The alkyne-anion promoted ring-contraction of hexachlorotropone: Synthesis and structure of [trimethylsilyl(pentachlorobenzoyl)ethyne]- hexacarbonyldicobalt

Article abstract of DOI:10.1016/j.jorganchem.2003.09.033

The reaction of hexachlorotropone with trimethylsilylethynyl-lithium leads to contraction of the seven-membered ring via a semibenzilic acid rearrangement to yield pentachlorophenyl trimethylsilylethynyl ketone, 4. Treatment of 4 with dicobalt octacarbonyl yields the corresponding alkyne-dicobalt hexacarbonyl, 6, which has been characterized by X-ray crystallography.

Full text of DOI:10.1016/j.jorganchem.2003.09.033

Journal of Organometallic Chemistry 689 (2004) 8–13
www.elsevier.com/locate/jorganchem
The alkyne-anion promoted ring-contraction of
hexachlorotropone: synthesis and structure of
[trimethylsilyl(pentachlorobenzoyl)ethyne]-hexacarbonyldicobalt
a
James A. Dunn , Laura E. Harrington , Michael J. McGlinchey
a
a,b,
*
a
Department of Chemistry, McMaster University, 1280 Main St. W., Hamilton, Ontario, Canada L8S 4M1
b
Department of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland
Received 30 May 2003; accepted 11 September 2003
Abstract
The reaction of hexachlorotropone with trimethylsilylethynyl-lithium leads to contraction of the seven-membered ring via a
semibenzilic acid rearrangement to yield pentachlorophenyl trimethylsilylethynyl ketone, 4. Treatment of 4 with dicobalt octa-
carbonyl yields the corresponding alkyne-dicobalt hexacarbonyl, 6, which has been characterized by X-ray crystallography.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Hexachlorotropone; Alkyne dicobalt hexacarbonyl; Ring contraction
1. Introduction
rotropylium cation, C7Cl7þ, which is readily synthesiz-
able [12] but whose structure has not, to our knowledge,
been reported. Moreover, the closely analogous hexa-
chlorotropone, C7Cl6O 1, has also been crystallo-
graphically characterized as a shallow boat [13,14];
apparently, the bulky chloro substituents do not permit
the ring to maintain the electronically favoured planar
geometry. If the canonical contributor 10 is considered,
then the geometries exhibited by C7Cl6O and C7Cl7þ
may be in close accord. If the polychlorinated tropylium
cation is indeed puckered, then it would be of interest to
probe the electronic demand that such a species would
impose upon a metal center.
Our goal was to prepare a system of the type
[(C7Cl6)MLn]þ, where MLn is an organometallic moiety,
such as (RCBC)Co2(CO)6 or ferrocenyl, both of which
are known to stabilize a neighbouring carbocationic site
[8,15]. In cases where the ligand itself is stable as a
cation, such as in the (C6H5)Cr(CO)3-pyrylium salt, 2
[16], or the ferrocenyl-tropylium cation, 3 [17], the cyclic
p system retains its planarity since it has no need for
additional stabilization from the metal center. We here
report an attempt to prepare an alkynyl-dicobalt hexa-
carbonyl derivative of the hexachlorotropylium cation,
and describe the structure of the resulting product.
Previous reports from this laboratory have described
the syntheses, structures and dynamics of a variety of
metal-stabilized carbocations. These have encompassed
cobalt and molybdenum complexes of formally non-
classical systems, such as the 2-bornyl or 2-fenchyl
cations [1–5], cobalt and iron derivatives of the cyclo-
pentadienyl, indenyl and fluorenyl cations which possess
4p, 8p and 12p electrons, respectively, and are formally
antiaromatic [6], and also chromium-stabilized benzyl
cations [7]. In such cases, the cationic carbon leans to-
ward the relatively electron-rich metal so as to alleviate
the electron deficiency by direct overlap with a filled
metal orbital [8].
These concepts have now been applied to another
aspect of our research interests, namely sterically
crowded molecules [9], in particular to chlorocarbon
chemistry [10]. Since we have recently shown that the
C7Phþ7 cation adopts a shallow boat-like structure [11],
one might envisage similar behaviour for the perchlo-
* Corresponding author. Tel.: +353-1-716-2880; fax: 353-1-716-2127.
E-mail address: michael.mcglinchey@ucd.ie (M.J. McGlinchey).
0022-328X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2003.09.033

J.A. Dunn et al. / Journal of Organometallic Chemistry 689 (2004) 8–13
9
O
O
Cl
Cl
Cl
Cl
+
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
1
1'
Me
+
+
O
Fe
Me
Cr
CO
CO
CO
2
3
2. Results and discussion
remained intact. These data indicated that the molecule
had undergone a semibenzilic acid rearrangement [18]
with concomitant ring contraction of the original seven-
membered ring, as indicated in Scheme 2.
As depicted in Scheme 1, the standard route to al-
kynyl-dicobalt hexacarbonyl stabilized carbocations in-
volves (i) synthesis of an appropriate ketone, (ii)
nucleophilic attack by an alkynyl anion to form the
precursor alcohol, (iii) incorporation of the organome-
tallic moiety, and (iv) protonation to generate the de-
sired carbocation.
Consequently, hexachlorotropone was treated with
trimethylsilylethynyl-lithium at )78 °C and then hy-
drolysed with water. The resulting product, 4, obtained
in 51% yield, was not the anticipated 7-trimethylsilyl-
ethynyl-1,2,3,4,5,6-hexachlorocycloheptatrien-7-ol, 5,
since the isotopic distribution pattern of the parent ion
in the mass spectrum clearly indicated the presence of
only five chlorines. The 13C NMR spectrum exhibited
four resonances at d 137.5, 135.7, 132.9, and 128.8 with
the intensity ratio 1:1:2:2, as well as a signal at d 173.5
(IR mCO at 1683 cmꢀ1), suggesting the formation of a
benzenoid product whereby the ketone functionality had
To verify that this rearrangement had indeed oc-
curred, the alkyne unit in 4 was allowed to react with
dicobalt octacarbonyl, and X-ray quality crystals of 6
were obtained. The resulting structure appears as Fig. 1,
and unambiguously verifies the identity of the precursor
4. The molecule exhibits bond lengths and angles that
are within the normal ranges (Table 1), and the crys-
tallographic collection and refinement parameters are
summarized in Table 2. There are several interesting
features of the structure, in particular the effects of steric
hindrance on the orientation of the substituents. The
carbonyl groups of the Co(CO)3 fragments are almost
perfectly eclipsed, each having one CO aligned so as to
bisect the CBC bond. Moreover, to avoid steric con-
gestion, the TMS group adopts a conformation in which
two of the methyl groups are directed away from the
carbonyl groups. The pentachlorophenyl ring is twisted
R2
OH
1. R3
Li
R3
R2
O
+
1
H O
2.
R
3
R1
Co2 (CO)8
R2
R1
OH
R3
R2
+
H
+
Co
R3
(OC)3
1
R
Co
Co
Co
Scheme 1.

10
J.A. Dunn et al. / Journal of Organometallic Chemistry 689 (2004) 8–13
Scheme 2.
Table 1
ꢀ
Selected bond lengths (A) and angles (°)
C–Cl(average)
C(7)–O(7)
C(7)–C(8)
1.719(10)
1.199(12)
1.449(13)
1.341(13)
2.476(2)
C(8)–C(9)
Co(1)–Co(2)
C(8)–Co(1)
C(8)–Co(2)
1.951(10)
1.970(9)
C(9)–Co(1)
C(9)–Co(2)
1.998(10)
1.977(9)
Si(1)–CH3(average)
C(9)–Si(1)
1.856(11)
1.866(10)
1.133(12)
1.389(15)
C@O (average)
Aromatic C–C (average)
C(7)–C(8)–C(9)
Si(1)–C(9)–C(8)
148.1(9)
148.7(7)
Si(1)–C(9)–C(8)–C(7)
C(1)–C(7)–C(8)–C(9)
11(3)
32(2)
chlorophenyl-2-oxo-ethanoate [19], as depicted in
Scheme 3. Attempts to prepare 5 by an alternative route
are continuing.
Fig. 1. X-ray crystal structure of cobalt carbonyl complex 6.
at an angle of 22° from the plane formed by the two
cobalt atoms and the center of the CBC bond, an ar-
rangement that minimizes interactions of the chlorine
atoms with the carbonyl groups and the methyl groups
of the TMS.
3. Experimental
3.1. General methods
In seeking precedents for such behaviour, we note
that hexachlorotropone has been reported to undergo
ring contraction to give methyl pentachlorobenzoate
when treated with methanol [12,13]. Analogously, 1 re-
acts with dimethoxycarbene to yield methyl 2-penta-
All reactions were carried out under an atmosphere
of dry nitrogen. Silica gel (particle size 20–45 lm) was
1
used for flash chromatography. H and 13C NMR data
were obtained on Bruker DRX 500 or AM 200 spec-

J.A. Dunn et al. / Journal of Organometallic Chemistry 689 (2004) 8–13
11
Table 2
Crystallographic collection and refinement parameters for 6
Empirical formula
Molecular weight (g molꢀ1
Description
Size (mm3)
Temperature (K)
C18H9Cl5Co2O7Si
660.45
)
Red plate
0.05 ꢁ 0.10 ꢁ 0.15
213(2)
ꢀ
Wavelength (A)
0.71073
Monoclinic
Crystal system
Space group
P21=c
7.8366(3)
ꢀ
a (A)
ꢀ
b (A)
31.5681(18)
20.0254(11)
90
ꢀ
c (A)
a (°)
b (°)
c (°)
91.535(3)
90
4952.2(4)
3
ꢀ
Volume (A )
Z
8
1.772
Calculated density (Mg/m3)
Scan mode
/- and x-scans
1.964
2608
Absorption coefficient (mmꢀ1
F(0 0 0)
)
h range for data collection (°)
Index ranges
2.13–24.00
)4 6 h 6 8
)36 6 k 6 35
)22 6 l 6 21
27466
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
7751 [R(int) ¼ 0.1263]
7751/0/596
0.995
Final R indices ½I > 2rðIÞꢂ
R1 ¼ 0:0773, wR2 ¼ 0:1775
R1 ¼ 0:1550, wR2 ¼ 0:2190
1.32402
R indices (all data)
Transmission (ratio of max. to min.)
ꢀ3
ꢀ
Largest diff. peak and hole (e A
)
1.253 and )1.033
Microanalytical data are from Guelph Chemical Labo-
ratories, Guelph, Ontario, Canada.
3.2. Hexachlorotropone (1)
C7Cl6O was synthesized according to the literature
procedure [12] as a yellow solid, mp 82–83 °C (82.5 lit)
and identified by conventional spectroscopic techniques.
13C NMR (50.3 MHz, CDCl3) d 177.5, 136.6, 134.5,
132.4; MS (DEI; m=z): 310 [M]þ, 282 [M-CO]þ (100%),
247 [C6Cl5]þ, 212 [C6Cl4]þ, 177 [C6Cl3]þ, 142 [C6Cl2]þ,
107 [C6Cl]þ; (DCI m=z): 328 [M+NH4]þ.
3.3. Pentachlorophenyl trimethylsilylethynyl ketone (4)
n-Butyllithium (0.75 ml of a 1.44 M hexane solution,
1.08 mmol) was added dropwise over a 15 min period to
a solution of trimethylsilylacetylene (0.14 ml, 0.99
mmol) in ether at )78 °C via cannula and the solution
was allowed to warm to 0 °C. After stirring for 1.5 h, the
solution was then cooled to )78 °C and C7Cl6O (201
mg, 0.64 mmol) dissolved in ether (40 ml) was added
dropwise. The solution was slowly warmed to room
temperature, stirred for a further 24 h, quenched with
distilled water and extracted with ether to yield a white
Scheme 3.
trometers. Mass spectra were acquired on a Finnigan
4500 spectrometer by direct electron impact (DEI), or
direct chemical ionization (DCI) with NH3. All masses
quoted are for the lowest value of m=z in the envelope
and correspond to 35Cl; the observed signal intensities
correctly matched the isotopic pattern required for
the number of chlorine atoms in the fragment ion.
1
powder (123 mg, 0.33 mmol, 51%), mp 128–130 °C. H

12
J.A. Dunn et al. / Journal of Organometallic Chemistry 689 (2004) 8–13
NMR (500 MHz, CDCl3) d 0.24 (Me3Si); 13C NMR
(125 MHz, CDCl3) d 173.5, 137.5, 135.7, 132.9, 128.8,
105.0, 100.5, )1.0; MS (DEI, m=z): 275 ([C6Cl5CO]þ,
247 [C6Cl5]þ, 212 [C6Cl4]þ, 177 [C6Cl3]þ, 142 [C6Cl2]þ,
107 [C6Cl]þ; (DCI m=z): 390 [M+NH4]þ. Calc. for
C12H9Cl5SiO: C, 38.48; H, 2.42. Found: C, 38.56; H,
2.30.
may be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ (Fax:
+44-1223-336033; E-mail: deposit@ccdc.cam.ac.uk or
http://www.ccdc.cam.ac.uk.
Acknowledgements
3.4. (Pentachlorophenyl trimethylsilylethynyl ketone)
dicobalt hexacarbonyl (6)
Financial support from the Natural Sciences and
Engineering Council of Canada (NSERC) is gratefully
acknowledged. JAD and LEH thank NSERC for
graduate fellowships. Mass spectra were acquired
courtesy of Dr Kirk Green of the McMaster Regional
Mass Spectrometry Centre.
Me3SiACBCAC(@O)C6Cl5, 4, (70 mg, 0.19 mmol)
was dissolved in THF (20 ml) and added dropwise over
a 45 min period to dicobalt octacarbonyl (64 mg, 0.19
mmol) dissolved in THF (20 ml). The solution was then
allowed to stir for 24 h at room temperature. After re-
moval of solvent, the residue was subjected to flash
chromatography, using hexanes as eluent, to give dark
red crystals of 6 in essentially quantitative yield. 1H
NMR (200 MHz, CD2Cl2) d 0.27; 13C NMR (125 MHz,
CD2Cl2) d 199.0, 191.4, 140.2, 135.0, 132.8, 128.5, 106.0,
98.2, 1.2; MS (DEI, m=z): 658 [M]þ, 275 [C7Cl5O]þ.
Calc. for C18H9Cl5Co2O7: C, 32.73; H, 1.37. Found: C,
32.98; H, 1.02. The cobalt complex was recrystallized
from hexane/CH2Cl2 (9:1) to give crystals of X-ray dif-
fraction quality.
References
[1] M.F. DÕAgostino, C.S. Frampton, M.J. McGlinchey, J. Organo-
met. Chem. 394 (1990).
[2] M. Gruselle, H. El Hafa, M. Nikolski, G. Jaouen, J. Vaissermann,
L. Li, M.J. McGlinchey, Organometallics 12 (1993) 4917.
[3] H. El Hafa, C. Cordier, M. Gruselle, Y. Besace, G. Jaouen, M.J.
McGlinchey, Organometallics 13 (1994) 5149.
[4] J.H. Kaldis, P. Morawietz, M.J. McGlinchey, Organometallics 22
(2003) 1293.
[5] M. Kondratenko, M. Gruselle, H. El Hafa, J. Vaissermann,
G. Jaouen, M.J. McGlinchey, J. Am. Chem. Soc. 117 (1995)
6907.
3.5. Crystallographic data for 6
[6] J.A. Dunn, R. Ruffolo, S.S. Rigby, M.A. Brook, M.J. McGlin-
chey, Organometallics 18 (1999) 3372.
X-ray crystallographic data for 6 were collected from
a suitable sample mounted with epoxy on the end of a
thin glass fiber. Data were collected on a P4 Bruker
diffractometer equipped with a Bruker SMART 1 K
CCD area detector (employing the program SMART)
[20] and a rotating anode utilizing graphite-monochro-
[7] P.A. Downton, B.G. Sayer, M.J. McGlinchey, Organometallics 11
(1992) 3281.
[8] (a) M.J. McGlinchey, L. Girard, R. Ruffolo, Coord. Chem. Rev.
143 (1995) 331;
(b) H. El Amouri, M. Gruselle, Chem. Rev. 96 (1996) 1077;
(c) A.J.M. Caffyn, K.M. Nicholas, in: G. Wilkinson, F.G.A.
Stone, E.W. Abel (Eds.), Comprehensive Organometallic Chem-
istry II, vol. 12, Pergamon Press, Oxford, UK, 1995, pp. 685–702
(Chapter 7.1);
ꢀ
mated MoKa radiation (k ¼ 0:71073 A). Data pro-
cessing was carried out by use of the program SAINT
[21], while the program SADABS [22] was utilized for
the scaling of diffraction data, the application of a decay
correction and an empirical absorption correction based
on redundant reflections. The structure was solved by
using the direct-methods procedure in the Bruker
SHELXL [23] program library and refined by full-ma-
trix least-squares methods on F2. All non-hydrogen
atoms were refined using anisotropic thermal parame-
ters, and hydrogen atoms were added as fixed contrib-
utors at calculated positions with isotropic thermal
parameters based on the carbon atom to which they are
bonded.
(d) G.G. Melikyan, S. Bright, T. Monroe, K.I. Hardcastle, J.
Ciurash, Angew. Chem., Int. Ed. Engl. 37 (1998) 161;
(e) T.A. Albright, R. Hoffmann, P. Hofmann, Chem. Ber. 111
(1978) 1591;
(f) S. Lupon, M. Kapon, M. Cais, F.H. Herbstein, Angew.
Chem., Int. Ed. Engl. 11 (1972) 1025;
(g) U. Behrens, J. Organomet. Chem. 182 (1979) 89.
[9] S. Brydges, L.E. Harrington, M.J. McGlinchey, Coord. Chem.
Rev. 233–234 (2002) 75.
[10] J.A. Dunn, H.K. Gupta, A.D. Bain, M.J. McGlinchey, Can. J.
Chem. 74 (1996) 2258.
[11] S. Brydges, J.F. Britten, L.C.F. Chao, H.K. Gupta, M.J.
McGlinchey, D.L. Pole, Chem. Eur. J. 4 (1998) 1199.
[12] (a) R. West, K. Kusada, J. Am. Chem. Soc. 90 (1968) 7354;
(b) K. Kusada, R. West, V.N.M. Rao, J. Am. Chem. Soc. 93
(1971) 3627.
[13] R.P. Dodge, R.J. Sime, D.H. Templeton, personal communi-
cation cited in K.V. Scherrer Jr., J. Am. Chem. Soc. 90 (1968)
7352.
4. Supplementary material
Crystallographic data for the structural analysis have
been deposited at the Cambridge Crystallographic Data
Centre, CCDC no. 208710. Copies of this information
[14] The seven-membered ring in (g5-C5H5)Co(g4-C7Cl6O) also adopts
€
a shallow boat conformation: R. Sunkel, J. Organomet. Chem.
391 (1990) 247.

J.A. Dunn et al. / Journal of Organometallic Chemistry 689 (2004) 8–13
13
[15] (a) L.W. Haynes, R. Pettit, in: G.A. Olah, P.v.R. Schleyer (Eds.),
Carbonium Ions, vol. 5, Wiley, New York, 1976, pp. 2049–
2133;
(b) The semibenzilic rearrangement is sometimes referred to as the
quasi Favorskii rearrangement.
[19] J.A. Dunn, J.P. Pezacki, M.J. McGlinchey, J. Warkentin, J. Org.
Chem. 64 (1999) 4344.
(b) L.E. Harrington, N. Reginato, I. Vargas-Baca, M.J. McG-
linchey, Organometallics 22 (2003) 663, and references therein.
ꢁ ꢁ
[16] K.L. Malisza, S. Top, J. Vaissermann, B. Caro, M.-C. Senechal-
[20] G.M. Sheldrick, SMART, Release 4.05, Siemens Energy and
Automation Inc., Madison, WI, 1996.
ꢁ ꢁ
Tocquer, D. Senechal, J.-Y. Saillard, S. Triki, S. Kahlal, J.F.
Britten, M.J. McGlinchey, G. Jaouen, Organometallics 14 (1995)
[21] G.M. Sheldrick, SAINT, Release 4.05, Siemens Energy and
Automation Inc., Madison, WI, 1996.
5273.
[17] S.K. Braunstein, E.J. Gabe, R.C. Hynes, Can. J. Chem. 70 (1992)
1011.
[22] G.M. Sheldrick, SADABS (Siemens Area Detector Absorption
Corrections), Siemens Energy and Automation Inc., Madison,
WI, 1996.
[18] (a) L.M. Harwood, Polar Rearrangements, Oxford University
Press, Oxford, UK, 1991, p. 39;
[23] G.M. Sheldrick, SHELXTL, Version 5.03, Siemens Crystallo-
graphic Research Systems, Madison, WI, 1994.