Peprah Frank, Tarantola Grace E, Plaman Alyson S, Vu Emily L, Huynh Alyssa B, Durr Christopher B
Department of Chemistry, Amherst College, 25 East Drive, Amherst, Massachusetts 01002, USA.
Dalton Trans. 2024 Apr 23;53(16):7073-7080. doi: 10.1039/d4dt00422a.
The synthesis, characterization, and ring-opening polymerization (ROP) activity of a family of niobium and tantalum alkoxide catalysts was studied. The final catalysts are made in a two-step synthesis, first by reacting the desired homoleptic metal ethoxide with a phenolketoimine ligand to form a series of synthetic intermediates, followed by reaction with catechol to produce a catalytic platform with a single ethoxide initiator. By using two separate ligands, the electronic properties of the catalyst can be tuned, and the molecular weight of the polymer can be increased. It was found that synthetic intermediates adopted a geometry both in solution and in the solid state. This geometry was retained for the final catechol derivatives, however in one case, where catechol was substituted for 3-methoxycatechol, the molecule adopted a highly distorted geometry. Catalytic ROP activity of the synthetic intermediates and final catechol derivatives with ε-caprolactone was studied through a kinetic analysis. In all seven cases studied the reactions proceeded through the expected coordination-insertion mechanism, following pseudo first-order kinetics and increasing in linearly . conversion. The single-initiator catechol derivatives increased the by three times compared to that of the three-initiator synthetic intermediates with little decrease in the overall reaction rate. Both the nature of the ligand and metal were found to impact the rate of reaction in these systems. By switching from an electron donating ligand to an electron withdrawing ligand, the rate was found to nearly double. Tantalum species were faster than their niobium counterparts by ∼3 times in the synthetic intermediates and ∼1.5 times in the catechol derivatives. This observed periodicity supports recent literature findings in this area.
研究了一系列铌和钽醇盐催化剂的合成、表征及开环聚合（ROP）活性。最终的催化剂通过两步合成制备，首先使所需的均配金属乙醇盐与酚酮亚胺配体反应形成一系列合成中间体，然后与邻苯二酚反应生成具有单一乙醇盐引发剂的催化平台。通过使用两种不同的配体，可以调节催化剂的电子性质，并提高聚合物的分子量。研究发现，合成中间体在溶液和固态中均采用一种几何构型。这种几何构型在最终的邻苯二酚衍生物中得以保留，然而在一种情况下，用3-甲氧基邻苯二酚取代邻苯二酚时，分子采用了高度扭曲的几何构型。通过动力学分析研究了合成中间体和最终邻苯二酚衍生物与ε-己内酯的催化ROP活性。在所研究的所有七种情况下，反应均通过预期的配位插入机理进行，遵循准一级动力学，且转化率呈线性增加。与具有三个引发剂的合成中间体相比，单一引发剂的邻苯二酚衍生物使转化率提高了三倍，而总反应速率几乎没有降低。研究发现，配体和金属的性质均会影响这些体系中的反应速率。通过从给电子配体切换到吸电子配体，发现反应速率几乎翻倍。在合成中间体中，钽物种的反应速度比铌物种快约3倍，在邻苯二酚衍生物中快约1.5倍。这种观察到的周期性支持了该领域最近的文献研究结果。
