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

A family of pyridine, 2,2'-bipyridine, 1,10-phenanthroline, 3,6-bis(2-pyrid yl) pyridazine ligands bearing two nitronyl nitroxide radicals was synthesi zed and characterized. The pincer ligands having two radical in ortho posit ions relative to the nitrogen atoms form stable nickel(ll) and copper(II) c omplexes. The pyridine based 2,6- and 2,5-biradicals crystallized in the mo noclinic space group P2(1)/c while the bipyridine 5,5'-biradical crystalliz ed in the monoclinic space group P2(1)/n (radical is 4,4,5,5 tetramethylimi dazoline 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 degree s), followed by the 2,5-biradical (ca. 38 degrees) and then by the bipyridi ne,6'- (ca. 27 degrees) and 5,5'-biradicals (ia. 24 degrees). All the birad icals 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, 3.3-3.8 G). In some cases t hese nine lines are reduced to five:lines (A, 7;5-7.6 G) at low temperature due to a weakening of the spin-spin exchange interaction J which becomes s maller than the isotropic coupling constant; The chi T product for all radi cals measured at 300 K lies in the range of 0.70-0.77 emu K mol(-1); For th e 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 int ramolecular interactions persist when the radicals are dispersed in a polys tyrene matrix. For the bipyridine 6,6'- and 5,5'-biradicals, the experiment al data were fitted using a Bleaney-Bowers expression for two interacting s pins and the values of J/k(B), = - 24 to - 1.9 K, respectively, are represe ntative of an antiferromagnetic through-space interaction between two neigh boring 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-electr on quasi-reversible process to the nitrosonium dication and irreversibly re duced to the dianions between -1.80 and -1.40 V. In the Cu(II) and Ni(rr) c omplexes of the bipyridine-6,6'- and phenathroline-2,9-biradical reduction of the nitroxides to the hydroxylamine anions is quasi-reversible and is fa cilitated by ca. 1.20 V versus the 'free' ligands, and the second redox cou ple corresponds to reduction of the bridging bipyridine or phenanthroline f ragments. In the nickel based complex of the pyridine-2,6-biradical two set s of equivalent nitroxides are successively; reduced by a dielectronic proc ess while the monoelectronic reductions of the two bridging pyridines are s eparated by 330 mV.