DIIRON(III) COMPLEXES OF SOME RELEVANCE TO THE PURPLE ACID-PHOSPHATASES

Three diiron(III) complexes of the tetradentate tripodal ligand N-(o-h ydroxybenzyl)-N,N-bis(2-pyridylmethyl)amine (HDP), [Fe-2(HDP)(2)O(O(2) CPh)]BPh(4) (1), [Fe-2(HDP)(2)O{O2P(OPh)(2)}]BPh(4) (2), and [Fe-2(HDP )(2){O2P(OPh)(2)}(2)](BPh(4))(2) (3), have been synthesized as models for the active site of the purple acid phosphatases. Single crystals o f 1 (P2(1)2(1)2(1), a = 17.728(3) Angstrom, b = 18.204(4) Angstrom, c = 19.383(4) Angstrom, Z = 4, V = 6255 Angstrom(3)) and 3 (P (1) over b ar, a = 14.316(11) Angstrom, b = 15.136(17) Angstrom, c = 13.303(6) An gstrom, alpha = 97.04(7)degrees, beta = 104.48(5)degrees, gamma = 115. 51(8)degrees, Z = 1, V = 2430(6) Angstrom(3)) were obtained and subjec ted to X-ray diffraction analysis. Complex 1 has a (mu-oxo)(mu-benzoat o)diiron(III) core, while complex 3 has a bis(mu-phosphato)diiron(III) core, the tetradentate HDP completing the coordination sphere about e ach iron center in both complexes. Due to differences in their core st ructures, the Fe-Fe separations in 1 and 3 are 3.217(11) Angstrom and 4.819(1) Angstrom, respectively. Complex 2 is presumed to have a struc ture analogous to 1 with phosphate replacing the benzoate bridge. Both 1 and 2 exhibit strong antiferromagnetic coupling due to the presence of the oxo bridge (J = -111 and -96 cm(-1) respectively; H = -2JS(1) . S-2). Interestingly, the iron(III) centers in 3 are not coupled at a ll; 3 exhibits Curie behavior throughout the entire temperature range studied (6-300 K) with a magnetic moment commensurate with two high-sp in iron(III) centers. The three complexes exhibit visible absorption m axima at 522, 516, and 605 nm, respectively, arising from phenolate-to -iron(III) charge transfer transitions. The shifts in the lambda(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 pur ple acid phosphatases.