Migratory insertion in N-heterocyclic carbene-containing Fe carbonyl complexes: An experimental and theoretical study

Article abstract of DOI:10.1039/b516328e

The compound [Fe(η-C₅H₅)(CO)₂(Me)] reacts thermally with N-heterocyclic carbenes (L) to give both alkyl, [Fe(η-C₅H₅)(L)(CO)(Me)], and acyl, [Fe(η-C ₅H₅)(L)(CO)(COMe)], derivative

Full text of DOI:10.1039/b516328e

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/dalton | Dalton Transactions
Migratory insertion in N-heterocyclic carbene-containing Fe carbonyl
complexes: an experimental and theoretical study
Simon A. Llewellyn,* Malcolm L. H. Green, Jennifer C. Green and Andrew R. Cowley
Received 17th November 2005, Accepted 6th March 2006
First published as an Advance Article on the web 17th March 2006
DOI: 10.1039/b516328e
The compound [Fe(g-C
5
H
5
)(CO)
2
(Me)] reacts thermally with N-heterocyclic carbenes (L) to give both
)(L)(CO)(COMe)], derivatives. The reaction
alkyl, [Fe(g-C )(L)(CO)(Me)], and acyl, [Fe(g-C
5
H
5
5
H
5
temperature has been shown to affect the product distribution. The alkyl and acyl derivatives exist in an
equilibrium that is more easily perturbed than in the tertiary phosphine analogues. DFT studies on the
reactivity of [Fe(g-C
CO exchange is energetically favoured for IH, and energetically disfavoured for PH
CO-induced migratory insertion, [Fe(g-C )(L)(CO)(COMe)], are more stable than the parent alkyl,
Fe(g-C
5
H
5
)(CO)
2
(Me)] with PH
3
and dihydroimidazole-2-ylidene (IH) have shown that
3
. The products of
5
H
5
[
5
H
5
)(L)(CO)(Me)], compounds. This stabilisation is larger when L = IH than when L = PH
3
.
Stabilisation of the transition state by agostic interactions was seen in both instances, but this was
significantly more pronounced for L = IH.
Introduction
Results and discussion
Transition-metal complexes of N-heterocyclic carbenes NHCs
have been synthesised by a number of different synthetic
routes, including deprotonation of the imidazolium salt by
Synthesis
Addition of 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene,
IMes, to a solution of [Fe(g-C )(CO) (Me)] in toluene gave
the compound [Fe(g-C )(IMes)(CO)(Me)], 1, after prolonged
1,2
3
4
5
H
5
2
metal anions, ligand displacement, oxidative addition, and
5
5
H
5
silver carbene transmetallation. Transition-metal N-heterocyclic
stirring at room temperature. Heating a solution of IMes with
carbene compounds have also been synthesised by carbonyl
group displacement. Thus the reaction of the simple car-
◦
[
[
Fe(g-C
Fe(g-C
5
H
H
5
)(CO)
2
(Me)] in toluene at 60 C gave the compound
)(IMes)(CO)(COMe)], 2, after prolonged stirring. The
5
5
bonyl compounds Fe(CO)
5
, Ni(CO)
4
, M(CO)
6
(M = Cr,
compound 2 was observed to slowly decarbonylate in solution,
forming 1. The compound 2 may also be synthesised by the car-
bonylation of 1 as described below. This reactivity is summarised
in Scheme 1. The compound 1 is stable in solution. Both 1 and
Mo, W) with N-heterocyclic carbenes gave the corresponding
3
carbene complexes. Buchgraber et al. have synthesised N-
heterocyclic carbene-containing iron piano-stool compounds, by
treating the compound [Fe(g-C
trimethylphenyl)imidazol-2-ylidene, (IMes) and 1,3-bis(2,4,6-
trimethylphenyl)imidazolin-2-ylidene (H -IMes) to give the ionic
-IMes).
5
H
5
)(CO)
2
(I)] with 1,3-bis(2,4,6-
2
are soluble in common polar organic solvents and essentially
insoluble in non-polar organic solvents, such as pentane and
2
1
+
−
hexane. Compounds 1 and 2 have been characterised by H,
compounds [Fe(g-C
5
H
5
)(L)(CO)
2
] [I] , (L = IMes, H
2
1
3
1
C{ H} NMR and IR spectroscopies, by mass spectrometry and
Subsequent light-induced decarbonylation afforded the neutral
6
by elemental analysis. Compound 1 has also been characterised
compounds [Fe(g-C
5
H
5
)(L)(CO)(I)] (L = IMes, H
2
-IMes).
by X-ray crystal structure determination.
Extensive work by Brown and Davies on systems of the
7
type [Fe(g-C
5
H
5
)(L)(CO)(Me)] (L = tertiary phosphine) has
demonstrated the application of such systems for alkyl ho-
mologation. Our interests focus on developing new templates
Crystallography
8,9
for alkyl homologation, with a view to the development of
homogeneous Fischer–Tropsch catalysts. In this paper we de-
scribe the synthesis and structure of new N-heterocyclic carbene
Crystals of [Fe(g-C H )(IMes)(CO)(Me)], 1, suitable for X-ray
structure determination, were grown by slow diffusion of pentane
into a saturated diethyl ether solution of 2 (presumably formed by
5
5
derivatives of [Fe(g-C
5
H
5
)(L)(CO)(R)] (R = Me, COMe), and their
decarbonylation in solution). An ORTEP representation of this
structure is shown in Fig. 1. Selected bond lengths ( A˚ ) and angles
carbonylation/decarbonylation chemistry. A theoretical study at
the DFT level has been conducted to explain the differences in
behaviour of these systems when contrasted with their tertiary
phosphine analogues.
◦
( ) are given in Table 1. The asymmetric unit cell contains four
crystallographically distinct molecules, all of which show similar
conformations. These form two pairs, (Fe(1)–O(1)/Fe(2)–O(2)
and Fe(3)–O(3)/Fe(4)–O(4)), that are related by an approximate
non-crystallographic centre of inversion. The crystal is formed by
a stacking of alternate layers running perpendicular to the c axis,
each set of which is composed entirely of one of these pairs. Despite
the presence of both enantiomers, the crystal adopts a chiral space
10
Inorganic Chemistry Laboratory, South Parks Road, Oxford, UK OX1 3QR
group, and refinement of the Flack enantiopole parameter gave
This journal is © The Royal Society of Chemistry 2006
Dalton Trans., 2006, 2535–2541 | 2535

View Article Online
Fig. 1 ORTEP view of 1. Thermal ellipsoids are drawn at 40% probability.
Hydrogen atoms have been omitted for clarity.
pounds [Fe(g-C
Fe(g-C I)(PPh
of 2.066(6) and 2.060 A, respectively. The Fe–CO bond length of
5
H
5
)(PPh
2
N(CH
3
)CH(CH
3
)Ph)(CO)(Me)] and
[
5
H
4
3
)(CO)(Me)], which have reported bond lengths
˚
˚
1
.728(4) A lies in the range typical for Fe carbonyl alkyl complexes.
˚
However, the C–O bond length, at 1.162(5) A, is longer than is
typical for Fe carbonyl alkyl compounds. Interestingly, the Fe(1)–
C(28)–O(1) bond angle, in 1, is 170.6 (average of four molecules).
Scheme 1 Synthesis of Fe N-heterocyclic carbene complexes.
◦
◦
This represents a bend of the bond, away from the mesityl ligand.
This may be due to a steric interaction between the carbonyl group
and the IMes ligand. Both mesityl groups in the carbene ligand
are twisted about the N–C bond with respect to metal–heterocycle
plane, by 90.18 and 103.25 . This may be attributed to a steric
interaction between the mesityl groups and the cyclopentadienyl
Table 1 Selected bond lengths ( A˚ ) and angles ( ) for [Fe(g-C
5
H )-
5
(
IMes)(CO)(Me)] (average for four molecules)
Fe(1)–C(1)
Fe(1)–C(22)
Fe(1)–C(28)
C(28)–O(1)
1.945(4)
2.034(4)
1.728(4)
1.162(5)
C(1)–N(1)
C(1)–N(2)
C(8)–C(9)
1.380(4)
1.376(5)
1.386(6)
◦
ligand. A similar effect is observed in the crystal structure of [Fe(g-
C(22)–Fe(1)–C(28)
C(1)–Fe(1)–C(22)
C(1)–Fe(1)–C(28)
C(1)–N(1)–C(4)
C(1)–N(2)–C(13)
86.3(19)
91.4(15)
97.9(16)
128.9(3)
128.6(3)
Fe(1)–C(28)–O(1)
Fe(1)–C(1)–N(1)
Fe(1)–C(1)–N(2)
N(1)–C(1)–N(2)
170.6(3)
130.1(3)
128.2(3)
101.6(3)
6
C
5
5
H )(IMes)(CO)(I)].
NMR Spectroscopy
1
The H NMR spectra of the compounds 1 and 2 show three singlet
resonances assignable to the methyl substituents of the mesityl
groups. The chemical inequivalence of the 2,6 methyl groups may
be attributed to the chirality of the molecule. The 3,5 aromatic
hydrogen atoms are seen as two distinct resonances. Conversely,
only a single resonance was observed for the heterocyclic ring
a value of 0.008(12), showing that the crystal as a whole consists
of a single enantiomer.
˚
Selected bond lengths (A) for similar [Fe(g-C
5
5
H )]-containing
compounds are listed in Table 2. The Fe–Ccarbene bond length
in Fischer-carbene complexes of Fe are in the range 1.82–
protons (HCN). In the compound 1, the Fe–CH group is observed
3
1
5
˚
˚
1
.97 A. Thus, the 1.945(4) A bond length found for 1,
at d 0.12, the high field value reflecting direct attachment to a metal
atom. By contrast, in the compound 2 the methyl group, now an
acyl, (COMe), is observed at d 2.52, the large downfield shift
being attributed to the electron withdrawing carbonyl group. The
cyclopentadienyl resonances are observed at similar values in both
˚
lies at the higher end of this range, and is 0.035 A shorter
than that of the related compound [Fe(g-C )(IMes)(CO)(I)].
5
H
5
˚
The Fe–Cmethyl bond length of 2.034(4) A is significantly
shorter than those reported in the related PPh
3
-containing com-
Table 2 Comparison of bond lengths ( A˚ )
Bond length
Compound
Fe–Ccarbene
Fe–Cmethyl
Fe–Ccarbonyl
C–O
Ref.
[
[
[
[
[
[
Fe(g-C
Fe(g-C
Fe(g-C
Fe(g-C
Fe(g-C
Fe(g-C
5
5
5
5
5
5
H
H
H
H
H
H
5
5
5
5
5
4
)(IMes)(CO)(Me)]
)(IMes)(CO)(I)]
1.945(4)
1.980(5)
1.857(6)
1.859(6)
2.034(4)
—
—
1.728(4)
1.641(9)
1.766(8)
1.769(7)
1.684(9)
1.162(5)
1.077(8)
—
This work
6
11
12
13
14
2
)(CO){g -C(OMe)-o-C
6
H
H
4
4
Cl}][OTf]
2
)(CO){g -C(OMe)-o-C
6
OMe}][OTf]
—
1.145(8)
1.186(9)
)(PPh
I)(PPh
2
N(CH
3
)CH(CH
3
)Ph)(CO)(Me)]
2.066(6)
a
a
a
3
)(CO)(Me)]
—
2.060
1.753
1.143
a
Esd not given.
2
536 | Dalton Trans., 2006, 2535–2541
This journal is © The Royal Society of Chemistry 2006

View Article Online
compounds, at ca. d 4.0. However, a slightly higher field value was
The chemical behaviour described above contrasts with the
1
3
1
recorded for 1. The C{ H} NMR spectra showed the carbene
carbon resonances at d 206.75 and 199.50 in the compounds 1
and 2, respectively. Resonances attributable to the COterminal group
were observed at ca. d 220 in both instances. In compound 2, a
resonance attributable to COacyl was observed at d 270.59.
analogous reactivity of [Fe(g-C
phines. These typically require elevated temperatures, affording
acyl complexes of the form [Fe(g-C
)(L)(CO)(COMe)] (L =
tertiary phosphine), along with traces of the decarbonylation
products of the type [Fe(g-C
)(L)(CO)(Me)] (L = tertiary
phosphine). The acyl compounds [Fe(g-C )(L)(CO)(COMe)]
5
H
5
)(CO)
2
(Me)] with tertiary phos-
17
5
H
5
5
H
5
5
H
5
Carbonylation/decarbonylation
(L = tertiary phosphine) are typically stable with respect to
◦
decarbonylation at temperatures of up to 80 C. Experimen-
A
30 mg sample of the compound [Fe(g-C
5
5
H )(IMes)-
tal studies on the decarbonylation of the compound [Fe(g-
(
CO)(COMe)], 2, was dissolved in d -benzene, and the ensuing
6
C
5
H
5
)(PPh
3
)(CO)(COMe)] have shown that this thermal decar-
1
reaction was monitored by H NMR spectroscopy. After 44 h in
◦
bonylation process, that is observed at temperatures of 98 C,
solution, 26% of the sample had decarbonylated to give [Fe(g-
occurs via the dissociation of PPh
3
to give [Fe(g-C
5
H
5
)(CO) (Me)].
2
C
1
5
H
5
)(IMes)(CO)(Me)], 1. This eventually equilibrated to give a
: 1 mixture of the compounds 1 : 2. This was somewhat surprising,
given that the related complex [Fe(g-C )(PPh )(CO)(COMe)] is
The free PPh
product of decarbonylation, [Fe(g-C
bonylation of [Fe(g-C )(PPh
3
subsequently displaces one CO group to give the
18
5
H
5
)(PPh
3
)(CO)(Me)]. Car-
5
H
5
3
5
H
5
3
)(CO)(Me)] is known to require a
stable with respect to decarbonylation at temperatures of up to
combination of elevated CO pressures (5.5 atm), and very polar
◦
ca. 100 C. This decarbonylation reaction was repeated, but this
16
solvents such as nitromethane.
time the NMR tube was charged with a 1 atm pressure of carbon
monoxide. Decarbonylation was observed again, but was retarded,
equilibrating to a 0.4 : 1 (1 : 2) mixture after one week.
Theoretical calculations
To understand the causes of the facile carbonylation/de-
carbonylation of the new NHC-containing compounds, when
compared with the tertiary phosphine analogues, a series of
DFT calculations was undertaken. For computational expediency,
the bulky 1,3-bis(2,4,6,trimethylphenyl)imidazol-2-ylidene ligand
The carbonylation of 1 to give 2 was examined by dissolving
3
0 mg samples of [Fe(g-C
5
5
H )(IMes)(CO)(Me)] in nitromethane
or dichloromethane. These solutions were stirred under an atmo-
sphere of CO. After removal of solvents under reduced pressure,
1
the residues were then examined by H NMR spectroscopy,
was replaced with dihydroimidazol-2-ylidene (IH), and PPh
replaced with PH
3
was
to determine the amount of [Fe(g-C
5
5
H )(IMes)(CO)(COMe)]
.
formed. The reaction conditions used for these carbonylations
and the percent conversion are summarised in Table 3. Conversion
yields were estimated by integration of the cyclopentadienyl and
3
Geometry optimisations
1
alkyl proton environments in the H NMR spectrum.
The molecular structures of [Fe(g-C
5
H
5
)(IH)(CO)(Me)], I, [Fe(g-
)(PH )(CO)(Me)], III,
)(CO)(COMe)], IV, have been optimised.
The carbonylation of 1 at 1.5 atm of CO represents a facile car-
C
5
H
5
)(IH)(CO)(COMe)], II, [Fe(g-C
5
H
5
3
bonylation, when compared with [Fe(g-C
The formation of 2 from 1, albeit in low yield, in dichloromethane
is surprising, given that [Fe(g-C )(PPh )(CO)(Me)] is resistant
to carbonylation in dichloromethane, even under a 5 bar pressure
5
H
5
)(PPh
3
)(CO)(Me)].
and [Fe(g-C )(PH
5
H
5
3
Selected data for the calculated bond distances are given in
Table 4, along with experimentally derived values for similar
compounds. Good agreement is seen between these calculated and
experimentally derived values.
5
H
5
3
16
of carbon monoxide.
Table 3 Reaction conditions and yields for the carbonylation of [Fe(g-
Migratory insertion
C
5
H
5
)(IMes)(CO)(Me)] to give [Fe(g-C
5
H
5
)(IMes)(CO)(COMe)]
In order to model CO-induced migratory insertion, a series of Lin-
ear Transit (LT) calculations was performed in which the carbon
atom of the methyl group was incrementally stepped towards the
carbon atom of the terminal carbonyl group (Scheme 2). Maxima
and minima on the LT energy surface were subsequently optimised
Reaction conditions
Yield (%)
92
75
15
5
1
1
atm CO, rt, 12 h, nitromethane
.5 atm CO, rt, 72 h, nitromethane
.5 atm CO, rt, 24 h, dichloromethane
Table 4 Selected bond lengths ( A˚ ) for calculated and experimentally derived structures
Bond length
Compound
Fe–L
Fe–Cacyl/alkyl
Fe–Ccarbonyl
C–Oterminal
Ref.
[
[
[
[
[
[
[
Fe(g-C
Fe(g-C
Fe(g-C
Fe(g-C
Fe(g-C
5
5
5
5
5
H
H
H
H
H
5
5
5
5
5
)(IMes)(CO)(Me)] 1
)(IH)(CO)(Me) I
1.945(4)
1.902
1.872
2.034(4)
2.048
1.908
1.728(4)
1.725
1.697
1.162(5)
1.178
1.171
This work
This work
This work
This work
This work
14
)(IH)(CO)(COMe)] II
)(PH
)(PH
3
)(CO)(Me)] III
2.092
2.007
1.703
1.167
3
)(CO)(COMe)] IV
2.113
1.925
2.060
1.939
1.711
1.753
1.805
1.166
1.143
1.134
a
a
a
a
Fe(C
Fe(C
5
H
H
4
I)(PPh
3
)(CO)(Me)]
2.195
a
a
a
a
5
5
)(PPh )(CO)(COMe)]
3
2.187
19
a
Values taken from CIF.
This journal is © The Royal Society of Chemistry 2006
Dalton Trans., 2006, 2535–2541 | 2537

View Article Online
For both species, the geometries of the calculated transition
states and intermediates more closely resemble the product acyl,
rather than the starting alkyl compounds, i.e. in both cases the
transition states for methyl-migration are late.
This is particularly emphasised for the Calkyl–CCO bond length.
˚
Short Fe–H (ca. 1.800 A) bond distances and elongated C–H (ca.
˚
1.155 A) bonds were calculated for both TS and INT. These data
Scheme 2 Linear Transit trajectory.
are suggestive of agostic interactions between one hydrogen atom
ꢀ
of the methyl group, and the metal centre. The results for TS and
to transition states and intermediates, respectively. Optimised
ꢀ
ꢀ
INT were similar, though the longest C–H bonds in INT and
geometries for the transition states (L = IH, TS and L = PH
3
,
ꢀ
TS were longer, and the Fe–H bonds shorter than were calculated
ꢀ
ꢀ
TS ) and intermediates (L = IH, INT and L = PH
3
, INT ), are
for INT and TS (see Table 5). This indicates that whilst agostic
given in Fig. 2 and Fig. 3, and selected bond lengths are given in
Table 5. Frequency calculations were performed to confirm these
assignments. For TS, a single imaginary frequency was calculated
ꢀ
ꢀ
interactions were also important in INT and TS , they are weaker
than in TS and INT.
−
1
at i170 cm . This corresponded to a shortening of the CH -
carbon to CO-carbon bond, and the rotation of the CH
3
Energetics
3
group
ꢀ
ꢀ
The relative energies of I, II, INT and TS and III, IV, INT
to give bring the agostic-H closer to the metal. For TS , a single
ꢀ
−
1
and TS are summarised in Table 6, and shown graphically
imaginary frequency was calculated at i61 cm . This frequency
also corresponded to a shortening of the CH -carbon to CO-
on Fig. 4. In each series, the energies have been normalised
3
to that of [Fe(g-C
the values shows that the formation of the acyl compounds
Fe(g-C , IV),
)(L)(CO)(COMe)] (L = IH, II, and L = PR
from the parent alkyl compounds [Fe(g-C
)(L)(CO)(Me)] (L =
IH, I, and L = PR , III), is energetically favourable (−85 and
68 kJ mol , respectively). This carbonylation reaction is a more
5
H
5
)(L)(CO)(Me)], i.e. I or III. Inspection of
carbon bond. A shortening of the agostic H bond length also
occurred.
[
5
H
5
3
5
H
5
3
−
1
−
favourable process for I than for III.
The activation energies for the carbonylation of [Fe(g-
−
1
−1
C
(
5
H
5
)(L)(CO)(Me)] were 75 kJ mol (L = PH
L = IH). Thus, the activation energy barrier is lower when L =
PH
than when L = IH. This fails to explain the more facile
carbonylation of [Fe(g-C )(IMes)(CO)(Me)] compared with
Fe(g-C )(PPh )(CO)(Me)], observed experimentally.
From the transition state calculations described, it is suggested
3
) and 93 kJ mol
3
5
H
5
[
5
H
5
3
Fig. 2 Geometry optimised structure of TS and INT.
that the more facile carbonylations when L = IH may be due to a
Table 6 Relative energies (kJ mol−
1
)
Relative
energy
Zero-point corrected
relative energy
[
Fe(g-C
5 5
H )(L)(CO)(Me)] Structure
L = IH
I
0
86
86
0
93
94
−85
TS
INT
II
−117
L = PH
3
IIIꢀ
TS
0
78
78
−92
0
75
76
−68
INTꢀ
IV
ꢀ
ꢀ
Fig. 3 Geometry optimised structure of TS and INT .
Table 5 Selected bond lengths ( A˚ ) for TS, INT, TS and INT
ꢀ
ꢀ
Bond length
Compound
Fe–L
Fe–Cacyl/alkyl
Fe–Ccarbonyl
Fe–H
C–H
C
alkyl–CCOterminal
[
[
[
[
[
[
Fe(g-C
Fe(g-C
Fe(g-C
Fe(g-C
Fe(g-C
Fe(g-C
5
5
5
5
5
5
H
H
H
H
H
H
5
5
5
5
5
5
)(IH)(CO)(Me)] TS
)(IH)(CO)(Me)] INT
)(IH)(CO)(Me)] I
1.861
1.863
1.902
1.872
2.096
2.096
2.131
2.135
2.048
1.908
2.143
2.139
1.753
1.747
1.725
1.697
1.750
1.759
1.790
1.800
—
3.029
1.928
1.880
1.159, 1.101, 1.099
1.155, 1.100, 1.096
1.106, 1.101, 1.102
1.102, 1.102, 1.104
1.127, 1.096, 1.099
1.135, 1.096, 1.099
1.524
1.528
2.741
1.497
1.649
1.620
)(IH)(CO)(COMe)] II
ꢀ
)(PH
)(PH
3
)(CO)(Me)] TS
)(CO)(Me)] INT
ꢀ
3
2
538 | Dalton Trans., 2006, 2535–2541
This journal is © The Royal Society of Chemistry 2006

View Article Online
Fig. 4 Energies for migratory insertion in I and III.
ꢀ
stronger agostic interaction in TS when compared with TS . The
steric differences between the model and experimental compounds
may explain the discrepancy between the calculated activation
energies, and the observed reactivity.
The relative energies of the complexes [Fe(g-C
Fe(g-C )(L)(CO)(Me)] and [Fe(g-C )(L)(CO)(COMe)] are
shown in Scheme 3 (L = IH) and Scheme 4 (L = PH ). Inspection
of these values shows that the energy change on replacement
5
H
5
)(CO)
2
(Me)],
3
Scheme 4 Energies of PH complexes.
[
5
H
5
5
H
5
3
showed that, while CO exchange in [Cr(CO) ] was favourable for
6
NHCs, it was unfavoured for tertiary phosphines.
The decarbonylation of acyl compounds of the form [Fe(g-
H )(L)(CO)(COMe)] may proceed via one of two routes (see
−
1
of a carbonyl ligand with the IH is −11 kJ mol , and hence is
energetically favoured.
C
5
5
Scheme 5). This may occur via dissociation of L, so that after
the reverse migratory insertion reaction, the compounds [Fe(g-
C
5
H
5
)(CO)
2
(Me)], and non-coordinated L are formed. Displace-
ment of CO by L would then lead to the observed product of
18
decarbonylation, [Fe(g-C
5
H )(L)(CO)(Me)]. Alternatively, this
5
may occur by loss of CO, followed by a reverse migratory insertion
reaction. This represents the exact reverse of the migratory
insertion mechanism that has been discussed above.
Scheme 3 Energies of NHC complexes.
Scheme 5 Decarbonylation mechanisms.
The magnitude of this stabilisation is small, but helps to ra-
tionalise the formation of [Fe(g-C
temperature reaction of IMes with [Fe(g-C
contrast, displacement of a CO ligand in [Fe(g-C
by PH
5
H
5
)(IH)(CO)(Me)] in the room
)(CO) (Me)]. By
)(CO) (Me)]
The activation energy for the formation of [Fe(g-
5
H
5
2
C
5
H
5
)(CO)(COMe)] + L from [Fe(g-C
5
H
5
)(L)(CO)(COMe)], was
−
1
5
H
5
2
calculated as 86 kJ mol for L = PH
3
. This compares well with
−
1
21
3
requires an energy change of +76 kJ mol , and is
a study by Cao et al., on PH
3
-induced migratory insertion in
thus energetically disfavoured. This can be rationalised in terms
of a stronger Fe–Ccarbene bond with respect to Fe–P bond. This
[Fe(g-C
5
H
5
)(CO) (Me)], (i.e. the reverse process), who reported a
2
−
1
rate-determining step of 74 kJ mol . Cao et al. used the Gaussian
20
finding is consistent with those of Lee and Hu who performed
calculations on a series of transition-metal phosphine and N-
heterocyclic carbene complexes and noted that NHC–metal bonds
were significantly stronger than metal–phosphine bonds. They
94 program. The activation energy for this decarbonylation process
−
1
when L = IH is much higher, at 195 kJ mol . This higher energy
may be attributed to the greater Fe–carbene bond strength when
compared with the Fe–phosphine bond strength.
This journal is © The Royal Society of Chemistry 2006
Dalton Trans., 2006, 2535–2541 | 2539

View Article Online
Thus in the complex [Fe(g-C
5
H
5
)(IH)(CO)(COMe)] the ac-
trometer or a Perkin-Elmer 1600 FTIR instrument in the range
−
1
tivation energy barrier to decarbonylation is similar ca.
4000–400 cm . Samples were prepared as Nujol mulls between
−
1
−1
2
00 kJ mol , for both the CO dissociation and carbene
dissociation mechanisms. By contrast, in the complex [Fe(g-
)(PH )(CO)(COMe)] the energies for the two different
KBr plates. Data are quoted in wavenumbers (cm ). Samples for
mass spectrometry were recorded on a Micromass GCT GC/GC-
MS. Elemental analyses were performed by the Microanalytical
Department of the Inorganic Chemistry Laboratory, University
of Oxford.
C
5
H
5
3
dissociations vary significantly. The activation energy bar-
rier to decarbonylation via tertiary phosphine dissociation, at
−
1
8
4 kJ mol , is much higher than that to decarbonylation
−
1
via CO dissociation, 170 kJ mol . This suggests that while
decarbonylation via phosphine dissociation will be favoured
Materials
22
The compounds [Fe(g-C
5
H
5
)(CO)
2
(Me)] and 1,3-bis(2,4,6-
in [Fe(g-C
experimentally for [Fe(g-C
)(IH)(CO)(COMe)] decarbonylation via N-heterocyclic car-
5
H
5
)(PH
3
)(CO)(COMe)]—indeed this has been shown
trimethylphenyl)imidazol-2-ylidene were prepared as reported
18
5
H
5
)(PPh )(CO)(COMe)], in [Fe(g-
3
23
previously.
C
5
H
5
bene dissociation will not be favoured with respect to the CO
dissociation mechanism.
[Fe(g-C H )(IMes)(CO)(Me)]. A solution of 1,3-bis(2,4,6-
5
5
trimethylphenyl)imidazol-2-ylidene (500 mg, 1.64 mmol) in
toluene (10 ml) was added to a solution of [Fe(g-C )(CO) (Me)]
627 mg, 3.26 mmol) in toluene (10 ml). The resulting solution
5
H
5
2
(
Conclusion
was then stirred for 72 h to give an orange–brown suspension.
Filtration, followed by removal of solvents under reduced pressure,
afforded an oily orange solid. This was washed with pentane
(3 × 10 ml) to leave 1 as an orange powder. Crystals suitable
for single crystal X-ray diffraction were grown by slow diffusion
of pentane into a saturated diethyl ether solution of [Fe(g-
The compounds [Fe(g-C
)(L)(CO)(COMe)] have been synthesised. DFT studies on
the reactivity of [Fe(g-C )(CO) (Me)] with PH and IH have
shown that CO exchange is energetically favoured for IH, and
energetically disfavoured for PH . The products of CO-induced
migratory insertion, [Fe(g-C )(L)(CO)(COMe)] are more sta-
ble with respect to the parent alkyl [Fe(g-C )(L)(CO)(Me)]
compounds. This stabilisation is larger when L = IH than
when L = PH . The activation energy to carbonylation of [Fe(g-
)(L)(CO)(Me)] is similar, though larger when L = IH than
. Stabilisation of the transition state by agostic
5
H
5
)(IMes)(CO)(Me)], and [Fe(g-
C
5
H
5
5
H
5
2
3
3
1
5
H
5
C H )(IMes)(CO)(COMe)]. Yield of 1 = 81.0 mg (10.1%). H
5
5
5
H
5
NMR (C D ): d 0.12 [s, 3H, FeCH ], 1.98 [s, 6H, o-CH C H ],
6
6
3
3
6
2
2.04 [s, 6H, o-CH C H ], 2.05 [s, 6H, p-CH C H ], 3.92 [s, 5H,
3
6
2
3
6
2
3
g-C H ], 6.20 [s, 2H, NCHCHN], 6.79 [s, 2H, ArH], 6.85 [s, 2H,
5
5
1
3
1
C
5
H
5
ArH]; C{ H} NMR: d 0.140 [s, FeCH ] 18.40 [s, o-CH C H ],
3
3
6
2
when L = PH
3
18.80 [s, o-CH C H ], 21.08 [s, p-CH C H ], 82.58 [s, g-C H ],
3 6 2 3 6 2 5 5
interactions was seen in both instances, but this was significantly
123.94 [s, NCHCHN], 129.40 [s, Mes C3 or 5], 129.30 [s, Mes C3 or 5],
136.52 [s, Mes C ], 137.51 [s, Mes C ], 138.87 [s, Mes C ], 206.75
more pronounced for L = IH.
2
.6
4
1
[
4
s, NCN], 223.28 [s, FeCO]. IR (Nujol, KBr): 1886s. m/z (FI+)
+
+
+
68 (80%) [M] , 440 (25%) [M − CO] , 305 [IMesH] . Anal. (%).
Experimental
Found (calc.): C 71.5 (71.8) H 6.9 (6.9) N 6.0 (6.0).
Fe(g-C )(IMes)(CO)(COMe)]. A solution of 1,3-bis(2,4,6-
trimethylphenyl)imidazol-2-ylidene (500 mg, 1.644 mmol) in
toluene (10 ml) was added to solution of [Fe(g-
All manipulations of air- and/or moisture-sensitive compounds
were carried out under an inert atmosphere of dinitrogen, in
either an inert-atmosphere box or using standard Schlenk line
techniques. Dinitrogen was purified by passage through columns
containing 4A molecular sieves and either manganese(II) oxide
suspended on vermiculite (vacuum line), or BASF catalyst (dry
box). Solvents and solutions were transferred by passage through
stainless steel cannulae, using a positive pressure of dinitrogen.
Filtrations of air- and/or moisture-sensitive compounds were
achieved by using modified stainless steel cannulae, fitted with
glass fibre filter discs at one end. All glassware and stainless steel
[
5
H
5
a
C
5
H
5
)(IMes)(CO)
2
(Me)] (627 mg, 3.26 mmol) in toluene (10 ml).
◦
The resulting solution was stirred at 60 C for 72 h to give
an orange–brown suspension. Filtration, followed by removal of
solvents under reduced pressure, afforded an oily orange solid.
This was washed with pentane (3 × 10 ml) to leave 2 as an orange
1
powder. Yield of 2 = 302 mg (37.0%). H NMR (C
6
D ): d 2.03
], 2.31 [s, 6H, p-
], 6.12 [s, 2H,
6
[
s, 6H, o-CH
3
C
6
H
2
], 2.13 [s, 6H, o-CH
3
C
6
H
2
CH ], 2.48 [s, COCH
3
C
6
H
2
3
], 4.14 [s, 5H, g-C
5
H
5
◦
cannulae were dried overnight at 150 C. Solvents used in the
1
3
1
NCHCHN], 6.82 [s, 1H, ArH], 6.85 [s, 1H, ArH]; C{ H} NMR: d
preparation of air- and/or moisture-sensitive compounds were
dried by passage through an alumina column under a positive
pressure of dinitrogen. Dinitrogen was passed through the dried
solvents for twenty minutes before use. Deuterated solvents were
degassed using three freeze-pump-thaw cycles and were vacuum
distilled from potassium or sodium.
1
5
8.30 [s, o-CH
3
C
6
H
2
], 18.80 [s, o-CH
3
C
6
H
2
], 20.90 [s, p-CH
3
C
6
H
2
],
0.50 [s, COCH
3
], 83.50 [s, g-C ], 127.60 [s, HCN], 128.20 [s,
5
H
5
Mes C3 or 5], 129.20 [s, Mes C3 or 5], 136.50 [s, Mes C2.6], 137.50 [s,
Mes C ], 138.80 [s, Mes C ], 199.50 [s, NCN], 221.86 [s, FeCO],
70.52 [s, COCH
4
1
2
(
(
3
]. IR (Nujol, KBr): 1632s, 1924s. m/z (FI+) 496
+ +
+
100%) [M] , 468 (20%) [M − CO] , 440 (10%) [M − 2CO] . Anal.
Solution NMR spectra were recorded with either a Varian
%). Found (calc.): C 69.0 (70.2) H 6.7 (6.5) N 5.8 (5.6).
1
13
UNITYplus ( H: 500.0 MHz, C: 125.7 MHz) or a Varian
1
13
Mercury 300 ( H: 300.2 MHz, C: 75.5 MHz). The spectra were
Decarbonylation of [Fe(g-C
tion of [Fe(g-C )(IMes)(CO)(COMe)] (30 mg, 0.060 mmol) in
-benzene (0.4 ml) was introduced into a Young’s Tap NMR
5
5
H )(IMes)(CO)(COMe)]. A solu-
1
internally referenced relative to the residual protio-solvent ( H)
5
H
5
1
3
1
13
and solvent ( C) resonances relative to trimethylsilane ( H, C,
d = 0 ppm). Chemical shifts are reported in parts per million (d). IR
spectra were recorded on either a Perkin-Elmer 1710 FTIR spec-
d
6
tube. The subsequent decarbonylation reaction was monitored
1
by H NMR spectroscopy. After 44 h in solution, 26% of the
2
540 | Dalton Trans., 2006, 2535–2541
This journal is © The Royal Society of Chemistry 2006

View Article Online
sample had decarbonylated to give [Fe(g-C
5
H
5
)(IMes)(CO)(Me)].
After 1 month H NMR data indicated that the solution had
equilibrated to a 1 : 1 mixture of [Fe(g-C )(IMes)(CO)(COMe)]:
Fe(g-C )(IMes)(CO)(Me)].
Acknowledgements
1
We thank the EPSRC for a grant (S. A. L) and the Oxford
Supercomputing Centre for the use of their facilities. We also thank
Nilay Hazari for his help.
5
H
5
5
H
5
Carbonylation of [Fe(g-C
5
5
H )(IMes)(CO)(Me)]. A solution of
[
Fe(g-C )(IMes)(CO)(Me)] (30 mg, 0.064 mmol) in either ni-
5
H
5
References
tromethane or dichloromethane (see Table 3) was introduced into
a Fisher–Porter bottle. This was purged five times with CO. The
solution was then stirred under an atmosphere of CO (either 1.5
or 5 atm). After removal of solvents under reduced pressure,
the residues were then examined by H NMR spectroscopy
to determine the amount of [Fe(g-C
formed. The reaction conditions used for these carbonylations and
the percentage conversion are summarised in Table 3. Conversion
yields were estimated by integration of the cyclopentadienyl/alkyl
1 K. Oefele, J. Organomet. Chem., 1968, 12, P42.
2
H. W. Wanzlick and H. J. Schoenherr, Angew. Chem., Int. Ed. Engl.,
1
968, 7, 141.
3
K. Oefele, W. A. Herrmann, D. Mihalios, M. Elison, E. Herdtweck, W.
1
Scherer and J. Mink, J. Organomet. Chem., 1993, 459, 177.
4 D. S. McGuinness, K. J. Cavell, B. F. Yates, B. W. Skelton and A. H.
5
H
5
)(IMes)(CO)(COMe)]
White, J. Am. Chem. Soc., 2001, 123, 8317.
5
6
H. M. J. Wang and I. J. B. Lin, Organometallics, 1998, 17, 972.
P. Buchgraber, L. Toupet and V. Guerchais, Organometallics, 2003, 22,
5144.
1
7 S. L. Brown and S. G. Davies, J. Chem. Soc., Chem. Commun., 1986,
environments in the H NMR spectra.
8
4.
8
9
S. A. Llewellyn, M. L. H. Green and A. R. Cowley, J. Organomet.
Chem., 2005, 690, 2358.
Calculations
G. D. W. Anderson, O. J. Boys, A. R. Cowley, J. C. Green, M. L. H.
Green, S. A. Llewellyn, C. M. von Beckh, S. I. Pascu and I. C. Vei,
J. Organomet. Chem., 2004, 689, 4407.
Optimised geometries of [Fe(g-C
)(IH)(CO)(COMe)] II, [Fe(g-C
Fe(g-C )(PH )(CO)(COMe)] IV and [Fe(g-C
were calculated using Vosko, Wilke and Nusairs local functional
5
H
5
)(IH)(CO)(Me)] I, [Fe(g-
)(PH )(CO)(Me)] III,
)(CO) (Me)]
C
5
H
5
5
H
5
3
1
0 H. D. Flack, Acta Crystallogr., Sect. A: Fundam. Crystallogr., 1983, 39,
[
5
H
5
3
5
H
5
2
876.
2
4
11 G. Poignant, S. Nlate, V. Guerchais, A. J. Edwards and P. R. Raithby,
Organometallics, 1997, 16, 124.
25
26
with the Becke88 and the Perdew86 exchange and correlation
gradient corrections, on ADF version 2000.02.
used were uncontracted triple-zeta Slater-type orbitals (STOs).
The cores of atoms were frozen, C and O up to the 1s level, P and
Fe up to the 2p level. Transition states and reactive intermediates
were confirmed by performing frequency calculations.
1
2 K. Ferre, G. Poignant, L. Toupet and V. Guerchais, J. Organomet.
Chem., 2001, 629, 19.
13 H. Brunner, B. Hammer, I. Bernal and M. Draux, Organometallics,
27–33
The basis sets
1
983, 2, 1595.
1
4 E. J. M. Meier, W. Kozminski, A. Linden, P. Lustenberger and W. von
Philipsborn, Organometallics, 1996, 15, 2469.
15 V. Guerchais, Eur. J. Inorg. Chem., 2002, 783.
1
1
6 T. C. Forschner and A. R. Cutler, Organometallics, 1985, 4, 1247.
7 M. Green and D. J. Westlake, J. Chem. Soc. A, 1971, 367.
Crystallography
18 K. H. Pannell and J. R. Rice, J. Organomet. Chem., 1988, 341, 415.
1
9 I. Bernal, H. Brunner and M. Muschiol, Inorg. Chim. Acta, 1988, 142,
235.
A crystal was selected under an inert atmosphere, encased in
perfluoro-polyether oil, and mounted on the end of a glass
fibre. Data collections were performed using an Enraf-Nonius
KappaCCD diffractometer (graphite-monochromated Mo-Ka
2
2
2
0 M.-T. Lee and C.-H. Hu, Organometallics, 2004, 23, 976.
1 Z. Cao, S. Niu and M. B. Hall, J. Phys. Chem. A, 2000, 104, 7324.
2 T. S. Piper and G. Wilkinson, J. Inorg. Nucl. Chem., 1956, 3, 104.
23 A. J. Arduengo, III, H. V. R. Dias, R. L. Harlow and M. Kline, J. Am.
˚
Chem. Soc., 1992, 114, 5530.
radiation, k = 0.71073 A). Intensity data were processed using the
34
24 S. H. Vosko, L. Wilk and M. Nusair, Can. J. Phys., 1990, 58, 1200.
2
2
27 L. Verluis and T. Ziegler, J. Chem. Phys., 1988, 88, 322.
2
DENZO-SMN package. Formula: C28
H
32FeN
2
O, M = 468.42,
5 A. D. Becke, Phys. Rev. A: At., Mol., Opt. Phys., 1988, 38, 2398.
6 J. Perdew, Phys. Rev. B: Condens. Matter, 1986, 33, 8822.
orthorhombic, space group P2
1
2
1
2
1
, T = 150 K, a = 17.0569(2),
3
˚
˚
b = 17.5875(2), c = 32.3450(4) A, V = 9703.1(2) A , Z = 16, 69648
8 G. Te Velde, F. M. Bickelhaupt, E. J. Baerends, C. Fonseca Guerra,
reflections measured, 20964 unique, l = 0.644, Rint = 0.050, wR =
S. J. A. Van Gisbergen, J. G. Snijders and T. Ziegler, J. Comput. Chem.,
0.0402.
2
001, 22, 931.
The crystal structure was solved using direct-methods program
29 B. te Velde and E. J. Baerends, J. Comput. Phys., 1992, 99, 84.
3
35
0 C. F. Guerra, J. G. Snijders, G. Te Velde and E. J. Baerends, Theor.
Chem. Acc., 1998, 99, 391.
SIR92, which located all non-hydrogen atoms. Subsequent
full-matrix least-squares refinement was carried out using the
3
1 C. Fonseca Guerra, J. G. Snijider, B. te Velde and E. J. Baerends, Theor.
Chem. Acc., 1998, 99, 391.
36
CRYSTALS program suite. Coordinates and anisotropic thermal
parameters of all non-hydrogen atoms were refined. Hydrogen
atoms were positioned geometrically after each cycle of refine-
ment. A three-term Chebychev polynomial weighting scheme
was applied. Crystal structure diagrams were produced using the
32 E. J. Baerends, E. G. Ellis and P. Ros, Chem. Phys., 1973, 2, 41.
3
3 S. ADF2002.01, Theoretical Chemistry, Vrije Universiteit, Amsterdam,
The Netherlands, http://www.scm.com, ADF2002.01, Theoretical
Chemistry, Vrije Universiteit, The Netherlands.
34 Z. Otwinowski and W. Minor, Methods Enzymol., 1997, 276, 307.
35 A. Altomare, G. Carascano, C. Giacovazzo and A. Guagliardi, J. Appl.
Crystallogr., 1993, 26, 343.
36
CRYSTALS program suite.
CCDC reference number 290045.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b516328e
3
6 D. J. Watkin, C. K. Prout, J. R. Carruthers, P. W. Betteridge and R. I.
Cooper, CRYSTALS Issue 11, Chemical Crystallography Laboratory,
Oxford, UK, Oxford, 2001.
This journal is © The Royal Society of Chemistry 2006
Dalton Trans., 2006, 2535–2541 | 2541