Despite being implicated as important intermediates, iron(V) compounds have proven very challenging to isolate and characterize. Here, we report the preparation of the iron(V) nitrido complex, [PhB((t)BuIm)(3)Fe(V)≡N]BAr(F24) (PhB((t)BuIm)(3)(-) = phenyltris(3-tert-butylimidazol-2-ylidene)borato, BAr(F24) = B(3,5-(CF(3))(2)C(6)H(3))(4)(-)), by one electron oxidation of the iron(IV) nitrido precursor. Single-crystal x-ray diffraction of the iron(V) complex reveals a four-coordinate metal ion with a terminal nitrido ligand. Mößbauer and electron paramagnetic resonance spectroscopic characterization, supported by electronic structure calculations, provide evidence for a d(3) iron(V) metal center in a low spin (S = 1/2) electron configuration. Low-temperature reaction of the iron(V) nitrido complex with water under reducing conditions leads to high yields of ammonia with concomitant formation of an iron(II) species.

The low-temperature (<-35 °C) reduction of the trivalent uranium monoarene complex [{((Ad,Me) ArO)3 mes}U] (1), with potassium spheres in the presence of a slight excess of 2.2.2-cryptand, affords the quantitative conversion of 1 into the uranium(II) monoarene complex [K(2.2.2-crypt)][(((Ad,Me) ArO)3 mes)U] (1-K). The molecular and electronic structure of 1-K was established experimentally by single-crystal X-ray diffraction, variable-temperature (1) H NMR and X-band EPR spectroscopy, solution-state and solid-state magnetism studies, and optical absorption spectroscopy. The electronic structure of the complex was further investigated by DFT calculations. The complete body of evidence confirms that 1-K is a uranium(II) monoarene complex with a 5f (4) electronic configuration supported by δ backbonding and that the nearly reversible, room-temperature reduction observed for 1 at -2.495 V vs. Fc/Fc(+) is principally metal-centered.

Sterically pressured mid- to high-valent uranium complexes with an aryloxide substituted triazacyclononane ligand scaffold, [(((R)ArO)3tacn)(3-)], were studied for carbon dioxide activation and transformation chemistry. The high valent uranium(V) imido species [(((R)ArO)3tacn)U(NR)] (R = (t)Bu, R' = 2,4,6-trimethylphenyl (2-(t)Bu); R = Ad, R' = 2,4,6-trimethylphenyl (2-Ad); R = (t)Bu, R' = phenyl (3-(t)Bu)) were synthesized and spectroscopically characterized. X-ray crystallography of the tert-butyl mesityl imido derivative, 2-(t)Bu , reveals coordination of a bent imido fragment with a relatively long U-N bond distance of 2.05 A. The mesityl imido complexes reacted with carbon dioxide, readily extruding free isocyanate to produce uranium(V) terminal oxo species, [(((R)ArO)3tacn)U(O)] (R = (t)Bu (4-(t)Bu), Ad (4-Ad)), presumably through multiple bond metathesis via a uranium(V) carbimate intermediate. Using the smaller phenyl imido fragment in 3-(t) Bu slowed isocyanate loss, allowing the uranium(V) carbimate intermediate to undergo a second metathesis reaction, ultimately producing the diphenyl ureate derivative, [(((tBu)ArO)3tacn)U(NPh2)CO] (5-(t)Bu). Single crystal X-ray diffraction studies were carried out on both uranium(V) terminal oxo complexes and revealed short U-O bonds (1.85 A) indicative of a formal UO triple bond. The electronic structure of the oxo U(V) complexes was investigated by electronic absorption and EPR spectroscopies as well as SQUID magnetization and DFT studies, which indicated that their electronic properties are highly unusual. To obtain insight into the reactivity of CO2 with U-N bonds, the reaction of the uranium(IV) amide species, [(((R)ArO)3tacn)U(NHMes)] (R = (t)Bu (6-(t)Bu), Ad (6-Ad) with carbon dioxide was investigated. These reactions produced the uranium(IV) carbamate complexes, [(((R)ArO)3tacn)U(CO2NHMes)] (R = (t)Bu (7-(t)Bu), Ad (7-Ad)), resulting from insertion of carbon dioxide into U-N(amide) bonds. The molecular structures of the synthesized uranium carbamate complexes highlight the different reactivities due to the steric pressure introduced by the alkyl derivatized tris(aryloxide) triazacyclononane ligand. The sterically open tert-butyl derivative creates a monodentate eta(1)-O bound carbamate species, while the sterically more bulky adamantyl-substituted compound forces a bidentate kappa(2)-O,O coordination mode of the carbamate ligand.

High on nitride: Discrete iron nitride complexes stabilized by N‐anchored tris(carbene) ligands have been synthesized (see picture). These high‐valent FeIVN complexes are stable at room temperature, which allows their full spectroscopic and—for the first time—crystallographic characterization.

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