Supramolecular polymer of Schiff base gadolinium complex: Synthesis, crystal structure and spectroscopic properties

Inorganica Chimica Acta 430 (2015) 108–113

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Supramolecular polymer of Schiff base gadolinium complex: Synthesis, crystal structure and spectroscopic properties Małgorzata T. Kaczmarek ⇑, Renata Jastrza˛b, Maciej Kubicki, Mateusz Gierszewski, Marek Sikorski ´ , Poland Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznan

a r t i c l e

i n f o

Article history: Received 26 November 2014 Received in revised form 19 February 2015 Accepted 21 February 2015 Available online 5 March 2015 Keywords: Lanthanides Gadolinium Schiff base Supramolecular polymer Self-assembly Salicylaldimines

a b s t r a c t The new gadolinium(III) nitrate complex [Gd(H2L)2(NO3)3(EtOH)]MeOH where H2L is N,N0 -bis(5-methylsalicylidene)-4-methyl-1,3-phenylenediamine was obtained in template condensation reaction of 5-methylsalicylaldehyde with 4-methyl-1,3-phenylenediamine in presence of gadolinium(III) nitrate hexahydrate. The structure of the complex was determined by single-crystal X-ray diffraction analysis and by physicochemical methods. The central cation is nine-coordinated, and the coordination resembles distorted tricapped trigonal prism and only oxygen atoms are involved in coordination. In the crystal structure there is uncoordinated solvent – methanol molecule. The methanol molecule plays an important role in the crystal packing by hydrogen bonding and it connects the complex molecules into infinite chains. Electron paramagnetic resonance studies confirmed complex formation of gadolinium ion because internal coordination sphere of gadolinium was changed. The absorption and fluorescence properties of [Gd(H2L)2(NO3)3(EtOH)]MeOH were studied in different solvents: ethyl acetate, 1-propanol, methanol, ethanol and acetonitrile. Fluorescence quantum yields (UF) were calculated for the complex in all solvents and the values, of UF are rather low, the exact values depend on solvent, being between 0.0005 (in acetonitrile and ethyl acetate) and 0.0014 (in 1-propanol). Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Salen-type Schiff bases derived from salicylaldehyde and diamines have numerous applications, such as fluorogenic agents, pesticides, herbicidal agents [1], and ion-selective electrodes for the determination of anions in analytical samples [2]. The interest in Schiff base ligands has grown recently, because of their antitumor, antibacterial, antivirus and antifungal activity improved by coordinating ligands to a metal ion [3]. Transition metal complexes with salen-type ligands have applications in heterogenous and homogenous catalysis [4], diagnostic pharmaceuticals and laser technology [5]. The interest in Schiff base complexes have been also increased because of their magnetic and optical properties [6]. Schiff base ligands and their complexes have been used as building blocks for mesogenic materials [7]. Lanthanide ions show high coordination numbers, variable and flexible coordination environments therefore they have great potential in the synthesis of novel unusual crystal structures [8]. Among these unusual crystal structures, supramolecular polymers involving lanthanide ions ⇑ Corresponding author. Tel.: +48 61829 1553; fax: +48 61829 1555. E-mail addresses: [email protected] (M.T. Kaczmarek), [email protected] (R. Jastrza˛b), [email protected] (M. Kubicki), [email protected] (M. Gierszewski), [email protected] (M. Sikorski). http://dx.doi.org/10.1016/j.ica.2015.02.026 0020-1693/Ó 2015 Elsevier B.V. All rights reserved.

which possess numerous infinite networks have been the subject of growing interest. Their organic ligands have unique properties allowing to form supramolecular polymers [9]. The self-assembly process, which lead to organize the simple molecules in the more complicated structures includes non-covalent interactions, such as electrostatic, p–p interactions, Van der Waals, donor–acceptor and hydrogen bonding [10]. These non-covalent interactions and intrinsic properties of the molecules to organize and form selfassembled architectures such as, host–guest receptors and nanomolecular devices or supramolecular polymers [11]. Among the supramolecular interactions, hydrogen bonding is one the most effective instruments that organize building blocks into supramolecular structures. The number of hydrogen bonds and arrangement of the donor and acceptor groups determines the strength of hydrogen bonded complexes. Many of structures based on multiple hydrogen bonds [12], however, the network polymeric complex structures have also been built based on a single H-bond between acceptor and donor [13]. Supramolecular polymers became a very attractive research subject, due to their fascinating architecture and potential applications in the field of luminescence [14], magnetism [15] and catalysis [16]. Such properties of salentype lanthanide complexes and potential applications, especially good biological activities [17] of gadolinium complexes and the continuation of our previous studies encouraged us further to

M.T. Kaczmarek et al. / Inorganica Chimica Acta 430 (2015) 108–113

study of new lanthanide complexes of Schiff base ligands. Herein, we introduced the synthesis and structural characterization of the new gadolinium complex with Schiff base ligand N,N0 -bis(5-methylsalicylidene)-4-methyl-1,3-phenylenediamine (H2L, Scheme 1). In the crystal structure of the complex there are two solvent molecules: coordinated ethanol and uncoordinated methanol molecules. Uncoordinated methanol molecule plays an important role in the crystal packing by hydrogen bonding, it connects the complex molecules into infinite chains.

2. Experimental 2.1. Materials and methods Gadolinium(III) nitrate, 5-methylsalicylaldehyde and 4-methyl1,3-phenylenediamine were obtained from Aldrich Chemical Company. 4-Methyl-1,3-phenylenediamine was purified by recrystallization from n-heptane. The compound was characterized using microanalyses (CHN), IR, ESI-MS, EPR, UV–Vis absorption and emission, and single crystal X-ray structural analysis. IR spectra was recorded using CsI pellets in the range of 4000–200 cm1 on a Bruker IFS 66v/S spectrophotometer. Mass spectra was performed using electrospray ionization (ESI) techniques. Electrospray mass spectra were determined in methanol using a Waters Micromass ZQ spectrometer. Microanalyses (CHN) were obtained using a Perkin-Elmer 2400 CHN microanalyzer. X-band electron paramagnetic resonance (EPR) spectra were recorded on an SE/X 2547 Radiopan spectrometer using WinEPR Bruker software. EPR studies were carried out at room temperature using capillary glass tubes (volume 130 lm3). UV–Vis absorption spectra were recorded on a JASCO V-650 spectrophotometer, whereas the emission spectra were recorded on a Jobin Yvon-Spex Fluorolog 3-22 spectrofluorometer. Fluorescence quantum yields were calculated using lumichrome in acetonitrile as a reference ðUst F ¼ 0:028Þ [18]. Fluorescence quantum yields were calculated according to the equation below:

R

UF ¼ UstF R

F X ð1  10Ast Þ ðnX Þ2 F st ð1  10AX Þ ðnst Þ2

Fk

ð1Þ

R Here, FX is the area under the emission curve of the sample, Fst is the area under the emission curve of the standard, and AX and Ast are the absorbance of the sample and standard at an excitation wavelength, respectively, nX – the solvent refractive index for the sample, nst – the solvent refractive index for the standard, Fk – constant describing the instrumental factors, including geometry and other parameters, Ust F is the value of fluorescence quantum yield of the standard.

109

2.2. Crystal structure determination Diffraction data were collected at room temperature by the xscan technique on Agilent Technologies four-circle SuperNova with Atlas CCD detector, equipped with Nova microfocus Cu Ka radiation source (k = 1.54178 Å). The data were corrected for Lorentzpolarization as well as for absorption effects [19]. Precise unit-cell parameters were determined by a least-squares fit of 9560 reflections of the highest intensity, chosen from the whole experiment. The structures were solved with SIR92 [20] and refined with the full-matrix least-squares procedure on F2 by SHELXL-2013 [21]. The scattering factors incorporated in SHELXL97 were used. All nonhydrogen atoms were refined anisotropically, N–H hydrogen atoms were found in difference Fourier maps, all others were placed in idealized positions and all refined as ‘riding model’ with isotropic displacement parameters set at 1.2 (1.5 for methyl or hydroxyl groups) times Ueq of appropriate carrier atoms. Crystal data: C48H50GdN7O14CH4O, Mr = 1138.24, monoclinic, Cc space group, a = 25.542(4)Å, b = 11.103(2)Å, c = 21.463(4)Å, b = 121.01(3)°, V = 5217(2)Å3, Z = 4, dx = 1.45 g cm3, l(Cu Ka) = 8.83 mm1, 13 287 reflections collected, 6894 symmetry independent (Rint = 3.06%), 6392 with I > 2r(I). Final R(I > 2r(I)) = 3.42%, wR2(I > 2r(I)) = 9.75%, R(all reflections) = 3.72%, wR2(all reflections) = 10.06%, S = 1.00, Dqmaximum/Dqminimum = 0.58/0.65 e Å3.

2.3. Synthesis of complex [Gd(H2L)2(NO3)3(EtOH)]MeOH To a mixture of gadolinium(III) nitrate hexahydrate (45 mg, 0.1 mmol) and 5-methylsalicylaldehyde (108 mg, 0.8 mmol) in methanol/ethanol (5/5 mL), 4-methyl-1,3-phenylenediamine (36 mg, 0.3 mmol) in methanol/ethanol (5/5 mL) was added dropwise with stirring. The reaction was carried out for 48 h at room temperature. The solution volume was then reduced to 5 mL by roto-evaporation and an orange single crystals suitable for X-ray diffraction analysis were formed by slow evaporation of solvent. Yield: (67 mg) 61% (1138.29). Elemental analysis for [Gd(C23H22N2O2)2(NO3)3(EtOH)]MeOH calc.: C, 51.70; H, 4.78; N, 8.61. Found: C, 51.38; H, 4.69; N, 8.79%. IR (KBr): m (cm1) = 3366 (mOH), 2426 (hydrogen bond), 1621 (mC@N), 1580, 1563, 1228, 1153, 1040, 749, (mC@C), 1319 (mC–O), 484 (mGd–O), 1768, 1487– 1332, 1384, 805 (mNO). ESI-MS (MeOH): m/z = 314 [Gd(C23H22N2O2)2(C2H5OH)(CH3OH)]3+, 359 [C23H22N2O2 + H]+.

R

Scheme 1. N,N0 -bis(5-methylsalicylidene)-4-methyl-1,3-phenylenediamine (H2L).

3. Results and discussion Schiff base gadolinium complex was formed between Gd3+ ions and N,N0 -bis(5-methylsalicylidene)-4-methyl-1,3-phenylenediamine (H2L, Scheme 1). The compound was obtained in a one-pot template reaction of gadolinium(III) nitrate hexahydrate, 5-methylsalicylaldehyde and 4-methyl-1,3-phenylenediamine. Orange single crystals suitable for X-ray diffraction analysis were formed by slow evaporation of the solvent. Its composition [Gd(H2L)2(NO3)3(EtOH)]MeOH has been characterized using single-crystal X-ray diffraction analysis, microanalysis (CHN), IR, ESI-MS, EPR, and also UV–Vis absorption and emission properties were studied. The ligand N,N0 -bis(5-methylsalicylidene)-4-methyl-1,3phenylenediamine (H2L) has been obtained in our research group by condensation reaction of 5-methylsalicylaldehyde and 4-methyl-1,3-phenylenediamine using method described earlier [22]. Single-crystal X-ray diffraction analysis reveals that [Gd(H2L)2(NO3)3(EtOH)]MeOH crystallizes in the monoclinic, Cc space group. Fig. 1 shows the perspective view of the complex

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Fig. 1. Perspective view of the [Gd(H2L)2(NO3)3(EtOH)]MeOH complex; ellipsoids are drawn at the 30% probability level, hydrogen atoms are depicted as spheres of arbitrary radii. Hydrogen bonds are shown as dashed blue lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

[Gd(H2L)2(NO3)3(EtOH)]MeOH, Table 1 lists the relevant geometrical parameters. The central Gd cation is nine-coordinated (as is quite typical for rare earth complexes [23]), and the coordination resembles distorted tricapped trigonal prism. Interestingly, only oxygen atoms are involved in coordination (one oxygen from each of the ligands, six oxygen atoms from three NO3 groups, and one from the coordinated ethanol molecule. Interestingly, in both ligand molecule only one oxygen and none of the nitrogen atoms are involved in coordination. The second OH group is involved in intramolecular O–H  N hydrogen bonds (Table 2 lists the hydrogen bond data). Both ligands are only slightly twisted; dihedral angles between the planes of the terminal phenyl rings are 27.5(3)° and 28.8(4)° in molecules A and B, respectively. Analysing the dihedral angles between the subsequent planar fragments (Table 1), we conclude that it is the ring with the coordinated oxygen atom that makes almost all of the twit in the molecules. Additionally weak p  p interactions can add to the stability of the complex (the interplanar distances are around 3.6 Å). In the crystal structure there is also another, uncoordinated solvent (methanol) molecule. It plays an important role in the crystal packing by hydrogen bonding (Table 2) it connects the complex molecules into infinite chains along the [1 0 1] direction (Fig. 2). The neighbouring chains interact, besides van der Waals contacts

Table 1 Selected geometrical parameters (Å, °) with s.u.’s in parentheses. A, B and C, D and E denote the planes of subsequent planar fragment: C1–C6 ring, C6  C9 linker, C9–C14 ring, C11  C17 linker and C17–C22 ring, respectively. Gd1–O1A Gd1–O2C Gd1–O2D G1–O2E Gd1–O1F O1B–Gd1–O1F O2D–Gd1–O2E O2D–Gd1–O2C

A/B B/C C/D D/E

2.291(6) 2.490(5) 2.463(7) 2.476(7) 2.419(7) 152.2(3) 148.1(3) 152.2(2)

Gd1–O1B Gd1–O3C G1–O3D Gd1–O3E

2.284(6) 2.466(6) 2.482(7) 2.517(7)

O1A–Gd1–O3C O2E–Gd1–O3D

152.4(2) 140.36(15)

Ligand A

Ligand B

8.2(9) 26.4(6) 6.4(8) 1.5(8)

5.3(8) 20.1(5) 9.3(10) 2.0(11)

Table 2 Hydrogen bond data (Å, °) with s.u.’s in parentheses. D

H

A

D–H

H  A

D  A

D–H  A

N8A N8B O18A O18B O1F O1G

H8A H8B H18A H18B H1F H1G

O1A O1B N15A N15B O1G O18Ai

0.88 0.91 0.82 0.82 0.91 0.85

2.06 2.02 2.06 1.99 1.81 2.23

2.659(9) 2.658(9) 2.634(11) 2.551(11) 2.708(13) 3.002(15)

124 126 126 125 169 152

Symmetry code: i1/2 + x, 1/2 + y, z.

and electrostatic interactions, only by means of weak p  p interactions, with interplanar distances of 3.59 Å and 3.49 Å. The formation of the Schiff base is confirmed by a strong IR band at 1621 cm1 attributable to the C@N stretching mode and the absence of characteristic bands of aldehyde and amine groups of the starting materials. The absence of the shift of the band at 1621 cm1 compared to that of the free ligand shows that gadolinium ion is not coordinated to the nitrogen atoms of imino groups. The bands at 1580 cm1, 1563 cm1, 1228 cm1, 1153 cm1, 1040 cm1 and 749 cm1 were assigned to the stretching vibrations of C@C bonds in the aromatic rings. The shift band of the C–O group toward higher wavelengths, to 1319 cm1 compared to that of the free ligand (1282 cm1) suggests the involvement of phenolic oxygen in the metal–ligand coordination. The band at the 3366 cm1 in the IR spectra of the complex corresponds to the OH vibration in the alcohol molecules. The existence of the hydrogen bonds that connects the complex molecules into infinite chains and intramolecular O–H  N hydrogen bond between the hydroxyl hydrogen and the nitrogen atoms of the azomethine groups were confirmed by the appearance of the band at 2429 cm1 and at 3223 cm1, respectively which is in good agreement with the crystal structure. The Gd–O stretching band appears at 484 cm1 additionally confirming participation of the phenolic oxygen in the metal–ligand coordination. The three bands of complex at the 1487, 1332 cm1 and the 805 cm1 are assigned to vibrations of coordinated nitrate groups. The frequency separation between 1487 cm1 and 1332 cm1 bands may be used to discriminate between these binding states. The difference between 1487 cm1 and 1332 cm1 is 155 cm1 which is consistent with the presence of nitrate groups coordinated in the bidentate fashion

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Fig. 2. Fragment of the hydrogen-bonded chain along the [1 1 0] direction; green – complex, blue – solvent molecules. The relevant hydrogen bonds are shown as red dashed lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

0.5 ethyl acetate 1-propanol methanol ethanol acetonitrile

0.4

Absorbance

[24]. The observations confirm the formation of complex with a neutral form of the ligands and coordination of gadolinium ion only to oxygen atoms which is consistent with the crystallographic data. The positive ESI-MS spectrum of [Gd(H2L)2(NO3)3(EtOH)] MeOH showed peak [M+1]+ at m/z = 314 for [Gd(C23H22N2O2)2 (C2H5OH)(CH3OH)]3+ and m/z = 359 for [C23H22N2O2 + H]+ which is in good agreement with the molecular formula. Differences between electron paramagnetic resonance (EPR) spectra of Gd(NO3)3 and [Gd(H2L)2(NO3)3(EtOH)]MeOH (Fig. 3) confirmed that internal coordination sphere of gadolinium was changed. The experimental value obtained was giso = 1.99. Solute–solvent interactions can change the geometry, the electronic structure, and the dipole moment of a solute in a ground and electronically excited states. To discuss solute–solvent interaction we present UV–Vis absorption or/and emission properties of [Gd(H2L)2(NO3)3(EtOH)]MeOH in different polar and non-polar, protic and aprotic solvents, including ethyl acetate, 1-propanol, methanol, ethanol and acetonitrile. Absorption spectra of [Gd(H2L)2(NO3)3(EtOH)]MeOH in selected organic solvents are presented in Fig. 4, whereas Table 3 summarizes absorption parameters for [Gd(H2L)2(NO3)3(EtOH)]MeOH. The studied compound has two characteristic absorption bands in the lowest-energy part of the spectrum (k1 and k2). According to data presented in Fig. 4 and Table 3, the two lowest-energy absorption maxima for [Gd(H2L)2(NO3)3(EtOH)]MeOH are located at about 355 nm (k1) and 274 nm (k2), however, the exact positions are slightly dependent on the solvent polarity (see Table 3 for more details). These two bands may be attributed to the two independent p ? p⁄ electronic transitions located at the aromatic ring of the ligand. In addition, we observe low intensity absorption at k > 400 nm for the gadolinium complex in all solvents. This may be result of the n ? p⁄ electronic transitions from the lone pair of N-atom in C@N group [25].

0.3 0.2 0.1 0.0 250

300

350

400

450

500

λ / nm Fig. 4. Absorption spectra of [Gd(H2L)2(NO3)3(EtOH)]MeOH in ethyl acetate, 1-propanol, methanol, ethanol and acetonitrile.

Table 3 Absorption, emission parameters and fluorescence [Gd(H2L)2(NO3)3(EtOH)]MeOH in selected organic solvents. Solvent

Ethyl acetate 1-Propanol Methanol Ethanol Acetonitrile

kabs

quantum

yield

for

UF

kem

k2 (nm)

k1 (nm)

k1 (nm)

k2 (nm)

274 274 274 274 275

357 357 355 356 355

482 478 481 430 483

576 568 574 571 576

0.0005 0.0014 0.0009 0.0011 0.0005

ethyl acetate 1-propanol methanol ethanol acetonitrile

Intensity / a.u.

Gd complex

Gd(NO3)3

400

450

500

550

600

650

700

750

λ / nm Fig. 5. Emission spectra of [Gd(H2L)2(NO3)3(EtOH)]MeOH in ethyl acetate, 1propanol, methanol, ethanol and acetonitrile (kexc = 370 nm). 0

200

400

600

[mT]

Fig. 3. Powder EPR spectrum of Gd(III) complex (dashed line) and Gd(NO3)3 (black line) at room temperature.

In Fig. 5, we show the emission spectra of [Gd(H2L)2(NO3)3(EtOH)]MeOH in selected organic solvents at the 370 nm excitation wavelength. Table 3 presents the emission parameters including the emission maxima and the fluorescence

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quantum yields of the complex. In addition to the excitation wavelength at 370 nm, we used 350 nm and 300 nm excitation wavelengths in all solvents. Using kexc = 350 nm we obtained the emission spectra, which are similar in the shape to the spectra shown in Fig. 5, however, their intensity was reduced. When we employed kexc = 300 nm, the characteristic emission band for the lanthanide in the complex was not observed in the shortwavelength part of the emission spectra any more. Typical emission spectra of the complex in all solvents show two bands. The first, located in the short-wavelength part of the spectra, is rather weak, with its maxima located in the range from about 430 nm (in ethanol) to about 483 nm (in acetonitrile). The second emission band, with higher intensity, is located from about 568 nm (in 1-propanol) to about 576 nm (in acetonitrile and ethyl acetate). The solvent polarity affects the exact position of both emission maxima; however this effect is rather small. Interestingly, comparing the localization of the k2 emission bands in alcohols, we conclude that the more polar alcoholic solvents caused bathochromic shift of this band. Other authors found that two emissions were present in the fluorescence spectrum of the Gd complex with 2-(2-benzothiazol-2-yl)phenolate ligand in THF solution (kexc = 405 nm). One fluorescence band was located at 430–440 nm (the shortwavelength band) and another one at 515 nm (the longwavelength band). They state that the long-wavelength emission band may be associated with the emission of intramolecular p ? p stacked fragments [26]. Similar behaviour was observed for N,N-bis(3-methoxysalicylidene)(propylene-2-ol)-1,3-diamine ligand in acetonitrile. The excitation of free ligand at 373 nm produced emission with a broad band located at kmax = 518 nm [27]. We calculate the fluorescence quantum yield (UF) for [Gd(H2L)2(NO3)3(EtOH)]MeOH in all solvents studied (Table 3). Generally, the values of UF are rather low, with the exact values depending on the solvent and ranging between 0.0005 (in acetonitrile and ethyl acetate) and 0.0014 (in 1-propanol).

4. Conclusion A novel gadolinium Schiff base complex using a template condensation reaction between 5-methylsalicylaldehyde and 4-methyl-1,3-phenylenediamine in presence of gadolinium(III) nitrate hexahydrate was successfully synthesized and crystal structure was determined. According to the data of elemental analysis (CHN), spectroscopic data (IR, ESI-MS, EPR, UV–Vis absorption and emission) and X-ray crystallographic structure determination, complex have a metal to Schiff base stoichiometry of 1:2 and three nitrate groups are bounded in bidentate manner to the gadolinium central ion. Additionally, the salen-type ligands act as neutral undeprotonated ligands. It is worth to note that Schiff bases generally act as deprotonated tetradentate ligands with the N2O2 set of donor atoms capable of effective coordination in the planar fashion [28]. The obtained Schiff base gadolinium complex is the next example of unique series of structurally defined salen-type lanthanide complexes in which salicylaldimine Schiff bases act as neutral undeprotonated ligands [23]. Recently, Radecka-Paryzek and co–workers have found that the nature of ligand spacers (aromaticity, flexibility) plays important role in form unusual crystal structures. Using as a spacers of ligand flexible aliphatic diamine has led to obtain infinite one- and two-dimensional coordination polymers [29]. On the other hand, when they used rigid aromatic diamines we obtained the finite monomeric complexes of N,N0 -bis(salicylidene)-4-methyl-1,3phenylenediamine with lanthanide ions. In these complexes the weak interaction between the complex and additional ligand molecule (N,N0 -bis(salicylidene)-4-methyl-1,3-phenylenediamine) has

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