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 [(RnCp)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 22+/1+, 72+/1+, and 102+/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 dz2 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.