Thermodynamic Hydricities of Biomimetic Organic Hydride Donors

Abstract

Thermodynamic hydricities (Δ G) in acetonitrile and dimethyl sulfoxide have been calculated and experimentally measured for several metal-free hydride donors: NADH analogs (BNAH, CN-BNAH, Me-MNAH, HEH), methylene tetrahydromethanopterin analogs (BIMH, CAFH), acridine derivatives (Ph-AcrH, MeN-AcrH, T-AcrH, 4OH, 2OH, 3NH), and a triarylmethane derivative (6OH). The calculated hydricity values, obtained using density functional theory, showed a reasonably good match (within 3 kcal/mol) with the experimental values, obtained using "potential p K" and "hydride-transfer" methods. The hydride donor abilities of model compounds were in the 48.7-85.8 kcal/mol (acetonitrile) and 46.9-84.1 kcal/mol (DMSO) range, making them comparable to previously studied first-row transition metal hydride complexes. To evaluate the relevance of entropic contribution to the overall hydricity, Gibbs free energy differences (Δ G) obtained in this work were compared with the enthalpy (Δ H) values obtained by others. The results indicate that, even though Δ H values exhibit the same trends as Δ G, the differences between room-temperature Δ G and Δ H values range from 3 to 9 kcal/mol. This study also reports a new metal-free hydride donor, namely, an acridine-based compound 3NH, whose hydricity exceeds that of NaBH. Collectively, this work gives a perspective of use metal-free hydride catalysts in fuel-forming and other reduction processes.

Kinetics of Hydride Transfer from Metal-Free Hydride Donors to CO2

“…26 We also demonstrated that the benzoimidazole-based hydrides can be efficiently regenerated electrochemically by optimizing the conditions for proton-coupled electron transfer, reporting the first proof-of-principle demonstration toward metal-free and selective CO2 reduction catalysis. 24,27 Despite the successful demonstration of HT to CO2, the kinetics of the transformation were quite slow in our studies, on the timescale of several hours.…”

“…22 Following in nature's footsteps, our group recently developed hydrides that mimic NADPH activity, and that are sufficiently hydridic to perform a direct reduction of CO2. [23][24][25][26][27][28] Our work has shown how the thermodynamic hydricities of metal-free hydrides can be tuned using aromatization, delocalization and hyperconjugation mechanisms and that the benzoimidazole-based hydride C4H ( Figure 1) can efficiently reduce CO2 to formate via the HT mechanism. 26 We also demonstrated that the benzoimidazole-based hydrides can be efficiently regenerated electrochemically by optimizing the conditions for proton-coupled electron transfer, reporting the first proof-of-principle demonstration toward metal-free and selective CO2 reduction catalysis.…”

Utilizing Selective Chlorination to Synthesize New Triangulenium Dyes with Redshifted Absorption and Emission

“…We also expect that this strategy will be useful for tuning of redox potentials important for photocatalysis, organic hydride donors and electrogenerated chemiluminescence applications where these cationic dye systems recently have shown promising results. [8][9][26][27] Finally, the chlorinated systems are obvious starting points for coupling reactions aiming for -system extension. [28][29][30] Experimental section: Synthetic Methods and Materials.…”

Conformational analysis of diols: Role of the linker on the relative orientation of hydroxyl groups

“…Our group is interested in metal‐free motifs capable of catalyzing electrochemical oxidation and reduction reactions . In our studies of the oxygen evolution reaction (OER), catalytic behavior was observed in an electrode‐assisted mechanism, where selected electrodes, such as glassy carbon and platinum, exhibited improved oxygen evolution performance in the presence of iminium or oxonium cations .…”

“…[10] Our group is interested in metal-free motifs capable of catalyzing electrochemical oxidation and reduction reactions. [11][12][13][14][15][16] In our studies of the oxygen evolution reaction (OER), catalytic behavior was observed in an electrodeassisted mechanism, where selected electrodes, such as glassy carbon and platinum, exhibited improved oxygen evolution performance in the presence of iminium or oxonium cations. [14][15][16] These results lead us to hypothesize that a fully homogeneous catalyst, composed of covalently linked organic cations (R + , such as iminium and oxonium ions), can catalyze the water oxidation process via alcohol (R-OH) and peroxide (R-O-O-R) intermediates, by the following sequence: (a) 2R…”