Cooperative binding of phosphate anion and a neutral nitrogen donor to alkaline-earth metal ions. Investigation of group 2 metal-organophosphate interaction in the absence and presence of 1,10-phenanthroline

Abstract

Alkaline-earth metal phosphates containing nitrogen-donor ligands have been prepared by the reaction of alkaline-earth metal acetates M(OAc)₂·xH₂O (M = Mg, Ca, Sr, Ba) with 2,6-diisopropylphenyl phosphate (dippH₂) in the absence and presence of 1,10-phenanthroline (phen). Interaction of strontium or barium acetate with dippH₂ in methanol at room temperature leads to the isolation of ionic phosphates [{M₂(μ-H₂O)₄(H₂O)₁₀}{dipp}₂]·4L [M = Sr, L = CH₃OH (1); M = Ba, L = H₂O (2)]. The addition of a bidentate nitrogen-donor phen to these reactions leads to the isolation of dinuclear metal phosphates [Mg(dipp)(phen)(CH₃OH)₂]₂ (3) and [M(dippH)₂(phen)₂(H₂O)]₂ [M = Ca (4), Sr (5), Ba (6)]. While ionic phosphates 1 and 2 are soluble in water, the predominately covalent dimeric compounds 3-6 are insoluble in all common solvents including water. The new compounds have been characterized in the solid state by elemental analysis, IR, UV-vis, and emission spectroscopy, and single-crystal X-ray diffraction studies. The cationic part in 1 and 2 is a {M₂(μ-H₂O)₄(H₂O)₁₀} unit, where each metal ion is surrounded by four bridging and five terminal water molecules as ligands. The dipp anion does not directly bind to the metal ions but is extensively hydrogen-bonded to the cationic unit through the phosphate oxygen and water hydrogen atoms to result in an infinitely layered structure where the hydrophobic aryl group protrudes out of the hydrophilic layer formed by the cationic part and -PO₃^2- units. In contrast, compounds 3-6 are discrete dimeric molecules built around a central M₂O₄P₂ eight-membered ring. While the dippH₂ ligand exists in a doubly deprotonated form in 3, two monodeprotonated dippH₂ ligands are present per metal ion in compounds 4-6. While 3 prefers only one phen ligand in the metal coordination sphere, two phen ligands chelate each metal ion in 4-6. The conformations of the eight-membered rings in 3-6 vary significantly from each other depending on the size of the cation and the coordination number around the metal. Further, intermolecular hydrogen bonding involving the phenanthroline C-H linkages result, in a gridlike structure in 1, one-dimensional chains in isostructural 2 and 3, and a two-dimensional layer arrangement in 4. Compounds 3-6 are the only examples of alkaline-earth metal phosphate complexes with neutral M-N donor bonds. The thermal behavior of compounds 1-6 has been examined with the help of thermogravimetric analysis and differential scanning calorimetry and also by bulk thermolysis followed by powder X-ray diffraction measurements. While compounds 1 and 2 yield M₂P₂O₇, decomposition of 4-6 results in the formation of M(PO₃)₂, consistent with the M-P ratio in the precursor complexes.