Reactivity of a (μ-Oxo)(μ-hydroxo)diiron(III) Diamond Core with Water, Urea, Substituted Ureas, and Acetamide

Abstract

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.

Ligand Topology Effects on Olefin Oxidations by Bio‐Inspired [FeII(N2Py2)] Catalysts

“…For bpmen, there is crystallographic evidence for only the cis-a topology. [18,19,20,21] This is exemplified by the structure of (6), in which the Nmethyl groups are oriented anti with respect to each other. In solution, the complex exhibits a low-spin/high-spin equilibrium, but the C 2 symmetry of the complex is maintained, as indicated by NMR spectroscopy.…”

“…[18] The two chloride ligands are cis to each other, the two pyridine rings are coordinated trans to each other, and the relative disposition of the N-methyl groups is anti. The reaction of bpmpn with iron(II) salts of more weakly coordinating anions,…”

“…O 2 is used, the epoxide product shows only partial incorporation of the 18 O label, indicating some involvement of radical processes, particularly for 9, that generate product with air oxidation. While these results clearly exclude a wa mechanism for the epoxidation, the nature of the active oxidizing species cannot be ascertained since no intermediate has been trapped in the reactions of either 9 or 10 with H 2 O 2 .…”

“…These observations suggest that different active species may be formed for complexes with cis-a topology depending on the presence or absence of water in the reaction media. 18 O-labeling experiments provide clues into the nature of the active species for olefin oxidation catalyzed by 6 or 7. Consistent with our previous work on 6,…”

“…The epoxide product also incorporates some 18 O label from water, suggesting that the wa pathway also contributes to the epoxidation of cyclooctene. However the dramatic differences in the D/E ratios that depend on the presence of water suggest that a different active species is responsible for the chemistry in the absence of added water.…”