Central Sn Atom Research Articles

Synthesis and Characterization of an Air‐Stable Tin(IV) β‐Tetracyanoisophlorin Complex: Enhanced Antiaromaticity through Metal Complexation

Isophlorins, which are two‐electron reduced porphyrins, have garnered significant interest because of their potential antiaromaticity and unique electronic properties. However, the isolation of stable antiaromatic isophlorins remains challenging because they are prone to oxidation or reduction to the corresponding aromatic molecules, or are structurally distorted, resulting in loss of antiaromaticity. Here, we report the synthesis and characterization of a Sn(IV) β‐tetracyanoisophlorin complex obtained through two‐electron reduction of the corresponding Sn(IV) β‐tetracyanoporphyrin complex. This Sn(IV) complex exhibits enhanced antiaromaticity owing to its highly planar structure, facilitated by the central Sn atom. X‐ray crystal diffraction analysis revealed that the Sn(IV) isophlorin complex contains two methoxy groups as axial ligands and that it exists as a divalent anion, demonstrating the oxophilic nature of the Sn(IV) isophlorin complexes. Notably, this Sn(IV) isophlorin complex is stable under air, distinguishing it from previously reported Si(IV) and Ge(IV) isophlorin complexes without cyano groups. This study thus presents the first isolation of an air‐stable metal complex of antiaromatic isophlorin, opening new possibilities for exploring the unique properties of antiaromatic porphyrinoids for various applications, including catalysis and materials science.

Synthesis of organotin(IV) heterocycles containing a xanthenyl group by a Barbier approach via ultrasound activation: synthesis, crystal structure and Hirshfeld surface analysis.

A series of organotin heterocycles of general formula [{Me2C(C6H3CH2)2O}SnR2] [R= methyl (Me, 4), n-butyl (n-Bu, 5), benzyl (Bn, 6) and phenyl (Ph, 7)] was easily synthesized by a Barbier-type reaction assisted by the sonochemical activation of metallic magnesium. The 119Sn{1H} NMR data for all four compounds confirm the presence of a central Sn atom in a four-coordinated environment in solution. Single-crystal X-ray diffraction studies for 17,17-dimethyl-7,7-diphenyl-15-oxa-7-stannatetracyclo[11.3.1.05,16.09,14]heptadeca-1,3,5(16),9(14),10,12-hexaene, [Sn(C6H5)2(C17H16O)], 7, at 100 and 295 K confirmed the formation of a mononuclear eight-membered heterocycle, with a conformation depicted as boat-chair, resulting in a weak Sn...O interaction. The Sn and O atoms are surrounded by hydrophobic C-H bonds. A Hirshfeld surface analysis of 7 showed that the eight-membered heterocycles are linked by weak C-H...π, π-π and H...H noncovalent interactions. The pairwise interaction energies showed that the cohesion between the heterocycles are mainly due to dispersion forces.

Vacancy–assisted exposed Sn atoms enhancing NO2 room temperature sensing of SnSe2 nanoflowers

NO2 is a hazardous gas extremely harmful to the ecosystem and human health, so effective detection of NO2 is critical. SnSe2 is a promising candidate for gas sensors owing to its unique layered configuration that facilitates the diffusion of gas molecules. Here, ultrathin self–assembled nanoflowers F–SnSe2 rich in defects were synthesized by a simple solvothermal method. It exhibits excellent gas sensing performances for NO2 at room temperature (25 °C), with a high gas sensing response of 8.6 for 1 ppm NO2 and a lower detection limit as low as 200 ppb, capable of sensitively detecting ppb–level NO2. DFT calculations revealed that the presence of Se vacancies assists the central Sn atoms to break through the shielding effect of the surface Se atoms and become exposed active sites. The higher reactivity leads to more charge transfer and higher adsorption energy, which strongly promoted the adsorption of NO2. This work verifies the important role of vacancies for the exposed active sites and provides new guidance for defect engineering to modulate the gas sensing performances of SnSe2.

Single-Metal-Encapsulated Double-Cage [Pt@Sn17 ]4- : An Exception from Group 14 Endohedral Clusters.

Group 14 endohedral clusters containing a metal center inside usually possess a single cage topological structure, but here an unexpected single-metal-filled double-cage cluster, [Pt@Sn17 ]4- (1 a) is reported. It can be seen as a combination of the more extended Pt-filled [Pt@Sn9 ] cage and hollow [Sn9 ] cage sharing a central Sn atom, which is offset from the central position. This double-cage species represents the largest group 14 intermetalloid cluster encapsulating a single transition metal atom. DFT calculations show that the capsule-like architecture of [Sn17 ]4- , similar to that found in [Pt2 @Sn17 ]4- , is unstable if filled with a single Pt atom and collapses to the title cluster 1 a upon geometry optimization. Deviation of the central Sn atom occurs due to the vibronic coupling as a consequence of pseudo-Jahn-Teller distortion leading to the bent Cs -symmetrical structure, in contrast to the more symmetrical D2d cage previously reported in [Ni2 @Sn17 ]4- .

Dual-cluster model of Sn-based binary eutectics and solders

Sn-based solders are typically binary eutectic-based and understanding of the eutectic compositions at atomic level is important to guide the development of improved solder alloys. In the present work, Sn-based eutectics of Sn-(Zn, Bi, Pb, Ag, Au, Mg) are investigated utilizing a dual-cluster model, which states that a eutectic liquid is composed of two chemical units from relevant eutectic phases and each unit is expressed by a cluster-plus-glue-atom model, [cluster](glue atoms). By introducing Friedel oscillation, the number of atoms in the chemical unit is calculated. Results include determining that the β-Sn phase being formulated as [Sn-Sn10]Sn5, i.e., the central Sn atom is nearest-neighbored by ten Sn atoms and glued by five others. The Sn-(Zn, Bi, Pb, Ag, Au, Mg) eutectics are well defined by dual-cluster formulism, which is applied to examine Sn-based multi-element commercial solder alloys. The results emphasize that the alloying elements correspond to integer substitutions of Sn in the basic units. The present method provides a quantitative approach towards developing a practical composition interpretation and design tool for Sn-based eutectic solders.

Novel diorganotin(IV) carboxylates based on 2-benzoylbenzoic acid: Synthesis, characterization, molecular structures and luminescent activities

Two novel diorganotin(IV) carboxylates: Ph2SnL2 1 and Cy2SnL2 2 (HL = 2-benzoylbenzoic acid), were assembled by dialkyltin oxide (alkyl = phenyl or cyclohexyl) and HL with 1:2 molar ratio. The synthesized complexes have been structurally characterized by elemental analysis, IR, 1H, 13C, 119Sn NMR spectroscopy and X-ray crystallography diffraction analyses. Both 1 and 2 are mono-nuclear dialkyltin carboxylates. The carboxylate groups of HL in 1 and 2 adopt bidentate mode to coordinate with the central Sn atom. 1D, 2D and 3D supramolecular structures formed by intermolecular hydrogen bonds, C-H···π and π···π interactions are discussed in detail. Furthermore, the preliminary luminescent properties of 1 and 2 have been studied.

An Atoms-in-Molecules Theory Interpretation for the Structure of Di-n-Butyltin(IV) Derivative of Glycylvaline

The topological and energetic properties of the electron density distribution ρ\( (\vec{r}) \) for the tin–ligand interaction in n-Bu2SnL, the geometric configuration of which was optimized at B3LYP/6-31G(d,p)/LANL2DZ(Sn) level of theory, have been theoretically calculated at the bonds around the central Sn atom in terms of atoms-in-molecules (AIM) theory using AIMAll (Version 16.01.09, standard). In n-Bu2SnL, the formation of a (3, − 1) critical point in the internuclear region between tin atom and bonded/coordinated atoms provided an evidence of a bonding interatomic interaction, and calculated bond path angles indicated a distorted trigonal bipyramidal geometry. The calculated topological and energetic parameters suggested a weak closed-shell interaction in all the bonded/coordinated bonds to Sn atom. This interaction possessed covalent character in Sn–Npeptide, Sn–Cα and Sn–Cα′, whereas an electrostatic interaction was observed in Sn–Namino and Sn–Ocarboxyl bonds. The calculated atomic charges suggested negatively charged centers around the central Sn atom.

Electronic Structure Explanation for the Structure and Reactivity of di-n-Butyltin(IV) Derivative of Glycylphenylalanine

The density functional theory (DFT)-based quantum-chemical calculations have been performed on di-n-butyltin(IV) derivative of glycylphenylalanine (H2L) using the Gaussian 09 software package. The molecular geometry of n-Bu2SnL was optimized at B3LYP/6-31G(d,p)/LANL2DZ(Sn) level of theory without any symmetry constraint. The harmonic vibrational frequencies were computed at the same level of theory to find the true potential energy surface (PES) minima. The various geometrical and thermochemical parameters for the studied complex are obtained in the gas phase. The atomic charges at all the atoms were calculated using Mulliken Population Analysis, Hirshfeld Population Analysis and Natural Population Analysis. The charge distribution within the studied complex is explained on the basis of molecular electrostatic potential maps, the frontier molecular orbital analysis and conceptual-DFT-based reactivity (global as well as local) descriptors, using the finite difference approximation method. The nature of O–Sn, N–Sn, N → Sn and C–Sn bonds is discussed in terms of the conceptual-DFT-based reactivity descriptors. The structural analysis of the studied complex has been carried out in terms of the selected bond lengths and bond angles. The structural and atomic charge analysis suggests a distorted trigonal bipyramidal arrangement consisting of negatively charged centres around the positively charged central Sn atom.

Organotin(IV) complexes derived from hydrazone Schiff base: Synthesis, crystal structure, in vitro cytotoxicity and DNA/BSA interactions

Four new organotin(IV) complexes Me2SnL1, Ph2SnL1, Me2SnL2 and Ph2SnL2 (H2L1=5-chlorosalicylaldehyde isonicotinoyl hydrazone and H2L2=2-hydroxy-4-methoxybenzaldehyde isonicotinoyl hydrazone) have been synthesized and characterized by elemental analyses, FT-IR, NMR (1H, 13C and 119Sn) and single crystal X-ray diffraction techniques. The crystal structure analysis shows that complexes 1 and 2 are trinuclear compounds, of which the enolic hydrazone Schiff base ligand chelate to tin center in the ONO tridentate mode. In contrast, the complexes 3 and 4 are uninuclear compounds with the central Sn atoms adopting five-coordinated distorted trigonal bipyramid geometry. Cytotoxicity of the four complexes was studied against two human cancer cell lines (A549 and HeLa) by the MTT assay method. The results demonstrate that the four complexes exhibited good anticancer activity, and mononuclear complex 4 was found to be more effective towards HeLa cancerous cells. The interactions between all complexes with calf thymus DNA (CT-DNA) and bovine serum albumin (BSA) were investigated using various spectroscopic techniques. The viscosity measurements, thermal temperature (Tm) and UV–Vis spectra data suggested intercalative binding between DNA and the four organotin (IV) complexes. In addition, all complexes could quench the intrinsic fluorescence of BSA in a static quenching process, which was also confirmed by the docking study.

A density functional theory insight into the structure and reactivity of diphenyltin(IV) derivative of glycylphenylalanine

Abstract The quantum-chemical calculations based on density functional theory (DFT) have been performed on the diphenyltin(IV) derivative of glycyl-phenylalanine (H2L) at the B3LYP/6-31G(d,p)/LANL2DZ(Sn) level of theory without any symmetry constraint. The harmonic vibrational frequencies were computed at the same level of theory to find the true potential energy surface minima. The various geometrical and thermochemical parameters for the studied complex are obtained in the gas phase. The atomic charges at all the atoms were calculated using the Mulliken population analysis, the Hirshfeld population analysis, and the natural population analysis. The charge distribution within the studied complex is explained on the basis of molecular electrostatic potential maps, frontier molecular orbital analysis, and conceptual DFT-based reactivity (global and local) descriptors, using the finite difference approximation method. The nature of O-Sn, N-Sn, N→Sn, and C-Sn bonds is discussed in terms of the conceptual DFT-based reactivity descriptors. The structural analysis of the studied complex has been conducted in terms of the selected bond lengths and bond angles. The structural and the atomic charge analyses suggest a distorted trigonal bipyramidal arrangement consisting of negatively charged centers around the positively charged central Sn atom.

Organotin(IV) scorpionates – X-ray structure and crystal packing of TpSn(Cl)2(n-Bu) [Tp=hydrotris(pyrazol-1-yl)borate

Abstract Reaction of potassium hydrotris(pyrazol-1-yl)borate (KTp) and n-BuSnCl3 at room temperature led to the quantitative formation of TpSn(Cl)2(n-Bu) (1). Colorless single crystals were obtained from a mixture of toluene/dichloromethane and were characterized as TpSn(Cl)2(n-Bu)·½ toluene. Complex 1 crystallizes in the triclinic P1̅ space group, with Z=2, a=8.0496(8) Å, b=9.4867(9) Å, c=14.2710(14) Å, α=90.365(3)°, β=104.479(3)°, γ=95.066(3)°, and V=1050.60(18) Å3. Its structure exhibits a piano stool conformation involving a distorted octahedral geometry around the central Sn(IV) atom. In the crystal and from a supramolecular point of view, 1 is organized in pairs according to a V-shaped arrangement implicating pyrazolyl rings and through weak intermolecular interactions (π-π, C-H···π, and C-H···Cl). The structural analysis of 1 was completed by infrared and solution NMR spectroscopy measurements as well as elemental analysis, which confirm the X-ray elucidation.

A Density Functional Theory (DFT) study on di-n-Butyltin(IV) Derivative of Glycyltryptophane

The density functional theory (DFT) based quantum-mechanical calculations have been performed on di-nbutyltin(IV) derivative of glycyl-tryptophane (H2L) using the Gaussian09 software package. The molecular geometry of n-Bu2SnL was optimized with B3LYPfunctionalwith the standard 3–21G basis set for all the atoms, except the tin(IV) atom which was described by LANL2DZ basis set along with the effective core potential, without any symmetry constraint. The harmonic vibrational frequencies were computed at the same level of theory to find the true potential energy surface (PES) minima. The various geometrical and thermo chemical parameters for n-Bu2SnL have been obtained in gas phase and in the solvent field. The atomic charges at all the atoms were calculated using Mulliken Population Analysis, Hirshfeld Population Analysis and Natural Population Analysis. The charge distribution within the studied complex is explained on the basis of molecular electrostatic potential maps and conceptual-DFT based reactivity (global as-well-as local) descriptors, using the finite difference approximation method. The calculated parameters suggest a distorted tetrahedral geometry around the central Sn atom. The nature of O—Sn, N—Sn, N→Sn and C—Sn bonds is discussed in terms of the conceptual-DFT based reactivity descriptors. The structural analysis of the studied complex has been carried out in terms of the selected bond lengths and bond angles. The vibrational analysis of characteristic infrared vibrational frequencies of the studied complex has also been carried out.

Organotin(IV) complexes derived from Schiff base N’-[(1E)-(2-hydroxy-3-methoxyphenyl)methylidene]pyridine-3-carbohydrazone: Synthesis, in vitro cytotoxicities and DNA/BSA interaction

Five organotin(IV) hydrazone compounds have been synthesized from N’-[(1E)-(2-hydroxyl-3-methoxyphenyl)methylidene]pyridine-3-carbohydrazone and the corresponding organotin(IV). Solid state structures of five complexes were determined by elemental analysis, IR, NMR spectroscopy and for 1, 2, 3, 4 and 5 single crystal X-ray diffraction analysis. For compounds 4 and 5, structural analysis shows that each complex is a monomer with the tin atom five-coordinated in a distorted trigonal bipyramid chelating by the Schiff base ligand in an enolic tridentate fashion. Fascinatingly, in compounds 1, 2 and 3, structural analysis of them shows that each complex is a centrosymmetric trimers, in which the central Sn atom adopts six-coordinated geometry and the other Sn atom adopts five-coordinated geometry. In vitro cytotoxicities of five compounds on three human drug resistant tumor cells lines (A549, HeLa and MCF-7) were assessed by MTT assay. The interaction of the complexes with calf thymus DNA (CT-DNA) has been explored by absorption titration method, which revealed that complexes interact with CT-DNA through groove-binding and partial intercalation of the extended planar ligand with the DNA base stack. Furthermore, the protein fluorescence quenching studies reveal that there are strong binding interactions between compounds and bovine serum albumins (BSA). Studies reveal that di-n-butyltin(IV) complexes 2 and 4 with significant antiproliferative effects in the cells show stronger DNA/BSA interaction than the other complexes.

Extended X-ray absorption fine structure investigation of Sn local environment in strained and relaxed epitaxial Ge1−xSnx films

We present an extended X-ray absorption fine structure investigation of the local environment of Sn atoms in strained and relaxed Ge1−xSnx layers with different compositions. We show that the preferred configuration for the incorporation of Sn atoms in these Ge1−xSnx layers is that of a α-Sn defect, with each Sn atom covalently bonded to four Ge atoms in a classic tetrahedral configuration. Sn interstitials, Sn-split vacancy complexes, or Sn dimers, if present at all, are not expected to involve more than 2.5% of the total Sn atoms. This finding, along with a relative increase of Sn atoms in the second atomic shell around a central Sn atom in Ge1−xSnx layers with increasing Sn concentrations, suggests that the investigated materials are homogeneous random substitutional alloys. Within the accuracy of the measurements, the degree of strain relaxation of the Ge1−xSnx layers does not have a significant impact on the local atomic surrounding of the Sn atoms. Finally, the calculated topological rigidity parameter a** = 0.69 ± 0.29 indicates that the strain due to alloying in Ge1−xSnx is accommodated via bond stretching and bond bending, with a slight predominance of the latter, in agreement with ab initio calculations reported in literature.

A tetranuclear organotin compound consisting of a core of three fused Sn2O2rings

The crystal structure of the tetranuclear organotin compound 1,1,3,3,5,5,7,7-octabutyl-7-((E)-2-cyano-3-{9-[2-(2-methoxyethoxy)ethyl]-9H-carbazol-3-yl}prop-2-enoyloxy)-4,8-dimethyl-2,4,6,8-tetroxa-1,3,5,7-tetrastannatricyclo[4.2.0.0(2,5)]oct-3-yl (E)-2-cyano-3-{9-[2-(2-methoxyethoxy)ethyl]-9H-carbazol-3-yl}prop-2-enoate, [Sn4(C4H9)8(C21H19N2O4)2(CH3O)2(μ3-O)2], consists of a core of three fused Sn2O2 rings. The central ring consists of two Sn atoms each coordinated by two n-butyl chains, two μ3-bridging O atoms and one μ2-bridging methanolate O atom. The peripheral Sn2O2 rings consist of one of the central ring Sn atoms, and an Sn coordinated by one μ3-bridging O atom, one μ2-bridging methanolate O atom, two n-butyl chains and the carboxylate O atom of a (E)-2-cyano-3-{9-[2-(2-methoxyethoxy)ethyl]-9H-carbazol-3-yl}prop-2-enoate ligand (L). Despite an apparent centrosymmetric nature, the complex does not crystallize about an inversion centre. The structure packs with π-π interactions between carbazole moieties on adjacent molecules.

Reactivity of Dicoordinated Stannylones (Sn0) versus Stannylenes (SnII): An Investigation Using DFT‐Based Reactivity Indices

The reactivity of dicoordinated Sn(0) compounds, stannylones, is probed using density functional theory (DFT)-based reactivity indices and compared with the reactivity of dicoordinated Sn(II) compounds, stannylenes. For the former compounds, the influence of different types of electron-donating ligands, such as cyclic and acyclic carbenes, stannylenes and phosphines, on the reactivity of the central Sn atom is analyzed in detail. Sn(0) compounds are found to be relatively soft systems with a high nucleophilicity, and the plots of the Fukui function f(-) for an electrophilic attack consistently predict the highest reactivity on the Sn atom. Next, complexes of dicoordinated Sn compounds with different Lewis acids of variable hardness are computed. In a first part, the double-base character of stannylones is demonstrated in interactions with the hardest Lewis acid H(+). Both the first and second proton affinities (PAs) are high and are well correlated with the atomic charge on the Sn atom, probing its local hardness. These observations are also in line with electrostatic potential plots that demonstrate that the tin atom in Sn(0) compounds bears a higher negative charge in comparison to Sn(II) compounds. Stannylones and stannylenes can be distinguished from each other by the partial charges at Sn and by various reactivity indices. It also becomes clear that there is a smooth transition between the two classes of compounds. We furthermore demonstrate both from DFT-based reactivity indices and from energy decomposition analysis, combined with natural orbitals for chemical valence (EDA-NOCV), that the monocomplexed stannylones are still nucleophilic and as reactive towards a second Lewis acid as towards the first one. The dominating interaction is a strong σ-type interaction from the Sn atom towards the Lewis acid. The interaction energy is higher for complexes with the cation Ag(+) than with the non-charged electrophiles BH(3), BF(3), and AlCl(3).

Synthesis and spectral characterization of stannocanes of the type [O(CH2CH2S)2SnR2](R=Me1,Bun2,Ph3)

Abstract The stannocanes of the type [O(CH2CH2S)2SnR2](R=Me1,Bun2,Ph3) have been synthesized in an improved method by the reaction of R2SnCl2 with 2, 2′-oxydiethanethiol O(CH2CH2SH)2 in molar ratio of 1:1 at the presence of sodium ethoxide in anhydrous ethanol. The reactions are carried out under inert atmosphere. These compounds have been extensively characterized by FT-IR, UV-Vis spectrophotometry, multi-nuclear (1H, 13C, 119Sn) NMR, elemental analysis and mass spectrometry. The obtained data clearly indicates that, there is a strong interaction between oxygen atom of the ligand as a donor and Sn atom of the organotin species as a Lewis acid acceptor. Therefore, the resulted dithiostannocanes possess a transannular secondary bonding and hypervalency at the central Sn atom which leads to an increase in the coordination number of tin from four to five-coordinated tin.

Di-μ2-isopropanolato-octamethylbis(μ-4-methyl-5-sulfanylidene-4,5-dihydro-1H-1,2,4-triazol-1-ido-κ2N1:N2)di-μ3-oxido-tetratin(IV)

The tetra­nuclear title compound, [Sn4(CH3)8(C3H7O)2O2(C3H4N3S)2], lies about a center of inversion; the mol­ecule features a three-rung-staircase Sn4O4 core in which two SnIV atoms are bridged by the 4-methyl-5-sulfanyl­idene-4,5-dihydro-1H-1,2,4-triazol-1-ide group. The negatively charged N atom of the group binds to the terminal SnIV atom at a shorter distance [Sn—N = 2.236 (2) Å] compared with the neutral N atom that binds to the central SnIV atom [Sn← N = 2.805 (2) Å]. The terminal SnIV atom is five-coordinate in a cis-C2SnNO2 trigonal–bipyramidal geometry [C—Sn—C = 136.4 (1)°], whereas the central SnIV atom is six-coordinate in a C2SnNO3 skew-trazepoidal bipyramidal geometry [C—Sn—C = 145.4 (1)°]. The C atoms of the isopropoxy group are disordered over two positions in a 0.591 (7):0.409 (7) ratio.

New Supramolecular Triorganotin(IV) Dithiocarboxylates as Potential Antibacterial Agents

ABSTRACTFour triorganotin derivatives of general formula C10H10NS2SnR3,, where R = CH3 (1), C4H9 (2), C6H11 (3), and C6H5 (4), have been synthesized by the metathesis reaction of 1,2,3,4‐tetrahydroisoquinolnium salt of ligand with triorganotin(IV) chloride in the 1:1 ratio. These complexes were characterized by elemental analysis, Raman, IR, multinuclear NMR (1H, 13C, and 119Sn), and mass spectrometry. The crystal structure confirmed a supramolecular zig‐zag chain structure mediated by S–H (2.968 Å) for complex 4 with the central Sn atom exists in a distorted trigonal bipyramidal geometry. A subsequent antibacterial study indicates that the compounds are biologically active.

Evidences of the formation of a tin(IV) complex in citric–citrate buffer solution: A study based on voltammetric, FTIR and ab initio calculations

Abstract We studied the Sn(IV)-complex, [Sn(C 6 H 4 O 7 ) 2 ] 2− formed after anodic dissolution of a tin surface through its passive oxide film in citric–citrate aqueous solution buffer pH 3. Tin-complex was experimentally characterized using infrared spectroscopy while ab initio calculations were made to study its structure and vibrational properties. These calculations gave us a precise knowledge of the normal modes of vibration taking into account that in the complex the molecule comprises a system of two citrate ions that are attached to a central Sn(IV) atom. The coordination about Sn(IV) consists of a distorted octahedral SnO 6 where two tridentate citrates are bonding to the central metal. The calculated harmonic vibrational frequencies are consistent with the experimental vibrational spectra. The theoretical calculations of the wavenumbers allowed us to obtain a tentative assignment of the observed spectral features. The nature of the different Sn–O and Sn ← O bonds in the complex and their topological properties were investigated by means of natural bond orbital (NBO) analysis and Bader’s atoms in the molecule (AIM) theory, respectively.