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

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(mu-L2-) 1/2 (S=1, acac=acetylacetonate) and [{(bpy)2OsII}2(mu-L2-)](ClO4)2 [3](ClO4)2/[(pap)2OsII}2(mu-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 approximate to 7 angstrom stabilized selectively in meso(Delta Lambda)/rac(Delta Delta/Lambda Lambda) 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 pi-pi interactions, no such non-covalent interaction was perceived in 1. RuIIIRuIII derived 1/2 (S=1, 2.63/2.57 mu B 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. Noninnocence of deprotonated epindolidione in selectively designed diruthenium/diosmium set up has been reaffirmed irrespective of metal ions and ancillary ligands (acac, bpy, pap). Multiple redox sensitive constituents in the complex frameworks introduced a competitive scenario with respect to their electronic forms, which has been addressed by experimental and DFT calculations.image