Gradual solid-state redox-isomerism in the lanthanide series

Oxidation of [(Ar^BIG-bian)^2-Yb²⁺(dme)] (1) (Ar^BIG-bian = 1,2-bis[(2,6-dibenzhydryl-4-methylphenyl)imino]acenaphthene; dme = 1,2-dimethoxyethane) by 0.5 equivalent of Me₂NC(S)S-S(S)CNMe₂ in dme at ambient temperature affords a mixture of two products, [(Ar^BIG-bian)^2-Yb³⁺{SC(S)NMe₂}^1-(dme)] and [(Ar^BIG-bian)^1-Yb²⁺{SC(S)NMe₂}^1-(dme)], which represent two redox-isomers (2a and 2b, respectively). Their ratio in solution depends on the solvent as well as on the temperature. In the solid state, a decrease of temperature (350 → 100 K) caused an electron transfer from the Yb²⁺ ion to the Ar^BIG-bian radical-anion in isomer 2b to afford isomer 2a. Accordingly, the ratio of isomers 2a and 2b changes from 1 : 1 (350 K) to 3 : 1 (100 K). In contrast, in the dimer [(dme)(dpp-bian)^1-Yb²⁺(μ-Cl)₂Yb³⁺(dpp-bian)^2-(dme)] (dpp-bian = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene), which is the sole example of a lanthanide complex that reveals solid-state redox-isomerism (valence tautomerism) reported so far, the electron transfer from the Yb²⁺ ion to the dpp-bian radical-anion takes place at around 150 K and is completed within a temperature interval of ca. 7 K.