Mass spectrometric and spectroscopic studies on hydrolysis of phosphoesters by bis(μ-acetato)-μ-phenolato dinuclear metal(II) complexes (metal = Mn, Co, Ni, and Zn)

Dinuclear metal(II) complexes of 2,6-bis{N-[(2-dimethylamino)ethyl] iminomethyl}-4-methylphenol (HL), [Mn ₂(L)(AcO) ₂(NCS)] (1), [Co ₂(L)(AcO) ₂]BPh ₄ (2), [Ni ₂(L)(AcO) ₂(MeOH)]BPh ₄ (3), and [Zn ₂(L)(AcO) ₂]BPh ₂ (4), have been examined as regards their hydrolytic activity toward tris(p-nitrophenyl) phosphate (TNP) and hydrogen bis(p-nitrophenyl) phosphate (HBNP) by means of mass spectrometric methods as well as UV-visible spectroscopic methods. All the complexes hydrolyze TNP to BNP ^- in the relative activity of 4 > 2 > 1 ≫ 3. It is found that one AcO ^- group of [M ₂(L)(AcO) ₂] ⁺ is replaced with BNP ^-, arising from the hydrolysis of TNP, affording [M ₂(L)(AcO)(bnp)] ⁺ in an equilibrium with [M ₂(L)(AcO) ₂] ⁺: [M ₂ (L)(AcO) ₂] ⁺ + BNP ^- ⇄ [M ₂(L)(AcO)(bnp)] ⁺ + AcO ^-. In the reaction of HBNP with 1-4, [M ₂(L)(AcO)(bnp)] ⁺ is produced in the equilibrium with [M ₂(L)(AcO) ₂] ⁺, and the bound BNP ^- is slowly hydrolyzed in the case of M = Mn and Co. The bound BNP ^- of [Ni ₂(L)(AcO)(bnp)] ⁺ is barely hydrolyzed and the bound BNP ^- of [Zn ₂(L)(AcO)(bnp)] ⁺ is practically not hydrolyzed. The relevance of the complexes to phosphotriesterase and phosphodiesterase is discussed.