Hydrogenase Enzymes and Their Synthetic Models: The Role of Metal Hydrides

Schilter, David,Camara, James M.,Huynh, Mioy T.,Hammes-Schiffer, Sharon,Rauchfuss, Thomas B. American Chemical Society 2016 Chemical reviews Vol.116 No.15

<P>Hydrogenase enzymes efficiently process H-2 and protons at organometallic FeFe, NiFe, or Fe active sites. Synthetic modeling of the many H(2)ase states has provided insight into H(2)ase structure and mechanism, as well as afforded catalysts for the H-2 energy vector. Particularly important are hydride-bearing states, with synthetic hydride analogues now known for each hydrogenase class. These hydrides are typically prepared by protonation of low-valent cores. Examples of FeFe and NiFe hydrides derived from H-2 have also been prepared. Such chemistry is more developed than mimicry of the redox-inactive monoFe enzyme, although functional models of the latter are now emerging. Advances in physical and theoretical characterization of H(2)ase enzymes and synthetic models have proven key to the study of hydrides in particular, and will guide modeling efforts toward more robust and active species optimized for practical applications.</P>

Nickel‐Molybdenum and Nickel‐Tungsten Dithiolates: Hybrid Models for Hydrogenases and Hydrodesulfurization

Schilter, David,Fuller, Amy L.,Gray, Danielle L. Wiley-VCH 2015 European journal of inorganic chemistry Vol.2015 No.28

<P><B>Abstract</B></P><P>The heterobimetallic complexes [(dppe)Ni(pdt)Mo(CO)<SUB>4</SUB>], [(dcpe)Ni(pdt)Mo(CO)<SUB>4</SUB>], [(dppe)Ni(pdt)W(CO)<SUB>4</SUB>] and [(dcpe)Ni(pdt)W(CO)<SUB>4</SUB>] {dppe = Ph<SUB>2</SUB>P(CH<SUB>2</SUB>)<SUB>2</SUB>PPh<SUB>2</SUB>; dcpe = Cy<SUB>2</SUB>P(CH<SUB>2</SUB>)<SUB>2</SUB>PCy<SUB>2</SUB>; pdt<SUP>2–</SUP> = <SUP>–</SUP>S(CH<SUB>2</SUB>)<SUB>3</SUB>S<SUP>–</SUP>} have been prepared and structurally characterized. The internuclear separation in these Ni<SUP>II</SUP>M<SUP>0</SUP> species is highly sensitive to diphosphine basicity and is smallest in the dppe complexes wherein the planar Ni centres are electron‐poor and require interaction with the group 6 metal. The Ni<SUP>II</SUP>W<SUP>0</SUP> species [(dppe)Ni(pdt)W(CO)<SUB>4</SUB>] was converted into its stable conjugate acid [(dppe)Ni(pdt)HW(CO)<SUB>4</SUB>]<SUP>+</SUP>, a rare example of a nickel–tungsten hydride. This Ni<SUP>II</SUP>HW<SUP>II</SUP> complex is an electrocatalyst for H<SUP>+</SUP> reduction and is relevant to both biological and synthetic H<SUB>2</SUB>‐processing catalysts.</P>

Sodide and Organic Halides Effect Covalent Functionalization of Single-Layer and Bilayer Graphene

Biswal, Mandakini,Zhang, Xu,Schilter, David,Lee, Tae Kyung,Hwang, Dae Yeon,Saxena, Manav,Lee, Sun Hwa,Chen, Shanshan,Kwak, Sang Kyu,Bielawski, Christopher W.,Bacsa, Wolfgang S.,Ruoff, Rodney S. American Chemical Society 2017 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.139 No.11

<P>The covalent functionalization of single and bilayer graphene on SiO2 (300 nm)/Si was effected through sequential treatment with the alkalide reductant [K(15-crown-5)(2)]Na and electrophilic aryl or alkyl halides, of which the iodides proved to be the most reactive. The condensation reactions proceeded at room temperature and afforded the corresponding aryl- or alkyl-appended graphemes. For each sample, Raman and X-ray photoelectron spectroscopies were used to evaluate the degrees and uniformities of functionalization. Statistical analyses of the Raman data revealed that the introduction of the organic moieties was accompanied by sp(3)-rehybridization of the basal plane atoms. When bilayers consisting of C-13 and C-12 layers were treated, both the top and bottom sheets were decorated with organic groups. The reaction was followed using Raman spectroscopy, and the mechanism was studied by theoretical calculations. Indicative of its structure and reactivity, 4-pyridyl-decorated single-layer graphene was readily benzylated and appears to be an ideal platform to develop functional materials.</P>

<i>N</i>,<i>N</i>′-Diamidocarbenes: Isolable Divalent Carbons with Bona Fide Carbene Reactivity

Moerdyk, Jonathan P.,Schilter, David,Bielawski, Christopher W. American Chemical Society 2016 Accounts of chemical research Vol.49 No.8

<P>Since the first reported isolation of a carbene just over a quarter century ago, the study of such compounds including stable derivatives has flourished. Indeed, N-heterocyclic carbenes (NHCs), of which imidazolylidenes and their derivatives are the most pervasive subclass, feature prominently in organocatalysis, as ligands for transition metal catalysts, and as stabilizers of reactive species. However, imidazolylidenes (and many other NHCs) typically lack the reactivity characteristic of electrophilic carbenes, including insertion into unactivated C-H bonds, participation in [2 + 1] cycloadditions, and reaction with carbon mohoxide. This has led to debates over whether NHCs are truly carbenic in nature or perhaps better regarded as ylides. The fundamental and synthetic utility of transformations that involve electrophilic carbenes has motivated our group and others to expand the reactivity of NHCs and other stable carbenes to encompass electrophilic carbene chemistry. These efforts have led to the develophient of the diamidocarbenes (DAcs), a stable and unique subset of the NHCs that feature carbonyl groups inserted into the N-heterocyclic scaffold. To date, crystalline five-, six-, and seven-membered DACs have been prepared and studied. Unlike imidaiolylidenes, which are often designated as prototypical NHCs, the DACs 'exhibit a reactivity profile similar to that of bona fide carbenes, reactive species that are less 'tamed' by heteroatom Pi conjugation. The DACs engage in [2 + 1] cycloadditions with electron-rich or -poor alkenes, aldehydes, alkynes, and nitriles, and doing so 'in a reversible manner in some cases. They also react with isonitriles, reversibly couple to CO, and mediate the dehydrogenation of hydrocarbons. Such rich chemistry may be rationalized in terms of their ambiphilicity: DACs are nucleophilic, as required for some of the reactions above, yet also have electrophilic character, as evidenced by their insertions into unactivated N-H and CH bonds, including nonacidic derivatives. As will become clear, such reactivity is unique among isolable carbenes. DAC chemistry is expected to find applications in synthesis, dynamic covalent chemistry, and catalysis: For example, the hydrolysis of DAC-derived diamidocyclopropanes and -propenes affords carboxylic acids and cyclopropenones, respectively. These new hydrocarboxylation and carbonylation methodologies are significant in that they represent alternatives to processes that typically involve precious metals and gaseous carbon monoxide. Future efforts in this area may involve modifications that transform the stoichiometric conversions facilitated by DACs into catalytic variants. In this context, the reversible binding of CO to DACs is an indication that the latter. ay serve as a blueprint for the development of more electrophilic, stable carbenes with the capacity to activate other challenging small molecules.</P>

Birch-Type Hydrogenation of Few-Layer Graphenes: Products and Mechanistic Implications

Zhang, Xu,Huang, Yuan,Chen, Shanshan,Kim, Na Yeon,Kim, Wontaek,Schilter, David,Biswal, Mandakini,Li, Baowen,Lee, Zonghoon,Ryu, Sunmin,Bielawski, Christopher W.,Bacsa, Wolfgang S.,Ruoff, Rodney S. American Chemical Society 2016 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.138 No.45

<P>Few-layer graphenes, supported on Si with a superficial oxide layer, were subjected to a Birch-type reduction using Li and H2O as the electron and proton donors, respectively. The extent of hydrogenation for bilayer graphene was estimated at 1.6-24.1% according to Raman and X-ray photoelectron spectroscopic data. While single-layer graphene reacts uniformly, few-layer graphenes were hydrogenated inward from the edges and/or defects. The role of these reactive sites was reflected in the inertness of pristine few-layer graphenes whose edges were sealed. Hydrogenation of labeled bilayer (C-12/C-13) and trilayer (C-12/C-13/C-12) graphenes afforded products whose sheets were hydrogenated to the same extent, implicating passage of reagents between the graphene layers and equal decoration of each graphene face. The reduction of few-layer graphenes introduces strain, allows tuning of optical transmission and fluorescence, and opens synthetic routes to long sought-after films containing sp(3)-hybridized carbon.</P>