Ferro- and antiferromagnetic coupling between metal ions and pyridine-substituted nitronyl nitroxides

The synthesis and crystal structure of two novel adducts of manganese(I1) and nitronyl nitroxide radicals of formula Mn- (hfac)z(NITpPy)z (I) and MnCI,(NITpPy), (II), where hfac = hexafluoroacetylacetonate and NlTpPy = 2-(4-pyridyl)- 4,4,5,5-tetramethyI-4,5-dihydroH-l-i midazolyl-1 - oxy1 3-oxide, are reported. Compound I crystallizes in the monoclinic space group P 2 , / n with a = 14.037 ( 2 ) A, b = 6.500 (7) A, c = 22.180 (5) A, 0 = 99.39 (I)', and Z = 2. Compound I1 crystallizes in the triclinic space group P i with a = 7.205 (4) A, b = 13.613 (2) A, c = 13.630 (4) A, a = 89.74 (2)O, p = 78.69 (4)O, y = 86.88 ( 4 ) O , and Z = 1. In both cases the complex is mononuclear with the manganese ion octahedrally coordinated and bound to the nitronyl nitroxide radicals through the nitrogen atom of the pyridine ring. The magnetic susceptibility of both compounds suggests that the manganese ion and the radicals are ferromagnetically coupled with a coupling constant of about 1 cm-I. The nickel(I1) analogues, Ni(hfac),(NITpPy), (Ill) and NiCI2(NITpPy), (IV), were also synthesized, and IV was found to be isomorphous with 11. The magnetic behavior of the nickel derivatives suggests that the metal ion is antiferromagnetically coupled to the radical through the pyridine ring with a coupling constant of about IO cm-I. The mechanism of exchange was analyzed in terms of orbital pathways involving the K* molecular orbital of the NITpPy radical containing the unpaired electron, the pyridine nitrogen lone pair, and the metal orbitals.