Seven Reversible Redox Processes in a Self-Assembled Cobalt Pentanuclear Bis(triple-stranded helicate): Structural, Spectroscopic, and Magnetic Characterizations in the CoICoII4, CoII5, and CoII3CoIII2 Redox States

Abstract

We report on the synthesis and structural characterization of the cobalt pentanuclear helicate complex from the rigid tetradentate bis(2-pyridyl)-3,5-pyrazolate ligand bpp^–, namely, [{Co^II(μ-bpp)₃}₂Co^II₃(μ₃-OH)]³⁺ (1³⁺), in which a trinuclear {Co^II₃(μ₃-OH)} core is wrapped by two {Co^II(μ-bpp)₃} units. The cyclic voltammogram of 1³⁺ in CH₃CN revealed seven successive reversible one-electron waves, in the 0 and −3.0 V potential range, highlighting the remarkable stability of such architecture in several redox states. Two mixed-valent states of this complex, the two-electron-oxidized Co^II₃Co^III₂ (1⁵⁺) and the one-electron-reduced species Co^ICo^II₄ (1²⁺), were generated by bulk electrolyses and successfully characterized by single-crystal X-ray diffraction among the eight redox levels between Co^I₅ and Co^II₃Co^III₂ that can be accessed under electrochemical conditions. Because of the crystallographic characterization of 1⁵⁺ and 1²⁺, the five reduction processes located at E_1/2 values of −1.63 (1^3+/2+), −1.88 (1^2+/+), −2.14 (1^+/0), −2.40 (1^0/–), and −2.60 V (1^–/2–) versus Ag/AgNO₃ were unambiguously assigned to the successive reduction of each of the five Co(II) ions to Co(I), starting with the three ions located in the central core followed by the two apical ions. The two other redox events at E_1/2 values of −0.21 (1^4+/3+) and −0.11 V (1^5+/4+) are assigned to the successive oxidation of the apical Co(II) ions to Co(III). The Co(I) complexes are rare, and the stabilization of a Co(I) within a trinuclear μ-hydroxo core in the reduced species, 1²⁺, 1⁺, 1⁰, 1^–, and 1^2–, is probably the result of the particular structure of this complex in the presence of the two apical sites that maintain the trinuclear core through the six bridging bpp^– ligands. The spectroscopic characteristics of 1²⁺, 1³⁺, and 1⁵⁺ (ultraviolet–visible and X-band electron paramagnetic resonance) are also described as well as their magnetic properties in the solid state.