Journal article
Fundamental factors controlling the exchange of multidentate ligands: displacement of 12-crown-4 and triglyme from complexes with divalent alkaline earth cations
International journal of mass spectrometry, Vol.195, pp.639-652
01/21/2000
DOI: 10.1016/S1387-3806(99)00199-2
Abstract
In an effort to shed light on the factors that influence the recognition of alkaline earth cations in natural systems, we have studied intrinsic recognition of these cations by well-ordered synthetic ionophores such as crown ethers (12-crown-4 [C4] and 18-crown-6 [C6]) as well as the acyclic analog of C4, triglyme (TG), in the gas phase. We have employed electrospray ionization (ESI) to generate gas phase crown and glyme alkaline earth complexes, and have used Fourier transform ion cyclotron resonance mass spectrometry to measure rate constants for displacement of the original ligands by C6. ESI of mixtures of C4 and TG with alkaline earths primarily produces sandwich complexes of the doubly charged cations, (C4)
2M
2+, (C4)(TG)M
2+, and (TG)
2M
2+. We find that the ligand exchange reactions are generally very efficient, with rates approaching or exceeding the Langevin collision rate in most cases. Trends in rates as metal size varies can be understood in terms of the degree of encapsulation of the metal by the ligands when the coordination shell is partially filled (smaller metals are more thoroughly encapsulated and tend to react more slowly) and in terms of the polarizing power of the metal cation when the metals are either “bare” or completely coordinated (smaller metals have greater charge density and tend to react more rapidly). Efficiencies for most of the reactions studied fall off in the order Mg
2+ > Ca
2+ > Sr
2+ > Ba
2+, consistent with decreasing charge density as the cation radius increases. Interestingly, TG is displaced more efficiently than C4 by C6, despite the fact that the total binding energy of the glyme is greater than that of the crown. This is consistent with a mechanism wherein the rate-limiting step involves breaking O–M
2+ electrostatic bonds, and where the bonds to the oxygens of TG can be broken one at a time, whereas the more rigid ring structure of C4 requires concerted breaking of multiple bonds. Molecular dynamics simulations of this process for complexes where M
2+ = Ca
2+ give support to this interpretation: in all observed dissociation events, TG oxygens were removed from the metal one at a time, whereas displacement of C4 oxygens occurred in pairs.
Details
- Title: Subtitle
- Fundamental factors controlling the exchange of multidentate ligands: displacement of 12-crown-4 and triglyme from complexes with divalent alkaline earth cations
- Creators
- Nanzhu Shen - Brigham Young UniversityR.Marshall Pope - Brigham Young UniversityDavid V. Dearden - Brigham Young University
- Resource Type
- Journal article
- Publication Details
- International journal of mass spectrometry, Vol.195, pp.639-652
- Publisher
- Elsevier B.V
- DOI
- 10.1016/S1387-3806(99)00199-2
- ISSN
- 1387-3806
- eISSN
- 1873-2798
- Language
- English
- Date published
- 01/21/2000
- Academic Unit
- Medicine Administration
- Record Identifier
- 9984627195802771
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