Axially Symmetric U−O−Ln‐ and U−O−U‐Containing Molecules from the Control of Uranyl Reduction with Simple f‐Block Halides

Abstract The reduction of UVI uranyl halides or amides with simple LnII or UIII salts forms highly symmetric, linear, oxo‐bridged trinuclear UV/LnIII/UV, LnIII/UIV/LnIII, and UIV/UIV/UIV complexes or linear LnIII/UV polymers depending on the stoichiometry and solvent. The reactions can be tuned to give the products of one‐ or two‐electron uranyl reduction. The reactivity and magnetism of these compounds are discussed in the context of using a series of strongly oxo‐coupled homo‐ and heterometallic poly(f‐block) chains to better understand fundamental electronic structure in the f‐block.

The d 0 f 0 uranyl ion, [UO 2 ] 2+ ,i st he most common form exhibited by uranium in molecular complexes;i ti sawatersoluble ion with linear, strongly bonded, and unreactive oxo groups. [1] Thereduction to [U V O 2 ] + and then insoluble U IV by metals,m inerals,o rm icrobes is important for keeping it immobilized in the environment and out of groundwater,but the mechanisms for this are still not clear. [2] TheU V uranyl ion can also provide better models for the highly radioactive and more oxo-basic f 1 and f 2 neptunyl and plutonyl ions found in nuclear waste. [3] Tw oseminal, coincident reports of the potassium-reduced coordination polymer {[UO 2 (py) 5 ][KI 2 (py) 2 ]} 1 (A)s howed that anaerobic uranyl(V) complexes are stable against disproportionation; [4] since then, significant advances have been made in isolating uranyl(V) complexes.T he reductive oxo functionalization with Group 1, [5] Group 2,[1c] d-block, [1c, 6] rare-earth, [6b, 7] actinide, [3a] and main-group metals [5c, 8,9] as well as silicon [1d, 5b,10] is also possible.T he linearity of the f 1 uranyl also provides ar are opportunity in f-block chemistry to control the orientation of the magnetic vector, and uranyl(V)/ transition-metal single-molecule magnets have been made through this oxo-bridging strategy. [3a, 6a, 7, 11] Lanthanide/actinide complexes are promising candidates for single-molecule magnetism, [11a] and it has been proposed that an even higher degree of axial symmetry should produce exceptional molecular magnets. [12] However,a ccess to this new chemistry has almost exclusively been achieved through deployment of complicated polydentate ligands to saturate the equatorial coordination sphere of the uranyl ion. [10c] Theonly examples of oxo functionalization of simple systems involve the treatment of uranyl dichloride with aGroup 1reagent, namely areduction with either K(Hg) or K[C 5 R 5 ](R= H, Me) to make A [4b] and ar eduction with NaCH 2 SiMe 3 (followed by quenching with Me 3 SiCl) to make [U IV (OSiMe 3 ) 2 I 2 (OPPh 3 ) 2 ](B). [13] Herein, we report how simple low-oxidation-state lanthanide and actinide salts can be used to oxo-coordinate and reduce simple uranyl(VI) salts to make new classes of stable, highly symmetric,4 fa nd 5f ion oxo-bridged uranyl(V) and uranium(IV) complexes.
Once formed, polymeric 2-Sm is no longer soluble,b ut FTIR spectra of the solid show that both oxo groups of the uranyl are now Ln-coordinated as the uranyl stretch is significantly weakened to 722 cm À1 .Unfortunately,the stron-ger reductant DyI 2 reacts with MeCN, [15] so we were unable to target 2-Dy.
Preliminary studies showed that the equatorial ligands can be exchanged.   [1c,3a, 15, 9] Thea verage U À Ob ond length of 1.859 in 1-Sm is longer than that of 1.838 in the coordination polymer {[UO 2 (py) 5 ][KI 2 (py) 2 ]} 1 (A). [4] Them ean U V =Oa nd SmÀOb ond lengths in 2-Sm (Figure 1b TheX -ray structures of 3-Dy and 4 confirmed that the double reduction and oxo coordination remove any "uranyl ion" character,b ut form strong M À O À M' interactions.W e propose that the preferential coordination of iodide instead of pyridine to the central U IV ion is af urther indication of the loss of uranyl character as the use of the majority of valence orbitals in forming U=Ob onds is ac haracteristic actinyl ion bonding feature.The binding of Cl trans to the oxo unit in 4 is ascribed to its stronger inverse trans influence (ITI) compared to that of iodide. [16] Thestructures of 3-Dy·4 py and 4·4 py have pseudo-D 2h or  (5) ), whereas the UÀIb onds in 3-Dy are significantly longer than those in 4 (3.1425(4)-3.1618(4) vs.3.0436(8)-3.0572(8) ). Furthermore,t he DyÀOb onds in 3-Dy (2.126(3) and 2.119(3) )a re significantly shorter than those in 1-Dy (2.270(5) ), which is consistent with reduction of U V to U IV and am ore symmetric oxo bridging in 3-Dy.I n4,t he outer U1ÀO1 bond is much shorter than the central U2ÀO1 bond (2.042(5) vs.2 .166(5) ), but both are shorter than the average UÀObond in the CSD (2.361 ).
Anticipating that the linear geometry of these complexes could generate interesting magnetic behavior, the dc magnetic susceptibilities c of 1-Sm and 4 were recorded as afunction of the temperature T (Figure 3).
Thesusceptibility is significantly larger for 4 than for 1-Sm over the whole investigated temperature range,af act that is at first counterintuitive given that the latter complex is made up of three Kramers ions whereas the former contains three non-Kramers ions.T his is explained by considering that the free-ion effective magnetic moment m eff of U IV is much larger than those of both U V and Sm III ,and that the high-symmetry, linear geometry of 4 cannot completely remove the degeneracyo ft he low-energy ligand-field levels (and therefore cannot isolate an on-magnetic singlet as the ground state). Furthermore,whereas the susceptibility for 1-Sm is essentially featureless,t he cT versus T plot for 4 shows ac lear upturn between 8a nd 6K followed by as harp drop upon further decreasing the temperature.S uch an upturn is usually considered as as ignature of ferromagnetic coupling,b ut it is more likely that each of the two outer U IV ions interacts antiferromagnetically with the central one,w hilst also carrying al arger magnetic moment. Indeed, fitting the hightemperature part of the inverse susceptibility of 4 ( Figure S6)   and an egative Curie-Weiss temperature of V = À36.2 K, which indicates the presence of antiferromagnetic interactions.T he effective magnetic moment is only slightly smaller than that for two U IV ions and much lower than for three such ions.Asimilar situation was found in the trinuclear Np VI/V neptunyl complex [(Np VI O 2 Cl 2 ){Np V O 2 Cl(THF) 3 } 2 ], [17] including the sharp downturn in cT at very low temperature, which was attributed to magnetic saturation. As this Np complex is also asingle-molecule magnet with ah igh energy barrier to magnetic relaxation (about 100 cm À1 ), we investigated the ac susceptibility of 4,b ut did not find any significant slowing down of the magnetic relaxation processes ( Figure S7).
An apriori prediction of which Ln or An salts would be capable of one-or two-electron uranyl reduction is complicated by the presence of strongly coordinating solvents, additional halides,a sw ell as the known propensity of these oxophilic metals to rapidly change coordination geometry, number, and their Lewis acidity.F or example,t he coordination of Lewis acidic Li + ,b oranes,o r4 fcations to the U VI uranyl oxo group shifts the U VI reduction by as much as + 600 mV, [5a,10d] and uranyl iodides are easier to reduce than their chloride analogues;U O 2 I 2 (py) 3 can be reduced by the organic anions of the normally stable organometallic U(h-C 8 H 8 ) 2 to form ahexa-uranium cluster U 6 O 8 I 8 (py) 10 . [18] In conclusion, we have demonstrated that redox reactions between readily accessible lanthanide and actinyl halides and related simple salts can produce linear oxo-coupled 4f/5f oligomers or infinite chains.T hese can be tuned for one-or two-electron reduction, and mono-or dioxo-functionalization. Thereductants can be 4f (exemplified by Sm II and Dy II ) or 5f cations (exemplified by U III ). Some postsynthetic modifications have been demonstrated through simple L donor ligand exchange.T he complexes are thermally stable, and most can be dissolved in av ariety of aprotic solvents. Given their inherent symmetry and simplicity,t hese complexes may be an interesting and flexible system from which spectroscopic studies can elucidate more of the fundamental electronic structural details that are still missing for the f-block, in particular for the f 1 actinyl ions;p reliminary magnetic analyses show strong antiferromagnetic coupling in the all-U system. Work is in progress to extrapolate these methods to the transuranic neptunyl and plutonyl cations,and to other combinations to explore the electronic structures of these systems.