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Highly Reactive Uranium(III) Polypyrrolide Complexes:  Intramolecular C−H Bond Activation, Ligand Isomerization, and Solvent Deoxygenation and Fragmentation

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posted on 2001-04-26, 00:00 authored by Ilia Korobkov, Sandro Gambarotta, Glenn P. A. Yap
The reaction of UX3(THF)4 (X = Cl, I) with the tetraanion of {[(−CH2−)5]4-calix[4]tetrapyrrole} gave different compounds depending on the uranium halide, the alkali-metal cation (Li vs K), the stoichiometric ratio of ligand to uranium, and the solvent used. Reaction of the potassium salt of the ligand with uranium iodide in THF and in the ratio 1:1 afforded the dinuclear, tetravalent species {[{[(−CH2−)5]4-calix[4]tetrapyrrole}UK(THF)3]22-O)}·2THF (1). The source of the bridging oxygen atom is a THF deoxygenation process. A reaction carried out under identical conditions but with the tetralithium salt of the calix[4]tetrapyrrole afforded instead intractable material unless a stoichiometric ratio of two ligands per uranium was employed. In this event, a new species, the dinuclear tetravalent species [{[(−CH2−)5]4-calix[4]pyrrole}ULi(THF)2]2·hexane (2), was isolated. In this complex, the β-C atom of one of the pyrrole rings of the macrocycle was deprotonated and metalated by uranium of a second identical unit, thus assembling the dinuclear structure. The reaction is not accompanied by loss of hydrogen gas, while the excess ligand is acting as a Brønsted base. An identical reaction carried out by using uranium trichloride afforded instead the mononuclear tetravalent species ({[(−CH2−)5]4-calix[4]tetrapyrrole}ULi(OC2H5)(THF)2) (3). In this compound one pyrrole ring was isomerized by shifting the attachment of the chain from the α- to the β-position. The LiOCH2CH3 unit was generated by another pathway of THF fragmentation. Finally, a reaction carried out in dimethoxyethane with the purpose of preventing oxidative attack to the metal center, afforded the mononuclear trivalent complex [{[(−CH2−)5]4-calix[4]tetrapyrrole}U(DME)][K(DME)] (4).

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