Magnetic Properties of a Novel Fe(II) \textit{meso}-tetra(4-pyridyl)porphyrin Network

Magnetic properties of Fe(II) \textit{meso}-tetra(4-pyridyl)porphyrin [Fe(TpyP)], a newly discovered metalloporphyrin network, were investigated using magnetic susceptibility $M(T)$/$H$, isothermal magnetization $M(H)$, and heat capacity $C(T)$ measurements. The crystal structure of Fe(TpyP) at room temperature is orthorhombic with a space group \textit{Cmca} (No. 64). And this network gives rise to an unprecedented two-dimensional paddle-wheel-like pattern (a 4$^{4}$ topology). The results of $M(T)$/$H$ on powder samples of Fe(TpyP) show that the effective moment \textit{$\mu $}$_{eff}$ of Fe$^{2+}$ is 5.52 \textit{$\mu $}$_{B}$ at 340 K, close to the expected value for a high spin Fe$^{2+}$ ($S$ = 2) ion. \textit{$\mu $}$_{eff}$ increases to 5.90 \textit{$\mu $}$_{B}$ as $T$ decreases to the range of 170 K $< \quad T \quad <$ 220 K, and then decreases monotonically as $T$ decreases further. The behavior of \textit{$\mu $}$_{eff}$ in the high temperature region may be attributed to a positive Fe$^{2+}$-Fe$^{2+}$ coupling. The low field ($H \quad \le $ 4 kG) $M(T)$/$H$ data reveal an anomalous behavior of this compound in the low temperature region which seems spin-glass-like. A small hysteresis was observed in $M(H)$ measured at 2 K. No anomaly was seen in the $C(T)$ data from 1.6 to 25 K.