Enhanced substitutional lability of [Fe(CO)2{P(OPh)3}2(η2- PhCCPh)]: Facile insertion of CO and organoisocyanides into iron-alkyne bonds

Article abstract of DOI:10.1039/b009849n

The η²-alkyne complex [Fe(CO)₂{P(OPh)₃}₂(η²- PhCCPh)] 1 reacts readily with PR₃ to give [Fe(CO)₂{P(OPh)₃}-(PR₃)(η²- PhCCPh)], and then [Fe(CO)₂(PR₃)₂(η²-PhCCPh)] (R = OMe, 2a; OEt, 2b; OⁿBu, 2c; Me, 2d; ⁿBu, 2e; Ph, 2f). The ability of the alkyne ligand to act as a four-electron donor to a ligand-dissociated 16-electron reaction intermediate promotes site-specific replacement of the axial phosphite ligands; there is no evidence for equatorial CO replacement. Reaction of 1 with CO affords six-coordinate ferracyclopent-3-ene-2,5-dione (maleoyl) complexes [Fe(CO)_m{P(OPh)₃}_n {η¹:η¹-C(O)C(Ph)C(Ph)C(O)}] (m = 3, n = 1, 3; m = n = 2, 4a) in which CO groups have inserted into each of the Fe-C(alkyne) bonds. Analogues of 4a, [Fe(CO)₂{P(OR)₃}₂ {η¹:η¹-C(O)C(Ph)C(Ph)C(O)}] (R = Me, 4b; Et, 4c), are similarly obtained as the sole products from the reaction of CO with 2a and 2b, respectively. The mechanism proposed for this reaction depends on replacement of an axial phosphite ligand by CO which is then susceptible to migratory attack by C(alkyne), whereas the existing equatorial CO ligands are not. The crystal structure of 3 has been determined: the complex exhibits a distorted octahedral geometry about the iron centre with a facial arrangement of the three CO ligands and the remaining coordination sites occupied by the P(OPh)₃ ligand and by the two C(O) groups of the maleoyl moiety. Reaction of 1 and CNR (R = Me and Ph) also proceeds via alkyne-CNR coupling to give [Fe(CO)₂{P(OPh)₃} {η¹:η¹:η¹:η¹- C(=NR)=C(Ph)C(Ph)=C(=NR)}] which is proposed to have a square pyramidal geometry on the basis of spectroscopic data.

Full text of DOI:10.1039/b009849n

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Enhanced substitutional lability of [Fe(CO)2{P(OPh)3}2(ꢀ2-
PhCCPh)]: facile insertion of CO and organoisocyanides into
iron–alkyne bonds
Maureen Barrow,a Natalie L. Cromhout,*a Anthony R. Manning*a and John F. Gallagherb
a Department of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland
b School of Chemical Sciences, Dublin City University, Glasnevin, Dublin 9, Ireland
Received 8th December 2000, Accepted 2nd March 2001
First published as an Advance Article on the web 30th March 2001
The η2-alkyne complex [Fe(CO)2{P(OPh)3}2(η2-PhCCPh)] 1 reacts readily with PR3 to give [Fe(CO)2{P(OPh)3}-
(PR3)(η2-PhCCPh)], and then [Fe(CO)2(PR3)2(η2-PhCCPh)] (R = OMe, 2a; OEt, 2b; OnBu, 2c; Me, 2d; nBu, 2e;
Ph, 2f). The ability of the alkyne ligand to act as a four-electron donor to a ligand-dissociated 16-electron reaction
intermediate promotes site-specific replacement of the axial phosphite ligands; there is no evidence for equatorial
CO replacement. Reaction of 1 with CO affords six-coordinate ferracyclopent-3-ene-2,5-dione (maleoyl) complexes
[Fe(CO)m{P(OPh)3}n{η1 : η1-C(O)C(Ph)C(Ph)C(O)}] (m = 3, n = 1, 3; m = n = 2, 4a) in which CO groups have
inserted into each of the Fe–C(alkyne) bonds. Analogues of 4a, [Fe(CO)2{P(OR)3}2{η1 : η1-C(O)C(Ph)C(Ph)C(O)}]
(R = Me, 4b; Et, 4c), are similarly obtained as the sole products from the reaction of CO with 2a and 2b, respectively.
The mechanism proposed for this reaction depends on replacement of an axial phosphite ligand by CO which is
then susceptible to migratory attack by C(alkyne), whereas the existing equatorial CO ligands are not. The crystal
structure of 3 has been determined: the complex exhibits a distorted octahedral geometry about the iron centre with
a facial arrangement of the three CO ligands and the remaining coordination sites occupied by the P(OPh)3 ligand
and by the two C(O) groups of the maleoyl moiety. Reaction of 1 and CNR (R = Me and Ph) also proceeds via
alkyne–CNR coupling to give [Fe(CO) {P(OPh) }{η1 : η1 : η1 : η1-C(᎐NR)᎐C(Ph)C(Ph)᎐C(᎐NR)}] which is
᎐
᎐
᎐
᎐
2
3
proposed to have a square pyramidal geometry on the basis of spectroscopic data.
change from dark orange to bright yellow with P(OR)3
(R = Me, Et or nBu) or orange with PR3 (R = Me or nBu). The
reactions were complete within seconds and afforded
[Fe(CO)2(L)2(η2-PhCCPh)] 2 in good yield. There was spectro-
scopic evidence [ν(CO) bands] for the formation of mixed
ligand derivatives [Fe(CO)2{P(OPh)3}(L)(η2-PhCCPh)] in some
reactions, but attempts to isolate these in the pure state failed.
The reaction of 1 with PPh3 followed a similar course, but the
reaction is an equilibrium and required a 15-fold excess of PPh3
to drive it to completion; a pure product could not be isolated.
These observations suggest that the reactions may be
described by inter-connecting equilibria (Scheme 1). For most
Introduction
The reactions of alkynes with iron carbonyls have been exten-
sively studied since the first such report1 in 1953 and a wealth
of both mononuclear and cluster derivatives is now known.2
However, examples of the simplest type of iron–alkyne inter-
action viz. η2-alkyne coordination to a single iron centre,
remain relatively rare. Such complexes are plausible reaction
intermediates but activation of the metal-bound alkyne
towards subsequent coupling and insertion reactions militates
against their isolation. Thus, the parent molecule [Fe(CO)4(η2-
HCCH)] was only reported in 1997 and is unstable to further
reaction at temperatures greater than ca. Ϫ60 ЊC.3 As expected,
however, substitution of CO ligands by phosphine or phos-
phite donors stabilises the Fe(η2-alkyne) fragment and a few
[Fe(CO)2(PR3)2(η2-RЈCCRЈ)] complexes are known and crys-
tallographically characterised.4 We have recently described
the synthesis and structure of the η2-diphenylacetylene com-
pound [Fe(CO)2{P(OPh)3}2(η2-PhCCPh)] 1: photolysis of
[Fe(CO)3{P(OPh)3}2] results in CO loss and the formation
of an orthometallated iron hydride [HFe(CO)2{P(OPh)3}-
5
᎐
{(PhO) POC H }] which reacts with PhC᎐CPh in the presence
᎐
2
6
4
of a Lewis acid to afford 1.6 In this paper the extremely facile
substitution reactions of 1 with two-electron donor ligands
are described and rationalised. Reaction of 1 with carbon
monoxide or organo isocyanides affords insertion products; to
the best of our knowledge, these are the first examples of CO
and CNR insertion into a pre-existing iron–alkyne bond.
Scheme 1 Reaction of complex 1 with PIII ligands.
of the ligands studied the equilibria lie far to the right. These
ligands are either smaller than P(OPh)3 [viz. P(OMe)3, P(OEt)3
and P(OnBu)3 which have cone angles7 ca. 106–110Њ]; are com-
parable in size to P(OPh)3 (cone angle 128Њ) but are much more
nucleophilic (PnBu3, cone angle 132Њ); or are both smaller and
more nucleophilic (PMe3, 118Њ). It is only when L = PPh3 (145Њ)
that the incoming ligand is significantly more bulky than
P(OPh)3 and is not particularly nucleophilic. This suggests that
Results and discussion
Reaction with PIII ligands
The addition of PIII ligands to a solution of [Fe(CO)2{P(OPh)3}2-
(η2-PhCCPh)] 1 in toluene at room temperature resulted in a
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J. Chem. Soc., Dalton Trans., 2001, 1352–1358
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b009849n

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the reaction equilibra are controlled in part by steric effects
with the smaller ligands replacing the larger, and in part by
electronic effects with the stronger nucleophiles replacing the
weaker.
Under the same reaction conditions [Fe(CO)3{P(OPh)3}2]
and the alkene-derivative [Fe(CO)2{P(OPh)3}2(C2H2C2O3)]
(C2H2C2O3 = maleic anhydride) fail to react with any of the
above PIII ligands.
The yields and characterisation data for complexes 2a–f are
compiled in Table 1; the IR and 13C NMR spectroscopic data
for compound 1 are included for comparison.
The spectroscopic data for 2 are consistent with the trigonal
bipyramidal geometry previously established for the parent
complex 1 in which the PIII ligands occupy mutually trans apical
sites and the CO and alkyne moieties are in the equatorial plane
of the molecule.6 This geometry has been confirmed for 2a by
X-ray crystallographic analysis.4b The IR spectra of 1 and 2
exhibit three absorption bands in the 2000–1750 cmϪ1 region
(Table 1): two of these bands are assigned to ν(CO) vibrations;
the third is much weaker and is assigned to the ν(CC) vibration
᎐
of the coordinated PhC᎐CPh ligand. The frequencies of all
᎐
three depend on the PIII ligand and decrease along the series
L = P(OPh)3 ӷ P(OMe)3 > P(OnBu)3 > P(OEt)3 > PPh3 ӷ PMe3
> PnBu3. This suggests that the increasing electron-
richness of the metal centre in 2 not only brings about
increasing back-bonding into the CO π* orbitals but also into
the CC π* orbitals of the η2-alkyne although the frequency
changes are greater for the ν(CO) modes than for ν(CC). In the
strong back-bonding limit the Fe–alkyne interaction is more
properly described by a metallacyclopropene structure.
The 13C NMR spectra of these complexes exhibit the
expected triplet resonances for both the carbonyl and acet-
ylenic C atoms (Table 1). The chemical shifts of both groups
are a function of L and correlate inversely with the corre-
sponding IR stretching frequencies: thus, δ(CO) and δ(CC)
increase along the series L = P(OPh)3 < P(OMe)3 < P(OnBu)3 ≅
P(OEt)3 < PPh3 < PMe3 < PnBu3. The
δ values reflect the
σ-donor capacity of the phosphane ligand and therefore the
extent of π-back-bonding from the iron centre to the CO and
alkyne ligands.
Reaction with CO
A solution of 1 in chloroform reacted rapidly with CO to give
a mixture of two compounds which were identified by
comparison with literature data6 as the ferracyclopent-3-ene-
2,5-dione complexes [Fe(CO)m{P(OPh)3}n{η1 : η1-C(O)C(Ph)-
C(Ph)C(O)}] (3, m = 3, n = 1; 4a, m = 2, n = 2) in which the
five-membered maleoyl-type metallacycle arises from CO
insertion into the Fe–alkyne bonds of the precursor. When
13CO was used in place of normal CO, the same products
were obtained but with 13CO distributed over all CO sites. Simi-
lar reactions occurred between CO and [Fe(CO)2{P(OR)3}2-
(η2-PhCCPh)] (R = Me or Et) but only analogues of 4a were
isolated: [Fe(CO)2{P(OR)3}2{η1 : η1-C(O)C(Ph)C(Ph)C(O)}]
(4b, R = Me; 4c, R = Et).
The spectroscopic data of 3 and 4 are consistent with the
structures shown in Fig. 1; the structure of 3 has also been
established by X-ray crystallography (following section). The
IR spectra exhibit two ν(CO) absorption bands the shapes and
relative intensities of which are consistent with the presence of
a fac-M(CO)3 and a cis-M(CO)2 moiety, respectively. Their fre-
quencies show the expected effects on changing the number and
types of phosphite ligands. If the CO ligands are replaced by
13CO, the frequencies of these bands decrease by ca. 44–47 cmϪ1
as anticipated.
The IR spectra also show a number of bands at ca. 1600
cmϪ1. Those complexes which contain a P(OPh)3 ligand have
a band at ca. 1590 cmϪ1 which does not change frequency on
replacement of CO by 13CO and which is not present in the
J. Chem. Soc., Dalton Trans., 2001, 1352–1358
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Fig. 1 Structure of the complexes 3 and 4.
spectra of the P(OMe)3 and P(OEt)3 derivatives. It is assigned
to an internal vibration of P(OPh)3. All of the complexes show
two absorption bands which are attributed to the ν(CO)/ν(CC)
vibrations of the –C(O)–C᎐C–C(O)– moiety. For 3 they lie at
᎐
1651 and 1636 cmϪ1, and for 4a at 1642 and 1616 cmϪ1. If CO is
replaced by 13CO these frequencies fall to 1633 and 1604 cmϪ1
,
and 1617 and 1578 cmϪ1, respectively. These observations are
consistent with the conclusions that there is mixing of the
ν(CO) and ν(CC) vibrations of the maleoyl ligand, that the 1651
(3) and 1642 cmϪ1 (4a) bands have more ν(CO) character than
the 1636 and 1604 cmϪ1 bands, respectively, and that the
Fig. 2 The molecular structure of complex 3. Selected bond lengths
(Å) and angles (Њ): Fe–P1 2.1937(19), Fe–C1 1.781(7), Fe–C2 1.859(6),
Fe–C3 1.846(6), Fe–C6 2.017(5), Fe–C7 2.003(5), C4–C5 1.356(6), C4–
C6 1.496(7), C5–C7 1.507(7), C6–O6 1.215(5), C7–O7 1.208(5), C1–Fe–
P1 165.3(2), C2–Fe–C6 171.2(2), C3–Fe–C7 172.7(2), Fe–C1–O1
175.2(6), Fe–C2–O2 175.0(6), Fe–C3–O3 175.0(6), P1–Fe–C2
98.01(19), P1–Fe–C3 94.66(19), P1–Fe–C6 85.15(16), P1–Fe–C7
87.73(16), C1–Fe–C2 93.8(3), C1–Fe–C3 92.9(3), C1–Fe–C6 82.0(2),
C1–Fe–C7 83.4(3).
–C(O)–C(Ph)᎐C(Ph)–C(O)– group acts as a π-acceptor ligand
᎐
whose C᎐O and C᎐C bond orders are reduced when the
᎐
᎐
Fe(CO)3{P(OPh)3} moiety is replaced by Fe(CO)2{P(OPh)3}2.
All of the anticipated resonances are present in the 13C NMR
spectra of 3 and 4 and their all-13CO counterparts, although the
latter are complicated by 13C–13C coupling. The carbonyl region
of the 13C NMR spectrum of 4a comprises a single triplet
resonance at δ 205.2 (JCP = 22.1 Hz) whereas that of 3 consists
of two doublet signals at δ 205.6 and 202.1 in a 1 : 2 ratio. The
less intense resonance exhibits the larger coupling (JCP = 30.7
Hz) and is assigned to the unique CO ligand trans to P(OPh)3;
the higher field signal is attributed to the equatorial CO ligands
cis to P(OPh)3. In both 3 and 4 the acyl CO groups of the
maleoyl ligand are strongly deshielded giving rise to resonances
at ca. δ 250–262 which couple to the ligated 31P atoms (JCP
=
ca. 31 Hz). Singlet resonances at ca. δ 167 are assigned to the
central C atoms of the maleoyl ligands.
Crystal structure of complex 3
Single crystals of 3 were obtained from CDCl3–hexane solution
and analysed by X-ray diffraction. The molecular structure is
shown in Fig. 2 together with selected bond lengths and angles.
The coordination geometry about the iron atom is approx-
imately octahedral: the three CO ligands span one face and
the remaining coordination sites are occupied by the P(OPh)3
ligand and by the two C(O) groups of the ferracyclopentene-
dione moiety. The structure of 3 compares closely with that of
[Fe(CO)2{P(OPh)3}2{η1 : η1-C(O)C(Me)C[CH(OEt)2]C(O)}] 5
which is obtained directly from the reaction of [HFe(CO)2-
Fig. 3 Packing of the molecules in the crystal structure of complex 3.
[P1–Fe–C1 165.3(2)Њ]. The extent of this displacement may
be indicated by the angles P1–Fe ؒ ؒ ؒ Cg1 84.90(9)Њ and
C1–Fe ؒ ؒ ؒ Cg1 80.7(2)Њ where Cg1 is the centroid of the met-
allacycle. This inclination of the axial ligands is also present
in complex 5 where the trans phosphites make an angle of
167.89(4)Њ with the iron centre and it has been observed in
structural determinations of related metal–maleoyl complexes.9
An analysis of the crystal structure of 3 reveals no strong
hydrogen bonding interactions; the most significant such
intermolecular interaction comprises C14–H14 ؒ ؒ ؒ O7i (i = Ϫx,
1 Ϫ y, 2 Ϫ z) with H ؒ ؒ ؒ O 2.49 Å and C–H ؒ ؒ ؒ O 144Њ. Two
molecules of 3 associate about an inversion centre forming a
6
᎐
{P(OPh) }{(PhO) POC H }] with MeC᎐CCH(OEt) . As in 5,
᎐
3
2
6
4
2
the metallacycle in 3 is essentially planar (r.m.s. deviation from
planarity is 0.02 Å) and there is no evidence for bond delocal-
isation within the ring: C4–C5 [1.356(6) Å] is a typical C–C
double bond; C4–C6 [1.496(7) Å] and C5–C7 [1.507(7) Å]
are single bonds. The Fe–C(O) bond lengths of 2.017(5) and
2.003(5) Å are slightly, but not significantly, shorter than the
average value reported for FeII–C(sp2) bonds (2.05 Å).8 The
distances C6–O6 [1.215(5) Å] and C7–O7 [1.208(5) Å] are
similar to the average C᎐O bond length of 1.21 Å.9 The struc-
᎐
tural data imply that the π-acceptor function of the maleoyl
ligand (deduced above from the IR spectra) must be small.
Distortions from ideal octahedral geometry arise from the
constrained bite of the maleoyl ligand which imposes an angle
of 81.0(2)Њ at the metal centre, and results in a correspondingly
larger opposite angle C2–Fe–C3 95.6(3)Њ, and from a significant
displacement of the axial ligands in the direction of the ring
hydrogen bonded motif with graph set R2 (20) as depicted in
2
Fig. 3. These pairs stack along the a-axis associated through
weak C–H ؒ ؒ ؒ O and C–H ؒ ؒ ؒ π(arene) interactions. The latter
form through C45–H45 ؒ ؒ ؒ π{C31, . . . ,C36}ii with H ؒ ؒ ؒ Cg2ii
2.83 Å and C–H ؒ ؒ ؒ Cg2ii 146Њ where Cg2 is the centroid of the
{C31, . . . ,C36} arene ring system and the symmetry operator
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J. Chem. Soc., Dalton Trans., 2001, 1352–1358

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Fig. 4 Variable-temperature 13C NMR spectra for complex 6b.
ii = (x Ϫ 1, y, z). This effect results in the two phenyl rings of
the maleoyl ligand interacting with a phenoxy ring of a
symmetry-related phosphiteii forming what can be described as
a triple embrace reminiscent of the hextuple embraces observed
in PPh3 derivatives.10 The other two phenoxy groups of
phosphiteii do not participate in this association. The closest
hydrogen bonding interaction involving O1 is O1 ؒ ؒ ؒ H35–C35iii
(iii = x Ϫ 1, 1 ϩ y, z) with O ؒ ؒ ؒ H 2.72 Å and O ؒ ؒ ؒ H–C 138Њ.
However, three other contacts also arise: these are depicted
in Fig. 3. The volume element around O1 is such that these
contacts, although weak, facilitate bending in the Fe–C1–O1
angle and contribute to the observed deviation from linearity in
the direction of the maleoyl group.
Fig. 5 Proposed structure for complexes 6a and b.
to intense Ph resonances) two signals in the carbonyl region
and broad resonances centred at δ 193.8 and 79.4. A variable
temperature 13C NMR experiment was therefore undertaken;
the data obtained are depicted in Fig. 4. From the above
compositional and spectroscopic data and, in particular, the
low-temperature 13C NMR spectrum of 6b [Fig. 4(d)] we
propose the structure shown in Fig. 5 in which the iron atom
Reaction with CNR (R ؍ Me or Ph)
A very rapid reaction occurred on addition of methyl or phenyl
isocyanide (CNR) to a toluene solution of 1. However, the
isolated products 6a (R = Me) and 6b (R = Ph) were not the
expected simple analogues of 3 or 4. Elemental analysis
indicated that both new compounds comprise 2 : 1 adducts
of CNR and the fragment Fe(CO)2{P(OPh)3}(Ph2C2). The IR
spectra of 6a,b exhibited two ν(CO) absorption bands but
signals attributable to terminal isocyanide ligands or to the
is in a square pyramidal environment and the RN᎐C᎐
᎐ ᎐
C(Ph)C(Ph)C᎐C᎐NR organic ligand acts as an η4, four-electron
᎐ ᎐
donor to the metal centre. Such a ligand is not without
precedent in the literature: Stone and co-workers have
reported11 the crystal structure of the square pyramidal
complex [Fe(CNtBu) {η1 : η1 : η1 : η1-C(᎐NtBu)᎐C(Ph)C(Ph)᎐
2
᎐
ν(C᎐C) stretch of an η -alkyne were not detected. Instead,
᎐
absorptions at 1715 (6a) and 1686 cmϪ1 (6b) suggested the
᎐
᎐
᎐
3
presence of imine and/or C᎐C functionalities. The room-
C(᎐NtBu)}] which was obtained from the interaction of free
᎐
᎐
temperature 13C NMR spectrum of 6b exhibited (in addition
diphenylacetylene with precoordinated isocyanide ligands in
J. Chem. Soc., Dalton Trans., 2001, 1352–1358
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[Fe(CNtBu)5], in opposite sense to the reaction we report here.
The room-temperature 13C NMR chemical shifts observed for
6b compare with those reported by Stone and co-workers for
the bis(tert-butylimino)diphenylbuta-1,3-diene ligand: δ 203.3
[tBuN᎐CC(Ph)–] and δ 72.0 [tBuN᎐CC(Ph)–].11
labilisation of the CO ligands in [M(CO)4(η2-alkyne)] (M = Fe,
Ru, Os) which undergo substitution reactions at rates that are
much greater than those of their [M(CO)5] precursors.15 This
was attributed to stabilisation of a ligand-dissociated 16-
electron intermediate by donation from the filled π⊥ alkyne
orbital to the vacated metal dσ orbital [see Fig. 4 in ref. 15(a)].
This suggestion implies that it is an axial ligand which is
labilised and our work establishes that this is the case since only
the axial P(OPh)3 ligands of [Fe(CO)2{P(OPh)3}2(η2-PhCCPh)]
are replaced; no CO substitution is observed. A parallel may
be drawn between the substitutional behaviour of 1 and the
labilisation of the axial PR3 ligands in Fe(CO)2(PR3)2(η2-CS2).16
In these complexes, the CS2 ligand acts in many respects as
᎐
᎐
The low-temperature 13C NMR spectrum of 6b is consistent
with the unsymmetrical structure shown in Fig. 5 in which the
phosphite ligand occupies a basal position. In the related com-
plexes [Fe(CO)2(L)(η4-diene)] (L = phosphine or phosphite) the
P–CO coupling constants have been shown to have the order
J(Pbasal–CObasal) ӷ J(Paxial–CObasal) ≅ J(Pbasal–COaxial).12
The
doublet resonance at δ 209.5 in the carbonyl region of the low-
temperature 13C NMR spectrum [Fig. 4(d)] is therefore assigned
to C1 [J(Pbasal–CObasal) = 12.3 Hz] and the singlet at δ 209.7
ϩ
Ϫ
᎐
though it were an S ᎐C–S two-electron donor through the
S ᎐C bond.
᎐
᎐
ϩ
᎐
is assigned to C2 assuming that the smaller J(Pbasal–COaxial
)
coupling is not resolved at this temperature. The doublet
resonances at δ 194.4 (JCP = 13.3 Hz) and δ 193.9 (JCP = 23.1
Hz) are assigned to C3 and C6, respectively, since C6 is
expected to exhibit the larger coupling to the pseudo-trans P
atom. Similarly, the singlet resonance at δ 79.3 and the doublet
at δ 79.0 (JCP = 4.8 Hz) are assigned to C4 and C5, respectively,
where P coupling to C4 is not resolved.
The reaction of 1 with CO is now explicable. The first
step is replacement of the labilised axial phosphite ligand by
CO (Scheme 2) followed by migration of one alkyne C atom to
On warming to room temperature the 13C NMR signals of
the organic ligand converge: C3 and C6 coalesce at ca. 19 ЊC
and C4 and C5 at ca. 22 ЊC. However, coalescence of the CO
resonances is not observed in the temperature range studied. At
90 ЊC these ligands give rise to two sharp singlets at δ 211.0 and
210.9 whereas the signals due to C3/C6 and C4/C5 appear as
somewhat broadened singlets at δ 191.8 and 79.5, respectively.
The data suggest a fluxional process which interchanges only
the basal phosphite and basal CO ligand. The result is an
inversion of chirality at the iron centre which equilibrates the
backbone carbon atoms of the bis(phenylimino)diphenylbuta-
1,3-diene ligand but does not effect scrambling of the axial and
basal CO groups.
In contrast to 6b, the variable temperature 13C NMR spectra
of the methyl derivative 6a are consistent with a dynamic
process which interchanges apical and basal sites and leads to
complete CO scrambling in the fast-exchange regime. Such
exchange processes are well established for the complexes
[Fe(CO)2(L)(η4-diene)] (L = CO, PR3, CNR) and occur via
either a sequential Berry pseudorotation or a turnstile rotation
of the [Fe(CO)2(L)] fragment relative to the diene.12,13 Thus,
at 90 ЊC the 13C NMR spectrum of 6a exhibits one doublet
resonance at δ 213.3 (JCP = 14.9 Hz) attributable to two equiv-
alent CO ligands, and slightly broadened singlets at δ 188.0 and
78.2 assigned to C3/C6 and C4/C5, respectively. On cooling, the
CO resonances collapse to a broad singlet which decoalesces at
ca. 10 ЊC; the resonances due to the backbone carbon atoms of
the organic ligand decoalesce in the range 2–0 ЊC. At the lowest
temperature accessible to us (Ϫ20 ЊC) the gross features of the
low temperature spectrum of 6b are reproduced i.e. two signals
due to the CO ligands and four signals for the four backbone
carbon atoms consistent with the unsymmetrical structure of
Fig. 5. However, all resonances are broad and coupling to P is
not resolved.
Scheme 2 Proposed mechanism for the reaction of complex 1 with
CO.
the axial CO ligand leaving behind a vacant site which is then
occupied by a second incoming CO. The second C atom of the
alkyne can migrate to this to give an intermediate which may be
formulated as a 16-electron species with an η2-maleoyl ligand
acting as a two-electron donor or a rather labile 18-electron
species with the maleoyl ligand acting as an η4, four-electron
donor. This takes up CO or P(OPh)3 to give the final products 3
and 4a, respectively. This scheme accounts for the observed
incorporation of 13CO into Fe–CO as well as maleoyl CO sites.
A similar mechanism may be invoked for the reaction of 1 with
organoisocyanides.
The reaction of 1 with CO was monitored by IR spec-
troscopy in an attempt to detect the intermediates proposed in
Scheme 2. The addition of a sub-stoichiometric quantity of CO
to a chloroform solution of 1 under nitrogen afforded partial
conversion to a species in equilibrium with 1 which was identi-
fied as [Fe(CO)3{P(OPh)3}(η2-PhCCPh)] 7 by comparison of
the IR spectrum [ν(CO) 2043s, 1983m, 1949s; ν(CC) 1844w
cmϪ1] with that reported for [Os(CO)3(PMe3)(η2-HCCH)]
[ν(CO) 2056s, 1977m, 1947s cmϪ1; ν(CC) not given].15b Complex
7 is unstable in the absence of CO and on a TLC plate and was
not isolable: on removal of the solvent or purging the solution
with a stream of nitrogen gas, the starting material 1 was
cleanly regenerated. However, under a low partial pressure
of CO, 7 is long-lived and was observed to decay only slowly
(ca. hours) to the final products 3 and 4a. No other inter-
mediates were detected. The isocyanide analogue of 7, viz.
[Fe(CO)2{P(OPh)3}(CNR)(η2-PhCCPh)], was not observed in
Reaction mechanisms
Complex 1 undergoes facile P(OPh)3 substitution whereas
neither the parent complex [Fe(CO)3{P(OPh)3}2] nor the alkene
derivative [Fe(CO)2{P(OPh)3}2(maleic anhydride)] react under
the same conditions. We suggest that the ability of the alkyne
to act as a four-electron donor as well as a two-electron
donor promotes the loss of a P(OPh)3 ligand by stabilising
the formally 16-electron intermediate [Fe(CO)2{P(OPh)3}(η2-
PhCCPh)]. Caulton and co-workers have similarly proposed
that four-electron donation from PhCCPh may contribute to
the ease of phosphine dissociation in [Ru(CO)2(PtBu2Me)2(η2-
PhCCPh)].14 Takats and co-workers have observed enhanced
1356
J. Chem. Soc., Dalton Trans., 2001, 1352–1358

View Article Online
the reaction of 1 with CNR; for this system, reaction appeared
to be complete within the time of mixing and no intermediates
were detected by IR spectroscopy.
Complex 1 is stable with respect to CO insertion into the
Fe–C(alkyne) bond except in the presence of added CO. This
may be rationalised on the basis that the bite angle of the η2-
alkyne is small and therefore the equatorial CO groups, though
formally cis to the Fe–C bonds, are actually too distant from
The reaction of [Fe(CO)2{P(OPh)3}2(η2-PhCCPh)] with PPh3
required a 15-fold excess of the ligand. The reaction mixture
was stirred as above and the solvent removed. The residue was
extracted with hot pentane to give an orange solution, which
was filtered and evaporated to dryness at reduced pressure.
Complex 2f was obtained as an orange oil.
[Fe(CO)3{P(OPh)3}{ꢀ1 : ꢀ1-C(O)C(Ph)C(Ph)C(O)}]
3 and
them to allow C migration to take place. In the structure of 1
[Fe(CO)2{P(OR)3}2{ꢀ1 : ꢀ1-C(O)C(Ph)C(Ph)C(O)}] (R ؍ Ph,
4a; Me, 4b; Et, 4c). [Fe(CO)2{P(OPh)3}2(η2-PhCCPh)] (0.5 g,
0.55 mmol) was dissolved in chloroform (20 ml) and this
solution was injected via a rubber septum into a 50 ml flask
filled with CO. The flask was shaken for 30 min. The solvent
was removed and the residue was purified by column chrom-
atography on silica. Elution with 3 : 1 hexane–chloroform
afforded orange [Fe(CO)3{P(OPh)3}{η1 : η1-C(O)C(Ph)C(Ph)-
C(O)}] 3 and subsequent elution with 1 : 1 hexane–chloroform
gave yellow [Fe(CO)2{P(OPh)3}2{η1 : η1-C(O)C(Ph)C(Ph)-
C(O)}] 4a. Both compounds were recrystallised from dichloro-
methane–hexane.
᎐
the distance from a C᎐C carbon atom to the carbon of the
᎐
(nearest) equatorial CO ligand is 3.07 Å; the distance to the P
atom of the axial phosphite ligands is 2.90 Å.6 In complex 7,
[Fe(CO)3{P(OPh)3}(η2-PhCCPh)], assuming a standard Fe–CO
bond distance of 1.78 Å,8 the distance to the axial CO carbon
is calculated at 2.64 Å. The axial CO ligand is thus ca. 0.4 Å
᎐
closer to the C᎐C group and susceptible to migratory attack by
᎐
C(alkyne).
Conclusions
The stabilisation of the Fe(η2-alkyne) fragment by coordination
of phosphite donor ligands has enabled a study of the chem-
istry of the [Fe(CO)2{P(OPh)3}2(η2-PhCCPh)] complex. The
reactivity of 1 is characterised by rapid phosphite substitution;
this is atypical of saturated 18-electron transition-metal com-
plexes, which often tend to be kinetically inert. It is attributed to
the capacity of the alkyne ligand to act as a four-electron donor
which stabilises the formally 16-electron [Fe(CO)2{P(OPh)3}-
(η2-PhCCPh)] transition state and which specifically promotes
labilisation of the axial ligands, as observed. When 1 is reacted
with excess CO the initial substitution product [Fe(CO)3-
{P(OPh)3}(η2-PhCCPh)] affords ferracyclopentenedione com-
plexes in which CO has inserted into both Fe–alkyne bonds.
Since iron–carbonyl mediated alkyne–CO coupling reactions
have always been assumed to proceed through undetected
[Fe(CO)4(η2-alkyne)] intermediates, the above is an important
observation. A similar insertion reaction is observed on
treatment of 1 with organoisocyanide but here alkyne–CNR
coupling affords an unusual bis(imino)buta-1,3-diene ligand
which is proposed to act as a four-electron donor to an
Fe(CO)2{P(OPh)3} fragment.
[Fe(CO)2{P(OR)3}2{η1 : η1-C(O)C(Ph)C(Ph)C(O)}] (4b, R =
Me; 4c, R = Et) were similarly obtained as the sole products
from the reaction of [Fe(CO)2{P(OR)3}2(η2-PhCCPh)] (R = Me
or Et) with CO.
Complex 3. Yield: 41%, mp 126–127 ЊC. Found: C, 64.8; H,
3.7; P, 4.4. C37H25FeO8P requires C, 64.9; H, 3.7; P, 4.5%. IR
(CH2Cl2, cmϪ1) ν(C᎐O) 2081vs, 2024vs (br); ν(C᎐O) 1651w;
᎐
᎐
᎐
ν(C᎐C) 1636m; ν(POPh) 1590m; δC(CDCl3) 251.4 (d, 2JCP = 30.8
᎐
2
Hz, C᎐O), 205.6 (d, 2J = 30.7 Hz, C᎐O), 202.1 (d, JCP = 22.2
᎐
᎐
᎐
᎐
CP
᎐
Hz, C᎐O), 167.4 (s, C᎐C), 151.4–121.1 (Ph); δ (CDCl ) 140.6.
᎐
P
3
Complex 4a. Yield: 25%, mp 141–142 ЊC. Found: C, 66.7; H,
4.0; P, 6.3. C54H40FeO10P2 requires C, 67.1; H, 4.1; P, 6.4%. IR
(CH2Cl2, cmϪ1) ν(C᎐O) 2044vs, 1992s; ν(C᎐O) 1642w; ν(C᎐C)
᎐
᎐
᎐
᎐
2
1616m; ν(POPh) 1590m; δC(CDCl3) 259.6 (t, JCP = 30.8 Hz,
C᎐O), 205.2 (t, JCP = 22.1 Hz, C᎐O), 168.3 (s, C᎐C), 151.4–
2
᎐
᎐
᎐
᎐
121.2 (Ph); δP (CDCl3) 145.1.
Complex 4b. Yield: 49%, mp 123–124 ЊC. Found: C, 49.6;
H, 4.8; P, 10.8. C24H28FeO10P2 requires C, 48.6; H, 4.8; P, 10.4%.
IR (CH2Cl2, cmϪ1) ν(C᎐O) 2035vs, 1979s; ν(C᎐O)/ν(C᎐C)
᎐
᎐
᎐
᎐
2
1618m (br); δC(CDCl3) 261.9 (t, JCP = 31.0 Hz, C᎐O), 207.9
᎐
(t, 2JCP = 23.8 Hz, C᎐O), 166.3 (s, C᎐C), 134.3–127.1 (Ph), 51.8
᎐
᎐
᎐
(vt, N = 2.2 Hz, Me).
Complex 4c. Yield: 51%, mp 122–125 ЊC. Found: C, 52.9; H,
Experimental
General procedures
5.9; P, 8.8. C30H40FeO10P2 requires C, 53.1; H, 6.0; P, 9.1%. IR
(CH2Cl2, cmϪ1) ν(C᎐O) 2033vs, 1980s; ν(C᎐O)/ν(C᎐C) 1621m
᎐
᎐
᎐
᎐
[Fe(CO)2{P(OPh)3}2(η2-PhCCPh)] was prepared as described
previously.6 Other chemicals were purchased from Aldrich and
used without further purification. All reactions were carried out
at room temperature under an atmosphere of nitrogen in dried
and deoxygenated solvents. IR spectra were recorded on a
Perkin-Elmer Paragon 1000 FTIR spectra. 13C NMR spectra
were obtained at 25 ЊC, unless otherwise stated, on a Varian
INOVA 500 MHz spectrometer operating at 125.7 MHz; 31P
NMR spectra were obtained at 30 ЊC on a Varian INOVA 300
MHz spectrometer operating at 121.4 MHz. Analyses were
determined by the Analytical Laboratory, University College
Dublin.
(br); δC(CDCl3) 260.9 (t, JCP = 30.9 Hz, C᎐O), 207.6 (t,
2
᎐
2
᎐
JCP = 22.1 Hz, C᎐O), 165.9 (s, C᎐C), 134.3–121.4 (Ph), 62.2
᎐
᎐
(d, 2JCP = 6.8 Hz, CH2), 16.1 (d, 3JCP = 5.1 Hz, Me).
[Fe(CO)2{P(OPh)3}{ꢀ1 : ꢀ1 : ꢀ1 : ꢀ1-C(NR)C(Ph)C(Ph)-
C(NR)}] (6a, R ؍ Me; 6b, R ؍ Ph). A 0.1 M solution of methyl
or phenyl isocyanide in toluene was added dropwise to a
solution of [Fe(CO)2{P(OPh)3}2(η2-PhCCPh)] (0.2 g, 0.22
mmol) in toluene (20 ml). The reaction was monitored by
IR spectroscopy and the addition was halted when the IR
absorbances of the starting material had disappeared. The
solvent was removed and the residue was purified by col-
umn chromatography on alumina. Elution with 1 : 1 hexane–
dichloromethane removed P(OPh)3; elution with acetone then
afforded a yellow material which was recrystallised from
acetone–hexane to give the product as a yellow solid.
Preparations
[Fe(CO)2(PR3)2(ꢀ2-PhCCPh)]. The PIII exchange products
(R = OMe, 2a; OEt, 2b; OnBu, 2c; Me, 2d; Bu, 2e) were pre-
n
pared by addition of the appropriate PIII ligand to a solution
of [Fe(CO)2{P(OPh)3}2(η2-PhCCPh)] (0.25 g, 0.27 mmol) in
toluene (20 ml) in a 2 : 1 molar ratio. The resulting mixture was
stirred for 15 min and the solvent was then removed at reduced
pressure. The residue was recrystallised from dichloromethane–
hexane (R = OMe, OEt or OnBu) or diethyl ether–methanol
(R = Me or nBu) to afford the product as a yellow-orange
crystalline solid.
Complex 6a. Yield: 53%, mp 129–130 ЊC. Found: C, 66.4; H,
5.0; N, 3.8; P, 4.7. C38H31FeN2O5P requires C, 66.9; H, 4.6; N,
4.1; P, 4.5%. IR (CH2Cl2, cmϪ1) ν(CO) 2006vs, 1957s; ν(C᎐C)/
᎐
ν(C᎐N) 1715m; ν(POPh) 1591m; δ (d-toluene; 90 ЊC) 213.3 (d,
᎐
C
2JCP = 14.9 Hz, CO), 188.0 (s, N᎐C᎐C), 151.4–121.1 (Ph), 78.2
᎐ ᎐
(s, N᎐C᎐C), 44.6 (s, Me); δ (CD Cl ; Ϫ20 ЊC) 214.6 (br s, CO),
᎐ ᎐
C
2
2
209.4 (br s, CO), 190.7 (br s, N᎐C᎐C), 188.9 (br s, N᎐C᎐C), 80.1
᎐ ᎐ ᎐ ᎐
(br s, N᎐C᎐C), 74.9 (br s, N᎐C᎐C), 44.6 (m, Me); δ (CDCl ;
᎐ ᎐
᎐ ᎐
H
3
J. Chem. Soc., Dalton Trans., 2001, 1352–1358
1357

View Article Online
Organometallic Chemistry, ed. G. Wilkinson, F. G. A. Stone and
30 ЊC) 7.37–6.97 (25 H, m, Ph), 3.43 (6 H, s, Me); δP(CDCl3;
30 ЊC) 156.7.
E. W. Abel, Pergamon, Oxford, 1983, vol. 4, p. 545.
3 J. Cooke and J. Takats, J. Am. Chem. Soc., 1997, 119,
11088.
Complex 6b. Yield: 47%, mp 163 ЊC (decomp.). Found: C,
71.1; H, 4.6; N, 3.6; P, 3.6. C48H35FeN2O5P requires C, 71.5; H,
4.4; N, 3.5; P, 3.8%. IR (CH2Cl2, cmϪ1) ν(CO) 2025vs, 1976s;
ν(C᎐C)/ν(C᎐N) 1686m; ν(POPh) 1592m; δ (d-toluene; 90 ЊC)
4 (a) [Fe(CO)2(PEt3)2(η2-Me3SiCCSiMe3)] and [Fe(CO)2(PEt3)2(η2-
Me3SiCCC2SiMe3)]: C. Gauss, D. Veghini and H. Berke, Chem. Ber.,
1997, 130, 183; (b) [Fe(CO)2{P(OMe)3}2(η2-PhCCPh)]: F. Meier-
Brocks, R. Albrecht and E. Weiss, J. Organomet. Chem., 1992, 439,
65; (c) [Fe(CO)2(PEt3)2(η2-HCCH)]: R. Birk, H. Berke, G. Huttner
and L. Zsolnai, Chem. Ber., 1988, 121, 471; (d) [Fe(CO)2-
{P(OMe)3}2(η2-PhCCH)]: R. Birk, U. Grossmann, H.-U. Hund and
H. Berke, J. Organomet. Chem., 1988, 345, 321.
᎐
᎐
C
211.0 (s, CO), 210.9 (s, CO), 191.8 (br s, N᎐C᎐C), 152.0–120.6
᎐ ᎐
(Ph), 79.5 (br s, N᎐C᎐C); δ (CDCl ; Ϫ20 ЊC) 209.7 (s, C2O),
᎐ ᎐
C
3
2
2
209.7 (d, JCP = 12.3 Hz, C1O), 194.4 (d, JCP = 13.3 Hz,
N᎐C3᎐C), 193.9 (d, 2J = 23.1 Hz, N᎐C6᎐C), 151.9–120.0 (Ph),
᎐
᎐
᎐
᎐
CP
79.3 (s, N᎐C᎐C4), 79.0 (d, J = 4.8 Hz, N᎐C᎐C5); δ (CDCl ;
2
᎐ ᎐
30 ЊC) 152.3.
᎐ ᎐
CP
P
3
5 S. M. Grant and A. R. Manning, J. Chem. Soc., Dalton Trans., 1979,
1789.
6 M. Barrow, N. L. Cromhout, D. Cunningham, A. R. Manning and
P. McArdle, J. Organomet. Chem., 2000, 612, 61.
7 C. A. Tolman, Chem. Rev., 1977, 77, 313.
Crystal structure determination of complex 3
8 A. G. Orpen, L. Brammer, F. H. Allen, O. Kennard, D. G. Watson
and R. Taylor, in Structure Correlation, ed. H. B. Bürgi and J. D.
Dunitz, VCH, Weinheim, 1994, vol. 2, appendix 2.
X-Ray data were collected on a Bruker P4 diffractometer
(Mo-Kα radiation, λ = 0.71073 Å) at 294(2) K. C37H25FeO8P
(yellow plate 0.32 × 0.30 × 0.03 mm) crystallised in the triclinic
9 Fe(CO)4[η1 : η1-C(O)C(Et)C(Et)C(O)]:S.Aime,L.Milone,E.Sappa,
A. Tiripicchio and A. M. Manoti Lanfredi, J. Chem. Soc., Dalton
Trans., 1979, 1664; Fe(CO)4[η1 : η1-C(O)C(Me)C(C2Me)C(O)]: R. C.
Petterson and R. A. Levenson, Acta Crystallogr., Sect. B, 1976, 32,
723.
¯
space group P1 with unit cell parameters: a = 9.508(3),
b = 10.766(3), c = 16.973(7) Å; α = 83.97(3), β = 84.64(2),
γ = 66.84(2)Њ; V = 1586.0(9) Å3; Z = 2; µ = 0.580 mmϪ1. 5932
reflections were collected in the range 2 ≤ θ ≤ 25Њ (h, 0–11;
k, Ϫ11–12; l, Ϫ20–10); 5572 independent reflections [2863
with I > 2σ(I)] and 424 parameters were used in full-matrix
least-squares (SHELXL9717a). Data were not corrected for
absorption (four ψ-scan reflections with 5Њ increments did not
afford any significant difference in intensity). In the final
refinement cycles (SHELXS9717b) all non-hydrogen atoms
were refined with anisotropic displacement parameters. Final
R = 0.068 [I > 2σ(I)], wR2 = 0.137 (all data), goodness-of-
fit = 0.99. The max./min. residual electron density was 0.30/
Ϫ0.29 e ÅϪ3 and the final shift/error ratio was <0.001.
10 I. G. Dance and M. L. Scudder, J. Chem. Soc., Dalton Trans., 2000,
1587.
11 J.-M. Bassett, M. Green, J. A. K. Howard and F. G. A. Stone,
J. Chem. Soc., Dalton Trans., 1980, 1779.
12 J. A. S. Howell, G. Walton, M.-C. Tirvengadum, A. D. Squibb,
M. G. Palin, P. McArdle, D. Cunningham, Z. Goldschmidt,
H. Gottlieb and G. Strul, J. Organomet. Chem., 1991, 401, 91.
13 P. McArdle, J. Skelton and A. R. Manning, J. Organomet. Chem.,
1997, 538, 9; K. S. Claire, O. W. Howarth and A. McCamley,
J. Chem. Soc., Dalton Trans., 1994, 2615; M. A. Busch and R. J.
Clark, Inorg. Chem., 1975, 14, 226.
14 M. Ogasawara, S. A. Macgregor, W. E. Streib, K. Folting,
O. Eisenstein and K. G. Caulton, J. Am. Chem. Soc., 1996, 118,
10189.
CCDC reference number 154303.
See http://www.rsc.org/suppdata/dt/b0/b009849n/ for crystal-
lographic data in CIF or other electronic format.
15 (a) J. Pearson, J. Cooke, J. Takats and R. B. Jordan, J. Am. Chem.
Soc., 1998, 120, 1434; (b) T. Mao, Z. Zhang, J. Washington, J. Takats
and R. B. Jordan, Organometallics, 1999, 18, 2331.
16 P. Conway, S. M. Grant and A. R. Manning, J. Chem. Soc., Dalton
Trans., 1979, 1920.
17 (a) G. M. Sheldrick, SHELXL97, Program for Crystal Structure
Refinement, University of Göttingen, Germany, 1997; (b) G. M.
Sheldrick, Acta Crystallogr., Sect. A, 1990, 46, 467.
References
1 W. Reppe and H. Vetter, Ann. Chem. Justus Liebig, 1953, 582, 133.
2 E. Sappa, J. Organomet. Chem., 1999, 573, 139 and references
therein; W. R. Fehlhammer and H. Stolzenberg, in Comprehensive
1358
J. Chem. Soc., Dalton Trans., 2001, 1352–1358