Oxygen and nitrogen Lewis base adducts of [UO2(OTf)2]. Crystal structures of polypyridine complexes with out-of-plane uranyl equatorial coordination

Abstract

Dissolution of [UO₂(OTf)₂] (1) in anhydrous thf, dme or py led to the formation of the complexes [UO₂(OTf)₂(thf)₃] (2), [UO₂(OTf)₂(dme)] (3) and [UO₂(OTf)₂(py)₃] (4), respectively. Compounds 2 and 4 are neutral monomers in the solid state as well as the chloride [UO₂Cl₂(py)₃] (5) which was prepared in a similar way as for 4 from the dimer [{UO₂Cl₂(thf)₂}₂]. Addition of 4 equivalents of triphenylphosphine oxide (tppo) to 1 afforded, in pyridine, the dicationic species [UO₂(tppo)₄][OTf]₂ (6). The bi- or terdentate nitrogen molecules 2,2′-bipy, phen or terpy reacted with 1 in acetonitrile or pyridine to give [UO₂(OTf)₂(bipy)₂] (7), [UO₂(phen)₃][OTf]₂ (8), [UO₂(OTf)₂(terpy)] (9) and [UO₂(terpy)₂][OTf]₂ (10), respectively. The hydroxide compound [{UO₂(OH)(terpy)}₂][OTf]₂ (11) was obtained by hydrolysis in air of 1 in a mixture of acetonitrile and ethanol in the presence of terpyridine. The X-ray crystal structures of 7, 8 and 10 reveal a novel coordination geometry for the uranyl ion, the uranium atom being in a rhombohedral environment; the six coordinating ligands atoms of the {UO₂}²⁺ ion are separated into two parallel and staggered equilateral triangles and the UO₂ axis is perpendicular to these triangles, passing through their centre. The structures of the mono(terpy) complexes 9 and 11 show the uranium atoms in a distorted pentagonal bipyramidal configuration with the nitrogen atom of the central pyridine ring of the terpy ligand significantly displaced from the equatorial plane.