Formation and structural characterization of a five-membered zirconacycloallenoid

Article abstract of DOI:10.1039/c3dt51497h

A conjugated enyne reacts with in situ generated zirconocene to yield a five membered zirconacycloallenoid that was characterized by X-ray diffraction.

Full text of DOI:10.1039/c3dt51497h

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Formation and structural characterization of a
five-membered zirconacycloallenoid†‡
Cite this: Dalton Trans., 2013, 42, 14673
Received 5th June 2013,
Accepted 11th July 2013
Georg Bender, Gerald Kehr, Constantin G. Daniliuc,§ Birgit Wibbeling§ and
Gerhard Erker*
DOI: 10.1039/c3dt51497h
www.rsc.org/dalton
A conjugated enyne reacts with in situ generated zirconocene to
yield a five membered zirconacycloallenoid that was characterized
by X-ray diffraction.
Zirconocene complexes of conjugated dienes, enynes or diynes
strongly favour metallacyclic five-membered ring structures
over their conventional π-complex resonance forms, which
indicates a very pronounced backbonding component for
these Group 4 bent metallocene complexes.1 Rosenthal’s
metallacyclocumulenes2 (1) and Suzuki’s metallacycloalkynes3
(3) are very typical examples. Recently, we4 and shortly there-
after Suzuki5 had described the formation and structural
characterization of two substituted examples of the zircona-
cycloallenoids (2) that were taking a position in between com-
plexes 1 and 3 in this unique general class of compounds6
(Scheme 1).
Scheme 1
metallacycloalkynes (3) was still missing. We have now pre-
pared a suitable example of a five-membered zirconacycloallen-
oid bearing only simple substituents and have characterized it
by X-ray diffraction. We also carried out a few reactions of this
compound (actually with an unusual outcome) that will be
described in this paper.
We treated Cp2ZrCl2 with two molar equivalents of n-butyl-
magnesium chloride in the presence of the enyne 4 (5.5 : 1
trans–cis-mixture, one equiv.). After keeping the mixture for
1 h at 60 °C, workup eventually gave compound 2c as yellow-
orange crystals in 58% yield.¶
Conjugated diene zirconocenes can be synthesised by react-
ing the respective substituted butadiene magnesium reagents
7
with, e.g., Cp2ZrCl2 or by reductive coupling of bis(alkenyl)-
zirconocenes.8 A convenient method is the trapping of a conju-
gated diene ligand by in situ generated zirconocene.1,9 Both
the latter reaction types had also been used to prepare
‘Rosenthal-zirconacyclocumulenes’ as well.2 Suzuki and sub-
sequently we had prepared examples of the zirconacycloallen-
oids by generating zirconocene by variants of the Negishi
method10 in the presence of conjugated enynes,11 and we had
used that to carry out typical subsequent reactions.12 However,
to the best of our knowledge, a simple derivative of the
general complex class of the zirconacycloallenoids (2) had
so far not been characterized by X-ray diffraction so that an
ample structural description of these conceptual links between
the well characterized metallacyclocumulenes (1) and the
The X-ray crystal structure analysis shows that the enyne 4
has become η4-coordinated to the in situ generated zircono-
cene unit. It shows a central unit that is probably best
described as a resonance hybrid between a Cp2Zr(enyne)
π-complex and
a five-membered metallacycloallene (see
Scheme 2). The bonds between the zirconium atom and
the pair of former acetylene carbon atoms are short (Zr1–C1
2.304(8) Å, Zr1–C2 2.321(7) Å). The Zr–C(olefin) linkages are
also rather short (Zr1–C3 2.446(8) Å, Zr1–C4 2.461(7) Å) but
markedly longer than the former pair of Zr–C bonds (for com-
parison, the Zr–C(Cp) bond lengths of compound 2c are in a
range between 2.488(9) Å and 2.55(2) Å). The C1–C2 bond is
still short at 1.269(10) Å, while the C3–C4 bond is slightly
longer (1.415(11) Å), both reminding us of their acetylenic and
Organisch-Chemisches Institut der Universität Münster, Corrensstrasse 40, 48149
Münster, Germany. E-mail: erker@uni-muenster.de
†Dedicated to Professor Carl Krüger on the occasion of his 80th birthday.
‡Electronic supplementary information (ESI) available: Experimental and
analytical details. CCDC 938715–938717. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c3dt51497h
§X-ray crystal structure analyses.
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Scheme 2
Fig. 2 Molecular structure of complex 6 (thermal ellipsoids are shown with
30% probability).
We generated Cp2Zr in the presence of 0.5 molar equivalent
of the enyne 4 and isolated a different compound, namely the
alkynyl/alkenyl-bridged bis-zirconocene 6. It was isolated in ca.
50% yield. We assume that in this reaction the zirconacyclo-
allenoid 2c is formed first and that this can then react with an
additional Cp2Zr equivalent via its (σ-alkenyl, σ-alkynyl)ZrCp2
isomer 5 formed by C–C bond cleavage. Similar reactions
had previously been observed in bis(alkynyl)zirconocene/
‘Rosenthal-zirconacyclocumulene’ chemistry.13
Complex 6 was characterized by NMR spectroscopy and
X-ray diffraction. The structural and spectroscopic features of the
(alkyne)ZrCp2 and (alkene)ZrCp2 structural subunits of 6 are
markedly different from those of the respective zirconacyclo-
allenoid 2c. The molecular structure of complex 6 shows an
alkynyl ligand that is σ-bonded to Zr1 (Zr1–C2 2.181(4) Å) and
π-bonded to Zr2 (Fig. 2). The bond lengths of this unit amount
to 1.242(6) Å (C1–C2), 2.376(4) Å (Zr2–C1) and 2.403(4) Å (Zr2–
C2). The C2–C1–C5 “substituent angle” of complex 6 was found
at 140.8(5)° (Zr1–C2–C1 172.8(4)°). Zirconium atom Zr2 has
the trans-styryl-ligand σ-bonded to it (Zr2–C4 2.258(4) Å,
C3–C4 1.396(6) Å, θ = Zr2–C4–C3–C31 −162.2(4)°). This ligand
is π-coordinated to Zr1 (Zr1–C3 2.576(5) Å, Zr1–C4 2.514(4) Å,
angles Zr2–C4–C3 142.2(3)°, C4–C3–C31 128.6(4)°).
Fig. 1
A view of the molecular structure of the zirconacycloallenoid 2c
(thermal ellipsoids are shown with 30% probability).
olefinic origin, respectively. However, their connecting bond
is remarkably short at an intermediate value between the
former C–C pair at C2–C3 1.383(11) Å, as one would probably
have expected it for a structure featuring some allenoid charac-
ter (see Fig. 1). The central framework of the zirconacycloallenoid
2c is non-planar.4,5 It features
a trans-arrangement of
the substituents at the former carbon–carbon double bond
(C3–C4): θ = C1–C2–C3–C4 −68.2(2)°, θ = C2–C3–C4–C21
−177.6(7)° (angles C2–C3–C4 121.7(8)°, C3–C4–C21 123.8(7)°,
Zr1–C4–C21 133.9(5)°). The former acetylene moiety features a
marked transoid distortion (θ = C3–C2–C1–C5 −173.1(13)°,
angles C3–C2–C1 150.5(8)°, C2–C1–C5 132.8(7)°, Zr1–C1–C5
151.4(6)°).
In solution (C6D6), complex 2c shows characteristic
13C NMR resonances of the framework at δ 162.5 (C1), 116.7
(C2), 92.8 (C3) and 68.5 (C4). The corresponding 4-H H NMR
signal was observed at δ 2.72 and the adjacent 3-H signal at
δ 5.04 (AX, JHH = 13.9 Hz). These values indicate a marked
participation of the metallacyclic allenoid resonance structure
as it is often observed for such zirconocene complexes. The Cp
NMR features of complex 2c occur at δ 5.39 and 4.87 (1H, each
5H; 13C: δ 104.8, 102.7) and the 1H NMR tBu signal is at δ 1.38.
Compound 6 shows sharp NMR signals in solution at 213 K
(C7D8). The 1H/13C NMR features of the pair of σ/π-ligands in 6
are quite different from those of the corresponding subunits
of 2c. Compound 6 shows the 13C NMR resonances of the
σ/π-acetylide moiety at δ 215.0 (C2) and δ 170.9 (C1), respect-
ively. The 1H NMR signals of the σ/π-trans-styryl ligand occur at
3
δ 9.66 and 5.41 (AX, JHH = 19.7 Hz) with corresponding
13C NMR resonances at δ 209.6 (C4) and 105.6, respectively.
The chiral bis(zirconocene) complex 6 exhibits two pairs of dia-
stereotopic Cp-ligands; consequently we have observed four
1
3
1
sharp H NMR Cp-signals (δ 5.43, 5.09, 5.02, 4.62). Increasing
the monitoring temperature leads to a pairwise coalescence of
the 1H NMR Cp signals. This probably indicates a mutual
exchange of the σ-alkenyl/σ-alkynyl pair of ligands between
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moiety was located at δ 193.4; the adjacent CvC bond shows
its 13C NMR resonances at δ 205.2 (ZrCv) and 148.7, respect-
ively. Complex 8 shows a sharp 1H NMR Cp resonance at δ 5.83
and the tBu 1H NMR signal at δ 1.42.
Conclusions
Our study has provided us with some detailed structural infor-
mation about the very special class of the metallacycloallenoid
complexes. It has revealed some interesting new reactivities, be
it in its reaction with an additional zirconocene equivalent or
the addition of acetonitrile. The metallacycloallenoids
2
occupy an interesting position between Rosenthal’s metalla-
cyclocumulenes 1 and Suzuki’s metallacycloalkynes 3. They
seem to show an interesting chemical behavior towards un-
saturated organic and organometallic reagents, as shown by
the examples described in this study.
Scheme 3
Acknowledgements
Financial support from the Deutsche Forschungsgemeinschaft
is gratefully acknowledged.
Notes and references
¶Preparation of complex 2c: N-butylmagnesium chloride (0.68 ml, 2 M diethyl
ether solution, 1.36 mmol, 2 eq.) was added to a solution of bis(η5-cyclopenta-
dienyl)zirconium dichloride (200 mg, 0.68 mmol, 1 eq.) and enyne 4 (153 mg,
0.83 mmol, 1.2 eq.) in THF (8 ml) at −78 °C. After heating for 1 h at 60 °C, the
volatiles of the deep red mixture were removed in vacuo and the residue was
extracted with diethyl ether (3 × 10 ml). The red coloured filtrate was concen-
trated and cooled to −30 °C. Compound 2c was obtained as yellow-orange crys-
Fig. 3 Molecular structure of complex 8 (thermal ellipsoids are shown with
30% probability).
the pair of zirconocene units9 (see Scheme 2) [ΔG≠rearr(273 K) =
12.1 0.2 kcal mol−1 (for details see the ESI‡)].
tals (161 mg, 58%). 1H NMR (500 MHz, 299 K, [d6]-benzene): δ = 7.26 (m, 2H,
3
m-Ph), 7.21 (m, 2H, o-Ph), 6.98 (m, 1H, p-Ph), 5.39 (s, 5H, CpA), 5.04 (d, JHH
=
13.9 Hz, 1H, 3-H), 4.87 (s, 5H, CpB), 2.72 (d, 3JHH = 13.9 Hz, 1H, 4-H), 1.38 (s, 9H,
tBu). 13C{1H} NMR (126 MHz, 299 K, [d6]-benzene): δ = 162.5 (C-1), 146.4 (i-Ph),
128.9 (m-Ph), 123.6 (o-Ph), 123.0 (p-Ph), 116.7 (C-2), 104.8 (CpA), 102.7 (CpB),
92.8 (C-3), 68.5 (C-4), 36.2 (tBu), 33.4 (tBu).
We12b and others14 had previously shown that five-mem-
bered zirconacycloallenoids often insert organic nitriles
into the Zr–C bond15 with the formation of the corresponding
seven-membered metallacycloallenoid products. Complex 2c
behaves differently. It seems that complex 2c reacts with aceto-
nitrile by means of a metallacyclic cycloaddition reaction via
its reactive η2-alkyne isomer 7 (see Scheme 3). Treatment of 2c
with acetonitrile overnight at r.t. gave product 8. The X-ray
crystal structure analysis shows the presence of the doubly
unsaturated five-membered metallaheterocyclic cores (Zr1–
N1A 2.247(5) Å, N1A–C9A 1.286(8) Å, C9A–C1A 1.498(9) Å, C1A–
C2A 1.347(9) Å, Zr1–C2A 2.402(7) Å) to which the remaining
trans-styryl substituent is bonded (C3A–C4A 1.337(10) Å,
θ = C1A–C2A–C3A–C4A 69.1(11)°, θ = C2A–C3A–C4A–C21A
173.3(7)°, see Fig. 3). The overall structure of compound 8 is
dimeric16 with monomeric entities being connected by means
of intramolecular Zr–N coordination (Zr1–N1# 2.359(5) Å,
angle Zr1–N1A–Zr1# 109.7(2)°).
1 G. Erker, G. Kehr and R. Fröhlich, Adv. Organomet. Chem.,
2004, 51, 109.
2 (a) U. Rosenthal, A. Ohff, W. Baumann, R. Kempe,
A. Tillack and V. V. Burlakov, Angew. Chem., Int. Ed. Engl.,
1994, 33, 1605; (b) P.-M. Pellny, V. V. Burlakov, P. Arndt,
W. Baumann, A. Spannenberg and U. Rosenthal, J. Am.
Chem. Soc., 2000, 122, 6317; (c) U. Rosenthal,
V. V. Burlakov, P. Arndt, W. Baumann and A. Spannenberg,
Organometallics, 2005, 24, 456; (d) S. Roy, E. D. Jemmis,
M. Ruhmann, A. Schulz, K. Kaleta, T. Beweries and
U. Rosenthal, Organometallics, 2011, 30, 2670.
3 (a) N. Suzuki, M. Nishiura and Y. Wakatsuki, Science, 2002,
295, 660; (b) N. Suzuki, N. Aihara, H. Takahara,
T. Watanabe, M. Iwasaki, M. Saburi, D. Hashizume and
T. Chihara, J. Am. Chem. Soc., 2004, 126, 60; (c) N. Suzuki,
N. Aihara, M. Iwasaki, M. Saburi and T. Chihara, Organo-
metallics, 2005, 24, 791.
In solution the typical 1H NMR features of the trans-styryl
3
substituent have been observed (δ 7.36, 5.80, AX, JHH
=
15.8 Hz). The 13C NMR resonance of the endocyclic NvC
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4 (a) J. Ugolotti, G. Dierker, G. Kehr, R. Fröhlich, S. Grimme
and G. Erker, Angew. Chem., Int. Ed., 2008, 47, 2622;
(b) J. Ugolotti, G. Kehr, R. Fröhlich, S. Grimme and
G. Erker, J. Am. Chem. Soc., 2009, 131, 1996.
5 N. Suzuki, D. Hashizume, H. Koshino and T. Chihara,
Angew. Chem., Int. Ed., 2008, 47, 5198.
6 See also: R. Rosenthal, Angew. Chem., Int. Ed., 2004, 43,
3882.
7 (a) H. Yasuda, Y. Kajihara, K. Mashima, K. Nagasuna,
K. Lee and A. Nakamura, Organometallics, 1982, 1, 388;
(b) H. Yasuda and A. Nakamura, Angew. Chem., Int. Ed.
Engl., 1987, 26, 723.
2012, 51, 8846; (b) G. Bender, G. Kehr, R. Fröhlich,
J. L. Petersen and G. Erker, Chem. Sci., 2012, 3, 3534;
(c) G. Bender, G. Kehr, C. G. Daniliuc, Q. M. Dao,
S. Ehrlich, S. Grimme and G. Erker, Chem. Commun., 2012,
48, 11085.
13 (a) G. Erker, W. Frömberg, R. Mynott, B. Gabor and
C. Krüger, Angew. Chem., Int. Ed. Engl., 1986, 25, 463;
(b) G. Erker, W. Frömberg, R. Benn, R. Mynott,
K. Angermund and C. Krüger, Organometallics, 1989, 8,
911; (c) U. Rosenthal, A. Ohff, W. Baumann, R. Kempe,
A. Tillack and V. V. Burlakov, Organometallics, 1994, 13,
2903.
8 R. Beckhaus and K.-H. Thiele, J. Organomet. Chem., 1984, 14 S. Zhang, W.-X. Zhang, J. Zhao and Z. Xi, J. Am. Chem. Soc.,
268, C7. 2010, 132, 14042.
9 (a) G. Erker, J. Wicher, K. Engel, F. Rosenfeldt, W. Dietrich 15 See for a comparison: J. Ugolotti, G. Kehr, R. Fröhlich and
and C. Krüger, J. Am. Chem. Soc., 1980, 102, 6344; G. Erker, Chem. Commun., 2009, 6572.
(b) G. Erker, C. Krüger and G. Müller, Adv. Organomet. 16 (a) H. Takaya, M. Yamakawa and K. Mashima, J. Chem.
Chem., 1985, 24, 1.
10 E. Negishi, F. E. Cederbaum and T. Takahashi, Tetrahedron
Lett., 1986, 27, 2829.
11 N. Suzuki, T. Shimura, Y. Sakaguchi and Y. Masuyama,
Pure Appl. Chem., 2011, 83, 1781.
12 (a) G. Bender, T. Wiegand, H. Eckert, R. Fröhlich,
C. G. Daniliuc, C. Mück-Lichtenfeld, S. Ndambuki,
T. Ziegler, G. Kehr and G. Erker, Angew. Chem., Int. Ed.,
Soc., Chem. Commun., 1983, 1283; (b) G. Erker, U. Dorf,
P. Czisch and J. L. Petersen, Organometallics, 1986, 5, 668;
(c) S. L. Buchwald, A. Sayers, B. T. Watson and J. C. Dewan,
Tetrahedron Lett., 1987, 28, 3245; (d) G. Erker,
U. Hoffmann, R. Zwettler, P. Betz and C. Krüger,
Angew. Chem., Int. Ed. Engl., 1989, 28, 630;
(e) R. Choukroun, C. Lorber and L. Vendier, Organometal-
lics, 2007, 26, 3784.
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