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

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 Fe^III(μ-O)(μ-OH)Fe^III core and forming intermediates with a (HO)Fe^III(μ-O)Fe^III(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.