Supramolecular assemblies of Cu(II) with a tetraphenylethene-imidazole ligand for tuning photocatalytic CO2 reduction

Cu(II) supramolecular assemblies [Cu2(tipe)2(H2O)2](NO3)4·2.5H2O (CuN4) and [Cu2Cl4(tipe)(CH3CN)]·H2O (CuN2Cl2) (tipe = 1,1,2,2-tetrakis(4-(imidazole-1-yl)phenyl)ethene) were synthesized and utilized for photocatalytic CO2 reduction. CuN4 exhibits CO production of up to 891 μmol gcat-1 with a selectivity of 79.9%, while CuN2Cl2 gives low CO production of 206 μmol gcat-1 but with a high selectivity of >99.9% in 5 h. The experimental and DFT calculation results indicate that the coordination environment and non-covalent interactions within the assemblies have a great impact on the photocatalytic CO2 reduction behavior. This work provides useful insights on Cu(II) assembly catalyzed CO2 photoreduction.

Tweaking Photo CO2 Reduction by Altering Lewis Acidic Sites in Metalated-Porous Organic Polymer for Adjustable H2 /CO Ratio in Syngas Production

Herein, we have specifically designed two metalated porous organic polymers (Zn-POP and Co-POP) for syngas (CO+H2 ) production from gaseous CO2 . The variable H2 /CO ratio of syngas with the highest efficiency was produced in water medium (without an organic hole scavenger and photosensitizer) by utilizing the basic principle of Lewis acid/base chemistry. Also, we observed the formation of entirely different major products during photocatalytic CO2 reduction and water splitting with the help of the two catalysts, where CO (145.65 μmol g-1  h-1 ) and H2 (434.7 μmol g-1  h-1 ) production were preferentially obtained over Co-POP & Zn-POP, respectively. The higher electron density/better Lewis basic nature of Co-POP was investigated further using XPS, XANES, and NH3 -TPD studies, which considerably improve CO2 activation capacity. Moreover, the structure-activity relationship was confirmed via in situ DRIFTS and DFT studies, which demonstrated the formation of COOH* intermediate along with the thermodynamic feasibility of CO2 reduction over Co-POP while water splitting occurred preferentially over Zn-POP.

Photocatalytic CO2 Conversion Using Metal-Containing Coordination Polymers and Networks: Recent Developments in Material Design and Mechanistic Details

International guidelines have progressively addressed global warming which is caused by the greenhouse effect. The greenhouse effect originates from the atmosphere’s gases which trap sunlight which, as a consequence, causes an increase in global surface temperature. Carbon dioxide is one of these greenhouse gases and is mainly produced by anthropogenic emissions. The urgency of removing atmospheric carbon dioxide from the atmosphere to reduce the greenhouse effect has initiated the development of methods to covert carbon dioxide into valuable products. One approach that was developed is the photocatalytic transformation of CO2. Photocatalysis addresses environmental issues by transferring CO2 into value added chemicals by mimicking the natural photosynthesis process. During this process, the photocatalytic system is excited by light energy. CO2 is adsorbed at the catalytic metal centers where it is subsequently reduced. To overcome several obstacles for achieving an efficient photocatalytic reduction process, the use of metal-containing polymers as photocatalysts for carbon dioxide reduction is highlighted in this review. The attention of this manuscript is directed towards recent advances in material design and mechanistic details of the process using different polymeric materials and photocatalysts.

Metal center regulation of the porphyrin unit in covalent organic polymers for boosting the photocatalytic CO2 reduction activity

Regulating the porphyrin's metal center of metalloporphyrin(MPor)/Ru(ii)-pincer complex(RuN3) covalent organic polymers (COPs) effectively boosted the CO2 photoreduction by promoting charge separation and sacrificial electron donor oxidation.

Effect of the Defect Modulator and Ligand Length of Metal-Organic Frameworks on Carbon Dioxide Photoreduction

The nature of defects and organic ligands can fine-tune the absorption energy (E_abs) of metal-organic frameworks (MOFs), which is crucial for photocatalytic reactions; however, the relevant studies are in their infancy. Herein, a series of typical MOFs of the UiO family (UiO-6x-NH₂, x = 8, 7, and 6) with ligands of varied lengths and amino-group-modified defects were synthesized and employed to explore their performance for photocatalytic CO₂ reduction. Sample UiO-66-NH₂-2ABA (2ABA = 3,5-diamino-benzoate) with the shortest dicarboxylate ligand and two amino-group-modified defects exhibits superior photocatalytic activity due to the lowest E_abs. The CO yield photocatalyzed by UiO-66-NH₂-2ABA is 17.5 μmol g^-1 h^-1, which is 2.4 times that of UiO-68-NH₂-BA (BA = benzoate) with the longest ligand and no amino group involved in the defects. Both the experiments and theoretical calculations show that shorter dicarboxylate ligands and more amino groups result in smaller E_abs, which is favorable for photocatalytic reactions. This study provides new insights into boosting the photocatalytic efficiency by modulating the defects and ligands in MOFs.

Coordination-driven discrete metallo-supramolecular assembly for rapid and selective photochemical CO2 reduction in aqueous solution

A discrete metallo-supramolecular assembly composed of six iron(ii) cations and twelve redox-active terpyridine fragments has been developed for the highly efficient visible-light-driven reduction of CO2 to CO with a TON of 14 956 and 99.6% selectivity in the presence of an organic thermally activated delayed fluorescence (TADF) photosensitizer 4CzIPN in aqueous solution. The photochemical system proceeds rapidly with a turnover frequency (TOF) of 276 min-1. It is demonstrated that the redox-active terpyridine fragments in the assembly are reduced by the photosensitizer which could further act as an electron reservoir for CO2 reduction, resulting in the highly efficient reduction of CO2. This work shows that discrete metallo-supramolecular assemblies could be used for robust photochemical CO2 reduction.

Water-Assisted Highly Efficient Photocatalytic Reduction of CO2 to CO with Noble Metal-Free Bis(terpyridine)iron(II) Complexes and an Organic Photosensitizer

Photocatalytic CO₂ reduction reaction is believed to be a promising approach for CO₂ utilization. In this work, a noble metal-free photocatalytic system, composed of bis(terpyridine)iron(II) complexes and an organic thermally activated delayed fluorescence compound, has been developed for selective reduction of CO₂ to CO with a maximum turnover number up to 6320, 99.4% selectivity, and turnover frequency of 127 min^-1 under visible-light irradiation in dimethylformamide/H₂O solution. More than 0.3 mmol CO was generated using 0.05 μmol catalyst after 2 h of light irradiation. The apparent quantum yield was found to be 9.5% at 440 nm (180 mW cm^-2). Control experiments and UV-vis-NIR spectroscopy studies further demonstrated that water strongly promoted the photocatalytic cycle and terpyridine ligands rather than Fe(II) were initially reduced during the photocatalytic process.

Metallo-Supramolecular Coordination Polymers Based on Amidopyridine Derivatives of Pillar[5]arene and Cu(II) and Pd(II) Cations: Synthesis and Recognition of Nitroaromatic Compounds

Decasubstituted pillar[5]arenes containing amidopyridine fragments have been synthesized for the first time. As was shown by UV-vis spectroscopy, the pillar[5]arenes with p-amidopyridine fragments form supramolecular associates with Cu(II) and Pd(II) cations in methanol in a 2:1 ratio. Using a sol-gel approach these associates are transformed into metallo-supramolecular coordination polymers (supramolecular gels) which were characterized as amorphous powders by scanning electron microscopy (SEM) and dynamic light scattering (DLS). The powders are able to selectively adsorb up to 46% of nitrophenols from water and were incorporated into an electrochemical sensor to selectively recognize them in aqueous acidic solution.

An iron-nitrogen doped carbon and CdS hybrid catalytic system for efficient CO2 photochemical reduction

Iron porphyrin and carbon black (CB) were utilized to fabricate an iron-nitrogen doped carbon (Fe-N-C) catalyst to create a new heterogeneous catalytic system with CdS to drive CO2 reduction to CO under UV/vis light (AM 1.5G) irradiation. The system delivers a high CO production yield of 111 mmol gcat-1 and a large turnover number (TON) of 1.22 × 103 in 8 h with a selectivity of 85%, all of which are competitive with state-of-the-art systems. The mechanism of the system was investigated by experimental and theoretical methods indicating that the high affinity between the iron active center and the *COOH intermediate facilitates the brilliant catalytic performance. This work provides a new direction for constructing heterogeneous CO2 photoreduction systems.

Single- and mixed-metal–organic framework photocatalysts for carbon dioxide reduction

This review focuses on the important roles of varied metal types over MOF-based photocatalysts. The basic principles, types of MOF photocatalysts and roles of the reaction system to achieve efficient MOFs for CO₂ photoreduction are discussed.

Dinuclear uranium(VI) salen coordination compound: an efficient visible-light-active catalyst for selective reduction of CO2 to methanol

A new dinuclear uranyl salen coordination compound, [(UO2)2(L)2]·2MeCN [L = 6,6'-((1E,1'E)-((2,2-dimethylpropane-1,3-diyl)bis(azaneylylidene))-bis(methaneylylidene))bis(2-methoxyphenol)], was synthesized using a multifunctional salen ligand to harvest visible light for the selective photocatalytic reduction of CO2 to MeOH. The assembling of the two U centers into one coordination moiety via a chelating-bridging doubly deprotonated tetradentate ligand allowed the formation of U centers with distorted pentagonal bipyramid geometry. Such construction of compounds leads to excellent activity for the photocatalytic reduction of CO2, permitting a production rate of 1.29 mmol g-1 h-1 of MeOH with an apparent quantum yield of 18%. Triethanolamine (TEOA) was used as a sacrificial electron donor to carry out the photocatalytic reduction of CO2. The selective methanol formation was purely a photocatalytic phenomenon and confirmed using isotopically labeled 13CO2 and product analysis by 13C-NMR spectroscopy. The spectroscopic studies also confirmed the interaction of CO2 with the molecule of the title complex. The results of these efforts made it possible to understand the reaction mechanism using ESI-mass spectrometry.