Preparation and magnetic properties of Mn(hfac)(2)-complexes of 2-(5-pyrimidinyl)- and 2-(3-pyridyl)-substituted nitronyl nitroxides

Mn(hfac)(2) complexes of 2-(5-pyrimidinyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H- imidazoline-1-oxyl 3-oxide] (1) and its 2-(3-pyridyl) analogue (2) were prepared. Both complexes formed similar dimer structures. However, their packing patterns were considerably different. The pyrimidine dimers were aligned to form a linear chain structure, and each dimer was weakly bound by two sets of O6-C2 short contacts. In the pyridine dimer complex, two structurally similar but independent dimers were alternatively arranged, and two dimer-dimer contacts, O6-C2 (3.13 A) and O6-C3 (3.30 A), were observed. The pyrimidine complex showed strong antiferromagnetic behavior in the high temperature region (150-300 K) and weak ferromagnetic behavior below 100 K. Two models were used to analyze these magnetic properties. One is a quintet-septet thermal equilibrium model with mean-field approximation, which can reproduce the round minimum observed at about 150 K in chi(p)T plots (J(1)/k(B) = -148 +/- 2 K with theta = +2.5 +/- 0.1 K). The other is a ferromagnetic S = 2 chain model to fit the chi(p)T values in the lower temperature region (J(S=2)/k(B) = +0.31 +/- 0.01 K). The pyridine complex showed antiferromagnetic interactions both in the high and low temperature regions. The magnetic behavior was similarly analyzed with the following parameters: J(1)/k(B) = -140 +/- 2 K with theta = -0.55 +/- 0.05 K, and J(S=2)/k(B) = -0.075 +/- 0.003 K. The ligand-ligand interactions for both of the complexes were theoretically analyzed. The calculated results agreed well with the experiments. The stronger antiferromagnetic behavior observed in both the complexes at high temperatures was attributed to the magnetic interaction between the Mn(II) and the coordinating nitroxide oxygen atom. The weaker ferromagnetic interaction, J(S=2)/k(B) = +0.31 +/- 0.01 K, in the pyrimidine complex was attributed to the coulombic O6-C2 contact. Antiferromagnetic interaction J(S=2)/k(B) = -0.075 +/- 0.003 K in the pyridine complex was attributed to the O6-C3 contact.