Distorted Equatorial Coordination Environments and Weakening of UO Bonds in Uranyl Complexes Containing NCN and NPN Ligands

Abstract

Treatment of [UO(2)Cl(2)(thf)(3)] in thf with 2 equiv of Na[PhC(NSiMe(3))(2)] (Na[NCN]) or Na[Ph(2)P(NSiMe(3))(2)] (Na[NPN]) gives uranyl complex [UO(2)(NCN)(2)(thf)] (1) or [UO(2)(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 [UO(2)(NCN)(2)] (2) with in-plane N donor ligands. Addition of 3 equiv of Na[NCN] gives the tris complex [Na(thf)(2)PhCN][[UO(2)(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)UO(2)(NCN)(2)](2)(micro(2)-O) (6), a complex with the lowest reported O=U=O symmetrical stretching frequency (nu(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)][UO(2)(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 (nu(1)) 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.

Equatorial coordination of uranyl: Correlating ligand charge donation with the Oyl-U-Oyl asymmetric stretch frequency

“…Because protons generally bind to one nucleophilic site in a molecule, 8 this unexpected bidentate coordination of tmtu to uranium is exemplary as to why PAs should not necessarily correlate well with MCAs when a second coordinating electron donor such as N is present in the ligand. The computed structure of UO 2 (tmtu) 4 …”

“…[1][2][3][4]7 Whereas the library of MCAs is sparse, PAs have been that one of the three ligands is bidentate such that even greater electron donation, and a greater  3 red-shift than predicted from PA[tmtu]-which corresponds to monodentate coordination of a proton to the carbonyl sulfur 32 -would be expected, the opposite of the observed deviation from the linear fit in Figure 5. These examples reveal that caution must be exercised in applying an apparently good correlation that appears as valid over a broad range of parameters to particular complexes that have relatively similar values of these parameters.…”

“…[1][2][3][4] The origins of this phenomenon are generally attributed to partial reduction of U(VI) with concomitant occupation of 5f orbitals, although other explanations have been proposed. 5 However, understanding the underlying basis for the effects of coordinating ligands on uranium-oxygen bonding in uranyl is complicated by the many competing and uncertain interactions in condensed phases.…”

Designing the Ideal Uranyl Ligand: a Sterically Induced Speciation Change in Complexes with Thiophene-Bridged Bis(3-hydroxy-N-methylpyridin-2-one)

“…Exceptions to this behavior typically involve bulky ligands (e.g. Cp 5 or large NCN or NPN ligands 6 ) in which the uranyl cation may deviate more than 11° from linearity and coordinating atoms distort out of the equatorial coordination plane. The apical oxo moieties are essentially non-reactive and are typically only observed to interact with Lewis acids in the solid state and in appropriately designed macrocyclic systems.…”

The Influence of Linker Geometry in Bis(3‐hydroxy‐N‐methyl‐pyridin‐2‐one) Ligands on Solution Phase Uranyl Affinity

“…[11] The equatorial coordination plane displays little to no orbital-dictated directionality, with observed coordination geometries ranging from trigonal-through hexagonal-planar, depending on ligand sterics and chelating ability. [12][13][14][15] Additionally, the terminal oxo atoms of the actinyl cations (especially in their +6 oxidation state) display poor Lewis basicity, although they have been observed to interact with metal cations in the solid state. [12,14,[16][17][18][19][20] The uranyl cation (UO 2 2+ ) is the most common actinyl species studied because uranium is the second most abundant naturally-occuring actinide as well as the one most frequently employed in nuclear power generation.…”