Relating structure and composition with accessibility of a single catalyst particle using correlative 3-dimensional micro-spectroscopy

To understand how hierarchically structured functional materials operate, analytical tools are needed that can reveal small structural and chemical details in large sample volumes. Often, a single method alone is not sufficient to get a complete picture of processes happening at multiple length scales. Here we present a correlative approach combining three-dimensional X-ray imaging techniques at different length scales for the analysis of metal poisoning of an individual catalyst particle. The correlative nature of the data allowed establishing a macro-pore network model that interprets metal accumulations as a resistance to mass transport and can, by tuning the effect of metal deposition, simulate the response of the network to a virtual ageing of the catalyst particle. The developed approach is generally applicable and provides an unprecedented view on dynamic changes in a material's pore space, which is an essential factor in the rational design of functional porous materials

Relating structure and composition with accessibility of a single catalyst particle using correlative 3-dimensional micro-spectroscopy

To understand how hierarchically structured functional materials operate, analytical tools are needed that can reveal small structural and chemical details in large sample volumes. Often, a single method alone is not sufficient to get a complete picture of processes happening at multiple length scales. Here we present a correlative approach combining three-dimensional X-ray imaging techniques at different length scales for the analysis of metal poisoning of an individual catalyst particle. The correlative nature of the data allowed establishing a macro-pore network model that interprets metal accumulations as a resistance to mass transport and can, by tuning the effect of metal deposition, simulate the response of the network to a virtual ageing of the catalyst particle. The developed approach is generally applicable and provides an unprecedented view on dynamic changes in a material's pore space, which is an essential factor in the rational design of functional porous materials

High-Valent Manganese–Oxo Valence Tautomers and the Influence of Lewis/Brönsted Acids on C–H Bond Cleavage

The addition of Lewis or Brönsted acids (LA = Zn­(OTf)₂, B­(C₆F₅)₃, HBAr^F, TFA) to the high-valent manganese–oxo complex Mn^V(O)­(TBP₈Cz) results in the stabilization of a valence tautomer Mn^IV(O-LA)­(TBP₈Cz^•+). The Zn^II and B­(C₆F₅)₃ complexes were characterized by manganese K-edge X-ray absorption spectroscopy (XAS). The position of the edge energies and the intensities of the pre-edge (1s to 3d) peaks confirm that the Mn ion is in the +4 oxidation state. Fitting of the extended X-ray absorption fine structure (EXAFS) region reveals 4 N/O ligands at Mn–N_ave = 1.89 Å and a fifth N/O ligand at 1.61 Å, corresponding to the terminal oxo ligand. This Mn–O bond length is elongated compared to the Mn^V(O) starting material (Mn–O = 1.55 Å). The reactivity of Mn^IV(O-LA)­(TBP₈Cz^•+) toward C–H substrates was examined, and it was found that H^• abstraction from C–H bonds occurs in a 1:1 stoichiometry, giving a Mn^IV complex and the dehydrogenated organic product. The rates of C–H cleavage are accelerated for the Mn^IV(O-LA)­(TBP₈Cz^•+) valence tautomer as compared to the Mn^V(O) valence tautomer when LA = Zn^II, B­(C₆F₅)₃, and HBAr^F, whereas for LA = TFA, the C–H cleavage rate is slightly slower than when compared to Mn^V(O). A large, nonclassical kinetic isotope effect of <i>k</i>_H/<i>k</i>_D = 25–27 was observed for LA = B­(C₆F₅)₃ and HBAr^F, indicating that H-atom transfer (HAT) is the rate-limiting step in the C–H cleavage reaction and implicating a potential tunneling mechanism for HAT. The reactivity of Mn^IV(O-LA)­(TBP₈Cz^•+) toward C–H bonds depends on the strength of the Lewis acid. The HAT reactivity is compared with the analogous corrole complex Mn^IV(O–H)­(tpfc^•+) recently reported (<i>J. Am. Chem. Soc.</i> <b>2015</b>, 137, 14481–14487)

Uncoupling binding of substrate CO from turnover by vanadium nitrogenase.

Biocatalysis by nitrogenase, particularly the reduction of N2 and CO by this enzyme, has tremendous significance in environment- and energy-related areas. Elucidation of the detailed mechanism of nitrogenase has been hampered by the inability to trap substrates or intermediates in a well-defined state. Here, we report the capture of substrate CO on the resting-state vanadium-nitrogenase in a catalytically competent conformation. The close resemblance of this active CO-bound conformation to the recently described structure of CO-inhibited molybdenum-nitrogenase points to the mechanistic relevance of sulfur displacement to the activation of iron sites in the cofactor for CO binding. Moreover, the ability of vanadium-nitrogenase to bind substrate in the resting-state uncouples substrate binding from subsequent turnover, providing a platform for generation of defined intermediate(s) of both CO and N2 reduction

Uncoupling binding of substrate CO from turnover by vanadium nitrogenase

Biocatalysis by nitrogenase, particularly the reduction of N(2) and CO by this enzyme, has tremendous significance in environment- and energy-related areas. Elucidation of the detailed mechanism of nitrogenase has been hampered by the inability to trap substrates or intermediates in a well-defined state. Here, we report the capture of substrate CO on the resting-state vanadium-nitrogenase in a catalytically competent conformation. The close resemblance of this active CO-bound conformation to the recently described structure of CO-inhibited molybdenum-nitrogenase points to the mechanistic relevance of sulfur displacement to the activation of iron sites in the cofactor for CO binding. Moreover, the ability of vanadium-nitrogenase to bind substrate in the resting-state uncouples substrate binding from subsequent turnover, providing a platform for generation of defined intermediate(s) of both CO and N(2) reduction

Setting an Upper Limit on the Myoglobin Iron(IV)Hydroxide p<i>K</i>_a: Insight into Axial Ligand Tuning in Heme Protein Catalysis

To provide insight into the iron­(IV)­hydroxide p<i>K</i>_a of histidine ligated heme proteins, we have probed the active site of myoglobin compound II over the pH range of 3.9–9.5, using EXAFS, Mössbauer, and resonance Raman spectroscopies. We find no indication of ferryl protonation over this pH range, allowing us to set an upper limit of 2.7 on the iron­(IV)­hydroxide p<i>K</i>_a in myoglobin. Together with the recent determination of an iron­(IV)­hydroxide p<i>K</i>_a ∼ 12 in the thiolate-ligated heme enzyme cytochrome P450, this result provides insight into Nature’s ability to tune catalytic function through its choice of axial ligand