Redox Levels of a <i>closo</i>-Osmaborane: A Density Functional Theory, Electron Paramagnetic Resonance and Electrochemical Study

Abstract

A <i>closo</i>-type 11-vertex osmaborane [1-(η⁶-pcym)-1-OsB₁₀H₁₀] (pcym = <i>para</i>-cymene) has been synthesized and characterized by single-crystal X-ray diffraction and elemental analysis, as well as by ¹¹B and ¹H NMR, UV–visible, and mass spectrometry. The redox chemistry has been probed by dc and Fourier transformed ac voltammetry and bulk reductive electrolysis in CH₃CN (0.10 M (<i>n</i>-Bu)₄NPF₆) and by voltammetry in the ionic liquid <i>N</i>-butyl-<i>N-</i>methylpyrrolidinium bis­(trifluoromethylsulfonyl)­amide (Pyrr_1,4-NTf₂), which allows the oxidative chemistry of the osmaborane to be studied. A single-crystal X-ray diffraction analysis has shown that [1-(η⁶-pcym)-1-OsB₁₀H₁₀] is isostructural with other metallaborane compounds of this type. In CH₃CN (0.10 M (<i>n</i>-Bu)₄NPF₆), [1-(η⁶-pcym)-1-OsB₁₀H₁₀] undergoes two well-resolved one-electron reduction processes with reversible potentials separated by ca. 0.63–0.64 V. Analysis based on a comparison of experimental and simulated ac voltammetric data shows that the heterogeneous electron transfer rate constant (<i>k</i>⁰) for the first reduction process is larger than that for the second step at GC, Pt, and Au electrodes. <i>k</i>⁰ values for both processes are also larger at GC than metal electrodes and depend on the electrode pretreatment, implying that reductions involve specific interaction with the electrode surface. EPR spectra derived from the product formed by one-electron reduction of [1-(η⁶-pcym)-1-OsB₁₀H₁₀] in CH₃CN (0.10 M (<i>n</i>-Bu)₄NPF₆) and electron orbital data derived from the DFT calculations are used to establish that the formal oxidation state of the metal center of the original unreduced compound is Os^II. On this basis it is concluded that the metal atom in [1-(η⁶-pcym)-1-OsB₁₀H₁₀] and related metallaboranes makes a 3-orbital 2-electron contribution to the borane cluster. Oxidation of [1-(η⁶-pcym)-1-OsB₁₀H₁₀] coupled to fast chemical transformation was observed at 1.6 V vs ferrocene^0/+ in Pyrr_1,4-NTf₂. A reaction scheme for the oxidation involving formation of [1-(η⁶-pcym)-1-OsB₁₀H₁₀]⁺, which rearranges to an unknown electroactive derivative, is proposed, and simulations of the voltammograms are provided