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 C4′ 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 pz 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.