Complexation of bis(trimethylsilyl)acetylene by decamethylhafnocene to give the hafnacyclopropene Cp*2Hf(η2-Me 3SiC2SiMe3): An unusually strong metal-alkyne interaction

Article abstract of DOI:10.1021/om0609259

The complexes Cp₂Hf(PMe₃)(η²-Me ₃SiC₂SiMe₃) (3) and Cp* ₂Hf(η²-Me₃SiC₂SiMe₃) (4), as the first examples of well-defined hafnium alkyne complexes with a simple intact alkyne, are described; 4 displays an unusually strong interaction of the alkyne with hafnium, in comparison to analogous compounds of titanium and zirconium.

Full text of DOI:10.1021/om0609259

© Copyright 2007
Volume 26, Number 2, January 15, 2007
American Chemical Society
Communications
Complexation of Bis(trimethylsilyl)acetylene by
Decamethylhafnocene To Give the Hafnacyclopropene
Cp*2Hf(η2-Me3SiC2SiMe3): An Unusually Strong Metal-Alkyne
Interaction†
Torsten Beweries, Vladimir V. Burlakov,‡ Marc A. Bach, Perdita Arndt,
Wolfgang Baumann, Anke Spannenberg, and Uwe Rosenthal*
Leibniz-Institut fu¨r Katalyse e. V. an der UniVersita¨t Rostock, Albert-Einstein-Strasse 29a,
D-18059 Rostock, Germany
ReceiVed October 9, 2006
Summary: The complexes Cp2Hf(PMe3)(η2-Me3SiC2SiMe3) (3)
and Cp*2Hf(η2-Me3SiC2SiMe3) (4), as the first examples of well-
defined hafnium alkyne complexes with a simple intact alkyne,
are described; 4 displays an unusually strong interaction of
the alkyne with hafnium, in comparison to analogous compounds
of titanium and zirconium.
than Zr-C bonds.2 Very recently the impressive consequences
of such high reactivity were shown by the Chirik group, who
reported the functionalization of molecular nitrogen by a
hafnocene complex. In contrast, the analogous zirconocene
compound did not functionalize N2.3
To the best of our knowledge, only two well-defined hafnium
alkyne complexes have been found, but neither of them bears
the starting intact acetylene (Me3SiCtCSiMe3 in the case of
2) as the ligand (Chart 1).
Complex 1 was reported by the Erker group, resulting from
the reaction of dipropynylhafnocene Cp2Hf(CtCMe)2 with
B(C6F5)3.4 Compound 2 was obtained by Shur and co-workers
as a product of the reaction of the hydride Cp2HfH2 with Me3-
SiCtCSiMe3.5
We report here the synthesis and characterization of the first
hafnium complexes containing the original alkyne reactand, Cp2-
Hf(PMe3)(η2-Me3SiC2SiMe3) (3)6 (Scheme 1) and Cp*2Hf(η2-
Me3SiC2SiMe3) (4)7 (Scheme 2).
In group 4 chemistry the complexation of bis(trimethylsilyl)-
acetylene by metallocenes leads to the metallacyclopropenes
Cp′2M(η2-Me3SiC2SiMe3) (Cp′ ) substituted or unsubstituted
η5-cyclopentadienyl). Such titanium and zirconium compounds
show a very broad chemistry.1 Surprisingly, all attempts to
obtain analogous well-defined hafnium complexes have failed
so far. The reason for this is not obvious but might be found in
the enhanced reactivity of hafnium compounds compared to
those of zirconium. Erker et al. described in case of (s-cis-η4-
diene)metallocene complexes the σ to π ratio of the diene
bonding to be shifted to a larger σ character for hafnium
compared to zirconium, giving Hf-C bonds that are shorter
(2) (a) Kru¨ger, C.; Mu¨ller, G.; Erker, G.; Dorf, U.; Engel, K. Organo-
metallics 1985, 4, 215. (b) Erker, G.; Kru¨ger, C.; Mu¨ller, G. AdV.
Organomet. Chem. 1985, 24, 1.
(3) Bernskoetter, W. H.; Olmos, A. V.; Pool, J. A.; Lobkovsky, E.; Chirik,
P. J. J. Am. Chem. Soc. 2006, 128, 10696.
(4) Ahlers, M.; Temme, B.; Erker, G.; Fro¨hlich, R.; Fox, T. J. Organomet.
Chem. 1997, 527, 191.
(5) Strunkina, L. I.; Minacheva, M. Kh.; Petrovskii, P. V.; Klemenkova,
Z. S.; Lokshin, B. V.; Burlakov, V. V.; Shur, V. B. Russ. Chem. Bull. 1997,
46, 820.
* To whom correspondence should be addressed. Tel: ++49-381-1281-
176. Fax: ++49-381-1281-51176. E-mail: uwe.rosenthal@catalysis.de.
† This work is dedicated to Professor Gerhard Erker on the occasion of
his 60th birthday.
‡ On leave from the A. N. Nesmeyanov Institute of Organoelement
Compounds, Russian Academy of Sciences, Vavilov St. 28, 117813,
Moscow, Russia.
(1) Rosenthal, U.; Burlakov, V. V. In Titanium and Zirconium in Organic
Synthesis; Marek, I., Ed.; Wiley-VCH: Weinheim, Germany, 2002; p 355.
10.1021/om0609259 CCC: $37.00 © 2007 American Chemical Society
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248 Organometallics, Vol. 26, No. 2, 2007
Communications
Cp*2Hf(η2-Me3SiC2SiMe3) (4) (Scheme 2). Obviously it was
the presence of THF that caused the problems encountered in
the attempted synthesis of the hafnium complexes using
magnesium in THF.
Chart 1. Known Hafnium Alkyne Complexes 1 and 2
Scheme 2. Formation of Complex 4
Scheme 1. Formation of the Cp Complexes 2 (by a New
Method) and 3
In contrast, in the synthesis of the analogous complexes Cp*2-
Ti(η2-Me3SiC2SiMe3) and Cp*2Zr(η2-Me3SiC2SiMe3) the THF
apparently acts as a stabilizing agent during the complex
formation reaction. All complexes of this type were synthesized
in THF.1 Additionally, THF stabilizes the resulting alkyne
complexes: e.g., in Cp2Zr(THF)(η2-Me3SiC2SiMe3). If the
interaction of hafnocene with THF is stronger, this could lead
to other reaction pathways and products.
Scheme 3. Ring-Opening Reaction of THF
Our first experiments showed that the method used for the
successful preparation of similar titanium and zirconium com-
plexes failed in the attempted synthesis of a hafnocene alkyne
complex. The reaction of Cp2HfCl2 with Me3SiCtCSiMe3 using
magnesium as the reducing agent in THF gave a mixture of
products which we could not identify. When this reaction was
conducted in the presence of PMe3, complex 3 was formed in
51% yield. When metallic lithium was used as the reducing
agent in toluene without added phosphine, complex 2, which
was identical to Shur’s product,5 was obtained by a new method
(Scheme 1) (see the Supporting Information).
To investigate details of the THF reactions with hafnocene,
the reactions of Cp*2HfCl2 with magnesium in THF with and
without Me3SiCtCSiMe3 were performed. Without the alkyne
a ring-opening reaction of THF occurred (Scheme 3), giving
the dinuclear complex Cp*2Hf(Cl)OCH2CH2CH2CH2(Cl)-
HfCp*2 (5). This product was only formed in the presence of
magnesium; heating of Cp*2HfCl2 in THF gave no reaction at
all. In benzene solution 5 underwent disproportionation to Cp*2-
HfCl2 and the 1-hafna-2-oxacyclohexane 6, as indicated by
NMR. This complex is very similar to the analogous 1-zircona-
2-oxacyclohexane which had been prepared earlier by another
procedure.8 The compounds 5 and 6 indicate clearly that during
the reaction with magnesium in THF ring opening instead of
alkyne complexation takes place. This is the reason why the
synthesis failed and lithium in toluene is preferred.
Complex 3 is highly sensitive toward oxygen and moisture.
One can consider 3 as a hafnacyclopropene, as can be seen from
its IR spectrum, which shows ν(CtC) at 1551 cm-1. The alkyne
is coordinated unsymmetrically, showing two singlets in the 1H
NMR at 0.32 and 0.52 ppm for the SiMe3 protons and two
doublets in the 13C NMR at 182.6 and 212.8 ppm. In the crystal
structure of 3 (Figure 1) the distance between the carbon atoms
of the complexed alkyne (1.311(4) Å) is in the range of a double
bond and two different C-Hf distances for the alkyne ligand
(2.198(3) and 2.277(3) Å) were observed.
However, when decamethylhafnocene dichloride, Cp*2HfCl2,
was used, reaction with Me3SiCtCSiMe3 and lithium in toluene
solvent resulted in formation of the desired type of complex,
(6) Preparation of Cp2Hf(PMe3)(Me3SiCtCSiMe3) (3): to a suspension
of Cp2HfCl2 (3.0 g, 7.90 mmol) and Mg turnings (0.13 g, 8.02 g atom) in
50 mL of THF were added at room temperature bis(trimethylsilyl)acetylene
(1.85 mL, 8.19 mmol) and PMe3 (8.0 mL of a 1.0 M solution in THF, 8.00
mmol). The colorless mixture became deep red as it was stirred for 16 h.
All volatiles were removed under vacuum, and the residue was extracted
with 4 × 15 mL of n-hexane. The solvent was again removed from the
obtained solutions, giving the red crude product material, which was
recrystallized from n-hexane/THF to give yellow prisms of 3: yield 2.24 g
(51%); mp 118 °C dec under Ar. Anal. Calcd for C21H37HfPSi2 (555.15):
C, 45.43; H, 6.72. Found: C, 40.27; H, 5.84 (because of partial dissociation
of PMe3, better data could not be obtained). IR (Nujol mull, cm-1): 1240
(SiMe3), 1551 (CtC). NMR (400 MHz, 298 K, C6D6): 1H, δ 0.32 (s, 9 H,
SiMe3), 0.52 (s, 9 H, SiMe3), 1.12 (d, JP,H ) 5.8 Hz, 9 H, PMe3), 5.07 (d,
JP,H ) 1.7 Hz, 10 H, Cp); 13C, δ 3.1 (s, SiMe3), 3.4 (s, SiMe3), 19.2 (d,
JP,C ) 18 Hz, PMe3), 101.8 (s, Cp), 182.6 (d, JP,C ) 8 Hz, CtC), 212.8 (d,
JP,C ) 5 Hz, CtC); 31P, δ -8.9 (s, PMe3). MS (70 eV, m/z): 480 [Cp2-
Hf(Me3SiCtCSiMe3)]+, 310 [Cp2Hf]+.
(7) Preparation of 4: a suspension of Cp*2HfCl2 (1.773 g, 3.41 g atom),
finely sliced lithium wire (0.103 g, 14.8 mmol), and bis(trimethylsilyl)-
acetylene (0.80 mL, 3.56 mmol) in 15 mL of toluene was stirred for 10
days at 60 °C. The resulting dark blue solution was filtered and evaporated
to dryness under vacuum. The residue was extracted with 15 mL of n-hexane
at 55 °C. The resulting blue solution was filtered and concentrated under
vacuum to a total volume of 10 mL. After the concentrated solution stood
for 24 h at -78 °C, dark blue crystals formed, which were separated, washed
with cold n-hexane, and dried under vacuum to give complex 4: yield 0.951
g (45%); mp 242-243 °C under Ar. Anal. Calcd for C28H48HfSi2: C, 54.30;
H, 7.81. Found: C, 54.54; H, 7.97. IR (Nujol mull, cm-1): 1470 (ν(Ct
C)). NMR (400 MHz, 298 K, C6D6): 1H, δ 0.26 (s, 18H, SiMe3), 1.81 (s,
30H, Cp*); 13C, δ 4.7 (SiMe3), 11.6 (C5Me5), 118.0 (C5Me5), 283.4 (Ct
C). MS (70 eV, m/z): 620 [M]+.
In complex 4 the alkyne is coordinated symmetrically to the
metal. The most important property of this compound is the
strong interaction of the alkyne with the hafnium, which can
(8) Mashima, K.; Yamakawa, M.; Takaya, H. J. Chem. Soc., Dalton
Trans. 1991, 2851.

Communications
Organometallics, Vol. 26, No. 2, 2007 249
atoms of the complexed alkyne, 283.4 ppm, is shifted far
downfield compared to those for the analogous titanium and
zirconium complexes Cp*2M(η2-Me3SiC2SiMe3) (Ti, 248.5
ppm;9 Zr, 260.5 ppm10). The molecular structure (Figure 2) of
complex 4 shows the two Cp* ligands and the alkyne coordi-
nated at the hafnium center.
The distance of 1.337(4) Å between the alkyne carbon atoms
is in the range of a double bond and also clarifies the
aforementioned tendency of group 4 alkyne complexes, in which
the electron density of the π system is reduced (Ti, 1.309(4)
Å;9 Zr, 1.320(3)10 Å) compared to the free alkyne (C1-C2 )
1.208 Å).11 For these complexes the trend of M-C(alkyne) bond
distances (Ti-C ) 2.122(3)/2.126(3) Å; Zr-C ) 2.216(2)/
2.221(2) Å; Hf-C ) 2.188(3)/2.184(3) Å) is the same as that
found in the complexes Cp2MMe2 (Ti-C ) 2.181(2)/2.170(2)
Å;12 Zr-C ) 2.280(5)/2.273(5) Å;13 Hf-C ) 2.240(12)/2.233-
(12) Å13).
This approach, resulting from the comparison of the spec-
troscopic and structural data, also should be reflected in the
chemical reactivity of these complexes. For example, in the
series of complexes Cp*2M(η2-Me3SiC2SiMe3) the titanium
species did not react with boiling acetone, whereas acetone
inserted into the ZrC2 ring of the zirconium complex easily at
room temperature.1 On reaction with carbon dioxide, the alkyne
was completely displaced from the titanium complex, whereas
in the case of zirconium CO2 insertion into the M-C bond of
the ZrC2 ring was observed.1 On this basis interesting new
insertion reactions are expected for the hafnium complex. These
will be the subject of further detailed investigations.
Figure 1. Molecular structure of complex 3. Hydrogen atoms are
omitted for clarity. The thermal ellipsoids correspond to 30%
probability. Selected bond lengths (Å) and angles (deg): C1-C2
) 1.311(4), Hf1-C1 ) 2.198(3), Hf1-C2 ) 2.277(3), C1-Si1 )
1.850(3), C2-Si2 ) 1.848(3), Hf1-P1 ) 2.660(1); C1-Hf1-C2
) 34.02(9), C1-C2-Si2 ) 126.8(2), C2-C1-Si1 ) 143.5(2).
Acknowledgment. We thank our technical stafff, in par-
ticular Regina Jesse and Petra Bartels, for assistance. Support
by the Deutsche Forschungsgemeinschaft (SPP 1118, Project
No. Ro 1269/6) and the Russian Foundation for Basic Research
(Project code 05-03-32515) is acknowledged.
Supporting Information Available: CIF files giving crystal-
lographic data, including bond lengths and angles, of compounds
3 and 4 and text giving experimental details for compounds 2, 5,
and 6. This material is available free of charge via the Internet at
http://pubs.acs.org.
Figure 2. Molecular structure of complex 4. Hydrogen atoms are
omitted for clarity. The thermal ellipsoids correspond to 30%
probability. Selected bond lengths (Å) and angles (deg): C1-C2
) 1.337(4), Hf1-C1 ) 2.188(3), Hf1-C2 ) 2.184(3), C1-Si1 )
1.860(3), C2-Si2 ) 1.862(3); C1-Hf1-C2 ) 35.60(11), C1-
C2-Si2 ) 134.5(3), C2-C1-Si1 ) 134.0(3).
OM0609259
(9) Burlakov, V. V.; Polyakov, A. V.; Yanovsky, A. I.; Struchkov, Y.
T.; Shur, V. B.; Vol’pin, M. E.; Rosenthal, U.; Go¨rls, H. J. Organomet.
Chem. 1994, 476, 197.
(10) Hiller, J.; Thewalt, U.; Polasek, M.; Petrusova, L.; Varga, V.;
Sedmera, P.; Mach, K. Organometallics 1996, 15, 3752.
(11) Bruckmann, J.; Kru¨ger, C. Acta Crystallogr., Sect. C: Cryst. Struct.
Commun. 1997, 53, 1845.
(12) Thewalt, U.; Worthle, T. J. Organomet. Chem. 1994, 464, C17.
(13) Hunter, W. E.; Hrncir, D. C.; Vann Bynum, R.; Penttila, R. A.;
Atwood, J. L. Organometallics 1983, 2, 750.
be derived from the spectroscopic and structural data. The
observed ν(CtC) value of 1470 cm-1 for complex 4 is
indicative of strong complexation of the alkyne to the metal
center, resulting in the hafnacyclopropene structure (ν(CtC)
for the Ti analogue, 1563/1596 cm-1 9
;
ν(CtC) for the Zr
analogue, 1516 cm-1 10). The 13C NMR signal for the carbon