Computational Insights into the Mechanisms of H2 Activation and H2/D2 Isotope Exchange by Dimolybdenum Tetrasulfide Complexes

The mechanisms for H2 activation by [Cp*Mo]2(μ- S)2(μ-S2) (1-a, Cp* = pentamethylcyclopentadienyl) and its reaction product [Cp*Mo]2(μ-S)2(μ-SH)2 (2) have been investigated by DFT methods. The reaction of 1-a involves the homolytic addition of H2 to its μ-S ligands, followed by the cleavage of the S–S bond of the μ-S2 ligand in a subsequent step. Complex 2 can adopt five conformations that only differ in the stereochemistry of the μ-SH and μ-S ligands; although an isomer with adjacent μ-S ligands (2-a) is formed initially, it then isomerises into the experimentally observed 2-d. This species promotes H/ D scrambling in H2/D2 mixtures, and the mechanism of the process has also been studied. Notably, all of the computed pathways for the addition of D2 to 2-d present prohibitive barriers; instead, only those isomers with adjacent μ-S ligands are able to react further. The homolytic activation of D2 by these leads to isomers of [Cp2Mo2(μ-SH)2(μ-SD)2], the interconversion of which is the rate-determining step

Electrocatalytic hydrogen evolution by an iron complex containing a nitro-functionalized polypyridyl ligand

Iron polypyridyl complexes have recently been reported to electrocatalytically reduce protons to hydrogen gas at -1.57 V versus Fc(+)/Fc. A new iron catalyst with a nitro-functionalized polypyridyl ligand has been synthesized and found to be active for proton reduction. Interestingly, catalysis occurs at -1.18 V versus Fc(+)/Fc for the nitro-functionalized complex, resulting in an overpotential of 300 mV. Additionally, the complex is active with a turnover frequency of 550 s(-1). Catalysis is also observed in the presence of water with a 12% enhancement in activity. (C) 2015 Elsevier Ltd. All rights reserved

Judicious Ligand Design in Ruthenium Polypyridyl CO2 Reduction Catalysts to Enhance Reactivity by Steric and Electronic Effects

A series of RuII polypyridyl complexes of the structural design [RuII(R−tpy)(NN)(CH3CN)]2+ (R−tpy=2,2′:6′,2′′-terpyridine (R=H) or 4,4′,4′′-tri-tert-butyl-2,2′:6′,2′′-terpyridine (R=tBu); NN=2,2′-bipyridine with methyl substituents in various positions) have been synthesized and analyzed for their ability to function as electrocatalysts for the reduction of CO2 to CO. Detailed electrochemical analyses establish how substitutions at different ring positions of the bipyridine and terpyridine ligands can have profound electronic and, even more importantly, steric effects that determine the complexes’ reactivities. Whereas electron-donating groups para to the heteroatoms exhibit the expected electronic effect, with an increase in turnover frequencies at increased overpotential, the introduction of a methyl group at the ortho position of NN imposes drastic steric effects. Two complexes, [RuII(tpy)(6-mbpy)(CH3CN)]2+ (trans-[3]2+; 6-mbpy=6-methyl-2,2′-bipyridine) and [RuII(tBu−tpy)(6-mbpy)(CH3CN)]2+ (trans-[4]2+), in which the methyl group of the 6-mbpy ligand is trans to the CH3CN ligand, show electrocatalytic CO2 reduction at a previously unreactive oxidation state of the complex. This low overpotential pathway follows an ECE mechanism (electron transfer–chemical reaction–electron transfer), and is a direct result of steric interactions that facilitate CH3CN ligand dissociation, CO2 coordination, and ultimately catalytic turnover at the first reduction potential of the complexes. All experimental observations are rigorously corroborated by DFT calculations

Molybdenum/cobalt/sulfur clusters: Models and precursors for hydrodesulfurization (HDS) catalysts

Sulfido clusters which incorporate molybdenum and a late transition metal, e.g. iron, cobalt or nickel, are readily prepared by the reactions of Cp 2 Mo 2 S 4 , Cp 2 Mo 2 S 2 (SR) 2 or Cp 2 Mo 2 (CO) 2 (SR) 2 with Fe 2 (CO) 9 , Co 2 (CO) 8 , Ni(CO) 4 , Cp 2 Ni, etc. The homogeneous reactions of the cluster Cp 2 Mo 2 Co 2 S 3 (CO) 4 with thiols, thiophene, and phosphines are reviewed, as are some reactions of the clusters with metal oxide surfaces to produce heterogeneous catalysts for CO hydrogenation or hydrodesulfurization.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/38296/1/590060504_ftp.pd

Metal-free electrocatalytic hydrogen oxidation using frustrated Lewis pairs and carbon-based Lewis acids

Whilst hydrogen is a potentially clean fuel for energy storage and utilisation technologies, its conversion to electricity comes at a high energetic cost. This demands the use of rare and expensive precious metal electrocatalysts. Electrochemical-frustrated Lewis pairs offer a metal-free, CO tolerant pathway to the electrocatalysis of hydrogen oxidation. They function by combining the hydrogen-activating ability of frustrated Lewis pairs (FLPs) with electrochemical oxidation of the resultant hydride. Here we present an electrochemical–FLP approach that utilises two different Lewis acids – a carbon-based N-methylacridinium cation that possesses excellent electrochemical attributes, and a borane that exhibits fast hydrogen cleavage kinetics and functions as a “hydride shuttle”. This synergistic interaction provides a system that is electrocatalytic with respect to the carbon-based Lewis acid, decreases the required potential for hydrogen oxidation by 1 V, and can be recycled multiple times

Kinetic and DFT Studies on the Mechanism of C−S Bond Formation by Alkyne Addition to the [Mo3S4(H2O)9]4+ Cluster

Reaction of [Mo3(μ3-S)(μ-S)3] clusters with alkynes usually leads to formation of two C−S bonds between the alkyne and two of the bridging sulfides. The resulting compounds contain a bridging alkenedithiolate ligand, and the metal centers appear to play a passive role despite reactions at those sites being well illustrated for this kind of cluster. A detailed study including kinetic measurements and DFT calculations has been carried out to understand the mechanism of reaction of the [Mo3(μ3-S)(μ-S)3(H2O)9]4+ (1) cluster with two different alkynes, 2-butyne-1,4-diol and acetylenedicarboxylic acid. Stoppedflow experiments indicate that the reaction involves the appearance in a single kinetic step of a band at 855 or 875 nm, depending on the alkyne used, a position typical of clusters with two C−S bonds. The effects of the concentrations of the reagents, the acidity, and the reaction medium on the rate of reaction have been analyzed. DFT and TD-DFT calculations provide information on the nature of the product formed, its electronic spectrum and the energy profile for the reaction. The structure of the transition state indicates that the alkyne approaches the cluster in a lateral way and both C−S bonds are formed simultaneously