The synthesis, characterization and some reactions of cationic η2-(α,ω-diene) complexes of iron

Article abstract of DOI:10.1016/S0277-5387(00)00649-5

The cationic iron η²-(α,ω-diene) complexes of the type. [(η⁵-C₅H₅)Fe(CO)₂{η ²-(α,ω-diene)}]PF₆, (diene = 1,3-butadiene: 1.4-pentadiene; 1.5-hexadiene; and 1.7-octadiene) were prepared by the reaction of η¹-alkenyl compounds of the type [(η⁵C₅H₅)Fe(CO)₂(η ¹-alkenyl)] with the so-called trityl salt. (Ph₃CPF₆). The reactions of the η²-diene complexes with a number of nucleophiles give a range of products resulting from nucleophillic addition to the coordinated diene and/or displacement of the diene. The reaction with triethylamine results in deprotonation leading to the formation of [(η⁵-C₅H₅)Fe(CO)₂(η ¹-allyl)] complexes.

Full text of DOI:10.1016/S0277-5387(00)00649-5

www.elsevier.nl/locate/poly
Polyhedron 20 (2001) 467–476
The synthesis, characterization and some reactions of cationic
h2-(a,v-diene) complexes of iron
Dalene Dooling, Genevieve Joorst 1, Selwyn F. Mapolie *
Department of Chemistry, Uni6ersity of the Western Cape, Post Bag X17, Bell6ille 7535, South Africa
Received 3 July 2000; accepted 21 November 2000
Abstract
The cationic iron h2-(a,v-diene) complexes of the type, [(h5-C5H5)Fe(CO)2{h2-(a,v-diene)}]PF6, (diene=1,3-butadiene; 1,4-
pentadiene; 1,5-hexadiene; and 1,7-octadiene) were prepared by the reaction of h1-alkenyl compounds of the type [(h5-
C5H5)Fe(CO)2(h1-alkenyl)] with the so-called trityl salt, (Ph3CPF6). The reactions of the h2-diene complexes with a number of
nucleophiles give a range of products resulting from nucleophillic addition to the coordinated diene and/or displacement of the
diene. The reaction with triethylamine results in deprotonation leading to the formation of [(h5-C5H5)Fe(CO)2(h1-allyl)]
complexes. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Diene complexes; Cationic dienes; h2-Dienes; Olefinic complexes; Iron-dienes
1. Introduction
analogues are sparse. Only a few examples of these
types of complexes are known and very little has been
reported on the reactivity of these species. The reactiv-
ity of the monosubstituted h2-alkene iron complexes of
the type, [h5-C5H5)Fe(CO)2(h2-CH2ꢀCHR)]+, (R=H,
CH3, Ph), has been extensively studied and reported in
the literature [2,4]. Some reactions of the cationic [(h5-
C5H5)Fe(CO)2(butadiene)]+ complex with nucleophiles
have been briefly reported by Rosenblum and co-work-
ers [5]. However, no reports on the reactivity of other
acyclic [(h5-C5H5)Fe(CO)2(h2-diene)]+ complexes have
been encountered. These compounds could be impli-
cated in organic synthesis [4,6–8] and could also serve
as potential model compounds for intermediates in
some catalytic reactions [7,9]. They could for example
play a role in the polymerization and dimerization of
dienes. It is in light of this that we have embarked on a
preliminary study of some of these complexes. Conse-
quently we have prepared a series of compounds with
the general formula [(h5-C5H5)Fe(CO)2(CnH2n−2)](PF6)
and we have investigated the reactivity of these with
nucleophiles. Some of these results together with char-
acterization details of the new compounds are reported
in this paper.
Cationic h2-(a,v-diene) complexes are compounds
which have a diene ligand p bonded to a cationic metal
centre via only one of the CꢀC double bonds. The
general structure for such cationic diene complexes is
shown in Fig. 1.
Although cationic h2-alkene complexes of iron [1–4]
have been extensively studied, reports on the diene
Fig. 1. General structure for h2-diene complexes.
* Corresponding author. Tel./fax: +27-21-9593056.
E-mail address: smapolie@uwc.ac.za (S.F. Mapolie).
1 Present address: SASTECH Ltd., PO Box 1, Sasolburg, South
Africa.
0277-5387/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0277-5387(00)00649-5

468
D. Dooling et al. / Polyhedron 20 (2001) 467–476
2. Experimental
2.1.2. Reactions of 1 and 2 with NaCl and NaI
A solution of compound 1 (0.20 g, 0.53 mmol) in dry
acetone (5 ml) was added dropwise to a solution of
sodium chloride (0.03 g, 0.53 mmol) dissolved in dry
acetone (5 ml). The reaction mixture was allowed to stir
at room temperature for 3 days. The yellow reaction
mixture turned an intense orange colour during this
period. The solvent was removed on a rotatory evapo-
rator leaving an orange solid. The solid was dissolved
in a minimum amount of dichloromethane and the
solution was chromatographed over alumina using
CH2Cl2 as eluent. An orange band was collected, which
on the removal of the solvent gave an orange solid,
identified by its IR and 1H NMR spectra as [(h5-
C5H5)Fe(CO)2Cl] (86% yield). The corresponding iron-
pentadiene compound, 2 undergoes a similar reaction,
yielding the same product in 89% yield. Analogous
results were obtained using NaI in the place of NaCl.
2.1. General procedures
All experiments, unless otherwise stated, were carried
out under nitrogen using typical Schlenk line tech-
niques. [(h5-C5H5)Fe(CO)2]2, the n-bromo-1-alkenes,
triphenylcarbenium hexaflourophosphate, triphenyl-
phoshine, trimethylphospine, triethylamine and diethyl-
malonate were used as purchased without further
purification. Tetrahydrofuran was distilled from
sodium/benzophenone
ketyl
under
nitrogen.
Dichloromethane was dried by refluxing over P2O5 and
distilling under nitrogen. All solvents were maintained
over molecular sieves, and degassed prior to use. All
column chromatography was performed using deacti-
vated alumina 90 (70–230 mesh) purchased from
Merck. Infrared spectra were recorded on a Perkin-
Elmer Paragon 1000 FT-IR spectrophotometer using
either NaCl solution cells or as DRIFTS spectra in a
KBr matrix. NMR spectra were recorded on a Varian
Gemini 2000 spectrometer operating at 200 MHz for
1H NMR and 50.30 MHz for 13C NMR, using te-
tramethylsilane as an internal standard. Elemental
analyses were performed at the micro-analytical labora-
tory of the University of Western Cape.
2.1.3. Reactions of 1–4 with tertiary phosphines
A solution of compound 1 (0.20 g, 0.53 mmol) in dry
acetone (5 ml) was added dropwise to a solution of
triphenylphosphine (0.14 g, 0.53 mmol) dissolved in dry
acetone (5 ml). The resulting pale yellow solution was
stirred under a constant flow of nitrogen for 1 h at
room temperature after which the solvent was removed
by rotatory evaporation. The resulting pale yellow crys-
talline solid obtained was recrystallized by dissolving in
a minimum amount of acetone (6 ml) followed by the
dropwise addition of diethyl ether (20 ml). The product
was collected on a Hirsch funnel as fine pale yellow
crystals and identified as [(h5-C5H5)Fe(CO)2-
(PPh3)][PF6] in a 92% yield. Similar reactions were
carried out using compounds 2–4 as the starting mate-
rial resulting in the same product in 90–95% yields. The
above reactions were repeated in a sealed Schlenk tube
previously flushed with nitrogen and the reaction time
extended to 1 day. Compounds 2–4 yielded the same
product viz. [(h5-C5H5)Fe(CO)2(PPh3)][PF6]. Com-
pound 1 however yielded a brown oil identified as
[(h5-C5H5){CH2CHꢀCHCH2PPh3}]+, compound 5.
With PMe3 a complex mixture of h1-alkenyl complexes
were formed on reaction with 1, evident by the appear-
ance of a number of Cp resonances (| 4.7–5.1) in the
1H NMR spectrum of the reaction product.
The [(h5-C5H5)Fe(CO)2{h1-alkenyl}] compounds
were prepared from [Fe(h5-C5H5)(CO)2] using literature
methods [10]. The h1-alkenyl products were isolated as
yellow oils and were used immediately after purification
by column chromatography to prepare the h2-(a,v-di-
ene) complexes.
2.1.1. General method for the preparation of the diene
complexes, [(p5-C5H5)Fe(CO)2(CnH2n−2)]PF6; where
n=4–6, 8 (1–4)
The preparation of the h2-butadiene complex 1 is
described below to illustrate the general procedure
employed.
A
solution
of
[(h5-C5H5)Fe(CO)2{CH2CH2-
CHꢀCH2}] (0.21g, 1.03 mmol) in CH2Cl2 (5 ml) was
added drop-wise to a solution of Ph3CPF6 (0.45g, 1.59
mmol) dissolved in CH2Cl2 (10 ml). The initial orange/
yellow solution turned black with a yellow tint. The
mixture was allowed to stand for 18h at room tempera-
ture. During this time a yellow crystalline material
precipitated out of solution. Acetone (10 ml) was added
to the mixture upon which the solid dissolved. The slow
addition of diethyl ether (40 ml) afforded a yellow
solid, which was collected on a Hirsch funnel and
allowed to dry in air. The product was recrystallized by
dissolving in a minimum amount of acetone, followed
by the addition of diethyl ether. The resulting yellow
solid was obtained in 49% yield.
The corresponding iron-pentadiene compound 2 un-
dergoes a similar reaction with PMe3, also yielding a
complex mixture of products.
2.1.4. Reactions of 2–4 with triethylamine
The reaction of compound 2 with triethylamine is
described here but the procedure has also been applied
to compounds 3 and 4, which gave similar results.
A suspension of compound 2 (0.20 g, 0.51 mmol) in
10 ml CH2Cl2 was treated dropwise with triethylamine

D. Dooling et al. / Polyhedron 20 (2001) 467–476
469
Scheme 1.
Table 1
Yields, micro-analytical and infrared spectral data for compounds, [(h5-C5H5)Fe(CO)2{h2-CHꢀCH(CH2)nCHꢀCH2}] a
Analysis (%) b
IR data c
n
Yield(%)
C
H
(C)
(H)
w(CO) (cm−1
)
w(CꢀC) (cm−1
)
1
2
3
4
0
1
2
4
49
46
40
50
35.12
36.94
38.63
41.69
2.93
3.34
3.72
4.46
35.14
36.95
38.64
41.69
2.95
3.36
3.74
4.43
2080(s)
2080(s)
2075(s)
2077(s)
2049(s)
2048(s)
2043(s)
2045(s)
1624(m)
1639(m)
1639(m)
1640(m)
1517(m)
1518(m)
1519(m)
1521(m)
a n=0−2; 4.
b Calculated values in parentheses.
c Measured as DRIFTS in a KBr matrix, s=strong; m=medium.
(0.07 ml, 0.51 mmol). The yellow solution turns brown
immediately and the iron-diene complex goes into solu-
tion upon addition of Et3N. The mixture was allowed
to stir at room temperature for 45 min. The solvent was
removed from the reaction mixture by rotatory evapo-
ration, leaving a brown residue. The residue was ex-
tracted with ether, filtered and the solvent removed
from the filtrate. The remaining brown oil was dis-
solved in a minimum amount of hexane and chro-
matographed on an alumina column using hexane as
eluent. Only one yellow band was observed, which
upon removal of the solvent yielded a yellow–brown oil
identified as the product [(h5-C5H5)Fe(CO)2(CH2-
CHꢀCHCHꢀCH2)], (compound 6). In a similar manner
compounds 7 and 8 were isolated in 40 and 45% yield,
respectively, using compounds 3 and 4 as starting mate-
rials.
2.1.5. Reactions of 1–4 with diethylmalonate
The reaction of compound 1 with diethylmalonate is
described below. Again the procedure was also ex-
tended to compounds 2–4.
To a solution of Li[N(SiMe3)2] in THF at −78°C
was added diethylmalonate (0.08 ml, 0.51 mmol). After
15 min, the resultant clear solution of the diethyl-
malonate anion was added to a suspension of com-
pound 1 (0.20 g, 0.51 mmol) in THF (10 ml) at −78°C.
The reaction mixture was stirred at this temperature
and then allowed to warm to room temperature over a
period of 3 h. The solution remains orange/yellow

470
D. Dooling et al. / Polyhedron 20 (2001) 467–476
throughout the reaction. The solvent was removed by
rotatory evaporation leaving a yellow/brown oil. The
oil was dissolved in a minimum amount of CH2Cl2 and
the solution chromatographed on a neutral alumina
column. Elution with CH2Cl2 gave a yellow band,
which on the removal of the solvent, yielded the
product as a yellow/brown oil. This was identified as an
h1-alkenyl compound, with a malonate substituent at
the b position of the chain (compound 9, 84% yield).
Products 10–12 were isolated in 80–86% yields start-
ing from the corresponding compounds 2–4 and using
a similar procedure.
Table 2
1H and 13C NMR. data for the compounds 1–4
1H NMR (l a/ppm)
13C NMR (l a/ppm)
52.71 Fe(h2-C
88.05 Fe(h2-CH2ꢀC
1
3.85 (d, 1H, J3,2,trans=14 Hz)
6 H2ꢀCH)
Fe(h2-CH2
6
ꢀCH)
4.04 (d, 1H, J3,1,cis=7.2 Hz)
Fe(h2-CH2
ꢀCH)
6
H)
6
5.54 (m, 1H)
90.22 C6 5H5
Fe(h2-CH2ꢀCH)-CH
6
ꢀCH2
5.90 (m, 8H)
124.66
Fe(h2-CH2ꢀCH)CHꢀC
137.71
Fe(h2-CH2ꢀCH)C
Fe(h2-CH2ꢀCH
6
)-CHꢀCH
6
2, C5H5
6
6
H2
6
HꢀCH2
209.33 C
6
O
O
211.22 C
6
3. Results and discussion
2
2.50 (m, 1H)
40.15 Fe(h2-CH2ꢀCH)-
H2CHꢀCH2
54.72 Fe(h2-C
Fe(h2-CH2ꢀCH)-CH
6
2CHꢀCH2
C6
3.1. Diene complexes
3.18 (m, 1H)
6
H2ꢀCH)
Fe(h2-CH2ꢀCH)-CH
6
2CHꢀCH2
3.68 (d, 1H, J3,2 trans=14.6 Hz)
Fe(h2-CH2
ꢀCH)
4.15 (d, 1H, J3,1 cis
Fe(h2-CH2
ꢀCH)
,
85.82 Fe(h2-CH2ꢀC
6 H)
3.1.1. Preparation of diene complexes
The preparation of the complexes, [(h5-C5H5)Fe-
(CO)2{h2-(a,v-diene)}]PF6, was carried out by the reac-
tion of the appropriate [(h5-C5H5)Fe(CO)2{h1-alkenyl}]
[10,11] with triphenylcarbenium–hexaflourophosphate,
(Ph3CPF6). The overall reaction scheme for the prepa-
ration of the h2-diene complexes is outlined in Scheme
1.
6
,
=8.4 Hz)
90.26 C
6 5H5
6
5.19 (m, 3H)
118.02 Fe(h2-CH2ꢀCH)-
Fe(h2-CH2ꢀCH
6
)-CH2CHꢀCH2
6
CH2CHꢀCH2
137.07 Fe(h2-CH2ꢀCH)-
CH2CHꢀCH2
209.33 C
211.22 C
6
5.85 (m, 1H)
Fe(h2-CH2ꢀCH)-CH2CH
6
ꢀCH2
6
5.91 (s, 5H) C5H5
6
6
O
O
6
Compounds 1 and 4 have previously been reported.
The authors however prepared these compounds via a
different synthetic route [6,12–14], which involved a
direct ligand exchange reaction of the compound, [(h5-
C5H5)Fe(CO)2(THF)]+ with the free diene. In addition,
the yields of the compounds prepared by this route are
quite low. Also complete characterisation data were not
given.
All the compounds were isolated as yellow air stable
solids, which show thermal decomposition in the range
150–160°C. These compounds were fully characterized,
by NMR and IR spectroscopy, the data for which are
presented in Tables 1–3.
The infrared spectral data for compounds 1–4 are
summarized in Table 1. All the compounds show two
strong w(CO) bands at 2077 cm−1 and 2040 cm−1 for
the terminal carbonyls. The w(CO) bands for com-
pounds 1–4 appear at higher frequencies, relative to
those of the corresponding h1-alkenyl compounds, [(h5-
C5H5)Fe(CO)2{(CH2)nCHꢀCH2}], (n=2–4, 6). The lat-
ter compounds have two strong w(CO) bands at 2000
cm−1 and 1940 cm−1. All the compounds with the
exception of compound 1 give IR spectra which exhibit
two bands of medium intensity at ꢀ1518 and ꢀ1640
cm−1. These can be assigned to w(CꢀC) of the coordi-
nated and uncoordinated CꢀC double bonds of the
diene, respectively. In the case of compound 1 the
corresponding bands are observed at 1517 and 1624
cm−1. The distinctive difference in the w(CꢀC) of the
uncoordinated CꢀC double bond for compound 1 can
3
1.65–2.62 (4 x m, 4H)
Fe(h2-CH2ꢀCH)-(CH2)2CHꢀCH2 Fe(h2-CH2ꢀCH)(C
CHꢀCH2
55.26 Fe(h2-C
36.39; 36.93
6
H2)2-
3.64 (d, 1H, J3,2 trans
,
=14.4 Hz)
6
H2ꢀCH)
Fe(h2-CH2
4.07 (d, 1H, J3,1 cis
Fe(h2-CH2
ꢀCH)
5.04 (m, 2H)
Fe(h2-CH2ꢀCH)-(CH2)2CHꢀCH2
6 ꢀCH)
,
=8.4 Hz)
87.61 Fe(h2-CH2ꢀC
6 H)
6
90.09 C6 5H5
6
5.24 (m, 1H)
116.40
)-(CH2)2CHꢀCH2 Fe(h2-CH2ꢀCH)(CH2)2-
CHꢀCH2
137.39
ꢀCH2 Fe(h2-CH2ꢀCH)(CH2)2-
HꢀCH2
Fe(h2-CH2ꢀCH
6
6
5.81 (m, 1H)
Fe(h2-CH2ꢀCH)-(CH2)2CH
6
C6
5.91 (s, 5H) C5H5
6
209.21 C
6
O
O
211.42 C
6
4
1.50–2.70 (3 x m, 8H)
Fe(h2-CH2ꢀCH)-(CH2)4CHꢀCH2 Fe(h2-CH2ꢀCH)(C
CHꢀCH2
)=14.6 Hz) 54.88 Fe(h2-C
30.73–36.77
6
H2)4-
3.64 (d, 1H, J3,2 trans
Fe(h2-CH2
ꢀCH)
4.06 (d, 1H, J3,1 cis
Fe(h2-CH2
ꢀCH)
4.95 (m, 2H)
Fe(h2-CH2ꢀCH)-(CH2)4CHꢀCH2
,
6
H2ꢀCH)
6
,
)=8.2 Hz)
88.88 Fe(h2-CH2ꢀC
6 H)
6
90.07 C6 5H5
6
5.31 (m, 1H)
114.95
)-(CH2)4CHꢀCH2 Fe(h2-CH2ꢀCH)(CH2)4-
CHꢀCH2
139.18
ꢀCH2 Fe(h2-CH2ꢀCH)(CH2)4-
HꢀCH2
209.27 CO
211.50 C
Fe(h2-CH2ꢀCH
6
6
5.81 (m,1H)
Fe(h2-CH2ꢀCH)-(CH2)4CH
6
C6
5.90 (s, 5H) C5H5
6
6
O
a Acetone-d6 as solvent.

D. Dooling et al. / Polyhedron 20 (2001) 467–476
471

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D. Dooling et al. / Polyhedron 20 (2001) 467–476

D. Dooling et al. / Polyhedron 20 (2001) 467–476
473
be ascribed to the fact that the two double bonds are
conjugated. The band assigned to w(CꢀC) at 1518 cm−1
is absent in the IR spectra of the parent alkenyl
complexes and we propose that this peak is due to the
double bond of the diene ligand, which is bonded to the
metal in an h2-fashion.
intense peak at lꢀ90 ppm observed in all the spectra
is due to the cyclopentadienyl resonance.
3.2. Reacti6ity of diene complexes
The reactions of cationic dicarbonyl h5-cyclopentadi-
enyl(olefin) complexes with nucleophiles have previ-
ously been reported to proceed via one of the following
pathways: (a) addition to the olefin double bond; (b)
addition to the carbonyl carbon; and (c) displacement
of the olefin. In addition to these reactions, allylic
deprotonation with tertiary amines (d) is another possi-
ble reaction. This leads to the formation of h1-allyl
complexes [1,2,16]. The displacement of the coordinat-
ing olefin and the formation of the iron dimer, [(h5-
C5H5)Fe(CO)2]2 were found to compete effectively with
the desired addition reaction. Treatment of the cationic
diene complexes 1–4, with various nucleophiles were
found to proceed via pathways (a), (c) or (d). The
analytical and spectral data of the products of these
reactions are summarized in Table 3 and are discussed
below.
1
The H NMR spectra for all the compounds show
signals, which are characteristic of species containing a
vinylic functionality. The assignments of the vinylic
protons for compounds 1–4 are based on the structure
in Fig. 2 and are listed in Table 2.
The spectra of all the compounds show two doublets
in the region l3.6–3.9 ppm and l4.0–4.2 ppm. These
signals were assigned to the coordinated vinylic protons,
H1 and H2, respectively. These protons have trans (J3,2
)
and cis (J3,1) coupling constants of ꢀ14 and 8 Hz,
respectively. The chemical shift difference between
protons H1 and H2 are ꢀ0.4 ppm in all the compounds
except compound 1. The butadiene compound shows a
lower chemical shift difference of 0.2 ppm for the
coordinated vinylic protons. Similar results for other
p-coordinated a-olefins were reported previously.
Rosenblum et al. ascribed the large chemical shift
differences to the anisotropic effects associated with the
cyclopentadienyl ring currents. However effects of the
CO ligands may also be significant [15]. The signals in
the region l4.9–6.0 ppm, with the exception of the
sharp broad peak around l5.9 ppm, were assigned to
the protons H5 and H6 of the uncoordinated vinylic
protons. The sharp broad peak around l5.9 ppm
observed in all the spectra were assigned to the
cyclopentadienyl protons. The corresponding peak for
the cyclopentadienyl protons of the [(h5-C5H5)Fe-
(CO)2{h1-alkenyl}] compounds appears around l4.7
ppm. This downfield chemical shift is again due to the
influence of the cationic metal centre, which draws
electrons from the neighboring Cp ring, hence leading to
the deshielding of the Cp protons.
3.2.1. Reactions with halides
3.2.1.1. Reactions of 1 and 2 with NaCl and NaI. The
reaction of 1 and 2 with NaCl in an acetone solution
leads to the exclusive formation of CpFe(CO)2Cl (5).
The product was isolated as a red, air stable crystalline
solid. The reaction was easily monitored by IR spec-
troscopy by following the replacement of the carbonyl
bands for the Fp-h2-(a,v-diene) complexes, 1 and 2 at
2045 and 2080cm−1 with two strong carbonyl stretch-
ing frequencies at 2040 and 2000 cm−1 for the product.
Using this method, it was found that the reaction was
1
complete after 3 days. The H NMR of the product
exhibits a single strong peak at l5.25 ppm, which is
assigned to the cyclopentadienyl protons. This corre-
lates well with data obtained from an authentic sample
of CpFe(CO)2Cl. Analogous results were obtained with
sodium iodide in the place of the chloride, with
CpFe(CO)2I being formed in good yields.
The 13C NMR spectra for the series of [(h5-C5H5)Fe-
(CO)2{h2-(a,v-diene)}]PF6 complexes, (1–4), were all
recorded in acetone and are briefly discussed here. The
assignment of the carbonyls and vinylic carbons are
based on the structure in Fig. 3.
Two carbonyl resonances are observed at l209 ppm
and 201 ppm. It has previously been reported that
monosubstituted olefin complexes exhibit two CO
resonances due to steric interactions between the
carbonyl group and the olefin ligand resulting from a
conformation in which the olefin is near the CpFe(CO)2
moiety. This causes the CO to have different chemical
environments [15].
The p-coordinated vinylic carbons Ca and Cb were
assigned to the peaks in the region l52–56 ppm and
l85–89 ppm, respectively. The uncoordinated vinylic
carbons were assigned to the peaks in the region of
l114–125 ppm and l137–139 ppm, respectively. The
3.2.2. Reactions with phosphines
3.2.2.1. Reaction of 1–4 with triphenylphosphine. For
the reactions of compounds 1–4 with PPh3 under a
constant flow of nitrogen, the displacement product,
[CpFe(CO)2(PPh3)][PF6], was isolated in all cases. These
reactions take place rapidly at room temperature and
the product was isolated as a pale yellow, air stable
crystalline solid after a reaction time of 2 h.
Repeating the above reactions using a sealed system,
gave a different result for compound 1, the reactions of
compounds 2–4 giving the same results as obtained
previously. In the case of compound 1, the reaction

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D. Dooling et al. / Polyhedron 20 (2001) 467–476
Fig. 2. Labelling of protons to assist 1H NMR assignments for the
complexes 1–4.
Scheme 3.
3.2.2.2. Reactions of 1 and 2 with PMe3. The reactions
of 1 and 2 with PMe3 take place rapidly at room
temperature resulting in a complex mixture of h1-
alkenyl species, evident by the appearance of a number
of Cp resonances (| 4.7–5.1ppm) in the 1H NMR
spectra of the reaction products. The appearance of
vinylic protons suggests that the reactions proceed via
the addition of the nucleophile to the coordinated
diene. Our observations are in agreement with reports
in the literature, where it was found that the Cp signals
for the products of addition reactions appear in the
range between l4.9–5.1 ppm [2]. Due to the complex
Fig. 3. Labelling of the carbon atoms to assist 13C NMR assignments
for the complexes 1–4.
proceeds via an addition mechanism resulting in the
formation of the cationic species, Fp-CH2CHꢀ
CHCH2−+PPh3, (compound 5), which was isolated as
1
an unstable yellow/brown oil. The H NMR spectrum
of this yellow brown oil shows two distinct signals in
the vinylic region, at l5.0 ppm and l6.2 ppm, respec-
tively. The signals resonate for one proton each. Of the
possible products of addition of triphenylphosphine to
1, compound 5 is the only one which has an internal
CꢀC functionality with each carbon having a single
proton. The above reaction was also followed in a
sealed NMR tube and this yielded similar results. Mon-
1
nature of the H NMR spectra, it was not possible to
identify unequivocally any of the products.
3.2.3. Reactions with triethylamine
1
itoring the reaction using H NMR revealed that the
The treatment of the diene compounds (2–4) with
triethylamine results in the deprotonation of the diene
ligand producing Fp-(h1-allyl) complexes as shown in
Equation (1). The reactions proceed fairly rapidly at
room temperature in dichloromethane and are complete
within 45 min. The progress of the reaction was easily
monitored by following the replacement of the carbonyl
bands for the Fp-h2-(a,v-dienes) (2–4) at ꢀ2045 and
2080 cm−1 and comparing it with those of the neutral
addition product was formed via a rearrangement pro-
cess as shown in Scheme 2. Unfortunately compound 5
was found to be highly unstable with the result that we
were unable to isolate it in a pure state. We were thus
not able to fully characterise it.
We are also not entirely sure why a different reaction
product is obtained when the reaction is carried out in
a sealed system as opposed to doing the reaction under
a constant flow of nitrogen. It is however speculated
that the butadiene complex undergoes some dissocia-
tion in solution to produce the free diene ligand (some
NMR evidence for this). Butadiene being volatile is
then removed in the nitrogen stream aiding further
dissociation of the starting diene complex. This leads to
a lowering of the possibility of the addition product
being formed. In the sealed system the equilibrium in
Scheme 3 is more to the left leading to the addition
reaction being favoured.
Fp-h1-(a,v-allyl) products at ꢀ1950 and 2005 cm−1
.
The products were isolated as relatively unstable yel-
low/brown oils.
(1)
The reactions of the Fp-h2-(a,v-dienes) (2–4) with
triethylamine are in agreement with those reported for
the related alkene complexes. [(h5-C5H5)Fe(CO)2(h2-
Scheme 2.

D. Dooling et al. / Polyhedron 20 (2001) 467–476
475
alkene)]+ complexes were reported to undergo deproto-
nation in the presence of a tertiary amine base, yielding
cis and trans (h1-allyl) complexes. The deprotonation
reaction is highly regiospecific and proceeds via prefer-
ential removal of an allylic proton trans to the metal-
olefin bond to give mixtures of cis and trans isomers
[1,4,8,16]. This is best illustrated by the reaction of
[(h5-C5H5)Fe(CO)2(h2-butenyl)] with triethylamine as
shown in Equation (2).
tained from direct addition at Cb as shown in Equation
(3). They exhibit two strong w(CO) bands for the
terminal carbonyls at 2004 and 1944 cm−1
.
(3)
The assignments of the signals in the NMR spectra
of compounds 9–12 are based on the structure in Fig.
5.
The 1H NMR spectra of all the products exhibit
signals for the three vinylic protons of Cc and Cd in the
region between l5.0–6.0 ppm. These are typical of
what is observed for the h1-alkenyl compounds and
also correspond to the position of the uncoordinated
vinylic protons for the cationic Fp-h2-(a,v-diene) com-
pounds 1–4. The two single proton multiplets at l1.0
and l1.7 ppm are due to the to Fp-CH2 protons at Ca.
The intense peak at l4.7 ppm, observed in all the
spectra was assigned to the cyclopentadienyl protons.
The addition of enolates to Fp-(olefin) cations has
been reported to be facile with uncomplicated side
reactions. Nucleophillic addition to the butadiene com-
plex 1 was reported to take place either by direct or
conjugate addition to the coordinated double bond.
With lithium dimethylmalonate, a single product was
formed in 86% yield derived from direct addition at C2
[6]. In our hands similar results were obtained for the
reactions of the cationic diene complexes 1–4 with
diethylmalonate.
(2)
The cyclopentadienyl signals for the cis and trans
[(h5-C5H5)Fe(CO)2(h1-butenyl)] isomers in the above
reaction were reported to appear at l4.65 and l4.58
ppm, respectively. Also, two protons in the region
l4.8–5.9 ppm were observed for the cis and trans
methylene protons at C [16]. A similar pattern was
found in the H NMR spectra of compounds 6–8. The
two distinct singlets in the region at l4.65–4.75 ppm,
assigned to the cyclopentadienyl protons for the h1-
(a,v-diene)iron complexes 6–8 are suspected to be due
to the cis and trans isomers. The two resonances at l5.0
ppm and l5.3 ppm could possibly be assigned to the cis
and trans methylene protons. A single multiplet at
ꢀl2.12 ppm was observed for the FpCH2 protons in
the Fp-h1-(a,v-diene) complexes. A multiplet at ꢀ
l5.7 ppm, obscured by the proton for Cd, was assigned
to the proton at Cb. The terminal vinylic protons at Cd
and Ce remain unaffected, as expected (Fig. 4).
ꢀ
1
4. Conclusion
3.2.4. Reactions with lithium diethylmalonate
The diethylmalonate anion, generated by treatment
of diethylmalonate with lithium bis(trimethylsilyl)amide
Li[N(SiMe3)2], reacts readily with a suspension of the
cationic dienyl complexes 1–4 in THF at −78°C. The
products were isolated in high yields as amber oils and
identified as products 9–12. The compounds are ob-
A series of cationic iron h2-(a,v-diene) complexes of
the type, [(h5-C5H5)Fe(CO)2{h2-(a,v-diene)}]PF6 have
been successfully prepared. These compounds were iso-
lated as yellow air stable solids, which decompose in the
range 150–160°C. Treatment of these cationic species
1–4, with nucleophiles were found to proceed via two
general routes: (a) addition to the coordinated diene
and (b) ligand substitution (displacement) of the diene.
Reactions with triethylamine lead to the deprotonation
1
of the diene. Microanalyses, FTIR, H NMR, and 13C
NMR spectroscopy were employed to characterize all
new compounds.
Fig. 4. Labelling of proton atoms to assist 1H NMR assignments for
the h1-(a,v-diene)iron complexes 6–8.
Acknowledgements
We gratefully acknowledge the financial support of
Polifin Ltd., SASOL Ltd., The National Research
Foundation of South Africa and the Research Commit-
tee of the University of the Western Cape.
Fig. 5. Labelling of proton atoms to assist 1H NMR assignments for
complexes 9–12.

476
D. Dooling et al. / Polyhedron 20 (2001) 467–476
[8] D.A. White, Organomet. Chem. Rev. 3 (1968) 497.
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