Synthesis, structural characterization and chromotropism of a copper(II) complex containing a bidentate ligand

Article abstract of DOI:10.1016/j.poly.2014.09.035

A new coordination compound, [Cu(L)₂(OH₂)](ClO₄)₂ where L = N-(pyridin-2-ylmethyl)propane-2-amine, was prepared and characterized by elemental analysis, molar conductance and IR and UV-Vis spectroscopic techniques. X-ray crystal analysis of the complex confirmed that the copper(II) ion has a distorted square pyramidal environment. The complex is chromotropic in solution. The chromotropic properties of the complex, including solvato-, thermo-, halo- and ionochromism, were investigated in detail. The complex displayed strongly pronounced reversible thermochromism in solution due to dissociation and of re-coordination of a water molecule.

Full text of DOI:10.1016/j.poly.2014.09.035

Accepted Manuscript
Synthesis, structural characterization and chromotropism of a copper(II) com-
plex containing a bidentate ligand
Hamid Golchoubian, Mahbobeh Tarahomi, Ehsan Rezaee, Giuseppe Bruno
PII:
S0277-5387(14)00642-1
DOI:
http://dx.doi.org/10.1016/j.poly.2014.09.035
POLY 11004
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
19 July 2014
16 September 2014
Please cite this article as: H. Golchoubian, M. Tarahomi, E. Rezaee, G. Bruno, Synthesis, structural characterization
and chromotropism of a copper(II) complex containing a bidentate ligand, Polyhedron (2014), doi: http://dx.doi.org/
10.1016/j.poly.2014.09.035
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Synthesis, structural characterization and chromotropism of a copper(II)
complex containing a bidentate ligand
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Hamid Golchoubian*1, Mahbobeh Tarahomi1, Ehsan Rezaee1, Giuseppe Bruno2
1Department of Chemistry, University of Mazandaran, Babol-sar; Iran. Postal Code 47416-
95447.
2 Dipartimento di Chimica Inorganica, Vill. S. Agata, Salita Sperone 31, Università di Messina
98166 Messina, Italy e-mail: gbruno@unime.it
Reprint requests to Dr. H. Golchoubian. Tel/Fax: (98)-1125342350, E-mail:
h.golchobian@umz.ac.ir
Abstract
A
new coordination compound, [Cu(L)2(OH2)](ClO4)2 where
L
=
N-(pyridin-2-
ylmethyl)propane-2-amine, was prepared and characterized by elemental analysis, molar
conductance and IR and UV-Vis spectroscopic techniques. X-ray crystal analysis of the complex
confirmed that the copper(II) ion has a distorted square pyramidal environment. The complex is
chromotropic in solution. The chromotropic properties of the complex, including solvato-,
thermo-, halo- and ionochromism, were investigated in detail. The complex displayed strongly
pronounced reversible thermochromism in solution due to dissociation and of re-coordination of
a water molecule.
Keywords: Diamine ligand; Copper(II) complex; Thermochromism; Solvatochromism;
Ionochromism; Halochromism
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1. Introduction
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Recently, the chromotropic behaviors of inorganic complexes have been investigated due to their
potentially versatile applications [1-3]. Chromotropism is a reversible change in the color of a
substance caused by some form of stimulus. One of the stimuli is temperature
(thermochromism), which involves a change in color of a compound induced by temperature
change. This phenomenon has many applications, such as temperature sensors, thermochromic
pigments, temperature indicators, security and novelty printing, coating, as well as in
thermography [4-7]. Solvatochromism is another important chromotropic property, which is
defined as a reversible color change of a transition metal complex due to variation of a solvent‟s
properties [8]. Recent studies have demonstrated that solvatochromic materials can be used as a
Lewis-acid base color indicator [9] and to develope of optical sensor materials to monitor
pollutant levels in the environment [10,11]. Solvatochromism also provides a quantitative
method to identify the solvent behavior for different solvents and the function of solvents in
physicochemical studies [12-23]. In addition to thermochromism and solvatochromism,
ionochromism, a reversible color change caused by the addition of ions, is another important
property of transition metal complexes which has attracted great attention due to their important
applications in analytical chemistry and in high technology fields such as carbonless copying
paper, direct thermal printing and sensor materials [7]. The pH effect on the visible absorption
spectrum is defined as halochromism and can be regarded as one of the simplest external
stimulations and can induce switching properties such as visible absorption for pH sensors. pH
sensors are especially attractive since various living organisms, like enzymes, function within a
narrow pH range, and their actions or behaviors can be explained as being “on-off” switched as a
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function of pH [24]. The visual detection of CO2 gas [25] and the preparation of smart and
interactive textiles [26] are other applications of halochromic dyes. However, the majority of pH
sensors are based on organic molecules and less attention has been focused on metal complexes
[27]. Among the transition metal complexes, copper(II) ions with a combination of chelate
ligands have been recognized as the most encouraging candidates for practical applications of
chromotropism due to their high thermodynamic stabilities, unsaturated coordination number and
also the existence of simple and regular changes in their electronic spectra in accord to the power
of the stimuli imposed on the system. In this work the new bidentate ligand N-(pyridin-2-yl-
methyl)propane-2-amine and its copper(II) complex, as shown in Scheme 1, were prepared and
fully characterized. The chromotropic behaviors of the complex, including thermochromism,
solvatochromism, ionochromism and halochromism, were investigated. Based on our knowledge
after our first report [28], this is the second time that a compound has shown all these properties
collectively.
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<scheme 1>
2. Experimental
2.1. Materials and methods
All solvents were spectral-grade and all other reagents were used as received. Caution!
Perchlorate salts are potentially explosive and should be handled with appropriate care.
Elemental analyses were performed on a LECO CHN-600 elemental analyzer. Absolute
metal percentages were determined with a Varian-spectra A-30/40 atomic absorption-flame
spectrometer. The sample was dried to a constant weight under a high vacuum prior to analysis.
Conductance measurements were made at 25 °C with a Jenway 400 conductance meter on
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concentrations of 10.0 × 10-4, 6.00 × 10-4, 4.00 × 10-4 and 2.00 × 10-4 M of samples in selected
solvents. Then for each solvent, a curve was plotted by drawing the molar conductance versus
the concentration of the sample. The curve was then extrapolated to an infinitely dilute solution
to obtain the molar conductance value. The infrared spectrum (potassium bromide disk) was
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recorded using a Bruker FT-IR instrument. H and 13C NMR spectra were measured with a
Bruker 400 DRX Fourier Transform spectrometer at room temperature. The electronic
absorption spectra were measured using Braic2100 model UV-Vis spectrophotometers. The
buffer solution of pH = 7.2 for the ionochromic study was prepared by mixing 100 mL of
ammonium chloride (1.1 M) and 100 mL of aqueous ammonia solution (0.05 M). The following
solvents were used for the solvatochromic study: nitromethane (NM), nitrobenzene (NB),
benzonitrile (BN), acetone (AC), acetonitrile (AN), propionitrile (PN), methanol (MeOH),
ethanol (EtOH), water (H2O), dimethylformamide (DMF), dimethylsulfoxide (DMSO) and
hexamethylphosphorictriamide (HMPA).
2.2. Synthesis
2.2.1. N-(pyridin-2-ylmethyl)propane-2-amine (L)
A mixture of pyridine-2-carbaldehyde (2.7 ml, 30 mmol), tert-butylamine (3.5 ml, 30 mmol) and
a few drops of acetic acid in methanol (35 ml) was prepared and refluxed for 10 min. The solvent
was then evaporated under reduced pressure. NaBH4 (1.7 g, 45 mmol) was added gradually to
the resultant yellow oil dissolved in methanol (30 mL). The resulting mixture was allowed to
stand overnight. After heating the solution to near the boiling point, HCl (10 ml, 17 M) was
added to it, while placing the solution in an ice bath, and then it was heated again to the boiling
temperature. The mixture was then made alkaline by adding NaOH (4 M). The sodium borate
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precipitate was removed by filtration. Evaporation of the solvent from the filtrate resulted in a
yellow oil which was subsequently extracted with dichloromethane (3  15 mL). The combined
CH2Cl2 fractions were dried over anhydrous Na2SO4. Evaporation of the solvent under reduced
pressure resulted in the desired product as a brown oil. The yield was 45%. Selected IR data
(ν/cm-1 using KBr disk): 3308 (m, N–H str.), 2964 (s, C–H str. aliphatic), 1595 (s, C=N str.),
1433 (s, CC str. aromatic), 1227 (s, NC str. aliphatic). 1H NMR (400 MHz, CDCl3) : 1.15 (s,
9H, ((CH3)3C-), 1.92 (s, br, H, tert-Butyl-NH), 3.83 (s, 2H, CH2Py), 7.80 (d,d, J = 5.2, 1.6
Hz, H, Py), 7.27 (s, H, Py), 7.57 (t,d, J = 7.6, 2.0 Hz, H, Py), 8.48 (d, J = 4.4 Hz, H, Py). When
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one drop of D2O was added to the H NMR sample, the broad signal at 1.92 ppm disappeared.
13C NMR (100 MHz in CDCl3) : 29.06 (CH3CN), 48.53 (CH3CN), 50.48
(NHCH2Py), 121.71, 122.46, 136.41, 149.08, 160.46 (PyC).
2.2.2. Preparation of [Cu(L)2(OH2)](ClO4)2
To a solution of the ligand L (0.328 g, 2 mmol) in ethanol (6 mL) was slowly added
Cu(ClO4)2·6H2O (0.37 g, 1 mmol) in ethanol (6 mL). The resultant purple color mixture was
stirred for 2 h at room temperature. The desired compound precipitated from the solution as a
blue solid. The compound was recrystallized by diffusion of diethylether into an acetonitrile
solution. The typical yield was 43%. The crystals were suitable for X-ray crystallography. Anal.
calcd for C20H34N4CuCl2O9 (MW= 608.96 g mol-1): C, 39.45; H, 5.63; N, 9.20; Cu, 10.44;
Found: C, 39.72; H, 5.62; N, 9.22; Cu, 10.53 %. Selected IR data (ʋ/cm-1 using KBr disk): 3448
(m, O-H str.), 3275 (m, N-H str), 2974 (m C-H str.), 1649 (s, C=N str.), 1476 (m, N-H bend.),
1112 (s, O-ClO3 str.), 625 (s, O-ClO3 bend.), 427 (w, Cu-O str.), 449 (w, Cu-N str.).
2.3.1. X-ray structure determination
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The X-ray measurements of the complex were made on a Bruker Apex-II CCD single crystal
diffractometer at room temperature using graphite-monochromated Mo-Kα radiation. Data were
collected and reduced by SMART and SAINT software [29] in the Bruker packages. The
structure was solved by direct method [30,31] and subsequent Fourier difference techniques and
refined using the full-matrix weighted least-squares procedure on F2 [32] with anisotropic
thermal parameters for all non-hydrogen atoms. Several hydrogen atoms were located on the
final difference map, the ligand H atoms were included in the refinement via the "riding model"
method with the X---H bond geometry and the H isotropic displacement parameter depending on
the parent X atom. The oxygen displacement ellipsoids are very large due to a significant
rotational disorder of the perchlorate anion, but each attempt to treat it by a suitable overlap of
two rotated conformations was unsuccessful. The last difference Fourier maps have shown the
largest electron density residuals around the disordered perchlorate anion. A summary of the
crystal data and structure determination is reported in Table 1.
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<Table 1>
3. Results and discussion
3.1 Synthesis
The ligand was prepared by the condensation of equimolar amounts of tert-butylamine and
pyridine-2-carbaldehyde and further reduction of the resulting diimine by sodium borohydride.
The copper(II) complex was synthesized by mixing Cu(ClO4)·6H2O and the ligand in a molar
ratio of 1:2, respectively, in ethanol. Thermochromism and solvatochromism in coordination
compounds are sensitive to the counter ion used, for example, the bis(N,N-
diethylethylenediamine)copper(II) complex showed distinctive thermochromism only with the
perchlorate counter ion amongst the seventeen counter ions tested. [33-34]. Moreover, our
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pervious works demonstrated that solvatochromism is also sensitive to the nature of the counter
ion used [1,15,20]. As a result, in this research the perchlorate anion was used as the counter ion.
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3.2. Characterization
The IR spectrum of the free ligand has indicator bands which are apparent with minor shifts in
the complex. The presence of a sharp signal at 3449 cm-1 in the complex can be assigned to the
O-H stretching frequency of the coordinated water molecule. Dependence on coordination is
also exhibited by an intense and narrow band occurring at 3280 cm-1 which is associated with the
N-H vibration; this band is broader and observed at around 3309 cm-1 in the free ligand. As the
lone pairs of electrons of the donor nitrogen atoms become involved in the metal-ligand bond,
the transfer of electron density to the metal and the subsequent polarization of the ligands
involves electron depopulation of the N-H bond, which culminates in a shift to lower frequencies
[35]. The presence of the ClO4 group was declared by an intense band at around 1100 cm-1 and a
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medium band at 630 cm-1, which are attributed to the anti-symmetric stretching and anti-
symmetric bending vibration modes, respectively [36]. The molar conductance of the complex
measured in some solvents with different polarities is presented in Table 2. The standard values
of 1:2 electrolytes in the respective solvents are shown in the same table [37]. The conductivity
data shows that the complex is almost consistent with a 1:2 electrolyte.
<Table 2>
3.2.3. X-ray structure
The complex was crystallized in the monoclinic space group P21/n. A perspective view together
with the atom labelling scheme for the complex is given in Fig. 1 and selected bond parameters
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are given in Table 3. The molecular structure of the complex consists of the [CuL2(H2O)]2+
cation and two perchlorate anions. According to Addison et al., [38] the distortion of the square
pyramidal geometry towards trigonal bipyramidal can be described by geometrical parameter  =
 - /60, ( = 0 for ideal SP and 1 for ideal TBP), where  and  are the bond angles involving
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the trans donor atoms in the basal plane. The  value is 0.053 in the complex, which indicates
that the coordination geometry around the copper(II) ion is only slightly distorted from square-
pyramidal towards a trigonal bipyramidal structure. The four coordinating atoms making up the
basal plane are two nitrogen atoms of the amine moieties (N(1) and N(3)) and two nitrogen
atoms of the pyridyl groups (N(2) and N(4)), while the apical site is occupied by one oxygen
atom of a water molecule. The basal atoms are nearly coplanar; the deviations from the least-
squares plane through the CuN4 atoms are N(1) -0.032, N(2) 0.032, N(3) -0.032, N(4) 0.032 and
Cu(1) 0.148 Å. The two unsymmetrical chelate ligands oriented in trans positions. Due to steric
hindrance of the tert-butyl groups, the water molecule in the apex is tilted (O(1)-Cu-N(1) =
85.42(9) °). The bite angles of the five-membered rings N(3)-Cu-N(4) and N(1)-Cu-N(2) are
81.47(9) and 80.98(9) , respectively. The bond lengths of amine moieties (Cu-N(1) and Cu-
N(3)) are slightly longer than to the corresponding values of the copper(II) amine complexes
CuL1 and CuL2 (L1 = N,N′-dimethyl-bis(pyridine-2-ylmethyl)-1,2-diaminoethane, L2= N,N′-
dimethyl-bis(pyridine-2-ylmethyl)-1,2-diaminopropane) [39,40]. According to the CSD [41], the
average bond distances are 1.971 and 2.031 Å for Cu(II)-amines. This lengthening obviously
originates from steric crowding of the tert-butyl groups attached to N(1) and N(3). The torsion
o
angles of N(1)-C(5)-C(6)-N(2) and N(3)-C(15)-C(16)-N(4) are 25.63 and 26.86 , respectively.
The apical bond length, Cu-O(H2O), of 2.382(3) Å is much longer than usual due to the Jahn-
Teller effect for a d9 electronic configuration.
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<Figure 1>
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<Table 3>
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There are four O-H···O and Cl(ClO4) and two N-H···O(ClO4) hydrogen bonds in the crystal
structure of the complex (Table 4). The O-H of the water molecule and N-H of the amine
moieties act as proton donors, while uncoordinated oxygen atoms of the perchlorates act as
proton acceptors. The compound is assembled into a 3D architecture by these hydrogen bonds,
as shown in Figure 2
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<Figure 2>
<Table 4>
3.5. Thermochromism
Room temperature UV-vis absorption spectra of the complex in solution demonstrate two bands,
one in the UV region which is assigned to a ligand-to-metal charge transfer (LMCT) involving
the pyridine ligand transition, and the second one in the visible region that is due to a d-d
transition of the copper(II) ion. The latter band is solvent dependent, as will be discussed in the
subsequent section, but the former in not. The complex is thermochromic in solvents with high
boiling points (above 80 °C), such as DMSO, DMF, acetonitrile and benzonitrile. The visible
spectra of the complex were studied at temperature of 25 to 100 C in DMSO and up to 90 C in
DMF and acetonitrile. In the solvents DMF and acetonitrile, the complex demonstrates reversible
thermochromism, such that the original blue color of the solution gradually turns to green (see
Fig. S1 and S2 in supplementary content). However, the reversible thermochromism of the
compound in DMSO involves a gradual change in color from blue to green up to 90 C and to
brown by increasing the temperatures to 100 C, as illustrated in Fig. 3. The color change occurs
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along with the appearance of a new absorption band at about 480 nm together with an isosbestic
point at 621 nm in DMSO, 566 nm in DMF and 570 nm in acetonitrile. The intensity of the new
band increases with the rise of temperature, while the d-d band becomes weaker. It is very
probable that this phenomena corresponds to loss of the coordinated water molecule from the
copper(II) complex. When the complex is dissolved in a solvent at room temperature the empty
coordination site of the complex is occupied rapidly by a solvent molecule, evidenced by being
solvatochromic. Upon heating the solution, the weakly coordinated water molecule is replaced
by solvent molecules. However, this process is reversible and after cooling the solution to room
temperature the water molecule that exists in the solution re-coordinates to the copper ion and
causes the reappearance of original blue color. The reverse process is much slower than the
forward reaction and it takes several days for the full development of the original color. The
decline in the rate of the reverse process could be due to the presence of a large number of
solvent molecules in the solution which hinders the re-coordination of the water molecule to the
copper center. This is supported by this fact that the compound is not thermochromic in aqueous
solution. Moreover, addition of a few drops of water into the heated solution causes a fast
redevelopment of the original blue color. The presence of a distinct isosbestic point suggests that
this phenomenon is due to the simple solvation equilibrium presented in Scheme 2. Appearance
of the green color is due to the emergence of the new band at 480 nm, which is almost
irrespective of the nature of solvent used. This band might be attributed to the charge transfer to
the solvent molecule (LMCT). However, the development of a brown color at 100 C in DMSO
is simply the result of the variation in the band width at 480 nm.
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<Scheme 2>
<Figure 3>
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3.4. Solvatochromism
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The compound is soluble in a variety of solvents and is solvatochromic. The color of the
complex changes from blue to green, as depicted in Fig. 4 and Table 5. The origin of the color
change is due to a change in the d–d band of the Cu(II) center. The shift in the d–d band by
alteration of the solvent indicates a change in the coordination geometry around the copper ion. It
also indicates that the solvent molecule coordinates to the empty coordination site of the central
metal ion and thus the ligand field around the metal changes, which results in the shift of the
absorption wavelength. As changing the solvent molecules changes the color of the complex, it
can be concluded that the solvent molecules are weakly coordinated due to a strong Jahn–Teller
effect of the Cu(II) center with a d9 configuration.
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<Figure 4>
<Table 5>
The spectrum was also measured in the solid-state (powder in Nujol mull), which showed a
broad maximum at 560 nm. This spectral pattern is typical of five-coordinate copper(II)
complexes with a SP-based geometry [42], which is coincident with its stereochemistry in the
crystalline state.
3.6. Ionochromism
The interactions of sodium halide (Cl-, Br- and I-,) and sodium pseudo halide (CN-,OCN-, SCN-
-
and N3 ) with the complex were investigated by visible spectroscopy in buffer solution (pH =
7.2). Upon addition of CN-, the complex showed the evidently ionochromic behavior and the
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color of the solution changed from blue to colorless, while the solution turned green when N3
was added. The other ions induced basically a weak spectral change, as shown in Fig. 5. These
results make the complex promising for application in cyanide and azide sensors. The interaction
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between the complex and a mixture of anions were also investigated. An aqueous solution of a
mixture of the aforementioned anions at a concentration of 0.06 mol L-1 was added to a buffer
solution of the complex (310-3 mol L-1) and its visible spectrum was recorded. The color of the
mixture turned colorless, indicating that none of the anions were able to compete effectively with
the CN- ion, in accordance with the decolorization of the resulting solution. In another
experiment, the reaction was carried out under the same reaction conditions, but excluding the
cyanide ion. It was found that the color of the original solution turned from blue to green within
a few seconds. These results indicate that the complex is highly sensitive and selective towards
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-
CN- and N3 anions. The color changes might be due to substitution of the weakly coordinated
water molecule by the anions which causes change in the ligand field strength around the
copper(II) ion. However, in the case where the cyanide ion was added, a hypochromic shift
(decrease in intensity) was observed and the d-d band of the complex disappeared. This is due to
the strong coordination power of the cyanide anion, which destroys the original complex by
formation of a copper(II) cyanide complex that decomposes quickly into copper(I) cyanide and
cyanogen [43].
<Figure 5>
3.7. Halochromism
The complex is halochromic, so that the original blue color of the aqueous solution is
decolorized upon addition of an acid (HClO4, 0.1 M), and this is reversibly returned by addition
of a base. As the pH of the solution was decreased from 7.9 to 5.8, the absorption band at 622 nm
shifted to longer wavelength (703 nm) with the emergence of an isosbestic point at 462 nm, as
shown in Fig. 6. A further decrease in the pH of the solution resulted in a bathochromic and
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7
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9
hypochromic shift of the d-d band (Fig 7). Spectrophotometric titration of the complex with
perchloric acid at max = 622 nm (inset of Fig. 7) in the pH range 7.2-5.8 corresponded to the
consumption of an equivalent proton, which is possibly due to protonation of the coordinated
water molecule. The decolorization of the solution below pH of 5.8 has arisen from the
formation of the protonated ligand and copper(II) perchlorate. This was confirmed by
comparison of its acidified visible spectrum with that of copper(II) perchlorate. When the pH of
the original solution was increased to 11.6 by addition of a base (NaOH, 0.1 M), no obvious
visible color change was observed, but two isobathic points at 462 and 372 nm were observed, as
is depicted in Fig 8. This might be due to deprotonation of the coordinated water molecule and
formation of an hydroxo group. However, upon addition of perchloric acid (0.1 M) to the
resulting aqueous solution (pH = 7.2), the original spectrum was restored.
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<Figure 6>
<Figure 7>
<Figure 8>
4. Conclusion
A new chelate ligand and its copper(II) complex were synthesized and characterized. In solution
the complex is soluble in a variety of solvents and is solvatochromic. The compound
demonstrates blue → green → brown color changes when a DMSO solution is heated to 100 C,
reverting back to the original color upon cooling the solution to room temperature. The
mechanism for this reversible color change is due to the elimination and re-coordination of the
coordinated water molecule. Visible absorption spectra of the complex change reversibly over
the pH range 2.2-11.4. The complex functions as a pH-induced off–on–off absorption switch
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through reversible protonation and deprotonation of the coordinated ligand and also dissociation
and re-coordination of the chelate ligand in aqueous solution. In addition, the compound is
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highly sensitive and selective towards CN- and N3 anions in the presence of other halide and
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pseudo-halide ions. Therefore, the complex responds to a combination of motivations (heat,
solvent, pH and anion).
Supplementary data:
CCDC 1012222 contains the supplementary crystallographic data for this paper. These data can
be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223
1
13
336033; or e-mail: deposit@ccdc.cam.ac.uk. Supporting Information. Assigned H NMR, C
NMR and IR spectra for the ligand and temperature dependence spectra of the complex in DMF
and acetonitrile solutions.
Acknowledgement
We are grateful for the financial support of university of Mazandaran of the Islamic Republic of
Iran.
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Table 1. Crystal data and structure refinement of [Cu(L2)(OH2)](ClO4)2
Empirical formula
Formula weight
Colour
Temperature (K)
Wavelength (Å)
Crystal system
Space group
C20H34CuN4O·2ClO4
608.95
Needle, blue
293(2)
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31
32
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34
35
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40
41
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48
49
50
51
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59
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61
62
63
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65
0.71069
Monclinic
P2(1)/n
Crystal size (mm)
0.23× 0.10× 0.10
Unit cell dimensions
a (Å)
11.763(5)
b (Å)
19.142(5)
12.318(5)
c (Å)
102.774(5)
 (°)
V (Å3)
2705.0(17)
Z
4
Calculated density (Mg/m3)
 (mm-1)
1.459
1.059
F (0 0 0)
1268
2.07 – 30.91
 range for data collection (˚)
Index ranges
-16 ≤ h ≤ 16
-27 ≤ k ≤ 27
-17 ≤ I ≤ 17
Reflections collected / unique
Completeness to  = 27.50
Absorption correction
118107 / 8441 [R(int) = 0.0286]
98.8%
Multi-scan
Refinement method
Full-matrix least- square on F2 a
8441 / 0 / 332
R1 = 0.0547, wR2 = 0.1736
Data/restraints/parameters
R indicesa [I > 2σ (I)]b
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Goodness-of-fit on F2c
R indices (all data)
1.034
R1 = 0.0704, wR2 = 0.1917
7
8
9
Largest diff. peak and hole, e Å-3
0.974 and -0.755
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31
32
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41
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2
2 2
2
a R =(||Fo| - |Fc||)/||Fo b wR = {[([Fo  Fc2)2]/w[(Fo ) ]}1/2 c S = [ w(Fo  Fc2)2 / (Nobs

Nparam
)
Table 2. Molar conductivity data (Λm) of the complex (Ω-1 cm2 mol-1) at 25 C in different
solvents.
solvent
Complex
CH3CN
277
CH3NO2
140
150-180
MeOH
168
H2O
175
130-170
DMF
159
Acetone
177
2:1 electrolytes 220-300
160-220
130-170
160-200
Table 3. Selected bond lengths (Å) and angles (°) for the complex.
Bond distances
Cu-N(1)
Cu-N(3)
Cu-O(1)
2.100(2)
2.083(2)
2.382(3)
Cu-N(2)
Cu-N(4)
2.004(2)
1.985(2)
Bond angles
N(1)-Cu-N(2)
N(1)-Cu-N(4)
N(2)-Cu-N(4)
O(1)-Cu-N(1)
O(1)-Cu-N(3)
2.382(3)
98.92(9)
173.33(10)
85.42(9)
104.46(9)
N(1)-Cu-N(3)
N(2)-Cu-N(3)
N(3)-Cu-N(4)
N(2)-Cu-O(1)
N(4)-Cu-O(1)
170.11(8)
97.48(9)
81.47(9)
99.89(10)
86.74(9)
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Table 4. Hydrogen bonds for [Cu(L2)(OH2)](ClO4)2 (D, donor atom; A, acceptor atom).
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D-H···A
d(D-H)
(Å)
0.85
0.85
0.91
0.91
d(H···A)
(Å)
d(D···A)
(Å)
2.808(4)
2.841(4)
3.136(7)
3.091(5)
D-H·· A
()
166.0
156.6
141.7
144.4
O(1)-H(1A)···O(7)#1
O(1)-H(1B)···O(2)#1
N(1)-H(1)···O(5)#1
N(3)-H(3)···O(6)
1.98
2.04
2.37
2.31
Symmetry transformations used to generate equivalent atoms: #1 x+1/2,-y+1/2,z+1/2
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Table 5. The electronic absorption maxima of the complex in various solvents.
6
7
8
Solvent
λmax/nm (ε/lit cm-1 mol-1)
9
Solid state
560
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13
14
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16
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19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
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613 (103)
591 (83)
NM
NB
BN
DMSO
PN
598 (161)
662 (129)
658 (123)
591 (43)
AC
HMPA
DMF
MeOH
EtOH
AN
762 (176)
646 (121)
613 (129)
612 (77)
614 (166)
622 (65)
H2O
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Caption for Schemes and Figures
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Scheme 1. The complex under study
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Scheme 2. Interconversion of the complex triggered by solvent and temperature in aqueous
solution.
Fig. 1. ORTEP view of [Cu(L2)(OH2)](ClO4)2.
Fig. 2. Hydrogen bonding of [Cu(L2)(OH2)](ClO4)2 along the „a‟ axis.
Fig. 3. Temperature dependence of the visible absorbance of a DMSO solution of the complex.
Fig. 4. Absorption spectra of the complex in various solvents.
Fig. 5. The absorption spectra of the complex (3.0×10-3 mol L-1) upon addition of NaCl, NaBr,
NaI, NaCN, NaOCN, NaSCN and NaN3 (0.06 mol L-1) in aqueous solution.
Fig. 6. The pH-dependent visible spectra of the complex in aqueous solution at 25 C in the pH
range 7.2-5.8. The inset graph shows the decrease of the 590 nm absorbance on titration
with HClO4.
Fig. 7. The pH dependent visible spectra of the complex in aqueous solution at 25 °C in the pH
range of 5.8-2.2.
Fig. 8. Visible absorption spectra of the complex in alkaline solution (4.0×10-3 mol dm-3) at 25
C. The pH values are denoted in the figure. The inset graphs show the isosbestic points
observed in the pH range 7.2-11.4.
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Scheme 1
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Scheme 2
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Figure 1
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Figure 2
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Figure 3
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800
700
600
500
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300
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100
0
20 °C
40 °C
50 °C
60 °C
70 °C
80 °C
90 °C
100 °C
20°C
100°C
90°C
621 nm
375
475
575
675
775
875
λ(nm)
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Figure 4
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35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
BN
acetone
ETOH
NM
An
DMF
H2O
HMPA
MEOH
PN
0
400
450
500
550
600
650
700
750
800
850
λ(nm)
28

1
2
3
4
5
Figure 5
6
7
8
9
400
350
300
250
200
150
100
50
NaN3
Original soln.
NaI
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
NaCL
NaCN
Original
soln.
NaSCN
NaBr
NaN3
NaBr,
NaCl,
NaI,
NaCN
NaSCN
0
400
450
500
550
600
650
700
750
800
850
λ(nm)
29