Pyrazolylcyclotriphosphazene containing pendant polymers: Synthesis, characterization, and phosphate ester hydrolysis using a Cu(II)-metalated cross-linked polymeric catalyst

Abstract

A multi-pyrazolyl cyclotriphosphazene containing polymerizable group N₃P₃(3,5-Me₂Pz)₅(O-C₆H₄-p-C₆H₄-p-CH=CH₂) (2) has been prepared from the corresponding chloro derivative N₃P₃Cl₅(O-C₆H₄-p-C₆H₄-p-CH=CH₂) (1). The X-ray structures of 1 and 2 have been determined. Compound 2 undergoes ready metalation with CuCl₂ to afford N₃P₃(3,5-Me₂Pz)₅(O-C₆H₄-p-C₆H₄-p-CH=CH₂)·CuCl₂ (3). Model compound N₃P₃(3,5-Me₂Pz)₅(O-C₆H₄-p-CHO)·CuCl₂ (6) has been prepared and characterized by spectroscopy and X-ray crystallography. In this compound, the coordination around copper is distorted trigonal bipyramidal, and the cyclotriphosphazene coordinates in a non-gem N₃ mode. Compound 2 has been copolymerized with divinylbenzene to afford cross-linked multisite coordinating polymer CPPL which is readily metalated with CuCl₂ to afford copper-containing polymer CPPL-Cu. The coordination environment around copper in CPPL-Cu has been evaluated by obtaining its EPR, optical, and IR spectra and comparing them with those of model compounds 3 and 6. The utility of CPPL-Cu as a heterogeneous catalyst has been demonstrated in the phosphate ester hydrolysis involving three model phosphate esters: p-nitrophenyl phosphate (pNPP), bis(p-nitrophenyl) phosphate (bNPP), and 2-(hydroxypropyl)-p-nitrophenyl phosphate (hNPP). In all of these reactions, a significant rate enhancement of ester hydrolysis is observed. Detailed kinetic analyses to evaluate Michaelis-Menten parameters have also been carried out along with experiments to elucidate the effect of pH, solvent, and temperature on the rate of hydrolysis. Recycling experiments on the hydrolysis of pNPP with CPPL-Cu shows that it can be recycled several times over without affecting the rates.