Synthesis, UV/vis spectra and electrochemical characterisation of arylthio and styryl substituted ferrocenes

Article abstract of DOI:10.2478/s11532-011-0040-8

Two series of substituted ferrocenes were synthesised using either the Horner-Wadswor th-Emmons reaction or monolithiation of ferrocene. The series consist of arylthio- and styryl-ferrocenes with different substituents in the para position of the aryl rings of the systems. The electronic communication was investigated by comparing the substituent effects in absorption spectroscopy and in cyclic voltammetry. A small substituent effect was found in the electronic transitions of the styryl substituted ferrocenes. The oxidation of the ferrocene derivatives showed clear substituent effects as illustrated by the linear Hammett plots. The effect was shown to be an order of magnitude larger in the arylthio-systems than in the styryl systems. It is suggested that the reason behind the large effect is a direct sulfur-iron orbital overlap. Versita Sp. z o.o.

Full text of DOI:10.2478/s11532-011-0040-8

Cent. Eur. J. Chem. • 9(4) • 2011 • 610-618
DOI: 10.2478/s11532-011-0040-8
Central European Journal of Chemistry
Synthesis, UV/vis spectra and electrochemical
characterisation of arylthio and styryl
substituted ferrocenes
Research Article
*
Thomas J. Sørensen , Merete F. Nielsen
Nano-Science Center and Department of Chemistry,
University of Copenhagen, 2100 København Ø, Denmark
Received 5 January 2011; Accepted 27 February 2011
Abstract: Two series of substituted ferrocenes were synthesised using either the Horner-Wadsworth-Emmons reaction or monolithiation
of ferrocene. The series consist of arylthio- and styryl-ferrocenes with different substituents in the para position of the aryl rings
of the systems. The electronic communication was investigated by comparing the substituent effects in absorption spectroscopy
and in cyclic voltammetry. A small substituent effect was found in the electronic transitions of the styryl substituted ferrocenes.
TheoxidationoftheferrocenederivativesshowedclearsubstituenteffectsasillustratedbythelinearHammettplots. Theeffectwasshown
to be an order of magnitude larger in the arylthio-systems than in the styryl systems. It is suggested that the reason behind the large
effect is a direct sulfur-iron orbital overlap.
Keywords: Electrochemistry • Lithiation • Ligand effects • Ferrocene
©
Versita Sp. z o.o.
1
. Introduction
cheap starting materials. Direct functionalisation of
ferrocene is possible by many methods, but few are
Ferrocene is a quite unique molecule. Among its many so selective that only the monosubstituted derivative is
interesting properties some stand out, as they have obtained. Direct monolithiation of ferrocene has been
been thoroughly investigated and exploited. Ferrocene reported by several groups, but no generally applicable
is a stable yet easily oxidised molecule, which undergo method has so far been reported [7-10]. Here, we
chemical reactions like a simple organic aromatic report a method for monolithiation of ferrocene that in
compound. Therefore, advanced redox active molecular our hands is shown to be reproducible with good to
structures can be made using ferrocene as a building excellent yields upon quenching with electrophiles.
block. Of the many examples, where the facile synthesis This method of making monolithiumferrocene is used
of ferrocene derivatives has been used in combination to synthesise arylthio-ferrocenes, a class of compounds
with electrochemistry, the use of ferrocene as redox which previously has been prepared using primarily
reporterinself-assembledmonolayersandinbiomolecule organomercury chemistry [11]. The arylthio-ferrocenes,
conjugates have received much attention [1-4]. Another and the series of styrylferrocenes synthesised using
specialpropertyofferroceneisitsstructure.Themolecular the Arbuzov [12] and Horner-Wadsworth-Emmons [13]
shape is that of a cylinder or barrel as shown in Fig. 1. reactions, were investigated using UV/vis spectroscopy
A fact that has been exploited in chiral organometallic and cyclic voltammetry. The results show that the sulphur
catalysis, where ligands incorporating ferrocene have bridged compounds have a higher degree of electronic
high enantioselectivity and catalytic efficiency [5,6].
communication in the oxidation of ferrocene than the
The synthesis of ferrocene derivatives has been ethylene bridged compounds. On the other hand, the
widely explored. In this work, the focus is on direct electronic transitions do not involve the ligand in the
monofunctionalisation of ferrocene, either from ferrocene arylthio-ferrocenes, whereas the absorption spectra of
itself or from ferrocenecarbaldehyde, which are both the styryl-ferrocenes are highly perturbed by the ligand.
*
E-mail: TJS@chem.ku.dk
610

T. J. Sørensen, M. F. Nielsen
with petrolether as the eluent. By removal of the solvent
the product was isolated as an orange powder in a yield
of 12% (450 mg). Mass Spectrometry: GC-MS m/z =
294. Elementary Analysis: Calculated C: 65.32; H: 4.80.
Found: C: 65.40; H: 4.88. Mp: 110-111°C.
Figure 1.Theferrocene-ferroceniumredoxcoupleand aspacefilling
model of ferrocene.
2.3. (4-chloro-phenyl)-ferrocenyl sulphide
The product was isolated as an orange powder in a yield
1
2
. Experimental Procedure
of 40% (1.9 g). H-NMR (400 MHz, CDCl ) δ 7.13 (d, J =
3
8.6, 2H), 6.96 (d, J = 8.6, 2H), 4.40 (s, 2H), 4.36 (s, 2H),
All compounds and solvents were used as received 4.28 (s, 5H). Mass Spectrometry: GC-MS m/z = 328.
unless otherwise noted. THF was dried by distillation
from Na/benzoephenone. NMR spectra were recorded 2.4. (4-bromo-phenyl)-ferrocenyl sulphide
on a Unity 400 MHz and are reported using the solvent The product was isolated as an orange powder in a yield
1
residual peak as reference. Mass spectrometry was of 68% (3.8 g). H-NMR (400 MHz, CDCl ) δ 7.27 (m,
3
done on a HP GC-MS and a Micromass quadrupole- 2H), 6.90 (m, 2H), 4.39 (s, 2H), 4.36 (s, 2H), 4.27 (s, 5H).
13
TOF ESI-MS, all compounds analysed by GC-MS were
C-NMR (100 MHz, CDCl ) δ 7.34 (d, 2H), 6.96 (d, 2H),
3
found to consist of only the desired product. Melting 4.40 (m, 4H), 4.27 (s, 5H). Mass Spectrometry: ESP+
points were determined on a Kofler heat stage and (CH CN+TFA): m/z = 371.9 & m/z=373.9. MALDI-TOF+
3
are corrected with benzil (mp = 95°C) and phenacetin (dithranol): m/z=372 & m/z=374. Elementary Analysis:
(mp = 134.5°C). Elementary analysis was performed Calculated C: 51.51; H: 3.51. Found: C: 51.87; H: 3.48.
for C, H and N on a FlashEA 1112. Electrochemistry Mp: 134-136°C
was carried out in acetonitrile with a minimum of 100:1
electrolyte/analyte concentration on a CH-Instruments 2.5. 4-tolyl-ferrocenyl sulphide [17]
Model 400A potentiostat with a three electrode set-up The product was isolated as a dark yellow powder in a
1
using a platinum working and counter electrode and a yield of 70% (3.2 g). H-NMR (400 MHz, CDCl ) δ 6.98
3
silver/silver chloride reference electrode; the reference (m, 4H), 4.49 (s, 2H), 4.42 (s, 2H), 4.32 (s, 4H), 4.27 (s,
electrode was calibrated with the ferrocene/ferrocenium 1H), 4.26 (s, 2H). Mass Spectrometry: GC-MS 100% of
couple between each sample. Additional data can be m/z = 308. Elementary Analysis: Calculated: C: 66.25;
found as supplemental information.
H: 5.23. Found: C: 66.52; H: 5.33. Mp: 94-96°C.
2
.1. General procedure to phenyl disulfides (1) 2.6. (4-carbonyl-phenyl)-ferrocenyl sulphide
The synthesis of disulphides 1a-d was accomplished by 20 mL of dry THF was cooled on an ice bath and
a method adopted from Hirano et al. [14].
n-butyllithium 0.45 mL (1.5 mmol, 2.5 M in hexane) was
added. 4-Ferrocenylsulfanyl-brombenzene (2c) 0.5 g
(1.3 mol) dissolved in 20 mL dry THF was added drop-
2
.2. Phenyl-ferrocenyl sulphide [15-17]
Ferrocene 4.7 g (25 mmol) was dissolved in 30 mL wise and the reaction mixture was stirred for 15 min.
dry THF and was cooled on an ice bath. t-butyllithium 3 mL DMF (dried over molecular sieves) was then added
13.3 mL (20 mmol, 1.5 M in hexane) was added and the ice bath was removed. After 75 min of stirring the
drop-wise during a period of 10 min. The reaction mixture reaction mixture was poured onto 100 mL of water and
was stirred for 30 min at 0°C and a solution of 15 mmol the product was extracted with two 100 mL portions of
diaryl disulphide (1) was added. The reaction mixture ethyl acetate. The combined extracts were washed with
was allowed to warm to room temperature and after water and dried over magnesium sulphate. The product
2
1
hours of stirring the reaction was quenched with was isolated by dry column vacuum chromatography
5 mL of water, the phases were separated and the with methylene chloride as eluent in a yield of 75%
1
solvent was evaporated in vacuum. The crude product (0.31 g). H-NMR (400 MHz, CDCl ) δ 9.86 (s, 1H), 7.65
was dissolved in petrolether and flashed through a silica (m, 2H), 7.11 (m, 2H), 4.41 (s, 4H), 4.28 (s, 5H). C-NMR
3
13
plug, with fractions of 50 mL of solvent. The fractions (100 MHz, CDCl ) δ 191.0, 150.3, 132.9, 129.7, 124.7,
3
were investigated using TLC and the fractions containing 74.8, 73.1, 70.6, 69.7. Mass Spectrometry: GC-MS:
a surplus of ferrocene were discarded. The remaining m/z = 322. Elementary Analysis: Calculated: C: 63.37;
fractions were combined and evaporated onto a celite H: 4.38. Found: C: 63.56; H: 4.22. Mp: 128-129°C.
support and purified by vacuum flash chromatography
6
11

Synthesis, UV/vis spectra and electrochemical
characterisation of arylthio and styryl substituted ferrocenes
δ 9.97 (s, 1H), 7.83 (d, 2H), 7.55 (d, 2H), 7.07 (d, 1H),
2
.7. General procedure to benzyl phosphonates
13
6.72 (d, 1H), 4.53 (s, 2H), 4.37 (s, 2H), 4.17 (s, 5H). C-
(3) [18-27]
NMR (100 MHz, CDCl ) δ 191.5, 144.0, 134.6, 131.5,
3
The benzyl bromide (Aldrich) 0.2 mol and
triethylphosphite, 41.5 g (0.25 mol) were dissolved in
130.3, 126.1, 124.7, 69.94, 69.60, 69.07, 67.47. Mass
Spectrometry: MALDI-TOF: m/z = 316. Elementary
Analysis: Calculated: C: 72.18; H: 5.10. Found: C: 72.12;
H: 5.13. Mp: 162-166°C.
200 mL of 1,4-dioxane and heated to reflux under N2.
The reaction mixture was stirred at reflux for 20 hours.
The product was isolated as a clear oil by kugelrohr
distillation (airbath). The reaction is quantitative with a
yield approaching 100%. The product dissociates on 2.10. 4-tert-butylsulfanyl-β-(ferrocenyl)-
column materials. For use in HWE type reactions the
styrene
surplus of triethylphophite does not have to be totally Ferrocene-carboxaldehyde200mg(1mmol)and(4-(tert-
removed. The compound slowly hydrolyses when butylsulfanyl)-benzyl)-phosphonic acid diethyl ester (3h)
exposed to air resulting in the low values of the results 300 mg (1 mmol) were dissolved in THF and cooled on
of elementary analysis. Boiling points and analysis are an icebath. Sodium hydride 160 mg (2.5 mmol, 60% in
given as supplemental information.
mineral oil) was added to the solution and the solution
was stirred 1 hour at room temperature. The reaction
2
.8. General procedure to styryl-ferrocenes mixture was poured onto 300 mL of water, the THF
was removed in vacuum and the product was filtered
4
a-h [28-34]
Ferrocenecarboxaldehyde 1.7
g
(8 mmol) and off. The crude product was washed off the filter with
benzylphosphonic acid diethyl ester (3a) 10 mmol were acetone and dried over magnesium sulphate. Removal
dissolved in 25 mL dry THF and cooled on an ice bath. of the solvent and recrystallization from acetonitrile
Potassium t-butoxide 2 g (18 mmol) suspended in afforded the product as orange crystals in a yield of 27%
1
3
0 mL dry THF was added drop-wise to the cooled (100 mg.). H-NMR (400 MHz, CDCl ) δ 7.48 (s,
3
solution with vigorous stirring. When the addition was 4H), 6.99 (d, 1H), 6.73 (d, 1H), 4.50 (s, 2H), 4.28 (s,
complete, the ice bath was removed and the reaction 2H), 4.12 (s, 5H), 1.29 (s, 9H). Mass Spectrometry:
mixture was stirred at ambient temperature for 2 hours. ESP+ (CH CN+TFA): m/z=376. Elementary Analysis:
3
The reaction mixture was then poured onto 100 mL Calculated: C: 70.21; H: 6.43. Found: C: 69.82; H: 6.40.
of water and the product was extracted with 3×40 mL Mp: 173-174°C
portions of toluene. The combined extracts were washed
with water and dried over magnesium sulphate. The
solvent was removed in vacuum and the crude product 3. Results and Discussion
was taken up in methylene chloride and flashed through
a silica plug. Yields, appearance and analysis are given
as supplemental information.
3
.1. Synthesis
In the work presented here, five differently substituted
arylthio-ferrocenes were prepared. The substituent
2
.9. 4-((E)-2-ferrocenyl-vinyl)-benzaldehyde para to the sulphur on the aryl ring was varied,
and derivatives with para H, Cl, Br, CH3 and CHO
-((E)-2-ferrocenyl-vinyl)-benzonitrile(4b)1.5g(5mmol) were synthesised. An analogues series of ten styryl
[28,29,35]
4
was dissolved in 50 mL of dry THF and the solution was substituted ferrocenes was prepared. In this series the
cooled on an ice bath. DIBAL 10 mL (10 mmol, 1 M in groups para to the vinylene bridge in the styrene system
t
cyclohexane) was added and the reaction was stirred were H, F, Cl, Br, CH , OCH , SCH , SBu, CN and CHO
3
3
3
on the ice bath for 3 hours. The reaction was monitored respectively. The two series of compounds reported
with TLC (CH Cl ). The reaction was quenched with here have previously been synthesised. In the case of
2
2
1
00 mL of 1 M HCl(aq) and the two phase system was the arylthio-ferrocenes by using a similar route, with the
stirred vigorously for 1 hour. The organic solvents were key step being monolithiation of ferrocene [36]. In the
removed in vacuum and the product was extracted case of the styryl-ferrocenes, they have previously been
with 3 100-mL portions of ethyl acetate, the combined synthesised using both Wittig type chemistry and Heck
organic extracts were washed with brine and dried coupling [29,35,37].
over magnesium sulphate. The product was isolated by
In this study, the monolithiation of ferrocene has
dry column vacuum chromatography, using graduated been revisited. A method for preparing monolithium-
elution from hexane to ethyl acetate, as red crystals ferrocene (FcLi) has been reported by several groups
1
in a yield of 14% (0.2 g). H-NMR (400 MHz, CDCl ) [8-10,17,36,38-40]. In this work, fourteen different
3
612

T. J. Sørensen, M. F. Nielsen
methods for preparing FcLi were tested [7-10,40], and (2a-d) is shown in Scheme 1. The reaction gives the
the results were evaluated by quenching with DMF and yields shown in Table 1, the yield was low in case of 2a,
p-bromophenyl disulfide (1c).
but this is believed to be due to the low vapour pressure
The two key issues in the preparation of monolithium- of the disulphide. The diaryl disulphides were prepared
ferrocene are selectivity of the reaction and the stability using a method reported by Hirano et al [14].
of FcLi under the applied conditions. FcLi is under many
The styryl-ferrocenes (or 1-aryl-2-ferrocenyl-
conditions produced in a high yield, and it is the reaction ethenes, 4a-j) were prepared as shown in Scheme 1.
time and the temperature of the following quenching Earlier reported syntheses of similar compounds have
with an electrophile that determines the yield. The half- used either Pd-coupling chemistry or the Wittig reaction
life of FcLi is reported to be around 40 minutes in THF to prepare the compounds [29,35,37]. Here, the Horner-
at -20°C, but it is considerably longer in ether [40]. This Wadsworth-Emmons reaction was chosen for two
makes ether an attractive solvent. However, the problem reasons: i) the phophonate reagents are easily prepared
is that the selectivity in ether is bad, as mostly dilithiated on a large scale as they can be purified by distillation,
ferrocene is obtained. So, a compromise between and ii) where the Wittig reaction in most cases give a
selectivity and stability must be found. Here, FcLi was mixture of cis and trans isomers, the HWE reaction
prepared using a method adopted from Kagan and co- selectively gives the trans isomer in most cases [13].
workers [7]. Multiple solvent combinations, temperatures The reactions to give 3 were performed on a 50-100 g
and reaction times were tested in order to find the best scale and ran with complete conversion. The reaction
conditions for the reaction of FcLi with the two selected from 3 to 4 was carried out with varying success. The
electrophiles. The result was tested multiple times yields of 4a-j are reported in Table 1. As these easily
with the two selected electrophiles and was found to obtained reagents were available in large quantities and
give yields from 50-70% on up to a 2 gram scale. The the reactions were run on a gram scale, the reaction
reaction pathway used to obtain the arylthio-ferrocenes conditions were not further optimised.
Scheme 1. Synthetic route to arylthio and styryl substituted ferrocenes.
Figure 2. Absorption spectra of substituted ferrocenes in acetonitrile.
613
613

Synthesis, UV/vis spectra and electrochemical
characterisation of arylthio and styryl substituted ferrocenes
of the π-π* transition lifts the second d-d transition in all
cases, but might, in the cyano and carboxaldehyde case,
also lift the first d-d- transition. The π-π* transitions in
the styrylferrocenes give rise to intense bands between
3
.2. Absorption spectra
The absorption spectra of metallocenes have been
treated extensively elsewhere [41]. For ferrocene the
spectra consists of three main bands: two low intensity
d-d transitions at 440 nm and 325 nm, with absorption
300 and 340 nm.
-1
-1
coefficients of 96 and 55 M cm , respectively, and a third
CT transition in the deep UV. The absorption spectra of 3.3. Electrochemistry
the arylthio-ferrocenes (2) and styryl-ferrocenes (4) are The electrochemistry of the substituted ferrocenes was
shown in Fig. 2, and the data are compiled in Table 1. The investigated in acetonitrile using cyclic voltammetry.
two compound classes show very different absorption
A platinum working and counter electrode were
properties. The arylthio substituents affect the electronic used with a silver/silver chloride reference electrode.
transitions in ferrocene by lowering the intensity of the Voltammograms were recorded with a minimum of three
d-d transitions, whereas the energies of the transitions scan rates (v). The peak current was shown to depend
½
are not affected. Compared to ferrocene a new band linearly with v and the peak separation was within the
is seen in the near UV, and this transition is ascribed range expected for fully reversible redox processes
to the local excitation of the arylthio substituent. On the [42]. In other words, the electrochemical behaviour of
other hand, the electronic transitions of styryl substituted the alkylthio- and styryl-substituted ferrocenes was
ferrocenes differ significantly from those of ferrocene. similar to that of ferrocene itself. Cyclic voltammograms
Both the intensity and the energy of the d-d transitions of p-carboxaldehydephenylthio-ferrocene (2e) and
are greatly affected by the addition of the styryl group p-cyanostyrylferrocene (4b) are shown in Fig. 3. The
and, to a lesser degree, by the nature of the para-group oxidation potentials of the compounds vary with both
of the substituent. The styryl substituent increases the the bridge and the para substituent. The changes
intensity of the first d-d transition by the factor of four and are shown in Fig. 4 and the electrochemical data are
-1
induces a redshift of 15 nm (750 cm ). There is a small compiled in Table 2. The arylthio substituents clearly
increase in the absorption coefficient with the number affect the oxidation potential more than the styryl
of electrons in the system. The transition energy falls in substituents. Furthermore, the effect of changing the
the case of electron withdrawing groups and increases para substituent is larger in the arylthio systems than in
slightly for the electron donating groups. These effects the styryl systems. The phenylthio system is oxidised at
are very small for all but the strong electron withdrawing 60 mV higher potential than ferrocene, whereas styryl-
groups. The cyano and carboxaldehyde substituted ferrocene is oxidised at basically the same potential as
styryls show a marked increase in the transition intensity ferrocene. The change in the oxidation potential on going
and a decrease in the transition energy. This can be from hydrogen in the para position to carboxaldehyde is
a real effect, but can also be due to the intense π-π* 118 mV for the sulphur bridge and only 40 mV for the
transitions in the cyclopentadienyl-styryl system. The tail ethylene bridge.
Table 1. The different substitution patterns, yields and characteristic electronic transitions of the synthesised ferrocenes, see Scheme 1 for the
definition of Z and for the molecular structures.
Compound
Bridge
Z
Yield
d-d transition
d-d transition
π-π* transition
-1
-1
-1
-1
-1
-1
λ, nm
ε, cm
M
λ, nm
ε, cm
M
λ, nm
ε, cm
M
2
2
2
2
2
4
4
4
4
4
4
4
4
4
4
a
b
c
d
e
a
b
c
d
e
f
S
-H
-Cl
12%
40%
68%
70%
75%
43%
80%
55%
54%
65%
44%
22%
89%
14%
27%
440
439
439
439
443sh
455
472
457
458
458
455
452
454
480
440
96
42
325
-
55
-
-
S
-
248
257
250
306
306
325
303
309
310
307
307
320
338
-
3914
S
-Br
35
-
-
4570
3800
5790
3990
4770
4400
4800
5200
4860
5100
5200
4590
-
S
S
-CH3
-CHO
-H
35
-
-
48
-
-
-CH =CH -
183
440
200
250
280
211
200
262
550
96
361sh
392sh
363sh
371sh
372sh
367sh
366sh
-
410
630
430
510
560
440
440
-
2
2
-CH =CH -
-CN
-F
2
2
-CH =CH -
2
2
-CH =CH -
-Cl
2
2
-CH =CH -
-Br
2
2
-CH =CH -
-CH3
-OCH3
-SCH3
-CHO
-StBu
2
2
g
h
i
-CH =CH -
2 2
-CH =CH -
2
2
-CH =CH -
400sh
325
760
55
2
2
j
-CH =CH -
2 2
614
614

T. J. Sørensen, M. F. Nielsen
Table 2. Electrochemical properties of the synthesised ferrocenes in 0.1 M TBAPF in acetonitrile. The values are averages of three to four scans
6
-1
at four scanrates. The experiments were done at ambient pressure at 24°C. ΔE -values are obtained at v = 1 V s .
p
Compound
Bridge
Z
σHammet
Ered vs NHE
Ered vs Fc
ΔEp
V
mV
0
mV
95
Fc
-
-
-H
-
0.366
0.425
0.468
0.499
0.543
0.367
0.402
0.362
0.394
0.379
0.353
0.34
2
2
2
2
4
4
4
4
4
4
4
4
4
d
a
c
e
a
b
c
d
e
f
S
-0.17
0
59
102
133
177
1
117
88
S
-Br
S
S
-CH3
-CHO
-H
0.23
0.42
0
77
78
-CH =CH -
68
2
2
-CH =CH -
-CN
-F
0.66
0.06
0.23
0.23
-0.17
-0.27
0
36
-4
91
2
2
-CH =CH -
80
2
2
-CH =CH -
-Cl
28
13
-13
-26
-2
112
75
2
2
-CH =CH -
-Br
2
2
-CH =CH -
-CH3
-OCH3
-SCH3
-CHO
105
79
2
2
g
h
i
-CH =CH -
2 2
-CH =CH -
0.364
0.407
111
61
2
2
-CH =CH -
0.42
41
2
2
Figure 3. Cyclic voltammogram of 0.5 mM 2e (right) and 4b (left) in 0.1 M TBAPF6 in acetonitrile, recorded with a platinum working electrode.
It has previously been attempted to show linear
correlations between the oxidation potentials of styryl-
ferrocenes and other properties [15]. Fig. 5 shows plots
of the oxidation potential of the arylthio- and styryl-
ferrocenes versus the Hammett parameters for the
different para-substituents. A good linear relationship
is seen in both cases. The reaction constant quantifies
the observation that the substituent effect is much
larger in the arylthio systems than in the styryl systems.
This can be due to i) a through bond effect where the
communication in the arylthio system is superior to that
of the styryl system or ii) a through space effect where
the arylthio systems brings the electrons of aromatic
system closer to the active iron center.
Figure 4. Electrochemical potentials for selected ferrocenes in
0
.1 M TBAPF6 in acetonitrile. ■ is arylthioferrocenes, ●
is styrylferrocenes, and ▲ is litterature values from [45]
and [46].
Nagy et al. have previously determined the reaction
constant for the oxidation of phenyl-ferrocenes to 0.45 styryl-ferrocenes influences the oxidation less than the
43]. This value in combination with the two values arylthio substituent. Interestingly, direct substitution onto
[
determined here, gives a measure for the electronic the ferrocene system gives a lower reaction constant
communication from the ligand to the redoxcenter. at only 0.38 [44]. Thus, supporting the notion that the
The direct attachment in the phenyl-ferrocens has the through space communication is larger than the through
largest effect, where the conjugated system of the bond communication.
615
615

Synthesis, UV/vis spectra and electrochemical
characterisation of arylthio and styryl substituted ferrocenes
Figure 5. Hammett plots of the oxidation potential of the synthesised arylthio- (■) and styryl- (●) ferrocenes in 0.1 M TBAPF6 in acetonitrile.
The applied version of the Hammett equation, from [46], is given below. Sigma values from [47].
The styryl systems have a restricted geometry and and transition energy was found in the styryl system
a high degree of conjugation within the single π-system only. The oxidation potentials of the arylthio- and styryl-
of the styryl-cyclopentadienyl ligand, vide supra. Thus, ferrocenes were determined in acetonitrile. A substituent
the effect of the para substituents working on the effect on the oxidation potential was found for both
cyclopentadienyl ring through conjugation is believed arylthio- and styryl-ferrocenes. The effect is an order
to be larger in this system, than in the arylthio system. of magnitude larger in the arylthio systems as shown
Therefore, a more direct mechanism of communication directly by the oxidation potentials and via a Hammett
between the arylthio substituent and the iron core of plot. This is explained by a mechanism of direct
ferrocene must be operating.
communication between the arylthio-substituent and the
The ferrocene system is very compact, and sulphur iron core of ferrocene involving the sulphur atom lone-
is relatively large. The C-S-C bond angle in sulphides pairs and the iron orbitals. The absorption properties
is around 103°, which in turn imply that the sulphur and the electrochemistry of the two series do not appear
lone-pairs take up a large space. Thus, the better to be correlated. The conclusion is that the absorption
communication between the para groups in the arylthio properties are governed by the electronic properties of
derivatives can be due to either a close contact of ferrocene, where the frontier orbitals are affected by
the aryl group and the iron orbitals or communication electronic perturbations in the cyclopentadienyl ligands.
through an overlap of the sulphur lone-pairs and empty On the other hand the oxidation potential is less sensitive
iron orbitals. The latter is most likely due to the strict to changes in the cyclopentadienyl ligand. Hence, it must
steric demands of the former. This conclusion is also be concluded that another communication mechanism
supported by the fact that direct substitution of ferrocene is operating. In this work a through space mechanism,
has less influence than substitution on the phenyl ring of working on the empty d-orbitals of ferrocene is found
phenyl-ferrocene. The direct electronic communication to the most likely. The conclusion is supported by the
between the phenyl substituent and the iron atom must large substituent effect observed in phenyl-ferrocenes
be larger than the effect of the smaller substituent, [43] and in arylthio-ferrocenes.
which must be transmitted through the cyclopentadienyl
ligand.
4
. Conclusions
Arylthio- and styryl-ferrocenes were synthesised
with many different para-groups in the aryl rings. The
absorption spectra were obtained, and an effect of the
para-substituent on the ferrocene d-d transition intensity
616
616

T. J. Sørensen, M. F. Nielsen
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