Paramagnetic Shifts and Guest Exchange Kinetics in CoFe Metal-Organic Capsules.

We investigate the magnetic resonance properties and exchange kinetics of guest molecules in a series of hetero-bimetallic capsules, [CoFeL] ( = 1-3), where L = 4,4'-bis[(2-pyridinylmethylene)amino]-[1,1'-biphenyl]-2,2'-disulfonate. H bond networks between capsule sulfonates and guanidinium cations promote the crystallization of [CoFeL]. The following four isostructural crystals are reported: two guest-free forms, (C(NH))[CoFeL]·69HO () and (C(NH))[CoFeL]·73HO (), and two Xe- and CFCl-encapsulated forms, (C(NH))[(Xe)CoFeL]·69HO () and (C(NH))[(CFCl)CoFeL]·73HO (), respectively. Structural analyses reveal that Xe induces negligible structural changes in , while the angles between neighboring phenyl groups expand by ca. 3° to accommodate the much larger guest, CFCl, in . These guest-encapsulated [CoFeL] molecules reveal Xe and F chemical shift changes of ca. -22 and -10 ppm at 298 K, respectively, per substitution of low-spin Fe by high-spin Co. Likewise, the temperature dependence of the Xe and F NMR resonances increases by 0.1 and 0.06 ppm/K, respectively, with each additional paramagnetic Co center. The optimal temperature for hyperpolarized (hp) Xe chemical exchange saturation transfer (hyper-CEST) with [CoFeL] capsules was found to be inversely proportional to the number of Co centers, , which is consistent with the Xe chemical exchange accelerating as the portals expand. The systematic study was facilitated by the tunability of the [ML] capsules, further highlighting these metal-organic systems for developing responsive sensors with highly shifted Xe resonances.