Dinuclear iron(III)–metal(II) complexes as structural core models for purple acid phosphatases †

Abstract

A series of mixed-valent iron and mixed-metal FeIII–MII (M = Zn, Cu, Ni or Co) complexes of the phenolate-hinged dinucleating ligand 2,6-bis{[bis(2-pyridylmethyl)amino]methyl}-4-tert-butylphenolato(1–), bpbp– have been prepared and characterized. Both exogenous bidentate bridging groups and different terminal ligands bound to each different metal ion at the exogenous site were identified. The structure of the mixed-valence complex [(bpbp)Fe2(F)2(H2O)2][BF4]2 confirms that it is a rare example of a dimetallic complex of a single-atom hinged acyclic dinucleating ligand with a ‘non-bridged’ arrangement at the exogenous bridging site. Mössbauer spectroscopy indicates valence trapping in this complex with the parameters, ΔEQ 3.242 mm s–1, δ 1.169 mm s–1 and ΔEQ 0.221 mm s–1, δ 0.460 mm s–1, respectively for the high spin Fe2+ and high spin Fe3+ ions. Crystals of [(bpbp)Fe2(F)2(H2O)2][BF4]2·4H2O are triclinic, space group P (no. 2), with a = 12.695(1), b = 19.197(2), c = 10.202(1) Å, α = 102.95(1), β = 97.61(1), γ = 93.76(1)°, Z = 2. The structure was refined to R = 0.1009 on F using 4338 reflections with I > 2σ(I) (wR2 on all data and F 2 = 0.3522). The FeII and FeIII atoms are bridged asymmetrically by the phenolic oxygen atom of bpbp– with FeII–O 2.175(6) Å and FeIII–O 2.033(6) Å with a FeIII · · · FeII distance of 3.726(2) Å. The two terminal fluoride ions are bound to the FeIII atom and strongly hydrogen bonded to two water molecules bound to the adjacent FeII atom. This complex may model the mode in which fluoride ions bind to the active site of the purple acid phosphatases (PAPs) thereby inhibiting the activity of these enzymes. Tetrahedral oxo anions are known also to inhibit PAPs and to mimic this inhibition a FeIII–ZnII complex incorporating molybdate bridging groups was prepared. Crystals of [(bpbp)FeZn(MoO4)2]·C3H7OH· 2H2O are monoclinic, space group P21/n with a = 11.773(13), b = 21.394(7), c = 17.001(11) Å and β = 90.98(7)°, Z = 4. The structure was refined to R = 0.0434 on F using 3758 reflections with I > 2σ(I) (wR2 on all data and F 2 = 0.1339). The FeIII · · · ZnII distance is 3.819(4) Å. A series of acetate-bridged complexes were prepared by the novel method of diffusing ethyl acetate or isopropyl acetate into mixtures of Hbpbp and iron perchlorate in the presence and absence of a second type of metal ion. The acetate bridging groups are the result of the hydrolysis of the alkyl acetate. These complexes have the general formulation [(bpbp)FeM(CH3CO2)2][ClO4]2. Crystals of [(bpbp)FeCu(CH3CO2)2][ClO4]2·0.5CH3OH are monoclinic, space group P21/n