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) n PO3H2 (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)} n 4 n + 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)} n 4 n + 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