Azo Appended Modified Lawsone Derived Ru-Systems with Multi-Redox Entities. Competitive Electronic Form and the Question of Azo Reduction

The article dealt with the ruthenium complexes of redox active azo appended modified lawsone L₁ ^- (HL₁ : (E)-2-hydroxy-3-(p-tolyldiazenyl)naphthalene-1,4-dione))/L₂ ^- (HL₂ :5-hydroxy-6-p-tolylazobenzo[a]phenazine) derived [Ru^III (acac)₂ (L₁ ^- )]/[Ru^III (acac)₂ (L₂ ^- )] 1/5, [Ru^II (bpy)₂ (L₁ ^- )]ClO₄ /[Ru^II (bpy)₂ (L₂ ^- )]ClO₄ [2]ClO₄ /[6]ClO₄ , ctc-[Ru^II (pap)₂ (L₁ ^- )]ClO₄ /ctc-[Ru^II (pap)₂ (L₂ ^- )]ClO₄ [3]ClO₄ /[7]ClO₄ and [Ru^II (CO)(H)(PPh₃ )₂ (L₁ ^- )]/[Ru^II (CO)(Cl)(PPh₃ )₂ (L₂ ^- )] 4/8 (acac=acetylacetonate, bpy=2,2'-bipyridine, pap=2-phenylazopyridine). The ligands L₁ ^- and L₂ ^- differed with respect to the para-quinone versus phenazine moieties linked to the azo function. Structural analysis of the complexes established unreduced state of the azo (N=N) group of coordinated L₁ ^- /L₂ ^- or pap as well as unprecedented para-quinone form of L₁ ^- . The involvement of selective redox center(s) towards the accessible redox steps of the complexes encompassing multiple redox active entities i. e. Ru, phenolate (L₁ ^- /L₂ ^- ), para-quinone (L₁ ^- ), phenazine (L₂ ^- ), azo (L₁ ^- /L₂ ^- , pap), diimine (bpy) was analyzed by combined experimental and DFT calculations. It revealed that under the prevailing competitive scenario oxidation was mostly dominated by the phenolate group of L₁ ^- /L₂ ^- (phenolate→phenoxide), while successive reductions were taken place either at the para-quinone/phenazine units of L₁ ^- /L₂ ^- or azo/diimine functions of pap/bpy. Though the azo function of pap in 3⁺ /7⁺ underwent facile reduction, the same azo function associated with L₁ ^- /L₂ ^- conspicuously remained unreduced in all occasions. The frontier molecular orbital analysis revealed that the propensity of pap for the azo reduction with special reference to that in L₁ ^- /L₂ ^- could be correlated with its relatively lower energy π* orbital (LUMO). Complexes displayed intense LMCT (1/5) and bpy (2⁺ /6⁺ ), pap (3⁺ /7⁺ ), L (4/8) targeted MLCT transitions in the visible region.