Expanded ligands : bis(2,2′:6′,2″-terpyridine carboxylic acid)ruthenium(II) complexes as metallosupramolecular analogues of dicarboxylic acids

Ligands in which multiple metal-binding domains are linked by a metal-containing moiety rather than a conventional organic group are described as ``expanded ligands``. The use of 4,4`-difunctionalised Ru(tpy)(2) units provides a linear spacer between metal-binding domains and we have extended this motif to expanded ligands containing two carboxylic acid metal-binding domains. In this paper, we describe the synthesis and structural characterisation of ruthenium(II) complexes of 2,2`: 6`, 2``-terpyridine-4`-carboxylic acid and 4`-carboxyphenyl-2,2`: 6`, 2``-terpyridine. The ability of the ruthenium( II) centre to charge compensate deprotonation of the carboxylic acid leads to Zwitterionic complexes and three representative compounds have been structurally characterised.