Structural Versatility of Uranyl(VI) Nitrate Complexes That Involve the Diamide Ligand Et2N(C=O)(CH2)n(C=O)NEt2 (0 ≤ n ≤ 6)

The interaction of uranyl nitrate with the series of diamides Et2N(C=O)(CH2)n(C=O)NEt2 (0 <= n <= 6) was investigated to evaluate systematically the effect of the (CH2)n spacer on the solid-state structures of the corresponding uranyl complexes. Under aerobic conditions, [UO2(NO3)(2).6H(2)O] reacted with an excess amount of these diamides (L) in organic solvents to yield [UO2(kappa(2)-NO3)(2)(L)] {1 [n = 0, tetraethyloxalamide (TEOA)], 2 [n = 1, tetraethylmalonamide (TEMA)], 3 [n = 2, tetraethylsuccinamide (TESA)], 5 [n = 3, tetraethylglycolamide (TEGA)], 6 [n = 4, tetraethyladipicamide (TEAA)], 7 [n = 5, tetraethylpimelicamide (TEPA)], and 8 [n = 6, tetraethylsubericamide (TESUA)]}, which were isolated and characterized by 1H NMR, ESI-MS, IR, and Raman spectroscopy. Under anhydrous and anaerobic conditions, [UO2(OTf)2] (OTf = trifluoromethanesulfonate) was treated with an excess amount of L to give [UO2(L)(2)][OTf](2), which was isolated for n = 1 (9) and n = 2 (10). The crystal structures of 2, 5, 6, 7, 8, and of the peroxido-bridged complex [{UO2(kappa(2)-NO3)(L)}2(mu,kappa(2),kappa(2)-O-2)] (4; n = 2) are presented. In all cases, the uranium ion is eight-coordinated with a classical hexagonal-bipyramidal configuration. The number of CH2 groups in the diamide central chain has considerable influence on the dimensionality of the complexes. They are monometallic (n = 1, 2), dimeric (n = 3, 4), or polymeric (n = 5, 6) with either a helical or zigzag structure, depending on the coordination mode of the bidentate diamide, which can be bridging or not, and the position cis or trans of the nitrate ions. DFT calculations in the gas phase show that the mono- and kappa(2)-bidentate coordination modes of the diamide (1 = n = 5) onto [UO2(kappa(2)-NO3)(2)] are energetically similar. By using the continuum solvation model, the binding affinity of the kappa(2)-bidentate diamide gradually decreases with the increase in n, but the most stable bidentate uranyl complex is obtained for n = 1 (TEMA). The structural differences in the series of [UO2(kappa(2)-NO3)(2){Et2N(C=O)(CH2)n(C=O)NEt2}] complexes are directly related to the length of the (CH2)n spacer.