Triol-Ligand
Modification and Structural Transformation of Anderson–Evans
Oxomolybdates via Modulating Oxidation State of Co-Heteroatom
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Abstract
Various covalent decorations and
structural transformations of a series of R–C(CH<sub>2</sub>OH)<sub>3</sub> triol ligands on Co<sup>II</sup>/Co<sup>III</sup>-centered Anderson–Evans polyoxomolybdates were studied in
aqueous and/or organic solution. Single-crystal X-ray structural analysis
demonstrated that four manners, including (1) double-sided χ/χ-isomer
and (2) double-sided δ/χ-isomer with Co<sup>II</sup> central
heteroatom, and (3) single-sided δ-isomer and (4) double-sided
δ/δ-isomer with Co<sup>III</sup> central heteroatom, appeared
on the decoration of the mother polyanion [Co<sup>II/III</sup>(OH)<sub>6</sub>Mo<sub>6</sub>O<sub>18</sub>]<sup>4–/3–</sup>. Through careful manipulation of the reaction conditions, a gradual
oxidation process from Co<sup>II</sup> to Co<sup>III</sup> was disclosed
to happen in air, and accompanying that process, the Co<sup>II</sup>-centered double-sided δ/χ-isomer that formed in the
initial stage transformed into a Co<sup>III</sup>-centered double-sided
δ/δ-isomer. When the electron-withdrawing group −NO<sub>2</sub> substituted triol ligand was used to replace CH<sub>3</sub>C(CH<sub>2</sub>OH)<sub>3</sub>, the oxidation process accelerated
spontaneously. A further investigation showed that this oxidation
process can be blocked by acid. While the triol ligands’ decoration
on Anderson–Evans polyoxometalates (POMs) took place along
with the oxidation of the central heteroatom, that codecoration of
other components was also accomplished. In the presence of excess
acetic acid, an unprecedented single-sided δ-isomer with an
acetate ligand covalently attaching on the other side by replacing
one μ<sub>3</sub>-O atom was obtained in aqueous solution. The
reaction of the mother cluster [(<i>n</i>-C<sub>4</sub>H<sub>9</sub>)<sub>4</sub>N]<sub>3</sub>[Co(OH)<sub>6</sub>Mo<sub>6</sub>O<sub>18</sub>] and triol ligands in methanol resulted in a codecoration
of methanol with the double-sided δ/δ-isomer, which has
never been reported in B-type Anderson–Evans POMs. The obtained
results revealed that the oxidation state of the heteroatom has a
significant impact on the triol ligand decoration styles, such as
the χ/χ- or δ/χ-isomer for the divalent heteroatom
and the δ- or δ/δ-isomer for the trivalent heteroatom
