Ferrocene-Cyclam: A redox-active macrocycle for the complexation of transition metal ions and a study on the influence of the relative permittivity on the coulombic interaction between metal cations

Article abstract of DOI:10.1002/1521-3765(20010702)7:13<2848::AID-CHEM2848>3.0.CO;2-P

The reaction of 1,1′-ferrocene-bis(methylenepyridinium) salt with 1,4,8,11-tetraazacyclotetradecane-5,12-dione, followed by LiAlH₄ reduction results in the formation of FcCyclam. Metal complexes of FcCyclam with M²⁺ = Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺ were synthesized from FcCyclam and the respective metal triflates. The complexation of Cu²⁺ and FcCyclam in CH₃CN is preceeded by a rapid electron transfer, followed by a slower complex formation reaction and a reverse electron transfer. The protonation constants of FcCyclam and the stability constants for the Cu²⁺ complex of FcCyclam (logK = 9.26(4) for the formation of the [Cu(FcCyclam)]²⁺ complex) were determined in 1,4-dioxane/water 70:30 v/v, 0.1 mol dm^-3, KNO₃, 25°C. By using FcCyclam one can selectively sense the presence of Cu²⁺ ions in the presence of Ni²⁺, Zn²⁺, Cd²⁺, Hg²⁺, and Pb²⁺ with a very large ΔE≈200 mV, depending on pH. The X-ray crystal structures of FcCyclam and of complexes with Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺ were determined and Fe-M²⁺ distances obtained: Fe-Co²⁺ 395.9, Fe-Ni²⁺ 385.4, Fe-Cu²⁺ 377.7, and Fe-Zn²⁺ 369.0 pm. The redox potential of FcCyclam is influenced in a characteristic manner by the complexation of M²⁺. A linear correlation of 1/r?ΔE [r = distance Fe-M²⁺ from crystal data, ΔE = E_1/2([M(FcCyclam)]²⁺)-E_1/2(FcCyclam)] was found; this is indicative of a mainly Coulomb type interaction between the two metal centers. The nature of the Fe...M²⁺ interaction was also investigated by determining ΔE in several solvents (mixtures) of different dielectric constants ε. The expected relation of ΔE?1/ε was only found at very high values of ε. At ε < 40 increased ion-pairing appears to reduce the effective positive charge at M²⁺ leading to progessively smaller values of ΔE with lowered ε. The dependence of ΔE and ε can be calculated semiquantitatively by combining the Fuoss ion-pairing theory with the Coulomb model. Wiley-VCH Verlag GmbH, 2001.

Full text of DOI:10.1002/1521-3765(20010702)7:13<2848::AID-CHEM2848>3.0.CO;2-P

FULL PAPER
Ferrocene ± Cyclam: A Redox-Active Macrocycle for the Complexation of
Transition Metal Ions and a Study on the Influence of the Relative Permittivity
on the Coulombic Interaction between Metal Cations
Herbert Plenio,*[a] Clemens Aberle,[a] Youseff Al Shihadeh,[b] Jose Manuel Lloris,[b]
RamoÂn Martínez-MaÂnÄez,*[b] Teresa Pardo,[b] and Juan Soto[b]
3
Abstract: The reaction of 1,1'-ferro-
cene-bis(methylenepyridinium) salt
0.1 moldm , KNO3, 258C. By using
FcCyclam one can selectively sense the
presence of Cu2‡ ions in the presence of
Ni2‡, Zn2‡, Cd2‡, Hg2‡, and Pb2‡ with a
very large DE ꢀ 200 mV, depending on
pH. The X-ray crystal structures of
FcCyclam and of complexes with Co2‡,
Ni2‡, Cu2‡, and Zn2‡ were determined
and Fe M2‡ distances obtained:
Fe Co2‡ 395.9, Fe Ni2‡ 385.4, Fe Cu2‡
377.7, and Fe Zn2‡ 369.0 pm. The redox
potential of FcCyclam is influenced in a
characteristic manner by the complex-
ation of M2‡. A linear correlation of
1/r DE [rˆ distance Fe M2‡ from crys-
tal data, DE ˆ E1/2([M(FcCyclam)]2‡)
E1/2(FcCyclam)] was found; this is in-
dicative of a mainly Coulomb type inter-
action between the two metal centers.
The nature of the Fe ´´´ M2‡ interaction
was also investigated by determining DE
in several solvents (mixtures) of differ-
ent dielectric constants e. The expected
relation of DE  1/e was only found at
very high values of e. At e < 40 increased
ion-pairing appears to reduce the effec-
tive positive charge at M2‡ leading to
progessively smaller values of DE with
lowered e. The dependence of DE and e
can be calculated semiquantitatively by
combining the Fuoss ion-pairing theory
with the Coulomb model.
with 1,4,8,11-tetraazacyclotetradecane-
5,12-dione, followed by LiAlH4 reduc-
tion results in the formation of FcCy-
clam. Metal complexes of FcCyclam
with M2‡ ˆ Co2‡, Ni2‡, Cu2‡, and Zn2‡
were synthesized from FcCyclam and
the respective metal triflates. The com-
plexation of Cu2‡ and FcCyclam in
CH3CN is preceeded by a rapid electron
transfer, followed by a slower complex
formation reaction and a reverse elec-
tron transfer. The protonation constants
of FcCyclam and the stability constants
for the Cu2‡ complex of FcCyclam
(logK ˆ 9.26(4) for the formation of the
[Cu(FcCyclam)]2‡ complex) were deter-
mined in 1,4-dioxane/water 70:30 v/v,
Keywords: coordination chemistry ´
electronic communication ´ ferro-
cene
chemistry
´
macrocycles
´
redox
ing,[10±12] anion sensing,[13±16] biomimetic chemistry,[17±19] or
redox-switching.[20] Numerous publications in recent years
have dealt with the phenomenon of redox-switching.[21]
Compounds possessing such properties comprise redox-active
components such as ferrocene,[22±24] cobaltocene,[25] cyman-
trene,[26] tetrathiafulvene,[27] or others[28±32] and a macrocyclic
ligand suited for the complexation of metal ions. In the
oxidized state the binding of metal cations by a redox-active
macrocyclic ligand is weak, due to repulsive charge inter-
actions, whereas in the reduced state the binding is strong. In
this manner the ability of the macrocyclic ligand to coordinate
metal ions can be switched on or off, depending on the redox
state of the redox-active component.[33] An efficient redox-
switched ligand is characterized by a large difference of the
redox potentials between the ligand and the respective ligand
with a coordinated metal cation.
Introduction
Macrocycles based on the 1,4,8,11-tetraazacyclotetradecane
(cyclam) framework are among the most popular ligands for
d- and f-block metal ions.[1] Consequently numerous deriva-
tives of cyclam have been synthesized,[2] while many metal
complexes have found applications in areas as diverse as
catalysis,[3±7] magnetic or radio imaging,[8, 9] tumor target-
[a] Prof. Dr. H. Plenio, Dipl.-Chem. C. Aberle
Institut für Anorganische Chemie, TU Darmstadt
Petersenstr. 18, 64287 Darmstadt (Germany)
Fax : (‡49)6151-166040
E-mail: plenio@tu-darmstadt.de
 Ä
[b] Dr. R. Martínez-Manez, Y. Al Shihadeh, Dr. J. M. Lloris, Dr. T. Pardo,
Dr. J. Soto
Â
Departamento de Química, Universidad Politecnica de Valencia
Camino de Vera s/n, 46071 Valencia (Spain)
E-mail: rmaez@qim.upv.es
We are interested in accomplishing a better understanding
of the factors governing the behavior of ferrocene-based
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Chem. Eur. J. 2001, 7, No. 13

2848±2861
redox switches, which will be important for the optimization
of their sensing properties on the way towards producing
immobilized sensors or redox-switch devices. In this vein the
nature of the interaction between the positively charged metal
ion coordinated by a macrocyclic ring system and the redox-
active ferrocene unit has to be elucidated. We have recently
studied the influence of protonation of a ferrocenylamine on
the redox potentials of the ferrocene unit.[34] A significant
result of these studies was that the positively charged nitrogen
and the iron of ferrocene communicate through electrostatics;
despite several approximations and simplifications, Cou-
lombꢁs law appeared adequate to describe such interactions.
chemistry of such complexes in solution hopefully will provide
us with an improved understanding of the metal ± metal
interactions. Consequently, we wish to report here in detail
about investigations of the M2‡ complexes of a new ligand, a
redox-active cyclam we term FcCyclam.[42]
Results and Discussion
Synthesis
Ligands: 1,8-N,N-substituted cyclams are less easily available
but nonetheless interesting starting materials for modified
metal ± cyclam complexes and consequently several groups
are working towards the selective functionalization of cy-
clam.[43±49] Alternatively cyclams with NH groups of different
reactivity can be synthesized from simple starting materials
like 1,4,8,11-tetraazacyclotetradecane-5,12-dione, whose syn-
thesis has been described in the patent literature. Conse-
quently, we set out to repeat the synthesis by Tomalia and
Wilson, which is not described in great detail in the patent.[50]
The reaction of ethylenediamine and methylacrylate initially
results in the quantitative formation N-(2-aminoethyl)-b-
‡
In another related study we correlated the reciprocal Fe Na
distances obtained from X-ray data of the ferrocene/crown
complexes with the shifts of the redox potentials observed
‡ [36]
upon complexation of Na .
It was already recognized
then that this approach involves simplifications; first of all
the assumption of identical structures of the ferrocene/
crown/sodium complexes in the solid state and in solution.
The coordination of alkali metal ions by crown ethers
typically leads to highly dynamic complexes and bearing in
mind the comparatively low-binding strength[37] in such
species, reliable structural information in solution species is
not easily obtained. The problem of how to predict the
communication between the metal centers led Martínez-
1
alanine as evidenced by the H NMR spectrum of the crude
reaction mixture (Scheme 1). The intermediate was dissolved
in water to give an approximately 50% solution and left for
Â
Ä
Manez et al. to propose an empirical equation based on a
Coulomb charge model, which
accounts for the maximum
oxidation shift as a function of
the charge of the oxidized elec-
troactive
framework,
the
charge of the substrate, the
number of electrons involved
in the redox reaction, the dis-
tance between the redox-active
groups, and the macroscopic
relative permittivity of the me-
dium.[38]
Scheme 1.
48 h. In contrast to the information given in the patent, we
never observed product precipitation from aqueous solutions
at this point. To isolate product from the mixture, the aqueous
solution was continuously extracted with CHCl3 for several
hours. In this manner about 10% of the crude product were
extracted into the organic solvent. The extract was composed
of the desired product (cyclic dimer) together with cyclic
monomer and trimer and contained between 10 ± 15% of
1,4,8,11-tetraazacyclotetradecane-5,12-dione based on the
1H NMR integral of the amide proton resonances at d ˆ 9.0.
After removal of CHCl3 and redissolution of the oily residue
in hot water, the desired product slowly precipitated upon
cooling. From ten attempts, yields of 1,4,8,11-tetraazacyclo-
tetradecane-5,12-dione varying between 1 ± 1.5% were at-
tained. This certainly is a very poor yield, nonetheless a single
large batch generates between 14 ± 19 g of product, since the
weakly exothermic reaction itself requires no solvent and can
be performed on a large scale owing to the low price of the
starting materials.
In contrast to crown ethers, cyclams typically form very
stable and, depending on the steric bulk of the ligand,
sometimes inert complexes with 3d metal(ii) ions (ˆ M2‡) in
which the four nitrogen donors are arranged such that they
form a square-planar environment for the metal cation.
Depending on the donor strength of counterions or solvent
molecules, weaker axial contacts between a metal ion and two
additional ligands may arise, leading in the extreme to an
octahedral coordination sphere.[39] The stable and spatially
well-defined bonding of M2‡ by cyclam make it an ideal
candidate for a detailed investigation of the Fe M2‡ inter-
actions in solution. Several cyclams with appended ferrocenes
have been described,[40, 41] but these compounds are not
suitable for our study as the distance Fe M2‡ is rather large
and ill-defined due to a flexible framework of the ligand.
Consequently it was our target to first synthesize a molecule
consisting of a ferrocene and a cyclam unit in close proximity,
to prepare metal complexes of such a ligand in which a single
M2‡ ion is held in close proximity to the ferrocene subunit, to
structurally characterize such compounds, and to study their
coordination chemistry. An extensive study of the electro-
1,4,8,11-tetraazacyclotetradecane-5,12-dione was treated
with 1,1'-ferrocene-bis(methylenepyridinium) salt in refluxing
acetonitrile (Scheme 2) to produce several macrocyclic com-
Chem. Eur. J. 2001, 7, No. 13
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H. Plenio et al.
FULL PAPER
strong oxidant in CH3CN.[53] Evidently upon mixing of the
starting materials, instead of the complexation of copper there
first appears to be a fast redox reaction between FcCyclam
‡
and Cu2‡ leading to the intermediate formation of FcCyclam
‡
and Cu , which is later followed by a slower complex-
formation/electron-transfer reaction that leads to the final
product [Cu(FcCyclam)]2‡. There is clear evidence for this in
the UV/Vis spectra (Figure 1).
We therefore propose the following to occur during
complex formation: As stated before, the mixing of FcCyclam
‡
and Cu2‡ gives rise to the very fast formation of FcCyclam
‡ [54]
‡
and Cu .
Cu certainly is not an ideal candidate for
complexation by FcCyclam as it is too large. We therefore
‡
Scheme 2.
pounds. The main product of
this reaction (ca. 50% yield) is
the 1‡1 addition product ferro-
cene ± cyclamdione. Apart from
the expected macrocycle, high-
er oligomers are also produced
in significant amounts. The 2‡2
addition product can be isolat-
ed in yields of up to 25%
together with some 3‡3 prod-
uct (yield up to 5%). In some
reactions of 1,4,8,11-tetraazacy-
clotetradecane-5,12-dione with
the
1,1'-ferrocenebis(methy-
lenepyridinium) salt, the chro-
matographic separation of the
Figure 1. UV/Vis traces (e vs l) of the reaction of FcCyclam and Cu2‡ in CH3CN.
products resulted in another
mobile band after the 3‡3
‡
believe, that Cu is first coordinated peripherally by the two
product, which may well be the 4‡4 product. However, we
did not attempt to isolate and characterize this material. The
reduction of the initial 1‡1 ferrocene ± cyclamdione is
effected by treatment with LiAlH4 in CH2Cl2/THF mixtures
to afford FcCyclam in 86% yield, following a protocol used by
Gokel et al. for the reduction of less reactive amides.[51]
secondary nitrogen atoms of FcCyclam. This should lower the
redox potential of Cu sufficiently to allow the back transfer
‡
‡
‡
of an electron from Cu to FcCyclam . Our proposal can be
summarized as: Cu2‡ ‡ FcCyclam ! Cu ‡ FcCyclam !
‡
‡
‡
‡
Cu ´´´ FcCyclam ! Cu2‡ ´´´ FcCyclam ! [Cu(FcCyclam)]2‡.
To lend more support to this idea we have attempted to
obtain information on the oxidation states of the two metal
Metal complexes: FcCyclam can be expected to produce
stable complexes with transition metal ions. However, in
contrast to cyclam itself, only the metal complexes of
FcCyclam and Cu2‡ and Zn2‡ in CH3CN are readily formed
at room temperature, whereas prolonged heating of the
respective metal triflates and FcCyclam is required to effect
the complexation of Co2‡ and Ni2‡. The relative inertness of
FcCyclam towards metal complexation is attributed to the low
degree of conformational flexibility of the tetraazamacrocycle
bridged by a 1,1'-ferrocenedimethylene unit.[52]
ions, that is, whether [Cu(FcCyclam)]2‡ consists of Fe3‡/Cu
‡
or of Fe2‡/Cu . The X-ray crystal structure data, Mössbauer
‡
data,[55] and an ESR spectrum leave no doubt about a Fe2‡/
Cu2‡ couple.
X-ray crystal structures: The crystal structures of a whole
series of metal ± FcCyclam complexes (Co2‡, Ni2‡, Cu2‡, Zn2‡)
as well as that of FcCyclam itself were determined (Table 1,
Table 2) in order to obtain the respective metal ± metal
distances in the solid state. In Figure 2 all complexes can be
viewed normal to the plane formed by the four nitrogen atoms
of the cyclam macrocycle. In Figure 3 the projection is such
that the five carbon atoms belonging to one cyclopentadienyl
ring are within the paper plane. The most important
parameter obtained from the solid-state structures of met-
al ± FcCyclam complexes are the distances between the iron of
ferrocene and the respective metal ion coordinated within the
macrocyclic unit; these are listed in Table 1. It can be seen that
the Fe M2‡ distances decrease in the series Co2‡, Ni2‡, Cu2‡,
Mechanism of [Cu(FcCyclam)]2‡ formation: Upon closer
inspection, the synthesis of [Cu(FcCyclam)]2‡ in CH3CN turns
out to be more complicated than anticipated. We observed
that the initially orange solution of FcCyclam turns blue
immediately following the addition of Cu(CF3SO3)2 solution
and that during the next couple of minutes the color changes
from blue to the dark pink of the [Cu(FcCyclam)]2‡ complex.
Two facts are important here: Blue is a typical color for a
ferrocenium ion, while it is also well known that Cu2‡ is a
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Chem. Eur. J. 2001, 7, No. 13

Ferrocene ± Cyclam Complexes
2848±2861
[a]
Table 1. Bond lengths, angles, and nonbonded distances in FcCyclam, [Co(FcCyclam)]2‡, [Ni(FcCyclam)]2‡
,
[Cu(FcCyclam)]2‡, and [Zn(FcCyclam)]2‡
.
FcCyclam
FcCyclam Co2‡
FcCyclam Ni2‡
FcCyclam Cu2‡
FcCyclam Zn2‡
Fe M2‡ [pm]
Cp Cp' [8]
M N [pm]
395.9(10)
7.3
200.2(5), 197.7(5),
200.4(5), 197.6(5)
385.4(11)
6.4
195.9(5) 194.8(4)
377.7(11)
9.8
203.0(5), 202.3(5),
202.4(5), 201.6(5)
369.0(9)
4.9
208.0(4), 208.0(4)
203.5(4), 205.5(4)
4.6
N-M-N [8]
N1-M-N2
N2-M-N3
N3-M-N4
N4-M-N1
N1-M-N3
N2-M-N4
93.92(18)
87.12(19)
94.27(18)
87.16(19)
160.23(17)
172.80(19)
95.1(2)
87.0(2)
95.7(2)
86.4(2)
96.4(2)
86.0(2)
165.3(2)
162.0(2)
87.91(18)
102.93(18)
87.99(17)
101.79(17)
160.87(17)
115.04(18)
166.1(3)
162.4(3)
cis-N N [pm][b]
295.4, 307.6,
295.3, 308.2
290.8, 274.3,
291.7, 274.2
Co N5 224.7
269.0, 288.3
300.5, 277.1,
301.1, 276.0
Cu O5 283.5
285.7, 321.9,
287.2, 320.9
M X [pm][c]
Ni O2 320.8
[a] Data from ref. [42]. [b] Non-bonded inter-nitrogen distances, the longer distances are over (CH2)3 bridges and the shorter ones over (CH2)2 units.
[c] Axial interactions of the metal ion with another donor atom, for Co2‡ this is CH3CN, for Ni2‡ this is (CH3)2CO, and for Cu2‡ this is CF3SO3
.
Table 2. Summary of X-ray crystal data of FcCyclam and the respective complexes with Co2‡, Ni2‡, Cu2‡, and Zn2‡
.
FcCyclam
[Co(FcCyclam)]
(CF3SO3)2 ´ CH3CN
[Ni(FcCyclam)]
[Cu(FcCyclam)]
[Zn(FcCyclam)]
(CF3SO3)2
(CF3SO3)2 ´ Me2CO[a] (CF3SO3)2 ´ 2Me2CO
empirical formula
C22H34FeN4
410.38
183
C26H37CoF6FeN5O6S2
805.48
293
C27H40F6FeN4NiO7S2 C30H44CuF6FeN4O8S2
C24H34F6FeN4O6S2Zn
773.89
213
1
Mr [gmol
T [K]
]
825.3
874.11
213
293
crystal system
space group
a [pm]
b [pm]
c [pm]
a [8]
b [8]
g [8]
V [Š3]
orthorhombic
Pbca
9.612(2)
16.040(3)
26.304(5)
90
90
90
4055.5(14)
8
1.344
0.758
orthorhombic
P212121
10.394(2)
29.826(6)
10.416(2)
90
tetragonal
P43212
10.5735(15)
10.5735(15)
30.251(6)
90
triclinic
P1
monoclinic
P21/c
18.741(4)
10.679(2)
15.392(3)
90
102.48(3)
90
3007.7(19)
4
Å
10.328(8)
12.974(7)
14.460(3)
102.311(15)
105.458(19)
102.51(5)
1745.8(16)
2
90
90
90
90
3229.1(11)
4
1.657
1.175
1648
3382.0(10)
8
1.621
1.192
1704
Z
3
1calcd [gcm
m [mm
F(000)
]
1.663
1.231
890
1.709
1.502
1584
1
]
1760
crystal size [mm]
q range [8]
index range (hkl)
reflections
0.25 Â 0.20 Â 0.12
0.76 Â 0.51 Â 0.25
0.4 Â 0.4 Â 0.2
0.5 Â 0.4 Â 0.4
3.6 ± 25.0
12,12; 15,15; 13,17
6107/5832
0.9 Â 0.7 Â 0.7
2.9± 26.0
22,23; 0,13; 18,0
6124/5889
1.6 ± 28.3
1.4 ± 23.5
2.7 ± 26.0
12,12; 20,17; 31,33
24559/4888
11,11; 33,33; 11,11 0,12; 13,0; 37,0
20297/4672
3524/3137
collected/independent
data/parameters
GooF
4888/252
0.796
4672/430
1.066
3137/219
1.057
5832/426
1.060
5889/397
1.095
Final R indices
R1/wR2 [I > 2s(I)]
R1/wR2 (all data)
largest peak/hole [eŠ
0.0438/0.0773
0.1125/0.089
0.46/ 0.35
0.0438/0.1076
0.0482/0.1090
0.49/ 0.63
0.0452/0.1101
0.1175/0.1323
0.42/ 0.37
0.0811/0.2075
0.1129/0.2300
0.98/ 0.88
0.0679/0.1925
0.0895/0.2040
3.12/ 1.32
3
]
[a] Data from ref. [42].
Zn2‡ even though the respective radii of M2‡ increase.[56]
Upon comparing the shifts of the redox potentials DEˆ
E1/2([M(FcCyclam)]2‡) E1/2(FcCyclam) and the Fe M2‡ cat-
ion distances r(Fe M2‡) an evident correlation between the
two values can be observed, that is, a decreased r(Fe M2‡)
results in a larger DE. This phenomenon will be discussed in
more detail in the electrochemistry section.
The geometry of the ferrocene unit itself is unspectacular in
all complexes. The two cyclopentadienyl rings are almost
coplanar (tilt angle between 4.6 ± 9.88). In the Co2‡, Ni2‡, and
Cu2‡ complexes of FcCyclam the coordination geometry
around the metal ions can be best described as square-planar,
when refering to the cyclam unit alone. The environment of
the Co2‡ metal ion is better described as square-pyramidal
owing to the weak coordination of an additional solvent
molecule (CH3CN). Apart from coordination by the four
cyclam nitrogen atoms, several metal ions display additional
(very) weak contacts to solvent molecules (Co2‡ NCCH3,
Ni2‡ OC(CH3)2) or counterions (Cu2‡ O3SCF3) (Table 1).
No such contacts are seen in the case of [Zn(FcCyclam)Zn]2‡.
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H. Plenio et al.
FULL PAPER
Figure 2. X-ray crystal structures of FcCyclam, [Co(FcCyclam)]2‡
,
[Ni(FcCyclam)]2‡, [Cu(FcCyclam)]2‡, and [Zn(FcCyclam)]2‡. All struc-
tures are viewed normal to the plane formed by the four nitrogen atoms of
the cyclam ligand.
Figure 4. Root mean square fit of the X-ray crystal structures of FcCyclam
and [Zn(FcCcylam)]2‡
.
the macrocycle adopting the trans I configuration. Again the
Zn2‡ complex is different in that the NH units are pointing
away from the ferrocene towards the periphery of the
complex, resulting in the trans V configuration. The metal ±
nitrogen distances observed are in the normal range for
metal ± cyclam complexes.[57]
UV-visible spectroscopy: The UV-visible spectra of the Co2‡,
Ni2‡, and Cu2‡ complexes of FcCyclam are dominated by the
d ± d transitions of the respective M2‡ ions, whereas the
spectra of [Zn(FcCyclam)]2‡ and FcCyclam itself display the
much weaker ferrocene transitions (Figure 5). The lmax and
Figure 3. X-ray crystal structures of FcCyclam, [Co(FcCyclam)]2‡
,
[Ni(FcCyclam)]2‡, [Cu(FcCyclam)]2‡, and [Zn(FcCyclam)]2‡. All struc-
tures are viewed normal to the plane formed by five carbon atoms of a
cyclopentadienyl ring. Only the nitrogen atom of the CH3CN molecule
weakly coordinated to Co2‡ is shown.
Evidently this metal ion is well shielded within the cavity close
to the ferrocene. The coordination sphere of Zn2‡ is some-
where between tetrahedral and square-planar, since the
ferrocene unit linking the two tertiary nitrogen atoms
enforces a near linear arrangement of the N-Zn2‡-N unit,
whereas the two secondary amines are more flexible resulting
in a much smaller N-Zn2‡-N angle (Table 2). Alternatively the
geometrical environment around Zn2‡ may be viewed as
trigonal pyramidal with one of the equatorial ligands missing.
It is quite interesting to compare the conformations of the
ligand in the different metal complexes; only the structure of
the ligand in [Zn(FcCyclam)]2‡ is quite similar to that of
FcCyclam itself, as can be seen in the residual-mean-square fit
of the two structures in Figure 4.
One interpretation is that the Fccylam ligand is perfectly
suited for the complexation of Zn2‡. Another more convinc-
ing idea is that the d10 ion Zn2‡, which is rather flexible with
respect to coordination geometries, is most willing to accept
the geometry provided by the ligand. Another similarity
between FcCyclam complexes of Co2‡, Ni2‡, and Cu2‡ is the
orientation of the NH units of the respective two secondary
amines, which are pointing towards the ferrocene unit, with
Figure 5. UV/Vis spectra (e vs l) of FcCyclam and [M(FcCyclam)]2‡
3
complexes (M2‡ ˆ Co, Ni, Cu, Zn) as a 10 3 moldm solution in CH3CN.
the e values of FcCyclam and [M(FcCyclam)]2‡ (M ˆ Co, Ni,
Cu, Zn) are as follows: FcCyclam 434 (95), [Co(FcCyclam)]2‡
460 (446), [Ni(FcCyclam)]2‡ 490 (305), [Cu(FcCyclam)]2‡ 520
1
(316), [Zn(FcCyclam)]2‡ 425 nm (125 Lm 1 cm ).
Solution studies
Protonation constants: Solution studies directed toward the
determination of protonation constants and stability constants
for the formation of complexes of FcCyclam and (FcCyclam)2
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Chem. Eur. J. 2001, 7, No. 13

Ferrocene ± Cyclam Complexes
2848±2861
with Cu2‡ and phosphate in 1,4-dioxane/water (70:30 v/v,
0.1 moldm KNO3) have been carried out. Other solvents
In a similar manner the difference between the logarithms
of the first and second protonation processes (DlogK ˆ 3.15)
compares well with those found in related compounds.[59] It is
well known that the last protonation processes in small cyclic
polyazacycloalkanes are much less basic due to large electro-
static repulsion between polyammonium groups.[60] This effect
is also observed in FcCyclam with a logarithm of the stability
constant for the third protonation of logK ˆ 3.14. In addition
the last protonation process is too acidic to be observed under
the working experimental conditions. The (FcCyclam)2 ligand
behaves as a hexaprotic base in 1,4-dioxane/water mixtures.
The difference between the logarithm of the first and second
protonation process is very small (DlogK ˆ 0.76), suggesting
that the second protonation takes place in the cyclam ring that
is still unprotonated. The third and fourth protons should
attack cyclam units that are already monoprotonated with the
resulting diminution of the protonation constant. The fifth
and sixth protonations are much less basic; these two
protonations should take place near two already protonated
ammonium groups. The last two protonations for (FcCyclam)2
were not observed.
3
such as water or DMSO/water mixtures were not used due to
insolubility of the receptors. Protonation constants of FcCy-
clam and (FcCyclam)2 are shown in Table 3, whereas Table 4
‡
reports the stability constants found for the FcCyclam/H /
‡
Cu2‡ and (FcCyclam)2/H /Cu2‡ systems. Figure 6 shows the
Table 3. Stability constants (logK)[a] in 1,4-dioxane/water (70:30 v/v,
0.1 moldm 3 potassium nitrate, 258C) for FcCyclam and (FcCyclam)2.
Reaction
L ˆ FcCyclam
9.21(2)
15.27(1)
18.46(2)
±
±
±
6.06
3.14
±
±
±
L ˆ (FcCyclam)2
‡
‡
L ‡ H > [HL]
11.46(2)
21.86(3)
30.73(4)
37.06(6)
40.18(7)
42.40(7)
10.40
8.87
6.33
3.12
2.22
‡
L ‡ 2H > [H2L]2‡
L ‡ 3H‡>[H3L]3‡
‡
L ‡ 4H > [H4L]4‡
‡
L ‡ 5H > [H5L]5‡
‡
L ‡ 6H > [H6L]6‡
‡
‡
[HL] ‡ H > [H2L]2‡
[H2L]2‡ ‡ H > [H3L]3‡
‡
[H3L]3‡ ‡ H > [H4L]4‡
‡
[H4L]4‡ ‡ H > [H5L]5‡
‡
[H5L]5‡ ‡ H > [H6L]6‡
‡
[a] Values in parentheses are standard deviations on the last significant
digit.
Metal and anion complexation: The Cu2‡ complexes of
FcCyclam and (FcCyclam)2 in 1,4-dioxane-water are readily
formed at room temperature, and we have not found any
evidence in this aqueous ± organic mixture of the mechanism
of FcCyclam Cu2‡ formation as described for CH3CN.
Potentiometric studies in the presence of Cu2‡ therefore
allowed us to determine the complex stability constants with
the FcCyclam and (FcCyclam)2 receptors. The FcCyclam
forms the mononuclear complexes [Cu(HFcCyclam)]3‡,
Table 4. Stability constants (logK)[a] for the formation of Cu2‡ complexes
3
of FcCyclam and (FcCyclam)2 in 1,4-dioxane/water (70:30 v/v, 0.1 moldm
potassium nitrate, 258C).
Reaction
L ˆ FcCyclam
±
L ˆ (FcCyclam)2
Cu2‡ ‡ L ‡ 3H > [Cu(H3L)]5‡
37.78(7)
33.65(6)
27.51(7)
19.62(7)
8.83(8)
‡
Cu2‡ ‡ L ‡ 2H > [Cu(H2L)]4‡
±
‡
Cu2‡ ‡ L ‡ H > [Cu(HL)]3‡
13.85(5)
9.26(4)
3.31(6)
‡
‡
Cu2‡ ‡ L > [Cu(L)]2‡
[Cu(FcCyclam)]2‡, and [Cu(FcCyclam)(OH)] . The loga-
Cu2‡ ‡ L ‡ H2O > [Cu(L)(OH)]
‡
rithm of the formation constant of the [Cu(FcCyclam)]2‡
complex (Cu2‡ ‡ FcCyclam ˆ [Cu(FcCyclam)]2‡) is logK ˆ
9.26. This stability constant is almost 1018 times smaller than
the stability constants of the [Cu(cyclam)]2‡ complex in
water[61] (cyclam ˆ 1,4,8,11-tetraazacyclotetradecane) and
1010 times smaller than the stability constant of the complex
[Cu(Fc4cyclam)]2‡ in THF/water (70:30 v/v). (Fc4cyclam ˆ
1,4,8,11-tetrakis(ferrocenylmethyl)-1,4,8,11-tetraazacyclotetra-
decane).[3]
‡
‡ H
[a] Values in parentheses are standard deviations on the last significant
digit.
The stability constants of Cu2‡ with the receptor (FcCy-
clam)2 (1:1 molar ratio) have also been determined by using
potentiometric methods. The mononuclear complexes
[Cu(H3(FcCyclam)2)]5‡, [Cu(H2(FcCyclam)2)]4‡, [Cu(H(Fc-
Cyclam)2)]3‡, [Cu((FcCyclam)2)]2‡, and [Cu((FcCyclam)2)-
‡
(OH)] were found. Solution studies with (FcCyclam)2 and
Cu2‡ were carried out by using 1:1 metal-to-ligand rations and,
therefore, only mononuclear complexes were found. Studies
with metal-to-ligand 2:1 ratio will probably lead to found
dinuclear complexes, but these experiments were not carried
out.
‡
Figure 6. Distribution diagram of the FcCyclam-H -Cu2‡ system.
The logarithm of the stability constants of the [Cu-
{(FcCyclam)2}]2‡ complex (Cu2‡ ‡ (FcCyclam)2 ˆ [Cu{(Fc-
Cyclam)2}]2‡) is logK ˆ 19.62. This value is 108 times smaller
than that found for cyclam in water (Cu2‡ ‡ cyclam ˆ
[Cu(cyclam)]2‡, logK ˆ 27.2),[6] but is quite similar to that for
[Cu(Me4cyclam)]2‡ (logK ˆ 18.3, Me4cyclam ˆ 1,4,8,11-tetra-
methyl-1,4,8,11-tetraazacyclotetradecane) and that for
‡
distribution diagram of the FcCyclam/H /Cu2‡ system. Only
three protonation processes have been found for FcCyclam in
1,4-dioxane/water (70:30 v/v). The logarithm of the first
protonation process (logK ˆ 9.21) is close to that found in
analogous functionalized polyazaalkanes in this medium.[58]
Chem. Eur. J. 2001, 7, No. 13
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2853

H. Plenio et al.
FULL PAPER
[Cu(Fc4cyclam)]2‡.[3] The (FcCyclam)2 receptor forms much
more stable complexes with Cu2‡ than the related FcCyclam
ligand.
We have also carried out potentiometric studies of the
interaction of FcCyclam and (FcCyclam)2 molecules with
phosphate. The FcCyclam and (FcCyclam)2 receptors are
polybases; they form charged protonated species at neutral
and acid pH and are good candidates for anion recognition
through protonated ammonium ± anion electrostatic and
hydrogen-bonding interactions.[62] Tables 5 and 6 list the
Under these experimental conditions complexes of the type
[LHj(PO4)2]j 6 have been found. Although the determination
of the nature of the existing species in solution taking only
into account potentiometric data is difficult, we have tenta-
tively assigned the complexes [LH3(PO4)2]3 [LH4(PO4)2]2 ,
,
‡
[LH5(PO4)2] , [LH6(PO4)2], [LH7(PO4)2] , and [LH8(PO4)2]2‡
‡
2
2
to the interaction between the species HL HPO4 HPO4
,
2
2
2
2
H2L2‡ HPO4 HPO4
,
H3L3‡ HPO4 HPO4
,
H3L3‡
and
2
HPO4 H2PO4 ,
H3L3‡ H2PO4 H2PO4 ,
H4L4‡ H2PO4 H2PO4 . That gives logarithms of the stability
constant for these interactions of 15.44, 17.42, 17.81, 17.30,
14.92, and 12.6, respectively.
Table 5. Stability constants (logK)[a] for the interaction of FcCyclam with
phosphate in 1,4-dioxane/water (70:30 v/v, 0.1 moldm 3 potassium nitrate,
258C).
Electrochemical behavior: One of the most promising appli-
cations of redox-functionalized molecules is their incorpora-
tion to amperometric sensing devices for target receptors.[63]
To achieve this goal, the development of systems that show
selectivity coupled to a large electrochemical shifts is of great
interest. The FcCyclam molecule has a promising architecture
with the metal-binding cavity very close to the iron atom of
the ferrocenyl group. Consequently, the shift of the redox
potential of the ferrocenyl groups in FcCyclam and (FcCy-
clam)2 in the presence of several metal ions and anions has
been studied.
Reaction[b]
L ˆ FcCyclam
L ‡ 2H ‡ PO4 > LH2PO4
L ‡ 3H ‡ PO4 > LH3PO4
L ‡ 4H ‡ PO4 > LH4PO4
L ‡ 5H ‡ PO4 > LH5PO4
HL ‡ HPO4 > LH2PO4
HL ‡ H2PO4 > LH3PO4
H2L ‡ H2PO4 > LH4PO4
H3L ‡ H2PO4 > LH5PO4
25.33(2)
34.00(2)
40.14(2)
43.56(1)
4.38
4.75
4.83
5.11
[a] Values in parentheses are the standard deviations in the last significant
digit. [b] Charges have been omitted for clarity.
First the electrochemical behavior of the FcCyclam and
(FcCyclam)2 receptors was studied in 1,4-dioxane/water
3
(70:30 v/v, 0.1 moldm KNO3) as a function of the pH and
in the presence of the metal ions Ni2‡, Cu2‡, Zn2‡, Cd2‡, Pb2‡,
and Hg2‡. Each metal ion was studied separately. The results
are presented in Figures 7 and 8 for FcCyclam and (FcCy-
Table 6. Stability constants (logK)[a] for the interaction of (FcCyclam)2
3
with phosphate in 1,4-dioxane/water (70:30 v/v, 0.1 moldm potassium
nitrate, 258C).
Reaction[b]
(FcCyclam)2
L ‡ 3H ‡ 2PO4 > LH3(PO4)2
L ‡ 4H ‡ 2PO4 > LH4(PO4)2
L ‡ 5H ‡ 2PO4 > LH5(PO4)2
L ‡ 6H ‡ 2PO4 > LH6(PO4)2
L ‡ 7H ‡ 2PO4 > LH7(PO4)2
L ‡ 8H ‡ 2PO4 > LH8(PO4)2
LH ‡ 2HPO4 > LH3(PO4)2
H2L ‡ 2HPO4 > LH4(PO4)2
H3L ‡ 2HPO4 > LH5(PO4)2
H4L ‡ 2HPO4 > LH6(PO4)2
H5L ‡ 2HPO4 > LH7(PO4)2
H6L ‡ 2HPO4 > LH8(PO4)2
50.40(8)
62.76(8)
72.02(9)
79.81(7)
85.73(8)
89.74(8)
15.46
17.42
17.81
19.27
22.07
23.86
[a] Values in parentheses are the standard deviations in the last significant
digit. [b] Charges have been omitted for clarity.
‡
‡
Figure 7. E1/2 versus pH for the FcCyclam/H and FcCyclam/H /M2‡
systems (M2‡ ˆ Ni2‡, Cu2‡, Zn2‡, Cd2‡, Hg2‡, and Pb2‡) in 1,4-dioxane/
water (70:30 v/v).
logarithms of the stability constants for the interaction of
FcCyclam and (FcCyclam)2 with this anion in 1,4-dioxane/
3
water (70:30 v/v, 0.1 moldm potassium nitrate, 258C).
Bearing in mind the values of the protonation constants of
phosphate and the receptor FcCyclam, the different species
clam)2, respectively. As already observed in related poly-
azaalkanes attached to ferrocenyl groups, there is a steady
anodic shift of the redox potential of the ferrocenyl groups
from basic to acidic pH due to the stepwise protonation of the
amino groups.[64] In the presence of stoichiometric amounts of
Ni2‡, Zn2‡, Cd2‡, Pb2‡, or Hg2‡ metal ions the E1/2 versus pH
curve for FcCyclam ligand (E1/2 ˆ half-wave potential from
rotating disc electrode (RDE) techniques) is not modified
significantly. In contrast, when the electrochemical studies
were carried out in the presence of Cu2‡ there is a remarkable
and selective anodic oxidation potential shift up to 210 mV at
‡
[LH2PO4] , [LH3PO4], [LH4PO4] , and [LH5PO4]2‡ existing
in solution can tentatively be assigned to the interaction of
‡
2
‡
2‡
LH HPO4
,
LH H2PO4 ,
LH2 H2PO4 ,
and
3‡
LH3 H2PO4 species, respectively. Assuming these inter-
actions, the logarithms of the stability constants for the
(i 3)
interaction the LHjj‡ ‡ HiPO4
species to give the
3)
[LHi‡jPO4](i‡j
complexes are 4.38, 4.75, 4.83 and 5.11.
The formation of complexes between phosphate and the
receptor (FcCyclam)2 (molar ratio 2:1) has also been studied.
2854
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Ferrocene ± Cyclam Complexes
2848±2861
highly charged species in solution, no significant variation of
the oxidation potential of the ferrocenyl groups has been
observed in the presence of the studied anions. Therefore,
both FcCyclam and (FcCyclam)2 are anion-insensitive. This
contrasts with the observed behavior of the related receptor
Fc4Cyclam that gives cathodic shifts up to 40 mVat low pH in
the presence of ATP.[65]
Influence of the solvent on the electrochemical shift: The
redox-active ligand FcCyclam offers a unique opportunity to
studyÐthrough the ferrocene redox potentialÐin some detail
the influence of more subtle effects, such as solvent variations
on the through-space electronic interactions between the iron
of ferrocene and a metal ion coordinated within the cyclam
unit. This is possible as a reuslt of several favorable properties
of the ligand. First of all large oxidation potential changes can
be observed upon complexation of metal ions; this renders it
possible to precisely probe the influence of minor effects on
the redox potentials. Furthermore the tetraazamacrocyclic
unit provides a thermodynamically stable and spatially well-
defined bonding situation for a coordinated metal ion. This
‡
‡
Figure 8. E1/2 versus pH for the (FcCyclam)2/H and (FcCyclam)2/H /M2‡
systems (M2‡ ˆ Ni2‡, Cu2‡, Zn2‡, Cd2‡, Hg2‡, and Pb2‡) in 1,4-dioxane/water
(70:30 v/v).
pH ꢀ 5. This is, to the best of our knowledge, the largest
electrochemical shift ever reported in electrochemical cation
recognition in an aqueous medium with related receptors.
In general, there is good agreement between the potentio-
metric and electrochemical results. The [Cu(FcCyclam)]2‡
complex exists in a percentage higher than 90% in the 6.5 ±
12 pH range (see distribution diagram in Figure 6). The
electrochemical behavior shows that in this pH range an
almost constant oxidation potential of about 600 mV versus
SCE is observed. When the complex [Cu(HFcCyclam)]3‡ is
formed the oxidation potential is anodically shifted. At
allows a reliable comparison of the Fe M2‡ distances M2‡
ˆ
Co2‡, Ni2‡, Cu2‡, and Zn2‡ as determined by X-ray crystal
structure analysis (see respective section) and the bonding
situation in solution, which control the redox potentials.
In a previous paper the electrochemical shift DE found in
acetonitrile for the complexes [M(FcCyclam)]2‡(CF3SO3)2
(M2‡ ˆ Co2‡, Ni2‡, Cu2‡, and Zn2‡) were measured (DE is
defined as E1/2 of [M(FcCyclam)]2‡(CF3SO3)2 E1/2 of the free
FcCyclam ligand).[42] Now, the precise distances between the
iron atom and the metal ions coordinated in the cyclam cavity
have been obtained from crystal structure determinations.
The first conclusion drawn from the X-ray data and
electrochemical measurements is that there is a good corre-
lation between the DE value of various metal ± FcCyclam
complexes and the reciprocal distance of the respective
coordinated metal to the iron atom. In fact, the shorter the
intermetallic distance, the larger is DE. Figure 9 plots the
linear dependence of DE versus 1/r for the [M(FcCy-
clam)](CF3SO3)2 complexes in acetonitrile. This is clear
evidence for a primarily Coulomb type interaction between
the different metal centers Fe M2‡.
‡
pH > 11 [Cu(FcCyclam)(OH)] exists with a consequent
decrease of the oxidation potential. In conclusion, the higher
the positive charge of the complex the higher is the oxidation
potential of the ferrocenyl groups.
The (FcCyclam)2 ligand is also able to selectively sense the
presence of Cu2‡ in 1,4-dioxane/water mixtures (see Figure 8)
electrochemically. However, in this case the maximum
electrochemical shift found in the presence of Cu2‡ is about
80 mV at pH 10. This displacement of the oxidation potential
of the ferrocenyl groups is close to that found for the parent
receptor Fc4cyclam in the presence of Cu2‡, which displayed
an anodic shift of 90 mV at pH ꢀ 9 in THF/water (70:30 v/v)
mixtures.[65] Despite the presence in both FcCyclam and
(FcCyclam)2 receptors of a ferrocenyl electro-signalling sub-
unit and cyclam binding sites, the maximum electrochemical
shift found in the presence of Cu2‡ is higher for the former
receptor than for the latter; this points to the importance of
the molecular architecture in the sensing event. This has to be
related to the closer proximity between the Cu2‡ cation and
the iron atom of the ferrocenyl group in FcCyclam upon metal
coordination.
Studies on the interaction of FcCyclam and (FcCyclam)2
with anions have also been carried out in 1,4-dioxane/water
(70:30 v/v). The anions phosphate, sulfate, adenosine-5'-
triphosphate (ATP), chloride, and bromide can hardly be
expected to enter the cavity and therefore the shift of the
oxidation potential of the ferrocenyl group should be small. In
fact, although FcCyclam forms complexes with phosphate, the
E1/2 versus pH curve of the receptor in the presence of those
anions does not differ from that of the free receptor. With
respect to the (FcCyclam)2 ligand, and despite the larger
number of amino groups in this molecule and the formation of
Figure 9. DE versus 1/r for the [M(FcCyclam)](CF3SO3)2 complexes
(M2‡ ˆ Zn2‡, Cu2‡, Ni2‡, and Co2‡) in CH3CN/THF mixtures (I 30:10; II
30:20; III 30:30; IV 20:30; V 10:30).
Chem. Eur. J. 2001, 7, No. 13
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2855

H. Plenio et al.
FULL PAPER
We wish to report here a detailed study on the influence of
solvent, namely its dielectric constant, on the electrochemical
shift of [Zn(FcCyclam)](CF3SO3)2. Compared to the FcCy-
clam complexes of Co2‡, Ni2‡, and Cu2‡, the Zn2‡ complex
appears best suited for such an investigation as the X-ray
crystal structure shows that Zn2‡ is coordinatively saturated
and localized inside the macrocyclic cavity. Consequently,
there is a good chance that the geometry of [Zn(FcCyclam)]2‡
will be relatively invariant to solvent changes. For our study a
number of suitable solvents have been chosen in order to
cover a wide range of dielectric constant values (from 64.6 to
5.0). The dielectric constant for a mixture of solvents was
Figure 10. DE versus 1/e for the [Zn(FcCyclam)](CF3SO3)2 complex in
various solvents as desrcibed in Table 7.
P
calculated as e ˆ Xiei, where Xi and ei are the molar fraction
i
and permittivity of solvent i. This certainly is no more than a
association process between anions and cations known as ion-
pairing. The formation of ion pairs involves the creation of
new species in solution that can be depicted as
FcCyclam Zn2‡ ´´´ X and FcCyclam Zn2‡ ´´´ 2X , in which
X is whatever anion present in the solution (ClO4 from the
supporting electrolyte or CF3SO3 from the M2‡ salt). The
anions do not have to bind to Zn2‡ (Zn2‡ is coordinatively
saturated as stated above), but may simply interact electro-
statically with the positively charged side of the
[Zn(FcCyclam)]2‡ complex. It is assumed here that the Fe ±
Zn2‡ distance (r) does not change upon ion-pair formation,
even though this can not be excluded.[70] When the dielectric
constant of the medium is high, ion-pairing will be negligible
and the difference DE between the oxidation potential of the
free-ligand FcCyclam and that of the respective Zn2‡ complex
can be understood by using a coulombic model, with the
electric work of removing an electron from the ferrocene in
the presence of two positive charges of the metal cation Zn2‡
at a certain distance (r). Accordingly, for solvents with 37.5 <
e < 64.6 the dependence of DE
reasonable approximation, but a more accurate calculation of
the dielectric constant of binary mixtures is not straightfor-
ward.[66] Some simple formulas to obtain such data based on
the permittivity of the respective pure solvents have been
suggested, but appear to work satisfactorily only for solvents
of similar e and with one solvent being the major compo-
nent.[67, 68] However, there is a large number of measurements
on binary mixturesÐdone mainly in the 1920s and 1930sÐin
which a good linear relationship between the molar fractions
of the individual components and e has been observed.[69] This
justifies our approach of calculating the permittivities of the
binary mixtures. The results together with other electro-
chemical data are listed in Table 7, whereas Figure 10 shows
the electrochemical shift as a function of the reciprocal
permittivity. Assuming the law of Coulomb to apply, one
would have expected a monotonous function of the type
DE  1/e, which is evidently not what is found in Figure 10.
In order to come up with explanations one should be aware
that in solvents with low dielectric constants there is an
and 1/e is as expected, since ion-
pairing does not occur to a
Table 7. Electrochemical data for the [M(FcCyclam)](CF3SO3)2 complexes in certain solvents
significant extent. However, at
e ꢀ 40 the trend of DE is re-
versed: upon decreasing e, DE
also decreases. We attribute this
to the reduced polarity of the
solvent; this increasingly favors
ion-pairing and results in a
decrease of the real positive
charge of the [Zn(FcCyclam)]2‡
complexes due to its association
with negatively charged coun-
terions. Consequently the ob-
served DE will also be small-
er.[71] A third segment of the
curve is found in the range of
very low e solvents. Evidently at
lowered e the initially linear
relation tails off towards a lim-
iting value of DE ꢀ 270 mV.
The curve obtained at e lower
than 40 can be fitted by a first-
order exponential decay. In or-
der to understand this, one has
Solvent
e
[Zn(FcCyclam)]2‡ [Cu(FcCyclam)]2‡ [Ni(FcCyclam)]2‡ [Co(FcCyclam)]2‡
ethanol
CH3CN
24.3 350
37.5 409
64.4 380
7.58 276
±
±
±
399
317
283
propylene carbonate
THF
CH2Cl2/acetone 30:30 v/v 14.4 297
CH2Cl2/acetone 20:30 v/v 15.6 299
CH2Cl2/acetone 10:30 v/v 17.4 305
THF/CH3CN 30:2 v/v
THF/CH3CN 30:4 v/v
THF/CH3CN 30:6 v/v
THF/CH3CN 30:8 v/v
THF/CH3CN 30:10 v/v
THF/CH3CN 30:20 v/v
THF/CH3CN 30:30 v/v
THF/CH3CN 20:30 v/v
THF/CH3CN 10:30 v/v
CH3CN/water 30:3 v/v
CH3CN/water 30:5 v/v
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
10.3 280
12.7 285
14.7 287
16.4 294
17.7 325
22.7 333
25.7 354
28.4 367
32.2 390
46.8 400
50.9 388
±
±
±
±
±
±
±
±
±
±
±
±
265
235
204
283
260
212
295
261
223
315
273
238
343
292
260
±
±
±
±
±
±
CH3CN/water 30:7.5 v/v 54.8 383
±
±
±
CH3CN/water 30:10 v/v
CH3CN/water 30:20 v/v
r [pm][c]
57.7 382
64.6 374
±
±
±
±
±
±
369.0(10)
377.7(11)
385.4(11)
395.9(9)
[a] DE [mV] ˆ E1/2([M(FcCyclam)]2‡
)
E1/2(FcCyclam). [b] e for a mixture of solvents have been calculated as
P
e ˆ iXiei, in which Xi and ei are the molar fraction and permittivity of solvent i, respectively. [c] rˆ distances
M2‡ iron from crystallographic data.
2856
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Chem. Eur. J. 2001, 7, No. 13

Ferrocene ± Cyclam Complexes
2848±2861
to realize that the dielectric constants given for the various
solvents (mixtures) in Table 7 are based to the dielectric value
of the respective pure solvents. However, it must be kept in
mind that during the electrochemical measurements these
iron distance in Š, e is the relative macroscopic permittivity
of the solvent, and DE is the electrochemical shift in mV.
ꢀ ꢁ
ꢂ
ꢃ
14 398 2
loge
2
r2
DE ˆ EM2‡
ˆ
‡ 39000
(6)
e
r
e
3
solvents contain a significant amount of salt (0.1 moldm
nBu4NClO4) as a supporting electrolyte. This polar admixture
must have a significant influence on the real e of the solution
in an otherwise low e solvent. Based on the data in Table 7
(see also Figure 10) it is reasonable to assume that in solvents
with low dielectric constant the presence of supporting
electrolyte prevents the real e from going below values of
ꢀ10.
At low e values the formation of the species FcCyclam
Zn2 ´´´ X and FcCyclam Zn2‡ ´´´ 2X have to be considered.
These species would show oxidation potentials of EM2‡
and
X
EM2‡
, respectively, that are generally different to that
2X
shown by the complex [Zn(FcCyclam)]2‡ (EM2‡). The oxida-
tion potential of the ion pairs are related to the work of
removing an electron from the ferrocene in the presence of a
metal ion M2‡ at a distance r and the presence of one or two
anions X at a distance a from the [Zn(FcCyclam)]2‡ cation.
In order to simplify the equations it is assumed that both
An alternative explanation of the observed electrochemical
behavior could be based on the binding of certain solvents to
the Zn2‡ ion; this is possible only after a re-organization of the
coordination environment of Zn2‡ in the FcCyclam complex.
If that was the mechanism, one should be able to find a
correlation between the Gutman donor number for a family
of solvents and the electrochemical shift DE. However, a plot
of DE versus the donor number for pure solvents such as
acetonitrile, DMF, ethanol, and propylene carbonate provides
no evidence for such an idea.
In order to account for the actual behavior, at least in a
semiquantitative manner, we have tried to combine both a
Coulomb model (that accounts for the electrostatic interac-
tions) with the Fuoss theory (that describes the formation of
ion pairs).[72] The electrochemical shift DE is defined in
Equation (1), in which Ecomplex and Eligand are the oxidation
potentials of the corresponding FcCyclam Zn2‡ complex and
that of the free receptor, respectively. Additionally Ecomplex can
also be defined as in Equation (2), in which WM2‡ is the electric
work of removing an electron from ferrocene in the presence
of two positive charges of the metal ion at a certain r distance.
This in turn leads to Equations (3) and (4).
anions
X
are to the same distance (a) from the
[Zn(FcCyclam)]2‡ cation and that the distance between both
anions is 2a. Using Equation (5) we can calculate the
oxidation potentials of EM2‡
and EM2‡
[Eqs. (7) and (8)].
2X
X
"
#
ꢀ
ꢀ
ꢁꢀ
ꢁ
14 398
2
1
lne
2
1
a2
EM2‡
ˆ
ˆ
‡ 39000
‡ 39000
(7)
2X
2X
2
e
rji
a
e
r
ji
"
#
ꢁꢀ
ꢁ
14 398
2
2
lne
2
2
EM2‡
(8)
e
rji
a
e
r2ji a2
In the above equations r describes the distance between the
iron atom of the ferrocene and the Zn2‡ cation, whereas a is
related with the distance between the [Zn(FcCyclam)]2‡
cation and the anion.
For a certain solvent the actual observed oxidation
potential shift DE would be given by Equation (9):
DE ˆ SXiEi ˆ XM2‡EM2‡ ‡ XM2‡ EM2‡
‡ XM2‡
EM2‡
2X
(9)
X
X
2X
in which Xi are the molar fractions of the [Zn(FcCyclam)]2‡
complex and the ions pairs FcCyclam Zn2 ´´´ X and
FcCyclam Zn2‡ ´´´ 2X . Xi can be obtained from the stability
constants of the equilibria given in Equations (10) ± (12):
DE ˆ Ecomplex Eligand
nFEcomplex
nFEligand ‡ WM2‡
(1)
(2)
(3)
(4)
ˆ
nFDE ˆ WM2‡ nFEM2‡
DE ˆ EM2‡
‡
‡
TBA ‡ X , TBA ´´´ X
KA
KA
KA
(10)
(11)
(12)
0
1
2
[Zn(FeCyclam)]2‡ ‡ X , [Zn(FeCyclam)]2‡ ´´´ X
[Zn(FcCyclam)]2‡ ‡ 2X , [Zn(FcCyclam)]2‡ ´´´ 2X
It has been recently reported that this oxidation potential
difference between the ligand-substrate species and that of
the free ligand can be calculated by using Equation (5).
‡
in which TBA is the tetrabutylammonium cation from the
supporting electrolyte. KA and KA can be evaluated from
ꢀ
ꢁ
0
1
zazbe2NA
1
1
X X
X X
1
DE ˆ
‡ B
(5)
Fuossꢁ equation [Eq. (13)], which for a charge of the
[Zn(FcCyclam)]2‡ ion of ‡2, a charge of ‡1 for the
tetrabutylammonium and a charge of the anion of 1 at
Tˆ 298 K can be written as:
jn 4pe0eF
rji
rji
j
i
j
i
In Equation (5) za is the charge of the oxidized electro-
active group, zb is the charge of the substrate, j is the number
of redox groups, and i the number of substrates at a certain rji
distance from the redox centres. B is an empirical parameter
that takes into account the fact that the permittivity is not
constant but a function of the distance. From previous data
KA ˆ 0.002523a3  10243.16/ea
(13)
in which a is the cation ± anion distance in Š.
By taking only electrostatic considerations into account,
and those reported in this work we have observed that B ˆ
39000ln e
.
Additionally bearing in mind that e2NA/4pe0eF ˆ
[73]
KA can be calculated as from KA by using Equation (14):
2
1
e
14398/e, the above equation can be rewritten in our case (za ˆ
1, zb ˆ 2, j ˆ 1, i ˆ 1) as Equation (6), in which r is the Zn2‡
±
RTlnKA ˆ 2RTlnKA ‡ W
(14)
2
1
Chem. Eur. J. 2001, 7, No. 13
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0947-6539/01/0713-2857 $ 17.50+.50/0
2857

H. Plenio et al.
FULL PAPER
in which W is the electric work of bring the X anion from the
infinite to the distance a, bearing in mind the repulsive
interactions of this anion with the anion X that is already
forming the ion-pair FcCyclam Zn2‡ ´´´ X . As Wˆ nFE
and bearing in mind Equation (5) we get the following
solvent permittivity for intramolecular interactions, further-
more the Coulomb model describes the interaction of point
charges, which is certainly only a crude approximation for the
interaction of iron with a transition metal M2‡.
The electrochemical behavior the [M(FcCyclam)]2‡ com-
plexes (M ˆ Cu2‡, Ni2‡ and Co2‡) have also been studied in
certain solvents. The oxidation potential shifts observed are
listed in Table 7. In general, a similar behavior to that shown
by the [Zn(FcCyclam)]2‡ complex was found.
equation [Eq. (15)] for KA :
2
"
#
ꢂ
ꢀ
ꢁ
ꢀ
ꢁ
ꢃ
F
14 398
1
log e
1
logKA ˆ logKA
‡ 39000
(15)
2
1
2
2:3 RT 1000
e
2a
e
ꢁ2a†
in which it is assumed, as stated above, that the distance
between the two anions is 2a.
Equation (9) is only a function of the relative permittivity
(e) and the anion ± cation distances (a) in ion pairs. In our
system there are two a distances corresponding to the
Conclusion
The properties of FcCyclam have been studied in detail and
the ligand shown to form complexes with M2‡ ˆ Co2‡, Ni2‡
Cu2‡, Zn2‡, which are less stable than complexes of cyclam
and the respective M2‡. The most unusual property of this
macrocycle is the redox activity of the ferrocene unit; this is
influenced in a characteristic manner by the coordination of
M2‡ by the macrocycle. The X-ray crystal structures of
FcCyclam and of all four [M(FcCyclam)]2‡ complexes have
been determined. The most important parameter obtained
from these studies is the distance r Fe ´´´ M2‡, which decreases
in the order Co2‡ (395.9 pm), Ni2‡ (385.4 pm), Cu2‡ (377.0 pm),
Zn2‡ (369.0 pm), while DE ꢂ {E1/2([M(FcCyclam)]2‡)
E1/2(FcCyclam)} increases in the same sequence of complexes:
DE ˆ ‡283 mV, ‡317 mV, ‡399 mV, ‡409 mV, respectively.
The relationship DE  1/r is in accord with a Coulomb type
interaction between Fe ´´´ M2‡ even though this model in-
volves several debatable simplifications for intramolecular
interactions. The stable and spatially well-defined bonding of
M2‡ within FcCyclam offers also the opportunity to study in
some detail the influence of solvents, that is, a variable
dielectric constant, on the propagation of the through space
interaction between the charged metal centers. The simple
relation of DE  1/e according to Coulomb only holds true for
solvents with a very high e with a lower limtit of around 40.
Upon further lowering the permittivity, ion-pairing as de-
scribed by the Fuoss theory takes place and the effective positive
charge of [M(FcCyclam)]2‡ is reduced and accordingly the
values of DE. The decrease is linear only initially and is best
described by a first-order exponential decay to a limiting
value of approximately 270 mV. The asymptotic behavior of
the curve is attributed to the fact that the real permittivity of
the solvents used for the experiments on the low polarity side
can not be reduced further due to the presence of
‡
distances between the [Zn(FcCyclam)]2‡ and TBA ions and
the corresponding anion (probably ClO4 from the supporting
electrolyte). A nonlinear regression fit of the experimental
DE versus e allowed us to determine these two anion ± cation
distances (a). These values were 420 and 480 pm, respectively,
which are reasonable anion ± cation distances in ion pairs.[74]
Figure 11 shows the good agreement between calculated and
Figure 11. Fit of the DE versus e plot by applying the combined Fuoss and
Coulomb theory.
experimental DE values for the [Zn(FcCyclam)](CF3SO3)2
complex. The fit is fairly good down to a dielectric constants
of e ꢀ 10.[73] As stated above, at lower values of e the
supporting electrolyte seems to impose a residual value of
the permittivity.
The model also predicts that the oxidation potential DE
increases as the dielectric constant decreases until e ꢀ 40
from which DE decreases when the permittivity diminishes.
Therefore, in solvents or solvent mixtures with dielectric
constants of about 40, the maximum electrochemical shift will
be observed. In this context, it is remarkable that many studies
seeking the electrochemical recognition of substrates have
been carried out in acetonitrile (e ˆ 37.5); the solvent for
which the model described above predicts the electrochemical
shift would be maximum. Solvents with lower dielectric
constants will not increase the electrochemical shift, as it
should be expected, but a decrease will be observed due to
ion-pair formation. Despite the apparent success of merging
Coulomb and Fuoss theory for the explanation of the data,
one should be aware that our approach involves at least two
simplifications. First of all it is debatable to use the bulk
3
0.1 moldm nBu4NClO4 of supporting electrolyte in the
solvents needed for the cyclic voltammetry experiments. The
dependence of DE and e can be described semiquantitatively
by combining the Fuoss theory with the Coulomb model.
In conclusion, we have synthesized a new redox-active
macrocyclic ligand, whose ligating properties can be influ-
enced by the redox behavior of the ferrocene unit in a highly
characteristic and controllable manner. In order to effect a
maximum response of a redox-active receptor as well as a
maximum redox-switching effect, the permittivity of the
solvent should be chosen so as to bring about the best
compromise between low e and a low degree of ion-pairing;
CH3CN appears to be a very good choice.
2858
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Chem. Eur. J. 2001, 7, No. 13

Ferrocene ± Cyclam Complexes
2848±2861
4H; CH2), 8.97 (brs, 2H; CONH); 13C NMR (CDCl3): d ˆ 35.42, 38.44,
Experimental Section
45.60, 48.37, 172.39.
Ferrocene ± cyclamdione:
1,1'-ferrocene-bis(methylenepyridinium)tosyl
General: Commercially available solvents and reagents were purified
according to literature procedures.[75] All reactions were carried out under
an atmosphere of argon and especially the Pd-catalyzed coupling reactions
require strict exclusion of oxygen. Chromatography was performed over
silica MN60 (63 ± 200 mm). TLC was done with on Merck plates coated with
silica gel 60F254. NMR spectra were recorded at 300 K with a Bruker
chloride (5.12 g, 9.02 mmol), 1,4,8,11-tetraazacyclotetradecane-5,12-dione
(2.06 g, 9.02 mmol), and Na2CO3 (7.0 g) were added to acetonitrile
(400 mL) and heated under reflux for 60 h. The cold reaction mixture
was filtered, and the filtrate evaporated to dryness. The residue was
purified by chromatography (CHCl3/CH3OH ˆ 10:1) to yield ferrocene ±
cyclamdione (2.0 g, 50%), 2‡2 product (0.94 g, 24%), and 3‡3 product (ca.
0.20 g, 5%).
1
Avance (1H NMR 200 MHz, 13C NMR 50.3 Mhz) spectrometer. H NMR
1
were referenced to residual H impurities in the solvent and 13C NMR to
the solvent signals: CDCl3 (d ˆ 7.26 (1H), 77.0 (13C)), C6D6 (d ˆ 7.16 (1H),
128.0(13C)), CD3CN (d ˆ 1.93 (1H), 1.30 (13C)). For the purpose of 1H NMR
signal assignment CpH denotes a proton attached to the sp2 carbon of
cyclopentadiene or to the carbon of a ferrocene h5-cyclopentadienyl ring.
Mass spectra were recorded on a Finnigan MAT3800 instrument. IR
spectra were recorded on a Bruker IFS-25 spectrometer in CHCl3. UV/Vis
spectra were measured on a JASCO-UV-570 spectrometer in CH2Cl2.
Potentiometric titrations were carried out in 1,4-dioxane/water (70:30 v/v,
0.1 moldm 3 KNO3) for FcCyclam and (FcCyclam)2 using a reaction vessel
water-thermostatted at 25.0 Æ 0.18C under nitrogen atmosphere. Exper-
imental potentiometric details have been published previously.[76] The
concentration of the Cu2‡ and the anion phosphate were determined by
using standard methods. The computer program SUPERQUAD[77] was
used to calculate the protonation and stability constants. The titration
curves for each system (ca. 250 experimental points corresponding to at
least three titration curves, pH ˆ log[H] range investigated ca. 2.5 ± 10,
concentration of the ligand being ca. 1.2 Â 10 3 moldm 3) were treated
either as a single set or as separated entities without significant variations in
the values of the stability constants. Finally the data sets were merged
together and treated simultaneously to give the stability constants.
Electrochemical data were carried out in several solvents, with a program-
mable function-generator Tacussel IMT-1, connected to a Tacussel PJT
120 ± 1 potentiostat. The working electrode was platinum with a saturated
calomel reference electrode separated from the test solution by a salt
bridge containing the solvent/supporting electrolyte. The auxiliary elec-
trode was platinum wire. The solvents used in the electrochemical
experiments were rigorously purified prior use. The supporting electrolyte
1
(1‡1) product: H NMR (CDCl3): d ˆ 2.11 ± 2.29 (m, 6H), 2.58 ± 3.36 (m,
10H), 3.82 ± 4.08 (m, 12H), 8.97 (s, NH), 9.02 (s, NH); 13C NMR (CDCl3):
d ˆ 36.11, 53.46, 54.62, 67.54, 69.29, 71.03, 88.51, 127.60, 173.24; MS-EI: m/z:
‡
438 [M] .
(2‡2) product: 1H NMR (CDCl3): d ˆ 2.13 ± 4.17 (m, 56H; CyclamH/FcH/
FcCH2), 8.40 ± 8.50 (m, 4H; OCNH); 13C NMR (CDCl3): d ˆ 29.22, 30.34,
31.78, 35.10, 48.25, 49.08, 50.48, 51.11, 51.46, 68.69, 69.34, 69.94, 71.05, 81.21,
‡
126.67, 171.84; MS-EI: m/z: 877 [M] .
(3‡3) product: In the 1H NMR (CDCl3) a complex collection of over-
‡
lapping multiplets between d ˆ 1.0 ± 5.1 is observed MS-EI: m/z: 1316 [M] .
FcCyclam: Ferrocene ± cyclamdione (4.4 g, 10 mmol) was dissolved in a
mixture of THF (100 mL) and CH2Cl2 (200 mL). LiAlH4 (7.6 g, 200 mmol)
was added to the ice-cooled solution and stirring continued for 48 h. After
careful hydrolysis of excess LiAlH4 with water (70 mL), NaOH (200 mL;
15% aq.) was added, and the product extracted with CHCl3. The organic
layer was separated, dried over MgSO4 and filtered, and the filtrate
evaporated to dryness. The residue was purified by chromatography
(CH3OH/Et2NH ˆ 10:1) to yield 3.5 g (86%) of product. 1H NMR
(CDCl3): d ˆ 1.40 (d, J ˆ 14 Hz, 2H), 1.85 (d, J ˆ 12 Hz, 2H), 2.14 (d, J ˆ
11 Hz, 4H), 2.46 ± 2.95 (m, 14H), 3.22 (td, J ˆ 7, 3 Hz, 2H), 3.73 (s, 2H),
3.93 ± 4.08 (m, 10H); 13C NMR (CDCl3): d ˆ 25.64, 47.02, 51.55, 53.40,
53.49, 58.20, 65.37, 65.76, 67.40, 70.26, 88.35; 1H NMR (CD3CN): d ˆ 1.83 ±
3.07 (m, 20H, CyclamH) 3.62 (s, 4H, FcCH2), 3.95 ± 3.97 (m, 8H, FcH);
13C NMR (CD3CN): d ˆ 26.77, 30.88, 47.86, 52.45, 54.35, 54.69, 59.47, 66.45,
‡
66.88, 68.38, 71.23, 89.54; MS-FD: m/z: 411 [M] .
Metal complexes of FcCyclam: Metal complexes of FcCyclam were
synthesized by an equimolar mixture of ligand and metal triflate in CH3CN
heating under reflux for 1 h. After evaporation of the solvent, the complex
was recrystallized from CH3CN/Et2O.
3
in all the electrochemical experiments was 0.1 moldm tetrabutylammo-
nium perchlorate. Elemental analyses were perfomrmed by the Mikro-
analytisches Laboratorium der Chemischen Laboratorien, Universität
Freiburg. Starting materials were available comercially or prepared
according to literature procedures: 1,1'-ferrocene-bis(methylenepyridi-
nium)tosyl chloride.[78]
[Co(FcCyclam)](CF3SO3)2: elemental analysis calcd (%) for C24H32CoF6-
FeN4O6S2 (765.44): C 37.56, H 4.47, N 7.30; found C 37.66, H 4.21, N 7.29.
[Ni(FcCyclam)](CF3SO3)2: 1H NMR (CD3CN) : d ˆ 1.81 ± 2.23 (m, 10H;
CyclamH), 2.47 ± 2.81 (m, 4H; CyclamH), 3.08 ± 3.34 (m, 6H; CyclamH),
3.78 ± 4.02 (m, 4H; FcCH2), 4.53 ± 4.70 (m, 8H; FcH); 13C NMR (CD3CN):
d ˆ 23.58, 51.00, 52.46, 58.26, 59.32, 59.81, 69.06, 70.70, 84.14; elemental
analysis calcd (%) for C24H32F6FeN4NiO6S2 (765.20): C 37.57, H 4.47, N 7.30;
found C 36.67, H 4.21, N 7.22.
1,4,8,11-tetraazacyclotetradecane-5,12-dione: This procedure was modified
from that of Tomalia and Wilson.[50] Methyl acrylate (688 g, 8.0 mol) was
added dropwise to neat ethylenediamine (680 g, 8.0 mol). The addition was
conducted at such a rate that the reaction temperarure did not exceed
358C; addition time was approximately 8 h. The crude colorless inter-
mediate formed in quantitative yield is N-(2-aminoethyl)-b-alanine
[1H NMR (CDCl3): d ˆ 1.26 (3H; NH), 2.51 (td, J ˆ 8.5, 2.0 Hz, 2H;
CH2), 2.71 (cm, 4H; CH2), 2.89 (td, J ˆ 8.5, 2.0 Hz, 2H,; CH2), 3.68 (s, 3H;
CH3)]. The intermediate was dissolved in water to give a 50% solution and
left standing for 48 h. The resulting solution was continuously extracted
with CHCl3 for 8 hours. After this time about 10% of the total mass of the
intermediate had been extracted by the organic solvent. The desired 14-
membered macrocycle was then extracted quantitatively from the crude
aqueous mixture with CHCl3. The extract is composed of the 1‡1 product
(seven-membered ring), the 2‡2 product (14-membered ring), and the 3‡3
product (21-membered ring). The composition of this mixture was
determined by 1H NMR spectroscopy, since the shift of the amide
hydrogen is highly characteristic for the different products [1H NMR
(CDCl3): d ˆ 6.75 (CONH of 1‡1 product), 8.35 (CONH of 3‡3 product),
9.0 (CONH of 2‡2 product)]. A typical extract contained about 72% of the
1‡1 product, 14% of the 2‡2 product, and 14% of the 3‡3 product. The
extract was evaporated to dryness, and the remaining oil dissolved in hot
water to give a concentration of ca. 25% by weight. This solution was
stored at 48C, whereupon the desired 2‡2 product crystallized slowly (over
several days) from the mixture. The product was filtered off, washed with
cold water and dried at 1008C (0.1 Torr) for several hours. Yield: 1 ± 1.4%
(13.7 ± 19.1 g). 1H NMR (CDCl3): d ˆ 1.2 (brs, 2H; NH), 2.39 (m, 4H;
CH2), 2.80 (t, J ˆ 5.3 Hz, 4H; CH2), 2.92 (t, J ˆ 5.5 Hz, 4H; CH2), 3.38 (m,
[Cu(FcCyclam)](CF3SO3)2: elemental analysis calcd (%) for C24H32CuF6-
FeN4O6S2 (770.0): C 37.43, H 4.19, N 7.28; found C 37.34, H 4.44, N 7.30.
[Zn(FcCyclam)](CF3SO3)2: 1H NMR (CD3CN): d ˆ 1.92 ± 2.33 (m, 4H;
NH), 2.77 ± 3.73 (m, 20H; CyclamH, FcCH2), 3.92 ± 4.74 (m, 8H; FcH);
13C NMR (CD3CN): d ˆ 25.34, 46.71, 50.72, 56.45, 57.30, 62.36, 64.70, 68.68,
69.28, 72.55, 85.98; elemental analysis calcd (%) for C24H32F6FeN4O6S2Zn
(771.90): C 37.25, H 4.43, N 7.24; found C 36.34, H 4.18, N 7.26.
X-ray crystal structure determination: Crystals suitable for X-ray crystal
structure determinations were grown in the following manner: FcCyclam:
recrystallization from CH3CN; [Co(FcCyclam)](CF3SO3)2 and [Zn(FcCy-
clam)](CF3SO3)2: slow diffusion of Et2O into an CH3CN solution of the
complex; [Ni(FcCyclam)](CF3SO3)2 and [Cu(FcCyclam)](CF3SO3)2: slow
diffusion of Et2 O into an Me2CO solution of the complex. Suitable crystals
were mounted on top of a glass fiber. X-ray data were collected on an
Enraf-Nonius CAD4 diffractometer or on a Siemens AED diffractometer
with MoKa radiation (71.069 pm) and a graphite monochromator. All
structure calculations were made by using SHELX-97,[79] the structures
were refined against F 2 with anisotropic temperature coefficients for all
non-hydrogen atoms. Hydrogen atoms were refined with fixed isotropic
temperature coefficients and fixed site occupancy factors, but the
coordinates were free to refine. An empirical absorption correction based
on psi scans was applied.[35] Definition of R factors and goodness-of-fit
Chem. Eur. J. 2001, 7, No. 13
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2859

H. Plenio et al.
FULL PAPER
(GooF): wR2 ˆ {S2s(I)[w(Fo2 Fc2†2]/S2s(I)[w(Fo2†2]}1/2, R1 ˆ S2s(I) j jFo j
j Fc j j/S2s(I) j Fo j ; the goodness-of-fit is based on F 2: GooF ˆ {S2s(I)
[w(Fo2 Fc2†2]/(n p)}1/2, in which n is the number of reflections and p is the
total number of parameters refined.
[31] F. Osterloh, W. Saak, D. Haase, S. Pohl, Polyhedron 1999, 18, 1957.
[32] P. Forlano, A. R. Parise, J. A. Olabe, Inorg. Chem. 1998, 37, 6406.
[33] The term ªswitchingº refers to the change of oxidation state between
reduced or oxidized and the corresponding decrease or increase of
binding strength of a receptor. The redox-switched complexation of
metal ions is not an on-off situation as it is (of course!) governed by
the laws of thermodynamics. Instead the equilibrium constant K for
the complexation of a metal ion by a redox-active ligand changes from
higher to lower values or vice versa upon oxidation or reduction. The
efficiency of a redox switch is characterized by the ratio of the
Acknowledgements
This work was supported by the DFG and by the Fonds der Chemischen
Industrie. We wish to thank Prof. Dr. Vahrenkamp for access to the CCD
detector at the Institut für Anorganische und Analytische Chemie/
University of Freiburg and Dipl.-Chem. A. Trösch and Dr. B. Kersting
for the collection of the X-ray data sets. Experimental support concerning
the stopped-flow kinetics was kindly provided by Dr. T. Poth and T.
Schwalm. We wish to thank Prof. Dr. Dinse (TU Darmstadt) for recording
an EPR spectrum of [Cu(FcCyclam)]2‡. We wish to thank the DGICYT
(proyecto PB98-1430-C02-02, 1FD97-0508-C03-01, and AMB99-0504-C02-
01) for support.
respective Kred and Kox
.
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[35] Crystallographic data (excluding structure factors) for the structures
reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication no.
CCDC-144468 (FcCyclam), CCDC-144469 ([Cu(FcCyclam)]),
CCDC-144470 ([Zn(FcCyclam)]), and CCDC-144471 ([Co(FcCy-
clam)]). Copies of the data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax:
(‡44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).
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ion complexes are weak.
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2860
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Chem. Eur. J. 2001, 7, No. 13

Ferrocene ± Cyclam Complexes
2848±2861
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Received: December 4, 2000 [F2911]
Chem. Eur. J. 2001, 7, No. 13
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0947-6539/01/0713-2861 $ 17.50+.50/0
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