Identification of an X-Band Clock Transition in Cp'3Pr- Enabled by a 4f25d1 Configuration.

Molecular qubits offer an attractive basis for quantum information processing, but challenges remain with regard to sustained coherence. Qubits based on clock transitions offer a method to improve the coherence times. We propose a general strategy for identifying molecules with high-frequency clock transitions in systems where a d electron is coupled to a crystal-field singlet state of an f configuration, resulting in an MJ = ±1/2 ground state with strong hyperfine coupling. Using this approach, a 9.834 GHz clock transition was identified in a molecular Pr complex, [K(crypt)][Cp'3PrII], leading to 3-fold enhancements in T2 relative to other transitions in the spectrum. This result indicates the promise of the design principles outlined here for the further development of f-element systems for quantum information applications.

Metal-Halide Covalency, Exchange Coupling, and Slow Magnetic Relaxation in Triangular (CpiPr5)3U3X6 (X = Cl, Br, I) Clusters.

The actinide elements are attractive alternatives to transition metals or lanthanides for the design of exchange-coupled multinuclear single-molecule magnets. However, the synthesis of such compounds is challenging, as is unraveling any contributions from exchange coupling to the overall magnetism. To date, only a few actinide compounds have been shown to exhibit exchange coupling and single-molecule magnetism. Here, we report triangular uranium(III) clusters of the type (CpiPr5)3U3X (1-X; X = Cl, Br, I; CpiPr5 = pentaisopropylcyclopentadienyl), which are synthesized via reaction of the aryloxide-bridged precursor (CpiPr5)2U2(OPhtBu)4 with excess Me3SiX. Spectroscopic analysis suggests the presence of covalency in the uranium-halide interactions arising from 5f orbital participation in bonding. The dc magnetic susceptibility data reveal the presence of antiferromagnetic exchange coupling between the uranium(III) centers in these compounds, with the strength of the exchange decreasing down the halide series. Ac magnetic susceptibility data further reveal all compounds to exhibit slow magnetic relaxation under zero dc field. In 1-I, which exhibits particularly weak exchange, magnetic relaxation occurs via a Raman mechanism associated with the individual uranium(III) centers. In contrast, for 1-Br and 1-Cl, magnetic relaxation occurs via an Orbach mechanism, likely involving relaxation between ground and excited exchange-coupled states. Significantly, in the case of 1-Cl, magnetic relaxation is sufficiently slow such that open magnetic hysteresis is observed up to 2.75 K, and the compound exhibits a 100-s blocking temperature of 2.4 K. This compound provides the first example of magnetic blocking in a compound containing only actinide-based ions, as well as the first example involving the uranium(III) oxidation state.

The Counterintuitive Relationship between Orbital Energy, Orbital Overlap, and Bond Covalency in CeF62- and CeCl62.

The 4f orbitals of Ce(IV) have shown appreciably enhanced covalent mixing with ligand orbitals relative to those of Ce(III). Here, X-ray spectroscopy, magnetic susceptibility measurements, and theoretical methods are used to investigate 4f covalency in CeF62- and CeCl62-. These techniques show covalent mixing between Ce 4f and F 2p orbitals to be about 25% less than mixing between Ce 4f and Cl 3p orbitals, placing CeF62- among the most ionic Ce(IV) compounds to-date. However, ligand field analysis using the experimental data shows significantly higher 4f orbital overlap with the F 2p orbitals compared to the Cl 3p. This result is counterintuitive since the Ce-F bonds display less 4f covalency despite their higher orbital overlap, and greater overlap is traditionally associated with enhanced bond covalency. The weaker covalency is attributed to the large energy gap between Ce 4f and F 2p orbitals strongly counteracting the higher orbital overlap. These results highlight that only a concerted consideration of both atomic orbital overlap and energy matching in f-element systems leads to an accurate picture of their bonding.

Analyzing the Intensities of K-Edge Transitions in X2 Molecules (X = F, Cl, Br) for Use in Ligand K-Edge X-ray Absorption Spectroscopy.

Ligand K-edge X-ray absorption spectroscopy (XAS) is regularly used to determine the ligand contribution to metal-ligand bonds. For quantitative studies, the pre-edge transition intensities must be referenced to an intensity standard, and pre-edge intensities obtained from different ligand atoms cannot be compared without standardization due to different cross sections at each absorption edge. In this work, the intensities of the 1s → σ* transitions in F2, Cl2, and Br2 are analyzed for their use as references for ligand K-edge XAS. We show that the intensities of these transitions are equal to the intensities of the 1s → np transitions in the unbound halogens. This finding is supported by a comparison between the normalized experimental intensities for the molecules and the calculated oscillator strengths for the atoms. These results highlight the potential for these molecules to be used as intensity standards in F, Cl, and Br K-edge XAS experiments.

Synthesis and Characterization of a Bridging Cerium(IV) Nitride Complex.

Complexes featuring lanthanide-ligand multiple bonds are rare and highly reactive. They are important synthetic targets to understand 4f/5d-bonding in comparison to d-block and actinide congeners. Herein, the isolation and characterization of a bridging cerium(IV)-nitride complex: [(TriNOx)Ce(Li2µ-N)Ce(TriNOx)][BArF4] is reported, the first example of a molecular cerium-nitride. The compound was isolated by deprotonating a monometallic cerium(IV)-ammonia complex: [CeIV(NH3)(TriNOx)][BArF4]. The average Ce═N bond length of [(TriNOx)Ce(Li2µ-N)Ce(TriNOx)][BArF4] was 2.117(3) Å. Vibrational studies of the 15N-isotopomer exhibited a shift of the Ce═N═Ce asymmetric stretch from ν = 644 cm-1 to 640 cm-1, and X-ray spectroscopic studies confirm the +4 oxidation state of cerium. Computational analyses showed strong involvement of the cerium 4f shell in bonding with overall 16% and 11% cerium weight in the σ- and π-bonds of the Ce═N═Ce fragment, respectively.

Versatile Fe-Sn Bonding Interactions in a Metallostannylene System: Multiple Bonding and C-H Bond Activation.

The metallostannylene Cp*(iPr2MeP)(H)2Fe-SnDMP (1; Cp* = η5-C5Me5; DMP = 2,6-dimesitylphenyl), formed by hydrogen migration in a putative Cp*(iPr2MeP)HFe[Sn(H)DMP] intermediate, serves as a robust platform for exploration of transition-metal main-group element bonding and reactivity. Upon one-electron oxidation, 1 expels H2 to generate the coordinatively unsaturated [Cp*(iPr2MeP)Fe═SnDMP][B(C6F5)4] (3), which possesses a highly polarized Fe-Sn multiple bond that involves interaction of the tin lone pair with iron. Evidence from EPR and 57Fe Mössbauer spectroscopy, along with DFT studies, shows that 3 is primarily an iron-based radical with charge localization at tin. Upon reduction of 3, C-H bond activation of the phosphine ligand was observed to produce Cp*HFe(κ2-(P,Sn)═Sn(DMP)CH2CHMePMeiPr) (5). Complex 5 was also accessed via thermolysis of 1, and kinetics studies of this thermolytic pathway indicate that the reductive elimination of H2 from 1 to produce a stannylyne intermediate, Cp*(iPr2MeP)Fe[SnDMP] (A), is likely rate-determining. Evidence indicates that the production of 5 proceeds through a concerted C-H bond activation. DFT investigations suggest that the transition state for this transformation involves C-H cleavage across the Fe-Sn bond and that a related transition state where C-H bond activation occurs exclusively at the tin center is disfavored, illustrating an effect of iron-tin cooperativity in this system.

Activations of all Bonds to Silicon (Si-H, Si-C) in a Silane with Extrusion of [CoSiCo] Silicide Cores.

The [BP3 iPr]Co(I) synthon Na(THF)6{[BP3 iPr]CoI} (1, [BP3 iPr] = κ3-PhB(CH2P iPr2)3-) reacts with PhSiH3 or SiH4 to form unusual {[BP2 iPr](SiH2R)CoH2}═Si═{H2Co[BP3 iPr]} species (R = Ph, 2a; R = H, 2b; [BP2 iPr] = κ2-PhB(CH2P iPr2)2) that result from activation of all Si-H and Si-C bonds in the starting silanes. Solution-spectroscopic data (multinuclear NMR, IR) for 2a,b, and the solid-state structure of 2a, indicate substantial Co═Si═Co multiple bonding and minimal interaction of the core Si atom with nearby hydride ligands. In the presence of 4-dimethylaminopyridine (DMAP), 1 reacts with PhSiH3 to give [BP3 iPr](H)2CoSiHPh(DMAP) (3). Complexes 2a,b eliminate RSiH3 upon thermolysis in the presence of DMAP to generate {[BP2 iPr]Co(NC5H3NMe2)}═Si═{H2Co[BP3 iPr]} (4).

Base-Free Iron Hydrosilylene Complexes via an α-Hydride Migration that Induces Spin Pairing.

Two new base-free hydrosilylene complexes of iron were synthesized using the novel starting material Cp*( iPr2MeP)FeMes. These Cp*( iPr2MeP)Fe(H)SiHR (R = DMP, Trip) complexes are in equilibrium with the corresponding iron silyl complexes, Cp*( iPr2MeP)FeSiH2R, which can be trapped and characterized for R = Trip. Unlike the Ru analogues, the Fe silylene complex with R = DMP is observed to undergo an intramolecular C-H activation involving formal addition of a benzylic C-H bond across the Fe-Si bond. This increased activity for bond activations is also observed for reactions with hydrogen, where Fe reacts faster than a Ru analog to form the hydrogenation product, Cp*( iPr2MeP)H2FeSiH2DMP.

Expanded Helicenes: A General Synthetic Strategy and Remarkable Supramolecular and Solid-State Behavior.

A divergent synthetic strategy allowed access to several members of a new class of helicenes, the "expanded helicenes", which are composed of alternating linearly and angularly fused rings. The strategy is based on a three-fold, partially intermolecular [2+2+n] (n = 1 or 2) cycloaddition with substrates containing three diyne units. Investigation of aggregation behavior, both in solution and in the solid state, revealed that one of these compounds forms an unusual homochiral, π-stacked dimer via an equilibrium that is slow on the NMR time scale. The versatility of the method was harnessed to access a selenophene-annulated expanded helicene that, in contrast to its benzannulated analogue, exhibits long-range π-stacking in the solid state. The new helicenes possess low racemization barriers, as demonstrated by dynamic 1H NMR spectroscopy.

Formation of uranium disulfide from a uranium thioamidate single-source precursor.

A single-source-precursor approach was developed to synthesize uranium-based materials outside of the typically-studied oxides. This approach allows for shorter reaction times, milder reaction conditions, and control over the chemicals present in synthesis. To this end, the first homoleptic uranium thioamidate complex was synthesized as a precursor for US2 materials. Pyrolysis of the thioamidate results in decomposition via an alkene elimination pathway and formation of γ-US2, which has historically been hard to access without the need for a secondary sulfur source. Despite the oxophilicity of uranium, the method successfully forms US2 without the inclusion of oxygen in the bulk final product. These findings are supported by simultaneous thermal analysis, elemental analysis, powder X-ray diffraction, and uranium L3-edge X-ray absorption fine-structure spectroscopy. This work represents the first example of a single-source precursor approach to target and synthesize actinide materials other than the oxides.

Realization of Organocerium-Based Fullerene Molecular Materials Showing Mott Insulator-Type Behavior.

Electron-rich organocerium complexes (C5Me4H)3Ce and [(C5Me5)2Ce(ortho-oxa)], with redox potentials E1/2 = -0.82 V and E1/2 = -0.86 V versus Fc/Fc+, respectively, were reacted with fullerene (C60) in different stoichiometries to obtain molecular materials. Structurally characterized cocrystals: [(C5Me4H)3Ce]2·C60 (1) and [(C5Me5)2Ce(ortho-oxa)]3·C60 (2) of C60 with cerium-based, molecular rare earth precursors are reported for the first time. The extent of charge transfer in 1 and 2 was evaluated using a series of physical measurements: FT-IR, Raman, solid-state UV-vis-NIR spectroscopy, X-ray absorption near-edge structure (XANES) spectroscopy, and magnetic susceptibility measurements. The physical measurements indicate that 1 and 2 comprise the cerium(III) oxidation state, with formally neutral C60 as a cocrystal in both cases. Pressure-dependent periodic density functional theory calculations were performed to study the electronic structure of 1. Inclusion of a Hubbard-U parameter removes Ce f states from the Fermi level, opens up a band gap, and stabilizes FM/AFM magnetic solutions that are isoenergetic because of the large distances between the Ce(III) cations. The electronic structure of this strongly correlated Mott insulator-type system is reminiscent of the well-studied Ce2O3.

4f-Orbital mixing increases the magnetic susceptibility of Cp'3Eu.

Traditional models of lanthanide electronic structure suggest that bonding is predominantly ionic, and that covalent orbital mixing is not an important factor in determining magnetic properties. Here, 4f orbital mixing and its impact on the magnetic susceptibility of Cp'3Eu (Cp' = C5H4SiMe3) was analyzed experimentally using magnetometry and X-ray absorption spectroscopy (XAS) methods at the C K-, Eu M5,4-, and L3-edges. Pre-edge features in the experimental and TDDFT-calculated C K-edge XAS spectra provided unequivocal evidence of C 2p and Eu 4f orbital mixing in the π-antibonding orbital of a' symmetry. The charge-transfer configurations resulting from 4f orbital mixing were identified spectroscopically by using Eu M5,4-edge and L3-edge XAS. Modeling of variable-temperature magnetic susceptibility data showed excellent agreement with the XAS results and indicated that increased magnetic susceptibility of Cp'3Eu is due to removal of the degeneracy of the 7F1 excited state due to mixing between the ligand and Eu 4f orbitals.

Efficient and selective alkene hydrosilation promoted by weak, double Si-H activation at an iron center.

Cationic iron complexes [Cp*(iPr2MeP)FeH2SiHR]+, generated and characterized in solution, are very efficient catalysts for the hydrosilation of terminal alkenes and internal alkynes by primary silanes at low catalyst loading (0.1 mol%) and ambient temperature. These reactions yield only the corresponding secondary silane product, even with SiH4 as the substrate. Mechanistic experiments and DFT calculations indicate that the high rate of hydrosilation is associated with an inherently low barrier for dissociative silane exchange (product release).

Gauging aromatic conjugation and charge delocalization in the aryl silanes PhnSiH4-n (n = 0-4), with silicon K-edge XAS and TDDFT.

Si K-edge X-ray absorption spectra (XAS) have been measured experimentally and calculated using time-dependent density functional theory (TDDFT) to investigate electronic structure in aryl silanes, PhnSiH4-n (n = 0-4). Adding aryl groups to SiH4 splits the Si-H σ-antibonding orbitals into new orbitals with Si-Ph π-bonding (πb) and π-antibonding (π*) character. Greater aryl substitution is reflected by increasingly intense Si 1s → πb and Si 1s → π* transitions, and weaker transitions into the Si-H and Si-C σ* orbitals. These observations are consistent with known trends in the hydride donor ability of aryl silanes, which is driven in part by the composition of the LUMOs and the accessibility of pathways for electron delocalization through aromatic conjugation. Methodology developed for liquid-phase Si K-edge XAS measurements on PhSiH3 and Ph2SiH2 will enable dynamic studies of chemical transformations involving silicon-containing catalysts, intermediates, and substrates.

Coordination and structural properties of encumbering 6-mesityl-2-picolinate complexes.

In an effort to enforce a sterically hindered environment in transition-metal and main-group 2-picolinate complexes, the synthesis of the encumbering derivative 6-mesityl-2-picolinate ((Mes)pic) is presented. The coordination and structural properties of (Mes)pic are demonstrated with a range of transition-metal and main-group fragments. The 6-position mesityl group of (Mes)pic is shown to alter both the primary and secondary coordination spheres of metal centers relative to the ubiquitous and unencumbered parent 2-picolinate anion.