Probing valence and spin situations in selective ruthenium-iminoquinonoid frameworks: an experimental and DFT analysis

Abstract

The ruthenium-iminoquinone complexes, [Ru(tpm)(Cl)(Q)]⁺ [tpm = tris(1-pyrazolyl)methane, Q = 3,5-di-tert-butyl-N-aryl-1,2-benzoquinonemonoimine, where aryl = C₆H₅, [1]⁺; m-(OCH₃)₂C₆H₃, [2]⁺; m-(Cl)₂C₆H₃, [3]⁺] have been synthesized. The sensitive bond distances of "Q" in [1](ClO4) and [2](ClO4), C-O: 1.294(8), 1.281(2) Å ; C-N: 1.352(8), 1.335(2) Å ; and C-C(meta): 1.366(10)/1.367(9) Å , 1.364(2)/1.353(2) Å , respectively, and other analytical as well as theoretical (DFT) events suggest the valence configuration of [Ru^III(tpm)(Cl)(Q_Sq^-)]⁺ for [1]⁺-[3]⁺. The paramagnetic [1]⁺-[3]⁺ show sharp ¹H NMR spectra with strikingly small J of 1.8-3.0 Hz. The DFT calculations on [1]⁺ predict that the triplet (S = 1) state exists above (1004 cm^-1) the singlet (S = 0) ground state. [1]⁺ exhibits μ = 2.2 BM at 300 κ which diminishes to 0.3 BM near 2 κ due to the steady decrease in the ratio of triplet to singlet population with the lowering of temperature. [1]⁺-[3]⁺ exhibit one oxidation and two successive reductions each in CH₃CN. Experimental and DFT analyses collectively establish the valence configurations at the non-innocent {Ru-Q} interface along the redox chain as [(tpm)(Cl)Ru^III(Q_Q^o)]²⁺ ([1]²⁺-[3]²⁺) → [(tpm)(Cl)Ru^III(Q^Sq-)]⁺ ([1]⁺-[3]⁺) →[(tpm)(Cl)Ru^II(Q_Sq^-)] ↔ [(tpm)(Cl)Ru^III(Q_Cat)] (1-3) → [(tpm)(Cl)Ru^II(Q_Cat)]^- ([1]^--[3]^-). The spectral features of [1]ⁿ-[3]ⁿ (n = +2, +1, 0) have been addressed based on the TD-DFT calculations on [1]ⁿ.