Synthesis and properties of an optically active helical bis-cobaltocenium ion

Article abstract of DOI:10.1021/ja00061a019

The optically active helical bis-cobaltocenium salt 6 is synthesized, as are two related monocobaltocenium salts, 29 and 30. The structure of 6 is analyzed by X-ray diffraction, which shows that the metals are separated by 8.49 ?. Reducing 6 either electrochemically or with K(Hg) produces species that absorb near 920 nm, but the absorption is not an intervalence transition. It originates instead from isolated Co(II) centers. This is demonstrated by the reduction product of 29, which has only one cobalt, also absorbing at a similar wavelength (λ_max, = 957 nm). The optical and ESR spectra imply that the unpaired electron in monoreduced 6 is largely localized on cobalt and that direduced 6 is essentially a Co(II)Co(II) diradical. The difference between two Co(III)/Co(II) reduction potentials of 6, 130 mV, is shown to be appropriate for a conjugated dimetallocene with metals so distant. Crystal data for 6: M = 1275.02; orthorhombic, space group P2₁2₁2₁; Z = 4; a = 11.560(4), b = 12.244(3), and c = 41.349(17) ?; V = 5852.5 ?³; R = 0.1137 for 4653 reflections having F_o ≥ nσ(F_o) (n = 7.5).