Phosphine and Arsine MOFs with Stabilized Diosmium(I) Carbonyl Sawhorse Pillars

A new metal-organic framework (MOF) with low-valent, σ-bonded Os^I-Os^I pillars has been prepared using [Os₂(ER₃)₂(μ₂-O₂CH)₂(CO)₄] sawhorse complexes (ER₃ = {E(C₆H₄CO₂)}^3-; E = P or As). This is the first example of the inclusion of Os^I into a MOF and is a rare example of a Lewis acidic {M-M}²⁺ structural motif, which provides broader insights into new synthetic methods for the preparation of MOFs containing catalytically relevant metal species. The isostructural phosphine and arsine MOFs reported here, PCM-201 and AsCM-201, respectively, achieve stabilization of the Os₂ cores compared to molecular examples; combined experimental and DFT studies show that the MOF rigidity prevents cleavage of the Os^I-Os^I bond, which gains stability in the solid state. In contrast, UV irradiation of single crystals at 254 and 365 nm results in a release of CO ligands from Os^I centers. The calculated HOMO and LUMO electronic states are found to be markedly different in the MOF versus the free complexes, in addition to significant lattice strain effects, which have important implications for the design of other low-valent MOFs for catalysis.