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 ¹H and ³¹P{¹H} 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.