Nickel complexes salicylaldiminato

Most recently, Grubbs group demonstrated that some neutral salicylaldiminato nickel(II) complexes, whose skeleton structure appears as lb in Figure 1, show catalytic activities rivaling those of the bisimine complexes [9], This potentially opens the door to a new class of catalysts as the active sites derived from these nickel complexes are neutral, thus reducing the ion-pairing problems encountered in the current catalysts. 

Most nickel(II) complexes with the various Schiff bases derived from salicylaldehyde and different amines have usually been easily prepared by three general methods (i) the reaction of a nickel(II) salt, usually hydrated nickel(II) acetate, with the preformed Schiff base using water, EtOH, MeOH or their mixtures as reaction medium (ii) the condensation reaction of bis(salicylaldehydato)nickel(II) with the appropriate amine in refluxing EtOH or H20/EtOH mixture (iii) the template reaction of the aldehyde with the appropriate amine in the presence of a nickel salt. In Table 97 the formulas, synthetic methods and some physicochemical properties for a number of nickel(II) salicylaldiminato complexes are reported. Examples of dinuclear complexes formed with Schiff bases are specifically reported in Section 50.5.8.5. 

Of the four-coordinate complexes of Ni those with the square planar stereochemistry are the most numerous. They include the yellow [Ni(CN)4] , the red bis(A(-methyl-salicylaldiminato)nickel(ll) and the well-known bis(dimethylglyoximato)nickel lI) (Fig. 27.6b and c) obtained as a floccuicnt red precipitate in gravimetric determinations of nickel. Actually, in the solid state, this last compound consists of planar molecules stacked above each other. so that Ni-Ni interactions occur (Ni-Ni 325 pm), and the nickel atoms should therefore be described as octahcdrally coordinated. However, in non-coordinating solvents it dissociates into the square-planar monomer, while in bis-(ethylmethylglyoximato)nickel(Il) a much longer Ni Ni separation (475 pm) indicates that even in the solid it must be regarded as square planar. 

Very few nickel-NHC catalysts have been shown to catalyze the polymerization of styrene. In 2005, Shen reported the details of a salicylaldiminato-Ni-NHC complex (64) (Figure 31.13), which is still among the most efficient Ni-NHC polystyrene catalysts known today [94]. Activation of the catalyst with NaBPh4 facilitated the cationic polymerization of styrene in quantitative yield after 12 h at 80 °C in toluene, and afforded a polymer with M of 17.6 kDa and a PDI of... 

The dynamics of the square-planar - octahedral isomerization of a nickel(II) complex with N-(2-piperidinoethyl)salicylaldiminato ligands has been studied by ultrasonic relaxation methods- O This rearrangement can be represented as follows. Scheme 11.8 ... 

---