Article pubs.acs.org/Organometallics
Substituent Effects on the Properties of Borafluorenes Mallory F. Smith, S. Joel Cassidy, Ian A. Adams, Monica Vasiliu, Deidra L. Gerlach, David A. Dixon,* and Paul A. Rupar* Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487-0036, United States S Supporting Information *
ABSTRACT: A series of substituted 9-borafluorenes were studied both experimentally and computationally in order to assess substituent effects on the optical and electronic properties and the stability of 9-borafluorenes. The previously unknown 9substituted-9-borafluorenes MesFBF (MesF = 2,4,6-tris(trifluoromethyl)phenyl), TipBF(OMe)2 (Tip = 2,4,6-tris(triisopropyl)phenyl, (OMe)2= methoxy at the borafluorene 3 and 6 positions), and iPr2NBF (iPr2N = diisopropylamino) were synthesized and structurally characterized. The previously reported TipBF, ClBF (9-chloro-9-borafluorene) and tBuOBF (9-(tert-butoxy)-9borafluorene) were also included in this study. All of the aryl borafluorenes (TipBF, TipBF(OMe)2, MesFBF), and tBuOBF are moderately air-stable. Both iPr2NBF and ClBF degrade rapidly in air. Cyclic voltammogram measurements and density functional theory (DFT) calculations reveal that (a) borafluorenes have higher electron affinities relative to comparable boranes and (b) substituents have a strong influence on the lowest unoccupied molecular orbital (LUMO) levels of borafluorenes but less influence over the highest occupied molecular orbital (HOMO) levels. The DFT calculations show that, in general, borafluorenes exhibit low electron reorganization energies, a predictor of good electron mobility. However, the MesF group, which is finding popularity as a stabilizing group in borane chemistry, significantly increases the electron reorganization energy of MesFBF compared to the other borafluorenes. The Lewis acidities of the borafluorenes were probed using Et3PO as a Lewis base (the Gutmann−Beckett method) and found to be dictated primarily by steric considerations. Calculated fluoride affinities (Lewis acidities) correlate with the LUMO energies of the borafluorenes. UV−visible and fluorescence spectroscopic measurements showed that compared to the Tip substituent, the MesF, Cl, and methoxy groups only cause subtle changes to the optical properties of the borafluorenes. The absorption spectra of both iPr2NBF and tBuOBF are blue-shifted due to substituent πbackbonding with the p-orbital on boron. The results of this study provide insights into substituent effects on conjugated boron systems and will help in the design of future boron containing materials.
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of the empty p-orbital on boron with the biphenylene π* system, borafluorenes have lower energy LUMOs compared to the related fluorenes, carbazoles, and silafluorenes.4 As a consequence, the UV−vis absorption spectra of borafluorenes typically extend into the visible range, whereas related fluorenes, carbazoles, and silafluorenes are colorless. Borafluorenes are generally considered to be stronger Lewis acids than comparable nonannulated boranes, a phenomenon that has been attributed to both the strained C−B−C bond angle in borafluorenes (∼105°)5 and the presence of the 4π electron antiaromatic borole core.6
INTRODUCTION The incorporation of main group elements into conjugated molecules is an established technique used to alter the optoelectronics of these systems.1 The inclusion of three coordinate boron into conjugated frameworks is of special interest as the empty p-orbital on boron increases the electron affinity and the Lewis acidity of the molecule.2 Given their unique properties, boron containing conjugated materials, both polymers and small molecules, have found applications as catalysts, electron transporting materials, emitters in OLEDs, and as sensors.3 9-Borafluorenes are the boron congeners of fluorene and have a number of properties that make them promising for use in organic-based electronic materials. Because of the interaction © XXXX American Chemical Society
Received: July 1, 2016
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DOI: 10.1021/acs.organomet.6b00537 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Borafluorenes have been studied both from fundamental and applications standpoints (Figure 1). Rivard demonstrated 4-
Figure 2. (A) Borafluorenes novel to this work. (B) Borafluorenes previously reported.
Figure 1. Selected examples of borafluorenes.
coordinate borafluorenes as blue fluorescence emitters.7 Piers has exploited the Lewis acidity of borafluorenes for activation of metallocenes used in olefin polymerization.5 Wagner extensively studied the in situ generation and reactivity of the parent 9H-borafluorene (HBF),8 has utilized the electron affinity of borafluorenes to form compounds containing one-electron bonds9 and has studied the interactions of borafluorenes with metal centers.10 Yamaguchi and co-workers have developed borafluorenes as fluoride sensors and showed that borafluorenes are strong electron-acceptors when in conjugation with electron-rich thiophenes and aryl amines.11 Importantly, Yamaguchi demonstrated that steric protection can render borafluorenes air-stable, a significant development as boranes are often reactive to ambient conditions.11 Building upon Yamaguchi’s work on conjugated borafluorene small molecules, we recently reported on the synthesis of borafluorene homopolymer P9BF and a vinylene copolymer. Both of these conjugated polymers have estimated LUMO energies approaching that of common electron-accepting materials (Figure 1).12 In the context of our recent report on the synthesis and properties of P9BF, we desired to study the effects of substituents on borafluorenes in an effort to manipulate their optoelectronic properties and their stability. Although many borafluorenes are known, only a few have been extensively studied optically and electronically.7,10,11,13 In this report, we examine the impact of substituent variation on the properties of a series of borafluorenes using a combination of NMR spectroscopy, electrochemistry, UV−visible and fluorescence spectroscopies, and electronic structure calculations. We also qualitatively assess how changes in substitution patterns influence the stability of the borafluorenes toward ambient conditions. Since there are relatively few systematic studies of substituent effects on conjugated boranes, we expect that these results will be of interest in the design of novel borane containing materials.
was chosen as a prototypical air-stable borafluorene from which other borafluorenes will be compared.11b TipBF(OMe)2 was synthesized to assess the influence of the electron-rich groups on an aryl substituted borafluorene, while MesFBF (MesF = 2,4,6-tris(trifluoromethyl)phenyl) was prepared to determine the effects of electron withdrawing groups. iPr2NBF and t BuOBF14 were both examined as lone-pair containing heteroatom substituted borafluorenes. Finally, ClBF15 was chosen as a simple borafluorene. Note that the parent, 9H-9borafluorene (HBF), is inherently unstable, making it unsuitable for use in this study.8b The borafluorenes were synthesized via substitution reactions with ClBF or by reaction of a 2,2′-dilithiobiphenyl with a boronic ester. We first synthesized ClBF by reaction of 2,2′dilithiobiphenyl with BCl3, using a modified version of the procedure reported by Bettinger (Scheme 1).16 The aryl substituted MesFBF, as well as the previously reported TipBF,11b were prepared by the addition of the appropriate aryl-lithium to ClBF. The air-stable MesFBF and TipBF were purified by column chromatography. TipBF(OMe)2 was synthesized by first dilithiating 2,2′-dibromo-5,5′-dimethoxy1,1′-biphenyl17 with BuLi and then combining the dilithiobiphenyl with TipB(OMe)2 (Scheme 1). TipBF(OMe)2 was subsequently purified by column chromatography, followed by recrystallization in hexanes. iPr2NBF was synthesized by the reaction of diisopropylamine with ClBF in the presence of triethylamine, whereas tBuOBF14 was formed by combining t BuONa with ClBF (Scheme 1). Both iPr2NBF and tBuOBF were purified by recrystallization from hexanes. We also attempted to synthesize borafluorenes substituted with 4-anisyl (AniBF) and 4-cyanophenyl (CyPhBF)18 groups through a variety of techniques, but were ultimately unsuccessful (Figure 3). In the case of AniBF, we were able to obtain an 1H NMR spectrum consistent with the structure of AniBF, but the product was unstable toward decomposition despite rigorous exclusion of moisture and oxygen. Structural Characterization. Single crystal X-ray diffraction studies were performed on iPr2NBF, MesFBF, and TipBF(OMe)2; we were unable to grow crystals of tBuOBF suitable for single crystal X-ray diffraction and the structures of TipBF11b and ClBF16 are known. The structures are depicted in Figure 4, and selected metrics are tabulated in Table 1. The
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RESULTS AND DISCUSSION Synthesis. The borafluorenes studied are shown in Figure 2; Figure 2A depicts borafluorenes novel to this report and Figure 2B depicts previously known borafluorenes. These borafluorenes were selected to highlight both steric and electronic contributions of the substituents on the properties of borafluorenes. TipBF (Tip = 2,4,6-tris(triisopropyl)phenyl) B
DOI: 10.1021/acs.organomet.6b00537 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Scheme 1. Synthesis of the Borafluorenes
around the three coordinate boron atoms are planar (Σ < C− B−C ≈ 360°) and the aryl rings are rotated orthogonal to the plane of the borafluorene. For TipBF(OMe)2, the methoxy groups appear to have little influence on the structural metrics of the borafluorene compared to TipBF. The biphenylene moiety of TipBF(OMe)2 does show a slight deviation of planarity, but this is likely due to crystal packing effects, a hypothesis supported by the DFT optimized geometry which shows a completely planar borafluorene moiety. The NMR spectra of MesFBF and TipBF(OMe)2 are as expected (Figures S1−S7). The borafluorene core of iPr2NBF shows little variation from that of the aryl borafluorenes with the boron center remaining trigonal planar. The B−N distance of 1.396(3) Å for iPr2NBF is similar to that reported by Yamaguchi19 for the related dimethyl substituted compound and is typical of an amino-
Figure 3. CyPhBF and AniBF.
structures of the borafluorenes in Figure 2 were also optimized at the DFT B3LYP/DZVP2 level (see the Supporting Information). In cases where X-ray structure data is available, the DFT optimized geometries are in excellent agreement with the X-ray structures (Table 1). The structures of MesFBF and TipBF(OMe)2 are similar to that of the previously characterized TipBF; the environments
Figure 4. OTREP drawings of (A) MesFBF, (B) iPr2NBF, and (C) TipBF(OMe)2 (50% ellipsoids). C
DOI: 10.1021/acs.organomet.6b00537 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
toward atmospheric conditions by first placing the borfluorenes in dry CDCl3 and then exposing these solutions to air. The decomposition of the borafluorenes was monitored by 1H NMR spectroscopy using the residual CHCl3 signal as a standard. Unsurprisingly, both iPr2NBF and ClBF are extremely sensitive to moisture and decompose rapidly, showing greater than 50% decomposition within 1 h in solution. This sensitivity limits borafluorenes such as iPr2NBF and ClBF from being used as functional materials. In contrast, tBuOBF is moderately air-stable and shows
