What a Difference Ancillary Thienyl Makes: Unexpected Additional Stabilization of the Diruthenium(III,II) but Not the Diosmium(III,II) Mixed-Valent State in Tetrazine Ligand-Bridged Complexes

The Ru2III,II mixed-valent state is strongly stabilized in [(bpy)2Ru(-bttz)Ru(bpy)2]^5+ (3^5+), bttz = 3,6-bis(2-thienyl)-1,2,4,5-tetrazine, as evident from lowered oxidation potentials and isolability, a strongly increased comproportionation constant Kc = 10^16.6, and a high-energy intervalence charge transfer band at 10100 cm^-1. Curiously, no such effects were observed for the diosmium(III,II) analogue, whereas the related systems [(bpy)2M(bmptz)M(bpy)2]^5+, bmptz = 3,6-bis(4-methyl-2-pyridyl)-1,2,4,5-tetrazine, exhibit conventional behavior, i.e., a slightly higher Kc value of the Os2III,II analogue. EPR signals were observed at 4 K for 3^5+ but not for the other mixed-valent species, and high-frequency (285 GHz) EPR was employed to study the diruthenium(II) radical complexes 2^3+ and 3^3+.