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.