P4 Coordination and Reduction at Alkaline Earth Metal Centers

The simplest allotrope of phosphorus, white phosphorus, P4, is shown to coordinate to main-group metal centers, forming stable complexes, for the first time. Coordinatively unsaturated, Lewis acidic magnesium(II) complexes, [Mg(EtNONAr)] (EtNONAr; Ar = TCHP, 4,5-bis(2,4,6-tricyclohexylanilido)-2,7-diethyl-9,9-dimethyl-xanthene, or Ar = DCHP, 4,5-bis(2,6-dicyclohexylanilido)-2,7-diethyl-9,9-dimethyl-xanthene) react with P4 to form 1:1 and 2:1 Lewis adducts, [Mg(EtNONTCHP)(η2-P4)] and [{Mg(EtNONDCHP)}2(μ-η2:η2-P4)], respectively. The related arene-capped complexes of the heavier alkaline earth metal complexes, [M(EtNONTCHP)(η6-arene)] (M = Ca, Sr) also engage in coordination with P4 to give [M(EtNONTCHP)(η6-arene)(η1-P4)] (arene = benzene or toluene). These alkaline earth metal-P4 complexes appear to reversibly bind P4 in aliphatic solvents, and dissociate P4 upon dissolution in aromatic solvents, thereby showing their capacity to act as storage vessels for P4. Reduction of P4 by magnesium(I) complex [K2{Mg(tBuNONTrip)}2] (tBuNONTrip = 4,5-bis(2,4,6-triisopropylanilido)-2,7-di(tert-butyl)-9,9-dimethyl-xanthene) and a magnesium(I) synthon [K2{Mg(EtNONTCHP)}2(μ-N2)] provides entry to the planar, aromatic P42– dianion in [K2{Mg(RNONAr)}2(μ-P4)] (R = tBu, Ar = Trip; R = Et, Ar = TCHP). Hydrolysis of these products releases PH3, a useful precursor for phosphorus-containing fine-chemicals.