Evidence for a carbon-carbon coupling reaction to proceed through a planar-tetracoordinate carbon intermediate

Article abstract of DOI:10.1021/om0495628

Treatment of bis(propynyl)zirconocene with BC₆F ₅)₃ results in a linear C-C coupling of the alkynyl ligands to form the zwitterionic complex 3. Its treatment with excess RC≡N yields an organometallic methylenecyclopropene derivative (6). This reaction topologically requires a very endothermic substituted butenyne to methylenecyclopropene cyclization to become energetically feasible by suitable stabilization effects. A DFT study has revealed that the 3 → 6 conversion is probably triggered by nitrile addition to the metal with formation of a planar-tetracoordinate carbon intermediate, featuring coordination of the three-membered carbocycle through one of its carbon-carbon σ bonds.

Full text of DOI:10.1021/om0495628

Organometallics 2004, 23, 4391-4395
4391
Evid en ce for a Ca r bon -Ca r bon Cou p lin g Rea ction To
P r oceed th r ou gh a P la n a r -Tetr a coor d in a te Ca r bon
In ter m ed ia te
Gerhard Erker,* Sabine Venne-Dunker, Gerald Kehr, Nina Kleigrewe,
Roland Fro¨hlich,† Christian Mu¨ck-Lichtenfeld,‡ and Stefan Grimme*,‡
Universita¨t Mu¨nster, Organisch-Chemisches Institut, Corrensstrasse 40,
D-48149 Mu¨nster, Germany
Received J une 15, 2004
Treatment of bis(propynyl)zirconocene with B(C6F5)3 results in a linear C-C coupling of
the alkynyl ligands to form the zwitterionic complex 3. Its treatment with excess RCtN
yields an organometallic methylenecyclopropene derivative (6). This reaction topologically
requires a very endothermic substituted butenyne to methylenecyclopropene cyclization to
become energetically feasible by suitable stabilization effects. A DFT study has revealed
that the 3 f 6 conversion is probably triggered by nitrile addition to the metal with formation
of a planar-tetracoordinate carbon intermediate, featuring coordination of the three-
membered carbocycle through one of its carbon-carbon σ bonds.
Sch em e 1
In tr od u ction
Planar-tetracoordinate carbon is sp2 hybridized and
thus contains a strongly electron deficient σ system (only
six electrons making four bonds), with the remaining
electron pair residing in the perpendicular p orbital.
Conjugative interaction of the p electron pair plus the
attachment of strongly σ donating substituents may
stabilize this very unusual coordination geometry of
tetravalent carbon.1,2 Computational studies have iden-
tified a variety of systems with favored planar-tetraco-
ordinate carbon geometries, mostly stabilized by metal
substituents or by a confinement in a rigid cage struc-
ture.3 A small number of compounds, stable at room
temperature, have actually been isolated and character-
ized so far that feature such electronically stabilized
tetracoordinate carbon atoms within an organometallic
framework.4,5 The complexes 1 are typical examples
(Scheme 1). In such systems an electron-rich carbon to
metal σ bond is coordinated to a second electron-poor
metal center inside a rigid framework. This results in
the formation of a three-center-two-electron bonding
situation in the σ plane with a conventionally stabilized
π system perpendicular to it.5,6
Although an increasing number of such examples
have been reported, planar-tetracoordinate carbon com-
pounds have remained more a structural curiosity
rather than being of importance as stabilized minimum
structures in chemical reactions. We have now found
an example where the electronic stabilization of a
planar-tetracoordinate-carbon-containing reactive in-
termediate may have helped significantly in making an
unusual carbon-carbon coupling reaction take place
under rather mild conditions.
* To whom correspondence should be addressed. E-mail: erker@uni-
muenster.de (G.E.).
† X-ray crystal structure analyses.
‡ DFT calculations.
(1) Hoffmann, R.; Alder, R. W.; Wilcox, C. F., J r. J . Am. Chem. Soc.
1970, 92, 4992. Hoffmann, R. Pure Appl. Chem. 1971, 28, 181.
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v. R.; Seeger, R.; Pople, J . A. J . Am. Chem. Soc. 1976, 98, 5419.
Lammertsma, K.; Schleyer, P. v. R.; Schwarz, H. Angew. Chem. 1989,
101, 1313; Angew. Chem., Int. Ed. Engl. 1989, 28, 1321. Li, X.; Zhang,
H.-F.; Wang, L.-S.; Geske, G. D.; Boldyrev, A. I. Angew. Chem. 2000,
112, 3776; Angew. Chem., Int. Ed. 2000, 39, 3630. Wang, L.-S.;
Boldyrev, A. I.; Li, X.; Simons, J . J . Am. Chem. Soc. 2000, 122, 7681
and references therein.
Resu lts a n d Discu ssion
We had previously shown that bis(propynyl)zir-
(3) Rasmussen, D. R.; Radom, L. Angew. Chem. 1999, 111, 3052;
Angew. Chem., Int. Ed. 1999, 38, 2876. Wang, Z.-X.; Schleyer, P. v. R.
J . Am. Chem. Soc. 2001, 123, 994; 2002, 124, 11979.
(4) Gunale, A.; Pritzkow, H.; Siebert, W.; Steiner, D.; Berndt, A.
Angew. Chem. 1995, 107, 1194; Angew. Chem., Int. Ed. Engl. 1995,
34, 1111. Hyla-Kryspin, I.; Gleiter, R.; Rohmer, M.-M.; Devemy, J .;
Gunale, A.; Pritzkow, H.; Siebert, W. Chem. Eur. J . 1997, 3, 294. Sahin,
Y.; Pra¨sang, C.; Hofmann, M.; Subramanian, G.; Geiseler, G.; Massa,
W.; Berndt, A. Angew. Chem. 2003, 115, 695; Angew. Chem., Int. Ed.
2003, 42, 671. See also: Kickham, J . E.; Gue´rin, F.; Stewart, J . C.;
Stephan, D. W. Angew. Chem. 2000, 112, 3406; Angew. Chem., Int.
Ed. 2000, 39, 3263.
conocene (2) and related systems react with B(C6F5)3
(6) Erker, G.; Ro¨ttger, D. Angew. Chem. 1993, 105, 1691; Angew.
Chem., Int. Ed. Engl. 1993, 32, 1623. Ro¨ttger, D.; Erker, G.; Fro¨hlich,
R.; Grehl, M.; Silverio, S. J .; Hyla-Kryspin, I.; Gleiter, R. J . Am. Chem.
Soc. 1995, 117, 10503. Binger, P.; Sandmeyer, F.; Kru¨ger, C.; Erker,
G. Tetrahedron 1995, 51, 4277. Ro¨ttger, D.; Pflug, J .; Erker, G.; Kotila,
S.; Fro¨hlich, R. Organometallics 1996, 15, 1265. Schottek, J .; Erker,
G.; Fro¨hlich, R. Eur. J . Inorg. Chem. 1998, 551. See also: Schottek,
J .; Erker, G.; Fro¨hlich, R. Angew. Chem. 1997, 109, 2585; Angew.
Chem., Int. Ed. 1997, 36, 2475. Schottek, J .; Ro¨ttger, D.; Erker, G.;
Fro¨hlich, R. J . Am. Chem. Soc. 1998, 120, 5264. Hogenbirk, M.; Schat,
G.; Akkerman, O. S.; Bickelhaupt, F.; Schottek, J .; Fro¨hlich, R.; Erker,
G.; Kooijman, H.; Spek, A. L. Eur. J . Inorg. Chem. 2004, 1175.
(5) Reviews: Ro¨ttger, D.; Erker, G. Angew. Chem. 1997, 109, 840;
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28, 307.
10.1021/om0495628 CCC: $27.50 © 2004 American Chemical Society
Publication on Web 08/19/2004

4392 Organometallics, Vol. 23, No. 19, 2004
Erker et al.
unusual reaction pathway that this remarkable chemi-
cal transformation takes.
Treatment of 3 with pivalonitrile in d8-toluene at -60
°C resulted in a spontaneous formation of a 1:1 adduct
(4a ). Its 13C NMR spectrum indicates that the alkyne
moiety of the central µ-hexadiyne ligand in 4a is
probably no longer coordinated to the d0 zirconium
center (13C NMR: δ(3)7 108.5/105.0 (C5/4) vs δ(4a )
103.3/68.2). Slowly warming the mixture to room tem-
perature in the dark revealed that the formation of the
adduct from 3 and R1CN was reversible and that
concurrent equilibration with the metallacyclocumulene
product 59 and R1CN-B(C6F5)3 occurred. Eventually out
of the 3 a 4a a 5 equilibrium mixture the irreversible
and complete formation of the organometallic methyl-
enecyclopropene product 6a took place.
Treatment of the betaine 3 with excess benzonitrile
at low temperature gave the adduct 4b , which sub-
sequently reacted further to give the metalated methyl-
enecyclopropene derivative 6b. For the determination
of the rate of the 4b f 6b conversion, the adduct 4b
was generated in situ by treatment of complex 3 with a
ca. 10-fold excess of benzonitrile in d8-toluene. At 243
K the consumption of 4b was monitored under pseudo-
first-order conditions using an added quantity of fer-
rocene as the internal standard. This gave a rate
F igu r e 1. Molecular structure of 3′ (experimentally
determined by single-crystal X-ray diffraction). Selected
bond lengths (Å) and angles (deg) (averaged over the two
independent molecules), with the corresponding DFT-
calculated values of 3 given in brackets for comparison:
Zr-C2, 2.175(3) [2.200]; Zr-C3, 2.643(3) [2.575]; Zr-C4,
2.376(3) [2.337]; Zr-C5, 2.718(3) [2.762]; C1-C2, 1.497(4)
[1.493]; C2-C3, 1.358(4) [1.367]; B1-C3, 1.684(5) [1.777];
C3-C4, 1.456(4) [1.432]; C4-C5, 1.208(5) [1.236]; C5-C6,
1.468(5) [1.463]; C1-C2-Zr, 141.0(2) [142.4]; C3-C2-Zr,
94.0(2) [89.3]; C1-C2-C3, 125.1(3) [128.3]; C2-C3-C4,
118.5(3) [122.7]; C2-C3-B, 125.2(3) [125.0]; C4-C3-B,
116.0(3) [112.1]; C3-C4-C5, 175.4(3) [178.8]; C4-C5-C6,
174.1(4) [168.2].
constant of kchem ) kobs/[PhCN] of 3.58 × 10-3 s-1
L
mol-1, which corresponds to a Gibbs activation energy
of ∆Gqchem(243 K) ) 16.8 ( 0.1 kcal mol-1 for the second-
order reaction (4b f 6b) under standard conditions (i.e.
1 M benzonitrile).10
by rapid coupling of the alkynyl ligands to yield the
zwitterionic product 3,7 A related hafnium complex was
characterized by X-ray diffraction: however, not with
high precision. We are now able to characterize complex
3′ by an X-ray crystal structure analysis (see Figure 1).
Complex 3′ (R ) CH3) contains a planar central organic
σ/π ligand framework, which was formed by coupling
of a pair of propynyl ligands that connects the zir-
conocene unit with the B(C6F5)3 moiety. Inside the
µ-Me2C4 ligand the C4-C5 bond length is very short
(1.208(5) Å), corresponding to a carbon-carbon triple
bond that is unsymmetrically η2 coordinated to zirco-
nium (Zr-C4 ) 2.376(3) Å, Zr-C5 ) 2.718(3) Å). The
newly formed C3-C4 bond (1.456(4) Å) is adjacent to a
C2-C3 double bond (1.358(4) Å). Inside this ligand
framework the C2-C3-C4 angle is close to the expected
sp2 value (118.5(3)°). The adjacent η2-acetylene unit is
close to linear (C3-C4-C5 ) 175.4(3)°, C4-C5-C6 )
174.1(4)°).
A detailed theoretical DFT study of the hypersurface
of 3 with acetonitrile instead of PhCN has located the
same types of structures as local minima. Their energies
relative to the reference 3/CH3CN are listed in Scheme
2 in brackets. The good agreement between the experi-
mental (X-ray diffraction, 3′) and calculated structural
parameters of the betaine systems 3 shows the suit-
ability of the DFT method for this organometallic
system.
Topologically, the formation of 6 from 3 requires an
intramolecular alkyne insertion into the zirconium to
carbon σ bond to take place rapidly under the reaction
conditions. However, calculations show that the under-
lying hexenyne to dimethyl-methylenecyclopropene
isomerization is endothermic by ca. 26.6 kcal mol-1
.
(DFT-BLYP; QCISD(T), +25.2 kcal mol-1). Therefore,
we had to identify the essential stabilizing factors that
allowed this type of rearrangement to occur under the
prevailing conditions of our experiment.
Complex 3 undergoes a remarkable reaction when
treated with an organic nitrile reagent. A total of 2 equiv
of RCN is taken up to form the methylenecyclopropene
derivatives 6. Several examples of these unusually
structured compounds have been characterized by X-ray
diffraction.8 We have now carried out a combined
theoretical/experimental study that probably reveals the
The DFT calculation produced a rather unusual
Cp2Zr/B(C6F5)3 substituted methylenecyclopropene struc-
ture (8) of the direct alkyne insertion product. The Zr-
C5-C4 angle in 8 is extremely reduced, namely from
an expected 120° to 82°. This brings the methylenecy-
(7) Temme, B.; Erker, G.; Fro¨hlich, R.; Grehl, M. Angew. Chem.
1994, 106, 1570; Angew. Chem., Int. Ed. Engl. 1994, 33, 1480. Ahlers,
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1997, 527, 191.
(8) (a) Temme, B.; Erker, G.; Fro¨hlich, R.; Grehl, M. J . Chem. Soc.,
Chem. Commun. 1994, 1713. Erker, G.; Ahlers, W.; Fro¨hlich, R. J . Am.
Chem. Soc. 1995, 117, 5853. Ahlers, W.; Erker, G.; Fro¨hlich, R.; Zippel,
F. Chem. Ber./ Recl. 1997, 130, 1079. Ahlers, W.; Erker, G.; Fro¨hlich,
R. J . Organomet. Chem. 1998, 571, 83. Ahlers, W.; Erker, G.; Fro¨hlich,
R.; Peuchert, U. J . Organomet. Chem. 1999, 578, 115. Venne-Dunker,
S.; Ahlers, W.; Erker, G.; Fro¨hlich, R. Eur. J . Inorg. Chem. 2000, 1671.
(b) For a remotely related reaction see, e.g.: Pragliola, S.; Milano, G.;
Guerra, G.; Longo, P. J . Am. Chem. Soc. 2002, 124, 3502.
(9) (a) Rosenthal, U.; Pellny, P.-M.; Kirchbauer, F. G.; Burlakov, V.
V. Acc. Chem. Res. 2000, 33, 119. Rosenthal, U.; Burlakov, V. V. In
Titanium and Zirconium in Organic Synthesis; Marek, I., Ed.; Wiley-
VCH: Weinheim, Germany, 2002; p 355. (b) The state of the 3 a 4 a
5 equilibrium can be very dependent on the substituents at the
acetylide units: Rosenthal, U.; Arndt, P.; Baumann, W.; Burlakov, V.
V.; Spannenberg, A. J . Organomet. Chem. 2003, 670, 84.
(10) (a) Dahlmann, M.; Erker, G.; Bergander, K. J . Am. Chem. Soc.
2000, 122, 7986. (b) Moore, J . W.; Pearson, R. G. In Kinetics and
Mechanism; Wiley: New York, 1981; pp 178-181. Robinson, P. J . J .
Chem. Educ. 1978, 55, 509.

A Planar-Tetracoordinate Carbon Intermediate
Organometallics, Vol. 23, No. 19, 2004 4393
Sch em e 2a
a
Legend: R1 ) CMe3 (a ), Ph (b), Me (c); RCp ) C5H5 MeC5H4
(3′). Calculated relative energies (in kcal/mol) are given in
brackets.
clopropene C(sp2)-C(sp2) σ bond into a direct bonding
contact with the strongly electrophilic zirconium cen-
ter.11,12 The core of the molecule has remained com-
pletely planar. The overall bonding situation in 8 is in
principle very similar to that in the organometallic
systems 1, depicted in Scheme 1, except that in 8 an
electron-rich carbon-carbon σ bond from a strained
all-sp2-carbon three-membered carbocycle is interacting
with the available metal acceptor orbital in the metal-
locene σ ligand plane. The resulting very distorted
structure of 8 featuring short Zr-C4 (2.396 Å) and
Zr-C2 (2.609 Å) contacts consequently contains a
planar-tetracoordinated carbon atom (C4) of a type
similar to that found in the experimental structures of
the complex types 1 (see Figure 2).
F igu r e 2. DFT-calculated structure of the internal alkyne
insertion product 8.
Sch em e 3. P ossible Rea ctive In ter m ed ia tes of th e
Con ver sion 3 f 6a
The internal σ coordination of the methylenecyclo-
propene carbon-carbon σ bond to the strongly electro-
philic zirconium center in 8 constitutes a major stabi-
lizing factor, making the product of internal alkyne
insertion (8) more favorable by ca. 7 kcal mol-1 than
the hypothetical, not C4-tetracoordinated isomer 7,
which could not be obtained as a stable minimum
in our computations. Nevertheless, the structurally
attractive system 8 is probably still too high in energy
(∆E(3/8) ≈ +19 kcal mol-1) to serve as a reactive
intermediate in the observed reaction under the actual
experimental conditions. However, we have located a
similarly structured likely intermediate of the 3 f 6
conversion on this hypersurface, which already con-
tained 1 equiv of the nitrile added. The DFT calculated
structure of 9 (see Scheme 3 and Figure 3) is only +6.7
kcal mol-1 in energy above the 3 plus acetonitrile
reference. It contains acetonitrile coordinated to zirco-
nium (d(Zr-N) ) 2.329 Å), and it has the completed
a
DFT-calculated relative energies (in kcal/mol) are given
in brackets. [Zr] denotes Cp2Zr, and [B] denotes B(C6F5)3.
methylenecyclopropene moiety located in the central σ
ligand plane of the bent metallocene unit. Again, the
three-membered carbocycle is side-on coordinated to the
metal center, but this essential σ-C-C coordination is
much more unsymmetrical than in 8. In 9 the Zr-C5-
C4 angle amounts to 88.3° and the Zr‚‚‚C2 distance has
opened to 3.150 Å, but the Zr-C4 bond has remained
strong at 2.496 Å. Carbon atom C4 is planar tetracoor-
dinate. It features four close contacts in a single plane
(C4-C5 ) 1.356 Å, C4-C3 ) 1.469 Å, C4-C2 ) 1.477
Å, and C4-Zr ) 2.496 Å).
It seems likely that the topologically necessary in-
tramolecular alkyne insertion into the C(sp2)-Zr bond
of 3 is triggered by nitrile, a feature that is probably
thermodynamic in origin to help to overcome the
enormous endothermicity of the underlying hexenyne
to dimethyl-methylenecyclopropene cyclization within
the zwitterionic framework of the zirconium/boron be-
taine system 3. The calculated likely intermediate (9)
of this unusual reaction seems to contain a planar-
(11) For related examples of σ interactions at unusually structured
bent metallocene complexes see, e.g.: (a) Suzuki, N.; Nishiura, M.;
Wakatsuki, Y. Science 2002, 295, 660. Suzuki, N:; Aihara, N.; Taka-
hara, H.; Watanabe, T.; Iwasaki, M.; Saburi, M.; Hashizume, D.;
Chihara, T. J . Am. Chem. Soc. 2004, 126, 60. (b) Lam, K. C.; Lin, Z.
Organometallics 2003, 22, 3466. J emmis, E. D.; Phukan, A. K.; J iao,
H.; Rosenthal, U. Organometallics 2003, 22, 4958.
(12) See for a comparison: Sorger, K.; Schleyer, P. v. R.; Stalke, D.
J . Chem. Soc., Chem. Commun. 1995, 2279. Sorger, K.; Schleyer, P. v.
R.; Fleischer, R. S. Stalke, D. J . Am. Chem. Soc. 1996, 118, 6924.
J ensen, T. R.; Marks, T. J . Macromolecules 2003, 36, 1775.

4394 Organometallics, Vol. 23, No. 19, 2004
Erker et al.
solution. Crystal data are as follows: formula C36H20BF15Zr,
Mr ) 839.55, colorless crystal 0.35 × 0.15 × 0.10 mm, a )
15.6168(1) Å, b ) 20.0336(2) Å, c ) 20.2833(2) Å, â )
90.012(1)°, V ) 6345.85(10) Å3, Fcalcd ) 1.758 g cm-3, µ ) 4.63
cm-1, empirical absorption correction (0.855 e T e 0.955), Z
) 8, monoclinic (pseudo-orthorhombic with the twinning law
100, 0,-1,0, 0,0,-1), space group P21/c (No. 14), λ ) 0.71073
Å, T ) 198 K, ω and æ scans, 48 557 reflections collected ((h,
(k, (l), (sin θ)/λ ) 0.65 Å-1, 13 397 independent (Rint ) 0.059)
and 11 026 observed reflections (I g 2σ(I)), 964 refined
parameters, R1 ) 0.038, wR2 ) 0.073, maximum residual
electron density 0.37 (-0.55) e Å-3, hydrogens calculated and
refined as riding atoms, ratio of the twins refined to 0.629(1):
0.371(1), two almost identical molecules in the asymmetric
unit. The data set was collected with a Nonius KappaCCD
diffractometer, equipped with a rotating anode generator
Nonius FR591. Programs used: data collection COLLECT
(Nonius BV, 1998), data reduction Denzo-SMN,13 absorption
correction SORTAV,14 structure solution SHELXS-97,15 struc-
ture refinement SHELXL-97,16 graphics SCHAKAL.17 Crystal-
lographic data (excluding structure factors) for the structures
reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication
CCDC-229224. Copies of the data can be obtained free of
charge on application to The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, U.K. (fax, int. code +44(1223)336-033;
e-mail, deposit@ccdc.cam.ac.uk).
Rea ction of th e Beta in e System 3 w ith P iva lon itr ile.
(a ) F or m a tion of th e Ad d u ct 4a a t 213 K. An NMR tube
was charged with complex 3 (5.00 mg, 6.20 µmol). d8-Toluene
(1 mL) was added and the solution cooled to -78 °C. At this
temperature 1.40 µL (1.03 mg, 12.4 µmol) of pivalonitrile was
added. The system was mixed and directly monitored by NMR
at -60 °C. This revealed that the adduct 4a was selectively
formed under these conditions. 1H NMR (toluene-d8, 599.9
MHz, 213 K): δ 5.54, 5.27 (each s, each 5H, Cp), 1.95 (s, 3H,
1-H), 1.21 (s, 3H, 6-H), 0.61 (s, 9H, 9-H). 13C{1H} NMR
(toluene-d8, 150.4 MHz, 213 K): δ 198.2 (C2), 120.5 (C7), 110.8,
110.3 (Cp), 103.3 (C5), 68.2 (C4), 28.1 (C8), 25.2 (C1), 25.0 (C9),
7.5 (C6). The one-dimensional 13C data are extracted from the
two-dimensional GHSQC and GHMBC experiments. GHSQC
(toluene-d8, 150.4 MHz/599.9 MHz, 213 K): δ(13C)/δ(1H) 110.8,
110.3/5.54, 5.27 (Cp/Cp), 25.2/1.95 (C1/1-H), 7.5/1.21 (C6/6-H),
25.0/0.61 (C9/9-H). GHMBC (toluene-d8, 150.4 MHz/599.9
MHz, 213 K): δ(13C)/δ(1H) 198.2/1.95 (C2/1-H), 120.5/0.61 (C7/
9-H), 103.3/1.21 (C5/6-H), 68.2/1.21(C4/6-H), 28.1/0.61 (C8/9-
H).
F igu r e 3. DFT-calculated structure of 9 featuring a
planar-tetracoordinate carbon atom (C4).
tetracoordinate carbon atom. Stable organometallic
systems exhibiting such unusual coordination geom-
etries of carbon had found interest mostly because of
their unusual structural and bonding features.5 Our
example shows that forming planar-tetracoordinate
carbon geometries might also help to stabilize reactive
intermediates and, thus, “anti-van’t Hoff/LeBel” carbon
compounds may in some cases play a much more
important role than previously thought.
Exp er im en ta l Section
Reactions and handling of the organometallic reagents and
products was carried out in an inert atmosphere (argon) using
Schlenk-type glassware or in a glovebox. Solvents (including
the deuterated solvents used for spectroscopic characterization)
were dried and distilled under argon prior to use. For ad-
ditional general information, including a list of instruments
used for spectroscopic and physical characterization of the
compounds, see ref 10a. The bis(propynyl)zirconocene com-
plexes 2 (Cp2Zr(CtCCH3)2) and 2′ ((MeC5H4)2Zr(CtCCH3)2)
and the betaine system 3 were prepared according to literature
procedures.7,8
Rea ction of Bis(η5-m eth ylcylop en ta d ien yl)bis(p r op y-
n yl)zir con iu m (2′) w ith Tr is(p en ta flu or op h en yl)bor a n e:
Syn th esis of Com p lex 3′. Complex 2′ (2.00 g, 6.11 mmol)
was mixed with tris(pentafluorophenyl)borane (3.13 g, 6.11
mmol). At -78 °C cold toluene (20 mL) was added. After 1 h
at low temperature the mixture was warmed to room temper-
ature and stirred overnight. The precipitated orange colored
product 3′ was collected by filtration, washed with pentane,
and dried in vacuo. Yield: 4.16 g (81%). Anal. Calcd for
(b) F or m a tion of th e Meta lla cyclocu m u len e 5 a n d
Su bsequ en tly of th e In ser tion P r od u ct 6a . The reaction
mixture described above was slowly warmed to room temper-
ature. NMR spectroscopy showed that first complex 5 was
formed, which was subsequently consumed during several
hours at ambient temperature to eventually give 6a . Data for
complex 5 are as follows. 1H NMR (toluene-d8, 599.9 MHz, 213
K): δ 5.01 (s, 10H, Cp), 2.77 (s, 6H, CH3); borane-nitrile
adduct δ 0.58 (s, 9-H, CH3). 13C{1H} NMR (toluene-d8, 150.4
MHz, 213 K): δ 170.1 (C1), 103.8 (C2), 102.7 (Cp), 17.6 (CH3);
borane-nitrile adduct δ 119.1 (CN), 26.1 (CH3). The CMe3
carbon signal was not observed.
(c) P r ep a r a tion of th e P iva lon itr ile In ser tion P r od u ct
6a . Pivalonitrile (0.25 mL, 188 mg, 2.26 mmol) was added to
a suspension of 600 mg (0.74 mmol) of complex 3 in 30 mL of
(13) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307-
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(14) Blessing, R. H. Acta Crystallogr. 1995, A51, 33-37. Blessing,
R. H. J . Appl. Crystallogr. 1997, 30, 421-426.
(15) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467-473.
(16) Sheldrick, G. M. SHELXL-97; Universita¨t Go¨ttingen, Go¨ttingen,
Germany, 1997.
C
36H20BF15Zr (839.6): C, 51.49; H, 2.38. Found: C, 51.73; H,
1
2.60. H NMR (d6-benzene, 200 MHz, 300 K): δ 5.23 (br, 8H,
C5H4), 2.26 (s, 3H, CH3), 1.39 (br s, 6H, C5H4CH3), 1.09 (s, 3H,
CC-CH3). 11B NMR (d6-benzene, 64.2 MHz, 300 K): δ -13.3.
X-r a y Cr ysta l Str u ctu r e An a lysis of Com p lex 3′. Single
crystals were grown over several months from a toluene
(17) Keller, E. SCHAKAL; Universita¨t Freiburg, Freiburg, Germany,
1997.

A Planar-Tetracoordinate Carbon Intermediate
Organometallics, Vol. 23, No. 19, 2004 4395
toluene at -78 °C. The mixture was slowly warmed to room
temperature and then stirred for a further 6 h. Volatiles were
removed in vacuo to give the product 6a (564 mg, 78%) as a
yellow solid as a 4:1 mixture of two isomers. Mp 238 °C dec.
Anal. Calcd for C44H34BF15N2Zr (987.9): C, 54.05; H, 3.50; N,
Deter m in a tion of th e Rea ction Ra te of th e 3 + Ben -
zon itr ile Rea ction To Yield 6b. Complex 3 was dissolved
with a defined quantity of ferrocene (used as an internal
standard) in d8-toluene at room temperature in an NMR tube.
The solution was cooled to -78 °C, and 7.5 µL of pivalonitrile
was added slowly via syringe. After the system was carefully
mixed inside the cooling bath, the NMR tube was transferred
to the NMR spectrometer at -50 °C. The reaction was then
started by raising the temperature to -30 °C, and the
measurement was also started. The decrease of the concentra-
tion of the educt (which was the corresponding benzonitrile
adduct 4b) was monitored using the corresponding 1H NMR
Cp resonances at δ 5.67 and 5.38 (each s, each 5H), and the
increase of the concentration of the product (6b) was followed
1
2.87. Found: C, 54.00; H, 3.81; N, 3.35. H NMR (toluene-d8,
599.9 MHz, 213 K): major isomer, δ 5.49 (s, 10H, Cp), 2.23 (s,
3H, 6-H), 1.62 (s, 3H, 5-H), 1.07 (s, 9H, 9-H), 0.70 (s, 9H,
12-H); minor isomer, δ 5.51 (s, 10H, Cp), 2.26 (s, 3H, 6-H),
1.77 (s, 3H, 5-H), 1.04 (s, 9H, 9-H), 0.65 (s, 9H, 12-H). 13C{1H}
NMR (toluene-d8, 150.4 MHz, 213 K): major isomer, δ 187.2
(C7), 166.0 (C1), 149.8 (C3), 145.6 (C2), 148.9 (dm, 1J CF ) 244
1
Hz, o-B(C6F5)3), 139.5 (dm, J CF ) 246 Hz, p-B(C6F5)3), 147.7
1
(dm, J CF ) 258 Hz, m-B(C6F5)3), 141.0 (C10), 122.8 (broad,
1
ipso-B(C6F5)3), 110.3, 110.2 (Cp), 83.8 (C4), 38.9 (C8), 30.2 (C9),
28.1 (C11), 26.2 (C12), 22.5 (C6), 11.5 (C5); minor isomer, δ
187.2 (C7), 166.0 (C1), 147.9 (C3), 145.4 (C2), 140.3 (C10),
111.6, 111.5 (Cp), 82.1 (C4), 41.5 (C8), 29.9 (C9), 29.4 (C11),
26.3 (C12), 21.3 (C6), 11.5 (C5). The 13C resonances of C1 were
not observed; the 13C resonances of the C6F5 unit were
superposed. 11B{1H} NMR (toluene-d8, 64.2 MHz, 300 K): δ
-17.3 (ν1/2 ) 3 Hz). 19F NMR (toluene-d8, 282.4 MHz, 213 K):
major isomer, δ -134.3 (o-B(C6F5)3), -163.0 (p-B(C6F5)3),
-167.5 (m-B(C6F5)3); minor isomer, δ -133.8 (o-B(C6F5)3),
-162.3 (p-B(C6F5)3), -167.1 (m-B(C6F5)3).
by integration of the corresponding H NMR Cp resonance at
δ 5.49 (s, 10H). Four independent series of measurements were
carried out (for details see the Supporting Information) to give
an averaged value of ∆Gqchem(243 K) ) 16.8 ( 0.1 kcal mol-1
.
DF T Ca lcu la tion s. All quantum-chemical calculations
have been performed with the TURBOMOLE suite of pro-
grams.18a The structures have been fully optimized at the
density functional (DFT) level employing the B-LYP func-
tional.18b A Gaussian AO basis of valence-triple-ú quality,
including polarization functions (TZVP),18c was used for heavy
atoms, except for the C6F5 substituents, for which a split-
valence basis set with polarization functions (SVP)18d was used.
One additional f-polarization function (R ) 0.988 993) from the
TURBOMOLE basis set library and a relativistic pseudo-
potential (small core)18e were employed for zirconium. The RI
approximation was used for the two-electron integrals.18f
Rea ction of Com p lex 3 w ith Ben zon itr ile a t 213 K.
Gen er a tion of th e Ad d u ct 4b. An NMR tube was charged
with 5.00 mg (6.20 µmol) of complex 3 and 1 mL of d8-toluene
and then cooled to -78 °C. At this temperature 1.30 µL (1.28
mg, 1.24 µmol) of benzonitrile was added and the system mixed
and characterized by NMR spectroscopy at -60 °C. At low
temperature, only the formation of the adduct 4b was ob-
Ackn owledgm en t. Financial support from the Deut-
sche Forschungsgemeinschaft and the Fonds der Che-
mischen Industrie is gratefully acknowledged.
1
served. H NMR (toluene-d8, 599.9 MHz, 213 K): δ 6.67, 6.49
(each m, 5H, Ph), 5.62, 5.36 (each s, each 5H, 2 Cp), 1.96 (s,
3H, 1-H), 1.24 (s, 3H, 6-H). 13C{1H} NMR (toluene-d8, 150.4
MHz, 213 K): δ 198.0 (C2), 131.8, 131.8, 128.4 (Ph), 111.6
(Phipso), 110.6, 110.3 (Cp), 106.4 (C5), 63.8 (C4), 7.5 (C1), 1.4
(C6) (C3 resonance was not observed). GHSQC (toluene-d8,
150.4 MHz/599.9 MHz, 213 K): δ(13C)/δ(1H) 131.8, 131.8. 128.4/
6.67, 6.49 (Ph/Ph), 110.6, 110.3/5.62, 5.36 (Cp/Cp), 7.5/1.96 (C1/
1-H), 1.4/1.24 (C6/6-H). GHMBC (toluene-d8, 150.4 MHz/599.9
MHz, 213 K): δ(13C)/δ(1H) 198.0/1.96 (C2/1-H), 131.8, 128.4/
6.67, 6.49 (Ph/Ph), 111.6/6.49 (Phipso/Ph), 110.6, 110.3/5.36, 5.62
(Cp/Cp), 106.4/1.24 (C5/6-H), 63.8/1.24 (C4/6-H).
Su p p or tin g In for m a tion Ava ila ble: Text, tables and
figures giving details of the DFT calculations, additional
spectroscopic data, and details of the kinetic study and the
X-ray crystal structure analysis; crystallographic data are also
given as a CIF file. This material is available free of charge
via the Internet at http://pubs.acs.org.
OM0495628
When the mixture was warmed to room temperature, the
(18) (a) Ahlrichs, R.; Ba¨r, M.; Baron, H.-P.; Bauernschmitt, R.;
Bo¨cker, S.; Ehrig, M.; Eichkorn, K.; Elliott, S.; Furche, F.; Haase, F.;
Ha¨ser, M.; Horn, H.; Huber, C.; Huniar, U.; Kattannek, M.; Ko¨lmel,
C.; Kollwitz, M.; May, K.; Ochsenfeld, C.; O¨ hm, H.; Scha¨fer, A.;
Schneider, U.; Treutler, O.; von Arnim, M.; Weigend, F.; Weis, P.;
Weiss, H. TURBOMOLE (Version 5.3); Universita¨t Karlsruhe, Karl-
ruhe, Germany, 2000. (b) Becke, A. D. Phys. Rev. A 1988, 38, 3098.
Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (c) Scha¨fer,
A. Huber, C.; Ahlrichs, R. J . Chem. Phys. 1994, 100, 5829. (d) Scha¨fer,
A.; Horn, H.; Ahlrichs, R. J . Chem. Phys. 1992, 97, 2571. (e) Andrae,
D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim.
Acta 1990, 77, 123. (f) Eichkorn, K.; Treutler, O.; O¨ hm, H.; Ha¨ser, M.;
Ahlrichs, R. Chem. Phys. Lett. 1995, 240, 283.
formation of the previously described product 6b7,8 was
1
monitored. H NMR (toluene-d8, 599.9 MHz, 298 K): δ 6.88,
6.79, 6.64 (each m, 10H, Ph), 5.52 (s, 10H, Cp), 1.85 (s, 3H,
5-H), 1.28 (s, 3H, 6-H). 13C{1H} NMR (toluene-d8, 150.4 MHz,
1
213 K): δ 181.6 (C7), 173.5 (C3), 153.7 (C2), 149.0 (dm, J CF
1
) 241 Hz, o-B(C6F5)3), 145.7 (C8), 139.6 (dm, J CF ) 246 Hz,
1
p-B(C6F5)3), 137.4 (dm, J CF ) 247 Hz, m-B(C6F5)3), 133.9,
133.5, 132.8, 130.6, 129.8, 129.5, 128.5, 127.8 (Ph), 112.6 (Cp),
87.6 (C4), 15.9 (C5), 9.0 (C6). The 13C resonances of C1 and
ipso-B(C6F5)3 were not observed.