Stable NMR- and EPR-Active 17-Electron Chromium(III) Half-Sandwich Compounds

Formerly elusive 17-electron half-sandwiches of chromium(III) were isolated after stabilization with isocyanide ligands by using different approaches. Thus, reaction of Cp₂Cr with CF₃SO₃H and CNCH₃ gave [CpCr(CNCH₃)₄]²⁺ (7²⁺) as a triflate, while [(R_nCp)CrCl₂]₂ (R = Me, H (3 and 5, respectively), n = 5; R = t-Bu, n = 1 (8)) reacted with Tl(CF₃SO₃) to give 7²⁺, [(C₅Me₅)Cr(CNCH₃)₄]²⁺ (2²⁺), and [(t-BuCp)Cr(CNCH₃)₄]²⁺ (10²⁺) again as triflates in up to 59% yield. The precursor 8 was synthesized from (t-BuCp)Na and CrCl₃(THF)₃. The dication 2²⁺ was also obtained electrochemically from [(C₅Me₅)Cr(CNCH₃)₄]⁺[PF₆]^- (2⁺[PF₆]^-), which was synthesized by reaction of [(C₅Me₅)Cr(C₆H₃Me₃)]⁺[PF₆]^- with CNCH₃ in 86% yield. Conversely, the dication 7²⁺ could be reduced to 7⁺ obtained as [PF₆]^- and [B(C₆H₅)₄]^- salts. The crystal structure of 10²⁺[CF₃SO₃]^-₂ confirmed the four-legged piano-stool structure of 10²⁺; no interaction of the chromium centers 10²⁺ with the two triflates per formula unit could be detected. Cyclic voltammetry showed a reversible conversion between the cations 2^2+/1+, 7^2+/1+, and 10^2+/1+. By contrast, further oxidation to the trication and reduction to the neutral molecules proved to be irreversible. The dications were shown by NMR and EPR spectroscopies to contain one unpaired electron in the chromium d_z² orbital. With ¹H and ²H NMR spectroscopy the remote protons of substituted Cps and (after labeling) the deuterons of Cp and methylisocyanide ligands, respectively, were detected. The signs of the NMR signal shifts established negative spin density in the Cp π systems.