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

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journal contribution
posted on 2004-11-01, 00:00 authored by Sonia Taktak, Sergey V. Kryatov, Elena V. Rybak-Akimova
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 [Fe2(μ-O)(μ-OH)(BPMEN)2](ClO4)3 (1) contains a (μ-oxo)(μ-hydroxo)diiron(III) diamond core. Complex [Fe(BPMEN)(urea)(OEt)](ClO4)2 (2) is a rare example of a mononuclear non-heme iron(III) alkoxide complex. Complexes [Fe2(μ-O)(μ-OC(NH2)NH)(BPMEN)2](ClO4)3 (3) and [Fe2(μ-O)(μ-OC(NHMe)NH)(BPMEN)2](ClO4)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 FeIII(μ-O)(μ-OH)FeIII core and forming intermediates with a (HO)FeIII(μ-O)FeIII(L) core. The entropy and enthalpy for urea binding through oxygen are ΔH° = −25 kJ mol-1 and ΔS° = −53.4 J mol-1 K-1 with an equilibrium constant of K1 = 37 L mol-1 at 25 °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 K1. An opening of the hydroxo bridge in the diamond core complex [Fe2(μ-O)(μ-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 −CONH2 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.