Zaheer Masood

Redox reactions at the interface of metal oxides and protic electrolytes almost always involve protons and electrons in equal amounts. Given the stoichiometry, these proton-coupled electron transfer (PCET) reactions are thermochemically equivalent to net H-atom transfer (HAT) reactions. The correlation between the chemical nature of solid catalysts and HAT kinetics has been employed for decades as the design principle for energy-relevant reactions (e.g., reactions of 2H⁺/H₂). More recently, chemists have experimentally determined that a change in liquid electrolytes that alters the microenvironment at the redox-active sites has an equally profound impact on electrocatalysis involving PCET/HAT. Yet, precise correlations between the chemical nature of electrolytes and the PCET kinetics are, to date, rare in the literature. Herein, we report our findings using the Ce-based metal-organic framework, Ce-MOF-808, as a model system. Each Ce₆(μ₃-O)₄(μ₃-OH)₄(OH)₆(H₂O)₆ node of this MOF undergoes a 1H⁺/1e^- redox reaction. Using chronoamperometry and the Cottrell analysis, we have determined that the PCET hopping kinetics within the pores of Ce-MOF-808 can change by orders of magnitude by altering the buffer species and the proton activity of the electrolyte. Furthermore, in all buffers, reductive reactions were ∼3-10 times faster in kinetics than the reverse oxidative reaction with the same electrochemical driving force, suggesting that the system, at first glance, violates the principle of microscopic reversibility. Isothermal titration calorimetry (ITC) and computational simulations corroborated that the buffer-node binding thermodynamics are quite distinct, depending on the chemical nature of the buffer and the oxidation state of the node. Together, these results suggest that the substrate and the product during the oxidative vs reductive reaction of Ce-MOF-808 are chemically different species, which explains the apparent 'microscopic irreversibility.' Thus, the rational modulation of electrolytes can dramatically enhance PCET kinetics, even though the solid electrodes remain identical. Implications of these findings are contrasted with the electrochemical/electrocatalytic behavior of other redox-active MOFs, heterogeneous catalysts, and enzymatic systems at the solid-liquid interface.

Herein, we report the discovery, structural and photophysical characterization of a new zero-dimensional (0D) lead-free all-inorganic halide, Rb₄CuSb₂Cl₁₁, which adopts a new structure type. Single-crystal X-ray diffraction (SCXRD) shows that the structure consists of isolated, distorted seesaw [SbCl₄]^- and trigonal planar [CuCl₃]^2- units, separated by Rb⁺ cations that provide charge balance. Optoelectronic measurements and density functional theory (DFT) calculations indicate an indirect band gap of 2.89 eV, making it a candidate for wide-bandgap optoelectronic applications. Electrical resistivity was measured at 1.29 × 10¹⁰ Ω·cm, and the trap-state density (n_trap) was found to be 7.44 × 10¹⁰ cm^-3. Attempts to synthesize substitution analogs of Rb₄CuSb₂Cl₁₁ led to the synthesis of Rb₂In_0.91(0.2)Sb_0.09Cl₅·H₂O, which was erroneously reported as Rb₂SbCl₅O in a previous study. Rb₂In_0.91(0.2)Sb_0.09Cl₅·H₂O adopts a vacancy-ordered perovskite structure and exhibits broad-band yellow emission under UV excitation. The measured photoluminescence quantum yield (PLQY) for Rb₂In_0.91(0.2)Sb_0.09Cl₅·H₂O is 18.2%. These findings add to the growing class of quaternary metal halides with multiple cation and anion compositions, expanding the chemical phase space for the discovery of new materials with functional properties.

Redox reactions at the interface of metal oxides and protic electrolytes almost always involve protons and electrons in equal amounts. Given the stoichiometry, these proton-coupled electron transfer (PCET) reactions are thermochemically equivalent to net H-atom transfer (HAT) reactions. The correlation between the chemical nature of solid catalysts to HAT kinetics has been employed for decades as the design principle for energy-relevant reactions (e.g., reactions of 2H+/H2). More recently, chemists have experimentally determined that a change in liquid electrolytes that alters the microenvi-ronment at the redox-active sites has an equally profound impact on electrocatalysis involving PCET/HAT. Yet, precise correlations between the chemical nature of electrolytes and the PCET kinetics are, to date, rare in the literature. Herein, we report our findings using Ce-based metal–organic framework, Ce-MOF-808, as a model system. Each Ce6(μ3–O)4(μ3–OH)4(OH)6(H2O)6 nodes of this MOF undergo a 1H+/1e- redox reaction. Using chronoamperometry and the Cottrell analy-sis, we have determined that the PCET kinetics within the pores of Ce-MOF-808 can change by orders of magnitude by altering the buffer species and the proton activity of the electrolyte. Furthermore, in all buffers, reductive reactions were ~3-10 times faster in kinetics than the reverse oxidative reaction with the same electrochemical driving force, suggesting that the system, at first glance, violates the principle of microscopic reversibility. Isothermal titration calorimetry (ITC) and computational simulations corroborated that the buffer-node binding thermodynamics are quite distinct, depending on the chemical nature of the buffer and the oxidation state of the node. Together, these results suggest that the substrate and the product during the oxidative vs. reductive reaction of Ce-MOF-808 are chemically different species, which ex-plains the apparent ‘microscopic irreversibility.’ Thus, the rational modulation of electrolytes can dramatically enhance PCET kinetics; therefore, energy-relevant electrocatalysis even though the solid catalysts remain identical. Implications of these findings are contrasted with the electrochemical/electrocatalytic behavior of other redox-active MOFs, heteroge-neous catalysts, and enzymatic systems at the solid-liquid interface.

Coupling CO ₂ with ethylene to produce acrylic acid is valuable for manufacturing various industrial fine chemicals. Currently, this reaction is catalyzed by transition metal molecular complexes, which show low activity, limiting their industrial application. Therefore, the development of highly active heterogeneous catalysts for this reaction is desirable. Herein, metal dimers introduced into graphene are assessed as heterogeneous catalysts for the CO ₂ −ethylene coupling reaction using density functional theory calculations. It is found that Pd, Rh, Ni, and Cu dimers facilitate the βH transfer steps, whereas Zn, Ru, and Os facilitate the CC coupling steps. The variation of the overall energetic barrier based on the energetic span model is rather narrow, as these two elementary steps are anticorrelated with each other. Furthermore, the stability of metallalactones is found to be a key descriptor for the activation energies of both the CC coupling and the βH transfer steps. Using Crystal Orbital Hamilton Population analysis, it is identified that the strength of the C _β C _γ bond in metallalactones can serve as an electronic‐level descriptor for activation energies. It is anticipated that the insights gained from this study will guide the development of heterogeneous catalysts for the CO ₂ −ethylene coupling for producing acrylic acid.

Lewis acidic metal oxides, including zirconia (ZrO₂), are catalytically active toward oxidative reactions in the presence of sacrificial oxidants like t-butyl hydroperoxide (TBHP). The structural ambiguity and heterogeneity of the ZrO₂ surface impose challenges to chemists in understanding the reaction mechanism down to atomic-level precision. The inorganic, Zr-oxo nodes of many crystalline metal-organic frameworks (MOFs) structurally mimic ZrO₂. Herein, we report three novel findings: (A) Zr-based MOF, Zr-MOF-808 is catalytically competent in activating TBHP to induce oxygen atom transfer (OAT) reactions to a model substrate, thioanisole, at room temperature, (B) its reaction mechanism can be derived with greater structural precision owing to the crystallinity of the MOF, and (C) the node-binding agent and other reaction conditions significantly impact the selectivity between the singly oxidized methyl phenyl sulfoxide vs the doubly oxidized sulfone. These findings suggest that both the activity and selectivity of OAT reactions within Zr-MOF-808 are governed by the chemistry occurring at the interface of the node and the surrounding reaction medium. Implications of these findings in OAT reactions and other MOF/metal oxide-catalyzed relevant catalysis are discussed.

Lewis acidic metal oxides, including zirconia (ZrO2), are catalytically active towards oxidative reactions in the presence of sacrificial oxidants like t-butyl hydroperoxide (TBHP). The structural ambiguity and heterogeneity of the ZrO2 surface impose challenges to chemists in understanding the reaction mechanism down to atomic-level precision. The inorganic, Zr-oxo nodes of many crystalline metal–organic frameworks (MOFs) structurally mimic ZrO2. Herein, we report three novel findings: (A) Zr-based MOF, Zr-MOF-808 is catalytically competent in activating TBHP to induce oxygen atom transfer (OAT) reactions to a model substrate, thioanisole, at room temperature, (B) its reaction mechanism can be derived with greater structural precision owing to the crystallinity of the MOF, and (C) the node-binding agent and other reaction conditions significantly impact the selectivity between the singly-oxidized methyl phenyl sulfoxide vs. the doubly-oxidized sulfone. These findings suggest that both the activity and selectivity of OAT reactions within Zr-MOF-808 are governed by the chemistry occurring at the interface of the node and the surrounding reaction medium. Implications of these findings in OAT reactions and other MOF/metal oxide-catalyzed relevant catalysis are discussed.