Counter-Anion-Regulated Mixed-Valency of Cobalt(II/III) Centers in a Metallosupramolecular Framework

A diamagnetic Au^I ₄ Co^III ₂ hexanuclear complex, [Au₄ Co₂ (dppe)₂ (l-nmc)₄ ]²⁺ ([1_{L -nmc} ]²⁺ ; dppe=1,2-bis(diphenylphosphino)ethane, l-H₂ nmc=N-methyl-l-cysteine), was newly synthesized by the reaction of [Co(l-nmc)₂ ]^- with [Au₂ Cl₂ (dppe)] and crystallized with different inorganic anions (X=ClO₄ ^- , NO₃ ^- , Cl^- , SO₄ ^2- ) to produce ionic solids ([1_{L -nmc} ]X_n ). Single-crystal X-ray analysis revealed that all the solids crystallize in the chiral space group F432 with a face-centered-cubic lattice structure consisting of supramolecular octahedra of complex cations. The paramagnetic nature of all the solids was evidenced by magnetic susceptibility measurements, showing the variation of the oxidation states of two cobalt centers in [1_{L -nmc} ]ⁿ⁺ from Co^II _1.00 Co^III _1.00 for X=ClO₄ ^- or NO₃ ^- to Co^II _0.67 Co^III _1.33 for X=Cl^- , via Co^II _0.83 Co^III _1.17 for X=SO₄ ^2- . The difference in the Co^II/III mixed-valences was explained by the difference in sizes and charges of counter anions accommodated in lattice interstices with a fixed volume.