Electrochemistry and UV/visible spectroscopy of phosphino-substituted bis(η5-indenyl)iron(II) complexes

Article abstract of DOI:10.1016/j.jorganchem.2005.09.044

Six phosphino-functionalized diindenyl ferrocenes have been characterized by UV/vis spectroscopy and cyclic voltammetry in dichloromethane. The complexes contain the following ligands: 1-diphenylphosphino- (1), 1-diphenylphosphino-2-methyl- (2), 1-dipheny

Full text of DOI:10.1016/j.jorganchem.2005.09.044

Journal of Organometallic Chemistry 691 (2006) 643–647
www.elsevier.com/locate/jorganchem
Electrochemistry and UV/visible spectroscopy of
phosphino-substituted bis(g5-indenyl)iron(II) complexes
*
Owen J. Curnow , Glen M. Fern, Elizabeth M. Jenkins
Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch, New Zealand
Received 10 May 2005; received in revised form 23 September 2005; accepted 26 September 2005
Available online 2 November 2005
Abstract
Six phosphino-functionalized diindenyl ferrocenes have been characterized by UV/vis spectroscopy and cyclic voltammetry in
dichloromethane. The complexes contain the following ligands: 1-diphenylphosphino- (1), 1-diphenylphosphino-2-methyl- (2), 1-diph-
enylphosphino-3-methyl- (3), 1-diphenylphosphino-3-trimethylsilyl- (4), 1-diphenylphosphino-2,3-dimethyl- (5), and 1-diphenylphosph-
ino-4,7-dimethyl-indenide (6). The cyclic voltammetry shows an approximately additive relationship between oxidation potential and
the type of substituent and its ring position, but with increasing substitution leading to lower than otherwise expected oxidation
potentials. The UV/vis spectra show two absorptions with the low energy band moving to lower energy with increasing substitution
on the C5 ring.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Ferrocene; Indenyl; UV/vis spectroscopy; Cyclic voltammetry; Phosphine
1. Introduction
in C2H4Cl2 show essentially reversible behavior, but are
followed by a fast chemical reaction. This reaction may
be a dimerization, a reaction with adventitious water, or
reaction with perchlorate electrolyte [9,10]. Other ferroc-
enes containing a phosphine directly attached to the ring
generally show unusual electrochemistry also [4,11], with
notable exceptions being (diphenylphosphino)ferrocene
[11–14], the highly substituted ferrocenephanes [(C5Me4)-
CH2PPh(C5H4)Fe] [15] and [{(C5Me4)2PPh}Fe] [16] and
the octamethyl analogue of dppf, [(C5Me4PPh2)2Fe] [16].
Given the generally complicated electrochemistry of phos-
phine-substituted ferrocenes and the tendency of indenyl
complexes to be more sensitive than their cyclopentadienyl
analogues (due to their ability to undergo facile ring-slip-
page reactions), we expected the electrochemistry of the
diindenyl analogue of dppf to be similarly complicated: at
least reversible followed by a fast chemical reaction, if not
irreversible – especially since a cationic oxidation product
should be more susceptible to nucleophilic attack and
subsequent decomposition processes via ring-slippage reac-
tions. This paper reports on those studies. Additionally,
in comparison to the analogous dicyclopentadienyliron(II)
The electrochemistry of ferrocene and its derivatives
have been extensively studied and the vast majority of fer-
rocenes have been found to exhibit a single reversible oxida-
tive process [1]. However, the introduction of a phosphine
substituent almost invariably introduces complications,
whether the phosphine is directly attached to the cyclopen-
tadienyl ring or a number of bonds away [2–4]. 1,10-
Bis(diphenylphosphino)ferrocene (dppf) has been inten-
sively studied by cyclic voltammetry. Early studies reported
that the complex undergoes an irreversible oxidation, how-
ever, subsequent studies have found the reversibility to be
highly solvent, temperature and scan rate dependent when
compared to the vast majority of ferrocenes [5]: in MeCN
solvent, the electrochemistry has been reported as irrevers-
ible [6], except by Housecroft et al. [7] who reported revers-
ible behavior at scan rates of 20–200 mV sꢀ1. All studies in
CH2Cl2 have found irreversible behavior [8] whereas studies
*
Corresponding author. Fax: +64 3 364 2110.
E-mail address: owen.curnow@canterbury.ac.nz (O.J. Curnow).
0022-328X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.09.044

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O.J. Curnow et al. / Journal of Organometallic Chemistry 691 (2006) 643–647
complexes, diindenyliron(II) complexes have barely been
investigated [17–21]. We have recently reported systematic
studies of the electrochemistry and UV/vis spectroscopy
of a variety of methyl- and TMS-substituted diindenyl fer-
rocenes: the methyl-substituted complexes were found to be
well behaved with (i) an additive relationship between the
number of methyl groups and their ring position on the oxi-
dation potential of the ferrocene and (ii) only minor effects
on the UV/vis spectra [17]. The TMS derivatives, however,
showed unpredictable oxidation potentials and variable
UV/vis spectra [18]. Their behavior was attributed to inter-
actions involving the p-acceptor orbitals on the TMS
groups. Therefore, we sought to investigate this further by
looking at the related p-acceptor phosphine derivatives to
see what trends they might exhibit. In this paper, we report
on the surprisingly reversible cyclic voltammetry of a series
of phosphino-substituted diindenyl ferrocenes as well as
their UV/vis spectra.
effects and transitions involving the SHOMO and SLUMO
(and potentially other MOs), as well as changes in relative
orbital energies and electron configurations, can signifi-
cantly complicate the situation to the extent that one can
generally only get meaningful information by comparing
closely related compounds [25,26]. Although group 4 half-
sandwich complexes have been investigated by cyclic vol-
tammetry and UV/vis spectroscopy [26–29], the processes
and orbitals involved are quite different from those ob-
served in diindenyl ferrocene systems: firstly, oxidation of
the group 4 complexes occurs from ligand-based orbitals
rather than metal-based d orbitals [26,29,30], and, secondly,
although the UV/vis spectroscopy provides information on
the HOMO–LUMO gap in both systems, it is an LMCT
process from the indenyl to a d0 metal centre in the group
4 complexes [26,27,29] rather than a d–d transition of the
d6 Fe atom [19,31]. One must be careful, therefore, when
making comparisons between these systems.
Diindenyl ferrocenes typically exhibit two UV/vis
absorption bands (with extinction coefficients of between
300 and 900 L molꢀ1 cmꢀ1), one near 420 nm and the other
near 555 nm [17,18,20]. These bands are likely to be
d–d transitions since the HOMO is largely metal-based
[19,31] and the extinction coefficients are less than 1,000
L molꢀ1 cmꢀ1. It has been observed that for methyl substi-
tution the UV/vis spectra vary by only 12 nm (416–426 and
548–560 nm) [17,20]. For mono-TMS-substitution on the
C5 ring, we found that the UV/vis absorption peaks change
by 10–22 nm whereas for disubstitution, all of the UV/vis
absorption peaks shift to longer wavelength by 40–62 nm
[12–14]. Apparently, the r* orbitals of the TMS groups
have a significant influence on the indenide ring MOs, thus
affecting the bands in the UV/vis spectra. Table 1 gives the
absorptions for the phosphino derivatives illustrated in
Scheme 1 and typical UV/vis spectra (rac- and meso-1)
are shown in Fig. 1. It can be seen from Fig. 1 that the
rac and meso isomers both exhibit two bands in the UV/
vis spectra and that these occur at similar energies. How-
ever, the band intensities for the two isomers, for 1 at least,
are different: the low energy band is a little weaker for the
rac isomer whereas the higher energy band is weaker for
the meso isomer, even though this band also has an
encroaching charge transfer band. Although some extinc-
tion coefficients for the high energy band are over 1000
L molꢀ1 cmꢀ1, it should be noted that these absorptions
overlap with a charge transfer band. The high energy
absorptions are otherwise similar to those observed for the
methyl-only derivatives. Although the low energy absorp-
tions for 1 and 6 (566 and 546 nm, respectively) are similar
to diindenyliron(II) and its methyl derivatives, somewhat
curiously, further methyl or TMS substitution on the C5 ring
(2–4) shifts the absorption to longer wavelength (572, 600
and 591 nm, respectively). Unfortunately, we were unable
to obtain meaningful UV/vis spectra for the sterically con-
gested, and consequently sensitive, complex 5.
2. Results and discussion
The synthesis and characterization of the ferrocenes
illustrated in Scheme 1 have all been reported previously
by us [22,23]. They can all be prepared by treatment of
the indenide (formed by deprotonation of the indene with
BuLi) with anhydrous ferrous chloride in THF. The bispla-
nar chiral nature of these complexes allows the possibility
for the formation of rac and meso isomers. Complex 1 is
the only ferrocene studied here in which we have isolated
both isomers. Complex 2 is formed in a 1:1.4 isomeric ratio
with the major isomer, probably meso, being isolated. Elec-
trochemical and spectroscopic techniques were applied to
this isomer. Complex 3 forms as a 3:1 mixture of isomers
from which the major isomer (rac-3) was isolated and the
electrochemical and spectroscopic techniques applied.
Complex 4 is initially formed in a 3:2 ratio of isomers with
one of the isomers decomposing more readily than the
other and the major isomer (probably rac) subsequently
being isolated and studied by electrochemical and spectro-
scopic techniques. Complexes 5 and 6 were run as mixtures
of isomers (2:5 and 1:3, respectively).
To a crude approximation, the electrochemical oxidation
potential is equivalent to the energy of the HOMO, the
reduction potential is equivalent to the LUMO, and
the long wavelength absorption energy corresponds to the
energy of the HOMO–LUMO gap [24]. However, solvent
R7
R2
H
R3
H
R4
H
R7
H
R4
R4
PPh2
R2
R2
1
2
3
4
5
6
R3
R3
Me
H
H
H
H
Fe
Me
SiMe3
Me
H
H
H
PPh2
H
H
H
R7
Me
H
H
H
Me
Me
(1)
Cyclic voltammetry of methyl- and TMS-substituted
diindenyl ferrocenes show a single-electron reversible redox
Scheme 1. Phosphino-substituted diindenyl ferrocenes.

O.J. Curnow et al. / Journal of Organometallic Chemistry 691 (2006) 643–647
645
Table 1
CV and UV/vis data for phosphino-substituted diindenyl ferrocenes in CH2Cl2
Compound
rac:meso Ratio
E0/mVa
DEp/mV
k
max/nm (e/L molꢀ1 cmꢀ1
)
k )
max/nm (e/L molꢀ1 cmꢀ1
1-PPh2 (rac-1)
1-PPh2 (meso-1)
1:0
0:1
0:1
1:0
1:0
2:5
1:3
ꢀ140
ꢀ140
ꢀ230
ꢀ235
ꢀ220
ꢀ350
ꢀ230
120
120
90
90
90
420 (870)
565 (265)
555 (330)
570 (200)
600 (360)
590 (210)
–
435 (660, sh)b
415 (1150)
420 (1340)
450 (540, sh)b
–
1-PPh2-2-Me (meso-2)
1-PPh2-3-Me (rac-3)
1-PPh2-3-SiMe3 (rac-4)
1-PPh2-2,3-Me2 (5)
1-PPh2-4,7-Me2 (6)
100
120
435 (810, sh)b
545 (530)
a
Versus the Fc/Fc+ couple.
Data reported for the point of inflection.
b
Fig. 1. Typical UV/vis spectra of phosphino-substituted diindenyl ferro-
cene complexes in CH2Cl2: compound rac-1 (solid line) and meso-1
(dashed line).
Fig. 2. Typical cyclic voltammogram of a phosphino-substituted diinde-
nyl ferrocene – compound 2 at a scan rate of 100 mV sꢀ1 is shown.
process [17,18]. A typical cyclic voltammogram of a phos-
phino-substituted diindenyl ferrocene in CH2Cl2 is shown
in Fig. 2 and the CV data for complexes 1–6 are given in
Table 1. Data collected at scan rates from 20 to 2000
mV sꢀ1 were found to be of a similar magnitude, and to
increase in a similar fashion, to ferrocene under our condi-
tions. The large DEp values are consistent with a large IR
drop that is commonly observed for organic solvents such
as dichloromethane. The ratios of the forward and back-
ward currents at various scan rates were also found to be
similar to that observed for ferrocene under the same con-
ditions, confirming that the process is reversible. Only one
redox process was observed in each of the rac/meso mix-
tures. Like the other diindenyl ferrocenes that have been
reported [17–21], the redox process is reversible. This is
in stark contrast to that of dppf in CH2Cl2, however, for
which irreversible processes are observed [8]. For complex
1, we found that the oxidation potential has increased by
138 mV from diindenyliron(II) (ꢀ140 versus ꢀ278 mV,
respectively). This compares to an increase of 140 mV for
adding only one diphenylphosphino group to ferrocene in
[Cp(C5H4PPh2)Fe] [13] and 183 mV (in dichloroethane sol-
vent) for adding two diphenylphosphino groups to give
dppf [10]. In contrast, adding a methyl group to each ring
decreases the oxidation potential by about 97 mV while a
single TMS group has very little nett effect on oxidation
potential (+3 mV) in this solvent. The PPh2 group is a
stronger electron-withdrawing group than both Me and
SiMe3, and this can be attributed to both r and p effects
[4i,32,33]. Addition to 1 of a methyl group in the 2 or 3 po-
sition on each ring (2 and 3, respectively) decreases the oxi-
dation potential by 90 and 95 mV, respectively. This
compares to 77 and 97 mV, respectively, when going from
the unsubstituted diindenyl ferrocene [11]. Addition to 1 of
a TMS group in the 3 position (4) gives a decrease of
80 mV and, although this decrease is much larger than
when adding a TMS group to each ring of diindenyliron(II)
(ꢀ278 mV for [(C9H7)2Fe] and ꢀ275 mV for [(1-C9H6Si-
Me3)2Fe], a difference of only 3 mV), the decrease is very
similar to that found when adding a second TMS group
on each ring to give [{1,3-C9H5(SiMe3)2}2Fe] (ꢀ358 mV,
a decrease of 83 mV) [18]. This large increase in the
ease of oxidation for adding a bulky TMS group to 1 or
[(1-C9H6SiMe3)2Fe] can be attributed to steric repulsion

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O.J. Curnow et al. / Journal of Organometallic Chemistry 691 (2006) 643–647
between the rings favoring oxidation since inter-ring sepa-
ration is greater in the resultant ferrocenium [33]. A similar
effect has been observed for multiple TMS additions to
ferrocene [33]. In some further support of this, the oxida-
tion potential of 5 (ꢀ350 mV) is also lower than otherwise
expected: a simple additive relationship based on the oxida-
tion potentials of diindenyliron(II) and 1–3 would have
predicted an oxidation potential of ꢀ323 mV.
Celite column. All other reagents were purchased from
Aldrich or Sigma Chemical Companies. UV/vis spectra
were obtained on a Hewlett-Packard 8452A Diode Array
(2 nm resolution) spectrometer using 1 cm cuvettes. Cyclic
voltammetry was performed using a PAR 173 Potentiostat
coupled to a PAR 175 Universal Programmer and a
Graphtec WX 1200 chart recorder. All electrochemical
measurements were made in CH2Cl2 solvent with 0.1 M
[Bu4N]PF6 electrolyte and using a three-electrode cell com-
prising of a platinum-disk working electrode (1 mm diam-
eter), a platinum-wire auxiliary electrode, and a Ag|Ag+
(0.01 M AgNO3, 0.1 M [Bu4N]PF6–CH2Cl2) reference elec-
trode. Sample concentrations were 1 mM. All potentials
are reported versus the ferrocene/ferrocenium (Fc/Fc+)
couple after referencing to in situ ferrocene. Before use,
the electrodes were polished with 1 lm diamond paste
and cleaned with acetone and distilled water. Electroche-
mical measurements were made at ambient temperature
under an inert atmosphere. Electrochemical and UV/vis
results are given in Table 1.
As might be expected, the influence of methyl groups in
the 4 and 7 positions is smaller than for substitution on the
C5 ring and 6 is found to have an oxidation potential
90 mV less than 1. This compares to a decrease of 65 mV
on methylation of diindenyliron(II) and the difference
might again be attributed to an increase in steric repulsion
between the indenyl rings.
The reason for the remarkable reversibility of the redox
processes for 1–6 is not obvious. A reasonable suggestion
would be that the steric bulk of the ferrocenes is hindering
nucleophilic attack and subsequent decomposition, how-
ever, we have previously reported the ring-flipping isomer-
ization of meso-1 to rac-1 at ambient temperatures in THF
via an intermediate involving coordination of THF [34], so
this seems unlikely. Pilloni and coworkers [9,10] have pro-
posed a dimerization mechanism for the decomposition of
dppf upon oxidation, and it may be that the steric bulk of
the indenyl complexes prevents this decomposition route.
Kirss and coworkers [14] have provided good evidence that
[Cp(C5H4PPh2)Fe]+ does not contain a ferrocenium center
but may instead have a phosphorous cationic radical cen-
ter. The reason for the stability of this compound is also
not clear.
Acknowledgments
We wish to thank Dr. Alison J. Downard (University of
Canterbury) for the use of electrochemical equipment and
Dr. Paula Brooksby for assistance with the collection of
electrochemical data.
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We have characterized six phosphino-substituted
bis(g5-indenyl)iron(II) complexes by UV/vis spectroscopy
and cyclic voltammetry in dichloromethane. The UV/vis
spectra show two absorptions with the low energy band
moving to lower energy with increasing substitution on
the C5 ring. The cyclic voltammetry shows a surprisingly
reversible oxidative process and an approximately addi-
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substituent and its ring position. Increasing substitution,
however, was found to give lower than otherwise ex-
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effects. Phosphino groups were found to increase the oxi-
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