CO2 Complex Research Articles

Helical Inversion of Peptide‐based Supramolecular Co2+ Complexes

Herein, we report the helical inversion of supramolecular polymeric complexes of Co2+ containing a peptide‐based ligand comprising one alanine and three glycine moieties and an achiral terpyridine group. The helicity of the peptide‐based supramolecular polymer is controlled via strain‐induced chirality at different stoichiometric ligand/Co2+ ratios. The supramolecular polymer S‐1 adopts octahedral geometry with right‐handed helicity (P‐type) in the presence of <0.7 equiv of Co2+. In contrast, it adopts coexisting octahedral and square‐pyramidal geometries in the presence of 1.0 equiv of Co2+ and presents left‐handed helicity (M‐type). The helicities of the supramolecular polymer R‐1 with Co2+, prepared using the chirally opposite ligand, were completely opposite to those of S‐1. Furthermore, the circular dichroism intensities of supramolecular polymers S‐1 and R‐1 in the presence of Co2+ were 900–1500 times higher than those of free S‐1 and R‐1. In addition, the helical inversion was completely reversible and controllable by the addition of more Co2+ or ligand.

Lewis Acid Enhancement of Proton Induced CO2 Cleavage: Bond Weakening and Ligand Residence Time Effects.

Though Lewis acids (LAs) have been shown to have profound effects on carbon dioxide (CO2) reduction catalysis, the underlying cause of the improved reactivity remains unclear. Herein, we report a well-defined molecular system for probing the role of LA additives in the reduction of CO2 to carbon monoxide (CO) and water. Mo(0) CO2 complex (2) forms adducts with a series of LAs, demonstrating CO2 activation that correlates linearly with the strength of the LA. Protons induce C-O cleavage of these LA adducts, in contrast to the CO2 displacement primarily observed in the absence of LA. CO2 cleavage shows dependence on both bond activation and the residence time of the bound small molecule, demonstrating the influence of both kinetic and thermodynamic factors on promoting productive CO2 reduction chemistry.

Theoretical studies for the infrared spectra of Ar–CO2 complex: Fundamental and combination bands

Two potential energy surfaces (PESs) were constructed for the ground and excited states of Ar–CO2 complex at the rigid rotor approximation. Besides the notable T-shape structure, two equivalent linear structures were also found on the PES for the first time. Based on the PESs of ground and excited states, the bound state calculations were performed to determine the rotational energy levels for the ground and excited states. In combination of the experimental spectroscopic parameters of ground state and the differences of rotational energy levels, we give a theoretical prediction of infrared spectra including one fundamental band and two combination bands for two isotopomers Ar–12C16O2 and Ar–12C18O2 in the ν3 region of CO2 monomer. The predicted transition frequencies and spectroscopic parameters of excited states are in excellent agreement with the available experimental data, and these results can also be used as a guide to perform the further investigation for the infrared spectra of combination bands experimentally.

A mechanistic study of CO oxidation over spinel MnFe2O4 surface during chemical-looping combustion

Spinel MnFe2O4 is a potential oxygen carrier owing to its low cost, non-toxicity and good thermal stability during the chemical-looping combustion (CLC) redox cycles. The reactivity of MnFe2O4 and the mechanism of CO oxidation over MnFe2O4 surface were investigated by using the density functional theory (DFT) method. It is found that CO chemisorbs preferably at the octahedral Fe3+ site with C-down orientation. Particularly, a Mn atomic layer lying on the top of Mn-terminated surface occupies some active O sites and weakens the surface activity. The adsorbed CO then reacts with a bridging surface O to generate a surface CO2 complex, revealing the key intermediate for CO2 production. CO2 desorption ultimately takes place by surmounting an energy barrier. Based upon the DFT-derived energetics, CO oxidation over the spinel MnFe2O4 surface is identified as a three-step process involving CO adsorption, CO diffusion and CO2 desorption. The pathway analysis illustrates that the surface O with three Fe3+ ligands shows superior reactivity than that with two Fe3+ and one Mn2+ ligands. Further, it is noted that the consumption of lattice O primarily occurs around Fe sites, which indicates that Fe3+ can be easily reduced to Fe2+ accompanying with MnFe2O4 transformation to (Fe, Mn)O. These results are well consistent with the experimental kinetics.

Highly Efficient Synthesis of Quinazoline-2,4(1H,3H)-diones from CO2 by Hydroxyl Functionalized Aprotic Ionic Liquids

Ionic liquids can be designed by varying a great deal of anions and cations offering efficient CO2 capture or CO2 utilization. Generally, the anion played a key role, but the cation did not have a significant impact. Here, a strategy for rational design of functionalized ionic liquids for efficient synthesis of quinazoline-2,4(1H,3H)-diones from CO2 has been developed through tuning the cations of aprotic ionic liquids. The basicity of cation affects its catalytic activity dramatically and the hydrogen bond from cation can promote this reaction. Then, hydroxyl functionalized ionic liquid [Ch][Im] was designed, which exhibited the best catalytic activity in this reaction. Through the combination of quantum-chemical calculations, NMR spectroscopic investigations, and controlled experiments, the results indicate that in situ generated [Ch][Im]-CO2 complex is the real catalyst. Furthermore, aprotic IL [Ch][Im] exhibits good generality and reuseability. Remarkably, quinazoline-2,4(1H,3H)-dione can also be obta...

Complexes of CO₂ with the Azoles: Tetrel Bonds, Hydrogen Bonds and Other Secondary Interactions.

Ab initio MP2/aug’-cc-pVTZ calculations have been performed to investigate the complexes of CO2 with the azoles pyrrole, pyrazole, imidazole, 1,2,3- and 1,2,4-triazole, tetrazole and pentazole. Three types of complexes have been found on the CO2:azole potential surfaces. These include ten complexes stabilized by tetrel bonds that have the azole molecule in the symmetry plane of the complex; seven tetrel-bonded complexes in which the CO2 molecule is perpendicular to the symmetry plane; and four hydrogen-bonded complexes. Eight of the planar complexes are stabilized by Nx···C tetrel bonds and by a secondary interaction involving an adjacent Ny-H bond and an O atom of CO2. The seven perpendicular CO2:azole complexes form between CO2 and two adjacent N atoms of the ring, both of which are electron-pair donors. In three of the four hydrogen-bonded complexes, the proton-donor Nz-H bond of the ring is bonded to two C-H bonds, thereby precluding the planar and perpendicular complexes. The fourth hydrogen-bonded complex forms with the strongest acid pentazole. Binding energies, charge-transfer energies and changes in CO2 stretching and bending frequencies upon complex formation provide consistent descriptions of these complexes. Coupling constants across tetrel bonds are negligibly small, but 2hJ(Ny-C) across Nz-H···C hydrogen bonds are larger and increase as the number of N atoms in the ring increases.

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.

Mono- and Binuclear Cobalt(II) Complexes Supported by Quinoline-2-imidate Ligands: Synthesis, Characterization, and 1,3-Butadiene Polymerization

A series of mono- and binuclear Co(II) complexes (Co1–Co7) supported by quinoline-2-imidate ligands were synthesized and thoroughly characterized. Measured by single crystal X-ray crystallography, complexes Co1 and Co3 adopted distorted tetrahedral structures around the cobalt center. Upon activation by ethylaluminium sesquichloride (EASC), these cobalt complexes exhibited high catalytic activity and cis-1,4-selectivity towards 1,3-butadiene polymerization. The effects of ligand environment, polymerization temperature, and cocatalyst types on the polymerization were investigated in detail. Interestingly, the binuclear Co(II) complexes exhibited high thermal stability, and the polymer yields were up to 97.2% even at a high temperature of 70 °C.

A non-innocent salen naphthalene ligand and its Co 2+ , Ni 2+ and Cu 2+ metal complexes: Structural, electrochemical, and spectroscopic characterization and computational studies

Abstract Three mononuclear metal complexes of Co2+, Ni2+ and Cu2+ with the Salen type ligand ((2,2′-[(1-methyl-1,2-ethanediyl) bis(nitriloethylidyne)] bis(1-naphthalenol)), H2Naphpren or L) were synthesized and structurally characterized. All metal complexes have square planar geometries and crystallize in the P21/c space group. Structural characterization of the ligand shows contribution of the bis(ketone) tautomer in the crystalline phase, in agreement with density functional theory (DFT) computations. However, according to the UV-vis spectrum, there is a stronger influence of the bis(phenol) tautomer in solution. Cyclic voltammetry experiments showed that the ligand undergoes a single irreversible oxidation process at 0.434 V (vs Fc/Fc+) while the complexes with Co2+, Ni2+ and Cu2+ suffer electroxidative process on the surface of the working electrode in addition to solution-assigned redox processes. The CoL oxidation is localized on the metal center, however, the CuL and NiL oxidations are considered as metal-ligand based processes. DFT calculations confirmed our experimental findings. The UV-vis spectra of the complexes illustrate ligand characteristic bands: in the case of the Cu2+ complex, the metal-to-ligand charge transfer (MLCT) band is present at 385 nm, while in the case of Co2+ complex this same type of MLCT band appears at 377 nm.

Elucidation of hydrated metal ions using flocculation-surface enhanced Raman scattering

Hydrated metal ions such as Na+, Mg2+, and Cu2+ in addition to surface species like citrate, p-mercaptobenzoic acid (PMBA), and halide anions were identified by exploiting coupled localized surface plasmons of flocculated silver nanoparticles (AgNPs). Indeed, SERS spectra provided clear evidence for the formation of citrate complexes of Co2+ and Cu2+ as well as ion pairs of metal ions and PMBA− anions on AgNPs. For the first time, we have detected pronounced Raman bands from hydrated metal ions in flocculates of halide-covered AgNPs assigned to restricted rotation and translation at 600–200 cm−1 and OH stretching bands at 3500–3600 cm−1. Interestingly, the peak frequencies of OH stretching mode from hydrated metal ions are sensitive to metal and halide species, proving the observed water molecules are sandwiched between trapped metal ions and halide ions on adjacent AgNPs while forming a solvent shared ion pair.

Construction of Complex Co3 O4 @Co3 V2 O8 Hollow Structures from Metal-Organic Frameworks with Enhanced Lithium Storage Properties.

A novel metal-organic-framework-engaged strategy is demonstrated for the preparation of multishelled Co3 O4 @Co3 V2 O8 hybrid nanoboxes. This strategy relies on the unique reaction of zeolitic imidazolate framework-67 with the vanadium source of vanadium oxytriisopropoxide. Benefitting from the synthetic versatility, a series of nanostructures can be realized including triple-shelled and double-shelled Co3 O4 @Co3 V2 O8 nanoboxes and single-shelled Co3 V2 O8 nanoboxes. When evaluated as electrode materials for lithium-ion batteries, these unique hollow structures demonstrate remarkable lithium storage properties. For example, the triple-shelled Co3 O4 @Co3 V2 O8 nanoboxes retain a high capacity of 948 mAh g-1 after 100 cycles at 100 mA g-1 .

Spectroscopic characterization of the complex of vinyl radical and carbon dioxide: Matrix isolation and ab initio study.

We report on the preparation and vibrational characterization of the C2H3⋯CO2 complex, the first example of a stable intermolecular complex involving vinyl radicals. This complex was prepared in Ar and Kr matrices using UV photolysis of propiolic acid (HC3OOH) and subsequent thermal mobilization of H atoms. This preparation procedure provides vinyl radicals formed exclusively as a complex with CO2, without the presence of either CO2 or C2H3 monomers. The absorption bands corresponding to the ν5(C2H3), ν7(C2H3), ν8(C2H3), ν2(CO2), and ν3(CO2) modes of the C2H3⋯CO2 complex were detected experimentally. The calculations at the UCCSD(T)/L2a level of theory predict two structures of the C2H3⋯CO2 complex with Cs and C1 symmetries and interaction energies of -1.92 and -5.19 kJ mol-1. The harmonic vibrational frequencies of these structures were calculated at the same level of theory. The structural assignment of the experimental species is not straightforward because of rather small complexation-induced shifts and matrix-site splitting of the bands (for both complex and monomers). We conclude that the C1 structure is the most probable candidate for the experimental C2H3⋯CO2 complex based on the significant splitting of the bending vibration of CO2 and on the energetic and structural considerations.

Carbon-Carbon Bonding between Nitrogen Heterocyclic Carbenes and CO2.

Ab initio MP2/aug'-cc-pVTZ calculations were performed to identify equilibrium complexes and molecules and the transition structures that interconvert them, on the potential energy surfaces of a series of seven binary systems that have nitrogen heterocyclic carbenes (NHCs) as the electron-pair donors to CO2. Seven of the NHCs form complexes stabilized by C···C tetrel bonds, and six of these seven are also stabilized by a secondary interaction between an O of CO2 and the adjacent N-H group of the carbene. Six of the seven NHCs also form stable molecules with C-C covalent bonds, and with one exception, these molecules have binding energies that are significantly greater than the binding energies of the complexes. Charge-transfer stabilizes all of the NHC:CO2 complexes and occurs from the C lone pair of the carbene to the CO2 molecule. The six complexes that have secondary stabilizing interactions are also stabilized by back-donation of charge from the O to the adjacent N-H group of the carbene. Transition structures present barriers to the interconversion of complexes and molecules. With one exception, the barrier for converting a molecule to a complex is much greater than the barrier for the reverse reaction. Atoms in Molecules bonding parameters, shifts of IR C-O stretching and O-C-O bending frequencies, changes in NMR 13C chemical shieldings, and changes in C-C and C-O coupling constants as 1tJ(C-C) and J(C-O) for complexes and transition structures become 1J(C-C) and 2J(C-O) for molecules, are all consistent with the changing nature of the C···C tetrel bond in the complex through the transition state to a covalent C-C bond in the molecule.

The first potential energy surface and microwave spectra of the Hg–CO2 complex

The first ab initio potential energy surface of the Hg-CO2 complex was calculated using the single- and double-excitation coupled-cluster theory with a noniterative perturbation treatment of triple ex-citations [CCSD(T)]. The potential had a global minimum with energy −284.48 cm−1 for a T-shaped geometry with R=6.90a0. Using the potential, the bound states for five isotopomers of Hg-CO2 complexes were calculated. The theoretical rotational spectra and molecular constants are all in good agreement with the experimental counterparts.

Azines as Electron-Pair Donors to CO2 for N···C Tetrel Bonds.

Ab initio MP2/aug'-cc-pVTZ calculations were performed to investigate tetrel-bonded complexes formed between CO2 and the aromatic bases pyridine, the diazines, triazines, tetrazines, and pentazine. Of the 23 unique equilibrium azine:CO2 complexes, 14 have planar structures in which a single nitrogen atom is an electron-pair donor to the carbon of the CO2 molecule, and 9 have perpendicular structures in which two adjacent nitrogen atoms donate electrons to CO2, with bond formation occurring along an N-N bond. The binding energies of these complexes vary from 13 to 20 kJ mol-1 and decrease as the number of nitrogen atoms in the ring increases. For a given base, planar structures have larger binding energies than perpendicular structures. The binding energies of the planar complexes also tend to increase as the distance across the tetrel bond decreases. Charge transfer in the planar pyridine:CO2 complex occurs from the N lone pair to a virtual nonbonding orbital of the CO2 carbon atom. In the remaining planar complexes, charge transfer occurs from an N lone pair to the remote in-plane π*C-O orbital. In perpendicular complexes, charge transfer occurs from an N-N bond to the adjacent π*O-C-O orbital of CO2. Decreases in the bending frequency of the CO2 molecule and in the 13C chemical shielding of the C atom of CO2 upon complex formation are larger in planar structures compared to perpendicular structures. EOM-CCSD spin-spin coupling constants 1tJ(N-C) for complexes with planar structures are very small but still correlate with the N-C distance across the tetrel bond.

Structural Characterization and Redox Activity of a Uranyl Dimer and Transition-Metal Complexes of a Tetradentate BIAN Ligand

The synthesis and characterization of a new ligand system combining the redox-active backbone of Ar-BIANs (N,N′-bis[(aryl)imino]acenaphthenes) and a mixed-donor O–N–N–O salen-type binding pocket is reported. Complexes of Co2+, Ni2+, and UO22+ were prepared and characterized through single-crystal X-ray diffraction and electrochemical studies. The Ni2+ and Co2+ complexes have been used as references against which to compare the unique behaviors exhibited by the uranyl (UO22+) complex, as the latter forms two distinct solid-state structures with unusual oxo contacts to CH2Cl2 and CHCl3 and displays a rich electrochemical profile that indicates a wide range of accessible metal oxidation states through the formation of mixed-valent U(VI)/U(V) and U(V)/U(IV) species in solution.

Photocatalytic removal of rhodamine B under irradiation of visible light using Co1−xCuxFe2O4 (0 ≤ x ≤ 0.5) nanoparticles

Co1−xCuxFe2O4 (0≤x≤0.5) nanoparticles were prepared by the microwave combustion technique. The obtained samples were characterized using X-ray diffraction (XRD) analysis, X-ray photoelectron spectroscopy (XPS), high resolution scanning electron microscopy (HR-SEM), energy dispersive X-ray (EDX) analysis, UV–visible diffuse reflectance spectroscopy, photoluminescence (PL) spectroscopy, Fourier transformed infrared (FT-IR) spectroscopy and vibrating sample magnetometry. The XRD result confirms the formation of cubic spinel structure and the average crystallite size was observed to be in the range of 46–31nm calculated by Debye Scherrer’s equation. XPS analysis was carried out to study the chemical elements and oxidation states of the synthesized Co1−xCuxFe2O4 (0≤x≤0.5) nanoparticles. The HR-SEM image showed agglomerated spherical nanoparticles and the elemental composition of copper doped cobalt ferrite was attained from energy dispersive X-ray analysis. The band gap energy was calculated by diffuse reflectance spectroscopy. Photoluminescence spectra showed the rate of recombination of electron-hole pairs and presence of defects. The band at 661 and 569cm−1 is due to the intrinsic stretching vibrations of octahedral group complex of Co2+–O2− and Fe3+–O2− at tetrahedral sites. Remanence magnetization (Mr), coercivity (Hc) and saturation magnetization (Ms) were deduced from the hysteresis curves. The obtained Co1−xCuxFe2O4 (0≤x≤0.5) nanoparticles were assessed for the photocatalytic degradation of RhB under visible light. A possible mechanism responsible for photocatalytic degradation process is discussed.

Highly Efficient CO2 Capture by Imidazolium Ionic Liquids through a Reduction in the Formation of the Carbene–CO2 Complex

A strategy for improving CO2 capture by imidazolium ionic liquids (ILs) through a reduction in the formation of carbene–CO2 complex was reported. The carbene–CO2 complex content in CO2 capture by imidazolium ILs was determined by a quantitative NMR method, and the carbene–CO2 complex formation was decreased through a reduction in the basicity of the anion and an enlargement in the steric hindrance of the cation. Thus, both enhanced absorption capacity and improved desorption were achieved, where an ideal IL, [Ipmim][Triz], exhibited a very high capacity of 0.21 g of CO2/g of IL and excellent reversibility.

The CO2-O2 van der Waals complex

Using the explicitly correlated CCSD(T)-F12 method with cc-pVXZ-F12 (VXZ-F12) and aug-cc-pVXZ (AVXZ) (up to X=Q) basis sets, various structures of the triplet CO2-O2 complex were investigated. Accordingly, the planar slipped-tilted isomer is most stable, with a binding energy (ΔE) of 230cm−1, followed by the cross-shaped C2v-X isomer, having ΔE=202cm−1 and by the in-plane T-shaped C2v-Ta isomer with ΔE=181cm−1 (energies obtained from counterpoise corrected optimizations using the VQZ-F12 basis set). Binding energies using the AVQZ basis set are similar, with 238, 211 and 188cm−1, respectively. Frequency calculations showed that C2v-Ta is a first-order saddle point. The binding energies are in line with values obtained for related CO2 complexes, although the slipped-tilted structure of the most stable isomer of the CO2-O2 complex is somewhat unexpected. To our knowledge, this is the first theoretical study of CO2-O2 complexes.

Carbenes as Electron-Pair Donors To CO2 for C···C Tetrel Bonds and C-C Covalent Bonds.

Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to identify stable complexes and molecules and the transition structures that interconvert them on the potential surfaces of ten singlet carbene bases acting as electron-pair donors to CO2. The carbene bases include cyclic C(NHCH)2 or NHC, C(NH2)2, an oxygen heterocyclic carbene C(OCH)2 or OHC, C(OH)2, C(CH3)2, cyclic C3H2, CCCH2, CCl2, CCH2, and CF2. Carbene:CO2 complexes stabilized by C···C tetrel bonds have been found on all potential surfaces, whereas carbene-CO2 molecules stabilized by C-C covalent bonds have been found on eight surfaces. Three of these molecules have open structures with C2v symmetry, whereas the remaining have cyclic three membered C-O-C rings with Cs symmetry. The transition structures which connect the complex and the molecule are bound on three of the potential surfaces. Whether the transition structure is bound or unbound relative to the carbene and CO2 depends on the relationship among C-C distances at the three stationary points on the surface. Charge-transfer interactions stabilize carbene:CO2 complexes. The primary charge transfer in complexes arises from electron donation from the carbene lone-pair to the CO2 molecule. There is also back-donation of charge from CO2 to the carbene in three complexes. Systematic changes in bonding properties occur as complexes go through transition structures and become molecules. EOM-CCSD inter- and intramolecular C-C and C-O spin-spin coupling constants have been computed and compared for complexes and molecules. A search of the CSD database found the (NH2)2C-CO2 structure and 17 NHC-CO2 derivatives. Computed bond distances and angles have been compared with experimental data.