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.

本工作阐述了一种新型策略，旨在通过构建两性离子金属-有机框架（MOFs）来生成非配位阴离子。通过将阴离子无机次级构建单元（SBUs）[In­(CO2)4]−与阳离子金属卟啉基有机连接体进行组装，我们制备了两性离子MOFs，其内部电荷的完全分离有效地阻止了反离子与阳离子金属中心的潜在结合。我们在三种代表性的电环化反应中展示了MnIII-和FeIII-卟啉在两性离子MOFs中的增强路易斯酸性：烯炔的[2 + 1]环异构化、氮杂环丁烷和烯烃的[3 + 2]环加成，以及醛与二烯的[4 + 2]杂-Diels-Alder环加成。本研究为设计可调化学催化用功能MOFs开辟了新的途径。