Structural and computational study of some new nano-structured Hg(II) compounds: a combined X-ray, Hirshfeld surface and NBO analyses

Abstract

In this study, the synthesis of some new five coordinated mercury(II) complexes of a nitrogen donor Schiff base ligand is reported. The complexes were characterized by FT-IR, ¹H NMR, ¹³C NMR, UV-visible, ESI-mass spectra and molar conductivity measurements. The nanoparticles of mercury(II) complexes were prepared by a sonochemical process and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Based on spectral data, the general formula of HgLX₂ (X = Cl⁻, Br⁻, I⁻, N₃⁻ and SCN⁻) was proposed for the complexes. The results of single crystal X-ray experiments for the HgLI₂ and HgL(SCN)₂ complexes confirm the spectral data and show distorted square-pyramidal geometries for metal centers. The packing of the molecules in the crystal structure is influenced by different intermolecular interactions such as N–H⋯I, C–H⋯I, C–H⋯O and C–H⋯π in HgLI₂ complex and C–H⋯N, S⋯N, C–H⋯O and N⋯O in the HgL(SCN)₂ complex. Hirshfeld surface analyses, especially d_norm surface and fingerprint plots, were used for decoding intermolecular interactions in the crystal network. The results indicate that in both complexes, the anions connected to the metal center, aromatic rings and their nitro substituent play an important role in the construction of the 3D architecture. Optimized molecular geometries were derived from density functional theory (DFT) calculations and the results were compared with experimental ones. NBO analysis was applied for investigation of intra and inter molecular bonding and conjugative interaction in molecular systems. Information about the size, shape, charge density distribution and site of chemical reactivity of the molecules has been obtained by mapping electron density with molecular electrostatic potential (MEP). The energies of frontier molecular orbitals (E_HOMO and E_LUMO), the energy band gap (E_HOMO–E_LUMO), chemical potential (μ), chemical hardness (η), global electrophilicity index (ω) and dipole moments (D) were calculated.

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1. Introduction

Structural topology of coordination compounds can effect their properties and applications. This factor depends on the self-assembly of the compound so that the extended molecular structure in the solid state is based on non-covalent interactions. These weak intermolecular interactions are generally considered responsible for the construction of solid networks.^1,2 Due to the strength and directionality of hydrogen bonds, these intermolecular interactions have a key role on the solid state packing from a supramolecular point of view.^3–5 It is well known that hydrogen bonding is very important in self-assembling and shaping of the biomolecules.⁶ Besides the conventional hydrogen bonds that include the N, O and F atoms as bond acceptors and donors, some non-conventional ones such as CH⋯O/N/C type have been found in which carbon atom acts as hydrogen bond donor.⁷ CH⋯X (X = Cl, Br and I) interactions are another non-conventional hydrogen bonding that stabilizes supramolecular chains in some solid state structures.^8–10 CH⋯π hydrogen bonds are the other weak intermolecular interactions that may conduct the structure to special supramolecular structure.¹¹ Non-covalent S⋯X (X = O, N, S, C, etc.) interactions have been reported by some researchers. These interactions have important role in stabilization of protein structure. For instance, in the bio-molecular structure, S⋯X interactions are formed in the vicinity of the active site of enzyme for stabilization of its structure after binding of the substrate to active site.¹² Current studies in crystal engineering field are concentrated for obtaining more information about new weak intermolecular interactions.

In recent years, the coordination chemistry of mercury has received considerable attention because mercury compounds exhibit high toxicity to living organisms.¹³ The increased concentration of mercury in the biosphere and the presence of mercury in industrial wastes have attracted interest to the mercury coordination chemistry, because these compounds form aqueous insoluble mercurial solids and provide a means for mercury removal from ground water and liquid industrial wastes.^14,15 The spherical d¹⁰ configuration of Hg(II) is associated with a flexible coordination environment, thus the geometries of these complexes can vary from linear to octahedral or even distorted hexagonal bipyramidal, and severe distortions from ideal coordination polyhedra occur easily.¹⁶ The variety of mercury coordination compounds is influenced by other factors such as the ligand structure, counter anions, experimental conditions and solvent molecules.^17–20 On the other hand, Schiff base ligands are among the most fundamental chelating systems in coordination chemistry and form complexes with both transition and p-block metals.^21,22 These compounds have proven to be particularly versatile, since they form strong coordination bonds and allow for a variety of weak, multiple and lateral non-covalent interactions.^23,24 Nanostructure compounds are of interest because of unique properties of them with respect to bulk ones. Nano-structured coordination materials are found as more suitable candidates in many applied fields, such as catalysis, molecular adsorption, magnetism, nonlinear optics, luminescence, and molecular sensing than bulk ones. Ultrasonic process as a fast, convenient, and economical method has been widely used to generate nanostructure coordination compounds.^25,26

In continuation of our previous research works^27–31 on structural characterization of group XII coordination compounds and effective intermolecular interactions in the crystal packing of them, in the present study, synthesis and spectral characterization of some new mercury complexes are described. Furthermore, the crystal structures of two Hg(II) complexes of (E)-N1-((E)-3-(2-nitrophenyl)allylidene)-N2-(2-((E)-((E)-3-(2 nitrophenyl)allylidene)amino) ethyl) ethane-1,2-diamine have been analyzed and compared to the previously reported structures of some other complexes. X-ray diffraction analysis of these new mercury coordination compounds gives new insight into the effect of various kinds of intermolecular interactions such as N–H⋯I, C–H⋯I, C–H⋯O and C–H⋯π, C–H⋯N, S⋯N, C–H⋯O and N⋯O on the geometry around the central mercury ion and formation of 2 or 3D supramolecular network in solid state. Moreover, the structural descriptions of HgLI₂ and HgL(SCN)₂ complexes have been also corroborated with Hirshfeld surfaces analysis. Special attention has been given to surfaces mapped over d_norm and 2D fingerprints plots.

2. Materials and measurements

The starting compounds (E)-3-(2-nitrophenyl)acrylaldehyde, diethylenetriamine, mercury salts and solvents were purchased from commercial suppliers (Sigma-Aldrich, Merck and others). All chemical reagents were used without further purification. Mercury azide and thiocyanate were freshly prepared based on our previous report.³² The FT-IR spectra were recorded on a JASCO-680 spectrophotometer in the range 4000–400 cm⁻¹. The samples were prepared as KBr pellets. The ¹³C and ¹H NMR data were recorded on a Bruker 400 DPX FT/NMR spectrometer. The chemical shifts are reported in ppm relative to tetramethylsilane (TMS) in deuterated dimethyl sulphoxide (DMSO-d₆) solution. The electronic spectra of the compounds in dichloromethane were obtained from a Perkin-Elmer lambda 25 spectrometer in the range of 200–800 nm. Melting points or decomposition temperatures of the compounds were recorded on a Kruss instrument. The ESI-MS of the compounds were scanned by a linear ion-trap mass spectrometer (LXQ, Thermo Scientific). The molar conductance of the compounds was measured using a SelectaLab ECW 312 microprocessor conductometer. All measurements were performed at room temperature using DMF (10⁻³ M) as a solvent. X-ray powder diffraction (XRD) measurements were performed using a STOE type STIDY-MP-Germany X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). Scanning electron microscopy (SEM) images were obtained on a Hitachi S-1460 field emission scanning electron microscope using Ac voltage of 30 kV. Transmission electron micrograph (TEM) image was recorded on instruments of Philips CM-10 TEM microscope operated at 100 kV.

2.1. Single-crystal structure determination

A complete X-ray data set of a suitable crystal of zinc iodide complex was recorded on a Bruker Apex2 CCD diffractometer with MoKα radiation (graphite monochromator) at T = 173(2) K. LP corrections were performed with the APEX SUITE software³³ and a numerical absorption correction was applied using SADABS.³⁴ Structure solution and refinement were performed with SHELXS/L-97.³⁵

2.2. Synthesis of ligand (L)

A mixture of diethylenetriamine (1 mmol, 0.1032 g) and (E)-3-(2-nitrophenyl)acrylaldehyde (2 mmol, 0.3543 g) in ethanol (30 mL) was severely stirred for 4 h in temperature room. After completion of the reaction, the reaction mixture was poured into cooled water (60 mL). Then the yellowish-white precipitate was filtered, washed and recrystallized from dichloromethane/ethanol mixture to obtain pure ligand. Color: honey colored. Yield: 58%. m.p.: 72 °C. UV-Vis in CH₂Cl₂ [λ_max(ε; cm⁻¹ M⁻¹)]: 253(30 416), 328(sh)(7090) nm. Λ^°_M(DMF): 1.70 cm² Ω⁻¹ mol⁻¹. Selected FT-IR data (KBr, cm⁻¹): 3235(w), 3064(w), 2921(w), 2892(w), 2843(w), 1635(s), 1614(m), 1521(vs), 1350(vs), 1162(m), 972(s), 858(m), 783(s), 742(s), 675(w). ¹H-NMR (in DMSO): 8.14(d, 1H_e, J = 8.5 Hz), 7.8–8.05(m, 4H_kk′hh′), 7.74(t, 1H_i, J = 6.5 Hz), 7.67(t, 1H_i′, J = 7.5 Hz), 763–7.43(m, 5H_jj′e′), 7.33(d, 1H_g, J = 15.76 Hz), 7.32(d, 1H_g′, J = 14.83 Hz), 6.91 (dd, 2H_ff′, J = 13.7, 9.2 Hz), 3.57(m, 2H_aa′), 3.13(m, 1H_c), 2.93(m, 2H_bb′), 2.79(m, 2H_cc′), 2.67(1H_NH), 2.35(m, 1H_c′). ESI-MS: m/z = 422(M + 1), 379, 375, 289, 246, 232, 218, 199, 171.

2.3. Synthesis of mercury complexes (bulk)

A general method has been followed for the preparation of the complexes using the Schiff base ligand and metal salts: a solution of the Schiff base ligand (1 mmol in 20 mL ethanol) gradually added to ethanolic solution of mercury halide, thiocynate or azide salts (1 mmol in 20 mL ethanol). The mixture was severely stirred at room temperature for 2–3 h. After this time, the complexes were obtained as cream precipitate that were filtered and washed with ethanol and then recrystallized via dichloromethane/ethanol mixture (1 : 1) and finally dried on the vacuum apparatus.

The ¹H NMR and/or ESI mass spectral data of the ligand and its mercury complexes based on Scheme 1 are listed as seen in following:

Scheme 1 Proposed structure for Schiff based ligand and its mercury complexes.

[HgLCl₂]. Color: cream. Yield: 83%. m.p.: 196 °C. UV-Vis in CH₂Cl₂ [λ_max; nm (ε; cm⁻¹ M⁻¹)]: 259(27 616), 332(sh)(19 264). Molar conductivities in DMF [Λ^°_M; cm² Ω⁻¹ mol⁻¹]: 5.30. Selected FT-IR data (KBr, cm⁻¹): 3188(s), 3043(w), 2935(w), 2906(w), 1641(vs), 1602(w), 1510(vs), 1350(vs), 1166(s), 1002(m), 976(m), 856(m), 791(m), 744(s), 507(w), 436(w). ESI-MS: m/z = 693(M + 1), 675, 661, 647, 519, 505, 420, 377, 305, 291.

[HgLBr₂]. Color: cream. Yield: 87%. m.p.: 187 °C. UV-Vis in CH₂Cl₂ [λ_max; nm (ε; cm⁻¹ M⁻¹)]: 253(24 344), 327(sh)(18 269). Molar conductivities in DMF [Λ^°_M; cm² Ω⁻¹ mol⁻¹]: 6.32. Selected FT-IR data (KBr, cm⁻¹): 3193(s), 3056(w), 2902(w), 2873(w), 2838(w), 1640(vs), 1601(w), 1511(vs), 1350(vs), 1162(s), 989(m), 975(w), 856(m), 788(m), 744(s), 509(w), 447(w). ESI-MS: m/z = 781(M + 1), 763, 745, 623, 499, 485, 472, 397, 305.

[HgLI₂]. Color: cream. Yield: 91%. m.p.: 155 °C. UV-Vis in CH₂Cl₂ [λ_max; nm (ε; cm⁻¹ M⁻¹)]: 246(36 060), 329(sh)(11 040). Molar conductivities in DMF [Λ^°_M; cm² Ω⁻¹ mol⁻¹]: 7.01. Selected FT-IR data (KBr, cm⁻¹): 3201(s), 3035(w), 2921(w), 2885(w), 2852(w), 1639(vs), 1601(w), 1516(vs), 1350(vs), 1163(s), 979(s), 856(m), 785(m), 744(s), 505(w), 437(w). ¹H NMR (DMSO): 8.39 (d, 2H_c,c′, J = 8.72 Hz), 8.06(d, 2H_i,i′, J = 8.16 Hz), 7.83(dt, 2H_h,h′, J = 7.48 Hz, J = 7.32 Hz), 7.79(d, 2H_f,f′, J = 7.28 Hz), 7.64(dt, 2H_g,g′, J = 7.80 Hz, J = 7.28 Hz), 7.51(d, 2H_e,e′, J = 15.84 Hz), 7.18(dd, 2H_d,d′, J = 15.72 Hz, J = 8.88 Hz), 3.71(bs, 4H_a,a′ and 1H_NH), 2.95(s, 4H_b,b′) ppm. ESI-MS: m/z = 877(M + 1), 858, 844, 829, 688, 563, 420, 353, 263.

[HgL(N₃)₂]. Color: cream. Yield: 64%. m.p.: 158 °C. UV-Vis in CH₂Cl₂ [λ_max; nm (ε; cm⁻¹ M⁻¹)]: 265(37 390), 329(sh)(12 404). Molar conductivities in DMF [Λ^°_M; cm² Ω⁻¹ mol⁻¹]: 10.01. Selected FT-IR data (KBr, cm⁻¹): 3207(m), 3068(w), 2903(w), 2866(w), 2035(vs), 1639(vs), 1516(vs), 1348(vs), 1163(s), 977(m), 856(m), 794(m), 745(s), 505(w), 467(w). ¹H NMR (DMSO): 8.52(d, 2H_c,c′, J = 8.60 Hz), 8.06(d, 2H_i,i′, J = 8.08 Hz), 7.87(d, 2H_f,f′, J = 7.72 Hz), 7.78(dt, 2H_h,h′, J = 7.44 Hz, J = 7.64 Hz), 7.64(dt, 2H_g,g′, J = 7.96 Hz, J = 7.56 Hz), 7.54(d, 2H_e,e′, J = 15.40 Hz), 6.98(dd, 2H_d,d′, J = 14.78 Hz, J = 8.68 Hz), 3.71(s, 4H_a,a′ and 1H_NH), 3.04(s, 4H_b,b′) ppm. ESI-MS: m/z = 706(M + 1), 650, 530, 420, 395, 232.

[HgL(SCN)₂]. Color: cream. Yield: 79%. m.p.: 164 °C. UV-Vis in CH₂Cl₂ [λ_max; nm (ε; cm⁻¹ M⁻¹)]: 256(34 266), 329(sh)(10 850). Molar conductivities in DMF [Λ^°_M; cm² Ω⁻¹ mol⁻¹]: 7.54. Selected FT-IR data (KBr, cm⁻¹): 3272(m), 3066(w), 2927(w), 2863(w), 2117(vs), 2084(vs), 1632(vs), 1519(vs), 1342(vs), 1163(s), 994(s), 858(m), 782(m), 741(s), 502(w), 452(w). ¹H NMR (DMSO): 8.52(d, 2H_c,c′, J = 8.50 Hz), 8.07(d, 2H_i,i′, J = 8.04 Hz), 7.92(d, 2H_f,f′, J = 7.72 Hz), 7.78(dt, 2H_h,h′, J = 7.56 Hz, J = 7.64 Hz), 7.65(dt, 2H_g,g′, J = 7.72 Hz, J = 8.04 Hz), 7.59(d, 2H_e,e′, J = 15.72 Hz), 7.07(dd, 2H_d,d′, J = 15.60 Hz, J = 8.92 Hz), 3.76(s, 4H_a,a′ and 1H_NH), 3.11(bs, 4H_b,b′) ppm. ESI-MS: m/z = 739(M + 1), 720, 603, 471, 443, 318.

2.4. Synthesis of nano-structured mercury complexes

For the synthesis of nano-structure mercury complexes, an ethanolic solution of mercury salts (1 mmol in 20 mL) was positioned under ultrasonic treatment. Then the Schiff base ligand (1 mmol in 20 mL of ethanol) was drop wisely added to it and the reaction mixture was kept in the ultrasonic bath for 60 min. The obtained precipitates were filtered, subsequently washed with ethanol and dried at 70 °C.

2.5. Computational details

The geometrical structures of mercury(II) complexes were optimized at a density functional theoretical level with B3LYP/LANL2DZ as basis set using Gaussian 03W program package with the aid of the GaussView visualization program. No symmetry constrains were applied during the geometry optimization. The vibrational frequencies are performed at the B3LYP/LANL2DZ level for the verification of the optimized geometries. NBO calculations³⁶ were performed using NBO 3.1 program as implemented in the GAUSSIAN 03W package at the DFT/B3LYP/LANL2DZ level in order to understand various second order interactions between the filled orbital of one subsystem and vacant orbital of another subsystem, which is a measure of the intermolecular and intra-molecular delocalization or hyper conjugation. The Hirshfeld surface analyses of HgLI₂ and HgL(SCN)₂ complexes were done to quantify the interactions with the help of Crystal Explorer.³⁷

3. Results and discussion

3.1. Physical properties and analytical data

The physical properties and analytical data of compounds are shown in Section 2. The mercury(II) complexes were readily formed in good yield on reacting with the same molar amount of the Schiff base ligand. The synthesized complexes are insoluble in water, methanol and ethanol but low soluble in acetone, DMF and DMSO. The molar conductivity values of 10⁻³ M solutions of all complexes in DMF solvent were found to be in the range of 5.30–10.01 cm² Ω⁻¹ M⁻¹ at room temperature, indicating the non-electrolytic nature of these complexes.³⁸

3.2. Characterization of nanostructure mercury complexes

The average size of particles was estimated from Scherer's equation, , where D is the average grain size, k is Blank's constant (0.891), λ is the X-ray wavelength (0.15405 nm), θ and β are the diffraction angle and full-width at half maximum of an observed peak, respectively.³⁹ The average size of the particles for mercury bromide, iodide and thiocynate is 32, 35 and 21 nm, respectively. Fig. 1 shows the XRD pattern of compounds prepared by the sonochemical process and compares the simulated XRD pattern from single crystal X-ray data of HgLI₂ and HgL(SCN)₂ complexes with XRD pattern of their nano-particles. Acceptable matches, observed between the simulated and experimental powder X-ray diffraction patterns indicate that the compound obtained by sonochemical process as nano-particles is identical to that obtained by single crystal diffraction. The significant broadening of the peaks is indicating small particles size of the prepared compounds. The morphology and size of synthesized complexes were checked by scanning electron microscopy (SEM). The SEM images are shown in Fig. 1. Based on these images, the particles are generally agglomerated so those show a non-uniform sizes.

Fig. 1 [Left] The XRD patterns of nano-particles of (A) HgLBr₂ complex (B) HgLI₂ complex (C) HgL(SCN)₂ complex and [right] the SEM image of (D) HgLBr₂ complex, (E) HgLI₂ complex and (F) HgL(SCN)₂ complex.

Moreover, transmission electron microscopy (TEM) was used to investigate the particles size of the mercury chloride complex (Fig. 2) which well confirms the nano-structure size for this complex. The TEM/SEM images display the nanoplate shape for HgLCl₂ and HgL(SCN)₂ complexes and nanospherical shape for HgLBr₂ and HgLI₂.

Fig. 2 The TEM image of HgLCl₂ powder prepared by the sonochemical process.

3.3. FT-IR and electronic spectra

The main infrared bands of the compounds and their assignments are listed in Table 2. In IR spectra of the Schiff base ligand, the absence of ν(C=O) and ν(NH₂) stretching modes of the aldehyde and amine educts, and the appearance a strong new band at 1635 cm⁻¹, assigned to the azomethine group, confirm the successful condensation of starting material.⁴⁰ In order to study the bonding mode of the ligand to the metal ions, the IR spectra of the free ligand and its complexes are compared. The shifting of ν(C=N) vibration (1635 cm⁻¹) to higher frequencies by 2–6 cm⁻¹ and also the enhancement of its intensity suggest that the ligand coordinates to the metal ion through nitrogen atoms of the azomethine groups.⁴¹ This is also supported by the presence of new bands in the spectra of the complexes at 430–570 cm⁻¹ corresponding to νM–N vibrations.⁴² Participation of the secondary amine nitrogen in the chelation was confirmed by the displacement of the stretching frequency related to the N–H vibration (3235 cm⁻¹) in the free ligand to lower (halides and azide complexes) and higher (thiocyanate complex) frequencies in the spectra of the complexes. The IR spectrum of HgL(N₃)₂ shows a very strong band at 2035 cm⁻¹ corresponding to ν_as(N₃), which is consistent with a mercury(II) complex containing two terminal azide ligands.⁴³ In the IR spectrum of the thiocyante complex, the stretching CN frequency of the thiocyanate ion observed at 2089 cm⁻¹ and 2117 cm⁻¹ indicates the coordination through the sulfur atom. The stretching mode ν(CN) in this complex is close to the reported value.⁴⁴

As mentioned in experimental section, the electronic spectrum of the free ligand exhibits two bands, one intense band at 253 nm and a shoulder at 328 nm assigned to intra-ligand transitions (π–π*) due to π-system of phenyl rings and olefinic and azomethine bonds.²⁸ In the complexes, these bands are red or blue shifted by 12 nm confirming coordination through azomethine nitrogen and the formation of Schiff base metal complexes.⁴⁵

3.4. Mass spectra

The mass spectra of all compounds showed the molecular ion peaks m/z [M + 1] at 422, 693, 781, 877 and 706, 739 for ligand, HgLCl₂, HgLBr₂, HgLI₂, HgL(N₃)₂, HgL(SCN)₂ respectively which are consistent with the formula weights and support the identity of the structures presented in Scheme 1.

3.5. ¹H-NMR spectra

The chemical shift (δ, ppm) of the different protons of the Schiff base ligand and their Hg(II) complexes have been listed in the experimental section. The resulting data from the ¹H-NMR spectrum of the ligand suggests the formation of a Schiff base-imidazolidine compound as shown in Scheme 1(A). The formation of a Schiff base ligand was approved by appearance of an iminic proton (H_e) at 8.14 ppm as a doublet signal with a coupling constant of 8.5 Hz. Aromatic hydrogen signals of the ligand arose in the range of 7.43–8.05 ppm as follows: hydrogen atoms of kk′ and hh′ as multiplet at 7.8–8.05 ppm, hydrogen atoms of i and i′ as two distinguishable triplets at 7.74 and 7.67 ppm, respectively, and hydrogen atoms of j and j′ along with H_e′ as multiplet signal at 7.43–7.63 ppm. H_aa′, H_cc′, H_bb′, H_dd′ are aliphatic hydrogen atoms and their signals appear as multiplets in the range of 2.79–3.57 ppm. Finally, the characteristic peak of the secondary amine hydrogen atom (H_NH) is observed at 2.67 ppm. ¹H-NMR spectra of the mercury complexes shows that the ligand have a symmetric structure and is coordinated as a tridentate Schiff base ligand to mercury center. Based on the suggested structure in Scheme 1(B), the iminic hydrogen atoms (H_cc′) of the complexes appear in the range of 8.39–8.59 ppm. The chemical shifts of these protons to down fields with respect to free ligand confirm the coordination through the azomethine nitrogen atoms. In the spectra of the complexes, the aromatic protons have new positions as follows: H_i,i′ as doublet in the range of 8.06–8.07 ppm, H_f,f′ as doublet in the range of 7.83–7.92 ppm, H_h,h′ and H_g,g′ as two asymmetric doublets of a triplet(dt) in the range of 7.64–7.79 ppm. Alkene hydrogen atoms of H_e,e′ and H_d,d′ in the complex ¹H NMR spectra are observed as distinguishable doublet(d) and doublet of doublet(dd) signals in the ranges of 7.51–7.59 ppm and 6.98–7.18 ppm, respectively. In the 2–4 ppm region, two distinct singlet signals are found, one attributed to four aliphatic protons of H_a,a′ along with secondary amine hydrogen(H_NH) and the other assigned to four aliphatic hydrogen atoms of 4H_b,b′. Therefore, the ¹H NMR spectra well confirm formation of the mercury complexes.

3.6. Crystal structures

The ORTEP diagram of the HgLI₂ and HgL(SCN)₂ complexes with atomic numbering scheme are shown in Fig. 3 and 6. The crystal data collection and refinement of complexes are given in Table 1. Selected bond lengths and bond angles of compounds are given in Table 2. The intermolecular interactions of compounds are collected in Table 3.

Fig. 3 The ORTEP diagram of the HgLI₂ complex with atomic numbering scheme. The atoms are represented by 50% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity.

Fig. 4 1D expansion of N–H⋯I interaction in HgLI₂ complex along the c axis.

Fig. 5 3D supramolecular structure induced by intermolecular interactions in HgLI₂ complex. Interactions are shown as dash lines.

Fig. 6 The ORTEP diagram of the HgL(SCN)₂ complex with atomic numbering scheme. The atoms are represented by 50% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity.

Table 1 Crystal data and structure refinement for mercury iodide and thiocyanate complexs

Compound	HgLI2	HgL(SCN)2
Empirical formula	C22H23HgI2N5O4	C24H23HgN7O4S2
Formula weight	875.84	738.20
Temperature (K)	173(2)	173(2)
Wavelength (Å)	0.71073	0.71073
Crystal system	Triclinic	Monoclinic
Space group	P1̄	P21/n
Unit cell dimensions	a = 7.1329(3) Å	a = 8.4523(8) Å
	b = 7.3046(3) Å	b = 11.6846(12) Å
	c = 14.7198(7) Å	c = 27.114(3) Å
Volume (Å^3)	660.03(5)	2676.3(5)
Z	1	4
Calculated density (Mg m^−3)	2.203	1.832
F(000)	408	1440
Crystal size (mm)	0.096 × 0.062 × 0.034	0.260 × 0.114 × 0.076
Reflections collected	7494	29 879
Rint	0.0278	0.0651
Completeness to theta (%)	94.4	100
Final R indices [I > 2σ(I)]	R1 = 0.0326, wR2 = 0.0767	R1 = 0.0822, wR2 = 0.2136
Largest diff. peak and hole (e Å^−3)	1.744 and −1.024	4.827 and −4.178

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Table 2 Bond lengths (Å) and bond angles (°) experimental and optimized of mercury iodide and thiocyanate complexes by using LANL2DZ basis set

HgLI2 complex			HgL(SCN)2 complex
Bond length (Å)	X-ray	B3LYP	Bond length (Å)	X-ray	B3LYP
Hg–N2	2.618(10)	2.65849	Hg1–N2	2.455(14)	2.54354
Hg–N3	2.385(9)	2.57349	Hg1–N3	2.343(14)	2.53100
Hg–N4	2.610(10)	2.65854	Hg1–N4	2.433(14)	2.57378
Hg–I1	2.683(9)	2.89694	Hg1–S1	2.630(5)	2.73478
Hg–I2	2.713(9)	2.91835	Hg1–S2	2.480(5)	2.72965
Bond angle (°)
N2–Hg–N3	70.9(3)	69.11035	N2–Hg1–N3	72.4(5)	71.68357
N2–Hg–N4	142.0(3)	136.07238	N2–Hg1–N4	144.6(5)	141.55771
N2–Hg–I1	97.2(3)	97.18668	N2–Hg1–S1	94.6(3)	96.19140
N2–Hg–I2	102.4(3)	98.67969	N2–Hg1–S2	106.7(4)	109.94379
N3–Hg–N4	71.5(3)	69.11098	N3–Hg1–N4	72.6(5)	69.98759
N3–Hg–I1	121.4(2)	91.95328	N3–Hg1–S1	116.8(4)	114.41879
N3–Hg–I2	122.2(2)	131.37683	N3–Hg1–S2	139.4(4)	125.20388
N4–Hg–I1	96.8(3)	97.17713	N4–Hg1–S1	96.4(4)	96.79964
N4–Hg–I2	102.6(2)	98.68429	N4–Hg1–S2	103.1(4)	94.51071
I1–Hg–I2	116.36(3)	136.66989	S1–Hg1–S2	103.80(16)	119.56343
			Hg1–S1–C1S	99.4(7)	90.10451
			Hg1–S2–C2S	94.7(7)	96.03512
			S1–C1S–N1S	174.4(19)	178.38570
			S2–C2S–N2S	179(2)	179.26995

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Table 3 Intermolecular interactions in structure of mercury iodide and thiocyanate complexes

Interaction	D–H⋯A	D–H (Å)	H⋯A (Å)	D⋯A (Å)	A⋯H–D (°)	Symmetry operation
HgLI2 complex
N–H⋯I	N3–H3A⋯I1	1.000	2.745	3.687	157	1 + x, y, z
C–H⋯I	C4–H4⋯I2	0.950	3.137	3.960	146	−1 + x, y, z
	C11–H11B⋯I2	0.989	3.174	3.892	145	x, −1 + y, z
	C19–H19⋯I2	0.953	3.141	3.962	131	−1 + x, y, z
C–H⋯O	C14–H14⋯O1	0.948	2.695	3.436	136	x, −1 + y, −1 + z
	C16–H16⋯O2	0.949	2.715	3.356	125	−1 + x, −1 + y, −1 + z
	C9–H9⋯O3	0.949	2.705	3.426	133	x, y, −1 + z
	C2–H2⋯O4	0.949	2.696	3.331	125	x, −1 + y, −1 + z
	C7–H7⋯O4	0.949	2.696	3.334	125	x, y, −1 + z
C–H⋯π	C9–H9⋯Cg(1)	0.949	3.316	3.942	125	x, −1 + y, z
	C10–H10B⋯Cg(1)	0.992	3.303	4.118	141	x, −1 + y, z
	C13–H13A⋯Cg(2)	0.988	3.343	4.123	137	x, −1 + y, z
	C14–H14⋯Cg(2)	0.948	3.349	3.958	124	x, −1 + y, z
HgL(SCN)2 complex
C–H⋯O	C13–H13B⋯O1	0.991	2.520	3.221	130	3/2 − x, −1/2 + y, 1/2 − z
	C11–H11A⋯O2	0.990	2.647	3.374	128	1/2 − x, −1/2 + y, 1/2 − z
	C21–H21⋯O2	0.951	2.477	3.399	163	1 − x, −y, 1 − z
	C4–H4⋯O3A	0.950	2.431	3.331	158	2 − x, 1 − y, 1 − z
	C13A–H13A⋯O4A	0.990	2.576	3.257	126	5/2 − x, 1/2 + y, 1/2 − z
C–H⋯N	C14–H14⋯N1S	0.948	2.676	3.587	161	−1 + x, y, z
	C2–H2⋯N2S	0.951	2.597	3.322	133	1 − x, −y, 1 − z
	C19–H19⋯N2S	0.952	2.722	3.338	123	2 − x, 1 − y, 1 − z
C–H⋯π	C22–H22⋯Cg(1)	0.949	3.399	3.926	117	−1 − x, y, z

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Structural description of HgLI₂ complex. The HgLI₂ complex crystallizes in the triclinic system with space group P1̄. The Hg(II) metal center is five-coordinated and surrounded by a N₃I₂ environment. The coordination sphere of pentacoordinated compounds can be described either as square-pyramidal or trigonal bipyramidal based on the parameter τ₅ = (β − α)/60 (α and β are the two largest bond angles around the metal ion). The values of τ₅ are zero and unity for ideal square pyramidal and trigonal bipyramidal geometry, respectively.⁴⁶ In this complex, β = N2–Hg1–N4 = 142.0(3)° and α = N3–Hg1–I2 = 122.2(2)° and so the τ value is calculated to be 0.33. This value is closer to distorted square pyramidal geometry around the mercury center. The equatorial plane is defined by the three nitrogen atoms of Schiff base ligand (N2, N3 and N4) and one iodide atom (I2). The axial site is occupied by the other iodide atom (I1). The equatorial bond angles N2–Hg–N3, N3–Hg–N4, N2–Hg–I2 and N4–Hg–I2 are 70.9(3), 71.5(3), 102.4(3), and 102.6(2), respectively. All angles are deviated from the ideal value of 90°. This deviation could be due to the restriction induced by the two five-membered chelate rings of Hg–N2–C10–C11–N3 and Hg–N3–C12–C13–N4. Trans angles N2–Hg–N4 and N3–Hg–I2 have values of 142.0(3)° and 122.2(2)°, respectively and are far from the ideal value of 180°. The distance of the mercury atom from the mean basal plane [N2, N3, N4 and I2] is 0.789 Å. The average bond length of Hg–I (2.698 Å) is similar and comparable to its average value in mercury(II) complexes with a HgI₂N₃ coordination environment (2.697 Å in [Hg(terpy)I₂] and 2.658 Å in Hg(TpzT)I₂·H₂O).^47,48 The average bond distance of Hg–N_imine (2.618 Å) is much longer than its value in two Hg(II) complexes with the same chromophore group (2.369 Å in C₁₂H₁₉HgI₂N₃ and the average value 2.406 Å in C₁₈H₂₃HgI₂N₃).⁴⁹ The Hg–N_amine bond length (2.385(9) Å) is shorter than Hg–N_imine indicating a stronger bond of amininc nitrogen to the Hg(II) ion. The length of this bond is close to observed value in two Hg(II) complexes where diethylenetriamine is coordinated as a tridentate ligands.⁵⁰ The bond length of axial Hg–I (2.713(9) Å) is elongated relative to the equatorial Hg–I (2.618(10) Å). The dihedral angle between the phenyl rings of the ligand (L) is 34.69°.

The packing of the molecules in the crystal structure is influenced by different intermolecular interactions such as N–H⋯I, C–H⋯I, C–H⋯O and C–H⋯π. Table 3 lists all of these interactions. The iodine atom acts as an acceptor in two interaction types. The hydrogen atoms of the secondary amine (H3A) is linked to the I1 atom an part of a N3–H3A⋯I1 [symmetry code: 1 + x, y, z] interaction. This interaction combines the complex molecules to a 1-D chain (Fig. 4). Three C–H⋯I interactions are induced by other iodide atoms (I2) with aliphatic carbon (C11) and two aromatic carbons (C4 and C19) acting as hydrogen donors. The hydrogen-bond length in the N–H⋯I interaction (2.745 Å) is shorter than the hydrogen-bond lengths in C–H⋯I interactions (average, 3.15 Å). The hydrogen-bond distance correlates well with hydrogen-bond energy and is often considered as a criterion for judging hydrogen-bond strength.⁵¹ Due to the strength of N–H⋯I interaction and therefore the close distance between adjacent molecules, each I2 atom locates between two phenyl rings of adjacent molecule and induces C4–H4⋯I2⋯H19–C19 hydrogen bonds. The nitro substituent of aromatic rings forms several C–H⋯O interactions with iminic, olefinic and aromatic hydrogens from neighbor molecules. These interactions connect any molecule to eight adjacent molecules and have an effective role in the 3-D expansion of the crystal structure (Fig. 5). Moreover, there are several C–H⋯π interactions that help to stabilize the supramolecular structure.

Structural description of HgL(SCN)₂ complex. Single crystal X-ray analysis revealed that HgL(SCN)₂ complex crystallizes in the monoclinic system with space group of P2₁/n and contains four complex formula units per unit cell. The Hg(II) metal center is five-coordinated by the N₃-tridentate Schiff base ligand and the S atoms of two thiocyanate ligands. In the present structure, β = N2–Hg1–N4 = 144.6(5)° and α = N3–Hg1–S2 = 139.4(4)° and so the calculated τ₅ index is 0.09 that indicates a distorted square-pyramidal geometry for the metal center. N2, N3 and N4 atoms of Schiff base ligand and S2 constitute the base in the pyramid and S1 occupies the apical position. The Hg(II) metal ion found 0.514 Å above the basal plane defined by N2, N3, N4 and S2. Two basal atoms of N2 and N4 are deviated 0.311 and 0.023 Å towards the axial thiocyanate (S1), respectively, while the other basal atoms (N3 and S2) deviate 0.432 and 0.200 Å opposite to the axial thiocyanate (S1), respectively. All angles around the metal ion deviate from the expected angles for perfect square pyramidal geometry presumably due to the small bite angles N2–Hg1–N3 [72.4(5)°] and N3–Hg1–N4 [72.6(5)°]. The equatorial bond angles range from 72.4(5)° to 106.7(4)°. Trans angles N2–Hg–N4 and N3–Hg–S2 have values of 144.6(5)° and 139.4(4)°, respectively. The value of trans angles for mercury complexes involving HgN₃S₂ chromophores and distorted square pyramidal geometry is 140.84(9)° and 126.09(7)° in C₂₄H₂₆Hg₂N₁₀S₄ and 143.0(2)° and 125.70(15)° in C₁₉H₂₁HgN₇S₂.^52,53 The comparison of bond lengths in the two present complexes indicates that the Hg–N_amine bond length in both complexes is shorter than Hg–N_imine and moreover, Hg–N_imine bond lengths in HgL(SCN)₂ complex (Hg1–N2 = 2.455(14) Å and Hg1–N4 = 2.433(14) Å) are shorter than them values in HgLI₂ complex (Hg–N2 = 2.618(10) Å and Hg–N4 = 2.610(10) Å). These bonds are longer as compared to the five-coordinated mercury thiocyanate complexes with N-donor Schiff base ligands (2.289(3) Å in C₂₄H₂₆Hg₂N₁₀S₄ and 2.367(5) in C₂₀H₂₃HgN₅S₂).^52,54 The Hg–S1 and Hg–S2 bond distances are 2.630(5) and 2.480(5) Å, respectively, and comparable to similar structures.^55,56 The bond angles of S-donor thiocyanates [S1–C1S–N1S = 174.4(19)° and S2–C2S–N2S = 179(2)°] indicates that two thiocyanate groups are quasi-linear and coordinated to the metal center in a bending way [Hg1–S1–C1S = 99.4(7) and Hg1–S2–C2S = 94.7(7)]. Two phenyl rings are almost parallel with dihedral angle 8.03°.

In the packing of HgL(SCN)₂, interactions related to thiocyanate groups and phenyl rings along with their nitro substituent act as control factors of the crystal structure (Fig. 7). The thiocyanate groups participate in the formation of non-classical C–H⋯N hydrogen bonds and S⋯N interactions (Fig. 8). There are three C–H⋯N interactions between non-coordinated nitrogen atoms of the thiocyanate groups as acceptor and aromatic (H2 and H19) and aliphatic (H14) hydrogens. Moreover, the coordinated S2 atom is involved in S⋯N interaction with the N1S atom of the thiocyanate ligand of an adjacent molecule [S2⋯N1S = 3.314 Å, symmetry code: 3/2 − x, 1/2 + y, 1/2 − z]. On the other hand, the nitro substituent of the aromatic rings connect each molecule to six other molecules via C–H⋯O and N⋯O intermolecular interactions (Fig. 9). The C13–H13 (A, B) donors from the ethylenic part of Schiff base ligand form interactions with O1 and O4A atoms from nitro groups of two adjacent molecule, and oxygen atom O2 is hydrogen-bonded to the aliphatic (H11A) and aromatic (H21) carbon atoms. The O3A atom also forms a contact with hydrogen (H4) located in the para position with respect to nitro group of adjacent molecule. This interaction has the largest difference (−0.289 Å) to the sum of the van der Waals radii of hydrogen and oxygen. The nitro groups can also take part in non-hydrogen bonding O⋯N interactions. Theoretical studies showed that the binding energy of this interaction is similar to that of a weak C–H⋯O hydrogen bond.⁵⁷ Each molecule is linked to two molecules via O⋯N interactions (O⋯N distance = 3.023 Å, symmetry code: −1 + x, −1 + y, z). In these interactions, the oxygen and nitrogen atoms are electron donor and acceptor, respectively.

Fig. 7 Supramolecular structure of HgL(SCN)₂ complex induced by intermolecular interactions.

Fig. 8 C–H⋯N and N⋯S interactions of thiocyanate groups in HgL(SCN)₂ complex.

Fig. 9 C–H⋯O and N⋯O interactions of nitro groups in HgL(SCN)₂ complex.

3.7. Hirshfeld surface analysis

The Hirshfeld surfaces of HgLI₂ and HgL(SCN)₂ complexes have been displayed in Fig. 10, showing surfaces that have been mapped over the normalized contact distance (d_norm). The surfaces have been displayed as transparent to permit visualization of the underlying complex molecules. The information related to intermolecular interactions presented in Table 3 makes clear that the patterns of intermolecular interactions are similar in both complexes, which is a good reason to investigate the contributions of the weak non covalent forces in the crystal packing. The weak interactions discussed in crystal structure section are summarized effectively in the spots, with the large circular depressions (deep red) visible on the d_norm surfaces indicative of hydrogen bonding contacts and other weak contacts. The small extent of area and light color on the surface indicates other weaker and longer contacts. Two deep red spots in the front and back views of d_norm surface of HgLI₂ complex are due to the N–H⋯I bonding contacts. The other small light red spots are due to C–H⋯I contacts. The dominant red spots on the d_norm surface of HgL(SCN)₂ complex are due to C–H⋯N_thiocyanate and C–H⋯O_nitro bonding contacts. Other small light red spots are related to N⋯O contact of nitro groups and S⋯N contact of thiocynate groups of adjacent molecules.

Fig. 10 Hirshfeld d_norm surfaces and 2D fingerprint plots of (left) HgLI₂ complex and (right) HgL(SCN)₂ complex.

The 2D fingerprint plots (Fig. 10) show the intermolecular interactions involved within the structures and can be decomposed to separate of the individual contribution of one interaction from different interaction types which overlap in the full fingerprint.⁵⁸ Complementary regions are visible in the fingerprint plot where one molecule acts as a donor (d_e > d_i) and the other as an acceptor (d_i > d_e). The 2D fingerprint plots reveal that H⋯H (32.8%) and C–H⋯O–N (20.9%) contacts are the main intermolecular interactions in HgLI₂ complex, while H⋯H, C⋯H and C–H⋯O–N contacts with almost identical distributions comprise the major section of intermolecular interactions in HgL(SCN)₂ complex.

C–H⋯O–N intermolecular interactions are represented by two spikes in the 2D fingerprint plots of complexes (the sharper spikes in HgL(SCN)₂) indicating that H-atoms interact with O-atoms of the nitro substituent of aromatic rings. The proportion of O⋯H/H⋯O interactions comprising 20.9% of the Hirshfeld surfaces for each molecule of HgLI₂ complex, whereas that in HgL(SCN)₂ complex is 21.1%. The proportion of N⋯H/H⋯N interactions comprising 1.8% and 13.3% of the Hirshfeld surfaces for each molecule in HgLI₂ and HgL(SCN)₂, respectively. Significant difference between proportion of these interactions in the title complexes is related to interaction of non-coordinated nitrogens of thiocyanate groups with H-atoms in HgL(SCN)₂ complex. These interactions are appeared as significant red spots in d_norm surface of HgL(SCN)₂ complex. H⋯H interactions are appeared by a spike (sharp in HgL(SCN)₂ and broad in HgLI₂) in the 2D fingerprint plots. This contact is the main intermolecular interactions in HgLI₂ (32.8%) but the replacing of iodide with thiocyanate leads to 11.5% reduction of its proportion in HgL(SCN)₂. This reduction is due to involvement of H-atoms with each three atoms of thiocyanate groups. C⋯H/H⋯C intermolecular interactions comprise 15.4% and 23.5% of the surface for each molecule in HgLI₂ and HgL(SCN)₂, respectively. In spite of the high proportion, C⋯H interactions does not appear as short contacts (red spots) on the d_norm surface, instead appearing as medium or longer range contacts (H⋯C > 3.3 Å).

The inspection of contacts between other atom types pointed out that there are also contacts between the nitro groups as N⋯O/O⋯N interactions for both complexes (1.1% in HgLI₂ and 0.9% in HgL(SCN)₂). This interaction is observed as a light red spot in d_norm surface of HgL(SCN)₂ complex, while in HgLI₂ is as the blue areas indicating the higher strength of this interaction in HgL(SCN)₂ complex. In fingerprint plot of both complexes, O⋯N and N⋯O interactions are appeared in upper (d_e > d_i) and lower (d_e < d_i) area, respectively, indicating that oxygen acts as donor to nitrogen atom. Finally, in fingerprint plot of each complex, there is a specific interaction of that complex. In HgLI₂ complex, I⋯H/H⋯I interactions comprise 18.2% of total Hirshfeld surface and are appeared as wings on the top left (H⋯I interaction) and bottom right (I⋯H) of the 2D fingerprint plot. This contact is related to N–H⋯I and C–H⋯I intermolecular interactions. N–H⋯I interaction is appeared as two deep red spots in d_norm surface, while C–H⋯I interactions are as white to blue areas. In HgL(SCN)₂ complex, S⋯H/H⋯S interactions comprise 9.7% of total Hirshfeld surface but this proportion is not reflecting of the strength and structure-directing properties of the interaction because sulfur atoms are coordinated to metal center and can't form strong hydrogen bonds. Sulfur atoms are also in contact with nitrogen atoms of thiocyanate groups of adjacent molecules and form N⋯S/S⋯N interactions. These interactions comprise only a small fraction of the Hirshfeld surface (1.4%) but they are observed as two red spots in d_norm surface of HgL(SCN)₂ complex. The appearance of N⋯S and S⋯N interactions in top (d_e > d_i) and down (d_i > d_e) areas of fingerprint plot, respectively, indicates that nitrogen acts as donor.

3.8. DFT calculations

Geometry optimization. The some results of geometry optimization of mercury iodide and thiocyanate complexes in comparison with X-ray structural data are given in Table 2. According to these data, some discrepancies occur between structural parameters from DFT calculations and those from experimental results. The maximum deviation concerning bond lengths is 0.21 Å for the Hg1–I2 bond in the HgLI₂ complex and 0.25 Å for the Hg1–S2 bond in the HgL(SCN)₂ complex. For bond angles, the maximum deviation is 29.45° for the N3–Hg–I1 angle in HgLI₂ and 15.76° for the S1–Hg1–S2 angle in HgL(SCN)₂. The dihedral angle between the phenyl rings changes from 34.69° (experimental) to 15.51° (theoretical) in HgLI₂ and from 8.03° (experimental) to 58.96° (theoretical) HgL(SCN)₂. The discrepancies observed between experimental and theoretical values can originate from the selected calculation method, the negligence of intermolecular interactions and the effects of crystal packing.⁵⁹ Based on DFT data, the value of τ₅ is 0.01 and 0.27 for HgLI₂ and HgL(SCN)₂ complexes, respectively which is consistent with the distorted square-pyramidal geometry suggested by X-ray data.

The results of geometry optimization of the other mercury complexes are found in Table S2 (ESI† file). For three complexes, the calculated τ₅ index is 0.33, 0.24 and 0.29 for HgLCl₂, HgLBr₂ and HgL(N₃)₂ respectively that suggests a distorted square-pyramidal geometry for metal center. The dihedral angle between the phenyl rings is 40.31°, 24.22° and 54.73° in HgLCl₂, HgLBr₂ and HgL(N₃)₂, respectively.

Frontier molecular orbitals. Frontier molecular orbitals (FMO), i.e. the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) play a key role in the investigation of the electrical, optical and chemical properties.⁶⁰ The molecular orbital pictures of Hg(II) complexes obtained from their optimized geometries are plotted in Fig. 11. In all complexes, the electron density of HOMO is mainly located on one of the anions connected to the metal, whereas the LUMO electron density is localized on the parts of Schiff base ligand distributed from the nitro group of the aromatic ring to the iminic nitrogen atoms on two sides. The energies of the frontier molecular orbitals (E_HOMO and E_LUMO), the energy band gap (E_HOMO–E_LUMO), the chemical potential (μ), the chemical hardness (η), the global electrophilicity index (ω) and the dipole moments (D)^61,62 are listed in Table 4. The FMO and their adjacent orbitals have negative energy indicating the stability of the prepared complexes. The energy gaps (E_HOMO–E_LUMO) are used to characterize the kinetic stability, chemical reactivity and chemical hardness–softness of a molecule.⁶³ Molecules with a small energy gap can be considered as soft and those with a large energy gap as hard molecules.⁶⁴ Chemical hardness (η) is a measure of the resistance of a chemical species to change its electronic configuration, whereas the chemical potential (μ) characterizes the tendency of electrons to escape from the equilibrium system⁶⁵ related to the electronic charge rearrangement associated to any chemical process. The electrophilicity index is one of the most important quantum chemical descriptors in terms of reactivity and site selectivity.⁶⁶ This index measures the stabilization in energy when the system acquires an additional electronic charge from the environment. HgLCl₂ has the smallest value for the chemical potential (μ = −5.089 eV), global electrophilicity (ω = 8.114 eV) and dipole moment (4.616 debye), and the highest value for the chemical hardness index (η = 1.596 eV), while HgL(N₃)₂ has the highest value for the chemical potential (μ = −4.65 eV) and global electrophilicity (ω = 9.899 eV) and the lowest value for the chemical hardness index (η = 1.092 eV). The highest value of dipole moment (6.792 debye) belongs to HgLI₂. The chemical hardness index (η) follows the order: HgLCl₂ > HgL(SCN)₂ > HgLBr₂ > HgLI₂ > HgL(N₃)₂. Interestingly, HgL(N₃)₂ containing hard nitrogen donor atoms has the lowest hardness value, while HgL(SCN)₂ with soft sulfur atoms as donors is the second hard compound.

Fig. 11 [Left] Highest occupied molecular orbital (HOMO), [middle] lowest unoccupied molecular orbital (LUMO) and [right] molecular electrostatic potential (MEP) of (A) HgLCl₂, (B) HgLBr₂, (C) HgLI₂, (D) HgL(N₃)₂ and (E) HgL(SCN)₂.

Table 4 Calculated E_HOMO, E_LUMO, energy band gap (E_gap), chemical potential (μ), chemical hardness (η), electrophilicity index (ω) and dipole moment (D) for mercury(II) complexes

Compound	HOMO (eV)	LUMO (eV)	Egap (eV)	μ (eV)	η (eV)	ω (eV)	D (debye)
HgCl2	−6.685	−3.493	3.191	−5.089	1.596	8.114	4.616
HgBr2	−6.365	−3.487	2.878	−4.926	1.439	8.43	5.594
HgI2	−5.992	−3.433	2.559	−4.712	1.28	8.677	6.792
Hg(N3)2	−5.742	−3.558	2.184	−4.65	1.092	9.899	5.443
Hg(SCN)2	−6.446	−3.536	2.911	−4.991	1.455	8.558	6.207

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Molecular electrostatic potential (MEP). The molecular electrostatic potential (MEP) is used to identify chemical reactivity as well as the presence of intra and intermolecular interactions in the skeleton of the compound.⁶⁷ MEP is a plot of the electrostatic potential mapped onto the constant electron density surface and simultaneously displays molecular size and shape as well as positive, negative and neutral electrostatic potential regions in terms of color grading and is very useful in research of molecular structure with its physicochemical property relationship. The maximum positive region, i.e. the preferred site for nucleophilic attack, is indicated in blue and the maximum negative region, preferred site for electrophilic attack, in red. The different values of the electrostatic potential at the surface are shown by different colors, the potential increases in the order red < orange < yellow < green < blue, where blue shows the strongest attraction and red shows the strongest repulsion.^68,69 Fig. 11 shows 3D plots of molecular electrostatic potential (MEP) of mercury(II) complexes. It can be observed from Fig. 11, regions having negative potentials (deepest red colored areas) are anions linked to metal center and nitro groups of the aromatic rings. These regions are the potential positions for accepting of hydrogen bonds and other intermolecular interactions consistent with X-ray crystallography and Hirshfeld surface analysis. The regions having a positive potential (deepest blue colored area) are mainly distributed over the aliphatic and iminic hydrogen atoms.

Natural bond orbital (NBO) analysis. The natural bond orbital analysis is a powerful technique for examining intra and inters molecular bonding and also provides a convenient basis for the investigation of charge transfer or conjugative interaction in molecular systems.⁷⁰ NBO analysis is carried out by considering all possible interactions between occupied Lewis-type (bond or lone pair) NBO orbitals and formally unoccupied (antibond or Rydgberg) non-Lewis NBO orbitals and estimating their energetic importance by second-order perturbation theory. For each donor NBO(i) and acceptor NBO(j), the stabilization energy E⁽²⁾ associated with electron delocalization between donor and acceptor is estimated as.^71,72
Where q_i is the occupancy of the donating orbital, ε_i and ε_j are the energies of the donating and accepting orbitals, and F_ij is the off-diagonal element of the Fock matrix in the NBO basis.⁷³ Large E⁽²⁾ values show intensive interactions between electron-donors and electron-acceptors and the extent of conjugation of the whole system. The results of second-order perturbation theory analysis of the Fock Matrix at B3LYP/LanL2DZ level of theory are presented in Table 5.

Table 5 Second order perturbation theory analysis of Fock matrix in NBO basis for mercury(II) complexes

Compound	Donor NBO (i)	ED (i)(e)	Acceptor NBO (j)	ED (j)(e)	E^(2) (kcal mol^−1)	Ej − Ei (a.u.)	F (i,j) (a.u.)
HgLCl2	π(C5–C14)	1.61633	π*(O2–N4)	0.65374	32.83	0.11	0.058
	π(C43–C52)	1.61632	π*(N53–O54)	0.65374	32.83	0.11	0.058
	n(O3)	1.43586	π*(O2–N4)	0.65374	163.43	0.12	0.128
	n(O55)	1.43585	π*(N53–O54)	0.65374	163.44	0.12	0.128
	π*(C5–C14)	0.44389	π*(C6–C8)	0.28862	227.58	0.01	0.081
	π*(C5–C14)	0.44389	π*(C10–C12)	0.26909	161.03	0.02	0.082
	π*(C5–C14)	0.44389	π*(C15–C17)	0.10864	61.31	0.03	0.069
	π*(C43–C52)	0.44390	π*(C39–C41)	0.10864	61.31	0.03	0.069
	π*(C43–C52)	0.44390	π*(C44–C46)	0.26908	161.03	0.02	0.082
	π*(C43–C52)	0.44390	π*(C48–C50)	0.28862	227.57	0.01	0.081
	n(Cl56)	1.77309	n*(Hg1)	0.54039	44.92	0.21	0.094
	n(Cl57)	1.78414	n*(Hg1)	0.54039	41.29	0.23	0.094
HgLBr2	n(C12)	0.47757	π*(C8–C10)	0.16432	35.24	0.14	0.109
	n(C14)	0.60054	π*(C5–C6)	0.19131	35.29	0.13	0.103
	n(O55)	0.71707	π*(N53–O54)	0.32689	81.65	0.12	0.128
	π*(C5–C6)	0.19131	π*(C8–C10)	0.16432	132.96	0.01	0.085
	π*(C43–C52)	0.22209	π*(C39–C41)	0.05496	30.99	0.03	0.070
	π*(C43–C52)	0.22209	π*(C44–C46)	0.13476	81.15	0.02	0.082
	π*(C43–C52)	0.22209	π*(C48–C50)	0.14403	111.96	0.01	0.081
HgLI2	n(I3)	0.83169	n*(Hg1)	0.33763	37.17	0.13	0.096
	n(O57)	0.71691	π*(N55–O56)	0.32792	80.37	0.12	0.127
	π*(C7–C8)	0.19114	π*(C10–C12)	0.16341	137.74	0.01	0.084
	π*(C45–C54)	0.22097	π*(C46–C48)	0.13694	86.76	0.02	0.082
	π*(C45–C54)	0.22097	π*(C50–C52)	0.14383	115.58	0.01	0.081
HgL(N3)2	π(C38–C39)	1.64355	π*(O1–N7)	0.65215	32.05	0.11	0.058
	π(C48–C49)	1.63908	π*(N9–O12)	0.65343	32.02	0.11	0.059
	n(O4)	1.43882	π*(O1–N7)	0.65215	163.15	0.12	0.128
	n(O5)	1.43652	π*(N9–O12)	0.65343	163.41	0.12	0.128
	π*(C38–C39)	0.37935	π*(C37–C45)	0.34962	203.85	0.01	0.078
	π*(C38–C39)	0.37935	π*(C41–C43)	0.31995	224.53	0.01	0.084
	π*(C47–C55)	0.35879	π*(C33–C35)	0.10760	91.72	0.02	0.071
	π*(C48–C49)	0.38279	π*(C47–C55)	0.35879	272.86	0.01	0.079
	π*(C48–C49)	0.38279	π*(C51–C53)	0.32827	263.36	0.01	0.085
	n(N10)	1.47799	π*(N57–N58)	0.50337	149.91	0.17	0.142
	n(N10)	1.46790	π*(N57–N58)	0.41058	106.30	0.20	0.136
	n(N10)	1.46790	σ*(N11–Hg61)	0.32432	30.77	0.13	0.060
	σ(N11–Hg61)	1.56743	π*(N59–N60)	0.43564	84.17	0.25	0.130
	n(N11)	1.43545	π*(N59–N60)	0.54318	164.42	0.16	0.144
HgL(SCN)2	n(S2)	1.81450	n*(Hg1)	0.52759	62.64	0.16	0.095
	n(S2)	1.92328	π*(C3–N4)	0.15641	34.68	0.33	0.096
	n(S5)	1.66757	n*(Hg1)	0.52759	74.64	0.20	0.115
	n(S5)	1.81311	π*(C6–N7)	0.15833	36.20	0.32	0.097
	π(C45–C47)	1.79190	π*(N42–C43)	0.19404	30.44	0.26	0.079
	n(O9)	1.42595	π*(O8–N10)	0.63342	174.38	0.14	0.141
	n(O60)	1.44360	π*(N59–O61)	0.61859	108.57	0.16	0.120
	π*(C18–C20)	0.37960	π*(C21–C23)	0.11432	79.43	0.02	0.064
	π*(C49–C57)	0.35748	π*(C45–C47)	0.10973	50.30	0.02	0.061

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The NBO analysis for mercury(II) complexes shows identical interactions in all complexes. There are strong intramolecular hyperconjugative interactions in two sides of the Schiff base ligand expanded from the nitro substituents of the aromatic ring to the imine group. The intramolecular hyperconjugative interactions are induced by the orbital overlap between π (C–C, C–N and N–O) and π* (C–C, C–N and N–O) bond orbitals which results in intra molecular charge transfer (ICT) causing a stabilization of the system. These interactions increase electron density (ED) of C–C, C–N and N–O antibonding orbitals weakening the respective bonds. The π* → π* interactions are the important interaction in all complexes excepted for HgL(SCN)₂. In the HgLCl₂ complex, the π*(C5–C14) → π*(C6–C8) and π*(C43–C52) → π*(C48–C50) interactions give the strongest stabilization to the system by 227.58 and 227.57 kcal mol⁻¹, respectively. The high ED (0.44e) in these two donor antibonding orbitals [(C5–C14) and (C43–C52)] is due to π→ π* interactions from C6–C8, C10–C12 and C15–C17 to π*(C5–C14) and C44–C46, C48–C50 and C39–C41 to π*(C43–C52) which clearly demonstrates strong delocalization leading to an energetic stabilization in the range of 16.43–22.95 kcal mol⁻¹. The other π* → π* interactions from these two orbitals result in high stabilization energies of 163.03 and 61.31 kJ mol⁻¹. π* → π* interactions in other complexes show a similar trend. In all complexes, n → π* interactions due to electron donation of the oxygen lone pairs to the anti-bonding acceptor π*(N–O) provide large stabilization to the system by 163.44, 81.65, 80.37 and 163.41 kcal mol⁻¹ in HgLCl₂, HgLBr₂, HgLI₂ and HgL(N₃)₂ respectively. In HgL(SCN)₂, this interaction give the strongest stabilization to the system by 174.38 kcal mol⁻¹ for n(O9) → π*(O8–N10) and 108.57 kcal mol⁻¹ for n(O60) → π*(O59–N61). In all complexes, n → n* interactions due to electron donation of lone pairs of two anions connected to metal center to the anti-bonding acceptor n*(Hg) are observed to give a stabilization by energies in the range from 9.84 and 21.54 kcal mol⁻¹ in HgL(N₃)₂ to 62.64 and 74.64 kcal mol⁻¹ in HgL(SCN)₂. These results indicate the higher tendency of mercury to interact with sulfur as compared to nitrogen. n → π* interactions due to electron donation of lone pairs of coordinated nitrogen and sulfur atoms to the anti-bonding acceptor π*(N–N) and π*(C–N) contribute to a strong stabilization by azide and thiocyanate groups by 149.91 and 164.42 kcal mol⁻¹ in HgL(N₃)₂ and 34.68 and 36.20 kcal mol⁻¹ in HgL(SCN)₂.

4. Conclusion

In summary some new nano-sized crystals of Hg(II) complexes with the general formula HgLX₂ (X = Cl⁻, Br⁻, I⁻, N₃⁻ and SCN⁻) have been synthesized using a sonochemical process with a tridentate Schiff base ligand based on diethylenetriamine. HgLI₂ and HgL(SCN)₂ complexes were structurally characterized by single crystal X-ray diffraction. The crystal structures of the two complexes show that the Hg(II) ions are coordinated by three nitrogen atoms of the Schiff base ligand (two imine nitrogens and one amine nitrogen) and two I⁻ or SCN⁻, resulting in a fivefold coordination with HgN₃I₂ and HgN₃S₂ chromophores in HgLI₂ and HgL(SCN)₂ complexes, respectively. The packing of the molecules in the crystal structure is influenced by different intermolecular interactions such as N–H⋯I, C–H⋯I, C–H⋯O and C–H⋯π in HgLI₂ and C–H⋯N, S⋯N, C–H⋯O and N⋯O in HgL(SCN)₂ giving rise to a three-dimensional supramolecular structure. Apart from traditional structure descriptions, crystal structures of the two complexes were additionally explored by using the Hirshfeld surface analysis. The dominant red spots on the d_norm surfaces are due to the N–H⋯I bonding contacts in HgLI₂ and the C–H⋯N_thiocyanate and C–H⋯O_nitro bonding contacts in HgL(SCN)₂. The 2D fingerprint plots reveal that H⋯H (32.7%) and C–H⋯O–N (20.9%) contacts are the main intermolecular interactions in HgLI₂, while H⋯H, C⋯H and C–H⋯O–N contacts with almost identical distributions comprise the major section of intermolecular interactions in HgL(SCN)₂. The NBO analysis for the mercury(II) complexes shows that there are strong intramolecular hyperconjugative interactions in two sides of the Schiff base ligand expanded from nitro substituent of aromatic ring to imine group. The molecular electrostatic potential (MEP) of the complexes showed that the regions having negative potential are anions linked to metal center and nitro groups of aromatic rings while the regions having positive potential are distributed mainly over the aliphatic and iminic hydrogen atoms.

Acknowledgements

Partial support of this work by Yasouj University is acknowledged.

References

1. H. R. Khavasi, A. R. Salimi, H. Eshtiagh-Hosseini and M. M. Amini, CrystEngComm, 2011, 13, 3710–3717.
2. S. L. Price, Acc. Chem. Res., 2009, 42, 117–126.
3. Y. Xia, S. Li, B. Wu, Y. Liua and X. J. Yanga, CrystEngComm, 2011, 13, 5763–5772.
4. P. Dechambenoit, S. Ferlay, M. W. Hosseini and N. Kyritsakas, Chem. Commun., 2007, 4626–4628.
5. I. Goldberg, Chem. Commun., 2005, 1243–1254.
6. C. R. Jones, P. K. Baruah, A. L. Thompson, S. Scheiner and M. D. Smith, J. Am. Chem. Soc., 2012, 134, 12064–12071.
7. O. Takahashi, Y. Kohno and M. Nishio, Chem. Rev., 2010, 110, 6049–6076.
8. M. B. Mariyatra, B. Kalyanasundari, K. Panchanatheswaran and A. E. Goeta, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2003, 59, o255–o257.
9. X. Zhang, P. Zhong, M. Hu, D. Lin and J. Lin, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2004, 60, o1200–o1201.
10. C. Evans, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2010, 66, o502–o503.
11. M. Baby Mariyatra, K. Panchanatheswaran, J. N. Low and C. Glidewel, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 2004, 60, o682–o685.
12. M. Iwaoka and N. Isozumi, Molecules, 2012, 17, 7266–7283.
13. I. Onyido, A. R. Norris and E. Buncel, Chem. Rev., 2004, 104, 5911–5929.
14. X. Wang and L. Andrews, Inorg. Chem., 2005, 44, 108–113.
15. J. G. Melnick and G. Parkin, Science, 2007, 317, 225–227.
16. A. Morsali and M. Y. Masoomi, Coord. Chem. Rev., 2009, 253, 1882–1905.
17. M. Mirzaei, H. Eshtiagh-Hosseini, M. Mohammadi Abadeh, M. Chahkandi, A. Fronterac and A. Hassanpoor, CrystEngComm, 2013, 15, 1404–1413.
18. G. Mahmoudi, A. A. Khandar, J. K. Zareba, M. J. Białek, M. S. Gargari, M. Abedi, G. Barandika, D. V. Derveer, J. Mague and A. Masoumi, Inorg. Chim. Acta, 2015, 429, 1–14.
19. H. R. Khavasi and M. Esmaeili, CrystEngComm, 2014, 16, 8479–8485.
20. G. Mahmoudi, A. Morsali and M. Zeller, Solid State Sci., 2008, 10, 283–290.
21. D. A. Atwood and M. J. Harvey, Chem. Rev., 2001, 101, 37–52.
22. C.-M. Che and J.-S. Huang, Coord. Chem. Rev., 2003, 242, 97.
23. S. Chattopadhyay, K. Bhar, S. Das, S. Satapathi, H.-K. Fun, P. Mitra and B. K. Ghosh, Polyhedron, 2010, 29, 1667–1675.
24. S. Satapathi, S. Chattopadhyay, S. Roy, K. Bhar, P. Mitra and B. K. Ghosh, J. Mol. Struct., 2012, 1030, 138–143.
25. S. Hojaghani, M. Hosaini Sadr and A. Morsali, Ultrason. Sonochem., 2015, 26, 305–311.
26. M. Khanpour, A. Morsali and P. Retailleau, Polyhedron, 2010, 29, 1520–1524.
27. M. Montazerozohori, S. Mojahedi Jahromi, A. Masoudiasl and P. McArdle, Spectrochim. Acta, Part A, 2015, 138, 517–528.
28. M. Montazerozohori, S. Yadegari, A. Naghiha and S. Veyseh, J. Ind. Eng. Chem., 2014, 20, 118–126.
29. M. Montazerozohori, A. Nazaripour, A. Masoudiasl, R. Naghiha, M. Dusekc and M. Kucerakova, Mater. Sci. Eng., C, 2015, 55, 462–470.
30. M. Montazerozohori, S. A. Musavi, A. Masoudiasl, A. Hojjati and A. Assoud, Spectrochim. Acta, Part A, 2015, 147, 139–150.
31. M. Montazerozohori, S. A. Musavi, A. Masoudiasl, A. Naghiha, M. Dusek and M. Kucerakova, Spectrochim. Acta, Part A, 2015, 137, 389–396.
32. M. Montazerozohori and S. A. R. Musavi, J. Coord. Chem., 2008, 61, 3934–3942.
33. Apex Suite (V. 2012.4), Bruker AXS Inc., Madison, WI, USA, 2012.
34. G. M. Sheldrick, SADABS, Program for Empirical Absorption Correction of Area Detector Data, University of Göttingen, Göttingen, Germany, 2002.
35. G. M. Sheldrick, SHELXS/L-97, Programs for Crystal Structure Determination, University of Göttingen, Göttingen, Germany, 1997.
36. G. Keresztury, S. Holly, J. Varga, G. Besenyei, A. Y. Wang and J. R. Durig, Spectrochim. Acta, Part A, 1993, 49, 2007–2026.
37. S. K. Wolff, D. J. Grimwood, J. J. McKinnon, M. J. Turner, D. Jayatilaka and M. A. Spackman, CrystalExplorer, University of Western Australia, 2012.
38. U. M. Rabie, A. S. A. Assran and M. H. M. Abou-El-wap, J. Mol. Struct., 2008, 872, 113–122.
39. J. Yang, C. Lin, Z. Wang and J. Lin, Inorg. Chem., 2006, 45, 8973–8979.
40. L. J. Bellamy, The Infrared Spectra of Complex Molecules, Methuen, London, 3rd edn, 1966.
41. J. M. Sece, M. Quiros and M. J. G. Garmendia, Polyhedron, 2000, 19, 1005.
42. D. X. West and A. A. Nassar, Transition Met. Chem., 1999, 24, 617–621.
43. L. K. Das, R. M. Kadam, A. Bauza, A. Frontera and A. Ghosh, Inorg. Chem., 2012, 51, 12407–12418.
44. S. Das, K. Bhar, S. Chattopadhyay, P. Mitra, V. J. Smith, L. J. Barbour and B. K. Ghosh, Polyhedron, 2012, 38, 26–35.
45. A. Garg and J. P. Tondon, Transition Met. Chem., 1987, 12, 212–214.
46. A. W. Addision, T. N. Rao, J. Reedijik, J. V. Rijn and G. C. Verschoor, J. Chem. Soc., Dalton Trans., 1984, 1349–1356.
47. H.-P. Zhou, X.-P. Gan, X.-L. Li, Z.-D. Liu, W.-Q. Geng, F.-X. Zhou, W.-Z. Ke, P. Wang, L. Kong, F.-Y. Hao, J.-Y. Wu and Y.-P. Tian, Cryst. Growth Des., 2010, 10, 1767.
48. C. Yan, Q. Chen, L. Chen, R. Feng, X. Shan, F. Jiang and M. Hong, Aust. J. Chem., 2011, 64, 104–118.
49. S. Kundu, K. Bhar, S. Satapathi, S. Choubey, R. Ghosh and B. K. Ghosh, J. Indian Chem. Soc., 2013, 90, 763–769.
50. G. K. Patra and I. Goldberg, Polyhedron, 2002, 21, 2195–2199.
51. S. J. Grabowski, J. Phys. Org. Chem., 2003, 17, 18–31.
52. S. Chattopadhyay, K. Bhar, S. Das, S. Satapathi, H.-K. Fun, P. Mitra and B. K. Ghosh, Polyhedron, 2010, 29, 1667–1675.
53. C. Hopa, R. Kurtaran, A. Azizoglu, M. Alkan, V. B. Arslan and C. Kazak, Z. Anorg. Allg. Chem., 2011, 637, 1238–1245.
54. S. Satapathi, S. Das, K. Bhar, R. K. Kumar, T. K. Maji and B. K. Ghosh, Polyhedron, 2011, 30, 387–396.
55. A. N. Kharat, A. Bakhoda and S. Zamanian, Polyhedron, 2011, 30, 1134–1142.
56. H. Zhou, J. Wang, F. Zhou, D. Xu, Y. Cao, G. Liu, L. Kong, J. Yang, J. Wu and Y. Tian, Dyes Pigm., 2012, 95, 723.
57. J. A. Platts, S. T. Howard and K. Woźniak, Chem. Phys. Lett., 1995, 232, 479–485.
58. M. A. Spackman and P. G. Byrom, Chem. Phys. Lett., 1997, 267, 215–220.
59. S. A. Zarei, K. Akhtari, K. Hassanzadeh, M. Piltan, S. Saaidpour and M. Abedi, J. Korean Chem. Soc., 2013, 57, 311–315.
60. I. Fleming, Frontier Orbitals and Organic Chemical Reactions, Wiley, London, 1976.
61. R. G. Pearson, J. Org. Chem., 1989, 54, 1423–1430.
62. J. Padmanabhan, R. Parthasarathi, V. Subramanian and P. Chattaraj, J. Phys. Chem. A, 2007, 111, 1358–1361.
63. B. Kosar and C. Albayrak, Spectrochim. Acta, Part A, 2011, 78, 160–167.
64. M. Karabacak, A. M. Asiri, A. O. Al-Youbi, A. H. Qusti and M. Cinar, Spectrochim. Acta, Part A, 2014, 120, 144–150.
65. R. G. Pearson, J. Mol. Struct.: THEOCHEM, 1992, 255, 261–270.
66. V. Balachandran, G. Santhi, V. Karpagam and A. Lakshmi, Spectrochim. Acta, Part A, 2013, 110, 130–140.
67. I. Sheikhshoaie, S. Y. Ebrahimipour, M. Sheikhshoaie, H. A. Rudbari, M. Khaleghi and G. Bruno, Spectrochim. Acta, Part A, 2014, 124, 548–555.
68. J. S. Murray and K. Sen, Molecular Electrostatic Potentials, Concepts and Applications, Elsevier, Amsterdam, 1996.
69. E. Scrocco and J. Tomasi, in Advances in Quantum Chemistry, ed. P. Lowdin, Academic Press, New York, 1978.
70. M. Snehalatha, C. Ravikumar, I. Hubert Joe, N. Sekar and V. S. Jayakumar, Spectrochim. Acta, Part A, 2009, 72, 654–662.
71. D. W. Schwenke and D. G. Truhlar, J. Chem. Phys., 1985, 82, 2418–2427.
72. M. Gutowski and G. Chalasinski, J. Chem. Phys., 1993, 98, 4728–4738.
73. A. E. Reed, L. A. Curtiss and F. Weinhold, Chem. Rev., 1988, 88, 899–926.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1434497 and 1434498. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra22899a

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