Intermolecular binding modes in a novel [1 + 1] condensation 1 H -pyrazole azamacrocycle: A solution and solid state study with evidence for CO2 fixation

Article abstract of DOI:10.1021/ic400645t

The synthesis of a novel cyclophane (L1) consisting of a 1H-pyrazole moiety linked through methylene groups to a 1,5,9,13-tetraazadecane chain is described. As far as we know, this is one of the first reported syntheses of a [1 + 1] condensation 1H-pyrazole azamacrocyclic ligand. The crystal structures of the complexes [Cu₂(H(H_-1L1))(H_-1L1)] (ClO₄)₃·3.75H₂O (1) and ([Cu ₂(H(H_-1L1))_0.5(H_-1L1) _1.5]₂(ClO₄)₃Br₂·4. 2H₂O (2) show that Cu²⁺ coordination leads to formation of 2:2 Cu²⁺:L dinuclear dimeric complexes in which the 1H-pyrazole units lose a proton behaving as bis(monodentate) bridging ligands. Unlike previously reported complexes of [2 + 2] pyrazole azamacrocycles, the pyrazolate units in 1 are pointing outward from the macrocyclic cavity to bind the Cu ²⁺ ions. Inner coordination with formation of 1:1 Cu²⁺:L complexes is however observed in [1 + 1] pyridine azamacrocycles as shown by the crystal structure here presented of the complex [CuL2](ClO₄) ₂ (3). Crystals of [Cu₃(H_-1L1) ₂(CO₃)(H₂O)](ClO₄) ₂·8H₂O (4) grown by evaporating aqueous solution at pH 9 containing Cu²⁺ and L1 in 3:2 molar ratio show the presence of a further Cu²⁺ coordinated to the two free amine groups found in structures 1 and 2. The metal ion fills its coordination sphere capturing atmospheric CO₂ as a η¹,η²-bidentate carbonate anion placed in the equatorial position and an axial water molecule. pH-metric data, UV-vis spectroscopic data, EPR measurements, and HR-ESI-MS data support that the outer coordination mode with formation of 2:2 dinuclear dimeric and 3:2 trinuclear complexes is preserved in aqueous solution.

Full text of DOI:10.1021/ic400645t

Article
pubs.acs.org/IC
Intermolecular Binding Modes in a Novel [1 + 1] Condensation
1H‑Pyrazole Azamacrocycle: A Solution and Solid State Study with
Evidence for CO2 Fixation
Raquel Belda,† Javier Pitarch-Jarque,† Conxa Soriano,‡ Jose M. Llinares,†,‡ Salvador Blasco,†
́
Jesus Ferrando-Soria,§ and Enrique García-Espana*,†
́
̃
†Instituto de Ciencia Molecular, Universidad de Valencia, C/Catedrat
‡Departamento de Química Organ
ica, Facultad de Farmacia, Universidad de Valencia, Avda. Vicente Andres
Burjassot Valencia, Spain
́
ico Jose
́
Beltran
́
Martínez, no. 2, 46980 Paterna, Valencia, Spain
Estelles s/n 46100,
́
́
́
§Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States
S
* Supporting Information
ABSTRACT: The synthesis of a novel cyclophane (L1)
consisting of a 1H-pyrazole moiety linked through methylene
groups to a 1,5,9,13-tetraazadecane chain is described. As far as
we know, this is one of the first reported syntheses of a [1 + 1]
condensation 1H-pyrazole azamacrocyclic ligand. The crystal
structures of the complexes [Cu2(H(H−1L1))(H−1L1)]-
(ClO4)3·3.75H2O (1) and ([Cu2(H(H−1L1))0.5(H−1L1)1.5]2-
(ClO4)3Br2·4.2H2O (2) show that Cu2+ coordination leads to
formation of 2:2 Cu2+:L dinuclear dimeric complexes in which the 1H-pyrazole units lose a proton behaving as bis(monodentate)
bridging ligands. Unlike previously reported complexes of [2 + 2] pyrazole azamacrocycles, the pyrazolate units in 1 are pointing
outward from the macrocyclic cavity to bind the Cu2+ ions. Inner coordination with formation of 1:1 Cu2+:L complexes is
however observed in [1 + 1] pyridine azamacrocycles as shown by the crystal structure here presented of the complex
[CuL2](ClO4)2 (3). Crystals of [Cu3(H−1L1)2(CO3)(H2O)](ClO4)2·8H2O (4) grown by evaporating aqueous solution at pH 9
containing Cu2+ and L1 in 3:2 molar ratio show the presence of a further Cu2+ coordinated to the two free amine groups found in
structures 1 and 2. The metal ion fills its coordination sphere capturing atmospheric CO2 as a η1,η2-bidentate carbonate anion
placed in the equatorial position and an axial water molecule. pH-metric data, UV−vis spectroscopic data, EPR measurements,
and HR-ESI-MS data support that the outer coordination mode with formation of 2:2 dinuclear dimeric and 3:2 trinuclear
complexes is preserved in aqueous solution.
Scheme 1. Coordination Modes of Pyrazole
INTRODUCTION
■
Organic compounds with 1H-pyrazole units have raised a great
deal of interest in the past years due to the interesting hydrogen
bonding and coordination properties of this small hetero-
cycle.1−3 Indeed, 1H-pyrazole in its neutral form can behave
both as a hydrogen bond acceptor and donor, while in its
pyrazolate anionic form it can accept two hydrogen bonds from
appropriate donor groups. With respect to metal coordination,
neutral 1H-pyrazole typically acts as a monodentate ligand
while as a pyrazolate anion it behaves typically as a bridging
bis(monodentate) or exobidentate ligand (Scheme 1).1−4
Many ligands have been built with a pyrazole unit spacing
two either equivalent or different coordination sites with the
idea of placing two interconnected metal ions in close
proximity in order to explore their electronic and magnetic
interactions (see Scheme 2).3,5−8 Other spacers containing two
adjacent nitrogen atoms able to provide similar organizations
are piridazines,9,10 1,2,4,5-tetraazines,11 1,2,4-triazoles,12−14
1,3,4-thiadiazoles, and 1,3,4- oxadiazoles.15 In recent years
some of us have contributed to this topic with the preparation
and study of the supramolecular and coordination chemistry of
a series of azamacrocycles formed either by [2 + 2] or [3 + 2]
condensation of 1H-pyrazole-3,5-dicarbaldehyde with different
triamines or with the tripodal tetraamine tris(2-aminoethyl)-
amine, respectively.16−20 These studies evidenced interesting
binding capabilities toward metal ions and organic molecules of
biological relevance, such as amino acids or dopamine. With
respect to metal ion coordination, the preferred coordination
Received: March 15, 2013
© XXXX American Chemical Society
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Scheme 2. Kinds of Polynuclear Complexes of Pyrazole
Ligands
RESULTS AND DISCUSSION
■
Synthetic Procedure. The synthesis of L1 and L2 was
performed according to a modification of the well-known
Richman−Atkins procedure,23 which is based on a macro-
cyclation reaction of the tosylated polyamine with either 1H-
3,5-bis(chloromethyl)pyrazole or with 2,6-bis(bromomethyl)-
pyridine in CH3CN using K2CO3 as a base. However, in the
case of L1, to avoid side reactions, it was first necessary to
protect the nucleophilic NH group of pyrazole by reaction with
3,4-dihydroxypyran in CH2Cl2.24 1H-3,5-Bis(chloromethyl)-
pyrazole was prepared from 1H-3,5-pyrazoledicarboxylate in a
three-step procedure which consists first of esterification in
ethanol, followed by reduction with di(isobutyl)aluminum
hydride (DIBAL) to obtain the diol and then treatment with
thyonyl chloride (Scheme 3).25 An alternative synthesis of the
macrocycle L2 had been previously reported by I. Dierk et al.22
Scheme 3. Synthesis of L1
mode of pyrazole was as a bis(monodenate) pyrazolate anion
bridging two metal ions in the interior of the macrocycle
(Scheme 2).
However, in spite of all these interesting chemical features, as
far as we know [1 + 1] pyrazole azamacrocycles had not yet
been reported. This kind of macrocycle can give rise to novel
binding modes due to the fact that an outer orientation of the
pyrazole nitrogen atoms favors intermolecular organizations
with the metal ion linking more than one ligand (Scheme 2).
Moreover, these metal complexes might behave as receptors for
additional metal ions which in their turn can bind other target
substrates through the formation of mixed metal complexes.
Such an outer orientation has been somehow evidenced by the
tetranuclear macrocyclic cage formed when two 1H-pyrazole
units are condensed with two 1,5-diazapentane (cadaverine)
diamines in the presence of Cu2+ (Scheme 2).21
Here we report the synthesis of the first macrocycle of this [1
+ 1] series (L1) (Chart 1). L1 is made by linking the polyamine
1,5,9,13-tetraazatridecane through methylene groups to the 3,5-
positions of a 1H-pyrazole ring.
Acid−Base Properties. The basicity constants of L1 and
L2 are shown in Table 1. Both ligands present four protonation
Table 1. Logarithms of the Basicity Constants for
Chart 1
Compounds L1 and L2 Determined in 0.15 M NaClO4 at
a
298.1 K
b
reaction
L1
L2
c
L + H ⇆ HL
HL + H ⇆ H2L
H2L + H ⇆ H3L
H3L + H ⇆ H4L
log β
10.63(2)
9.35(1)
7.24(2)
5.58(2)
32.80(3)
10.60(3)
8.57(3)
6.73(3)
5.27(4)
31.17(5)
a
Logarithms of the stepwise (KH L) and cumulative [β = (ΠKH L)]
j
j
b
c
protonation constants are reported. Charges omitted. Values in
parentheses are standard deviations in the last significant figure.
We have also studied the Cu2+ coordination of L1.
Formation of mononuclear, binuclear complexes with 2:2
Cu2+:L1 stoichiometry and trinuclear ones with 3:2 Cu2+:L1
stoichiometry has been evidenced through different solution
and solid state techniques. Furthermore, we show that the 3:2
Cu2+:L1 complexes are able to fix atmospheric carbon dioxide
in water with remarkable affinity. For comparison, we have also
studied the Cu2+ coordination chemistry of a related macro-
cycle (L2) containing a 2,6-dimethylpyridine linker connecting
the ends of the same polyamine chain.22
steps in the pH range available for the potentiometric
measurements (pH 2.5−11.0). Except for the first one, L1
displays higher basicity than L2 for all the protonation steps.
Moreover, although the pyrazole or pyridine units may bear
themselves an additional protonation step, we have not
detected such a protonation constant. UV−vis spectra of L1
recorded at variable pH do not show any significant change
below pH 4 supporting that the pyrazole ring does not
protonate in this pH range (see Supporting Information, Figure
B
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S1). However, a plot of the absorbance at 225 nm overlapped
with the species distribution diagram (Figure 1a) denotes that
Pyridinophane L2 has comparable basicity to L1 in its first
protonation and lower basicity in the following three next steps.
In agreement with what has been observed for related pyridine
macrocycles,27,28 protonation of the pyridine is not observed in
the 2−11 pH range in which the potentiometric measurements
were carried out. As shown in Supporting Information Figure
S2, the largest hypochromicity and hypsochromic shifts in the
UV spectra are observed from pH 9 to 5, suggesting that, in this
case, the second and third protonation steps are mostly
affecting the secondary nitrogen atoms closest to the pyridine
spacer.
Cu2+ Coordination. Crystal Structure of [Cu2(H(H−1L1))-
(H−1L1)](ClO4)3·3.75H2O (1). Crystals of [Cu2(H(H−1L1))-
(H−1L1)](ClO4)3·3.75H2O (1) were grown by slow evapo-
ration of aqueous solutions containing L1·4HBr·H2O and
Cu(ClO4)2·6H2O in 1:1 mol ratio at pH 9. The crystral
structure of 1 consists of [Cu2(H(H−1L1))(H−1L1)]3+
−
dinuclear dimeric cations, ClO4 anions, and lattice water
molecules. Table 2 collects selected bond distances and angles,
Table 2. Selected Bond Distances (Å) and Angles (deg) for
[Cu2(H(H−1L1))(H−1L1)](ClO4)3·3.75H2O (1)
bond distance (Å)
bond angle (deg)
Cu1
Cu1
Cu1
Cu1
Cu1
Cu1
Cu2
Cu2
Cu2
Cu2
Cu2
N12
N6
1.955(3)
N12
N12
N6
N6
N7
N7
N1
N1
N1
N8
N8
N8
N8
N11
N10
N10
N4
N4
N4
N9
N9
N9
N9
93.84(14)
79.12(14)
165.43(15)
142.67(15)
80.21(15)
97.59(16)
113.78(15)
96.98(15)
97.53(15)
103.53(14)
94.18(14)
166.25(16)
78.90(14)
79.88(15)
140.10(17)
97.73(16)
95.27(15)
117.06(15)
98.46(15)
102.80(16)
1.955(4)
2.070(4)
2.136(4)
2.221(4)
1.956(4)
1.965(3)
2.054(4)
2.163(4)
2.202(4)
Cu1
Cu1
Cu1
Cu1
Cu1
Cu1
Cu1
Cu1
Cu1
Cu2
Cu2
Cu2
Cu2
Cu2
Cu2
Cu2
Cu2
Cu2
Cu2
N7
N1
N12
N6
N8
N5
N7
N11
N10
N4
N12
N6
N7
N9
N1
N5
N5
N11
N5
N11
N10
N5
N11
N10
N4
Figure 1. Plot of the species distribution diagram of L1 along with (a)
the variation in the absorption at 225 nm with pH (pH values 2.26,
2.35, 2.49, 2.66, 2.88, 3.07, 3.39, 3.68, 4.00, 4.62, 4.83, 5.17, 5.89, 6.56,
6.84, 7.05, 7.43, 8.00, 8.75, 8.93, 9.49, and 10.19) and (b) variations
with pH of the chemical shifts of 13C NMR signals of C5 (red) and C9
(blue) (pH values 3.09, 4.05, 4.96, 5.81, 6.70, 8.07, 9.73, and 10.89).
and Supporting Information Table S1 details of the crystal
structure refinement. In the dimeric [Cu2(H(H−1L1))-
(H−1L1)]3+ complex units the metal ions are connecting two
different L1 macrocycles (Figure 2a,b). In both macrocycles the
pyrazole units are deprotonated behaving as bis(monodentate)
ligands. Unlike most of the previously reported pyrazole
azamacrocycles resulting from [2 + 2] and [3 + 2]
condensations in which the pyrazolate units adopted an inner
orientation to bind the metal ions,18,30 in the present [1 + 1]
macrocycle the pyrazolate anion displays an outer orientation
which permits the binding of the metal ions in cooperation with
a second macrocycle. As above-mentioned, a somewhat similar
case was observed in the formation of the metallocage Cu2+
tetranuclear complex reported in ref 21.
significant changes occur following the second and fourth
protonation steps (H2L12+ and H4L14+ species, respectively),
suggesting that in such steps the amine groups closest to the
pyrazole ring are the ones protonating at a larger extent.
The changes with pH of the 13C NMR labeled as C5 and C9
(see Figure 1b) support also the major participation of the
nitrogen atoms closest to the pyrazole ring at this stage. While
C5, which is placed in β-position with respect to amine groups
N1 and N2,26 experiences a continuous upfield shift upon
protonation of L1, C9, which is just placed in β-position with
respect to N2, only moves significantly upfield in correspond-
ence with the first and third protonation steps of L1.
The metal ions in 1 make a sort of hinge between the
macrocycles which dispose the central parts of their polyamine
chains toward opposite sides of the plane defined by the metal
ions and the pyrazole units (Figure 2b). The Cu2+ metal ions
C
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Figure 2. (a) View of the dinuclear dimeric complex [Cu2(H-
(H−1L1))(H−1L1)]3+ of 1. Ellipsoids are shown at 50% probability
level. Hydrogen atoms omitted except the one of the ammonium
group involved in intramolecular hydrogen bonding. (b) Two-colored
view of the complex [Cu2(H(H−1L1))(H−1L1)]3+ of 1 highlighting
the connection between the macrocycles.
Figure 3. (a) ORTEP view of 2.Counter ions are not shown. Ellipsoids
are shown at 50% probability level. (b) Stick plots highlighting the two
different dispositions of the noncoordinated lateral chain in the
[Cu2(H(H‑1L1))0.5(H‑1L1)1.5]2.5+ cation.
have in both sites distorted square pyramidal geometry with
strong trigonal bipyramidal component (τ = 0.38 and τ = 0.43
for Cu1 and Cu2, respectively).29 The equatorial plane is
defined by two nitrogen atoms belonging to pyrazolate
fragments of different macrocycles and by two contiguous
secondary nitrogen atoms, also of different macrocycles. The
elevation of both Cu2+ ions above the mean plane defined by
the equatorial nitrogen atoms is ca. 0.46 Å. The distance
between the copper atoms (3.9496(8) Å) is fixed by the nature
of the pyrazolate bridges and is close to that usually observed in
Cu2+ complexes of [2 + 2] pyrazole macroycles with a similar
bridging scheme.18,30 The axial positions are occupied by two
amine groups at the central part of the polyamine chain, but in
this case belonging to the same macrocyclic unit (Figure 2a).
The equivalent amine groups of the other macrocycle remain
uncoordinated, one of them being protonated at the pH values
where these crystals evolved (Figure 2). Formation of an
intramolecular hydrogen bond between the protonated (N3)
and nonprotonated (N2) amine groups of this noncoordinating
bridge portion is also observed. The N2−N3 distance is
2.661(9) Å. This pattern extends intermolecularly to connect
the complexes through a relay of hydrogen bonds involving the
protonated amine group, water molecules, perchlorate anions,
and a coordinated amine group (see Supporting Information
Figure S3). Hydrogen bonding contacts are collected in
Supporting Information Table S2.
Crystal Structure of [CuL2](ClO4)2 (3). Compound 3 is
formed by [CuL2]2+ monomeric cationic complexes and
perchlorate counteranions. The crystal structure shows that
Cu2+ is coordinated in a regular square pyramidal geometry (τ
= 0.036). The pyridine nitrogen (N1), the two nitrogen atoms
closest to the aromatic spacer (N2 and N5), and one of the
nitrogen atoms (N3) of the central part of the chain
surrounded by six-membered chelate rings conform the
equatorial plane. The slightly distorted axial position is
occupied by the remaining nitrogen atom (N4) (Figure 4).
Crystal Structure of [Cu2(H(H−1L1))0.5(H−1L1)1.5]2(ClO4)3Br2·
4.2H2O (2). When slowly evaporating solutions of Cu(ClO4)2·
6H2O and L1·4HBr·H2O at pH 10, crystals of [Cu2(H-
(H−1L1))0.5(H−1L1)1.5]2(ClO4)3Br2·4.2H2O (2) appeared. De-
tails of the data collection, selected bond distances, and
hydrogen bonding contacts are included in the Supporting
Information (Tables S1, S3, and S4). All the bond distances
and angles in the coordination sphere of the metal ions are
close to those found in 1 (Figure 3 and Table S3). In this case,
there are two bromide anions and three perchlorate anions
shared by two dimeric complexes. One perchlorate is
disordered in two possible positions. The main difference
with respect to the previously discussed structure is that only
one out of every four ligands has a protonated amine group in
the noncoordinated chain. This part of the chain is disordered
being observed two arrangements for the C4N2C5C6C7 set of
atoms.
Figure 4. Molecular structure showing the cationic complex [CuL2]2+.
Ellipsoids are shown at the 50% probability level. Hydrogen atoms
omitted.
Table 3 collects selected bond distances and angles. The
shortest bond distance in the equatorial plane is that with the
pyridine nitrogen, while the longest one is that with one of the
contiguous secondary nitrogen atoms (N5), which is sharing
the corner between five-membered and six-membered chelate
rings. The location of this longest equatorial bond distance can
be attributed to the aromatic spacer that prevents a closer
D
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and hydrogen bonding contacts are collected in Table 4 and in
Supporting Information Tables S6 and S7). The axial position
Table 3. Selected Bond Distances (Å) and Angles (deg) of
Complex 3
bond distance (Å)
bond angle (deg)
Table 4. Selected Bond Distances (Å) and Angles (deg) of
Cu3 Coordination Site in 4
Cu1
Cu1
Cu1
Cu1
Cu1
N1
N3
N2
N5
N4
1.971(6)
N1
N1
N3
N1
N3
N2
N1
N3
N2
N5
Cu1
Cu1
Cu1
Cu1
Cu1
Cu1
Cu1
Cu1
Cu1
Cu1
N3
N2
N2
N5
N5
N5
N4
N4
N4
N4
155.5(3)
80.6(3)
89.4(3)
80.5(3)
100.2(2)
153.3(3)
111.6(2)
92.8(2)
110.1(2)
94.4(2)
2.028(5)
2.087(7)
2.125(6)
2.214(6)
bond distance (Å)
bond angle (deg)
Cu3
Cu3
Cu3
Cu3
Cu3
O1C
O2C
N2
1.988(3)
O1C
O2C
O1C
N2
Cu3
O2C
N2
N3
N3
O1
O1
O1
O1
66.17(12)
98.94(13)
93.94(13)
100.26(14)
92.72(12)
101.85(12)
91.52(12)
91.38(13)
1.998(3)
1.999(3)
2.023(3)
2.315(3)
Cu3
Cu3
Cu3
Cu3
Cu3
Cu3
Cu3
N3
O1
O1C
O2C
N2
N3
approach. The elevation of the copper atom above the nitrogen
atoms in the equatorial plane is 0.41 Å. Unlike compound 1, in
this case only a few weak intermolecular hydrogen bonds have
been observed (Table S5).
of the square pyramid (τ = 0.15) is occupied by a water
molecule. The narrow angle imposed by the carbonate ligand
provides a rather rhombic geometry to the system as evidenced
by the EPR spectra of the crystals (see Figure S4). Since
carbonate or carbon dioxide was not introduced as a reactant in
the reaction flask, CO2 should have been fixed from the
atmosphere. Carbonate fixation is reinforced in the solid state
by an extensive hydrogen bond network; while the coordinated
oxygen atoms O1C and O2C hydrogen bond to a lattice water
molecule and to the coordinated water molecule of another
complex, O3C forms a trifurcated hydrogen bond with an
amine group and two lattice water molecules (Figure 5c).
Such a CO2 fixation induced by an unsaturated Cu2+
coordination site forming part of a larger preorganized complex
entity puts into evidence one of the potential application of the
ligand L1 reported in this work.
Crystal Structure of [Cu3(H−1L1)2(CO3)(H2O)](ClO4)2·8H2O
(4). Slow evaporation at pH 8 of aqueous solutions containing
L1·4HBr·H2O and Cu(ClO4)2·6H2O in 3:2 molar ratio led to
the formation of crystals of [Cu3(H−1L1)2(CO3)(H2O)]-
(ClO4)2·8H2O (4) composition suitable for X-ray diffraction
analysis. The structure reproduces the essential features of 1
and 2 with two macrocycles connected by two Cu2+ metal ions,
but it has an additional Cu2+ bound to the two amine groups
that remained free in one of the bridges of 1 or 2 (Figure 5a,b).
Interestingly, this third Cu2+ completes its equatorial plane with
a carbonate anion bound in a η1,η2-bidentate fashion with an
O−Cu−O angle of 66.2(2)° (selected bond distances, angles,
Cu2+ Complex Formation in Solution. The stability
constants for the systems Cu2+−L1 and Cu2+−L2 determined
in 0.15 mol dm−3 NaClO4 at 298.1 K have been included in
Table 5, and the distribution diagrams in Figure 6 and
Table 5. Stability Constants for the Systems Cu2+−L1 and
a
Cu2+−L2
b
reaction
L1
L2
c
Cu + 2H + L ⇆ CuH2L
Cu+ H + L ⇆ CuHL
26.76(1)
26.88(8)
23.29(7)
20.08(1)
Cu + L ⇆ CuL
2Cu+ H + 2L ⇆ Cu2HL2
2Cu + 2L ⇆ Cu2L2
43.00(1)
37.491(8)
29.81(2)
20.23(5)
27.92(2)
2Cu + 2L ⇆ Cu2H−1L2 + H
2Cu + 2L ⇆ Cu2H−2L2 + 2H
3Cu + 2L ⇆ Cu3H−2L2 + 2H
3Cu + 2L ⇆ Cu3H−2L2(OH) + 3H
20.20(4)
a
b
c
Logarithms of the constants are reported. Charges omitted. Values
in parentheses are standard deviations in the last significant figure.
Supporting Information Tables S5 and S6. In order to select the
best speciation model, particularly for the system Cu2+:L1,
different titrations were carried out varying not only the
Cu2+:L1 molar ratio but also the concentration of the ligand
and metal ion. The speciation study shows in both systems for
1:1 Cu2+:L molar ratios the formation of complexes of 1:1
Cu2+:L stoichiometry. However, as the X-ray studies evidenced
the formation of 2:2 Cu2+:L1 dimeric dinuclear complexes, we
carried out ESI mass spectrometry and EPR measurements to
ascertain the nature of the complexes formed at the
Figure 5. (a) ORTEP plot of the [Cu3(H−1L1)2(CO3)(H2O)]2+.
Ellipsoids are shown at the 50% probability level. Hydrogen atoms
omitted. (b) View highlighting with different colors the two
macrocycles present in 4. (c) View of the carbonate binding site
and hydrogen bonding network.
E
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netic coupling occurring through the pyrazolate bridges (see
below). Therefore, these experiments suggest that the
stoichiometry of the species existing in equilibrium above pH
5 should be 2:2 instead of 1:1. This is also supported by the
fitting of the pH-metric titrations. This means that, as it
happened in the solid state, also in solution the pyrazolate
subunits have to be disposed outward from the macrocyclic
cavity, which makes a clear-cut difference with L2 containing a
pyridine ring or with other ligands containing pyrazole units.32
The analysis of the UV−vis spectra agrees with the formation of
only one type of complexes at this molar ratio. As early as the
dinuclear [Cu2(L1)2]4+ species starts forming in solution (pH
ca. 4, Figure 6a), the intensity of the peaks at 376 and 600 nm
increases reaching a maximum at pH 6 accordingly with the full
formation of the dinuclear species. Since then on, dimeric
dinuclear species prevail throughout all the pH range, and no
further changes in the UV−vis spectra are observed (Figure
6b).
When speciation studies were performed for solutions with
molar ratios Cu2+:L1 over 1, complexes of higher nuclearity
needed to be introduced in the model to obtain a satisfactory
fitting of the system. Although in principle a [Cu2(H−1L)-
(OH)]2+ species could provide a reasonable fitting, the HR-ESI
mass and EPR spectra and the previously discussed crystal
structure 4 did not support this stoichiometry. EPR spectra
showed the presence of a paramagnetic mononuclear Cu2+
center (Figure S4) while the HR-ESI mass spectra of solutions
presented a peak at 408.5914 that could be ascribed to the
species [Cu3(H−1L1)2Cl2]2+ (Figure S9). Therefore, we
introduced in our model species of this stoichiometry finding
the best fit for the species [Cu3(H−1L1)2]4+ and
[Cu3(H−1L1)2(OH)]3+ in agreement with the X-ray and mass
spectra. It has to be emphasized that while distribution
diagrams calculated for 1:1 Cu2+:L molar ratio do not show
formation of complexes of 3:2 Cu2+:L stoichiometry,
distribution diagrams calculated for 3:2 Cu2+:L molar ratio
show that such species predominate above pH 7 (Figure S5).
In order to estimate the stability of the ternary complexes we
Figure 6. (a) UV−vis spectra of aqueous solutions containing Cu2+:L1
in 1:1 molar ratio recorded as a function of pH (pH values: 3.9, 4.3,
4.5, 4.6, 5.1, 6.0, 7.2, 8.2, 9.2, 9.9, 10.4, 10.7, and 11.0). (b) Plot of the
distribution diagram of the system Cu2+:L1 for 1:1 molar ratio ([Cu2+]
= 1 × 10−3 M, I = 0.15 mol dm−3 NaClO4, T = 298.1 0.1 K, charges
omitted) along with the absorbance values at 376 (blue) and 600 nm
(red). Absorbances are normalized with respect to the maximum
values at each wavelength.
carried out titrations of solution containing Cu2+, L1, and CO3
2‑
in 3:2:3 molar ratios from basic to acidic pH values. Data
obtained in the pH range 10.0−7.0 were treated with the
HYPERQUAD program33 giving a value of log β = 32.40(4) for
2‑
the reaction 3Cu2+ + 2L1 + CO3 ⇆ [Cu3(H−1L1)2(CO3)]2+.
The distribution diagram for this system (see Figure S10)
provides evidence that the ternary complex predominates from
pH 10.0 to 7.5, which indicates an important ability for fixing
CO2.
In the case of L2, the value obtained for the stability constant
of the [CuL2]2+ complex compares well with the stability values
reported for the open-chain pentaamine 1,5,8,11,15-pentaazate-
tradecane (log KCuL = 21.28)34 or with a related pyridinophane
in which this polyamine subunit had been connected to a 2,6-
concentration range used in the titrations (ca. 10−4−10−3 M).
HR-ESI-MS spectra recorded in the positive mode of aqueous
solutions containing Cu(ClO4)2·6 H2O and L1·4HBr·H2O
adjusted to pH 7 and 9 show basically the existence in solution
of dimeric dinuclear 2:2 species as denoted by the m/z peaks at
342.1597, 719.2853, 783.2653, and 795.2643 which correspond
to the species [Cu2(H−1L1)2]2+, [Cu2(H−1L1)2(Cl)]+,
[Cu2(H−1L1)2(ClO4)]+, and [Cu2(H−1L1)2(CH3OH)(Br)]+,
respectively (see Figure S7). A very good agreement between
the experimental and simulated isotopic distributions was
obtained in all the cases.
pyridine spacer through methylene groups (log KCuL
=
20.44),32 suggesting that the pentacoordination observed in
the crystal structure of 3 is also kept in solution.
Moreover, in the EPR spectra of solutions containing Cu2+
and L1 recorded at pH values 2.3, 3.8, 5.5, 7.5, 9.5, and 11, it
can be seen that while at pH values 2.3 and 3.8 the signal of free
aqueous Cu2+ ion appears, the other solutions are EPR silent
(see Supporting Information Figure S8). Taking into account
the magnetic susceptibility study and previous reports on
pyrazolate bridged complexes,31 this can be ascribed to the
formation of dinuclear complexes with a strong antiferromag-
Magnetic Behavior. The magnetic properties of 1 are
shown in Figure 7 in the form of χMT versus T, χM being the
magnetic susceptibility per two Cu2+ ions. The value of χMT at
room temperature is 0.44 cm3 mol−1 K, a value that is
significantly lower than that expected for two uncoupled Cu2+
ions. When the sample has been cooled, χMT continuously
decreases, vanishing at around 70 K. This magnetic behavior is
F
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Inorganic Chemistry
Article
EXPERIMENTAL SECTION
■
1H-3,5-Bis(chloromethyl)pyrazole and 3,5-bis-(chloromethyl)-1-(tet-
rahydropyran-2-yl)-pyrazole were prepared as described in refs 24 and
25.
1,5,9,13-Tetra-(p-tolylsulfonyl)-1,5,9,13-tetraazatridecane.
N,N′-Bis(3-aminopropyl)-1,3-propanediamine (10.0 g, 53.1 mmol)
was dissolved in 400 mL of THF, and K2CO3 (53.6 g, 387.7 mmol) in
100 mL of water was placed in a three-necked round-bottom flask
provided with mechanical stirring. Then, a solution of p-tolylsulfonyl
chloride (55.4 g, 290.8 mmol) in 100 mL of THF was dropwise added
over 1 h. The solution was kept under stirring for one day, and the
organic phase was separated and vacuum evaporated to dryness. The
residue was suspended in ethanol and refluxed for two hours. Then, it
was filtrated and washed exhaustively with ethanol (yield: 89%). Mp:
128−130 °C. 1H NMR (300 MHz, CDCl3): δ 7.73 (d, 4H, J = 8 Hz),
7.62 (d, 4H, J = 8 Hz), 7.32−7.26 (m, 8H), 5.39 (t, 2H, J = 6 Hz), 3.13
(t, 4H, J = 8 Hz), 3.08 (t, 4H, J = 8 Hz), 2.98 (td, 4H, J1 = 8 Hz, J2 = 6
Hz), 2.42 (s, 6H), 2.41 (s, 6H), 1.91−169 (m, 6H). 13C NMR (300
MHz, CDCl3): δ 143.8, 143.5, 136.9, 135.7, 130.0, 129.9, 127.3, 127.1,
47.5, 46.6, 40.3, 29.5, 28.7, 21.6.
Figure 7. χMT vs T plot for compound 1.
11-Tetrahydropyran-2-yl-3,7,11,15-p-toluensulfonyl-1(3,5)-
pyrazolacyclohexadecaphane. 1,5,9,13-Tetra-(p-tolylsulfonyl)-
1,5,9,13-tetraazatridecane (2.65 g, 3.29 mmol) and K2CO3 (4.21 g,
32.9 mmol) were suspended in CH3CN (250 mL). To this mixture,
3,5-bis-(chloromethyl)-1-(tetrahydropyran-2-yl)-1H-pyrazole (820
mg, 3.29 mmol) in 150 mL was added dropwise over 2 h. The
suspension was refluxed for further 20 h and then filtered off. The
solution was vacuum evaporated to dryness. Purification was achieved
by column chromatography (silica gel, chloroform/acetone = 25/1)
characteristic of an important antiferromagnetic interaction
between the Cu2+ ions with singlet spin ground state.
Given that the interaction between dinuclear subunits is
expected to be very weak, it is reasonable to assume that the
magnetic properties are determined by the coupling within
dinuclear subunits. For this reason we have interpreted the
magnetic data according to the actual dinuclear nature of the
complex through the spin Hamiltonian H = −JS1S2. The fit of
the experimental data to the appropriate susceptibility
expression derived from the van Vleck formula, and with the
assumption that the g factors for both Cu2+ are identical, leads
to J = −299(5) cm−1 and g = 2.06(1). The observed J values are
within the range found for related bis(μ-pyrazolato)-bridged
dinuclear complexes.30,31
1
(yield: 34%). Mp: 78−80 °C. H NMR (400 MHz, CDCl3): δ 7.73−
7.63 (m, 8H), 7.40−7.28 (m, 8H), 6.27 (s, 1H), 5.58 (dd, 1H, J1 = 10
Hz, J2 = 2), 4.40 (d, 1H, J = 15 Hz), 4.27 (d, 1H, J = 15 Hz), 4.12 (d,
1H, J = 7 Hz), 4.07 (d, 1H, J = 7 Hz), 3.92−3.83 (m, 1H), 3.78−3.68
(m, 1H), 3.23−2.84 (m, 12H), 2.47 (s, 3H), 2.43 (s, 3H), 2.42 (s,
3H), 2.41 (s, 3H), 2.39−2.37 (m, 2H), 1.88−1.42 (m, 10H). 13C
NMR (400 MHz, CDCl3): δ 148.9, 144.3, 143.5, 138.5, 135.7, 134.6,
130.2, 129.9, 127.5, 127.4, 127.3, 100.1, 84.4, 68.4, 48.3, 48.2, 47.9,
47.8, 47.5, 30.1, 29.7, 29.6, 21.7. ESI-MS observed m/z 1003.44; calcd
for [M + Na]+ 1003.32.
The pathway for the magnetic exchange is propagated
through the bridging pyrazolate moieties. From the molecular
structure of 1, one can conclude that the unpaired electron in
3,7,11,15-Tetraaza-1(3,5)-pyrazolacyclohexadecaphane tet-
rahydrobromide (L1·4HBr·H2O). 11-Tetrahydropyran-2-yl-
3,7,11,15-p-toluensulfonyl-1(3,5)-pyrazolacyclohexadecaphane. (2.64
g, 2.69 mmol) and phenol (12.18 g, 129 mmol) were suspended in
HBr−AcOH 33% (130 mL). The mixture was stirred at 90 °C for 24 h
and then cooled. The resulting residue was filtered off and washed
several times with dry ethanol to give the product L1 as its
2
2
each metal center is clearly described by a dx ‑y magnetic orbital
which is coplanar with the pyrazolate skeleton. The significant
overlap between these magnetic orbitals accounts for the strong
antiferromagnetic coupling observed.
CONCLUSIONS
■
1
pentahydrobromide salt (yield: 65%). Mp: 250−257 °C. H NMR
The synthesis of a [1 + 1] condensation pyrazole azamacro-
cycle is described. At least in the pH range 2−11, neither
protonation nor deprotonation of the pyrazole unit is observed
in this ligand. However, in the presence of an equivalent of
Cu2+, the pyrazole unit readily deprotonates to form 2:2
dimeric Cu2+ complexes with the pyrazolate moieties pointing
outward hinging the two macrocyclic units. This makes an
important difference with related cyclophanes containing
pyridine spacers. The presence of the metal ions and free
amine and ammonium groups within the same structural unit
provides appealing characteristics for the recognition of guest
species at this site of the molecule. As a matter of fact, we have
shown that a third Cu2+ ion can be bound to the two free amine
groups of the polyamines giving rise to an unsaturated
coordination site which prompts this metalloreceptor for
capturing further substrates. This is clearly demonstrated by
the crystal structure of [Cu3(H−1L1)2(CO3)(H2O)](ClO4)2·
8H2O (4) in which an atmospheric CO2 molecule is efficiently
fixed at this site. Future studies will be performed to address the
possibility of using these complexes as catalysts in CO2
chemistry.
(400 MHz, D2O): δ 6.84 (s, 1H), 4.42 (s, 4H), 3.23−3.17 (m, 12H),
2.15−2.07 (m, 6H). 13C NMR (400 MHz, D2O): δ 138.9, 109.9, 43.8,
43.5, 43.4, 22.0, 21.9. ESI-MS observed m/z 281.24 calcd for [M + H]+
281.24. Anal. Calcd for C14H28N6·4HBr·H2O: C, 27.18; H, 5.54; N,
13.59. Found: C, 27.0; H, 5.5; N, 13.6.
3,7,11,15-Tetraaza-3,7,11,15-p-toluensulfonyl-1-(2,6)-pyridi-
nacyclohexadecaphane. 1,5,9,13-Tetra-(p-tolylsulfonyl)-1,5,9,13-
tetraazatridecane (1.5 g, 1.86 mmol) and K2CO3 (2.57 g, 18.6
mmol) were suspended in CH3CN (250 mL). To this mixture was
added 2,6-bis(bromomethyl)pyridine (0.49 g, 1.86 mmol) in 150 mL
CH3CN dropwise over 2 h. The suspension was refluxed for further 20
h and then filtered off. The solution was vacuum evaporated to dryness
and the residue suspended in refluxing ethanol. Then, it was filtrated to
1
give a white solid (yield: 88%). Mp: 157−160 °C. H NMR (400
MHz, CDCl3): δ 7.73 (d, 4H, J = 8 Hz), 7.62 (d, 4H, J = 8 Hz), 7.42−
7.28 (m, 11H), 4.19 (s, 4H), 3.18 (t, 4H, J = 7 Hz), 2.98 (t, 4H, J = 7
Hz), 2.83 (t, 4H, J = 7 Hz), 2.46 (s, 6H), 2.42 (s, 6H), 1.61 (quin, 4H,
J = 7 Hz), 1.09 (quin, 2H, J = 7 Hz). 13C NMR (400 MHz, CDCl3): δ
144.1, 143.9, 138.3, 135.5, 130.3, 130.2, 127.6, 127.5, 54.4, 48.9, 48.8,
48.6, 30.4, 30.2, 21.5, 21.4.
3,7,11,15-Tetraaza-1-(2,6)-pyridinacyclohexadecaphane tet-
rahydrobromide (L2·4HBr·2H2O). 3,7,11,15-Tetraaza-3,7,11,15-p-
toluensulfonyl-1-(2,6)-pyridinecyclohexadecaphane (1.78 g, 2.96
G
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Inorganic Chemistry
Article
1
mmol) and phenol (9.4 g, 99 mmol) were suspended in HBr−AcOH
33% (100 mL). The mixture was stirred at 90 °C for 24 h and then
cooled. The resulting residue was filtered off and washed several times
with dry ethanol to give the product as its tetrahydrobromide salt
as a reference standard (δ = 1.24 ppm for H and δ = 70.36 ppm for
13C).40 Adjustments to the desired pH were made using drops of DCl
or NaOD solutions. The pD was calculated from the measured pH
values using the correlation pH = pD − 0.4.41
1
(yield: 65%), which decomposes at 250 °C. H NMR (400 MHz,
X-ray Analysis. Single crystals of 1 and 4 were measured in an
Oxford diffraction Supernova diffractometer using Mo Kα radiation
(λ= 0.710 73 Å) at 293 and 120 K, respectively. Single crystals of 2 and
3 were measured in an Enraf Nonius Kappa CCD diffractometer using
Mo Kα radiation at 293 and 120 K, respectively. Structures were
solved by direct methods using SIR9742 and refined by full-matrix
least-squares on all F2 using SHELXL9743 with the WinGX suite.44
The non-hydrogen atoms were refined anisotropically, and hydrogen
atoms were placed in calculated positions and refined using a riding
model. Table S1 summarizes crystal data and final refinement
parameters. Hydrogen bonds have been analyzed with SHELXL97.
Molecular drawings were produced with the MERCURY program.45
Magnetic Susceptibility Measurements. Magnetic susceptibil-
ity measurements on polycrystalline samples of compound [Cu2(H-
(H−1L1))(H−1L1)](ClO4)3·3.75H2O were carried out in the 1.9−
300.0 K temperature range with a Quantum Design SQUID
magnetometer under an applied magnetic field of 5000 G. The
experimental data were corrected for the diamagnetic contributions of
the constituent atoms and the sample holder as well as for the
temperature-independent paramagnetism of the Cu2+ ion (60 × 10−6
cm3 mol−1).
D2O): δ 7.92 (t, 1H, J = 8 Hz), 7.47 (d, 2H, J = 8 Hz), 4.53 (s, 4H),
3.31−3.39 (m, 12H), 2.15−2.30 (m, 6H). 13C NMR (400 MHz,
D2O): δ 150.2, 139.5, 122.9, 50.55, 44.4, 42.1, 41.9, 22.1, 19.3. ESI-MS
observed m/z 292.2, calcd for [M + H]+ 292.24; observed m/z 314.2,
calcd for [M − Na]+ 314.2. Anal. Calcd for C16H29N5·4HBr·2H2O: C,
29.51; H, 5.73; N, 10.76. Found: C, 29.4; H, 5.7; N, 10.6.
[Cu2(H(H−1L1))(H−1L1)](ClO4)3·3.75H2O (1). Crystals of 1 were
grown by slow evaporation of aqueous solutions containing L1·4HBr·
H2O and Cu(ClO4)2·6H2O in 1:1 mol ratio at pH 9 in an open vessel.
After several days, blue crystals of 1 suitable for X-ray diffraction were
obtained. Anal. Calcd for C28H62,5Cl3Cu2N12O15,75: C, 31.94; H, 5.98;
N, 15.96. Found: C, 31.8; H, 6.1; N, 15.8.
[Cu2(H(H−1L1))0.5(H−1L1)1.5]2(ClO4)3Br2·4.2H2O (2). Crystals of 2
were obtained as those of 1 but with pH of 10. Anal. Calcd for
C56H117.4Br2Cl3Cu4N24O16.2: C, 35.27; H, 6.20; N, 17.63. Found: C,
35.5; H, 6.2; N, 17.6.
[CuL2](ClO4)2 (3). Crystals of 3 were obtained by slow evaporation
of an aqueous solution containing Cu(ClO4)2·6H2O and L2·4HBr·
2H2O at pH 8 with an excess of NaClO4. Anal. Calcd for
C16H29Cl2CuN5O8: C, 34.70; H, 5.28; N, 12.64. Found: C, 34.9; H,
5.6; N, 12.7.
Spectrometric Measurements. HR-ESI mass spectra of solutions
(water/methanol 50/50 vol/vol) containing L1 and Cu(ClO4)2·6H2O
in 1:1 and 2:3 molar ratio were acquired in the positive ion mode using
a Triple TOF 5600 hybrid quadrupole time-of-flight (TOF) mass
spectrometer. N2 was used as a curtain and nebulizing gas. The
experiments were performed at a voltage of 5300 V and GS1 and GS2
(35 psi) ion source gas at 723.15 K. The AB SCIEX Peak View
software was used for the analysis of the data.
[Cu3(H−1L1)2(CO3)(H2O)](ClO4)2·8H2O (4). Crystals of 4 were grown
by slow evaporation of aqueous solutions containing L1·4HBr·H2O
and Cu(ClO4)2·6H2O in 2:3 mol ratio at pH 9 in an open vessel. After
several days, blue crystals of 1 suitable for X-ray diffraction were
obtained. Anal. Calcd for C29H72Cl2Cu3N12O20: C, 29.75; H, 6.20; N,
14.35. Found: C, 29.8; H, 6.3; N, 14.3.
Electromotive Force Measurements. The potentiometric
Spectroscopic Measurements. Spectra of the samples contain-
ing the ligand and the complexes in 1:1 and 2:3 molar ratio (1 × 10−3
M) were recorded with an Agilent 8453 spectrometer UV−vis spectra
at 293.15 K.
Electron Paramagnetic Resonance Measurements. X-band
EPR spectra (ν = 9.47 GHz) of the frozen-matrix aqueous solutions at
titrations were carried out at 298.1
0.1 K using NaClO4 0.15 M
as supporting electrolyte. The experimental procedure (buret,
potentiometer, cell, stirrer, microcomputer, etc.) has been fully
described elsewhere.35 The acquisition of the emf data was performed
with the computer program PASAT.36 The reference electrode was a
Crison 52 40 Ag/AgCl electrode in 0.5 M NaCl solution. A Wilhelm
bridge filled with 0.5 M NaCl was used to separate the glass and the
reference electrode. The glass electrode (Crison 52 50 Ag/AgCl) was
calibrated as a hydrogen-ion concentration probe by titration of
previously standardized amounts of HCl with CO2-free NaOH
solutions and determination of the equivalent point by Gran’s
method,37 which gives the standard potential, E°′, and the ionic
product of water (pKw = 13.73(1)).
70
K and Q-band EPR spectra (ν = 34.03 GHz) of
[Cu3(H−1L1)2(CO3)(H2O)](ClO4)2·8H2O at 9 K were recorded in
nonsaturating conditions on a Bruker ER 200 D spectrometer
equipped with a helium cryostat.
ASSOCIATED CONTENT
■
S
The computer program HYPERQUAD33 was used to fit the
protonation and stability constants. Solutions containing the ligand
salts with Cu2+:L molar ratios varying from 2:1 to 1:2 were titrated
with NaOH. In the case of the system Cu2+−L1 nine independent
titrations with Cu2+ concentrations ranging from 2.0 × 10−4 M to 1.1
× 10−3 M with a total of 769 experimental data points were carried
out. The different titration curves for each ligand were treated as
separated curves without significant variations in the values of the
stability constants. Finally, the sets of data were merged together and
treated simultaneously to give the final stability constants. For
estimating the constant for the formation of the carbonate complexes,
two titrations of solutions containing Cu2+, L1, and Na2CO3 in 3:2:3
molar ratio were performed from basic to acidic pH values. The
constants for the protonation of L1 and carbonate, for the formation
of binary Cu2+−L1 complexes, were introduced as fixed parameters,
* Supporting Information
CIF data, crystallographic data, distribution diagrams, UV−vis
spectra, EPR spectra, ESI-mass spectra, and experimental
details. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: enrique.garcia-es@uv.es.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
2‑
and then the constants for the formation of ternary Cu2+−L1−CO3
Financial support by the Spanish MINECO, MECD, and
FEDER funds of the E.U. (Projects CTQ2010-14892 and
CONSOLIDER INGENIO 2010 CSD2010-00065) and
Generalitat Valenciana (PROMETEO 2011/008) is gratefully
acknowledged. The authors would like to thank Prof. K.
Rissanen and G. de Munno for their help with the X-ray
analysis. R.B. and J.P.-J. want to thank MECD and MINECO
for their Ph.D. grants.
complexes calculated. When more than one model fit the experimental
data, the most reliable chemical model was chosen by performing F
tests at the 0.05 confidence level.38,39 More details on data treatment
are included in the Supporting Information.
NMR Measurements. The 1H and 13C NMR spectra were
recorded on Bruker Advance DPX 300 MHz and Bruker Advance
DPX 400 MHz spectrometer operating at 299.95 and 399.95 MHz for
1H and at 75.43 and 100.58 MHz for 13C. tert-Butyl alcohol was used
H
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Inorganic Chemistry
Article
(29) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor,
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