CO2 Hydrogenation Catalyzed by a Ruthenium Protic N-Heterocyclic Carbene Complex

Ligand-Bound CO2 as a Nonclassical Route toward Efficient Photocatalytic CO2 Reduction with a Ni N-Confused Porphyrin

Implementing the synergistic effects between the metal and the ligand has successfully streamlined the energetics for CO2 activation and gained high catalytic activities, establishing the important breakthroughs in photocatalytic CO2 reduction. Herein, we describe a Ni(II) N-confused porphyrin complex (NiNCP) featuring an acidic N-H group. It is readily deprotonated and exists in an anion form during catalysis. Owing to this functional site, NiNCP gave rise to an outstanding turnover number (TON) as high as 217,000 with a 98% selectivity for CO2 reduction to CO, while the parent Ni(II) porphyrin (NiTPP) was found to be nearly inactive. Our mechanistic analysis revealed a nonclassical reaction pattern where CO2 was effectively activated via the attack of the Lewis-basic ligand. The resulting ligand-bound CO2 adduct could be further reduced to produce CO. This new metal-ligand synergistic effect is anticipated to inspire the design of highly active catalysts for small molecule activations.

Ru(II) Complexes with Protic- and Anionic-Naked-NHC Ligands for Cooperative Activation of Small Molecules

A set of ruthenium(II)-protic-N-heterocyclic carbene complexes, [Ru(NNCH )(PPh3 )2 (X)]Cl (1, X=Cl and 2, X=H) and their deprotonated forms [Ru(NNC)(PPh3 )2 (X)] (1', X=Cl and 2', X=H), in which NNC is a new unsymmetrical pincer ligand, are reported. The four complexes are interconvertible by simple acid-base chemistry. The combined theoretical and spectroscopic investigations indicate charge segregation in anionic-NHC complexes (1' and 2') and can be described from a Lewis pair perspective. The chemical reactivity of deprotonated complex 1' shows cooperative small molecule activation. Complex 1' activates H-H bond of hydrogen, C(sp3 )-I bond of iodomethane, and C(sp)-H bond of phenylacetylene. The activation of CO2 using anionic NHC complex 1' at moderate temperature and ambient pressure and subsequent conversion to formate is also described. All the new compounds have been characterized using ESI-MS, 1 H, 13 C, and 31 P NMR spectroscopy. Molecular structures of 1, 2, and 2' have also been determined with single-crystal X-ray diffraction. The cooperative small molecule activation perspective broadens the scope of potential applications of anionic-NHC complexes in small molecule activation, including the conversion of carbon dioxide to formate, a much sought after reaction in the renewable energy and sustainable development domains.

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.

Effect of Micropores of a Porous Coordination Polymer on the Product Selectivity in RuII Complex-catalyzed CO2 Reduction

To develop an efficient CO2 reduction catalyst, hybridizing a molecular catalyst and a porous coordination polymer (PCP) is a promising strategy because it can combine both advantages of the precise reactivity control of the former and the CO2 adsorption property of the latter. Although several PCP hybrid catalysts have been reported to date, the CO2 sorption behavior and the CO2 reduction reactivity have been investigated separately, and the CO2 enrichment during the catalysis is still unclear. We report CO2 photoreduction under different temperatures and pressures using a PCP-RuII complex hybrid catalyst. The product selectivity (CO or HCOOH) varied depending on the reaction conditions. The altered selectivity could be interpreted in terms of the CO2 capture in the micropores of a PCP.

Metal-Ligand Cooperation in Cp*Ir-Pyridylpyrrole Complexes: Rational Design and Catalytic Activity in Formic Acid Dehydrogenation and CO2 Hydrogenation under Ambient Conditions

Interconversion between CO₂ + H₂ and FA/formate is the most promising strategy for the fixation of carbon dioxide and reversible hydrogen storage; however, FA dehydrogenation and CO₂ hydrogenation are usually studied separately using different catalysts for each reaction. This report describes of the catalysis of [Cp*Ir(N∧N)(X)]ⁿ⁺ (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; X = Cl, n = 0; X = H₂O, n = 1) bearing a proton-responsive N∧N pyridylpyrrole ligand for both reactions. Complex 2-H₂O catalyzes FA dehydrogenation at 90 °C with a TOF_max of 45 900 h^-1. Its catalysis is more active in aqueous solution than in neat solution under base-free conditions. These complexes also catalyze CO₂ hydrogenation in the presence of base to formate under atmospheric pressure (CO₂/H₂ = 0.05 MPa/0.05 MPa) at 25 °C with a TOF value of 4.5 h^-1 in aqueous solution and with a TOF value of 29 h^-1 in a methanol/H₂O mixture solvent. The possible mechanism is proposed by intermediate characterization and KIE experiments. The extraordinary activity of these complexes are mainly attributed to the metal-ligand cooperative effect of the the pyrrole group to accept a proton in the dehydrogenation of formic acid and assist cooperative heterolytic H-H bond cleavage in CO₂ hydrogenation.