Binding and activation of N2O at transition-metal centers: recent mechanistic insights

Abstract

No laughing matter, nitrous oxide's role in stratospheric ozone depletion and as a greenhouse gas has stimulated great interest in developing and understanding its decomposition, particularly through the use of transition-metal promoters. Recent advances in our understanding of the reaction pathways for N(2)O reduction by metal ions in the gas phase and in heterogeneous, homogeneous, and biological catalytic systems have provided provocative ideas about the structure and properties of metal N(2)O adducts and derived intermediates. These ideas are likely to inform efforts to design more effective catalysts for N(2)O remediation.

Figure 1

Bonding descriptions for N₂O. Top: The major contributing resonance structures and interatomic distances. Bottom: The degenerate pairs of N₂O frontier orbitals computed at the mPW1PW level of theory with the MIDI! basis set, visualized at a contour level of 0.02 a.u. (Computations performed and orbitals drawn by C. J. Cramer).

Figure 2

Possible modes of bonding of N₂O to a transition metal (M).

Figure 3

Calculated energy profile for N₂O reduction mediated by Fe⁺, with the numbers being relative stabilities (in kcal/mol) with respect to the separated reactants N₂O + Fe⁺(⁶D). Reproduced from ref. [18] with permission from the American Chemical Society.

Figure 4

Bonding interactions for an N-coordination of N₂O to a metal ion (M) showing synergistic σ-donation to M from the N₂O 7σ orbital (only lobe on terminal N shown) and π-backbonding from M into the 3π* of N₂O.

Figure 5

Calculated reaction paths for N₂O activation and decomposition on Rh(111) into N₂ and absorbed O (left) vs. NO and adsorbed N (right), where the zero energy level corresponds to the non-interacting N₂O molecule in the gas phase. Reproduced from ref. [41] with permission from Elsevier, Inc.

Figure 6

Calculated transition state structure for N₂O decomposition at a [Fe^II₂(OH)(O)]⁺ site in ZSM-5. Small gray balls are H, small blue balls are Si, green ball is Al, red balls are O, and large gray balls are N; bond lengths are in angstroms. Figure reproduced from ref. [52] with permission from the American Chemical Society.

Figure 7

Proposed mechanism for the reduction of N₂O by N₂OR,[–72] with the structure of the active site derived from X-ray crystallography shown in the box.

Figure 8

Core structural motifs identified by X-ray crystallography for copper-sulfur complexes supported by N-donor ligands (not shown).

Figure 9

Generalized summary of reactions of transition metal complexes in solution with N₂O.

Figure 10

General mechanism proposed[89] for reactions of transition metal complexes ([M]) with N₂O proceeding according to path A (Figure 9), including a previously suggested[90] N-nitrosoimide bonding alternative.

Figure 11

Reaction coordinate for N–O bond cleavage computed at the M06L DFT level for the dicopper species derived from [(Me₃tacn)₃Cu₃S₂]²⁺. Free energies are in kcal/mol and selected bond distances are in angstroms. Carbon and hydrogen atoms of Me₃tacn ligands are not shown for clarity. Free energies correspond to values calculated after correcting for solvation. Color key for atoms: green = Cu, blue = N, yellow = S, and red = O. Figure reproduced from ref. [75] with permission from the American Chemical Society.