Pyrazol-4-yl-substituted α-nitronyl and α-imino nitroxide radicals in solution and solid states (original) (raw)
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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.
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.
European Journal of Inorganic Chemistry, 2003
Treatment of the ligands 1-(3-thia-5-hydroxypentyl)-3,5dimethylpyrazole (thpd) and 1-(3-thia-6-hydroxyhexyl)-3,5-dimethylpyrazole (thhd) with [PdCl 2 (CH 3 CN) 2 ], [PtCl 2 (CH 3 CN) 2 ], and NiCl 2 ·6H 2 O produces the complexes [MCl 2 (L)] [M= Pd, L = thpd (1), thhd (2); M = Pt, L = thpd (3), thhd (4); M = Ni, L = thpd (5), thhd (6)], which have been characterised by elemental analyses, conductivity, IR, electronic spectra, and NMR spectroscopy (when possible). The crystal structures of 2, 3, and 4, determined by single-crystal X-ray diffraction, each consist of discrete [MCl 2 (L)] units. Each M II atom is coordinated by the pyrazolyl nitrogen and [a] plexation of these ligands with [MCl 2 (CH 3 CN) 2 ] (M ϭ Pd II , Pt II ) and NiCl 2 ·6H 2 O is also reported, to give the complexes: [MCl 2 (L)] [Mϭ Pd, L ϭ thpd (1), thhd (2); M ϭ Pt, L ϭ thpd (3), thhd (4); M ϭ Ni, L ϭ thpd (5), thhd (6)]. Treatment of 5 with non-anhydrous alcohols produces [NiCl(H 2 O) 2 (thpd)]Cl ([7]Cl). These complexes have been characterised by elemental analyses, conductivity, IR spectroscopy and, when possible, 1 H NMR, 13 C{ 1 H} NMR, and 195 Pt{ 1 H} NMR spectroscopy. The X-ray crystal structures of complexes 2, 3, 4, and [7]Cl are also presented.
Molecules
In contrast to diradicals connected by alternant hydrocarbons, only a few studies have addressed diradicals connected by nonalternant hydrocarbons and their heteroatom derivatives. Here, the synthesis, structure, and magnetic properties of pyrrole-2,5-diyl–linked bis(nitronyl nitroxide) and bis(iminonitroxide) diradicals are described. The diradicals show characteristic electron spin resonance spectra in dilute glassy solutions, from which conclusions about the presence of distinct conformations, their symmetry, and interspin distance were made. X-ray diffraction analysis of the diradicals revealed that paramagnetic moieties lie in the plane of the pyrrole ring, because of the formation of an intramolecular hydrogen bond, ONO…H−N, with O…H distances of 2.15–2.23 Å. The N–O groups participating in the formation of H-bonds have greater bond lengths (~1.29 Å) as compared with nonparticipating groups (~1.27 Å). The nitronyl nitroxide and iminonitroxide diradicals showed an intramolecula...
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. #