Inside or outside the box? Effect of substrate location on coordination-cage based catalysis

Area of Science:

• Supramolecular Chemistry
• Catalysis
• Coordination Chemistry

Background:

• Coordination cages are investigated as catalysts for ester hydrolysis.
• Substrate binding location (cavity vs. exterior) influences catalytic activity.
• Hydroxide ion concentration near the cage surface is crucial for catalysis.

Purpose of the Study:

• To compare and contrast the hydrolysis of diacetyl fluorescein (DAF) and 4-nitrophenyl acetate (4NPA) using a Co8 coordination cage catalyst.
• To investigate the role of substrate binding location (cage exterior vs. interior) on catalytic efficiency.
• To evaluate the potential for general exterior-surface catalysis and negative cavity catalysis.

Main Methods:

• Utilized an octanuclear cubic Co8 coordination cage as a catalyst.
• Compared hydrolysis rates of DAF and 4NPA under varying conditions.
• Determined binding constants (K) for substrate-cage interactions.
• Measured catalytic rate acceleration (kcat/kuncat).

Main Results:

• DAF, too large for the cavity, showed accelerated hydrolysis (≈50-fold) via exterior surface interaction.
• 4NPA, binding inside the cavity, exhibited inhibited hydrolysis due to unfavorable geometry or steric constraints.
• 4NPA in solution underwent exterior surface catalysis, but limited by low exterior binding affinity.
• Exterior surface catalysis demonstrated effectiveness without size/shape limitations.

Conclusions:

• Exterior-surface catalysis by coordination cages is effective and potentially general.
• Cavity binding can lead to negative catalysis, inhibiting substrate hydrolysis.
• The location of substrate interaction with the catalyst is critical for determining catalytic outcome.