Coupling Solid-State NMR with GIPAW ab Initio Calculations in Metal Hydrides and Borohydrides

An integrated experimental–theoretical approach for the solid-state NMR investigation of a series of hydrogen-storage materials is illustrated. Seven experimental room-temperature structures of groups I and II metal hydrides and borohydrides, namely, NaH, LiH, NaBH₄, MgH₂, CaH₂, Ca­(BH₄)₂, and LiBH₄, were computationally optimized. Periodic lattice calculations were performed by means of the plane-wave method adopting the density functional theory (DFT) generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional as implemented in the Quantum ESPRESSO package. Projector augmented wave (PAW), including the gauge-including projected augmented-wave (GIPAW), methods for solid-state NMR calculations were used adopting both Rappe–Rabe–Kaxiras–Joannopoulos (RRKJ) ultrasoft pseudopotentials and new developed pseudopotentials. Computed GIPAW chemical shifts were critically compared with the experimental ones. A good agreement between experimental and computed multinuclear chemical shifts was obtained

A Unified Electro- and Photocatalytic CO₂ to CO Reduction Mechanism with Aminopyridine Cobalt Complexes

A mechanistic understanding of electro- and photocatalytic CO2 reduction is crucial to develop strategies to overcome catalytic bottlenecks. In this regard, for a new CO2-to-CO reduction cobalt aminopyridine catalyst, a detailed experimental and theoretical mechanistic study is herein presented toward the identification of bottlenecks and potential strategies to alleviate them. The combination of electrochemistry and in situ spectroelectrochemistry together with spectroscopic techniques led us to identify elusive key electrocatalytic intermediates derived from complex [LN4Co­(OTf)2] (1) (LN4 = 1-[2-pyridylmethyl]-4,7-dimethyl-1,4,7-triazacyclononane) such as a highly reactive cobalt­(I) (1(I)) and a cobalt­(I) carbonyl (1(I)-CO) species. The combination of spectroelectrochemical studies under CO2, 13CO2, and CO with DFT disclosed that 1(I) reacts with CO2 to form the pivotal 1(I)-CO intermediate at the 1(II/I) redox potential. However, at this reduction potential, the formation of 1(I)-CO restricts the electrocatalysis due to the endergonicity of the CO release step. In agreement with the experimentally observed CO2-to-CO electrocatalysis at the CoI/0 redox potential, computational studies suggested that the electrocatalytic cycle involves striking metal carbonyls. In contrast, under photochemical conditions, the catalysis smoothly proceeds at the 1(II/I) redox potential. Under the latter conditions, it is proposed that the electron transfer to form 1(I)-CO from 1(II)-CO is under diffusion control. Then, the CO release from 1(II)-CO is kinetically favored, facilitating the catalysis. Finally, we have found that visible-light irradiation has a positive impact under electrocatalytic conditions. We envision that light irradiation can serve as an effective strategy to circumvent the CO poisoning and improve the performance of CO2 reduction molecular catalysts

A Unified Electro- and Photocatalytic CO₂ to CO Reduction Mechanism with Aminopyridine Cobalt Complexes

A mechanistic understanding of electro- and photocatalytic CO2 reduction is crucial to develop strategies to overcome catalytic bottlenecks. In this regard, for a new CO2-to-CO reduction cobalt aminopyridine catalyst, a detailed experimental and theoretical mechanistic study is herein presented toward the identification of bottlenecks and potential strategies to alleviate them. The combination of electrochemistry and in situ spectroelectrochemistry together with spectroscopic techniques led us to identify elusive key electrocatalytic intermediates derived from complex [LN4Co­(OTf)2] (1) (LN4 = 1-[2-pyridylmethyl]-4,7-dimethyl-1,4,7-triazacyclononane) such as a highly reactive cobalt­(I) (1(I)) and a cobalt­(I) carbonyl (1(I)-CO) species. The combination of spectroelectrochemical studies under CO2, 13CO2, and CO with DFT disclosed that 1(I) reacts with CO2 to form the pivotal 1(I)-CO intermediate at the 1(II/I) redox potential. However, at this reduction potential, the formation of 1(I)-CO restricts the electrocatalysis due to the endergonicity of the CO release step. In agreement with the experimentally observed CO2-to-CO electrocatalysis at the CoI/0 redox potential, computational studies suggested that the electrocatalytic cycle involves striking metal carbonyls. In contrast, under photochemical conditions, the catalysis smoothly proceeds at the 1(II/I) redox potential. Under the latter conditions, it is proposed that the electron transfer to form 1(I)-CO from 1(II)-CO is under diffusion control. Then, the CO release from 1(II)-CO is kinetically favored, facilitating the catalysis. Finally, we have found that visible-light irradiation has a positive impact under electrocatalytic conditions. We envision that light irradiation can serve as an effective strategy to circumvent the CO poisoning and improve the performance of CO2 reduction molecular catalysts

Decoding the CO₂ Reduction Mechanism of a Highly Active Organometallic Manganese Electrocatalyst: Direct Observation of a Hydride Intermediate and Its Implications

A detailed mechanistic study of the electrochemical CO2 reduction catalyzed by the fac-[MnI(CO)3(bis-MeNHC)MeCN]+ complex (1-MeCN+) is reported herein by combining in situ FTIR spectroelectrochemistry (SEC), synthesis and characterization of catalytic intermediates, and DFT calculations. Under low proton concentrations, 1-MeCN+ efficiently catalyzes CO2 electroreduction with long catalyst durability and selectivity toward CO (ca. 100%). The [Mn‑I(CO)3(bis-MeNHC)]− anion (1–) and the tetracarbonyl [MnI(CO)4(bis-MeNHC)]+ complex (1-CO+) are key intermediates of the catalytic CO2-to-CO mechanism due to their impact on the selectivity and the reaction rate, respectively. Increasing the proton concentration increases formate production (up to 15% FE), although CO remains the major product. The origin of formate is ascribed to the competitive protonation of 1– to form a Mn(I) hydride (1-H), detected by SEC in the absence of CO2. 1-H was also synthesized and thoroughly characterized, including by X-ray diffraction analysis. Stoichiometric reactivity studies of 1-H with CO2 and labeled 13CO2 indicate a fast formation of the corresponding neutral Mn(I) formate species (1-OCOH) at room temperature. DFT modeling confirms the intrinsic capability of 1-H to undergo hydride transfer to CO2 due to the strong σ-donor properties of the bis-MeNHC moiety. However, the large potential required for the HCOO– release from 1-OCOH limits the overall catalytic CO2-to-HCOO– cycle. Moreover, the experimentally observed preferential selectivity for CO over formate is dictated by the shallow kinetic barrier for CO2 binding to 1– compared to the Mn–H bond formation. The detailed mechanistic study highlights the reduction potential, pKa, and hydricity of the metal hydride intermediate as crucial factors affecting the CO2RR selectivity in molecular systems

Decoding the CO₂ Reduction Mechanism of a Highly Active Organometallic Manganese Electrocatalyst: Direct Observation of a Hydride Intermediate and Its Implications

A detailed mechanistic study of the electrochemical CO2 reduction catalyzed by the fac-[MnI(CO)3(bis-MeNHC)MeCN]+ complex (1-MeCN+) is reported herein by combining in situ FTIR spectroelectrochemistry (SEC), synthesis and characterization of catalytic intermediates, and DFT calculations. Under low proton concentrations, 1-MeCN+ efficiently catalyzes CO2 electroreduction with long catalyst durability and selectivity toward CO (ca. 100%). The [Mn‑I(CO)3(bis-MeNHC)]− anion (1–) and the tetracarbonyl [MnI(CO)4(bis-MeNHC)]+ complex (1-CO+) are key intermediates of the catalytic CO2-to-CO mechanism due to their impact on the selectivity and the reaction rate, respectively. Increasing the proton concentration increases formate production (up to 15% FE), although CO remains the major product. The origin of formate is ascribed to the competitive protonation of 1– to form a Mn(I) hydride (1-H), detected by SEC in the absence of CO2. 1-H was also synthesized and thoroughly characterized, including by X-ray diffraction analysis. Stoichiometric reactivity studies of 1-H with CO2 and labeled 13CO2 indicate a fast formation of the corresponding neutral Mn(I) formate species (1-OCOH) at room temperature. DFT modeling confirms the intrinsic capability of 1-H to undergo hydride transfer to CO2 due to the strong σ-donor properties of the bis-MeNHC moiety. However, the large potential required for the HCOO– release from 1-OCOH limits the overall catalytic CO2-to-HCOO– cycle. Moreover, the experimentally observed preferential selectivity for CO over formate is dictated by the shallow kinetic barrier for CO2 binding to 1– compared to the Mn–H bond formation. The detailed mechanistic study highlights the reduction potential, pKa, and hydricity of the metal hydride intermediate as crucial factors affecting the CO2RR selectivity in molecular systems

Electro- and Photoinduced Interfacial Charge Transfers in Nanocrystalline Mesoporous TiO₂ and TiO₂/Iron Porphyrin Sensitized Films under CO₂ Reduction Catalysis

Electro- and photochemical CO2 reduction (CO2R) is the quintessence of modern-day sustainable research. We report our studies on the electro- and photoinduced interfacial charge transfer occurring in a nanocrystalline mesoporous TiO2 film and two TiO2/iron porphyrin hybrid films (meso-aryl- and β-pyrrole-substituted porphyrins, respectively) under CO2R conditions. We used transient absorption spectroscopy (TAS) to demonstrate that, under 355 nm laser excitation and an applied voltage bias (0 to −0.8 V vs Ag/AgCl), the TiO2 film exhibited a diminution in the transient absorption (at −0.5 V by 35%), as well as a reduction of the lifetime of the photogenerated electrons (at −0.5 V by 50%) when the experiments were conducted under a CO2 atmosphere changing from inert N2. The TiO2/iron porphyrin films showed faster charge recombination kinetics, featuring 100-fold faster transient signal decays than that of the TiO2 film. The electro-, photo-, and photoelectrochemical CO2R performance of the TiO2 and TiO2/iron porphyrin films are evaluated within the bias range of −0.5 to −1.8 V vs Ag/AgCl. The bare TiO2 film produced CO and CH4 as well as H2, depending on the applied voltage bias. In contrast, the TiO2/iron porphyrin films showed the exclusive formation of CO (100% selectivity) under identical conditions. During the CO2R, a gain in the overpotential values is obtained under light irradiation conditions. This finding was indicative of a direct transfer of the photogenerated electrons from the film to absorbed CO2 molecules and an observed decrease in the decay of the TAS signals. In the TiO2/iron porphyrin films, we identified the interfacial charge recombination processes between the oxidized iron porphyrin and the electrons of the TiO2 conduction band. These competitive processes are considered to be responsible for the diminution of direct charge transfer between the film and the adsorbed CO2 molecules, explaining the moderate performances of the hybrid films for the CO2R

Decoding the CO₂ Reduction Mechanism of a Highly Active Organometallic Manganese Electrocatalyst: Direct Observation of a Hydride Intermediate and Its Implications

A detailed mechanistic study of the electrochemical CO2 reduction catalyzed by the fac-[MnI(CO)3(bis-MeNHC)MeCN]+ complex (1-MeCN+) is reported herein by combining in situ FTIR spectroelectrochemistry (SEC), synthesis and characterization of catalytic intermediates, and DFT calculations. Under low proton concentrations, 1-MeCN+ efficiently catalyzes CO2 electroreduction with long catalyst durability and selectivity toward CO (ca. 100%). The [Mn‑I(CO)3(bis-MeNHC)]− anion (1–) and the tetracarbonyl [MnI(CO)4(bis-MeNHC)]+ complex (1-CO+) are key intermediates of the catalytic CO2-to-CO mechanism due to their impact on the selectivity and the reaction rate, respectively. Increasing the proton concentration increases formate production (up to 15% FE), although CO remains the major product. The origin of formate is ascribed to the competitive protonation of 1– to form a Mn(I) hydride (1-H), detected by SEC in the absence of CO2. 1-H was also synthesized and thoroughly characterized, including by X-ray diffraction analysis. Stoichiometric reactivity studies of 1-H with CO2 and labeled 13CO2 indicate a fast formation of the corresponding neutral Mn(I) formate species (1-OCOH) at room temperature. DFT modeling confirms the intrinsic capability of 1-H to undergo hydride transfer to CO2 due to the strong σ-donor properties of the bis-MeNHC moiety. However, the large potential required for the HCOO– release from 1-OCOH limits the overall catalytic CO2-to-HCOO– cycle. Moreover, the experimentally observed preferential selectivity for CO over formate is dictated by the shallow kinetic barrier for CO2 binding to 1– compared to the Mn–H bond formation. The detailed mechanistic study highlights the reduction potential, pKa, and hydricity of the metal hydride intermediate as crucial factors affecting the CO2RR selectivity in molecular systems

Decoding the CO₂ Reduction Mechanism of a Highly Active Organometallic Manganese Electrocatalyst: Direct Observation of a Hydride Intermediate and Its Implications

A detailed mechanistic study of the electrochemical CO2 reduction catalyzed by the fac-[MnI(CO)3(bis-MeNHC)MeCN]+ complex (1-MeCN+) is reported herein by combining in situ FTIR spectroelectrochemistry (SEC), synthesis and characterization of catalytic intermediates, and DFT calculations. Under low proton concentrations, 1-MeCN+ efficiently catalyzes CO2 electroreduction with long catalyst durability and selectivity toward CO (ca. 100%). The [Mn‑I(CO)3(bis-MeNHC)]− anion (1–) and the tetracarbonyl [MnI(CO)4(bis-MeNHC)]+ complex (1-CO+) are key intermediates of the catalytic CO2-to-CO mechanism due to their impact on the selectivity and the reaction rate, respectively. Increasing the proton concentration increases formate production (up to 15% FE), although CO remains the major product. The origin of formate is ascribed to the competitive protonation of 1– to form a Mn(I) hydride (1-H), detected by SEC in the absence of CO2. 1-H was also synthesized and thoroughly characterized, including by X-ray diffraction analysis. Stoichiometric reactivity studies of 1-H with CO2 and labeled 13CO2 indicate a fast formation of the corresponding neutral Mn(I) formate species (1-OCOH) at room temperature. DFT modeling confirms the intrinsic capability of 1-H to undergo hydride transfer to CO2 due to the strong σ-donor properties of the bis-MeNHC moiety. However, the large potential required for the HCOO– release from 1-OCOH limits the overall catalytic CO2-to-HCOO– cycle. Moreover, the experimentally observed preferential selectivity for CO over formate is dictated by the shallow kinetic barrier for CO2 binding to 1– compared to the Mn–H bond formation. The detailed mechanistic study highlights the reduction potential, pKa, and hydricity of the metal hydride intermediate as crucial factors affecting the CO2RR selectivity in molecular systems

Toward the Understanding of the Structure–Activity Correlation in Single-Site Mn Covalent Organic Frameworks for Electrocatalytic CO₂ Reduction

The encapsulation of organometallic complexes into reticular covalent organic frameworks (COFs) represents an effective strategy for the immobilization of molecular electrocatalysts. In particular, well-defined polypyridyl Mn sites embedded into a crystalline COF backbone (COFbpyMn) were found to exhibit higher selectivity and activity toward electrochemical CO2 reduction compared to the parent molecular derivative noncovalently immobilized on carbon electrodes. In situ mechanistic studies revealed that the electronic and steric features of the reticular framework strongly affect the redox mechanism of the Mn sites, stabilizing the formation of a mononuclear Mn(I) radical anion intermediate over the most common off-cycle Mn0–Mn0 dimer. Herein, we report the study of a Mn-based COF (COFPTMn), introducing a larger phenanthroline building block, to explore how tuning the structural and electronic properties of the lattice may affect the catalytic CO2 reduction performance and the mechanism at the molecular level of the reticular system. The Mn sites encapsulated into the reticular COFPTMn exhibited a remarkable enhancement in the intrinsic catalytic CO2 reduction activity at near-neutral pH compared to that of the corresponding noncovalently immobilized molecular derivative. On the other hand, the poor crystallinity and porosity of COFPTMn, likely introduced by the lattice expansion and spatial dynamics of the phenanthroline linker, were found to limit its catalytic performances compared to those of the bipyridyl COFbpyMn analogue. ATR-IR spectroelectrochemistry revealed that the higher spatial mobility of the Mn sites does not completely suppress the Mn0–Mn0 dimerization upon the electrochemical reduction of the Mn sites at the COFbpyMn. This work highlights the positive role of the reticular structure of the material in enhancing its catalytic activity versus that of its molecular counterpart and provides useful hints for the future design and development of efficient reticular frameworks for electrocatalytic applications