A series of complexes with [Fe(II)(2)(mu-OH)(2)] cores has been synthesized with N3 and N4 ligands and structurally characterized to serve as models for nonheme diiron(II) sites in enzymes that bind and activate O(2). These complexes react with O(2) in solution via bimolecular rate-limiting steps that differ in rate by 10(3)-fold, depending on ligand denticity and steric hindrance near the diiron center. Low-temperature trapping of a (mu-oxo)(mu-1,2-peroxo)diiron(III) intermediate after O(2) binding requires sufficient steric hindrance around the diiron center and the loss of a proton (presumably that of a hydroxo bridge or a yet unobserved hydroperoxo intermediate). The relative stability of these and other (mu-1,2-peroxo)diiron(III) intermediates suggests that these species may not be on the direct pathway for dioxygen activation.

Regioselective hydroxylation of aromatic acids with hydrogen peroxide proceeds readily in the presence of iron(II) complexes with tetradentate aminopyridine ligands [Fe(II)(BPMEN)(CH(3)CN)(2)](ClO(4))(2) (1) and [Fe(II)(TPA)(CH(3)CN)(2)](OTf)(2) (2), where BPMEN=N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)-1,2-ethylenediamine, TPA=tris-(2-pyridylmethyl)amine. Two cis-sites, which are occupied by labile acetonitrile molecules in 1 and 2, are available for coordination of H(2)O(2) and substituted benzoic acids. The hydroxylation of the aromatic ring occurs exclusively in the vicinity of the anchoring carboxylate functional group: ortho-hydroxylation affords salicylates, whereas ipso-hydroxylation with concomitant decarboxylation yields phenolates. The outcome of the substituent-directed hydroxylation depends on the electronic properties and the position of substituents in the molecules of substrates: 3-substituted benzoic acids are preferentially ortho-hydroxylated, whereas 2- and, to a lesser extent, 4-substituted substrates tend to undergo ipso-hydroxylation/decarboxylation. These two pathways are not mutually exclusive and likely proceed via a common intermediate. Electron-withdrawing substituents on the aromatic ring of the carboxylic acids disfavor hydroxylation, indicating an electrophilic nature for the active oxidant. Complexes 1 and 2 exhibit similar reactivity patterns, but 1 generates a more powerful oxidant than 2. Spectroscopic and labeling studies exclude acylperoxoiron(III) and Fe(IV)=O species as potential reaction intermediates, but strongly indicate the involvement of an Fe(III)--OOH intermediate that undergoes intramolecular acid-promoted heterolytic O-O bond cleavage, producing a transient iron(V) oxidant.

Three novel iron(II) complexes with pyridine-containing macrocycles bearing an aminopropyl pendant arm were synthesized and characterized. Crystal structures of two of the complexes revealed high-spin iron(II) centers coordinated to the five ligand nitrogen atoms with no coordination of either the solvent molecules or anions, resulting in an unusual square-pyramidal geometry. Related tetradentate ligand CRH formed a low-spin iron(II) complex (meso form was structurally characterized) with a planar arrangement of the four nitrogen atoms from the macrocycle and two axial acetonitrile molecules. Similarly to the corresponding nickel and copper complexes of the pentadentate ligands, the protonation of the amino group on the ligand arm in iron(II) complexes was found to be reversible. Spectral changes and magnetic susceptibility measurements indicated that a change in the geometry and spin state of the metal center is associated with this acid-base process. In the presence of noncoordinating acids (e.g., triflic acid), these complexes, as well as their nonmethylated analogue, can efficiently catalyze the epoxidation of cyclooctene and 1-decene under mild conditions, using hydrogen peroxide as the oxidant. However, in the deprotonated form or in the presence of coordinating acids like HCl, no epoxidation occurs.

The iron-assisted hydroxylation of benzoic acid to salicylic acid by 1/H2O2 has been achieved in good yield under mild conditions (where is [Fe(II)(BPMEN)(CH3CN)2](ClO4)2 and BPMEN =N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)ethane-1,2-diamine); the product of this reaction is a novel mononuclear iron(III) complex with a chelating salicylate.

A series of iron(III) complexes of the tetradentate ligand BPMEN (N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)ethane-1,2-diamine) were prepared and structurally characterized. Complex [Fe(2)(mu-O)(mu-OH)(BPMEN)(2)](ClO(4))(3) (1) contains a (mu-oxo)(mu-hydroxo)diiron(III) diamond core. Complex [Fe(BPMEN)(urea)(OEt)](ClO(4))(2) (2) is a rare example of a mononuclear non-heme iron(III) alkoxide complex. Complexes [Fe(2)(mu-O)(mu-OC(NH(2))NH)(BPMEN)(2)](ClO(4))(3) (3) and [Fe(2)(mu-O)(mu-OC(NHMe)NH)(BPMEN)(2)](ClO(4))(3) (4) feature N,O-bridging deprotonated urea ligands. The kinetics and equilibrium of the reactions of 1 with ligands L (L = water, urea, 1-methylurea, 1,1-dimethylurea, 1,3-dimethylurea, 1,1,3,3-tetramethylurea, and acetamide) in acetonitrile solutions were studied by stopped-flow UV-vis spectrophotometry, NMR, and mass spectrometry. All these ligands react with 1 in a rapid equilibrium, opening the four-membered Fe(III)(mu-O)(mu-OH)Fe(III) core and forming intermediates with a (HO)Fe(III)(mu-O)Fe(III)(L) core. The entropy and enthalpy for urea binding through oxygen are DeltaH degrees = -25 kJ mol(-1) and DeltaS degrees = -53.4 J mol(-1) K(-1) with an equilibrium constant of K(1) = 37 L mol(-1) at 25 degrees C. Addition of methyl groups on one of the urea nitrogen did not affect this reaction, but the addition of methyl groups on both nitrogens considerably decreased the value of K(1). An opening of the hydroxo bridge in the diamond core complex [Fe(2)(mu-O)(mu-OH)(BPMEN)(2)] is a rapid associative process, with activation enthalpy of about 60 kJ mol(-1) and activation entropies ranging from -25 to -43 J mol(-1) K(-1). For the incoming ligands with the -CONH(2) functionality (urea, 1-methylurea, 1,1-dimethylurea, and acetamide), a second, slow step occurs, leading to the formation of stable N,O-coordinated amidate diiron(III) species such as 3 and 4. The rate of this ring-closure reaction is controlled by the steric bulk of the incoming ligand and by the acidity of the amide group.