Metal Ion Promoted Hydrolysis of Polyphosphates

Abstract

Various Co^III amine aqua hydroxo complexes have been evaluated for their ability to hydrolyse pyrophosphate to phosphate rapidly. Two very effective reagents have emerged, [Co( trpn )(OH)(OH₂)]²⁺ ( trpn = 3,3′,3″-nitrilotris(propan-1-amine)) and [Co( tamen )(OH)-OH₂)]²⁺ ( tamen = 6-(4-amino-2-azabutyl)-6-methyl-1,4-diazacycloheptane), and the latter has been examined closely to assess details of the mechanistic path for hydrolysis. The study shows substitution of the reagent on the substrate is fast and not rate determining. Two moles of the reagent are required to bind to [(en)₂Co(P₂O₇)]^- (en = ethane-1,2-diamine) to effect rapid hydrolysis. One supplies the coordinated OH^- nucleophile to cleave the phosphorus anhydride, and the other binds as a chelate to the leaving group to assist bond rupture. An unproductive side path, to form a symmetrical dichelate of the form [(en)₂Co(P₂O₇)Co( tamen )]²⁺, becomes rate limiting. However, if this path can be reduced in significance or eliminated, the study implies that polyphosphates and phosphate esters should hydrolyse on the subsecond time scale.