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(kappa(6)-{((t-ArO)-Ar-Bu-Ar-2)(2)Me-2-cyclam})I][I] (H-2((t-ArO)-Ar-Bu-Ar-2)(2)Me-2-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-ArO)-Ar-Bu-Ar-2)(2)Me-2-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(kappa(4)-{((t-ArO)-Ar-Bu-Ar-2)(2)Me-2-cyclam})(NPh)(2)] (1) and trans-oxido-imido [U(kappa(4)-{((t-ArO)-Ar-Bu-Ar-2)(2)Me-2-cyclam})(O)(NPh)] (2) complexes. Natural bond orbital (NBO) analysis allowed us to determine the mixing covalency parameter lambda, 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 N-15-labeled complexes [(U(kappa(4)-{((t-ArO)-Ar-Bu-Ar-2)(2)Me-2-cyclam})((NPh)-N-15)(2)] (1-N-15) and [U(kappa(4)-{((t-ArO)-Ar-Bu-Ar-2)(2)Me-2-cyclam})(O)((NPh)-N-15)] (2-N-15) were prepared and analyzed by solution N-15 NMR spectroscopy. The calculated and experimental N-15 chemical shifts are in good agreement, displaying the same trend of delta(N )(1-N-15) > delta(N) (2-N-15) and reveal that the N-15 chemical shift may serve as a probe for the covalency of the U=NR bond.