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) UIII complex [((nP,MeArO)3tacn)­UIII]
(1) reacts with mesityl azide and CO2 to form
mesityl isocyanate and the dinuclear bis­(μ-oxo)-bridged UV/UV complex [{((nP,MeArO)3tacn)­UV}2(μ-O)2] (3). This reaction proceeds via the mononuclear UV imido
intermediate [((nP,MeArO)3tacn)­UV(NMes)] (2), which has been synthesized and fully characterized
independently. The dimeric UV oxo species shows rich redox
behavior: complex 3 can be reduced by one and two electrons,
respectively, yielding the mixed-valent UIV/UV bis­(μ-oxo) complex [K­(crypt)]­[{((nP,MeArO)3tacn)­UIV/V}2(μ-O)2]
(7) and the UIV/UIV bis­(μ-oxo)
complex K2[{((nP,MeArO)3tacn)­UIV}2(μ-O)2] (6). In
addition, complex 3 can be oxidized to provide the mononuclear
uranium­(VI) oxo complexes [((nP,MeArO)3tacn)­UVI(O)eq(OTf)ax] (8) and
[((nP,MeArO)3tacn)­UVI(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,MeArO)3tacn)­UV(O)­(O-pyridine)] (4) and [((nP,MeArO)3tacn)­UV(O)­(O-NMe3)] (5) were synthesized and fully characterized. The
magnetic study revealed an unusually strong antiferromagnetic exchange
coupling between the two UV ions in 3. Examination
of the 18O-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.