CO Dissociation Mechanism Research Articles

Kinetics and substitution mechanism in the reaction of the seven-coordinate complexes bis-μ-diphenylphosphido- and bis-μ-dimethylphosphido-octacarbonyldimolybdenum (MoMo) with tri-n-butyl-phosphine

The dinuclear phosphido-bridged molybdenum complexes [(CO) 4 Mo(μ-PR 2) 2M o(CO) 4] (R=Ph or Me) react with P(n-Bu) 3 to give the corresponding mono and bis-phosphine derivatives. A kinetic study of the first substitution in decalin indicates a CO-dissociative mechanism involving the coordinatively unsaturated intermediate [(CO 4) Mo(μ-PR 2) 2M o(CO) 3]. The overall substitution rate depends on the rate of CO dissociation, k 1, and on the rate of bimolecular attack by CO, k −1, and by P(n-Bu) 3, k 2, on the reactive intermediate. The nature of the substituents at the phosphido bridge markedly affects the value of k 1, which is higher for the phenyl compared with the methyl group. This is mainly due to a lower activation enthalpy (Δ H 1*=125 (Ph) versus 141 (Me) kJ mol −1), which may reflect a weakening of the MoCO bond in the presence of a more electron-withdrawing ligand in trans position. The values of the competition rate ratio k −1/ k 2, always largely greater than unity, show that attack of the small CO is favoured with respect to the large P(n-Bu) 3; this suggests that the steric crowding observed on the molecular structure of the starting seven-coordinate complexes should play an important role also on the reactivity of their six-coordinate intermediates.

Density Functional Study on the Preactivation Scenario of the Dötz Reaction: Carbon Monoxide Dissociation versus Alkyne Addition as the First Reaction Step

Two different mechanistic proposals have been investigated theoretically for the initial steps of the chromium-assisted Dötz benzannulation reaction at the nonlocal density functional level of theory. The energy requirements needed for the reaction to start through the usually assumed CO-dissociative mechanism are calculated to be 144.7 kJ mol-1 for (CO)5CrC(OH)(C2H3) (1). Under the mild thermal experimental conditions, the decarbonylation step may be sometimes a serious bottleneck for the reaction to proceed, even to the extent of completely blocking the formation of the subsequent tetracarbonyl compound (CO)4CrC(OH)(C2H3) (5), which is supposed to add an alkyne molecule in the second step. An alternative path is suggested, where the alkyne reacts directly with the saturated metal−carbene complex 1 instead of competing with a CO molecule for a position in the coordination sphere of the metal center in 5. Our calculations reveal that if alkyne addition takes place in 1 before CO loss, then the initial process becomes clearly exothermic (−163.4 kJ mol-1). Moreover, this alternative path makes it easier for CO dissociation to occur as the second step because at this point the complex has more internal energy than 1 to expel a CO ligand. From a comparison of both mechanistic routes, it is concluded that the Dötz reaction better proceeds initially through an associative step.

Ultrafast dissociation dynamics studied

Researchers at Harvard University have obtained femtosecond infrared spectra of carbon monoxide dissociating from the heme prosthetic group in myoglobin, a protein that serves as a reserve oxygen source in muscle. The study pushes the sensitivity and time-resolution limits of IR spectroscopy and provides answers to long-standing questions about the mechanism of CO dissociation under physiological conditions. The work, carried out by graduate students Manho Lim and Timothy A. Jackson and chemistry professor Philip A. Anfinrud, reveals two detailed routes by which CO can dissociate from heme and shows the role played by myoglobin conformational changes in the process. Myoglobin's normal physiological function is to bind oxygen, but it is capable of binding other ligands, such as CO. Anfinrud and coworkers used a laser to photodissociate CO from myoglobin, and then obtained timeresolved IR spectra of the departing ligand. The researchers found that, upon dissociation of CO from its heme binding site, the lig...

Metal-to-Ligand Charge Transfer (MLCT) Photochemistry offac-Mn(Cl)(CO)3(H-DAB): A Density Functional Study

The title compound has low-energy Mn−3d to 1,4-diaza-1,3-butadiene (H-DAB) π* metal-to-ligand charge transfer (MLCT) excited states, which are not, by their electronic nature, Mn−CO dissociative. Their potential energy curves (PEC) exhibit Mn−COeq and Mn−COax bonding minima around Re. Loss of an equatorial CO ligand upon MLCT excitation is explained by a radiationless transition from the MLCT states to the dissociative continuum of the electronic ground state. According to the calculated PEC of the ground state, the complex will undergo a strong structural rearrangement upon equatorial CO dissociation, during which the chloride shifts to the equatorial open site. This rearrangement explains the experimentally found formation of mer-Mn(Cl)(CO)3(α-diimine) complexes upon back-reaction of their CO-loss product with CO. This mechanism of equatorial CO dissociation is very different from the usual photochemical dissociation directly from a dissociative ligand-field state or through crossing of the photoactive excited state by such a ligand-field state. In contrast, axial CO dissociation, which does not occur readily, does not give rise to structural rearrangement and is predicted to produce the fac complex upon back-reaction with CO.

Reactivity of Co 4(CO) 12 with polydentate ligands. Fragmentation kinetic studies of the substituted derivatives

The reaction of Co 4(CO) 12 with the polydentate phosphines (Ph 2Ppy (2-(dihenylphosphino)pyridine), t-dppv ( trans-1,2-vinylenebis(diphenylphosphine) and dpmp (bis((diphenylphosphino)methyl)phenylphosphine) yield the tetrasubstituted Co 4(CO) 8L 4 complexes (L  Ph 2Ppy and t-dppv) in which the ligand behaves as monodentate, trisubstituted Co 4(CO) 9(dpmp) complex in which the ligand behaves as tridentate, and pentasubstituted Co 4(CO 7(dpmp) 2 in which the ligands behave as di- and tridentate. All the compounds have been characterized by elemental analysis and IR, 1H and 31P NMR spectroscopy. The kientics of fragmentation in DCE (dichloroethane) of Co 4(CO) 8(PPh 2Py) 4 to form Co 2(CO) 6(Ph 2Ppy) 2 has been studied under a N 2 or CO atmosphere. The rate constants are independent of the concentration of Ph 2Ppy and are lower under CO. This suggests that the reaction proceeds by a CO-dissociative (D) mechanism. The complexes Co 4(CO) 8( t-dppv) 4, Co 4(CO) 9(dpmp) and Co 4(CO) 7(dpmp) 2 are stable towards fragmentation.

Co dissociation and recombination reactions on Ni(100)

The kinetics of atomic carbon and oxygen buildup on a Ni(100) surface exposed to carbon monoxide at high temperatures have been investigated by Auger electron spectroscopy. The experimental data, taken at different sample temperatures (453 ⩽, T ⩽ 573 K) and at different CO partial pressures (3 ×10 −7 ⩽, P co ⩽, 3 ×10 −1 mbar) allowed the identification of the CO dissociation mechanism. By fitting the experimental data with a set of rate equations describing CO dissociation, CO reduction of surface oxygen, and C and O recombination, we have been able to determine the pre-exponential factors and the activation energies of these processes.

CO adsorption and dissociation on Rh(111)

Using the atom superposition and electron delocalization molecular orbital method, we studied the adsorption of C, O, and CO on different sites of Rh(111) represented by a small cluster of 29 Rh atoms. By calculating the activation energy of several possible reaction paths, the most probable mechanism of CO dissociation can be found. A stepped Rh(111) is simulated by a ridge of six extra Rh atoms. The influence of this ridge on both CO adsorption and dissociation will be discussed.

Kinetics of some Reactions of Ru 3(CO) 10(μ-bisdiphenylphosphinomethane)

The kinetics of substitution of Ru 3(CO) 10(dppm) with several relatively weak nucleophiles to form Ru 3(CO) 9(dppm)(L) in benzene have been studied over the temperature fange 30–60°C. At lower temperatures the observed first order rate constants for reaction under N 2 or air are independent of the nature or concentration of the nucleophile but reactions are retarded by CO in a way characteristic of a simple CO dissociative mechanism. It is, however, also possible that the rate determining step is a form of isomerization of the cluster and some possibilities are discussed. In terms of △ H * the dppm substituent is found to be more labilizing than two PPh 3 substitutents, possibly because of a greater strain in the cluster caused by the bridging dppm ligand. At higher temperatures another reaction path appears to be available for reaction with PPh 3, and this accounts for ca. 30% of the rate at 50 °C. A similar path has been reported for reaction with dppm but some difficulties arise which suggest that the mechanism proposed for that reaction is at least oversimplified.

Addition and elimination of hydrogen and ligand substitution on triosmium clusters. Kinetics and mechanism

The kinetics of the reversible reaction HOs 3(μ-COMe)(CO) 10 + H 2 ⇌ H 3Os 3(μ 3-COMe)(CO) 9 + CO has been investigated. The reaction of HOs 3(μ-COMe)(CO) 10 with hydrogen involves dissociation of a CO ligand prior to the rate-determining step, which is proposed to be the oxidative addition of molecular hydrogen. The reaction of H 3Os 3(μ 3-COMe)(CO) 9 with CO involves rate-limiting hydrogen loss. The equilibrium constant and the competition ratio for hydrogen and CO for the unsaturated intermediate were determined. The mechanism of substitution by AsPh 3 on HOs 3(μ-COMe)(CO) 10 also involves a CO dissociative mechanism. Based upon relative rate constants for CO, AsPh 3, and hydrogen addition to HOs 3(COMe)(CO) 9, CO dissociation and hydrogen addition are proposed to occur at different metal sites.

Mechanisms of dirhenium decacarbonyl substitution reactions: crossover experiments with dirhenium-185 decacarbonyl and dirhenium-187 decacarbonyl

/sup 185/Re/sub 2/(CO)/sub 10/ and /sup 187/Re/sub 2/(CO)/sub 10/ were prepared separately and then utilized in combination in crossover experiments to probe for fragmentation to mononuclear rhenium species in thermal and photochemically initiated substitution reactions. For the CO-Re/sub 2/(CO)/sub 10/ exchange reaction, a reaction separately analyzed for /sup 13/CO-/sup 12/CO interchange, no crossover was observed at 150/sup 0/C after 14 half-lives of reaction (14 h). Similarly, the thermal reaction sequences of Re/sub 2/(CO)/sub 10/ + P(C/sub 6/H/sub 5/)/sub 3/ reversible Re/sub 2/(CO)/sub 9/P(C/sub 6/H/sub 5/)/sub 3/ + CO and Re/sub 2/(CO)/sub 9/P(C/sub 6/H/sub 5/)/sub 3/ + P(C/sub 6/H/sub 5/)/sub 3/ reversible Re/sub 2/(CO)/sub 8/(P(C/sub 6/H/sub 5/)/sub 3/)/sub 2/ + CO were examined at 150/sup 0/C (maintaining a CO pressure of approx. 560 to 640 mm). No crossover was detectable in either Re/sub 2/(CO)/sub 10/ or Re/sub 2/(CO)/sub 9/P(C/sub 6/H/sub 5/)/sub 3/ (relative to blank experiments). Hence, phosphine substitution reactions proceed without a detectable formation of mononuclear rhenium species. These observations support a CO dissociative mechanism. A model based upon this mechanism can accurately reproduce the mass spectra observed during /sup 13/CO-/sup 12/CO interchange. In the absence of a CO atmosphere, /sup 185/Re/sub 2/(CO)/sub 10/ and /sup 187/Re/sub 2/(CO)/sub 10/ formedmore » /sup 185/Re/sup 187/Re(CO)/sub 10/, and this interchange was nearly complete at 150/sup 0/C within 14 to 16 half-lives. All photochemically initiated reactions with the two labeled decacarbonyls led to complete crossover within short reaction times. It appears then that the primary mode of reaction for Re/sub 2/(CO)/sub 10/ under photolysis conditions involves Re-Re bond scission as an early elementary step. Also, the reversible steps leading to the precursor(s) to Re/sub 2/(CO)/sub 10/ decomposition include scission of the rhenium-rhenium bond.« less

Reaction mechanisms of metal–metal-bonded carbonyls. Part 13. Substitution reactions of µ-diphenylacetylene-bis(tricarbonylcobalt)-(Co–Co)

The kinetics have been studied of substitution (in decalin at 55 °C) of one carbonyl ligand in the complex [Co2(C2Ph2)(CO)6] by several phosphorus-donor ligands covering a wide range of nucleophilic character {L = PBun3, PPh3, PEt2Ph. PEtPh2, P(OEt)3, P(OPh)3, and 4-ethyl-2.6,7-trioxa-l-phosphabicyclo[2.2.2]octane (etpb)}. Reactions under an atmosphere of argon follow the rate equation kobs, =k1+k2[L], and the dependence of log k2 on the basicity of L shows that bond making (probably to the metal) plays an important part in the transition state for the bimolecular path. The effect of carbon monoxide on the path governed by the limiting first-order rate constant k1 has been studied and found to be consistent with. but not to prove, a simple CO-dissociative mechanism for substitution by this path. If this is the mechanism, the data for reactrons under carbon monoxide provide relative rate constants for bimolecular attack by the various ligands at the vacant co-ordination site. This would be the first example where relative nucieophilicities have been determined towards a fully co-ordinated complex and a co-ordinatively unsaturated form of the same complex.

Reaction of di-µ-phenylthio-bis(tricarbonyliron)(Fe–Fe) with triphenyl-phosphine: a detailed kinetic and mechanistic study

Di-µ-phenylthio-bis(tricarbonyliron)(Fe–Fe) undergoes a two-step CO substitution reaction with triphenyl-phosphine in decalin. The substitution does not go to completion in the presence of carbon monoxide and the kinetics of the forward and reverse reaction for each step have been studied. The unsubstituted complex undergoes direct attack by PPh3 either on the predominant anti-form or on the very reactive syn-form which is produced in the rate-determining anti–syn-isomerisation. The monosubstituted complex, which is also present mainly in the anti-form in equilibrium with a reactive syn-form, reacts with carbon monoxide through an SN2 mechanism, but a CO-dissociative mechanism is involved in its reaction with the bulkier PPh3. The bis(phosphine) complex so obtained is unable to undergo SN2 displacement even by carbon monoxide and must previously lose one molecule of phosphine. Relative rate constants for bimolecular attack on the co-ordinatively unsaturated intermediate by carbon monoxide and triphenylphosphine have been obtained. Equilibrium and activation parameters for these reactions are reported.

Reaction mechanisms of metal–metal bonded carbonyls. Part IV. The substitution reaction of µ-diphenylacetylene-bis(tetracarbonylcobalt) with tri-n-butylphosphine

µ-Diphenylacetylene-bis(tetracarbonylcobalt) undergoes two successive substitution reactions with tri-n-butylphosphine in decalin and the kinetics of these reactions have been studied. Each step shows pseudo-first-order behaviour with kobs=k1+k2[PBu3] under an atmosphere of argon. Activation parameters have been obtained for each path in each step and the effect of carbon monoxide on the paths governed by k1 is consistent with a CO-dissociative mechanism. Relative rate constants for bimolecular attack on the co-ordinatively unsaturated intermediates by carbon monoxide and tributylphosphine have been obtained. The rate parameters for the two dissociative paths are quite similar but the second bimolecular path is governed by a much higher value of ΔH‡, and a much less negative value of ΔS‡, than the first.

Kinetic studies on the reactions of the tricarbonyl(1–3,6-η-cyclo-octadiene)-iron and -ruthenium complexes with tertiary phosphines and phosphites

The complex [(1–3,6-η-C8H12)Fe(CO)3](1–3,6-η-C8H12= 1–3,6-η-cyclo-octadiene), (Ia), when treated with tertiary phosphines L (L = triphenylphosphine or triphenyl phosphite) in n-heptane (40–70 °C) produces the monosubstituted derivative [(1–3,6-η-C8H12)Fe(CO)2L], (IIa), via a CO-dissociative mechanism. Observed pseudo-first-order rate constants for reaction with a range of more nucleophilic phosphorus ligands (for example methyl phosphite) are given by the relation kobs=k1+k2[L]. The ligand-independent term corresponds to formation of the carbonyl-substituted derivative and the ligand-dependent term to formation of the complexes [Fe(CO)2L3], (IIa), and the trans-annular ketone C9H12O, (III). Studies of the analogous ruthenium system show that dissociation of CO (at 40 °C) is ca. 40 times greater than that of the iron complex. This results from a more favourable enthalpy of activation. However, no second-order reaction of this complex has been detected.

Rates and mechanism of substitution reactions of dimanganesedecacarbonyl and some of its derivatives

The rates of substitution reactions of Mn 2(CO) 10 and some of its derivatives are reported. The rate of reaction of Mn 2(CO) 10 with various reagents is first-order in carbonyl and zero-order in reagent concentrations. For poor nucleophiles the rate of reaction is largely decreased with the addition of CO and the activation parameters for reaction are ΔH * = 37 kcal/mole and ΔS * = +20 e.u. These observations suggest that substitution reactions of Mn 2(CO) 10 in p-xylene solution take place by a CO dissociation mechanism. Similar results were obtained for the reaction of monosubstituted derivatives Mn 2(CO) 9L, but the kinetic behavior of Mn 2(CO) 8[P(C 6H 5) 3] 2 is different.