Electronic structure and near-IR transitions of FcC2R and FcC4R dyads

Article abstract of DOI:10.1016/j.poly.2006.07.002

This paper describes the orbital configuration which provides a basis for the understanding of the electronic structure and spectroscopic properties of 17e and 18e FcC₂R and FcC₄R dyads, where R is H, 1-naphthyl, 9-anthryl, 3-pyrenyl, perylenyl. DFT calculations show that destabilisation of the ferrocenyl π orbitals upon binding a C{triple bond, long}CR group to a Cp ring leads to the metal-based a₁ orbital dropping below the e₁-a so that the frontier orbital configuration is (e₂^′ -a, π)² (e₂^′ -b,metal)² (e₁^′ -a, π)², (a₁, metal)². The contribution of the aryl group to the π e₂-a and e₁-a orbitals varies with the annelation of the ring. The LUMO is aryl based. The calculations are consistent with the spectroscopic data for the 18e species. Oxidation to the 17e cations does not change the orbital configuration but the orbital energies are lowered by the positive charge centred on the Fe. A strongly solvatochromic transition in the near-IR, a signature for the 17e cations, is best described as an LMCT transition but the contribution of C₂R and C₄R to the donor and acceptor levels depends on the ionization energy of the aryl π orbital. LMCT energies decrease from FcC₂R to FcC₄R dyads.

Full text of DOI:10.1016/j.poly.2006.07.002

Polyhedron 26 (2007) 448–455
www.elsevier.com/locate/poly
Electronic structure and near-IR transitions of FcC2R and FcC4R dyads
a
b
b
b
c
A.H. Flood , C.J. McAdam , K.C. Gordon , H.G. Kjaergaard , A.M. Manning ,
b,
b
B.H. Robinson *, J. Simpson
a
Chemistry Department, Indiana University, 800 East Kirkwood Avenue, Bloomington, IN 47401, USA
b
Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand
c
Department of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland
Received 16 May 2006; accepted 6 July 2006
Available online 14 July 2006
Abstract
This paper describes the orbital configuration which provides a basis for the understanding of the electronic structure and spectro-
scopic properties of 17e and 18e FcC2R and FcC4R dyads, where R is H, 1-naphthyl, 9-anthryl, 3-pyrenyl, perylenyl. DFT calculations
show that destabilisation of the ferrocenyl p orbitals upon binding a C„CR group to a Cp ring leads to the metal-based a1 orbital drop-
2
2
2
ping below the e1-a so that the frontier orbital configuration is ðe02-a;pÞ ðe20 -b; metalÞ ðe10 -a;pÞ , (a1,metal)2. The contribution of the aryl
group to the p e2-a and e1-a orbitals varies with the annelation of the ring. The LUMO is aryl based. The calculations are consistent with
the spectroscopic data for the 18e species. Oxidation to the 17e cations does not change the orbital configuration but the orbital energies
are lowered by the positive charge centred on the Fe. A strongly solvatochromic transition in the near-IR, a signature for the 17e cations,
is best described as an LMCT transition but the contribution of C2R and C4R to the donor and acceptor levels depends on the ionization
energy of the aryl p orbital. LMCT energies decrease from FcC2R to FcC4R dyads.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Electronic structure; DFT calculations; Ferrocenylaryl dyads; Ferroceniumaryl dyads; NIR spectra
1. Introduction
motif [12] have not received the same attention. Barlow
and Marder [2] have commented that a D5h/5d metallocene
can interact with a p-bridge both via a cyclopentadienyl
ring and through a metal-based orbital when an organic
acceptor was involved. Whether this situation also pertains
in electron-rich p-linked dyads, and/or with ferrocenium
analogues, is the thrust of this paper. This was of interest
to us as any change in the electronic structure, either in
the neutral or oxidized molecule, may enable the classical
quenching effect of the metallocene to be modified if a
fluorophore is bound to a cyclopentadienyl ring via a
p-conjugated bridge. This has been achieved where the
fluorophore is a strong electron acceptor [13–15].
Many aspects of the electronic structure of closed shell
metallocenes, especially ferrocene [1–5], have been investi-
gated, both theoretically and experimentally. With the cur-
rent focus on ferrocenyl materials with second-order NLO
properties [6,7], these studies have extended into ferrocenyl
derivatives where electron acceptors are bound to one or
two cyclopentadienyl rings, especially via a p-linkage
[2,8–10]. A detailed description of the electronic structure
for this class of compound has evolved, the salient features
being an understanding of the ferrocenyl-(p-link)-acceptor
energy levels and consequential optical transitions. The
electronic structure of compounds with strong donors on
a Cp ring [11] or dyads with a ferrocenyl-(p-link)-donor
In a previous paper [16] we reported the synthesis and
molecular structure of the ferrocenyl dyads 1a–f and 2a–f
and [FcC@C–C@CR]. Spectroscopic and structural data
indicated that there was little mutual perturbation of the
Fc and aryl groups via the unsaturated link in the ground
*
Corresponding author. Tel.: +64 3 4797912; fax: +64 3 4797906.
E-mail address: brobinson@alkali.otago.ac.nz (B.H. Robinson).
0277-5387/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.07.002

A.H. Flood et al. / Polyhedron 26 (2007) 448–455
449
state of the dyad. But this was not true when the ferrocenyl
group was oxidized to the ferrocenium analogue. Absorp-
tion bands in the 400–700 nm region of the visible spectrum
were not typical of ferrocenium compounds and all oxidized
dyads had weak bands in the near-IR. Similar NIR bands
have been reported for a number of ferrocenium derivatives
[10,11,17] and the effects of donors on the ferrocenium
LMCT band have been documented [17]. We subsequently
found that these near-IR bands were also a feature of dyads
with FcC„C– or FcC@C-group, even when R was a strong
acceptor (e.g. naphthalimide [16]). They were also observed
in oxidized dyads of general type, Mc+–C„C–R or Mc+–
C„C–R, where Mc is a donor metallocene fragment such
as the isolobal g-C5H4Co(Ph4C4). In order to gain an
insight into the origin of the NIR bands, DFT calculations
were performed on the dyads 1a–f [18] and 2a–f. The p link
between the ferrocenyl and aryl groups significantly per-
turbs the ferrocenyl energy levels leading to a different orbi-
tal configuration to that of ferrocene. This one-electron
configuration provides an explanation of the interesting
spectroscopic properties and electrochromism displayed
by these dyads
R = 2a H
R =
H
1a
b Ph
b Ph
napthyl
anthryl
pyrenyl
perylenyl
C=CR
c napthyl
C
CR
c
d
e
d anthryl
Fe
Fe
e pyrenyl
f perylenyl
f
2. Results and discussion
2.1. DFT calculations on 18e dyads
Density functional theory (DFT), which is well-
performed for ferrocene [4,5] and derivatives [9b], was used
to probe the influence of the –C„C and aryl group on the
frontier orbitals of ferrocene. B3LYP calculations were
carried out on 1a–f and 2a–f. Iso-surfaces for the frontier
orbitals for 1d are represented in Fig. 1a; surfaces for the
other molecules are available from the authors. Calculated
orbital energies are given in Table 1 and schematically rep-
resented in Fig. 2. B3LYP/6.31G(d) calculations of the
optimised geometries were in good agreement with those
found in the X-ray crystal structures [16]. An eclipsed
geometry for the Cp rings was found for all dyads so the
appropriate ferrocenyl point group is D5h but the subgroup
D5 is adequate. For the dyads the symmetry is C2v.
Ferrocene (D5) has an e02 ðx2 ꢀ y2; xyÞ HOMO, which is
predominately a d bond and metal in character. The e02 lies
0.35 eV above the metal-based a1 ðdz2 Þ orbital. The
e01 ðCp þ dxz=yzÞ is the strongest bonding interaction. Ioniza-
tion potentials for the e2, a1, e1 frontier orbitals of ferro-
cene are, respectively, 6.88, 7.23 and 8.72 eV [4]. The
addition of a C„C to an aromatic ring raises the ioniza-
Fig. 1. (a) Frontier orbitals of 1d; (b) comparison of HOMO and
HOMO ꢀ 2 frontier orbitals for 1b and 1d.
tion energy of the p HOMO and for HC„CR the IP’s
range from 9.12 eV for ethynylbenzene to 8.12 eV for the
larger annelated rings [19]. Consequently, the p orbitals
of the C„CR group are ideally placed for a strong p inter-
action with ferrocene orbitals, with the aromatic HOMO
having a stronger interaction with the e2 orbital as the ring
size increases; this was confirmed by the B3LYP calcula-
tions (vide infra). The DFT calculations show that the
introduction of a conjugated substituent does indeed have
a significant impact on the relative energies of the ferrocene
orbitals, particularly the strongly bonding e01 level (Fig. 2).

450
A.H. Flood et al. / Polyhedron 26 (2007) 448–455
Table 1
Molecular orbital energiesa
Molecule
FcH
HOMO ꢀ 3
HOMO ꢀ 2
a10
HOMO ꢀ 1
e20
HOMO
e20
LUMO
e10
e10
ꢀ0.2487
e01-a=a1
ꢀ0.22228
e01-a=a1
ꢀ0.18981
e02-b
ꢀ0.18980
e02-a
0.00784
1a
2a
ꢀ0.23194
ꢀ0.2257
ꢀ0.22827
ꢀ0.2219
e01-a
ꢀ0.19687
ꢀ0.1922
e02-b
ꢀ0.19560
ꢀ0.1900
e02-a
ꢀ0.00892
ꢀ0.0120
a1
1b
1c
1d
1e
ꢀ0.22892
ꢀ0.22958
ꢀ0.23850
ꢀ0.22985
ꢀ0.21229
ꢀ0.20519
ꢀ0.19965
ꢀ0.19986
ꢀ0.19605
ꢀ0.19699
ꢀ0.19763
ꢀ0.19704
ꢀ0.19184
ꢀ0.19011
ꢀ0.18173
ꢀ0.18435
ꢀ0.03386
ꢀ0.05012
ꢀ0.06893
ꢀ0.06393
e10
a10
e20
e20
e10
[FcH]+b
ꢀ0.4281(a)
ꢀ0.4281(a)
ꢀ0.3916(b)
ꢀ0.3916(b)
ꢀ0.2862(b)
a
Energies in au.
b
Energies are given for the a or b one-electron orbitals.
18e Dyads
Ph
Fc
Naph
Anth
Pery
H
''
e
1
LUMO
HOMO
443nm
'
e
2
HOMO-1
HOMO-2
'
a
1
HOMO-3
'
e
1
17e Dyads
Naph
Fc
Ph
Anth
Pery
H
LUMO
HOMO
470nm
HOMO-1
HOMO-2
LMCT 630nm
HOMO-3
Fig. 2. Calculated orbital energies for 18e and 17e dyads compared to ferrocene.

A.H. Flood et al. / Polyhedron 26 (2007) 448–455
451
For ease of tracing the perturbation of the ferrocenyl orbi-
tals, the dyad frontier orbitals arising from the degenerate
ferrocenyl orbitals in lower symmetry will be designated as
e02-a=e20 -b and e01-a=e10 -b rather than a formal C2v designa-
tion. One of the degenerate e02 or e01 orbitals has the correct
nodal symmetry (e02-a and e01-a) to interact with the HOMO
of the C„CR substituent.
Calculations show that the LUMO in all the dyads is
entirely aryl-based and this is consistent with experimental
data. Their reduction potentials were similar to the free
arene in the same solvent and simulation of the EPR spec-
tra of the radical anions 1(b–f)ꢀÅ and 2(a–e)ꢀÅ was achieved
using the values obtained from EPR spectra of the
HC„CRꢀÅ and HC@CRꢀÅ radical anions.
We first consider the effect of adding a –C„CH group
to the Cp ring of ferrocene. The energies of both orbital
pairs, e02-a=e20 -b and e01-a=e01-b, are lowered relative to the
parent ferrocene orbital. There is an orbital mismatch for
the ferrocenyl-p-substituent interaction (Fig. 1) and the
orbital involved in the p interaction, the e02-a, is slightly
destabilised relative to the e02-b orbital which is entirely
metal-based. This is the reverse of that found when the
C1 substituent is an electron-withdrawing group [9b]. Sim-
ilarly the strongly interacting e01-a orbital is higher in
energy than the metal-based e01-b. This destabilisation of
the p e01-a orbital has an extremely important consequence
for the relative energy of the e01-a and a1 ðdz2 Þ orbitals. For
1a, mixing of these two orbitals occurs with the result that
the one-electron HOMO ꢀ 2 and HOMO ꢀ 3 both have
dz2 and dxz character. The orbital configuration for 1b–f
is more simple. For these dyads the increase in energy of
the p ðe01-aÞ orbital leads to a crossover of energy such that
the HOMO ꢀ 2 orbital is now the e01-a combination and the
HOMO ꢀ 3 the a1 (see Fig. 2). The frontier orbital config-
3. Electronic spectra
Identification of the lowest energy bands in the UV–Vis
spectra was hampered by the overlap with the very intense
p–p* transition of the aryl group. Nevertheless, the weak
lowest energy band does not change significantly in energy
as the annelation of the ring increases; the energies are all
22500 200 cmꢀ1 compared to 22700 cmꢀ1 for ferrocene.
These bands can be given an assignment analogous to the
lowest energy spin-allowed d–d transition (1A1 ! aE1) of
ferrocene [1] and the invariant character of this transition
is consistent with the relatively constant HOMO–LUMO
band gap across the series. Resonance Raman spectra
(kexc = 457.9 nm) supported this assignment for the low-
energy band as there was a weak enhancement of the mC„C
band, the ferrocenyl ring breathing mode at 1100 cmꢀ1 and
mFe–C at 298 cmꢀ1 and aryl modes. As noted previously
[16], the small red-shift in the p–p* transition of the C„CR
group shows that there was not strong perturbation by the
ferrocenyl moiety.
2
2
2
2
uration for 1b–f is thus ðe02-aÞ , ðe20 -bÞ , (a1)2, ðe10 -aÞ , ðe01-bÞ
(Figs. 2 and 5).
Electrochemical oxidation of 1a–f and 2a–f at the poten-
tial of their [Fc]+/0 couple generated the respective green-
blue [1a–f]+ and green [2a–f]+ 17e cations. The C4 species,
[Fc-(CH@CH)2R]+ (R = Ph, naph, pyr, anthryl), were also
produced to study the effect of extending the unsaturated
link. For [1b–f]+ constant isosbestic points, complete spec-
tral reversibility when the potential is cycled in an OTTLE
experiment, and the fact that identical spectra were
obtained by controlled chemical oxidation, attest to the
stability of the 17e species. There was spectroscopic evi-
dence that the lifetime of 1a+ was much shorter due to
decomposition to the butadiene species (FcC„C)2. Repre-
sentation of the cations as Fc+C„CR in the ground state is
vindicated by the vibrational data which show that shifts in
the ferrocenyl vibrational bands are similar to those that
occur with the formation of a ferrocenium ion [20]. For
example, the Raman spectrum of 1d+ shows a shift of the
Fc ring-breathing mode of +7 cmꢀ1 and there is a typical
Fc+ band at 1113 cmꢀ1 in the infrared spectrum.
There is a progressive destabilisation of the HOMO e02-a p
orbital as the annelation of the aryl ring increases. Since
there is little variation in the energy of e02-b, the energy differ-
ence between HOMO and HOMO ꢀ 1 increases as the ion-
ization energy of the aryl decreases, 0.035 eV (1a) to 0.43 eV
(1d), although it is still relatively small. More significant is
the increasing destabilisation of the strongly bonding
HOMO ꢀ 2 ðe01-aÞ orbital such that the HOMO ꢀ 2/
HOMO ꢀ 3 separation increases from 0.45 eV for 1a to
1.06eV for 1e; in fact, the HOMO ꢀ 1/HOMO ꢀ 2 orbitals
for 1e are almost degenerate (separation 0.05 eV) (Table
1). Concomitant with the increase in energy of the HOMO,
there is an increasing –C„CR contribution to the HOMO;
by 1d the HOMO has a large –C„CR component. This
trend nicely correlates with the decreasing ionization poten-
tial of the C„CR group. Conversely, there is an increase in
the metal character of HOMO ꢀ 2 ðe01-aÞ (Fig. 1b).
Calculations on the ethenyl dyads 2a, 2c and 2d indicated
2
2
that the orbital configuration ðe02-a;pÞ ðe20 -b; metalÞ -
In the ferrocenium cation [1,21–23], the 2E2 state is
2
ðe1-a;pÞ , (a1, metal)2 also held for the ethenyl series. An
0.46 eV more stable than A1 state and the ground state
2
ethenyl group interacts more strongly with the ferrocene
moiety than an ethynyl group and this is reflected in the
higher energy of the p orbitals for the ethenyl series. As a
consequence, the HOMO for ferrocene and 2a have almost
the same energy which accounts for the observation that the
potentials for the [Fc]+/0 couple for ferrocene and 2a are
similar (0.55 V and 0.52 V, respectively) whereas that the
ethynyl dyad 1a is more difficult to oxidise (0.63 V).
configuration is e23 ðxy; x2 ꢀ y2Þa21 ðz2Þ. Previous calcula-
tions on ferrocenium derivatives [9b], and PE data indicate
that the orbital ranking does not change from that in the
18e parent. Our DFT calculations on [1a–b]+, and preli-
minary results for the ethenyl series, also confirmed that
the frontier orbital energy sequence was also unchanged
1
2
2
2
upon oxidation, ½ðe20 -aÞ ðe20 -bÞ ðe1-aÞ ; ða1Þ ꢁ, and that aryl
contribution to the one-electron orbitals in the 17e and

452
A.H. Flood et al. / Polyhedron 26 (2007) 448–455
18e species was similar. Overall, the energies of the occu-
pied orbitals are lower than in the 18e analogues due to
the coulombic effect of the positive charge. The HOMO–
LUMO gap is significantly smaller than for the corre-
sponding neutral species and it decreases from Fc+ to
1a+ and as the ring size increases.
it has the opposite effect, implying that longer conjugated-
chain ferrocenium dyads could have significantly more
intense NIR bands. The anomalous behaviour of 2e+ in
the trend data, discussed elsewhere [16], is probably due
to steric hindrance involving the ethenyl a-H atoms
restricting the p interaction.
Broad (m1/2 ꢂ 2100 cmꢀ1), weak bands (e 700–
1550 Mꢀ1cmꢀ1 ), with energies from 7600 to 14300 cmꢀ1
(band A, Fig. 3, Table 2), were a feature of the electronic
spectra of all 17e C2 and C4 dyads. The energies for the eth-
enyl compounds are red-shifted relative to the ethynyl ser-
ies (Table 2) indicative of a stronger p interaction.
Interestingly, the intensity and energy of these bands
increased from the C2 to the C4 analogue; that is, with
the interpolation of another –C@C group between the
donor arene and acceptor (Table 2). So rather than dis-
tance reducing the increase in the energy of the e1-a orbital,
Support for an LMCT assignment for these NIR bands
came from several sources. (1) The energy of A was depen-
dent on the annelation of the aryl group (Table 2) and plots
of these energies against the first vertical ionization energy
of the respective aryl group are linear. The intensity also
increases as the IP of the aryl group decreases. (2) A large
negative solvatochromic shift DEsolv of 900–1200 cmꢀ1
between CH2Cl2 (e = 9.1) to MeCN (e = 37.5) is indicative
of a larger dipole in the ground state of the cations than
the excited state. This is consistent with a CT assignment
as in the excited state configuration of the cations have the
411
.4
476
.3
.2
A
B
1118
.1
711
0
400
600
800
1000
1200
1400
1600
Nanometers
Fig. 3. OTTLE spectrum of 1d, typical of the aryl dyads; 0.7 V on Pt, CH2Cl2, 293 K.
Table 2
Representative data for LMCT (A) and HOMO–LUMO (B) transitionsa
R
Fc+–C„C–R
max(A)
Fc+–CH@CH–R
max(A)b
Fc+(CH@CH)2R
max(A)b
k
e(A)
k(B)
k
e(A)
k
e(A)
H
Phenyl
14.3
12.6
11.5
11.8
11.1
8.9b
9.8c
8.3
17.2
17.4
17.1
17.2
13.9
11.1
10.5
10.4
10.5
9.52
9.0
7.6
17.1
17.1
17.2
17.2
0.7
0.8
0.7
0.7
0.9
1.3
1.4
0.6
0.7
1.3
1.3
0.7
1.6
1.5
9.5
9.1
0.5
0.4
1-Naphthyl
2-Naphthyl
Phenanthryl
Anthryl
Pyrenyl
Perylenyl
a
17.0
d
17.2
d
9.3
8.3
1.2
1.9
17.5
d
17.5
d
k
max(A), e(A), kmax(B), refer to the data for bands A and B labelled in Fig. 3. kmax · 10ꢀ2 cmꢀ1; e · 10ꢀ2 Mꢀ1 cmꢀ1. Recorded in CH2Cl2.
b
c
In MeCN, 9.7.
In MeCN, 10.6.
d
Obscured by p–p* tail.

A.H. Flood et al. / Polyhedron 26 (2007) 448–455
453
positive charge spread throughout the p system; in the
ground state it is localised on the ferrocenyl acceptor. (3)
For a CT transition the energy should be related to the elec-
trochemical parameters of the donor and acceptor moieties;
this is indeed the case (Fig. 4). (4) The Raman spectrum res-
onant with these bands shows an enhancement of modes
associated with both Fc+ and aryl units. The ring-breathing
modes of the Fc/Fc+ component show an increase in reso-
nance enhancement on going from 1b–f ! [1b–f]+ whereas
there is a threefold decrease in enhancement in the equiva-
lent modes for FcH to FcH+. These data imply that the
Fc-based modes are deriving enhancement from an addi-
tional electronic transition that is not isolated on the
metallocene.
LUMO
-1
17300cm
'-
(e a)
2
'-
(e b)
2
-1
10,200cm
'-
(e a)
1
')
(a
1
Assignment of the lowest energy transition at
15900 cmꢀ1 (in CH2Cl2) in the visible spectrum of the
ferricenium ion and ferrocenium derivatives to a LMCT
Fig. 5. NIR band assignment for 1d+; CH2Cl2, 15000–8000 cmꢀ1
.
2
(2E2 ! E1), corresponding to a e1 ! e2 one-electron transi-
tion, is well-founded [1,21,22]. Clearly, the same one-elec-
tron assignment e1-a ! e2-a (Fig. 5) can be made for the
lowest energy transition observed for these 17e dyads. What
makes the LMCT energies so remarkable is the magnitude
a C„C–R group to the ferrocenyl ring. It is reasonable to
assign the donor energy level as one related to the e1-a orbi-
tal and the energy of this will be heavily dependant on the
ionization potential of the C„CR group. In the excited
state the acceptor energy level, approximated by the
[Fc]+/0 potential, while perturbed strongly by the incorpora-
tion of the C„C, is relatively invariant with aryl group.
Therefore, the linear correlation of the CT energies seen
in Fig. 4 is consistent with an assignment of a HOMO ꢀ 3
(donor) ! HOMO (acceptor) CT transition. While a
description as an LMCT transition is appropriate it is
important to recognise that the contribution of the C„CR
p orbital to the donor and acceptor levels depends on the
ionization energy of this p orbital. For the small ring aryl,
the donor level has a significant input from the C„CR p
orbital; the reverse holds for the acceptor level.
2
of their red-shift from the 2E2 ! E1 transition in Fc+
(e.g. 8200 cmꢀ1 for 2f+). Previous work has found that
the ‘ferrocenium’ transition is red-shifted by donor Cp-
substituents [17]. Substitution by two butyl groups causes
a red-shift of 800 cmꢀ1 for example [20], and with the strong
donor, N(p-tol)2 the red-shift is 3250 cmꢀ1 in CH3CN with
negative solvatochromism DEsolv = 427 cmꢀ1 for CH2Cl2/
CH3CN [11]. Aryl groups are not strong donors so we need
to look to the electronic configuration for an explanation of
the large red shifts. From the DFT calculations both the sin-
gly occupied HOMO and, especially the HOMO ꢀ 2, are
the strongly perturbed orbitals because of conjugation of
Correlation of CT Energies
13000
12500
12000
11500
11000
10500
10000
9500
ethenyl
ethynyl
9000
8500
8000
1
1.2
1.4
1.6
1.8
2
2.2
2.4
Aryl oxidation potential
Fig. 4. Correlation of CT energies with oxidation potential of aryl group; CH2Cl2.

454
A.H. Flood et al. / Polyhedron 26 (2007) 448–455
New bands in the OTTLE UV–Vis spectra above
metry and for Osteryoung square-wave voltammetry,
square-wave step heights of 1–5 mV, a square amplitude
of 15–25 mV with a frequency of 30–240 Hz. All potentials
are referenced to decamethylferrocene; E1/2 for sublimed
ferrocene was 0.55 V. EPR spectra were obtained using a
Bruker EMX X-band spectrometer equipped with a Bruker
variable temperature accessory, a Systron-Donner micro-
wave frequency counter and a Bruker gaussmeter; ca
5 mM THF/TBAPF6 solutions of the compound were
reduced electrochemically in situ.
22000 cmꢀ1 were generally shoulders on the intense aryl
transitions and were not studied in detail nor were the weak
transitions which were observed (Fig. 3) from 20000 cmꢀ1
to the LMCT band. The energy of some was dependent on
the annelation of the ring and were solvatochromatic. Of
greatest interest were the weak bands at ꢂ17200 cmꢀ1
the energy of which varied little with aryl and were not sol-
vatochromic (Table 2). These weak bands are assigned to
HOMO–LUMO transitions (Fig. 5) but because both
energy levels have considerable aryl character they cannot
be described as spin-allowed d–d transitions as is the case
for FcH+ [1,21,22,24]. The calculated HOMO–LUMO
gap for the 17e species is significantly smaller than for
the corresponding 18e, consistent with the lower energy
for the HOMO–LUMO transition in the 17e species.
Acknowledgements
We thank Dr. A Rieger and the late Prof. P Rieger
(Brown University) for assistance in recording and analy-
sing the epr spectra and Dr. B. James (University of Auck-
land) for attempting to record UPS spectra of the dyads.
4. Conclusion
References
Perturbation of a ferrocenyl moiety by an unsaturated
RC2 or RC4 (R = aryl) group leads to an increase in the
energy of the ferrocenyl p bonding orbitals such that the
frontier orbital configuration for both 17e and 18e species
has the dyad p orbitals at HOMO and HOMO ꢀ 2. The rel-
ative importance of the aryl contribution to the p orbitals is
dependent on the ionization potential of the –C„CR group
and the HOMO has a significant aryl contribution with large
annelated rings. Both the HOMO and HOMO ꢀ 2 provide a
delocalised system for communication between the ferroce-
nyl and aryl groups but the experimental data suggest that
there is not a strong p interaction in the ground state.
Low-energy LMCT are ubiquitous in 17e (metallocenyl)+-
CnR systems; R is a formal p donor and includes aromatic
derivatives such as naphthalimide [13] generally regarded
as acceptors. A wide range of metallocenyl groups, e.g. g-
C5H4Ni(PPh3) [25], g-C5H4Co(Ph4C4) [26] can be involved.
As the LMCT energies range from 12500 to 4000 cmꢀ1 these
systems represent a tunable electrochromic series. The chal-
lenge is to increase the intensity of the LMCT bands.
[1] Y.S. Sohn, D.N. Hendrickson, H.B. Gray, J. Am. Chem. Soc. 92
(1970) 3233.
[2] S. Barlow, S.R. Marder, J. Chem. Soc., Chem. Commun. (2000) 1555.
[3] (a) S. Evans, M.L.H. Green, B. Jewitt, A.F. Orchard, C.F. Pygall, J.
Chem. Soc., Faraday Trans. 2 (68) (1972) 1847;
(b) C. Cauletti, J.C. Green, M.R. Kelly, P. Powell, J. van Tilburg, J.
Robbins, J.C. Smart, J. Electron. Spectrosc. Rel. Phenom. 19 (1980)
327.
[4] M. Lein, J. Frunzke, A. Timoshkin, G. Frenking, Chem. Eur. J. 7
(2001) 4155.
[5] Z-F. Xu, Y. Xie, W-L. Feng, H.F. Schafer III, J. Phys. Chem. A 107
(2003) 2716.
[6] (a) N.J. Long, Angew. Chem., Int. Ed. Engl. 34 (1995) 21;
(b) A. Togni, T. Hayashi, Ferrocenes, VCH, Weinheim, 1995.
[7] I.R. Whitall, A.M. McDonagh, M. Samoc, M.G. Humphrey, Adv.
Organometal. Chem. 42 (1998) 291.
[8] S. Barlow, H.E. Bunting, C. Ringham, J.C. Green, G.U. Bublitz, S.G.
Boxer, J.W. Perry, S.R. Marder, J. Am. Chem. Soc. 121 (1999) 3715.
[9] (a) A.R. Pike, L.C. Ryder, B.R. Horrocks, W. Clegg, M.R.J.
Elsegood, B.A. Connolly, A. Houlton, Chem. Eur. J. 8 (2002) 2891;
(b) M. Hocek, P. Stepnicka, J. Ludvik, I. Cisarova, I. Votruba, D.
Reha, P. Hobza, Chem. Eur. J. 10 (2004) 2058;
(c) L. Lin, A. Berces, H-B. Kraatz, J. Organometal. Chem. 556 (1998)
11.
5. Experimental
[10] M. Malaun, R. Kowallick, A.M. McDonagh, M. Marcaccio, R.L.
Paul, I. Asselberghs, K. Clays, A. Persoons, B. Bildstein, C. Fiorini, J-
M. Nunzi, M.D. Ward, J.A. McCleverty, J. Chem. Soc., Dalton
Trans. (2001) 3025.
[11] (a) A. Mendiratta, S. Barlow, M.W. Day, S.R. Marder, Organomet-
allics 18 (1999) 454;
(b) J. Ruiz, F. Ogliaro, J-Y. Saillard, J-F. Halet, F. Varret, D. Astruc,
J. Am. Chem. Soc. 120 (1998) 1169.
[12] S-G. Liu, I. Perez, N. Martin, L. Echegoyen, J. Org. Chem. 65 (2000)
9092.
[13] C.J. McAdam, J.L. Morgan, B.H. Robinson, J. Simpson, P.H.
Rieger, A.L. Rieger, Organometallics 22 (2003) 5126.
[14] (a) C.J. McAdam, B.H. Robinson, J. Simpson, Organometallics 19
(2000) 3644;
(b) E.M. McGale, B.H. Robinson, J. Simpson, Organometallics 22
(2003) 931.
[15] (a) S. Fery-Forgues, B. Delavaux-Nicot, J. Photochem. Photobiol. A
132 (2000) 137;
The synthesis and characterisation of the compounds
discussed herein have been published elsewhere [16]. Calcu-
lations: All ab initio calculations were performed using the
B3LYP hybrid density functional theory with a 6-31G(d)
basis set. All calculations were run within Spartan ’02 and
then QChem quantum module. The unrestricted B3LYP
was used for calculations on the non-18 electron systems.
Spectroelectrochemistry: Cyclic and square wave voltamme-
try in CH2Cl2 were performed for all compounds using a
three-electrode cell with a polished disk, Pt (2.27 mm2) as
the working electrode; solutions were ꢂ10ꢀ3 M in electroac-
tive material and 0.10 M in supporting electrolyte (triply
recrystallised TBAPF6). Data were recorded on an EG&G
PAR 273A computer-controlled potentiostat. Scan rates
of 0.05–1 V sꢀ1 were typically employed for cyclic voltam-
(b) M.D. Ward, J.A. McCleverty, J. Chem. Soc., Dalton Trans.
(2002) 275;

A.H. Flood et al. / Polyhedron 26 (2007) 448–455
455
(c) M. Murata, M. Yamada, T. Fujita, K. Kojima, M. Kurihara, K.
[20] P.K. Dutta, M.A. Thomson, Chem. Phys. Lett. 131 (1986) 435.
[21] R. Prins, Chem. Commun. (1970) 280.
[22] R. Prins, Mol. Phys. 19 (1970) 603.
[23] (a) C. Kollmar, O. Khan, J. Chem. Phys. 96 (1992) 2988;
(b) A. Berces, T. Ziegler, L. Fan, J. Phys. Chem. 98 (1994) 1584.
Kubo, Y. Kobayashi, H. Nishihara, J. Am. Chem. Soc. 123 (2001) 12903.
[16] L. Cuffe, R.D.A. Hudson, J.F. Gallagher, S. Jennings, C.J. McAdam,
R.B.T. Connelly, A.R. Manning, B.H. Robinson, J. Simpson,
Organometallics 24 (2005) 2051.
[17] S. Toma, A. Gaplovsky, M. Hudecek, Z. Langfelderova, Monatsh.
fur Chemie 116 (1985) 357.
[24] C. Levanda, K. Bechgaard, D.O. Cowan, J. Org. Chem. 41 (1976)
2700.
[18] A low-level calculation on 1a has been reported T. Lixandru, C.
Ghirvu, L. Tataru, I. Mazilu, Buletin. Institut. Politehnic din Iasi,
Chimie 23 (1977) 65.
[25] C.J. McAdam, A.R. Manning, B.H. Robinson, J. Simpson, Inorg.
Chim. Acta 358 (2005) 1673.
[26] C.J. McAdam, A.R. Manning, B.H. Robinson, J. Simpson, unpub-
lished work.
[19] E.B. Ledesma, M.J. Wornat, Anal. Chem. 72 (2000) 5437.