SYNTHESIS AND CHARACTERIZATION OF THE BENZOYLFORMATO FERROUS COMPLEXES WITH THE HINDERED TRIS(PYRAZOLYL)BORATE LIGAND AS A STRUCTURAL MODELFOR MONONUCLEAR NONHEME IRON ENZYMES

By using a hindered tripodal ligand, ydrotris(3-tert-butyl-5-isopropyl pyrazo-1-y)borate HB(3-tBu-5-iPrpz)(3), a series of monomeric ferrous complexes having acetate, hydroxide, and benzoylformate ligands were s ynthesized. Reaction of KHB(3-tBu-5-iPrpz)(3) with anhydrous Fe(OAc)(2 ) yielded acetato complexes Fe(OAc)[HB(3-tBu-5-iPrpz)(3)] (1) and Fe(O Ac)[HB(3-tBu-5-iPrpz)(3)](3-iPr-5-tBup (2). A hydroxo complex Fe(OH)[H B(3-tBu-5-iPrpz)(3)] (3) was prepared by the treatment of 1 or 2 with aqueous NaOH. The geometry of Fe(II) in 3 is a slightly distorted tetr ahedron as determined by X-ray crystallography. The hydroxo complex 3 reacted with benzoylformic acid to give the benzoylformato complex Fe( O2CC(O)Ph)[HB(3-tBu-5-iPrpz)(3)] (4), which showed thermochromism whic h depended on the coordination geometry of the benzoylformate ligand. The Fe(LT) ion in the colorless form of 4 isolated at 4 degrees C is c oordinated by a tetrahedral N3O1 ligand donor set including the uniden tate benzoylformato ligand. On the other hand, the bluish purple form of 4 isolated ar -20 degrees C has a five-coordinate trigonal bipyrami dal Fe(IT) center. The benzoylformate ligand in this bluish purple for m works as a chelate ligand through coordination of the unidentate car boxylate oxygen atom as well as the ketonic oxygen atom. A benzoylform ato complex containing an additional pyrazole, e(O2CC(O)Ph)[HB(3-tBu-5 -iPrpz)(3)](3-iPr-5-tBupzH) (5), was obtained by the reaction of 3 wit h benzoylformic acid in the presence of 3-tert-butyl-5-isopropylpyrazo le. The iron atom in 5 is coordinated by an N4O1 ligand donor set with trigonal bipyramidal geometry. A hydrogen-bonding interaction between the carboxylate oxygen atom and the additional pyrazole's NH proton i n 5 is suggested from the short distance between O-carboxylate and N-p yrazole observed in the X-ray structure and the absence of the nu NH v ibration in the LR spectrum.