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Electrochemistry and Catalysis with Vitamin B12 Hexacarboxylate in an Insoluble Surfactant Film1* Chang-ling Miaw,lb Naifei Hu,lCJames M. Bobbitt, Zhenkun Ma,ld Maryam F. Ahmadi, and James F. Rusling’ Department of Chemistry (U-601, University of Connecticut, Storrs, Connecticut 06269-3060 Received August 26,1992. In Final Form: October 9,1992 Vitamin B12hexacarboxylate, &aminocob(III)yrinicacid c-lactam, was used to incorporate catalytic activity into insoluble liquid crystal f h of a cationic surfactant on carbon electrodes. The most incorporationinto caet f h of didodecyldimethylammoniumbromide (DDAB)from aqueous buffers was found at pH 4.9 in solutions containing CN-. This pH is close to the pK. of carboxylic acid group on the corrin ring. Co(III)B12 hexacarboxylate can exist in two intercowertable forms in DDAB filme in solutionscontainingCN-. The dicyanoCo(IL1) complex, reduced irreversiblyat -1.2 V va SCE, is the main speciesat equilibrium. Repetitive cyclic voltammetryslowly converta dicyanoBlzCo(III)hexacarboxylata to monocyanoB&o(III) hexacarboxylate, which gives a chemically reversible two electron reduction to the four-coordinateCo(1) form of the complex (-0.66 V at pH 4.7). Upon standing in solutionscontaining CN-, themonocyano form of the complex in DDABf h convertsback to dicyanoB&o(III)hexacarboxyhte. DDAB f h loaded with B&o(III) hexacarboxylate catalyzed reductions of trichloroacetic acid and 1,2dibromoethane at potentials of Co(1) formation. Thus,vitamin B12 hexacarboxylatehas s i m i i catalytic activity to vitamin B12 (aquocobalamine) which cannot be incorporated into films at cationic sitea. The cobalt corrin complex vitamin B1%, or aquocob(III)alamine, is related to adenosylcob(III)alamine, an enzymecofactorwhich catalyzesavariety of group transfer and other unique reactions in living systems? The electrochemistry of vitamin Bla and its derivatives has been studied extensively,*6 and there has been a great deal of interest in its catalytic properties. An increasing number of synthetic reactions have been elaborated, predominantly by Scheffold and cO-workers,blo using vitamin B12 for catalytic (mediated) electrochemical and photoelectrochemicalsyntheses. Such reactions have been used to form carbon-carbon bonds6and to make lactones? insect pheromones? prostoglandins,’* esters of terpenoid alcohols,’Ob C-glycosides,’b optically active olefins and alcohols,6Joaand bi- and tricyclic hydrocarbonsP We are interested in usingvitamin B12 and related metal macrocycles in films on electrodes designed to convert organohalide pollutants to hydrocarbonsin contaminated water and other environmental materials. Additionally, such electrodes should have a variety of applications in synthesis and as sensors. Alkyl vicinal dihalides, which have been used as soil fumigantdl and are models for more complex pesticides, are rapidly reduced to olefins by electrochemical catalysis using vitamin B12. Rate constants12J3of 1to 6 X 108M-l (1) (a) Part 11 of the wriee Electrocatalyaiein Organized Assemblies. (b) Southern Illinois University, Carbondale, IL. (e) Permanent address: Beijhg Normal University, Beijing, China. (2) Dolphin, D., Ed. BIZ;Wiley: New York, 1982; Vol. 2. (3) Lexa, D.; Saveant, J. M. Acc. Chem. Res. 1983,16,235-243, and references therein. (4) (a) Lexa,D.;Saveant, J. M.; Zickler,J. J.Am. Chem. SOC.1980,102, 2855-2663. (b) Amatore, C.; Lexa, D.; Saveant, J. M. J. Electmnal. Chem. 1980, I l l , 81-89. (c) Le=, D.; Saveant, J. M.; Soufflet, J. P. J.
Electroanal. Chem. 1979,100,159-172. (5) (a) Scheffold, R. In Modern Synthetic Methods; Scheffold, R., Ed.;Wiley: NewYork,1983;Vol.3,pp366-440. (b)Scheffold,R.;Abrecht, 5.;Orlineki, R.; Ruf, H.-R.; Stamouli, P.; Tinembart, 0.;Walder, L.; Weymuth, C. Pure Appl. Chem. 1987,59,383-372. (6)Scheffold, R. Chimia 1986,39, 203-211. (7) (a)Bruurto, S.;Tinembart, 0.; Zhang, Z.;Scheffold,R. Tetrahedron 1990,46,3155-3166. (b)Abrecht, S.;Scheffold,R. Chimia 1S86,39,211212. (8) Wi, S.; Scheffold, R. Chimia 1991,46, 30-32. (9) Steiger, B.;Walder, L.; Scheffold, R. Chimia 1986,40,93-97. (10) (a) Su, H.; Walder, L.; Zhaug, Z.;Scheffold,R.Helu. Chim. Acta 1988,71,1073-1078. (b) Lee,E. R.; Lakomy,I.; Bigler, P.; Scheffold, R. Helu. Chim. Acta 1991, 74, 148-162.
8’ at 26 OC were measured in weakly acidic acetonitrilewater for reaction of B&o(I) with ethylene dibromide, l,2-dibromobutane, and tras-l,2-dibromocyclohexane. Trichloroaceticacid, a byproduct of water chlorination, is dechlorinated by B12Co(I) with rate constant14 of 1.6 X 106M-l s-l. Unlike simple l-haloalkanes,which formstable organocobaltcomplexeswith B12C0(1),~4 vicinaldihalidea and a-halocarboxylic acids are dehalogenated at the potential at which B12Co(I) is formed at the electrode. The rate determining step involves inner sphere electron transfer between B&o(I) and the organ0halide.l2-l4 A key feature in catalytic films is the means to incorporate the catalyst in the film on the electrode. We are currently developing water-insolublefilmsof cationic surfactants for electrochemical catalysis.1b18 Such films are fluid in their liquid crystal states and can preconcentrate neutral and polar substrates from water.18b Coulombicinteractionswith cationicheadgroups intheae films can be used to bind multianionic catalyst4~~5Je However, vitamin B12 binds very weakly to cationic surfactant aggregatas13Jg and cannot be incorporated into cationic surfactant films by ion exchange. Vitamin B12 has been derivatized for several specific applications. For example, a lipophilic vitamin B12 was prepared as a selective carrier for anions- and used in a nitrite-selective electrode.mb An epoxy resin form of (11) (a) Woo, Y. T.; Lai,D. Y.; Arms, J. C.; Argw, M. F. Chemical Induction of Cancer. Vol. IIIB. Aliphatic and Polyhalogetwted Carcinogem; Academic: Orlando, FL,1986. (b) O b ,D. F.; Pelizratti, E.; Serpone, N. Environ. Sci. Technol. 1991,26,1523. (12) Connore, T. F.; Arena, J. V.; Ruding, J. F. J. Phys. Chem. 1988, 92,2810-2816. (13) Owlia, A.; Wang, Z.;Ruling, J. F. J. Am. Chem. Soc. 1989,111, 5901-5908. (14) Ruling, J. F.; Miaw, C. L.; Couture, E. C. Inorg. Chem. 1990,29, 2025-2027. (15) Rueling, J. F.; Zhmg, H. Longmuir 1991, 7,1791-1798. , (16) (a)Rueling,J.F.;Hu,N.;Zhaug,H.;Hwe,J.D.;Miaw,C.;Couture, E. C. In Electrochemistry in Colloids and DbperaioM; M a c k , R. A, Texter, J., Eda.;VCH Publiehere: Deerfield Beach, FL, 1992; pp 303318. (b) Hu, N.; Howe, J. D.; Ahmadi,M. F.; Ruling, J. F.A d . Chcm.,
in prese. (17) Hu, N.; Ruling, J. F. Anal. Chem. 1991,69,2163-2188. (18) (a) Ruling, J. F.; Ahmadi, M.F.; Hu, N. Longmuir 1992,8,2349. (b) Zhmg, H.; Rueling, J. F. Talonto, in p m . (19) Fendler, J. H.; Nome, F., Van Woert, H. C. J. Am. Chem. Soc. 1974, S, 6745-8753.
0743-7463/93/2409-0315$04.00/0Q 1993 American Chemical Society
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HmC
c o o n 200, Figure 1. Structure of 8-aminocob(III)yrinic acid c-lactam as cyanochloro complex.
vitaminBIZwm coatedonto electrodes and used to catalyze carbon-carbon bond formation.21 Tris(2,2'-bipyridyl)ruthenium linked to a cob(II)yrinate ester was used as a photoredoxcatalyst for organic cyclizations.22In all these derivatives, the cobalt(1II) conin retains the essential electrochemical properties of vitamin BIZ. Ourgoalwmtointroducethecatalyticactivityofvitamin Bl2 into cationic surfactant films. A derivative with multiple negative charges on the corrin ring should bind to catalytic surfactant films, while retaining the catalytic activity of ita parent compound. Vitamin BIZhexacarboxylate, 8-aminocobyrinic acid c-lactam,a* is prepared by alkaline hydrolysisof vitamin BIZ,converting carboxamides to carboxylic acid groups, and cleaving the benzimidazole moiety. Also, a lactam forms at the "c"ring of the corrin ligand (Figure 1). We show here that this molecule readily binds to films of the cationicsurfactantdidodecyldimethylammoniumbromide (DDAB).In the presence of cyanide, a chemically reversible Co(III)/Co(I) reduction was observed by cyclic voltammetry when vitamin BIZ hexacarboxylate was incorporated in DDAB films. Catalysis of organohalide reductions by Bl2 hexacarboxylate in DDAB films was qualitativelysimilartothat of vitamin BIZin homogeneous solutions.
Experimental Section Chemicals. Vitamin B12 was obtained as cyanocob(II1)alamine (Rhone-Polenc Co., 99%1. Didodecyldimethylammonium bromide (DDAB) (Eastman, 99+ % ) was used as received. Phenol (Aldrich)was liquefied by adding 8%water. Dowex 1x2 resin (Aldrich) in the chloride form (50-100 mesh) was washed with 2% NaCl solution. All other chemicals were reagent grade and wed as received. Preparation and Purification of 8-AminocobyrinicAcid c-Lactam. Alkaline hydrolysis was done as described by Bonnett," with somemodificationsin the purification procedure. Cyanocobalamiin (2g) was dissolved in water (160 mL), and solid d u m hydroxide(50 g) was added slowly. Stirringwas continued for 2.5 h. The mixture was heated to reflux in an oil bath at 150 "C for 1 h. The resulting dark brown solution became red on cooling and was acidified to pH 2. This solution was diluted with water to 600 mL and then extracted with liquefied phenol (4 x 100mL) until the aqueow phase was colorless. The combined phenolic extracts were washed with distilled water (2 X 100 mL), (20) (a) S c h u l h , P.;Ammann, D.;Simon,W.;Cadem, C.;Stepauek, R.; Krautler, B. Helu. Chim. Acta 1984,67,1026-1032. (b) Schultheee, P.;A"ann,D.;Krautler,B.;Cader~,C.;Stepaaek,R.;Simon,W.Anol.
Chem. 1986,67,1397-1401. (21) R u b , A.; Walder, L.; Scheffold, R. Helu. Chim. Acta 1986, 68, 1301-1311. (22) Steiger, B.;Eichenkrger, E.; Walder, L. Chimia 1991,45,32-37. (23) Bonnett, R.;Cannon, J. R.;J o h n , A. W.; Todd, A. J. Chem. SOC.1967, 114J3-llMI. (U)(a) "hh complex played a key historical role in etructural elucidation of vitamin BIZ,be' the fmt corrinoid without the nucleotide to be isolated M c r y a t a b . z 1 b 3 ) Bonnett, R. In BIZ;Dolphin, D., Ed.; Why: New York, 1982; Vol. 1, pp 201-243.
and any colored material in the aqueous layer was re-extracted with phenol. The combined salt-free phenol solutions were diluted with three volumes of ether and shaken with 0.2 N aqueous ammonia until no more color was removed from the organiclayer. Several different vitamin Biz carboxylic acids and the cleaved benzimidazolenucleotidewere thus isolated in the aqueousphase as ammonium salts. This aqueous phase was washed with ether (2 X 100 mL), and water was removed on a rotary evaporator. The last traces of water were removed by freeze drying, giving a dark red solid product mixture (1.6 g). The different vitamin Biz carboxylic acids were isolated by ion exchangechromatography. A glass column (75 X 4.5 cm i.d.1 filled with Dowex 1x2 resin (50-100 mesh) in the chloride form was washed with 0.02% aqueous HCN (100 mL). The dark red product mixture was dissolved in 0.8% aqueous HCN (50 mL) and added to the column. The column was washed with 0.8% HCN (200 mL). The benzimidazolenucleotide was eluted first with 0,05 N acetic acid (750mL) containing 0.02%HCN. Eluant was then changed to 0.02%aqueous HCN (100 mL) followed by 0.3 M NaCl containing 0.02% aqueous HCN at a flow rate of 17 mL/min. An initial portion (800 mL) was discarded, and subsequent 0.5-L fractions were collected. The concentration of sodium chloride in the NaCl/HCN eluant mixture was slowly increased to 1 M as Fractions 6-8 were combined and extracted by the phenol method describedabove. After re-extractioninto 0.2 N ammonia, the water was evaporated, and the residue was dissolved in 0.8% aqueous HCN. This solution was separated on the column described above except after it was washed with 1 L of 0.5% NaCl solution, 1 L of 2% NaOH solution, 2 L of distilled water, and finally 500 mL of 2% NaCl solution. The vitamin Blz carboxylicacid solution was eluted with 0.30.6 M NaCl solutions containing 0.02% HCN at a flow rate of 17 mL/min. Each fraction was collected until the red color became light pink, then the concentration of the eluant solution was increased. Resulting fractions were analyzed by thin layer chromatography (TLC) on silica gel 60 Fm sheets (E. Merck) with butanol/acetic acid/2% HCN (41:5) as eluant. T w o main spots were obtained, with Rf values of 0.28 and 0.39. Fractions 9-15 gave only a spot with Rf 0.39. These were combined and extracted with phenol, but the last step employed a 0.02% HCN solution for the re-extraction into the aqueous phase instead of 0.2 N aqueous ammonia. After the water was evaporated and the sample was dried under vacuum at 40 OC, 0.8 g of a red powder was obtained, 40% isolated yield. Hydrogen Cyanide Solutions. Care must be taken in preparation and handling of poisonous HCN gas. All operations were conducted in a fume hood in a closed vessel with gaseous effluent directed into a trap f i e d with 3 M NaOH. Concentrated sulfuric acid was added dropwise to a saturated potassiumcyanide (50g of KCN/100 mL of water) in an ice bath to decrsaee the heat generation. The HCN gas was then passed through a known volume of distilled water (200 mL) to make a 4 % stock solution. Structural Analysis. The carboxylated product with TLC Rf0.39was dried at 1 mmHg, 40 OC for 48 h before analysis. The acid did not melt below 288 "C (lit. for &aminocobyrinic acid c-lactam chloride cyanide, >300 0C).23 The IR spectrum of the product (KBr pellet) showed maxima at 3420,2931,2123,1703, 1583,1508,1402,1369,and 1155 cm-l and was in agreement with the spectrum reported previously for Saminocobyrinic acid c-lactam. The UV-visible spectrum in water was identical to that reported earlier for hminocobyrinic acid c - l a ~ t a m .An ~ average of thrw elementalanalysesgave the following C, 52.40%; H, 5.80%;N, 7.84%;C12.92. Theory for hminocobyrinic acid c-lactam chloride cyanide: C, 53.35%;H, 5.85%;N, 8.12%;C1 3.43% (Degree of hydration is assumed the same as found by X-ray diffraction.)a NMR was done on an IBM/Bruker AF 270 MHz spectrometer (spectra are available in ref 25). The sample wan dissolved in DzO with acetonitrile as internal standard. lH NMR (DpO) d 5.65 (1 H, a), 3.80-3.50 (2 H, m), 3.14 (1 H, m), 2.71 (1 H, m), 2.5CF2.30 (4 H, m), 2.29 (4 H, d, J = 5.41, 2.15-2.20 (18 H, m),
.
(25) Miaw, Chang4igPh.D. Thegis,UnivemityofConnecticubstom, CT, 1991.
Langmuir, Vol. 9, No. 1, 1993 317
Vitamin B12 Hexacarboxylate Catalysis 1.53 (3 H, a), 1.48 (6 H, e), 1.45-1.30 (9 H, m), 1.27 (3 H, 81, 1.25-1.10 (1 H, m), 1.13 (3 H, a), 1.01 (3 H, a), 1.104.9 (1 H, m). Ban& between 5 and 8 ppm could not be assigned precisely. Multiple bands at 2.15-2.20 ppm may be attributed to the six saturated methyl groups. Multiple bands at 2.50-2.30 ppm and a doublet band at 2.29 may be attributed to the 8-methylene groups of the four propionic acid chains. The integral of the hydrogens was the aame as the theoretical molecular formula hydrogen number of 64. WNMRresulte were6 179.2,178.3,177.1,176.8,168.7,163.2, 161.83,118.9, 114.1, 112.7, 104.11, 87.4, 86.8,85.11,82.83, 75.5, 74.1,71.3,60.9,58.5, 56.4, 53.2,50.5,47.1,45.8,44.6,44.1,40.2, 35.5, 34.2, 32.7, 31.8,31.4,30.9, 28.1, 25.9, 25.0,21.8, 19.4, 18.5, 17.6,16.3, 16.0, and 14.6 ppm. The four high field signals at 6 179.2, 178.3, 177.1, and 176.8 ppm are assigned to carboxylic groups; signals at 179.2 and 176.8 ppm are doublet peaks and may be assigned to six carboxylic carbons. Those at 6 168.7, 163.2, and 161.83 ppm are assigned to three C-N carbons; 6 118.9,114.1,112.7,and 104.11ppmareassignedtoCECcarbons; the peaks at 6 87.4,86.8,85.11, and 82.83 ppm are assigned to CH-N carbons; those at 6 75.5,74.1, and 71.3 ppm are assigned to C-N carbons; those at 60.9,58.5,56.4,53.2, 50.5,47.1,45.8, 44.6, and 44.1 ppm are assigned to CH2COOH;those at 6 40.2, 35.5,34.2,32.7,31.8,31.4,30.9,28.1,25.9,25.0,and21.8ppmare assigned to secondary carbons; and those at 6 19.4, 18.5, 17.6, 16.3,16.0, and 14.6 ppm are assigned to CH:, type carbons. The carboxylic group signals show that the molecule does have carboxylic acid groups, and the total number of signals match the molecular formula with the carbon number of 46. Fast atom bombardment mass spectra confirmedthe structure of the 8-aminocobyrinicacid c-lactam containing six carboxylic acid groups. Spectra were obtained by using the first two sectors (MS-1) of a JEOL HXllOlHX110 tandem mass spectrometer. The primary beam was 25 keV Cs+. Acceleratingvoltage was 10 kV, with 20-kV postacceleration at the detector. The sample (Rf 0.39) was dissolved in water using two matrices, glycerol and 0-benzyl and negative and positive ion spectra were obtained. The calculated molecular weight of unhydrated W ~ & O l ~ C o Wisl 998.32. No ion correspondingto this species was detected in any of the spectra, mainly because of dissociation of axial ligands. A brief summary of major mlz peaks is given below: glycerol, negative ion mode, mlz 934 from loss of (HCN, HC1) from molecular (M - H)- mlz 997; benzyl glycerol, negative ion mode, mlz 934 as above, mlz 888 from loss of HCOOH from mlz 934, m/z 956 and 978 from exchange of Na for H and 2 Na for 2 H; glycerol, positive ion mode, mlz 935 as above, mlz 961 loss of HC1 and H from molecular species, and change in Co oxidation state of lose of HZfrom protonated molecular species. ElectrochemicalApparatus. Cyclic (CV) and normal pulse (NPV) voltammetry were done in a three-electrode cell using glassy carbon disk and basal plane pyrolytic graphite (Union Carbide HPG) disks as working electrodes. Platinum wire was used as a counter electrode with an aqueous saturated calomel electrode (SCE) as reference, or in solutions containing 0.1 M KBr, a Ag/AgBr wire as reference. The measured conversion factor for E vs AglAgBr (0.1 M KBr) to E vs SCE is addition of -0.089 V. All potentials are reported vs SCE. A Bioanalytical System BAS-100 electrochemical analyzer was used for voltammetry. Potentiometric titrations were done with a glass pH electrode, an SCE, and a Coming Model 130 pH meter. Glamy carbon electrodes (A = 0.071 cm2) were constructed, poliihed, and cleaned ultrasonically as described previously.z7 Pyrolyticgraphite (PG) cylinderswere machined from PG blocks. PG dike (geometric A = 0.20 cm2, ht. 3 mm) were cleaved and sealed into polypropylene holders as described previously." PG electrodes were abraded to a rough finish with 600 grit S i c paper on a poliihing wheel and then ultrasonicatsd in distilled water. Burfaatant Films. PG electrodes were coated with 20 pL of 0.1 M DDAB in chloroform by dropping with a small syringe onto a freshly poliihed PG disk. Solvent was evaporated (26) Staempfli, A. A.; Schlunegger, U. P.Rapid Commun. Mass Spectrom. 1991,6, 30-31. (27) Kamau, 0. N.;Willin, W.5.;Rurrling, J. F.Anal. Chem. 1986,57, 546-661.
a
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Figure 2. Cyclic voltammograms at 0.1 V 8-1 in 1 mM vitamin Bl2 hexacarboxylicacid solutions: (a) PG electrode, 0.1 M KBr, 0.01 M phosphate buffer, pH 2.2; (b) GC electrode, 0.2 M phosphate buffer, pH 2.5 + 7 mM HCN. overnight after fitting electrodes tightly into small glass bottles. This slowed evaporation and gave more uniform films.1s Procedures. Vitamin BIZ hexacarboxylate solutions containing 0.2 M phosphate buffer with 7 mM HCN were stable for up to 1 month at ambient temperatures when kept in the dark, as shown by CV and UV-vis spectroscopy. Oxygen was removed from solutions before voltammetry by bubbling with purified nitrogen for 5 min or more. Ohmic drop of the electrochemical cell was fully compensated by the BAS-100. All experimente were done at ambient temperature (ca. 23 f 1 "C). UV-vis spectra were obtained with a Perkin-Elmer A6 spectrophotometer. For spectra of f i ,DDAB was cast onto quartz plates, and films were equilibrated with vitamin B12 hexacarboxylate solutions overnight. After equilibration, films were washed gently with several portions of water before recording spectra.
Results Acid-Base Equilibria, Potentiometric microtitrations of vitamin BIZhexacarboxylate in water with 0.100 N NaOH gave a single sharp end point indicating reaction of 5.2 f 0.3 weak acid groups per molecule. This suggests that at least five of the carboxylic acid groups have very similar pKa values. Although not investigated in detail, it is possible that this result is within experimental error of 6, the number of acid groups per molecule. Alternatively, the nonequivalent carboxylic acid group close to the c-ring in the structure may have a larger PKa. The apparent pKaestimated from the pH of half-neutralization was 4.9 0.1. Electrochemistry in Water. Initial survey of the electrochemistry of vitamin B12 hexacarboxylate in water on bare PG and GC electrodes showed that at pH >> pK,, electrode reactions occurred at potentials quite negative of -1 V. Since we wish to develop catalytic f h with relatively positive reduction potentials, further studies were restricted to pH
