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Using molten salts to probe outer-coordination sphere effects on lanthanide()/() electron-transfer reactions
Ist Teil von
Dalton transactions : an international journal of inorganic chemistry, 2021-11, Vol.5 (43), p.15696-1571
Ort / Verlag
Cambridge: Royal Society of Chemistry
Erscheinungsjahr
2021
Quelle
Alma/SFX Local Collection
Beschreibungen/Notizen
Controlling structure and reactivity by manipulating the outer-coordination sphere around a given reagent represents a longstanding challenge in chemistry. Despite advances toward solving this problem, it remains difficult to experimentally interrogate and characterize outer-coordination sphere impact. This work describes an alternative approach that quantifies outer-coordination sphere effects. It shows how molten salt metal chlorides (MCl
n
; M = K, Na,
n
= 1; M = Ca,
n
= 2) provided excellent platforms for experimentally characterizing the influence of the outer-coordination sphere cations (M
n
+
) on redox reactions accessible to lanthanide ions; Ln
3+
+ e
1−
→ Ln
2+
(Ln = Eu, Yb, Sm; e
1−
= electron). As a representative example, X-ray absorption spectroscopy and cyclic voltammetry results showed that Eu
2+
instantaneously formed when Eu
3+
dissolved in molten chloride salts that had strongly polarizing cations (like Ca
2+
from CaCl
2
)
via
the Eu
3+
+ Cl
1−
→ Eu
2+
+ ½Cl
2
reaction. Conversely, molten salts with less polarizing outer-sphere M
1+
cations (
e.g.
, K
1+
in KCl) stabilized Ln
3+
. For instance, the Eu
3+
/Eu
2+
reduction potential was >0.5 V more positive in CaCl
2
than in KCl. In accordance with first-principle molecular dynamics (FPMD) simulations, we postulated that hard M
n
+
cations (high polarization power) inductively removed electron density from Ln
n
+
across Ln-Cl M
n
+
networks and stabilized electron-rich and low oxidation state Ln
2+
ions. Conversely, less polarizing M
n
+
cations (like K
1+
) left electron density on Ln
n
+
and stabilized electron-deficient and high-oxidation state Ln
3+
ions.
Molten salt matrices were used to evaluate outer-coordination sphere effects on lanthanide redox chemistry. Results were rationalized by correlating the polarization power of the outer-sphere cation with shifts in the Ln
3+
/Ln
2+
reduction potentials.