A Quantitative Approach of the Interaction between Metal Triflates and Organic Ligands Using Electrospray Mass Spectrometry

Abstract

The interaction between two Lewis "superacid" catalysts Zn(OTf) 2 and In(OTf) 3 and series of amide and phosphate ligands is quantitatively characterized by electrospray ionization mass spectrometry (ESI-MS). A specific feature of the ESI-MS spectra of the mixture of metal triflates and Lewis bases is the formation of ionic adducts resulting from the displacement of one triflate anion by two neutral ligands. A ligand competition model is developed, which describes the relative intensities of the ionic adducts as a function of relative ligand concentrations. The relative affinities deduced from the ligand competition method are combined in an affinity scale for the metal triflate.

Quantitative Ligand Affinity Scales for Metal Triflate Salts: Application to Isomer Differentiation

“…The interesting “superacidic” properties of the triflates and related triflimides were examined in previous studies by using ESI‐MS . Based on the competition between two ligands to bind a metal center in solution, this method was recently introduced to quantify the relative affinity of In(OTf) 3 and Zn(OTf) 2 catalysts to amides and phosphates, and was also applied to several other metal triflates . When these metal salts are mixed with sufficiently basic organic molecules, a displacement of one anion by two equivalents of the Lewis base is observed.…”

“…In general, this reaction was observed with a high efficiency when using nitromethane as a solvent, the mixed metal adducts [(OTf) n −1 ML 2 ] + being the most intense in the positive mode (ESI + ) spectra. On the basis of the OTf − displacement by two different ligands (L 1 and L 2 ), Equations (2 a)– c), a methodology for building quantitative scales of relative affinities of ligands for the metal cation centers was set up …”

Catalytic Activation of Olefins by Metal Triflates and Triflimides: Application to Fragrance Chemistry

“…The attachment of two neutral ligands or substrates L opens the possibility to study competitive adduct formation between two different substrates and thus get some information on the selectivity of coordination of two different functional groups to a particular metal centre. Orders of coordination to several metal triflates were obtained for series of amide and phosphate ligands, indicating some different basicity order of the ligands as a function of the nature of the metal cation 60. These mass spectrometry experiments may provide further insight into more quantitative data on relative substrate‐catalysts interactions, allowing a better and more rational choice of a precise catalyst for a given reaction.…”

Bond Strength and Reactivity Scales for Lewis Superacid Adducts: A Comparative Study with In(OTf)3 and Al(OTf)3

“…The details of the synthesis and characterization, detailed experimental procedures and NMR spectra of the amide ligands 7,8,9,10 and 12 are provided in the Supporting Information. The other ligands were purchased.…”

“…In the quest for information regarding Lewis‐base interactions, some aspects of donor/acceptor adduct formation were studied using MS . Several experiments on metal triflates were conducted by ESI‐MS, and the major ions observed during the electrospray process were frequently due to the displacement of one triflate anion by two neutral ligands L ( m =2), leading to singly charged ions [Eq. ]. …”