Superexchange-Coupled Electron-Spin Pairs in Iron Tetraphenylporphyrin Chloride at Low Temperatures

Magnetic susceptibility, heat capacity, and EPR studies reported here show that close to 50% of the high-spin ${\mathrm{Fe}}^{3+}$ ions in this compound form isolated spin pairs each of which shows a highly anisotropic superexchange coupling. Direct EPR transitions between the superexchange-split energy levels have been observed.