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

Abstract

Three diiron(III) complexes of the tetradentate tripodal ligand N-(- ohydroxybenzyl)- N, N-bis(2-pyridylmethyl)amine (HDP), [Fe 2(HDP) 2O(O 2CPh)BPh 4 ( 1), [Fe 2(HDP) 2O{O 2P(OPh) 2}]BPh 4 ( 2), and [Fe 2(HDP) 2{O 2(OPh) 2} 2](BPh 4) 2 ( 3), have been synthesized as models for the active site of the purple acid phosphtes. Single crystals of 1 ( P2 12 12 1, a=17.728(3) A ̊ , b=18.204(4) A ̊ c=19.3383(4) A ̊ , Z=4, V=6255 A ̊ 3 ) and 3 ( P 1, a=14.316(11) A ̊ , b=15.136(17) A ̊ , c=13.303(6) A ̊ , α=97.04(7)°, β=104.48(5)°, γ=115.51(8)°, Z=1, V=2430(6) A ̊ 3 ) 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=−2 JS 1· S 2). 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 λ max 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.