Metal-organic frameworks (MOFs) incorporating low-valent transition metals are attractive materials for applications including catalysis and separations. These metals have traditionally been appended to MOF structures by adding soft Lewis basic sites to standard MOF linkers (e.g. polycarboxylates). Interestingly, very few examples exist of MOF structures with low-valent metals incorporated directly as structural nodes. Because phosphine ligands are widely used to stabilize low-valent metal complexes, polytopic phosphine ligands are well suited for the synthesis of such materials. In this work, the first examples of three-dimensional Pd(0) and Pt(0) MOF structures are reported and fully characterized, which also represent the first examples of three-dimensional MOFs constructed using only metal−phosphine coordination bonds. A synthetic strategy was employed using triphenylphosphine as a modulator to give crystalline materials, which were characterized by single-crystal X-ray diffraction. A total of five new Pd(0) and Pt(0) MOFs are realized using four different tetratopic phosphine linkers. These materials obey the principles of reticular chemistry that are canonical to the MOF field, and their luminescent properties give insight into the electronic properties that govern their formation and stability. This approach has been extended to Rh(I), yielding two new materials with distinct topologies compared to the Pd(0)/Pt(0) MOFs. This demonstrates that polytopic phosphine ligands, coupled with this unique phosphine-modulated synthetic method, may be broadly applied to the synthesis of low-valent MOFs with tunable catalytic and materials properties.

Low-valent metal-organic frameworks constructed using polytopic phosphine ligands

ACS Spring 2022

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