Bidentate formate

The bidentate formate ligand of OsH(K2-02CH)(CO)(P,Pr3)2 is converted into a monodentate group by carbonylation. Thus, the reaction of this compound with carbon monoxide gives 0sH K1-0C(0)H (C0)2(P Pr3)2. Similarly, the addition of a stoichiometric amount of trimethylphosphite yields 0sH k -0C(0)H (C0) P(OMe)3 (P Pr3)2, and the addition of a stoichiometric amount of ethyne di-carboxylic methyl ester leads to 0sH K1-0C(0)H (C0)(r 2-Me02CC=CC02Me) (P Pr3)2, which in solution partially dissociates the alkyne. As is shown in... 

In a follow-up study, the authors498 probed the mechanism over Pt/Ti02 rutile and anatase catalysts. The authors could see OCO stretching bands clearly upon CO adsorption, but had difficulty observing the C-H stretching bands to identify them as bidentate formates (rutile—OCO bands at 1374 cm-1 for symmetric and 1599 cm-1 for asymmetric anatase—OCO bands at 1362 cm-1 for symmetric and 1561 cm-1... 

The v(OD) peak at 2706 cm 1 on an OD-covered ZnO surface attributable to linear OD groups on two-coordinated Zn ions, decreased by reaction with CO at 473 K and accompanied with the appearance of uLS(OCO) and vs(OCO) peaks for surface bidentate formats (DCOO ) at 1586 and 1342 cm1, respectively, suggesting that the OD groups react with CO to produce the bidentate formates. The formates (DCOO-) react with the D atoms of bridge (2682 cm ) or threefold-hollow (2669 cm-1) OD groups at 573 K as monitored by FT-IR, evolving D2, COz, D20, and CO in the gas phase. 

It was found that the rate constant of the forward decomposition of the surface bidentate formate (DCOO ) to produce D2 and C02 increased from 0.34X10 4 sec-1 under vacuum to 5.3 X10-4 sec-1 under ambient water. Electron donors such as NH3, CH3OH, pyridine, and THF also increased the decomposition rate the rate constants of the forward decomposition of the surface formates at 553 K were determined to be 28.0X10 4, 7.7X10 4, 8.1X10-4, and 6.0X10 4 sec-1 under NH3, methanol, pyridine, and THF vapors (0.4 kPa), respectively. It is likely that the driving force for the forward decomposition of the formate is electron donation of the adsorbed molecule to the Zn ion on which the bidentate formate adsorbs. The reactant-promoted mechanism for the catalytic WGS reaction on ZnO is illustrated in Scheme 8.2. 

The WGS reaction is a reversible reaction that is, the WGS reaction attains equilibrium with the reverse WGS reaction. Thus, the fact that the WGS reaction is promoted by H20 (a reactant), in turn implies that the reverse WGS reaction may also be promoted by a reactant, H2 or C02. In fact, the decomposition of the surface formates produced from H2+C02 was promoted 8-10 times by gas-phase hydrogen. The WGS and reverse WGS reactions conceivably proceed on different formate sites of the ZnO surface unlike usual catalytic reaction kinetics, while the occurrence of the reactant-promoted reactions does not violate the principle of microscopic reversibility. The activation energy for the decomposition of the formates (produced from H20+CO) in vacuum is 155 kJ/mol, and the activation energy for the decomposition of the formates (produced from H2+C02) in vacuum is 171 kJ/mol. The selectivity for the decomposition of the formates produced from H20+ CO at 533 K is 74% for H20 + CO and 26% for H2+C02, while the selectivity for the decomposition of the formates produced from H2+C02 at 533 K is 71% for H2+C02 and 29% for H20+C0 as shown in Scheme 8.3. The drastic difference in selectivity is not presently understood. It is clear, however, that this should not be ascribed to the difference of the bonding feature in the zinc formate species because v(CH), vav(OCO), and v/OCO) for both bidentate formates produced from H20+C0 and H2+C02 show nearly the same frequencies. Note that the origin (HzO+CO or H2+C02) from which the formate is produced is remembered as a main decomposition path under vacuum, while the origin is forgotten by coadsorbed H20. 

Scheme 8.3 The reactivity of bidentate formates (reaction intermediates) in normal and reverse WGS reactions on ZnO, and the promotion of the reactivity by H20 and H2 respectively, accompanied with change in the selectivity.

In the catalytic WGS reaction on Rh/Ce02, linear OH groups reacted with CO to produce bidentate formates. In vacuum, 65% of the surface formates decomposed backwardly to H20+C0, and 35% of them decomposed forwardly to H2+C02. When water vapor coexisted, 100% of the formates decomposed forwardly to H2+C02 as shown in Table 8.1. The activation energy for the forward decomposition of the formate decreased from 56kJ/mol in vacuum to 33 kJ/mol due to the presence of water(D20) vapor. By addition of a small amount of Rh (0.2 wt%) to Ce02, the rate of the WGS reaction increased tremendously, and the value of the forward decomposition rate constant (k ) was promoted about 100-fold by the coexistence of gas-phase water (Table 8.1). 

The isotope effect is observed with the hydrogen atom of the formate and not with the hydrogen atom of the water molecule. The result is similar to that observed on ZnO, where the ratedetermining step of the formate decomposition is suggested to be dissociation of the CH bond of the bidentate formate. In summary, the reaction mechanism for the catalytic WGS reaction on Rh/Ce02 is essentially the same as that on ZnO. 

The same distortion, however, is not required for chelation of transA, -cyclohexanediamine (chxn) molecular models show that the two NH2 groups are now ideally situated for bidentate formation to a metal ion (Fig. 8). 7rart -l,2-cyclohexanediamine differs from the ethylenediamine system in that, when it is coordinated, only one chair conformation of the cyclohexane ring is possible. If the other conformer is assumed, the NH2... 

Formic acid NH -Y, H-Y Unidentate and bidentate formate species (480)... 

Temperature-programmed methanol decomposition was observed with DRIFT (Fig. 3). At 50 C monodentate formate, formaldehyde and methoxy were observed at 1600, 1130 and 1080cm, respectively. As temperature increased, the bands due to monodentate formate and methoxy decreased slowly. The bands at 1580 and 1370cm due to bidentate formate increased with temperature, but disappered over 270°C. Gas phase products fi om the decomposition of adsorbed methanol were analyzed by mass spectrometer(Fig. 4). Methanol began to appear at around 70°C and disappeared about 200°C, which supported the IR results... 

The bidentate formate ligand in (11) thermally decarboxylates in the presence of PPh3 to give hydride. 

In 1993 Shido and Iwasawa [93] investigated the mechanism of the WGSR over Rh/Ce02 catalysts. They made the following conclusions. The reaction occurs at Rh/Ce02 interface and not on Rh metallic particles. OH groups form bidentate formates with ceria. Then, bidentate formate decomposes and forms H2 and unidentate. Water molecules promote the desorption of formate into CO2. Electron donor-acceptor interaction between Ce-0 and H2O is an important factor for the mechanism. 

The photocatalytic conversion of CO in the presence of is proposed to proceed by an Eley-Rideal mechanism or Langmnir-Hinshelwood mechanism when carried out over typical basic oxides such as ZrO and MgO, or Ga203, respectively. In the case of ZrO and MgO, the intermediate derived from CO is a bidentate formate, whereas a monodentate bicarbonate species is formed in the case of Ga Oj. The bidentate formate intermediate formed on ZiO and MgO does not decompose directly to yield CO, but acts as a reductant and converts a second CO molecule to gaseous CO under photoirradiation. In contrast, the monodentate bicarbonate intermediate formed on Ga Oj decomposes directly. H, which acts as a reductant, cannot be adsorbed on ZrO and MgO but is dissociatively adsorbed on Ga Oj. Ga-H and Ga-OH species are formed via heteroly tic adsorption of Under photoirradia-... 

The molar ratio of coordinating carboxylate to RE(III) is usually noted as carboxylate/RE. rare earth formates are polymeric from La to Lu when carboxylate/RE = 3. They become mononuclear when carboxylate/RE increases from 3 to 6 or 8, where the metal centers are eight-coordinated either by 4 unidentate() ) and 2 bidentate( ) formates, or by 8 unidentate( ) formates. 4 ... 

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