ABSTRACT
Phosphoric acid forms two distinct coordination compounds with manganese salts in aqueous media, a two-dimensional layered structure, [Mn(HPO4).(H2O)3], 1, under ambient conditions, and a three-dimensional synthetic mineral, [Mn5(mu-OH2)2(HPO4)2(PO4)2(H2O)2], 2, under hydrothermal procedures, at 120 degrees C. In compound 1, the oxygen atom of the doubly deprotonated phosphoric acid interconnects the metal centers to form a layered structure. The neutral hydrophilic layers of 1 are separated by 5.5 A and may potentially intercalate hydrophilic organic guest molecules. The metal centers in 2 are octahedral and bridged by PO4(3-), HPO4(2-), and OH2 groups to form a complex three-dimensional network. XPS analysis on 1 and 2 confirms that manganese exists in the +2 oxidation state. Compound 2 is a poor ion exchanger, but some improvement is observed on partial dehydration. The magnetic properties of both 1 and 2 were studied in detail to examine the amplitude of the magnetic interactions through phosphate ligand bridges. While 1 reveals dominant antiferromagnetic interactions between the spin carriers, a complete investigation of the magnetic properties of 2 revealed its true nature to be a glassy magnet.
ABSTRACT
Nitrilotris(methylenephosphonic acid) (NTP, [N(CH(2)PO(3)H(2))(3)]) recently has been found to form three-dimensional porous structures with encapsulation of templates as well as layered and linear structures with template intercalation. It was, therefore, of interest to examine the type of organic-inorganic hybrids that would form with metal cations. Mn(II) was found to replace two of the six acid protons, while a third proton bonds to the nitrilo nitrogen, forming a zwitter ion. Two types of compounds were obtained. When the ratio of acid to Mn(II) was less than 10, a trihydrate, Mn[HN(CH(2)PO(3)H)(3)(H(2)O)(3)] (2) formed. Compound 2 is monoclinic P2(1)/c, with a = 9.283(2) A, b = 16.027(3) A, c = 9.7742(2) A, beta = 115.209(3) degrees, V = 1315.0(5) A(3), and Z = 4. The Mn atoms form zigzag chains bridged by two of the three phosphonate groups. The third phosphonate group is only involved in hydrogen bonding. The metal atoms are octahedrally coordinated with three of the sites occupied by water molecules. Adjacent chains are hydrogen-bonded to each other through POH and HN donors, and the additional participation of all the water hydrogens in H-bonding results in a corrugated sheet-like structure. Use of excess NTP at a ratio to metal of 10 to 1 yields an anhydrous compound Mn[HN(CH(2)PO(3)H)(3)] (1), P2(1)/n, a = 9.129(1) A, b = 8.408(1) A, c = 13.453(1) A, beta = 97.830(2) degrees, V = 1023.0(2) A(3), and Z = 4. Manganese is five coordinate forming a distorted square pyramid with oxygens from five different phosphonate groups. The sixth oxygen is 2.85 A from an adjacent Mn, preventing octahedral coordination. All the protonated atoms, three phosphonate oxygens and N, form moderately strong hydrogen bonds in a compact three-dimensional structure. The open-structured trihydrate forms a series of isostructural compounds with other divalent transition metal ions as well as with mixed-metal compositions. This is indicative that the hydrogen bonding controls the type of structure formed irrespective of the cation.
