Lanthanum Complexes with a Diimine Ligand in Three Different Redox States

The reduction of 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (dpp-Bian) with an excess of La metal in the presence of iodine (dpp-Bian/I₂ = 2/1) in tetrahydrofuran (thf) or dimethoxyethane (dme) affords lanthanum(III) complexes of dpp-Bian dianion: deep blue [(dpp-Bian)^2-LaI(thf)₂]₂ (1, 84%) was isolated by crystallization of the product from hexane, while deep green [(dpp-Bian)LaI(dme)₂] (2, 93%) precipitated from the reaction mixture in the course of its synthesis. A treatment of complex 1 with 0.5 equiv of I₂ in thf leads to the oxidation of the dpp-Bian dianion to the radical anion and results in the complex [(dpp-Bian)^1-LaI₂(thf)₃] (3). Addition of 18-crown-6 to the mixture of 1 and NaCp* (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl) in thf affords ionic complex [(dpp-Bian)^2-La(Cp*)I][Na(18-crown-6)(thf)₂] (4, 71%). In the absence of crown ether the alkali metal salt-free complex [(dpp-Bian)^2-LaCp*(thf)] (5, 67%) was isolated from toluene. Reduction of complex 1 with an excess of potassium produces lanthanum-potassium salt of the dpp-Bian tetra-anion {[(dpp-Bian)^4-La(thf)][K(thf)₃]}₂ (6, 68%). Diamagnetic compounds 1, 2, 4, 5, and 6 were characterized by NMR spectroscopy, while paramagnetic complex 3 was characterized by the electron spin resonance spectroscopy. Molecular structures of 2-6 were established by single-crystal X-ray analysis.