Multiple‐Bond Metathesis Mediated by Sterically Pressured Uranium Complexes

Abstract

In recent years, sterically encumbering ligands have been used increasingly to synthesize coordinatively unsaturated, highly reactive metal complexes.[1] The steric constraints imposed by spectator ligands and chelators often translate into molecular and electronic structural changes as they directly impact the coordination mode and metal-ligand orbital interactions; the latter being particularly important in atom-transfer chemistry. Relative to complexes with less customized ligands, unsaturated metal ions with molecularly engineered ligand environments often show altered and increased reactivity as a result of the increased steric pressure. Sterically tailored, highly reactive transition-metal complexes achieve a wide variety of small-molecule activation and atom-transfer chemistry very successfully.[2-6] The strategy of applying steric pressure, however, is not always sufficient to achieve certain desired reactivities. The most recent class of transition-metal-based nitrogen-transfer reagents (aziridination catalysts), for example, takes advantage of high-valent transition-metal nitrido complexes that readily transfer the imidoacyl entity to olefins upon activation with trifluoroacetic acid anhydride (TFAA). [7][8][9] The MN activation with TFAA is indispensable due to the highly covalent dp-pp interaction, thus resulting in very strong metal-nitrido triple bonds. [10][11][12] In uranium chemistry, however, the valence f orbitals do not participate in bonding to the same extent as metal d orbitals. As a result, the UN formal triple bond in the uranium imido and the elusive nitrido species is considerably more ionic in nature and can be described as U(d. Accordingly, we expected sterically pressured uranium imido and/or nitrido complexes with custom-designed chelators to exhibit increased nucleophilicity toward organic substrates. We therefore aimed to synthesize high-valent uranium imido and nitrido complexes of varying steric demand to explore possible applications for nitrogen-and group-transfer chemistry.We recently reported the reaction of the trivalent precursor complex [(( tBu ArO) 3 tacn)U] (1, ( tBu ArOH) 3 tacn = 1,4,7-tris(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane) [13]

Formation and Reactivity of the Terminal Vanadium Nitride Functionality

“…A few exemplary and unique reactions mediated by metal nitrides include direct insertion into C-H bonds, [12-14a] bioinspired hydrogenation to form the parent metal amide and ammonia, [6,15,16] complete and incomplete N-atom transfer to form C-N bonds from π-acids (CO, CNR), [16][17][18][19][20] and multi-electron reductive coupling reactions to render metal-dinitrogen complexes. [13,21] Moreover, a new and synthetically important application of molybdenum nitrides is the generation of molybdenum alkylidyne catalysts for alkyne metathesis geared towards the formation of carbon-carbon triple bonds for the synthesis of polymers and other organic commodity reagents.…”

Homologation and functionalization of carbon monoxide by a recyclable uranium complex

“…Organometallic uranium complexes can effect reductive homologation of CO to C n -oligomers (n ¼ 2-4) (26)(27)(28), and a small number of uranium triamide and triaryloxide complexes also reductively dimerize CO (29,30). More recently, an organometallic uranium complex was shown to effect the simultaneous reduction and hydrogenation of CO to give a coordinated methoxide that could be liberated as CH 3 OSiMe 3 (31); however, there have been no reports of successful homologation, functionalization, and removal of C n -oligomers (n ≥ 2) under mild conditions with retention of a metal complex that may be recycled for reuse in a synthetic cycle as has been demonstrated for CO 2 (32, 33), P 4 (34), and carbodiimides (35). Thus, the direct homologation of carbon monoxide into oxycarbons in viable, simple, and straightforward synthetic cycles is yet to be accomplished (36).…”

The Coupling of Pyridine and Dichloromethane Mediated by UO2Cl2

“…Halide abstraction by a number of uranium molecules were noted, e.g. by laser ablated uranium atoms [13] or metal powder [14] and sterically bulky U III compounds [15], whilst in the context of destruction of chlorine containing volatile organic compounds uranium oxide supported catalysts were reported [16].…”