Revisiting Borylanilines: Unique Solid-State Structures and Insight into

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Revisiting Borylanilines: Unique Solid-State Structures and Insight into Photophysical Properties Pagidi Sudhakar, Sanjoy Mukherjee, and Pakkirisamy Thilagar* Department of Inorganic & Physical Chemistry, Indian Institute of Science, Bangalore-560012, India S Supporting Information *

ABSTRACT: The structure and photophysical properties of two known borylanilines, 4-(dimesitylboryl)aniline (1) and 4(dimesitylboryl)-3,5-dimethylaniline (2), have been investigated. 1 and 2 have similar donor and acceptor centers but differ in their molecular conformations. Compounds 1 and 2 have been structurally characterized, and they exhibit a rare form of intermolecular N−H- - -π electrostatic interactions. The structure and photophysical properties of 1 and 2 are discussed in the context of computational results.

T

we used a modification of Asher’s9 procedure for the synthesis of 1 and 2 (see the Supporting Information and Scheme 1).

riarylboranes1 have been continuously gaining much attention owing to their excellent photophysical properties. Because of its inherent Lewis acidity, the three-coordinate boron in triarylboranes has been extensively used as a receptor for fluoride and cyanide and neutral molecules such as pyridines. With a suitable donor site attached, triarylboranes exhibit intense intramolecular charge transfer (ICT) character, and this phenomenon has been well exploited in linear/ nonlinear optics and electroluminescence (EL).1,2 In the 1970s, Williams studied extensively the photophysical/chemical properties of several N,N-disubstituted borylanilines.3−5 Since then, several groups reported numerous D−A systems based on N,N-(bis-aryl/alkyl)borylanilines.2,6,12 Although the synthetic utility of (4-aminophenyl)dimesitylborane has been well documented in the recent and past literature, surprisingly there have been no data reported on the crystal structures and photophysical properties of unsubstituted borylanilines (4(dimesitylboryl)aniline (1) and 4-(dimesitylboryl)-3,5-dimethylaniline (2)).2b,5,8 Recently, we became interested in investigating the effect of the molecular conformation on the photophysical properties of D−A systems.7 As part of this program, we have investigated the effects of molecular conformation on the solid-state structures and photophysical properties of borylanilines 1 and 2, and the results are reported here. As reported elsewhere, compounds 1 and 2 can be conveniently prepared by the Williams method.3,4,8 To avoid the use of a heavy metal (Pd) and to reduce the reaction time, © 2013 American Chemical Society

Scheme 1. Synthesis of 1 and 2

First, the reaction of bromoanilines with 2 equiv of nBuLi in THF at −78 °C selectively generated a dianion at the nitrogen center, which was trapped with ClSiMe3 to give the intermediate N,N-bis(trimethylsilyl) bromobenzenes. The second step of the process is the selective metal−halogen exchange with nBuLi and addition of Mes2BF to afford the desired product in good yield after deprotection of the −SiMe3 group by methanolysis. The structure of borylanilines 1 and 2 were confirmed by NMR (1H and 13C), HRMS, and singlecrystal X-ray crystallography. In 1H NMR, the −NH2 resonance of 1 (4.01 ppm) is shifted slightly downfield with respect to that of 2 (3.61 ppm). It is worth mentioning that the 1H Received: December 13, 2012 Published: May 3, 2013 3129

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resonance (3.87 ppm) of the −NH2 group in (E)-4-(2(dimesitylboryl)vinyl)benzenamine reported in ref 11 falls between the resonances of 1 and 2. Interestingly, the aryl C−H protons of the 3,4-Me2-C4H2-NH2 moiety of 2 gives raise to two distinct resonances at 6.73 and 6.72 ppm in CDCl3. From variable-temperature NMR experiments in toluene-d8 (see the Supporting Information, Figure S4), we conclude that the two aryl protons are nonequivalent. At 223 K, the two protons are observed at 6.67 and 6.69 ppm; as the temperature is raised, these two signals begin to merge and become a broad singlet (Tc = 313 K and ΔG = 20.26 kcl/mol). From the single-crystal structure, it is found that the dihedral angle between the 3,4Me2-C4H2-NH2 unit and the −B(mes)2 unit is 52.6°; presumably this twisted arrangement is retained in solution and renders the two C−H protons of the former unit nonequivalent. The molecular structures of 1 and 2 were determined (Figure 1 and Table 1), and the crystallographic data are given in the

Supporting Information (Table S1). The boron atom in both 1 and 2 has a trigonal-planar configuration with the sum of angles around boron being 360°. The B−C(aryl) bond in 1 (1.538(2) Å) is shorter than the bond length observed in 2 (1.578(3) Å) and 4-(dimesitylboryl)-N,N-dimethylaniline10 (1.545(2) Å). In contrast, the B−C(mes) bond length follows the opposite trend (Table S2, Supporting Information). The C−C bond lengths of the aniline moiety of 1 show more quinoidal distortion than for 2. A similar feature has been observed for 4-(dimesitylboryl)N,N-dimethylaniline by Marder et al.10 The dihedral angle between the phenyl moiety of H2NPh and the BC3 plane in 1 (12.2°) is significantly less than that in 2 (52.6°). The presence of two additional methyl groups at ortho positions of the dimesitylboryl group twisted the H2NPh unit significantly in 2. However, the nitrogen atoms in 1 and 2 are planar, with the sum of angles around nitrogen being 360°. The C(aryl)−N bond length in 1 (1.363(2) Å) is comparatively shorter than the C(aryl)−N length in 2 (1.502(3) Å). On the basis of these results, one can tentatively conclude that the donor−acceptor interaction in 1 is stronger than that in 2 and 4(dimesitylboryl)-N,N-dimethylaniline.10 Interestingly, the crystal structures of 1 and 2 reveal a rare form of intermolecular N−H- - -π interaction11 (Figure 1). Though N−H- - -π interactions are widely prevalent in a number of protein crystal structures, such intermolecular N− H- - -π interactions were not clearly demonstrated by X-ray crystal structures previously for D−A molecules, to the best of our knowledge.2,12 A view of the N−H- - -π hydrogen-bonding interaction is shown in Figure 1. The electron-deficient boryl unit and the electron-donating −NH2 moieties of neighboring molecules are close to each other (Figure 1). In the solid state the adjacent molecules pack together to form a onedimensional chain (Figure 1). The N−H- - -π distances (π is the centroid of the mesityl moiety) are 3.02 and 4.08 Å for 1 and 2, respectively. This difference in N−H- - -π distances can be attributed to the structural constraint imposed by the additional methyl groups at the ortho positions of the dimesitylboryl group in 2. The positive and negative electrostatic potential surfaces at the two ends of the dyads 1 and 2 clearly demonstrate the head-to-tail arrangement of D−A units, which can be a stabilizing factor for these dyads in the solid state (Figure 1). In solution both 1 (270 and 325 nm) and 2 (325 and 370 nm) exhibit two characteristic absorption bands (Figure 2). Since 1 and 2 have a D−A architecture,2,12 solvent polarity dependent absorption studies of both compounds were carried out with the objective to rationalize the absorption process involved in these dyads. For both compounds the shorter wavelength band does not shift irrespective of the solvent polarity; however, the lower energy band shows a huge red shift

Figure 1. Crystal structures of 1 (top) and 2 (middle), ORTEP diagrams of 1 and 2 with ellipsoids at the 50% probability level (bottom left), and ESP surfaces of 1 and 2 (bottom right) as obtained from DFT computations (isovalue 0.0004).

Table 1. Comparison of Mean Bond Lengths (Å) and Selected Torsion Angles (deg) in 1, 2 and Me2NC6H4− B(mes)2a 1 B−C(aryl) B−C(Mes) N−C(aryl) C1−C2 C2−C3, C5−C6 C1−B−C7−C8 C1−B−C13−C14 C2−C1−B1−C7 a

2

Bond Lengths 1.538(2) 1.578(3) 1.589(1) 1.575(2) 1.363(2) 1.505(3) 1.412(1) 1.413(2) 1.381(1) 1.394(2) Torsion Angles −63.8 −51.1 −63.8 −51.1 −12.2 −52.6

Me2NC6H4−B(mes)2 1.545(2) 1.586(2) 1.368(1) 1.409(2) 1.382(2) 66.8 50.5 20.0

See page S8 in the Supporting Information. Figure 2. Absorption spectra of 1 (left) and 2 (right) in different solvents, obtained at 10 −5 M. 3130

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with increasing solvent polarity. Thus, the high-energy band can be ascribed to π−π* transitions, whereas the lower energy band can be attributed to a charge transfer transition (ICT) between the amine group and the boron center. On the basis of the NH2 chemical shift, the dihedral angle between the D and A units and the short N−C bond distances, one may expect that the ICT band in 1 should be more sensitive to the solvent polarity than 2. In contrast, the latter exhibits a more bathochromic shift than the former. To understand the apparent anomaly, DFT calculations were performed using the B3LYP hybrid functional and 6-31G(d) basis set for all of the atoms as incorporated in Gaussian 09 software.13 Optimization of molecular structures with consequent frequency test provided structures with close similarity to the X-ray crystallographic geometries. The FMOs (frontier molecular orbitals) of 1 and 2 are shown in Figure 3. It is understandable that the differences in Figure 4. Emission spectra of 1 (top left) and 2 (top right) in different solvents (10 −5 M; for 1 λex 340 nm and for 2 λex 365 nm).

and literature reports one can assign the corresponding emitting states of these two emission bands as local excited state (LE) and twisted intramolecular charge transfer (TICT) state, respectively.14 The TICT emission only appears in polar solvents due to the preferential stabilization of the TICT state. To validate the above argument, solvent viscosity dependent PL studies were carried out, because it is known that the emission of the TICT fluorophore is sensitive to solvent viscosity variations.15 The PL studies were carried out in highly viscous nonpolar (paraffin) and polar (ethylene glycol) solvents. The dependency of peak emission intensity on the viscosity of the solvent can be seen in Figure 5. In viscous nonpolar solvent, 1

Figure 3. Schematic diagram of FMOs (isovalue 0.04) of 1 and 2 as obtained from DFT computations. Hydrogens are omitted for clarity.

the charge-transfer characteristics of 1 and 2 arise mainly because of the specific localization of the HOMO and LUMO. For example, because of the twisted conformation of 2, the antibonding interaction of the boron pπ orbital with the aryl ring is diminished; as a result, the energy of the LUMO in 2 is slightly lower than in 1. In addition, the twisted arrangement of D−A units in 2 limits the bonding interaction between the aniline and the boryl units; hence, the HOMO is destabilized in 2. On the other hand, the planar arrangement of D−A units in 1 facilitates the bonding interaction in the whole molecule; as a result the HOMO is stabilized to a greater extent (0.28 eV lower than HOMO of 2). Thus, the overall HOMO−LUMO gap is lower in 2 (3.89 eV) than in 1 (4.26 eV). These results directly support why compound 2 absorbs and emits at longer wavelength in comparison to 1. In summary, the DFT results clearly indicate the CT nature of the HOMO−LUMO transition and also provide useful information about the effect of steric perturbation on the electronic band gap. The solution PL spectra of 1 and 2 in different solvents are shown in Figure 4. The emission bands of 1 and 2 are redshifted and weakened in intensity with an increase in the solvent polarity (Figure 4). The large Stokes shifts and weak fluorescence characteristics of 1 and 2 in polar solvents suggest that a polar CT state is emitting in both compounds (Tables S3 and S4, Supporting Information). Compounds 1 and 2 exhibit a considerable solvatochromism of fluorescence from blue to orange colors (Figure 4). These spectral characteristics are comparable with the behavior of tris(phenylethynylduryl)borane-based D−A systems reported by Yamaguchi et al.14 In hexane both 1 and 2 exhibit a single emission band at 375 and 415 nm, respectively. In contrast, the DMSO/EtOAc solutions of 1 and 2 exhibit two emission bands (1, 400 and 520 nm; 2, 400 and 540 nm). On the basis of the computational results

Figure 5. Emission spectra of 1 (left) and 2 (right) in paraffin and ethylene glycol (10 −5 M; for 1 λex 340 nm and for 2 λex 365 nm).

and 2 show only LE emission bands (Figure 5). In viscous polar solvent the TICT peaks increases with viscosity but the increase is less in 1 in comparison to that in 2. This result is consistent with the twisted arrangement of D−A units in 2. Both polarity and viscosity significantly influence the emission process, and these spectral characteristics of 1 and 2 are consistent with the behavior of other TICT luminogens.15 The bright solution luminescence and the characteristic solid-state packing prompted us to study the solid-state luminescence of compounds 1 and 2. Thin films of both the compounds were prepared by spin coating on quartz substrates from their DCM solutions (1 × 10−2 M). The thin-film photoluminescence spectra of compounds 1 and 2 retain the spectral feature of the solution state (DCM) with a slight red shift of the emission wavelength (Figure 6). The bathochromic shift in the solid state is presumably due to the N−H- - -π intermolecular interactions observed for these compounds in the solid state. However, the peak positions and intensities of emission bands in dyads 1 and 2 are independent of excitation 3131

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was added, and the reaction mixture was warmed to room temperature over 12 h. The solvent was evaporated, and the crude residue was dissolved in methanol and refluxed for 6 h. The reaction mixture was cooled to room temperature and extracted using ethyl acetate. The combined extracts were stored over anhydrous Na2SO4, and the solvent was removed under reduced pressure. The crude product was purified by column chromatography (5% EtOAc in petroleum ether). Yield: 85%. 1H NMR (400 MHz, CDCl3, 25 °C): δ (ppm) 7.41 (d, J = 8 Hz, 2H), 6.83 (s, 4H), 6.61 (d, J = 8.4, 2H), 4.02 (s, 2H), 2.32 (s, 6H), 2.07 (s, 12H); 13C NMR (100 MHz, CDCl3, 25 °C): δ (ppm) 151, 141.2, 140.4, 138.2, 128.4, 114.3, 23.9, 21.7. ESI-MS (positive ion mode): Mcalcd(C24H28BN), 341.2315 Da; found, 342.2393 Da [M + H]+. 4-(Dimesitylboryl)-3,5-dimethylaniline (2). Compound 2 was prepared following a procedure similar to that used for compound 1. The quantities involved and characterization data are as follows: 4bromo-3,5-dimethylaniline (1 g, 5 mmol), n-BuLi (6.25 mL, 10 mmol) ClSi(CH3)3 (1.24 g, 11.5 mmol). Yield: 98%. 1H NMR (400 MHz, CDCl3, 25 °C): δ (ppm) 6.68 (s, 2H), 2.34 (s, 6H), 0.06 (s, 18H). Step-2: N-(4-bromo-3,5-dimethylphenyl)-1,1,1-trimethyl-N(trimethylsilyl)silanamine (1g, 2.90 mmol), n-BuLi (2 mL, 3.19 mmol), Mes2BF (0.95 g, 3.48 mmol), colorless solid, 80% yield. 1H NMR (400 MHz, CDCl3, 25 °C): δ (ppm) 6.73 (d, J = 4 Hz, 4H), 6.28 (s, 2H), 3.66 (s, 2H), 2.26 (s, 6H), 2.02 (s, 6H), 1.95−1.91 (m, 12H). 13C NMR (100 MHz, CDCl3, 25 °C): δ (ppm) 148.09, 145.26, 143.65, 140.89, 140.76, 139.08, 128.93, 128.83, 114.92, 23.70, 23.45, 23.19, 21.74. ESI-MS (positive ion mode): Mcalcd(C26H32BN), 369.2628 Da; found, 370.2706 Da [M + H]+.

Figure 6. Solid state emission and excitation spectra of 1 (left) and 2 (right) (λex 300 nm). The inset shows the thin film photoluminescence color of 1 and 2.

wavelength. This indicates that the emission is originating from an excited state which is populated irrespective of the excitation wavelength.

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CONCLUSIONS We have followed a facile synthetic route for the synthesis of borylanilines. The exclusion of a palladium-catalyzed hydrogenation step would be highly useful for synthesizing materials containing alkylynic, vinylic, and other functional groups which are extensively utilized in organoboron chemistry. The two borylanilines reported here display dual emission and strong solvatochromic effects, which are explained on the basis of molecular conformation dependent charge transfer. Detailed studies on the structures, conformation, and photophysical properties of a series of borylanilines are under investigation.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Text, figures, tables, and CIF files giving crystal structures, experimental procedures, characterization data, and DFT computational data. This material is available free of charge via the Internet at http://pubs.acs.org.

EXPERIMENTAL SECTION

All reactions were carried under an atmosphere of purified nitrogen using standard Schlenk techniques. THF was distilled over sodium. The 400 MHz 1H NMR and 100 MHz 13C NMR spectra were recorded on a Bruker Advance 400 MHz NMR spectrometer. All solution 1H and 13C spectra were referenced internally to the solvent signal. Electronic absorption spectra were recorded on a Perkin-Elmer LAMBDA 750 UV/visible spectrophotometer. Solutions were prepared using a microbalance (±0.1 mg) and volumetric glassware and then charged in quartz cuvettes with sealing screw caps. Fluorescence emission studies were carried out on a Horiba JOBIN YVON Fluoromax-4 spectrometer. Single-crystal X-ray diffraction studies were carried out with a Bruker SMART APEX diffractometer equipped with a three-axis goniometer. The crystals were kept under a steady flow of cold dinitrogen during the data collection. Details regarding the data collection and refinement for compounds 1 and 2 are given in Table S1 (Supporting Information). The data were integrated using SAINT, and an empirical absorption correction was applied with SADABS. The structures were solved by direct methods and refined by full-matrix least squares on F2 using SHELXTL software.16 All of the non-hydrogen atoms were refined with anisotropic displacement parameters, while the hydrogen atoms were refined isotropically on the positions calculated using a riding model. 4-(Dimesitylboryl)aniline (1). To a solution of 4-bromoaniline (4 g, 23.25 mmol) in THF (40 mL) at −78 °C was added dropwise nBuLi (29 mL of a 1.6 M solution in hexanes, 46.5 mmol), and the reaction mixture was stirred for 1 h. Trimethylsilyl chloride (5.8 g, 53.5 mmol) was added, and the reaction mixture was warmed to room temperature over 6 h. Volatiles were removed in vacuo, and the product (N-(4-bromophenyl)-1,1,1-trimethyl-N-(trimethylsilyl)silanamine) was distilled under reduced pressure. Yield: 99%. 1H NMR (400 MHz, CDCl3, 25 °C): δ (ppm) 7.38 (d, J = 8.8 Hz, 2H), 6.84 (d, J = 8.8 Hz, 2H), 0.12 (s, 18H). To a solution of N-(4bromophenyl)-1,1,1-trimethyl-N-(trimethylsilyl)silanamine (1 g, 3.2 mmol) in THF (30 mL) at −78 °C was added dropwise n-BuLi (2.2 mL of a 1.6 M solution in hexanes, 3.5 mmol), and the reaction mixture was stirred for 1 h. Mes2BF (1 g, 3.8 mmol) in THF (2 mL)

Corresponding Author

*P.T.: tel, +91-80-2293-3353; fax, (+91) 8023601552, e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.T. thanks the Department of Science and Technology (DST) New Delhi, IISc-Bangalore, and CSIR New Delhi for financial support. P.S. thanks the IISc, and S.M. thanks the CSIR for SPMF.

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