Cobalt(III)-promoted hydrolysis of 4-nitrophenyl phosphate: the role of dinuclear species

The reactions which follow on mixing aqueous solutions of CoIII(tn)2(aq) and 4-nitrophenyl phosphate (NPP) have been reexamined using spectrophotometry and 31P NMR (here ‘(aq)’ refers to (H2O)2, (H2O)(OH), or (OH)2 depending on pH; charges omitted). When solutions of CoIII(tn)2(aq) and NPP are mixed with pH maintained in the neutral region, equilibrium formation of the mononuclear monodentate complex CoIII(tn)2(OH2)(NPP) (cis and trans) is established in a few minutes. This 1:1 complex (or its deprotonated hydroxo form) undergoes NPP hydrolysis by two parallel paths: (a) direct rate determining hydrolysis to produce 4-nitrophenol(ate) (NP) and Co(tn)2PO4; and (b) reaction with additional CoIII(tn)2(aq) to form a dinuclear complex of NPP, which never builds up to directly detectable levels and which undergoes hydrolysis to produce NP and the dinuclear complex [(tn)2Co]2(μ4-PO4)3+. Path (b) is favored by high reactant concentrations and by high cobalt to NPP ratios. Additional dinuclear complex of orthophosphate is produced by reaction of Co(tn)2PO4 with CoIII(tn)2(aq). At pH 7.3 and at 25°C: for path (a) k=1.75×10−4 s−1 (a rate enhancement of ∼3×104 over the unpromoted NPP hydrolysis rate); for path (b), the first order rate constant for hydrolysis of the dinuclear complex of NPP is estimated to be >5.8×10−3 s−1 (a rate enhancement of at least 1.0×106 over the unpromoted NPP hydrolysis rate).