Experimental and Computational Study of a Tetraazamacrocycle Bis(aryloxide) Uranyl Complex and of the Analogues...

The reaction of [U(κ⁶-{(ArO)₂Me₂-cyclam})I][I] (H₂{(ArO)₂Me₂-cyclam} = 1,8-bis(2-hydroxy-3,5-di-tert-butyl)-4,11-dimethyl-1,4,8,11-tetraazacyclotetradecane) with 2 equiv of NaNO₂ in acetonitrile results in the isolation of the uranyl complex [UO₂{(ArO)₂Me₂-cyclam}] (3) in 31% yield, which was fully characterized, including by single-crystal X-ray diffraction. Density functional theory (DFT) computations were performed to evaluate and compare the level of covalency within the U═E bonds in 3 and in the analogous trans-bis(imido) [U(κ⁴-{(ArO)₂Me₂-cyclam})(NPh)₂] (1) and trans-oxido-imido [U(κ⁴-{(ArO)₂Me₂-cyclam})(O)(NPh)] (2) complexes. Natural bond orbital (NBO) analysis allowed us to determine the mixing covalency parameter λ, showing that in 2, where both U-O_oxido and U-N_imido bonds are present, the U-N_imido bond registers more covalency with regard to 1, and the opposite is seen for U-O_oxido with respect to 3. However, the covalency driven by orbital overlap in the U-N_imido bond is slightly higher in 1 than in 2. The ¹⁵N-labeled complexes [U(κ⁴-{(ArO)₂Me₂-cyclam})(¹⁵NPh)₂] (1-N) and [U(κ⁴-{(ArO)₂Me₂-cyclam})(O)(¹⁵NPh)] (2-N) were prepared and analyzed by solution ¹⁵N NMR spectroscopy. The calculated and experimental ¹⁵N chemical shifts are in good agreement, displaying the same trend of δ_N (1-N) > δ_N (2-N) and reveal that the ¹⁵N chemical shift may serve as a probe for the covalency of the U═NR bond.