Structure, magnetism and catecholase activity of the first dicopper(II) complex having a single μ-alkoxo bridge

Abstract

Pyridine-2,6-dimethanol (H₂L), which is a versatile donor has a neutral, monoanionic and dianionic coordination behaviour with two different coordination modes viz. tridentate and bidentate towards Cu(II), leading to three different geometric environments around Cu(II) centers. The single crystal X-ray structure reveals two different Cu(II) complexes with H₂L: in the first case ClO₄⁻ forms a polymer through intermolecular H-bonding, [Cu(H₂L)(HL)]ClO₄ (1) while the other one is the first report of a mono μ-alkoxo bridged dicopper(II) complex, [Cu(H₂L)(HL)Cu(L)]⁺ (2). The crystallographic asymmetric unit of 2 contains a mononuclear cation [Cu(H₂L)(HL)]⁺ and a dinuclear [Cu(H₂L)(HL)Cu(L)]⁺ unit along with two perchlorate anions, formulated as [Cu(H₂L)(HL)][Cu₂(H₂L)(HL)(L)] (ClO₄)₂. The magnetic characterization of 2 shows a very weak antiferromagnetic coupling between the Cu(II) centers in the dinuclear unit. This is the first report on the magnetic characterization of the first mono alkoxo-bridged dinuclear Cu(II) complex and, hence needs further examples to firmly establish magneto-structural correlations. The efficient catecholase activity of 2 (k_cat = 2.08 × 10⁴ h⁻¹) is attributed to the presence of a mono μ-alkoxo bridge, which holds two Cu(II) centers ∼3.26 Å apart and possibly facilitates the binding of catecholate during the electron transfer reaction. Moreover, the Cu2 center is square planar, and provides one free position for coordination by the substrate molecule.

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Introduction

Molecules with higher unpaired electrons are the source of new magnetic materials i.e. they function as magnetizable magnets¹ below a critical temperature, and also as a collection of extremely small magnetic particles of uniform size, in contrast to metal oxide nanoparticles, which have a range of particle sizes. The last two decades have witnessed an explosive growth in polynuclear complexes/clusters of 3d-metal ions with primarily oxygen- and/or nitrogen donor ligands. The coordination ability of pyridyl alcohols² and pyridyl oximes/dioximes³ leads to metal clusters, which inspired us to employ pyridine-2,6-dimethanol (H₂L), a well-known precursor for the synthesis of 3d-metal clusters and small molecule magnets (SMMs). Herein, we report two different Cu(II) complexes with H₂L: in the first case ClO₄⁻ forms a polymer through intermolecular H-bonding, [Cu(H₂L)(HL)]ClO₄ (1) while the other one is a dicopper(II) complex, [Cu(H₂L)(HL)Cu(L)]⁺ (2) having a mono μ-alkoxo bridge, which is observed for the first time, and the magnetic properties of which were studied. However, the already reported dinuclear Cu(II) complexes bearing a mono μ-hydroxo⁴ or μ-phenoxo^5–10 bridge show three possible geometric arrangements (Scheme 1): a bridge at the apical–apical position (Type I), basal–apical position (Type II) and basal–basal position (Type III). It is interesting to note that all of the reported Type III complexes exhibit strong anti-ferromagnetic coupling.

Scheme 1 Three possible geometries for dinuclear Cu(II) complexes having a mono μ-hydroxo or μ-phenoxo bridge. “A” represents vacant positions and/or different hetero atoms. “X” represents other donor atoms.

On the other hand, Cu(II) containing enzymes¹¹ viz. hemocyanin (O₂ transport), tyrosinase (hydroxylation of monophenols and oxidation of catechols) and catechol oxidase (oxidation of catechols) regulate the rate of several biochemical reactions in living systems. They are Type-III copper proteins and have magnetically coupled binuclear Cu(II) centers at their active sites. These metallo-proteins are very efficient catalysts for the selective oxidation of many organic compounds by the activation of dioxygen.¹² Tyrosinase oxidizes phenols into catechols (cresolase activity), and then catechols into o-quinones (catecholase activity), whereas catechol oxidase oxidizes catechols to o-quinones, which are highly reactive compounds that undergo auto-polymerization to produce melanin, a brown pigment, responsible for protecting the tissues of higher plants from damage against pathogens and insects.¹³

Recently, the X-ray crystal structure¹⁴ of the met form of the catechol oxidase of sweet potato reveals that the active site consists of a hydroxo bridged dicopper(II) centre in which each copper(II) centre is coordinated to three histidine nitrogens to adopt an almost trigonal pyramidal environment with one nitrogen at the apical site.

The catecholase activity depends upon several factors such as metal–metal distance, electrochemical properties, exogenous bridging ligand, ligand structure and pH of the medium.¹⁵ Recently, it was documented that the presence of a positive charge center close to the metal center(s) may enhance activity. Non-planar mononuclear, as well as binuclear Cu(II) complexes with Cu–Cu distances ranging from 2.9–3.3 Å show good catecholase activity. The binuclear complexes have generally been found to be more reactive than the former, pointing towards the possibility of a dicopper–catecholate adduct as an intermediate in the process. The coordination of catechol to Cu(II) favours the intra-molecular electron transfer that results in the release of o-quinone and the reduction of the Cu(II) to dicopper(I), which subsequently reacts with O₂ to restore the active form of the enzyme.^16,17

All of these facts inspired us to investigate and report the catecholase activity of 2.

Results and discussion

Single X-ray crystal structure of 1

Single crystals of 1, which were suitable for X-ray structure determination, were obtained from methanol solution. An ORTEP representation of 1, formulated as 2[Cu(H₂L)(HL)]ClO₄ is shown in Fig. 1. Its crystallographic asymmetric unit contains two hydrogen bonded cationic mononuclear complexes [Cu(H₂L)(HL)]⁺ and two perchlorate ions.

Fig. 1 An ORTEP view of 1. Hydrogen atoms and perchlorate anions are omitted for clarity (50% thermal ellipsoids).

It is interesting to note that each Cu(II) is coordinated to two 2,6-dihydroxymethyl pyridines, acting as tridentate (O,N,O-donor), however, one is in its neutral form (H₂L) and the other one is in its monoanionic mode (HL⁻). Accordingly, the geometry around Cu1 and Cu2 is distorted octahedral, due to the Jahn–Teller effect. In the coordination polyhedron of Cu2, the donor atoms N1, N2, O3 and O4 form the basal plane and O1 and O2 are situated at the apical positions. Similarly, N3, N4, O5 and O6 form the basal plane of the coordination polyhedron of Cu1, while O7 and O8 are situated at the apical positions. Moreover, noteworthy is the fact that between the two [Cu(H₂L)(HL)]⁺ cationic complexes, a very short hydrogen bond (O4⋯H–O6, ∼2.360 Å) exists, which is one of the shortest hydrogen bonds between oxygen atoms.¹⁸ Similarly, a very short hydrogen bond exists between the O-site of ClO₄⁻ and H-site of two different HL (Fig. 2). Fig. 3 shows that probably, hydrogen bonding is responsible for 32 single unit complexes to be present in one unit cell. This type of hydrogen bonded polymerization in a Cu(II) complex is also very rare. Selected bond angles and distances are collected in Table 1.

Fig. 2 Inter-molecular H-bonding between the O-center of ClO₄⁻ and H-center of HL.

Fig. 3 Unit cell of 1 along the (a) a-axis, (b) b-axis and (c) c-axis.

Table 1 Selected bond lengths [Å] and angles [°] for 1

Cu1–N3	1.916(2)	Cu2–N1	1.979(3)
Cu1–N4	2.009(3)	Cu2–N2	1.911(3)
Cu1–O5	2.015(2)	Cu2–O1	2.367(3)
Cu1–O6	1.979(16)	Cu2–O2	2.410(3)
Cu1–O7	2.237(7)	Cu2–O3	2.021(2)
Cu1–O8	2.395(3)	Cu2–O4	1.956(2)
O6–H6O	0.83(2)	O1–H1O	0.794(18)
O7–H7O	0.83(3)	O2–H2O	0.73(4)
O8–H8O	0.88(5)	O3–H3O	0.74(3)
Cl3–O13	1.428(3)	Cl4–O9	1.414(3)
Cl3–O14	1.412(3)	Cl4–O10	1.454(5)
Cl3–O15	1.432(2)	Cl4–O11	1.493(9)
Cl3–O16	1.408(3)	Cl4–O12	1.390(6)
N3–Cu1–N4	169.16(12)	N2–Cu2–N1	176.47(12)
O6–Cu1–O5	160.7(5)	O4–Cu2–O3	162.65(9)
O7–Cu1–O8	152.2(2)	O1–Cu2–O2	150.92(9)
N3–Cu1–O7	113.0(2)	N2–Cu2–O2	107.84(10)

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Single X-ray crystal structure of 2

Single crystals of 2, which were suitable for X-ray structure determination, were obtained from methanol solution. An ORTEP view of 2 (Fig. 4) shows that its crystallographic asymmetric unit contains a mononuclear cation [Cu(H₂L)(HL)]⁺ and a dinuclear [Cu(H₂L)(HL)Cu(L)]⁺ unit along with two perchlorate anions, formulated as [Cu(H₂L)(HL)][Cu₂(H₂L)(HL)(L)] (ClO₄)₂. Here, 2,6-dihydroxymethyl pyridine acts as a neutral, monoaninic or dianionic donor.

Fig. 4 An ORTEP view of 2. Hydrogen atoms and perchlorate anions are omitted for clarity (40% thermal ellipsoids).

The cationic mononuclear complex [Cu(H₂L)(HL)]⁺ has an N₂O₃ environment around Cu3, which is provided by a monoanionic tridentate O,N,O-donor and a neutral bidentate N,O-donor. Although a weak interaction between Cu3⋯O38 (∼2.575 Å) was detected, it is too long to be a true coordination bond.¹⁹ Thus, the geometry around Cu3 is distorted square pyramidal (τ = 0.23), in which the alcohol oxygen center O40 occupies the apical position. Selected bond angles and distances are noted in Table 2. On the other hand, in the cationic dinuclear complex [Cu(H₂L)(HL)Cu(L)]⁺, Cu1 is coordinated to two ligands, both acting as tridentate ones (NNO), but one in neutral form while the other one is in the monoanionic mode. Accordingly, the geometry around Cu1 is distorted octahedral, due to the Jahn–Teller effect. In this octahedron, the donor atoms N1, N11, O8 and O10 form the basal plane and O18 and O20 are in the apical positions. Cu2 is square planar, three of the coordination positions are occupied by a tridentate dianionic ligand and the fourth site is filled by O8, which acts as a bridge between Cu1 and Cu2. The square plane shows an umbrella like distortion with Cu2 0.194 Å above the mean calculated plane (distances of the O atoms to the mean calculated plane: O8 = −0.092 Å, O28 = −0.106 Å, O30 = −0.105 Å, N11 = −0.119 Å). Again, the weak interaction between Cu2⋯O38 (about 2.491 Å) is too long to be a true coordination bond and is best described as a secondary intermolecular interaction.^20,21 It is interesting to note that Cu1 and Cu2 are singly bridged by the alkoxo O8 center, with a Cu1–O8–Cu1 angle of about 114.7°. Thus, in the cationic dinuclear complex [Cu(H₂L)(HL)Cu(L)]⁺, the copper centers present different geometric environments, octahedral for Cu1 and square planar for Cu2, with an angle 89.17° between the calculated basal plane of the octahedron and the square plane.

Table 2 Selected bond lengths [Å] and angles [°] for 2

Cu1–N1	1.911(10)	Cu1–O10	1.995(8)
Cu1–N11	2.006(10)	Cu1–O20	2.348(9)
Cu1–O8	1.990(7)	Cu1–O18	2.371(9)
Cu3–N31	1.984(10)	Cu3–O48	1.936(8)
Cu3–N41	1.903(10)	Cu3–O50	1.998(8)
Cu3–O40	2.273(9)	Cu3⋯O38	2.575(9)
Cu2–O8	1.889(7)	Cu2–O30	1.968(8)
Cu2–N21	1.915(9)	Cu2–O28	1.984(9)
O8–Cu2–N21	161.9(3)	Cu2⋯O38	2.491(7)
O8–Cu2–O30	96.9(4)	N21–Cu2–O28	81.5(4)
N21–Cu2–O30	83.1(4)	O30–Cu2–O28	164.4(3)
N1–Cu1–O8	81.4(3)	N41–Cu3–O48	82.0(4)
N1–Cu1–N11	179.0(4)	N41–Cu3–N31	176.7(4)
O8–Cu1–N11	97.6(4)	O48–Cu3–N31	95.2(4)

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Magnetic studies

The magnetic properties of 2 were investigated in the temperature range, 2–300 K. A plot of χ_MT versus T (Fig. 5) reveals that χ_MT is 1.18 cm³ K mol⁻¹ at 300 K and that this product monotonously decreases with the decreasing temperature until ca. 14 K. Below this temperature, a more pronounced fall in the χ_MT value is observed, reaching a value of ca. 0.35 cm³ K mol⁻¹ at 2 K. This behavior indicates an overall antiferromagnetic coupling.

Fig. 5 Plot of χ_MT vs. T for 2: □: experimental results; solid line: best fit.

As previously discussed, the structure of 2 reveals that the unit cell contains three copper(II) ions, two of them being part of a dinuclear complex, with a unique O-bridge, and one belonging to a mononuclear compound. Accordingly, the susceptibility curve was treated with the MAGPACK program,²² in which the exchange spin Hamiltonian was expressed as H = −2ΣJ_ijS_iS_j, which choose the model for a dinuclear plus a mononuclear complex. The best fit of the data (Fig. 5, solid line) renders the following parameters: 2J = −6.3 cm⁻¹, g = 1.92, TIP = 5.0 × 10⁻⁴ cm³ mol⁻¹ (R = 5.35 × 10⁻⁴), in which 2J denotes the intra-molecular magnetic coupling in the dinuclear complex, [Cu(H₂L)(HL)Cu(L)]⁺.

The reliability of the 2J variable was analyzed by considering the Cu1 and Cu2 centers in [Cu(H₂L)(HL)Cu(L)]⁺ as octahedral and square planar, respectively, which are singly bridged by an O-donor, in the basal-basal position (Type III, Scheme 1). The magnetic orbital of Cu^II is d_x²_−y², thus this bridge may transmit a relatively important magnetic interaction. Accordingly, attempts have been made to compare the observed 2J value with those reported for mono O-bridged dinuclear Cu(II) complexes.^8–10,20 In mono-hydroxo bridged Cu(II) complexes with a square-planar environment, some magneto-structural correlations have been established.^4,23 It is found that antiferromagnetic coupling increases with the Cu–O–Cu angle, Cu⋯Cu distance and Cu–O–Cu/Cu⋯Cu ratios. These factors also seem to play an important role in the magnetic exchange coupling for mono phenoxo bridged Cu(II) complexes (Table 3).^5–10 In addition, Table 3 reveals that the dihedral angle between the basal planes influences the exchange coupling, as already demonstrated for dinuclear complexes with a Cu₂O₂ core.²⁴ Thus, it seems that the wider the dihedral angle, the stronger the antiferromagnetic coupling will be.

Table 3 Comparison of some magneto-structural parameters for Cu(μ-OPh)Cu complexes^c

Complex^a	Cu–O–Cu (°)	Cu⋯Cu (Å)	2J^b (cm^−1)	Ratio^d	θ^e
a Ligands L^x (x = 1–6) are 2,6-R-phenol derivatives, with different R podant arms.b 2J values referred to Hamiltonian H = −2JS1S2.c Different types of μ-phenoxo bridge shows in Scheme 1.d Cu–O–Cu/Cu⋯Cu ratio.e Dihedral angle between basal coordination planes; *average values. Excluded [Cu2(L^1)(Cl)2]^+, which is Type I and [Cu2(L^2)(H2O)2]^3+ and H[Cu2(L^4)(H2O)2] which are Type II, all of the remaining complexes are Type III.f nd: not described.
[Cu2(L^1)(Cl)2]^+ (ref. 5)	nd^f	nd	0
[Cu2(L^2)(H2O)2]^3+ (ref. 6)	nd	3.785	+4.2
H[Cu2(L^4)(H2O)2]^7	127.9	3.726	∼0
[Cu2(L^3)(N3)3][Cu2(L^3)(N3)2]^+ (ref. 13)	133.4*	3.630	−280	36.7	83.6
[Cu2(L^3)(NO2)2(H2O)2]^+ (ref. 13)	130.1	3.603	−244	36.1	83.7
Na[Cu2(L^4)(Py)2]^8	123.7	3.531	−168	35.0	nd
[Cu2(L^5)(SCN)(H2O)]^+ (ref. 9)	111.5	3.291	−109.8	33.9	81.2
[Cu2(L^5)(NCO)(H2O)]^+ (ref. 9)	109.6	3.244	−95.9	33.8	81.8
[Cu2(L^5)(N3)(H2O)]^+ (ref. 9)	109.3	3.244	−103.9	33.7	81.7
[Cu2(L^6)(NO3)(MeOH)0.3(H2O)0.7]^+ (ref. 10)	109.96*	3.20	−84.2	34.4	80.1

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From magnetic point of view, the present case is equivalent to the square-planar environment around Cu(II) (Type III, Scheme 1), the 2J value appears to be too low, given that for the Cu–O–Cu angle, 114.7° and Cu⋯Cu distance, 3.266 Å (ratio = 35.1), a value of about −100 cm⁻¹ should be expected.

In view of the low 2J parameter in 2, the chosen magnetic model was reappraised considering the possibility of a magnetic interaction between the Cu2 and Cu3 centers through the oxygen donor O38. Accordingly, a new attempt to set the data with a 2 J-model was performed using the MAGPACK program. With this model, the best fit gives a nearly superimposed curve with that of the 1 J-model but renders identical 2J₁ and 2J₂ (−4.0 cm⁻¹) parameters, with a g value of 1.92. This result seems unrealistic, given that 2J₁ represents a single-O basal–basal magnetic interaction and 2J₂ represents a single-O apical–apical magnetic coupling, with both apical Cu–O having very long distances.

Therefore, some theoretical calculations utilising the crystallographic coordinates of 2 were performed with the CACAO program,²⁵ to have deeper insight on the structural and magnetic situation of 2. The frontier orbitals of 2 are depicted in Fig. 6. These studies reveal that 2, as already described, is a dinuclear plus a mononuclear complex, with no bridges between Cu2 and Cu3. Thus, the correct magnetic interpretation is the first one, with the low 2J value of −6.3 cm⁻¹. Therefore, this is the lowest antiferromagnetic coupling ever reported for a single-O bridge dinuclear Cu(II) complex, which indicates that in addition to the reported magnetostructural correlations for this type of complex, other structural parameters must play an important role. Accordingly, maybe the –CH₂– moiety attached to the bridging oxygen atom significantly influences the magnetic exchange and more examples of alkoxo single bridged complexes are necessary in order to explain this low antiferromagnetic coupling.

Fig. 6 Frontier molecular orbitals for 2.

Electrochemical studies

The cyclic voltammogram of complex 1 (Fig. S1, ESI†) displays one irreversible peak (E_1/2) at +0.038 V in methanol vs. SCE due to the Cu(II)/Cu(I) couple, which is associated with the ΔE_P value of 564 mV indicates the irreversible redox process.

On the other hand, complex 2 (Fig. S1, ESI†) displays two redox processes, one reversible peak (E_1/2) at +0.0035 V associated with the ΔE_P value of 1 mV and another irreversible peak (E_1/2) at +0.447 V with a ΔE_P of 115 mV.

Kinetic studies for catechol oxidase activity

Among different catechols used for model studies, 3,5-di-tert-butylcatechol (DTBC) is the most widely used because of its low redox potential, i.e. easily oxidized to quinone (DTBQ), having high stability with maximum absorbance at 401 nm in methanol.¹⁶ At first, the ability of both of the Cu(II) complexes to oxidize 3,5-DTBC was evaluated. For this purpose, 1 × 10⁻⁴ mol dm⁻³ solutions of complexes 1 and 2 were treated with 1 × 10⁻² mol dm⁻³ (100 equivalents) of 3,5-DTBC under aerobic conditions in methanol. For complex 2, the absorbance of the quinone band increases with time (Fig. 7).

Fig. 7 Increase in the quinone band at 400 nm after the addition of 100 fold 3,5-DTBC (1 × 10⁻² M) to a methanol solution of complex 2 (1 × 10⁻⁴ M). The spectra were recorded at an interval of 5 min.

The complex 1 also behaves similarly (Fig. S2, ESI†). Then, the kinetics of DTBC oxidation was monitored by the initial rate method, measuring the increase in absorbance of the product 3,5-DTBQ at 400 nm in methanol at 25 °C. Maintaining the concentration of Cu(II) complex at a specific value, 1 × 10⁻⁴ mol dm⁻³, the concentration of the substrate, DTBC was varied from 1 × 10⁻³ to 1 × 10⁻² mol dm⁻³. Then, 2 mL substrate solution was quickly mixed with 0.04 mL of 5 × 10⁻³ mol dm⁻³ Cu(II) complex, such that final concentration of Cu(II) complex in the mixture became 1 × 10⁻⁴ mol dm⁻³.

The rate constant was determined from the log[Aα/(Aα − At)] vs. time plot. To evaluate various kinetic parameters, as well as rate dependency on the substrate concentration, solutions of 1 and 2 were treated with different concentrations of DTBC (from 10 to 100 equiv.) under aerobic conditions. The results are given in Table S1 (ESI†). At low DTBC concentration, a first-order dependence on the substrate concentration is observed but at higher concentrations, saturation kinetics is found for both the complexes 1 and 2 (Fig. 8 and S3, ESI†).

Fig. 8 Rate vs. substrate concentration plot for complex 2. Inset: Lineweaver–Burk plot.

The rate constants vs. substrate concentration results were then analyzed using the Michaelis–Menten approach of enzymatic kinetics to obtain the Lineweaver–Burk (double reciprocal) plot. The binding constant (K_M), maximum velocity (V_max), and rate constant (i.e., turnover number, k_cat) are calculated for both the complexes 1 and 2 using the Lineweaver–Burk graph: 1/V vs. 1/[S] (Fig. 8 and S2, ESI†), using the equation 1/V = {K_M/V_max} × {1/[S]} + 1/V_max. The kinetic parameters are presented in Table 4.

Table 4 Kinetic parameters of complex 1 and 2

Complex	Solvent	Vmax (M s^−1)	KM (M)	kcat (h^−1)
1	Methanol	2.53 × 10^−4	2.62 × 10^−3	9.11 × 10^3
2	Methanol	5.77 × 10^−4	1.20 × 10^−3	2.08 × 10^4

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Although the real mechanism of the reaction may be rather complicated, the data obtained from the Lineweaver–Burk plot are satisfactory for catalytic activity measurements. Complex 2 provides the k_cat (2.08 × 10⁴ h⁻¹), which is comparable to the value reported by the Das group²⁶ (1.08 × 10⁴ h⁻¹ to 3.24 × 10⁴ h⁻¹), however, significantly higher than the value reported by Krebs et al.²⁷ (4 h⁻¹ to 214 h⁻¹). The higher initial rate value for complex 2 may be due to the presence of a mono μ-alkoxo bridge,^27a holding the Cu(II) centers ∼3.26 Å apart, which possibly facilitates the binding of catecholate during the electron transfer reaction. Moreover, in complex 2, Cu2 is square planar resulting in one free position for the coordination of the substrate molecule.¹⁷

Conclusions

The synthesis, structural as well as magnetic characterization and catecholase activity of a Cu(II) complex containing a dinuclear plus a mononuclear motif in a single unit cell are presented. The single crystal X-ray structure reveals several interesting features, such as three different geometric environments around Cu(II) centers, two different coordination modes (tridentate and bidentate) of the H₂L ligand, and the versatility of the donor sites to act as neutral, monoanionic and dionionic. The magnetic characterization shows a very weak antiferromagnetic coupling between the Cu(II) centers in the dinuclear unit of 2. It is remarkable that, to the best of our knowledge, the present study provides the magnetic analysis of the first mono alkoxo-bridged dinuclear Cu(II) complex. The unexpected results make it necessary for further examples of this type to firmly establish magneto-structural correlations. The efficient catecholase activity of 2 (k_cat = 2.08 × 10⁴ h⁻¹) is attributed to the presence of a mono μ-alkoxo bridge, holding two Cu(II) centers ∼3.26 Å apart, facilitating the binding of catecholate during the electron transfer reaction. Moreover, the Cu2 center is square planar, and provides one free position for coordination by the substrate molecule.

Experimental

Materials and methods

All experiments were carried out in aerobic conditions. High-purity pyridine-2,6-dimethanol and Cu(ClO₄)₂·6H₂O (Merck, India) were used. Spectroscopic grade solvents were used. Other analytical reagent grade chemicals were used without further purification except when specified. A Shimadzu Multi Spec 1501 spectrophotometer was used to record UV-vis spectra. FTIR spectra are recorded on a Perkin Elmer FTIR (model RX1) spectrophotometer at 25 °C using KBr pellets. Elemental analyses were performed using a Perkin Elmer CHN analyser with the first 2000-analysis kit. Mass spectra were obtained using a QTOF Micro YA 263 mass spectrometer in the ES positive mode. Cyclic voltammetry (CV) experiments were recorded on a CHI620D potentiometer using the three-electrode configuration: a platinum disk working electrode, a platinum wire auxiliary electrode and a calomel reference electrode. The electrochemical measurements were carried out in methanol solution, extensively purged with N₂ prior to the measurements using 0.1 M TBAP as the supporting electrolyte. All potentials were determined at a scan rate of 100 mV s⁻¹ at room temperature. Thermal studies were performed with a Perkin-Elmer Diamond TG/DT analyzer in the temperature range from 30 °C to 700 °C under nitrogen. The magnetic susceptibilities were measured on pulverised crystalline samples with a Quantum Design SQUID MPMS-XL susceptometer operating in the 2–300 K temperature range under magnetic fields of 500 G (2–30 K) and 10 000 G (2–300 K). The diamagnetic corrections are evaluated from Pascal's tables. The agreement factor is based on the function R = Σ(χ_MT_exp − χ_MT_cal)²/Σ(χ_MT_exp).²

Synthesis of the compounds

Synthesis of 1. A methanol solution of Cu(ClO₄)₂·6H₂O (133.1 mg, 0.35 mmol, 5 mL) was slowly added to a solution of H₂L (100.0 mg, 0.71 mmol, 5 mL) in methanol under stirring and the reaction was continued for 30 minutes. Then, the mixture was maintained at room temperature to allow slow evaporation of methanol to give blue crystals after 5 days. Yield: 60%. Anal. calcd (%): C, 38.19; H, 3.89; N, 6.36; found: C, 38.27; H, 3.74; N, 6.49. ESI-MS (m/z): calcd 440.29; found, 440.42. UV-vis (Fig. S4, ESI†): λ (nm) in MeOH (ε, M⁻¹ cm⁻¹), 265 (15 725).

Single crystal X-ray structure analysis confirms the structure of the complex. FTIR of 1 also supports the presence of ClO₄⁻ (Fig. S5, ESI†); υ = 1014.5, 1047.3, 1141.8. Fig. S6 (ESI†) shows the thermal stability and decomposition pattern for 1.

Synthesis of 2. A methanol solution of Cu(ClO₄)₂·6H₂O (157.8 mg, 0.42 mmol, 5 mL) was added slowly to a solution of H₂L (100.0 mg, 0.71 mmol, 5 mL) in methanol under stirring and the reaction was continued for 30 minutes. Then, the mixture was maintained at room temperature to allow slow evaporation of methanol to give blue crystals after 10 days. Yield: 65%. Anal. calcd (%): C, 37.88; H, 3.50; N, 7.41; found: C, 38.07; H, 3.65; N, 7.57. ESI-MS (m/z): calcd 1081.26; found, 1081.25. UV-vis (Fig. S4, ESI†): λ (nm) in MeOH (ε, M⁻¹ cm⁻¹), 265 (25 829); 386 (810).

Single crystal X-ray structure analysis confirms the structure of the complex. FTIR spectrum of 2 also supports the presence of ClO₄⁻ (Fig. S7, ESI†); υ = 1090.1, 1111.9, 1141.8. Fig. S8 (ESI†) shows the thermal stability and decomposition pattern for complex 2.

X-ray crystallography

The diffraction data for 1 and 2 were collected on a Bruker X8 APEXII CCD diffractometer at 100(2) K, using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). Some significant crystal parameters and refinement data are summarized in Table S2 (ESI†). Data were processed and corrected for Lorentz and polarization effects. Structures were solved by the standard direct methods²⁸ and refined by full matrix least squares on F².²⁹ All non-hydrogen atoms were anisotropically refined. Hydrogen atoms were included in the structure factor calculation in geometrically idealized positions, with thermal parameters depending on the parent atom, using a riding model.

Acknowledgements

S. Adhikari and S. Nandi are thankful to Department of Environment, Govt. W. B. and UGC, New Delhi for fellowships. A. Banerjee is thankful to CSIR for fellowship. Thanks to Sandip Mandal for his generous help.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Crystal parameters, rate of catecholase activities, UV-vis, FTIR spectra, cyclic voltammograms and thermograms of 1 and 2. CCDC 1033744 and 953617. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra14603d

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