Open AccessParamagnetic Shifts and Guest Exchange Kinetics in ConFe4–n Metal–Organic Capsules
Author(s) -
Kang Du,
Serge D. Zemerov,
Patrick J. Carroll,
Ivan J. Dmochowski
Publication year - 2020
Publication title -
inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.348
H-Index - 233
eISSN - 1520-510X
pISSN - 0020-1669
DOI - 10.1021/acs.inorgchem.0c01816
Subject(s) - chemistry , isostructural , bimetallic strip , paramagnetism , molecule , crystallography , chemical shift , metal , biphenyl , heteronuclear molecule , nuclear magnetic resonance spectroscopy , stereochemistry , crystal structure , organic chemistry , physics , quantum mechanics
We investigate the magnetic resonance properties and exchange kinetics of guest molecules in a series of hetero-bimetallic capsules, [Co n Fe 4- n L 6 ] 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 n Fe 4- n L 6 ] 4- . The following four isostructural crystals are reported: two guest-free forms, (C(NH 2 ) 3 ) 4 [Co 1.8 Fe 2.2 L 6 ]·69H 2 O ( 1 ) and (C(NH 2 ) 3 ) 4 [Co 2.7 Fe 1.3 L 6 ]·73H 2 O ( 2 ), and two Xe- and CFCl 3 -encapsulated forms, (C(NH 2 ) 3 ) 4 [(Xe) 0.8 Co 1.8 Fe 2.2 L 6 ]·69H 2 O ( 3 ) and (C(NH 2 ) 3 ) 4 [(CFCl 3 )Co 2.0 Fe 2.0 L 6 ]·73H 2 O ( 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, CFCl 3 , in 4 . These guest-encapsulated [Co n Fe 4- n L 6 ] 4- molecules reveal 129 Xe and 19 F 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 129 Xe and 19 F 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) 129 Xe chemical exchange saturation transfer (hyper-CEST) with [Co n Fe 4- n L 6 ] 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 [M 4 L 6 ] 4- capsules, further highlighting these metal-organic systems for developing responsive sensors with highly shifted 129 Xe resonances.