PAPER www.rsc.org/dalton | Dalton Transactions Expanded ligands: bis(2,2 ′ :6 ′ ,2 ′′ -terpyridine carboxylic acid)ruthenium(II) complexes as metallosupramolecular analogues of dicarboxylic acids† Edwin C. Constable,* a Emma L. Dunphy, a Catherine E. Housecroft, a Markus Neuburger, a Silvia Schaffner, a Frank Schaper a,b and Stuart R. Batten c Received 22nd June 2007, Accepted 19th July 2007 First published as an Advance Article on the web 14th August 2007 DOI: 10.1039/b709557k 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. Introduction We have recently been developing the concept of “expanded ligands” in which metal-binding domains are linked through metal-containing scaffolds. 1–8 Typical scaffolds are {M(bpy) 3 } 9 and {M(tpy) 2 } 10–12 moieties (bpy = 2,2 ′ -bipyridine, tpy = 2,2 ′ :6 ′ ,2 ′′ - terpyridine), which possess different topological and topographi- cal properties as well as potentially different numbers of attached metal-binding domains. 13,14 a Department of Chemistry, University of Basel, Spitalstrasse 51, 4056 Basel, Switzerland. E-mail: edwin.constable@unibas.ch; Fax: +41 61 267 1018; Tel: +41 61 267 1001 b Current address: Department of Chemistry, University of Montreal, CP 6128, succ. Centre-ville, Montreal, Que, Canada, H3C 3J7 c School of Chemistry, Monash University 3800, Australia †CCDC reference numbers 651821–651823. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b709557k Fig. 1 The concept of an expanded ligand exemplified by the comparison of oxalic and terephthalic acids with the complex cations bis[Ru(H1) 2 ] 2+ and [Ru(H2) 2 ] 2+ . This paper is concerned with ruthenium complexes of the carboxylic acid ligands H1 and H2. The origin of the term “expanded ligands” is seen in Fig. 1 which presents the ideal carboxylic acid C ··· C distances in oxalic acid, terephthalic acid, and the complexes [Ru(H1) 2 ] 2+ and [Ru(H2) 2 ] 2+ . The C ··· C distance increases by about 20 A ˚ or over 1000% on moving from oxalic acid to [Ru(H2) 2 ] 2+ . This journal is © The Royal Society of Chemistry 2007 Dalton Trans., 2007, 4323–4332 | 4323