Perspectives on atmospheric CO2 fixation in inorganic and biomimetic structures

Chemical fixation of atmospheric CO2 in tricopper(II)-carbonato complexes with tetradentate N-donor ligands: reactive intermediates, probable mechanisms, and catalytic and magneto-structural studies

In the present era, the fixation of atmospheric CO2 is of significant importance and plays a crucial role in maintaining the balance of carbon and energy flow within ecosystems. Generally, CO2 fixation is carried out by autotrophic organisms; however, the scientific community has paid substantial attention to execute this process in laboratory. In this report, we synthesized two carbonato-bridged trinuclear copper(II) complexes, [Cu3(L1)3(μ3-CO3)](ClO4)3 (1) and [Cu3(L2)3(μ3-CO3)](ClO4)3 (2) via atmospheric fixation of CO2 starting with Cu(ClO4)2·6H2O and easily accessible pyridine/pyrazine-based N4 donor Schiff base ligands L1 and L2, respectively. Under very similar reaction conditions, the ligand framework embedded with the phenolate moiety (HL3) fails to do so because of the reduction of the Lewis acidity of the metal center, inhibiting the formation of a reactive hydroxide bound copper(II) species, which is required for the fixation of atmospheric CO2. X-ray crystal structures display that carbonate-oxygen atoms bridge three copper(II) centers in μ3syn-anti disposition in 1 and 2, whereas [Cu(HL3)(ClO4)] (3) is a mononuclear complex. Interestingly, we also isolated an important intermediate of atmospheric CO2 fixation and structurally characterized it as an anti-anti μ2 carbonato-bridged dinuclear copper(II) complex, [Cu2(L2)2(μ2-CO3)](ClO4)2·MeOH (2-I), providing an in-depth understanding of CO2 fixation in these systems. Variable temperature magnetic susceptibility measurement suggests ferromagnetic interactions between the metal centers in both 1 and 2, and the results have been further supported by DFT calculations. The catalytic efficiency of our synthesized complexes 1-3 was checked by means of catechol oxidase and phenoxazinone synthase-like activities. While complexes 1 and 2 showed oxidase-like activity for aerobic oxidation of o-aminophenol and 3,5-di-tert-butylcatechol, complex 3 was found to be feebly active. ESI mass spectrometry revealed that the oxidation reaction proceeds through the formation of complex-substrate intermediations and was further substantiated by DFT calculations. Moreover, active catalysts 1 and 2 were effectively utilized for the base-free oxidation of benzylic alcohols in the presence of air as a green and sustainable oxidant and catalytic amount of TEMPO in acetonitrile. Various substituted benzylic alcohols smoothly converted to their corresponding aldehydes under very mild conditions and ambient temperature. The present catalytic protocol showcases its environmental sustainability by producing minimal waste.

Nickel(II) Complexes of Tripodal Ligands as Catalysts for Fixation of Atmospheric CO2 as Organic Carbonates

The fixation of atmospheric CO₂ into value-added products is a promising methodology. A series of novel nickel(II) complexes of the type [Ni(L)(CH₃ CN)₂ ](BPh₄ )₂ 1-5, where L=N,N-bis(2-pyridylmethyl)-N', N'-dimethylpropane-1,3-diamine (L1), N,N-dimethyl-N'-(2-(pyridin-2-yl)ethyl)-N'-(pyridin-2-ylmethyl) propane-1,3-diamine (L2), N,N-bis((4-methoxy-3,5-dimethylpyridin-2-ylmethyl)-N',N'-dimethylpropane-1,3-diamine (L3), N-(2-(dimethylamino) benzyl)-N',N'-dimethyl-N-(pyridin-2-ylmethyl) propane-1,3-diamine (L4) and N,N-bis(2-(dimethylamino)benzyl)-N', N'-dimethylpropane-1,3-diamine (L5) have been synthesized and characterized as the catalysts for the conversion of atmospheric CO₂ into organic cyclic carbonates. The single-crystal X-ray structure of 2 was determined and exhibited distorted octahedral coordination geometry with cis-α configuration. The complexes have been used as a catalyst for converting CO₂ and epoxides into five-membered cyclic carbonates under 1 atmospheric (atm) pressure at room temperature in the presence of Bu₄ NBr. The catalyst containing electron-releasing -Me and -OMe groups afforded the maximum yield of cyclic carbonates, 34% (TON, 680) under 1 atm air. It was drastically enhanced to 89% (TON, 1780) under pure CO₂ gas at 1 atm. It is the highest catalytic efficiency known for CO₂ fixation using nickel-based catalysts at room temperature and 1 atm pressure. The electronic and steric factors of the ligands strongly influence the catalytic efficiency. Furthermore, all the catalysts can convert a wide range of epoxides (ten examples) into corresponding cyclic carbonate with excellent selectivity (>99%) under this mild condition.

A DFT-design of single component bifunctional organocatalysts for the carbon dioxide/propylene oxide coupling reaction

The aim of this work is to develop single-component bifunctional organic catalysts capable of effective coupling reactions between CO2 and propylene epoxide (PO) under mild conditions using density functional theory (DFT) calculations. The dual functionalities of the target catalysts come from their inclusion of a hydroxyl-containing electrophile and the nucleophilicity of iodide ion. In this respect, a series of hydroxyl-functionalized quaternary onium-based ionic liquids were studied using M062X-D3/def2-TZVP//M062X-D3/def2-SVPP model chemistry. The design of catalysts was based on tailoring two structural factors; the first one is the onium center of pnictogens (N, P, As, Sb and Bi), and the second one is the number of hydrogen bond donor groups (n = 1-3). The proposed catalysts were examined by investigation of their catalytic mechanisms to afford the cyclic carbonate. Additionally, the highest active transition state, along with the potential energy difference, was examined using non-covalent interaction (NCI) analysis. Also, the activation strain model (ASM) was used to explain the kinetic behavior of PO activation. The findings showed that the ring-opening step of PO is always the critical step of the reaction. Among the suggested catalysts, the results indicated that the dihydroxyl ammonium-based catalyst (2OH-NI) is a good choice for this catalysis under mild and solvent-free conditions.

The Importance of Precursors and Modification Groups of Aerogels in CO2 Capture

The rapid growth of CO₂ emissions in the atmosphere has attracted great attention due to the influence of the greenhouse effect. Aerogels' application for capturing CO₂ is quite promising owing to their numerous advantages, such as high porosity (~95%); these are predominantly mesoporous (20-50 nm) materials with very high surface area (>800 m²∙g^-1). To increase the CO₂ level of aerogels' uptake capacity and selectivity, active materials have been investigated, such as potassium carbonate, K₂CO₃, amines, and ionic-liquid amino-acid moieties loaded onto the surface of aerogels. The flexibility of the composition and surface chemistry of aerogels can be modified intentionally-indeed, manipulated-for CO₂ capture. Up to now, most research has focused mainly on the synthesis of amine-modified silica aerogels and the evaluation of their CO₂-sorption properties. However, there is no comprehensive study focusing on the effect of different types of aerogels and modification groups on the adsorption of CO₂. In this review, we present, in broad terms, the use of different precursors, as well as modification of synthesis parameters. The present review aims to consider which kind of precursors and modification groups can serve as potentially attractive molecular-design characteristics in promising materials for capturing CO₂.

Water Breakup at Fe2O3-Hematite/Water Interfaces: Influence of External Electric Fields from Nonequilibrium Ab Initio Molecular Dynamics

The dynamical properties of physically and chemically adsorbed water molecules at pristine hematite-(001) surfaces have been studied by means of nonequilibrium ab initio molecular dynamics (NE-AIMD) in the NVT ensemble at room temperature, in the presence of externally applied, uniform static electric fields of increasing intensity. The dissociation of water molecules to form chemically adsorbed species was scrutinized, in addition to charge redistribution and Grotthus proton hopping between water molecules. Dynamical properties of the adsorbed water molecules and OH- and H3O+ ions were gauged, such as the hydrogen bonds between protons in water molecules and the bridging oxygen atoms at the hematite surface, as well as the interactions between oxygen atoms in adsorbed water molecules and iron atoms at the hematite surface. The development of Helmholtz charge layers via water breakup at Fe2O3-hematite/water interfaces is also an interesting feature, with the development of protonic conduction on the surface and more bulk-like water.

Rapid atmospheric carbon dioxide fixation by nickel(II) complexes: meridionally coordinated diazepane-based 3N ligands facilitate fixation

Octahedral complexes of the type [Ni(L)(H2O)3](ClO4)2 (1 and 2), where L is the tridentate 3N ligand 4-methyl-1-(pyrid-2-ylmethyl)-1,4-diazacycloheptane (L1, 1), or 4-methyl-1-(N-methylimidazolyl)-1,4-diazacycloheptane (L2, 2), have been isolated and characterized using elemental analysis, ESI-MS and electronic absorption spectroscopy. The DFT optimized structures of 1 and 2 reveal that the tridentate 3N ligands are coordinated meridionally constituting a distorted octahedral coordination geometry around nickel(ii). In methanol solution, the complexes, upon treatment with triethylamine, generate the reactive red colored low-spin square planar Ni-OH intermediate [Ni(L1/L2)(OH)]+ (1a and 2a), as characterized by ESI-MS and electronic absorption spectroscopy, and energy minimized structures. The latter when exposed to the atmosphere rapidly absorbs atmospheric CO2 to produce the carbonate bridged dinickel(ii) complexes [Ni2(L1/L2)2(μ-CO3)(H2O)2](ClO4)2 (3 and 4), as characterized by elemental analysis and the IR spectral feature (∼1608 cm-1) characteristic of bridging carbonate. The single crystal X-ray structure of 3 reveals the presence of a dinickel(ii) core bridged by a carbonate anion in a symmetric mode. Both the Ni(ii) centers are identical to each other with each Ni(ii) possessing a distorted octahedral coordination geometry constituted by a meridionally coordinated 3N ligand, a carbonate ion and a water molecule. The decay kinetics of the red intermediates generated by 1 (kobs, 7.7 ± 0.1 × 10-5 s-1) and 2 (kobs, 5.8 ± 0.3 × 10-4 s-1) in basic methanol solution with atmospheric CO2 has been determined by absorption spectroscopy. DFT studies illustrate that meridional coordination of the 3N ligand and the electron-releasing imidazole ring as in 2 facilitate fixation of CO2. The carbonate complex 3 efficiently catalyzes the conversion of styrene oxide into cyclic carbonate by absorbing atmospheric and pure CO2 with excellent selectivity.

Fixation of atmospheric CO2 as C1-feedstock by nickel(ii) complexes

The development of molecular catalysts for the activation and conversion of atmospheric carbon dioxide (CO₂) into a value-added product is a great challenge. A series of nickel(ii) complexes, [Ni(L)(CH₃CN)₃](BPh₄)₂, 1-4 of diazepane based ligands, 4-methyl-1-[(pyridin-2-yl-methyl)]-1,4-diazepane (L1), 4-methyl-1-[2-(pyridine-2-yl)ethyl]-1,4-diazepane (L2), 4-methyl-1-[(quinoline-2-yl)-methyl]-1,4-diazepane (L3) and 1-[(4-methoxy-3,5-dimethyl-pyridin-2-yl)methyl]-4-methyl-1,4-diazepane (L4), have been synthesized and characterized as catalysts for the activation of atmospheric CO₂. The single-crystal X-ray structure of 1 shows a distorted octahedral geometry with a cis-β configuration around the NiN₆ coordination sphere. All the complexes are used as catalysts for the conversion of atmospheric CO₂ and epoxides into cyclic carbonates at 1 atmosphere (atm) pressure and in the presence of Et₃N. Catalyst 4 was found to be the most efficient catalyst and showed a 31% formation of cyclic carbonates with a TON of 620 under 1 atm air as the CO₂ source. This yield was enhanced to 94% with a TON of 1880 under 1 atm pure CO₂ gas and it is the highest catalytic efficiency known for nickel(ii)-based catalysts. Catalyst 4 enabled the transformation of a wide range of epoxides (eight examples) into corresponding cyclic carbonates with excellent selectivity (>99%) and yields of 59-94% and 11-31% under pure CO₂ and atmospheric CO₂, respectively. The catalytic efficiency is strongly influenced by the electronic nature of the complexes. The CO₂ fixation reactions without an epoxide substrate led to the formation of the carbonate bridged dinuclear nickel(ii) complexes [(LNi^II)₂CO₃](BPh₄)₂1a-4a, which are speculated as catalytically active intermediates. The formation of these species was accompanied by the formation of new absorption bands around 592-681 nm and was further confirmed by the ESI-MS and IR spectral studies. The molecular structures of these carbonate-bridged key intermediates were determined by X-ray analysis. The structures contain two Ni²⁺-centers bridged via a carbonate ion that originated from CO₂. Distorted square pyramidal geometries are adopted around each Ni(ii) center. All these results support that CO₂ fixation reactions occur via CO₂-bound nickel key intermediates.

Hydrogen and Deuterium Molecular Escape from Clathrate Hydrates: "Leaky" Microsecond-Molecular-Dynamics Predictions

It is predicted herewith that the leakage of both hydrogen (H2) and deuterium (D2) from sII clathrate hydrates, borne of guest chemical-potential equalization driving enhanced nonequilibrium intercage hopping, should be observable experimentally. To this end, we have designed simulations to realize and study this process by microsecond molecular dynamics within the temperature range of 150-180 K-for which the hydrate lattice was found to be stable. In this pursuit, we considered initial large-cage (51264) guest occupancies of 1-4, with single occupation of 512 cavities. Examining transient, nonequilibrium intercage hopping, we present a lattice-escape activation energy for the four nominal large-cage occupancies (1-4), by fitting to the hydrate-leakage rate. The intercage hopping of H2 and D2 was studied using Markov-chain models and expressed at different temperatures and large-cage occupancies. The free energy of guest "binding" in the large and small cages was also computed for all of the occupancies. Toward equilibrium, following the majority of H2/D2 escape via leakage, the percentage of occupancies was calculated for both H2 and D2 for all of the systems for all initial nominal large-cage occupancies; here, not unexpectedly, double occupancies occurred more favorably in large cages and single occupancies dominated in small cages.

Oxidative addition of carbon dioxide into mesoionics

This work examines the prospect of making stable mesoionic compounds of the type mesomeric betaine R+-CO2- from direct oxidative additions of carbon dioxide to suitably-delocalized singlet carbene moieties, with bold objectives of carbon sequestration and overall energy storage. A set of possible candidates for such mesoionic compounds is theoretically explored through DFT calculations, inspecting coupling paths, thermodynamic and kinetic stabilities, and geometric and electronic structural features. Among others, the addressed cationic parts include aromatic rings in their broader sense, phenalene systems, and odd linear polyenic chains. Various structurally-close neutral alternatives such as oxiranones or carbene-acid forms are also considered. In the linear polyenic chain family, there is stark contrast between 4N + 1 and 4N - 1 lengths, with ensuing substantial consequences for stabilities and structures. Amino substitutions can favor mesoionic arrangements through their cation-stabilizing π-donor properties, further supported by possible strong intramolecular hydrogen bonds, but they can also contribute to weaken their kinetic stability through the existence of stable neutral imino alternatives. All in all, mesoionics including tropylium, phenalene, or 4N + 1 odd polyene frames as cationic parts could be reasonable targets.

CO2 fixation and transformation on a thiolate-bridged dicobalt scaffold under oxidising conditions

CO2 fixation and conversion promoted by a thiolate-bridged dicobalt complex in the presence of an oxidant.

Pressure-Induced Densification of Ice Ih under Triaxial Mechanical Compression: Dissociation versus Retention of Crystallinity for Intermediate States in Atomistic and Coarse-Grained Water Models

Molecular-dynamics (MD) simulation of triaxially pressurized ice I_h up to 30 kbar at 240 K (with sudden mechanical pressurization from its ambient-pressure structure) has been carried out with both the single-particle mW and atomistic TIP4P-Ice water potentials on systems of up to ∼1 million molecules, for times of the order of 100 ns. It was found that the TIP4P-Ice systems adopted a high-density liquid state above ∼7 kbar, while densification of the mW systems retained essentially crystalline order, owing to a failure for the tetrahedral network to break down appreciably from its ice I_h lattice structure. Both are intermediate states adopted along the path toward respective thermodynamically stable states (and with pressure removal show reversion to I_h for mW and to supercooled liquid for TIP4P-Ice), similar to recent ice electro-freezing simulations in "No Man's Land". Densification kinetics showed faster mW-system adaptation.

C-C Bond Formation of Mg- and Zn-Activated Carbon Dioxide

Gas-phase activation of CO₂ by chloride tagged metal atoms, [ClM]^- (M=Mg, Zn), has been investigated by mass spectrometry and high-level quantum chemistry. Both metals activate CO₂ with significant bending of the CO₂ moiety to form complexes with the general formula [ClM,CO₂ ]^- . The structure of the metal-CO₂ complex depends on the method of formation, and the energy landscapes and reaction dynamics have been probed by collisional induced dissociation and thermal ion molecule reactions with isotopically labeled species. Having established these structural relationships, the gas-phase reactivity of [ClM(κ² -O₂ C)]^- with acetaldehyde (here considered a carbohydrate mimic) was then studied. Formation of lactate and enolate-pyruvate complexes are observed, showing that CO₂ fixation by C-C bond formation takes place. For M=Zn, even formation of free pyruvate ([C₃ H₃ O₃ ]^- ) is observed. Implications of the observed CO₂ reactivity for the electrochemical conversion of carbon dioxide, and to biochemical and artificial photosynthesis is briefly discussed. Detailed potential energy diagrams obtained by the quantum chemical calculations offer models consistent with experimental observation.

Synthesis, Structure, and Reactivity of a Terminal Magnesium Hydride Compound with a Carbatrane Motif, [TismPriBenz]MgH: A Multifunctional Catalyst for Hydrosilylation and Hydroboration

The tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl)]methyl ligand, [Tism^PriBenz], has been employed to form the magnesium carbatrane compound, [Tism^PriBenz]MgH, which possesses a terminal hydride ligand. Specifically, [Tism^PriBenz]MgH is obtained via the reaction of [Tism^PriBenz]MgMe with PhSiH₃. The reactivity of [Tism^PriBenz]MgMe and [Tism^PriBenz]MgH allows access to a variety of other structurally characterized carbatrane derivatives, including [Tism^PriBenz]MgX [X = F, Cl, Br, I, SH, N(H)Ph, CH(Me)Ph, O₂CMe, S₂CMe]. In addition, [Tism^PriBenz]MgH is a catalyst for (i) hydrosilylation and hydroboration of styrene to afford the Markovnikov products, Ph(Me)C(H)SiH₂Ph and Ph(Me)C(H)Bpin, and (ii) hydroboration of carbodiimides and pyridine to form N-boryl formamidines and N-boryl 1,4- and 1,2-dihydropyridines, respectively.

A Simple Zinc Catalyst for Carbamate Synthesis Directly from CO2

Several zinc salts were employed as catalysts for the synthesis of carbamates directly from aromatic amines, CO₂ , and silicate esters. Zn(OAc)₂ offered the best performance among the salts tested. The addition of an N-donor ligand such as 1,10-phenanthroline increased the yield. The best catalytic performance of Zn(OAc)₂ can be explained by carboxylate-assisted proton activation. The interaction between the substrate and the catalyst can be observed by chemical shifts in ¹ H and ¹⁵ N NMR spectra. Isocyanate was a key intermediate, which was generated from amine and CO₂ . Silicate ester was finally converted to siloxane, which was determined by ²⁹ Si NMR. The commercially available catalyst system could be reused. The yield of isolated carbamate could reach up to 96 % with various substrates, and the catalytic reaction was amine-selective in the presence of other functional groups.

Perspectives on metal–organic frameworks with intrinsic electrocatalytic activity

This highlight article focuses on the rapidly emerging area of electrocatalytic metal–organic frameworks (MOFs) with a particular emphasis on those systems displaying intrinsic activity.

Solvent influence on the thermodynamics for hydride transfer from bis(diphosphine) complexes of nickel

Hydride transfer from a molecular nickel hydride to CO₂ changes from unfavourable in organic solvent to favourable in aqueous solution.

Closing the carbon cycle through rational use of carbon-based fuels

In this paper, a brief overview is presented of natural gas as a fuel resource with subsequent carbon capture and re-use as a means to facilitate reduction and eventual elimination of man-made carbon emissions. A particular focus is shale gas and, to a lesser extent, methane hydrates, with the former believed to provide the most reasonable alternative as a transitional fuel toward a low-carbon future. An emphasis is placed on the gradual elimination of fossil resource usage as a fuel over the coming 35 to 85 years and its eventual replacement with renewable resources and nuclear power. Furthermore, it is proposed that synthesis of chemical feedstocks from recycled carbon dioxide and hydrogen-rich materials should be undertaken for specific applications in the transport sector which require access to high energy density fuels. To achieve the latter, carbon dioxide capture is imperative and possible synthetic routes for chemical feedstock production are briefly reviewed.

Magneto-structural properties of carbonato-bridged copper(ii) complexes: fixation of atmospheric CO2

Weak ferromagnetic [Cu₃(dmMePEA)₃(μ₃-CO₃)(ClO₄)₃]ClO₄ (1) and strong antiferromagnetic [Cu₂(iptren)₂(μ₂-CO₃)](ClO₄)₂·H₂O (2) carbonato-bridged Cu(ii) complexes were synthesized via atmospheric fixation of CO₂ in slightly basic solutions.

Synthesis and characterization of copper(ii) complexes with multidentate ligands as catalysts for the direct hydroxylation of benzene to phenol

Like iron(iii) complexes, copper(ii) complexes catalyse the direct hydroxylation of benzene to phenol with H₂O₂ as the oxidant and their catalytic efficiency correlates with the reduction potentials of the copper(ii) complexes.

Synthesis and coordination chemistry of 1,1,1-tris-(pyrid-2-yl)ethane

The complexes [ML₂]ⁿ⁺(Mⁿ⁺= Fe²⁺, Co²⁺, Co³⁺, Cu²⁺and Ag⁺), [PdCl₂L] and [CuI(L)] are described. [AgL₂]⁺is an unusual square planar silver(i) centre (left). Exposure of [CuI(L)] to air affords mono- or dinuclear copper(ii)/carbonato products. Two of the copper complexes form crystalline hydrates with complicated hydrogen bond networks (right).

Theoretical studies of separation of cis-trans isomers using dinuclear (Cu(2+)- and Zn(2+)-based) cryptates

Theoretical investigations have been carried out using density functional theory, with the M06L functional and SMD solvent-modeling, to study the binding interactions of geometrical cis-trans isomers of fumarate (fum(2-)), maleate (male(2-)) and 1,2-cyclopropane dicarboxylate with Cu(2+)- and Zn(2+)-based dinuclear cryptates. It was found that cis-trans isomers of these groups of compounds bind strongly to metal cryptates, and the binding ability of the cryptates is controlled by the shape of the 'incoming' isomer (cis or trans); trans isomers bind more strongly than cis. Due to the size and shape of the cis and trans isomers, the cryptates can bind selectively and, to a large extent, 'recognize' the various cis-trans isomers, suggesting the tantalising possibility of isomeric separation/purification and recognition.

Fixation of CO2 in bi-layered coordination networks of zinc tetra(4-carboxyphenyl)porphyrin with multi-component [Pr2Na3(NO3)(H2O)3] connectors

This report demonstrates for the first time a uniquely elegant application of porphyrin-based MOF materials in fixation of atmospheric CO₂ in a multi-component reaction.