Ru(II) complexes of polyarylated terpyridines: unexpected side-chain C-metallation and photosensitization of electron transfer

The new ligands 4,4″-diphenyl-6,6″-di(4-ethoxycarbonylphenyl)-2,2′:6′,2″-terpyridine (H4), its non-carboxylated 4,6,4″,6″-tetraphenyl analogue (H5) and the 4,4″-diphenyl-6,6″-di(4-methoxyphenyl) analogue were prepared in high yields via double Kröhnke reactions. Reactions with activated (ttpy)RuCl3 (ttpy is 4′-p-tolyl-2,2′:6′,2″-terpyridine) provided the symmetrical N6-coordinated complexes, [Ru(H4)(ttpy)](PF6)2 and [Ru(H5)(ttpy)](PF6)2, along with the novel, unsymmetrical C-metallated analogues [Ru(4)(ttpy)]PF6 and [Ru(5)(ttpy)]PF6, in which an unprecedented side-chain metallation occurred in lieu of incomplete substitution. A crystallographic analysis of [Ru(5)(ttpy)]NO3 confirmed the N5C donor set in these products and revealed a distorted binding of 5− as well as ‘π-stacking’ between its uncoordinated pyridine and the ttpy ligand. The diester [Ru(H4)(ttpy)](PF6)2 was hydrolyzed to the diacid complex [Ru(H23)(ttpy)](PF6)2 (H23 is 4,4″-diphenyl-6,6″-di(4-carboxyphenyl)-2,2′:6′,2″-terpyridine). Measurements of the photogeneration of methyl viologen cation radical with [Ru(H5)(ttpy)](PF6)2 and [Ru(H23)(ttpy)](PF6)2 as sensitizers showed that the presence of carboxyl groups in the latter provided a distinct benefit, owing to the formation of the neutral form [Ru(3)(ttpy)]0 but this was insufficient to overcome the detrimental effect of inter-ligand repulsions.