Probing the Self-Assembly Mechanism of Lanthanide-Containing Sandwich-Type Silicotungstates [{Ln(H<sub>2</sub>O)<sub><i>n</i></sub>}<sub>2</sub>{Mn<sub>4</sub>(B-α-SiW<sub>9</sub>O<sub>34</sub>)<sub>2</sub>­(H<sub>2</sub>O)<sub>2</sub>}]<sup>6–</sup> Using Time-Resolved Mass Spectrometry and X‑ray Crystallography

The reaction of [γ-SiW<sub>10</sub>O<sub>36</sub>]<sup>8–</sup> with Mn<sup>2+</sup> and Ln<sup>3+</sup> in an aqueous solution led to the isolation of a series of new lanthanide-containing sandwich-type polyoxometalates (POMs) [{Ln­(H<sub>2</sub>O)<sub><i>n</i></sub>}<sub>2</sub>­{Mn<sub>4</sub>(B-α-SiW<sub>9</sub>O<sub>34</sub>)<sub>2</sub>­(H<sub>2</sub>O)<sub>2</sub>}]<sup>6–</sup> (<b>1</b>–<b>5a</b>) (Ln = La (<b>1</b>), Nd (<b>2</b>), Gd (<b>3</b>), Dy (<b>4</b>), Er (<b>5</b>); <i>n</i> = 5, 6), which crystallize in the space groups <i>C</i>2/<i>c</i> with <i>a</i> = 33.0900(2)–32.9838(15) Å, <i>b</i> = 12.8044(10)–12.7526(6) Å, <i>c</i> = 22.8273(17)–22.6368(11) Å, <i>V</i> = 9669.2(12)–9519.7(8) Å<sup>3</sup>, <i>Z</i> = 2 (<b>1</b>, <b>2</b>); <i>P</i>1̅ with <i>a</i> = 11.9502(4)–11.8447(6) Å, <i>b</i> = 13.2203(4)–13.1164(5) Å, <i>c</i> = 15.8291(5)–15.8524(7) Å, <i>V</i> = 2221.25(13)–2189.95(18) Å<sup>3</sup>, <i>Z</i> = 1 (<b>3</b>, <b>4</b>, <b>5</b>), respectively. X-ray diffraction analysis reveals that they consist of two-dimensional networks based on a sandwich-type polyanion [Mn<sub>4</sub>(B-α-SiW<sub>9</sub>O<sub>34</sub>)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>]<sup>12–</sup> (<b>6a</b>, {Mn<sub>4</sub>(SiW<sub>9</sub>)<sub>2</sub>}) and lanthanide cations (Ln<sup>3+</sup>), which are further connected into three-dimensional frameworks by potassium cations for <b>3</b>, <b>4</b>, and <b>5</b>. The unprecedented combination of time-resolved electrospray ionization mass spectrometry (ESI-MS) studies and X-ray crystallography allows us not only to directly observe the in-solution rearrangement of divant anion [γ-SiW<sub>10</sub>O<sub>36</sub>]<sup>8–</sup> into the sandwich-type POM <b>6a</b> via an intermediate species [Mn<sub>3</sub>(B-β-SiW<sub>8</sub>O<sub>30</sub>(OH))­(B-β-SiW<sub>9</sub>O<sub>33</sub>(OH))­(H<sub>2</sub>O)]<sup>12–</sup> (<b>7a</b>, {Mn<sub>3</sub>(SiW<sub>8</sub>)­(SiW<sub>9</sub>)}) from ESI-MS results, but also to gain the solid-state structures of intermediate and final product isolated from reaction solutions from X-ray crystallography results, from which the self-assembly mechanism of the lanthanide-containing sandwich-type POMs <b>1</b>–<b>5a</b> was proposed.