Journal of Chromatography A, 1363 (2014) 11–26

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Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma

Review

Homochiral metal–organic frameworks and their application in chromatography enantioseparations Paola Peluso a,∗ , Victor Mamane b , Sergio Cossu c a b c

Istituto di Chimica Biomolecolare ICB CNR—UOS di Sassari, Traversa La Crucca 3, Regione Baldinca, Li Punti, I-07100 Sassari, Italy Institut de Chimie de Strasbourg, UMR 7177, Equipe LASYROC, 1 rue Blaise Pascal, BP 296 R8, 67008 Strasbourg Cedex, France Dipartimento di Scienze Molecolari e Nanosistemi, Università Ca’ Foscari di Venezia, Dorsoduro 2137, I-30123 Venezia, Italy

a r t i c l e

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Article history: Received 15 April 2014 Received in revised form 10 June 2014 Accepted 19 June 2014 Available online 25 June 2014 Keywords: Chiral stationary phases GC Enantioseparation HPLC Metal-organic framework

a b s t r a c t The last frontier in the chiral stationary phases (CSPs) field for chromatography enantioseparations is represented by homochiral metal-organic frameworks (MOFs), a class of organic-inorganic hybrid materials built from metal-connecting nodes and organic-bridging ligands. The modular nature of these materials allows to design focused structures by combining properly metal, organic ligands and rigid polytopic spacers. Intriguingly, chiral ligands introduce molecular chirality in the MOF-network as well as homochirality in the secondary structure of materials (such as homohelicity) producing homochiral nets in a manner mimicking biopolymers (proteins, polysaccharides) which are characterized by a definite helical sense associated with the chirality of their building blocks (amino acids or sugars). Nowadays, robust and flexible materials characterized by high porosity and surface area became available by using preparative procedures typical of the so-called reticular synthesis. This review focuses on recent developments in the synthesis and applications of homochiral MOFs as supports for chromatography enantioseparations. Indeed, despite this field is in its infancy, interesting results have been produced and a critical overview of the 12 reported applications for gas chromatography (GC) and high-performance liquid chromatography (HPLC) can orient the reader approaching the field. Mechanistic aspects are shortly discussed and a view regarding future trends in this field is provided. © 2014 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metal-organic frameworks (MOFs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Homochiral MOFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Enantioseparation on homochiral MOFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Chiral stationary phases for gas chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Chiral stationary phases for liquid chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

12 12 13 14 15 16 24 25

Abbreviations: H2 asp, aspartic acid; H2 bda, 2,2 -dihydroxy-1,1 -binaphthalene-6,6 -dicarboxylic acid; H2 bdc, 1,4-benzenedicarboxylic acid; H2 bpdc, 4,4 biphenyldicarboxylic acid; bpe, trans-1,2-bis(4-pyridyl)ethylene; bpy, bipyridine; H3 btb, benzene-1,3,5-tribenzoic acid; H2 cam, camphoric acid; CEC, capillary electrochromatography; CSP, chiral stationary phase; dabco, 1,4-diazabicyclo[2.2.2]octane; DCM, dichloromethane; def, N,N-diethylformamide; dma, N,N-dimethylacetamide; dmeda, dimethylethylenediamine; dmf, N,N-dimethylformamide; eda, ethylenediamine; FR, flow rate; GC, gas chromatography; hex, n-hexane; HPLC, high-performance liquid chromatography; i.d., inner diameter; IPA, isopropyl alcohol; H-isn, isonicotinic acid; l-H2 lac, l-lactic acid; MKD, minimum kinetic diameter; MOF, metal-organic framework; MP, mobile phase; NPLC, normal phase liquid chromatography; PCP, porous coordination polymer; POST, Pohang University of Science and Technology; PSM, post-synthetic modification; H2 sala, N-(2-hydroxybenzyl)-l-alanine; SBU, secondary building unit; TLC, thin layer chromatography; tmdpy, trimethylenedipyridine; UMCM, University of Michigan Crystalline Material. ∗ Corresponding author. Tel.: +39 079 2841218; fax: +39 079 2841299. E-mail address: [email protected] (P. Peluso). http://dx.doi.org/10.1016/j.chroma.2014.06.064 0021-9673/© 2014 Elsevier B.V. All rights reserved.

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P. Peluso et al. / J. Chromatogr. A 1363 (2014) 11–26

1. Introduction Chirality is an important property of nature, producing and regulating fundamental phenomena in living matter. More than 150 years ago, Louis Pasteur observed the spontaneous resolution of racemic sodium ammonium tartrate tetrahydrate into enantiomorphous crystals. Moreover, he reported that the dextro form of ammonium tartrate was more rapidly destroyed by the mold Penicillium glaucum than the levo isomer [1]. This observation led Pasteur to the recognition of the role stereochemistry plays in the basic mechanisms of life, generating a fundamental relationship between molecular dissymmetry and living matter. Particularly, life is essentially constructed using l-amino acids as building blocks and, consequently, it is governed by homochirality [2]. The term homochiral was introduced by Lord Kelvin in 1893 when chirality was defined as a relational term and two relations, homochiral and heterochiral, are possible [3]. On this basis, it appeared evident that chiral molecules could recognize other chiral molecules through suitable mechanisms and, as declared by J. M. Lehn, recognition is not a mere binding but it is binding with a purpose [4]. Besides its original meaning, since the 1980s, the term homochiral has been employed to indicate that a compounds or a sample consists of only one enantiomer [5,6]. In keeping with the importance of chirality, with the challenging aim to produce pure enantiomers, scientists have spent research efforts for developing methods to resolve enantiomers, reproducing homochirality in xenobiotic chemical contexts and understanding the mechanisms which govern the chiral discrimination processes. In general, two approaches can be carried out, namely stereoselective asymmetric synthesis of just one of the enantiomers or the resolution of a racemic mixture [7]. Nowadays, gas chromatography (GC) and high-performance liquid chromatography (HPLC) on chiral support are largely used by academic, industrial and pharmaceutical analysts for the enantiomeric purity measurement of chiral compounds as well as for resolving racemic mixtures. The evolution of the techniques as well as the development of chiral columns of high efficiency made chromatography a strategic tool for the chemical sciences. Inside the chiral column, the enantioseparation event is driven by the ability of the enantiopure molecular system characterizing the chiral stationary phase to discriminate between the two introduced enantiomers by forming two energetically and kinetically distinguishable transient diastereoisomeric complexes. The understanding of the chiral recognition mechanisms is a fundamental tool to design a successful enantioseparation. Thus, to date the development of the chiral chromatography science [8–17] occurred parallel to the advancements in understanding the chiral recognition mechanisms of the enantioselective biological and chemical processes [18]. Indeed, in keeping with the pioneering works of Fisher [19], Easson and Stedman on biological enantioselectivity [20], Dalgliesh invoked the three-point interaction to explain the enantioselective separation of amino acids achieved by chromatography on cellulose paper [8]. This work represented the first successful experiment of chiral chromatography which proved the possibility to resolve racemic mixture of chiral compounds by using chiral supports. Nowadays cyclodextrin- and polysaccharide-based chiral stationary phases (CSPs) are the most versatile and used for GC and HPLC, respectively. In particular, the high versatility of polysaccharidebased selectors is mainly due to their polymeric structure where molecular, conformational and supramolecular chirality cooperate to determine the separation outcome [18]. However, understanding the intermolecular interactions involved in the retention and discrimination processes inside the polysaccharide-based CSPs is an unsolved problem which still continues to stimulate the research into the field. Indeed, only a few aspects of the enantiorecognition mechanism have been clarified because of a) the lack of adequate

crystallographic models to be employed in docking studies, b) the complexity of the three-dimensional structure of the selectors and c) the presence of multiple active sites involved in the enantiorecognition process [21]. Despite this issue is still open, several works devoted to this topic allowed to overcome the pioneering three point interaction model through the identification of repulsive interaction [10], conformational adjustment of the analyte to promote the steric fit into the chiral groove [22] and, more recently, the possibility to use stereoselective halogen bonds [23] as a new tool to drive chiral recognition besides the well-known hydrogen bonding, ␲–␲ and dipole–dipole interactions. The last frontier in the chiral CSPs field is represented by homochiral metal-organic frameworks (MOFs) [13–15]. This topic is in its infancy and, to date, a limited number of enantioselective separations are reported in the literature. However, the availability of homochiral MOF materials represents a promising challenge for the future of chiral chromatography. Indeed, MOFs consist of polymeric networks, thus the homochiral material can be characterized by molecular, conformational and supramolecular chirality. Moreover this type of chiral supports can be designed, synthesized and structurally characterized and, consequently, the chiral environment related to their specific enantioselective adsorption can be potentially predicted or decoded for understanding operative chiral recognition mechanisms. In the last years, the application of nanomaterials in the field of separation has been covered in some significant reviews [24–28], but a comprehensive overview of the research field on homochiral MOF-based CSPs for chromatographic enantioseparations is missing. Indeed, the majority of studies were developed starting from 2013 [29]. In this review, after an introduction of the topic by summarizing seminal and more recent studies in the field, we focus on the use of homochiral MOFs as CSPs for GC and HPLC applications by highlighting synthetic strategy, structural features and chromatographic results.

2. Metal-organic frameworks (MOFs) MOFs, also known as porous coordination polymers (PCPs) [30], are solids with permanent porosity which are built from nodes (metal ions, clusters or secondary building units) bridged by organic linkers to form one- (1D), two- (2D), or three-dimensional (3D) coordination networks (Fig. 1). The concept of secondary building units (SBUs) was introduced by Yaghi and co-workers to classify MOF structures into different topologies [31]. SBUs are described as molecular entities (complexes or clusters) in which ligand coordination modes and metal coordination environments can serve for the transformation of these fragments into extended porous networks using polytopic linkers. The interest in MOFs started around 1990, when Hoskins and Robson focused on the assembly of organic and inorganic building blocks in order to build porous structures [32]. Later, Yaghi and coworkers prepared the first robust and highly porous MOF, MOF-5, based on Zn4 O(CO2 )6 octahedral SBUs each linked by six chelating 1,4-benzenedicarboxylate (bdc) units to give a cubic framework [33]. The advantages of the modular nature of MOFs were highlighted in a recent study of Fedin and co-workers in which the structural features of copper- and nickel-containing MOFs could be tuned by changing the length of the rigid spacers from 4,4 bipyridines (bpy) to trans-1,2-bis(4-pyridyl)ethylene (bpe) [34]. Many variations of nodes and linkers are available allowing tuning the structure of MOFs for applications in a variety of areas [35,36], including gas storage [37], molecular separations [26,28] and catalysis [38]. These applications are commonly based on adsorption and/or recognition properties which are related

P. Peluso et al. / J. Chromatogr. A 1363 (2014) 11–26

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Fig. 1. Schematic representation of MOFs and their current uses.

to structure, topology, pore size and surface area of the MOFs. Therefore, highly guest selective adsorption and high separation performances can be attained either during the MOF preparation by varying the synthetic conditions [39] or by post-synthetic modification (PSM) of the MOF material [40]. It is worthy of note that the permanent porosity of MOFs is one of most important properties of these materials. Among them, mesoporous coordination polymers (pore size = 2–50 nm) are considerably rarer than microporous coordination polymers (pore size