C–H···BF2O2 Interactions in Crystals: A Case for Boron Hydrogen

Communication pubs.acs.org/crystal

C−H···BF2O2 Interactions in Crystals: A Case for Boron Hydrogen Bonding? Pere Alemany,*,§ Anthony D’Aléo,‡ Michel Giorgi,# Enric Canadell,⊥ and Frédéric Fages*,‡ §

Departament de Química Física and Institut de Química Teórica i Computacional (IQTCUB), Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain ‡ Aix-Marseille Université, CNRS, CINaM UMR 7325, Campus de Luminy, Case 913, 13288 Marseille, France # Aix-Marseille Université, CNRS, Spectropole FR 1739, Campus Scientifique de Saint Jérome, 13397 Marseille, France ⊥ Institut de Ciència de Materials de Barcelona (CSIC), Campus de la UAB, 08193 Bellaterra, Spain S Supporting Information *

ABSTRACT: Molecules bearing the borondifluoride unit represent an important class of dyes. We report the intriguing occurrence of short H···B contacts in the crystal structure of compounds featuring the BF2O2 fragment. Using density functional theory-based calculations, we discovered that, in contrast to what is suggested by the structural observation, the driving force behind these short H···B contacts is not a direct H···B interaction but it relies on the existence of a combination of single, bifurcated, or trifurcated hydrogen bond paths that are practically isoenergetic within a broad range of C−H···B interaction orientations. The BF2O2 group as a whole displays a very isotropic hydrogen bonding ability and, in turn, a strong adaptability to structural requirements in the solid state.

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eak intermolecular interactions are of utmost importance in the fields of crystal engineering, molecular tectonics, and supramolecular chemistry.1 They encompass a palette of classical and nonconventional noncovalent forces that provide essential tools for the control of solid-state packing.2 The ubiquitous hydrogen bond is certainly one of the most important, and its definition has been extended to a broad range of interactions.3 In particular, nonconventional hydrogen bonds have recently been identified to play a fundamental role in determining the conformational choice as well as the packing orientation in molecular crystals.4 We present herein a description of the weak interaction between the hydrogen atom and the BF2O2 unit and show for the first time that this unconventional H-bond can be a determinant of the crystal packing. Compounds containing the BF2XY group (where X, Y = N, O) form an important class of organic dyes.5−8 Controlling their crystal arrangement is of paramount importance for the generation of materials and devices with photonic, electronic, and photovoltaic applications. This study focuses on those compounds, such as 1 and 2, that are based on monoanionic bidentate ligands containing two coordinating oxygen atoms (X = Y = O). The X-ray diffraction analysis of 1 (Table S1, Figures S1 and S2) reveals the striking occurrence of intermolecular nonbonded C−H···B short contacts. Compared to other 2′hydroxychalcones,9 the conformation of the π-system of 1 strongly deviates from planarity, which causes the two independent molecules of the asymmetric unit to fit into a centrosymmetric dimer (Figure 1). The meso hydrogen atom of the anthracene fragment of one molecule points toward the © 2014 American Chemical Society

Figure 1. Dimeric aggregates of 1 found in the crystal structure. Blue lines highlight the most relevant noncovalent interactions.

boron center of the second one, the H···B distance (3.108 Å) being much shorter than the sum of the van der Waals radii (3.28 Å). The F···H distances (2.708 and 3.046 Å) are longer than the van der Waals distance (2.55 Å). Dimer stabilization further involves two symmetrical C···H short contacts. A search in the Cambridge Structural Database (CSD) for molecular crystals of BF2O2-containing compounds reveals the frequent occurrence of C−H···B short contacts similar to those found in 1 (Table S2 and Figure 2). Yet this feature has not received attention in the literature. The H−C bond can belong either to a saturated alkyl moiety, usually a methyl group, or to an aromatic nucleus as in the case of 1. C−H···B interactions generally coexist with other noncovalent interactions, but they Received: June 11, 2014 Revised: July 9, 2014 Published: July 16, 2014 3700

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Chart 1. Two BF2 Complexes Considered in This Work

Figure 3. Potential energy surface (PES) for a CH4 molecule interacting with C3H3O2BF2. The x, y coordinates indicate the position of the C atom of CH4 with respect to the position of the B atom at the coordinate origin. Minima on the PES are highlighted using red arrows. The units in the color scale refer to energies (in kcal/mol) of the complex relative to the two separated fragments. The inset shows the minimum energy path joining the three minima highlighted with red arrows.

Figure 2. Histogram giving the number of structures for different domains of H···B bond distances.

are often overwhelmed by the presence of a large body of interactions, which makes it difficult to decipher which is their real contribution to the cohesive energy of the crystals. We thus selected compound 2 because in its crystal structure B···H−C interactions seem to play a well-defined role.10 Indeed, in the crystals of 2 each molecule interacts with two nearest neighbors via C−H···B contacts (3.109 Å), forming an infinite onedimensional chain (Figure S3). Alignment of those chains leads to flat layers which, in turn, π-stack into a lamellar threedimensional structure. The distances between the two hydrogen atoms and the nearest fluorine atoms (2.712 and 2.803 Å) are again significantly longer than the sum of the corresponding van der Waals radii. A detailed analysis of the data retrieved from the CSD indicates that there are two main recurrent patterns for these C−H···B interactions. Either the direction of the H···B interaction bisects the F−B−F angle or the H atom points toward the boron atom through a triangular face of the BF2O2 tetrahedron, being almost aligned with the boron atom and the heteroatom in the anti position, an oxygen atom in most cases. The former possibility gives short H···F distances, hinting at hydrogen bonding with fluorine atoms,11 while the latter is illustrated by compounds 1 and 2. We used simplified theoretical models to reproduce the essential features of a C−H···B interaction, which provided useful pieces of information on its nature and strength, as well as on the factors affecting it. According to the conclusions of our CSD search, we have considered interactions of the BF2O2 group with either a methane or benzene molecule as models for the two cases involving the H−C bond of alkyl or aromatic moieties, respectively. We took C3H3O2BF2 as minimal fragment (Figure 3) representing the molecule containing the BF2O2 group and calculated the interaction energy as a function of θ and dCB (Figure 3) applying geometrical restrictions in the optimization procedure. We imposed a C2v symmetry to the C3H3O2BF2 fragment in which the plane containing the C, H, O, and B atoms is perpendicular to that containing the two F atoms. We forced a C3v symmetry for the methane fragment, allowing the C−H bond length for the interacting H atom to be different from the other three. A C2v geometry was imposed for

the interacting benzene molecule using either coplanar or perpendicular orientations of the benzene ring relative to the C3O2B plane. Except for these restrictions all structural parameters were optimized. To test the possible influence of electron delocalization in the electronic distribution in the BF2O2 group, we have also investigated the case with an additional aromatic ring using the 2,2-difluoro-2H-benzo[1,3,2]dioxaborine unit. All calculations reported in the following were of the M062X/6-31++G(d,p) type (see Supporting Information).12 Figure 3 shows the potential energy surface (PES) obtained for a mesh of (θ, dCB) values in the case of the interaction between CH4 and C3H3O2BF2. From these data, three most relevant aspects emerge: (a) the steep repulsive potential for B···C distances below ∼3.5 Å (B···H distances below ∼2.5 Å), (b) the presence of two shallow minima for θ ∼125° (∼235°) and 180°, and a deeper minimum at ∼80° (∼280°), (c) the weak dependence of the interaction energy on the direction of the approaching methane molecule for 100° < θ < 260°. The two minima for θ = 125° and 180° are practically isoenergetic and separated by a low barrier of ∼0.02 kcal/mol. The B(F)···H distances for θ = 125° and 180° are 3.14 (3.01) and 3.47 (2.97) Å, respectively. Remarkably, these two geometries of minimal energy match those identified from the CSD survey. The deeper minimum at 80°, with a relative energy around −0.8 kcal/mol is due to hydrogen bonding with the O atom and will not be considered further. Introduction of an aromatic ring in C3H3O2BF2 does not influence the PES noticeably except for the anticipated effect of the steric hindrance with the hydrocarbon skeleton which shifts the energy minimum for the C−H···O interaction from 80° to 95°. As a consequence, the shallow minimum at 125° becomes a slight shoulder. Thus, the model with the benzodioxaborine fragment has not been further considered in the following. 3701

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estimated contribution of each C−H···π and π stacking interaction are −1.42 and −4.46 kcal/mol, respectively. The strength of the C−H···B interaction can be estimated from comparison of the three models a1−a3. As shown by a1, the interaction of a hydrogen in a benzene molecule with the C3H3O2BF2 fragment taking the solid-state geometry is −2.22 kcal/mol, a value consistent with the previous model approach. Extension of the conjugation in the B-containing fragment (a2) has a very small effect, whereas passing to anthracene (a3) gives an interaction energy twice as large (−4.45 kcal/mol) as for benzene. Although for obvious reasons, the addition of these contributions does not give exactly the total interaction energy in the real dimer, we can estimate that the stabilization energy afforded to the dimer by the two C−H···B interactions amounts to ∼55% of the total interaction, whereas C−H···π and π···π interactions represents the remaining ∼17% and ∼28%, respectively. In the case of 2, the analysis of the short contacts with the H atoms suggests that the interaction between two adjacent molecules should be purely due to that interaction. Let us note that the C−H···B contact distances in 1 and 2 are practically identical (∼3.109 Å). The calculated stabilization energy for a dimer of 2 with the same geometry as in the solid is −6.47 kcal/mol, i.e., around −3.2 kcal/mol per C−H···B interaction. These results show that the C−H···B interaction provides a clear contribution to the stabilization of the crystal structure of 1 and 2. A study for 13 additional selected compounds of the CSD search with alkyl and aromatic C−H···B contacts with different geometries of interaction always led to non-negligible stabilization energies in the range −1.7 to −6.7 kcal/mol. Up to now we have discussed the C−H···B interactions from a purely structural point of view by comparing the B(F)···H distances with the sum of the corresponding van der Waals radii. From this perspective, it is tempting to infer the existence of a hydrogen bond with the boron atom in BF2O2-containing compounds such as 1 and 2, but this possibility is at odds with the positive charge borne by boron in that class of molecules,13 raising the question about the ultimate nature of the interaction detected in the structural analysis and confirmed by the calculations. The answer to this question can be given by looking at the critical points of the charge density. According to the “atoms in molecules” theory,14 should a boron hydrogen bond occur, a bond critical point would appear in between the two interacting atoms. The results of such analysis for the three minima in the PES of Figure 3 are summarized in Figure 5. As shown in Figure 5a (θ = 180°), there are two critical points in between the two H···F lines, suggesting the presence of a bifurcated fluorine hydrogen bond.15 When moving to the second minimum (θ ∼125°; Figure 5b), the situation evolves toward a set of three critical points associated with H and the set of two F and one O atoms on one side of the BF2O2 tetrahedron, i.e., a typical trifurcated hydrogen bond. In contrast, as shown in Figure 5c, the situation for the third minimum (θ ≈ 80°) is completely different and corresponds to a typical oxygen hydrogen bond. Thus, it is clear that, although from the purely structural point of view it looks as if there is a B···H interaction for a broad series of θ values, such interaction does actually not exist, and we instead note a progressive adaptation to the hydrogen bonding ability of the BF2O2 group taken as a whole. Thus, the broad region of ∼110° around the main axis of the BF2O2 group is associated with a series of bifurcated and trifurcated hydrogen bond situations with almost no energy preference for any specific orientation.

The interaction with benzene shows similar features, that is, a more or less flat PES with two shallow minima for θ ∼125° and 180°, a minimum due to a C−H···O interaction at θ ∼80°, and a steep increase for θ < 75°. However, the average interaction in the 100° < θ < 180° region is significantly stronger for benzene (∼ −1.1 and ∼ −1.5 kcal/mol for the coplanar and perpendicular approaches, respectively) than for methane (∼ −0.5 kcal/mol). Due to the larger size of benzene as compared with methane, steric hindrance for low approaching angles is more important for benzene, especially in the coplanar geometry for which the minimum at θ ∼80° is reduced to a shoulder in the curve. The stronger interaction of the BF2O2 group with an aromatic hydrocarbon leads to shorter C−H···B distances, which are similar both in the coplanar and the perpendicular interaction geometries. For the minimum at θ ∼125° the optimal C−H···B distance is ∼2.8 Å, significantly shorter than the distance of ∼3.2 Å found for the interaction with methane. For the minimum at θ = 180° the optimal distance is also shorter than for methane (∼3.5 Å) but there is a significant difference between the optimal C−H···B distance for the coplanar (3.2 Å) and the perpendicular (∼3.0 Å) orientations of benzene. Note that the use of the counterpoise method to correct for the basis set superposition error slightly reduces the absolute values of the interaction energies but does not change any of the previous observations. With these data in mind, we now consider molecules 1 and 2 in the solid state. As shown in Figure 1, the dimeric unit, which is the building block for the crystal structure of 1, contains two of these C−H···B interactions and two additional C−H···π contacts. In this relatively simple case, we could quantify the strength of both interactions and disclose their different contributors. The calculations are again of the M062X/6-31+ +G(d,p) type with the basis set superposition error corrected through the counterpoise method and using the solid state structure. The calculated interaction energy for the dimer is −18.77 kcal/mol. To separate the contribution of the C−H···B and C−H···π interactions, we carried out calculations for the different fragments of the structure as shown in Figure 4a,b, respectively. A realistic estimation of the contribution of the C−H···π interaction involves model b1 from which the contribution of pure π stacking (b2) is subtracted. The

Figure 4. Analysis of the different contributions to the interaction energy of the dimer structure (Figure 1): (a) C−H···B interaction and (b) C−H···π interaction. Energies are given in kcal/mol. 3702

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engineering. We foresee a growing number of crystal structures for which such so far unidentified interaction plays a nonnegligible role.

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ASSOCIATED CONTENT

S Supporting Information *

Syntheses, X-ray crystallography data, survey of the Cambridge Structural Database, and computational details. CCDC number of 1 873153. This material is available free of charge via the Internet at http://pubs.acs.org

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a MINECO (Spain) through Grants FIS2012-37549-C05-05 and CTQ2011-23862-C02-02, Generalitat de Catalunya (2009 SGR 1459 and 2014 SGR 301) and XRQTC. We thank Drs. E. Molins and I. Mata for a useful discussion. Aix Marseille Université and CNRS are acknowledged for financial support.

Figure 5. (a−c) Critical points analysis of the three minimum structures in Figure 3. Bond critical points are shown as small blue spheres.

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REFERENCES

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Remarkably, we found that when CH4 or benzene in the θ = 180° geometry are allowed to move out of the plane so that the C−H approaches one of the F atoms of the BF2O2 group directly along the B−F direction, the interaction energy is in both cases practically identical to those obtained above. However, the H···F distance in this case becomes considerably shorter (∼2.75 Å for CH4 and ∼2.40 Å for benzene) and, as expected, indicative of the presence of a classical C−H···F hydrogen bond2d as revealed by the existence of a bond critical point between the H and F atoms. In conclusion, the interaction between the hydrogen atom and the BF2O2 unit cannot be reduced to a classical fluorine− hydrogen bond , but it is not due to a direct B···H interaction either, as misleadingly suggested by the short contacts between these two atoms prevailing in the crystal structures. The present study shows instead that the BF2O2 group behaves uniquely in that its H-bonding ability is the result of a unique combination of single, bifurcated, and trifurcated interactions with the vertices of the BF 2O 2 tetrahedron. Remarkably, these interactions act cooperatively over a broad region around the BF2O2 group, forming a shell of constant H-bond strength that endows the BF2O2 unit as a whole with a very isotropic interaction ability. The result is a lack of directionality for the interaction between the hydrogen atom and the BF2O2 fragment, which imparts the latter with a remarkable adaptability to structural requirements in the solid state. Clearly, when direct hydrogen bonding with the O atom is not possible, for steric grounds, for instance, this region of cooperative hydrogen bonding will take the lead in the control of the crystal structure with a constant strength roughly equivalent to that of a single F···H hydrogen bond. Therefore, the “apparent” C−H···B interaction observed in the crystal structures is shown to stem from synergetic effects within a group of atoms, allowing a broad range of orientations, which makes the BF2O2 unit a versatile building block for crystal 3703

dx.doi.org/10.1021/cg5008473 | Cryst. Growth Des. 2014, 14, 3700−3703