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（zwitterionic metal–organic frameworks, MOFs）制备非配位阴离子的全新策略。通过将阴离子型无机次级结构单元（secondary building units, SBUs）([In(CO₂)₄]⁻)与阳离子型金属卟啉基有机连接体进行组装，我们成功制备出两性离子MOFs，该材料内部实现了完全电荷分离，可有效阻止抗衡阴离子与阳离子金属中心发生潜在结合。我们在三类典型电环化反应中验证了两性离子MOFs中三价锰卟啉与三价铁卟啉的增强路易斯酸性：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。本研究为设计可调控化学催化的功能性MOFs开辟了全新路径。