Schiff bases polydentate

The ancillary N-donor and O-donor ligands are also important in the formation and stabilization of oxomanganese clusters. N-donor ligands are especially common ranging and range from bidentate to predesigned polydentate and include aliphatic, cyclic, Schiff base, and polypyridyl systems (Figures 2 and 3). 

Most of the polydentate ligands that stabilize manganese(III) are Schiff bases. In this section, complexes containing various polydentate ligands are summarized (see (Figures 3, 18 and 19) for... 

Eichhom and his co-workers have thoroughly studied the kinetics of the formation and hydrolysis of polydentate Schiff bases in the presence of various cations (9, 10, 25). The reactions are complicated by a factor not found in the absence of metal ions, i.e, the formation of metal chelate complexes stabilizes the Schiff bases thermodynamically but this factor is determined by, and varies with, the central metal ion involved. In the case of bis(2-thiophenyl)-ethylenediamine, both copper (II) and nickel(II) catalyze the hydrolytic decomposition via complex formation. The nickel (I I) is the more effective catalyst from the viewpoint of the actual rate constants. However, it requires an activation energy cf 12.5 kcal., while the corresponding reaction in the copper(II) case requires only 11.3 kcal. The values for the entropies of activation were found to be —30.0 e.u. for the nickel(II) system and — 34.7 e.u. for the copper(II) system. Studies of the rate of formation of the Schiff bases and their metal complexes (25) showed that prior coordination of one of the reactants slowed down the rate of formation of the Schiff base when the other reactant was added. Although copper (more than nickel) favored the production of the Schiff bases from the viewpoint of the thermodynamics of the overall reaction, the formation reactions were slower with copper than with nickel. The rate of hydrolysis of Schiff bases with or/Zw-aminophenols is so fast that the corresponding metal complexes cannot be isolated from solutions containing water (4). 

The Schiff-base complex 17 of ruthenium was developed by Scott et al. [51] and shows substantially high efficiency for ACP. The cyclopropanes derived from 4-nitrostyrene and EDA were obtained in 92% yield with a 99 1 trans-to-cis ratio and 98% ee for the trans form [51]. In most cases, in order to attain high trans-cis stereoselectivity, bulky ester groups of diazoesters were effective. Nevertheless, Nguyen et al. [52] reported in 2002 that the reaction of the smaller and common EDA with styrene assisted by Ru-salen-pyridine complexes 18 (1 mol %) at room temperature produced the cyclopropane products in high yield, 90-96%, and 98-99% ee for the trans form and 95-96% ee for the cis form. Zhang et al. [53] reported that a N,0-mixed polydentate ligand pro-... 

Myriad polydentate aza-macrocycles have been reported 41. The extent of the subject forces limitation of this discussion to only macrocycles containing a pyridine or dipyridine subunit. Most of these coronands have been synthesized by a SchifF base condensation of an aldehyde or ketone with a hfc-primary amine in the presence of a metal ion. The metal ion acts as a template, resulting in dramatic increases in yield of the desired cyclic product over linear polymerization products42 46. Lindoy and Busch45 have described this effect in two ways, kinetic and thermodynamic. If the metal ion controls the steric course of a series of stepwise reactions, the template effect is considered to be kinetic. If the metal ion influences an equilibrium in an organic reaction sequence by coordination with one of the reactants, the template effect is termed thermodynamic. It is the kinetic effect that is believed to be operative in most metal ion-assisted (in situ) syntheses of... 

A large number of polydentate Schiff base ligands have been reported, and of particular interest are the iron(II) complexes of ligands such as (42) (R = H, Ph) because they are low-spin, even though they contain only two diimine units.578 The rate of acid aquation of [Fe(42)]2+ is slow, but this depends on acid concentration. Reaction with hydroxide ion is second order, as is that with cyanide ion. The product of the last reaction is not [Fe(CN)6]4-, but [Fe(CN)4(42)]2 that has only one diimine bound to the metal. Related ligands have been prepared and much of the early work has been reviewed.54615"5464... 

---