Specific formation of isocyanide iron complexes by reaction of primary carbamoyl ferrates with oxalylchloride

Article abstract of DOI:10.1016/S0020-1693(02)01553-0

Reaction of primary carbamoyl ferrates {(CO)₄Fe[C(O)NHR]} ^- (R=Me, Et, allyl, decyl, cyclohexyl, t-butyl, benzyl, phenyl) with 1/2 equiv. of oxalylchloride affords cis-bis-carbamoyl intermediates: (CO) ₄Fe[C(O)NHR]₂ which thermally give rise, in good yields, to the mono-isocyanide complexes (CO)₄Fe(CNR). The mechanism of the reaction is discussed. Via a similar process, an alkoxycarbamoyl intermediate (CO)₄Fe[C(O)NHR](CO₂Me) affords Fe(CO)₅ and 1,3-dialkylurea.

Full text of DOI:10.1016/S0020-1693(02)01553-0

Inorganica Chimica Acta 350 (2003) 656ꢃ660
/
www.elsevier.com/locate/ica
Note
Specific formation of isocyanide iron complexes by reaction of
primary carbamoyl ferrates with oxalylchloride
Denis Luart, Jean-Yves Salaun *, Ve´ronique Patinec, Rene´ Rumin, 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 Cedex, France
Received 8 July 2002; accepted 16 September 2002
Dedicated in honor of Professor Pierre Braunstein
Abstract
Reaction of primary carbamoyl ferrates {(CO)4Fe[C(O)NHR]}ꢀ (Rꢁ
Me, Et, allyl, decyl, cyclohexyl, t-butyl, benzyl, phenyl)
/
with 1/2 equiv. of oxalylchloride affords cis-bis-carbamoyl intermediates: (CO)4Fe[C(O)NHR]2 which thermally give rise, in good
yields, to the mono-isocyanide complexes (CO)4Fe(CNR). The mechanism of the reaction is discussed. Via a similar process, an
alkoxycarbamoyl intermediate (CO)4Fe[C(O)NHR](CO2Me) affords Fe(CO)5 and 1,3-dialkylurea.
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Iron; Isocyanide; Carbamoyl; Dialkylurea
1. Introduction
mediates [4]. This paper describes (1) our attempts to
obtain complexes of the cis-(CO)4Fe[C(O)NHR]2 series
Bis-alkoxycarbonyl organometallic species have been
proposed as intermediates in catalytic processes afford-
ing oxalates by oxidative carbonylation of alcohols. For
this reason, numerous complexes bearing two alkoxy-
carbonyl ligands have been prepared and their reactivity
widely investigated [1]. Conversely, the chemistry of
their bis-carbamoyl homologues is scarcely documented.
To date, only a few h1-bis-carbamoyl complexes of Hg
[2a], Ru [2b], Pt [2c] or Pd [2d] stabilized by strong
electron donor ancillary ligands and h2-bis-carbamoyl
of U and Th [2e] have been characterized. {W[C(O)Ni-
Pr2]2(CO)4}2ꢀ [3a] {(CO)3Fe[C(O)NR2]2}{Sn(CH3)2}
[3b] and {(CO)3Fe[C(O)NR2]2}2{Ti} [3c,3d] are better
described as carbene complexes. For our part, we
recently reported the rapid thermal transformation of
displaying primary carbamoyl ligands with labile NÃ
/
H
bonds by reaction of the appropriate ferrate:
{(CO)4Fe[C(O)NHR]}ꢀ with 1/2 equiv. of oxalylchlor-
ide and (2) the transformation of these complexes into
mono-isocyanide compounds (CO)4Fe(CNR).
2. Experimental
2.1. Material and methods
Fe(CO)5, BuÃ
/
Li and the primary amines were pur-
chased commercially and used without further purifica-
tion. (CO)4Fe(CO2Me)2 was prepared as described
elsewhere [5]. Infrared spectra were obtained in solution
cis-bis-secondary
(CO)4Fe[h1-C(O)NR2]2 (Rꢁ
metallacarbenes
CÃ
ligands of (CO)3Fe[h1-C(O)NR2][h2-C(O)NR2] inter-
carbamoyl
iron
complexes
1
on a Nicolet Nexus spectrometer. H and 13C NMR
/Me, Et, n-Pr) into cyclo-
spectra were recorded on Brucker AC 300 and AMX 3-
400 spectrometers. Chemical shifts are reported in d
units (parts per million) downfield from TMS in CH2Cl2
(external reference) (1H) or from the solvent resonances
as external references (13C). Gas chromatography stu-
dies were performed on a Hewlett-Packard 5890 instru-
ment using a CP Sil 25 m capillary column. Elemental
analyses were performed by the Centre de Microana-
by a
/
O coupling performed between the two carbamoyl
* Corresponding author. Tel.: ꢂ
6594.
E-mail address: jean-yves.salaun@univ-brest.fr (J.-Y. Salaun).
/
33-2-98-01 6286; fax: ꢂ33-2-98-01
/
¨
0020-1693/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0020-1693(02)01553-0

D. Luart et al. / Inorganica Chimica Acta 350 (2003) 656ꢃ
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660
657
lyses du CNRS at Lyon-Solaize, France. 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. Yields of reactions are
evaluated from recrystallized products and relative to
ClC(O)C(O)Cl.
3.92 ppm (1H, m). 13C NMR 213.5, 153.9, 56.0, 32.4,
25.1, 22.8 ppm.
Complex 4f: Rꢁdecyl, yield 60% (807 mg). Anal.
/
calc. for FeC15H21NO4: C, 53.60; H, 6.31. Found: C,
53.82; H, 6.38%. IR: 2187, w (nCN); 2059, m, 1994, m,
1
1967 cmꢀ1, s (nCO). H NMR 3.66 (2H, m); 0.60ꢃ
/
1.75
ppm (19H, m). 13C NMR 213.4, 154.3, 45.9, 32.0, 29.6,
29.5, 29.4, 29.1, 28.8, 26.4, 22.9, 14.1 ppm.
Complex 4g: Rꢁbenzyl, yield 65% (740 mg). Anal.
/
2.2. Syntheses of (CO)4Fe(CNR) complexes 4
calc. for FeC12H7NO4: C, 50.55; H, 2.48. Found: C,
50.49; H, 2.39%. IR: 2182, m (nCN); 2061, m, 1999, m,
1
To 8 mmol of primary amine solution in 30 ml of
1972 cmꢀ1, s (nCO). H NMR 7.4 (5H, m); 4.95 ppm
THF at ꢀ
of n-butyllithium. After 30 min, the temperature of the
solution was raised to ꢀ10 8C and 1.57 ml (12 mmol) of
/
80 8C, was added 3.2 ml of a 2.5 M solution
(2H, s). 13C NMR 213.3, 157.7, 132.4, 129.4, 128.9,
126.8, 49.2 ppm.
/
Complex 4h: Rꢁphenyl, yield 63% (683 mg). Anal.
/
Fe(CO)5 solution in 5 ml of THF was introduced
dropwise. After 30 min, the temperature of the solution
calc. for FeC11H5NO4: C, 48.75; H, 1.86. Found: C,
48.65; H, 1.89%. IR: 2155, w (nCN); 2061, m, 1999, m,
1
was lowered to ꢀ
/
30 8C and 4 mmol (0.35 ml) of
1972 cmꢀ1, s (nCO). H NMR 7.5 ppm (5H, m). 13C
NMR 212.9, 166.8, 129.8, 129.7, 128.1, 125.7 ppm.
oxalylchloride was added. The orange-brown solution
rapidly turned yellow. After 1 h, the solvent was
evaporated to dryness (for MeNH2, EtNH2, AllylNH2,
and t-BuNH2, the solvent was trapped with liquid
nitrogen). For C6H5NH2, C6H5CH2NH2, C10H21NH2
and C6H11NH2, the formed amine was extracted from
2.3. Exchange reactions from (CO)4Fe(CO2Me)2 1
To a stirred solution of 3 mmol of 1 (858 mg) in THF,
12 mmol of primary amine (900 ml of allylamine or 6 ml
of a 2 M solution of methyl or ethylamine in THF) was
added at 25 8C. After 1 h, the solvent was evaporated to
dryness and the residue extracted with two portions of a
1/5 dichloromethane/hexane mixture. The formed 1,3-
dialkylurea was identified either by gas chromatography
or by 13C by comparison with authentic samples.
the residue by two portions of 5 ml of hexane at ꢀ
/
40 8C. The presence of amines was evidenced by gas
chromatography and 13C NMR. Complexes 4 were
extracted from the residue by two portions of 20 ml of
hexane at 0 8C. Crystals were obtained from these
solutions after 1 day at ꢀ
/
30 8C.
Complex 4a: RꢁMe, yield 68% (570 mg). Anal. calc.
/
for FeC6H3NO4: C, 34.49; H, 1.45. Found: C, 34.42; H,
1.51%. IR: 2192, m (nCN); 2058, m, 1998, m, 1969
cmꢀ1, s (nCO). 1H NMR 3.43 ppm (s). 13C NMR 213.0,
151.8, 31.0 ppm.
3. Results and discussion
3.1. Reactions of (CO)4Fe[C(O)NHR]ꢀ with
oxalylchloride
Complex 4b: RꢁEt, yield 65% (580 mg). Anal. calc.
/
for FeC7H5NO4: C, 37.70; H, 2.26. Found: C, 37.81; H,
2.32%. IR: 2186, m (nCN); 2059, m, 1995, m, 1968
We already reported the synthesis of the bis-alkox-
ycarbonyl complexes (CO)4Fe(CO2R)2 1 [5] and the
achievement of the very unstable bis-carbamoyl
(CO)4Fe[C(O)NR2]2 2 intermediates [4] by reaction of
1/2 equiv. of oxalylchloride with the appropriate alkox-
ycarbonyl or carbamoyl ferrates {(CO)4Fe[C(O)R]}ꢀ
?
1
cmꢀ1, s (nCO). H NMR 3.72 (2H, q, Jꢁ
/7.2 Hz), 1.43
ppm (3H, t, Jꢁ
18.9 ppm.
Complex 4c: Rꢁ
for FeC8H5NO4: C, 40.89; H, 2.15. Found: C, 40.96; H,
2.17%. IR: 2180, m (nCN); 2060, m, 1997, m,1972 cmꢀ1
s (nCO). 1H NMR 5.87 (1H, m), 5.30 (1H, m), 5.25 (1H,
m), 3.56 ppm (2H, d, Jꢁ
2.5 Hz). 13C NMR 213.4,
157.8, 128.6, 118.0, 47.8 ppm.
Complex 4d: Rꢁt-Bu, yield 75% (753 mg). Anal.
/
7.2 Hz). 13C NMR 212.0, 154.6, 40.4,
/allyl, yield 62% (583 mg). Anal. calc.
,
RꢁOR?, NR2, themselves obtained by nucleophilic
/
addition of RNa or Rli on a terminal carbonyl of
Fe(CO)5. As mentioned above, complexes 2 were found
to give, at low temperature, metallacarbenes 3 (Scheme
1).
/
/
calc. for FeC9H9NO4: C, 43.05; H, 3.62. Found: C,
43.12; H, 3.58%. IR: 2171, m (nCN); 2056, m, 1994, m,
The same reaction performed from {(CO)4Fe[C-
(O)N(H)R]}ꢀ bearing a primary carbamoyl was then
supposed to afford cis-(CO)4Fe[C(O)N(H)R]2 2? which
only differ from 2 by the presence of mobile hydrogen
on nitrogen of carbamoyl ligands. Surprisingly, the
process was found to afford specifically and in good
yield (from 60 to 80%) the iron isocyanide complexes
1
1967 cmꢀ1, s (nCO). H NMR 1.48 ppm s. 13C NMR
213.4, 152.2 (t, Jꢁ/18 Hz), 59.2, 30.1 ppm.
Complex 4e: Rꢁ/cyclohexyl, yield 70% (775 mg).
Anal. calc. for FeC11H11NO4: C, 47.68; H, 4.01. Found:
C, 47.55; H, 4.15%. IR: 2178, m (nCN); 2057, m, 1994,
1
m,1966 cmꢀ1, s (nCO). H NMR 1.45ꢃ
/2.10 (10H, m);
(CO)4Fe(CNR) (RꢁMe, Et, phenyl, benzyl, allyl, t-
/

658
D. Luart et al. / Inorganica Chimica Acta 350 (2003) 656ꢃ660
/
troscopic characteristics identical to those already
reported for analogous compounds [6]. In IR, in
addition to nCÄ
and 2192 cmꢀ1, three nCÅ
nCÅ
O frequencies area (C3v symmetry). 1H NMR
/N stretching frequency between 2155
/O bands are observed in the
/
spectra of 4 are also in agreement with the proposed
structures. It can be noted that the observed resonances
of the ligands are found downfield relative to the free
isonitriles. In 13C NMR, the resonance of the terminal
carbonyls is observed as a single signal near 213 ppm.
Variable-temperature NMR studies performed on these
complexes have shown the occurrence, even at ꢀ70 8C,
/
of rapid exchanges between these terminal carbonyls.
The isonitrile ligating carbon resonance, generally ob-
served as a broad signal, is found between 151.8 and
166.8 ppm. On the spectrum of the t-Bu isocyanide
complex this signal, which should appear as a 1.1.1
triplet from 13CÃ
/
14N spin coupling, is observed as a
18 Hz) with a more intense central
triplet (J(13CÃ14N)ꢁ
/
/
Scheme 1.
peak probably resulting from quadrupolar relaxation of
the coupling.
butyl, decyl and cyclohexyl (4)) together with the
corresponding amine (Scheme 2).
It has been known for a long time that isocyanide
complexes 4 can be prepared from Fe(CO)5 and
isocyanides or with isocyanide precursors such as
isocyanates, phosphine-imines, isocyanide dichlorides
or metal trimethylsilyl amides; these reactions often
present a low selectivity and then very low yields.
3.2. Alkoxy/amine exchanges from cis-
(CO)4Fe(CO2Me)2 1
Another method which was supposed suitable for the
preparation of bis-carbamoyl complexes 2? was the
alkoxy/amine exchange performed from bis-alkoxycar-
bonyl complexes (CO)4Fe(CO2R)2 (1). This type of
exchanges affording carbamoyl ligands has been ob-
served for numerous alkoxycarbonyl complexes [9]. For
example, this reaction has been used for the preparation
of the bis-carbamoyl compound: Ru(dppe)(CO)2-
However,
a
specific method of preparation of
1ꢃ5), catalyzed by CoCl2×
Fe(CO)5ꢀn(CNR)n (nꢁ
/
/
/
H2O has been reported from Fe(CO)5 and RCN [6].
The transformation of the carbamoyl ligand of
CpFe(CO)2{C(O)N(H)R} into an isocyanide ligand
has been performed by phosgene in the presence of an
excess of tertiary amines [7] and, more recently, the
phosphoramidate conversion of terminal carbonyl li-
gands into isocyanide ligands has been achieved [8].
The new formation of iron isocyanide complexes (4)
by the reaction of carbamoyl ferrates with oxalylchlor-
ide seems general as it has been performed from primary
amines of different electronic or steric effects (MeNH2,
[C(O)NiÃ/Pr2]2 [2b] and, performed from 1 and second-
ary amines, the process is another suitable method of
synthesis of the unstable cis-(CO)4Fe[C(O)NR2]2 inter-
mediates [4].
When the exchange was carried out by reacting 1 with
an excess (4 equiv.) of primary amine (methyl, ethyl or
allylamine), the reaction was found to afford quantita-
tively Fe(CO)5 and, respectively, 1,3-dimethyl, diethyl or
diallylurea identified by gas chromatography or 13C
NMR by comparison with authentic samples (Scheme
3).
EtNH2, CH2Ä
/
CHÃ
/
CH2NH2, t-BuNH2, C6H11NH2,
C6H5NH2, C6H5CH2NH2). Complexes 4 display spec-
The formation of isocyanide complexes by reaction of
a carbamoyl ferrate with oxalylchloride and that of
dialkylurea by alkoxy/amine exchanges from bis-alkox-
Scheme 2.
Scheme 3.

D. Luart et al. / Inorganica Chimica Acta 350 (2003) 656ꢃ
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660
659
ycarbonyl complex 1 may be explained by the following
considerations.
intermediates in the reaction of formation of isocyanide
complexes by the well-known nucleophilic addition of
primary amines to thiocarbonyl ligand of Mo [12a], W
[12a,12b,12c], Ru [12d] or Fe [12e,12f] complexes. The
preparation and the study of these carbenes have
allowed to confirm their transformation into isocyanide
3.3. Mechanisms of the reactions
Similar to the processes performed with {(CO)4-
Fe[CO2R]}ꢀ and {(CO)4Fe[C(O)NR2]}ꢀ, the reaction
of the primary carbamoyl ferrates {(CO)4Fe-
[C(O)NHR]}ꢀ with 1/2 equiv. of oxalylchloride very
complexes thus CpFe(CO)2[Ä
C(NHR)(NHR?)]ꢂ was
/
found to give rise to CpFe(CO)2[(CNR?)]ꢂ [13]. These
reactions involving elimination of H2S, H2O or RNH2
and affording isocyanide ligands are the reverse of the
classic reaction of isocyanide complexes with pronu-
cleophile reagents.
In conclusion, it can be noted that if the two steps of
the mechanism described in Scheme 4 have already been
observed separately, their simultaneous occurrence and
then the transformation of two carbamoyl ligands into
an isocyanide complex and an amine has never been
reported.
As reported above, though supposed to also give rise
to the bis-carbamoyl intermediates 2?, the alkoxy/amine
exchanges performed from 1 (Scheme 3) do not afford
the same results than the reaction of carbamoyl ferrates
with oxalylchloride. We have already shown that, on
complex 1, the second alkoxy/secondary amine exchange
giving 2 occurs more slowly than the first one. It then
becomes credible that the intermediate cis-
(CO)4Fe(CO2Me)[C(O)NR2] [4] could be stable enough
to present, as observed for 2, a proton exchange,
between its two organic ligands. Such a reaction has
already been observed for a ruthenium homologue [10].
As shown in Scheme 5, this proton migration would
afford an isocyanate and an hydroxy alkoxycarbene
whose transformation could give rise to alcohol and
Fe(CO)5. A further reaction between isocyanate and
amine in excess in solution gives 1,3-dialkylurea.
likely
affords
bis-carbamoyl
complexes
cis-
(CO)4Fe[C(O)N(H)R]2 2?. These unstable entities would
be responsible for the formation of the isocyanide
complexes 4 (Scheme 2). A mechanism that could
account for the formation of 4 from 2? is displayed in
Scheme 4.
An intramolecular migration of the labile proton of
the amino group of a first carbamoyl ligand of 2?
towards oxygen of the second carbamoyl would give rise
to an hydroxy amino carbene 5 and to isocyanate.
Further elimination of water from 5 would afford the
isocyanide complexes 4 and a last reaction between
water and isocyanate affords carbamic acid which by
decarboxylation gives the amine. Though different
(formation of Ru(dppe)(CO)3 and 1,3-diisopropylurea),
the thermal transformation of the close complex cis-
(dppe)(CO)2Ru[C(O)N(H)i-Pr]2 has been reported to
follow a similar mechanism. However, a specific elim-
ination of i-propylamine from an intermediate analo-
gous to 5 affords Ru(dppe)(CO)3 and not an isocyanide
complex formed by elimination of water [10].
The migration of an hydrogen between the two
carbamoyl ligands of 2? is not surprising as such an
interaction between the proton of a first carbamoyl and
the nitrogen of a second carbamoyl has been observed in
the solid state and in solution for cis-(dppe)(CO)2R-
u[C(O)N(H)i-Pr]2 [2b]. The transformation, involving
an intermolecular proton migration, of a carbamoyl
ligand inducing the formation of isocyanate is also
documented: for example, this reaction has been per-
formed by reaction of CpW(CO)3[C(O)N(H)CH3] with
triethylamine which here plays the part of proton
acceptor. Methylisocyanate was then formed together
with CpW(CO)3ꢀNH(C2H5)3ꢂ. However, under the
same conditions, no reaction was observed for
CpFe(CO)2[C(O)N(H)R] [11]. Amino mercapto car-
benes analogous to 5 have already been suggested as
4. Conclusion
This work reports the first description of the thermal
evolution of cis-bis-primary carbamoyl intermediates
into isocyanide compounds. The reaction is a specific
and convenient method for the preparation of
(CO)4Fe(CNR) complexes starting from a large variety
Scheme 4.
Scheme 5.

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D. Luart et al. / Inorganica Chimica Acta 350 (2003) 656ꢃ
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660
[4] N. Le Gall, D. Luart, J.Y. Salaun, H. des Abbayes, L. Toupet, J.
¨
of primary amines which are more easily available than
the corresponding isocyanides required by other meth-
ods. Furthermore, this one-pot process does not require
dangerous and air- or moisture-sensitive reagents.
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/
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1069.
[7] W.P. Fehlhammer, A. Mayr, Angew. Chem. Int. Ed. 14 (1975)
757.
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