Carbon Dioxide Activation with Sterically Pressured Mid- and High-Valent Uranium Complexes

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.