The hydrolysis of 4-nitrophenylphosphate promoted by [(N) 4Co(OH/H)(OH/H)] +/2+/3+ species ((N) 4=abap, tren); use of 31P NMR to uncover mechanistic complexity

Abstract

Rate studies of the reaction of [(N) 4Co(OH/H)(OH/H)] +/2+/3+ species ((N) 4=abap ( 5- and 6-isomers), tren) with sodium 4-nitrophenylphosphate (Na 2NPP) in aqueous solution (15, 25 or 40 °C, I=0.50 M, NaClO 4) have been carried out using UV–Vis spectrophotometry and 31P NMR spectroscopy. Under weakly acidic conditions (pH ca. 5, excess Co(III) reagent) anation is faster than hydrolysis, and the first formed [(N) 4Co(OH 2)(NPP/H)] 1+/2+ complex reacts largely with [(N) 4Co(OH 2)OH] 2+ to produce a host of dimeric Co(III) products plus 4-nitrophenol. Under less acidic conditions (pH 6) rates of anation approach those of hydrolysis, and 4-nitrophenol production increasingly occurs via the intramolecular reaction of [(N) 4Co(OH)NPP]. For [Co] T=0.040 M (abap system) this pathway account for 4% of the observed products at pH 5, 47% at pH 7 and 96% at pH 9. Rate laws for the processes are described. Under no pH condition does the pseudo first-order rate constant (or apparent pseudo first-order rate constant) determined from spectrophotometry directly relate to that for intramolecular hydrolysis in [(N) 4Co(OH)NPP]. The X-ray crystal structure of 6-[(abap)Co(O 2PO 2)]·6H 2O is reported, and confirms the presence of chelated phosphate in this species.