Molecular and electronic structure of dinuclear uranium bis-μ-oxo complexes with diamond core structural motifs.

In a multiple-bond metathesis reaction, the triazacyclononane (tacn)-anchored methyl- and neopentyl (nP)-substituted tris(aryloxide) U(III) complex [(((nP,Me)ArO)3tacn)U(III)] (1) reacts with mesityl azide and CO2 to form mesityl isocyanate and the dinuclear bis(μ-oxo)-bridged U(V)/U(V) complex [{(((nP,Me)ArO)3tacn)U(V)}2(μ-O)2] (3). This reaction proceeds via the mononuclear U(V) imido intermediate [(((nP,Me)ArO)3tacn)U(V)(NMes)] (2), which has been synthesized and fully characterized independently. The dimeric U(V) oxo species shows rich redox behavior: complex 3 can be reduced by one and two electrons, respectively, yielding the mixed-valent U(IV)/U(V) bis(μ-oxo) complex [K(crypt)][{(((nP,Me)ArO)3tacn)U(IV/V)}2(μ-O)2] (7) and the U(IV)/U(IV) bis(μ-oxo) complex K2[{(((nP,Me)ArO)3tacn)U(IV)}2(μ-O)2] (6). In addition, complex 3 can be oxidized to provide the mononuclear uranium(VI) oxo complexes [(((nP,Me)ArO)3tacn)U(VI)(O)eq(OTf)ax] (8) and [(((nP,Me)ArO)3tacn)U(VI)(O)eq]SbF6 (9). The unique series of bis(μ-oxo) complexes also shows notable magnetic behavior, which was investigated in detail by UV/vis/NIR and EPR spectroscopy as well as SQUID magnetization studies. In order to understand possible magnetic exchange phenomena, the mononuclear terminal oxo complexes [(((nP,Me)ArO)3tacn)U(V)(O)(O-pyridine)] (4) and [(((nP,Me)ArO)3tacn)U(V)(O)(O-NMe3)] (5) were synthesized and fully characterized. The magnetic study revealed an unusually strong antiferromagnetic exchange coupling between the two U(V) ions in 3. Examination of the (18)O-labeled bis(μ-oxo)-bridged dinuclear complexes 3, 6, and 7 allowed for the first time the unambiguous assignment of the vibrational signature of the [U(μ-O)2U] diamond core structural motif.