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: The final distribution of the divalent cations [Fe(L)3]2+ developed from the ligand combinations L2 and L4, L2 and L3, and L1 and L4 after 24 h of equilibration also provided informative data (Figure 6). In the majority of ligand combinations with the counter tetrafluoroborates and perchlorate salts, the distribution of products proved to be similar to that seen with L1 and L3, which is consistent with equilibrium having been achieved in all but one case; the distribution for the ester combination of L2 and L4 with Fe(BF4)2 was still dominated by peaks assigned to [Fe(L2)3]2+ and [Fe(L4)3]2+. The ligand combination L2 with L3 in the presence of FeCl2 does not follow the expected 1:3:3:1 distribution of [Fe(L2)3]2+, [Fe(L2)2(L3)]2+, [Fe(L2)(L3)2]2+, and [Fe(L3)3]2+, with the observed dominance of [Fe(L2)3]2+ although it would appear that in this case the distribution of products has come to equilibrium given the depletion of [Fe(L3)3]2+ from the system. This is further exaggerated with ligands L1 and L4, with complexation of the amide L1 dominating the coordination of the metal centre, with the inference being that the chloride anion “templates” the formation of the homoleptic amide species, presumably through the hydrogen bonding interactions of the amidic protons with the chloride in keeping with the findings of the previously reported ruthenium(II) complexes.8 This affect is also observed with the iron(II) complexes formed from FeBr2 and the L1 and L4 ligand system, but to a lesser extent.
Affiliation: School of Chemistry and Chemical Engineering, Queen's University Belfast, UK.