Pyrazol-4-yl-substituted α-nitronyl and α-imino nitroxide radicals in solution and solid states (original) (raw)
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.
2,6-Bis(pyrazolyl)pyridine Functionalised with Two Nitronylnitroxide and Iminonitroxide Radicals
European Journal of Organic Chemistry, 2004
During the last two decades, growing interest in the field of material chemistry has been focused on the synthetic design of organic high-spin molecules for use as novel magnetic materials. Conjugated radicals based on the bis(pyrazolyl)pyridine core are very attractive in principle, since they per se combine optical, chelating and magnetic properties on a single molecular entity. The functionalisation of this core via the bis(4Ј-formyl) derivatives with Ullman radicals results in novel functional biradicals, which may further be used for supramolecular architectures. In this paper we describe the synthesis, structure and properties of 2,6-bis[4Ј-(3oxo-1-oxyl-4,4,5,5-tetramethylimidazolin-2-yl)pyrazol-1Јyl]pyridine (Pz 2 PyNN) and 2,6-bis [4-(1-oxyl-4,4,5,5-tetramethylimidazolin-2-yl)pyrazolyl]pyridine (Pz 2 PyIN). The two biradicals can clearly be differentiated by UV/Vis, IR and EPR spectroscopy. The optical absorptions of the blue [a]
Inorganic Chemistry
Transition metal complexes of 2-[4′-(2,2′:6′,2′′-terpyridyl)]-(4,4,5,5-tetramethylimidazolinyl-3-oxide-1-oxyl) (terpy-NIT) and 2-[4′-(2,2′:6′,2′′-terpyridyl)]-(4,4,5,5-tetramethylimidazolinyl-1-oxyl) (terpy-IM) have been prepared. Whereas the pyridyl fragments of the free ligands are in an anti conformation, the complexes are obtained by coordination of two terpyridines in a syn conformation, forming a distorted octahedron around the metal center: [M(terpy-NIT) 2 ](ClO 4) 2 (M) Ni(II) 1, Zn(II) 2, Cu(II) 3) and [M(terpy-IM) 2 ](ClO 4) 2 (M) Ni(II) 4, Zn(II) 5). The ligands and their complexes have been characterized by FAB-MS, UV-vis, FT-IR spectroscopies, elemental analysis, and by EPR spectroscopy and susceptibility measurements. Single-crystal X-ray diffraction have been performed on the terpy-NIT ligand and on complexes 1, 4, and 5 giving following crystal data: terpy-NIT, monoclinic, P2 1 /c, Z) 4, a) 14.2186(5), b) 12.9129(6), c) 11.704(1) Å,) 108.615(4)°; 1, orthorhombic, P(n a 2 1), Z) 4, a) 23.6367(6), b) 8.7836(1), c) 24.2748(7) Å; 4, monoclinic, P2 1 , Z) 1, a) 8.738(1), b) 25.010(1), c) 11.704(1) Å,) 102.849(3)°; 5, monoclinic, P2 1 , Z) 1, a) 8.7463(2), b) 25.0833(5), c) 11.8168(3) Å,) 102.757(3)°. For complexes 1 and 3, an antiferromagnetic behavior has been found and parametrized by considering a symmetric magnetic trimer, highlighting a strong intramolecular coupling between the metal and the radicals (average values 2J M-NIT)-19.6 K for M) Ni and-22.8 K for M) Cu). In the case of compound 4, an asymmetric magnetic trimer has been used to model the antiferromagnetic interactions (2J Ni-IM1)-13.0 K, 2J Ni-IM2)-5.6 K). The shape of the EPR spectra of complexes 2, 3, and 5 in solution indicates the intermediate exchange limit, of the order of a few mK, between the two nitroxide radicals through the pyridyl-metal-pyridyl fragment.
Journal of Coordination Chemistry
Two mononuclear complexes [(Et 3 NH)[M(hfac) 2 L] (M = Ni, 1 Zn, 2) have been synthesized using a nitronyl-nitroxide radical substituted nitrophenol, i.e., 2-(2-Hydroxy-3-methoxy-5nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-3-oxide-1-oxyl, HL, as a proligand. The crystal structures of the two compounds have been solved and indicate an octahedral coordination geometry of the metal ions. The magnetic behavior for compound 1 is characterized by a strong antiferromagnetic metal-radical interaction (J =-351 ± 1 cm-1 ; H =-JS Ni S Rad). This exchange interaction was rationalized by DFT calculations. The EPR spectra recorded both in solution and solid state at 120 K confirm the S = ½ ground state for compound 1.
A Crystallographic Study of a Novel Tetrazolyl-Substituted Nitronyl Nitroxide Radical
Crystals
Spin-labelled compounds are widely used in chemistry, physics, biology, and material sciences, but the directed synthesis of some functionalized organic radicals is still a challenge. We succeeded in the preparation of a tetrazolyl-substituted nitronyl nitroxide radical in pure crystalline form. According to the single-crystal X-ray data, intra- (NH…O, 2.43 Å) and inter-molecular hydrogen bonds (NH…O, 1.91 Å) are formed between NH groups of the tetrazole cycles and O atoms of the paramagnetic moieties. The intermolecular H-bonds connect the molecules forming chains along the a-axis. Moreover, there are short intermolecular contacts between the O atoms (3.096 Å) and between the O and C atoms (3.096 Å) of the nitronyl nitroxide moieties within the chain. The spin-unrestricted broken-symmetry calculations performed at the BS-UB3LYP/def2-TZVP level of theory predicted a sufficient ferromagnetic interaction (J ≈ 20 cm–1) between the adjacent radicals inside the chain, but a weak antiferr...
Synthesis of α-aryl N-adamant-1-yl nitrones and using them for spin trapping of hydroxyl radicals
Bioorganic & Medicinal Chemistry Letters, 1998
a-Aryl N-adamant-l-yl nitrones were synthesized and evaluated with respect to the stability of the hydroxyl radical adduct. The polarity and water solubility of nitrones were altered with changing the o~-aryl groups. Introduction of adamantane ring instead of tert-butyl group resulted in a reasonable good stability of hydroxyl radical adduct for biological measurements.
A CW-EPR and ESEEM spectroscopic study of the dithiadiazolyl radicals p-XC6F4CNSSN (X = CN, Br)
Applied Magnetic Resonance, 2001
Two dithiadiazolyl radicals, p-NCC6F4CNSSN and p-BrC 6 F4CNSSN, have recently been found to be paramagnetic in the solid state. While the (3-phase of the first one exhibits spontaneous magnetization below 36 K, the second one shows a paramagnetic character in the solid state. The spin density distribution in these radicals is examined through continuous-wave electron paramagnetic resonance and electron spin echo envelope modulation spectroscopies. Hyperfine correlation sublevel spectroscopy provides information about the interaction of the unpaired spin with F and N nuclei. A signal coming from the interaction with Br nucleus is also detected. The superhyperfine coupling constants of the unpaired electron with the magnetic nuclei are obtained and values of the corresponding spin densities, f and f -f, can be estimated in the isolated radicals. Spin density distribution has also been calculated in both molecules with density functional theory, being in excellent agreement with those determined from the spectra. The spin density is mainly concentrated in the dithiadiazolyl ring, but some spin density is observed on the fluorinated aromatic rings. They also provide a strong basis to understand the differences of the magnetic behavior of both molecules in terms of their respective packing in the solid state.
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.
Synthesis and Magnetic Properties of the Novel Dithiadiazolyl Radical, p-NCC6F4C6F4CNSSN
Molecules, 2004
The dithiadiazolyl radical p-NCC 6 F 4 C 6 F 4 CNSSN • (4) retains its monomeric nature in the solid state with molecules linked together into chains via supramolecular CN···S interactions. Variable temperature magnetic studies on 4 show that it behaves as a near-ideal Curie paramagnet (|θ| less than 0.1 K), indicating negligible intermolecular exchange. The effective magnetic moment (1.78 µ B ) is temperature independent and in excellent agreement with the value expected for an S = ½ paramagnet with g = 2.01 (1.74µ B ). The lack of exchange coupling between radicals is attributed to the absence of significant orbital overlap between radical centres.