Ruthenium(II) complexes of some new polynucleating ligands incorporating terpyridyl and macrocyclic aza-crown binding sites

Abstract

A series of five new compounds has been prepared in which terpyridyl fragments are linked to hexadentate aza-crown macrocycles. Reaction of 1-aza-18-crown-6 (1,4,7,10,13-pentaoxa-16-azacyclooctadecane) and 1,10-diaza-18-crown-6 (1,4,10,13-tetraoxa-7,16-diazacyclooctadecane) with 4′-bromo-2,2′:6′,2″-terpyridine afforded macrocycles L¹–L³ in which the aza-crown macrocycles are linked directly to the C^4′ position of the terpyridyl groups via the macrocyclic N atoms. Compounds L¹ and L² contain a single macrocycle (1-aza-18-crown-6 or 1,10-diaza-18-crown-6 respectively) attached to a terpyridyl group, whereas L³ contains two terpyridyl groups attached to either end of a central 1,10-diaza-18-crown-6-unit. Alternatively, the aza-crown macrocycles reacted with 4′-[(4-bromomethyl)phenyl]terpyridine to give L⁴ and L⁵ in which the aza-crown fragments are separated from the terpyridyl fragments by tolyl spacers: L⁴ contains one macrocycle attached to a terpyridyl core, whereas L⁵(like L³) contains two terpyridyl binding sites attached to either end of a central 1,10-diaza-18-crown-6 unit. Reaction of these with [Ru(terpy)Cl₃] afforded the complexes [Ru(terpy)L¹][PF₆]₂, [Ru(terpy)L²][PF₆]₂, [{Ru(terpy)}₂(µ-L³)][PF₆]₄, [Ru(terpy)(HL⁴)][PF₆]₃ and [{Ru(terpy)}₂(µ-H₂L⁵)][PF₆]₆, the last two having the aliphatic amine groups of the macrocycles protonated. The crystal structure of [(terpy)Ru(L²)Na(BF₄)₂][PF₆]·1.5Me₂CO {grown by recrystallising [Ru(terpy)L²][PF₆]₂ from a medium containing traces of NaBF₄} shows that the pendant N₂O₄-donor macrocyclic group contains a sodium cation co-ordinated by five of the six macrocyclic donor atoms, and two additional monodentate BF₄^– ligands in axial positions. Significantly, the N atom of the macrocycle which is attached to the terpyridyl fragment is sp²-hybridised with trigonal geometry, which permits the lone pair, in a p_z orbital, to conjugate with the π system of the terpyridyl fragment. This macrocylic amine group is therefore a poor base and is not co-ordinated to the sodium cation. The crystal structure of [{Ru(terpy)}₂(µ-H₂L⁵)][PF₆]₆·2MeCN confirms that the central macrocycle is doubly protonated, with the extra protons inside the macrocyclic cavity consistent with an endocyclic disposition of lone pairs. The electrochemical and UV/VIS spectroscopic properties of the complexes were also examined: in the dinuclear complexes there is no electrochemical interaction between the remote metal centres across the saturated bridging group.