Ligand Schiff-base

Manganese(V) complexes of the Schiff base ligand salen and its derivatives have been the focus of intense study. These complexes have one additional ligand, either an 0x0, 0 , or a nitrido, group and much of the interest arises because these oxygen or nitrogen atoms are readily 

The geometries and spin multiplicities of models of the Mn -salen catalyst and the Mn -oxo intermediate have been studied using DFT. The Mn complexes have quintet ground states, while the nature of the salen ligand influences whether quintet, triplet, or singlet ground states are lowest in energy for the Mn -oxo intermediates. 

Most complexes with catalytic properties are tetradentate derivatives of salen but other ligands, e.g., A-(hydroxyphenyl)salicyldimine and A-(hydroxyphenyl)pyridine-2-carboxaldimine, form ternary complexes with bipy and CU ((14) and (15), respectively) that are catalytically competent. 

Nitrido complexes of Mn with porphyrin ligands were first prepared by Hill and Hollander and independantly by Buchler et al. in 1982 . These are described in Section 5.1.7.1.1. Nitrido Mn complexes with Schiff base ligands were first reported in 1996. The nitrido ligand 

Schiff base complexes to aromatic silyl enol ethers/ olefins/ sugars,Mn ° Schiff base complexes, and rhenium(III) complexes has been reported. Similar reactions are observed with the Mn (N)porphyrin complexes. The reaction products are dependant on the nature of the Schiff base ligand, both the yield and the enantiomeric excess being affected. Salen nitridomanganese(V) complexes have been incorporated into Zeolite 

Another spectroscopic study has been concerned with the influence of substituents in the Schiff base ligand of the complexes VO(Schiff base) on spectroscopic properties. Two series of Schiff bases were used substituted NN -bis(2-hydroxybenzophene)ethylenediamine and NN -bis(salicylidene)phenylenediamine. A linear correlation was established between the Hammett a-factors of the substituents and r(V=0). 

A linear correlation was also found between the a-factors and the isotropic e.p.r. A values of the complexes. Changes in A are ascribed primarily to changes in the covalency of the metal-ligand bond. A computer analysis of the electronic spectra indicated xy xz, yz x — y z as the most probable order of metal d-orbital energies. Difficulties in correlating spectroscopic and structural data on oxovanadium chelates of even closely-related ligands are illustrated by the observation that within the series of Schiff bases polymeric, dimeric, and monomeric structures are encountered (see also p. 52). 

An extension to the range of metal-ion template reactions is provided by the condensation of salicylaldehyde with salicylaldehyde aminoquinazine which occurs 

Okamoto, J. Hidaka, and Y. Shimura, Bull. Chem. Soc. Japan, 1973, 46, 475. 

Reaction of TCNQ (104) with [Co(acacen)] in pyridine gives low-spin [Co(acacen)(py)j],TCNQ. and e.s.r. measurements have shown this to contain Co and (TCNQC not Co and TCNQ. Thus the complex should be formulated as [Co(acacen)(py)2] (TCNQ) . The 2 1 complex [Co(acacen)(py)2]2,-TCNQ has also been isolated and is formulated as [Co (acacen)(py)2]2- 

Calligaris, G. Nardin, and L. Randaccio, Inorg. Nuclear Chem. Letters. 1972, 8, 477 J.C.S. Dalton 1972, 1433. 

A number of M-(salen) complexes, where M = Co(II), Fe(II), Cu(II), or Mn(II), have been polymerized onto a microelectrode that proved to be suitable for amper-ometric quantitative determination of dissolved NO [117]. Also, in view of the unusual nature of the metal center, polymers obtained from [Pd-(salen)]-type complexes, electrosynthesised and characterized in Ref. [118], deserve to be cited. 

Abundant literature also exists about Schiff base ligands linked, by the N atoms, to the p positions of Th or Py rings, for the development of particular type 2 metallopolymers. Thanks to the versatility of the Schiff bases as ligands, different metal ions can be coordinated according to the scheme in Fig. 3.27a. Alternatively, the phenyl residue of the outermost portion of the Schiff base is bound to a-positions of the heterocycle (see Fig. 3.27b). By polymerization of monomers like that reported in Fig. 3.27a, metallopolymers are obtained. They show a structure that classifies them as type 2 metallopolymers, with possible interesting coupling of electronic and redox conduction, by redox matching. On the other hand, type 3 metallopolymers are obtained from monomers reported in Fig. 3.27b. 

Peculiar behavior has been observed for a Th derivative similar to those reported in Fig. 3.28, in which the two phenyl rings are connected to each other by a crown ether bridge comprising four oxygen atoms. The resulting modified electrode (see 

Salen and additional Schiff base ligands complexing Cu ions are functionalized by pseudo-crowns , according to the authors definition. They are electropo-lymerized and identified as suitable to detect, by cyclic voltammetry, the occurrence of complexation with Ba(II) ions [121]. 

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SOLID PHASE EXTRACTION AND DETERMINATION OF TRACE AMOUNT OF Co + BY MODIFIED ANALCIME ZEOLITE WITH A NEW SCHIFF-BASE LIGAND... 

The first example of asymmetric catalytic ring-opening of epoxides with sp2-hybridized carbon-centered nucleophiles was reported by Oguni, who demonstrated that phenyllithium and a chiral Schiff base ligand undergo reaction to form a stable system that can be used to catalyze the enantioselective addition of phenyllithium to meso-epoxides (Scheme 7.24) [48]. Oguni proposed that phenyllithium... 

Fe(acac2trien)]N03. For the iron(III) complex of the hexadentate Schiff-base ligand acac2trien, the barrier heights have been determined from ultrasonic relaxation [94] as AG[h = 6-28 kcalmoD = 2196 cm and AGJil = 5.85 kcalmol= 2046 cm The difference of zero-point energies has been obtained from equilibrium studies as AG° = 0.43 kcal mol =... 

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]. 

In 2000, Gennaii et al. discovered a new family of chiral Schiff-base ligands, with the general structure, Af-alkyl-p-(A -salicylideneamino)alkanesulfonamide, depicted in Scheme 2.28. These ligands were successfully implicated in the copper-catalysed conjugate addition of ZnEt2 to cyclic enones (Scheme 2.28) and, less efficiently, to acyclic enones such as benzalacetone (50% ee) or chalcone... 

Scheme 2.28 Cu-catalysed 1,4-additions of ZnEt2 to cyclic enones with Schiff-base ligands.

John E, Bott A, Green M. Preparation and biodistribution of copper-67 complexes with tetradentate Schiff-base ligands. J Pharm Sci 1994 83 587-590. 

Infrared and thermal studies also add to our knowledge of these polymers. For example, the aromatic C-0 stretch of the Schiff-base ligands is shifted above 1300 cm-1 when coordinated, and coordinated carboxylate shows no CO stretch above 1600 cm-1 when bidentate. The... 

Electrogenerated monovalent Co complexes of the well-known open chain N202 Schiff base ligands salen (8), salphen (9), and their substituted derivatives undergo oxidative additions with alkyl halides. Reactions of the complex with substrates within the series RBr (R = Pr, Bu, t-Bu) proceed at different rates. The reaction occurs by an inner-sphere alkyl-bridged electron transfer, with a Co1- R+- X-transition state, which is sensitive to distortions of the complex in different configurations.124... 

The presence of N-donors in pendent arms makes Schiff base ligands very effective for the formation of bimetallic complexes with two Ni11 ions. Dinuclear complexes of compartmental Schiff base ligands are covered in Section 6.3.4.12. 

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. 

Belokon et al.83 have investigated the formation of the homo-and bimetallic titanium complexes with di-Schiff base ligands, by means of FT NMR spectroscopy. The ligands have been shown to adapt the ds-p configuration in titanium (IV) complexes. Analysis of the 1H NMR spectra has allowed determination of the population of the homobimetallic complexes derived from two different Ti(IV) complexes [34],... 

A range of tetradentate Schiff-base ligands have also been employed to prepare discrete aluminum alkoxides. The most widely studied system is the unsubstituted parent system (256), which initiates the controlled ROP of rac-LA at 70 °C in toluene. The polymerization displays certain features characteristic of a living process (e.g., narrow Mw/M ), but is only well behaved to approximately 60-70% conversion thereafter transesterification causes the polydispersity to broaden.788 MALDI-TOF mass spectroscopy has been used to show that even at low conversions the polymer chains contain both even and odd numbers of lactic acid repeat units, implying that transesterification occurs in parallel with polymerization in this system.789... 

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