Evaluating molecular cobalt complexes for the conversion of N2 to NH3

Abstract

Well-defined molecular catalysts for the reduction of N2 to NH3 with protons and electrons remain very rare despite decades of interest and are currently limited to systems featuring molybdenum or iron. This report details the synthesis of a molecular cobalt complex that generates superstoichiometric yields of NH3 (>200% NH3 per Co-N2 precursor) via the direct reduction of N2 with protons and electrons. While the NH3 yields reported herein are modest by comparison to those of previously described iron and molybdenum systems, they intimate that other metals are likely to be viable as molecular N2 reduction catalysts. Additionally, a comparison of the featured tris(phosphine)borane Co-N2 complex with structurally related Co-N2 and Fe-N2 species shows how remarkably sensitive the N2 reduction performance of potential precatalysts is. These studies enable consideration of the structural and electronic effects that are likely relevant to N2 conversion activity, including the π basicity, charge state, and geometric flexibility.

Figure 1

Cyclic voltammagram of (TPB)Co(N₂) (1) scanning oxidatively (left) and reductively (right) at 100 mV/sec in THF with 0.1 M TBAPF₆ electrolyte.

Figure 2

Solid-state crystal structures of 2 (left) and 3 (right; also see SI). Thermal ellipsoids shown at 50% probability. Countertions, solvent molecules, and H atoms omitted for clarity.

Figure 3

Temperature dependence of the magnetic susceptibility of [(TPB)Co][BAr^F₄] (3) as measured by SQUID magnetometry.

Figure 4

(left) Cyclic voltammagram of (CP₃)Co(N₂) (5) scanning oxidatively at 100 mV/sec in THF with 0.1 M TBAPF₆ electrolyte. (middle) UV-vis spectra of 6 under 1 atm N₂ (solid line) and under static vacuum (dotted line: after three freeze-pump-thaw cycles). Spectra were collected on a 1 mM solution of 6 in THF at 298 K. (right) X-band EPR spectrum of 6 collected under 1 atm N₂ in 2-Me-THF at 80 K. No low-field features were detected.

Figure 5

Vibrational spectroscopy, electrochemistry, and catalytic competence data for select [(P₃E)M(N₂)]⁻ complexes. Data for M = Fe, E = B is from refs. and ; data for M = Fe, E = C is from ref. ; data for M = Fe, E = Si is taken refs. and ; and data for M = Co, E = B is from this work. ^aIR from solid state samples ^bOxidation potentials determined by cyclic voltammetry in THF. Note: NH₃ yields based on the addition of ~ 50 equiv [H•(OEt₂)₂][BAr^F₄] and ~ 60 equiv KC₈ in Et₂O (see refs. provided for specific details).

Figure 6

Electrostatic potential maps of anionic 2 and neutral 5 (isovalue = 0.015, color map in Hartrees), and atomic charges for N_β.

Scheme 1

Chemical oxidation and reduction of (TPB)Co(N₂).

Scheme 2

Synthesis and oxidation of (CP₃)Co(N₂)