Molecular Structure of a Heptadentate Cogwheel: C7Me7+ Is Not Planar

Full text of DOI:10.1021/jo000958c

J . Org. Chem. 2000, 65, 6795-6797
6795
Molecu la r Str u ctu r e of a Hep ta d en ta te
+
Cogw h eel: C7Me7 Is Not P la n a r †
Matthias Tamm,* Bernd Dreâel, and Roland Fro¨hlich‡
Anorganisch-Chemisches Institut, Westfa¨lische
Wilhelms-Universita¨t, Wilhelm-Klemm-Strasse 8,
48149 Mu¨nster, Germany
mtamm@uni-muenster.de
Received J une 26, 2000
F igu r e 1. Schematic presentation of CnRn complexes (n ) 5,
6, 7).
Sch em e 1a
η-Cyclopentadienyl complexes, (η-C5R5)M, as well as
η-arene complexes, (η-C6R6)M, are among the most
important classes of compounds in organotransition
metal chemistry.1 In particular, the widespread use of
cyclopentadienes as organometallic ligands is due to the
large number of ring-substituted derivatives that have
been synthesized since the serendipitous synthesis and
discovery of ferrocene. Succesive replacement of hydrogen
atoms by alkyl or aryl substituents alters the steric and
electronic properties of the Cp ring and gives rise to
increased steric bulk, solubility and stability of the
resulting complexes.2 Consequently, the introduction of
persubstituted derivatives such as the pentamethyl-
cyclopentadienyl ligand Cp* has resulted in the synthesis
of a large number of novel complexes and in the discovery
of new reactivity patterns.1,3 This also holds true in the
area of ring-substituted arene ligands, and the per-
methylated analogue hexamethylbenzene has been em-
ployed in the synthesis of many transition metal com-
plexes (Figure 1).1,4
a
Key: (a) CH2Cl2, N2CHCH3; (b) CCl4, PCl5; (c) AcOH, HBF4.
In continuation of our work on transition metal com-
plexes with novel cycloheptatrienyl ligands,9 we became
interested in the synthesis of salts containing the hep-
tamethylcycloheptatrienyl cation, C7Me7+, and in the use
of these compounds as ligands in organotransition metal
chemistry. Whereas our attempts to realize the latter are
still under investigation, we report herein an improved
protocol for the synthesis of heptamethylcycloheptatriene
2 from hexamethylbenzene 1 together with the synthesis
and structural characterization of (C7Me7)BF4 3.
A multistep synthesis of heptamethylcycloheptatriene
2 has been reported by Knoche.10 The first step involves
ring-expansion of C6Me6 1 with ethyl diazoacetate, a
reaction which does not only give the desired carbene
addition product ethoxycarbonylhexamethylcyclohepta-
triene and its isomers, but also a significant amount of
the C-H insertion product pentamethyldihydrocinnamic
ethyl ester. Separation of this mixture followed by
reduction of the ethoxycarbonyl group to a methyl group
via the sequence RCOOEt f RCH2OH f RCH2Cl f
RCH3 does only allow the isolation of 2 in very low yield
(3.8% based on ethyl diazoacetate).10
In contrast, cycloheptatrienyl complexes, (η-C7R7)M,
with sterically demanding peralkylated or perarylated
ligands are unknown despite the fact that the aromatic
+
+
cycloheptatrienyl cations C7Me7 and C7Ph7 have been
known for decades.5,6 The molecular structure of C7Ph7
however, has been determined only recently and repre-
sents the first structurally cycloheptatrienyl cation with
seven substituents prior to this work.7 Attempts to
coordinate transition metals to the central seven-
membered ring of this perphenylated π-perimeter have
not been successful so far.8
,
+
† The title refers to ref 7: “The Structure of a Seven-Bladed
+
Propeller: C7Ph7 is Not Planar”.
‡ Organisch-Chemisches Institut, Westfa¨lische Wilhelms-Universi-
ta¨t, Corrensstrasse 40 Wilhelm-Klemm-Str. 8, 48149 Mu¨nster, Ger-
many.
(1) (a) Comprehensive Organometallic Chemistry; Wilkinson, G.,
Stone, F. G. A., Abel, E. W., Eds; Pergamon Press: Oxford, 1982. (b)
Comprehensive Organometallic Chemistry II; Abel, E. W., Stone, F.
G. A., Wilkinson, G., Eds; Chapman and Hall: Oxford, 1995. (c)
Metallocenes; Togni, A., Halterman, R. L., Eds; Wiley-VCH: Weinheim,
1998.
(2) (a) J aniak, C.; Schumann, H. Adv. Organomet. Chem. 1991, 33,
331. (b) Okuda, J . Top. Curr. Chem. 1991, 160, 99.
(3) King, R. B.; Bisnette, M. B. J . Organomet. Chem. 1967, 8, 287.
(4) Fischer, E. O.; O¨ fele, K.; Essler, H.; Fro¨hlich, W.; Martensen, J .
P.; Semmlinger, W. Chem. Ber. 1958, 91, 2763.
Therefore, the development of a coordination chemistry
based on the C7Me7 system required a much better
protocol for the synthesis of 2. According to Scheme 1, 2
can be synthesized in one step from hexamethylbenzene11
1 by ring-expansion with diazoethane which is generated
(5) Takeuchi, K.; Yokomichi, Y.; Okamoto, K. Chem. Lett. 1977,
1177.
(6) Battiste, M. A. J . Am. Chem. Soc. 1961, 83, 4101.
(7) Brydges, S.; Britten, J . F.; Chao, L. C. F.; Gupta, H. K.;
McGlinchey, M. J . Chem. Eur. J . 1998, 4, 1201.
(8) (a) Chao, L. C. F.; Gupta, H. K.; Hughes, D. W.; Britten, J . F.;
Rigby, S. S.; Bain, A. D.; McGlinchey, M. J . Organometallics 1995, 14,
1139. (b) Gupta, H. K.; Brydges, S.; McGlinchey, M. J . Organometallics
1999, 18, 115.
(9) Tamm, M.; Bannenberg, T.; Baum, K.; Fro¨hlich, R.; Steiner, T.;
Meyer-Friedrichsen, T.; Heck, J . Eur. J . Inorg. Chem. 2000, 1161 and
references therein.
(10) Knoche, H. Chem. Ber. 1966, 99, 1097.
(11) Scha¨fer, W. Angew. Chem. 1966, 78, 716; Angew. Chem., Int.
Ed. Engl. 1966, 5, 669.
10.1021/jo000958c CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/14/2000

6796 J . Org. Chem., Vol. 65, No. 20, 2000
Notes
from N-nitroso-N-ethylurea.12 The conversion of 1 into 2
proceeds in only modest yield (30%). The unreacted
starting material, however, can be completely re-isolated
and used again without any further purification step
giving a nearly 100% yield based on re-isolated 1.13
Hydride abstraction from 2 to form the C7Me7+ cation
has proven to be rather difficult. The use of complex salts
containing the triphenylmethyl cation, Ph3C+, were
completely unsuccessful, although triphenylmethane was
obtained as the result of the hydride abstraction.10
However, 3 can be synthesized from 2 according to the
procedure published by Takeuchi et al.5 Hence, treatment
of 2 with PCl5 followed by dissolution of the intermediate
precipitates, presumably a mixture of chloride and
hexachlorophosphate salts, in acetic acid/acetic anhydride
and addition of aqueous HBF4 gives 3 in moderate yield.14
+
F igu r e 2. Molecular structure of C7Me7
.
The 1H NMR spectrum of 3 in CD3CN exhibits one
resonance for the methyl groups at 2.62 ppm. The 13C
NMR resonances for the methyl and ring carbon atoms
are observed at 24.4 and 160.2 ppm, respectively. The
latter is observed at only sligthly lower field compared
to the 13C NMR resonance of the unsubstituted tropylium
cation and adopts an intermediate position between
Ta ble 1. Selected Bon d Len gth s (Å) a n d An gles (Deg) in
3‚1/3CH3COOHa
molecule A
molecule B
molecule C
C1-C2
1.315(10)
1.321(10)
1.332(10)
1.392(11)
1.527(13)
1.429(12)
1.356(10)
1.38
1.399(7)
1.397(7)
1.409(8)
1.408(8)
1.392(8)
1.404(7)
1.401(7)
1.40
1.410(7)
1.420(7)
1.390(8)
1.395(7)
1.389(6)
1.423(6)
1.375(7)
1.40
C1-C7
C2-C3
C3-C4
+
+ 15
C7H7 and C7Ph7
.
C4-C5
C5-C6
After the addition of diethyl ether 3 crystallized
directly from the reaction mixture affording single crys-
tals of 3‚1/3CH3COOH suitable for X-ray structure deter-
mination.16 The asymmetric unit contains three inde-
pendent molecules, and the molecular structure of one
cation (molecule B) is shown in Figure 2.
Due to disorder of the incorporated acetic acid the
structure could only be refined with reduced accuracy.
Molecule A is most strongly affected by this disorder as
indicated by larger standard deviations of the respective
bond lengths and angles (Table 1). Therefore, a meaning-
C6-C7
L C-C
C1-C2-C3
C2-C3-C4
C3-C4-C5
C4-C5-C6
C5-C6-C7
C6-C7-C1
C7-C1-C2
φ
127.5(7)
126.5(6)
122.3(5)
122.6(6)
123.3(5)
129.6(5)
127.0(6)
30.6
125.5(4)
126.2(5)
126.6(5)
124.9(4)
125.6(4)
127.2(4)
125.2(4)
28.8
128.1(4)
124.7(4)
127.2(4)
124.3(4)
125.7(4)
126.1(4)
122.9(4)
31.8
γ
21.7
21.4
19.1
a
Standard deviations are given in parentheses.
ful discussion of the bonding parameters must be con-
fined to molecules B and C. The intra-ring bond distances
in these cations do not alter significantly, and a mean
value of 1.40 Å is observed which is similar to the average
C-C distance found for the C7Ph7+ cation (L ) 1.41 Å).7
Figure 2 shows an alternative presentation of the
cation and reveals that the seven-membered ring is not
planar but rather adopts a boat conformation. Similar
(12) (a) Arndt, F.; Scholz, H. Angew. Chem. 1933, 46, 47. (b) Werner,
E. A. J . Chem. Soc. 1919, 1093.
(13) A solution of diazoethane was prepared by the addition of solid
N-ethyl-N-nitrosourea (25 g, 210 mmol) to a mixture of KOH (23 g,
410 mmol), water (60 mL) and CH2Cl2 (180 mL) at -15 °C over 45
min. The organic phase was separated using a cooled separatory funnel
and was dried over KOH pellets for 1 h at -78 °C. The diazoethane
solution was added via a cooled (-78 °C) dropping funnel to a refluxing
mixture of 1 (4.94 g, 30 mmol), CuBr (10 g, 74 mmol) and CH2Cl2 (30
mL) over 75 min. CuBr was removed by filtration over Celite, and all
volatiles were removed in vacuo. 2 was extracted from the residue with
petroleum ether (3 × 15 mL). Evaporation afforded 1.6 g of 2 in
essentially pure form: 1H NMR (600 MHz, CDCl3): δ 1.98 (q, J ) 7.8
Hz, 1H, 7-CH), 1.94 (s, 6H, CH3), 1.76 (m, 6H, CH3), 1, 74 (s br, 6H,
CH3), 1.20 (d, 3H, 7-CH3); 13C NMR (151 MHz, CDCl3): δ 134.9, 131.5,
126.3 (CdC), 39.4 (7-CH), 17.7, 16.1, 14.2, 14.1 (CH3).
+
boat forms have been observed for the C7Ph7 cation as
well as for the seven-membered rings in [7]circulene17
and in the bis[tris(bicyclo[2.2.2]octeno)tropylium]acetylene
dication.18 The bend angles φ and γ for molecules A, B
and C have been determined and are summarized in
Table 1. These results are in agreement with semiem-
pirical MO calculations which predict a ground-state Cs-
optimized structure with interplanar angles of 22° (φ) and
18° (γ) to be almost 15 kcal mol-1 more stable than its
planar C7h conformer.7 Comparison with the molecular
strucuture of the perphenylated cation C7Ph7+ (φ ) 10.7°,
γ ) 12.8°)7 confirms the prediction that nonplanar
substituents unlike the phenyl group would increase the
distortion of the central C7 ring (Figure 3).
(14) Anal. Calcd for C14H21BF4 (Mr ) 276.12): C, 60.89; H, 7.66.
Found: C, 59.20; H, 7.85.
(15) The 13C NMR spectra (in CD3CN) of (C7H7)BF4 and (C7Ph7)-
BF4 exhibit resonances for the ring carbon atoms at 156.3 and 167.2
ppm, respectively.
(16) X-ray crystal structure analysis of 3‚1/3CH3COOH: formula
C14H21BF4‚1/3CH3COOH, M ) 296.14, yellow crystal, 0.30 × 0.30 ×
0.25 mm, a ) 14.982(2), b ) 9.063(1), c ) 17.975(4) Å, â ) 106.70(1)°,
V ) 2337.7(7) Å3, Fcalcd ) 1.262 g cm-3, F(000) ) 940 e, µ ) 1.07 cm-1
,
absorption correction (0.969 e T e 0.974), Z ) 6, monoclinic, space
group P21 (No. 4), λ ) 0.71073 Å, T ) 198 K, ω and æ scans, 9838
reflections collected ((h, (k, (l), [(sin θ)/λ] ) 0.65 Å-1, 9838 indepen-
dent (Rint ) 0.000) and 5953 observed reflections [I g 2 σ(I)], 552 refined
parameters, R ) 0.096, wR2 ) 0.254, max. residual electron density
0.52 (-0.40) e Å-3, hydrogens calculated and refined as riding atoms,
three chemically identical molecules with different conformation in the
asymmetric unit. The anisotropic displacement parameters of molecule
A indicate some unrefined positional disorder. Data set was collected
with a Nonius KappaCCD diffractometer, equipped with a rotating
anode generator. Programs used: data acquisition COLLECT, data
reduction DENZO-SMN, absorption correction SORTAV, structure
solution SHELXS-86, structure refinement SHELXL-97, graphics
DIAMOND.
In contrast to C7Me7+, the corresponding, iso-π-
electronic perimeters C5Me5- and C6Me6 exist as planar
(17) Yamamoto, K.; Harada, T.; Okamoto, Y.; Chikamatsu, H.;
Nakazaki, M.; Nakao, T.; Tanaka, M.; Harada, S.; Kasai, N. J . Am.
Chem. Soc. 1988, 110, 3578.
(18) Kagayama, A.; Komatsu, K.; Nishinaga, T.; Takeuchi, K.;
Kabuto, C. J . Org. Chem. 1994, 59, 4999.

Notes
J . Org. Chem., Vol. 65, No. 20, 2000 6797
Me7)M incorporating early-transition metals in (formally)
low-oxidation states where strong back-bonding from the
metal centers can be expected which will increase the
intra-ring C-C-bond distances significantly. In addition,
-
Umpolung and generation of the anti-aromatic C7Me7
anion seems to be a feasible goal in view of the related
chemistry of the C7Ph7 system.19
+
F igu r e 3. Presentation of the boat-conformation of C7Me7
.
Ack n ow led gm en t. This work was supported by the
Deutsche Forschungsgemeinschaft, the Fonds der Che-
mischen Industrie, and the government of North Rhine-
Westphalia (Bennigsen-Foerder-Preis 2000 to M. T.). We
wish to thank Prof. Dr. F. E. Hahn for his generous
support.
aromatic species1 despite the fact that increasing the
ringsize lengthens the radial distance of the external
methyl groups from the center of the molecule. This,
however, is more than compensated by substantially
decreasing the angle beween adjacent methyl groups and
the center of the internal ring from 72° and 60° to 51.4°
(360°/n, n ) 5,6,7).
The strong distortion together with the electrophilicity
of the heptamethylcycloheptatrienyl cation might impede
applications of the C7Me7 ligand to the synthesis of
Su p p or tin g In for m a tion Ava ila ble: X-ray crystal struc-
ture determination of 3‚1/3CH3COOH. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
J O000958C
+
organometallic derivatives. Nevertheless, we are cur-
rently focusing on the preparation of complexes (η-C7-
(19) (a) Battiste, M. A. J . Am. Chem. Soc. 1962, 84, 3780. (b)
Breslow, R.; Chang, H. W. J . Am. Chem. Soc. 1965, 87, 2200.