Arylphosphanide Complexes of the Alkaline-Earth Metals Magnesium, Calcium, Strontium, and Barium of the Formula (THF)nM[P(H)Ph]2 and Formation of Potassium Diphenylphosphinomagnesiates

Abstract

The reaction of diethylmagnesium with diphenylphosphane yields [(THF)Mg(Et)PPh₂]_∞ (1; THF = tetrahydrofuran) with bridging PPh₂ ligands and average Mg−P bond lengths of 262.2 pm. The metalation reaction of MgEt₂ with HPPh₂ and H₂PPh with a 1:2 stoichiometry gives [(THF)₄Mg(PPh₂)₂] (2) and [(THF)₆Mg₄{P(H)Ph}₈] (3), respectively. Tetranuclear 3 contains three chemically different phenylphosphanide groups with characteristic P−H stretching frequencies at 2261, 2286, and 2310 cm⁻¹. The metathesis reaction of potassium phenylphosphanide with CaI₂ yields oligomeric (THF)₃Ca[P(H)Ph]₂ (4). A similar reaction with SrI₂ and BaI₂ gives polymeric [(THF)₂Sr{P(H)Ph}₂]_∞ (5) and [(THF)Ba{P(H)Ph}₂]_∞ (6), respectively, showing one stretching frequency at 2285 cm⁻¹. These compounds crystallize polymeric with bridging phenylphosphanide substituents. The addition of Et₂O to a mixture of KPPh₂ and Mg(PPh₂)₂ in THF initiates the crystallization of (Et₂O)K[(THF)Mg(PPh₂)₃] (7) with a strand structure and (Et₂O)_x(THF)_yK₂[Mg(PPh₂)₄] (8) with a layer structure depending on the stoichiometry. The crystals of 8 easily lose THF and Et₂O and, therefore, the content of these ethers varies. Recrystallization of 8 from hot 1,4-dioxane (diox) yields (diox)₂K₂[Mg(PPh₂)₄] (9) with a layer structure comparable to that of 8. The central structural units are eight-membered K₂Mg₂P₄ rings that are interconnected by P−K−P bridges. In a THF solution, the magnesiates 7−9 dissociate into the homometallic derivatives KPPh₂ and Mg(PPh₂)₂, as can be seen from NMR experiments.