Reactivity of a (μ-Oxo)(μ-hydroxo)diiron(III) diamond core with water, urea, substituted ureas, and acetamide

A series of iron(Ill) 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)](ClO4)(3) (1) contains a (eta-oxo)(mu-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 [Fe-2(mu-O)(mu-OC(NH2)NH)(BPMEN)(2)](ClO4)(3) (3) and [Fe-2(mu-O)(mu-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(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 DeltaHdegrees = -25 kJ mol(-1) and DeltaSdegrees = -53.4 J mol(-1) K-1 with an equilibrium constant of K-1 = 37 L mol(-1) at 25 degreesC, 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 -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(Ill) 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.