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

Abstract

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, 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(2)O(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(2)O(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(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.

Homologation and functionalization of carbon monoxide by a recyclable uranium complex

“…The high reduction potentials of low valent f-element complexes suggests a promising approach for the reductive activation of C 1 -small molecules such as and CO 2 (23); however, only a handful of f-element complexes can affect the reductive homologation of CO under ambient conditions (24, 25). Organometallic uranium complexes can effect reductive homologation of CO to C n -oligomers (n ¼ 2-4) (26-28), 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).…”

Computational study of An–X bonding (An = Th, U; X = p-block-based ligands) in pyrrolic macrocycle-supported complexes from the quantum theory of atoms in molecules and bond energy decomposition analysis

“…31 Finally for our pruposes, the delocalisation index between atoms A and B δ(A,B) gives the number of electrons shared between two atomic basins and is a measure of the bond order between two atoms. 32 The QTAIM has been previously applied to a range of actinide-ligand and other metal-ligand bonds 18,[33][34][35][36][37][38][39] and it has been concluded that while An bonding is predominantly ionic, covalency differences across the 5f series can be distinguished.…”

Molecular Pnictogen Activation by Rare Earth and Actinide Complexes

“…[(η ; however, recent studies by Arnold and co-workers have shown that N-N bond lengths determined using X-ray diffraction experiments can be underestimated and so may not provide the best way of assessing the level of dinitrogen reduction [87,88]. Both the Sm-N/C bond lengths and the Since this landmark discovery, the methodology of using reducing divalent rare earth metal complexes to activate N2 has resulted in analogous cyclopentadienyl complexes of Dy (5, 6) [74] and Tm (7,20) [75].…”

“…For reference, N-N bond lengths obtained from single crystal X-ray diffraction experiments, N-N stretching frequencies (obtained by IR or Raman spectroscopy) and 14/15 N-NMR spectroscopic data of actinide N2 complexes are summarised in Table 4. The first example of dinitrogen activation by an actinide complex was reported by Scott and co-workers; the trivalent uranium complex {N(CH2CH2NSiMe2 Arnold and co-workers reported that the trivalent uranium aryloxides U(OAr)3 (Ar = 2,6- [88]. Though 74 was obtained as a minor product, the more sterically hindered 75 was formed in quantitative yield and was stable under dynamic vacuum and in the presence of coordinating solvents and polar small molecules (CO and CO2) under ambient conditions.…”