Cai S, Walker F A, Licoccia S
Department of Chemistry, University of Arizona, Tucson, Arizona 85721-0041, USA.
Inorg Chem. 2000;39(16):3466-78. doi: 10.1021/ic990784l.
The chloroiron corrolates of 2,3,7,8,12,13,17,18-octamethyl- and 7,13-dimethyl-2,3,8,12,17,18-hexaethylcorrole ([(Me8C)FeCl] and [(7,13-Me2Et6C)FeCl], respectively) and their bisimidazole complexes have been investigated by NMR spectroscopy as a function of temperature, and by EPR spectroscopy at 4.2 K. Magnetic susceptibilities were measured by the modified Evans method. It is found that the electron configuration of the chloroiron corrolates is that of a S = 3/2 Fe(III) center coupled to a corrolate pi radical, where one electron has been removed from the pi system of the corrolate. This pi radical is antiferromagnetically coupled to the unpaired electrons of the iron to yield an overall S = 1 complex, as evidenced by the very large positive shifts of the meso-H resonances (183 and 172 ppm). That this antiferromagnetic coupling is very strong is supported by the near-Curie behavior of the 1H chemical shifts. For the chloroiron corrolates in the presence of imidazole, imidazole-d4, and N-methylimidazole at temperatures of -50 degrees C and below, the mono- and bisligand complexes are formed. The NMR spectra can be assigned on the basis of chemical exchange between the chloroiron(III) parent complex and the bisligand complex at -30 degrees C, and between the bisligand complex and the monoligand complex at -50 degrees C. The bisimidazole complexes show pyrrole CH2 and CH3 resonances characteristic of low-spin Fe(III) centers (S = 1/2), but with strongly upfield-shifted meso-H resonances (delta values of -95 and -82.5 ppm for the octamethyl complex and -188 and -161 ppm for the dimethylhexaethyl complex at 203 K) characteristic of the presence of a macrocycle-centered unpaired electron. The magnetic moments of these bisligand complexes are somewhat lower than expected for overall S = 1 systems, and decrease as the temperature is lowered. The lower apparent magnetic moments (2.0-1.8 mu B between -50 and -90 degrees C) are believed to be caused by a combination of weak or no magnetic coupling between the metal and macrocycle electrons and decreasing solubility of the complex as the temperature is lowered. The non-Curie behavior of the 1H chemical shifts observed in the low-temperature (-50 to -90 degrees C) NMR spectra likely arises from a combination of the effects of weak antiferromagnetic coupling of metal and macrocycle spins, a low-lying electronic excited state, and ligand binding/loss equilibria at the highest temperatures studied (-50 degrees C).
通过核磁共振光谱研究了2,3,7,8,12,13,17,18-八甲基卟啉铁氯络合物和7,13-二甲基-2,3,8,12,17,18-六乙基卟啉铁氯络合物（分别为[(Me8C)FeCl]和[(7,13-Me2Et6C)FeCl]）及其双咪唑配合物随温度的变化情况，并在4.2 K下通过电子顺磁共振光谱进行了研究。采用改进的埃文斯方法测量了磁化率。研究发现，卟啉铁氯络合物的电子构型是一个S = 3/2的Fe(III)中心与一个卟啉π自由基耦合，其中一个电子已从卟啉的π体系中移除。这个π自由基与铁的未成对电子反铁磁耦合，产生一个总S = 1的配合物，中氢共振的非常大的正位移（183和172 ppm）证明了这一点。1H化学位移的近居里行为支持了这种反铁磁耦合非常强的观点。对于在-50℃及以下温度下存在咪唑、咪唑-d4和N-甲基咪唑的卟啉铁氯络合物，会形成单配体和双配体配合物。核磁共振光谱可以根据氯铁(III)母体配合物与双配体配合物在-30℃时以及双配体配合物与单配体配合物在-50℃时的化学交换来进行归属。双咪唑配合物显示出低自旋Fe(III)中心（S = 1/2）特有的吡咯CH2和CH3共振，但中氢共振强烈向高场位移（203 K时，八甲基配合物的δ值为-95和-82.5 ppm，二甲基六乙基配合物的δ值为-188和-161 ppm），这是大环中心存在未成对电子的特征。这些双配体配合物的磁矩略低于总S = 1体系的预期值，并随温度降低而减小。较低的表观磁矩（-50至-90℃之间为2.0 - 1.8 μB）被认为是由金属与大环电子之间弱的或无磁耦合以及配合物在温度降低时溶解度降低共同导致的。在低温（-50至-90℃）核磁共振光谱中观察到的1H化学位移的非居里行为可能是由金属和大环自旋的弱反铁磁耦合、低电子激发态以及在所研究的最高温度（-50℃）下配体结合/解离平衡的综合影响引起的。
