Small Molecule Activation by Uranium Tris(aryloxides): Experimental and Computational Studies of Binding of N2, Coupling of CO, and Deoxygenation Insertion of CO2under Ambient Conditions

Previously unanticipated dinitrogen activation is exhibited by the well-known uranium tris (aryloxide) U (ODtbp) (3), U (OC (6) H (3) -Bu (t) (2) -2, 6) (3), and the tri-tert-butyl analogue U (OTtbp) (3), U (OC (6) H (2) -Bu (t) (3) -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, KU (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 (2) O (2) ), in direct analogy with the reductive coupling recently shown to afford UN (SiMe (3) ) (2) (3) (2) (μ-η (1): η (1) -C (2) O (2) ). The related U (III) complexes UN (SiPhMe (2) ) (2) (3) and UCH (SiMe (3) ) (2) (3) however do not show CO coupling chemistry in our hands. Of the aryloxide complexes, only the U (OC (6) H (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 (2) COC (6) H (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 π 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 (OAr) (3) (2) (μ-C (2) O (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.