Computational studies on the hydride transfer barrier for the catalytic hydrogenation of CO2 by different Ni(II) complexes

DFT insight into asymmetric alkyl-alkyl bond formation via nickel-catalysed enantioconvergent reductive coupling of racemic electrophiles with olefins

A DFT study has been conducted to understand the asymmetric alkyl–alkyl bond formation through nickel-catalysed reductive coupling of racemic alkyl bromide with olefin in the presence of hydrosilane and K₃PO₄. The key findings of the study include: (i) under the reductive experimental conditions, the Ni(ii) precursor is easily activated/reduced to Ni(0) species which can serve as an active species to start a Ni(0)/Ni(ii) catalytic cycle. (ii) Alternatively, the reaction may proceed via a Ni(i)/Ni(ii)/Ni(iii) catalytic cycle starting with a Ni(i) species such as Ni(i)–Br. The generation of a Ni(i) active species via comproportionation of Ni(ii) and Ni(0) species is highly unlikely, because the necessary Ni(0) species is strongly stabilized by olefin. Alternatively, a cage effect enabled generation of a Ni(i) active catalyst from the Ni(ii) species involved in the Ni(0)/Ni(ii) cycle was proposed to be a viable mechanism. (iii) In both catalytic cycles, K₃PO₄ greatly facilitates the hydrosilane hydride transfer for reducing olefin to an alkyl coupling partner. The reduction proceeds by converting a Ni–Br bond to a Ni–H bond via hydrosilane hydride transfer to a Ni–alkyl bond via olefin insertion. On the basis of two catalytic cycles, the origins for enantioconvergence and enantioselectivity control were discussed.

The enantioconvergent alkyl–alkyl coupling involves two competitive catalytic cycles with nickel(0) and nickel(i) active catalysts, respectively. K₃PO₄ plays a crucial role to enable the hydride transfer from hydrosilane to nickel–bromine species.

Towards H2O catalyzed N2-fixation over TiO2 doped Run clusters (n = 5, 6): a mechanistic and kinetic approach

H2O driven N2 fixation is known as the best alternative pathway to synthesise NH3 under ambient conditions. The thermodynamic non-spontaneous reaction can be accomplished by a photocatalytic water splitting reaction over a TiO2 supported surface with oxygen vacancies. Previous experiments have also shown N2 activation over a neutral Ru cluster whose catalytic activity was remarkably enhanced by TiO2 doping. In this article, we have investigated the detailed mechanism and kinetics of the H2O catalyzed nitrogen reduction reaction (NRR) over bare and TiO2 doped Ru5 clusters in conjunction with DFT and TST calculations. The lack of photochemical activity of the small model cluster provoked us to explore an alternative route of NH3 formation via H2O catalysis. For this, we have considered H2 as co-reactant. The partial reduction of N2 into NH3 or N2H4 could be achieved by a H2O oxidation reaction, however, catalytic regeneration requires additional H2 which effectively makes the overall reaction catalyzed by H2O. Above all, the present investigation suggests that NH3 is most favorably produced through the distal mechanism. Analysis of the rate constants demonstrates that the doping with TiO2 accelerates the kinetics of NRR by a few orders of magnitude. Furthermore, an increase of the size of the metal cluster would not significantly enhance the overall performance of NRR.

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.

Hydrogenation and hydration of carbon dioxide: a detailed characterization of the reaction mechanisms based on the reaction force and reaction electronic flux analyses

A computational DFT study of the reaction mechanism of hydrogenation and hydration of carbon dioxide is presented. It has been found that hydrogenation and hydration are endoenergetic reactions that are carried out in two steps, passing by a stable intermediate that is surrounded by energy barriers of 70 kcal/mol and 10 kcal/mol for hydrogenation and 50 kcal/mol and 10 kcal/mol for hydration. Using the reaction force analysis, we were able to characterize the physical nature of the activation barriers and found that activation energies are mostly due to structural rearrangements. An interesting difference in the reaction mechanisms disclosed by the reaction force and electronic flux analyses is that while in the hydrogenation reaction the mechanisms is conditioned by the H₂ cleavage with a high energy barrier, in the hydration reaction the formation of a transient four member ring structure favoring an attractive local hydrogen bond interaction pushes the reaction toward the product with a considerably lower energy barrier.

Unambiguous hydrogenation of CO2 by coinage-metal hydride anions: an intuitive idea based on in silico experiments

Coinage metal hydride anions, especially AgH⁻, can effectively and deterministically hydrogenate CO₂ to HCO₂⁻.