Reactivity of CuIN4 Flattened Complexes: Interplay between Coordination Geometry and Ligand Flexibility
Reactivity of CuIN4 Flattened Complexes: Interplay between Coordination Geometry and Ligand Flexibility
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DOI：
10.1021/acs.inorgchem.0c02037
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发表时间：
2020-10-19
影响因子：
4.6
通讯作者：
Lemus, Luis
中科院分区：
文献类型：
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作者：
Llanos, Leonel;Vera, Cristian;Lemus, Luis
（CuN4）-N-I配合物的氧化还原活性与配位几何之间的关系表明，结构越扁平，反应性越强。这种配体的预组织使复杂的几何形状更接近于过渡状态，这在金属蛋白中被称为“内在”状态，最近将这一概念扩展到铜配合物中。然而，Cu-I配合物的氧化还原化学的许多方面不能仅仅用平坦化来解释。例如，配体灵活性在这种情况下的作用现在是一个公开的辩论。为了分析这一点，我们研究了一系列五种单金属Cu-I希夫碱配合物[Cu-I(L-n)]（+）的氧化性能，这些配合物跨越了从扭曲的方形平面（n = 3）到扭曲的四面体（n = 6,7）的一系列几何形状。这种对Cu-I原子周围结构的阶梯式控制使我们能够通过该系列探索平坦化畸变对电子和氧化还原性质的影响。基于密度泛函理论计算的理论分析补充了实验研究。正如预期的那样，氧化有利于扁平结构，整个系列跨越370 mV的宽电位窗口。在四氯乙烷（TCE）的还原脱卤反应中测试了这种有序行为。动力学研究表明，TCE氧化Cu-I的速度越快，金属的压扁变形越大，氧化电位越低。然而，最活跃的配合物并不是更平面的，这与氧化电位预测的趋势相矛盾。这种不规则性的起源与配体的柔韧性及其与原子/电子转移反应路径的联系有关，突出了需要考虑除平坦扭曲之外的影响，以更好地理解这类重要配合物的反应性。
The relation between redox activity and coordination geometry in (CuN4)-N-I complexes indicates that more flattened structures tend to be more reactive. Such a preorganization of the ligand confers to the complex geometries closer to a transition state, which has been termed the "entatic" state in metalloproteins, more recently extending this concept for copper complexes. However, many aspects of the redox chemistry of Cu-I complexes cannot be explained only by flattening. For instance, the role of ligand flexibility in this context is an open debate nowadays. To analyze this point, we studied oxidation properties of a series of five monometallic Cu-I Schiff-base complexes, [Cu-I(L-n)](+), which span a range of geometries from a distorted square planar (n = 3) to a distorted tetrahedron (n = 6, 7). This stepped control of the structure around the Cu-I atom allows us to explore the effect of the flattening distortion on both the electronic and redox properties through the series. Experimental studies were complemented by a theoretical analysis based on density functional theory calculations. As expected, oxidation was favored in the flattened structures, spanning a broad potential window of 370 mV for the complete series. This orderly behavior was tested in the reductive dehalogenation reaction of tetrachloroethane (TCE). Kinetic studies show that Cu-I oxidation by TCE is faster as the flattening distortion is higher and the oxidation potentials of the metal are lower. However, the most reactive complex was not the more planar, contradicting the trend expected from oxidation potentials. The origin of this irregularity is related to ligand flexibility and its connection with the atom/electron transfer reaction path, highlighting the need to consider effects beyond flattening distortion to better understand the reactivity of this important class of complexes.