Selectivity of C−H vs. C−F Bond Oxygenation by Homo- and Heterometallic Fe_4, Fe_3Mn, and Mn_4 Clusters

A series of tetranuclear [LM_3(HFArPz)_3OM'][OTf]_2 (M, M' = Fe or Mn) clusters that displays 3-(2-fluorophenyl)pyrazolate (HFArPz) as bridging ligand is reported. With these complexes manganese is demonstrated to facilitate C(sp^2)−F bond oxygenation via a putative terminal metal-oxo species. Moreover, the presence of both ortho C(sp^2)−H and C(sp^2)−F bonds in proximity provides an opportunity to investigate the selectivity of intramolecular C(sp^2)−X bond oxygenation (X = H or F) in these isostructural compounds. With iron as the apical metal center (M' = Fe) C(sp^2)−F bond oxygenation occurs almost exclusively, whereas with manganese (M' = Mn) the opposite reactivity is preferred

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

The dangler manganese center in the oxygen-evolving complex (OEC) of photosystem II plays an important role in the oxidation of water to dioxygen. Inspired by the structure of the OEC, we synthesized a series of site-differentiated tetra-manganese clusters [LMn_3(PhPz)_3OMn][OTf]_x (2: x= 2; 3: x = 1) that features an apical manganese ion—distinct from the others—that is appended to a trinuclear manganese core through an μ4-oxygen atom bridge. This cluster design was targeted to facilitate studies of high-valent Mn-oxo formation, which is a proposed step in the mechanism for water oxidation by the OEC. Terminal Mn-oxo species—supported by a multinuclear motif—were targeted by treating 2 and 3 with iodosobenzene. Akin to our previously reported iron complexes, intramolecular arene hydroxylation was observed to yield the C–H bond oxygenated complexes [LMn3(PhPz)_2(OArPz)OMn][OTf]x (5: x = 2; 6: x = 1). The fluorinated series [LMn_3(F_2ArPz)_3OMn][OTf]_x (8: x = 2; 9: x = 1) was also synthesized to mitigate the observed intramolecular hydroxylation. Treatment of 8 and 9 with iodosobenzene results in intramolecular arene C–F bond oxygenation as judged by electrospray ionization mass spectrometry. The observed aromatic C–H and C–F hydroxylation is suggestive of a putative high-valent terminal metal-oxo species, and it is one of the very few examples capable of oxygenating C–F bonds

Accelerated Oxygen Atom Transfer and C−H Bond Oxygenation by Remote Redox Changes in Fe_3Mn-Iodosobenzene Adducts

We report the synthesis, characterization, and reactivity of [Lfe_3(PhPz)_3OMn(^sPhIO)][OTf]_x (3: x=2; 4: x=3), where 4 is one of very few examples of iodosobenzene–metal adducts characterized by X-ray crystallography. Access to these rare heterometallic clusters enabled differentiation of the metal centers involved in oxygen atom transfer (Mn) or redox modulation (Fe). Specifically, ^(57)Fe Mössbauer and X-ray absorption spectroscopy provided unique insights into how changes in oxidation state (Fe^(III)_2Fe^(II)Mn^(II) vs. Fe^(III)_3Mn^(II)) influence oxygen atom transfer in tetranuclear Fe_3Mn clusters. In particular, a one-electron redox change at a distal metal site leads to a change in oxygen atom transfer reactivity by ca. two orders of magnitude

Toward Models for the Full Oxygen-Evolving Complex of Photosystem II by Ligand Coordination To Lower the Symmetry of the Mn_3CaO_4 Cubane: Demonstration That Electronic Effects Facilitate Binding of a Fifth Metal

Synthetic model compounds have been targeted to benchmark and better understand the electronic structure, geometry, spectroscopy, and reactivity of the oxygen-evolving complex (OEC) of photosystem II, a low-symmetry Mn_4CaO_n cluster. Herein, low-symmetry Mn^(IV)_3GdO_4 and Mn^(IV_)3CaO_4 cubanes are synthesized in a rational, stepwise fashion through desymmetrization by ligand substitution, causing significant cubane distortions. As a result of increased electron richness and desymmetrization, a specific μ_3-oxo moiety of the Mn_3CaO_4 unit becomes more basic allowing for selective protonation. Coordination of a fifth metal ion, Ag+, to the same site gives a Mn_3CaAgO_4 cluster that models the topology of the OEC by displaying both a cubane motif and a “dangler” transition metal. The present synthetic strategy provides a rational roadmap for accessing more accurate models of the biological catalyst

Effects of Lewis Acidic Metal Ions (M) on Oxygen-Atom Transfer Reactivity of Heterometallic Mn_3MO_4 Cubane and Fe_3MO(OH) and Mn_3MO(OH) Clusters

The modulation of the reactivity of metal oxo species by redox inactive metals has attracted much interest due to the observation of redox inactive metal effects on processes involving electron transfer both in nature (the oxygen-evolving complex of Photosystem II) and in heterogeneous catalysis (mixed-metal oxides). Studies of small-molecule models of these systems have revealed numerous instances of effects of redox inactive metals on electron- and group-transfer reactivity. However, the heterometallic species directly involved in these transformations have rarely been structurally characterized and are often generated in situ. We have previously reported the preparation and structural characterization of multiple series of heterometallic clusters based on Mn_3 and Fe_3 cores and described the effects of Lewis acidity of the heterometal incorporated in these complexes on cluster reduction potential. To determine the effects of Lewis acidity of redox inactive metals on group transfer reactivity in structurally well-defined complexes, we studied [Mn_3MO_4], [Mn_3MO(OH)], and [Fe_3MO(OH)] clusters in oxygen atom transfer (OAT) reactions with phosphine substrates. The qualitative rate of OAT correlates with the Lewis acidity of the redox inactive metal, confirming that Lewis acidic metal centers can affect the chemical reactivity of metal oxo species by modulating cluster electronics

Variable Pathways for Oxygen Atom Insertion into Metal-Carbon Bonds: The Case of Cp*W(O)₂(CH₂SiMe₃)

Article discussing variable pathways for oxygen atom insertion into metal-carbon bonds and the case of Cp*W(O)2(CH2SiMe3)

Assessment of Adsorbate π‑Backbonding in Copper(I) Metal–Organic Frameworks via Multinuclear NMR Spectroscopy and Density Functional Theory Calculations

We assess the binding of C2H4 to the coordinately unsaturated copper(I) sites of the metal-organic frameworks Cu(I)-ZrTpmC* and Cu(I)-MFU-4l via 13C solid-state nuclear magnetic resonance spectroscopy, density functional theory (DFT), and natural localized molecular orbital analysis. Using these methods, forward-donation and back-donation contributions between C2H4 and the exposed Cu(I) are delineated, and high binding enthalpies are contextualized as a function of electronic changes upon site modification and adsorption. With the infrastructure for DFT and solid-state 13C NMR becoming more routine for scientists, we envision that these results will support the study of exposed electron-rich metal sites in a variety of chemical applications

Assessment of Adsorbate π-backbonding in Copper(I) Metal-Organic Frameworks via Multinuclear NMR Spectroscopy and Density Functional Theory Calculations

We assess the binding of C2H4 to the coordinately unsaturated copper(I) sites of the metal-organic frameworks Cu(I)-ZrTpmC* and Cu(I)-MFU-4l via 13C solid-state nuclear magnetic resonance spectroscopy, density functional theory (DFT), and natural localized molecular orbital (NLMO) analysis. Using these methods, forward-donation and back-donation contributions between C2H4 and the exposed Cu(I) are delineated and high-binding enthalpies are contextualized as a function of electronic changes upon site modification and adsorption. With the infrastructure for DFT and solid-state 13C NMR becoming more routine for scientists, we envision these results will support the study of exposed electron-rich metal sites in a variety of chemical applications

Prediction of Multiple Hydrogen Ligation at a Vanadium(II) Site in a Metal-Organic Framework

Densifying hydrogen in a metal-organic framework (MOF) at moderate pressures can circumvent challenges associated with high-pressure compression. The highly tunable structural and chemical composition in MOFs affords vast possibilities to optimize binding interactions. At the heart of this search are the nanoscale characteristics of molecular adsorption at the binding site(s). Using density functional theory (DFT) to model binding interactions of hydrogen to the exposed metal site of cation-exchanged MFU-4l, we predict multiple hydrogen ligation of H2 at the first coordination sphere of V(II) in V(II)-exchanged MFU-4l. We find that the strength of this binding between the metal site and \ce{H2} molecules can be tuned by altering the halide counterion adjacent to the metal site and that the fluoride-containing node affords the most favourable interactions for high-density H2 storage. Using energy decomposition analysis, we delineate electronic contributions that enable multiple hydrogen ligation and demonstrate its benefits for hydrogen adsorption and release at modest pressures