Parallel and perpendicular stacking of ferrocene rings. Syntheses, X-ray structures, and electrochemistry of 1,8-bis-[1-(1′-phenylthio)ferrocenyl] naphthalene and 8,8′-bis-[1-(1′-phenylthio)ferrocenyl]-1,1′- binaphthyl

Article abstract of DOI:10.1016/S0020-1693(02)01573-6

The parallel and perpendicular stacking of ferrocene rings in 1,8-bis-[1-(1′-phenylthio)ferrocenyl]naphthalene (1) and 8,8′-bis-[1-(1′-phenylthio)ferrocenyl]-1,1′-binaphthyl (2), respectively, were investigated. They were synthesized from the Suzuki coupling of 2,4,6-tris(1′-phenylthio-1-ferrocenyl)boroxin (3) with 1,8-diiodonaphthalene and 1,8-diiodo-1,1'-binaphthyl, respectively. Their structures were confirmed by single-crystal X-ray analyses. The two cyclopentadienyl (Cp) rings (C9-C13 and C19-C23) of the two respective ferrocenes Fe(1) and Fe(2) of 1 are not parallel, but are 21.3° away from parallel. The distance between the centroids of these two Cp rings is 3.336 A. The two Cp rings (C9-C13 and C9′-C13′) of the two respective ferrocenes Fe(1) and Fe(2) of 2 are not perpendicular, but have a dihedral angle of 49.9°, and the distance between two centroids of these two Cp rings is 5.255 A. The electrochemical behaviour of 1 and 2 in dichloromethane solution shows that they undergo reversibly two ferrocene-centred one-electron oxidations to the corresponding dications. The separation between the two anodic processes is greater for 1 with respect to 2, but the absolute value of the separation significantly depends upon the nature of the supporting electrolyte. In the presence of the classical [NBu ₄][PF₆] the ΔE°′ value is 0.15 V for 1 and 0.07 V for 2. In the presence of the new [NBu₄][B(C ₆F₅)₄] the ΔE°′ value increases to 0.45 V for 1 and 0.13 V for 2. In view of the low ion-pairing ability of the [B(C₆F₅)₄]^- counteranion, it is conceivable that the increased intramolecular electronic communication might essentially arise from the increased electrostatic repulsion (or through-space interaction) following the 0/+/2+ redox changes rather than from through-bond electron delocalization.

Full text of DOI:10.1016/S0020-1693(02)01573-6

Inorganica Chimica Acta 350 (2003) 259ꢂ265
/
www.elsevier.com/locate/ica
Parallel and perpendicular stacking of ferrocene rings.
Syntheses, X-ray structures, and electrochemistry of
1,8-bis-[1-(1?-phenylthio)ferrocenyl]naphthalene and
8,8?-bis-[1-(1?-phenylthio)ferrocenyl]-1,1?-binaphthyl
Duy H. Hua a,*, James W. McGill a, Kaiyan Lou a, Akiharu Ueki a, Brian Helfrich a,
John Desper a, Piero Zanello b,*, Arnaldo Cinquantini b, Maddalena Corsini b,
Marco Fontani b
a Department of Chemistry, Kansas State University, Manhattan, KS 66506, USA
b Dipartimento di Chimica, University of Siena, Via Aldo Moro, 53100 Siena, Italy
Received 25 June 2002; accepted 24 October 2002
Dedicated to Pierre Braunstein in recognition of his long and outstanding passion for chemical research
Abstract
The parallel and perpendicular stacking of ferrocene rings in 1,8-bis-[1-(1?-phenylthio)ferrocenyl]naphthalene (1) and 8,8?-bis-[1-
(1?-phenylthio)ferrocenyl]-1,1?-binaphthyl (2), respectively, were investigated. They were synthesized from the Suzuki coupling of
2,4,6-tris(1?-phenylthio-1-ferrocenyl)boroxin (3) with 1,8-diiodonaphthalene and 1,8-diiodo-1,1’-binaphthyl, respectively. Their
structures were confirmed by single-crystal X-ray analyses. The two cyclopentadienyl (Cp) rings (C9Ã
/
C13 and C19Ã
/
C23) of the two
respective ferrocenes Fe(1) and Fe(2) of 1 are not parallel, but are 21.38 away from parallel. The distance between the centroids of
˚
these two Cp rings is 3.336 A. The two Cp rings (C9Ã
/
C13 and C9?ÃC13?) of the two respective ferrocenes Fe(1) and Fe(2) of 2 are
/
˚
not perpendicular, but have a dihedral angle of 49.98, and the distance between two centroids of these two Cp rings is 5.255 A. The
electrochemical behaviour of 1 and 2 in dichloromethane solution shows that they undergo reversibly two ferrocene-centred one-
electron oxidations to the corresponding dications. The separation between the two anodic processes is greater for 1 with respect to
2, but the absolute value of the separation significantly depends upon the nature of the supporting electrolyte. In the presence of the
classical [NBu4][PF6] the DE8? value is 0.15 V for 1 and 0.07 V for 2. In the presence of the new [NBu4][B(C6F5)4] the DE8? value
increases to 0.45 V for 1 and 0.13 V for 2. In view of the low ion-pairing ability of the [B(C6F5)4]ꢀ counteranion, it is conceivable
that the increased intramolecular electronic communication might essentially arise from the increased electrostatic repulsion (or
through-space interaction) following the 0/ꢁ
/
/2ꢁ redox changes rather than from through-bond electron delocalization.
/
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Face-to-face ferrocene triads; Perpendicular stacking ferrocenes; Crystal structures; Electrochemistry; Intramolecular electronic
communication
1. Introduction
increasing relevance in the study of intramolecular
electron transfer processes in that they allow a quick
experimental evaluation of the extent of electronic
interaction between the two redox active centres as a
function of the nature and the molecular complexity of
the interposed spacers. In particular, p-conjugated
bridging systems are very frequently used in these
studies in that they are electronically predisposed to
Dating from years 70 electrochemical investigations
on diferrocenes have assumed a more and more
* Corresponding authors. Tel.: ꢁ
234 233.
E-mail address: zanello@unisi.it (P. Zanello).
/
39-0577-298 009; fax: ꢁ39-0577-
/
0020-1693/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0020-1693(02)01573-6

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D.H. Hua et al. / Inorganica Chimica Acta 350 (2003) 259ꢂ265
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1
favour the intramolecular communication [1]. Not for
nothing, ferrocene polymers have been studied for the
development of highly conductive materials [2].
120ꢂ
2H), 7.76 (d, Jꢃ
7.19ꢂ6.93 (a serious of m, 10H), 4.19 (m, 4H), 4.06 (m,
/
122 8C; H NMR (CDCl3): d 8.20 (d, Jꢃ
/
7.6 Hz,
/
7.6 Hz, 2H), 7.47 (t, Jꢃ7.6 Hz, 2H),
/
/
4H), 4.01 (m, 4H), 3.98 (m, 4H); 13C NMR (CDCl3): d
140.8, 135.5, 132.1, 128.53, 128.5, 127.3, 125.8, 125.7,
124.7, 124.4, 76.3, 72.3, 72.25, 68.5. Compound 5 (the
solid was not suitable for X-ray analysis): 1H NMR
In this picture, diferrocenes containing naphthalene
spacers [3a-g], as well as polymers of 1,8-diferrocenyl-
naphthalene [2a,3h] have attracted significant attention,
also in view of the peculiarity that the stacking
interactions of the ferrocene rings inside such molecules
may further contribute their internal electronic
mobility. Our interest in charge-transfer complexes [4],
therefore, led us to synthesize 1,8-bis-[1-(1?-phe-
nylthio)ferrocenyl]naphthalene (1) and 8,8?-bis-[1-(1?-
phenylthio)ferrocenyl]-1,1?-binaphthyl (2), in that 1,8-
bisferrocenylnaphthalenes exhibit parallel stacking
(face-to-face metallocene triads) [3a,b,e,g], whereas
8,8?-bisferrocenyl-1,1?-binaphthyls give rise to perpendi-
cular stacking of ferrocene rings. In order to avoid
intermolecular stacking of the ferrocene rings they have
been functionalized with phenylsulfenyl moieties.
(CDCl3): d 8.0ꢂ
/
6.9 (a serious of m, 11H), 5.0ꢂ4.0 (a
/
serious of m, 8H); 13C NMR (CDCl3): d 140.7, 135.3,
132.4, 130.6, 128.6, 128.5, 127.9, 127.5, 126.3, 126.0,
125.8, 125.1, 124.7, 124.2, 73.2, 72.6, 72.3, 68.8, 68.2.
2.3. 8,8?-Bis-[1-(1?-phenylthio)ferrocenyl]-1,1?-
binaphthyl (2)
To a mixture of 0.100 g (0.197 mmol) of 8,8?-diiodo-
1,1?-binaphthyl (6) [6], 0.100 g (0.296 mmol; calculation
based on boronic acid) of boroxin 3, and 0.015 g (0.020
mmol) of 1,1?-bis(diphenylphosphino)ferrocenylpalla-
dium dichloride [PdCl2(dppf)] under argon, were added
1 ml of DME and 0.20 ml of aqueous 3 N NaOH. The
solution was stirred under reflux for 1 day, cooled to
25 8C, neutralized with aqueous HCl, and extracted
twice with Et2O. The combined ether layer was washed
with aqueous NH4Cl, and brine, dried (MgSO4), con-
centrated, and column chromatographed on silica gel
using hexane and Et2O as eluants to give 0.058 g (35%
yield) of bisferrocenylbinaphthyl 2 and 0.022 g (17%
yield) of monoferrocenylbinaphthyl 7 [4], Crystallization
of 2 in dichloromethane gave crystals suitable for X-ray
2. Experimental
2.1. General methods
Nuclear magnetic resonance spectra were obtained at
400 MHz for 1H and 100 MHz for 13C in deuterio-
chloroform, and reported in ppm. Davisil silica gel,
grade 643 (200ꢂ425 mesh), was used for the flash
/
column chromatographic separation. Tetrahydrofuran
and Et2O were distilled over sodium and benzophenone
before use. Dichloromethane was distilled over CaH2
and toluene and C6H6 were distilled over LiAlH4.
Chemicals and reagents were purchased either from
Aldrich Chemical Company or Fisher Chemical Com-
pany, and were used without purification.
analysis. Compound 2 (yellow crystals): m.p. 181ꢂ
/
1
184 8C; H NMR (CDCl3): d 7.62 (d, Jꢃ
7.6 (d, Jꢃ6.8 Hz, 2H), 7.51 (d, Jꢃ8 Hz, 2H), 7.36 (t,
Jꢃ7.2 Hz, 2H), 7.30 (t, Jꢃ7.6 Hz, 2 H), 7.09 (t, Jꢃ7.6
Hz, 6H), 6.98 (d, Jꢃ7.2 Hz, 2H), 6.91 (d, Jꢃ7.6 Hz,
/
7.6 Hz, 2H),
/
/
/
/
/
/
/
2.2. 1,8-Bis-[1-(1?-phenylthio)ferrocenyl]naphthalene
(1)
4H), 3.99 (m, 6H), 3.90 (m, 2H), 3.56 (m, 2H, 3.38 (m,
2H), 3.29 (m, 4H); 13C NMR (CDCl3): d 141.5, 134.8,
134.7, 132.2, 131.1, 128.9, 128.5 (6C), 125.9, 124.7,
124.5, 123.9, 94.4, 75.1, 70.9, 65.0.
Both crystals 1 and 2 contain half molecule of
dichloromethane (a solvent in the crystallization) (Table
1).
To a mixture of 0.387 g (1.10 mmol; calculation based
on boronic acid) of boroxin 3, 0.190 g (0.50 mmol) of
1,8-diiodonaphthalene (4) [5], and 0.023 g (0.020 mmol)
of palladium acetate under argon, were added 5 ml of
1,2-dimethoxyethane (DME) and 0.42 ml of 3 N NaOH
solution. The solution was refluxed for 1 day, cooled to
25 8C, neutralized with aqueous HCl, and extracted
twice with Et2O. The combined ether layer was washed
with aqueous NH4Cl, and brine, dried (MgSO4), con-
centrated, and column chromatographed on silica gel
using C6H14 as eluant to give 0.109 g (33% yield) of
bisferrocenylnaphthalene 1 and 0.044 g (16% yield) of
monoferrocenylnaphthalene 5. Crystallization of 1 in a
mixture of Et2O and CH2Cl2 gave crystals suitable for
X-ray analysis. Compound 1 (yellow crystals): m.p.
2.4. Materials and apparatus for electrochemistry
Materials and apparatus for electrochemistry have
been described elsewhere [10] Electrochemical grade
[NBu4][PF6] Fluka was used as supporting electrolyte,
whereas [NBu4][B(C6F5)4] was prepared according to
published procedure [11]. All the potential values are
referred to the saturated calomel electrode (SCE).

D.H. Hua et al. / Inorganica Chimica Acta 350 (2003) 259ꢂ
/
265
261
Table 1
Crystal data of 1 and 2
Similarly reaction of boroxin 3 and 1,8-diiodo-1,1?-
binaphthyl (6) [6] using 1,1?-bis(diphenylphosphino)fer-
rocenylpalladium dichloride [PdCl2(dppf)] as catalyst
afforded a 35% yield of bisferrocenylbinaphthyl 2 and
17% yield of monoferrocenylbinaphthyl 7 [4]. Crystal-
lization of 1 and 2 separately in a mixture of diethyl
ether and dichloromethane and dichloromethane, re-
spectively, gave single crystals.
Compound
1
2
Chemical formula
(C42H32Fe2S2)ꢁ
(C1H2Cl2)0.5
203
/
C52H38S2Fe2ꢁ
(C1H2Cl2)2
203
/
Temperature (K)
Crystal system
Space group
monoclinic
P21/c
Bruker
monoclinic
C2/c
Bruker
Instrumentation
SMART 1000
Mo Ka
graphite
SMART 1000
Mo Ka
graphite
3.2. X-ray structure
Radiation source
Radiation treatment
The molecular structures of 1 and 2 are firmly
established by single-crystal X-ray analyses, and Figs.
1 and 2 show the thermal ellipsoid drawings of 1 and 2,
respectively.
Although single-crystal X-ray structures of 1,8-difer-
rocenylnaphthalene and its related metallocene [3a,b,e]
had been reported, 1 and 2 represent two new crystal
structures of naphthalene and binaphthyl diferrocenes.
The asymmetric unit of 1 contains one molecule of 1 and
1/2 molecule of CH2Cl2. In molecule 1, the ferrocene
moiety containing Fe(1) is essentially eclipsed [the
dihedral angle of the centroid of cyclopentadienyl (Cp)
monochromator
SMART [7]
monochromator
SMART [7]
Data collection
software
Color, habit
yellow needle
8.145(3)
28.114(10)
16.826(6)
90
yellow prism
28.452(7)
8.189(2)
19.743(5)
90
˚
a (A)
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
Volume (A )
Z
m (mmꢀ1
112.358(10)
90
103.091(4)
90
3
˚
3563(2)
4
1.04
4480(2)
4
1.02
)
Dimensions (mm3)
Detn’ of space group
0.08ꢄ/0.01ꢄ/0.01 0.50ꢄ
/
0.20ꢄ0.20
/
ring (C9Ã
C18)ÃC16 is 1.18], while in ferrocene moiety containing
Fe(2) is staggered [the dihedral angle of the centroid of
Cp ring (C19ÃC23)ÃC23 and the centroid of Cp ring
(C24ÃC28)ÃC27 is 41.28]. A significant conformational
feature of molecule 1 is that the two Cp rings, C9ÃC13
and C19ÃC23, of respectively Fe(1) and Fe(2), are not
parallel, but are 21.38 (the angle between the plane of Cp
ring C9ÃC13 and the plane of Cp ring C19ÃC23) away
from parallel. The distance between the centroids of
/
C13)Ã
/
C10 and the centroid of Cp ring (C14Ã
/
systematic
absences
SAINT [8]
5037
systematic
absences
SAINT [8]
14 307
/
Data integration software
Reflections collected
Unique reflections
Observed reflections
2U max/min
/
/
/
/
1117
524
5106
2891
/
2.90/27.44
2.94/56.98
/
h max/min
k max/min
ꢀ
/
5, 5
ꢀ
/
35/36
10/10
24/23
ꢀ
/
18, 18
ꢀ
/
/
/
l max/min
Rint (%)
ꢀ
/
11, 11
ꢀ/
13.2
4.42
˚
these two Cp rings is 3.336 A, a short distance that may
allow an efficient electron-transfer process to occur. The
˚
Absorption correction multi-scan
Transmission max/min 0.578/0.552
Solution/refinement software SHELXTL [9]
multi-scan
1.00/0.78
SHELXTL [9]
distance of C1Ã
/
C8 is 2.643 A.
Molecule 2 sits on a crystallographic twofold axis,
Structure solution
Structure refinement
direct methods
full-matrix least-
squares on F2
4.56
direct methods
full-matrix least-
squares on F2
4.57
which bisects the C1Ã
also contains one molecule of 2 and 1/2 molecule of
dichloromethane. The ferrocene moieties are eclipsed
/
C1? axis. The asymmetric unit of 2
R1 (observed data) (%)
wR2 (all data) (%)
8.81
0.804
14.02
0.935
[the dihedral angle of the centroid of Cp ring (C9Ã
/
S
Difference peak/hole (eꢀ A ) 0.214/ꢀ
/0.233
0.607/ꢀ0.529
/
3
˚
C13)ÃC10 and the centroid of Cp ring (C14ÃC18)Ã
/
/
/
C16 is 2.58]. The two naphthyl rings do not lie
orthogonally, instead, an angle of 61.38 is resulted
from the two planes. The dihedral angle of the two Cp
3. Results and discussion
rings, (C9Ã
/
C13) and (C9?ÃC13?), of respective Fe(1) and
/
Fe(2) is 49.98, and the distance between two centroids of
3.1. Synthesis
˚
Cp ring (C9Ã
/
C13) and Cp(C9?ÃC13?) is 5.255 A.
/
Some selected bond lengths and bond angles of 1 and
2 are listed in Tables 2 and 3, respectively.
Suzuki coupling reactions [4,12] were used to synthe-
size 1 and 2 from 2,4,6-tris(1?-phenylthio-1-ferroce-
nyl)boroxin (3) [13] and aryl diiodides (Scheme 1).
1?-Phenylthio-1-ferroceneboronic acid apparently un-
dergoes dehydration to give the trimeric boroxin [13].
Treatment of 3 with 1,8-diiodonaphthalene (4) [5], 0.02
3.3. Electrochemistry
Fig. 3 illustrates the redox aptitude of complexes 1
and
[NBu4][PF6] as supporting electrolyte as it results from
cyclic and square wave voltammetric techniques.
2
in dichloromethane solution containing
equiv. of Pd(PPh3)4, and NaOH in DMEꢂH2O at 85 8C
gave a 33% yield of bisferrocenylnaphthalene 1 along
with a small amount of monoferrocenenaphthalene 5.
/

262
D.H. Hua et al. / Inorganica Chimica Acta 350 (2003) 259ꢂ265
/
Scheme 1.
Table 2
Selected geometric parameters (A, 8) of 1
˚
Bond lengths
Bond angles
C1Ã
C1Ã
C8Ã
C8Ã
C9Ã
/
C2
C9
1.394(3)
1.459(3)
1.475(3)
1.483(3)
1.416(2)
1.378(3)
1.410(2)
1.755(3)
1.396(3)
C2Ã
C2Ã
C1Ã
C10Ã
C14Ã
C14Ã
C19Ã
C20Ã
S2ÃC35Ã
/
C1Ã
C1Ã
C2Ã
/
C9
121.5(3)
115.9(3)
121.7(3)
104.7(2)
105.9(2)
103.0(1)
109.7(2)
112.3(3)
116.1(2)
/
/
/C8A
/
C19
C8A
C10
/
/C3
/
/
C9Ã
C15Ã
S1ÃC29
C20Ã
C21Ã
/C13
/
/
/
C16
C10Ã
C15Ã
S1Ã
C29Ã
/C11
/
/
/C16
/
/
C21
C22
/C29
/
/
Fig. 1. The molecular structure and atom-numbering scheme for 1
with displacement ellipsoids at the 30% probability level.
/C30
/
/C36
Table 3
Selected geometric parameters (A, 8) of 2
˚
Bond lengths
Bond angles
C1Ã
C1Ã
C1Ã
C8Ã
C9Ã
C14Ã
C14Ã
C19Ã
C19Ã
/
C2
1.376(5)
1.440(5)
1.499(7)
1.497(5)
1.432(5)
1.428(5)
1.761(4)
1.769(4)
1.400(5)
C1Ã
C1Ã
C7Ã
C6Ã
C9Ã
C14Ã
C14Ã
C19Ã
C20Ã
/
C2Ã
/
C3
123.1(4)
/
C8A
C1?
C9
/
C8AÃ
/C4A
117.0(3)
117.4(3)
121.6(4)
108.4(3)
107.7(3)
104.42(18)
119.3(4)
116.2(3)
/
/
C8Ã
/
C9
/
/
C7Ã
/
C8
/
C10
/
C10Ã
C15Ã
S1ÃC19
C20Ã
C19Ã
/C11
/C15
/
/C16
/S1
/
/
/S1
/
/C21
/C20
/
/S1
As seen, both the diferrocenes undergo two sequential
oxidations featuring chemical reversibility. In 1,8-bis-[1-
(1?-phenylthio)ferrocenyl]naphthalene (1) the two pro-
Fig. 2. The molecular structure and atom-numbering scheme for 2
with displacement ellipsoids at the 30% probability level.
cesses appear sufficiently well separated (DE8?ꢃ
/
150

D.H. Hua et al. / Inorganica Chimica Acta 350 (2003) 259ꢂ
/
265
263
It is useful to look at the separation between the two
oxidations for the different diferrocenes in that such a
parameter allows a quick evaluation of the nature of the
relative mixed-valent species through the measurement
of the comproportionation constant, Kc [15]. From a
routine viewpoint, [1]ꢁ and [9]ꢁ would belong to the
partially delocalized Robin-Day Class II, whereas [2]ꢁ
and [10]ꢁ should be classified as localized Class I
systems [15b]. These data preliminarily point out that
either the lack of transannular interactions (as happens
for 10) or the remarkable elongation of the p-conjugated
spacer (as happens for 2 with respect to 1) notably
reduces the electronic communication between the two
outer ferrocene units. It is however important to under-
line that the binaphthyl conjugated system in 2 seems to
allow a better electronic interaction than the simplest
Fig. 3. Cyclic (solid line) and Osteryoung square wave (dashed line)
voltammetric responses recorded at a platinum electrode on CH2Cl2
solutions of: (a) 1 (0.7ꢄ
dmꢀ3). [NBu4][PF6] (0.2 mol dmꢀ3) supporting electrolyte. Scan rates:
CV 0.2; OSWV 0.1 Vsꢀ1
/
10ꢀ3 mol dmꢀ3); (b) 2 (0.6ꢄ10ꢀ3 mol
/
(CHÄ
FcÃ(CHÄ
rapidly to approach with increasing n. As a matter of
fact, they are almost overlapped for nꢃ3 (DE8?ꢃ0.09
V) and then completely overlapped for n ]4 [1a].
/CH)n conjugation, in that in diferrocenylpolyenes
.
/
/
CH)n ÃFc the two ferrocenyl oxidations tend
/
mV), whereas in 8,8?-bis-[1-(1?-phenylthio)ferrocenyl]-
1,1?-binaphthyl (2) the two processes are partially over-
/
/
/
lapped (DE8?ꢃ
tually centred on the naphthalene spacers have been
detected up to ꢀ2 V.
As far as 1 is concerned, step-by-step controlled
potential coulometry (Ewꢃ 0.7 V) confirms the gra-
/70 mV). No reduction processes even-
At this point it is however noted that the above
discussion (as well as pertinent literature data) on the
electronic communication and the consequent nature of
/
the mixed valent ferroceneꢂferrocenium species has to
/
/
ꢁ
/
be somewhat revised in the light of the recent use of new
supporting electrolytes containing weakly coordinating
and weakly ion-pairing anions, in particular
[NBu4][B(C6F5)4] [11]. As a matter of fact, Fig. 4 shows
the voltammetric behaviour of 1 and 2 in dichloro-
methane solution containing [NBu4][B(C6F5)4] as sup-
porting electrolyte.
Comparison with Fig. 3 makes immediately evident
that the separation between the two oxidation processes
is significantly increased, thus outlining a new classifica-
tion of the corresponding mixed-valent congeners. As
deducible from Table 5, the cation [1]ꢁ would have now
to be reclassified as completely localized Class III,
whereas [2]ꢁ should be classified as partially localized
dual consumption of two electrons per molecule. After
the removal of the first electron the original yellow
solution (lmax
ꢃ460 nm) turns brown, giving rise to a
/
charge transfer band at 1000 nm. The removal of the
second electron does not cause further changes of
colour. As a token of the chemical reversibility of the
overall anodic process, cyclic voltammetry on the
exhaustively oxidized solution affords profiles quite
complementary to the original ones.
A qualitatively similar behaviour is exhibited by the
yellow solution of 2. In this case the brown solution
resulting from exhaustive two-electron oxidation dis-
plays a broad band at 830 nm and it also affords cyclic
voltammetric responses complementary to the original
ones.
Table 4
Formal electrode potentials (V, vs. SCE), peak-to-peak separations
(mV) and difference in redox potentials (V) for the two one-electron
oxidations exhibited by the present diferrocenylnaphthalene complexes
and related species
Stated that iron-centred ferrocenium solution are
commonly blue-to-green coloured, the fact that in
both cases the oxidized solutions are brown coloured
suggests that the HOMO levels of the two derivatives
are significantly contributed by the different ligands
appended to the ferrocendiyl cyclopentadienyl rings. In
this connection it must be considered that the chemically
reversible one-electron oxidation of the yellow solution
of 1-phenylthioferrocene (8) [14] generates the corre-
a
a
Complex E8?
DEp
E8?
DEp
DE8? Kc
0.15 3.4ꢄ
0/ꢁ
ꢁ/2ꢁ
1
ꢁ
/
0.43
85
ꢁ0.58
/
70
/
102
10
103
10
2
ꢁ
/
0.39
0.45 b
0.64
0.57
0.39
ꢁ
/
0.46
0.65
0.74
0.07 1.5ꢄ
0.20 2.4ꢄ
0.10 4.9ꢄ
/
9
ꢁ
/
ꢁ
/
/
10
8
ꢁ
/
ꢁ/
/
ꢁ
/
92
80
sponding orangeꢂbrown monocation.
/
FcH
ꢁ/
The formal electrode potentials of the above discussed
redox changes are compiled in Table 4, together with
those of the related complexes 1,8-diferrocenylnaphtha-
lene (9) [3a] and 1,5-diferrocenylnaphthalene (10) [3d].
Dichloromethane solution containing [NBu4][PF6] (0.2 mol dmꢀ3
as supporting electrolyte.
)
a
Measured at 0.1 V sꢀ1
.
b
[NBu4]BF4 (0.2 mol dmꢀ3) supporting electrolyte.

264
D.H. Hua et al. / Inorganica Chimica Acta 350 (2003) 259ꢂ265
/
electron oxidations to the corresponding dications. Even
if, as expected, such a separation is greater for 1 than 2,
it is noted that the electronic interaction between the
iron centres is still present in 2 is spite of the interposi-
tion of two naphthyl groups. Given for granted that the
extended carbon/carbon double bond conjugation inside
the spacers must contribute the through-bond compo-
nent of the intramolecular communication, the fact that
in both cases the low ion-pairing supporting electrolyte
[NBu4][B(C6F5)4] significantly increases the separation
of the two one-electron oxidations with respect to the
classical [NBu4][PF6] suggests that the through-space
interaction is also highly active. Chemical oxidation of
these molecules along with the synthesis of polymers
containing repeating units of 1 and 2 are being studied
and will be reported in due course.
Fig. 4. Cyclic (solid line) and Osteryoung square wave (dashed line)
voltammetric responses recorded at a platinum electrode on CH2Cl2
solutions of: (a) 1 (0.7ꢄ
dmꢀ3). [NBu4][B(C6F5)4] (0.1 mol dmꢀ3) supporting electrolyte. Scan
rates: CV 0.2; OSWV 0.1 V sꢀ1
/
10ꢀ3 mol dmꢀ3); (b) 2 (0.6ꢄ10ꢀ3 mol
/
.
4. Supplementary material
Class II [15b]. In reality, stated the low ion-pairing
ability of the counteranion [B(C6F5)4]ꢀ, it seems con-
ceivable that the increased separation might arise from
the increased coulombic effects associated with the
Crystallographic data for structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC No. 195041 and 195042, com-
pounds 1 and 2, respectively. Copies of this information
can be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK
sequential oxidations 0/ꢁ
/
/2ꢁ rather than from varia-
/
tions of charge delocalization.
Hence, it must be considered that, since the Kc value is
contributed by different factors (i.e. electron delocaliza-
tion, electrostatic interactions, structural and magnetic
stabilizations, statistical effects) [16], the electrochemical
measurements look like particularly sensitive to the
electrostatic contribution (through-space interaction),
or to evaluate the overall intramolecular electronic
communication without a special aptitude to discrimi-
nate the remaining contributions (in particular, the
through-bond charge delocalization).
In conclusion, a face-to-face ferrocene and a perpen-
dicular ferrocene stacking systems, 1 and 2, respectively
were synthesized. Their single-crystal X-ray structures
show that the two ferrocene rings in 1 are 21.38 away
from parallel, and the two Cp rings of ferrocenes Fe(1)
and Fe(2) in 2 have a dihedral angle of 49.98. The
electrochemical studies indicate that these two molecules
undergo reversibly two separated ferrocene-centred one-
(fax: ꢁ44-1223-336-033; e-mail: deposit@ccdc.cam.ac.
/
uk or www: http://www.ccdc.cam.ac.uk).
Acknowledgements
D.H.H. gratefully acknowledges financial support
from the National Science Foundation (CHE-
0078921), National Institutes of Health (National Can-
cer Institute, CA86842), American Heart Association,
Heartland Affiliate (0051658Z, 9951177Z), NASA-Bio-
Serve Space Technologies (NAGW-1197), and Great
Plains Diabetes Research, Inc. P.Z. gratefully acknowl-
edges the financial support of the University of Siena
(PAR 2001).
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Table 5
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