Carbon monoxide reaction with zirconium compounds

The thermodynamic equilibria for the reactions of zirconium with oxygen, water vapor, carbon monoxide, carbon dioxide, and nitrogen have been discussed elsewhere (27). All these reactions can occur in the temperature range of 800° to 1200°C. and down to pressures of 10-8 mm. of mercury. In this range the rate of solution of the compounds formed is sufficient to maintain the zirconium surface in a film-free condition provided the reaction rate is maintained below the rate of solution. At very low pressures the reaction rate is probably proportional to the pressure of the gases present. The critical conditions for the reactions are the pressure and temperature at which the rate of formation of the compound equals the rate of solution in the metal. Although we have not determined these conditions precisely, experience has shown that the metal remains in the proper film-free condition at 800° to 1200°C. at pressures of the order of 1 X 10 mm. of mercury and less. 

Bis(i -cyclopentadienyl)titanium or titanocene, (Tj-C5H5)2Ti (1), and bis(i7-cyclopentadienyl)zirconium or zirconocene, (i7-C5H5)2Zr (2), although frequently referred to in the literature, have never actually been isolated as discrete chemical compounds. However, these molecules have been implicated as highly reactive intermediates in a wide variety of chemical reactions with olefins, hydrogen, carbon monoxide, and dinitrogen. In recent years some discrete, well-characterized bis(7j-cyclopenta-dienyl) and bis(Tj-pentamethylcyclopentadienyl) complexes of low-valent titanium and zirconium have been isolated and studied, and it has become possible to understand some of the reasons for the remarkable reactivity of titanocene- and zirconocene-related organometallics toward small unsaturated molecules. 

Reaction of dialkyl derivatives of zirconium(IV) of formula Zr(j -C5Me5)2R2 with carbon monoxide gives a mononuclear ene-diolato compound resulting from carbon-carbon coupling of the product of monoinsertion into both zirconium-carbon bonds ... 

Tetracarbonylnickel and other nickel(O) compounds, as well as palladium complexes, catalyze the [2 + 2 + 1] cycloaddition of allylic systems with alkenes or alkynes and carbon monoxide to form cyclopentanones or cyclopentenones. This reaction type resembles stoichiometric zirconium- and cobalt-mediated [2 + 2 + 1] cycloadditions (vide supra), mechanistically, however, it proceeds via transition metal 7r-allyl complexes. 

An even more striking reaction of the alkylzirconium compounds formed by hydrozirconation is insertion of carbon monoxide into the C—Zr bond to form zirconium-acyl complexes. For example, 1-hexene or an internal hexene is treated in benzene with (1) to form the alkylzirconium complex (6), which is then allowed to react under 20 psi of carbon monoxide at room temperature. The colorless acyl complex (7) is formed within several hours. An intermediate zirconium carbonyl species is not detected. The acyl complex (7) is converted into an aldehyde (8) on protonolysis, into a carboxylic acid (9) by treatment first with aqueous NaOH and then 30% H2O2, or into an ester (10) by treatment with NBS and then with an alcohol. This procedure differs significantly from... 

The conversion of CO + H2 (syn-gas) to hydrocarbons and oxygenates (Fischer-Tropsch chemistry)" is of considerable industrial importance and recently the activation and fixation of carbon monoxide in homogeneous systems has been an active area for research. The early transition elements and the early actinide elements, in particular zirconium and thorium," " supported by two pentamethylcyclopentadienyl ligands have provided a rich chemistry in the non-catalytic activation of CO. Reactions of alkyl and hydride ligands attached to the Cp M centers with CO lead to formation of reactive -acyl or -formyl compounds." " These may be viewed in terms of the resonance forms (1) and (2) shown below. 

Carbonyls of these metals which do not contain any other ligands in the coordination sphere are not stable (Table 2.21). Presently known are Ti(CO) ( = l-6) carbonyls, which were prepared by reaction of atomic titanium with carbon monoxide at low temperatures. However, the following compounds are stable [Ti(CO)2Cp2], [Zr(CO)2Cp2], and [Hf(CO)2cp2]. Titanium and zirconium compounds are obtained by the following reactions ... 

In reaction with carbon monoxide, as well as many other Lewis acid-base type reactions, the vacant orbital is often the initial site of reaction. Thus the reaction between zirconium dihydride and carbon monoxide initially forms a carbonyl metal dihydride complex about the zirconium metal atom concluding in the formation of the ethoxide derivative after hydrogenation as shown in scheme 1 This reaction is important because it leads to the formation of a variety of compounds that have M-O bonds. But, it is also important because it signals a probable mechanistic mode for attack by a Lewis base. 

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