Synthesis and Characterization of Two New p-tert-Butylcalix[4]-arene Schiff Bases

Abstract

Synthesis and characterization of two new Schiff bases of p-tert-buthylcalix[4]arene (H₂L¹ and HL²) is described. The synthesis of H₂L¹ and HL² has been achieved by the condensation of salicylaldehyde with the amine group of upper rim monoamine p-tert-butylcalix[4]arene in ethanol. These compounds have been characterized on the basis of elemental analysis and spectral data. Solvatochromicity and fluorescence properties were observed and measured for H₂L¹ and HL². Solvatochromicity of these ligands indicates their potential for NLO applications.

Introduction

Calix[4]arenes can be easily functionalized both at the phenolic OH groups (lower rim) and, after partial removal of tert-butyl groups, at the para positions of the phenol rings (upper rim) [1,2,3]. The vast majority of these modified calixarenes exist in the cone conformation in which there is a cavity suitable for reception of different ionic and neutral species [4]. Furthermore, the most significant feature of the chemistry of these molecules is their ability to bind selectively alkali and alkaline earth cations [5,6]. Compared to the number of reports on the binding of alkali metal ions with calixarenes, reports on the binding of transition metal ions are still limited [7,8,9]. From this point of view calixarene Schiff base ligands are in the center of interest [10,11,12]. Monofunctionalized calixarenes are potentially excellent starting materials for the selective design of new materials. Reinhoudt et al. [13] reported the ipsonitration of p-tert-buthylcalix[4]arenes for the preparation of nitrocalix[4]arenes. In this work we used the selectively ipsonitrated p-tert-buthylcalix[4]arenes as starting materials for the preparation of two monoamine p-tert-butylcalix[4]arenes functionalized at the upper rim and studied their conversion to the salicylaldehyde Schiff bases, 4 and 7.

Results and Discussion

Synthesis of the Schiff Bases

Schiff bases are potentially capable of forming stable complexes with metal ions [9,14,15,16]. In the present work the synthesis of (4) and (7) according to the Scheme 1 is described.

The cone mononitro-p-tert-butylcalix[4]arene 2 was obtained from the mono ipsonitration of monohydroxycalixarene using a modified method [17]. The mononitro derivatives 2 and 5 were reduced to the corresponding monoamines by hydrogenation over a palladium-charcoal catalyst. The condensation of compounds 3 and 6 with salicylaldehyde gave the Schiff base ligands H₂L¹ and HL² as NO donors with a p-tert-butylcalix[4]arene moiety (Scheme 1). The ¹H-NMR spectrum of the ligands indicated the calixarene to be in a cone conformation. The conclusion that H₂L¹ and HL² exist in cone conformations was deduced from the presence of two sets of characteristic AB systems (figure 1 and figure 2) as described in the Experimental Section [18]. The analytical results of the isolated solid ligands with their melting points and colors are compiled in Table 1.

Table 1. Colors, yields, melting points and analytical results of H₂L¹ and HL²

Compound	Formula Weight	Color	m.p, (oC)	Yield, %	Calcd. (Found) %
					C	H	N
H2L1.H2O (C56H73NO6)	838.17	Yellow	192	86	80.25(79.42)	8.54(8.69)	1.67(1.44)
HL2 (C59H77NO5)	880.25	Yellow	172	84	80.50(80.74)	8.82(8.79)	1.59(1.88)

Table 1. Colors, yields, melting points and analytical results of H₂L¹ and HL²

Compound	Formula Weight	Color	m.p, (oC)	Yield, %	Calcd. (Found) %
					C	H	N
H2L1.H2O (C56H73NO6)	838.17	Yellow	192	86	80.25(79.42)	8.54(8.69)	1.67(1.44)
HL2 (C59H77NO5)	880.25	Yellow	172	84	80.50(80.74)	8.82(8.79)	1.59(1.88)

IR Spectra

The characteristic IR absorptions are given in Table 2. The observed microanalytical data for C, H, and N atoms shows that H₂L¹ contains a water molecule that is identified by broad O-H absorptions around 3547-3400 cm^-1.

Table 2. Characteristic IR bands of the H₂L¹ and HL² as KBr Pellets ( cm^-1 )

Compound	ν (H2O)	ν (O-H)	ν (C-H)	ν (C=N)
H2L1.H2O	3420 mbr	3547	2960, 2874 s	1620
HL2	-	3540	2960, 2875 s	1620

Table 2. Characteristic IR bands of the H₂L¹ and HL² as KBr Pellets ( cm^-1 )

Compound	ν (H2O)	ν (O-H)	ν (C-H)	ν (C=N)
H2L1.H2O	3420 mbr	3547	2960, 2874 s	1620
HL2	-	3540	2960, 2875 s	1620

Electronic Spectra

The electronic spectra were recorded in chloroform and acetonitrile (Table 3). An important property for distinguishing potential NLO materials is the existence of solvatochromicity [19], i.e., the solvent dependent shift of the absorption bands in the UV/vis spectra. Both H₂L¹ and HL² display strong negative solvatochromicity as shown in Table 3. Negative solvatochromicity can be attributed to the stabilization of polar ground states in polar solvents.

Table 3. electronic spectra of H₂L¹ and HL²

Compound	ν ( cm-1)		Δν( cm-1)	λex( nm ) (excitation)	λem.( nm) (emission)
	CHCl3	CH3CN
H2L1	277	281	400	390	526
HL2	287	290	300	390	522

Table 3. electronic spectra of H₂L¹ and HL²

Compound	ν ( cm-1)		Δν( cm-1)	λex( nm ) (excitation)	λem.( nm) (emission)
	CHCl3	CH3CN
H2L1	277	281	400	390	526
HL2	287	290	300	390	522

As a result these Schiff bases are good candidates for NLO chromophores due to their strong solvatochromicity. UV/Vis fluorescence of H₂L¹ and HL² was observed when they were irradiated at a wavelength of 390 nm whereby they emitted a light with a wavelength of 526 and 522 nm, respectively.

¹H-NMR Spectra

¹H-NMR spectra of H₂L¹ and HL² are shown in Figure 1 and Figure 2, respectively. Assignments of ¹H-NMR signals can be found in the Experimental Section. The downfield signal of the proton of hydroxy group of the salicylaldehide moiety, the salicylidene part of H₂L¹ and HL², justifies the existence of intramolecular hydrogen bonding between the hydrogen atom of the hydroxy group and the nitrogen atom of the imine.

Fig.1. ¹H-NMR Spectra of H₂L¹

Fig.1. ¹H-NMR Spectra of H₂L¹

Fig. 2. ¹H-NMR Spectra of HL²

Fig. 2. ¹H-NMR Spectra of HL²

Conclusions

In this paper we present the preparation of two Schiff bases of p-tert-buthylcalix[4]arene derivatives. Both these Schiff base ligands have flourescence properties which suggest their potential for analytical applications. Also the solvent dependent UV/Vis spectra and solvatochromicity of these compounds show their potential for NLO applications.

Experimental

General 

Melting points are taken on a Büchi SMP-20 apparatus and are uncorrected. ¹H-NMR spectra were recorded on a Bruker AM-400MHz in CDCl₃ with Me₄Si as an internal standard. Elemental analysis were recorded on Carlo-Erba-Analysor Model 1104. IR spectra were recorded on Bruker IFS 25. Compound 1, p-tert-buthylcalix[4]tripropoxyarene, was prepared according to a literature procedure [20].

Preparation of H₂L¹ and HL² 

According to the Scheme 1, mononitro derivatives were reduced to the corresponding monoamines by hydrogenation over palladium-charcoal catalyst and then used for the preparation of the H₂L¹ and HL² as follows: salicylaldehyde (170 mg, 1.36mmol) was added to a solution of 1.36 mmol of corresponding monoamine, 3 or 6, in ethanol (30 mL) and the mixture was refluxed for 24h. After cooling the reaction mixture, the yellow colored H₂L¹ product was precipitated by addition of water but HL² was precipitated without addition of water. Both were recrystallized from ethanol, yields 86% for H₂L¹ and 84% for HL¹.

¹H-NMR spectra of H₂L¹: δ 13.85 (1H, s, H-O, sal.), 8.65 (1H, s, H-C=N), 7.44 (2H, dd, Ar-H) 7.35 (4H, m, Ar-H, sal), 7.13 (2H, dd, Ar-H), 6.55 (4H, dd, Ar-H), 5.95 (1H, s, O-H), 4.38 and 3.25 (4H, dd, Ar-CH₂-Ar, J = 12.9 Hz), 3.85 (2H, t, OCH₂), 3.75 (4H, t, OCH₂), 4.33 and 3.19 (4H, dd, Ar-CH₂-Ar, J = 13.8 Hz), 2.3 (2H, m, CH₂), 1.95 (4H, m, CH₂), 1.35 (9H, s, C(CH₃)₃), 1.1 (6H, t, 2CH₃), 0.95 (3H, t, CH₃), 0.85 (18H, s, C(CH₃) ₃).

¹H-NMR spectra of HL²: δ 13.20 (1H, s, H-O, sal.), 8.15 (1H, s, H-C=N), 7.29 (2H, dd, Ar-H) 7.25 (4H, m, Ar-H, sal), 7.11 (2H, dd, Ar-H), 6.40 (4H, dd, Ar-H), 4.47 and 3.15 (4H, dd, Ar-CH₂-Ar, J = 13.0 Hz), 4.05 (2H, t, OCH₂), 4.00 (2H, t, OCH₂) 3.70 (4H, t, OCH₂), 4.42 and 3.12 (4H, dd, Ar-CH₂-Ar, J = 13.5 Hz), 2.07 (4H, m, CH₂), 1.90 (4H, m, CH₂), 1.31 (18H, s, C (CH₃) ₃), 1.5 (6H, t, 2CH₃), 0.92 (6H, t, CH₃), 0.60 (9H, s, C(CH₃) ₃).