How Metal Ion Lewis Acidity and Steric Properties Influence the Barrier to Dioxygen Binding, Peroxo O-O Bond Cleavage, and Reactivity

Abstract

Herein we quantitatively investigate how metal ion Lewis acidity and steric properties influence the kinetics and thermodynamics of dioxygen binding versus release from structurally analogous Mn-O₂ complexes, as well as the barrier to Mn peroxo O-O bond cleavage, and the reactivity of Mn oxo intermediates. Previously we demonstrated that the steric and electronic properties of Mn^III-OOR complexes containing N-heterocyclic (N^Ar) ligand scaffolds can have a dramatic influence on alkylperoxo O-O bond lengths and the barrier to alkylperoxo O-O bond cleavage. Herein, we examine the dioxygen reactivity of a new Mn^II complex containing a more electron-rich, less sterically demanding N^Ar ligand scaffold, and compare it with previously reported Mn^II complexes. Dioxygen binding is shown to be reversible with complexes containing the more electron-rich metal ions. The kinetic barrier to O₂ binding and peroxo O-O bond cleavage is shown to correlate with redox potentials, as well as the steric properties of the supporting N^Ar ligands. The reaction landscape for the dioxygen chemistry of the more electron-rich complexes is shown to be relatively flat. A total of four intermediates, including a superoxo and peroxo species, are observed with the most electron-rich complex. Two new intermediates are shown to form following the peroxo, which are capable of cleaving strong X-H bonds. In the absence of a sacrificial H atom donor, solvent, or ligand, serves as a source of H atoms. With TEMPOH as sacrificial H atom donor, a deuterium isotope effect is observed (k_H/k_D = 3.5), implicating a hydrogen atom transfer (HAT) mechanism. With 1,4-cyclohexadiene, 0.5 equiv of benzene is produced prior to the formation of an EPR detected Mn^IIIMn^IV bimetallic species, and 0.5 equiv after its formation.

Figure 1.

Low temperature reaction between ${\mathbf{1}}^{\mathbf{\text{Mepy}}}$ and ${\text{O}}_2$ affords ${\mathbf{6}}^{\mathbf{\text{Mepy}}}$ via observable superoxo ${\mathbf{2}}^{\mathbf{\text{Mepy}}}$ and peroxo ${\mathbf{3}}^{\mathbf{\text{Mepy}}}$ intermediates.

Figure 2.

Electronic absorption spectrum of putative peroxo ${\mathbf{3}}^{\mathbf{\text{Quino}}}$ (green) generated via the addition of ${\mathrm{O}}_2$ to 1.0 mM ${\mathbf{1}}^{\mathbf{\text{Quino}}}$ in ${\text{CH}}_2{\text{Cl}}_2$ at −73 °C, and ${\mathbf{3}}^{\mathbf{\text{Mepy}}}$ (black) generated via the addition of ${\mathrm{O}}_2$ to 0.9 mM ${\mathbf{1}}^{\mathbf{\text{Mepy}}}$ and ${\mathrm{O}}_2$ in ${\text{CH}}_3\text{CN}$ at −40 °C.

Figure 3.

Time-dependent density functional theory (TD DFT) calculated electronic absorption spectrum of peroxo-bridged ${\mathbf{3}}^{\mathbf{\text{Quino}}}$ and its DFT optimized structure (using a B3LYP spin-unrestricted hybrid functional, and 6-311G basis set).

Figure 4.

Dioxygen reactivity of ${\text{Mn}}^{\text{II}}{\mathbf{1}}^{\mathbf{\text{Quino}}}$, showing rate constant labeling scheme, as well as observed versus unobserved intermediates.

Figure 5.

Eyring plot for cleavage of the peroxo O–O bond of ${\mathbf{3}}^{\mathbf{\text{Quino}}}$ and its conversion to mono oxo-bridged ${\mathbf{6}}^{\mathbf{\text{Quino}}}$ in ${\text{CH}}_3\text{CN}$, from which activation parameters $\Delta {H_3}^{‡\text{Quino}}=13\left(1\right)\text{kJ}{\text{mol}}^{-1}$ and $\Delta {S_3}^{‡\text{Quino}}=-220\left(4\right)\mathrm{J}{\text{mol}}^{-1}{\mathrm{K}}^{-1}$, were obtained. First-order rate constants ${k_3}^{\text{Quino}}$ were obtained from stopped-flow experiments.

Figure 6.

ORTEP diagram of mono oxo bridged $6^{\text{MeOpy}}$ formed in the reaction between $1^{\text{MeOpy}}$ and ${\mathrm{O}}_2$. Hydrogens have been omitted for clarity.

Figure 7.

Electronic absorption spectrum of peroxo intermediate $3^{\mathbf{\text{MeOpy}}}$ formed in the reaction between $1^{\mathbf{\text{MeOpy}}}$ and ${\mathrm{O}}_2$ (5 mL dry ${\mathrm{O}}_2$ gas) in ${\text{CH}}_3{\text{CH}}_2\text{CN}$ at −73 °C.

Figure 8.

DFT optimized structure of peroxo-bridged $3^{\mathbf{\text{MeOpy}}}.\mathrm{O}–\mathrm{O}=1.518Å,\text{Mn}-{\mathrm{O}}_{\text{avg}}=1.86Å,\text{Mn}-{\mathrm{S}}_{\text{avg}}=2.33;\text{Mn}\left(1\right)\cdots \text{Mn}\left(2\right)=4.361Å,\text{Mn}-\mathrm{O}-{\mathrm{O}}_{\text{avg}}={106}^{\circ }$.

Figure 9.

Time-resolved electronic absorption spectra, for dioxygen binding to $1^{\mathbf{\text{MeOpy}}}$ in ${\text{CH}}_3{\text{CH}}_2\text{CN}$ at −80 °C, obtained using a stoppedflow instrument, under pseudo-first-order conditions with excess ${\mathrm{O}}_2(\left[1^{\mathbf{\text{MeOpy}}}\right]=0.375\text{mM},\left[{\mathrm{O}}_2\right]=8\text{mM}$ after mixing). Inset: kinetic trace obtained at ${\lambda }_{\text{max}}=504$ and 612 nm.

Figure 10.

Dioxygen reactivity of 6-MeO-pyridine $1^{\mathbf{\text{MeOpy}}}$, showing rate constant labeling scheme, and observed intermediates.

Figure 11.

Correlation between the activation barrier $\left(E_{\mathrm{a}}\right)$ to ${\mathrm{O}}_2$ binding and the cathodic peak potential, $E_{\mathrm{p},\mathrm{c}}$.

Figure 12.

Steric properties of the ligand influence metal ion (shown in purple) accessibility, as shown by space filling diagrams of ${\mathbf{1}}^{\mathbf{\text{Mepy}}}$ (left), ${\mathbf{1}}^{\mathbf{\text{Quino}}}$ (center), and $1^{\text{MeOpy}}$ (right).

Figure 13.

Correlation between cathodic peak potential, $E_{\mathrm{p},\mathrm{c}}$ and the kinetic barrier to the conversion of superoxo 2 to peroxo 3.

Figure 14.

Conversion of peroxo $3^{\text{MeOpy}}$ to a metastable species, $4^{\text{MeOpy}}$, with ${\lambda }_{\text{max}}=805\text{nm}$ monitored by transient absorption spectroscopy in ${\text{CH}}_3{\text{CH}}_2\text{CN}$ at −44 °C. $\left[{\mathrm{O}}_2\right]=8.8\text{mM},\left[1^{\mathbf{\text{MeOpy}}}\right]=0.375\text{mM}$, after mixing.

Figure 15.

Low temperature (6.6 K) perpendicular-mode X-band (9.645 GHz) EPR spectrum of the Mn(III)Mn(IV) intermediate, 15, observed following peroxo 12. Microwave power = 0.2 mW, modulation amplitude = 0.75 mT.

Figure 16.

Comparison of the rates of formation and yield of $6^{\mathbf{\text{MeOpy}}}$ in ${\text{CH}}_2{\text{Cl}}_2$, both in the absence (green) and presence of sacrificial H atom donors CHD (red) or TEMPOH (blue).

Figure 17.

Deuterium isotope effect observed when TEMPOH(D) is used as a sacrificial H atom donor for the conversion of $5^{\mathbf{\text{MeOpy}}}$ to $6^{\mathbf{\text{MeOpy}}}$.

Scheme 1.

Low Temperature Reaction between 1 and ${\mathbf{O}}_2$ Affords Mono Oxo Bridged 6 via Observable Superoxo, 2, and Peroxo, 3, Intermediates^{a a}L= py, Mepy, MeOpy, and Quino.