Supramolecular assemblies of organotin(IV)–diphosphoryl adducts: insights from X-rays and DFT

Abstract

Mono-, di- and tri-phenyltin(IV) adducts containing a neutral diphosphoryl ligand, Ph₂P(O)OC₆H₄OP(O)Ph₂, were synthesized and characterized. These 2 : 1 adducts of two trans-organotin(IV) moieties and the bridging ligand all crystallize around a centre of inversion. The influence of tin-substituents on the building up of multidimensional architectures, as well as, strength of the Sn–O interactions are discussed in terms of geometrical parameters, Hirshfeld surface analysis and theoretical calculations. The CH⋯Cl interactions make a substantial contribution to directing 1D self-assembly of the tin adducts. Replacement of Cl atoms with Ph groups on tin(IV) in these complexes has increased the probability of CH⋯π and H⋯H interactions and weakens the Sn–O bonds. QTAIM and NBO analyses reveal that the Sn–O interactions are principally electrostatic in nature (donor–acceptor like) with only a small amount of covalence increasing from the tri- to the mono-phenyltin adduct.

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Introduction

Supramolecular chemistry of organotin(IV) complexes are attracting more and more attention due to their wide applications in different areas from industrial to bioinorganic chemistry^1–4 and also their structural diversity.^5,6 The molecular structures as well as the architectures of self-assembled species depends on a combination of several factors including the type of donor ligands, tin-R groups, the geometry of tin coordination, metal to ligand molar ratio and reaction conditions.

Adduct formation with organotin halides, particularly dimethyltin dichloride, and bidentate bases, such as diphosphoryl ligands, has afforded chelated 1 : 1 six-coordinated complexes, binuclear and polymeric arrangements with octahedral or trigonal bipyramidal coordination.^7–9 For ligands mentioned above, various structures have been obtained by altering the length of the aliphatic spacer between two P=O groups. Although, the coordination diversity of these compounds has been investigated, we have not found any precise study on the effect of all non-covalent interactions (including coordinating linkages and intermolecular interactions) on the structure of organotin(IV)–diphosphoryl complexes. A clear understanding intermolecular recognition between molecular building blocks is very important to design and control of the molecular crystal packing with desired properties.

Very recently, in our study of organotin(IV) chemistry of phosphoryl-containing ligands,^10,11 some triphenyltin(IV) adducts with a family of diphosphoryl ligands, RR′P(O)–X–P(O)RR′ have been introduced.¹² Steric and electronic parameters of the ligands manipulated by modifications of the spacer, X, as well as the terminal phosphorus substituents, R and R′, with the aim of studying the influence on the organization and stabilization of the resultant crystal structures. In the following, we began a systematic investigation of the interaction between several organotin(IV) halides (SnPh_nCl_4−n, n = 1–3) and the invariant diphosphoryl ligand, Ph₂P(O)OC₆H₄OP(O)Ph₂ (L), in order to explore the effects of replacing tin-bonded Ph groups with chlorine atoms on the tin–ligand coordination and the intermolecular interactions directing multidimensional self-association of the complexes. This work will provide detailed insight into the molecular and supramolecular structures of these systems using X-ray diffraction. An investigation of intermolecular interactions via Hirshfeld analysis is also presented. Moreover, DFT calculations were performed to fathom the electronic, bonding and energy aspect of the tin–ligand as well as the intermolecular interactions, in these structures.

Results and discussion

Solid-state structures

Molecular structures. Single crystals of ligand and its complexes were obtained after slow evaporation of mentioned solutions (see Experimental section) at room temperature. Crystal data and details of the X-ray analysis are given in Table S1,† whereas selected bond lengths and angles are summarized in Table 1. Intermolecular interaction data are represented in Tables 2 and 3. A view of the structures is shown in Fig. 1.

Table 1 Selected bond lengths (Å), angles (°) and binding energies (kcal mol⁻¹) for crystalline compounds

Compounds	d(P–CR)	d(P–OX)	d(P=O)	d(Sn–O)	ΔESn–O	d(Sn–Clax)	d(Sn–Cleq.)	d(Sn–C)	∠O–Sn–Cl	∑∠C/Cleq.–Sn–C/Cleq.
L	1.7921(17)	1.6030(12)	1.4755(12)	—	—	—	—	—	—	—
C1	1.7792(17)	1.5870(13)	1.4876(14)	2.2144(14)	−40.0	2.3970(7)	2.3309(6)	2.1209(19)	170.32(4)	357.798
	1.7799(19)						2.3290(7)
C2	1.7854(16)	1.5900(13)	1.4915(12)	2.3711(12)	−29.6	2.4639(5)	2.3329(5)	2.1191(17)	177.40(3)	357.792
	1.7835(16)							2.1182(16)
C3	1.7908(11)	1.6003(8)	1.4820(9)	2.4663(8)	−25.3	2.4721(3)	—	2.1255(12)	177.32(2)	356.37
	1.7851(11)							2.1235(12)
								2.1216(11)

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Table 2 Non-classic hydrogen bonding interactions (all distances in Å, angles in ° and binding energies in kcal mol⁻¹)

Compounds	DH⋯A	D⋯A	D–H	H⋯A	∠D–H–A	ΔECH⋯A	ΔEdimer	Symm. codes
L	C3–H3⋯O1	3.074(2)	0.95	2.30	139	−3.8	—	2 − x,1 − y, −z
C1	C30–H30⋯Cl1	3.843(3)	0.93	2.91	177	−5.4	−21.6	1 + x,−1 + y,z
C2	C30–H30⋯Cl1	3.653(2)	0.93	2.78	158	−9.0 −ΔECH⋯π	−20.7	−1 + x,y,z
	C27–H27⋯Cl2	3.864(7)	0.93	2.98	161	—	−9.1	−x,2 − y,1 − z
C3	C30–H30⋯Cl1	3.712(1)	0.95	2.93	141	−5.9	−22.4	x,1 + y,z

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Table 3 CH⋯π and π⋯π intra*- and inter-molecular interactions (all distances in Å, angles in ° and binding energies in kcal mol⁻¹)^a

Compd.	Interaction	D⋯A	Cg⋯Cg	∠P–P	Slippage	C⋯Cg	H⋯Cg	∠C–H–Cg	ΔE	Symm. codes
a Cg stands for the centre of gravity of the mentioned ring. For L: Cg1: C1–C3, C1^(i)–C3^(i); Cg2: C4–C9; Cg3: C10–C15; for C1: Cg1: C1–C6; Cg3: C25–C30; for C2: Cg1: C1–C6; Cg3: C19–C24; Cg4: C25–C30; Cg5: C31–C33, C31^(i)–C33^(i); for C3: Cg1: C1–C6; Cg2: C7–C12; Cg3: C13–C18; Cg4: C19–C24; Cg5: C25–C30.
L	π–π	Cg2⋯Cg2	3.7802(10)	0	1.732	—	—	—	−8.1	1 − x,2 − y,−z
	CH⋯π	C7–H7⋯Cg1	—	—	—	3.9570(4)	3.15	144		1 − x,2 − y,−z
	CH⋯π	C8–H8⋯Cg3	—	—	—	3.7524(18)	2.90	149	−3.1	1 − x,1 − y,−z
	CH⋯π	C12–H12⋯Cg2	—	—	—	3.6356(18)	2.81	145	−3.3	2 − x,1 − y,−z
C1	π–π	Cg1⋯Cg1	3.9811(17)	0	1.882	—	—	—	—	−x,2 − y,−z
	CH⋯π	C22–H22⋯Cg3	—	—	—	3.8843(6)	3.032	149	−3.9	1 − x,−y,1 − z
C2	π–π*	Cg2⋯Cg3	4.0226(15)	10.53(12)	2.142,1.722	—	—	—	—	x,y,z
	π–π	Cg4⋯Cg4	3.8496(13)	0	1.654	—	—	—	−8.3	−x,1 − y,1 − z
	CH⋯π	C28–H28⋯Cg5	—	—	—	3.567(2)	2.70	155		−1 + x,y,z
	CH⋯π	C9–H9⋯Cg4	—	—	—	3.883(3)	2.97	169	−1.9	1 + x,y,z
	CH⋯π	C29–H29⋯Cg1	—	—	—	3.815(2)	2.89	173	−9.0 −ΔECH⋯Cl	x,−1 + y,z
C3	π–π	Cg1⋯Cg1	3.6671(8)	0	1.464	—	—	—	−4.2	2 − x,1 − y,−z
	CH⋯π	C3–H3⋯Cg4	—	—	—	3.8905(1)	3.05	159	−2.0	2 − x,2 − y,−z
	CH⋯π	C9–H9⋯Cg3	—	—	—	3.5129(15)	2.89	124	−5.2	2 − x,1 − y,1 − z
	CH⋯π	C23–H23⋯Cg5	—	—	—	3.5530(13)	2.66	157	−3.3	2 − x,2 − y,−z
	CH⋯π*	C26–H26⋯Cg3	—	—	—	3.6899(13)	2. 90	142	—	x,y,z
	CH⋯π	C28–H28⋯Cg4	—	—	—	3.5305(16)	2.67	150	−3.1	1 + x,y,z
	CH⋯π*	C32–H32⋯Cg2	—	—	—	3.6136(14)	2.69	165	—	x,y,z

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Fig. 1 The structures L, C₁, C₂ and C₃ (50% probability level) in their crystals.

All four compounds crystallize as dimers in the space group P1̄; the two organotin(IV) fragments of the 2 : 1 adducts are related by the inversion and O,O′-bridged by the diphosphoryl ligand. The P=O donors in the complexes are approximately antiparallel to each other (an anti-conformation) as they are in the free form of ligand. The distorted trigonal bipyramidal configuration of the tin centers has been formed by the chlorine and oxygen atoms in the axial positions and the phenyl carbon atoms as well as other chlorines (in C₁ and C₂) in the equatorial sites.

The Sn–C and Sn–Cl distances lie in the range corresponding to normal covalent radii, while the Sn–O bonds are in accordance with the coordinate bond.¹³ The stepwise shortening of the Sn–O bond distances from C₃ to C₁ (the strengthening Sn–O donor–acceptor interaction) increasing with the Lewis acidity of tin. As a result of their coordination to Sn(IV), the P=O distances (1.4820(9)–1.4915(12) Å) in the complexes are longer than that found in the free diphosphine dioxide (1.4755(12) Å) due to polarization effects and electron donation to tin(IV). In contrast, P–O and P–C bonds are slightly shortened in the complexes (Table 1). The geometrical reorganization in the ligand upon complexation is reflected in a significant increase for P=O vibrational frequency and decreasing υ(P–O). The other bond distances and angles within the ligand do not vary significantly in the coordinated structures.

Crystal packing. The cohesion of the ligand L is due to weak interactions. Non-classical CH⋯O_P=O hydrogen bonds build up [100] chains consisting of alternating phenyl and R²₂ (12) rings. (The graph-set-notation R^X_Y (Z) designates a Z-membered ring generated by X hydrogen bonds between Y donor–acceptor units¹⁴). These chains are laterally held together by one π–π stacking and three CH⋯π interactions which complete a 3D network (Fig. 2 and Tables 2 and 3).

Fig. 2 (a) Illustration of R²₂ (12) graph set in L (b) the overview of its crystal packing.

Common feature of supramolecular associations in the complexes are intermolecular CH⋯Cl hydrogen bonds, which link neighboring molecules together into chains along the b-axis (Table 2); that seems to be effective in directing the crystal packing. These chains are further reinforced by π–π stacking contacts in crystal C₁. On the other hand, weaker CH⋯π linkages connect these 1D chains together to form (101) layers (Fig. 3 and Table 3).

Fig. 3 Representation of intermolecular interactions in C₁, resulting in 2D arrangements.

CH⋯π interactions in the structure of C₂ (Fig. 4a and Table 3), reinforces the [010] chains, formed by CH⋯Cl1 interactions. Interestingly, close packing of chlorine atoms in the crystal lattice of C₂ leads to the type I Cl⋯Cl contacts with trans geometry (dCl2⋯Cl2 = 3.4094(7) Å and θ = 131.80°) within the chains. Moreover, each Cl2 atom is involved in another CH⋯Cl interaction (Table 2), leading to the formation of 2D layers which are directed along the ab-plane. Weaker CH⋯π and π–π interactions contribute to further stabilization of the 2D arrangements (Fig. 4b and Table 3).

Fig. 4 Representation of (a) CH⋯Cl, (b) CH⋯π and π–π interactions in C₂, resulting in 2D arrangements.

Compound C₃ has a 3D network formed by intermolecular interactions including: (i) π–π stacking, (ii) several CH⋯π and (iii) weaker H⋯H interactions in addition to the mentioned CH⋯Cl hydrogen bonds (Fig. 5 and Table 3).

Fig. 5 Representation of (a) intramolecular CH⋯π interactions and (b) crystal packing fragment of C₃, showing intermolecular interactions.

Replacement of Cl atoms with Ph groups on tin(IV) in these complexes has increased the probability of CH⋯π and H⋯H interactions and weakens the Sn–O bonds. Notably, replacement of Cl substituents with Ph rings, decreases the experimental crystal density from C₁ to C₃ (1.675, 1.554 and 1.465 Mg m⁻³, respectively), in accordance with the order of melting points (197, 192 and 172 °C, respectively).

Hirshfeld analysis. The intermolecular interactions in the crystal structures were quantified via Hirshfeld surface analysis¹⁵ using Crystal Explorer 3.0.¹⁶ Hirshfeld surface analysis is a powerful tool for visualization and understanding of intermolecular interactions. The contribution percentage of different interactions to the Hirshfeld surface area is shown in Fig. 6. The histogram indicates that in all cases, the H⋯H interactions are indeed the dominant ones and form C₁ to C₃, by replacing Cl atoms with Ph rings, the contribution of the H⋯H (from 38.1% to 52.4%) and C⋯H (from 22.3% to 32.8%) interactions increase, while the contribution of Cl⋯H decreases (from 32.4% to 11.1%). Other interactions in C₁, include Cl⋯C, Cl⋯O and Sn⋯H ones (3.9, 0.4 and 0.1%, respectively). It should be noted, the small percentage of Cl⋯Cl interaction to the Hirshfeld surface area in C₂ (0.8%) reveals the negligible contribution of this contact in the crystal packing. Fig. 7 illustrates full fingerprint plots of all the compounds along with the decomposed ones for the selected contacts.

Fig. 6 Relative contributions of various intermolecular contacts to the Hirshfeld surface area in compounds.

Fig. 7 2D fingerprint plots, full and resolved into H⋯H, C⋯H and Cl⋯H (or O⋯H for L) contacts, showing percentages of contacts contributed to the total Hirshfeld surface area of C₁, C₂, C₃ and L, from left to right. d_e and d_i refers to distances from the surface to the nearest nucleus exterior and interior to the surface, respectively.

Theoretical aspects

Non-covalent binding energies. It was thought of interest to further investigate the intermolecular and also coordination binding energies to characterize the strength of these interactions in the crystal structures. ΔE_Sn–O values, from −25.27 to −40.03 kcal mol⁻¹, represent a weak to medium-strong Sn–O interaction for this series of complexes and decrease in order C₁ > C₂ > C₃ (Table 1). The CH⋯Cl interactions have a substantial contribution in directing 1D self-assembly of the tin adducts. Neighboring complexes in these 1D chains are connected by −20.68 to −22.42 kcal mol⁻¹, while contribution of each CH⋯Cl interaction individually is 5.38 to −8.99 kcal mol⁻¹ (Table 2). CH⋯π and π-stacking interactions further stabilize the 3D structure of C₃ and L by about 2–4 kcal mol⁻¹ for each interaction (Table 3). It should be noted; the estimated ΔE_π–π in structures L and C₂ doesn't exhibit the energy of π–π stacking interaction exclusively; but reflect an overall binding energy from a combination of π–π and two equal CH⋯π interactions (C7–H7⋯Cg and C28–H28⋯Cg, respectively; where Cg is phenylene ring) (Table 3). Although π-stacking, CH⋯π, C⋯C and H⋯H interactions are weak, sum of these interactions and cooperative effects between them seem to have an important role in stability of the crystal structures. On the other hand, the estimated weak interaction energy (−0.8 kcal mol⁻¹) for the Cl⋯Cl contact in C₂ clearly indicates the negligible contribution of this interaction in maintaining favorable packing interactions in this complex.

QTAIM and NBO analysis. Detailed information about the electronic characteristics and nature of bonding in the tin–ligand interaction, as a function of tin substituents, was obtained by QTAIM and NBO analysis on the optimized structures. Corresponding parameters are listed in Table 4.

Table 4 QTAIM parameters (in au) and delocalization energies (from NBO analysis) for Sn–O bonds, at B3LYP/6-311+G*/LANL2DZ

Compounds	Sn–O			E^(2) (kcal mol^−1)
	ρ	Δ^2ρ	H(r)		LP(OP=O) to LP*(Sn)
C1	0.057	0.248	−0.007		74.03
C2	0.042	0.169	−0.003		36.98
C3	0.033	0.127	−0.001		29.10

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The obtained Sn–O critical points have the typical properties of the closed-shell interactions. Explicitly, the ρ values are small and the corresponding ∇²ρ values are all positive. Despite falling in a region of charge depletion with ∇²ρ > 0, the small negative values of H(r) shows that the Sn–O interactions are principally electrostatic in nature with a partial amount of covalent contribution. Notably, replacing Ph substituents on Sn with the Cl ones increases the values of ρ and ∇²ρ at the Sn–O BCPs (from C₃ to C₁) in line with strengthening of the tin–ligand interaction. Furthermore, the related H(r) terms amount to more negative values on going from C₃ to C₁ as the tin centers become more acidic.

The P=O → Sn interaction was found to be of donor (oxygen lone pair, LP(O)) to acceptor (Sn s, p; LP*(Sn)) character within the NBO framework. The stabilizing energies, E⁽²⁾, for the electronic delocalization LP(O) → LP*(Sn) rise from C₃ to C₁ consistent with increased Lewis acidity of the organotin compounds as the substituents on the tin become more electronegative.

It should be noted here, dipole moment of optimized tin reactants (3.56, 4.82 and 5.09 Debye) is in agreement with the increased acceptor power along the series SnPh₃Cl < SnPh₂Cl₂ < SnPhCl₃. The observed trend for the energy of lowest unoccupied molecular orbitals (E_LUMO: SnPh₃Cl > SnPh₂Cl₂ > SnPhCl₃) that is mainly located at the tin centers can further support the obtained results.

Conclusion

In this work, mono-, di- and tri-phenyltin(IV) adducts of a neutral bidentate ligand, Ph₂P(O)OC₆H₄OP(O)Ph₂, are presented. X-ray crystallography confirms the binuclear structures for the tin(IV) adducts, with two trans organotin(IV) moieties O,O′-ligated by the diphosphoryl ligand. The crystal packing studies reveal that the importance of CH⋯Cl interactions as a main factor controlling the 1D supramolecular architectures by about 5–6 kcal mol⁻¹ for each interaction. Replacement of Cl atoms with Ph groups on tin(IV) increases the probability of CH⋯π and weaker H⋯H interactions, while decreases the contribution of CH⋯Cl linkages, as well as, leads to weakening of the Sn–O bonds and also. Based on the parameters derived from QTAIM, the nature of tin–ligand are mainly electrostatic with a small amount of covalent overlap, increasing as Ph-substituents on tin are decreased. The same trend was observed for LP(O) → LP*(Sn) delocalization energies within the NBO framework.

Experimental

Materials and methods

All chemicals and solvents are commercially available and were used as received without further purification. ¹H, ³¹P and ¹¹⁹Sn NMR spectra were recorded on a Bruker Avance DRS 300 spectrometer at 300.13, 121.50 and 111.86 MHz, respectively. ¹H chemical shifts were determined relative to Si(CH₃)₄. ³¹P and ¹¹⁹Sn chemical shifts were measured relative to 85% H₃PO₄ and Sn(CH₃)₄ as external standard respectively. Infrared (IR) spectra were recorded on KBr disk using a Shimadzu model IR-60 spectrometer.

Crystallization of compounds

General method for synthesis and characterization data of ligand and its complexes have been reported in our other work.¹⁷ Crystallization procedure of them are described in the following.

1,4-[(C₆H₅)₂P(O)O]₂C₆H₄ (L). As reported earlier,¹⁷ the ligand was synthesized from the reaction of 2 mmol Ph₂P(O)Cl with 1 mmol of 1,4-benzendiol in presence of Et₃N as HCl scavenger in CH₂Cl₂ or CH₃CN at room temperature. After stirring for 24 h, the solvent was evaporated and the residue was washed with distilled water and dried. Suitable crystal for X-ray diffraction was obtained from slow evaporation of the solution MeOH–H₂O at room temperature.

μ-{1,4-[(PhO)₂P(O)O]₂C₆H₄}[SnPhCl₃]₂ (C₁). Toluene solutions of 2 eq. SnPh₃Cl and 1 eq. the ligand were mixed and stirring for 3–4 hours on the heater. Crystals suitable for X-ray diffraction were obtained from slow evaporation of the solution at room temperature.

μ-{1,4-[(PhO)₂P(O)O]₂C₆H₄}[SnPh₂Cl₂]₂ (C₂). Chloroform solutions of 2 eq. SnPh₂Cl₂ and 1 eq. the ligand were mixed and n-heptane was added dropwisely. Crystals suitable for X-ray diffraction were obtained from slow evaporation of the solution at room temperature.

μ-{1,4-[(PhO)₂P(O)O]₂C₆H₄}[SnPh₃Cl]₂ (C₃). Methanol–chloroform solutions of 2 eq. SnPh₃Cl and 1 eq. the ligand were mixed. Crystals suitable for X-ray diffraction were obtained from slow evaporation of the solution at room temperature.

Crystal structure determination

X-ray crystallography for L. X-ray intensities of L were collected on a Bruker SMART APEXII CCD diffractometer with graphite-monochromatized MoKα radiation (λ = 0.71073 Å) at T = 100 K. Cell refinement and data reduction were performed with the help of the programs APEX2 and SAINT.¹⁸ The structure was solved with the Olex2.solve¹⁹ and refined with the program SHELXL-97 ²⁰ by full-matrix least-squares on F².

All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were located from geometric considerations. Then the C–H atoms were refined in riding model. All hydrogen atoms were refined with the U_iso(H) parameters equal to 1.2U_eq.(A), where U(A) is the equivalent thermal parameter of the atom to which the corresponding H atom is bonded.

X-ray crystallography for C₁ and C₂. Bragg-intensities of C₁ and C₂ were collected on a Stoe IPDS II diffractometer with graphite-monochromatized MoKα radiation (λ = 0.71073 Å) at room temperature. Cell refinement, data reduction and a numerical absorption correction were performed with the help of the programs X-AREA (1.62)²¹ and XRED32 (1.31).²² The structures were solved with direct methods using the program SHELXL-97 (ref. 20) and refined using the program SHELXL-2014/7 by full-matrix least-squares on F².¹⁷ All non-hydrogen atoms and the hydrogens were refined anisotropically and isotropically, respectively.

X-ray crystallography for C₃. X-ray intensities of complex C₃ was collected on a Bruker APEX-II CCD diffractometer with graphite-monochromatized MoKα radiation (λ = 0.71073 Å) at T = 100 K. Cell refinement and data reduction were performed with the help of the programs APEX2 and SAINT.²³ The structure was solved and refined with the help of the program SHELXTL (5.1)²⁴ by full-matrix least-squares against F² in anisotropic approximation (for non-hydrogen atoms). The H atoms were placed in calculated positions and included in the refinement within riding model with fixed isotropic displacement parameters (U_iso(H) = 1.2U_eq.(C)). The U_eq.(C) are the equivalent isotropic displacement parameters of the carbon atoms, to which corresponding H atoms are bonded.

The crystallographic and refinement data are summarized in Table S1† for all compounds. Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Center, CCDC no. 1049490–1049493 for L, C₁, C₂, C₃, respectively.†

Computational details

Quantum chemical calculations were carried out using the Gaussian 09 suite of programs²⁵ in the gas phase. All of the complexes and the free ligands were fully optimized at the B3LYP/6-311G* level of theory²⁶ for all atoms except the tin(IV) which were described by the LanL2DZ basis set.²⁷ The electronic features of Sn–O interactions in the optimized structures were evaluated by Quantum theory of atoms in molecules (QTAIM)^13,28 and natural bond orbital (NBO)²⁹ analyses at the B3LYP/6-311+G*/LANL2DZ level of theory.

In order to investigate the intermolecular interactions in the solid state, the X-ray structures were used as starting points for the geometrical calculation (B3LYP/6-311G*/LANL2DZ). The two selected fragments were cut out directly from the CIF data. We optimized only the hydrogen atoms of these systems, and kept all other atoms frozen during the process. The intermolecular binding energies were calculated, at M062X/6-311G*/LANL2DZ level, based on the energy difference between the total energy of the system and its fragments, as represented in the equation ΔE = E_total − (E_frag1 + E_frag2). The interaction energies have been corrected for the basis set superposition error (BSSE) using counterpoise (CP) procedure.³⁰

Acknowledgements

Financial support of this work by Tarbiat Modares University is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The crystal data and the details of the X-ray analysis. CCDC 1049490–1049493. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra15645a

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