Matrix assisted laser desorption/ionisation (MALDI)-TOF mass spectrometry of supramolecular metalloporphyrin assemblies: a survey

Abstract

Analysis of a representative series of metalloporphyrins and of their supramolecular assemblies using laser desorption/ionisation time of flight mass spectrometry (LDI-TOF MS) revealed that non-covalent metal–ligand interactions in the complexes remain effective in the gas phase. Detectable assemblies range from simple ruthenium( II) porphyrin complexes with N- or P-donor ligands, or tin( IV) and zirconium( IV) porphyrin complexes with O-donor ligands, up to trimeric arrays. Crucial to successful recording of intact complexes is avoiding protic matrices which interfere with Lewis acidic analytes and which may induce ligand exchange reactions to form complexes with the matrix itself. Also described is the in situ generation of a series of ruthenium( II) porphyrin dimers via laser induced vaporisation of the corresponding monomers and the suppression of dimerisation by nitrogen and phosphorus ligands. Careful adjustment of instrument parameters such as incident laser energy requires special attention. Relative affinities in complexation reactions are found to be consistent with liquid phase chemistry, therefore LDI-MS can provide a fast and simple, yet effective screening method for reactivity exploration. However, it also induces side-reactions such as iodine abstraction/transfer reactions. Furthermore, structurally different porphyrins show variations up to 90% in relative ion formation. Extreme caution is therefore required in the interpretation, and particularly in the quantitation, of such mass spectra.