Designing Biomimetic NADH-Analogs for Artificial Photosynthesis

Over the past few decades, numerous homogenous organometallic catalysts have been studied for electrocatalytic CO2RR. While superior in their activity, they often suffer from low selectivity and durability. Metal-free hydrides are emerging as promising cost- effective substitutes for selective CO2 reduction, where the selectivity is achieved via hydride transfers. Initial studies on metal-free CO2 reduction focused on B-H and Si-H based hydrides, but despite being successful in conversion to the formate-stage and beyond, the oxophilic nature and consequent formation of Si-O and B-O bonds limit them to the application as stoichiometric reagents. This work focuses on less oxophilic reagents, carbon-based hydride donors – NADH analogs, as potential catalysts in CO2 reduction. NADH analogs are designed to be strong enough hydride donors, capable of performing a hydride transfer reaction to CO2. The structural design is governed by the concept of thermodynamic hydricity, which is obtained using DFT calculations and then tested using experimental methods. Apart from the hydride donor ability, an important requirement for any type of catalyst is to have a relatively easy regeneration of the active form. To test this, we employ electrochemical methods (such as cyclic voltammetry and preparative electrolysis) often coupled with spectroscopic techniques to gain insights on regeneration pathways. The principles obtained here give a perspective of utilizing metal- free motifs in electrocatalytic selective CO2 reduction

Thermodynamic Hydricities of Biomimetic Organic Hydride Donors

Thermodynamic hydricities (Δ<i>G</i>_H^–) 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, Me₂N-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<i>K</i>_a” 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<i>G</i>_H^–) obtained in this work were compared with the enthalpy (Δ<i>H</i>_H^–) values obtained by others. The results indicate that, even though Δ<i>H</i>_H^– values exhibit the same trends as Δ<i>G</i>_H^–, the differences between room-temperature Δ<i>G</i>_H^– and Δ<i>H</i>_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 NaBH₄. Collectively, this work gives a perspective of use metal-free hydride catalysts in fuel-forming and other reduction processes

Benzimidazoles as Metal-Free and Recyclable Hydrides for CO2 Reduction to Formate

We report a novel metal-free chemical reduction of CO2 by a recyclable benzimidazole-based organo-hydride, whose choice was guided by quantum chemical calculations. Notably, benzimidazole-based hydride donors rival the hydride-donating abilities of noble metal-based hydrides such as [Ru(tpy)(bpy)H]+ and [Pt(depe)2H]+. Chemical CO2 reduction to the formate anion (HCOO) was carried out in the absence of biological enzymes, a sacrificial Lewis acid, or a base to activate the substrate or reductant. 13CO2 experiments confirmed the formation of H13COO by CO2 reduction with the formate product characterized by 1H-NMR and 13C-NMR spectroscopies, and ESI-MS. The highest formate yield of 66% was obtained in the presence of potassium tetrafluoroborate under mild conditions. The likely role of exogenous salt additives in this reaction is to stabilize and shift the equilibrium towards the ionic products. After CO2 reduction, the benzimidazole-based hydride donor was quantitatively oxidized to its aromatic benzimidazolium cation, establishing its recyclability. In addition, we electrochemically reduced the benzimidazolium cation to its organo-hydride form in quantitative yield, demonstrating its potential for electrocatalytic CO2 reduction. These results serve as a proof of concept for the electrocatalytic reduction of CO2 by sustainable, recyclable and metal-free organo-hydrides

Role of Surface-Grafted Polymers on Mechanical Reinforcement of Metal–Organic Framework–Polymer Composites

Utilizing metal–organic frameworks (MOFs) as reinforcing fillers for polymer composites is a promising strategy because of the low density, high specific modulus, and tunable aspect ratio (AR). However, it has not been demonstrated for the MOF-reinforced polymer composite using MOFs with high AR and polymer-grafted surface, both of which are extremely important factors for efficient load transfer and favorable particle–matrix interaction. To this end, we designed an MOF–polymer composite system using high AR MOF PCN-222 as the mechanical reinforcer. Moreover, we developed a synthetic route to graft poly(methyl methacrylate) (PMMA) from the surface of PCN-222 through surface-initiated atomic transfer radical polymerization (SI-ATRP). The successful growth of PMMA on the surface of PCN-222 was confirmed via proton nuclear magnetic resonance and infrared spectroscopy. Through thermogravimetric analysis, the grafting density was found to be 0.18 chains/nm2. The grafted polymer molecular weight was controlled ranging from 50.3 to 158 kDa as suggested by size exclusion chromatography. Finally, we fabricated MOF–polymer composite films by the doctor-blading technique and measured the mechanical properties through the tension mode of dynamic mechanical analysis. We found that the mechanical properties of the composites were improved with increasing grafted PMMA molecular weight. The maximum reinforcement, a 114% increase in Young’s modulus at 0.5 wt % MOF loading in comparison to pristine PMMA films, was achieved when the grafted molecular weight was higher than the matrix molecular weight, which was in good agreement with previous literature. Moreover, our composite presents the highest reinforcement measured via Young’s modulus at low weight loading among MOF-reinforced polymer composites due to the high MOF AR and enhanced interface. Our approach offers great potential for lightweight mechanical reinforcement with high AR MOFs and a generalizable grafting-from strategy for porphyrin-based MOFs