Redox Tuning of Metals with High Coordination Numbers: Quantifying Systematic Charge Density Effects of Co-encapsulated Cations

Heavy element cations with large ionic radii naturally tend to adopt high coordination numbers (C.N. ≥ 8), endowing them with unique chemical and physical properties. Rational control of the redox chemistry or electronic properties of such systems with high C.N. values must overcome these ions’ high degree of structural freedom. Here, a tailored tripodal ligand has been used to establish stereochemical control over co-encapsulated cerium centers with C.N. = 7–9 and secondary monovalent (Li, Na, K) or divalent cations (Ca, Sr, Ba) with C.N. = 6–9. Spectroscopic and electrochemical studies reveal that Ce(III) and Ce(IV) forms of the complexes are accessible, enabling correlation of the characteristics manifested by the cerium cores with the properties of the incorporated secondary cations. The Ce(IV/III) reduction potential can be shifted systematically across a span of > 600 mV, representing a higher sensitivity to cation identity than in any tunable heterobimetallic nd- and 5f-element complexes studied to date. A structural model for understanding these effects was formulated on the basis of comprehensive X-ray diffraction analysis, revealing well-defined roles for both the ionic radius (position) and Lewis acidity (charge density) of the co-encapsulated secondary metal ions in influencing the cerium core. Formation of shared triangular faces between coordination polyhedra drives enables the high tuning sensitivity by generating an internuclear axis upon which the secondary cations adopt unique positions that are dictated by their size and coordination preferences rather than their charge states alone.