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

Abstract

AbstractThe 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.