A new dipyridine-containing cryptand for both proton and Cu(II) encapsulation. A solution and solid state study

Article abstract of DOI:10.1039/b200486k

Synthesis and characterization of the new polyamine cryptand 2,5,8-triaza-5-methyl-2,8-(N-methyl-dipropylamino)[9]-2,2′- dipyridinophane (L) and its macrocyclic precursor 2,5,8-triaza-5-methyl[9]-2,2′-dipyridinophane (L1) are reported. Ligand L1 contains

Full text of DOI:10.1039/b200486k

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A new dipyridine-containing cryptand for both proton and Cu(II)
encapsulation. A solution and solid state study
Carla Bazzicalupi, Anna Bellusci,† Andrea Bencini,* Emanuela Berni, Antonio Bianchi,*
Samuele Ciattini, Claudia Giorgi and Barbara Valtancoli
Department of Chemistry, University of Florence, Via della Lastruccia 5,
50019 Sesto Fiorentino, Firenze, Italy. E-mail: andrea.bencini@unifi.it
Received 14th January 2002, Accepted 1st March 2002
First published as an Advance Article on the web 12th April 2002
Synthesis and characterization of the new polyamine cryptand 2,5,8-triaza-5-methyl-2,8-(N-methyl-
dipropylamino)[9]-2,2Ј-dipyridinophane (L) and its macrocyclic precursor 2,5,8-triaza-5-methyl[9]-2,2Ј-
dipyridinophane (L1) are reported. Ligand L1 contains a diethylenetriamine chain linking the 6,6Ј positions
of a 2,2Ј-dipyridine moiety; in L an N-methyldipropylamine bridge links the two benzylic nitrogens of L1.
Protonation and Cu() coordination have been studied in aqueous solution by means of potentiometric and
UV-vis spectrophotometric measurements. Considering proton binding, cryptand L behaves as a proton sponge,
i.e., the first protonation constant is too high to be measured in aqueous solution. Both ligands form 1 : 1 Cu()
complexes in aqueous solutions. The crystal structure of [CuL1]2ϩ reveals that the metal is coordinated by the five
ligand donors in a strongly distorted square pyramidal geometry. Almost the same coordination environment is
found in the protonated complex with L, [CuLH]3ϩ, while the nitrogen of the N-methyldipropylamine bridge is
protonated. In marked contrast, in the [CuL]2ϩ complex this nitrogen is involved in metal coordination. Both
the solution and structural results account for the high rigidity of the macrocyclic moiety of L defined by the
dipyridine unit and the diethylenetriamine chain, while coordination of the more flexible
N-methyldipropylamine unit is modulated by complex protonation.
Introduction
There is much current interest in the development of new
polyamine macrobicyclic receptors. Macropolycyclic poly-
amines containing appropriate binding sites and cavities of
a suitable size and shape may be designed to form selective
inclusion complexes in aqueous solution. Actually, the mole-
cular topology of the host molecule can be synthetically
modulated in order to bind many different chemical species
from inorganic or organic cations,1–10 to anionic species.10–17
Structural factors, such as ligand rigidity, type of donor atoms
and their disposition, have been shown to play significant roles
in determining the binding features of macrocycles toward
metal cations.1–13 Heteroaromatic subunits, such as 2,2Ј-dipyr-
idine or 1,10-phenanthroline, are often introduced as integral
parts of the host molecules.17–20 These units are rather rigid,
and, at the same time, provide two aromatic nitrogens whose
unshared electron pairs may act cooperatively in binding cat-
ions. Furthermore, incorporation of these moieties into macro-
cyclic structures may allow the combination, within the same
species, of the special complexation features of macrocycles
Results and discussion
Ligand synthesis
with the photophysical and photochemical properties displayed
by the metal complexes of these heterocycles.20 In the course
of our investigation of the cation binding capabilities of
phenanthroline-21 and dipyridine-containing22 polyazamacro-
cycles, we have synthesized the new cryptand L, which con-
tains two different polyamine chains linked by a 2,2Ј-dipyridine
moiety. In this paper we report on the synthesis, basicity and
Cu() binding by cryptand L and by its synthetic precursor L1.
Cryptand L and its macrocyclic precursor L1 were obtained by
following the synthetic procedure depicted in Scheme 1. The
disodium salt of the tosylated amine 1,23 was obtained accord-
ing to the general procedure of Richman and Atkins.24
Reaction of 6,6Ј-bis(bromomethyl)-2,2Ј-dipyridine25 2 with 1,
carried out in anhydrous DMF, afforded, after separation by
column chromatography, the tosylated macrocycles 3 and, in a
lower yield, 4. (3, 29%; 4, 10%).
The tosylated compounds 3 and 4 were finally deprotected in
a 33% HBr/CH3CO2H mixture, according to a previously
† Present address: Department of Chemistry, Università della Calabria,
87030 Arcavacata di Rende, Cosenza, Italy.
DOI: 10.1039/b200486k
J. Chem. Soc., Dalton Trans., 2002, 2151–2157
This journal is © The Royal Society of Chemistry 2002
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Table 1 Protonation constants (log K) of L and L1 (Me4NCl 0.1 M,
298.1 K)
Log K
Reaction
L1
L
[L] ϩ Hϩ = [HL]ϩ
9.59(1)
8.47(1)
2.64(1)
[HL]ϩ ϩ Hϩ = [H2L]2ϩ
[H2L]2ϩ ϩ Hϩ = [H3L]3ϩ
[H3L]3ϩ ϩ Hϩ = [H4L]4ϩ
10.42(1)
3.47(2)
1.70(7)
suitable to give a 1 ϩ 1 reaction with respect to “longer”
polyamines, thus also giving rise to the 2 ϩ 2 cyclization
product.
L1 is a versatile precursor for the assembly of macropoly-
cyclic structures, since it contains two secondary amine groups
which can be connected by appropriate bridging moieties.
Actually, reaction of L1 with N,N-bis[3-(methanesulfonyloxy)-
propyl]-N-methylamine 6,26 in the presence of K2CO3 as a base,
gave the cryptand L (Scheme 1). Separation by column chrom-
atography (neutral alumina, CHCl3) afforded the macrobicycle
L as its monochloride salt (LؒHCl), which was characterized by
1
means of elemental analysis, H and 13C NMR and ESI mass
spectroscopy.
Ligand protonation
The protonation equilibria of L and L1 were studied in 0.1 mol
dmϪ3 NMe4Cl aqueous solution at 298.1 0.1 K by means of
potentiometric measurements in the pH range 2.5–12, and the
results are reported in Table 1. Ligands L and L1 bind up to
four and three protons in aqueous solution, respectively. The
first protonation constant of L is too high to be determined by
means of potentiometric measurements (see below). In the case
of L1 a marked grouping of the first two protonation constants
and a sharp decrease in basicity between the second and the
third protonation step (more than six logarithm units) is
observed. A marked drop between the second and the third
protonation constant is also found in L. This behavior, com-
mon in polyamine compounds,27 is generally explained in
terms of minimization of the electrostatic repulsions between
protonated amine groups. Dipyridine nitrogens, however, are
characterized by far lower basicity than amine nitrogens and,
therefore, it is expected that at least the first protonation steps
take place on the aliphatic polyamine chains. Protonation of
the heteroaromatic nitrogens can be easily monitored by using
UV-vis spectrophotometric titrations, since protonation of
dipyridine is accompanied by a marked red shift of the UV
band of dipyridine at 290 nm with the appearance in the UV-vis
spectra of a new red-shifted band at ca. 305 nm.22e The free
amine L1 displays a band in the UV-vis spectrum at 288 nm and
these spectral features are not changed by ligand protonation in
the pH range 11–2, indicating that protonation occurs on the
aliphatic amine groups. Most likely, in the [H2L1]2ϩ species
the two acidic protons are localized on the two benzylic amine
groups, separated by an unprotonated amine group and by the
dipyridine spacer. The third protonation step would occur on
the methylated nitrogen, adjacent to the two already protonated
benzylic amines, with a consequent increase of the electrostatic
repulsions, thus explaining the markedly lower log K value of
the third protonation constant. Similar considerations can be
made in the case of L, where a remarkable decrease of the log K
values is found between the second and the third protonation
step. The UV band at 290 nm of L does not bear significant
change in the pH range 11–3, indicating that the first three
protonation equilibria involve the aliphatic amine groups.
Below pH 3, the formation of the tetraprotonated [H4L]4ϩ
species is accompanied by a red shift of the UV band of dipyr-
idine (λmax = 307 nm at pH 1.5) indicating that this protonation
step occurs on the heteroaromatic moiety.
Scheme 1
reported procedure,22d to afford ligands L1 and 5 as hydro-
bromide salts.
The most interesting finding is the formation, in the critical
cyclization step, of compound 4, derived from a 2 ϩ 2 cycliz-
ation. The same synthetic procedure, carried out with tosylated
tetra-, penta- or hexa-amines, allowed the obtention of only the
1 ϩ 1 cyclization product, while the formation of the 2 : 2
macrocycles was not observed. Most likely, their longer more
flexible chain allowed them to easily achieve a better con-
formation to react with the two bromo-methyl functions of 2,
which are instead separated by the rigid and smaller dipyridine
moiety. In the present case, the short chain of 1 would be less
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Table 2 Selected bond lengths (Å) and angles (Њ) for [CuL1](ClO4)2
The most interesting finding in Table 1 is the remarkable
higher basicity of cryptand L with respect to its macrocyclic
precursor L1, at least in the first two protonation steps. The first
protonation constant is too high to be determined by means
of potentiometric measurements. However, in the [HL]ϩ species
the proton cannot be removed even in more strongly alkaline
solutions: the 1H NMR spectrum recorded in aqueous solution
at pH 13 does not show significant differences with respect to
that of [HL]ϩ. Extractions with chloroform of a 3 M NMe4OH
aqueous solution of the cryptand, in the presence of chloride,
leads to the isolation of the [HL]Cl salt. All these experimental
observations suggest that L displays a “proton sponge”
behavior, i.e., the acidic proton of the [HL]ϩ species cannot be
removed by varying the pH in aqueous solutions. This behavior
has been also observed in small aza-cryptands and was gener-
ally attributed to encapsulation of the acidic protons inside the
cavity, allowing the formation of an intramolecular hydrogen
bond network involving the protonated N–Hϩ ammonium
function and the other amine groups of the cryptand.9 A sim-
ilar assumption can also be made in the present case. Actually,
Cu(1)–N(2)
Cu(1)–N(1)
Cu(1)–N(3)
1.945(4)
1.956(4)
2.053(4)
Cu(1)–N(5)
Cu(1)–N(4)
2.122(5)
2.226(5)
N(2)–Cu(1)–N(1)
N(2)–Cu(1)–N(3)
N(1)–Cu(1)–N(3)
N(2)–Cu(1)–N(5)
N(1)–Cu(1)–N(5)
79.57(18)
80.92(18)
160.5(2)
158.9(2)
81.1(2)
N(3)–Cu(1)–N(5)
N(2)–Cu(1)–N(4)
N(1)–Cu(1)–N(4)
N(3)–Cu(1)–N(4)
N(5)–Cu(1)–N(4)
118.4(2)
110.60(18)
102.27(17)
83.78(17)
81.7(2)
geometry for the Cu() ion can be best described as a distorted
square pyramid. The aromatic N(1) and N(2) and the benzylic
N(3) and N(5) nitrogens define the basal plane (max. deviation
0.081(6) Å for N(5)), while the methylated nitrogen N(4) occu-
pies the apical position. The bond angles for the metal coordin-
ation geometry are significantly different from their theoretical
values (Table 2). The metal atom lies 0.0772(8) Å apart from the
basal plane shifted toward N(4), with the Cu(1)–N(4) bond
forming an angle of 19.1(1)Њ with the normal to the basal plane.
The O(23) oxygen atom of a perchlorate anion gives rise to a
weak interaction with the metal (Cu ؒ ؒ ؒ O(23), 2.998(8) Å).
Considering this interaction, the coordination environment
could be described as a distorted octahedron, with the O(23)
oxygen occupying the sixth position of the coordination
polyhedron.
1
the H NMR spectrum of [HL]Cl in CDCl3 shows the signal
of a highly deshielded N–Hϩ proton at 10.8 ppm, supporting
the hypothesis that the acidic proton is encapsulated inside the
cryptand cavity, deshielded by strong intramolecular hydrogen
bonds.
Crystal structure of [CuL1](ClO4)2
The planes defined by the two pyridine rings are almost
coplanar (dihedral angle 5.7(2)Њ). The ligand assumes a folded
conformation with a dihedral angle of 77.8(2)Њ between the
mean plane defined by the two pyridine moieties and by the
benzylic nitrogens (max. deviation 0.135(5) for N(3)) and that
formed by the amine groups N(3), N(4) and N(5). The values of
the C–C–N and C–N–C bond angles (mean value 111Њ) for the
aliphatic chains indicate that this part of the ligand is rather
strained. A similar bent conformation has already been found
in the metal complexes with ligand L2, where a phenanthroline
unit replaces the dipyridine moiety of L1. This suggests that the
insertion of a phenanthroline or dipyridine unit as an integral
part of a rather small cyclic framework leads to an increase in
the rigidity of the macrocycle. Most likely, the ligand rigidity
accounts for the observed distorted coordination polyhedron.
Finally, as far as the crystal packing is concerned (Fig. 1b),
symmetry related macrocyclic complexes are coupled by
π-stacking interactions between the dipyridine moieties. The
aromatic systems are parallel, with a plane to plane distance of
3.4 Å.
The crystal structure of [CuL1](ClO4)2 consists of [CuL1]2ϩ
cations and perchlorate anions. Fig. 1a shows an ORTEP28
drawing of the complex cation with atom labelling and Table 2
lists selected bond angles and distances. The coordination
Crystal structure of [CuLH](ClO4)3ؒ2H2O
The crystal structure of [CuLH](ClO4)3ؒ2H2O consists of a
[CuLH]3ϩ complex cation, perchlorate anions and water solvent
molecules. The ORTEP28 drawing of the cation (Fig. 2) shows
the metal atom coordinated by five of the six donor atoms of
the ligand, in a distorted square pyramidal coordination
environment. The heteroaromatic and the bridgehead nitrogens
N(1), N(2), N(3) and N(5), (max. deviation 0.026(5) Å for N(5))
define the basal plane, while N(4) occupies the apical position.
Cu() is shifted toward the apical position, 0.2369(8) Å away
from the basal plane. The apical bond gives rise to an angle of
20.7(1)Њ with the normal to the plane. Table 3 lists bond angles
and distances.
The N(6) nitrogen is protonated and lies 4.688(5) Å away
from Cu(1). This nitrogen atom, not involved in metal coordin-
ation, gives rise to an H-bond interaction with the O(100) oxy-
gen of a water molecule (N(6)–H(100) ؒ ؒ ؒ O(100) 2.14(5) Å).
The most interesting finding is the fact that the coordination
geometry of the metal is very similar to that found in the L1
complex (see above). In fact, the Cu() ion is coordinated
by the smaller macrocyclic subunit defined by the dipyridine
unit and the two ethylenic chains N(3)–C(12)–C(13)–N(4)
Fig. 1 ORTEP drawing of the [CuL1]2ϩ complex (a) and crystal
packing (b). The perchlorate anions are omitted for clarity.
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Table 3 Selected bond lengths (Å) and angles (Њ) for [CuLH](ClO4)3ؒ
2H2O
Table 4 Stability constants (log K) of the Cu() complexes with L and
L1 (Me4NCl 0.1 M, 298.1 K)
Cu(1)–N(2)
Cu(1)–N(1)
Cu(1)–N(5)
1.937(4)
1.939(5)
2.085(5)
Cu(1)–N(3)
Cu(1)–N(4)
2.107(5)
2.183(5)
Log K
Reaction
L1
L
L ϩ Cu2ϩ = [CuL]2ϩ
19.8(1)
>20.6
11.42(5)
2.82(2)
N(2)–Cu(1)–N(1)
N(2)–Cu(1)–N(5)
N(1)–Cu(1)–N(5)
N(2)–Cu(1)–N(3)
N(1)–Cu(1)–N(3)
80.6(2)
160.4(2)
82.5(2)
81.59(19)
158.0(2)
N(5)–Cu(1)–N(3)
N(2)–Cu(1)–N(4)
N(1)–Cu(1)–N(4)
N(5)–Cu(1)–N(4)
N(3)–Cu(1)–N(4)
112.15(19)
110.5(2)
114.2(2)
85.65(19)
84.1(2)
[HL]ϩ ϩ Cu2ϩ = [CuLH]3ϩ
[CuL]2ϩ ϩ Hϩ = [CuLH]3ϩ
[CuL]2ϩ ϩ OHϪ = [CuLOH]ϩ
3.4(1)
3.8(1)
Fig. 2 ORTEP drawing of the [CuLH]3ϩؒH2O complex.
and N(4)–C(14)–C(15)–N(5) The overall conformation of this
cyclic moiety is similar to that found for L1 in the [CuL1]2ϩ
complex, with the two pyridine units almost coplanar (dihedral
angle 5.2(3)Њ) and an angle of 83.9(2)Њ between the mean plane
defined by the heteroaromatic unit and the bridgehead amines
and that formed by the N(3), N(4) and N(5) nitrogens. As in
[CuL1]2ϩ, this part of the ligand is affected by a severe con-
formational strain, with large deviations of the C–C–N and
C–N–C angles from their theoretical values (mean value 112Њ).
These observations further confirm the high rigidity of this
macrocyclic subunit.
The cryptand adopts a symmetrical conformation, with a
non-crystallographic plane passing through the two methylated
nitrogens and the metal ion and bisecting the C(1)–C(2) bond
of the dipyridine unit. The tetraaza macrocyclic subunit defined
by N(3), N(4), N(5) and N(6) is rather flat (max. deviation from
the mean plane defined by N(3), N(4), N(5) and N(6) 0.123(6)
Å for N(4)). The dipyridine unit bridges the two N(3) and
N(5) nitrogens, defining a plane (max. deviation 0.001(7) Å
for C(7)) almost perpendicular (84.5(2)Њ) to that of the tetra-
azamacrocycle.
Fig. 3 Distribution diagrams of the species present in the systems:
L1/Cu() (a) and L/Cu() (b) (NMe4Cl 0.1 mol dmϪ3, 298.1 K, [L] =
[L1] = [Cu2ϩ] = 1 × 10Ϫ3 M).
can assume that the log K value for the equilibrium L ϩ Hϩ
=
LHϩ is greater than 13 log. units. On this basis we can estimate
log K > 20.6 for the formation of the [CuL]2ϩ complex. There-
fore, both ligands form rather stable 1 : 1 complexes with Cu().
Considering ligand L1, its Cu() complex forms a protonated
complex at acidic pHs and a monohydroxo species [ML1(OH)]ϩ
at alkaline pH values. It is to be noted, however, that the stabil-
ity constants of the Cu() complexes with L1 is remarkably
lower than those found for the corresponding complexes with
L3, where an ethylenediamine linkage replaces the dipyridine
unit (log K = 26.73 for the equilibria Cu2ϩ ϩ L3 = [CuL3]2ϩ).29
At the same time, the [CuL3]2ϩ complex does not form any
hydroxylated complex at alkaline pHs and shows a very low
tendency to protonate, giving rise to the [CuL3H]3ϩ species only
at strongly acidic pH values (log K = 0.96 for the equilibrium
([CuL3]2ϩ ϩ Hϩ = [CuL3H]3ϩ). These observations suggested
that in this complex the metal is “enveloped” by the ligand,
achieving an almost saturated coordination sphere. The large
drop in stability observed for the L1 complex with respect to L3
cannot be ascribed to a different binding ability of aliphatic
secondary amine groups with respect to heteroaromatic ones,
since 2,2Ј-dipyridine forms a [CuL]2ϩ complex with an almost
equal stability constant with respect to N,NЈ-dimethylethylene-
diamine (log K = 9.0 and 9.54 for the equilibrium Cu2ϩ ϩ L =
[CuL]2ϩ, where L = 2,2Ј-dipyridine and N,NЈ-dimethylethyl-
enediamine, respectively).30 The lower stability of the [CuL1]2ϩ
complex can be reasonably explained in terms of a lower overall
interaction between the donor atoms of L1 and the metal.
Cu(II) coordination in aqueous solution
The formation of the Cu() complexes with L and L1 has been
investigated by means of potentiometric measurements in
aqueous solution (NMe4Cl 0.1 M, 298.1 K) and the determined
stability constants of the complexes are reported in Table 4.
On the basis of these equilibrium data, the distribution of
individual metal complexes can be calculated as a function of
pH and Fig. 3 displays the distribution diagrams for the systems
Cu()/L1 and Cu()/L. Unfortunately, the unavailability of a
value for the first protonation constant of L did not allow us to
determine the formation constant of the [CuL]2ϩ complex.
However, according to the potentiometric and NMR results we
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Actually, the UV-vis spectrum of the [CuL1]2ϩ complex dis-
plays one band at 617 nm (ε = 109 molϪ1 dm3 cmϪ1), ca. 30 nm
red shifted with respect to that of [CuL3]2ϩ (λmax = 585 nm, ε =
181 molϪ1 dm3 cmϪ1), indicating a weaker ligand field operating
on the Cu2ϩ center in the L1 complex. At the same time, the
reflectance spectrum of the solid [CuL1](ClO4)2 complex shows
similar spectral features (a band at 620 nm) to those found for
[CuL1]2ϩ in aqueous solution, suggesting a similar coordination
for the Cu() ion both in solution and in the solid state.
These observations point out that cryptand L is composed of
a rigid cyclic pentaaza moiety, where the metal is lodged, and by
a flexible dipropylamine bridge, whose involvement in metal
coordination is modulated by complex protonation.
Concluding remarks
The particular cage-like molecular architecture of L strongly
affects both its protonation and Cu() binding behavior. Cryp-
tand L behaves as a proton sponge, i.e., it has a very high
basicity in the first protonation step, which is not measurable in
aqueous solution. Most likely, the high stability of the mono-
protonated species [HL]ϩ is due to the involvement of the
N–Hϩ proton in a hydrogen bond network inside the tri-
dimensional cavity, which stabilizes this cation. In the Cu()
complexes with L the metal is enveloped inside the cryptand
cavity. In the protonated [CuLH]3ϩ complex the metal is
coordinated by the pentaaza moiety defined by the dipyridine
unit and by two short ethylenediamine linkages, with a
coordination geometry almost equal to that found in the Cu()
complex with the precursor macrocycle L1. The similar con-
formation of this binding unit in both [CuL]2ϩ and [CuLH]3ϩ
accounts for the high rigidity of the monocyclic part of the
cryptand. In [CuLH]3ϩ the proton is localized on the N(6)
nitrogen of the dipropylamine bridge. Deprotonation of the
complex, however, takes place at acidic pHs and is accom-
panied by coordination of N(6) to the metal, indicating a
higher flexibility of the dipropylamine linkage with respect to
the remaining cyclic framework of the cryptand.
Actually, the crystal structure of the [CuL1]2ϩ cation shows
the metal coordinated by the five nitrogens of L1 in a strongly
distorted square-planar coordination geometry, with two amine
groups bound at a longer distance (Cu–N(4), 2.226 Å; Cu–N(5)
2.122(5) Å). These seem to be peculiar structural features of the
first row transition metal complexes with phenanthroline and
dipyridine-containing polyazamacrocycles,21,22 due to the inser-
tion of these heteroaromatic moieties which leads to a stiffening
of the macrocyclic framework. Most likely, the rigidity of these
large heteroaromatic units, together with the short ethylenic
linkage connecting the aliphatic amine groups, does not allow a
simultaneous optimal arrangement of the nitrogen donors
around the metal ion. A weak interaction of the set of donors
with the metal may also explain the different acid–base charac-
teristics of the [CuL1]2ϩ complex with respect to those of
[CuL3]2ϩ. [CuL1]2ϩ shows a higher tendency to form a mono-
protonated [CuHL1]3ϩ species, as expected considering that
protonation can occur on weakly interacting amine groups. At
the same time, the crystal structure of the [CuL1]2ϩ cation dis-
plays a large zone on the metal not saturated by the ligand
donors, occupied by a weakly bound oxygen atom of a
perchlorate anion. Most likely, this anion is replaced by a
water molecule in aqueous solution and deprotonation of
the coordinated water takes place at alkaline pHs, giving the
monohydroxo [CuL1(OH)]ϩ complex.
Experimental
General procedures
N-Methyl-2,2Ј-ditosylimino-bis(ethylamine) disodium salt (1),23
6,6Ј-bis(bromomethyl)-2,2Ј-dipyridine (2)25 and N,N-bis[3-
(methanesulfonyloxy)propyl]-N-methylamine (6)26 were pre-
As far as cryptand L is concerned, the formation of a mono-
protonated [CuLH]3ϩ complex takes place at pH > 2.5, with
almost simultaneous deprotonation of a nitrogen donor to give
the [CuL]2ϩ complex. As shown in Fig. 3b, only a minor
amount of the protonated complex [CuLH]3ϩ is formed in
aqueous solution. However, the protonated [CuLH](ClO4)3
complex can be crystallized by slow evaporation of an aqueous
solution containing Cu(ClO4)2 and L in equimolar ratios at pH
3. Crystallization from an aqueous solution at neutral pH,
instead, leads to the isolation of the unprotonated [CuL](ClO4)2
complex. The potentiometric measurements also point out that
no hydroxylated species are formed in the pH range investigated
(2.5–12). This suggests a more saturated coordination environ-
ment for the metal with respect to the L1 complex. Although
the [CuL](ClO4)2 complex was not characterized by X-ray
analysis, comparison of the UV-vis of the Cu() complexes
with L and L1 may allow us to gain further insight on the
coordination environment of this complex. The reflectance
spectrum of the protonated [CuLH](ClO4)3 solid complex dis-
plays a band at 615 nm, almost equal to that found for
[CuL1](ClO4)2, as expected considering the similar coordin-
ation geometry of the metal displayed by the crystal structures
of the two complexes. Interestingly, the deprotonated [CuL]-
(ClO4)2 complex, both in aqueous solution and in the solid
state, displays a 25 nm blue-shifted band (λmax = 590 nm ε =
120 molϪ1 dm3 cmϪ1 at pH 7) with respect to the protonated
[CuLH](ClO4)3 solid complex. The stronger ligand field experi-
enced by Cu2ϩ in [CuL]2ϩ can be reasonably ascribed to the
involvement of the methylated nitrogen N(6) in metal coordin-
ation, confirming the hypothesis, made on the basis of the
potentiometric results, of a more saturated coordination sphere
for the Cu() ion.
1
pared as previously described. 200.0 MHz H and 50.32 MHz
13C NMR spectra in D2O or CDCl3 solutions were recorded at
1
298 K on a Bruker AC-200 spectrometer. In H NMR spectra
peak positions are reported relative to TMS (CDCl3 solutions)
or to HOD at 4.75 ppm (D2O solutions). Dioxane was used as
the reference standard in 13C NMR spectra (δ = 67.4 ppm) in
D2O solutions. UV-vis spectra were recorded on a Shimadzu
UV-2101PC spectrophotometer.
Synthesis of the ligands and their Cu(II) complexes
2,8-Ditosyl-2,5,8-triaza-5-methyl[9]-2,2Ј-dipyridinophane (3)
and 2,8,23,29-tetratosyl-2,5,8,23,26,29-hexaaza-5,26-dimethyl-
[24]-10,21,31,42-bis-dipyridinophane (4). A solution of sodium
ethanolate, obtained by addition of small amounts (ca. 200 mg
each) of sodium (1.5 g, 0.062 mol) in dry ethanol (100 cm3), was
added to a suspension of N-methyl-2,2Ј-ditosylimino-bis(ethyl-
amine) (12.45 g, 0.029 mol) in ethanol (400 cm3). The resulting
mixture was refluxed for ca. 30 min and the solvent was then
removed under reduced pressure. The solid residue 1 was dis-
solved in dry DMF (500 cm3) and Na2CO3 (8 g, 0.075 mol) was
added. To the resulting suspension, heated at 115 ЊC, was added
2 (10 g, 0.029 mol) in dry DMF (300 cm3) over a period of ca.
6 h. The reaction mixture was kept at 115 ЊC for 2 h. After being
cooled at room temperature, the suspension was filtered out
and the solvent was evaporated to dryness. The crude oil was
chromatographed on neutral alumina eluting with CHCl3. The
eluted fractions containing 3 (Rf = 0.3) and 4 (Rf = 0.6) were
collected separately and evaporated to dryness affording 3 and
4 as white solid compounds. 3: Yield 5.1 g (8.4 mmol, 29%).
1H NMR (CDCl3): δ 1.80 (s, 6H), 1.86 (s, 3H), 3.02 (t, 4H), 3.63
(t, 4H), 4.66 (s, 4H), 7.46 (d, 4H), 7.59 (d, 4H), 7.87 (m, 12H),
7.99 (t, 4H) ppm. 13C NMR (CDCl3): δ 21.9, 22.1, 43.7, 47.8,
Coordination of N(6) to the metal suggests a higher flexi-
bility of the dipropylamine bridge with respect to the remaining
pentaaza-macrocycle, due to the longer propylenic linkages
connecting N(6) to the bridgehead nitrogen donors.
J. Chem. Soc., Dalton Trans., 2002, 2151–2157
2155

View Article Online
Table 5 Crystal data and structure refinement for [CuL1](ClO4)2 (a)
and [CuLH](ClO4)3ؒ2H2O (b)
54.7, 56.5, 121.3, 124.1, 127.5, 130.1, 137.3, 138.3, 143.6, 156.0,
157.4 ppm. Anal. calc. for C31H35N5S2O4: C, 61.46; H, 5.82;
N, 11.56. Found: C, 61.42; H, 5.70; N, 11.5%. 4: Yield 1.8 g
[CuL1](ClO4)2
[CuLH](ClO4)3ؒ2H2O
1
(1.5 mmol, 10%). H NMR (CDCl3): δ 1.74 (s, 12H), 1.77 (s,
6H), 2.55 (m, 16H), 4.76 (s, 8H), 7.41 (d, 8H), 7.61 (d, 4H), 7.74
(d, 8H), 7.87 (m, 8H) ppm. 13C NMR (CDCl3): δ 21.8, 22.0,
46.9, 56.7, 65.1, 120.8, 126.1, 129.1, 129.9, 135.9, 137.7, 145.0,
148.9, 155.5 ppm. Anal. calc. for C62H70N10S4O8: C, 61.46;
H, 5.82; N, 11.56. Found: C, 61.32; H, 5.79; N, 11.60%.
Empirical formula
Formula weight
Crystal system
Space group
a/Å
b/Å
c/Å
C17H23Cl2CuN5O8
559.84
Monoclinic
P21/c
12.1570(10)
8.8210(10)
20.685(3)
100.973(9)
2177.6(4)
4
1.54180
4.186
298
4100
3034
C24H41Cl3CuN6O14
807.52
Monoclinic
C2/c
29.500(7)
17.825(7)
13.781(3)
104.193(10)
7025(4)
8
1.54180
3.606
298
11338
4936
2,5,8-Triaza-5-methyl[9]-2,2Ј-dipyridinophane trihydrobrom-
ide (L1ؒ3HBr). Compound 3 (5.1 g, 0.0082 mol) and phenol
(40.0 g, 0.41 mol) were dissolved in 33% HBr/CH3CO2H
(300 cm3). The reaction mixture was continuously stirred at
90 ЊC for 22 h until a precipitate was formed. The solid was
filtered out and washed several times with CH2Cl2. The tri-
hydrobromide salt was recrystallized from an EtOH–water 2 : 1
β/Њ
U/Å3
Z
λ/Å
µ/mmϪ1
T /K
Reflections collected
Independent reflections
R(int)
1
mixture. Yield 3.0 g (0.0055 mol, 67%). H NMR (D2O, pH =
0.0743
0.0684
0.2012
0.0105
0.078
0.2302
R(F)a (I > 2σ(I ))
WR(F2)a
11): δ 2.28 (3H, s), 3.66 (4H, t), 3.78 (4H, t), 4.78 (4H, s), 7.68
(4H, d), 8.21 (4H, t), 8.32 (4H, d) ppm.13C NMR (D2O pH =
11): δ 43.5, 45.6, 52.2, 56.3, 125.8, 127.1, 143.5, 154.0, 155.8
ppm. MS m/z 298 ([M ϩ H]ϩ). Anal. calc. for C17H26N5Br3:
C, 37.80; H, 4.85; N, 12.97. Found: C, 37.6; H, 4.8; N, 13.0%.
a R1 = Σ||Fo| Ϫ |Fc||/Σ|Fo|; wR2 = [Σw(Fo2 Ϫ Fc2)2/ΣwFo ]1/2
.
4
2,5,8,23,26,29-Hexaaza-5,26-dimethyl[24]-10,21,31,42-bis-
dipyridinophane octahydrobromide (5ؒ8HBr). This compound
was synthesized from 4 (2.28 g, 2 mmol) following the pro-
cedure reported for L1 obtaining pure 5ؒ8HBr as a colorless
solid. Yield: 1.1 g (0.9 mmol, 44%). 1H NMR (D2O, pH = 11):
δ 2.47 (s, 6H), 3.11 (t, 8H), 3.52 (t, 8H), 4.47 (s, 8H), 7.29 (d,
8H), 7.66 (t, 8H), 7.90 (d, 8H) ppm. 13C NMR (D2O, pH = 11):
δ 41.4, 43.0, 51.1, 52.8, 122.6, 125.0, 140.7, 150.4, 153.8 ppm.
MS m/z 5.95 ([M ϩ H]ϩ). Anal. calc. for C34H54N10Br8: C, 32.87;
H, 4.38; N, 11.28. Found: C, 32.7; H, 4.4; N, 11.1%.
solution (10 cm3) containing LؒHCl (11 mg, 0.025 mmol). The
pH was adjusted to 3 with 0.01 M HClO4. NaClO4ؒH2O (70 mg)
was added and the resulting solution was stirred for 5 h at room
temperature. Blue crystals of the complex suitable for X-ray
analysis were obtained by slow evaporation of this solution.
Yield: 10 mg (50%). Anal. calc. for C24H41Cl3CuN6O14: C,
35.70; H, 5.12; N, 10.41. Found: C, 35.6; H, 5.2; N, 10.3%.
[CuL](ClO4)2. This complex was synthesized following the
procedure reported for [CuLH](ClO4)3ؒ2H2O, adjusting the
solution pH to 7. Yield: 14 mg (84%). Anal. calc. for
C24H36Cl2CuN6O8: C, 42.96; H, 5.41; N, 12.52. Found: C, 42.8;
H, 5.4; N, 12.5%.
2,5,8-Triaza-5-methyl-2,8-(N-methyldipropylamino)[9]-2,2Ј-
dipyridinophane monohydrochloride (LؒHCl). A solution of
6ؒHCl (1.25 g, 3.7 mmol) in dry CH3CN (100 cm3) was added
over a period of 64 h to a refluxing and vigorously stirred
suspension of L1ؒ3HBr (2 g, 3.7 mmol) and Na2CO3 (3.8 g,
37 mmol) in CH3CN (300 cm3). After the addition was com-
pleted, the solution was refluxed for additional 48 h. The result-
ing suspension was filtered out and the solution was vacuum
evaporated to give a crude solid. The product was chromato-
graphed on neutral alumina (CHCl3). The eluted fractions were
collected and vacuum evaporated to afford LؒHCl as a colorless
solid, which was recrystallized from a CHCl3–cyclohexane 1 : 1
mixture. Yield: 0.9 g (2 mmol, 55%). 1H NMR (CDCl3): δ 1.43
(m, 4H), 1.58 (m, 4H), 2.11 (s, 3H), 2.24 (s, 3H), 2.45 (t, 4H),
2.60 (m, 4H), 3.26 (t, 4H), 3.77 (s, 4H), 7.12 (d, 2H), 7.65 (m,
4H) ppm. 13C NMR (CDCl3): δ 27.2, 40.8, 41.1, 52.1, 53.0, 55.5,
56.4, 60.2, 119.8, 121.2, 157.0, 161.4 ppm. MS m/z 446 ([M ϩ
H]ϩ). Anal. calc. for C24H37N6Cl: C, 64.77; H, 8.38; N, 18.88;
Cl, 7.97. Found: C, 64.6; H, 8.5; N, 18.70; Cl, 8.1%.
X-Ray crystallography
Analyses on prismatic blue single crystals of [CuL1](ClO4)2 (a)
and [CuLH](ClO4)3ؒ2H2O (b) were carried out on a Siemens P4
X-ray diffractometer. A summary of the crystallographic data
is reported in Table 5. No loss of intensity was observed during
data collections. Empirical absorption corrections (ψ-scan
method) were applied. Both structures were solved by direct
methods (SIR-97).31 Refinements were performed by means of
full-matrix least-squares using the SHELXL-97 program.32
In both structures all non-hydrogen atoms were aniso-
tropically refined while the hydrogen atoms were introduced in
calculated position and their coordinates and thermal factors
were refined according to the linked atom.
In [CuL1](ClO4)2, the H(1)–N(3) and H(1)–N(5) hydrogen
atoms, bound to the benzylic secondary nitrogens, were
localized in the ∆F map, introduced in the calculation and
isotropically refined.
[CuL1](ClO4)2. A solution of Cu(ClO4)2ؒ6H2O (7 mg,
0.019 mmol) in water (5 cm3) was slowly added to an aqueous
solution (10 cm3) containing L1ؒ3HBr (10 mg, 0.019 mmol).
The pH was adjusted to 7 with 0.01 M NaOH and then NaClO4
(30 mg) was added. Blue crystals of the complex suitable for
X-ray analysis were obtained by slow evaporation at room
temperature. Yield: 7 mg (66%). Anal. calc. for C17H23-
Cl2CuN5O8: C, 36.47; H, 4.14; N, 12.50. Found: C, 36.3; H, 4.2;
N, 12.3%.
In [CuLH](ClO4)3ؒ2H2O, the ∆F map carried out in the last
refinement step allowed us to locate the H100 acidic proton
which was then introduced in the calculation and isotropically
refined. Two perchlorate anions (Cl(1) and Cl(2) are located in
special position on C2 crystallographic axes (symmetry oper-
ation: Ϫx ϩ 1, y, Ϫz ϩ 0.5 for Cl(1) and Ϫx ϩ 1, y, Ϫz ϩ 1.5
for Cl(2)). In particular, in the Cl(1) perchlorate the crystallo-
graphic axis passes along the Cl(1)–O(11) bond. This is due to
the rotational disorder affecting this perchlorate. Actually, two
different tetrahedrons, sharing the Cl(1)–O(11) bond and
related by the C2 axis, have been used to resolve such disorder.
CCDC reference numbers 177616 and 177617.
CAUTION! Perchlorate salts of organic ligands and their
metal complexes are potentially explosive; these compounds
must be handled with great care.
[CuLH](ClO4)3ؒ2H2O. A sample of Cu(ClO4)2ؒ6H2O (9 mg,
See http://www.rsc.org/suppdata/dt/b2/b200486k/ for crystal-
lographic data in CIF or other electronic format.
0.025 mmol) in water (5 cm3) was slowly added to an aqueous
2156
J. Chem. Soc., Dalton Trans., 2002, 2151–2157

View Article Online
G. Mathis, Angew. Chem., Int. Ed. Engl., 1987, 26, 266; (c)
Potentiometric measurements
M. Cesario, J. Guilhem, E. Pascard, E. Anklam, J. M. Lehn and
M. Pietraskiewicz, Helv. Chim. Acta, 1991, 74, 1157; (d )
J. Bkouche-Waksmann, J. Guilhem, E. Pascard, B. Alpha,
R. Deschenaux and J.-M. Lehn, Helv. Chim. Acta, 1991, 75, 1163;
(e) J. M. Lehn and J. B. Regnouf de Vains, Helv. Chim. Acta, 1992,
75, 1221; ( f ) C. Roth, J.-M. Lehn, J. Guilheim and C. Pascard, Helv.
Chim. Acta, 1995, 78, 1895 and references therein.
All the pH metric measurements (pH = Ϫlog [Hϩ]) were carried
out in degassed 0.1 mol dmϪ3 NMe4Cl solutions, at 298.1 K, by
using previously reported equipment and procedures.33 The
combinated Ingold 405 S7/120 electrode was calibrated as a
hydrogen concentration probe by titrating known amounts of
HCl with CO2-free NMe4OH solutions and determining the
equivalent point by the Gran’s method34 which allows
determination of the standard potential E o, and the ionic
product of water (pKw = 13.83(1) at 298.1 K in 0.1 mol dmϪ3
NMe4Cl). At least three potentiometric titrations were per-
formed for each system in the pH range 2.5–11. A ligand
concentration of [L] = 1 × 10Ϫ3 mol dmϪ3 and a metal concen-
tration of [M] = 0.8[L] were adopted in the complexation
experiments (L = L or L1). Due to the high stability of the
Cu() complexes with L1, competition between protonation of
the free ligands and complex formation is not significant
enough to derive the values of the stability constants, and the
use of EDTA as an auxiliary ligand was necessary. The relevant
emf data were treated by means of the computer program
HYPERQUAD35 which furnished the equilibrium constants
reported in Tables 1 and 4.
19 (a) P.-L. Vidal, B. Divisia-Blohorn, G. Bidan, J.-M. Kern, J.-P.
Sauvage and J.-L. Hazemann, Inorg. Chem., 1999, 38, 4203–4210;
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B. Valtancoli, Inorg. Chem., 1998, 37, 941; (b) C. Bazzicalupi,
A. Bencini, V. Fusi, C. Giorgi, P. Paoletti and B. Valtancoli, J. Chem.
Soc., Dalton Trans., 1999, 393; (c) C. Bazzicalupi, A. Bencini,
A. Bianchi, C. Giorgi, V. Fusi, B. Valtancoli, M. A. Bernardo and
F. Pina, Inorg. Chem., 1999, 38, 3806; (d ) C. Bazzicalupi,
A. Beconcini, A. Bencini, C. Giorgi, V. Fusi, A. Masotti and
B. Valtancoli, J. Chem. Soc., Perkin Trans. 2, 1999, 1675; (e)
A. Bencini, M. A. Bernardo, A. Bianchi, V. Fusi, C. Giorgi, F. Pina
and B. Valtancoli, Eur. J. Inorg. Chem., 1999, 1911.
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P. Paoletti, B. Valtancoli, N. Navon and D. Meyerstein, J. Chem.
Soc., Dalton Trans., 2000, 2383; (b) C. Bazzicalupi, A. Bencini,
A. Bianchi, C. Giorgi, V. Fusi, A. Masotti, B. Valtancoli, A. Roque
and F. Pina, Chem. Commun., 2000, 561; (c) A. Bencini, A. Bianchi,
C. Lodeiros, A. Masotti, A. J. Parola, J. S. Melo, F. Pina and
B. Valtancoli, Chem. Commun., 2000, 1639; (d ) A. Bencini,
A. Bianchi, V. Fusi, C. Giorgi, A. Masotti and P. Paoletti, J. Org.
Chem., 2000, 65, 7686; (e) C. Lodeiro, A. J. Parola, F. Pina,
C. Bazzicalupi, A. Bencini, A. Bianchi, C. Giorgi, A. Masotti and
B. Valtancoli, Inorg. Chem., 2001, 40, 2968; ( f ) C. Bazzicalupi,
A. Bencini, E. Berni, A. Bianchi, C. Giorgi, V. Fusi, B. Valtancoli,
C. Lodeiro, A. Roque and F. Pina, Inorg. Chem., 2001, 40, 6172; (g)
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6383.
Acknowledgements
This work has been supported by the Italian Ministero
dell’Università e della Ricerca Scientifica e Tecnologica
(MURST, Rome) within the program COFIN 2000. We thank
the “Centro Interdipartimentale di Spettrometria di Massa”
(University of Florence) for ESI mass spectra.
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2157