Coordination Properties of New Bis(1,4,7-triazacyclononane) Ligands: A Highly Active Dizinc Complex in Phosphate Diester Hydrolysis

Abstract

The synthesis and characterization of three new bis([9]aneN₃) ligands, containing respectively 2,2‘-bipyridine (L¹), 1,10-phenanthroline (L²), and quinoxaline (L³) moieties linking the two macrocyclic units, are reported. Proton binding and Cu(II), Zn(II), Cd(II), and Pb(II) coordination with L¹−L³ have been studied by potentiometric titrations and, for L¹ and L², by spectrophotometric UV−vis measurements in aqueous solutions. All ligands can give stable mono- and dinuclear complexes. In the case of L¹, trinuclear Cu(II) complexes are also formed. The stability constants and structural features of the formed complexes are strongly affected by the different architecture and binding properties of the spacers bridging the two [9]aneN₃ units. In the case of the L¹ and L² mononuclear complexes, the metal is coordinated by the three donors of one [9]aneN₃ moiety; in the [ML²]²⁺ complexes, however, the phenanthroline nitrogens are also involved in metal binding. Finally, in the [ML³]²⁺ complexes both macrocyclic units, at a short distance from each other, can be involved in metal coordination, giving rise to sandwich complexes. In the binuclear complexes each metal ion is generally coordinated by one [9]aneN₃ unit. In L¹, however, the dipyridine nitrogens can also act as a potential binding site for metals. The dinuclear complexes show a marked tendency to form mono-, di-, and, in some cases, trihydroxo species in aqueous solutions. The resulting M−OH functions may behave as nucleophiles in hydrolytic reactions. The hydrolysis rate of bis(p-nitrophenyl)phosphate (BNPP) was measured in aqueous solution at 308.1 K in the presence of the L² and L³ dinuclear Zn(II) complexes. Both the L² complexes [Zn₂L²(OH)₂]²⁺ and [Zn₂L²(OH)₃]⁺ and the L³ complex [Zn₂L³(OH)₃]⁺ promote BNPP hydrolysis. The [Zn₂L³(OH)₃]⁺ complex is ca. 2 orders of magnitude more active than the L² complexes, due both to the short distance between the metal centers in [Zn₂L³(OH)₃]⁺, which could allow a bridging interaction of the phosphate ester, and to the simultaneous presence of single-metal bound nucleophilic Zn−OH functions. These structural features are substantially corroborated by semiempirical PM3 calculations carried out on the mono-, di-, and trihydroxo species of the L³ dizinc complex.