Structure, Equilibrium and Ribonuclease Activity of Copper(II) and Zinc(II) Complexes Formed with a Dinucleating Bis‐Imidazole Ligand

The syntheses, crystal structures, solution equilibria and ribonuclease activity are reported for copper(II) and zinc(II) complexes of a new potentially dinucleating, bis‐imidazole ligand N , N ′‐bis(5‐methylimidazol‐4‐ylmethyl)‐1,3‐diaminopropan‐2‐ol ( bimido ). The zinc(II) ion in [Zn(bimido)Cl]NO 3 is coordinated in a slightly distorted square pyramidal environment, with the four N atoms of bimido in the basal positions and the Cl − ion in the axial one. The two copper(II) ions in [Cu 2 (bimido −1H )(DPP)(ClO 4 )(CH 3 OH)]ClO 4 ·1/2 H 2 O (DPP = diphenyl phosphate) are bridged by the deprotonated alkoxo group of bimido and by the phosphate group of DPP in a 1,3‐bridging mode. Depending on the [M]/[L] ratio, the ML and M 2 L −2H species are present in solution in the neutral pH range, having analogous structures as described above for the crystalline complexes. The zinc(II)‐bimido (2/1) system, in a 65% EtOH‐H 2 O mixed solvent, shows an important increase of hydrolytic activity, parallel with the formation of the Zn 2 L −2H species, with a sigmoidal pH‐rate profile modelling both steps of RNA hydrolysis. The k cat value for the transesterification of 2‐hydroxypropyl‐ p ‐nitrophenyl phosphate by Zn 2 L −2H , determined from saturation kinetic measurements ( T = 298 K), corresponds to a maximum rate acceleration ( k cat / k uncat ) of ca. 10 4 . The observed pseudo‐first order rate constant for the hydrolysis of uridine 2′,3′‐cyclic monophosphate, under nearly physiological conditions ( T = 310 K, pH = 8, [Zn 2 L −2H ] = 3.6 m M , k obs = 2·10 −5 s −1 ) reflects a higher hydrolytic activity of Zn 2 L −2H towards this biologically relevant substrate ( k obs / k uncat indicates ca. 10 4 fold rate acceleration). The proposed mechanisms include bifunctional Lewis‐acid and general base catalysis.