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.

The new neopentyl (Neop)-substituted tris(aryloxide) U(III) complex [(((Neop,Me)ArO)(3)tacn)U(III)] reacts with CO(2) to form CO and the bridging carbonate complex [{(((Neop,Me)ArO)(3)tacn)U(IV)}(2)(μ-CO(3))]. The uranium(IV) bridging oxo [{(((Neop,Me)ArO)(3)tacn)U(IV)}(2)(μ-O)] has been determined to be the intermediate in this reaction. For the first time, both U(iv) complexes can be reduced back to the U(III) starting material. Thus, with KC(8) as reductant, [(((Neop,Me)ArO)(3)tacn)U(III)] engages in a synthetic cycle, in which CO(2) is converted to CO and CO(3)(2-).

Monomeric lithium bis(diphenylphosphino)amide, LiN(PPh2)2 (5), solvated with tetrahydrofuran, 5·3THF, was characterized by single-crystal X-ray analysis. The pentacoordinated lithium environment, a distorted trigonal bipyramid, includes two THF oxygens and the nitrogen of the bis(phosphino)amide in the equatorial positions; the third oxygen and a phosphorus are apical. 31P and 6Li NMR spectroscopy show that the monomeric structure of 5 in THF solution is similar to the X-ray structure of solid 5·3THF. Dynamic 31P NMR spectroscopy gave an 8.1 kcal/mol rotation barrier around the PN bonds. The 6Li−CP/MAS spectrum of 5·3THF has a single 6Li line, whereas the 31P CP/MAS spectrum reflects the chemical nonequivalence of the phosphorus sites observed by X-ray analysis. The appearance of two 31P signals in the solid-state NMR spectrum at +25 °C suggests a minimum activation barrier of the P,P-exchange process of ΔG ⧧ > 12.6 kcal/mol in the solid state. Ab initio calculations on the simplified, unsolvated models, PH2NH2, (PH2)2NH, PH2NH-, (PH2)2N-, PH2NHLi, and (PH2)2NLi, show that the strong stabilization of the amino anions by the α-phosphino substituents is due to negative hyperconjugation and phosphorus polarization. The presence of the metal counteracts much of the α-substituent stabilizing effect in the free anions. The metal cations in lithiated model compounds show little tendency to bridge.

The first sulfite [{(((nP,Me) ArO)3 tacn)U(IV) }2 (μ-κ(1) :κ(2) -SO3 )] (tacn=triazacyclononane) and dithionite [{(((nP,Me) ArO)3 tacn)U(IV) }2 (μ-κ(2) :κ(2) -S2 O4 )] complexes of uranium from reaction with gaseous SO2 have been prepared. Additionally, the reductive activation of CO2 was investigated with respect to the rare oxalate [{(((nP,Me) ArO)3 tacn)U(IV) }2 (μ-κ(2) :κ(2) -C2 O4 )] formation. This ultimately provides the unique S2 O4 (2-) /C2 O4 (2-) and SO3 (2-) /CO3 (2-) complex pairs. All new complexes were characterized by a combination of single-crystal X-ray diffraction, elemental analysis, UV/Vis/NIR electronic absorption, IR vibrational, and (1) H NMR spectroscopy, as well as magnetization (VT SQUID) studies. Moreover, density functional theory (DFT) calculations were carried out to gain further insight into the reaction mechanisms. All observations, together with DFT, support the assumption that SO2 and CO2 show similar (dithionite/oxalate) to analogous (sulfite/carbonate) activation behavior with uranium complexes.

A series of uranium tritylimido complexes with structural continuity across complexes in different oxidation states, namely U(IV), U(V), and U(VI), is reported. This series was successfully synthesized by employing the trivalent uranium precursor, [(((nP,Me)ArO)3tacn)U(III)] (1) (where ((nP,Me)ArO)3tacn(3-) = trianion of 1,4,7-tris(2-hydroxy-5-methyl-3-neopentylbenzyl)-1,4,7-triazacyclononane), with the organic azides Me3SiN3, Me3SnN3, and Ph3CN3 (tritylazide). While the reaction with Me3SiN3 yields an inseparable mixture of both the azido and imido uranium complexes, applying the heavier Sn homologue yields the bis-μ-azido complex [{(((nP,Me)ArO)3tacn)U(IV)}2(μ-N3)2] (2) exclusively. In contrast to this one-electron redox chemistry, the reaction of precursor 1 with tritylazide solely leads to the two-electron oxidized U(V) imido [(((nP,Me)ArO)3tacn)U(V)(N-CPh3)] (3). Oxidation and reduction of 3 yield the corresponding U(VI) and U(IV) complexes [(((nP,Me)ArO)3tacn)U(VI)(N-CPh3)][B(C6F5)4] (4) and K[(((nP,Me)ArO)3tacn)U(IV)(N-CPh3)] (5), respectively. In addition, the U(V) imido 3 engages in a H atom abstraction reaction with toluene to yield the closely related amido complex [(((nP,Me)ArO)3tacn)U(IV)(N(H)-CPh3)] (6). Complex 6 and the three tritylimido complexes 3, 4, and 5, with oxidation states ranging from +IV to +VI and homologous core structures, were investigated by X-ray diffraction analyses and magnetochemical and spectroscopic studies as well as density functional theory (DFT) computational analysis. The series of structurally very similar imido complexes provides a unique opportunity to study electronic properties and to probe the uranium imido reactivity solely as a function of electron count of the metal-imido entity. Evidence for the U-N bond covalency and f-orbital participation in complexes 3-6 was drawn from the in-depth and comparative DFT study. The reactivity of the imido and amido complexes with CO2 was probed, and conclusions about the influence of the formal oxidation state are reported.