Click Heterogenization of Phosphines Furnishes Recyclable Hydroformylation Catalysts that Reproduce Homogeneous Performance

Heterogeneous catalysts
confer notable practical advantages for
large-scale reactions, while homogeneous catalysts permit targeted
performance optimization. A rapid and general method for the heterogenization
of molecular transition-metal catalysts without loss of performance
would thus permit facile translation of optimized homogeneous catalysts
into practical heterogeneous catalysts. Here, we show how a wide variety
of phosphines carrying anionic substituents can be charge-tethered
to the walls of the spacious supercages of a metal–organic
framework (MOF) to provide an adaptable heterogeneous ligand set in
a single synthetic step. The addition of Co2(CO)8 to MOF-heterogenized phosphine ligands provides recyclable, heterogeneous
hydroformylation catalysts that faithfully reproduce the performance
of the molecular analogues in both activity and selectivity. Key to
the solution-like reactivity of the click heterogenized phosphines
is their high degree of mobility, which was directly demonstrated
by 31P NMR analysis and which enables them to effectively
accommodate cobalt complexes with three distinct oxidation states
and coordination geometries. While the lack of directionality of the
ionic interaction between the ligand and the host permits the phosphines
to effectively reproduce homogeneous catalytic cycles, the strength
of the ionic interaction ensures that phosphine leaching remains below
0.05 ppm.