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.