The ability of certain transition metals to mediate the reduction of N2 to NH3 has attracted broad interest in the biological and inorganic chemistry communities. Early transition metals such as Mo and W readily bind N2 and mediate its protonation at one or more N atoms to furnish M(NxHy) species that can be characterized and, in turn, extrude NH3. By contrast, the direct protonation of Fe-N2 species to Fe(NxHy) products that can be characterized has been elusive. Herein we show that addition of acid at low temperature to [(TPB)Fe(N2)][Na(12-crown-4)] results in a new S = 1/2 Fe species. EPR, ENDOR, Mössbauer, and EXAFS analysis, coupled with a DFT study, unequivocally assign this new species as [(TPB)Fe≡N-NH2]+, a doubly protonated hydrazido(2-) complex featuring an Fe-to-N triple bond. This unstable species offers strong evidence that the first steps in Fe-mediated nitrogen reduction by [(TPB)Fe(N2)][Na(12-crown-4)] can proceed along a distal or `Chatt-type' pathway. A brief discussion of whether subsequent catalytic steps may involve early or late stage cleavage of the N-N bond, as would be found in limiting distal or alternating mechanisms, respectively, is also provided.

dinitrogen activation ͉ high-valent iron ͉ nitrogenase ͉ high-valent iron nitride ͉ spectroscopy

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 16 O 2 -this band shifts to 837 cm -1 when 18 O 2 is used in the preparation. Moreover, electrospray ionization mass spectra contain a strong ion at an m/z of 576.2703 for the 16 O-isotopomer that shifts to 580.2794 in the 18 O-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.The binding and activation of dioxygen is an essential process in synthetic and biological chemistry. 1 The activation processes are often proposed to involve formation of peroxometal complexes, as exemplified by bleomycin and the mono-oxygenases cytochromes P450. 2 It is generally agreed that the initial steps in the O 2 binding/activation process in these enzymes involve a superoxoiron(III) intermediate that converts to a hydroperoxoiron(III) species through addition of an electron and proton. In this report, we demonstrate that a similar O 2 to peroxo conversion is operable in a synthetic manganese system. The observation of synthetic monomeric peroxometal complexes is frequently difficult because of their inherent reactivity. This is especially true for peroxomanganese complexes, where the Mn IV 2 (μ-1,2-peroxo) complex of Wieghardt is the only O 2 -derived system that has been structurally characterized. Preparation of the precursor 1 is outlined in Figure 1. 10 Treating H 5 bupa with 3 equivalents (equiv) of KH in dimethylacetamide (DMA) followed by one equiv of Mn(OAc) 2 afforded K [1] and 2 equiv of KOAc.The molecular structure of 1 determined by X-ray diffraction shows a five-coordinate Mn II complex, having a distorted trigonal bipyramidal geometry. 10 The trigonal plane is defined by the deprotonated urea and pyridyl nitrogen atoms of [H 2 bupa] 3-; the apical N1 atom and carbonyl oxygen O1 from the deprotonated carboxamide occupy the axial positions. The remaining portions of the urea groups form the scaffolding of a cavity, in which NH groups are positioned inward toward atom O1. However, the N6(N7)⋯O1 distances are greater than 3.2 Å, distances that are too long for intramolecular H-bonds.A new green species (2) is formed in approximately 50% yield 11 when ...

Manganese transport regulator (MntR) is a member of the diphtheria toxin repressor (DtxR) family of transcription factors that is responsible for manganese homeostasis in [4295][4296][4297][4298][4299][4300][4301][4302][4303], and generally follow the Irving-Williams series. Direct detection of the dinuclear Mn 2+ site in MntR with EPR spectroscopy is presented, and the exchange interaction was determined, J = -0.2 cm -1 . This value is lower in magnitude than most known dinuclear Mn 2+ sites in proteins and synthetic complexes and is consistent with a dinuclear Mn 2+ site with a longer Mn···Mn distance (4.4 Å) observed in some of the available crystal structures. MntR is found to have a surprisingly low binding affinity (∼160 μM) for its cognate metal ion Mn 2+ . Moreover, the results of DNA binding studies in the presence of limiting metal ion concentrations were found to be consistent with the measured metal-binding constants. The metalbinding affinities of MntR reported here help to elucidate the regulatory mechanism of this metaldependent transcription factor.Bacteria handle the delicate issue of metal ion homeostasis using a class of transcription factors known as metalloregulatory proteins (metalloregulators). In some systems, these metal-sensing proteins are involved in mediating the removal of toxic metals, while in other systems they are central to maintaining the required levels of essential metals. To date, a large number of metalloregulatory proteins have been identified that respond to a variety of metal ions (1-3). The subject of how metal binding is translated into an ability to control transcription via a † This work was supported by the University of California, San Diego, the Hellman Family fund, a Cottrell Scholar Award from the Research Corporation (S.M.C.), and the National Institutes of Health GM077387 (M.P.H.). * Author to whom correspondence should be adddressed. (M.P.H.) Telephone: (412) 268-1058; fax: (412) NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript metalloregulator has become a prominent subject of investigation (4-10). Indeed, many metalloregulators are reported to bind several metal ions in vitro, while only eliciting a specific transcriptional response when they are bound to the cognate metal in vivo (5)(6)(7)9). This observation naturally leads to the question: how does a metalloregulator selectively respond to its cognate metal ion as opposed to other available metal ion activators? The ability of a metalloregulator to respond selectively to a metal ion may depend on several factors, including the availability of the requisite metal ion, the binding affinity for the metal ion, the charge on the metal ion, and the coordination geometry/number assumed by the metal ion upon binding. Determining which of these factors are most important for a given metalloregulator is essential for gaining a better understanding of how these proteins elicit transcriptional control.The manganese transport regulator MntR 1 is found in Bacillus subtilis and is ...