Schiff base complexes of nickel

Although the term dynamical combinatorial chemistry has only been coined at the end of the last century, the template effect has been used for a much longer time. Some of the best known macrocyclic ligands such as cyclam probably would not have been known so well if their synthesis had not implied the template effect. Probably the first example of a metal ion template for the formation of imino-macrocycles N. F. Curtis, D. A. House, Structure of some aliphatic Schiff-base complexes of nickel (II) and copper (II), Chem. and End., 1961, 42, 1708-1709. 

Fox et al.101-103 have studied the structure of Ni(II) complexes being derivatives of frans-l,2-diaminocyclohexane, using 1H NMR spectroscopy. Chemical shift differences (A<5) between di-Schiff base and its nickel (II) complexes observed for aromatic protons were attributed to the ring currents [40]. 

It was first suggested that the reaction of an alkyl halide with a nickel(I) Schiff base complex yields an alkylnickel(III) intermediate (Equation (56)). Homolytic cleavage of RBr to give an alkyl radical R and a nickel(II) complex (Equation (57)) or, alternatively, one-electron dissociative reduction leading to R (Equation (58)) are possible pathways.254 A mechanism based on the formation of R via dissociative electron transfer of Ni -salen to RX (Equation (59)) has also been proposed.255... 

A range of L-cysteine derivatives bearing a 1,2,4-triazolyl residue on the sulfur atom has been prepared by the asymmetric Michael addition of 4,5-dialkyl-3-mercapto-l,2,4-triazoles to a nickel Schiff base complex. The enantiomeric excesses of the product aminoacids were measured and found to be greater than 98.5% in some cases <2004TA705, 2004RCB932, 2004IZV894>. 

Palladium(II) and nickel(II) Schiff-base complexes such as 67 catalyze hydrogenation of 1-hexene in dimethylformamide at ambient conditions (542). 

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... 

Nickel(II) complexes are particularly suitable for investigation by means of TlNMR spectroscopy. Numerous studies have been carried out on Schiff base complexes.2333,2342-2347... 

The reaction of 2,2 -bisbenzothiazoline with hydrated nickel acetate results in the Schiff base complex (332 Scheme 32). Complex (332) can be reduced to either monoanionic or dianionic species polarographically. The reduction to the monoanionic species can also be accomplished via chemical reduction with BU4NBH4 or sodium analgam.2414... 

There have been very few reports of the Raman spectra of spin-equilibrium complexes. In one experiment the presence of both high-spin and low-spin isomers of an iron(II) Schiff base complex was observed by the resonance Raman spectra of the imine region (11). The temperature dependence of the spectra was recorded for both solid and solution samples. Recently differences were described in the resonance Raman spectra of four- and six-coordinate nickel(II) porphyrin complexes which undergo coordination-spin equilibria. These studies are extensions of a considerable literature on spin state effects on the Raman spectra of iron porphyrins and hemes. There are apparently no reports of attempts to use time-resolved Raman spectra for dynamics experiments. 

Vanadyl and copper(n) ions catalyse the /J-elimination reaction of O-phospho-threonine in the presence of pyridoxal.429 Equilibrium spectroscopic studies of the threonine-metal ion-pyridoxal system have identified a metal-ion complex of the amino-acid-pyridoxal Schiff base. The catalytic effect of the metal is ascribed to its electron-with drawing effecCIt was suggested that the specific catalytic effect of Cu2 + and V02+ arises from their reluctance to co-ordinate the phosphate in an axial position. Other metal ions such as nickel can also form the Schiff base complex but probably stabilize the phosphothreonine system by chelate formation. 

The reaction of benzothiazoline 148 with RhCl(PPh3)3 gives rhodium(III) complex 149 as determined by X-ray diffraction <03CL1058>. The nickel(II) Schiff base complex 150 is also synthesized from the corresponding benzothiazoline <03BCJ127>. 

In some of the earliest experiments involving in situ macrocyclic ligand synthesis, it was shown that the reaction of 2,6-diacetylpyridine with certain polyamines in the presence of metal ions leads to the preparation of new macrocyclic complexes. As is often the case with Schiff-base condensations of the type, the addition of a small amount of acid catalyzes the reaction. Thus treatment of 2,6-diacetylpyridine with 3,3 -diaminodipropylamine [A-(3-aminopropyl)-l,3-pro-panediamine] in the presence of nickel(II) leads to the isolation of nickel complexes of the macrocyclic ligand (Me2-Pyo[14] trieneN4). " ... 

Catalytic oxidation of alcohols by nickel (11) Schiff base complexes containing triphenylphosphine using NaOCl has been studied by Bhat et al. [25],... 

Several nickel(II) complexes have been reported with Schiff bases derived from the condensation of salicylaldehyde and various amino acids. The structures of the complexes were investigated by means of electronic and HNMR spectra as well as X-ray crystallography.Recently the X-ray structure of complex (321), prepared by the reaction of pyridoxal-HCl, o-phospho-DL-threonine and Ni(N03)2-6H20 at pH 5, has been reported. Nickel(II) complexes are particularly suitable for investigation by means of HNMR spectroscopy. Numerous studies have been carried out on Schiff base complexes. ... 

Ni(salen)SnCl4 R ] (R = Ph, Me n = 1, 2) (324)Similar complexes were also reported with PbCl2Ph2 and TlCl2Ph, SbXs (X = C1, Br) and Mn(CO)3X. Di- and tri-nuclear complexes of nickel(II) were obtained by reacting Ni(N03)2 hydrate with Schiff b e complexes of nickel(Il). The donor-acceptor molecular complexes between 1,3,5-trinitrobenzene and bidentate Schiff base complexes were also investigated. ... 

The coordination geometry of the dihydrosalen complex Ni [H2]L is shown in Figure 2. The arrangement of the donor atoms around the nickel is again practically square-planar, but the asymmetry within the tetradentate ligand is obvious. The Schiff base part of the ligand is flat and lies within the coordination plane, whereas the hydrogenated part is much more distorted. 

Losada, J., I. Del Peso, and L. Beyer (1998). Redox and electrocatalytic properties of electrodes modified by films of polypyrrole nickel(II) Schiff-base complexes. J. Electroanal. Chem. 447, 147-154. 

In this paper, the electrochemical reduction of cobalt and nickel complexes of the ligand N,N -l,2-phenylenebis(salicylideneiminato] (salophen = L) and its relation to the electrochemical activation of CO2 is discussed. These complexes have been investigated as potential electrocatalysts of CO2 reduction. Indeed, cobalt and nickel complexes containing tetraazamacrocycles or tetradentate Schiff base ligands have been recognized as powerful catalysts in the electrochemical reduction of CO2 [4,5]. Bifunctional fixation of CO2 by nucleophilic CofQ-Schiff base complexes assisted by alkali cations coordinated to the same ligand has oeen reported [6. ... 

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