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Exploration of the reduction chemistry of the 2,2’‐bipyridine (bipy) lanthanide metallocene complexes Cp*₂LnCl(bipy) and Cp*₂Ln(bipy) (Cp* = C₅Me₅) resulted in the isolation of a series of complexes with unusual composition and structure including complexes with a single Cp* ligand, multiple azide ligands, and bipy ligands with close parallel orientations. These results not only reveal new structural types, but they also show the diverse chemistry displayed by this redox‐active platform. Treatment of Cp*₂NdCl(bipy) with excess KC₈resulted in the formation of the mono‐Cp* Nd(III) complex, [K(crypt)]₂[Cp*Nd(bipy)₂],1, as well as [K(crypt)][Cp*₂NdCl₂],2, and the previously reported [K(crypt)][Cp*₂Nd(bipy)]. A mono‐Cp* Lu(III) complex, Cp*Lu(bipy)₂,3, was also found in an attempt to make Cp*₂Lu(bipy) from LuCl₃, 2 equiv. of KCp*, bipy, and K/KI. Surprisingly, the (bipy)¹⁻ligands in neighboring molecules in the structure of3are oriented in a parallel fashion with intermolecular C⋅⋅⋅C distances of 3.289(4) Å, which are shorter than the sum of van der Waals radii of two carbon atoms, 3.4 Å. Another product with one Cp* ligand per lanthanide was isolated from the reaction of [K(crypt)][Cp*₂Eu(bipy)] with azobenzene, which afforded the dimeric Eu(II) complex, [K(crypt)]₂[Cp*Eu(THF)(PhNNPh)]₂,4. Attempts to make4from the reaction between Cp*₂Eu(THF)₂and a reduced azobenzene anion generated instead the mixed‐valent Eu(III)/Eu(II) complex, [K(crypt)][Cp*Eu(THF)(PhNNPh)]₂,5, which allows direct comparison with the bimetallic Eu(II) complex4. Mono‐Cp* complexes of Yb(III) are obtained from reactions of the Yb(II) complex, [K(crypt)][Cp*₂Yb(bipy)], with trimethylsilylazide, which afforded the tetra‐azido [K(crypt)]₂[Cp*Yb(N₃)₄],6, or the di‐azido complex [K(crypt)]₂[Cp*Yb(N₃)₂(bipy)],7 a, depending on the reaction stoichiometry. A mono‐Cp* Yb(III) complex is also isolated from reaction of [K(crypt)][Cp*₂Yb(bipy)] with elemental sulfur which forms the mixed polysulfido Yb(III) complex [K(crypt)]₂[Cp*Yb(S₄)(S₅)],8 a. In contrast to these reactions that form mono‐Cp* products, reduction of Cp*₂Yb(bipy) with 1 equiv. of KC₈in the presence of 18‐crown‐6 resulted in the complete loss of Cp* ligands and the formation of [K(18‐c‐6)(THF)][Yb(bipy)₄],9. The (bipy)¹⁻ligands of9are arranged in a parallel orientation, as observed in the structure of3, except in this case this interaction is intramolecular and involves pairs of ligands bound to the same Yb atom. Attempts to reduce further the Sm(II) (bipy)¹⁻complex, Cp*₂Sm(bipy) with 2 equiv. of KC₈in the presence of excess 18‐crown‐6 led to the isolation of a Sm(III) salt of (bipy)²⁻with an inverse sandwich Cp* counter‐cation and a co‐crystallized K(18‐c‐6)Cp* unit, [K₂(18‐c‐6)₂Cp*]₂[Cp*₂Sm(bipy)]₂ ⋅ [K(18‐c‐6)Cp*],10.

 

The fine-tuning of topologically protected states in quantum materials holds great promise for novel electronic devices. However, there are limited methods that allow for the controlled and efficient modulation of the crystal lattice while simultaneously monitoring the changes in the electronic structure within a single sample. Here, we apply significant and controllable strain to high-quality HfTe₅samples and perform electrical transport measurements to reveal the topological phase transition from a weak topological insulator phase to a strong topological insulator phase. After applying high strain to HfTe₅and converting it into a strong topological insulator, we found that the resistivity of the sample increased by 190,500% and that the electronic transport was dominated by the topological surface states at cryogenic temperatures. Our results demonstrate the suitability of HfTe₅as a material for engineering topological properties, with the potential to generalize this approach to study topological phase transitions in van der Waals materials and heterostructures.

 

The salt metathesis reaction between one equivalent of SmI2(THF)2 and two equivalents of K(C5Me4H) in THF afforded single crystals of the unusual, toluene-soluble, and asymmetric bimetallic Sm(II)/Sm(II) complex, (C5Me4H)2SmII(μ-η3:η5-C5Me4H)SmII(C5Me4H)(THF)2, instead of the expected product, (C5Me4H)2SmII(THF)2. The toluene-insoluble products of this reaction can be worked up in 1,2-dimethoxyethane (DME) to provide X-ray quality crystals of the monomeric Sm(II) metallocene, (C5Me4H)2SmII(DME). (C5Me4H)2SmII(DME) can also be synthesized directly by the reaction between one equivalent of SmI2(THF)2 and two equivalents of K(C5Me4H) in neat DME. The isolation and characterization of the bimetallic Sm(II)/Sm(II) complex provides supporting evidence for the possible oligomerization that may occur during the synthesis of Sm(II) complexes with cyclopentadienyl ligands that are less sterically bulky and less solubilizing than (C5Me5)1−.