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

Hydrogenase enzymes efficiently process H2 and protons at organometallic FeFe, NiFe, or Fe active sites. Synthetic modeling of the many H2ase states has provided insight into H2ase structure and mechanism, as well as afforded catalysts for the H2 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 H2 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 H2ase 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.

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

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 graphenes. 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 sp3-rehybridization of the basal plane atoms. When bilayers consisting of 13C and 12C 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.

N,N'-Diamidocarbenes: Isolable Divalent Carbons with Bona Fide Carbene Reactivity.

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 monoxide. 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 development 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 imidazolylidenes, 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 π 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 C-H 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 may serve as a blueprint for the development of more electrophilic, stable carbenes with the capacity to activate other challenging small molecules.

Synthetic Models for Nickel-Iron Hydrogenase Featuring Redox-Active Ligands.

The nickel-iron hydrogenase enzymes efficiently and reversibly interconvert protons, electrons, and dihydrogen. These redox proteins feature iron-sulfur clusters that relay electrons to and from their active sites. Reported here are synthetic models for nickel-iron hydrogenase featuring redox-active auxiliaries that mimic the iron-sulfur cofactors. The complexes prepared are NiII(µ-H)FeIIFeII species of formula [(diphosphine)Ni(dithiolate)(µ-H)Fe(CO)2(ferrocenylphosphine)]+ or NiIIFeIFeII complexes [(diphosphine)Ni(dithiolate)Fe(CO)2(ferrocenylphosphine)]+ (diphosphine = Ph2P(CH2)2PPh2 or Cy2P(CH2)2PCy2; dithiolate = -S(CH2)3S-; ferrocenylphosphine = diphenylphosphinoferrocene, diphenylphosphinomethyl(nonamethylferrocene) or 1,1'-bis(diphenylphosphino)ferrocene). The hydride species is a catalyst for hydrogen evolution, while the latter hydride-free complexes can exist in four redox states - a feature made possible by the incorporation of the ferrocenyl groups. Mixed-valent complexes of 1,1'-bis(diphenylphosphino)ferrocene have one of the phosphine groups unbound, with these species representing advanced structural models with both a redox-active moiety (the ferrocene group) and a potential proton relay (the free phosphine) proximal to a nickel-iron dithiolate.

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

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 (12C/13C) and trilayer (12C/13C/12C) 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 sp3-hybridized carbon.

Nickel-Iron Hydrogenases: High-Resolution Crystallography Resolves the Hydride, but Not the Debate.

An (X-ray) eye for detail: Modern high-resolution protein crystallography allows H atoms to be located. Applied to nickel-iron hydrogenase, X-ray structural analysis has finally confirmed the presence of an active-site hydride and thiol, as well as unveiling the intricate pathways that protons take to and from the active site.

N-Substituted Derivatives of the Azadithiolate Cofactor from the [FeFe] Hydrogenases: Stability and Complexation.

Experiments are described that probe the stability of N-substituted derivatives of the azadithiolate cofactor recently confirmed in the [FeFe] hydrogenases (Berggren, G., et al. Nature 2013, 499, 66). Acid-catalyzed hydrolysis of bis(thioester) BnN(CH2SAc)2 gives [BnNCH2SCH2]2 rather than azadithiol BnN(CH2SH)2. Treatment of BnN(CH2SAc)2 with NaO(t)Bu generates BnN(CH2SNa)2, which was trapped with NiCl2(diphos) (diphos = 1,2-C2H4(PR2)2; R = Ph (dppe) and Cy (dcpe)) to give fully characterized complexes Ni[(SCH2)2NBn](diphos). The related N-aryl derivative Ni[(SCH2)2NC6H4Cl](diphos) was prepared analogously from 4-ClC6H4N(CH2SAc)2, NaO(t)Bu, and NiCl2(dppe). Crystallographic analysis confirmed that these rare nonbridging [adt(R)](2-) complexes feature distorted square planar Ni centers. The analogue Pd[(SCH2)2NBn](dppe) was also prepared. (31)P NMR analysis indicates that Ni[(SCH2)2NBn](dppe) has basicity comparable to typical amines. As shown by cyclic voltammetry, the couple [M[(SCH2)2NBn](dppe)](+/0) is reversible near -2.0 V versus Fc(+/0). The wave shifts to -1.78 V upon N-protonation. In the presence of CF3CO2H, Ni[(SCH2)2NBn](dppe) catalyzes hydrogen evolution at rate of 22 s(-1) in the acid-independent regime, at room temperature in CH2Cl2 solution. In contrast to the instability of RN(CH2SH)2 (R = alkyl, aryl), the dithiol of tosylamide TsN(CH2SH)2 proved sufficiently stable to allow full characterization. This dithiol reacts with Fe3(CO)12 and, in the presence of base, NiCl2(dppe) to give Fe2[(SCH2)2NTs](CO)6 and Ni[(SCH2)2NTs](dppe), respectively.

Protonation of nickel-iron hydrogenase models proceeds after isomerization at nickel.

Theory and experiment indicate that the protonation of reduced NiFe dithiolates proceeds via a previously undetected isomer with enhanced basicity. In particular, it is proposed that protonation of (OC)3Fe(pdt)Ni(dppe) (1; pdt(2-) = (-)S(CH2)3S(-); dppe = Ph2P(CH2)2PPh2) occurs at the Fe site of the two-electron mixed-valence Fe(0)Ni(II) species, not the Fe(I)-Ni(I) bond for the homovalence isomer of 1. The new pathway, which may have implications for protonation of other complexes and clusters, was uncovered through studies on the homologous series L(OC)2Fe(pdt)M(dppe), where M = Ni, Pd (2), and Pt (3) and L = CO, PCy3. Similar to 1, complexes 2 and 3 undergo both protonation and 1e(-) oxidation to afford well-characterized hydrides ([2H](+) and [3H](+)) and mixed-valence derivatives ([2](+) and [3](+)), respectively. Whereas the Pd site is tetrahedral in 2, the Pt site is square-planar in 3, indicating that this complex is best described as Fe(0)Pt(II). In view of the results on 2 and 3, the potential energy surface of 1 was reinvestigated with density functional theory. These calculations revealed the existence of an energetically accessible and more basic Fe(0)Ni(II) isomer with a square-planar Ni site.

Multicopper models for the laccase active site: effect of nuclearity on electrocatalytic oxygen reduction.

Cu complexes of 2,2'-dipicolylamine (DPA) were prepared and tested as electrocatalysts for the oxygen reduction reaction (ORR). To study the effect of multinuclearity on the ORR, two Cu-DPA units were connected with a flexible linker, and a third metal-binding pocket was installed in the ligand framework. ORR onset potentials and the diffusion-limited current densities of di- and tricopper complexes of DPA derivatives were found to be comparable to those of the simpler Cu-DPA system. Electrochemical analyses, crystallographic data, and metal-substitution studies suggested that Cu complexes of DPA derivatives reacted with O2 via a binuclear intermolecular pathway but that the Cu center in the third binding site did not participate in the ORR process. This study highlights the viability of Cu-DPA complexes to mimic the T3-site of laccase, and serves as a guide for designing future laccase models.

Sphaeropsidin A C15-C16 Cross-Metathesis Analogues with Potent Anticancer Activity.

We recently discovered that sphaeropsidin A (SphA), a fungal metabolite from Diplodia cupressi, overcomes apoptosis resistance in cancer cells by inducing cellular shrinkage by impairing regulatory volume increase. Previously, we prepared a pyrene-conjugated derivative of SphA by a cross-metathesis reaction involving the phytotoxin's C15,C16-alkene. This derivative's evaluation in a cancer cell panel revealed a significant increase in potency, with the IC50 values 5-10× lower than those displayed by the original natural product. Herein, we describe the preparation and anticancer evaluation of fifteen novel C15,C16-alkene cross-metathesis analogues in which the pyrene moiety was replaced with other aromatic or non-aromatic hydrophobic groups. The idea for this replacement was to prepare a family of compounds that would not be predicted to be mutagenic compared with the original pyrene analogue. We predict several of our new compounds to be non-mutagenic, while retaining the high potency of the original pyrene-containing analogues. Examples of these potential lead compounds included those containing pentamethylphenyl and triphenylethylene pendant groups. As an additional feature of the current investigation, we prepared several deuterated pyrene-containing compounds to overcome intellectual property issues associated with non-patentability of the original pyrene derivative.

And the winner is...azadithiolate: an amine proton relay in the [FeFe] hydrogenases.

A victory in the pocket: An international team of chemists and biophysicists have resolved the long-standing question of the structure of the active site of the [FeFe] hydrogenases by assembling the active enzyme with a version of the active site synthesized in vitro (see scheme; HydF is a scaffold protein, HydA1 is a natural hydrogenase). The protein incorporating the diiron complex 2 showed similar activity to that of the natural enzyme.

Connecting [NiFe]- and [FeFe]-hydrogenases: mixed-valence nickel-iron dithiolates with rotated structures.

New mixed-valence iron-nickel dithiolates are described that exhibit structures similar to those of mixed-valence diiron dithiolates. The interaction of tricarbonyl salt [(dppe)Ni(pdt)Fe(CO)(3)]BF(4) ([1]BF(4), where dppe = Ph(2)PCH(2)CH(2)PPh(2) and pdt(2-) = -SCH(2)CH(2)CH(2)S-) with P-donor ligands (L) afforded the substituted derivatives [(dppe)Ni(pdt)Fe(CO)(2)L]BF(4) incorporating L = PHCy(2) ([1a]BF(4)), PPh(NEt(2))(2) ([1b]BF(4)), P(NMe(2))(3) ([1c]BF(4)), P(i-Pr)(3) ([1d]BF(4)), and PCy(3) ([1e]BF(4)). The related precursor [(dcpe)Ni(pdt)Fe(CO)(3)]BF(4) ([2]BF(4), where dcpe = Cy(2)PCH(2)CH(2)PCy(2)) gave the more electron-rich family of compounds [(dcpe)Ni(pdt)Fe(CO)(2)L]BF(4) for L = PPh(2)(2-pyridyl) ([2a]BF(4)), PPh(3) ([2b]BF(4)), and PCy(3) ([2c]BF(4)). For bulky and strongly basic monophosphorus ligands, the salts feature distorted coordination geometries at iron: crystallographic analyses of [1e]BF(4) and [2c]BF(4) showed that they adopt "rotated" Fe(I) centers, in which PCy(3) occupies a basal site and one CO ligand partially bridges the Ni and Fe centers. Like the undistorted mixed-valence derivatives, members of the new class of complexes are described as Ni(II)Fe(I) (S = ½) systems according to electron paramagnetic resonance spectroscopy, although with attenuated (31)P hyperfine interactions. Density functional theory calculations using the BP86, B3LYP, and PBE0 exchange-correlation functionals agree with the structural and spectroscopic data, suggesting that the spin for [1e](+) is mostly localized in a Fe(I)-centered d(z(2)) orbital, orthogonal to the Fe-P bond. The PCy(3) complexes, rare examples of species featuring "rotated" Fe centers, both structurally and spectroscopically incorporate features from homobimetallic mixed-valence diiron dithiolates. Also, when the NiS(2)Fe core of the [NiFe]-hydrogenase active site is reproduced, the "hybrid models" incorporate key features of the two major classes of hydrogenase. Furthermore, cyclic voltammetry experiments suggest that the highly basic phosphine ligands enable a second oxidation corresponding to the couple [(dxpe)Ni(pdt)Fe(CO)(2)L](+/2+). The resulting unsaturated 32e(-) dications represent the closest approach to modeling the highly electrophilic Ni-SI(a) state. In the case of L = PPh(2) (2-pyridyl), chelation of this ligand accompanies the second oxidation.

Mixed-valence nickel-iron dithiolate models of the [NiFe]-hydrogenase active site.

A series of mixed-valence nickel-iron dithiolates is described. Oxidation of (diphosphine)Ni(dithiolate)Fe(CO)(3) complexes 1, 2, and 3 with ferrocenium salts affords the corresponding tricarbonyl cations [(dppe)Ni(pdt)Fe(CO)(3)](+) ([1](+)), [(dppe)Ni(edt)Fe(CO)(3)](+) ([2](+)) and [(dcpe)Ni(pdt)Fe(CO)(3)](+) ([3](+)), respectively, where dppe = Ph(2)PCH(2)CH(2)PPh(2), dcpe = Cy(2)PCH(2)CH(2)PCy(2), (Cy = cyclohexyl), pdtH(2) = HSCH(2)CH(2)CH(2)SH, and edtH(2) = HSCH(2)CH(2)SH. The cation [2](+) proved unstable, but the propanedithiolates are robust. IR and EPR spectroscopic measurements indicate that these species exist as C(s)-symmetric species. Crystallographic characterization of [3]BF(4) shows that Ni is square planar. Interaction of [1]BF(4) with P-donor ligands (L) afforded a series of substituted derivatives of type [(dppe)Ni(pdt)Fe(CO)(2)L]BF(4) for L = P(OPh)(3) ([4a]BF(4)), P(p-C(6)H(4)Cl)(3) ([4b]BF(4)), PPh(2)(2-py) ([4c]BF(4)), PPh(2)(OEt) ([4d]BF(4)), PPh(3) ([4e]BF(4)), PPh(2)(o-C(6)H(4)OMe) ([4f]BF(4)), PPh(2)(o-C(6)H(4)OCH(2)OMe) ([4g]BF(4)), P(p-tol)(3) ([4h]BF(4)), P(p-C(6)H(4)OMe)(3) ([4i]BF(4)), and PMePh(2) ([4j]BF(4)). EPR analysis indicates that ethanedithiolate [2](+) exists as a single species at 110 K, whereas the propanedithiolate cations exist as a mixture of two conformers, which are proposed to be related through a flip of the chelate ring. Mössbauer spectra of 1 and oxidized S = 1/2 [4e]BF(4) are both consistent with a low-spin Fe(I) state. The hyperfine coupling tensor of [4e]BF(4) has a small isotropic component and significant anisotropy. DFT calculations using the BP86, B3LYP, and PBE0 exchange-correlation functionals agree with the structural and spectroscopic data, suggesting that the SOMOs in complexes of the present type are localized in an Fe(I)-centered d(z(2)) orbital. The DFT calculations allow an assignment of oxidation states of the metals and rationalization of the conformers detected by EPR spectroscopy. Treatment of [1](+) with CN(-) and compact basic phosphines results in complex reactions. With dppe, [1](+) undergoes quasi-disproportionation to give 1 and the diamagnetic complex [(dppe)Ni(pdt)Fe(CO)(2)(dppe)](2+) ([5](2+)), which features square-planar Ni linked to an octahedral Fe center.

Experience of total knee arthroplasty using a novel navigation system within the surgical field.

BACKGROUND: With the aim of improving component alignment and outcome in total knee arthroplasty (TKA), several computer-assisted devices (CAD) have been developed. METHODS: In February 2014, the present unit started to use a new imageless navigation system with accelerometric pods within the surgical field for all primary TKAs; there was no need for optical trackers or cameras. This paper presents the results of the first 72 TKAs using this iAssist system in 71 prospectively collected and retrospectively analyzed patients. It analyzed component positioning in standard and full-length leg x-rays. RESULTS: The mean age of the patients was 70years (range 52-88). The center of hip, knee and ankle (mechanical axes) deviated on average 0.5° (standard deviation (SD) of 1.8) valgus from the targeted straight alignment. Three TKAs had >3° deviation (i.e. four degree varus, five degree and seven degree valgus). The frontal tibial tray alignment was an average of 89.9° (range 86.4-100.1°, SD ±2.0) with the target being 90°, and the sagittal slope was as targeted at 85.0° (range 78.4-88.8°, SD ±1.7). CONCLUSIONS: This CAD facilitated good mechanical alignment and reproducible accuracy in component positioning. Pods clipped onto cutting jigs within the surgical field provided simple and accurate navigation, with little extra time needed for calibration and no need for optical trackers or pre-operative imaging.