Search

The allyl complexes (C5Me5)2Ln(η3-C3H5) (Ln = Y, Dy, Tb) react with benzoxazole, C7H5NO, by deprotonation and ring-opening to yield dark red complexes identified by X-ray crystallography as the 2-i...

Reduction of (C5Me5)2UIII(NR2) and (C5Me5)UIII(NR2)2 (R = SiMe3) with potassium graphite in the presence of 2.2.2-cryptand (crypt) generates dark solutions that have UV–visible spectra consistent w...

Californium (Cf) is currently the heaviest element accessible above microgram quantities. Cf isotopes impose severe experimental challenges due to their scarcity and radiological hazards. Consequently, chemical secrets ranging from the accessibility of 5f/6d valence orbitals to engage in bonding, the role of spin–orbit coupling in electronic structure, and reactivity patterns compared to other f elements, remain locked. Organometallic molecules were foundational in elucidating periodicity and bonding trends across the periodic table1–3, with a twenty-first-century renaissance of organometallic thorium (Th) through plutonium (Pu) chemistry4–12, and to a smaller extent americium (Am)13, transforming chemical understanding. Yet, analogous curium (Cm) to Cf chemistry has lain dormant since the 1970s. Here, we revive air-/moisture-sensitive Cf chemistry through the synthesis and characterization of [Cf(C5Me4H)2Cl2K(OEt2)]n from two milligrams of 249Cf. This bent metallocene motif, not previously structurally authenticated beyond uranium (U)14,15, contains the first crystallographically characterized Cf–C bond. Analysis suggests the Cf–C bond is largely ionic with a small covalent contribution. Lowered Cf 5f orbital energy versus dysprosium (Dy) 4f in the colourless, isoelectronic and isostructural [Dy(C5Me4H)2Cl2K(OEt2)]n results in an orange Cf compound, contrasting with the light-green colour typically associated with Cf compounds16–22.

Electrochemical measurements on tris(cyclopentadienyl)thorium and uranium compounds in the +2, +3, and +4 oxidation states are reported with C5H3(SiMe3)2, C5H4SiMe3, and C5Me4H ligands. The reduction potentials for both U and Th complexes trend with the electron donating abilities of the cyclopentadienyl ligand. Thorium complexes have more negative An(iii)/An(ii) reduction potentials than the uranium analogs. Electrochemical measurements of isolated Th(ii) complexes indicated that the Th(iii)/Th(ii) couple was surprisingly similar to the Th(iv)/Th(iii) couple in Cp′′-ligated complexes. This suggested that Th(ii) complexes could be prepared from Th(iv) precursors and this was demonstrated synthetically by isolation of directly from UV-visible spectroelectrochemical measurements and reactions of with elemental barium indicated that the thorium system undergoes sequential one electron transformations., Electrochemical determination of the reduction potentials for a variety of tris(cyclopentadienyl)uranium and thorium complexes, including data on U(ii) and Th(ii) complexes.

Reduction of Cp′3ThCl, Cp′3ThBr, and Cp′3ThI (Cp′ = C5H4SiMe3) with potassium graphite generates dark blue solutions with reactivity and spectroscopic properties consistent with the formation of Cp′3Th. The EPR and UV-visible spectra of the solutions are similar to those of crystallographically-characterized tris(cyclopentadienyl) Th(III) complexes: [C5H3(SiMe3)2]3Th, (C5Me4H)3Th, (C5tBu2H3)3Th, and (C5Me5)3Th. Density functional theory (DFT) analysis indicates that the UV-visible spectrum is consistent with Cp′3Th and not [Cp′3ThBr]1−. Although single crystals of Cp′3Th have not been isolated, the blue solution reacts with Me3SiCl, I2, and HCCPh to afford products expected from Cp′3Th, namely, Cp′3ThCl, Cp′3ThI, and Cp′3Th(CCPh), respectively. Reactions with MeI give mixtures of Cp′3ThI and Cp′3ThMe. Evidence for further reduction of the blue solutions to a Cp′-ligated Th(II) complex has not been observed. The crystal structures of Cp′3ThMe and (Cp′3Th)2(μ-O) were also determined as part of these studies.