Empirical Estimation of the Molecular Weight of Gold Complexes in Solution by Pulsed-Field Gradient NMR

Homogenous gold catalysis has emerged as a powerful tool in organic synthesis, but many mechanistic questions in that area remain unanswered. The ability of diffusion-ordered NMR spectroscopy (DOSY) to investigate interactions between substrates, to study molecular assemblies, and to characterize reactive intermediates makes it a powerful technique to get a better understanding of gold-catalyzed processes. In this article, we show that a simple empirical approach makes it possible to derive estimates of the molecular weight of gold complexes from their translational diffusion coefficients measured with DOSY. This approach is based on external calibration curves (ECC), modified to take into account the molecular density of gold complexes. The estimated molecular weights in turn provide information on interactions, as notably illustrated here with ion pairing, and have the potential to provide key information to rationalize the reactivity of reagents in gold-catalyzed reactions.