α-β Alkenyl isomerisation at diiron centres

Article abstract of DOI:10.1039/b007977o

α-Substituted alkenyl complexes [Fe2(CO)6(μ-RC=CH2)(μ-PPh2)] 1a-1e (R=Ph, Me, n-Pr, n-Bu or t-Bu) have been prepared via regioselective hydrodimetallation of primary alkynes by [Fe2H(CO)7(μ-PPh2)]. Thermolysis in toluene results in isomerisation to the β-substituted complexes [Fe2(CO)6(μ-HC=CHR)(μ-PPh2)] 2a-2e. Isomerisation is accelerated in the presence of a range of tertiary phosphines yielding [Fe2(CO)5(PR'3)(μ-HC=CHR)(μ-PPh2)] 3a-3j in which the phosphine is coordinated to the ?-bound metal centre and lies trans to the phosphido-bridge. Addition of trimethyl phosphite to [Fe2(CO)6(μ-PhC=CH2)(μ-PPh2)] 1a at 60-70 deg C affords mono- and di-substituted α-alkenyl complexes [Fe2(CO)5{P(OMe)3}(μ-PhC=CH2)(μ-PPh2)] 4 and [Fe2(CO)4{P(OMe)3}(μ-PhC=CH2)(μ-PPh2)] 5 respectively. Above 100 deg C, conversion into β-substituted isomers [Fe2(CO)5{P(OMe)3}(μ-HC=CHPh)(μ-PPh2)] 6 and [Fe2(CO)4{P(OMe)3}2(μ-HC=CHPh)(μ-PPh2)] 7 occurs cleanly. Crystallographic studies have been carried out on 1a, [Fe2(CO)6(μ-n-PrC=CH2)(μ-PPh2)] 1c, [Fe2(CO)6(μ-HC=CHPh)(μ-PPh2)] 2a, [Fe2(CO)5(PPh3)(μ-HC=CHPh)(μ-PPh2)] 3a, 6 and 7 and compared with related structures. Different structural characteristics are seen between isomers, β isomers being characterised by a longer Fe_?-C_β interaction and a more obtuse Fe_?-C_α-C_β angle. The mechanisms of alkyne addition to [Fe2H(CO)7(μ-PPh2)] and alkenyl isomerisation have been probed using PhC2D. Hydrodimetallation results in an equal distribution of the deuterium over both β sites in 1a, and since they do not in exchange on the NMR timescale suggests that a radical process in operating. Thermolysis of this mixture leads to a 3:1 mixture of [Fe2(CO)6(μ-DC=CHPh)(μ-PPh2)] 2a-d¹_α and [Fe2(CO)6(μ-HC=CDPh)(μ-PPh2)] 2a-d¹_β. A mechanism for α-β alkenyl isomerisation is proposed in which oxidative additions of the trans (exo) and cis (endo) β-protons to terminal sites on the diiron centre, giving a parallel alkyne intermediate, are in competition.

Full text of DOI:10.1039/b007977o

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ꢀ–ꢁ Alkenyl isomerisation at diiron centres
M. Kamal Anwar,a Graeme Hogarth,a* Ozan S. Senturk,ab William Clegg,c
Simon Dohertyc and Mark R. J. Elsegoodc
a Chemistry Department, University College London, 20 Gordon Street, London,
UK WC1H 0AJ. E-mail: g.hogarth@ucl.ac.uk
b Department of Chemistry, Ege University, Bornova 35100, Izmir, Turkey.
E-mail: sanli@fenfak.ege.edu.tr
c Department of Chemistry, University of Newcastle, Bedson Building, Newcastle-upon-Tyne,
UK NE1 7RU. E-mail: simon.doherty@ncl.ac.uk
Received 3rd October 2000, Accepted 1st December 2000
First published as an Advance Article on the web 12th January 2001
α-Substituted alkenyl complexes [Fe (CO) (µ-RC᎐CH )(µ-PPh )] 1a–1e (R = Ph, Me, Prn, Bun or But) have been
᎐
2
6
2
2
prepared via regioselective hydrodimetallation of primary alkynes by [Fe2H(CO)7(µ-PPh2)]. Thermolysis in toluene
results in isomerisation to the β-substituted complexes [Fe (CO) (µ-HC᎐CHR)(µ-PPh )] 2a–2e. Isomerisation is
᎐
2
6
2
accelerated in the presence of a range of tertiary phosphines yielding [Fe (CO) (PRЈ )(µ-HC᎐CHR)(µ-PPh )]
᎐
2
5
3
2
3a–3j in which the phosphine is coordinated to the σ-bound metal centre and lies trans to the phosphido-bridge.
Addition of trimethyl phosphite to [Fe (CO) (µ-PhC᎐CH )(µ-PPh )] 1a at 60–70 ЊC affords mono- and di-substituted
᎐
2
6
2
2
α-alkenyl complexes [Fe (CO) {P(OMe) }(µ-PhC᎐CH )(µ-PPh )] 4 and [Fe (CO) {P(OMe) } (µ-PhC᎐CH )(µ-PPh )]
᎐
᎐
2
5
3
2
2
2
4
3
2
2
2
5 respectively. Above 100 ЊC, conversion into the β-substituted isomers [Fe (CO) {P(OMe) }(µ-HC᎐CHPh)(µ-PPh )]
᎐
2
5
3
2
6 and [Fe (CO) {P(OMe) } (µ-HC᎐CHPh)(µ-PPh )] 7 occurs cleanly. Crystallographic studies have been carried out
᎐
2
4
3
2
2
on 1a, [Fe (CO) (µ-PrnC᎐CH )(µ-PPh )] 1c, [Fe (CO) (µ-HC᎐CHPh)(µ-PPh )] 2a, [Fe (CO) (PPh )(µ-HC᎐CHPh)-
᎐
᎐
᎐
2
6
2
2
2
6
2
2
5
3
(µ-PPh2)] 3a, 6 and 7 and compared with related structures. Different structural characteristics are seen between
isomers, β isomers being characterised by a longer Feπ–Cβ interaction and a more obtuse Feσ–Cα–Cβ angle. The
mechanisms of alkyne addition to [Fe2H(CO)7(µ-PPh2)] and alkenyl isomerisation have been probed using PhC2D.
Hydrodimetallation results in an equal distribution of the deuterium over both β sites in 1a, and since they do not in
exchange on the NMR timescale suggests that a radical process is operating. Thermolysis of this mixture leads to a
3:1 mixture of [Fe (CO) (µ-DC᎐CHPh)(µ-PPh )] 2a-d1 and [Fe (CO) (µ-HC᎐CDPh)(µ-PPh )] 2a-d1 . A mechanism
᎐
᎐
2
6
2
α
2
6
2
β
for α–β alkenyl isomerisation is proposed in which oxidative additions of the trans (exo) and cis (endo) β-protons to
terminal sites on the diiron centre, giving a parallel alkyne intermediate, are in competition.
shown that cis-[Re (CO) (µ-H)(µ-HC᎐CHR)] (R = Me, Et)
converts into the thermodynamically stable trans isomers both
Introduction
᎐
2
8
Alkenyl ligands have been proposed as key intermediates in the
Fischer–Tropsch process.1 Evidence comes from a series of
labeling studies carried out by Maitlis and co-workers, which
suggest that chain growth results via methylene insertion
into the metal–carbon σ bond of a surface-bound alkenyl. This
generates an allyl moiety, which subsequently undergoes
isomerisation via a 1,3-proton shift to give a new β-substituted
alkenyl. As a result of this proposal, the chemistry of alkenyl
ligands has been studied at a variety of metal centres, emphasis
being placed on carbon–carbon bond forming reactions.2–4
The proposed surface-bound alkenyl ligands are monosubsti-
tuted. Such species can adopt three possible isomeric forms,
shown for a binuclear centre as I–III, with the substituent on
thermally and photochemically (eqn. 1). While the thermal
(1)
transformation is clean, upon photolysis a photostationary
equilibrium state results. Similarly, we have previously shown6
that photolysis of the disubstituted alkenyl complex [Fe2-
(CO) (µ-RC᎐CHR)(µ-PPh )(µ-dppm)] (R = CO Me), in which
᎐
4
2
2
the ester groups lie cis to one another, results in isomerisation at
the β-carbon, a process which is reversed thermally (eqn. 2).
(2)
either the α- (I) or β- (II,III) carbon, the latter being either trans
(II) or cis (III) to the metal centre. At a binuclear centre, inter-
conversion of cis and trans β isomers (II,III) has been noted in
a number of instances. For example, Nubel and Brown5 have
Until recently there was little evidence for α–β isomerisation
in alkenyl complexes; that is, conversion of I to II,III. On the
basis of NMR results, Knox and co-workers7 have suggested
DOI: 10.1039/b007977o
J. Chem. Soc., Dalton Trans., 2001, 341–352
This journal is © The Royal Society of Chemistry 2001
341

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that propenyl cations [Fe Cp (CO) (µ-CO)(µ-HC᎐CHMe)]ϩ
ing two signals between δ 3.2 and 2.0 assigned to inequivalent
᎐
2
2
2
and [Fe Cp (CO) (µ-CO)(µ-MeC᎐CH )]ϩ may be in equi-
β-protons of the alkenyl ligand. Carty and co-workers11 had
previously noted high regioselectivity in reactions of alkynes
and diynes with [Fe2H(CO)7(µ-PPh2)], although for some
primary alkynes (R = Ph, SiMe3 or C6H4C2Ph) they reported
the formation of mixtures of α- and β-substituted isomers.
Hydrometallation reactions generally proceed in a cis fashion
via initial coordination of the alkyne to the metal centre
followed by cis addition of the hydride via a planar transition
state.17–19 In a number of instances trans addition products have
been isolated20–22 and this is accounted for either by postulating
rapid isomerisation of the initially formed cis addition
product,20 or a free-radical pathway.21,22 In order to probe the
hydrodimetallation further we examined the reaction with
᎐
2
2
2
2
librium, however, this could not be shown conclusively. In
the same paper a reference to unpublished results8 suggests
that α–β propenyl isomerisation occurs in dimolybdenum
cations. Thus, protonation of the µ-allene complex [Mo2Cp2-
(CO) (µ-H C᎐C᎐CH )] affords β-substituted [Mo Cp (CO) -
᎐ ᎐
4
2
2
2
2
4
(µ-HC᎐CHMe)]ϩ rather than the α isomer which would be
᎐
expected to be formed initially. Similar evidence for α–β
propenyl isomerisation comes from our work on reactions
of [Fe2(CO)4(µ-H)(µ-CO)(µ-PR2)(µ-dppm)] (R = Ph or Cy)
with allene, which result in a mixture of α- and β-propenyl
complexes.9,10
A number of synthetic routes to alkenyl complexes are
known, one widely utilised being the hydrometallation of
alkynes. Here, primary alkynes can give either α- or
β-substituted isomers depending upon the regioselectivity of
the insertion, with the former generally predominating since
they result from Markovnikov addition. Carty and co-workers
have previously reported that hydrodimetallation of PhC2H
by [Fe2H(CO)7(µ-PPh2)] afforded a mixture of α-[Fe2(CO)6-
PhC D. Somewhat surprisingly, this afforded [Fe (CO) (µ-PhC᎐
᎐
2
2
6
CHD)(µ-PPh2)] 1a-d1 as a 1:1 mixture of cis and trans alkyne
addition products.
(µ-PhC᎐CH )(µ-PPh )] 1a and β-[Fe (CO) (µ-HC᎐CHPh)-
᎐
᎐
2
2
2
6
(µ-PPh2)] 2a substituted phenylethenyl complexes in a 4:1
ratio.11 As part of our work on σ–π alkenyl reactivity and
fluxionality at the diiron centre,9,10,12–14 we have used 1a as an
entry into dppm-bridged alkenyl complexes since the diphos-
phine ligand acts as a convenient NMR probe of this process.12
In repeating its synthesis we found that room temperature
work-up gave only the α isomer [Fe (CO) (µ-PhC᎐CH )-
᎐
2
6
2
1
(µ-PPh2)] 1a, as shown by H NMR spectroscopy. However,
if the temperature is raised to 110 ЊC, then α–β isomerisation
occurs cleanly, a process which is accelerated in the presence of
a phosphine. Herein we follow up on our preliminary com-
munication15 concerning phenylethenyl complexes to show that
α–β isomerisation is a general process in these complexes and
that ground-state differences in the binding of the alkenyl
ligand in α- and β-substituted complexes may be the origin of
different thermodynamic stabilities.
Initially this was considered to result from the inter-
conversion of the two β-proton sites in complex 1a and such a
process has been shown to occur in related triosmium com-
1
plexes in the presence of pyridine.23 However, a H NOESY
spectrum clearly showed that this was not the case, implying
that either cis–trans isomerisation occurs during the reaction or
a free-radical pathway is operative. While we cannot definitely
rule out the former, the observation of a 1:1 mixture (within
experimental error) points strongly towards the latter. The most
likely source of free radicals is homolytic cleavage of the iron–
iron bond and it may be that the phosphido-bridge acts to hold
the two metal centres together during this process. However,
Results and discussion
Synthesis of ꢀ-alkenyl complexes [Fe (CO) (ꢂ-RC᎐CH )-
(ꢂ-PPh2)] 1
᎐
2
6
2
Hydrodimetallation of primary alkynes by [Fe2H(CO)7-
(µ-PPh2)] leads to regioselective formation of α-substituted
alkenyl complexes [Fe (CO) (µ-RC᎐CH )(µ-PPh )]11,16 1a–1e,
᎐
2
6
2
2
Baker et al.24 have reported that oxidation of Na[Fe2(CO)7/8
-
while with PhC2Me a single isomer, namely, [Fe2(CO)6-
(µ-PhC᎐CHMe)(µ-PPh )]16 1f also results. All are the expected
(µ-PPh2)] by ferrocenium affords stable paramagnetic 33-
electron [Fe2(CO)7(µ-PPh2)] and we cannot discount the possi-
bility that this radical is generated during our procedure.
᎐
2
ꢀ–ꢁ Alkenyl isomerisation: synthesis of ꢁ-alkenyl complexes
[Fe (CO) (ꢂ-HC᎐CHR)(ꢂ-PPh )] 2a–2e
᎐
2
6
2
Heating toluene solutions of complexes 1a–1e at 110 ЊC for 45
min to 4 h resulted in isomerisation to the β-alkenyl complexes
[Fe (CO) (µ-HC᎐CHR)(µ-PPh )] 2a–2e. Yields varied [2a
᎐
2
6
2
(R = Ph), 95%; 2c (R = Prn), 28%] as did reaction times. Thus,
formation of 2d (R = Bun) and 2e (R = But) required 45 min,
2a (R = Ph) 1 h, while conversion of 1b (R = Me) took 4 h.
Characterisation was made on the basis of H NMR spectra,
the newly generated α-proton appearing as a low-field doublet
Markovnikov type addition products. Only with propyne a
small amount (≈ 10%) of the β isomer [Fe (CO) (µ-HC᎐CHMe)-
᎐
2
6
1
(µ-PPh2)] 2b was formed. Characterisation of 1a–1e was
1
straightforward and based on H NMR spectra, each display-
342
J. Chem. Soc., Dalton Trans., 2001, 341–352

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1
of doublets between δ 8.9 and 7.9. Monitoring by H NMR
spectroscopy showed that the transformations of 1a and 1e
were first order in complex concentration.
Acceleration of ꢀ–ꢁ alkenyl isomerisation by tertiary phosphines:
synthesis of [Fe (CO) (PRЈ )(ꢂ-HC᎐CHR)(ꢂ-PPh )] 3a–3j
᎐
2
5
3
2
Heating toluene solutions of complex 1a (R = Ph) and a range
of tertiary phosphines at 110 ЊC resulted in the rapid (≈ 10 min)
formation of the phosphine-substituted β-alkenyl complexes
[Fe (CO) (PRЈ )(µ-HC᎐CHPh)(µ-PPh )] 3a–3g. In no instance
᎐
2
5
3
2
stant of 63.4 Hz suggesting that the phosphorus containing
ligands lie cis to one another. On the basis of these data, and
in comparison with data for isomer (A) of the related complex
[Fe (CO) {P(OMe) }(µ-PhC᎐CH )(µ-PPh )] 4 (see later), we
᎐
2
5
3
2
2
assign the structure shown in which the phosphine is coordin-
ated to the π-bound iron centre and lies trans to the metal–
metal bond. Heating the mixture of 1b, 3j, 3jЈ and PPh3 further
resulted in clean conversion to 3j showing that 3jЈ is an inter-
mediate in the transformation. These results suggest that, in all
cases, phosphine substitution is rate-determining and that
α–β alkenyl isomerisation occurs rapidly for the phosphine-
substituted α-alkenyl complexes.
was there any evidence for formation of the α-alkenyl isomer.
Characterisation was straightforward, in each case the 1H
NMR spectrum containing a characteristic low-field doublet
of doublet of doublets for the α-proton with couplings of
approximately 6 Hz to the phosphido-bridge, 13 Hz to the
β-proton and 28 Hz to the phosphine ligand. The latter sug-
gested that the phosphine was coordinated to the σ-bound iron
centre and this was confirmed by X-ray crystallography (see
later). In the 31P NMR spectra only one isomer was detected,
appearing as a pair of low- and high-field doublets assigned to
the phosphido-bridge and phosphine respectively, the coupling
constant of around 94 Hz being indicative of a trans arrange-
ment of phosphorus atoms. All were temperature independent
indicating that the two phosphorus centres are static on the
NMR timescale as is the alkenyl ligand. In a separate experi-
ment, heating a toluene solution of 2a and PPh3 at 110 ЊC for
10 min resulted in quantitative formation of [Fe2(CO)5(PPh3)-
Synthesis of ꢀ-alkenyl phosphite complexes [Fe2(CO)5{P(OMe)3}-
(ꢂ-PhC᎐CH )(ꢂ-PPh )] 4 and [Fe (CO) {P(OMe) } -
᎐
2
2
2
4
3 2
(ꢂ-PhC᎐CH )(ꢂ-PPh )] 5
᎐
2
2
Heating a toluene solution of complex 1a and P(OMe)3 at
65–75 ЊC resulted in formation of both mono- and di-substi-
tuted phosphite derivatives [Fe (CO) {P(OMe) }(µ-PhC᎐CH )-
᎐
2
5
3
2
(µ-PPh )] 4 and [Fe (CO) {P(OMe) } (µ-PhC᎐CH )(µ-PPh )] 5
᎐
2
2
4
3
2
2
2
(µ-HC᎐CHPh)(µ-PPh )] 3a showing that phosphine substi-
᎐
2
tution of the β-alkenyl complexes is facile. Further phosphine
substitution did not occur even under forcing conditions. Thus,
refluxing a toluene solution of 3a with a large excess of PPh3
for over one week resulted only in the isolation of starting
materials.
Alkenyl isomerisation in complex 1a was also effected in
the presence of AsPh3 leading to the formation of [Fe2(CO)5-
(AsPh )(µ-HC᎐CHPh)(µ-PPh )] 3h in 14% yield. Further,
᎐
3
2
the course of the reaction of 1a with tolylphosphines was
dependent upon the position of methyl substitution. Thus,
while P(C6H4Me-p)3 and P(C6H4Me-m)3 gave the expected
phosphine-substituted β-alkenyl complexes 3b and 3c respect-
ively after 10 min, the more sterically demanding P(C6H4Me-o)3
and likewise PCy3 simply gave 2a after 1 h.
Thermolysis of complex 1d (R = But) and PPh3 resulted in a
significant rate enhancement (from 45 to 10 min) leading to
formation of [Fe (CO) (PPh )(µ-HC᎐CHBut)(µ-PPh )] 3i in
᎐
2
5
3
2
57% yield. With 1b (R = Me) and PPh3 rate enhancement was
also observed (from 4 to 1 h), [Fe (CO) (PPh )(µ-HC᎐CHMe)-
᎐
2
5
3
(µ-PPh2)] 3j being formed cleanly (93%). However, when this
reaction was stopped after 30 min a second product was
clearly seen by NMR, formulated as the phosphine-substituted
α-alkenyl complex [Fe (CO) (PPh )(µ-MeC᎐CH )(µ-PPh )] 3jЈ.
᎐
2
5
3
2
2
In the 1H NMR spectrum the two β-protons are seen at δ 2.29
(dd, J 7.9, 3.0, Hcis) and 2.07 (ddd, J 15.3, 6.7, 3.0 Hz, Htrans).
1
Comparison with the H NMR spectrum of 2b shows that the
only coupling of the β-protons to PPh3 is that of 6.7 Hz to
Htrans. In the 31P NMR spectrum two doublets are observed at
δ 168.5 and 64.1, the phosphorus–phosphorus coupling con-
J. Chem. Soc., Dalton Trans., 2001, 341–352
343

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isolated in 47 and 34% yields respectively. A 1H NMR spectrum
of the crude reaction mixture revealed that no α–β isomerisa-
tion had occurred under these conditions.
Like the α isomers, both complexes 6 and 7 have highly tem-
perature dependent NMR spectra. At low temperature (223 K)
the 31P NMR spectrum of 6 shows a pair of doublets at δ 182.6
and 177.7, the phosphorus–phosphorus coupling constant of
154.7 Hz clearly indicating that the two phosphorus atoms lie
Complex 4 was shown by 31P NMR to be a 2:1 mixture of
isomers at 213 K. Both appeared as two sharp doublets; one at
δ 186.6 and 165.0 (J 70.2 Hz) due to the major isomer (A), the
and other at δ 188.1 and 152.5 (J 52.7 Hz) due to minor isomer
(B). The magnitude of the phosphorus–phosphorus coupling
constants suggests that in both isomers the phosphite lies cis to
the phosphido-bridge, as a trans orientation is expected to have
a much larger coupling constant. Further, on the basis that the
β-protons of isomer (A) couple with the phosphite (JPH = 3.3
Hz), while those of isomer (B) do not, we propose that (A) has
the phosphite coordinated to the π-bound iron centre, while in
(B) it is located on the σ-bonded metal.
1
trans to one another. In the H NMR spectrum at the same
temperature the α-proton is seen at δ 8.73 (ddd, J 31.8, 12.7, 8.1
Hz). The large coupling constant (JPH 31.8 Hz) is that to the
phosphite ligand, suggesting that substitution has occurred at
the σ-bound iron centre. This conclusion was later confirmed
by X-ray crystallography.
Disubstituted complex 7 exists at low temperature as a mix-
ture of two isomers (A:B) in an approximate 6:1 ratio. At 223
K the 31P NMR spectrum clearly shows this, the major isomer
(A) being seen as a pair of doublets at δ 188.6 (J 145.7) and
180.3 (J 59.8 Hz) assigned to the phosphites, while the
phosphido-bridge appears as a doublet of doublets at δ 165.4.
The minor isomer (B) has a similar spectrum, with two phos-
phite resonances at δ 196.8 (d, J 45.5) and 181.0 (d, J 150.7 Hz)
and the phosphido-bridge at δ 159.0 (dd, J 150.7, 45.0 Hz). In
Disubstituted complex 5 exists as a single isomer which
appears in the 31P NMR spectrum at 213 K as a pair of sharp
doublets at δ 189.8 (J 47.0) and 189.1 (J 71.6 Hz) assigned to the
inequivalent phosphite ligands, and a broad doublet of doub-
lets at δ 145.9 assigned to the phosphido-bridge. Given the simi-
larity of these coupling constants to those of the two isomers of
4 it seems reasonable to suggest that similar sites are occupied;
that is, one on each iron centre with both lying cis to the
phosphido-bridge. While we cannot be certain, we favour
coordination at the sites trans to the metal–metal bond. This is
supported by the observation that in the 13C NMR spectrum
neither Cα nor Cβ shows a significant coupling to either phos-
phite ligand. Also, in the related thiolate-bridged complex
1
the H NMR spectrum (223 K) two low-field resonances are
apparent at δ 8.74 (ddd, J 30.2, 13.0, 8.1 Hz) and 8.63 (br)
assigned to isomers (A) and (B) respectively. Resonances due to
(B) are not well resolved and often overlap with those of the
more abundant (A) making assignment of this isomer difficult.
Assignment of the major isomer (A) was made on the basis of a
crystallographic study (see later). The phosphite at the σ-bound
metal centre lies trans to the phosphido-bridge, while that at the
π-bound iron lies trans to the metal–metal vector. On the basis
of this assignment, and since the two isomers interconvert, we
tentatively assign isomer (B) as that having the same phosphite
orientation at the σ-bound centre but with the phosphite at the
π-bound centre lying trans to the β-carbon of the alkenyl
ligand.
[Fe (CO) {P(OMe) } (µ-SBut)(µ-PhC᎐CH )]25 the two phos-
᎐
2
4
3
2
2
phite ligands have been shown by X-ray crystallography to
occupy sites trans to the metal–metal bond.
Phosphite accelerated ꢀ–ꢁ alkenyl isomerisation: synthesis of
[Fe (CO) {P(OMe) }(ꢂ-HC᎐CHPh)(ꢂ-PPh )] 6 and
᎐
2
5
3
2
[Fe (CO) {P(OMe) } (ꢂ-HC᎐CHPh)(ꢂ-PPh )] 7
᎐
2
4
3
2
2
Mechanism of ꢀ–ꢁ alkenyl isomerisation
Heating a toluene solution of complex 4 at 100 ЊC resulted
in quantitative conversion into the β-alkenyl isomer
While α–β alkenyl isomerisation could occur via a direct 1,2-
proton shift, we disfavour such a process on the grounds that it
is difficult to rationalise the rate acceleration by phosphines and
phosphites. Knox and co-workers7 have suggested previously
that it probably involves a temporary hydrogen shift to a bond-
ing site on the dimetal centre. If we accept this supposition
we still need to consider whether both β-hydrogens undergo
addition or if the process is selective. In α isomers 1 the trans
proton lies exo to the phosphido-bridge, while the cis proton
lies endo to it. From the crystal structures of both 1a and 1c it is
seen that the endo proton lies closer than the exo proton to the
π-bound metal [1a (1c), Feπ–Hexo 2.814 (2.791), Feπ–Hendo 2.563
(2.657) Å], which suggests that cis (endo) proton addition might
be favourable.
[Fe (CO) {P(OMe) }(µ-HC᎐CHPh)(µ-PPh )]
min, and under the same conditions 5 cleanly afforded
6
within 10
᎐
2
5
3
2
Some insight was gained from a deuterium labeling study. At
110 ЊC the 1:1 cis–trans mixture of [Fe (CO) (µ-PhC᎐CHD)-
᎐
᎐
2
6
(µ-PPh )] 1a-d1 was transformed into [Fe (CO) (µ-DC᎐CHPh)-
2
2
6
(µ-PPh2)] 2a-d1 and [Fe (CO) (µ-HC᎐CDPh)(µ-PPh )] 2a-d1
᎐
α
2
6
2
β
[Fe (CO) {P(OMe) } (µ-HC᎐CHPh)(µ-PPh )] 7. In a separate
᎐
2
4
3
2
2
experiment, approximately equal amounts of 4 and 5 were
warmed to 100 ЊC in d8-toluene whilst monitoring by 31P NMR
spectroscopy. Below 90 ЊC no significant conversion of either
into the β-isomer occurred, however at 100 ЊC both 6 and 7
grew in at approximately equal rates.
344
J. Chem. Soc., Dalton Trans., 2001, 341–352

View Article Online
Scheme 1
in a 3:1 ratio and this remained constant upon further heat-
and the unfavourable addition site. Thus, 1a-d1
should
trans
ing. The latter observation confirms that the alkenyl protons in
β-substituted complexes, like those in the α isomers, do not
exchange. Thus, the redistribution of deuterium does not occur
in either of the ground states.
afford 2a-d1α exclusively. In 1a-d1cis the deuterium is now in the
preferred position for oxidative addition and thus addition of
the trans proton becomes competitive. The observed 3:1 ratio
for 2a-d1α : 2a-d1β can be accounted for if the rates of conversion
of 1a-d1cis into both are approximately equal.
This result is difficult to interpret unambiguously but, assum-
ing that the β-protons do not interconvert (as shown by
NMR and supported by literature precedent), then it does show
that both of the β-protons undergo transfer. For example, if
only addition of the trans (exo) proton occurred then complex
Acceleration of α–β alkenyl isomerisation in the presence of
phosphines and phosphites may simply be a consequence of the
increased basicity of the metal upon substitution leading to an
increase in the rate of oxidative addition. Phosphites are poorer
donors than phosphines and affect the rate to a lesser extent.
It is difficult to assess the extent of the rate acceleration as
a function of the phosphine, as it is undoubtedly carbonyl
substitution that is rate-limiting. Detection of the intermediate
1a-d1 would give 2a-d1 and 1a-d1
would give 2a-d1
cis
α
trans
β
exclusively, and while their rates of formation might be different
(as a result of the different rates of C᎐H vs. C᎐D addition) the
1:1 ratio should be maintained. The same follows for addition
of the cis (endo) proton and it is only a combination of the two,
together with a kinetic isotope effect, which can account for the
observed 3:1 ratio.
[Fe (CO) (PPh )(µ-MeC᎐CH )(µ-PPh )] 3jЈ by NMR and
᎐
2
5
3
2
2
isolation of [Fe (CO) {P(OMe) }(µ-PhC᎐CH )(µ-PPh )]
4
᎐
2
5
3
2
2
shows that the preferred site of ligand substitution for α-alkenyl
complexes 1 lies cis to the phosphido-bridge, probably at the
π-bound iron centre. The former may give rise to unfavourable
steric interactions in the ground state which are relieved upon
α–β alkenyl isomerisation as the phosphido-bridge and phos-
phine adopt a relative trans configuration. If the phosphine
is too sterically encumbered then coordination to the diiron
centre cannot occur and no rate acceleration is found.
The rate of alkenyl isomerisation is sensitive to the nature of
the alkenyl substituent being fastest for the large groups
(R = Ph, Bun or But) and slowest for the small methyl substitu-
ent. This suggests that there may be unfavourable steric inter-
actions between the phosphido-bridge and alkenyl substituent
in the α isomers which are relieved upon oxidative addition of
a β-proton. The nature of these is difficult to see and certainly
none is apparent from the crystal structure determinations.
A further point for consideration is whether proton addition
occurs to a bridging or terminal site on the diiron centre. In
order to maintain the EAN of 34 electrons, a parallel 2-electron
donor alkyne intermediate seems likely to be generated. We
assume that the phosphido-bridge and hydrocarbyl fragments
retain their relative cis disposition throughout and note that in
related zwitterionic diiron complexes of this type this is the
case.26 Oxidative addition of either proton might occur to a
terminal site but it is only the trans (exo) proton which can add
across the metal–metal bond as the phosphido-bridge blocks
the site of addition of the cis (endo) proton. Addition to a
terminal site necessitates proton mobility to either the bridging
site or a terminal site on the more sterically encumbered iron
atom. While we cannot differentiate unambiguously between
these possible reaction pathways, given that both protons must
be able to undergo addition, we favour initial addition to a
terminal site. The observed ratio of isotopomers can then be
rationalised in terms of a difference in the rates of addition of
the two protons combined with a kinetic isotope effect. Scheme
1 shows this based on the assumption that the closer cis (endo)
proton adds faster than the more distant trans (exo) proton. For
1a-d1trans, oxidative addition of the cis proton should proceed
Oxidative addition of methyl to the diiron centre: synthesis of
[Fe (CO) (ꢂ-HC᎐CHCH Ph)(ꢂ-PPh )] 2f
᎐
2
6
2
2
Thermolysis of [Fe (CO) (µ-PhC᎐CHMe)(µ-PPh )] 1f at
᎐
2
6
2
110 ЊC for 1 h resulted in the isolation of a new alkenyl complex
[Fe (CO) (µ-HC᎐CHCH Ph)(µ-PPh )] 2f in 42% yield. The
᎐
2
6
2
2
rapidly to generate 2a-d1 , while in contrast addition of the
mass spectrum clearly showed that isomerisation had occurred.
α
trans deuterium will be slow, based on a kinetic isotope effect
In the 1H NMR spectrum the newly generated α-proton
J. Chem. Soc., Dalton Trans., 2001, 341–352
345

View Article Online
appeared at δ 8.10 (dd, J 12.4, 6.5 Hz) and the β-proton as a
quintet at δ 3.34 (J 6.5 Hz), being coupled approximately
equally to the phosphido-bridge, α-proton and the two
inequivalent benzylic hydrogens. The latter appeared as a broad
singlet at room temperature, but upon cooling to 233 K separ-
ated into a pair of distinct signals at δ 3.35 (dd, J 14.0, 7.3) and
2.66 (dd, J 14.0, 7.0 Hz).
Although its mode of formation is unclear, a plausible route
is shown in Scheme 2. Addition of the cis (endo) proton to the
Scheme 2
metal centre potentially leads to the formation of isomeric
[Fe (CO) (µ-MeC᎐CHPh)(µ-PPh )]. While minor unidentified
᎐
2
6
2
products were isolated from the reaction this was not one of
them. It may be that 1f is thermodynamically preferred over this
isomer and that addition of the cis proton is reversible. In the
absence of a trans (exo) β-proton, addition of one of the methyl
protons to the metal centre would give a dimetallacyclopentene
complex. Related diiron phosphido-bridged species have
recently been characterised.27 A 1,3-phenyl migration to the
methylene to give a benzyl-substituted vinylidene, followed by
proton transfer to the metal-bound carbon, would generate the
observed β-alkenyl complex. Carrying out the thermolysis of
1f in the presence of PPh3 failed to yield any characterisable
products.
Fig. 1 Molecular structures of (a) complex 1a and (b) 1c.
substituent also adopts an exo configuration. The iron–iron
bond length varies over a short-range (0.032 Å), being longest
[2.6427(8) Å] in 3a and shortest in molecule C of 7 [2.611(2) Å].
However, on closer inspection a number of more subtle
and potentially important differences appear between α- and
β-substituted complexes (Table 1). These become evident when
considering two parameters, ∆Cα and ∆Feπ, defined as {(Feπ–
Cα) Ϫ (Feσ–Cα)} and {(Feπ–Cα) Ϫ (Feπ–Cβ)} respectively. ∆Cα is
a measure of how symmetrically the α-carbon is bound to the
diiron centre. For α-substituted complexes, ∆Cα is around 0.10
Å, while β isomers are characterised by values of 0.12 Å. A
bigger difference is seen in ∆Feπ, where α isomers have values
around 0.06 Å, and β isomers nearer 0.14 Å. Thus, in β isomers,
binding of the α-carbon to the diiron centre and the carbon–
carbon double bond of the alkenyl ligand to iron are more
asymmetric than in the α isomers. The origin of the larger
values of ∆Feπ for β isomers is the elongation of the Feπ–Cβ
bond. This is further reflected in the Feσ–Cα–Cβ bond angle
which is more acute in α [1a, 121.3(4); 1c, 124.4(8)Њ] versus
β isomers [128.9(3)–133.4(8)Њ]. A test of the significance of
these regular, but small, differences comes with the three
independent molecules of 7. While the parameters do vary,
there is no overlap with the values found for α isomers. Isomers
1a and 2a differ only in the mode of substitution at the alkenyl
ligand. Here the difference in all three parameters is consistent
with the general trend; ∆Cα (1a, 0.102; 2a, 0.123 Å), ∆Feπ (1a,
0.056; 2a, 0.150 Å) and Feσ–Cα–Cβ [1a, 121.3(4); 2a, 128.9(3)Њ].
We have previously crystallographically characterised
Crystallographic studies
In order to probe ground-state differences between α- and
β-alkenyl complexes, crystallographic studies were carried out
on [Fe (CO) (µ-PhC᎐CH )(µ-PPh )] 1a, [Fe (CO) (µ-PrnC᎐
᎐
᎐
2
6
2
2
2
6
CH )(µ-PPh )] 1c, [Fe (CO) (µ-HC᎐CHPh)(µ-PPh )] 2a, [Fe -
᎐
2
2
2
6
2
2
(CO) (PPh )(µ-HC᎐CHPh)(µ-PPh )] 3a, [Fe (CO) {P(OMe) }-
᎐
5
3
2
2
5
3
(µ-HC᎐CHPh)(µ-PPh )] 6 and [Fe (CO) {P(OMe) } (µ-HC᎐
᎐
᎐
2
2
4
3 2
CHPh)(µ-PPh2)] 7. Figs. 1–3 give perspective views of each of
the molecules, while the results are summarised in Table 1. For
7 there are three independent molecules in the unit cell and
data are given for each.
In all, the diiron centre is bridged by both phosphido and
alkenyl ligands, the latter binding in a σ,π manner. In complex
3a the PPh3 ligand is coordinated to the σ-bound iron centre
ligand and lies trans to the phosphido-bridge [P(1)–Fe(1)–P(2)
162.08(5)Њ], while in 6 the phosphite coordinates in a similar
fashion [P(1)–Fe(2)–P(2) 157.83(6)Њ]. All three independent
molecules of 7 have the same general structure, that is with
the phosphite at the σ-bound metal centre lying trans to the
phosphido-bridge [P–Fe–P 157.86(14)–166.81(13)Њ] and the
phosphite at the π-bound centre lies cis to the phosphido-bridge
[P–Fe–P 97.39(12)–100.24(13)Њ] and trans to the metal–metal
bond [P–Fe–Fe 150.70(11)–152.31(12)Њ].
Gross structural details of the Fe2(µ-PPh2)(µ-alkenyl) core
for all crystallographically characterised complexes are similar.
The two bridging ligands lie cis to one another, with the
α-carbon of the alkenyl group exo and the β-carbon endo to the
phosphido-bridge, while, in both α and β isomers, the alkenyl
isomeric [Fe (CO) (µ-dppm)(µ-PhC᎐CH )(µ-PPh )] α-I and
᎐
2
4
2
2
[Fe (CO) (µ-dppm)(µ-HC᎐CHPh)(µ-PPh )] β-I and found
᎐
2
4
2
346
J. Chem. Soc., Dalton Trans., 2001, 341–352

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Table 1 Selected bond lengths (Å) and angles (Њ) for complexes 1a, 1c, 2a, 3a, 6 and 7
7
1a
1c
2a
3a
6
Molecule A
Molecule B
Molecule C
Feσ–Cα
Fe–Fe
Feπ–Cα
Feπ–Cβ
Cα–Cβ
Feσ–PPh2
Feπ–PPh2
Feσ–PR3
Feπ–PR3
2.007(5)
2.6153(12)
2.109(5)
2.165(6)
1.393(8)
2.229(2)
2.252(2)
2.027(11)
2.629(2)
2.126(12)
2.188(11)
1.402(14)
2.260(3)
2.295(3)
1.996(4)
2.6166(11)
2.119(4)
2.269(3)
1.405(5)
2.2508(12)
2.2940(11)
1.955(4)
2.6427(8)
2.079(4)
2.228(4)
1.405(5)
2.2054(11)
2.2524(12)
2.2558(11)
1.996(5)
2.6367(11)
2.121(5)
2.261(5)
1.417(8)
2.246(2)
2.295(2)
2.230(2)
1.972(10)
2.641(2)
2.093(11)
2.235(11)
1.40(2)
2.214(3)
2.270(3)
2.184(3)
2.168(4)
1.984(10)
2.633(2)
2.110(9)
2.225(9)
1.405(13)
2.206(3)
2.279(3)
2.183(3)
2.177(3)
1.981(10)
2.611(2)
2.087(11)
2.211(11)
1.397(14)
2.224(3)
2.270(3)
2.186(3)
2.166(3)
Fe–PPh2–Fe
Fe–Cα–Fe
Feσ–Cα–Cβ
Cα–Cβ–Feπ
71.43(5)
78.8(2)
121.3(4)
68.8(3)
70.49(9)
78.5(4)
124.4(8)
68.6(6)
70.29(4)
78.91(13)
128.9(3)
65.6(2)
72.71(4)
81.79(14)
128.5(3)
65.3(2)
70.98(5)
79.6(2)
130.5(4)
65.9(3)
72.15(10)
81.0(4)
132.6(9)
65.7(6)
71.89(10)
80.0(3)
133.4(8)
66.7(5)
71.03(10)
79.8(4)
129.8(8)
66.3(6)
∆Cα
∆Feπ
0.102
0.056
0.099
0.062
0.123
0.150
0.124
0.149
0.125
0.140
0.121
0.142
0.126
0.115
0.106
0.124
Fig. 3 Molecular structures of (a) complex 6 and (b) 7 (Molecule A).
Fig. 2 Molecular structures of (a) complex 2a and (b) 3a.
isomeric pairs. In α-I and β-I, binding of the alkenyl ligand to
the π-bound iron centre (Feπ) varies greatly. For α-I it is bound
almost symmetrically to both Cα and Cβ (∆Feπ Ϫ 0.015 Å),
while in β-I the α-carbon lies closer by some 0.180 Å. The trend
in ∆Feπ is consistent across all three isomeric pairs; that is,
β isomers are more asymmetrically bound. This is especially
apparent in the dppm-bridged complexes I where ∆∆Feπ is
0.195 Å as compared to 0.094 Å between 1a/2a and 0.135 Å in
significant structural differences.9,13 Structural characterisation
of 1a and 2a provides another isomeric pair for comparison,
while a third set of phenylethenyl isomers, thiolate-bridged
[Fe (CO) (µ-PhC᎐CH )(µ-SBut)]25 α-II and [Fe (CO) (µ-HC᎐
᎐
᎐
2
6
2
2
6
CHPh)(µ-SBut)]28 β-II have also been characterised crystallo-
graphically. Table 2 gives a comparison of some key structural
data for diiron alkenyl complexes and allows a comparison of
J. Chem. Soc., Dalton Trans., 2001, 341–352
347

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Table 2 Selected bond lengths (Å) for diiron alkenyl complexes (esds omitted for clarity)
Complex
Cα–Cβ
Feσ–Cα
Feσ–Cα
Feπ–Cβ
∆Cα
∆Feπ
Ref.
[Fe2(CO)6(µ-PhC᎐CHPh)(µ-COEt)]
1.423
1.424
1.412
1.440
1.400
1.366
1.379
1.393
1.405
1.416
1.397
1.392
1.437
1.424
1.359
1.410
1.394
2.011
2.039
2.000
2.000
1.959
1.966
1.965
2.007
1.996
1.998
1.979
2.004
2.009
1.990
1.951
1.979
1.972
2.059
2.099
2.114
2.088
2.091
2.075
2.084
2.109
2.119
2.156
2.106
2.088
2.074
2.129
2.075
2.130
2.104
2.185
2.223
2.254
2.201
2.159
2.181
2.170
2.165
2.269
2.141
2.286
2.158
2.279
2.201
2.249
2.241
2.262
0.048
0.060
0.114
0.088
0.132
0.109
0.119
0.102
0.123
0.158
0.127
0.084
0.065
0.139
0.124
0.151
0.132
0.126
0.124
0.140
0.113
0.068
0.094
0.086
0.056
0.150
Ϫ0.015
0.180
0.070
0.205
0.072
0.174
0.111
0.158
4(a)
29
30
31
10
32
11
᎐
[Fe2(CO)6(µ-PhC᎐CHPh)(µ-PPh2)]
᎐
[Fe2(CO)6(µ-PhC᎐CHPh)(µ-AuPPh3)]
᎐
[Fe2(CO)6(µ-PhC᎐CHPh)(µ-CH2Ph)]
᎐
[Fe2(CO)4(µ-dppm)(µ-HC᎐CH2)(µ-PCy2)]
᎐
[Fe2(CO)6(µ-HC᎐CH2)(µ-SPh)]
᎐
]Fe2(CO)6(µ-HC᎐CH2)(µ-PPh2)]
᎐
[Fe2(CO)6(µ-PhC᎐CH2)(µ-PPh2)]
This work
This work
᎐
[Fe2(CO)6(µ-HC᎐CHPh)(µ-PPh2)]
᎐
[Fe2(CO)4(µ-dppm)(µ-PhC᎐CH2)(µ-PPh2)]
9
13
25
33
34
35
36
10
᎐
[Fe2(CO)4(µ-dppm)(µ-HC᎐CHPh)(µ-PPh2)]
᎐
[Fe2(CO)6(µ-PhC᎐CH2)(µ-SBut)]
᎐
[Fe2(CO)6(µ-HC᎐CHPh)(µ-SBut)]
᎐
[Fe2(CO)6(µ-HC᎐CHPh)(µ-Cl)]
᎐
[Fe2(CO)6(µ-HC᎐CHPh)(µ-SEt)]
᎐
[Fe2(CO)6(µ-HC᎐CHPh)(µ-SePh)]
᎐
[Fe2(CO)4(µ-dppm)(µ-HC᎐CHPh)(µ-PCy2)]
᎐
thiolate-bridged II. Clearly, the trans influence of the dppm
ligand plays a part in amplifying the effect. Similar values of
∆Feπ are seen for the other β-substituted isomers (3a, 0.149; 6,
0.140; 7, 0.115–0.142 Å), while α-substituted 1c is far more
symmetrically bound (∆Feπ 0.062 Å). Thus, the origin of the
larger values of ∆Feπ for β vs. α isomers is the elongation of the
Feπ–Cβ bond and it is this which differentiates α and β isomers.
As the differences discussed above are based on a small
sample size we have also examined a range of related crystallo-
graphically characterised diiron µ-alkenyl complexes (Table
2). Phenyl substitution at the α-carbon has little effect on the
Feπ–Cβ bond. Thus, values of 2.159–2.181 Å (∆Feπ 0.068–0.113
Å) in the unsubstituted alkenyl complexes can be compared
with those of 2.141–2.165 Å (∆Feπ Ϫ0.015 to 0.070 Å). In
contrast, substitution at the β-carbon results in significant
elongation to 2.201–2.286 Å (∆Feπ 0.072–0.205 Å) and this is
maintained in complexes with phenyl substituents on both
α- and β-carbons (2.185–2.254 Å). From this structural data
it is clear that there are significant differences between
phenyl-substituted alkenyl complexes. Unfortunately, the lack
of crystallographic data for alkyl substituted diiron µ-alkenyl
complexes does not allow us to extend this conclusion.
While the structural studies clearly show that there are
significant differences in the binding of the α- and β-substituted
alkenyl moieties to the diiron centre, why this should lead to
enhanced thermodynamic stability is still unclear. Carty and
co-workers11 have also distinguished α- and β-substituted
diiron alkenyl complexes on the basis of 13C NMR chemical
shifts. Thus, for α-alkenyl complexes, Cα is shifted downfield
(δ 178–220) with respect to its position for α-substituted or
ethenyl complexes (δ 145–164), while Cβ is shifted upfield
(α, δ 48–75; β, δ 72–94). Specifically, 1a (2a) are characterised by
chemical shifts of δ 189.0 (145.7) and 66.2 (95.6) for Cα and Cβ
respectively. These data suggest that α-substituted complexes
have a greater contribution from a metallacyclic resonance
structure (B) than the metallaolefin formulation (A), and this
character seen in β isomers which leads to their greater thermo-
dynamic stability.
Conclusion
This work has shown unequivocally for the first time that
α–β alkenyl isomerisation can occur at the binuclear centre.
Kinetically stable α-alkenyl complexes [Fe (CO) (µ-RC᎐CH )-
᎐
2
6
2
(µ-PPh2)] 1, resulting from regioselective hydrodimetallation of
primary alkynes, rearrange to the thermodynamically stable
β isomers [Fe (CO) (µ-HC᎐CHR)(µ-PPh )] 2 upon heating.
᎐
2
6
2
Isomerisation is believed to occur via oxidative addition of a
β-proton to the diiron centre and is accelerated in the presence
of phosphines and phosphites. Detection of the intermediate
phosphine-substituted α-alkenyl complex [Fe2(CO)5(PPh3)-
(µ-MeC᎐CH )(µ-PPh )] 3jЈ by NMR and isolation of
᎐
2
2
[Fe (CO) {P(OMe) }(µ-PhC᎐CH )(µ-PPh )] 4 and [Fe (CO) -
᎐
2
5
3
2
2
2
4
{P(OMe) } (µ-PhC᎐CH )(µ-PPh )]
5 show that carbonyl
᎐
3
2
2
2
substitution precedes alkenyl isomerisation and it is the former
which is rate-limiting. Acceleration of α–β alkenyl isomeris-
ation upon substitution may result from an increase in electron
density at the metal centre, making oxidative addition more
favourable. However, steric effects cannot be ruled out and
carbonyl substitution in α-alkenyl complexes may lead to
adverse steric interactions between the alkenyl ligand, the
phosphido-bridge and the incoming ligand which are relieved
upon isomerisation.
In earlier work we have described the reactivity of [Fe2-
(CO) (µ-PhC᎐CH )(µ-PPh )] 1a towards the bidentate phos-
᎐
6
2
2
phines dppm12 and dppe.14 For both, phosphine substitution
imparts a degree of sp3-like character to Cβ. This view concurs
with the structural data, in that as the β-carbon develops more
sp3 character then the Feπ–Cβ bond length is expected to
decrease. Hence it appears that it is the decrease in metallacycle
and CO insertion into the alkenyl ligand occur at 90 ЊC giving
bridging α–β unsaturated acyl complexes [Fe (CO) {µ-O᎐
᎐
2
4
CC(Ph)᎐CH }(µ-PPh )(µ-dppm)] and [Fe (CO) {µ-O᎐CC(Ph)=
᎐
᎐
2
2
2
n
CH2}(µ-PPh2)(dppe)] (n = 3 or 4) respectively. The key feature
348
J. Chem. Soc., Dalton Trans., 2001, 341–352

View Article Online
of these experiments is that phosphine substitution has
occurred without α–β alkenyl isomerisation. Clearly, carbonyl
insertion into the alkenyl ligand becomes favourable with
respect to alkenyl isomerisation. Interestingly, however, pro-
1a (R = Ph) (80% yield): ν(CO)(C6H14) 2060m, 2028s,
1994m, 1980m and 1964w cmϪ1 1H NMR (CDCl3) δ 7.8–
6.7 (m, 15H, Ph), 3.12 (dd, J 14.8, 2.8, 1H, Hβ) and 2.33
(dd, J 10.4, 2.8 Hz, 1H, Hβ); 31P NMR (CDCl3) δ 180.3 (s,
µ-PPh2).
;
longed thermolysis of [Fe (CO) {µ-O᎐CC(Ph)=CH }(µ-PPh )-
᎐
2
4
2
2
(µ-dppm)] results in both carbonyl loss and α–β alkenyl isomer-
1b (R = Me) (34% yield): ν(CO)(C6H14) 2061m, 2026s,
1
isation giving [Fe (CO) (µ-HC᎐CHPh)(µ-PPh )(µ-dppm)] in
1992m, 1980m and 1969w cmϪ1; H NMR (CDCl3) δ 7.5–7.0
᎐
2
4
2
high yield.12 This provides further evidence that alkenyl
isomerisation proceeds via oxidative addition of a proton
to a terminal site on the metal centre as the two bridging sites
cis to the alkenyl ligand are blocked by the phosphido and
diphosphine ligands.
(m, 10H, Ph), 2.91 (dd, J 15.2, 2.2, 1H, Hβ), 2.76 (s, 3H, CH3)
and 2.13 (dd, J 9.9, 2.2 Hz, 1H, Hβ); 31P NMR (CDCl3) δ 179.5
(s, µ-PPh2).
1c (R = Prn) (62% yield): ν(CO)(C6H14) 2062m, 2026s,
1991s, 1979s, 1965m and 1953w cmϪ1 1H NMR (CDCl3)
;
δ 7.6–7.0 (m, 10H, Ph), 3.31 (m, 1H, CH2), 2.89 (d, J 15.3,
1H, Hβ), 2.08 (d, J 9.8, 1H, Hβ), 2.07 (m, 1H, CH2), 1.90 (m,
1H, CH2), 1.61 (m, 1H, CH2) and 0.96 (t, J 7.1 Hz, 3H, CH3);
31P NMR (CDCl3) δ 180.0 (s, µ-PPh2); mass spectrum (FAB)
m/z 534 (Mϩ), 506, 478, 450, 422, 394 and 366 (Mϩ Ϫ 6CO);
calc. for C23H19Fe2O6P C 51.68, H 3.54; found C 51.83, H
3.32%.
1d (R = Bun) (34% yield): ν(CO)(C6H14) 2062m, 2025vs,
1991s, 1979s, 1965s and 1954m cmϪ1 1H NMR (CDCl3)
;
α–β Alkenyl isomerisation in diiron phosphido-bridged com-
plexes may be more common than previously thought. Thus we
note that, while a large number of α-substituted diiron hexa-
carbonyl alkenyl complexes are known, phosphine substituted
derivatives remain elusive. This suggests that alkenyl isomeris-
ation and its acceleration by phosphines may be a common trait
in this area of chemistry. Fässler and Huttner25 have reported
the synthesis of thiolate-bridged trimethyl phosphite complexes
δ 7.8–7.2 (m, 10H, Ph), 3.45 (dt, J 15.0, 4.3, 1H, C2H2),
3.04 (dd, J 15.3, 2.5, 1H, Hβ), 2.27 (m, 1H, C2H2), 2.12
(dd, J 12.9, 4.3, 1H, Hβ), 1.97 (m, 1H, C4H2), 1.74 (m,
1H, C4H2), 1.57 (m, 1H, C3H2), 1.48 (m, 1H, C3H2) and 1.03 (t,
J 7.3 Hz, 3H, CH3); 31P NMR (CDCl3) δ 180.0 (s, µ-PPh2);
13C NMR (CDCl3) δ 212.5 (s, 4CO), 211.63 (d, J 21.7,
CO), 211.57 (d, J 20.1, CO), 192.5 (d, J 19.8, Cα), 139.0 (d,
J 44.1, Cipso), 135.4 (d, J 28.0, Cipso), 133.8 (d, J 6.4, Ph), 133.4
(d, J 7.4, Ph), 132.3 (s, Ph), 130.5 (d, J 12.5, Ph), 130.1 (s, Ph),
130.0 (s, Ph), 128.7 (d, J 11.0, Ph), 128.3 (d, J 9.8, Ph), 70.1 (d,
J 13.6 Hz, Cβ), 60.0 (s, CH2), 38.5 (s, CH2), 22.6 (s, CH2) and
14.0 (s, CH3).
[Fe (CO) {P(OMe) }(µ-PhC᎐CH )(µ-SBut)] and [Fe2(CO)4-
᎐
2
5
3
2
{P(OMe) } (µ-PhC᎐CH )(µ-SBut)], analogues of
4 and 5
᎐
3
2
2
respectively, at 60 ЊC, and it remains to be seen whether they
undergo facile alkenyl isomerisation. Indeed, given the previous
results of Knox and co-workers,7,8 α–β alkenyl isomerisation at
a binuclear centre may be widespread.
1e (R = But) (70% yield): ν(CO)(C6H14) 2053m, 2019vs,
1982s, 1974s, 1965m and 1954w cmϪ1 1H NMR (CDCl3)
;
δ 7.8–7.2 (m, 10H, Ph), 3.38 (dd, J 16.2, 4.4, 1H, Hβ), 2.06 (dd,
J 13.1, 4.4 Hz, 1H, Hβ) and 1.46 (s, 9H, CH3); 31P NMR
(CDCl3) δ 179.9 (s, µ-PPh2).
Experimental
General
[Fe (CO) (µ-PhC᎐CHMe)(µ-PPh )] 1f (85% yield): ν(CO)-
᎐
2
6
2
All reactions were carried out under nitrogen using standard
vacuum line techniques and dried and degassed solvents.
Chromatography was carried out on deactivated alumina
(6% w/w distilled water) wet packed with light petroleum (bp
40–60 ЊC) unless otherwise stated. The solution to be separated
was added to alumina (3–5 g) and the solvent removed under
reduced pressure. The resulting solids were then deposited on
top of the prepared column and separation effected by elution
with progressively more polar solvents. Alkynes and phosphines
were purchased from Aldrich or Strem and used as supplied
while Na[Fe2(CO)7/8(µ-PPh2)]ؒ1.5THF37 was prepared via the
literature method.
The IR spectra were recorded on a Nicolet 205 FTIR spec-
trometer, NMR spectra on Varian VXR-400, Bruker AMX400
and Avance500 spectrometers internally referenced to residual
solvent peaks (1H, 13C) or externally to P(OMe)3 (31P), mass
spectra on VG 7070 high resolution and VG Analytical ZAB2F
spectrometers. Elemental analyses were performed in house.
(C6H14) 2058s, 2022vs, 1990s, 1980s and 1966m cmϪ1; 1H NMR
(CDCl3) δ 7.8–7.2 (m, 15H, Ph), 3.15 (dq, J 12.1, 6.0, 1H, Hβ)
and 1.28 (d, J 6.0 Hz, 3H, Me); 13C NMR (CDCl3) δ 216–218
(br, CO), 184.8 (d, J 20.4, Cα), 153.1 (s, Cipso), 138.7 (d, J 46.1,
Cipso), 134.7 (d, J 30.4, Cipso), 134–126 (m, Ph), 80.7 (d, J 15.6,
Cβ) and 23.1 (d, J 5.1 Hz, Me); 31P NMR (CDCl3) 173.3 (s,
µ-PPh2); mass spectrum (FAB) m/z 554 (M Ϫ CO), 526
(M Ϫ 2CO), 498 (M Ϫ 3CO), 470 (M Ϫ 4CO), 442 (M Ϫ 5CO)
and 414 (M Ϫ 6CO); calc. for C27H19Fe2O6P C 55.67, H 3.26;
found, C 55.77, H 3.26%.
[Fe (CO) (ꢂ-HC᎐CHR)(ꢂ-PPh )] 2. Thermolysis of a toluene
᎐
2
6
2
solution (15 cm3) of complex 1 (70 mg) for between 45 min and
4 h resulted in a slight lightening of the solution. After removal
of volatiles chromatography gave a yellow band eluting with
light petroleum which yielded 2a–2e.
2a (R = Ph) (95% yield after 1 h): ν(CO)(C6H14) 2064m,
2029sm, 1999m and 1984m and 1971w cmϪ1; 1H NMR (CDCl3)
δ 8.85 (dd, J 13.5, 6.2, 1H, Hα), 7.8–7.1 (m, 13H, Ph), 6.87 (d, J
7.0, 2H, Ph) and 4.06 (dd, J 13.5, 5.0 Hz, 1H, Hβ); 31P NMR
(CDCl3) δ 180.3 (s, µ-PPh2).
Syntheses
[Fe (CO) (ꢂ-RC᎐CH )(ꢂ-PPh )] 1. A dichloromethane solu-
᎐
2
6
2
2
tion (20 cm3) of Na[Fe2(CO)7/8(µ-PPh2)]ؒ1.5THF (0.55 g, 0.83
mmol) was cooled to Ϫ78 ЊC and HBF4ؒEt2O (0.15 cm3) added
dropwise by syringe. The resulting solution was stirred for
20 min and then allowed to warm slowly to room temperature.
To this was added a slight excess of alkyne (ca. 0.1 cm3) and
stirring at room temperature for 16 h resulted in the formation
of a bright orange solution. Volatiles were removed under
reduced pressure and chromatography gave a yellow-orange
band eluting with light petroleum (bp 40–60 ЊC) which afforded
complexes 1a–1e.
2b (R = Me) (45% yield after 4 h): ν(CO)(C6H14) 2064m,
2026s, 1997m, 1982m and 1969w cmϪ1 1H NMR (CDCl3)
;
δ 7.96 (dd, J 12.4, 6.2, 1H, Hα), 7.6–7.1 (m, 10H, Ph), 3.21 (dq,
J 6.2, 5.4, 1H, Hβ) and 1.52 (d, J 5.4 Hz, 3H, CH3); 31P NMR
(CDCl3) δ 181.1 (s, µ-PPh2).
2c (R = Prn) (28% yield after 3 h): ν(CO)(C6H14) 2063s,
2026vs, 1996s, 1981s, 1965s and 1953m cmϪ1; 1H NMR (CDCl3)
δ 7.9 (br, 1H, Hα), 7.6–7.0 (m, 10H, Ph), 3.28 (br, 1H, Hβ), 1.76
(br, 2H, CH2), 1.31 (br, 2H, CH2) and 0.71 (br, 3H, CH3); 31P
NMR (CDCl3) δ 180.9 (s, µ-PPh2).
J. Chem. Soc., Dalton Trans., 2001, 341–352
349

View Article Online
2d (R = Bun) (38% yield after 45 min): ν(CO)(C6H14) 2063s,
2026s, 1995s, 1981m, 1965s and 1954m cmϪ1; 1H NMR (CDCl3)
δ 7.95 (dd, J 12.7, 5.9, 1H, Hα), 7.8–7.2 (m, 10H, Ph), 3.16 (m,
1H, Hβ), 1.65 (m, 2H, CH2), 1.16 (m, 2H, CH2), 0.83 (m, 2H,
CH2) and 0.71 (t, J 7.2 Hz, 3H, CH3); 31P NMR (CDCl3) δ 180.8
(s, µ-PPh2).
(m, 25H, Ph), 6.58 (d, J 7.1, 2H, Ph), 4.08 (dd, J 12.8, 5.4 Hz,
1H, Hβ) and 2.37 (s, 9H, Me); 31P NMR (CDCl3) δ 180.6 (d,
J 94.4, µ-PPh2) and 70.6 (d, J 94.4 Hz, PAr3); 13C NMR
(CDCl3) δ 219.0 (t, J 15.4, CO), 214.8 (br, 2CO), 212.9 (dd,
J 20.8, 17.2, CO), 210 (vbr, CO), 154.0 (t, J 24.6, Cα), 142.4 (s,
Cipso), 140.2 (d, J 42.1, Cipso), 138.0 (d, J 9.1), 137.7 (d, J 9.8,
Cipso), 137.4 (d, J 8.5), 134.5 (s), 134.1 (s), 133.6 (d, J 5.8), 132.4
(s), 131.0 (d, J 25.9), 130.8 (s), 129.4 (d, J 18.3), 128.9 (s), 128.8
(s), 125.9 (d, J 21.4), 125.0 (s), 89.4 (d, J 15.3, Cβ) and 21.6 (s,
CH3); mass spectrum (FAB) m/z 844 (Mϩ), 788 (M Ϫ 2CO),
760 (M Ϫ 3CO), 732 (M Ϫ 4CO) and 704 (M Ϫ 5CO); calc. for
C46H38Fe2O5P2 C 65.40, H 4.50; found C 65.22, H 4.49%.
3d (R = Ph, RЈ = p-FC6H4) (62% yield after 10 min):
2e (R = But) (70% yield after 45 min): ν(CO)(C6H14) 2062s,
2024s, 1995s, 1980s, 1966m and 1953m cmϪ1; 1H NMR (CDCl3)
δ 7.91 (dd, J 14.0, 5.6, 1H, Hα), 7.8–7.2 (m, 10H, Ph), 3.39 (dd,
J 14.0, 5.2 Hz, 1H, Hβ) and 0.82 (s, 9H, But); 31P NMR (CDCl3)
δ 180.0 (s, µ-PPh2).
[Fe (CO) (µ-HC᎐CHCH Ph)(µ-PPh )] 2f. Thermolysis of a
᎐
2
6
2
2
toluene solution (15 cm3) of complex 1f (60 mg) for 1 h resulted
in consumption of starting material as shown by IR spectro-
scopy. After removal of volatiles chromatography gave a yellow
band eluting with light petroleum which yielded 2f (42%).
ν(CO)(C6H14) 2063s, 2028s, 1997s, 1988m, 1982s and 1970w
ν(CO)(CH2Cl2) 2035s, 1985vs, 1952m and 1918m cmϪ1 1H
;
NMR (CDCl3) δ 8.20 (ddd, J 34.8, 12.9, 6.3, 1H, Hα), 7.8–7.0
(m, 25H, Ph), 6.58 (d, J 6.9, 2H, Ph) and 4.11 (dd, J 13.0, 5.5
Hz, 1H, Hβ); 31P NMR (CDCl3) δ 182.5 (d, J 95.8, µ-PPh2) and
71.9 (d, J 95.8, PAr3); 13C NMR (CDCl3) δ 218.5 (t, J 15.8, CO),
214.1 (br, 2CO), 212.5 (t, J 21.0, CO), 209.8 (br, CO), 163.9 (d, J
252.9, CipsoF), 152.9 (t, J 24.6, Cα), 141.8 (s, Cipso), 139.9 (d, J
42.7, Cipso), 136.9 (d, J 23.2, Cipso), 135.9 (d, J 18.9), 135.9 (s),
135.2 (d, J 5.2), 133.5 (s), 130.2 (s), 129.6 (d, J 25.9), 128.3 (d, J
27.8), 126.3 (s), 124.9 (s), 116.0 (dd, J 21.4, 10.5) and 89.9 (d, J
15.9 Hz, Cβ); mass spectrum (FAB) m/z 856 (Mϩ), 800
(M Ϫ 2CO), 772 (M Ϫ 3CO), 744 (M Ϫ 4CO) and 716
(M Ϫ 5CO); calc. for C43H29F3Fe2O5P2 C 60.28, H 3.39; found
C 59.84, H 3.35%.
1
cmϪ1; H NMR (CDCl3) δ 8.10 (dd, J 12.4, 6.5, 1H, Hα), 7.53
(m, 2H, Ph), 7.4–7.2 (m, 11H, Ph), 6.74 (d, J 2.6, 1H, Ph), 6.73
(s, 1H, Ph), 3.34 (quintet, J 6.5 Hz, 1H, Hβ) and 3.03 (br, 2H,
CH2); (233 K) δ 8.11 (dd, J 12.3, 6.2, 1H, Hα), 7.53 (m, 2H, Ph),
7.45–7.15 (m, 11H, Ph), 6.68 (m, 2H, Ph), 3.35 (dd, J 14.0, 7.3,
1H, CH2), 3.29 (quintet, J 6.8, 1H, Hβ) and 2.66 (dd, J 14.0, 7.0
Hz, 1H, CH2); 31P NMR (CDCl3) 180.8 (s, µ-PPh2); mass
spectrum (FAB) m/z 582 (Mϩ), 554 (M Ϫ CO), 526 (M Ϫ 2CO),
498 (M Ϫ 3CO), 470 (M Ϫ 4CO), 442 (M Ϫ 5CO) and 414
(M Ϫ 6CO).
3e (R = Ph, RЈ = CH2CH2CN) (98% yield after 20 min):
[Fe (CO) (PRЈ )(ꢂ-HC᎐CHR)(ꢂ-PPh )] 3. A toluene solu-
ν(CO)(CH2Cl2) 2036m, 1982vs, 1952m and 1922m cmϪ1 1H
;
᎐
2
5
3
2
tion (20 cm3) of complex 1a (80 mg, 0.14 mmol) and PPh3 (0.040
g, 0.15 mmol) was heated to reflux for 10 minutes, during which
time a change from orange to red occurred. After cooling to
room temperature, chromatography on alumina gave a red
band with diethyl ether–light petroleum (2:3) which afforded
3a as a dry red solid. Recrystallisation from a dichloromethane–
methanol mixture gave thin orange plates suitable for X-ray
crystallography. All other complexes were prepared in an
analogous fashion.
NMR (CDCl3) δ 8.08 (ddd, J 27.5, 12.8, 6.0, 1H, Hα), 7.7–7.1
(m, 13H, Ph), 6.80 (d, J 7.6, 2H, Ph), 4.03 (dd, J 12.8, 5.5, 1H,
Hβ), 2.87 (q, J 7.4, 6H, CH2) and 2.50 (dquin, J 49.9, 7.4 Hz,
PCH2); 31P NMR (CDCl3) δ 176.2 (d, J 89.2, µ-PPh2) and 56.6
(d, J 89.2 Hz, PRЈ3); mass spectrum (FAB) m/z 705 (M Ϫ CO),
677 (M Ϫ 2CO), 649 (M Ϫ 3CO) and 593 (M Ϫ 5CO); calc. for
C34H29Fe2N3O5P2 C 55.66, H 3.96, N 5.73; found, C 55.58, H
3.91, N 5.57%.
3f (R = Ph, RЈ = Me) (65% yield after 10 min): ν(CO)-
1
3a (R = Ph, RЈ = Ph) (95% yield after 10 min): ν(CO)
(CH2Cl2) 2026m, 1978vs, 1943s and 1918sh cmϪ1; H NMR
1
(CH2Cl2) 2034m, 1983vs, 1947s, 1917m and 1884w cmϪ1; H
(CDCl3) δ 8.30 (br, 1H, Hα), 7.8–7.0 (m, 13H, Ph), 6.88 (d, J 6.7
Hz, 2H, Ph), 3.99 (br, 1H, Hβ) and 1.64 (br, 9H, CH3); 31P NMR
(CDCl3) δ 175.2 (d, J 92.8, µ-PPh2) and 27.3 (d, J 92.8 Hz,
PMe3); 13C NMR (CDCl3) δ 217.7 (br t, J 14, CO), 214.4 (br,
3CO), 212.1 (vbr, CO), 148.7 (br, Cα), 142.2 (s, Cipso), 140.2 (d,
J 42.5, Cipso), 137.3 (d, J 21.0, Cipso), 133.8 (s), 133.5 (d, J 6.1),
129.5 (d, J 36.6), 129.0 (s), 128.2 (s), 128.0 (s), 126.2 (s), 125.2
(s), 91.0 (br, Cβ) and 18.2 (d, J 36.1 Hz, CH3); mass spectrum
(FAB) m/z 616 (Mϩ), 560 (M Ϫ 2CO), 532 (M Ϫ 3CO), 504
(M Ϫ 4CO) and 476 (M Ϫ 5CO).
NMR (CDCl3) δ 8.35 (ddd, J 27.9, 12.9, 6.5, 1H, Hα), 7.9–6.4
(m, 30H, Ph) and 4.12 (dd, J 12.9, 5.4 Hz, 1H, Hβ); 31P NMR
(CDCl3) δ 181.2 (d, J 93.4, µ-PPh2) and 73.0 (d, J 93.4 Hz,
PPh3); 13C NMR (CDCl3) 218.9 (t, J 14.5, CO), 214.5 (br, 2CO),
212.7 (dd, J 21.5, 17.5, CO), 209.8 (br, CO), 153.6 (t, J 24.7, Cα),
142.2 (s, Cipso), 140.5 (d, J 42.6, Cipso), 137.3 (d, J 19.8, Cipso),
134.6 (s), 134.0 (d, J 9.2), 133.6 (s), 130.1 (s), 129.5 (d, J 25.0),
128.4 (d, J 8.6), 128.2 (d, J 19.4), 126.0 (s), 125.0 (s) and 89.4
(d, J 15.2 Hz, Cβ); mass spectrum (FAB) m/z 802 (Mϩ),
746 (M Ϫ 2CO), 718 (M Ϫ 3CO), 690 (M Ϫ 4CO) and 662
(M Ϫ 5CO); calc. for C43H32Fe2O5P2 C 64.34, H 3.99; found C
65.02, H 4.67%.
3g (R = Ph, PRЈ3 = PPh2Me) (66% yield after 10 min):
ν(CO)(CH2Cl2) 2030m, 1978vs, 1949m and 1916m cmϪ1 1H
;
NMR (CDCl3) δ 8.12 (ddd, J 27.3, 12.9, 6.3, 1H, Hα), 7.8–6.9
(m, 23H, Ph), 6.52 (d, J 7.0, 2H, Ph), 3.99 (dd, J 12.9, 5.4, 1H,
Hβ) and 2.18 (d, J 7.5 Hz, 3H, CH3); 31P NMR (CDCl3) δ 178.7
(d, J 92.5, µ-PPh2) and 54.4 (d, J 92.5 Hz, PPh2Me); mass spec-
trum (FAB) m/z 740 (Mϩ), 684 (M Ϫ 2CO), 656 (M Ϫ 3CO),
628 (M Ϫ 4CO) and 600 (M Ϫ 5CO).
3b (R = Ph, RЈ = p-tolyl) (71% yield after 10 min): ν(CO)
1
(CH2Cl2) 2030m, 1982vs, 1946s, 1917m and 1885w cmϪ1; H
NMR (CDCl3) δ 8.35 (ddd, J 27.8, 12.9, 6.5, 1H, Hα), 7.8–7.0
(m, 25H, Ph), 6.62 (d, J 7.5, 2H, Ph), 4.13 (dd, J 12.9, 5.3 Hz,
1H, Hβ) and 2.36 (s, 9H, Me); 31P NMR (CDCl3) δ 180.6 (d,
J 93.9, µ-PPh2) and 72.0 (d, J 93.9, PAr3); 13C NMR (CDCl3)
δ 219.1 (t, J 15.6, CO), 214.6 (br, 2CO), 212.9 (t, J 21.3, CO),
210 (vbr, CO), 154.1 (t, J 26.2, Cα), 142.3 (s, Cipso), 140.3 (d,
J 33.0, Cipso), 140.1 (s), 137.4 (d, J 20.7, Cipso), 133.9 (d, J 10.0),
133.5 (s), 131.3 (d, J 40.6), 129.5 (s), 129.1 (d, J 9.5), 128.8 (s),
128.1 (d, J 22.9), 125.8 (s), 125.0 (s), 89.3 (d, J 17.1 Hz, Cβ)
and 21.3 (s, CH3); mass spectrum (FAB) m/z 844 (Mϩ),
788 (M Ϫ 2CO), 760 (M Ϫ 3CO), 732 (M Ϫ 4CO) and 704
(M Ϫ 5CO); calc. for C46H38Fe2O5P2 C 65.40, H 4.50; found, C
65.64, H 4.83%.
3h (R = Ph, ERЈ3 = AsPh3) (14% yield after 1 h): ν(CO)
1
(CH2Cl2) 2034m, 1981vs, 1952m and 1918m cmϪ1; H NMR
(CDCl3) δ 8.39 (dd, J 13.1, 4.7, 1H, Hα), 7.7–7.2 (m, 24H,
Ph), 7.04–6.97 (m, 4H, Ph), 6.51 (d, J 7.9, 2H) and 4.06 (dd,
J 13.2, 5.3 Hz, 1H, Hβ); 31P NMR (CDCl3) δ 183.2 (s, µ-PPh2);
mass spectrum (FAB) m/z 846 (Mϩ), 790 (M Ϫ 2CO), 762
(M Ϫ 3CO), 734 (M Ϫ 4CO) and 706 (M Ϫ 5CO).
3i (R = But, RЈ = Ph) (57% yield after 10 min): ν(CO)
(CH2Cl2) 2030s, 1987vs, 1969s, 1951s and 1921s cmϪ1; 1H NMR
(CDCl3) δ 7.8–7.1 (m, 26H, Ph ϩ Hα), 3.34 (dd, J 13.4, 5.8 Hz,
1H, Hβ) and 0.70 (s, 9H, But); 31P NMR (CDCl3) δ 180.4 (d,
J 93.9, µ-PPh2) and 73.7 (d, J 93.9 Hz, PPh3); mass spectrum
(FAB) m/z 782 (Mϩ), 726 (M Ϫ 2CO), 670 (M Ϫ 4CO) and 642
3c (R = Ph, RЈ = m-tolyl) (93% yield after 10 min): ν(CO)
1
(CH2Cl2) 2033m, 1982vs, 1970sh, 1949m and 1915m cmϪ1; H
NMR (CDCl3) δ 8.31 (ddd, J 27.8, 12.9, 6.5, 1H, Hα), 7.8–7.0
350
J. Chem. Soc., Dalton Trans., 2001, 341–352

View Article Online
(M Ϫ 5CO); calc. for C41H36Fe2O5P2 C 62.91, H 4.60; found C
62.80, H 4.60%.
NMR (CDCl3) (223 K) δ 8.73 (ddd, J 31.8, 12.7, 8.1, 1H, Hα),
7.8–6.8 (m, 15H, Ph), 3.79 (d, J 11.1, 9H, Me) and 3.73 (t,
J 14.0, 1H, Hβ); (d8-toluene) (353 K) δ 8.91 (ddd, J 21.4, 13.4,
7.3, 1H, Hα), 7.68 (m, 4H, Ph), 7.0–6.7 (m, 11H, Ph), 4.21 (dd,
J 13.4, 5.2, 1H, Hβ) and 3.36 (d, J 10.9 Hz, 9H, Me) 31P NMR
(CDCl3) (223 K) δ 182.6 (d, J 154.7, P(OMe)3), 177.7 (d,
J 154.7, PPh2); (d8-toluene) (353 K) δ 177.3 (d, J 125.9) and
172.4 (d, J 125.9 Hz). Mass spectrum (FAB): m/z 664 (Mϩ), 636
(M Ϫ CO), 608 (M Ϫ 2CO), 580 (M Ϫ 3CO), 552 (M Ϫ 4CO)
and 524 (M Ϫ 5CO). Calc. for C28H26Fe2O8P2 C 50.60, H 3.92;
found, C 49.99, H 4.01%.
3j (R = Me, RЈ = Ph) (93% yield after 1 h): ν(CO)(C6H14)
2034s, 1989vs, 1973s, 1955m and 1923m cmϪ1 1H NMR
;
(CDCl3) δ 7.6–7.2 (m, 26H, Ph ϩ Hα), 3.11 (sextet, J 5.7, 1H,
Hβ) and 1.34 (d, J 5.7 Hz, 3H, CH3); 31P NMR (CDCl3)
δ 177.3 (d, J 93.5, µ-PPh2) and 67.6 (d, J 93.5 Hz, PPh3); mass
spectrum (FAB) m/z 740 (Mϩ), 684 (M Ϫ 2CO), 656
(M Ϫ 3CO), 628 (M Ϫ 4CO) and 600 (M Ϫ 5CO); calc. for
C38H30Fe2O5P2ؒ0.75CH2Cl2 C 57.85, H 3.92; found C 58.01,
H 4.08%.
[Fe (CO) (PPh )(ꢂ-MeC᎐CH )(ꢂ-PPh )] 3jЈ. Heating com-
᎐
2
5
3
2
2
[Fe (CO) {P(OMe) } (ꢂ-HC᎐CHPh)(ꢂ-PPh )] 7. A toluene
᎐
2
4
3
2
2
solution (10 cm3) of complex 5 (25 mg) was heated to reflux for
30 minutes, during which time the yellow solution darkened
slightly. After cooling to room temperature, chromatography on
alumina gave a yellow band with diethyl ether–light petroleum
(1:5) which afforded 7 (23 mg, 92%) as a dry orange solid.
Recrystallisation from a dichloromethane–ethanol mixture
gave orange crystals suitable for X-ray crystallography. Two
isomers A:B in 6:1 ratio at 223 K: ν(CO)(CH2Cl2) 1995s,
plex 1b and PPh3 at 110 ЊC for 30 min gave an inseparable
1
mixture of 1b, 3j and 3jЈ (approximate ratio 1:2:2). H NMR
(CDCl3) δ 7.6–7.1 (m, Ph), 2.29 (dd, J 7.9, 3.0, 1H, Hβ), 2.19
(s, J 5.7, 3H, CH3) and 2.07 (ddd, J 15.3, 6.7, 3.0, 1H, Hβ): 31
P
NMR (CDCl3) δ 168.5 (d, J 63.4, µ-PPh2) and 64.1 (d, J 63.4
Hz, PPh3).
[Fe (CO) {P(OMe) }(ꢂ-PhC᎐CH )(ꢂ-PPh )] 4 and [Fe (CO) -
᎐
2
5
3
2
2
2
4
1
1963vs, 1930s and 1910m cmϪ1. H NMR (CDCl3) (223 K)
{P(OMe) } (ꢂ-PhC᎐CH )(ꢂ-PPh )] 5. A toluene solution (20
᎐
3
2
2
2
cm3) of complex 1a (0.110 g, 0.19 mmol) and P(OMe)3 (50 mg,
0.40 mmol) was heated at 65 ЊC for 5 minutes, during which
time a change from orange to red occurred. After cooling to
room temperature, chromatography on alumina gave an orange
band with diethyl ether–light petroleum (1:9) which afforded 4
(60 mg, 47%) as a dry orange solid, while further elution with
diethyl ether–light petroluem (1:5) gave a second orange band
which afforded 5 (50 mg, 34%) as a dry orange solid.
δ 8.74 (ddd, J 30.2, 13.0, 8.1, 1H, Hα, A), 8.63 (br, 1H, Hα, B),
7.85–6.8 (m, 15H, Ph), 3.96 (br, 1H, Hβ, B), 3.79 (d, J 11.2,
9H, Me, A), 3.73 (d, J 11.2, 9H, Me, B), 3.50 (d, J 11.5, 1H,
Hβ, A), 3.32 (d, J 11.0, 9H, Me, A) and 3.28 (br, 9H, Me, B);
(d8-toluene) (353 K) δ 8.99 (dddd, J 24.9, 13.6, 11.5, 2.1, Hα),
7.87 (t, J 10.1, 2H, Ph), 7.63 (m, 2H, Ph), 7.07–6.61 (m, 11H,
Ph), 4.01 (dd, J 13.5, 4.7, 1H, Hβ) and 3.34 (d, J 10.9 Hz, 18H,
Me). 31P NMR: (CDCl3) (223 K) δ 196.8 (d, J 45.5, µ-P(OMe)3,
B), 188.6 (d, J 145.7, P(OMe)3, A), 181.0 (d, J 150.7, P(OMe)3,
B), 180.3 (d, J 59.8 P(OMe)3, A), 165.4 (dd, J 145.7, 59.8, PPh2,
A) and 159.0 (dd, J 150.7, 45.0, PPh2, B); (d8-toluene) (353 K)
δ 180.9 (d, J 45.1) and 160.6 (t, J 45.1 Hz).
4 (two isomers A:B in 2:1 ratio at 213 K): ν(CO)(CH2Cl2)
2035s, 1987vs, 1971m and 1958s cmϪ1 1H NMR (CDCl3)
;
δ 7.8–7.1 (m, Ph), 3.65 (d, J 11.2, Me, 9H, A), 3.60 (d, J 11.2,
Me, 9H, B), 3.16 (d, J 15.2, 1H, B), 3.01 (dt, J 15.4, 3.3, 1H, A),
2.28 (dt, J 12.1, 3.3, 1H, A) and 2.24 (d, J 10.5 Hz, 1H, B); 31
P
NMR (CDCl3) (213 K) δ 188.1 (d, J 52.7, P(OMe3), B), 186.6
(d, J 70.2, P(OMe3), A), 165.0 (d, J 70.2, PPh2, A) and 152.5
(d, J 52.7, PPh2, B); (d8-toluene) (383 K) δ 164 (br) and 181.8
(d, J 46.8 Hz); 13C NMR (CDCl3) 215.7 (t, J 14.6, 2CO, B),
215.5 (d, J 24.4, 2CO, B), 215.1 (s, 2CO, A), 213.2 (s, CO, B),
210.9 (s, 3CO, A), 189.5 (d, J 19.9, Cα, A), 188.9 (d, J 19.3, Cα,
X-Ray data collection and solution
For complexes 1a, 1c, 2a, 6 and 7 single crystals were mounted
on glass fibres and all geometric and intensity data taken
using an automated four-circle diffractometer (Nicolet R3mV)
equipped with Mo-Kα radiation (λ = 0.710 73 Å) at 293 2 K.
The lattice parameters were identified by application of
the automatic indexing routine of the diffractometer to the
positions of a number of reflections taken from a rotation
photograph and centred by the diffractometer. The ω–2θ or
ω technique was used to measure reflections and three standard
reflections (remeasured every 97 scans) showed no significant
loss in intensity during data collection. The data were corrected
for Lorentz and polarisation effects and empirically for absorp-
tion. The unique data with I у 2σ(I) were used to solve and
refine the structures. For 3a measurements were made on a
Bruker AXS SMART CCD area-detector diffractometer using
graphite-monochromated Mo-Kα radiation (λ = 0.710 73 Å)
at 160 2 K. Intensities were corrected semiempirically for
absorption.
Structures were solved by direct methods and developed by
using alternating cycles of least-squares refinement and Fourier-
difference synthesis. All non-hydrogen atoms were refined
anisotropically. Hydrogens were generally placed in idealised
positions (C–H 0.96 Å) and assigned a common isotropic
thermal parameter (U = 0.08 Å2). The hydrogens on the alkenyl
ligands and all hydrogens in complex 3a were located in differ-
ence maps. In 3a there is a disordered diethyl ether molecule.
This was modeled with restraints on geometry and anisotropic
displacement parameters. Structure solution used SHELXTL
PLUS program packages.38 Selected crystallographic data is
given in Table 3.
B), 158.6 (s, Cipso, B), 156.1 (s, Cipso, A), 141.1 (d, J 43.3, Cipso
,
A), 140.6 (d, J 44.4, Cipso, B), 136.2–125.4 (m), 66.8 (d, J 14.6,
Cβ, B), 62.6 (dd, J 12.0, 5.3 Hz, Cβ, A) and 52.2 (m, OCH3,
A ϩ B); calc. for C28H26Fe2O8P2 C 50.60, H 3.92; found C
50.23, H 3.79%.
1
5: ν(CO)(CH2Cl2) 1997vs, 1962s and 1932m cmϪ1; H NMR
(CDCl3) δ 7.8–7.1 (m, 15H, Ph), 3.55 (d, J 11.2, 9H, Me), 3.51
(d, J 11.2, 9H, Me), 3.00 (d, J 16.1, 1H) and 2.15 (d, J 10.8 Hz,
1H); 31P NMR (CDCl3) (213 K) 189.8 (d, J 47.0, P(OMe)3),
189.1 (d, J 71.6) and 145.9 (br, PPh2); (d8-toluene) (383 K)
δ 160.4 (t, J 44.9) and 183.8 (br d); 13C NMR (CDCl3) (223 K)
δ 218.0 (d, J 8.6, CO), 216.6 (t, J 24.4, CO), 214.3 (m, br, CO),
211.3 (br, CO), 185.3 (br d, J 10.3, Cα), 159.7 (s, Cipso), 142.5 (d,
J 43.0, Cipso), 136.8 (d, J 21.6, Cipso), 136.2 (s), 134.3 (s), 130.7
(s), 128.7 (s), 127.5(s), 126.4 (d, J 26.5 Hz), 125.8 (s), 62.4 (br s,
Cβ), 52.3 (s, OCH3) and 52.0 (s, OCH3); mass spectrum (FAB)
m/z 760 (Mϩ), 704 (M Ϫ 2CO), 676 (M Ϫ 3CO) and 648
(M Ϫ 4CO); calc. for C30H35Fe2O10P2 C 47.37, H 3.95; found C
47.35, H 4.59%.
[Fe (CO) {P(OMe) }(ꢂ-HC᎐CHPh)(ꢂ-PPh )] 6. A toluene
᎐
2
5
3
2
solution (10 cm3) of complex 4 (25 mg) was heated to reflux for
30 minutes, during which time the yellow solution darkened
slightly. After cooling to room temperature, chromatography on
alumina gave a yellow band with diethyl ether–light petroleum
(1:10) which afforded 6 (23 mg, 92%) as a dry orange solid.
Recrystallisation from a dichloromethane–ethanol mixture
gave orange crystals suitable for X-ray crystallography. ν(CO)
CCDC reference number 186/2286.
See http://www.rsc.org/suppdata/dt/b0/b007977o/ for crystal-
lographic files in .cif format.
(CH2Cl2) 2037s, 1989vs, 1976s, 1956s and 1928m cmϪ1 1H
:
J. Chem. Soc., Dalton Trans., 2001, 341–352
351

View Article Online
Table 3 Selected crystallographic data for complexes 1a, 1c, 2a, 3a, 6 and 7
1a
1c
2a
3a
6
7
Formula
Formula weight
Crystal system
Space group
a/Å
b/Å
c/Å
C26H17Fe2O6P
568.07
Monoclinic
P21/n
14.066(3)
12.748(3)
14.593(3)
C23H19Fe2O6P
534.05
Monoclinic
P21/c
13.444(3)
8.120(2)
22.698(5)
C26H17Fe2O6P
568.07
Monoclinic
P21/n
9.993(2)
14.641(3)
18.690(4)
C47H42Fe2O6P2
876.45
Monoclinic
P21/c
16.8886(11)
12.9963(8)
19.5996(13)
C28H26Fe2O8P2
664.13
C90H105Fe6O30P9
2280.57
Triclinic
Triclinic
¯
¯
P1
P1
10.954(2)
11.645(2)
13.745(2)
109.43(3)
99.92(3)
102.79(3)
1553.7(5)
2
10.81
5692
5373
361
11.462(2)
17.092(3)
27.804(6)
80.91(3)
88.26(3)
81.62(3)
5321(2)
2
10.04
11664
10.951
1216
α/Њ
β/Њ
110.74(3)
91.55(3)
104.49(3)
104.807(2)
γ/Њ
V/Å3
2447.1(9)
4
12.90
4248
4067
316
0.052 (0.123)
0.089 (0.160)
2476.9(10)
4
12.70
4350
4157
289
0.097 (0.244)
0.167 (0.342)
2647.5(9)
4
11.93
4945
4659
316
0.049 (0.130)
0.068 (0.156)
4159.0(5)
4
8.23
25146
9775
548
0.066 (0.128)
0.115 (0.147)
Z
µ(Mo-Kα)/cmϪ1
Data measured
Unique data used
No. parameters
R (all data)
Rw (all data)
0.065 (0.167)
0.087 (0.195)
0.071 (0.188)
0.108 (0.239)
20 G. E. Herberich, B. Hessner and J. Okuda, J. Organomet. Chem.,
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Acknowledgements
We thank the Royal Society for an award allowing O. S. S. to
spend a summer at University College, London, Dr Abil E.
Aliev (UCL) for help with NMR experiments and Dr Mark
Waugh (Newcastle) for synthesis of some starting materials.
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J. Chem. Soc., Dalton Trans., 2001, 341–352