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, [Co_nFe_4-nL₆]^4- (n = 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_nFe_4-nL₆]^4-. The following four isostructural crystals are reported: two guest-free forms, (C(NH₂)₃)₄[Co_1.8Fe_2.2L₆]·69H₂O (1) and (C(NH₂)₃)₄[Co_2.7Fe_1.3L₆]·73H₂O (2), and two Xe- and CFCl₃-encapsulated forms, (C(NH₂)₃)₄[(Xe)_0.8Co_1.8Fe_2.2L₆]·69H₂O (3) and (C(NH₂)₃)₄[(CFCl₃)Co_2.0Fe_2.0L₆]·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 [Co_nFe_4-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 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_nFe_4-nL₆]^4- capsules was found to be inversely proportional to the number of Co^II 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.

Figure 1.

Preparation schemes and structures for 1 and 2 (a), 3 (b), and 4 (c). Disordered H₂O molecules and H atoms are omitted for clarity. Green, purple, yellow, cyan, red, blue, and gray spheres represent Co/Fe, Cl, S, F, O, N, and C atoms, respectively.

Figure 2.

Variable-temperature ¹⁹F NMR spectra for an aqueous sample containing 5 mM 4. Legend indicates the experimental temperature in the unit of K. Dotted boxes indicate the ¹⁹F peak assignments for [(CFCl₃)M^II₄L₆]^4– molecules and * suggests the position for free CFCl₃, which is used as reference.

Figure 3.

Stacked ¹²⁹Xe NMR spectra for aqueous samples containing 200 μM 1 (black and red), 2 (blue), and 100 μM 5 (orange) collected at 278 K, with Eburp1-shaped pulses (660 Hz width) applied at –22.6, –49.7, –72.8, and –94.4 ppm, respectively.

Figure 4.

Z-spectra of an aqueous solution containing 10 μM 1 collected at 298 K with 2.3 s presaturation pulses applied at 5 ppm intervals with an averaged B₁ = 30 μT. Highlighted regions in pink, gray, red, blue, and orange indicated the resonances at +15, –20, –45, –70, and –90 ppm.

Figure 5.

Top: Variable-temperature z-spectra for an aqueous sample containing 10 μM 1 collected with 2.3 s presaturation pulses applied at 2 ppm intervals with an averaged saturation power B₁ = 12 μT. The legend indicates the experimental temperatures in the unit of K. Circles and solid lines represent experimental data and Lorentzian fits, respectively, except for 278 K, where the purple solid line represents connection of experimental data points. Bottom: Temperature-dependent ¹²⁹Xe chemical shifts for [(Xe)CoFe₃L₆]^4– (black), [(Xe)Co₂Fe₂L₆]^4– (red), and [(Xe)Co₃FeL₆]^4– (blue), which are summarized from variable-temperature CEST experiments. The chemical shifts are referenced to ¹²⁹Xe(aq) signal.