Synthesis, Structure Characterization, and Reversible Transformation of a Cobalt Salt of a Dilacunary γ‑Keggin Silicotungstate and Sandwich-Type Di- and Tetracobalt-Containing Silicotungstate Dimers

A cobalt salt of a γ-Keggin
dilacunary silicotungstate, {CoL5}2[γ-SiW10O34L2] [Co-SiW10; L = N,N-dimethylformamide (DMF) or H2O], could be synthesized by the cation-exchange reaction of TBA4[γ-H4SiW10O36] (TBA
= tetra-n-butylammonium) with 2 equiv of Co­(NO3)2 with respect to TBA4[γ-H4SiW10O36] in a mixed solvent of DMF
and acetone (97% yield). Each Co-SiW10 was linked by
water molecules via a hydrogen-bonding network. Besides Co-SiW10, various kinds of isostructural M-SiW10 could be synthesized
via the same procedure as that for Co-SiW10 (M = Mn2+, Fe2+, Ni2+, Cu2+, Zn2+, and Cd2+). By the reaction of Co-SiW10 with 1 equiv of TBA6[γ-H2SiW10O36] in acetone, a silicotungstate dimer pillared by two
cobalt cations with a significantly slipped dimer configuration, TBA6[Co2(γ-H3SiW10O36)2]·3H2O (Co2), could
be synthesized. By the reaction of Co-SiW10 with 3 equiv
of TBAOH in acetone, a tetracobalt-containing sandwich-type silicotungstate,
TBA6[{Co­(H2O)}2(μ3-OH)2{Co­(H2O)2}2(γ-H2SiW10O36)2]·5H2O (Co4), could be synthesized. Compound Co4 possessed the tetracobalt–oxygen core, [{Co­(H2O)}2(μ3-OH)2{Co­(H2O)2}2]6+, identical with those of
previously reported Weakley-type sandwich polyoxometalates, [Co4(H2O)2(XM9O34)2]n− (X = P5+, Si4+, Ge4+, As5+ or V5+; M = Mo6+ or W6+). The reversible transformation
between these three compounds (Co-SiW10 ⇆ Co2, Co-SiW10 ⇆ Co4, and Co2 ⇆ Co4) took place by the addition
and/or subtraction of required components in appropriate solvents,
affording the desired products in high yields (71–93% yields).