Paramagnetic Organocobalt Capsule Revealing Xenon Host–Guest Chemistry

We investigated Xe binding in a previously reported paramagnetic metal–organic tetrahedral capsule, [Co₄L₆]^4–, where L^2– = 4,4′-bis­[(2-pyridinylmethylene)­amino]­[1,1′-biphenyl]-2,2′-disulfonate. The Xe-inclusion complex, [XeCo₄L₆]^4–, was confirmed by ¹H NMR spectroscopy to be the dominant species in aqueous solution saturated with Xe gas. The measured Xe dissociation rate in [XeCo₄L₆]^4–, k_off = 4.45(5) × 10² s^–1, was at least 40 times greater than that in the analogous [XeFe₄L₆]^4– complex, highlighting the capability of metal–ligand interactions to tune the capsule size and guest permeability. The rapid exchange of ¹²⁹Xe nuclei in [XeCo₄L₆]^4– produced significant hyperpolarized ¹²⁹Xe chemical exchange saturation transfer (hyper-CEST) NMR signal at 298 K, detected at a concentration of [XeCo₄L₆]^4– as low as 100 pM, with presaturation at −89 ppm, which was referenced to solvated ¹²⁹Xe in H₂O. The saturation offset was highly temperature-dependent with a slope of −0.41(3) ppm/K, which is attributed to hyperfine interactions between the encapsulated ¹²⁹Xe nucleus and electron spins on the four Co^II centers. As such, [XeCo₄L₆]^4– represents the first example of a paramagnetic hyper-CEST (paraHYPERCEST) sensor. Remarkably, the hyper-CEST ¹²⁹Xe NMR resonance for [XeCo₄L₆]^4– (δ = −89 ppm) was shifted 105 ppm upfield from the diamagnetic analogue [XeFe₄L₆]^4– (δ = +16 ppm). The Xe inclusion complex was further characterized in the crystal structure of (C­(NH₂)₃)₄[Xe_0.7Co₄L₆]·75 H₂O (1). Hydrogen bonding between capsule-linker sulfonate groups and exogenous guanidinium cations, (C­(NH₂)₃)⁺, stabilized capsule–capsule interactions in the solid state and also assisted in trapping a Xe atom (∼42 ų) in the large (135 ų) cavity of 1. Magnetic susceptibility measurements confirmed the presence of four noninteracting, magnetically anisotropic high-spin Co^II centers in 1. Furthermore, [Co₄L₆]^4– was found to be stable toward aggregation and oxidation, and the CEST performance of [XeCo₄L₆]^4– was unaffected by biological macromolecules in H₂O. These results recommend metal–organic capsules for fundamental investigations of Xe host–guest chemistry as well as applications with highly sensitive ¹²⁹Xe-based sensors.