Exploring secondary-sphere interactions in Fe-N x H y complexes relevant to N2 fixation

Abstract

Hydrogen bonding and other types of secondary-sphere interactions are ubiquitous in metalloenzyme active sites and are critical to the transformations they mediate. Exploiting secondary sphere interactions in synthetic catalysts to study the role(s) they might play in biological systems, and to develop increasingly efficient catalysts, is an important challenge. Whereas model studies in this broad context are increasingly abundant, as yet there has been relatively little progress in the area of synthetic catalysts for nitrogen fixation that incorporate secondary sphere design elements. Herein we present our first study of Fe-N _x H _y complexes supported by new tris(phosphine)silyl ligands, abbreviated as [SiPNMe3] and [SiPiPr2P^NMe], that incorporate remote tertiary amine hydrogen-bond acceptors within a tertiary phosphine/amine 6-membered ring. These remote amine sites facilitate hydrogen-bonding interactions via a boat conformation of the 6-membered ring when certain nitrogenous substrates (e.g., NH₃ and N₂H₄) are coordinated to the apical site of a trigonal bipyramidal iron complex, and adopt a chair conformation when no H-bonding is possible (e.g., N₂). Countercation binding at the cyclic amine is also observed for anionic {Fe-N₂}^- complexes. Reactivity studies in the presence of proton/electron sources show that the incorporated amine functionality leads to rapid generation of catalytically inactive Fe-H species, thereby substantiating a hydride termination pathway that we have previously proposed deactivates catalysts of the type [EPR3]FeN₂ (E = Si, C).

Fig. 1. Schematic of the nitrogenase FeMo cofactor; His-195 may interact via hydrogen-bonding with the active site.

Fig. 2. Select previously reported systems incorporating hydrogen-bonding or proton-responsive ligands for the binding and/or conversion of nitrogenous substrates. (a) Treatment of a Cr(0)–(N₂)₂ complex within a scaffold bearing tertiary amines with acid produces ammonia and hydrazine. (b) The participation of a proton-responsive ligand is invoked in the disproportionation of hydrazine by Fe(ii). (c) Fe(ii) and Fe(iii) complexes of ammonia show multiple hydrogen-bonding interactions with ligand.

Scheme 1. Synthesis of ligand arm L₀ .

Scheme 2. Synthesis of ligands L₁ and L₂ .

Scheme 3. Synthesis of Fe(ii) and Fe(i) precursor complexes.

Fig. 3. Structures of 1′ (left, two views) and 2′ (right, with space-filling view down N–N–Fe axis). Solvent molecules and hydrogen atoms omitted for clarity. Thermal ellipsoids are shown at 50% probability.

Scheme 4. Synthesis of cationic NH₃ and N₂H₄ adducts of LFe(ii).

Fig. 4. Structure of ammonia, hydrazine, and amide complexes 3, 3′, 4, and 5. BArF4 counteranions, solvent molecules, and carbon-bound hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at 50% probability.

Scheme 5. Synthesis of a parent amide complex (5).

Scheme 6. Synthesis of Fe(0)–N₂ complexes, and their reaction profiles with HBArF4·2Et₂O.

Fig. 5. Structures of complexes 6 and 6′. For 6, the intermolecular interaction (dashed lines) between the sodium cation and the tertiary amine group of a neighboring molecule (N1′) is shown, as well as the interaction with the sodium countercation of another neighbor (Na′). Thermal ellipsoids are shown at 50% probability, and hydrogen atoms and uncoordinated solvent are omitted for clarity. Coordinated THF molecules are truncated to show only the oxygen atom bound to Na.

Fig. 6. Kinetic and thermodynamic protonation sites of {[SiPR3]FeN₂}^– anions.