Diruthenium Complexes of p -Benzoquinone-Imidazole Hybrid Ligands: Innocent or Noninnocent Behavior of the Quinone Moiety

Diverse Coordination Modes and Bidirectional Noninnocence of Pyridyl-β-diketonate on Ruthenium Platforms as a Function of Coligands

The article demonstrated diverse binding modes of deprotonated 1,3-di(2-pyridinyl)-1,3-propanedione (HL) (κ2-[O,O]-, κ2-[N,O]-, and μ-bis-κ2-[N,O]-) on selective ruthenium platforms: Ru(acac)2 (dimeric [1]ClO4), Ru(bpy)2 (monomeric [2]ClO4), Ru(pap)2 (isomeric monomeric [3]ClO4/[4]ClO4, dimeric [5](ClO4)3), and Ru(PPh3)2(CO) (monomeric 6, isomeric dimeric [7]ClO4/[8]ClO4) (acac = acetylacetonate, bpy = 2,2'-bipyridine, pap = 2-phenylazopyridine). Structural authentication of the complexes revealed (i) diverse binding mode of L- including its unprecedented bridging mode in [8]ClO4, (ii) varying degrees of nonplanarity of L-, and (iii) development of 1D polymeric chains or dimeric/tetrameric forms via intermolecular π-π interactions. The preferential binding feature of L- in the complexes could also be corroborated by their calculated relative energies. The analysis of the multiredox steps of the complexes suggested severe mixing of metal-ligand frontier orbitals, which in effect pinpointed the involvement of L- in both the oxidative and reductive processes along the redox chain, suggesting its bidirectional noninnocence under the present coordination situations. Though α-diketone or β-diketiminate was reported to activate O2 on the selective Ru(acac)2 platform, the inability of analogous β-diketonate-derived [1]ClO4 could be attributed to its calculated greater HOMO-LUMO energy gap, which disfavored electron exchange at the metal(RuIII)-ligand(L-) interface to introduce the required unpaired spin at the ligand backbone toward the 3O2 activation event.

On the non-innocence and reactive versus non-reactive nature of α-diketones in a set of diruthenium frameworks

Slightly modified ligand designs in diruthenium setups have major impacts on the reactivity/stability of coordination complexes. The 1,2-bis(2-hydroxyphenyl)ethane-1,2-dione bridge is also potentially redox non-innocent.