Molybdophosphonate Clusters As Building Blocks In The Oxomolybdate-Organodiphosphonate/Cobalt(II)-Organoimine System: Structural Influences Of Secondary Metal Coordination Preferences And Diphosphonate Tether Lengths

In press, in the journal Inorganic Chemistry. This work includes a first attempt at an oxomolybdate crystal cell optimization using periodic boundary conditions (the same types of calculations I do for vibrational spectroscopy studies). Optimization issues related to the treatment of the high-spin cobalt atoms in DMol³ (a problem still being examined here) kept the unit cells from converging to structures with the coordination sphere of the cobalt found in the crystal cell, leaving the majority of the computational study to focus on the isolated-molecule analysis of the {Mo₅O₁₅(O₃PCH₃)₂}^4- cluster, the goal being an understanding of preferred binding modes to the oxomolybdate cluster by organic and organometallic groups (it seems to be based in sterics, not electronic structure). An interested in-progress result of the solid-state studies involved the calculation of hydrogen-bonding interactions within the crystal cell and their predicted importance in directing MoO₄ orientations relative to the isolated oxometal cluster, a result found by performing an optimization on a "first guess" hydrogen position set (from one set of nearest-neighbor oxygen atom distances, Cell #1) followed by a re-optimization at "second guess" hydrogen positions that better preserve the crystal cell symmetry (Cell #2). The utility of deuteration and neutron diffraction studies are not lost on solid-state quantum chemists working with X-ray crystallographers.

N. Gabriel Armatas^a, Damian G. Allis^a, Andrew Prosvirin^b, Gabriel Carnutu^c, Charles J. O'Connor^c, Kim Dunbar^b, and Jon Zubieta^a,*

a. Department of Chemistry, Syracuse University, Syracuse, NY 13244
b. Department of Chemistry, Texas A&M University, College Station, TX 77843
c. Department of Chemistry, University of New Orleans, New Orleans, LA 70148

Abstract: Hydrothermal conditions have been exploited in the preparation of a series of organic-inorganic hybrid materials of the cobalt-molybdophosphonate family. The reactions of MoO₃, cobalt(II) acetate or cobalt(II) acetylacetonate, tetra-2-pyridyl pyrazine (tpyprz) and organodiphosphonic acids H₂O₃P(CH₂)_nPO₃H₂ (n = 1-5 and 9) of varying tether lengths yielded compounds of the general type {Co₂(tpyprz)(H₂O)_m}⁴⁺/Mo_xO_y{O₃P(CH₂)_nPO₃}_z. The recurring theme of the structural chemistry is the entrainment of {Mo₅O₁₅(O₃PR)₂}^4- clusters as molecular building blocks observed in the structures of nine phases (compounds 2-9 and 11). As noted previously, minor variations in reaction conditions can have profound influence on the structures of these bimetallic molybdophosphonates. Thus, for the propylene diphosphonate ligand, four unique structures 4-7 are observed, including two distinct three-dimensional architectures for compounds 5 and 6 whose formulations differ only in the number of water molecules of crystallization. It is noteworthy that with pentyldiphosphonate, a second phase 10 is obtained which exhibits a unique cluster building block, the hexamolybdate [Mo₆O₁₈{O₃P(CH₂)₅PO₃}]^4-. In the case of methylenediphosphonic acid, the lack of spatial extension of the ligand precludes tethering of adjacent molybdate clusters and the formation of the common penta-molybdophosphonate building block. Consequently, a third structural motif, the trinuclear {(Mo₃O₈)(O₃PCH₂PO₃)}^2- subunit, is observed in compound 1. Trinuclear subunits have been described previously for the corresponding methylenediphosphonate phases of the copper-molybdophosphonate and nickel-molybdophosphonate families.

The cobalt series of compounds shares with the nickel phases a tendency toward catenation of the M(II)-tpyprz subunits, as manifested in compounds 2 and 7. However, the structural chemistry of compounds 1-11 of this study is quite distinct from that of the {Ni₂(tpyprz)(H₂O)m}⁴⁺/Mo_xO_y{O₃P(CH₂)_nPO₃}_z family, as well as that of the copper based family. The structural diversity of this general class of materials reflects 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 are quite uniform, the attachment of {M₂(tpyprz)}⁴⁺ subunits to the molybdophosphonates appears to be largely determined by steric, coulombic and packing factors.

Substitution of 2,2'-bipyridine, o-phenanthroline or terpyridine for the binucleating tpyprz ligand results in the one-dimensional structures of 12–14. Compounds 12 and 13 are unusual examples of "undecorated" molybdophosphonate chains, containing protonated variants of the pentamolybdate cluster.

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