Reactions of a neutral silylene ruthenium complex with heterocumulenes: C=o hydrosilylation of isocyanates vs c=s bond cleavage of isothiocyanate

Article abstract of DOI:10.1021/om2010854

Reactions of the neutral silylene ruthenium complex Cp*(CO)(H)Ru= Si(H){C(SiMe₃₎₃} (2) with heterocumulenes were investigated. Treatment of 2 with ArNCO (Ar = Mes, Ph; Mes = 2,4,6-trimethylphenyl) resulted in hydrosilylation of ArNCO selectively at the C=O bond at room temperature to give the five-membered chelate complexes Cp*(CO)Ru[κ2N,Si-N(Ar)=C(H) OSi(H)- {C(SiMe₃₎₃}] (3a, Ar = Mes; 3b, Ar = Ph). In contrast, the reaction of 2 with MesNCS led to cleavage of the C=S double bond to give the isocyanide complex Cp*(CO)Ru(CNMes){SSiH2C(SiMe₃₎₃} (4). The structures of both types of products were unambiguously determined by X-ray crystallography.

Full text of DOI:10.1021/om2010854

Communication
pubs.acs.org/Organometallics
Reactions of a Neutral Silylene Ruthenium Complex with
Heterocumulenes: CO Hydrosilylation of Isocyanates vs CS Bond
Cleavage of Isothiocyanate
Mitsuyoshi Ochiai, Hisako Hashimoto,* and Hiromi Tobita*
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan
S
* Supporting Information
ABSTRACT: Reactions of the neutral silylene ruthenium
complex Cp*(CO)(H)RuSi(H){C(SiMe3)3} (2) with
heterocumulenes were investigated. Treatment of 2 with
ArNCO (Ar = Mes, Ph; Mes = 2,4,6-trimethylphenyl) resulted
in hydrosilylation of ArNCO selectively at the CO bond at
room temperature to give the five-membered chelate
complexes Cp*(CO)Ru[κ2N,Si-N(Ar)C(H)OSi(H)-
{C(SiMe3)3}] (3a, Ar = Mes; 3b, Ar = Ph). In contrast, the reaction of 2 with MesNCS led to cleavage of the CS double
bond to give the isocyanide complex Cp*(CO)Ru(CNMes){SSiH2C(SiMe3)3} (4). The structures of both types of products
were unambiguously determined by X-ray crystallography.
reactions and structural characterization of two types of
products and discuss possible mechanisms for formation of
them.
Addition of 1 equiv of MesNCO (Mes = mesityl = 2,4,6-
trimethylphenyl) to 2 in C6D6 at room temperature gave
Cp*Ru(CO)[κ2N,Si-N(Mes)C(H)OSi(H){C(SiMe3)3}]
(3a) instantaneously in 74% yield by NMR spectroscopy.
Complex 3a was isolated in 52% yield as yellow crystals by a
preparative-scale reaction in hexane (Scheme 1). The formation
n the chemistry of transition-metal carbene complexes,
reactions with unsaturated organic substrates are particularly
I
important and well investigated because they are extremely
useful for the syntheses of various exotic organic compounds.1
In contrast, although transition-metal silylene complexes have
been postulated to play an important role in metal-mediated
transformations of organosilicon substrates,2 the fundamental
reactivity of isolated silylene complexes, especially toward
unsaturated organic substrates, have scarcely been reported
until recently. This is mainly because the metal−silicon double
bonds in most of the isolated silylene complexes are excessively
protected by bulky groups and thus become nonreactive toward
most regular-sized organic or inorganic substrates. One distinct
exception is the cationic silylene complex [Cp*(PMe3)2Ru
SiMe2]+, which has small methyl groups on the silicon atom
and underwent [2 + 2] cycloaddition with RNCO (R = Me,
Ph) to give Ru−Si−N−C four-membered-ring complexes.3
Recently, another type of silylene complex in which the metal−
silicon double bonds are less sterically protected, i.e. silylene
complexes having hydrogen atoms on both silicon and metal
centers, has been synthesized.4−6 These silylene complexes
have been demonstrated to show reactivity far higher than that
of most of the previously reported silylene complexes. For
example, our group reported that the neutral silylene complexes
Cp*(CO)2(H)WSi(H){C(SiMe3)3} (1)4 and Cp*(CO)-
(H)RuSi(H){C(SiMe3)3} (2)5 reacted cleanly with nitriles,
ketones, and α,β-unsaturated carbonyl compounds. This time,
we examined the reactions of 2 with ArNCO and ArNCS. As a
result, in contrast with the aforementioned cationic silylene
complex, the reaction of 2 with ArNCO led to hydrosilylation
of ArNCO at the CO bond to give five-membered Ru−Si−
O−C−N chelate complexes, while reaction with ArNCS led to
the CS double bond cleavage of ArNCS to give isocyanide
complexes at room temperature. We report here these new
Scheme 1. Reactions of 2 with ArNCO and ArNCS
of 3a means that the CO bond of MesNCO was
hydrosilylated with 2. The phenyl-substituted analogue Cp*Ru-
(CO)[κ2N,Si-N(Ph)C(H)OSi(H){C(SiMe3)3}] (3b) was
also obtained in a similar procedure but in rather low yield
(12%) because of its thermal instability. These products were
Received: November 5, 2011
Published: January 5, 2012
© 2012 American Chemical Society
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dx.doi.org/10.1021/om2010854 | Organometallics 2012, 31, 527−530

Organometallics
Communication
characterized by NMR, IR, and mass spectroscopy, and
elemental analysis. The crystal structure of 3a was determined
by X-ray crystallography.
Si(sp3)−H protons. Similarly, the 29Si and 1H signals for SiH of
3b are observed at 95.9 and 6.12 ppm, respectively. In addition,
1
the H signal for NCH appears at 7.18 ppm for 3a and 7.65
ppm for 3b, which is comparable with the chemical shifts of the
corresponding protons of siloxyimines (PhNCHO-
SiR1R2R3).11 In relation to the present results, we have
reported very recently that the germylene analogue of tungsten
silylene complex 1, Cp*(CO)2(H)WGe(H){C(SiMe3)3},
underwent hydrogermylation of PhNCO or RNCS to give a
similar complex having a five-membered W−Ge−O−C−N or
W−Ge−N−C−S ring kinetically, respectively.12
The ORTEP drawing of 3a depicted in Figure 1 (left) shows
that 3a has a new five-membered Ru−Si−O−C−N ring.7 The
In contrast to the reactions of 2 with isocyanates, the reaction
of 2 with an isothiocyanate led to the cleavage of the CS
double bond. Thus, the reaction of 2 with MesNCS in C6D6
proceeded at room temperature to afford Cp*Ru(CO)-
(CNMes){SSiH2C(SiMe3)3} (4) quantitatively, which was
clearly observed by NMR spectroscopy. Complex 4 was
isolated as yellow crystals in 78% yield by a preparative-scale
reaction in hexane after recrystallization (Scheme 1). In this
reaction, the CS bond of the incoming MesNCS was cleaved
and converted into an isocyanide ligand (CNMes) and a
silanethiolato ligand {SSiH2C(SiMe3)3} in 4. Although the C
S bond cleavage of isothiocyanates induced by Cp*Co-
(H2C=CH2)2,13 [RhIr(CO)2(μ-η1:η2-C2Ph)(dppm)2][X] (X =
BF4, SO3CF3; dppm = Ph2PCH2PPh2),14 and [Mo(P4)-
(dppm)] (P4 = meso-o-C6H4(PPhCH2CH2PPh2)2)15 has
previously been reported, this is the first example of CS
bond cleavage induced by a silylene complex.
Figure 1. ORTEP drawings of 3a (left) and 4 (right) showing thermal
ellipsoids at the 50% probability level. Selected bond lengths (Å) and
angles (deg) are as follows. 3a: Ru(1)−Si(1) = 2.365(2), Ru(1)−N(1)
= 2.164(6), Si(1)−O(1) = 1.761(6), O(1)−C(1) = 1.308(9), C(1)−
N(1) = 1.282(10), Si(1)−C(11) = 1.928(7); Si(1)−Ru(1)−N(1) =
79.25(17), Ru(1)−N(1)−C(1) = 118.8(5), Ru(1)−Si(1)−O(1) =
99.87(18), Si(1)−O(1)−C(1) = 117.2(5), N(1)−Ru(1)−C(21) =
98.7(3). 4: Ru(1)−S(1) = 2.442(2), Ru(1)−C(1) = 1.939(6), Ru(1)−
C(21) = 1.893(7), Si(1)−S(1) = 2.092(2), C(1)−N(1) = 1.167(7),
N(1)−C(2) = 1.394(7); S(1)−Ru(1)−C(1) = 84.6(2), S(1)−Ru(1)−
C(21) = 95.7(2), Ru(1)−S(1)−Si(1) = 105.25(8), S(1)−Si(1)−
C(11) = 115.6(2), Ru(1)−C(1)−N(1) = 174.4(6), C(1)−N(1)−
C(2) = 170.9(6).
The X-ray crystal structure of 4 (Figure 1, right) clearly
shows that 4 bears a CNMes and a SSiH2C(SiMe3)3 ligand.8
The Ru(1)−S(1) and S(1)−Si(1) bond lengths are 2.442(2)
and 2.092(2) Å, respectively, and the Ru(1)−S(1)−Si(1) bond
angle is 105.25(8)°. The Ru(1)−S(1) bond length is
comparable with that reported for CpRu(PPh3)2(SSiiPr3)
(2.461 Å).16 The S(1)−Si(1) bond length and the Ru(1)−
S(1)−Si(1) bond angle are in the range of known silanethiolato
complexes (S−Si, 2.05−2.18 Å; M−S−Si, 85.7−131.5°).10 The
Ru(1)−C(1)−N(1) linkage is almost linear (174.4(6)°). The
Ru(1)−C(1) bond length is 1.939(6) Å, which is comparable
with those of typical isocyanide ruthenium complexes (1.86−
2.09 Å, average 1.98 Å).10 The N(1)−C(1) bond length is
1.167(7) Å, which is also in the range of known isocyanide
ruthenium complexes (1.07−1.27 Å).10
The NMR data also support this structure of 4. For example,
the signals of two diastereotopic hydrogen atoms on silicon are
observed at 5.15 and 5.30 ppm in the 1H NMR spectrum. The
13C{1H} NMR signal of the CN group appears at 165.9 ppm,
which is a chemical shift characteristic of isocyanide ligands.
The IR spectrum exhibits strong bands at 1950 and 2112 cm−1,
which are assigned to the stretching vibrations of CO and CN,
respectively.17
N(1)−C(1) bond length is 1.282(10) Å, which is in the range
of NC double-bond lengths (1.15−1.36 Å),8 whereas the
Si(1)−O(1) bond length (1.761(6) Å) is significantly longer
than the usual Si−O single-bond length (1.63−1.65 Å) and
rather close to the partially dative Si−O bond length of alkoxy-
bridged bis(silylene) complexes (1.78−1.86 Å).9 On the other
hand, the Ru(1)−Si(1) bond length (2.365(2) Å) is relatively
short as a typical Ru−Si single-bond length (2.35−2.55 Å) and
is in the range of RuSi double-bond lengths (2.26−2.41 Å)
of base-stabilized silylene ruthenium complexes.9 The sum of
the bond angles at Si(1), except for the Si(1)−O(1) bond, i.e.
Ru(1)−Si(1)−C(11), Ru(1)−Si(1)−H(1), and C(11)−Si(1)−
H(1), is 348°. This value is between the median of 328° for a
typical sp3-hybridized silicon atom and 360° for an sp2-
hybridized atom and is slightly closer to the latter value.
Therefore, it is suggested that there is a significant contribution
of canonical form II (silylene complex) in addition to form I
(silyl complex) illustrated in Chart 1.
Chart 1
Possible mechanisms for the reactions of 2 with ArNCO and
ArNCS are illustrated in Scheme 2. In the reaction with
ArNCO, coordination of the oxygen atom to the electron-
deficient silylene silicon atom occurs as denoted by A. This
coordination in turn enhances the electrophilicity of the
isocyanate carbon and induces hydride migration from Ru to
the carbon atom to form intermediate B. Subsequent rotation
of the C−O bond in B and coordination of the nitrogen atom
to Ru gives product 3. In the reaction with ArNCS, there are at
least two possible routes: I and II. Route I is started by
coordination of the less sterically hindered sulfur atom of
ArNCS to the silylene ligand. However, presumably this S-
This resonance contribution of form II is also suggested on
the basis of NMR data. For example, the 29Si{1H} NMR
spectrum of 3a shows a signal for SiH at 93.1 ppm. This
downfield shift is comparable with those of base-stabilized
silylene complexes.10 The 1H signal for SiH is observed at 6.47
ppm, which is shifted downfield compared with that for usual
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Organometallics
Communication
Scheme 2. Possible Mechanisms for Reactions of 2 with
ArNCO and ArNCS
ACKNOWLEDGMENTS
■
This work was supported by the Ministry of Education,
Culture, Sports, Science and Technology of Japan (Grants-in-
Aid for Scientific Research Nos. 18064003, 20038002, and
21550054).
REFERENCES
■
(1) For recent reviews of carbene complexes. see: (a) Tonka, T. M.;
Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18−29. (b) Schrock, R. R.
Chem. Rev. 2002, 102, 145−179. (c) Schrock, R. R.; Hoveyda, A. H.
Angew. Chem., Int. Ed. 2003, 42, 4592−4633. (d) Grubbs, R. H. Angew.
Chem., Int. Ed. 2006, 45, 3760−3765. (e) Whited, M. T.; Grubbs, R. H.
Acc. Chem. Res. 2009, 42, 1607−1616. (f) Schrock, R. R. Chem. Rev.
2009, 109, 3211−3226. (g) Hoveyda, A. H.; Malcolmson, S. J.; Meek,
S. J.; Zhugralin, A. R. Angew. Chem., Int. Ed. 2010, 49, 34−44 and
references cited therein.
(2) (a) For recent reviews of silylene complexes, see: (a) Eisen, M. S.
In The Chemistry of Organic Silicon Compounds; Rappoport, Z.,
Apeloig, Y., Eds.; Wiley: New York, 1998; Vol. 2, Chapter 35.
(b) Ogino, H. Chem. Rec. 2002, 2, 291−306. (c) Okazaki, M.; Tobita,
H.; Ogino, H. Dalton Trans. 2003, 493−506. (d) Tilley, T. D. Acc.
Chem. Res. 2007, 40, 712−719 and references cited therein.
(3) Mitchell, G. P.; Tilley, T. D. J. Am. Chem. Soc. 1997, 119, 11236−
11243.
(4) (a) Watanabe, T.; Hashimoto, H.; Tobita, H. Angew. Chem., Int.
Ed. 2004, 43, 218−221. (b) Watanabe, T.; Hashimoto, H.; Tobita, H.
J. Am. Chem. Soc. 2006, 128, 2176−2177. (c) Watanabe, T.;
Hashimoto, H.; Tobita, H. J. Am. Chem. Soc. 2007, 129, 11338−
11339. (d) Hashimoto, H.; Ochiai, M.; Tobita, H. J. Organomet. Chem.
2007, 692, 36−43.
(5) (a) Ochiai, M.; Hashimoto, H.; Tobita, H. Angew. Chem., Int. Ed.
2007, 46, 8192−8194. (b) Ochiai, M.; Hashimoto, H.; Tobita, H.
Dalton Trans. 2009, 1812−1814.
(6) (a) Feldman, J. D.; Peters, J. C.; Tilley, T. D. Organometallics
2002, 21, 4065−4075. (b) Glaser, P. B.; Tilley, T. D. J. Am. Chem. Soc.
2003, 125, 13640−13641. (c) Calimano, E.; Tilley, T. D. J. Am. Chem.
Soc. 2008, 130, 9226−9227. (d) Calimano, E.; Tilley, T. D. J. Am.
Chem. Soc. 2009, 131, 11161−11173.
coordination is so weak that the central carbon is not
sufficiently polarized to accept hydride migration from Ru.
Instead, [2 + 2] cycloaddition between the RuSi and CS
bonds leads to D, from which reductive elimination of the Si−
H bond and coordination of sulfur to the metal center occur to
generate E. The weak C−S bond of E cleaves easily to give 4.
The alternative route II involves coordination of ArNCS to the
ruthenium atom of the unsaturated silyl complex produced by
1,2-H migration from Ru to Si in 2. Subsequent silyl migration
to the sulfur atom in F produces E and then 4. At present, we
do not have any concrete evidence to distinguish these two
routes. It should be noted that the aforementioned reaction of
[Cp*(Me3P)3RuSiMe2]+ with RNCO gave the N-silylated
product [Cp*(Me3P)2Ru(SiMe2NRCO)]+, having a Ru−Si−
N−C four-membered ring instead of an O-silylated product. In
this case, possibly the steric repulsion between the relatively
bulky Cp*(Me3P)2Ru fragment and the R group on RNCO
prevents [2 + 2] cycloaddition between the RuSi and CO
bonds accompanied by O-silylation.
In summary, we have found new reactions of neutral silylene
complex 2 with isocyanates and an isothiocyanate. The former
resulted in hydrosilylation of the CO bond to form a novel
Ru−Si−O−C−N five-membered ring, while the latter resulted
in CS double-bond cleavage. These reactions demonstrate
the existence of new types of bond-forming and -breaking
processes induced by a silylene complex. Further reactivity
studies of 2 are underway.
(7) Crystal data (150 K) for 3a: C31H55NO2Si4Ru; fw 687.19;
monoclinic; P21/c; a = 9.354(5) Å; b = 23.530(14) Å; c = 16.460(10)
Å; β = 91.667(2)°, V = 3621(4) Å3; calcd density 1.273 Mg/m3; Z = 4.
Final R indices: R1 = 0.0783, wR2 = 0.2191 for 5276 reflections with I >
2σ(I). Crystal data (150 K) for 4: C31H55NOSSi4Ru; fw 703.25;
triclinic; P1; a = 11.7791(6) Å; b = 12.1471(5) Å; c = 14.9347(9) Å;
̅
α = 86.458(2)°; β = 74.9512(17)°; γ = 63.014(4)°; V = 1835.02(16) Å3;
calcd density 1.273 Mg/m3; Z = 2. Final R indices: R1 = 0.0703, wR2 =
0.1717 for 6515 reflections with I > 2σ(I). Crystallographic information
has been deposited with the Cambridge Crystallographic Data Centre:
CCDC 837728 (3a), CCDC 804700 (4).
(8) (a) Kato, M.; Kawano, M.; Taniguchi, H.; Funaki, M.; Moriyama,
H.; Sato, T.; Matsumoto, K. Inorg. Chem. 1992, 31, 26−35. (b) Field,
L. D.; Shaw, W. J.; Turner, P. Chem. Commun. 2002, 46−47. (c) Field,
L. D.; Shaw, W. J.; Turner, P. Organometallics 2001, 20, 3491−3499.
(9) Based on a survey of the Cambridge Structural Database, CSD
version 5.27 (November 2005).
ASSOCIATED CONTENT
■
(10) (a) Ueno, K.; Tobita, H.; Shimoi, M.; Ogino, H. J. Am. Chem.
Soc. 1988, 110, 4092−4093. (b) Tobita, H.; Ueno, K.; Shimoi, M.;
Ogino, H. J. Am. Chem. Soc. 1990, 112, 3415−3420. (c) Takeuchi, T.;
Tobita, H.; Ogino, H. Organometallics 1991, 10, 835−836. (d) Tobita,
H.; Sato, T.; Okazaki, M.; Ogino, H. J. Organomet. Chem. 2000, 611,
314−322.
S
* Supporting Information
Text giving experimental details and CIF files giving and X-ray
crystallographic data of 3b and 4. This material is available free
of charge via the Internet at http://pubs.acs.org.
(11) Otter, J. C.; Adamson, C. L.; Yoder, C. H.; Rheingold, A. L.
Organometallics 1990, 9, 1557−1562.
AUTHOR INFORMATION
■
Corresponding Author
(12) Hashimoto, H.; Fukuda, T.; Tobita, H. New J. Chem. 2010, 34,
1723−1730.
*Tel: +81-22-795-6539. Fax: +81-22-795-6543. E-mail:
hhashimoto@m.tohoku.ac.jp (H.H.); tobita@m.tohoku.ac.jp
(H.T.).
(13) Beaumont, I. R.; Begley, M. J.; Harrison, S.; Wright, A. H. Chem.
Commun. 1990, 1713−1715.
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Organometallics
Communication
(14) George, D. S. A.; Hilts, R.; McDonald, W.; Cowie, R. M. Inorg.
Chim. Acta 2000, 300−302, 353−368.
(15) Ohnishi, T.; Seino, H.; Hidai, M.; Mizobe, Y. J. Organomet.
Chem. 2005, 690, 1140−1146.
(16) Kovacs, I.; Pearson, C.; Shaver, A. J. Organomet. Chem. 2000,
596, 193−203.
(17) (a) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry,
5th ed.; Wiley: New York, 1988; p 62. (b) Collman, J. P.; Hegedus, L.
S.; Norton, J. R.; Finke, R. G. Principles and Applications of
Organotransition Metal Chemistry; University Science Books: Mill
Valley, CA, 1987; p 148.
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