[(C5H5)Co{P(O)(OH)2}3H]: A novel organometallic tris-phosphonic acid that dissolves glass to form a six-coordinate silicon complex

The sodium salt of the tripodal oxygen ligand Na[(C5H5)Co{P(O)(OMe)2}3] (1) reacts with gaseous hydrogen chloride in dichloromethane to give sodium chloride and [(C5H5)Co{P(O)(OMe)2}3H2]Cl (2). Addition of an equimolar amount of 1 leads to precipitation of sodium chloride again to yield the acid form of the ligand [(C5H5)Co{P(O)(OMe)2}3H] (3). Its solid-state structure contains a very short intramolecular OH⋅⋅⋅O hydrogen bond. The compound hydrolyses in boiling water to give methanol and the title complex [(C5H5)Co{P(O)(OH)2}3H] (4). This complex is a rather strong tris-phosphonic acid (pK 2.0, pK 4.0, pK 6.3, and pK 9.6). Attempts to grow single crystals of this highly water-soluble yellow crystalline compound lead to the silicon complex [{(C5H5)Co[P(O)(OH)2]2[P(O)2(OH)]}2Si]⋅8 H2O (5). The crystal structure determination shows that it is a molecular complex of two tris-phosphonic acids that each act as a tris-chelating ligand. Additional water molecules form a complex net of hydrogen bonds between the complexes of 5. The SiO6 octahedron is only insignificantly distorted with Si−O bond lengths of 1.78 A. Complex 4 reacts with stoichiometric amounts of potassium carbonate to yield the potassium salt K[(C5H5)Co{P(O)(OH)2}3] (6). The crystal structure of 6 has also been determined. In the solid state it is a two-dimensional coordination polymer with each potassium ion being coordinated by twelve oxygen atoms of six tris-phosphonic acids.