Organometallics 1996, 15, 4667-4668
4667
Syn th esis of a n Alk yn e-Br id ged Deca m eth ylh a fn ocen e
Dim er a n d Rela ted Alk yn e-Su bstitu ted Mon om er s
Glen E. Southard, M. David Curtis,* and J effrey W. Kampf
Department of Chemistry, Willard H. Dow Laboratory, The University of Michigan,
Ann Arbor, Michigan 48109-1055
Received J uly 16, 1996X
Sch em e 1
Summary: Reaction of Hf*Cl2 (1; Hf* ) decamethyl-
hafnocene), with excess sodium acetylide at room tem-
perature results in the synthesis of Hf*(CCH)2 (2). The
identical reaction in refluxing THF results in the dimeric
product [(µ2-C2)(Hf*CCH)2] (4). Subsequent reaction of
2 with lithium diisopropylamide (LDA) in the presence
of trimethyltin chloride resulted in the trimetallic
Hf*(CCSnMe3)2 (3).
In recent years, much attention has been given to
metal-containing polymers as sources of novel materials
in terms of their electronic,1 optical,2 and magnetic
properties3 or as precursors to ceramics by thermal
decomposition.4 Initially, we desired to synthesize a
group 4 R,ω-metallocene alkynyl polymer via trans-
metalation of the group 4 metallocene dichloride with
a dilithiated alkyne, such as dilithioacetylene. The
resulting polymer would consist entirely of the group 4
metal, carbon, and hydrogen, which would be desirable
as a polymeric metal carbide ceramic precursor.4b Ad-
ditionally, it was hoped that a group 4 metallocene
polymer with alkynyl and/or aromatic bridging ligands
would exhibit enhanced NLO properties. EHMO cal-
culations of H(CCHf*CCsCC)Hf*CCH indicated that its
LUMO exhibits extensive conjugation along the oligo-
mer backbone with good overlap between the alkynyl
spacer π orbitals and the hafnium d orbitals. The
LUMO may be accessed by an optically pumped excita-
tion, by a chemical reduction of the oligomer, or by
substitution of the hafnium (d0) with a group 5 (d1)
metal to give an “inherently doped” oligomer. Thus, the
main chain conjugation was expected to show interest-
ing electronic properties.
The reactions of Hf*Cl2 (1) with lithium acetylides
were investigated as models for polymerization reac-
tions.5,6 The alkyl-substituted Cp is expected to impart
better hydrolytic stability and solubility to the mono-
mers and polymers once synthesized. It was found,
however, that reactions of 1 with stoichiometric amounts
of lithium p-tolylacetylide resulted in the incomplete
substitution of chloride ligands with the acetylide
groups. Thus, synthesis of high-molecular-weight
alkynyl-hafnocene polymers under similar conditions
is not feasible.
It was possible to obtain acceptable yields of the
disubstituted products by using a 3-fold excess of
lithium p-tolylacetylide.7 While not useful in itself for
making high-MW polymers, this reaction does give
access to new polymer precursors. Synthesis of 2, as
shown in Scheme 1, was realized in >50% yield from
the reaction of 1 with excess sodium acetylide in THF
or pyridine at room temperature for 1 day. Pyridine
was used to decrease the possibility of oxygen abstrac-
tion by the oxophilic Hf from THF. Compound 2 is a
X Abstract published in Advance ACS Abstracts, October 1, 1996.
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(7) Syn th esis of Hf*(CC-C6H4CH3)2: p-Tolylacetylene (138.5 mg,
1.20 mmol) and n-butyllithium (0.75 mL, 1.6 M in hexanes, 1.20 mmol)
were placed into THF (25 mL) via syringe, and the mixture was stirred
for 1 h. The light yellow solution was transferred via cannula to a
solution of Hf*Cl2 (100 mg, 0.19 mmol) in THF (5 mL). The reaction
mixture was heated to reflux and stirred for 2 h before the THF was
removed and the residue extracted with hexanes. The resulting bronze
solution was filtered and concentrated, giving orange crystals (101 mg,
0.15 mmol, 78% yield). 1H NMR (300 MHz, 25 °C, C6D6): δ 7.50 (d,
4H, CH tolyl), 6.91 (d, 4H, CH tolyl), 2.14 (s, 30H, C5Me5), 2.01 (s, 6H,
Me tolyl).
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