Article pubs.acs.org/JPCC
Synthesis of Pyridine− and Pyrazine−BF3 Complexes and Their Characterization in Solution and Solid State Etienne Chénard,†,‡ Andre Sutrisno,§ Lingyang Zhu,§ Rajeev S. Assary,†,# Jeffrey A. Kowalski,†,% John L. Barton,†,% Jeffery A. Bertke,∥ Danielle L. Gray,∥ Fikile R. Brushett,†,% Larry A. Curtiss,†,# and Jeffrey S. Moore*,†,‡,⊥ †
Joint Center for Energy Storage Research, ‡School of Chemical Sciences, §NMR/EPR Laboratory, School of Chemical Sciences, G.L. Clark X-ray Facility, School of Chemical Sciences, and ⊥Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States # Materials Science Division, Argonne National Laboratory, Lemont, Illinois 60439, United States % Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ∥
S Supporting Information *
ABSTRACT: Following the discovery of the redox-active 1,4bis-BF3-quinoxaline complex, we undertook a structure− activity study with the objective to understand the active nature of the quinoxaline complex. Through systematic synthesis and characterization, we have compared complexes prepared from pyridine and pyrazine derivatives, as heterocyclic core analogues. This paper reports the structural requirements that give rise to the electrochemical features of the 1,4-bis-BF3-quinoxaline adduct. Using solution and solidstate NMR spectroscopy, the role of aromatic ring fusion and nitrogen incorporation in bonding and electronics was elucidated. We establish the boron atom location and its interaction with its environment from 1D and 2D solution NMR, X-ray diffraction analysis, and 11B solid-state NMR experiments. Crystallographic analysis of single crystals helped to correlate the boron geometry with 11B quadrupolar coupling constant (CQ) and asymmetry parameter (ηQ), extracted from 11B solid-state NMR spectra. Additionally, computations based on density functional theory were performed to predict electrochemical behavior of the BF3−heteroaromatic complexes. We then experimentally measured electrochemical potential using cyclic voltammetry and found that the redox potentials and CQ values are similarly affected by electronic changes in the complexes.
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degradation of the LiBF4.2 Compared to quinoxaline, this newly generated bis-BF3-quinoxaline complex demonstrated enhanced redox activity and stability in propylene carbonate (PC).1 Inspired by this finding, and to gain fundamental comprehension in the structural requirements necessary to produce electrochemically active and stable BF3−heterocyclic complexes, we undertook a systematic study with different heteroaromatic−BF3 systems (Figure 1). To probe parameters influencing the stability of 1,4-bis-BF3quinoxaline, we separately evaluated core and substituent components, starting with mono-BF3−heteroaromatic adducts. Specifically, we prepared, isolated, and characterized complexes from pyridine (1), 4-picoline (2), quinoline (3), and isoquinoline (4), as illustrated in Figure 1. Then, to elucidate the influence of the second nitrogen in the heteroaromatic ring,
INTRODUCTION
Our interest in high capacity redox-active materials for energy dense nonaqueous flow batteries led us to the discovery of 1,4bis-BF3-quinoxaline adducts (Scheme 1).1 We hypothesized that the 1,4-bis-BF3-quinoxaline is the electrochemically active species when quinoxaline is reduced in solution with LiBF4 as supporting salt. The adduct forms from an in situ reaction of quinoxaline and boron trifluoride (BF3), generated from the Scheme 1. LiBF4 Decomposition Produces 1,4-Bis-BF3Quinoxaline
Received: January 26, 2016 Revised: March 31, 2016 Published: March 31, 2016 © 2016 American Chemical Society
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DOI: 10.1021/acs.jpcc.6b00858 J. Phys. Chem. C 2016, 120, 8461−8471
Article
The Journal of Physical Chemistry C
chemical activity of the complex. We then wanted to extract structural information using single-crystal X-ray analysis and supplement X-ray analysis with 11B solid-state NMR (ssNMR) study of the boron environment in BF3 adducts. The observed satellite transition (ST) powder patterns and NMR tensor calculations yield important information regarding the local bonding and geometry at the boron centers of our materials. Also, we performed computational predictions of the redox potential of adducts using density functional theory (DFT) and conducted electrochemical analysis by cyclic voltammetry.
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EXPERIMENTAL METHODS Synthesis of the BF3−Heteroaromatic Complexes. Gram quantities of complexes were prepared by adding, in a dropwise manner, a solution of BF3·etherate (2 equiv) to a solution of the heterocyclic starting material in diethyl ether at 0 °C. Upon addition of the Lewis acid, the desired product precipitated from the solution. After complete addition of the BF3·etherate solution, the mixture was allowed to warm to room temperature and stand overnight to ensure maximal conversion to the BF3 complexes. The reaction vessel was transferred into a glovebox. The suspension was filtered, washed with dry diethyl ether, and dried under vacuum to provide products with good to excellent purities. Complexes from pyrazine and quinoxaline were prepared using Martin’s protocol.1,21 Solution NMR spectra (1H, 13C, 11B, 19F, 15N, COSY, HSQC, HMBC, and 19F−1H NOE) were recorded on either Varian Unity Inova 400, 500, 600, or VNMRS 750 MHz spectrometers. All the 11B solid-state NMR experiments were conducted on a Varian Unity Inova 300 MHz and VNMRS 750 MHz spectrometers. No background signal in the 11B ssNMR spectra was detected when adequate control were performed. All the 11B ssNMR spectra were processed using NUTS NMR processing software, and the simulated spectra were generated using the DMFIT22 and WSOLIDS123 simulation packages. Ab initio calculations (11B EFG NMR tensors and reduction potential prediction) were also conducted using the Gaussian 09 program. The NMR tensor parameters were then extracted directly from the Gaussian output files using the EFGShield program.24 The X-ray data were collected on a Bruker D8 Venture equipped with a four-circle kappa diffractometer (with Photon 100 detector) and an X8 Bruker Kappa four-circle diffractometer equipped with an APEXII CCD detector. Additional metrical details and CIF files are provided in the Supporting Information. CCDC 1446928 (N-BF3-picoline 2), 1446930 (NBF3-quinoline 3), and 1446929 (1,4-bis-BF3-pyrazine 5) also contain the supplementary crystallographic data for this paper. All electrochemical experiments were performed in a threeelectrode cell in an argon-filled glovebox (MBraun Labmaster) using a VSP-300 potentiostat (BioLogic). For further details, refer to the Supporting Information.
Figure 1. Structures of pyridine (1 to 4) and pyrazine (5 and 6) derivatives.
pyrazine and 2-methylquinoxaline were reacted with BF3− etherate to synthesize complexes 5 and 6, respectively (Figure 1). We also consider the effect of the fused benzo group by comparing complexes 1 and 5, with 3, 4, and 1,4-bis-BF3quinoxaline. We found few literature reports on the redox activity of heteroaromatic complexes, with limited characterization on BF3-adducts, most of which discussed the pyridine system.3−14 For example, Meulen and co-workers3 first synthesized the NBF3−pyridine complex 1 and determined the notably strong N−B bond energy dissociation in the solid state.15 Since then, pyridine−BF3 complex 1 was used in a study to probe donor/ acceptor interactions.4 Another report correlated and compared bond strength between boron and nitrogen of different boron Lewis acids.5 Miller and Hartman analyzed different nitrogencontaining heteroaromatics such as pyridine and 4-picoline complexes, with different examples of boron Lewis acids.16−18 Some more fundamental studies showed how the rotation along the N−B bond in complex 1 behaves in its solid state or in gas phase.19,20 So far, two applications have been identified for heterocyclic BF3 complexes. Temin was the first to find the formulation of an epoxy polymer with complex 1 as an additive to catalyze the curing process of epoxy resin.7 In 2012, Shved et al. followed up on the polyepoxy curing additive using complexes made from aniline and piperidine.8 They also tabulated N-BF3-isoquinoline 4 but omitted any discussion and characterization regarding this heteroaromatic complex. Recently, Dahn et al. reported complex 1 as an additive to reduce the loss of capacity of Li-ion battery upon cycling.12−14 Complex 2 was studied along with 1.5 Like N-BF3-pyridine, the in situ formation of N-BF3-4-picoline facilitated the functionalization of the 2-position of the pyridine ring via lithiation.9−11 The same approach was used to prepare intermediates from quinoline and isoquinoline,9 but no direct analysis of coordinated intermediates was reported. In a subsequent report of the pyridinium additive, Dahn et al. similarly applied bis-BF3pyrazine 5 to reduce the Li-ion batteries capacity decrement.13,14 Finally, Martin and co-workers were the first to prepare and isolate 1,4-bis-BF3-pyrazine along with other pyrazine analogues.21 Of all the molecules mentioned, only N-BF3-pyridine and N-BF3-4-picoline had characterization involving 1H, 13C, 11B, and 19F solution NMR spectroscopy. In the present work, we synthesized, isolated, and characterized mono- and bis-adducts using different solution NMR techniques. Our first objective was to test for a relationship between boron environment and the electro-
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RESULTS AND DISCUSSION Straightforward synthetic procedures resulted in the preparation and isolation of complexes in gram quantities. Each of the complexes was structurally and electrochemically characterized by a range of techniques. We first evaluated conversion and purity using solution NMR. Structure Assignment from Solution NMR Spectroscopy. We first compared 1H NMR spectra of starting materials 8462
DOI: 10.1021/acs.jpcc.6b00858 J. Phys. Chem. C 2016, 120, 8461−8471
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The Journal of Physical Chemistry C
Figure 2. 1H NMR spectral comparison between pyrazine and 1,4-bis-BF3-pyrazine in MeCN-d3. X corresponds to signal from moisture in MeCNd3.
Figure 3. 19F−1H NOE of N-BF3-pyridine 1 in CDCl3.
Table 1. Comparison of the 1JC2−H and 15N Chemical Shift between the Free and BF3−Heterocyclic Molecules N NMR δ (ppm)
1
15
JC2−H (Hz)
structure
no BF3
BF3 complex
no BF3
quantum correlation (HSQC), and heteronuclear mulBF3 complex
pyridine 4-picoline quinoline isoquinoline pyrazine quinoxaline 2-methylquinoxaline
177a 177 178a 178 for C1 and C3a 183a 184g 184
187 188 186 185 (C1)/189(C3) 196 195 193
−67.7b −76.7b −73.7d −77.3d −47.8e,f −51.3e,h −52.8 and −60.3c
−143.3c −150.5c −154.8c −151.0c −136.9e −142.5 and −36.2c
a Reference 25. bSee ref 27. cAnalysis was performed in CDCl3. 15N ssNMR of complex 1 showed a signal at −189 ppm. dReference 28. eAnalysis was performed in CD3CN. fPyrazine δ −42.8 ppm in CDCl3; see ref 29. gReference 30. hQuinoxaline δ is reported in CDCl3; see ref 29. All measurements were adjusted vs MeNO2 at 0 ppm. The blank entry refers to 15N signal that was too broad to be observed.
(1H−15N HMBC) also demonstrated a large upfield shift for the complex 1 compared to its free form (Table 1).26,27 These last results combined with NOE observations support the conclusion of BF3 being linked to nitrogen and agree with previous reports.6,12 N-BF3-4-picoline 2 was also soluble in deuterated chloroform. The 13C NMR spectra showed a JC2−H of 188 Hz (Table 1), which is similar to the value observed with the N-BF3pyridine 1. A chemical shift of −150.5 ppm was noted by 1 H−15N HMBC. 19F−1H NOE corroborated the proximity of the fluoride with hydrogens positioned at the C2 and C2′. Analysis of the N-BF3-quinoline 3 with 19F−1H NOE experiment showed interrelation between fluorides bonded to BF3 and hydrogens from C2 and C8. Additional evidence of the formation of the N-BF3 adduct comes from the increase of the coupling constant of the C−H bond at C2 (JC2−H = 186 Hz) by around 8 Hz, as a result of the formation of a cationic nitrogen (free quinoline JC2−H = 178 Hz) (Table 1). Quinoline has chemical shift of −73.7 ppm by 15N NMR.28 When complex to BF3, the quinoline 15N signal is shifted to −154.8 ppm on 1 H−15N HMBC (Table 1). Also, well-resolved quartets due to
and products to establish the conversion to the BF3 complexes (Figure 2). The significant changes in chemical shift, especially from the hydrogens neighboring the nitrogen−boron bond, were reliable indications that the expected products were formed (Figure 2). We then proved structures with different 1D and 2D NMR spectroscopy, analyzing 1H, 11B, 13C, 15N, and 19 F nucleus. N-BF3-pyridine 1 was soluble and stable in deuterated chloroform. Analysis using 1H, 11B, 13C, 15N, 19F, and 19F−1H nuclear Overhauser effect (NOE), also known as 19F{1H} heteronuclear Overhauser effect spectroscopy (HOESY), confirmed the structure. 19F−1H NOE interactions effectively demonstrate the close proximity of the fluoride nuclei with the hydrogens of the C2 and C2′ (for a representative 19F−1H NOE spectrum, see Figure 3). 13C NMR gave a C−H Jcoupling of 187 Hz at the 2-position (C2) when the 1H decoupler was turned off. Typical C−H coupling of C2 for a free pyridine is 177 Hz.25 This larger J-coupling constant is the result of the formation of a pyridinium-like entity (Table 1). For example, the corresponding N-hydropyridinium has a coupling constant of 191 Hz.25 As expected, 15N NMR 8463
DOI: 10.1021/acs.jpcc.6b00858 J. Phys. Chem. C 2016, 120, 8461−8471
Article
The Journal of Physical Chemistry C a coupling between C and F with J = 5.3 and 4.1 Hz were found in 13C NMR for C2 and C9, respectively. 2D NMR experiments such as correlation spectroscopy (COSY), heteronuclear single quantum correlation (HSQC), and heteronuclear multiple bond correlation (HMBC) were used to assign other hydrogens and carbons (see Supporting Information). Finally, the solid was recrystallized in chloroform, and crystallographic analysis confirmed the structures (Figure S3). We assigned the N-BF3-isoquinoline adduct 4 using the same NMR techniques as 3. We found through-space interactions between fluorides and hydrogens with chemical shifts at 9.42 and 8.50 ppm. They respectively correspond to H1 and H3, and one bond coupling with C1 and C3 was found by HSQC. The proton coupled HSQC spectrum indicates the coupling for JC1−H = 185 Hz and JC3−H = 189 Hz, and we observed proton coupling with the nitrogen peak at −151.0 ppm by 1H−15N HMBC (Table 1). In 13C NMR, C1 and C3 show as quartets from fluoride coupling and chemical shifts at 148.0 ppm (JC−F = 2.5 Hz) and 134.6 ppm (JC−F = 1.9 Hz), respectively. All hydrogens and carbons in 1,4-bis-BF3-pyrazine 5 are equivalent; therefore, 1H and 13C NMR spectra show only one signal (see Figure 2 for 1H NMR of 5). Compared to the free pyrazine, the hydrogen signal of complex 5 was shifted downfield by 0.58 ppm. 19F−1H NOE was also detected for this compound and allowed the determination of the proximity of fluoride to hydrogen. A very broad signal in the 13C NMR with CD3CN made the measurement of JC−H impossible. Switching the solvent to CD3NO2 gave sufficient chemical shift dispersion to observe a C−H coupling of 196 Hz for the complex 5 (Table 1). Also, complex 5 provided a diffraction quality crystal in MeNO2 and was submitted to X-ray analysis to provide the first reported crystallographic structure of this complex. No signal was observed, and no comparison with pyrazine29 was possible in our attempt to perform 15N and 1 H−15N HMBC NMR spectroscopy. We previously reported on the 1,4-bis-BF3-quinoxaline redox activity and stability and characterized the complex using the typical 1H, 13C, 11B, and 19F.1 In this work, we wanted to provide further spectroscopic evidence of the putative structure. In the 13C NMR spectra, the signal at 143.0 ppm (C2 and C3) showed a coupling with hydrogen of 195 Hz (vs 1JC2−H = 184 Hz for the free quinoxaline).30 A peak at −136.9 ppm was observed in 15 N NMR (Table 1). The singlet peak corresponding to H2 (H3) at 9.32 ppm and the multiplet peak at 8.60 ppm (H5 (H8)) showed interactions with fluorides from BF3 by 19F−1H NOE experiment. For 4-BF3-2-methylquinoxaline 6, COSY exhibits no coupling for the signal at 8.96 ppm, and this signal appeared to interact with fluorine by 19F−1H NOE. HMQC proved that the hydrogen at 8.96 ppm coupled with a carbon that has a signal at 141 ppm and JC3−H of 193 Hz (Table 1). We assigned this hydrogen and carbon to position 3 of the 4-BF3-2methylquinoxaline. The hydrogen signal at 8.50 ppm interacts with fluoride, as suggested by 19F−1H NOE, and therefore corresponds to H5. Two distinct peaks were observed in the 15 N NMR. The first signal with a chemical shift of −36.2 ppm corresponds to the uncomplexed nitrogen, whereas the peak at −142.5 ppm belongs to the nitrogen complex with BF3. These last results are in agreement with all solution NMR experiments performed with complex 6, which confirm that 2-methylquinoxaline only bears one BF3 unit. Overall, we used a combination of solution NMR techniques to characterize the molecules for this study. Their correspond-
ing individual structures were confirmed by 1D and 2D NMR experiments. The use of 19F−1H NOE provided a fast and effective tool to locate the position of fluoride and, by the same token, confirmed that in solution boron is bonded to nitrogen. It also served to distinguish which nitrogen of the 2methylquinoxaline reacted with BF3, in the formation of 6. The C−H J-couplings were indicative of the formation of cationic heteroaromatics. The cationic heteroaromatics formation were also evidenced by the 15N NMR chemical shift of complexes measured using 15N NMR or 2D {1H−15N} HMBC experiment. Spectral comparison of the free nitrogen vs the complex generally showed a 70−80 ppm shift in the high field direction. A recent study comparing pyridine and hydropyridinium demonstrated a similar effect on chemical shift.26 11 B Solid-State NMR Spectroscopy and Single Crystal X-ray Diffraction. Solution NMR analyses proved the bonding interactions of the BF3 complexes; for those complexes that yielded diffraction-quality crystals, we extracted information from their X-ray structures. Interestingly, disorder caused by N−B rotation was not observed in the X-ray structures. These finding were in good agreement with the NMR study performed by Fyfe and co-workers.19 Not all of the complexes yielded suitable diffraction quality crystals. Hence, crystallographic analysis alone was not a viable method for all the studied complexes, thwarting our desire to obtain a complete set of structure−property correlations. However, the structural and electronic properties for all BF3 complexes were ascertained by extracting the electric field gradient (EFG) tensors from 11B ssNMR spectroscopy.31,32 There are two parameters generally used to describe the quadrupolar interaction: (a) the quadrupolar coupling constant, CQ, and (b) asymmetry parameter, ηQ, describing the magnitude and asymmetry of the EFG tensor components (which are related to the spherical and axial symmetries of the molecule, respectively). The known boron chemical shifts for three- and four-coordinated species cover ca. 200 ppm altogether.33 Three-coordinated species chemical shifts vary from +90 to −20 ppm with respect to BF3·OEt2 at 0 ppm, whereas the fourcoordinated species have chemical shifts ranging from +20 to −120 ppm.34 Recently, Bryce and co-workers analyzed twocoordinated borinium cations by 11B ssNMR, which exhibit a chemical shift range of +100 to −30 ppm.35 The overlap between chemical shift regions makes it difficult to distinguish coordination geometries. However, the two- and threecoordinated boron has a much larger CQ (1.0−5.5 MHz), whereas the four-coordinated boron possesses a relatively smaller CQ (
