Schiff bases multidentate

Iron(III) Spin Crossover Systems of Multidentate Schiff Base-Type Ligands 285... 

Abstract In this chapter, selected results obtained so far on Fe(III) spin crossover compounds are summarized and discussed. Fe(III) spin transition materials of ligands containing chalcogen donor atoms are considered with emphasis on those of M,f T-disubsti-tuted-dithiocarbamates, Ar,N-disubstituted-XY-carbamates (XY=SO, SSe, SeSe), X-xan-thates (X=0, S), monothio-/J-diketonates and X-semicarbazones (X=S, Se). In addition, attention is directed to Fe(III) spin crossover systems of multidentate Schiff base-type ligands. Examples of spin inter-conversion in Fe(III) compounds induced by light irradiation are given. 

Another class of ligands for ATH is represented by multidentate Schiff bases and their derivatives. Zassinovich and Mestroni reported on the effective reduction of alkyl aryl ketones catalyzed by a series of lr(l) complexes with chiral bidentate pyridylaldimines, of the form [lr(cod)(NNR )]C104 (76a-f see Scheme 4.31). It was observed that both the activity and selectivity depended heavily on the nature of the subshtuents at the chiral center of the ligand, and also at the prochiral center of the substrate. Optical yields of up to 50% (R-isomer) at 100% conversion were obtained in the ATH of BuC(0)Ph and PhCH2C(0)Ph using [lr(cod)(PPEl)]C104 as the precatalyst (0.1% mol, 83 °C, PrOH, KOH) [66]. 

Multicoordinated metal complex comprising a multidentate Schiff base ligand with ruthenium, (I) and (II), were prepared by Schaubroeck et al. (1) and Verpoort et al. (2), respectively, and used to prepare poly(dicyclopentadiene). 

A Schiff base is the common name for the (usually acyclic) imine product of the reaction of a primary aryl amine with an aldehyde or ketone. These imines are stable if there is at least one aryl group on the imino nitrogen or on the imino carbon (54). A cyclidene is generically a cyclic, multidentate imine. 

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. 

As observed above, many Schiff bases have not been characterized in the free, i.e. uncomplexed, state. This is particularly so for Schiff bases with more than four donor atoms. Without entering into the details of the structures or spectroscopic properties of the metal complexes of such multidentate ligands, a brief survey of compounds will be presented here in order to exemplify the ligand ability of such Schiff bases. 

The importance of the chelate effect combined with the construction of multidentate ligands is well known in lanthanide chemistry. This is expressed in the rich coordination chemistry of / -diketonates [88] or complexes with Schiff bases [89] and macrocyclic polyethers [90] where lanthanide cations achieve steric saturation by high coordination numbers. Entrapment of the cation in a macrocyclic cavity results in greater complex stability. However, simply functionalized ligands such as ethanolamines can also supply a suitable ligand sphere [91-93],... 

Cun-dinuclear, multidentate Schiff bases variable temperature magnetism 817... 

Multidentate Macrocyclic Ligands 49.5.6.1 Schiff base ligands... 

In this way, alkoxides, glycolates, catecholates, carboxylates, or Schiff base complexes have been obtained.37,38 Other metal chelates have been synthesized by electro-oxidation of the bare metal electrode phosphido, azolates, phthalocyanines, semicarbazides, Schiff bases, etc.37 In addition to the syntheses of monometallic complexes, electrochemistry has been proven to yield polynuclear complexes by using assembling or multidentate ligands.37... 

Stabilizing the vanadium with auxiliary ligands makes it possible to form complexes with various functionalities including the alkoxy functionality. Such classes of ligands include pyrazolyl borates (15)102 and Schiff bases, and will be described in the appropriate multidentate ligand section. 

The formation of peraza-crown maeroeycles containing Schiff base functional groups were among the first syntheses of macrocyclic multidentate compounds (Busch, 1967 Lindoy, 1975). These cyclic reactions between carbonyl and primary amine compounds were the first to use a template cation (Black and Hartshorn, 1972-1973). The unsaturated Schiff bases were then... 

The synthesis, structures, and luminescence properties of polynuclear lanthanide complexes and lanthanide coordination polymers, which are formed from multidentate oxygen and nitrogen-containing Schiff-base ligands, are described in this chapter. 

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