All-Inorganic Networks and Tetramer Based on Tin(II)-Containing Polyoxometalates: Tuning Structural and Spectral Properties with Lone-Pairs

Two MOF-like but all-inorganic polyoxometalate-based networks, [Na7X2W18Sn9Cl5O68·(H2O)m]n (1, X = Si, m = 35; 2, X = Ge, m = 41), and the molecular tetramer Na6[{Na­(μ-OH2)­(OH2)2}6{Sn6(B-SbW9O33)2}2]·50H2O (3) have been prepared and characterized by X-ray diffraction and spectroscopic methods. All three compounds exhibit unique structural features, and networks 1 and 2 incorporate the highest nuclearity of Sn­(II)-containing POMs to date. Tetramer 3 comprises bridging Sn­(II) ions with [B-SbW9O33]9– units and exhibits two highly unusual features, a long-range Sb···Sb interaction and an intramolecular charge-transfer transition involving donation of the lone-pair electron density on both Sb­(III) and Sn­(II) to the POM. The electronic structure and excited-state dynamics have been studied by transient spectroscopy, spectroelectrochemistry, DFT calculations, and resonance Raman spectroscopy. The synergistic effect of two types of stereoactive lone-pairs on Sb­(III) and Sn­(II) is critical for the charge-transfer absorption feature in the visible.