A Crystallographic Study of a Novel Tetrazolyl-Substituted Nitronyl Nitroxide Radical (original) (raw)
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2007
Two paramagnetic building blocks, 2-(4-ethynyl-1-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (3) and 2-(5ethynyl-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 (Ns O‚‚‚HsCtCs type) chain structures with O‚‚‚C distances of 3.181 and 3.155 Å, respectively. High-resolution isotropic liquid state (c e 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, NsO) to the acetylenic proton (donor, HsCtCs), which is mediated by the π-conjugated backbone. The influence of the heteroatom (pyridine nitrogen-14 N) in the ESR hyperfine splitting pattern was clearly seen in radical 4, with an additional number of lines appearing in the M I ) 0 line of the total five-line spectrum. The solution state paramagnetic 1 H NMR investigation of radicals 2-(4-trimethylsilylethynyl-1-phenyl)-4,4,5,5tetramethylimidazoline-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/K B 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/K B values of -0.62 ( 0.02 and -2.21 ( 0.13 K for 3 and 4, respectively.
Structure - Magnetism relationships in α-nitronyl nitroxide radicals
The crystal packing of α-nitronyl nitroxide radicals that have dominant ferromagnetic or antiferromagnetic interactions is analyzed in order to test if there are characteristic orientations of their functional groups that can be associated with these magnetic interactions. From a large crystalline structural database of compounds containing α-nitronyl nitroxide radical units (143 structures), 23 representative cases with dominant intermolecular ferromagnetic interactions, and 24 cases exhibiting dominant antiferromagnetic interactions were selected. The spatial distribution of the N-O ⋯ O-N, C(sp3)-H ⋯ ON, and C(sp2)-H ⋯ ON contacts whose distance is smaller than 10 Å was analyzed, with special emphasis on the 0-5 À region for the N-O ⋯ O-N contacts and 0-3.8 Å for the C-H ⋯ O-N contacts. No correspondence is found between the presence of intermolecular ferro- or antiferromagnetic interactions and the geometry of any of the previous isolated contacts. Therefore, there is a need to c...
Synthetic Metals, 2001
The atomic spin population of all the a-nitronyl nitroxide radicals whose crystals present dominant ferro or antiferromagnetic properties is computed at the B3LYP/6-31G(d) level, searching for changes and similarities in their spin distribution. The quality of the computed values is previously tested on the a-phenyl-nitronyl nitroxide radical using various basis sets. It is found that computed atomic spin population presents small changes with the basis set, when the computation is done using the B3LYP density functional. The B3LYP/6-31G(d) atomic spin population of the radicals presenting dominant ferromagnetic interactions is practically independent of the system. The same is true for the antiferromagnetic systems. The two sets present also similar values. This fact is of great help when analyzing the magnetic properties of the crystals of these radicals. #
Polyhedron, 2009
The effects of acceptor-donor interactions in thienyl substituted benzimidazole-nitronyl nitroxides (TBNN) on the absorption spectroscopy, spin density distribution, magnetic behavior, and crystallographic packing were explored through spectroscopy, computation, and characterization of structure and magnetic properties in the crystalline phase. The electronic spectra of the radicals exhibit a strong broad absorption in the NIR (k max $ 1000 nm) that exhibits solvatochromism consistent with charge transfer between the thienyl (donor) and benzonitronyl nitroxide (acceptor) dyads. Computational analysis allowed assignment of the transition as a HOMO-SOMO transition (TD-DFT UB3LYP/6-31G**). The TBNN radicals form highly disordered slipped p-stacks in the solid state that give rise to antiferromagnetic interactions consistent with 1D chain interactions. The magnetic behavior was well-fit to a Bonner-Fisher model to give exchange parameters of J = À2 to À10 cm À1 depending on substitution. The weak exchange parameters are attributed to the degree of solid-state disorder, and the observed properties can be rationalized by the effects of substitution on the electronic structure and topology of the radicals.
Structure-Magnetism Relationships inα-Nitronyl Nitroxide Radicals
Chemistry-a European Journal, 1999
oxidized species of the compounds gave rise to only one sub-gap electronic transition near 1.25 (pentamer) and 1.15 eV (hexamer). More detailed experimental conditions of electronic spectral measurements can be found elsewhere [9b,14].
Journal of Molecular Structure, 2011
Two pairs of nitronyl nitroxide radicals with specific molecular structures in which the imidazole radical moiety was directly bound to their chiral center have been prepared in their enantiopure forms. In these specific molecular structures, chirality may directly affect the unpaired spin electronic cloud. The X-ray crystal structures of them have been solved, which reveal that supermolecule helical chains are formed by intermolecular hydrogen bond interactions in the L-NNP crystal and the L-NNNBP is a 3D supermolecule structure shaped by means of intermolecular hydrogen bond interactions to link homochiral chains. Circular dichroism spectroscopy of the two pairs of chiral a-nitronyl nitroxides shows significant Cotton effects between 200 and 400 nm. The magnetic properties of the two pairs of nitronyl nitroxide radicals in the solid state were characterized by EPR spectroscopy and magnetic susceptibility measurements, respectively. Magnetic susceptibility measurements of the solids reveal antiferromagnetic interactions in all cases. However, in the L-NNNBP, the exchange interaction abruptly changes from untiferromagnetic to a ferromagnetic interaction at 40.0 k.
Assembly of Imidazolyl-Substituted Nitronyl Nitroxides into Ferromagnetically Coupled Chains
Crystals
New nitronyl nitroxides, namely, 2-(4,5-dimethylimidazol-2-yl)- and 2-(4,5-dichloroimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-3-oxide-1-oxyl, were prepared in crystalline form. According to single-crystal X-ray data, intra- and intermolecular hydrogen bonds are formed between NH groups of the imidazole cycles and O atoms of the nitroxide moieties. The intermolecular H-bonds contribute to the alignment of molecules into chains along the a-axis; this alignment causes short intrachain contacts between O and C atoms carrying spin density of opposite signs. Such an arrangement of nitroxides induces ferromagnetic intrachain interactions (J ≈ 10 cm−1) between neighboring radicals.
Journal of Organometallic Chemistry, 2001
The synthesis, X-ray structure and magnetic properties of two new ferrocene substituted nitroxide monoradicals are described. The magnetic susceptibility data was nicely fitted in both cases to the Curie-Weiss law yielding the Weiss constants of q= −1.8 and −1.1 K for radicals 1 and 2, respectively. This result evidenced the presence of very weak antiferromagnetic intermolecular interactions between neighbor molecules. Moreover, the X-ray structure of the key precursor 2,3-bis(hydroxyamino-)-2,3dimethylbutane (3) is also included.
Inorganic Chemistry, 2002
Four new mononuclear complexes of formula Cd(PN) 4 (NCS) 2 (A), Cd(PNN) 4 (N 3 ) 2 (B), Zn(PNN) 4 (N 3 ) 2 (C), and Zn(PNN) 2 (NCS) 2 (D), where PNN stands for 2-(4-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide and PN for 2-(4-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl, were synthesized and structurally and magnetically characterized. The X-ray structures of compounds B and C were also determined at 90 K. Compounds A−C crystallize in the triclinic space group P1 h (No. 2), and D crystallizes in the monoclinic space group P2 1 /m (No. 11). A−C adopt a centrosymmetric distorted octahedral geometry in which the metal ions are bonded to four radical ligands through the nitrogen atom of the pyridyl rings and the azido or thiocyanato ligands occupy the apical positions. Compound D adopts a distorted tetrahedral geometry in which the zinc ion is bonded to two radicals and two thiocyanato ligands. As suggested by their magnetic behavior, the low-temperature X-ray structures of B and C show that these compounds undergo a clear structural change with respect to the room-temperature structures. The experimental magnetic behaviors were perfectly reproduced by a dimer model for A−C and an alternating chain model for D while the sudden breaks observed in the M T versus T curves for B and C were well accounted for by the highand low-temperature X-ray structures. For all these complexes the crystal structures favor significant overlap between molecular magnetic orbitals leading to rather strong intermolecular antiferromagnetic interactions. A criticism of the McConnell theory can be found in the following: Deumal, M.; Novoa, J. J.; Bearpark, M. J.; Celani, P.; Olivucci, M.; Robb, M. A.