Structural and electrochemical studies on uranyl(VI) complex with pentadentate Schiff base ligand: A guide to stable uranyl(V)

Uranium(V) is normally instable in solution because of its disproportionation. On the other hand, U(V) has one unpaired electron in the 5f orbital, i.e., 5f1 configuration., and hence chemistry of U(V) is essential for the systematic understanding of actinide chemistry. Previously, we studied electrochemical behavior of U(VI) complexes in non-aqueous solvents, and obtained insight that multidentate ligands may stabilize U(V).1 From this point of view, we have found that two stable U(V) complexes [UVO2(salophen)DMSO]– (salophen = N,N’-disalicylidene-o-phenylenediaminate) and [UVO2(dbm)2DMSO]– (dbm = dibenzoylmethanate) are stable in DMSO.1-4 However, dissociation of a unidentate ligand (L) such as DMSO in [UVO2(salophen)L]– was also observed at lower L concentration. This prevents preparation and observation of a “pure” U(V) complex. On the basis of this knowledge, we obtained a hint that it is better to exclude L from a U(V) complex for its stability. Normally, U(VI) (UO22+) has 3–6 coordination sites in its equatorial plane, and probably most prefers 5 even in a bulky ligand.5 In this study, we selected N,N’-disalicylidenediethylenetriaminate (saldien2–)6 as a pentadentate ligand to satisfy this request. The U(VI) complex with saldien2– was characterized by single crystal X-ray analysis and X-ray absorption fine structure (XAFS) spectroscopy, and its electrochemical behavior in DMSO and N,N-dimethylformamide (DMF) was studied.
The obtained U(VI)-saldien2– complex recrystallized from DMSO was identified as orthorhombic UVIO2(saldien)•DMSO by single crystal X-ray analysis (Fig. 1). It should be noted that all coordination sites in the equatorial plane of UO22+ are occupied by saldien2–, and that any unidentate ligands are excluded as desired. A DMF solution of UVIO2(saldien) shows k3-weighted U LIII-edge EXAFS spectrum similar to that in solid state, indicating that the structure of UVIO2(saldien) remains even in the solution. This is supported by structural parameters from EXAFS curve fit.
Redox behavior of UVIO2(saldien) in DMSO and DMF was studied by using cyclic voltammetry. As a result, quasi-reversible redox waves were observed around E°’ = –1.582 ± 0.005 V vs. Fc/Fc+ (Ep = 0.080–0.170 V at v = 0.010–0.500 V•s–1) in DMSO and E°’ = –1.632 ± 0.003 V vs. Fc/Fc+ (Ep = 0.076–0.141 V at v = 0.010–0.500 V•s–1) in DMF. UV-Vis-NIR absorption spectral changes with the electrochemical reduction of UVIO2(saldien) were recorded using the spectroelectrochemical technique.1,4 The result of the DMSO solution is shown in Fig. 2. From the absorbance change, the electron stoichiometry in the reduction of UVIO2(saldien) was determined as 0.92 in DMSO and 0.82 in DMF using the Nernstian relationship. This quantity close to unity reveals that the following reaction occurs in both solutions.

UVIO2(saldien) + e– = [UVO2(saldien)]–

As we expected, it was found that UVIO2(saldien) without unidentate ligands results in the stable U(V) complex, [UVO2(saldien)]–, in DMSO and DMF. This U(V) species also shows the characteristic absorption bands of U(V) at 620, 700, 830, 1390, and 1890 nm as well as other U(V) species, [UVO2(salophen)DMSO]– and [UVO2(dbm)2DMSO]–.1,4