Ytterbium and Europium Complexes of Redox-Active Ligands: Searching for Redox Isomerism

The reaction of (dpp-Bian)­Eu^II(dme)₂ (<b>3</b>) (dpp-Bian is dianion of 1,2-bis­[(2,6-diisopropylphenyl)­imino]­acenaphthene; dme is 1,2-dimethoxyethane) with 2,2′-bipyridine (bipy) in toluene proceeds with replacement of the coordinated solvent molecules with neutral bipy ligands and affords europium­(II) complex (dpp-Bian)­Eu^II(bipy)₂ (<b>9</b>). In contrast the reaction of related ytterbium complex (dpp-Bian)­Yb^II(dme)₂ (<b>4</b>) with bipy in dme proceeds with the electron transfer from the metal to bipy and results in (dpp-Bian)­Yb^III(bipy)­(bipy^–̇) (<b>10</b>) – ytterbium­(III) derivative containing both neutral and radical-anionic bipy ligands. Noteworthy, in both cases dianionic dpp-Bian ligands retain its reduction state. The ligand-centered redox-process occurs when complex <b>3</b> reacts with <i>N</i>,<i>N</i>′-bis­[2,4,6-trimethylphenyl]-1,4-diaza-1,3-butadiene (mes-dad). The reaction product (dpp-Bian)­Eu^II­(mes-dad)­(dme) (<b>11</b>) consists of two different redox-active ligands both in the radical-anionic state. The reduction of 3,6-di-<i>tert</i>-butyl-4-(3,6-di-<i>tert</i>-butyl-2-ethoxyphenoxy)-2-ethoxycyclohexa-2,5-dienone (the dimer of 2-ethoxy-3,6-di-<i>tert</i>-butylphenoxy radical) with (dpp-Bian)­Eu^II(dme)₂ (<b>3</b>) caused oxidation of the dpp-Bian ligand to radical-anion to afford (dpp-Bian)­(ArO)­Eu^II(dme) (ArO = OC₆H₂-3,6-<i>t</i>Bu₂-2-OEt) (<b>12</b>). The molecular structures of complexes <b>9</b>–<b>12</b> have been established by the single crystal X-ray analysis. The magnetic behavior of newly prepared compounds has been investigated by the SQUID technique in the range 2–310 K. The isotropic exchange model has been adopted to describe quantitatively the magnetic properties of the exchange-coupled europium­(II) complexes (<b>11</b> and <b>12</b>). The best-fit isotropic exchange parameters are in good agreement with their density functional theory-computed counterparts