Synthesis, Characterization, and Reactivity Studies of Heterodinuclear Complexes Modeling Active Sites in Purple Acid Phospatases

To model the heterodinuclear active sites in plant purple acid phosphatases, a mononuclear synthon, [Fe(III)(H2IPCPMP)(Cl2)][PF6] (1), has been generated in situ from the ligand 2-(N-isopropyl-N-((2-pyridyl)methyl)aminomethyl)-6-(N-(carboxylmethyl)-N-((2-pyridyl)methyl)amino methyl)-4-methylphenol (IPCPMP) and used to synthesize heterodinuclear complexes of the formulas [Fe(III)M(II)(IPCPMP)(OAc)2(CH3OH)][PF6] (M = Zn (2), Co (3), Ni (4), Mn (5)), [Fe(III)Zn(II)(IPCPMP)(mpdp)][PF6] (6) (mpdp = meta-phenylene-dipropionate), and [Fe(III)Cu(II)(IPCPMP) (OAc)}2(μ-O)][PF6] (7). Complexes 2−4, 6, and 7 have been crystallographically characterized. The structure of 6 is a solid state coordination polymer with heterodinuclear monomeric units, and 7 is a tetranuclear complex consisting of two heterodinuclear phenolate-bridged Fe(III)Cu(II) units bridged through a μ-oxido group between the two Fe(III) ions. Mössbauer spectra confirm the presence of high spin Fe(III) in an octahedral environment for 1, 3, and 5 while 2 and 4 display relaxation effects. Magnetic susceptibility measurements indicate weak antiferromagnetic coupling for 3, 4, and 5 and confirm the assignment of the metal centers in 2−5 as high spin Fe(III)-M(II) (M = Zn, Co (high spin), Ni (high spin), Mn (high spin)). Complexes 2−5 are intact in acetonitrile solution as indicated by IR spectroscopy (for 2−4) and electrospray ionization mass spectrometry (ESI-MS) but partly dissociate to hydroxide species and a mononuclear complex in water/acetonitrile solutions. UV−vis spectroscopy reveal pH-dependent behavior, and species that form upon increasing the pH have been assigned to μ-hydroxido-bridged Fe(III)M(II) complexes for 2−5 although 2 and 3 is further transformed into what is propsed to be a μ-oxido-bridged tetranuclear complex similar to 7. Complexes 2−5 enhance phosphodiester cleavage of 2-hydroxy-propyl-p-nitrophenyl phosphate (HPNP) and bis(2,4-dinitrophenyl)phosphate (BDNPP), but the reactivities are different for different complexes and generally show strong pH dependence.