Paramagnetic Shifts and Guest Exchange Kinetics in ConFe4-n Metal-Organic Capsules.

Abstract

We investigate the magnetic resonance properties and exchange kinetics of guest molecules in a series of hetero-bimetallic capsules, [ConFe4-nL6]4- (n = 1-3), where L2- = 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 [ConFe4-nL6]4-. The following four isostructural crystals are reported: two guest-free forms, (C(NH2)3)4[Co1.8Fe2.2L6]·69H2O (1) and (C(NH2)3)4[Co2.7Fe1.3L6]·73H2O (2), and two Xe- and CFCl3-encapsulated forms, (C(NH2)3)4[(Xe)0.8Co1.8Fe2.2L6]·69H2O (3) and (C(NH2)3)4[(CFCl3)Co2.0Fe2.0L6]·73H2O (4), respectively. Structural analyses reveal that Xe induces negligible structural changes in 3, while the angles between neighboring phenyl groups expand by ca. 3° to accommodate the much larger guest, CFCl3, in 4. These guest-encapsulated [ConFe4-nL6]4- molecules reveal 129Xe and 19F chemical shift changes of ca. -22 and -10 ppm at 298 K, respectively, per substitution of low-spin FeII by high-spin CoII. 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 CoII center. The optimal temperature for hyperpolarized (hp) 129Xe chemical exchange saturation transfer (hyper-CEST) with [ConFe4-nL6]4- capsules was found to be inversely proportional to the number of CoII centers, n, 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.