Paramagnetic Shifts and Guest Exchange Kinetics in Co_<i>n</i>Fe_4–<i>n</i> Metal–Organic Capsules

Abstract

We investigate the magnetic resonance properties and exchange kinetics of guest molecules in a series of hetero-bimetallic capsules, [Co_<i>n</i>Fe_4-<i>n</i>L₆]^4- (<i>n</i> = 1-3), where L^2- = 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 [Co_<i>n</i>Fe_4-<i>n</i>L₆]^4-. The following four isostructural crystals are reported: two guest-free forms, (C(NH₂)₃)₄[Co_1.8Fe_2.2L₆]·69H₂O (<b>1</b>) and (C(NH₂)₃)₄[Co_2.7Fe_1.3L₆]·73H₂O (<b>2</b>), and two Xe- and CFCl₃-encapsulated forms, (C(NH₂)₃)₄[(Xe)_0.8Co_1.8Fe_2.2L₆]·69H₂O (<b>3</b>) and (C(NH₂)₃)₄[(CFCl₃)Co_2.0Fe_2.0L₆]·73H₂O (<b>4</b>), respectively. Structural analyses reveal that Xe induces negligible structural changes in <b>3</b>, while the angles between neighboring phenyl groups expand by ca. 3° to accommodate the much larger guest, CFCl₃, in <b>4</b>. These guest-encapsulated [Co_<i>n</i>Fe_4-<i>n</i>L₆]^4- molecules reveal ¹²⁹Xe and ¹⁹F chemical shift changes of ca. -22 and -10 ppm at 298 K, respectively, per substitution of low-spin Fe^II by high-spin Co^II. Likewise, the temperature dependence of the ¹²⁹Xe and ¹⁹F NMR resonances increases by 0.1 and 0.06 ppm/K, respectively, with each additional paramagnetic Co^II center. The optimal temperature for hyperpolarized (hp) ¹²⁹Xe chemical exchange saturation transfer (hyper-CEST) with [Co_<i>n</i>Fe_4-<i>n</i>L₆]^4- capsules was found to be inversely proportional to the number of Co^II centers, <i>n</i>, which is consistent with the Xe chemical exchange accelerating as the portals expand. The systematic study was facilitated by the tunability of the [M₄L₆]^4- capsules, further highlighting these metal-organic systems for developing responsive sensors with highly shifted ¹²⁹Xe resonances.