Crystal structures and magnetic properties of acid–base molecular complexes, (p-pyridyl nitronylnitroxide)₂X (X=hydroquinone, fumaric acid and squaric acid)

Abstract

The reactions of 2-(4-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-N-oxide (or p-pyridyl nitronylnitroxide, abbreviated as p-PYNN) with the three dibasic organic acids, X [=fumaric acid (FA), squaric acid (SA) and hydroquinone (HQ)], result in the formation of hydrogen-bonding complexes of ( p-PYNN)₂ X composition. In their crystals, the organic acids make selective hydrogen bonds to the two kinds of hydrogen-bond accepting sites in p-PYNN; (a) the oxygen atom in the NO group and (b) the nitrogen atom of the pyridyl ring. p-PYNN₂HQ crystallizes in the monoclinic P2₁ /n space group. The HQ molecule bridges two p-PYNNs, and selects site (a) in p-PYNN as the hydrogen bond acceptor [i.e. ( p-PYNN)NOHO(HQ)OHON( p-PYNN)]. p-PYNN₂ FA crystallizes in the monoclinic P2₁ /n space group. The FA molecule connects two p-PYNN molecules with an intermolecular hydrogen bond to site (b) [( p-PYNN)NHO(FA)OHN( p-PYNN)]. The 2:1 compound of p-PYNN and SA crystallizes with the crystal solvent, 1,4-dioxane (abbreviated as diox), in the formula for p-PYNN₂ SAdiox. The crystal belongs to the triclinic P1̄ space group. The SA molecule occupies the space between two p-PYNNs, making contact with site (b), as FA does in the p-PYNN₂ FA crystal. However the structure of SA indicates that it is a dianion in which the two protons are missing and, thus, the hydrogen bond is ionic [( p-PYNN)NH⁺ O^–(SA)O^– H⁺N( p-PYNN)]. The selectivity and features of the hydrogen bonds can be qualitatively understood in terms of competition between the electrostatic and charge-transfer terms in the hydrogen-bonding energy, which is governed by the acidity of the organic acids and the proton accepting abilities of the two sites in p-PYNN. The three molecular compounds exhibited different antiferromagnetic properties, which depend on the mutual arrangement of p-PYNN in the crystals. The intermolecular interactions were interpreted based on the McConnell’s spin polarization mechanism.

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