J. Phys. Chem. B 2009, 113, 4555–4559
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4-Isopropylpyridine Hydroperoxide Crystals Resulting from the Aerobic Oxidation of a 4-Isopropylpyridine/4-Propylpyridine Mixture Evgenia Vaganova,*,† Ellen Wachtel,‡ Gregory Leitus,‡ David Danovich,§ and Shlomo Yitzchaik*,† Department of Organic Chemistry and the Farkas Center for Light-Induced Processes, The Hebrew UniVersity of Jerusalem, 91904, Jerusalem, Israel; Chemical Research Support Unit, Weizmann Institute of Science, 76100, RehoVot, Israel; and Department of Organic Chemistry and the Lisa Meitner Center for Computational Quantum Chemistry, The Hebrew UniVersity of Jerusalem, 91904, Jerusalem, Israel ReceiVed: September 28, 2008; ReVised Manuscript ReceiVed: February 9, 2009
We model the interaction of side-chain and end-chain groups of poly(4-vinylpyridine) by a 5:1 molar ratio mixture of 4-isopropylpyridine (side-chain model) and 4-propylpyridine (end-chain model). We find that the 4-isopropylpyridine in the mixture is oxidized in a slow air flow to produce 4-isopropylpyridine hydroperoxide which in turn precipitates as lamellar crystals with monoclinic structure. The fact that the peroxide group is exchanged for the hydrogen of the tertiary carbon demonstrates the high activity of the latter and gives strong support for its involvement in the self-protonation mechanism proposed earlier for the poly(4-vinylpyridine)/ pyridine gel. Introduction
Experimental Methods
In order to clarify the self-protonation mechanism proposed earlier for poly(4-vinylpyridine) dissolved in pyridine,1-5 we have studied the interaction of side-chain and end-chain groups of poly(4-vinylpyridine). As a model for this interaction we use a mixture of 4-isopropylpyridine (model of the side-chain group) and 4-propylpyridine (model of the end-chain group) in molar ratio 5:1. To confirm the correct choice of models, we compared the photoinduced optical behavior of the 4-isopropylpyridine/ 4-propylpyridine mixture with that of the poly(4-vinylpyridine)/ pyridine gel and, as we shall show below, they are in agreement. In addition, as with the gel, the photoinduced properties of the mixture were reversible during storage in the dark.6
Materials. 4-Isopropylpyridine and 4-propylpyridine was purchased from Aldrich. Both chemicals were subjected to NMR and GS/MS analysis. The results of the analysis showed that the purity of the 4-propylpyridine was 92% and that of the 4-isopropylpyridine, 90% (the nature of the contaminants was not determined). For the experiments described below, both compounds were used without further purification; however, the effect of vacuum distillation of 4-isopropylpyridine was investigated. Fresh samples of 4-isopropylpyridine and 4-propylpyridine were mixed at a molar ratio of 5:1 and placed on a precleaned glass substrate inside a laminar flow hood (PV-Plast, Israel). The airflow rate was 0.6 m/s (Sureflow, TSI, Israel). The process of evaporation of the mixture of 4-propylpyridine and 4-isopropylpyridine in this airflow is observed to be slow, perhaps due to the high viscosity of the 4-propylpyridine which retards movement of molecules to the sample surface. In contrast to the individual solutions, the evaporation of the mixture, placed on a precleaned glass substrate, produced large transparent single crystals and a small amount of a yellow colored product. Uv-Vis Spectroscopy. Excitation and photoluminescence (PL) spectra were measured on a Shimadzu RF-5301PC spectrofluorimeter. The data were collected at right angles to the excitation beam. The resolution of the emission and excitation spectra was 1.0 nm. Multiwavelength UV irradiation (λ ) 385 ( 10 nm, intensity 5.3 mW/cm2) was accomplished using a xenon short arc lamp (Ushio) inside the Shimadzu RF5301PC instrument. X-ray Structure Analysis. Data Collection and Processing. Bruker Appex2 KappaCCD diffractometer, Mo KR (λ ) 0.710 73 Å), graphite monochromator, 17 539 reflections collected, -15 e h e 15, -12 e k e 12, -19 e l e 19, frame scan width ) 0.5°, scan speed 1.0° per 40 s, typical peak mosaicity 0.67°, 3755 independent reflections (R(int) ) 0.0511). The data were processsed with APEX2. Solution and refinement: structure solved by direct methods with SHELXS. Full matrix least-squares refinement based on F2 with SHELXL-97. 144
The ground-state interaction of the model compounds at room temperature unexpectedly produced crystalline phases which precipitated as the solvent mixture slowly evaporated. The major fraction of the precipitate was shown to consist of 4-isopropylpyridine hydroperoxide (4-IPPHP) crystals, the structure of which we have now determined using X-ray single-crystal analysis. 4-Isopropylpyridine hydroperoxide has been known since 1960, when it was synthesized7,8 as a homologue of cumene hydroperoxide.9,10 However, the molecule has not been widely investigated except for its thermal stability.11 Here we present the crystal structure, 1H and 13C NMR data, and the optical properties of 4-IPPHP. In addition, we show that the finding of 4-IPPHP as a product of the solvent mixture can provide an explanation for the mechanism of self-protonation of the polymer’s pyridine units in the poly(4-vinylpyridine)/ pyridine gel. * Corresponding author. Tel: 972 2658 6971. Fax: 972 2658 5319. E-mail:
[email protected] (E.V.);
[email protected] (S.Y.). † Department of Organic Chemistry, and the Farkas Center for LightInduced Processes, The Hebrew University of Jerusalem. ‡ Weizmann Institute of Science. § Department of Organic Chemistry and the Lisa Meitner Center for Computational Quantum Chemistry, The Hebrew University of Jerusalem.
10.1021/jp808587d CCC: $40.75 2009 American Chemical Society Published on Web 03/10/2009
4556 J. Phys. Chem. B, Vol. 113, No. 14, 2009 parameters with 0 restraints, final R1 ) 0.0382 (based on F2) for data with I > 2σ(I) and, R1 ) 0.0474 on 3755 reflections, goodness of fit on F2 ) 1.041, largest electron density peak ) 0.635 e Å-3, deepest hole ) -0.195 e Å-3. Crystal data: C8H11N1O2, colorless, 0.5 × 0.3 × 0.03 mm3, monoclinic, P21/c (No.14), a ) 9.334(2) Å, b ) 7.725(2) Å, c ) 11.499(2) Å, β ) 112.29(3)°, from 20 degrees of data, T ) 100(2) K, V ) 767.2(3) Å3, Z ) 4, Fw ) 153.18 g/mol, Dc ) 1.326 Mg.m-3, µ ) 0.096 mm-1. Crystallographic data have been deposited in the Cambridge Crystallographic Data Center under accession #701960. GS/MS. GC equipment was Agilent 6890N; mass selective detector was Agilent 5793 N; nonpolar column DB-5 (J&W Scientific Folsom, CA). The injector temperature was set to 50 °C for 5 min, then programmed at a rate of 10 °C/min to reach 120 °C, followed by a rate of 15°/min to reach 280 °C. The temperature was then held constant at 280 °C for 3 min. Ethanol was used as a solvent. NMR Spectroscopy. All NMR measurements were performed with a Bruker DRX 400 or Bruker Avance II 500 spectrometer. 400 MHz NMR spectra were recorded at 298 ( 0.5 K and 500 MHz spectra at 303 ( 0.5 K, the higher temperature being more convenient, especially for 13C NMR, due to extra heating from the decoupler. DFT Calculations. The calculations of the ground states of the molecules have been carried out with the use of the Perdew-Wang 91 (PW91) density functional in conjunction with Dunning’s correlation consistent polarized valence triple-ζ (cc-pVTZ) basis sets as implemented in the Gaussian 03 program package.12 Equilibrium geometries of the neutral molecules were fully optimized without symmetry constraints. Total energies included zero point vibrational energy (ZPVE) corrections. For the purpose of comparison of DFT results with highlevel ab initio data, perturbation theory [MP2] method has been also applied. Results 1. Excited-State Interaction. To investigate the photoinduced interaction of the 4-isopropylpyridine and 4-propylpyridine molecules, a drop of the solvent mixture (5:1 molar ratio) was placed between two quartz slides. The quartz slides were held in a homemade holder inside the spectrophotometer. The absorption spectrum of the mixture displayed the characteristic band of the pyridine ring at λmax ) 254 nm13 and a high-intensity absorption tail in the ultraviolet till 200 nm (the wavelength limit of the equipment) due to the methyl and ethyl groups (data not shown). The sample was then placed in the spectrofluorimeter to determine its emission and photophysical properties. Samples were irradiated at 385 nm and during irradiation, absorption and emission properties were recorded every 30 min. Prior to irradiation, two emission bands were observed: an intense band at 470 nm, excitation at 390 nm; and a relatively more intense green emission at 540 nm, excitation at 486 nm. After 1 h of irradiation, the intensity of the green emission at 540 nm was significantly increased and a new red emission at 580 nm (excitation at 520 nm) appeared (Figure 1). For comparison, the emission peaks of the P4VP/pyridine gel following irradiation are also presented on the graph. Irradiationinduced changes in the absorption spectrum of the solvent mixture (Figure 2) demonstrate the formation of new centers, with an absorption band centered at 450 nm and a much weaker absorption at 380 nm. The intensity of the absorption at 450 nm, as well as at that at 380 nm, is linearly related to the time
Vaganova et al.
Figure 1. Normalized emission (right curves) and excitation spectra (left curves) of the 4-isopropylpyridine/4-propylpyridine mixture after irradiation during 1 h at λ ) 385 nm; green emission was also present before irradiation (black curve); red emission (red curve) only appeared after irradiation. Blue dashed curves: green (λmax ) 522 nm) and red (λmax ) 580 nm) normalized emission spectra of the poly(4-vinylpyridine)/pyridine gel by excitation at 466 and 540 nm, respectively, after irradiation during 1 h at λ ) 385 nm.
Figure 2. Room temperature absorption spectra of the 4-isopropylpyridine/4-propylpyridine mixture before irradiation (dashed black curve); following 30 min of irradiation (red curve); following 60 min of irradiation (blue curve); and after storage during 16 h (solid black curve).
of exposure (Figure 2). The red-shifted emission at 580 nm, the intensity of the green emission at 540 nm, and the corresponding absorption bands at 380 and 450 nm were not stable in the dark (Figure 2). The reversibility of photoinduced changes in optical properties during storage in the dark is another property shared by the 4-isopropylpyridine/4-propylpyridine mixture and the P4VP/pyridine gel.6 The photophysical properties of the gel have been found to be stable during at least 3 months of storage in the dark. 2. Ground-State Interaction. Investigation of the groundstate interaction of 4-isopropylpyridine and 4-propylpyridine revealed an unexpected chemical reaction, which resulted in the production of new crystalline phases. When a mixture of the two solutions (100-150 µL) at molar ratio 5:1 was placed on a precleaned glass substrate and very slowly evaporated during 16-18 h in a flow of air at room temperature, crystals began to grow. A polarized optical micrograph (Figure 3) demonstrates at least two different crystalline forms, one colorless and the second colored yellow. The large, transparent,
4-Isopropylpyridine Hydroperoxide Crystals
Figure 3. Polarized light microscope image of the crystals precipitated during evaporation of the 4-isopropylpyridine/4-propylpyridine mixture in air flow at room temperature.
Figure 4. ORTEP drawing of 4-isopropylpyridine hydroperoxide crystals (thermal ellipsoids at 50% probability).
colorless crystals were examined by X-ray diffraction. X-ray diffraction of the single crystals revealed that they contain the molecule 4-isopropylpyridine hydroperoxide (4-IPPHP) in a monoclinic, P21/c (No.14) lattice (Figure 4) (CCDC #701960). The most important interatomic distances are as follows: O(1)-C(6) ) 1.4383(8) Å; O(1)-O(2) ) 1.4629(7) Å; C(5)C(6) ) 1.5207(9)) Å; C(6)-C(7) ) 1.5226(10) Å; C(6)-C(8) ) 1.5299(9) Å; O(2)-H(2O) ) 0.910(15) Å; the angle C(6)O(1)-O(2) ) 108.66(5)°, and the angle O(1)-O(2)-H(2O) ) 98.7(9)°. Parameters for the hydrogen atoms were included in the refinement cycles but did not change the stability. The interatomic distances and bond angles obtained are in the range of values observed for corresponding bonds in such compounds as 2-(5,5,6-trimethyl-6-phenyl-1,2-dioxan-3-yl)propan-2-yl hydroperoxide.14 The 1H and 13C NMR spectra are consistent with the X-ray structure. 13C resonances are as follows: C1, C2 at 150.12 ppm; C3, C4 120.36 ppm; C5 153.96 ppm; C6 82.32 ppm; C7, C8 25.32 ppm. The positions of the proton resonances are as follows: H1, H2 8.51 ppm; H3, H4 6.94 ppm; H7A, H7B, H7C, H8A, H8B, H8C 1.24 ppm; H20 0.55 ppm. Due to the small amounts of yellow crystals available (X-ray powder diffraction
J. Phys. Chem. B, Vol. 113, No. 14, 2009 4557
Figure 5. Room temperature absorption spectrum of 4-isopropylpyridine hydroperoxide dissolved in acetonitrile. The coefficient of extinction at 254 nm is 2.25 × 105 L/(mol cm).
shows a mixture of crystalline forms), we were not able to completely characterize all the products of the reaction. As controls, we monitored the slow evaporation of the individual liquids from a precleaned glass surface under a slow flow of air. Slow evaporation of 4-propylpyridine did not produce any solid precipitate; evaporation of either nonpurified or distilled 4-isopropylpyridine resulted in a white powder precipitate. In neither case were any crystals obtained. 3. Optical Properties of 4-Isopropylpyridine Hydroperoxide Crystals. We investigated the optical properties of 4-IPPHP. The absorption spectrum of 4-IPPHP dissolved in acetonitrile is presented in Figure 5. The coefficient of extinction of the molecule in acetonitrile was estimated to be 2.23 × 105 L/(mol cm) at 254 nm. Analysis of the emission and excitation spectra revealed properties of charge transfer emission: blue emission at 470 nm, excitation at 385 nm (data not shown). It was shown by Pierola et al.15 that emission at 470 nm with excitation at 385 nm can be assigned to a protonated species. 4-IPPHP also exhibits a strong emission at 525 nm under excitation at 470 nm. 4. Proposed Mechanim of the Aerobic Oxidation of the 4-Isopropylpyridine/4-Propylpyridine Mixture. To help elucidate the mechanism of the reaction between 4-isopropylpyridine and 4-propylpyridine which resulted in crystal formation, we investigated the acid-base properties of the solution which remained during evaporation. 20 µL of the crystallizing solution was taken every 10 min and dissolved in 20 mL of the TDW (triple distilled water), and the pH of each solution was measured. Figure 6 presents the changes in pH of the solutions during 1 h of solid-state precipitation. A significant decrease in pH (from the intial value 9.1 ( 0.1 to 7.2 ( 0.2) occurred during the first 10 min; during the next 10 min the pH increased to 8.6 ( 0.2 and then slowly decreased until the saturation value of 8.3 ( 0.15 was reached. Under this final pH, crystals were continuously deposited. The measurement was repeated not less than three times in order to prove reproducibility. The decrease in pH signals the appearance of dissolved hydrogen ions (H+) in the solution. We interpret the appearance of free hydrogen ions as being due to the degradation of one of the compounds. Since the crystal structure is built on the basis of 4-isopropylpyridine, the negative ion must be formed on this molecule. With increasing concentration of the oxidation products, includ-
4558 J. Phys. Chem. B, Vol. 113, No. 14, 2009
Figure 6. Dependence of the pH of 20 µL of the crystallizing solution mixed with 20 mL of TDW during the first hour of crystal formation (three experiments).
ing the hydroperoxide group, the pH is then observed to increase. By analogy, it is known that cumene hydroperoxide is formed by air oxidation (autoxidation) of the tertiary carbon of the cumene (isopropylbenzene).16,17 5. Ground-State Modeling. Modeling of the oxidation of 4-isopropylpyridine was performed by density functional theory (DFT) calculations. The direct reaction of 4-isopropylpyridine with O2 (S)1, 3Σg-) is spin-forbidden and probably will be kinetically slow. Nevertheless, according to PW91 DFT calculations it is thermodynamically favorable.
Vaganova et al. -0.124e. According to MP2/cc-pVTZ calculations, there is also considerable difference in the Milliken charge distribution of the tertiary carbon in 4-isopropylpyridine (-0.09e) and 4-propylpyridine (-0.209e). Questions concerning the interaction between 4-propylpyridine and 4-isopropylpyridine and the unique role of 4-propylpyridine in the aerobic oxidation of 4-isopropylpyridine remain open and will be subjects for future research. In conclusion, in the present study we have investigated the interaction of 4-isopropylpyridine and 4-propylpyridine as a model of the interactions between side-chain and end-chain groups of poly(4-vinylpyridine) in pyridine. We show that in air, 4-isopropylpyridine is oxidized to 4-isopropylpyridine hydroperoxide in the presence of 4-propylpyridine. We suggest that the pathway of this reaction is via negative ion formation on the tertiary carbon of the CH group of 4-isopropylpyridine. Further details of the mechanism, particularly the mandatory role of 4-propylpyridine, are under investigation. The activity of the tertiary hydrogen of the CH group of 4-isopropylpyridine in the presence of 4-propylpyridine clearly raises the possibility of the liberation of the tertiary hydrogen on the poly(4vinylpyridine) polymer chain due to oxidation by residual oxygen and as a consequence of side-/end-chain polymer group interaction. Acknowledgment. E.V. gratefully acknowledges financial support from the Israel Ministry for Immigrant Absorption; Dr. Shmuel Kohen for supporting X-ray diffraction measurements; Dr. Roy Hoffman and Dr. Yair Ozery for the NMR measurements; and Prof. O. Lev and Dr. R. Shelkov for GC/MS measurements. This work was partly supported by EC through contract FP6-029192. References and Notes
The reaction with triplet O2 is exothermic with ∆E ) 17.55 kcal/mol according to PW91 calculations with Dunning’s correlation consistent triple-ζ quality (cc-pVTZ) basis set. MP2/ cc-pVTZ calculation predicts ∆E ) 21.96 kcal/mol. The reaction with singlet O2 (S)0, 1∆g) is also exothermic with ∆E ) 56.48 kcal/mol (51.23 kcal/mol according to MP2/cc-pVTZ calculation), where ∆ES-T for the O2 molecule is 38.93 kcal/mol. MP2 and DFT results show reasonable agreement. We did not investigate the reaction mechanism in the present study. However, we propose that 4-propylpyridine can serve as a freeradical initiator. The explanation is based on the following: (1) 4-propylpyridine is a stronger base than 4-isopropylpyridine; the pKb of 4-propylpyridine is estimated to be 3.53, and that of 4-isopropylpyridine 6.52 (calculations of basicity as in ref 18); (2) 4-propylpyridine is more hydrophobic than 4-isopropylpyridine; the contact angle of water on the 4-isopropylpyridine film, placed on quartz, was 17.5°, and that for 4-propylpyridine 46.4°. In addition, the Mulliken charge distribution calculated with PW91/cc-pVTZ showed a practically neutral tertiary carbon (-0.03e (e ) charge of the electron)) for 4-isopropylpyridine, while the same atom in 4-propylpyridine is strongly negative,
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