A Simple Route to Calcium and Strontium Hydride Clusters

Area of Science:

• Inorganic Chemistry
• Organometallic Chemistry
• Materials Science

Background:

• Metal hydrides are crucial compounds in chemistry and industry.
• Strontium and calcium hydrides have been less explored compared to other alkaline earth metals.
• Understanding the bonding and structure of novel metal hydrides is essential for developing new materials and catalytic processes.

Purpose of the Study:

• To synthesize and characterize the first strontium hydride complex.
• To investigate the structural, bonding, and electronic properties of the novel strontium and analogous calcium hydride clusters.
• To explore the thermal stability and decomposition pathways of these metal hydride complexes.

Main Methods:

• Synthesis of strontium and calcium hydride complexes via reaction of metal amide precursors with phenylsilane in the presence of PMDTA.
• Single-crystal X-ray diffraction for structural determination of the Sr complex.
• Nuclear Magnetic Resonance (NMR) spectroscopy (1H NMR) to study solution behavior and structural integrity.
• Density Functional Theory (DFT) calculations to investigate electronic structure and bonding in the Ca complex.
• Atoms in Molecules (AIM) analysis to quantify bonding characteristics.

Main Results:

• Successful synthesis and isolation of the first strontium hydride complex, Sr₆H₉[N(SiMe₃)₂]₃·(PMDTA)₃, exhibiting an "inverse cryptand" structure.
• The complex features an interstitial hydride surrounded by a Sr₆H₈⁴⁺ cage, with amide and PMDTA ligands.
• Analogous Ca complex was synthesized; both retain solid-state structures in solution, confirmed by 1H NMR spectroscopy up to 90°C.
• DFT and AIM analyses of the Ca cluster revealed unexpectedly low ionicity and significant hydride-hydride bonding interactions.
• Thermal decomposition leads to larger, undefined metal hydride aggregates.

Conclusions:

• The synthesis of the strontium hydride complex represents a significant advancement in alkaline earth metal hydride chemistry.
• The "inverse cryptand" structure and low ionicity challenge conventional bonding models for metal hydrides.
• The observed hydride-hydride bonding suggests covalent character, necessitating a re-evaluation of bonding in related systems.
• These findings open new avenues for exploring the chemistry and potential applications of early transition metal hydrides.