Redox and Spectroscopic Orbitals in Ru(II) and Os(II) Phenolate Complexes

A detailed spectroscopic and electrochemical study of a series of novel phenolate bound complexes, of general
formulas [M(L-L)2(box)](PF6), where M is Os and Ru, L-L is 2,2-bipyridine or 2,2-biquinoline, and box is 2-(2-hydroxyphenyl)benzoxazole, is presented. The objectives of this study were to probe the origin of the LUMOs and
HOMOs in these complexes, to elucidate the impact of metal and counter ligand on the electronic properties of the
complex, and to identify the extent of orbital mixing in comparison with considerably more frequently studied quinoid
complexes. [M(L-L)2(box)](PF6) complexes exhibit a rich electronic spectroscopy extending into the near infrared
region and good photostability, making them potentially useful as solar sensitizers. Electrochemistry and spectroscopy
indicate that the first oxidation is metal based and is associated with the M(II)/(III) redox states. A second oxidative
wave, which is irreversible at slow scan rates, is associated with the phenolate ligand. The stabilities of the oxidized
complexes are assessed using dynamic electrochemistry and discussed from the perspective of metal and counter
ligand (LL) identity and follow the order of increasing stability [Ru(biq)2(box)]+ < [Ru(bpy)2(box)]+ < [Os(bpy)2(box)]+. Electronic and resonance Raman spectroscopy indicate that the lowest energy optical transition for the
ruthenium complexes is a phenolate (π) to L-L (π*) interligand charge-transfer transition (ILCT) suggesting the
HOMO is phenolate based whereas electrochemical data suggest that the HOMO is metal based. This unusual
lack of correlation between redox and spectroscopically assigned orbitals is discussed in terms of metal−ligand
orbital mixing which appears to be most significant in the biquinoline based complex.