DINUCLEAR IRON(III)-METAL(II) COMPLEXES AS STRUCTURAL CORE MODELS FORPURPLE 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 dylme thyl)amino]methyl}-4-tert-butylphenolato(1-), bpbp(-) have been prepar ed and characterized. Both exogenous bidentate bridging groups and dif ferent terminal ligands bound to each different metal ion at the exoge nous 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 l igand with a 'non-bridged' arrangement at the exogenous bridging site. Mossbauer spectroscopy indicates valence trapping in this complex wit h the parameters, Delta E-Q 3.242 mm s(-1), delta 1.169 mm s(-1) and D elta E-Q 0.221 mm s(-1), delta 0.460 mm s(-1), respectively for the hi gh spin Fe2+ and high spin Fe3+ ions. Crystals of [(bpbp)Fe-2(F)(2)(H2 O)(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 stru cture 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 ato ms 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-II I... Fe-II distance of 3.726(2) Angstrom. The two terminal fluoride io ns are bound to the Fe-III atom and strongly hydrogen bonded to two wa ter 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 enz ymes. Tetrahedral oxo anions are known also to inhibit PAPs and to mim ic this inhibition a Fe-III-Zn-II complex incorporating molybdate brid ging 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. Th e 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-I I distance is 3.819(4) Angstrom. A series of acetate-bridged complexes were prepared by the novel method of diffusing ethyl acetate or isopr opyl acetate into mixtures of Hbpbp and iron perchlorate in the presen ce and absence of a second type of metal ion. The acetate bridging gro ups are the result of the hydrolysis of the alkyl acetate. These compl exes have the general formulation [(bpbp)FeM(CH3CO2)(2)][ClO4](2). Cry stals 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 evid ent in the bis(molybdate)-bridged Fe-Zn complex and the bis(acetate)-b ridged Fe-Cu complex, however it is significantly less pronounced comp ared with the 'non-bridged' fluoride containing Fe-III-Fe-II complex. While the fluoride and molybdate complexes may model aspects of the bi nding of these ions in inhibited PAPs, the generation of acetate compl exes from the hydrolysis of alkyl esters may indeed model part of the reactivity of PAPs. However, we have unfortunately not been able to as certain that this reaction is promoted by the metal complexes.