Review
The development of compartmental macrocyclic Schiff bases and related polyamine derivatives

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Abstract

The design and synthesis of [1 + 1], [2 + 2], [3 + 3] or [3 + 2] macrocyclic or macrobicyclic Schiff bases, the multiple self-condensation procedure between appropriate polyformyl- and polyamine-precursors or the templating capability of different metal ions in directing the synthetic pathway toward specific compounds are reported together with the use of transmetalation reactions of particular complexes with a different metal salt in order to obtain not otherwise accessible complexes.

The reduction of the cyclic Schiff base or the reductive demetalation of the related complexes to the polyamine homologues is also considered.

These systems can form mononuclear, homo- and heterodinuclear (or polynuclear) complexes, when reacted with appropriate metal salts. Attention is especially devoted to the physico-chemical and structural aspects of the resulting systems, especially the magneto-structural relationships arising from the interaction of paramagnetic ions, coordinated inside a unique moiety.

The role of compartmental ligands, i.e. their ability to bind two or more metal ions in close proximity into two identical or different compartments, the presence of bridging groups inside these coordination moieties and their relevance in modulating the type and the extent of mutual interaction between the metal ions inside the adjacent chambers is also reviewed.

The insertion of asymmetry into these ligands provides important diversification of the coordinating sites and allows for different and well defined recognition processes involving specific cations and/or anions at the adjacent sites.

Introduction

Schiff bases have been extensively employed in the understanding of molecular processes occurring in biochemistry, material science, catalysis, encapsulation, activation, transport and separation phenomena, hydrometallurgy, etc. [1], [2].

A large variety of [1 + 1] and [2 + 2] macrocyclic ligands have been synthesized to ascertain correctly the role of the different donor atoms, their relative position, the number and size of the chelating rings formed, the flexibility and the shape of the coordinating moiety on the selective binding of charged or neutral species and on the properties arising from these aggregations [3], [4].

The evolution of these Schiff bases has produced macrobicyclic ligands obtained in one-step multiple condensation reactions [4], [5]; the cyclic [3 + 2] Schiff base condensation represents the extension of the [2 + 2] macrocyclic coordination systems into the third dimension.

The introduction of specific functionalities at the periphery of the coordinating moiety gives rise to quite sophisticated systems capable of contemporary multi-recognition processes, specific separation and transport processes across membranes or activation and catalysis in ecocompatible solvents.

For macrocyclic receptors the hole size represents an additional parameter which may influence greatly the ability to discriminate among the different charged or neutral species to be recognized. The progressive enlargement of the coordinating moiety allowed studies aimed at a deep understanding of physico-chemical properties arising from the simultaneous presence of two or more metal ions in close proximity within the same coordinating moiety.

Using compartmental ligands, binuclear complexes have been synthesized, where the two metal centers, if paramagnetic, interact with each other through the bridging donor atoms of the ligands in a ferromagnetic or antiferromagnetic way. By changing the type of the ligand, the distance between the two chambers and/or the paramagnetic centers, it is possible to vary considerably the magnetic interaction and, with particular complexes ferromagnetic interactions have been observed. Thus, these complexes may be good building blocks for the preparation of molecular magnets.

Complexes in which a single ligand organizes more than two metal centers into some predetermined arrangement, giving rise to unique behaviour, have been also designed, synthesized and fully characterized. They can be obtained by simple self-condensation of suitable formyl- or keto- and primary amine-precursors. Multiple self-condensation processes give rise to planar or tridimensional compounds in one step. Alternatively, they can be obtained by template procedure: this synthetic pathway directly gives the designed complexes. Moreover, these complexes can undergo transmetalation reactions when reacted with a different metal salt, allowing the formation of not otherwise accessible complexes. Template and transmetalation reactions quite often give rise to the designed complexes in high yield and in a satisfactory purity grade.

The macrocyclic systems can be functionalized by inserting appropriate groups in the aliphatic and/or aromatic chains of the formyl- or keto- and amine-precursors.

Furthermore, the Schiff bases can be reduced to the related polyamine derivatives, containing the same cyclic complexity, by reaction with an appropriate reducing agent. Similarly the related complexes can give rise to reductive decomplexation reactions when treated with appropriate reductants with the consequent formation of the corresponding polyamine derivatives. These polyamine compounds are less sensitive to hydrolysis and more flexible. Furthermore, they contain NH groups which may be further functionalized by appropriate synthetic procedures.

Depending on the shape of the formyl- and amine precursors and/or the metal ion template used, differing ring sizes can be formed during the cyclization reaction. If one dicarbonyl moiety reacts with one diamine moiety a [1 + 1] macrocycle results; when two dicarbonyl precursors react with two diamines then a [2 + 2] macrocycle results while if three dicarbonyl precursors react with two tripodal triamines a [3 + 2] macrobicycle is formed, and so on. This synthetic procedure may form [3 + 3] or [4 + 4] superior macrocyclic homologues or [1 + 1] polymeric species. Similarly one tricarbonyl precursor reacts with a tripodal triamine to form a [1 + 1] macrobicyclic compound while three dicarbonyl precursors react with a tripodal amine to form [3 + 2] macrobicyclic compounds (Scheme 1).

The success of these cyclization reactions is usually identified by analysis of the infrared spectrum of the product: this should contain no absorptions due to the primary amine or carbonyl groups but should have a new band due to the presence of the imine bonds. NMR and mass spectrometry measurements offer further useful indication about the cyclic product obtained. When suitable crystals are available, X-ray diffractometric investigations give the final proof of the macrocyclic nature and the molecular complexity of these systems.

This comprehensive list in addressed to review the most recent results obtained with compartmental macrocyclic or macropolycyclic Schiff bases and related reduced polyamines together with the homo- and/or heterodinuclear complexes mainly arising from their reaction with d- and/or f-metal ions and published between 2002 and 2005. The previous results on the same topics were covered by recent reviews [3], [5]. Scheme 2 contains the diformyl-, diketo- or tripodal triformyl-precursors and the linear or tripodal polyamine derivatives especially used in the Schiff base condensation reactions, reported in the literature reviewed in the present article.

The preparation and properties of other, more sophisticated precursors and related macrocycles are reported in the appropriate sections together with their complexes.

The formyl-precursors can be derived into two classes: (i) those containing a bridging group between the formyl moieties; (ii) those containing a spacer group between the formyl moieties without bridging groups.

The employed amine precursors can bear additional donor groups in the lateral chains.

Section snippets

Phenolate-based systems

Variously diformyl precursors have been prepared respectively by the oxidation of 2,6-dimethanol-4-substitued phenol with MnO2 in CHCl3, (H-1a⋯H-1c) the reaction of 2,3-dihydroxy-benzaldehyde with the appropriate ditosilate derivative in the presence of NaH and in anhydrous dimethylsulphoxide (H2-3a⋯H2-3c), by the Mannich reaction of 5-substituted salicylaldehyde with the appropriate diamine and formaldehyde in alcoholic solution (H2-4⋯H2-7) or by reaction of the appropriate

Macrobicyclic ligands and related complexes

[1 + 1] or [3 + 2] macrobicyclic Schiff bases and related reduced polyamine analogues have been prepared according to Scheme 12. In particular the [1 + 1] cryptands have been synthesized by the condensation of equimolar amounts of methanolic solutions of the desired tripodal trialdehyde with a suitable tripodal triamine; the further reduction of the resulting Schiff base with NaBH4 affords the related polyamine derivative [151]. The [3 + 2] tripodal Schiff bases have been prepared by condensation of

Conclusion and future perspectives

The considerable number of papers published in the period of time considered (2002–2005) reveals the great interest toward the synthesis, characterization and possible applications of compartmental Schiff bases, their reduced homologues and the related homo- and/or heterodinuclear complexes.

In these years special effects have been devoted to the synthesis of cyclic polyamine derivatives, obtained by reduction of the related Schiff bases, by NaBH4, LiAlH4, etc., or by reductive demetalation of

Acknowledgments

We thank Mrs. G. Bonato and Mr. A. Aguiari, for the valuable assistance in the collecting and classifying the data and in preparing drawings and the manuscript. Also we thank Progetto FIRB RBNE019H9K, MIUR for financial support.

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