The transient titanocene(II): Direct synthesis from solvated titanium(II) chloride and cyclopentadienylsodium and ensuing interception with diphenylacetylene as 1,1-bis(cyclopentadienyl)-2,3,4,5- tetraphenyltitanacyclopentadiene

Article abstract of DOI:10.1002/ejic.200600766

For the first time the unstable titanocene(II) has been directly synthesized by the Wilkinson metallocene approach, namely the interaction of a THF-soluble form of titanium(II) chloride with two equivalents of cyclopentadienylsodium in THF solution at 0°-25°C. Because of the transient existence of the titanocene(II) thereby obtained, it could only be chemically trapped in high yield as 1,1-bis(cyclopentadienyl)-2,3,4,5- tetraphenyltitanacyclopentadiene by two equivalents of diphenylacetylene, if the acetylene was added at 25°C, without removal of the by-product LiCl and NaCl. If the addition of the acetylene was delayed, in order to filter off the LiCl and NaCl from the reaction mixture, then no trace of the titanacyclopentadiene derivative was found upon hydrolytic workup. Instead, a significant portion of the acetylene was found to have undergone hydrotitanation. This finding is clear evidence that the titanocene(II) had undergone a precedented rearrangement to a known dimer having the structure of a titanocene(III) hydride with a fulvalene bridge between the titanium centers. We suggest that the LiCl and NaCl present in the unfiltered reaction mixture form a dichloro complex with titanocene(II) and thereby retard its dimerizing rearrangement to the titanocene(III) hydride. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200600766

SHORT COMMUNICATION
DOI: 10.1002/ejic.200600766
The Transient Titanocene(II): Direct Synthesis from Solvated Titanium(II)
Chloride and Cyclopentadienylsodium and Ensuing Interception with
Diphenylacetylene as 1,1-Bis(cyclopentadienyl)-2,3,4,5-
tetraphenyltitanacyclopentadiene[‡]
John J. Eisch,*[a] Adetenu A. Adeosun,[a] and John M. Birmingham[b]
Keywords: Titanocene(II) / Wilkinson metallocene synthesis / Epititanation / 1,1-Bis(cyclopentadienyl)titanacycloprop-
enes / 1,1-Bis(cyclopentadienyl)titanacyclopentadienes
For the first time the unstable titanocene(II) has been directly
synthesized by the Wilkinson metallocene approach, namely
the interaction of a THF-soluble form of titanium(II) chloride
with two equivalents of cyclopentadienylsodium in THF
solution at 0°–25 °C. Because of the transient existence of the
titanocene(II) thereby obtained, it could only be chemically
trapped in high yield as 1,1-bis(cyclopentadienyl)-2,3,4,5-tet-
raphenyltitanacyclopentadiene by two equivalents of di-
phenylacetylene, if the acetylene was added at 25 °C, with-
out removal of the by-product LiCl and NaCl. If the addition
of the acetylene was delayed, in order to filter off the LiCl
and NaCl from the reaction mixture, then no trace of the tit-
anacyclopentadiene derivative was found upon hydrolytic
workup. Instead, a significant portion of the acetylene was
found to have undergone hydrotitanation. This finding is
clear evidence that the titanocene(II) had undergone a pre-
cedented rearrangement to a known dimer having the struc-
ture of a titanocene(III) hydride with a fulvalene bridge be-
tween the titanium centers. We suggest that the LiCl and
NaCl present in the unfiltered reaction mixture form a
dichloro complex with titanocene(II) and thereby retard its
dimerizing rearrangement to the titanocene(III) hydride.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
sodium (3)[8a,8b] or the α-elimination of the two groups G
from Cp2TiG2 (4) (G = Cl, or CH3) by an active metal for
G = Cl,[9] dihydrogen,[10] heat,[11] or light[12] for G = CH3
(Scheme 1). None of the socalled “titanocene” products
thus generated could be isolated in a crystalline form suit-
able for single-crystal X-ray structure analysis.
Introduction
Shortly after the serendipitous synthesis and discovery of
ferrocene through the independent routes of Pauson and
Kealy[2] and of Miller and co-workers[3] the systematic syn-
thesis of analogous cyclopentadienyl derivatives of transi-
tion metals was undertaken with great success by both the
Wilkinson and the Fischer research groups.[4] From group
4 tetrahalides the bis(cyclopentadienyl)metal(IV) dihalides
were the most readily formed and isolated. Although the
preparation of bis(cyclopentadienyl)titanium(III) halides
was achieved by the activated metal reduction of Cp2TiX2
in donor, nonprotic solvents,[5,6] the preparation of bis(cy-
clopentadienyl)titanium(II) or titanocene(II) (2) has re-
ceived the attention of many researchers during the period
of 1956–1976 with an overall indecisive outcome.[7]
Scheme 1.
A range of different “titanocene” products have been ac-
cordingly isolated, varying in color (green, brown, violet or
The attempted preparations of titanocene(II) (2) have in- black) and in spectroscopic as well as magnetic properties.
volved either the direct reaction of polymeric TiCl2 (1a) in It is generally agreed that none of these products is titano-
ethereal suspension with two equiv. of bis(cyclopentadienyl)- cene (2) itself, which should be very reactive due to its car-
benoid nature.[13,14] Therefore, in order to take into account
the anticipated high reactivity of authentic titanocene (2),
provision was made to capture 2 as it was purportedly
formed by the reduction of Cp2TiCl2 with the sodium–
naphthalene adduct in THF.[9a] When diphenylacetylene (5)
was included as the trapping agent in the reaction, the titan-
acyclopentadiene 6 was isolated in low yield. This finding
was taken as convincing evidence that titanocene (2) had
[‡] Organic Chemistry of Subvalent Transition Metal Complexes,
39. Part 38: Ref.[1]
[a] State University of New York at Binghamton, Department of
Chemistry,
P. O. Box 6000, Binghamton, NY 13902-6000, USA
Fax: +1-607-777-4865
E-mail: jjeisch@binghamton.edu
[b] The Boulder Scientific Company,
Mead, CO, USA
Eur. J. Inorg. Chem. 2007, 39–43
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39

J. J. Eisch, A. A. Adeosun, J. M. Birmingham
SHORT COMMUNICATION
Scheme 2.
been generated [4 (G = Cl)Ǟ2] and had been captured by employing the insoluble, polymeric form, [TiCl2]x (1a), ob-
diphenylacetylene (5) (path a, Scheme 1, 2Ǟ6).[15] How- tainable by the high-temperature disproportionation of tita-
ever, the conclusion that titanocene (2) has been captured nium(III) chloride, a newly discovered, THF-soluble form
in this experiment is ambiguous and thus inconclusive. It of titanium(II) chloride (1b), [TiCl2·THF]n, generated by
has long been recognized that alkali metals in general and the alkylative reduction of titanium(IV) chloride,[20] has
lithium metal in particular can effect the reductive di- been used to great advantage [Equations (1) and (2)]. The
merization of diphenylacetylene in ethers (5Ǟ7, degree of aggregation (n) of 1b is not yet known with cer-
Scheme 2).[16,17] The 1,4-dilithiobutadiene of proven (E,E)- tainty but the existence of the paramagnetic open-chain,
configuration has been successfully employed for the syn- trimeric biradical, [Ti(OiPr)2]3, supports the proposal that
thesis of 6 and several other metallacyclopentadienes.[18,19] the diamagnetic 1b may form a cyclic trimer in which a core
Thus the titanocycle 6 isolated from the reaction of of 3 titanium atoms is bonded as a triangular structure.[1]
Cp2TiCl2 and diphenylacetylene with sodium could equally
as well have been formed from path b in Scheme 2
(5Ǟ7Ǟ6) and would not have required the presence of free
(1)
titanocene (2), as in path a.
Results and Discussion
In the present research the direct reaction of titanium(II)
chloride with bis(cyclopentadienyl)sodium (3) was again in-
(2)
vestigated but with one significant difference. Instead of
Scheme 3.
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The Transient Titanocene(II)
SHORT COMMUNICATION
A THF solution of 0.10 mmol of freshly prepared 1b, and thereby retarding its rearrangement to 7 [Equation (4)].
still containing the LiCl by-product, was treated at 0 °C A relevant precedent is the complex 12 formed from 1a and
with 0.22 mmol of bis(cyclopentadienyl)sodium (3). The NaCl as shown in Equation (5).[22]
black reaction mixture, presumably containing titano-
cene(II) (2), was brought to 25 °C and the LiCl and NaCl
solids filtered off. Treating the filtrate with diphenylacety-
lene (5), however, produced no trace of 1,1-bis(cyclopenta-
dieny)-l-2,3,4,5-tetraphenyltitanacyclopentadiene (6). This
conclusion was reached after protolysis of the reaction mix-
ture with acetic acid was shown to produce no (E,E)-
1,2,3,4-tetraphenyl-1,3-butadiene (8) (Scheme 3), the ex-
pected protolysis product that would have arisen from 6,
which would have formed by the trapping of 2 by diphenyl-
Conclusions
From the foregoing experiments we have now shown that
the transient titanocene(II) can also be prepared by the
classic method introduced by the Wilkinson group some 50
years ago. Its existence, although short, has been firmly es-
tablished by its chemical trapping with diphenylacetylene as
the titanacyclopentadiene 6.[23]
acetylene (5) (path a, Scheme 2). That on the other hand
cis-stilbene (10) was isolated as the sole protolysis product
Experimental Section
is consistent with the view that any 2 formed from 1b and
3 had undergone the known rearrangement to 9 before it
could be intercepted by 5.[7b] The resultant 9 could then
have hydrotitanated acetylene 5 and hence have generated
cis-olefin 10, upon protolysis.
Starting Materials, Reaction Conditions and Instrumentation: The
starting TiCl4 employed was of at least 98% purity as commercially
available and the n-butyllithium used was as a 1.60  solution in
hexane. The cyclopentadiene was obtained from the thermolysis of
commercial dicyclopentadiene under argon and collected in an ice
bath. The sample for the preparation of cyclopentadienylsodium
resulted from a redistillation of the monomer just before use.[24]
The titanium(II) chloride in THF, containing 2 equiv. of the by-
product, LiCl, was obtained from the treatment of TiCl4 in THF
with 2 equiv. of n-butyllithium at low temperatures according to
the published procedure.[20] In the same reference are given experi-
mental details for removing LiCl from the product and obtaining
analytically pure TiCl2·THF.[20] However, for its reaction here with
cyclopentadienylsodium, the titanium(II) chloride/lithium chloride
mixture was used directly. Finally, the diphenylacetylene used in
In our further attempt to intercept any titanocene(II) (2)
formed from 1b and 2, we repeated the reaction between 1b
and 2 at 0 °C, brought the reaction mixture to 25 °C, but
this time immediately added the diphenylacetylene (5) with-
out attempting to filter off the suspended LiCl and NaCl. A
nonhydrolytic workup involving removal of any solids from
the hexane extracts of the final reaction mixture led, via
epititanation to titanacyclopropene 5a,[14] to the isolation
1
of 6 in 83% yield. Its identity was confirmed by H NMR
and IR spectral comparisons with an authentic sample of 6
prepared according to a previously outlined method,[21]
whose details are published here for the first time [Equa-
tion (3)]. Furthermore, the sample of 6 isolated from 1b and
2 was smoothly protolyzed to yield solely 8 in high yield
(Scheme 3).
1
trapping reactions was 99% pure and was shown by TLC and H
NMR spectroscopy to be free of cis- or trans-stilbene. All reactions,
transfers and separations were carried out under a positive pressure
of anhydrous, oxygen-free argon.[25] All solvents employed, mainly
tetrahydrofuran, heptane and toluene, with these air- and moisture-
sensitive reagents were dried and distilled from a sodium metal
benzophenone ketyl mixture prior to use.[25]
1
The H and 13C NMR spectra were recorded with a Bruker spec-
trometer, model AM-360, with tetramethylsilane (Me4Si) as the in-
ternal standard. The chemical shifts reported are expressed on the
δ-scale in parts per million (ppm) from the Me4Si reference signal.
(3)
1,1-Bis(cyclopentadienyl)-2,3,4,5-tetraphenyltitanacyclopentadiene
(6): Although an authentic sample of this known compound can
be prepared by a number of previous published methods,[12,15]
a
The success of this present procedure for generating 2
from 1b and 3 and in trapping titanocene(II) (2) before it
could rearrange to 9 we ascribe to two factors: 1) the
shorter time the THF solution of 2 stood at 25 °C before
acetylene 5 was introduced; and 2) the presence of LiCl and
NaCl, which furnish an ample source of Lewis basic chlo-
method developed in our laboratory proved most convenient,[21]
because its starting material, 11 is available through a verified ex-
perimental procedure.[26,27] A 100-mL quartz phototube, having
ground-glass joints at the upper end for attachment to the argon
source and for a rubber septum, respectively, was repeatedly evacu-
ated and filled with argon gas. Then a solution of η3-allylbis(cyclo-
ride ions, capable of coordinating with monomeric Cp2Ti pentadienyl)titanium(III) (365 mg, 1.67 mmol) prepared under ar-
gon in anhydrous, deoxygenated cyclohexane (25 mL) was intro-
duced into the tube through the septum, after which a similarly
prepared solution of diphenylacetylene (594 mg, 3.34 mmol) in
(4)
10 mL of purified cyclohexane was introduced. The reaction solu-
tion was irradiated for 48 h in a Rayonet photochemical reactor,
model 100, equipped with a cylindrical array of phototubes emit-
ting radiation at 254 nm. The cylindrical cavity of the lamp bank
had a diameter of 24 cm. During this time the initial purple color
(5)
of the reaction mixture became greenish-yellow. The cyclohexane
Eur. J. Inorg. Chem. 2007, 39–43
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41

J. J. Eisch, A. A. Adeosun, J. M. Birmingham
SHORT COMMUNICATION
was removed from the photolysate under reduced pressure and the
dark green residue was extracted under argon with 3ϫ10-mL por-
tions of hexane. Such combined extracts were filtered and then con-
centrated. Storage of the hexane solution at 0 °C led to the deposi-
tion of between 339 mg to 491 mg of dark green 6 (38–55%) in
two different runs; m.p. (capillary under argon) 148.5–150 °C [lit.
150 °C];[17] 1H NMR (C6D6): δ = 6.06(s, 10 H), 6.4–6.9 (m, 20 H)
repeated with glacial O-deuterioacetic acid (99%), the resulting 8
lacked the 1H NMR singlet at δ = 6.40 ppm, showing that this
sample of 8 was deuteriated at the C1 and C4 sites.
Acknowledgments
ppm. IR (KBr): ν = 697 cm–1 (vs), 725 (m), 750 (m), 770 (s), 780
˜
The authors are grateful to the Boulder Scientific Company, Mead,
Colorado, for the generous support of their research with transi-
tion-metal metallocenes and to the Alexander von Humboldt
Foundationfor a Senior Scientist Award to the first-cited author,
renewed in 2005 for studies in the laboratory of Professor W. A.
Herrmann, Institute of Inorganic Chemistry, Technische Uni-
versität München, Germany.
(m), 800 (vs), 807 (vs), 830 (sh), 840 (w), 905 (vw), 945 (vw), 1015
(s), 1075 (s), 1095 (sh), 1145 (sh), 1155 (w), 1180 (w), 1245 (w),
1260 (w), 1362 (w), 1370 (sh), 1440 (s), 1480 (s), 1590 (s), 3040 (w),
3060 (w), 3080 (w) cm–1.
Interaction of Bis(cyclopentadienyl)sodium (3) with Titanium(II)
Chloride–Bis(tetrahydrofuran) (1b) in Tetrahydrofuran
1) Reaction at 0 °C, Removal of Alkali Metal Salts by Filtration and
Reaction of the Titanocene Filtrate with Diphenylacetylene (5): A
solution of titanium(IV) chloride (1.90 g, 10 mmol) in toluene
(10 mL) was added to dry THF (60 mL) and the mixture cooled
to –78 °C to yield a yellow suspension. Then n-butyllithium
(20 mmol) in hexane (2.50 ) was introduced into the yellow slurry
dropwise. Allowing this reaction mixture to come to 25 °C over 1 h
produced a black slurry of TiCl2·THF (1b) in solution with sus-
pended LiCl by-product, as has been established by a published
procedure.[22] Then an orange solution of the cyclopentadienyl-
sodium 3 (22 mmol), previously prepared from sodium metal dis-
persion (0.51 g, 22 mmol) and freshly distilled cyclopentadiene
(1.50 g, 23 mmol) in THF (50 mL) was added rapidly to the solu-
tion of 1b maintained at 0 °C. The resulting reaction mixture was
allowed to stand at 25 °C for 2 h before the suspended salts (LiCl
and NaCl) were filtered off on a glass frit under argon. The fil-
tration step required about 90 min. The green-black filtrate was
then treated with a solution of diphenylacetylene (5) (3.56 g,
20 mmol) in THF (40 mL). This final mixture was allowed to stand
at room temp. for 10 h and then heated at reflux for 6 h. Treatment
of an aliquot of the reaction mixture with glacial acetic acid and
usual hydrolytic workup of the dried organic extract showed that
35% of the diphenylacetylene initially introduced had been con-
verted into cis-stilbene (10) (7.0 mmol.). Equally noteworthy was
the absence of any (E,E)-1,2,3,4-tetraphenyl-1,3-butadiene (8), the
protolysis product expected to arise if any 6 had been formed from
the reaction of the titanocene(II) 2 with the added diphenylacety-
lene.
[1] J. J. Eisch, J. N. Gitua, D. C. Doetschman, Eur. J. Inorg. Chem.
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[2] T. J. Kealy, P. L. Pauson, Nature 1951, 168, 1039–1040.
[3] S. A. Miller, J. A. Tebboth, J. F. Tremaine, J. Chem. Soc. 1952,
632–635.
[4] A comprehensive survey of the various syntheses of cyclopen-
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[5] J. M. Birmingham, A. K. Fischer, G. Wilkinson, Naturwissen-
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[6] P. C. Wailes, R. S. P. Coutts, H. Weigold, Organometallic Chem-
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[7] A detailed recounting of the “Titanocene Saga” has appeared
in two reviews: a) An empirical summary of the several experi-
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nometallic Chemistry of Titanium, Zirconium, and Hafnium,
Academic Press, New York, 1974, pp. 228–239; b) A more re-
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[8] a) A. K. Fischer, G. Wilkinson, J. Inorg. Nucl. Chem. 1956,
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[9] a) G. W. Watt, L. J. Baye, F. O. Drummond Jr., J. Am. Chem.
Soc. 1966, 88, 1138–1140 (sodium naphthalenide); b) J. J. Salz-
mann, P. Mosimann, Helv. Chim. Acta 1967, 50, 1831–1836
(sodium dispersion); c) G. P. Pez, J. Am. Chem. Soc. 1976, 98,
8072–8074 (potassium naphthalenide to give by XRD a struc-
ture, CpTi-TiCp2 unit, whose Ti centers have a C5H4 bridging
ring π-bonded to the Ti on the left and σ-bonded to the Ti on
the right).
2) Reaction at 25 °C, Followed by the Immediate Addition of Diphen-
ylacetylene (5) without the Removal of Alkali Metal Salts: Parallel
to the foregoing procedure, a freshly prepared black slurry of
TiCl2·THF (1b, 10 mmol) and LiCl (20 mmol) in THF (60 mL) was
treated dropwise at 0 °C with 3 (22 mmol) in THF (50 mL). The
resulting mixture was allowed to stand for 2 h at 20–25 °C, after
which time a solution of diphenylacetylene (5) (3.56 g, 20 mmol) in
THF (40 mL) was added rapidly. The final reaction mixture was
kept at room temp. for 10 h and then heated at reflux for 6 h. After
removal of all volatiles under reduced pressure the residue was ex-
tracted with 4ϫ15-mL portions of warm hexane, the combined
extracts filtered and the filtrate concentrated to about half its vol-
ume. Storage of the hexane solution at 0 °C deposited 4.27 g (83%)
of dark green crystals of 6, m.p. 148–150 °C, whose 1H NMR
(C6D6) and IR (KBr) spectra were identical with those of the au-
thentic sample. Treatment of a sample of such a product with hot
glacial acetic acid, followed by the usual hydrolytic workup, af-
forded pure (E,E)-1,2,34-tetraphenyl-1,3-butadiene (8), m.p. 181–
183 °C, as the only nonvolatile organic product. 1H NMR (CDCl3):
δ = 6.40 (s, 2 H), 6.98–7.38 (m, 20). When the protolysis of 6 was
[10] K. Clauss, H. Bestian, Justus Liebigs Ann. Chem. 1962, 654, 8–
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[11] G. A. Razuvaev, V. N. Latyaeva, L. I. Vishinskaya, A. M. Rabi-
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[12] H. Alt, M. D. Rausch, J. Am. Chem. Soc. 1974, 96, 5936–5937.
[13] H. H. Brintzinger, L. S. Bartell, J. Am. Chem. Soc. 1970, 92,
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[14] J. J. Eisch, J. Organomet. Chem. 2001, 617–618, 148–157.
[15] G. W. Watt, F. O. Drummond Jr., J. Am. Chem. Soc. 1970, 92,
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[16] L. I. Smith, H. H. Hoehn, J. Am. Chem. Soc. 1941, 63, 1184–
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[17] J. J. Eisch, Organometallic Syntheses, vol. 2, Academic Press,
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[18] F. C. Leavitt, T. A. Manuel, F. Johnson, L. U. Matternas, D. S.
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[19] E. H. Braye, W. Hübel, I. Caplier, J. Am. Chem. Soc. 1961, 83,
4406–4413.
42
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The Transient Titanocene(II)
SHORT COMMUNICATION
[20] J. J. Eisch, X. Shi, J. Lasota, Z. Naturforsch., Teil B 1995, 50,
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[21] J. J. Eisch, S. I. Pombrik, X. Shi, S. C. Wu, Macromol. Symp.
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[22] G. W. A. Fowles, T. E. Lester, R. A. Walton, J. Chem. Soc. A
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[24] R. B. King, in: Organometallic Syntheses (Eds.: J. J. Eisch, R. B.
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[26] H. A. Martin, F. Jellinek, J. Organomet. Chem. 1967, 8, 115–
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[23] For a leading reference to other recent chemical trapping meth-
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N. Avarvari, D. Bourissou, F. Mathey, P. LeFloch, Organome-
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[27] M. P. Boleslawski, J. J. Eisch, Organometallic Syntheses (Eds.:
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Received: August 17, 2006
Published Online: November 17, 2006
Eur. J. Inorg. Chem. 2007, 39–43
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