The design of earth abundant metal catalysts for nitrous oxide-based oxidations: Part I. N2O coordination and oxygen-transfer to metal

Abstract

Nitrous oxide (N₂O) is thermodynamically unstable, but kinetically unreactive, and thus presents a fundamental challenge for catalysis. It is also a potent greenhouse gas, yet it is underutilized as a chemical feedstock and thus provides a practical opportunity for catalysis as well. A computational design approach is employed in this first part of a two-part study to evaluate the potential of selected first row transition metal complexes to activate N₂O as an oxidant for various substrates. Density Functional Theory (DFT) calculations are employed on several ligated transition metal fragments LM, M = V(III), Fe(II), Mn(II), Cr(II), Cu(I) and second row Ru(II) with 4/5-coordinate geometries to assess their N₂O binding affinity and energetics for N–O cleavage to LMO. The effects of the ligand, coordination number, the metal, its spin state, charge, etc. are addressed. The key points from the DFT analysis and energy profiles are: (1) favorable N₂O binding (ΔG_a < 0) is correctly predicted for known N₂O complexants: (N-3Pyr)V, [Al(OR_F)₄]Cu and (NH₃)₅Ru²⁺; (2) for a range of unproven complexants, the N₂O-affinity ranges from moderate (ΔG_a −5 to +2 kcal mol⁻¹) to low (ΔG_a > +5 kcal) in the order Cr(II) > Ru(II) ≥ Fe(II) ≅ Mn(II) ≅ Cu(I); (3) M–N binding to N₂O is generally more stable than M–O binding, but is nearly isoenergetic for many high spin metal fragments; (4) N–O cleavage via monometallic complexes, LM–ON₂ (M = Fe(II), Mn(II), Cu(I)) requires moderate-to-high activation energies, (23–40 kcal mol⁻¹), but the barriers are much lower for LCr(II) and LRu(II) species (3–17 kcal mol⁻¹); (5) N–O cleavage is facilitated by pyramidal and penta-coordinating ligands; (6) from bent N₂O-bimetallics is very low: 1–5 kcal mol⁻¹; and (7) the total activation energy barriers for bimetallic-N₂O complexes, ca. 20–30 kcal mol⁻¹, are 5–8 kcal mol⁻¹ lower than for mono-metallics, providing a bimetallic advantage for N₂O scission to oxido-metals, LMO.