Microwave-assisted synthesis of cyclopentadienone iron tricarbonyl complexes: molecular structures of [{η4-C4R2C(O)C4H8}Fe(CO)3] (R = Ph, 2,4-F2C6H3, 4-MeOC6H4) and attempts to prepare Fe(II) hydroxycyclopentadienyl–hydride complexes

Article abstract of DOI:10.1007/s11243-018-0229-1

Abstract: Microwave irradiation of 1,6-diynes, RC≡C(CH₂)₄C≡CR, with Fe(CO)₅ in dimethylether leads to the facile and clean formation of cyclopentadienone complexes [{η⁴-C₄R₂C(O)C₄H₈}Fe(CO)₃] in good yields resulting from a [2 + 2 + 1] cycloaddition. The molecular structures of three examples (R = Ph, 2,4-F₂C₆H₃, 4-MeOC₆H₄) have been obtained. The addition of HBF₄ leads to the clean and reversible formation of cationic hydroxycyclopentadienyl complexes [{η⁵-C₄R₂C(OH)C₄H₈}Fe(CO)₃][BF₄]. Sequential addition of hydroxide and acid has also been carried out in an attempt to prepare hydroxycyclopentadienyl–hydride complexes. These were largely unsuccessful but in one case a Shvo-type complex with a bridging hydride was detected by ¹H NMR spectroscopy. Reasons for the differing behaviour of [{η⁴-C₄(SiMe₃)₂C(O)C₄H₈}Fe(CO)₃] and the related aryl-functionalised derivatives are considered. Graphical Abstract: Microwave irradiation of 1,6-diynes, RC≡C(CH₂)₄C≡CR, with Fe(CO)₅ gives cyclopentadienone complexes [{η⁴-C₄R₂C(O)C₄H₈}Fe(CO)₃], the molecular structures of three (R = Ph, 2,4-F₂C₆H₃, 4-MeOC₆H₄) being carried out. Sequential addition of hydroxide and acid was carried out in an attempt to prepare hydroxycyclopentadienyl–hydride complexes, and while largely unsuccessful, in one case a Shvo-type complex with a bridging hydride was suggested by ¹H NMR spectroscopy.

Full text of DOI:10.1007/s11243-018-0229-1

Transition Metal Chemistry
https://doi.org/10.1007/s11243-018-0229-1
Microwave‑assisted synthesis of cyclopentadienone iron tricarbonyl
complexes: molecular structures of [{η4‑C4R2C(O)C4H8}Fe(CO)3]
(R = Ph, 2,4‑F2C6H3, 4‑MeOC6H4) and attempts to prepare Fe(II)
hydroxycyclopentadienyl–hydride complexes
Christian J. Richard1,2 · Derek Macmillan1 · Graeme Hogarth2
Received: 9 January 2018 / Accepted: 16 March 2018
© The Author(s) 2018
Abstract
Microwave irradiation of 1,6-diynes, RC≡C(CH2)4C≡CR, with Fe(CO)5 in dimethylether leads to the facile and clean forma-
tion of cyclopentadienone complexes [{η4-C4R2C(O)C4H8}Fe(CO)3] in good yields resulting from a [2+2+1] cycloaddition.
The molecular structures of three examples (R=Ph, 2,4-F2C6H3, 4-MeOC6H4) have been obtained. The addition of HBF4
leads to the clean and reversible formation of cationic hydroxycyclopentadienyl complexes [{η5-C4R2C(OH)C4H8}Fe(CO)3]
[BF4]. Sequential addition of hydroxide and acid has also been carried out in an attempt to prepare hydroxycyclopentadi-
enyl–hydride complexes. These were largely unsuccessful but in one case a Shvo-type complex with a bridging hydride was
detected by 1H NMR spectroscopy. Reasons for the difering behaviour of [{η4-C4(SiMe3)2C(O)C4H8}Fe(CO)3] and the
related aryl-functionalised derivatives are considered.
Introduction
example, the most efective and efcient class of catalysts for
the enantioselective hydrogenation of pro-chiral ketones to
chiral alcohols are the ruthenium(II) diphosphine–diamine-
based complexes developed by Noyori et al. [18, 19], and
recent practical [20, 21] and theoretical [22, 23] work has
suggested that related iron complexes may be viable alterna-
tives. The dimeric ruthenium complex (Scheme 1), widely
known as the Shvo catalyst [24–31], acts as a catalyst for
hydrogen transfer reactions between alcohols and aldehydes/
ketones, a process resulting from the scission of the Shvo
catalyst into saturated 18-electron and unsaturated 16-elec-
tron mononuclear fragments (Scheme 1).
The replacement of precious metal catalysts with cheaper
and more readily available earth-abundant metal species that
can function in a similar manner presents major challenges.
Iron catalysts are particularly attractive since not only is this
metal abundant but it is also biocompatible which limits
the concerns of toxicity and environmental impact. Con-
sequently, the development of homogeneous iron catalysts
has become a topic of intense interest with some signifcant
breakthroughs being made over the past few years [1–17].
An obvious place to start with respect to the development
of iron catalysts is the replacement of its heavier conge-
ners ruthenium and osmium. Ruthenium catalysts are used
in a number of asymmetric hydrogen transfer systems. For
Over the past few years, related iron complexes have
received some attention, with Casey and Guan reporting that
the hydroxycyclopentadienyl–hydride complex B, formed
upon sequential addition of hydroxide and proton sources to
cyclopentadienone 1 (Scheme 2), can catalyse ketone hydro-
genation and transfer hydrogenation [12]. More recently,
many new catalytic applications of cyclopentadienone 1 and
related complexes have been reported [32–45].
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11243-018-0229-1) contains
supplementary material, which is available to authorized users.
* Graeme Hogarth
The synthesis of cyclopentadienone complexes of this
type is typically achieved via the [2 + 2 + 1] cycloaddi-
tion of 1,6-hexadiynes and iron carbonyls [14–17, 46–54].
Such processes require high temperatures and long reaction
times and generally provide the desired cyclopentadienone
graeme.hogarth@kcl.ac.uk
1
Department of Chemistry, University College London, 20
Gordon Street, London WC1H 0AJ, UK
2
Department of Chemistry, King’s College London, Britannia
House, 7 Trinity Street, London SE1 1DB, UK
Vol.:(0123456789)
1 3

Transition Metal Chemistry
Ph
Ph
Ph
Ph
Ph
Ph
H
Ph
Ph
Ph
Ph
Ph
CO
O
OH
O
O
+
Ph
Ph
H
Ru
Ph
Ru
Ph
Ru
OC
OC
Ph
OC
OC
OC
Ru
H
CO
OC
Scheme 1 Proposed equilibrium between dimeric Shvo catalyst and mononuclear species
SiMe3
O
SiMe3
O
SiMe3
O
Na
H
H3PO4
NaOH
- CO2
SiMe3
H
SiMe3
CO
SiMe3
H
Fe
OC
OC
Fe
OC
OC
Fe
OC
OC
1
A
B
Scheme 2 Sequential conversion of cyclopentadienone 1 into hydroxycyclopentadienyl–hydride complex B
complexes in only low-to-moderate yields. Further, they are
often carried out in sealed tubes, thus requiring specialist
handling techniques in order to minimise risks. The use of
microwave reactors to accelerate organic transformations is
now well documented [55, 56], and there is an increasing
literature concerning the use of microwaves in organometal-
lic synthesis [57–60]. In order to develop the chemistry of
cyclopentadienone iron tricarbonyl complexes as catalysts
for hydrogen transfer reactions, we sought a simple, efcient
and high-yielding synthesis of range of these complexes and
consequently sought to develop a microwave-assisted syn-
thesis. Herein, we report the success of this route together
with the molecular structures of three of these complexes
with the aim of better understanding the correlation between
structural parameters and catalytic efficiency. We also
report attempts to prepare Shvo-type hydroxycyclopentadi-
enyl–hydride complexes upon sequential addition of hydrox-
ide and proton sources. These were largely unsuccessful,
but in one case an iron hydride was identifed by 1H NMR
spectroscopy.
of Fe(CO)5 in toluene at 125–130 °C for 24 h under 100 psi
of carbon monoxide [46–48]. Over the past 20 years, few
developments have been made to this procedure, with Wills
and co-workers recently reporting the use of similar condi-
tions for the low–moderate-yield synthesis of some related
complexes [17], although a CO pressure was not required.
We have been interested in using such complexes as cata-
lysts for a range of organic transformations but considered
the harsh reaction conditions required for their synthesis to
be detrimental to our ability to readily fne-tune the nature
of the substituents on both the cyclopentadienone ring and
also the saturated backbone. Consequently, we decided to
attempt their synthesis under microwave irradiation. For
the synthesis of cyclopentadienone iron tricarbonyl com-
plexes 1–5, we found that dimethylether provided a suitable
medium. Heating a 1:2 mixture of the diyne and Fe(CO)5 in
a microwave reactor tube for 15 min resulted after cooling,
venting, fltration and fash chromatography in the isolation
of the desired cyclopentadienone complexes [{η4-C4R2C(O)
C4H8}Fe(CO)3] (1–5) as yellow crystalline solids in 70–80%
yields (Scheme 3). Yields of 1 and 2 are signifcantly higher
than those of 57 and 52%, respectively, reported previously
[46–48], suggesting that the signifcantly lower reaction
times lead to less product decomposition. While we gen-
erally carried out reactions using 0.8 mmol of diyne, we
noted similar yields when the reaction scale was doubled
along with the irradiation time. The workup is simple and
rapid, meaning that gram quantities of these materials can
be prepared and purifed within an hour.
Results and discussion
Synthesis and characterisation
of cyclopentadienone complexes
The synthetic methodology developed by Pearson and co-
workers in the early 1990s for the conversion of 1,6-diynes,
RC≡C(CH2)nC≡CR (n=3–5) into cyclopentadienone iron
tricarbonyl complexes involves heating with a fvefold excess
Characterisation was straightforward, IR spectra being
particularly characteristic with each showing (in CH2Cl2)
1 3

Transition Metal Chemistry
considerably downfeld, appearing at 181.6 ppm. This sug-
gests that while the metal centres in the aryl (2–5)-func-
tionalised and trimethylsilyl (1)-functionalised complexes
are electronically comparable, the nature of the ketonic car-
bonyl difers which may account for observed diferences
in reactivity.
Structural studies
In an attempt to correlate binding parameters with sub-
stituents, the crystal structures of 2–4 were determined,
the results of which are summarised in Fig. 1 and Table 1.
Complex 4 contained two independent molecules in the
asymmetric unit, and pertinent bond lengths and angles
are given for both. Gross structural features are compa-
rable with those found in related iron cyclopentadienone
complexes [17, 46–48, 61, 62]. Thus, the shortest interac-
tions between the iron atom and the cyclopentadienone
ring [Fe–C 2.074–2.099 Å] are to the carbons C(6) and
C(7) which are also part of the six-membered backbone,
distances to the aryl-bound carbon atoms C(4) and C(8)
[Fe–C 2.107–2.151 Å] being on average 0.05 Å longer.
Scheme 3 Microwave-accelerated syntheses of cyclopentadienone
complexes 1–5
three strong absorptions between 2073 and 1987 cm−1
associated with the metal-bound carbonyls and a fourth at
1610–1638 cm−1 attributed to the ketonic carbonyl. NMR
spectra were in accordance with the proposed structures,
notable features being the observation of ketonic and metal-
bound carbonyl groups at 169–170 and 208–210 ppm,
respectively, in the 13C spectra of 2–5. It is perhaps notewor-
thy that while the metal-bound carbonyls in 1 also appear
within the expected range, the ketonic carbonyl was shifted
(b)
(a)
(c)
Fig. 1 Molecular structures of a 2, b 3 and c 4
1 3

Transition Metal Chemistry
Table 1 Selected bond lengths (Å) and angles (o) for cyclopentadien-
SiMe3
O
SiMe3
O
one complexes
h , MeCN, -30 oC
ν
2
3
4
6c
SiMe3
Fe
SiMe3
CO
Fe
- 2 CO
Fe–C(4)O
Fe–C(5)R
Fe–C(8)R
Fe–C(6)CH2
Fe–C(7)CH2
C(4)–O(4)
C(4)–C(5)
C(4)–C(8)
C(5)–C(6)
C(7)–C(8)
C(6)–C(7)
2.419 (2) 2.386 (2) 2.373 (4)
2.412 (4)
2.362 (9)
CO
MeCN
MeCN
OC
OC
2.137 (2) 2.107 (2) 2.141 (4)
2.114 (9)
2.131 (9)
2.057 (9)
2.053 (9)
1.237 (10)
1.484 (13)
1.482 (12)
1.440 (12)
1.418 (13)
1.408 (12)
105.2 (9)
0.257
6
1
2.151 (4)
2.122 (2) 2.140 (2) 2.139 (4)
2.147 (4)
Scheme 4 Synthesis of bis(acetonitrile) adduct of 1
2.094 (2) 2.080 (2) 2.076 (4)
2.084 (4)
2.074 (2) 2.099 (2) 2.092 (4)
2.086 (4)
where the C(6)–C(7) bond length of 1.430(2) Å is shorter
than the C(7)–C(8) interaction of 1.446(2) Å but compa-
rable to the C(5)–C(6) vector of 1.429(2) Å. This possibly
refects the better electron-withdrawing properties of the
methoxy- and fuoro-substituted arene substituents result-
ing in stronger back-bonding in these complexes.
1.223 (2) 1.228 (2) 1.240 (5)
1.237 (5)
1.490 (2) 1.480 (2) 1.487 (5)
1.502 (5)
1.492 (2) 1.494 (2) 1.496 (6)
1.487 (5)
1.429 (2) 1.437 (2) 1.453 (6)
1.444 (5)
As shown in Fig. 1, in all three complexes the aryl
rings are rotated out of the plane of the cyclopentadienone
moiety. This can be quantifed by considering the torsion
angles of the aryl rings relative to the diene unit which
vary between 17.4 and 73.0°, the smallest and largest
values both being associated with the independent mol-
ecules of 4 (Table 1). In 2 and 4 (both molecules), there
are signifcant diferences in the positions of the two aryl
rings, while in 3 both are rotated by ca. 40° with respect
to the cyclopentadienone group. In all cases, the two rings
are rotated in opposite directions. Closer inspection of
the structures shows that in each there is a close contact
between an ortho-proton on the aryl ring and the ketonic
oxygen atom, O(4). Thus in 1, O(4) lies close to both
H18A and H20A [O–H 2.573 and 2.681 Å], while in other
molecules O–H contacts range from 2.284 to 2.649 Å. In
3 there is a close contact with one proton [O(4)–H(24A)
2.325 Å] and also one of the fuorine substituents on the
other ring [O(4)–F(1) 2.968 Å].
1.446 (2) 1.440 (2) 1.439 (5)
1.432 (5)
1.430 (2) 1.429 (2) 1.421 (6)
1.425 (6)
C(5)–C(4)–C(8) 103.3 (1) 103.6 (1) 104.9 (3)
104.3 (3)
0.246
0.276
C(4)a
0.308
0.639
25.5
0.280
0.570
40.5
O(4)a
0.507
0.477
0.577
C(13)–C(18)b
C(19)–C(24)b
29.8
17.4
–
59.9
40.0
73.0
–
50.1
aDistance above C(5)–C(8) plane
bTorsion angle with respect to the C(5)–C(8) plane
c[{η4-C4(SiMe3)2C(O)C4H8}Fe(CO)(NCMe)2] (6)
The molecular structure of the trimethylsilyl deriva-
tive 1 has not been reported, but a carbonyl substitution
product, [{η4-C4(SiMe3)2C(O)C4H8}Fe(CO)(NCMe)2] (6)
(Scheme 4), is in the literature [46]. Bonding parameters
(Table 1) are very similar to those seen for the aryl-sub-
stituted complexes, suggesting that there are no signifcant
gross structural diferences between these diferent vari-
ants of the iron(0) cyclopentadienone complexes.
The carbonyl carbon, C(4), is not formally bound to the
iron centre, and iron–carbon distances [Fe–C 2.373–2.419
Å] refect this as does the bending of the carbonyl out
of the plane of the C4 ring [C(4) 0.246–0.308 Å; O(4)
0.507–0.639 Å]. Carbon–carbon distances within the
cyclopentadienone ring are more variable within the series.
In all three complexes, the bonds from C(5) and C(8) to the
carbonyl group are the longest [Fe–C 1.480–1.502 Å]. In
other complexes of this type, it is generally the central car-
bon–carbon bond, C(6)–C(7), which is the shortest within
the ring. This is also the case for 4 (both independent mol-
ecules) and 3 [C(6)–C(7) 1.421–1.429 Å; C(5)–C(6) and
C(7)–C(8) 1.432–1.453 Å] and has been attributed to the
back-bonding interaction between the metal centre and
the diene LUMO [46–48]. The situation is less clear in 2
In a number of studies, the trimethylsilyl complex 1 has
been shown to perform as a better catalyst than the phenyl-
substituted 2, and this might be attributable to the greater
steric presence of the substituents in 1 vs. those in 2. In
order to explore this, we compared space-flling diagrams
of 2 and [{η4-C4(SiMe3)2C(O)C4H8}Fe(CO)(NCMe)2] (6)
(Fig. 2). These show that the ketonic group in the latter
is more sterically encumbered than that in 2, four of the
1 3

Transition Metal Chemistry
Fig. 2 Space-flling diagrams for a 2 and b [{η4-C4(SiMe3)2C(O)C4H8}Fe(CO)(NCMe)2] (6)
2.51, 1.97 and 1.83 and similar changes were noted for other
cations. The hydroxyl proton could not be located, possibly
since it was in equilibrium with the free acid. The 13C{1H}
NMR spectrum of 7 showed the expected loss of the ketonic
carbonyl resonance at 169.7 ppm, while the metal-bound
carbonyl singlet was shifted up-feld by ca. 5 ppm, reso-
nating at 204.5 ppm. Cations 7–9 are stable in the CH2Cl2
solutions for at least 1 week. Addition of NEt3 did, however,
result in the rapid reformation of the neutral complexes,
showing that the process is fully reversible.
Scheme 5 Protonation of cyclopentadienone complexes 2–4
methyl groups lying in close proximity. In turn, this might
suggest that the ketone group in 2 is more exposed and
able to form secondary interactions with a proton, allow-
ing the close contact of two iron monomers.
Attempts to prepare Shvo‑type
hydroxycyclopentadienyl–hydride complexes
A key feature of the ability of 1 to act as hydrogen trans-
fer catalyst is its conversion to the hydroxycyclopentadie-
nyl–hydride B formed upon sequential addition of hydroxide
and proton sources (Scheme 2) [12, 13, 52]. The frst step
in this sequence is the nucleophilic attack of hydroxide at
the metal-bound carbonyl followed by subsequent elimina-
tion of CO2 in a Hieber base reaction [64–66]. The resulting
hydrido-anion, A, is then protonated at the ketonic carbonyl.
As seen above, the latter is facile for all neutral cyclopenta-
dienone complexes and thus should not vary upon changing
substituents. Nucleophilic attack of hydroxide at the metal-
bound carbonyl is the frst step and should be controlled by
the electrophilic nature of the metal-bound carbon, which in
turn should be refected by the position of these carbons in
the 13C NMR spectrum. As the chemical shift of the carbon-
yls in aryl-substituted 2–5 (208.8–209.6 ppm) are in accord
with those in 1 (209.1 ppm), it seems reasonable to expect
a similar degree of electrophilicity across these complexes.
We have attempted to prepare related hydroxycyclopentadi-
enyl–hydride complexes starting from the aryl-substituted
cyclopentadienone complexes.
Protonation experiments
An established reactivity trait of cyclopentadienone iron
tricarbonyl complexes frst being noted by Hübel et al. [63]
is their facile protonation to aford hydroxycyclopentadi-
enyl iron tricarbonyl cations. The addition of HBF4.Et2O to
CH2Cl2 solutions of 2–4 resulted in a rapid lightening of the
solution and the clean formation of cyclopentadienyl cations
7–9, respectively (Scheme 5). The nature of the protona-
tion processes was most easily probed by IR spectroscopy;
in all cases, the ketonic carbonyl peak disappeared and the
metal-bound carbonyls were shifted to higher frequencies
consistent with the development of positive charge at the
metal centre. Thus for 2, new metal-carbonyl absorptions
appeared at 2099 and 2046 cm−1, an average shift to higher
wavenumbers of ca. 40 cm−1 consistent with a signifcant
degree of positive charge localised at the iron centre. Proto-
nation could also be monitored by NMR spectroscopy, with
the addition of a slight excess of HBF4.Et2O to a CD2Cl2
solution of 2 leading to the clean formation of 7. The most
notable change to the 1H NMR spectrum was the “splitting”
of the methylene signals into two separate signals, 7 being
characterised by four equal-intensity multiplets at δ 2.73,
Following a procedure developed by Knölker et al. [52],
the addition of aqueous NaOH (0.8 mol dm−3) to THF solu-
tions of 2–5 gave red solutions containing small amounts of a
1 3

Transition Metal Chemistry
yellow precipitate. An IR spectrum of the solution generated
from 2 showed almost complete consumption of the start-
ing material and formation of new metal-bound carbonyls
at 2004 and 1942 cm−1, together with a strong absorption at
1712 cm−1. The latter corresponds to that for the free cyclo-
pentadienone which is reported to be a red oil [67, 68] and
suggests that decomplexation of the organic ligand is either
a secondary consequence of hydroxide attack or competes
with it. The free cyclopentadienones of 3–5 have not previ-
ously been reported, but the similar formation of red–purple
solutions and an IR absorption between 1707 and 1711 cm−1
suggest some decomplexation occurs in each case. Knölker
has previously reported that cyclopentadienone decomplexa-
tion is mediated by photolysis of acetonitrile solutions in air
[46] or containing Me3NO [49–51]. We also fnd that mild
heating of THF–ether solutions of 2–5 over extended periods
(12 h) leads to the formation of deep red solutions which
consist mainly of unreacted iron complex but also small
amounts (ca. 5%) of free cyclopentadienone. Decomplexa-
tion can be accelerated by microwave irradiation; heating
toluene solutions of 2–5 in the presence of four equivalents
of Me3NO results in similar amounts of free cyclopentadien-
one being generated. Thus, we conclude that the appearance
of bright red solutions is a result of the formation of small
amounts of the free ligands. A 1H NMR spectrum (CD2Cl2)
of the reaction mixture generated from 5 showed three
hydride signals at δ − 8.16, − 11.87 and − 19.48 in a ca.
1:5:3 ratio. All were broad, as was the remainder of the spec-
trum, and little further information could be gleaned, but
these signals are indicative of mixtures of terminal (δ − 8
and −11) and bridging (δ −19) hydrides. For comparison,
Knölker reports hydride signals for A and B (Scheme 2) at δ
−13.05 and −11.62, respectively (in C6D6) [52], while the
bridging hydride in the Shvo-dimer (Scheme 1) appears at δ
−17.75 in the same solvent [26]. We can also compare the
IR spectra with those reported for A in MeOH (1997, 1970,
1937 and 1904 cm−1) and B in KBr (1991 and 1932 cm−1)
[52] and conclude that at this stage the reaction mixtures are
likely to consist of three components: the aryl equivalents of
A and B and a dimeric product, most likely of a Shvo-type.
We have no further information regarding the latter and can
only speculate on its structure, with that shown in Scheme 6
appearing to be a sensible guess.
The addition of H3PO4 or HBF4 (used in NMR studies)
to the reaction mixture at this stage resulted in formation
of a clear red solution in each case. IR spectra showed the
Ar
Ar
Ar
O
O
Na
Ar
O
NaOH
H
+
Ar
Ar
Ar
Fe
Fe
Fe
- CO2
CO
H
OC
OC
OC
OC
H
OC
OC
Ar
Na
H
O
O
+
Ar
Ar
Fe
CO
Fe
CO
OC
OC
H3PO4 or HBF4.Et2O
Ar
H
Ar
O
O
Ar
Ar
Fe
CO
Fe
OC
OC
CO
H
Scheme 6 Proposed transformations upon sequential addition of NaOH and acids to cyclopentadienone complexes 2–5
1 3

Transition Metal Chemistry
generation of two new carbonyl bands coming at 1996 and
1946 cm−1 for 2. While clearly not the same species seen
in solution prior to protonation, the small shifts (− 8 and
+ 4 cm−1) and similar pattern suggest Fe(CO)2X unit(s).
that upon addition of acetone to the crude reaction mixture
in d8-toluene a signal was observed at δ −22.36 in the 1H
NMR spectrum which is the same species as we observed
(at δ −22.32 in CD2Cl2). Guan suggests that the observed
diferences between 1 and 2 are likely due to the propensity
of the bulky trimethylsilyl groups to block the formation of
dimeric products. We would broadly concur with this view,
while also noting the clear diference in stability between the
iron(II) cyclopentadienyl–hydride B and the related species
[(η5-C5H5)Fe(CO)2H] studied by Baird and co-workers [70].
The latter is rapidly degraded in the presence of oxidants via
a free-radical chain process which generates hydrogen and
the iron(I) dimer [(η5-C5H5)Fe(CO)2]2. Hence, it might be
that the stability of iron(II) hydride complexes of the general
type [(η5-cyclopentadienyl)Fe(CO)2H] is very sensitive to
the nature of the substituents on the cyclopentadienyl ligand.
Further studies are required to fully ascertain this.
1
The H NMR spectrum of the crude material after addi-
tion of acid was complex, and little information could be
obtained; however, there were no signals associated with ter-
minal hydrides, but a sharp singlet was observed at δ −22.3
attributed to a bridging hydride. Attempts were made to
separate reaction products by chromatography on silica but
without much success. Most notably, from the reaction of 5
a red band was isolated with carbonyl resonances at 2018vs,
1985 m, 1961 cm−1 in the IR spectrum, while the three
hydride resonances prior to addition of acid (see above) were
replaced by a single sharp resonance at δ −22.87. Further, in
the aromatic region of the 1H NMR spectrum, a pair of AB
doublets shows that the two aromatic groups remain equiva-
lent, while in the aliphatic region signals associated with
the methylene and methyl resonances were observed. We
suggest that these observations are consistent with conver-
sion of all three components of the reaction mixture prior to
acidifcation into Shvo-type complexes (Scheme 6). Unfor-
tunately, all our attempts to isolate pure products have been
unsuccessful.
Summary and conclusions
In this contribution, we have shown that microwave irradia-
tion of 1,6-diynes, RC≡C(CH2)4C≡CR, with Fe(CO)5 pro-
vides a simple and high-yielding route to cyclopentadienone
complexes [{η4-C4R2C(O)C4H8}Fe(CO)3] (1–5) and crystal
structures of three aryl derivatives (2–4) are reported. Proto-
nation results in the facile and reversible conversion of the
cyclopentadienone ligand into a hydroxyl-cyclopentadienyl
moiety (7–9). Sequential addition of hydroxide and acid was
carried out in an attempt to prepare hydroxycyclopentadi-
enyl–hydride complexes but was largely unsuccessful, and
these results are in line with those of Guan et al. [16]. We
also fnd that mild heating of aryl-substituted cyclopentadi-
enone complexes 2–5 over slowly leads to elimination of the
free cyclopentadienone, a process that is accelerated upon
microwave irradiation in the presence of the decarbonyla-
tion agent, Me3NO. Thus, while the trimethylsilyl deriva-
tive [{η4-C4(SiMe3)2C(O)C4H8}Fe(CO)3] (1) and related
silyl-substituted complexes fnd widespread use in cataly-
sis, seemingly similar aryl derivatives apparently generate
less stable 16-electron dicarbonyl and 18-electron hydroxy-
cyclopentadienyl–hydride species and thus are less useful
in a catalytic context. Very recently, Wills and co-workers
[71] have reported that 2 and related aryl-substituted iron
cyclopentadienone complexes are competent catalysts for
ketone reductions and alcohol oxidations. Ketone reduction
takes place under a H2 atmosphere, and in a model reaction
(4 bar H2 and UV irradiation) on a phosphine derivative,
hydroxycyclopentadienyl–hydride complexes (two diaste-
Discussion
From these studies, we believe that the aryl-substituted
cyclopentadienone iron tricarbonyl complexes 2–5 undergo
a broadly similar Hieber base reaction upon addition of
sodium hydroxide to that described by Knölker et al. for
1 [52]. A competing reaction in the case of the aryl com-
plexes is the decomplexation of the cyclopentadienone
ligand which leads to the formation of a strong red coloura-
tion of the reaction mixture and some precipitate. This is,
however, a relatively minor side reaction and may also occur
to some extent with 1, but since the free cyclopentadienone
is yellow [69], its formation is not easily detected. A second
competing reaction appears to be the “back reaction” of the
generated hydride anion with unreacted cyclopentadienone
complex to aford, after CO loss, a dimeric complex with a
bridging hydride. Such a “dimerisation” may be precluded in
the case of the trimethylsilyl derivative on the basis of steric
considerations (Fig. 2). This also becomes important upon
protonation, which in the case of 1 yields the desired hydrox-
ycyclopentadienyl–hydride B, but for the aryl derivatives
generates Shvo-type complexes instead. Guan et al. have
described similar attempts to generate an iron(II) hydride
from 2 [16]. They reported that “rapid decomposition reac-
tions precluded purifcation and full characterisation of
the desired” hydride, which at frst sight appears to con-
tradict our own observations. However, they also reported
1
reoisomers) were clearly observed by H NMR (hydride
signals at δ −12.11 and −12.18). Clearly, such species are
accessible and may be catalytically active.
1 3

Transition Metal Chemistry
(CH2), 81.8 (C–CH2), 100.3 (CAr), 113.7, 123.54, 130.9,
159.2 (Ar), 169.56 (C=O), 209.6 (CO). IR (CH2Cl2) 2062 s
Experimental
(CO), 2005 s (CO), 1991 s (CO), 1627 m (C=O) cm−1
(THF) 2057 s, 2001 s, 1981 s, 1643 m cm−1.
;
All reactions were carried out under a nitrogen atmosphere
in dried degassed solvents unless otherwise stated. Diynes
were prepared by standard methods, and Fe(CO)5 was pur-
chased from Aldrich and used as supplied. NMR spectra
were run on Bruker AC300 or AMX400 spectrometers and
referenced internally to the residual solvent peak. Infrared
spectra were run on Nicolet 205 or Shimadzu 8700 FTIR
spectrometers in a solution cell ftted with calcium fuoride
plates, subtraction of the solvent absorptions being achieved
by computation. Fast atom bombardment mass spectra were
recorded on a VG ZAB-SE high-resolution mass spectrom-
eter, and elemental analyses were performed in-house.
Microwave irradiation was carried out in a CEM 150-W
microwave reactor.
5 1H NMR (400 MHz, CDCl3): δ 1.79 (m, 4H, CH2), 2.34
(s, 6H, Me), 2.48 (m, 2H, CH2), 7.11 (d, J 7.6, 4H, Ar), 7.31
(d, J 7.6, 4H, Ar). IR (CH2Cl2) 2064 s (CO), 2006 s (CO),
1992 s (CO), 1631 m (C=O) cm−1.
1
7 H NMR (400 MHz, CD2Cl2): δ 1.83 (m, 2H, CH2),
1.97 (m, 2H, CH2), 2.51 (d, J 16.0, 2H, CH2), 2.73 (d, J 16.0,
2H, CH2), 7.58 (s, 6H, Ph), 7.62 (s, 4H, Ph). 13C NMR δ
21.4 (CH2), 21.9 (CH2), 87.5 (C–CH2), 103.3 (CPh), 124.4,
129.5, 130.6, 130.6 (Ph), 144.0 (C–OH), 209.5 (CO). IR
(CH2Cl2) 2099 s (CO), 2046 s (CO) cm−1.
8 1H NMR (400 MHz, CD2Cl2): δ 1.80 (m 2H, CH2), 1.99
(m, 2H, CH2), 2.38 (m, 2H, CH2), 2.68 (m, 2H, CH2), 7.10
(br, 4H, Ar), 7.50 (br, 1H, Ar), 7.70 (m, 1H, Ar). 19F NMR
δ −101.4 (s), −105.6 (s), −150.8 (br). IR (CH2Cl2) 2104 s
(CO), 2053 s (CO) cm−1.
Synthesis and spectroscopic data
1
9 H NMR (400 MHz, CD2Cl2): δ 1.82 (m, 2H, CH2),
1.96 (m, 2H, CH2), 2.55 (m, 2H, CH2), 2.72 (m, 2H, CH2),
3.82 (s, 6H, OMe), 7.09 (d, J 7.6, 4H, Ar), 7.58 (d, J 7.6, 4H,
Ar). IR (CH2Cl2) 2097 s (CO), 2046 s (CO) cm−1.
Diyne (0.8 mmol), Fe(CO)5 (200 μL, 1.6 mmol) and anhy-
drous dimethylether (2.5 ml) were added to a microwave
reactor tube. The tube was fushed with nitrogen and irradi-
ated for 15 min at 200 W and 140 °C in the microwave reac-
tor. The cooled reaction mixture was vented in a fume hood,
fltered and the solvent removed in vacuo. Flash chromatog-
raphy of the residue on silica gel (10% ethyl acetate/hexane)
gave two bands, the second yielding 1–5 as dry yellow solids
in 70–80% yields. Complex 1 was characterised by compari-
son with literature spectroscopic data [12, 13]. Protonation
studies were carried out by adding a slight excess of HBF4.
Et2O to CH2Cl2 or CD2Cl2 solutions of 2–4 in air. The addi-
tion of NEt3 resulted in regeneration of the cyclopentadi-
enone complexes. Attempts to generate Shvo-type species
resulted from addition of aqueous NaOH (0.8 mol dm−3) to
THF solutions of 2–5 which was followed by addition of a
slight excess of H3PO4 or HBF4 (for NMR studies).
Crystallography
Single crystals of 2–4 suitable for X-ray difraction were
grown by slow difusion of hexane into a dichlorometh-
ane solution at 4 °C. All geometric and crystallographic
data were collected at 150 K on a Bruker SMART APEX
CCD difractometer using MoKα radiation (λ = 0.71073
Å). Data reduction and integration were carried out with
SAINT + [72], and absorption corrections were applied
using the program SADABS. Structures were solved by
direct methods and developed using alternating cycles of
least-squares refnement and diference Fourier synthesis.
All non-hydrogen atoms were refned anisotropically. For 2
hydrogen atoms were located in diference maps and refned
independently; for 3–4 hydrogen atoms were placed in the
calculated positions and their thermal parameters linked
to those of the atoms to which they were attached (riding
model). The SHELXTL PLUS V6.10 program package was
used for structure solution and refnement [73]. Final dif-
ference maps did not show any residual electron density of
stereochemical signifcance. The details of the data collec-
tion and structure refnement are given in Table 2.
1
2 H NMR (400 MHz, CD2Cl2): δ 1.95 (m, 4H, CH2),
2.77 (m, 2H, CH2), 7.36–7.48 (m, 6H, Ph), 7.72 (dd, J 8.0,
0.8 Hz, 4H, Ph). 13C NMR δ 22.2 (CH2), 23.5 (CH2), 81.9
(C–CH2), 100.99 (CPh), 127.87, 128.33, 129.87, 131.59
(Ph), 169.71 (C=O), 209.24 (CO). IR (CH2Cl2) 2066 s (CO),
2009 s (CO), 1994 s (CO), 1632 m (C=O) cm−1.
1
3 H NMR (400 MHz, CD2Cl2): δ 1.84 (br, 4H, CH2),
2.59 (m, 2H, CH2), 6.97 (m, 4H, Ar), 7.60 (q, J 10.8, 2H,
Ar). 19F NMR δ −103.6 (s), −110.1 (s). 13C NMR δ 22.5
(CH2), 23.0 (CH2), 77.3 (C–CH2), 102.6 (CAr), 104.7 (t, J
26.2, Ar), 112.0 (d, J 21.0, Ar), 115.2 (d, J 16.0, Ar), 134.8
(d, J 4.0, Ar), 168.0 (C=O), 208.8 (CO). IR (CH2Cl2) 2072 s
(CO), 2016 s (CO), 2003 s (CO), 1638 m (C=O) cm−1.
Supplementary material
1
Crystallographic data for the structural analyses have been
deposited with the Cambridge Crystallographic Data Center,
CCDC Nos. 1520535 (2), 1520536 (3) and 1520539 (4).
4 H NMR (400 MHz, CD2Cl2): δ 1.63 (br, 4H, CH2),
2.77 (m, 2H, CH2), 3.85 (s, 6H, OMe), 6.96 (d, J 8.0, 4H,
Ar), 7.71 (d, J 8.0, 4H, Ar). 13C NMR δ 22.5 (CH2), 23.7
1 3

Transition Metal Chemistry
Table 2 Crystallographic
data and structure refnement
information for 2–4
2
3
4
Empirical formula
Formula weight (Å)
C24H18O4Fe
426.23
C24H14O4F4Fe
498.20
C26H22O6Fe
1945.14
Crystal system
Space group
Triclinic
P 1
Monoclinic
P21/n
Triclinic
P 1
̄
̄
a (Å)
b (Å)
c (Å)
α (°)
9.137 (1)
9.715 (1)
11.565 (1)
80.552 (2)
70.070 (2)
77.609 (2)
937.99 (18)
2
12.954 (2)
10.543 (1)
14.940 (2)
90
98.511 (2)
90
2018.4 (4)
4
1.639
11.332 (3)
13.616 (3)
16.431 (4)
105.536 (4)
92.801 (4)
110.047 (4)
2266.8 (9)
4
β (°)
γ (°)
Volume (Å3)
Z
Dcalc (Mg/m3)
Absorption coefcient (mm−1
F(000)
1.509
0.833
440
1.425
0.705
1008
)
0.815
1008
Crystal size (mm)
θ Range for data collection (°)
Index ranges
0.32×0.20×0.12
2.72–28.27
−11≤h≤12,
−12≤k≤12,
−14≤1≤14
7957
0.42×0.40×0.20
2.25–28.29
−16≤h≤17,
−13≤k≤13,
−19≤1≤19
16,599
0.18×0.18×0.03
1.67–28.25
−14≤h≤14,
−18≤k≤17,
−21≤1≤20
18,738
Refections collected
Independent refections
Data/restraints/parameters
Goodness of ft on F2
Final R indices [I>2σ(I)]
4238 [Rint =0.0211]
4238/0/334
1.050
4813 [Rint =0.0223]
4813/0/299
1.052
10,022 [Rint =0.0548]
10,022/0/595
0.963
R1 =0.0350,
R1 =0.0322,
R1 =0.0716,
wR2 =0.0979
wR2 =0.0917
wR2 =0.1794
R indices (all data)
R1 =0.0374,
wR2 =0.1000
0.528 and −0.400
R1 =0.0345,
wR2 =0.0930
0.429 and −0.412
R1 =0.1075,
wR2 =0.2050
0.942 and −1.122
Largest dif. peak and hole(e. Å−3
)
4. Enthaler S, Hagemann B, Erre G, Junge K, Beller M (2006) Chem
Asian J 1:598
Copies of this information can be obtained free of charge
from The Director, CCDC, 12 Union Road, Cambridge, CB2
1FZ, UK (E-mail: deposit@ccdc.cam.ac.uk, or on the Web
at http://www.ccdc.ac.uk).
5. Enthaler S, Junge K, Beller M (2008) Angew Chem Int Ed
47:3317
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Dalton Trans 1659
7. Bianchini C, Farnetti E, Graziani M, Peruzzini M, Polo A (1993)
Organometallics 12:3753
Open Access This article is distributed under the terms of the Crea-
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1 3