Varying Electronic Structures of Diosmium Complexes from Noninnocently Behaving Anthraquinone-Derived Bis-chelate Ligands
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Inorganic Chemistry 54 (16): 7936--7944 (2015)

(Bpy)2OsII(m-L21-)OsII(bpy)2(ClO4)2 (1(ClO4)2) and (pap)2OsII(m-L21-)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(m-L2.bul.-)OsII(bpy)2(ClO4)3 (3(ClO4)3) and (pap)2OsII(m-L22-)OsII(pap)2(ClO4)2 (4(ClO4)2) (H2L2 = 1,4-diamino-9,10-anthraquinone) were anal. identified as the meso and rac diastereoisomers, resp. The paramagnetic 3(ClO4)3 was also characterized by crystal structure detn. In CD3CN soln., 3(ClO4)3 displays rather narrow but widely split (13 \textgreater d \textgreater -8 ppm) resonances in the 1H NMR spectrum, yet no EPR signal was obsd. down to 120 K. Cyclic voltammetry and differential pulse voltammetry reveal several accessible redox states on oxidn. and redn., showing that the replacement of 1,4-oxido by imido donors causes cathodic shifts and that the substitution of bpy by the stronger p-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 consts. Kc of $\sim$103 and long-wavelength near-IR absorptions; an EPR signal with appreciable g splitting (1.84, 1.96, and 2.03) was only obsd. for 43+, which exhibits the smallest spin d. on the osmium centers. An oxidn. state formulation OsIII(m-L.bul.3-)OsIII3+ with some OsII(m-L2-)OsIII3+ contribution was found to best describe the electronic structures. UV-visible-NIR absorption spectra were recorded for all accessible states by OTTLE spectroelectrochem. and assigned from TD-DFT calcns. These results and addnl. EPR measurements suggest rather variegated oxidn. state situations, e.g., the pap ligands competing with the bridge L for electrons, while the oxidn. produces mixed spin systems with variable metal/ligand contributions. on SciFinder(R)
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