Mechanism of Co–C Bond Photolysis in the Base-On Form of Methylcobalamin

A mechanism of Co–C bond photodissociation in the base-on form of the methylcobalamin cofactor (MeCbl) has been investigated employing time-dependent density functional theory (TD-DFT), in which the key step involves singlet radical pair generation from the first electronically excited state (S₁). The corresponding potential energy surface of the S₁ state was constructed as a function of Co–C and Co–N_axial bond distances, and two possible photodissociation pathways were identified on the basis of energetic grounds. These pathways are distinguished by whether the Co–C bond (path A) or Co–N_axial bond (path B) elongates first. Although the final intermediate of both pathways is the same (namely a ligand field (LF) state responsible for Co–C dissociation), the reaction coordinates associated with paths A and B are different. The photolysis of MeCbl is wavelength-dependent, and present TD-DFT analysis indicates that excitation in the visible α/β band (520 nm) can be associated with path A, whereas excitation in the near-UV region (400 nm) is associated with path B. The possibility of intersystem crossing, and internal conversion to the ground state along path B are also discussed. The mechanism proposed in this study reconciles existing experimental data with previous theoretical calculations addressing the possible involvement of a repulsive triplet state

Beyond Metal-Hydrides: Non-Transition-Metal and Metal-Free Ligand-Centered Electrocatalytic Hydrogen Evolution and Hydrogen Oxidation

A new pathway for homogeneous electrocatalytic H₂ evolution and H₂ oxidation has been developed using a redox active thiosemicarbazone and its zinc complex as seminal metal-free and transition-metal-free examples. Diacetyl-bis­(<i>N</i>-4-methyl-3-thiosemicarbazone) and zinc diacetyl-bis­(<i>N</i>-4-methyl-3-thiosemicarbazide) display the highest reported TOFs of any homogeneous ligand-centered H₂ evolution catalyst, 1320 and 1170 s^–1, respectively, while the zinc complex also displays one of the highest reported TOF values for H₂ oxidation, 72 s^–1, of any homogeneous catalyst. Catalysis proceeds via ligand-centered proton-transfer and electron-transfer events while avoiding traditional metal-hydride intermediates. The unique mechanism is consistent with electrochemical results and is further supported by density functional theory. The results identify a new direction for the design of electrocatalysts for H₂ evolution and H₂ oxidation that are not reliant on metal-hydride intermediates

Translation of Ligand-Centered Hydrogen Evolution Reaction Activity and Mechanism of a Rhenium-Thiolate from Solution to Modified Electrodes: A Combined Experimental and Density Functional Theory Study

The homogeneous, nonaqueous catalytic activity of the rhenium-thiolate complex ReL₃ (L = diphenylphosphinobenzenethiolate) for the hydrogen evolution reaction (HER) has been transferred from nonaqueous homogeneous to aqueous heterogeneous conditions by immobilization on a glassy carbon electrode surface. A series of modified electrodes based on ReL₃ and its oxidized precursor [ReL₃]­[PF₆] were fabricated by drop-cast methods, yielding catalytically active species with HER overpotentials for a current density of 10 mA/cm², ranging from 357 to 919 mV. The overpotential correlates with film resistance as measured by electrochemical impedance spectroscopy and film morphology as determined by scanning and transmission electron microscopy. The lowest overpotential was for films based on the ionic [ReL₃]­[PF₆] precursor with the inclusion of carbon black. Stability measurements indicate a 2 to 3 h conditioning period in which the overpotential increases, after which no change in activity is observed within 24 h or upon reimmersion in fresh aqueous, acidic solution. Electronic spectroscopy results are consistent with ReL₃ as the active species on the electrode surface; however, the presence of an undetected quantity of catalytically active degradation species cannot be excluded. The HER mechanism was evaluated by Tafel slope analysis, which is consistent with a novel Volmer–Heyrovsky–Tafel-like mechanism that parallels the proposed homogeneous HER pathway. Proposed mechanisms involving traditional metal-hydride processes vs ligand-centered reactivity were examined by density functional theory, including identification and characterization of relevant transition states. The ligand-centered path is energetically favored with protonation of cis-sulfur sites culminating in homolytic S–H bond cleavage with H₂ evolution via H atom coupling

Polarized XANES Monitors Femtosecond Structural Evolution of Photoexcited Vitamin B₁₂

Ultrafast, polarization-selective time-resolved X-ray absorption near-edge structure (XANES) was used to characterize the photochemistry of vitamin B₁₂, cyanocobalamin (CNCbl), in solution. Cobalamins are important biological cofactors involved in methyl transfer, radical rearrangement, and light-activated gene regulation, while also holding promise as light-activated agents for spatiotemporal controlled delivery of therapeutics. We introduce polarized femtosecond XANES, combined with UV–visible spectroscopy, to reveal sequential structural evolution of CNCbl in the excited electronic state. Femtosecond polarized XANES provides the crucial structural dynamics link between computed potential energy surfaces and optical transient absorption spectroscopy. Polarization selectivity can be used to uniquely identify electronic contributions and structural changes, even in isotropic samples when well-defined electronic transitions are excited. Our XANES measurements reveal that the structural changes upon photoexcitation occur mainly in the axial direction, where elongation of the axial Co–CN bond and Co–N_Im bond on a 110 fs time scale is followed by corrin ring relaxation on a 260 fs time scale. These observations expose features of the potential energy surfaces controlling cobalamin reactivity and deactivation