Organometallics 1995, 14, 992-999
992
Gas-Phase Reactivity of Lanthanide Cations with Hydrocarbons Hans H. Cornehl, Christoph Heinemann, Detlef Schroder, and Helmut Schwarz" Institut fur Organische Chemie der Technischen Universitat Berlin, 0-10623 Berlin, FRG Received August 17, 1994@ The gas-phase reactions of all lanthanide cations Ln+ (Ln = La-Lu, with the exception of Pm+)with several linear, branched, and cyclic alkanes, cyclopropane, and alkenes have been examined by Fourier transform ion cyclotron resonance mass spectrometry. This series of substrates allows to evaluate estimates for the relative reactivities of Ln+ cations with respect to C-H and C-C bond activation of hydrocarbons. None of the Ln+ cations was found to react with methane, in accord with the unfavorable thermochemical situation for formation of a cationic carbene complex LnCHZ+ from Ln+ and CH4. Very slow single dehydrogenation of ethane is observed for La+ and Ce+. All acyclic alkanes larger than ethane, as well as cyclopropane and cyclohexane, are only activated by La+, Ce+, and Gd+, and the reaction rates approach the collisional limit with increasing polarizability. The nonreactivity of all other lanthanide cations toward alkanes provides experimental support for Schilling and Beauchamp's suggestion that a minimum of two non-f valence electrons is required for the activation of C-H or C-C bonds. In addition to La+, Ce+, and Gd+, Pr+ and Tb+, the two of the 6s14P" configurated lanthanide cations with the lowest excitation energies to states with at least two non-f valence electrons, also activate propene but are unreactive with cyclopropane. The occurrence of C-H bond activation of propene by Pr+ and Tb+is described in terms of a curve-crossing model in which an electronically excited asymptote of a state with two non-f electrons is involved en route to the products. With 1-butene also Nd+, Dy+, Ho+, and Er+ cations mediate dehydrogenation, and only Sm+, Eu+, Tm+, and Yb+ are unreactive with this substrate; these are precisely those lanthanide cations which exhibit the largest excitation energies to states with at least two non-f electrons. Furthermore, the relative rates for the iodmolecule reactions are in qualitative agreement with the curvecrossing model proposed for the reaction of Pr+ and Tb+ with propene. Finally, with 1,4-cyclohexadiene as substrate even Sm+, Eu+, and Tm+ mediate C-H activation to yield the corresponding benzene/ln+ complexes.
Introduction The long-neglectedlanthanide elements have become of considerable importance in many areas of modern techno1ogy.l Subjects as diverse as heterogeneous catalysis, superconductivity, or advanced materials for optical, electronic, magnetic, and biomedical applications benefit from the unusual properties found among these elements. Parallel to the increasing industrial use, the organometallic chemistry of lanthanide elements developed as a timely and fascinating research topic2 As hallmarks, we mention the remarkable activity of organometallic lanthanide complexes in the activation of methane,3 the polymerization of olefin^,^ and the development of highly volatile organolanthanide
complexes with perspectives for chemical vapor deposit i ~ n . Furthermore, ~ a variety of organolanthanide complexes can be used as efficient and selective catalysts and, therefore, became powerful tools in synthetic organic chemistry.6 Within the last decade, thorough and extensive investigationson the gas-phase chemistry of "bare" and ligated transition-metal ions brought about a comprehensive body of information with respect to reaction mechanisms of metal-mediated C-H and C-C bond activation and the correlation of the intrinsic reactivities of the metal ions with the corresponding electronic structures.' While there has been some work8 on the gas-phase chemistry of La+, it is somewhat surprising that the other lanthanide ions Ln+ (in this study, this term denotes all 4f elements from cerium to lutetium
Abstract published in Advance ACS Abstracts, November 15,1994. (1)(a)Industrial Applications ofRare Earth Elements; Gschneider, R A,, Jr., Ed.; American Chemical Society: Washington, DC, 1981. (b) 2nd International Conference on f-Elements; University of Helsinki, Aug 1-6, 1994. (2) (a)Marks, T. J.; Ernst, R. D. In Comprehensive Organometallic Chemistry; Wilkinson, G.; Stone, F. G. A., Ebel, E. W., Eds.; Pergamon Press: Oxford, U.K., 1982; Chapter 21. (b) Schumann, H.; MeeseMarktscheffel, J. A.; Esser, L. Chem. Rev., submitted for publication. (3) (a)Watson, P. L.; Parshall, G. W. Acc. Chem. Res. 1986,18,51. (b) Otsura, K.; Shimizu, Y.; Komatsu, T. Chem. Lett. 1987, 1835. ( c ) Ashcroft, T.; Cheetham, A. K.; Foord, J. S.; Green, M. L. H.; Grey, C. P.; Murrell, A. J.: Vernon, P. D. F. Nature 1990,344,319. (d) Watson, P.L. In Selective Hydrocarbon Activation; Davies, J. A., Watson, P. L., Greenberg, A., Liebman, J. F., Eds.; VCH: New York, 1990. Capitan, M. J.; Malet, J.; Centeno, M. A,; Munoz-Paez, A,; Carrizosa, I.; Odriozola, J. A. J.Phys. Chem. 1993, 97, 9223.
(4)(a) Jeske, G.; Lauke, H.; Mauermann, H.; Swepston, P. N.; Schumann, H.; Marks, T. J. J.Am. Chem. SOC. 1985,107,8091. (b) Jeske, G.; Schock, L. E.; Swepston, P. N.; Schumann, H.; Marks, T. J . J. Am. Chem. Soc. 1985, 107, 8103. ( c ) Jeske, G.; Lauke, H.; Mauermann, H.; Schumann, H.; Marks, T. J. J.Am. Chem. SOC.1986, 107, 8111. (5)For recent reports in this field, see: (a) Herrmann, W. A.; Anwander, R.; Denk, M. Chem. Ber. 1992, 125, 2399. (b) Benelli, C.; Caneschi, D.; Gatteschi, D.; Sessoli, R. Adv. Mater. 1992, 4 , 504. ( c ) Barnhart, D. M.; Clark, D. L.; Huffman, J. C.; Vincent, R. L.; Watkin, J. G. Inorg. Chem. 1993, 32, 4077. (d) Bradley, D. C.; Chudzynska, A.; Hurthouse, M. B.; Motevalli, M.; Wu, R. Polyhedron 1994, 13, 1. (e) For an excellent overview, see: Anwander, R. PhD Thesis, Technische Universitat Miinchen, 1993; and references cited therein. (6) For a recent review, see: Molander, G. A. Chem. Reu 1992,92, 29.
@
0276-733319512314-0992$09.00/0
0 1995 American Chemical Society
Organometallics, Vol. 14,No. 2, 1995 993
Reactivity of Lanthanide Cations with Hydrocarbons as well as lanthanum itself) have scarcely been addressed yet.g In the most comprehensive study carried out so far, Schilling and Beauchamp investigated reactions of Pr+, Eu+, and Gd+ with several hydrocarbons A general concluand oxygen-contqining sion from this work was that only lanthanide cations possessing a t least two non-f valence electrons in their electronic ground state configuration give rise t o C-H and C-C bond activation processes. In this paper, we present a general survey of gasphase reactions of all lanthanide cations Ln+ (Ln = LaLu, except for Pm which does not have a stable isotope). Several linear and branched aliphatic and alicyclic alkanes as well as the unsaturated hydrocarbons propene, 1-butene, and l,4-cyclohexadiene serve as substrates to probe the occurrence of C-H and C-C bond activation processes. The implication of these particular substrates is to study the relative reactivity of Ln+ cations with respect to C-H and C-C bond activation, while lowering the kinetic and thermodynamic restrictions associated with these processes. For example, the activation of methane, a small symmetrical molecule with low polarizability and four equally strong bonds should be difficult, whereas on the other hand dehydrogenation of 1,4-cyclohexadiene is expected to be rather facile, since its allylic C-H bonds are much weaker as compared to those of methane and the concomitant formation of a benzene molecule increases the thermodynamic driving force for the overall process.
Experimental Section The experiments were performed using a Spectrospin-CMS47X Fourier transform ion cyclotron resonance mass spec-
trometer; the instrument and its operational details have been described elsewhere.1° Lanthanide ions Ln+ were generated by laser desorptiodaser ionization’l in the external ion source by focusing the beam of a Nd:YAG laser (Spectron Systems; ,A = 1064 nm) onto pure metal pieces (’99%; Dy, Tb, Tu, and Yb from Heraeus; La, Ho, Er, Eu, Gd, Lu, Nd, Pr, and Sm from Strem Chemicals). Most of the targets need not be stored under protective conditions, since the surface oxide layer is easily removed in the laser ablation process under the conditions of ion generation. However, Ce and Eu suffer from fast oxidation upon exposure to air. Therefore, a flintstone of a conventional lighter was used as a Ce sourcegd(ca. 70%Ce; undesired isotopes of Fe, La, Pr, Nd, and Gd were ejected from (7) Recent reviews: (a) Armentrout, P. B. Annu. Reu. Phys. Chem. 1990, 41, 313. (b) Martinho SimBes, J. A.; Beauchamp, J. L. Chem. Rev. 1990, 90, 629. (c) Eller, K.; Schwarz, H. Chem. Rev. 1991, 91, 1121. (d) Weishaar, J. C. ACC.Chem. Res. 1993,26, 213. (8) (a) Huang, Y.; Wise, M. B.; Jacobsen, D. B.; Freiser, B. S. Organometullics 1987,6,346. (b) Hettich, R. L.; Freiser, B. S. J . Am. 1987,109,3543. (c) Sunderlin, L. S.; Armentrout, P. B. J . Chem. SOC. Am. Chem. SOC. 1989, 111, 3845. (d) Huang, Y.; Hill, Y. D.; Freiser, 1991, 113, 840. (e) Ranasinghe, Y. A.; B. S. J . Am. Chem. SOC. MacMahon, T. J.; Freiser, B. S. J . Am. Chem. SOC. 1992,114,9112. (0 Ranasinghe, Y. A.; Freiser, B. S. Chem. Phys. Lett. 1992,200, 135. (9) (a) Schilling, J. B.; Beauchamp, J. L. J . Am. Chem. SOC.1988, 110, 15. (b) Reference 8c. (c) Azarro, M.; Breton, S.; Decouzon, M.; Geribaldi, S. Int. J.Mass Spectrom. Zon Processes 1993, 128, 1. (d) Heinemann, C.; Schroder, D.; Schwarz, H. Chem. Ber. 1994,127,1807. (e) Yin, W. W.; Marshall, A. G.;Marqalo, J.; Pires de Matos, A. J . Am. Chem. Soc., in press. (0For activation of carbon-fluorine bonds by “bare” and ligated Pr+ cations, see: Heinemann, C.; Goldberg, N.; Tornieporth-Oetting, I. C.; Klapotke, T. M.; Schwarz,H. Angew. Chem., in press. (10) (a) Eller, K.; Schwarz, H. Int. J . Muss Spectrom.Zon Processes 1989,93,243. (b) Eller, K.; Zummack, W.; Schwarz, H. J . Am. Chem. SOC. 1990, 112, 621. (11) (a) Freiser, B. S. Talanta 1985,32,697. (bj Freiser, B. S. Anal. Chim. Acta 1985,178, 137.
the ICR cell as described below), and the Eu target was permanently stored under viscous mineral oil. After being generated, the ions were extracted from the source and via a system of electrostatic potentials and lenses transferred into the cylindrical ICR cell, which is located in the field of a superconductingmagnet (Oxford Instruments, maximum field strength 7.05 T). Then, the respective elements’ most abundant isotope was isolated using FERETS,12 a computercontrolled ejection protocol which combines frequency sweeps and single-frequency pulses to optimize ejection of all undesired ions by resonant excitation. Prior t o any chemical reaction, Ln+ions were thermalized by allowing them to collide with repeatedly pulsed-in argon (maximum pressure ca. 5 x mbar, maximum pulse time ca. 20 ms, ca 100 collisions per ion). This procedure serves for the cooling of the translational energy of the ions t o the temperature of the collision gas (300 K); furthermore, if the Ar/Ln+ interaction potentials allow for an appropriate relaxation mechanism, depopulation of electronically excited states also takes place. Thorough thermalization of the ions was evaluated by the reproducibility of the reaction kinetics as well as the nonoccurrence of endoergic rea~ti0ns.I~ Reactants were admitted to the cell via mbar, as a leak valve at a stationary pressure of (2-8) x measured by an uncalibrated ion gauge (BALZERS IMG070). Rate constants were determined from the pseudo-first-order decay of the reactant ions and are reported as percentage of ) an estimated error of the theoretical collision rate ( K ~ o with f 30%.14 Since we cannot exclude that electronically excited Ln+*ions are not completely quenched by multiple collisions with argon, each reaction was followed until the reactant ion intensity accounted for less than 20% of the sum of all ion intensities in order to ensure that mainly ground state Ln+ ions were involved in the process of interest. Unfortunately, most of the Ln+ ions (Ln = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, and Er) undergo fast side reactions with oxygen sources present as contaminants in the background of the machine (Le. mainly HzO and 0 2 ) . In those cases where the undesired processes accounted for more than 5% of the products, the overall reaction rates were multiplied by the initial quota of C-H- and C-C-bond activation to obtain the correct rates for these processes.15 If reactions of interest were not observed, an upper bound for the overall reaction rate can be derived from the pressure of the reactant gas, the reaction time, and the signal-to-noise-ratio. Due to side reactions with oxygen-containingcompounds, in the present study the upper bound is relatively high as compared t o previous studies from cm3 molecule-’ our laboratory and amounts t o ca. 5 x S-1.
Results and Discussion An overview of the observed C-H- and C-C-bond activations of alkanes induced by the Ln+ cations La+, Ce+, and Gd+ as well as branching ratios and relative reaction rates are given in Scheme 1 and Table 1. None of the lanthanide cations was found to react with methane. The energetically least demanding activation of methane by a bare Ln+ cation would lead to the corresponding carbene complex via dehydrogenation (Scheme 1).l6The minimum metal-methylene cation bond dissociation energy (BDE) required for the formation of MCH2+ H2 from M+ CHI amounts to
+
+
(12) Forbes, R. A.; Laukien, F. H.; Wronka J. Znt. J . Mass Spectrom. Ion Processes 1988, 83, 23. (13)For example, see: Schroder, D.; Fiedler, A.; Ryan, M. A,; Schwarz, H. J . Phys. Chem. 1994,98, 68. (14) (a) For ADO theory, see: Su, T.; Bowers, M. T. Int. J . Muss Spectrom. Zon Phys. 1973, 12, 347. (b) For calibration, see: Lin, Y.; Ridge, D. P.; Munson, B. Org. Mass Spectrom. 1991, 26, 550. (15)Blum, 0.; Stockigt, D.; Schroder, D.; Schwarz, H. Angew. Chem., Int. Ed. Engl. 1992, 31, 603.
994 Organometallics, Vol. 14,No. 2, 1995
Ln'
t
CH,
Ln' + C,H,
Ln'
t
-
C,H,
Scheme 1
Ln++ CC,H,
Ln'
t
E
E
c-C,H,-
Ln=
La
Ce
Gd
Ha
100
100
-
LnC,H,'+
H,
60
80
70
LnC,H,'+
ZH,
30
20
30
C,H,
10
LnC,H,'+
H,
30
20
35
LnC.H,'+
ZH,
70
70
45
LnC,H,'+
C,H,
