Tracking “Apolar” NMe4+ Ions within Two Polyoxothiomolybdates that Have the Same Pores: Smaller Clathrate and Larger Highly Porous Clusters in Action

Two nanosized polyoxothiometalates were synthesized based on linking oxomolybdate building blocks with {Mo2O2S2}2+ groups. Remarkably, both compounds are formed selectively primarily upon changing the related concentrations in a logical way; they exhibit common structural features based on the same {Mo9O6S3}-type pores, which result in connections between {Mo6O21} pentagons and {Mo2O2S2}2+ linkers. Whereas the much larger spherical Mo132-type Keplerate contains twenty pores, the smaller Mo63-type cluster remarkably contains only two. The two compounds and a similar Keplerate exhibit interesting supramolecular properties related to interactions with the unusual predominantly apolar NMe4+ cations. Structural characterization of the Mo63-type compound reveals in the solid state a clathrate-like species that contains four NMe4+ cations embedded in two types of structurally well-adapted pockets. Related NMR spectroscopic investigations in solution using NMe4+ as the NMR spectroscopic probe are in agreement with the solid-state description. 1H NMR spectroscopic experiments (1D variable-temperature, 2D total correlation spectroscopy (TOCSY), exchange spectroscopy (EXSY), and diffusion-ordered spectroscopy (DOSY)) feature firmly immobilized and mobile NMe4+ ions in relationship with the type of host–guest arrangements. The use of the 1H NMR DOSY spectroscopic methodology has been successfully applied to track the interactions of the NMe4+ cations with the {Mo9O6S3} pores of a sulfurated Keplerate, thereby allowing the first quantitative analysis of this type of plugging process. The stability constant K=(210±20) mol−1 L is discussed related to the character of the process.