Synthesis and Characterization of Heteronuclear Copper ( II )-Lanthanide ( III ) Complexes of N , N-1 , 3-Propylenebis ( Salicylaldiminato ) Where Lanthanide ( III ) = Gd or Eu

Three complexes, namely, [Cu(salbn)] (1), [Cu(salbn)Gd(NO 3 ) 3 ⋅H 2 O] (2), and [Cu(salbn)Eu(NO 3 ) 3 ⋅H 2 O] (3) where salbn =N,N1,3-propylenebis (salicylaldiminato) have been synthesized and characterized by elemental analyses, ICP-AES, IR, UV, NMR, MS, EDX, powder XRD, and EPR spectroscopies. The EDX results suggest the presence of two different metal ions in heteronuclear complexes (2) and (3). The ligand(salbn), complex (1), and complex (3) crystallize in triclinic system while complex (2) crystallizes inmonoclinic system.TheEPR studies suggest that [Cu(salbn)] complex is tetragonally coordinatedmonomeric copper(II) complex with unpaired electron in the d x 2 −y 2 orbital and spectral features that are the characteristics of axial symmetry while complex (2) in DMF solution at liquid nitrogen temperature exhibits an anisotropic broad signal around g∼ 2.03 which may suggest a weak magnetic spin-exchange interaction between Gd(III) and Cu(II) ions. The fluorescence intensity of Eu(III) decreased markedly in the complex (3).


Introduction
The spectroscopic and unique properties of copper(II)lanthanide(III) complexes have become a subject of intense research interest with coordination chemists because of their wide arrays of applications in electroluminescent devices, biomedicine, MRI, magneto magnets, and many more [1][2][3][4][5].The coordination chemistry of lanthanide(III) ions is also increasing day by day, owing to the relevance of these compounds in basic and applied research in various fields to chemistry, material science, life science, and so forth [6][7][8].Lanthanide complexes possessing higher stability are especially important in two different fields of research where inert complexes are potentially useful, namely, for the design of Gd(III) contrast agents for NMR imaging and for the separation of the lanthanides as a set of metals [9].Most of the heteronuclear copper(II)-lanthanide(III) complexes have been synthesized with heterodonor ligands of N and O which are coordinated to copper and lanthanide atoms.The structural chemistry of the lanthanides is interesting as they have a strong tendency to form complexes with higher coordination numbers up to twelve.Because of their large size and their tendency to form ionic bonds rather than covalent bonds, lanthanide(III) ions may form complexes having higher coordination numbers with monodentate, simple bidentate or polydentate ligands possessing small chains [10].The structural versatility arises from the lack of strong crystal efforts for the 4f electronic configurations as well as from the large ionic radii of these lanthanide metal ions, which change markedly with atomic number or oxidation number of the lanthanides.The chemistry of lanthanides with N,N/N,O donor ligands and their thermal properties has been intensively investigated [11].Vey recently, Akine made a review on design of novel ion recognition systems based on salen (H 2 salen = N,N  -disalicylideneethylenediamine) or related ligands [12].These ligands are versatile and important compounds that have been widely used in coordination chemistry.In general, salen-type ligands coordinate to a transition or typical metals as a doubly form in a tetradentate fashion [13].Heteronuclear copper(II)-lanthanide(III) complexes have attracted increasing interest because of their different coordination capabilities.Thus, heteronuclear copper(II)-lanthanide(III) compounds have commonly been synthesized with heterodonor ligands of N and O which are good building blocks for the formation of different lanthanide coordination compounds [14][15][16].

Physical Measurements.
FT-IR spectra were recorded on a Shimadzu FT-IR 8400S model spectrometer in the range of 4000-400 cm −1 using KBr disks.Electronic spectra of the ligand and complexes were recorded at room temperature on a Perkin Elmer UV-Vis Lambda 35 spectrophotometer.The fluorescence spectra were recorded by a Perkin Elmer LS55 fluorescence spectrophotometer.The elemental analyses (C, H, N) were performed on a Perkin Elmer 2400 series II analytical instrument.Lanthanid(III) metals were estimated from Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) by using ARCOS Spectrometer (Germany).MS spectra were recorded on MS Waters ZQ-4000.Energy-dispersive X-ray spectroscopy (EDX) spectra were recorded on JEOL and JEM-7600F models.The powder X-ray diffraction pattern was recorded on PAN analytical Philips diffractometer with Cu-k radiation of wavelength 1.54056 Å operating at a voltage of 40 kV and a current of 20 mA.The 2 range used was from 5 ∘ to 65 ∘ at a step of 0.02 ∘ .The peak positions of the various complexes were obtained by fitting Lorentzian function.The peak positions so obtained were used to index the pattern and compute the cell parameters using the programme DICVOL and P-INDEX.Thermogravimetric analyses (TGA/DTA) of the compounds were carried out in the temperature from 40 ∘ C to 900 ∘ C with Perkin Elmer STA 6000.The experimental conditions were platinum crucible, nitrogen atmosphere with 20 mL/min flow rate, and a heating rate 5 ∘ C/min.Magnetic moment at room temperature ( eff ) was measured using Sherwood Scientific susceptibility balance (MSB).EPR experiments were conducted on a VARIAN-E-112 spectrometer X-band frequency (9.5 GHz) with sensitivity of 5 × 10 10 ΔH spins using tetracyanoethylene (TCNE). 1 H NMR and 13 C NMR spectra were recorded on FT-NMR spectrometer BRUKER AVANCE II 400 in CDCl 3 solution, using TMS as the internal standard chemical shifts, and were reported in  units downfield from TMS.
[Cu(salbn)] (1) was synthesized by adding the methanolic solution of copper acetate (0.01 mole) to the methanolic solution of the yellow ligand (0.01 mole) under constant stirring.The reaction mixture was subsequently allowed to stir and refluxed for 5 hours giving green precipitate.After cooling, the green precipitate was filtered and washed with methanol and finally dried in air.[Cu(salbn)] complex is found to be soluble in DMF, chloroform, and ethanol solvents.
[Cu(salbn)Ln(NO 3 ) 3 ⋅H 2 O] where Ln = Gd or Eu was synthesized by slowly adding a methanolic solution of Ln(III) nitrate salts to a solution of [Cu(salbn)] dissolved in hot chloroform under constant stirring.The reaction mixture was subsequently allowed to stir and refluxed for 5 hours giving dark green precipitate.Then the volume of the dark green precipitate solution was reduced to half of the initial volume and recrystallized in hot chloroform-methanol mixture.The dark green precipitate so obtained was filtered and finally dried in air.[Cu(salbn)Gd(NO 3 ) 3 ⋅H 2 O] (2) and [Cu(salbn)Eu(NO 3 ) 3 ⋅H 2 O] (3) complexes were soluble in chloroform, DMF, methanol, and ethanol solvents.The following reaction scheme (Scheme 1) affords the synthesis of [Cu(salbn)] and [Cu(salbn)Ln(NO 3 ) 3 ⋅H 2 O] where Ln = Gd or Eu complexes from the ligand(salbn).

Characterization of the Complexes. Some analytical data of the ligand(salbn), [Cu(salbn)], and [Cu(salbn)
Ln(NO 3 ) 3 ⋅H 2 O] where Ln = Gd or Eu are tabulated in Table 1 which are in close agreement with theoretical and experimental values.All the complexes are soluble in chloroform, DMF, ethanol, and methanol.

X-Ray Diffraction
Studies.Since single crystal of the complexes could not be obtained, the exact crystal structure could not be determined.In order to provide a typical idea about the single phasic nature of the complexes, we have carried out powder X-ray diffraction pattern for the ligand, [Cu(salbn)] ( 1), [Cu(salbn)Gd(NO 3 ) 3 ⋅H 2 O] (2), and [Cu(salbn)Eu(NO 3 ) 3 ⋅H 2 O] (3) complexes as shown in Figure 1.Lattice parameters of the XRD spectra were determined using a set of DICVOL and P-INDEX programs [18].

Electronic Spectral Study.
The electronic spectra of the free ligand(salbn) exhibits two charge transfer (CT) bands at 257 and 310 nm attributed to  →  * and n →  * transitions within the ligand.In the spectrum of the complex, the CT band at 257 nm remains intact, in agreement with the  →  * transition of the ligand.Another band at ca. 342 nm is observed in the spectrum of complex (1).The band at 310 nm observed in the spectrum of free ligand is red shifted to 342 nm in the form of ligand to metal charge transfer transition (LMCT).Unlike the spectrum of the free  ligand, broadband at ca. 597 nm is observed in the spectrum of complex (1) (see SFigure 1 in Supplementary Material available online at http://dx.doi.org/10.1155/2013/281270).The d-d transition at ca. 597 nm may be attributed to structurally well characterised square-planar copper(II) complexes [19].The electronic spectrum of complex (1) in DMF exhibits a higher energy band at 597 nm, which has been shifted to lower energy at ca. 615 nm (SFigure 2) for the complex (2) seems to be due to a distortion of geometry occurring at the copper centre.Other f-f transitions, which are expected to appear, may be concealed by d-d, charge-transfer, or intraligand transitions.The electronic spectra taken in DMF have the higher absorbance and shifted to longer wavelength compared to the spectra taken in ethanol (EtOH) and methanol (MeOH) of complex (2) (SFigure 2).

Infrared Spectral Study.
A strong sharp absorption band around 1638 cm −1 in the spectrum of the ligand may be assigned to the C=N stretching.In the complex, this band is shifted to 1627 cm −1 upon complexation with the metal, which may be attributed to the coordination of the imine nitrogen to the metal centre [20][21][22][23].The ligand shows band at 3539 cm −1 due to O-H stretching from phenolic group which disappears in the complexes (1), (2), and (3), indicating the deprotonation of the ligand upon complexation [5].The strong phenolic C-O absorption band at 1212 cm −1 observed in the spectrum of ligand shifts to lower frequency at 1017 cm −1 , supporting the coordination of the deprotonated phenolic oxygen atoms to the metal centres in the complexes [22].A broadband at ca. 3300-3450 cm −1 indicates that coordinated water is present in the heteronuclear Cu(II)-Ln(III) complexes which is also supported by thermal data.
The IR spectra of the complexes ( 2) and (3) show weak bands at 860 and 897 cm −1 for   and at 550 cm −1 for  wagg of coordinated water [21].The ligand coordination to the metal centre is substantiated by a band appearing at 470 cm −1 which is mainly attributed to  Cu-N in the complexes.
The spectra of the heteronuclear copper(II)-lanthanide(III) complexes (2) (SFigure 3) and (3) exhibit characteristic vibrational frequencies of coordinated nitrate group, which is supported by two bands observed at ca. 1485 and 1301 cm −1 [24].The medium band at 1030 cm −1 due to the  2 vibration of the nitrate group (C 2V ) stands as additional evidence for the presence of coordinated nitrate group.The difference in wavenumbers between the two highest frequency bands ( 4 - 1 ) of nitrate (C 2V ) is about 184 cm −1 , indicating the nature of bidentate nitrate bridging [21][22][23][24][25][26].In the 1 H NMR spectrum of the free ligand no broad peak was observed due to free amine proton in the region  The FAB mass spectra of complex, [Cu(salbn)] with ligand (SFigures 7 and 8), play an important role in confirming the monomeric Cu(II) complex.The molecular ion peak of [Cu(salbn)] complex was observed at m/z 344, which is equivalent to its molecular weight and that of ligand(salbn) was observed at m/z 282 which is also equivalent to its molecular weight.The other peaks like 336, 344, 346, 368, 369, 382, and 384 in [Cu(salbn)] complex correspond to various fragments.The stoichiometry of metal to ligand in the ratio 1 : 2 was supported by the observed molecular ion peaks in spectrum of complex (1).These observations confirm the stoichiometric composition of the complex formation.This is also supported by the FAB mass spectra of the other complexes [17].The data of mass spectra was found in complete agreement with that of obtained via elemental analysis.

NMR and Mass Spectral Studies
3.6.Energy-Dispersive X-Ray Spectroscopy Study.The composition of the heteronuclear copper(II)-lanthanide(III) complexes that is, complexes, (2) and (3), was defined by energy-dispersive X-ray spectrometer (EDX) analysis as shown in Figures 2 and 3, respectively.The analysis of the International Journal of Inorganic Chemistry EDX spectra indicates the presence of two kinds of different metals ions in heteronuclear complexes (2) and (3).Thus, from the EDX analysis, it can be proved that complexes (2) and (3) are heterobinuclear complex [29].corresponds to decomposition in the Ln 2 O 3 oxides in temperature around 680 ∘ C. The residue obtained after heating up to 820 ∘ C to constant weight is very close to those expected for the lanthanide(III) oxides [5,29,30].

VSM Study.
The magnetic hysteresis loop of the complex (3) taken at room temperature is shown in Figure 4. Saturation magnetization is not observed as the applied magnetic field is raised, and hence there is no coercivity and remanence in the hysteresis loop of complex (3) [31].
3.9.Fluorescence Study.In order to investigate the effect of an adjacent metal ion on fluorescence of Eu(III), fluorescence spectra of complex (3) were recorded.The complex (3) showed no significant fluorescence in the range of 550-750 nm by the maximum excitation wavelength, while [Eu(NO 3 ) 3 ⋅5H 2 O] gave the well-known five typical fluorescence bands at ∼592, ∼618, ∼649, ∼685, and ∼698 nm when excitation monitored at 395 nm corresponding to characteristic emission transitions 5 D 0 → 7 F  ( = 0, 1, 2, 3, 4) [32,33].This significant decrease in fluorescence intensity may be due to the energy transfer from the excited Eu(III) to the adjacent Cu(II) complex moiety (intramolecular energy transfer), followed by the radiationless energy loss through the Cu(II) complex moiety [34].

Magnetic Moment and EPR Studies of the Complex.
The magnetic susceptibility measurement of the [Cu(salbn)] complex showed a magnetic moment value of 1.73 B.M (Table 1) corresponding to one unpaired electron, indicating that the Cu(II) complex is mononuclear.The heteronuclear copper(II)-lanthanide(III) complexes ( 2) and (3) under study showed a magnetic moment,  eff value at room temperature of 8.06 and 3.84 B.M (Table 1), respectively.The magnetic moments observed are compared with the theoretical spin orbital coupling values (the Hund's values) and the values calculated from Van Vleck formula of the respective lanthanide ion.These values agree well with each other for the copper(II)-lanthanide(III) complexes [35].
The solid and DMF solution EPR spectra of [Cu(salbn)] complex were recorded at 300 K and liquid nitrogen temperature by using tetracyanoethylene ( = 2.0027) as reference.According to  Cu = 3/2 and calculation expression (2 + 1), four hyperfine splitting lines can be expected.Indeed four weak peaks are observed giving parameters  ⊥ = 2.072 and  ‖ = 2.183 which is a normal axial symmetric EPR spectrum as shown in Figure 5.This is typical of tetragonally coordinated monomeric copper(II) complex with unpaired electron in the d  2 − 2 orbital and spectral features which are the characteristics of axial symmetry [36].The  ‖ and  ⊥ values are >2.0023indicating that the complexes are largely covalent [37].
EPR spectra of complex (2) were recorded at X-band in the solid and solution state at room temperature and liquid nitrogen temperature.The room temperature polycrystalline powder EPR spectrum of complex (2) shows a unique quasiisotropic broad signal centred at  = 2.1 (Figure 6) but shows no clear characteristic peak in the  = 2 region [17,38].Little information is obtained from the room temperature polycrystalline powder EPR spectrum.The EPR spectrum of liquid nitrogen temperature in DMF solution of complex (2) exhibits an anisotropic broad signal around  ∼ 2.03 which may suggest a weak magnetic spin-exchange interaction between Gd(III) and Cu(II) ions [39] as shown in Figure 7.
In the EPR spectrum of complex (2) in DMF solution at liquid nitrogen temperature, the fine structure exhibits the seven expected transitions around 2380, 2680, 3000, 3220, 3300, 3480, and 4000 G which may be generated by zero field splitting [17,40].This spectrum corresponds to the superposition of the signals of the copper(II) and gadolinium(III) ions.These results suggest that the copper(II) ( = 1/2) and gadolinium(III) ( = 7/2) spins are coupled to yield the total spin states  = 3 (low lying excited state) and  = 4 (ground state) which is a weak interaction between copper(II) and gadonium(III) ions [41].