Magnetic Interactions in Supramolecular NO···HC⋮C Type Hydrogen-Bonded Nitronylnitroxide Radical Chains

Abstract

Two paramagnetic building blocks, 2-(4-ethynyl-1-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (3) and 2-(5-ethynyl-2-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (4) were synthesized and crystallized. Single crystal X-ray studies of 3 and 4 show the formation of supramolecular head-to-tail one-dimensional H-bonded (NO···HC⋮C type) chain structures with O···C distances of 3.181 and 3.155 Å, respectively. High-resolution isotropic liquid state (c ≤ 10-4 M) electron spin resonance (ESR) spectroscopy studies of the well-isolated molecules confirmed the intramolecular spin polarization from the nitronylnitroxide radical group (acceptor, NO) to the acetylenic proton (donor, HC⋮C), which is mediated by the π-conjugated backbone. The influence of the heteroatom (pyridine nitrogen−14N) in the ESR hyperfine splitting pattern was clearly seen in radical 4, with an additional number of lines appearing in the MI = 0 line of the total five-line spectrum. The solution state paramagnetic 1H NMR investigation of radicals 2-(4-trimethylsilylethynyl-1-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (1) and 3 clearly support the intramolecular spin density propagation from the acceptor to the donor groups as well as the proton hyperfine coupling (hfc) values of the conjugated backbone determined by ESR studies. Bulk magnetic investigations of the polycrystalline chain compounds (3 and 4) in the temperature range 300 down to 4.5 K display antiferromagnetic exchange interactions at very low temperature. The experimental bulk magnetic data were found to be fit by using the dimer model with exchange coupling 2J/KB values of −3.10 ± 1.16 and −8.00 ± 3.83 K for 3 and 4, respectively, as well as by adopting the Heisenberg-chain model with 2J/KB values of −0.62 ± 0.02 and −2.21 ± 0.13 K for 3 and 4, respectively.