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

• Published in: Organometallics 2002-08, Vol.21 (17), p.3572-3579
• Main Authors: Heigl, Oliver M, Herdtweck, Eberhardt, Grasser, Stefan, Köhler, Frank H, Strauss, Werner, Zeh, Harald
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om020229i

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+ (7 2+) as a triflate, while [(R n Cp)CrCl2]2 (R = Me, H (3 and 5, respectively), n = 5; R = t-Bu, n = 1 (8)) reacted with Tl(CF3SO3) to give 7 2+, [(C5Me5)Cr(CNCH3)4]2+ (2 2+), and [(t-BuCp)Cr(CNCH3)4]2+ (10 2+) again as triflates in up to 59% yield. The precursor 8 was synthesized from (t-BuCp)Na and CrCl3(THF)3. The dication 2 2+ 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 7 2+ could be reduced to 7 + obtained as [PF6]- and [B(C6H5)4]- salts. The crystal structure of 10 2+[CF3SO3]- 2 confirmed the four-legged piano-stool structure of 10 2+; no interaction of the chromium centers 10 2+ 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 z 2 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.