Solid-State Phosphine Ligand Engineering via Postsynthetic Modification of Amine-Functionalized Metal-Organic Frameworks

Postsynthetic phospha-Mannich condensation has been investigated for the design of solid-state phosphine ligands using amine-functionalized metal-organic frameworks (MOFs). Hydroxymethylphosphine precursors Ph2P(CH2OH), PhP(CH2OH)(2), and CyP(CH2OH)(2) readily condense at the 2-aminoterephthalate linkers of MIL-101(Al)-NH2 and IRMOF-3 to generate the phosphine-functionalized MOFs MIL/IRMOF-PPh2-x, MIL/IRMOF-PPh-x, and MIL/IRMOF-PCy-x, respectively, where x denotes the phosphine loading per amine site. Solution-state H-1 and P-31{H-1} nuclear magnetic resonance spectra of base-digested MOFs reveal that PhP(CH2OH)(2) and CyP(CH2OH)(2) react at the amine groups of the adjacent linkers, resulting in intraframework cross-linking. The phosphinated MOFs have been investigated as solid-state ligands for the Ir-catalyzed C-H borylation of arenes. MIL-PPh-0.1 and MIL-PCy-0.1 exhibit good activity for the benchmark C-H borylation of toluene when metalated with [Ir(OMe)(cod)](2) (cod = 1,5-cyclooctadiene). MIL-PPh2-0.1 and the IRMOF-3 derivatives show little or no catalytic turnover under the same conditions, revealing that the phosphine connectivity and MOF topology and pore size are critical factors in solid-state ligand design.