Single B-vacancy enriched α1-borophene sheet: an efficient metal-free electrocatalyst for CO2 reduction

By employing first principles calculations, we have studied the electronic structures of pristine (α1) and different defective (α1-t1, α1-t2) borophene sheets to understand the efficacy of such systems as metal-free electrocatalysts for the CO2 reduction reaction. Among the three studied systems, only α1-t1, the defective borophene sheet created by removal of a 5-coordinated boron atom, can chemisorb and activate a CO2 molecule for its subsequent reduction processes, leading to different C1 chemicals, followed by selective conversion into C2 products by multiple proton coupled electron transfer steps. The computed onset potentials for the C1 chemicals such as CH3OH and CH4 are low enough. On the other hand, in the case of the C2 reduction process, the C-C coupling barrier is only 0.80 eV in the solvent phase which produces CH3CHO and CH3CH2OH with very low onset potential values of -0.21 and -0.24 V, respectively, suppressing the competing hydrogen evolution reaction.

Transforming CO2 into Methanol with N-Heterocyclic Carbene-Stabilized Coinage Metal Hydrides Immobilized in a Metal-Organic Framework UiO-68

By synergizing the advantages of homogeneous and heterogeneous catalysis, single-site heterogeneous catalysis represents a highly promising opportunity for many catalytic processes. Particularly, the unprecedented designability and versatility of metal-organic frameworks (MOFs) promote them as salient platforms for designing single-site catalytic materials by introducing isolated, well-defined active sites into the frameworks. Herein, we design new MOF-supported single-site catalysts for CO₂ hydrogenation to methanol (CH₃OH), a reaction of great significance in CO₂ valorization. Specifically, N-heterocyclic carbene (NHC), a class of excellent modifiers and anchors, is used to anchor coinage metal hydrides M(I)-H (M = Cu, Ag, and Au) onto the organic linker of UiO-68. The strong metal-ligand interactions between NHC and M(I)-H verify the robustness and feasibility of our design strategy. On the tailor-made catalysts, a three-stage sequential transformation is proposed for CH₃OH synthesis with HCOOH and HCHO as the transit intermediates. A density functional theory-based comparative study suggests that UiO-68 decorated with NHC-Cu(I)-H performs best for CO₂ hydrogenation to HCOOH. This is further rationalized by three linear relationships for the Gibbs energy barrier of CO₂ hydrogenation to HCOO intermediate, the first with the NBO charge of the hydride in NHC-M(I)-H, the second with the electronegativity of M, and the third with the gap between the lowest unoccupied molecular orbital of CO₂ and the highest occupied molecular orbital of the catalyst. It is confirmed that the high efficiency of MOF-supported NHC-Cu(I)-H for CO₂ transformation to CH₃OH is via the proposed three-stage mechanism, and in each stage, the step involving heterolytic dissociation of H₂ together with product generation is the most energy-intensive. The rate-limiting step in the entire mechanism is identified to be H₂ dissociation accompanying with simultaneous HCHO and H₂O formation. Altogether, the tailor-made UiO-68 decorated with NHC-Cu(I)-H features well-defined active sites, enables precise manipulation of reaction paths, and demonstrates excellent reactivity for CO₂ hydrogenation to CH₃OH. It is also predicted to surpass a recently reported MOF-808 catalyst consisting of neighboring Zn²⁺-O-Zr⁴⁺ sites. The designed MOFs as well as the proposed strategy here establish a new paradigm and can be extended to other hydrogenation reactions.

Reactivity of Coinage Metal Hydrides for the Production of H2 Molecules

The formation of molecular hydrogen as well as the possibility of using coinage metal hydrides as a prospective complex to produce hydrogen was presented in this work. Therefore, the reactions involving the interaction between two coinage metal hydrides, MH (M=Cu, Ag and Au, homo and heterodimers), were studied. The free energy profiles corresponding to aforementioned complexation were analysed by means of ab initio methods of quantum chemistry. The characteristics of these intermediates, final complexes and the electron density properties of the established interactions were discussed.

Activation of CO2 by Alkaline-Earth Metal Hydrides: Matrix Infrared Spectra and DFT Calculations of HM(O2CH) and (MH2)(HCOOH) Complexes (M = Sr, Ba)

Reactions of MH₂ (M = Sr, Ba) with CO₂ were explored in pure parahydrogen at 3.5 K using matrix isolation infrared spectroscopy and quantum chemical calculations. The formate complex HM(η²-O₂CH) and formic acid complex (MH₂)(HCOOH) were trapped and identified by isotopic substitutions and density functional theory (DFT) frequency calculations. Natural population analysis and the CO₂ reduction mechanism demonstrate that hydride ion transfer from a metal hydride to a CO₂ moiety facilitates the stabilization of such complexes.

CO2 activation by ligand-free manganese hydrides in a parahydrogen matrix

The reaction of MnH₂ with CO₂ gave insertion product HMn(η²-O₂CH) by concerted hydride ion transfer.

Rivalry between Regium and Hydrogen Bonds Established within Diatomic Coinage Molecules and Lewis Acids/Bases

A theoretical study of the complexes formed by Ag₂ and Cu₂ with different molecules, XH (FH, ClH, OH₂ , SH₂ , HCN, HNC, HCCH, NH₃ and PH₃ ) that can act as hydrogen-bond donors (Lewis acids) or regium-bond acceptors (Lewis bases) was carried out at the CCSD(T)/CBS computational level. The heteronuclear diatomic coinage molecules (AuAg, AuCu, and AgCu) have also been considered. With the exception of some of the hydrogen-bonded complexes with FH, the regium-bonded binary complexes are more stable. The AuAg and AuCu molecules show large dipole moments that weaken the regium bond (RB) with Au and favour those through the Ag and Cu atoms, respectively.

UV/Vis Spectroscopy of Copper Formate Clusters: Insight into Metal-Ligand Photochemistry

The electronic structure and photochemistry of copper formate clusters, Cu^I₂(HCO₂)₃⁻ and Cu^II_n(HCO₂)_2n+1⁻, n≤8, are investigated in the gas phase by using UV/Vis spectroscopy in combination with quantum chemical calculations. A clear difference in the spectra of clusters with Cu^I and Cu^II copper ions is observed. For the Cu^I species, transitions between copper d and s/p orbitals are recorded. For stoichiometric Cu^II formate clusters, the spectra are dominated by copper d–d transitions and charge‐transfer excitations from formate to the vacant copper d orbital. Calculations reveal the existence of several energetically low‐lying isomers, and the energetic position of the electronic transitions depends strongly on the specific isomer. The oxidation state of the copper centers governs the photochemistry. In Cu^II(HCO₂)₃⁻, fast internal conversion into the electronic ground state is observed, leading to statistical dissociation; for charge‐transfer excitations, specific excited‐state reaction channels are observed in addition, such as formyloxyl radical loss. In Cu^I₂(HCO₂)₃⁻, the system relaxes to a local minimum on an excited‐state potential‐energy surface and might undergo fluorescence or reach a conical intersection to the ground state; in both cases, this provides substantial energy for statistical decomposition. Alternatively, a Cu^II(HCO₂)₃Cu⁰⁻ biradical structure is formed in the excited state, which gives rise to the photochemical loss of a neutral copper atom.

Mechanistic insights into the non-bifunctional hydrogenation of esters by Co(ii) pincer complexes: a DFT study

A recent experiment has revealed that additive free ester hydrogenation by Co-pincer complexes might follow an unusual non-bifunctional mechanism, however, the detailed mechanistic pathway is missing. It has been predicted that several intermediates and transition states are involved, having their essential role in the catalytic performances. Detailed theoretical studies are therefore essential in this regard for achieving more efficient ester hydrogenation catalysts. On the basis of first principles calculations, performed over Co(PNP)/(PNN) complexes, we present here the energetics and mechanistic details, showing the distinct orientations of different possible intermediates and transition states, and find the minimum energy pathway for the conversion of esters to alcohols. In the way, we find that some intermediates must undergo structural distortion for achieving the lowest potential energy barrier which must have a severe impact on the catalytic turnover frequency.