Unsymmetrical dizinc complexes as models for the active sites of phosphohydrolases

Abstract

The unsymmetrical dinucleating ligand 2-(N-isopropyl-N-((2-pyridyl)methyl)aminomethyl)-6-(N-(carboxylmethyl)-N-((2-pyridyl)methyl)aminomethyl)-4-methylphenol (IPCPMP or L) has been synthesized to model the active site environment of dinuclear metallohydrolases. It has been isolated as the hexafluorophosphate salt H₄IPCPMP(PF₆)₂·2H₂O (H₄L), which has been structurally characterized, and has been used to form two different Zn(II) complexes, [{Zn₂(IPCPMP)(OAc)}₂][PF₆]₂ (2) and [{Zn₂(IPCPMP)(Piv)}₂][PF₆]₂ (3) (OAc = acetate; Piv = pivalate). The crystal structures of 2 and 3 show that they consist of tetranuclear complexes with very similar structures. Infrared spectroscopy and mass spectrometry indicate that the tetranuclear complexes dissociate into dinuclear complexes in solution. Potentiometric studies of the Zn(II) : IPCPMP system in aqueous solution reveal that a mononuclear complex is surprisingly stable at low pH, even at a 2 : 1 Zn(II) : L ratio, but a dinuclear complex dominates at high pH and transforms into a dihydroxido complex by a cooperative deprotonation of two, probably terminally coordinated, water molecules. A kinetic investigation indicates that one of these hydroxides is the active nucleophile in the hydrolysis of bis(2,4-dinitrophenyl)phosphate (BDNPP) enhanced by complex 2, and mechanistic proposals are presented for this reaction as well as the previously reported transesterification of 2-hydroxypropyl p-nitrophenyl phosphate (HPNP) promoted by Zn(II) complexes of IPCPMP.