Organometallics 1996, 15, 3095-3098
3095
Cp *Ru Alk oxid es w ith σ-Br id gin g P h en oxo Gr ou p s.
X-r a y Str u ctu r e of [Cp *Ru (µ-OMe)(µ-OC6H3-2,4-(t-Bu )2)]
Karin Bu¨cken, Ulrich Koelle,* Roland Pasch, and Beate Ganter
Institute for Inorganic Chemistry, Technical University at Aachen,
Prof.-Pirlet-Strasse 1, D-52074 Aachen, Germany
Received February 15, 1996X
Summary: [Cp*RuOMe]2 (1) reacts with phenols by
successive exchange of one and two OMe groups for the
phenolato group, forming mixed µ-methoxy-µ-phenolato
complexes (5) and bis(µ-phenolato) dimeric complexes (6)
successively. The latter, by an intramolecular rear-
rangement, convert into Cp*Ru(η5-oxocyclohexadienyl)
complexes (7). With phenol and 2-methyl-5-isopropyl-
phenol (carvacrol) the stepwise reactions were followed
by NMR spectroscopy. Using 2,4-di-tert-butylphenol,
both the mono(phenolato) (5c) and bis(phenolato) (6c)
complexes have been isolated. The latter complex is
obtained best from Cp*Ru(proline) (8) and 1. Complex
5c was characterized by an X-ray structure. The mo-
lecular geometry is very similar to the one found for 1.
detection of σ-intermediates. In addition to phenol (2),
alkyl-substituted phenols, i.e., 5-isopropyl-2-methylphe-
nol (carvacrol; 3) and 2,4-di-tert-butylphenol (4), were
investigated with the aim of sterically slowing the σ f
π rearrangement.
Resu lts
When 1 was mixed with phenol in a 3:1 Cp*Ru:phenol
molar ratio, the 1H NMR spectrum showed, apart
from 1, signals at δ 4.85 (bridging OMe) and 1.43 (Cp*)
(Table 1) due to the mixed σ-bridged complex Cp*Ru-
(µ-OMe)(µ-OPh)RuCp* (5a ), as well as weak signals due
to the symmetrical complex 6a . After addition of phenol
up to the ratio Cp*Ru:Ph ) 1:2, the signals of 1
disappeared and the NMR spectrum indicated a mixture
of Cp*Ru(µ-OPh)2RuCp* (6a ) and Cp*Ru(η5-C6H5O)
(7a ). When the mixture stood for 1 day further at
ambient temperature, the rearrangement to 7a was
complete. The method is thus convenient for the
generation of pure 7a .
If [Cp*Ru(µ-OMe)]2 (1) is treated with an alcohol more
acidic than MeOH, the exchange equilibrium (Scheme
1) generally places OR groups of the latter in the bridge
position of the complex.1 Phenols, which are more acidic
than aliphatic alcohols, however, apart from the σ-bridg-
ing function analogous to aliphatic alcohols, can complex
to a Cp*Ru moiety as π-ligands, either η6-phenol or η5-
oxocyclohexadienyl, depending on the availability of
protons. Reactions of 1 with a variety of different
phenols have led invariably to π-phenol or oxocyclo-
hexadienyl sandwich complexes as the only isolable
products.2 This was found not only for phenol itself,
An analogous experiment was performed with the
alkyl-substituted phenol carvacrol (3). A mixture of 1
and 3 with Cp*Ru:3 stoichiometry slightly below 1:1
gave after 1 h at ambient temperature an NMR spec-
trum of almost pure [Cp*Ru(µ-carvacrol)]2 (6b) with
some 10% of the mixed complex 5b also present.
Compound 6b in turn rearranged with a half-life of
about 2 days to the π-complex Cp*Ru(η5-5-i-Pr-2-Me-
C6H3O) (7b). After the rearrangement was complete,
i.e., after no signals due to 6b were left, the NMR
spectrum still contained a small methyl signal at δ 2.52,
which, by comparison with spectra obtained with less
carvacrol (Table 1), can be confidently assigned to 5b.
This observation shows that only the doubly phenol-
bridged complexes 6 undergo rearrangement to the
π-complex and that the mixed σ-complexes 5 are stable
in the absence of excess phenol.
which reacted with
1
to yield Cp*Ru(η5-C6H5O)‚
2HOC6H5, a complex with a planar oxocyclohexadienyl
ligand firmly hydrogen-bonded to two molecules of
phenol, but also with potentially chelating phenols such
as salicylaldehyde or o-hydroxyacetophenone. Even
pentafluorophenol gave exclusively the π-complex Cp*Ru-
(η5-C6F5O).3 Similarly, Tilley and co-workers reacted
[Cp*Ru(µ-Cl)]4 with 2,6-di-tert-butylphenolate and iso-
lated Cp*Ru(η5-2,6-di-tert-butyl-1-oxocyclohexadienyl) in
high yield.4 From these experiments it appeared that
phenols have a high preference to act as π-ligands
toward the Cp*Ru moiety and that bridged σ-intermedi-
ates are not easily obtained.
Investigation of the exchange equilibrium between 1
and 2,4-di-tert-butylphenol in benzene-d6 showed, 1 h
after mixing, the mono(σ-2,4-di-tert-butylphenol) com-
plex 5c as the major component even in the presence of
an excess of phenol. The equilibrium is shifted toward
the bis(σ-2,4-di-tert-butylphenol) complex 6c over the
course of 24 h. Rearrangement of 6c to 7c requires
about 2 weeks at ambient temperature in benzene
solution to go to completion. An alternative preparative
route leading to pure 6c was found in the reaction of
the proline complex 85 with 2,4-di-tert-butylphenol
(Scheme 2). Interestingly, the reaction of [Cp*RuCl]4
with lithium 2,4-di-tert-butylphenolate, as in the case
Later work showed that the polarity of the solvent
plays an important role in the reaction course and that
σ-complexes show increased stability in less polar
solvents. Since most of the earlier experiments were
performed in acetone or diethyl ether, we have reinves-
tigated the reaction between 1 and phenols in benzene-
d6 by means of NMR spectroscopy, which allowed the
X Abstract published in Advance ACS Abstracts, J une 1, 1996.
(1) Hoernig, A.; Englert, U.; Koelle, U. J . Organomet. Chem. 1993,
453, 255.
(2) Koelle, U.; Wang, M. H.; Raabe, G. Organometallics 1991, 10,
257.
(3) Koelle, U.; Hoernig, A.; Englert, U. Organometallics 1994, 13,
4064.
(4) Loren, S. D.; Campion, B. K.; Heyn, R. H.; Tilley, T. D.; Bursten,
B. E.; Luth, K. W. J . Am. Chem. Soc. 1989, 111, 4712.
(5) Koelle, U.; Bu¨cken, K.; Englert, U. Organometallics 1996, 15,
1376.
S0276-7333(96)00121-5 CCC: $12.00 © 1996 American Chemical Society