Oligopyridine bis(nitronyl nitroxides): synthesis, structures, electrochemical, magnetic and electronic properties

Abstract

A family of pyridine, 2,2′-bipyridine, 1,10-phenanthroline, 3,6-bis(2-pyridyl)pyridazine ligands bearing two nitronyl nitroxide radicals was synthesized and characterized. The pincer ligands having two radicals in ortho positions relative to the nitrogen atoms form stable nickel(II) and copper(II) complexes. The pyridine based 2,6- and 2,5-biradicals crystallized in the monoclinic space group P2 ₁ /c while the bipyridine 5,5′-biradical crystallized in the monoclinic space group P2 ₁ /n (radical is 4,4,5,5-tetramethylimidazoline 3-oxide 1-oxyl). The highest dihedral angle between the radicals and the pyridine ring is found in the pyridine 2,6-biradical (ca. 47°), followed by the 2,5-biradical (ca. 38°) and then by the bipyridine 6,6′- (ca. 27°) and 5,5′-biradicals (ca. 24°). All the biradicals exhibit similar EPR spectra, in solution and at room temperature, with nine lines due to coupling of the unpaired electrons with four equivalent nitrogen nuclei (hyperfine coupling constant A _N 3.3-3.8 G). In some cases these nine lines are reduced to five lines (A _N 7.5-7.6 G) at low temperature due to a weakening of the spin-spin exchange interaction J which becomes smaller than the isotropic coupling constant. The χT  product for all radicals measured at 300 K lies in the range of 0.70-0.77 emu K mol ^–1 . For the pyridine 2,6- and 2,5-biradicals, magneto-structural correlation reveals through bond ferro- and antiferro-magnetic interaction (J/k _B =+9.4 and –39 K, respectively) due to a favorable spin polarization effect. These intramolecular interactions persist when the radicals are dispersed in a polystyrene matrix. For the bipyridine 6,6′- and 5,5′-biradicals, the experimental data were fitted using a Bleaney-Bowers expression for two interacting spins and the values of J/k _B =–24 to –1.9 K, respectively, are representative of an antiferromagnetic through-space interaction between two neighboring radicals. No intramolecular magnetic interactions were found in the oligopyridine based compounds. In all of these biradicals, both nitroxides are oxidized at the same potential (ca. +0.45 V vs. Ag/Ag ⁺ ) by a two-electron quasi-reversible process to the nitrosonium dication and irreversibly reduced to the dianions between –1.80 and –1.40 V. In the Cu(II) and Ni(II) complexes of the bipyridine-6,6′- and phenanthroline-2,9-biradical reduction of the nitroxides to the hydroxylamine anions is quasi-reversible and is facilitated by ca. 1.20 V versus the