Cobalt(III) and copper(II) hydrides at the crossroad of catalysed chain transfer and catalysed radical termination: a DFT study
Cobalt(III) and copper(II) hydrides at the crossroad of catalysed chain transfer and catalysed radical termination: a DFT study
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钴(III)和铜(II)氢化物处于催化链转移和催化自由基终止的十字路口：DFT研究
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发表时间：
2016
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影响因子：
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通讯作者：
R. Poli
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作者：
S. Rahaman;K. Matyjaszewski;R. Poli
介导自由基聚合的金属配合物也可能导致催化链转移（CCT）或催化自由基终止（CRT），这两个过程都是通过相同类型的氢化物中间体发生的。目前还不清楚是什么导致这些中间体更喜欢与单体反应，产生 CCT，或与自由基反应，产生 CRT。我们在此报告了两种不同的氢化物配合物 [(TMP)CoIII(H)]（TMP = 四均卟啉）和 [(TPMA)CuII(H)]+ (TPMA = 三(2-吡啶基甲基)胺)（由 [CoII(TMP)] 和 [CuI(TPMA)]+ 生成）与单体和自由基的反应性比较的 DFT 研究，使用˙CH(CH3)(COOCH3) 和 ˙C(CH3)2(COOCH3) 自由基作为不断增长的 PMA 和 PMMA 自由基链的模型。未取代的卟啉被用作全量子力学 (QM) 计算的模型，但也通过混合 QM/MM 方法对全 TMP 系统进行了选定的计算，在分子力学 (MM) 水平上处理异丙基取代基。这些计算为合理化实验观察到的钴体系在催化链转移（CCT）聚合中的强活性提供了基础，迄今为止尚未报道催化自由基终止（CRT）的活性，而铜体系导致CRT但不促进CCT。本质上，钴体系有利于 CCT 的关键因素是 H 转移到单体的势垒非常低，并且单体相对于自由基的浓度要高得多，从而产生 vCCT > vCRT。另一方面，对于铜体系，H 向单体转移的更大势垒使得 CCT 速率慢得多，而 CRT 猝灭路径有利地通过电子无势垒路径发生，在长 C⋯H 距离下具有初期稳定性。两个系统的不同自旋状态（Co 系统沿 CCT 路径自旋猝灭，Cu 系统沿 CRT 路径自旋猝灭）使观察到的行为合理化。新获得的理解应该有助于设计更高效的系统。
Metal complexes that mediate radical polymerisation may also lead to catalysed chain transfer (CCT) or to catalysed radical termination (CRT), both processes occurring via the same type of hydride intermediate. What leads these intermediates to prefer reacting with the monomer, leading to CCT, or with radicals, leading to CRT, was unclear. We report here a DFT investigation of the comparative reactivity of two different hydride complexes, [(TMP)CoIII(H)] (TMP = tetramesitylporphyrin) and [(TPMA)CuII(H)]+ (TPMA = tris(2-pyridylmethyl)amine), generated from [CoII(TMP)] and [CuI(TPMA)]+, versus the monomer and radical, using the ˙CH(CH3)(COOCH3) and ˙C(CH3)2(COOCH3) radicals as models for the growing PMA and PMMA radical chains. The unsubstituted porphyrin was used as a model for full quantum mechanical (QM) calculations, but selected calculations on the full TMP system were also carried out by the hybrid QM/MM approach, treating the mesityl substituents at the molecular mechanics (MM) level. The calculations provide a basis for rationalizing the experimentally observed strong activity of the cobalt system in catalysed chain transfer (CCT) polymerization without a reported activity so far for catalysed radical termination (CRT), whereas the copper system leads to CRT but does not promote CCT. In essence, the key factors in favour of CCT for the cobalt system are a very low barrier for H transfer to the monomer and the much greater concentration of the monomer relative to the radical, yielding vCCT > vCRT. For the copper system, on the other hand, the greater barrier for H transfer to the monomer makes the CCT rate much slower, while the CRT quenching pathway favourably takes place through an electronically barrierless pathway with incipient stabilization at long C⋯H distances. The different spin states of the two systems (spin quenching along the CCT pathway for the Co system and along the CRT pathway for the Cu system) rationalize the observed behavior. The new acquired understanding should help design more efficient systems.