Crystal Structures and Magnetic Properties of m-Phenylenebis(imidazole) Derivatives Having Two Nitronyl Nitroxide or Iminyl Nitroxide Radicals. The Two Kinds of Antiferromagnetic Interaction Alternating along One-Dimensional Chains

Abstract

1,3-Bis{2-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)imidazol-1-yl}benzene (1), 1,3-bis{2-(1oxyl-4,4,5,5-tetramethylimidazolin-2-yl)imidazol-1-yl}benzene (2), and their chloro derivatives (3 and 4) were prepared. Both diradicals 1, 2 and 3, 4 crystallized in orthorhombic, space group Pbcn and in monoclinic, space group P2 1 /c, respectively. The molecular structures of diradicals 1 and 2 were similar to each other and had C 2 symmetry. Intra-and intermolecular average distances between the two NO groups of the nitronyl nitroxide (NN) moieties in 1 and those of the N-N and NO-NO of the iminyl nitroxide (IN) moieties in 2 were found to be 4.26 and 3.49 Å for 1 and 4.38 and 3.54 Å for 2, respectively, suggesting the formation of magnetically linear chain structures along the c axis. An alternating antiferromagnetic one-dimensional chain model (H ) -2J Σ{S A2i S A2i-1 + RS A2i S A2i+1 }) was applied to the spin systems 1 and 2 and a theoretical equation derived therefrom was fitted to the observed χ mol T vs T plots to give the exchange coupling parameters, J/k B ) -158 ( 2 and -239 ( 8 K and R ) 0.22 ( 0.02 and 0.11 ( 0.04, respectively. The molecular structures of 3 and 4 were also similar to each other, in which two NN and IN groups are far away from each other by the nearest intramolecular average distance between radical centers being 4.54 and 4.72 Å for 3 and 4, respectively. One of the latter has a disorder with respect to the position of the oxygen atom in IN. One of the two NNs and INs in both diradicals had intermolecular short distances between the radical centers in the neighboring molecules; average distances 3.56 Å for 3 and 3.82 Å for 4, to form magnetic dimer structures. The exchange coupling parameters for 3 and 4 were determined to be J/k B ) -4.85 ( 0.15 and -37.3 ( 0.3 K (one triplet species for the singlet-triplet model; H ) -2JS 1 S 2 , and isolated two doublets in a ratio of 1:1), respectively.

Regiospecificity in the Exchange Coupling of the Spins of Copper(II) Ion Coordinated with the Ring Nitrogen Atoms and N-tert-Butylaminoxyl Radical Attached as a Substituent on the Pyridine and N-Phenylimidazole Rings

“…The relatively large dihedral angle (39°) between the plane of N - tert -butylaminoxyl moiety and the pyridine ring is considered to weaken the interaction between the aminoxyl and the copper ion whereas the short distance of 2.45 Å between one aminoxyl group and another one belonging to its nearest neighbor molecule strengthens the through-space antiferromagnetic interaction between the two aminoxyl groups. Recently, we found strong intermolecular antifferomagnetic interactions with J / k B = − 158 and −239 K in nitronylnitroxide ( NN ) and iminylnitroxide ( IN ) imidazole derivatives, respectively, in which the relative geometry of the p-orbitals between two aminoxyl radical centers were ideal for overlap; these centers were separated by a short distance of 3.49 and 3.54 Å . As represented in Figure b, the observed geometry of the two aminoxyl radical centers in this work was closer to ideal than for the imidazole derivatives.…”

Synthesis of Intrinsically Blue-Colored bis-Nitronyl Nitroxide Peptidomimetic Templates and Their Conformational Preferences as Revealed by a Combined Spectroscopic Analysis

“…In this Figure we show the simulation calculated with g = 2.0068 and J = 10 mT. Also, these data agree closely with those published for similar bis -imidazolinyl nitronyl nitroxides in solvents of low polarity. ,− …”

“…Some bis -imidazolinyl nitronyl nitroxide compounds have previously been prepared and their chemical, spectroscopic, and magnetic properties investigated. ,− However, there are no published reports so far on two such nitronyl nitroxides covalently linked to the same peptidomimetic scaffold. In this article, we reported the syntheses by solution methods and full characterizations of two blue-colored peptidomimetics, each bearing two pendant, chiral, imidazolinyl nitronyl nitroxide free radical units.…”

“…Another quite interesting, related class of organic, stable mono -radicals is that of the usually intrinsically blue-colored imidazolinyl nitronyl nitroxides, introduced by Ullman and co-workers almost 50 years ago − [for the only other class known of intrinsically blue-colored peptides, based on the Trp side-chain analog β-(1-azulenyl)- l -alanine, see ref ]. Only a limited number of peptides have been ( mono )­labeled with this type of radical, either at the N-terminal amino function or covalently linked to a few α-amino acid side chains with the purpose of investigating their biological activities and radical scavenging properties. − Moreover, published organic compounds containing nitronyl nitroxide bi -radicals are relatively rare ,− (specifically, those based on a peptide template have not been reported to date). The properties of such systems have been studied by use of UV–vis absorption, EPR, NMR, magnetic susceptibility, and electrochemistry, and in a few cases their ability to function as EPR-active sensors for metal cations has been noted. , Nitroxide and nitronyl nitroxide free-radical probes, rather than being considered an alternative, are complementary.…”

Manganese (II) Coordination Complexes Involving Nitronyl Nitroxide Radicals

“…The high-temperature (300−50 K) decrease of χ M T can be well reproduced by a pair model of S = 1 / 2 spins to yield an exchange parameter J = −130 cm -1 . Related rather strong through-space antiferromagnetic interactions between aminoxyl radicals have been described recently …”