Structural and Electronic Diversity as a Function of Metal Ions and Co‐ligands in Deprotonated Epindolidione Bridged Diruthenium and Diosmium Set‐up

Abstract

The article dealt with the structurally validated L2− (H2L=epindolidione=5,11‐dihydroquinolino[3,2‐b]quinoline‐6,12‐dione, a heteroatom‐modified tetracene and a structural isomer of indigo dye) bridged {(acac)2RuIII}2(μ‐L2−) 1/2 (S=1, acac=acetylacetonate) and [{(bpy)2OsII}2(μ‐L2−)](ClO4)2 [3](ClO4)2/[(pap)2OsII}2(μ‐L2−)](ClO4)2 [4](ClO4)2 (S=0, bpy=2,2′‐bipyridine, pap=2‐phenylazopyridine). 1/2 and [3](ClO4)2/[4](ClO4)2 with a metal‐metal separation of ≈7 Å stabilized selectively in meso(ΔΛ)/rac(ΔΔ/ΛΛ) and meso diastereomeric forms, respectively, in spite of calculated minor difference in energy between the diastereomers in each case. Though 2, [3](ClO4)2/[4](ClO4)2) exhibited intermolecular π–π interactions, no such non‐covalent interaction was perceived in 1. RuIIIRuIII derived 1/2 (S=1, 2.63/2.57 μB at 300 K) attributed to non‐interacting two Ru(III) (S=1/2) spins with an intermolecular through space weak antiferromagnetic exchange interaction below 12 K and its variable temperature 1H NMR revealed Fermi and pseudo‐contact shifts. Multiple redox steps of the complexes varied as a function of metal ion (Ru versus Os), its oxidation state (III versus II) and ancillary ligand (acac versus bpy versus pap). A collective consideration of DFT calculated MOs and Mulliken spin densities suggested delicate electronic forms along the redox chain due to the participation of bridge largely in the oxidation processes as a consequence of its redox non‐innocent feature. Reductions were however mostly centered around the metal (RuIII to RuII) and ancillary ligands (bpy/pap) in 1/2 and 32+/42+, respectively. 1n/2n, [3]2+/[4]2+ displayed mixed metal/ligand based weak low‐energy (800‐1400 nm) transitions in addition to intense visible bands as corroborated by TD‐DFT calculations.