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

Abstract

A series of mixed-valent iron and mixed-metal Fe^III–M^II (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, ΔE_Q 3.242 mm s^–1, δ 1.169 mm s^–1 and ΔE_Q 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 Fe^II and Fe^III atoms are bridged asymmetrically by the phenolic oxygen atom of bpbp^– with Fe^II–O 2.175(6) Å and Fe^III–O 2.033(6) Å with a Fe^III · · · Fe^II distance of 3.726(2) Å. The two terminal fluoride ions are bound to the Fe^III atom and strongly hydrogen bonded to two water molecules bound to the adjacent Fe^II 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 Fe^III–Zn^II 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 Fe^III · · · Zn^II 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