Structural flexibility of carboxylate bridging exemplified by a series of mu-acetato dizinc complexes

Abstract

A series of closely related dizinc acetato complexes [LZn(2)(OAc)](ClO(4))(2) has been prepared from an assortment of known (HL(1) - HL(3)) and new (HL(4) - HL(6)) binucleating compartmental pyrazolate-based ligands that provide two tripodal tetradentate {N(4)}-type binding pockets but differ by the type of N-donor within the chelate side arms. Six complexes 1a, 1b, 2 - 5 have been characterized by X-ray crystallography. In all cases the two zinc ions are five-coordinate, are nested in the adjacent ligand compartments as anticipated, and are spanned by a mu(1,3)-bridging acetate that is hosted within the bimetallic pocket. Significant flexibility of the mu(1,3)-acetato ligand is inferred from the structural data, resulting in dislocations of the metal ions out of the pyrazolate plane as well as severe twisting and tilting of the acetate bridge with respect to the bimetallic scaffold. These different orientations of the bridging acetate cannot be distinguished by their IR spectra, in contrast to the different acetate binding modes known from the dynamic behaviour termed "carboxylate shift". However, the high flexibility of the mu(1,3)-acetate even without changes in its coordination mode may well be a prerequisite for reactivity patterns of acetate-bridged bimetallic arrangements.