Schiff-bases coordination numbers

In Sn(II) complexes, the tin atom was located above the di-Schiff base coordination plane, while in Sn(IV) complexes, it was coplanar with the imine coordination framework. The position of the metal was supported by X-ray data. For the compounds studied, the 119Sn chemical shift values varied from 501.4 to —1015.9 ppm. Increase in the coordination number from Sn(II) to Sn(IV) led to an increase in the tin shielding. The differences of up to 3.0 ppm between 5119Sn values for the complexes, being derivatives of R,R and S,S 1,2-diaminocyclohexane, were observed. 

The first soluble Schiff base coordination polymer was reported by Archer et al. in 1985,31 which was prepared by condensation of 1,2,4,5-tetra-amino benzene (TAB) with tetrakis(salicyladehydato)zirconium(IV) (Zr(sal)4) in dry dimethyl sulfoxide (Fig. 15). The polymer has a number-average molecular weight of up to 4.9 X 104 based on inherent viscosity, gel permeation chromatography, and elemental analyses and shows no decomposition at temperature <500°C. 

Pyridine derivatives with additional donor functions and sterically demanding substituents have been used with the intention of producing complexes of Cd (and of other metals) with low coordination number one of these ligands is the tridentate, planar-bonding 2,6-bis[(2,6-dimethyl-phenylimino)methyl] pyridine (pydim a Schiff base derived from 2,6-pyridine dialdehyde), which with Cd(BF4)2 and thiocyanate gives a dinuclear complex [(pydim)Cd(/x-NCS-S,N)]2(BF4)2 with N-dominated coordination sphere.191 As centrosymmetric Plijc, Z= 2), the complex has an antiparallel /x-1,3 NCS double bridge with Cd—N and Cd—S bonds (224.6 pm and 255.5 pm, respectively) the Cd—N(py) bond is clearly shorter than the Cd—N(imino) bonds (225.6 pm and 245.0 pm,... 

Schiff bases provide useful mixed donor sets. The carbonyl function of the most frequently used ligands is derived from either 1,3-dicarbonyl compounds or salicylaldehyde. Favourable combinations involve O-, N- and S-donor atoms. A range of technetium and rhenium complexes exist with bi-, tri-, tetra- and pentadentate ligands. The geometry of these complexes depends on the number and type of coordinating atoms as well as on the chain length between the donor atoms in the SchifF-base ligands. 

Analysis of the 1H and 13C NMR spectra of the tin complexes, being derivatives of various di-Schiff bases, [46] has revealed the dependence of the position of tin centre on the type of linker between the two imine units and the metal coordination number.110... 

Schiff base-type systems are the second most widespread class of ligands which have been used to obtain Fe(III) spin crossover materials. These ligands may be classified according to the number of donor atoms available for coordination to the Fe(III) ion. In Sects. 3.1 to 3.4 attention is drawn to tri-, tetra-, penta- and hexadentate Schiff base-type ligands, severally. Section 3.5 focuses on spin crossover in iron(III) induced by light irradiation, whereas Sect. 3.6 is devoted to recent developments in the field of materials science with the objective of incorporation of Fe(III) spin crossover materials in devices. 

In order to give the usual overview of nickel complexes at increasing coordination numbers we begin with the usual square planar complexes of the Schiff bases salen and saloph.149,150 As an example, Figure 98 shows the molecular structure of [Nin(salen)]. 

The Schiff bases have contributed to a large extent to the development of the coordination chemistry of nickel(II), particularly those obtained by the condensation of salicylaldehyde with various mono- and poly-dentate amines. A large number of nickel(II) complexes have been synthesized with these ligands exhibiting all of the possible coordination geometries, spin states and spectromagnetic properties. The early complexes of nickel(II) with different salicylal-dimines have been covered extensively in a number of review articles.2246-2252... 

A number of representative nickel(II) complexes prepared with Schiff bases derived from pyridine-2-carbaldehyde, pyridine-2,6-dicarbaldehyde and related species are summarized in Table 98, together with some of their distinctive physicochemical properties and preparative routes. All of these complexes involve N and either O or S as donor atoms and exhibit various coordination numbers and geometries depending on the denticity of the ligands and on their steric and electronic requirements. 

Amino acids and their derivatives undergo a wide range of reactions, e.g. racemization, peptide bond formation, ester hydrolysis, aldol-type condensation, Schiff base formation and redox reactions, which are catalyzed by coordination to a metal centre. A number of reviews are available which cover some of these reactions.48,69,70... 

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