Synthesis, structures, and magnetic properties of a series of lanthanum(III) and gadolinium(III) complexes with chelating benzimidazole-substituted nitronyl nitroxide free radicals.: Evidence for antiferromagnetic GdIII-radical interactions

This paper reports the synthesis, crystal structures, and magnetic properties of a series of lanthanide complexes with nitronyl nitroxide radicals of general formula {[Ln(III)(radical)(4)].(ClO4)(3).(H2O)(x).(THF)(y)} (1-4) and [Ln(III)(radical)(2)-(NO3)(3)] (5, 6) [Ln = La (compounds 1, 3, 5) or Gd (compounds 2, 4, and 6); radical = 2-(2'-benzymidazolyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (NITBzlmH, compounds 1, 2, 5, 6) or 2-{2'-[(6'-methyl)benzymidazolyl]}-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (NITMeBzlmH, compounds 3, 4)], (1) C64H88Cl3LaN16O24, fw = 1710.76, orthorhombic, Fddd, a = 11.0682(8) Angstrom, b = 34.240(3) Angstrom, c = 42.787(3) Angstrom, V = 16215(2) Angstrom(3), Z = 8, R = 0.0876, R-w = 0.2336. (2) C64H88Cl3GdN16O24, fw = 1729.10, tetragonal, P (4) over bar 2c, a = 16.0682(4) Angstrom, b = 16.0682(4) Angstrom, c = 18.7190(6) Angstrom, V = 4833.0(2) Angstrom(3), R = 0.0732, R-w = 0.2218. (3) C68H94Cl3LaN16O23, fw = 1742.80, tetragonal, P (4) over bar2(1)m, a = 21.125(3) Angstrom, b = 21.125(3) Angstrom, c = 10.938(2) Angstrom, V = 4881.5(14) Angstrom(3), R = 0.1017, R-w = 0.3126. (5) C28H34LaN11O13, fw = 871.57, orthorhombic, Pna2(1), a = 19.5002(12) Angstrom, b = 13.0582(8) Angstrom, c = 14.5741(g) Angstrom, V = 3711.1(4) Angstrom(3), P = 0.0331, R-w = 0.1146. (6) C28H34GdN11O13, fw = 889.91, orthorhombic, Pna2(1), a = 19.1831 (10) Angstrom, b = 13.1600(7) Angstrom, c = 14.4107(7) Angstrom, V = 3638.0(3) Angstrom(3), Z = 4, R = 0.0206, R-w = 0,0625. Compounds 1-4 consist of [M-III(radical)(4)](3+) Cations, uncoordinated perchlorate anions, THF, and water crystallization molecules. In these complexes, the coordination number around the lanthanide ion is eight, and the polyhedron is either a distorted dodecahedron (1) or a distorted cube (2, 3). The crystal structures of 5 and 6 consist of independent [M-III(radical)(2)(NO3)(3)] entities in which the lanthanide is ten-coordinated and has a distorted bicapped square antiprism coordination polyhedron, For the lanthanum(III) complexes, the temperature dependence of the magnetic susceptibility indicates that radical-radical magnetic interactions are negligible either for compounds 1 and 3, while for compound 5 it is simulated considering dimers of weakly antiferromagnetically coupled radicals (J(rad-rad) = -1.1 cm(-1)). In the case of the gadolinium(III) compounds (2, 4, 6), each magnetic behavior gives unambiguous evidence of antiferromagnetic Gd-III-radical interaction (2, J(Gd-rad) = -1.8 cm(-1); 4, J(Gd-rad) = -3.8 cm(-1); 6, J(Gd-rad1) = -4.05 cm(-1) and J(Gd-rad2) = -0.80 cm(-1)), in contrast to the ferromagnetic case generally observed. The nature of the Gd-III-radical interaction is explained in relation to the donor strength of the free radical ligand.