Polyhedron 29 (2010) 1035–1040

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Polyhedron

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Synthesis and characterization of a (1+1) cyclic Schiff base of a lower rim1,3-diderivative of p-tert-butylcalix[4]arene and its complexes of VO2+,UO2þ

2 , Fe3+, Ni2+, Cu2+ and Zn2+

Amjad Ali a, Roymon Joseph a, Bernard Mahieu b, Chebrolu P. Rao a,*

a Bioinorganic Laboratory, Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, Indiab Department de Chemie, Catholic University of Louvain, CSTR, Belgium

a r t i c l e i n f o a b s t r a c t

Article history:Received 8 November 2009Accepted 7 December 2009Available online 23 December 2009

Keywords:Calix[4]arene based Schiff baseMetal complexMössbauer spectroscopyEPR spectroscopy

0277-5387/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.poly.2009.12.020

* Corresponding author. Tel.: +91 22 2576 7162; faE-mail addresses: cprao@iitb.ac.in, cprao@chem.iit

A (1+1) macrocyclic calix[4]arene based Schiff base derivative has been synthesized and was subjected tocomplexation with different ions or ionic species, viz., VO2+, UO2þ

2 , Fe3+, Ni2+, Cu2+ and Zn2+. Both the con-jugate and the complexes were characterized using various spectral techniques, viz.; FTIR, 1H, 13C NMRand FAB mass. The complexes have been further characterized by UV–Vis, EPR and magnetic susceptibil-ity, while the iron complex was studied further by Mössbauer spectroscopy. On the basis of all these stud-ies, the VO2+ and UO2þ

2 complexes were found to be mononuclear, whereas all the other complexes werefound to be dinuclear. Based on the studies, the iron complex was found have a distorted octahedral highspin Fe(III) center with an antiferromagnetically coupled dinuclear core in the complex.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Modified calix[4]arene derivatives having additional bindingsites at the lower rim augment the binding capabilities of the par-ent calixarene [1]. Calixarene conjugates provide a platform to linkvarious binding subunits onto a single molecule. There are a vari-ety of calixarene derivatives and metal ion complexes of calixare-nes in the literature as catalysts for a variety of organictransformations [2]. Even an upper rim functionalized copper com-plex of calix[4]arene has been used for the selective cleavage of oli-gonucleotides [2l]. Besides this, a large number of metal complexesof calixarene derivatives with alkali, alkaline earth, transition,main group, lanthanides and actinides have been reported in theliterature and reviewed [3]. Calix[4]arene derivatives can also actas good candidates for metalloenzyme models [4]. There are onlya few reports regarding the synthesis, isolation and characteriza-tion of transition metal ion complexes of calix[4]arene based Schiffbases in the literature [5]. Similarly cyclic ligands containing Schiffbases and other donor moieties formed on a calix[4]arene platformhave been used for cationic recognition [5–7]. There are few re-ports in the literature regarding metal ion complexes of (1+1) cy-clo-addition products of a lower rim 1,3-di-conjugate. Thus thepresent paper deals with the synthesis and characterization of a ca-lix[4]arene based macrocyclic Schiff base and its complexes withVO2+, UO2þ

2 , Fe3+, Ni2+, Cu2+ and Zn2+.

ll rights reserved.

x: +91 22 2572 3480.b.ac.in (C.P. Rao).

2. Experimental

The molecule H6L was synthesized in three steps as shown inScheme 1, starting from p-tert-calix[4]arene (C4A) via its dinitrile(C4A-dinitrile) to its diamine (C4A-diamine) and then to the targetmolecule (H6L) using known literature procedures [7c,8]. Six me-tal ion complexes of this derivative have been synthesized usingthe precursors VO(acetylacetonato)2, UO2(CH3CO2)2, Fe(ClO4)3,Ni(CH3CO2)2, Cu(CH3CO2)2 and Zn(CH3CO2)2 to result in the forma-tion of [VO(H4L)] (1), [UO2(H4L)] (2), [Fe2(H2L)](ClO4)2 (3), [Ni2

(H4L)(CH3CO2)2] (4) [Cu2(H4L)(CH3CO2)2] (5) and [Zn2(H4L)(CH3CO2)2] (6), respectively.

2.1. Synthesis and characterization of H6L

C4A-diamine (147 mg, 0.2 mmol) was dissolved in 80 ml etha-nol and 2-hydroxy-5-methyl-1,3-benzenedicarboxaldehyde(32.8 mg, 0.2 mmol) in 10 ml ethanol was added to it. The resultingreaction mixture was stirred at room temperature for 12 h to give ayellow precipitate. The product was separated by filtration andwas dried under vacuum. Yield (90 mg) 52%. Anal. Calc. forC114H140N4O10 (1725.06): C, 79.31; H, 8.17; N, 3.25. Found: C,79.72; H, 8.12; N, 3.10%. FTIR: (KBr, cm�1): 1640 (mHC@N), 3416(mOH). 1H NMR: (CDCl3, d ppm): 13.55 (s, 2H, OH), 8.71 (s, br, 4H,HC@N), 7.02 (s, 12H, calix-Ar–H and pendant-Ar–H), 6.73 (s, 8H,calix-Ar–H), 4.05–4.15 (m, 24H, OCH2CH2 and Ar–CH2–Ar), 3.25(d, J = 12.70, 8H, Ar–CH2–Ar), 1.79 (s, 6H, CH3), 1.29 (s, 36H,C(CH3)3), 0.91 (s, 36H, C(CH3)3). 13C NMR (CDCl3, d ppm): 19.52

OH

R

OH

R

OH

R

OH

R

O

R

OH

R

O

R

OH

R

O

R

OH

R

O

R

OH

R

NH2 H2N

C4A-dinitrile C4A-diamine

i iiiii

O

R

OHR

OR

OHR

N

N

H6L

OR

HO R

OR

HO R

N

N

OH

CH3

OH

CH3

C4A

N N

Scheme 1. Synthesis of H6L. (i) K2CO3, ClCH2CN, CH3CN, reflux; (ii) LiAlH4, C2H5OC2H5, reflux; (iii) 2-hydroxy-5-methylbenzene-1,3-dialdehyde, EtOH, stirred at RT for 3 h.R = tert-butyl.

1036 A. Ali et al. / Polyhedron 29 (2010) 1035–1040

(CH3), 30.96 (C(CH3)3, 31.55 (Ar–CH2–Ar), 31.73 (C(CH3)3), 33.78,33.86 (C(CH3)3), 59.80 (NCH2), 74.95 (OCH2), 124.85, 125.39,127.58, 127.87, 132.46, 132.99, 141.08, 146.82, 149.57, 150.72 (cal-ixarene and pendant-Ar–C), 158.50 (HC@N). FAB-MS: m/z (inten-sity (%), fragment) 1727 (100, [M+1]+). UV–Vis spectral data(CHCl3) k, nm (e, mol lit�1 cm�1): 350 (51 138), 449 (2020).

2.2. Metal ion complexes of H6L

H6L was subjected to metal ion complexation reactions with thesalts of VO2+, Fe(III), Ni(II), Cu(II), Zn(II) and UO2þ

2 . H6L and theappropriate metal salt in 1:2 ratios were added to a CHCl3/MeOH(1:4; v/v) solvent mixture and stirred at room temperature. Theprecipitate obtained in each case was isolated and characterized.

2.2.1. Vo(H4L) (1)Yield (65 mg) 65%. M.p. > 200 �C. FTIR: (KBr, cm�1): 925 (mV@O),

1560, 1627 (mHC@N), 3394 (mOH). kmax/nm, (e, mol lit�1 cm�1); 461(5416), 384 (26 950). Anal. Calc. for C114H138N4O11V (1790.98): C,76.44; H, 7.77; N, 3.13; V, 2.84. Found: C, 76.89; H, 8.01; N, 3.52;V, 2.92%. FAB-MS: m/z (intensity (%), fragment) 1792 (40, [M+1]+).

2.2.2. {Uo2(H4L)} (2)Yield (70 mg) 60%. M.p. > 200 �C. FTIR: (KBr, cm�1): 925 (mU@O)

1630, 1643 (mHC@N), 3420 (mOH). kmax/nm, (e, mol lit�1 cm�1); 417(26 295), 258 (68 069). Anal. Calc. for C114H138N4O12U (1993.08):C, 68.65; H, 6.97; N, 2.81; U, 11.94. Found: C, 68.98; H, 6.66; N,3.21; U, 11.54%. FAB-MS: m/z (intensity (%), fragment) 1996 (50,[M+H]+).

2.2.3. [{Fe2(H2L)}(ClO4)2] (3)Yield (85 mg) 65%. M.p. > 200 �C. FTIR: (KBr, cm�1): 1100 (mClO4 ),

1636, 1655 (mHC@N), 3394 (mOH). kmax/nm, (e, mol lit�1 cm�1); 564(220), 449 (56 250), 330 (10 050), 272 (7400). Anal. Calc. forC114H138N4O10Fe2�2ClO4 (2032.80): C, 67.29; H, 6.83; N, 2.75; Fe,5.49. Found: C, 67.58; H, 6.49; N, 2.85; Fe, 5.14%. FAB-MS: m/z(intensity (%), fragment) 1861 (10, [M+Na+2H–2ClO4]+).

2.2.4. {Ni2(H4L)(CH3COO)2} (4)Yield (60 mg) 53%. M.p. > 200 �C. FTIR: (KBr, cm�1): 1562 (mC@O),

1643 (mHC@N), 3424 (mOH). kmax/nm, (e, mol lit�1 cm�1); 398(26 950), 285 (68 085). Anal. Calc. for C118H144N4O14Ni2

(1956.94): C, 72.32; H, 7.41; N, 2.86; Ni, 5.99. Found: C, 71.98; H,7.29; N, 2.55; Ni, 5.65%. FAB-MS: m/z (intensity (%), fragment)1838 (50, [M�2H�2CH3COO]+).

2.2.5. {Cu2(H4L)(CH3COO)2} (5)Yield (60 mg) 50%. M.p. > 200 �C. FTIR: (KBr, cm�1): 1563 (mC@O),

1626 (mHC@N), 3420 (mOH). kmax/nm, (e, mol lit�1 cm�1); 412(47 940), 321 (29 871); Anal. Calc. for C118H144N4O14Cu2

(1966.93): C, 71.96; H, 7.37; N, 2.84; Cu, 6.45. Found: C, 71.58;H, 7.79; N, 2.47; Cu, 6.75%. FAB-MS: m/z (intensity (%), fragment)1851 (45, [M�H�2CH3COO]+).

2.2.6. {Zn2(H4L)(CH3COO)2} (6)Yield (70 mg) 59%. M.p. > 200 �C. FTIR: (KBr, cm�1): 1545 (mC@O),

1622 (mHC@N), 3445 (mOH). kmax/nm, (e, mol lit�1 cm�1); 411(261 367), 258 (67 657). Anal. Calc. for C118H144N4O14Zn2

(1968.93): C, 71.83; H, 7.36; N, 2.84; Zn, 6.63. Found: C, 71.98; H,7.63; N, 2.44. Zn, 6.70%. FAB-MS: m/z (intensity (%), fragment)1857 (25, [M+2H�2CH3COO]+).

3. Results and discussions

3.1. Characterization

The calix[4]arene conjugate H6L as well as its metal ion com-plexes have been well characterized by using routine analyticaland spectral methods, viz., FTIR, UV–Vis, NMR, FAB mass and ele-mental analysis, and the corresponding data has been given inthe Section 2. These complexes have also been characterized byconductivity and room temperature magnetic susceptibility stud-ies. Compound 3 has been studied by variable temperature mag-netic susceptibility as well as by Mössbauer spectroscopy, andthe EPR spectra of compounds 1, 3, 4 and 5 were studied.

3.2. Elemental analysis and conductivity studies

Elemental analyses for the metal ion complexes fit well with a1:1 metal to ligand ratio in the case of the vanadyl and uranylcomplexes, viz. [VO(H4L)] (1), [UO2(H4L)] (2), while the remainingcomplexes, viz. [Fe2(H2L)](ClO4)2 (3), [Ni2(H4L)(CH3CO2)2] (4),[Cu2(H4L)(CH3CO2)2] (5) and [Zn2(H4L)(CH3CO2)2] (6), were foundto be consistent with a 2:1 ratio. Elemental analysis data sup-ports the presence of ClO4

- moieties in the case of 3, and CH3COO-

moieties in the case of 4, 5 and 6. Based on the conductivitymeasurements, it was found that all the complexes were non-ionic except for the iron complex, 3, which was found to be a1:2 electrolyte (kM = 122.0 cm2 mol�1). As no conductivity wasobserved, it is reasonable to conclude that in the case of 4, 5and 6, the acetate ions were bound to the metal ion center inthe coordination sphere.

3.3. FTIR studies

H6L shows a strong band at 1640 cm�1 due to the imine groupvibrations. After complexation, this band shifts to a lower fre-quency, in the range 1560–1630 cm�1, supporting the binding ofthe imine nitrogen with the metal ions. Both the ligand as wellas its complexes shows a broad band at �3450 cm�1 due to pheno-

A. Ali et al. / Polyhedron 29 (2010) 1035–1040 1037

lic –OH group vibrations. In the case of the Fe(III) complex, thestrong band observed at 1100 cm�1 supports the presence of theperchlorate moiety. A characteristic vibrational band observed at925 cm�1 in the case of [(UO2)H4L] is assignable to the asymmetricstretching vibration of trans-UO2þ

2 . Similarly, for the vanadiumcomplex [VO(H4L)], the band observed at 925 cm�1 is characteris-tic of a V@O vibration.

(i) (ii)

300 400 5000

1

2

3

Abs

orba

nce

Wavelength (nm)

H6L

(c)

(b)

(a)

500 600 700 800

0.1

0.2

0.3

H6L

Wavelength (nm)

(a)

(b)

(c)

0

1

2

3

Fig. 1. UV–Vis spectra of ligand H6L and its metal ion complexes in the range of 325–500

Fig. 2. FAB mass spectra of (a) 1, (b

3.4. UV–Vis studies

The electronic absorption spectra of the ligand H6L and its com-plexes were recorded in CHCl3, except in the case of the iron com-plex which was recorded in the DMSO, over the 220–800 nm range(Fig. 1). The calixarene based macrocyclic ligand H6L shows twobands at 350 and 450 nm associated with phenolate and azome-

300 400 500

Wavelength (nm)

H6L

(f)

(e)

(d)

500 600 700 800

0.1

0.2

0.3

H6L

Wavelength (nm )

(d)

(e)

(f)

(iii) (iv)

nm [(i) and (iii)] and 500–800 nm [(ii) and (iv)]. (a) 1, (b) 3, (c) 4, (d) 5, (e) 6 and (f) 2.

) 3, (c) 4, (d) 5, (e) 6 and (f) 2.

Fig. 3. EPR spectra of the vanadium complex (1) recorded in the solid state: (a) at300 K and (b) at 77 K, and in CHCl3: (c) at 300 K and (d) at 77 K.

Fig. 4. EPR spectra of the Cu-dinuclear complex 5, recorded in the solid state: (a) at300 K and (b) at 77 K.

0 100 200 3000

2

4

2

4

6

χ MT

(cm

3 mol

-1K

)

μ eff (

μ Β)

Temperature (K)

χMTμeff (μΒ)

(a)

Fig. 5. (a) Plots of leff and vMT for the iron complex (per di-iron) vs temperature. (b) Mrepresents the theoretical fitting.

1038 A. Ali et al. / Polyhedron 29 (2010) 1035–1040

thine p ? p* transitions, respectively. The band at 550 nm in 3 canbe assigned to a LMCT transition. All the other metal complexesshowed weak d ? d transition bands, except the Zn(II) complex,in the range 500–600 nm. The strong band observed in the range350–500 nm is associated with the imine transition in all the metalcomplexes. Thus, a comparison of the absorption data of the com-plexes with that of the ligand clearly indicates the formation of ametal complex in each case.

3.5. NMR studies

1H and 13C NMR spectra of the ligand were recorded in CDCl3. Apeak that appeared at �9 ppm in the 1H NMR spectrum clearlysupports the presence of an imine (–HC@N) moiety and the sameis supported by the presence of a peak at �160 ppm in the 13CNMR spectrum. The appearance of a pair of doublets for the meth-ylene bridged protons with a splitting constant of �13 Hz supportsthe cone conformation of the calixarene in the ligand. The 1H NMRspectra for the metal complexes have also been recorded, but wecould not obtain interpretable spectra owing to their paramagneticnature. Even, the Zn(II) and UO2þ

2 complexes resulted in broadspectra and hence detailed assignments could not be done. A broadpeak observed around 13.5–14.0 ppm is indicative of the presenceof free phenolic –OH groups.

3.6. FAB mass studies

The molecular weights of all the complexes were establishedfrom the molecular ion peaks observed in the corresponding FABmass spectra. Most of the complexes showed additional peaks cor-responding to the fragments formed due to the loss of the metalion. Representative FAB mass spectra of complexes are shown inFig. 2.

3.7. EPR studies

EPR spectra have been recorded for the complexes 1, 3, 4 and 5at room temperature as well as at liquid nitrogen temperature(77 K). However, the iron (3) and the nickel (4) complexes werefound to be EPR silent. EPR spectra of the vanadyl complex 1, asshown in Fig. 3, remain the same when recorded at room temper-ature as well as at low temperature (77 K), whether the compoundis in the polycrystalline state or in the solution state. It shows atypical eight line pattern (I = 7/2) with anisotropic behavior. Thebehavior of g|| < g\ and A|| > A\ is characteristic of an axially com-pressed d1 configuration. The corresponding EPR data isgıı = 1.951, g\ = 1.983, A|| = 156G, A\ = 55G. The data is reminiscentof a square pyramidal complex.

The X-band polycrystalline EPR spectra of the copper complex 5were recorded at 300 and 77 K as shown in Fig. 4. The spectra are

150 200 250 300

2

4

5

χ MT

(cm

3 mol

-1K

)

Temperature (K)

(b)

3

agnetic moment of the iron complex as a function of temperature. The solid line

Fig. 6. 57Fe Mössbauer spectra of 3: (a) at 300 K and (b) at 78 K.

A. Ali et al. / Polyhedron 29 (2010) 1035–1040 1039

characteristic of a triplet spin state with an anisotropic zero fieldtensor [9]. The normally observed feature for a Cu(II) dimer as ahalf field signal due to the forbidden DMS = 2 transition is noticedin the present case at g = 4.14 at RT as well as at the liquid nitrogentemperature.

3.8. Magnetic susceptibility measurements

Room temperature magnetic susceptibility measurement car-ried out for the VO2+ complex exhibits a magnetic moment of1.65 BM, which is consistent with a mononuclear vanadium com-plex. The same measurement for the Cu(II) dinuclear complex 5has been found to be 2.52 BM, indicating the presence of two Cu(II)centers in the complex having one unpaired electron each whichare weakly coupled antiferromagnetically.

The Fe3+ compound 3 was found to be EPR silent at room tem-perature as well as at 77 K. VTMS data for this complex were col-lected in the temperature range 5–300 K and the resulting dataare shown in Fig. 5a. As evident from the plot, it shows a strongtemperature dependence [10] as the magnetic moment per dimerdrops from a value of 5.98 BM at 300 K to 3.10 BM at 5 K. The

Table 157Fe Mössbauer fitting parameters for 3.

T (K) d (mms�1) a,b DEQ, (mms�1)b C (mms�1) b,c (%) area

300 0.31(1) 0.88(1) 0.43(1) 730.54(1) 0.91(1) 0.28(2) 27

78 0.43(1) 0.92(3) 0.49(2) 760.65(1) 0.95(1) 0.34(1) 24

a Isomer shift relative to Fe metal.b Error in last significant figure given in parentheses.c Full-width at half-maximum listed in order of increasing velocity of the peak.

V=O

Ophe

OpheNim

Nim

Nim NimOphe

OpheNim Nim

Nim Nim

(a) (b)

O=U=O

H3COCO

Ocali

Fig. 7. Binding core in the complexes as deduced based on the studies reported: (a)calix[4]arene-cyclic Schiff base. Ocalix = calix-oxygen, OPhe = bridged phenolic oxygen, OC

experimental value of vMT/dimer (1.203 cm3 K�1 mol�1) obtainedfor this complex is well described for S = 1 spin state at 5 K [11].The distorted octahedral geometry (where the perchlorate ion isin the sixth coordination site) around each iron center is differentas the complex shows two doublets in its Mössbauer spectra. Thusthe distortion in the geometry leads to assign the spin state as 3/2and 5/2 for the two iron centers present in the dinuclear complexand the S = 1 state can be generated with spin–spin coupling of thetwo iron centers (S1 = 5/2 and S2 = 3/2) [12]. The temperaturedependence of the magnetic measurements has been fitted withthe HDVV model by using the following equations [13], consider-ing two interacting (S1 = 5/2, and S2 = 3/2) Fe(III) centers

H ¼ �2JðS1 � S2Þ ðiÞ

leff ¼ 2:828ðvMTÞ1=2 ðiiÞ

v ¼ ðNg2b2=kTÞ½f2þ 10e4x þ 28e10x þ 60e18xg=f3þ 5e2x

þ 7e6x þ 9e18xg� ðiiiÞ

where x = J/kT.The fit shown in Fig. 5b was obtained for J = �16.4 cm�1 with

g = 2.0. This equation has provided a best fit in the temperaturerange 250–160 K. At low temperature, the data does not show agood fit, probably due to more distortion in the Fe(III) geometrymaking the zero field splitting very high [12].

3.9. 57Fe Mössbauer studies

Mössbauer spectra for the iron complex (3) were recorded atroom temperature as well as at liquid nitrogen temperature(78 K), as shown in Fig. 6, and the best fit values of the isomer shift(d) and the quadrupole splitting (DEQ) are summarized in Table 1.The observed values can be fitted with two doublets with intensity

Cu Cu

Ophe

OpheNim Nim

Nim Nim

x

Ocalix

OCOCH3

(c)

M = Fe X = ClO4M = Ni, Zn X = OCOCH3

M M

Ophe

OpheNim Nim

Nim Nim

Ocalix

Ocalix

(d)

X

X

vanadyl, (b) uranyl (III), (c) copper, (d) iron, nickel and zinc complexes of thelO4

= perchlorate oxygen and Nim = imine nitrogen.

1040 A. Ali et al. / Polyhedron 29 (2010) 1035–1040

ratios of �1:3. The d values, as well DEQ, are consistent with a highspin Fe(III) in a distorted octahedral geometry as a mixture of spinstates [14].

4. Conclusions and correlations

Synthesis and characterization data of the macrocyclic Schiffbase of calix[4]arene (H6L) have been reported in this paper. Theligand H6L was subjected to metal complexation reactions withVO2+, UO2þ

2 , Fe3+, Ni2+, Cu2+ and Zn2+. On the basis of various spec-tral and analytical studies, including FAB mass spectra, the VO2+

and UO2þ2 complexes were found to be mononuclear. EPR and room

temperature magnetic measurement studies support the presenceof vanadium in +4, iron in +3 (high spin), and nickel and copper in+2 oxidation states. The EPR data is also suggestive of a squarepyramidal VO2+ complex. Based on the variable temperature mag-netic susceptibility measurements, it could be noted that the twoiron centers in the dinuclear complex are antiferromagneticallycoupled. Further the magnetic studies and 57Fe Mössbauer spec-troscopy support the presence of two different spin states of a dis-torted octahedral geometry in the dinuclear iron complex. Thus,the present studies allow us to deduce the following (Fig. 7) metalcores in case of the complexes.

Acknowledgements

CPR acknowledges the financial support from the DST, BRNSand CSIR. AA and RJ acknowledge SRF fellowship from CSIR andUGC, respectively. We thank CDRI, Lucknow for FAB Mass and SAIF,IIT Bombay for some spectral measurements.

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