Co2+ translocation in a terpyridine-cyclam ditopic receptor

Article abstract of DOI:10.1002/1099-0682(200105)2001:5<1227::AID-EJIC1227>3.0.CO;2-0

The coordination behaviour of the difopic receptor of 1-[p-(2,2′:6′,2″-terpyrid-4′-yl) tolyl]-1,4,8,11-tetraazacyclo-tetradecane (L¹) towards Co²⁺ in acetonitrile/water (70:30, v/v) has been investigated. At acidic pH values, the L¹-H⁺-Co²⁺ system shows an oxidation wave at 150 mV vs. SCE, characteristic of Co²⁺ in a bis(terpyridine) environment. Upon addition of OH^-, a reduction in the intensity of the wave at 150 mV is observed and a new oxidation wave appears at 1.30 V. This new wave is close to that found for [Co(cyclam)]²⁺ under similar working conditions. The electrochemical data thus suggest that there is a pH-controlled translocation of the Co²⁺ ion from the bis(terpyridine) to the cyclam environment. A similar inference can be made from the results of UV/Vis studies. A thermodynamic characterization of the L¹-H⁺-Co²⁺ system has also been carried out. Stability constants have been determined in acetonitrile/water (70:30, v/v, containing 0.1 mol dm^-3 nBu₄NClO₄ at 25°C) using potentiometric techniques. The following species were found: [Co(L¹)₂H₄]⁶⁺, [Co(L¹)₂H₃]⁵⁺, [Co(L¹)H]³⁺, [Co(L¹)]²⁺, and [Co(L¹)(OH)]⁺. The 2:1 ligand-to-metal species [Co(L¹)₂H₄]⁶⁺ and [Co(L¹)₂H₃]⁵⁺ are assigned to complexes where Co²⁺ is coordinated by two terpyridine units. The [Co(L¹)H]³⁺, [Co(L¹)]²⁺, and [Co(L¹)(OH)]⁺ species are assigned to complexes where the Co²⁺ cation is in the cyclam binding domain. The pH-induced Co²⁺ jumping betwee coordination sites is discussed in terms of the different basicities of cyclam and terpyridine. Co²⁺ jumping between coordination sites has also been studied by the sequential acidification of basic mixtures of L¹ and Co²⁺. Under these conditions, the translocation was observed at a different pH value than when an acid-to-basic path was followed. The effect of anions such as phosphate and chloride on the translocation process has also been studied. In the presence of phosphate, translocation occurs at pH = 11 rather than at pH ≈ 8. The phosphate-L¹ and phosphate-L¹-Co²⁺ systems have been characterized by potentiometry and a remarkably good agreement has been found between the electrochemical and potentiometric data.

Full text of DOI:10.1002/1099-0682(200105)2001:5<1227::AID-EJIC1227>3.0.CO;2-0

FULL PAPER
Co2؉ Translocation in a Terpyridine؊Cyclam Ditopic Receptor
[a]
[a]
[a]
[a]
´
´
´
´
˜
Miguel E. Padilla-Tosta, Jose M. Lloris, Ramon Martınez-Manez,* Teresa Pardo,
[a]
[a]
[b]
´
Felix Sancenon, Juan Soto, and M. Dolores Marcos
Keywords: Translocation / Ditopic receptors / Electrochemistry / Cobalt complexes
The coordination behaviour of the ditopic receptor of 1-[p-
(2,2Ј:6Ј,2ЈЈ-terpyrid-4Ј-yl)tolyl]-1,4,8,11-tetraazacyclo-
tetradecane (L1) towards Co2+ in acetonitrile/water (70:30,
v/v) has been investigated. At acidic pH values, the
L1−H+−Co2+ system shows an oxidation wave at 150 mV vs.
SCE, characteristic of Co2+ in a bis(terpyridine) environment.
Upon addition of OH−, a reduction in the intensity of the
[Co(L1)]2+, and [Co(L1)(OH)]+. The 2:1 ligand-to-metal spe-
cies [Co(L1)2H4]6+ and [Co(L1)2H3]5+ are assigned to com-
plexes where Co2+ is coordinated by two terpyridine units.
The [Co(L1)H]3+, [Co(L1)]2+, and [Co(L1)(OH)]+ species are
assigned to complexes where the Co2+ cation is in the cyclam
binding domain. The pH-induced Co2+ jumping between co-
ordination sites is discussed in terms of the different basicit-
wave at 150 mV is observed and a new oxidation wave ap- ies of cyclam and terpyridine. Co2+ jumping between coor-
pears at 1.30 V. This new wave is close to that found for dination sites has also been studied by the sequential acid-
[Co(cyclam)]2+ under similar working conditions. The elec- ification of basic mixtures of L1 and Co2+. Under these condi-
trochemical data thus suggest that there is a pH-controlled
translocation of the Co2+ ion from the bis(terpyridine) to the
cyclam environment. A similar inference can be made from
tions, the translocation was observed at a different pH value
than when an acid-to-basic path was followed. The effect of
anions such as phosphate and chloride on the translocation
the results of UV/Vis studies. A thermodynamic characteriza- process has also been studied. In the presence of phosphate,
tion of the L1−H+−Co2+ system has also been carried out.
translocation occurs at pH = 11 rather than at pH ഠ 8. The
Stability constants have been determined in acetonitrile/ phosphate−L1 and phosphate−L1−Co2+ systems have been
water (70:30, v/v, containing 0.1 mol dm−3 nBu4NClO4 at
25 °C) using potentiometric techniques. The following spe-
characterized by potentiometry and a remarkably good
agreement has been found between the electrochemical and
potentiometric data.
cies were found: [Co(L1)2H4]6+, [Co(L1)2H3]5+, [Co(L1)H]3+
,
Introduction
(a polytopic ligand), a guest (organic or inorganic) capable
of being bound by the binding domains, and a suitable
switching mechanism to induce jumping of the guest be-
tween sites on the polytopic receptor. For instance, several
examples have been reported where bound cations shift
from one site to another in ditopic receptors as a result of
redox triggering.[5] In these systems, the shift arises as a
result of the dependence of the binding preference of the
metal ion on its oxidation state. Besides redox triggering,
ionically controlled translocation is also possible. We de-
scribe herein a translocation process found in the interac-
tion of Co2ϩ with the ditopic ligand 1-[p-(2,2Ј:6Ј,2ЈЈ-terpy-
rid-4Ј-yl)tolyl]-1,4,8,11-tetraazacyclotetradecane (L1) pos-
sessing 2,2Ј:6Ј,2ЈЈ-terpyridine and 1,4,8,11-tetraazacyclote-
tradecane (cyclam) coordination domains. We have found
the metal ion jumping between these two coordination sites
to be ionically governed by protons. This pH-induced jump-
ing is discussed in terms of the basicities of the cyclam and
terpyridine units and the values of the formation stability
constants. The effect of certain anions on the translocation
process has also been studied.
The appropriate arrangement of suitably predisposed in-
dividual molecular elements has recently resulted in the de-
velopment of systems such as sensors, switches, molecular
devices, etc.[1] This has led to the exploration of interesting
new effects arising from particular molecular organisations
or specific assemblies of individual components with the
aim of mimicking the more complex functions observed in
biological systems.[2] Among known molecular functions is
that of translocation.[3] It has been argued that the translo-
cation process (positional changes of atoms in a molecule)
resembles mechanical processes at the macroscopic level but
is operative on the molecular scale.[4] Compounds used for
such studies generally have in common the property of bi-
or multistability. Thus, mechanical microscopically based
translocation processes involve interconversion between two
structurally different states in bi- or multistable systems. To
construct such systems, several suitable components have to
be coupled in a specific manner. The prerequisites of the
final device are a molecule containing multiple binding sites
[a]
´
´
Departamento de Quımica, Universidad Politecnica de
Valencia,
Results and Discussion
Camino de Vera s/n, 46071 Valencia, Spain
Fax: (internat.) ϩ 34-96/387-7349
E-mail: rmaez@qim.upv.es
The design of artificial systems for studying translocation
processes requires careful choice of each individual com-
ponent. We have assembled two coordination sites (terpyri-
[b]
`
Institut de Ciencia dels Materials de la Universitat de
`
Valencia (ICMUV),
P. O. Box 2085, 46071 Valencia, Spain
Eur. J. Inorg. Chem. 2001, 1227Ϫ1234
WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001
1434Ϫ1948/01/0505Ϫ1227 $ 17.50ϩ.50/0
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FULL PAPER
Acidified solutions of Co2ϩ and L1 (molar ratio L1/
Co2ϩ ϭ 2:1) show a reversible oxidation wave at 150 mV
with a ratio between the cathodic and anodic peak intensit-
ies, ipc/ipa, of ca. 1. This value is very close to the oxidation
potential found for the complex [Co(MePhtpy)2]2ϩ
[MePhtpy ϭ 4Ј-(4-methylphenyl)-2,2Ј:6Ј,2ЈЈ-terpyridine] un-
der similar working conditions. However, whereas the ox-
idation wave of the [Co(MePhtpy)2]2ϩ complex does not
change as a function of the pH value, the addition of OHϪ
to the Co2ϩϪL1 system leads to a decrease in the intensity
of the wave at 150 mV and the appearance of an oxidation
process at higher anodic potential (1.35 V). The new wave
is similar to that found for the [Co(cyclam)]2ϩ complex in
acetonitrile/water. Figure 1 shows the electrochemical beha-
viour of the Co2ϩϪL1 system at certain pH values. The
data suggest that a translocation occurs,[8] which is schem-
atically represented in Scheme 2. Figure 2 shows a plot of
the intensity of the oxidation wave of the
Co2ϩϪbis(terpyridine) core as a function of the pH value
for the L1ϪCo2ϩ system.
The large difference in intensity between the 150 mV and
1.35 V waves merits comment. This is not an unexpected
behaviour and can be attributed to the low diffusion coeffi-
cient of the bis(terpyridine) complex and to adsorption pro-
cesses observed for the oxidation of the cyclamϪCo2ϩ spe-
cies. Moreover, the oxidation wave at 1.35 V is cathodically
Scheme 1
dine and cyclam), both of which are capable of coordinat- shifted when the pH value is very basic, probably due to
ing transition metal ions.[6] The use of an appropriate medi- the formation of hydroxo complexes.
ator (i.e. protons) makes it possible to control the relative
The jumping of Co2ϩ between the terpyridine and cyclam
affinity of each binding domain towards a certain metal environments can also be observed by UV/Vis spectropho-
cation. Terpyridine and cyclam components were chosen in tometry. The visible spectrum of Co2ϩϪL1 is pH-depend-
view of the large difference in basicity between their nitro- ent. At acidic pH values, a poorly defined band with a max-
gen atoms. Besides this difference, the terpyridine-cyclam imum centred at 515 nm is found. When the solution is bas-
couple also has to meet an important thermodynamic re- ified, a dramatic decrease in its intensity is observed. The
quirement; the stability constants of each binding domain spectra of the Co2ϩϪL1 system under acidic and basic con-
with a certain metal ion must be adequately complemented ditions closely resemble those found for the complexes
by the basicities of the binding atoms in order to observe, [Co(MePhtpy)2]2ϩ and [Co(cyclam)]2ϩ, respectively.
when the pH value is modulated, jumping between coor-
dination sites (see below).
The electrochemical and UV/Vis experiments thus sug-
gest that there is a proton-induced jumping of the cobalt
Terpyridine and cyclam were covalently linked by reac- cation between the two different sites on the ditopic recep-
tion of 4Ј-[4-(bromomethyl)phenyl]-2,2Ј:6Ј,2ЈЈ-terpyridine tor L1. In addition, we have completed the study with a
with an excess of 1,4,8,11-tetraazacyclooctadecane (cyclam) thermodynamic characterization of the system. The ther-
in dichloromethane in the presence of triethylamine as a modynamic behaviour of polyamines such as cyclam can be
base, to give the ditopic receptor L1 (see Scheme 1). Details adequately studied by potentiometry. However, this tech-
of the synthesis have been published elsewhere.[7] L1 is insol- nique is not generally suitable for solution studies on polyp-
uble in water and therefore the coordination studies were yridines. Potentiometry, as a technique for deducing
carried out in acetonitrile/water (70:30, v/v) mixtures. The stability constants in polyamines, is based on the competi-
use of other solvent mixtures such as dioxane/water or tion between the metal ion and protons to bind to the N
DMSO/water was also precluded by inadequate solubility.
donor atoms. The nitrogen atoms in polyamines are suffi-
Once the ditopic ligand had been synthesized, the next ciently basic to react with protons even in the presence of
step was the choice of a suitable metal ion. Co2ϩ was se- metal ions to give the protonated free receptors. Thus, a
lected in view of the fact that its coordination environment, requirement for the deduction of stability constants by me-
in both the cyclam and terpyridine moieties, can easily be ans of potentiometry is that both the free receptor and the
monitored by electrochemical techniques. Co2ϩ in a bis(ter- complex must be observable in the pH range studied (usu-
pyridine) coordination environment displays a very charac- ally 2Ϫ10). This is generally the case with polyamines, but
teristic oxidation process at ca. 200 mV vs. SCE, whereas not with terpyridines due to their lower basicities. However,
when coordinated by cyclam the oxidation to Co3ϩ be- although simple titrations of terpyridine in the presence of
comes much more difficult.
a metal ion would not lead to accurate determinations of
1228
Eur. J. Inorg. Chem. 2001, 1227Ϫ1234

Co2ϩ Translocation in a TerpyridineϪCyclam Ditopic Receptor
FULL PAPER
Figure 1. Electrochemical behaviour of the L1ϪCo2ϩ system at various pH values using rde and differential pulse voltammetry
Figure 2. Intensity of the oxidation potential of the
Co2ϩϪbis(terpyridine) core at 150 mV vs. SCE as a function of the
pH value and in the presence of phosphate for the L1ϪCo2ϩ system
outlined above; terpyridine is covalently coupled to a poly-
amine that, as suggested by the electrochemical experiments,
Scheme 2
strongly reduces the affinity of the Co2ϩ for the coordina-
the stability constants, more complex experiments can be tion environment of the former. Therefore, one should be
designed for the thermodynamic characterization of able to carry out standard potentiometric experiments to
terpyridineϪmetal mixtures. Thus, a terpyridineϪmetal so- thermodynamically characterize the L1ϪHϩϪCo2ϩ system.
lution might be treated with a ligand (appropriate for po-
The first step involved the determination of the pro-
tentiometric study) that strongly competes with terpyridine tonation constants of L1 by titrating previously acidified
in binding the metal cation. At this point, it is important solutions of the ligand in acetonitrile/water (70:30, v/v, 0.1
to state that receptor L1 does indeed meet the requirement mol dmϪ3 tetrabutylammonium perchlorate) with KOH.
Eur. J. Inorg. Chem. 2001, 1227Ϫ1234
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R. Martınez-Man˜ez et al.
FULL PAPER
Although L1 has seven possible protonation sites, only five Table 2. Stability constants (logK) for the formation of Co2ϩ com-
plexes of L1 in acetonitrile/water (70:30, v/v) containing 0.1 mol
constants could be measured, indicating that two pro-
tonation processes are too acidic to be determined (see
dmϪ3 nBu4NClO4 at 25 °C
Table 1).
Reaction
logK
Table 1. Protonation constants (logK) of L1, mftpy, and cyclam
Co2ϩ ϩ 2 L1 ϩ 4 Hϩ Ǟ [Co(L1)2H4]6ϩ
ǟ
47.20(5)[a]
42.81(4)
18.82(4)
12.76(3)
0.83(5)
ǟ
Co2ϩ ϩ 2 L1 ϩ 3 Hϩ Ǟ [Co(L1)2H3]5ϩ
[mftpy ϭ 4Ј-(4-methylphenyl)-2,2Ј:6Ј,2ЈЈ-terpyridine; cyclam
ϭ
1,4,8,11-tetraazacyclooctadecane] in acetonitrile/water (70:30, v/v)
Co2ϩ ϩ L1 ϩ Hϩ Ǟ [Co(L1)H]3ϩ
ǟ
containing 0.1 mol dmϪ3 tetrabutylammonium perchlorate at 25 °C
Co2ϩ ϩ L1 Ǟ [Co(ǟL1)]2ϩ
ǟ
Ǟ
Co2ϩ ϩ L1 ϩ H2O [Co(L1)(OH)]ϩ ϩ Hϩ
Reaction
L1
logK
mftpy
logK
cyclam
logK
[a] Values in parentheses are standard deviations in the last signific-
ant figure.
L ϩ Hϩ Ǟ (HL)ϩ [a]
ǟ
9.75(4)[b]
15.40(4)
19.71(3)
23.33(4)
26.21(5)
5.65
4.35(2)
6.80(2)
8.05(1)
11.62(2)
21.85(3)
23.22(2)
ǟ
L ϩ 2 Hϩ Ǟ (H2L)2ϩ
ǟ
L ϩ 3 Hϩ Ǟ (H3L)3ϩ
ǟ
L ϩ 4 Hϩ Ǟ (H4L)4ϩ
ǟ
The 2:1 ligand-to-metal species [Co(L1)2H4]6ϩ and
[Co(L1)2H3]5ϩ are attributed to complexes where the co-
balt(II) is coordinated by two terpyridine units. The addi-
tional presence of protons in these complexes indicates that
the peripheral cyclam groups are protonated.
L ϩ 5 Hϩ Ǟ (H5L)5ϩ
ǟ
(HL)ϩ ϩ Hϩ Ǟ (H2L)2ϩ
ǟ
2.45
1.25
10.23
1.37
(H2L)2ϩ ϩ Hϩ Ǟ (H3L)3ϩ
ǟ
4.31
(H3L)3ϩ ϩ Hϩ Ǟ (H4L)4ϩ
ǟ
3.62
2.88
(H4L)4ϩ ϩ Hϩ Ǟ (H5L)5ϩ
[a]
[b]
The 1:1 ligand-to-metal complexes [Co(L1)H]3ϩ
,
Charges have been omitted for the sake of clarity. Ϫ
Values
in parentheses are standard deviations in the last significant figure.
[Co(L1)]2ϩ, and [Co(L1)(OH)]ϩ are attributed to species
where the cobalt ion is coordinated by the cyclam binding
unit. Scheme 3 shows the assignments of the various com-
In order to compare the protonation behaviour of the
ditopic receptor with those of the individual components,
the stepwise protonation constants of the cyclam as well as
those of 4Ј-[(4-methyl)phenyl]-2,2Ј:6Ј,2ЈЈ-terpyridine were
also determined in acetonitrile/water (70:30, v/v) mixtures
(see Table 1). Three and two protonation processes were ob-
served for the cyclam and terpyridine units, respectively.
The protonation constants obtained in acetonitrile/water
(70:30, v/v) solutions are in good agreement with the ana-
logous constants reported in water.[9] Cyclam displays two
very basic protonation processes in water with logK values
for the processes cyclam ϩ Hϩ ǟǞ Hcyclamϩ and Hcyclamϩ
ϩ Hϩ Ǟ H2cyclam2ϩ of 11.5 and 10.5, respectively. In con-
ǟ
trast, the third and fourth protonation processes are much
less basic with logK values of 1.5 and 0.9. Terpyridine
shows less basic character in water. Only two protonation
constants could be determined, with logK values for the
processes Hϩ ϩ tpy ǟǞ Htpyϩ and Htpyϩ ϩ Hϩ ǟǞ H2tpy2ϩ
of 4.65 and 3.5. It can be concluded that the difference in
basicity found between the terpyridine and the cyclam in
water is retained in acetonitrile/water and probably also in
the ditopic ligand L1. Comparison of the basicity constants
of L1 with those of the individual components (see Table 1)
suggests that the first two protonation processes occur at
the cyclam core, the third and fourth protonations at the
terpyridine, whereas the final protonation process relates to
the attack of a proton at the cyclic polyamine.
Potentiometric studies aimed at determining the nature
of the L1ϪCo2ϩ complexes in solution have also been car-
ried out in acetonitrile/water (70:30, v/v) by titrating previ-
ously acidified solutions containing the ligand and the
metal ion in a 2:1 molar ratio with KOH. The potentiomet-
ric data were fitted with a model where the following com-
plexes were found: [Co(L1)2H4]6ϩ
,
[Co(L1)2H3]5ϩ
,
[Co(L1)H]3ϩ [Co(L1)]2ϩ and [Co(L1)(OH)]ϩ. Table 2
,
,
shows the formation stability constants for these complexes. Scheme 3
1230
Eur. J. Inorg. Chem. 2001, 1227Ϫ1234

Co2ϩ Translocation in a TerpyridineϪCyclam Ditopic Receptor
FULL PAPER
both of which can either be protonated or form a complex
with a certain metal ion M. Let us assume that the stability
Ǟ
ǟ
constant for the A ϩ M AM process is larger than that
Ǟ
ǟ
for B ϩ M
BM. Under these circumstances, only when
Ǟ
ǟ
the protonation constant for A ϩ H
AH is larger than
that for B ϩ H Ǟǟ BH can a complete pH-controlled translo-
cation be observed. Therefore, to observe ionically governed
pH-induced jumping of the metal ion between coordination
sites, a requirement that has to be fulfilled is a difference in
basicity and binding strength towards the metal ion in fa-
vour to the more basic binding sites. The translocation pro-
cess then works as follows: When the proton concentration
is low (basic pH values), the metal ion is coordinated by
the more basic centre (stronger complex); when the proton
concentration is increased, the more basic centre becomes
protonated, inducing the jumping of the metal ion to the
less basic binding domain. The CoϪL1ϪHϩ system is sim-
Figure 3. Distribution diagram of the various species derived from
the L1ϪHϩϪCo2ϩ system and a plot of the overall percentages of
the Co2ϩϪcyclam (s) and Co2ϩϪbis(terpyridine) complexes (•) as
a function of the pH value
plexes existing in solution. Figure 3 shows the distribution ilar to that described above, the only difference being the
diagram for the L1ϪHϩϪCo2ϩ system. The Co2ϩϪcyclam presence of 2:1 ligand-to-metal complexes. To observe
complexes have formation stability constants very similar translocation in this case, the formation stability constant
to those reported for the Co2ϩϪcyclamϪHϩ system. For for complexation of the metal ion at the less basic centre
instance, the stability constant for the formation of the (terpyridine) has to be less than twice that for complexation
complex [Co(cyclam)]2ϩ is logK ϭ 12.71,[9] which is very at the more basic coordination environment (cyclam). Bear-
Ǟ
ǟ
close to the value found for the process Co2ϩ ϩ L1
ing in mind that this requirement is fulfilled in the
[Co(L1)]2ϩ, i.e. logK ϭ 12.76. This indicates that the pres- Co2ϩϪL1 system, one would expect pH-induced jumping
ence of a bulky terpyridylphenyl group attached to the cy- between coordination sites, as is indeed observed {Co2ϩ
ϩ
clam core and the use of acetonitrile/water solutions instead L1 Ǟ [Co(L1)]2ϩ; logK ϭ 12.71 for Co2ϩ in the cyclam en-
ǟ
of pure water does not significantly modify the coordina- vironment, and Co2ϩ ϩ 2 L1 Ǟ [Co(L1)2]2ϩ; logK ഠ 16.4
ǟ
tion behaviour of the cyclic tetraamine. The logK value for for Co2ϩ in the bis(terpyridine) environment}. At basic pH
the formation of the bis(terpyridine)Ϫcobalt(II) complex, values, Co2ϩ jumping occurs from the bis(terpyridine) en-
Co2ϩ ϩ 2 tpy
[Co(tpy)2]2ϩ, is 18.6.[9] The analogous vironment to the more basic cyclam binding domain. This
Ǟ
ǟ
stability constant for Co2ϩ ϩ 2 L1 Ǟ [Co(L1)2]2ϩ cannot be process can be represented by the chemical reaction
ǟ
obtained from the potentiometric titrations of the [Co(L1)2H3]5ϩ ǟǞ [Co(L1)]2ϩ ϩ L1 ϩ 3 Hϩ; logK ϭ Ϫ30.05.
Co2ϩϪHϩϪL1 system. However, it can be approximately As can be seen, the [Co(L1)]2ϩ/[Co(L1)2H3]2ϩ ratio is con-
deduced from the stability constant for the process Co2ϩ
ϩ
trolled by the proton concentration. The more basic the so-
2 L1 ϩ 4 Hϩ Ǟ [Co(L1)2H4]6ϩ, assuming that the process lution, the greater the proportion of Co2ϩ in the cyclam.
ǟ
[Co(L1)2]2ϩ ϩ 4 Hϩ Ǟ [Co(L1)2H4]4ϩ would have an ap-
We also synthesized the receptor L2 by reaction of 4Ј-
ǟ
proximate value of 30.80 [logK ϭ 30.80 is calculated as [(4-bromomethyl)phenyl]-2,2Ј:6Ј,2ЈЈ-terpyridine and 4,7,10-
twice logK ϭ 15.40 corresponding to the L1 ϩ 2 Hϩ
ϭ
trioxa-1-azacyclododecane. L2 is similar to L1 in the sense
(H2L1)2ϩ process]. This gives a stability constant for the that it also contains two coordination sites of different basi-
process Co2ϩ ϩ 2 L1 ǟǞ [Co(L1)2]2ϩ of ca. 16.40, which sug- cities. Although we have not carried out potentiometric
gests
that
logK
for
the
formation
of
the studies on L2 in acetonitrile/water, we have found in the
Co2ϩϪbis(terpyridine) complex of L1 in acetonitrile/water literature that cobalt forms 1:1 complexes with the ligand
is of the same order of magnitude as that reported for the 1,7-dioxa-4,10-diazacyclododecane (L3) with a stability
formation of the [Co(tpy)2]2ϩ complex.
constant for the process Co2ϩ ϩ L3 ǟǞ [Co(L3)]2ϩ of logK ϭ
Figure 3 also shows a plot of the overall percentages of 6.01.[10] A similar order of magnitude is presumed for the
the Co2ϩϪcyclam and Co2ϩϪbis(terpyridine) species as a process Co2ϩ ϩ L2 Ǟ [Co(L2)]2ϩ. Therefore, as the value of
ǟ
function of the pH value. A pH-dependent coordination the stability constant of the bis(terpyridine) complex
pattern can be seen, with cyclam complexes as the main (logK ഠ 16) is greater than twice the formation stability
species at basic pH values and bis(terpyridine) complexes constant of Co2ϩ with the oxa-aza ligand, for the
being present at a level of 80% or more at pH Ͻ ca. 8. Co2ϩϪL2ϪHϩ system one would expect the bis(terpyrid-
Remarkably, the translocation process represented in Fig- ine) species to be predominant over the entire pH range.
ure 3 is in good agreement with the electrochemical data Indeed, the electrochemical data reveal a unique oxidation
presented in Figure 2.
process at ca. 150 mV attributable to the presence of Co2ϩ
To fully understand how translocation operates one in a bis(terpyridine) coordination environment and the in-
should be aware of the relationship between stability con- tensity of the electrochemical wave does not change over
stants and ion jumping. The easiest system one can picture the pH range studied. There is no evidence for any translo-
is a receptor containing two coordination sites (A and B), cation process. This highlights the importance of a careful
Eur. J. Inorg. Chem. 2001, 1227Ϫ1234
1231

´
´
R. Martınez-Man˜ez et al.
FULL PAPER
choice of each element in the design of systems displaying Table 3. Stability constants (logK) for the formation of phosphate
complexes of L1 in acetonitrile/water (70:30, v/v) containing 0.1
pH-modulated translocations.
Ionically controlled metal ion jumping has been demon-
mol dmϪ3 nBu4NClO4 at 25 °C
strated for the interaction a terpyridineϪcyclam ditopic re-
ceptor with Co2ϩ. In addition, we have studied some factors
that might have an effect on the translocation.
The first effect concerns the fact that demetallation from
cyclam is slower than the process of metal incorporation L1 ϩ PO4 ϩ 5 Hϩ Ǟ [L1H5PO4]2ϩ
Reaction
logK
L1 ϩ PO4 ϩ 2 Hϩ Ǟ [L1H2PO4]Ϫ
26.37(4)[a]
35.63(3)
43.21(3)
47.25(3)
3Ϫ
ǟ
L1 ϩ PO4 ϩ 3 Hϩ Ǟ [L1H3PO4]
3Ϫ
ǟ
L1 ϩ PO4 ϩ 4 Hϩ Ǟ [L1H4PO4]ϩ
3Ϫ
ǟ
3Ϫ
ǟ
into the macrocycle. This is a well-known effect and is gen-
[a] Values in parentheses are standard deviations in the last signific-
ant figure.
erally operative in cyclic polyamines. The influence of this
effect on the Co2ϩ jumping between coordination sites has
been studied by monitoring the electrochemical behaviour
of basic mixtures of L1 and Co2ϩ upon their stepwise acid- terpyridine to the cyclam environment in the presence of
ification. Under these conditions, the Co2ϩ ion remains in phosphate occurs at pH ഠ 11, quite different from the pH
the cyclam core until pH ഠ 6. Further acidification induces value (8) at which ion jumping is observed in the absence
translocation to give bis(terpyridine) species. At neutral pH of this anion. In contrast, chloride does not affect the pH
value, the most stable complex is that involving the bis(ter- value of translocation. It is noteworthy that although some
pyridine) and Co2ϩϪcyclam species are transformed to this molecular systems have been described where a transloca-
thermodynamically favoured arrangement. However, this tion effect is induced by redox triggering or by the pH
process is very slow; at pH ϭ 7, only after two weeks was value, there have been very few examples reported in the
a partial translocation of the Co2ϩ from the cyclam to the literature where ion jumping is modulated by the presence
bis(terpyridine) observed.
of anions.[13]
The operation of this kinetic process gives a hysteresis
We also carried out potentiometric studies to thermodyn-
loop. Hysteresis phenomena in the solid state are well docu- amically characterize the L1ϪphosphateϪHϩ and
mented.[11] In contrast, studies of hysteresis loops in solu- L1ϪCo2ϩϪphosphateϪHϩ systems. Table 3 shows the
tion are, to the best of our knowledge, scarce.[12] This hys- stability constants for the formation of the phosphate com-
teresis phenomenon arises as a consequence of the existence plexes of L1 in acetonitrile/water (70:30, v/v). Potentiomet-
of a bistable system coupled with a large kinetic activation ric techniques give information about the stoichiometries of
barrier. Although the system is far from comparable with the formed complexes, but do not give direct information
other well-known systems and is far too simple to be of any concerning the nature of the species. Therefore, it is not
application, it complies with the very basic requirements for straightforward to deduce whether the protons in the com-
information storage: the existence of two metastable states plexes are bonded to the phosphate or to L1. Nevertheless,
(Co2ϩ in cyclam and Co2ϩ in terpyridine) for a given envir- bearing in mind the pH ranges of existence of the various
onmental condition (neutral solutions) that can be readily phosphate species and the pH ranges of existence of the
interconverted by the variation of an external parameter protonated (HjL1)jϩ species, we can tentatively assign the
(pH).
complex [(L1)H2(PO4)]Ϫ to the interaction of (L1H)ϩ with
A different approach that might affect the pH value at HPO42Ϫ, and the complexes [(L1)H3(PO4)], [(L1)H4(PO4)]ϩ,
which translocation occurs involves the addition of anions. and [(L1)H5(PO4)]2ϩ to the interaction of the anion
If the relative affinities of a certain anionic guest towards H2PO4Ϫ with (L1H)ϩ, (L1H2)2ϩ, and (L1H3)3ϩ, respectively.
Co2ϩϪcyclam and Co2ϩϪterpyridine complexes are differ-
Table 4 shows the stability constants for the formation of
ent, then a modulation of the translocation might be the complexes of the L1ϪCo2ϩϪphosphateϪHϩ system.
achieved. To examine this attractive possibility, we studied The following complexes were found: [Co(L1)2H5(PO4)]4ϩ
the Co2ϩ jumping between coordination sites in the pres- [ C o ( L 1 ) 2 H 4 ( P O 4 ) ] 3 ϩ , [ C o ( L 1 ) 2 H 3 ( P O 4 ) ] 2 ϩ
,
,
ence of phosphate and chloride anions. A plot of the intens- [Co(L1)2H2(PO4)]ϩ, and [Co(L1)(OH)(PO4)]2Ϫ. Taking into
ity of the Co2ϩ/Co3ϩ oxidation wave in the bis(terpyridine) account the pH ranges of existence of the PO43Ϫ, HPO4
,
2Ϫ
Ϫ
environment as a function of the pH value in the presence and H2PO4 species and the distribution diagram of the
of phosphate is shown in Figure 2. Translocation from the L1ϪCo2ϩϪphosphateϪHϩ system in Figure 4, we tentat-
Table 4. Stability constants (logK) for the formation of complexes of L1 with Co2ϩ and phosphate in acetonitrile/water (70:30, v/v)
containing 0.1 mol dmϪ3 nBu4NClO4 at 25 °C
Reaction
logK
2 L1 ϩ Co2ϩ ϩ PO4 ϩ 5 Hϩ Ǟ [Co(L1)2H5(PO4)]4ϩ
68.12(5)[a]
62.22(4)
54.65(4)
46.00(4)
9.30(4)
3Ϫ
ǟ
2 L1 ϩ Co2ϩ ϩ PO4 ϩ 4 Hϩ Ǟ [Co(L1)2H4(PO4)]3ϩ
3Ϫ
ǟ
2 L1 ϩ Co2ϩ ϩ PO4 ϩ 3 Hϩ Ǟ [Co(L1)2H3(PO4)]2ϩ
3Ϫ
ǟ
2 L1 ϩ Co2ϩ ϩ PO4 ϩ 2 Hϩ Ǟ [Co(L1)2H2(PO4)]ϩ
3Ϫ
ǟ
L1 ϩ Co2ϩ ϩ PO4 ϩ H2O [Co(L1)(OH)(PO4)]2Ϫ ϩ Hϩ
3Ϫ
Ǟ
ǟ
[a]
Values in parentheses are standard deviations in the last significant figure.
1232
Eur. J. Inorg. Chem. 2001, 1227Ϫ1234

Co2ϩ Translocation in a TerpyridineϪCyclam Ditopic Receptor
FULL PAPER
Experimental Section
The ditopic ligand L1 was prepared according to a published
method;[7] L2 was synthesized according to ref.[14]; all other re-
agents were commercially available and were used as received. Ϫ
Potentiometric titrations were carried out in acetonitrile/water
(70:30, v/v, 0.1 mol dmϪ3 tetrabutylammonium perchlorate) under
nitrogen in a water-thermostatted (25.0 Ϯ 0.1 °C) reaction vessel.
The titrant was added by means of a Crison 2031 microburette.
The potentiometric measurements were made using a Crison 2002
pH meter and a combined glass electrode. The titration system was
automatically controlled by a PC using a program that monitors
the e.m.f. values and the volume of titrant added. The electrode
was calibrated as a hydrogen ion concentration probe by titration
of precise amounts of HCl with CO2-free KOH solution and deter-
mining the equivalent point by Gran’s method,[15] which gives the
Figure 4. Distribution diagram of the various species derived from
the L1ϪHϩϪCo2ϩϪphosphate system and a plot of the overall
percentages of the Co2ϩϪcyclam (᭡) and Co2ϩϪbis(terpyridine)
complexes (•) as a function of the pH value
standard potential EЈ° and the ionic product of water (KЈw
ϭ
[Hϩ][OHϪ]). The concentrations of the metal ion solutions were
determined using standard methods. The computer program SU-
PERQUAD[16] was used to calculate the protonation and stability
constants. The titration curves for each system (ca. 250 experi-
mental points corresponding to at least three titration curves; pH ϭ
Ϫlog[Hϩ] range investigated 2Ϫ10; concentration of the ligand and
metal ion ca. 1.2·10Ϫ3 mol dmϪ3) could be treated either as a single
set or as separate entities without significant variations in the
values of the stability constants. Finally, the data set was merged
and treated as one to give the stability constants. Electrochemical
experiments were carried out in acetonitrile/water (70:30, v/v, 0.1
mol dmϪ3 tetrabutylammonium perchlorate) using a Tacussel IMT-
1 programmable function generator connected to a Tacussel PJT
120-1 potentiostat. The working electrode was platinum with a sat-
urated calomel reference electrode separated from the solution un-
der study by a salt bridge containing the solvent/supporting electro-
lyte. The auxiliary electrode was a platinum wire.
ively assign the complex [Co(L1)2H2(PO4)]ϩ to the interac-
2Ϫ
tion of HPO4 with [Co(L1)2H]3ϩ, and the complexes
[Co(L1 )2 H5 (PO4 )]4 ϩ , [Co(L1 )2 H4 (PO4 )]3 ϩ , and
Ϫ
[Co(L1)2H3(PO4)]2ϩ to the interactions of H2PO4 with
[Co(L1)2H3]5ϩ, [Co(L1)2H2]4ϩ, and [Co(L1)2H]3ϩ, respect-
ively. On this basis, the stability constant for the process
Ϫ Ǟ
ǟ
[Co(L1)2H3]5ϩ ϩ H2PO4
stance, logK ϭ 3.79.
[Co(L1)2H5(PO4)]4ϩ is, for in-
The 2:1 ligand-to-metal species [Co(L1)2H5(PO4)]4ϩ
,
[Co(L1)2H4(PO4)]3ϩ, [Co(L1)2H3(PO4)]2ϩ, and [Co(L1)2-
H2(PO4)]ϩ are attributed to complexes where the cobalt(II)
is in a bis(terpyridine) environment, whereas in the complex
[Co(L1)(OH)(PO4)]2Ϫ the Co2ϩ cation is coordinated by the
cyclam domain. Figure 4 also shows a plot of the overall
percentages of the Co2ϩϪcyclam and Co2ϩϪterpyridine
species as a function of the pH value. There is a remarkable
agreement between this plot and the electrochemical data
presented in Figure 2. The modulation induced in the trans-
location process by the presence of phosphate suggests that
this anion has a greater affinity for Co2ϩϪterpyridine than
for Co2ϩϪcyclam complexes. The net result is that translo-
cation occurs at higher pH values than in the absence of
phosphate.
Acknowledgments
We would like to thank the DGICYT (proyecto PB98-1430-C02-
02, 1FD97-0508-C03-01, and AMB99-0504.C02-01) for support.
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In conclusion, we have studied the ionically induced
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1234
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