Polish J. Chem., 74, 813–822 (2000)
Stability and ESR Spectra of Ni(III) Tetraazamacrocyclic
Complexes in Nitrate and Chloride Solutions
by J. Taraszewska1*, J. Sad³o2, J. Michalik2 and B. Korybut-Daszkiewicz3
1
Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
2
Institute of Nuclear Chemistry and Technolgy, Dorodna 16, 03-195 Warsaw, Poland
3
Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
(Received January 5th, 2000)
Complexes [NiIII(cyclam)Cl2]Cl (1), [NiIII(cyclam)(NO3)2]ClO4 (2) and [NiIII(2-methyl-cyclam)(NO3)2]ClO4 (3) were isolated and the stability of Ni(III) was studied by
UV-VIS spectrophotometry as a function of NaCl and NaNO3 concentration. In complexes 2 and 3 the decay of Ni(III) followed the first order kinetics in aqueous and in nitrate solutions up to 1 mol/dm3. In complex 1 the first order kinetics was observed only in
aqueous and saturated NaCl solutions. With increase in NaNO3 concentration the stability of Ni(III) in complexes 2 and 3 increased however, in complex 3 it was lower than in
complex 2. Stability of Ni(III) in complex 1 increased also with increasing NaCl concentration but it diminished starting from 2 mol/dm3 NaCl. The forms of complexes depending on the salt concentration were characterized by the ESR technique.
Key words: Ni(III) complexes, Ni(III) ESR, tetraazamacrocyclic complexes, synthesis
The chemistry of Ni(III) complexes attracts continuously much attention due to
its role in catalytic oxidation reactions [1–7] and in biological systems [8–10]. Numerous tetraazamacrocyclic Ni(III) complexes have been prepared by chemical,
radiolytic or electrochemical oxidation of the corresponding Ni(II) species and characterized by UV-VIS and ESR techniques [11–13]. Among various factors, such as
pH, nature of solvent and complex structure, which exert considerable influence on
the stability of Ni(III) azamacrocyclic complexes in solutions, an important role
plays also the type of anion apically coordinated to the Ni(III) centre. Since the discovery of Gore and Busch [14] that the axial coordination of simple ligands stabilizes
Ni(III) complexes dramatically, a lot of work has been done on this subject. Meyerstein and coworkers in a series of papers [15–20] have shown that simple inorganic
anions, coordinated axially to the Ni centre, are able to stabilize the Ni(III) oxidation
state in complexes with 14-membered N4 macrocyclic ligands. Their results for
meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane indicated a sig–
2nificant effect of SO 24 , C 8 H 4O 4 (phthalate), H 2 PO 4 and Cl ions on the stability of
Ni(III) in this complex. Also stability of Ni(III) in cyclam increased significantly in
acidic (pH = 1.6) sulfate solutions. Recently, the stabilization of Ni(III) in 1,8dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane by axial binding of anions in neu* Corresponding author; E-mail: [email protected]
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J. Taraszewska et al.
tral aqueous solutions has been studied by Zilbermann et al. [20]. Lampeka and
Rosoha [21] compared the stabilizing effect of some inorganic anions on the life time
of Ni(III) in macrocyclic and non-cyclic tetraamines. Recently, we have studied the
kinetic and thermodynamic stability of Ni(III) in polyazacyclotetradecane complexes as a function of pH, NaClO4 concentration and solvent [22].
The aim of this work was the synthesis of complexes [NiIII(cyclam)Cl2]Cl (complex 1), [Ni III (cyclam)(NO 3 ) 2 ]ClO 4 (complex 2), and [Ni III (2-methyl-cyclam)(NO3)2]ClO4 (complex 3), where (cyclam = 1,4,8,11-tetraazacyclotetradecane),
and the study of Ni(III) stability in pure aqueous solutions and as a function of NaCl
and NaNO3 concentration. The questions to be answered concern also the form of
complexes present as well as their decomposition products. The stability of Ni(III)
was followed by the decay of UV-VIS spectra and the forms of complexes in solutions were characterized by the ESR technique.
EXPERIMENTAL
Materials: All chemicals were Merck p.a. grade reagents. Triply distilled water was used. The third
distillation was carried out from an all-quartz still.
Synthesis: Macrocyclic tetraamine nickel(II) complexes have been prepared by mixing equimolar
amounts of NiCl2×6H2O and appropriate ligand in methanol. 2-Methylcyclam was obtained by analogy to
known cyclam [23] synthesis with 2-methylglioxal instead of glioxal [24].
1,4,8,11-Tetraazacyclotetradecanenickel(III) trichloride [Ni [14]aneN4Cl2]Cl, (1): 1 g of 1,4,8,11tetraazacyclotetradecanenickel(II) dichloride was dissolved in 50 cm3 of methanol containing 2.5 cm3 of
concentrated hydrochloric acid. After addition of 2.5 cm3 of 30% hydrogen peroxide the mixture was
stirred for 1 hour at room temperature. Precipitated brown crystalline product (1.10 g) was filtered off,
crystallized from methanol-acetone solution and dried under reduced pressure. Anal. calcd. for
C10H24N4Cl3Ni: C, 32.9; H, 6.6; N, 15.3; Cl, 29.1; found: C, 32.6; H, 6.9; N, 15.2; Cl, 29.0; IR (Nujol):
3137 cm–1 (nN–H).
2-Methyl-1,4,8,11-tetraazacyclotetradecanenickel(III) trichloride [Ni 2-Me[14]ane N4Cl2]Cl×H2O
was obtained by H2O2/HCl oxidation of [Ni 2-Me[14]aneN4]Cl2 according to the same procedure and was
crystallized from methanol-acetone solutions (yield 90%). Anal. calcd. for C11H26N4Cl3Ni×H2O: C, 33.2;
H, 7.1; N, 14.1; Cl, 26.8; found: C, 32.4; H, 7.3; N, 14.2; Cl, 27.5; IR (Nujol): 3131 cm–1 (nN–H).
1,4,8,11-Tetraazacyclotetradecanenickel(III) dinitrate perchlorate [Ni 14]aneN4(NO2)2]ClO4, (2):
Silver nitrate (1 g) was added to solution of [Ni [14]aneN4Cl2]Cl (1 g) in minimal amount of water. The
precipitated silver chloride was filtered off and 72% perchloric acid (2 cm3) was added to the solution.
Green precipitate (0.6 g) was filtered off, washed with methanol and dried under vacuum. Anal. calcd. for
C10H24N6ClO10Ni: C, 24.9; H, 5.0; N, 17.4; Cl, 7.4; found: C, 25.0; H, 5.2; N, 17.4; Cl, 7.4; IR (Nujol):
3184 cm–1 (nN–H), 1276 and 1028 (nNO2), 1092 (nClO4) and 625 (dClO4).
2-Methyl-1,4,8,11-tetraazacyclotetradecanenickel(III) dinitrate perchlorate [Ni 2-Me[14]aneN4(NO2)2]ClO4 (3) has been obtained from [Ni 2-Me[14]aneN4Cl2]Cl by the same procedure.
Anal. calcd. for C11H26N6ClO10Ni: C, 26.6; H, 5.3; N, 16.9; Cl, 7.1; found: C, 26.3; H, 5.5; N, 16.7; Cl, 7.4;
IR (Nujol): 3179 cm–1 (nN–H), 1280 and 1031 (nNO2), 1092 (nClO4) and 625 (dClO4).
Decomposition of [Ni [14]aneN4Cl2]Cl: [Ni [14]aneN4Cl2]Cl (1.0 g) and Na2CO3 (0.2 g) were dissolved in 100 cm3 of water maintained at room temperature. After 4 h the mixture was absorbed on a SP
Sephadex C-25 column, washed with water and eluted with 0.3 mol/dm3 NaClO4 solution. Two fractions
were collected, concentrated and left for crystallization. The first very small fraction (0.01 g) was characterized by 1H and 13C NMR in CD3NO2 solution and was identified as Ni(II) complex of 1,4,8,11tetraazacycloteradeca-7-ene. 13C NMR (d in ppm, 50.4 MHz): 175.9 (7C), 27.6 (13C), 33.2 (6C), 45.1,
50.3, 50.6, 52.4, 52.4, 53.2, 61.3; 1H NMR in CD3NO2 (d in ppm, 200 MHz): 7.81, 1H broad doublet
Stability and ESR spectra of Ni(III) tetraazamacrocyclic...
815
(7-CH); 1.98, 1H broad doublet and 1.47, 1H quartet of triplets (13-CH2 group, J = 16 Hz), 2.1–3.8 (remaining protons). The second major fraction contained [Ni [14]aneN4](ClO4)2 (0.7 g).
Instrumentation: The UV-VIS spectra were measured by Cary 1E (Varian) spectrophotometer.
Measurements were made at 25±0.2°C in non-deareated solutions. ESR spectra were recorded by Bruker
ESP 300 X-band spectrometer with a maximum microwave power of 200 mV. The magnetic field was
modulated at 100 kHz, and measured using a Bruker ER035M Gaussmeter. For absolute g-value determination, a calibration using DPPH (diphenylpicrylhydrazyl) at 0.1 mW (g = 2.0036) was done. IR spectra
(paraffin oil mulls) were recorded on a Perkin Elmer 1600 FTIR spectrometer. NMR spectra were measured using Varian-Gemini 200 spectrometer.
Procedure: The absorbance changes of the green solutions of Ni(III) complexes were monitored at
the wavelengths 306–310 nm. In solutions, in which the decay of Ni(III) followed a first-order kinetics,
plots of the absorbance changes vs. time were approximated by A(t) = Aoexp(–kt) up to 3 or 4 half-times.
Samples of complexes for ESR measurements were frozen 10 min. after dissolution.
RESULTS AND DISCUSSION
Stability of complex 1: The kinetic stability of Ni(III) in complex 1 was studied
in pure aqueous solution and in function of NaCl concentration up to saturated solution by monitoring the absorbance changes at the wavelength 306–310 nm. The decay
of Ni(III) was described by a first-order rate law only in pure aqueous and in saturated
NaCl solutions (t1/2 » 7 h, and 27 h, respectively). The absorption spectrum recorded
in pure aqueous solution is shown in Fig. 1. It exhibits a maximum at 306 nm, shoulder at 362 nm, and an isosbestic point at 236 nm. In solutions containing other NaCl
concentrations the kinetics of Ni(III) decomposition did not obey a simple first or second order rate law. These results are in agreement with observations of Zeigerson et
al. [19], who studied the stability of Ni(III) in meso-5,7,7,12,14,14-hexamethyl1,4,8,11-tetraazacyclotetradecane in solutions containing chloride ions up to 0.3
mol/dm3. The decay of absorption band at 308–310 nm for complex 1, measured in
function of NaCl concentration and additionally at two HCl concentrations, is presented in Table 1. In agreement with literature Ni(III) is very stable in acidic solutions. Its stability is also relatively high in NaCl solutions in the range 0.1–1 mol/dm3.
In more concentrated NaCl solutions the stability of Ni(III) starts to diminish. The
data in Table 1 are presented only to point out the relative stabilities of Ni(III) in complex 1 and to point out that they are relatively long lived.
Table 1. The decay (in %) of absorption band at 308–310 nm with time for complex 1 in dependence on NaCl
and HCl concentration.
medium [mol/dm3]
0.1 NaCl
0.5 NaCl
1.0 NaCl
2.0 NaCl
3.0 NaCl
0.1 HCl
1.0 HCl
5h
9
11
9
22
20
3
3
24 h
24
20
18
30
38
6
4
75 h
40
30
35
47
12
6
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J. Taraszewska et al.
Figure 1. The UV-VIS spectrum of complex 1 in pure aqueous solution: curves 1–8 recorded every 1 h,
curve 9 recorded after 11 h, curve 10 recorded after 23 h.
The mechanism of decomposition of Ni(III) macrocyclic tetraamine complexes
was intensively investigated [25–28]. The first step of decomposition is connected
with dehydrogenation of one of the four equivalent amine groups in the macrocyclic
ligand. In the presence of axially coordinated to Ni(III) anions, the acidity of protons
at the amine groups is expected to decrease [17]. This may be the reason of increasing
stability of Ni(III) in such type of complexes.
To determine the decomposition products, complex 1 was decomposed by addition of sodium carbonate solution of pH = 8. As final decomposition products were
isolated [NiII(cyclam)](ClO4)2 and its monoimine analogue, although not in quantitative amounts (see Experimental).
Stability of complexes 2 and 3: After the dissolution of complex 2 in water the
UV-VIS spectrum was similar to that recorded for [NiIII(cyclam)(H2O)2](ClO4)3 produced by electrolysis [22]. It is shown in Fig. 2. The rate of Ni(III) decay in this complex followed the first order kinetics with the value of t1/2 » 3 h. With increase in
NaNO3 concentration the stability of Ni(III) in complex 2 increased. In 0.4 mol/dm3
NaNO3 t1/2 was about 5 h and in 1 mol/dm3 NaNO3 increased to about 8 h. With rise in
NaNO3 concentration the shape of the spectrum did not change, however, the disturbing influence of NO -3 anions was observed in the range 200–240 nm. Stability of
Ni(III) in complex 3 was lower. In pure aqueous solution t1/2 was » 1.8 h and in 0.2
mol/dm3 NaNO3 it increased to 3 h. This is consistent with our previous results [22],
and with the observation of Lovecchio et al. [29]. The lower kinetic stability of com-
Stability and ESR spectra of Ni(III) tetraazamacrocyclic...
817
Figure 2. The UV-VIS spectrum of complex 2 in pure aqueous solution: curves 1–8 recorded every 1 h,
curve 9 recorded after 23 h.
plex with axially oriented methyl group on a 6-membered chelate ring may be due to
the steric repulsion between this substituent and axially coordinated to Ni(III) anions.
The presented results show that an increase in NaNO3 concentration stabilizes the
Ni(III) species. This is opposite to our previous results [22], where an increase in
NaClO4 concentration caused destabilization of Ni(III) in cyclam complex with two
axially coordinated water molecules produced by electrolysis.
ESR spectra: ESR spectra at 77 K of complex 1 in pure aqueous solution and in
solutions with various concentrations of NaCl are presented in Fig. 3. Spectrum in
pure frozen aqueous solution displays a single anisotropic line A with axial symmetry
of g values: g^ = 2.189 and gçç = 2.025 (Fig. 3a). For NaCl solutions in the concentration range 0.1–2 mol/dm3 the parallel component of singlet B is clearly splitted in
seven lines with hyperfine splitting Bêê = 26 G (Fig. 3b). The perpendicular component does not exhibit any splitting, however, the line width between its maximum and
minimum is distinctly larger (34 G) than that for aqueous solution (25 G). Both chlorine isotopes 35Cl and 37Cl have spin I = 3/2 and natural abundances of 75.4% and
24.6%, respectively. Although they have different magnetic moments µ(35Cl) = 0.821
and µ(37Cl) = 0.683, we were unable to detect lines from the two isotopes. In 3
mol/dm3 and more concentrated NaCl solutions (Fig. 3c) the splitting of the parallel
component disappears, although the g^ and gúú values do not change. For solid sample
818
J. Taraszewska et al.
Figure 3. The ESR spectra at 77 K of complex 1 dissolved in: a) H2O, b) 0.1–2 mol/dm3 NaCl,
c) 3 mol/dm3 NaCl.
of complex 1 we recorded the structureless singlet with g^ = 2.192 and gçç = 2.023 as a
dominant ESR signal.
The ESR spectra of complex 1, dissolved in solutions with different concentrations of NaNO3, are shown in Fig. 4. In 0.1 mol/dm3 NaNO3 solution the dominant
signal is an anisotropic singlet B (g^ = 2.196, gúú = 2.092) with parallel component
splitted to seven lines (Fig. 4a). Broad lines denoted as D possibly belong to an ESR
singlet with the orthorhombic symmetry (gx = 2.189, gy = 2.132, gz = 2.092). The
spectrum with similar g values was recorded by Desideri et al. [30] after treatment of
[NiIII(cyclam)Cl2]Cl complex in DMSO with small amount of AgClO4 and was assigned to monochloro-Ni(III)-cyclam complex. In 0.2 mol/dm3 NaNO3 solution dramatic spectral changes are seen (Fig. 4b). Signal D disappears completely and the
observed spectrum is a superposition of three different singlets: B – with parallel
component, splitted to seven lines, E – with g^ = 2.21, and gúú = 2.023 splitted to four
lines and F – with g^ = 2.234 shifted apparently to low field and with the unchanged
gúú = 2.023. Both line components of singlet F are narrow. They do not show any splitting and their intensity grows with the increase of NaNO3 concentration. Singlet F is
dominant in 1.5 mol/dm3 NaNO3 solution but signal E is still recorded (anisotropic
singlet E is clearly seen in Fig. 4c, when signal B completely disappears).
Stability and ESR spectra of Ni(III) tetraazamacrocyclic...
819
Figure 4. The ESR spectra at 77 K of complex 1 dissolved in: a) 0.1 mol/dm3 NaNO3, b) 0.2 mol/dm3
NaNO3, c) 1.5 mol/dm3 NaNO3.
The low temperature ESR spectra of complex 3, dissolved in water and in NaNO3
solutions, are shown in Fig. 5. In aqueous solution a broad singlet with Hpp » 160 G
and non-symmetrical shape was recorded (Fig. 5a). The intensity of this signal was
substantially reduced, when complex 3 was dissolved in 0.04 mol/dm3 NaNO3. At the
same time the anisotropic singlet F with g values (g^ = 2.235, gúú = 2.022), similar to
the signal presented in Fig. 4c, was recorded. This singlet appears as the unique ESR
signal in NaNO3 solutions with the concentration range 0.1–1.5 mol/dm3.
The presented results have shown a significant influence of the nature and the
concentration of electrolyte on the kinetic stability of Ni(III) and the forms of complexes present in solution. It was found that when complex 1 is dissolved in NaCl solutions with the concentration range 0.1–2 mol/dm3 the parallel component of the
ESR spectrum is splitted into seven lines. The Coulomb interaction between Ni(III)
and Cl– shortens the cation-axial ligand distance resulting in the localization of the
spin density at Cl– ions. This manifests in the ESR spectrum by the hyperfine splitting
of the parallel line component and the broadening of the perpendicular component.
The spectrum shown in Fig. 3b was nicely simulated with the following set of parameters: g^ = 2.182, gúú = 2.025 and A^ =8 G, Aúú = 28 G assuming interaction with two
equivalent chlorine nuclei I = 3/2 and Gaussian lineshape.
820
J. Taraszewska et al.
Figure 5. The ESR spectra at 77 K of complex 3 dissolved in: a) H2O, b) 0.04 mol/dm3 NaNO3, c) 0.1–1.5
mol/dm3 NaNO3.
Similar spectra were observed by Desideri et al. [30] for trans-dichloro Ni(III)
cyclam dissolved in dimethylsulfoxide and by Di Casa et al. [31] for dichloro Ni(III)
complex with N-cetylcyclam in methylene chloride. In both cases the spectra were assigned to Ni(III) cation coordinated in equatorial plane by four nitrogens and by two
chloride anions along the z axis. Complex of Ni(III) with two axially coordinated Cl–
ions is very stable (Table 1). Stability of such kind of complex was stated by Haines
and McAuley [32] and by Di Casa et al. [31].
The splitting of the parallel component in ESR spectra was not observed in 3
mol/dm3 and more concentrated NaCl solutions. Similar changes were observed by
McAuley and Subramanian [33] in the ESR spectra of dinuclear Ni(III)-6,6¢-spirobicyclam, when the concentration of HCl was changed from 1 mol/dm3 to 6 mol/dm3.
They speculated that the disappearance of hyperfine structure on parallel component
is related to the formation of the chloro-bridged polymer. We suppose that in the case
of complex 1 an increase in NaCl concentration to 3 mol/dm3 and higher may weaken
the coordination of Cl– ions to Ni(III) center, due to the competition of Cl– ions between Ni(III) and Na+ ions. Weaker coordination of Cl– ions may be the reason for the
decreasing the kinetic stability of Ni(III) in these solutions. In such concentrated
NaCl solutions one can also expect the formation of ion pairs [NiIII(cyclam)Cl2]×Cl.
The similarity of the ESR spectrum to that of the solid sample supports such an as-
Stability and ESR spectra of Ni(III) tetraazamacrocyclic...
821
sumption. Formation of ion pairs has been postulated by Fabbrizzi et al. [34] for
Ni(III)-N-cetylcyclam in CH2Cl2 in the presence of chloride ions. This may be an additional reason that the Ni(III) kinetic stability decreases in very concentrated NaCl
solutions.
The spectral changes presented in Fig. 4 can be explained by the gradual exchange of apically coordinated ligands in complex 1. In 0.1 mol/dm3 NaNO3 (Fig. 4a)
two forms of complexes may co-exist – complex 1 with two apical Cl– ligands and
complex with Cl– in one apical position and probably H2O in the second one. We postulate that in 0.2 mol/dm3 NaNO3 (Fig. 4b) three different complexes co-exist:
dichloro complex 1, monochloro complex having Cl– ion in one apical position and
NO -3 anion in another (hyperfine splitting of parallel component of signal E to four
lines unambiguously indicates the interaction with one Cl nucleus), and the dinitrate
complex with NO -3 anions in both apical positions represented by signal F. An increase in the intensity of this signal with increase in NaNO3 concentration strongly
supports the assignment of signal F to [NiIII(cyclam)(NO3)2]+ complex. In 1.5
mol/dm3 NaNO3 (Fig. 4c) this complex co-exists with monochloro complex having in
the second apical position the NO -3 anion (signal E).
The broad singlet presented in Fig. 5 for complex 3 in aqueous solution we assign
tentatively to the complex with different ligands in two apical positions, i.e. H2O and
NO -3 ions. Such a complex with a low symmetry should exhibit an ESR singlet with
orthorhombic character, however, when line components are very broad, the resultant
spectrum could appear as a singlet. The appearance of singlet F, for solutions containing NaNO3, confirms that it represents the complex with NO -3 anions in both apical
positions. The presence of 0.1 mol/dm3 NaNO3 is sufficient to prevent the exchange
of apical NO -3 ligands for H2O.
The interpretation of the ESR spectrum of complex 1 in pure aqueous solution (g^
= 2.189 and gúú = 2.025) is less straightforward. The lack of any hyperfine splitting of
the parallel component indicates that the unpaired electron does not interact with neither the nickel nor chlorine nuclei, what suggests that the axially coordinated Cl– ions
are replaced by H2O molecules. However, when we added an excess of AgClO4 to
aqueous solution of complex 1, a new spectrum with g^ = 2.238 and gúú = 2.023 was recorded. A similar spectrum was presented by Fabbrizzi et al. [35] for Ni(III) complex
with N-(aminoethyl)cyclam (g^ = 2.23, gúú = 2.02), in which in acidified aqueous solutions the coordinating to Ni(III) centre amino group was protonated and both apical
positions were occupied by water molecules. Therefore, taking into account the difference in g^ factor of ESR spectra discussed above, we suppose that in pure aqueous
solution the Cl– anions are not fully replaced by water but their interaction with
Ni(III) centre is very weak. This may explain, that the half life time of Ni(III) in this
complex is a little bit higher than in [NiIII(cyclam)]3+ with two apically coordinated
water molecules [22] and can be described by the first order rate law.
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J. Taraszewska et al.
Acknowledgments
We thank Mrs. S. Paw³owska for assistance in UV-VIS measurements.
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