Ru(xantsil)(CO)(PCy3): Facile generation of a coordinatively unsaturated ruthenium(II) complex bearing 14 valence electrons [xantsil = (9,9-dimethylxanthene-4,5-diyl)bis(dimethylsilyl)]

Article abstract of DOI:10.1021/om049379e

Treatment of Ru(xantsil)(CO)(η⁶-toluene) (1) [xantsil = (9,9-dimethylxanthene-4,5-diyl)bis(dimethylsilyl)] with PCy₃ led to the formation of Ru(xantsil)(CO)(PCy₃) (3), in which the xantsil ligand is bound to the ruthenium center in K³(Si,Si,O) fashion. The highly coordinatively unsaturated nature of 3 is indicated by the reaction with CO to give Ru(xantsil)(CO)₃(PCy₃) (5).

Full text of DOI:10.1021/om049379e

Organometallics 2004, 23, 4531-4533
4531
Ru (xa n tsil)(CO)(P Cy3): F a cile Gen er a tion of a
Coor d in a tively Un sa tu r a ted Ru th en iu m (II) Com p lex
Bea r in g 14 Va len ce Electr on s [xa n tsil )
(9,9-d im eth ylxa n th en e-4,5-d iyl)bis(d im eth ylsilyl)]
Masaaki Okazaki,*,† Nobukazu Yamahira, J im J osephus Gabrillo Minglana, and
Hiromi Tobita*
Department of Chemistry, Graduate School of Science, Tohoku University,
Sendai 980-8578, J apan
Received August 11, 2004
Summary: Treatment of Ru(xantsil)(CO)(η6-toluene) (1)
[xantsil ) (9,9-dimethylxanthene-4,5-diyl)bis(dimethyl-
silyl)] with PCy3 led to the formation of Ru(xantsil)(CO)-
(PCy3) (3), in which the xantsil ligand is bound to the
ruthenium center in κ3(Si,Si,O) fashion. The highly
coordinatively unsaturated nature of 3 is indicated by
the reaction with CO to give Ru(xantsil)(CO)3(PCy3) (5).
Coordinatively unsaturated organotransition metal
complexes are known to be key intermediates in metal
complex-mediated homogeneous catalytic reactions. The
preparation and isolation of these species as a stable
form requires the use of ligands with steric and/or
electronic properties capable of stabilizing the com-
plexes.1 It was shown by Heyn et al. that strongly
σ-donating groups such as silyl and hydrido ligands can
be effective in stabilizing coordinatively unsaturated
F igu r e 1. Molecular structure of 2 with thermal ellipsoids
at the 50% probability level. Selected interatomic distances
(Å) and angles (deg): Ru-C1 1.865(3), Ru-P1 2.4006(9),
Ru-P2 2.4004(8), Ru-P3 2.3930(8), Ru-Si1 2.5235(8),
Ru-Si2 2.5277(8), C2‚‚‚C25 3.250(5), C4‚‚‚C28 3.298(5),
Si1-Ru-Si2 91.02(3), C1-Ru-P1 174.89(9), P2-Ru-Si1
173.54(3), P3-Ru-Si2 173.75(3).
complexes.2 Recently, our group has developed a new
type of bis(silyl) chelating ligand, (9,9-dimethylxan-
thene-4,5-diyl)bis(dimethylsilyl), or “xantsil”, which
can be coordinated to transition metals in κ2(Si,Si) or
κ3(Si,Si,O) fashion.3 The complexation of two electron-
releasing silyl groups and an ether oxygen atom in the
xanthene core is expected to stabilize the coordinatively
unsaturated species generated in the catalytic cycle.
This communication reports on the reactions of Ru-
(xantsil)(CO)(η6-toluene) (1) with tertiary phosphines
PR3 to give Ru(xantsil)(CO)(PR3)n [R ) Me (n ) 3), Cy
(n ) 1)]. The highly coordinatively unsaturated nature
of the latter is indicated by the reaction of Ru(xantsil)-
(CO)(PR3) with 2 equiv of CO to give Ru(xantsil)(CO)3-
(PR3), the structure of which was determined by X-ray
diffraction study.
The η6-toluene ligand in 1 was easily substituted by
three PMe3 molecules in dichloromethane at room
temperature to give Ru(xantsil)(CO)(PMe3)3 (2) in 84%
yield (eq 1). The molecular structure of 2 was deter-
mined by X-ray diffraction study (Figure 1). Complex 2
takes a slightly distorted octahedral geometry in which
three incoming PMe3 ligands occupy the fac-position.
The xanthene core is bent and the dihedral angle
between the least-squares planes of two aromatic rings
becomes 38.47(13)°. The ruthenium-silicon bonds Ru-
Si1 (2.5235(8) Å) and Ru-Si2 (2.5277(8) Å) are unusu-
ally long, a structural feature that was also observed
in Ru(xantsil)(CO)4 (av 2.565 Å)3a and which can be
explained similarly by the steric requirement of the
xantsil ligand. That is, the rigid xanthene core forces
two methyl groups (C3 and C5) to be within an ex-
tremely short interatomic distance (3.224(6) Å) com-
pared to the sum of the van der Waals radii of the two
methyl groups (4.0 Å). The steric repulsion causes the
other methyl groups (C2 and C4) on the silicon atoms
to move closer to the methyl groups on the P2 and P3
* To whom correspondence should be addressed. E-mail: mokazaki@
scl.kyoto-u.ac.jp (M.O.); tobita@mail.tains.tohoku.ac.jp (H.T.).
† Present address: International Research Center for Elements
Science, Institute for Chemical Research, Kyoto University, Uji, Kyoto
611-0011, J apan.
(1) Coordinatively unsaturated complexes: (a) Campion, B. K.;
Heyn, R. H.; Tilley, T. D. J . Chem. Soc., Chem. Commun. 1988, 278.
(b) Ogasawara, M.; Macgregor, S. A.; Streib, W. E.; Folting, K.;
Eisenstein, O.; Caulton, K. G. J . Am. Chem. Soc. 1996, 118, 10189. (c)
Gemel, C.; Mereiter, K.; Schmid, R.; Kirchner, K. Organometallics
1997, 16, 5601. (d) Tenorio, M. J .; Mereiter, K.; Puerta, M. C.; Valerga,
P. J . Am. Chem. Soc. 2000, 122, 11230. (e) Yamaguchi, Y.; Nagashima,
H. Organometallics 2000, 19, 725.
(2) Heyn, R. H.; Macgregor, S. A.; Nadasdi, T. T.; Ogasawara, M.;
Eisenstein, O.; Caulton, K. G. Inorg. Chim. Acta 1997, 259, 5.
(3) (a) Tobita, H.; Hasegawa, K.; Minglana, J . J . G.; Luh, L.-S.;
Okazaki, M.; Ogino, H. Organometallics 1999, 18, 2058. (b) Minglana,
J . J . G.; Okazaki, M.; Tobita, H.; Ogino, H. Chem. Lett. 2002, 406. (c)
Okazaki, M.; Minglana, J . J . G.; Yamahira, N.; Tobita, H.; Ogino, H.
Can. J . Chem. 2003, 81, 1350.
10.1021/om049379e CCC: $27.50 © 2004 American Chemical Society
Publication on Web 09/02/2004

4532 Organometallics, Vol. 23, No. 20, 2004
Communications
phosphines (C2‚‚‚C25 3.250(5) Å, C4‚‚‚C28 3.298(5) Å),
leading to considerable stretching of the ruthenium-
silicon bonds.
F igu r e 2. Optimized structures of 3a (a) and 3b (b) by
the DFT calculation (B3LYP) with the use of the basis set,
i.e., SCC for Ru, 6-31G(d) for Si and P, and 6-31G for the
rest.
The steric repulsion between the xantsil and PMe3
ligands is reflected in the high lability of the PMe3
ligands. In CD2Cl2, an equilibrium is achieved between
2, the dissociated PMe3, and 2′ formed through the
dissociation of PMe3 (eq 2). This is supported by the
observation that when colorless 2 was dissolved in
highly coordinative unsaturation (14 valence electrons
for 3a ). Single crystals of 3 suitable for X-ray diffraction
study could not be obtained.
1
CD2Cl2, the solution turned yellow and the H nuclear
The coordinative unsaturation of 3 was clarified by
reaction of 3 with CO. Introduction of CO gas into an
NMR tube containing the CD2Cl2 solution of 3 at room
temperature caused the solution to turn from orange
to dark green and then finally colorless (eq 3). Accord-
ingly, the 31P NMR spectrum indicated the existence of
the intermediate 4 (δ 37.6), which subsequently led to
the formation of the single product 5 (δ 39.3). The
corresponding large-scale experiment allowed the isola-
tion of 5 in 84% yield.
magnetic resonance (NMR) spectrum exhibited several
signals assignable to resonances of 2′ and a broad signal
due to free PMe3 in addition to the peaks due to 2. In
the 31P NMR spectrum, three new broad signals were
observed at δ -59.4, -6.5, and 27.4. The first peak was
assigned to dissociated PMe3, while the last two were
assigned to 2′. The addition of excess PMe3 to the yellow
solution shifted the equilibrium in eq 2 to the left to
form 2 predominantly, causing the yellow color to
disappear. These results are consistent with the forma-
tion of Ru(xantsil)(CO)(PMe3)2, although the geometry
remains unclear.
- PMe3
Ru(xantsil)(CO)(PMe3)3
{
}
+ PMe3
2
Ru(xantsil)(CO)(PMe3)2
(2)
2’
Treatment of complex 1 with PCy3 gave Ru(xantsil)-
(CO)(PCy3) (3) in 76% yield. In contrast to 2, elemental
analysis clearly showed that the ruthenium center
binds only one PCy3 molecule. All spectroscopic data in
CD2Cl2 are also in good agreement with the formation
of 3: The 29Si NMR spectrum exhibits a doublet signal
at δ 48.6 coupled with the 31P nuclei (J SiP ) 39.0 Hz),
whereas the 31P NMR spectrum displays a singlet signal
Recrystallization from CH2Cl2/hexane gave colorless
crystals of 5 suitable for X-ray diffraction study (Figure
3). Complex 5 adopts a slightly distorted octahedral
geometry in which three carbonyl ligands are located
in a mer-relationship. The PCy3 ligand occupies the
trans position of the silyl group (Si1). Unusual stretch-
ing of two ruthenium-silicon bonds [Ru-Si1 2.5366(9)
Å, Ru-Si2 2.5730(9) Å] is also observed and can be
explained similarly to that for 2. The significant differ-
ence in the length of the two ruthenium-silicon bonds
is attributable to electronic and/or steric effects: Steric
repulsion between one of the Cy groups (C17-C22) and
the methyl group (C4) [C4‚‚‚C17 3.546(5) Å, C4‚‚‚C22
3.775(5) Å] lengthens the Ru-Si2 bond. Moreover, the
strongly electron-releasing PCy3 ligand, located at the
trans position of the Si1 atom, effectively increases the
back-donation from Ru (dπ) to Si1 (σ*),4 resulting in
shortening of the Ru-Si1 bond.
1
at δ 25.0. In the H NMR spectrum, four signals due to
methyl groups appear at δ 0.20 (6H, SiMe), 0.51 (6H,
SiMe), 1.51 (3H, 9-Me), and 1.80 (3H, 9-Me). No signals
were observed in the high-field region typical of the
metal-hydrido fragment. The infrared (IR) spectrum
includes a strong band at 1876 cm-1 assigned to CO
stretching vibration. The structure of the model com-
plex, Ru(xantsil)(CO)(PiPr3), was optimized by density
functional theory (DFT) calculations. Complexes 3a and
3b were identified as stationary points in the potential
energy surface (PES) (Figure 2). The interatomic dis-
tances of Ru-O (xanthene) in 3a and 3b are 3.264 and
2.284 Å, respectively, indicating the existence of the
Ru-O bond in the latter. Accordingly, the former takes
a distorted tetrahedral geometry, while the latter takes
a distorted square pyramidal geometry with a silyl
ligand (Si2) in the apical position. Complex 3b is lower
in energy (∆E ) -14.5 kcal mol-1) than 3a . Coordina-
tion of the ether oxygen atom would compensate for the
As demonstrated in the reaction of 3 with CO, without
regard to the existence of the ruthenium-oxygen bond,
(4) (a) Eisen, M. S. In The Chemistry of Organic Silicon Compounds;
Rappoport, Z., Apeloig, Y., Eds.; Wiley: New York, 1998; Vol. 2,
Chapter 35, p 2037. (b) Tilley, T. D. In The Silicon-Heteroatom Bond;
Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1991; Chapters 9,
10, pp 245, 309.

Communications
Organometallics, Vol. 23, No. 20, 2004 4533
through the presence of two easily accessible, low-lying
empty orbitals that can be used in reactions involving
successive binding of substrates, oxidative addition, or
migration, etc. The catalytic activity of 3 is therefore of
great interest.
Ack n ow led gm en t. This work was supported by
Grants-in-Aid for Scientific Research (Nos. 14204065,
14044010, and 14078202) from the Ministry of Educa-
tion, Culture, Sports, Science and Technology of J apan.
One of authors (M.O.) wishes to thank the Nissan
Science Foundation, the Inoue Foundation for Science,
and the Tokuyama Science Foundation.
Su p p or tin g In for m a tion Ava ila ble: Experimental de-
tails of the synthesis and characterization of all compounds
and theoretical study of 3 (PDF), and full details of the X-ray
diffraction study in CIF format. This material is available free
of charge via the Internet at http://pubs.acs.org.
F igu r e 3. Molecular structure of 5 with thermal ellipsoids
at the 50% probability level. Selected interatomic distances
(Å) and angles (deg): Ru-Si1 2.5366(9), Ru-Si2 2.5730-
(9), Ru-P 2.4935(8), C1‚‚‚C3 3.4181(6), C2‚‚‚O3 3.089(5),
C4‚‚‚C17 3.546(5), C4‚‚‚C22 3.775(5), P-Ru-Si1 167.93-
(3), C25-Ru-Si2 175.93(11), C26-Ru-C24 159.89(14).
OM049379E
(5) (a) Ogasawara, M.; Huang, D.; Streib, W. E.; Huffman, J . C.;
Gallego-Planas, N.; Maseras, F.; Eisenstein, O.; Caulton, K. G.; J . Am.
Chem. Soc. 1997, 119, 8642. (b) Huang, D.; Streib, W. E.; Bollinger, J .
C.; Caulton, K. G.; Winter, R. F.; Scheiring, T. J . Am. Chem. Soc. 1999,
121, 8087. (c) Huang, D.; Bollinger, J . C.; Streib, W. E.; Folting, K.;
Young, V., J r.; Eisenstein, O.; Caulton, K. G. Organometallics 2000,
19, 2281. (d) Watson, L. A.; Ozerov, O. V.; Pink, M.; Caulton, K. G. J .
Am. Chem. Soc. 2003, 125, 8426.
complex 1 could become a synthetic equivalent of the
Ru(II) species with 14 valence electrons.5 Such 14-
electron species are expected to exhibit higher catalytic
activity than the corresponding 16-electron species6
(6) Crabtree, R. H. In Activation and Functionalization of Methane;
Davies, J . A., Ed.; VCH: New York, 1990; p 69.