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.