Probing the local coordination environment and nuclearity of uranyl(vi) complexes in non-aqueous media by emission spectroscopy

Abstract

We describe the synthesis, solid state and solution properties of two families of uranyl(VI) complexes that are ligated by neutral monodentate and anionic bidentate P=O, P=NH and As=O ligands bearing pendent phenyl chromophores. The uranyl(VI) ions in these complexes possess long-lived photoluminescent LMCT (3)Π(u) excited states, which can be exploited as a sensitive probe of electronic structure, bonding and aggregation behaviour in non-aqueous media. For a family of well defined complexes of given symmetry in trans-[UO(2)Cl(2)(L(2))] (L = Ph(3)PO (1), Ph(3)AsO (2) and Ph(3)PNH (3)), the emission spectral profiles in CH(2)Cl(2) are indicative of the strength of the donor atoms bound in the equatorial plane and the uranyl bond strength; the uranyl LMCT emission maxima are shifted to lower energy as the donor strength of L increases. The luminescence lifetimes in fluid solution mirror these observations (0.87-3.46 μs) and are particularly sensitive to vibrational and bimolecular deactivation. In a family of structurally well defined complexes of the related anion, tetraphenylimidodiphosphinate (TPIP), monometallic complexes, [UO(2)(TPIP)(thf)] (4), [UO(2)(TPIP)(Cy(3)PO)] 5), a bimetallic complex [UO(2)(TPIP)(2)](2) (6) and a previously known trimetallic complex, [UO(2)(TPIP)(2)](3) (7) can be isolated by variation of the synthetic procedure. Complex 7 differs from 6 as the central uranyl ion in 7 is orthogonally connected to the two peripheral ones via uranyl → uranium dative bonds. Each of these oligomers exhibits a characteristic optical fingerprint, where the emission maxima, the spectral shape and temporal decay profiles are unique for each structural form. Notably, excited state intermetallic quenching in the trimetallic complex 7 considerably reduces the luminescence lifetime with respect to the monometallic counterpart 5 (from 2.00 μs to 1.04 μs). This study demonstrates that time resolved and multi-parametric luminescence can be of value in ascertaining solution and structural forms of discrete uranyl(VI) complexes in non-aqueous solution.

Synthesis, crystal structure, and spectra of the Np(V) terephthalate [C(NH2)3]3[NpO2(C6H4(COO)2)2]·5H2O and An(VI) terephthalates [AnO2(C6H4(COO)2)] (An = U, Np, Pu)

“…However, tetragonal bipyramids constituted exclusively by O atoms are untypical of the coordination surrounding of the An(VI) atoms. Such polyhedron usually occurs in compounds with bulky organic ligands, e.g., with O=PPh 3 [17][18][19][20][21], O=P(NMe 2 ) 3 [22], and (O=PPh 2 ) 2 N - [23], or with ligands containing long alkyl chains [24][25][26]. …”

Modulation of the Structure and Properties of Uranyl Ion Coordination Polymers Derived from 1,3,5-Benzenetriacetate by Incorporation of Ag(I) or Pb(II)

“…Uranyl emission is commonly considered characterized by the vibronic progression corresponding to the S 11 → S 00 and S 10 → S 0ν (ν = 0−4) electronic transitions, 38 and it is common to observe only five peaks. However, the positions of the emission bands are sensitive to the nature and geometry of the uranyl coordination sphere, 4 and the overall intensity of the spectra is often affected by quenching effects, in particular, when d-block transition metal ions are present as well. 5b,6,12h,39 It is also well-established that the bandwidth of emission at room temperature can mask considerable complexity involving multiple, overlapping vibronic progressions resolvable only at low temperature, 40 such measurements on some simple halouranates providing a recent example.…”

Synthesis, structure and photophysical properties of [UO2X2(OPPh3)2] (X = Cl, Br, I)

“…24 1, 3 and 4 allow an opportunity to explore the influences of the halide on the photophysical properties of the uranyl ion, although it should be noted that 1 has previously been reported. 24 Also included in our study is the ] system, a red shift in the O yl →U LMCT band in line with the increased donor strength was observed and a shift from 517 nm to 529 nm to 531 nm for L = Ph 3 PvNH, Ph 3 PvO and Ph 3 AsvO respectively was reported in DCM. This observed red shift was postulated to signify a decrease in uranyl oxo bond order as a consequence of increased electron donation from the ancillary ligand in the equatorial plane to the uranium centre.…”