Crown Ether Schiff bases and their Complexes : Recent Advances ( A Review )

In latterly years, an interest in the synthesis of crown ether Schiff bases and their Complexes has been increased, due to the significance and broadly the uses of these compounds in various fields. In the present review paper outline extensive recent advances literature survey on the crown ether including azomethine gathering and their complexes with the tough and easy granter atoms, has been reconsidered. Assertiveness has been done on element complexation with crown ether holding Schiff bases to enable the researchers to procure valuable information of the chelating activity of crown ether containing azomethine gathering and their enforcement.


INTRODUCTION
The first set of crown ethers compounds prepared by Pedersen assumes an essential part in the growth of supramolecular chemistry [1][2][3][4] .In the area of supramolecular chemistry, crown ethers are suited to the gathering of most famous host group, since their implication complexes supply high practical applications [5][6] .Macrocycles offer the thrilling potential to build supramolecular congregations that are fit for performing very particular atomic capacities.Molecular enforcement through these composition and their geustes, essentially transition elements ions or bio-molecules (nucleic acids, proteins, etc.), supply a high chance for seeking key side of supramolecular chemistry, which are also considered in a diversity of directions including, biology, medicine, physics, chemistry and related science and technology [7][8][9] .The best notable character of crown ethers is their potential to obtain compounds electively with ionic hosts, such as cations of elements of the first group and the second group of periodic table, the various crown ether hole bulk can be elected to coordinate selectively with a given cation 10 .Crown ether ligands with extra donor atom in the side chain have been prepared orderly to vary the ion bonding capability, sensitivity and selectivity of the origin crown ethers.Crown ethers [11][12][13] along with Schiff base [14][15][16] consideration of coordination chemists.Collection of these fractions in one molecule superiority to ambis-dentate chelating systems able to figuration of both coordinate with d metals of intermediate hardness (Ni +2 , Zn +2 ,Cu +2 ) coordinating via the azomethine part and crown ether complexes with toughs s-metal ions (Li + , K + , Na + , Ba +2 ) 17,18 .The coordination with d and s metals can be complete effectively in various arrangements to produce single-and di-nuclear compounds.The steadiness of the generate Macrocyclic metal buildings influenced basically by a few variables including the symmetry of the crown ether, the unit and sort of donners show in the ligand, variety and helpful impacts of neighbouring restricting destinations, relative size of the metal particle as for the crown ether hole estimate, the sign on the ion, adaptation of the ether cycle, the specific framework of the outlined ligands, their comparative location inside the macrocyclic shell, number and space of the coordinated cycle framed on complexation and the dissolvable, it is additionally conceivable to tailor-make distinctive kinds of atoms for particular uses, ion selective electrodes 19 and in photosensitive frameworks 20,21,22 .The specific capacity of crown ethers to complex cations 23 has been utilized to survey an extensive number of uses, for example, the creation of sensors and the particular removing of cations or the ionic transmit in velums 24 .Crown ethers-containing Schiff bases are surveyed deficiently [25][26][27][28] .Since these ligands are imperative for the advancement of chemosensors and photograph triggers, and also for extraction of particles 1,2 .Polyfunctional materials synthesized on the principle of metal buildings of azomethines [29][30][31][32] are appeared by luminescent [33][34][35] and magneto dynamic 36,37 materials, catalysts 38 , sensors [39][40][41] , metal-containing polymers 42 .In recent years, there has been a major attention in the chemistry of antipyrine Schiff base crown ether [43][44][45] .Crown ether azomethine group of 4-antipyrine and its buildings have an assortment of utilization in the natural, clinical, logical and pharmacological regions 46 .Despite the fact that ambidentate ligands were analyzed in various investigations, just a single report has given an account of benzo-crown ether-4-antipyrine containing Schiff bases 47 .

Aza-Crown Ether Schiff base
In recent years an interest in the research of aza-crown ethers containing azomethine groupmetal complexes have been increased.Essentially, Crown ethers utilize as the substituents have encounter considerable interest, because of their coordinating capability to ions and specific arranging.Wei and his co-workers prepared some aza-crown ether containing single and di-azomethine groups (Scheme 1), orderly to examine the action of the aza-crown ring have stereo arranging an assignment on diver's remarkable characteristic, namely, the strength for complexation with the metal ion and bio-mimetic performance 48 .

Scheme 1. Preparation n of Aza-crown ether holding mono and azome ethane gathering.
Lu 49 and co-workers 2003 has been effectively prepared a range of Cobalt(II) complexes with aza-crown ether exhibit salicylaldimine azomethine moiety (Figure 1).The cation chelating quality of the complexes and the steadiness constant with first and second metal cation have been studied.The synthesis of aza-crown ether Schiff bases illustrated in (Scheme 2,3 and 4).Dioxygen consanguinity of azomethine Cobalt (II) Complexes with aza-crown ethers have been reported by Li 50 and co-workers 2010.Set of aza-crown ether Schiff bases were synthesized (Fig. 2).The result of the study shows that the existence of a pendant crown ether gathering in the imine ligand remarkably evolves the O 2-binding abilities of the complexes.with that of crown free analogues 53 (Figure 4).Novel Series of mono-azomethine group crown-ether and their metal compounds has been synthesized (Fig. 5), and their catalytic execution in PNPP hydrolysis, and the kinetics and the mechanism of PNPP catalytic hydrolysis in buffer solution have been investigated 54 .Cobalt (II) Salen Compounds with diazacrown pendants were perfectly prepared beginning with benzo-10-aza-15-crown-5(Scheme 7) and (Figure 6).
The modification of O 2-binding abilities by pendant substituents was studied, and contrast with the origin azomethine gathering compounds CoL 1 (Co Salen).The results signalizes that the dioxygen consanguine of CoL has been highly promoted by aza-crown pendants compared with that by morpholino pendants, and the O 2-linking abilities of CoL 1 and CoL 2 with aza-crown pendants will also be promoted by insert alkali metal cations 55 .
The complexation of a set of novel azomethine compounds holding the N-phenylaza-15-crown-5 moiety with first and second groups of periodic table metal ions (incl.Be +2 and Mg +2 ), was prepared (Fig. 7) by Antonov and their co-workers 2001.The position of the N-phenylaza-15-crown-5 moiety in the ligand structure and the properties of the metal ion was studied 56 .
New uniform di-azomethine group Mn(III) and Co(II) compounds with benzo-10-aza-crown ether pendants (MnL 1 Cl, CoL 1 ), and their similarities with morpholino pendants (MnL 2 Cl, CoL 2 ), have been prepared and utilized as design to imitative hydrolase in p-nitrophenyl picolinate by Li and Co-workers 2006 (Scheme 8).
Furthermore, the effect of temperature on the synthesized complexes signalizes that compounds are stead in the domain of the temperature studied in the work 57 .
Khandar 2007 reported the preparation of Ni(II) compounds 16-membrane mixed-donor macrocyclic azomethine ligand, potentially hexa-dentate, holding two pendant alcohol groups (Scheme 9).The Ultra-violet and Visible spectroscopy, conductivity determination and X-ray limitation show that the compounds are a distorted octahedral shape 58 .The synthesized Chemosensor was used as electro-electrode for uncovering Hg +2 .Chemosensor 16 display noteworthy high ability to distinguish between Hg +2 and structurally similar ions in coupling with a visible colorimetric and fluorometricchange 60 .

Aza-Crown Ether Schiff base as Catalyst
Zeng et al., 2002 reported the preparation of Schiff base group containing aza-crown ether cycle and their transition elements compounds.The produced complexes were identified using different spectroscopy methods.The prepared Schiff bases were used as catalytic oxidation.They found that aromatic ring moiety in crown ether observed remarkably dioxygen affinities and biomimetic catalytic compared to uncrowned moiety 61 (Scheme 10).
More investigations about the synthesis of azomethine containing aza-crown ether cycles were provided.Zeng et al., 2006 reported the synthesis of salicylaldimine imine containing aza-crown ether cycle and their Mn (III) metal complexes (Figure 10).
The prepared imine complexes were examined for their catalytic oxidation of styrene, they found that adding of the element of the first and the second group of periodic elements to the oxidation system increases the transformation of styrene to benzaldehyde 62 The investigation illustrated that elective oxidation of para-xylene to para-toluic acid can effectively happen in the existing of azomethine Mn (III) complexes with pendant aza-crown group, which show a more best catalytic action than the azomethine Mn (III)compounds with pendant morpholino groupdo 63 .

Scheme 10. Route for the key intermediate ligands
Investigating the continuity of oxygenation and catalytic oxidation, Zeng et al., 2004 reported the synthesis of mono-Schiff base containing crown ether cycles and their metal complexes (Fig. 11).The result of the study concluded that the azomethine compounds holding crown ether cycles effected on the dioxygen affinities and biomimetic catalytic oxidation 64 .

Fig. 11. The Skeleton of crown ether complexes
Li 65 and his research team 2006, reported the synthesis of novel mono-azomethine cobalt complexes containing aza-crown moiety.The influence of Crown ether on the adjustment, of O 2-coordinating ability and the catalytic oxidation of styrene were studied.The conclusion suggests that the dioxygen consanguine of the cobalt complexes are too high performances by aza-crown pendants than that with morpholino pendants, and the O 2-linking abilities of cobalt complexes with aza-crown pendants can also be promoted by inserting alkali metal cations (Figure 12).A set of new aza-crown ether azomethine have been prepared and their host-guest complexation with C 60 , have been investigated in toluene using absorption spectroscopic method 67 .All Produced compounds show stable complexes with 1:1 stoichiometry (Scheme 13).
New complexes from cobalt(II) and manganese (III) with aza-crown ether containing salen and salophen substituents were prepared beginning from benzo-10-aza-15-crown-5 (Fig. 13).The adjustment of O 2-binding abilities and catalytic oxidation by these pendant substituent's in the complexes were studied and resemble with the complexes ML 5 and ML 6 .The results point out that the dioxygen consanguine and catalytic oxidation affectivity of these complexes have been Novel azomethine Mn(III) and cobalt(II) compounds with each of benzo-10-aza-crown ether pendants (MnL 1 , CoL 1 ) or morpholino pendants (MnL 2 2Cl, CoL 2 ) have been utilize das patterns for hydrolase enzymes by mediate the kinetics of their hydrolysis reactions with p-nitrophenyl picolinate (PNPP)(Scheme 15) (Fig. 14).Result of the study described the effect of pH on the average of catalytic PNPP hydrolysis, which increased by increasing pH 70 .) and morpholino pendants (MnL 2 2Cl, CoL 2 ) were prepared and utilized as patterns for hydrolase enzymes to enhance the hydrolysis of p-nitrophenyl picolinate (PNPP) in the buffered CTAB solution (Fig. 15).The result of the study indicates that In differentiation, with the non-crowned compounds, the crowned azomethine compounds display higher catalytic effectiveness or enhance PNPP hydrolysis 71 .
Hu et al., 2010, illustrated the synthesis of two new cobalt(II) complexes of the azomethine containing aza-crown moiety (Fig. 16).The prepared Schiff base complexes were investigated by the action of surfactant micelles on the activity of the hydrolysis of PNPP.The result appears that the azomethine cobalt (II) compounds and their metallo-micelles as hydrolase imitate display useful catalytic effective and identical catalytic properties to normal enzyme 72 .
Novel cobalt compounds with N-crown ether azomethine containing substituted salicylaldimine were prepared and utilized as design to imitate hydrolase in catalytic hydrolysis of the ester (Fig. 17).kinetic mechanism of PNPP hydrolysis have been studied.The kinetic mathematical design for PNPP cleavage catalyzed by the cobalt compounds has been suggested.The result indicates that the bis(aza crown ether) c obalt complexes show more activity in the PNPP catalytic hydrolysis 73 .

Crown ether containing azomethine groupand their Metal Complexes
N ovel Mn (II) azomethine complexes group N-crown ether were prepared and identified using spectroscopy methods.The prepared complexes were used as catalysts for oxidation of cyclohexene and cyclo-octene by oxone (Scheme 16).The action of crown-ether cycle connected to imine catalysts on the qualification of these catalysts was examined, the catalytic performance of these catalysts arises when an aromatic azo-base were added to the reaction 74 .
Kandaz et al., reported the synthesis of new soluble symmetrical phthalocyanine containing macrocycles (Scheme 17).The synthesized ligands were characterized using different spectroscopy methods.The author of this work achieved success in alkali-metal binding to the endocyclic cap of crown ether by metal-ion 75 .

Scheme 16. Synthesis route of ligands and their complexes MnCIL
Parkin and Menon 76 reported novel crown ether azomethine synthesized from the reaction of different aldehydes with di-amino-18-crown-6.The compounds which have been separated were purified and identified using various Spectroscopy method.All prepared Compounds appear liquid crystal properties.
Zinc complexes of crown azomethine were synthesized and characterized using various Method of analysis (Scheme 18).The data of analysis indicate that in correspondence with the notion of hard and soft acids and bases, the hard lithium cation bonded to the crown moiety with the figuration of mono- (Li) and binuclear (Li, Zn) complexes 77 .
Safin et al., 2012, reported the preparation of 15-Crown-ether-exhibit N-salicylidene aniline compounds by reacting of 4-amino-15-crown-5 with salicylaldehyde (Fig. 18).The produced crown ether Schiff base was characterized using a different method of analysis.In addition of that, the optical properties of ligands were investigated 78 .
Jogani et al., 2000, reported the preparation of a bi-branches crown ether possessed of two cation binding side chains containing an azomethine chelating derived from the reaction of crown ether, l8-crown-6 with aldehyde (Scheme 19) (Fig. 19).The complexes of azomethine with metal ions have been prepared and their chelating action was investigated 79 .Azomethine ligands of 4-antipyrine and their complexes have a variety of applications in the biological, medical, analytical and pharmacological areas.Although bi-dentate chelating compounds were investigated in plentiful studies, only one of them has been reported on benzo crown ether-4-antipyrine containing Schiff base group.Regarding this property, Hayvali 80 , 2009, reported the preparation of a novel crown antipyrine azomethine (Figure 20).Fer nando and his team 2011, has been successfully prepared di-metallated crown azomethine Palladacycles complexes (Fig. 22).They tried to use tetradentate [C,N,N,C] crown ether azomethine to prepare di-cyclo-metallated complexes having a polymeric Skeleton.Furthermore, the activity of such ligands toward tertiary mono-and diphosphines have been tested orderly to detect new, and full of hope unforeseen, results 83 .Four novels Mn (III) complexes with a crown containing salicylaldimine azomethine ligand have been successfully prepared 86 and utilized as design to imitate hydrolase in the hydrolysis of 4-nitrophenylpicolinate (PNPP).The kinetics and mechanism of catalytic PNPP hydrolysis have been studied.The results signalize that the crown azomethine metal complexes appeared more catalytic power compared with complexes without crown ether Schiff base (Figure 24).

Scheme 23. Synthesis of crown ether Schiff base and their complexes
Novel crown ether azomethine from an N-salicylidenaniline compound with 15-crown-5 moiety, was prepared 88 .Coordination of prepared ligands with some metal ion with Na + or/and Zn +2 encourage evident changes in fluorescence spectra.Crown ether moiety after chelating with metal ion, the electron-rich metals in crown moiety will be coordinated.Hence, the PET action from crown giver across the fluorophore is closed and high fluorescence will be formed (Scheme 24 and 25).

Fig. 2 .
Fig. 2. The Structure of Schiff base Cobalt (II) ComplexesNew salicylaldimine mono-Schiff bases with aza-crown pendant have been produced by condensing reaction of 3-[(benzo-10-aza-15crown-methyl] salicylaldehye with substituted aniline (Scheme 5).The action of the coordination aza-crown ether moiety in the complexation with metal ions have been studied51 .Fur ther more, work in Synthesis of N-(4-Salicylideneiminoaryl) mono aza crown ethers and Dioxygen consanguinity of their cobalt (II) compounds have been provided byZeng et al.,  2003.The authors of this research designed and synthesized N-pivot lariat ethers with azomethine gathering as side chain, and predictable their cobalt (II) compounds to have enhanced dioxygen consanguinity52 (Scheme 6) and (Fig.3).

Sheme 5 .Fig. 3 .
Scheme 6.The synthetic route and structure of synthesized Schiff base crown ether

Fig. 10 .
Fig. 10.The structure of Mn(II)azomethine ligands and their complexes A set of new unsymmetrical azomethine manganese (III) compounds with morpholino moiety have been prepared and investigated as catalysts in the respiratory oxidation of xylene to toluic acid (Scheme 11).
More works about the preparation of new mono azomethine Mn(III) Compounds containing aza-crown moiety have been provided by Yan et al., 2006.The synthesized Complexes are used as catalyst in the respiratory oxidation of 4-xylene to 4-toluic acid (Scheme 12).Result of the study indicates that in the inserting of alkali metal addition the transformation of 4-xylene to 4-toluic acid 66 .

Scheme 22 .Fig. 23 .
Scheme 22.The route of synthesis of poly crown ether Schiff base

Moutet 89
Scheme 24.The molecular structutes of ligand and their complexes with Na+ and Zn+2