Experimental and Computational Study of a Tetraazamacrocycle Bis(aryloxide) Uranyl Complex and of the Analogues {E=U=NR}²⁺(E = O and NR)

The reaction of [U(κ6-{(t-Bu2ArO)2Me2-cyclam})I][I] (H2{(t-Bu2ArO)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{(t-Bu2ArO)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-{(t-Bu2ArO)2Me2-cyclam})(NPh)2] (1) and trans-oxido-imido [U(κ4-{(t-Bu2ArO)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-Ooxido and U-Nimido bonds are present, the U-Nimido bond registers more covalency with regard to 1, and the opposite is seen for U-Ooxido with respect to 3. However, the covalency driven by orbital overlap in the U-Nimido bond is slightly higher in 1 than in 2. The 15N-labeled complexes [U(κ4-{(t-Bu2ArO)2Me2-cyclam})(15NPh)2] (1-15N) and [U(κ4-{(t-Bu2ArO)2Me2-cyclam})(O)(15NPh)] (2-15N) 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 δN (1-15N) > δN (2-15N) and reveal that the 15N chemical shift may serve as a probe for the covalency of the U=NR bond.