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

A synthetic strategy has been devised for the preparation of the mononuclear complex [Ru(tpy)(L-2)][PF6](2) (where L-2 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 L-1, in which there is one terpyridyl group pendant from the macrocycle, and the second NH site of the macrocycle is not alkylated. Reaction of L-1 with [Ru(tpy)Cl-3] gave mononuclear [Ru(tpy)(L-1)][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)(L-2)][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(L-2)}(2)M][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(L-2)}(2)Fe][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)(L-2)][PF6](2). 0.7 HPF6. 0.3Et(2)O . MeCN . 1.5H(2)O 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.