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

Abstract

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 [RuIII (acac)2 (L1 - )]/[RuIII (acac)2 (L2 - )] 1/5, [RuII (bpy)2 (L1 - )]ClO4 /[RuII (bpy)2 (L2 - )]ClO4 [2]ClO4 /[6]ClO4 , ctc-[RuII (pap)2 (L1 - )]ClO4 /ctc-[RuII (pap)2 (L2 - )]ClO4 [3]ClO4 /[7]ClO4 and [RuII (CO)(H)(PPh3 )2 (L1 - )]/[RuII (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.