Synthesis and characterization of Zr(IV) and Hf(IV) tropocoronand complexes

Zirconium(IV) and hafnium(IV) complexes of the tropocoronands, [MY2(TC-n,m)] (M = Zr (IV), Hf(IV); Y = Cl,CH2Ph,CH2SiMe3) have been prepared and structurally characterized. The compounds exhibit variable stereochemistry depending on the number of methylene units, n and m, in the polymethylene linker chains connecting the two aminotroponiminate rings, although in all cases the cis stereoisomer is formed. With the smallest ligand (TC-3,3), the dialkyl and dichloro complexes display slightly distorted trigonal prismatic structures; increasing the size of the macrocyclic ring substantially shifts the geometry at the metal center toward octahedral stereochemistry. Concomitant with the reorientation toward octahedral symmetry, the stability of the complexes decreases and the metal center can attack the C-H bonds in the alkyl linker arms of the ligand. Substitution of the latter with xylyl groups eliminates this undesired decomposition pathway while still affording strained complexes. Finally, when the binding cavity of the ligand becomes too large, dinuclear species such as {[Hf(CH2SiMe3)(3)](2)(TC-5,5)} are obtained. (C) 1997 Elsevier Science S.A.