Oxo-anion recognition by mono- and bisurea pendant-arm macrocyclic complexes

Article abstract of DOI:10.1021/ic501527k

The novel macrocyclic copper(II) complexes [2]²⁺ and [3]²⁺, carrying one or two (nitrophenyl)urea fragments appended to an azacyclam or diazacyclam framework, exploit the hydrogen-bond-forming abilities of the urea subunits, along wi

Full text of DOI:10.1021/ic501527k

Article
pubs.acs.org/IC
Oxo-Anion Recognition by Mono- and Bisurea Pendant-Arm
Macrocyclic Complexes
Massimo Boiocchi, Maurizio Licchelli,* Michele Milani, Antonio Poggi, and Donatella Sacchi
†
,‡
‡
‡
‡
†
‡
Centro Grandi Strumenti and Dipartimento di Chimica, Universita
S Supporting Information
̀
di Pavia, via T. Taramelli 12, 27100 Pavia, Italy
*
ABSTRACT: The novel macrocyclic copper(II) complexes
2+
2+
[
2] and [3] , carrying one or two (nitrophenyl)urea fragments
appended to an azacyclam or diazacyclam framework, exploit the
hydrogen-bond-forming abilities of the urea subunits, along with
the metal−ligand interaction, in the recognition of anionic
2
+
species. Equilibrium studies in acetonitrile performed on [2]
and [3]2 show that (nitrophenyl)urea pendant arms strongly
interact with anionic species such as carboxylates and phosphates,
which display both coordinating tendencies toward copper(II)
and good affinity toward urea subunits. Stability constants of the
adducts are considerably higher than those determined for the
interaction of the same anions with a “plain urea” reference
compound, confirming the synergistic action of metallomacro-
+
2
+
cyclic and urea subunits. Complex [2] forms 1:1 adducts with acetate, benzoate, hydrogendiphosphate, and dihydrogen
2
+
phosphate, while complex [3] interacts with the same anions according to both 1:1 and 1:2 stoichiometries, with the exception
of hydrogendiphosphate, which forms only the 1:1 adduct with a distinctly high association constant (log K > 7).
Spectrophotometric investigations suggest that oxoanionic species interact with the complexes according to a “bridged” mode,
inducing the macrocyclic systems to adopt a scorpionate-like conformation, as confirmed by crystallographic studies on the
2+
[
3] /succinate adduct.
INTRODUCTION
Among hydrogen-bonding donor groups, urea and thiourea
have been frequently employed as anion binding fragments. In
fact, they are able to form two hydrogen bonds in a parallel
fashion, displaying an enhanced complementarity toward Y-
■
Molecular recognition of anionic species has attracted
increasing interest during the last decades
1
−3
because of the
important role played by anions in biological and environ-
shaped oxo anions (e.g., carboxylates). Seminal papers by
2
,4,5
mental processes.
A very large number of artificial systems
24,25
Wilcox and Hamilton
showed the excellent anion complex-
for recognition and sensing of anions have been designed and
synthesized by properly exploiting supramolecular con-
ation properties displayed by urea-containing molecular
receptors. Since then, a large number of receptors and sensors
containing one or more urea/thiourea units in their structure
6
−12
cepts.
Recognition of anionic species is often accomplished by
electrostatic and/or hydrogen-bonding interactions. In their
pioneering work, Lehn and Schmidtchen
positively charged cagelike receptors able to incorporate
inorganic anions. In these kinds of receptors, selectivity is
mainly based on the matching of the geometrical features
26−33
have been reported.
A large number of anion receptors based on metal centers
1
3,14
developed
34−38
have also been reported.
In these systems, metal ions can
play a merely structure-organizing role or be directly involved
in the anion binding. In the first case, for instance, the
coordination of properly functionalized ligand(s) around a
metal ion can result in the formation of a highly organized
system displaying one or more sites suitable for anion binding
(
shape and size) of both the anion and the host’s cavity.
Hydrogen bonding, despite its fundamentally electrostatic
nature, has a directional character and, in principle, allows
better discrimination between anions with different structural
and electronic features than genuine electrostatic interactions.
Therefore, a variety of molecular receptors have been
developed that perform recognition of their anionic targets by
3
4,35
through electrostatic or hydrogen-bonding interactions.
In
the second case, metal−ligand interactions are directly involved
in anion binding and recognition, exploiting the definite
coordinating tendencies of this kind of substrate toward
15
36−38
metal ions.
Coordinative interactions are usually stronger
8
,16−19
establishing a complementary set of hydrogen bonds.
than electrostatic interactions (including the hydrogen bond);
Some classes of compounds, for instance, cyclic and acyclic
polyammonium receptors, interact with anions by combining
pure electrostatic and hydrogen-bonding interactions.
Received: June 27, 2014
20−23
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ic501527k | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
moreover, they display a more definite directional character and
may impose further geometrical constraints, which contribute
to increasing selectivity in the recognition process. A metal−
ligand subunit included in the structure of an anion receptor
should be coordinatively unsaturated, thus leaving one (or
more) binding site(s) available for the anionic substrate, and
should establish with it kinetically labile interactions to provide
a fast and reversible interaction (as requested for the
recognition process). On the other hand, metal centers should
be firmly retained in the receptor structure in order to prevent
release of the metal ion and loss of the anion binding ability.
Macrocyclic complexes fulfill these requirements because (i)
they are usually characterized by high thermodynamic stability
considerably increases their binding ability toward the oxo
anions.
EXPERIMENTAL SECTION
■
General Procedures and Materials. All reagents for syntheses
were purchased from Sigma-Aldrich and used without further
purification.
N-tert-Butoxycarbonyldiaminoethane, en-BOC, was prepared ac-
4
6
cording to the literature. The copper complex of 1,8-bis(2-
aminoethyl)-1,3,6,8,10,13-esaazacyclotetradecane was prepared ac-
cording to the literature method and isolated as the diammonium
4
7
tetraperchlorate salt [5](ClO ) .
4
4
Carbon, hydrogen, and nitrogen elemental analyses were carried out
using a Costech ECS 4010 instrument at the Department of
Chemistry, Materials, and Chemical Engineering “G. Natta”,
39−41
and are kinetically inert
and (ii) they may interact at the
axial position(s) with additional ligand(s) and this interaction is
kinetically labile.
1
13
Politecnico di Milano, Milan, Italy. H and C NMR spectra were
acquired on a Bruker Avance 400 spectrometer. Mass spectrometry
In spite of the large number of examples concerning anion
recognition processes based on either hydrogen bonds or
coordinative interactions, a relatively small number of synthetic
receptors have been reported, which exploit both interactions at
(MS) spectra were acquired using a Thermo-Finnigan ion trap LCQ
Advantage Max instrument equipped with an electrospray ionization
(ESI) source. IR spectra were run on a PerkinElmer Spectrum X100
Fourier transform infrared (FT-IR) instrument equipped with an
attenuated total reflectance (U-ATR) apparatus. UV−vis spectra were
recorded using a Varian Cary 50 or Cary 100 spectrophotometer with
a quartz cuvette (path length: 1 or 0.1 cm).
4
2,43
the same time.
In particular, a few years ago we reported
the synthesis and solution behavior of a couple of receptors for
4
4
anions characterized by a macrocyclic complex framework.
2
+
2+
All titrations were performed at 25 °C on solutions of [2](ClO4)
2
Copper complexes [1a] and [1b] incorporating a urea
subunit are able to recognize oxo anions exploiting the
cooperative effect because of the proximity of a coordinatively
unsaturated metal center and a hydrogen-bonding donor group.
and [3](ClO4)2 in MeCN. Aliquots of freshly prepared standard
−
−
−
−
solutions of [Bu N]X (X = CH COO , C H COO , NO , H PO ,
4
3
6
5
3
2
4
−
and HSO ) and [Bu N] HP O in MeCN were added, and a UV−vis
4
4
3
2
7
spectrum of the resulting solution was taken after every addition.
Spectral intensities were corrected for the dilution factor. Solutions of
succinate and glutarate in MeCN were prepared by the addition of
2
+
2+
It should be noted that compounds [1a] and [1b] were
obtained by a template reaction in which ureas were used as
4
1,45
stoichiometric amounts of [Et N]OH to solutions of the correspond-
“locking fragments”;
therefore, a nitrogen of the urea group
4
ing dicarboxylic acid. Titration data were processed with the
is incorporated into the macrocyclic azacyclam framework. This
prevents full exploitation of the tendencies of urea moieties to
form several hydrogen bonds at the same time (in particular
toward Y-shaped anions).
4
8
Hyperquad software package to determine the equilibrium constants.
Care was taken that in each titration the p parameter (p =
[
concentration of complex]/[maximum possible concentration of
complex]) was lower than 0.8, a condition required for the safe
determination of a reliable equilibrium constant.
4
9
Safety note! Perchlorate salts of metal complexes are potentially
explosive and should be handled with care. In particular, they should never
5
0
be heated as solids.
Synthesis of [4](ClO ) . N,N′-Bis(2-aminoethyl)propane-1,3-
4
3
diamine (2,3,2-tet; 0.4 g, 2.5 mmol), en-BOC (0.4 g, 2.5 mmol),
6.5% aqueous formaldehyde (2.0 mL, 26.5 mmol), and triethylamine
3
(0.75 mL, 5.4 mmol) were added to a solution of Cu(CH COO) ·
3 2
H O in methanol (50 mL). The resulting mixture was magnetically
2
stirred and heated at reflux for 24 h. After cooling at room
temperature, 65% aqueous HClO (20 mL) was added and a pink
4
solid precipitated. It was recovered by filtration and recrystallized from
0
.1
M aqueous HClO4. Yield: 30%. Anal. Calcd for
C H Cl CuN O (607.3): C, 21.8; H, 4.8; N, 13.8. Found: C,
11
29
3
6
12
2
1.5; H, 4.6; N, 13.6. ESI-MS (MeCN): m/z 508 (100%; [M −
− +
ClO ] ). FT-IR (solid, ATR): 3525 (m), 3247 (m), 3180 (m), 2970
4
(
9
(
w), 2888 (w), 1602 (m), 1504 (m), 1472 (m), 1428 (m), 1052 (vs),
−1
−1
−1
98 (s), 620 (s) cm . UV−vis [H O; λmax, nm (ε, M cm )]: 500
2
67).
Synthesis of [2](ClO ) . A solution of (4-nitrophenyl)isocyanate
(0.27 g, 1.65 mmol) in anhydrous MeCN (20 mL) was added
dropwise under a dinitrogen atmosphere to a round-bottomed flask
4
2
We report here the synthesis of two novel macrocyclic
compounds, [2]2+ and [3] , in which one or two genuine urea
fragments are appended onto an azacyclam or diazacyclam
structure, respectively. In principle, these compounds could
fully exploit the hydrogen-bond-forming abilities of urea
subunits, along with the metal−ligand interaction, in the
recognition process. Equilibrium studies in acetonitrile
2+
containing a solution of [4](ClO
) (1 g, 1.65 mmol), triethylamine
4 3
(
0.92 mL, 6.6 mmol), and pyridine (0.53 mL, 6.6 mmol) in anhydrous
MeCN (30 mL). The resulting mixture was magnetically stirred at
room temperature for 5 h. The pink solid that precipitated during the
reaction was isolated by filtration, washed with cold MeCN (10 mL),
and dried in vacuo. Yield: 71%. Anal. Calcd for C H Cl CuN O
2
+
2+
18 32
2
8
11
(
MeCN) performed on [2] , [3] , and “plain urea” reference
(
670.9): C, 32.2; H, 4.8; N, 16.7. Found: C, 32.4; H, 4.9; N, 16.5. ESI-
compound 6 have shown that (nitrophenyl)urea pendant
arm(s) strongly interact with carboxylate and phosphate anions
and that the proximate copper(II) macrocycle subunit
− +
MS (MeCN): m/z 572 (100%; [M − ClO ] ), 235 (25%; [M −
4
−
2+
2ClO ] ). FT-IR (solid, ATR): 3425 (w), 3388 (m), 3230 (m),
4
2951 (w), 2887 (w), 1700 (s), 1655 (w), 1608 (m), 1594 (m), 1534
B
dx.doi.org/10.1021/ic501527k | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
3
(
s), 1505 (s), 1485 (s), 1446 (m), 1425 (m), 1365 (w), 1316 (vs),
1584.9(8) Å , Z = 1, 7859 measured reflections, 5553 unique
reflections (Rint = 0.021), 3490 strong data [I > 2σ(I )], R1 = 0.0641
1
1
295 (s), 1207 (s), 1170 (m), 1154 (m), 1104 (s), 1081 (s), 1071 (s),
054 (vs), 997 (s), 954 (s), 876 (m), 838 (m), 743 (m), 684 (m), 616
O
O
and wR2 = 0.1056 for strong data, and R1 = 0.1669 and wR2 = 0.1933
for all data. Positional disorder affects perchlorate counterions and
dimethyl sulfoxide solvent molecules [see the Supporting Information
(SI) for details].
−1
−1
−1
(
5
s) cm . UV−vis [CH CN; λmax, nm (ε, M cm )]: 335 (17050),
3
01 (68).
Synthesis of [3](ClO ) . [5](ClO ) (1.00 g 1. 33 mmol) was
4
2
4 4
dissolved in anhydrous MeCN (30 mL) under a dinitrogen
atmosphere, and then triethylamine (0.74 mL, 5.32 mmol) and
pyridine (0.43 mL, 5.32 mmol) were added to the mixture. A solution
of (4-nitrophenyl)isocyanate (0.44 g, 2.66 mmol) in anhydrous MeCN
Crystal data for 2([3]C H O )·4DMF: C H Cu N O , M =
4
4
4
72 116
2
28 24
̅
1885.02, triclinic, P1 (No. 2), a = 9.329(1) Å, b = 12.550(1) Å, c =
18.982(2) Å, α = 92.63(1)°, β = 93.56(1)°, γ = 101.92(1)°, V =
3
2166.2(4) Å , Z = 1, 15742 measured reflections, 7592 unique
(
20 mL) was slowly added, and the resulting mixture was stirred for 5
reflections (Rint = 0.016), 6306 strong data [I > 2σ(I )], R1 = 0.0380
O O
h at room temperature. A pink precipitate was filtered off, washed with
cold MeCN (10 mL), and dried in vacuo. Yield: 80%. Anal. Calcd for
C H Cl CuN O (879.1): C, 35.5; H, 4.6; N, 19.1. Found: C, 35.5;
and wR2 = 0.0464 for strong data, and R1 = 0.1046 and wR2 = 0.1117
for all data.
26
40
2
12 14
+
H, 4.7; N, 19.2. ESI-MS (MeOH): m/z 780 (50%; [M − ClO ] ), 340
RESULTS AND DISCUSSION
Synthesis and Crystal Structure of the Macrocyclic
Complexes. The copper complexes [2] and [3] were
4
■
−
2+
(
100%; [M − 2ClO ] ). FT-IR (solid, ATR): 3433 (w), 3394 (m),
4
3
(
1
235 (m), 2952 (w), 2885 (w), 1705 (s), 1661 (w), 1613 (m), 1600
m), 1542 (s), 1510 (s), 1490 (s), 1456 (m), 1431 (m), 1374 (w),
325 (vs), 1301 (s), 1272 (m), 1213 (s), 1176 (m), 1159 (m), 1109
s), 1090 (s), 1075 (s), 1059 (vs), 1000 (s), 960 (s), 932 (w), 883
2+
2+
prepared by a two-step procedure (see Scheme 1) involving (i)
(
(
λ
−1
Scheme 1. Synthesis of Complexes [2]2+ and [3]2+
m), 843 (m), 749 (m), 688 (m), 619 (s) cm . UV−vis [CH CN;
max
3
−1
−1
, nm (ε, M cm )]: 336 (32800), 498 (69).
Synthesis of N-(4-Nitrophenyl)-N′-propylurea (6). A solution
of (4-nitrophenyl)isocyanate (0.22 g, 1.33 mmol) in anhydrous MeCN
20 mL) was slowly added under a dinitrogen atmosphere to a
solution containing propylamine (0.078 g, 1.33 mmol), triethylamine
0.37 mL, 2.66 mmol), and pyridine (0.21 mL, 2.66 mmol) in
(
(
anhydrous MeCN (10 mL). After 5 h of magnetic stirring, a yellow
precipitate formed. It was separated by filtration, washed with MeCN,
−
and dried in vacuo. Yield: 82%. ESI-MS: m/z 222 (100%; [M − H] ).
1
H NMR [400 MHz, dimethyl sulfoxide (DMSO)-d , 25 °C]: δ 0.88
6
(
t, 3H, CH ), 1.45 (m, 2H, CH ), 3.06 (m, 2H, CH ), 6.45 (m, 1H,
3 2 2
13
NH) 7.62 (d, 2H, arom), 8.15 (d, 2H, arom), 9.23 (d, 1H, NH).
C
NMR (100 MHz, DMSO-d , 25 °C): δ 12.12 (s, 1C, CH ), 23.61 (s,
6
3
1
C, CH ), 41.78 (s, 1C, CH ), 117.61 (s, 2C, arom) 125.98 (s, 2C,
2
2
arom), 141.18 (s, 1C, arom), 148.10 (s, 1C, arom), 155.23 (s, 1C,
−
1
−1
CO). UV−vis [CH CN; λmax, nm (ε, M cm )]: 336 (16200).
3
X-ray Crystallographic Studies. Diffraction data for [3](ClO )
red, 0.35 × 0.22 × 0.12 mm ) and [3](ClO ) ·6DMSO (pale red,
.75 × 0.60 × 0.45 mm ) crystals were collected by means of a
4
2
3
(
4
2
3
0
the metal-templated synthesis of the macrocyclic framework,
conventional Enraf-Nonius CAD4 four-circle diffractometer. Diffrac-
tion data for a crystal of succinate-containing complex 2([3]C H O )·
affording the protonated mono- and diamino intermediates,
3+
4+
4
4
4
[4] and [5] , respectively, and (ii) the reaction with (4-
3
4
DMF (violet, 0.50 × 0.42 × 0.33 mm ; DMF N,N-dimethylforma-
nitrophenyl)isocyanate to convert primary amino groups into
mide) were collected by means of a Bruker AXS CCD-based three-
circle diffractometer. Both instruments work at ambient temperature
with graphite-monochromatized Mo Kα X-radiation (λ = 0.71073 Å).
Data reductions for intensities collected with the conventional
3+
ureas. Azacyclam complex [4] was obtained by a template
reaction in which the copper(II) complex of an open
tetraamine (2.3.2-tet) is converted into a macrocyclic
compound in the presence of formaldehyde and monop-
rotected ethylenediamine, which behaves as a “locking
fragment”.
51
diffractometer were performed with the WinGX package; absorption
52
effects were evaluated with the ψ-scan method, and absorption
correction was applied to the data. Data reduction for frames collected
5
3
4+
by the CCD-based system were performed with the SAINT software;
absorption effects were empirically evaluated by the SADABS
The diazacyclam complex [5] was also prepared according
47
to a template method in which ethylenediamine acts both as
the starting ligand for copper(II) and as the locking fragment.
54
software, and absorption correction was applied to the data. All
5
5
crystal structures were solved by direct methods (SIR 97) and
3+
4+
Both intermediates [4] and [5] were isolated, after the
addition of excess perchloric acid, as tri- and tetraperchlorate
salts, respectively, with the primary amino groups protonated.
The addition of acid induces at the same time deprotection of
the primary amino group of the mono-pendant-arm azacyclam
derivative. The reaction with (4-nitrophenyl)isocyanate was
carried out in anhydrous MeCN in the presence of
2
refined by full-matrix least-squares procedures on F using all
56
reflections (SHELXL 97). Anisotropic displacement parameters
were refined for all non-H atoms. H atoms bonded to C atoms were
placed at calculated positions with the appropriate AFIX instructions
and refined using a riding model; H atoms bonded to secondary
amines were located in the final ΔF maps, and their positions were
successively refined, restraining the N−H distance to be 0.96 ± 0.01 Å.
Crystal data for [3](ClO ) : C H Cl CuN O , M = 879.15,
5
7,58
4
2
26 40
2
12 14
pyridine.
triclinic, P1
̅
(No. 2), a = 8.153(4) Å, b = 9.129(6) Å, c = 13.618(7) Å,
The mono- and bisurea derivatives [2]2+ and [3]2+ were
3
α = 76.59(5)°, β = 75.01(5)°, γ = 68.05(4)°, V = 897.7(9) Å , Z = 1,
satisfactorily characterized by elemental analyses and by FT-IR
3415 measured reflections, 3162 unique reflections (Rint = 0.036),
2+
4+
and ESI-MS. The electronic spectra of complexes [2] −[5]
2
321 strong data [IO > 2σ(I )], R1 = 0.0837 and wR2 = 0.1185 for
O
as well as of the reference urea derivative 6 were measured in
strong data, and R1 = 0.1743 and wR2 = 0.2039 for all data.
Crystal data for [3](ClO ) ·6DMSO: C H Cl CuN O S , M =
−
5
MeCN. Solutions with different concentrations (about 10
4
2
38 76
2
12 20 6
and 10− M) of macrocycle/urea conjugates were examined to
3
1
347.98, triclinic, P1
̅
(No. 2), a = 10.259(3) Å, b = 12.451(3) Å, c =
14.554(4) Å, α = 66.14(2)°, β = 74.18(2)°, γ = 70.85(2)°, V =
accurately detect both the intense bands in the UV range and
C
dx.doi.org/10.1021/ic501527k | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
the weak bands in the visible range. Absorption data of all of
the mentioned compounds and of the reference macrocyclic
2
+
copper complexes of methylazacyclam, [7] , and dimethyldia-
2+
zacyclam, [8] , are reported in Table 1.
Table 1. UV−Vis Spectral Data for Copper(II) Aza- and
Diazacyclam Complexes and Related Reference Compounds
in MeCN
ε (M− cm−1
1
)
compound
λ (nm)
[
[
[
[
2]2+
335
17050
68
5
01
336
98
500
3]2+
32800
69
4
4]3+
Figure 1. ORTEP view (30% probability level) of the copper(II)
67
a
a
diazacyclam complex salt [3](ClO ) . Selected bond lengths (Å) and
4 2
5
10
76
angles (deg): Cu(1)−N(1) 2.009(6), Cu(1)−N(3) 1.999(5), N(1)−
Cu(1)−N(3) 94.0(2), N(1)−Cu(1)−N(3)′ 86.0(2). Symmetry code ′
5]4+
496
336
505
498
68
6
16500
=
−x, 1 − y, 1 − z. Dashed lines indicate hydrogen bonds.
7]2+
8]2+
75
76
a
,
b
[
[
a,
c
a
b
c
In water. Reference 60. Reference 61.
II
mean Cu −N distance is 2.00(1) Å, slightly shorter than the
63
value of 2.03 Å reported for the strain-free Cu−N distance.
The strong absorption in the UV region (335−336 nm) can
be ascribed to a charge-transfer (CT) transition due to the
nitrophenyl)urea moieties on the pendant arms of compounds
The two axial positions corresponding to a rather elongated
octahedral coordination for the metal center are occupied by
two symmetrically equivalent O(6) atoms of two perchlorate
counterions. The observed Cu(1)−O(6) distance of 2.71(1) Å
suggests that only a very weak coordinative interaction occurs.
In particular, each perchlorate counterion profits from this weak
interaction and from an additional hydrogen-bond interaction
provided by the N(3) secondary amine, which acts as a H-atom
donor. Features of the N−H···O interaction are N(3)···O(7)
3.11(1) Å, H(3N)···O(7) 2.27(5) Å, and N(3)−H(3N)···O(7)
146(4)°.
(
2+
2+ 15,44
[2] and [3] .
This band displays a similar intensity in the
2
+
spectra of complex [2] and of metal-free urea derivative 6,
2+
while it is about 2-fold more intense in the spectrum of [3] , in
accordance with the presence of two (nitrophenyl)urea
chromophores appended on the macrocyclic complex.
The weak bands observed in the visible region (around 500
nm) must be considered an envelope of the three possible d−d
2
2
2
2
2
2
2
transitions (xz, yz → x − y ; z → x − y ; xy → x − y ) in the
copper(II) tetramine chromophore with square-planar coordi-
The tertiary amines show a trigonal-pyramidal geometry with
N(2) located at only 0.26(1) Å from the basal plane defined by
the three C atoms bonded with N. This suggests a pronounced
sp character for the N(2) atom, which is also excluded by any
coordinative interaction with the metal center, due to the
Cu(1)−N(2) distance of 3.31(1) Å.
The (nitrophenyl)urea pendant arms are almost coplanar,
with dihedral angles of 9.7(8)° between the urea group and
phenyl ring and 10.3(19)° between the phenyl ring and
terminal nitro group. The dihedral angle between the urea and
nitro groups is 19.2(15)°.
4
4,59
nation geometry.
The d−d absorptions in the spectra of
2+
2+
[
2] and [3] are comparable to those previously reported for
2
+
2+
2
plain copper(II) azacyclam, [7] , and diazacyclam, [8] ,
6
0,61
complexes
and to those exhibited by the mono- and
3
+
4+
bis(ethylammonium) derivatives [4] and [5] .
Red crystals of complex salt [3](ClO ) suitable for X-ray
4
2
diffraction studies were obtained by slow diffusion of diethyl
ether in an MeCN solution. In a separate experiment, slow
diffusion of ethyl acetate in a DMSO solution provided pale-red
crystals of [3](ClO ) ·6DMSO, whose structure has been
4
2
discussed in the SI (see Figures S1 and S2 and Table S1).
The molecular structure of the copper(II) diazacyclam
complex [3]2 determined on a single crystal of its perchlorate
salt is shown in Figure 1.
The NH groups of the urea moieties act as H-atom-donor
groups of weak N−H···O interactions (D···A > 3.2 Å) involving
the O atoms of perchlorate counterions as H-atom-acceptor
species. A complete description of hydrogen-bond interactions
in the solid state is reported in the SI.
Interaction of [2] with Oxo Anions. Spectrophoto-
metric Studies. The behavior of anion receptors containing
binding groups inclined to form hydrogen bonds, such as urea
and thiourea, is usually investigated in aprotic media in order to
prevent solvent competition. The present study was performed
+
The molecular structure exhibits C symmetry with the metal
i
2
+
center placed on a crystallographic center of inversion; the
macrocyclic diazacyclam ligand adopts the trans-III(R,R,S,S)
6
2
configuration commonly observed in copper(II) cyclam metal
II
complexes. The Cu center shows a square-planar coordination
and lies on the plane defined by the four secondary amines. The
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in MeCN, in which system [2]2+ (as well as [3] ) displays a
reasonable solubility.
2+
increase of the molar absorbance from 17050 to 19600 M
−1
−1
cm . The titration profile obtained by plotting the absorption
intensity at 370 nm versus the equivalent ratio (see Figure 2b)
indicates a 1:1 stoichiometry for the receptor−acetate
interaction. The log K value determined for the equilibrium
In particular, the interaction of the copper complex/urea
conjugate with anionic species was investigated by spectropho-
tometric titration experiments in which solutions of tetrabuty-
lammonium salts of the selected anions (usually about 2 × 10
M) were added to a solution of [2] (about 5 × 10 M).
−3
[2]2 + CH COO ⇄ [2(CH COO)]
+
−
+
2
+
−5
3
3
(
1)
The urea−anion interaction was monitored by observing the
changes in the absorption band corresponding to the CT
transition (335 nm). This transition generates a partial negative
charge on the nitro substituent and a partial positive charge on
the NH group belonging to the urea moiety. Therefore, the
interaction of a negatively charged species with the urea subunit
is expected to stabilize the excited state and, as a consequence,
to reduce the transition energy. Thus, the anion−urea
interaction should result in a red shift of the absorption
band: the stronger the interaction, the larger the resulting Δλ.
Nonlinear least-squares treatment of the spectrophotometric
titration data allowed evaluation of the stability constants for
complex/anion adducts. The corresponding log K values are
reported in Table 2.
is 6.03 ± 0.04.
It should be noted that a monourea pendant-arm receptor,
2+
[
2] , was isolated as perchlorate salt and, as a consequence,
−
ClO4 anions are present in solution when spectrophotometric
titration is performed. Therefore, the receptor−anion associa-
tion constant determined by the above-described experiment
should be related, in principle, to an “exchange” equilibrium:
+
−
+
−
[
2(ClO )] + CH COO ⇄ [2(CH COO)] + ClO
4
3
3
4
(2)
However, because of the extremely poor basic and coordinating
properties of perchlorate, competition with this anion should be
considered negligible.
These data show that the interaction between complex [2]2+
and acetate involves the urea pendant arm, as evidenced by the
red shift of the (nitrophenyl)urea CT absorption band. Because
the low intensity of the d−d band at low concentration levels
prevents a clear observation of possible changes, no information
about a cooperative effect of the proximate metal center in the
anion binding process can be provided by this experiment.
Therefore, the titration was repeated on a more concentrated
solution of the azacyclam complex [2]2+ in MeCN.
Table 2. Stability Constants (log K), Determined from UV−
Vis Spectral Data, for the Interaction of Anions with
Copper(II) Aza- and Diazacyclam Complexes [2] and [3]2+
2+
and Urea Reference Compound 6 in MeCN
anion
[2]2+
[3]2+
6
−
CH COO
6.03 ± 0.04 log K = 6.47 ± 0.09, log K
4.48 ± 0.03
3
1
2
2
=
5.24 ± 0.09
−
C H COO
5.14 ± 0.01 log K = 5.25 ± 0.04, log K
4.10 ± 0.01
6
5
1
Figure 3 shows d−d spectra taken during the titration of a
=
4.71 ± 0.01
−3
2+
1
0
M solution of [2] with [Bu N]CH COO. The band
−
4 3
H PO4
a
a
3.92 ± 0.01
4.27 ± 0.01
<2
2
centered at 501 nm undergoes a shift to higher wavelength (λ =
3−
HP O7
5.36 ± 0.02 7.43 ± 0.06
−1
−1
2
5
1
44 nm) and an intensity increase (up to 102 M cm ), until
equiv of acetate is added. This behavior suggests that the
NO3−
<2
<2
<2
<2
HSO4−
<2
interaction between the anion and metal center also takes place,
inducing a change in the coordination geometry and, as a
consequence, in the d−d absorption band. In particular, a
change from the square-planar geometry to five-coordination,
probably with a square-pyramidal geometry, is caused by the
first 1 equiv of added acetate. In fact, the approach of a fifth
donor atom along the z axis is expected to raise the energy of
the d orbitals with a z component, with a consequent decrease
in the energy of the d−d transition envelope, and to cause a red
shift of the corresponding band. Moreover, the interaction of an
a
Precipitation occurred during the titration experiment.
At first, a common Y-shaped oxo anion such as acetate was
considered because of its well-known tendency to interact with
urea derivatives. Spectra taken during the titration of a solution
of [2](ClO ) with a standard solution of tetrabutylammonium
4
2
acetate are reported in Figure 2.
−
Upon the addition of CH COO , the band at 335 nm
3
progressively shifts to higher wavelengths up to 353 nm with an
Figure 2. (a) Family of spectra taken in the course of the titration of a 5.4 × 10−5 M solution in [2](ClO ) with a 2.2 × 10 M solution of
−3
4
2
[
Bu N]CH COO at 25 °C (solid bold line, 0 equiv; dashed bold line, 4 equiv of added acetate). (b) Titration profile at 370 nm indicating the
4 3
formation of a 1:1 adduct.
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Figure 3. (a) Spectra taken over the course of the titration of a 1.01 × 10 M solution of the copper(II) azacyclam complex [2]2+ in MeCN with a
−3
−2
2
.02 × 10 M solution of [Bu N]CH COO in MeCN (solid bold line, 0 equiv; dashed bold line, 1 equiv; short-dashed bold line, 3.5 equiv of added
4 3
acetate). (b) Titration profile at 570 nm.
“axial” ligand induces the loss of the center of symmetry in the
Spectral data suggest that the carboxylate group interacts at
azacyclam complex, a circumstance that makes d−d transitions
the same time with both the metal ion and urea pendant arm
4
4,64
43a,65,66
“
less” forbidden and increases the absorbance.
and a scorpionate-like
structure could be assumed for
+
The titration profile clearly shows that the band intensity
the [2(CH COO)] complex, as sketched in Scheme 2.
3
increases until 1 equiv of acetate has been added and then
moderately decreases upon the addition of a second 1 equiv (82
Scheme 2. Sketch of a Hypothesized Structure of the
−
1
−1
+
M
cm at 544 nm). This result could be explained by the
[
2(CH COO)] Complex
3
interaction of a further anion with the metal center, causing a
coordination change from a square-pyramidal to an elongated
octahedral geometry. The partial symmetry recovery results in a
moderate intensity decrease of the d−d absorption. It should be
noted that the second 1 equiv of anion interacts only with the
2
+
metallomacrocyclic subunit of receptor [2] because no
detectable variations of the band corresponding to the urea
subunit were observed after the addition of the first 1 equiv of
[
Bu N]CH COO.
4 3
Therefore, UV−vis titration experiments performed at
different concentrations indicate that acetate interacts at the
same time with both the urea and the metallomacrocyclic
2
+
subunits of the receptor [2] . The presence of a metal ion
close to the urea groups may favor the binding process not only
for electrostatic reasons but also because of the active part
played in the recognition process, by establishing coordinative
interactions with the anionic substrate.
An analogous investigation was performed on the urea model
compound 6 in order to assess the anion binding properties of
the (4-nitrophenyl)urea subunit in the absence of any possible
cooperative effect due to presence of the metal center. Figure
S4 in the SI reports the spectra taken during the titration of a
solution of 6 with a standard solution of [Bu N]CH COO and
It should be noted that the carboxylate group interacts with
the monourea pendant-arm receptor [2]2+ according to a
“bridged” mode, while it is expected to behave as a “Y-shaped”
anion in the interaction with plain urea. However, the existence
of a dynamic equilibrium between different species containing
acetate interacting via bridged and Y-fashioned modes cannot
be ruled out in solution.
4
3
the corresponding titration profiles.
The reference system also interacts with acetate, as shown by
the red shift of the CT band (from 336 to 364 nm) observed
during the addition of the anion solution. The isosbestic point
observed along the titration experiment at 344 nm suggests that
Analogous investigations were carried out with a variety oxo
−
−
−
−
3−
anions (C H COO , NO , HSO , H PO , and HP O ).
6
5
3
4
2
4
2
7
The addition of tetrabutylammonium benzoate to a MeCN
solution of [2]2+ induced spectral changes very similar to those
observed during the titration experiment with acetate. In
particular, the maximum of the band at 335 nm underwent a
red shift of 15 nm, and an isosbestic point at 336 nm was
observed (Figure S5 in the SI). The log K value (5.14 ± 0.01)
determined for the equilibrium
only two species are present at the equilibrium (6 and [6·
−
CH COO] ). The best fit of the spectrophotometric titration
3
48
data with a nonlinear least-squares procedure over the interval
90−390 nm was obtained by assuming a 1:1 equilibrium with
a log K = 4.48 ± 0.03. A comparison of the stability constant
2
−
+
values determined for [6·CH COO] and [2(CH COO)]
2+
−
+
3
3
[
2] + C H COO ⇄ [2(C H COO)]
(3)
2+
6
5
6
5
adducts clearly indicates that the proximity of the Cu /
macrocycle subunit connected to the urea group distinctly
increases its binding properties toward acetate.
by nonlinear least-squares treatment of the spectral data is
slightly lower than that obtained from titration with acetate (see
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Table 2). It can be ascribed to the lower basicity of benzoate
and its consequent lower tendency to accept hydrogen bonds if
intermolecular fashion with two binding sites (namely, urea
and metal center) located on different receptor molecules at the
same time. Therefore, the formation of receptor/anion adducts
with different stoichiometric ratios (e.g., 2:1 or 3:2) could be
expected at low concentrations of anion (receptor/anion molar
ratio >1), as also suggested by the titration profile displaying an
inflection point at an anion/receptor molar ratio of close to 0.5.
The best fit of the titration data was obtained by assuming the
1
5,29
compared with acetate.
The adduct formed by benzoate
2
+
with receptor [2] is considerably more stable than the adduct
formed with model compound 6 (log K = 4.10 ± 0.01), as
already observed in the case of acetate. This confirms that the
proximity of a metal center to the urea subunit increases the
stability of the receptor/anion adduct.
Less basic (and, therefore, poorly coordinating) anions such
as nitrate and hydrogen sulfate affected to a lower extent the
formation of 1:1 species, and the log K value for the
−
[
2(HP O )] adduct was 5.36 ± 0.02.
2
7
2
+
spectral properties of receptor [2] than carboxylate anions. As
an example, Figure S6 in the SI displays the spectra obtained
upon titration with hydrogen sulfate. The band at 335 nm
underwent a very small red shift along the titrations with
An analogous titration experiment performed on the urea
model compound 6 showed that pyrophosphate interacts with
(
nitrophenyl)urea according to 1:1 stoichiometry (see Figure
3
−
S9 in the SI) with log K = 4.27 ± 0.01. The [6·HP O ]
2
7
[
Bu N]NO and [Bu N]HSO (by 4 and 7 nm, respectively)
4 3 4 4
adduct is distinctly less stable than the 1:1 complex formed by
suggesting that only a weak interaction of anionic substrates
with the (4-nitrophenyl)urea subunit takes place (receptor−
anion association constants lower than 2 log units). Moreover,
the d−d band at 501 nm, monitored during analogous titration
experiments on more concentrated solutions of the receptor,
did not show any significant red shift. The small intensity
increase may be ascribed to a precipitation process, as
suggested by the baseline increase (Figure S6b in the SI).
Titration with dihydrogen phosphate induced considerable
changes in the absorption spectrum of the monourea derivative
2+
[
2] with pyrophosphate, confirming again the relevant
contribution of the proximal metal center to the anion
recognition process.
Titration with [Bu N] HP O was performed also on a more
4
3
2
7
2+
concentrated MeCN solution of [2] in order to monitor the
changes experienced by the d−d band and to assess the
interaction of pyrophosphate with the metal center. The band
centered at 501 nm underwent a red shift (Δλ = 35 nm) and an
−1
intensity increase in the course of the titration (ε = 106 M
max
−1
cm at 536 nm after the addition of 1 equiv of anion), as
2+
[
2] . In particular, the band at 335 nm underwent a red shift
shown in Figure S10a in the SI. The titration profile suggests
that an interaction according to an 1:1 stoichiometry takes
place, although a precipitation process, which occurred in the
presence of excess anion, did not allow determination of the
corresponding equilibrium constant (see Figure S10b in the
SI). Variations of the d−d band are consistent with a change of
the copper(II) coordination geometry from square-planar to
square-pyramidal and clearly indicate that the metal ion is
directly involved in the recognition process.
(
to 348 nm) and a progressive intensity decrease, while a
simultaneous increase of the baseline was observed (signaling
incipient precipitation), which prevented completion of the
titration experiment (Figure S7 in the SI). The shift of the CT
band to higher wavelength suggests that an interaction of the
anion with the (nitrophenyl)urea subunit takes place, as already
observed in the case of acetate and benzoate. On the other
hand, an absorption intensity decrease and a spectrum baseline
increase are usually consistent with the formation of a poorly
soluble species during the titration (for instance, the 1:2 neutral
complex). The precipitation lowers the complex concentration
and absorption intensity and generates turbidity of the solution
and a shift of the spectrum baseline. Therefore, the spectral
data obtained by this titration experiment did not allow an
accurate determination of the adduct stoichiometry and of the
corresponding stability constant value. Any attempt to perform
a similar experiment at a lower concentration did not provide
Changes of both the CT and d−d bands observed during the
titration experiments suggest that one anion interacts at the
same time with the (nitrophenyl)urea group and metal-
lomacrocyclic subunit. Therefore, it may be assumed that
pyrophosphate is bound according to a “bridged mode”,
providing a scorpionate-like structure. Unfortunately, although
there have been several attempts to isolate the adduct in
crystalline form, no crystals suitable for X-ray crystallographic
studies were obtained and the hypothesis of the scorpionate-
better results.
2
+
In order to have better insight into the affinity of [2]2+
type adducts formed by the monourea derivative [2] cannot
be supported by structural data directly concerning this
toward anions of the phosphate family, the receptor’s behavior
in solution was also investigated in the presence of
pyrophosphate. It should be noted that di- and polyphosphate
anions, which participate in several bioenergetic and metabolic
complex.
−
Titration experiments with selected oxo anions (CH COO ,
3
−
−
3−
HSO , H PO , and HP O ) were also performed on
4
2
4
2
7
2+
60
5
,67
reference macrocyclic complex [7] , in order to estimate the
processes,
represent relevant targets in anion recognition
contribution of the coordinative interaction involving the
and sensing. Spectra collected during the titration experiment
2
+
metalloazacyclam subunit on the stability of the receptor [2] /
anion adduct. It should be noted that these experiments were
carried out on solutions of ca. 10− M of the complex to allow
monitoring of the low-intensity d−d band. Unfortunately, the
addition of any of the considered anions induced precipitation,
preventing determination of the constant values associated with
the formation of adducts. As an example, spectra collected
with [Bu N] HP O and the corresponding titration profile are
4
3
2
7
reported in Figure S8 in the SI. In this case also, the band at
3
335 nm was shifted to higher wavelengths (λmax = 360 nm) and
its intensity increased (εmax = 19700), suggesting a strong
interaction of the oxo anion with the urea moiety. The molar
absorbance versus equivalent ratio plot clearly indicates that the
2
+
monourea pendant-arm complex [2] interacts with pyro-
phosphate according to a 1:1 stoichiometry even if no isosbestic
point is observed in the absorption spectra during the titration
experiment. The presence of more than one complex species in
solution can be explained with the structural features of the
dimeric phosphate anion, which could interact in an
2
+
during titration of the copper(II) azacyclam complex [7] with
3
−
HP O7 are reported in Figure S11 in the SI.
2
2
+
Interaction of [3] with Oxo Anions. Titration experi-
ments in MeCN analogous to those described in the previous
section were also performed to investigate the solution behavior
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−
5
Figure 4. (a) Family of spectra taken in the course of the titration of a 2.5 × 10 M solution of [3](ClO ) in MeCN with a standard solution (2.1
×
4
2
−3
10 M) of [Bu N] HP O at 25 °C (solid bold line, 0 equiv; dashed bold line, 5.2 equiv of added anion). (b) Titration profile at 370 nm.
4 3 2 7
of bisurea pendant-arm complex [3]2+ in the presence of
different oxo anions.
scorpionate-like structures analogous to the one proposed for
+
[2(CH COO)] and sketched in Scheme 2. In particular,
3
The addition of acetate considerably affected the absorption
spectrum of the complex: the strong absorption band at 336
nm undergoes a red shift (Δλ= 25 nm) and an intensity
mono- or discorpionate structures could be expected for 1:1
and 1:2 adducts, respectively.
−
The poorly coordinating and nonbasic anions NO and
3
increase (ε from 32800 to 38900 M− cm ) in the course of
the titration (see Figure S12a in the SI). The spectral changes
are observed until 2 equiv of anion was added to the complex
solution. The 1:2 (receptor/anion) stoichiometry of the final
adduct was confirmed by the titration profiles reported in the
SI. Moreover, the best fit of the titration data was obtained by
assuming the formation of 1:1 and 1:2 complex species
according to two stepwise processes:
1
−1
HSO4− did not significantly affect the CT and d−d absorption
2+
bands of receptor [3] , confirming their low tendency to
interact with both the (nitrophenyl)urea moiety and copper(II)
macrocyclic complex subunit, as already observed for the
2+
mono-pendant-arm analogue [2] .
The addition of dihydrogen phosphate induced a progressive
precipitation with a consequent absorbance decrease and
baseline increase; therefore, the titration experiment could
not be completed (see Figure S14 in the SI). However, a
change of the CT band could be observed before complete
precipitation, indicating that hydrogen-bond interaction with
urea subunits takes place. The poor quality of the spectral data
did not allow assessment of the stoichiometry of the receptor/
anion adduct or the accurate determination of the correspond-
ing association constant(s).
3]2+ + CH COO ⇄ [3(CH COO)]
−
+
[
[
(4)
(5)
3
3
+
−
3(CH COO)] + CH COO ⇄ [3(CH COO) ]
3
3
3
2
The log K values determined for these equilibria are 6.47 ±
0
.09 and 5.24 ± 0.09, respectively.
The interaction with acetate also modifies the d−d
−
Different from H PO , pyrophosphate did not induce
2
4
absorption band, as shown by the titration experiment
performed on a more concentrated (10 M) solution of
−4
precipitation at low concentration levels of the complex (<10
−3
2+
M) and titration experiments were successfully performed in
order to monitor the CT band. The distinct red shift of the
band at 336 nm (Δλ = 27 nm) again supports hydrogen-bond
interactions involving the (nitrophenyl)urea pendant arms
[3] . The band at 498 nm undergoes a red shift and an
intensity increase, as already observed for the monourea-
functionalized complex (see Figure S13 in the SI). Unfortu-
nately, precipitation occurs even during the addition of the first
(
Figure 4). The changes of the d−d band at 498 nm could not
1
equiv of anion, preventing completion of the titration
be completely monitored because of a progressive precipitation
during the titration experiment performed on a more
experiment.
2
+
The behavior of [3] in the presence of benzoate is quite
similar to that found for acetate. Spectral changes observed
during the titration experiments indicate that the overall
receptor−anion interaction can be described also in this case by
a stepwise process analogous to that described by eqs 4 and 5,
but the corresponding association constants are smaller (log K1
2+
concentrated solution of [3] . Nevertheless, the variations
observed after the first additions of anion (intensity increase
and shift to higher wavelength) indicate that the coordinative
environment of copper(II) undergoes a drastic change due to
pyrophosphate coordination.
Quite surprisingly, the titration profiles determined at
different wavelength values seem to indicate that a 1:1 adduct
is formed in solution. In addition, the best fit of the spectral
data was obtained when the formation of only the 1:1 adduct
was assumed. The corresponding log K value was 7.43 ± 0.06.
This behavior can be explained by assuming that the formation
of the 1:2 species might be prevented by an electrostatic
=
5.25 ± 0.04; log K = 4.71 ± 0.01) if compared to those
2
obtained for acetate, as expected on the basis of the lower
intrinsic basicity of benzoate.
15,29
The spectrophotometric titration experiments performed
with acetate and benzoate indicate that the bisurea pendant-arm
complex [3]2 interacts in solution with 1 or 2 equiv of
carboxylate, depending on its concentration. Each carboxylate
group establishes both hydrogen bonds with urea groups on the
pendant arms and coordinative interactions at the apical
positions of the metal center. These simultaneous interactions
should induce the folding of the pendant arm(s), generating
+
−
repulsion due to the high negative charge (3 ) on each
pyrophosphate anion.
The copper(II) diazacyclam complex [8] , which can be
2+
61
considered as a reference compound for the study of receptor
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Figure 5. (a) Spectra taken over the course of the titration of a MeCN solution of [3](ClO ) (2.5 × 10 M) with a standard solution (2.0 × 10−3
−
5
4
2
M) of [Bu N] (succinate) at 25 °C (solid bold line, 0 equiv; dashed bold line, 4.4 equiv of added anion). (b) Titration profile at 350 nm.
4
2
2
+
[
3] −anion interaction, was also investigated by titration
different interaction modes involving at the same time two
carboxylate groups (from the anion side) and three binding
sites (metal center and two urea subunits from the receptor
side) can be hypothesized. Titration experiments performed at
higher concentration (1.0 × 10− M) in order to monitor the
d−d band variations induced by dicarboxylate addition could
not be completed because precipitation occurred in all cases
before the first 1 equiv of anion was added. However, the red
shift and intensity increase of the band centered at 498 nm,
which were observed before precipitation occurred, suggest
again that the metal center directly contributes to the
interaction with the anionic species by establishing axial
coordinative bond(s) (see Figure S18 in the SI).
experiments with oxo anions. As already observed for the
2
+
azacyclam analogue [7] (see above), insoluble material
formed upon the addition of anions and spectral data could
not be used to calculate the stability constants of the adducts.
As an example, spectra registered during the titration of [8]2+
with acetate are reported in Figure S15 in the SI.
3
Interaction of [3]2+ with Dicarboxylates. Spectrophoto-
2
+
2+
metric titration experiments performed on [2] and [3]
clearly indicate that carboxylate and phosphate anions interact
with both complexes by establishing at the same time a
coordinative bond with the metal ion and hydrogen bonds with
the urea subunit(s) on the pendant arm(s). On the basis of the
spectrophotometric studies in solution, we can assume that
carboxylate as well as phosphate groups behave as ditopic
species and interact according to a bridging mode, as sketched
in Scheme 2. However, other interaction modes cannot be
ruled out, particularly in the case of the bisurea derivative [3] ,
and, in principle, several equilibria involving different species
could take place in solution.
In order to investigate the behavior of [3] in the presence
of genuine ditopic anionic species, titration experiments with
selected dicarboxylate compounds were performed. In partic-
ular, spectrophotometric titrations were carried out with
2
+
More information about the structure of dicarboxylate/[3]
adducts was provided by crystallographic studies. In fact, upon
slow diffusion of diethyl ether into a DMF/MeCN solution
(1:1 by volume) containing equimolar amounts of
[Bu N] (succinate) and [3](ClO ) , violet crystals of the
2+
4
2
4 2
resulting 1:1 adduct, suitable for X-ray diffraction studies, were
obtained.
2
+
The crystal contains two symmetrically independent
II
2+
[Cu (3)] molecular cations, each interacting with two
succinate anions and four additional DMF solvent molecules:
2([3]C H O )·4DMF (C H O = succinate). One of the two
4
4
4
4
4
4
−
−
II
2+
malonate, CH (COO ) , succinate, (CH ) (COO ) , and
glutarate, (CH ) (COO ) . The addition of dicarboxylate to
very similar but nonsymmetrically equivalent [Cu (3)]
2
2
2 2
2
−
molecular cations and the interacting oxo anions are shown
in Figure 6 (see Figure S19 in the SI for the ORTEP view of the
other molecular cation). Both nonsymmetrically equivalent
2
3
2
the solution of the bisurea pendant-arm macrocyclic complex
induces a progressive shift of the band at 335 nm to higher
wavelengths, as already observed in the titration experiment
with acetate. Similar families of spectra were obtained for the
three anionic species, with large Δλ values (24, 25, and 24 nm
for malonate, succinate, and glutarate, respectively) and
moderate intensity increase. Figure 5a reports the spectra
taken during the titration with succinate, while Figures S16 and
S17 in the SI display the families of spectra obtained upon
titration with malonate and glutarate, respectively.
copper(II) diazacyclam subunits exhibit C symmetry: the two
i
II
Cu metal centers are placed on two crystallographic centers of
inversion, and both copper ions lie in the plane of the four-
amine donor set. Both diazacyclam ligands adopt the trans-
III(R,R,S,S) configuration. The mean values for the Cu−N
bonds are slightly longer than those observed in the structure of
the bisurea diazacyclam complex [3]2+ (without oxoanions):
the mean value of 2.02(1) Å for both the Cu(1) and Cu(2)
centers in the succinate adduct should be compared to the
mean value of 2.00(1) Å observed in the structure shown in
Figure 1.
Plots of molar absorbance versus equivalent ratio at
significant wavelengths indicate that the interaction of [3]2+
with all of the considered dicarboxylates occurs according to a
1
:1 stoichiometry. In Figure 5b, the plot obtained for succinate
Very interestingly, each carboxylate group of the succinate
2+
at 350 nm is reported as an example.
ions interacts with receptor [3] according to a “bridged”
−
Although the titration experiments suggest that 1:1 adducts
are formed in solution, nonlinear least-squares treatment of the
spectral data do not allow calculation of accurate values of
association constants. This should be due to the rather large
number of equilibria that can take place in solution because
mode: one O atom of −COO is located on an axial position of
II
the elongated octahedral coordination exhibited by the Cu
center, while the other O atom acts as a H-atom acceptor of
three hydrogen bonds, two of which are provided by the two
NH groups of urea.
I
dx.doi.org/10.1021/ic501527k | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 3. Features of Hydrogen Bonds in the
II
2
([Cu (3)](C H O ))·4DMF Crystal
4
4
4
D···A
(Å)
H···A
(Å)
D−H···A
acceptor
atom A
donor group D
(deg)
N(1)−H(1N)
N(4)−H(4N)
N(5)−H(5N)
N(3)−H(3N)
N(9)−H(9N)
N(10)−H(10N)
N(11)−H(11N)
N(7)−H(7N)
3.01(1)
3.02(1)
2.83(1)
3.17(1)
2.98(1)
3.13(1)
2.77(1)
2.92(1)
2.13(1)
2.16(2)
1.90(2)
2.45(2)
2.09(1)
2.29(2)
1.83(2)
2.13(2)
156(1)
148(2)
166(2)
133(1)
159(1)
147(2)
173(2)
141(1)
O(8)
O(8)
O(8)
O(11)
O(10)
O(10)
O(10)
O(12)
Figure 6. ORTEP view (30% probability level) of one [3]2+ molecular
cation interacting with succinate oxo anions. Selected bond lengths
(
Å) and angles (deg): Cu(1)−N(1) 2.008(2), Cu(1)−N(3) 2.032(2),
Cu(1)−O(7) 2.425(2); N(1)−Cu(1)−N(3) 93.7(1), N(1)−Cu(1)−
N(3)′ 86.3(1), N(1)−Cu(1)−O(7) 91.2(1), N(3)−Cu(1)−O(7)
8
9.8(1). Symmetry code ′: 1 − x, 1 − y, 1 − z. Dashed lines indicate
hydrogen bonds.
The direct interaction of carboxylates with the metal center is
very similar for the two nonequivalent CuII centers: the
observed distances are Cu(1)−O(7) 2.43(1) Å and Cu(2)−
O(9) 2.44(1) Å.
The “bridged” interaction mode of each carboxylic group is
determined by two asymmetric N−H···O bonds provided by
the urea group on a pendant arm and by a single N−H···O
bond provided by a secondary amine of the macrocycle. The
two asymmetric H···O distances for the hydrogen bonds
involving urea NH groups are 1.90(2) and 2.16(2) Å and
Figure 7. Simplified sketch of the hydrogen bonds in 2([3]C H O )·
4
4
4
4
DMF crystal. Atom names identify the independent hydrogen-bond
1.83(2) and 2.29(2) Å for the two independent molecular
interactions.
complexes, respectively, with the shortest H···O distance being
provided by the urea NH group close to the terminal
nitrophenyl group. The H···O distances of the single N−H···
O bond provided by the secondary amine of the macrocycle are
−
the bifurcated hydrogen bonds with the −COO moiety, as
shown by the scorpionate-like structure hypothesized in
Scheme 2. This conformation can be adopted by complex
2
.13(1) and 2.09(1) Å for the two independent molecular
complexes, respectively. These values are intermediate between
those observed for the urea NH groups.
2+
2+
[3] , and similarly by [2] , even in the recognition in solution
of simpler anionic species such as acetate and related
monocarboxylates.
Because each succinate ion interacts at the same time with
the metal centers of two nonsymmetically equivalent adjacent
molecules, molecular chains extending along the c-axis direction
originate in the solid state. A portion of a molecular chain is
shown in Figure 6, and the geometrical features of the
hydrogen-bond interactions are reported in Table 3.
CONCLUSIONS
■
This paper has confirmed the role of metal complexes, in
particular those with functionalized macrocyclic ligands, as
useful platforms in the design of synthetic receptors for anions.
Among macrocyclic complexes, aza- and diazacyclam systems
can be easily prepared by template syntheses and functionalized
with different groups. In particular, the introduction of one or
two urea subunits on the macrocyclic framework of the ligands
described in this paper provides multicomponent systems that
can interact with anionic substrates by both coordinative
interactions (metal center) and hydrogen bonds (urea
subunits).
2
+
Crystallographic studies performed on the [3] /succinate
adduct show that ditopic anions such as aliphatic dicarboxylates
preferentially establish “intermolecular” interactions with the
investigated macrocyclic systems. However, the structure
represented in Figure 7 describes the most stable (or one of
the most stable) molecular arrangement in the solid state, and
other different structures could be hypothesized in solution.
On the other hand, it is important to point out that the
II
simultaneous interaction of carboxylate species with the Cu
ion and the urea group belonging to the same macrocyclic
complex is structurally allowed. In particular, the urea-
containing pendant arms can be folded in order to establish
The investigated copper(II) complexes [2]2+ and [3]2+,
carrying one and two urea pendant arms, respectively,
recognize anionic species displaying a coordinating tendency
J
dx.doi.org/10.1021/ic501527k | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
toward copper(II) and a good affinity toward urea subunits at
ACKNOWLEDGMENTS
■
the same time, namely, carboxylates and phosphates.
Financial support of the Italian Ministry for Education,
Universities and Research (MIUR), is gratefully acknowledged
Project PRIN2010 010CX2TLM_009). Thanks are due to Dr.
2
+
In particular, [2] forms 1:1 adducts with acetate, benzoate,
hydrogendiphosphate, and dihydrogen phosphate, although in
the latter case, an insoluble product is obtained. Stability
constants of the adducts are considerably higher than the ones
determined for the interaction of the same anions with plain
urea reference compound 6, confirming the synergistic action
of metallomacrocyclic and urea subunits. Complex [3]2+
interacts with the same anions according to 1:1 and 1:2
stoichiometries, with the exception of hydrogendiphosphate,
which forms just the 1:1 adduct with a distinctly high
association constant (log K > 7).
(
Luca Pasotti for assistance with the NMR measurements.
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AUTHOR INFORMATION
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■
*
Corresponding Author
E-mail: maurizio.licchelli@unipv.it.
Notes
(
The authors declare no competing financial interest.
K
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