Synthesis and Crystallographic Characterization of a Reduced Bimetallic Yttrium ansa-Metallocene Hydride Complex, [K(crypt)][(μ-CpAn)Y(μ-H)]2 (CpAn = Me2Si[C5H3(SiMe3)-3]2), with a 3.4 Å Yttrium-Yttrium Distance

Fluoride Clusterfullerenes: Tuning Metal-Metal Bonding and Magnetic Properties via Single Fluorine Atom Doping

Endohedral fullerenes are known for their exceptional ability to host metal clusters that contain unique bonding motifs. In this study, we report a facile strategy to synthesize a new family of clusterfullerenes, fluoride clusterfullerenes (FCFs). This work demonstrates that actinides and rare earth metals as well as alkaline earth metals can be encapsulated within a variety of fullerene cages, and these fullerenes can be obtained in their pristine form without additional functionalization methods. Notably, Th2F@Ih(7)-C80 and CaScF@Cs(6)-C82 were isolated and their molecular structures and magnetic properties were characterized by X-ray single-crystal diffraction and multiple spectroscopic techniques as well as DFT calculations. These findings reveal that the unique internal addition of a single fluorine atom significantly alters the metal-metal bonding interactions of Th-Th and Ca-Sc. While Th2@Ih(7)-C80 hosts a σ2 Th-Th bond, an unprecedented actinide-actinide (Th-Th) single electron metal-metal bond is formed inside Th2F@Ih(7)-C80 upon the internal addition of fluoride. Similarly, while a Ca-Sc single electron bond exists in CaSc@Cs(6)-C82, which exhibits excellent molecular qubit properties, the addition of fluoride transforms the compound into a singlet. The present study not only highlights the successful synthesis of a novel family of FCFs, which will likely be an extensive family, it also shows that fluorine doping can induce novel metal-metal bonding motifs leading to potentially intriguing magnetic properties.

A Delocalized Mixed-Valence Dinuclear Ytterbium Complex That Displays Intervalence Charge Transfer

The analysis of intervalence charge transfer (IVCT) in mixed-valence compounds can help understand electron transfer processes that are important in diverse applications such as molecular electronics and artificial photosynthesis. While mixed-valence complexes of the lanthanides are more difficult to access than their transition metal analogues, they have shown IVCT phenomena derived from Robin-Day Class II localized valency or even electronic transitions due to d-d metal-metal bonding. In contrast, we report here the synthesis, characterization, and computational analysis of a rare, Robin-Day Class III, singly reduced dinuclear Yb complex, which is best viewed as having delocalized oxidation states. In this case, no metal-metal bonding occurs and, for the first time, IVCT in a Robin-Day Class III complex resulting from f-f transitions is observed.

Synthesis and reduction of [(C5H4SiMe3)2Ln(μ-OR)]2 (Ln = La, Ce) complexes: structural effects of bridging alkoxides

Alcoholysis of Cp'3Ln (Ln = La, Ce; Cp' = C5H4SiMe3) generate high-yielding (72-97%) bimetallic LnIII complexes of [Cp'2Ln(μ-OR)]2 [R = Et, iPr, or C6H4-4-tBu]. Single-crystal X-ray diffraction of these complexes reveal unexpected decreases in Ln⋯Ln distances, increasing Cpcent-Ln-Cpcent angles, and increasing intermolecular C⋯C contacts with bulkier bridging alkoxides, in line with structural control driven by significant dispersion forces. 1H NMR spectroscopy of [Cp'2Ce(μ-OEt)]2 and [Cp'2Ce(μ-OiPr)]2 revealed significantly upfield resonances assigned as methylene and methine moieties of -43.74 and -70.85 ppm, respectively. 2D 1H DOSY NMR experiments of [Cp'2Ce(μ-OiPr)]2 in C6D6 supported a dimeric structure in solution, including in the presence of a Lewis base (i.e., THF). Reduction of [Cp'2La(μ-OiPr)]2 using KC8 in the presence of 2.2.2-cryptand at -78 °C generated a purple solution and X-band EPR spectroscopy revealed an eight-line hyperfine pattern indicative of a LaII species.

f-Block hydride complexes - synthesis, structure and reactivity

Complexes formed between the heaviest and lightest elements in the periodic table yield the f-block hydrides, a unique class of compounds with wide-ranging utility and interest, from catalysis to light-responsive materials and nuclear waste storage. Recent developments in syntheses and analytics, such as exploiting low-oxidation state metal ions and improvements in X-ray diffraction tools, have transformed our ability to understand, access and manipulate these important species. This perspective brings together insights from binary metal hydrides, with molecular solution phase studies on heteroleptic complexes and gas phase investigations. It aims to provide an overview of how the f-element influences hydride formation, structure and reactivity including the sometimes-surprising power of co-ligands to tune their behaviour towards a variety of applications.

δ-Bonding modulates the electronic structure of formally divalent nd1 rare earth arene complexes

Landmark advances in rare earth (RE) chemistry have shown that divalent complexes can be isolated with non-Aufbau 4f n {5d/6s}1 electron configurations, facilitating remarkable bonding motifs and magnetic properties. We report a series of divalent bis-tethered arene complexes, [RE(NHAriPr6 )2] (2RE; RE = Sc, Y, La, Sm, Eu, Tm, Yb; NHAriPr6 = {N(H)C6H3-2,6-(C6H2-2,4,6-iPr3)2}). Fluid solution EPR spectroscopy gives g iso < 2.002 for 2Sc, 2Y, and 2La, consistent with formal nd1 configurations, calculations reveal metal-arene δ-bonding via mixing of nd(x 2-y 2) valence electrons into arene π* orbitals. Experimental and calculated EPR and UV-Vis-NIR spectroscopic properties for 2Y show that minor structural changes markedly alter the metal d(x 2-y 2) contribution to the SOMO. This contrasts 4f n {5d/6s}1 complexes where the valence d-based electron resides in a non-bonding orbital. Complexes 2Sm, 2Eu, 2Tm, and 2Yb contain highly-localised 4f n+1 ions with no appreciable metal-arene bonding by density functional calculations. These results show that the physicochemical properties of divalent rare earth arene complexes with both formal nd1 and 4f n+1 configurations are nuanced, may be controlled through ligand modification, and require a multi-pronged experimental and theoretical approach to fully rationalise.

Intervalence Charge Transfer in Nonbonding, Mixed-Valence, Homobimetallic Ytterbium Complexes

There are several reports of compounds containing lanthanide ions in two different formal oxidation states; however, there are strikingly few examples of intervalence charge transfer (IVCT) transitions observed for these complexes, with those few occurrences limited to extended solids rather than molecular species. Herein, we report the synthesis, characterization, and computational analysis for a series of ytterbium complexes including a mixed-valence Yb25+ complex featuring a remarkably short Yb···Yb distance of 2.9507(8) Å. In contrast to recent reports of short Ln···Ln distances attributed to bonding through 5d orbitals, the formally Yb25+ complex presented here displays clear localization of Ln2+ and Ln3+ character and yet still displays an IVCT in the visible spectrum. These results demonstrate the ability to tune the electronic structure of formally mixed oxidation state lanthanide complexes: the high exchange stabilization of the Yb2+ 4f14 configuration disfavors the formation of a 5d1 bonding configuration, and the short metal-metal distance enforced by the ligand framework allows for the first observed lanthanide IVCT in a molecular system.

Alkaline earth metals: homometallic bonding

The study of alkaline earth metal complexes is undergoing a renaissance. Stable molecular species featuring Mg-Mg bonds were reported in 2007 and their reactivity has since been intensively investigated. Motivated by this work, efforts have also been devoted to the synthesis of complexes featuring Be-Be and Ca-Ca bonds. These collective endeavours have revealed that the chemistry of the group 2 metals is richer and more complex than had previously been appreciated. Here, a discussion of the nature of homometallic alkaline earth bonding is presented, recent synthetic advances are described, and future directions are considered.