Laser Chemistry of Organometallics - American Chemical Society

Chapter 9

UV Photodissociation and Energy-Selective Ionization of Organometallic Compounds in a Molecular Beam

Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: June 8, 1993 | doi: 10.1021/bk-1993-0530.ch009

Jeffrey A. Bartz, Terence M. Barnhart, Douglas B. Galloway, L. Gregory Huey, Thomas Glenewinkel-Meyer, Robert J. McMahon, and F. Fleming Crim Department of Chemistry, University of Wisconsin—Madison, Madison, WI 53706

This chapter introduces the application of molecular beams, ultraviolet lasers, energy-selective ionization, and time-of-flight mass spectrometry to the general study of the Laser Chemistry of Organometallics. It briefly contrasts energy-selective vacuum ultraviolet ionization and time-of-flight mass spectrometry with other laser methods used in investigating the photodecomposition of organometallic molecules. After a description of the experimental apparatus, a summary of results from photodissociation studies of 5

dicarbonyl(η-cyclopentadienyl)(1-propyl)iron appears. LASERS AND ORGANOMETALLIC MOLECULES Lasers can provide useful information about the primary photodissociation pathways of organometallic compounds. Techniques such as chemical trapping, transient infrared spectroscopy, and mass spectrometry explore the photolyses of organometallic molecules in a "solvent-free" environment. These gas phase investigations generate information on energy disposal andfragmentationpathways for volatile organometallic systems. A notable example of early work on the gas phase photochemistry of organometallic molecules is that of Yardley and coworkers (1,2). They used PF to trap the fragmentsfromthe 248-nm photodissociation of Fe(CO). They found a distribution of coordinatively unsaturated iron carbonyl fragments, but, because collisions between different iron carbonylfragmentsrandomize the extent of unsaturation, they did not unambiguously determine the detailed distribution of the initial fragments. Direct spectroscopic methods can also detect the primary photodissociationfragmentsfromorganometallic compounds. Weitz and coworkers studied the photodissociation of Fe(CO) bytime-resolvedinfrared spectroscopy (TRIS) (3,4) and observed the CO stretchingfrequenciesoffragmentsformed by ultraviolet (UV) laser photolysis. They detected the same fragments trapped by Yardley and coworkers (1,2). Another method for detecting productsfromthe laser photodissociation of organometallic molecules is mass spectrometry using electron impact or multiphoton 3

5

5

0097-6156/93/0530-0116$06.00/0 © 1993 American Chemical Society Chaiken; Laser Chemistry of Organometallics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

9.

BARTZ E T AL.

UV Photodissociation and Ionization

ionization. The ions observed in a mass spectrum help identify the primary photodissociation pathways of the molecules in a collision-free environment. As one example, Mikami et al. (5) used a molecular beam, multiphoton ionization (MPI), and time-of-flight mass spectrometry (TOFMS) to detect metal atoms and other fragments from the photodissociation of a series of metal acetylacetonates. Multiphoton ionization is a useful technique for identifying products, although many of the species come from secondary photolyses. Thus, the observed masses reflect more than just the primary photodissociation event. A more conventional ionization method, electron impact, can detect all products from a photodissociation of an organometallic compound. For example, Vernon and coworkers studied the 248-nm photodissociation of Fe(CO) (6), Z n ( C H ) (7) and M ( C O ) (8) (M = Cr, Mo, W) by electron impact mass spectrometry. The typical electron energies used for mass spectrometry both ionize the primary photodissociation products and extensively fragment those same ions. We have implemented an energy-selective method that avoids many of the complications of secondary dissociations in the ionization step of mass spectrometry detection. We use vacuum ultraviolet (VUV) ionization and TOFMS to identify the primary fragments from the photodissociation of a variety of molecules, including organometallic compounds. This method is energy-selective as it ionizes only the chemical species with ionization potentials of 9.9 eV or lower, which for most of them is near their ionization threshold. Because these ions have little excess energy from the V U V photon, they undergo secondary dissociations less extensively compared to those formed by MPI or electron impact ionization. Our approach is similar to the single photon ionization work of Van Bramer and Johnston (9), although we generate our V U V light differently. V U V ionization coupled with U V laser photodissociation and TOFMS is a valuable tool for determining decomposition pathways in a collision-free environment and is a means of identifying the important intermediates in chemical vapor deposition or in condensed phase photolysis. 5


117

2

5

2

6

METHOD Figure 1 shows a portion of our experimental molecular beam apparatus, which has two differentially pumped chambers separated by a skimmer. The organometallic compound resides in a sample holder, inside the molecular beam source chamber, approximately 5 cm behind the nozzle of the pulsed valve. Thermal coaxial cable heats this holder, which has an inlet for the external carrier gas, He, and an outlet that connects to the nozzle. Typically, the source chamber operates at a temperature of 80 °C and the carrier gas pressure is about 300 torr. The nozzle pulses at 20 Hz and produces 1 us-long bursts of gas. The valve sits in an aluminum holder, wrapped in thermal coaxial cable. Typically, the pulsed valve operates at 90 °C, a higher temperature than the sample holder, to reduce condensation or sublimation of the organometallic compound in the nozzle assembly. The seeded carrier gas travels out of the nozzle and expands, producing a molecular beam that passes through a skimmer and travels into the interaction region, about 3 cm from the nozzle. In the interaction region, 280-nm U V light crosses the beam of organometallic molecules and photolyzes it. After a short time delay (0-1000 ns), 125-nm V U V light ionize the resulting photofragments. An extraction field accelerates the ions into the field-free region of a TOFMS where they separate by mass. To remove any contributions from either the U V or V U V photons alone, the mass spectrum presented later is the result of subtracting the UV-only and VUV-only signal contributions from the signal acquired by the method described above. In general, these background contributions are negligible in the mass region presented.

Chaiken; Laser Chemistry of Organometallics ACS Symposium Series; American Chemical Society: Washington, DC, 1993.



O

M

o

o

o

O

H

w Pd o K W g

00

9.

BARTZETAL.

119

UV Photodissociation and Ionization


LASER W A V E L E N G T H S AND ENERGETIC CONSIDERATIONS The apparatus uses two separate Nd:YAG-pumped dye laser systems. The first produces U V light for photolysis (280 nm) by frequency-doubling the dye laser light. The second generates V U V light by four-wave mixing in mercury. Light from the second dye laser (625 nm) passes through a K D P crystal where a portion of the beam doubles in frequency. The two wavelengths (625 nm + 312.5 nm) travel co-linearly through a lens and into a Hg heat pipe where the combination of two 312.5-nm U V photons and one 625-nm visible photon produces a single 125-nm (9.9 eV) V U V photon through resonant four-wave mixing. The details of the heat pipe design appear elsewhere (10). As with electron impact and multiphoton ionization, small ionization cross-sections and secondary dissociations can obscure the pathways we hope to observe. Furthermore, the energy of the V U V photons limits the fragments detected to those with ionization potentials less than 9.9 eV. Published ionization potentials (11,12) and previous results from the photodissociation of 1-nitropropane (10) in this laboratory guide our assignment of primary photodissociation fragments. In the photolysis of ( n ^ 5 H ) F e ( C O ) ( C H C H O H ) described below, 9.9-eV photons ionize all the products, excluding CO, which has an ionization potential (14.0 eV) (11) greater than the energy of the V U V ionization photons (9.9 eV). 5

5

2

2

2

3

5

(ri -C5H5)Fe(CO)2(CH CH CH3) R E S U L T S 2

2

5

Solution phase photodissociation of (r| -C5H5)Fe(CO) (alkyl) 1 produces alkanes (13) and alkenes (14) (Scheme 1). The production of alkanes apparently 2

Scheme 1

Fe o c " I H

> -

R

begins with Fe-alkyl bond homolysis, and the resulting alkyl radical 3 abstracts a hydrogen atom from solvent to yield an alkane 4. The proposed primary step in the production of alkenes involves the photochemical generation of a vacant coordination site, either through C O loss to give 5 or through ring slippage. Subsequent insertion of the metal into a P-C-H bond (P-hydride insertion) produces a metal-alkene hydride complex 6. Alkane production occurs most readily in fluid solution, where geminate


120

LASER CHEMISTRY O F ORGANOMETALLICS

propyl C3H7

FeC H 5

FeC H


5

40

60

120

100

80

6

5

Mass 5

Figure 2. TOF mass spectrum of (7? -C H )Fe(CO) (CH CH CH ) following 280-nm photolysis. 5

5

2

2

2

3

radical pairs diffuse apart rapidly (13,15,16), while alkene production dominates in matrices at low temperature (14,17). The partitioning between alkane and alkene pathways is unknown eitKer in solution or in the gas phase. The measurements described here provide direct evidence for both pathways occurring in the gas phase. Figure 2 shows the mass spectrum obtained upon photodissociation of (rj C5H5)Fe(CO)2(CH CH2CH ) (1, R=CH ) at 280 nm. The presence of propyl and propene as primary photoproducts, following the absorption of one U V photon, provides strong evidence for the two separate photodissociation pathways. The products containing iron, FeCsHs from Fe-R bond homolysis and FeCsH6 from (3hydride elimination, also one-photon products, are an additional indication of two decomposition channels. Moreover, the products C5H5 and C5H6, which come from multiphoton dissociation, are also consistent with those photopathways. (To determine the number of photons required to produce any feature in the mass spetrum we measure the variation in product yield as a function of laser power.) The number of carbonyls attached to any Fe-containing intermediates following 280-nm photolysis remains unclear from our data. The detected FeCsHs and FeCsH6 ions may not correspond to the residual Fe-containing species following alkyl loss or alkene production because of excess energy from the ionizing V U V photon. With the absorption of a 9.9-eV photon, secondary loss of C O may occur for any product containing Fe-CO bonds. For example, the parent molecule 1, with an ionization potential between 7 and 8 eV (72), dissociates both carbonyls following V U V ionization. Despite these secondary dissociations, two distinct primary photodissociation pathways appear from the absorption of a 280-nm photon: Fe-alkyl bond homolysis and direct production of alkenes. These results agree qualitatively with those of condensed phase experiments (18) which produce alkenes (14,17) and, to a lesser extent, alkyl radicals (14-16). We are preparing a more detailed analysis of these data. 5

2

3

3


9.

BARTZ ET AL.

UV Plwtodissociation and Ionization

121

CONCLUSIONS The laser chemistry of organometallic compounds in a molecular beam provides clues to the mechanisms for photodecomposition of metal-containing species. The combination of U V photolysis, single-photon V U V ionization, and TOFMS, can observe many species that may also appear in solution or in laser-assisted chemical vapor deposition. In the case of dicarbonyl(r| -cyclopentadienyl)(l-propyl)iron, the measurements clearly show that both alkyl radical and alkene production are primary processes. 5


ACKNOWLEDGMENTS J.A.B. wishes to thank the U.S. Department of Education for fellowship support, and T.G.M. gratefully acknowledges the support of the Alexander von Humboldt Foundation by a Feodor-Lynen Fellowship. The Army Research Office and the National Science Foundation support this work.

Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)

Nathanson, G.; Gitlin, B.; Rosan, A. M.; Yardley, J. T. J. Chem. Phys. 1981, 74, 361-369. Yardley, J. T.; Gitlin, B.; Nathanson, G.; Rosan, A. M. J. Chem. Phys. 1981, 74, 370-378. Ouderkirk, A. J.; Weitz, E. J. Chem. Phys. 1983, 79, 1089-1091. Ryther, R. J.; Weitz, E. J. Phys. Chem. 1992, 96, 2561-2567. Mikami, N.; Ohki, R.; Kido, H. Chem. Phys. 1990, 141, 431-440. Ray, U.; Brandow, S. L.; Bandukwalla, G.; Venkataraman, B. K.; Zhang, Z.; Vernon, M. J. Chem. Phys. 1988, 89, 4092-4101. Hou, H.; Zhang, Z.; Ray, U.; Vernon, M. J. Chem. Phys. 1990, 92, 1728-1746. Venkataraman, B.; Hou, H.; Zhang, Z.; Chen, S.; Bandukwalla, G.; Vernon, M. J. Chem. Phys. 1990, 92, 5338-5362. Van Bramer, S. E.; Johnston, M. V. Anal. Chem. 1990, 62, 2639-2643. Huey, L. G. Ph. D. Thesis, University of Wisconsin - Madison, 1992. Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. Gas-Phase Ion and Neutral Thermochemistry; Supplement 1 ed.; American Chemical Society and American Institute of Physics: New

(12) (13) (14) (15) (16) (17) (18)

York, 1988; Vol. 17. Lichtenberger, D. L.; Fenske, R. F. J. Am. Chem. Soc. 1976, 98, 50-63. Alt, H. G.; Herberhold, M.; Rausch, M. D.; Edwards, B. H. Z. Naturforsch. 1979, 34b, 1070-1077. Kazlauskas, R. J.; Wrighton, M. S. Organometallics 1982, 1, 602-611. Blaha, J. P.; Wrighton, M. S.J.Am. Chem. Soc. 1985, 107, 2694-2702. Hudson, A.; Lappert, M. F.; Lednor, P. W.; MacQuitty, J. J.; Nicholson, B. D. J. Chem. Soc. Dalton Trans. 1981, 2159-2162. Mahmoud, K. A.; Rest, A. J.; Alt, H. G. J. Chem. Soc. Dalton. Trans. 198 1365. Pourreau, D. B.; Geoffroy, G. L. In Advances in Organometallic Chemistry; F. G. A. Stone and R. West, Ed.; Academic Press: New York, 1985; Vol. 24; pp

249-352.

RECEIVED

January 25, 1993


Laser Chemistry of Organometallics - American Chemical Society

Recommend Documents