Preparation and reactivity of the versatile uranium(IV) imido complexes U(NAr)Cl2(R2bpy)2 (R = Me, (t)Bu) and U(NAr)Cl2(tppo)3.

Uranium tetrachloride undergoes facile reactions with 4,4'-dialkyl-2,2'-bipyridine, resulting in the generation of UCl4(R2bpy)2, R = Me, (t)Bu. These precursors, as well as the known UCl4(tppo)2 (tppo = triphenylphosphine oxide), react with 2 equiv of lithium 2,6-di-isopropylphenylamide to provide the versatile uranium(IV) imido complexes, U(NDipp)Cl2(L)n (L = R2bpy, n = 2; L = tppo, n = 3). Interestingly, U(NDipp)Cl2(R2bpy)2 can be used to generate the uranium(V) and uranium(VI) bisimido compounds, U(NDipp)2X(R2bpy)2, X = Cl, Br, I, and U(NDipp)2I2((t)Bu2bpy), which establishes these uranium(IV) precursors as potential intermediates in the syntheses of high-valent bis(imido) complexes from UCl4. The monoimido species also react with 4-methylmorpholine-N-oxide to yield uranium(VI) oxo-imido products, U(NDipp)(O)Cl2(L)n (L = (t)Bu2bpy, n = 1; L = tppo, n = 2). The aforementioned molecules have been characterized by a combination of NMR spectroscopy, X-ray crystallography, and elemental analysis. The chemical reactivity studies presented herein demonstrate that Lewis base adducts of uranium tetrachloride function as excellent sources of U(IV), U(V), and U(VI) imido species.

Catalytic reduction of dioxygen to water with a monomeric manganese complex at room temperature.

There have been numerous efforts to incorporate dioxygen into chemical processes because of its economic and environmental benefits. The conversion of dioxygen to water is one such example, having importance in both biology and fuel cell technology. Metals or metal complexes are usually necessary to promote this type of reaction and several systems have been reported. However, mechanistic insights into this conversion are still lacking, especially the detection of intermediates. Reported herein is the first example of a monomeric manganese(II) complex that can catalytically convert dioxygen to water. The complex contains a tripodal ligand with two urea groups and one carboxyamidopyridyl unit; this ligand creates an intramolecular hydrogen-bonding network within the secondary coordination sphere that aids in the observed chemistry. The manganese(II) complex is five-coordinate with an N(4)O primary coordination sphere; the oxygen donor comes from the deprotonated carboxyamido moiety. Two key intermediates were detected and characterized: a peroxo-manganese(III) species and a hybrid oxo/hydroxo-manganese(III) species (1). The formulation of 1 was based on spectroscopic and analytical data, including an X-ray diffraction analysis. Reactivity studies showed dioxygen was catalytically converted to water in the presence of reductants, such as diphenylhydrazine and hydrazine. Water was confirmed as a product in greater than 90% yield. A mechanism was proposed that is consistent with the spectroscopy and product distribution, in which the carboxyamido group switches between a coordinated ligand and a basic site to scavenge protons produced during the catalytic cycle. These results highlight the importance of incorporating intramolecular functional groups within the secondary coordination sphere of metal-containing catalysts.

Role of the secondary coordination sphere in metal-mediated dioxygen activation.

Alfred Werner proposed nearly 100 years ago that the secondary coordination sphere has a role in determining the physical properties of transition-metal complexes. We now know that the secondary coordination sphere impacts nearly all aspects of transition-metal chemistry, including the reactivity and selectivity in metal-mediated processes. These features are highlighted in the binding and activation of dioxygen by transition-metal complexes. There are clear connections between control of the secondary coordination sphere and the ability of metal complexes to (1) reversibly bind dioxygen or (2) bind and activate dioxygen to form highly reactive metal-oxo complexes. In this Forum Article, several biological and synthetic examples are presented and discussed in terms of structure-function relationships. Particular emphasis is given to systems with defined noncovalent interactions, such as intramolecular H-bonds involving dioxygen-derived ligands. To further illustrate these effects, the homolytic cleavage of C-H bonds by metal-oxo complexes with basic oxo ligands is described.

The effects of hydrogen bonds on metal-mediated O2 activation and related processes.

Hydrogen bonds stabilize and direct chemistry performed by metalloenzymes. With inspiration from enzymes, we will utilize an approach that incorporates intramolecular hydrogen bond donors to determine their effects on the stability and reactivity of metal complexes. Our premise is that control of secondary coordination sphere interactions will promote new function in synthetic metal complexes. Multidentate ligands have been developed that create rigid organic structures around metal ions. These ligands place hydrogen bond (H-bond) donors proximal to the metal centers, forming specific microenvironments. One distinguishing attribute of these systems is that site-specific modulations in structure can be readily accomplished, in order to evaluate correlations with reactivity. A focus of this research is consideration of dioxygen binding and activation by metal complexes, including developing structure-function relationships in metal-assisted oxidative catalysis.

A monomeric Mn(III)-peroxo complex derived directly from dioxygen.

The binding and activation of dioxygen by transition metal complexes is a fundamentally and practically important process in chemistry. Often the initial steps involve formation of peroxometal species that is difficult to observe because of their inherent reactivity. The interaction of dioxygen with a manganese(II) complex (1) of bis[(N'-tert-butylurealy)-N-ethyl]-(6-pivalamido-2-pyridylmethyl)amine was investigated, leading to the detection of a new intermediate that is a peroxomanganese(III) complex (2). This complex is high-spin (S = 2) with a g value of 8.2 and D = -2.0(5) as determined by parallel-mode electron paramagnetic resonance spectroscopy. The coordination of a peroxo ligand was established using Fourier transform infrared spectroscopy that reveals a new signal at 885 cm-1 for 2 when formed from 16O2-this band shifts to 837 cm-1 when 18O2 is used in the preparation. Moreover, electrospray ionization mass spectra contain a strong ion at an m/z of 576.2703 for the 16O-isotopomer that shifts to 580.2794 in the 18O-isotopomer. Complex 2 also is capable of oxidatively deformylating aldehydes, which is a known reaction of peroxometal complexes. The similarities of 2 to the peroxo intermediates in cytochrome P450 are noted.