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

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-2(II), triangular Cr-3(III), tetrahedral Zn-4(II), and octahedral Zr-6(IV) have been extensively studied, the linear trinuclear SBUs (herein denoted as M-3), though frequently encountered, are rarely discussed as a class. A literature survey reveals that M-3 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 M-3-based MOFs are also sufficiently robust upon guest removal and exchange or multi-step post-modifications to enable catalytic mechanism elucidation. Some of these M-3-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.