Synthesis and spectral characterization of macrocyclic lanthanide(III) Complexes of an 18-Membered Tetraaza Tetraimine Schiff Bases

2.1 Materials and methods

4,5-Dimethyl-1,2-phenylenediamine (Aldrich) was used as such for the synthesis of complexes. 2,6-Diformyl-4-methylphenol was synthesized by the literature methods [33]. The lanthanide(III) nitrates, namely, Pr(NO₃)₃.6H₂O (99.8%), Nd(NO₃)₃.6H₂O (99.8%), Sm(NO₃)₃.6H₂O (99.8%), Eu(NO₃)₃.6H₂O (99.8%), Tb(NO₃)₃.6H₂O (99.8%), and Dy(NO₃)₃.6H₂O (99.8%) (Indian Rare Earth Ltd.), and Pr(NO₃)₃.6H₂O (99.8%), Gd(NO₃)₃.6H₂O (99.8%), Ho(NO₃)₃.6H₂O (99.8%), Er(NO₃)₃.6H₂O (99.8%), and Y(NO₃)₃.6H₂O (99.8%) (Aldrich) were used as such for the synthesis of complexes. p-Toluenesulfonyl chloride, p-cresol, tetraethylammonium perchlorate (Fluka), lithium hydroxide monohydrate, sodium dichromate dihydrate (LOBA), sodium carbonate, paraformaldehyde, formic acid, sodium hydroxide, sodium acetate, sodium sulfate anhydrous, and calcium chloride anhydrous (E. Merck) were used as purchased. Formaldehyde (38% v/v), glacial acetic acid, hydrochloric acid, sulfuric acid and fuming nitric acid (AR, E. Merck), were used as such. CDCl₃ (Aldrich), KBr (FT-IR grade) (Aldrich) were used for recording NMR and IR spectra respectively. Acetonitrile, N,N-dimethylformamide, chloroform, dichloromethane benzene, toluene (E. Merck), and methanol (SD’s) were reagent grade and purified according to the literature method [34]. Diethyl ether, dimethyl sulfoxide, acetone and petroleum ether (60–80C) (E. Merck) were used as purchased. Double deionized water was obtained by distilling, distilled water over alkaline potassium permanganate. Absolute ethanol was prepared by double distilling rectified spirit over lime and the fraction boiling at 78C was collected. Super dry ethanol was prepared by distilling absolute ethanol over magnesium turnings activated by iodine and the fraction boiling at 78C was collected. Super dry methanol was also prepared by the same method by collecting the fraction boiling at 65C. N,N-dimethylformamide, distilled over P₂O₅ and stored over 4Å molecular sieves, was used for the electrochemical studies.

2.2 Physical measurements

Electronic absorption spectra were recorded on a Perkin-Elmer Lambda 3B UV-Vis spectrophotometer attached to a PC AT-286 and the spectra were recorded in the 900–190 nm range using PECSS software. The spectra of the complexes and the ligand were recorded in acetonitrile at 25C using a matched pair of Teflon stoppered quartz cells of path length 1 cm. IR spectra were recorded in a Perkin-Elmer RX-I FT-IR spectrometer in the range of 4000–400 cm^-1 using KBr pellets. CHN microanalyses were carried out on a Perkin-Elmer 2400 CHNS/O analyzer and AD-6 Auto balance. Conductivity measurements of the complexes were carried out at 25±1C in N,N-dimethylformamide using Elico CM-180 conductivity meter and Elico Type CC-03 conductivity cell (cell constant 1.02 cm^-1). FAB mass spectra were recorded on a JEOL-SX 102/DA 600 mass spectrometer/data system using argon (6 kV, 10 mA). The accelerating voltage was 10 kV and the spectra were recorded at room temperature. m-Nitrobenzyl alcohol was used as the matrix solvent and the mass spectrometer was operated in the positive ion mode. The EPR spectra were recorded on a Varian E-LINE Century series EPR spectrometer at 77 K. Magnetic susceptibility measurements were carried out on an EG & G PAR MODEL 155 vibrating sample magnetometer at 25C. The instrument was calibrated using pure nickel. ¹ H NMR spectra were recorded on JEOL 200 MHz ¹H NMR spectrometer in CDCl₃. Electrochemical studies of the complexes were performed on a EG & G PARC model 273A potentiostat/galvanostat at 25C. A three-electrode configuration consisting of glassy carbon disc working electrode, a platinum wire auxiliary electrode, and Ag/AgCl reference electrode was used to record the cyclic voltammograms. The CVs were recorded using 10^-3 to 10^-4 M solutions of the complexes in dimethylsulfoxide containing 50 to 100 fold excess of tetraethylammonium perchlorate as the supporting electrolyte. All electrochemical studies were carried out in an atmosphere of oxygen free argon and the test solutions were flushed through argon for 30 min. before the voltammograms were recorded. Fluorescence study of the complexes were carried out on a Hitachi 650–40 Fluorescence Spectrophotometer in acetonitrile or N,N-dimethylformamide. Thermogravimetric studies were carried out using Stanton Redcroft STA-780 simultaneous thermal analyzer. The measurements were carried out on the platinum crucibles on sample size between 5–15 mg with high purity alumina as the reference. The heating rate was fixed at 10C/min.

2.3 Synthesis of the 18-membered dioxatetraaza diamine-diimine macrocycle (L1)

To a solution of 2,6-diformyl-4-methylphenol (1.64 g, 10 mmol) in 100 mL of methanol was added a solution of 4,5-dimethyl-1,2-phenylenediamine (1.36 g, 10 mmol) in 100 mL of methanol and the resulting red-orange solution was refluxed for 4 h, a shiny maroon red compound was separated out. The solution was cooled to room temperature (30C) and the product was filtered, washed with methanol, and dried under vacuum over anhydrous CaCl_2 . 

2.4 Characterization of the diamine-diimine macrocycle (L1)

Yield 1.47 g (56%) mp 302–304C. Analytically Calculated for C₃₄H₃₆N₄O₂: C, 76.6; H, 6.81; N, 10.52. Found C, 76.52; H, 6.77; N, 10.43. EI-MS (m/z, species): 532 M+ ion. ¹H NMR (200 MHz, CDCl₃) (ppm from TMS) a doublet at 4.4 ppm and a triplet at 6.2 ppm due to CH₂ and N-H protons, respectively. The singlets at 2.35, 8.55, and 13.57 ppm are due to the aromatic methyl, azomethine and phenolic protons, respectively. ¹³C NMR (200 MHz, CDCl₃) (ppm from TMS): 163.7, 155.7, 142.0, 136.2, 135.7, 132.5, 130.9, 129.5, 128.9, 126.6, 124.6, 118.2, 114.4, 47.2, 24.6, and 17.8. IR FT (KBr, cm^-1): 1615 s, ν(C=N): 3404 sp ν(NH); 3386 s, ν(OH); 2847 w, ν(CH). UV-Vis (CH₃CN) [λ_max (nm)(ɛ (Lmol^-1 cm^-1))]: 374 (15) 539), 349 (14 786), 265 (33 969).

2.5 Attempted synthesis of complexes of L1

The metal-free macrocyclic ligand L1 does not coordinate with lanthanide(III) metal ions either at ambient temperature or under refluxing conditions in nitromethane, acetonitrile, methanol, chloroform or ethanol. When the reaction is carried out in refluxing ethanol or methanol the free macrocyclic ligand isolates out from the reaction medium on cooling to room temperature [41].

2.6 General procedure for the synthesis of lanthanide(III) complexes of L2

To a solution of 2,6-diformyl-4-methyl-phenol (1.64 g, 10 mmol) and the respective lanthanide(III) nitrate hydrate (5 mmol) in 50 mL of methanol was added a solution of 4,5-dimethyl-1,2-phenylenediamine (1.36 g, 10 mmol) in 50 mL of methanol at room temperature (30C) and the solution was stirred on a magnetic stirrer. The color of the solution gradually changed from yellow to orange and then to deep red. Black crystals started depositing on the sides of the flask within 30 min. and the stirring was continued for 4 h. The solid product, which separated out, was filtered, washed with methanol followed by chloroform, and dried under vacuum over anhydrous CaCl₂. All the complexes described below were isolated as black crystalline compounds and were soluble in DMF and DMSO (Scheme 1).

OPEN IN VIEWER

Scheme 1

Synthesis of the ligand (L1) and the Lanthanide(III) complexes of L2.

3.1 Infra-Red spectral data of lanthanide(III) complexes of L2

The infrared spectra of the complexes of L2 contain no absorption band characteristic of -NH₂ or >C=O functions. Infrared spectra of the lanthanide(III) complexes of L2 exhibit absorption bands characteristic of the ligand framework. The ν(OH) vibration occurring in the infrared spectrum of the diamine-diimine macrocycle L1 at 2916 cm^-1 is observed at ca. 3386–3390 cm^-1 in all the complexes of L2. The higher energy of the ν(OH) vibration of L2 in the complexes indicates that the OH protons are not involved in hydrogen bonding. This absorption is very broad in the complexes when compared to that of L1. This may probably be due to the coupling of the ν(OH) vibrations of the coordinated L2 with that of the coordinated and/or lattice water [35]. The strong and sharp absorption band occurring at 1625 cm^-1 in the infrared spectra of the complexes are attributed to the ν(C=N) [36] vibration of the coordinated L2. The ν(C=N) vibration is shifted to lower energy (ca. 10 cm^-1) in the complexes of L2 compared to that of the free macrocycle L1. The ν(N-H) vibration of L1 [37] occurring at 3386 cm^-1 as a strong band is absent in the complexes of L2. This is an additional evidence for the formation of complexes of the tetraimine macrocycle L2 during the template synthesis. The absorption bands occurring in 1482–1488, 1283–1294, and 1031–1036 cm^-1 regions are assignable to the ν(N=O) (ν₁), ν_a(NO₂) (ν₅), ν_s(NO₂) (ν₂), and ν_s(NO) vibrations respectively, of the bidentate chelating nitrate ion [38]. The ν(NO) absorption band observed at 824 cm^-1 is characteristic of chelating bidentate nitrate ion. The large separation of 197–203 cm^-1 between the two highest frequency bands (ν₁ and ν₅) indicates strong interaction of the oxygen donors of the nitrate ion with the lanthanide ions and is typical of bidentate coordination [39]. The absorption band at 1384 cm^-1 is characteristic of ionic nitrate present in the outer-coordination sphere [38]. The sharp and weak absorption bands occurring at 471 and 546 cm^-1 are due to the rocking and wagging modes of vibration, respectively, or coordinated water molecule and the absorption at 495 cm^-1 is due to the rocking mode of vibration of lattice water [38]. Characteristic infrared absorption band of the complexes of the tetraimine macrocycle L2 are given in Table 1.

The molar conductivity of the lanthanide(III) complexes of L2 were measured in DMF. The molar conductivities of the complexes show that the complexes are 1:2 electrolytes [40].

3.3 FAB mass spectra of the complexes of L2

The FAB mass spectra of the complexes contain peaks due to the species [Ln(L2)(NO₃)(H₂O)]⁺, [Ln(L2)(NO₃)]⁺, [Ln(L2)(H₂O)]⁺, [Ln(L2)]⁺, [Ln(L2)₂)]⁺, [(L2)+H]⁺, and [H₂L2 + H]⁺. The molecular cation [Ln(L2)(NO₃)(H₂O)]⁺ loses the exocyclic ligands (nitrate or water) resulting in the formation of the species [Ln(L2)(NO₃)]⁺ and [Ln(L2)(H₂O)]⁺. Both these fragments further lose the exocyclic ligands to form the species [Ln(L2)]⁺ which undergoes demetallation to form [L2 + H]⁺ and/or [H₂L2 + H]⁺. Nitrate ion is removed from [Ln(L2)(NO₃)(H₂O)]⁺ as HNO₃ with a mass loss of 63. In the FAB mass spectra of the complexes there is an intense peak at m/z 529 corresponding to the species [L2 + H]⁺. In the FAB mass spectra of the complexes there is an intense peak at m/z 531 corresponding to the species [H₂L2 + H]⁺. This indicates that the species [Ln(L2)]⁺ undergoes demetallation to give the triimine-amine macrocycle H₂L2 under FAB conditions. The triimine-amine macrocycle formation is more pronounced in the complexes of Tb, Dy, Ho, and Y. The FAB mass spectrum of the representative Sm(III) complex showed a molecular ion peak at m/z 758 which is equivalent to the species [Sm(L2)(NO₃)(H₂O)]⁺. The FAB mass spectrum of [Sm(L2)(NO₃)(H₂O)](NO₃)₂.H₂O is depicted in Fig. 1. Peaks corresponding to the sandwich complexes of the type [Ln(L2)₂]⁺ (Ln(III) = Sm, Eu, Gd, Dy, Ho, and Y); [Pr(L2)(H ₂ L2)]⁺ and [Tb(L2)(H ₂ L2)]⁺ are observed in the FAB mass spectra. Peaks due to sandwich complex of [Sm(L2)(NO₃)(H₂O)](NO₃)₂.H₂O at m/z 1206 is depicted in Fig. 1.

OPEN IN VIEWER

Fig.1

FAB Mass Spectrum of [Sm(L2)(NO₃)(H₂O)](NO₃)₂.H₂O.

These sandwich complexes have been formed during the FAB fragmentation process. The peaks due to the sandwich complexe species are observed in the FAB mass spectra of [Sm(L2)(NO₃)(H₂O)] (NO₃)₂.H₂O,[Eu(L2)(NO₃)(H₂O)](NO₃)₂.2H₂O, [Gd(L2)(NO₃)(H₂O)](NO₃)₂.H₂O, and [Y(L2) (NO₃)(H₂O)](NO₃)₂.2H₂O. The macrocycle L2 in the complexes undergoes deprotonation of the phenolic protons under FAB conditions. The peak at m/z 526 in the complexes [Dy(L2)(NO₃)(H₂O)](NO₃)₂.2H₂O and [Y(L2)(NO₃)(H₂O)](NO₃)₂.2H₂O indicate the deprotonation of the phenolic protons. A prominent and intense peak at m/z 529, due to the formation of [L2 + H]⁺ in the FAB mass spectrum, is shown in Fig. 1.

Electronic absorption spectrum of L1 and the complexes of L2 were recorded at 300K using suitable solvents. The free ligand L1 exhibits three absorptions at 374, 349 and 265 nm corresponding to the π-π* and n-π* transitions. The absorption band observed at 28 448 L mol^-1cm^-1 for L2 is assigned to the π-π* transition of the azomethine chromophore. On complexation, this band shifted to lower wavelength, suggesting coordination of azomethine nitrogen with lanthanides [41]. Complexes of L2 exhibit two absorptions one around at 295 and the other in 400–440 nm region. All the complexes exhibit a shoulder around 245 nm which may be due to charge transfer transition. Electronic absorption spectra of the complexes, recorded immediately after preparing the solution and after standing the complexes in solution over a period of one day, one week, and one month, are the same. There is no apparent loss of the metal ion in solution indicating the kinetic inertia of these complexes toward the metal ion release.

3.5 The EPR spectrum of [Gd(L2)(NO₃)(H₂O)](NO₃)₂.2H₂O

The EPR spectrum of Gd(III) complex, recorded at liquid nitrogen temperature, is a typical spectrum of Gd(III) ion with a 4f⁷ electronic configuration (⁸S_7/2). The EPR spectrum consists of a singly very broad line of g = 2.004, which is an indicative of strong Zeeman interactions and a weak crystal field with highly anisotropic g-factor [42]. The strong crystal field splits the eightfold spin degeneracy of the Gd(III) free ion into four doubly degenerate levels. As a result of transition of unpaired electron between these eight levels, additional line of g < 2.0 is observed. The broad EPR spectrum without any additional lines shows that there is a strong spin-spin interaction and the behavior is not due to strong crystal field [42, 43]. The EPR spectrum of [Gd(L2)(NO₃)(H₂O)](NO₃)₂.H₂O is shown in Fig. 2.

OPEN IN VIEWER

Fig.2

The EPR Spectrum of [Gd(L2)(NO₃)(H₂O)](NO₃)₂.H₂O.

The cyclic voltammogram of the europium complex of L2, [Eu(L2)(NO₃)(H₂O)] NO₃)₂.3H₂O (Fig. 3) in DMF consists of a reduction wave and an oxidation wave with E_pc = –1.292 V and E_pa = –0.936 V (vs Ag/AgCl). The large peak separation, (ΔE_p = 356 mV) and the ratio of the peak currents (i_pa/i_pc = 2.674) indicate that the Eu(III)/Eu(II) redox couple is irreversible. The samarium complex [Sm(L2)(NO₃)(H₂O)](NO₃)₂.3H₂O consists of an anodic wave at E_pa = –0.84 V and a cathodic wave at E_pc = –1.294 V (vs Ag/AgCl). The redox behavior is very similar to that of europium complex with large peak separation (ΔE_p = 454 mV) and ratio of the peak currents differ significantly from unity indicating that the Sm(III) couple undergoes reduction irreversibly with E_1/2 = –1.067 V. The cyclic voltammogram of terbium complex [Tb(L2)(NO₃)(H₂O)]NO₃)₂.2H₂O consists of a reduction wave at E_pc = –1.252 V and an oxidation at E_pa = –0.936 V leading to irreversible redox reaction. The E_pa and E_pc values of the europium, samarium, and terbium redox couples are at less negative potentials than the reduction potential of the free metal ion indicating that the Ln(III) ions are stabilized by the macrocycles.

OPEN IN VIEWER

Fig.3

Cyclic Voltammogram of [Eu(L2)(NO₃)(H₂O)] NO₃)₂.3H₂O.

The magnetic moments of the lanthanide(III) complexes show that they are paramagnetic in nature. The μ_eff values of the complexes of Pr(III), Gd(III), Tb(III), and Dy(III) are 3.49, 7.94, 9.56, and 10.58 B.M respectively. These values are very close to the magnetic moments of the free metal ions [44].

3.8 Thermal stability of the lanthanide(III) complexes of L2

The TG-DTA studies of the ligand L1 and the complex, [Gd(L2)(NO₃)(H₂O)] (NO₃)₂.H₂O, was carried out under Ar-atmosphere with a heating rate of 10 C/min. Here, the ligand is thermally stable upto 450C. After which it starts decomposing with an endothermic peak maximum at 590C and ends at 650C. The complete decomposition involved a mass loss equivalent to that of the ligand. However, thermal behavior of the ligand in Gd complex of L2 shows ligand stability only upto 350C and then the complete decomposition takes place with an endothermic peak maximum at 450C. On comparing the stability of the free ligand with that of the stability of the Gd complex, the free ligand in its diamine-diimine form is much more thermally stable than the tetraimine form of the ligand in the complex. The diamine-diimine macrocycle is not only thermally stable but also resistant to complete reduction to tetraimine as confirmed from the chemical reactions.

3.9 Fluorescence studies

The emission spectra of the mononuclear complexes were recorded in acetonitrile and DMF. The excitation spectrum of Eu(L2)(NO₃)(H₂O)](NO₃)₂.2H₂O contains an intense broad excitation peak at 358 nm, and a medium intense band at 433 nm. Upon excitation at the excitation maxima it exhibits a very broad emission band at 690 nm characteristic of the europium(III) emission [45, 46]. The excitation spectrum of [Tb(L2)(NO₃)(H₂O)](NO₃)₂.2H₂O contains bands at 369 and 388 nm. Upon excitation it exhibits characteristic typical Tb(III) emission at 590 nm due to the ⁵D₄ ⟶ ⁷F₅ emission [46].