Uranium alkoxide chemistryV. Synthesis, characterization and interconversion of uranium(IV) tert-butoxide complexes

Abstract

The reaction of [(Me3Si)2N]2UN(SiMe3)SiMe2CH2 with ≥4.5 equivalents of t-butanol in toluene provides a green crystalline solid formulated as U2(OBut)8 (HOBut) on the basis of elemental analysis, 1H NMR, IR and UV-vis spectroscopy. U2(OBut)8(HOBut) reacts with KOBut (or KH) to give the potassium salt KU2(OBut)9, with dry dioxygen to give U2(OBut)9, and with H2O to give U3O(OBut)10. Temperature-dependent magnetic susceptibility measurements (2-250 K) on solid U3O(OBut)10 show Curie-Weiss behaviour from 90 to 250 K. Below 90 K the susceptibility becomes temperature independent with no evidence of magnetic ordering. The history of "U(OBut)4" dates back over 30 years. Early reports on the synthesis of this compound failed to provide conclusive proof for its existence.2-4 In our opinion, Cotton and co-workers have stablished beyond reasonable doubt that the compound originally prepared in Gilman's laboratory was actually the potassium salt, KU2(O-But)9. 5 We have shown here that the alcohol adduct, U2(O-But)8(HOBut, can be synthesized from well characterized starting materials, and have obtained spectroscopic data which support its formulation. We suggest that this is the same compound prepared by Perego and co-workers from U(C3H5)4 and excess t-butanol.4 We have also shown that Cotton's KU2(OBut)9 can be prepared from U2(OBut)8(HOBut), or directly from UCl4 and potassium t-butoxide. KU2(OBut)9 and U2(OBut)8 (HOBut) exhibit similar reactivity toward oxygen and water, and their UV-vis spectroscopic signatures indicate that they have similar structures. Finally, we have developed a reproducible route to the previously reported trinuclear uranium(IV) complex, U3O(OBut)10, via hydrolysis of either U2(OBut)8(HOBut) or KU2(OBut)9. U3O(OBut)10 is a promising starting material for further explorations of uranium(IV) cluster chemistry. Magnetic susceptibility data obtained on U3O(OBut)10 indicate that there is no magnetic ordering of its uranium(IV) centres down to 2 K. It seems clear to us that U(OBut) has not yet been reported in the literature. Since uranium(IV) compounds with sterically non-demanding ligands favour high coordination numbers (≥ 6), U(OBut)4, assuming it can be isolated, will surely adopt an oligomeric structure, [U(OBut)4]n. Efforts to prepare this compound and related anologues are continuing in our laboratory.17. © 1990.