To rationalise mechanistically the intriguing regio- and chemoselectivity patterns for different substrates of the non-haem iron/2-oxoglutarate dependent halogenase SyrB2, it is crucial to elucidate the structure of the pivotal [Fe[double bond, length as m-dash]O] intermediate. We have approached the problem by a combination of classical and QM/MM modelling. We present complete atomistic models of SyrB2 in complex with its native substrate l-threonine as well as l-α-amino butyric acid and l-norvaline (all conjugated to the pantetheine tether), constructed by molecular docking and extensive MD simulations. We evaluate five isomers of the [Fe[double bond, length as m-dash]O] intermediate in these simulations, with a view to identifying likely structures based on a simple "reaction distance" measure. Starting from models of the resting state, we then use QM/MM calculations to investigate the formation of the [Fe[double bond, length as m-dash]O] species for all three substrates, identifying the intermediates along the O activation/decarboxylation pathway on the S = 1, 2, and 3 potential-energy surfaces. We find that, despite differences in the detailed course of the reaction, essentially all pathways produce the same [Fe[double bond, length as m-dash]O] structure, in which the oxido is directed away from the substrate.

Mixed-metal oxides, e.g., V-Mo and Bi-Mo, are promising selective oxidation catalysts. Yet, their intricate chemical composition and electronic structure often confound DFT methods. This study addresses problems arising from the simultaneous presence of two kinds of transition metals, by probing eight functionals-five hybrid functionals (MN15, M06, PBE0-D3, B3LYP-D3, and TPSSh-D3), the meta-GGA functional M06-L-D3, the range-separated functional ωB97XD, and the GGA functional PBE-D3. We examine the ability of these functionals to localize reducing electrons, and to reproduce reaction energies from CCSD(T) calculations. Accordingly, hybrid functionals containing 20% or more exact exchange perform considerably better in both tests. The B3LYP-D3 approach exhibits the lowest overall mean absolute deviation of reaction energies (OMAD), 21 kJ mol, and gave electron distributions as expected from the local lattice structure according to the pseudo-Jahn-Teller effect. MN15 and PBE0-D3 reproduced the electron distributions, but bore slightly higher OMAD values, at 31 and 32 kJ mol. Despite acceptable OMAD values, M06 (28 kJ mol) and TPSSh (23 kJ mol) in some cases did not yield the expected electron distributions. The range-separated functional ωB97XD experienced the opposite problem, yielding correct electron distributions but a poor OMAD of 41 kJ mol. M06-L-D3 and PBE-D3 performed relatively poorly, regarding the electron distribution and the OMAD values, 39 and 65 kJ mol, respectively.

The fluorine-NHC gauche effect: a structural and computational study. Tetrahedron .

With hybrid DFT calculations applied to periodic models of the bulk MoVNbTeO M1 catalyst, we examined how [TeO]2+ species in the hexagonal channels of this material stabilize nearby reduced metal centers. In particular, an S2(Mo) site, with adjacent [TeO]2+ moieties at both sides, is calculated to be reduced to Mo5+. The modeling study presented offers insight into how the redox behavior of V and Mo centers, a crucial aspect of the M1 catalyst for the selective partial oxidation of small hydrocarbons, may be fine-tuned via TeO moieties at various distances from the metal centers. Graphic Abstract TeO moieties in hexagonal channels, adjacent on either side of an S2(Mo) center, stabilize a gap state at the Mo center, facilitating its reduction to Mo5+.