Copper(II) Complexes of Cyclams Containing Nitrophenyl Substituents: Push-Pull Behavior and Scorpionate Coordination of the Nitro Group

Article abstract of DOI:10.1021/acs.inorgchem.5b01273

The three nitrophenyl-cyclam derivatives (nitrocyclams): 1-(4-nitrophenyl)-1,4,8,11-tetraazacyclotetradecane (2), 1-(2-nitrophenyl)-1,4,8,11-tetraazacyclotetradecane (3), and 1-(2,4-dinitrophenyl)-1,4,8,11-tetraazacyclotetradecane (4), in an MeCN solution

Full text of DOI:10.1021/acs.inorgchem.5b01273

Article
pubs.acs.org/IC
Copper(II) Complexes of Cyclams Containing Nitrophenyl
Substituents: Push−Pull Behavior and Scorpionate Coordination of
the Nitro Group
Massimo Boiocchi,† Carlo Ciarrocchi,‡ Luigi Fabbrizzi,*,‡ Maurizio Licchelli,‡ Carlo Mangano,‡
Antonio Poggi,‡ and Miguel Vazquez Lopez§
́
́
‡
†Centro Grandi Strumenti and Dipartimento di Chimica, Universita
§Centro Singular de Investigacion
en Química Bioloxica e Materiales Moleculares (CIQUS) and Departamento de Química
Inorganica, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain
̀
di Pavia, 27100 Pavia, Italy
́
́
́
S
* Supporting Information
ABSTRACT: The three nitrophenyl−cyclam derivatives
(nitrocyclams): 1-(4-nitrophenyl)-1,4,8,11-tetraazacyclotetra-
decane (2), 1-(2-nitrophenyl)-1,4,8,11-tetraazacyclotetrade-
cane (3), and 1-(2,4-dinitrophenyl)-1,4,8,11-tetraazacyclote-
tradecane (4), in an MeCN solution, specifically incorporate
the CuII ion according to an irreversible process signaled by
disappearance of the yellow color for a concentration c < 1 ×
10−4 M and by a yellow-to-red color change for c ≥ 1 × 10−3,
and must be considered efficient and specific dosimeters of
copper(II) salts. When present in the ortho position of the
nitrophenyl substituent, the −NO2 group coordinates the CuII according to a scorpionate mode, while the metallocyclam system
exhibits a trans-I configuration. In an MeCN solution the red trans-I-[CuII(3)]2+ and trans-I-[CuII(4)]2+ scorpionate complexes
slowly convert into the violet trans-III scorpionate complexes. Kinetic aspects of the trans-I-to-trans-III configurational
rearrangement were investigated in detail for the [CuII(4)]2+ system. In particular, the conversion is spectacularly accelerated by
catalytic amounts of Cl−, NCO−, and F−. While for Cl− and NCO− the effect can be associated with the capability of the anion to
stabilize through coordination a possible dissociative intermediate, the amazingly powerful effect of F− must be related to the
preliminary deprotonation of one N−H fragment of the macrocycle, driven by the formation of the HF2− ion. Most of the metal
complex species studied in solution were isolated in a crystalline form, and their molecular structures were elucidated through X-
ray diffraction studies. This study documents the first examples of effective metal coordination by the nitro group.
The optical transition is therefore defined charge transfer
(CT) transition, characterized by an intense absorption band in
the visible region (λmax = 390 nm, ε = 20 600 M−1 cm−1, EPA,
77 K). The highly polar nature of the excited state accounts for
the distinctive solvatochromic behavior. The more polar the
solvent, the more pronounced the red shift of the absorption
band.
■. INTRODUCTION
N,N-Dimethyl-(4-nitro)-aniline (1a, DMNA) is a classical
push−pull system D−Ph−A, in which the dimethylamino
donor group D and the nitro acceptor group A are connected via
the π-conjugated system of a phenyl ring (Ph). The molecule
exhibits a pronounced polar character, with μ0 = 5.92 D. On
light absorption, the system is excited to the state 1b, which
involves a substantial transfer of electron density from D to A,
as depicted in Scheme 1.
Scheme 1. CT Optical Transition in DMNA
We had previously appended the DMNA subunit to the
framework of cyclam and observed that the resulting macro-
cycle 2 was able to incorporate irreversibly and selectively the
copper(II) ion.1 Coordination of Cu2+ to the aniline nitrogen
Received: June 5, 2015
© XXXX American Chemical Society
A
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atom of 2 makes the intensity of the dipole decrease. As a
consequence, the CT band is remarkably blue-shifted (from
386 to 266 nm) and, on CuII addition, under diluted conditions
(<1 × 10−4 M), the yellow-orange solution of 2 turns colorless.
Macrocyle 2 can be therefore considered an exclusive optical
dosimeter of CuII.2 Moreover, structural studies on the
[CuII(2)](ClO4)2 complex showed that the Cu2+ ion is fully
encircled by the macrocycle according to a square planar
geometry.1
Quite interestingly, the macrocycle adopts the R,S,R,S
configuration (diastereoisomer trans-I, see Scheme 2), rather
Scheme 2. Trans-I and trans-III Configurational Isomers of
Metal Cyclam Complexes
Figure 1. Absorption spectra of macrocycles 2 (4-nitro in the legend),
3 (2-nitro), and 4 (2,4-dinitro) taken in MeCN solution, at 25 °C.
Macrocycles 2 and 4, in MeCN solution, uptake one CuII ion
to give a substitutionally inert complex, from which the metal
ion cannot be extruded even under drastic conditions (addition
of excess of competing ligands, of strong acid). Figure 2a shows
the spectra taken over the course of the titration of an MeCN
solution of 4 with CuII. Complexation induces a substantial
blue-shift of the absorption band assigned to the CT transition
from the anilino group to the 4-nitro substituent (from 385 to
280 nm), a behavior previously observed also in the case of
derivative 2 (blue-shift from 365 to 260 nm, in MeCN).1
The nature of the blue shift is illustrated in Figure 3, where
the 4-nitrophenyl derivative is specifically considered. It is
suggested that the excited state of the CuII complex is highly
destabilized due to the formal electrostatic repulsion between
the metal ion and the anilino nitrogen atom, made positive by
the CT (see Figure 3b). This makes hνCT distinctly larger than
observed for the uncomplexed macrocycle (Figure 3a).
Titration profiles at selected wavelengths, shown in Figure
2b, indicate the 1:1 stoichiometry of the complex formed in
MeCN. Addition of any other divalent 3d metal to an MeCN
solution of 4 did not cause any spectral modification. Thus,
receptor 4 appears as a specific dosimeter for CuII, in the same
way as the mononitro derivative 2, but displaying a higher
sensitivity due to the higher intensity of the absorption band.
The substantial blue-shift makes the yellow diluted solution of
the macrocycle (<1 × 10−4 M) turn colorless. A yellow-to-red
color change is observed at a higher concentration (≥1 × 10−3
M), due to the significant appearance of d−d bands (vide
infra).
unusual for metal complexes of monosubstituted cyclam
derivatives and typically observed with metal complexes of
N,N′,N″,N‴-tetrasubstituted cyclams.3 However, unsubstituted
or mono N-substituted cyclam complexes usually adopt an
R,R,S,S configuration (trans-III; see Scheme 2).4
We wish now to extend the study to the copper(II)
complexes of cyclam derivatives with a 2-nitrophenyl (3) and
with a 2,4-dinitrophenyl substituent (4). This could allow us to
evaluate (i) how CuII coordination modifies the properties of
the push−pull chromophore and (ii) if the 2-nitro group can
bind the metal center from the top, according to a scorpionate
mode. Factors affecting the coordination of the nitro group
were elucidated thanks to the determination of the crystal and
molecular structures of seven complex salts of macrocycles 2, 3,
and 4 (nitrocyclams). They are the only structures available for
metal complexes of cyclam derivatives with a nitrophenyl
substituent directly linked to a nitrogen atom, along with the
precursor complex described in ref 1. Noticeably, the −NO2
group was observed to coordinate the CuII ion only in the
nitronate form, in a macrocycle in which the 2-nitrophenyl
fragment was appended through an ethylenic chain to a
nitrogen atom of cyclam.5
This study has demonstrated that nitrocyclams are effective
and specific dosimeters for copper(II) and that, in the
corresponding CuII complexes, (i) the nitrobenzene pendant
group exhibits a scorpionate effect depending on placement of
the nitro groups and (ii) a trans-I to trans-III configurational
conversion takes place, catalyzed by coordinating anions and
influenced by base and fluoride.
Complexation of CuII by the 2-nitro derivative 3 in MeCN
causes a distinctly different behavior. Figure 4a shows the
spectra taken over the course of the titration of a MeCN
solution of 3 with a standard MeCN solution of
CuII(CF3SO3)2.
It is observed that metal complexation provokes a moderate,
still distinct red shift of the band at 250 nm (shifted to 275 nm
on addition of excess CuII). The nature of the red shift is
tentatively illustrated in Figure 5.
In particular, the excited state of the metal complex (Figure
5b) may be stabilized by the attractive electrostatic interaction
between one negatively charged oxygen atom of the nitronate
group on one side and (i) the close iminium group and (ii) the
Cu2+ ion on the other. These interactions cannot be established
when the nitro substituent is in para position.
■. RESULTS AND DISCUSSION
Macrocycles 2, 3, and 4 as Specific Dosimeters of
Copper(II). Figure 1 shows the absorption spectra in MeCN of
the three macrocycles 2, 3, and 4.
Macrocycles 2 (nitrophenyl substituent in para-, yellow-
orange color) and 4 (nitrophenyl substituents in ortho- and
para-, orange) show an intense absorption band at 365 nm (ε =
7950 M−1 cm−1) and at 382 nm (ε = 15 900 M−1 cm−1),
respectively, which can be ascribed to the CT transition
illustrated in Scheme 1. Such a band is absent for the
macrocycle 3, which does not bear a nitro substituent in the 4-
position of the phenyl ring and shows a pale yellow color.
The titration profile shown in Figure 4b clearly indicates the
incorporation of copper(II) ion by the macrocycle according to
a 1:1 stoichiometry. Also in the present case, metal uptake is
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Figure 2. (a) Spectra taken over the course of the titration of an MeCN solution of 4 with a standard MeCN solution of CuII(CF3SO3)2; (b)
titration profiles (ε vs equiv of CuII) at 390 and 280 nm), indicating the formation of a complex of 1:1 stoichiometry.
Figure 5. Energy diagrams illustrating the CT transitions in (a) the
uncomplexed macrocycle 3, and (b) macrocycle 3 incorporating CuII.
added metal, which indicate no metal−ligand interaction. Thus,
nitrocyclams can be considered as specific dosimeters of CuII.
Conformational Aspects: the Existence of a Scorpio-
nate Effect. On preparation of crystalline metal complexes
salts of macrocycles 3 and 4 products of different color were
obtained depending upon the chosen experimental conditions.
The structural features of the different complex salts were
clearly defined through single-crystal X-ray diffraction studies.
Complexes of 4. On addition of an MeOH solution of
copper(II) perchlorate (pale blue) to an MeOH solution of 4
(yellow) a red precipitate formed, while the supernatant
solution took a violet color. Red crystals suitable for X-ray
diffraction studies were obtained on diffusion of diethyl ether
vapor on an MeCN solution of the red solid. The crystal and
molecular structure of the [CuII(4)](ClO4)2 complex salt is
shown in Figure 6.
Figure 3. Energy diagrams illustrating the CT transitions in (a) the
uncomplexed macrocycle 2, and (b) macrocycle 2 incorporating CuII.
irreversible, thus defining 3 as an optical dosimeter of CuII.
Note that in the titration profile at 300 nm in Figure 4b a
discontinuity exists at the addition of 0.5 equiv of CuII. Such a
feature could be tentatively ascribed to the formation of an
intermediate species of 1:2 metal/ligand ratio, in which each
macrocycle molecule contributes to the coordination with 2 (or
3) secondary nitrogen atoms.
MeCN solutions of the three investigated nitrocyclams (2, 3,
4) were titrated with solutions of other divalent 3d metal ions
(MnII, FeII, CoII, NiII, ZnII). No spectral modifications were
observed even in the presence of a large excess (10 equiv) of
The complex is five-coordinate, according to a square
pyramidal geometry: the four amine nitrogen atoms of the
Figure 4. (a) Spectra taken over the course of the titration of an MeCN solution of 3 with a standard MeCN solution of CuII(CF3SO3)2; (b)
titration profiles (ε vs equiv of CuII) at 300 nm), which monitors the formation of the 1:1 metal complex.
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Figure 6. Crystal and molecular structure of the complex salt trans-I-
[CuII(4)](ClO4)2, red color. Hydrogen atoms and counteranions, not
involved in metal coordination, were omitted for clarity.
Figure 8. Crystal and molecular structure of the complex salt trans-III-
[CuII(4)(ClO4)2·MeCN], violet color. Hydrogen atoms and counter-
anions, not involved in metal coordination, were omitted for clarity.
macrocycle occupy the corners of the square, while an oxygen
atom of the nitro group in ortho position of the phenyl
substituent occupies the apical site. The cyclam framework
exhibits a trans-I configuration. The presence of a tertiary
nitrogen atom in the cyclam framework induces some
distortion in the coordination polyhedron: (i) the CuII−Ntert
distance is 214(1) pm versus the 199(1)−200(1) pm CuII−NH
distances (while the CuII−O distance is 233(1) pm) and (ii)
the O−CuII−Ntert angle is 80.7(1)°.
are 241(1) and 240(1) pm for CuII−O and CuII−N,
respectively.
If diethyl ether vapor is allowed to diffuse on an MeCN
solution of the red complex salt [CuII(4)](ClO4)2, in the
presence of [Bu4N]Cl, blue-violet crystals of the salt CuII(4)-
Cl2·H2O·1/2Et2O form, which were investigated through X-ray
diffraction studies. The corresponding crystal and molecular
structure is shown in Figure 9.
On dissolution of the red complex salt trans-I-[CuII(4)]-
(ClO4)2 in MeCN, a red solution is obtained, whose absorption
spectrum is shown in Figure 7 (red). The red solution turns
Figure 9. Crystal and molecular structure of the complex salt trans-III-
[CuII(4)Cl]Cl·H2O·1/2(C2H5)2O. The chloride counterion, not
involved in coordination, as well as solvational molecules were
omitted for clarity.
Figure 7. (red) Spectrum of an MeCN solution of the red form of the
[CuII(4)](ClO4)2 complex salt (trans-I configuration). (violet)
Spectrum of the same solution taken after several weeks.
It is observed that the complex cation [CuII(4)Cl]+ shows a
square pyramidal coordination geometry, with the chloride ion
occupying the apical position. The nitro group in ortho
position is well far away from the metal, while the macrocycle
adopts the trans-III configuration.
violet over a period of weeks. The spectrum of the solution,
taken when no further modifications were observed, is shown in
Figure 7 (violet). On slow evaporation of the violet solution,
the violet complex salt CuII(4)(ClO4)2·MeCN was obtained in
a crystalline form, whose molecular structure was determined
through X-ray diffraction studies. The corresponding structure
is shown in Figure 8.
The complex shows an elongated octahedral geometry, with
a scorpion-type coordination of the nitro group in the phenyl
ortho position and the nitrogen atom of an MeCN molecule
occupying the opposite axial position. Noticeably, the cyclam
framework exhibits a trans-III configuration. Also in the present
case, the N4 square is distorted, as the CuII−Ntert distance
(215(1) pm) is distinctly longer than the CuII−NH distances
(ranging from 201(1) to 203(1) pm). The two axial distances
Complexes of 2. We have now observed that on diffusion of
diethyl ether on an MeCN solution of trans-I-[CuII(2)](ClO4)2
in the presence of an excess of [Bu4N]]X (X = Cl, NCO), the
following complex salts can be obtained in a crystalline form,
suitable for X-ray diffraction studies: 2trans-III-[CuII(2)Cl]Cl)·
MeCN·5/2H2O (Figure 10b) and trans-III-[CuII(2)(NCO)]-
NCO·2H2O (Figure 10c). For comparative purposes, Figure
10a shows the crystal and molecular structures of the complex
salts of the parent macrocycle 2: trans-I-[CuII(2)](ClO4)2.1
Notice that, in the presence of a coordinating anion like Cl−
and NCO−, the complex moves from the square planar
geometry (Figure 10a) to a square pyramidal arrangement
(Figure 10b,c), while cyclam configuration changes from trans-I
to trans-III. In the three complexes, the CuII−Ntert distances
remain a bit longer than the CuII−NH distances. However, the
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Figure 10. Crystal and molecular structure of the complex salts: (a)
trans-I-[CuII(2)](ClO4)2, red color;1 (b) 2(trans-III-[CuII(2)Cl]Cl)·
MeCN·5/2H2O, blue color; (c) trans-III-[CuII(2)(NCO)]NCO·2-
(H2O), blue color. Hydrogen atoms, counterions, and solvational
molecules were omitted for clarity. Crystal of b contains two similar
but not symmetrically equivalent [CuII(2)Cl]+ molecular cations.
Figure 11. Crystal and molecular structure of the complex salts: (a):
trans-I-[CuII(3)](ClO4)2·MeCN, red color; and (b): trans-III-
[CuII(3)](ClO4)2, violet color. Hydrogen atoms, unbound perchlorate
ions and solvational molecules were omitted for clarity.
As noted for 4, the red form corresponds to the trans-I
configuration of the cyclam macrocycle, while the violet
complex shows the trans-III arrangement. In contrast to what
was observed for the analogous complexes of 4, crystals of the
red trans-I-[CuII(3)]2+ complex contain an MeCN molecule,
which is not bonded to the metal, whereas the violet trans-III
[CuII(3)]2+ species does not keep in the solid state any MeCN
molecule. In particular, the apical site opposite to the
coordinated nitro group of both trans-I and trans-III CuII
complexes of 3 is occupied by a perchlorate anion, whose
loosely bound oxygen atom is positioned at 277(1) pm for
trans-I [CuII(3)]2+ species and 272(1) pm for trans-III
[CuII(3)]2+ species.
Other structural features are in full agreement with the
principles outlined in the previous Section. In particular, both
trans-I and trans-III complexes of 3 show CuII−Ntert distances
(210(1) and 209(1) pm) longer than the CuII−NH distances
(in the range of 199(1)−203(1) pm). The axial CuII−O
distances are 236(1) pm and 224(1) pm in trans-I [CuII(3)]2+
and trans-III [CuII(3)]2+ complexes, respectively.
Nitromethane and nitrobenzene are classical noncoordinat-
ing solvents, typically used to handle in solution undissociated
metal complex salts in which anions either coordinate the metal
ion or interact electrostatically with the complex according to
an outer sphere mode. Coordination of the nitro group can be
achieved in copper(II) complexes of nitrocyclams, thanks to the
scorpionate effect.
Mechanism of the trans-I to trans-III Conversion. It has
been mentioned that the trans-I complex of both 3 and 4
macrocycles slowly and spontaneously converts in solution to
the thermodynamically stable trans-III complex, which keeps a
scorpionate arrangement. Figure 7 showed that the two
configurational isomers exhibit clearly distinct spectral features
in the d−d region of the absorption spectrum, which allows
their detection in solution. Thus, the kinetic aspects of the
trans-I to trans-III conversion could be investigated spectro-
photometrically.
distortions for the N4 squares of 2 in the three complexes are
less pronounced than in complex trans-I-[CuII(2)](ClO4)2.
Actually, the CuII−Ntert distances range from 207(1) pm in the
trans-I-[CuII(2)](ClO4)2 to 209(1) pm in the trans-III-
[CuII(2)(NCO)]NCO·2H2O, whereas the CuII−NH distances
are in the range of 200(1)−204(1) pm. The axial distances are
253(1) and 257(1) pm for CuII−Cl in the two independent
molecules of 2trans-III-[CuII(2)Cl]Cl)·MeCN·5/2H2O and
218(1) pm for CuII−NNCO in complex trans-III-[CuII(2)-
(NCO)]NCO·2H2O. Notice that on halide/pseudohalide
coordination the CuII ion, only 3(1) pm over the mean N4
plane in the trans-I-[CuII(2)](ClO4)2, is raised by 18(1)/23(1)
pm from the plane toward the anion.
On the basis of the structural data described above, the
following conclusions can be drawn:
1. The presence of the hindering phenyl substituent favors
the trans-I configuration.
2. Coordination of a further ligand (an anion, an MeCN
molecule, the oxygen atom of the nitro group) favors the
conversion to the trans-III configuration: the higher the
coordinating tendencies of the exotic ligand, the easier
the trans-I to trans-III conversion.
Thus, whatever the position the nitro group is in, whether
ortho- or para-, the kinetically less-complicated pathway leads
to the formation of a complex in a trans-I configuration. In the
case of the complex of macrocycle 4, the five-cordinate
scorpionate complex forms. However, this is not the
thermodynamically stable species, and the trans-I scorpionate
species (red) slowly converts in MeCN to the octahedral
scorpionate trans-III form (violet), with an MeCN molecule
occupying the axial position opposite to the coordinated
oxygen atom of the nitro group. If a coordinating anion is
present (Cl−, NCO−), the trans-III complex prefers to bind the
anion according to a square pyramidal geometry, diverting the
−NO2 oxygen from the axial site and releasing the bound
MeCN molecule to the solution. The metal profits at most
from the anion coordination by raising itself 19(1) pm from the
N4 plane (a behavior observed also with complexes of 2 with
chloride and cyanate).
Figure 12a shows the decay of the concentration of the trans-
I complex (red ●), which was calculated from the absorbance
at 460 nm, corrected for the absorbance of the trans-III
complex, of an MeCN solution 1 × 10−3 M in the trans-I
[CuII(4)](ClO4)2 complex salt (red; values at longer times not
reported in the diagram). In Figure 12b, ln[trans-I] was plotted
versus time (h). The linear behavior indicates the first-order
nature of the process, to which a half-time t1/2 ≈ 12 d
corresponds.
Complexes of 3. Red solvate complex salt [CuII(3)](ClO4)2·
MeCN and violet complex salt [CuII(3)](ClO4)2 were obtained
through reaction of Cu(ClO4)2 with 3 in MeCN in the same
crop. Trans-I and trans-III complex were separated by elution
through a cation exchange column. The crystal and molecular
structures of the two forms are shown in Figure 11.
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Figure 12. Kinetics studies on the trans-I to trans-III conversion of the trans-I [CuII(3)](ClO4)2 complex in an MeCN solution at 25 °C (1 × 10−3
M). The decay of the trans-I complex concentration is monitored through the decrease of the absorbance of the absorption band at 460 nm: (a)
conversion in the absence of anions and in the presence of a catalytic amount of a [Bu4N]X salt (1 × 10−5 M, X− = Cl−, NCO−, F−); (b) ln[trans-I]
vs time plots, disclosing a first-order behavior.
Figure 13. (a) Spectra taken over the course of the titration of an aqueous solution 1 × 10−4 M in trans-I-[CuII(4)](ClO4)2 and 0.1 M in NaClO4;
(b) titration profiles based on the decreasing ε at 275 nm, monitoring the concentration of the [CuII(4)]2+ complex (blue ; right vertical axis) and
▼
●
the increasing ε at 400 nm, due to the demetalated-deprotonated macrocyle 4 (red ; left vertical axis).
The presence of anions in catalytic amounts (i.e., 1 × 10−5
M) makes the trans-I-to-trans-III conversion considerably faster.
In this connection, Figure 12a shows the decay of the
absorbance at 460 nm for an MeCN solution 1 × 10−3 M in
[CuII(4)]2+ and 1 × 10−5 M in [Bu4N]Cl. The linear ln[trans-I]
versus t(h) in Figure 12b discloses a t1/2 = 5.5 h. We can
tentatively suggest that the trans-I to trans-III conversion may
take place through a dissociative mechanism (detachment of an
amine nitrogen atom, as a preliminary step to the configura-
tional reorganization). The chloride ion is assumed to
temporarily occupy the empty coordination site, thus
decreasing the energy of the transition state. Accordingly,
anions of higher coordinating tendencies than chloride should
make the conversion process faster. Indeed, in the presence of
1/100 equiv of cyanate, which follows chloride in the
spectrochemical series, the rate of the configurational rearrange-
ment increases distinctly, and the half-time is substantially
reduced (t1/2 = 0.5 h).
properties rather than to the coordinating properties of F−, as
will be discussed in detail in the next Section.
Base Effect on the trans-I-to-trans-III Conversion. An
aqueous solution of the trans-I-[CuII(4)](ClO4)2 complex salt,
containing excess strong acid and 0.1 M in NaClO4, was titrated
with a standard solution of NaOH, at 25 °C. Figure 13a shows
the spectra taken during the titration over the 7.0−12.5 pH
interval.
It is observed that at pH ≥ 10 the absorbance of the band at
275 nm, pertinent to the [CuII(4)]2+ complex, decreases
according to a sigmoidal profile. At the same pH a new band
develops, at 360 nm, with a well-defined shoulder at 400 nm,
still showing a sigmoidal profile. It is hypothesized that
[CuII(4)]2+ (= [CuII(HL)]2+), behaves as a monoprotic acid,
according to the equilibrium 1, which involves simultaneous
demetalation:
−
[CuII(HL)]2+ + OH− ⇆ L + Cu2+ + H2O
(1)
From the two intersecting curves in Figure 13b, a value of pKa
≈ 11.8 can be roughly estimated. Thus, the band at 360 nm
with a shoulder at 400 nm should pertain to the deprotonated
macrocycle L− (HL = 4). In Figure 13a, the spectrum of the
metal-free macrocycle 4 (HL) in MeCN is also displayed for
comparative purposes (dotted black line), which originates
It is therefore surprising that the highest acceleration is
exerted by the less-coordinating halide ion, fluoride (see empty
circles in Figure 12a), to which a half-time t1/2 = 13 min
corresponds from the linear plot in Figure 12b. Such an
unexpected behavior must be related to the Brønsted base
F
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Figure 14. (a) Spectra recorded over the course of the titration of a 1 × 10−4 M solution of the trans-I-[CuII(4)](ClO4)2 complex salt in MeCN with
a solution of [Bu4N]F in MeCN (optical path 1 cm); (b) titration profiles at selected wavelengths, illustrating the occurrence of a two-step process,
at 1 and 2 equiv of added fluoride.
Figure 15. (a) Spectra recorded over the course of the titration of a 1 × 10−4 M solution of the trans-I-[CuII(4)](ClO4)2 complex salt in MeCN with
a solution of [Bu4N]F in MeCN (optical path 10 cm); addition of 0.01 equiv of F− to the red solution favors the quick formation of the trans-III
complex, and the solution turns violet; (b) titration profiles at selected wavelengths; the red triangle on the left vertical axis pertains to the trans-I
complex prior the addition of F−.
from one single transition. The fact that the absorption at pH ≥
10 results from two distinct transitions suggests that the L−
form exists as an equilibrium mixture of two species, probably
differing for the position of the deprotonated amine group,
whether closer or farther from the phenylamine nitrogen atom.
To the two isomeric species, dipoles of different intensity
correspond, as well as transitions of different energy.
The occurrence of demetalation in a strongly basic solution
was demonstrated by the following experiment: an aqueous
solution of the trans-I-[CuII(4)](ClO4)2 complex salt was
brought to pH = 14 with NaOH; the yellow solution was then
extracted with CHCl3, to which the yellow color was
transferred. One drop of the CHCl3 layer was dissolved in
MeOH, and the electrospray ionization (ESI) mass of the
solution was determined: a peak m/z 367.2 (M+H+, 100%) was
observed; no peaks with the typical isotopic pattern of CuII
were detected.
observed in a strongly basic aqueous solution and pertaining to
the deprotonated macrocycle L− (reported as a black dotted
line in Figure 14a for comparative purposes). In particular, the
band in the visible region shows two clearly distinct transitions
at 360 and 430 nm. Thus, the F− ion in the MeCN solution acts
as a strong base, taking one proton from a N−H group of the
macrocycle and inducing demetalation. However, the stoi-
chiometry of the process is not straightforward. In particular,
titration profiles at different wavelengths (e.g., at 209 and 307
nm; see Figure 14b) indicate discontinuity after the additions of
1 and 2 equiv of F−, suggesting the occurrence of a two-step
process. In particular, it is proposed that in the first step the
fluoride ion coordinates the metal to give a five-coordinate
complex, analogous to the [CuII(4)Cl]+ species, whose
structure is illustrated in Figure 9. In the second step,
deprotonation of one N−H fragment of the macrocycle takes
−
place with formation of the HF2 hydrogen bonding complex
Then, we titrated a 1 × 10−4 M solution of the trans-I-
[CuII(4)](ClO4)2 complex salt in MeCN with a solution of
[Bu4N]F in MeCN. The recorded family of spectra is shown in
Figure 14a.
and demetalation. The two equilibria are described below:
[CuII(HL)]2+ + F− ⇆ [CuII(HL)F]+
(2)
−
[CuII(HL)F]+ + F− ⇆ Cu2+ + L + HF −
(3)
2
It is observed that on fluoride addition the band at 250 nm,
corresponding to the copper(II) complex, decreases in
intensity, while a new two-transition band develops at 350−
450 nm. Such a band reaches a limiting intensity on addition of
excess of fluoride, and its shape is strongly reminiscent of that
In fact, if one F− ion in an aprotic solvent is not an especially
strong base, two F− ions exert a strongly basic behavior due to
−
the formation of the hydrogen bonding complex HF2 , for
which the highest HB energy in the gas phase has been
G
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determined (38.6 kcal mol−1).6 Fluoride-induced deprotona-
tion of an acidic N−H fragment had been previously observed
in the case of urea derivatives (BH),7−10 when investigated as
anion receptors, according to the two stepwise equilibria
reported below:
positioned in an ortho- position, copper(II) incorporation
results in a red-shift response. In any case, all the investigated
nitrophenylcyclam derivatives accumulate the dose and should
be considered colorimetric dosimeters. They are specific for
copper(II) as they do not complex any other metal ion under
the same conditions, at room temperature.
BH + F− ⇆ [BH···F]−
(4)
When in the ortho- position of the phenyl substituent
(macrocycles 3 and 4), the nitro group can apically coordinate
the CuII ion, according to a scorpionate mode. Nitro
coordination influences the configuration of the cyclam-like
macrocycle. In fact, the [CuII(2)]2+ complex (one nitro group
in para-, scorpionate coordination prevented) is obtained in the
trans-I form, a configuration that is indefinitely maintained. The
trans-III form is obtained only in the presence of stoichiometric
amount of coordinating anions (e.g., Cl−), with formation of a
square-pyramidal complex. The CuII complex of 4 (one nitro
group in ortho-) is obtained as a perchlorate salt of a
scorpionate in a trans-I configuration, which slowly converts to
the trans-III arrangement, maintaining the apical coordination
of the nitro fragment. The conversion is dramatically
accelerated by catalytic amounts of Cl−, NCO−, and F−.
While for Cl− and NCO− the effect can be associated with the
capability of the anion to stabilize through coordination to any
dissociative intermediate, the very powerful effect exerted by F−
seems to be related to the deprotonation of one N−H fragment
of the macrocycle, driven by the formation of the highly stable
[BH···F] + F− ⇆ B− + HF −
(5)
2
First, the hydrogen bonding complex [BH···F]− forms, eq 4,
and then, on addition of the second fluoride ion, deprotonation
of one N−H urea fragment takes place, according to an
−
equilibrium displaced to the right by the formation of HF2 , eq
5.11
This may explain the strong acceleration of the trans-I-to-
trans-III conversion induced by a catalytic amount of fluoride
(see Figure 12). In fact, it may happen that traces of fluoride
provoke deprotonation of a microscopic amount of the
complex, inducing demetalation, conformational rearrange-
ment, new protonation, and new metal complexation. At this
stage, the fluoride ion can begin a new cycle.
The spectrophotometric titration experiment illustrated in
Figure 14, performed in a cuvette of 1 cm optical pathway, was
also performed in a cuvette of 10 cm optical path to detect
spectral changes in the visible region and monitor the
occurrence of d−d transitions.
−
HF2 complex ion.
The red line in Figure 15a indicates the spectrum of the
solution before anion addition and corresponds to the red
trans-I-[CuII(HL)]2+ complex. On addition of 0.01 equiv of F−,
a substantial spectral change quickly takes place, and the
spectrum indicated by the blue line forms: this is the spectrum
of the trans-III-[CuII(HL)]2+ complex, whose formation was
made fast by the presence of 0.01 equiv of fluoride. Further sub-
stoichiometric additions of F− induced a steady red shift of the
d−d band, as typically observed on axial coordination of an
additional ligand to a copper(II)-tetramine complex. The
process can be followed through the absorbance profiles shown
in Figure 15b. The decrease of ε at 530 nm corresponds to the
disappearance of the trans-III-[CuII(HL)]2+ scorpionate com-
plex, while the increase at 600 nm is related to the formation of
the trans-III-[CuII(HL)F]+ species of presumably square-
pyramidal geometry. However, after the addition of 1 equiv
of anion, a neat discontinuity of the two profiles is observed,
due to the incipient decomposition of the macrocyclic complex.
A completely similar behavior was observed with the
corresponding nonscorpionate complex of macrocycle 2 (see
Supporting Information, Figure S9).
This work was undertaken to design specific and irreversible
receptors for CuII, by coupling the high coordinating tendencies
of cyclam and the unique chromophoric properties of the
nitrophenyl substituent. While the synthesized ligands display
the aimed analytical properties, novel features of interest in
basic inorganic and coordination chemistry were discovered,
which include the binding capability of the nitro group
according to a scorpionate mode, the factors affecting the
thermodynamic and kinetic relative stability of the trans-I and
trans-III configurations of complexed cyclam, the role exerted
by the fluoride ion in the deprotonation of a coordinated N−H
fragment.
■. EXPERIMENTAL SECTION
Physical Measurements. UV−vis spectra were recorded on a
Varian CARY 100 spectrophotometer with quartz cuvettes of the
appropriate path length (0.1, 1, 10 cm). In any case, the concentration
of the chromophore and the optical path were adjusted to obtain
1
spectra with AU ≤ 1. H NMR spectra were obtained on a Bruker
Avance 400 spectrometer (400 MHz) operating at 9.37 T. Mass
spectra were acquired on a Thermo-Finnigan ion trap LCQ Advantage
Max instrument, equipped with an ESI source. Elemental analyses
were performed on a PerkinElmer 2400 Series II CHNS/O Elemental
Analyzer.
The otherwise slow trans-I-to-trans-III conversion in CuII
complexes of nitrocyclams 3 and 4 can be made fast by catalytic
effects exerted either by a coordinating anion, which reduces
the energy of the dissociative intermediate, or by bases,
including fluoride, through deprotonation of a coordinated
amine group of the macrocycle.
Syntheses. All the macrocycles were prepared through a multistep
synthesis involving a reaction of triprotected cyclam 1,4,8-tris(tert-
butoxycarbonyl)-1,4,8,11-tetraazacyclotetradecane (tri-BOCcyclam)1
with the appropriate nitroaryl fluoride, followed by deprotection of
the intermediate to yield the desired product. The mononitro
derivatives 2 (4-nitrophenyl) and 3 (2-nitrophenyl) were synthesized
according to literature procedure,1 with some modifications, whereas
the dinitro derivative 4 (2,4-dinitrophenyl) was obtained under milder
conditions.
1,4,8-Tris(tert-butoxycarbonyl)-11-(4-nitrophenyl)-1,4,8,11-tet-
raazacyclotetradecane (5). A solution of tri-BOCcyclam (2.5 g., 5
mmol) in anhydrous dimethyl sulfoxide (DMSO; 30 mL) was added
to cesium carbonate (6 g., 18.42 mmol) and heated under stirring to
100 °C under a dinitrogen atmosphere; 1-fluoro-4-nitrobenzene (1.06
■. CONCLUSION
It has been shown that appending of a nitrophenyl substituent
at a nitrogen atom seriously modifies the spectral properties of
cyclam and of its copper(II) complexes. In particular,
nitrophenylcyclam derivatives 2 and 4, containing a nitro
group in para- position, in an MeCN solution incorporate CuII
according to an irreversible process signaled by a neat color
change (yellow-to-red), associated with a significant blue-shift
of the CT band. For macrocycle 3, whose nitro substituent is
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mL, 10 mmol) was then added dropwise, and the resulting suspension
was left to react for 24 h, added to an additional amount of cesium
carbonate (3 g, 9.12 mmol) and 1-fluoro-4-nitrobenzene (0.53 mL, 5
mmol) and kept until no more increase in the concentration of the
product could be detected by MS-ESI (3 d). The reaction mixture was
left to cool to room temperature and taken up with ethyl acetate (100
mL). The organic solution was then washed with a saturated sodium
hydrogen carbonate solution (2 × 30 mL) and brine (1 × 30 mL, left
overnight on Na2SO4, filtered, and evaporated) to give an oil. The oil
was purified through silica gel chromatography (n-hexane/ethyl acetate
3:2) to give the desired product as a yellow oil (0.99 g., yield: 31.8%),
which was satisfactorily characterized through mass spectroscopy (ESI
(CH3OH): m/z 623.4 (M+H+, 100%) and used as such for the
following step.
1-(4-Nitrophenyl)-1,4,8,11-tetraaza-cyclotetradecane (2). Com-
plex 5 (0.99 g, 1.6 mmol) was dissolved in dichloromethane (17 mL),
and trifluoroacetic acid (1.7 mL) was added dropwise. The solution
was stirred at room temperature under a dinitrogen atmosphere until
the reaction was complete (∼20 h, checked by MS-ESI). The solvent
was then removed under vacuum to give a dark yellow precipitate,
which was dissolved in 5 mL of ethyl acetate and then precipitated
with diethyl ether (75 mL). The solid (a trifluoroacetate adduct) was
separated through filtration, taken up with dichloromethane (20 mL),
and washed with an equal volume of aqueous NaOH 5%. The organic
solution was kept overnight on Na2SO4, filtered, and brought to
dryness under vacuum to give the pure macrocycle 2 (0.45 g., yield:
88.24%). MS (CH3OH, ESI): m/z 322,2 (M+H+, 100%).
C16H27N5O2, MW 321.42, % calc: C, 59.8; H, 8.5; N, 21.8; %
found: C, 59.7; H, 8.4; N, 21.9; 1H NMR (CD3CN, 400 MHz): δ 8.15
(2H, d, J = 8.3 Hz), 6.96 (2H, d, J = 8.3 Hz), 3.69 (2H, br t, J = 5.3
Hz), 3.51 (2H, br t, J = 6.5 Hz), 3.31 (2H, br t, J = 6.8 Hz), 3.24 (2H,
br t, J = 5.3 Hz), 3.17 (4H, br), 2.88 (2H, br t, J = 5.3 Hz), 2.80 (2H,
br t, J = 5.3 Hz), 1.81 (2H, br). A signal is visible at 2.13 ppm.
However, it was not integrated due to the overlapping by the nearby
intense H2O peak.
1-(2-Nitrophenyl)-1,4,8,11-tetraaza-cyclotetradecane (3). The
synthetic procedure is same as described above for macrocycle 2,
the only difference being the replacement of cesium carbonate with
potassium carbonate. Thus, the reaction of tri-BOCcyclam (2.5 g, 5
mmol) in anhydrous DMSO (30 mL) with potassium carbonate (2.6
g., 18.8 mmoli) and 1-fluoro-2-nitrobenzene (1.06 mL, 10 mmol) at
100 °C for 3 d (including a further addition of potassium carbonate,
1.3 g, and 1-fluoro-2-nitrobenzene, 0.53 mL) followed by workup and
chromatographic separation (silica gel, n-hexane/ethyl acetate 3:2,
gradient) afforded 1,4,8,11-tris(tert-butoxycarbonyl)-11-(2-nitrophen-
yl)-1,4,8,11-tetraazacyclotetradecane (1.91 g, 3.1 mmol, yield: 61.4%)
as an oil, which solidified on standing. Treatment with trifluoroacetic
acid (3.4 mL) in dichloromethane (33 mL) gave the pure product (0.8
g, yield: 81.2%). MS (CH3OH, ESI): m/z 322,2 (M+H+, 100%).
C16H27N5O2, MW 321.42, % calc C, 59.8; H, 8.5; N, 21.8; % found C,
59.6; H, 8.5; N, 21.7; 1H NMR (CD3CN, 400 MHz): δ 7.82 (1H, d, J
= 8.0), 7.78−7.72 (2H, m), 7.51 (1H, t, J = 7.5 Hz), 3.39 (2H, br t, J =
5.6 Hz), 3.24 (2H, br t, J = 5.6 Hz), 3.17 (4H, m), 3.09 (2H, br t, J =
6.6 Hz), 2.98 (4H, m), 2.89 (2H, br t, J = 5.8 Hz), 1.90 (2H, br quin, J
= 5.8 Hz), 1.80 (br quin, J = 5.8 Hz).
1-(2,4-Dinitrophenyl)-1,4,8,11-tetraaza-cyclotetradecane (4). A
solution of 1-fluoro-2,4-dinitrobenzene (0.93 g, 5 mmol) in anhydrous
tetrahydrofuran (50 mL) was added dropwise to a boiling solution of
1,4,8-tris(tert-butoxycarbonyl)-1,4,8,11-tetraazacyclotetradecane (2.5 g,
5 mmol) and N,N-diisopropylethylamine (0.86 mL, 5 mmol) in the
same solvent (150 mL). The solution was refluxed under a dinitrogen
atmosphere for 24 h, when an additional amount of reagents (0.47 g of
1-fluoro-2,4-dinitrobenzene and 0.43 mL of N,N-diisopropylethyl-
amine) was added, and then left to react for additional 20 h. The
solvent and the excess of N,N-diisopropylethylamine were removed in
vacuo, and the residue was purified through liquid chromatography
(silica gel, ethyl acetate/hexane 1:1, gradient) to give a yellow oil (3 g,
4.49 mmol, yield: 89.8%), which was satisfactorily characterized
through mass spectroscopy (ESI (CH3OH): m/z 668,4 (M+H+,
100%). The tri-BOC derivative was then dissolved in dichloromethane
I
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Inorganic Chemistry
Article
(47 mL), added of trifluoroacetic acid (4.7 mL), and left to react at
room temperature until complete removal of the tert-butoxycarbonyl
groups (∼27 h). The volatiles were distilled under vacuum, and the
residual orange-yellow oil was dissolved in ethyl acetate (40 mL); the
resulting solution was then treated with diethyl ether (200 mL) to
form a sticky, yellow solid. The solid was taken up with aqueous
NaOH 10% (50 mL) to give a mixture, which was extracted with
toluene (3 × 50 mL). The combined organic phases were then washed
with water (3 × 50 mL), dried overnight on Na2SO4, filtered, and
distilled to dryness under vacuum to give the pure product (0.95 g.,
2.58 mmol, yield: 57.4%). ESI (CH3OH): m/z 369,4 (M+H+, 100%).
C16H26N6O4, MW 366.42, % calc C, 52.5; H, 7.2; N, 22.9; % found C,
52.4; H, 7.1; N, 23.0; 1H NMR (CD3CN, 400 MHz): δ 8.48 (1H, d, J
= 2.8 Hz), 8.17 (1H, dd, J = 9.5, 2.8 Hz), 7.31 (1H, d, J = 9.5 Hz), 3.65
(2H, t, J = 6.9 Hz), 3.30 (2H, t, J = 5.1 Hz), 2.83 (2H, t, J = 5.3 Hz),
2.67 (4H, m), 2.61 (2H, m), 2.57−2.53 (4H, m), 1.66−1.58 (4H, m).
trans-I-[CuII(4)](ClO4)2. Ligand 4 (18.4 mg, 0.0501 mmol) was
dissolved in 3 mL of CH2Cl2, to which a solution of 18.2 mg (0.0491
mmol) of Cu(ClO4)2·6H2O in 0.5 mL of MeCN was added at room
temperature. A red-brown solution formed, to which 5 mL of diethyl
ether was added. A dark-brown gummy precipitate formed, which was
suspended in 1 mL of MeOH and sonicated. A red-orange powder
formed, which was filtered on a Hirsch funnel, washed with iced
MeOH, dried, and recrystallized from MeCN through slow diffusion of
diethyl ether. Red crystals were obtained (21.8 mg, yield 71%. ESI-MS:
m/z 214.5 (M2+); MW C16H26Cl2CuN6O12, 628.86, % calc C, 30.6; H,
4.2; N, 13.4; % found C, 30.4; H, 4.1; N, 13.3.
effects were evaluated with the ψ-scan method,13 and absorption
correction was applied to the data. Data reduction for frames collected
by the CCD-based system was performed with the SAINT software;14
absorption effects were empirically evaluated by the SADABS
software,15 and absorption correction was applied to the data. All
crystal structures were solved by direct methods (SIR 97)16 and
refined by full-matrix least-squares procedures on F2 using all
reflections (SHELXL 97).17 Anisotropic displacement parameters
were refined for all non-hydrogen atoms, excluding some atom sites
partly populated and belonging to disordered solvent molecules.
Hydrogens bonded to C atoms were placed at calculated positions
with the appropriate AFIX instructions and refined using a riding
model; hydrogens bonded to secondary amines of the macrocycle
moiety and to water molecules (when possible) were located in the
final ΔF maps and their positions were successively refined restraining
the N−H or O−H distance to be 0.90
0.01 Å. Crystal data are
reported in Table 1.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b01273.
Plots showing thermal ellipsoids for the studied
molecular crystals. Details on the spectrophotometric
titrations in MeCN. (PDF)
trans-III-[CuII(4)](ClO4)2. Violet crystals suitable for X-ray diffraction
studies were obtained by dissolving the red trans-I-[CuII(4)](ClO4)2
complex salt in MeCN (∼1 × 10−3 M). Then, 0.01 equiv of [Bu4N]F
was added. The solution was kept at room temperature for 1 d. On
slow diffusion of diethyl ether, violet crystals formed.
X-ray crystallographic information. (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: luigi.fabbrizzi@unipv.it.
■
trans-I-[CuII(3)](ClO4)2 and trans-III-[CuII(3)](ClO4)2. A similar
procedure as for trans-I-[CuII(4)](ClO4)2 was followed. The dark-
brown gummy precipitate that formed was charged on a Sephadex SP
C-25 column and eluted with aqueuous NaClO4. A red and a violet
fraction were separated (the red one eluted first). Slow evaporation at
room temperature of the two fractions gave the crystalline materials
studied through X-ray diffraction. Analytical data for trans-I-[CuII(3)]-
(ClO4)2: ESI-MS: m/z 192.0 [M]2+ (100%). C16H27Cl2CuN5O10, MW
583.87, % calc C, 32.9; H, 4.7; N, 12.0; % found C, 32.9; H, 4.6; N,
11.8.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The financial support of the Italian Ministry of University and
Research (PRIN−InfoChem) is gratefully acknowledged.
■
trans-I-[CuII(2)](ClO4)2. The complex salt was prepared as previously
described.1 ESI-MS: m/z 192.2 [M]2+ (52%), 383.2 [M − H]+
(100%), 483.2 [M + ClO4]+ (13%). C16H27Cl2CuN5O10, MW
583.87; % calc C, 32.9; H, 4.7; N, 12.0; % found C, 33.0; H, 4.7;
N, 11.9.
Complexes containing chloride and cyanate were obtained through
crystallization of MeCN solutions of the trans-I perchlorate complex
salt in the presence of an excess of the corresponding [Bu4N]X salt (X
= Cl, NCO).
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