Guest Molecules in a Layered Microporous Tin(IV) Phosphonate-Phosphate Material: Solid State NMR Studies

There is little systematic understanding of pore surfaces in layered microporous metal(IV) phosphate-phosphonate materials and their interactions with guest molecules. In this paper, we show how to probe the mobility of guest molecules in such poorly crystalline systems using multinuclear solid-state NMR and relaxation time measurements. Anisotropic motions of benzene- d₆ molecules absorbed on the pore walls of material Sn(O₃PC₆H₄PO₃)_0.85(O₃POH)_0.33 (1) have been recognized as the fast in-plane C₆ rotation due to metal-π interactions with pore walls. The benzene- d₆ absorption enthalpy due to Sn···π interactions has been determined as -Δ H = 5.9 kcal/mol. Specific interactions between pyridine and the pore walls of 1 have been observed as immobile pyridine, the population of which grows strongly at low temperatures to show thermodynamic parameters -Δ H of 5.0 kcal/mol and Δ S of -11.0 e.u. It has been suggested that these parameters characterize N···H-OP hydrogen bonding as a driving force for accumulation of immobile pyridine molecules in pores of compound 1.