Cyclization of the pyruvyl(methoxycarbonyl)tetracarbonyliron complex cis-(CO)4Fe[C(O)C(O)CH3](CO2CH3) induced by pronucleophile reagents

Article abstract of DOI:10.1021/om010989d

Two iron complexes, cis-(CO)₄Fe[C(O)C(O)CH₃](CO₂CH₃) (1a) and cis-(CO)₄Fe[C(O)CO₂-CH₂] [C(O)CH₂] (3), are reported. Each complex can be considered as a λ-keto ester with a metal atom inserted into the chain linking its two organic groups. 1a is the only one found to evolve thermally, similarly to organic λ-keto esters, into the methoxy metallalactone (CO)₄FeC(O)C(CH₃)(OCH₃)OC(O) (2a). This reaction is not reversible and does not require any acid or alkaline catalysis. The process does not result from a spontaneous dissociation of the methoxycarbonyl ligand of la but from an associative intra or intermolecular mechanism. A similar process can also be induced by pronucleophile reagents, and it affords a series of substituted metallalactones: (CO)₄FeC(O)C(CH₃)(Nu)OC(O) (Nu = OCH₃ (2a), OC₂H₅ (2b), SC₂H₅ (2c), P(C₆H₅)₂ (2d), P(C₆H₁₁)₂ (2e)). Complex 2e has been characterized structurally. These substituted lactones are formed by nucleophilic attack of the reagent at the pyruvyl β-carbonyl followed by carbonylate attack on the alkoxycarbonyl ligand.

Full text of DOI:10.1021/om010989d

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2196 Organometallics 2002, 21, 2196-2202
Cycliza tion of th e
P yr u vyl(m eth oxyca r bon yl)tetr a ca r bon ylir on Com p lex
cis-(CO)4F e[C(O)C(O)CH3](CO2CH3) In d u ced by
P r on u cleop h ile Rea gen ts
Patrice Cabon, Murielle Sellin, J ean-Yves Salau¨n,* Ve´ronique Patinec, and
Herve´ des Abbayes
Laboratoire de Chimie, Electrochimie Mole´culaires et Chimie Analytique, UMR CNRS 6521,
UFR Sciences et Techniques, Universite´ de Bretagne Occidentale, 6, avenue le Gorgeu,
BP 809, 29285 Brest Ce´dex, France
Marek M. Kubicki
Laboratoire de Synthe`se et Electrosynthe`se Organome´talliques, UMR CNRS 5632,
Universite´ de Bourgogne, 9, Avenue A. Savary, B.P. 47870, 21078 Dijon Ce´dex, France
Received November 14, 2001
Two iron complexes, cis-(CO)4Fe[C(O)C(O)CH3](CO2CH3) (1a ) and cis-(CO)4Fe[C(O)CO2-
CH3][C(O)CH3] (3), are reported. Each complex can be considered as a γ-keto ester with a
metal atom inserted into the chain linking its two organic groups. 1a is the only one found
to evolve thermally, similarly to organic γ-keto esters, into the methoxy metallalactone
(CO)4FeC(O)C(CH3)(OCH3)OC(O) (2a ). This reaction is not reversible and does not require
any acid or alkaline catalysis. The process does not result from a spontaneous dissociation
of the methoxycarbonyl ligand of 1a but from an associative intra or intermolecular
mechanism. A similar process can also be induced by pronucleophile reagents, and it affords
a series of substituted metallalactones: (CO)4FeC(O)C(CH3)(Nu)OC(O) (Nu ) OCH3 (2a ),
OC2H5 (2b), SC2H5 (2c), P(C6H5)2 (2d ), P(C6H11)2 (2e)). Complex 2e has been characterized
structurally. These substituted lactones are formed by nucleophilic attack of the reagent at
the pyruvyl â-carbonyl followed by carbonylate attack on the alkoxycarbonyl ligand.
Sch em e 1
In tr od u ction
Complexes bearing one acyl, aroyl, or alkoxycarbonyl
ligand have been widely investigated due to their
involvement in catalytic carbonylation processes,1 and
the difference in reactivity of carbonylated ligands and
organic ketones or esters has been well-established. On
the other hand, complexes displaying two carbonylated
ligands are often unstable and thus are less common.
For this reason, no comparison between their chemical
properties and those of diketones, keto acids, or keto
esters has been described. Thus, though studied for
more than 30 years, the well-known ring-chain isomer-
ization process, γ-hydroxy or γ-alkoxy lactone h γ-keto
or γ-aldehydo acids or esters,2 has no real organo-
metallic counterpart. This reversible organic process,
which is of importance in carbohydrate chemistry,3
proceeds from an intramolecular addition of the oxygen
of the ketone group at the carbon of the carbonyl of the
acid or ester functions, followed by migration of the
alkoxy (or the hydroxy) group (Scheme 1). Except for
keto acid compounds, this process requires acid or base
catalysis.
In organometallic chemistry, only a few iron,4 nickel,5
or molybdenum6,7 complexes present reactions that can
be compared to this organic transformation. However,
in these processes the organometallic complex acts as
the migrating group (R1). Such a cyclization has also
been described for a ligand in a tungsten complex whose
* To whom correspondence should be addressed. Fax: (+33)-
298016594. E-mail: jean-yves.salau¨n@univ-brest.fr.
(1) Tkatchenko, I. In Comprehensive Organometallic Chemistry;
Pergamon Press: Oxford, U.K., 1983; Vol. 8, p 101.
(2) Valters, R. E.; Flitsh, W. Ring-chain Tautomerism; Plenum: New
York, 1985.
(4) (a) Ru¨ck-Braun, K.; Martin, T.; Mikulas, M. Chem. Eur. J . 1999,
5, 1028. (b) Ru¨ck-Braun, K.; Mo¨ller, C. Chem. Eur. J . 1999, 5, 1038.
(5) Fukuoka, S.; Ryang, M.; Tsutsumi, S. J . Org. Chem. 1968, 33,
2973.
(6) Bottrill, M.; Green, M. J . Chem. Soc., Dalton Trans. 1979, 820.
(7) Green, M.; Nyathi, J . Z.; Scott, C.; Stone, F. G. A.; Welch, A. J .;
Woodward, P. J . Chem. Soc., Dalton Trans. 1978, 1067.
(3) David, S. Chimie Mole´culaire et Supramole´culaire des Sucres;
Inter Editions/CNRS Editions: Paris, 1995.
10.1021/om010989d CCC: $22.00 © 2002 American Chemical Society
Publication on Web 05/03/2002

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Cyclization of cis-(CO)4Fe[C(O)C(O)CH3](CO2CH3)
Sch em e 2
Organometallics, Vol. 21, No. 11, 2002 2197
Sch em e 3
organometallic part, inserted into the R2 group, is not
directly involved in the cyclization process and may only
increase the polarity of the double bond.8 To our
knowledge, only some platinum complexes are found to
display a metal center inserted into the link bridging
γ-positioned keto and ester groups.9 However, these
compounds, when they do not display trans geometries
unfavorable for achievement of cyclizations,9a are not
reported to thermally induce such reactions.9b
Sch em e 4
In a preliminary work10 we reported the thermal
behavior of a complex which can be considered as a
γ-keto ester with a metal inserted into its organic chain.
This compound, cis-pyruvyl(alkoxycarbonyl)tetracar-
bonyliron Fe(CO)4[C(O)C(O)CH3](CO2CH3) (1a ), affords
thermally the alkoxy γ-metallalactone 2a (Scheme 2)
by a reaction similar to the chain-ring isomerization of
organic γ-keto esters.
This irreversible process, which requires no catalysis,
formally involves the formation of a metallacycle by
C-O coupling between the oxygen of the â-carbonyl of
the pyruvyl and the carbon of the alkoxycarbonyl of 1a
and the migration of the methoxy of this last ligand to
the pyruvoyl â-carbonyl. In the present paper we report
full results of our investigations concerning the mech-
anism of formation of substituted metallalactones analo-
gous to 2a obtained by inducing the reaction with
various nucleophiles.
[C(O)CH3]10 (4) (see Scheme 3). 4 can also be prepared
in low yield (15%)10 by reaction of acetyl chloride with
[Na][Fe(CO)4(CO2CH3)].11
Like 1a and 3, complex 4 displays four IR bands in
the ν(CtO) region. In 13C NMR, the resonances of the
terminal carbonyls are found in 2/1/1 intensities at
respectively 201.4, 199.8, and 198.8 ppm; the signal of
the acetyl carbonyl is found at 242.3 ppm and that of
the alkoxycarbonyl at 195.3 ppm.
The difference in reactivity of complexes 1a and 3 may
result from the difference of mobility observed between
the alkoxy groups of alkoxycarbonyl and alkyloxalyl
ligands. The high mobility of the alkoxy of alkoxycar-
bonyl ligands is well-known; it has been shown to occur
either by a spontaneous dissociation of the ligand in Fe-
(Cp)[C(O)R](CO)[P(Ph)3] (R ) OCH3, OC6H5, SCH3,
SC6H5)12 or by an associative pathway,13 via the tri-
functionalized intermediate H[Fe(CO)3(CO2R)3] in cis-
Fe(CO)4(CO2R)2.14
Resu lts a n d Discu ssion
As mentioned above, thermolysis of 1a affords the
methoxy metallalactone 2a (60% yield). The cis geom-
etry of the complex is established in IR by the presence
of four ν(CtO) bands at 2125, 2085, 2060, and 2050
cm-1 and in 13C NMR (where two different substituents
on the sp3 carbon of the metallacycle induce the in-
equivalency of the two axial carbonyls) by four signals
at 199.8, 199.7, 199.2, and 198.7 ppm in the terminal
carbonyl resonance area. The resonance of the metal-
lacyclic sp3 carbon is observed at 109.9 ppm. The
structure attributed to 2a is confirmed by an X-ray
diffraction study10 showing the envelope shape of the
metallacycle, which displays an out-of-plane sp3 carbon
and a methoxy group in a pseudoaxial position.
To probe a potential contribution of a spontaneous
dissociation of 1a to its process of thermal cyclization,
we synthesized the cation {Fe(CO)5[C(O)C(O)CH3]}+
Thermolysis of the closely related complex Fe(CO)4-
[C(O)CO2CH3][C(O)CH3]10 (3) shows completely differ-
ent behavior. The observed reactions are a carbon-
carbon coupling, affording methyl pyruvate, and a
decarbonylation, giving rise to cis-Fe(CO)4(CO2CH3)-
15
(5a ) by reacting 1a with HBF4 (Scheme 4). The
reaction performed in THF at -50 °C afforded a white
(11) McLean, J . L. Ph.D. Thesis, New York University, 1975.
(12) (a) Grice, N.; Kao, S. C.; Pettit, R. J . Am. Chem. Soc. 1979,
101, 1627. (b) Liu, L. K.; Eke, U. B.; Mesubi, M. A. Organometallics
1995, 14, 3958.
(13) Sellin, M.; Luart, D.; Salau¨n, J . Y.; Laurent, P.; des Abbayes,
H. J . Organomet. Chem. 1998, 562, 183.
(14) Sellin, M.; Luart, D.; Salau¨n, J . Y.; Laurent, P.; Toupet, L.; des
Abbayes, H. Organometallics 1996, 15, 521.
(15) Salau¨n, J . Y.; Le Gall, G.; Laurent, P.; des Abbayes, H. J .
Organomet. Chem. 1992, 441, 99.
(8) Seino, H.; Ishii, Y.; Hidai, M. Inorg. Chem. 1997, 36, 161.
(9) (a) Chen, J . T.; Tzeng, W. H.; Tsai, F. Y.; Cheng, M. C.; Wang,
Y. Organometallics 1991, 10, 3954. (b) Chen, J . T.; Yeh, Y. S.; Yang,
C. S.; Tsai, F. Y.; Huang, G. L.; Shu, B. C.; Huang, T. M.; Chen, Y. S.;
Lee, G. H.; Cheng, M. C.; Wang, C. C.; Wang, Y. Organometallics 1994,
13, 4804.
(10) Preliminary communication: Sellin, M.; Luart, D.; Salau¨n, J .
Y.; Laurent, P.; Toupet, L.; des Abbayes, H. J . Chem. Soc., Chem.
Commun. 1996, 857.

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2198 Organometallics, Vol. 21, No. 11, 2002
Sch em e 5
Cabon et al.
intermolecular nature of the formation of 2a by thermal
evolution of 1a .
When performed in the presence of 100 equiv of
ethanol, the cyclization of 1a was found to afford in 3 h
at 0 °C the ethoxy metallacyclolactone 2b, whose
spectroscopic characteristics (see Experimental Section)
are very close to those displayed by 2a . However, a
careful 13C NMR monitoring of the reaction leading to
2b revealed the transitory formation of an intermediate
species presenting the same characteristics as Fe(CO)4-
(CO2C2H5)[C(O)C(O)CH3] (1b) already obtained by reac-
tion of C2H5ONa with the cationic complex 5a . The
formation of 1b from 1a results from a rapid methoxy/
ethanol exchange, which once more makes more obvious
the mobility of alkoxy groups of alkoxycarbonyl ligands
and shows the more electrophilic sites of 1a to be the
terminal carbonyls and not the â-carbonyl of the pyru-
vyl. Indeed, as already mentioned, the alkoxy/alcohol
exchange on this complex is thought to occur by an
associative pathway via the formation of the trifunc-
tionalized intermediate fac-{(CO)3Fe[C(O)C(O)CH3](CO2-
CH3)(CO2C2H5)}H, formed by addition of EtOH on a
terminal carbonyl of 1a .13,14 The observation of this
methoxy/ethanol exchange prior to the cyclization, af-
fording the ethoxy metallalactone 2b, raises the ques-
tion of whether the formation of the ethoxycarbonyl
complex 1b is essential for the formation of the ethoxy
lactone 2b.
precipitate whose characteristics are in good agreement
with the proposed structure (see Experimental Section).
The cationic nature of the complex was confirmed in 13C
NMR by a shift of its carbonyl resonances toward higher
fields. The two signals of 4/1 intensities at 196.1 and
190.4 ppm in the terminal carbonyl resonance area
together with the resonances of the pyruvyl group at
231.5 and 189.5 ppm suggest the presence of a single
organic ligand on the metal center (the pyruvyl) and
rules out the possible formation of the cyclic isomer 5b,
which would require in 13C NMR three resonances of
2/1/1 intensities for its terminal carbonyls and an
additional signal near 100 ppm.
The stability of 5a shows that a spontaneous cycliza-
tion by a nucleophilic attack of the oxygen of the
â-carbonyl of the pyruvyl on a terminal carbonyl to
afford 5b does not occur. To check the possible formation
of 2a by a nucleophilic addition on 5a of MeO-, which
would be released in solution by a spontaneous dissocia-
tion of 1a , the cationic complex 5a was allowed to react
with 1 equiv of alcoholate (C2H5ONa). The reaction,
performed at low temperature (-20 °C), afforded a new
complex whose characteristics (see Experimental Sec-
tion) are very close to those displayed by 1a . This
compound was characterized as cis-Fe(CO)4(CO2C2H5)-
[C(O)C(O)CH3] (1b) and can also be prepared by reac-
tion of ClC(O)C(O)CH3 with [Fe(CO)4(CO2C2H5)]-. Its
formation results from a nucleophilic addition of C2H5O-
on a terminal carbonyl of 5a . This shows that the more
electrophilic sites of this cation are the terminal carb-
onyls and not the â-carbonyl of the pyruvyl ligand and
rules out the possible intervention of a dissociative
mechanism in the process of formation of 2a .
As we have previously shown that exchanges between
alkoxycarbonyl ligands of iron complexes and thiols do
not occur,16 the reaction of 1a with C2H5SH was thought
to give an answer to this question. In presence of 80
equiv of ethanethiol 1a gives rise over 48 h at 0 °C to
90% of the ethylthio metallalactone 2c. A careful
monitoring of the reaction by 13C NMR confirms that
no exchange occurs between the methoxycarbonyl ligand
of 1a and C2H5SH. Though slow, the reaction of 1a with
C2H5SH only produces 5% of the methoxy metallalac-
tone 2a as a byproduct whose formation probably results
from a nucleophilic addition at the pyruvyl â-carbonyl
of 1a of the methanol released in solution at the first
stages of the formation of 2c. The new complex 2c
displays IR and NMR spectra similar to those of 2a and
2b (see Experimental Section). The carbons linked to
the sulfur atom, however, exhibit 13C NMR resonances
at higher fields than those observed for their homo-
logues of the ethoxy complex 2b.
A second reaction pathway that could account for the
formation of the metallacyclic complex 2a is an inter-
or an intramolecular nucleophilic addition of the meth-
oxy group of 1a on the carbon atom of the â-carbonyl of
the pyruvyl of the same complex (Scheme 5). To check
this assumption, we attempted to perform the cycliza-
tion of 1a in the presence of an excess of pronucleophile
reagents (NuH ) CH3OH, C2H5OH, C2H5SH, HP-
(C6H5)2, HP(C6H11)2). In the presence of a large excess
of methanol (used as solvent), 1a was found to afford
the metallacyclic complex 2a in 4 h at 0 °C (compared
to 24 h at 15 °C under thermolysis conditions). Per-
formed in the presence of 1 equiv of MeOH, the process
requires longer reaction times, however (20 h at 0 °C).
Such an enhancement in the cyclization rate lends
support to a possible induction of formation of the
metallacycle by a nucleophilic addition of methanol at
the â-carbonyl of the pyruvyl ligand. However, this
result does not give any information about the intra- or
Several attempts to bring about the cyclization of 1a
with an excess of amines (diethylamine, aniline) as
pronucleophile reagents failed. These reactions do not
afford any metallacyclic lactones; Fe(CO)5 and pyruva-
mides are the only products observed in the reaction
mixtures. As these last compounds are also obtained by
-
reaction of the carbamyl ferrates Fe[C(O)NR2](CO)4
with pyruvyl chloride, their formation may be due to a
fast thermal evolution of Fe(CO)4[C(O)NR2][C(O)C(O)-
CH3] intermediates formed from 1a by a rapid amine/
alkoxy exchange.17 Fast evolutions of carbamyl pyruvyl
intermediates are not unexpected, since iron complexes
carrying two carbamyl ligands or a carbamyl ligand and
(16) Salau¨n, J . Y.; Ollivier, G. Unpublished results.
(17) Ford, P. C.; Rokicki, A. Adv. Organomet. Chem. 1988, 28, 139.

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Cyclization of cis-(CO)4Fe[C(O)C(O)CH3](CO2CH3)
Organometallics, Vol. 21, No. 11, 2002 2199
a carbonylated organic ligand in cis positions have
already been found to be unstable.18
Soft nucleophiles such as secondary phosphines are
also found to induce the cyclization of 1a . The reaction
performed at 0 °C in the presence of 1 equiv of the
reagent affords after 1 h (HPPh2) or 4 h (HP(C6H11)2)
two new metallalactonic complexes (2d ,e) displaying the
same structure as 2a -c with a PR2 substituent on the
sp3 carbon of their metallacycle. Similarly to the met-
allalactones already synthesized, 2d ,e present in their
IR spectra four ν(CtO) bands and, in 13C NMR, four
signals for the resonances of their terminal carbonyls.
The signals of the sp3 carbon of their metallacycle are
found at 100.1 ppm (2d ) and 99.2 ppm (2e). Due to the
presence of a PR2 group (R ) C6H5 (2d ), C6H11 (2e)) on
this carbon, large 13C-31P coupling constants (35.6 Hz
for 2d and 37.3 Hz for 2e) are observed. Smaller
interactions between the phosphorus and the ketonic
carbon of each metallacycle (8.7 Hz, 2d ; 23.2 Hz, 2e)
are also detected. 31P-1H NMR HMBC sequences show
F igu r e 1. ORTEP representation of 2e. Hydrogens have
been omitted for clarity. (the thermal ellipsoids are drawn
at the 50% probability level)
Ta ble 1. Cr ysta l Da ta a n d Str u ctu r e Refin em en t
Deta ils for 2e
3
for both complexes J interactions between the protons
of the methyl substituent and the phosphorus. In 31P
NMR, 2d ,e each display one signal, respectively, at 10.9
and 14.1 ppm; these values fall within the range of the
resonances observed for organic phosphines presenting
a large cone angle19 and do not correspond to phosphines
bound to a metal. The absence of interaction between
the terminal carbonyls and the phosphorus also con-
firms the noncoordination of the phosphine to the
metal.13 13C{1H,31P} NMR spectra of the two complexes
also reveal the presence of 8 signals in the aromatic
carbon resonance area of 2d and of 12 resonances for
the cyclohexyls of 2e (see Experimental Section). In both
complexes, the two phosphorus substituents (phenyls
or cyclohexyls) are then not equivalent. The presence
of 8 signals for the phenyl carbons of 2d suggests fast
rotations of these groups around the P-C bonds, while
the detection of 12 signals for the cyclohexyls of 2e is
indicative of slow rotations of these groups on the scale
of NMR detection. This difference of behavior is cer-
tainly due to a greater steric hindrance of the cyclohexyl
groups.
The molecular structure of 2e was determined by a
single-crystal X-ray study. Monocrystals of 2e were
grown from a hexane/dichloromethane solution at -30
°C. An ORTEP drawing of the complex is depicted in
Figure 1, crystallographic data are gathered in Table
1, and selected bond lengths and angles are listed in
Table 2. As revealed by this study, the metal center of
this complex is part of a five-membered metallacycle.
The coordination polyhedron of the iron atom in 2e is a
distorted octahedron built of four terminal carbonyls
and the chelating ligand. As generally observed for iron
five-membered metallacycles,10,20 the C(5)-Fe-C(7) bite
angle of 81.8° is smaller than 90°. The two axial
carbonyls are bent toward the metallacyclic ring (C(4)-
compd
2e
chem formula
C
20H25FeO7P
fw
464.22
color; dimens, mm
pale orange; 0.30 × 0.30 × 0.17
cryst syst
space group
a, Å
monoclinic
P21/n (No. 14)
10.6941(3)
10.9020(3)
18.8707(7)
90.097(1)
2200.08(12)
4
b, Å
c, Å
â, deg
V, Å3
Z
F
calcd, g cm-3
1.402
968
F(000)
radiation, Å
λ(Mo KR) 0.710 73
temp, K
110
2.16-32.03
0.795
none
35
6156
2240
θ range, deg
linear abs coeff µ, mm-1
abs cor
detector to cryst dist, mm
no. of rflns measd
no. of rflns with I > 2σ(I)
no. of variables
final R indices (I > 2σ(I))
final R indices (all data)
GOF
263
R1 ) 0.095, wR2 ) 0.130
R1 ) 0.143, wR2 ) 0.140
1.005
F
max/Fmin, e Å-3
1.03/-0.41
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for 2e
Fe-C(1)
Fe-C(2)
Fe-C(3)
Fe-C(4)
Fe-C(5)
Fe-C(7)
1.855(4)
1.861(5)
1.877(3)
1.812(3)
1.991(3)
1.999(4)
P-C(6)
1.905(3)
1.378(4)
1.433(4)
1.545(4)
1.517(4)
O(6)-C(5)
O(6)-C(6)
C(6)-C(7)
C(6)-C(8)
C(1)-Fe-C(2)
C(1)-Fe-C(3)
C(1)-Fe-C(4)
C(1)-Fe-C(5)
C(1)-Fe-C(7)
C(2)-Fe-C(3)
C(2)-Fe-C(4)
C(2)-Fe-C(5)
89.44(16)
94.41(13)
169.67(14)
87.71(12)
88.94(15)
98.28(16)
96.98(14)
90.37(15)
C(2)-Fe-C(7)
C(3)-Fe-C(4)
C(3)-Fe-C(5)
C(3)-Fe-C(7)
C(4)-F--C(5)
C(4)-Fe-C(7)
C(5)-Fe-C(7)
172.05(13)
92.67(13)
171.10(15)
89.60(14)
84.17(12)
83.59(13)
81.79(12)
(18) Luart, D.; Le Gall, N.; Salau¨n, J . Y.; Toupet, L.; des Abbayes,
H. Organometallics 1998, 17, 2680.
(19) Rahman, M. M.; Liu, H. Y.; Eriks, K.; Prock, A.; Giering, W. P.
Organometallics 1989, 8, 1.
C(5)-O(6)-C(6)
Fe-C(5)-O(6)
118.5(2)
115.80(19)
O(6)-C(6)-C(7)
Fe-C(7)-C(6)
108.7(3)
114.0(2)
(20) (a) Aime, S.; Milone, L.; Sappa, E.; Tiripicchio, A.; Manotti
Lanfredi, A. M. J . Chem. Soc., Dalton Trans. 1979, 1664. (b) Hoffmann,
K.; Weiss, E. J . Organomet. Chem. 1977, 128, 399. (c) Karel, K. J .;
Tulip, T. H.; Ittel, S. D. Organometallics 1990, 9, 1276. (d) Pettersen,
R. C.; Cihonski, J . L.; Young, F. R.; Levenson, R. A. J . Chem. Soc.,
Chem. Commun. 1975, 370.
Fe-C(1) ) 169.7(1)°). As in the structure of 2a , the five-
membered ring is slightly puckered with deviations from
the best plane formed by five atoms varying from