JRP CALL information
Supported By

European Commission

Short description of the work

This project was a continuation of a previous JRP grant (TALI-C02-07) and the prior research work of Michal S. Dutkiewicz (SyntCryst4NS) on a set of reactions that represent the first use of a redox reaction between two actinide complexes to make heterobimetallic complexes. The complex [(UVIO2)(thf)(H2LEt)], in which the uranyl(VI) is supported by a calix[4]pyrrole Schiff base macrocycle, H4LEt reacts with the potential reductants AnIII An(Cp)3 (An = U, Np, Cp = C5H5-) was studied. The reaction between U(Cp)3 with the uranyl(VI) complex results in one-electron reduction to uranyl(V) and concomitant binding to one uranyl oxo group forming [(Cp)3UIVOUVO(thf)(H2LEt)]; the first selective functionalization of the uranyl oxo by another actinide ion. However, when the analogous reaction with Np(Cp)3 was carried out a novel trimetallic oxo‑neptunium(IV) compound, [(Cp)3Np(μ‑O)Np(Cp)2(μ-O)Np(Cp)3]. This unusual tri-Np complex, for which no other early An analogue exists is an interesting candidate for magnetic analyses as there should be strong coupling of the ions through the oxo bridges.
Further, we have investigated the effects of Pacman ligand modification on reactivity, solubility and stability, to help with optimization and ‘scale-up’ syntheses prior to SQUID and AC magnetometry studies of the resulting materials. To our surprise, simple alkyl group substitution in the peripheries of the ligand scaffold, not only yielded two isostructural AnIV-O-UV complexes but also changed their packing in the crystal lattice from previously observed P-1 to P21/n space group. This improved the isolation process and enhanced the purity and chemical yields for both compounds.
Thus samples of the new compounds [(Cp)3AnIVOUVO(thf)(H2LOct)], An = U, Np, complexes have now been isolated, purified, and characterized in the solid state by single crystal X-ray diffractometry, ATR‑IR and in a solution with 1H, 1H-1H NMR and UV-Vis-nIR spectroscopies, confirmative of their structural identity and the oxidation states as NpIV-UV and UIV-UV. Using these well characterized model compounds, we have explored the 5f-ion magnetic couplings mediated by a uranyl oxo-group in the first direct correlation of an actinide based Kramers–Kramers ion pair with a ‘non-Kramers’–Kramers ion couple.
Due to their air sensitivity some percentage of by-products were also observed, forming the known AnIV complexes (μ‑O)[An(Cp)3]2, and the UVI uranyl starting material, [(UVIO2)(thf)(H2LOct)] arising from oxidation with trace impurities. This suggests the unusual trimetallic oxo-neptunium(IV) complex is a product arising from the reaction of either reagent Np(Cp)3 or product [(Cp)3NpIVOUVO(thf)(H2LEt)] with trace O2 impurities. Some reactions to target partial oxygenation of solutions containing Np(Cp)3 confirmed this hypothesis, although the co-precipitation of (μ‑O)[Np(Cp)3]2 often occurrs, unfortunately precluding the isolation of a sample suitable for physicochemical measurements.
Magnetic characterization of both AnIV-O-UV complexes indicated that slow magnetic relaxation merely occurs for [(Cp)3UIVOUVO(thf)(H2LOct)]. The temperature and frequency dependence of its ac magnetic susceptibility are compatible with single-molecule-magnet behaviour. Due to the fact that UIV (5f2, 3H4) ion is often nonmagnetic at low temperatures, it is tempting to attribute this to the single-ion magnetism of the Kramers UV (5f1, 2F5/2) center. On the other hand, preliminary measurements performed on the NpIV (5f3, 4I9/2) analogue have not indicated any occurrence of the slow magnetic relaxation. If confirmed, this result would suggest the interaction between the two actinide sites is crucial to explain the magnetic properties. The detailed measurements are still ongoing.

Main visitor contact data
Name: Michal Seweryn Dutkiewicz
Organisation: University of Edinburgh

JRP Identification
JRP nr: TALI-C04-07
JRP title: Covalency and CCI studies through reduction and oxo-transfer phenomena in transuranic molecules
JRP scope: Scope 3: Actinide materials

Visited Associated Pooled Facility
Visited APF during the stay: JRC-ITU - Labs & Hot-cells
Name of the APF Contact Person: Prof. Roberto G. M. Caciuffo and Dr. Olaf Walter

Other APF and organisation involved in the JRP
Other organisations involved: University of Edinburgh (Prof. Dr. Polly L. Arnold)
Other APF involved in the project:

Description of the work done at the associated pooled facility
Start date of the stay: 4/21/2015
End date of the stay: 8/6/2015
Quantity of access: 60
Access Unit: Days