Dihydrogen to dihydride isomerization mechanism in [(C5Me 5)FeH2(Ph2PCH2CH2PPh 2)]+ through the experimental and theoretical analysis of kinetic isotope effects

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,^‡,Agust´ı 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 Qu´ımica, 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)(η

²-H₂)]⁺, generated in situ by low-temperature protonation of

Cp*Fe(dppe)H with either HBF₄or CF₃COOH, to the dihydride tautomer trans-[Cp*Fe(dppe)(H)₂]⁺is irreversible

1

and follows first-order kinetics in the

−10 to

+15

°

C range with

∆

H^q

)

21.6

±

0.8 kcal mol^-and

∆

S^q

)

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

[Cp*Fe(dppe)(

²-HD)]⁺complex establishes a very rapid isotope redistribution equilibrium with the

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 k_HH, k_HD, and k_DDfor the three

isotopomers, yielding kinetic isotope effects: k_HH/k_HD1.24 0.01 and k_HD/k_DD1.58 0.01 (and, consequently,

_HH/k_DD1.96

0.02). The analysis of the DFT-calculated frequencies, using the [Cp*Fe(dhpe)H₂]⁺model system,

for the [Cp*Fe(dhpe)(

²-XY)]⁺isotopomers as well as transition states for the “direct” (TS_dir) and “via Cp” (TS_rot

pathways (X H, D) allowed the computation of the expected isotope effects. A comparison with the experiment

+

CF₃COOD and Cp*Fe(dppe)D

+

CF₃COOD systems at 273 K. The

η

²-H₂and ²-D₂

η

)

±

)

±

)

±

k

)

±

η

)

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

initial stages of their interaction with dihydrogen.¹Subse-

quently, 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.^2-7For the group 8 half-sandwich systems such as

It is nowadays well established that most 16-electron L_nM

fragments form a dihydrogen complex L_nM(η²-H₂) in the

* To whom correspondence should be addressed. E-mail: poli@

lcc-toulouse.fr (R.P.), agusti@klingon.uab.es (A.L.), shu@ineos.ac.ru (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 Qu´ımica 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.

(1) Kubas, G. J. Metal Dihydrogen and σ-Bond Complexes; Kluwer

Russian Academy of Sciences.

Academic/Plenum Press: New York, 2001.

10248 Inorganic Chemistry, Vol. 45, No. 25, 2006

10.1021/ic061428n CCC: $33.50

Published on Web 11/15/2006

Analysis of Kinetic Isotope Effects

Scheme 1

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.³A theoretical study of this isomer-

ization 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.^10,11This

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 intra-

molecular mechanism is suggested by an identical rate when

using proton donors of different strength at the same

temperature. The alternative reversible deprotonation, fol-

lowed by a slower protonation at the metal site, is incon-

sistent with this experimental observation.¹⁰Our attempts

to carry out an Eyring analysis of this isomerization step

were complicated by the reversibility of the preceding proton-

transfer 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 intermedi-

ate 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).

[CpRuLL′H₂]⁺(Cp ) cyclopentadienyl; L ) tertiary phos-

phine)^3,4and iron analogues,^6,7this 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-legged-

piano-stool dihydrogen complex to a transoid square-based

four-legged-piano-stool dihydride (Scheme 1).⁸Thus, 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)(η²-

H₂)]⁺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 com-

plexes.^3,9Reported activation enthalpies range from 16 to

21 kcal mol^-1, with small and negative activation entropies.

The highest values have been obtained for bidentate phos-

phines with sterically demanding ligands.⁹The small ∆S^q

supports an intramolecular mechanism in which the dihy-

drogen ligand has lost rotational freedom in the transition

state.³A dissociative mechanism has been proposed when

bulky and good electron-donor ligands are present⁹but was

discarded for a simpler system on the basis of kinetic

evidence (no retardation effect was observed in the presence

of excess ligand).³The 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.^3,9Finally, an alternative

intramolecular mechanism proceeding by transfer of an H

atom from the dihydrogen ligand to the Cp ligand, thereby

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

-

BF₄or CF₃COO^-. These investigations include the deter-

mination of the kinetic isotope effects (KIEs) k_HH/k_HDand

k

_HD/k_DD, obtained from the comparative isomerization rates

of complexes [Cp*Fe(dppe)(H₂)]⁺(k_HH), [Cp*Fe(dppe)(HD)]⁺

(k_HD), and [Cp*Fe(dppe)(D₂)]⁺(k_DD). 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 inves-

tigation 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,

(2) Parkin, G.; Bercaw, J. E. J. Chem. Soc., Chem. Commun. 1989, 255-

257.

(3) Chinn, M. S.; Heinekey, D. M. J. Am. Chem. Soc. 1990, 112, 5166-

5175.

(4) Jia, G.; Lough, A. J.; Morris, R. H. Organometallics 1992, 11, 161-

171.

(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, 1045-

1054.

i

Me; R′ ) H, Ph, Pr), separating electronic and steric

contributions. Second, it explores the intimate details of the

possible intramolecular isomerization mechanisms. In par-

ticular, discrimination between two pathways showing very

(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, 5467-

5476.

(9) De Los R´ıos, I.; Jime´nez-Tenorio, M.; Padilla, J.; Puerta, M. C.;

Valerga, P. Organometallics 1996, 15, 4565-4574.

(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 10249

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.

Computational Details

QM calculations were performed with the Gaussian 98

package¹²at the density functional theory (DFT) B3PW91

level.^13-15Furthermore, coupled-cluster^16,17single-point cal-

culations on the DFT-optimized structures, using both single

and double substitutions^18-21and including triple excitations

noniteratively,²²were performed to obtain more reliable

energies. Core electrons of the Fe and P atoms were

described using the effective-core pseudopotentials of Hay-

Wadt,^23,24and valence electrons were described with the

standard LANL2DZ basis set associated with the ECP.¹²In

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.²⁶The two H atoms bonded to the Fe atom

were described with a 6-31G(d,p) basis set of functions.²⁷

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.⁷NMR

characterization of the deuteride complex in C₆D₆: ²H NMR δ

-16.9 (t, J_PD) 10.4 Hz); ³¹P{¹H} NMR δ 111.5 (t, J_PD) 10.3

1

Hz). In addition, the H 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.

Mixed QM/MM calculations were performed with the

IMOMM method²⁸with a program built from modified

versions of two standard programs: Gaussian 98¹²for the

QM part and mm3(92)²⁹for the MM calculations. The

IMOMM approach will be used to point out the effect of

The NMR studies were carried out in standard 5-mm NMR tubes

1

containing solutions of the complexes in CD₂Cl₂. The H and ³¹P

NMR data were collected with a Bruker AMX 400 spectrometer

operating at 400.13 and 161.98 MHz, respectively. The low-

temperature kinetic experiments with ³¹P{¹H,²H} NMR moni-

toring were carried out on a Bruker AV500 spectrometer equipped

with a 5-mm triple-resonance inverse probe with dedicated ³¹P

(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.

1

channel operating at 500.33 MHz for H 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

(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.

1

decoupling. For ³¹P and H NMR, a typical pulse width corre-

(17) Bartlett, R.; Purvis, G. Int. J. Quantum Chem. 1978, 14, 561-81.

(18) Cizek, J. AdV. Chem. Phys. 1969, 14, 35.

sponding 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% H₃PO₄sample. The

temperature was regulated with a Bruker TV-3000 unit. Temper-

ature 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.

(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, 2257-

2261.

(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.

10250 Inorganic Chemistry, Vol. 45, No. 25, 2006

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)'

a

run

complex

acid

HBF₄

CF₃COOH

CF₃COOD

[HA/Fe]

T (°C)

10⁴k_HH(s^-1

)

10⁴k_HD(s^-1

)

10⁴k_DD(s^-1

)

1

2

3

4

5

6

7

8

Cp*Fe(dppe)H

Cp*Fe(dppe)D^a

Cp*Fe(dppe)H

1

3

1

-10

0

5

15

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

0

1.73 ( 0.12

1.56 ( 0.01

3.07 ( 0.02

2.48 ( 0.01

^aThe 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. ^bThe 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 geo-

metrical 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 Gauss-

ian 03.³⁰

using CF₃COOH or HBF₄in 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 dichloro-

methane. 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.

Solvent effects were taken into account by means of

polarized continuum model calculations^31,32using standard

options.³⁰Individual 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)_nCH_3-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 an-

other 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.¹⁰This phenomenon is

not observed when using the (CF₃)_nCH_3-nOH proton donors,

even in large excess amounts, whereas it is observed when

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 tem-

peratures (-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

1

final dihydride complex resonance were monitored by H

NMR spectroscopy. The sum of the integral, after normaliza-

tion relative to the solvent peak that was used as an internal

standard, was relatively constant throughout the reaction,

showing limited decomposition (<10% when using 1 equiv

of a strong acid; a maximum 17% decomposition during 153

min was found for run 5, where a greater excess of acid

was used). However, because the acid-catalyzed decomposi-

tion occurs only for the classical product and because the

isomerization is irreversible,¹⁰the decay rate of the non-

classical reagent is an accurate measure of the isomerization

rate.

(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, 227-

261.

The nonclassical resonance decay gives an excellent fit

to the first-order rate law at each temperature (examples are

shown in Figure 1, above), yielding the rate constants k_HH

Inorganic Chemistry, Vol. 45, No. 25, 2006 10251

Baya et al.

Figure 2. Eyring analysis of the isomerization rate constant k_HH

.

2. Role of the Electronic and Steric Effects in the

Relative Stabilities of the Dihydrogen and Dihydride

Complexes. Before analysis of the possible rearrangement

pathways in detail from the computational point of view, it

is important to gain a full appreciation of the model effect

on the relative energies. As a general trend in computational

chemistry, models are used to represent the real molecular

systems. This is mainly done in order to deal with affordable

computation time and system resources, but this procedure

also allows one to introduce the complexity of the real

systems in a gradual way, analyzing at each step the changes

suffered by the system. This approach will be used to separate

electronic and steric contributions to the relative stabilities

of the dihydrogen and dihydride forms. Usually, the system

is simplified by replacing substituents with H atoms. If we

apply this general procedure to the [(C₅R₅)Fe(R′₂PCH₂CH₂-

PR′₂)H₂]⁺complexes, we can define four models, as depicted

in Chart 1, together with the real complexes. The smallest

model is the [CpFe(dhpe)H₂]⁺complex 1 (dhpe ) PH₂CH₂-

CH₂PH₂), where both the methyl groups of the Cp* ligand

and the phenyl or isopropyl substituents of the phosphine

ligand are replaced by H atoms. In the second model [Cp*Fe-

(dhpe)H₂]⁺(2), only the phosphine substituents are replaced

by H atoms, keeping the whole Cp*. Two additional models

can be generated by introducing the real phosphine substit-

uents while keeping the model Cp ligand: [CpFe(dppe)H₂]⁺

(3) for R′ ) Ph and [CpFe(dippe)H₂]⁺(5) for R′ ) ⁱPr. The

four models can be related to the two experimentally reported

complexes [Cp*Fe(dppe)H₂]⁺(4)^6,33and [Cp*Fe(dippe)H₂]⁺

(6).³⁴The dihydride form of all of the species considered in

the calculations and the numbering scheme are presented in

Chart 1. The cisoid (dihydrogen) and transoid (dihydride)

isomers will be labeled c and t, respectively.

Figure 1. ¹H NMR monitoring of the [Cp*Fe(dppe)(η²-H₂)]⁺resonance

-

decay (BF₄salt in CD₂Cl₂).

reported in Table 1. The three different kinetic runs carried

out at 0 °C (runs 2, 5, and 6) show that the isomerization

rate does not significantly depend on the acid concentration

(compare runs 2 and 5) or on its nature (compare runs 2 and

6). The latter observation confirms our previous findings of

identical isomerization rates using different (CF₃)_nCH_3-nOH

proton donors.¹⁰A comparison of runs 6 and 7, on the other

hand, shows a significant KIE. Monitoring of run 7 could

not be carried out conveniently by ¹H NMR because the Cp*

and dppe peaks of the reactant and product are not

significantly spread apart. The quantitative determination was

based on the integration of the inverse-gated doubly de-

coupled ³¹P{¹H,²H} NMR resonance (see the Experimental

Part). It should be remarked that the ca. 25% of the

Cp*FeH(dppe) isotopomer in the deuteride starting material

may slightly affect the resulting rate constant value, given

that the ¹H-containing nonclassical impurity, [Cp*Fe(η²-HD)-

(dppe)]⁺, isomerizes faster (vide infra). Therefore, the value

of k_DDreported in Table 1 should be considered as an upper

estimate of the isomerization rate constant for complex

[Cp*Fe(dppe)(η²-D₂)]⁺(a more accurate value for k_DDis

available from an alternative approach, vide infra). The

Eyring analysis of the rate constants k_HHof runs 1-4 (Figure

2) yields the activation parameters ∆H^q) 21.6 ( 0.8 kcal

mol^-1and ∆S^q) 5 ( 3 eu. These values are similar to those

previously reported for related ruthenium derivatives.^3,9The

enthalpy value resulting from this analysis is useful for a

comparison with the computed energies for the transition

states of different potential rearrangement pathways (vide

infra).

The optimized structures of the isomers for the model

complex 1 are depicted in Figure 3. The structural features

of the dihydrogen and dihydride isomers for the systems 1-6

are analyzed in detail in the Supporting Information. The

relative energies of the dihydrides 1t-6t with respect to the

corresponding dihydrogen isomers 1c-6c are given in Table

2. Our computed values for 1 and 4 (+6.6 and -3.9 kcal

(33) Hamon, P.; Toupet, L.; Hamon, J.-R.; Lapinte, C. Organometallics

1992, 11, 1429-1431.

(34) Jime´nez-Tenorio, M.; Puerta, M. C.; Valerga, P. Organometallics 1994,

13, 3330-3337.

10252 Inorganic Chemistry, Vol. 45, No. 25, 2006

Analysis of Kinetic Isotope Effects

Chart 1

Figure 3. Optimized geometries of the dihydrogen (left) and dihydride

(right) isomers of 1.

Table 2. Relative QM Energies (in kcal mol^-1; QM/MM Values in

Parentheses) of the Dihydride Complexes, with Respect to the

Corresponding Dihydrogen Isomer

complex

∆E

[CpFe(dhpe)H₂]⁺, 1

[Cp*Fe(dhpe)H₂]⁺, 2

[CpFe(dppe)H₂]⁺, 3

[Cp*Fe(dppe)H₂]⁺, 4

[CpFe(dippe)H₂]⁺, 5

[Cp*Fe(dippe)H₂]⁺, 6

6.6

4.9 (5.9)^a

1.4 (5.7)^b

-3.9 (0.1)^b

-3.6 (0.9)^c

-8.6 (-6.2)^c

^aThe methyl groups of the Cp* ligand are in the MM part. ^bThe phenyl

substituents of the dppe ligand are in the MM part. ^cThe isopropyl

substituents of the dippe ligand are in the MM part.

of the protonation of the Cp*Fe(dppe)H complex,^6,10,11we

expect that it must be more stable than the related non-

classical dihydrogen complex. However, the nonclassical

dihydrogen complex is the most stable isomer for all of the

models except when the computation is carried out on the

real complex [Cp*Fe(dppe)(H)₂]⁺. It is clear that in our case

model systems cannot account for the experimental trends

on the relative stabilities of the two isomers.

The substituents tune the relative stabilities of the dihy-

drogen and dihydride forms, and thus the ability of the metal

fragment to break the H-H bond, by a combination of

electronic and steric effects. We will apply a simple analysis

already used on other systems to separate and quantify both

contributions on the relative energies of the two isomers.^36-38

To this aim, we have carried out additional QM/MM

computations. The approach relies on two basic assump-

tions: (1) the full QM calculation on the whole system

describes correctly the experimental behavior and thus

includes both electronic and steric contributions; (2) the QM/

MM calculation describes only the steric effects of the groups

included in the MM part, leaving out their electronic effects.

From the small model 1, the substituents have been added

in order to build up the real complexes, but in these QM/

MM calculations, they have been introduced at the MM level.

By this scheme, only the steric contributions of the substitu-

ent are responsible for the changes in the relative stabilities

of the two isomers.

mol^-1, respectively) compare well with those previously

reported using a different functional (+4.3 and -4.4 kcal

mol^-1, respectively).⁶To check the accuracy of our meth-

odology, we have recalculated the energy of the dihydrogen/

dihydride couple of the smallest system, 1, with the highly

correlated CCSD(T) method. There is a reasonable agreement

between both methods because the dihydride lies 9.4 kcal

mol^-1above the dihydrogen complex at the CCSD(T) level.

Moreover, the experimental trend on the stabilities is well

reproduced by our calculations. The dihydrogen complex was

found to be more stable than the dihydride only for the Cp/

dppe set of ligands.^6,10,33-35We have also checked whether

solvation affects the dihydrogen/dihydride relative stabilities

by calculating the energy difference between the dihydrogen

and dihydride forms of 2 in dichloromethane. The gas-phase

energy difference (4.9 kcal mol^-1in favor of the dihydrogen

complex) is only slightly decreased to 4.5 kcal mol^-1in

dichloromethane. Therefore, solvation plays a very minor

role in this equilibrium. On the contrary, the equilibrium is

strongly influenced by the Cp and phosphine substituents.

The introduction of the methyl substituents in the Cp ring

favors the dihydride, reversing the stabilities. The preference

for the dihydride isomer is considerably enhanced by the

presence of the isopropyl substituents in the phosphine.

Because the classical dihydride is the thermodynamic product

The relative energy between the two isomers computed

with model 1 is the reference. No substituents are present,

(36) Barea, G.; Maseras, F.; Jean, Y.; Lledo´s, A. Inorg. Chem. 1996, 35,

6401-6405.

(37) Bustelo, E.; Carbo, J.; Lledo´s, A.; Mereiter, K.; Puerta, M.; Valerga,

P. J. Am. Chem. Soc. 2003, 125, 3311-3321.

(38) Maresca, O.; Maseras, F.; Lledo´s, A. New J. Chem. 2004, 28, 625-

630.

(35) Jia, G.; Ng, W. S.; Yao, J.; Lau, C.-P.; Chen, Y. Organometallics

1996, 15, 5039-5045.

Inorganic Chemistry, Vol. 45, No. 25, 2006 10253

Baya et al.

the extra stabilization of the dihydride isomer. The branched

isopropyl groups have a more important steric hindrance than

the planar phenyl groups, rendering the dihydride the favored

isomer, even with the Cp ligand (1 vs 5).

At a first glance, it could seem surprising that the

coordination number increase associated with the dihydrogen

f dihydride interconverson could be favored on steric

grounds. However, the destabilizing steric interactions are

not developed between the piano-stool legs but rather

between the phosphine substituents and the C₅ring. They

are signaled by the notable increase in the [C₅ring-

(centroid)]-Fe-[P₂(centroid)] (R) angle on going from the

dihydrogen (R of about 150°) to the dihydride structures (R

of about 180°); see the Supporting Information. In a thorough

study of the η²-dihydrogen complexes of ruthenium, it was

already pointed out that the interaction of phosphine sub-

stituents with the Cp or Cp* ring may be significant in

determining the position of the dihydrogen f dihydride

equilibrium.³Our study fully confirms this hypothesis and

quantifies the magnitude of such an interaction.

The electronic effects also stabilize the dihydride isomer.

The largest effect is found with the phenyl groups. The role

of the phenyl substituents is particularly important in the

preferential stabilization of the isomer with the highest

oxidation state. The omission of the phenyl substituents in

the model complexes is the main reason for the poor results

they give for the dihydrogen/dihydride relative energies.

3. Computational Study of the Dihydrogen f Dihy-

dride Isomerization. We now turn to the analysis of the

isomerization mechanism. As mentioned in the Introduction,

several mechanisms have been suggested for the isomeriza-

tion of the related dihydrogen ruthenium derivatives:^3,9,39,40

(1) a direct intramolecular rearrangement through a trigonal-

bipyramidal transition state; (2) a dissociative process with

the partial or total phosphine ligand decoordination; (3) a

relay of the migrating hydride via the Cp ligand; (4) a

deprotonation of the cis isomer followed by protonation of

the metal at the trans position. The first three possibilities

are intramolecular processes, while the latest involves a

proton-donor molecule. Because we have accumulated

evidence, including the present study, that the isomerization

rate of 4 is independent of the nature of the proton donor,^6,10,11

only intramolecular mechanisms have been explored. In

addition, the new experimental results shown in this paper

set an experimental value to calibrate the highest enthalpy

point along the computed reaction coordinate.

Figure 4. Substituent steric and electronic effects on the stabilization of

the classical dihydride isomer for the complexes drawn in Chart 1.

neither on the C₅ring nor on the phosphine ligand, and the

dihydrogen is 6.6 kcal mol^-1more stable than the dihydride.

This value is reduced to 4.9 kcal mol^-1when performing a

full QM calculation of the [Cp*Fe(dhpe)H₂] system 2,

whereas a QM/MM calculation of the same system with the

methyl substituents of the Cp* considered only at the MM

level gives the dihydrogen 5.9 kcal mol^-1more stable than

the dihydride. The total contribution of the methyls is thus

1.7 kcal mol^-1, of which 0.7 kcal mol^-1is attributed to the

steric effect. Afterward, the electronic effects of the methyl

groups (1.0 kcal mol^-1) can be estimated by subtracting

(1.7-0.7 kcal mol^-1) the steric effect from the total contribu-

tion. Of course, the electronic effect could also be evaluated

directly as the difference (5.9-4.9 kcal mol^-1) between the

relative QM and QM/MM energies reported in Table 2, with

an identical result. The same scheme can be applied to the

phenyl and isopropyl substituents. The results are sum-

marized in Figure 4.

From 1 to 2, both the electronic and steric effects are quite

small and are of the same order of magnitude. However, the

effect of the phenyl groups (from 1 to 3) is more pronounced.

The dihydride isomer is stabilized about 6.0 kcal mol^-1, but

this energy is not enough to reverse the thermodynamic

preference for the dihydrogen. The extra stabilization is

mainly due to electronic effects, with the steric effects not

being relevant. When we compare the relative energies of

the real systems containing the bulky phosphines and the

Cp* ligand, a stronger influence of the Cp* methyl groups

can be appreciated: 5.3 and 5.0 kcal mol^-1for the dppe and

dippe phosphines, respectively. In both cases, the substitution

of Cp by Cp* favors the dihydride complex. Assuming that

the electronic effect of the methyls will be similar in the

three cases, stabilizing the dihydride by about 1.0 kcal mol^-1,

it is evident that steric factors of the ring substituents work

in favor of the dihydride in the real complexes. Phenyl and

isopropyl substituents have a major influence in the ther-

modynamic preference for the dihydride isomer. Changing

the H model substituents with phenyls in the Cp*-containing

complexes (2 vs 4) favors the dihydride by 8.8 kcal mol^-1,

with a similar contribution of electronic and steric effects.

The influence of the isopropyls (2 vs 6) in favoring the

dihydride is even higher (13.5 kcal mol^-1), although in this

case, the weight of the steric effects is the main cause for

To keep the computational demands affordable, the study

of the reaction mechanisms was performed at a full QM level

only for the 1 and 2 model complexes and at the QM/MM

level with the IMOMM method for the real 4 complex (with

phenyl groups at the MM level). Additional single-point

energy computations at the full QM level on the real 4

complex were performed on the geometry of the intermedi-

ates and transition states located along the QM/MM potential

energy surface (QM/IMOMM).

(39) Cayuela, E.; Jalon, F. A.; Manzano, B. R.; Espino, G.; Weissensteiner,

W.; Mereiter, K. J. Am. Chem. Soc. 2004, 126, 7049-7062.

(40) Ryan, O. B.; Tilset, M. J. Am. Chem. Soc. 1991, 113, 9554-9561.

10254 Inorganic Chemistry, Vol. 45, No. 25, 2006

Analysis of Kinetic Isotope Effects

Figure 5. Direct (right) and “via Cp” (left) pathways for the cis-dihydrogen f trans-dihydride isomerization. The optimized geometries are shown for the

2 system (the Cp* and dhpe H atoms have been omitted for the sake of clarity), whereas the energies (in kcal mol^-1) correspond to the QM calculations on

the QM/MM optimized 4 system.

Table 3. Main Geometrical Parameters (Distances in Angstroms and

Direct Intramolecular Rearrangement. As discussed

Angles in Degrees) of the Transition States for the Two Reported

Dihydrogen f Dihydride Interconversion Mechanisms of 2^a

above, in these piano-stool compounds, the cis-dihydrogen

f trans-dihydride isomerization implies two processes:

oxidative addition of dihydrogen and structural rearrangement

of the piano-stool legs. These two processes may proceed

in one or two steps. The simple cleavage of the H-H bond

in the dihydrogen structure leads to a cis-dihydride. Although

there are no reports of cis-dihydride [(C₅R₅)FeH₂(PP)]⁺

complexes, either isolated or spectroscopically detected, we

considered also the possibility of a transient cis-dihydride

intermediate. Starting from the dihydrogen complex and

taking as a reaction coordinate the H-H distance, we

calculated the energy curve for the H-H breaking. The

energy was continuously increasing, and no stationary point

corresponding to a cis-dihydride intermediate could be

located. Thus, the reaction appears to be taking place in a

single step.

2c

TS_dir

TS_rot

2t

Fe-H

1.576

1.580

0.869

2.220

2.208

2.121

2.159

144.8

85.2

32.0

1.482

1.490

2.282

2.149

2.203

2.115

2.202

1.483

1.644

2.274

2.165

2.183

2.104

2.238

1.482

2.698

2.166

2.167

2.105

2.135

H_f-H_m

Fe-P

Fe-C(Cp)_range

R^b

158.9

162.3

85.1

179.3

P₁-Fe-P₂

H_f-Fe-H_m

Cp_c*-Fe-H_f-H_m

84.0

100.3

100.7

151.8

153.6

119.0

91.5

131.0

179.7

178.8

134.0

c

36.9

c

Cp_c*-Fe-P₁-P₂

115.8

126.0

128.0

c

Cp_c*-Fe-P₁

c

Cp_c*-Fe-P₂

^aValues for the initial dihydrogen and the final dihydride complexes

are also included. ^bR ) angle [C₅ring(centroid)]-Fe-[P₂(centroid)]. ^cCp_c*

) Cp* centroid.

Then, we explored the potential energy surface for the

intramolecular rearrangement taking the H-Fe-H angle as

the reaction coordinate. While the two hydride positions are

symmetry-equivalent in the starting dihydrogen complex,

they become inequivalent in the transition state. They will

be identified as the “fixed” (H_f) and “migrating” (H_m)

positions, respectively. The H_f-Fe-H_mangle must undergo

a large change during the process: from 32° in the dihy-

drogen complex to 131° in the dihydride complex. Starting

from the maximum of this monodimensional energy profile,

we were able to locate the transition state for the direct

intramolecular rearrangement (TS_dir). This presents a single

imaginary frequency (298i cm^-1), with the opening of the

H_f-Fe-H_mangle as the main component of its associated

eigenvector. The mechanism of the process is illustrated in

Figure 5 (right-hand side), together with the structure of the

TS_dirfor the 2 system. The main geometric parameters of

the TS_dirfor the Cp*-dhpe system are given in Table 3,

while the full list of calculated frequencies as given as the

Supporting Information. Similar structural values, not re-

ported for the sake of brevity, are obtained for the Cp-dhpe

and Cp*-dppe systems.

The transition state has a dihydride nature. The H-H bond

appears totally broken, as reflected by the long H-H

separation, whereas the two Fe-H bonds are already formed.

Consequently, the oxidative addition process is already

completed at the transition state level; the structure can be

Inorganic Chemistry, Vol. 45, No. 25, 2006 10255

Baya et al.

Table 4. Relative Energies (∆E, kcal mol^-1) of the Species Involved in the Two Mechanisms

dihydrogen

TS_Cpcis

I_Cpcis

TS_Rot

TS_dir

I_Cptrans

TS_Cptrans

12.5

dihydride

[CpFe(dhpe)H₂]⁺

[Cp*Fe(dhpe)H₂]⁺

[Cp*Fe(dppe)H₂]^+a

[Cp*Fe(dppe)H₂]^+b

0.0

30.1

28.9

24.5

21.3

6.6

4.9

0.1

15.3

14.9

12.1

11.2

30.0

23.9

22.2

11.5

5.9

3.4

-3.9

^aQM/MM calculation with the phenyl groups in the MM part. ^bFull QM calculation at the QM/MM-optimized geometries.

The transition state TS_dirlies 30.1 and 28.9 kcal mol^-1

above the dihydrogen for 1 and 2, respectively (Table 4). In

these model systems, the effect of the Cp methyl substituents

is to decrease slightly the potential energy barrier. We have

also computed the enthalpy and free energy of activation,

namely, ∆H^q) 27.1 kcal mol^-1and ∆G^q) 30.7 kcal mol^-1

for 2 at T ) 298.15 K. The small, almost zero, activation

entropy (∆S^q) -3.9 eu) is not inconsistent with the small

value observed in this work for the iron complex (vide supra)

and with the experimentally reported ∆S^q(-3.3 ( 0.7 eu)

in related ruthenium complexes.³As was already found for

the dihydrogen-dihydride equilibrium, the energy barrier of

the rearrangement is only slightly affected by solvent effects.

The computed barrier in dichloromethane for the 2 system

is 28.6 kcal mol^-1, very close to the gas-phase value (28.9

kcal mol^-1).

As reported for the relative stabilities of the dihydrogen

and dihydride complexes, the effects of the substituents

(electronic and steric) are also major on the transition-state

energy. The stabilization found for the Cp* transition state

is similar to those found for the minima, in agreement with

the dihydride nature of the transition state. The same trend

could by inferred for the phenyl effects, and this is indeed

the case. The potential energy barrier for the real system 4,

with the phenyl groups described at the QM/MM level, is

computed as 24.5 kcal mol^-1, and full QM energy computa-

tions at the fixed QM/MM minima and transition state further

lower the energy barrier to 21.3 kcal mol^-1. Thus, the effect

of the phenyl substituents is to considerably decrease the

isomerization barrier. The calculated barrier is very close to

the experimental value of 21.6 ( 0.8 kcal mol^-1(see above)

and is also close to the barrier of the related isomerization

of the [Cp*Ru(dippe)(H₂)]⁺complex.⁹

Chart 2

described as an iron(IV) dihydride. Nevertheless, the R angle

is closer to its initial value in the dihydrogen complex than

to the 180° value in the final dihydride. The phosphine ligand

is also experiencing a significant repositioning. The dihedral

angle formed by the C₅ring(centroid), the Fe atom, and the

two P atoms takes a midway value (about 150°) between

the 115° in the dihydrogen and the 180° in the trans-

dihydride. The movement of the phosphine ligand can also

be appreciated from the two Cp_c*-Fe-P angles. These are

close to each other in both the starting dihydrogen and final

dihydride complexes, and the values do not change much

on going from one to the other (126° and 128° for the

dihydrogen complex and 134° and 134° for the dihydride

complex). However, one angle is opened to 150°, whereas

the other one is closed to 120°, in the transition state. This

illustrates the swing of the entire diphosphine ligand toward

one side of the molecule, which is necessary to allow passage

of one hydride ligand from the side, to move from the front

to the back of the metal center.

Given that both the transition state and the final trans-

dihydride are four-legged-piano-stool complexes, they can

be analyzed in relation to the mechanisms invoked for the

CpMo(CO)₂LR cis-trans interconversion.^3,41Within this

model, the two trigonal-bipyramidal-like transition states

presented in Chart 2 can be envisaged. The geometric

parameters of the transition states are in closer agreement

with a structure of type B, describing a movement in which

one of the phosphine arms has moved down to allow the

migration of the hydride ligand toward its final position. The

generation of a four-legged-piano-stool product with a

transoid disposition of the E and H ligands resulting from

the oxidative addition of a E-H bond by a (C₅R₅)ML₂

fragment is a relatively common result, not well explained

yet. A transition state closely related to TS_dirhas been

reported for the cis-trans isomerization of a hydridoaryl

complex of rhenium.⁴²It could also be at work for the

oxidative addition of an alkyne C-H bond to yield a trans-

alkynyl hydride.³⁷

Phosphine Dissociation Mechanism. We have also

considered a dissociative isomerization mechanism, in which

one of the P atoms in the [Cp*Fe(dhpe)(H₂)]⁺complex

dissociates from the metal, yielding an unsaturated interme-

diate where the diphosphine acts as a monodentate ligand.

The oxidative addition would subsequently take place in this

intermediate. However, the nonclassical dihydrogen complex

with one phosphine arm fully decoordinated is 30.0 kcal

mol^-1above the saturated dihydrogen complex. This high

Fe-P binding energy value led us to discard this mechanism

for the systems under study.

Isomerization through the Cp Ring. Some examples of

hydrogen migration involving the metal center and the Cp

ring can be found in the literature for iron compounds.

Examples are provided by [η⁴-C₅H₅(exo-D)]Fe(CO)₃,⁴³

(41) Faller, J. W.; Anderson, A. S. J. Am. Chem. Soc. 1970, 92, 5852-

5860.

(42) Clot, E.; Oelckers, B.; Klahn, A. H.; Eisenstein, O.; Perutz, R. N. J.

Chem. Soc., Dalton Trans. 2003, 4065-4074.

(43) Whitesides, T. H.; Shelly, J. J. Organomet. Chem. 1975, 92, 215-

226.

10256 Inorganic Chemistry, Vol. 45, No. 25, 2006

Analysis of Kinetic Isotope Effects

Scheme 3

for the simpler Cp model complex, but this search was

unsuccessful. All of our attempts to obtain such species

reverted to the dyhdrogen complex. Evidently, the simpler

Cp ligand is not basic enough for this system to be protonated

by the dihydrogen. We must point out that, for those CpFe

complexes where the hydrogen migration from the metal

center to the Cp ring has been experimentally observed, this

process occurs under more drastic conditions. In terms of

the currently examined mechanism, replacing Cp by Cp*

should facilitate the migration, making it possible under

milder conditions.

The relative energies of the intermediates and transition

states for the “via Cp” mechanism for the 2 and 4 systems

are given in Table 4. In the model system, the I_Cpcis

intermediate lies 14.9 kcal mol^-1above the dihydrogen and

the transition state that connects both is 0.4 kcal mol^-1above

the intermediate. The trans-Cp*-protonated complex is 3.4

kcal mol^-1more stable than the cis-protonated derivative.

The transition state for the rotation of the protonated ring is

found to be 30.0 kcal mol^-1above the initial dihydrogen

complex in the gas phase and 30.8 kcal mol^-1in dichloro-

methane. The energy barrier for the Cp* rotation is 15.1 kcal

mol^-1. The presence of the strong agostic bond, which is

preserved along the rotation, is the main reason for this high

value for a Cp* rotational process. Therefore, the rate-

determining step of the “via Cp” mechanism is the rotation

of the protonated Cp*. The computed activation enthalpy

and free energy are ∆H^q) 29.2 kcal mol^-1and ∆G^q) 30.7

kcal mol^-1. This intramolecular mechanism also implies a

small negative activation entropy (∆S^q) -5.0 eu). In the

gas phase, the energy barriers of the “direct” and “via Cp”

mechanisms for the dihydrogen f trans-dihydride intercon-

version in the model 2 system are very similar (about 30

kcal mol^-1). Dichloromethane slightly favors the direct

mechanism, increasing the difference between both barriers

from 1.1 to 2.2 kcal mol^-1. Next, we examine how the

presence of the phenyl substituents in the real system affects

this result.

For the 4 complex, we have not been able to find the

transition states TS_Cpcisand TS_Cptransbecause of the flatness

of the potential energy surface around the intermediates. The

main effect of the phenyl groups is to stabilize all of the

species involved in the mechanism with respect the dihy-

drogen complex (see Table 4). Given that the phenyl groups

are described at the MM level, the explanation for this

behavior should be in the steric effects. On the basis of the

results presented above on the relative stabilities of the

dihydrogen and dihydride forms, the main reason for this

behavior could be attributed to a destabilization of the

dihydrogen complex by the steric interaction between phenyl

and methyl groups. Very close values of the energy barriers

for the “direct” and “via Cp” mechanisms are found also

for the real system at the QM/MM level: 24.5 kcal mol^-1

(“direct”) and 23.9 kcal mol^-1(“via Cp”).

Fe(CO)₃(η⁴-C₆H₇Ph),⁴⁴and (η⁵-C₅H₅)FeH(triphos) (triphos

) Ph₂PCH₂CH₂PPh-CH₂CH₂PPh₂).⁴⁵These isomerizations

take place under more drastic conditions relative to the

dihydrogen f dihydride interconversion examined here. In

addition, this mechanism was briefly contemplated for the

analogous ruthenium system by Chinn and Heinekey³but

considered unlikely on the basis of bond strength consider-

ations. During the thorough exploration of the potential

energy surface for the dihydrogen f dihydride rearrangement

in the [Cp*Fe(dhpe)(H₂)]⁺system, we discovered the exist-

ence of low-lying intermediates with a protonated C₅ring.

This finding prompted us to fully explore the “via Cp”

mechanism.

This mechanism appears as a multistep pathway in which

the H_matom migrates from the cis to the trans position via

a successive series of H jumps from and to the metal, together

with a rotation of the protonated C₅ring. The mechanism of

the process is illustrated in Figure 5 (left-hand side), along

with the structure of all intermediates for the 2 system. The

most relevant geometrical parameters of the TS_rot(one

imaginary frequency with 87i cm^-1) for the Cp*-dhpe

system are given in Table 3, while the full list of calculated

frequencies are given as Supporting Information. The two

intermediates I_Cpcisand I_Cptransare formally unsaturated, 16-

electron dieneiron(II) complexes, [(η⁴-C₅Me₅H)Fe(dppe)H]⁺.

However, they are stabilized by an agnostic interaction

established by the newly created σ C-H bond and the Fe

center, which keeps the C₅ring essentially planar. The C-H

and Fe-H distances (1.248 and 1.661 Å in I_Cpcis; 1.216 and

1.757 Å in I_Cptrans) confirm the presence of this interaction.

The H-H bond is completely broken in I_Cpcis(H-H ) 2.005

Å). In the rotation transition state TS_rot, the proton is midway

between the two positions. The H-H distance is almost

constant along the rotation. The TS_rotalso presents a σ C-H

agostic interaction with the Fe center (C-H) 1.179 Å;

Fe-H ) 1.483 Å). The transition states TS_Cpcisand TS_Cptrans

have geometries very near those of the corresponding

intermediates, I_Cpcisand I_Cptrans, respectively. The optimized

geometries of all stationary points are available as Supporting

Information.

This first step can be viewed as an intramolecular

heterolytic splitting of the H-H bond of the dihydrogen

ligand. In cationic complexes, the η²-H₂ligand can be acidic

enough to protonate a basic cis ligand; see Scheme 3,⁴⁶

whereas the basic properties of a Cp* ligand are well-known.

We have searched the related ring-protonated intermediate

To introduce the electronic effects of the phenyl substit-

uents, we have performed full QM computations at the fixed

QM/MM structures of the optimized intermediates and

transition state of the 4 system. The relative energies of all

(44) Whitesides, T. H.; Neilan, J. P. J. Am. Chem. Soc. 1973, 95, 5811-

5813.

(45) Davies, S. G.; Felkin, H.; Watts, O. J. Chem. Soc., Chem. Commun.

1980, 159-160.

(46) Kubas, G. J. AdV. Inorg. Chem. 2004, 56, 127-177.

Inorganic Chemistry, Vol. 45, No. 25, 2006 10257

Baya et al.

Scheme 4

Figure 6. Example of a ³¹P NMR spectrum obtained during the kinetic

run of the isomerization experiment starting from Cp*FeH(dppe) +

CF₃COOD (1:1) at 0 °C in CD₂Cl₂.

process do not scramble the hydride ligands. The latter

statement was verified by an independent experiment, where

a sample of [Cp*Fe(dppe)(H)₂]⁺was generated and then

treated with an equivalent amount of CF₃COOD. No

deuterium incorporation occurred over 2 h at 273 K (the

isomerization process is extensive over this time scale, as

can be seen in Figure 1).

of the intermediates and TS_rotslightly decrease relative to

the dihydrogen (Table 4). At this level, the energy barrier

for the “via Cp” pathway remains again close to that

computed for the “direct” pathway and to the experimentally

determined values. Thus, the possibility of a dihydrogen f

dihydride interconversion via hydrogen migration to the Cp*

ring and involving an heterolytic H-H bond breaking

appears as a feasible and competing pathway, together with

the expected direct oxidative addition mechanism in these

cationic iron half-sandwich complexes. Therefore, the com-

putational work carried out so far does not allow a clear-cut

distinction of the two alternative mechanisms. For this reason,

we have decided to carry out a more detailed analysis of the

KIE.

4. Isomerization Kinetics of [Cp*Fe(dppe)(η²-HD)]⁺.

The isomerization kinetics starting from the hydride complex

Cp*Fe(dppe)H and the deuterated trifluoroacetic acid,

CF₃COOD, in a 1:1 ratio provided additional information

on the KIE. We carried out this experiment hoping to be

able to measure the isomerization rate constant of the pure

η²-HD complex (k_HD) and compare this with the results of

the rate constants given in Table 1 for the η²-H₂(k_HH) and

η²-D₂(k_DD) complexes. As it turns out, the outcome of this

experiment was more complex and interesting than expected

because the nonclassical species obtained by low-temperature

protonation establishes a rapid equilibrium with the mixture

of the other two isotopomers. This means that, although the

proton transfer to the hydride site is effectively quantitative

when using a strong acid such as CF₃COOD, the process is

still subject to rapid reversibility, generating both CF₃COOH

and Cp*Fe(dppe)D, which further lead to the other two

nonclassical complexes as shown in Scheme 4. There-

fore, subsequent isomerization yields all three classical

products.

[Cp*Fe(dppe)(η²-H₂)]⁺+ [Cp*Fe(dppe)(η²-D₂)]⁺a

2[Cp*Fe(dppe)(η²-HD)]⁺(1)

The information on the individual concentrations of the

three nonclassical and the three classical species from a

kinetic run carried out at 0 °C, starting with approximately

equimolar amounts of Cp*FeH(dppe) and CF₃COOD, was

obtained by integration of the inverse-gated doubly decoupled

2

³¹P{¹H, H} NMR resonances. An example of a recorded

spectrum is shown in Figure 6. The double decoupling from

proton and deuterium is essential to generating sufficiently

sharp resonances and allowing a satisfactory deconvolution

of the individual components, especially for the nonclassical

isotopomers. The inverse-gated procedure, together with long

relaxation times and the use of a small pulse angle (see details

in the Experimental Section), allowed one to remove or at

least minimize the distortion of the integrated intensities

caused by the nuclear Overhauser effect and by an incom-

plete magnetization recovery. Visual inspection of Figure 6

shows that the classical structure experiences a much greater

isotope shift than the nonclassical one. This is quite expected

because the isotope effect in the former is transmitted from

the hydride ligands to the detected ³¹P nucleus through

stronger Fe-H covalent bonds.

The evolution of the signal intensities is shown in Figure

7. The presence of a KIE is already quite evident from a

qualitative analysis of the experimental data. In fact, the

concentration of the η²-H₂complex is always below that of

the η²-D₂complex, whereas the classical dihydride product

always has a greater concentration than the dideuteride

product. Fitting the calculated decays of the three nonclassical

species and the calculated growth of the three classical

species to the experimental data gave the solid lines indicated

However, useful kinetics information could still be ob-

tained from this experiment because (i) the equilibration

between the three nonclassical isotopomers according to the

equilibrium shown in eq 1 is much more rapid than the

isomerization rate, (ii) the isomerization is irreversible, and

(iii) the classical dihydride products of the isomerization

10258 Inorganic Chemistry, Vol. 45, No. 25, 2006

Analysis of Kinetic Isotope Effects

From the results in Table 1 (run 8), we can derive k_HH

/

k_HD) 1.24 ( 0.01 and k_HD/k_DD) 1.58 ( 0.01. Isotope

effects have been previously reported for the oxidative

addition of dihydrogen to transition metals but only for

reactions between a metal complex and free dihydrogen, to

the best of our knowledge. Furthermore, they only relate to

the addition of dihydrogen versus dideuterium. Examples (k_H/

k_Din parentheses) are the additions to Fe(CO)₄(1.1 ( 0.1

at 24 °C),⁴⁷to Ir(PPh₃)₂(CO)Cl (1.09 at 25 °C),^48,49to

[Rh(PPh₃)₂Cl] (1.5 at 23 °C),⁵⁰to [Cr(CO)₅(C₆H₁₂)] (1.9 at

23 °C),⁵¹and to [W(PMe₃)₃I₂] (1.2 ( 0.2 at 60 °C).⁵²While

the oxidative addition process may take place in a single

step, a local energy minimum for an intermediate dihydrogen

complex may also occur. In the latter case, when the slow

step is the intramolecular rearrangement of the dihydrogen

ligand, the observed isotope effect results from the combina-

tion of an EIE for the dihydrogen coordination step and a

KIE for the rearrangement. The dihydrogen coordination

equilibrium constant has been measured for a number of

systems and is always characterized by an inverse EIE.

Examples (with the K_H/K_Din parentheses) are [Ir(PCy₃)₂-

HCl₂] (0.50 at -13 °C),⁵³[Ir(PtBu₂Me)₂H₂Cl] (0.37 at -13

°C),⁵⁴[Os(PⁱPr₃)₂(CO)(Cl)H] (0.35 at 85°C),⁵⁵[Cr(CO)₃-

(PCy₃)₂] (0.65 ( 0.15 at 22 °C),⁵⁶and [W(PMe₃)₄I₂] (0.63

( 0.05 at 60 °C).⁵²Thus, a greater normal KIE for the

isomerization process would be necessary to yield an

observed normal KIE for the oxidative addition. Our

determined k_HH/k_DDvalue is 1.96 ( 0.02 at 273 K, i.e.,

comparable to the highest reported value for an oxidative

addition process, namely, 1.9 for [Cr(CO)₅(C₆H₁₂)].⁵¹The

present report appears to be the first direct investigation of

the KIE for the rearrangement process of a dihydrogen

intermediate to the final dihydride product and the first

complete analysis encompassing the H₂, HD, and D₂isoto-

pomers. Note, however, that the rearrangement does not

consist of a simple H-H bond breaking completed by the

formation of the two M-H bonds with little change in the

rest of the coordination sphere because it occurs in the

majority of the other cases. A cis to trans rearrangement also

takes place. Thus, the current results should not be extrapo-

lated in general.

Figure 7. Evolution of the concentrations of (a) nonclassical and (b)

classical complexes during the isomerization process following the reaction

of Cp*Fe(dppe)H with CF₃COOD at 0 °C in CD₂Cl₂: diamonds, dihydride;

squares, monohydride monodeuteride; triangles, dideuteride. The solid lines

are the best fit on the basis of the kinetic model, as outlined in the text and

in the Supporting Information.

in Figure 7 for the optimized rate constant values reported

in Table 1 (run 8). The details of the data analysis, which

required the numerical integration of the coupled differential

equations and a global least-squares fitting procedure, are

available as Supporting Information. Note that the optimized

value for k_HHis identical within experimental error to that

independently (and more directly) obtained from the

CF₃COOH protonation (Table 1, run 6). In addition, the

optimized value of k_DDis slightly lower than that obtained

directly from the CF₃COOD protonation of the deuteride

sample (Table 1, run 7), which must be considered an upper

estimate, as discussed above.

The data analysis (see the Supporting Information) used

the assumptions that the mixture of the three nonclassical

isotopomers is instantaneously equilibrated by virtue of eq

1, with an equilibrium constant taken as the average

calculated from the experimental data (K ) 3.3 ( 0.3). Note

that this value is significantly different from that expected

for a statistical distribution without an isotope effect (K )

4). When the fitting procedure was repeated by manually

altering this K value to 4 or to other values, the obtained

residual was greater, confirming the significance of this

deviation. Such observations prove that, besides the KIE

affecting the dihydrogen-dihydride isomerization, there is

an additional equilibrium isotope effect (EIE) on the isotope

redistribution equilibrium among the three nonclassical

species. The reason for this EIE will be analyzed in the next

section.

We can conclude that the isomerization process from

[Cp*Fe(dppe)(η²-H₂)]⁺to 4 has a small isotope effect and

(47) Wang, W.; Narducci, A. A.; House, P. G.; Weitz, E. J. Am. Chem.

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Am. Chem. Soc. 1985, 107, 8049-8054.

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Chem. 1993, 97, 5890-5896.

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Soc., Chem. Commun. 1985, 30-32.

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R. A.; Parkin, G. J. Am. Chem. Soc. 1999, 121, 11402-11417.

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Chim. Acta 1990, 177, 115-120.

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116, 208-214.

(55) Bakhmutov, V. I.; Bertra´n, J.; Esteruelas, M. A.; Lledo´s, A.; Maseras,

F.; Modrego, J.; Oro, L. A.; Sola, E. Chem.sEur. J. 1996, 2, 815-

825.

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Capps, K. B.; Hoff, C. D. J. Am. Chem. Soc. 1997, 119, 9179-9190.

Inorganic Chemistry, Vol. 45, No. 25, 2006 10259

Baya et al.

Table 5. Frequency Analysis of the EIE for the Isotope Redistribution

Equilibrium in Complex [Cp*Fe(dhpe)(η²-XY)]⁺(2-XY) (X, Y ) H, D)

at 273 K

that this is not symmetric relative to the two hydride ligands.

This is not surprising because the two hydride ligands are

inequivalent in the rearrangement process (one migrates to

the new position, and the second one remains on the starting

site). Both KIEs and the EIE will now be analyzed from a

theoretical point of view, in terms of the two mechanistic

pathways that were found energetically compatible with the

measured activation enthalpy.

ν/cm^-1

2-H₂2-HD 2-D₂

assignment

MMI EXC ZPE

K

2762.7 2423.5 1956.3 ν(H-H)

1810.3 1561.0 1283.9 ν_as(Fe-H)

1.09 0.95 0.71 2.94

1.05 0.98 0.93 3.84

0.95 1.03 1.22 4.78

1096.9 903.1 785.6

ν_sym(Fe-H)

680.4 651.5 512.9 δ(Fe-H₂)_out-of-plane1.22 1.00 0.75 3.65

585.5 533.2 534.6 δ(Fe-H₂)_in-plane

468.1 370.6 359.4 τ(Fe-H₂)

total

0.91 1.00 1.15 4.19

0.82 1.00 1.26 4.10

0.97 0.96 0.88 3.30

5. Theoretical Analysis of the EIE. On the basis of the

energy-optimized structure of the nonclassical model com-

plex [Cp*Fe(dhpe)(η²-H₂)]⁺, i.e., system 2, frequency cal-

culations were run for the HD and D₂analogues. For the

sake of clarity, the three isotopomers have been named 2-H₂

(nondeuterated), 2-HD (monodeuterated), and 2-D₂(dideu-

terated). The EIE can be obtained directly from the calculated

∆G of the isotope redistribution equilibrium (see thermo-

chemical data in the Supporting Information). This is

obtained from a statistical mechanics treatment that uses

unscaled normal-mode frequencies. The resulting value of

the equilibrium constant for eq 1 is 3.81. The slight

disagreement with the experiment is expected, given the

various approximations used by the calculation.

Table 6. Calculated Activation Free Energies in kcal mol^-1[and

Relative ∆∆G^qValues with Respect to ∆G^q(HH) in Parentheses] for the

“Direct” and “via Cp” Mechanisms at 273 K (Model 2 System)

mechanism

∆G^q(HH)

∆G^q(H_mD_f)

∆G^q(D_mH_f)

∆G^q(DD)

direct

29.26 (0.00) 29.41 (0.15) 29.54 (0.28)

29.70 (0.44)

via Cp

31.27 (0.00) 31.41 (0.14) 31.15 (-0.12) 31.31 (0.04)

the experimental value. As Table 5 shows, the EIE is

dominated by the ZPE term, and the most significant

contribution to the reduction of K from the statistical value

of 4 is given by the ν(H-H) vibration. However, the

contribution of the other isotope-sensitive modes cannot be

neglected.

From the knowledge of the individual frequencies, the

equilibrium constant may also be calculated through the

classical Bigeleisen-Mayer treatment, where the constant

is given by the expression in eq 2. For the definition of the

6. KIE on the Dihydrogen-Dihydride Isomerization.

Analogous frequency calculations were run on the isoto-

pomers of the two transition states corresponding to the two

mechanisms found compatible with the experimental activa-

tion barrier. We must also consider that the symmetry of

the dihydrogen ligand is broken on going from the initial

nonclassical system to the transition state. While the two

hydride positions are symmetry-equivalent in the starting

dihydrogen complex, they become inequivalent in the

transition state. They will be identified as the “fixed” (H_f/

D_f) and “migrating” (H_m/D_m) positions, respectively (see

Figure 5). This implies that two pathways for each mecha-

nism must be considered in the case of the monodeuterated

species, depending on whether the migrating atom is H or

D.

As for the EIE analysis, the KIE can be derived directly

from the calculated free energies, in this case the activation

free energies. These are reported in Table 6, and the

corresponding ZPE and thermal enthalpy values are included

in the Supporting Information. Clearly, the migration of

hydrogen is favored over the migration of deuterium in the

mixed 2-HD species for the “direct” mechanism, whereas

the reverse is true for the “via Cp” mechanism. The resulting

calculated isotope effects are given in Table 7. The ∆∆G^q

values for the two different transition states of the 2-HD

systems (H_mD_fand D_mH_f) are small in each mechanism

(-0.117 kcal mol^-1for TS_dirand 0.264 kcal mol^-1for TS_rot),

allowing the calculation of k(H_mD_f)/k(D_mH_f) ) 1.241 for the

“direct” and 0.615 for the “via Cp” mechanism. Thus, a

considerable fraction of the HD sample will isomerize by

the less favorable pathway (H_mD_f/D_mH_f) 55.4:44.6 for the

“direct” and 38.1:61.9 for the “via Cp” mechanism). The

observed KIE must take this into account. Thus, the

calculated effective rate of isomerization for the 2-HD

K ) 4(MMI)(EXC)(ZPE)

(2)

MMI (mass moment of inertia), EXC (excitations of vibra-

tional energy levels), and ZPE (zero-point vibrational energy)

terms, see the Supporting Information and the literature.^57,58

The factor of 4 takes into account the doubled concentration

of the HD species in the equilibrium expression. The

advantage of the Bigeleisen-Mayer treatment is to pinpoint

the isotope-sensitive modes that are mostly responsible for

the observed effect. Related analyses have been previously

reported for the secondary isotope effects in the coordination

of C₂H₄vs C₂D₄to the Os₂(CO)₈moiety⁵⁹and for the primary

isotope effects in the H₂/D₂, CH₄/CH₃D, and CH₄/CD₄

oxidative additions to trans-Ir(PR₃)₂(CO)X,⁶⁰in the H₂/D₂

coordination to W(CO)₃(PCy₃)₂,⁵⁶and in the H₂/D₂/T₂

coordination and oxidative addition to W(CO)₅.⁶¹The

isotopic redistribution between H₂, HD, and D₂complexes

has never been analyzed to the best of our knowledge.

This type of analysis was carried out for our system on

the basis of the selected, most isotope-sensitive frequencies,

reported in Table 5 (no scaling factors have been used). Only

these normal modes are sensitive to the isotope nature and

contribute significantly to the EIE. A full list of vibrational

frequencies is provided in the Supporting Information section.

The calculated K value is 3.30 at 273 K, in agreement with

(57) Bigeleisen, J.; Mayer, M. G. J. Chem. Phys. 1947, 15, 261-267.

(58) Melander, L. Isotope effects on reaction rates; The Ronal Press Co.:

New York, 1960.

(59) Bender, B. R. J. Am. Chem. Soc. 1995, 117, 11239-11246.

(60) Abu-Hasanayn, F.; Krogh-Jespersen, K.; Goldman, A. S. J. Am. Chem.

Soc. 1993, 115, 8019-8023.

(61) Janak, K.; Parkin, G. Organometallics 2003, 22, 4378-4380.

10260 Inorganic Chemistry, Vol. 45, No. 25, 2006

Analysis of Kinetic Isotope Effects

Table 7. Frequency Analysis of the KIE for the Irreversible

Isomerization of [Cp*Fe(dhpe)(η²-AB)]⁺to [Cp*Fe(dhpe)(A)(B)]⁺(A,

B ) H, D) at 273 K

Table 8. Isotope-Sensitive Frequencies (cm^-1) for the Transition States

of the “Direct” (TS_dir) and “via Cp” (TS_rot) Mechanisms

(a) “Direct” Mechanism

b

c

X/Y^a

VP*

EXC*

ZPE*

k_X/k_Y

TS_dir-H₂TS_dir-H_mD_fTS_dir-D_mH_fTS_dir-D₂

assignment

(a) “Direct” Mechanism

-298.3

507.5

536.5

587.0

591.7

654.5

667.5

677.0

778.7

1025.3

1921.0

2027.7

-296.8

601.2

523.6

593.8

568.5

466.1

545.4

668.4

671.4

1024.9

1369.7

2027.5

-227.2

507.1

536.3

586.8

589.0

651.2

662.6

675.6

783.1

719.3

1921.3

1448.9

-226.6

598.8

522.6

593.7

567.3

465.1

544.3

664.7

668.9

721.3

1369.0

1449.4

ν*

HH/H_mD_f

HH/D_mH_f

H_mD_f/DD

D_mH_f/DD

0.938

0.753

0.772

0.961

1.050

1.126

1.093

1.019

1.313

1.380

1.521

1.447

1.304

1.619

1.693

1.364

1.299

1.536

1.680

1.421

(b) “Via Cp” Mechanism

δ(Fe-H_f)_in-plane

HH/H_mD_f

H_mD_f/DD

1.018

0.997

1.018

1.038

1.027

1.100

1.068

0.998

1.150

0.742

0.824

1.277

1.282

0.788

0.826

1.344

1.215

0.816

0.897

1.336

δ(Fe-H_m)_out-of-plane

ν(Fe-H_f)

ν(Fe-H_m)

^aThe m and f labels refer to the “moving” and “fixed” atoms involved

in the rearrangement. ^bFrom the ∆G^qanalysis (see the text). ^cFrom eq 3

and the selected frequencies in Table 8.

(b) “Via Cp” Mechanism

TS_rot-H₂TS_rot-H_mD_fTS_rot-D_mH_fTS_rot-D₂

species will be k_HD) 0.554k(H_mD_f) + 0.446k(D_mH_f) for the

“direct” pathway and k_HD) 0.381k(H_mD_f) + 0.619k(D_mH_f)

for the “via Cp” pathway. This leads to the calculation of

the following values for the KIE:

-86.8

364.8

394.7

440.1

469.1

531.2

537.0

548.6

637.3

686.8

768.6

772.9

801.6

819.1

823.7

846.6

929.9

983.2

1059.8

1083.3

1289.7

1408.1

1924.1

1952.3

-85.9

352.1

394.3

411.6

448.5

530.6

544.0

542.0

529.4

686.7

756.2

776.7

802.3

603.3

822.4

834.0

929.9

982.9

1059.7

1083.2

1289.7

1408.3

1926.1

1390.2

-86.6

364.3

382.1

439.3

467.6

519.6

535.4

546.1

637.1

681.4

768.3

772.8

834.4

813.6

821.3

847.3

699.9

1121.7

998.1

1075.1

906.9

1353.3

1386.2

1950.4

-85.7

351.8

381.7

411.5

447.8

516.8

543.2

539.6

529.1

681.4

755.9

776.6

834.7

602.6

817.5

832.6

700.0

1121.7

997.7

1075.0

906.6

1353.3

1381.3

1396.4

ν*

(a) direct

(b) via Cp

k

_HH/k_HD) 1.428

_HD/k_DD) 1.547

k_HH/k_HD) 0.924

k_HD/k_DD) 1.147

δ(Fe-H_f)

The computed values are in much better agreement with the

experimental ones (1.24 ( 0.01 and 1.58 ( 0.01, respec-

tively) for the “direct” mechanism. This result must, there-

fore, be considered in strong support of the occurrence of

the “direct” mechanism and against the “via Cp” mechanism.

As for the EIE analysis, the KIEs can also be calculated

by an extension of the Bigeleisen-Mayer method,^58,62with

the relevant relationship becoming that of eq 3. The

δ(Fe-H_m-C)

ν

_sym(Fe-H_m;Fe-C)

ν_as(Fe-H_m;Fe-C)

_sym(Fe-H_m;C-H_m)

ν(Fe-H_f)

KIE ) k_X/k_Y) (VP*)(EXC*)(ZPE*)

(3)

ν

expressions of the VP*, EXC*, and ZPE* terms are based

on the starting materials and transition states rather than on

the starting materials and products. They are again available

in the Supporting Information and in the literature.

around TS_dir. On the other hand, the “via Cp” mechanism

involves a transition state TS_rotconnecting two agostic

intermediates and having itself a significant agostic interac-

tion. The C-H_mand Fe-H_mbonds undergo little changes

around TS_rot, resulting in essentially isotope-independent

imaginary frequencies.

Table 8 contains only those frequencies that are sensitive

to the isotope nature and contribute significantly to the KIE.

The full list of frequencies is available in the Supporting

Information. The frequency analysis in this case is not as

straightforward as that for the EIE, especially for the “via

Cp” mechanism, because the Fe-X/Y and X-Y stretching

and bending motions are heavily mixed with other molecular

motions in several normal modes. Only those normal modes

having a major component from the Fe-X/Y bonds have

been explicitly assigned in Table 8. The calculation of the

various terms of eq 3 leads to the values collected in Table

7. It can be seen that the ZPE* term contributes the most

but the VP* term also gives an important contribution,

especially for the “direct” mechanism when comparing

different moving atoms (H_m/D_m). This is mostly due to the

ν*_X/ν*_Yterm contained in VP* (ratio of the imaginary

frequencies; see the Supporting Information) because large

changes at the level of the Fe-H_mand H_f-H_mbonds occur

The corresponding KIE values are again listed in Table

7. We can grossly attribute this difference to the variation

of the chemical environment of the migrating atom, H_m/D_m,

on going from the starting dihydrogen complex and the

transition state. For the “direct” mechanism, the strong H-H

and two weak Fe-H bonds (in-phase and out-of-phase

stretching vibrations at 1096.9 and 1810.3 cm^-1for the 2-H₂

species, respectively) are transformed into two stronger

Fe-H bonds, yielding stretching vibrations at 1921.0 (for

H_f) and 2027.7 (for H_m) cm^-1. However, the loss of the strong

H-H bond in the starting material (ν_HH) 2762.7 cm^-1)

dominates, yielding a normal KIE. The fact that KIE(HH/

HD) is greater when the migrating atom is D (1.536, primary

isotope effect) and lower when it is H (1.299, secondary

isotope effect) is not solely related to the bond stretching

vibrations because ν(Fe-X_m) > ν(Fe-X_f) for both X ) H

and D. Thus, the bending modes play a dominant role. The

(62) Bigeleisen, J. J. Chem. Phys. 1949, 17, 675-678.

Inorganic Chemistry, Vol. 45, No. 25, 2006 10261

Baya et al.

Chart 3

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 (k_HH, k_HD, and

k

_DD). The resulting KIEs at 273 K are k_HH/k_HD) 1.24 (

0.01 and k_HD/k_DD) 1.58 ( 0.01 (and, consequently, k_HH

/

k_DD) 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

D₂isotopomers, is unprecedented to the best of our knowl-

edge. 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.

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-H_fbond (its stretching vibration at 1952.3 cm^-1

in TS_rotis similar to that calculated in TS_dir) and an agostic

Fe‚‚‚H_m-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-H_mand Fe‚‚‚H_mvibrations, 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 H_matom

and a weakening for the H_fatom, leading to an inverse

primary KIE (migrating D) and a normal secondary KIE

(migrating H).

Acknowledgment. We thank the European Commission

through the HYDROCHEM program (Contract No. HPRN-

CT-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 05-03-

32415) and the Division of Chemistry and Material Sciences

of RAS (Russia), and from MEC (Project CTQ2005-09000-

CO2-01; 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.

Conclusions

We have reported here a detailed investigation of the

irreversible isomerization process of the dihydrogen inter-

mediate [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 mech-

anism: a “direct” pathway (∆E^q) 21.3 kcal mol^-1) and a

“via Cp” pathway (∆E^q) 22.2 kcal mol^-1), never suggested

previously, which involves Cp*H intermediates (cf. 21.6 (

0.8 kcal mol^-1from the experiment). Sorting between these

pathways was possible through the investigation of isotope

effects. Generation of the [Cp*Fe(dppe)(η²-HD)]⁺complex

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

10262 Inorganic Chemistry, Vol. 45, No. 25, 2006

Article Doi

Dihydrogen to dihydride isomerization mechanism in [(C5Me 5)FeH2(Ph2PCH2CH2PPh 2)]+ through the experimental and theoretical analysis of kinetic isotope effects

DOI: 10.1021/ic061428n

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Article abstract of DOI:10.1021/ic061428n

Full text of DOI:10.1021/ic061428n

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