SYNTHESIS OF THE POTENTIALLY PENTADENTATE LIGAND 6,6''-BIS(2-HYDROXYPHENYL)-2,2'6',2''-TERPYRIDINE (H(2)L) AND THE CRYSTAL-STRUCTURE AND MAGNETIC-PROPERTIES OF [(CU(HL))(2)][PF6](2)CENTER-DOT-5MECN

The new potentially pentadentate (ONNNO donor) ligand 6,6 ''-bis(2-hyd roxyphenyl)-2,2':6',2 ''-terpyridine (H(2)L) reacted with Cu-II to for m the complex [{CU(HL)}(2)][PF6](2). X-Ray structural analysis of [{Cu (HL)}(2)] [PF6](2).5MeCN revealed that the complex exists as centrosym metric d imers. The monomeric unit is four-co-ordinate CU(HL)(+), in w hich the copper(II) ion is co-ordinated by one phenolate oxygen and th ree pyridyl nitrogen atoms of HL, with the remaining phenol group prot onated and not co-ordinated but involved in hydrogen-bonding interacti ons with [PF6](-) ions or lattice MeCN molecules. The geometry is best approximated as square planar within the constraints imposed by the l igand. Two of these units are stacked such that the copper(II) centre of one monomeric unit is co-ordinated axially by the phenolate ligand of the other, forming a Cu-2(mu-O)(2) core with (approximately)elongat ed square-pyramidal copper(II) centres. This results in aromatic stack ing between the two ligands. Low-temperature magnetic susceptibility m easurements indicate a weak antiferromagnetic coupling between the met als. The EPR spectrum (CH2Cl2-dimethylformamide glass at 77 K) is a ty pical tripler with a well resolved double-quantum transition and a sep tet hyperfine coupling pattern, showing that the dimer remains intact in the solvent mixture used. However in the presence of pyridine the d imeric units break up via axial ligation of pyridine, resulting in an EPR spectrum characteristic of mononuclear copper(II) species.