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 [RuIII(tpm)(Cl)(QSq−)]+ for [1]+–[3]+. The paramagnetic [1]+–[3]+ show sharp 1H NMR spectra with strikingly small J of 1.8–3.0Hz. The DFT calculations on [1]+ predict that the triplet (S=1) state exists above (1004cm−1) the singlet (S=0) ground state. [1]+ exhibits μ=2.2 BM at 300K which diminishes to 0.3 BM near 2K 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)RuIII(QQo)]2+ ([1]2+–[3]2+)→[(tpm)(Cl)RuIII(QSq−)]+ ([1]+–[3]+)→[(tpm)(Cl)RuII(QSq−)]↔[(tpm)(Cl)RuIII(QCat)] (1–3)→[(tpm)(Cl)RuII(QCat)]− ([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.