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 L 1 - (HL 1 : (E)-2-hydroxy-3-(p-tolyldiazenyl)naphthalene-1,4-dione))/L 2 - (HL 2 :5-hydroxy-6-p-tolylazobenzo[a]phenazine) derived [Ru III (acac) 2 (L 1 - )]/[Ru III (acac) 2 (L 2 - )] 1/5, [Ru II (bpy) 2 (L 1 - )]ClO 4 /[Ru II (bpy) 2 (L 2 - )]ClO 4 [2]ClO 4 /[6]ClO 4 , ctc-[Ru II (pap) 2 (L 1 - )]ClO 4 /ctc-[Ru II (pap) 2 (L 2 - )]ClO 4 [3]ClO 4 /[7]ClO 4 and [Ru II (CO)(H)(PPh 3 ) 2 (L 1 - )]/[Ru II (CO)(Cl)(PPh 3 ) 2 (L 2 - )] 4/8 (acac=acetylacetonate, bpy=2,2'-bipyridine, pap=2-phenylazopyridine). The ligands L 1 - and L 2 - 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 L 1 - /L 2 - or pap as well as unprecedented para-quinone form of L 1 - . The involvement of selective redox center(s) towards the accessible redox steps of the complexes encompassing multiple redox active entities i. e. Ru, phenolate (L 1 - /L 2 - ), para-quinone (L 1 - ), phenazine (L 2 - ), azo (L 1 - /L 2 - , 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 L 1 - /L 2 - (phenolate→phenoxide), while successive reductions were taken place either at the para-quinone/phenazine units of L 1 - /L 2 - 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 L 1 - /L 2 - 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 L 1 - /L 2 - 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.