Alkyne Hydrogenation Catalysis across a Family of Ga/In Layered Zintl Phases

Zintl Ions and Phases Promote the Catalytic Hydrophosphination of Alkynes, Alkenes, and Imines

Although Zintl ions and phases have been known for more than a century, their application as tools to build organic molecules is underdeveloped. Here, a range of Zintl ions and phases were surveyed in the hydrophosphination of alkynes, alkenes, and imines with diphenylphosphine to afford useful organophosphine products. Further investigations with diphenylphosphine in the absence of the unsaturated organic substrates revealed the formation of the diphenylphosphide anion, allowing for the conclusion that the role of the Zintl species is as an initiator in these transformations.

Zintl Phases: From Curiosities to Impactful Materials

The synthesis of new compounds and crystal structures remains an important research endeavor in pursuing technologically relevant materials. The Zintl concept is a guidepost for the design of new functional solid-state compounds. Zintl phases are named in recognition of Eduard Zintl, a German chemist who first studied a subgroup of intermetallics prepared with electropositive metals combined with main-group metalloids from groups 13-15 in the 1930s. Unlike intermetallic compounds, where metallic bonding is the norm, Zintl phases exhibit a combination of ionic and covalent bonding and are typically semiconductors. Zintl phases provide a palette for iso- and aliovalent substitutions that can each contribute uniquely to the properties. Zintl electron-counting rules can be employed to interrogate a structure type and develop a foundation of structure-property relationships. Employing substitutional chemistry allows for the rational design of new Zintl compounds with technological properties, such as magnetoelectronics, thermoelectricity, and other energy storage and conversion capabilities. Discovering new structure types and compositions through this approach is also possible. The background on the strength and innovation of the Zintl concept and a few highlights of Zintl phases with promising thermoelectric properties in the context of structural and electronic design will be provided.

Lucky Number 13: A 13-Layer Polytype of the Alkyne Hydrogenation Catalyst CaGaGe

Polytypism, the ability of materials to form crystal structures with different stacking sequences, occasionally causes materials with the same stoichiometry and similar local structures to have profoundly different properties. Herein, we discover a metastable 13-layer trigonal (13T) polytype of CaGaGe, a layered intermetallic phase comprised of [GaGe]^2- honeycombs separated by Ca²⁺. 13T-CaGaGe is synthesized from arc-melting the elements, and its structure is elucidated via neutron powder diffraction. Air-stable 13T-CaGaGe has one misaligned [GaGe]^2- layer for every 13 and transforms into the more stable 4-layer hexagonal (4H) CaGaGe polytype after annealing at 500 °C. Transition-metal-free 13T-CaGaGe shows remarkable activity in the catalytic hydrogenation of phenylacetylene to styrene and ethylbenzene, much higher than the 4H polytype. This work identifies the first 13-layer polytype for any crystal structure and further establishes the influence of polytypism on catalysis.