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,MeArO)₃tacn)­U^III] (<b>1</b>) reacts with mesityl azide and CO₂ to form mesityl isocyanate and the dinuclear bis­(μ-oxo)-bridged U^V/U^V complex [{((^nP,MeArO)₃tacn)­U^V}₂(μ-O)₂] (<b>3</b>). This reaction proceeds via the mononuclear U^V imido intermediate [((^nP,MeArO)₃tacn)­U^V(NMes)] (<b>2</b>), which has been synthesized and fully characterized independently. The dimeric U^V oxo species shows rich redox behavior: complex <b>3</b> can be reduced by one and two electrons, respectively, yielding the mixed-valent U^IV/U^V bis­(μ-oxo) complex [K­(crypt)]­[{((^nP,MeArO)₃tacn)­U^IV/V}₂(μ-O)₂] (<b>7</b>) and the U^IV/U^IV bis­(μ-oxo) complex K₂[{((^nP,MeArO)₃tacn)­U^IV}₂(μ-O)₂] (<b>6</b>). In addition, complex <b>3</b> can be oxidized to provide the mononuclear uranium­(VI) oxo complexes [((^nP,MeArO)₃tacn)­U^VI(O)_eq(OTf)_ax] (<b>8</b>) and [((^nP,MeArO)₃tacn)­U^VI(O)_eq]­SbF₆ (<b>9</b>). 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,MeArO)₃tacn)­U^V(O)­(O-pyridine)] (<b>4</b>) and [((^nP,MeArO)₃tacn)­U^V(O)­(O-NMe₃)] (<b>5</b>) were synthesized and fully characterized. The magnetic study revealed an unusually strong antiferromagnetic exchange coupling between the two U^V ions in <b>3</b>. Examination of the ¹⁸O-labeled bis­(μ-oxo)-bridged dinuclear complexes <b>3</b>, <b>6</b>, and <b>7</b> allowed for the first time the unambiguous assignment of the vibrational signature of the [U­(μ-O)₂U] diamond core structural motif