Distorted equatorial coordination environments and weakening of U=O bonds in uranyl complexes containing NCN and NPN ligands

Treatment of [UO2Cl2(thf)(3)] in thf with 2 equiv of Na[PhC(NSiMe3)(2)] (Na[NCN]) or Na[Ph2P(NSiMe3)(2)] (Na[NPN]) gives uranyl complex [UO2(NCN)(2)(thf)] (1) or [UO2(NPN)(2)] (3), respectively. Each complex is a rare example of out-of-plane equatorial nitrogen ligand coordination; the latter contains a significantly bent O=U=O unit and represents the first example of a uranyl ion within a quadrilateral-faced monocapped trigonal prismatic geometry. Removal of the thf in 1 gives [UO2(NCN)(2)] (2) with in-plane N donor ligands. Addition of 3 equiv of Na[NCN] gives the tris complex [Na(thf)(2)PhCN][[UO2(NCN)(3)](4(.)PhCN) with elongation and weakening of one U=O bond through coordination to Na+. Hydrolysis of 4 provides the oxo-bridged dimer [Na(thf)UO2(NCN)(2)](2)(mu(2)-O) (6), a complex with the lowest reported O=U=O symmetrical stretching frequency (v(1) = 757 cm(-1)) for a dinuclear uranyl complex. The anion in complex 4 is unstable in solution but can be stabilized by the introduction of 18-crown-6 to give [Na(18-crown-6)][UO2(NCN)(3)} (5). The structures of 1-4 and 6 have been determined by crystallography, and all except 2 show significant deviations of the N ligand atoms from the equatorial plane, driven by the steric bulk of the NCN and NPN ligands. Despite the unusual geometries, these distortions in structure do not appear to have any direct effect on the bonding and electronic structure of the uranyl ion. The main influences toward lowering the U=O bond stretching frequency (v(1)) are the donating ability of the equatorial ligands, overall charge of the complex, and U=(ONa)-Na-...-type interactions. The intense orange/red colors of these compounds are because of low-energy ligand-to-metal charge-transfer electronic transitions.