Tuning the selectivity of P4 reduction at alkaline-earth metal centres

Abstract

Reduction of P₄ with β-diketiminate Mg^I complexes, (BDI)MgMg(BDI), depends strongly on the bulk of the ligand. Whereas superbulky BDI ligands gave selective reduction to P₄²⁻ in a butterfly conformation, reduction with a less bulky ligand gave various products among which P₈⁴⁻ had a realgar-type structure. The selectivity of P₄ reduction can also be controlled by metal choice. Reduction of P₄ with Ca^I synthons of general type (BDI*)Ca–X–Ca(BDI*) in which BDI* is a superbulky ligand and X is a bridging dianion (C₆H₆²⁻ < p-xylene²⁻ < N₂²⁻) led to reduction of P₄ to the very common, stable Zintl anion P₇³⁻. Monitoring this process with ³¹P NMR shows that cyclo-P₄²⁻ is an intermediate en route to P₇³⁻. Conversion rates increase with increasing reducing power: X = C₆H₆²⁻ < p-xylene²⁻ < N₂²⁻. A complex with the weakly reducing DBA²⁻ dianion led to selective P₄ reduction to cyclo-P₄²⁻ (DBA = 9,10-dimethyl-diboraanthracene). DBA inhibits P₄-to-P₇ conversion, most likely by capturing the electron needed for further P₄ reduction by radical processes. Experimental investigations are supported by crystal structure determinations and a computational DFT study which also shows that the nature of metal–P₄ bonding (covalent or ionic) determines the preference for formation of butterfly-shaped P₄²⁻ or planar 6π-electron aromatic cyclo-P₄²⁻.