Unexpected Mechanism for Substitution of Coordinated Dihydrogen in trans-[FeH(H2)(DPPE)2]+

Article abstract of DOI:10.1021/ic970493h

Substitution reactions of the type trans-[FeH(H₂)(DPPE)₂]⁺ + L → trans-[FeHL(DPPE)₂]⁺ + H₂ (L = MeCN, PhCN, DMSO) occur in a single measurable kinetic step. Although the observed rate constants, k_obs, in THF solution show a saturation behavior in [L] with a curvature that is sensitive to the steric requirements of L, the limiting rate constant is almost independent of L (ca. 7 × 10^-3 s^-1 at 30°C) and agrees with the values obtained under solvolytic conditions. The rate law in acetone solutions is simpler, with k_obs being independent of [L] and very close to the limiting value in THF. The thermal and pressure activation parameters for the limiting rate constants of the reaction with MeCN have been determined in THF, acetone and neat acetonitrile. The values of ΔH^? are close to 80 kJ/mol for the three solvents while ΔS^? is slightly negative in all cases. The activation volumes are very negative and solvent dependent: -23 ± 1 cm³/mol (THF), -18 ± 1 cm³/mol (acetone), and -35 ± 2 cm³/mol (acetonitrile). As a whole, the kinetic and activation parameters do not agree with a mechanism in which a direct substitution of H₂ for the incoming ligand takes place; instead a mechanism is proposed in which the initial opening of a DPPE chelate ring leads to an intermediate containing a monodentate DPPE and a weakly bound solvent molecule. Thus, the rate-determining step is an associative attack of L to this intermediate to form a species containing both coordinated L and H₂. The final substitution product is formed in a rapid intramolecular attack of the dangling PPh₂ arm followed by a cis/trans isomerization.

Full text of DOI:10.1021/ic970493h

Inorg. Chem. 1998, 37, 1623-1628
1623
Unexpected Mechanism for Substitution of Coordinated Dihydrogen in
trans-[FeH(H2)(DPPE)2]+
Manuel G. Basallote,*,† Joaqu´ın Dura´n,† M. Jesu´s Ferna´ndez-Trujillo,† Gabriel Gonza´lez,‡
M. Angeles Ma´n˜ez,† and Manuel Mart´ınez*,‡
Departamento de Ciencia de los Materiales e Ingenier´ıa Metalu´rgica y Qu´ımica Inorga´nica,
Universidad de Ca´diz, Apartado 40, E-11510 Puerto Real, Ca´diz, Spain, and Departament de Qu´ımica
Inorga`nica, Universitat de Barcelona, Diagonal 647, E-08028 Barcelona, Spain
ReceiVed April 29, 1997
Substitution reactions of the type trans-[FeH(H2)(DPPE)2]+ + L f trans-[FeHL(DPPE)2]+ + H2 (L ) MeCN,
PhCN, DMSO) occur in a single measurable kinetic step. Although the observed rate constants, kobs, in THF
solution show a saturation behavior in [L] with a curvature that is sensitive to the steric requirements of L, the
limiting rate constant is almost independent of L (ca. 7 × 10-3 s-1 at 30 °C) and agrees with the values obtained
under solvolytic conditions. The rate law in acetone solutions is simpler, with kobs being independent of [L] and
very close to the limiting value in THF. The thermal and pressure activation parameters for the limiting rate
constants of the reaction with MeCN have been determined in THF, acetone and neat acetonitrile. The values of
∆Hq are close to 80 kJ/mol for the three solvents while ∆Sq is slightly negative in all cases. The activation
volumes are very negative and solvent dependent: -23 ( 1 cm3/mol (THF), -18 ( 1 cm3/mol (acetone), and
-35 ( 2 cm3/mol (acetonitrile). As a whole, the kinetic and activation parameters do not agree with a mechanism
in which a direct substitution of H2 for the incoming ligand takes place; instead a mechanism is proposed in
which the initial opening of a DPPE chelate ring leads to an intermediate containing a monodentate DPPE and
a weakly bound solvent molecule. Thus, the rate-determining step is an associative attack of L to this intermediate
to form a species containing both coordinated L and H2. The final substitution product is formed in a rapid
intramolecular attack of the dangling PPh2 arm followed by a cis/trans isomerization.
Introduction
in some catalytic cycles.1,3,5,7 Given the fact that both experi-
mental determinations1,8-10 and theoretical calculations11,12 show
Following the initial efforts that were concentrated in the
synthesis and characterization of dihydrogen complexes, interest
has been focused more recently in the chemical properties and
reactivity of these complexes and their relevance to catalytic
processes.1-6 Consequently, a great amount of information is
available but little is known about the mechanisms of the
reactions involved.
that M-H2 bonds are not very strong (ca. 28-80 kJ/mol), a
limiting dissociative mechanism involving the existence of a
coordinatively unsaturated intermediate has been proposed
frequently for these reactions.13-18 This assignment has been
reinforced by the existence of several 16-electron complexes
resulting from H2 dissociation or able to add H2 to form stable
18-electron dihydrogen complexes.19-21 However, some of
these presumed intermediates can only be obtained using drastic
conditions or do not add H2 under the mild conditions in which
substitution reactions occur.2,19,22 Alternative pathways are, in
fact, found more effective in cases as [FeH(H2)(PP3)]+ (PP3)
tris(2-diphenylphosphinoethyl)phosphine), where a mechanism
involving the opening of a chelate ring adjacent to coordinated
Substitution of coordinated H2 is one of the simplest reactions
of dihydrogen complexes and it plays surely an important role
† Universidad de Ca´diz.
‡ Universitat de Barcelona.
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(2) Bautista, M. T.; Cappellani, E. P.; Drouin, S. D.; Morris, R. H.;
Schweitzer, C. T.; Sella, A.; Zubkowski, J. J. Am. Chem. Soc. 1991,
113, 4876.
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Esteruelas, M. A.; Oro, L. A. Organometallics 1992, 11, 138.
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Inorg. Chem. 1991, 30, 279. (b) Bianchini, C.; Peruzzini, M.; Zanobini,
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A.; Oro, L. A.; Peruzzini, M. Organometallics 1992, 11, 3837. (d)
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C. D.; Crabtree, R. H. J. Am. Chem. Soc. 1990, 112, 9643.
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S0020-1669(97)00493-X CCC: $15.00 © 1998 American Chemical Society
Published on Web 03/14/1998

1624 Inorganic Chemistry, Vol. 37, No. 7, 1998
Basallote et al.
H2 has been invoked3 to explain the catalytic properties of the
complex, despite the known existence of the unsaturated
compound [FeH(PP3)]+.23 Moreover, thermodynamic and ki-
netic studies of H2 dissociation from iridium complexes have
been carried out and the data suggest that the mechanism is not
so simple; strong evidence for concerted formation of an agostic
bond, or an L f M π-bond, in the resulting unsaturated
complexes have been found.8,9,24,25
Although it seems clear that H2 dissociation from these
complexes is not necessarily the first step in the substitution
processes of coordinated H2,1 it is evident that more kinetic work
is required to elucidate the mechanisms that operate in these
reactions. Following our interest in the tuning of the electronic
and steric characteristics that influence the substitution processes
of metal phosphine and other type of complexes,26,27 we have
carried out a systematic kinetic study on the substitution of H2
in one of the best known dihydrogen complexes, trans-[FeH-
(H2)(DPPE)2]+ (DPPE ) 1,2-bis(diphenylphosphino)ethane).2,28
The results presented in this paper include the effect of the
solvent and the incoming ligand on the kinetics of substitution
as well as the determination of thermal and pressure activation
parameters; in this case ∆Vq has been a decisive tool to elucidate
the mechanism, as in other substitution29 and oxidative addition
reactions.30 The data obtained as a whole clearly shows that
substitution of H2 in this complex does not occur in a simple
dissociative process and an alternative mechanism is required
to explain the kinetic results.
Figure 1. 31P{1H} NMR spectra recorded during the reaction of trans-
[FeH(H2)(DPPE)2]+ with MeCN excess showing the conversion to
trans-[FeH(MeCN)(DPPE)2]+. Spectra were recorded at 128 s intervals,
but some of them have been omitted for clarity. The experiment was
carried out in acetone-d6 at 10 °C under an argon atmosphere. The
Results and Discussion
concentrations of iron complex and MeCN were close to 1 × 10-3
M
Coordinated dihydrogen in trans-[FeH(H2)(DPPE)2]BF4 is
substituted easily by MeCN, PhCN, DMSO, and other mono-
and 1.2 M, respectively.
(19) Cappellani, E. P.; Drouin, S. D.; Jia, G.; Maltby, P. A.; Morris, R.
H.; Schweitzer, C. T. J. Am. Chem. Soc. 1994, 116, 3375.
(20) Rocchini, E.; Mezzetti, A.; Ru¨eger, H.; Burckhardt, U.; Gramlich, V.;
Del Zotto, A.; Martinuzzi, P.; Rigo, P. Inorg. Chem. 1997, 36, 711.
(21) (a) Mezzetti, A.; Del Zotto, A.; Rigo, P.; Farnetti, E. J. Chem. Soc.,
Dalton Trans. 1991, 1525. (b) Ogasawara, A. K.; Saburi, M.
Organometallics 1993, 12, 3393. (c) Jime´nez-Tenorio, M.; Puerta, M.
C.; Valerga, P. J. Am. Chem. Soc. 1993, 113, 9794. (d) Field, L. D.;
Hambley, T. W.; Yau, B. C. K. Inorg. Chem. 1994, 33, 2009. (e)
Chin, B.; Lough, A. J.; Morris, R. H.; Schweitzer, C. T.; D’Agostino,
C. Inorg. Chem. 1994, 33, 6278. (f) Kranenburg, M.; Kamer, P. C. J.;
van Leuwen, P. W. N. M.; Chaudret, B. J. Chem. Soc., Chem.
Commun. 1997, 373.
(22) Bautista, M. T.; Earl, K. A.; Maltby, P. A.; Morris, R. H. J. Am. Chem.
Soc. 1988, 110, 4056.
(23) Bianchini, C.; Laschi, F.; Peruzzini, M.; Ottaviani, F. M.; Vacca, A.;
Zanello, P. Inorg. Chem. 1990, 29, 3394.
(24) Zhang, K.; Gonza´lez, A. A.; Mukerjee, S. L.; Chou, S.; Hoff, C. D.;
Kubat-Martin, K. A.; Barnhart, D.; Kubas, G. J. J. Am. Chem. Soc.
1991, 113, 9170.
(25) Hauger, B. E.; Gusev, D.; Caulton, K. G. J. Am. Chem. Soc. 1994.
116, 208.
dentate ligands. The reactions are clean and take place without
any side product formation. Thus, Figure 1 shows the changes
observed in the phosphorus NMR spectra for the reaction with
acetonitrile in acetone-d6. No intermediates are detected, and
the intensities of both signals can be fitted to single exponentials;
the rate constants thus obtained agree within experimental error.
Similar conclusions are obtained when the incoming ligand is
PhCN or DMSO instead of MeCN. The time required for the
reactions to go to completion is adequate for a kinetic study
using conventional spectrophotometry; accordingly, the moni-
toring wavelength was selected in every case from preliminary
spectral scanning experiments, details are given in the Experi-
mental Section. When L is present in an excess, the absorbance
Versus time traces in THF solution for the substitution reactions,
trans-[FeH(H2)(DPPE)2]+ + L f trans-[FeHL(DPPE)2]+ + H2
(L ) MeCN, PhCN, DMSO)
(26) (a) Ma´n˜ez, M. A.; Puerta, M. C.; Valerga, P.; Basallote, M. G. J.
Chem. Soc., Dalton Trans. 1992, 1291. (b) Ma´n˜ez, M. A.; Ferna´ndez-
Trujillo, M. J.; Basallote, M. G. J. Chem. Soc., Dalton Trans. 1994,
1717. (c) Ma´n˜ez, M. A.; Ferna´ndez-Trujillo, M. J.; Basallote, M. G.
Polyhedron 1995, 14, 1865. (d) Ma´n˜ez, M. A.; Ferna´ndez-Trujillo,
M. J.; Basallote, M. G. Polyhedron 1996, 15, 2305.
behave as single exponentials, producing observed rate constants
kobs which are independent of the concentration of the iron
complex but show saturation kinetics on [L] as indicated in eq
1, Figure 2, and Tables S1 and S2 (Supporting Information).
(27) (a) Mart´ınez, M.; Muller, G. J. Chem. Soc., Dalton Trans. 1989, 1669.
(b) Gonza´lez, G.; Mart´ınez, M.; Rodriguez, E. J. Chem. Soc., Dalton
Trans. 1995, 891. (c) Estevan, F.; Gonza´lez, G.; Lahuerta, P.; Mart´ınez,
M.; Peris, E.; van Eldik, R. J. Chem. Soc., Dalton Trans. 1996, 1045.
(d) Go´mez, M.; Granell, J.; Mart´ınez, M. Organometallics 1997, 16,
2539.
(28) (a) Morris, R. H.; Sawyer, J. F.; Shiralian, M.; Zubkowski, J. D.; J.
Am. Chem. Soc. 1985, 107, 5581. (b) Bautista, M.; Earl, K. A.; Morris,
R. H.; Sella, A. J. Am. Chem. Soc. 1987, 109, 3780. (c) Ricci, J. S.;
Koetzle, T. F.; Bautista, M. T.; Hofstede, T. M.; Morris, R. H.; Sawyer,
J. F. J. Am. Chem. Soc. 1989, 111, 8823.
ab[L]
kobs
)
(1)
1 + b[L]
In acetone solution, though, the values of kobs for the same
reactions are independent of [L], that is kobs ) a. The values
for the parameters a and b for all the reactions are collected in
Table 1, which also includes the data for the corresponding
solvolysis reaction (i.e. in neat coordinating solvent). These
values correspond within reasonable error to the limiting value
of kobs for the saturation kinetics (i.e. a). The values found for
a are always close to 7 × 10-3 s-1 at 30 °C, and they are little
(29) Gonza´lez, G.; Moullet, B.; Mart´ınez, M.; Merbach, A. E. Inorg. Chem.
1994, 33, 2330.
(30) Crespo, M.; Mart´ınez, M.; de Pablo, E. J. Chem. Soc., Dalton Trans.
1997, 1321.

Dihydrogen Substitution in trans-[FeH(H2)(DPPE)2]+
Inorganic Chemistry, Vol. 37, No. 7, 1998 1625
Chart 1
K
k
k –
k
k
correspond to the discriminating ability of the pentacoordinated
intermediate between L and H2; since the concentration of
dihydrogen is in all cases lower than 1 × 10-4 M, our results
would suggest that the attack of H2 to the intermediate is several
orders of magnitude faster, in clear disagreement with known
data.33 Even so, in that case the concentration of H2 in solution
would increase during the reaction and the kinetic traces would
not be well-behaved.
Figure 2. Plots of kobs versus ligand concentration for the reaction of
trans-[FeH(H2)(DPPE)2]+ with MeCN (a), PhCN (b), and DMSO (c)
(THF solvent, 30 °C, argon atmosphere).
Although the pentacoordinated [FeH(DPPE)2]+ species ex-
ists,2,34 and several coordinatively unsaturated [MX(diphos-
phine)2]+ (M ) Fe, Ru, Os) species react with H2 to form stable
H2 complexes,19-21 this existence does not imply necessarily
that it is formed in the H2 substitution processes. In fact, the
formation of [FeH(DPPE)2]+ from trans-[FeH(H2)(DPPE)2]+
has not been observed, and, although the related [FeH(DTFE)2]+
complex (DTFE ) 1,2-bis[bis(p-trifluoromethylphenyl)phos-
phino]ethane) reacts readily with H2, the reverse process can
only be achieved under vacuum at 170 °C.19
Table 1. Kinetic Parameters at 30 °C for the Dihydrogen
Substitution Reactions Studied According to Eq 1 (Values in
Brackets Indicate the Values of kobs Determined in Neat L)
solvent
THF
L
103a/s-1
b/M-1
MeCN
PhCN
DMSO
MeCN
PhCN
6.7 ( 0.2 (6.7 ( 0.1)
7.8 ( 0.2 (7.3 ( 0.1)
7.6 ( 0.9 (6.4 ( 0.1)
8.7 ( 0.3 (6.7 ( 0.1)
7.2 ( 0.3 (7.3 ( 0.1)
32 ( 3
4.0 ( 0.3
0.9 ( 0.2
acetone
Table 2. Activation Parameters for the Dihydrogen Susbtitution
Reaction of trans-[FeH(H2)(Dppe)2]+ with MeCN
Another possible explanation is the existence of an as-
sociatively activated interchange mechanism:
solvent
∆Hq/kJ mol-1
∆Sq/J K-1mol-1
∆Vq/cm3mol-1
trans-[FeH(H2)(DPPE)2]+ + L {Kos}
MeCN
THF
acetone
78 ( 3
80 ( 4
86 ( 7
-29 ( 8
-25 ( 13
-4 ( 21
-35 ( 2
-23 ( 1
-18 ( 1
{trans-[FeH(H2)(DPPE)2]+, L}
9
k8
trans-[FeHL(DPPE)2]+ + H2
influenced by the nature of the entering ligand or solvent used.
In contrast, it is evident that the values for b are highly sensitive
to the steric characteristics of the ligand L (increasing from 0.9
to 32 M-1 for DMSO and MeCN, respectively) as well as to
the electronic characteristics of the solvent (a lower limit of
103 M-1 can be estimated from the absence of curvature in the
plots in acetone, a better coordinating solvent). The temperature
and pressure activation parameters for the limiting rate constant,
a, in the reaction with MeCN have been determined and results
are collected in Table 2. Runs carried out at elevated pressure
and different concentrations of MeCN in THF indicate the
absence of any measurable pressure effect on the curvature of
kobs vs[MeCN] plots at the high concentration used for the
determination of activation volumes. From data in Table 2, it
is clear that the values for the activation enthalpy are the same
within error for all the solvents, and the values for the activation
entropy are slightly negative but very close to zero in all cases.
Nevertheless the activation volumes are significantly negative,
and the value found in neat MeCN is clearly more negative.
All the kinetic information clearly indicates that substitution
of coordinated dihydrogen in trans-[FeH(H2)(DPPE)2]+ does
not occur through the limiting D mechanism proposed both for
reactions of polyhydride and dihydrogen complexes,13-18 and
the related low-spin Fe(II)-phosphine complexes.31 In that
case, the limiting rate constant a would correspond to H2
dissociation and the value of ∆Vq should be positive and close
to +20 cm3/mol, as observed for the reductive elimination of
H2.32 Furthermore, if that were the case the values of b should
where a ) k and b ) Kos; the very negative values found for
the activation volume associated with k suggest a definite
associative character for these substitution reactions. Neverthe-
less, the values for Kos are expected to be larger for solvents
with lower dielectric constants, contrarily to the experimental
facts. Furthermore, although 20 electron species have been
claimed as reasonable transition states for substitution reactions
in organometallic complexes, all of them seem to operate for
large metal centers.35 In our case, the small low-spin Fe(II)
metal center and the bulky phenyl substituents on the bidentate
ligand are difficult to associate with an increased coordination
sphere for the transition state.
A point that merits some comment is the fluxional behavior
of the trans-[FeH(H2)(DPPE)2]+ complex that makes all H
atoms homotopic;2 as a consequence, all hydrogens acquire a
certain hydridic character that stabilizes the interaction with the
iron center and makes much more difficult their dissociation as
H2. In all, it seems clear that direct substitution (associatively
or dissociatively activated) is rather inadequate due both to the
bulky ligands on a small metal center and the strength of the
bond with the leaving ligand in the starting complex.
(33) (a) Church, S. P.; Grevels, F. W.; Hermann, H.; Schaffner, K. J. Chem.
Soc., Chem. Commun. 1985, 30. (b) Hodges, P. M.; Jackson, S. A.;
Jake, J.; Poliakoff, M.; Turner, J. J.; Grevels, F. W. J. Am. Chem.
Soc. 1990, 112, 1234. (c) Hudson, R. H. E.; Po¨e, A. J.; Sampson, C.
N.; Siegel, A. J. Chem. Soc., Dalton Trans. 1989, 2235.
(34) Aresta, M.; Giannoccaro, P.; Rossi, M.; Sacco, A. Inorg. Chim. Acta
1971, 5, 115.
(31) Henderson, R. A. J. Chem. Soc., Dalton Trans. 1988, 509.
(32) Anhaus, J.; Bajaj, H. C.; van Eldik, R.; Nevinger, L. R.; Keister, J. B.
Organometallics 1989, 8, 2903.
(35) (a) Zhang, S.; Zhang, V.; Dobson, G. R.; van Eldik, R. Inorg. Chem.
1991, 30, 355. (b) Reddy, K. B.; van Eldik, R. Organometallics 1990,
9, 1418.

1626 Inorganic Chemistry, Vol. 37, No. 7, 1998
Basallote et al.
{trans-[FeH(H2)(η2-DPPE)2]+, L} a
The experimental rate law (eq 1) can be interpreted according
to one of the reaction sequences shown in Chart 1. For a
mechanism of Type I, the limiting rate constant would be k2
and intermediate B accumulates in the presence of a large excess
of L. As both the NMR and UV-vis spectra recorded
immediately after mixing do not differ significantly from those
obtained in the absence of L, this kind of mechanism requires,
unreasonably, that B have NMR and electronic spectral proper-
ties very similar to those of the starting complex (to be, for
example, an outer-sphere complex). The very negative activa-
tion volumes found for step k2 requires the previous conversion
of B to some kind of coordinatively unsaturated intermediate
bound to have different spectral characteristics, thus precluding
this mechanism.
{[FeH(H2)(η2- DPPE)(η1-DPPE)]+, L}
K1, fast (3)
{[FeH(H2)(η2- DPPE)(η1-DPPE) ]+, L} a
[FeH(H2)L(η2- DPPE)(η1-DPPE) ]+
k2 (4)
[FeH(H2)L(η2-DPPE)(η1-DPPE) ]+ a
trans-[FeHL(η2-DPPE)2 ]+ + H2
fast (5)
B (similar to A with eqs 2 and 3 replaced by eq 6)
For a mechanism of Type II, the limiting rate constant should
correspond to k1 and no buildup of intermediate will be expected.
It is difficult, though, to imagine an intramolecular process
involving a very large volume decrease unless there is a previous
conversion of trans-[FeH(H2)(DPPE)2]+ to an unsaturated
intermediate. Thus, to accommodate all the experimental
observations, both mechanisms require a further additional step
to convert the starting compound to a reaction intermediate in
very low concentration capable of undergoing an associative
substitution. Given the fact that no structural information is
available for this intermediate, the experimental data are
consistent with any intramolecular transformation of the starting
complex (or with participation of the solvent or the counterion).
Once the simplest interpretations (H2 dissociation and direct
associative attack of L) can be ruled out by the reasons quoted
above other possibilities have to be explored. Although an
isomerization process is possible, it is not expected to involve
a volume decrease as that determined experimentally; hydrogen
trans-[FeH(H2)(η2-DPPE)2]+ + L a
{[FeH(H2)(η2- DPPE)(η1-DPPE)]+, L}
k1, k-1 (6)
For reaction sequence A, the rate law would be given by eq
1 with a ) K1k2 and b ) Kos, assuming that the equilibrium
constant K1 for the opening of the chelate ring is small. The
major initial product in the presence of a large L excess would
be {trans-[FeH(H2)(DPPE)2]+, L}, with spectral properties
similar to the starting complex. The value of k2, corresponding
to L attack to a little discriminating unsaturated intermediate,
has to be little affected by the nature of both the solvent and
the incoming ligand. The experimental activation volumes
q
would then correspond to ∆V1° + ∆V2 . The value of ∆V1° is
expected to be small because the partial volume increase caused
by the existence of a free PPh2 group is compensated by the
volume decrease due to a reduced coordination sphere and
probably also by intramolecular hydrogen bonding between
uncoordinated phosphorus and the acidic H2 ligand. Therefore,
the major contribution to the very negative activation volumes
bonding of trans-[FeH(H2)(DPPE)2]+ to BF4 is possible, but
-
the anion concentration is very small relative to both L and
solvent; furthermore, its effect would be that of stabilizing the
starting complex; proton dissociation is also possible, but it
would lead to cis-[FeH2(DPPE)2], which does not react with
MeCN. An attractive possibility is that the limiting rate constant
corresponds to an associative process that takes place once the
opening of one of the chelate rings to form an intermediate
species containing a monodentate DPPE has occurred. In fact,
the dissociation of a spectator ligand has been suggested to
explain both the catalytic properties3 of trans-[FeH(H2)(PP3)]+
and the conversion of [Ru(H2)(η5-C5H5)(PPh3)(CNBut)]+ to its
HD isotopomer,7 as well as for the initial step in H2-D2
equilibration catalyzed by Pt-Au clusters.36 Moreover, the
opening of a chelate ring has been also proposed to explain the
kinetics of protonation of trans-[FeHX(diphosphine)2] (diphos-
phine ) DPPE, DEPE; X ) Cl, Br)31 and the substitution
reactions of trans-[{FeCl(DPPE)2}2N2].37 If the unsaturated
intermediate is assumed to be formed in a chelate ring-opening
process, substitution reactions of trans-[FeH(H2)(DPPE)2]+
could occur through any of the following mechanisms:
q
is due to ∆V2 , which corresponds to an associative attack of L
to the unsaturated intermediate. Nevertheless, the strong solvent
dependence of ∆Vq suggests some kind of solvent interaction
with the [FeH(H2)(η2-DPPE)(η1-DPPE)]+ species; thus, if
solvent and incoming ligand are the same (solvolysis), the
volume decrease associated with k2 would be more negative
because of the lack of solvent molecule release. Furthermore,
the values of b for a given solvent have to reflect the differences
in Kos due to steric requirements of the incoming ligand, and
the larger b values in acetone show that equilibrium Kos does
not only involve electrostatic interactions but that there must
be also some kind of solvent dependent specific interactions.
If reaction sequence B is considered, application of the
improved steady-state38 approximation also leads to eq 1, with
a ) k2 and b ) k1/(k-1 + k2). The activation volumes
determined from the limiting rate constants would correspond
directly to the associatively activated k2 step as defined above,
and most comments about sequence A would also apply,
including those relative to the role of the solvent. Furthermore,
the value of b would be the result of several contributions and
A
(36) (a) Aubart, M. A.; Chandler, B. D.; Gould, R. A. T.; Krogstad, D. A.;
Schoondergang, M. F. J.; Pignolet, L. H. Inorg. Chem. 1994, 33, 3724.
(b) Rubinstein, L. I.; Pignolet, L. H. Inorg. Chem. 1996, 35, 6755. (c)
Krogstad, D. A.; Konze, W. V.; Pignolet, L. H. Inorg. Chem. 1996,
35, 6763.
(37) Henderson, R. A. J. Chem. Soc., Dalton Trans. 1988, 515.
(38) Espenson, J. H. Chemical Kinetics and Reaction Mechanisms, 2nd ed.;
McGraw-Hill: New York, 1995; pp 86-90.
trans-[FeH(H2)(η2-DPPE)2]+ + L a
{trans-[FeH(H2)(η2-DPPE)2]+, L}
Kos, fast (2)

Dihydrogen Substitution in trans-[FeH(H2)(DPPE)2]+
Inorganic Chemistry, Vol. 37, No. 7, 1998 1627
the curvature in the plots should reflect, as it does, the steric
effects of L and the ability of the solvent to favor formation of
the intermediate. In this case, the steady-state approximation
applied also accounts for the lack of spectral differences between
the starting material and the reaction mixture at very short
reaction times.
W complexes leads to a pentacoordinate intermediate with a
value of ∆Hq several kJ/mol larger than the M-H2 bond
energy.8,9 A mechanism involving concerted dissociation of the
leaving ligand and formation of an agostic bond with a
cyclohexyl group has been proposed to explain the kinetics of
these reactions.24 Thus, despite the reduced number of kinetic
studies, a variety of mechanisms have been proposed for
substitution of coordinated dihydrogen. Although reactions may
proceed in some cases through simple dissociative mechanisms,
it is evident that more work is required to understand the reasons
that lead to the operation of alternative mechanisms and their
relevance to catalyzed hydrogenations.
Thus, although the nature of the reaction intermediate is not
defined by experimental data, both reaction sequences, A and
B, satisfactorily explain the kinetic results, and it is clear at
this time that the rate-determining step for substitution of
coordinated H2 in trans-[FeH(H2)(DPPE)2]+ corresponds to a
rather unexpected process involving an associatively activated
transition state. As pointed out before, the fluxional process
can be responsible for a stronger H-Fe-H2 interaction that
hinders H2 dissociation and favors the chelate ring-opening
pathway. However, there is also the possibility that the Fe-
P(chelate) bond is inherently weaker than the Fe-H2 bond, and
so it is dissociated preferentially. Actually, there is evidence37
of a chelate ring-opening process in the protonation of the related
complex trans-[FeHCl(DPPE)2], which lacks the fluxional
processes that stabilize the leaving ligand. However, the vacant
coordination site created does not seem to be used in the
substitution reactions of that complex.37 Another question that
arises from the mechanism proposed for substitution of H2 in
trans-[FeH(H2)(DPPE)2]+ is the reason that makes substitution
of H2 in the intermediate species [FeHL(H2)(η2-DPPE)(η1-
DPPE)]+ faster than in the starting material. Recent works have
revealed a significant labilization of H2 in complexes containing
a π-donor ligand in cis,10,18,20,25 which would account for the
faster substitution in the intermediates formed by all the entering
ligands, better π-donors than the PPh2 group. Nevertheless,
important changes in the electronic and steric characteristics of
the complexes are also associated to the opening of the DPPE
chelate ring and may contribute to labilization of H2. That is,
both the relief of steric constraints on going from an η2- to an
η1-diphosphine and the decrease in electron density on the metal
once phosphorus is dissociated can easily enhance associative
substitution.
Experimental Section
Products. All the preparative and kinetic work was carried out under
an argon atmosphere using Schlenk and syringe techniques. NMR
spectra were obtained with a Varian Unity 400 spectrometer. The
complex trans-[FeH(H2)(DPPE)2](BF4) was prepared from the corre-
sponding dihydride complex according to literature procedures2 and
its purity was checked by 1H and 31P NMR. FeCl2, DPPE, and PhCN
were obtained from Aldrich. DMSO, acetonitrile, acetone, and
tetrahydrofuran were obtained from SDS (Solvants Documentation
Syntheses). The solvents were dried by refluxing them from appropriate
drying agents and deoxygenated by bubbling argon through them
immediately before use. All the products of substitution reactions
showed a singlet in the phosphorus spectra at positions typical of trans-
[FeHX(DPPE)2]+ complexes:40 84.3 ppm (X ) MeCN), 83.5 ppm (X
) PhCN), and 103.1 ppm (X ) DMSO).
Kinetics. Kinetic experiments at room pressure were carried out
under pseudo-first-order conditions (ligand excess) by adding weighed
amounts of solid trans-[FeH(H2)(DPPE)2](BF4) to previously thermo-
stated solutions containing the desired concentration of incoming ligand.
The resulting solutions, with a concentration of iron complex of 5 ×
10-4 M, were rapidly transferred with a Teflon tube to a thermostated
flow cell. The time required to carry out the transfer is not significant
with respect to the reaction time. Absorbance versus time traces were
obtained with a Perkin-Elmer Lambda 3B spectrophotometer interfaced
to a PC computer and were analyzed by conventional least-squares
procedures. The monitoring wavelength was selected for every reaction
from preliminary spectral scanning experiments: 445 nm (MeCN), 405
nm (PhCN), and 370 nm (DMSO). The first-order dependence of the
rate on the concentration of iron complex was confirmed in some
preliminary experiments that led to observed rate constants independent
of the concentration of trans-[FeH(H2)(DPPE)2]+. Thermal activation
parameters were obtained from kinetic data at different temperatures
using the standard Eyring plots.
From the previous discussion, it is evident that the mechanism
proposed can only be considered a first approximation to
substitution mechanisms in this kind of dihydrogen complexes
and more work is in progress to gain insight into the many
questions that arise from the results obtained for trans-[FeH-
(H2)(DPPE)2]+. In any case, it seems clear at this time that
experimental observations cannot be explained with a single
mechanism involving direct substitution of H2 for L. Due to
the limited kinetic data available, it is not possible to decide if
the mechanism proposed is valid for reactions of other dihy-
drogen complexes. In fact, the rate law is similar for all the
complexes studied to date (the observed rate constants are
independent of the concentration of incoming ligand, or they
show saturation behavior), and the thermal activation parameters
are also similar to those in Table 2 (∆Hq ranging from 28 to 80
kJ/mol, and ∆Sq usually close to zero).8,10,13,14,18,25,40 Although
the value of the M-H2 bond dissociation energy has been
calculated to be between 40 and 82 kJ/mol for the model
compound trans-[FeH(H2)(PH3)4]+ 11 and other complexes,12
covering a range very similar to that of the values found for
∆Hq, these are also similar to those found in classical associative
substitutions. Thus, there is the possibility that substitution of
coordinated H2 in some other complexes occur through a
mechanism similar to that proposed for trans-[FeH(H2)-
(DPPE)2]+. Consideration of [M(CO)3(PCy3)2(H2)] complexes
requires special attention; elimination of H2 from the Cr and
For runs at elevated pressure, a previously described pressurizing
system41 and cylindrical cell fitted with a plunger were used.27d The
solutions were made directly in the pressure cell by mixing the necessary
amounts of solid product, or nonreacting THF or acetone solution, with
the entering MeCN ligand. In these cases the absorbance versus time
traces were recorded at a fixed wavelength on a Shimadzu UV1230
instrument fitted with fiber optics; alternatively, a TIDAS instrument
measuring the full spectrum has also been used. Rate constants were
derived from exponential least-squares fitting by the standard routines
at a fixed wavelength or evaluating all the spectral changes.
Acknowledgment. Financial support from the Junta de
Andaluc´ıa (Universidad de Ca´diz) and the DGICYT (PB94-
0857) and the Direccio´ General d’Universitats (Universitat de
Barcelona) is gratefully acknowledged.
(39) Zhang, K.; Gonza´lez, A. A.; Hoff, C. D. J. Am. Chem. Soc. 1989,
111, 3627.
(40) Ittel, S. D.; Tolman, C. A.; Krusic, P. J.; English, A. D.; Jesson, J. P.
Inorg. Chem. 1978, 17, 3432.
(41) (a) Spitzer, M.; Gartig, F.; van Eldik, R. ReV. Sci. Instrum. 1988, 59,
2092. (b) Fleischman, F. K.; Conze, E. G.; Stranks, D. R.; Kelm, H.
ReV. Sci. Instrum. 1974, 45, 1427.

1628 Inorganic Chemistry, Vol. 37, No. 7, 1998
Basallote et al.
Supporting Information Available: Tables of observed rate
constants as a function of solvent, entering ligand, temperature, pressure
and concentration of entering ligand, and limiting rate constants as a
function of solvent, entering ligand, temperature and pressure. Figure
including plots of ln a, as defined in eq 1, versus P for the reaction of
trans-[FeH(H2)(DPPE)2]+ with MeCN in (a) neat MeCN, (b) acetone,
and (c) THF (15.1 °C, argon atmosphere) (3 pages). Ordering
information is given on any current masthead page.
IC970493H