Reactions of the five-membered hafnacyclocumulene Cp2Hf(- 4- t -Bu-C4- t- Bu) with the lewis acids tris(pentafluorophenyl)borane and diisobutylaluminum hydride

Article abstract of DOI:10.1021/om100208g

The previously described hafnacyclocumulene Cp₂Hf(- ⁴-t-Bu-C₄-t-Bu) (1) reacts with B(C₆F ₅)₃ to give the dinuclear complex 2, which consists of a cationic part with two hafnocene centers bridged unsymmetrically by butadiyne and acetylide moieties and an alkynyl boranate representing the anionic part. In the reaction of 1 with i-Bu₂AlH the central C - C bond of the hafnacyclocumulene ring is weakened, resulting in the formation of an intermediate zwitterion, which then forms the hafnium bis-acetylide Cp ₂Hf(C - C-t-Bu)₂. Subsequent hydroalumination of one of the C - C bonds with i-Bu₂AlH yields the final complex Cp ₂Hf(C - C-t-Bu)[C(Al-i-Bu₂)=CH(t-Bu)] (3). Reaction of the titanacyclocumulene 4 with catalytic amounts of B(C₆F ₅)₃ results in the formation of the previously described dinuclear complex 5, which then undergoes further reaction with B(C ₆F₅)₃ to give the known zwitterionic Ti(III) species 6 and the ene-yne t-BuCH - CHC - C-t-Bu.

Full text of DOI:10.1021/om100208g

Organometallics 2010, 29, 2367–2371 2367
DOI: 10.1021/om100208g
Reactions of the Five-Membered Hafnacyclocumulene
Cp2Hf(η4-t-Bu-C4-t-Bu) with the Lewis Acids
Tris(pentafluorophenyl)borane and Diisobutylaluminum Hydride
Vladimir V. Burlakov,† Torsten Beweries,‡ Katharina Kaleta,‡ Vyacheslav S. Bogdanov,†
Perdita Arndt,‡ Wolfgang Baumann,‡ Anke Spannenberg,‡ Vladimir B. Shur,† and
Uwe Rosenthal*,‡
†A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov St. 28,
‡
119991 Moscow, Russia, and Leibniz-Institut fu€r Katalyse e. V. an der Universitat Rostock,
Albert-Einstein-Str. 29a, D-18059 Rostock, Germany
€
Received March 18, 2010
The previously described hafnacyclocumulene Cp2Hf(η4-t-Bu-C4-t-Bu) (1) reacts with B(C6F5)3 to
give the dinuclear complex 2, which consists of a cationic part with two hafnocene centers bridged
unsymmetrically by butadiyne and acetylide moieties and an alkynyl boranate representing the anionic
part. In the reaction of 1 with i-Bu2AlH the central CdC bond of the hafnacyclocumulene ring is
weakened, resulting in the formation of an intermediate zwitterion, which then forms the hafnium bis-
acetylide Cp2Hf(CtC-t-Bu)2. Subsequent hydroalumination of one of the CtC bonds with i-Bu2AlH
yields the final complex Cp2Hf(CtC-t-Bu)[C(Al-i-Bu2)=CH(t-Bu)] (3). Reaction of the titanacyclo-
cumulene 4 with catalytic amounts of B(C6F5)3 results in the formation of the previously described
dinuclear complex 5, which then undergoes further reaction with B(C6F5)3 to give the known
zwitterionic Ti(III) species 6 and the ene-yne t-BuCHdCHCtC-t-Bu.
Scheme 1. Formation of the Hafnacyclocumulene 1
Introduction
The investigation of hafnocene complexes in comparison to
the corresponding titanocene and zirconocene complexes with
respect totheir spectroscopicand structural properties and the
application of such compounds in unusual bond activation
reactions has been a topic of current interest in group 4
organometallic chemistry.1 For example, in our group, the
first hafnocene alkyne complexes of the type Cp02Hf(L)(η2-
RC2SiMe3) (Cp0 = substituted or unsubstituted cyclopenta-
dienyl; R = SiMe3, Ph; L = PMe3) have been synthesized and
investigated.2 During the formation of Cp*2Hf(η2-Me3Si-
C2SiMe3) unprecedented C-H and Si-C bond activations
occurred which have not been observed for the titanium
and zirconium analogues.3 Very recently we found that the
reaction of di-n-butylhafnocene with di-tert-butylbutadiyne
yields the first structurally characterized hafnacyclocumulene,
Cp2Hf(η4-t-Bu-C4-t-Bu) (1) (Scheme 1).4 Most remarkably,
and in contrast to Negishi’s Zr reagent,5 the hafnocene source
Cp2Hf(n-Bu)2 can be isolated and does not have to be
prepared freshly prior to its application.
The corresponding titanium and zirconium complexes of the
type Cp2M(η4-t-Bu-C4-t-Bu) were the first examples of the same
rather unusual structural motif of five-membered metallacyclo-
cumulenes (i.e., 1-metallacyclopenta-2,3,4-trienes).6,7 For the
corresponding complexes with pentamethylcyclopenta-
dienyl ligands and different substituents Cp*2Zr(η4-RC4R)
*To whom correspondence should be addressed. Tel: þþ49-381-1281-
176. Fax: þþ49-381-1281-51176. E-mail: uwe.rosenthal@catalysis.de.
(1) Examples: (a) Bernskoetter, W. H.; Olmos, A. V.; Pool, J. A.;
Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128, 10696.
(b) Bernskoetter, W. H.; Lobkovsky, E.; Chirik, P. J. Angew. Chem., Int.
Ed. 2007, 46, 2858. (c) Beweries, T.; Fischer, C.; Peitz, S.; Burlakov, V. V.;
Arndt, P.; Baumann, W.; Spannenberg, A.; Heller, D.; Rosenthal, U. J. Am.
Chem. Soc. 2009, 131, 4463. (d) Knobloch, D. J.; Benito-Garagorri, D.;
Bernskoetter, W.; Keresztes, I.; Lobkovsky, E.; Toomey, H.; Chirik, P. J.
J. Am. Chem. Soc. 2009, 131, 14903. (e) Zirngast, M.; Flock, M.; Baumgartner,
J.; Marschner, C. J. Am. Chem. Soc. 2009, 131, 15952.
(2) (a) Beweries, T.; Burlakov, V. V.; Bach, M. A.; Arndt, P.;
Baumann, W.; Spannenberg, A.; Rosenthal, U. Organometallics 2007,
26, 247. (b) Beweries, T.; Burlakov, V. V.; Peitz, S.; Arndt, P.; Baumann, W.;
Spannenberg, A.; Rosenthal, U. Organometallics 2008, 27, 3954.
(3) Beweries, T.; Burlakov, V. V.; Bach, M. A.; Peitz, S.; Arndt, P.;
Baumann, W.; Spannenberg, A.; Rosenthal, U.; Pathak, B.; Jemmis,
E. D. Angew. Chem., Int. Ed. 2007, 46, 6907.
(4) (a) Burlakov, V. V.; Bogdanov, V. S.; Lyssenko, K. A.; Petrovskii,
P. V.; Beweries, T.; Arndt, P.; Rosenthal, U.; Shur, V. B. Russ. Chem.
Bull. Int. Ed. 2008, 57, 1319. (b) Burlakov, V. V.; Beweries, T.; Bogdanov,
V. S.; Arndt, P.; Baumann, W.; Spannenberg, A.; Lyssenko, K. A.; Shur, V. B.;
Rosenthal, U. Organometallics 2009, 28, 2864.
(5) (a) Negishi, E.; Cederbaum, F. E.; Takahashi, T. Tetrahedron
Lett. 1986, 27, 2829. (b) Negishi, E.; Takahashi, T. Bull. Chem. Soc. Jpn.
1998, 71, 755. (c) Negishi, E.; Hou, S. In Titanium and Zirconium in
Organic Synthesis; Marek, I., Ed.; Wiley-VCH: Weinheim, Germany, 2002;
p 1 and references cited therein.
(6) Burlakov, V. V.; Ohff, A.; Lefeber, C.; Tillack, A.; Baumann, W.;
Kempe, R.; Rosenthal, U. Chem. Ber. 1995, 128, 967.
(7) Rosenthal, U.; Ohff, A.; Baumann, W.; Kempe, R.; Tillack, A.;
Burlakov, V. V. Angew. Chem., Int. Ed. Engl. 1994, 33, 1605.
r
2010 American Chemical Society
Published on Web 04/01/2010
pubs.acs.org/Organometallics

2368 Organometallics, Vol. 29, No. 10, 2010
Scheme 2. Reaction of the Hafnacumulene 1 with B(C6F5)3
Burlakov et al.
(R = Me3Si, Me, Ph) the reactivity toward the Lewis acids
B(C6F5)38 and i-Bu2AlH9 and the behavior of the products in
the polymerization of ethylene and ε-caprolactone were inves-
tigated in detail.9,10 A very strong influence of the substituents
on the reaction products was found; for the smaller titanium
such complexes Cp*2M(η4-RC4R) (R = Me3Si, Me, Ph) do
not exist and, therefore, a comparison of the influence of the
metals is not possible. However, having a complete series of Cp-
substituted complexes of the type Cp2M(η4-t-Bu-C4-t-Bu)
(M = Ti, Zr, Hf) in hand, an investigation of the influence of
the different metals on the products of the reactions with the
Lewis acids B(C6F5)3 and i-Bu2AlH is now possible.
Results and Discussion
The hafnacyclocumulene Cp2Hf(η4-t-Bu-C4t-Bu) (1) reacts
with B(C6F5)3 in toluene to give the cationic complex 2, which
was obtained as yellow crystals in 62% yield (Scheme 2).
In the dinuclear complex 2 the cation consists of two hafno-
cenes which are bridged unsymmetrically by a butadiyne and an
acetylide ligand, whereas the anion is represented by an alkynyl
boranate. A similar zirconocene complex, which was formed by
reaction of Cp2Zr(CtCMe)2 with [Ph3C]þ[BPh4]-, was pre-
viously reported by Erker et al., who found a C-C coupling of
the σ-propynyl ligands at zirconium and the formation of a
bridging μ-2,4-hexadiyne ligand.11 In addition to the asymmetric
complexation of the hexadiyne, which was found in the solid
state, an interpretation of this species as a complex with a sym-
metric η2:η2 coordination of the diyne ligand is also possible.
Figure 1. Molecular structure of the cation of complex 2. Thether-
mal ellipsoids correspond to 30% probability. Hydrogen atoms are
˚
omitted for clarity. Selected bond lengths (A) and angles (deg):
C1-C2=1.333(5), C2-C3=1.433(5), C3-C4=1.206(5), Hf1-
C1=2.166(3), Hf1-C2=2.420(3), Hf2-C2=2.375(3), Hf2-C3=
2.478(3), Hf1-C5 = 2.413(4), Hf2-C5 = 2.278(4); Hf1-C5-
Hf2 = 96.68(13), Hf2-C2-Hf1 = 93.98(11).
The molecular structure and NMR behavior of this hafno-
cene complex 2 resemble those of the zirconocene complex.11
The complex appears to have C2v symmetry at ambient tem-
perature in solution, as its spectra display one single resonance
due to the four Cp ligands and two due to the tert-butyl groups
(ratio 2:1). The coordinated carbon atoms of the 1,3-butadiyne
are not observable directly, due to exchange broadening, but
can be detected indirectly (1H-13C HMBC experiment). The
average 13C NMR shifts of 198.3 ppm (C1/C4, cf. Figure 1) and
89.2 ppm (C2/C3) indicate a strong complexation of the
butadiyne between the two hafnocene centers with a preserved
C2-C3 bond; this chemical shift pattern is similar to that found
for complex 5(see refs 17a,17c for further discussion). Scheme 3
shows the exchange of binding modes of the butadiyne to the
hafnocene centers which is responsible for the apparent sym-
metry of the cation, as has been already described11,14 for the
aforementioned [(Cp2Zr)2(μ-MeC2C2Me)(μ-C2Me)]þ. The
NMR data for the anion agree well with the known boranate
[Ph3B-CtC-t-Bu]NMe4;15 however, the 13C NMR signals for
the perfluorophenyl rings and the carbon adjacent to the boron
€
However, Erker and Rottger found for similar structures bear-
ing planar tetracoordinated carbon atoms (i.e., asymmetric
coordination of the ligand) that this structural motif is energeti-
cally more favored than the corresponding symmetric binding
mode.12
The mechanism of the formation of this complex is unclear
and requires a special study. One may suggest that the process
proceeds stepwise via zwitterionic intermediates. Most likely, in
the last step of the reaction the resulting bis(acetylide)hafno-
cene complex Cp2Hf(CtC-t-Bu)2 reacts with the cationic
fragment [Cp2Hf-CtC-t-Bu]þ to give the product cation; this
possibility was also considered by Erker et al. for the formation
of the corresponding zirconocene complex.11,13
(8) Burlakov, V. V.; Arndt, P.; Baumann, W.; Spannenberg, A.;
Rosenthal, U. Organometallics 2004, 23, 5188.
(9) Burlakov, V. V.; Arndt, P.; Baumann, W.; Spannenberg, A.;
Rosenthal, U. Organometallics 2004, 23, 4160.
(10) Burlakov, V. V.; Bach, M. A.; Klahn, M.; Arndt, P.; Baumann,
W.; Spannenberg, A.; Rosenthal, U. Macromol. Symp. 2006, 236, 48.
(14) Unfortunately, NMR studies at variable temperature in order to
freeze out the process were not possible due to the low solubility of
complex 2 in toluene-d8, which caused precipitation at low temperatures.
The use of more polar solvents such as THF-d8 resulted in reaction with
the solvent to form a polymer. Thus, we discuss this exchange of binding
modes based on literature data11 and on the observation of a symmetric
data set in 1H NMR and the asymmetric solid-state structure.
€
(11) Ahlers, W.; Temme, B.; Erker, G.; Frohlich, R.; Zippel, F.
Organometallics 1997, 16, 1440.
€
(12) Erker, G.; Rottger, D. Angew. Chem., Int. Ed. Engl. 1993, 32, 1623.
(13) The authors’ assumption finds support in the observation of
the alkyne H3CCtCCPh3 in 1H NMR. This species was formed by
σ-propynyl abstraction from thþe bis(acetyliþde) complex Cp2Zr-
-
(CtCMe)2 and reaction with Ph3C from Ph3C BPh4
.
(15) Ishida, N.; Miura, T.; Murakami, M. Chem. Commun. 2007, 4381.

Article
Organometallics, Vol. 29, No. 10, 2010 2369
Scheme 3. Molecular Dynamics of the Cation of Complex 2 in Benzene Solution According to NMR Spectra
Scheme 4. Formation of Complexes 5 and 6 by Reaction of 4 with B(C6F5)3
Scheme 5. Reaction of 1 with i-Bu2AlH
atom were not observed at room temperature, due to low
solubility and insufficient sensitivity.
The molecular structure of the cation of 2 is depicted in
Figure 1. As discussed before for the similar zirconium com-
plex, the binding parameters of the Hf-C(tC-t-Bu)-Hf
˚
˚
unit (Hf1-C5 = 2.413(4) A, Hf2-C5 = 2.278(4) A, C5-
˚
C6 = 1.206(5) A, Hf1-C5-Hf2 = 96.68(13)°) are indicative
of a σ-bridging acetylide ligand.16 The butadiyne ligand
connects the two hafnocene centers unsymmetrically; the
bridging functionality is formed mainly by one triple bond,
which connects both metals in an η1:η2 fashion with the
planar tetracoordinated carbon atom C2. The Hf2-C bond
distances and angles of the remaining alkynyl group in the
proved by X-ray analysis (6) and by GC-MS (t-BuCHd
CHCtC-t-Bu, [M]þ at m/z 164), respectively.
Most likely, in the first step of the reaction of 4 with
B(C6F5)3, a zwitterionic titanacyclopentadiene is obtained,
which subsequently forms the boranate [t-BuCtCB-
(C6F5)3]- and the cationic titanocene fragment [Cp2Ti(Ct
C-t-Bu)]þ. These two species recombine to give the bis-
alkynyl complex Cp2Ti(CtC-t-Bu)2 and the borane. In the
final step, 2 equiv of Cp2Ti(CtC-t-Bu)2 forms the free buta-
diyne t-BuC4-t-Bu and the product complex 5.17
Reaction of complex 1 with the Lewis acid i-Bu2AlH re-
sults in the formation of complex 3 in 53% yield (Scheme 5).
Recrystallization from n-hexane gave yellow crystals which
were suitable for X-ray analysis.
˚
˚
butadiyne (Hf2-C3 = 2.478(3) A, Hf2-C4 = 3.11 A, C2-
C3-C4 = 178.8(4)°) are different from those of the Erker
˚
compound (2.453(5), 2.763(6) A and 164.2(6)°, respec-
tively).11 However, complex 2 is best described by the asym-
metric binding mode shown in Scheme 3.
These findings motivated us to study the reaction behavior
of the corresponding titanocene complex with B(C6F5)3. The
interaction of the titanacyclocumulene Cp2Ti(η4-t-Bu-C4-t-
Bu) with catalytic amounts of the Lewis acid at 20 °C results
in the liberation of di-tert-butylbutadiyne and the well-known
dinuclear complex 5 (Scheme 4),17 which were unequivocally
identified by NMR spectroscopy.
Addition of further B(C6F5)3 to complex 5 and heating of
the reaction mixture at 100 °C results in electrophilic sub-
stitution of one of the hydrogen atoms in one of the Cp rings
of each “Cp2Ti“ fragment to give the previously reported
zwitterionic and paramagnetic Ti(III) complex CpTi[η5-
C5H4-B(C6F5)3] (6)18 and the ene-yne t-BuCHdCHCtC-t-
Bu (Scheme 4). The formation of these compounds was
Most likely, complex 3 is formed by an initial hydroalu-
mination step, as it was proven for the Cp*2Zr complex with
Me instead of t-Bu (this hydroalumination product9 was
stabilized by another molecule of i-Bu2AlH). Moreover, in
the case of the reaction of Cp*2Zr(η4-Me3SiC4SiMe3) with
1
i-Bu2AlH, Me3SiCtCAl-i-Bu2 was detected by H NMR,
which is indicative of cleavage of the hydroalumination
intermediate into this species and an alkyne π-complex of
the type [Cp*2Zr(η2-HCtCSiMe3)]. These steps are likely
for the formation of complex 3 as well; however, we have no
experimental support for this.
(16) Bridging σ-acetylide ligands in zirconocene complexes: (a) Erker,
Interestingly, complex 3 did not show any reductive
alkenyl-alkynyl coupling to the ene-yne t-BuCtCC(Al-i-
Bu2)dCH-t-Bu), which would be capable of forming a five-
membered metallacycloallenoid, as described very recently
by Erker et al. for the reaction of (MeCp)2M(CtCR)2 with
HB(C6F5)2.19 Most likely, the stabilization by aluminum
€
€
G.; Fromberg, W.; Benn, R.; Mynott, R.; Angermund, K.; Kruger, C.
Organometallics 1989, 8, 911. (b) Niu, S.; Derecskei-Kovacs, A.; Hall, M. B.
J. Organomet. Chem. 2007, 692, 4760.
€
(17) (a) Rosenthal, U.; Ohff, A.; Tillack, A.; Baumann, W.; Gorls, H.
J. Organomet. Chem. 1994, 468, C4. (b) Pulst, S.; Arndt, P.; Heller, B.;
Baumann, W.; Kempe, R.; Rosenthal, U. Angew. Chem., Int. Ed. Engl.
1996, 35, 1112. (c) Baumann, W.; Pellny, P.-M.; Rosenthal, U. Magn. Reson.
Chem. 2000, 38, 519.
(18) (a) Burlakov, V. V.; Troyanov, S. I.; Letov, A. V.; Mysov, E. I.;
Furin, G. G.; Shur, V. B. Russ. Chem. Bull. Int. Ed. 1999, 48, 1012.
(b) Burlakov, V. V.; Troyanov, S. I.; Letov, A. V.; Furin, G. G.; Rosenthal, U.;
Shur, V. B. J. Organomet. Chem. 2000, 598, 243.
€
(19) (a) Ugolotti, J.; Dierker, G.; Kehr, G.; Frohlich, R.; Grimme, S.;
Erker, G. Angew. Chem., Int. Ed. 2008, 47, 2622. (b) Ugolotti, J.; Kehr, G.;
Frohlich, R.; Grimme, S.; Erker, G. J. Am. Chem. Soc. 2009, 131, 1996.
€

2370 Organometallics, Vol. 29, No. 10, 2010
Burlakov et al.
Table 1. Crystal Data and Structure Refinement Parameters for
2 and 3
2
3
cryst syst
space group
triclinic
P1
triclinic
P1
˚
a (A)
10.1365(3)
15.8667(5)
19.3646(6)
83.164(3)
76.599(3)
89.156(3)
3007.86(16)
2
1.691
3.537
200(2)
38 900
10.1020(3)
17.5528(4)
18.3908(4)
69.777(2)
78.672(2)
79.446(2)
2976.74(13)
4
1.368
3.548
200(2)
45 795
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
density (g cm-3
)
Figure 2. Molecular structure of complex 3. The thermal ellipsoids
correspond to 30% probability. Hydrogen atoms, except the hy-
drogen at C8, and the second molecule of the asymmetric unit are
μ(Mo KR) (mm-1
T (K)
)
no. of rflns (measd)
no. of rflns (indep)
no. of rflns (obsd)
no. of params
˚
omitted for clarity. Selected bond lengths (A) and angles (deg):
11 168
8536
773
0.941
12 631
10 417
600
0.932
Hf1-C1=2.261(3), Hf1-C7=2.093(3), Al1-C1=2.182(3), Al1-
C7=2.122(3), Al1-C23 = 1.982(3), Al1-C27 = 1.982(3), C1-
C2 = 1.216(4), C2-C3 = 1.477(5), C7-C8 = 1.330(4), C8-
C9=1.529(4); C1-C2-C3=174.5(3), C7-C8-C9 = 127.9(3),
Hf1-C1-C2 = 176.1(2), Hf1-C7-C8 = 164.8(2), Al1-C1-
Hf1 = 84.19(10), Al1-C7-Hf1 = 89.91(10), C27-Al1-C23 =
116.81(15).
GOF on F2
R1 (I>2σ(I))
wR2 (all data)
0.0245
0.0452
0.0226
0.0509
complex 4. As a consequence of this behavior, the butadiyne
is cleaved in the hafnium compound but is eliminated un-
changed from Cp2Ti(η4-t-BuC4-t-Bu). Moreover, the bis-
alkynyl species Cp2M(CtC-t-Bu)2 is less stable in the case
of titanium and thus dimerizes to give complex 5, whereas
this intermediate can react with the cationic fragment Cp2Hf-
(CtC-t-Bu)þ to give the dinuclear complex 2. This is in
agreement with very recently published effects for group
4 metallocene alkyne complexes and the correspond-
ing theoretical calculations, which did show shorter M-C
bond distances for hafnium compared to titanium and
zirconium.22
in complex 3 prevents this coupling reaction to form
t-BuCtCC(Al-i-Bu2)dCH(t-Bu).
In solution complex 3 exhibits Cs symmetry and thus gives
rise to a single 1H/13C NMR Cp resonance at 5.68/105.4 ppm
(benzene-d6). The 13C NMR spectrum of 3 shows inter alia
three signals due to quaternary carbon atoms and a reso-
nance at 158.7 ppm, which originates from C8 (1JC,H = 147
Hz). Thus, C7-C8 may be regarded as an olefinic double
bond, whereas C1-C2 largely retains its triple-bond charac-
ter. This is evident from the chemical shifts of Cq within the
tert-butyl groups; for 2-t-Bu it is found at much lower
frequency (30.6 ppm) due to the magnetic anisotropy of
the triple bond.
Experimental Section
General Considerations. All operations were carried out under
argon with standard Schlenk techniques or in a glovebox. Prior
to use, non-halogenated solvents (including benzene-d6) were
freshly distilled from sodium tetraethylaluminate and stored
under argon. Cp2HfCl2 was purchased from Sigma Aldrich and
MCAT (Metallocene Catalysts & Life Science Technologies,
Konstanz, Germany) and used without further purification.
n-Butyllithium, 1,4-di-tert-butylbutadiyne, and diisobutylalu-
minum hydride were purchased from Sigma Aldrich, and
B(C6F5)3 was obtained from Strem Inc. Complexes 1 and 4 were
synthesized according to published procedures.4,6 Mass spectra
were obtained with a MAT 95-XP spectrometer and NMR
spectra with Bruker AV 300 and AV 400 spectrometers. Che-
mical shifts (1H, 13C, 19F) are given relative to SiMe4 and are
referenced to signals of the solvent used: benzene-d6 (δH 7.16, δC
128.0). The spectra were assigned with the help of DEPT.
Melting points were obtained in sealed capillaries on a Buchi
535 apparatus. Elemental analyses were obtained with a Leco
TruSpec Micro elemental analyzer.
Diffraction data were collected on a STOE IPDS II diffracto-
meter using graphite-monochromated Mo KR radiation. The
structures were solved by direct methods (SIR 200423 and
SHELXS-97,24 respectively) and refined by full-matrix least-
squares techniques on F2 (SHELXL-9724). XP (Bruker AXS)
The molecular structure of complex 3 was determined by
X-ray analysis and shows the hafnocene unit and the alumi-
num atom connected by an alkynyl and an alkenyl moiety
(Figure 2). In complex 3 the C1-C2 bond distance is 1.216(4)
˚
A, which is in the range of a free CtC triple bond, while the
˚
C7-C8 bond length (1.330(4) A) is close to that of a double
bond.20 The bond distances Al1-C1 = 2.182(3) A and
˚
˚
Al1-C7 = 2.122(3) A are in the expected range, as described
before by Erker et al. for Cp2Zr(μ-η1:η2-RCtCMe)(μ-
CtCR)AlMe2 complexes.21
Conclusion
The hafnacyclocumulene Cp2Hf(η4-t-Bu-C4-t-Bu) reacts
with the Lewis acids B(C6F5)3 and i-Bu2AlH to give the
complexes 2 and 3. Both compounds are formed by interac-
tion of the Lewis acidic center with the electrons of the central
double bond of the metallacycle. Apparently, a competition
between the metallocene unit and the reaction partner to
interact with the electrons of the double bond takes place; the
interaction with the butadiyne seems to be stronger for the
hafnacyclocumulene 1 than for the corresponding titanium
€
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Article
Organometallics, Vol. 29, No. 10, 2010 2371
was used for graphical representations. Crystal data and struc-
ture refinement parameters for 2 and 3 are given in Table 1.
Preparation of Complex 2. B(C6F5)3 (0.679 g, 1.33 mmol) was
dissolved in 15 mL of warm toluene, and the obtained solution
was added to a solution of complex 1 (0.625 g, 1.33 mmol) in
5 mL of toluene. The resulting yellow-orange mixture was filtered
and allowed to stand at room temperature. After 10 days the
solution had turned yellow-brown and light yellow crystals and an
orange oil had formed. The crystals were separated from the
mother liquor by decanting, washed twice with toluene, and dried
under vacuum to give 0.684 g (62%) of complex 2. Crystals of 2
suitable for X-ray analysis were obtained from a saturated ben-
zene-d6 solution. Mp: 201-203 °C under Ar. Anal. Calcd for C62-
106.4 (C7 (or C1)), 156.1 (C2), 158.7 (1JCH = 147 Hz, C8), 279.0
(C1 (or C7)). IR (Nujol mull, cm-1): 1582 (CdC), 2059 (CtC).
MS (70 eV, m/z): 614 [M]þ, 557 [M - i-Bu]þ, 499 [M - 2 i-Bu]þ,
391 [Cp2HfC2t-Bu]þ, 310 [CpHfC2t-Bu - 2 H]þ.
Preparation of Complex 5. B(C6F5)3 (0.029 g, 0.057 mmol)
was dissolved in 1 mL of toluene and subsequently added to a
solution of complex 4 (0.390 g, 1.15 mmol) in 5 mL of toluene.
The resulting green mixture was allowed to stand at room
temperature, and dark green crystals of 5 started to form within
10 min. After 3 h the crystals were separated from the mother
liquor by decanting, washed with cold toluene, and dried under
vacuum to give 0.245 g (83%) of complex 5. Mp: 276-277 °C
dec under Ar (lit.17a mp 277 °C dec under Ar). Anal. Calcd for
C32H38Ti2: C, 74.14; H, 7.39. Found: C, 73.80; H, 7.17. 1H NMR
(benzene-d6, 297 K): δ 1.22 (s, 18 H, C(CH3)3), 5.33 (s, 20 H, Cp).
13C{1H} NMR (benzene-d6, 297 K): δ 32.6 (C(CH3)3), 42.1
(C(CH3)3), 106.3 (Cp), 124.5 (CdC-t-Bu); 233.9 (CdC-t-Bu).
The mother liquor was evaporated to dryness under vacuum to
give a solid mixture, which contained mainly t-BuCtCCt
H56BF15Hf2 C6D6: C, 53.10; H, 4.46. Found: C, 53.82; H, 4.73.
3
1H NMR (benzene-d6, 300 MHz, 297 K): δ 0.99 (s, 9 H, 6-t-Bu),
1.08 (s, 18 H, 1- and 4-t-Bu), 1.51 (s, 9 H, t-Bu anion), 5.42 (s, 20 H,
Cp). 13C{1H} NMR (benzene-d6, 75 MHz, 297 K): δ 28.6
(C(CH3)3 anion), 28.8 (6-CH3), 29.3 (1- and 4-CH3), 29.9 (6-
C(CH3)3), 32.2 (CH3 anion), 37.8 (1- and 4-C(CH3)3), 89.2 (C2/3),
102.3 (CtC anion), 108.3 (Cp), 111.0 (C5), 148.7 (C6), 198.3
(C1/4); resonances of the C6F5 groups were not observed. The
signals of CtC were detected directly by polarization transfer
(INEPT) or indirectly (HMBC; see the Supporting Information).
19F NMR (benzene-d6, 282 MHz, 297 K): δ -130.5 (o-C6F5),
-166.7 (m-C6F5), -163.8 (p-C6F5). IR (Nujol mull, cm-1): 1513,
1642 (CdC), 2017, 2033 (CtC).
1
C-t-Bu (by NMR). H NMR (benzene-d6, 297 K): δ 1.06 (s,
18H, C(CH3)3). 13C{1H} NMR (benzene-d6, 297 K): δ 28.0
(C(CH3)3), 30.6 (C(CH3)3), 65.3 (CtC), 86.3 (CtC).
Preparation of Complex 6. B(C6F5)3 (0.470 g, 0.92 mmol) was
dissolved in 10 mL of toluene and subsequently added to a
solution of complex 5 (0.238 g, 0.46 mmol) in 20-25 mL of
warm toluene. The resulting blue-green mixture was warmed to
100 °C for 30 h, filtered, and allowed to stand at room
temperature. Within 2 days dark blue crystals of 6 had formed,
which were separated from the green mother liquor by decant-
ing, washed with toluene, and dried under vacuum to give
Preparation of Complex 3. Complex 1 (0.823 g, 1.75 mmol)
was dissolved in 20 mL of n-hexane, and i-Bu2AlH (3.5 mL of a
1.0 M solution in toluene, 3.5 mmol) was added. After 24 h all
volatiles were removed from the yellow solution under vacuum,
followed by addition of n-hexane (3-4 mL) to the oily residue.
The solution was filtered and allowed to stand at -78 °C. After
3 days yellow crystals had formed, which were separated from the
mother liquor by decanting, washed twice with cold n-hexane, and
dried under vacuum to give 0.492 g (53%) of complex 3. Mp:
176-178 °C under Ar. Anal. Calcd for C30H47AlHf: C, 58.76; H,
7.73. Found: C, 58.72; H, 7.77. 1H NMR (benzene-d6, 300 MHz,
297 K): δ 0.38 (dd, 2JH,H = 13.8 Hz, 3JH,H = 6.6 Hz, 2 H, CH2-i-
Bu), 0.54 (dd, 2JH,H = 13.8 Hz, 3JH,H = 6.7 Hz, 2 H, CH2-i-Bu),
1.10 (s, 9 H, 8-t-Bu), 1.14(s, 9H, 2-t-Bu), 1.28 (d, 3JH,H =6.5 Hz, 6
H, Me-i-Bu), 1.29 (d, 3JH,H = 6.5 Hz, 6 H, Me-i-Bu), 2.20 (m, 2 H,
CH-i-Bu), 5.68 (s, 10 H, Cp), 7.32 (s, 1 H, 8-H). 13C{1H} NMR
(benzene-d6, 75 MHz, 297 K): δ 27.0 (CH-i-Bu), 27.2 (br, CH2-i-
Bu), 28.9 (CH3-i-Bu), 28.9 (CH3-i-Bu), 29.8 (8-CH3), 30.4 (2-
CH3), 30.4 (2-CMe3), 38.1 (8-CMe3), 105.4 (1JCH = 173 Hz, Cp),
0.222 g (62%) of complex 6 C6H5CH3. GC/MS analysis of
3
the mother liquor revealed the presence of t-BuCHdCHCt
C-t-Bu (m/z 164 [M]þ).
Acknowledgment. We thank our analytical and tech-
nical staff for assistance. Support by the Deutsche For-
schungsgemeinschaft (No. GRK 1213) and the Russian
Foundation for Basic Research (Project code 09-03-
00503) is acknowledged.
Supporting Information Available: Figures giving NMR
spectra and the structure of complex 3 and CIF files giving
crystallographic data for compounds 2 and 3. This material is
available free of charge via the Internet at http://pubs.acs.org.