731
Inorg. Chem. 1987, 26, 731-736
Contribution No. 7376 from the Chemical Laboratories, California Institute of Technology, Pasadena, California 9 1 125
Alcohol Electrooxidation Catalysts from Degraded Polyanionic Chelating Ligand Complexes. The Uncertainty for Catalyst Identification That Accompanies a Decomposing Catalytic System Fred C. Anson, Terrence J . Collins,*' Stephen L. Gipson, and Terry E. Krafft Received April 21, 1986
Electrooxidation of tran~-Os(v~-CHBA-Et)(py),, H,CHBA-Et = 1,2-bis(3,5-dichloro-2-hydroxybenzamido)ethane,in the presence of alcohols led to the discovery of several catalytic systems for the electrooxidation of alcohols. The catalytic systems are formed via the oxidative and hydrolytic degradation of the ethylene unit of the polyanionic chelating (PAC) ligand [v4-CHBA-EtI4- in a sequence that involves 15 observable species. The principal species in the catalytically active solutions have been identified. The uncertainty for catalyst identification that is inherent with decomposing catalyst systems is discussed. At high alcohol concentrations, primary alcohols are oxidized to aldehydes. Pure aldehydes are not oxidized. However, it has not been possible to study the catalytic oxidations at high concentrations of both primary alcohols and aldehydes. Cyclohexanol is oxidized to cyclohexanone. All the species in the degradation sequence are closely related. It is suggested that the principal species in the catalytic solutions might be able to carry out rapid inner-sphere oxidations of alcohols because of a higher affinity for alcohols than the noncatalysts in the degradation sequence.
v
Introduction Catalyzed electrochemical oxidations of organic substrates such as alcohols2 are receiving increased attention. In the process of developing new polyanionic chelating (PAC) ligands t h a t are compatible with highly oxidizing metal center^,^-^ we conducted controlled-potential electrolysis experiments on solutions of primary and secondary alcohols in t h e presence of trace quantities of
trans-Os(~~-CHBA-Et)(py)~ (5) to search for alcohol electrooxidation catalysts. Electrocatalysts were indeed discovered, b u t early on in t h e work, it became obvious t h a t the initial compound, 5, was not a catalyst. We discovered that 5 undergoes a multistep Alfred P. Sloan Research Fellow, 1986-1988; Dreyfus Teacher-Scholar, 1986-1990. See, for example: (a) Shono, T.Tetrahedron Lett. 1979,20, 3861-3864. (b) Shono, T.; Matsumura, Y.; Hayashi, J.; Mizoguchi, M. Ibid. 1979, 20, 165-168. (c) Shono, T.; Matsumura, Y.; Hayashi, J.; Mizoguchi, M. Ibid. 1980, 21, 1867-1870. (d) Leonard, J. E.; Scholl, P. C.; Steckel, T. P.; Lentsch, S. E.; Van De Mark, M. R. Ibid. 1980,21, 4695-4698. (e) Moyer, B. A,; Thompson, M. S.; Meyer, T.J. J . Am. Chem. SOC. 1980,102,2310-2312. ( f ) Thompson, M. S.; DeGiovani, W. F.; Moyer, B. A,; Meyer, T. J. J . Org. Chem. 1984, 25, 4972-4977. (g) Meyer, T. J. J . Electrochem. SOC.1984, 131, 221C-228C. (h) Yoshida, J. I.; Nakai, R.; Kawabata, N. J . Org. Chem. 1980, 45, 5269-5273. (i) Masui, M.; Ueshima, T.; Ozaki, S. J . Chem. SOC.,Chem. Commun. 1983, 479-480. 6 ) Semmelhack, M. F.; Chou, C. S.; Cortes, D. A. J . Am. Chem. SOC.1983,105,4492-4494. (k) Dvonch, W.; Mehltretter, C. L. J . Am. Chem. SOC:1952, 74, 5522-5523. (I) Horowitz, H. H.; Horowitz, H. S.; Longo, J. M. In Proceedings of the Symposium on Electrocatalysis; OGrady, W. E., Ross, P. N. J., Will, F. G., Eds.; The Electrochemical Society: Pennington, NJ, 1981; pp 285-288. (m) Baizer, M. M. Organic Electrochemistry; Marcel Dekker: New York, 1973; pp 821-838. For examples without catalysts, see: (n) Scholl, P. C.; Lentsch, S. E.; Van De Mark, M. R. Tetrahedron 1976, 32, 303-307. (0)Brown, D. R.; Chandra, S.; Harrison, J. A. J . Electroanal. Chem. Interfacial Electrochem. 1972, 38, 185. (p) Mayeda, E. A,; Miller, L. L.; Wolf, J. F. J . Am. Chem. SOC.1972, 94, 6812. (4) Horanyi, G.; Vertes, G.; Koning, P. Natunvissenschaften 1970, 60, 519-520. (a) Anson, F. C.; Christie, J. A,; Collins, T. J.; Coots, R. J.; Furutani, T. T.; Gipson, S. L.; Keech, J. T.; Krafft, T. E.; Santarsiero, B. D.; Spies, G. H. J . Am. Chem. SOC.1984, 106, 4460-4472. (b) Anson, F. C.; Collins, T. J.; Coots, R. J.; Gipson, S. L.; Keech, J. T.; Krafft, T. E.; Santarsiero, B. D.; Spies, G. H. Inorg. Chem., in press. Christie, J. A.; Collins, T. J.; Krafft, T.E.; Santarsiero, B. D.; Spies, G. H. J . Chem. SOC.,Chem. Commun. 1984, 198-199. Collins, T.J.; Santarsiero, B. D.; Spies, G. H. J . Chem. Soc., Chem. Commun. 1983. 681-682. Anson, F. C.; Collins, T. J.; Coots, R. J.; Gipson, S. L.; Richmond, T. G. J . ~~. A m . Chem. 106. . . SOC.1984.,~ ..,.5037-5038. Collins, T. J.; Richmond, T. G.; Santarsiero, B. D.; Treco, B. G.R. T. J . Am. Chem. SOC.1986, 108, 2088-2090. (a) Collins, T. J.; Coots, R. J.; Furutani, T. T.; Keech, J. T.; Peake, G. T.; Santarsiero, B. D. J . Am. Chem. SOC.1986, 108, 5333-5339. (b) Anson, F. C.; Collins, T. J.; Gipson, S . L.; Keech, J. T.; Krafft, T. E.; Peake, G . T. J . Am. Chem. SOC.1986, 108, 6593-6605. Anson, F. C.; Collins, T. J.; Gipson, S. L.; Keech, J. T.; Krafft, T. E. Inorg. Chem., in press. ~
~~~
ligand oxidation/hydrolysis sequence when electrooxidized in the presence of alcohols; t h e ethylene unit linking the two organic amide nitrogen atoms is degraded by sequential oxidation and hydrolysis events as shown in Scheme L3,Io I t has been possible t o show that t h e early species in t h e sequence are not catalysts. The catalytic systems eventually degrade to inactive material. We decided to study t h e degradation sequence and the catalytic reactions because this system provides an excellent illustration of the complexity t h a t can be associated with a catalytic system that is undergoing degradation during t h e course of catalysis. In such circumstances, serious limitations c a n occur for catalyst identification (vide infra). The principal species in t h e catalytically active solutions have been identified. The characterizations of t h e compounds in the degradation sequence,3including detailed studies of the unusual isomerizations t h a t result in the branching of the sequence into two parallel diastereomeric paths: have been described previously. Experimental Section All compounds were synthesized as previously de~cribed.~ UV-visible spectra were recorded on a Hewlett-Packard 8450A diode array spectrophotometer. Electrochemical Procedures. Dichloromethane (MCB or Mallinckrodt) used in electrochemical experiments was reagent grade and was further purified by passage over a column of activated alumina (Woelm N. Akt. I). TBAP (tetrabutylammonium perchlorate) supporting electrolyte (Southwestern Analytical Chemicals) was dried, recrystallized twice from acetone/ether, and then dried under vacuum. The TBAP concentration in all solutions was 0.1 M. Alcohols were reagent or spectrophotometric grade and were used as received. Cyclic voltammetry was performed with a Princeton Applied Research Model 173 potentiostat equipped with positive-feedback IR compensation
~~
0020-166918711326-073 1$01.50/0
(10) Because of the complexity of the degradation sequence the compound numbering scheme is retained from prior work to assist with comparisons of papers.) (11) Shriner, R. L.; Fuson, R. C.; Curtin, D. Y.; Morrill, T. C. The Systematic Identification of Organic Compounds, 6th ed.; Wiley: New York, 1980; p 162.
0 1987 American Chemical Society
732 Inorganic Chemistry, Vol. 26, No. 5, 1987
Anson et al.
Scheme I"
0'
CI
CI
t I
9'
Ron, b o
\
L
t
-2r' -2"
/
CI
CI
-2c'
-2
10'
CI
CI
yo
m -2"
ROH, -2"
-2"
/
L
11
2 4
1s' ci
An asterisk denotes structures characterized by X-ray crystallography. and a Model 175 universal programmer. Current-voltage curves were recorded on a Houston Instruments Model 2000 X-Yrecorder. Standard two- and three-compartment electrochemical cells were used. When necessary, solutions were purged with argon to remove oxygen. The working electrode was a 0.1 7 cm2 basal-plane pyrolytic graphite (BPG, Union Carbide Co.) disk mounted in a glass tube with heat-shrinkable tubing. The counter electrode was a platinum wire. Various reference electrodes were used including SCE, SSCE, Ag/AgCl, and a silver-wire quasi-reference electrode. For each compound studied at least one experiment was performed in the presence of ferrocene as internal potential standard. All potentials are quoted with respect to the formal potential of the ferrocenium/ferrocene couple, In CH2C12under these conditions we have consistently measured this couple as +0.48 V vs. SCE. The formal potentials of all the couples were taken as the average of the anodic and cathodic peak potentials. Controlled-potential electrolysis experiments were performed with the PAR Model 173 potentiostat equipped with a Model 179 digital coulometer using positive-feedback IR compensation. Electrolyses in dichloromethane were performed in a standard three-compartment H-cell with a platinum-gauze counter electrode in one compartment and the reference electrode and a 1.7 X 4.5 X 0.07 cm BPG working electrode in the third compartment. The reference electrode used for controlledpotential oxidations with the catalyst, compounds was a Corning Ag/ AgCl double-junction reference electrode. The product solutions containing decomposed catalysts were found to cause the potential of the usual single-junction Ag/AgCI reference electrode to drift by as much as 500 mV. Therefore, a double-junction reference electrode was used to maintain proximity of the reference electrode to the working electrode and minimize IR compensation but, at the same time, isolate the refer-
ence electrode from the catalyst solution. The inner compartment of the double-junction reference electrode contained an aqueous solution of saturated KCI and AgC1. The outer compartment contained this solution layered over CH2CI2containing 0.5 M TBAP. This arrangement gave potentials almost identical with those of the single-compartment electrode but was not susceptible to drift during the electrolyses. Most electrolyses were performed in the presence of Na2CO3,added to neutralize the acid formed by alcohol oxidation. All experiments were performed at room temperature, 22 i 2 OC. HPLC analyses for aldehydes and ketones were performed on an IBM LC 9533 ternary gradient LC equipped with an octadecyl column, an LC 9522 254-nm UV detector and a Hewlett-Packard 3390A integrator. The eluting solvent was 3:2 methanol/water (v/v). All analyses were performed immediately after the electrolyses to minimize the effects of air oxidation of the alcohols. The following procedure was used to prepare all samples. All of the anolyte solution was collected and filtered into a 100-mL flask. Benzoylhydrazine (50 mg) and trifluoroacetic acid (2 drops) were added and the solution was heated under reflux for 1 h. The CH2C12was then removed on a rotary evaporator at 50 "C. The resulting oil was dissolved in methanol and diluted to 100 mL. The concentration of the product in the sample was determined by comparison of its HPLC peak area with that of the standard solution of the corresponding hydrazone of similar concentration. The hydrazone samples used for preparing the standard solutions were synthesized by reaction of benzoylhydrazine with the appropriate aldehyde or ketone in boiling ethyl acetate. The products were recrystallized several times from methanol and thoroughly dried. No residual benzoylhydrazine was visible upon TLC analysis. The analysis was tested on a sample of benzaldehyde (
