An unusually weak intervalence transition in a very stable bis chelate analog of the ruthenium mixed-valent Creutz-Taube ion. UV/visible/near-IR and EPR spectroelectrochemistry of (NH3)4Ru(m-bptz)Ru(NH3)4n+ (bptz = 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine; n = 3 - 5)
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Inorganic Chemistry 32 (12): 2640--2643 (1993)

The pentacationic form 25+ of the title complex, which exhibits an electrochem. stability const. Kc of 1015 in MeCN, shows much formal similarity to the mixed-valent (RuII/RuIII) pyrazine-bridged Creutz-Taube ion 15+. However, the flanking of the central p-accepting tetrazine ring of the bridging ligand bptz by 2 coordinating pyridyl groups results in a rigid chelate arrangement with the Ru-ammine bonds situated parallel and perpendicular to the p system of bptz. This conformation is essentially different from the staggered arrangement in 15+, the difference being reflected not by the shape or max. of the band due to the intervalence transition (IT) at 1450 nm but by a decrease of its intensity by 1 order of magnitude from e 5000 (15+) to 500 M-1 cm-1 for 25+. While the IT band max. is essentially solvent insensitive, the near-IR spectrum in D2O/DCl reveals some structuring of the IT band with a spacing of $\sim$900 cm-1. In addn. to UV/visible/near-IR spectroelectrochem., the EPR data confirm the metal-based spin in the 5+ form (g1 = 2.019, g2 = 2.418, g3 = 2.913) and a predominantly ligand-centered unpaired electron in the 3+ state (g‖ = 2.022, g$\bot$ = 1.989), which is accessible here due to the good p-acceptor properties of bptz. Nevertheless, the observation of the EPR signal only \textless70 K indicates a stronger metal contribution in 23+ than in the related radical complex (bpy)2Ru(m-bptz)Ru(bpy)23+. on SciFinder(R)
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