Rare-Earth Metal Triflates Inclusion Complexes with Cobalt(II) Hemicryptophane Tren Cage

Abstract

Rare-earth metal triflates can modulate the structure, redox properties, and reactivity of transition-metal complexes when confined in supramolecular hosts. In this paper, we have investigated the inclusion of rare-earth metal triflates (Ln(OTf)₃, Ln = Dy, Yb, La, Lu) into a cobalt(II) hemicryptophane tren cage (CoCage^HCPT) using high-resolution electrospray ionization mass spectrometry (ESI-MS), cyclic traveling wave ion mobility mass spectrometry (cIM-TWIMS), host–guest exchange experiments, and examined the electrochemical properties using cyclic voltammetry. ESI-MS reveals the formation of dicationic inclusion complexes [Co^II(Cage^HCPT)(Ln(OTf)₃)]²⁺ with additional solvated adducts. Ion mobility measurements revealed the conformational flexibility of the empty cage complex, showing a broad distribution of isomers. In contrast, guest encapsulation yields defined geometries with only two dominant isomers, each with similar abundance and a close collisional cross-section in the range of 371–374 Ų. The DFT calculations revealed that the cap of the cage creates an attractive pocket for the lanthanum ion, while the side openings of the cage accommodate the coordinated triflate ions. In addition, one triflate anion is bridging, anchoring La(OTf)₃ to the cobalt center. This rigid structure leads to the formation of two diastereoisomers of the inclusion complex, corresponding to the experimentally detected isomers. Equilibration experiments demonstrated fast host–guest exchange kinetics (k_MS ≥ 0.5 s^–1), indicating dynamic binding of Ln(OTf)₃ within the cage cavity. Furthermore, cyclic voltammetry studies showed anodic shifts of the Co^II/I (0.6 V) and Co^I/0 (0.5 V) reduction waves upon Dy(OTf)₃ inclusion, indicating strong Lewis-acid effects on the cobalt center. Overall, the combined results reveal that lanthanoid triflates form dynamic yet structurally defined inclusion complexes that substantially tune the redox properties of the cobalt cage system.