Stabilizing Ruthenium(I) Radicals with N-Heterocyclic Carbene and Arene Ligands

Abstract

Ru(I) radicals with a 4d⁷ electronic configuration are sought after for their unique electronic structure and synthetic utility in chemical transformations. However, due to their high reactivity, ligand scaffolds effecting the stabilization of Ru(I) species are exceedingly rare. In this study, we present readily accessible, persistent Ru(I) radicals supported by N-heterocyclic carbenes (NHCs) and p-cymene, [RuCl(NHC)(η⁶-p-cymene)]. These halide complexes are easily prepared via single-electron reduction of their Ru(II) dichloride precursors. Integrated single-crystal X-ray diffraction, EPR spectroscopy, magnetic measurements, and DFT studies establish an S = 1/2 ground state with spin density primarily localized on ruthenium. Frontier molecular orbital analysis indicates that the stabilization of these Ru(I) radicals originates from the synergistic interplay of the strong σ-donation from the NHC ligand and δ-acceptance of the arene ligand. Accordingly, voltammetry study showes a reversible one-electron oxidation at relatively high potential (E_1/2 = –0.32 V vs. SCE) and an irreversible one-electron reduction at very low potential (E_p = –2.17 V vs. SCE). Furthermore, the chloride ligand in these Ru(I) complexes allows chloride-to-phosphine exchange reaction as demonstrated by the preparation of the cationic Ru(I) complex [Ru(PMe₃)(IMes)(η⁶-p-cymene)][BPh₄] from the reaction of [RuCl(IMes)(η⁶-p-cymene)] with PMe₃ and NaBPh₄, underscoring their utility as tunable precursors for catalysis.

Keywords

Ruthenium / N-Heterocyclic carbene / Metalloradical / Redox reaction / Arene ligand / DFT calculation / EPR spectroscopy / Spin density

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