Synthesis and Computational Studies of a New Class of Lanthanide Niobate Cluster : [Ln\u3csub\u3e4\u3c/sub\u3e(H\u3csub\u3e2\u3c/sub\u3eO)\u3csub\u3e8\u3c/sub\u3e(SO\u3csub\u3e4\u3c/sub\u3e)\u3csub\u3e5\u3c/sub\u3e(NbO\u3csub\u3e3\u3c/sub\u3e)\u3csub\u3e2\u3c/sub\u3e]+3H\u3csub\u3e2\u3c/sub\u3eO; Ln= Dy, Tb

Polyoxoniobates (PONbs) are a small family of highly electron-rich clusters. The development of new solids composed of these clusters have applications in green energy and electronics. However, the high charge environment of PONbs typically requires alkaline synthetic conditions that are unsuitable for introducing other metals and organic molecules, making synthesis of new systems difficult. To date, very few transition metals and organic ligands have been incorporated into these PONb solids, and lanthanide metal inclusion, which generally improves photoconductivity due to longlived f-orbital excitations, has not yet been fully realized. Here, the synthesis of a new class of lanthanide niobate cluster [Ln4(H2O)8(SO4)5(NbO3)2]·3H2O; Ln= Dy, Tb under acidic conditions is reported. Structures were determined by crystallography and time-dependent density functional theory (TD-DFT) was used to provide insight into photo-induced electronic transitions. Supporting computational methods that are currently being developed for modeling these emerging cluster systems are described

Computational modeling of electronically excited states in cobalamin-dependent reactions.

The current understanding of the photolytic properties of Vitamin B12 derivatives or cobalamins are summarized from a computational point of view. The focus is on two non-alkylcobalamins, cyanocobalamin (CNCbl) and hydroxocobalamin (HOCbl), two alkylcobalamins, methylcobalamin (MeCbl) and adenosylcobalamin (AdoCbl), as well as the stable cob(II)alamin radical. Photolysis of alkylcobalamins involves low-lying singlet excited states where photo-dissociation of the Co-C bond forms singlet-born alkyl/cob(II)alamin radical pairs (RPs). Potential energy surfaces (PESs) of low-lying excited states as functions of both axial bonds provide the most reliable tool for analysis of photochemical and photophysical properties. Due to the size limitations associated with the cobalamins, the primary method for calculating ground state properties is density functional theory (DFT), with time-dependent DFT (TD-DFT) mainly used for electronically excited states. The energy pathways on the lowest singlet surfaces of the alkylcobalamins, connect metal-to-ligand charge transfer (MLCT) and ligand field (LF) minima associated with photo-homolysis of the Co-C bond observed experimentally. Additionally, energy pathways between minima and seams associated with crossing of S1/S0 surfaces are the most efficient for internal conversion (IC) to the ground state. Depending on the specific cobalamin, such IC may involve simultaneous elongation of both axial bonds (CNCbl), or detachment of axial base coupled with corrin ring distortion (MeCbl). The possible involvement of triplet RPs is also discussed, and a mechanism of intersystem crossing based on Landau-Zener theory is presented

Mercury Methylation by Cobalt Corrinoids: Relativistic Effects Dictate the Reaction Mechanism

The methylation of HgII(SCH3)2 by corrinoid‐based methyl donors proceeds in a concerted manner through a single transition state by transfer of a methyl radical, in contrast to previously proposed reaction mechanisms. This reaction mechanism is a consequence of relativistic effects that lower the energies of the mercury 6p1/2 and 6p3/2 orbitals, making them energetically accessible for chemical bonding. In the absence of spin–orbit coupling, the predicted reaction mechanism is qualitatively different. This is the first example of relativity being decisive for the nature of an observed enzymatic reaction mechanism.Of relative importance: The methylation of HgII(SCH3)2 by corrinoid‐based methyl donors proceeds in a concerted manner through a single transition state by transfer of a methyl radical. This reaction mechanism is a consequence of relativistic effects, and constitutes the first example of relativity being decisive for the nature of an enzymatic reaction mechanism. SOC=spin–orbit coupling.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137374/1/anie201606001-sup-0001-misc_information.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137374/2/anie201606001.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137374/3/anie201606001_am.pd

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(N-4-methyl-3-thiosemicarbazone) and zinc diacetyl-bis(N-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

Mechanism of Co–C Bond Photolysis in Methylcobalamin: Influence of Axial Base

A mechanism of Co–C bond photolysis in the base-off form of the methylcobalamin cofactor (MeCbl) and the influence of its axial base on Co–C bond photodissociation has been investigated by time-dependent density functional theory (TD-DFT). At low pH, the MeCbl cofactor adopts the base-off form in which the axial nitrogenous ligand is replaced by a water molecule. Ultrafast excited-state dynamics and photolysis studies have revealed that a new channel for rapid nonradiative decay in base-off MeCbl is opened, which competes with bond dissociation. To explain these experimental findings, the corresponding potential energy surface of the S₁ state was constructed as a function of Co–C and Co–O bond distances, and the manifold of low-lying triplets was plotted as a function of Co–C bond length. In contrast to the base-on form of MeCbl in which two possible photodissociation pathways were identified on the basis of whether the Co–C bond (path A) or axial Co–N bond (path B) elongates first, only path B is active in base-off MeCbl. Specifically, path A is inactive because the energy barrier associated with direct dissociation of the methyl ligand is higher than the barrier of intersection between two different electronic states: a metal-to-ligand charge transfer state (MLCT), and a ligand field state (LF) along the Co–O coordinate of the S₁ PES. Path B initially involves displacement of the water molecule, followed by the formation of an LF-type intermediate, which possesses a very shallow energy minimum with respect to the Co–C coordinate. This LF-type intermediate on path B may result in either S₁/S₀ internal conversion or singlet radical pair generation. In addition, intersystem crossing (ISC) resulting in generation of a triplet radical pair is also feasible

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

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

Pieter Boddaert to Carl Linnaeus

Pieter Boddaert to Carl Linnaeu