This work reports on the generation of a graphite-conjugated diimine macrocyclic Co catalyst (GCC-CoDIM) that is assembled at o-quinone edge defects on graphitic carbon electrodes. X-ray ...photoelectron spectroscopy and X-ray absorption spectroscopy confirm the existence of a new Co surface species with a coordination environment that is the same as that of the molecular analogue, Co(DIM)Br2+. GCC-CoDIM selectively reduces nitrite to ammonium with quantitative Faradaic efficiency and at a rate that approaches enzymatic catalysis. Preliminary mechanistic investigations suggest that the increased rate is accompanied by a change in mechanism from the molecular analogue. These results provide a template for creating macrocycle-based electrocatalysts based on first-row transition metals conjugated to an extreme redox-active ligand.
The presence of redox innocent metal ions has been proposed to modulate the reactivity of metal ligand multiple bonds; however, insight from structure/function relationships is limited. Here, alkali ...metal reduction of the Fe(III) imido complex Ph2B( t BuIm)2FeNDipp (1) provides the series of structurally characterized Fe(II) imido complexes Ph2B( t BuIm)2FeNDippLi(THF)2 (2), Ph2B( t BuIm)2FeNDippNa(THF)3 (3), and Ph2B( t BuIm)2FeNDippK2 (4), in which the alkali metal cations coordinate the imido ligand. Structural investigations demonstrate that the alkali metal ions modestly lengthen the FeN bond distance from that in the charge separated complex Ph2B( t BuIm)2FeNDippK(18-C-6)THF2 (5), with the longest bond observed for the smallest alkali metal ion. In contrast to 5, the imido ligands in 2–4 can be protonated and alkylated to afford Fe(II) amido complexes. Combined experimental and computational studies reveal that the alkali metal polarizes the FeN bond, and the basicity of imido ligand increases according to 5 < 4 ≈ 3 < 2. The basicity of the imido ligands influences the relative rates of reaction with 1,4-cyclohexadiene, specifically by gating access to complex 5, which is the species that is active for HAT. All complexes 2–4 react with benzophenone form metastable Fe(II) intermediates that subsequently eliminate the metathesis product Ph2CNDipp, with relative rates dependent on the alkali metal ion. By contrast, the same reaction with 5 does not lead to the formation of Ph2CNDipp. These results demonstrate that the coordination of alkali metal ions dictate both the structure and reactivity of the imido ligand and moreover can direct the reactivity of reaction intermediates.
ConspectusTransition metal complexes featuring an M═NR bond have received great attention as critical intermediates in the synthesis of nitrogen-containing compounds. In general, the properties of ...the imido ligand in these complexes are dependent on the nature of the metal center. Thus, the imido ligand tends to be nucleophilic in early transition metal complexes and electrophilic in late transition metal complexes. Nonetheless, the supporting ligand can have a dramatic effect on its reactivity. For example, there are sporadic examples of nucleophilic late transition metal imido complexes, often based on strongly donating supporting ligands. Building on these earlier works, in this Article, we show that the imido ligand in a low-coordinate high-spin bis(carbene)borate Fe(II) complex is able to access previously unknown reaction pathways, ultimately leading to new catalytic transformations. We first focus on the synthesis, characterization, and stoichiometric reactivity of a highly nucleophilic Fe(II) imido complex. The entry point for this system is the intermediate-spin three-coordinate Fe(III) imido complex, which is generated from the reaction of an Fe(I) synthon with an organic azide. Alkali metal reduction leads to a series of M+ (M = Li, Na, K) coordinated and charge-separated (M = K(18-C-6)) high-spin Fe(II) imido complexes, all of which have been isolated and fully characterized. Combined with the electronic structure calculations, these results reveal that the alkali ions moderately polarize the Fe═N bond according to K+ ≈ Na+ < Li+. As a result, the basicity of the imido ligand increases from the charged separated complex to K+, Na+, and Li+ coordinated complexes, as validated by intermolecular proton transfer equilibria. The impact of the counterion on imido ligand reactivity is demonstrated through protonation, alkylation, and hydrogen atom abstraction reactions. The counterion also directs the outcome of 2 + 2 reactions with benzophenone, where alkali coordination facilitates double bond metathesis. Building from here, we describe how the unusual nucleophilicity of the high-spin Fe(II) imido complex revealed in stoichiometric reactions can be extended to new catalytic transformations. For example, a 2 + 2 cycloaddition reaction serves as the basis for the catalytic guanylation of carbodiimides under mild conditions. More interestingly, this complex also exhibits the first ene-like reactivity of an M═NR bond in reactions with alkynes, nitriles, and alkenes. These transformations form the basis of catalytic alkyne and nitrile α-deuteration and pKa-dictated alkene transposition reactions, respectively. Mechanistic studies reveal the critical role of metal-ligand cooperativity in facilitating these catalytic transformations and suggest the new avenues for transition metal imido complexes in catalysis that extend beyond classical nitrene transfer chemistry.
The cobalt pyridinophane complex Co(
N4)Cl
(
N4 = 3,7-diaza-1,5(2,6)-dipyridinacyclooctaphane) is converted under catalytic conditions to an electrode-adsorbed species. Aqueous Co
solutions ...similarly deposit a species under these conditions. Surface characterization reveals the formation of Co nanoparticles. These nanoparticles are active in the electrocatalytic redution of aqueous nitrate.
We present molecular dating analyses for land plants that incorporate 33 fossil calibrations, permit rates of molecular evolution to be uncorrelated across the tree, and take into account ...uncertainties in phylogenetic relationships and the fossil record. We attached a prior probability to each fossil-based minimum age, and explored the effects of relying on the first appearance of tricolpate pollen grains as a lower bound for the age of eudicots. Many of our divergence-time estimates for major clades coincide well with both the known fossil record and with previous estimates. However, our estimates for the origin of crown-clade angiosperms, which center on the Late Triassic, are considerably older than the unequivocal fossil record of flowering plants or than the molecular dates presented in recent studies. Nevertheless, we argue that our older estimates should be taken into account in studying the causes and consequences of the angiosperm radiation in relation to other major events, including the diversification of holometabolous insects. Although the methods used here do help to correct for lineage-specific heterogeneity in rates of molecular evolution (associated, for example, with evolutionary shifts in life history), we remain concerned that some such effects (e.g., the early radiation of herbaceous clades within angiosperms) may still be biasing our inferences.
Reduction of the three-coordinate iron(III) imido Ph2B( t BuIm)2FeNDipp (1) affords Ph2B( t BuIm)2FeNDippK(18-C-6)THF2 (2), a rare example of a high-spin (S = 2) iron(II) imido complex. ...Unusually for a late metal imido complex, the imido ligand in 2 has nucleophilic character, as demonstrated by the reaction with DippNH2, which establishes an equilibrium with the bis(anilido) complex Ph2B( t BuIm)2Fe(NHDipp)2K(18-C-6)THF2 (3). In an unusual transformation, formal insertion of i PrNCN i Pr into the FeN(imido) bond yields the guanidinate Ph2B( t BuIm)2Fe( i PrN)2CNDippK(18-C-6)THF2 (4). Reaction of 4 with excess DippNH2 provides 3, along with the guanidine ( i PrNH)2CNDipp. As suggested by these stoichiometric reactions, 2 is an efficient catalyst for the guanylation of carbodiimides, converting a wide range of aniline substrates under mild conditions.
Down syndrome is associated with genome-wide perturbation of gene expression, which may be mediated by epigenetic changes. We perform an epigenome-wide association study on neonatal bloodspots ...comparing 196 newborns with Down syndrome and 439 newborns without Down syndrome, adjusting for cell-type heterogeneity, which identifies 652 epigenome-wide significant CpGs (P < 7.67 × 10
) and 1,052 differentially methylated regions. Differential methylation at promoter/enhancer regions correlates with gene expression changes in Down syndrome versus non-Down syndrome fetal liver hematopoietic stem/progenitor cells (P < 0.0001). The top two differentially methylated regions overlap RUNX1 and FLI1, both important regulators of megakaryopoiesis and hematopoietic development, with significant hypermethylation at promoter regions of these two genes. Excluding Down syndrome newborns harboring preleukemic GATA1 mutations (N = 30), identified by targeted sequencing, has minimal impact on the epigenome-wide association study results. Down syndrome has profound, genome-wide effects on DNA methylation in hematopoietic cells in early life, which may contribute to the high frequency of hematological problems, including leukemia, in children with Down syndrome.
Nitrogen lifts iron to hexavalence
The myriad ways that iron can interact with oxygen have been amply studied in biochemical and geochemical contexts. More recently, chemists have explored the extent ...to which nitrogen can likewise stabilize iron in high oxidation states. Martinez
et al.
now report that an iron center coordinated by carbene ligands can react with an organic azide to form a pentavalent bis(imido) complex with two Fe=N bonds. One-electron oxidation then accessed the Fe(VI) oxidation state. Both compounds were sufficiently stable for crystallographic characterization.
Science
, this issue p.
356
Multiple bonds to nitrogen can stabilize iron in an unusually high oxidation state.
High-valent iron species are key intermediates in oxidative biological processes, but hexavalent complexes apart from the ferrate ion are exceedingly rare. Here, we report the synthesis and structural and spectroscopic characterization of a stable Fe(VI) complex (
3
) prepared by facile one-electron oxidation of an Fe(V) bis(imido) (
2
). Single-crystal x-ray diffraction of
2
and
3
revealed four-coordinate Fe centers with an unusual “seesaw” geometry.
57
Fe Mössbauer, x-ray photoelectron, x-ray absorption, and electron-nuclear double resonance (ENDOR) spectroscopies, supported by electronic structure calculations, support a low-spin (
S
= 1/2) d
3
Fe(V) configuration in
2
and a diamagnetic (
S
= 0) d
2
Fe(VI) configuration in
3
. Their shared seesaw geometry is electronically dictated by a balance of Fe-imido σ- and π-bonding interactions.
Double deprotonation of the salt Ph2B(PMe3)2OTf (1) provides access to a bis(ylide)diphenylborate ligand that is readily transferred in situ to iron(II). Depending on the reaction stoichiometry, ...both the “ate” complex Ph2B(Me2PCH2)2Fe(μ-Cl)2Li(THF)2 (2) and the homoleptic complex Ph2B(Me2PCH2)22Fe(3) can be prepared from FeCl2(THF)1.5. Further reaction of 3 with FeCl2(THF)1.5 produces the chloride-bridged dimer Ph2B(Me2PCH2)2Fe(μ-Cl)2Fe(CH2PMe2)2BPh2(4). Attempts to reduce or alkylate 4 provide 3 as the only isolable product, likely a consequence of the low steric hindrance of the bis(ylide)diphenylborate ligand. On the other hand, reaction of 4 with the strong field ligand CN t Bu provides the six-coordinate, diamagnetic complex Ph2B(Me2PCH2)2Fe(CN t Bu)4Cl(5). Electronic structure calculations for the bis(ylide)diphenylborate ligand and homoleptic complex 3 suggest that the C(ylide) atoms are strong σ-donors with little π-bonding character. These initial results suggest the potential for this bis(ylide)diphenylborate ligand in coordination chemistry.
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