Infrared Matrix-Isolation and Theoretical Studies of the Reactions of

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Infrared Matrix-Isolation and Theoretical Studies of the Reactions of Ferrocene with Ozone Roger W. Kugel, Laura F. Pinelo, and Bruce S. Ault* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221, United States S Supporting Information *

ABSTRACT: The reactions between ferrocene (Cp2Fe) (2a) and ozone (O3) were studied using low-temperature matrixisolation techniques coupled with theoretical density functional theory (DFT) calculations. Co-deposition of Ar/Cp2Fe and Ar/O3 gas mixtures onto a cryogenically cooled CsI window produced a dark-green charge-transfer complex, Cp2Fe−O3, that photodecomposed upon red (λ ≥ 600 nm) and infrared (λ ≥ 1000 nm) irradiation but was stable to green or blue irradiation. Products of photodecomposition were characterized by FT-IR, oxygen-18 labeling, and DFT calculations using the B3LYP functionals and the 6-311G++(d,2p) basis set. Likely, photochemical products included four structures having the molecular formula C10H10FeO, identified by DFT calculations based on their calculated infrared spectra and 18O isotope shifts. Each of these calculated molecules had one intact and fully coordinated η5-C5H5 cyclopentadienyl (Cp) ring and (1) an η5C5H5O cyclic ether (pyran ring) (2b), (2) an η4-C5H5O linear aldehyde (2c), (3) a bidentate cyclic aldehyde with a sevenmembered ring including the iron atom (2d), or (4) an Fe−O bond and an η2-C5H5 (Cp) ring (2e). No conclusive evidence for a gas-phase thermal reaction between ferrocene and ozone was observed under the conditions of these experiments. However, strong evidence for a surface-catalyzed thermal reaction was observed in merged-jet experiments wherein the gases were premixed before deposition. Surface-catalyzed ferrocene−ozone reaction products included a thin film of Fe2O3 observed on the walls of the merged tube as well as cyclopentadiene (C5H6), cyclopentadienone (C5H4O), and further oxidation products observed in the matrix. Possible mechanisms for both the photochemical and the thermal reactions are discussed.

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INTRODUCTION The reaction between oxygen or ozone and ferrocene has drawn interest in recent years as a means of creating iron(III) oxide thin films on various substrates through the processes of chemical vapor deposition (CVD)1,2 or atomic layer deposition (ALD).3−9 CVD involves the reaction of a gas mixture at or near the surface of a substrate that deposits a solid reaction product as a thin film on the surface. ALD, on the other hand, involves exposing the substrate surface alternately to each gasphase reactant in repeating cycles. This exposure causes selflimiting surface reactions to occur and results in a monolayer of reaction product(s) building up in a thin film on each cycle. CVD is faster and cheaper, whereas ALD affords more control of film thickness and allows for more conformal coatings on rough surfaces with high aspect ratios. Iron(III) oxide (αFe2O3) thin films can be made by either deposition method, depending on the application or end use. Iron(III) oxide thin films have been used in batteries and in photoelectrochemical cells for the solar energy conversion of water to hydrogen.2,10−14 Despite its importance in making thin films for electronics applications, relatively little is known about the chemistry of the reaction between ferrocene and ozone. Koda et al.15 studied the similar reaction between ferrocene and atomic oxygen (O3P) produced by a microwave discharge through oxygen gas. They observed a “brown colored metallic species” precipitate on the © XXXX American Chemical Society

reactor walls, and they isolated a number of volatile organic species, including 1,3-butadiene (C4H6), propyne (C3H4), cyclopentadiene (C 5 H 6 ), acetylene (C 2 H 2 ), propylene (C3H6), 1-butyne (C4H6), allene (C3H4), and vinylacetylene (C4H4). Bulgakov et al.16 studied the ferrocene−ozone reaction in CCl4 solution and observed chemiluminescence of some reaction intermediates. They proposed a mechanism involving cyclopentadienone (C 5 H 4 O) and cyclopentenedione (C5H4O2), but their results are complicated by the involvement of solvent in the reactions, and their conclusions, although consistent with their data, are not definitive. More recently, Martinson et al.3 used mass spectrometry to analyze the gasphase products formed during the ALD cycles of the ferrocene−ozone reaction. They detected cyclopentadiene (m/z = 66) and cyclopentadienone (m/z = 80) during the ferrocene cycle, and carbon dioxide (m/z = 44) and water (m/z = 18) during the ozone cycle. It is noteworthy that cyclopentadiene had been observed in previous ALD experiments with metallocenes,17,18 but cyclopentadienone, which is known to be reactive,19 had not been previously observed under these reaction conditions. Special Issue: Markku Räsänen Festschrift Received: July 25, 2014 Revised: September 9, 2014

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The frozen matrices formed during deposition were studied by FT-IR spectroscopy (PerkinElmer Spectrum One) at 1 cm−1 resolution and were subsequently photolyzed and/or annealed to see the effects of light and/or temperature on the observed reaction intermediates and products. Light sources for photochemical reactions included a medium-pressure mercury arc lamp (Karl Zeiss #470, Germany), a red laser diode (λ ≈ 650 nm), and a 100-W incandescent bulb either with glass “cutoff” filters (Corning red glass filter #2418, transmits λ ≥ 600 nm; Schott infrared glass filter RG1000, transmits λ ≥ 1000 nm) or with a 1 mm silicon wafer (λ ≥ 1100 nm). Quantum calculations were carried out with the Gaussian 09 suite of programs20 using density functional theory (DFT) and B3LYP functionals with the 6-311G++ (d,2p) basis set. Gaussian 09 calculations were carried out online at the Ohio Supercomputer Center (OSC) in Columbus, OH.

In the present work, the reactions of ferrocene with ozone were investigated using a low-temperature matrix-isolation infrared spectroscopic technique. The technique has the potential of trapping and characterizing the reaction intermediates and/or products isolated in a solid argon matrix to help elucidate the gas-phase or surface-catalyzed reaction mechanisms. It also allows for these frozen matrices to be irradiated and/or annealed to observe possible photochemical and/or low-activation-energy thermal reactions occurring in solid argon. Using this technique we obtained evidence presented in the following sections for both photochemical and surface-catalyzed thermal reactions between ferrocene and ozone.

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EXPERIMENTAL SECTION The oxygen (O2) and argon (Ar) gases were supplied by Wright Brothers, Inc., Cincinnati, OH. Oxygen-18-isotopically enriched oxygen (18O2, 99 atom-percent 18O) was supplied by Aldrich Chemical Co., Milwaukee, WI. Ozone (O3 or 18O3) was produced by a high-frequency Tesla coil discharge through oxygen (O2 or 18O2) gas while being condensed with liquid nitrogen. To minimize the ozone explosion hazard, the pressure of ozone gas at room temperature was never allowed to exceed 5.0 in. of Hg (127 Torr). A stock supply of ferrocene (Cp2Fe) from Eastman Kodak, Rochester, NY, was purified by sublimation. Deuterated ferrocene (Cp2Fe-d10) was obtained from CDN Isotopes, Pointe-Claire, Quebec, Canada, and purified by sublimation. The reactions between ozone (O3) and ferrocene (Cp2Fe) were studied using low-temperature matrix-isolation/FT-IR techniques. Gas mixtures of ozone with argon and of ferrocene with argon (at mole ratios of approximately Ar/O3 ≈ 250 and Ar/Cp2Fe ≈ 250) were prepared using standard manometric techniques. The argon/ozone mixtures were premixed, whereas the argon/ferrocene mixtures were prepared by sublimation of the ferrocene (at ∼45 °C) through an Ultra-Torr tee into a flowing stream of pure argon during deposition. Both gas mixtures were deposited slowly using stainless steel needle valves onto a cryogenically cooled CsI window held at about 13 K with a closed-cycle helium cryostat (CTI Cryogenics). Matrix formation took place over approximately 24 h at an overall gas deposition rate of about 2.3 mmol/h. Gases were deposited using one of three different deposition geometries: 1. Merged-jet (MJ): the Ar/O3 and Ar/Cp2Fe gas mixtures were combined at an Ultra-Torr tee on their way to the cold window and flowed together through a Teflon merged tube whose length could be varied. This geometry was useful for studying later intermediates and stable products of the thermal gas-phase (or surface mediated) reactions. 2. Concentric-jet (CJ): the Ar/O3 and Ar/Cp2Fe gas mixtures were combined concentrically with a 1/8 in. OD Teflon tube flowing inside of a 1/4 in. OD Teflon tube. The longitudinal distance between the tube ends could also be varied. This geometry was useful for studying less stable intermediates. 3. Twin-jet (TJ): the Ar/O3 and Ar/Cp2Fe gas mixtures were fed to the cold window through separate ports in the cold head. This geometry minimized the gas-phase mixing time of the two gas mixtures and was useful for trapping the most unstable, early intermediates in the gas-phase reactions.

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RESULTS AND DISCUSSION Prior to matrix experiments using mixtures of ozone and ferrocene with argon, “blank” deposition experiments were run with argon alone, argon plus ozone, and argon plus ferrocene. These blank deposition experiments showed infrared spectral features for argon matrix-isolated ozone and argon matrixisolated ferrocene that were consistent with literature spectra,21,22 and they were used as references for the experiments in which ozone and ferrocene were combined in the same argon matrix. Note that the experimental infrared spectra of argon matrix-isolated ferrocene and deuterated ferrocene are given in Figures S8 and S9 and in Table S1 in the Supporting Information section available on line. Note also that DFT calculations showed that the eclipsed conformation of ferrocene was 0.58 kcal/mol more stable than the staggered form. Finally, the observed ferrocene spectrum was found to be more consistent with the calculated spectrum for the eclipsed conformer. (See note (b) in Table S1, Supporting Information.) Results of these low-temperature, matrix-isolation FT-IR experiments on the ozone (O3)/ferrocene (Cp2Fe) system indicated evidence for both photochemical reactions and surface-mediated thermal reactions. The photochemistry was first observed in early twin jet experiments when the matrix deposit was formed while the cold window was exposed to the beam of the infrared spectrometer. The beam itself (either light from the IR source or light from the He/Ne calibration laser (λ = 632 nm) or both) caused observable photochemical changes in the depositing matrix. As a result, all subsequent experiments were performed in the dark, with the IR beam and all room light blocked from the depositing matrix during deposition. Under dark-deposition conditions with twin-jet (TJ) or concentric-jet (CJ) deposition geometries, no new infrared peaks attributable to chemical reaction products were observed in freshly formed matrices of Ar/O3/Cp2Fe at 500:1:1 mol ratios. These deposits did have a dark green color that we attributed to a Cp2Fe−O3 charge-transfer complex, but we could not determine whether this complex formed in the gas phase or, more likely, formed in the nascent matrix upon deposition. Evidence for the fact that a Cp2Fe−O3 chargetransfer complex is forming in these experiments includes the intense green color, suggesting strong red and blue absorptions in the visible spectrum, and slightly red-shifted infrared ozone absorptions at 700, 1027, and 2099 cm−1, shifted from 704, 1034, and 2108 cm−1, respectively, characteristic of perturbed ozone molecules.23 B

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Table 1. New Experimental Infrared Frequencies (Intensitiesa) and Oxygen-18 Isotope Shifts Observed upon Red/Infrared Photolysisb of Ar Matrices at 10−15 K Containing Ozone (O3) and Ferrocene (Cp2Fe) with Calculated Frequencies (Intensitiesc), Oxygen-18 Isotope Shifts, and Proposed Assignmentsd experimental (cm−1)

18

18

O shift (cm−1)

calculated (cm−1)

403(0.03)

−1

406(24)

0

421(0.03)

−1

431(14)

0

449(0.22)

−3

456(11)

−3

506(10)

−2

515(9)

−1

554(14) 586(2)

−9 −5

489(0.58)

O shift (cm−1)

522(0.17) 534(0.03) 550(0.36) 567(0.02)*

−2 −1 −8 −4

592(0.02)*

0

593(3)

0

599(0.06)*

0

596(0)

0

614(0.06) 625(0.11) 634(0.06) 714(0.03)*

−2 −3 −3 −5

642(2) 648(1) 707(1)

−3 −3 −4

738(0.17)* 750(0.06)* 772(0.05)

0 0 −3

720(13) 752(131) 777(7)

0 0 −1

777(7)

−1

781(0.15)

0

810(0.59)*

−1

816(39)

0

810(0.59) 851(0.18)

−1 −1

820(62) 855(3)

0 0

851(0.18)

−1

844(14)

0

867(0.04) 892(0.17) 899(0.03) 921(0.06) 931(0.90) 946(0.13) 1021(0.11)

0 −3 −10 −4 −11 −14 −1

868(9) 916(33)

0 −4

916(33) 961(42) 965(6) 1017(21)

−4 −10 −10 −1

cis-aldehyde (2c) cis-aldehyde (2c) cis-aldehyde (2c) cis-aldehyde (2c) pyran (2b) pyran (2b) ring-aldehyde (2d) ring-aldehyde (2d) ring-aldehyde (2d) pyran (2b) pyran (2b) ring-aldehyde (2d) Fc−O (2e) Fc−O (2e) cis-aldehyde (2c) cis-aldehyde (2c) ring-aldehyde (2d) pyran (2b) cis-aldehyde (2c) cis-aldehyde (2c) cis-aldehyde 2c pyran (2b) pyran pyran pyran pyran

18

18

O shift (cm−1)

calculated (cm−1)

1079(0.23) 1117(0.10) 1142(0.24) 1167(0.07) 1248(0.03)*

−5 −4 −7 −3 −5

1100(14) 1100(14) 1144(6) 1173(22) 1236(2)

−4 −4 −2 −7 −5

pyran (2b) pyran (2b) pyran (2b) pyran (2b) ring-aldehyde (2d)

1264(0.04) 1322(0.06)*

0 0

1323(139)

−3

ring-aldehyde (2d)

1361(0.09) 1372(Si−O−FeCp plus C5H6(g). It is reasonable to postulate that the chemisorption of ferrocene on hematite occurs by the similar mechanism shown in reaction 3. The oxidation step, reaction 4 in this scheme, is complex and must certainly occur through multiple substeps. It involves 10 mol of electrons transferring to chemisorbed oxygen, 2 mol from the oxidation of 2 mol of iron(II) to iron(III), and 8 mol from the oxidation of 2 mol of cyclopentadienyl anions to cyclopentadienone and water. The detailed mechanism of this complex redox reaction remains unknown. Finally, the overall process represented in reaction 6 illustrates the true autocatalytic nature of the reaction. All the water and surface sites consumed in reactions 1−3 are regenerated in reactions 4 and 5, and the overall product Fe2O3(s) provides additional catalytic surfaces. The gas-phase products of the overall reaction 6, cyclopentadiene (C5H6) and cyclopentadienone (C5H4O), re-enter the gas stream that still contains excess ozone, and they are subject to further reaction by gas-phase ozonolysis on their way to the cold window. Previous work in our laboratory32 focused on the reaction of ozone with cyclopentadiene and identified early intermediates and stable products of this reaction. The results obtained in the present work are consistent with these earlier results. Reaction 6 represents the creation of 30 mol of unpaired electrons (3 mol of Fe2O3 contain 6 mol of high-spin iron(III) ions each with five unpaired electrons). Reaction 1 taken five times generates 30 mol of unpaired electrons because 5 mol of ozone adsorbs to the catalyst surface as 15 mol of O (3P) atoms, each with two unpaired electrons. Thus, the oxidation reaction 4 occurs with conservation of electron spin.

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SUMMARY AND CONCLUSIONS The observation of a dark-green-colored matrix deposit and infrared peaks attributable to perturbed ozone upon twin-jet (TJ), dark-deposited mixtures of argon, ozone, and ferrocene provides evidence for the formation of a Cp2Fe−O3 chargetransfer complex isolated in an argon matrix. However, it could not be determined whether this complex formed in the gas J

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oxidation products of the surface-catalyzed reactions between the adsorbed reactants. 8. The products of the surface-catalyzed reaction of ozone with ferrocene were identified from the infrared bands observed in the argon matrix. These products included cyclopentadiene (C5H6), cyclopentadienone (C5H4O), 3-butenal (C 4 H 6 O), 2-butenal (crotonaldehyde) (C4H6O), γ-butyrolactone (C4H6O), 3-butenoic acid (C4H6O2), 2-butenoic acid (C4H6O2), acetic acid (C 2 H 4 O 2 ), formic acid (CH 2 O 2 ), acetaldehyde (C2 H4 O), formaldehyde (CH2 O), carbon dioxide (CO2), carbon monoxide (CO), and water (H2O).

ASSOCIATED CONTENT

S Supporting Information *

Supporting Information includes the full citation for ref 20, additional infrared spectra from twin-jet deposition and red photolysis experiments of ozone and ferrocene isolated in an argon matrix showing more evidence of red (λ ≥ 600 nm) lightinitiated photochemistry, and additional spectra from the long path (121 in.) merged-jet experiment showing more evidence of the surface-mediated thermal reaction of ozone with ferrocene. Further, an additional energy-level diagram and calculated molecular structures show high activation energies for two direct, thermal gas-phase reactions of ozone with ferrocene and help explain why the thermal gas-phase reaction was not observed. Finally, infrared spectra of argon matrixisolated ferrocene and deuterated ferrocene are given. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*B. S. Ault. Phone: 513-556-9238. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Hairong Guan and Anna Gudmundsdottir for helpful discussions. The authors also thank the National Science Foundation for funding under Grant CHE 1110026. Finally, R.W.K. is grateful to Saint Mary’s University of Minnesota for sabbatical funding during the 2011−2012 academic year.

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REFERENCES

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