Phosphinoborane interception at magnesium by borane-assisted phosphine-borane dehydrogenation

Area of Science:

• Organometallic Chemistry
• Boron Chemistry
• Coordination Chemistry

Background:

• The synthesis and reactivity of alkaline-earth metal complexes with phosphidoborane ligands are underexplored.
• Boron-based Lewis acids like B(C6F5)3 are valuable reagents for activating small molecules.

Purpose of the Study:

• To investigate the reactions of B(C6F5)3 with β-diketiminato (BDI) alkaline-earth phosphidoborane complexes.
• To characterize the resulting phosphinodiboronate complexes and elucidate their reactivity.
• To understand the contrasting behavior of calcium and magnesium in these reactions.

Main Methods:

• Synthesis of novel phosphinodiboronate complexes of calcium and magnesium.
• NMR spectroscopy for structural characterization and reactivity studies.
• Density Functional Theory (DFT) calculations to probe reaction mechanisms.
• X-ray crystallography to determine the solid-state structures of key compounds.

Main Results:

• Formation of phosphinodiboronate complexes [(BDI)Ca(η6-toluene){H3B·PPh2·B(C6F5)3}] (4a) and [(BDI)Mg{H3B·PPh2·B(C6F5)3}] (4b).
• Calcium complex 4a exhibits stability in aromatic solvents, while magnesium complex 4b undergoes transformation to a hydridoborate and an N,P,N'-ligated species.
• DFT calculations suggest a mechanism involving B(C6F5)3 migration and hydride abstraction for the magnesium complex.
• Crystal structures of 4a and the N,P,N'-ligated species 5 were determined.

Conclusions:

• The reactivity of BDI-supported phosphidoborane complexes with B(C6F5)3 is element-dependent.
• Magnesium complex 4b demonstrates unique reactivity including hydride abstraction and ligand backbone interception.
• The Ca-π(toluene) interaction significantly stabilizes calcium complex 4a, leading to different outcomes.
• The study provides insights into the activation and stabilization of boron- and phosphorus-containing species by alkaline-earth metals.