Communication pubs.acs.org/crystal
Coordinating Benzenes Stack Stronger than Noncoordinating Benzenes, even at Large Horizontal Displacements Dušan P. Malenov,† Jovan Lj. Dragelj,† Goran V. Janjić,§ and S. D. Zarić*,†,∥ †
Department of Chemistry, University of Belgrade, Studentski trg 12-16, 11000 Belgrade, Serbia ICTM, University of Belgrade, Njegoševa 12, 11000 Belgrade, Serbia ∥ Department of Chemistry, Texas A&M University at Qatar, P.O. Box 23874, Doha, Qatar §
S Supporting Information *
ABSTRACT: Stacking interactions between two benzene molecules that coordinate transition metal ions within organometallic sandwich and half-sandwich compounds were investigated by performing Cambridge Structural Database (CSD) search and DFT-D calculations. Calculations of interaction energies revealed that stacking interactions between coordinating benzenes of sandwich (−3.69 kcal/ mol) and half-sandwich compounds (−3.29 kcal/mol) are significantly stronger than the stacking interaction between noncoordinating benzenes (−2.73 kcal/mol). At large horizontal displacements (offset r = 5.0 Å), these sandwich∥sandwich interactions are remarkably strong (−3.03 kcal/mol), while half-sandwich∥half-sandwich interactions are significantly weaker (−1.27 kcal/mol). The results of calculations are in good agreement with the data in the crystal structures from the CSD, where 76% of sandwich∥sandwich contacts have large horizontal displacements, which is significantly more than 46% of halfsandwich∥half-sandwich contacts arranged in this fashion. The study provides valuable information about interactions of aromatic molecules relevant to crystal engineering, materials design, and molecular recognition.
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substantially stronger than stacking of two noncoordinating benzene molecules (−4.22 kcal/mol and −2.73 kcal/mol, respectively).29 In this paper we present our results on parallel interactions between two coordinating benzene molecules of sandwich and half-sandwich compounds, obtained by analyzing crystal structures from the CSD and by quantum chemical calculations. To the best of our knowledge, this is the first study of parallel interactions between two benzenes of organometallic sandwich and half-sandwich compounds. Crystal structures archived in the CSD (v 5.37, November 2015)30 were first seached for organometallic compounds with (η6) coordinating benzene molecules. Then we searched for contacts between two (η6) coordinating benzene molecules with parallel orientation (angle between benzene ring planes smaller than 10°). Initially, the search yielded contacts between two coordinating benzene molecules with distance d between their centers lower than 8.0 Å (Figure 1). The contacts were considered interactions if they are within the area corresponding to the ellipsoid defined by offset r of 7.5 Å and normal distance R of 4.0 Å. We found 166 contacts between benzenes of half-sandwich compounds and 46 contacts between benzenes
romatic molecules are ubiquitous in nature and are of great importance in molecular systems. Aromatic− aromatic interactions play very important roles in processes of chemical recognition and supramolecular chemistry.1−8 Benzene dimer is the model system most frequently used for investigating aromatic interactions by means of quantum chemical calculations.9,10 In our and other studies, it was revealed that two parallel aromatic molecules prefer large horizontal displacements in crystal structures from Cambridge Structural database (CSD) and Protein Data Bank (PDB).11−15 This is in agreement with calculated substantial interaction energy of −2.0 kcal/mol at large horizontal displacement (r = 4.5 Å, R = 2.8 Å),11 which is more than 70% of the strongest interaction energy in stacking benzene dimer.9 The similar energetic profile was calculated for two parallel pyridine molecules.12 Coordination compounds containing aromatic ligands are common in the fields of catalysis, materials science, crystal engineering, and drug design.16−28 Interactions between coordinating and noncoordinating benzene molecules have been investigated by Mutter and Platts.29 It was shown that, in comparison to tilted-T benzene dimer (interaction energy of −2.84 kcal/mol), benzene of half-sandwich (benzene)tricarbonylchromium is a somewhat weaker CH/π acceptor (−2.38 kcal/mol) and significantly stronger CH/π donor (−5.85 kcal/mol). Additionally, stacking interaction between noncoordinating and coordinating benzene molecules is © XXXX American Chemical Society
Received: October 26, 2015 Revised: May 29, 2016
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DOI: 10.1021/acs.cgd.5b01514 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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normal distances are larger than 3.0 Å, with majority of them at values of 3.3 to 3.5 Å, which are typical for stacking interactions (Figure 2b). However, at larger horizontal displacements (r > 4.0 Å), normal distances are smaller than 3.0 Å, and below 2.0 Å at r > 5.5 Å (Figure 2b). Interaction energies between coordinating benzene molecules were calculated on two model moleculesbis(benzene)chromium for sandwich∥sandwich interactions and (benzene)tricarbonylchromium for half-sandwich∥half-sandwich interactions (Figure 3). The monomer geometries were kept rigid,
Figure 1. Model-system used for CSD search of parallel contacts between two coordinating benzene molecules. M is transition metal, Ω1 and Ω2 are centers of coordinating benzenes, while Ω2′ is the projection of Ω2 to the plane of benzene of Ω1; d is the distance between Ω1 and Ω2, horizontal displacement (or offset) r is the distance between Ω1 and Ω2′, and normal distance R is the distance between Ω2 and Ω2′.
of sandwich compounds. For more details on the search, see Supporting Information. In our previous work, the high preference for large horizontal displacements was observed for two noncoordinating benzene molecules, where the vast majority of the contacts had horizontal displacements larger than 4.5 Å.11 For sandwich∥sandwich contacts, the distribution of offset values (Figure 2a)
Figure 3. Model systems used for DFT-D calculations: two (benzene)tricarbonylchromium molecules for half-sandwich compounds (a) and two bis(benzene)chromium molecules for sandwich compounds. The geometries with r = 3.0 Å and R = 3.0 Å are presented.
while normal distances (R) were systematically varied for the series of offset values (r = 0.0−6.0 Å) in order to find the preferred geometries. The energies of parallel interactions of coordinating benzene molecules were calculated in program ORCA (v 2.9.1),31 using TPSS-D2 functional32,33 and def2TZVP basis set.34,35 Namely, the TPSS-D2/def2-TZVP level of theory without correction for basis set superposition error (BSSE) gives the energy for parallel interaction between noncoordinating benzene and benzene of (benzene)tricarbonylchromium that is in good agreement with very accurate CCSD(T)/CBS energy (−4.12 and −4.22 kcal/mol, respectively).29 Moreover, for benzene−benzene interaction TPSS-D2/def2-TZVP is in good agreement with CCSD(T)/ CBS level (−2.76 and −2.73 kcal/mol, respectively).9 The calculations showed that the strongest interaction for both model systems is at offset value of 1.5 Å, with normal distance of 3.3 Å for sandwich∥sandwich system and 3.2 Å for half-sandwich∥half-sandwich system. The interaction in the sandwich∥sandwich system (−3.69 kcal/mol) is stronger than interaction in half-sandwich∥half-sandwich system (−3.29 kcal/ mol) (Figure 4a). Both of these interactions are weaker than benzene∥coordinating-benzene stacking (−4.22 kcal/mol);29 however, they are significantly stronger than benzene∥benzene stacking (−2.73 kcal/mol).9 It was previously shown that at offsets of 4.5−5.0 Å the benzene∥benzene interaction is substantially strong, with interaction energy of about −2.0 kcal/mol,11 which is more than 70% of the strongest interaction energy between two benzenes (at r = 1.5 Å). The energy of half-sandwich∥halfsandwich interactions at larger horizontal displacements almost linearly decreases, being only −1.27 kcal/mol at r = 5.0 Å and R = 2.4 Å (about 40% of the strongest energy, Figure 4a). However, the energy of sandwich∥sandwich parallel interaction at horizontal displacement of 5.0 Å and R = 2.3 Å is −3.03 kcal/ mol, which is more than 80% of the strongest interaction energy (Table S1, Supporting Information). The energy of the sandwich∥sandwich interaction at large horizontal displacement is even stronger than the most stable benzene∥benzene
Figure 2. Geometrical parameters for contacts between two coordinating benzene molecules of half-sandwich and sandwich compounds: (a) percentage distribution of offset values (r), (b) graph showing normal distances (R) for different offset values (r).
shows that 76% of the contacts are with horizontal displacements larger than 4.5 Å. Half-sandwich∥half-sandwich parallel interactions are with large horizontal displacements in 46% of the contacts, which is significantly less than sandwich∥sandwich parallel interactions (Figure 2a). Regarding the normal distances, it was noted that for offsets smaller than 4.0 Å B
DOI: 10.1021/acs.cgd.5b01514 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 5. Two views of electrostatic potential maps of (a) (benzene)tricarbonylchromium and (b) bis(benzene)chromium. Electrostatic potential scale: V(r) < −6.3 kcal/mol, blue; −6.3−0.0 kcal/ mol, green; 0.0−6.3 kcal/mol, yellow; >6.3 kcal/mol, red.
attractive electrostatic interaction only in the sandwich∥sandwich system at r = 5.0 Å (Table 1), which is in agreement with electrostatic potential maps (Figure 5). If it had not been for this electrostatic attraction, interaction in the sandwich∥sandwich system at large horizontal displacements would not be as attractive (−3.03 kcal/mol at r = 5.0 Å and R = 2.3 Å, Figure 4). Sandwich compounds, forming remarkably stronger parallel interactions at large offsets than half-sandwich compounds (Figure 4), also have larger fractions of structures in crystals with contacts at larger offsets (Figure 2). Parallel interactions at large horizontal displacements are favorable in supramolecular structures (see Supporting Information, Figure S2 and Figure S3), since molecules can form additional interactions, which provide further stabilization.11,12 However, only systems with substantial interaction energies at large offsets can form these interactions. Sandwich compounds can form remarkably strong parallel interactions at large offsets (ΔE = −3.03 kcal/mol, at r = 5.0 Å and R = 2.3 Å), which results in very high occurrence of such contacts in crystal structures (76% of all the contacts). Half-sandwich compounds, on the other hand, should not profit from parallel interactions at large horizontal displacements as much as sandwich compounds, since the interaction is not strong (ΔE = −1.27 kcal/mol, at r = 5.0 Å and R = 2.4 Å). Data from the CSD shows that sandwich compounds have very large tendency to form parallel interactions at large offsets, while half-sandwich compounds have smaller tendencies (Figure 2a). In conclusion, the energies of parallel interactions between coordinating benzene molecules of half-sandwich and sandwich compounds calculated at DFT-D level revealed that they are significantly stronger (−3.29 and −3.69 kcal/mol, respectively) than parallel interaction between noncoordinating benzene molecules (−2.73 kcal/mol). Calculations at large horizontal displacements showed remarkable interaction energies between two benzenes of sandwich compounds (−3.03 kcal/mol, at r = 5.0 Å and R = 2.3 Å), which are also stronger than stacking of two noncoordinating benzenes. The calculated energies are in agreement with the data in crystal structures from the CSD, where sandwich compounds prefer forming parallel interactions at large offsets much more than half-sandwich compounds. Our results provide further insight into interactions of aromatic molecules and can be very important in the fields of crystal engineering, materials design, and molecular recognition.
Figure 4. Results of calculations at TPSS-D2/def2-TZVP level for benzene∥benzene, half-sandwich∥half-sandwich and sandwich∥sandwich model systems: (a) interaction energies (ΔE) for different offset values (r) and fixed normal distance different for each r, presented in (b); (b) normal distances (R) for the given offset values (r), corresponding to interaction energies (ΔE) in (a). For every individual offset value, normal distances were systematically varied in order to find the normal distance with the strongest interaction.
stacking interaction (−2.73 kcal/mol).9 For all of the calculated systems, the normal distances decrease with increasing of offset values (Figure 4b). The calculated normal distances are in good agreement with the data from crystal structures (Figure 2b), being higher than 3.0 Å for smaller offsets and lower than 3.0 Å for larger offsets (Figure 4b). Maps of electrostatic potentials for bis(benzene)chromium and (benzene)tricarbonylchromium were calculated using the WFA program (v 1.0),36 at the outer contour of electron density of 0.001 au (Figure 5).37 At the benzene surface of (benzene)tricarbonylchromium, the map shows only positive potential as a consequence of withdrawal of electron density by carbonyl ligands and chromium, implying that only dispersion can be attractive force in this system, while electrostatics should be repulsive at all offset values. On the other hand, at the face of benzene of bis(benzene)chromium, there is negative potential, which gradually shifts toward positive potential at the edges, implying the possibility of an attractive electrostatic component at large offset values, where surfaces of two coordinating benzenes can overlap with potentials of opposite signs. This is supported by the energy decomposition analysis described by Morokuma,38 which was performed at MP2/6-311G(d,2p) level of theory.39,40 The decomposition shows significant contribution of correlation (mostly dispersion) at all offset values (in particular, at r = 1.5 Å) for both systems, but C
DOI: 10.1021/acs.cgd.5b01514 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Table 1. Energy Decomposition Analysis (EDA) of Half-Sandwich∥Half-Sandwich and Sandwich∥sandwich Interaction Energies at MP2/6-311G(d,2p) Level; CORR Denotes Correlation Component, ES is Electrostatic Component, While ER is Exchange-Repulsion Component of Interaction Energy; the Energies Are Given in kcal/mol r = 1.5 A
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half-sandwich∥half-sandwich sandwich∥sandwich
r = 5.0 A
CORR
ES
ER
CORR
ES
ER
−11.25 −10.71
+1.46 +2.83
+6.13 +4.11
−3.88 −4.38
+0.32 −0.95
+2.16 +2.76
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01514. Structures of monomers used for DFT-D calculations; Details of CSD search; DFT energies with and without dispersion correction; supramolecular structures of sandwich compounds (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Ministry of Science, Education and Technological Development of the Republic of Serbia (grant 172065). The authors are grateful to Dr. Horst Borrmann from Max Planck Institute for Chemical Physics of Solids in Dresden, Germany, for his support.
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DOI: 10.1021/acs.cgd.5b01514 Cryst. Growth Des. XXXX, XXX, XXX−XXX
