Small Molecule Activation by Uranium Tris(aryloxides): Experimental and Computational Studies of Binding of N_2, Coupling of CO, and Deoxygenation Insertion of CO_2 under Ambient Conditions;

摘要

Previously unanticipated dinitrogen activation is exhibited by the well-known uranium tris(aryloxide) U(ODtbp)_3, U(OC_6H_3-Bu~t_2-2,6)_3, and the tri-tert-butyl analogue U(OTtbp)_3, U(OC_6H_2-Bu~t_2-2,4,6)_3, in the form of bridging, side-on dinitrogen complexes [U(OAr)_3]_2(μ-η~2:η~2-N_2), for which the tri-tert-butyl N_2 complex is the most robust U_2(N_2) complex isolated to date. Attempted reduction of the tris(aryloxide) complex under N_2 gave only the potassium salt of the uranium(III) tetra(aryloxide) anion,K[U(OAr)_4], as a result of ligand redistribution. The solid-state structure is a polymeric chain formed by each potassium cation bridging two arenes of adjacent anions in an η~6 fashion. The same uranium tris(aryloxides) were also found to couple carbon monoxide under ambient conditions to give exclusively the ynediolate [OCCO]~(2-) dianion in [U(OAr)_3]_2(μ-η~1:η~1-C_2O_2), in direct analogy with the reductive coupling recently shown to afford [U{N(SiMe_3)_2}_3]_2(μ-η~1:η~1-C_2O_2). The related U~III complexes U{N(SiPhMe_2)_2}_3 and U{CH(SiMe_3)_2}_3 however do not show CO coupling chemistry in our hands. Of the aryloxide complexes, only the U(OC_6H_2-Bu~t_3-2,4,6)_3 reacts with CO_2 to give an insertion product containing bridging oxo and aryl carbonate moieties, U_2(OTtbp)_4(μ-O)(μ-η~1:η~1-O_2COC_6H_2-Bu~t_3-2)4,6)_2, which has been structurally characterized. The presence of coordinated N_2 in [U(OTtbp)_3]_2(N_2) prevents the occurrence of any reaction with CO_2, underscoring the remarkable stability of the N_2 complex. The di-tert-butyl aryloxide does not insert CO_2, and only U(ODtbp)_4 was isolated. The silylamide also reacts with carbon dioxide to afford U(OSiMe_3)_4 as the only uranium-containing material. GGA and hybrid DFT calculations, in conjunction with topological analysis of the electron density, suggest that the U—N_2 bond is strongly polar, and that the only covalent U—N_2 interaction is n backbonding, leading to a formal (U~IV)_2(N_2)~(2-) description of the electronic structure. The N—N stretching wavenumber is preferred as a metric of N_2 reduction to the N—N bond length, as there is excellent agreement between theory and experiment for the former but poorer agreement for the latter due to X-ray crystallographic underestimation of r(N—N). Possible intermediates on the CO coupling pathway to [U(OAt)_3]_2(μ-C_2O_2) are identified, and potential energy surface scans indicate that the ynediolate fragment is more weakly bound than the ancillary ligands, which may have implications in the development of low-temperature and pressure catalytic CO chemistry.