Abhishek Mandal

Analysis of Redox Series of Unsymmetrical 1,4-Diamido-9,10-anthraquinone-Bridged Diruthenium Compounds

The unsymmetrical diruthenium complexes [(bpy)2Ru(II)(μ-H2L(2-))Ru(III)(acac)2]ClO4 ([3]ClO4), [(pap)2RuII(μ-H2L(2-))Ru(III)(acac)2]ClO4 ([4]ClO4), and [(bpy)2Ru(II)(μ-H2L(2-))Ru(II)(pap)2](ClO4)2 ([5](ClO4)2) have been obtained by way of the mononuclear precursors [(bpy)2Ru(II)(H3L(-))]ClO4 ([1]ClO4) and [(pap)2Ru(II)(H3L(-))]ClO4 ([2]ClO4) (where bpy = 2,2′-bipyridine, pap = 2-phenylazopyridine, acac(-) = 2,4-pentanedionate, and H4L = 1,4-diamino-9,10-anthraquinone). Structural characterization by single-crystal X-ray diffraction and magnetic resonance (nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR)) were used to establish the oxidation state situation in each of the isolated materials. Cyclic voltammetry, EPR, and ultraviolet-visible-near-infrared (UV-vis-NIR) spectroelectrochemistry were used to analyze the multielectron transfer series of the potentially class I mixed-valent dinuclear compounds, considering the redox activities of differently coordinated metals, of the noninnocent bridge and of the terminal ligands. Comparison with symmetrical analogues [L2′Ru(μ-H2L)RuL2′](n) (where L′ = bpy, pap, or acac(-)) shows that the redox processes in the unsymmetrical dinuclear compounds are not averaged, with respect to the corresponding symmetrical systems, because of intramolecular charge rearrangements involving the metals, the noninnocent bridge, and the ancillary ligands.

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

The new compounds [(bpy)2Os(II)(μ-L1(2-))Os(II)(bpy)2](ClO4)2 ([1](ClO4)2) and [(pap)2Os(II)(μ-L1(2-))Os(II)(pap)2](ClO4)2 ([2](ClO4)2) (H2L1 = 1,4-dihydroxy-9,10-anthraquinone, bpy = 2,2(/)-bipyridine, and pap = 2-phenylazopyridine) and [(bpy)2Os(II)(μ-L2(•-))Os(II)(bpy)2](ClO4)3 ([3](ClO4)3) and [(pap)2Os(II)(μ-L2(2-))Os(II)(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 (1)H 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 3(n) 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 10(3) and long-wavelength near-infrared absorptions; an EPR signal with appreciable g splitting (1.84, 1.96, and 2.03) was only observed for 4(3+), which exhibits the smallest spin density on the osmium centers. An oxidation state formulation [Os(III)(μ-L(•3-))Os(III)](3+) with some [Os(II)(μ-L(2-))Os(III)](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.

Ru-Complex Framework toward Aerobic Oxidative Transformations of β-Diketiminate and α-Ketodiimine

The impact of the {Ru(acac)2} (acac- = acetylacetonate) framework on the transformations of C-H and C-H/C-C bonds of coordinated β-diketiminate and ketodiimine scaffolds, respectively, has been addressed. It includes the following transformations involving {Ru(acac)2} coordinated β-diketiminate in 1 and ketodiimine in 2 with the simultaneous change in metal oxidation state: (i) insertion of oxygen into the C(sp2)-H bond of β-diketiminate in 1, leading to the metalated ketodiimine in 2 and (ii) Bronsted acid (CH3COOH) assisted cleavage of unstrained C(sp2)-C(sp2)/C═N bonds of chelated ketodiimine (2) with the concomitant formation of intramolecular C-N bond in 3, as well as insertion of oxygen into the C(sp3)-H bond of 2 to yield -CHO function in 4 (-CH3 → -CHO). The aforesaid transformation processes have been authenticated via structural elucidation of representative complexes and spectroscopic and electrochemical investigations.