Site specific redox properties in ligand differentiated di-nickel complexes inspired by the acetyl CoA synthase active site.

Bimetallic transition metal complexes with site-specific redox properties offer a versatile platform for understanding electron polarization, intramolecular electron transfer processes, and customizing electronic and magnetic properties that might impact reactivity and catalyst design. Inspired by the dissymmetric nickel sites in the Acetyl CoA Synthase (ACS) Active Site, three new bimetallic Ni(N2S2)-Ni(S2C2R2) complexes based on Ni(N2S2) metalloligand donor synthons, Nid, in mimicry of the nickel site distal to the redox-active iron sulfur cluster of ACS, and nickel dithiolene receiver units, designated as Nip, the nickel proximal to the 4Fe4S cluster, were combined to explore the influence of ligand environment on electronic structure and redox properties of each unit. The combination of synthons gave a matrix of three S-bridged dinickel complexes, characterized by X-ray crystallography, and appropriate spectroscopies. Computational modeling is connected to the electronic characteristics of the nickel donor and receiver units. This study demonstrated the intricacies of identifying sites of electrochemical redox processes, within multi-metallic systems containing non-innocent ligands.

A Prospective Open-Label Study for Treatment of Infraorbital Hollows Using a Volumizing Hyaluronic Acid Filler.

This study aimed to prospectively evaluate the effectiveness, patient satisfaction, and early adverse events of using the hyaluronic acid filler VYC-20L for the treatment of infraorbital hollowing. A total of 21 participants underwent injection of VYC-20L. FACE-Q satisfaction surveys before and after treatment along with early adverse events surveys were conducted. Pre- and posttreatment photos were graded, and the Allergan Infraorbital Hollows Scale was used to assess the difference in infraorbital hollowing. The results showed infraorbital hollowing improvement with VYC-20L was significant (p < 0.001). FACE-Q Satisfaction with Eyes scores on average were 27 points higher after treatment (p < 0.001). The mean FACE-Q Satisfaction with Decision score was 74.1%. The most common short-term adverse symptoms were tenderness (67%), swelling (62%), and bruising (52%). This study concludes that VYC-20L is an effective nonsurgical treatment option for infraorbital hollowing with high patient satisfaction.

Mechanism of Coupling of Methylidene to Ethylene Ligands in Dimetallic Assemblies; Computational Investigation of a Model for a Key Step in Catalytic C1 Chemistry.

Methylidene complexes often couple to ethylene complexes, but the mechanistic insight is scant. The path by which two cations [(η5-C5H5)Re(NO)(PPh3)(═CH2)]+ (5+) transform (CH2Cl2/acetonitrile) to [(η5-C5H5)Re(NO)(PPh3)(H2C═CH2)]+ (6+) and [(η5-C5H5)Re(NO)(PPh3)(NCCH3)]+ is studied by density functional theory. Experiments provide a number of constraints such as the second-order rate in 5+; no prior ligand dissociation/exchange; a faster reaction of (S)-5+ with (S)-5+ than with (R)-5+ ("enantiomer self-recognition"). Although dirhenium dications with Re(µ-CH2)2Re cores represent energy minima, they are not accessible by 2 + 2 cycloadditions of 5+. Transition states leading to ReCH2CH2Re linkages are prohibitively high in energy. However, 5+ can give non-covalent SRe/SRe or SRe/RRe dimers with π interactions between the PPh3 ligands but long ReCH2···H2CRe and H2CRe···H2CRe distances (3.073-3.095 Å and 3.878-4.529 Å, respectively). In rate-determining steps, these afford [(η5-C5H5)Re(NO)(PPh3)(µ-η2:η2-H2C···CH2)(Ph3P)(ON)Re(η5-C5H5)]2+ (132+), in which one rhenium binds the bridging ethylene more tightly than the other (2.115-2.098 vs 2.431-2.486 Å to the centroid). In the SRe/RRe adduct, Dewar-Chatt-Duncanson optimization leads to unfavorable PPh3/PPh3 contacts. Ligand interactions are further dissected in the preceding transition states via component analyses, and ΔΔG‡ (1.2 kcal/mol, CH2Cl2) favors the SRe/SRe pathway, in accordance with the experiment. Acetonitrile then displaces 6+ from the more weakly bound rhenium of 132+. The formation of similar µ-H2C···CH2 intermediates is found to be rate-determining for varied coordinatively saturated M═CH2 species [M = Fe(d6)/Re(d4)/Ta(d2)], establishing generality and enhancing relevancy to catalytic CH4 and CO/H2 chemistry.

Cooperative redox and spin activity from three redox congeners of sulfur-bridged iron nitrosyl and nickel dithiolene complexes.

The synthesis of sulfur-bridged Fe-Ni heterobimetallics was inspired by Nature's strategies to "trick" abundant first row transition metals into enabling 2-electron processes: redox-active ligands (including pendant iron-sulfur clusters) and proximal metals. Our design to have redox-active ligands on each metal, NO on iron and dithiolene on nickel, resulted in the observation of unexpectedly intricate physical properties. The metallodithiolate, (NO)Fe(N2S2), reacts with a labile ligand derivative of [NiII(S2C2Ph2)]0, NiDT, yielding the expected S-bridged neutral adduct, FeNi, containing a doublet {Fe(NO)}7. Good reversibility of two redox events of FeNi led to isolation of reduced and oxidized congeners. Characterization by various spectroscopies and single-crystal X-ray diffraction concluded that reduction of the FeNi parent yielded [FeNi]-, a rare example of a high-spin {Fe(NO)}8, described as linear FeII(NO-). Mössbauer data is diagnostic for the redox change at the {Fe(NO)}7/8 site. Oxidation of FeNi generated the 2[FeNi]+⇌[Fe2Ni2]2+ equilibrium in solution; crystallization yields only the [Fe2Ni2]2+ dimer, isolated as PF6- and BArF- salts. The monomer is a spin-coupled diradical between {Fe(NO)}7 and NiDT+, while dimerization couples the two NiDT+ via a Ni2S2 rhomb. Magnetic susceptibility studies on the dimer found a singlet ground state with a thermally accessible triplet excited state responsible for the magnetism at 300 K (χMT = 0.67 emu·K·mol-1, µeff = 2.31 µB), and detectable by parallel-mode EPR spectroscopy at 20 to 50 K. A theoretical model built on an H4 chain explains this unexpected low energy triplet state arising from a combination of anti- and ferromagnetic coupling of a four-radical molecular conglomerate.

Whole genome deep sequencing analysis of cell-free DNA in samples with low tumour content.

BACKGROUND: Circulating cell-free DNA (cfDNA) in the plasma of cancer patients contains cell-free tumour DNA (ctDNA) derived from tumour cells and it has been widely recognized as a non-invasive source of tumour DNA for diagnosis and prognosis of cancer. Molecular profiling of ctDNA is often performed using targeted sequencing or low-coverage whole genome sequencing (WGS) to identify tumour specific somatic mutations or somatic copy number aberrations (sCNAs). However, these approaches cannot efficiently detect all tumour-derived genomic changes in ctDNA. METHODS: We performed WGS analysis of cfDNA from 4 breast cancer patients and 2 patients with benign tumours. We sequenced matched germline DNA for all 6 patients and tumour samples from the breast cancer patients. All samples were sequenced on Illumina HiSeqXTen sequencing platform and achieved approximately 30x, 60x and 100x coverage on germline, tumour and plasma DNA samples, respectively. RESULTS: The mutational burden of the plasma samples (1.44 somatic mutations/Mb of genome) was higher than the matched tumour samples. However, 90% of high confidence somatic cfDNA variants were not detected in matched tumour samples and were found to comprise two background plasma mutational signatures. In contrast, cfDNA from the di-nucleosome fraction (300 bp-350 bp) had much higher proportion (30%) of variants shared with tumour. Despite high coverage sequencing we were unable to detect sCNAs in plasma samples. CONCLUSIONS: Deep sequencing analysis of plasma samples revealed higher fraction of unique somatic mutations in plasma samples, which were not detected in matched tumour samples. Sequencing of di-nucleosome bound cfDNA fragments may increase recovery of tumour mutations from plasma.

Nickel-Borolide Complexes and Their Complex Electronic Structure.

Borolides (BC42-) can be considered as dianionic heterocyclic analogues of monoanionic cyclopentadienides. Although both are formally six-π-electron donors, we herein demonstrate that the electronic structure of their corresponding transition metal complexes differs significantly, leading to altered properties. Specifically, the 18-electron sandwich complex Ni(iPr2NBC4Ph2)2 (1) features an ∼90° angle between the Ni-B-N planes and is best described as a combination of three limiting resonance structures with the major contribution stemming from a formally Ni2+ species bound to two monoanionic radical (BC4•-) ligands. Compound 1 displays two sequential one-electron oxidation events over a small potential range of <0.2 V, which strikingly contrasts the large potential separations between redox partners in the family of metallocenes, and the potential reasons for this unusual observation are discussed.

Self-Assembled Nickel-4 Supramolecular Squares and Assays for HER Electrocatalysts Derived Therefrom.

Solid-state structures find a self-assembled tetrameric nickel cage with carboxylate linkages, [Ni(N2S'O)I(CH3CN)]4 ([Ni-I]40), resulting from sulfur acetylation by sodium iodoacetate of an [NiN2S]22+ dimer in acetonitrile. Various synthetic routes to the tetramer, best described from XRD as a molecular square, were discovered to generate the hexacoordinate nickel units ligated by N2Sthioether, iodide, and two carboxylate oxygens, one of which is the bridge from the adjacent nickel unit in [Ni-I]40. Removal of the four iodides by silver ion precipitation yields an analogous species but with an additional vacant coordination site, [Ni-Solv]+, a cation but with coordinated solvent molecules. This also recrystallizes as the tetramer [Ni-Solv]44+. In solution, dissociation into the (presumed) monomer occurs, with coordinating solvents occupying the vacant site [Ni(N2S'O)I(solv)]0, ([Ni-I]0). Hydrodynamic radii determined from 1H DOSY NMR data suggest that monomeric units are present as well in CD2Cl2. Evans method magnetism values are consistent with triplet spin states in polar solvents; however, in CD2Cl2 solutions no paramagnetism is evident. The abilities of [Ni-I]40 and [Ni-Solv]44+ to serve as sources of electrocatalysts, or precatalysts, for the hydrogen evolution reaction (HER) were explored. Cyclic voltammetry responses and bulk coulometry with gas chromatographic analysis demonstrated that a stronger acid, trifluoroacetic acid, as a proton source resulted in H2 production from both electroprecatalysts; however, electrocatalysis developed primarily from uncharacterized deposits on the electrode. With acetic acid as a proton source, the major contribution to the HER is from homogeneous electrocatalysis. Overpotentials of 490 mV were obtained for both the solution-phase [Ni-I]0 and [Ni-Solv]+. While the electrocatalyst derived from [Ni-Solv]+ has a substantially higher TOF (102 s-1) than [Ni-I]0 (19 s-1), it has a shorter catalytically active lifespan (4 h) in comparison to [Ni-I]0 (>18 h).

Theoretical Analysis of Competing Pathways for Carbon-Hydrogen Activation of Cyclopentadienyl-Triphenylphosphine-Iridium in Benzene.

Density functional theory (DFT) calculations are used to evaluate alternative reaction mechanisms when the photochemically produced (η5-C5Me5)IrPPh3 oxidatively adds a C-H bond from either a benzene solvent molecule or a phenyl group of the phosphine ligand. Experimentally, the ortho-metalated complexes produced from intramolecular C-H activation and the hydridophenyl complexes produced from intermolecular C-H activation form in a ratio of 53:47 (Janowicz, A. H.; Bergman, R. G. J. Am. Chem. Soc. 1982, 104, 352-354). Both products are predicted to be thermodynamically stable such that the back reaction, reductive elimination, is predicted to be exceedingly unfavorable. Thus, the product ratio must be under kinetic control. The DFT calculations predict the initial formation of π-bound intermediates, η2-C6H5X (X = H or PPh2), with the intermolecular π-intermediate 2.0 kcal/mol more stable in free energy than the intramolecular π-intermediate. From these intermediates, the two competing reactions have slightly different free-energy barriers. The intramolecular activation of a phenyl C-H bond to yield ortho-metalated complexes has a barrier of 13.4 kcal/mol, and the intermolecular oxidative addition of solvent molecule C6H6 to form (η5-C5Me5)Ir(PPh3)(Ph)H has a barrier of 15.9 kcal/mol. We propose that exchange between the π-intermediates is faster than the oxidative additions, so the dominant early intermediate is the intermolecular π-intermediate. Hence, from this intermediate the two reaction paths have free-energy barriers of 15.4 (intramolecular) and 15.9 (intermolecular) kcal/mol. Thus, within the accuracy of DFT, the free-energy barriers for the intramolecular pathway and the intermolecular pathway are very compatible with the 53:47 product ratio. However, the calculations cannot completely exclude a higher interchange barrier, which would mean that the final product ratio must result from the nearly equal distribution (53:47) of the two π-intermediates that then proceed toward their own products. Further calculations on the less sterically crowded (η5-C5H5)IrPPh3 predict that the intermolecular product should dominate the ratio.

Mössbauer Spectroscopy and Theoretical Studies of Iron Bimetallic Complexes Showing Electrocatalytic Hydrogen Evolution.

Mössbauer spectroscopy and density functional theory (DFT) calculations are reported for the mononuclear Fe-nitrosyl complex [Fe( N, N'-bis(2-mercaptoethyl)-1,4-diazacycloheptane)NO] {[Fe(bme-dach)(NO)] (1)} and the series of dithiolate-bridged dinuclear complexes M-Fe(CO)Cp [M = Fe(bme-dach)(NO) (1-A), Ni(bme-dach) (2-A), and Co(bme-dach)(NO) (3-A)], in which M is a metallo-ligand to Fe(CO)Cp+ (Fe'Cp). The latter is an organometallic fragment in which Fe is coordinated by one CO and one cyclopentadienyl ligand. Complexes 1-A and 2-A were previously shown to have electrocatalytic hydrogen evolution activity. Mononuclear {Fe-NO}7 complex 1, with overall spin of 1/2, has an isomer shift of 0.23(2) mm/s [Δ EQ = 1.37(2) mm/s] and magnetic hyperfine couplings of {-38 T, -26.8 T, 8.6 T}. In complexes 2-A and 3-A, Fe'(CO)Cp+ has a diamagnetic ground state and δ = 0.33(2) mm/s (Δ EQ ≈ 1.78 mm/s), consistent with a low-spin FeII site. In contrast, in complex 1-A, M = Fe(bme-dach)(NO) (i.e., complex 1) the magnetic hyperfine interactions of both metallo-ligand, M, and low-spin Fe'Cp are perturbed and Fe'Cp exhibits small magnetic hyperfine interactions, although its isomer shift and quadrupole splittings are largely unaltered. The DFT calculations for 1-A are in agreement with the paramagnetism observed for the Fe'(CO)Cp+ iron site.

Influence of chelate ring type on chelate-chelate and chelate-aryl stacking: the case of nickel bis(dithiolene).

Chelate-aryl and chelate-chelate stacking interactions of nickel bis(dithiolene) were studied at the CCSD(T)/CBS and DFT levels. The strongest chelate-aryl stacking interaction between nickel bis(dithiolene) and benzene has a CCSD(T)/CBS stacking energy of -5.60 kcal mol-1. The strongest chelate-chelate stacking interactions between two nickel bis(dithiolenes) has a CCSD(T)/CBS stacking energy of -10.34 kcal mol-1. The most stable chelate-aryl stacking has the benzene center above the nickel atom, while the most stable chelate-chelate dithiolene stacking has the chelate center above the nickel atom. Comparison of chelate-aryl stacking interactions of dithiolene and acac-type nickel chelate shows similar strength. However, chelate-chelate stacking is stronger for dithiolene nickel chelate than for acac-type nickel chelate, which has a CCSD(T)/CBS interaction energy of -9.50 kcal mol-1.

Novel Use of a Volumizing Hyaluronic Acid Filler for Treatment of Infraorbital Hollows.

IMPORTANCE: Hyaluronic acid filler can be safely used as a soft-tissue filler for correction of infraorbital hollowing. It has a high overall patient satisfaction profile among patients. OBJECTIVE: To report safety and patient satisfaction outcomes of Juvéderm Voluma XC for correction of infraorbital hollows. DESIGN, SETTING, AND PATIENTS: This was a retrospective observational study performed at a private ambulatory facial plastic and reconstructive surgery practice. Participants were all patients 21 to 85 years old who presented to our practice and underwent Juvéderm Voluma XC treatment for correction of infraorbital hollows as a singular intervention from February 2016 to March 2017. INTERVENTIONS: Injection of Juvéderm Voluma XC to the tear trough, nasojugal fold, and/or palpebromalar groove. MAIN OUTCOMES AND MEASURES: Primary outcome measures include the number of recorded short- and long-term adverse events, need for additional treatment, and patient questionnaire FACE-Q scores. RESULTS: A total of 202 eyes were treated in 101 patients with a mean follow-up of 12 months. Patients were principally female (90 [89%]) with an average age of 54 years (range, 21-85 years). Most patients (99) had Fitzpatrick grade 1 to 4 skin type (98%) and had an infraorbital hollows score of 2 to 4 (89 [88%]). The average initial treatment volume was 1 mL with 18 patients (18%) requiring additional treatment within 3 months. The average time until additional treatment was 35.7 days. Adverse effects include bruising (in 10 [10%], contour irregularities (2 [2%]), swelling (3 [3%]), and Tyndall effect (1 [1%]). Hyaluronidase was required in 3 patients (3%). Forty-one patients completed the FACE-Q Satisfaction With Eyes survey, and 42 patients completed the FACE-Q Satisfaction With Decision survey (41% and 42%). Overall mean (SD) patient satisfaction (based on FACE-Q scores) was 71.1% (27.3) and 65.6% (31.3), respectively. CONCLUSIONS AND RELEVANCE: Juvéderm Voluma XC has a high patient satisfaction profile and an acceptable safety profile for the correction of infraorbital hollowing. LEVEL OF EVIDENCE: 4.

Nickel Fluorocarbene Metathesis with Fluoroalkenes.

Alkene metathesis with directly fluorinated alkenes is challenging, limiting its application in the burgeoning field of fluoro-organic chemistry. A new nickel tris(phosphite) fluoro(trifluoromethyl)carbene complex ([P3 Ni]=CFCF3 ) reacts with CF2 =CF2 (TFE) or CF2 =CH2 (VDF) to yield both metallacyclobutane and perfluorocarbene metathesis products, [P3 Ni]=CF2 and CR2 =CFCF3 (R=F, H). The reaction of [P3 Ni]=CFCF3 with trifluoroethylene also yields metathesis products, [P3 Ni]=CF2 and cis/trans-CFCF3 =CFH. However, unlike reactions with TFE and VDF, this reaction forms metallacyclopropanes and fluoronickel alkenyl species, resulting presumably from instability of the expected metallacyclobutanes. DFT calculations and experimental evidence established that the observed metallacyclobutanes are not intermediates in the formation of the observed metathesis products, thus highlighting a novel variant of the Chauvin mechanism enabled by the disparate four-coordinate transition states.

Biomimetics of [NiFe]-Hydrogenase: Nickel- or Iron-Centered Proton Reduction Catalysis?

The [NiFe] hydrogenase (H2ase) has been characterized in the Ni-R state with a hydride bridging between Fe and Ni but displaced toward the Ni. In nearly all of the synthetic Ni-R models reported so far, the hydride ligand is either displaced toward Fe, or terminally bound to Fe. Recently, a structural and functional [NiFe]-H2ase mimic ( Nat. Chem. 2016 , 8 , 1054 - 1060 ) was reported to produce H2 catalytically via EECC mechanism through a Ni-centered hydride intermediate like the enzyme. Here, a comprehensive DFT study shows a much lower energy route via an E[ECEC] mechanism through an Fe-centered hydride intermediate. Although catalytic H2 production occurs at the potential corresponding to the complex's second reduction, a third electron is needed to induce the second proton addition from the weak acid. The first two-electron reductions and a proton addition produce a semibridging hydride with a short Fe-H bond like other structured [NiFe]-biomimetics, but this species is not basic enough to add another proton from the weak acid without the third electron. The calculated mechanism provides insight into the origin of this structure in the enzyme.

Unexpected Importance of Aromatic-Aliphatic and Aliphatic Side Chain-Backbone Interactions in the Stability of Amyloids.

The role of aromatic and nonaromatic amino acids in amyloid formation has been elucidated by calculating interaction energies between ß-sheets in amyloid model systems using density functional theory (B3LYP-D3/6-31G*). The model systems were based on experimental crystal structures of two types of amyloids: (1) with aromatic amino acids, and (2) without aromatic amino acids. Data show that these two types of amyloids have similar interaction energies, supporting experimental findings that aromatic amino acids are not essential for amyloid formation. However, different factors contribute to the stability of these two types of amyloids. In the former, the presence of aromatic amino acids significantly contributes to the strength of interactions between side chains; interactions between aromatic and aliphatic side chains are the strongest, followed by aromatic-aromatic interactions, while aliphatic-aliphatic interactions are the weakest. In the latter, that is, the amyloids without aromatic residues, stability is provided by interactions of aliphatic side chains with the backbone and, in some cases, by hydrogen bonds.

Sinonasal malignancy: What to do with an unexpected pathology result?

BACKGROUND: Endoscopic sinus surgery has become the mainstay in surgical treatment of sinusitis and nasal polyps. While rare, diagnostic discrepancies or pathological contamination during routine specimen analysis has been described. Thus, an accurate diagnosis and indication for surgery are mandatory before proceeding with surgical intervention. METHODS: We present the case of a 40-year-old female patient who underwent endoscopic sinus surgery (ESS) for chronic sinusitis without nasal polyposis and fragments of squamous cell carcinoma (SCC) were found in the pathology specimen. RESULTS: We propose an algorithm to help guide physicians presented with a tissue diagnosis that does not match the clinical scenario. Moreover, we discuss strategies to help prevent medical errors and the importance of DNA genetic analysis in this situation. CONCLUSION: When an unexpected diagnosis occurs, the pathology slides should be reviewed for a second opinion. If the unexpected diagnosis is confirmed, the tissue should undergo STR genetic analysis to ensure against tissue contamination.

Facile P-C/C-H Bond-Cleavage Reactivity of Nickel Bis(diphosphine) Complexes.

Unusual cleavage of P-C and C-H bonds of the P2 N2 ligand, in heteroleptic [Ni(P2 N2 )(diphosphine)](2+) complexes under mild conditions, results in the formation of an iminium formyl nickelate featuring a C,P,P-tridentate coordination mode. The structures of both the heteroleptic [Ni(P2 N2 )(diphosphine)](2+) complexes and the resulting iminium formyl nickelate have been characterized by NMR spectroscopy and single-crystal X-ray diffraction analysis. Density functional theory (DFT) calculations were employed to investigate the mechanism of the P-C/C-H bond cleavage, which involves C-H bond cleavage, hydride rotation, Ni-C/P-H bond formation, and P-C bond cleavage.

Positive Pressure Bronchoscopy Technique: Case Report.

OBJECTIVE: Foreign body aspiration into the tracheobronchial tree continues to be a challenging problem for otolaryngologists. This is especially true in patients with poor pulmonary reserve. METHODS: We describe a novel technique in which an endotracheal sheathed bronchoscope is used as a means to provide positive pressure ventilation simultaneously during foreign body extraction. RESULTS: This new technique afforded the bronchoscopist more time during retrieval of the foreign body where previous attempts were limited by rapid desaturations and the overall nature of the foreign body. CONCLUSION: The endotracheal sheathed bronchoscope is a safe and efficacious technique for challenging airway foreign bodies complicated by a patient's limited pulmonary reserve.

Cyanide-bridged iron complexes as biomimetics of tri-iron arrangements in maturases of the H cluster of the di-iron hydrogenase.

Developing from certain catalytic processes required for ancient life forms, the H2 processing enzymes [NiFe]- and [FeFe]-hydrogenase (H2ase) have active sites that are organometallic in composition, possessing carbon monoxide and cyanide as ligands. Simple synthetic analogues of the 2Fe portion of the active site of [FeFe]-H2ase have been shown to dock into the empty carrier (maturation) protein, apo-Hyd-F, via the bridging ability of a terminal cyanide ligand from a low valent FeIFeI unit to the iron of a 4Fe4S cluster of Hyd-F, with spectral evidence indicating CN isomerization during the coupling process (Berggren, et al., Nature, 2013, 499, 66-70). To probe the requirements for such cyanide couplings, we have prepared and characterized four cyanide-bridged analogues of 3-Fe systems with features related to the organoiron moiety within the loaded HydF protein. As in classical organometallic chemistry, the orientation of the CN bridge in the biomimetics is determined by the precursor reagents; no cyanide flipping or linkage isomerization was observed. Density functional theory computations evaluated the energetics of cyanide isomerization in such [FeFe]-CN-Fe ⇌ [FeFe]-NC-Fe units, and found excessively high barriers account for the failure to observe the alternative isomers. These results highlight roles for cyanide as an unusual ligand in biology that may stabilize low spin iron in [FeFe]-hydrogenase, and can act as a bridge connecting multi-iron units during bioassembly of the active site.

Addition of ethylene to a π-conjugated two-dimensional nickel-based organometallic framework with implications for olefin separation.

We have investigated the mechanism for the reversible addition of ethylene to a periodic 2D network of square-planar nickel centers ([Ni(SC)4]n) using density functional theory. The mechanism is analogous to that for homogeneous olefin binding to metal bis(dithiolene) complexes. We considered periodic boundary calculations (PBC) as well as clusters containing up to 12 nickel atoms using a screened hybrid density functional to obtain accurate reaction barriers. Several different products were analyzed. The lowest-energy route begins with the addition of ethylene across the nickel-sulfur bond, which has previously not been considered for this system. The effect of the addition of several alkene molecules simultaneously on the surface was investigated to determine the potential efficiency of this material. This material is a candidate for alkene purification as it exhibits similar reactivity to its molecular analog in terms of the relative stability of products, energy barriers, and molecular efficiency while also providing the intrinsic technical benefits of heterogeneous catalysis.

Intramolecular iron-mediated C-H bond heterolysis with an assist of pendant base in a [FeFe]-hydrogenase model.

Although many metalloenzymes containing iron play a prominent role in biological C-H activation processes, to date iron-mediated C(sp(3))-H heterolysis has not been reported for synthetic models of Fe/S-metalloenzymes. In contrast, ample precedent has established that nature's design for reversible hydrogen activation by the diiron hydrogenase ([FeFe]-H2ase) active site involves multiple irons, sulfur bridges, a redox switch, and a pendant amine base, in an intricate arrangement to perform H-H heterolytic cleavage. In response to whether this strategy might be extended to C-H activation, we report that a [FeFe]-H2ase model demonstrates iron-mediated intramolecular C-H heterolytic cleavage via an agostic C-H interaction, with proton removal by a nearby pendant amine, affording Fe(II)-[Fe'(II)-CH-S] three-membered-ring products, which can be reduced back to 1 by Cp2Co in the presence of HBF4. The function of the pendant base as a proton shuttle was confirmed by the crystal structures of the N-protonated intermediate and the final deprotonated product in comparison with that of a similar but pendant-amine-free complex that does not show evidence of C-H activation. The mechanism of the process was backed up by DFT calculations.