Kinetics of ligand exchange in solution: a quantitative mass spectrometry approach

Complex speciation and exchange kinetics of labile ligands are critical parameters for understanding the reactivity of metal complexes in solution. We present a novel approach to determine ligand exchange parameters based on electrospray ionization mass spectrometry (ESI-MS). The introduction of isotopically labelled ligands to a solution of metal host and unlabelled ligands allows the quantitative investigation of the solution-phase equilibria. Furthermore, ion mobility separation can target individual isomers, such as ligands bound at specific sites. As a proof of concept, we investigate the solution equilibria of labile pyridine ligands coordinated in the cavity of macrocyclic porphyrin cage complexes bearing diamagnetic or paramagnetic metal centres. The effects of solvent, porphyrin coordination sphere, transition metal, and counterion on ligand dissociation are discussed. Rate constants and activation parameters for ligand dissociation in the solution can be derived from our ESI-MS approach, thereby providing mechanistic insights that are not easily obtained from traditional solution-phase techniques. Delayed reactant labelling can provide thermodynamic parameters (& UDelta;G, & UDelta;H, and & UDelta;S) of solution equilibria using ESI-MS monitoring. As a proof of concept, we investigated the equilibria of pyridine ligands with macrocyclic porphyrin cage complexes.

Automated summary

We present a new approach to determine ligand exchange parameters based on electrospray ionization mass spectrometry (ESI-MS). This method allows for the quantitative investigation of the solution-phase equilibria by introducing isotopically labelled ligands to a solution of metal host and unlabelled ligands. Our ESI-MS approach provides rate constants and activation parameters for ligand dissociation in the solution, providing mechanistic insights that are not easily obtained from traditional solution-phase techniques.