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

The reaction of [U(κ-{(ArO)Me-cyclam})I][I] (H{(ArO)Me-cyclam} = 1,8-bis(2-hydroxy-3,5-di--butyl)-4,11-dimethyl-1,4,8,11-tetraazacyclotetradecane) with 2 equiv of NaNO in acetonitrile results in the isolation of the uranyl complex [UO{(ArO)Me-cyclam}] () 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 and in the analogous -bis(imido) [U(κ-{(ArO)Me-cyclam})(NPh)] () and -oxido-imido [U(κ-{(ArO)Me-cyclam})(O)(NPh)] () complexes. Natural bond orbital (NBO) analysis allowed us to determine the mixing covalency parameter λ, showing that in , where both U-O and U-N bonds are present, the U-N bond registers more covalency with regard to , and the opposite is seen for U-O with respect to . However, the covalency driven by orbital overlap in the U-N bond is slightly higher in than in . The N-labeled complexes [U(κ-{(ArO)Me-cyclam})(NPh)] () and [U(κ-{(ArO)Me-cyclam})(O)(NPh)] () were prepared and analyzed by solution N NMR spectroscopy. The calculated and experimental N chemical shifts are in good agreement, displaying the same trend of δ () > δ () and reveal that the N chemical shift may serve as a probe for the covalency of the U═NR bond.