Inorg. Chem. 2006, 45, 10248−10262
Dihydrogen to Dihydride Isomerization Mechanism in [(C5Me5)FeH2(Ph2PCH2CH2PPh2)]+ through the Experimental and Theoretical Analysis of Kinetic Isotope Effects Miguel Baya,†,‡,§ Olivier Maresca,‡ Rinaldo Poli,*,† Yannick Coppel,† Feliu Maseras,‡,| Agustı´ Lledo´s,*,‡ Natalia V. Belkova,⊥ Pavel A. Dub,⊥ Lina M. Epstein,⊥ and Elena S. Shubina*,⊥ Laboratoire de Chimie de Coordination, UPR CNRS 8241 lie´ e par conVention a` l’UniVersite´ Paul Sabatier et a` l’Institut National Polytechnique de Toulouse, 205 Route de Narbonne, 31077 Toulouse Cedex, France, Departament de Quı´mica, Edifici cn, UniVersitat Auto` noma de Barcelona, 08193 Bellaterra, Catalonia, Spain, and NesmeyanoV Institute of Organoelement Compounds, Russian Academy of Sciences, VaViloV Street 28, 119991 Moscow, Russia Received July 29, 2006
The isomerization of complex [Cp*Fe(dppe)(η2-H2)]+, generated in situ by low-temperature protonation of Cp*Fe(dppe)H with either HBF4 or CF3COOH, to the dihydride tautomer trans-[Cp*Fe(dppe)(H)2]+ is irreversible and follows first-order kinetics in the −10 to +15 °C range with ∆Hq ) 21.6 ± 0.8 kcal mol-1 and ∆Sq ) 5 ± 3 eu. The isomerization rate constant is essentially independent of the nature and quantity of a strong acid. Density functional theory (DFT) calculations on various models, including the complete system at both the quantum mechanics/ molecular mechanics (QM/MM) and full QM levels, probe the relative importance of steric and electronic effects for the relative stability of the nonclassical and classical isomers and identify two likely isomerization mechanisms: a “direct” pathway involving simultaneous H−H bond breaking and cis−trans isomerization and a “via Cp” pathway involving agostic C5Me5H intermediates. Both pathways are characterized by activation energies in close correspondence with the experimental value (21.3 and 22.2 kcal mol-1, respectively). Further kinetic studies were carried out for the Cp*Fe(dppe)H + CF3COOD and Cp*Fe(dppe)D + CF3COOD systems at 273 K. The [Cp*Fe(dppe)(η2-HD)]+ complex establishes a very rapid isotope redistribution equilibrium with the η2-H2 and η2-D2 analogues. The equilibrium constant value (K ) 3.3 ± 0.3) indicates a significant equilibrium isotope effect. Simulation of the rate data provides access to the individual isomerization rate constants kHH, kHD, and kDD for the three isotopomers, yielding kinetic isotope effects: kHH/kHD ) 1.24 ± 0.01 and kHD/kDD ) 1.58 ± 0.01 (and, consequently, kHH/kDD ) 1.96 ± 0.02). The analysis of the DFT-calculated frequencies, using the [Cp*Fe(dhpe)H2]+ model system, for the [Cp*Fe(dhpe)(η2-XY)]+ isotopomers as well as transition states for the “direct” (TSdir) and “via Cp” (TSrot) pathways (X ) H, D) allowed the computation of the expected isotope effects. A comparison with the experiment strongly suggests that the mechanism occurs via the “direct” pathway for the present system, although the small difference in the calculated energy barriers suggests that the “via Cp” pathway may be preferred in other cases.
Introduction It is nowadays well established that most 16-electron LnM fragments form a dihydrogen complex LnM(η2-H2) in the * To whom correspondence should be addressed. E-mail: poli@ lcc-toulouse.fr (R.P.),
[email protected] (A.L.),
[email protected] (E.S.S.). Fax: +33-561553003 (R.P.). † UPR CNRS 8241 lie ´ e par convention a` l’Universite´ Paul Sabatier et a` l’Institut National Polytechnique de Toulouse. ‡ Universitat Auto ` noma de Barcelona. § Current address: Departamento de Quı´mica Inorga ´ nica, Instituto de Ciencia de Materiales de Arago´n, Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain. | Current address: Institute of Chemical Research of Catalonia (ICIQ), 43007 Tarragona, Catalonia, Spain. ⊥ Russian Academy of Sciences.
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initial stages of their interaction with dihydrogen.1 Subsequently, depending on the properties of the metal and ligands, an oxidative addition process may take place, leading to a dihydride, or the dihydrogen complex may remain as the stable final product. An equilibrium between the two isomeric forms may also be observed. Dihydrogen complexes are also sometimes obtained by protonating hydride precursors under kinetically controlled conditions, preceding in some cases the rearrangement to the thermodynamic classical dihydride product.2-7 For the group 8 half-sandwich systems such as (1) Kubas, G. J. Metal Dihydrogen and σ-Bond Complexes; Kluwer Academic/Plenum Press: New York, 2001.
10.1021/ic061428n CCC: $33.50
© 2006 American Chemical Society Published on Web 11/15/2006
Analysis of Kinetic Isotope Effects Scheme 1
[CpRuLL′H2]+ (Cp ) cyclopentadienyl; L ) tertiary phosphine)3,4 and iron analogues,6,7 this interconversion implies an important stereochemical change. In these systems, two different mutual arrangements of two ligands are possible: cisoid and transoid. Because there are no reports of the coexistence of dihydrogen and cis-dihydride isomers, the interconversion must entail passing from the three-leggedpiano-stool dihydrogen complex to a transoid square-based four-legged-piano-stool dihydride (Scheme 1).8 Thus, the cleavage of the H-H bond is accompanied by a cis-trans rearrangement, which involves the migration of one H atom from one side of the molecule to the opposite one. The isomerization of the dihydrogen complex [Cp*Fe(dppe)(η2H2)]+ is particularly interesting because the chelate nature of the diphosphine ligand may impose an extra obstacle to the H migration to yield the observed trans structure for the dihydride product. The isomerization kinetics have been studied by NMR spectroscopy for the cationic ruthenium dihydrogen complexes.3,9 Reported activation enthalpies range from 16 to 21 kcal mol-1, with small and negative activation entropies. The highest values have been obtained for bidentate phosphines with sterically demanding ligands.9 The small ∆Sq supports an intramolecular mechanism in which the dihydrogen ligand has lost rotational freedom in the transition state.3 A dissociative mechanism has been proposed when bulky and good electron-donor ligands are present9 but was discarded for a simpler system on the basis of kinetic evidence (no retardation effect was observed in the presence of excess ligand).3 The possibility of an intermolecular deprotonation followed by reprotonation of the neutral hydride intermediate was ruled out for the ruthenium system on the basis of the absence of an isotope scrambling for a selectively generated (HD) system.3,9 Finally, an alternative intramolecular mechanism proceeding by transfer of an H atom from the dihydrogen ligand to the Cp ligand, thereby (2) Parkin, G.; Bercaw, J. E. J. Chem. Soc., Chem. Commun. 1989, 255257. (3) Chinn, M. S.; Heinekey, D. M. J. Am. Chem. Soc. 1990, 112, 51665175. (4) Jia, G.; Lough, A. J.; Morris, R. H. Organometallics 1992, 11, 161171. (5) Papish, E. T.; Rix, F. C.; Spetseris, N.; Norton, J. R.; Williams, R. D. J. Am. Chem. Soc. 2000, 122, 12235-12242. (6) Hamon, J. R.; Hamon, P.; Toupet, L.; Costuas, K.; Saillard, J. Y. C. R. Chim. 2002, 5, 89-98. (7) Roger, C.; Hamon, P.; Toupet, L.; Rabaaˆ, H.; Saillard, J.-Y.; Hamon, J.-R.; Lapinte, C. Organometallics 1991, 10, 10451054. (8) Ontko, A. C.; Houlis, R. J. F.; Schnabel, C.; Roddick, D. M.; Fong, T. P.; Lough, A. J.; Morris, R. H. Organometallics 1998, 17, 54675476. (9) De Los Rı´os, I.; Jime´nez-Tenorio, M.; Padilla, J.; Puerta, M. C.; Valerga, P. Organometallics 1996, 15, 4565-4574.
affording an intermediate cyclopentadiene complex, followed by ring rotation and transfer back to the metal center, has also been contemplated but discarded on the basis of indirect bond energy arguments.3 A theoretical study of this isomerization process has not been tackled yet, and no reports on the transition states or a comparison of energy barriers for different mechanisms can be found. We have recently reported experimental investigations, including a stopped-flow kinetic study, of the hydrogen bonding and proton transfer to Cp*Fe(dppe)H with a variety of proton donors (HA) of different acid strength.10,11 This study has highlighted the presence of a hydrogen-bonded intermediate involving the hydride site (and not the metal site), Cp*Fe(dppe)H‚‚‚HA, the reversibility of the proton transfer step (see Scheme 2), and the need of a second proton donor molecule to trigger the proton transfer process. An accompanying theoretical investigation has mostly focused on the first step, i.e., proton transfer to the hydride ligand.11 Concerning the subsequent isomerization process, an intramolecular mechanism is suggested by an identical rate when using proton donors of different strength at the same temperature. The alternative reversible deprotonation, followed by a slower protonation at the metal site, is inconsistent with this experimental observation.10 Our attempts to carry out an Eyring analysis of this isomerization step were complicated by the reversibility of the preceding protontransfer step, which resulted in strong coupling between two key rate constants in the mathematical fitting procedure, with the first rate constant leading from the dihydrogen intermediate to the final dihydride product (k2, irreversible step) and the second one leading from the same intermediate back to the starting monohydride complex (k-1; see the Supporting Information of ref 11). In this report, we present a new experimental study that has led to the determination of the accurate rate constant and activation parameters for the isomerization process of the [Cp*Fe(dppe)H2]+ system when the counterion is either BF4- or CF3COO-. These investigations include the determination of the kinetic isotope effects (KIEs) kHH/kHD and kHD/kDD, obtained from the comparative isomerization rates of complexes [Cp*Fe(dppe)(H2)]+ (kHH), [Cp*Fe(dppe)(HD)]+ (kHD), and [Cp*Fe(dppe)(D2)]+ (kDD). To the best of our knowledge, the experimental determination of the isotope effects for the transformation of a nonclassical [(H2)/(HD)/ (D2)] to a classical [(H)2/(H)(D)/(D)2] dihydride complex is unprecedented. This is accompanied by a theoretical investigation whose purpose is 2-fold. First, it seeks to determine the subtle factors tuning the relative stability between the nonclassical dihydrogen and the classical dihydride isomers of the [(C5R5)Fe(R′2PCH2CH2PR′2)H2]+ complexes (R ) H, Me; R′ ) H, Ph, iPr), separating electronic and steric contributions. Second, it explores the intimate details of the possible intramolecular isomerization mechanisms. In particular, discrimination between two pathways showing very (10) Belkova, N. V.; Revin, P. O.; Epstein, L. M.; Vorontsov, E. V.; Bakhmutov, V. I.; Shubina, E. S.; Collange, E.; Poli, R. J. Am. Chem Soc. 2003, 125, 11106-11115. (11) Belkova, N. V.; et al. Chem.sEur. J. 2005, 11, 873-888.
Inorganic Chemistry, Vol. 45, No. 25, 2006
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Baya et al. Scheme 2
similar computed activation barriers (both close to the experimental value) will be possible through the analysis of the KIEs. The computational work involves pure quantum mechanics (QM) and mixed QM/molecular mechanics (MM) methods. Experimental Part All manipulations were carried out under an argon atmosphere by standard Schlenk techniques. Complexes Cp*Fe(dppe)H and Cp*Fe(dppe)D were synthesized according to the literature.7 NMR characterization of the deuteride complex in C6D6: 2H NMR δ -16.9 (t, JPD ) 10.4 Hz); 31P{1H} NMR δ 111.5 (t, JPD ) 10.3 Hz). In addition, the 1H NMR spectrum shows contamination by the hydride complex Cp*Fe(dppe)H (25%, by integration of the hydride resonance against the Cp* resonance). The acids HBF4‚OEt2, CF3COOH, and CF3COOD were purchased from Aldrich and used as received. The NMR studies were carried out in standard 5-mm NMR tubes containing solutions of the complexes in CD2Cl2. The 1H and 31P NMR data were collected with a Bruker AMX 400 spectrometer operating at 400.13 and 161.98 MHz, respectively. The lowtemperature kinetic experiments with 31P{1H,2H} NMR monitoring were carried out on a Bruker AV500 spectrometer equipped with a 5-mm triple-resonance inverse probe with dedicated 31P channel operating at 500.33 MHz for 1H NMR, 202.54 MHz for 31P NMR , and 76.80 MHz for 2H NMR. For the purpose of quantitative concentration measurements for the kinetic runs, the 31P NMR spectra were recorded with inverse-gated 1H and 2H decoupling. For 31P and 1H NMR, a typical pulse width corresponding to a 30° flip angle and 30-s relaxation delays was used to obtain reliable integration data. In the case of overlapping resonances, the quantitative evaluation was realized by peak deconvolution (see the Supporting Information). All chemical shifts for 1H and 2H NMR are relative to tetramethylsilane using a residual peak of the solvent as a secondary standard. 31P NMR chemical shifts were referenced to an external 85% H3PO4 sample. The temperature was regulated with a Bruker TV-3000 unit. Temperature calibration was determined using a methanol chemical shift thermometer. The temperature accuracy and stability was (1 K. All mixings between the acid and hydride complexes were performed at -80 °C.
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Computational Details QM calculations were performed with the Gaussian 98 package12 at the density functional theory (DFT) B3PW91 level.13-15 Furthermore, coupled-cluster16,17 single-point calculations on the DFT-optimized structures, using both single and double substitutions18-21 and including triple excitations noniteratively,22 were performed to obtain more reliable energies. Core electrons of the Fe and P atoms were described using the effective-core pseudopotentials of HayWadt,23,24 and valence electrons were described with the standard LANL2DZ basis set associated with the ECP.12 In the case of the P atoms, a set of d-type functions was added.25 C and H atoms nonbonded to the metal were described with a 6-31G basis set.26 The two H atoms bonded to the Fe atom were described with a 6-31G(d,p) basis set of functions.27 Mixed QM/MM calculations were performed with the IMOMM method28 with a program built from modified versions of two standard programs: Gaussian 9812 for the QM part and mm3(92)29 for the MM calculations. The IMOMM approach will be used to point out the effect of (12) Frisch, M. J.; et al. Gaussian 98; Gaussian, Inc.: Pittsburgh, PA, 1998. (13) Perdew, J. P. In Electronic Structure of Solids; Ziesche, P., Eschrig, H., Eds.; Akademie Verlag: Berlin, 1991; Vol. p∧11. (14) Perdew, J. P. Phys. ReV. B: Condens. Matter Mater. Phys. 1986, 33, 8822-8824. (15) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652. (16) Pople, J. A.; Krishnan, R.; Schlegel, H. B.; Binkley, J. S. Int. J. Quantum Chem. 1978, 14, 545-560. (17) Bartlett, R.; Purvis, G. Int. J. Quantum Chem. 1978, 14, 561-81. (18) Cizek, J. AdV. Chem. Phys. 1969, 14, 35. (19) Purvis, G. D.; Bartlett, R. J. J. Chem. Phys. 1982, 76, 1910-1918. (20) Scuseria, G. E.; Janssen, C. L.; Schaefer, H. F. J. Chem. Phys. 1988, 89, 7382-7387. (21) Scuseria, G. E.; Schaefer, H. F. J. Chem. Phys. 1989, 90, 3700-3703. (22) Pople, J. A.; Head-Gordon, M.; Raghavachari, K. J. Chem. Phys. 1987, 87, 5968-5975. (23) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284-298. (24) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270-283. (25) Ho¨llwarth, A.; et al. Chem. Phys. Lett. 1993, 208, 237-240. (26) Hehre, W.; Ditchfie, R.; Pople, J. J. Chem. Phys. 1972, 56, 22572261. (27) Harihara, P.; Pople, J. Theor. Chim. Acta 1973, 28, 213-222. (28) Maseras, F.; Morokuma, K. J. Comput. Chem. 1995, 16, 1170-1179. (29) Allinger, N. L. mm3(92); Quantum Chemistry Program Exchange: Bloomington, IN, 1992.
Analysis of Kinetic Isotope Effects Table 1. Results of the Isomerization Kinetics of [Cp*Fe(dppe)(η2-X2)]+ to [Cp*Fe(dppe)(X)2]+ (X ) H, D)' run
complex
acid
[HA/Fe]
T (°C)
104kHH (s-1)a
1 2 3 4 5 6 7 8
Cp*Fe(dppe)H Cp*Fe(dppe)H Cp*Fe(dppe)H Cp*Fe(dppe)H Cp*Fe(dppe)H Cp*Fe(dppe)H Cp*Fe(dppe)Da Cp*Fe(dppe)H
HBF4 HBF4 HBF4 HBF4 HBF4 CF3COOH CF3COOD CF3COOD
1 1 1 1 3 1 1 1
-10 0 5 15 0 0 0 0
0.70 ( 0.09 3.21 ( 0.07 5.92 ( 0.15 28.3 ( 0.7 3.53 ( 0.22 3.10 ( 0.12 3.07 ( 0.02
104kHD (s-1)a
104kDD (s-1)a
2.48 ( 0.01
1.73 ( 0.12 1.56 ( 0.01
a The standard deviations reported for runs 1-6 were calculated keeping in mind an estimated 10% error in the NMR integration. For run 7, see the Supporting Information. b The starting material was contaminated by ca. 25% of CpFe(dppe)H.
the substituents located both on the Cp group and on the phosphine ligand. The substituents are methyl, phenyl, and isopropyl groups treated in the MM part through the MM3 force field. The same basis set as that in the pure quantum computations was used at the same level of theory for the QM part. In the IMOMM computations, all of the geometrical parameters were optimized except for the distances between the atoms linking the QM and MM parts. The KIEs were calculated as described in the Results and Discussion section using harmonic vibrational frequencies obtained from frequency calculations. The frequencies of each isotopically substituted system were obtained at the fixed geometry previously optimized for the corresponding unsubstituted isotopomer. These calculations were carried out with Gaussian 03.30 Solvent effects were taken into account by means of polarized continuum model calculations31,32 using standard options.30 Individual solvation cavities were added on the H atoms directly bonded to the Fe atom. The free energies of solvation were computed in dichloromethane ( ) 8.93) at the geometries optimized in the gas phase. Results and Discussion 1. Isomerization Kinetics of [Cp*Fe(dppe)(η2-X2)]+ (X ) H, D). As outlined in the Introduction, the accurate determination of the isomerization rate constant for the process shown in Scheme 2 at different temperatures was thwarted by the reversibility of the proton-transfer step when a mildly acidic proton donor (i.e., (CF3)nCH3-nOH with n ) 1-3) was used. In an attempt to circumvent this problem, we have carried out stopped-flow studies using stronger acids (CF3COOH and HBF4), to force the proton-transfer step to quantitative conversion and then measure the subsequent isomerization under clean pseudo-first-order conditions. Unfortunately, these experimental conditions introduce another complication. The final dihydride product is unstable in dichloromethane in the presence of an excess of strong acids, leading to the oxidized chloride complex [Cp*Fe(dppe)Cl]+, as described previously.10 This phenomenon is not observed when using the (CF3)nCH3-nOH proton donors, even in large excess amounts, whereas it is observed when (30) Frisch, M. J.; et al. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (31) Tomasi, J.; Persico, M. Chem. ReV. 1994, 94, 2027-2094. (32) Amovilli, C.; Barone, V.; Cammi, R.; Cance`s, E.; Cossi, M.; Menucci, B.; Pomelli, C. S.; Tomasi, J. AdV. Quantum Chem. 1998, 32, 227261.
using CF3COOH or HBF4 in greater than a 2-fold excess. Thus, the system is not amenable to a clean kinetic study in dichloromethane. A change of solvent has been envisaged, but no ideal system has so far been found: tetrahydrofuran reduces considerably the thermodynamic drive to proton transfer (because of the free acid stabilization by hydrogen bonding with the solvent molecules) and polymerizes under strongly acidic conditions; toluene cannot keep the produced salts in solution at sufficient concentrations. A convenient solution to this problem was found by recurring to the NMR technique. The advantage in this case is that the relative amounts of dihydrogen and dihydride complexes can be measured directly and independently, even in the presence of any amount of the [Cp*Fe(dppe)Cl]+ decomposition product (a paramagnetic complex). Therefore, the rate at which the intermediate dihydrogen complex decays is directly accessible with accuracy. In addition, working at low temperatures (thereby slowing down the isomerization process to a suitable time scale for NMR monitoring) reduces the impact of the acid-catalyzed decomposition in dichloromethane. The sum of the integrated intensities for the dihydrogen and dihydride complexes, monitored against that of an internal standard, provides a direct gauge of the system stability. After generation of the intermediate dihydrogen complex quantitatively in situ by low-temperature (-80 °C) addition of the appropriate amount of a strong acid (see Table 1 for details), monitoring was carried out at four different temperatures (-10, 0, +5, and +15 °C, runs 1-4, respectively). Both the decay of the dihydrogen complex resonance (examples are shown in Figure 1, top) and the growth of the final dihydride complex resonance were monitored by 1H NMR spectroscopy. The sum of the integral, after normalization relative to the solvent peak that was used as an internal standard, was relatively constant throughout the reaction, showing limited decomposition ( ν(Fe-Xf) for both X ) H and D. Thus, the bending modes play a dominant role. The Inorganic Chemistry, Vol. 45, No. 25, 2006
10261
Baya et al. Chart 3
same argument rationalizes why KIE(HD/DD) is greater when the migrating atom is H. For the “via Cp” mechanism, on the other hand, the two hydride ligands yield a relatively strong Fe-Hf bond (its stretching vibration at 1952.3 cm-1 in TSrot is similar to that calculated in TSdir) and an agostic Fe‚‚‚Hm-C moiety. The strength of the C-H bond is reduced from that of a regular C-H bond, but the combination of the C-Hm and Fe‚‚‚Hm vibrations, which are mixed with an Fe-C component, yields three relatively high-frequency normal modes, as shown in Chart 3. The overall effect is a strengthening of the bonding environment for the Hm atom and a weakening for the Hf atom, leading to an inverse primary KIE (migrating D) and a normal secondary KIE (migrating H). Conclusions We have reported here a detailed investigation of the irreversible isomerization process of the dihydrogen intermediate [Cp*Fe(dppe)(η2-H2)]+, obtained by low-temperature protonation of Cp*Fe(dppe)H, to the dihydride complex trans-[Cp*Fe(dppe)(H)2]+. The computational tool illustrates the need to include the steric and electronic effects of the Cp and diphosphine substituents to quantitatively reproduce the relative stabilities of nonclassical and classical isomers and the activation barrier of the isomerization process. However, the calculations also indicate that two different pathways are likely candidates for the isomerization mechanism: a “direct” pathway (∆Eq ) 21.3 kcal mol-1) and a “via Cp” pathway (∆Eq ) 22.2 kcal mol-1), never suggested previously, which involves Cp*H intermediates (cf. 21.6 ( 0.8 kcal mol-1 from the experiment). Sorting between these pathways was possible through the investigation of isotope effects. Generation of the [Cp*Fe(dppe)(η2-HD)]+ complex
10262 Inorganic Chemistry, Vol. 45, No. 25, 2006
by protonation of Cp*Fe(dppe)H with CF3COOD results in a rapid isotope redistribution equilibrium, yielding complexes [Cp*Fe(dppe)(η2-H2)]+ and [Cp*Fe(dppe)(η2-D2)]+; each of the three isotopomers isomerizes to the corresponding classical product with its own rate constant (kHH, kHD, and kDD). The resulting KIEs at 273 K are kHH/kHD ) 1.24 ( 0.01 and kHD/kDD ) 1.58 ( 0.01 (and, consequently, kHH/ kDD ) 1.96 ( 0.02). This type of isotope effect analysis, for a rearrangement process of a dihydrogen complex to the isomeric dihydride and encompassing the three H2, HD, and D2 isotopomers, is unprecedented to the best of our knowledge. The isotope redistribution EIE and the KIEs were also derived from the calculated normal-mode frequencies of the dihydrogen isotopomers and the transition states. The computed values are in excellent agreement with the experimental values, but only for the “direct” isomerization mechanism, providing strong support in favor of this rearrangement pathway and against the “via Cp” pathway for this system. However, given the closeness of the calculated activation barriers, it seems reasonable to think that subtle modifications in the metallic system could favor a mechanism of the “via Cp” type. Acknowledgment. We thank the European Commission through the HYDROCHEM program (Contract No. HPRNCT-2002-00176) for support of this work. Additional bilateral (PICS, France-Russia) and national support from the CNRS (France), from the RFBR (Grants 05-03-22001 and 05-0332415) and the Division of Chemistry and Material Sciences of RAS (Russia), and from MEC (Project CTQ2005-09000CO2-01; Spain) is also gratefully acknowledged. A.L. thanks the Generalitat de Catalunya for a Distincio´ per a la Promocio de la Recerca Universita`ria. N.V.B. thanks the Russian Science Support Foundation for an individual grant. M.B. thanks the Spanish Ministerio de Educacio´n y Ciencia for a postdoctoral fellowship. Supporting Information Available: Details of the kinetic analysis for the Cp*Fe(dppe)H + CF3COOD reaction, optimized geometries (Cartesian coordinates) for the calculated species, and calculated normal-mode frequencies used for the EIE and KIE analyses. This material is available free of charge via the Internet at http://pubs.acs.org. IC061428N
