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)2PhCN][[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(μ2-O) (6), a complex with the lowest
reported OUO symmetrical stretching frequency (ν1 = 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 (ν1) are the donating ability of the equatorial ligands, overall charge of the complex, and
UO···Na-type interactions. The intense orange/red colors of these compounds are because of low-energy ligand-to-metal charge-transfer electronic transitions.