Synthesis and characterization of high-spin [(CO)3FeII(CO2R)3]2Fe II complexes formed by thermolysis of cis-(CO)4Fe(CO2R)2 (R = Me, t-Bu, allyl, 1,1′-dimethylallyl). X-ray crystal structure of the allyl derivative

Article abstract of DOI:10.1021/om011097g

Instead of the expected carbon-carbon coupling into oxalates, thermolysis at 30 °C of the cis-bis(alkoxycarbonyl) monomers (CO)₄Fe(CO₂R)₂ (1) affords the novel trimetallic compounds [(CO)₃Fe(μ,η²-CO₂R)₃]₂ Fe (R = Me (2a), t-Bu (2b), allyl (2c), 1,1′-dimethylallyl (2d)). As shown by ¹H and ¹³C NMR, these complexes 2, which can be described as a central Fe(II) surrounded by two [(CO)₃Fe(CO₂R)₃]^- ligands, are paramagnetic. A Mo?ssbauer study of 2a and 2d revealed that these complexes display a high-spin configuration of their central iron atom and a low-spin configuration of the two lateral iron atoms. The easy formation of these complexes 2 by reacting fac-[(CO)₃Fe(CO₂R)₃]^- anions with FeCl₂ suggests that formation of 2 by thermal evolution of 1 could occur via an associative mechanism, giving rise to the [(CO)₃Fe(CO₂R)₃] pattern. Further thermolysis of 2 at 50 °C affords alcohols, Fe(CO)₅, carbon monoxide, and bis(alkyl carbonates).

Full text of DOI:10.1021/om011097g

Organometallics 2002, 21, 1775-1781
1775
Syn th esis a n d Ch a r a cter iza tion of High -Sp in
[(CO)3F eII(CO2R)3]2F eII Com p lexes F or m ed by
Th er m olysis of cis-(CO)4F e(CO2R)2 (R ) Me, t-Bu , Allyl,
1,1′-Dim eth yla llyl). X-r a y Cr ysta l Str u ctu r e of th e Allyl
Der iva tive
Nathalie Le Gall, Denis Luart, J ean-Yves Salau¨n,* J ean Talarmin, 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
Lo¨ıc Toupet
Groupe Matie`re Condense´e et Mate´riaux, UMR CNRS 6626, Universite´ de Rennes I,
Campus de Beaulieu, 35042 Rennes ce´dex, France
Nieves Menendez and Franc¸ois Varret
Laboratoire de Magne´tisme et d’Optique, CNRS-Universite´ de Versailles (UMR 8634),
78035 Versailles ce´dex, France
Received December 28, 2001
Instead of the expected carbon-carbon coupling into oxalates, thermolysis at 30 °C of the
cis-bis(alkoxycarbonyl) monomers (CO)4Fe(CO2R)2 (1) affords the novel trimetallic compounds
[(CO)3Fe(µ,η2-CO2R)3]2Fe (R ) Me (2a ), t-Bu (2b), allyl (2c), 1,1′-dimethylallyl (2d )). As shown
by 1H and 13C NMR, these complexes 2, which can be described as a central Fe(II) surrounded
by two [(CO)3Fe(CO2R)3]- ligands, are paramagnetic. A Mo¨ssbauer study of 2a and 2d
revealed that these complexes display a high-spin configuration of their central iron atom
and a low-spin configuration of the two lateral iron atoms. The easy formation of these
complexes 2 by reacting fac-[(CO)3Fe(CO2R)3]- anions with FeCl2 suggests that formation
of 2 by thermal evolution of 1 could occur via an associative mechanism, giving rise to the
[(CO)3Fe(CO2R)3] pattern. Further thermolysis of 2 at 50 °C affords alcohols, Fe(CO)5, carbon
monoxide, and bis(alkyl carbonates).
In tr od u ction
the synthesis of cis-bis(alkoxycarbonyl)iron com-
pounds displaying a metal center known for its low
catalytic activity. These syntheses were carried out by
reaction, at low temperature, of 0.5 equiv of oxalyl
chloride on tetracarbonyl(alkoxycarbonyl)iron anions
([(CO)4Fe(CO2R)]-) obtained by nucleophilic addition of
alcoholates on Fe(CO)5. Though the mechanism of this
reaction remains unclear, the process enabled us to
prepare cis-Fe(CO2R)2(CO)4 complexes specifically in
good yields. These compounds (1a , R ) Me; 1b, R )
t-Bu; 1c, R ) allyl; 1d , R ) 1,1′-dimethylallyl) were
The syngas route to ethylene glycol is an important
industrial reaction. A crucial step of this process is the
production of oxalates, which are subsequently reduced
by H2 to afford ethylene glycol. An alternative method
for the preparation of oxalates is the oxidative carbo-
nylation of alcohols. The key intermediate of this
reaction, which has only been achieved with palladium-
based catalysts,1-3 is believed to be a bis(alkoxycarbo-
nyl) Pd species that could release oxalate by a reductive
carbon-carbon coupling between its two alkoxycarbonyl
ligands. However, this mechanism remains doubtful as,
among the bis(alkoxycarbonyl) complexes described in
the literature,4 only one has been claimed to induce
oxalate formation under thermolysis conditions.5
To study the possibility of performing such a reac-
tion with well-characterized complexes, we carried out
(4) (a) Hanson, B. E.; Merola, J . S.; Waller, F. J . Organometallics
1993, 12, 568. (b) Bryndza, H. E. Organometallics 1985, 4, 1686. (c)
Werner, K. V.; Beck, W. Chem. Ber. 1972, 105, 3947. (d) Vasopollo,
G.; Toniolo, L.; Cavinato, G.; Bigoli, F.; Lanfranchi, M.; Pellighelli, M.
A. J . Organomet. Chem. 1994, 481, 173. (e) Minghetti, G.; Bonati, F.;
Banditelli, G. Synth. Inorg. Met.-Org. Chem. 1973, 3, 415. (f) Fachinetti,
G.; Funaioli, T.; Masi, D.; Mealli, C. J . Organomet. Chem. 1991, 417,
C32. (g) Werner, H.; Hofmann, L.; Zolk, R. Chem. Ber. 1987, 120, 379.
(h) Milstein, D.; Huckaby, J . L. J . Am. Chem. Soc. 1982, 104, 6150. (i)
Burk, P. L.; Van Engen, D. V.; Campo, K. S. Organometallics 1984, 3,
493. (j) Gargulak, J . D.; Berry, A. J .; Noirot, M. D.; Gladfelter, W. L.
J . Am. Chem. Soc. 1992, 114, 8933. (k) Evans, E. W.; Howlader, M. B.
H.; Atlay, M. T. Inorg. Chim. Acta 1995, 230, 193.
* To whom correspondence should be addressed. E-mail: jean-
yves.salau¨n@univ-brest.fr.
(1) Fenton, D. M.; Steinwand, P. J . J . Org. Chem. 1974, 39, 701.
(2) Rivetti, F.; Romano, U. J . Organomet. Chem. 1978, 154, 323.
(3) Rivetti, F.; Romano, U. J . Organomet. Chem. 1979, 174, 221.
(5) Rivetti, F.; Romano, U. Chim. Ind. 1980, 62, 7.
10.1021/om011097g CCC: $22.00 © 2002 American Chemical Society
Publication on Web 03/23/2002

1776 Organometallics, Vol. 21, No. 9, 2002
Le Gall et al.
found to be relatively stable at room temperature and
only evolved thermally at 50 °C to afford dialkyl
carbonates (low yields), alcohols, and Fe(CO)5.6 No trace
of dialkyloxalates that would result from a carbon-
carbon coupling process between the two alkoxycarbonyl
ligands of these complexes was detected in the different
reaction mixtures. However, a recent reinvestigation of
these thermolysis reactions of Fe(CO2R)2(CO)4 led us to
show the transient formation of a series of complexes,
[Fe(CO)3(µ,η2-CO2R)3]2Fe (2), never described previously
in thermal evolution processes of bis(alkoxycarbonyl)
complexes.
In the present paper, we wish to report the synthesis
of these new poly(alkoxycarbonyl) trimetallic complexes
and their further preparation via a new synthetic
method. The structural study of the allyl derivative will
be developed, and a possible mechanism that could
account for the formation of these complexes by thermal
evolution of bis(alkoxycarbonyl) entities will also be
suggested.
Resu lts a n d Discu ssion
P r ep a r a tion a n d X-r a y Cr ysta llogr a p h ic Ch a r -
a cter iza tion of Com p lexes 2. IR monitoring of the
thermal evolution of the complexes cis-Fe(CO2R)2(CO)4
(1) performed in CH2Cl2 or in THF at 30 °C showed the
slow disappearance of the starting set of four bands in
the terminal carbonyl ν(CO) area at 2130, 2080, 2070,
and 2045 cm-1. The concomitant growth of two new
ν(CO) bands at 2080 and 2025 cm-1 was indicative of
the formation of a series of relatively stable new
organometallic compounds. The complexes thus formed,
2 (2a , R ) Me; 2b, R ) t-Bu; 2c, R ) allyl; 2d , R )
1,1′-dimethylallyl), were obtained after 15 h of ther-
molysis and isolated as pale yellow crystals (yields ca.
70% after recrystallization). Crystals suitable for an
X-ray crystallographic study were obtained for 2c. These
crystals grown from hexane at -30 °C allowed us to
assign to this complex the trimetallic structure [Fe(CO)3-
(µ,η2-CO2CH2CHdCH2)3]2Fe.
F igu r e 1. (a) Molecular structure of 2c, showing 50%
probability displacement ellipsoids and the numbering
scheme. (b) Crystal structure of 2c viewed along the Fe-
Fe-Fe axis.
The ORTEP plot of the molecule with its atom-
numbering scheme is displayed in Figure 1, and the
X-ray data conditions are summarized in Table 1, while
selected bond distances and bond angles are given in
Table 2.
Complex 2c crystallizes in the trigonal space group
R3h. The unit cell contains one molecule. The molecule
displays a linear chain composed of three iron atoms, it
is centrosymmetric, and the central Fe(1) atom is the
center of inversion. The two lateral iron atoms are
surrounded by three terminal carbonyls and three
bridging alkoxycarbonyl ligands bound to the metal by
the carbon of their carbonyl. The central iron is coor-
dinated to six bridging alkoxycarbonyl ligands via the
oxygen of their carbonyl. 2c can also be described as
two fac-[Fe(CO2CH2CHdCH2)3(CO)3]- entities acting as
3-fold axis along the Fe(2)-Fe(1)-Fe′(2) direction. Such
a structure makes the six terminal carbonyls and the
six bridging alkoxycarbonyls of 2c respectively equiva-
lent. In the solid state it is noteworthy that analogous
equivalencies of the three alkoxycarbonyl or the three
acyl ligands are not observed for the related complexes
[(CO)3Fe(CO2t-Bu)3][K]7 and [(CO)3Re(µ,η2-C(O)Me)3]-
[BX]8 (X ) Cl, Br).
The tripodal ligands [Fe(CO)3(CO2R)3]- formally dis-
play two alkoxycarbonyl groups and one carbenic group.
However, the equivalence of these groups strongly
suggests an electronic delocalization along the whole
ligand. This assumption is confirmed by the absence of
carbenoid character of these alkoxycarbonyls, which
tridentate ligands encapsulating a central Fe2+
.
Figure 1b shows the alignment of the three metals
and the staggered positions of the alkoxycarbonyls and
the terminal carbonyls, resulting in a minimization of
the interactions between these ligands and inducing a
(7) Sellin, M.; Luart, D.; Salau¨n, J . Y.; Laurent, P.; Toupet, L.; des
Abbayes, H. Organometallics 1996, 15, 521.
(8) (a) Lukehart, C. M. Acc. Chem. Res. 1981, 14, 109 and references
cited therein. (b) Baskar, A. J .; Lukehart, C. M. J . Organomet. Chem.
1983, 254, 149.
(6) Salau¨n, J . Y.; Le Gall, G.; Laurent, P.; des Abbayes, H. J .
Organomet. Chem. 1992, 441, 99.

High-Spin [(CO)3FeII(CO2R)3]2FeII Complexes
Organometallics, Vol. 21, No. 9, 2002 1777
Ta ble 1. Cr ysta llogr a p h ic Da ta , Collection
P a r a m eter s, a n d Refin em en t P a r a m eter s for 2c
Ta ble 3. Selected In fr a r ed a n d NMR Da ta for
Com p lexes 2
formula
fw
cryst syst
space group
a, Å
C30H30Fe3O18
846.10
trigonal
R3h
10.112(5)
103.85(8)
925(2)
NMR (δ (ppm); in CD2Cl2, 298 K)
IR (cm-1; in hexane)
νCtO νCOOR δCtO
13C
δCOOR
δR
1H
2a 2089 (m) 1590 245.1 494.3
2022 (s)
86.8
-14.1 (s, 3H)
R, deg
V, Å3
2b 2078 (m) 1592 249.9 460.0
2017 (s)
89.1
25.1
-18.4 (s, 9H)
Z
1
F(calcd), g cm3
cryst size, mm
2θmax, deg
1.800
2c 2085 (m) 1596 246.5 503.2
2032 (s)
123.9
118.4
101.5
-0.2 (m, 1H)
-3.1 (m, 1H)
-18.0 (m, 1H)
-20.5 (m, 2H)
-1.0 (m, 1H)
-8.3 (m, 1H)
-17.0 (s, 6H)
-17.6 (m, 1H)
0.28 × 0.28 × 0.32
50
h,k,l range
diffractometer
radiation
0-12; 0-12; -12 to +12
Enraf-Nonius CAD4
Mo KR
12.308
2d 2080 (m) 1597 250.2 463.0
127.8
103.7
87.7
2019 (s)
µ(Mo KR), cm-1
monochromator
T, K
graphite cryst
294
16.7
F(000)
432
no. of rflns measd
no. of rflns obsd (I > σ(I))
Rint (from merging equiv rflns)
R(isotropic)
R(anisotropic)
N(obs)/N(var)
final R
1432
912 (3σ)
0.014
0.09
0.050
912/78
0.036
0.036
0.692
0.297; 0.04
88.6°. These angles compare well with the same data
determined for [(CO)3Fe(CO2-t-Bu)3][K]7 (95.1, 88.6, and
87.5°, respectively), whose alkoxycarbonyls, in the solid
state, are not bound to the same counterion and suggest
that coordination of the tripodal group Fe(CO2CH2CHd
CH2)3 to the central iron does not induce significant
deformations of the (CO)3Fe(CO2CH2CHdCH2)3 pattern.
The geometry about the central Fe(1) atom can also
be described as a distorted octahedron with angles of
86.50(9)° for O(2)-Fe(1)-O(2) (between two alkoxycar-
bonyl ligands bound by their carbon to the same Fe(2))
and 93.50(9)° for O(2)-Fe(1)-O′(2) observed between
two alkoxycarbonyls linked by their carbon to two
different lateral Fe(2) atoms. The Fe(1)-O(2) (or Fe(1)-
O′(2)) bond length, 2.100(2) Å, is longer than those
observed for the bimetallic complexes Fe2(µ,η2-C(O)-
C6H5)2(CO)6 (1.967(5) Å)13 and Fe2(µ,η2-CO2t-Bu)2(CO)5-
(PPh3) (2.009(2) Å).9
To our knowledge, only a few examples of compounds
displaying structures related to 2c have been described
in the literature. We have already mentioned the
complexes [Fe(CO2-t-Bu)3(CO)3][K]7and [Re(µ,η2-C(O)-
Me)3(CO)3][BX] (X ) Cl, Br),8 which display the tripodal
ligand M(CO)3[C(O)R]3 linked to a second metal by the
oxygen of the carbonyl of their organic ligands. The
complexes [Fe(µ,η2-SC6H5)3(CO)3]2Fe14,15 and [Fe(µ,η2-
SeC6H5)3(CO)3]2M (M ) Zn, Cd, Ni, Fe),16 which exhibit
a three-metal linear arrangement similar to that ob-
served for 2c, must also be mentioned. These molecules
are centrosymmetric and display the same 3-fold axis
as 2c. They are respectively prepared by reaction of
Fe(CO)5 with C6H5SSC6H5 or by addition of the anion
[Fe(SeC6H5)3(CO)3]- to MX2 (X ) BF4 (Zn), NO3 (Cd,
Ni), ClO4 (Fe)). A series of complexes of the general
formula L2M (L ) CpCo[P(O)(OR)2]3; M ) Co, Ni, Cu,
Fe) has also been described.17 These last compounds,
like complexes 2, present two organometallic tripodal
ligands linked by their oxygens to a central metal atom.
In the IR spectra of complexes 2 (see Table 3) the
presence of two bands in the terminal carbonyl stretch-
ing region confirms the existence in the molecule of
a
Rw
∑w
max residual, e Å-3; ∆/σ
w )1/σ(Fo)2 - [σ2(I) + 0.04Fo2)2]-1/2
.
a
Ta ble 2. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for 2c
Bond Lengths
Fe(1)-O(2)
Fe(2)-C(1)
Fe(2)-C(2)
2.100(2)
1.820(4)
1.991(3)
C(1)-O(1)
C(2)-O(2)
C(2)-O(3)
1.121(4)
1.221(4)
1.349(4)
Bond Angles
C(1)-Fe(2)-C(1)
C(1)-Fe(2)-C(2)
C(1)-Fe(2)-C(2)
O(2)-Fe(1)-O(2)
94.7(2)
87.5(2)
175.5(1)
86.50(9)
O(2)-Fe(1)-O′(2)
O(2)-Fe(1)-O′(2)
C(2)-Fe(2)-C(2)
93.50(9)
180.0(1)
88.6(1)
show CdO (C(2)-O(2)) bond lengths of 1.221(4) Å,
very close to the values observed for the same bonds of
fac-[(CO)3Fe(CO2R)3][K] (average of 1.21(1) Å). These
values are well within the range of distances found
for CdO of the alkoxycarbonyl group of the neutral
complex Fe(CO2-t-Bu)2(CO)4 (average of 1.241(9) Å)
and much shorter than the C-O bonds observed for
iron carbenes substituted by OR groups: 1.394(5) Å
9
for (CO)3Fe(C(NEt2)OC(O)NEt2)10 and 1.336(3) Å for
(CO)4Fe(C(NEt2)OC2H5).11 Since the covalent radius of
iron is 1.43 Å,12 the Fe(1)-Fe(2) distance of 3.680(2) Å
observed for 2c rules out any direct metal-metal
interaction.
The ligand arrangement around the two lateral Fe(2)
atoms is that of a slightly distorted octahedral environ-
ment, as the C(1)-Fe(2)-C(1), C(1)-Fe(2)-C(2), and
C(2)-Fe(2)-C(2) angles are respectively 94.7, 87.5, and
(9) Luart, D.; Le Gall, N.; Salau¨n, J . Y.; Toupet, L.; des Abbayes,
H. Inorg. Chim. Acta 1999, 291, 166.
(10) (a) Luart, D.; Le Gall, N.; Salau¨n, J . Y.; Toupet, L.; des Abbayes,
H. Organometallics 1998, 17, 2680. (b) Le Gall, N.; Luart, D.; Salau¨n,
J . Y.; des Abbayes, H.; Toupet, L. J . Organomet. Chem. 2001, 617-
618, 483.
(11) Fischer, E. O.; Schneider, J .; Ackermann, K. Z. Naturforsch.
1984, 39b, 468.
(12) Putnik, C. F.; Welter, J . J .; Stucky, G. D.; D’aniello, M. J .;
Sosinski, B. A.; Kirner, J . F.; Muetterties, E. L. J . Am. Chem. Soc.
1978, 100, 4107.
(13) Lindley, P. F.; Mills, O. S. J . Chem. Soc. A 1969, 1279.
(14) Berger, U.; Stra¨hle, J . Z. Anorg. Allg. Chem. 1984, 516, 19.
(15) Walters, M. A.; Dewan, J . C. Inorg. Chem. 1986, 25, 4889.
(16) Liaw, W. F.; Chen, C. H.; Lee, C. M.; Lin, G. Y.; Ching, C. Y.;
Lee, G. H.; Peng, S. M. J . Chem. Soc., Dalton Trans. 1998, 353.
(17) (a) Kla¨ui, W.; Werner, H. Angew. Chem., Int. Ed. Engl. 1976,
15, 172. (b) El Murr, N.; Chaloyard, A.; Kla¨ui, W. Inorg. Chem. 1979,
18, 2629.

1778 Organometallics, Vol. 21, No. 9, 2002
Le Gall et al.
fac-FeX3(CO)3 groups of C3v symmetry. The values
measured for the frequencies of these ν(CO) bands
(2090 and 2015 cm-1), very close to those displayed
by the anions [Fe(CO2R)3(CO)3]- (2070, 2005 cm-1),7
suggest that complexes 2 are better described as two
[Fe(CO2R)3(CO)3]- anions linked to a central Fe2+ atom
rather than the bis(carbene) [(CO)3Fe(µ,η2-CO2R)2-
(µ,η2dC(OR)O)]2Fe. These conclusions are in good agree-
ment with those of the structural study of 2c, whose
alkoxycarbonyl ligands do not display any carbenic
character.
Complexes 2 are found to exhibit unusual 1H and 13C
NMR spectra with resonances well removed from the
diamagnetic region (see Table 3); such spectra suggest
the presence of a paramagnetic center in the molecule.
Performed at 298 K, the H NMR spectrum of 2d (R )
1,1′-dimethylallyl) displays broad signals at high fields.
1
F igu r e 2. Plot of H NMR chemical shifts obtained for
2d as a function of T-1
.
Ta ble 4. 13C NMR Reson a n ces (δ (p p m )) of 2d a t
Va r iou s Tem p er a tu r es
1
temp (K)
298
283
268
265
253
A resonance at -17.0 ppm (ω1/2 ) 80 Hz) is assigned
to the protons of the methyl groups, whereas the signal
of the internal allylic protons is observed at -17.6 ppm
(ω1/2 ) 110 Hz). As shown by a 1H-13C heteronuclear
correlation (HMQC), the signals of the methylenic
protons are found at -8.3 ppm (ω1/2 ) 55 Hz) and -1.0
ppm (ω1/2 ) 35 Hz). The large difference (∼7 ppm)
between the shifts of these two protons could be induced
by a great difference in through-space interactions with
the electronic spins. It is noteworthy that the protons
closest to the metals present the broadest and the most
shifted NMR signals.
In contrast to the 1H signals, the 13C NMR resonances
observed for 2d are shifted toward lower fields. At 298
K the carbonyl resonance of the alkoxycarbonyl ligands
is observed at 463.0 ppm (212 ppm for the diamagnetic
homologues7) and that of the terminal CtO at 250.2
ppm (210 ppm for CtO of a related diamagnetic
complex7). The signals observed for the carbons of the
1,1′-dimethylallyl groups are similar to those quoted for
the carbons of the same ligand on diamagnetic entities.
The 13C NMR resonances observed for the carbonyls
of complexes 2 are then very different from those
observed for the series of related complexes [Fe(CO2R)3-
(CO)3]M (R ) Me, M ) Na; R ) t-Bu, M ) K;7 signals
at 212 ppm for CdO and at 209 ppm for the terminal
CdO), from which complexes 2 can be considered to
differ by the nature of the counterion. These observa-
tions suggest that the paramagnetic center of 2 is the
central FeII, atom which could display a high-spin
configuration. Such an electronic structure with un-
paired electrons has already been reported for the
related complexes (CO)3Fe(µ-SPh)3Fe(µ-SPh)3Fe(CO)3,14,15
(CO)3Fe(µ-SePh)3Fe(µ-SePh)3Fe(CO)3,16 and {CpCo-
[P(O)(OR)2]3}2Fe,17 whose central irons are respectively
surrounded by six sulfide, selenium, and oxygen atoms.
C(O)OR
CtO
CHd
CH2
C(CH3)2
CH3
463.0
250.2
127.8
103.7
87.7
486.6
256.0
125.5
102.4
87.6
514.5
262.9
123.0
100.9
87.5
526.0
267.3
125.0
102.5
89.6
548.2
271.3
119.8
99.2
87.3
10.7
16.7
15.1
13.1
14.5
ously linear and show that the behavior of 2d does not
exactly fit the Curie-Weiss law.
In the same way, the 13C NMR spectra of 2d are found
to be temperature dependent. As the temperature is
lowered, the signals of the carbonyls are shifted toward
lower field (see Table 4). The resonance of the carbonyls
of the alkoxycarbonyl ligands is found at 548.2 ppm at
253 K (a difference of 85 ppm from the signal of the
same carbon observed at 298 K) and that of terminal
carbons at 271.3 ppm (vs 250.2 ppm at 298 K).
Though they are ESR silent in frozen solutions,
complexes 2 are found to display solid-state effective
magnetic moments. The values of this parameter are
determined as µeff ) 5.30 (2a ), 5.20 (2b), 5.41 (2c), 5.45
µB (2d ) at 298 K. As regular octahedral iron complexes
are supposed to show either S ) 0 or S ) 2 electronic
configurations, it is noteworthy that these values are
higher than that required for four unpaired electrons
(4.90 µB). The same observations are also made for the
magnetic moments determined in solution by the Evans’
method:18 2a , 5.0 µB; 2b, 5.20 µB; 2c, 5.80 µB; 2d , 6.90
µB. The occurrence of such high µeff values may be
explained either by an orbital angular momentum
contribution or by a distorted-octahedral complexation
around the central FeII.
As it is well-known that Mo¨ssbauer spectroscopy is a
very useful tool to identify FeII high- or low-spin
configurations in organometallic complexes, we have
undertaken such a study on 2a and 2d .
Mo1ssba u er Stu d y of Com p lexes 2a a n d 2d . At all
temperatures, the spectra of 2a and 2d can be resolved
into two subspectra: one single line and one quadru-
polar doublet which are, according to the typical values
of the hyperfine parameters,19 attributed to FeII in the
low-spin (LS) and high-spin (HS) states, respectively.
As expected for paramagnetic complexes, 2d shows,
at low temperature, broader and more greatly contact
shifted resonances. In 1H NMR, low temperatures
induce a shielding of the signals; thus, at 253 K one
methylenic proton of 2d is found at -5.1 ppm (ω1/2
)
96 Hz) and the second at -16.1 ppm (ω1/2 ) 80 Hz), the
methyl group protons display a broad resonance at
-27.0 ppm (ω1/2 ) 128 Hz), and the last ethylenic
(18) (a) Evans, D. F. J . Chem. Soc. 1959, 2003. (b) Crawford, T. H.;
Swanson, J . J . Chem. Educ. 1971, 48, 382. (c) Sur, S. K. J . Magn.
Reson. 1989, 82, 169.
(19) See for example: Claude, R.; Real, J . A.; Zarembowitch, J .;
Kahn, O.; Ouahab, H.; Grandjean, D.; Boukheddaden, K.; Varret, F.;
Dworkin, A. Inorg. Chem. 1990, 29, 4442.
hydrogen shows a broad signal at -30.8 ppm (ω1/2
)
144 Hz). However, as displayed in Figure 2, the varia-
tions of these chemical shifts versus T-1 are not rigor-

High-Spin [(CO)3FeII(CO2R)3]2FeII Complexes
Organometallics, Vol. 21, No. 9, 2002 1779
F igu r e 4. Cyclic voltammogram of 2d in THF-[Bu4N]-
[PF6] (glassy-carbon electrode, v ) 0.05 V/s, T ) 25 °C).
increase of the anodic-to-cathodic peak separation (∆Ep)
with increasing scan rates. In a typical experiment22a
∆Ep increased from 96 mV at v ) 0.025 V/s to 223 mV
at v ) 1 V/s (under similar conditions ∆Ep for the
reversible oxidation of ferrocene increased from 68 mV
(v ) 0.05 V/s) to 91 mV (v ) 0.5 V/s), showing the effect
of some uncompensated solution resistance). The rate
constant of the heterogeneous electron transfer, ks ) (3.0
( 0.5) × 10-3 cm/s, was calculated by measurements of
∆Ep at different scan rates and use of the equation22b,23
F igu r e 3. Selected Mo¨ssbauer spectra of 2d and 2a with
least-squares fitted Lorentzian lines.
Selected spectra are shown in Figure 3. The poor
statistics of the room-temperature spectra result from
the particularly low values of the f(T) factor (at 300 K,
the Mo¨ssbauer lines of 2d are 20 times smaller than at
77 K).
The relative proportions of the spectral contributions
were determined at low temperature (77-200 K) as
39(1) and 37(1)% HS in compounds 2d and 2a , respec-
tively. This determination of the low-temperature atomic
proportions enabled us to unambiguously assign the HS
state to the central iron. The two lateral iron atoms are
then low-spin. Such an assignment is similar to those
given for other trimetallic iron compounds displaying a
related structure.15,16 The central FeII atom is linked to
six oxygen atoms, which provide a weak ligand field
consistent with an HS state, while the lateral FeII atoms
are bound to six carbons (CO groups) which provide a
stronger ligand field, due to the presence of the LS
state.20 It is worth noting that the Mo¨ssbauer param-
eters of these lateral iron atoms coordinated to six CO
groups are very close to those observed in the 4-fold
coordinated Na2Fe(CO)4.21 At higher temperature (above
200 K), despite the smaller f factor of the HS species
and a decrease of the total area, an increase of the HS
percentage area is observed (47% for 2a and 46% for
2d at 300 K), suggesting, for both complexes, a gradual
but incomplete LS-HS spin transition. However, as no
sizable change is observed in the magnetic properties
of 2a and 2d in the temperature range suggested by
the Mo¨ssbauer data, the occurrence of such a spin
conversion can be ruled out.
ks ) ψ(πaD)1/2
where D is the diffusion coefficient of 2d , a ) Fv/RT,
and ψ is a dimensionless parameter related to ∆Ep. For
∆Ep values outside the range reported by Nicholson,23
ψ was calculated using the fact that ∆Ep vs log ψ is
linear for ∆Ep values larger than ca. 170 mV.23-25
Mech a n ism of F or m a t ion of Com p lexes 2. The
mechanism of formation of complexes 2 by thermal
evolution of cis-(CO)4Fe(CO2R)2 (1) remains unclear. The
formation of the two “(CO)3Fe(CO2R)3” patterns of 2
could, however, result from the particular mobility of
the alkoxy groups of the alkoxycarbonyl ligands of 1.
This great mobility is evidenced by rapid exchanges
between the alkoxy groups of 1 and alcohols.6 We have
already shown in a previous study that, for complexes
1, these exchanges take place via an associative mech-
anism, which includes a nucleophilic addition of an
alcohol on a terminal carbonyl of 1, giving rise to
trifunctionalized fac-[(CO)3Fe(CO2R)2(CO2R′)][H] inter-
mediates which, after elimination of ROH, afford cis-
(CO)4Fe(CO2R)(CO2R′).26 The same exchange occurs also
between the alkoxy groups of different bis(alkoxycar-
bonyl)iron complexes. The formation of these fac-tris-
(alkoxycarbonyl) intermediates could then occur during
Oxid a tive Electr och em istr y of 2d . The electro-
chemical behavior of complex 2d was investigated by
cyclic voltammetric and potential step experiments in
THF-[NBu4][PF6] at room temperature. In this elec-
trolyte, the trimetallic complex undergoes a single one-
electron oxidation (E1/2 ) -0.02 V) in the potential range
extending from -3.0 to ca. 1.2 V (see Figure 4). That
this oxidation is a quasi-reversible step is shown by the
(22) (a) The shape of the cyclic voltammograms (peak currents and
peaks separation) was found to be critically dependent on the surface
state of the vitreous carbon disk used throughout. Reproducible results
were obtained, provided the disk was polished with a wet felt tissue
before each individual CV scan. (b) The diffusion coefficients of the
neutral and oxidized forms of the trimetallic compounds were assumed
to be identical; the diffusion coefficient of the neutral species was
obtained from potential-step experiments in which the potential was
a
stepped at a value far beyond the anodic peak potential (Ep ) +500
mV).
(20) (a) Greatrex, R.; Greenwood, N. N. Discuss. Faraday Soc. 1969,
47, 126. (b) Greenwood, N. N.; Gibb, T. C. Mo¨ssbauer Spectroscopy;
Chapman and Hall: London, 1971; p 223.
(21) Greenwood, N. N.; Gibb, T. C. Mo¨ssbauer Spectroscopy; Chap-
man and Hall: London, 1971; p 169.
(23) Nicholson, R. S. Anal. Chem. 1965, 37, 1351.
(24) Geiger, W. E. Prog. Inorg. Chem. 1985, 33, 275.
(25) Tulyathan, B.; Geiger, W. E. J . Am. Chem. Soc. 1985, 107, 5960.
(26) Sellin, M.; Luart, D.; Salau¨n, J . Y.; Laurent, P.; Des Abbayes,
H. J . Organomet. Chem. 1998, 562, 183.

1780 Organometallics, Vol. 21, No. 9, 2002
Le Gall et al.
Sch em e 1
Sch em e 2
the thermolysis of 1 either by an intermolecular addition
of an alkoxy group of a first molecule on a terminal
carbonyl of a second molecule of 1 or by a partial
decomposition of the complex, releasing RO-, which
could attack a terminal carbonyl of 1 still present in
solution. Formation of 2 could result from a further
reaction of these trifunctionalized intermediates with
unidentified iron fragments. With the aim of clarifying
the mechanism of formation of 2, we performed the
synthesis of the fac-tris(tert-butoxycarbonyl)tricarbonyl-
iron anions by reacting complex 1b (R ) t-Bu) with
t-BuOK.7 The [(CO)3Fe(CO2t-Bu)3]- anion thus formed
was found to be stable as a potassium salt. It reacted
six alkoxycarbonyl groups of these ligands. As shown
1
by H and 13C NMR, the new trimetallic complexes are
paramagnetic and a Mo¨ssbauer study evidenced, at low
temperature, a central high-spin iron atom and two low-
spin lateral irons.
However, the mechanism of formation of these tri-
metallic complexes is not obvious. The [(CO)3Fe(CO2R)3]
pattern of complexes 2 could be formed from (CO)4Fe-
(CO2R)2 by an intermolecular nucleophilic addition of
an alkoxy group of a first molecule of 1 on the terminal
carbonyl of a second molecule of the same compound.
This reaction, leading to 2, would then display an
associative mechanism analogous to that proposed to
justify the alkoxy exchanges observed between cis-
(CO)4Fe(CO2R)2 complexes bearing different alkoxy
groups.27 The easy formation of 2 by reaction of [(CO)3Fe-
(CO2R)3][M] (M ) Na, K) with FeCl2 is also in good
agreement with the possible intervention of trifunction-
alized intermediates in the course of formation of 2 by
thermal evolution of 1. This study also shows that
thermolysis of complexes 2 does not afford dialkyl-
oxalates by a carbon-carbon coupling process between
two of their alkoxycarbonyl ligands. Again, these results
confirm the difficulty of performing the synthesis of
double carbonyl chaining by carbon-carbon coupling
between two carbonylated ligands and even bring up
the question of the real intervention of bis(alkoxycar-
bonyl) intermediates in the course of catalytic processes,
affording dialkyloxalates. Further experiments are in
progress to obtain an answer to this question.
1
rapidly at 0 °C with /2 equiv of FeCl2 to afford complex
2b in good yield (74%) (see Scheme 1). This process,
performed at low temperature, confirms the possible
intervention of tris(alkoxycarbonyl) intermediates in the
process of formation of 2 by thermal evolution of 1.
The easy preparation of 2 also shows the potentiality
of the anion (CO)3Fe(CO2R)3-, which has an isolobal
-
analogy with C5H5
.
Th er m a l Evolu tion of Com p lexes 2. As shown by
IR monitoring, when they are thermolyzed at 50 °C in
THF, complexes 2 undergo rapid evolutions. Analysis
of the resulting reaction mixtures by IR, 13C and 1H
NMR, and GC shows in each case the formation of
Fe(CO)5 (quantitative), alcohols (90%), and dialkyl
carbonates (5%). No formation of oxalates, which would
result from direct carbon-carbon couplings between two
alkoxycarbonyl ligands, is observed, confirming that
thermally induced carbon-carbon couplings between
two ligands linked to a metal center by two sp2 carbonyl
carbons do not occur easily.27 The nature of the alkoxy
groups seems to have no influence on the course of
the reaction. These results, similar to those already
observed by thermolysis of the bis(alkoxycarbonyl)
monomers 1,6 confirm that the thermal evolution of
these latter complexes into alcohols and dialkyl carbon-
ates occurs via the intermediate formation of complexes
2, as shown in Scheme 2.
Exp er im en ta l Section
All reactions were carried out under an atmosphere of argon
using standard Schlenk techniques, and all solvents were
purified by distillation under an inert atmosphere from an
appropriate drying agent.28 Infrared spectra were obtained
with a Perkin-Elmer 1430 spectrometer. 1H and 13C NMR
spectra were recorded on Bruker AC 300 and AMX 3-400
spectrometers. Chemical shifts are reported in δ units (parts
per million) downfield from tetramethylsilane in dichloro-
methane (external reference) (1H) or from the solvent reso-
nance as external reference (13C). Gas chromatography studies
were performed on a Hewlett-Packard 5890 instrument using
a CP SIL 25 m capillary column. Magnetic measurements were
carried out with a Gouy balance at room temperature. Mag-
netic moments were calculated following standard methods,29
Con clu sion
For the first time, thermolysis of cis-bis(alkoxycarbo-
nyl) complexes has been found to give rise to the series
of poly(alkoxycarbonyl) complexes [(CO)3Fe(µ,η2-CO2R)3]2-
Fe. These compounds display three metal centers and
can be described as a central iron linked to two (CO)3Fe-
(CO2R)3 groups by the oxygens of the carbonyls of the
(27) Salau¨n, J . Y.; Laurent, P.; des Abbayes, H. Coord. Chem. Rev.
1998, 178-180, 353.
(28) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals; Pergamon Press: Oxford, U.K., 1981.

High-Spin [(CO)3FeII(CO2R)3]2FeII Complexes
Organometallics, Vol. 21, No. 9, 2002 1781
and corrections for underlying diamagnetism were applied to
the data.30 Elemental analyses were performed at the Centre
de Microanalyses du CNRS at Lyon-Solaize, France. Com-
plexes 1 were prepared as described in the literature.6
P r ep a r a tion of th e Com p lexes (CO)3F e(µ,η2-CO2R)3F e-
(µ,η2-CO2R)3F e(CO)3 (2). Gen er a l P r oced u r e. Complexes
1 (4 mmol) dissolved in 25 mL of THF were stirred under argon
at 30 °C. The reactions were monitored by IR. After disap-
pearance of the ν(CO) bands of the starting complexes (ca. 15
h), the solvent was removed under reduced pressure. The
remaining brown residue was extracted with two 25 mL
portions of hexane (R ) t-Bu (2b), allyl (2c), 1,1′-dimethylallyl
(2d )) or with a 5/1 mixture of hexane and dichloromethane (2
× 25 mL) (R ) Me, 2a ). After filtration and concentration of
the solution, complexes 2 were obtained as pale yellow crystals
after one night at -30 °C. IR and room-temperature 1H and
13C NMR data of 2a -d are collected in Table 3.
set in theoretical positions. The whole structure was refined
by full-matrix least-squares techniques (use of F magnitude;
x, y, z, and âij for Fe, C, and O atoms and x, y, z fixed for H
atoms; 78 variables and 912 observations). Atom scattering
factors were obtained from ref 33.
Mo1ssba u er In vestiga tion s Ca r r ied Ou t on 2a a n d 2d .
The Mo¨ssbauer spectra were recorded on a constant-accelera-
tion spectrometer, with a 50 mCi 57Co source, giving a typical
instrumental line width of 0.215 mm/s. Thin absorbers were
prepared (∼20 mg/cm2), and the spectra were fitted by doublets
of Lorentzian lines. Isomer shift data were referred to metallic
iron at room temperature (also used for velocity calibration).
Ma gn etic In vestiga tion s. Using a MPS5 Design SQUID
magnetometer, we investigated the temperature dependence
of the magnetic properties of compounds 2a and 2d , a few
milligrams of which were taken from the Mo¨ssbauer samples.
Electr och em istr y. Electrochemical experiments were per-
formed in deaerated solutions under an inert atmosphere (N2
or Ar) using a Tacussel-Radiometer PJ T 120 potentiostat
connected via an IMT102 interface (Tacussel-Radiometer) to
a PC computer running the VM2 software (Tacussel-Radiom-
eter). The electrodes used were a vitreous carbon disk (working
electrode) and either a carbon rod or a stainless steel wire as
the secondary electrode. The reference electrode was a Ag/Ag+
electrode. The potentials are referred to the Fc+/Fc couple.
P r ep a r a tion of 2b by Rea ction of fa c-[(CO)3F e(CO2-t-
Bu )3][K] w ith F eCl2. fac-[(CO)3Fe(CO2-t-Bu)3][K] was pre-
pared as described elsewhere7 by nucleophilic addition of
t-BuOK on a terminal carbonyl of 1b.
2a (R ) Me). Yield: 63% (1.74 g). Anal. Calcd for C18H18
Fe3O18 (689.55): C, 31.32; H, 2.61. Found: C, 31.25; H, 2.67.
2b (R ) t-Bu ). Yield: 69% (2.60 g). Anal. Calcd for C36H54
Fe3O18 (941.55): C, 45.88; H, 5.73. Found: C, 45.95; H, 5.79.
2c (R ) Allyl). Yield: 73% (2.47 g). Anal. Calcd for C30H30
-
-
-
Fe3O18 (846.10): C, 42.58; H, 3.55. Found: C, 42.51; H, 3.63.
2d (R ) 1,1′-Dim eth yla llyl). Yield: 72% (2.91 g). Anal.
Calcd for C42H54Fe3O18 (1013.55): C, 49.73; H, 5.33. Found:
C, 49.65; H, 5.42.
NMR Stu d ies of Com p lexes 2. We only report the details
of the spectra obtained at different temperatures for 2d . The
resonances observed for this complex vs the temperature are
reported in Figure 2 (1H) or collected in Table 4 (13C).
Deter m in a tion of th e Un p a ir ed Electr on Nu m ber s. An
average of 10 mg of complexes 2 in solution in 0.35 mL of CD2-
Cl2 + 5% of TMS was introduced under argon into an NMR
tube together with a capillary tube containing a solution of
CD2Cl2 and 5% of TMS. The difference (∆f in Hz) between the
two 1H TMS signals allowed the determination of the magnetic
moment of the molecule by the Evans method. The chemical
shift of the 1H and 13C signals of complexes 2 were obtained
by the same method. They were measured using the resonance
of the TMS in solution in CD2Cl2 contained in a capillary tube
as external reference. For the 13C resonances the use of TMS
as internal reference did not induce an error exceeding 1.5
ppm.
Syn th esis of 2b. To 0.5 mmol of fac-[(CO)3Fe(CO2-t-Bu)3]-
[K] (241 mg) in solution at 0 °C in 20 mL of THF was added
0.25 mmol of FeCl2 (32 mg). The solution was left for 2 h at 0
°C with agitation before the solvent was removed. The residue
was extracted with two 20 mL portions of of a hexane-
dichloromethane mixture (4/1). After filtration the solution was
kept for 15 h at -30 °C. The brown precipitate that formed
was isolated by filtration (0.174 g; 74% yield).
Th er m olysis of Com p lexes 2. Gen er a l Meth od . A solu-
tion of 2 mmol of complex 2 in 30 mL of THF was heated with
stirring for 6 h at 50 °C. The reaction mixture was then cooled
to room temperature, and its composition was determined by
gas chromatography analysis (CP SYL capillary column), by
1
IR (Fe(CO)5), and by H and 13C NMR (dialkyl carbonates).
Complex 2a (R ) Me; 1.4 g) was found to give rise to 2% of
MeOC(O)OMe, 70% of MeOH, and 85% of Fe(CO)5.
Compounds formed by thermolysis of 2b (R ) t-Bu; 1.9 g):
t-BuOC(O)O-t-Bu, 5%; t-BuOH, 80%; Fe(CO)5, 70%.
Compounds formed by thermolysis of 2c (R ) allyl; 1.7 g):
(CH2dCHCH2O)2C(O), 3%; CH2dCH-CH2-OH, 70%; Fe(CO)5,
65%.
Compounds formed by thermolysis of 2d (R ) 1,1′-dimethyl-
allyl; 2 g): (CH2dCHC(CH3)2O)2C(O), 3%; CH2dCHC(CH3)2-
OH, 70%; Fe(CO)5, 75%.
X-r a y Cr ysta llogr a p h y for 2c. Crystals suitable for X-ray
diffraction studies were obtained from hexane at -30 °C. The
data were collected on a CAD-4 Enraf-Nonius diffractometer.
Calculations were performed on a Hewlett-Packard 9000-710
instrument for structure determination and on
a digital
Microwax 3100 computer with the MolEN package31 for
refinement and Ortep calculations. Crystal data collection and
refinement parameters are collected in Table 1. The cell
parameters were obtained by least-squares fitting of a set of
25 high-θ reflections. After Lorentz and polarization correc-
tions, the structure was solved by direct methods with the
program SHELX-86,32 which located all the non-hydrogen
atoms of the compound. After isotropic (R ) 0.09) and then
anisotropic refinement (R ) 0.05), all the hydrogen atoms were
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal-
lographic data, positional and thermal parameters, and bond
distances and angles for complex 2c. This material is available
free of charge via the Internet at http://pubs.acs.org.
(29) Mabbs, M. B.; Reardon, D. Magnetism and Transition Metal
Complexes; Chapman and Hall: London, 1973.
(30) Foese, G.; Gorter, C. J .; Smits, L. J . Constantes Se´lectionne´es
Diamagne´tisme, Paramagne´tisme, Relaxation Paramagne´tique; Mas-
son: Paris, 1957.
(31) Fair, C. K. MolEN: An Interactive Intelligent System for
Crystal Structure Analysis; Enraf-Nonius, Delft, The Netherlands,
1990.
OM011097G
(32) Sheldrick, G. M. In Crystallographic Computing 3: Data
Collection, Structure Determination, Protein and Database; Sheldrick,
G. M., Kru¨ger, C., Goddard, R., Eds.; Clarendon Press: Oxford, U.K.,
1985.
(33) International Tables for X-ray Crystallography; Kluwer Aca-
demic: Dordrecht, The Netherlands, 1992; Vol. C.