Synthesis and Reduction of Heteroleptic Bis(cyclopentadienyl) Uranium(III) Complexes

Abstract

Heteroleptic U(III) complexes supported by bis(cyclopentadienyl) frameworks have been synthesized to examine their suitability as precursors to U(II) complexes. The newly synthesized (C 5 Me 5 ) 2 U(OC 6 H 2 t Bu 2 -2,6-Me-4), (C 5 Me 5 ) 2 U-(OC 6 H 2 Ad 2 -2,6-t Bu-4) (Ad = 1-adamantyl), (C 5 Me 5 ) 2 U(C 5 H 5 ), and (C 5 Me 5 ) 2 U(C 5 Me 4 H) are compared with (C 5 Me 5 ) 2 U[N-(SiMe 3 ) 2 ], (C 5 Me 5 ) 2 U[CH(SiMe 3 ) 2 ], and (C 5 Me 5 )U[N(SiMe 3 ) 2 ] 2 . An improved synthesis of (C 5 Me 5 ) 2 U(μ-Ph) 2 BPh 2 was developed, which was used to synthesize (C 5 Me 5 ) 2 U(C 5 Me 4 H). Since the X-ray crystal structure of (C 5 Me 5 ) 2 U(OC 6 H 2 t Bu 2 -2,6-Me-4) contained two very different molecules in the asymmetric unit with 115.7(5)°and 166.0(5)°U−O−C ipso angles, the (C 5 Me 4 H) 2 U(OC 6 H 2 t Bu 2 -2,6-Me-4) and (C 5 Me 5 ) 2 Ce(OC 6 H 2 t Bu 2 -2,6-Me-4) analogues were synthesized and characterized by Xray diffraction for comparison. Electrochemical studies in THF with a 100 mM [ n Bu 4 N][BPh 4 ] supporting electrolyte showed U(IV)/U(III) and U(III)/U(II) redox couples for all the heteroleptic complexes except (C 5 Me 5 ) 2 U(C 5 H 5 ). Chemical reduction of all heteroleptic compounds formed dark blue solutions characteristic of U(II) when reacted with KC 8 at −78 °C, but none formed isolable U(II) complexes. The targeted U(II) complexes, [(C 5 Me 5 ) 2 U(OC 6 H 2 t Bu 2 -2,6-Me-4)] 1− , {(C 5 Me 5 ) 2 U[CH(SiMe 3 ) 2 ]} 1− , [(C 5 Me 5 ) 2 U(C 5 H 5 )] 1− , and [(C 5 Me 5 ) 2 U(C 5 Me 4 H)] 1− , were analyzed by density functional theory, and a 5f 3 6d 1 electron configuration was found to be the ground state in each case.

Reduction of CO2 and CS2 with Uranium(III) Metallocene Aryloxides

“…The U–O­(aryloxide) bond distances are 2.128(6) and 2.181(6) Å, while the U–O length bonding η 1 to U1 is 2.264(6) Å, and chelating oxygen distances to U2 are 2.435(6) and 2.423(7) Å. The U–O–C­(ipso) bond angles for each BHT ligand are 167.3(5)° with respect to U1 and 158.8(5)° for U2, which are similar to the 166.0(5)° U–O–C­(ipso) angle in 3 . All the C–O bond distances in the carbonate moiety are similar at 1.289(10), 1.311(11), and 1.272(11) Å.…”

“…Complex 2 can be synthesized independently from the reaction of [(C 5 Me 5 ) 2 UCl 2 ] with two equivalents of KO-4- t BuC 6 H 4 , Figure S7. The aryloxide with greater steric properties, [(C 5 Me 5 ) 2 U­(O-2,6- t Bu 2 -4-MeC 6 H 2 )], 3 , has recently been reported, and the 1 H NMR resonance for the (C 5 Me 5 ) 1– ligands does not follow the trend, but 3 does not have a THF molecule coordinated, so the comparison is not exact. The UV–vis–NIR spectra for 1 and 3 are consistent with U­(III) ions with absorptions in the visible region and several absorptions of weak intensity in the NIR region, which correspond to Laporte forbidden f–f transitions …”

“…All non-deuterated solvents used were dried by passing through a solvent purification system (MBraun, USA) and stored over sieves, and potassium metal for aromatic and aliphatic solvents or calcium hydride for ethereal solvents. [(C 5 Me 5 ) 2 UI-(THF)], 66 [(C 5 Me 5 ) 2 UCl 2 ], 67 and [(C 5 Me 5 ) 2 U(O-2,6-t Bu 2 -4-MeC 6 H 2 )], 3, 38 were prepared accordingly to their literature procedures. Carbon disulfide was purified by distillation from calcium hydride.…”

Does Reduction-Induced Isomerization of a Uranium(III) Aryl Complex Proceed via C–H Oxidative Addition and Reductive Elimination across the Uranium(II/IV) Redox Couple?

“…Additionally, C–H bond activation processes were found to occur at low temperatures in the putative uranium­(II) reduction products of Cp* 2 U­(N­(SiMe 3 ) 2 ) and Cp*U­(N­(SiMe 3 ) 2 ) 2 , leading to the isolation of cyclometalated uranium­(III) species . Together, these results suggest that C–H bond activation may be a feature of uranium­(II) chemistry; however, the mechanistic pathway(s) available for such processes has not been established …”

“…47 Together, these results suggest that C−H bond activation may be a feature of uranium(II) chemistry; however, the mechanistic pathway(s) available for such processes has not been established. 48 While mechanistically inconclusive, the results described above suggest that uranium(II) species may be capable of novel two-electron chemistry, such as oxidative addition reactions. Confirmation of this idea could lead to a shift in how we think about the reactivity of low-valent uranium, as the redox chemistry of this element typically occurs, in contrast, via one-electron steps.…”