Metal–Metal Bridging Using the DPPP Dye System: Electronic Configurations within Multiple Redox Series

Redox series [LnRu­(μ-DPPP)­RuLn]k, H2DPPP = 2,5-dihydro-3,6-di-2-pyridylpyrrolo­(3,4-c)­pyrrole-1,4-dione and L = 2,4-pentanedionato (acac–), 2,2′-bipyridine (bpy), and 2-phenylazopyridine
(pap), have been studied by voltammetry (CV, DPV), EPR, and UV–vis–NIR
spectroelectrochemistry, supported by TD-DFT calculations. Crystal
structure analysis and 1H NMR revealed oxidation states
[(acac)2RuIII(μ-DPPP2–)­RuIII(acac)2] and [(bpy)2RuII(μ-DPPP2–)­RuII(bpy)2]2+ for the corresponding precursors, isolated
as rac diastereomers. Oxidation was observed to occur
mainly at the bridging ligand (DPPP2– → DPPP•–), whereas the site of reduction (DPPP, Ru,
or L) depends on effects from the ancillary ligands L. The metal coordination
of a derivative of the pigment forming 2,5-dihydro-pyrrolo­(3,4-c)­pyrrole-1,4-dione (DPP) dyes and the analysis of corresponding
multistep redox series add to the previously recognized coordinative
and electron transfer potential of dye molecules of the azo, indigo,
anthraquinone, and formazanate type.