Raj Kumar Dubey, Meenakshi and Avadhesh Pratap Singh

Synthesis, spectroscopic [IR, (1H, 13C, 27Al) NMR] and mass spectrometric studies of aluminium(III) complexes containing O- and N-chelating Schiff bases

Open Access
De Gruyter | Published online: April 30, 2015

Abstract

Reactions of aluminium isopropoxide with 2-(o-hydroxyphenyl)-benzoxazole (pboxH) and salicylidene-4-chloroaniline (spcaH) in different molar ratios in benzene solution afforded the complexes of the types [(μ-OPri)2Al2(pbox)2(OPri)2] 1, [Al(OPri)(pbox)2] 2, [Al(pbox)3] 3, [(μ-OPri)2Al2(spca)2(OPri)2] 4, [Al(OPri)(spca)2] 5 and [Al(spca)3] 6. All these colored solid complexes are soluble in common organic solvents and were characterized by elemental (C, H, N and Al) analysis, spectroscopic [IR (1H, 13C and 27Al) NMR] and mass spectrometric studies. On the basis of these studies, plausible structures have been tentatively proposed for these complexes.

Introduction

During the past, considerable attention have been paid to the Schiff base complexes of aluminium(III) due to their catalytic activities such as polymerisation of ethylene (Camerson et al., 1999) and methacrylate (Camerson et al., 2000) as well as ring opening polymerisation of the heterocyclic complexes and organic light-emitting devices (OLED) [e.g., AlQ3 tris(8-quinolinolate)aluminium is one of the important complexes used in electroluminescent devices]. The coordination environment around the aluminium centre in Al(OPri)3 can be modified by the reaction of aluminium isopropoxide with the Schiff bases which provide useful steric and electronic properties (Atwood and Harvey, 2001) essential for the reactivity and fine tuning of structure. On the basis of their differences of response in fluorescence emission (Jeanson and Bereau, 2006), 2-(o-hydroxyphenyl)-benzoxazole, their metal complexes (Dubey et al., 2006), were potentially used as cytotoxic agents (Rodriguez et al., 1999; Davison and Corey, 2003; Don et al., 2005), cathapsin S inhibitors (Tully et al., 2006), HIV reverse transcriptase inhibitors (Grobler et al., 2007), estrogen receptor antagonists (Leventhal et al., 2006), selective peroxisome proliferate, activated receptor antagonists (Nishiu et al., 2006), anticancer agents (Easmon et al., 2006), orexin-1 receptor antagonists (Rasmussen et al., 2007). They have also found application as herbicide and as fluorescent whiting agent dyes (Leaver and Milligan, 1984). As in certain cases similar complexes (Thomas, 2009) have been reported for the ring opening polymerisation of lactide. Taking the above facts in view, we report in this paper the synthesis of aluminium(III) complexes containing O and N donor atoms and their characterisation by elemental (C, H, N and Al) analysis, spectral [IR (1H, 13C and 27Al) NMR] and mass spectrometric studies.

Results and discussion

Aluminium(III) complexes (16) have been synthesised by the interactions of aluminium isopropoxide with 2-(o-hydroxyphenyl)-benzoxazole and salicylidene-4-chloroaniline in different molar ratios (1:1, 1:2 and 1:3) in benzene as shown in Scheme 1. All these coloured solid complexes are soluble in common organic solvents such as CDCl3, DMSO and DMF. Some physical properties of these complexes are listed in Table 1.

Scheme 1: Synthetic route for preparation of aluminium(III) complexes.

Scheme 1:

Synthetic route for preparation of aluminium(III) complexes.

Table 1

Synthetic and analytical details of Al(III) complexes 16.

S.

no.
Reactant

(g, mmol)
Product (g, %)

yield

physical state
MP

(°C)
Refluxing time (h) Amount of PriOH in azeotrop found

(calcd.) g.
% Elemental analysis found

(calcd.)
C H N Cl Al
1 Al(OPri)3

(1.452, 7.10)
+ pboxH

(1.50, 7.10)
C38H44N2O8Al21

( 2.256, 89.27 )

Dirty green solid
342 7 0.498

(0.495)
64.17

(64.22)
6.20

(6.24)
3.85

(3.94)
_ 7.18

(7.59)
2 Al(OPri)3

(0.575, 2.81)
+ 2pboxH

(1.18 5.58)
C29H23N2O5Al 2

(1.20, 83.91)

Dirty green solid
350 9 0.34

(0.33)
68.35

(68.77)
4.23

(4.58)
5.42

(5.53)
_ 5.16

(5.33)
3 Al(OPri)3

(0.470, 2.30)
+ 3pboxH

(1.45, 6.86)
C39H24N3O6Al 3

(1.43, 94.70)

Brown solid
250 10 0.41

(0.41)
71.05

(71.23)
3.46

(3.68)
6.11

(6.39)
_ 3.90

(4.10)
4 Al(OPri)3

(0.903, 4.42)
+ spcaH

(1.02, 4.41)
C38H46N2O6Cl2Al24

(1.65, 99.75)

Light yellow solid
203 7 0.28

(0.26)
60.64

(60.72)
5.80

(6.17)
3.37

(3.73)
9.02

(9.43)
6.99

(7.18)
5 Al(OPri)3

(0.210, 1.02)
+ 2spcaH

(0.47, 2.02)
C29H25N2O3Cl2Al 5

(0.43, 76.78)

Light yellow solid
198 9 0.11

(0.12)
63.59

(63.63)
4.54

(4.60)
4.98

(5.12)
12.82

(12.95)
4.86

(4.93)
6 Al(OPri)3

(0.459, 2.20)
+ 3spcaH (1.53, 6.60) C39H27Cl3N3O3Al 6

(1.49, 94.30)

Yellow solid
180 10 0.37

(0.39)
64.91

(65.15)
3.69

(3.79)
5.71

(5.84)
14.02

(14.79)
3.47

(3.75)

Infrared spectral studies

The characteristic infrared frequency of aluminium(III) complexes is given in Table 2. The 2-(o-hydroxyphenyl)-benzoxazole and salicylidene-4-chloroaniline show strong and broad absorption band in the region 3420–3320 cm-1 due to hydrogen bonded OH stretching vibration (Dueke-Eze et al., 2011). This band was absent in the aluminium(III) complexes indicating the involvement of phenolic oxygen in the formation of metal-oxygen bond due to metallation of OH group (Dubey et al., 2011b). The strong intensity band observed in the region 1265–1248 cm-1 in the parent Schiff bases attribute to the phenolic C-O stretching vibration were shifted to higher frequency (Mishra and Singh, 2004) supporting the bonding through phenolic oxygen of the Schiff base to aluminium(III) atom, which was further supported by the appearance of new band in the region 733–680 cm-1 assignable to νAl-O (Jain et al., 2006). The coordination through azomethine nitrogen was supported by the shifting of νC=N towards lower frequencies with respect to that observed 1655–1623 cm-1 in free Schiff bases. The involvement of coordination of azomethine nitrogen was further supported by the appearance of a band in the 591- to 507-cm-1 region due to νAl-N stretching vibration. Infrared spectra of the newly synthesised derivatives exhibit structurally important frequencies in the region 1032–946 assigned to νC-O of the isopropoxide groups in complexes 1, 2, 4 and 5, whereas in complexes 1 and 4, an additional medium band was also observed at 770 and 756 cm-1, respectively, assigned as νAl-O-Al (Jain et al., 2006).

Table 2

Characteristic IR frequencies (cm-1) of Schiff base and their aluminium(III) complexes.

S.

no.
Complex ν(OH) ν(C=N) ν(C-O)

Phenolic
ν(C-O)

Alcoholic
ν(Al-O-Al) ν(Al-O) ν(Al-N)
1 pboxH 3420 1655 1248 _ _ _ _
2 [(μ-OPri)2Al2(pbox)2(OPri)2] 1 _ 1617 1271 1030,

946
770 680,729 591,564,

539,507
3 [(pbox)2Al(OPri)] 2 _ 1616 1270 1032 _ 639,730,

716
591,563,

541,507
4 [Al(pbox)3] 3 _ 1614 1262 733,710,

686
590,562,

542,508
5 spcaH 3320 1623 1265 _ _ _ _
6 [(μ-OPri)2Al2(spca)2(OPri)2] 4 _ 1616 1279 1016 756 698,696,

634
526
7 [(spca)2Al(OPri)] 5 _ 1613 1273 1006 _ 715,707 517
8 [Al(spca)3] 6 _ 1612 1272 _ 709,697 516

1H NMR spectral studies

1H NMR spectra (Table 3) of the complexes 16 have been recorded in CDCl3 using tetramethylsilane as an internal reference. The 1H NMR spectra of the complexes display the expected multiplicity of signals, and integrated proton ratios correspond to the stoichiometric formulae of complexes. A comparison of the spectra of parent Schiff bases with their corresponding aluminium(III) complexes revealed the following important features (i) absence of a signal assignable to phenolic OH in the 11.25- to 10.84-ppm region (Dubey et al., 2011a), (ii) appearance of a signal for azomethine H in the 8.91- to 8.48-ppm region, which is downfield shifted (Shen et al., 2000) compared to that observed for the corresponding Schiff base at 8.18 ppm. The methyl and methine protons of the isopropoxy groups were observed in the region 1.93–1.72 ppm and 3.90–3.81 ppm in the complexes 1-2 and 4-5, whereas in complexes 1 and 4, additional signals observed in the region 1.26–1.22 and 4.49–4.15 due to bridging the isopropoxy group (Sharma et al., 2003a,b).

Table 3

1H NMR data (ppm) for new complexes of aluminium (IV) 1–6.

S. no. Complex CH=N Ar-H Isopropoxy moiety
OCH CH3
1 [(μ-OPri)2Al2(pbox)2(OPri)2] 1 _ 7.95–6.433 (16H, m) 4.15 (2H, br)br

3.81(2H, br)t
1.26 (12H, d)br

1.93 (12H, d)t
2 [(pbox)2Al(OPri)] 2 _ 7.95–6.58 (16H, m) 3.90 (2H, br)t 1.85 (6H, d)t
3 [Al(pbox)3] 3 _ 7.94–6.58 (24H, m) _ _
4 [(μ-OPri)2Al2(spca)2(OPri)2] 4 8.48 (2H, s) 7.87–6.34 (16H, m) 4.49 (2H, br)br

3.89 (2H, br)t
1.22 (12H, d)br

1.72 (12H, d)t
5 [(spca)2Al(OPri)] 5 8.81 (2H, s) 8.02–6.55 (16H, m) 3.82 (2H, br)t 1.83 (6H, d)t
6 [Al(spca)3] 6 8.91 (2H, s) 8.15–6.63 (24H, m) _ _

13C NMR spectral studies

The 13C NMR spectra (Table 4) of three typical aluminium(III) complexes 1, 3 and 5 were recorded in CDCl3 and are also supportive of complex formation involving bonding through azomethine nitrogen and phenolic oxygen atom. The presence of a downfield shifted signal in the region 163.36–160.31 ppm with respect to that observed 153.56–151.53 ppm for the corresponding parent Schiff bases is in support of the coordination of azomethine nitrogen to aluminium atom (Dubey et al., 2014). The signal, which appears in the (160.34–158.59 ppm) region for the carbon atom adjacent to the phenolic oxygen in the spectra of free ligands, is observed in the complexes 165.58–163.39 ppm region, which is indicative of the bonding through phenolic oxygen (Dubey et al., 2014). Complexes 1 and 5 show signals for the methyl and methine carbon atoms in the regions 25.08–24.61 and 68.58–62.04 ppm, respectively, which is consistent with the presence of Al-OPri group.

Table 4

13C NMR data (ppm) for aluminium(III) complexes 1, 3 and 5.

S. no. Complex C-O phenolic CH=N Ar-C Isopropoxy moiety
OCH CH3
1 [(μ-OPri)2Al2(pbox)2(OPri)2] 1 165.58 163.36 127.94–109.05 68.58, 64.92 25.08, 24.61
2 [Al(pbox)3] 3 165.06 162.97 128.40–110.27 _ _
3 [(spca)2Al(OPri)] 5 163.39 160.31 132.97–115.11 62.04 25.04

27Al NMR spectral studies

The aluminium(III) complexes 2 and 3 atom exhibit a broad signal at 12.40 and 7.22 ppm, respectively, which is consistent with the penta- (Sharma and Singh, 2006) and hexa- (Hoveyda et al., 1993) coordinated, respectively. Interestingly, complex 4 exhibits two signals (Figure 1) at 70.20 ppm and 8.43 ppm, respectively, indicating the presence of both four and six coordination around the two aluminium atoms (Sharma et al., 2003b). Such type of dinuclear derivative with unsymmetrical geometry containing both four- and six-coordinated aluminium(III) atoms were reported previously (Sharma et al., 2002, 2003a).

Figure 1: 27Al NMR spectrum of complex [(μ-OPri)2Al2(spca)2(OPri).

Figure 1:

27Al NMR spectrum of complex [(μ-OPri)2Al2(spca)2(OPri).

Mass spectrometric studies

The QTOF LC-MS spectra of the complexes (1, 5 and 6) were recorded, and the fragmentation pattern (Schemes 2 and 3) with m/z have been suggested (Dubey et al., 2011b, 2014) for complexes 1 and 5. In the spectra of complexes 1 (Figure 2), 5 (Figure 3) and 6 molecular peaks observed at m/z 711.3 [C38H44Al2O8N2; calculated mass=710.27], 547.3 [C29H25AlO3N2Cl2; calculated mass=546.10] and 717.5 [C39H27AlO3N3Cl3, calculated mass 719.32], respectively, correspond to the complex 1 shows dimeric molecular composition, whereas complex 5 and 6 are found to be monomeric in nature. Furthermore, in these spectra, the base peak was observed due to the formation of fragment [C6H5O]+ about m/z 94.4. Other important peaks were observed at m/z 711.3, 652.1, 592.3, 507.3, 447.2, 388.8, 301.2, 245.1, 212.1, 121.2 complex 1 and complex 5 shows some prominent peaks at 547.3, 475.5, 447.2, 400.3, 301.2, 245.1 and 212.1 due to various fragmentations.

Scheme 2: Fragmentation pattern of complex [(μ-OPri)2Al2(pbox)2(OPri)2] 1.

Scheme 2:

Fragmentation pattern of complex [(μ-OPri)2Al2(pbox)2(OPri)2] 1.

Scheme 3: Fragmentation pattern of complex [(spca)2Al(Opri)] 5.

Scheme 3:

Fragmentation pattern of complex [(spca)2Al(Opri)] 5.

Figure 2: Q-TOF(LC-MS) spectrum of complex [(μ-OPri)2Al2(pbox)2(OPri)2] 1.

Figure 2:

Q-TOF(LC-MS) spectrum of complex [(μ-OPri)2Al2(pbox)2(OPri)2] 1.

Figure 3: Q-TOF(LC-MS) spectrum of complex [(spca)2Al(Opri)] 5.

Figure 3:

Q-TOF(LC-MS) spectrum of complex [(spca)2Al(Opri)] 5.

Conclusions

Reactions of aluminium isopropoxide with 2-(o-hydroxyphenyl)-benzoxazole (pboxH) and salicyledine-4-chloroaniline (spcaH) in 1:2 and 1:3 molar ratios in benzene solution afforded penta- and hexa- coordinated aluminium(III) complexes, whereas product obtained in equimolar reaction with both of the mentioned ligands give interesting complex, which shows both six and four coordination around the two aluminium atoms. These complexes were characterised by elemental (C, H, N and Al) analysis, spectral [IR (1H, 13C and 27Al) NMR] and mass studies.

Experimental

Materials

All the chemicals and reagents were of analytical grade. Salicylamide (loba chemie, Mumbai, India), o-aminophenol (loba chemie, Mumbai, India), salicylaldehyde (CDH, New Delhi, India) and p-chloroaniline (CDH, New Delhi, India) were used without further purification. Benzene (AR Merck, Germany) and n-hexane (AR Merck, Germany) were dried by refluxing over sodium-benzophenone, followed by distillation under anhydrous conditions (Armengo and Chai, 2009) and other solvents were purified by standard procedure. Isopropoxy contents were estimated by the oxidimetric method (Mehrotra, 1953) using N-K2Cr2O7 solution in 12.5% H2SO4. Aluminium was estimated as oxinate gravimetrically. The 2-(o-hydroxyphenyl)-benzoxazole (Walter and Freiser, 1952) and salicylidene-4-chloroaniline (Dubey et al., 2012) were synthesized according to literature methods.

Physical measurement

Elemental analyses for C, H and N were performed on a Haraceous Carlo Erba 1108 elemental analyser. Infrared spectra were recorded on a Perkin-Elmer 100 FT-IR spectrophotometer in the range 4000–400 cm-1. 1H, 13C and 27Al NMR spectra were recorded on Bruker Avance II 400 NMR spectrometer. Chemical shifts were given in ppm relative to Me4Si for carbon, hydrogen and Al(NO3)3 for aluminium in CDCl3 solvent. ESI-MS spectra were recorded on Agilent 6520 Q-T of LC-MS, MS/MS spectrometers in acetonitrile.

Synthesis of the complexes

A benzene solution (∼70 cm3) containing Al(OPri)3 (1.452 g, 7.10 mmol) and 2-(o-hydroxyphenyl)-benzoxazole (pboxH) (1.50 g, 7.10 mmol) was heated under reflux for ∼5 h. The liberated PriOH was fractionated azeotropically and estimated (0.23 g) periodically to check completion of the reaction. Removal of volatile components from the reaction mixture under reduced pressure yielded a green solid. Yield: 2.256 g (89%). The compound was dissolved in a minimum amount of chloroform and then started to add n-hexane when turbidity appeared, one to two drops of chloroform were added, and a clear solution was obtained and kept for ∼12 h. The compound was precipitated, and the remaining solvent was decanted off, and the solid obtained was washed two to three times with n-hexane to obtain the pure form.

A similar procedure was adopted to prepare complexes 2 and 3 by carrying out the reactions of Al(OPri)3 and pboxH in 1:2 and 1:3 molar ratios, respectively. Similarly, complexes of aluminium(III) (4–6) have been synthesized with salicylidene-4-chloroaniline in 1:1, 1:2 and 1:3 molar ratios.

27Al NMR data (CDCl3) ppm: 12.40 [(pbox)2Al(OPri)] 2; 7.22 [Al(pbox)3] 3; 70.20, 8.43[(μ-OPri)2Al2(spca)2(OPri)2] 4.

Acknowledgements

Meenakshi gratefully acknowledges the U.G.C. for a PhD fellowship and the Head of the Department of Chemistry for providing laboratory facilities. Special thanks to SAIF Chandigarh for IR, NMR, (1H, 13C, 27Al) and Q TOF LC-MS mass spectra of the complexes and CDRI Lucknow for providing elemental analysis.

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Received: 2014-10-9
Accepted: 2015-4-8
Published Online: 2015-4-30
Published in Print: 2015-3-1

©2015 by De Gruyter

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