Synthesis and properties of a silyl(silylene)ruthenium complex: activation barrier of the Ru=Si bond rotation and facile replacement of the methyl groups with alkoxy groups of a silyl ligand

Article abstract of DOI:10.1021/om900177z

A silyl(silylene)ruthenium complex, Cp* Ru(CO)(=SiMes ₂)SiMe₃ (2), was synthesized by the reaction of Cp*Ru(CO) (py)Me with HSiMe₂SiMeS₂Me (Mes = 2,4,6trimethylphenyl). In contrast to its iron analogue Cp*Fe(CO)

Full text of DOI:10.1021/om900177z

Organometallics 2009, 28, 3963–3965 3963
DOI: 10.1021/om900177z
Synthesis and Properties of a Silyl(silylene)ruthenium Complex:
Activation Barrier of the RudSi Bond Rotation and Facile Replacement of
the Methyl Groups with Alkoxy Groups of a Silyl Ligand
Hisako Hashimoto,* Jun Sato, and Hiromi Tobita*
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578,
Japan
Received March 5, 2009
Summary: A silyl(silylene)ruthenium complex, Cp*Ru(CO)-
(dSiMes2)SiMe3 (2), was synthesized by the reaction of
Cp*Ru(CO)(py)Me with HSiMe2SiMes2Me (Mes=2,4,6-
trimethylphenyl). In contrast to its iron analogue Cp*Fe(CO)-
(dSiMes2)SiMe3 (1), which we reported previously, 2 ex-
hibits a fluxional behavior that involves rotation around the
RudSi and Si-C(mesityl) bonds. The activation parameters
for the MdSi bond rotation were determined for the first time
using line-shape analysis of the variable-temperature 1H NMR
spectra of 2. Complex 2 underwent a facile alkoxylation
reaction on the silyl ligand with ROH (R=Me or Et) in the
presence of R0NC (R0 =tBu or Xyl=2,6-dimethylphenyl) to
afford Cp*Ru(CO)(CNR0)[Si(OR)3] and Mes2Si(OR)H
in high yields at room temperature, with cleavage of all three
Si-C bonds.
weakly reactive toward external organic substrates, we foc-
used our attention on the ruthenium analogue, Cp*Ru(CO)-
(dSiMes2)SiMe3 (2), which possesses a larger and a more
electron-rich metal center. Accordingly, the behavior of 2
was noticeably different from that of 1. First, the fluxional
behavior involved rotations around the RudSi double and
the Si-C(mesityl) single bonds; accordingly, line-shape anal-
ysis of the variable-temperature 1H NMR spectra of 2
allowed for the first experimental evaluation of the activat-
ion parameters for the MdSi bond rotation. Second, 2
exhibited facile replacement of all alkyl groups by alkoxy
groups on the silyl ligand, with cleavage of all three Si-C
bonds at room temperature. Such a complete alkoxylation
reaction on the silyl ligands under mild conditions is very
rare.6,7 As a representative example, Maitlis has reported the
alkoxylation of all ethyl groups of one of the silyl ligands on
Cp*RhH2(SiEt3)2 with alcohol under photochemical condit-
ions.6 Herein, we report the synthesis and X-ray structural
analysis of 2 along with details on its alkoxylation reaction.
The synthesis of 2 was based on the thermal synthetic
methodology of 1:4b Cp*Ru(CO)(py)Me (py=pyridine)8 re-
acted with HSiMe2SiMes2Me at room temperature to afford 2
as yellow crystals, which were isolated in 75% yield (eq 1).
Transition-metal silyl(silylene) complexes have been pos-
tulated as important intermediates for the scrambling of
substituents and dehydrogenative coupling of hydrosilanes.1
Recently, several base-free silyl(silylene) complexes of Pt,2
Pd,2 W,3a Fe,4 and Mo3b have been successfully isolated. Of
these, the isolated Fe complex Cp*Fe(CO)(dSiMes2)SiMe3
(1, Mes=mesityl)4 was employed as a probe to investigate
the mechanisms of the above reactions. Specifically, the
scrambling of substituents and Si-Si bond formation can
be attributed to the 1,2- and 1,3-group migrations on the silyl-
(silylene) complex.5 However, because 1 was found to be only
*To whom correspondence should be addressed. Tel: þ81-22-795-
6539. Fax: þ81-22-795-6543. E-mail: hhashimoto@mail.tains.tohoku.
ac.jp; tobita@m.tains.tohoku.ac.jp.
(1) (a) Tilley, T. D. In The Chemistry of Organic Silicon Compounds;
Patai, S., Rappoport, Z., Eds.; John Wiley & Sons: New York, 1989; Chapter
24.(b) Eisen, M. S. In The Chemistry of Organic Silicon Compounds Vol. 2;
Rappoport, Z., Apeloig, Y., Eds.; John Wiley & Sons: New York, 1998;
Chapter 35.(c) Ogino, H. Chem. Rec. 2002, 2, 291. (d) Okazaki, M.; Tobita,
H.; Ogino, H. J. Chem. Soc., Dalton Trans. 2003, 493, and references
therein.
(2) (a) Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F.; Maciejewski,
H. Organometallics 1998, 17, 5599. (b) Avent, A. G.; Gehrhus, B.;
Hitchcock, P. B.; Lappert, M. F.; Maciejewski, H. J. Organomet. Chem.
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lics 2002, 21, 1326. (b) Hirotsu, M.; Nunokawa, T.; Ueno, K. Organome-
tallics 2006, 25, 1554.
The 29Si NMR spectrum of 2 exhibited signals at 346.8 ppm,
of which the large downfield shift is characteristic of the
silylene ligand,9 and at 17.3 ppm, which corresponds to the
(6) Jose, R.; Maitlis, P. M. J. Chem. Soc., Chem. Commun. 1986, 862.
(7) For the related works, there are a few examples of photoinduced
mono- or dialkoxylation of alkyl groups with alcohol and hydroxylation
of alkyl groups on the silyl ligands with water or strong base under
heating conditions. For photoinduced alkoxylation reaction, see: (a)
Ueno, K.; Seki, S.; Ogino, H. Chem. Lett. 1993, 2159. For hydroxylation
reactions, see: (b) Akhrem, I. S.; Chistovalova, N. M.; Mysov, E. I.; Vol'pin,
M. E. J. Organomet. Chem. 1974, 72, 163. (c) Albrecht, M.; Kwok, W.-H.;
Lu, G.-L.; Rickard, C. E. F.; Roper, W. R.; Salter, D. M.; Wright, L. J. Inorg.
Chim. Acta 2005, 358, 1407.
(4) (a) Tobita, H.; Matsuda, A.; Hashimoto, H.; Ueno, K.; Ogino, H.
Angew. Chem., Int. Ed. 2004, 43, 221. (b) Hashimoto, H.; Matsuda, A.;
Tobita, H. Chem. Lett. 2005, 1374. (c) Hashimoto, H.; Matsuda, A.; Tobita,
H. Organometallics 2006, 25, 472.
(5) (a) Pannell, K. H.; Cervantes, J.; Hernandez; Cassias, J.;
Vincenti, S. Organometallics 1986, 5, 1056. (b) Tobita, H.; Ueno, K.;
Ogino, H. Chem. Lett. 1986, 1777. (c) Ueno, K.; Tobita, H.; Ogino, H.
Chem. Lett. 1990, 369. (d) Sharma, H. K.; Pannell, K. H. Chem. Rev.
1995, 95, 1351.
(8) Iwata, M.; Okazaki, M.; Tobita, H. Organometallics 2006, 25,
6115.
(9) (a) Straus, D. A.; Grumbine, S. K.; Tilley, T. D. J. Am. Chem. Soc.
1990, 112, 7801. (b) Grumbine, S. K.; Mitchell, G. P.; Straus, D. A.; Tilley, T.
D. Organometallics 1998, 17, 5607. (c) Dysard, J. M.; Tilley, T. D.
Organometallics 2000, 19, 4726. (d) Schmedake, T. A.; Haaf, M.; Paradise,
B. J.; Millevolte, A. J.; Powell, D. R.; West, R. J. Organomet. Chem. 2001,
636, 17. (e) Ochiai, M.; Hashimoto, H.; Tobita, H. Angew. Chem., Int. Ed.
2007, 46, 8192.
r
2009 American Chemical Society
Published on Web 06/25/2009
pubs.acs.org/Organometallics

3964 Organometallics, Vol. 28, No. 14, 2009
Hashimoto et al.
Figure 1. ORTEP drawing of 2 (50% thermal probability ellip-
˚
soids). Selected bond lengths (A) and angles (deg): Ru(1)-Si(1)
2.2335(7), Ru(1)-Si(2) 2.4224(7), Ru(1)-C(22) 1.835(3), Si(1)-
Ru(1)-Si(2) 93.85(2), C(1)-Si(1)-C(10) 105.43(12), C(1)-
Si(1)-Ru(1) 125.76(8), Ru(1)-Si(1)-C(10) 127.89(9).
silyl ligand. The remaining spectroscopic data are also con-
sistent with the structure of 2.10
1
Figure 2. Variable-temperature H NMR spectra of 2 in non-
ane-d20.
As shown in Figure 1, X-ray crystallographic studies11
revealed that, except for the lengths of the M-Si bonds, the
structural features of 2 were comparable to those of 1. The
and ΔGq298=69(5) kJ mol-1).13 The negative value for ΔSq
implies that the transition state is more sterically congested
than the ground state. Compared to the corresponding
calculated activation barriers for (OC)5CrdSiH(OH) (0.46 kJ
˚
Ru(1)-Si(1) bond length of 2.2335(7) A was within the range
of reported values for RudSi double bond lengths (2.22-
12
˚
mol-1 14a and (OC)5ModSiH2 (1.5 kJ mol-1),14b the sign-
)
2.34 A), while the Ru(1)-Si(2) bond length of 2.4224-
˚
ificantly higher value of ΔHq for 2 indicates the involvem-
ent of, besides the π-bonding energy for the RudSi bond, the
energy required to overcome the steric repulsion between the
mesityl groups and ligands on the ruthenium center. Several
related complexes such as [Cp*(Me3P)2RudSiMe2]þ,15a
Cp*(dmpe)ModSiEt2 (dmpe=Me2PCH2CH2PMe2),15b and
Cp*(Me3Si)(OC)2MdSiMes2 (M=W,3a Mo3b)havealso exhibited
fast rotation around the MdSi bonds, within the NMR time
scale, at room temperature, according to their NMR spectra;
unfortunately, detailed analysis have yet to be reported.
Furthermore, related kinetic studies for donor-stabilized bis-
(silylene) complexes have indicated that the MdSi bond rota-
tion and the SirO dative bond cleavage cannot be separated.16
Remarkably, the reaction of 2 with MeOH at room
temperature, in the presence of isonitrile, tBuNC, or XylNC
(Xyl=2,6-dimethylphenyl), affords the trimethoxysilyl com-
plex Cp*Ru(CO)(CNR0)[Si(OMe)3] (3a: R0 =tBu, 4a: R0 =
Xyl), together with Mes2Si(OMe)H (5a) (eq 2). For these
reactions, the isonitrile was added to trap the ruthenium
species generated during the reaction. Similar results were
also obtained with the use of EtOH instead of MeOH.
(7) A was typical for a Ru-Si single bond. The difference
between the M-Si bond lengths of 2 and 14a of about 0.08 A
˚
˚
corresponds almost exactly to the difference (0.09 A) be-
˚
˚
tween the atomic radii of Ru (1.33 A) and Fe (1.24 A). In
contrast to the FedSi bond of 1, the longer RudSi bond of 2
allows for increased exposure to attacks by external mole-
cules arising in its unique properties, as shown below.
1
As shown in Figure 2, the variable-temperature H NMR
spectroscopy (300 MHz) was employed to examine the flux-
ional behavior of 2. At 298 K, the mesityl groups exhibited
dissimilar signals for its four o-Me, four m-H, and two p-Me
groups. At higher temperatures, the coalescence of each set of
the o-Me, p-Me, and m-H signals into single signals verified the
rotations of, within the NMR time scale, (1) the silylene unit
around the RudSi double bond and (2) the two mesityl groups
around the Si-C(mesityl) single bond.
Line-shape analysis of the p-Me signals in the variable-
1
temperature H NMR spectra of 210 allowed for the deter-
mination of activation parameters for the RudSi bond
rotation (ΔHq=64(3) kJ mol-1, ΔSq=-18(9) J K-1 mol-1
,
(10) For the details, see Supporting Information.
(11) Crystal data (150 K) for 2: C32H46OSi2Ru; fw 603.94; mono-
˚
˚
˚
clinic; P21/c; a=10.5565(3) A, b=18.7387(7) A, c=15.7809(6) A, β=
3
3
˚
95.1401(13)°, V=3109.2(2) A , density (calcd) 1.290 Mg/m , Z=4. Final
R indices R=0.0420, Rw=0.1004 for all data, 6879 unique reflections.
Crystallographic Information has been deposited with the Cambridge
Crystallographic Data Centre: CCDC 698346 (2).
(12) Based on a survey of the Cambridge Structural Database, CSD
verison 5.29 (November 2007).
(13) Rotation barrier for the two mesityl groups around the Si-C
(mesityl) single bond could be derived from the analysis of the o-Me or
m-H signals. However, unfortunately, sufficient line-shape analysis was
not possible for the o-Me signals, because four o-Me signals coalesced
around 361 K, but, above this temperature, the very broad signal did not
become sharp enough for the analysis even at 380 K, which was the
upper limit of temperature due to the boiling point of the solvent. For the
m-H signals, we were not able to get good visual line fitting. It seems that
the long-range couplings between the m-protons in the mesityl groups
affect the line width, and we could not estimate it exactly, even though we
tested various coupling constants.
Attempts to purify 3 and 4 were unsuccessful, either by
chromatography (facile decomposition on silica gel and
(14) (a) Nakatsuji, H.; Usio, J.; Yonezawa, T. J. Organomet. Chem.
ꢀ
1983, 258, C1. (b) Marquez, A.; Sanz, J. F. J. Am. Chem. Soc. 1992, 114,
2903.
(15) (a) Grumbine, S. K.; Tilley, T. D.; Arnold, F. P.; Rheingold, A.
L. J. Am. Chem. Soc. 1994, 116, 5495. (b) Mork, B. V.; Tilley, T. D.; Schultz,
A. J.; Cowan, J. A. J. Am. Chem. Soc. 2004, 126, 10428.

Communication
Scheme 1. Possible Mechanism for the Reaction of 2
with Alcohol
Organometallics, Vol. 28, No. 14, 2009 3965
and 4 indicates that every Si-C bond on the SiMe3 ligand
was cleaved and functionalized at room temperature. In
contrast, the Fe analogue of 2 simply produced Cp*Fe-
(CO)(CNtBu)SiMe3 and 5a.10
A possible mechanism for the substitution of the Me
groups on the silyl ligand in 2 is shown in Scheme 1.
Initially, 1,2-addition of the alcohol to the RudSi double
bond affords A. Reductive elimination of Mes2Si(OR)H
(5), followed by the 1,2-Me migration, gives methyl(sily-
lene) complex B. Subsequently, addition of an alcohol to B,
reductive elimination of methane, and 1,2-Me migration
occur to produce D. The process from B to D repeats twice
to give E. Finally, the coordination of isonitrile affords 3 or
4. It is important to note that, for this reaction, partially
alkoxylated products were not observed. Moreover, in a
separate experiment, Cp*Ru(CO)(CNXyl)(SiMe3), which
could act as an intermediate for the reaction, was found to
be unreactive toward excess methanol, even at 80 °C.10
These results indicate that isonitrile does not coordinate to
the metal center until the end of the multistep conversion of
A to E.
alumina) or by recrystallization (similar solubilities between
3 or 4 and 5).10 Nevertheless, existence of the Si(OR)3 group,
instead of the SiMe3 group, was unambiguously confirmed
using 1H NMR and mass spectra of 3 and 4.10 The 1H NMR
spectrum of 4a exhibited, in addition to the signals for Cp*
and Xyl groups, a singlet at 3.65 ppm with an intensity of 9H
that was assignable to the Si(OMe)3 group. For reference, the
1H NMR spectrum of Cp*Ru(CO)(CNXyl)SiMe310 exhibits
a signal at 0.66 ppm for its SiMe3 group. The molecular ion
peak of 4a appeared at m/z=517, which is consistent with a
structure that bears a Si(OMe)3 group. The formation of 3
Acknowledgment. This work was supported by the
Ministry of Education, Culture, Sports, Science and
Technology of Japan [Grants-in-Aid for Scientific Research
Nos. 18350027, 18064003, 16750044,19550056, 19029003,
and 20038002] and Tokuyama Science Foundation.
Supporting Information Available: Experimental details
(PDF) and X-ray crystallographic data of 2 (CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
(16) (a) Ueno, K.; Masuko, A.; Ogino, H. Organometallics 1997, 16,
5023. (b) Wada, H.; Tobita, H.; Ogino, H. Chem. Lett. 1998, 993. (c) Ueno,
K.; Masuko, A.; Ogino, H. Organometallics 1999, 18, 2694.