Bridging CO Research Articles

Shedding Light on Proton and Electron Dynamics in [FeFe] Hydrogenases.

[FeFe] hydrogenases are highly efficient catalysts for reversible dihydrogen evolution. H2 turnover involves different catalytic intermediates including a recently characterized hydride state of the active site (H-cluster). Applying cryogenic infrared and electron paramagnetic resonance spectroscopy to an [FeFe] model hydrogenase from Chlamydomonas reinhardtii (CrHydA1), we have discovered two new hydride intermediates and spectroscopic evidence for a bridging CO ligand in two reduced H-cluster states. Our study provides novel insights into these key intermediates, their relevance for the catalytic cycle of [FeFe] hydrogenase, and novel strategies for exploring these aspects in detail.

Spectroscopic and Computational Evidence that [FeFe] Hydrogenases Operate Exclusively with CO-Bridged Intermediates.

[FeFe] hydrogenases are extremely active H2-converting enzymes. Their mechanism remains highly controversial, in particular, the nature of the one-electron and two-electron reduced intermediates called HredH+ and HsredH+. In one model, the HredH+ and HsredH+ states contain a semibridging CO, while in the other model, the bridging CO is replaced by a bridging hydride. Using low-temperature IR spectroscopy and nuclear resonance vibrational spectroscopy, together with density functional theory calculations, we show that the bridging CO is retained in the HsredH+ and HredH+ states in the [FeFe] hydrogenases from Chlamydomonas reinhardtii and Desulfovibrio desulfuricans, respectively. Furthermore, there is no evidence for a bridging hydride in either state. These results agree with a model of the catalytic cycle in which the HredH+ and HsredH+ states are integral, catalytically competent components. We conclude that proton-coupled electron transfer between the two subclusters is crucial to catalysis and allows these enzymes to operate in a highly efficient and reversible manner.

Design and synthesis of spherical-platelike ternary copper-cobalt-manganese catalysts for direct conversion of syngas to ethanol and higher alcohols

Novel CuCoMn ternary catalyst with a “spherical-platelike” (CuMn-Co) nanosized particles structure was designed and successfully performed in ethanol and higher alcohols (HA) production via heterogeneous CO hydrogenation. The “spherical-platelike” CuCoMn catalyst, achieved through a simple co-precipitation (CP) route followed by a calcination-reduction process, contained a CuMn-rich sphere structure and a Co-dominated nanosheet. The catalyst demonstrated a total alcohols selectivity of 46.2%, and the fraction of ethanol reached up to 45.4% among the total alcohols products, which is superior to the classical modified CuCo-based catalysts. The outstanding catalytic performance was attributed to the unique “spherical-platelike” structure, which altered the surface Cu+/Cu0 distribution and the dispersion of Co species. As revealed by in situ XRD, H2-TPR, in situ XPS, HAADF-STEM and in situ DRIFT spectra techniques, a strong electronic and geometric interaction between Cu and Mn species in optimized CuCoMn catalyst modified the chemical states of Cu species to present a higher proportion of surface Cu+/(Cu0 + Cu+) and, especially, enhanced the linear CO adsorption on Cu+ active sites, which provided a higher probability of CO insertion, and eventually contributed to promotion of catalytic performance. In addition, a higher probability of bridge CO adsorption on metallic Co was also observed over the CuCoMn catalyst, which was beneficial for the formation of CHx intermediates. It is concluded that a synergistic effect between Cu+ and Co species, promoted by the presence of manganese species, was responsible for CO hydrogenation to produce ethanol.

In-situ ATR-IR study of surface reaction during aqueous phase reforming of glycerol, sorbitol and glucose over Pt/γ-Al2O3

Surface reactions involved in aqueous phase reforming of glycerol, sorbitol and glucose over Pt/γ-Al2O3 are studied by in-situ attenuated total reflection infrared (ATR-IR) spectroscopy. In these experiments aqueous solutions of the reactants are flowed over a catalyst layer that is coated on the internal reflection element of the spectroscopic cell. Linearly bound CO (COL) and bridging CO (COB) are readily formed on the Pt particles at 24–72 °C. The relative abundance of CO in different environments and the rates of formation and conversion of these species provide insight into the reaction path of different oxygenates. Even at 24 °C, COB can be converted by the water-gas shift reaction. During the conversion of sorbitol and glucose, the formation and conversion of adsorbed CO is affected by the presence of co-adsorbed reactants or by-products. In addition to CO, surface bound alkoxy species are observed during the conversion of sorbitol and glucose.

Ethanol and Higher Alcohols Synthesis from Syngas over CuCoM (M=Fe, Cr, Ga and Al) Nanoplates Derived From Hydrotalcite‐Like Precursors

AbstractA series of CuCoM (M=Fe, Cr, Ga and Al) nanoplates derived from hydrotalcite‐like precursors were synthesized by co‐precipitation method and evaluated for ethanol and higher alcohols (HA) from syngas. The CuCoAl nanoplates demonstrated prominently improved HA selectivity of 54.9 %, and the fraction of ethanol/HA reached up to 55.9 % level. As revealed by XRD and HAADF‐STEM results, the presence of Al species in the CuCoAl nanoplates obviously enhanced the dispersion ability of Cu and Co species. Moreover, the abundance of basic sites (surface hydroxyl groups) on CuCoAl nanoplates resulted in the formation of formate species, which was a significant C1 species for the generation of CHx intermediates. More importantly, a high probability of CO bridge adsorption on metallic Co atoms was found over optimized CuCoAl catalyst, which provided a favorable effect for CO dissociation, leading to the formation of CHx intermediates. Therefore, more CHx intermediates were generated from the formate species and CO bridge adsorption on metallic Co atoms, which provided a beneficial role for C−C chain growth in the higher alcohols production in CO hydrogenation reaction. Furthermore, a moderate ratio of surface Cu/Co was observed over optimized CuCoAl nanoplates, which can exert a synergetic effect between Cu and Co species. Ultimately, the enhanced catalytic performance was attributed to the combination of the Cu and Co species and the basic property of the CuCoAl nanoplates.

An Approach to Carbide-Centered Cluster Complexes.

We report the first examples of the carbide ligand in (Cy3P)2Cl2Ru≡C (RuC) developing into a μ3 ligand toward metal centers. Conventionally, sterics exclude this coordination mode, but Fe2(CO)9 and Co2(CO)8 expel bridging CO ligands upon reaction with RuC to form trimetallic (Cy3P)2Cl2Ru═CFe2(CO)8 (RuCFe2) and (Cy3P)2Cl2Ru═CCo2(CO)7 (RuCCo2) complexes. Thus, the proximity offered by metal-metal associations in bimetallic carbonyl complexes allows the formation of trinuclear carbide complexes as verified by NMR, Mössbauer, and X-ray spectroscopic techniques.

Crystal structure of NiFe(CO)5[tris(pyridyl-meth-yl)aza-phosphatrane]: a synthetic mimic of the NiFe hydrogenase active site incorporating a pendant pyridine base.

The reaction of Ni(TPAP)(COD) {where TPAP = [(NC5H4)CH2]3P(NC2H4)3N} with Fe(CO)5 resulted in the isolation of the title heterobimetallic NiFe(TPAP)(CO)5 complex di-μ-carbonyl-tricarbon-yl[2,8,9-tris-(pyridin-2-yl-meth-yl)-2,5,8,9-tetra-aza-1-phosphabi-cyclo-[3.3.3]undeca-ne]ironnickel, [FeNi(C24H30N7P)(CO)5]. Characterization of the complex by 1H and 31P NMR as well as IR spectroscopy are presented. The structure of NiFe(TPAP)(CO)5 reveals three terminally bound CO mol-ecules on Fe0, two bridging CO mol-ecules between Ni0 and Fe0, and TPAP coordinated to Ni0. The Ni-Fe bond length is 2.4828 (4) Å, similar to that of the reduced form of the active site of NiFe hydrogenase (∼2.5 Å). Additionally, a proximal pendant base from one of the non-coordinating pyridine groups of TPAP is also present. Although involvement of a pendant base has been cited in the mechanism of NiFe hydrogenase, this moiety has yet to be incorporated in a structurally characterized synthetic mimic with key structural motifs (terminally bound CO or CN ligands on Fe). Thus, the title complex NiFe(TPAP)(CO)5 is an unique synthetic model for NiFe hydrogenase. In the crystal, the complex mol-ecules are linked by C-H⋯O hydrogen bonds, forming undulating layers parallel to (100). Within the layers, there are offset π-π [inter-centroid distance = 3.2739 (5) Å] and C-H⋯π inter-actions present. The layers are linked by further C-H⋯π inter-actions, forming a supra-molecular framework.

Bonding in Binuclear Carbonyl Complexes M2(CO)9 (M = Fe, Ru, Os).

Quantum-chemical density functional theory calculations using the BP86 functional in conjunction with a triple-ζ basis set and dispersion correction by Grimme with Becke-Johnson damping D3(BJ) were performed for the title molecules. The nature of the bonding was examined with the quantum theory of atoms in molecules (QTAIM) and natural bond order (NBO) methods and with the energy decomposition analysis in conjunction with the natural orbital for chemical valence (EDA-NOCV) analysis. The energetically lowest-lying form of Fe2(CO)9 is the triply bridged D3 h structure, whereas the most stable structures of Ru2(CO)9 and Os2(CO)9 are singly bridged C2 species. The calculated reaction energies for the formation of the cyclic trinuclear carbonyls M3(CO)12 from the dinuclear carbonyls M2(CO)9 are in agreement with experiment, as the iron complex Fe2(CO)9 is thermodynamically stable in these reactions, but the heavier homologues Ru2(CO)9 and Os2(CO)9 are not. The metal-CO bond to the bridging CO ligands is stronger than the bonds to the terminal CO ligands. This holds for the triply bridged D3 h structures as well as for the singly bridged C2 or C2 v species. The analysis of the orbital interactions with the help of the EDA-NOCV method suggests that the overall M→CO π backdonation is always stronger than the M←CO σ donation. The bridging carbonyls are more strongly bonded than the terminal CO ligands, and they are engaged in stronger σ donation and π backdonation, but the formation of bridging carbonyls requires reorganization energy, which may or may not be compensated by the stronger metal-ligand interactions. The lower-lying D3 h form of Fe2(CO)9 and C2 structures of Ru2(CO)9 and Os2(CO)9 are due to a delicate balance of several forces.

Unique Spectroscopic Properties of the H-Cluster in a Putative Sensory [FeFe] Hydrogenase.

Sensory type [FeFe] hydrogenases are predicted to play a role in transcriptional regulation by detecting the H2 level of the cellular environment. These hydrogenases contain the hydrogenase domain with distinct modifications in the active site pocket, followed by a Per-Arnt-Sim (PAS) domain. As yet, neither the physiological function nor the biochemical or spectroscopic properties of these enzymes have been explored. Here, we present the characterization of an artificially maturated, putative sensory [FeFe] hydrogenase from Thermotoga maritima (HydS). This enzyme shows lower hydrogen conversion activity than prototypical [FeFe] hydrogenases and a reduced inhibition by CO. Using FTIR spectroelectrochemistry and EPR spectroscopy, three redox states of the active site were identified. The spectroscopic signatures of the most oxidized state closely resemble those of the Hox state from the prototypical [FeFe] hydrogenases, while the FTIR spectra of both singly and doubly reduced states show large differences. The FTIR bands of both the reduced states are strongly red-shifted relative to the Hox state, indicating reduction at the diiron site, but with retention of the bridging CO ligand. The unique functional and spectroscopic features of HydS are discussed with regard to the possible role of altered amino acid residues influencing the electronic properties of the H-cluster.

Fe5Mo Cluster with Iron-Carbide and Molybdenum-Carbide Bonding Motifs: Structure and Selective Alkyne Reductions.

Herein we report the synthesis, X-ray structure, and characterization of the title pentairon (molybdo)carbido cluster. The reaction of the pentairon (μ5-carbido) dianion [Fe5(μ6-C)(μ2-CO)2(CO)12]2- (1) with [Mo(CO)3(chpt)] (chpt = cycloheptatriene) forms the heterohexanuclear cluster [K(benzo-18-crown-6)]2[Fe5Mo(μ6-C)(μ2-CO)3(CO)14] (2). The dianion exhibits a Fe5Mo(μ6-C) core structure supported by three bridging (νCO = 1788 cm-1) and terminal (νCO = 1943 cm-1) CO ligands. Cluster 2 provides the selective reduction of diphenylacetylene to cis-diphenylethylene via a spectroscopically observed cluster-hydride intermediate (1H NMR: δ -26).

Acetylene semi-hydrogenation over Pd-Zn/CeO2: Relevance of CO adsorption and methanation as descriptors of selectivity

The most selective Pd-Zn surfaces for the selective hydrogenation of acetylene to ethylene were recently proposed to be intermetallic in nature, exhibiting only Pd-Zn-Pd units and no Pd-Pd pairs, the later forming bridged CO upon exposure to CO. Ceria-supported Pd-Zn formulations apparently free of Pd-Pd ensembles were prepared and yet led to poor selectivity to ethylene. CO methanation was actually shown to be a better selectivity descriptor in the present case. Only the samples most selective for acetylene semi‑hydrogenation, which were obtained by reduction at temperatures equal or higher than 550°C, led to negligible CO methanation rates.

Synthesis of an Electron-Deficient Triruthenium Hydrido Complex Having a Bridging Carbonyl Ligand: Influence of a CO Ligand on the Properties and Reactivities of a Hydrido Cluster

A novel triruthenium hydrido cluster, (Cp*Ru)3(μ3-H)(μ-H)2(μ-CO) (4; Cp* = η5-C5Me5), having a bridging CO ligand was synthesized by the reaction of {Cp*Ru(μ-H)}3(μ3-O) (3) with methanol. Upon introduction of a CO ligand, the cyclic voltammogram of 4 demonstrated a significant shift of the redox potential in the positive direction in comparison to a pentahydrido complex, {Cp*Ru(μ-H)}3(μ3-H)2 (1), which adopts the same 44-electron configuration. Owing to its coordinatively unsaturated nature, 4 readily reacted with butadiene, terminal alkynes, and alkenes similarly to the parent pentahydrido complex 1. However, the reactivity was slightly different from that of 1, and the influence of the CO ligand at the triruthenium site was evaluated by the reaction of various unsaturated hydrocarbons. The reaction of 4 with 1,3-dienes yielded a diene adduct, {Cp*Ru(μ-H)}3(μ-η2:η2-s-cis-H2C═CRCH═CH2)(CO) (5a, R = H; 5b, R = Me) without elimination of dihydrogen. In the same manner as for 1, 4 reacted with phenylacetylene to yield a μ3-η2:η2(⊥)-alkyne complex, (Cp*Ru)3(μ-H){μ3-η2:η2(⊥)-PhCCH}(μ3-CO) (7a); in contrast, an isomeric μ3-pentenylidene complex, (Cp*Ru)3(μ-H){μ3-η2-C═C(nPr)H}(μ-CO) (9b), was obtained by reaction with 1-pentyne. A series of products was obtained by the reaction of 4 with ethylene molecules. A μ3-ethylidyne complex, (Cp*Ru)3(μ-H)2(μ3-CMe)(μ-CO) (11), was initially formed, accompanied by the formation of ethane at ambient temperature. The treatment of 11 with ethylene resulted in the removal of hydrido ligands, affording a μ3-vinylidene complex, (Cp*Ru)3(μ-H)(μ3-η2-C═CH2)(μ-CO) (9a). At higher temperatures, a second ethylene molecule was incorporated in the triruthenium plane and an equilibrated mixture of a μ3-ethylidyne−μ-ethylidyne complex, (Cp*Ru)3(μ-H)(μ-CMe)(μ3-CMe)(μ-CO) (13), and μ3-ethylidyne−μ-vinyl complex, (Cp*Ru)3(μ-H)(μ3-CMe)(μ-η2-CH═CH2)(μ-CO) (12), was obtained. The formation of a μ3-η3-C3 ring on the Ru3 plane was observed upon the thermolysis of the equilibrated mixture at 180 °C, which clearly demonstrates the coupling of the two C2 moieties placed on each face of the triruthenium plane.

CO adsorption on Pd(100) studied by multimodal ambient pressure X-ray photoelectron and infrared reflection absorption spectroscopies

The adsorption of CO on Pd(100) was investigated using simultaneous ambient pressure X-ray photoelectron spectroscopy (APXPS) and infrared reflection absorption infrared spectroscopy (IRRAS). The measurements were performed as a function of CO partial pressures from ultra-high vacuum to 0.5 Torr. Total CO coverages estimated from the complementary APXPS and IRRAS measurements are in good agreement. A signal for atop CO, which is uncommon for Pd(100), was observed in the IRRAS data and was used to identify the C 1 s binding energy of this species. Discerning this binding configuration of CO on the Pd(100) surface at elevated pressures has significance for catalytic reactions involving CO, where bridging CO is often the only configuration considered. We also detail the combined APXPS/IRRAS instrumentation and discuss ways to improve these multimodal measurements, which should have wide applicability across many areas of surface and interface science.

Role of N-Heterocyclic Carbenes as Ligands in Iridium Carbonyl Clusters

The low-energy isomers of Irx(CO)y(NHC)z (x = 1, 2, 4) are investigated with density functional theory (DFT) and correlated molecular orbital theory at the coupled cluster CCSD(T) level. The structures, relative energies, ligand dissociation energies, and natural charges are calculated. The energies of tetrairidium cluster are predicted at the CAM-B3LYP level that best fit the CCSD(T) results compared with the other four functionals in the benchmark calculations. The NHC's behave as stronger σ donors compared with CO's and have higher ligand dissociation energies (LDEs). For smaller isomers, the increase in the LDEs of the CO's and the decrease in the LDEs of the NHC's as more NHC's are substituted for CO's are due to π-back-bonding and electron repulsion, whereas the trend of how the LDEs change for larger isomers is not obvious. We demonstrate a μ3-CO resulting from the high electron density of the metal centers in these complexes, as the bridging CO's and the μ3-CO's can carry more negative charge and stabilize the isomers. Comparison of calculations for a mixed tetrairidum cluster consisting of two calixarene-phosphine ligands and a single calixarene-NHC ligand in the basal plane demonstrated good agreement in terms of both the ligand substitution symmetry (C3v derived), as well as the infrared spectra. Similar comparisons were also performed between calculations and experiment for novel monosubstituted calixarene-NHC tetrairidium clusters.

Individual Heat of Adsorption of Adsorbed CO Species on Palladium and Pd–Sn Nanoparticles Supported on Al2O3 by Using Temperature-Programmed Adsorption Equilibrium Methods

The present article is dedicated to the adsorption of CO on reduced 2% Pd/Al2O3 and 2% Pd-x% Sn/Al2O3 (weight %, x = 2 or 5 wt %) in the 300–713 K temperature range to study the geometric and electronic effects of Sn on the palladium adsorption sites. Using Fourier transform infrared (FTIR) spectroscopy, it is shown that the insertion of Sn leads to (a) the total disappearance of the Pd sites forming bridged CO species (denoted as “B”), which are the dominant species on Pd0 particles and (b) a significant increase in the Pd sites forming linear CO species (denoted as “L”). This is ascribed to a geometric effect of Sn that dilutes the superficial palladium sites. The measurement of the individual heats of adsorption of the different adsorbed CO species by using two original temperature-programmed adsorption equilibrium methods (denoted AEIR and TPAE) allows the estimation of the electronic effect of Sn on the Pd sites. On 2% Pd/Al2O3, in parallel to the formation of two strongly adsorbed B CO species, two linear L1Pd0 and L2Pd0 CO species are formed, which exhibit different heats of adsorption. For the dominant L1Pd0 CO species, the heat of adsorption decreases linearly, from 92 kJ/mol to 54 kJ/mol, as its coverage increases at coverage 0 and 1, respectively, while that of the L2Pd0 species is >165 kJ/mol at coverage 1. On the two Pd–Sn containing particles, two linear CO species are formed denoted L12Pd–xSn and L22Pd–xSn with x = 2 or 5. The L12Pd–xSn species dominates the CO adsorption on the two solids. It is shown that its heat of adsorption (which is slightly dependent on x) linearly varies with its coverage: ∼90 kJ/mol and ∼50 kJ/mol at low and high coverage, respectively. The comparison with the heat of adsorption of L1Pd0 indicates that the electronic effect of tin is very modest, compared to its geometric effect. This conclusion is consistent with literature data dedicated to DFT calculation. Moreover, (a) XRD and TEM/EDX analysis suggest that bimetallic particles such as Pd3Sn and Pd2Sn are present and (b) the impact of tin on the H2 chemisorption on Pd0 sites are presented.

Transformations in Transition-Metal Carbonyls Containing Arsenic: Exploring the Chemistry of [Et4N]2[HAs{Fe(CO)4}3] in the Search for Single-Source Precursors for Advanced Metal Pnictide Materials

The chemistry of [Et4N]2[HAs{Fe(CO)4}3] ([Et4N]2[I]) has been explored with the goal of preparing heterometallic compounds for use as single-source precursors to metal pnictide phases. Reaction of AgPF6 with [Et4N]2[I] in THF produces [Et4N][HAs{Fe2(CO)6(μ-CO)(μ-H)}{Fe(CO)4}] in 12% yield, but the yield is improved to 71% if the reaction is carried out with added [Et4N][HFe(CO)4]. The structure possesses rare As–H and {Fe2(CO)6(μ-CO)(μ-H)} fragments. The addition of [CPh3][BF4] to [Et4N]2[I] in THF gives [Et4N][Fe3(CO)9(μ-CO){μ3-AsFe(CO)4}] in 45% yield. This compound is also obtained as a minor product from oxidation of [Et4N]2[I] with 1 equiv of AgPF6, AsCl3, SbCl3, or BiCl3 in THF. The cluster consists of an Fe3As tetrahedron with an isolated Fe(CO)4 unit bound to the arsenic in a spiked-tetrahedral arrangement. Refluxing [Et4N]2[I] with 1 equiv of AgPF6 in THF produces [Et4N][(μ-H)2Fe3(CO)9){μ3-AsFe(CO)4}]. This cluster is similar to [Et4N][Fe3(CO)9(μ-CO){μ3-AsFe(CO)4}] with the bridging CO being repl...

DFT dissection of the reduction step in H2 catalytic production by [FeFe]-hydrogenase-inspired models: can the bridging hydride become more reactive than the terminal isomer?

Density functional theory has been used to study diiron dithiolates [HFe2(xdt)(PR3)n(CO)5-nX] (n = 0, 2, 4; R = H, Me, Et; X = CH3S(-), PMe3, NHC = 1,3-dimethylimidazol-2-ylidene; xdt = adt, pdt; adt = azadithiolate; pdt = propanedithiolate). These species are related to the [FeFe]-hydrogenases catalyzing the 2H(+) + 2e(-) ↔ H2 reaction. Our study is focused on the reduction step following protonation of the Fe2(SR)2 core. Fe(H)s detected in solution are terminal (t-H) and bridging (μ-H) hydrides. Although unstable versus μ-Hs, synthetic t-Hs feature milder reduction potentials than μ-Hs. Accordingly, attempts were previously made to hinder the isomerization of t-H to μ-H. Herein, we present another strategy: in place of preventing isomerization, μ-H could be made a stronger oxidant than t-H (E°μ-H > E°t-H). The nature and number of PR3 unusually affect ΔE°t-H-μ-H: 4PEt3 models feature a μ-H with a milder E° than t-H, whereas the 4PMe3 analogues behave oppositely. The correlation ΔE°t-H-μ-H ↔ stereoelectronic features arises from the steric strain induced by bulky Et groups in 4PEt3 derivatives. One-electron reduction alleviates intramolecular repulsions only in μ-H species, which is reflected in the loss of bridging coordination. Conversely, in t-H, the strain is retained because a bridging CO holds together the Fe2 core. That implies that E°μ-H > E°t-H in 4-PEt3 species but not in 4PMe3 analogues. Also determinant to observe E°μ-H > E°t-H is the presence of a Fe apical σ-donor because its replacement with a CO yields E°μ-H < E°t-H even in 4PEt3 species. Variants with neutral NHC and PMe3 in place of CH3S(-) still feature E°μ-H > E°t-H. Replacing pdt with (Hadt)(+) lowers E° but yields E°μ-H < E°t-H, indicating that μ-H activation can occur to the detriment of the overpotential increase. In conclusion, our results indicate that the electron richness of the Fe2 core influences ΔE°t-H-μ-H, provided that (i) the R size of PR3 must be greater than that of Me and (ii) an electron donor must be bound to Fe apically.

Regioselectivity in ligand substitution reactions on diiron complexes governed by nucleophilic and electrophilic ligand properties.

The discovery of a diiron organometallic site in nature within the diiron hydrogenase, [FeFe]-H2ase, active site has prompted revisits of the classic organometallic chemistry involving the Fe-Fe bond and bridging ligands, particularly of the (μ-SCH2XCH2S)[Fe(CO)3]2 and (μ-SCH2XCH2S)[Fe(CO)2L]2 (X = CH2, NH; L = PMe3, CN(-), and NHC's (NHC = N-heterocyclic carbene)), derived from CO/L exchange reactions. Through the synergy of synthetic chemistry and density functional theory computations, the regioselectivity of nucleophilic (PMe3 or CN(-)) and electrophilic (nitrosonium, NO(+)) ligand substitution on the diiron dithiolate framework of the (μ-pdt)[Fe(CO)2NHC][Fe(CO)3] complex (pdt = propanedithiolate) reveals the electron density shifts in the diiron core of such complexes that mimic the [FeFe]-H2ase active site. While CO substitution by PMe3, followed by reaction with NO(+), produces (μ-pdt)(μ-CO)[Fe(NHC)(NO)][Fe(CO)2PMe3](+), the alternate order of reagent addition produces the structural isomer (μ-pdt)[Fe(NHC)(NO)PMe3][Fe(CO)3](+), illustrating how the nucleophile and electrophile choose the electron-poor metal and the electron-rich metal, respectively. Theoretical explorations of simpler analogues, (μ-pdt)[Fe(CO)2CN][Fe(CO)3](-), (μ-pdt)[Fe(CO)3]2, and (μ-pdt)[Fe(CO)2NO][Fe(CO)3](+), provide an explanation for the role that the electron-rich iron moiety plays in inducing the rotation of the electron-poor iron moiety to produce a bridging CO ligand, a key factor in stabilizing the electron-rich iron moiety and for support of the rotated structure as found in the enzyme active site.

Catalytic Deoxygenation of Octanoic Acid over Supported Palladium: Effects of Particle Size and Alloying with Gold

Catalytic deoxygenation of octanoic acid (OA) to n-heptane was investigated over silica-supported Pd and PdAu catalysts at 260 °C and 1 atm in a fixed-bed microreactor. Pd/SiO2 catalysts were prepared by incipient wetness (IW) and ion exchange (IE). Bimetallic catalysts were prepared using an IE procedure that is known to produce supported PdAu nanoparticles. The Pd nanoparticles (7.5 nm average size) in the Pd/SiO2 (IW) catalyst exhibit well-defined (100) and (111) facets, as evidenced by high-resolution electron microscopy (HREM) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of adsorbed CO. As expected, the smaller nanoparticles (1.5 nm average size) in the Pd/SiO2 (IE) catalyst display strong linear and bridging CO DRIFTS bands. The PdAu/SiO2 (1/1 atomic ratio) catalyst contains 5 nm alloy nanoparticles with Pd-rich surfaces, as evidenced by HREM with energy-dispersive X-ray (EDX) analysis and in situ EXAFS spectroscopy. DRIFTS thermal desorption experiments demonstrated that...

Investigation of the Pt/MOR–Al2O3 Catalysts by IR Spectroscopy

Pt/MOR–Al2O3 catalysts, with platinum supported from H2PtCl6 and [Pt(NH3)4]Cl2 solutions are investigated in the work with the methods of IR spectroscopy. The received results testify about platinum fixing mainly in channels of a mordenite in case of supporting from an ammine, and its localization on Al2O3 surface when supporting from chloroplatinic acid. Small concentration of the absorption bands referred to bridging CO species on platinum testify about high dispersion of metal on samples. Data explain higher activity of the catalysts prepared with [Pt(NH3)4]Cl2 in reactions of a hydroisomerization of benzene and an isomerization of cyclohexane.