Diiron (III) complexes of some relevance to the purple acid phosphatases

Three diiron(III) complexes of the tetradentate tripodal ligand N-(-ohydroxybenzyl)-N,N-bis(2-pyridylmethyl)amine (HDP), [Fe2(HDP)2O(O2CPh)BPh4 (1), [Fe2(HDP)2O{O2P(OPh)2}]BPh4 (2), and [Fe2(HDP)2{O2(OPh)2}2](BPh4)2 (3), have been synthesized as models for the active site of the purple acid phosphtes. Single crystals of 1 (P212121, a=17.728(3) Å, b=18.204(4) Å c=19.3383(4) Å, Z=4, V=6255 Å3) and 3 (P1, a=14.316(11) Å, b=15.136(17) Å, c=13.303(6) Å, α=97.04(7)°, β=104.48(5)°, γ=115.51(8)°, Z=1, V=2430(6) Å3) were obtained and subjected to X-ray diffraction analysis. Complex 1 has a (μ-oxo)(μ-benzoato)Diiron(III) core, while complex 3 has a bis(μ-phosphato)diiron(III) core, the tetradentate HDP completing the coordination spere about each iron center in both complexes. Due to differences in their core structures, the FeFe separations in 1 and 3 are 3.217(11) Å and 4.819(1) Å, respectively. Complex 2 is presumed to have a structure analogous to 1 with phosphate replacing the benzoate bridge. Both 1 and 2 exhibit strong atiferromagnetic coupling due to the presence of the oxo bridge (J=−111 and −96 cm−1, respectively; H=−2JS1·S2). Interestingly, the iron(III) centers in 3 are not coupled at all; 3 exhibits Curie behavior throughout the entire temperature range studied (6–300 K) with a magnetic moment commensurate with two high-spin iron(III) centers. The three complexes exhibit visible absorption maxima at 522, 516, and 605 nm, respectively, arising from phenolate-to-iron(III) charge transfer transitions. The shifts in the λmax values can be rationalized on the basis of the Lewis acidities of the respective iron(III) centers. These are compared with those of the purple acid phosphatases.