C-H activation of benzene with Cp*Ru(CO)2CH3

Article abstract of DOI:10.1016/j.jorganchem.2008.05.013

Irradiation of Cp^*Ru(CO)₂CH₃ (1) in C₆D₆ at room temperature yields Cp^*Ru(CO)₂C₆D₅ and CH₃D (where Cp^* = n⁵-C₅Me

Full text of DOI:10.1016/j.jorganchem.2008.05.013

Journal of Organometallic Chemistry 693 (2008) 2700–2702
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
C–H activation of benzene with Cp*Ru(CO)2CH3
a
a
a
John R. Moss a, , Siyabonga Ngubane , Akella Sivaramakrishna , Banothile C.E. Makhubela ,
*
John E. Bercaw b, Jay A. Labinger b, Michael W. Day b, Lawrence M. Henling b, Hong Su a
a Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa
b Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, Pasadena, CA 91125, USA
a r t i c l e i n f o
a b s t r a c t
Irradiation of Cp*Ru(CO)2CH3 (1) in C6D6 at room temperature yields Cp*Ru(CO)2C6D5 and CH3D (where
Cp* = n5-C5Me5). Cp*Ru(CO)2CD3 (2) has also been prepared and similar irradiation in C6H6 yields
Cp*Ru(CO)2C6H5 (3) and CD3H. This latter reaction confirms that it is the methyl group bonded to ruthe-
nium that is involved in the C–H activation process and not the methyl groups on the Cp* ligand system.
The compound Cp*Ru(CO)2C6H5 (3) has been prepared for the first time in good yield by the reaction of
Cp*Ru(CO)2Br with NaBPh4. X-ray crystal structures of both Cp*Ru(CO)2CH3 (1) and Cp*Ru(CO)2C6H5 (3)
have been determined and the results are reported and discussed.
Article history:
Received 4 January 2008
Received in revised form 25 April 2008
Accepted 6 May 2008
Available online 13 May 2008
Keywords:
C–H bond activation of benzene
Ruthenium carbonyl complexes
Irradiation
Ó 2008 Elsevier B.V. All rights reserved.
Structural characterization
1. Introduction
reversible) activation of a ring C–H bond [9]. These various studies
prompted us to report our findings on the reaction of the related
The intermolecular activation of C–H bonds by metal complexes
is an important process, which may comprise the first step in the
catalytic conversion of alkanes into more useful products [1–3].
Platinum group metal compounds are particularly useful in this re-
gard [3]. Metal–alkyl complexes LmM—R in particular have been
shown to undergo a variety of C–H activation reactions; recent re-
views discuss the current understanding of the sigma-bond
metathesis reactions of LmMR with R0–H [4].
system Cp*Ru(CO)2CH3 with benzene under irradiation.
2. Results and discussion
Irradiation of a solution of Cp*Ru(CO)2CH3 in C6D6 results in the
clean formation of a new species, as judged by the disappearance
of the Ru–CH3 peak in the 1H NMR spectrum and the appearance
of a single new Cp* peak at d 1.46 ppm. The IR spectrum of this
There have been a number of recent studies of reactions of
LmRuR (with various ligand systems Lm; R = CH3 or C6H5) with are-
nes. Diversi et al. reported the thermolysis (48 h, 120 °C) of
Cp*Ru(PMe2Ph)2CH3 in benzene to give Cp*Ru(PMe2Ph)2C6H5 [5].
Oxgaard and Goddard have carried out calculations on the system
TpRu(CO)(MeCN)C6H5 which catalyzes hydroarylation via C–H
activation [6]. More recently, Cundari et al. have investigated the
use of complexes of the type TpRu(L)(MeCN)R (L = CO, PMe3;
R = Me or Ph; Tp = hydridotris(pyrazolyl borate) toward hydroary-
lation of isonitriles [7].
solution also showed the appearance of two new m(CO) peaks. High
resolution mass spectrometry was consistent with the new species
being Cp*Ru(CO)2C6D5. The 1H NMR spectrum of this reaction solu-
tion was also consistent with the main organic product being CH3D
(Eq. (1)).
*
*
Cp
Cp
hv
Ru
CH3
+
C6D6
Ru
C6D5
+
CH3D
OC
OC
CO
CO
There are few reports on C–H activation by species generated
from the ‘‘classical” organometallic complexes CpRu(CO)2CH3 or
Cp*Ru(CO)2CH3. Photolysis of [CpRu(CO)2]2 in toluene generates
(inter alia) CpRu(CO)2H, presumably via H-atom abstraction by
1
ð1Þ
A possible mechanism for this reaction involves loss of CO, oxi-
dative addition of a benzene C–D bond, elimination of CH3D and
then re-coordination of CO. Alternatively, the mechanism could
involve a concerted process, as has been suggested by others for
related systems [5,6]. There is also the possibility that the methane
is generated from one of the methyl groups on the pentamethylcy-
clopentadienyl ring. In order to rule this out, we made the
the 17-electron intermediate CpRu(CO)2 [8]. Photolysis of
a
bridged dicylopentadienyl-dirutheniun species leads to (thermally
* Corresponding author.
E-mail address: John.Moss@uct.ac.za (J.R. Moss).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.05.013

J.R. Moss et al. / Journal of Organometallic Chemistry 693 (2008) 2700–2702
2701
perdeuterated methyl compound Cp*Ru(CO)2CD3 and reacted it
with benzene; NMR spectra were consistent with the formation
of CD3H and Cp*Ru(CO)2C6H5 as shown in Eq. (2).
The reaction of Cp*Ru(CO)2CD3 with C6D6 yielded Cp*Ru(CO)2-
C6D5 and no protio methanes. It is quite conclusive that the CH3
of the Cp* is not being involved with the formation of methane
(Eq. (3)).
*
Cp
*
Cp
hv
Ru
CD3
+
C6H6
Ru
C6H5
+
CD3H
OC
OC
CO
CO
2
3
ð2Þ
*
*
Cp
Cp
hv
Ru
CD3
C6D6
+
Ru
CO
C6D5
+
CD4
OC
OC
CO
Fig. 2. Molecular structure of Cp*Ru(CO)2C6H5 (3) showing the atom numbering
2
scheme.
ð3Þ
Since the phenyl compound Cp*Ru(CO)2C6H5 (3) had not previ-
ously been reported, we prepared it independently by a method
which has been used for preparing related phenyl compounds
[9], shown in Eq. (4). The spectroscopic data obtained for com-
pound (3) was completely consistent with the Ru–phenyl com-
pound being produced as described in Eqs. (2) and (4).
Table 1
Comparison of X-ray structures of complexes 1 and 3
Cp*Ru(CO)2CH3
Cp*Ru(CO)2C6H5
Ru–CO(1)
Ru–CO(2)
Ru–R
Ru–C5Me5 (average)
C–O(1)
1.8774 (11)
1.8803 (11)
2.1441 (15)
2.2669 (12)
1.1443 (14)
1.1457 (14)
1.871 (4)
1.874 (4)
2.105 (3)
2.260 (3)
1.151 (4)
1.147 (4)
*
*
Cp
Cp
reflux
Ru
Br +
NaBPh4
+
C6H5
Other products
Ru
C–O(2)
OC
OC
Ru
CO
CO
90.32 (5)
87.92 (6)
89.76 (6)
92.68 (15)
90.70 (13)
90.71 (13)
(1)OC
(1)OC
(2)OC
CO(2)
R
3
Ru
Ru
ð4Þ
R
Since no crystal structures have been previously reported on
simple alkyl compounds of the type Cp*Ru(CO)2R, we determined
the crystal structures of both the methyl (Fig. 1) and phenyl
(Fig. 2) complexes for comparison (Table 1). The molecular struc-
tures of compounds 1 and 3 are similar; as one would expect,
the Ru–C bond length in the phenyl compound (2.105 (3) Å)
is shorter than that in the methyl compound (2.1441 (15) Å).
The Ru–C6H5 distance in (3) is very similar to that found in
Cp*Ru(PMe2Ph)2C6H5 (2.104 Å) [5]. The angles given in Table 1
are all bigger for the phenyl compound as compared with the
methyl compound, which may be due to steric crowding at the
metal centre.
3. Conclusions
We have shown that the complex Cp*Ru(CO)2CH3 can activate
the C–H bond of benzene to give the new Ru–phenyl complex
Cp*Ru(CO)2C6H5 and methane. We have also demonstrated that
the methyl group in the methane is derived from the methyl group
directly bonded to the Ru. The Ru–phenyl complex has also been
prepared by another route and successfully characterized. The
Ru–C6H5 distance of 2.105(3) Å is significantly shorter than Ru–
CH3 (2.144 (2) Å) in Cp*Ru(CO)2R.
4. Experimental
4.1. General
All preparations were carried out in dry nitrogen saturated sol-
vents using standard Schlenk techniques. Irradiations were carried
out with a Englehard Hanovia Lamp (125 W) at a distance of 16 cm
from the reaction vessel. Many reactions were carried out in J.
Young re-sealable NMR tubes. 1H and 13C NMR spectra were re-
Fig. 1. Molecular structure of Cp*Ru(CO)2CH3 (1) showing the atom numbering
scheme.

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J.R. Moss et al. / Journal of Organometallic Chemistry 693 (2008) 2700–2702
corded on a Varian Mercury 300 or Varian INOVA – 500 spectrom-
eters at room temperature. All 1H and 13C NMR chemical shifts are
reported relative to tetramethylsilane. IR spectra were recorded on
a Bio Rad Merlin spectrophotometer. Mass spectra were recorded
on a JEOL JMS 600 mass spectrometer.
4.4. X-ray crystal structure determinations
Cp*Ru(CO)2CH3 (1): Crystals of Cp*Ru(CO)2CH3 were obtained by
slow evaporation of the solvent from a cold pentane solution. X-ray
data was collected on a Bruker SMART 1000 diffractometer and de-
tails of the crystal, data collection and refinement are given in Ta-
ble 2 (see Supplementary Information).
4.2. Synthesis of compounds 1–3
Cp*Ru(CO)2C6H5 (3): Crystals of Cp*Ru(CO)2C6H5 were grown
from hexane solutions on cooling. X-ray data was collected on a
Nonius Kappa-CCD diffractometer using graphite-monochromated
Mo Ka radiation (k = 0.71073 Å) and details of the crystal, data col-
lection and refinement are given in Table 3 (see Supplementary
[Cp*Ru(CO)2]2 was prepared by the method of King et al. [10],
and Cp*Ru(CO)2CH3 (1) was prepared by the method of Malisch
et al. [11]. The mass spectrum of 1 shows a parent ion at m/e
308 P (16%) with the expected isotope pattern and other peaks at
293 P–CH3 (10%), 279 P–CO (40%), 264 P–CH3–CO (7%), 252 P–
Information).
2CO (98%) and 235 P–2CO–CH3 (77%). IR m(CO) (pentane) 2005
(s) and 1946 (s) cmꢀ1. Cp*Ru(CO)2CD3 (2) was also prepared by
the method of Malisch et al. [11] using CD3I. The product was char-
acterized by IR, 1H NMR and mass spectra.
Acknowledgements
We thank the University of Cape Town, the DST Centre of Excel-
*
lence in Catalysis, C Change, Anglo Platinum and BP Chemicals for
4.2.1. Preparation of Cp*Ru(CO)2C6H5 (3)
support and Johnson & Matthey for the loan of ruthenium
trichloride.
A solution of Cp*Ru(CO)2Br (0.101 g, 0.2713 mmol) was refluxed
with NaBPh4 (93.6 mg, 0.2735 mmol) in methanol for 4 h as re-
ported by Haines et al. [12]. The solvent was removed and the
product recrystallized from 3 ml of CH2Cl2/n-hexane (1:2) to give
pure (3) as off-white prisms in good yield (72 mg, 72%). Anal. Calc.
for C18H20O2Ru: C, 58.52; H, 5.46. Found: C, 58.36; H, 5.58%. IR
References
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C18.
1
The H NMR spectrum now showed the peaks of Cp*Ru(CO)2CH3
to have disappeared and a new Cp* peak appeared at d 1.46 ppm
corresponding to Cp*Ru(CO)2C6D5 as well as a singlet at d 0.14 cor-
responding to CH4 (23%) and a triplet centred at d 0.13 correspond-
ing to CH3D (77%). After removing the solvent, the residue in
pentane showed
m(CO) peaks in the IR spectrum at 2012 and
1956 cmꢀ1. A mass spectrum of this residue showed a parent ion
measured at m/e 375.0812 (calculated for C18H15D5O2Ru:
375.0821). The data is consistent with the product being Cp*Ru-
(CO)2C6D5.
[12] R.J. Haines, A.L. du Preez, J. Am. Chem. Soc. 93 (1979) 2820.