Probing the Self-Assembly Mechanism of Lanthanide-Containing Sandwich-Type Silicotungstates [{Ln(H₂O)_n}₂{Mn₄(B-α-SiW₉O₃₄)₂­(H₂O)₂}]^6– Using Time-Resolved Mass Spectrometry and X‑ray Crystallography

The reaction of [γ-SiW₁₀O₃₆]^8– with Mn²⁺ and Ln³⁺ in an aqueous solution led to the isolation of a series of new lanthanide-containing sandwich-type polyoxometalates (POMs) [{Ln­(H₂O)_n}₂­{Mn₄(B-α-SiW₉O₃₄)₂­(H₂O)₂}]^6– (1–5a) (Ln = La (1), Nd (2), Gd (3), Dy (4), Er (5); n = 5, 6), which crystallize in the space groups C2/c with a = 33.0900(2)–32.9838(15) Å, b = 12.8044(10)–12.7526(6) Å, c = 22.8273(17)–22.6368(11) Å, V = 9669.2(12)–9519.7(8) ų, Z = 2 (1, 2); P1̅ with a = 11.9502(4)–11.8447(6) Å, b = 13.2203(4)–13.1164(5) Å, c = 15.8291(5)–15.8524(7) Å, V = 2221.25(13)–2189.95(18) ų, Z = 1 (3, 4, 5), respectively. X-ray diffraction analysis reveals that they consist of two-dimensional networks based on a sandwich-type polyanion [Mn₄(B-α-SiW₉O₃₄)₂(H₂O)₂]^12– (6a, {Mn₄(SiW₉)₂}) and lanthanide cations (Ln³⁺), which are further connected into three-dimensional frameworks by potassium cations for 3, 4, and 5. 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-SiW₁₀O₃₆]^8– into the sandwich-type POM 6a via an intermediate species [Mn₃(B-β-SiW₈O₃₀(OH))­(B-β-SiW₉O₃₃(OH))­(H₂O)]^12– (7a, {Mn₃(SiW₈)­(SiW₉)}) 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 1–5a was proposed.