Carboxy-derived (tpy)2Ru2+ complexes as sub-units in supramolecular architectures: The solubilized ligand 4′-(4-carboxyphenyl)-4,4″-di-(tert-butyl)tpy and its homoleptic Ru(II) complex

We herein describe the synthesis and characterization of a series of homoleptic, Ru(II) complexes bearing peripheral carboxylic acid functionality based upon the novel ligand 4′-(4-carboxyphenyl)-4,4″-di-(tert-butyl)tpy (L1), as well as 4′-(4-carboxyphenyl)tpy (L2) and 4′-(carboxy)tpy (L3) (where tpy = 2,2′: 6′,2″-terpyridine). Inspection of the metal-based oxidations (E1/2 = 1.22–1.42 V) indicates an anodic shift (∼0.2 V) for (L3)2Ru2+ (3b) (E1/2 = 1.40 V) relative to (L2)2Ru2+ (2b) (E1/2 = 1.22 V). The metal-based oxidation (E1/2 = 1.22 V) and ligand-based reductions (E1/2 = −1.25 to −1.52 V) of (L1)2Ru2+ (1) are essentially invariant relative to those of the structural analogue 2b (PF6)2, which suggests no significant electronic effect caused by the tert-butyl groups. This is supported by invariance in the metal-to-ligand charge transfer bands in both the electronic absorption (494–489 nm) and emission spectra (654–652 nm). However, contrary to 2b, complex 1 is both very soluble and exhibits a highly porous solid-state structure with internal cavity dimensions of 15 Å × 14 Å due to the preclusion of inter-annular interactions by the bulky tert-butyl substituents.