Integrating ~(31)P DOSY NMR Spectroscopy and Molecular Mechanics as a Powerful Tool for Unraveling the Chemical Structures of Polyoxomolybdate-Based Amphiphilic Nanohybrids in Aqueous Solution

摘要

Novel organic-inorganic hybrids of various sizes were generated by reaction of 1,8-octanediphosphonic acid (ODP) and (NH_4)_6Mo_7O_(24) in aqueous solution. The formation of rodlike hybrids with variable numbers of covalently bound ODP and polyoxomolybdate (POM) units can be tuned as a function of increasing (NH_4)_6Mo_7O_(24) concentration at fixed ODP concentration. The chemical structure of the ODP/POM hybrids was characterized by ~1H ~(31)P, and ~(95)Mo NMR spectroscopy. Heteronuclear ~(31)P DOSY (diffusion-ordered NMR spectroscopy) and molecular mechanics (MM) calculations were applied to determine the size and shape of the nanosized hybrids generated at various ODP/POM ratios. For this purpose, the structures of ODP/POM hybrids with variable numbers of ODP and POM units were optimized by MM and then approximated as cylinder-shaped objects by using a recently described mathematical algorithm. The thus-obtained cylinder length and diameter were further used to calculate the expected diffusion coefficients of the ODP/POM hybrids. Comparison of the calculated and experimentally determined diffusion coefficients led to the most probable ODP/POM hybrid length for each sample composition. The ~(31)P DOSY results show that the length of the hybrids increases with increasing POM concentration and reaches a maximum corresponding to an average of 8ODP/7 POM units per chain at a sample composition of 20 mM ODP and 14 mM POM. With excess POM, above the latter concentration, the formation of shorter-chain hybrids terminated by Mo_7 clusters at one or both ends was evidenced on further increasing the POM concentration. The results demonstrate that the combination of ~(31)P DOSY and MM, although virtually unexplored in POM chemistry, is a powerful innovative strategy for the detailed characterization of nanosized organic-inorganic POM-based hybrids in solution.