Recent developments in first-row transition metal complex-catalyzed CO2 hydrogenation

The effect of Cu(I)-doping on the photoinduced electron transfer from aqueous CdS quantum dots

The doping of CdS quantum dots (QDs) with Cu(I) disrupts electron-hole correlation due to hole trapping by the dopant ion, post-photoexcitation. The present paper examines the effect of such disruption on the rate of photoinduced electron transfer (PET) from the QDs to methyl viologen (MV2+), with implications in their photocatalytic activity. A significantly greater efficiency of PL quenching by MV2+ is observed for the doped QDs than for the undoped ones. Interestingly, the Stern-Volmer plots constructed using PL intensities exhibit an upward curvature for both the cases, while the PL lifetimes remain unaffected. This observation is rationalized by considering the adsorption of the quencher on the surface of the QDs and ultrafast PET post-photoexcitation. Ultrafast transient absorption experiments confirm a faster electron transfer for the doped QDs. It is also realized that the transient absorption experiment yields a more accurate estimate of the binding constant of the quencher with the QDs, than the PL experiment.

Cyclopentadienone Diphosphine Ruthenium Complex: A Designed Catalyst for the Hydrogenation of Carbon Dioxide to Methanol

The development of homogeneous metal catalysts for the efficient hydrogenation of carbon dioxide (CO2) into methanol (CH3OH) remains a significant challenge. In this study, a new cyclopentadienone diphosphine ligand (CPDDP ligand) was designed, which could coordinate with ruthenium to form a Ru-CPDDP complex to efficiently catalyze the CO2-to-methanol process using dihydrogen (H2) as the hydrogen resource based on density functional theory (DFT) mechanistic investigation. This process consists of three catalytic cycles, stage I (the hydrogenation of CO2 to HCOOH), stage II (the hydrogenation of HCOOH to HCHO), and stage III (the hydrogenation of HCHO to CH3OH). The calculated free energy barriers for the hydrogen transfer (HT) steps of stage I, stage II, and stage III are 7.5, 14.5, and 3.5 kcal/mol, respectively. The most favorable pathway of the dihydrogen activation (DA) steps of three stages to regenerate catalytic species is proposed to be the formate-assisted DA step with a free energy barrier of 10.4 kcal/mol. The calculated results indicate that the designed Ru-CPDDP and Ru-CPDDPEt complexes could catalyze hydrogenation of CO2 to CH3OH (HCM) under mild conditions and that the transition-metal owning designed CPDDP ligand framework be one kind of promising potential efficient catalysts for HCM.

Activated Mn-MACHO Complexes Form Stable CO2 Adducts

Manganese(I) carbonyl complexes bearing a MACHO-type ligand (HN(CH2 CH2 PR2 )2 ) readily react in their amido form with CO2 to generate 4-membered {Mn-N-C-O} metallacycles. The stability of the adducts decreases with the steric demand of the R groups at phosphorous (R=isopropyl>adamantyl). The CO2 -adducts display generally a lower reactivity as compared to the parent amido complexes. These adducts can thus be interpretated as masked forms of the active amido catalysts and potentially play important roles as off-loop species or branching points in catalytic transformations of carbon dioxide.

Machine Learning-Enabled Predictions of Condensed Fukui Functions and Designing of Metal Pincer Complexes for Catalytic Hydrogenation of CO2

This research showcases the machine learning (ML)-enabled homogeneous catalyst discovery to be employed in carbon dioxide hydrogenation. To achieve the desired turnover frequency (TOF), the electrophilicity of the central metal atom is a crucial factor in transition metal pincer complexes. The condensed Fukui function is a direct measure of the catalytic performance of these pincer complexes. Herein, we demonstrate that machine learning is a convenient and effiecient method to calculate condensed Fukui functions of the central metal atom. The electrophilicity values of 202 pincer complexes were calculated by using density functional theory (DFT) to train the ML model. The test data of the experimentally established pincer complexes show a direct linkage between calculated electrophilicity and experimental TOF. Further, this data was used to develop an ML protocol to screen 2,84,062 catalyst complexes to get the electrophilicity values of the Mn, Fe, Co, and Ni transition metals encompassing various permutation combinations of PNP, PNN, NNN, and PCP pincer ligands. These findings validate the efficacy of machine learning in the rapid screening of metal pincer catalysts based on condensed Fukui functions.

Base Metal Catalyst for Indirect Hydrogenation of CO2

We herein report a novel Mn-SNS-based catalyst, which is capable of performing indirect hydrogenation of CO2 to methanol via formylation. In this domain of CO2 hydrogenation, pincer ligands have shown a clear predominance. Our catalyst is based on the SNS-type tridentate ligand, which is quite stable and cheap as compared to the pincer type ligands. The catalyst can also be recycled effectively after the formylation reaction without any significant change in efficiency. Various amines including both primary and secondary amines worked well under the protocol to provide the desired formylated product in good yields. The formed formylated amines can also be reduced further at higher pressures of hydrogen. As a whole, we have developed a protocol that involves indirect CO2 hydrogenation to methanol that proceeds via formylation of amines.

Hydrogenation of CO2 to MeOH Catalyzed by Highly Robust (PNNP)Ir Complexes Activated by Alkali Bases in Alcohol

Despite receiving significant attention, well-defined homogeneous complexes for hydrogenation of carbon dioxide (CO2) to methanol (MeOH) are scarce and suffer issues of low catalyst turnover numbers (TONs) at high catalyst concentrations and deactivation in the presence of CO and at elevated temperatures. Herein, we disclose a system deploying sterically demanded (PNNP)Ir complexes for a sustained activity for hydrogenation of CO2 to MeOH at temperatures ∼200 °C in an alcohol solvent. Through reaction optimization, we achieved a TON of ∼9000 for MeOH formation, which exceeds most active homogeneous systems reported to date, and robustness on par with or exceeding most reactive systems utilizing amine additives was demonstrated. The key to achieving sustained catalyst turnover for the system was utilizing a catalytic amount of an alkali base additive, which serves the dual purpose of facilitating more efficient outer-sphere reduction of CO2 and HCO2Et and enhancing the selectivity of MeOH over in situ formed CO.

Comparative study of CO2 insertion into pincer supported palladium alkyl and aryl complexes

The insertion of CO2 into metal alkyl bonds is a crucial elementary step in transition metal-catalyzed processes for CO2 utilization. Here, we synthesize pincer-supported palladium complexes of the type (tBuPBP)Pd(alkyl) (tBuPBP = B(NCH2PtBu2)2C6H4-; alkyl = CH2CH3, CH2CH2CH3, CH2C6H5, and CH2-4-OMe-C6H4) and (tBuPBP)Pd(C6H5) and compare the rates of CO2 insertion into the palladium alkyl bonds to form metal carboxylate complexes. Although, the rate constant for CO2 insertion into (tBuPBP)Pd(CH2CH3) is more than double the rate constant we previously measured for insertion into the palladium methyl complex (tBuPBP)Pd(CH3), insertion into (tBuPBP)Pd(CH2CH2CH3) occurs approximately one order of magnitude slower than (tBuPBP)Pd(CH3). CO2 insertion into the benzyl complexes (tBuPBP)Pd(CH2C6H5) and (tBuPBP)Pd(CH2-4-OMe-C6H4) is significantly slower than any of the n-alkyl complexes, and CO2 does not insert into the palladium phenyl bond of (tBuPBP)Pd(C6H5). While (tBuPBP)Pd(CH2CH3) and (tBuPBP)Pd(CH2CH2CH3) are resistant to β-hydride elimination, we were unable to synthesize complexes with n-butyl, iso-propyl, and tert-butyl ligands due to β-hydride elimination and an unusual reductive coupling, which involves the formation of new C-B bonds. This reductive process also occurred for (tBuPBP)Pd(CH2C6H5) at elevated temperature and a related process involving the formation of a new H-B bond prevented the isolation of (tBuPBP)PdH. DFT calculations provide insight into the relative rates of CO2 insertion and indicate that steric factors are critical. Overall, this work is one of the first comparative studies of the rates of CO2 insertion into different metal alkyl bonds and provides fundamental information that may be important for the development of new catalysts for CO2 utilization.

Recent developments on the transformation of CO2 utilising ligand cooperation and related strategies

A portfolio of value-added chemicals, fuels and building block compounds can be envisioned from CO₂ on an industrial scale. The high kinetic and thermodynamic stabilities of CO₂, however, present a significant barrier to its utilisation as a C1 source. In this context, metal-ligand cooperation methodologies have emerged as one of the most dominant strategies for the transformation of the CO₂ molecule over the last decade or so. This review focuses on the advancements in CO₂ transformation using these cooperative methodologies. Different and well-studied ligand cooperation methodologies, such as dearomatisation-aromatisation type cooperation, bimetallic cooperation (M⋯M'; M' = main group or transition metal) and other related strategies are also discussed. Furthermore, the cooperative bond activations are subdivided based on the number of atoms connecting the reactive centre in the ligand framework (spacer/linker length) and the transition metal. Several similarities across these seemingly distinct cooperative methodologies are emphasised. Finally, this review brings out the challenges ahead in developing catalytic systems from these CO₂ transformations.