Metal–organic frameworks of linear trinuclear cluster secondary building units: structures and applications

Abstract

Secondary building units (SBUs) in metal–organic frameworks (MOFs) are essential from both a structural and performance perspective. While a variety of SBUs, such as paddlewheel Cu^II₂, triangular Cr^III₃, tetrahedral Zn^II₄, and octahedral Zr^IV₆ have been extensively studied, the linear trinuclear SBUs (herein denoted as M₃), though frequently encountered, are rarely discussed as a class. A literature survey reveals that M₃ clusters are ubiquitous in discrete molecular entities as well as in MOFs. Unlike most other cluster types, however, they have an unprecedented metal diversity and ligand tolerance. The single-crystals of some M₃-based MOFs are also sufficiently robust upon guest removal and exchange or multi-step post-modifications to enable catalytic mechanism elucidation. Some of these M₃-based SBUs endow MOFs with stability under demanding conditions necessary, for example, in flue gas separation. Herein we review MOFs sustained by this common but under-appreciated class of SBUs and discuss applications of the resulting MOF motifs.