A half cleaved zirconacyclopentadiene

Article abstract of DOI:10.1016/j.inoche.2003.10.039

When the sterically strained bicyclic zirconacyclopentadiene 2 is reacted with two equivalents of iodine in the presence of copper(I) chloride, the diiodinated product 4 forms in the range of 10 h. However, when reacted only for 2 h, the monoiodinated zirconium complex 3 is isolated. 3 shows an unexpected stability towards hydrolysis and oxidation. The molecular structure of complex 3 has been determined by X-ray structural analysis.

Full text of DOI:10.1016/j.inoche.2003.10.039

Inorganic Chemistry Communications 7 (2004) 245–248
www.elsevier.com/locate/inoche
A half cleaved zirconacyclopentadiene q
*
Matthias Zeller
Department of Chemistry, Youngstown State University, One University Plaza, Youngstown, OH 44555, USA
Received 11 June 2003; accepted 14 October 2003
Published online: 9 December 2003
Abstract
When the sterically strained bicyclic zirconacyclopentadiene 2 is reacted with two equivalents of iodine in the presence of
copper(I) chloride, the diiodinated product 4 forms in the range of 10 h. However, when reacted only for 2 h, the monoiodinated
zirconium complex 3 is isolated. 3 shows an unexpected stability towards hydrolysis and oxidation. The molecular structure of
complex 3 has been determined by X-ray structural analysis.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Zirconacyclopentadienes; Half Cleavage; Alkenylzirconocenes; Alkenyliodides
1. Introduction
successfully used in situ for further reactions as de-
scribed by Takahashi et al. [3] (Scheme 2).
Bis(cyclopentadienyl)zirconium(IV) complexes were
discovered almost half a century ago, but their appli-
cations in organic and polymer chemistry continue to
expand at a rapid pace [1]. Processes such as hydro-
zirconation, carbozirconation, transmetallations and
bond insertions open up a wide range of different gate-
ways towards these zirconocene derivatives.
As 1,4-dianion precursors, zirconacyclopentadienes
are especially valuable. Using the Negishi reagent
Cp2Zr(butene), they can be easily prepared from two
alkynes in a one pot synthesis [2] (Scheme 1).
There are no reports of any attempts to isolate the
intermediate half cleaved zirconium complexes, imply-
ing that these compounds may be either quite sensitive
compounds or that they are thermally unstable, as may
be concluded from the formation of cyclobutadienes
upon treatment with CuCl [3d]. This contribution will
present the first successful isolated half cleaved zirco-
nium complex of this kind.
2. Results and discussion
Treatment of these zirconacycles with iodine, NCS or
NBS results in the oxidative cleavage of the carbon–
zirconium bonds [3]. The first bond reacts generally
much faster and to achieve full conversion to, e.g., the
diiodinated butadienes either a large excess of iodine
and long reaction times are required, or the reactions are
catalyzed with copper(I) chloride [3a]. With no catalyst
or only one equivalent of I2, NCS or NBS the mono-
halogenated butadienes are produced and have been
In an attempt to synthesize an 1,4-bisiodobutadiene
with an anellated cyclohexyl backbone, the sterically
strained bicyclic zirconacyclopentadiene 2 was reacted
with two equivalents of iodine in the presence of cop-
per(I) chloride. In contrast to reactions with less strained
zirconacyclopentadienes [4], the diiodinated product 4
was only isolated after prolonged reaction times in the
range of 10 h. When reacted only for 2 h, the monoio-
dinated product 3 was isolated as the only product by
aqueous workup with sodium thiosulfate and extraction
with ether as yellow air stable crystals (Scheme 3).
The halogen atom at the zirconium center has
been unambiguously assigned as a chlorine atom by
X-ray structural analysis and an exchange of the iodine
atom must have taken place after the cleavage reaction
q Supplementary data associated with this article can be found, in the
online version, at doi:10.1016/j.inoche.2003.10.039.
* Tel.: +1-330-941-7105; fax: +1-330-941-1579.
E-mail address: mzeller@cc.ysu.edu (M. Zeller).
1387-7003/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2003.10.039

246
M. Zeller / Inorganic Chemistry Communications 7 (2004) 245–248
R1
With no previous reports of any isolated half
R2
R3
cleaved zirconacyclopentadienes, the pronounced sta-
bility of zirconium complex 3 is unexpected and its
stability towards water and air during workup needs
some discussion.
Alkynes
BuLi
ZrCp2
R4
ZrCp2
Cp2ZrCl2
Scheme 1.
To some extend the stability of 3 can be explained by
the sterical shielding effect of the large substituents
surrounding the remaining carbon–zirconium bond. As
can be seen in the solid state structure (Fig. 1), this bond
is effectively enclosed by the cyclopentadienyl rings, the
bulky trimethylsilyl groups and the anellated cyclohexyl
ring. The presence of these bulky substituents may as
well further stabilize the complex towards reductive
elimination to form a cyclobutadiene derivative in the
(see Table 1). Similar halogen exchange reactions have
been observed by Erker and co-workers [5] for related
alkenylzirconocene complexes, and, taking the strained
cis conformation of the butadiene unit into account, the
driving force for the exchange will be not only the for-
mation of the stronger Zr–Cl bond but also relief of steric
strain.
R
R
R
Hal
Hal
R
2) 2nd halogenation
Hal
R
R
R
R
H
2) H3O+
R
R
R
R
R
R
1) 1 eq I2, NCS or NBS
ZrCp2
Hal
[Zr]
R
R
R
R
R
R
R
R
2) CuCl
I
R
R
R'
R
R
Hal
R'
R
R
Scheme 2.
SiMe3
I
SiMe3
ZrCp2
SiMe3
2 eq I2, CuCl,
-78˚C to rt, 2h
a) 2 n-BuLi, THF, -78˚C, 1h
Cp2ZrCl2
Cl
ZrCp2
85%
SiMe3
b)
SiMe3
Me3Si
2
1
3
-78˚C to rt
SiMe3
2 eq I2, CuCl,
-78˚C to rt, 10h
I
I
94%
SiMe3
4
Scheme 3.

M. Zeller / Inorganic Chemistry Communications 7 (2004) 245–248
247
Table 1
Selected bond lengths (A) and angles (deg) for 3
strain, the carbon–zirconium bond is expected to be
considerably weakened. Indeed, the Zr–C bond length is
ꢀ
ꢀ
with 2.3286(17) A rather long when compared with
those of other alkenylzirconocene complexes, which are
ꢀ
generally in the range of 2.209–2.258 A [5]. The bond
Zr–Cl
Zr–C1
C1–C2
C2–C3
C3–C4
C4–I
2.4502(6)
2.3286(17)
1.345(2)
1.504(2)
1.337(2)
2.1632(17)
length found here compares better with weaker carbon–
zirconium bonds as they are found in complexes having
an additional donor ligand such as an amine or phos-
phine [6]. Therefore, replacement of the Cp2ZrCl moiety
by hydrolysis should lead to much less strained mole-
cule. In contrast to that, the molecule is stable during
aqueous workup, implying an inherent stability of the
complex not derived from sterical protection alone.
Taking into account, that zirconacyclopentadienes are
easily synthesized in a one pot synthesis from readily
available alkynes, and that they have two functional
groups of opposite polarity, the surprising stability of the
complex presented here should be taken as a strong en-
couragement for further investigations of this class of
compounds. Especially if the zirconium complexes can be
isolated as pure materials rather than used in situ these
compounds may well find a much more extended use in
organic synthesis than possible with the present in situ
methods.
Zr–C1–C2
Zr–C1–Si1
Si1–C1–C2
C1–C2–C3
C1–C2–C8
C8–C2–C3
C2–C3–C4
C2–C3–C5
C5–C3–C4
C3–C4–I
121.13(13)
124.34(9)
114.49(13)
126.14(16)
127.88(17)
105.46(15)
124.78(16)
111.08(15)
122.38(17)
116.14(13)
108.64(8)
Si2–C4–I
C3–C4–Si2
135.22(14)
3. Experimental
3.1. 1,2-Bis(trimethylsilyl)octa-1,7-diyne (1)
1 was synthesized as described for 1,2-bis(trimethyl-
stannyl)octa-1,7-diyne [7]. 1 is distilled at 155–165
mTorr/58–65 °C to get 85% of the product as a clear
1
liquid, that solidifies slowly at 4 °C. H NMR (299.858
MHz, CDCl3, rt): d ¼ 0:157 (s, 18H, SiMe3), 1.385 (m,
4H, CH2), 1.935 (m, 4H, CH2).
Fig. 1. View of the molecular structure of 3 (50% probability ellip-
soids).
3.2. 1,6-Bis(iodotrimethylsilylmethylene)cyclohexane (4)
To a THF solution (100 ml) of Cp2ZrCl2 (2.5 g, 8.55
mmol) and 1 (2.14 g, 8.54 mmol), is added a hexane
solution of n-BuLi (6.1 ml, 2.8 M, 17.08 mmol) at )78
°C. After stirring for 30 min, the mixture is allowed to
warm up to room temperature and is stirred for 4 h. A
THF solution (100 ml) of iodine (5 g, 19.5 mmol) and
solid Cu(I)Cl (0.853 g, 8.54 mmol) are added at )78 °C.
After warming up to room temperature the mixture is
stirred for further 10 h. A saturated aqueous solution of
Na2S2O3 is added and the mixture is extracted with
pentanes. The combined extracts are washed with brine,
dried over MgSO4, filtered through a bed of celite and
condensed by rotary evaporation. Recrystallization
from pentanes at )30 °C yields 4 (4.06 g, 8.06 mmol,
presence of CuCl, as it has been observed for other less
sterically shielded complexes [3d].
On the other side, the replacement of the original
iodine atom at the zirconium indicates, that the metal
center is not totally inaccessible for further reactions.
Therefore, the insensitivity towards oxidation and hy-
drolysis seems to be at least in part due to some inherent
stability of the complex itself. This assumption seems to
be further confirmed by the considerable strain, which is
present in the molecule. The sterical demands of the
Cp2ZrCl moiety and the iodine atom are forcing the two
double bonds out of conjugation with the dihedral angle
being as large as 91.53°. The angles around the carbon
atoms C2 and C3 are differing considerably from the
idealized 120°. With 105.46° the angle C3–C2–C8 ex-
hibits the most extreme deviation. Due to this steric
1
94%). H NMR (299.858 MHz, CDCl3): d ¼ 0:325 (s,
18H, SiMe3), 1.26 (m, 2H, CH2), 1.5 (m, 2H, CH2), 2.09
(m, 2H, CH2), 2.73 (m, 2H, CH2).

248
M. Zeller / Inorganic Chemistry Communications 7 (2004) 245–248
3.3. 1-(iodotrimethylsilylmethylene)-6-{(chloro-di-g5-cy-
clopentadienyl-zirconium)trimethylsilylmethylene} cyclohex-
ane (3)
Acknowledgements
The author thanks Allen D. Hunter for granting ac-
cess to the diffractometer. The author was supported by
NSF grant 0111511 and the diffractometer was funded
by NSF grant 0087210, by Ohio Board of Regents grant
CAP-491, and by YSU.
As for 4, but the iodination reaction is stopped after 2
h at room temperature by addition of aqueous sodium
thiosulfate. The organic layer is extracted with ether, the
ether fractions are dried with MgSO4, and filtered
through a bed of celite. Upon concentration of the
ethereal solution and cooling to 4 °C, 3 is isolated as
bright yellow crystals (4.6 g, 7.26 mmol, 85%). 1H NMR
(399.905 MHz, CDCl3, rt): d ¼ 6:377 (s, 5H, Cp), 6.272
(s, 5H, Cp), 2.756 (m, 2H, CH2), 2.254 (m, 2H,
CH2),1.891 (m, 2H, CH2), 1.470 (m, 2H, CH2),0.299 (s,
9H, CH3), 0.291 (s, 9H, CH3); 13C NMR (100.565 MHz,
CDCl3): d ¼ 180:917 (@C), 168.027 (@C), 165.941
(@C), 113.091 (Cp), 112.818 (Cp), 103.683 (@C), 43.329
(CH2), 39.418 (CH2), 31.695 (CH2), 30.235 (CH2), 4.656
(CH3), 1.651 (CH3). Anal. calcd. for (C24H36Si2ZrClI):
C 45.45, H 5.72, total halogen (as Cl) 11.18. Found: C
45.31, H 5.67 total halogen (as Cl) 11.25.
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