Molybdophosphonate clusters as building blocks in the oxomolybdate-organodiphosphonate/M(II)-organoamine system: Structural influences of secondary metal, organoamine, and diphosphonate tether length (M=copper, nickel, cobalt)

Nathan Gabriel Armatas


This research focused on the development of organic-inorganic hybrid materials, with a particular focus on the factors that affected the formation of, and structural variation in the resulting solid materials. Using a bottom-up approach, two major components were formed and studied: the oxomolybdate-diphosphonate anionic building block and the M(II)/organoimine cationic charge compensating unit. Hydrothermal synthesis was used to produce various products in an attempt to better understand which factors influence their formation. Of particular importance were stoichiometry, pH, acidic component, temperature, and time. A typical reaction included four components: molybdenum as the primary metal within the cluster, an organodiphosphonic acid to produce the desired anionic cluster, a secondary metal (Co, Ni, Cu) provided charge compensation, and an organoimine completed the cationic unit and also increased dimensionality. This study observed how these four components could be modified to alter the final product; the major goal being to understand how these alterations influenced structure.

The reactions of MoO 3 , cobalt(II) acetate or cobalt(II) acetylacetonate, tetra-2-pyridylpyrazine (tpyprz), and organodiphosphonic acids H 2 O 3 P(CH 2 ) n PO 3 H 2 ( n = 1-5 and 9) of varying tether lengths yielded compounds of the general type {Co 2 (tpyprz)(H 2 O) m } 4+ /Mo x O y {O 3 P(CH 2 ) n PO 3 } z . The recurring theme of the structural chemistry was the incorporation of {Mo 5 O 15 (O 3 PR) 2 } 4- clusters as molecular building blocks observed in the structures of nine phases found within the study. The structural consequences of variations in reaction conditions were most apparent in the series with propylene diphosphonate, where four unique structures were observed, including two distinct three-dimensional architectures for compounds whose formulations differed only in the number of water molecules of crystallization. The structural chemistry of these compounds were quite distinct from that of the {Ni 2 (tpyprz)(H 2 O) m } 4+ /Mo x O y {O 3 P(CH 2 )nPO 3 } z family, as well as that of the copper-based family. The structural diversity of this general class of materials reflected the coordination preferences of the M(II) sites, the extent of aqua ligation to the M(II) sites, the participation of both phosphate oxygen atoms and molybdate oxo-groups in linking to the M(II) sites, and the variability in the number of attachment sites at the molybdophosphonate clusters. Since the charge densities at the peripheral oxygen atoms of the clusters were quite uniform, the attachment of {M 2 (tpyprz)} 4+ subunits to the molybdophosphonates appeared to be largely determined by steric, coulombic, and packing factors, as shown by extensive density functional theory calculations.

The second study covered they synthesis and physical characterization of eight different copper(II) bipyrimidine compounds. Altering the linking organonitrogen ligand had a profound effect on the resulting structure. Bipyrimidine produced two two-dimensional layers and six three-dimensional networks. Further analysis of these materials is being performed, but early results showed that bipyrimidine tightened up the network structures providing little pore volume, but closer metal-metal interactions provided some interesting magnetic communication between metal sites.

The focus of the previous projects was to develop hybrid materials using the [Mo 5 O 15 (PO 3 ) 2 ] 4- cluster as the building block. Providing the system a organophosphonate produced this result the majority of the time, with a few exceptions due to sterics or anomalous effects. In the final study the pnictide was altered to arsenic; due to its expanded atomic radii it expanded to produce a larger molybdenum unit, the cluster added an additional MoO 3 polyhedra to become {Mo 6 O 18 (O 4 As) 2 } 4- . Although it was only slightly larger, the products showed a significant change in the coordination. It should also be noted that the new cluster was symmetrical so some of the bending that was observed for the pentamolybdate cluster would be avoided.