Manganese(III) Compounds of Phenol-Pyrazole-Based Ligands: Synthesis, Crystal Structure, Magnetic, and Thermal Properties

Reaction of H(2)phpzR (R = H, Me, Ph; H(2)phpzH = 3(5)-(2-hydroxyphenyl)pyrazole, H(2)phpzMe = 3(5)-(2-hydroxyphenyl)-5(3)-methylpyrazole, and H(2)phpzPh = 3(5)-(2-hydroxyphenyl)-5(3)-phenylpyrazole)) with MnCl2 center dot 2H(2)O or MnBr2 center dot 4H(2)O in an acetonitrile-methanol mixture and in the presence of a small quantity of triethylatnine affords the compounds [Mn(HphpzR)(2)Cl] (R = H (1), Me (3), and Ph (5) and [Mn(HphpzR)(2)Br] (R = H (2), Me (4), and Ph (6)). This new family of compounds 1-6 with a library of phenol-pyrazole-based ligands was structurally characterized. All compounds are mononuclear manganese(III) coordination compounds in which the manganese(III) ion has a square-pyramidal geometry. In compounds 1-4 intermolecular hydrogen bonds are present between the N-H group of the pyrazole ring of a mononuclear entity and the halogen of a neighboring molecule, thus forming ladder-like chains. The presence of two crystallographically independent molecules in the unit cell of 5 and 6 precludes formation of ID structure and instead gives rise to a 2D structure with the independent molecules aligned in a face-to-face manner. Low-temperature magnetization studies evidence the presence of antiferromagnetic interactions between the manganese(III) ions in all compounds. Detailed temperature-dependent susceptibility and magnetic specific heat measurements on compounds 1 and 2 show the presence of antiferromagnetic S = 2 chains, the magnetic interaction being described by the anisotropic Heisenberg model with a predominantly planar (XY) type of crystal field anisotropy. In addition, the magnetic specific heat data evidence the occurrence of long-range magnetic ordering between chains in 1 and 2 below about 1.5 K. Compounds 3-6 show weaker antiferromagnetic interactions as compared with 1 and 2.