Intramolecular C-H and C-F Bond Oxygenation by Site-Differentiated Tetranuclear Manganese Models of the OEC

Electrochemical Properties and Reactivity Study of [MnV(O)(μ-OR-Lewis Acid)] Cores

A mononuclear manganese(V) oxo complex of a bis(amidate)bis(alkoxide) ligand, (NMe₄)[Mn^V(HMPAB)(O)] [2; H₄HMPAB = 1,2-bis(2-hydroxy-2-methylpropanamido)benzene], was synthesized and structurally characterized. A Mn-O_term distance of 1.566(4) Å was observed in the solid-state structure of 2, consistent with the Mn≡O formulation. The reaction of redox-inactive metal ions (Mⁿ⁺ = Li⁺, Ca²⁺, Mg²⁺, Y³⁺, and Sc³⁺) with 2 resulted in the formation of 2-Mⁿ⁺ species, which were characterized by UV-vis, ¹H NMR, cyclic voltammetry, and in situ IR spectroscopy. Theoretical calculations suggested that the alkoxide oxygen atoms of the ligand scaffold are energetically most favorable for coordinating the Mⁿ⁺ ions in 2. Complex 2 revealed one-electron-reduction potential at -0.01 V versus ferrocenium/ferrocene, which shifted anodically upon coordination of Mⁿ⁺ ions to 2, and such a shift became more prominent with stronger Lewis acids. The oxygen-atom transfer (OAT) reactivities of 2 and 2-Mⁿ⁺ species with triphenylphosphine were compared, which exhibited a systematic increase of the reaction rate with increasing Lewis acidity of Mⁿ⁺ ions, and a plot of log k₂ versus Lewis acidity of Mⁿ⁺ ions (ΔE) followed a linear relationship. It was observed that 2-Sc³⁺ was ca. 3200 times more reactive toward the OAT reaction compared to 2. Hammett analysis of 2 exhibited a V-shaped plot, indicating a change of the reaction mechanism upon going from electron-rich to electron-deficient Ar₃P substrates. In contrast, 2-Ca²⁺ and 2-Sc³⁺ showed an electrophilic nature toward the OAT reaction, thus demonstrating the role of the Lewis acid in controlling the OAT mechanism. The hydrogen-atom abstraction reaction of 2 and 2-Mⁿ⁺ adducts with 1-benzyl-1,4-dihydronicotinamide was investigated, and it was observed that the rate of reaction did not vary considerably with the Lewis acidity of Mⁿ⁺ ions. On the basis of Eyring analysis of 2 and 2-Mⁿ⁺ adducts, we hypothesized an entropy-controlled hydrogen-atom-transfer reaction for 2-Sc³⁺, which is different from the reaction mechanism of 2 and 2-Ca²⁺.

Mechanistic Insights into the ortho-Defluorination-Hydroxylation of 2-Halophenolates Promoted by a Bis(μ-oxo)dicopper(III) Complex

C-F bonds are one of the most inert functionalities. Nevertheless, some [Cu₂O₂]²⁺ species are able to defluorinate-hydroxylate ortho-fluorophenolates in a chemoselective manner over other ortho-halophenolates. Albeit it is known that such reactivity is promoted by an electrophilic attack of a [Cu₂O₂]²⁺ core over the arene ring, the crucial details of the mechanism that explain the chemo- and regioselectivity of the reaction remain unknown, and it has not being determined either if Cu^II₂(η²:η²-O₂) or Cu^III₂(μ-O)₂ species are responsible for the initial attack on the arene. Herein, we present a combined theoretical and experimental mechanistic study to unravel the origin of the chemoselectivity of the ortho-defluorination-hydroxylation of 2-halophenolates by the [Cu₂(O)₂(DBED)₂]²⁺ complex (DBED = N,N'-di-tert-butylethylenediamine). Our results show that the equilibria between (side-on)peroxo (P) and bis(μ-oxo) (O) isomers plays a key role in the mechanism, with the latter being the reactive species. Furthermore, on the basis of quantum-mechanical calculations, we were able to rationalize the chemoselective preference of the [Cu₂(O)₂(DBED)₂]²⁺ catalyst for the C-F activation over C-Cl and C-H activations.

A Terminal FeIII-Oxo in a Tetranuclear Cluster: Effects of Distal Metal Centers on Structure and Reactivity

Tetranuclear Fe clusters have been synthesized bearing a terminal Fe^III-oxo center stabilized by hydrogen-bonding interactions from pendant ( tert-butylamino)pyrazolate ligands. This motif was supported in multiple Fe oxidation states, ranging from [Fe^II₂Fe^III₂] to [Fe^III₄]; two oxidation states were structurally characterized by single-crystal X-ray diffraction. The reactivity of the Fe^III-oxo center in proton-coupled electron transfer with X-H (X = C, O) bonds of various strengths was studied in conjunction with analysis of thermodynamic square schemes of the cluster oxidation states. These results demonstrate the important role of distal metal centers in modulating the reactivity of a terminal metal-oxo.

Thermodynamics of Proton and Electron Transfer in Tetranuclear Clusters with Mn-OH2/OH Motifs Relevant to H2O Activation by the Oxygen Evolving Complex in Photosystem II

We report the synthesis of site-differentiated heterometallic clusters with three Fe centers and a single Mn site that binds water and hydroxide in multiple cluster oxidation states. Deprotonation of Fe^III/II₃Mn^II-OH₂ clusters leads to internal reorganization resulting in formal oxidation at Mn to generate Fe^III/II₃Mn^III-OH. ⁵⁷Fe Mössbauer spectroscopy reveals that oxidation state changes (three for Fe^III/II₃Mn-OH₂ and four for Fe^III/II₃Mn-OH clusters) occur exclusively at the Fe centers; the Mn center is formally Mn^II when water is bound and Mn^III when hydroxide is bound. Experimentally determined p K_a (17.4) of the [Fe^III₂Fe^IIMn^II-OH₂] cluster and the reduction potentials of the [Fe₃Mn-OH₂] and [Fe₃Mn-OH] clusters were used to analyze the O-H bond dissociation enthalpies (BDE_O-H) for multiple cluster oxidation states. BDE_O-H increases from 69 to 78 and 85 kcal/mol for the [Fe^IIIFe^II₂Mn^II-OH₂], [Fe^III₂Fe^IIMn^II-OH₂], and [Fe^III₃Mn^II-OH₂] clusters, respectively. Further insight of the proton and electron transfer thermodynamics of the [Fe₃Mn-OH _x] system was obtained by constructing a potential-p K_a diagram; the shift in reduction potentials of the [Fe₃Mn-OH _x] clusters in the presence of different bases supports the BDE_O-H values reported for the [Fe₃Mn-OH₂] clusters. A lower limit of the p K_a for the hydroxide ligand of the [Fe₃Mn-OH] clusters was estimated for two oxidation states. These data suggest BDE_O-H values for the [Fe^III₂Fe^IIMn^III-OH] and [Fe^III₃Mn^III-OH] clusters are greater than 93 and 103 kcal/mol, which hints to the high reactivity expected of the resulting [Fe₃Mn═O] in this and related multinuclear systems.

Remarkable increase in the rate of the catalytic epoxidation of electron deficient styrenes through the addition of Sc(OTf)3 to the MnTMTACN catalyst

Sc(OTf)₃ produces a remarkable enhancement in the activity of the MnTMTACN catalyst in the epoxidation of electron deficient styrenes.

Selectivity of C-H versus C-F Bond Oxygenation by Homo- and Heterometallic Fe4 , Fe3 Mn, and Mn4 Clusters

A series of tetranuclear [LM3 (HFArPz)3 OM'][OTf]2 (M, M'=Fe or Mn) clusters that displays 3-(2-fluorophenyl)pyrazolate (HFArPz) as bridging ligand is reported. With these complexes, manganese was demonstrated to facilitate C(sp2 )-F bond oxygenation via a putative terminal metal-oxo species. Moreover, the presence of both ortho C(sp2 )-H and C(sp2 )-F bonds in proximity of the apical metal center provided an opportunity to investigate the selectivity of intramolecular C(sp2 )-X bond oxygenation (X=H or F) in these isostructural compounds. With iron as the apical metal center, (M'=Fe) C(sp2 )-F bond oxygenation occur almost exclusively, whereas with manganese (M'=Mn), the opposite reactivity is preferred.