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

Introducing proton/electron mediators enhances the catalytic ability of an iron porphyrin complex for photochemical CO2 reduction

A novel iron porphyrin complex with hydroquinone moieties as proton/electron mediators at meso positions was designed and synthesised. The complex serves as an efficient catalyst for photochemical CO2 reduction, and its turnover frequency (TOF = 1.3 × 104 h-1) was the highest among those of comparable systems with sufficient durability.

Interfacial Self-Assembly of Organized Ultrathin Films of Tripodal Metal-Terpyridyl Coordination Polymers as Luminophores and Heterogeneous Catalysts for Photocatalytic CO2 Reduction

Metal-directed interfacial self-assembly of well-defined coordination polymer (CP) ultrathin films can control the metal complex arrangement and distribution at the molecular level, providing a convenient route for the design and fabrication of novel opto-electrical devices and heterogeneous catalysts. Here, we report the assembly of two series of CP multilayers with the transition-metal ions of Fe²⁺, Co²⁺, Zn²⁺ and Tb³⁺ as connectors and tripodal terpyridyl ligands of 4,4',4″-(1,3,5-triazine-2,4,6-triyl)tris(1-(4-([2,2':6',2″-terpyridin]-4'-yl)benzyl)pyridin-1-ium) (TerPyTa) and 4,4',4″-(benzene-1,3,5-triyl)tris(1-(4-([2,2':6',2″-terpyridin]-4'-yl)benzyl)pyridin-1-ium) (TerPyBen) as linkers at the air-water interface. The as-prepared Langmuir-Blodgett (LB) films display strong luminescence, with the emission wavelength and relative intensity dependent on both the metal ions and linkers; among them, the Zn-TerPyTa and Zn-TerPyBen CPs give off the strongest luminescent emission centered at about 370 nm with an emission lifetime of approximately 0.2-0.3 ns. The Tb-TerPyTa CPs can give off emission at approximately 490, 546, 586, and 622 nm, attributed to the ⁵D₄ to ⁷F_3-6 electron transitions of typical Tb³⁺ ions. Finally, these CP LB films can act as efficient heterogeneous photocatalysts for the CO₂ reduction to selectively produce CO. The catalytic efficiency can be optimized by adjusting the experimental conditions (light sensitizer, electron donor, and water content) and CP composition (metal ion and ligand) with an excellent yield of up to 248.1 mmol g^-1. In particular, it is revealed that, under the same conditions, the catalytic efficiency of the Fe-TerPyTa CP LB film is nearly 2 to 3 orders of magnitude higher than that of the other metalated complexes investigated in the homogeneous system. UV-vis spectroscopy and cyclic voltammetry studies demonstrated that the dual active sites of Fe-terpyridine and TerPyTa units contribute to the enhanced catalytic activity. This work provides an effective method to introduce the earth-abundant metal complexes into CP films to construct efficient noble-metal-free photocatalysts for the CO₂ reduction.

Photocatalytic CO2 reduction with aminoanthraquinone organic dyes

The direct utilization of solar energy to convert CO₂ into renewable chemicals remains a challenge. One essential difficulty is the development of efficient and inexpensive light-absorbers. Here we show a series of aminoanthraquinone organic dyes to promote the efficiency for visible light-driven CO₂ reduction to CO when coupled with an Fe porphyrin catalyst. Importantly, high turnover numbers can be obtained for both the photosensitizer and the catalyst, which has not been achieved in current light-driven systems. Structure-function study performed with substituents having distinct electronic effects reveals that the built-in donor-acceptor property of the photosensitizer significantly promotes the photocatalytic activity. We anticipate this study gives insight into the continued development of advanced photocatalysts for solar energy conversion.

Metal-Terpyridine Assembled Functional Materials for Electrochromic, Catalytic and Environmental Applications

Molecular assembly induced by metal-terpyridine-based coordinative interactions has become an emergent research topic due to its ease of synthesis and diverse applications. This article highlights recent significant developments in the metal-terpyridine-based supramolecular architectures. At first, the design aspect of the molecular building blocks has been described, followed by elaboration on how the ligand backbone plays an important role for achieving different dimensionalities of the resulting assemblies which exhibit a wide range of potential applications. After that, we discussed different synthetic approaches for constructing these assemblies, and finally, we focused on their significant developments in three specific areas, viz., electrochromic materials, catalysis and a new application in wastewater treatment. In the field of electrochromic materials, these assemblies made important advancements in various aspects like sub-second switching time (<1 s), low switching voltage (<1 V), increased switching stability (>10000 cycles), tuning of multiple colors by using multimetallic systems, fabrication of charge storing electrochromic devices, utilizing and storing solar energy etc. Similarly, the catalysis field witnessed application of the metal-terpyridine assemblies in C-H monohalogenation, heterogeneous Suzuki-Miyaura coupling, photocatalysis, reduction of carbon dioxide, etc. Finally, the environmental application of these coordination assemblies includes capturing Cr(VI) from waste water efficiently with high capture capacity, good recyclability, wide pH independency etc.

Dual electronic effects achieving a high-performance Ni(II) pincer catalyst for CO2 photoreduction in a noble-metal-free system

A carbazolide-bis(NHC) Ni^II catalyst (1; NHC, N-heterocyclic carbene) for selective CO₂ photoreduction was designed herein by a one-stone-two-birds strategy. The extended π-conjugation and the strong σ/π electron-donation characteristics (two birds) of the carbazolide fragment (one stone) lead to significantly enhanced activity for photoreduction of CO₂ to CO. The turnover number (TON) and turnover frequency (TOF) of 1 were ninefold and eightfold higher than those of the reported pyridinol-bis(NHC) Ni^II complex at the same catalyst concentration using an identical Ir photosensitizer, respectively, with a selectivity of ∼100%. More importantly, an organic dye was applied to displace the Ir photosensitizer to develop a noble-metal-free photocatalytic system, which maintained excellent performance and obtained an outstanding quantum yield of 11.2%. Detailed investigations combining experimental and computational studies revealed the catalytic mechanism, which highlights the potential of the one-stone-two-birds effect.

Exploiting consecutive photoinduced electron transfer (ConPET) in CO2 photoreduction

The consecutive photoinduced electron transfer (ConPET) process of 1,2,3,5-Tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) in CO₂ photoreduction to achieve powerful reducing species has been disclosed by activating a bis(terpyridine)ruthenium(II) complex bearing a high overpotential for selective light-driven reduction of CO₂ to CO in homogeneous solution.

Combination of Organic Dye and Iron for CO2 Reduction with Pentanuclear Fe2Na3 Purpurin Photocatalysts

Molecular photocatalysts designed with earth-abundant elements are rare and challenging in artificial photosynthesis study. Herein, we report a multimetallic Fe2Na3 purpurin (1) complex for the reduction of CO2 in DMF under visible-light irradiation. The photocatalytic system achieves 91% selectivity and 2625 ± 334 turnovers of CO in 120 h, which is among the highest reported for a noble-metal-free catalyst without an additional photosensitizer. UV-vis and electrochemical studies suggest that the mechanism involves subsequent reductions and protonations of 1 to generate [FeII2Na3((H)2PP)6]5- and [FeIII2Na3((H)2PP)6]3- as the active photocatalysts in CO2 reduction.

Noble metal-free bis-tridentate benzimidazole zinc(II) and iron(II) complexes for selective CO2 photoreduction

Three noble metal-free metal complexes [Fe(Me-bzimpy)₂]²⁺ (Fe1), [Fe(bzimpy)₂]²⁺ (Fe2) and [Zn(Me-bzimpy)₂]²⁺ (Zn1) were synthesized and studied in the visible light-driven CO₂ reduction, where ligands bzimpy and Me-bzimpy were 2,6-bis(1-methyl-1H-benzo[d]imidazol-2-yl)pyridine and 2,6-bis(1H-benzo[d]imidazol-2-yl)pyridine, respectively. It was found that Fe1 displayed the best photocatalytic performance with a turnover number (TON) of 878 and high selectivity up to 99.2% towards CO generation in the presence of an organic thermally activated delayed fluorescence (TADF) photosensitizer, which was more than 10 times that of Fe2 (TON_CO = 63) and Zn1 (TON_CO = 53). This is attributed to the much higher stability of Fe1 upon reduction, as proved by the cyclic voltammograms of the three complexes. These results highlight the cooperation of ligands and metals in molecular metal complexes for CO₂ photoreduction.

Synthesis of an Fe(terpy-cage)2 dumbbell

An azide masked amine is used to obtain a cage of lower symmetry that possess one terpy-group in an exo-position. This group can coordinate to iron(ii), yielding selectively an easy to purify Fe(terpy-cage)₂ dumbbell. The dumbbell can also be obtained in a one pot reaction, which proceeded without isolation of the exo-functionalized cage.

A masked amine building block is used to synthesize an organic cage that is exo-functionalized with one terpy group. Two exo-functionalized cages can be combined via iron-terpy coordination resulting in a cage dumbbell.