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

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)Fe-2(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. Mossbauer spectroscopy indicates valence trapping in this complex with the parameters, Delta E-Q 3.242 mm s(-1), delta 1.169 mm s(-1) and Delta E-Q 0.221 mm s(-1), delta 0.460 mm s(-1), respectively for the high spin Fe2+ and high spin Fe3+ ions. Crystals of [(bpbp)Fe-2(F)(2)(H2O)(2)][BF4](2) . 4H(2)O are triclinic, space group P (1) over bar (no. 2), with a = 12.695(1), b = 19.197(2), c = 10.202(1) Angstrom, alpha = 102.95(1), beta = 97.61(1), gamma = 93.76(1)degrees, Z = 2. The structure was refined to R = 0.1009 on F using 4338 reflections with I > 2 sigma(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) Angstrom and Fe-III-O 2.033(6) Angstrom with a Fe-III... Fe-II distance of 3.726(2) Angstrom. 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 . 2H(2)O are monoclinic, space group P2(1)/n with a = 11.773(13), b = 21.394(7), c = 17.001(11) Angstrom and beta = 90.98(7)degrees, Z = 4. The structure was refined to R = 0.0434 on F using 3758 reflections with I > 2 sigma(I) (wR2 on all data and F-2 = 0.1339). The Fe-III... Zn-II distance is 3.819(4) Angstrom. 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) . 0.5CH(3)OH are monoclinic, space group P2(1)/n with a = 12.677(2), b = 22.059(2), c = 16.269(2) Angstrom and beta = 94.184(1)degrees, Z = 4. The structure was refined to R = 0.0538 on F using 5097 reflections with I > 2 sigma(I) (wR2 on all data and F-2 = 0.2684). The Fe-III... Cu-II distance is 3.419(2) Angstrom. Asymmetrical bridging by the hinging phenolate group is evident in the bis(molybdate)-bridged Fe-Zn complex and the bis(acetate)-bridged Fe-Cu complex, however it is significantly less pronounced compared with the 'non-bridged' fluoride containing Fe-III-Fe-II complex. While the fluoride and molybdate complexes may model aspects of the binding of these ions in inhibited PAPs, the generation of acetate complexes from the hydrolysis of alkyl esters may indeed model part of the reactivity of PAPs. However, we have unfortunately not been able to ascertain that this reaction is promoted by the metal complexes.