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

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 P₄ ^2- in a butterfly conformation, reduction with a less bulky ligand gave various products among which P₈ ^4- 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 (C₆H₆ ^2- < p-xylene^2- < N₂ ^2-) led to reduction of P₄ to the very common, stable Zintl anion P₇ ^3-. Monitoring this process with ³¹P NMR shows that cyclo-P₄ ^2- is an intermediate en route to P₇ ^3-. Conversion rates increase with increasing reducing power: X = C₆H₆ ^2- < p-xylene^2- < N₂ ^2-. A complex with the weakly reducing DBA^2- dianion led to selective P₄ reduction to cyclo-P₄ ^2- (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 P₄ ^2- or planar 6π-electron aromatic cyclo-P₄ ^2-.