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The coordination chemistry of trimethyltriazacyclohexane and trimethyltriazacyclononane with rare-earth metal salts was investigated. Structural differences were found based on metal size and (pseudo)halide salt used.

1,3,5-Trimethyltriazacyclohexane, Me

The reaction of UI3 with NaC5Ph5 in THF generated (η5-C5Ph5)UI2(THF)2 which was characterized by X-ray crystallography. The complex has a four-legged piano stool structure with a 2.539 Å U– (C5Ph5 ring centroid) distance that is slightly longer than the 2.504 Å distance in (η5-C5Me5)UI2(THF)3 which has a larger formal coordination number. Although the synthesis of the mono-cyclopentadienyl complex was facile, no bis-cyclopentadienyl complexes were isolated from reactions using two equivalents of the pentaphenylcyclopentadienyl ligand per uranium. In one UI3/2NaC5Ph5 synthesis, the X-ray crystal structure of (THF)3Na(η5-C5Ph5) was obtained. Attempts to reduce (η5-C5Ph5)UI2(THF)2 did not give evidence for a UII product. KC8 reductions of the in situ reaction of NdI3 and NaC5Ph5, analogous to the synthesis of (η5-C5Ph5)UI2(THF)2, yielded an X-ray crystal structure of a solvent-separated ion pair (SSIP), [K(18-crown-6)(THF)2][C5Ph5].

The U(v) compound (C5Me5)2UVI(NSiMe3) was crystallographically characterized from the reaction of (C5Me5)2UIIII(THF) with N3SiMe3.

1,3,5-Trimethyl-1,3,5-triazacyclohexane (Me

Density functional theory (DFT) calculations on four known and seven hypothetical U(II) complexes indicate the importance of coordination geometry in favoring 5f36d1 versus 5f4 electronic ground states. The known [Cp″3U]-, [Cptet3U]-, and [U(NR2)3]- [Cp″ = C5H3(SiMe3)2, Cptet = C5Me4H, and R = SiMe3] anions were found to have 5f36d1 ground states, while a 5f4 ground state was found for the known compound (NHAriPr6)2U. The UV-visible spectra of the known 5f36d1 compounds were simulated via time-dependent DFT and are in qualitative agreement with the experimental spectra. For the hypothetical U(II) compounds, the 5f36d1 configuration is predicted for [U(CHR2)3]-, [U(H3BH)3]-, [U(OAr')4]2-, and [(C8H8)U]2- (OAr' = O-C6H2tBu2-2,6-Me-4). In the case of [U(bnz')4]2- (bnz' = CH2-C6H4tBu-4), a 5f3 configuration with a ligand-based radical was found as the ground state.

The utility of γ irradiation for generating unstable, low oxidation state molecular species containing rare-earth metal ions in frozen solution has been examined. The method was evaluated by irradiating Ln(III) precursors (Ln = Sc, Y, and La) in a solid matrix of 2-methyltetrahydrofuran at 77 K with a 700 keV

A new synthesis of Th(II) complexes has been identified involving addition of simple MX salts (M = Li, Na, K; X = H, Cl, Me, N

The U(V) imido complex (C

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...

The reduction potentials (reported vs. Fc+/Fc) for a series of Cp′3Ln complexes (Cp′ = C5H4SiMe3, Ln = lanthanide) were determined via electrochemistry in THF with [nBu4N][BPh4] as the supporting electrolyte. The Ln(III)/Ln(II) reduction potentials for Ln = Eu, Yb, Sm, and Tm (−1.07 to −2.83 V) follow the expected trend for stability of 4f7, 4f14, 4f6, and 4f13 Ln(II) ions, respectively. The reduction potentials for Ln = Pr, Nd, Gd, Tb, Dy, Ho, Er, and Lu, that form 4fn5d1 Ln(II) ions (n = 2–14), fall in a narrow range of −2.95 V to −3.14 V. Only cathodic events were observed for La and Ce at −3.36 V and −3.43 V, respectively. The reduction potentials of the Ln(II) compounds [K(2.2.2-cryptand)][Cp′3Ln] (Ln = Pr, Sm, Eu) match those of the Cp′3Ln complexes. The reduction potentials of nine (C5Me4H)3Ln complexes were also studied and found to be 0.05–0.24 V more negative than those of the Cp′3Ln compounds.

Eight cyclopentadienyl U(III) iodide complexes have been synthesized from UI3 and crystallographically characterized to show the variations that can occur with this ligand set and uranium in the +3 oxidation state: (C5Me5)2UI(THF), {[K(OEt2)2][(C5Me5)2U(µ–I)2]}2, [K(18-crown-6)][(C5Me5)2U(µ–I)2], [K(2.2.2-cryptand)][(C5Me5)2UI2], [Li(THF)4][(C5Me5)2UI2], [Li(THF)3][(C5Me5)2UI(µ–I)], {[K(18-crown-6)]2(µ–toluene)}{[C5H3(SiMe3)2]2UI(µ–I)}2 and [(C5Me5)U]3(µ3–I)2 (µ–I)3I(THF)2. The structures show that small modifications in synthetic and crystallization procedures lead to changes in the coordination mode of the isolated product such that many structural options are available that can only be defined by X-ray crystallography. These variations highlight the many low energy options available in the reaction chemistry of this combination of metal and ligands.

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...

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

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.

Formulating insightful questions and experiments is crucial to the advancement of science. The purpose of this Perspective is to encourage scientists in all areas of chemistry to ask more "What if?" questions: What if we tried this experiment? What if we used these conditions? What if that idea is not correct? To stimulate this thinking, a retrospective analysis of a specific field, in this case rare-earth metal chemistry, is presented that describes the "What if?" questions that could have and should have been asked earlier based on our current knowledge. The goal is to provide scientists with a historical perspective of discovery that exemplifies how previous views in chemistry were often narrowed by predominant beliefs in principles that were incorrect. The same situation is likely to exist today, but we do not realize the limitations! Hopefully, this analysis can be used as a springboard for posing important "What if?" questions that should be asked right now in every area of chemical research.

Density functional theory (DFT) calculations on four known and seven hypothetical U(II) complexes indicate the importance of coordination geometry in favoring 5f

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.