[2π+2π] Cycloaddition of Isocyanates to Uranium(IV) Imido Complexes for the Synthesis of U(IV) κ2‑Ureato Compounds

A new family of uranium­(IV) imido complexes of the form Tp*2U­(NR) (Tp* = hydrotris­(3,5-dimethylpyrazolyl)­borate; R = benzyl (Bn), para-tolyl (p-Tol), 2,6-diethylphenyl (detp), and 2,6-diisopropylphenyl (dipp)) have been generated by bibenzyl extrusion from Tp*2UBn. Tp*2U­(NBn), Tp*2U­(Np-Tol), and Tp*2U­(Ndetp), along with previously reported Tp*2U­(NPh) and Tp*2U­(NAd) (Ad = 1-adamantyl), readily undergo [2π+2π] cycloaddition with isocyanates and isothiocyanates to generate κ2-ureato and κ2-thioureato derivatives, respectively. These new uranium­(IV) complexes were characterized via multinuclear NMR, vibrational and electronic absorption spectroscopies, and, where possible, X-ray crystallography. The steric demands of the ligands were quantitatively assessed using computational modeling, and it was shown that cycloaddition only occurs for imido species where ligands occupy 90% or less of the coordination sphere.