Synthesis, spectroscopic and structural properties of an unusual series of homoleptic phosphine oxide complexes of the alkaline earth dications

Abstract

The distorted tetrahedral [M(OPPh 3) 4](SbCl 6) 2 (M = Mg, Ca, Sr) are obtained from MCl 2, OPPh 3 and SbCl 5 in suitable solvent conditions, while Ba 2+ gives [Ba(OPPh 3) 5](SbCl 6) 2. The chelating o-C 6H 4(P(O)Ph 2) 2 gives the tris complexes [M{ o-C 6H 4(P(O)Ph 2) 2} 3](SbCl 6) 2, while under similar conditions the stoichiometries of the products obtained using Ph 2P(O)CH 2P(O)Ph 2 vary with M; and products include the five-coordinate [Mg{Ph 2P(O)CH 2P(O)Ph 2} 2(H 2O)](SbCl 6) 2, the distorted octahedral [M{Ph 2P(O)CH 2P(O)Ph 2} 3](SbCl 6) 2 (M = Ca or Sr) and the seven-coordinate [Sr{Ph 2P(O)CH 2P(O)Ph 2} 3(EtOH)](SbCl 6) 2. Strontium is unusual in that both six- and seven-coordinate species are evident and may be isolated depending upon the reaction conditions. These are the first reported Group 2 complexes with bidentate phosphine oxides and there are no other phosphine oxide complexes with Sr or Ba. Crystal structures are reported for [Mg(OPPh 3) 4](SbCl 6) 2 · CH 2Cl 2, [Ca{Ph 2P(O)CH 2P(O)Ph 2} 3](SbCl 6) 2 · 2CH 2Cl 2 and [Sr{Ph 2P(O)CH 2P(O)Ph 2} 3(EtOH)](SbCl 6) 2 · 0.45CH 2Cl 2 The compounds isolated show significant dependence upon the charge:radius ratio of the metal and the steric bulk and rigidity of the ligand.