TRANSITION-METAL DERIVATIVES OF A CHELATING NITRONYL NITROXIDE LIGAND - NICKEL(II) AND MANGANESE(II) COMPLEXES

Complexes of the nitroxide ligand 2-(2-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxy 3-oxide (N1T2Py) with NiCl2 (1, C26H36Cl6N6NiO4) and MnCl2 (2, C26H36Cl6MnN6O4) have been structurally and magnetically characterized. Both complexes are centrosymmetric three-spin systems in which the nitroxyl group is coordinated to the metal ion. Metal-nitroxide interactions of -110 cm-1 for 1 and -80 cm-1 for 2 (H = -2JS(i)S(j)) are observed. These coupling constants are weaker than usually observed in previously reported examples (<-200 cm-1 for Ni and -1 50,-200 cm-1 for Mn). This result was rationalized by considering a reduced overlap of the magnetic orbitals due to a peculiar geometric arrangement in the chelate. Comparison to literature reported species is obtained from the homologous hexafluoroacetylacetonates, 3 (C22H18F12N3NiO6) and 4 (C22H18F12MnN3O6), which exhibit similar structural and magnetic properties. Coordination of the nitroxyl group in these derivatives illustrates the use of the chelate effect for designing exchange-coupled nitroxide derivatives of poor electrophilic metal centers. Relevant structural parameters are as follows: 1, monoclinic, P2(1)/c, a = 8.890(2) angstrom, b = 13. 957(2) angstrom, c = 13.802(2) angstrom, beta = 103.06(2)-degrees, Z = 2; 2, monoclinic, P2(1)/c, a = 9.018(2) angstrom, b = 13.815(3) angstrom, c = 14.095(2) angstrom, beta = 102.90(2)-degrees, Z = 2; 3, monoclinic, P2(1)/c, a = 11.866(3) angstrom, b = 15.173(3) angstrom, c = 17.282(3) angstrom, beta = 95.66(1)-degrees, Z = 4;4, monoclinic, P2(1)/c, a = 10.453(2) angstrom, b = 16.518(3) angstrom, c = 16.697(4) angstrom, beta = 94.08(1)-degrees, Z = 4.