ESR study of reduced mixed ligand complexes of Ru(II): 4,4'- and 5,5'-diester bipyridines with Ru(II)

The electron spin resonance spectra for a series of reduced tris (ML/sub 3//sup (2-n)+/) and mixed ligand complexes (ML/sub 2/L'/sup (2-n)+/), where L and L' are 2,2'-bipyridine (bpy), 4,4'-dicarbethoxy-2,2'-bipyridine (4-COOEt), and 5,5'-dicarbethoxy-2,2'-bipyridine (5-COOEt), exhibit a range of behavior. The one-electron (n = 1) spectra of (Ru(bpy)/sub 2/L/sup -/)/sup +/ where L = 4-COOEt and 5-COOEt give hyperfine structure with no temperature-dependent line broadening. While the n = 1 and n = 2 reduction products of the tris RuL/sub 3//sup 2 +/ complexes exhibit a temperature-dependent S = 1/2 signal from which an activation energy for intramolecular electron hopping can be obtained, the n = 3 product does not broaden with temperature. These results are consistent with the spatially isolated localized orbital model. The ESR for the n = 4 and n = 5 reduction products of (Ru(5-COOEt)/sub 3/)/sup 2 +/ give only an S = 1/2 signal that is not temperature dependent. The n = 6 species is diamagnetic, consistent with expectation. The hyperfine splitting constants (a/sub i/) obtained from the monocomplexes indicate a small spin density at the Ru/sup 2 +/ metal center.