Bidirectional noninnocence of hinge-like deprotonated bis-lawsone on selective ruthenium platform: a function of varying ancillary ligands

Abstract

The present work aimed to obtain discrete heavier metal complexes of unperturbed deprotonated bis-lawsone (hinge-like H₂L = 2,2′-bis(3-hydroxy-1,4-napthoquinone). This is primarily due to its limited examples with lighter metal ions (Co, Zn, and Ga) and the fact that our earlier approach with the osmium ion facilitated its functionalisation. Herein, we demonstrated the successful synthesis and structural characterisation of L²⁻-derived diruthenium [(bpy)₂Ru^II(μ-L²⁻)Ru^II(bpy)₂](ClO₄)₂ [1](ClO₄)₂ (S = 0), (acac)₂Ru^III(μ-L²⁻)Ru^III(acac)₂2 (S = 1) and monoruthenium (pap)₂Ru(L²⁻) 3 (S = 0) derivatives (bpy = 2,2′-bipyridine, acac = acetylacetonate, and pap = 2-phenylazopyridine). The crystal structures established that (i) O,O⁻/O,O⁻ donating five-membered bis-bidentate and O⁻,O⁻ donating seven-membered bidentate chelating modes of deprotonated L²⁻ in rac (ΔΔ/ΛΛ) diastereomeric [1](ClO₄)₂, 2 and 3, respectively. (ii) The L²⁻ bridging unit in [1](ClO₄)₂, 2 and 3 underwent twisting its two naphthoquinone rings with respect to the ring connecting C–C bond by 73.01°, 62.15° and 59.12°, respectively. (iii) Intermolecular π–π interactions (∼3.5 Å) between the neighbouring molecules. The paramagnetic complex 2 (S = 1) with two non-interacting Ru(III) (S = 1/2) ions exhibited weak antiferromagnetic coupling only at very low temperatures. In agreement with the magnetic results, 2 displayed typical Ru^III-based anisotropic EPR in CH₃CN (<g>/Δg: 2.314/0.564) but without any forbidden g_1/2 signal at 120 K. The complexes exhibited multiple redox processes in CH₃CN in the experimental potential window of ± 2.0 V versus SCE. The analysis of the redox steps via a combined experimental and theoretical (DFT/TD-DFT) approach revealed the involvement of L²⁻ to varying extents in both the oxidative and reductive processes as a consequence of its bidirectional redox non-innocent feature. The mixing of the frontier orbitals of the metal ion and L²⁻ due to their closeness in energy indeed led to the resonating electronic form in certain redox states instead of any precise electronic structural state.