Reduction of Methanol by Tetracarbonylcobalt Anion Assisted by

34

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Reduction of Methanol by Tetracarbonylcobalt Anion Assisted by Carbon Dioxide and Cobalt Cation Giuseppe Fachinetti and Tiziana Funaioli Dipartimento di Chimica e Chimica Industriale, University of Pisa, via Risorgimento 35, I-56126 Pisa, Italy

Fundamental electron-transfer reactions involving CH OH and CO (CO) are investigated. In a fully disproportionated CH OH solution of Co (CO) , in the presence of CO , a CH OH reduction by Co(CO) - occurs, under the intervention of the highly polarizing Co cations. In tetrahydrofuran as solvent under carbon monoxide atmosphere, the CH O nucleophile formed gives rise to the Co(I) species tetracarbonyl(methoxycarbonyl)cobalt. Under low carbon monoxide pressure, this Co(I) species disproportionates to Co(OCH ) and Co (CO) . Catalytic amounts of Co (CO) activate the carbonylation reaction of Co(OCH ) to dimethyl carbonate and tetracarbonyl(methoxycarbonyl)cobalt. 3

2

8

3

2

8

2

3

2+

4

3

-

3

2

8

2

3

2

8

2

V^ARB()NYL(METii()XYCARBONYL)(X)BALTS are believed to be key intermediates in important industrial and laboratory catalytic processes performed in C H j O H solutions of C o ( C O ) , namely olefin carbalkoxylation (I, 2) and CI 1 O H carbonylation (3) or homologation (4). This assumption is justified by the chemical properties of preformed tetracarbonyl(alkoxycarbonyl)cobalts. Nevertheless, the generation of these intermediates in C H O H solutions of C o ( C O ) requires the O - H bond activation of the C I I O H molecule, an elementary step that is still to be demonstrated in these solutions. We recently observed an O - I I bond activation of the H 0 molecule in a fully disproportionated C o ( C O ) wet ethereal solution (5). Such an activation consists of a C o -assisted reduction of I I 0 (to H and nucleophile 2

8

3

3

2

s

3

2

2

2 +

8

2

2

(X)65-2393/92/0230-0507$06.00/0 © 1992 American Chemical Society

Moser and Slocum; Homogeneous Transition Metal Catalyzed Reactions Advances in Chemistry; American Chemical Society: Washington, DC, 1992.

508

H O M O G E N E O U S TRANSITION M E T A L CATALYZED REACTIONS

hydroxide) by C o ( C O ) . We have now found that in a fully disproportionated C H 3 O H solution of C o ( C O ) in the presence of C 0 , the O - H bond activation of the C H 3 O H molecule consists of C o - and C0 -assisted C H O H reduction to H and nucleophile methoxide by Co(CO) ~. 4

2

8

2

2 +

2

2

3

4

Experimental Details All manipulations were carried out by standard Schlenk techniques under prepurified argon. Methanol was distilled from Mg(OCH ) , and tetrahydrofuran (THF) was distilled from L i A l H . C o ( C O ) was purchased from Strem Chemicals and sublimed (38 °C, 0.1 mm Hg) prior to use. IR spectra were recorded on a Perkin Elmer model 283 instrument (0.1 mm C a F cell). C o ( C O ) (3 g, 8.77 mmol) was dissolved in 50 mL of C H O H and stirred until gas evolution ceased. The resulting pink solution was transferred in a 120mL rocking steel autoclave and charged with C 0 up to 15 atm. After 4 h at 100 °C, gas chromatographic (GC) analysis revealed the presence of H and C O in the gas phase. After they were cooled, the gases were vented out and a 10mL portion of the solution was treated with a slight excess of bis(triphenvlphosphine)iminium chloride (PPNC1). The quantitative precipitation of Co(CO) yielded colorless crystals of PPNCo(CO) (1.095 g, 1.54 mmol, 88% yield). By potentiometric back titration of the mother liquor, 1.63 meq of base (methyl carbonate) was determined. Anhydrous C o C l (30 mg) was added to the remaining solution (40 mL). Upon evacuation, the 1615-cm" methyl carbonate band disappeared. The evacuation of the solution was prolonged until dryness, leaving a solid residue that was dissolved in 40 m L of T H F . The resulting dark green solution was transferred again in the 120-mL autoclave, pressurized with 80 atm of C O , and kept at 80 °C for 4 h. After the solution cooled, the gases were vented out. C H O C ( 0 ) C o ( C O ) and Co (CO) were quantitatively determined by measuring the infrared absorbances of the reaction mixture at 1684 and 1857 c m . }

4

2

2

8


2

2

8

3

2

2

4

4

2

1

3

4

2

8

1

Discussion As a consequence of the complete disproportionation reaction (6) (eq 1), a 0.15 M C H 3 O H solution of C o ( C O ) soon becomes pink and its IR spectrum 2

8

in the C O stretching region shows only the 1910-cm

1

band of the unper-

turbed C o ( C O ) . 4

CH3OH

I Co (CO) 2

[Co(CH OH)„] 3

s

2+

+ 2Co(CO).f + 4 C O

(1)

When this solution is warmed in a closed vessel at about 100 °C under an A r atmosphere, cobalt metal and C O are formed. Presumably, intermediates of such a Co(II)-Co(-I) synproportionation are endothermic Co ,Co(CO) homonuclear ion pairs (7, 8). However, when the same solution is warmed to 100 °C under 15 atm of C 0 , metal formation is suppressed and both H and C O evolve. The IR spectrum of the solution shows the 1910-cm" band of un reacted Co(CO) ~ and a new absorption (1615 2 +

4

2

2

1

4


34.

FACIIINETTI & FUNAIOLI

Methanol

and Tetracarbonylcobalt

509

Anion

cm" ), which was attributed to the methyl carbonate ion acting as bidentate 1

ligand for C o

cations (9). C O and H were detected in the evolved gases.

2 +

2

Quantitative determination of methyl carbonate and of unreacted C o ( C O )

4

indicates the stoichiometry of eq 2. 100 °C,

[Co(CH OH) ] 3

n

+ 2Co(CO)

2+

3Co + 8 C O (2)

4

y [Co0 COCH ] Downloaded by UNIV OF ARIZONA on May 18, 2017 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch034

2

3

2

3

/2Co(CO)

+

+

+ y H

4

4

2

+ 2CO

Thus, C 0 assists the C H Q H reduction by C o ( C O ) . Highly polarizing Co cations also play a crucial role in the reaction. Neither H nor methyl carbonate was formed when a 0.2 M solution of N a C o ( C O ) in C H O H , pressurized with C 0 at 15 atm, was warmed to 100 °C. 2

3

4

2 +

2

4

3

2

These results can be rationalized in terms of proton transfer from C o coordinated C H O H to C o ( C O ) . In the absence of C 0 the acid-base equilibrium is far to the left. In the presence of C 0 the formation of bidentate methyl carbonate ligand increases the steady-state concentration of H C o ( C O ) , whose decomposition accounts for the observed products (eq 3). 2+

3

4

2

2

4

[(CH OH)„CoO-H] 3

+ Co(CO)

+

*

4

I CH

3

[(CH OH) Co0 COCH ] 3

n

2

3

+

+ H C o ( C O ) -> 4

V4H + VfeCcfeiCOk 2

(3)

This finding constitutes a rare case in which a C 0 effect on electron transfers occurring in a carbonyl metal solution has been singled out. Furthermore, we have found that in the presence of catalytic amounts of halide ions, [ C o 0 C O C H ] is in equilibrium with [ C o O C H ] . Thus, a solution containing an equimolar mixture of C H 0 " and C o ( C O ) (as counteranions of C o ) was obtained on evacuating C 0 . 2

2

3

+

+

3

3

4

2 +

2

Investigation of the carbonylation reaction of a solution containing C o , C H 0 , and C o ( C O ) (1:1:1 molar ratio) confirmed that the C o - , C 0 assisted reduction of C H O H by C o ( C O ) " can constitute a step toward the formation of C H O C ( 0 ) C o ( C O ) . In C H O H the carbonylation to methyl formate masks the reaction of the cobalt-containing species. Therefore T H F was used as solvent instead of C H O H . This change of solvent alters the solution from pink to deep green. Correspondingly, IR analysis of the T H F 2 +

3

2 +

4

3

3

4

4

3

3


2

510

H O M O G E N E O U S TRANSITION M E T A L CATALYZED

REACTIONS

solution shows a characteristic pattern of four bands at 2042 (m), 1978 (s), 1952 (vs), and 1940 (vs) cm *. This pattern suggests the presence of a dimeric alkoxo-bridged [ C o O C H ] , C o ( C O ) homonuclear ion pair of the [(OC) CoCo(OR)L] type (10). The T H F solution containing the [ ( O C ) C o C o ( O C H ) T H F ] dimer can be carbonylated at 80 °C under carbon monoxide pressure (Pco) of 80 atm. Under these conditions tetracarbonyl(methoxycarbonyl)cobalt [v(CO) in T H F : 2118 (m), 2054 (s), 2043 (vs), 2032 (vs), and 1684 (m) cm" ] and C o ( C O ) were formed in substantially quantitative yields according to the stoichiometry of eq 4. +

3

4

4

2

4

3

2

1

2

8


C O , 80 atm

[(OC) CoCo(OCH )THF] 4

3

2

T H F , 80 °C

2CH OC(0)Co(CO) 3

4

+ Co (CO) 2

8

(4)

The formation of tetracarbonyl(methoxycarbonyl)cobalt according to this reaction sequence (Scheme I) suggests a plausible pathway for the formation of carbonyl(methoxycarbonyl)cobalts as intermediates in catalytic processes carried out in C H O H solutions of C o ( C O ) . The highly polarizing C o cations and C 0 assist the C H O H reduction by C o ( C O ) . The synproportionation reaction induced by high C O pressure gives the Co(I) species C H O C ( 0 ) C o ( C O ) , presumably through a dimeric, alkoxo-bridged, [ C o O C H ] , C o ( C O ) " homonuclear ion pair. 3

2

2

3

2 +

8

3

4

4

3

+

4

The formation of C H O C ( 0 ) C o ( C O ) at 80 °C (eq 4) is rather surprising because this compound has been reported to undergo thermal decomposition at room temperature (11). As a matter of fact, IR investigations under various C O pressures, gas volumetric measurements, and the characterization of the solid product C o ( O C H ) indicate that the thermal decomposition of C H O C ( 0 ) C o ( C O ) actually occurs according to the P -dependent equilibrium in eq 5. That fact was confirmed by reacting a mixture of preformed C o ( O C H ) (12) and C o ( C O ) in a 2:1 molar ratio with C O at 80 °C and 80 atm pressure. IR analyses of the resulting solution showed that C H O C ( 0 ) C o ( C O ) was the unique product of the carbonylation reaction. 3

3

3

4

2

4

3

(:o

2

2

3

8

4

2CH OC(0)Co(CO) 3

THF

Co(OCH )

4

3

2

1 + - Co (CO) Ù 2

8

4- 6 C O

(5)

Thus, C o ( O C H ) , which does not react with C O even under drastic conditions (13), is carbonylated to C H O C ( 0 ) C o ( C O ) in the presence of the stoichiometric amount of C o ( C O ) as electron source. 3

2

3

2

4

8

A completely different carbonylation of C o ( O C H ) was observed when only catalytic amounts of C o ( C O ) were added to a suspension of C o ( O C H ) in T H F . In this case, C O itself acts as reducing agent, and both dimethyl 3

2

2

8


3

2

FACHINETTI & FUNAIOLI

Methanol and Tetracarbonylcobalt Anion

CH OH, -CO »• 4 [ C o ( Œ O H ) ] 3

6Co (CO) 2

8

3

co

n

2 +

+ 8CO

2

-CO

+ 6[(CH OH) . Co02COCH ] + 6Co(CO)

2

3


3H

n

1

3

-co ,[X]-

+

+CO-,

2

6[(CH OH) . CoOCH ] + 6Co(CO) 3

n

1

3

THF

+

4

CH OH 3

[(OC) CoCo(OCH )THF] 4

3

2

CO

6CH OC(0)Co(CO) + SCo^COJg 3

-CO

4

+CO

3Co(OCH )2 + 3X20^(0% 3

[Co (CO) ], CO 2

8

3\2CH OC(0)OCH + 3CH OC(0)Co(CO) 3

3

3

4

Scheme I. Some C chemistry in CH OH solutions of Co^CO)st

3


4

512

H O M O G E N E O U S TRANSITION M E T A L CATALYZED REACTIONS

carbonate and C H O C ( 0 ) C o ( C O ) were formed in substantially quantitative yields (eq 6). 3

4

Co (CO) 2

2Co(OCH ) 3

2

+ 11CO

8

80 °C, 80 atm

CH OC(0)OCH 3

3

+ CH OC(0)Co(CO) 3

4

(6)


According to Scheme I, the summation of the elementary steps leading to dihydrogen and dimethyl carbonate gives the overall stoichiometry of eq 7. GWCOkCO,

2CH OH + C O

-

3

-

H + CH OC(0)OCH 2

3

(7)

3

The formation of dimethyl carbonate according to reaction 6 can be tentatively attributed to the reductive elimination of dimethyl carbonate from a tricarbonylbis(methoxycarbonyl)cobaltate anion. This hypothesis is substantiated by the fact that we found the alkali metal salts of triearbonylbis(methoxycarbonyl)cobaltate to reductively eliminate dimethyl carbonate at room temperature (14) (eq 8). THF

{[CH OC(0)] Co(CO) } 3

2

3

CH OC(0)OCH 3

3

+ Co(CO)

4

(8)

Scheme I reports some fundamental electron-transfer reactions that can be useful for a better understanding of known catalytic processes. In general, our findings indicate that, in metal carbonyl chemistry, transition metal cations formed in disproportionation reactions of neutral carbonyls are highly reactive species (15-17).

Acknowledgments We thank F. Calderazzo, L . Cassar, and F. Rivetti for helpful discussions. Support for this work by E N I C H E M is gratefully acknowledged.

References 1. Milstein, D.; Huckaby, J. L.J.Am. Chem. Soc. 1982, 104, 6150. 2. Milstein, D. Acc. Chem. Res. 1988, 21, 428. 3.Ungváry,F.; Markó, L. Organometallics 1983, 2, 1608. 4. Bartik, T.; Krümmling, T.; Markó, L.; Pályi, G. Gazz. Chim.Ital.1989, 119, 307. 5. Funaioli, T.; Biagini, P.; Fachinetti, G. Inorg. Chem. 1990, 29, 1440. 6. Hieber, W.; Mühlbauer, F.; Ehmann, A. Chem. Ber. 1932, 65, 1090.



34.

FACHINETTI & FUNAIOLI

Methanol and Tetracarbonylcohalt Anion

513

7. Fachinetti, G.; Fochi, G.; Funaioli, T. J. Organomet. Chem. 1986, 301, 91. 8. Fachinetti, G.; Fochi, G.; Funaioli, T.; Zanazzi, P. F. Angew. Chem., Int. Ed. Engl. 1987, 26, 680. 9. Yamamoto, T.; Kubota, M.; Yamamoto, A. Bull. Chem. Soc. Jpn. 1980, 53, 680. 10. Funaioli, T.; Biagini, P.; Zanazzi, P. F.; Fachinetti, G. Gazz. Chim. Ital. 1991, 121, 321. 11. Tasi, M.; Pályi, G. Organometallics 1985, 4, 1523. 12. Adams, R. W.; Bishop, E.; Martin, R. L.; Winter, G. Aust. J. Chem. 1966, 19, 207. 13. Saegusa, T.; Tsuda, T.; Isayama, K. J. Org. Chem. 1970, 35, 2976. 14. Fachinetti, G.; Funaioli, T.; Masi, D.; Mealli, C. J. Organomet. Chem. 1991, 417, C32. 15. Fachinetti, G.; Funaioli, T.; Marcucci, M. J. Organomet. Chem. 1988, 353, 393. 16. Fachinetti, G.; Fochi, G.; Funaioli, T.; Zanazzi, P. F. J. Chem. Soc., Chem. Commun. 1987, 89. 17. Mealli, C.; Proserpio, D. M.; Fachinetti, G.; Funaioli, T.; Fochi, G.; Zanazzi, P. F. Inorg. Chem. 1989, 28, 1122. RECEIVED for review October 19, 1990. ACCEPTED revised manuscript July 22, 1991.


Reduction of Methanol by Tetracarbonylcobalt Anion Assisted by

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