Comparison and Analysis of Zinc and Cobalt-Based Systems as Catalytic Entities for the Hydration of Carbon Dioxide

In nature, the zinc metalloenzyme carbonic anhydrase II (CAII) efficiently catalyzes the conversion of carbon dioxide (CO2) to bicarbonate under physiological conditions. Many research efforts have been directed towards the development of small molecule mimetics that can facilitate this process and thus have a beneficial environmental impact, but these efforts have met very limited success. Herein, we undertook quantum mechanical calculations of four mimetics, 1,5,9-triazacyclododedacane, 1,4,7,10-tetraazacyclododedacane, tris(4,5-dimethyl-2-imidazolyl)phosphine, and tris(2-benzimidazolylmethyl)amine, in their complexed form either with the Zn2+ or the Co2+ ion and studied their reaction coordinate for CO2 hydration. These calculations demonstrated that the ability of the complex to maintain a tetrahedral geometry and bind bicarbonate in a unidentate manner were vital for the hydration reaction to proceed favorably. Furthermore, these calculations show that the catalytic activity of the examined zinc complexes was insensitive to coordination states for zinc, while coordination states above four were found to have an unfavorable effect on product release for the cobalt counterparts.自然界中，锌基金属酶碳酸酐酶II（carbonic anhydrase II, CAII）可在生理条件下高效催化二氧化碳（carbon dioxide, CO₂）转化为碳酸氢根。诸多研究致力于开发可促进该过程、进而产生有益环境影响的小分子模拟物，但此类研究收效甚微。本文中，我们对四种模拟物——1,5,9-三氮杂环十二烷（1,5,9-triazacyclododedacane）、1,4,7,10-四氮杂环十二烷（1,4,7,10-tetraazacyclododecane）、三(4,5-二甲基-2-咪唑基)膦（tris(4,5-dimethyl-2-imidazolyl)phosphine）以及三(2-苯并咪唑基甲基)胺（tris(2-benzimidazolylmethyl)amine）——与锌离子（Zn²+）或钴离子（Co²+）形成的配合物开展了量子力学计算，并研究了它们用于CO₂水合反应的反应坐标。计算结果表明，配合物维持四面体几何构型并以单齿配位方式结合碳酸氢根的能力，是水合反应顺利进行的关键。此外，计算结果还显示，所考察的锌配合物的催化活性不受锌配位态的影响；而对于钴配合物，配位数高于4的配位态会对产物释放产生不利影响。