Valence and spin situations in isomeric [(bpy)Ru(Q′)2]n (Q′ = 3,5-di-tert-butyl-N-aryl-1,2-benzoquinonemonoimine). An experimental and DFT analysis

Abstract

The article deals with the ruthenium complexes, [(bpy)Ru(Q′)₂] (1–3) incorporating two unsymmetrical redox-noninnocent iminoquinone moieties [bpy = 2,2′-bipyridine; Q′ = 3,5-di-tert-butyl-N-aryl-1,2-benzoquinonemonoimine, aryl = C₆H₅ (Q′₁), 1; m-Cl₂C₆H₃ (Q′₂), 2; m-(OCH₃)₂C₆H₃ (Q′₃), 3]. 1 and 3 have been preferentially stabilised in the cc-isomeric form while both the ct- and cc-isomeric forms of 2 are isolated [ct: cis and trans and cc: cis and cis with respect to the mutual orientations of O and N donors of two Q′]. The isomeric identities of 1–3 have been authenticated by their single-crystal X-ray structures. The collective consideration of crystallographic and DFT data along with other analytical events reveals that 1–3 exhibit the valence configuration of [(bpy)Ru^II(Q′_Sq)₂]. The magnetization studies reveal a ferromagnetic response at 300 K and virtual diamagnetic behaviour at 2 K. DFT calculations on representative 2a and 2b predict that the excited triplet (S = 1) state is lying close to the singlet (S = 0) ground state with singlet–triplet separation of 0.038 eV and 0.075 eV, respectively. In corroboration with the paramagnetic features the complexes exhibit free radical EPR signals with g ∼2 and ¹HNMR spectra with broad aromatic proton signals associated with the Q′ at 300 K. Experimental results in conjunction with the DFT (for representative 2a and 2b) reveal iminoquinone based preferential electron-transfer processes leaving the ruthenium(II) ion mostly as a redox insensitive entity: [(bpy)Ru^II(Q′_Q)₂]²⁺ (1²⁺–3²⁺) [(bpy)Ru^II(Q^′_Sq)(Q^′_Q)]⁺ (1⁺–3⁺) [(bpy)Ru^II(Q^′_Sq)₂] (1–3) [(bpy)Ru^II(Q^′_Sq)(Q^′_Cat)]⁻/[(bpy)Ru^III(Q^′_Cat)₂]⁻ (1⁻–3⁻). The diamagnetic doubly oxidised state, [(bpy)Ru^II(Q′_Q)₂]²⁺ in 1²⁺–3²⁺ has been authenticated further by the crystal structure determination of the representative [(bpy)Ru^II(Q′₃)₂](ClO₄)₂ [3](ClO₄)₂ as well as by its sharp ¹H NMR spectrum. The key electronic transitions in each redox state of 1ⁿ–3ⁿ have been assigned by TD–DFT calculations on representative 2a and 2b.