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Article Cite This: J. Org. Chem. 2018, 83, 6142−6150

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Effects of Halogen and Hydrogen Bonding on the Electronics of a Conjugated Rotor Zachary R. Kehoe,† Garrett R. Woller,† Erin D. Speetzen,† James B. Lawrence,† Eric Bosch,‡ and Nathan P. Bowling*,† †

Department of Chemistry, University of WisconsinStevens Point, 2001 Fourth Avenue, Stevens Point, Wisconsin 54481, United States ‡ Department of Chemistry, Missouri State University, 901 South National Avenue, Springfield, Missouri 65897, United States S Supporting Information *

ABSTRACT: The electronic properties of a pyrazine-containing arylene ethynylene unit are influenced by hydrogen bond and halogen bond donors that are held in proximity of the pyrazine rotor. These interactions are evident with iodine- and bromine-centered halogen bonds and O−H- and C−H-based hydrogen bonds. Bathochromic shifts of UV−vis and fluorescence signals are the best indicators of this intramolecular attraction. The effects can be attenuated in solvents that are less favorable for intramolecular halogen or hydrogen bonding, such as 2-propanol, and amplified in solvents that are supportive, such as toluene. Intramolecular attractions promote planarity in the pyrazine ethynylene system, likely increasing the effective conjugation of the unsaturated backbone. Additionally, computations at the B3LYP and M062X levels of theory using 6-311++G(2d,p) and aug-cc-pVTZ basis sets suggest that the Lewis acidity of the halogen and hydrogen atoms influences electronic behavior even in the absence of conformational constraints.

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INTRODUCTION The relative rotation of phenyl rings on each tolane (diphenylacetylene) unit in a p-phenylene ethynylene (p-PE) oligomer or polymer is an important consideration in predicting optical and electronic properties (Chart 1).1,2 In

In solution, p-arylene ethynylenes (AEs) are best imagined as free rotors. The barrier to rotation between the coplanar and orthogonal conformers depicted in Figure 1 of a benzene−

Chart 1. Twisting of Tolane Subunits Is an Important Consideration in p-Phenylene Ethynylene Properties Figure 1. B3LYP/6-311++G(2d,p) comparison of the electronic properties of a coplanar and an orthogonal conjugated species.

pyrazine−benzene arylene ethynylene unit, for instance, is calculated to be only 3.59 kcal/mol at the B3LYP/6-311+ +G(2d,p) level of theory. Computed electronic spectra of these two conformers at this level of theory predict an 85 nm hypsochromic shift of the λmax of the orthogonal ring system compared to the coplanar form (Figure 1). Thus, one might expect that factors that bias the structure toward coplanarity might have significant impacts on the electronic properties. Conformational bias in these types of systems is illustrated in a prior study where Ag(I) and Pd(II) cations are introduced to AE 1 (Chart 2).16 Complexation of 1 with Ag(I) or Pd(II) leads to bathochromic shifts of 75 and 110 nm, respectively. There are two likely explanations for this electronic behavior.

most solid-state materials, where p-PEs are employed for their efficient charge-transport properties3−5 or sensing abilities,6−9 bond rotation is minimized by close packing. Coplanarity of the conjugated backbone can be encouraged via a number of tactics, such as intramolecular hydrogen bonding or metal coordination.10−18 Such features can be exploited in solution for the development of sensors or molecular switches.19 Conversely, steric impediments, covalent bridges, or intramolecular attractions that force twisting of the backbone can prevent access to the coplanar conformation.20−23 Additionally, efforts have been made to design materials in which crystal packing tendencies and low barriers to bond rotation are exploited to provide freely rotating units that function as molecular rotors in the solid state.24,25 © 2018 American Chemical Society

Received: May 2, 2018 Published: May 4, 2018 6142

DOI: 10.1021/acs.joc.8b01064 J. Org. Chem. 2018, 83, 6142−6150

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halogen bonding in solution and, therefore, is expected to display only very small electronic differences from 7. In the process of studying intramolecular halogen bonding in arylene ethynylenes, we have discovered that intramolecular C−H hydrogen bonding can also provide significant contributions when the C−H group is acted upon by strong electronwithdrawing groups.45 Studies of 5 should provide evidence of the relative strength of this attraction compared to halogen bonding (3 and 4) and conventional hydrogen bonding (2). Because 2 has less conjugation than the arene-substituted systems (3, 4, 5, and 7), terminal alkyne 6 was generated and studied for comparison.

Chart 2. Introduction of Transition Metals to 1 Leads to Significant Electronic Changes

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First, removing electron density via complexation to nitrogen heterocycles in a conjugated system is expected to result in a bathochromic shift.26 Second, as predicted in Figure 1, significant electronic differences are expected for a coplanar conformation compared with one in which the pyrazine is rotated out of plane. Bridging the central pyrazine unit of the benzene−pyrazine−benzene chromophore to the appended pyridine units via metal coordination forces the AE unit into coplanarity as opposed to the free rotation expected for 1. With this behavior in mind, we have identified this AE motif as an ideal template for studying different types of intramolecular attractions. Specifically, we expect the UV−vis absorption and fluorescence emissions of the benzene− pyrazine−benzene AE chromophores of compounds 2−7 to provide a semiquantitative metric for the strengths of halogen bonding (3 and 4), conventional hydrogen bonding (2), and C−H hydrogen bonding (5) in solution (Chart 3).

RESULTS AND DISCUSSION One of the best illustrations of the impact of intramolecular attractions in different solvents comes from comparisons of alcohol 2 with terminal alkyne 6. Alcohol 2 was generated from a known asymmetric aryldiyne precursor containing dodecyloxy solubilizing groups (Scheme 1).46 Deprotection of the alcoholbased protecting group with KOH in toluene yielded terminal alkyne 6 in quantitative yield.

Chart 3. Compounds 2−7 Were Generated to Study the Electronic Effects of Appended Groups

Intramolecular hydrogen bonding in 2 should be influenced somewhat by competitive interactions with the solvent. The intramolecular attraction can be maximized in a relatively nonpolar solvent, such as toluene, and minimized in a competitive solvent, such as 2-propanol. One would expect dichloromethane would lie between these two extremes. Experimental results are consistent with these expectations as the λmax of 415 nm in toluene is red-shifted compared to compound 2 in 2-propanol (λmax = 403 nm) and CH2Cl2 (λmax = 410 nm) (Figure 2). When compared to compound 6, in which the electronic spectra are essentially identical in three different solvents (λmax = 391−394 nm), it is apparent that hydrogen bonding is contributing to the electronic properties of 2 (Figure 2). It is also worth noting that the UV−vis spectra

Scheme 1. Generation of 2 and 6

Halogen bonding is the attraction between an electron donor and the electropositive region on a halogen atom.27−32 The utility of this interaction spans a variety of fields, including crystal engineering, catalysis, supramolecular chemistry, and biomolecular recognition.33−39 In prior studies, we40−42 and others43 have established that arylene ethynylene frameworks provide desirable distances and orientations for intramolecular halogen bonding. It is well known that halogen bond strength depends on the size of the halogen, with iodine providing the strongest attractions and fluorine usually providing no attraction at all.44 Consequently, our expectation is that the largest halogen bonding-based electronic effects will occur with 3 and that electronically similar but halogen bond deficient 7 would provide a suitable control (Chart 3). The bromotrifluoromethylbenzene unit of 4 typically only provides weak

Figure 2. UV−vis spectra in three different solvents illustrate a solvent dependence on electronic properties for 2, but not for 6 (0.8−0.9 μM). 6143

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values at 459 nm. With hydrogen-bonding contributions, the differences in λmax are altered dramatically. For example, the increased effective conjugation of 2 in dichloromethane compensates for the solvent effect observed for 6 in 2propanol, making the fluorescence spectra of 2 nearly identical in the two solvents (λmax = 498−499 nm). Similarly, the features of 2 in toluene (λmax = 467 nm) are significantly redshifted compared to 6 in toluene (λmax = 431 nm), suggesting significant contributions of hydrogen bonding in 2. Overall, the fluorescence spectra indicate hydrogen bonding effects in toluene and dichloromethane, but much less dramatic changes in the competitive 2-propanol solvent. The initial hypothesis for this study was that compounds displaying stronger intramolecular attractions would have the most apparent effect on the UV−vis and fluorescence spectra of the benzene−pyrazine−benzene AE chromophore, and this behavior would be amplified in noncompetitive solvents and minimized in competitive solvents. This is indeed what was witnessed in these studies, as seen in 2 and 6. The low solubility of the fluoroarene-functionalized systems (3, 4, 5, and 7) diminished our ability to study UV−vis and fluorescence behavior of these compounds in a wide range of solvents or by other techniques, such as NMR chemical shift comparisons. Introduction of 2-ethylhexoxy-solubilizing groups instead of dodecyloxy groups provided the opportunity to study these compounds at concentrations appropriate for UV−vis and fluorescence without aggregation. To this end, 1,2-bis((2-ethylhexyl)oxy)benzene47 was iodinated using a common protocol48 then converted to intermediate 9 using a Sonogashira coupling and TBAF deprotection (Scheme 2). Because the C−I bonds at the 2/5 positions of pyrazine are significantly more reactive than the

of 2 all display a bathochromic shift compared to 6, suggesting hydrogen bonding contributions to the electronic structure. Fluorescence studies indicate a similar trend to UV−vis studies (Figure 3). It is important to note that the luminescent

Figure 3. Effects of hydrogen bonding in 2 can be seen in the shifts of fluorescence spectra in toluene and dichloromethane compared to 6 (0.9 μM).

properties of compounds are naturally influenced by different solvent interactions with the ground and excited states. Fluorescence spectra of control compound 6, which are not influenced by intramolecular hydrogen bonding, display significantly different features. The λmax in 2-propanol of 493 nm is different than the λmax of 431 nm observed in toluene. As expected, the λmax in dichloromethane is in between these two Scheme 2. Generation of 3, 4, 5, and 7

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conclusions, with 3 and 7 displaying nearly identical maxima (λmax ≈ 489 nm), there is enough evidence of aggregation in these samples (e.g., spectral broadening and solubility challenges) that this data is unreliable. While compound 4 has the ability to form intramolecular halogen bonds, potentially altering the electronics of the AE chromophore, the strength of this bromine-based halogen bond is expected to be weaker than the iodine-based halogen bond in 3. This expectation is supported by the UV−vis spectra of 4 (Supporting Information), which displays a maximum (λmax = 407 nm) that is between those observed for 3 (λmax = 418 nm) and 7 (λmax = 399 nm) in toluene. As expected for a weaker halogen bond, the solvent dependence of 4 is less pronounced than in 3. In fact, the maximum for 4 remains around 407 nm in each of the three solvents. The diminished halogen bonding contributions of 4 are also apparent in fluorescence spectra where the emission maxima of λmax = 451 nm in toluene and λmax = 473 in DCM are between those observed for 3 and 7. Though the results for bromoarene 4 are consistent with expectations for relatively weak halogen bonding, the possibility of C−H hydrogen bonding from the electron-deficient haloarene cannot be eliminated from consideration. In prior studies, we have found this type of C−H hydrogen bonding attraction to be competitive with halogen bonding. To explore this in the context of the current study, compound 5 was generated and studied via UV−vis and fluorescence. The bathochromic shift of 5 (λmax = 419 nm) in toluene compared to 7 (λmax = 399 nm) is on par with that observed with strong halogen bonding 3 (λmax = 418 nm), suggesting a significant attraction in 5 (Figure 6). As expected for this type of

C−I bond of 9, this terminal alkyne could be selectively coupled to 2,5-diiodopyrazine in high yield. Coupling of 10 to TMSA yields 11, which can be quickly deprotected to give terminal alkyne 12, which is the common starting material for products 3, 4, 5, and 7. The effects of halogen bonding in 3 mirror those observed for hydrogen bonding in 2 (Figure 4). Specifically, halogen

Figure 4. Effects of intramolecular halogen bonding can be seen with comparisons of UV−vis spectra of 3 and 7 in different solvents. Samples are approximately 1 mg per 100 mL (or 0.7−0.9 μM). Absorbance normalized for visualization of λmax shift.

bonding of 3 in toluene (λmax = 418 nm) contributes to a redshift of 19 nm from control 7 (λmax = 399 nm). This difference is diminished in dichloromethane (DCM), a solvent less supportive of intramolecular halogen bonding, with a difference of only 13 nm between 3 (λmax = 414 nm) and 7 (λmax = 401 nm) in this solvent. The slight blue shift of 3 in dichloromethane versus toluene is consistent with the decrease in intramolecular attractions expected in a more competitive solvent. Fluorescence studies are consistent with UV−vis studies, showing a red shift of 3 (λmax = 467 nm) compared to 7 (λmax = 441 nm) in toluene (Figure 5). As with absorption studies, a smaller difference is observed between 3 (λmax = 480 nm) and 7 (λmax = 463 nm) in DCM as compared to toluene. Though the emissions observed in 2-propanol support these

Figure 6. Attractions of the C−H hydrogen to the central pyrazine of 5 result in a red-shift in UV−vis absorptions compared to 7. Samples are approximately 1 mg per 100 mL (0.9 μM). Absorbance normalized for visualization of λmax shift.

interaction, solvent competition diminishes this effect in DCM (λmax = 409). Fluorescence spectra are consistent with absorption studies, with emission maxima of λmax = 456 in toluene and λmax = 474 in DCM red-shifted from 7 (λmax = 441 in toluene, and λmax = 463 nm in DCM) (Figure 7). The intention of this study was to alter the electronics of an arylene ethynylene (AE) by affecting the pyrazine free-rotor via intramolecular attractions. An expected, bathochromic shift upon inducing coplanarity is predicted via calculation (Figure 1). Another explanation for the observed red-shifts, however, is that the arms of AEs 2−5 are acting as Lewis acids toward the central pyrazine unit. Even in the absence of conformational effects, these interactions are expected to have some influence

Figure 5. Effects of intramolecular halogen bonding can be seen with comparisons of emission spectra of 3 and 7 in different solvents. Samples are approximately 1 mg per 100 mL (or 0.7−0.9 μM). Emission normalized for visualization of shift. The apparent noisiness in emission spectra of 3 is from low emission intensity due to partial fluorescence quenching from the iodine atom. 6145

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B3LYP56,57 functional with the 6-311++G(2d,p) basis set for all atoms except iodine, which used the aug-cc-pVTZ basis set and corresponding pseudo potential.55 Single-point energy calculations were carried out using the M062X functional with the triple-ζ basis sets described above. Binding energies were calculated for the 1:1 adducts relative to the two optimized monomers, while for the 1:2 adducts binding energies were calculated relative to the 1:1 adduct and relaxed monomer, allowing us to determine sequential binding energies. As expected, creation of the 1:1 adduct removes electron density from the pyrazine, leading to weaker binding of the second ligand (Table 1). This effect is a modest one, decreasing the binding energy by 10% or less. In all cases, ligand binding causes a red shift in the predicted UV−vis spectra compared to the calculated UV−vis spectrum of 13 (λmax = 386 nm). The magnitudes of both calculated and experimental shifts are consistent with the expectation that the more strongly the ligand binds, the larger the change in λmax. For example, the traditional hydrogen bonding model 14 is expected to have the strongest binding to the pyrazine backbone (Table 1). Indeed, compound 2 shows the largest shifts in absorbance and emission values compared to a similarly conjugated control (6). Likewise, strong halogen bonding in 15 is predicted to have significant effects on the electronic spectra. Compound 3 confirms this behavior with significant red-shifting of absorbance and emission maxima. Consistent with this trend, the lower wavelength emission maxima of 5 and 4 are in line with the weaker binding predicted for 17 and 16, respectively.

Figure 7. Attractions of the C−H hydrogen to the central pyrazine of 5 result in a red-shift in fluorescence emissions compared to 7. Samples are approximately 1 mg per 100 mL (0.9 μM). Emission normalized for visualization of shift.

on the electronics of the AE. This is particularly true in fluorescence spectra where conformational considerations are less important due to the increased barrier to rotation in the excited state.49 To explore this effect further, calculations were used to not only predict the relative UV−vis shifts when conformational changes are not present, but also to estimate the relative strengths of the intermolecular forces. Because the introduction of 1 equiv of Lewis acid is expected to remove electron density from the pyrazine heterocycle, we also wanted to determine the relative energy of binding a second Lewis acid. In order to estimate the strengths of the intermolecular forces, compound 13 was used as a model AE system and the sequential binding of four different ligands (14−17) was examined to mimic experimental compounds 2−5 (Scheme 3).

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CONCLUSION The electronic properties of a conjugated arylene ethynylene (AE) backbone can be manipulated via attractions to tethered side groups. Both conventional hydrogen bonding from a nearby O−H and halogen bonding from a C−I activated by strong electron withdrawing groups can provide significant bathochromic shifts in favorable solvents, such as toluene. These effects can be negated somewhat in less favorable solvents, such as dichloromethane or 2-propanol. Weaker halogen bonding and nonconventional C−H hydrogen bonding can also be detected using this AE motif, though the effects are somewhat diminished with these weaker attractions. High-level calculations support these observations and indicate that the Lewis acidity of the tethered groups is possibly as important as the conformational bias provided by the intramolecular attractions.

Scheme 3. Modeling of 1:1 and 1:2 Adducts of 13

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The 1:1 adducts and 1:2 adducts (Scheme 3) were fully optimized in the gas-phase using the M062X50 functional with the 6-31+G*51−54 basis set on all atoms, except for iodine which used the aug-cc-pVDZ55 basis set and corresponding pseudopotential. UV−vis spectra were generated using the

EXPERIMENTAL DETAILS

Computational Details. All monomer, 1:1 adducts, and 1:2 adducts (Scheme 3) were fully optimized in the gas phase using the M062X50 functional with the 6-31+G*51−54 basis set on all atoms, except for iodine which used the aug-cc-pVDZ55 basis set and corresponding pseudopotential. All structures were verified to be local

Table 1. Binding Energies and λmax for 1:1 and 1:2 Adducts of 13 Compared to Experimental Values of 2-5 computed 1:1 adduct X=

ΔE(kcal/mol)

14 15 16 17

−7.90 −7.36 −2.42 −4.93

λmax = 386 nm for 13.

aabs

λmax (nm)a

abs

399 399 388 392 babs

computed 1:2 adduct ΔE (kcal/mol)

λmax (nm)

abs

−7.68 −6.98 −2.17 −4.82

λmax, = 391 nm for 6 and 403 nm for 7.

413 413 392 398

experimental (in toluene) sample

absorbance λmax (nm)c

emission λmax (nm)b

2 3 4 5

415 418 407 419

467 467 451 456

λmax = 431 nm for 6 and 441 nm for 7.

cem

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1,2-Bis((2-ethylhexyl)oxy)-4-ethynyl-5-iodobenzene (9). 1,2-Bis((2-ethylhexyl)oxy)-4,5-diiodobenzene (8, 1.329 g, 2.267 mmol) was dissolved in diisopropylamine in a storage tube. Argon was bubbled through this solution for 20 min before Pd(PPh3)4 (0.132 g, 0.114 mmol), CuI (0.0217 g, 0.114 mmol) and trimethylsilylacetylene (0.32 mL, 2.26 mmol) were added. The tube was sealed under argon and heated at 70 °C for 20 h. After being cooled to room temperature, the mixture was rinsed into a separatory funnel with CH2Cl2. The organic mixture was washed with DI water, dried with MgSO4, filtered, and concentrated to reveal an oil. This oil was dissolved in dry THF (8 mL). To this solution at room temperature was added 2.28 mL of 1.0 M TBAF solution in THF. After 5 min, NH4Cl solution was added to the reaction flask. The mixture was rinsed into a separatory funnel with ethyl acetate. The organic phase was separated and concentrated to reveal an oil. The compound was purified via flash chromatography (silica, 100% hexane, polarity very slowly increased to 2% CH2Cl2/ 98% hexane) to yield 0.557 g of colorless oil (1.15 mmol, 50.8% yield). 1 H NMR (400 MHz, CDCl3): δ 7.20 (s, 1H), 6.98 (s, 1H), 3.82 (m, 4H), 3.27 (s, 1H), 1.74 (m, 2H), 1.44 (m, 8H), 1.31 (m, 8H), 0.92 (m, 12H) ppm. 13C NMR (100 MHz, CDCl3): δ 150.7, 149.3, 122.5, 117.4, 89.4, 85.6, 78.9, 71.6, 39.4, 30.5, 29.1, 23.90, 23.88, 23.0, 14.1, 11.17, 11.15 ppm. HRMS (APCI-QTOF) m/z: [M + H]+ calcd for C24H37IO2 484.1838, found 484.1834. 2,5-Bis((4,5-bis((2-ethylhexyl)oxy)-2-iodophenyl)ethynyl)pyrazine (10). 2,5-Diiodopyrazine (0.160 g, 0.482 mmol), terminal alkyne 9 (0.466 g, 0.962 mmol), and 20 mL of triethylamine were added to a storage tube. Argon was bubbled through this mixture for 15 min before Pd(PPh3)4 (0.056 g, 0.048 mmol) and CuI (0.0090 g, 0.0.047 mmol) were added. The tube was sealed under argon and heated at 40 °C for 6 days. After being cooled to room temperature, the yellow mixture was rinsed into a separatory funnel with CH2Cl2. The organic mixture was washed with DI water, dried with Na2SO4, filtered, and concentrated. The product was purified via flash chromatography (silica, 40% hexane/60% CH2Cl2) to yield the product as a yellow solid (0.432 g, 0.413 mmol, 86% yield). 1H NMR (400 MHz, CDCl3): δ 8.78 (s, 2H), 7.26 (s, 2H), 7.12 (s, 2H), 3.86 (m, 8H), 1.76 (m, 4H), 1.48 (m, 16H), 1.33 (m, 16H), 0.92 (m, 24H) ppm. 13C NMR (100 MHz, CDCl3): δ 151.4, 149.4, 147.2, 137.7, 122.4, 119.7, 117.0, 97.6, 90.5, 88.0, 71.65, 71.60, 39.43, 39.40, 30.5, 29.08, 29.06, 23.92, 23.90, 23.05, 23.03, 14.1, 11.19, 11.16 ppm. HRMS (APCI-QTOF) m/z: [M + H]+ calcd for C52H75I2N2O4+ 1045.3816, found 1045.3836. 2,5-Bis((4,5-bis((2-ethylhexyl)oxy)-2-((trimethylsilyl)ethynyl)phenyl)ethynyl)pyrazine (11). 2,5-Bis((4,5-bis((2-ethylhexyl)oxy)-2iodophenyl)ethynyl)pyrazine (10, 0.656 g, 0.63 mmol) was dissolved in triethylamine in a storage tube. Argon was bubbled through this solution for 20 min before Pd(PPh3)4 (0.0275 g, 0.0628 mmol), CuI (0.012 g, 0.0628 mmol), and trimethylsilylacetylene (0.18 mL, 1.26 mmol) were added. The tube was sealed under argon and heated at 60 °C for 20 h. After being cooled to room temperature, the mixture was rinsed into a separatory funnel with CH2Cl2. The organic mixture was washed with DI water, dried with MgSO4, filtered, and concentrated to reveal an oil. The compound was purified via flash chromatography (silica, 70% hexane, 30% CH2Cl2) to yield 0.428 g yellow oil (0.441 mmol, 69.2% yield). 1H NMR (400 MHz, CDCl3): δ 8.71 (s, 2H), 7.06 (s, 2H), 6.96z (s, 2H), 3.87 (m, 8H), 1.77 (m, 4H), 1.48 (m, 16H), 1.33 (m, 16H), 0.92 (m, 24H), 0.29 (s, 18H) ppm. 13C NMR (100 MHz, CDCl3): δ 150.5, 149.7, 147.1, 137.8, 119.3, 116.8, 115.9, 115.8, 103.3, 97.5, 94.7, 88.6, 71.7, 71.5, 39.5, 39.4, 30.6, 29.12, 29.09, 24.0, 23.1, 14.1, 11.20, 11.17, 0.12 ppm. HRMS (APCI-QTOF) m/z: [M + H]+ calcd for C62H93N2O4Si2+ 985.6674, found 985.6688. 2,5-Bis((4,5-bis((2-ethylhexyl)oxy)-2-ethynylphenyl)ethynyl)pyrazine (12). 2,5-Bis((4,5-bis((2-ethylhexyl)oxy)-2-((trimethylsilyl)ethynyl)phenyl)ethynyl)pyrazine (11, 0.427g, 0.433 mmol) was dissolved in dry THF (12 mL) in a reaction flask. To this solution at room temperature was added 1.25 mL of 1.0 M TBAF solution in THF. After 10 min, NH4Cl solution was added to the reaction flask. The mixture was rinsed into a separatory funnel with ethyl acetate. The organic phase separated, dried with Na2SO4, filtered, and concentrated to reveal an oil. The compound was purified via flash chromatography (silica, 50% hexane, 50% CH2Cl2) to yield 0.204 g

minima through vibrational frequency analysis. The 1:1 adduct of 16 repeatedly optimized to a halogen-bonded structure in which the ring system of the ligand was orthogonal to that of 13. Since this geometry is not a reasonable model for the experimental system, to approximate the structure of the 1:1 adduct one ligand was removed from the fully optimized 1:2 adduct structure, with no subsequent optimization. UV−vis spectra were generated using the B3LYP56,57 functional with the 6-311++G(2d,p) basis set for all atoms except iodine, which used the aug-cc-pVTZ basis set and corresponding pseudopotential.55 Single-point energy calculations were carried out using the M062X functional with the triple-ζ basis sets described above. Binding energies were calculated for the 1:1 adducts relative to the two optimized monomers, while for the 1:2 adducts binding energies were calculated relative to the 1:1 adduct and relaxed monomer, allowing us to determine sequential binding energies. All calculations were carried out using revision D.01 of the Gaussian 09 suite of programs58 made available through XSEDE.59 Synthetic and Spectroscopic Details. All UV−vis and fluorescence data was collected on samples containing approximately 1 mg of sample per 100 mL of solvent. Except for fluoroarene samples in 2-propanol, spectral shapes and shifts were identical at different dilutions suggesting that aggregation was not a significant concern in reported spectra. The fluoroarenes were not soluble enough in 2propanol to provide reliable spectra, as evidenced by spectral broadening. Because no significant differences were observed between samples in rigorously dried and commercially available solvents, the latter was used for these studies. 4,4′-((Pyrazine-2,5-diylbis(ethyne-2,1-diyl))bis(3,4-bis(dodecyloxy)-6,1-phenylene))bis(2-methylbut-3-yn-2-ol) (2). The terminal alkyne, 4-(4,5-bis(dodecyloxy)-2-ethynylphenyl)-2-methylbut-3-yn-2-ol, was generated using a literature procedure.46 This alkyne starting material (0.119 g, 0.215 mmol) was dissolved in 10 mL of diisopropylamine and transferred to a storage tube. Argon was bubbled through this solution for 20 min before 2,5-diiodopyrazine (0.0355 g, 0.107 mmol), Pd(PPh3)4 (0.0124 g, 0.011 mmol), and CuI (0.0020 g, 0.011 mmol) were added. The tube was sealed under argon and heated at 70 °C for 20 h. After being cooled to room temperature, the mixture was rinsed into a separatory funnel with CH2Cl2. This organic mixture was washed with an NH4Cl solution, dried with MgSO4, filtered, and concentrated. The product was purified via flash chromatography (silica, 1% CH3OH/99% CH2Cl2) to reveal a fluorescent yellow solid (0.113 g, 0.095 mmol, 89% yield). 1H NMR (400 MHz, CDCl3): δ 8.67 (s, 2H), 7.01 (s, 2H), 6.91 (s, 2H), 4.01 (t, J = 6.8 Hz, 8H), 1.83 (p, J = 6.8 Hz, 8H), 1.64 (s, 12H), 1.46 (m, 8H), 1.27 (m, 64H), 0.88 (t, J = 6.8 Hz, 12H) ppm. 13C NMR (100 MHz, CDCl3): δ 150.5, 149.0, 146.6, 137.6, 120.5, 116.8, 115.6, 115.2, 98.6, 95.8, 89.1, 80.6, 69.4, 69.2, 65.4, 32.0, 31.5, 29.71, 29.68, 29.63, 29.4, 29.11, 29.07, 26.0, 14.1 ppm. HRMS (APCI-QTOF) m/z: [M + H]+ calcd for C78H121N2O6+ 1181.9225, found 1181.9212. 2,5-Bis((4,5-bis(dodecyloxy)-2-ethynylphenyl)ethynyl)pyrazine (6). To a flask containing diol 2 (0.113 g, 0.095 mmol) was added toluene (5 mL) and approximately 500 mg of freshly crushed pellets of KOH. This mixture was heated at 90 °C for 1 h, at which point it was complete by TLC. After the mixture was cooled to room temperature, NH4Cl solution was added to the reaction flask. The mixture was rinsed into a separatory funnel with CH2Cl2. After separation, a second CH2Cl2 extraction was performed. The combined organic layers were dried with MgSO4, filtered, and concentrated to reveal a red oil. This material was loaded onto a flash column (silica, 50% hexane/50% CH2Cl2) with a minimal amount of toluene. The polarity of the mobile phase was gradually increased to 40% hexane/60% CH2Cl2. Purification yielded a fluorescent yellow-green solid in quantitative yield (0.102 g, 0.095 mmol). 1H NMR (400 MHz, CDCl3): δ 8.72 (s, 2H), 7.07 (s, 2H), 7.00 (s, 2H), 4.02 (t, J = 6.6 Hz, 4 H), 4.01 (t, J = 6.6 Hz, 4 H), 3.34 (s, 2H), 1.83 (p, J = 6.6 Hz, 8 H), 1.47 (m, 8H), 1.27 (m, 64H), 0.88 (t, J = 6.6 Hz, 12H) ppm. 13C NMR (100 MHz, CDCl3): δ 150.3, 149.6, 147.3, 137.9, 118.4, 117.2, 116.7, 116.4, 94.2, 88.7, 82.0, 80.3, 69.4, 69.3, 32.0, 29.71, 29.68, 29.63, 29.38, 29.08, 26.0, 22.7, 14.1 ppm. HRMS (APCI-QTOF) m/z: [M + H]+ calcd for C72H109N2O4+ 1065.8387, found 1065.8387. 6147

DOI: 10.1021/acs.joc.8b01064 J. Org. Chem. 2018, 83, 6142−6150

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The Journal of Organic Chemistry yellow oil (0.242 mmol, 56% yield). 1H NMR (400 MHz, CDCl3): δ 8.73 (s, 2H), 7.08 (s, 2H), 7.01 (s, 2H), 3.88 (m, 8H), 3.37 (s, 2H), 1.78 (m, 4H), 1.49 (m, 16H), 1.33 (m, 1H), 0.94 (m, 24H) ppm. 13C NMR (100 MHz, CDCl3): δ 150.6, 149.9, 147.3, 137.8, 118.2, 117.0, 116.2, 116.0, 94.3, 88.7, 82.1, 80.2, 71.6, 71.5, 39.5, 30.6, 29.7, 29.1, 23.96, 23.94, 23.05, 14.1, 11.19, 11.17 ppm. HRMS (APCI-QTOF) m/ z: [M + H]+ calcd for C56H77N2O4+ 841.5883, found 841.5887. 2,5-Bis((4,5-bis((2-ethylhexyl)oxy)-2-((2,3,4,5-tetrafluoro-6iodophenyl)ethynyl)phenyl)ethynyl)pyrazine (3). 1,2-Diiodotetrafluorobenzene (0.0482 g, 0.120 mmol), Pd(PPh3)4 (0.001 g, 0.00087 mmol), and CuI (0.001 g, 0.0052 mmol) were added to a dry flask charged with argon. Deprotected alkyne 12 (0.010 g, 0.0121 mmol) was dissolved in triethylamine in an addition funnel. Argon was bubbled through both solutions for 30 min. To the flask was added dropwise the solution of 12 at room temperature (20 °C). Upon complete addition of 12, the reaction flask was heated at 60 °C for 24 h. After being cooled to room temperature, the mixture was rinsed into a separatory funnel with CH2Cl2. The organic mixture was washed with DI water, dried with Na2SO4, filtered, and concentrated to reveal an oil. The compound was purified via preparatory thin-layer chromatography (silica, 90% hexane, 10% CH2Cl2) yielding 0.004 g yellow solid (0.0029 mmol, 24%). 1H NMR (400 MHz, CDCl3): δ 8.71 (s, 2H), 7.11 (s, 2H), 7.05 (s, 2H), 3.93 (m, 8H), 1.79 (m, 4H), 1.50 (m, 16H), 1.35 (m, 16H), 0.94 (m, 24H) ppm. 13C NMR (100 MHz, CDCl3): δ 150.8, 150.6, 147.4, 137.6, 117.6, 116.8, 116.4, 116.0, 99.0, 95.1, 89.0, 85.6, 83.5, 71.7, 71.6, 39.53, 39.47, 30.64, 30.59, 29.7, 29.2, 29.1, 24.0, 23.1, 14.1, 11.3, 11.2 ppm (extensive C−F coupling in the fluoroarenes obscures carbon signals of those rings and the neighboring alkene carbon at achievable concentrations).19F NMR (376 MHz, CDCl3): δ −114.1 (ddd, J = 23.2, 10.3, 3.8 Hz), −131.0 (ddd, J = 20.7, 10.1, 3.4 Hz), −151.8 (ddd, J = 19.7, 3.7, 2.9 Hz), −154.3 (td, J = 19.7, 3.8 Hz). HRMS (APCI-QTOF) m/z: [M + H]+ calcd for C68H75F8I2N2O4+ 1389.3688, found 1389.3694. 2,5-Bis((2-((2-bromo-5-(trifluoromethyl)phenyl)ethynyl)-4,5-bis((2-ethylhexyl)oxy)phenyl)ethynyl)pyrazine (4). Deprotected alkyne 12 (0.051 g, 0.062 mmol) was dissolved in triethylamine in a storage tube. Argon was bubbled through this solution for 20 min before Pd(PPh3)4 (0.0057 g, 0.0049 mmol), CuI (0.001 g, 0.0052 mmol), and 1-bromo-2-iodo-4-trifluoromethylbenzene (0.0285 g, 0.0813 mmol) were added. The tube was sealed under argon and heated at 60 °C for 24 h. After being cooled to room temperature, the mixture was rinsed into a separatory funnel with CH2Cl2. The organic mixture was washed with DI water, dried with Na2SO4, filtered, and concentrated to reveal an oil. The compound was purified via preparatory thin layer chromatography (silica, 70% hexane, 30% CH2Cl2) to yield 0.036 g yellow oil (0.028 mmol, 45%). 1H NMR (400 MHz, CDCl3): δ 8.73 (s, 2H), 8.01 (d, J = 2.2 Hz, 2H), 7.73 (d, J = 8.4 Hz, 2H), 7.40 (dd, J = 8.4, 1.9 Hz, 2H), 7.13 (s, 2H), 7.06 (s, 2H), 3.93 (m, 8H), 1.80 (s, 4H), 1.51 (m, 16H), 1.35 (m, 16H), 0.95 (m, 25H). 13C NMR (100 MHz, CDCl3): δ 150.7, 150.3, 147.2, 137.8, 133.0, 130.5 (m), 129.9 (q, J = 33.2 Hz), 129.0 (m), 126.6, 125.6 (m), 123.5 (q, J = 272.7 Hz), 118.4, 117.0, 116.0, 115.6, 94.3, 94.2, 89.9, 89.2, 71.7, 71.5, 39.51, 39.47, 30.6, 29.15, 29.11, 24.0, 23.1, 14.1, 11.2, 11.2 ppm. 19F NMR (376 MHz, CDCl3) δ −62.8 ppm. HRMS (APCI-QTOF) m/z: [M + H]+ calcd for C70H81Br2F6N2O4+ 1287.4447, found 1287.4456. 2,5-Bis((4,5-bis((2-ethylhexyl)oxy)-2-((2,3,4,5-tetrafluorophenyl)ethynyl)phenyl)ethynyl)pyrazine (5). Deprotected alkyne 12 (0.0384 g, 0.0464 mmol) was dissolved in 15 mL of triethylamine in a storage tube. Argon was bubbled through this solution for 20 min before Pd(PPh3)4 (0.0026 g, 0.0023 mmol), CuI (0.0004 g, 0.0026 mmol), and 1,2,3,4-tetrafluoro-5-bromobenzene (15, 0.011 mL, 0.091 mmol) were added. The tube was sealed under argon and heated at 70 °C for 20 h. After being cooled to room temperature, the mixture was rinsed into a separatory funnel with CH2Cl2. The organic mixture was washed with DI water, dried with Na2SO4, filtered, and concentrated to reveal an oil. The compound was purified via preparatory thin layer chromatography (silica, 90% hexane, 10% CH2Cl2) to yield 0.0066 g of yellow oil (0.0058 mmol, 13% yield). 1H NMR (400 MHz, CDCl3) δ 8.75 (s, 2H), 7.50 (m, 2H), 7.12 (s, 2H), 7.03 (s, 2H), 3.93 (m, 8H), 1.80 (m, 4H), 1.50 (m, 16H), 1.34 (m, 16H), 0.95 (m, 24H) ppm. 13C

NMR (100 MHz, CDCl3) δ 150.8, 150.4, 146.9, 137.7, 118.1, 117.1, 115.8, 115.3, 114.7 (dd, J = 20.7, 3.1 Hz), 95.2 (d, J = 3.9 Hz), 94.1, 89.3, 83.2, 71.7, 71.6, 39.49, 39.45, 30.6, 29.7, 29.13, 29.10, 23.96, 23.1, 14.1, 11.22, 11.19 ppm (extensive C−F coupling in the fluoroarenes obscures some carbon signals of those rings at achievable concentrations). 19F NMR (376 MHz, CDCl3) δ −135.5 (m), −139.4 (m), −153.8 (m), −155.1 (t, J = 20.3 Hz) ppm. HRMS (APCI-QTOF) m/z: [M + H]+ calcd for C68H77F8N2O4+ 1137.5756, found 1137.5776. 2,5-Bis((4,5-bis((2-ethylhexyl)oxy)-2-((perfluorophenyl)ethynyl)phenyl)ethynyl)pyrazine (7). Deprotected alkyne 12 (0.038 g, 0.045 mmol) was dissolved in 10 mL of triethylamine in a storage tube. Argon was bubbled through this solution for 20 min before Pd(PPh3)4 (0.003 g, 0.003 mmol), CuI (0.003 g, 0.02 mmol), and iodopentafluorobenzene (0.013 mL, 0.097 mmol) were added. The tube was sealed and heated at 60 °C for 1 day. After being cooled to room temperature, the mixture was rinsed into a separatory funnel with CH2Cl2. The organic mixture was washed with water, dried with Na2SO4, filtered, and concentrated to reveal an oil. The product was purified via preparatory thin-layer chromatography (silica, 50% hexane, 50% CH2Cl2) to reveal the product as a yellow solid (28.8 mg, 0.025 mmol, 55%). 1H NMR (400 MHz, CDCl3): δ 8.74 (s, 2H), 7.14 (s, 2H), 7.04 (s, 2H), 3.92 (m, 8H), 1.80 (m, 4H), 1.50 (m, 16H), 1.34 (m, 16H), 0.95 (m, 24H) ppm. 13C NMR (100 MHz, CDCl3): δ 150.7, 147.2, 137.7, 117.2, 117.0, 116.1, 115.6, 100.5, 93.7, 89.2, 71.7, 71.6, 39.5, 39.4, 30.61, 30.58, 29.14, 29.09, 23.99, 23.96, 23.94, 23.1, 14.1, 11.23, 11.17 ppm (extensive C−F coupling in the fluoroarenes obscures carbon signals of those rings and the neighboring alkene carbon at achievable concentrations). 19F NMR (376 MHz, CDCl3): δ −136.1 (m), −152.8 (t, J = 20.7 Hz), −161.6 ppm. HRMS (APCIQTOF) m/z: [M + H]+ calcd for C68H75F10N2O4+ 1173.5567, found 1173.5589.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.8b01064. 1 H and 13C NMR spectra for all new compounds; UV− vis and fluorescence spectra of 4; computational coordinates and energies (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Nathan P. Bowling: 0000-0002-6930-1333 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation for support (CHE1606558 and CHE-1606556) and funding for NMR (CHE0957080) and HR-MS instrumentation (CBET-0958711). Computational work used the Extreme Science and Engineering Discovery Environment (XSEDE) resource Comet at the San Diego Supercomputing Center through allocation wis158. XSEDE is supported by the National Science Foundation Gratn No. ACI-1548562. Additional computational resources were provided by NSF-MRI awards CHE-1039925 and CHE0520704 through the Midwest Undergraduate Computational Chemistry Consortium--MU3C.

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REFERENCES

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DOI: 10.1021/acs.joc.8b01064 J. Org. Chem. 2018, 83, 6142−6150

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DOI: 10.1021/acs.joc.8b01064 J. Org. Chem. 2018, 83, 6142−6150