Solid-State Coordination Chemistry of the Oxomolybdate−Organodiphosphonate/Nickel−Organoimine System: Structural Influences of the Secondary Metal Coordination Cation and Diphosphonate Tether Lengths

The hydrothermal reactions of a molybdate source, a nickel(II) salt, tetra-2-pyridylpyrazine (tpyprz), and
organodiphosphonic acids H2O3P(CH2)nPO3H2 (n = 1−5) of varying tether lengths yielded a series of organic−inorganic hybrid materials of the nickel−molybdophosphonate family. A persistent characteristic of the structural
chemistry is the presence of the {Mo5O15(O3PR)2}4- cluster as a molecular building block, as noted for the one-dimensional materials [{Ni2(tpyprz)2}Mo5O15{O3P(CH2)4PO3}]·6.65H2O (6·6.65H2O) and [{Ni2(tpyprz)2}Mo5O15{O3P(CH2)5PO3}]·3.75H2O (8·3.75H2O), the two-dimensional phases [{Ni4(tpyprz)3}{Mo5O15(O3PCH2CH2PO3)}2]·23H2O (3·23H2O) and [{Ni3(tpyprz)2(H2O)2}(Mo5O15)(Mo2O4F2){O3P(CH2)3PO3}2]·8H2O (5·8H2O), and the three-dimensional structures [{Ni2(tpyprz)(H2O)3]Mo5O15{O3P(CH2)3PO3)}]·H2O (4·H2O) and [{Ni2(tpyprz)(H2O)2}Mo5O15
{O3P(CH2)4PO3}]·2.25H2O (7·2.25H2O). In the case of methylenediphosphonic acid, the inability of this ligand to
tether adjacent pentanuclear clusters precludes the formation of the common molybdophosphonate building block,
manifesting in contrast a second structural motif, the trinuclear {(Mo3O8)x(O3PCH2PO3)y} subunit of [{Ni(tpyprz)(H2O)2}(Mo3O8)2 (O3PCH2PO3)2] (1) which had been previously observed in the corresponding methylenediphosphonate phases of the copper−molybdophosphonate family. Methylenediphosphonic acid also provides a second
phase, [Ni2(tpyprz)2][Mo7O21(O3PCH2PO3)]·3.5H2O (9·5H2O), which contains a new heptamolybdate cluster
{Mo7O21(O3PCH2PO3)}4- and a cationic linear chain {Ni(tpyprz)}n4n+ substructure. The structural chemistry of the
nickel−molybdophosphonate series contrasts with that of the corresponding copper−molybdophosphonate materials,
reflecting in general the different coordination preferences of Ni(II) and Cu(II). Consequently, while the Cu(II)−organic complex building block of the copper family is invariably the binuclear {Cu2(tpyprz)}4+ subunit, the Ni(II)
chemistry with tpyprz exhibits a distinct tendency toward catenation to provide {Ni3(tpyprz)2]6+, {Ni4(tpyprz)3}8+,
and {Ni(tpyprz)}n4n+ building blocks as well as the common {Ni2(tpyprz)}4+ moiety. This results in a distinct structural
chemistry for the nickel(II)−molybdophosphonate series with the exception of the methylenediphosphonate derivative
1 which is isostructural with the corresponding copper compound [{Cu2(tpyprz)(H2O)2}(Mo3O8)2(O3PCH2PO3)] (2).
The structural chemistry of the nickel(II) series also reflects variability in the number of attachment sites at the
molybdophosphonate clusters, in the extent of aqua ligation to the Ni(II) tpyprz subunit, and in the participation of
phosphate oxygen atoms as well as molybdate oxo groups in linking to the nickel sites.