Solvent-assisted reversible proton transfer within an intermolecular dihydrogen bond and characterization of an unstable dihydrogen complex

Full text of DOI:10.1021/ic981320a

2
550
Inorg. Chem. 1999, 38, 2550-2551
Solvent-Assisted Reversible Proton Transfer within an Intermolecular Dihydrogen Bond and
Characterization of an Unstable Dihydrogen Complex
†
†
,†
‡
‡
Stephan Gr u1 ndemann, Stefan Ulrich, Hans-Heinrich Limbach,* Nikolai S. Golubev, Gleb S. Denisov,
§
|
|
Lina M. Epstein, Sylviane Sabo-Etienne, and Bruno Chaudret
Freie Universit a¨ t, Berlin, Takustrasse 3, D-14195 Berlin, Germany, Institute of Physics, St. Petersburg State University,
1
98904 St. Petersburg, Russian Federation, A. N. Nesmeyanov Institute of Organoelement Compounds,
Laboratory of Organometalic Compounds, Vavilov Street, 28, 117813 Moscow, Russian Federation,
and Laboratoire de Chimie de Coordination du CNRS (UPR 8241), 205, route de Narbonne, F-31077 Toulouse Cedex, France
ReceiVed NoVember 16, 1998
Transition-metal polyhydrides and dihydrogen complexes have
received much attention in the past few years as a result of their
rich chemistry1 and physicochemical properties involving in
Scheme 1
2
particular interesting intramolecular hydrogen exchange and
3
intermolecular proton-transfer processes. Up to date, little is
known about the details of the latter processes. Only recently has
it been recognized that transition-metal hydrides can form
hydrogen bonds to proton donors. However, the need for the
presence of a dihydrogen bond for the proton transfer to occur is
still an open question. Few examples of proton transfer within
this “dihydrogen bond” have been observed or proposed, the result
of which is the protonation of the hydride ligand to give a
dihydrogen species which is generally unstable as it may easily
2
Et O immediately led to extensive dihydrogen evolution and even
dehydrogenation of one cyclohexyl ring stabilized by agostic
4
C-H- - -Ru interactions.6
In this study we obtained further insights by NMR of the
mechanism of proton transfer to transition metal hydrides within
dihydrogen-bonded complexes by using the Freon mixture
2 3
CDCl F/CDF (2:1) as solvent. This solvent has proven to be
useful for NMR studies down to 100 K where the slow hydrogen
release H
2
as shown in Scheme 1.
7
bond regime can often be reached. Here, we took advantage of
During a study of the role of dihydrogen bonds on the chemistry
of polyhydride and hydrido dihydrogen complexes3k,4r,5 some of
us observed the particularly interesting case of the ruthenium
another property of this solvent, i.e., that its dielectric constant
8
increases strongly when the temperature is reduced, thus assisting
the protonation of 1.
trihydride complex Cp*RuH
3
(PCy
3
) (1), Cp* ≡ C
5 3 5
(CH ) which
exhibits exchange couplings and a classical exchange between
the hydride nuclei at low temperatures.2 In toluene solution 1
formed dihydrogen bonds with weak proton donors leading to a
(4) (a) Richardson, T. G.; de Gala, S.; Crabtree, R. H.; Siegbahn, P. E. M.
J. Am. Chem. Soc. 1995, 117, 12875. (b) (Crabtree, R. H.; Siegbahn, P.
E. M.; Eisenstein, O.; Rheingold, A. L.; Koetzle, T. F. Acc. Chem. Res.
b,e
1996, 29, 348. (c) Lee, J. C., Jr.; Peris, E.; Rheingold, A. L.; Crabtree,
5
considerable increase of the exchange couplings. By contrast,
R. H. J. Am. Chem. Soc. 1994, 116, 11014. (d) Peris, E.; Lee, J. C., Jr.;
Rambo, J. R.; Eisenstein, O.; Crabtree, R. H. J. Am. Chem. Soc. 1995,
the interaction of 1 with stronger proton donors such as HBF ‚
4
117, 3845. (e) Peris, E.; Wessel, J.; Patel, B. P.; Crabtree, R. H. J. Chem.
Soc., Chem. Commun. 1995, 2175. (f) Yuo, W.; Crabtree, R. H. Inorg.
Chem. 1996, 35, 3007. (g) Lough, A. J.; Park, S.; Ramachandran, R.;
Morris, R. H. J. Am. Chem. Soc. 1994, 116, 8356. (h) Park, S.;
Ramachandran, R.; Lough, A. J.; Morris, R. H. J. Chem. Soc., Chem.
Commun. 1994, 2201. (i) Park, S.; Lough, A. J.; Morris, R. H. Inorg.
Chem. 1996, 35, 3001. (j) Schlaf, M.; Lough, A. J.; Morris, R. J.
Organometallics 1996, 15, 4423. (k) Wessel, J.; Lee, Jr., J. C.; Peris,
E.; Yap, G. P.; Fortin, J. B.; Ricci, J. S.; Sini, G.; Albinati, A.; Koetzle,
T. F.; Eisenstein, O.; Rheingold, A. L.; Crabtree, R. H. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 2507. (l) Patel, B. P.; Yao, W.; Yap, G. P. A.;
Rheingold, A. L.; Crabtree, R. H. J. Chem. Soc., Chem. Commun. 1996,
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V.; Epstein, L. M.; Gusev, D. G.; Niedermann, M.; Berke, H. J. Am.
Chem. Soc. 1996, 118, 1105. (n) Belkova, N. V.; Shubina, E. S.; Ionidis,
A. V.; Epstein, L. M.; Jacobsen, H.; Messmer, A.; Berke, H. Inorg. Chem.
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J. Organomet. Chem. 1997, 536-537, 17. (r) Guari, Y.; Ayllon, J. A.;
Sabo-Etienne, S.; Chaudret, B.; Hessen, B. Inorg. Chem. 1998, 37, 640.
(5) Ayllon, J. A.; Sabo-Etienne, S.; Chaudret, B.; Ulrich, S.; Limbach, H.-
H. Inorg. Chim. Acta 1997, 259, 1.
*
Corresponding author. Fax: +49 30 838-5310. E-mail: limbach@
chemie.fu-berlin.de.
†
Freie Universit a¨ t.
St. Petersburg State University.
A. N. Nesmeyanov Institute of Organoelement Compounds.
CNRS (UPR 8241).
‡
§
|
(
1) (a) Jessop, P. G.; Morris, R. H. Coord. Chem. ReV. 1992, 121, 155. (b)
Heinekey, D. M.; Oldham, W. J., Chem. ReV. 1993, 93, 913. (c)
Esteruelas, M. A.; Oro, L. A. Chem. ReV. 1998, 98, 577.
(
2) Selected references: (a) Sabo-Etienne, S.; Chaudret, B. Chem. ReV. 1998,
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8, 2077. (b) Arliguie, T.; Border, C.; Chaudret, B.; Devillers, J.;
Poilblanc, R.; Organometallics 1989, 8, 1308. (c) Zilm, K. W.; Heinekey,
D. M.; Millar, J. M.; Payne, N. G.; Neshyba, S. P., Duchamp, J. C.;
Szcyrba, J. J. Am. Chem. Soc. 1990, 112, 920. (d) Guari, Y.; Sabo-
Etienne, S.; Chaudret, B. J. Am. Chem. Soc. 1998, 120, 4228. (e)
Limbach, H.-H.; Ulrich, S.; Gr u¨ ndemann, S.; Buntkowsky, G.; Sabo-
Etienne, S.; Chaudret, B.; Kubas, G. J.; Eckert, J. J. Am. Chem. Soc.
1
998, 120, 7929.
(
3) (a) Baker, M. V.; Field, L. D.; Young, D. J. J. Chem. Soc., Chem.
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Commun. 1989, 255. (c) Bampos, N.; Field, L. D. Inorg. Chem. 1990,
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9, 587. (d) Field, L. D.; Hambley, T. W.; Yau, B. C. K. Inorg. Chem.
994, 33, 2009. (e) Bullock, R. M.; Song, J.-S.; Szalda, D. J.
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F. J. Organometallics 1991, 10, 1888.
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Fern a´ ndez-Trujillo, M. J.; M a´ n˜ ez, M. A.; Rodr ´ı guez de la Torre, J. J.
Chem. Soc., Dalton Trans. 1998, 745. (g) Basallote, M. G.; Dur a´ n, J.;
Fern a´ ndez-Trujillo, M. J.; M a´ n˜ ez, M. A. J. Chem. Soc., Dalton Trans.
(7) (a) Denisov, G. S.; Golubev, N. S. J. Mol. Struct. 1981, 75, 311. (b)
Smirnov, S. N.; Golubev, N. S., Denisov; G. S., Benedict, H.; Schah-
Mohammedi, P.; Limbach, H. H., J. Am. Chem. Soc. 1996, 118, 4094.
(c) Shenderovich, I.; Smirnov, S.; Denisov, G. S.; Gindin, V.; Golubev,
N. S.; Dunger, A.; Reibke, R.; Kirpekar, S.; Malkina, O. L.; Limbach,
H.-H. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 422. (d) Golubev, N.
S.; Denisov, G. S.; Smirnov, S. N.; Shchepkin, D.; Limbach, H.-H. Z.
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492.
1
1
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H. Inorg. Chem. 1993, 32, 3628. (j) Shubina, E. S.; Krylov, A. N.;
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1
0.1021/ic981320a CCC: $18.00 © 1999 American Chemical Society
Published on Web 05/13/1999

Communications
Inorganic Chemistry, Vol. 38, No. 11, 1999 2551
If we increase the concentration of the alcohol to about 4.3
equiv, Jab increases only slightly indicating that 1 is present almost
fully as hydrogen bonded complex 1b (Figure 1f). By contrast,
the concentration of 2 increases indicating a classical slow
equilibrium between 1b and 2. This is consistent with the
assumption that a single alcohol molecule is sufficient for the
formation of 1b, but that the addition of at least a second alcohol
molecule is needed for the proton transfer to occur by solvating
the anion formed, as illustrated in Figure 1.
If a more acidic alcohol such as (CF
3 3
) COH is used the
hydrogen bonded species disappears and only 2 is visible (Figure
1g). Additional experiments show that the equilibrium between
1
and 2 is reversible. Longitudinal relaxation time measurements
at 500 MHz revealed a minimum value of T1min ) 18 ms at 170
K for 2. This value is in agreement with a dihydrido-dihydrogen
+
structure [Cp*RuH
2
(H
2
)(PCy
3
)] where dihydrogen and dihydride
nuclei exchange rapidly, although a bis-dihydrogen structure
cannot be excluded.10
The observation that the hydride chemical shift of 2 is
independent of the type of alcohol is an indication that this species
is no longer involved in hydrogen bonding to the anion formed.
The fact that 1 is protonated in Freons but not in toluene arises
8
from the dielectric properties of the solvent which favors the
ionic form only at low temperature where the solvent is ordered
around the ions formed. Because of the associated entropy
decrease, this protonation is not possible at high temperatures.
By contrast, the dielectric constant (2.5) of toluene remains
constant between room temperature and 190 K.11 Therefore, the
Freon mixture is used here for protonation under mild condi-
tions: the reactants can be mixed at room temperature but the
proton transfer only occurs when the temperature is sufficiently
lowered i.e., the dielectric constant has sufficiently increased.
1
Figure 1. Hydride regions of the 500 MHz H NMR spectra (200 K) of
.05 M solutions of Cp*RuH (PCy ) (1) in toluene-d (a-c) and CDClF
CDF (2:1) (d-g) in the presence of varying equivalents of (CF CHOH
and (CF COH.
0
3
3
8
2
/
3
3 2
)
3 3
)
2 2
Similar observations were made by using CD Cl as solvent which
1
also exhibits an increase of the dielectric constant from 9 at room
The results of the H NMR measurements are shown in Figure
temperature to about 17 at 170 K.12
1
. In the absence of an added alcohol the hydride nuclei of 1
In conclusion, we have obtained evidence that the protonation
of 1 occurs in its hydrogen bonded form to give the novel cationic
complex 2. This transformation is slow within the NMR time
scale. As 2 decomposes at room-temperature its synthesis was
2
give rise to an AB X spin system signal pattern due to coupling
to 1P. The H-H coupling constant Jab resulting from quantum
3
exchange is of the order of 80 Hz and similar whether measured
in toluene-d
similar in both solvents. When 3.9 equiv of (CF
as proton donor to a solution of 1 in toluene-d , the H-H coupling
ab is strongly increased to about 220 Hz (Figure 1b) as previously
8
or in CDF
3
/CDF
2
Cl. The chemical shifts are also
6
attempted so far without success. The method described here of
3 2
) CHOH is added
low-temperature protonation using a solvent whose polarity
increases at low temperatures could therefore be used to charac-
terize reactive intermediates of proton-transfer reactions and to
prepare otherwise unaccessible species.
8
J
5
reported. Only one signal set is observed which represents an
average over the fast exchanging monomer 1a and the hydrogen
bonded complex 1b. By contrast, when we use the mixture
CDClF /CDF (2:1) as solvent we also obtain an AB X signal
2 3 2
pattern with an increased exchange coupling Jab but, in addition,
we also observe a low-field singlet at -8.4 ppm (Figure 1e) which
we assign to the cation [Cp*RuH
to a CH
signal of 1 at δ 1.97 ppm. Quantitative P{ H}NMR experiments
at different temperatures and concentrations gave singlets at 79.8
ppm for 1 and at 65.9 ppm for 2, where the signal intensity ratios
Acknowledgment. This work was supported by the DFG,
Bonn, the EU programs INTAS and Human Capital & Mobility,
the Fonds der Chemischen Industrie, Frankfurt, and the CNRS,
France.
+
4
(PCy
3
)] (2). 2 gives also rise
3
signal at δ 2.03 ppm in addition to the corresponding
IC981320A
31
1
3 3
(9) Cp*RuH (PCy ) was prepared according to ref 2b. The samples were
2d,7
prepared using standard Schlenk and vacuum techniques employing
NMR tubes with an attached Teflon needle valve (Wilmad). The final
composition of the samples was checked by integration of the 500 MHz
spectra at 200 K.
I(1)/I(2) were the same as found for the corresponding methyl
group signals. The finding of values of /
peaks of 1 and 2 provides strong evidence that 2 is a tetrahydride
formed by protonation of 1.
3
4
I(1)/I(2) for the hydride
(
10) Shorter relaxation time would have been anticipated for a bis dihydrogen
3
1
structure (see ref 1b). Couplings of hydrides with P were not resolved.
An increase of the line width of the hydride signal e from 25 Hz for
2
-h
4 3
to 29 Hz for 2-hd was observed indicating nonresolved HD
3 2
(8) The dielectric constant of CHF /CHF Cl (1:1) increases from about 20
couplings which is compatible with both structures.
at 170 K to 45 at 95 K. A. P. Burtsev, G. S. Denisov, H. H. Limbach,
unpublished results.
(11) Isnardi, H. Z. Phys. 1922, 9, 153.
(12) Morgan, S. O.; Lowry, H. H. J. Phys. Chem. 1930, 34, 2385.