Schiff base ligands 2:1 condensation

The simultaneous observation of the two EPR spectra has been reported in particular for several tris(dithiocarbamato)iron(III) complexes [Fe(R2NC(S)S)3] where R = cyclohexyl [143], hydroxyethyl [144], and n-butyl [145, 146]. In addition, a considerable number of iron(III) complexes of the type [Fe" -N402] has been found which show EPR spectra of both the HS and LS isomers. These comprise [Fe(X-SalEen)2] Y2 where X-SalEen is the Schiff-base ligand obtained by condensation of X-substituted salicylaldehyde and IV-ethylethylenediamine [147] and similar compounds [100, 148, 149, 150, 151]. For the cobalt(II) complex [Co(terpy)2] (004)2, it is not completely clear whether the two observed EPR spectra are due to HS and LS states related by a spin-state transformation [152]. 

The formation of diimine systems by Schiff -base-type condensation of suitable aldehydes and primary amines has been widely applied. Those reported are mostly strong field systems and their relevance to the spin crossover field is generally in systems of the kind [Fe(diimine)2(NCS)2]. The effect of the incorporation of substituents likely to hinder coordination has been studied. Robinson and Busch noted a fundamental difference at room temperature in the electronic properties of the [Fe N6]2+ derivatives of 2-pyridi-nalmethylhydrazone and 2-pyridinal-dimethylhydrazone, those of the former being low spin and those of the latter high spin [49]. The temperature-dependence of the magnetism of the latter complex was not reported but may well be of interest. However, spin crossover [Fe(diimine)3]2+ systems have been characterised for systems where the incorporation of appropriate substituents has reduced the ligand field. 

Iron(II) systems based on hexadentate ligands where all the donor functions are imines will generally be low-spin. One well-known example is the Schiff base ligand obtained by condensing tren with 2-pyridinecarbaldehyde (22). 

The N402 ligand systems depicted in Fig. 18 have been shown to generate spin crossover in Fe(III). Two families of Schiff base ligands have been obtained from the 1 2 condensation of triethylenetetramine with derivatives of... 

PPhs, the active oxidant being probably a Os =0 species. trani-Dioxoosmium(VI) complexes of Schiff base ligands obtained from the condensation of /3-diketones and 1,2-diaminoethane are also known. The X-ray structure of tra 5-[0s (0)2 (BA)2en ] has been determined. The 0s=0 distances are 1.753 A and 1.736 A (average) for the two crystallographically independent... 

The condensation of o-aminobenzenethiol with pyridine-2,6-dicarbaldehyde does not form the corresponding Schiff base but a polyheterocyclic compound which behaves as a tridentate ligand towards nickel(II).2365 In contrast the reaction of 2-(2-pyridyl)benzothiazoline with nickel(II) results in the opening of the heterocyclic ring and in the formation of a Schiff base ligand which coordinates to nickel(II) as a mononegative tridentate ligand.2366,2367... 

Figure 28 shows another two examples of trivalent Schiff base ligands. The macrocyclic ligand was generated in a template condensation of 2,6-diacetyl-pyridine with l,3-diamino-2-hydroxypropane in the presence of lanthanide salts [187]. The La(N03)3 H20-reaction yielded a trinuclear complex of composition [La3U/ 3-0HX0H)(N03)4]-7H20 [188]. 

The Mn11 compounds, derived from the terdentate O—N—O or S—N—O donor set Schiff base ligands obtained from the condensation of both substituted412 and unsubstituted412 413 o-hydroxyan-iline, anthranilic acid412 or o-aminobenzenethiol413 with various salicylaldehyde residues, have the... 

The chemistry in this section is almost exclusively that of Schiff base ligands this reflects the predominance of the aldehyde/carbonyl condensation reaction with primary amines in the preparation of mixed donor atom ligands. 

Schiff-base ligands prepared form the condensation of salicylaldehyde with diamines offer [N2O2] donor sets that are capable of coordinating a wide range of metals. Tin(IV) acetate reacts with salenH2 and R2SnCl2 reacts with either (3-MeO)salphen (R = Ph, Bu , or the ligand... 

A tripod-type Schiff-base ligand, (127), was prepared by a 3 1 condensation reaction of 2-phenyl-4-formylimidazole in methanol.204 In two nickel(II) complexes, the screw coordination arrangement of the tripod ligand (127) around the metal ion induces chirality, resulting in a A (clockwise) or a A (anticlockwise) enantiomer.204 A multifunctional tripodal ligand possessing two different functionalities, such as pyridine and cyano moieties, (128), has been prepared by the reaction of 2-aminomethylpyridine, excess acrylonitrile, and glacial acetic acid. 

Several Schiff-base ligands derived from the condensation of diamines and 1,3-diones were employed as tetradentate ligands for Zr. For example, the complexes ZrCl2[OCRCHC-(Me)NCH2]2 (R = CF3, Me) were derivatized and subsequently the salt... 

All the solvents were purified by standard methods[10]. 2,5-dihydroxyterephthalaldehyde (dhtH2) was prepared from dimethylhydroquinone ether using the literature methods[ll]. Polymeric Schiff base ligand P-dhtenH2, [Poly(2,5-dihyeroxyterephthalaldehyde ethylenediamine diprotonated)] was prepared by condensation of 2,5-dihydroxyterephthalaldehyde with ethylenediamine using the reported procedure with some modifications[12]. Infrared spectra were recorded on a Bruker IPS 66V FT-IR and thermal analysis on Delta series TGA-7. 

Condensation of ketones or aldehydes with the methyl ester of dithiocarbazic acid produces ligands with S,N,0 or S,N donor sets. An example is (142), which acts as both a didentate and a tridentate to ReO. Nickel (II)-templated reactions of pentanedione or salicylaldehyde with aminoethanethiol produce N,0,S ligands such as (143) and (144) these form simple mononuclear complexes or else dinuclear species with thiolate bridging two Ni centers. Mixed donor Schiff-base ligands including other heteroatoms such as P (145) and Te (146) have appeared. 

Mn compounds of several types of terdentate Schiff base ligands are known.Of interest for supposed biological reasons are ligands of donor set O—N—O derived from condensation of (x-amino acids with salicylaldehyde," pyruvic acid/" pyridoval and substituted salicylal-dehydes/ " Although X-ray analysis " indicates that the compound [Mn (pyridoxal-valine)2] is octahedral, the structures of most of these compounds have not been fully elucidated. Claims of monomeric, dimeric and polymeric molecules are based mainly upon analytical results and upon room temperature magnetic susceptibility measurements. Nothing of special biological interest has come from this work. 

---