Porous polymer networks cross-linked by novel copper Schiff base complex: From synthesis to catalytic activity

Highlights

• A Copper Schiff base complex has been prepared.

• X-ray diffraction and NMR spectroscopy of paramagnetic organometallic complex has been implied to characterize copper Schiff base complex.

• Polyacetylene-type micro/mesoporous organometallic networks based on the copper Schiff base complex have been synthesized.

• The oxidation catalytic activity of the prepared organometallic networks has been proved.

Abstract

A new ethynylated Schiff base complex, [Cu(SALANI)₂], in which two N-salicylidene(3-ethynylaniline) (SALANI) ligands coordinated one Cu²⁺ ion, was prepared and characterized by single-crystal X-ray diffraction, FTIR and ¹H NMR spectroscopy. The ¹H NMR spectra were obtained and interpreted (with the help of DFT calculation) despite the paramagnetic character of the [Cu(SALANI)₂] complex. Chain-growth copolymerization of [Cu(SALANI)₂] with 4,4′-diethynylbiphenyl and 1,4-diethynylbenzene provided polyacetylene-type micro/mesoporous organometallic networks with [Cu(N-salicylideneaniline)₂] segments as network knots. Networks of this type were also prepared by postpolymerization metalation of polyacetylene networks containing N-salicylideneaniline segments as pendant groups. Postpolymerization metalation (leading to further cross-linking) enabled to transform nonporous parent networks into organometallic networks with permanent porosity. The new porous organometallic networks showed a specific surface area from 400 to 700 m²/g and a Cu²⁺ content of about 6 wt%. The networks prepared by both prepolymerization and postpolymerization metalation were active as heterogeneous catalysts of oxidation of linalool to linalool oxide.

Introduction

Schiff base complexes of metal ions are well known as versatile catalysts of many different reactions including oxidation, isomerization, epoxidation, condensation, polymerization and ring-opening/forming reactions [1], [2], [3]. In addition to imine CH=N group(s) the Schiff bases derived from salicylaldehyde also contain hydroxy group(s) suitably positioned for metal binding. The lone pair electrons of the N atom of the imine group(s) and the deprotonated hydroxy group(s) participate in the coordination of the metal ions in such complexes. The negative charge of the deprotonated OH group(s) fully or partially compensates the positive charge of the coordinated metal ion. Salen-type tetradentate ligands [various N,N′-ethylene-bis(salicylimine)s] containing two imine groups and two hydroxy groups are probably the most used ligands of catalytically active Schiff base complexes [4]. Complexes with bidentate N-substituted salicylimine ligands (e.g. of the N-salicylideneaniline-type) containing one pair of CH=N and OH groups are reported less frequently as the catalysts in the literature [1], [5]. On the other hand, various complexes with bidentate N-substituted salicylimine ligands were prepared and tested in non-catalytic applications. For example, Kargar et al. reported Cu and Zn complexes with bidentate Schiff-base ligands composed from ring-substituted salicylaldehyde and ring-substituted anilines or aminopyrazoles. The complexes were applied as antibacterial agents and their binding to DNA was studied [6], [7], [8], [9].Fig. 1..

Much attention has been paid to the heterogenization of Schiff base complexes aimed at the preparation of easily separable and potentially reusable catalysts [10], [11]. For example, Mn, Fe, and Cu salen complexes were anchored on surface-modified siliceous mesoporous molecular sieves of various types to form porous heterogeneous catalysts for the synthesis and transformation of epoxides [12], [13] and Henry reaction [14]. Efficient heterogeneous catalysts were also achieved by immobilizing Schiff base complexes on (i) polymer resins [15], (ii) graphene oxide [16], (iii) glass beads [17], (iv) magnetic nanoparticles [18] and other supports.

In several cases, heterogenization was carried out by covalent incorporating Schiff base complexes as building blocks into so-called Porous Organic Polymers (POPs) while maintaining the catalytic activity of these complexes. POPs are amorphous polymer networks with permanent micro/mesoporosity and a high specific surface area originating from the stiffness of the network segments combined with extensive cross-linking [19], [20]. A number of different polymerization and postpolymerization procedures have been developed for the preparation of POPs, which allowed to tune the covalent structure and textural parameters of these materials. This makes POPs very promising for a number of applications, especially in the field of reversible sorption [21], [22], [23], [24] and heterogeneous catalysis [25], [26], [27], [28], [29]. Xie et al. prepared a POP containing Co-salen segments by Sonogashira cross-coupling copolymerization of 1,3,5-triethynylbenzene and a dibromo-functionalized Co-salen complex. The prepared POP contained Co-salen segments as the struts between benzene-triyl knots and showed a Brunauer-Emmett-Teller specific surface area, S_BET, of 965 m²/g [29]. Co-salen containing hyper-cross-linked POPs with S_BET up to 663 m²/g were achieved by a knitting polymerization of the Co-salen complex with a 1,4-bis(bromomethyl)benzene cross-linker [30]. POPs containing Schiff base complexes with Al, Co and Ni ions were prepared by condensation polymerization of 2,3,6,7,14,15-hexaammoniumtriptycene with 2,6-diformyl-4-methylphenol in the presence of salts of respective metals [31]. One pot condensation reaction of 5-tert-butyl-4-hydroxyisophthalaldehyde, ethylenediamine, and copper(II) acetate provided POPs composed of Cu-salen and porphyrin segments [32]. The S_BET values up to 280 m²/g were achieved. Schiff base complexes containing POPs were active as heterogeneous catalysts for the Henry reaction [32] and for the conversion of epoxides and CO₂ to cyclic carbonates [29], [30], [31].

Linear substituted polyacetylenes, i.e. mostly soluble polymers of formulas [–HC=CR-]_n and [-R₁C=CR₂-]_n, are the first described conjugated polymers [33]. Polyacetylenes are prepared by transition metal-catalyzed coordination chain-growth polymerization of acetylenes, HCCR and R₁CCR₂ [34], [35]. In this way, not only hydrocarbon acetylenes but also monomers functionalized with a whole range of heteroatom groups, including imino [36], [37], [38] and hydroxy groups [39], [40], were successfully polymerized into linear polymers. Recently, we modified and optimized this polymerization to provide micro- and micro/mesoporous hyper-cross-linked networks of the POP-type (S_BET ∼ 1000 m²/g) [28], [35], [41]. In order to ensure extensive cross-linking of the products, aromatic acetylenes with two to four ethynyl groups per molecule were used as (co)monomers. The resulting POPs consisted of conjugated polyacetylene (polyene) main chains in which single and double bonds alternated. These chains were hyper-cross-linked by rigid arene-type cross-links. The polymerization was well compatible with different heteroatom groups of monomers, which enabled the preparation of functionalized POPs for specific sorption, catalytic and luminescence applications [24], [42], [43], [44], [45].

In this paper, we report new micro/mesoporous organometallic polyacetylene POPs containing molecules of N-salicylideneaniline complex with Cu²⁺ ions as network knots. These POPs were achieved by chain-growth polymerization of ethynylated precursors combined with either prepolymerization or postpolymerization metalation with copper(II) acetate. Both methods provided organometallic POPs with a high content of Cu²⁺ ions which were active as heterogeneous catalysts for oxidation of linalool.