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 (1)H NMR spectroscopy to be the dominant species in aqueous solution saturated with Xe gas. The measured Xe dissociation rate in [XeCo4L6](4-), koff = 4.45(5) x 10(2) 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 (129)Xe nuclei in [XeCo4L6](4-) produced significant hyperpolarized (129)Xe 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 (129)Xe 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 (129)Xe 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 (129)Xe NMR resonance for [XeCo4L6](4-) (delta = -89 ppm) was shifted 105 ppm upfield from the diamagnetic analogue [XeFe4L6](4-) (delta = +16 ppm). The Xe inclusion complex was further characterized in the crystal structure of (C(NH2)3)4[Xe0.7Co4L6].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 ( approximately 42 A(3)) in the large (135 A(3)) 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 (129)Xe-based sensors.