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

Synthesis and characterization of two new Schiff bases of p-tert-buthylcalix[4]arene (H2L1and HL2) is described. The synthesis of H2L1and HL2has 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 H2L1and 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.

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 H 2 L 1 and HL 2 as NO donors with a p-tert-butylcalix [4]arene moiety (Scheme 1). The 1 H-NMR spectrum of the ligands indicated the calixarene to be in a cone conformation. The conclusion that H 2 L 1 and HL 2 exist in cone conformations was deduced from the presence of two sets of characteristic AB systems (figures 1 and 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.

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

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 H 2 L 1 and HL 2 display strong negative solvatochromicity as shown in Table 3. Negative solvatochromicity can be attributed to the stabilization of polar ground states in polar solvents. As a result these Schiff bases are good candidates for NLO chromophores due to their strong solvatochromicity. UV/Vis fluorescence of H 2 L 1 and HL 2 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.

H-NMR Spectra
1 H-NMR spectra of H 2 L 1 and HL 2 are shown in Figures 1 and 2, respectively. Assignments of 1 H-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 H 2 L 1 and HL 2 , justifies the existence of intramolecular hydrogen bonding between the hydrogen atom of the hydroxy group and the nitrogen atom of the imine.

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.

Acknowledgements
We are grateful to the Research Council of Tabriz University for financial support. Generous support from Prof. Dr. J. Ipaktschi, Institute of Organic Chemistry, Giessen University is highly acknowledged. The Ministry of Science, Research and Technology of Iran is acknowledged for a grant to B. Shaabani.

Experimental General
Melting points are taken on a Büchi SMP-20 apparatus and are uncorrected. 1 H-NMR spectra were recorded on a Bruker AM-400MHz in CDCl 3 with Me 4 Si 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 H 2 L 1 and HL 2
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 H 2 L 1 and HL 2 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 H 2 L 1 product was precipitated by addition of water but HL 2 was precipitated without addition of water. Both were recrystallized from ethanol, yields 86% for H 2 L 1 and 84% for HL 1 .