Varying Electronic Structures of Diosmium Complexes from Noninnocently Behaving Anthraquinone-Derived Bis-chelate Ligands

The
new compounds [(bpy)2OsII(μ-L12–)­OsII(bpy)2]­(ClO4)2 ([1]­(ClO4)2) and
[(pap)2OsII(μ-L12–)­OsII(pap)2]­(ClO4)2 ([2]­(ClO4)2) (H2L1 = 1,4-dihydroxy-9,10-anthraquinone, bpy = 2,2/-bipyridine,
and pap = 2-phenylazopyridine) and [(bpy)2OsII(μ-L2•–)­OsII(bpy)2]­(ClO4)3 ([3]­(ClO4)3) and [(pap)2OsII(μ-L22–)­OsII(pap)2]­(ClO4)2 ([4]­(ClO4)2) (H2L2 = 1,4-diamino-9,10-anthraquinone)
have been analytically identified as the meso and rac diastereoisomers, respectively. The paramagnetic [3]­(ClO4)3 was also characterized by
crystal structure determination. In CD3CN solution, [3]­(ClO4)3 displays rather narrow but
widely split (13 > δ > −8 ppm) resonances in the 1H NMR spectrum, yet no EPR signal was observed down to 120
K. Cyclic voltammetry and differential pulse voltammetry reveal several
accessible redox states on oxidation and reduction, showing that the
replacement of 1,4-oxido by imido donors causes cathodic shifts and
that the substitution of bpy by the stronger π-accepting pap
ligands leads to a strong increase of redox potentials. Accordingly,
system 3n with the lowest
(2+/3+) potential was synthetically obtained in the mono-oxidized
(3+) form. The (3+) intermediates display small comproportionation
constants Kc of about 103 and
long-wavelength near-infrared absorptions; an EPR signal with appreciable g splitting (1.84, 1.96, and 2.03) was only observed for 43+, which exhibits the smallest spin density on
the osmium centers. An oxidation state formulation [OsIII(μ-L•3–)­OsIII]3+ with some [OsII(μ-L2–)­OsIII]3+ contribution was found to best describe the
electronic structures. UV–vis–NIR absorption spectra
were recorded for all accessible states by OTTLE spectroelectrochemistry
and assigned on the basis of TD-DFT calculations. These results and
additional EPR measurements suggest rather variegated oxidation state
situations, e.g., the pap ligands competing with the bridge L for
electrons, while the oxidation produces mixed spin systems with variable
metal/ligand contributions.