Synthesis, neutron structure, and reactivity of the bis(dihydrogen) complex RuH2(η2-H2)2(PCyp 3)2 stabilized by two tricyclopentylphosphines

Article abstract of DOI:10.1021/ja055126g

Treatment of Ru(η⁴-C₈H₁₂)(η⁶-C₈H₁₀) with 3 bar H₂ in the presence of 2 equiv of tricyclopentylphosphine (PCyp₃) in pentane resulted in the isolation of the new bis(dih

Full text of DOI:10.1021/ja055126g

Published on Web 11/19/2005
Synthesis, Neutron Structure, and Reactivity of the Bis(dihydrogen) Complex
RuH2(η2-H2)2(PCyp3)2 Stabilized by Two Tricyclopentylphosphines
Mary Grellier,† Laure Vendier,† Bruno Chaudret,† Alberto Albinati,‡ Silvia Rizzato,‡ Sax Mason,§ and
Sylviane Sabo-Etienne*,†
Laboratoire de Chimie de Coordination du CNRS, 205 route de Narbonne, 31077 Toulouse Cedex 04, France,
Department of Structural Chemistry (DCSSI), UniVersity of Milan, 21, Via G. Venezian, 20133 Milan, Italy, and
Institut Laue-LangeVin, 6 rue Jules Horowitz, BP 156, 38042 Grenoble Cedex 9, France
Received August 20, 2005; E-mail: sabo@lcc-toulouse.fr
Dihydrogen complexes were a curiosity 20 years ago when the
first paper by Kubas demonstrated that dihydrogen could coordinate
to a metal center without H-H bond breaking. The dihydrogen
bonding in W(η2-H2)(CO)3(PiPr3)2 was established unambiguously
by single-crystal neutron diffraction, showing a H-H bond distance
of 0.82 Å.1 A number of polyhydride species were then reformu-
lated, and the number of dihydrogen complexes grew rapidly.2 Most
complexes contain only one dihydrogen ligand in the octahedral
coordination sphere of a d6 metal. However, very few complexes
containing two dihydrogen ligands are known. Iridium,3 osmium,4
and very recently rhenium5 bis(dihydrogen) complexes have only
been characterized in solution by NMR spectroscopy, whereas
chromium6 bis(dihydrogen) complexes have been observed in noble-
gas matrices. The bis(dihydrogen) ruthenium complexes are the only
species that have been isolated as solids: the bis(phosphine) RuH2-
(η2-H2)2(PCy3)2 (1)7 and RuH2(η2-H2)2(PiPr3)2,8 the mixed phos-
phine(carbene) RuH2(η2-H2)2(PCy3)(IMes),9 and the Tp compounds7b
Figure 1. Neutron structure of RuH2(η2-H2)2(PCyp3)2 (2). Ellipsoids drawn
TpRuH(η2-H2)2 (Tp ) hydridotris(3,5-dimethylpyrazolyl)borate or
at 50% probability.
hydridotris(3-isopropyl-4-bromopyrazolyl)borate). The chemistry of
1, incorporating two tricyclohexylphosphines, has been extensively
1). The coordination geometry around the metal center is a distorted
studied by our group. Due to the presence of two labile dihydrogen
octahedron defined by the two phosphines in a trans configuration
ligands, a fascinating reactivity has been developed. For example,
(making an angle of 168.9(1)°), two cis dihydrogen ligands and
1 is used as an entry for the design of other σ complexes, silane
two hydrides trans to them, defining the equatorial plane. As already
and borane compounds.10,11 1 is also an efficient catalyst precursor
observed in similar complexes, the P atoms are slightly bent toward
for a wide variety of reactions.12 Remarkably, 1 may be used for
the two hydride ligands.17
hydrogen storage. Up to 10 hydrogen atoms can be abstracted from
1, and the reaction is reversible.13 This is the result of a partial
dehydrogenation of the cyclohexyl ring of the phosphine ligands,
Table 1. Selected Bond Distances (Å) and Angles (deg) for
RuH2(η2-H2)2(PCyp3)2 (2)
neutrona/X-rayb
neutrona/X-rayb
leading ultimately to the formation of the hydride complex RuH-
[(η3-C6H8)PCy2][(η2-C6H9)PCy2)]. In some catalytic reactions, the
formation of intermediate species leading to such partially dehy-
drogenated phosphine can be detrimental for the selectivity, due to
the competition of several catalytic cycles. Thus, we were interested
in preparing an analogous complex in which such reaction could
be limited. We assumed that the tricyclopentylphosphine, with a
more constrained ring, would be a good candidate. In this
Communication, we describe the synthesis of a new bis(dihydrogen)
complex, which displays remarkable activity toward H/D exchange.
Treatment of Ru(η4-C8H12)(η6-C8H10) with 3 bar H2 in the
presence of 2 equiv of tricyclopentylphosphine (PCyp3) in pentane
resulted in the isolation of a cream-colored solid, analyzed as RuH2-
(η2-H2)2(PCyp3)2 (2).14 The molecular structure of 2 was determined
by single-crystal X-ray and neutron diffraction (see Supporting
Information).15,16 This is the first report of a neutron structure
determination of a bis(dihydrogen) complex (see Figure 1 and Table
Ru-P1
Ru-P2
Ru-H1
Ru-H2
Ru-H11
Ru-H12
2.312(3)/2.3121(4) Ru-H21
2.307(3)/2,3110(4) Ru-H22
1.628(4)/1.58(4)
1.625(4)/1.54(4)
1.730(5)/1.66(3)
1.753(5)/1.75(4)
1.764(5)/1.75(4)
1.745(5)/1.69(3)
1.692(5)
1.702(5)
0.825(8)/0.70(4)
0.835(8)/0.78(4)
Ru-m11c
Ru-m22c
H11-H12
H21-H22
P1-Ru-P2
H1-Ru-H2
168.9(1)/168.64(2) H2-Ru-H21
81.9(3)/73(2) H2-Ru-H22
H1-Ru-H11 157.8(3)/156(2)
H1-Ru-H12 173.4(3)/176(1)
173.5(3)/174(1)
157.7(3)/157(2)
m11-Ru-m22 c 99.8(3)
a At 20 K. b At 100 K. c m11 and m22 are, respectively, the midpoints
of the H11-H12 and H21-H22 separations.
The two H-H separations are equal (at 0.825(8) and 0.835(8)
Å) and comparable, for example, to that found in [Ru(dppe)2(H)-
(H2)]+,18 a typical value for an “unstretched” dihydrogen ligand,
involving only weak π-back-bonding from the Ru to the dihydrogen
σ* orbital. Excellent agreement is also found with the results from
DFT calculations at the B3LYP level (H-H bond lengths of 0.853
Å).19 There is a slight asymmetry in the bonding of the two H2
† LCC, Toulouse, France.
‡ DCSSI, Milan, Italy.
§ ILL, Grenoble, France.
9
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J. AM. CHEM. SOC. 2005, 127, 17592-17593
10.1021/ja055126g CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Murai’s Reaction Catalyzed by 2 at Room
Temperature
(5) Ingleson, M. J.; Brayshaw, S. K.; Mahon, M. F.; Ruggiero, G. D.; Weller,
A. S. Inorg. Chem. 2005, 44, 3162.
(6) (a) Goff, S. E. J.; Nolan, T. F.; George, M. W.; Poliakoff, M.
Organometallics 1998, 17, 2730 and references therein. (b) Wang, X.;
Andrews, L. J. Phys. Chem. A 2003, 107, 570.
(7) (a) Chaudret, B.; Poilblanc, R. Organometallics 1985, 4, 1722. (b) Sabo-
Etienne, S.; Chaudret, B. Coord. Chem. ReV. 1998, 170-180, 381. (c)
Borowski, A. F.; Donnadieu, B.; Daran. J.-C.; Sabo-Etienne, S.; Chaudret,
B. Chem. Commun. 2000, 543.
(8) Abdur-Rashid, K.; Gusev, D. G.; Lough, A. J.; Morris, R. H. Organo-
metallics 2000, 19, 1652.
molecules, as can be judged from the Ru-(H2) distances (1.730-
(5), 1.753(5) Å and 1.745(5), 1.764(5) Å). Moreover, these two
ligands are tilted with respect to the coordination plane by 24.6-
(5)° and 22.8(5)°. The ruthenium-hydride separations are equal
and in the expected range for a classical M-H distance (1.628(4)
and 1.625(4) Å). The distances between each hydride and the cis
dihydrogen ligand are around 2.1 Å, ruling out the presence of any
cis interaction.20
(9) Giunta, D.; Holscher, M.; Lehmann, C. W.; Mynott, R.; Wiirtz, C.; Leitner,
W. AdV. Synth. Catal. 2003, 345, 1139.
(10) (a) Delpech, F.; Sabo-Etienne, S.; Daran, J.-C.; Chaudret, B.; Hussein,
K.; Marsden, C. J.; Barthelat, J.-C. J. Am. Chem. Soc. 1999, 121, 6668.
(b) Ayed, T.; Barthelat, J.-C.; Tangour, B.; Prade`re, C.; Donnadieu, B.;
Grellier, M.; Sabo-Etienne, S. Organometallics 2005, 24, 3824.
(11) Lachaize, S. Essalah, K.; Montiel-Palma, V.; Vendier, L.; Chaudret, B.;
Barthelat, J.-C.; Sabo-Etienne, S. Organometallics 2005, 24, 2935.
The NMR spectra show that 2 is fluxional down to 170 K. In
the 1H NMR spectrum measured at room temperature in C6D6, one
triplet is observed at δ -7.98 (JH-P ) 8.0 Hz) for the six hydrogens
in rapid exchange. H/D exchange between the Ru-H and the C-D
bonds of the deuterated benzene is observed within 1 h, leading to
the formation of various isotopomers RuHxD6-x(PCyp3)2 (with x
) 0-6). After 5.5 h, integration of the hydride and the cyclopentyl
proton signals shows 24% D incorporation. This is in remarkable
contrast with the behavior of 1, which displays no significant H/D
exchange even after 24 h. This observation prompted us to test the
activity of 2 toward the Murai reaction. Indeed, we have previously
demonstrated that 1 was, at room temperature, the best active
precursor for ethylene coupling to a functionalized arene.12d,e Now,
by using 2 as catalyst precursor, we found a 90% conversion of
acetophenone to 2-ethylacetophenone within 35 min (see Scheme
1), whereas 10 h was needed in the same conditions using 1 as the
catalyst precursor.
(12) See for example: (a) Borowski, A. F.; Sabo-Etienne, S.; Donnadieu, B.;
Chaudret, B. Organometallics 2003, 22, 4803. (b) Lachaize, S.; Sabo-
Etienne, S.; Donnadieu, B.; Chaudret B. Chem. Commun. 2003, 214. (c)
Montiel-Palma, V.; Lumbierres, M.; Donnadieu, B.; Sabo-Etienne, S.;
Chaudret, B. J. Am. Chem. Soc. 2002, 124, 5624. (d) Guari, Y.; Sabo-
Etienne, S.; Chaudret, B. J. Am. Chem. Soc. 1998, 120, 4228. (e) Guari,
Y.; Castellanos, A.; Sabo-Etienne, S.; Chaudret, B. J. Mol. Catal. 2004,
212, 77.
(13) Borowski, A. F.; Sabo-Etienne, S.; Christ, M. L.; Donnadieu, B.; Chaudret,
B. Organometallics 1996, 15, 1427.
(14) Ru(η4-C8H12)(η6-C8H10) (0.200 g, 0.63 mmol) and PCyp3 (316 µL, 0.63
mmol) were introduced into a Fischer-Porter bottle, and pentane (5 mL)
was added. Pressurization to 3 bar dihydrogen and stirring for 45 min led
to an orange solution. After the solution was cooled below - 30 °C, a
cream-colored solid was isolated after washing with cold pentane and
rapid drying under vacuum (70% yield). For long periods, 2 should be
kept at low temperature under argon. An easy access to large single crystals
was found by solubilization of 2 in pentane at room temperature and further
cooling at -35 °C for 2 h. Data for 2: 1H NMR (C6D6, 293 K, 250.13
2
MHz) δ -7.98 (t, JPH ) 8.0 Hz, 6H, RuH6), 1.6-2.0 (54H, PCyp3);
31P{1H} NMR (C6D6, 293 K, 101.25 MHz) δ 81.1 (s); T1 (C7D8, 170 K,
500 MHz) 95 ms for the hydride resonance (the minimum value could
not be reached). Anal. Calcd for C30H60P2Ru: C, 61.70; H, 10.30.
Found: C, 61.72; H, 10.64.
(15) The X-ray data collection was carried out at 100 K on a STOE image
plate diffractometer. Crystals are monoclinic, space group P21/n, a )
14.928(2) Å, b ) 9.4580(7) Å, c ) 21.713(2) Å, â ) 105.14(1)°, V )
2959.3(5) Å3, Z ) 4. A total of 22 631 data were collected (5526 unique),
R ) 0.0275, Rw ) 0.070. Neutron diffraction data were collected at the
ILL reactor, using the D19 diffractometer (equipped with three high-
pressure square position-sensitive gas detectors), on a crystal of 2.0 ×
2.0 × 1.1 mm3. The crystal was mounted, under argon, inside a thin-
walled quartz tube on a Displex cryorefrigerator and cooled (∼2 K/min)
to 20 K. No significant changes in the crystal mosaic or splitting of peaks
was observed during cooling. The space group P21/n was confirmed at
20 K. Unit cell dimensions (from fittings of the centroids in 3D of 1979
strong reflections) are a ) 14.8322(9) Å, b ) 9.4259(6) Å, c ) 21.679-
(1) Å, â ) 105.377(2)°, V ) 2922.4(3) Å3. A total of 12 878 reflections
were collected, yielding 6498 unique reflections (4979 observed with Iobs
g 2σ(I)). The starting structural model was based on the atomic
coordinates of the heavy atoms from the X-ray structure, while all
hydrogen atoms were located from Fourier difference maps. The structure
was refined by full matrix least squares using ADPs for all atoms (R )
0.0521, Rw ) 0.1207, GOF ) 1.070) and all the collected intensities. See
also the Supporting Information.
In summary, the use of an alkylphosphine with a C5 ring allows
the stabilization of a new bis(dihydrogen) complex, RuH2(η2-H2)2-
(PCyp3)2 (2). The single-crystal neutron diffraction study of 2 is
the first carried out for a bis(dihydrogen) complex, confirming the
nature of the hydrogens in the ruthenium coordination sphere and
showing the presence of two “unstretched”, asymmetrically bonded,
dihydrogen ligands. We have also shown that 2 is not only stable
at room temperature but also much more active for H/D exchange
and C-C bond coupling than the well-known complex RuH2(η2-
H2)2(PCy3)2 (1). We are currently exploring both the reactivity of
2 and its catalytic activity for a wide variety of reactions, with a
special focus on C-H activation, hydrogenation, and dehydroge-
nation reactions.
Acknowledgment. This work is supported by the CNRS. A.A.
(16) There is good agreement between the X-ray and the neutron diffraction
results (see Table 1). As expected, the distances obtained from X-ray data
are systematically shorter than those from the neutron diffraction with
much larger esd’s. However, even in the absence of neutron measurements,
we have shown that, in borane and silane chemistry, key information on
hydride location may be obtained by combining X-ray, DFT, and NMR
studies (see refs 10 and 11).
and S.R. acknowledge the support of a PRIN 2004 grant.
Supporting Information Available: X-ray and neutron crystal-
lographic files for 2 (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
(17) Eckert, J.; Jensen, C. M.; Koetzle, T. F.; Le Husebo, T.; Wu, P. J. Am.
Chem. Soc. 1995, 117, 7271.
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