Dehydration/Rehydration Processes and the Porous Structure of a Tris(Macrocyclic) Carboxylate Copper(II) Complex

Abstract

A. I. Prikhod'ko, L. V. Tsymbal, P. S. Yaremov, V. G. Il'in, and Ya. D. LampekaWe have established that the crystal hydrate of the tris(macrocyclic) complex of copper with carboxylate, with a porous structure, undergoes stepwise dehydration leading to a change in its crystal lattice structure; but even total dehydration does not lead to loss of porosity in the latter. We have studied the sorption characteristics of the dehydrated sample with respect to water, methanol, and hexane.Porous crystalline materials based on metal ions linked by organic ligands ("metal-organic frameworks," MOFs) have attracted considerable attention in recent years [1]. These materials are interesting both from the standpoint of basic research and because of their specific magnetic, optical, sorption, catalytic, and other properties [2], which makes them promising systems for practical application. Publications in recent years also suggest the possibility of using MOFs for sorption and storage of gases [3,4], including molecular hydrogen [4][5][6].A special case of MOFs is crystal hydrates constructed on the basis of azamacrocyclic complexes of copper(II) and nickel(II) and aromatic carboxylates. They form a wide range of anisotropic lattices of different dimensionalities with pores and/or channels [7]. Study of the thermal and sorption properties of such compounds has shown that many of them undergo reversible dehydration/hydration reactions with retention of the crystal lattice, and can selectively react with guest molecules. In the overwhelming majority of cases, in materials of this type the carboxylate anions act as bridging ligands linking the metal ions into one-dimensional chains or two-dimensional layers (coordination polymers). The only macrocyclic MOF with a nonpolymeric structure known today is [Ni(cl)(H 2 O) 2 ] 3 (btc) 2 ⋅24H 2 O (cl = 1,4,8,11-tetraazacyclotetradecane (cyclam); btc 3-= the anion of 1,3,5-benzenetricarboxylic acid). Interaction between the cation and the anion in the crystal lattice of this complex occurs via hydrogen bonds between water molecules coordinated to the nickel(II) ion and the ionic carboxylate [8]. It has been shown that this compound has specific sorption properties [9]. Recently we described another complex with a similar crystal structure, [Cu 3 L(H 2 O) 6 ](btc) 2 ⋅40H 2 O (I), where L is a tris(macrocyclic) ligand constructed on the basis of melamine (2,4,6-triamino-1,3,5-triazine) [10].

Influence of specific interactions on the sorption characteristics of porous complexes of 3d metals with derivatives of 4,4′-diazophenyl

“…Since the form of the isotherm of methanol and water by PCC does not correspond to the model of the volume of filled micropores, it is impossible to calculate the numerical characteristics of the interaction of methanol with these complexes. However, it was shown with the example of macrocyclic complexes with porous structures [7] that the characteristic energy (E) of the absorption of methanol may be considerably greater than E for water (by a factor of about 2, despite the fact that the value of E for the adsorption of methanol found in [7], seems somewhat too high to us).…”

“…In particular, PCC contain pores of strictly defined size and topology, but it is possible by varying the transition metal ions in the composition of the complex and/or the bonded bridging ligands, it is possible to obtain compounds with required sorbtive or catalytic properties [4]. The key role in determining the absorption characteristic of a PCC is frequently played by the specific interactions of the PCC with the sorbate, which can be determined by the ratio of the size of the pores of the adsorbent and the size of the molecule of the adsorbate (for example, a sample may absorb a large amount of H 2 and O 2 but absorb practically no N 2 and CO [5]) or as the difference in the bonding energy of the different adsorbates, i.e, the specific interaction of the adsorbate and the PCC [6,7]. The interaction of the adsorbate with the coordination compound, the energy of which is comparable with the energetic barrier for conformational reconstruction of the adsorbent, may be explained by the possibility of "breathing" of a complex compound with a porous structure [8,9].…”

“…The consequence is that this interaction may play a key role in determining the size of the limiting sorption volume. At the present time in most papers studying the properties of a PCC, the sorption characteristics are determined from the adsorption isotherm for a single sorbate (usually nitrogen), while the numbers of papers in which there is a systematic analysis of the interaction of sorbates of various types with a PCC is quite small [5][6][7][8][9]. In describing the sorption characteristics they are frequently divided into case 1) absorbed into pores formed by a "rigid" crystal structure, 2) absorbed into pores of a "breathing polymer" with a "labile" crystal structure [8], and 3) absorption in points of the crystal structures, formed by non-bonded elements [9].…”

“…Note that examples are known of complex compounds possessing a porous structure in which separate structural elements are not bonded [7,9,13,14]. The complexes discussed in this work may be provisionally separated into two groups: formed from 3d metal ions with a characteristic coordination number of 6 (Fe 3+ ,Co 2+ , Ni 2+ ) and from ions with a lower coordination number (Cu 2+ 5 or 4, Zn 2+ 4).…”