Synthesis of bis-(acyl, carbamoyl or alkoxycarbonyl) iron monomer or dimer complexes by reaction of oxalyl chloride with various tetracarbonyl ferrates: (Fe[C(O)R](CO)4)-

Article abstract of DOI:10.1016/s0020-1693(99)00024-9

The reaction of oxalyl chloride with various tetracarbonyl ferrates bearing a carbonylated organic ligand induces the formation of bis substituted monomers Fe[C(O)R]₂(CO)₄ or dimers (Fe[C(O)R](CO)₄)₂ and of bis-[μ,η²-C(O)R]Fe₂(CO)₆ bridged dimers. The orientation of the reaction depends on the nature of the organic ligand of the ferrates. Thus carbamoyls or alkoxycarbonyls ferrates bearing mobile R groups are found to induce the formation of bis substituted monomers while bulky alkoxycarbonyl ligands afford non-bridged dimers. Acyls and bulky carbamoyls give rise to dimers bearing bridging acyls or carbamoyls. The mechanisms of formation of these different complexes and the possible thermal interconversions of these compounds are investigated.

Full text of DOI:10.1016/s0020-1693(99)00024-9

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Inorganica Chimica Acta 291 (1999) 166–177
Synthesis of bis-(acyl, carbamoyl or alkoxycarbonyl) iron
monomer or dimer complexes by reaction of oxalyl chloride with
various tetracarbonyl ferrates: {Fe[C(O)R](CO)4}−
Denis Luart a, Nathalie le Gall a, Jean-Yves Salau¨n a,*, Lo¨ıc Toupet b,
Herve´ des Abbayes a
a Laboratoire de Chimie, Electrochimie Mole´culaires et Chimie Analytique, UMR CNRS 6521, UFR Sciences et Techniques,
Uni6ersite´ de Bretagne Occidentale, 6, a6enue le Gorgeu, BP 809, 29285 Brest Cedex, France
b Groupe Matie`re Condense´e et Mate´riaux, UMR CNRS 6626, Uni6ersite´ de Rennes I, Campus de Beaulieu, 35042 Rennes Cedex, France
Received 15 October 1998; accepted 24 December 1998
Abstract
The reaction of oxalyl chloride with various tetracarbonyl ferrates bearing a carbonylated organic ligand induces the formation
of bis substituted monomers Fe[C(O)R]2(CO)4 or dimers {Fe[C(O)R](CO)4}2 and of bis-[m,h2-C(O)R]Fe2(CO)6 bridged dimers.
The orientation of the reaction depends on the nature of the organic ligand of the ferrates. Thus carbamoyls or alkoxycarbonyls
ferrates bearing mobile R groups are found to induce the formation of bis substituted monomers while bulky alkoxycarbonyl
ligands afford non-bridged dimers. Acyls and bulky carbamoyls give rise to dimers bearing bridging acyls or carbamoyls. The
mechanisms of formation of these different complexes and the possible thermal interconversions of these compounds are
investigated. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Crystal structures; Iron complexes; Alkoxycarbonyl complexes; Carbamoyl complexes; Acyl complexes
pected reaction was not obvious, we suggested that the
process could occur via the formation of C(O)–C(O)
bridged intermediates 1 (Eq. (1)), analogous to the
species obtained by reaction of oxalyl chloride on man-
ganese or rhenium anions [15]. These intermediates 1,
after decarbonylation, were supposed to give rise to the
mono C(O) bridged homologues 2. Due to the great
mobility of the alkoxy of the alkoxycarbonyl ligands
[14,16], the migration of one of these alkoxy groups
1. Introduction
There is considerable interest in the chemistry of
bis-(alkoxycarbonyl) complexes of transition metals
owing to their recognition as intermediates in several
important catalytic processes such as carbonylation of
alcohols into dialkyl carbonates or dialkyl oxalates
[1–3]. The preparation of such bis-(alkoxycarbonyl)
complexes of Pd [4,5], Pt [6–8], Rh [9], Co [10,11] or
Ru [12,13] have generally been realised either by CO
insertion into metal alkoxy bonds or by nucleophilic
additions of alcoholates on terminal carbonyl ligands.
We recently published the synthesis of bis-(alkoxy-
carbonyl) tetracarbonyl iron complexes by reacting 0.5
equivalents of oxalyl chloride with alkoxycarbonyl te-
tracarbonyl ferrates: [Fe(CO2R)(CO)4]− themselves ob-
tained by addition of alcoholates on one carbonyl of
Fe(CO)5 [14]. However, as the mechanism of this unex-
* Corresponding author: Tel.: +33-298-016 286; fax: +33-298-
016 594.
E-mail address: jean-yves.salaun@univ-brest.fr (J.-Y. Salau¨n)
(1)
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved
PII: S0020-1693(99)00024-9

D. Luart et al. / Inorganica Chimica Acta 291 (1999) 166–177
167
towards either a bridging CO or a terminal carbonyl
of the second Fe atom was supposed to induce the
formation of Fe(CO2R)2(CO)4 (3) and Fe(CO)5. In this
paper we would like to report our recent results con-
cerning the influence of the nature of the carbonylated
organic ligand of different acyl, carbamoyl or alkoxy-
carbonyl ferrates on the reactions of these anions with
oxalyl chloride.
removed under vacuum and the brown residue was
extracted with two portions of 30 ml of hexane at
room temperature (r.t.). After filtration the hexane
solution was concentrated and 430 mg (47% yield) of
4a were obtained as a brownish oil. Anal. Found: C,
32.74; H, 1.67. Calc. for C10Fe2H6O8: C, 32.81; H,
1.67%.
2.3.2. By reaction of oxalyl chloride with
{Fe[C(O)N(iPr)2](CO)4}−Li+
2. Experimental
The preparation of LiN(iPr)2 was achieved by addi-
tion at −80°C of 2 ml of a 2.5 M solution of n-butyl
lithium in hexane to a solution of 5 mmol of HN(iPr)2
(500 mg; 700 ml) in 10 ml of THF. The reaction
realised as above afforded a red solution. The solvent
was removed at 0°C and the red residue was extracted
with 2×20 ml of hexane. After filtration and concen-
tration of the solution, 0.809 g of 4b (72% yield) were
obtained as a red powder. Anal. Found: C, 37.30; H,
3.17; N, 6.16. Calc. for C14Fe2H14N2O8: C, 37.36; H,
3.11; N, 6.23%.
2.1. Materials
All reactions were carried out with Schlenk tech-
niques under dry argon. All solvents were purified by
preliminary distillation from an appropriate drying
1
agent [17]. H (300 MHz) and 13C (75.47 MHz) NMR
spectra were obtained on a Bruker AC300 spectrome-
ter with chemical shifts reported in l values relative to
residual solvent (1H) or to the solvent resonance (13C).
Infrared spectra were recorded in hexane on a Perkin–
Elmer 1430 spectrometer. Analytical GLC analyses of
the organic products of the reactions were performed
on a Hewlett–Packard 5890 by using a 25m cp syl
capillary column. Elemental analyses were realised by
the Service Central d’Analyses du CNRS. All reagents
were commercially available and used as received from
commercial sources.
2.4. Preparation of bis-(alkoxycarbonyl tetracarbonyl
iron) by reaction of oxalyl chloride with bulky
alkoxycarbonyl ferrates at −30°C
2.4.1. R=tBu
The reaction was performed following the general
procedure (5 mmol of NaOtBu (480 mg), 10 mmol of
Fe(CO)5 (1.31 ml), 0.210 ml of ClC(O)C(O)Cl). After
addition of the oxalyl chloride, the reaction mixture
turned brown. After evaporation of the solvent, the
residue was extracted by 2×15 ml of hexane. After 12
h at −30°C 5a was obtained as orange crystals (805
mg, yield: 60%). Anal. Found: C, 40.09; H, 3.29. Calc.
for C18Fe2H18O12: C, 40.17; H, 3.35%.
2.2. Synthesis of acetyl, carbamoyl, or alkoxycarbonyl
tetracarbonyl ferrates
2.2.1. General procedure for the preparation of ferrates:
{Fe[C(O)R](CO)4}−
To a solution of 7.5 mmol of Fe(CO)5 in 30 ml of
THF at 0°C were added 5 mmol of MeLi, LiN(iPr)2,
or NaOR. The reaction was monitored by IR (appear-
ance of new w(CꢀO) bands at lower frequencies near
1900 cm−1). Ferrates were used in situ without further
purification.
2.4.2. R=C(Me)2CHꢁCH2
The reaction was realised as for R=tBu. 540 mg
(5 mmol) of CH2=CH–C(Me)2ONa were used. After
extraction with hexane, reduction of the solution to
5 ml, a brown oil was obtained after 12 h at −30°C.
This oil was found to exhibit spectroscopic and ana-
lytical data in accordance with the proposed struc-
ture 5b: yield: 57% (800 mg). Anal. Found: C,
42.65; H, 3.28. Calc. for C20Fe2H18O12: C, 42.73; H,
3.20%.
2.3. Achie6ement of bis-(v,p2-carbonylated ligands)
di-iron hexacarbonyl complexes
2.3.1. By reaction of oxalyl chloride with
{Fe[C(O)Me](CO)4}−Li+
To a solution of 5 mmol of {Fe[C(O)Me](CO)4}−
[18], prepared by addition at 0°C of 3.15 ml of a 1.6
M solution of MeLi in ether to 10 mmol of Fe(CO)5
(1.96 g, 1.31 ml) in 10 ml of THF, 2.5 mmol of
ClC(O)C(O)Cl (0.218 ml) were added dropwise. Very
rapidly the orange ferrate solution turned brown and
an evolution of CO was observed. The solution was
stirred for 30 min at −30°C. The disappearance of
the ferrate was confirmed by IR. The solvent was then
2.4.3. R=C(Me)2CO2Me
A total of 700 mg (5 mmol) of CO2MeC(Me)2ONa
was used. After extraction of the residue of the reac-
tion with 2×30 ml of hexane and evaporation of this
solvent, a brown oil was obtained. The 13C NMR
spectrum of this crude product clearly indicated that it
was composed of a mixture of products that we were
unable to identify.

168
D. Luart et al. / Inorganica Chimica Acta 291 (1999) 166–177
2.5. Preparation of bis-(alkoxycarbonyl) tetracarbonyl
iron complexes
trifluoromethyl sulfonate in suspension in 20 ml of
THF was added dropwise at −70°C. The temperature
of the solution was slowly warmed to 0°C and stirred
for an additional 2 h while monitoring by IR. This
monitoring showed the disappearance of the w(CꢀO)
bands of the ferrates and the formation of new bands
at 2000 and 1510 cm−1. The solvent was then removed
under vacuum and the brown residue extracted with
2×20 ml of hexane. After concentration of the solu-
tion at −30°C, 4c (R=tBu) and 4d (R=Me) were
obtained as reddish crystals. 4c: yield: 28% (337 mg).
Anal. Found: C, 39.82; H, 3.81. Calc. for
C16Fe2H18O10: C, 39.86; H, 3.74%. 4d yield: 24% (240
mg). Anal. Found: C, 30.23; H, 1.45. Calc. for
C10Fe2H6O10: C, 30.17; H, 1.51%.
2.5.1. By reactions of oxalyl chloride with small
alkoxycarbonyl ferrates
2.5.1.1. R=nBu, R=allyl, R=iPr. The reaction was
realised following the general procedure with 5 mmol of
alcoholate: nBuONa: 480 mg, allylONa: 400 mg, iPrONa:
410 mg. After addition of the oxalyl chloride, the solution
turned brown. After evaporation of the solvent and
extraction with hexane, 3a–c precipitated from this
solution after 12 h at −30°C and were obtained as
yellow crystals. Yields: 3a: 62% (574 mg), 3b: 66% (558
mg), 3c: 55% (470 mg). Anal. 3a Found: C, 45.58; H, 4.91.
Calc. for C14FeH18O8: C, 45.41; H, 4.86%. Anal. 3b
Found: C, 42.54; H, 3.02. Calc. for C12FeH10O8: C, 42.62;
H, 2.95%. Anal. 3c Found: C, 42.15; H, 4.10. Calc. for
C12FeH14O8: C, 42.10; H, 4.09%.
2.7. Influence of the electronic density on the ligand
migration
2.7.1. Preparation of the monophosphine dimer 7a
One equivalent of triphenyl phosphine (1.05 g) in 20
ml of THF was added at −5°C to a solution of 4 mmol
of 5a (2.151 g) in 30 ml of THF. The reaction mixture
was stirred at this temperature while monitoring by IR.
This monitoring showed the slow disappearance of the
w(CꢀO) of 5a and indicated that the reaction was
complete after 1 h. The solvent was then removed under
vacuum and the residue extracted with 2×30 ml of
hexane. After filtration, the solution was left at −30°C
and red crystals of 7a precipitated (yield: 62%, 1.90 g).
Anal. Found: C, 54.29; H, 4.35. Calc. for C35Fe2H33O11P:
C, 54.43; H, 4.28%.
2.5.2. By reactions of oxalyl chloride with bulky
aekoxycarbonyl ferrates performed at −70°C
2.5.2.1. R=tBu, C(Me)2CHꢁCH2, C(Me)2CO2Me. These
reactions were performed as above; the addition of
ClC(O)C(O)Cl was realised at −70°C. After 30 min at
this temperature, the solution was warmed until 0°C and
left at this temperature for 20 min. The solvent was then
removed and the residue extracted with two portions of
25 ml of hexane. After filtration the solution was cooled
for 12 h at −30°C. Yellow crystals were obtained for 3d
(R=tBu) and 3f (R=C(Me)2CO2Me); 3e (R=
C(Me)2CHꢁCH2) was isolated as a brown oil. 3d: 600 mg
(65% yield). Anal. Found: C, 45.52; H, 4.91. Calc. for
C14FeH18O8: C, 45.56; H, 4.87%. 3e: 690 mg (70% yield).
Anal. Found: C, 49.02; H, 4.72. Calc. for C16FeH18O8:
C, 48.73; H, 4.57%. 3f: 572 mg (50% yield). Anal. Found:
C, 42.04; H, 4.05. Calc. for C16FeH18O12: C, 41.92; H,
3.93%.
2.7.2. Preparation of the bis phosphine dimer 8a
The reaction was realised as above with four equiva-
lents of triphenyl phosphine (4.2 g). The monitoring of
the reaction by IR showed, as previously, the disappear-
ance of the w(CꢀO) of 5a. At the same time the formation
of a red precipitate was observed. After filtration, the so
formed powder was washed with 2×15 ml of cold THF
to give 8a as a red powder (yield 69%, 2.80 g). Anal.
Found: C, 61.93; H, 4.72. Calc. for C52Fe2H48O10P2: C,
62.05; H, 4.77%.
2.6. Study of the ligand migration
2.6.1. By thermolysis of 5a
A total of 2.5 mmol of 5a in solution in 30 ml of THF
was heated at 45°C. After 1 h at this temperature, the IR
spectrum of the solution showed the complete disappear-
ance of the w(CꢀO) bands of the starting complex. The
solvent was then removed and the residue extracted with
30 ml of hexane. After filtration and concentration of the
solution to ꢀ5 ml 4c which precipitated was obtained
as reddish crystals (1 g, 83% yield).
2.7.3. Thermolysis of complexes 7a and 8a
2.7.3.1. General procedure. The complexes in
a
solution of THF were stirred for 15 h at r.t. The re-
action was monitored by IR. After complete disappear-
ance of the w(CꢀO) of the starting complexes, the sol-
vent was removed and the residue extracted with
2×25, ml of hexane. The solution was filtered and then
cooled at −30°C. Red crystals were obtained after
12 h.
2.6.2. By oxidation of [Fe(CO2R)(CO)4]− ( formation
of 4c and 4d)
To a solution of 5 mmol of [Fe(CO2R)(CO)4]− (R=
tBu or Me) in 25 ml of THF, one equivalent of cooper

D. Luart et al. / Inorganica Chimica Acta 291 (1999) 166–177
169
2.7.3.2. Thermolysis of 7a. 3 mmol of 7a (2.315 g). Yield
of 9a: 67% (1.43 g).
from International Tables [21]. All the calculations were
performed on a Digital Micro VAX 3100 computer
with the
MolEN package Enraf-Nonius 1990 [22].
2.7.3.3. Thermolysis of 8a. Due to its low solubility in
THF, only 1 mmol of 8a was used. It was dissolved in
50 ml of THF. Yield of 9a: 54% (0.386 g). Anal. Found:
C, 55.22; H, 4.66. Calc. for C33Fe2H33O9P: C, 55.33; H,
4.61%.
3. Results and discussion
As mentioned in Section 1, the results obtained by
reaction of alkoxycarbonyl ferrates with oxalyl chloride
suggested that this process could be a convenient way
to realise the synthesis of bis-substituted tetracarbonyl
iron complexes. With the aim of confirming this as-
sumption, we have realised this reaction with various
acyl, carbamoyl and alkoxycarbonylferrates.
2.7.4. Thermolysis of 4c in the presence of triphenyl
phosphine
A total of 3 mmol of 4c (1.45 g) in solution in 60 ml
of hexane was stirred for 2 h at r.t. in the presence of
three equivalents of triphenyl phosphine (2.3 g). The
yellow solution slowly turned red as the monitoring of
the reaction showed the disappearance of the w(CꢀO)
bands of the starting complex. After 2 h the solution
was concentrated to 20 ml, filtered and cooled to
−30°C affording, after 12 h, red crystals of 9a (1.83 g,
yield: 86%).
3.1. Achie6ement of bis-(v,p2-carbonylated ligand)
di-iron hexacarbonyl complexes
3.1.1. Reaction of oxalyl chloride with an acyl
tetracarbonyliron anion {Fe[C(O)Me](CO)4}−
To our knowledge, bis-(acyl) tetracarbonyl iron com-
plexes Fe[C(O)R]2(CO)4 have never been described un-
2.8. Thermolysis of complexes 4c and 9a
2.8.1. General procedure
A solution of 1 mmol of 4c (0.480 mg) or of 9a (720
mg) in 30 ml of THF was heated for 6 h at 50°C. IR
spectra of the resulting solutions showed the decompo-
sition of the starting complexes giving rise to Fe(CO)5
for 4c or to Fe(CO)4(PPh3) (w(CꢀO) at 2050, 1973 and
1940 cm−1) and Fe(CO)3(PPh3)2 (w(CꢀO) at 1887
cm−1) [19] for 9a. These observations, together with
the presence of free CO, were confirmed by monitoring
Table 1
Details of structure analyses for 3d and 9a
Complex
3d
9a
Formula
C14FeH18O8
370.14
orthorhombic
Pnma
12.403(9)
23.178(6)
5.976(11)
C33Fe2H33O9P
716.29
monoclinic
P21/n
20.834(3)
11.380(3)
14.670(2)
97.51(1)
3448(1)
4
Molecular weight
Crystal system
Space group
t
the reaction by 13C NMR. The presence of BuOH and
t
t
the absence of BuOC(O)OtBu or BuOC(O)C(O)OtBu
in solution were determined by gas chromatography by
comparison with authentic samples.
,
a (A)
,
b (A)
,
c (A)
i (°)
3
,
V (A )
1718(3)
4
1.431
768
9.09
120
2.9. Structure determinations of 3d and 9a
Z
Dcalc. (g cm−3
F(000)
)
1.380
1480
9.34
294
Many of the details of the structure analyses carried
out on 3d and 9a are listed in Table 1.
v(Mo Ka) (cm−1
T (K)
)
X-ray diffraction measurements were made using an
Enraf-Nonius CAD4 diffractometer with graphite-
monochromatised Mo Ka radiation. The cell parame-
ters were obtained by fitting a set of 25 high-q
reflections. After Lorentz and polarisation corrections,
the structure was solved with a Patterson map, several
scale factor refinements and Fourier differences calcula-
tions. After isotropic refinement (3d: R=0.095; 9a:
R=0.105), an absorption correction was made with the
program DIFABS [20]. After anisotropic refinement (3d:
R=0.075; 9a: R=0.09), all the hydrogen atoms were
Crystal dimensions (mm)
Radiation
Max. 2q (°)
Range of h, k, l
No. reflections measured
No. reflections observed
(I\3|(I))
0.15×0.22×0.25 0.35×0.35×0.50
Mo Ka
50
Mo Ka
50
0.7, 0.15, 0.29
2198
−24.24, 0.13, 0.17
6659
887(3|)
3821(3|)
Rint (from merging equiv-
alent reflections)
N (obs.)/N (var.)
R
0.019
921/140
0.056
0.051
2.17
3821/505
0.034
0.031
1.496
0.52
a
Rw
GOF
Max. residual (e A
D/|
found after Fourier differences (3d between 0.65 and
−3
,
)
0.62
−3
0.33 e A ; 9a between 0.59 and 0.25 e A−3). The
whole structure was refined by the full-matrix least-
squares technique. Atomic scattering factors were taken
,
,
0.55
0.55
a w=1/|(Fo)2=[|2(I)+(0.04Fo2)2]−1/2
.

170
D. Luart et al. / Inorganica Chimica Acta 291 (1999) 166–177
less they are perfluorinated [23] or cyclic [24,25]. The
reaction of ClC(O)C(O)Cl with {Fe[C(O)Me](CO)4}−
was a good opportunity to synthesise such a series of
compounds. Unfortunately, the reaction was found to
give rise, in 40% yield, to a complex (4a, Eq. (2)) whose
spectroscopic characteristics were not found to fit with
those expected for Fe[C(O)R]2(CO)4.
bulky (and more electron-withdrawing) ligands
(C(O)NR2=C(O)NMe2, C(O)NEt2 and C(O)NnPr2)
which were found to induce the formation of metalla-
cyclic carbene monomers by fast evolutions of bis-car-
bamoyl intermediates (Eq. (3)) [33].
(3)
(2)
3.2. Formation of bis-(alkoxycarbonyl tetracarbonyl
iron) complexes by reactions of bulky alkoxycarbonyl
ferrates with ClC(O)C(O)Cl
These characteristics (four w(CꢀO) bands near 2000
cm−1 and a w(CꢁO) band at low frequency (1540
cm−1) in IR and four resonances of respective intensi-
ties 2:2:1:1 for the terminal carbonyls and a signal at
very low field: 293.6 ppm in 13C NMR) (see Table 2)
were identical to those displayed by the bis-(m,h2-acetyl)
di-iron hexacarbonyl complex already prepared by oxi-
dation of {Fe[C(O)Me](CO)4}− [18,26].
As already mentioned in a preliminary communica-
tion [34], the reaction of a bulky alkoxycarbonyl fer-
rate: [Fe(CO2tBu)(CO)4]− with 0.5 equivalent of oxalyl
chloride at −30°C gave rise to the bimetallic species
[Fe(CO2tBu)(CO)4]2 (5a) (Eq. (4)) whose formation
could result from a second decarbonylation of the
mono bridged carbonyl intermediate 2 (Eq. (1)).
These results suggest that oxalyl chloride could also
induce an oxidation of the acetyl ferrate anion.
3.1.2. Reaction of ClC(O)C(O)Cl with carbamoyl
ferrates
(4)
This structurally characterised complex [34] was found
to be composed of two Fe(CO2tBu)(CO)4 entities linked
The reaction of ClC(O)C(O)Cl with {Fe[C(O)N(iPr)2]-
(CO)4}− bearing a bulky (and not very electron-with-
drawing) carbamoyl ligand, gave rise in 70% yield to 4b
(Eq. (2)), which displayed the Fe2(CO)6[m,h2-
C(O)N(iPr)2]2 structure analogous to that of 4a. 4b
exhibited an IR spectrum (see Table 2) very close to that
of Fe2(CO)6[m,h2-C(O)N(Et)2]2, itself obtained by oxida-
tion of the corresponding ferrate [27]. Contrary to 4a
which displayed an acyl carbonyl resonance at very low
field (293.6 ppm) in 13C NMR, the signal of the carbonyl
of the carbamoyl ligands of 4b was observed in the same
region as for terminal carbonyl ligands (:215 ppm).
Similar observations have already been realised for
di-ruthenium compounds [28,29] and very recently for
the very close complex Fe2(CO)5(PPh3)[m,h2-C(O)NiPr2]2
[30]. These high field resonances are presumably due to
a reduced carbenic character of the carbons of the
carbonyls of m,h2-carbamoyl ligands. Restricted rotation
at the C–N(iPr)2 bond is probably responsible for the
,
by their metal centre (Fe–Fe distance: 2.840(1) A).
Each t-butyloxycarbonyl ligand was complexed in cis
position from the metal–metal bond and, probably due
to the electron-withdrawing effect of the t-butyloxycar-
bonyl ligands, the Fe–CO bonds trans to these ligands
,
were longer (1.852(5) A) than the other Fe–CO dis-
,
tances (from 1.806(8) to 1.819(5) A).
In order to confirm that the formation of the dimer
5a was induced by a steric effect of the t-butyloxycar-
bonyl ligand, another ferrate displaying a bulky alkoxy-
carbonyl ligand {Fe[C(O)OC(Me)2CHꢁCH2](CO)4}−
was reacted at −30°C with ClC(O)C(O)Cl. This first
reaction confirmed our assumption as the single so
obtained product (5b) displayed the Fe–Fe dimeric
structure (Table 2). However when realised with a
ferrate bearing a still bulky but more electron-with-
drawing alkoxycarbonyl ligand {Fe[C(O)OC(Me)2-
C(O)OMe](CO)4}−, the same procedure gave rise to a
mixture of unidentified complexes.
1
appearance in H and in 13C NMR of separate signals
for the two iso-propyl groups of each equivalent car-
bamoyl bridge. Such observations have already been
observed for an h2-carbamoyl monomer [31], for
bimetallic complexes [29,30] or for organic amides (N,N-
dimethyl formamide [32]). The non-equivalence of the
two methyls of the same iso-propyl group could arise
from the chirality of 4b.
3.3. Achie6ement of bis-(alkoxycarbonyl) tetracarbonyl
iron monomers
3.3.1. By reaction of oxalyl chloride with less hindered
alkoxycarbonyl ferrates
The process giving rise to 4b is quite different from
those observed by reaction of ferrates bearing less
We have already described that the reaction of oxalyl-
chloride with methoxy or ethoxycarbonyl tetracarbonyl

D. Luart et al. / Inorganica Chimica Acta 291 (1999) 166–177
171
Table 2
IR and NMR data for the new complexes
Complex
IR hexane (cm−1
)
NMR (CD2Cl2, −10°C), l (ppm), J(C–P) (Hz)
w(CꢀO)
w(CꢁO)
13C
1H
31P
CꢀO
J(C–P)
CꢁO
J(C–P)
R
3a
3b
3c
3d
3e
3f
2132 (w)
2080 (w, sh)
2076 (s)
1670 (m)
1668 (m)
1658 (m)
1663 (m)
1665 (m)
199.8:2
199.5:2
191.5
66.2
4.08 (t, 2H, 6.4 Hz)
1.58 (m, 2H)
31.2
19.5
1.35 (m, 2H)
2044 (s)
13.8
0.89 (t, 3H, 7.3 Hz)
5.89 (m, 1H)
5.30 (m, 1H)
5.25 (m, 1H)
4.56 (d, 2H, 6 Hz)
5.05 (m, 1H, 6.2 Hz)
1.16 (d, 6H, 6.2 Hz)
2100 (w)
2081 (w, sh)
2076 (s)
199.5:2
199.2:2
191.0
191.2
190.6
189.8
191.6
293.6:2
133.3
117.2
66.7
2048 (s)
2135 (w)
2076 (m, sh)
2070 (s)
200.0:2
199.6:2
69.1
21.9
2048 (s)
2126 (w)
2076 (m, sh)
2068 (s)
200.8:2
200.6:2
83.8
28.8
1.43 (s)
2040 (s)
2128 (w)
2075 (m, sh)
2066 (s)
200.3:2
200.1:2
143.7
112.0
83.9
27.0
173.5
79.9
52.2
24.6
48.0
5.92 (br, m, 1H)
5.32 (br, m, 1H)
5.28 (br, m, 1H)
1.56 (s, 6H)
3.72 (s, 3H)
1.62 (s, 6H)
2043 (s)
2138 (w)
2078 (s)
1668 (m)
1755 (m)
199.3:2
199.0:2
2050 (s)
4a
4b
2075 (m)
2030 (s)
1995 (m)
1980 (m)
2072 (m)
2022 (s)
1997 (m)
1938 (m)
1540 (m)
1522 (m)
214.6:2
211.0:2
210.0:1
195.1:1
215.7:2 a
215.2:2
215.1:2
211.1:1
196.5:1
2.62
51.2
48.3
21.5
21.2
21.0
20.8
85.5
28.3
3.40 (m, 1H)
3.37 (m, 1H)
1.32 (d, 3H, 6.2 Hz)
1.20 (d, 3H, 6.3 Hz)
1.15 (d, 3H, 6.2 Hz)
1.10 (d, 3H, 6.3 Hz)
1.38
4c b
4d b
2084 (m)
2046 (s)
2024 (m)
2005 (s)
2087 (m)
2050 (s)
2022 (m)
2007 (s)
1509 (s)
1512 (s)
215.0:2
212.6:2
206.9:1
194.6:1
214.1:2
211.8:2
206.4:1
194.6:1
225.6
226.0
54.3
3.73
5a
5b
2114 (w)
2068 (s)
2050 (s)
2024 (s)
2120 (w)
2072 (s)
2048 (s)
2025 (s)
1670 (s)
1667 (s)
212.0:2
205.7:1
204.7:1
194.1
193.0
83.4
28.6
1.39 (s)
210.0:2
204.5:1
203.1:1
143.4
111.9
83.8
6.02 (m, 1H)
5.29 (m, 1H)
5.22 (m, 1H)
1.47 (s, 6H)
26.5
7a
2118 (w)
2062 (s)
2044 (s)
2030 (s)
2015 (s)
2002 (w)
1993 (s)
1658 (m)
1622 (m)
216.8:2
214.2:2
212.0:1
207.6:1
205.2:1
13.1
13.1
203.0:1
195.0:1
23.3
82.3
81.3
28.7
28.5
7.25 (m)
1.37 (s)
1.06 (s)
61.3
8a
9a
1998 (s)
1980 (w)
1972 (s)
1610 (m)
1522 (w)
218.1:2
213.2:1
12.0
10.7
205.3
228.0
21.8
9.0
80.2
28.0
7.19 (m)
1.00 (s)
58.4
2055 (w)
1998 (s)
1994 (s)
1935 (w)
220.2:2
215.4:2
207.3:1
2.3
4.5
12.5
84.1
28.7
7.15 (m)
1.20
102.5
a Signals of the terminal carbonyls and of the carbonyl of the carbamoyl ligand.
b NMR (CD2Cl2, −40°C).

172
D. Luart et al. / Inorganica Chimica Acta 291 (1999) 166–177
Fig. 1. ORTEP representation of the structure of 3d (the thermal ellipsoids are drawn at the 50% probability level).
ferrates gave rise in good yield (ꢀ70%) (Eq. (1)) [14] to
cis bis-(alkoxycarbonyl) iron tetracarbonyl complexes.
In order to generalise the reaction, the process was
applied to other alkoxycarbonyl ferrates. As before the
reaction was shown to afford similar complexes
(n-butyloxy: 3a, allyloxy: 3b, iso-propyloxy: 3c)
characterised in IR by four w(CꢀO) from 2130 to 2040
cm−1 and one w(CꢁO) at ꢀ1665 cm−1 and by two
signals for the terminal carbonyls at ꢀ200 ppm and
one resonance at ꢀ191 ppm for the carbonyls of the
alkoxycarbonyl ligands in 13C NMR (Table 2).
bis-(alkoxycarbonyl) complexes. The ORTEP diagram
of 3d is displayed in Fig. 1 and a selection of bond
lengths and angles values of the molecule is collected
in Table 3.
This structure confirms the cis geometry of the two
t-butyloxycarbonyl ligands. The coordination about
the central atom can be described as a distorted octa-
hedron (angles values between ligands in cis positions
from 84.3(3) to 95.8(4)°). Contrary to the dimer 5a
which displays long Fe–CꢀO bonds in trans to the
alkoxycarbonyl ligands, these lengths are normal for
,
3d. Short Fe–CO2R distances (2.009(2) A) and long
,
,
CꢁO (1.241(9) A) and C–O (1.365(6) A) bonds could
suggest a carbenoid character of the alkoxycarbonyls,
however the resonance observed at 190.6 ppm in 13C
NMR for CO2tBu do not confirm this hypothesis.
3.3.2. By reaction of bulky alkoxycarbonyl ferrates with
ClC(O)C(O)Cl at low temperature
When the addition of ClC(O)C(O)Cl to the bulky
[Fe(CO2R)(CO)4]− (R=tBu, C(Me)2CHꢁCH2 or
C(Me)2CO2Me) was performed at low temperature
(−70°C), the products of the reactions were the bis-
(alkoxycarbonyl) monomers: Fe(CO2R)2(CO)4; R=
tBu: 3d; C(Me)2CHꢁCH2: 3e; C(Me)2CO2Me: 3f
(Table 2) analogous to 3a–c obtained previously. The
difference of reactivity observed at −30 and −70°C
for this bulky ferrates is a good argument in favour
of the intermediate formation of the species 2 (Eq.
(1)). Due to the reduced ability of migration of these
bulky alkoxy groups towards other carbonyls, a rapid
decarbonylation of 2 could occur at −30°C and in-
duce the formation of 5a or 5b. This decarbonylation
could be quenched at −70°C leaving the even slower
alkoxy migration process to occur, giving rise to 3.
The formation of these new bis-(alkoxycarbonyl)
monomers 3 allowed us to grow crystals of 3d which
were found suitable for a crystallographic study. This
study is, to our knowledge, the first realised on iron
Table 3
,
Selected bond lengths (A) and angles (°) for complex 3d
Fe–C(1)
Fe–C(2)
Fe–C(3)
Fe–C(4)
O(1)–C(1)
1.845(6)
1.801(8)
1.822(8)
2.013(6)
1.133(7)
O(2)–C(2)
O(3)–C(3)
O(4)–C(4)
O(5)–C(4)
O(5)–C(5)
1.15(1)
1.14(1)
1.234(7)
1.370(7)
1.488(7)
C(1)–Fe–C(1)
C(1)–Fe–C(2)
C(1)–Fe–C(3)
C(1)–Fe–C(4)
C(1)–Fe–C(4)
C(2)–Fe–C(3)
C(2)–Fe–C(4)
C(3)–Fe–C(4)
95.9(3)
94.2(3)
93.4(3)
87.8(2)
176.2(3)
168.7(4)
84.5(3)
87.4(3)
C(4)–Fe–C(4)
Fe–C(1)–O(1)
Fe–C(2)–O(2)
Fe–C(3)–O(3)
Fe–C(4)–O(4)
Fe–C(4)–O(5)
O(4)–C(4)–O(5)
C(4)–O(5)–C(5)
88.5(2)
176.0(5)
177.6(7)
176.7(7)
127.0(4)
111.0(4)
122.0(5)
120.2(4)

D. Luart et al. / Inorganica Chimica Acta 291 (1999) 166–177
173
In order to obtain more information on the mecha-
nism of formation of complexes 3–5 and to check if
these complexes are not different intermediates of a
single process, we have studied the thermal reactivity of
these compounds to evidence possible thermal inter-
conversions between these species.
eringtheseresults, itisclearthatthethreecarbonylligands
of one Fe(CO)3 moiety are exchangeable while the others
remainfixed. Thoughthebridgingcarbonylsaresupposed
to withdraw electron density from the carbon linked metal
centre and donate electron density to the oxygen bound
metal centre, we presume that the dynamic process could
be associated with this second atom. This exchange
reaction would then be initiated by the cleavage of one
Fe–O bond providing a five-coordinate intermediate
which could facilitate the exchange by a pseudo-rotation
pathway. If a number of studies of the fluxional behaviour
of carbonyls of molecules which present the Fe2(CO)6
pattern have appeared, very few of them concern the
different behaviour of two Fe(CO)3 units. Such a deco-
ordination of an ancillary ligand has, however, been
invoked to explain the mobility of the carbonyls of one
metal centre of Fe2(CO)6[m,h2-C(S)OR]2 [36] and
Fe2(CO)6[m,h3-C(CF3)C(CF3)S] [37].
3.4. Migration of one organic carbonylated ligand from
one metal to another by thermolysis of bis-(alkoxy-
carbonyl) complexes
3.4.1. Thermolysis of the monomers Fe(CO2R)2(CO)4 (3)
When heated at 30°C, monomers 3 were found to
afford tri-metallic compounds {(CO)3Fe[m,h2-C(O)-
OR]3}2Fe (6) (Eq. (5)) [35]. Owing to their relatively
high thermal stability, it is clear that complexes 3 are
not involved in the reaction of formation of 4 or 5.
The formation of complex 4c formally results from a
t-butyloxycarbonyl ligand migration from one metal to
another followed by a double decarbonylation. Such a
process has already been described for a di-rhodium
complex which however does not possess a metal–metal
bond [38]. A similar migration of an acyl ligand is also
probably involved in the formation of [m,h2(CO)5ReCH2-
CH2C(O)]2Fe2(CO)6 complex via the non-characterised
intermediate [CO)5ReCH2CH2C(O)]2Fe2(CO)8 [39]. Sim-
ilar to the mechanisms proposed for metal-to-metal acyl
migrations processes [40–44], the formation of 4c from
5a could occur via h2-coordination of the t-butyloxycar-
bonyl ligands through carbon and oxygen to one or to
two metal centres, followed by a nucleophilic attack of
one iron on the carbon of the carbonyl of the alkoxycar-
bonyl ligand linked to the second metal (Scheme 1). These
two pathways require, on each metal of 5a, an easily
removable ligand that could be the carbonyl trans to the
alkoxycarbonyl (long Fe–CꢀO distance). However, the
formation of 4c can also result either from a migration
of a t-butyloxy group toward a carbonyl linked to the
second metal followed by a double decarbonylation, or
(5)
t
3.4.2. Thermolysis of the dimer [Fe(CO2 Bu)(CO)4]2 (5a)
Complex 5a, heated at 45°C, gave rise to a new complex
4c isolated in 83% yield. According to its spectroscopic
characteristics (Table 2) very close to those displayed
by 4a or 4b, the bis-[m,h2-C(O)OtBu]Fe2(CO)6 structure
was attributed to 4c (Eq. (6)). This geometry was
confirmed by the formation in 23% yield of the same
complex as well as that of its homologue bis-[m,h2-
C(O)OMe] (4d) by a one electron oxidation process of
the appropriate alkoxycarbonyl ferrates achieved by
copper trifluoro methane sulfonate.
(6)
by an homolytic cleavage of the metal–metal bond of 5a
t
As observed for 4a and 4b, 4c and 4d show a w(CꢁO) at
very low frequency (ꢀ1510 cm−1) in IR and a resonance
at rather low field (ꢀ226 ppm) in 13C NMR, indicative
of a carbenoid character for the carbonyl of their bridging
ligands. The presence in 13C NMR of a single signal for
these two carbonyls confirms the equivalence of the two
alkoxycarbonyl bridges.
’
giving rise to [Fe(CO2Bu)(CO)4] radicals that would
rearrange into 4c by a process analogous to that observed
by oxidation of Fe(CO2tBu)(CO)4]−.
In order to show the mobility of the carbonyls trans
to the t-butyloxycarbonyl ligands of 5a and to study the
influence of the electron density of the metal centres on
the course of the reaction of ligand migration, we have
performed the thermolysis of 5a in the presence of
triphenylphosphine.
At −40°C the 13C NMR spectra of 4c and 4d show
four terminal carbonyl resonances with respective inten-
sity 2:2:1:1, which confirm the non-equivalence of the two
metal centres. This carbonyl region is temperature-depen-
dent: two of these signals gradually coalesce so that at
20°C the spectra appear as two signals with 2:1 intensity
analogous to the only and unexplained spectrum reported
for Fe2(CO)6[m,h2-C9H19C(O)]2 [26]. At a higher temper-
ature 4c and 4d undergo thermal decomposition. Consid-
3.5. Study of the influence of the electronic density of the
metal on the ligand migration
3.5.1. By thermolysis of the dimer 5a in the presence of
triphenylphosphine
When reacted at −5°C with one equivalent of PPh3,

174
D. Luart et al. / Inorganica Chimica Acta 291 (1999) 166–177
Scheme 1.
5a gave rise to the monophosphine complex 7a formed
in 65% yield (Eq. (7)). As expected, the absence of
symmetry of 7a resulting from the complexation of one
phosphine on one metal centre, induced in IR the
presence of seven w(CꢀO) and two w(CꢁO) bands (Table
2).
tuted by phosphines compared to the same reaction on
monomers 3 [45] confirms the lability of the CO trans to
the alkoxycarbonyls of this dimer and give substance to
a possible intervention of the symmetric intermediate
{(CO)3Fe[m,h2-C(O)OR]}2 (Scheme 1) in the process of
formation of 4c by thermal evolution of 5a.
3.5.2. Thermolysis of the mono- and bis-phosphine
bis-alkoxycarbonyl dimers 7a and 8a and reaction of
the bis-(v,p2-alkoxycarbonyl) di-iron hexacarbonyl
complex 4c with triphenylphosphine
The preparation of the phosphine substituted com-
plexes 7a and 8a gave us the opportunity to study the
influence of the electronic density of metal centres on the
process of thermal ligand migrations from one metal to
another.
When heated for 15 h at 25°C, 7a or 8a gave rise to
the same complex 9a (Eq. (8)); surprisingly, the same
product was also obtained by thermolysis of the unsub-
tituted dimer 4c in the presence of triphenylphosphine.
(7)
In 13C NMR, the resonances of the terminal carbonyls
of the non-substituted metal centre appeared as three
singlets of 2:1:1 intensity. Two doublets (intensity 2:1
J(C–P) 13.1 Hz) fitted with the carbonyls linked to the
metal bearing the phosphine and a singlet and a doublet
(J(C–P) 23.3 Hz) to the carbonyl of the alkoxycarbonyl
ligands. The important deshielding of the resonance of
one carbonyl of an alkoxycarbonyl ligand associated
with a large 13C–31P coupling constant (23.3 Hz) of this
carbon with the phosphine, strongly suggested mutual
trans positions of these two ligands.
When 5a was allowed to react under the same condi-
tions with an excess of PPh3, the formation of a second
product was rapidly observed. The 13C NMR spectrum
of this complex (three doublets of intensity 2:1:1 in the
CO resonance area) is indicative of the formation of the
symmetric bis-phosphine complex 8a (Eq. (7)). The
presence of three w(CꢀO) bands in IR (C26 symmetry of
the Fe(CO)3 groups) and the value (J(C–P) 21.8 Hz) of
the coupling constant observed for the signal of the
carbonyl of the alkoxycarbonyls confirmed that these last
ligands were trans to the phosphines.
(8)
The IR and the 13C NMR characteristics of 9a (Table 2)
suggested for this complex a structure with two bridging
alkoxycarbonyl ligands analogous to that displayed
by 4c and 4d. These data were also indicative of the
presence of a single phosphine on the molecule. In
The ease with which two carbonyls of 5a are substi-

D. Luart et al. / Inorganica Chimica Acta 291 (1999) 166–177
175
Fig. 2. ORTEP representation of the structure of 9a (the thermal ellipsoids are drawn at the 50% probability level).
13C NMR three signals of relative intensity 2:2:1 in the
terminal CO area showed that this phosphine was
complexed in trans from the metal–metal bond and
that the two alkoxycarbonyl were head-to-head bridged
between the two metals. Contrary to the observations
realised on the mono phosphine dimer 7a, all the
signals of the carbonyls of 9a were coupled with the
phosphine. This could result from the location of the
phosphine in trans from the metal–metal bond. This
geometry was also marked by an important J(13C–31P)
(12.5 Hz) exhibited by the CO trans to the metal–metal
bond.
However, these spectroscopic data did not tell
whether or not the phosphine was complexed to the
metal linked to the bridging alkoxycarbonyl ligands by
their carbon or by their oxygen. An answer to this
question was brought by the crystallographic study
realised on 9a.
complex 5a (2.840(1) A) [34] but falls between the range
of the lengths observed for the acyl or the carbamoyl
homologues [46,30]. The rather long Fe–O distances
,
,
observed for 9a (2.009(2) and 2.003(2) A) result from
the reduced nucleophilic character of the oxygens of the
alkoxycarbonyls compared to that of the acyl (1.97(1)
A) or the carbamoyl (1.977(2), 1.956(2) A) ligands. The
distances between the metal and the carbon of the
alkoxycarbonyl ligands (1.942(4), 1.932(8) A) of 9a are
,
,
,
shorter than their homologues of the monomer
Fe(CO2tBu)2(CO)4 (3d) (2.009(6) A) or of the dimer
,
t
,
[Fe(CO2Bu)(CO)4]2 (5a) (2.024(4) A). However, as the
CꢁO bonds of 9a (1.244(8), 1.245(4) A) are not signifi-
cantly longer than those of 3d (1.241(9) A), the carbenic
character of the bridging alkoxycarbonyls is not struc-
turally established. The influence of the phosphine
linked to Fe(2) is marked by reduced bond lengths
,
,
Suitable crystals of 9a were obtained by crystallisa-
tion in hexane at −30°C. The so obtained ORTEP
diagram of 9a and the selected bond lengths and angle
values of the molecule are displayed in Fig. 2 and
collected in Table 4, respectively.
The crystal structure confirmed the proposed geome-
try for 9a and revealed that the phosphine was linked,
in trans position from the metal–metal bond, to the
iron itself bound to the oxygens of the bridging alkoxy-
carbonyls. This structure is very close to those reported
for the bridged bis-m,h2-acyl [30,46] or bis-m,h2-car-
bamoyl iron complexes [30] but is, to our knowledge,
the first realised for bis-(m,h2-alkoxycarbonyl) di-iron
dimers. The molecule can be described as two square
pyramidal iron centres edge linked by two alkoxycar-
bonyls. Due to the presence of these two bridging
alkoxycarbonyl ligands, the Fe–Fe distance is shorter
Table 4
,
Selected bond lengths (A) and angles (°) for complex 9a
Fe(1)–Fe(2)
Fe(1)–C(1)
Fe(1)–C(3)
Fe(1)–C(6)
Fe(2)–P
2.5840(7)
1.809(4)
1.800(4)
1.942(4)
2.305(1)
Fe(2)–O(8)
Fe(2)–C(4)
C(6)–O(6)
O(6)–C(7)
O(9)–C(6)
2.009(2)
1.753(4)
1.330(4)
1.493(4)
1.244(4)
Fe(2)–Fe(1)–C(1)
Fe(2)–Fe(1)–C(3) 154.1(1)
97.8(1)
P–Fe(2)–C(4)
96.6(1)
85.4(1)
168.1(1)
90.7(1)
O(8)–Fe(2)–O(9)
O(8)–Fe(2)–C(4)
O(8)–Fe(2)–C(5)
C(4)–Fe(2)–C(5)
C(6)–O(6)–C(7)
Fe(2)–O(8)–C(11)
Fe(1)–C(6)–O(6)
Fe(1)–C(6)–O(9)
O(6)–C(6)–O(9)
Fe(2)–Fe(1)–C(6)
C(2)–Fe(1)–C(6)
C(1)–Fe(1)–C(6)
C(3)–Fe(1)–C(6)
C(6)–Fe(1)–C(11)
Fe(1)–Fe(2)–P
Fe(1)–Fe(2)–O(8)
Fe(1)–Fe(2)–C(4)
P–Fe(2)–O(8)
67.03(9)
166.5(2)
88.8(2)
94.0(2)
83.5(1)
161.73(3)
73.16(7)
95.0(1)
89.6(2)
123.0(3)
100.0(2)
120.5(2)
118.8(2)
102.7(3)
95.11(7)
,
(2.5840(7) A) than that measured for the non-bridged

176
D. Luart et al. / Inorganica Chimica Acta 291 (1999) 166–177
(1.753(4), 1.756(4)) between this atom and the terminal
carbonyls compared to those observed for the analogous
ligands bound to Fe(1) (1.809(4), 1.804(4)). This shorten-
ing of the Fe–CꢀO bonds is clearly induced by a more
efficient back-bonding towards these CꢀO resulting from
a higher electron density on Fe(2).
the dicarbonylated mono bridged intermediate
{Fe[C(O)R](CO)4}2COCO which, after a mono decar-
bonylation could give rise to {Fe[C(O)R](CO)4}2CO.
The intermediates of this type displaying very mobile R
groups could evolve, by a fast migration of R towards
a carbonyl (bridging or terminal) linked to the second
metal, into Fe[C(O)R]2(CO)4 and Fe(CO)5, while other
intermediates bearing less mobile R, after a second
decarbonylation, could induce the formation of
{Fe[C(O)R](CO)4}2. These Fe–Fe bridged dimers could
be stable only if they are substituted by rather electron-
withdrawing organic ligands (C(O)OtBu, C(O)OC(Me)2-
CHꢁCH2). For more electron-donor RC(O) ligands,
these complexes are found to give rise, even at low
temperature, to bis-[m,h2-RC(O)]Fe2(CO)6 dimers. The
results of this work suggest that this reaction probably
occurs via an homolytic cleavage of the metal–metal
The substitution of one carbonyl of 5a by a
triphenylphosphine makes easier the formation of the
bis-m,h2-alkoxycarbonyl complexes as the reaction is
finished after 15 h at 25°C for 7a and 8a and only after
24 h at the same temperature for the unsubstituted
complex 5a. As the presence of one or two phosphines
on the complexes should strengthen the Fe–CꢀO bonds,
this raises up the question of the real importance of the
decarbonylation step in the process of formation of 4c
and suggests a possible cleavage of the metal–metal
bond. This cleavage should lead to the forma-
tion of 17 electrons organometallic radicals [Fe(CO2tBu)-
’
bond affording {Fe[C(O)R](CO)4} which, by a process
’
(CO)4−n(PPh3)n] (n=0, 1), which could present en-
analogous to that observed by direct oxidation of
{Fe[C(O)R](CO)4}−, could induce the formation of the
bis-[m,h2-C(O)R]Fe2(CO)6 complexes. However, the for-
hanced mobility of their carbonyl or phosphine ligands
[47]. These radicals, similarly to those formed by oxida-
tion of [Fe(CO2tBu)(CO)4]− could induce, by a still
unknown mechanism, the formation of bis-(m,h2-alkoxy-
carbonyl) dimers.
’
mation of {Fe[C(O)R](CO)4} by direct oxidation of the
electron rich {Fe[C(O)R](CO)4}− by ClC(O)C(O)Cl
cannot be precluded and further experiments are under
progress to clear up such a possibility.
3.6. Thermolysis of the bis-v,p2-alkoxycarbonyl
complexes 4c and 9a
5. Supplementary material
Contrary to their bis-(m,h2-acyl) homologues, which
thermally were found to give rise to mono or di-carbony-
lated organic compounds (benzophenone and diphenyl
ethane dione) [48] by a carbon–carbon coupling between
their two bridging ligands and though displaying two
organic ligands very close to each other, 4c and 9a were
not found to induce such a carbon–carbon coupling.
After keeping 4c and 9a 3 h at 50°C, the only products
found in the reaction mixture were tBuOH and Fe(CO)5
or Fe(CO)4(PPh3) and Fe(CO)3(PPh3)2 but no trace of
Full tables of interatomic distances and bonds angles,
displacement parameters, and observed and calculated
structure amplitudes are available from the authors on
request.
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