Synthesis of an alkyne-bridged decamethylhafnocene dimer and related alkyne-substituted monomers

Article abstract of DOI:10.1021/om9605903

Reaction of Hf*Cl₂ (1; Hf* = decamethyl-hafnocene), with excess sodium acetylide at room temperature results in the synthesis of Hf*(CCH)₂ (2). The identical reaction in refluxing THF results in the dimeric product [(μ₂-C<

Full text of DOI:10.1021/om9605903

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.
(1) (a) Lhost, O.; Toussaint, J . M.; Bredas, J . L.; Wittman, J . F.;
Friend, R. H.; Fuhrman, K.; Khan, M. S.; Lewis, J . Synth. Met. 1993,
55-57, 4525. (b) Frapper, G.; Kertesz, I. M. Inorg. Chem. 1993, 32(5),
732. (c) Phillpot, S. R.; Rice, M. J ., Bishop, A. R.; Campbell, D. K. Phys.
Rev. B 1987, 36(3), 1735. (d) Corriu, R.; Gerbier, Ph.; Guerin, C.;
Henner, B.; Fourcade, R. J . Organomet. Chem. 1993, 449, 111-118.
(e) Nelson, J . M.; Lough, A. J .; Manners, I. Organometallics 1994, 13,
3703-3710. (f) Tolbert, L. M.; Zhao, X.; Ding, Y.; Bottomley, L. A. J .
Am. Chem. Soc. 1995, 117, 12891-12892. (g) Funaki, H.; Aramaki,
K.; Nishihara, H. Synth. Met. 1995, 74, 59-64. (h) Nguyen, M. T.; Diaz,
A. F.; Dement’ev, V. V.; Pannel, K. H. Chem. Mater. 1993, 5, 1389-
1394. (i) Wright, M. E.; Sigman, M. S. Macromolecules 1992, 25, 6055-
6058.
(2) (a) Guha, S.; Frazier, C. C.; Porter, P. L.; Kang, K.; Finberg, S.
E. Opt. Lett. 1989, 17(14), 952. (b) Wright, M. E.; Toplikan, E. G.
Macromolecules 1992, 25, 6050-6054. (c) Yuan, Z.; Stringer, G.; J obe,
I. R.; Kreller, D.; Scott, K.; Koch, L.; Taylor, N.; Marder, T. B. J .
Organomet. Chem. 1993, 452, 115-120. (d) Myers, L. K.; Langhoff,
C.; Thompson, M. E. J . Am. Chem. Soc. 1992, 114, 7560.
(5) (a) Schock, L. E.; Marks, T. J . J . Am. Chem. Soc. 1988, 110,
7781-7795. (b) Reid, A. F.; Shannon, J . S.; Swan, J . M.; Wailes, P. C.
Aust. J . Chem. 1965, 18, 173-181.
(6) (a) Manriquez, J . M.; McAlister, D. R.; Rosenberg, E.; Shiller,
A. M.; Williamson, K. L.; Chan, S. I.; Bercaw, J . E. J . Am. Chem. Soc.
1978, 100, 3078-3083. (b) Hillhouse, G. L.; Bercaw, J . E. J . Am. Chem.
Soc. 1985, 106, 5474.
(3) (a) LeNarvor, N.; Lapinte, C. Organometallics 1995, 14, 634-
639. (b) Manriquez, J . M.; Ward, M. D.; Reiff, W. M.; Calabrese, J . C.;
J ones, N. L.; Carrol, P. J .; Bunel, E. E.; Miller, J . S. J . Am. Chem.
Soc. 1995, 117, 6182-6193.
(4) (a) Ijadi-Maghsoodi, S.; Barton, T. J . Macromolecules 1990,
23(20), 4485. (b) Ijadi-Maghsoodi, S.; Pang, Y.; Barton, T. J . J . Polym.
Sci., Part A: Polym. Chem. 1990, 28, 955. (c) Corriu, R.; Devylder, N.;
Guerin, C.; Henner, B.; J ean, A. Organometallics 1994, 13, 3194-3202.
(d) Bunz, U. H. F.; Enkelmann, V.; Beer, F. Organometallics 1995,
14, 2490. (e) Altman, M.; Enkelmann, V.; Lieser, G.; Bunz, U. H. F.
Adv. Mater. 1995, 8(7), 726.
(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).
S0276-7333(96)00590-0 CCC: $12.00 © 1996 American Chemical Society

4668 Organometallics, Vol. 15, No. 22, 1996
Communications
Sch em e 2
yellow, moderately air-stable solid that is readily soluble
in ordinary organic solvents.
In order to determine if the terminal alkyne hydrogen
atoms in 2 could be replaced with metals, a reaction of
2 with trimethyltin chloride in the presence of LDA was
attempted. The addition of an LDA solution to a
solution of 2 and Me3SnCl in toluene at -78 °C gave
compound 3 (Scheme 1) as a moderately air stable, off-
white solid in 60% yield. This is the first example of a
formation of transient acetylide anion bonded to an
early transition metal.8,9
F igu r e 1. PLUTO drawing of 4. Selected bond distances
(Å) and angles (deg): Hf(1)-C(21) ) 2.17(2), Hf(1)-C(23)
) 2.25(2), C(21)-C(22) ) 1.25(3), C(23)-C(24) ) 1.24(3),
Hf(1)-(Cp* centroids) ) 2.228 and 2.241; C(21)-Hf(1)-
C(23) ) 94.9(10), C(22)-C(21)-Hf(1) ) 173(3), C(24)-
C(23)-Hf(1) ) 171(2).
There is a nearly right angle between the two acetylene
groups bonded to a hafnium atom: C(21)-Hf(1)-C(23)
) 94.9°.11
When 1 was allowed to react with excess sodium
acetylide in refluxing THF, 2 was observed to form
initially by 1H NMR, but its signals slowly disappeared
as two new peaks were seen to grow in slightly down-
field positions. The mixture was heated to reflux for 3
days, after which time the solvent was removed, and
the residue was dissolved in hexane and this solution
chromatographed on a silica gel column, affording 4
(Scheme 1) in 50% yield. Compound 4 is a light yellow
solid that is stable in air and in solution. Analytical
grade, diamond-shaped crystals were grown from hex-
ane by slow evaporation. The FT-IR spectrum exhibits
two absorptions at 1938 and 3280 cm-1, associated with
alkyne CC and CH stretching modes, respectively.
Compounds 2 and 4 are the first examples of an early-
transition-metal metallocene with a terminal σ-bound
(-CCH) acetylide ligand;12 the synthesis of 3 represents
the first evidence for the formation of a terminal
acetylide anion bound to an early transition metal.9 All
three compounds are potentially useful organometallic
polymer precursors via oxidative coupling of the termi-
nal alkyne groups or by Stille coupling through the
alkynyltrimethyltin ligand.13,14
Ack n ow led gm en t. We thank the donors of the
Petroleum Research Fund, administered by the Ameri-
can Chemical Society, for support of this research.
Compound 4 exhibits fluxional behavior in the 1H
NMR spectrum: the acetylenic protons appear at 3.04
ppm, and two inequivalent Cp* resonances are at 2.09
and 2.14 ppm, respectively, at room temperature. The
two Cp* signals coalesce at 105 °C, and cooling the
sample to room temperature resulted in the restoration
of the original spectrum. These data and molecular
modeling suggest that the dimer belongs to the chiral
C2 point group. The two sets of Cp* groups are assigned
to R-sites, adjacent to the acetylide group on the
neighboring hafnium atom, and â-sites that face away
(Scheme 2). The R- and â-sites exchange roles as the
ends of the molecule rotate about the Hf-C2-Hf axis,
as this motion interconverts the enantiomers of 4. The
barrier to rotation, 19.5 kcal/mol, was estimated from
the coalescence temperature and the separation of the
Cp* peaks in the slow exchange limit.10
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal-
lographic data, atomic positional parameters, thermal param-
eters, and bond distances and angles and an ORTEP drawing
for 4 and text giving details of the preparation of Hf*(CCH)2,
Hf*(CCSnMe3)2, and [(µ2-C2)(Hf*CCH)2] (14 pages). Tables of
observed and calculated structure factors are available from
the authors upon request. Ordering information is given on
any current masthead page.
OM9605903
(11) Crystals suitable for diffractometry were grown by allowing a
concentrated solution of hexane to stand at room temperature for
several days. Crystallographic data for 4 are as follows: Space group:
P1h (No. 2), a ) 9.927(6) Å, b ) 14.133(5) Å, c ) 15.488(9) Å, R ) 71.75-
(4)°, â ) 88.04(5)°, γ ) 88.63(4)°, V ) 2062(2) Å3, Z ) 2. The Cp* groups
on Hf2 are disordered. The disorder was modeled by placing two rigid
Cp* groups at variable distances in two slightly different orientations,
each at 50% occupancy, and refining with common isotropic thermal
parameters for all the Cp* carbon atoms. The Cp* groups are rotated
slightly with respect to one another, and their centroids are displaced
approximately 0.21 Å from each other. The Hf-ring centroid distances
are 2.228 and 2.241 Å for Hf(1) and 2.274, 2.278, 2.290, and 2.317 Å
for Hf(2).
(12) Substituted acetylides of early transition metals are known,
and Bercaw et al. have spectroscopically identified a terminal Sc-
acetylide at -80 °C: (a) St. Clair, M.; Bercaw, J . Organometallics 1991,
10, 525-527. (b) Lang, H.; Seyferth, D. Z. Naturforsch., B: Chem. Sci.
1990, 45(2), 212-220. (c) Lang, H.; Seyferth, D. Appl. Organomet.
Chem. 1990, 4(6), 599-606. (d) Lang, H.; Zsolnai, L. J . Organomet.
Chem. 1991, 406(1-2), C5-C8.
Crystals of 4 suitable for X-ray diffractometry were
grown by allowing a concentrated solution of hexane to
stand at room temperature for several days. The
PLUTO of 4 (Figure 1) features nearly linear Hf(1)-
C(23)-C(24) (171°) and C(23)-C(24)-Hf(2) linkages
(172°) between the two hafnium atoms and the acety-
lene spacer. The C(23)-C(24) bond length is 1.24 Å.
(8) Akita, M.; Moro-aka, Y. Bull. Chem. Soc. J pn. 1995, 68, 420.
(9) Brady, M.; Weng, W.; Gladysz, J . A. J . Chem. Soc., Chem.
Commun. 1994, 2655.
(10) Sandstrom, J . Dynamic Nuclear Magnetic Resonance Spectros-
copy; Academic Press: London, New York, 1982.
(13) Takahashi, S.; Murata, E.; Sonagashira, K.; Hagihara, N. J .
Polym. Sci., Polym. Chem. Ed. 1980, 18, 661-669.
(14) Khan, M. S.; Davies, S. J .; Kakkar, A. K.; Schwartz, D.; Lin,
B.; J ohnson, B. F. G.; Lewis, J . J . Organomet. Chem. 1992, 424, 87-
89.