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

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.