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The use of electrospray mass spectrometry to determine speciation in a dynamic combinatorial library for anion recognition.

Phillips HI, Chernikov AV, Fletcher NC, Ashcroft AE, Ault JR, Filby MH, Wilson AJ - Chemistry (2012)

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.

View Article: PubMed Central - PubMed

Affiliation: School of Chemistry and Chemical Engineering, Queen's University Belfast, UK.

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ESI-MS spectrum of ligands L1 and L3 with FeCl2 in acetonitrile 30 min after mixing (metal ion concentration=50 μm)
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fig03: ESI-MS spectrum of ligands L1 and L3 with FeCl2 in acetonitrile 30 min after mixing (metal ion concentration=50 μm)

Mentions: Equilibria in mixed ligand systems: The two amide-containing ligands L1 and L3 were selected for the initial studies on the composition of heteroleptic solutions given that the individual homoleptic metal complexes gave clean and interpretable mass spectra for the iron(II) complexes. The first step of the investigation was to establish whether the metal complexes were at equilibrium (Scheme 1) under the conditions required to provide interpretable spectra, and significantly whether the species being detected were representative of the solution composition. The two complexes [Fe(L1)3]Cl2 and [Fe(L3)3]Cl2 were prepared separately, and then mixed in a 1:1 ratio following the protocol outlined in preparation method A. An excess of both ligands was used (in this case, 6 equivalents) to each metal ion to minimize the presence of the bischelate species [Fe(L)2Cl]+. The spectra of the resulting pink solutions were then recorded over a period of 24 h establishing the time taken for the complexes to reach equilibrium at 298 K. With anhydrous FeCl2, using a little methanol to encourage solubility of the iron salt, the resulting mass spectra recorded after 30 min of mixing the two solutions demonstrated that the key regions of interest could be readily identified with peaks indicative of both bis- and tris-bipyridine species (Figure 3). Clusters of peaks are observed for the exchange complex species for [Fe(L1/L3)3]2+ at m/z 413–461, [Fe(L1/L3)2Cl]+ at m/z 606–669, and {[Fe(L1/L3)3]Cl}+ at m/z 862–958 ({[Fe(L1/L3)3]Cl}+. Over the course of the experiment, the relative intensities for both the [Fe(L1)/(L3)3]2+ (m/z 413, 429, 445, 461) and {[Fe(L1)/(L3)3]Cl}+ (m/z 862, 894, 926, 958) species were normalized and their relative change over time plotted (Figure 4). This simple system appears to follow first order kinetics (kCl=(1.7±0.1×10−4) s−1), with the system reaching equilibrium in the order of four hours demonstrating the anticipated speciation in the final products. Importantly the two species investigated, the divalent ion [Fe(L)3]2+ and the monovalent ion {[Fe(L)3]Cl}+, gave similar time plots despite having different relative intensities in the spectra analysed. Interestingly, the complexes formed from ligand L1 appears to be the more dominant over those composed of L3. Initially it was considered if this observation arose from the two different complexes having dissimilar ionization behaviour, resulting in lower detection of the species containing ligand L3, however the ion count for the complexes [Fe(L1)3]2+ and [Fe(L3)3]2+ under similar conditions were comparable indicating that the observed speciation in the heteroleptic system is probably representative of the composition of the solution, with ligand L1 having a greater thermodynamic stability on the metal centre than L3. One would have expected the more sterically demanding ligand to have shown the weaker relative binding but that does not appear to be the case.


The use of electrospray mass spectrometry to determine speciation in a dynamic combinatorial library for anion recognition.

Phillips HI, Chernikov AV, Fletcher NC, Ashcroft AE, Ault JR, Filby MH, Wilson AJ - Chemistry (2012)

ESI-MS spectrum of ligands L1 and L3 with FeCl2 in acetonitrile 30 min after mixing (metal ion concentration=50 μm)
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3569615&req=5

fig03: ESI-MS spectrum of ligands L1 and L3 with FeCl2 in acetonitrile 30 min after mixing (metal ion concentration=50 μm)
Mentions: Equilibria in mixed ligand systems: The two amide-containing ligands L1 and L3 were selected for the initial studies on the composition of heteroleptic solutions given that the individual homoleptic metal complexes gave clean and interpretable mass spectra for the iron(II) complexes. The first step of the investigation was to establish whether the metal complexes were at equilibrium (Scheme 1) under the conditions required to provide interpretable spectra, and significantly whether the species being detected were representative of the solution composition. The two complexes [Fe(L1)3]Cl2 and [Fe(L3)3]Cl2 were prepared separately, and then mixed in a 1:1 ratio following the protocol outlined in preparation method A. An excess of both ligands was used (in this case, 6 equivalents) to each metal ion to minimize the presence of the bischelate species [Fe(L)2Cl]+. The spectra of the resulting pink solutions were then recorded over a period of 24 h establishing the time taken for the complexes to reach equilibrium at 298 K. With anhydrous FeCl2, using a little methanol to encourage solubility of the iron salt, the resulting mass spectra recorded after 30 min of mixing the two solutions demonstrated that the key regions of interest could be readily identified with peaks indicative of both bis- and tris-bipyridine species (Figure 3). Clusters of peaks are observed for the exchange complex species for [Fe(L1/L3)3]2+ at m/z 413–461, [Fe(L1/L3)2Cl]+ at m/z 606–669, and {[Fe(L1/L3)3]Cl}+ at m/z 862–958 ({[Fe(L1/L3)3]Cl}+. Over the course of the experiment, the relative intensities for both the [Fe(L1)/(L3)3]2+ (m/z 413, 429, 445, 461) and {[Fe(L1)/(L3)3]Cl}+ (m/z 862, 894, 926, 958) species were normalized and their relative change over time plotted (Figure 4). This simple system appears to follow first order kinetics (kCl=(1.7±0.1×10−4) s−1), with the system reaching equilibrium in the order of four hours demonstrating the anticipated speciation in the final products. Importantly the two species investigated, the divalent ion [Fe(L)3]2+ and the monovalent ion {[Fe(L)3]Cl}+, gave similar time plots despite having different relative intensities in the spectra analysed. Interestingly, the complexes formed from ligand L1 appears to be the more dominant over those composed of L3. Initially it was considered if this observation arose from the two different complexes having dissimilar ionization behaviour, resulting in lower detection of the species containing ligand L3, however the ion count for the complexes [Fe(L1)3]2+ and [Fe(L3)3]2+ under similar conditions were comparable indicating that the observed speciation in the heteroleptic system is probably representative of the composition of the solution, with ligand L1 having a greater thermodynamic stability on the metal centre than L3. One would have expected the more sterically demanding ligand to have shown the weaker relative binding but that does not appear to be the case.

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.

View Article: PubMed Central - PubMed

Affiliation: School of Chemistry and Chemical Engineering, Queen's University Belfast, UK.

Show MeSH