Paramagnetic Organocobalt Capsule Revealing Xenon Host-Guest Chemistry.

We investigated Xe binding in a previously reported paramagnetic metal-organic tetrahedral capsule, [CoL], where L = 4,4'-bis[(2-pyridinylmethylene)amino][1,1'-biphenyl]-2,2'-disulfonate. The Xe-inclusion complex, [XeCoL], was confirmed by H NMR spectroscopy to be the dominant species in aqueous solution saturated with Xe gas. The measured Xe dissociation rate in [XeCoL], = 4.45(5) × 10 s, was at least 40 times greater than that in the analogous [XeFeL] complex, highlighting the capability of metal-ligand interactions to tune the capsule size and guest permeability. The rapid exchange of Xe nuclei in [XeCoL] produced significant hyperpolarized Xe chemical exchange saturation transfer (hyper-CEST) NMR signal at 298 K, detected at a concentration of [XeCoL] as low as 100 pM, with presaturation at -89 ppm, which was referenced to solvated Xe in HO. 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 Xe nucleus and electron spins on the four Co centers. As such, [XeCoL] represents the first example of a paramagnetic hyper-CEST (paraHYPERCEST) sensor. Remarkably, the hyper-CEST Xe NMR resonance for [XeCoL] (δ = -89 ppm) was shifted 105 ppm upfield from the diamagnetic analogue [XeFeL] (δ = +16 ppm). The Xe inclusion complex was further characterized in the crystal structure of (C(NH))[XeCoL]·75 HO (). Hydrogen bonding between capsule-linker sulfonate groups and exogenous guanidinium cations, (C(NH)), stabilized capsule-capsule interactions in the solid state and also assisted in trapping a Xe atom (∼42 Å) in the large (135 Å) cavity of . Magnetic susceptibility measurements confirmed the presence of four noninteracting, magnetically anisotropic high-spin Co centers in . Furthermore, [CoL] was found to be stable toward aggregation and oxidation, and the CEST performance of [XeCoL] was unaffected by biological macromolecules in HO. These results recommend metal-organic capsules for fundamental investigations of Xe host-guest chemistry as well as applications with highly sensitive Xe-based sensors.