A New Approach to Non-Coordinating Anions: Lewis Acid Enhancement of Porphyrin Metal Centers in a Zwitterionic Metal–Organic Framework

We describe a new strategy to generate non-coordinating anions using zwitterionic metal–organic frameworks (MOFs). By assembly of anionic inorganic secondary building blocks (SBUs) ([In­(CO2)4]−) with cationic metalloporphyrin-based organic linkers, we prepared zwitterionic MOFs in which the complete internal charge separation effectively prevents the potential binding of the counteranion to the cationic metal center. We demonstrate the enhanced Lewis acidity of MnIII- and FeIII-porphyrins in the zwitterionic MOFs in three representative electrocyclization reactions: [2 + 1] cycloisomerization of enynes, [3 + 2] cycloaddition of aziridines and alkenes, and [4 + 2] hetero-Diels–Alder cycloaddition of aldehydes with dienes. This work paves a new way to design functional MOFs for tunable chemical catalysis.我们报道了一种利用两性离子金属有机框架（MOFs）制备非配位阴离子的新策略。通过将阴离子型无机次级结构单元（SBUs，[In(CO₂)₄]⁻）与阳离子型金属卟啉基有机配体进行组装，我们合成了两性离子MOFs，其内部完全的电荷分离可有效阻止抗衡阴离子与阳离子金属中心发生潜在结合。我们针对三类典型电环化反应，验证了两性离子MOFs中Mn(III)和Fe(III)卟啉的路易斯酸性得到增强：分别为烯炔的[2+1]环异构化反应、氮丙啶与烯烃的[3+2]环加成反应，以及醛与二烯烃的[4+2]杂狄尔斯-阿尔德环加成反应。本研究为设计用于可调谐化学催化的功能化MOFs开辟了全新途径。