Biological applications of macrocyclic Schiff base ligands and their metal complexes: a survey of the literature (2005-2019)

This article aims to provide a survey of biological applications of Schiff base macrocycles and their metal complexes, with emphasis given to the synthesis of the compounds and to their uses as antibacterial and antifungal agents. The literature on the subject, published during the 2005-2019 period, is shortly reviewed. This is an informed report collecting information on the addressed topic in a concise systematic way, and can be expected to be useful as a fast literature catalogue for researchers working on this and related domains.

The usual amount of di-and polyamines present inside the cells of living systems is at the millimol level [15,16]. However, the concentration of free amines is small and maintained within a very narrow range (7-10% of the total amount) because the decrease in their concentration inhibits cell proliferation while excess is toxic [15,16]. Indeed, most of the di-and polyamines in the cells are bound to different polyanionic molecules, especially nucleic acids, but also proteins and phospholipids [13][14][15][16][17][18].
It has been noticed that an increase in the concentration of polyamines may be associated with rapidly proliferating cancer cells (in particular in the case of breast, colon, lung, prostate and skin tumors) [19,20], and this has been used as basis for diagnosing cancer and monitoring progresses during cancer treatment [6,19,[20][21][22]. In fact, the interaction of polyamines with nucleic acids and other biological molecules, in terms of their association with cancer, has become the focus of intensive research [1,[19][20][21][22][23][24].
Another relevant use of di-and polyamines is in the synthesis of Schiff base compounds, which can then receive applications in coordination chemistry. Indeed, complexation of Schiff base ligands (in particular of macrocyclic Schiff base derivatives) with metal ions has been shown to give rise to a considerable number of chemical systems exhibiting unique properties for application in several domains, like catalysis, materials sciences and medicine, for example [25,26]. This fact has been giving increased importance to the investigation on this particular type of compounds.

Schiff base ligands
Schiff bases are compounds with the general formula RR'C=NR'' (R'' ≠ H), and can be considered a sub-class of imines [27,28]. Depending on its structure, a Schiff base can be either a secondary aldimine (when R or R' is an H atom) or a secondary ketamine (when both R and R' are different from H). Their general designation (Schiff base) originates from Hugo Schiff, who first discover this type of compounds [29]. A number of other specific usual names exist for Schiff bases having particular structural elements. For example, a Schiff base derived from an aniline (where R'' is a phenyl or a substituted phenyl group) can be called an anil, while bis-Schiff base compounds are frequently designated as Salen-type compounds.
Schiff bases can be synthesized from an aliphatic or aromatic amine and a carbonyl compound by nucleophilic addition forming a hemiaminal, followed by dehydration to generate the imine.

Scheme 1 -Synthesis of a Schiff base from condensation of a carbonyl and an amine precursor.
The literature on Schiff bases is extensive and it is not purpose of this article to address this general topic. In here, we will focus on a specific type of Schiff bases, the ones designated currently by macrocyclic Schiff bases, which are the members of this family of compounds bearing at least one large (7 membered or larger) ring. An example of a naturally occurring macrocyclic Schiff base derivative is corrin, which is the parent macrocycle related to the substituted derivative found in vitamin B12 (cobalamine), but the majority of this type of compounds are produced in the laboratory.
Considerable effort has been made in the last two decades for developing metal-free methods for furnishing macrocycles starting from dicarbonyl compounds and diamines (Scheme 2) in addition to standard metal-templated protocols [30,31] (macrocyclic) ligands has been synthesized in order to ascertain correctly the role of the different coordinating atoms and of their relative position on the structures resulting from coordination of these ligands to metal ions [30][31][32][33][34][35][36][37][38][39][40][41]. These studies also aimed to understand the factors that most contribute to determine the number and size of the chelating rings formed upon complexation of Schiff base macrocyclic ligands, as well as the effect of the flexibility and shape of the coordinating moiety on binding [31,[42][43][44][45]. Progress on the chemistry for synthesis of these type of Schiff bases has allowed, for example, the preparation of macrobicyclic ligands via a one-step multiple condensation reactions procedure [30,[42][43][44][45]. .

Oligomers [2+2]-(Macrocycle)
Scheme 2 -Main possible condensation products of dicarbonyl compounds with diamines in different proportions. The R groups may be distinct from each other and X and Y represent generic fragments (in general alkyl chains).
The macrocyclic Schiff bases can also be reduced to the related polyamine derivatives, containing the same cyclic complexity, by reaction with an appropriate reducing agent [30][31][32][33].
Similarly, the related complexes can undergo reductive decomplexation reactions when treated with appropriate reductants, with the consequent formation of the corresponding polyamine derivatives [30][31][32][33]. These polyamine compounds are less sensitive to hydrolysis and more flexible than their imine analogous, what can be of interest for certain applications.
Introduction of specific functionalities at the periphery of the coordinating moiety has allowed the synthesis of macrocyclic systems capable of multi-recognition, and that may be used, for example, in specific separation [46][47][48], transport processes across membranes [49,50], or activation and catalysis in ecocompatible solvents [51]. In these areas, the presence in the molecules of pendant arms (two or more) attached to the proper positions of the macrocycle framework appears particularly interesting, since it allows for the generation of a pseudo-enclosed environment suitable for modeling of supramolecular molecule-receptor systems [52]. On the other hand, for macrocycles acting as receptors, the hole size represents an additional parameter which may greatly influence the ability to discriminate among the different species to be recognized.
Macrocyclic Schiff base derivatives are in general good chelating agents and prone to form different types of complexes with metal ions. In fact, one of the most popular methods for the synthesis of macrocyclic Schiff bases requires the active participation of a metal ion in the process, i.e., the metal-promoted one-step (template) condensation method [36][37][38][39][40]. In this synthetic procedure, a metal ion is used as template to induce orientation of the reacting groups of linear substrates in the required conformation for the ring to close [53]. In this way, one has direct access to macrocyclic Schiff base metal complexes, whose practical applications are, as already mentioned, relevant in many different domains [14,17,33,52]. In this article we will focus on their uses as bioactive systems, in particular as antibacterial and antifungal agents.

Macrocyclic Schiff base metal complexes
The majority of therapeutic drugs are organic compounds and, because of this, less attention has been given in this field to inorganic and coordination compounds. Nevertheless, the number of inorganic and coordination compounds shown to exhibit beneficial bioactive properties has been growing considerably during the last decades. They have been noticed to be particularly effective in cancer therapy, as a result of their ability to specifically interact with DNA [1,6,[19][20][21][22][23][24]54]. The prototype compound is cisplatin [cis-diaminedichloroplatinum(II)], which was approved as a chemotherapeutic drug for use in testicular and ovarian cancers by the U.S. Food and Drug Administration (FDA) in 1978 [55][56][57], and in Europe one year later [58]. Interestingly, despite the fact that thousands of cisplatin analogues have been synthesized since then, and tested as potential anticancer drugs, only two of them, carboplatin and oxaliplatin, have been used in the clinical treatment of neoplasic diseases, while the rest have remained inactive [54,[59][60][61][62][63][64].
Being common ligands in coordination chemistry, Schiff bases have been used extensively in this field, giving rise to a plethora of different types of complexes. Their coordination ability results mainly from the -acceptor properties of their constituting imine nitrogen atom. Accordingly, the coordination versatility of Schiff bases strongly increases when the ligand bears several imine groups, which may act as multiple coordination sites. Procedures have been developed successfully for preparation of mono-, di-or polymacrocycles, catenands, compartmental ligands and calixarenes based on Schiff bases [65].
An interesting observation is that complexation with a metal often improves the stability of the Schiff base ligand, while the majority of macrocyclic complexes have also been noticed to be both kinetically and thermodynamically more stable than the analogous compounds with non-cyclic ligands [66]. These facts have contributed to stimulate research on this type of compounds, which in turn has allowed elucidating aspects of the reactivity of Schiff base coordination compounds that would not be possible to investigate using the less stable analogous complexes of non-cyclic ligands. The interest in exploring metal ion complexes with macrocyclic Schiff base type ligands has received strong stimulus also owing to the recognition of the role played by this kind of structures in metalloproteins [65], and a broad variety of Schiff base macrocycles have been used for metal-biosites modeling of cationic, anionic or neutral receptors [30]. In addition, the progress on synthetic macrocyclic chemistry has also resulted in an increased understanding of the properties and function of naturally occurring biological macrocycles and their complexes [52,67].
Among the general type of systems considered in this review, those formed by functionally substituted macrocyclic Schiff bases ligands bearing additional donor groups are of particular interest. Such ligands represent one of the most important classes of heteropolydentate ligands capable of forming not only mononuclear complexes of different types with transition and/or nontransition metal ions, but also diverse bi-and polynuclear complexes with potential for application in several areas. The progressive availability of larger and more multifarious ligands exhibiting such structural characteristics has been facilitating also the design of macrocyclic Schiff base metal complexes with specific properties resulting from the simultaneous presence of metal ions in close proximity within the same coordinating moiety. For example, 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 introducing modifications in the ligands, one can adjust the distance between the two chambers and/or the paramagnetic centers as well as the chemical environment around them, thus allowing for the modulation of the magnetic interactions in order to achieve the desired materials' magnetic properties [66].
These types of complexes can be obtained by self-condensation of suitable formyl-or ketoprecursors and primary (poly)amines followed by coordination to the metal center via reaction with an appropriate metal salt [30,66]. Alternatively, they can be obtained in a single step by the already mentioned metal-template procedure [36][37][38][39][40]53]. In some cases, the initially synthesized complexes can be converted in other species through transmetalation reactions, leading to species otherwise not accessible to synthesis [66]. Template and transmetalation reactions quite often give rise to the desired complexes in high-yield and in a satisfactory purity grade.  PROTAC (proteolysis-targeting chimeras) type active agent [69]. Nevertheless, in spite of all accumulated favorable data, and the large list of publications stressing the potential of this type of compounds as medicines, the practical use of macrocyclic compounds in general as drugs is yet rather limited. Indeed, there is still a considerable resistance to the acceptance of macrocycles as pharmaceutical agents mostly because they did not fit the usual structural paradigm followed by the industry: small heterocyclic molecules with a reduced number of functional groups that most of times conformed to the Lipinski's rule of five [70]. Also, the synthetic challenge for macrocyclic structures is just recently been partially overcome and time is still required to fulfil the necessities of the library formats needed for the high throughput screening efforts essential to the majority of contemporary drug discovery programs. These facts give additional relevance to publications gathering together the most prominent recent results on the bioactivity of macrocyclic compounds (as those considered in this article), which appear as a privileged way to stimulate further fundamental research in this field and, in particular, market-targeted research by the specialized R&D laboratories of the pharmaceutical companies.

Literature survey (highlights 2005-2019)
In this section, the most relevant literature on biological applications of macrocyclic Schiff base ligands and their metal complexes appearing during the period 2005-2019 is reviewed in brief. The data have been arranged in the format of a table (Table 1), where the most relevant results and the systems investigated in the surveyed studies are concisely described. To facilitate the search for a specific chemical system, for each article mentioned in the Table an illustration is provided, which shows the schematic structure of a representative compound or type of compounds addressed in the cited publication. The articles are organized by year of publication instead of by subject.
Though the alternative possibility has also some advantages, the presentation of the articles by chronological order permits to follow in an easier way the progress over the time on a given matter and, more importantly, to interconnect this progress with developments taking place on related subjects. On the whole, 84 articles (besides those cited in the introduction) dealing with biological applications (in many cases better designated as biological potential applications) of macrocyclic Schiff base ligands and their metal complexes are described in this review, in particular those related to their ability to act as antibacterial or/and antifungal agents. This is an informed report gathering information on this topic in a concise systematic way, and will certainly be of great value for the researchers working on this and related domains.        The synthesis and carachterization of four Mg(II) complexes prepared via cyclocondensation of 2,6diformylpyridine and 2,6-diacetylpyridine with two hexadentate hexaamines, in the presence of Mg(II) ion, were reported. Their antibacterial and antifungal properties against E. coli and S. aureus, and C. albicans, respectively, were also described.    A new asymmetric heptaaza Schiff base macrocyclic bis (pendant donor) Mn(II) complex has been prepared and tested against E. coli, S. aureus and C. albicans. The optimized geometry of the prepared complex has been obtained from density functional method, using B3LYP/6-31G* basis set, and its structure discussed.    condensation of 2,6-diacetylpyridine with three different linear aromatic amines were investigated. The compounds were found to exhibit antibacterial activity against both Gram positive (S. aureus, B. cereus, C. xerosis) and Gram-negative bacteria (E. coli, K. pneumonia, P. vulgaris). It was also shown that the antibacterial activity of the investigated Cd(II) complexes is greater compared to that of the corresponding Zn(II) complexes.

S. Chandra, S. Agrawal
Complexes of Ce(III), Nd(III), Sm(III) and Eu(III) with a macrocyclic ligand obtained by the condensation of 2,6-diacetylpyridine with thiourea were prepared and characterized. The compounds were shown to exhibit in vitro moderate antifungal behavior against A. niger and F. oxysporum. Novel bisaldehyde-hydrazide Schiff bases were prepared as new macrocyclic compounds via condensation reactions of (2,2'-(ethane-1,2-diylbis (oxy))dibenzaldehyde) bisaldehyde and terephthaloor benzohydrazide. Their biological activities were tested in vitro against E. coli, P. vulgaris, B. subtilis and S. aurous bacteria, in order to assess their antimicrobial potential.    The synthesis of macrocyclic complexes of M= Cr(III), Mn(III) and Fe(III) with a Schiff base ligand obtained through the condensation of 1,4-dicarbonyl phenyl dihydrazide with 1,2-di(1H-indol-1yl)ethane-1,2-dione was reported. The antifungal and antibacterial activities of the complexes against Aspergillus sp., Rizoctonia sp. and Penicillium sp., and S. aureus, B. subtilis, P. aeruginosa, E. coli and S. typhi, respectively, were found to be considerably higher than those of the free ligand and of the reference drugs phenylbutazone (anti-inflammatory), imipenem (antibacterial) and miconazole (antifungi).  The macrocyclic ligand, (2E)-3,6,10,13-tetramethyl-2,7,9,14-tetraaza-1,8 (1,4)-dibenzenacyclotetradecaphane-2,6,9,13-tetraene and its Co(II), Ni(II) and Cu(II) complexes were isolated and characterized. The compounds were screened in vitro against pathogenic bacteria, and concluded to have better inhibition proprieties against Gram positive (S. pyogenes and S. epidermidis) than against Gram negative (P. vulgaris and Klebsiella sp.) bacteria. The complexes were also found to have some protective effect on DNA. Several macrocyclic Schiff base complexes with the homopiperazine skeleton were prepared via template approach, based on the condensation reaction of an amine containing homopiperazine moiety and 2,6diacetylpyridine or 2,6-pyridine dicarboxaldehyde, in the presence of M= Cd(II), Mn(II) and Zn(II) ions. The cytotoxic activity of the compounds was found to be stronger than that of doxorubicin, used as standard. Also, the bonding situation in the complexes, were analyzed by NBO and energy-decomposition analyses. The synthesis, characterization and antimicrobial properties of some novel macrocyclic complexes with general formula [M(C22H16N4O2)X]X2, where M= Fe(III), Co(III) and Cr(III), and X is either bromine or chlorine, were reported. The complexes were prepared by template condensation of ethylenediamine and 2,2-dihydroxyindane-1,3-dione using salts of the considered trivalent metals. The antimicrobial properties against Gram positive (B. subtilis and B. stearo thermophiles) and Gram negative (P. putida and E. coli) bacteria were investigating in comparison with standard antibiotics (streptomycin and chloramphenicol). The novel cobalt bromide and chromium chloride complexes were concluded to have significant antimicrobial activity.  [154]