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(κ 6 -{( t -Bu2 ArO) 2 Me 2 -cyclam})I][I] (H 2 {( t -Bu2 ArO) 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 NaNO 2 in acetonitrile results in the isolation of the uranyl complex [UO 2 {( t -Bu2 ArO) 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(κ 4 -{( t -Bu2 ArO) 2 Me 2 -cyclam})(NPh) 2 ] ( 1 ) and trans -oxido-imido [U(κ 4 -{( t -Bu2 ArO) 2 Me 2 -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 15 N-labeled complexes [U(κ 4 -{( t -Bu2 ArO) 2 Me 2 -cyclam})( 15 NPh) 2 ] ( 1- 15 N ) and [U(κ 4 -{( t -Bu2 ArO) 2 Me 2 -cyclam})(O)( 15 NPh)] ( 2- 15 N ) were prepared and analyzed by solution 15 N NMR spectroscopy. The calculated and experimental 15 N chemical shifts are in good agreement, displaying the same trend of δ N ( 1- 15 N ) > δ N ( 2- 15 N ) and reveal that the 15 N chemical shift may serve as a probe for the covalency of the U═NR bond.