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Structure-Reactivity Studies in Copper(II)-Catalyzed Phosphodiester Hydrolysis.
Eric L. Hegg1, Stephen H. Mortimore, Chin Li Cheung
1Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin 53706.
Inorganic Chemistry
|October 24, 2001
Summary
Copper complexes with increasing macrocycle size show enhanced phosphodiester hydrolysis. Larger rings decrease dimer formation, increasing active monomer concentration and catalytic rates.
Area of Science:
- Coordination Chemistry
- Catalysis
- Bioinorganic Chemistry
Background:
- Copper complexes are investigated for their catalytic activity in hydrolysis reactions.
- Macrocyclic ligands influence the structure and reactivity of metal complexes.
- Understanding structure-activity relationships is crucial for designing efficient catalysts.
Purpose of the Study:
- To investigate the hydrolysis of activated phosphodiesters using a series of copper complexes, Cu([9-11]aneN(3))X(2).
- To correlate the catalytic activity with the structural and electronic properties of these complexes.
- To elucidate the role of monomer-dimer equilibrium in the catalytic mechanism.
Main Methods:
- Synthesis and characterization of copper complexes with varying macrocycle sizes ([9]aneN(3), [10]aneN(3), [11]aneN(3)).
- X-ray crystallography to determine the solid-state structures of Cu([10]aneN(3))Br(2) and Cu([11]aneN(3))Br(2).
- Kinetic studies to measure the hydrolysis rates of bis(4-nitrophenyl) phosphate (BNPP) and ethyl 4-nitrophenyl phosphate (ENPP).
- Analysis of monomer-dimer equilibrium in solution.
Main Results:
- Crystal structures reveal a progression from square pyramidal to distorted trigonal bipyramidal geometry with increasing macrocycle size.
- Hydrolysis rate constants for BNPP increase by nearly an order of magnitude with increasing ligand ring size (9- to 11-membered).
- Catalyst activity is primarily governed by the monomer-dimer equilibrium, with smaller dimer formation constants (K(f)) leading to higher catalytic rates.
Conclusions:
- The size of the macrocyclic ligand significantly impacts the geometry and catalytic activity of copper complexes.
- Increased ligand size reduces dimer formation, favoring the catalytically active monomeric species.
- The dimerization equilibrium is a more critical factor in determining hydrolysis rates than Lewis acidity or steric constraints on substrate binding and P-O bond cleavage.