Tuning the Selectivity of P Reduction at Alkaline-earth Metal Centres

Reduction of P with β-diketiminate Mg complexes, (BDI)MgMg(BDI), depends strongly on the bulk of the ligand. Whereas superbulky BDI ligands gave selective reduction to P in a butterfly conformation, reduction with a less bulky ligand gave various products among which P had a realgar-type structure. The selectivity of P reduction can also be controlled by metal choice. Reduction of P with Ca synthons of general type (BDI*)Ca-X-Ca(BDI*) in which BDI* is a superbulky ligand and X is a bridging dianion (CH < -xylene < N ) led to reduction of P to the very common, stable Zintl anion P . Monitoring this process with P NMR shows that -P is an intermediate en route to P . Conversion rates increase with increasing reducing power: X = CH < -xylene < N . A complex with the weakly reducing DBA dianion led to selective P reduction to -P (DBA = 9,10-dimethyl-diboraanthracene). DBA inhibits P-to-P conversion, most likely by capturing the electron needed for further P 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-P bonding (covalent or ionic) determines the preference for formation of butterfly-shaped P or planar 6π-electron aromatic -P .