Schiff-base zinc complexes

The recognition of barium containing crown ether bridged chiral Schiff base zinc complex [44] with the rigid bidendate guest 1,4-diazobicyclo-[2,2,2]octane (DABCO) was studied by aH NMR titration.107... 

Other examples involved the use of chiral Schiff base-zinc complexes as catalysts [33 a] and polymer-supported chiral N-tritylaziridino alcohols as catalysts. The stereoselectivity was reported to be up to 97% ee for aUphatic and up to 96% ee for aromatic aldehydes [103]. 

Polymer-supported SchifF-base zinc complexes... 

The addition of diethylzinc to aldehydes produces secondary alcohols. This process can be stereoselectively catalyzed by chiral amino alcohols that form Schiff-base zinc complexes with the aldehyde and the metal. With the aim of simplifying the work-up of these reactions and to use continuous-flow processes, the polymer-supported amino alcohols 115 and 116 were synthesized (Scheme 21) [91]. The polymers were obtained by co-polymerization of the chiral monomer 117 and styrene 58 in the presence of divinylbenzene (118) or cross-tinldng agent 119 containing a flexible oxyethylene chain. The latter was used to ensure sufficient flexibility within the cross-linked network of the polymer and to further activate the nucleophile by coordination of the oxyethylene chain to the metal. 

Recently, N,N,0-tridentate Schiff-based zinc alkoxide complexes 52a-53e have been developed by our group [76]. All complexes efficiently initiate the polymerization of L-lactide at 25 °C with >90% conversion within 30-240 min, with only one exception, 52c which is inactive. The polymerization was well-controlled (PDI = 1.04—1.09) and showed that the reactivity decreases with an electron-withdrawing... 

Amino-2,6-di-terf-butylphenol reacts normally with saiicylaldehydes to form Schiff bases, and complexes of the type [ZnL lfHL = 18) have been isolated, and shown to generate radical species on treatment with Pb02. In contrast, reaction with bis(diketonato)zinc(II) complexes leads not to the Schiff bases, but to the monoiminoquinone complexes (19).326... 

A kinetic study of the hydrolysis of JV-salicylideneaniline (113) in the presence and absence of cobalt(II), nickel(II), copper(II) and zinc(II), using 10% ethanol-water as solvent, has been carried out by Dash and Nanda. The (1 1) Schiff base-metal complexes (ML ) were found to undergo acid-catalyzed hydrolysis at rates decreasing with the thermodynamic stabilities of the complexes, the most thermodynamically stable complexes undergoing the slowest rate of hydrolysis. More recent measurements using high copper(II) to ligand ratios have indicated that the copper(II)-imine is quite stable to hydrolysis at pH 5. 

Among the different techniques utilized for developing chemical sensors, those based on molecular fluorescence are the most sensitive. Ohveri and Di Bella [121] developed a powerful fluorescent probe based on an amphiphilic Schiff-base zinc (II) complex for the sensitive detection of alkaloids, including tropane and atropine. This complex exhibits fluorescent enhancement upon formation of 1 1 adducts with alkaloids. Its binding interaction, related to Lewis basicity, is strongly influenced... 

The stereocontrol observed in these reactions is rationalized on the basis of zinc complexes formed with the Schiff bases, as described in Section 1.4.4.2.2. The isocyanide, being a free and potent nucleophile, attacks this complex from the sterically less-hindered side, that is. from the side of the ring oxygen. 

Ferrocene-containing sulfur ligands have been used in complex formation with zinc. 3-Ferro-cenyl-3-mercaptopropenale and l,l -bis(3-mercaptopropenale)ferrocene (112) form stable complexes as do their Schiff base derivatives with aniline or 1,2-ethylenediamine. The Fen/Fein redox couple shows only minor metal dependence, this is attributed to tetrahedral distortion.884... 

Schiff bases with intramolecular charge transfer complexes such as 2,3-bis[(4-diethylamino-2-hydroxybenzylidene)amino]but-2-enedinitrile zinc (II) (BDPMB-Zn, 187) emit red fluorescence with fluorescent quantum yields up to 67%. OLEDs with a structure of ITO/TPD/ TPD BDPMB-Zn/Alq3 BDPMB-Zn/Alq3/Mg-Ag showed very bright saturated red emission with CIE (0.67, 0.32) with a luminance of 2260 cd/m2 at 20 V and a current efficiency of 0.46 cd/A (at 20 mA/cm2). In addition, the EL spectra do not change with the doping concentration in the range of 0.5—3% [229]. 

The intervention of a metal ion in the stoichiometry of a reaction has been illustrated several times previously. Reaction is forced to completion in ester hydrolysis since the carboxylate grouping forms a more stable complex than the ester moiety does. A similar driving force underlies the formation of macrocycles and the completion of transamination by formation of the metal-Schiff base complex. The latter is particularly relevant in dilute solution and at low pH. For example, the extent of aldimine formation between pyridoxal and alanine is undetectable at the physiological pH but occurs to the extent of = 10% in the presence of zinc... 

Schiff base ligands are a particularly suitable alternative in ROP of lactide because of their ease of preparation and the possibility to easily tunable steric and electronic properties. Chisholm and coworkers reported Schiff base-supported zinc amide and phenoxide complexes 51a, b (Fig. 10) [75], which catalyze the polymerization of L-lactide in benzene at room temperature yielding 90% conversion in 3 h for 51a and 72 h for 51b. The significant difference in reactivity between the catalysts can be taken from the rate of initiation, which is slower for 51a owing to presence of bulky 2,6-ferf-butylphenoxide leaving group. 

Fig. 10 Zinc and magnesium complexes based on Schiff base ligands...

Schiff bases, especially multidentate ones, are powerful ligands for metal ions and zinc and cadmium complexes are well represented in this area. The findings of structural studies are interesting in that the ligand can control the stereochemistry of the complex and provide us with numerous examples of unusual geometry about the central metal ion, thus serving to illustrate the coordination flexibility of these ions. 

The Schiff base from 2-aminomethylfuran and salicylaldehyde forms a zinc(II) complex which has been shown, by NMR and IR spectroscopy, to contain the ligand in the form (21), rather than as a 2-furylimine (22).330 The complex [Zn2La] (H2L = 23) has been described and considered to have the structure (24).331... 

The complex [ZnL -H20 of the quinquedentate Schiff base (25 H2L) has been prepared the zinc environment is a distorted trigonal bipyramid (Zn—O = 1.95 A Zn—N = 2.11-2.16 A)332 and IR and NMR studies on the Znl complex of the quadridentate Schiff base (26 H2L) indicate a tetrahedral rather than a planar configuration.333... 

In fact, several reports have appeared on the spectroscopic features of complexes of zinc and cadmium with Schiff bases. The electronic absorption spectra of various salicylaldimine complexes of zinc have been determined335 to clarify assignments made previously for similar copper(II) complexes. A band at ca. 41000 cm-1 has been reassigned as an n-n transition of the oxygen lone pair, rather than the o-3d transition proposed earlier. 

A charge-transfer complex [ZnLQ] (H2L = 28 Q = p-chloranil) has been characterized, and a weak n-n interaction between the ZnL and Q moieties demonstrated.338 A related complex, [ZnL] (H2L = 29), is formed from the template condensation of HCN tetramer, H2N(NC)C=C(CN)NH2 with salicylaldehyde in the presence of ZnCl2.339 In the absence of the zinc salt, unsymmetrical mono-Schiff bases are obtained. 

Template and capping reactions feature in recent work. A number of sterically hindered H20 and N4 ligands have been prepared in which a quadridentate Schiff base is capped by condensation of salicylaldehyde or pyrrole-2-carbaldehyde with a series of bis(8-aminonaphthyl)alkyl ethers. These ligands combine the versatility of the Schiff base with the protective features well known for certain model porphyrin systems, and appear to be of some interest. The zinc(II) complex and other transition metal complexes have been prepared.340... 

Also containing adjacent OH and CHO groups is pyridoxal, and zinc complexes of Schiff bases derived from histidine and pyridoxal have been investigated as models for the reactions of vitamin B6 analogues.343... 

Structural surprises are not confined to 2,6-DAP derivatives though zinc and cadmium complexes have been obtained with the quadridentate thioiminato Schiff base N,N -ethylenebis(monothioacetonimine) 347 uv and visible spectra indicate planar geometry, rather than the tetrahedral geometry observed for the salicylaldimine complexes. 

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