Synthesis of a zirconium sandwich complex and crystallographic characterization of its adduct with tetrahydrofuran

Article abstract of DOI:10.1021/ja035594v

Reductive elimination of alkane from a silylated bis-indenyl zirconium isobutyl hydride affords a zirconium sandwich complex with an unusual η⁶ indenyl ligand. Crystallographic characterization of its adduct with tetrahydrofuran has been achieved and reveals significant localization in the six-membered ring. Complexation of more potent ligands such as carbon monoxide and diphenylacetylene are effective in promoting haptotropic rearrangement of the η⁶ indenyl ligand and provides familiar bent zirconocene derivatives. The zirconium sandwich compound also undergoes oxidative addition of carbon-hydrogen bonds of pyridyl ligands, resulting in a crystallographically characterized dimethylamino pyridyl hydride complex of zirconium. Copyright

Full text of DOI:10.1021/ja035594v

Published on Web 06/12/2003
Synthesis of a Zirconium Sandwich Complex and Crystallographic
Characterization of Its Adduct with Tetrahydrofuran
Christopher A. Bradley, Emil Lobkovsky, and Paul J. Chirik*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell UniVersity, Ithaca, New York 14853
Received April 11, 2003; E-mail: pc92@cornell.edu
The discovery of metal sandwich compounds, more commonly
known as metallocenes, spawned the modern field of organometallic
chemistry.1 Interest in such molecules as related to group 4 transition
metals originated with Wilkinson’s attempt to prepare the parent
titanocene, [(η5-C5H5)2TiII], over 50 years ago2 and since that time
has expanded to include numerous examples of M(IV), bent
metallocene derivatives.3 Subsequent attempts to prepare the parent
sandwich compounds have been frustrated by their ability to activate
cyclopentadienyl C-H bonds4 or to bind relatively inert molecules
such as dinitrogen.5
Preparation and crystallographic characterization of a titanium
sandwich compound with sterically demanding cyclopentadienyl
ligands was first described by Lawless6 and more recently expanded
by Mach.7 Synthesis of the heavier group 4 congeners has not been
achieved, owing to the greater thermodynamic driving force for
Figure 1. ORTEP diagram of 3-THF with 30% probability ellipsoids.
these elements to attain their highest oxidation state.8 Numerous
catalytic processes9 have implicated the existence of zirconocene,
while other studies have successfully trapped the molecule with
donor ligands.10 In this communication, we report the synthesis of
amu, consistent with a monomeric bis-indenyl zirconium sandwich
complex (MWcalc ) 610 amu). The possibility of a dinitrogen com-
plex was ruled out by performing successful syntheses of 3
a zwitterionic, bis-indenyl zirconium sandwich complex, crystal-
lographic characterization of its adduct with tetrahydrofuran, and
preliminary reactivity studies.
both in an argon atmosphere and under vacuum. Elemental analysis
was also consistent with an empirical formula of [(C9H5-1,3-
(SiMe3)2)2Zr].
The 1H and 13C NMR spectra of 3 in benzene-d6 and cyclohex-
ane-d12 are nearly identical and display a total of 8 and 12 ligand
resonances, respectively, inconsistent with a C2V symmetric zirco-
nium sandwich complex. Instead, the NMR data indicate a Cs
symmetric molecule with inequiValent indenyl rings. Moreover, the
1H NMR spectrum in benzene-d6 displays benzo protons that are
shifted upfield to 4.00 and 5.28 ppm, indicative of coordination of
the six-membered ring to the zirconium center.13 On the basis of
these data and crystallographic characterization of its adduct with
THF (vide infra), the most plausible structure for 3 is a zirconium
sandwich in which one of the indenyl ligands has undergone a
haptotropic rearrangement (eq 1).14 Formally, 3 can be viewed as
a 14-electron, zwitterionic zirconium sandwich complex.
Addition of an excess (>3 equiv) of THF to a pentane solution
of 3 followed by solvent removal and recrystallization from heptane
at -35 °C afforded bright red crystals identified as 3-THF (eq 2).
Dissolution of pure 3-THF in benzene-d6 resulted in a 2:1 mixture
of 3-THF and 3 along with free THF. Complete (>95%) conversion
to 3-THF may be achieved by addition of more than 3 equiv of
THF to benzene-d6 solutions of 3.
Recent work in our laboratory has focused on the application of
early transition metal alkane reductive elimination reactions to the
11
hydrogenation of P4 and the activation of molecular nitrogen.12
Kinetic studies in the latter case demonstrated accelerated rates of
alkane loss with addition of [SiMe3] substituents to the cyclopen-
tadienyl ligands. Having obtained these results, we became
interested in studying the related bis-indenyl zirconium alkyl hydride
complexes and determining the rate and products of their alkane
reductive elimination reactions.
Synthesis of the desired zirconocene alkyl hydride complex,
(η5-C9H5-1,3-(SiMe3)2)2Zr(CH2CHMe2)H (2), was achieved by
addition of two equivalents of LiCH2CHMe2 to a toluene solution
of the corresponding dichloride complex, 1, at 22 °C (eq 1). One
equivalent of isobutene was also observed by 1H NMR spectroscopy
and confirmed by mass spectrometry. Benzene-d6 solutions of 2
undergo facile reductive elimination at ambient temperature, afford-
ing one equivalent of isobutane and a new burgundy zirconium
complex, 3. Rapid reductive elimination has thwarted all attempts
to isolate 2 in the solid state. However, 3 may be obtained in
multigram quantities by alkylation of 1, followed by filtration and
recrystallization from cold pentane.
X-ray diffraction on single crystals of 3-THF reveals a Cs
symmetric zirconium complex with a centrally bound THF ligand
(Figure 1). The molecule contains one commonly observed η5
indenyl ligand, while the other is bound through the benzo ring.
The coordinated six-membered ring is no longer planar with a
dihedral angle of 19° between the planes defined by C1-C2-
C3-C4 and C1-C6-C5-C4. Relatively short Zr-C1 and
Zr-C4 distances of 2.340(2) and 2.355(3) Å, respectively, are also
observed and are suggestive of localization in the benzo ring. This
Solution molecular weight determinations for 3 by cryoscopy
and ebulliometry in benzene both provided values of 570 ( 60
9
8110
J. AM. CHEM. SOC. 2003, 125, 8110-8111
10.1021/ja035594v CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Figure 3. ORTEP diagram of 6 with 30% probability ellipsoids.
Figure 2. ORTEP diagram of 4 with 30% probability ellipsoids.
In conclusion, alkane reductive elimination has provided the first
example of a zirconium sandwich compound that undergoes hap-
totropic rearrangement to yield a zwitterionic zirconium sandwich
with an η6 indenyl ligand. Reconstitution of η5 hapticity may be
achieved by addition of strong donor ligands or by oxidative addi-
tion. Further studies exploring the chemistry of this unique molecule
are currently under investigation.
coordination environment is similar to that observed previously
for a series of arene complexes of niobium15 and tantalum16 while
formal η2 hapticity for a Cp ring has been reported for both
titanium17 and tantalum18 complexes. Thus 3 and 3-THF may be
viewed as either Zr(II) complexes with an η6 indenyl ligand or as
Zr(IV) compounds with a bound η2,η1,η2,η1 six-membered ring.
Preliminary reactivity studies were carried out with 3 to establish
the molecule as a source of low-valent zirconium. Addition of one
atmosphere of CO to a pentane solution of 3 resulted in isolation
of forest green crystals identified as (η5-C9H5-1,3-(SiMe3)2)2Zr-
(CO)2 (4) (eq 2).19 Both NMR spectroscopic data and X-ray
Acknowledgment. We thank Cornell University, the donors of
the Petroleum Research Fund administered by the ACS, and the
National Science Foundation (CAREER award to P.J.C. and
predoctoral fellowship to C.A.B.) for financial support.
Supporting Information Available: Experimental procedures,
NMR spectra for selected compounds (PDF); crystallographic data for
3-THF, 4 and 6 (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
References
(1) Adams, R. D. J. Organomet. Chem. 2001, 637, 1.
(2) Fischer, A. K.; Wilkinson, G. J. Inorg. Nucl. Chem. 1956, 2, 149.
(3) Negishi, E. I. In The Metallocenes; Togni, A., Halterman, R. L., Eds.;
Wiley-VCH: Weinheim, 1998; Vol. 1, p 282.
(4) Brintzinger, H. H.; Bercaw, J. E. J. Am. Chem. Soc. 1970, 92, 6182.
(5) Fryzuk, M. D.; Johnson, S. A. Coord. Chem. ReV. 2000, 200, 379.
(6) (a) Hitchcock, P. B.; Kerton, F.; Lawless, G. A. J. Am. Chem. Soc. 1998,
120, 10264.
(7) Lukesova´, L.; Hora´cek, M.; Stepnicka, P.; Fejfarova´, K.; Gyepes, R.;
Cisorova´, I.; Kubista, J.; Mach, K. J. Organomet. Chem. 2002, 663, 134.
(8) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, 5th ed.; John
Wiley and Sons: New York, 1988; p 778.
(9) Negishi, E.; Takahashi, T. Acc. Chem. Res. 1994, 27, 124.
(10) Gell, K. I.; Schwartz, J. J. Am. Chem. Soc. 1981, 103, 2687.
(11) Chirik, P. J.; Pool, J. A.; Lobkovsky, E. Angew. Chem., Int. Ed. 2002,
41, 3463.
(12) Pool, J. A.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2003, 125,
2241.
diffraction studies (Figure 2) demonstrate η5 coordination of both
indenyl ligands. Likewise, reaction of 3 with one equivalent of
PhCtCPh afforded the alkyne adduct, (η5-C9H5-1,3-(SiMe3)2)2Zr-
(PhCCPh) (5), with η5 indenyl ligands as judged by its C2V sym-
metry in benzene-d6 solution (eq 3). Complexation of ligands more
potent than THF is effective in promoting haptotropic rearrange-
ment, resulting in familiar bent zirconocene derivatives.
(13) Fryzuk, M. D.; Kozak, C. M.; Mehrkhodavandi, P.; Morello, L.; Patrick,
B. O.; Rettig, S. J. J. Am. Chem. Soc. 2002, 124, 516.
(14) (a) Albright, T. A.; Hofman, P.; Hoffmann, R.; Lillya, C. P.; Dobosh, P.
A. J. Am. Chem. Soc. 1983, 105, 3396. (b) Wheeler, D. E.; Bitterwolf, T.
E. Inorg. Chim. Acta 1993, 205, 123.
Sandwich compound 3 also promotes the oxidative addition of
carbon-hydrogen bonds. One equivalent of (dimethylamino)-
pyridine (DMAP) reacts with 3 to provide golden yellow crystals
of (η5-C9H5-1,3-(SiMe3)2)2Zr(η2-cyclo-NC5H3-4-NMe2)H (6) (eq 3).
Both the solution- and solid-state structure (Figure 3) of 6 contain
η5 indenyl rings with central coordination of the pyridyl nitrogen
and a transoid arrangement of the C-H fragments arising from
oxidative addition.
(15) Yoon, M.; Lin, J.; Young, V. G.; Miller, G. J. J. Organomet. Chem. 1996,
507, 31.
(16) Wexler, P. A.; Wigley, D. E.; Koerner, J. B.; Albright, T. A. Organo-
metallics 1991, 10, 2319.
(17) Lucas, C. R.; Green, M.; Forder, R. A.; Prout, K. Chem. Commun. 1973,
97.
(18) Chirik, P. J.; Zubris, D. L.; Ackerman, L. J.; Henling, L.; Day, M. W.;
Bercaw, J. E. Organometallics 2003, 22, 172.
(19) Spectroscopic data can be found in the Supporting Information.
JA035594V
9
J. AM. CHEM. SOC. VOL. 125, NO. 27, 2003 8111