Controlling Product Selectivity During Dioxygen Reduction with Mn Complexes Using Pendent Proton Donor Relays and Added Base
Controlling Product Selectivity During Dioxygen Reduction with Mn Complexes Using Pendent Proton Donor Relays and Added Base
复制标题
使用悬垂质子供体继电器和添加碱在锰配合物还原过程中控制产物选择性
DOI：
10.1039/d3sc02611f
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发表时间：
2024
期刊：
影响因子：
8.4
通讯作者：
Machan, Charles W.
中科院分区：
文献类型：
--
作者：
Cook, Emma N.;Courter, Ian M.;Dickie, Diane A.;Machan, Charles W.
氧气（O2）的催化还原在生物能源转换和替代能源应用中是重要的。与Fe基和Co基系统相比，通过均相Mn基系统催化O2还原的实例相对较少。受这种知识缺乏的启发，已经开发了两种用于氧还原反应（ORR）的含有基于联吡啶的非卟啉配体框架的Mn基催化剂，以评估侧挂质子供体中继如何改变ORR的活性和选择性，其中Mn（p-tbudhbpy）Cl（1）用作对照络合物，Mn（nPrdhbpy）Cl（2）在次级配位球中含有侧挂-OMe基团。使用基于铵的质子源，N，N′-二异丙基乙基铵六氟磷酸盐，我们分析了ORR的催化活性：发现1对H2 O2的选择性为64%，2对H2 O2是定量的，其中O2与还原的Mn（II）中心结合是速率决定步骤。在加入共轭碱N，N′-二异丙基乙胺后，观察到的1和2的催化选择性均转变为H2O作为主要产物。有趣的是，虽然选择性的变化表明1和2的机制发生了变化，但在碱的存在下2的催化活性大大增强，并且速率决定步骤变成Mn-过氧化氢物种中O-O键的快速裂解。这些数据表明，引入侧基中继部分可以提高H2 O2的选择性，但以降低来自强氢键相互作用的反应速率为代价。此外，虽然催化速率增强，观察到的产物选择性的变化时，碱被添加到缓冲质子活性，悬垂继电器稳定二聚体中间体，限制了最大速率。
The catalytic reduction of dioxygen (O2) is important in biological energy conversion and alternative energy applications. In comparison to Fe- and Co-based systems, examples of catalytic O2 reduction by homogeneous Mn-based systems is relatively sparse. Motivated by this lack of knowledge, two Mn-based catalysts for the oxygen reduction reaction (ORR) containing a bipyridine-based non-porphyrinic ligand framework have been developed to evaluate how pendent proton donor relays alter activity and selectivity for the ORR, where Mn(p-tbudhbpy)Cl (1) was used as a control complex and Mn(nPrdhbpy)Cl (2) contains a pendent –OMe group in the secondary coordination sphere. Using an ammonium-based proton source, N,N′-diisopropylethylammonium hexafluorophosphate, we analyzed catalytic activity for the ORR: 1 was found to be 64% selective for H2O2 and 2 is quantitative for H2O2, with O2 binding to the reduced Mn(II) center being the rate-determining step. Upon addition of the conjugate base, N,N′-diisopropylethylamine, the observed catalytic selectivity of both 1 and 2 shifted to H2O as the primary product. Interestingly, while the shift in selectivity suggests a change in mechanism for both 1 and 2, the catalytic activity of 2 is substantially enhanced in the presence of base and the rate-determining step becomes the bimetallic cleavage of the O–O bond in a Mn-hydroperoxo species. These data suggest that the introduction of pendent relay moieties can improve selectivity for H2O2 at the expense of diminished reaction rates from strong hydrogen bonding interactions. Further, although catalytic rate enhancements are observed with a change in product selectivity when base is added to buffer proton activity, the pendent relays stabilize dimer intermediates, limiting the maximum rate.