CO Coordination Research Articles

Mutual Effects of Substrates and Inhibitors in Reactions Catalyzed by the Nitrogenase Iron–Molybdenum Cofactor outside the Enzyme

The inhibiting effects of CO and N2 on the ability of the nitrogenase iron–molybdenum cofactor (FeMoco) to catalyze acetylene reduction outside the protein were studied to obtain data on the mechanism of substrate reduction at the active center of the enzyme nitrogenase. It was found that CO and N2 reacted with FeMoco that was separated from the enzyme and reduced by zinc amalgam (E = –0.84 V with reference to a normal hydrogen electrode (NHE)) (I) or europium amalgam (E = –1.4 V with reference to NHE) (II). In system I, CO reversibly inhibited the reaction of acetylene reduction to ethylene with Ki = 0.05 atm CO. In system II, CO inhibited the formation of the two products of C2H2 reduction in different manners: the mixed-type or competitive inhibition of ethylene formation with Ki = 0.003 atm CO and the incomplete competitive inhibition of ethane formation with Ki = 0.006 atm CO. The fraction of C2H6 in the reaction products was higher than 50% at a CO pressure of 0.05 atm because of the stronger inhibiting effect of CO on the formation of C2H4. A change in the product specificity of acetylene-reduction centers under exposure to CO was explained by some stabilization of the intermediate complex [FeMoco · C2H2] upon the simultaneous coordination of CO to the catalytic cluster. Because of this, the fraction of the many-electron reduction product (ethane) increased. The experimental results suggest that several active sites in the FeMoco cluster reduced outside the protein can be simultaneously occupied by substrates and (or) inhibitors. The inhibition of both ethane and ethylene formation by molecular nitrogen in system II is competitive with Ki = 0.5 atm N2 for either product. That is, N2 and C2H2 as ligands compete for the same coordination site in the reduced FeMoco cluster. The inhibiting effects of CO and N2 on the catalytic behaviors of FeMoco outside the protein and as an enzyme constituent were compared.

Mutual Effects of Substrates and Inhibitors in Reactions Catalyzed by Isolated Iron–Molybdenum Cofactor of Nitrogenase

The inhibiting effects of CO and N2 on the ability of the nitrogenase iron–molybdenum cofactor (FeMoco) to catalyze acetylene reduction outside the protein were studied to obtain data on the mechanism of substrate reduction at the active center of the enzyme nitrogenase. It was found that CO and N2 reacted with FeMoco that was separated from the enzyme and reduced by zinc amalgam (E = –0.84 V relative to a normal hydrogen electrode (NHE)) (I) or europium amalgam (E = –1.4 V relative to NHE) (II). In system I, CO reversibly inhibited the reaction of acetylene reduction to ethylene with K i = 0.05 atm CO. In system II, CO inhibited the formation of the two products of C2H2 reduction in different manners: the mixed-type or competitive inhibition was found for ethylene formation with K i = 0.003 atm CO and the incomplete competitive inhibition was found for ethane formation with K i = 0.006 atm CO. The fraction of C2H6 in the reaction products was greater than 50% at a CO pressure of 0.05 atm because of the stronger inhibiting effect of CO on the formation of C2H4. The change in the product specificity of acetylene-reduction centers under influence of CO was explained by some stabilization of the intermediate complex [FeMoco · C2H2] upon the simultaneous coordination of CO to the catalytic cluster. Because of this, the fraction value of ethane as a multielectron reduction product increased. The experimental results suggest that several active sites at the FeMoco cluster reduced outside the protein can be simultaneously occupied by substrates and (or) inhibitors. The inhibition of both ethane and ethylene formation by molecular nitrogen in system II is competitive with K i = 0.5 atm N2 for either product. That is, N2 and C2H2 as ligands compete for the same coordination site at the reduced FeMoco cluster. The inhibiting effects of CO and N2 on the catalytic behaviors of both isolated FeMoco and that in the enzyme were compared.

Synthesis and crystal structures of carbonyl derivatives of chloride–tetramethylene sulfoxide–ruthenium(III) complexes: [RuCl 3(TMSO) 2(CO)] and [H(TMSO) 2] [RuCl 4(TMSO)(CO)

The synthesis, spectroscopic characterization and crystal structure of [RuCl 3(TMSO) 2(CO)] ( 1) and [H(TMSO) 2][RuCl 4(TMSO)(CO)] ( 2) are reported. These complexes are prepared by the reaction of the precursors [ mer-RuCl 3(TMSO) 3] and [H(TMSO)][RuCl 4(TMSO) 2] with carbon monoxide in the presence or absence of TMSO at ambient temperature and pressure. Complexes 1 and 2 represents the first examples of Ru(III) compounds bearing both tetramethylene sulfoxide and carbon monoxide ligands on the metal. Substitution of one of the two trans-S-bonded TMSO from the anion accompanied addition of one molecule of TMSO on the cation. Coordination of CO induces an S-to-O linkage isomerization of the TMSO ligands. The crystal of 1 is a red–orange needle, monoclinic, space group C2/ c, a=26.446(2) Å, b=8.8726(6) Å, c=15.1093(8) Å, β=107.427(4)°. The crystal of 2 is an orange needle, orthorhombic, space group Pnma, a=11.9820(9) Å, b=14.1930(11) Å, c=12.6500(15) Å.

Enhancement of CO Insertion into a Pd−C Bond in a Pd−Co Heterodinuclear Complex

A series of Pd-containing heterodinuclear methyl complexes, (dppe)(CH3)Pd−MLn (MLn = MoCp(CO)3, WCp(CO)3, Co(CO)4; dppe = 1,2-bis(diphenylphosphino)ethane), have been prepared by the metathetical reactions of Pd(CH3)(NO3)(dppe) with Na+[MLn]-, and the complexes were characterized by spectroscopic methods and/or X-ray structure analysis. Related Pt-containing dinuclear complexes were similarly prepared and characterized. The rate of CO insertion into a Pd−CH3 or Pt−CH3 bond was investigated using these complexes. The Pd−Co complex (dppe)(CH3)Pd−Co(CO)4 shows a high activity of CO insertion, giving (dppe)(CH3CO)Pd−Co(CO)4, and the initial rate is ca. 80 times higher than those of the analogous complex Pd(CH3)Cl(dppe). Whereas slow insertion of CO is observed in the Pt−Co complex (dppe)(CH3)Pt−Co(CO)4, no reaction takes place for Pt(CH3)Cl(dppe). It is revealed by using 13CO that CO in the Co(CO)4 fragment preferentially inserts into the Pd−CH3 bond over CO in the gas phase. The carbonyl insertion reaction in the Pd−Co heterodinuclear complex has been theoretically studied using B3LYP hybrid density functional theory to clarify its reaction mechanism and the electronic factors controlling the reaction. The calculations for a model complex, (H2PCH2CH2PH2)Pd(CH3)−Co(CO)4, have shown that the most favorable reaction path consists of methyl migration from the Pd to the Co atom, carbonyl insertion reaction on the Co atom, CO coordination to the Co atom, and acetyl migration from the Co atom to the Pd atom, which is in accord with the experimental results. The electron donation from the Pd d orbital to the bridging CO π* orbitals plays an important role in stabilizing the intermediates and transition states.

Theoretical study of carbon dioxide coordination in palladium complexes

Activation of the relatively inert CO 2 molecule to form higher organic molecules has been a focus of research for many years. Therefore, the coordination of CO 2 to transition metal fragments is of considerable interest. The coordination of CO 2 to transition metals may involve η 1 -O, η 1 -C, or η 2 -C,O bonding. The η 1 -C, or η 2 -C,O coordination with both early and late transition metal complexes are known. The ancillary ligands on the metal can play a significant role in effecting coordination of CO 2 to the metal center. Here we report a theoretical study of coordination modes of carbon dioxide to palladium complexes. The theoretical geometry and energies of a series of phosphine-substituted palladium carbon dioxide complexes, (PR 3 ) 2 Pd(CO 2 ), were investigated. The phosphine ligands included PH 3 , PMe 3 , PCy 3 , PMe 2 Ph, PMePh 2 , PPh 3 , P(OCH 3 ) 3 , P (CH=CH 2 ) 3 , PF 3 , PCl 3 , and PBr 3 . The electron-rich (PR 3 ) 2 Pd fragments can act as Lewis bases towards the CO 2 molecule. We have examined the phosphine ligand influence on the electron density of the palladium center. In all cases the η 2 -C,O bonding mode was lowest in energy. The interaction between metal fragment and CO 2 frontier orbitals will be presented in order to provide an understanding of Pd-CO 2 bonding.

Spectral Investigation of the Interaction of Carbon Monoxide and Dioxygen with Low-Temperature Sublimates of Co(II) meso-Tetraphenylporphyrinate: Evidences for Oxidative Catalysis

The interaction of carbon monoxide and dioxygen with low-temperature (T= 77 K) sublimates of cobalt(II) meso-tetraphenylporphyrinate (CoTPP) is studied by IR spectroscopy using isotopically labeled compounds (13CO and 18O2). The obtained data confirm the coordination of CO and O2with the formation of axial complexes CoTPP · L (L = O2, CO) and the mixed complex O2· CoTPP · CO. The heating of the complex containing CO and O2to 150 K leads to the appearance of lines in the IR spectrum corresponding to valence asymmetric (νasOCO) and degenerated deformation (δOCO) vibrations of carbon dioxide. This is evidence for the oxidation of CO. The evacuation of CO2at a higher, but still low, temperature and the addition of a new portion of CO and O2mixture lead to the formation of new portions of CO2. This demonstrates the catalytic character of the process. Only low-temperature sublimates have catalytic properties. Sublimates of CoTPP obtained on the surface at room or higher temperature demonstrate neither catalytic activity nor the ability to coordinate any amount of CO and O2sufficient for the detection by spectroscopy.

Carbon Monoxide Coordination by Iron(II) meso -Mono-4-Pyridyltriphenylporphyrinate (FeM4PyTPP) and Its Structure in Sublimed Layers

Interaction of CO with sublimed layers of iron(II) meso-mono-4-pyridyltriphenylporphyrinate (FeM4PyTPP) resulting in the formation of known mono- and dicarbonyl complexes was studied using IR spectroscopy. The frequency of the stretching vibration of the coordinated CO in the monocarbonyl complex was found to be ∼20 cm–1higher than in the complex with iron meso-tetraphenylporphyrinate (FeTPP), with the former complex being significantly more stable than the latter. The differences observed in CO coordination by porphyrins with close structures are explained by the formation, in the FeM4PyTPP sublimed layers, of oligomeric structures where the pyridyl group of one molecule is coordinated by the metal ion of the neighboring molecule. This conclusion is confirmed by a comparative analysis of IR spectra of FeM4PyTPP and FeTPP in the regions of structurally sensitive vibrations.

A dynamical density functional study of CO insertion into the metal–alkyl bond in Ti(Cp)2(CH3)2

The migratory insertion of CO into the titanium–methyl bond in Ti(Cp)2(CH3)2 has been investigated by both static and dynamic density functional calculations. CO coordination prior to insertion has been analysed considering both “lateral” and “central” approaches, finding the two pathways kinetically equivalent. The relative stability of the O-outside and O-inside η2-bound acyl complexes has been investigated, finding the O-outside isomer more stable by 4.0 kcal mol−1, with an energy barrier for the conversion from the O-outside isomer of 9.6 kcal mol−1. Dynamic simulations have also been performed on the Ti(Cp)2(CH3)2–CO adduct in order to study the detailed features of the CO migratory insertion and of the η1- → η2-acyl conversion, and show that migratory insertion takes place within 1 ps through detachment of the methyl group upon CO migration, leading to an O-inside η2-acyl complex.

Density Functional Study of CO Insertion into the Metal−Alkyl Bond in Bis(cyclopentadienyl)−Zr−(CH3)2

The migratory insertion reaction of CO into the zirconium−alkyl bond in bis(cyclopentadienyl)−Zr−(CH3)2 has been investigated by means of density functional calculations. CO coordination processes prior to insertion have been analyzed considering both the case of “lateral” and “central” coordination, finding lateral coordination, leading to a O-outside arrangement of the ensuming acyl complex, to be thermodynamically and kinetically favored. The relative stability of the O-outside and O-inside η2-bound acyl complexes has been investigated, finding the O-inside isomer to be the most stable by 2.9 kcal/mol, with an energy barrier for the conversion between the two acyl isomers of 12.2 kcal/mol, in excellent agreement with available experimental data. Finally, the insertion of the residual alkyl group into the acyl moiety, leading to a η2-bound ketone, has been investigated.

Field-dependent chemisorption of carbon monoxide and nitric oxide on platinum-group (111) surfaces: Quantum chemical calculations compared with infrared spectroscopy at electrochemical and vacuum-based interfaces

Density Functional Theory (DFT) is utilized to compute field-dependent binding energies and intramolecular vibrational frequencies for carbon monoxide and nitric oxide chemisorbed on five hexagonal Pt-group metal surfaces, Pt, Ir, Pd, Rh, and Ru. The results are compared with corresponding binding geometries and vibrational frequencies obtained chiefly from infrared spectroscopy in electrochemical and ultrahigh vacuum environments in order to elucidate the broad-based quantum-chemical factors responsible for the observed metal- and potential-dependent surface bonding in these benchmark diatomic chemisorbate systems. The surfaces are modeled chiefly as 13-atom metal clusters in a variable external field, enabling examination of potential-dependent CO and NO bonding at low coverages in atop and threefold-hollow geometries. The calculated trends in the CO binding-site preferences are in accordance with spectral data: Pt and Rh switch from atop to multifold coordination at negative fields, whereas Ir and Ru exhibit uniformly atop, and Pd hollow-site binding, throughout the experimentally accessible interfacial fields. These trends are analyzed with reference to metal d-band parameters by decomposing the field-dependent DFT binding energies into steric (electrostatic plus Pauli) repulsion, and donation and back-donation orbital components. The increasing tendency towards multifold CO coordination seen at more negative fields is due primarily to enhanced back-donation. The decreasing propensity for atop vs multifold CO binding seen in moving from the lower-left to the upper-right Periodic corner of the Pt-group elements is due to the combined effects of weaker donation, stronger back-donation, and weaker steric repulsion. The uniformly hollow-site binding seen for NO arises from markedly stronger back-donation and weaker donation than for CO. The metal-dependent zero-field DFT vibrational frequencies are in uniformly good agreement with experiment; a semiquantitative concordance is found between the DFT and experimental frequency-field (“Stark-tuning”) slopes. Decomposition of the DFT bond frequencies shows that the redshifts observed upon chemisorption are due to donation as well as back-donation interactions; the metal-dependent trends, however, are due to a combination of several factors. While the observed positive Stark-tuning slopes are due predominantly to field-dependent back-donation, their observed sensitivity to the binding site and metal again reflect the interplay of several interaction components.

Density Functional Study of the Reppe Carbonylation of Acetylene

The Reppe carbonylation of acetylene has been investigated by both static and dynamic density functional methods. Structures of all intermediates and transition states involved in each step of the catalytic cycle have been determined using gradient-corrected exchange−correlation potentials. Dynamic simulations have been performed on the migrative insertion of CO into the metal−vinyl bond after CO coordination by the metal and show that the insertion is preceded by a Berry pseudorotation of the initial pentacoordinated reagent and occurs via a simultaneous detachment of the vinyl group from the metal and formation of the vinyl−carbonyl bond. The overall thermodynamics and kinetics for the full catalytic cycle have been evaluated and have shown that the Reppe carbonylation of acetylene is a thermodynamically favored and kinetically easy process.

Coordination of Co2+ Cations inside Cavities of Zeolite MFI with Lattice Oxygen and Adsorbed Ligands

The redox chemistry of Co ions in cavities of zeolite MFI has been studied in materials prepared by solid-state ion exchange and displaying Co/Al ratios varying from 0.4 to 1.0. Fourier Transform Infrared (FTIR) spectroscopy, electron spin resonance (ESR), and diffuse reflectance UV−visible spectroscopy were used to identify the oxidation state and the coordination of the Co ions. In the dehydrated blue material, Co2+ ions in cationic exchange sites strongly interact with the zeolite framework; their coordination symmetry is tetrahedral. ESR reveals that the ions are in their high-spin state, detectable only below 60 K; no ESR signal was detected at 77 K. However, adsorption of ammonia ligating to the Co2+ cations gives rise to an ESR signal at 77 K, indicating a strong reconfiguration of the electronic states. After coadsorption of oxygen and ammonia, ESR reveals the presence of two kinds of cobalt: Co2+ in a high-spin state at g = 5.1, detected at 77 K, and low-spin (Co3+LxO2-)2+ adducts inside the zeo...

Spatial structure of ordered electrochemical adlayers from in situ scanning tunneling microscopy and infrared spectroscopy: single-site carbon monoxide binding on iridium(111) and comparisons with related systems

The spatial structure of compressed carbon monoxide adlayers on Ir(111) electrodes in aqueous solution is discussed on the basis of in situ scanning tunneling microscopy (STM) along with earlier infrared spectral data. Hexagonal CO adlattices are discerned from STM data with fractional coverages, ca. 0.65±0.05, consistent with electrochemical and infrared spectrophotometric measurements. In contrast to CO adlayers on Pt(111) and Rh(111) electrodes, the STM images display no uniform periodicities associated with a well-defined unit cell, although more irregular nanoscale variations in image intensity were discerned. This differing behavior is ascribed to the exclusive presence of atop and ‘near-atop’ CO coordination on Ir(111), as deduced from infrared spectra in both electrochemical and ultrahigh vacuum environments, contrasting the atop/multifold CO binding patterns on Pt(111) and Rh(111) electrodes.

1,2,3-Triphenylphosphirene derivatives of the iridium carbonyl clusters [HIr4(CO)9L(μ-PPh2)] (L = CO, PPh3) resulting from substitution, insertion and hydrometallation processes †

1,2,3-Triphenylphosphirene reacts with [HIr4(CO)10(μ-PPh2)] 1, at room temperature, to afford [Ir4(CO)8(μ3-η2-PhPCPhCPh)(μ-PhPCPhCHPh)(μ-PPh2)] 2 which contains the phosphametallacycle (μ3-η2-PhPCPhCPh) and the phosphidoalkenyl (μ-PhPCPhCHPh) ligands arising from insertion and hydrometallation processes respectively. In contrast, the PPh3 derivative of 1, [HIr4(CO)9(PPh3)(μ-PPh2)] 3, reacts selectively at room temperature with the phosphirene to give only CO substitution products, [HIr4(CO)9 − n(PPh3)(η1-PhPCPhCPh)n(μ-PPh2)] (n = 1, 4 and 2, 5) which are the first carbonyl cluster compounds containing intact η1-ligated phosphirene rings. High yield conversion of compound 4 into the phosphametallacycle species [HIr4(CO)7(PPh3)(μ3-η2-PhPCPhCPh)(μ-PPh2)] 6 is achieved under mild thermolytic conditions. An insight into the mechanism of formation of 2 was given by the reaction of the phosphirene ring with the anion [Ir4(CO)10(μ-PPh2)]−1a derived from 1, followed by protonation, which gave [HIr4(CO)8(μ3-η2-PhPCPhCPh)(μ-PPh2)] 7, which is analogous to 6 with a CO ligand replacing PPh3. Quantitative conversion of the hydride phosphametallacycle 7 into the labile phosphidoalkenyl cluster [Ir4(CO)11(μ-PhPCPhCHPh)(μ-PPh2)] 8 is easily achieved in the presence of CO (1 atm, RT, 2 h), as a result of the reductive elimination of a C–H group. Compound 8 undergoes facile CO dissociation and co-ordination of the phosphidoalkenyl CC bond to the metal centre to produce [Ir4(CO)9(μ3-η3-PhPCPhCHPh)(μ-PPh2)] 9. The molecular structures of compounds 2, 4, 6 and 9 were established by X-ray diffraction studies and the structures of all compounds in solution were investigated by a combination of 1H and 31P{1H} NMR studies.

Highly dispersed cobalt in CuCo/SiO 2 cluster-derived catalyst

A silica-supported CuCo catalyst has been prepared by anchoring a Cu–Co bimetallic complex. Evolution of the species under vacuum and CO treatments has been followed by in situ FTIR. A breakdown of the initial complex with CO at 373 K gives [Co(CO) 4] −. After total decarbonylation, a subsequent CO treatment followed by FTIR indicates the presence of metallic Cu and very small particles of cobalt. The catalyst obtained after an H 2-treatment at 473 K has been characterized by XRD, TEM and XPS techniques. The presence of large Cu particles and highly-dispersed subnanometric cobalt particles is inferred. Coordination of CO on these very small particles of metallic cobalt could give Co 4(CO) 12. Catalytic behaviour in the hydroformylation of ethylene and in the hydrogenation of CO is related to the highly dispersed cobalt, which probably interacts with Cu. An easy CO insertion into a metal–alkyl bond is favoured over this CuCo catalyst.

Coadsorbate vibrational interactions within mixed carbon monoxide-nitric oxide adlayers on ordered low-index platinum-group electrodes

In-situ infrared reflection-absorption spectra are described for saturated mixed carbon monoxide-nitric oxide adlayers in comparison with CO and NO adsorbed separately on selected ordered Pt-group electrodes in aqueous solution in order to assess the nature of the coadsorbate vibrational interactions and hence the adlayer structure and bonding. Both chemisorbates exhibit pronounced intramolecular vibrational fingerprints (νCO, νNO bands) that are sensitive to the local vibrational environment as well as the surface bonding geometry, enabling a microscopic-level assessment of the coadsorbate interactions in relation to CO and NO coadsorbed with water. The surfaces selected—Ir(110), Ir(111), Pt(100), Pt(111), Rh(100), Rh(111), and Pd(111)—yield near-exclusive molecular (rather than dissociative) NO adsorption, yet exhibit differing coverage-dependent NO, and especially CO, spectral and coordinative properties. Progressive displacement of NO by exposing NO-saturated electrodes to dilute CO solutions yielded chiefly molecularly intermixed CO/NO adlayers. This deduction is most straightforward (and quantitative) on Ir(110) and Ir(111), facilitated by the exclusively atop-like adsorption of CO and NO, as gleaned from the single νCO and νNO band frequencies. Both these features on Ir(110) redshift markedly (by 40–60 cm−1) upon dilution with either the other chemisorbate or with coadsorbed water. Such redshifts, along with the observed suppression of the νNO band intensity in the CO/NO mixtures, arise chiefly from composition-dependent dynamic-dipole coupling within the intermixed dipolar adlayers, and are diagnostic of microscopic coadsorbate structure. Similar νNO redshifts induced by dilution with coadsorbed CO are observed on each of the other surfaces. The composition-dependent analysis is facilitated by the observation of a lone coverage-dependent νNO band, associated probably with multifold NO coordination, in both the absence and presence of coadsorbed CO in most cases. While corresponding effects upon the νCO band frequencies are also observed, the spectra on Pt and Rh surfaces include a pair of νCO bands with frequencies, ca. 1800–1850 and 2000–2070 cm−1, suggestive of bridging and atop-like CO coordination, respectively. On Rh(100), Rh(111), Pt(111) and especially Pt(100), NO coadsorption induces CO ‘bridging’ to ‘atop’ site switching. This is largely consistent with the formation of molecularly intermixed CO/NO adlayers where the strong preference of NO for multifold sites shifts CO molecules into neighboring atop-like configurations. On Pt(111), Rh(111), and Pd(111), however, the νCO spectral fingerprints for the CO/NO adlayers suggest the additional presence of segregated compressed ‘CO-rich’ domains. Comparisons are made also with the behaviour of analogous CO/NO adlayers in ultrahigh vacuum.

MIXED DINUCLEAR Co(II) COMPLEXES WITH N,N′,N″,N″ -TETRAKIS-(2-PYRIDYLMETHYL)-l, 4, 8, 11-TETRAAZACYCLOTETRADECANE AND α - AND β-AMINOCARBOXYLATO LIGANDS

Four new mixed complexes of Co(II) with N,N′ N″,N′″-tetrakis(2-pyridylmethyl)-1, 4, 8, 11-tetraazacyclotetradecane (tpmc) and bridged α- or β-aminocarboxylato ligands of general formula [Co2(Y)tpmc](C1O4)3 zH2O where Y = glycinato, S-alaninato, S-aminobutyrato, β-aminobutyrato ion, and z = 0, 0.5 or 1 were isolated. The complexes were characterised by elemental analysis, electronic and IR spectroscopy, magnetic measurements and cyclic voltam-metry. A structure with μ-O, O′-coordination of the aminocarboxylato ligand, and exo coordination of Co(II) ions and tpmc is proposed. The complexes exhibit different electrochemical activities; glycinato and S-alaninato complexes are electrochemically active, whereas S-aminobutyrato and β-aminobutyrato complexes are electrochemically inactive under the given conditions.

Coverage-Dependent Infrared Spectroscopy of Carbon Monoxide on Palladium(100) in Aqueous Solution: Adlayer Phase Transitions and Electrooxidation Pathways

Infrared reflection−absorption spectra (IRAS) measured for the C−O stretch (νCO) of carbon monoxide dosed on ordered Pd(100) in aqueous 0.1 M HClO4 as a function of the coverage (θCO) and the electrode potential (E) are examined in comparison with extant IRAS and adlayer structural data on Pd(100) in ultrahigh vacuum (UHV). A noteworthy feature of the electrochemical adlayer, as for the UHV system, is the presence of a single νCO band blue shifting markedly (by ca. 100 cm-1) with increasing coverage up to saturation (θCO ≈ 0.8), indicative of uniformly bridging CO coordination. Extrapolation of the θCO-dependent νCO frequencies to surface potentials corresponding to the UHV-based system, however, yielded reasonable concordance only at high coverages. The discrepancies at lower θCO are suggested to arise from solvation effects on the local CO-site surface potentials. Examination of the θCO-dependent νCO peak frequencies ( ), integrated absorbances (Ai), and bandwidth (Δν1/2) for the dosed electrochemical a...

Synthesis, structure and electrochemistry of ferrocenylethynylsilanes and their complexes with dicobalt octacarbonyl

Coupling of lithium ethynylferrocene to alkyl and aryl chlorosilanes and to silicon tetrachloride leads to the formation of a variety of ferrocenylethynylsilanes, [( η 5-C 5H 5)Fe( η 5-C 5H 4C 2)] 4− n SiR n ( n=0–3, R=Me, Ph). An additional series of compounds [( η 5-C 5H 5)Fe( η 5-C 5H 4C 2)] 4− n Si( n-Bu) x R y ( n=0–3, x+ y= n, R=Me, Ph) result from competitive reactions of excess n-BuLi at the silicon centre. Dicobalt octacarbonyl reacts with the ferrocenylethynylsilanes to give dicobalt hexacarbonyl derivatives, but for the tris- and tetrakis-compounds coordination of Co 2(CO) 6 is limited to two of the alkyne units as a result of steric crowding around the silicon centre. The crystal and molecular structure of the complex [( η 5-C 5H 5)Fe( η 5-C 5H 4C–(Co 2(CO) 6)C] 2Si[(CC- η 5 -C 5H 4)Fe( η 5-C 5H 5)] 2 has been determined from X-ray data. Electrochemical investigation of the ferrocenylethynylsilanes and their dicobalt complexes shows that the equivalent ferrocenyl redox centres in these molecules are non-interacting.

Time-Resolved Infrared Spectroscopy in Supercritical Fluids

We have used fast Time-resolved Infrared Spectroscopy (TRIR) to probe organometallic reactions in supercritical fluids on the nanosecond time-scale. This has allowed us to identify, for the first time in solution at room temperature, organometallic noble gas complexes which are formed following irradiation of metal carbonyls in supercritical noble gas solution. We have found that these complexes are surprisingly stable and have comparable reactivity to organometallic alkane complexes. We have also studied the coordination of CO2 to metal centres in supercritical CO2 (scCO2) and provide the first evidence for the formation and reactivity of ɳ1-O bound metal CO2 complexes in solution at or above room temperature.