O-O Bond Formation and Liberation of Dioxygen Mediated by N5 -Coordinate Non-Heme Iron(IV) Complexes

Abstract

Formation of the O-O bond is considered the critical step in oxidative water cleavage to produce dioxygen. High-valent metal complexes with terminal oxo (oxido) ligands are commonly regarded as instrumental for oxygen evolution, but direct experimental evidence is lacking. Herein, we describe the formation of the O-O bond in solution, from non-heme, N₅ -coordinate oxoiron(IV) species. Oxygen evolution from oxoiron(IV) is instantaneous once meta-chloroperbenzoic acid is administered in excess. Oxygen-isotope labeling reveals two sources of dioxygen, pointing to mechanistic branching between HAT (hydrogen atom transfer)-initiated free-radical pathways of the peroxides, which are typical of catalase-like reactivity, and iron-borne O-O coupling, which is unprecedented for non-heme/peroxide systems. Interpretation in terms of [Fe^IV (O)] and [Fe^V (O)] being the resting and active principles of the O-O coupling, respectively, concurs with fundamental mechanistic ideas of (electro-) chemical O-O coupling in water oxidation catalysis (WOC), indicating that central mechanistic motifs of WOC can be mimicked in a catalase/peroxidase setting.

Keywords: O−O activation; bioinorganic chemistry; iron; nitrogen ligands; oxo ligands.

Scheme 1

Top: Structures of the pentadentate N₅ podands Bn‐TPEN and L and the iron(II) complex, which has dissociable MeCN at the sixth coordination site (X). Bottom: Phenomenology of oxoiron(IV) formation and decay as described here.

Figure 1

Oxygen evolution (Clark‐electrode system) from the oxoiron(IV) species [Fe^IV(L)(O)]²⁺, as synthesized in MeCN/water (1:4) from the reaction of [Fe^II(L)(MeCN)]²⁺ with a) 10 equiv mCPBA, b) 2 equiv PhIO, and c) 2 equiv PhIO followed by 10 equiv mCPBA; asterisks denote the addition of PhIO; arrows denote the addition of mCPBA.

Scheme 2

Gated formation of the O−O bond from the reaction of oxoiron(IV) with mCPBA; oxygen atoms susceptible to isotope labeling are highlighted in red.

Figure 2

Maximum amplitudes of dioxygen MS ion currents over “dry” MeCN solutions of [Fe^IV(L)(O)]²⁺ (presynthesized via 10 mm [Fe^II(L)(MeCN)]²⁺ + 2 equiv PhIO); left: [Fe^IV(L)(O)]²⁺ in native “dry” MeCN after addition of 10 equiv mCPBA, middle: [Fe^IV(L)(O)]²⁺ after labeling with 100 μL ¹⁸OH₂ for 30 min and addition of 10 equiv mCPBA; right: [Fe(Bn‐TPEN)(O)]²⁺ after labeling with 100 μL ¹⁸OH₂ for 30 min and addition of 10 equiv mCPBA (for ion current vs. time plots, see Figures S11–S13).

Figure 3

a) UV/Vis/NIR spectral dynamics of [Fe^II(L)(MeCN)]²⁺ (0.14 mm, MeCN, t=0; black curve) after addition of 20 equiv mCPBA (blue curve: t=10 min; gray curve t=140 h). b) ¹H NMR spectroscopic dynamics of [Fe^II(L)(MeCN)]²⁺ (10 mm; d₃‐MeCN; bottom) directly after addition of 10 equiv mCPBA (middle) and after 12 h (top); dashed lines are given to guide the eye.