Genuine Redox Isomerism in a Rare-Earth-Metai Complex

摘要

Redox isomerism, which is a reversible metal-to-ligand electron transfer, was first reported by Pierpont and Buchanan in 1980 for an o-benzoquinone complex of cobalt. A few years later, a similar effect was observed by Abakumov, Nevodchikov, and Cherkasov in a copper complex. To date, redox isomerism, which is also called valence tautomerism, is known for various d elements, including Co, Ru, Cr, Ni, Mn, Fe, Rh, Cu, and Ir, in solution and in the solid state. In 2007, the occurrence of redox isomerism was claimed for a complex of an f element, [(dpp-dad)Yb(C5Me5)(thf)] (dpp-dad = l,4-bis(2,6-diisopropylphenyl)diaza-l,3-buta-diene). Based on the solid-state magnetic susceptibility it was proposed that at 2 K the complex consists of Yb~(II) and a dpp-dad radical anion, while at 300 K it is present as Yb~(III) with a dpp-dad dianion. As far as we know, these redox preferences are opposite to almost all of the earlier reported cases, where the low-temperature form is represented by the species with a more oxidized metal M~(n+1) and the high-temperature isomer has a reduced metal ion M~n. More precisely, as redox isomerism is an entropy-driven process, the isomeric forms with the shorter M—L bonds, that is, with the more oxidized metal ion M, are usually stabilized at lower temperatures. Herein we report the synthesis and a combined X-ray crystallography plus SQUID study of the new complex [{(dpp-bian)Yb(μ-Cl)(dme)}2] (1; dpp-bian = l,2-bis[(2,6-di-isopropylphenyl)imino]acenaphthene), which turns out to be the first rare-earth-metal complex that exhibits genuine thermally induced redox isomerism in the solid state. Similar to its bromine analogue, [{(dpp-bian)Yb(μ,-Br)(dme)}2], complex 1 reveals intramolecular electron transfer between the metal and the dpp-bian ligand in solution as well; these data will be published elsewhere.