PHOSPHATE ESTER AND PHOSPHINATE BINDING TO THE (MU-OXO)DIIRON(III) CORE - SYNTHESIS AND CHARACTERIZATION OF [FE2O(O2P(OC6H5)2)2(HBPZ3)2] AND [FE2O(O2P(C6H5)2)2(HBPZ3)2]

To model the interaction of phosphate ligands with oxo-bridged diiron proteins, (μ-oxo)bis(μ-diphenyl phosphato)bis(hydrotris(1-pyrazolyl)borato)diiron(III) (1) and (μ-oxo)bis(μ-diphenylphosphinato)bis(hydrotris(1-pyrazolyl)borato)diiron(III) (2) were prepared. X-ray crystallographic studies reveal that the diiron(III) core is expanded in both compounds relative to dicarboxylate-bridged proteins and model compounds. Fe-O-Fe bond angles of 134.7 (2) and 130.6 (3)°, and Fe⃛Fe distances of 3.335 (1) and 3.292 (2) Å were observed for 1 and 2, respectively, and the symmetric Fe-O-Fe stretching vibrations at 478 and 485 cm−1 have lower energies than those of other triply-bridged diiron(III) compounds. The Fe-Ooxo bond distances in 1 and 2, 1.808 (3) and 1.812 (3) Å, are longer than observed in analogous dicarboxylate bridged compounds, and, consequently, the antiferromagnetic spin exchange coupling constants,-97.5 (1) and-93 (1) cm−1, are smaller in magnitude than usually found in such oxo-bridged diiron(III) compounds. Interactions with the paramagnetic metal centers shift the 1H NMR signals by up to 6.5 ppm downfield from free ligand values. NMR assignments were facilitated by determination of T1 relaxation times. As for purple acid phosphatases, no signals were observed in the 31P NMR spectra of either model compound. Mössbauer and electronic spectral parameters resemble those of dicarboxylate-bridged (μ-oxo)diiron(III) model compounds. The phosphate ester and phosphinate bridged model complexes do not exhibit the unusual spectroscopic features of purple acid phosphatases, suggesting that these proteins are unlikely to have a (μ-oxo)(μ-phosphato)diiron(III) center. © 1990, American Chemical Society. All rights reserved.