Pentacoordinated Organoaluminum Complexes: A Computational Insight

The geometry and the electronic structure of a series of organometallic pentacoordinated aluminum complexes bearing tri- or tetradentate N,O-based ligands have been investigated with theoretical methods. The BP86, B3LYP, and M06 functionals reproduce with low accuracy the geometry of the selected complexes. The worst result was obtained for the complex bearing a Schiff base ligand with a pendant donor arm, <i>aeimp</i>AlMe₂ (<i>aeimp</i> = <i>N</i>-2-(dimethylamino)­ethyl-(3,5-di-<i>tert</i>-butyl)­salicylaldimine). In particular, the Al–N_amine bond distance was unacceptably overestimated. This failure suggests a reasonably flat potential energy surface with respect to Al–N elongation, indicating a weak interaction with probably a strong component of dispersion forces. MP2 and M06-2X methods led to an acceptable value for the same Al–N distance. Better results were obtained with the addition of the dispersion correction to the hybrid B3LYP functional (B3LYP-D). Natural bond orbital analysis revealed that the contribution of the d orbital to the bonding is very small, in agreement with several previous studies of hypervalent molecules. The donation of electronic charge from the ligand to metal mainly consists in the interactions of the lone pairs on the donor atoms of the ligands with the s and p valence orbitals of the aluminum. The covalent bonding of the Al with the coordinated ligand is weak, and the interactions between Al and the coordinated ligands are largely ionic. To further explore the geometrical and electronic factors affecting the formation of these pentacoordianted aluminum complexes, we considered the tetracoordinated complex <i>imp</i>AlMe₂ (<i>imp</i> = <i>N</i>-isopropyl-(3,5-di-<i>tert</i>-butyl)­salicylaldimine)), analogous to <i>aeimp</i>AlMe₂, and we investigated the potential energy surface around the aluminum atom corresponding to the approach of NMe₃ to the metal center. At the MP2/6-31G­(d) level of theory, a weak attraction was revealed only when NMe₃ heads toward the metal center through the directions trans to the nitrogen atom. The analysis of the binding energies for this adducts revealed that the formation of the pentacoordinated derivative is a result of a subtle balance between the penalty paid to deform the <i>imp</i>AlMe₂ complex and energy gain resulting from interaction between the two fragments

Selectivity of Electrochemical CO₂ Reduction on Metal Electrodes: The Role of the Surface Oxidized Layer

In the past decade, density functional theory (DFT) calculations have been employed to study the mechanism of electrochemical CO2 reduction reactions. However, the lack of understanding of the CO2 chemisorption states, proton-coupled-electron-transfer (PCET) steps, and dynamic redox reactions of the electrode surface has limited the reliability of these simulations. The *OCHO and *COOH species are widely recognized as the key intermediates for the formic acid and carbon monoxide production, respectively. However, the comparison between the binding energies of *OCHO and *COOH cannot directly indicate the reaction trends. In this work, we propose that the energy difference between *COOH on the neutral and extra-electron substrates, in the form of [ΔG(*COOHe) – ΔG(*COOH)], can serve as a descriptor for the electrochemical CO2 reduction selectivity. In addition, the computational hydrogen electrode (CHE) model is revised by applying the previously studied charged species. The noninteger charge-transfer (NICT) model is used for the calculation of energy profile at a certain potential, which can have a good prediction of the potential-limiting step. The surface oxide of metal electrodes is found to play a key role in modulating the selectivity and improving the electron transfer to CO2

π-Face Donation from the Aromatic N-Substituent of N-Heterocyclic Carbene Ligands to Metal and Its Role in Catalysis

In this work, we calculate the redox potential in a series of Ir and Ru complexes bearing a N-heterocyclic carbene (NHC) ligand presenting different Y groups in the para position of the aromatic N-substituent. The calculated redox potentials excellently correlate with the experimental Δ<i>E</i>_1/2 potentials, offering a handle to rationalize the experimental findings. Analysis of the HOMO of the complexes before oxidation suggests that electron-donating Y groups destabilize the metal centered HOMO. Energy decomposition of the metal–NHC interaction indicates that electron-donating Y groups reinforce this interaction in the oxidized complexes. Analysis of the electron density in the reduced and oxidized states of representative complexes indicates a clear donation from the C_ipso of the N-substituents to an empty d orbital on the metal. In case of the Ru complexes, this mechanism involves the Ru–alkylidene moiety. All of these results suggest that electron-donating Y groups render the aromatic N-substituent able to donate more density to electron-deficient metals through the C_ipso atom. This conclusion suggests that electron-donating Y groups could stabilize higher oxidation states during catalysis. To test this hypothesis, we investigated the effect of differently donating Y groups in model reactions of Ru-catalyzed olefin metathesis and Pd-catalyzed C–C cross-coupling. Consistent with the experimental results, calculations indicate an easier reaction pathway if the N-substituent of the NHC ligand presents an electron-donating Y group

Determination of the Intrinsic Defect at the Origin of Poor H₂ Evolution Performance of the Monoclinic BiVO₄ Photocatalyst Using Density Functional Theory

The effects of intrinsic defects in monoclinic bismuth vanadate (BiVO₄) on its stability and optoelectronic properties for photochemical water splitting application were examined using density functional theory. Among the most favorable structures, only that associated with V-antisites on Bi with additional Bivacancies (Bi_{(1–5<i>x</i>)}V_{(1+3<i>x</i>)}O₄ with <i>x</i> = 0.0625) revealed narrower band gap energy by 0.5 eV compared to pristine material (calculated value is 2.8 eV) giving a value of 2.3 eV, which is very close to the experimentally reported ones (in the 2.4–2.5 eV range). The low electron mobility reported experimentally for this material was also confirmed by the relatively large electron effective masses obtained for the intrinsic defective Bi_{(1–5<i>x</i>)}V_{(1+3<i>x</i>)}O₄ (<i>x</i> = 0.0625) structure along the three principal crystallographic directions. The strongly localized nature of the accommodated electrons on the d-orbitals of the newly substituted V at Bi sites was also predicted to be at the origin of the poor H₂ evolution performance of this material

Application of Semiempirical Methods to Transition Metal Complexes: Fast Results but Hard-to-Predict Accuracy

A series of semiempirical PM6* and PM7 methods has been tested in reproducing relative conformational energies of 27 realistic-size complexes of 16 different transition metals (TMs). An analysis of relative energies derived from single-point energy evaluations on density functional theory (DFT) optimized conformers revealed pronounced deviations between semiempirical and DFT methods, indicating a fundamental difference in potential energy surfaces (PES). To identify the origin of the deviation, we compared fully optimized PM7 and respective DFT conformers. For many complexes, differences in PM7 and DFT conformational energies have been confirmed often manifesting themselves in false coordination of some atoms (H, O) to TMs and chemical transformations/distortion of coordination center geometry in PM7 structures. Despite geometry optimization with fixed coordination center geometry leading to some improvements in conformational energies, the resulting accuracy is still too low to recommend explored semiempirical methods for out-of-the-box conformational search/sampling: careful testing is always needed

Catalytic Role of Nickel in the Decarbonylative Addition of Phthalimides to Alkynes

Density functional theory calculations have been used to investigate the catalytic role of nickel(0) in the decarbonylative addition of phthalimides to alkynes. According to Kurahashi et al. the plausible reaction mechanism involves a nucleophilic attack of nickel at an imide group, giving a six-membered metallacycle, followed by a decarbonylation and insertion of an alkyne leading to a seven-membered metallacycle. Finally a reductive elimination process produces the desired product and regenerates the nickel­(0) catalyst. In this paper, we present a full description of the complete reaction pathway along with possible alternative pathways, which are predicted to display higher upper barriers. Our computational results substantially confirm the proposed mechanism, offering a detailed geometrical and energetical understanding of all the elementary steps

Application of Semiempirical Methods to Transition Metal Complexes: Fast Results but Hard-to-Predict Accuracy

A series of semiempirical PM6* and PM7 methods has been tested in reproducing relative conformational energies of 27 realistic-size complexes of 16 different transition metals (TMs). An analysis of relative energies derived from single-point energy evaluations on density functional theory (DFT) optimized conformers revealed pronounced deviations between semiempirical and DFT methods, indicating a fundamental difference in potential energy surfaces (PES). To identify the origin of the deviation, we compared fully optimized PM7 and respective DFT conformers. For many complexes, differences in PM7 and DFT conformational energies have been confirmed often manifesting themselves in false coordination of some atoms (H, O) to TMs and chemical transformations/distortion of coordination center geometry in PM7 structures. Despite geometry optimization with fixed coordination center geometry leading to some improvements in conformational energies, the resulting accuracy is still too low to recommend explored semiempirical methods for out-of-the-box conformational search/sampling: careful testing is always needed

Mechanistic Insights into the Organopolymerization of <i>N</i>‑Methyl <i>N</i>‑Carboxyanhydrides Mediated by <i>N</i>‑Heterocyclic Carbenes

We report on a DFT investigation of initiation, propagation, and termination in the organopolymerization of <i>N</i>-methyl <i>N</i>-carboxyanhydrides toward cyclic poly­(<i>N</i>-substituted glycine)­s, promoted by <i>N</i>-heterocyclic carbenes (NHC). Calculations support the experimentally based hypothesis of two competing initiation pathways. The first leading to formation of a zwitterionic adduct by nucleophilic addition of the NHC to one of the carbonyl groups of monomer. The second via acid–base reactivity, starting with the NHC promoted abstraction of a proton from the methylene group of the monomer, leading to an ion-pair-type adduct, followed by nucleophilic attack of the adduct to a new monomer molecule. Chain elongation can proceed from both the initiation adducts via nucleophilic attack of the carbamate chain-end to a new monomer molecule via concerted elimination of CO₂ from the carbamate chain-end. Energy barriers along all the considered termination pathways are remarkably higher that the energy barrier along the chain elongation pathways, consistent with the quasi-living experimental behavior. Analysis of the competing termination pathways suggests that the cyclic species determined via MALDI-TOF MS experiments consists of a zwitterionic species deriving from nucleophilic attack of the N atom of the carbamate chain-end to the CO group bound to the NHC moiety

Application of Semiempirical Methods to Transition Metal Complexes: Fast Results but Hard-to-Predict Accuracy

A series of semiempirical PM6* and PM7 methods has been tested in reproducing relative conformational energies of 27 realistic-size complexes of 16 different transition metals (TMs). An analysis of relative energies derived from single-point energy evaluations on density functional theory (DFT) optimized conformers revealed pronounced deviations between semiempirical and DFT methods, indicating a fundamental difference in potential energy surfaces (PES). To identify the origin of the deviation, we compared fully optimized PM7 and respective DFT conformers. For many complexes, differences in PM7 and DFT conformational energies have been confirmed often manifesting themselves in false coordination of some atoms (H, O) to TMs and chemical transformations/distortion of coordination center geometry in PM7 structures. Despite geometry optimization with fixed coordination center geometry leading to some improvements in conformational energies, the resulting accuracy is still too low to recommend explored semiempirical methods for out-of-the-box conformational search/sampling: careful testing is always needed

Major Difference in Visible-Light Photocatalytic Features between Perfect and Self-Defective Ta₃N₅ Materials: A Screened Coulomb Hybrid DFT Investigation

Relevant properties to visible-light overall water splitting reactions of perfect and self-defective bulk Ta₃N₅ semiconductor photocatalysts are investigated using accurate first-principles quantum calculations on the basis of density functional theory (DFT, including the perturbation theory DFPT) within the screened coulomb hybrid (HSE06) exchange-correlation formalism. Among the various explored self-defective structures, a strong stabilization is obtained for the configuration displaying a direct interaction between the created N- and Ta-vacancies. In the lowest-energy structure, each of the three created Ta-vacancies and the five created N-vacancies is found to be in aggregated disposition, leading to the formation of cages into the lattice. Although the calculated structural, electronic, and optical properties of the two materials are found to be very similar and in good agreement with available experimental works, their photocatalytic features for visible-light overall water splitting reactions show completely different behaviors. On the basis of calculated band edge positions relative to water redox potentials, the perfect Ta₃N₅ (calculated band gap of 2.2 eV) is predicted by HSE06 to be a good candidate only for H⁺ reduction while the self-defective Ta₃N₅ (calculated band gap of 2.0 eV) reveals suitable band positions for both water oxidation and H⁺ reduction similar to the experimental data reported on Ta₃N₅ powders. Its ability to reduce H⁺ is predicted to be lower than the perfect one. However, the strongly localized electronic characters of the valence band (VB) and conduction band (CB) edge states of the self-defective material only on the N 2p and Ta 5d orbitals surrounding the aggregated N- and Ta-vacancies are expected to strongly limit the probability of photogenerated carrier mobility through its crystal structure