Macrocyclic Heterodinuclear Zn^IIPb^II Complexes: Synthesis, Structures, and Hydrolytic Function toward Tris(p-nitrophenyl) Phosphate

A heterodinuclear Zn^IIPb^II complex ZnPb(L)(ClO₄)₂·2H₂O (1) has been obtained where (L)^2- is an unsymmetric macrocycle derived from the 2:1:1 condensation of 2,6-diformyl-4-methylphenol, ethylenediamine and diethylenetriamine and has the "salen"- and "saldien"-like metal-binding sites sharing the phenolic moiety. Its DMF adduct, ZnPb(L)(ClO₄)₂·MeOH·2DMF (1′), crystallizes in the triclinic space group P1 with a = 14.457(4) Å, b = 14.795(6) Å, c = 10.307(9) Å, α = 109.04(5)°, β = 96.24(5)°, γ = 102.56(3)°, V = 1995(2) ų, and Z = 2. The refinement converges with R = 0.058 and R_w = 0.060 for 3532 reflections with |F₀| > 3σ(|F₀|). It has a discrete heterodinuclear core with the Zn^II in the "salen" site and the Pb^II in the "saldien" site of the macrocycle (L)^2-. The Zn has a square-pyramidal geometry together with a methanol oxygen, and the Pb has a seven-coordinate geometry together with one DMF oxygen and one perchlorate oxygen. The complex 1 is converted into [ZnPb(L)(OH)ClO₄]H₂O (2) under a weak alkaline condition. Its anhydrous form, [ZnPb(L)(OH)]ClO₄ (2), crystallizes in the monoclinic space group C2/c wich a = 25.835(4) Å, b = 13.190(6) Å, c = 16.553 Å, β = 106.31(2)°, V = 5413(2) ų and Z = 8. The refinement converges with R = 0.038 and R_w = 0.029 for 3944 reflections with |F₀| > 3σ(|F₀|). It has a dimer structure of a dinuclear {ZnPb(L)(OH)}⁺ unit having the Zn^II in the "salen" site and the Pb^II in the "saldien" site of the macrocycle. The hydroxide is bound to the Zn^II to afford a square-pyramidal geometry about the metal. The dimeric core [ZnPb(L)(OH)]₂²⁺ is formed by the bridge of the hydroxide oxygen to the Pb* of the adjacent molecule and vice versa. The geometry about the Pb in the dimer structure is a pentagonal pyramid showing a distortion to "umbrella-like" structure, with the bridging hydroxide oxygen at the apex, in a DMSO solution, an equilibrium exists between the dimeric and monomeric species: [{ZnPb(L)(OH)}₂]²⁺ ⇌ 2[ZnPb(L)(OH)]⁺. On the basis of ³¹P NMR and visible spectra, 2 is shown to hydrolyze tris(p-nitorophenyl) phosphate (TNP) into bis(p-nitrophenyl) phosphate (BNP) in DMSO. 1 also exhibits a low activity to hydrolyze TNP into BNP due to the equilibrium [ZnPb(L)(H₂O)]²⁺ ⇌ [ZnPb(L)(OH)]⁺ + H⁺. From the reaction mixture with 2, a BNP complex [ZnPb(L)(BNP)]ClO₄ (3) has been isolated. 3 crystallizes in the triclinic space group P1̄ with a = 13.494(9) Å, b = 13.88(1) Å, c = 12.765(8) Å, α = 94.71(6)°, β = 97.02(6)°, γ = 61.68(5)°, V = 2088(2) ų, Z = 2. The refinement converges with R = 0.044 and R_w = 0.056 for 6070 reflections with |F₀| > 3σ(|F₀|). The BNP^- group bridges the pair of metal ions through its two oxygens, together with two phenolic oxygens of (L)^2-. On the basis of the above findings, a mechanistic scheme for the TNP hydrolysis by 2 is proposed; a TNP molecule is bound to the Pb center, and the hydroxide on the adjacent Zn ion attacks the phosphorus nucleus of TNP, leading to the formation of the BNP complex 3.