Substituent Effects on the Structure and Supramolecular Assembly of Bis(dioxaborole)s Derived from 1,2,4,5-Tetrahydroxybenzene Weijun Niu, Mark D. Smith, and John J. Lavigne* Department of Chemistry & Biochemistry, 631 Sumter Street, UniVersity of South Carolina, Columbia, South Carolina 29208
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 6 1274-1277
ReceiVed March 2, 2006
ABSTRACT: The synthesis and structural characterization of bis(dioxaborole)s 1-3, based on 1,2,4,5-tetrahydroxybenzene, are described. The target compounds result from the facile dehydration reaction between the bis-diol and substituted phenyl boronic acids. While substituents on the terminal phenyl rings do not influence the planarity of the extended π-system through the boroles, they do significantly influence the intermolecular stacking arrangements for these compounds in the solid state. A combination of several weak and medium intermolecular interactions, including traditional π-stacking, CH-O hydrogen bonding, and a consistent phenyl-boron-phenyl sandwich motif determine the nature of the supramolecular assembly for these bis(dioxaborole)s. Bis(dioxaborole)s are diboronate esters resulting from the condensation reaction connecting an aromatic bis(1,2-diol) with a monoboronic acid (1, 2, and 3) or a diboronic acid with an aromatic 1,2-diol (4 and 5) as depicted in Scheme 1.1 Recently, we described the crystal structures of bis(dioxaborole)s based on 1,4-benzene diboronic acid and catechol, both with and without alkyl substituents on the central phenyl ring (4 and 5, respectively).1 These materials exhibit planar conformations and display extended conjugation through the borole linkage. Interestingly, while the substituent on the phenyl ring does not greatly influence the planarity of the conjugated system, it does remarkably influence the supramolecular assembly in the solid state. Controlling the structural ordering for these compounds is of interest due to their potential use as organic thin film transistors, sensors, switches, and the like.2 Furthermore, they can serve as models to help develop an understanding of the structure and properties of polymeric materials derived from boronate ester linkages. These materials have received much attention recently because of the covalent yet reversible ester formation leading to self-repairing linear polymers3 as well as highly crystalline microporous extended networks.4 Remarkably, regardless of the variations in the solid-state assembly previously observed for bis(dioxaborole)s, these compounds consistently display a strong stacking preference wherein the electron-deficient, sp2-hybridized, trigonal planar boron atom is sandwiched between the π-clouds of adjacent phenyl rings. To investigate how substituents affect the intermolecular interaction between bis(dioxaborole)s and to further elucidate the nature of this phenyl-boron-phenyl sandwich, herein we report on the synthesis, structure, and solid-state self-assembly of bis(dioxaborole)s 1, 2, and 3 based on 1,2,4,5-tetrahydroxybenzene. In contrast to a previous analysis,1 additional substituents have been added to the external phenyl rings. Additionally, to the best of our knowledge, these structures represent the first solid-state structural analysis of boronate esters derived from 1,2,4,5-tetrahydroxybenzene. Bis(dioxaborole)s 1-3 were readily synthesized in high yields (71-95%) from a 2:1 mixture of the respective monoboronic acid and freshly prepared 1,2,4,5-tetrahydroxybenzene5 (Scheme 1) through the azeotropic removal of water with degassed toluene.6 Formation of the bis(dioxaborole)s was confirmed by the disappearance of the hydroxyl protons from both catechol and the diboronic acid in the 1H NMR spectra as well as by mass spectral * To whom correspondence should be addressed. Tel: 803-777-2295. Fax: 803-777-9521. E-mail:
[email protected].
analysis.7 In the synthesis of these bis(dioxaborole)s, it is important to work under an inert atmosphere since 1,2,4,5-tetrahydroxybenzene can be readily oxidized in air, to a form incapable of forming an ester with the boronic acid and thereby reducing yield. The 11B NMR of bis(dioxaborole)s 1-3, in CDCl3, shows a single peak around 30 ppm, indicating the presence of only sp2 boron in solution.1 Information about solid-state 11B NMR, however, is limited, yet the solid-state spectral data are important to aid in characterizing the hybridization state and structure of boron centers in insoluble, noncrystalline polymeric boronate networks.4,8 Therefore, comparison between solution and solid 11B NMR data for compounds with known single-crystal structures provides an opportunity to begin to establish correlation between structure and both solution and solid-state 11B NMR chemical shifts. The solid-state 11B NMR of bis(dioxaborole)s 1-5 were thus all recorded using boric acid as a solid-state reference at 19.3 ppm. The solid-phase spectra were referenced to solution phase analysis by relating boric acid in solution (chemical shift 19.3 ppm) versus the common boron reference, BF3‚(OEt2), at 0 ppm. By the use of this method, compounds 1-5 all showed chemical shift values between 28 and 30 ppm, which are consistent with solutionbased measurements for trigonal planar boron, thereby suggesting that solid-state characterization can be carried out using this method to predict hybridization and geometry at boron in the solid state. The X-ray structures of 1-3 were solved to further probe the hybridization at boron in this series as well as to investigate the degree of planarity found in crystalline dioxaboroles and to study any potential intermolecular interactions associated with this functional group.9 Colorless block-, plate- and needle-shaped crystals of bis(dioxaborole)s 1, 2, and 3, respectively, suitable for single-crystal X-ray analysis, were obtained through the slow evaporation of a dichloromethane/hexane solution (degassed, stored under argon). Similar to that of 4,1 bis(dioxaborole) 1 is centrosymmetric and only slightly deviates from planarity with a small dihedral angle of 2.7° between the central phenyl ring and the external phenyl ring (Figure 1). The summary of the three bond angles around the boron atoms (360.0°) is consistent with that expected for trigonal planar boron. As expected, introduction of the isopropyloxysubstituent on the two external phenyl rings in 2 and 3 did not significantly alter the planarity of the bis(dioxaborole) backbone (Figure 1). In the solid state, the dihedral angle between the borole and the external phenyl rings for 2 and 3 only deviate from planarity by 4.10(6)° and 1.48(4)°, respectively. The boron atoms of both 2
10.1021/cg0601148 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/20/2006
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Figure 1. Orthogonal views of the molecular structures for aromatic bis(dioxaborole)s 1, 2, and 3. All atoms are shown with 50% probability displacement ellipsoids. Hydrogen atoms are omitted for clarity. Atoms labeled with an asterisk (*) are related to their unstarred counterparts by inversion.
Figure 2. The supramolecular packing of aromatic bis(dioxaborole)s 1, 2, and 3 show off-set π stacks. The phenyl-boron-phenyl sandwich interactions are indicated by blue dashed lines. Hydrogen atoms are omitted for clarity.
and 3 also meet the expected bond angle summation (360°) to qualify as sp2-hybridized boron. Bis(dioxaborole)s 1-3 all show offset π-stacking interactions between parallel molecules (Figure 2). This interaction is the dominant assembly force leading to a columnar supramolecular structure in the crystal packing. One striking feature in these π-stacks is that the electron-deficient boron of the borole ring is sandwiched between the π-clouds of the central and terminal phenyl rings of adjacent layers.1 The perpendicular distance between adjacent parallel molecules in bis(dioxaborole) 1 is 3.40 Å (Figure 2), which is essentially the same as that of 4 and consistent
with typical π-stacking interactions.10 The perpendicular intermolecular distances between adjacent π-stacked molecules for bis(dioxaborole)s 2 and 3 are 3.49 and 3.31 Å, respectively (Figure 2). While all three bis(dioxaborole)s form similar π-stacked suprastructures, there is some variation in the intermolecular packing distances. The larger spacing in 2, as compared to 1 and 3, is likely due to steric repulsion between the para-isopropyloxy substituents on the two external phenyl rings of adjacent parallel layers. This steric interaction between meta-isopropyloxy substituents in 3 is absent due to a large lateral offset of πstacked molecules in the solid state. Bis(dioxaborole)s 1 and 2 have virtually no lateral offset in the π-stacked columns. This offset in 3 may in fact account for the smaller interlayer distance because the molecules are better oriented for π-π interactions. These variations demonstrate that the distance between adjacent π-stacked molecules can be modified by changing the substituents at different positions on the ring system. Such distinctions in intermolecular spacing between π-stacked layers can be important in tuning the conductive properties in electronic materials.11 The crystal packing for aromatic bis(dioxaborole) 1 adopts a classical herringbone geometry (Figure 3), which is analogous to 4.1 Adjacent columns are connected through CH-π interactions into supramolecular two-dimensional (2D) sheets with a CHcentroid distance of 3.535(2) Å, which is shorter than the CHcentroid distance of 3.618(2) Å found for 4. This is perhaps due to preferred CH-π interactions in 1 as compared to 4 given the packing geometry where the CH-donor in 1 points its hydrogen atom directly toward the π-donor’s π-cloud, whereas in 4, the CHdonor has the hydrogen atoms splayed toward the edges of the π-cloud. The columns of borole 1 in its crystal packing are stacked together with alternately changing tilt angles of approximately (70°; for comparison, bis(dioxaborole) 4 displays tilt angles of (75°.1 The assembly of 2 and 3 does not display the herringbone packing geometry found in 1 or 4. Nor did these molecules assemble
1276 Crystal Growth & Design, Vol. 6, No. 6, 2006 Scheme 1.
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Aromatic Bis(dioxaborole)s Used to Study the Substitute Effect on Intermolecular π-Interactions
into a supramolecular network through weak CH-O hydrogen bonding as shown in the crystal packing of 5.1 The isopropyloxy substituents disrupt the CH-π as well as the CH-O hydrogen bonding assembly mechanism while providing only van der Waals interactions as a driving force to align π-stacked columns (Figure 3). It is noteworthy to point out that in the crystal packing of 3, the methyl groups participate in weak CH-O interactions as indicated by these weak bonding interactions12 between C12(H12C)-O2 of 3.414 (2) Å long ( 2σ(I)). Crystal data for 3: C24H24B2O6, M ) 430.05, triclinic, space group P1h, a ) 6.3967(5), b ) 6.9843(6), c ) 12.217(1) Å, R ) 95.919(1), β ) 104.683(1), γ ) 96.679(1)°, V ) 519.39(7) Å3, T ) 150(1) K, Z ) 1, 1841 independent reflections measured, final R1 ) 0.0362, and wR2 ) 0.0951 (I > 2σ(I)). (a) Janiak, C. J. Dalton Trans. 2000, 3885-3896. (b) Chen, C.-L.; Su, C.-Y.; Cai, Y.-P.; Zhang, H.-X.; Xu, A.-W.; Kang, B.-S.; zur Loye, H.-C. Inorg. Chem. 2003, 42, 3738-3750. (a) Miao, Q.; Chi, X.; Xiao, S.; Zeis, R.; Lefenfeld, M.; Siegrist, T.; Steigerwald, M. L.; Nuckolls, C. J. Am. Chem. Soc. 2006, 128, 13401345. (b) Tulevski, G. S.; Miao, Q.; Afzali, A.; Graham, T. O.; Kagan, C. R.; Nuckolls, C. J. Am. Chem. Soc. 2006, 128, 1788-1789. (c) Curtis, M. D.; Cao, J.; Kampf, J. W. J. Am. Chem. Soc. 2004, 126, 4318-4328. (d) Ling, M. M.; Bao, Z. Chem. Mater. 2004, 16, 48244840. (e) Anthony, J. E.; Brooks, J. S.; Eaton, D. L.; Parkin, S. R. J. Am. Chem. Soc. 2001, 123, 9482-9483. (f) Roncali, J. Acc. Chem. Res. 2000, 33, 147-156. Steiner, T. Angew. Chem., Int. Ed. 2002, 141, 48-76.
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