Synthesis of the potentially pentadentate ligand 6,6″-bis(2-hydroxyphenyl)-2,2′ :6′,2″- terpyridine (H2L) and the crystal structure and magnetic properties of [{Cu(HL)}2][PF6]2·5MeCN

Abstract

The new potentially pentadentate (ONNNO donor) ligand 6,6″-bis(2-hydroxyphenyl)-2,2′ : 6′,2″-terpyridine (H₂L) reacted with Cu^II to form the complex [{Cu(HL)}₂][PF₆]₂. X-Ray structural analysis of [{Cu(HL)}₂][PF₆]₂·5MeCN revealed that the complex exists as centrosymmetric dimers. The monomeric unit is four-co-ordinate Cu(HL)⁺, in which the copper(II) ion is co-ordinated by one phenolate oxygen and three pyridyl nitrogen atoms of HL, with the remaining phenol group protonated and not co-ordinated but involved in hydrogen-bonding interactions with [PF₆]^– ions or lattice MeCN molecules. The geometry is best approximated as square planar within the constraints imposed by the ligand. 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 Cu₂(µ-O)₂ core with (approximately) elongated square-pyramidal copper(II) centres. This results in aromatic stacking between the two ligands. Low-temperature magnetic susceptibility measurements indicate a weak antiferromagnetic coupling between the metals. The EPR spectrum (CH₂Cl₂-dimethylformamide glass at 77 K) is a typical triplet with a well resolved double-quantum transition and a septet hyperfine coupling pattern, showing that the dimer remains intact in the solvent mixture used. However in the presence of pyridine the dimeric units break up via axial ligation of pyridine, resulting in an EPR spectrum characteristic of mononuclear copper (II) species.