AN UNUSUALLY WEAK INTERVALENCE TRANSITION IN A VERY STABLE BIS-CHELATE ANALOG OF THE MIXED-VALENT CREUTZ-TAUBE ION - UV VIS NEAR-IR AND EPRSPECTROELECTROCHEMISTRY OF [(NH3)4RU(MU-BPTZ)RU(NH3)4]N+ (BPTZ = 3,6-BIS(2-PYRIDYL)-1,2,4,5-TETRAZINE N = 3-5)/

The pentacationic form 2(5+) of the title complex, which exhibits an e lectrochemical stability constant K(c) of 10(15) in acetonitrile, show s much formal similarity to the mixed-valent (Ru(II)/Ru(III)) pyrazine -bridged Creutz-Taube ion 1(5+). However, the flanking of the central pi-accepting tetrazine ring of the bridging ligand bptz by two coordin ating pyridyl groups results in a rigid chelate arrangement with the r uthenium-ammine bonds situated parallel and perpendicular to the pi sy stem of bptz. This conformation is essentially different from the 'sta ggered'' arrangement in 1(5+), the difference being reflected not by t he shape or maximum of the band due to the intervalence transition (IT ) at 1450 nm but by a decrease of its intensity by 1 order of magnitud e from epsilon 5000 (1(5+)) to 500 M-1 cm-1 for 2(5+). While the IT ba nd maximum is essentially solvent insensitive, the near-IR spectrum in D2O/DCl reveals some structuring of the IT band with a spacing of abo ut 900 cm-1. In addition to UV/vis/near-IR spectroelectrochemistry, th e EPR data confirm the metal-based spin in the 5+ form (g1 = 2.019, g2 = 2.418, g3 = 2.913) and a predominantly ligand-centered unpaired ele ctron in the 3+ state (g(parallel-to) = 2.022, g(perpendicular-to) = 1 .989), which is accessible here due to the good pi-acceptor properties of bptz. Nevertheless, the observability of the EPR signal only below 70 K indicates a stronger metal contribution in 2(3+) than in the rel ated radical complex [(bPY)2Ru(mu-bptz)Ru(bpy)2]3+.