Orbitals For Chemical Valence Research Articles

Exploring the bonding and extraction separation effects of N donor ligands with different skeletons on Am(III) and Eu(III)

Designing ligands for effective separation of actinide(III)/lanthanide(III) is one of the key issues in the treatment of spent nuclear fuel. However, due to the chemical similarity of trivalent actinides and lanthanides, their separation faces a great challenge in the reprocessing of spent nuclear fuel. Nitrogen donor extractants are considered as promising reagents for separation of trivalent actinides and lanthanides. To the best of our knowledge, the pre-organized structure of the ligand has a strong influence on the extraction separation capacity. In this work, we designed four N-donor ligands (L1 and L2 with pyridine and bipyridine skeletons, respectively, and L3 and L4 both with phenanthroline skeletons) with different pre-organized structures and investigated their selective extraction for Am(III) and Eu(III) by using scalar relativistic density functional theory (DFT). The absolute minimum ESP values of these ligands and the higher HOMO level suggest that the ligand L2 may have a stronger affinity for Am(III)/Eu(III) ions than other ligands. The three average bond lengths of each complex indicate that the Am(III) and Eu(III) ions have larger bond lengths with N2 atoms than with N1 atoms. Various theoretical approaches have been used to evaluate the properties of the four ligands as well as the coordination structures, bonding properties and thermodynamic properties of the complexes of the four ligands with Am(III) and Eu(III). The WBIs, QTAIM, NBO, DOS and EDA analysis indicate that the metal–ligand bonds are mainly ionic in character, the Am-N bond has more covalency than the Eu-N bond because the Am 5f orbitals are more involved in bonding with ligand than Eu 4f orbitals. According to the extended transition state and natural orbital chemical valence theory (ETS-NOCV) analysis, electrons are obviously transferred from the lone pair of the ligand N atom to the hybrid s, d and f orbitals of the metal ion. The thermodynamic results show that the four ligands can coordinate well with the Am and Eu ions, and also have suitable separation capacity for Am and Eu. Among the four ligands, the L2 and L3 ligands have the strongest coordination ability for metals and the best extraction separation effect for Am and Eu. It can be seen that the bridging skeleton of the ligand and the number of N atoms on the five-membered ring and also different organic solvents have obvious effects on the extraction and separation capacity of the ligand. This study provides a theoretical basis for the efficient separation of Am(III)/Eu(III) by adjusting the preorganization level of ligands, and provides a theoretical basis for the design and screening of ligands by preorganization.

Torsional Rotation in Ditopic Receptor Host and its Complex Formation with Resorcinol Guest: A Computational Study.

Noncovalent interactions due to the presence of heteroatoms in supramolecular compounds have gained a lot of attention. These different heteroatom-based supramolecular compounds have inspired us to examine the noncovalent interaction in the isolated host and host-guest complexes. In view of this, in the current manuscript, we investigated the stability and torsional energy barrier of different conformers of the ditopic receptor host 1,6-bis(2,6-bis(benzothiazol-2-yl) pyridine-4-yloxy) hexane (bbh). The conformer that is accompanied by intramolecular C-H⋯N and C-H⋯S interactions is relatively more stable than the others. Due to torsional angle rotation within the host, the C-H⋯N and C-H⋯S interactions get disrupted and exhibit different binding sites for capturing guest molecules. In addition, we have extended the investigation to understand the interaction energy and nature of interaction in host-guest (1 : 1 and 1 : 2) complexes formed between the host (bbh) and guest (resorcinol) by using different DFT functionals. Extended transition state-natural orbital chemical valence (ETS-NOCV) analysis of complexes revealed that the electrostatic interaction significantly contributes to the host-guest interaction energy. The noncovalent (NCI) analysis provides the existence of intermolecular hydrogen bonding and other weak interactions within the complexes.

Probing into complexation and separation of chiral‐at‐uranium complex to chiral sulphur enantiomers R/S‐ethiproles

Chiral pesticides R/S‐ethiproles(R/S‐EPLs) have different biotoxicity. Due to the difficulty in the separation of chiral sulphur enantiomers, it is of great significance to study the selective recognition and separation of chiral sulphur enantiomers R/S‐ethiproles. Based on DFT method, we theoretically designed a novel asymmetrical ligand of 4‐amino‐6‐(9‐[5‐hydroxy‐1H‐indol‐6‐yl]‐4,7‐dimethoxy‐1,10‐phenanthrolin‐2‐yl)‐1H‐indol‐5‐ol (AHPIL). Then we further constructed the chiral‐at‐uranium complex, namely, Uranyl‐AHPIL, to recognize and separate the enantiomers R/S‐EPLs. Various analyses, including electrostatic potential (ESP), molecular geometric structures, Mayer bond order (MBO), electron density difference diagram (EDDM), independent gradient model based on Hirschfield partition (IGMH), quantum theory of atoms in molecules (QTAIM), molecular orbitals, the extended transition state‐natural orbitals for chemical valence (ETS‐NOCV), hard–soft acid–base (HSAB), relaxed force constants (RFCs) and binding free energy, furthermore unravelled the selective complexation and chiral separation ability of Uranyl‐AHPIL receptor to R/S‐EPLs guests. The results all indicated that the novel ligand could coordinate with uranyl to form a stable chiral‐at‐uranium complex, which more inclined selectively to bind with R/S‐EPLs through thiyl oxygen O22, and its complexation ability to S‐EPL was much greater than to R‐EPL. In solvents water, acetone, dichloromethane, benzene, cyclohexane and in gas phase, the separation factors of Uranyl‐AHPIL towards R/S‐EPL were in range of 189–633, and the enantioselectivity coefficients were all above 99%. These results provide a theoretical guidance for the complexation and separation of chiral pesticides and at the same time offer a new reference for the design of actinide complexes.

Biomimics of phenazine oxidase activity of a cobalt (III)‐dipyridylamine complex: Spectroscopic, structural, and computational studies†

AbstractThis manuscript demonstrates the synthesis, structural characterization, computational studies, and biomimics of the phenazine oxidase activity of a newly designed cobalt (III) complex, [Co (dpa)(dpa‐H+)(N3)2]Cl2 (1) [dpa = 2,2′‐dipyridylamine] under an aerobic condition. The crystal structure analysis reveals that the cobalt (III) center adopts an octahedral geometry, and the complex forms a beautiful supramolecular framework through noncovalent interactions. The cobalt (III) catalyst turns out to be a promising catalyst for the oxidative coupling of o‐phenylenediamine (OPD) in oxygen‐saturated methanol with an excellent turnover number, kcat = 7.85 × 103 h−1. Spectrophotometric, electrochemical, mass spectrometry, and computational analysis ensure that the course of catalysis undergoes through a catalyst‐substrate complexation, facilitating the development of cobalt‐iminobenzoquinone species in the solution. The computational calculations employing the density functional theory (DFT) throw a light to the mechanistic insights of the phenazine oxidase mimics. ETS‐NOCV (extended transition state‐natural orbitals for chemical valence) plots of the reactive intermediates portray the coordination‐driven depletion of electron density from the nitrogens of OPD to cobalt center leading to the enhancement of electrophilic character on para‐positioned C‐atoms with respect to N‐atoms of OPD, therby catalyzing the nucleophilic attack by second OPD to produce the oxidation product, 2,3‐diaminophenazine (DAP). Interestingly, we are able to isolate the oxidation product of the OPD oxidation reaction as a hydrated chloride salt, DAPH+Cl− ·3H2O (2). The crystal engineering perspectives of 2 attribute the intriguing fate of the secondary chlorides for the stabilization of the oxidation product in crystalline phase.

Stabilization of group 14 elements E = C, Si, Ge by hetero-bileptic ligands cAAC, MCOn with push-pull mechanism.

The stability and bonding of a series of hetero-diatomic molecules with general formula (cAAC)EM(CO)n , where cAAC = cyclic alkyl(amino) carbene; E = group 14 elements (C, Si, and Ge); M = transition metal (Ni, Fe, and Cr) have been studied by quantum chemical calculations using density functional theory (DFT) and energy decomposition analysis-natural orbital chemical valence (EDA-NOCV). The equilibrium geometries were calculated at the BP86/def2-TZVPP level of theory. The tri-coordinated group 14 complex (1a, 4a, and 7a) in which one of the CO groups is migrated to the central group 14 element from adjacent metal is theoretically found to be more stable when the central atom (E) is carbon. On the other hand, the two-coordinate group 14 element containing metal-complexes (2, 5, 8, 3, 6, and 9) are found to be more stable with their corresponding heavier analogues. The electronic structures of all the molecules have been analyzed by molecular orbital, topological analysis of electron density and natural bond orbital (NBO) analysis at the M06/def2-TZVPP//BP86/def2-TZVPP level of theory. The nature of the cAACE and EM bonds has been studied by EDA-NOCV calculations at BP86-D3(BJ)/TZ2P level of theory. The EDA analysis suggests that the bonding of cAACC(CO) can be best represented by electron sharing σ and π interactions, whereas, C(CO)M(CO)n-1 by dative σ and π interactions. On the other hand, EDA-NOCV calculations suggests both dative σ and π interactions for cAACE and EM(CO)n bonds of the corresponding Si and Ge analogues having stronger σ- and relatively weaker π-bonds. The topological analysis of electron density supports the closed-shell interaction for the Si and Ge complexes and open-shell interaction for the carbon complexes. The calculated proton affinity and hydride affinity values corroborated well with the present bonding description. This class of complexes might act as efficient future catalysts for different organic transformations due to the presence of electron rich group 14 element and metal carbonyl.

Through-Bond and Through-Space Interactions in [2,2]Cyclophanes

The interpretation of the distortions of the electron distribution in [2,2]cyclophanes (22-CPs) is controversial. Some studies indicate that there is an accumulation of electron density (ρ) outside the cavity of 22-CPs. The nature of through-space (ts) interaction is still under debate. The relative importance of ts and through-bond (tb) is an open question. In an attempt to clarify these points, we have investigated five 22-CPs and their corresponding toluene dimers by molecular orbitals analysis, electron density difference analysis, some topological analysis of ρ (quantum theory of atoms in molecules (QTAIM), electron localization function (ELF) and noncovalent interactions (NCI)), and energy decomposition analysis with natural orbitals for chemical valence (EDA-NOCV). ρ is concentrated inside the inter-ring region. All the analyses indicated that ts is predominant. The ts is composed by attractive dispersion and Pauli repulsion, with a small covalent contribution. Except for 1 and 6, all the compounds present inter-ring bond paths.

Interplay of Hückel and Möbius Aromaticity in Metal-Metal Quintuple Bonded Complexes of Cr, Mo, and W with Amidinate Ligand: Ab initio DFT and Multireference Analysis*.

The aromaticity of metal-metal quintuple bonded complexes of the type M2 L2 (M=Cr, Mo, and W; L=amidinate) are studied employing gauge including magnetically induced ring current (GIMIC) analysis and electron density of delocalized bonds (EDDB). It is found that the complexes possess two types of aromaticity: i) Hückel aromaticity through delocalization of ligand π electrons with metal-metal δ-bond-forming 6 conjugated electrons (4π and 2δ) ring; ii) Craig-Möbius aromaticity through delocalization of π electrons of both the ligands with metal d-orbitals in Craig type orientation forming 10π electrons ring with a double twist. Extended transition state natural orbital chemical valence (ETS-NOCV) and canonical molecular orbital natural chemical shielding (CMO-NCS) analysis confirm the Craig-Möbius type arrangement of the orbitals. Furthermore, the unprecedented Hückel and Möbius type aromaticity is confirmed from the plot of the current pathways using 3D line integral convolution (3D-LIC) plots. The metal-metal bond order also increases down the group as justified from the complete active space self-consistent field (CASSCF) analysis. Due to an increase in the π and δ electron conjugation, both the Hückel and Möbius aromaticity increase down the group.

Spectroscopic/Bond Property Relationship in Group 11 Dihydrides via Relativistic Four-Component Methods.

Group 11 dihydrides MH2– (M = Cu, Ag, Au, Rg) have been much less studied than the corresponding MH compounds, despite having potentially several interesting applications in chemical research. In this work, their main spectroscopic constants (bond lengths, dissociation energies, and force constants) have been evaluated by means of highly accurate relativistic four-component coupled cluster (4c-CCSD(T)) calculations in combination with large basis sets. Periodic trends have been quantitatively explained by the charge-displacement/natural orbitals for chemical valence (CD-NOCV) analysis based on the four-component relativistic Dirac–Kohn–Sham method, which allows a consistent picture of the nature of the M–H bond to be obtained on going down the periodic table in terms of Dewar–Chatt–Duncanson bonding components. A strong ligand-to-metal donation drives the M–H bond and it is responsible for the heterolytic (HM···H–) dissociation energies to increase monotonically from Cu to Rg, with RgH2– showing the strongest and most covalent M–H bond. The “V”-shaped trend observed for the bond lengths, dissociation energies, and stretching frequencies can be explained in terms of relativistic effects and, in particular, of the relativistically enhanced sd hybridization occurring at the metal, which affects the metal–ligand distances in heavy transition-metal complexes. The sd hybridization is very small for Cu and Ag, whereas it becomes increasingly important for Au and Rg, being responsible for the increasing covalent character of the bond, the sizable contraction of the Au–H and Rg–H bonds, and the observed trend. This work rationalizes the spectroscopic/bond property relationship in group 11 dihydrides within highly accurate relativistic quantum chemistry methods, paving the way for their applications in chemical bond investigations involving heavy and superheavy elements.

Periodic trends and complexation chemistry of tetravalent actinide ions with a potential actinide decorporation agent 5-LIO(Me-3,2-HOPO): A relativistic density functional theory exploration.

A relativistic density functional theory (DFT) study is reported which aims to understand the complexation chemistry of An4+ ions (An = Th, U, Np, and Pu) with a potential decorporation agent, 5-LIO(Me-3,2-HOPO). The calculations show that the periodic change of the metal binding free energy has an excellent correlation with the ionic radii and such change of ionic radii also leads to the structural modulation of actinide-ligand complexes. The calculated structural and binding parameters agree well with the available experimental data. Atomic charges derived from quantum theory of atoms in molecules (QTAIM) and natural bond order (NBO) analysis shows the major role of ligand-to-metal charge transfer in the stability of the complexes. Energy decomposition analysis, QTAIM, and electron localization function (ELF) predict that the actinide-ligand bond is dominantly ionic, but the contribution of orbital interaction is considerable and increases from Th4+ to Pu4+ . A decomposition of orbital contributions applying the extended transition state-natural orbital chemical valence method points out the significant π-donation from the oxygen donor centers to the electron-poor actinide ion. Molecular orbital analysis suggests an increasing trend of orbital mixing in the context of 5f orbital participation across the tetravalent An series (Th-Pu). However, the corresponding overlap integral is found to be smaller than in the case of 6d orbital participation. An analysis of the results from the aforementioned electronic structure methods indicates that such orbital participation possibly arises due to the energy matching of ligand and metal orbitals and carries the signature of near-degeneracy driven covalency.

Craig-Möbius type antiaromatic chelate ring

Gauge including magnetically induced ring current (GIMIC) calculations on the chelate rings of o-benzoquinonediimine (bqdi) in [M(bqdi)2F2] (M = Ru, Os) gives significant negative current. It implies an antiaromatic character. Excited transition state-natural orbital chemical valence (ETS-NOCV) gives a back donation from d-orbital to the ligand antibonding π* orbital. It shows Craig-Möbius type orbital arrangement containing (4n + 2)π electrons. The current pathways of the chelate rings are visualized using the line integral convolution (LIC) and streamline plots. To the best of my knowledge, this is the first report of an antiaromatic Craig-Möbius metal chelate ring by ab initio calculation.

ETS-NOCV decomposition of the reaction force for double-proton transfer in formamide-derived systems

The analysis of the electronic-structure changes along IRC paths for double-proton-transfer reactions in the formamide dimer (R1), formamide–thioformamide system (R2), and the thioformamide dimer (R3) was performed based on the extended-transition-state natural orbitals for chemical valence (ETS-NOCV) partitioning of the reaction force, considering the intra-fragments strain and the inter-fragments interaction terms, and further—the electrostatic, Pauli-repulsion and orbital interaction components, with the latter being decomposed into the NOCV components. Two methods of the system partitioning into the fragments were considered (‘reactant perspective’/bond-formation, ‘product perspective’ / bond-breaking). In agreement with previous studies, the results indicate that the major changes in the electronic structure occur in the transition state region; the bond-breaking processes are, however, initiated already in the reactant region, prior to entering the TS region. The electrostatic contributions were identified as the main factor responsible for the increase in the activation barrier in the order R1 < R2 < R3.

Electronic structure and bonding of the dinuclear metal M2(CO)10 decacarbonyls: applications of natural orbitals for chemical valence.

The nature of the chemical metal-metal bond in M2(CO)10 (M = Mn, Re, Tc) dinuclear decacarbonyls complexes was investigated for the first time using the natural orbital chemical valence (NOCV) approach combined with the extended transition state (ETS) for energy decomposition analysis (EDA). The optimized geometries carried out at different levels of theory BP86, BLYP, BLYPD and BP86D, showed that the latter method, i.e., BP86D, led to the best agreement with X-ray experimental measurements. The BP86D/TZP results revealed that the computed covalent contribution to the metal-metal bond are 60.5%, 54.1% and 52.0% for Mn-Mn, Re-Re and Tc-Tc, respectively. The computed total interaction energies resulting from attractive terms (ΔE orb and ΔE eles), correspond well to experimental predictions, based on bond lengths and energy interaction analysis for the studied complexes.

A Comparison of Donor-acceptor Interactions in Borane Complexes of Divalent Tetrylenes(II) and Divalent Tetrylones(0) using Energy Decomposition Analysis Method with Natural Orbital for Chemical Valence Theory

A scheme for chemical bond analysis by combining the energy decomposition analysis (EDA) method with natural orbitals for chemical valence (NOCV) theory has been investigated. Quantum chemical calculations at the BP86/TZ2P+ level of theory are performed for a comparison of chemical bond analysis between tetrylones [BH3-{E(PH3)2}] (B3-EP2) and tetrylenes [(BH3-{NHEMe}] (B3-NHE) (E = C to Pb). The EDA-NOCV results suggest that the B–E bond dissociation energies (BDEs) in tetrylones and tetrylenes decrease from the slighter to the heavier homologs. The decrease in the bond strengths from the lighter to the heavier homologs of B3-EP2 comes mainly from the decrease in the electrostatic attractions ΔEelstat and the orbital interactions ΔEorb, while the decrease in the bond strength from carbene B3-NHC to plumbylene B3-NHPb strongly correlates with the decrease in electrostatic term ΔEelstat.

Electronic propensity of Cu(II) versus Cu(I) sites in zeolites to activate NO — Spin- and orbital-resolved Cu–NO electron transfer

Electronic factors responsible for the notable decline of NO activation by Cu(II) with respect to Cu(I) sites in zeolites are investigated within spin-resolved analysis of electron transfer channels between the copper center and the substrate. The results of natural orbitals for chemical valence (NOCV) charge transfer analysis for a minimal model of Cu(II) sites in zeolite ZSM-5 ({T1Cu}+ NO) are compared with those for Cu(I)–NO and referenced to an interaction of NO with bare Cu+ cations. The bonding of NO, which is an open-shell and non-innocent ligand, gives rise to a noticeable nondynamical correlation in the adduct with Cu(II) (reflected in a broken-symmetry solution obtained at the density functional theory (DFT) level). Four distinct components of electron transfer between the copper and NO are identified: (i) donation of an unpaired electron from the NO π∥* antibonding orbital to the Cu species, (ii) backdonation from copper d⊥ to the NO antibonding orbital, (iii) “covalent” donation from NO π∥ and Cu d∥ orbitals to the bonding region, and (iv) donation from the nitrogen lone pair to Cus,d. Large variations in channel identity and significance may be noted among studied systems and between spin manifolds: channel i is effective only in the bonding of NO with either a naked Cu+ cation or Cu(II) site. Channel ii comes into prominence only for the model of the Cu(I) site: it strongly activates the NO bond by populating antibonding π*, which weakens the N–O bond, in contrast to channel i depopulating the antibonding orbital and strengthening the N–O bond. Channels iii and iv, however, may contribute to the strength of the bonding between NO and copper, and are of minor importance for the activation of the NO bond. This picture perfectly matches the IR experiment: interaction with either Cu(II) sites or a naked Cu+ cation imposes a comparable blue-shift of NO stretching frequency, while the frequency becomes strongly red-shifted for a Cu(I) site in ZSM-5 due to enhanced π* backdonation.