Organometallics 1996, 15, 345-349
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Gas Phase Chemistry of Bis(pentamethylcyclopentadienyl)samarium Joaquim Marc¸ alo* and Anto´nio Pires de Matos Departamento de Quı´mica, Instituto Tecnolo´ gico e Nuclear, P-2686 Sacave´ m Codex, Portugal
William J. Evans* Department of Chemistry, University of California at Irvine, Irvine, California 92717 Received June 8, 1995X
The gas phase chemistry of bis(pentamethylcyclopentadienyl)samarium, (C5Me5)2Sm, was studied by Fourier transform ion cyclotron resonance mass spectrometry (FTICR/MS). Positive electron impact (EI) spectra showed the formation of (C5Me5)2Sm+, (C5Me5)Sm+, and Sm+. All three ions reacted with (C5Me5)2Sm by charge transfer, as verified by doubleresonance techniques, and (C5Me5)Sm+ also formed the (C5Me5)3Sm2+ ion in a condensation reaction with neutral (C5Me5)2Sm. The laser desorption/ionization (LDI) spectra showed, in addition to (C5Me5)2Sm+, (C5Me5)Sm+, and Sm+, the formation of (C5Me4H)Sm+ and (C5Me4CH2)Sm+. The latter species most probably involves a tetramethylfulvenide ligand. Access to all of the ionic species cited here could also be obtained by reacting laser-desorbed Sm+ ions with pentamethylcyclopentadiene, C5Me5H. (C5Me4CH2)Sm+, (C5Me4H)Sm+, and (C5Me5)Sm+ were formed as primary products, and the metallocene ion (C5Me5)2Sm+ resulted from the rapid addition of C5Me5H to (C5Me4CH2)Sm+. Introduction Recently, considerable attention has been given to the previously unexplored gas phase chemistry of lanthanide cations. Gas phase reactions of lanthanide ions with arenes,1,2 alkanes,3,4 and alkenes3-5 have been reported and suggest that the reactivity along the lanthanide series is strongly dependent on the electronic configuration of each lanthanide ion and the magnitude of the promotion energies to reactive configurations. In an effort to further explore the chemistry of organolanthanide species and to begin to connect the gas phase results with condensed-phase chemistry, we have conducted FTICR/MS6-8 studies of the gas phase ion chemistry of the Sm/C5Me5 system. This system was chosen because (C5Me5)2Sm,9-11 with its strong reduction potential and unusual “bent sandwich” structure, has been shown to have a rich chemistry: it can Abstract published in Advance ACS Abstracts, November 15, 1995. (1) Yin, W. W.; Marshall, A. G.; Marc¸ alo, J.; Pires de Matos, A. J. Am. Chem. Soc. 1994, 116, 8666. (2) Leal, J. P.; Marc¸ alo, J.; Pires de Matos, A.; Marshall, A. G.; Yin, W. W.; Spirlet, J.-C. Proceedings of the 2nd European FTMS Workshop, Antwerp 1993; Van Vaeck, L., Ed.; University of Antwerp (UIA): Antwerp, Belgium, 1994; p 312. (3) Schilling, J. B.; Beauchamp, J. L. J. Am. Chem. Soc. 1988, 110, 15. (4) Cornehl, H. H.; Heinemann, C.; Schro¨der, D.; Schwarz, H. Organometallics 1995, 14, 992. (5) Heinemann, C.; Schro¨der, D.; Schwarz, H. Chem. Ber. 1994, 127, 1807. (6) Marshall, A. G.; Verdun, F. R. Fourier Transforms in NMR, Optical, and Mass SpectrometrysA User’s Handbook; Elsevier: Amsterdam, 1990. (7) Marshall, A. G.; Grosshans, P. B. Anal. Chem. 1991, 63, 215A. (8) Freiser, B. S. In Techniques for the Study of Ion Molecule Reactions; Farrar, J. M., Saunders, W. H., Jr., Eds.; Wiley: New York, 1988; p 61. (9) Evans, W. J.; Hughes, L. A.; Hanusa, T. P. J. Am. Chem. Soc. 1984, 106, 4270. (10) Evans, W. J.; Hughes, L. A.; Hanusa, T. P. Organometallics 1986, 5, 1285. (11) Evans, W. J.; Bloom, I.; Hunter, W. E.; Atwood, J. L. J. Am. Chem. Soc. 1981, 103, 6507. X
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bind substrates and effect reactions that traditionally were not thought to be possible with lanthanide compounds.12-14 As a result, extensive chemistry has developed for compounds containing the Sm/C5Me5 combination, which provides valuable data useful in interpreting the gas phase chemistry observed. In addition, since (C5Me5)2Sm is volatile, gas phase studies can be conducted starting from a molecular precursor, i.e., from (C5Me5)2Sm, as well as from the combination of metal-derived Sm+ with pentamethylcyclopentadiene. We report here the electron impact (EI) and laser desorption/ionization (LDI) spectra of (C5Me5)2Sm and connect them with the reactivity of Sm+ with C5Me5H. These experiments show how ligated samarium ions are formed in the gas phase and how the initially formed species react. Experimental Section All FTICR/MS experiments were performed with an FT/MS (Madison, WI) Model 2001-DT spectrometer, equipped with a 3.0 T superconducting magnet and interfaced to a SpectraPhysics Quanta-Ray GCR-11 pulsed Nd:YAG laser operating at the fundamental wavelength (1064 nm). The air-sensitive, volatile complex (C5Me5)2Sm9,10 was prepared by the published procedure and transferred to the automatic solids probe of the instrument under argon. EI spectra were obtained at a probe temperature of 120 °C, while no heating was used for LDI spectra. The samarium metal ions were produced by LDI of a commercially obtained pure metal piece mounted on the solids probe. C5Me5H, predried with CaH2, was introduced into the spectrometer through a leak valve. All experiments were carried out on the “source” side of the dual ion trap of the instrument. Neutral pressures, (12) Evans, W. J. Polyhedron 1987, 6, 803. (13) Evans, W. J.; Keyer, R. A.; Rabe, G. W.; Drummond, D. K.; Ziller; J. W. Organometallics 1993, 12, 4664 and references cited therein. (14) Schaverien, C. J. Adv. Organomet. Chem. 1994, 36, 283.
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measured by an ion gauge, were in the range of 3 × 10-8 to 3 × 10-7 Torr. Reaction pathways were established by double-resonance methods, and the isolation of the desired ions was achieved with the SWIFT technique.15 Rate constants were determined by observing the pseudo-first-order decay of the reactant ion intensities as a function of time at constant neutral reagent pressure. Pressure calibration was made by using the reaction of CH3COCH3+ with CH3COCH316 and included corrections for the ionization efficiencies of the neutral reagents;17 the molecular polarizability of C5Me5H was estimated by the method of Miller.18 For the experiments involving (C5Me5)2Sm, the Langevin rate constant (kL) estimated by Richardson and coworkers19a for the reaction couple (C5Me5)2Fe+/(C5Me5)2Fe (1.1 × 10-9 cm3 molecule-1 s-1) was used to obtain estimates of the molecular polarizability and ionization efficiency of (C5Me5)2Fe, which were then used for (C5Me5)2Sm. As a further test of the pressure calibration procedure in the case of samarocene, a few experiments with ferrocene were performed and were satisfactorily checked against reported kinetic data on the gas phase chemistry of this metallocene.19,20 In spite of the preceding precautions, we believe that the uncertainties in the pressure calibration procedure can lead to errors in the absolute rate constants that may be higher than 50% in the case of the experiments involving samarocene; in the case of the reactions of Sm+ ions with C5Me5H, the errors are probably lower. The relative rate constants for each type of experiment have estimated errors of (20%. Collisional cooling of the reactant ions was performed in all cases and involved 0.5-1 s collisional periods with argon introduced into the vacuum chamber through General Valve Corporation Series 9 pulsed solenoid valves to maximum pressures of ∼10-6 Torr. Collision-induced dissociation (CID) experiments were carried out by excitation of the ions in question to lab-frame energies of ∼10-50 eV and pulsed-valve introduction of argon.
Results and Discussion Electron Ionization and Positive Ion Chemistry of (C5Me5)2Sm. Positive EI spectra of (C5Me5)2Sm at 3 × 10-8 to 1 × 10-7 Torr and 70 eV electron beam energies showed the formation of (C5Me5)2Sm+, (C5Me5)Sm+, and Sm+ as major products and (C5Me4H)Sm+, SmCH3+, SmH+, and C5Me5H+ as minor products (
