Solid-State ²³Na NMR Study of Sodium Lariat Ether Receptors Exhibiting Cation−π Interactions

Abstract

Noncovalent cation-pi interactions are important in a variety of supramolecular and biochemical systems. We present a 23Na solid-state nuclear magnetic resonance (SSNMR) study of two sodium lariat ether complexes, 1 and 2, in which a sodium cation interacts with an indolyl group that models the side chain of tryptophan. Sodium-23 SSNMR spectra of magic-angle spinning (MAS) and stationary powdered samples have been acquired at three magnetic field strengths (9.4, 11.75, 21.1 T) and analyzed to provide key information on the sodium electric field gradient and chemical shift (CS) tensors which are representative of the cation-pi binding environment. Triple-quantum MAS NMR spectra acquired at 21.1 T clearly reveal two crystallographically distinct sites in both 1 and 2. The quadrupolar coupling constants, CQ(23Na), range from 2.92 +/- 0.05 MHz for site A of 1 to 3.33 +/- 0.05 MHz for site B of 2; these values are somewhat larger than those reported previously by Wong et al. (Wong, A.; Whitehead, R. D.; Gan, Z.; Wu, G. J. Phys. Chem. A 2004, 108, 10551) for NaBPh4, but very similar to the values obtained for sodium metallocenes by Willans and Schurko (Willans, M. J.; Schurko, R. W. J. Phys. Chem. B 2003, 107, 5144). We conclude from the 21.1 T data that the spans of the sodium CS tensors are less than 20 ppm for 1 and 2 and that the largest components of the EFG and CS tensors are non-coincident. Quantum chemical calculations of the NMR parameters substantiate the experimental findings and provide additional insight into the dependence of CQ(23Na) on the proximity of the indole ring to Na+. Taken together, this work has provided novel information on the NMR interaction tensors characteristic of a sodium cation interacting with a biologically important arene.

Synthesis of a New Diaazacrown Ether Compound Interconnected with an Azacrown Ether and Decorated with a Long Lipophilic Chain

“…Purification by column chromatography (acetone=NEt 3 ) afforded pure 5 as an orange oil (75 mg, 39%). 1 6, 69.1, 69.3, 69.4, 69.5, 70.0, 70.1, 70.4, 70.5, 70.6, 70.9, 71.3, 71.8 f7-[1-Aza-(15-crown-5)-1-(butyl-4-yl)]-1-docosylg-1,7-diaza-(15-crown-5) 1…”

“…[1] They have very important uses as synthetic receptors in molecular recognition processes [2] and anion complexation properties that are comparable to those of certain biological systems. [3] Other interesting uses of azacrown ether may be quoted as their use as catalyst in nucleophilic substitutions or oxidation reactions and for analytical applications.…”

Influence of Ag+ on the Magnetic Response of [2.2.2]Paracyclophane: NMR Properties of a Prototypical Organic Host for Cation Binding Based on DFT Calculations

“…In general, solution‐ and solid‐phase experimental studies of cation–π interactions frequently use NMR spectroscopic experiments . Among other nondestructive characterization methods, NMR spectroscopy is a valuable tool for assessing any variations in the chemical environment of an organic host that might occur upon complex formation with a metal center.…”

“…[19] In general, solution-a nd solid-phasee xperimental studies of cation-p interactions frequently use NMR spectroscopic experiments. [8,20,21] Among other nondestructivec haracterization methods, NMR spectroscopy is av aluable tool for assessing any variations in the chemical environment of an organic host that might occur upon complex formation with am etal center. The magnetic response at the nuclei, which account for usual NMR experiments, [22] can be conveniently generalized through space, leading to ag raphical representation of the short-a nd The complexation of metal cations into ah ost-guest situation is particularly well exemplified by [2.2.2]paracyclophane and Ag I ,which leads to astrong cation-p interaction with aspecific face of the hostm olecule.…”

Solid-State NMR Study of Halogen-Bonded Adducts

“…Among the advantages of solid-state NMR compared to diffraction techniques is the former's ability to provide detailed information on local structure even when long-range crystalline order is absent. NMR has proven to be a particularly useful site-specific probe of non-covalent interactions in solution and in the solid state, having provided fundamental insights into hydrogen bonding [1][2][3][4], cation-π interactions [5,6], and CH/π interactions [7], for example. Applications to the phenomenon of halogen bonding, where a halogen bond donor RX (X ¼ halogen) interacts with a halogen bond acceptor, Y (Y ¼ anion, Lewis base, π electrons, etc.…”