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.