CRYSTAL-STRUCTURES AND MAGNETIC-PROPERTIES OF ACID-BASE MOLECULAR-COMPLEXES, (P-PYRIDYL NITRONYLNITROXIDE)(2)X (X = HYDROQUINONE, FUMARIC-ACID AND SQUARIC ACID)

The reactions of 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3- N-oxide (or p-pyridyl nitronylnitroxide, abbreviated as p-PYNN) with t he three dibasic organic acids, X[= fumaric acid (FA), squaric acid (S A) and hydroquinone (HQ)], result in the formation of hydrogen-bonding complexes of (p-PYNN)(2) . X composition. In their crystals, the orga nic acids make selective hydrogen bonds to the two kinds of hydrogen-b ond accepting sites in p-PYNN; (a) the oxygen atom in the NO group and (b) the nitrogen atom of the pyridyl ring. p-PYNN2 . HQ crystallizes in the monoclinic P2(1)/n space group. The HQ molecule bridges two p-P YNNs, and selects site (a) in p-PYNN as the hydrogen bond acceptor [i. e. (p-PYNN)NO ... HO(HQ)OH ... ON(p-PYNN)]. p-PYNN2 . FA crystallizes in the monoclinic P2(1)/n space group. The FA molecule connects two p- PYNN molecules with an intermolecular hydrogen bond to site (b) [(p-PY NN)N ... HO(FA)OH ... N(p-PYNN)]. The 2:1 compound of p-PYNN and SA cr ystallizes with the crystal solvent, 1,4-dioxane (abbreviated as diox) , in the formula for p-PYNN2 . SA . diox. The crystal belongs to the t riclinic <P(1)over bar> space group. The SA molecule occupies the spac e 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 i s a dianion in which the two protons are missing and, thus, the hydrog en bond is ionic [(p-PYNN)NH+... O-(SA)O-... H+N(p-PYNN)]. The selecti vity and features of the hydrogen bonds can be qualitatively understoo d in terms of competition between the electrostatic and charge-transfe r terms in the hydrogen-bonding energy, which is governed by the acidi ty of the organic acids and the proton accepting abilities of the two sites in p-PYNN. The three molecular compounds exhibited different ant iferromagnetic properties, which depend on the mutual arrangement of p -PYNN in the crystals. The intermolecular interactions were interprete d based on the McConnell's spin polarization mechanism.