NMR and EPR investigations of iron correlates:: Iron(III) correlate π cation radicals or iron(IV) correlates?
NMR and EPR investigations of iron correlates:: Iron(III) correlate π cation radicals or iron(IV) correlates?
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DOI：
10.1021/ic990784l
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发表时间：
2000-08-07
影响因子：
4.6
通讯作者：
Licoccia, S
中科院分区：
文献类型：
--
作者：
Cai, S;Walker, FA;Licoccia, S
利用核磁共振光谱和4.2 K的EPR光谱研究了2,3,7,8,12,13,17,18-八甲基-和7,13-二甲基-2,3,8,12,17,18-六乙基罗([（Me8C)FeCl]和[(7,13- me2et6c)FeCl]）的氯铁相关物及其双咪唑配合物的温度函数。磁化率用改进的Evans法测定。发现氯铁相关物的电子构型是S = 3/2 Fe（LU）中心与相关的pi自由基耦合的电子构型，其中一个电子从相关物的pi系统中移除了。这个π自由基与铁的未配对电子反铁磁耦合，产生一个整体的S = 1配合物，正如介氢共振（183和172 ppm）的非常大的正位移所证明的那样。这种反铁磁耦合非常强，这是由H-1化学位移的近居里行为所支持的。对于氯铁相关物，在-50℃及以下的温度下，咪唑、咪唑-d(4)和n -甲基咪唑存在时，形成单配体和双配体配合物。根据氯铁（III）母体配合物与双配体配合物在-30℃、双配体配合物与单配体配合物在-50℃的化学交换，可以确定其核磁共振谱。双咪唑配合物表现出低自旋Fe（III）中心（S = 1/2）的吡咯CH2和CH3共振特征；在203 K时，八甲基配合物的δ值分别为-95和-82.5 ppm，二甲基六乙基配合物的δ值分别为-188和-161 ppm，这表明存在一个大环中心的未配对电子。这些双配体配合物的磁矩比整个S = 1体系的磁矩略低，并且随着温度的降低而减小。较低的视磁矩（2.0-1.8 mu(B)）在-50和-90℃之间)被认为是由于金属和大环电子之间的弱或无磁耦合以及配合物的溶解度随着温度的降低而降低。在低温（-50℃至-90℃）核磁共振光谱中观察到的H-1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、低空电子激发态以及在研究的最高温度（-50℃）下配体结合/损失平衡的综合影响引起的。
The chloroiron correlates 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 correlates is that of a S = 3/2 Fe(LU) center coupled to a correlate pi radical, where one electron has been removed from the pi system of the correlate. 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 H-1 chemical shifts. For the chloroiron correlates in the presence of imidazole, imidazole-d(4), 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 H-1 chemical shifts observed in the low-temperature (-50 to -90 OC) 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.).