Uranyl to Uranium(IV) Conversion through Manipulation of Axial and Equatorial Ligands

Abstract

The controlled manipulation of the axial oxo and equatorial halide ligands in the uranyl dipyrrin complex, UOCl(L), allows the uranyl reduction potential to be shifted by 1.53 V into the range accessible to naturally occurring reductants that are present during uranium remediation and storage processes. Abstraction of the equatorial halide ligand to form the uranyl cation causes a 780 mV positive shift in the U/U reduction potential. Borane functionalization of the axial oxo groups causes the spontaneous homolysis of the equatorial U-Cl bond and a further 750 mV shift of this potential. The combined effect of chloride loss and borane coordination to the oxo groups allows reduction of U to U by H or other very mild reductants such as Cp*Fe. The reduction with H is accompanied by a B-C bond cleavage process in the oxo-coordinated borane.

Interaction of Metal Oxido Compounds with B(C6F5)3

“…This is in agreement with similarobservations made in uranium dioxido chemistry where in U VI compounds (electron poor) one oxido moiety was coordinated by ab orane, whereas in U V and U IV species both oxido groups were ablet oc oordinate one molecule of B(C 6 F 5 ) 3 (see section 5,Figure7). [38][39][40] Ac ommonf eature of the Lewis adducts is the expected elongation of the metal oxido bond length by approximately 0.1 compared with the parentc ompound. This elongation is often accompanied by as hortening of the bonds to the other ligandst oc ompensate the electron-withdrawing effect of the M-O-B unit.…”

“…[69] Arnold and co-workersr eported the well-behaved uranyl(VI) complex [U VI O 2 Cl(dp)] coordinated by the monoanionic tetradentate dipyrrin ligand (dp), which upon reduction to the dinuclear species [{U V O 2 (dp)} 2 ], is capable of coordinating one or two molecules of B(C 6 F 5 ) 3 per uranium center, respectively,b y cleavage of the intermediately formed dimer (Figure 7). [40] The influence of B(C 6 F 5 ) 3 coordination at the uranyl oxido groups was also investigated by means of electrochemistry,r evealing an increase of the U V /U IV reduction potential upon addition of borane.…”

“…Molecular structures of uranyl(V) complexes with B(C 6 F 5 ) 3 molecules coordinated to one (45,left) or two (46,right) oxido moieties. [40] Scheme12. Lewis acid addition induced valence state change in amanganese oxido corrolazine compound (left, Goldberg) [74] and amanganese oxido corrole compound( right, Abu-Omar).…”

Oligonuclear Actinoid Complexes with Schiff Bases as Ligands—Older Achievements and Recent Progress

“…A quasi-reversible wave at −0.28 V (F C /F C + ) has been assigned as the U V /U VI oxidation. toluene (the yield was ≈65%); (L 30 ) − is the monoanion of the dipyrrin ligand HL 30 [111]. The near-IR electronic spectrum of 52 exhibits a band at ≈6800 cm −1 consistent with an 5f-5f transition and U V .…”

“…Complex 52 exhibits rich reactivity; for example, its reaction with 4 equiv. of B(C6F)3 yields the borane oxidofunctionalized compound [U V {OB(C6F5)3}2(L 30 )] [111]. Cyclic voltammetry studies of this complex in o-difluorobenzene suggests that no communication occurs between the two metal ions in the dimer and no ligand-based reduction is observed.…”