J. Am. Chem. Soc. 1998, 120, 3255-3256
3255
The Ansa-Effect in Zirconocene Chemistry: The
Syntheses and Interconversions of the Hydride
Scheme 1
Complexes {[Me2Si(C5Me4)2]Zr(H)(µ-H)}2,
[
Me2Si(C5Me4)2]ZrH2(PMe3), and
Me2Si(C5Me4)2]Zr(Ph)H
[
Hyosun Lee, Peter J. Desrosiers,* Ilia Guzei,†
‡
,‡,§
,†
,‡
Arnold L. Rheingold,* and Gerard Parkin*
Department of Chemistry, Columbia UniVersity
New York, New York 10027
Department of Chemistry, UniVersity of Delaware
Newark, Delaware 19716
ReceiVed December 15, 1997
5
Zirconocene complexes, (η -C
5
R
5
)
2
ZrX
2
, have long been the
subjects of considerable attention. Such interest stems not only
from their participation in the discovery of novel structures and
reactivity in organometallic chemistry, but also from their
1
applications in organic synthesis and their use as catalysts in
olefin polymerization.2 More recently, a large variety of ansa-
R
3
2 2
zirconocene complexes [ACp ]ZrX have been investigated.
These studies have chiefly been driven by efforts to improve
complex [Me
2
Si(C
5
Me
4
)
2
]Zr(Ph)H (Scheme 1).10 The molecular
structure of [Me
2
Si(C Me ) ]Zr(Ph)H has been determined by
5 4 2
polymer stereoregularity as a result of the configurational stability
X-ray diffraction,11 and the location of the phenyl group is
imparted by the ansa bridge.2 Despite numerous studies on ansa-
indicative of a â-agostic interaction.12
metallocene complexes, however, there has been relatively little
attention given to delineating the ansa-effect in well-defined
systems.4 In this paper, a detailed comparison of the chemistry
Despite the inability to isolate a dihydride complex for the ansa-
zirconocene system under the conditions used to synthesize Cp*2-
,5
6
ZrH , a related species may be obtained by reaction of [Me Si-
of [Cp*
2
Zr] and {[Me
2
Si(C
5
Me
4
)
2
]Zr} derivatives allows interest-
2
2
(C
5
Me
4
)
2
]ZrMe
2
with H
2
in cyclohexane. Interestingly, the
ing consequences of the [Me
2
Si] ansa bridge to be elucidated.
11
bridging dihydride so obtained, {[Me
reacts readily with benzene to give [Me
Scheme 1). Together with additional observations, this result
suggests that the formation of the phenyl-hydride complex upon
hydrogenation of [Me Si(C Me ]ZrMe in benzene, as compared
2
Si(C
5
Me
Si(C
4
)
2
]Zr(H)(µ-H)}
2
,
The formation of hydride complexes by hydrogenation of Zr-
alkyl bonds is a common reaction of zirconocene derivatives. For
example, Bercaw has reported the synthesis of the dihydride [Cp*]-
2
5
Me ]Zr(Ph)H
4 2
)
1
3
(
Me
Me
in toluene,7
ZrH upon
2
[
Cp ]ZrH
while Marks has described the formation of Cp*
in benzene.8 It is, therefore,
treatment of Cp* ZrMe with H
particularly striking that we have discovered that the correspond-
2
by reaction of [Cp*][Cp ]ZrMe with H
2
2
2
5
)
4 2
2
2
to the dihydride for the corresponding permethylzirconocene
system, is a consequence of the different equilibrium positions
for the reactions of the dihydrides with benzene, i.e. Kansa > KCp*
.
The dimeric structure of {[Me
of interest since it provides a contrast with the monomeric nature
2
2
2
14
9
2 5 4 2 2
ing reaction of the ansa-complex [Me Si(C Me ) ]ZrMe does
2 5 4 2 2
Si(C Me ) ]Zr(H)(µ-H)} is also
not yield a dihydride; rather, the reaction results in C-H bond
activation of benzene and the formation of the phenyl-hydride
15-17
of the permethylzirconocene analogue, Cp*
2 2
ZrH .
Dissocia-
‡
Columbia University.
†
University of Delaware.
(10) Another illustration of an ansa-effect in hydrogenation is the observa-
tion that hydrogenation of the ethylene-bridged complex [(C )Si(C Me ]-
Zr(X)CH
§
Permanent address: University of the Virgin Islands, St. Thomas, U.S.
2
H
2
4
5
4 2
)
t
t
Virgin Islands 00802.
(
Zr(X)CH
2
Bu (X ) F, Cl, Br) is more facile than that for Cp*
2
Bu .
1) (a) Negishi, E.; Takahashi, T. Synthesis 1988, 1-19. (b) Buchwald, S.
See: Wochner, F.; Brintzinger, H. H. J. Organomet. Chem. 1986, 309, 65-
L.; Broene, R. D. ComprehensiVe Organometallic Chemistry II; Abel, E. W.,
Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: New York, 1995; Vol. 12,
pp 771-784.
75.
(11) Crystal data: [Me Si(C Me ) ]Zr(Ph)H is orthorhombic, Pnma (No.
2 5 4 2
3
62), a ) 15.219(2) Å, b ) 11.201(4) Å, c ) 13.942(2) Å, V ) 2377(1) Å ,
Z ) 4, R ) 0.0423 [I < 2σ(I)]; {[Me Si(C Me ]Zr(H)(µ-H)} is ortho-
rhombic, Pbcn (No. 60), a ) 19.0228(2) Å, b ) 10.3503(1) Å, c )
(2) (a) Brintzinger, H. H.; Fischer, D.; M u¨ lhaupt, R.; Rieger, B.; Waymouth,
1
2
5
4
)
2
2
R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143-1170. (b) Grubbs, R.
H.; Coates, G. W. Acc. Chem. Res. 1996, 29, 85-93. (c) M o¨ hring, P. C.;
Coville, N. J. J. Organomet. Chem. 1994, 479, 1-29.
3
19.2320(1) Å, V ) 3787(1) Å , Z ) 4, R
Me ]ZrH (PMe ) is orthorhombic, Cmca (No. 64), a ) 14.338(3) Å, b )
3
1
2 5
) 0.0311 [I < 2σ(I)]; [Me Si(C -
)
4 2
2
3
(
3) The term ansa (meaning bent handle, attached at both ends) was first
11.787(2) Å, c ) 28.686(4) Å, V ) 4848(2) Å , Z ) 8, R
2σ(I)].
1
) 0.0473 [I <
introduced with respect to metallocene chemistry by Brintzinger. See: Smith,
J. A.; von Seyerl, J.; Huttner, G.; Brintzinger, H. H. J. Organomet. Chem.
(12) For example, the Zr-C-C bond angles [98.5(4)° and 146.0(5)°]
1
979, 173, 175-185.
deviate considerably from 120°.
(
4) (a) Smith, J. A.; Brintzinger, H. H. J. Organomet. Chem. 1981, 218,
(13) For example, Cp*
ZrH
2
Zr(Ph)H reacts with H
2
in benzene to give Cp*
]/[{Zr}H ][PhH].
2
-
1
59-167. (b) Dorer, B.; Diebold, J.; Weyand, O.; Brintzinger, H. H. J.
2
.
Organomet. Chem. 1992, 427, 245-255.
(14) Kansa and KCp* are defined as [{Zr}(Ph)H][H
2
2
(5) (a) Labella, L.; Chernega, A.; Green, M. L. H. J. Chem. Soc., Dalton
(15) Manriquez, J. M.; McAlister, D. R.; Sanner, R. D.; Bercaw, J. E. J.
Trans. 1995, 395-402. (b) Labella, L.; Chernega, A.; Green, M. L. H. J.
Organomet. Chem. 1995, 485, C18-C21. (c) Chernaga, A.; Cook, J.; Green,
M. L. H.; Labella, L.; Simpson, S. J.; Souter, J.; Stephens, A. H. H. J. Chem.
Soc., Dalton Trans. 1997, 3225-3243.
Am. Chem. Soc. 1978, 100, 2716-2724.
R
(16) Other dimeric {[Cp ]
2
Zr(H)(µ-H)}
2
complexes have, nevertheless, been
reported, but these are normally associated with less heavily substituted
Me
16a
t-Bu
2
cyclopentadienyl derivatives, e.g. [(Cp )
2
Zr(H)(µ-H)]
2
,
[(Cp
) Zr(H)-
16b
TMS
16c
(6) Marks has applied the [Me
2
Si(C
5
Me
4
)
2
] ansa ligand to lanthanide
(µ-H)]
2
,
and [(Cp
2 2
) Zr(H)(µ-H)] .
(a) Jones, S. B.; Petersen, J. L. Inorg.
chemistry, with the intention that the ansa bridge would create a more “open”
coordination sphere. See: Fendrick, C. M.; Schertz, L. D.; Day, V. W.; Marks,
T. J. Organometallics 1988, 7, 1828-1838 and references therein.
Chem. 1981, 20, 2889-2894. (b) Choukroun, R.; Dahan, F.; Larsonneur, A.-
M.; Samuel, E.; Petersen, J.; Meunier, P.; Sornay, C. Organometallics 1991,
10, 374-376. (c) Larsonneur, A.-M.; Choukroun, R.; Jaud, J. Organometallics
1993, 12, 3216-3224.
(
(
(
7) Wolczanski, P. T.; Bercaw, J. E. Organometallics 1982, 1, 793-799.
8) Schock, L. E.; Marks, T. J. J. Am. Chem. Soc. 1988, 110, 7701-7715.
(17) Dimeric hydride bridged complexes supported by the ansa ligand [Me
2
-
9) [Me
Si(C Me
a) Jutzi, P.; Dickbreder, R. Chem. Ber. 1986, 119, 1750-1754.
2
Si(C
5
Me
4
)
2
]ZrMe
2
2
and [Me
2
Si(C
5
Me
4
)
2
]Zr(CO)
2
are prepared from
Si(C
Si(C
be noted that for these elements the non-ansa derivatives, [Cp*
5
Me
4
4
) ] have also been reported for the larger elements, Y and Th: {[Me -
2
2
2
9
a
1
7
a
6
[
(
Me
2
5
4
)
2
]ZrCl
by standard methods. See Supporting Information.
5
Me
)
]Y(µ-H)}
2
,
2 5 4 2 2 2
and {[Me Si(C Me ) ]Th(µ-H) } . However, it should
17b
2
Y(µ-H)]
2
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Published on Web 03/19/1998