Hydrogen Elimination from a Hydroxycyclopentadienyl Ruthenium(II) Hydride: Study of Hydrogen Activation in a Ligand−Metal Bifunctional Hydrogenation Catalyst

At high temperatures in toluene, [2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(CO)2H (3) undergoes hydrogen elimination in the presence of PPh3 to produce the ruthenium phosphine complex [2,5-Ph2-3,4-Tol2-(η4-C4CO)]Ru(PPh3)(CO)2 (6). In the absence of alcohols, the lack of RuH/OD exchange, a rate law first order in Ru and zero order in phosphine, and kinetic deuterium isotope effects all point to a mechanism involving irreversible formation of a transient dihydrogen ruthenium complex B, loss of H2 to give unsaturated ruthenium complex A, and trapping by PPh3 to give 6. DFT calculations showed that a mechanism involving direct transfer of a hydrogen from the CpOH group to form B had too high a barrier to be considered. DFT calculations also indicated that an alcohol or the CpOH group of 3 could provide a low energy pathway for formation of B. PGSE NMR measurements established that 3 is a hydrogen-bonded dimer in toluene, and the first-order kinetics indicate that two molecules of 3 are also involved in the transition state for hydrogen transfer to form B, which is the rate-limiting step. In the presence of ethanol, hydrogen loss from 3 is accelerated and RuD/OH exchange occurs 250 times faster than in its absence. Calculations indicate that the transition state for dihydrogen complex formation involves an ethanol bridge between the acidic CpOH and hydridic RuH of 3; the alcohol facilitates proton transfer and accelerates the reversible formation of dihydrogen complex B. In the presence of EtOH, the rate-limiting step shifts to the loss of hydrogen from B

Hydrogen Elimination from a Hydroxycyclopentadienyl Ruthenium(II) Hydride: Study of Hydrogen Activation in a Ligand−Metal Bifunctional Hydrogenation Catalyst

At high temperatures in toluene, [2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(CO)2H (3) undergoes hydrogen elimination in the presence of PPh3 to produce the ruthenium phosphine complex [2,5-Ph2-3,4-Tol2-(η4-C4CO)]Ru(PPh3)(CO)2 (6). In the absence of alcohols, the lack of RuH/OD exchange, a rate law first order in Ru and zero order in phosphine, and kinetic deuterium isotope effects all point to a mechanism involving irreversible formation of a transient dihydrogen ruthenium complex B, loss of H2 to give unsaturated ruthenium complex A, and trapping by PPh3 to give 6. DFT calculations showed that a mechanism involving direct transfer of a hydrogen from the CpOH group to form B had too high a barrier to be considered. DFT calculations also indicated that an alcohol or the CpOH group of 3 could provide a low energy pathway for formation of B. PGSE NMR measurements established that 3 is a hydrogen-bonded dimer in toluene, and the first-order kinetics indicate that two molecules of 3 are also involved in the transition state for hydrogen transfer to form B, which is the rate-limiting step. In the presence of ethanol, hydrogen loss from 3 is accelerated and RuD/OH exchange occurs 250 times faster than in its absence. Calculations indicate that the transition state for dihydrogen complex formation involves an ethanol bridge between the acidic CpOH and hydridic RuH of 3; the alcohol facilitates proton transfer and accelerates the reversible formation of dihydrogen complex B. In the presence of EtOH, the rate-limiting step shifts to the loss of hydrogen from B

Protonated Aminocyclopentadienyl Ruthenium Hydride Reduction of Benzaldehyde and the Conversion of the Resulting Ruthenium Triflate to a Ruthenium Hydride with H₂ and Base

Reaction of N-phenyl-2,5-dimethyl-3,4-diphenylcyclopenta-2,4-dienimine (6) with Ru3CO12 formed two isomers of {[2,5-Me2-3,4-Ph2(η5-C4CNHPh)]Ru(CO)(μ-CO)}2 (8-trans and 8-cis). Photolysis of 8 under a H2 atmosphere led to the formation of the aminocyclopentadienyl ruthenium hydride [2,5-Me2-3,4-Ph2(η5-C4CNHPh)]Ru(CO)2H (9-H). 9-H reduced benzaldehyde slowly at 75 °C to give benzyl alcohol and 8. Protonation of 9-H with triflic acid produced {[2,5-Me2-3,4-Ph2(η5-C4CNH2Ph)]Ru(CO)2H}OTf (11-H), which reacted rapidly with benzaldehyde at −80 °C to give benzyl alcohol and [2,5-Me2-3,4-Ph2(η5-C4CNHPh)]Ru(CO)2OTf (9-OTf). Reaction of 9-OTf with H2 and base led to the re-formation of 9-H. These reactions provide the transformations required for a catalytic cycle for hydrogenation of aldehydes

Protonated Aminocyclopentadienyl Ruthenium Hydride Reduction of Benzaldehyde and the Conversion of the Resulting Ruthenium Triflate to a Ruthenium Hydride with H₂ and Base

Reaction of N-phenyl-2,5-dimethyl-3,4-diphenylcyclopenta-2,4-dienimine (6) with Ru3CO12 formed two isomers of {[2,5-Me2-3,4-Ph2(η5-C4CNHPh)]Ru(CO)(μ-CO)}2 (8-trans and 8-cis). Photolysis of 8 under a H2 atmosphere led to the formation of the aminocyclopentadienyl ruthenium hydride [2,5-Me2-3,4-Ph2(η5-C4CNHPh)]Ru(CO)2H (9-H). 9-H reduced benzaldehyde slowly at 75 °C to give benzyl alcohol and 8. Protonation of 9-H with triflic acid produced {[2,5-Me2-3,4-Ph2(η5-C4CNH2Ph)]Ru(CO)2H}OTf (11-H), which reacted rapidly with benzaldehyde at −80 °C to give benzyl alcohol and [2,5-Me2-3,4-Ph2(η5-C4CNHPh)]Ru(CO)2OTf (9-OTf). Reaction of 9-OTf with H2 and base led to the re-formation of 9-H. These reactions provide the transformations required for a catalytic cycle for hydrogenation of aldehydes

Hydrogen Transfer to Carbonyls and Imines from a Hydroxycyclopentadienyl Ruthenium Hydride: Evidence for Concerted Hydride and Proton Transfer

Reaction of {[2,5-Ph2-3,4-Tol2(η5-C4CO)]2H}Ru2(CO)4(μ-H) (6) with H2 formed [2,5-Ph2-3,4-Tol2(η5-C4COH)Ru(CO)2H] (8), the active species in catalytic carbonyl reductions developed by Shvo. Kinetic studies of the reduction of PhCHO by 8 in THF at −10 °C showed second-order kinetics with ΔH‡ = 12.0 kcal mol-1 and ΔS‡ = −28 eu. The rate of reduction was not accelerated by CF3CO2H, and was not inhibited by CO. Selective deuteration of the RuH and OH positions in 8 gave individual kinetic isotope effects kRuH/kRuD = 1.5 ± 0.2 and kOH/kOD = 2.2 ± 0.1 for PhCHO reduction at 0 °C. Simultaneous deuteration of both positions in 8 gave a combined kinetic isotope effect of kOHRuH/kODRuD = 3.6 ± 0.3. [2,5-Ph2-3,4-Tol2(η5-C4COSiEt3)Ru(CO)2H] (12) and NEt4+[2,5-Ph2-3,4-Tol2(η4-C4CO)Ru(CO)2H]- (13) were unreactive toward PhCHO under conditions where facile PhCHO reduction by 8 occurred. PhCOMe was reduced by 8 30 times slower than PhCHO; MeNCHPh was reduced by 8 26 times faster than PhCHO. Cyclohexene was reduced to cyclohexane by 8 at 80 °C only in the presence of H2. Concerted transfer of a proton from OH and hydride from Ru of 8 to carbonyls and imines is proposed

Hydrogen Transfer to Carbonyls and Imines from a Hydroxycyclopentadienyl Ruthenium Hydride: Evidence for Concerted Hydride and Proton Transfer

Reaction of {[2,5-Ph2-3,4-Tol2(η5-C4CO)]2H}Ru2(CO)4(μ-H) (6) with H2 formed [2,5-Ph2-3,4-Tol2(η5-C4COH)Ru(CO)2H] (8), the active species in catalytic carbonyl reductions developed by Shvo. Kinetic studies of the reduction of PhCHO by 8 in THF at −10 °C showed second-order kinetics with ΔH‡ = 12.0 kcal mol-1 and ΔS‡ = −28 eu. The rate of reduction was not accelerated by CF3CO2H, and was not inhibited by CO. Selective deuteration of the RuH and OH positions in 8 gave individual kinetic isotope effects kRuH/kRuD = 1.5 ± 0.2 and kOH/kOD = 2.2 ± 0.1 for PhCHO reduction at 0 °C. Simultaneous deuteration of both positions in 8 gave a combined kinetic isotope effect of kOHRuH/kODRuD = 3.6 ± 0.3. [2,5-Ph2-3,4-Tol2(η5-C4COSiEt3)Ru(CO)2H] (12) and NEt4+[2,5-Ph2-3,4-Tol2(η4-C4CO)Ru(CO)2H]- (13) were unreactive toward PhCHO under conditions where facile PhCHO reduction by 8 occurred. PhCOMe was reduced by 8 30 times slower than PhCHO; MeNCHPh was reduced by 8 26 times faster than PhCHO. Cyclohexene was reduced to cyclohexane by 8 at 80 °C only in the presence of H2. Concerted transfer of a proton from OH and hydride from Ru of 8 to carbonyls and imines is proposed

Economic and Environmental Trade-Offs of Simultaneous Sugar and Lignin Utilization for Biobased Fuels and Chemicals

Efficient lignin conversion is vital to the production of affordable, low-carbon fuels and chemicals from lignocellulosic biomass. However, lignin conversion remains challenging, and the alternative (combustion) can emit harmful air pollutants. This study explores the economic and environmental trade-offs between lignin combustion and microbial utilization for producing bisabolene as a representative biobased fuel or chemical. Results for switchgrass and clean pine-based biorefineries show that using lignin to increase fuel yields rather than combusting it reduces the capital expenditures for the boiler and turbogenerator if the facilities process more than 1100 bone-dry metric tons (bdt) feedstock/day and 560 bdt/day, respectively. No comparable advantage was observed for lower-lignin sorghum feedstock. Deconstructing lignin to bioavailable intermediates and utilizing those small molecules alongside sugars to boost product yields is economically attractive if the overall lignin-to-product conversion yield exceeds 11–20% by mass. Although lignin-to-fuel/chemical conversion can increase life-cycle greenhouse gas (GHG) emissions, most of the lignin can be diverted to fuel/chemical production while maintaining a >60% life-cycle GHG footprint reduction relative to diesel fuel. The results underscore that lignin utilization can be economically advantageous relative to combustion for higher-lignin feedstocks, but efficient depolymerization and high yields during conversion are both crucial to achieving viability

Computational Prediction and Experimental Validation of Signal Peptide Cleavages in the Extracellular Proteome of a Natural Microbial Community

An integrated computational/experimental approach was used to predict and identify signal peptide cleavages among microbial proteins of environmental biofilm communities growing in acid mine drainage (AMD). SignalP-3.0 was employed to computationally query the AMD protein database of >16,000 proteins, which resulted in 1,480 predicted signal peptide cleaved proteins. LC−MS/MS analyses of extracellular (secretome) microbial preparations from different locations and developmental states empirically confirmed 531 of these signal peptide cleaved proteins. The majority of signal-cleavage proteins (58.4%) are annotated to have unknown functions; however, Pfam domain analysis revealed that many may be involved in extracellular functions expected within the AMD system. Examination of the abundances of signal-cleaved proteins across 28 proteomes from biofilms collected over a 4-year period demonstrated a strong correlation with the developmental state of the biofilm. For example, class I cytochromes are abundant in early growth states, whereas cytochrome oxidases from the same organism increase in abundance later in development. These results likely reflect shifts in metabolism that occur as biofilms thicken and communities diversify. In total, these results provide experimental confirmation of proteins that are designed to function in the extreme acidic extracellular environment and will serve as targets for future biochemical analysis

Metal Affinity Enrichment Increases the Range and Depth of Proteome Identification for Extracellular Microbial Proteins

Many key proteins, such as those involved in cellular signaling or transcription, are difficult to measure in microbial proteomic experiments due to the interfering presence of more abundant, dominant proteins. In an effort to enhance the identification of previously undetected proteins, as well as provide a methodology for selective enrichment, we evaluated and optimized immobilized metal affinity chromatography (IMAC) coupled with mass spectrometric characterization of extracellular proteins from an extremophilic microbial community. Seven different metals were tested for IMAC enrichment. The combined results added ∼20% greater proteomic depth to the extracellular proteome. Although this IMAC enrichment could not be conducted at the physiological pH of the environmental system, this approach did yield a reproducible and specific enrichment of groups of proteins with functions potentially vital to the community, thereby providing a more extensive biochemical characterization. Notably, 40 unknown proteins previously annotated as “hypothetical” were enriched and identified for the first time. Examples of identified proteins includes a predicted TonB signal sensing protein homologous to other known TonB proteins and a protein with a COXG domain previously identified in many chemolithoautotrophic microbes as having a function in the oxidation of CO

Image_1_Microbial Community Structure and Functional Potential Along a Hypersaline Gradient.TIF

<p>Salinity is one of the strongest environmental drivers of microbial evolution and community composition. Here we aimed to determine the impact of salt concentrations (2.5, 7.5, and 33.2%) on the microbial community structure of reclaimed saltern ponds near San Francisco, California, and to discover prospective enzymes with potential biotechnological applications. Community compositions were determined by 16S rRNA amplicon sequencing revealing both higher richness and evenness in the pond sediments compared to the water columns. Co-occurrence network analysis additionally uncovered the presence of microbial seed bank communities, potentially primed to respond to rapid changes in salinity. In addition, functional annotation of shotgun metagenomic DNA showed different capabilities if the microbial communities at different salinities for methanogenesis, amino acid metabolism, and carbohydrate-active enzymes. There was an overall shift with increasing salinity in the functional potential for starch degradation, and a decrease in degradation of cellulose and other oligosaccharides. Further, many carbohydrate-active enzymes identified have acidic isoelectric points that have potential biotechnological applications, including deconstruction of biofuel feedstocks under high ionic conditions. Metagenome-assembled genomes (MAGs) of individual halotolerant and halophilic microbes were binned revealing a variety of carbohydrate-degrading potential of individual pond inhabitants.</p