J. Phys. Chem. 1994, 98, 68-70
68
Surprisingly Low Reactivity of Bare FeO+ in Its Spin-Allowed, Highly Exothermic Reaction with Molecular Hydrogen To Generate Fe+ and Water? Detlef SchrMer,* Andreas Fiedler, Matthew F. Ryan, and Helmut Schwarz* Institut flir Organische Chemie der Technischen Universitiit Berlin, D- 10623 Berlin, Germany Received: August 31, 1993; In Final Form: October 19, 1993'
+
-
+
The rate constant &R for the highly exothermic, spin-allowed reaction F e O + ( W ) H2 Fe+(6D) H 2 0 is 2 orders of magnitude smaller than the collision rate constant &L (&R = 1.6 X 10-11 cm3 molecule -l s-I; kL = 1.5 X cm3 molecule-' s-l). Labeling experiments using H D and D2 reveal that no strong intermolecular kinetic isotope effects are operative on the rate constant of this reaction. Extremely low reactivities are also observed for the reactions of Coo+ and NiO+ with H2 ( k =~ 1.2 and 2.1 X 10-12 cm3 molecule-l s-l), and there are no kinetic isotope effects within experimental error in the reactions of these metal oxides with D2 as well. The experimental data and theoretical considerations point to a reaction mechanism involving a multicentered transition structure in the rate-determining step.
SCHEME 1
Introduction The activation of C-H and C-C bonds of organic molecules by ionic metal oxides, in particular iron oxenoids,is of fundamental interest in organic chemistry as well as biochemistry.] Recently, the C-H bond activation of a series of hydrocarbons? including methane and benzene, by bare FeO+ in the gas phase3 was reported, and mechanistic insight in the elementary steps of these reactions was provided. Here, we describe experiments on the gas-phase reactions of FeO+ with molecular hydrogen. This process, which is analogous to the thoroughly studied reactions of the early transition-metal oxides MO+ (M = Sc, Ti, and V)4 with D2, is, for at least two reasons, of particular importance: (i) In thedetailed study from the Armentrout group4it is convincingly demonstrated that-in addition to reaction energetics-spin conservation is the prime factor in oxygenation processes. As the ground electronic state of FeO+ corresponds to the W state,5 one should expect that the exothermic, spin-allowed oxidation of molecular hydrogen (eq 1) is a facile process.
+
-
+
Fe0+(62+) H2 Fe'(6D) + H,O 36 kcal/mo16,' (1) (ii) If this supposition holds true, one can expect that the simple redox pair described in eq 1 forms the basis for a catalytic cycle (Scheme 1),8 in which molecular hydrogen is oxidized to water by means of FeO+, which itself is formed by reacting Fe+ with convenient oxidants, e.g., N Z O . ~Although ~ the combustion process H2 + ' / 2 0 2 H20 is highly exothermic, this reaction is, as is amply known, kinetically hampered. For the purpose of comparison we have also conducted some preliminary experiments with, in terms of electronic structure, the less properly characterized metal oxides MO+ (M = Co, Ni).
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Experimental Section The experiments were performed with a Spectrospin CMS 47X Fourier transform ion-cyclotron-resonance(FTICR) mass spectrometer; the experimental setup has been described in detail elsewhereO9In brief, the metal ions M+(M = Fe, Co. Ni) were generated by laser desorption/laser ionization by focusing the beam of a Nd:YAG laser onto a metal target. The cations were extracted from the source and transferred to the analyzer cell by a system of electric potentials and lenses. The isolation of the metal ions most abundant isotope and all subsequent isolations t Dedicated to Professor Wilhelm Pritzkow, Merseburg, on the occasion of his 65th birthday. Abstract published in Aduunce ACS Abstracts, December 1, 1993.
0022-3654/94/2098-0068%04.50/0
H2 + N 2 0
-
N2 + H 2 0 + 77.4 kcaI/mol
was performed by using FERETS (front-end-resolution enhancement using tailored sweeps).'O The metal oxides MO+ ions were generated by reacting M+with pulsed-in N2011and subsequently thermalized by collisions with pulsed-in argon gas. The degree of thermalization of, for example, FeO+was monitored by probing the rate constants of FeO+ with H2 as a function of the amount of pulsed-in argon gas and assumed to be complete if no further changein reactivity occursupon increasing the number of collisions with argonV2gGenerally, ca. 250 collisionswere sufficient to serve this purpose. In addition, the reactions were followed until all MO+ ions were consumed. Hydrogen and its isotopomers were admitted to the FTICR cell via leak valves (typical pressures ca. 5 X 1o-S mbar). The pseudo-first-order rate constants were derived from at least three independent measurements of the decays of the MO+ signalsand converted to absoluterateconstants by calibrating the ionization gauge measurements using rates of well-known ion/molecule processes.2gJ2 Due to the relatively low sensitivity of the ionization gauge towards hydrogen13 and the small magnitude of the rate constant, the error of the absolute rates amounts to 140%; however, for the relative rates in the comparison of H2, HD, and D2 the uncertainty amounts to 110% only. Double-resonance (DR)I4 experiments were performed, and endothermic ion/molecule reactions15 were induced by irradiation of the cyclotron resonance frquency of the ion of interest for a certain time. All functions of the instrument were controlled by a Bruker-Aspect 3000 minicomputer. Hydrogen (Linde AG, 99.999% purity) and deuterium (Linde AG, 99.995% purity, >99.8atom % D) were used without further purification. Hydrogen deuteride was synthesized on-line by gently warming a mixture of lithium deuteride (Janssen Chimica, >99 atom % D) and pure pentadecanoic acid. 0 1994 American Chemical Society
Reactivity of FeO+ and H2
The Journal of Physical Chemistry, Vol. 98, No. 1 , 1994 69
Results and Discussion When thermalized FeO+ is reacted with molecular hydrogen, the only reaction observed corresponds to the formation of Fe+, and on thermochemical grounds it is inferred that the neutral product formed corresponds to H2O. Although the formation of water (eq 1) is highly exothermic (Mr= -36 kcal/m016.~),the rate constant kR of this process is surprisingly small with respect to the maximum rate constant kL as derived from Langevin collision theory,16 i.e.: kR = 1.6 X lo-" cm3 molecule-' s-1; kL = 1.5 X 10-9 cm3 molecule-' s-l. Thus, approximately only 1 in 100 collisions of FeO+ with H2 results in product formation. Double resonance, i.e., continuous ejection of the adduct complex (H2)FeO+ from the ICR cell, did not affect the rate constant significantly;thus, the lifetime of the encounter complex is below the microsecond timeframe.14 As compared to the reaction rates of FeO+ with other substrates,2 the rate constant for H20 formation (eq 1) is quite small, pointing to the existence of a substantial kinetic barrier. We will discuss this aspect further below. In contrast to an earlier report,Ilb FeOH+ (eq 2) is hardly formed at all in the reaction of FeO+ with H2, provided FeO+ is properly thermalized.'' This observation is in keepingwith recent thermochemical data for this process (eq 2), which is predicted to be endothermic by 6 kcal/mol.6s7 FeO'
+ H,
-
+RH FeO+ + H 2 0 FeO+
FeOH'
+ H'
(2)
FeOH'
+ R'
(3)
(FeO)+H20
(4)
The absence of the hydrogen abstraction process (eq 2) suggests the design of a catalytic cyclic (Scheme 1) for the oxidation of H2 with a theoretically infinite turnover. However, in reality the turnover number of Fe+ to mediate the oxidation of H2 by N20 is limited due to the facile reaction of FeO+ with residual organic substrates RH (eq 3),17the slow formation of adduct complexes with background water (eq 4),'* and diffusive loss of ions from the ICR cell. In order to obtain further insight into the mechanism of water formation from FeO+ and H2, thermalized FeO+ was kinetically excited in the presence of hydrogen gas, and the product distribution was monitored as a function of the excitation energy.I5 Due to several uncertainties in the analysis of endothermic ion/ molecule reactions in an FTICR experiment, e.g., multiple collisions, discontinuousexcitation,collision-induceddissociation, etc., we refrain from a conversion of the excitation energy from the Ebb frame in a center-of-mas energy frame.15J9 However, the following qualitative trends can be derived from the analysis ofthe breakdowndiagram (Figure 1): At low excitation energies (region a), the formation of Fe+ (and concomitantly of H2O) decreases. Such behavior is expected for an exothermic ion/ molecule reaction and is rationalizedby the decreaseof the lifetime of kinetically excited encounter complexes. In addition, this observation suggests that the small efficiency of this reaction (eq 1) is not due to a situation in which the barrier associated with product formation is located near the energy of the entrance channel;rather, it points to the existenceof an entropic bottleneck. At medium collision energies (region b), formation of Fe+ (eq 1) as well as hydrogen atom abstraction to yield FeOH+ (eq 2) is observed. Indeed, within this energy regime FeOH+ formation can effective compete with that of Fe+; tht is in line with the descriptionof the hydrogen abstraction process as a continuously endothermic reaction.' At higher energies (region c) the Fe+ fragment predominates, which is due to reaction 1 as well as collision-induceddissociation of the FeO+ precursor. In addition, a signal corresponding to FeH+ arises which may point to the formation of an H-Fe+-OH intermediate analogous to the
100:
a
b
C
--
I
/ - FeOH+ 30
60
90
120
c-u+
1 so
(eV)
Figure 1. Products of the reaction of FeO+ with molecular hydrogen as a function of translational energy in the &,b scale (see text for details).
H-M+-OH speciesreported earlier for the endothermic processes of MO+ with H2 (M = Sc, Ti, V)? As the analysis of kinetic isotope effects has often served as an extremely meaningful tool to elucidate mechanistic details of gas-phase ion/molecule reactions,20we have also employed this approach in order to unravel the mechanism(s) underlying the process depicted in eq 1. On principal grounds, only interme lecular2I kinetic isotope effects can be measured in the present case since in any conceivable isotopic labeling experiment both atoms of the hydrogen molecule will end up in the water product. For the reactions of thermalized FeO+ with HD and D2 we obtain thefollowingrateconstants: FeO+/HDsystem, kR = 1.3 X 10-11 cm3 molecule-' s-1 ( k =~ 1.2 X l t 9 cm3 molecule-' s-1); FeO+/ D2 system, k~ = 1.1 X 10-11 cm3 molecule-' s-1 ( k =~ 1.1 X 10-9 cm3 molecule-' s-l). Obviously, the kinetic isotope effects associated with the formation of HDO and D2O from HD and D2 are extremely small, if there are any within the experimental error. This is at first sight surprising, as in these reactions the operation of primary kinetic isotope effects is self-evident. The absence of significant isotope effects on the rate constants can be rationalized in a straightforward manner. While the intrinsic primary kinetic isotope effect will slow down the rates for the reactions of FeO+ with HD and D2, the lifetimes of the encounter complexes will increase by substaitution of D for H,a such that both opposing effects almost cancel each other. Thus, in the simple reaction of FeO+ with molecular hydrogen, the analysis of the inremolecular kinetic isotope effects is not very conclusive except for the fact that the very small magnitudes of the kinetic isotope effects associated with HDO and D20 formations quite likely rule out any quantum mechanism tunneling.21b How can thelow reactivity of FeO+towards molecular hydrogen be explained? Selection rule arguments, which were shown to be crucial for the endothermic oxidation reactions of MO+ (M = Sc, Ti, V) with D2,4 cannot be applied, since the sextet ground state5 of FeO+ correlates with the electronic ground state of the product ion Fe+(6D), and the symmetry breaking through the approaching H2 molecule does not violate any spatial symmetry selection rules. Further, conceivable intermediates en route to products, for examplesH-Fe+-OH or Fe(H20)+, are substantially lower in energy than the entrance channel33 and the electronic state of these species also corresponds to a sextet.24 Thus, the reactive coordination of the hydrogen molecule to FeO+ is most likely the origin of the barrier of the higher exothermic oxidation of H2. From a chemical viewpoint there exist in principle three different directionalities4Js for H2 to approach a metal oxide MO+.(i) Side-one or end-on (not shown) coordination of H2 to the metal atom leads via oxidative addition to the intermediate 2 (Scheme 2, path A). As 2 formally corresponds to a high oxidation state (Fe(V)) of the metal, and in view of the electron
70 The Journal of Physical Chemistry, Vol. 98, No. 1, 1994
SCHEME 2
1-t-0-
E>ie-o
I
a
2
4
JL n-"
6
S
dmand of oxidative additionof molecular hydrogen to transitionmetal fragments,262 is not likely to play a role en route to product formation. (ii) Similarly, mechanism B to initially generate intermediate 3 can be dismissed because there is no acceptor orbital on the oxygen end of the FeO+ molecule. (iii) The most likely path, in our view, proceedsviaa multicenteredcoordination (intermediate 5) to generate 6. This intermediate enjoys two options to generate the products Fe+ and H20, in that either H20 is reductively eliminated or 6 first isomerizes to the Fe(H20)+ complex 4 from which the ligand is eventually detached. Obviously, reliable potential energy surface calculation^^^ are indicated to clarify this point and also to provide information about which stage in the overall process H2 + FeO+ 5 6 4 H20 Fe+ constitutes the bottleneck and is responsible for the unexpectedly small reaction rate of this highly exothermic, spin- and symmetry-allowed oxidation process. We have also performed some preliminary experiments using COO+and NiO+ as oxidants. These two metal oxides were chosen on the grounds that COO+has a bond dissociation energy close to that of FeO+ (75.9 kcal/mo12*for COO+versus 81.4 kcal/mo16 for FeO+), while for NiO+ the bond dissociation energy is much smaller (64.1 kcal/moP). Surprisingly,this energeticdifference is nor reflected in the rates of the reactions of these two metal oxides with H2, which give rise to the exclusive formation of H2O and M+. For COO+we obtain a quite small reaction rate ( k =~ 1.2 X 10-12cm3molecule-1 s-l), and for NiO+ the oxidation occurs with only an insignificantly faster rate ( k =~ 2.1 X cm3 molecule' s-1). Given the experimentalerror of the rate constant measurements, intermolecular kinetic isotope effects are practically nonexisting in the reactions of these metal oxides with D2O ( k ~ , / =k 1). ~ ~ Obviously, reaction energetics is also for these two metal oxides not the crucial factor, and state-of-the-art potential energy surface calculations are indicated to provide an understanding for the vanishingly small reactivity of these metal oxides in a fundamental redox process.
--
+
--
Acknowledgment. We are grateful to the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie for financialsupport,toDr. S.Haebel andDip1.-Chem. D. Stkkigt for technical assistance, and to Dr. J. HrUk and a reviewer for helpful comments. References and Notes (1) See, for example: (a) Shilov, A. E. Activurion of Sururured Hydrocurbons by Trunsirion Merhod Complexes; Reidel: Boston, 1984. (b) Ortizde Montellano, P. R. (Ed.)Cyrochrome P-450 Srrucrure, Mechanism und Biochemistry; Plenum: New York, 1986. (2) (a) SchrMer, D.; Schwarz, H. Helu. Chim. Acto 1990,73,380. (b) SchrMer, D.; Schwarz, H. Angew. Chem. Inr. Ed. Engl. 1990,29, 1431. (c) SchrMer, D.; Fiedler, A.; H d k , J.; Schwarz, H. J. Am. Chem. Soc. 1992, 114,1215. (d) SchrMer, D.;Schwarz, H. Helv. Chim. Acta 1992,75,1281. (e) S c h d e r , D.; Florencio, H.; Schwarz, H. Helv. Chim. Act 1992,75,1792. (0 S c h d e r , D.; H d k , J.; Schwarz, H. Helv. Chim. Acru 1992, 75,2215. (g) SchrMer, D. Ph.D. Thesis, Technische Universitgt Berlin, D83.1993. (h) Bccker, H.; SchrMer, D.; Zummack, W.;Schwarz, H. J . Am. Chem. Soc., submitted. (i) SchrMer, D.; Schwarz, H. A n w . Chem. Inr. Ed. Engl. 1993, 32, 1420. (j) For early work, see: Jackson, T.C.; Jacobson, D. B.; Freiser, B. S. J. Am. Chem. Soc. 1984,106. 1252. (3) For recent reviews on gas-phase transition-metal ion chemistry, scc: (a) E l k , KO; Schwarz, H. Chem. Rev. 1991,91, 1121. (b) Eller, K. Coord. Chem. Rev. 1993, 126, 93.
SchriMer et al. (4) Clemmer, D. E.; Aristov,N.; Armentrout,P. B. J.Phys. Chem. 1993, 97, 544. ( 5 ) Fiedler, A.; HruUk, J.; Koch, W.; Schwarz, H. Chem. Phys. Leu. 1993, 211, 242, and references therein. (6) A more recent value for AHdFeO+) is reported by: Loh, S. K.;Fisher, E. R.; Lian, L.; Schultz, R. H.; Armentrout, P. B. J. Phys. Chem. 1989,93, 3 159. (7) If not stated otherwise, thermochemical data were taken from: (a) Lias, S. G.; Bartmw, J. E.; Liebman, J. F.; Holms, J. L.; b i n , R. D.; Mallard, W. G. J. Phys. Chem. Ref. Dura 1988, 17, Suppl. 1. (8) For other examples of catalytic proasses in the gas-phasc, see: (a) Reference 2b,g. (b) Kappa, M.M.; Staley, R. H. J. Am. Chem. Soc. 1981, 103, 1286. (c) Buckner, S. W.;Freiser, B. S. J . Am. Chem. Soc. 1988,110, 6606. (d) Schnabel, P.; Inon, M. P.; Weil, K.G. J. Phys. Chem. 1991, 95, 9688. (e) Schnabel, P.; Irion, M.P.; Weil, K.G. Chem. Phys. Lerr. 1992, 190,255. (f) Irion, M.p. Inr. J. Muss Specrrom. Ion Processes 1992,121, 1. (g) Irion, M. P.;Schnabel, P. Be?. Bunsenges. Phys. Chem. 1992,96,1091. (h) Schnabel, P.; Irion, M. P. Ber. Bunsenges. Phys. Chem. 1992,96, 1101. (i) Schnabel, P.; Weil, K.G.; Irion, M. P. Angew. Chem. Inr. Ed. Engl. 1992, 104, 633. (9) (a) Eller, K.; Schwarz, H. Inr. J. Muss Specrrom. Ion Processes 1989,93,243. (b) Eller, K.; Zummack, W.;Schwarz, H. J. Am. Chem. Soc. 1990, 12,621. (10) Forbes, R. A.; Laukien, F. H.;Wronka, J. Inr. J. Muss Specrrom. Ion Processes 1988, 83, 23. (1 1) (a) Reference 8b. (b) Kappa, M.M.; Staley, R. H. J. Phys. Chem. 1981, 85, 942. (12) Lin, Y.; Ridge, D. P.; Munson, B. Org. Muss Specrrom. 1991, 26, 550.
(13) Bartmess. J. E.; Gaorgiadis, R. M.Vucuum 1983,33, 149. (14) Comisarow, M. B.; Grassi, V.; Parisod, G. Chem. Phys. Lor. 1978, 57, 413. (15) (a) McLuckey,S. A.; Sallans, L.; Cody, R. B.; Burnier, R. C.; Verma, S.;Freiser, B. S.; Cooks, R. G. Inr. J. Muss Specrrom. Ion Phys. 1982, 44, 215. (b) Bensimon, M.; Houriet, R. Int. J. Muss Specrrom. Ion Processes 1986, 72, 93. (c) Forbes, R. A.; Lech, L. M.; Feiser, B. S. Inr. J. Muss Specrrom. lon Processes 1987, 77, 107. (16) Su, T.; Bower, M. T. In. J. MussSpecrrom. Ion Phys. 1973,12,347. (17) Minor amounts of FeOHI+ (
