The use of electrospray mass spectrometry to determine speciation in a dynamic combinatorial library for anion recognition.
Bottom Line: The time taken for the complexes to come to equilibrium appears to be dependent on the counteranion, with chloride causing a rapid redistribution of two preformed heteroleptic complexes (of the order of 1 hour), whereas the time it takes in the presence of tetrafluoroborate salts is in excess of 24 h.Similarly the final distribution of products is dependent on the anion present, with the presence of chloride, and to a lesser extent bromide, preferring three amide-functionalized ligands, and a slight preference for an appended benzyl over a methoxyethyl group.Furthermore, for the first time, this study shows that the distribution of a dynamic library of metal complexes monitored by ESI-MS can adapt following the introduction of a different anion, in this case tetrabutylammonium chloride to give the most favoured heteroleptic complex despite the increasing ionic strength of the solution.
Affiliation: School of Chemistry and Chemical Engineering, Queen's University Belfast, UK.Show MeSH
Mentions: Identification of appropriate ligand systems: The primary objective of this study was to demonstrate that a combinatorial library of products can first be appropriately speciated by using routine electrospray spectroscopy, and then demonstrate that the library can be perturbed by the introduction of an external stimulus. Having selected labile iron(II) complexes of 2,2′-bipyridine and armed with the understanding that substitution at the 5-position of the ligands with amidic functions is known to give a relatively strong interaction with electronegative anions,6–8 we selected chloride as a suitable component to add to the combinatorial library, given its benign nature under normal electrospray conditions assuming that it does not cause either a significant perturbation in the ionic strength or precipitation. The problem with many 2,2′-bipyridine amide-functionalized ligand systems reported for the recognition of anions44 using the tris-chelating ruthenium(II) centre is their solubility.6, 45 To overcome this problem, and the possibility that inter-ligand steric interactions can dominate larger ligand systems around the iron(II) centre, 5-benzylamido-2,2′-pyridine (L1, Figure 1) was selected with the aromatic group separated from the amide by a flexible methylene spacer, and only one functional group per ligand. This ligand system has previously been shown to have a good response to the introduction of anions in the complex mer- and fac-[Ru(L1)3]2+.8
Affiliation: School of Chemistry and Chemical Engineering, Queen's University Belfast, UK.