Stepwise synthetic strategy for the preparation of trinuclear complexes of bis(terpyridyl) bridging ligands containing aza-crown macrocyclic spacer groups

Abstract

A synthetic strategy has been devised for the preparation of the mononuclear complex [Ru(tpy)(L²)][PF₆]₂ (where L² is a bridging ligand containing two chelating 2,2′∶6′,2″-terpyridyl fragments attached via tolyl spacers to the N atoms of a 1,10-diaza-18-crown-6 macrocycle), which avoids the separation of a statistical mixture of mono- and di-nuclear complexes which would arise from normal synthetic methods. Reaction of 1,10-diaza-18-crown-6 with one equivalent of 4′-[4-(bromomethyl)phenyl]terpyridine afforded L¹, in which there is one terpyridyl group pendant from the macrocycle, and the second NH site of the macrocycle is not alkylated. Reaction of L¹ with [Ru(tpy)Cl₃] gave mononuclear [Ru(tpy)(L¹)][PF₆]₂. Subsequent reaction of this with a second equivalent of 4′-[4-(bromomethyl)phenyl]terpyridine resulted in attachment of the second (vacant) terpyridyl chelating site by alkylation of the remaining secondary amine group in the macrocycle to give [Ru(tpy)(L²)][PF₆]₂. Assembly of two of these mononuclear ‘complex ligands’ around first-row transition-metal dications M²⁺ (M = Fe or Ni) afforded in high yield the linear trinuclear Ru–M–Ru complexes [{(tpy)Ru(L²)}₂M][PF₆]₆, in which the two terminal {Ru(tpy)₂}²⁺ and the central {M(tpy)₂}²⁺ fragments are separated by diaza-18-crown-6 units. Electrospray mass spectrometry proved a very useful characterisational tool in all cases, showing a variety of charged species arising from both loss of anions and protonation of the basic amine sites in the aza-crown macrocycles: for [{(tpy)Ru(L²)}₂Fe][PF₆]₆ for example intact complex cations were observed with charges of up to +9 (from loss of all six anions, and triple protonation). The mononuclear complexes [Ru(tpy)(HL¹)][ClO₄]₂[PF₆]·2MeCN·Et₂O·H₂O and [Ru(tpy)(L²)][PF₆]₂·0.7HPF₆·0.3Et₂O· MeCN·1.5H₂O were crystallographically characterised. Electrochemical and electronic spectroscopic studies show that the {Ru(tpy)₂}²⁺ and {M(tpy)₂}²⁺ components of the trinuclear complexes are essentially electronically isolated.