Ligand–Copper & Iron–O₂ Reactivity: Comprehensive Bullet-Note Summary

Redox sequence after O₂ binding to Cu(I)–L (L = N/O/S donor, bidentate–tetradentate)
- $\text{Cu(I)–L} + \text{O}2 \rightarrow \text{Cu(II)}–\text{O}2^{!*−}$
- End-on superoxo ⇆ side-on superoxo (ligand denticity & pocket size govern geometry).
- Proton/electron delivery → hydroperoxo: $\text{Cu(II)}–\text{O}_2^{!*−}+\text{H}^+ + e^- \rightarrow \text{Cu(II)}–\text{OOH}$
- Second Cu(I) equivalent yields dinuclear complexes:
- End-on µ-η¹:η¹ peroxo with tetradentate ligands.
- Side-on µ-η²:η² peroxo with tridentate ligands.
- Further O–O cleavage ⇒ bis(µ-oxo) $\text{Cu(III)}2(\mu\text{-O})2$ (high-valent “cupryl”).
- Entire manifold is in rapid equilibrium; subtle changes in donor type (aliphatic vs aromatic, N vs O/S) shift speciation.
Reactivity profile
- Superoxo = H-atom abstractor (nucleophilic character toward weak X–H).
- Peroxo/hydroperoxo = electrophilic oxidants (O-atom transfer, C–C/N cleavage).
- Synthetic models reproduce PHM/DBM chemistry: selective H-atom abstraction leading to N-dealkylation, substrate hydroxylation.
Spectroscopic handles
- UV-vis signatures: superoxo (~350–450 nm), peroxo (550–600 nm), bis-µ-oxo (~350 nm shoulder).
- Resonance Raman ( $\nu{\text{O–O}}$ shifts on $^{18}\text{O}2$ ), rapid-freeze/stop-flow capture (sub-second timescale).

Formal homolysis of $\text{Cu(II)}–\text{OOH}$ affords postulated cupryl $\text{Cu(III)}!=!O$ (or $\text{Cu(II)}–O^\bullet$ ) analogous to $\text{Fe(IV)}=O$ in heme enzymes.
Debate: in PHM/DBM active species could be superoxo, hydroperoxo, cupryl, or bis-µ-oxo; lack of crystallographic capture keeps discussion open.

Active site: heme-a₃ (Fe) + Cu_B (3His, Tyr-His cross-link).
Catalytic cycle
1. $\text{Fe}^{2+}$ + O₂ → $\text{Fe}^{3+}–O_2^{!*−}$ (pulled into porphyrin plane).
2. Electron from $\text{Cu}^{+}$ → µ-peroxo $\text{Fe}^{3+}–O_2^{2−}–\text{Cu}^{2+}$ .
3. Proton/electron steps cleave O–O → water, generating transient $\text{Fe}^{4+}=O$ / $\text{Cu}^{2+}–OH$ with Tyr•.
Synthetic side-on/side-on and side-on/end-on Fe–Cu peroxos characterized; protonation triggers O–O scission to $\text{Fe}^{4+}=O$ .

Resting $\text{Fe}^{3+}$ → O₂ adduct → peroxo $\text{Fe}^{3+}–O_2^{2−}$ .
Protonations → hydroperoxo (“Compound 0”) → heterolysis ⇒ Compound I \text{Fe}^{4+}=O\;\text{Porphyrin•^+} (formal $\text{Fe}^{5+}=O$ ).
Compound I abstracts H•, then “rebound” gives $R!−!OH$ (aliphatic) or installs heteroatoms.

Use H₂O₂ as oxidant (“peroxide shunt”), generating Compound I; peroxidases oxidize substrates, catalase disproportionates 2 H₂O₂ → 2 H₂O + O₂.

Fe(II) coordinated by “2-His-1-carboxylate” facial triad.
O₂ activation produces $\text{Fe(IV)}=O$ or di-Fe bis-µ-oxo species depending on system.

Cosubstrate α-KG binds bidentate; O₂ attack at keto carbon forms alkyl-peroxo; decarboxylation → $\text{Fe(IV)}=O$ + succinate.
$\text{Fe(IV)}=O$ abstracts H•; rebound gives hydroxylation (oxygenase) or, if Asp/Glu in triad is replaced by Cl⁻/Br⁻, halogen rebound dominates (halogenase).

Fe(III)–OOH → O–O cleavage (assisted by H₂O or RCO₂H) → $\text{Fe(V)}=O(OH)$ .
Predictable C–H selectivity: 3°>2°>1°; electron-withdrawing substituents or sterics invert/prevent reaction; carboxylate directing overrides innate bias.

Resting Fe(II)₂ → O₂ → µ-η¹:η¹ superoxo $\text{Fe(III)}–O_2^{!−}–\text{Fe(II)}$ (P).
Electron/proton steps → peroxo (P) $\text{Fe(III)}2!(\mu!−!O2^{2−})$ .
O–O cleavage → Q (bis-µ-oxo $\text{Fe(IV)}2(\mu!−!O)2$ , 720 nm band).
Q + CH₄ (k ≈ $10^4\;\text{M}^{−1}\text{s}^{−1}$ ) → CH₃OH + $\text{Fe(III)}2(\mu!−!OH)2$ .
Kinetic isotope effect large for “class I” substrates (C–H activation RDS); for bigger substrates diffusion to active site becomes rate-limiting.

Inorganic core: $\text{Mn}4\text{CaO}5$ (questioned due to X-ray damage).
Kok cycle S₀–S₄: sequential oxidation by Tyr_Z•; S₄ → O–O bond formation, release of O₂, reset to S₀.
Leading model: Mn(V)=O attacked by Ca–OH nucleophile yielding peroxo; alternatives consider µ-oxo coupling.
Experimental probes: EPR multiline (S₂), X-ray emission, X-ray free-electron laser snapshots to minimize radiation reduction.

Superoxo (one-electron reduced O₂) is generally a radical H-atom abstractor.
Peroxo/hydroperoxo (two-electron reduced) are electrophilic; protonation state tunes activity.
High-valent M═O / M–O• (Cu or Fe) perform the scission of strong C–H (up to \sim105\;\text{kcal·mol}^{−1} for CH₄).
Axial ligand charge modulates Fe–O bond: thiolate (P450) pushes e⁻ density, easing O–O cleavage and stabilizing Compound I.
Facial triad swap (Asp/Glu ↔ Cl⁻) diverts α-KG enzymes from hydroxylation to halogenation.
Spectroscopy cheat-sheet:
- $\text{Fe(IV)}=O$ (heme) → near-IR & Mossbauer $\delta\approx0.0$ mm s⁻¹.
- $\text{Fe(IV)}2(\mu!−!O)2$ Q → 720 nm UV-vis.
- $\text{Cu(II)}–O2^{!*−}$ end-on $\nu{\text{O–O}}$ ≈ 1120–1140 cm⁻¹.
Remember key stoichiometries in LaTeX:
- HCO overall: $4\,\text{Cu}^{+}/\text{Fe}^{2+}+\text{O}2+8\,\text{H}^+ \rightarrow 4\,\text{Cu}^{2+}/\text{Fe}^{3+}+2\,\text{H}2\text{O}$ .
- P450 monooxygenation: $\text{RH}+\text{O}2+\text{NADPH}+\text{H}^+ \rightarrow \text{ROH}+\text{H}2\text{O}+\text{NADP}^+$ .
- sMMO: $\text{CH}4+\text{O}2+2\,\text{H}^++2e^- \rightarrow \text{CH}3\text{OH}+\text{H}2\text{O}$ .
- OEC net (per cycle): $2\,\text{H}2\text{O} \xrightarrow{4\,h\nu} \text{O}2+4\,\text{H}^++4e^-$ .

Map each enzyme to: (i) metal site architecture, (ii) sequence of O₂-derived intermediates, (iii) principal oxidative step.
Correlate synthetic models to biological analogues to rationalize reactivity trends.
Practice electron-counting and formal charge assignments; convert $M=O$ ↔ $M^{n-1}–O^•$ representations fluently.
Use provided UV-vis/Resonance Raman bands as fingerprint identifiers in mechanism problems.