Piano-stool inversion in arene complexes of Ru(II): Modelling the transition state

Article abstract of DOI:10.1039/b202630a

In the solid-state, the [RuH(arene)(Binap)]CF₃SO₃, complexes, 1, arene = η⁶-benzene and 2, arene = η⁶-toluene, distort markedly from a classical three-legged piano-stool structure with the former having the P-Ru-P plane approximately perpendicular to the plane of the arene; this structure for 1 is what one would expect for a transition state leading from one diastereomer to another via inversion at ruthenium.

Full text of DOI:10.1039/b202630a

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Piano-stool inversion in arene complexes of Ru(II): modelling the
transition state
Tilmann J. Geldbach,a Paul S. Pregosin*a and Alberto Albinati*b
a Laboratory of Inorganic Chemistry, HCI ETH Hönggerberg, 8093 Zürich, Switzerland
b Chemical Pharmacy, University of Milan, I-20131 Milan, Italy
Received 14th March 2002, Accepted 30th April 2002
First published as an Advance Article on the web 16th May 2002
In the solid-state, the [RuH(arene)(Binap)]CF3SO3,
complexes, 1, arene ؍ ꢀ6-benzene and 2, arene ؍ ꢀ6-toluene,
distort markedly from a classical three-legged piano-stool
structure with the former having the P–Ru–P plane
approximately perpendicular to the plane of the arene;
this structure for 1 is what one would expect for a tran-
sition state leading from one diastereomer to another via
inversion at ruthenium.
The atropoisomeric chiral bidentate Binap is an excellent
chiral auxiliary and is often employed in homogeneously
catalysed reactions. It is perhaps best known in connection
with Ru-assisted homogeneous hydrogenation chemistry,1 and
consequently a number of solid-state structures of Ru-Binap
complexes have appeared.2
Often, the ruthenium precursor used in the catalysis involves
an 18-electron η6-arene complex of Ru(). These arene com-
pounds are known to have classical, but distorted, three-legged
piano-stool structures, e.g. [RuCl(arene)(Binap)]ϩ,3 [RuH-
(η6-benzene or η6-toluene)(PPh3)2]ϩ 4 or [RuH(η6-benzene)-
(dippe)]ϩ [dippe = bis(diisopropylphosphino)ethane].5 This
structural type has been considered6 via computational
methods and electronic effects are thought to contribute to the
distortion of the piano-stool. We show here that the Binap
complexes [RuH(η6-arene)(Binap)]CF3SO3, 1 and 2, both dis-
tort markedly from a classical three-legged piano-stool and, for
1, can reach a trigonal structure.
The complexes were prepared by reacting Ru(OAc)2(Binap)
with either benzene or toluene and triflic acid in methanol to
afford 1 and 2 respectively.7 The molecular structures for 1 and
2 were determined via X-ray diffraction methods8 and ORTEP9
views of these molecules are given in Fig. 1. Selected bond
distances and bond angles are given in the caption. In both
structures the bond lengths and angles are very similar and fall
in the expected range. The major differences between the two
structures lie in a) the values of the dihedral angle between the
planes defined by atoms P1–Ru–P2 and the C1–C6 ring
[89.7(1)Њ for 1 and 79.6(1)Њ for 2, respectively] and b) the
extremely large value of the anisotropic displacement par-
ameters (ADPs) for the Ru atom in 1. The two Ru–P distances
are equal in 1 and slightly different in 2.
In 1 the ruthenium ADPs show a very large amplitude of
displacement in one direction (≈0.2 Å) (see Figs. 1 and 2) com-
pared to compound 2, where a much smaller elongation of the
ADPs is observed (≈0.04 Å). This large displacement may be
the result of the metal atom undergoing a large amplitude
motion or it may be caused by static disorder due to the super-
imposition of two conformations each having the Ru sitting at
Fig. 1 (a) Selected bond lengths (Å) and angles (Њ) for compound 1ؒ
CH2Cl2: Ru–P1 2.290(1), Ru–P2 2.290(2), Ru–C11 2.247(6), Ru–C21
2.268(8), Ru–C31 2.271(7), Ru–C41 2.260(6), Ru–C51 2.275(6), Ru–
C61 2.262(6), P1–C6 1.843(6), P2–C6Ј 1.843(5); P1–Ru–P2 90.57(5) and
(b) for compound 2: Ru–P1 2.299(1), Ru–P2 2.280(1), Ru–C11 2.272(5),
Ru–C21 2.235(5), Ru–C31 2.245(5), Ru–C41 2.259(5), Ru–C51
2.282(5), Ru–C61 2.268(4), P1–C6 1.838(4), P2–C6Ј 1.835(4), Ru–H
1.62(7); P1–Ru–P2 90.58(3).
each end of the observed displacement. Thus the observed “tri-
gonal” geometry at the metal centre in 1 may be seen as an
average structure consistent with the molecule sitting in a very
shallow single minimum potential. For 1, the P-atoms, the
DOI: 10.1039/b202630a
J. Chem. Soc., Dalton Trans., 2002, 2419–2420
This journal is © The Royal Society of Chemistry 2002
2419

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Detailed 1H NMR studies suggest that the observed
dynamics stem from restricted rotation around P–C(phenyl)
bonds and not the inversion of eqn. (1). These NMR results are
consistent with non-equivalent but very similar P-donors. A
solid-state 31P NMR spectrum was obtained but, due to the
substantial line width, was not informative.
The chemistry of eqn. (1) can (but need not be) facile and
there is substantial discussion by Brunner and co-workers,10
amongst others,11 on this subject. Our result for 1 presents the
first example of a structure confirming that the barrier need not
be very high.
Acknowledgements
P. S. P. thanks the Swiss National Science Foundation and the
BBW for financial support. A. A. thanks MURST for partial
support. We also thank Johnson Matthey for the loan of
precious metals.
Notes and references
1 R. Noyori, Chimia, 1988, 42, 215; R. Noyori, Asymmetric Catalysis
in Organic Synthesis, John Wiley and Sons, Inc., 1994.
2 J. A. Wiles, C. E. Lee, R. McDonald and S. H. Bergens, Organo-
metallics, 1996, 15, 3782; J. A. Wiles and S. H. Bergens, J. Am. Chem.
Soc., 1997, 119, 2940; J. A. Wiles and S. H. Bergens, Organometal-
lics, 1998, 17, 2228; J. A. Wiles and S. H. Bergens, Organometallics,
1999, 18, 3709; D. D. Pathak, H. Adams, N. A. Bailey, P. J. King and
C. White, J. Organomet. Chem., 1994, 479, 237; N. Feiken, P. S.
Pregosin, G. Trabesinger, A. Albinati and G. L. Evoli, Organometal-
lics, 1997, 16, 5756; A. M. Joshi, I. S. Thorborn, S. J. Rettig and
B. R. James, Inorg. Chim. Acta, 1992, 198–200, 283.
3 K. Mashima, K. Kusano, T. Ohta, R. Noyori and H. Takaya,
J. Chem. Soc., Chem. Commun., 1989, 1208.
4 O. S. Sisodia, A. N. Sahay, D. S. Pandey, U. C. Agarwala, N. K. Jha,
P. Sharma, A. Toscano and A. Cabrera, J. Organomet. Chem., 1998,
560, 35.
5 I. de los Rios, M. J. Tenorio, M. A. J. Tenorio, M. C. Puerto and
P. Valerga, J. Organomet. Chem., 1996, 525, 57.
6 A. R. Siedle, R. A. Newmark, L. H. Pignolet, D. X. Wang and
T. A. Albright, Organometallics, 1986, 5, 38; P. Hofmann, Angew.
Chem., Int. Ed. Engl., 1977, 16, 536.
Fig. 2 (a) View of 1 from behind the arene looking towards the metal.
Only the P–Ru–P atoms of the Ru(Binap) fragment are shown. The
approximately perpendicular arrangement of the two planes is clear.
(b) View of 2 from behind the arene looking towards the metal. Only
the P–Ru–P atoms of the Ru(Binap) fragment are shown. The slight
deviation, ca. 10Њ, is noticable.
C-atoms of the Binap core and those of the η6-benzene do not
show the same large amplitude displacements of the Ru atom.
This is consistent with the observation that, in the fluxional
process, the ligands do not move. Thus it may be assumed that
the observed structure of 1 is in fact a static picture of the reac-
tion path leading from one piano-stool conformation to the
opposite through the trigonal transition state [see eqn. (1)].
7 G. Trabesinger, A. Albinati, N. Feiken, R. W. Kunz, P. S. Pregosin
and M. Tschoerner, J. Am. Chem. Soc., 1997, 119, 6315. CF3SO3H
was used instead of HBF4 and methanol instead of 2-propanol.
8 Crystal data for compound 1ؒCH2Cl2: C52H41Cl2F3O3P2RuS, M =
1037.83, monoclinic, space group P21/n (no. 14), a = 11.2603(5),
b = 30.441(1), c = 13.3700(6) Å, β = 100.537(2)Њ, U = 4505.7(3) Å3,
Z = 4, µ = 6.41 cmϪ1, T = 223 K. All atoms were refined aniso-
tropically by full-matrix least squares on F 2. Final agreement factors
are: R1 = 0.090 [for 7213 unique reflections having I > 2σ(I )], 0.126
(for all 10642 independent reflections). The terminal hydride was not
located unambiguously. Crystal data for compound 2: C52H40F3O3-
P2RuS, M = 964.91, monoclinic, space group P21/n (no. 14),
a = 11.7605(2), b = 21.0232(5), c = 17.2739(5) Å, β = 91.247(1)Њ,
U = 4269.9(2) Å3, Z = 4, µ = 5.50 cmϪ1, T = 200 K. All atoms were
refined anisotropically by full-matrix least squares on F 2. Final
agreement factors are: R1 = 0.060 [for 8198 unique reflections with
I > 2σ(I )], 0.073 (for all 9764 independent reflections). The hydride
ligand was located on a difference Fourier map and refined without
constraints using an isotropic temperature factor. CCDC reference
numbers 182445 and 182446. See http://www.rsc.org/suppdata/dt/
b2/b202630a/ for crystallographic data in CIF or other electronic
format.
(1)
In both compounds two P bonded phenyl rings are also
disordered but this may not be related to the fluxional process.
The 31P spectra for 1 and 2 in CD2Cl2 reveal dynamic
character. At ambient temperature one finds AB spectra which
collapse to broad singlets at ca. 240 K and ca. 260 K, respect-
ively, and then reappear as two new AB spectra upon lowering
the temperature. The two resonances for 1 are separated by
< 0.7 ppm between 195 K and 298 K. The 1H-hydride resonance
is sharp throughout the entire temperature range and appears
as the X part of an ABX spin system. One finds NOEs from the
hydride signals to the two non-equivalent sets of P-phenyl ortho
protons: one from PA (phenyl axial) and one from PB (phenyl
equatorial) i.e.,
9 ORTEP3 for Windows. L. J. Farrugia, J. Appl. Crystallogr., 1997, 30,
565.
10 H. Brunner, Angew. Chem., 1999, 111, 1248; H. Brunner and
T. Zwack, Organometallics, 2000, 19, 2423; H. Brunner, Eur. J. Inorg.
Chem., 2001, 905.
11 S. Attar, V. J. Catalano and J. H. Nelson, Organometallics, 1996, 15,
2932; H. H. Hansen, K. Maitra and J. H. Nelson, Inorg. Chem.,
1999, 38, 2150; N. Gül and J. H. Nelson, Organometallics, 1999, 18,
709.
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J. Chem. Soc., Dalton Trans., 2002, 2419–2420