Dinuclear Metal Complexes Based on all cis 2,4,6-triamino-1,3,5-trihydroxycyclohexane as Catalysts of Phosphate Esters Cleavage

Two dimetallic ligands 2 and 3 for transition metal ions were obtained by connecting two all-cis- 2,4,6-triamino-cyclohexane-1,3,5-triol (TACI, 1) subunits via 1,3- or 1,4-xylyl linkers. Their dimetallic Cu-II and Zn-II complexes were investigated as catalysts for the cleavage of the phosphate diesters 2,4-dinitrophenyl ethyl phosphate (DNPEP) and 2-hydroxypropyl p-nitrophenyl phosphate (HPNP) and the triester 2,4-dinitrophenyl diethyl phosphate (DNPDEP). The results of a comparative kinetic study using the monometallic complexes of TACI as a reference indicate that the Cu-II complexes of 2 and 3 are virtually inert; this finding is ascribed to the formation of intra-complex p-hydroxo bridges that prevent the required interactions with the substrate. On the other hand, the dimetallic Zn-II complexes produce remarkable accelerations, particularly in the case of the HPNP transesterification. The dimetallic systems are more efficient than the TACI.Zn-II complex in promoting the hydrolysis of HPNP at pH values close to neutrality (7.0-7.8). In this case, the effects of cooperativity between the two metal centers were highlighted in a detailed kinetic study; the catalytic efficiency seems to be related to the stronger binding of the substrate to the dimetallic Zn-II complexes. Additionally, in this case, although to a much lesser extent than in the case of the Cu-II counterparts, the formation of p-hydroxo bridges apparently hampers the catalytic efficiency, as indicated also by the observation that the activity of the dimetallic complexes increases as the distance between the two metal centers increases, which, thus, thwarts the formation of intermetallic bridges.