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Stronger #···# Interaction Leads to a Smaller Thermal Expansion in Some Charge Transfer Complexes Viswanadha G. Saraswatula, DURGAM SHARADA, and Binoy K. K. Saha Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01502 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 14, 2017

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Crystal Growth & Design

Stronger π⋅⋅⋅π Interaction Leads to a Smaller Thermal Expansion in Some Charge Transfer Complexes Viswanadha G. Saraswatula,‡ Durgam Sharada‡ and Binoy K. Saha* Department of Chemistry, Pondicherry University, Puducherry 605 014, India, E-mail: [email protected]. KEYWORDS. thermal expansion · charge transfer complex · π⋅⋅⋅π stacking · layered structure

ABSTRACT. Crystal structures and thermal expansion properties have been studied for hexamethylbenzene (HMB), picric acid (PIC), tetracyanobenzene (TCB), HMB-PIC complex and HMB-TCB complex. HMB-PIC and HMB-TCB form charge transfer complexes in solid state as well as in solution. From the UV-Vis spectroscopy study it has been found that HMBTCB forms a stronger π⋅⋅⋅π complex than HMB-PIC. On the other hand, HMB, in its crystal structure, forms a very weak π⋅⋅⋅π stacking interaction. Thermal expansion study shows that thermal expansion along the stacking direction is highest in HMB, which is followed by HMBPIC and then by HMB-TCB complexes. Therefore, this study shows that stronger π⋅⋅⋅π stacking interaction leads to a weaker thermal expansion in the materials.

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Thermal expansion is a property which depends upon several factors, such as packing, interaction energy, type of atoms etc.1-7 Atoms are known to vibrate more strongly perpendicular to the bonded axis compared to that along the axis. In a planar molecule, the atoms tend to vibrate strongly perpendicular to the plane rather than along the plane.8-10 Strength of the interactions is another parameter that can control the magnitude of thermal expansion. Stronger interactions are known to be less affected by the temperature change compared to the weaker interactions. Previously we have shown that stronger I···I interactions are less affected by temperature change compared to relatively weaker Br···Br interaction. Much weaker Cl···Cl interactions’ distances increase even much faster with increasing temperature.11 We also have shown that in a pair of polymorphic systems, the order of expansion along different types of contacts with respect to the temperature was O−H···N < C−H···O < weak van der Waals contacts.12 The strengths of these interactions are known to follow a reverse order. To the best of our knowledge no such systematic attempt has been made on π···π interactions. Here we have attempted to study the effect of temperature variation on π···π interactions in some π⋅⋅⋅π stacked systems.

π···π Interaction is one of the most fascinating interactions studied in solid-state.13-19 It has been established that this interaction is more favorable when one of the species is π-electron rich and the other one is π-electron deficient.20-21 One of the classic examples is the complex between benzene and hexafluorobenzene. This complex exhibits a higher melting point (297 K) compared to its individual components (benzene, 279 K and hexafluorobenzene, 278 K). The interaction energy in this complex has been estimated to be as high as −5.38 kcal/mol.22,23 There is a plethora of reports where the π···π interaction between the electron rich and electron deficient


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compounds causes charge transfer complexes.24-29 π···π interactions are also known to play a vital role in the biological systems.30-32 In this work we are interested to determine whether π···π interactions also follow the reverse relationship between the interaction strength and thermal expansion, i.e. whether the stronger π···π interaction between the electron rich and electron deficient compounds would show a smaller thermal expansion than the weaker π···π interaction between the electron rich compounds. To study the effect of temperature change on π···π interactions, we have chosen hexamethylbenzene (HMB) and its complexes with tetracyanobenzene (TCB) and picric acid (PIC). HMB has been particularly selected for this study because it is an π electron rich compound and it also forms layered structure. Therefore, it would allow us comparing the π···π stacking distances between the π-electron rich crystal structure of HMB and its π···π stacked complexes with the electron deficient compounds. Crystal structures of HMB, PIC, TCB and HMB-TCB complex were already reported in the literature.33-36 Here we describe these crystal structures in the context of thermal expansion. The crystal structure of the HMB-PIC complex is reported here for the first time. PIC crystallizes in the Pca21 space group with two molecules in the asymmetric unit. The molecules are mainly assembled via C−H⋅⋅⋅O hydrogen bonds, O⋅⋅⋅N interactions between the nitro groups and O⋅⋅⋅C interactions between electron rich O atom of the nitro group and electron deficient C atoms of the aromatic ring. In this crystal structure the molecules don’t form layered structure (Figure 1a). TCB crystallizes in P21/n space group with one molecule in the asymmetric unit. The molecules are assembled in a herringbone fashion via weak interactions, such as C−H⋅⋅⋅N, N⋅⋅⋅C between cyano groups and also between electron rich N atom of the cyano group and electron deficient C atoms of the aromatic ring. TCB also does not form layered


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structure (Figure 1b). The molecules, in the crystal structures of these two electron deficient aromatic ring containing compounds, are assembled in a “T” or a “V” shaped geometry rather than via π···π stacking. HMB, on the other hand, crystallizes in P1ത space group with a half molecule in the asymmetric unit. The HMB molecules form layered structure, where each HMB is surrounded by six other HMB in the 2D layer. These 2D layers are parallel to the (11−1) plane. The contacts between the molecules, within a layer, are only H···H type among the Me groups. These layers are stacked over each other with some off-set via C−H···π and π···π interactions to form the 3D structure (Figure 2). The molecular planes of the two consecutive stacked HMB are parallel to each other and the distance between these two planes is 3.684 Å (at 298 K). The Cg-Cg distance between these two HMB molecules is 5.312 Å and the Cg of HMB is off-set by 3.827 Å from the ideal stacking position. The 1:1 HMB-PIC complex crystallizes in a monoclinic space group, C2/c. The crystal structure of this complex consists of half molecule each of HMB and PIC in its asymmetric unit. This causes a high disorder in one of the NO2 groups, and the OH group is disordered over three positions. These electron rich and electron deficient compounds stack alternately along the c axis (Figure 3). The components form individual layers parallel to the ab plane. Similar to HMB, the 2D sheets in this complex are flat. Therefore, the arrangement of the HMB molecules in a layer, in this binary complex, is very much similar to that found in the HMB crystal structure. Each molecule in their respective layers is surrounded by six other identical molecules. The interplanar angle between the consecutive HMB and PIC aromatic rings is 4.61° (at 298 K) and the Cg-Cg distance between them is 3.691 Å. The off-set of the PIC Cg from its ideal position of π…π stacking is 1.378 Å which is much smaller than that found in HMB.


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The 1:1 HMB-TCB complex crystallizes in the P21/c space group. The asymmetric unit of this complex contains half molecule each of HMB and TCB. The HMB molecule is found to be disordered over two orientations. In this crystal structure, the two components stack alternately along the crystallographic a axis with some off-set (Figure 4). As a result, the HMB and TCB molecules also form individual corrugated layers parallel to the bc plane and these layers are stacked alternately along the a axis. The interplanar angle between the stacked HMB and TCB molecules is 3.39° (at 298 K) and the Cg-Cg distance is found to be 3.697 Å. The off-set of TCB Cg from its ideal position is 1.256 Å which is smallest among these three π⋅⋅⋅π stacked crystal structures. Though, the crystals of HMB and TCB are colorless and PIC is of pale yellow in color, the colors of HMB-TCB and HMB-PIC complexes are bright yellow and dark orange respectively (Figure 5). This color change is indicative of charge transfer nature of the π···π interactions in these complexes. This is also evident from the red shift of the absorption peak in visible region of the UV-Vis spectrum of 1:1 HMB-PIC complex even in the solution state (Figure S1, supporting information) and appearance of a new peak at 411 nm in the visible region in the solution state UV-Vis spectrum of 1:1 HMB-TCB complex (Figure S2, supporting information). Though there is practically no shift observed in the UV-Vis. spectrum of HMB-PIC in 1,4dioxane solution at 16 mmol concentration, a red shift of the absorption peak of the complex is clearly visible at concentration higher than 20 mmol. On the other hand, at 4 mmol concentration of HMB-TCB in 1,4-dioxane there is no new peak observed, but at higher than 8 mmol concentration a new peak starts appearing in the visible region of the UV-Vis. spectrum. This indicates that HMB-TCB complexation occurs even at higher dilution than in the case of HMB-


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PIC complex. Therefore, HMB-TCB is expected to form a stronger π⋅⋅⋅π interaction than the HMB-PIC complex. Six sets of single crystal of X-ray diffraction data were collected in the range of 118 K to 298 K at an interval of 36 K for each crystal of HMB, PIC, TCB, HMB-PIC and HMB-TCB to study the thermal expansion properties of these five systems (Table 1). We have already reported the five data sets in the range 118 K to 262 K for HMB in one of our previous works3 and here we are reporting the 298 K data collected from the same crystal. The % change in area of the 2D layer (Figure 3a) in HMB-PIC is found to be higher than that in HMB (Figure 2a) and HMBTCB (Figure 4a) crystal structures (Figure 6a). In the crystal structure of HMB-PIC, the molecules form individual layers and in these layers the aromatic rings are almost coplanar to the layers and each molecule is surrounded by six identical molecules. As a result, similar groups such as Me⋅⋅⋅Me in HMB and O⋅⋅⋅O in PIC layers come in proximity (Figure 3). This might be causing a relatively larger repulsion among the like molecules in the flat 2D sheets in these structures and hence larger change in the area. Similar feature is also found in the crystal structure of HMB, but here the interaction is only of weak Me⋅⋅⋅Me type (Figure 2). Therefore, the repulsion is relatively less compared to that in the HMB-PIC structure. On the other hand, in the HMB-TCB structure the molecules form corrugated sheets (Figure 4). As a result the molecules are bonded via several C−H⋅⋅⋅N type of interactions. Due to the attractive nature of these interactions, the thermal expansion of the area in this structure is least among these three systems. The interlayer interactions are mainly dictated by π···π interactions. When the average interlayer distances are compared in the layered structures of HMB, HMB-PIC and HMB-TCB, the change in interlayer distance, with increasing temperature, is more prominent in the HMB


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structure, whereas it is relatively less for the two complexes (Figure 6b). This is attributed to the stronger π···π interactions between the electron rich and electron deficient aromatic rings in the complexes compared to weaker π···π interactions between the electron rich aromatic rings in the HMB crystal structure. It should be noted that the volumetric thermal expansion coefficients (Table 1) of the HMB-PIC complex (245(5) MK-1), calculated using PASCal program,37 is very close to one of its components HMB (236(3) MK-1) but much higher than the other component PIC (188(9) MK-1), whereas this value for the HMB-TCB complex (168(7) MK-1) is significantly smaller than that of HMB but very close to that of TCB (171(3) MK-1). Therefore, the smaller change in interlayer distance in the complexes compared to that in HMB cannot be attributed to the consequence of overall volumetric thermal expansion of the complexes. Between the two complexes, the HMB-TCB system shows a relatively smaller change in interlayer distance with increasing temperature. It may be noted that the UV-Vis absorption study also suggest the formation of a stronger charge transfer complex in HMB-TCB compared to that in HMB-PIC, as manifested from the complex formation at a relatively dilute solution of HMB-TCB in 1,4dioxane with the appearance of a new prominent absorption peak in the visible region in comparison to the complex formation at a relatively concentrated solution of HMB-PIC in 1,4dioxane with only a small red shift of the absorption peak in the visible region.

In summary, we have investigated thermal expansion properties of HMB, PIC, TCB, HMBPIC and HMB-TCB crystals. HMB, HMB-PIC and HMB-TCB form layered structures. Among these three structures HMB forms the weakest π⋅⋅⋅π stacked crystal structure and the HMB-TCB forms the strongest π⋅⋅⋅π stacked crystal structure. This is also reflected in the amount of off-set in stacking geometries of these complexes and also from the UV-Vis spectroscopic study. The


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highest and the lowest off-set π⋅⋅⋅π stacking are found in the crystal structures of HMB and HMB-TCB respectively among the three structures. As a result, the thermal expansion along the interlayer stacking axis is found to be highest in the case of weakest π⋅⋅⋅π stacked system (HMB) and lowest in the case of strongest π⋅⋅⋅π stacked system (HMB-TCB).

(a)

(b) Figure 1. Crystal structures of (a) PIC and (b) TCB.


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(a)

(b) Figure 2. The top (a) and side (b) views of the planner crystal structure of HMB.

(a)


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(b) Figure 3. The top (a) and side (b) views of the π⋅⋅⋅π stacked planner packing structure of the HMB-PIC co-crystal.

(a)

(b) Figure 4. The top (a) and side (b) views of the π⋅⋅⋅π stacked layered packing structure of the HMB-TCB co-crystal. The side view shows the corrugated nature of the layers.


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Figure 5. Colors of the individual compounds and the co-crystals.

(a)

(b)


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Figure 6. (a) % Change in area of the 2D layers with respect to temperature in the HMB, HMBPIC and HMB-TCB crystals structures. (b) % Change of the interlayer distances with respect to temperature in the HMB, HMB-PIC and HMB-TCB crystals structures.

Table 1. Thermal expansion coefficients along the three principal axes (αX1, αX2 and αX3) and volumetric thermal expansion coefficients (αV) in the five systems. Complex

αX1(σ) [direction of the axis] HMB 41(4) [2-10] HMB-PIC 33(4) [010] HMB-TCB 15(4) [101] TCB 18(2) [90-2] PIC -39(8) [001]

αX2(σ) [direction of the axis] 65(3) [218] 66(8) [105] 38(2) [010] 50(4) [101] 88(4) [100]

αX3(σ) [direction of the axis] 124(7) [671] 142(6) [501] 112(5) [301] 100(3) [010] 136(14) [010]

αV(σ)

236(3) 245(5) 168(7) 171(3) 188(9)

ASSOCIATED CONTENT Supporting Information. X-ray crystallography, crystallization, crystallographic table and crystallographic data in CIF format for the structures with CCDC 1580507−1580531. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author [email protected]. Author Contributions ‡These authors have contributed equally.


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Funding Sources B.K.S. thanks Council of Scientific and Industrial Research, India (No. 01(2908)/17/EMR-II) for financial support. V.G.S. thanks UGC and D.S. thanks RGNF for fellowships. ACKNOWLEDGMENT B.K.S. thanks DST-FIST for single crystal X-ray diffractometer facility. REFERENCES 1 Das, D.; Jacobs, T.; Pietraszkob, A.; Barbour, L. J. Chem. Commun. 2011, 47, 6009–6011. 2 Bhattacharya, S.; Saha, B. K. Cryst. Growth Des. 2012, 12, 4716−4719. 3 Saraswatula, V. G.; Saha, B. K. Chem. Commun. 2015, 51, 9829−9832. 4 Bhattacharya, S.; Saha, B. K. CrystEngComm 2014, 16, 2340−2343. 5 Forni, A.; Metrangolo, P.; Pilati, T.; Resnati, G. Cryst. Growth Des. 2004, 4, 291−295. 6 Hutchins, K. M.; Groeneman, R. H.; Reinheimer, E. W.; Swensona, D. C.; MacGillivray L. R. Chem. Sci., 2015, 6, 4717–4722. 7 Saha, B. K. J. Indian Inst. Sci. 2017, 97, 177–191. 8 Haas, S.; Batlogg, B.; Besnard, C.; Schiltz, M.; Kloc, C.; Siegrist, T. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, (205203), 1−5. 9 Saha, B. K.; Rather, S. A.; Saha, A. Eur. J. Inorg. Chem. 2017, 3390–3394. 10 Lock, N.; Christensen, M.; Wu, Y.; Peterson, V. K.; Thomsen, M. K.; Piltz, R. O.; Cuesta, A. J. R.; McIntyre, G. J.; Norén, K.; Kutteh, R.; Kepert, C. J.; Kearley, G. J.; Iversen, B. B. Dalton Trans. 2013, 42, 1996−2007. 11 Saraswatula, V. G.; Saha, B. K. New J. Chem. 2014, 38, 897−901. 12 Bhattacharya, S.; Saha, B. K. Cryst. Growth Des. 2013, 13, 3299−3302.


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For Table of Contents Use Only

Stronger π⋅⋅⋅π Interaction Leads to a Smaller Thermal Expansion in Some Charge Transfer Complexes Viswanadha G. Saraswatula,‡ Durgam Sharada‡ and Binoy K. Saha*

Stronger π⋅⋅⋅π interaction between charge transfer aromatic systems leads to a smaller thermal expansion along the π⋅⋅⋅π stacking direction compared to weaker π⋅⋅⋅π interactions between electron rich aromatic systems.


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