Delle Chiaie K R, Biernesser A B, Ortuño M A, Dereli B, Iovan D A, Wilding M J T, Li B, Cramer C J, Byers J A
Eugene F. Merkert Chemistry Center, Department of Chemistry, Boston College, 2609 Beacon St., Chestnut Hill, Massachusetts 02467, USA.
Dalton Trans. 2017 Oct 14;46(38):12971-12980. doi: 10.1039/c7dt03067c. Epub 2017 Sep 21.
The reactivity of iron-based ring opening polymerization catalysts is compared when the catalyst is in three different oxidation states. Formally iron(i) monoalkoxide complexes 3a (p-methoxyphenoxide) and 3b (neopentoxide) supported by bis(imino)pyridine ligands were synthesized and investigated as catalysts for the ring opening polymerization and copolymerization of various monomers. For most monomers, 3a and 3b were superior catalysts compared to analogous, formally iron(ii) and iron(iii) complexes (1a/1b and 2a/2b, respectively) for the ring opening polymerization of various cyclic ester and cyclic carbonate monomers. Experimental and computational investigation into the electronic structures of 3a and 3b revealed that they are most accurately described as containing a high spin iron(ii) center that is antiferromagnetically coupled to a singly reduced bis(imino)pyridine ligand. This electronic structure leads to increased electron density near the metal center without modulating the apparent metal oxidation state, which results in superior catalytic performance for the more highly reduced 3a and 3b compared to the increasingly more oxidized complexes (i.e.1a/1b and 2a/2b, respectively) in ring opening polymerization reactions. These findings have significant ramifications for the emerging field of redox-switchable polymerization catalysis.
当铁基开环聚合催化剂处于三种不同氧化态时，对其反应活性进行了比较。合成了由双（亚氨基）吡啶配体支撑的形式上的铁（I）单醇盐配合物3a（对甲氧基苯氧基）和3b（新戊氧基），并研究了它们作为各种单体开环聚合和共聚反应的催化剂。对于大多数单体，在各种环状酯和环状碳酸酯单体的开环聚合反应中，与类似的形式上的铁（II）和铁（III）配合物（分别为1a/1b和2a/2b）相比，3a和3b是更优异的催化剂。对3a和3b电子结构的实验和计算研究表明，它们最准确的描述是含有一个高自旋铁（II）中心，该中心与一个单还原的双（亚氨基）吡啶配体反铁磁耦合。这种电子结构导致金属中心附近的电子密度增加，而不改变表观金属氧化态，这使得在开环聚合反应中，与氧化程度越来越高的配合物（即分别为1a/1b和2a/2b）相比，还原程度更高的3a和3b具有更优异的催化性能。这些发现对氧化还原可切换聚合催化这一新兴领域具有重要意义。
