Walker Ashlie N, Ayala Megan A, Mondal Somrita, Bergagnini Mackenzie C, Bui Phuong John D, Chidester Stephanie N, Doeden Chad I, Esjornson Louise, Sweany Brian R, Garcia Leslie, Krause Jeanette A, Oliver Allen G, Prior Timothy J, Hubin Timothy J
Department of Chemistry and Physics, Southwestern Oklahoma State University, Weatherford, OK 73096, USA.
Department of Chemistry, University of Cincinnati, Cincinnati, OH 45220, USA.
Molecules. 2023 Jan 16;28(2):895. doi: 10.3390/molecules28020895.
Tetraazamacrocycles, cyclic molecules with four nitrogen atoms, have long been known to produce highly stable transition metal complexes. Cross-bridging such molecules with two-carbon chains has been shown to enhance the stability of these complexes even further. This provides enough stability to use the resulting compounds in applications as diverse and demanding as aqueous, green oxidation catalysis all the way to drug molecules injected into humans. Although the stability of these compounds is believed to result from the increased rigidity and topological complexity imparted by the cross-bridge, there is insufficient experimental data to exclude other causes. In this study, standard organic and inorganic synthetic methods were used to produce unbridged dibenzyl tetraazamacrocycle complexes of Co, Ni, Cu, and Zn that are analogues of known cross-bridged tetraazamacrocycles and their transition metal complexes to allow direct comparison of molecules that are identical except for the cross-bridge. The syntheses of the known tetraazamacrocycles and the new transition metal complexes were successful with high yields and purity. Initial chemical characterization of the complexes was conducted by UV-Visible spectroscopy, while cyclic voltammetry showed more marked differences in electronic properties from bridged versions. Direct comparison studies of the unbridged and bridged compounds' kinetic stabilities, as demonstrated by decomposition using high acid concentration and elevated temperature, showed that the cyclen-based complex stability did not benefit from cross-bridging. This is likely due to poor complementarity with the Cu ion while cyclam-based complexes benefited greatly. We conclude that ligand-metal complementarity must be maintained in order for the topological and rigidity constraints imparted by the cross-bridge to contribute significantly to complex robustness.
四氮杂大环化合物是一种含有四个氮原子的环状分子，长期以来人们已知它能生成高度稳定的过渡金属配合物。研究表明，用双碳链对这类分子进行交叉桥连可进一步提高这些配合物的稳定性。这为将所得化合物应用于从水相绿色氧化催化到注入人体的药物分子等各种不同且要求苛刻的领域提供了足够的稳定性。尽管人们认为这些化合物的稳定性源于交叉桥连所赋予的更高刚性和拓扑复杂性，但尚无足够的实验数据排除其他原因。在本研究中，采用标准的有机和无机合成方法制备了钴、镍、铜和锌的未桥连二苄基四氮杂大环配合物，它们是已知交叉桥连四氮杂大环化合物及其过渡金属配合物的类似物，以便直接比较除交叉桥连外其他方面均相同的分子。已知四氮杂大环化合物和新型过渡金属配合物的合成均取得成功，产率和纯度都很高。通过紫外可见光谱对配合物进行了初步化学表征，而循环伏安法显示其电子性质与桥连形式相比有更显著的差异。通过使用高酸浓度和升高温度进行分解所证明的未桥连和桥连化合物动力学稳定性的直接比较研究表明，基于环四胺的配合物稳定性并未因交叉桥连而得到提升。这可能是由于与铜离子的互补性较差，而基于环戊二烯四胺的配合物则受益匪浅。我们得出结论，为了使交叉桥连所赋予的拓扑和刚性限制对配合物的稳定性有显著贡献，必须保持配体 - 金属的互补性。
