Zirconium complexes Schiff bases

Other examples of this synthetic strategy are known for example, a recent zirconium polymer by Illingsworth and Burke (8), who joined amine side groups of a zirconium bis(quadridentate Schiff-base) with an acid dianhydride to give amide linkages. Once again, caution is necesary, as Jones and Power (2) learned when they attempted to link metal bisO-diketonates) with sulfur halides that is, they obtained insoluble metal sulfides because the p-diketone complexes which they used were fairly labile and the insolubility drove the reactions to completion in the wrong direction. 

Zirconium and hafnium tetraalkoxides are highly reactive compounds. They react with water, alcohols, silanols, hydrogen halides, acetyl halides, certain Lewis bases, aryl isocyanates and other metal alkoxides. With chelating hydroxylic compounds HL, such as j8-diketones, carboxylic acids and Schiff bases, they give complexes of the type ML (OR)4 these reactions are discussed in the sections dealing with the chelating ligand. 

Zirconium(IV) forms a rather large number of complexes with ONO- and ONS-tridentate Schiff base ligands, ONNO- and SNNS-tetradentate Schiff base ligands, and related species such as diacyl hydrazines. Relatively few hafnium analogues have been reported thus far. 

Dibasic tridentate Schiff bases derived from salicylaldehydes and 2-aminobenzoic acid658 or l-amino-2-mercaptobenzene659 react with aqueous zirconium nitrate to give monomeric complexes of the type [Zr(0H)2(L)(H20)]. IR spectra of these compounds support an ONO-or ONS-tridentate attachment of the (L)2 ligands. 

The in situ formed basic cyclopentadienyl complex of titanium, ethylene-bis( f-tetrahydroindenyl)-titanium hydride (EBTHI)TiH) containing a planar element of chirality, is active for the reduction of a C=N double bond of both N-aryl-imines (Schiff bases) and unsubstituted imines (108,109). (EBTHl)TiX2 derivatives are used as precursors of the hydride analogue. The chiral zirconium analogues of the above-mentioned complexes provided a breakthrough in asymmetric polymerization of terminal oleflns (110). 

Zirconium Schiff-base complex modified SBA-15 (Zr-SBA-15) was used as catalyst in the synthesis of quinoxaline-based heterocycles such as pyridopyrazine, pyrazine, and quinoxaline derivatives [156]. 2,3-Disubstituted quinoxalines among others were prepared, in water media, in good-to-excellent yields. The Zr/SBA-15 catalyst showed an excellent reusability over seven successive runs xmder optimized conditions. 

R. Malakooti, G.R. Bardajee, H. Mahmoudi, N. Kakavand, Zirconium Schiff-base complex modified mesoporous silica as an efficient catalyst for the s)fnthesis of nitrogen containing pyrazine based heterocycles, Catal. Lett. 143 (2013) 853-861. 

---