D'Angelo P, Migliorati V, Gibiino A, Busato M. Direct Observation of Contact Ion-Pair Formation in La
3+ Methanol Solution.
Inorg Chem 2022;
61:17313-17321. [PMID:
36255362 PMCID:
PMC9627567 DOI:
10.1021/acs.inorgchem.2c02932]
[Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
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An approach combining molecular dynamics (MD) simulations
and X-ray
absorption spectroscopy (XAS) has been used to carry out a comparative
study about the solvation properties of dilute La(NO3)3 solutions in water and methanol, with the aim of elucidating
the still elusive coordination of the La3+ ion in the latter
medium. The comparison between these two systems enlightened a different
behavior of the nitrate counterions in the two environments: while
in water the La(NO3)3 salt is fully dissociated
and the La3+ ion is coordinated by water molecules only,
the nitrate anions are able to enter the metal first solvation shell
to form inner-sphere complexes in methanol solution. The speciation
of the formed complexes showed that the 10-fold coordination is preferential
in methanol solution, where the nitrate anions coordinate the La3+ cations in a monodentate fashion and the methanol molecules
complete the solvation shell to form an overall bicapped square antiprism
geometry. This is at variance with the aqueous solution where a more
balanced situation is observed between the 9- and 10-fold coordination.
An experimental confirmation of the MD results was obtained by La
K-edge XAS measurements carried out on 0.1 M La(NO3)3 solutions in the two solvents, showing the distinct presence
of the nitrate counterions in the La3+ ion first solvation
sphere of the methanol solution. The analysis of the extended X-ray
absorption fine structure (EXAFS) part of the absorption spectrum
collected on the methanol solution was carried out starting from the
MD results and confirmed the structural arrangement observed by the
simulations.
The formation of contact ion pairs between
the La3+ ions and the nitrate anions has been demonstrated
in diluted methanol
solution using a combined approach using Molecular Dynamics simulations
and X-ray absorption spectroscpy.
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