6 Search Results

A set of nine new robust, tunable cerium complexes supported by an ansa-bis(cyclopentadienyl) ligand, [Me₂Si(η⁵-Cp^R)₂]CeX (^anCp^R)CeX, are excellent homogeneous visible-light photocatalysts for the monodefluoroalkylation of trifluorotoluene with Mg(CH₂C₆H₅)₂THF₂ (R = Me₄, SiMe₃, X = N(SiMe₃)₂ (N″), X = CH(SiMe₃)₂ (R''), Cl, OC₆H₂^tBu₂-2,6,Me-4 (OAr)). The trends in photocatalytic activity within the series are explained by photophysical spectroscopic analyses. The aryloxide complex [Me₂Si(Cp^SiMe3)₂]CeOAr, which has the highest activity (95% substrate conversion in 27 h), shows the most negative (most reducing) excited-state reduction potential (-2.71 V vs Fc). The precatalyst excited-state lifetimes are also exceptionally long. Detailed photoluminescence, NMR spectroscopic, and kinetic studiesmore » on chloride [Me₂Si(Cp^Me4)₂]CeCl suggest that the "ate" complex [{Me₂Si(Cp^Me4)₂}₂Ce^IIIClBn][MgBn] is the active catalyst in the alkylation reaction to form PhCF₂CH₂Ph with high selectivity over PhCF₂H. Finally, the reaction rates are up to 30 times higher than previously reported for organometallic rare-earth photocatalysts for these Ce complexes and comparable to established Ir-based photoredox systems.« less

Isostructural, diamagnetic diluents are commonly used in magnetic investigations to eliminate interactions between neighboring electron spins. For divalent lanthanides, these diluents are currently limited to closed-shell ytterbium(II) complexes; ytterbium(II), however, is a poor size-match for the large early- and mid-lanthanides, precluding access to certain ligand frameworks. To aid in the study of lanthanide(II) complexes with unconventional electronic structures and magnetic properties, three strontium tris-cyclopentadienyl complexes, [K(2.2.2-cryptand)][SrCpR3], with common substituted cyclopentadienyl ligands, CpR = C5Me4H (Cptet), C5H4(SiMe3) (Cp'), and C5H3(SiMe3)2 (Cp″) were synthesized, characterized, and examined as diamagnetic diluents. En route to their synthesis, two new Sr metallocenes, [SrCpR2] CpR =more » Cptet, Cp', were also prepared. [K(2.2.2-cryptand)][SrCptet3] was used to dilute the new complex [K(2.2.2-cryptand)][EuIICptet3], and the EPR spectra of the diluted material shows narrower linewidths than those of pure [K(2.2.2-cryptand)][EuIICptet3], suggesting an increase in the spin-spin relaxation time that enables determination of the europium hyperfine coupling constant.« less

The study of actinide electronic structure and bonding within rigorously controlled environments is fundamental to advancing nuclear applications. Here, we report a new set of isostructural actinide organometallics; An(COT^big)₂, (An = Th, U, Np, and Pu), where COT^big is the bulky 1,4-bis(triphenylsilyl)-substituted cyclooctatetraenyl dianion (1,4-(Ph₃Si)₂C₈H₆)²-. The actinide(IV) metallocene sandwiches have a clam-shell structure, offering a new molecular symmetry to explore f-orbital contributions in bonding. Combined experimental and computational studies reveal that An(COT^big)₂ complexes strongly differ from the previously published coplanar An(COT)₂ sandwiches due to the bent geometry and electron-withdrawing nature of the substituents. While COT^big displays comparatively weaker electron donation,more » the low-energy f-f transitions in An(COT^big)₂ have increased molar absorptivity consistent with the removal of the parity selection rule and better energetic matching between ligand and actinide 5f orbitals as the series is traversed. For Pu(COT^big)₂, covalent mixing of donor 5f metal orbitals and the ligand-π orbitals is especially strong.« less

Understanding the chemistry of the inert actinide oxo bond in actinyl ions AnO2 2+ is important for controlling actinide behavior in the environment, during separations, and in nuclear waste (An=U, Np, Pu). The thioether calixarene TC4A (4-tert-butyltetrathiacalix[4]arene) binds equatorially to the actinyl cation forming a conical pocket that differentiates the two trans-oxo groups. The 'ate' complexes, [A]2[UO2(TC4A)] (A=[Li(DME)2], HNEt3) and [HNEt3]2[AnO2(TC4A)] (An=U, Np, Pu), enable selective oxo chemistry. Silylation of the UVI oxo groups by bis(trimethylsilyl)pyrazine occurs first at only the unencapsulated exo oxo and only one silylation is needed to enable migration of the endo oxo out of themore » cone, whereupon a second silylation affords the stable UIV cis-bis(siloxide) [A]2[U(OSiMe3)2(TC4A)]. Calculations confirm that only one silylation event is needed to initiate oxo rearrangement, and that the putative cis dioxo isomer of [UO2(TC4A)]2- would be stable if it could be accessed synthetically, at only 23 kcal.mol-1 in energy above the classical trans dioxo. Calculations for the transuranic cis[AnO2(TC4A)]2- (An=Np, Pu) are at higher energies, 30-35 kcal.mol-1, retaining the U complexes as the more obvious target for a cis-dioxo actinyl ion. The aryloxide (OAr) groups of the macrocycle are essential in stabilizing this as-yet unseen uranyl geometry as further bonding in the TC4A U-OAr groups stabilizes the U=O 'yl' bonds, explaining the stability of the putative cis[UO2(TC4A)]2- in this ligand framework.« less

Abstract Understanding the chemistry of the inert actinide oxo bond in actinyl ions AnO ₂ ²⁺ is important for controlling actinide behavior in the environment, during separations, and in nuclear waste (An=U, Np, Pu). The thioether calixarene TC4A (4‐ tert ‐butyltetrathiacalix[4]arene) binds equatorially to the actinyl cation forming a conical pocket that differentiates the two trans‐ oxo groups. The ‘ate’ complexes, [A] ₂ [UO ₂ (TC4A)] (A=[Li(DME) ₂ ], HNEt ₃ ) and [HNEt ₃ ] ₂ [AnO ₂ (TC4A)] (An=U, Np, Pu), enable selective oxo chemistry. Silylation of the U ^VI oxo groups by bis(trimethylsilyl)pyrazine occurs first at onlymore » the unencapsulated exo oxo and only one silylation is needed to enable migration of the endo oxo out of the cone, whereupon a second silylation affords the stable U ^IV cis‐ bis(siloxide) [A] ₂ [U(OSiMe ₃ ) ₂ (TC4A)]. Calculations confirm that only one silylation event is needed to initiate oxo rearrangement, and that the putative cis dioxo isomer of [UO ₂ (TC4A)] ²⁻ would be stable if it could be accessed synthetically, at only 23 kcal.mol ⁻¹ in energy above the classical trans dioxo. Calculations for the transuranic cis [AnO ₂ (TC4A)] ²⁻ (An=Np, Pu) are at higher energies, 30–35 kcal.mol ⁻¹ , retaining the U complexes as the more obvious target for a cis ‐dioxo actinyl ion. The aryloxide (OAr) groups of the macrocycle are essential in stabilizing this as‐yet unseen uranyl geometry as further bonding in the TC4A U‐O _Ar groups stabilizes the U=O ‘yl’ bonds, explaining the stability of the putative cis [UO ₂ (TC4A)] ²⁻ in this ligand framework.« less

Lanthanide sandwiches of the D _2h 1,3,4,6-tetraphenylpentalenide are reported. The Ph ₄ Pn dianion shows promise for new electron-rich f-block organometallics using the metal's d _{z ²} orbital, in contrast to the more widely used cyclooctatetraenyl dianion.