Manganese borohydride; synthesis and characterization.

Area of Science:

• Inorganic Chemistry
• Materials Science
• Solid-State Chemistry

Background:

• Transition metal borohydrides are crucial for energy storage applications.
• Existing synthesis methods for metal borohydrides often lack reproducibility and scalability.
• Manganese borohydride (Mn(BH4)2) synthesis has been challenging, limiting its study and application.

Purpose of the Study:

• To develop a reproducible solvent-based synthesis for manganese borohydride (Mn(BH4)2).
• To characterize the synthesized Mn(BH4)2 and identify new polymorphs.
• To explore the structural relationship between manganese and magnesium borohydrides.

Main Methods:

• Solvent-based synthesis using manganese chloride and lithium borohydride in a toluene/dimethylsulfide mixture.
• Characterization via synchrotron radiation-powder X-ray diffraction, infrared spectroscopy, and thermogravimetric analysis coupled with mass spectroscopy.
• Identification of a new nanoporous polymorph, γ-Mn(BH4)2, and its solvate precursor, Mn(BH4)2·1/2S(CH3)2.

Main Results:

• Successful synthesis of α-Mn(BH4)2 and a novel nanoporous polymorph, γ-Mn(BH4)2.
• γ-Mn(BH4)2 exhibits structural analogy to γ-Mg(BH4)2 with pores larger than 6.0 Å.
• Mn(BH4)2 decomposition begins at 160 °C, releasing hydrogen and diborane.

Conclusions:

• Established the first reproducible solvent-based synthesis for a stable, crystalline transition metal borohydride.
• Discovered a new nanoporous manganese borohydride polymorph with potential for hydrogen storage.
• Revealed structural analogies between Mg(BH4)2 and Mn(BH4)2, leading to the identification of ζ-Mg(BH4)2.