Construction of Chimeric Metal–Organic Frameworks with Symmetry-Mismatched Building Blocks

Chimeric metal–organic frameworks (CMOFs) composed
of symmetry-mismatched
inorganic and organic building blocks are rare because the interconnections
between these blocks are topologically demanding. Herein, an MOF with
symmetry-matched building blocks is used as a self-template for the
template-assisted synthesis of CMOFs. Specifically, the post-synthetic
transformation of the [Zn4O(COO)6] clusters
in the self-templating MOF into the [Fe3IIIO(COO)6]+ clusters with symmetry-mismatched trigonal prismatic
site symmetry affords isoreticular CMOFs while maintaining the template
morphology and crystallinity. The framework strain of CMOFs due to
the symmetry-mismatched linkages between their building blocks is
reduced by modulating the conformation of organic building blocks
with torsional degrees of freedom. The further transformation of [Fe3IIIO(COO)6]+-based CMOFs
through redox-facilitated metal exchange using Cr2+ ions
yields highly stable isostructural CMOFs containing [Cr3IIIO(COO)6]+ clusters and exhibiting
stabilities in strongly acidic and moderately basic media comparable
to those of prototypical [Cr3IIIO(COO)6]+-based MOFs with symmetry-matched building blocks. These
results hold significant potential and highlight the vast opportunities
for revisiting well-known yet fragile MOFs built upon the zinc acetate
clusters.

由对称性失配的无机与有机结构基元构成的嵌合金属有机框架（Chimeric metal–organic frameworks, CMOFs）较为罕见，因为此类结构基元间的连接面临严苛的拓扑要求。本文以具有对称性匹配结构基元的金属有机框架作为自模板，通过模板辅助法合成CMOFs。具体而言，将自模板金属有机框架中的[Zn4O(COO)6]簇进行合成后转化，得到具有对称性失配三角棱柱位点对称性的[Fe3IIIO(COO)6]+簇，由此获得同网格CMOFs，同时保留了模板的形貌与结晶性。针对结构基元间对称性失配连接所引发的CMOFs框架应变，通过调控具有扭转自由度的有机结构基元构象来予以缓解。进一步以Cr²+离子通过氧化还原辅助金属交换对基于[Fe3IIIO(COO)6]+簇的CMOFs进行转化，可得到含[Cr3IIIO(COO)6]+簇的高稳定同构CMOFs，其在强酸性与中等碱性介质中的稳定性可媲美经典的对称性匹配结构基元型[Cr3IIIO(COO)6]+基金属有机框架。本研究结果展现出巨大应用潜力，并凸显了重新审视以乙酸锌簇为基础的经典但稳定性欠佳的金属有机框架的广阔机遇。