cis amide/ammine complex [(bpyPy 2 Me)Fe(NH 2 )(NH 3 )] 2+ , followed by the onset of catalysis at E 2 . Theoretical calculations suggest the possibility of N−N bond formation via multiple thermodynamically plausible pathways, including both reductive elimination and ammonia nucleophilic attack. In sum, this study underscores that Fe, an earthabundant metal, is a promising metal for further development in metal-mediated AO catalysis by molecular systems.

Dinitrogen Binding Relevant to FeMoco of Nitrogenase: Clear Visualization of σ‐Donation and π‐Backdonation from Deformation Electron Densities around Carbon/Silicon‐Iron Site

“…The visualization of deformation densities provides a complete understanding of the flow of the electron densities (red to blue; Scheme 3) between the interacting fragments, leading to the formation of the complex in its ground state. The fluctuation of electron densities along E−Fe bond (Scheme 3; E=C, Si) is the focus of our study keeping in mind the efforts of other research works [10–34] relevant to N 2 bonded FeMoco of nitrogenase (Scheme 1; left). The σ‐donation of phosphine ligands and its effect on π‐backdonation from Fe to N 2 can be clearly viewed from the deformation densities (Scheme 3).…”

“…Hence, simplified mono‐nuclear Fe‐complexes are studied here by the EDA‐NOCV method. The research groups of Peters (complex A ) [17–18] and Hollard (complex B ) [7] separately came up with different model complexes (Scheme 1; c–d) to shed light on the structure and electronic properties of such complexes relevant to FeMoco [17–34] . Sulfur and phosphorus‐donor ligands are shown to stabilize Fe‐atoms in lower oxidation states.…”

“…have shown that the efficiency of the N 2 reduction to NH 3 is dependent on the light element (E=B, C, Si) bonded to the Fe‐center, in the order of P 3 B−Fe>P 3 C−Fe>P 3 Si−Fe [17–18] . The electronic influence of the E‐atom on the N 2 binding and its reduction are debated based on experimental data and theoretical calculations [10–23] . The positive isomeric shifts of the Mossbauer spectral data of complex A have followed the reverse trend when compared with the related complexes in the similar oxidation states of Fe‐atom [17] .…”

“…The report has given emphasis to the unusual shortening of Fe−N 2 bond length as a possible reason for the unexpected Mossbauer spectral feature [33] . Till now, decades‐long debate concerning the role of the light element (C) in the N 2 binding has not been demonstrated in an exact fashion [7,10–23,28,34a] . The EDA‐NOCV [35] (energy decomposition analysis‐natural orbital for chemical valence), which is known as the state‐of‐the‐art calculation for its ability to clearly show the flow of the electron densities (deformation densities; Scheme 3) when two or multiple hypothetical fragments are allowed to combine in their suitable excited states to form the compound/complex in its ground state under study.…”

“…The research groups of Peters (complex A) [17][18] and Hollard (complex B) [7] separately came up with different model complexes (Scheme 1; c-d) to shed light on the structure and electronic properties of such complexes relevant to FeMoco. [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] Sulfur and phosphorus-donor ligands are shown to stabilize Fe-atoms in lower oxidation states. Peters et al have shown that the efficiency of the N 2 reduction to NH 3 is dependent on the light element (E=B, C, Si) bonded to the Fe-center, in the order of P 3 BÀ Fe > P 3 CÀ Fe > P 3 SiÀ Fe.…”