CatalyticReductionofIodoethaneand2Iodopropane

CatalyticReductionofIodoethaneand2Iodopropane - Anal. Chem....

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Unformatted text preview: Anal. Chem. 1994, 66. 3117—3123 This Research Contribution is in Commemoration of the Life and Science of I. M. Kennett (1894—1993). Catalytic Reduction of Iodoethane and 2-Iodopropane at Carbon Electrodes Coated with Anodically Polymerized Films of Nickel(II) Salen ChrlstOpher E. Dahrn and Dennle G. Pators' Department of Chemistry. Indiana University, Btoomington. Indiana 47405 In acetonitrile containing tetramethylammonium tetrafluo- roborate. nickel(l.l) salen undergoes anodic polymerization onto a carbon electrode. Nickelfll) in the polymer film exhibits reversible one-electron reduction to form nickel“), which can catalyticall)r reduce iodoethanc or 2-iodopropane to form an ethyl or 2-propyl radical, respectively, and to regenerate nickel- (H ). Kinetics studies with the aid ofhydrodyna mic roltammetry indicate that the catalytic reduction of iodoetbane belongs to the ER regime of Savéant and co-worlters. whereas catalytic reduction of liodopropane is of the S classification. Controlled- potential electrolyses of iodoethane and 2-iodopropane at niche l- (11} salon-coated reticulated vitreous carbon cathodes give product distributions in accord with the relative importance of radical coupling and disproportionation. Direct reduction of iodoetha no at a bare cathode generates products via a carbunion mechanism. Products obtained from direct reduction of 2-iodopropane depend on the potential employed: at a potential corresponding to the first voltammetric ware, product distribu- tions are nearly identical with those obtained from the catalytic red action. whereas at a potential after the second voltammetric wave, the products are derived from the 2-propyl carbunion. A number of studies have dealt with the use of nickclfl) macrocyclic complexes as homogeneous catalysts for the reduction of alkyl halides.“15 In one of the earliest investiga- tions, Gosdcn, Healy, and Pletchcr' demonstrated that the electrochemically generated nickeKI) analogue of [[2,2’-[ l .2- cthanediylbistnitrilomcthylidyne]] bis[phcnolato]]-N,N10.0’}~ nickelfll). hereafter called nickelflll salon, catalytically reduces n-. sec, and terr-octyl bromides. and product distributions obtained from preparative-scale electrolyses of (1) Gordon. (2,; Ilcaly. K. P.: Fletcher. D. J. Chem. Soc. Dalton Trans. 1973. 9'i2—9'lfi. (2] Hcaly. K. F.'. Fletcher. D. J. Orgohomer. Chem. 1918. Mi. 109420. (3) Gordan. (2.; Fletcher. D. J. Orgnnomet. Chem. 1980. i86. dill-409. (4) Becker.J.Y.:Kern].3.:Plctchcr,D.:Rosas.R.J.Electroohoi.Chem.198]. HF. ill-99. [5) Gordon. (3.; Kerr. J. B.: Fletcher. D.: Roses, R. J'. Electroenct. Chem. 1981. H3". I‘ll—10T— (cl Bakes. a: Espcnmn. J. H. J. Am. Chem. Soc. 1986. ms. "HS—1'19. (1') Bakes, Ax, Espenson, J. H. J. Am. Chem. Soc. 1986, 1'08. Tits—1'23, [8) Ram. M. 5.; Eakac. A4 Espensort. J. H. incrg. Chem.1936.25. 326?—32?2. (9} Ram. M. 5.: Espcnson. J. H.; Bakac. A. inorg. Chem. 1936‘ 25. 4| 35—4115. {ID} Espenson. .l. H: Ram. M. 5.; Bakac. A. J. Am. Chem. Soc. 1987. 1'99. 6892~ 6393. (l 1) Rant. M. 3.; Bakac. As. Espensotl. J. H. incrg. Chem. 1986. 2?. 4231—4235. {12) Stolzcnbcrg. A. Md Stcrshic. M. T. Irwrg. Chem. I987. 26. 3082—3083. (13) Stolzenherg. A. M_:Stcrshic. M. T. J. Am. Chem. Soc. 1938. “0. 5391—5403. (14) Stolzcnbcrg. A. M4 Storahic. ELT. J.Am. Chem. Soc. 1983. Hi). 6391—6402. [15‘] Mubarak. hi. 3.; Peters. D. G. .l. Electroanal. Chem.1991. 332. 111-134. UDGG-ETODFQ‘JUSEB-al17$04.50!D 1994 Amarlcan Chomlcal Society these alkyl bromides were consistent with the intermediacy of an alkyl radical. In the mid-19805. Bakac, Espenson. and CO-WOl'kchfi_” used ultravioletrvisiblc spectros copy to explore the kinetics and mechanisms of the catalytic reactions between alkyl halides and electrochemically or photochemically gener- ated nickelfl) macrocyclic complexes. Stolzcnberg and Stershic'l'” have examined the catalytic reduction ofseveral alkyl halides with an electrogencrated nickclfl) tetrapyrrolc derivative to form the original nickehfill) tetrapyrrolc and an alkane. In recent work” in our laboratory, the reductive intramolecular cyclization of 6-iodo- and b-bromo-l-phenyl- l-hcxyne was induced with electrogeneratcd nickelfl) salon to form bcnzylidenecyclopcntane in yields ranging from 84% to 95%. in addition to studies of the homogeneous catalytic reduction of alkyl halides with nickelfl) complexes, several reports‘M‘l of the incorporation of nickeltll) species into polymer films on electrodes have been published. Horwitz and Murray” synthesized 4.4’odi(N-methyl-N-phenylami- nomcthyllnickclfll) salon and oxidativcly polymerized this compound onto the surface ofa platinum electrode. At about the same time. Goldsby and co-workers'llla oxidativcly polymerized nickelfll] salon onto platinum, as well as indium- dopcd, tin oxide-coated glass electrodes; the resulting polymer films were characterized with the aid of ultraviolet—visible spectroscopy and scanning electron microscopy. A few years later. Bcdioui and colleagues19 studied the oxidative polym- erization of nickelfll), cobaltfll), and manganesetIII) salon onto glassy carbon electrodes; most notably. the nickel(Il)— nickeltl) redo-x couple exhibited good reversibility within a polymer coating on an electrode. In the present work. we have investigated the catalytic reduction of iodocthane and 2—iodopropanc in acctonitrilc containing 0.050 M totramethylammonium tetrafluoroborate at glassy carbon electrodes coated with anodically polymerized films of nickelfll} salon. Cyclic voltammetric studies have been employed to establish that catalytic reduction of these alkyl iodides occurs at the filmed electrodes. Voltammetry with a polymer-coated rotating disk, in conjunction with theory developed by Savéant and coworkers?“-11 has allowed us to [15] I-lorwitz, (3.1).: Murray. R. W. Mot. Cry“. U9. Cry“. I988. J60. 389—404. [1?) Goidsby. K. As. Blaho. J. K: Hofcrlcamp. L. A. Polyhedron 1989. 8. 113—1 E5. (13) Hoferlramp. L. As. Goldshy1 K. A. Chem. Mater. 1989. l. 348—352. (19) Bedioui. F.: Lnbbc. E..' Gutierrez-Grenades. 5.: Dcvynck. J. J'. Electrochei. Chem. 1991. 301. 267—21“. Analytical Chemistry. Vol. 66, No. 19. October 1. 19.94 3117‘ Flgura 1. Electrochemical cell used tor oxidatiyo polymerization of nickeltlt} salon and for controlled-potential electrolysos determine the parameters which limit the catalytic processes. Controlled-potential electrolyses have been performed at polymer-coated reticulated vitrcooscarbon disks. in a specially designed electrolysis cell. to identify and quantitatc the produels derived from the catalytic reduction of icdocthane and 2-iodopropane. Finally. we have conducted experiments to determine the robustness of nickcllll} sated-coated rt:- ticulated vitreous carbon cathodes for preparative-settle clectrolyscs. EXPERIMENTAL SECTION Reagents. Acctonitrile { Burdick and Jackson high—purity “LIV” grade]. which was continuously refluxed over calcium hydride. and dimethylformamide {Bardick and Jackson “distilled in glass" reagent;- were employed as solvents for all electrochemical experiments. Tctt‘amcthylunintonium tet— rnl‘luoroborate (TMABFt: Aldrich. Wit.) used as supporting electrolyte was stored in a vacuum oven at £15 “C. All deacration procedures were carried out with Air Produce.- UHF—grade argon. Each of the following chemicals was used as received: le.2’-[ l .2-cthancdiylbislnitrilomethylldynelibis[phenolutoi l- t\-"..'\”.0.0’|nickci{ll] {nickeltln salon: Aldrich. 93W). ictit>~ ethane (Aldrich. 99%}. 2-iodopropanc (Aldrich, 99%], 2,3— dimcthylbutaec (Aldrich. 98%). ethane {Air Products, instrument grade).ethylcne (Air Products. instrument grade). propane (Air Products. instrument grade}. propylene [Air Products. instrument grade). arid n—butanc (Air Products. instrument grade]. An authentic sample of diethyl isopro— pylmalonatc was synthesized by means of an established procedure” from diethyl isopropylidcnemalonalc (Aldrich. 97'??- . Cells. Electrodes. and Instrumentation. Cells for cyclic voltammctry“ and for voltammctry with a Totaling disk electrode}1 have been described in earlier papers. Shown in Figure 1 is a cell designed for two purposes: {a} anodic Ill): AndrieunC P.;Dumes—Houehial.l. MLSMEMLJ. M J Elatimtmel t"th W31. “Ll 33. Ill] Andricus. C. P.; Savant. J. M. In Molecular Hat'ng of Electrode Surfaces; Hurray. R W.. Ed: Wiley: New York. E991: Chapter V. [11] Mullins. 3. J Org. Chem. I966. 3t. 630-622. 12]! ‘r'leira. K. E... fitters. D. G. J'. e'lrctrounot. Chem. £985. :96. 93—10»! 124! Sttntple. J. 2.. f’eters. l). (i J Electrnanui. can». [99“, 286. mil—12! 3113 Answer-eat Chemistry, Vol. 6‘6. No. 19, October I, 1994 polymerization of nickcll Ill salon onto a reticulated vitreous carbon working electrode and lb] preparative-scale controlled- potenlial catalytic reduction of an alkyl iodide at the polymer- cozttcd electrode. For this cell the ratio of working~electrodc area to solution volume is relatively large. and there are two opposing auxiliary electrodes which promote the formation of a uniform polymer film over tlte entire surface ol‘the working electrode, In use, the cell was held upright on u Styrofoam platform that was placed on a Tek-Pro Model R4139-5 variable-speed rotatortoensure adequate mixiagofthe solution in the central compartment. Controlled-potential clectrolysesli‘ and hydrodynamic vol- tarnrnetry3‘1 wore performed as previously described. For both cyclic and hydrtxiynamic voltammctry. we used the some disk- shapcd planar giassy carbon electrode {Model AFDD2UGC. Pine instrument Co.) with an area of 0.459 crnl. For controlled-potential elcctrolyses. reticulasz vitreous carbon disks {Energy Research and Generation}. I cut in thickness and 2.4 cm in diameter. Were used; the procedure employed to fabricate and clean the electrodes before an electrolysis has been published elsetti'ltcri.:.21$ All potentials are quoted with respect to a reference electrode consisting of a saturated cadmium amalgam in contact with dimethylfcrnmrnidc saturated with both cadmium chloride and sodium chloride: this electrode has a potential of “0.76 V Versus the aqueous saturated calomcl electrode at 25 “CPMS Preparation anti Handling of PolymehCosted Electrodes. We employed the same procedure to coat both the large reticulated Vitreous carbon and the small carbondisk electrodes with films of attodically polymerized nickelflll salon. A 2 rob-I solutionol'nickelfl ] ) salon in acctonitrile containing 0.050 M TMABF; was placed into the cell and was dcacratcd with argon for 2&3!) min. Afterthedeaeration period. the solution was allowed to become quiescent while the flow ofargon was continued over the surface of the solution. Then the potential of the carbon electrode was scanned cyclically between 0 and +2.26 V at a chosen rate. Both the scan rate and the number of potential cycles can be adju5ted to vary the thickness ofa film: in this paper. we use the term anodic polymeric-afloat iron to refer to one complete potential sweep from fl to +1.26 to (l V. Next. the eleCtrode was rinsed thoroughly with acctooitrile to remove all solution-soluble monomeric nickel- (ll) eaten. To remove the polymer film front the small carbon disk electrode alter voltarnrnctric studies. the electrode was cleaned with ODS-um alumina on a Master-Tex {Beuhlerl polishing pad. 0n the other hand. the large reticulated vitreous carbon disks were soaked in 6 M hydrochloric acid for l- 3 weeks and then cleaned as described beforetzfi soaking in hydrochloric neid protonates the film. which then becomes soluble in the solvents used in the cleaning process. Procedure for Controlled-Potential Electrolyses. After a polymer—coated reticulated vitreous. carbon electrode was prepared according to the preceding procedure, the electrode was rinsed repeatedly. without being retrieved from the cell. with pureacctonitrilc and with solvent—supporting electrolyte. iii]I’ritts.W.;\.;Vi-:m1.l£ l..: Peters. D. G. Ana! Cheri] 19%. 55. 2I45v2149 lltiIClcary..l.A.;Muh1ralt.M.5.;'\"icira. K. 1..:Andcrson.?t§. R: Peters. III. [3 J. fiffi'efluonal. Chem. 19?”. I03. Ill".r 424 (21": Marple. L. W. AMI. Chem. I967. 39'. “Matti. (as: Manning. C. W; i’urdy. W. C. Anal. {him Am 1970, it. llslvllfi Fresh solvent-supporting electrolyte was added to the cell and deaerated with argon for approximately 15 rnin. Next, the filmed electrode and solvent-supporting electrolyte were preelectrolyzed at a potential («1.00 V) where nickel[ll) is reduced to nickelU}, during which period the argon flow was continued and the current decayed to a steady-state back- ground level. When the prcelectrolysis was finished, the potential of the filmed electrode was reset to 0 V for approximately 10—20 min to ensure complete conversion of nickelfl) back to nickelUI}. Argon flow was then stopped, and the Teflon valve at the top ofthe cell was closed to create a gas-tight configuration. Known quantities of the starting materialand an electroinactiveintemalstandard wereinjected into the cathode compartment of the cell. Immediately, the electrolysis was begun; a typical time required for the current to attain its previous background level was 30-40 min for iodoethane and 15—25 min for 2-iodopropane. Electrolysis products were separated and quantitated according to an established procedure.25 Ethane, ethylene, and nvbutane derived from the electrolysis of iodoethane, and propane, propylene, and 2.3~dimethylbutane formed from the reduction of 2-iodopropane, were identified by comparison of gas chromatographic retention times for the suspected compounds with those of authentic samples. RESULTS AND DISCUSSION Cyclic Voltammetric Studies of Anodically Polymerized Nickelill) Salen. Figure 2A depicts the first thrEe sequential cyclic voltammograms for the anodic polymerization ofnickel- (ll) salen onto a glassy carbon disk in aeetonitrile containing 0.050 M TMABFs. Our observations are similar to those reported in previous investigations“?-19 As the potential is scanned for the first time in. the positive direction, two poorly resolved anodic waves are seen, the first corresponding to oxidation of nickel(ll) to nickelflll) and the second clue to oxidation of the salon ligand itself. On the first negative- going scan, asingle wave for reduction of nickelflll) to nickel- [11) appears. For the second and all succeeding positive- going scans, the polymer film becomes thicker so that the two anodic processes merge into a single wave. We believe that the anodic current is larger than the cathodic current for each complete cycle because the combined oxidations of nickelfII) and the salon ligand obviously require passage of more electricity than reduction of nickelflll} alone. As repetitive potential scans are made at I OOmV 5—1. the anodic andcathodic waves centered at approximately +].‘iI V grow until scans 12—15,at which pointthe thickness ofthefilmimpedes further electron transfer. In agreement with the findings of Hofer- kamp a nd Goldsby, ‘3 we have found that the apparent thickness ofthe polymer film on the electrodcsurfacc increases linearly as a function of the first several anodic scans. Anodic polymerization of nickelill) salon onto a carbon surface probably occurs in a manner similar to that ofthe oxidative carbon-carbon coupling of phenols. as discussed in the literature}9 and Goldsby, Blaho. and Hoferl-tarnp” have concluded that oxidative polymerization of unsubstituted nickelfll) salen takes place via coupling of monomers at the para positions. (29} Oyama. NJ, Ohsaka. T3. Ohnuki. Y.'. Suzuki, T. J. Eiectrochem. Soc. l931, H4. 3068—3073. 400 400 connEur, an 4200 +2.50 +2.00 +1.50 +1.00 +0.50 +0.00 POTENTIAL, V 150 100 chIFIENT. as 0.00 43.25 43.50 -0.T5 4.00 -1 .25 POTENTIAL, v Figure 2. {A} F lretthree cyclic voltammogmms for Boodle polymerization of 2.0 mM nlckeltll} salon onto a glassy carbon electrode (area, 0.459 cm?) In acetonltrlle containing 0.050 M TMABF. at a scan rate or use mV 5'1 from 0 to +2.26 to 0 V. (8} Cyclic voltammogram tor a nlckel— (II) salon-coated electrode (5 X 10"9 mo! cm") In acetonttnle contalnlnn 0.050 M TMABF; at a scan rate of we rn‘v' s" from —0.05 to 4.20 to —0.05 V. Figure 23 displays the cyclic voltarnrnctric behavior in acetonitrile containing 0.050 M TMABH ofa nickelfll) salen- coated electrode {5 X 10—9 molcn‘rz) which was formedthrough one anodic polymerization scan at 300 mV 5’1. As repetitive scans are made with this modified electrode, the current for reduction of nickel( II) to nickelU} slowly decreases. However. ifthe electrode is held att] Vfor 1—2 min, the originalbehavior is restored. Such characteristics have been observed with other polymer films and have been explained through loss of film salvation or through exclusion of electrolyte as the film is cycled through the neutral oxidation state.'6 An in situ ultraviolet—visible spectrum for a nickelfll) salen filrn polymerized onto an optically transparent, indium-doped, tin oxide-coated quartz electrode shows two prominent maxima at 320 and 410 nm. We have verified that the absorption maxima and spectral features fer the polymer-coated electrode are in accord with those obtained in separate experiments With solution-soluble nickelfll) salen and with the free salen ligand in acetonitrile containing 0.050 M TMABFr. In addition. preliminary spectrochemical measurements have demonstrated that nickeifl) is indeed formed upon reduction of nickelUI) in the polymer film and that the nickelllI]-— nickelU) interconversion can he followed in a cyclic vol- tamrnetric experiment. Anle’cai Chemistry, Vol. 6'6, No. 19, October 1, $994 3119 1200 5: son '2 LIJ h 3 400 s ' o — c“! 415 ~10 —1.5 -2.0 POTENTIAL. v 1200 5: E430 D I— 2 LL! E c a see 0 -o.5 -1.o -1.5 -2.o Porsmuv Flats-ea. Gydievohemman'oataecanretoof 100mVs-1for reduction of 5.0 In“ echelons of altyl lodides In acetenltrlle containlng 0.050 MTMABFn (Allied:th and [0) 2-hodopropane at a nickel“ l) salon-coath electrode (5 X 10-9 mol lam—7) and (B) Montana and {0) 24odopropene at a bare glassy carbon electrode. Cyclic Voltammetrie Studies of the Catalytic Reduction of Iodoetbane. Figure 3A exhibits a cyclic voltannnograrn recorded after a 5 mM concentration of iodoethane was introduced into solvent-supporting electrolyte containing a modified electrode. As the potential is swept negatively. an increase in cathodic current (in comparison with Figure ZB) is seen for the catalytic reduction of iodoethane. whereas the anodic current dis appears. When a second successive scan is performed. the catalytic current for reduction of iodoethane is lower. However. the full catalytic activity of the modified electrode is reestablished if its potential is held at 0 V for 5~10 min. One factor which could account for the decrease in catalytic current as repetitive scans are made is that the rate of diffusion of the alkyl halide into the polymer film is slew with respect to the time scale of a cyclic voltarnmetric experiment. A cyclic voltammogram for reduction of a 5 mM solution of iodoethane at a bare carbon electrode in acetonitrile containing0.050 M TMABF; is shown in Figure SE. A single irreversible wave with a peak potential of—l .50 V is observed. which is 500 mV more negative than the potential for the catalyzed reduction at a filmed electrode (Figure 3A). Because the peak current for reduction of l-iodobutane. which is knewn 312D AnaMlcal Chemistry. Vol. 66, Na. 1‘9. October 1. T994 to be reduced to a carbanion in a two-electron proceSS.3°‘33 is similar under identical experimental conditions to the peak current for lode-ethane in Figure 313. we conclude that iodoethaneundergoes a two-electron reduction at a bare carbon electrode. Cyclic Volummetric Studies of the Catalytic Reduction of Z-Iodopropane. Figure 3C is a cyclic voltammogram for the catalytic reduction of a 5 mM solution of 2-iodopropane at a carbon electrode coated with polymeric nickel(II) salen (5 X 10‘9 mol crn'z). We chose to compare the catalytic reductions of 2-iodopropane and iodoethane because Bakac and Espenson‘ reported that the former compound reacts 7.5 times faster than the latter alkyl iodide with nickel(l) macrocyclic species. We hoped to discover a significant difference between the catalytic reductions ofiodoethane and 2-iodopropane at a fihned electrode. For the second sequential cyclic voltammogram for reduction of 2-iodopropane at the modified electrode. the catalytic current was once again smaller than that for the first cathodic potential sweep. A cyclic voltammogram for the direct reduction of 2-iodopropane at a bare carbon electrode in acetonitrile containing 0.050 M TMABF; is shown in Figure 3D. It is interesting that. whereas iodoethane is reduced in a one-step. two-electron process. 2-iodopropane undergoes a pair of stepwise one-electron transfers with peak potentials of -I.30 and —l.80 V. yielding the'alkyl radical and then the alkyl carbanion. A similar mechanism has been observed in a previous study of the electrochemistry oi" 2-iodooctanc.26 Voltammetric Reduction of Iodoethane at a Rotating Poly mar-Coated Disk. To explore the kinetics of the catalytic reduction of iodoethane and 2-iodopropane in the polymer film. the analysis developed by Savéant and co-workersmiz' was employed. Two factors which contribute to the limiting current (:1) on the plateau of a voltammograrn obtained with a polyme r-coated rotating disk electrode are the Lev ich current {fir} and the current (1'12) arising from kinetics processes occurring inside the film, where i..- can depend on three rate- limiting events: (a) the rate of electron diffusion in the film. (b) the rate of diffusion of substrate within the film. and (c) the rate of the catalytic reaction taking place inside the film. To determine which ofthe three rate-limiting events, or which combination of these three events. controls the limiting current (:1) , one must construct a Koutecky—Levich plot and ascertain whether it is linear or nonlinear; in addition. one must determine whether a second wave for direct (noncatalytic) reduction of the substrate through the film appears at more negative potentials. Voltarnrnograms for the catalytic reduction of iodoethane at a rotating disk electrode coated with a nickelUI) salen film (5 X 10-9 mo] cm"1] revealed. as shown in Figure 4A, a second wave for direct [noncatalytic} reduction of iodoethane at the same potential that is required to reduce iodoethane at a bare electrode [Figure BB). This observation places the catalytic regime mos: likely into either the R or ER classification. To ———._—__.__________ (30) Andrieux. C. E; Gellsrdo, l.‘. Saveanl. J. M.: Su. K. B. J. Am. Chem. Soc. 1986.. I03. 638L641 (3| J Andricux. c. P.: Susana. .T. 54.; Su. K. 3.1.Pfilys. Chem. 1935. so, 3315- 382]. {31) Andrieux, C. P.', Gallarda. 1.; Say-dent. J. M. J. Am. Chem. Soc. [939. I”. 1620—1626. (33) Prilts. W. A.'. Peters. D. 6.1. Electroonol'. Chem. in press. 1200 A i '_. 500 Z w o: E u 400 0 0.0 -0.5 4.0 ~1.5 -2.0 POTENTiAL, V 1200 — €- 800 L; _ 2 to II: E U 400 “I 0 0.0 {1.5 *1 .0 -‘l .5 '2 .0 POTENTIAL. V Figure 4. Hydrodynamic vohamrnograms tor the catalytic reduction of a 10 mM solution oi {A} iodoethene at a rotatIOn rate of ‘00 rpm and (B) 2-iodopropane at a rotation rate of 2000 rpm in acetonitrlie containing 0.050 M TMABF; ate glassy carbon disk timed with poiymeric nickeitII] salon [5 X 10" mol cm?) at a scan rate of 100 mt.Ir s". differentiate between these two scenarios, the linearity of a KouteckyuLevich plot for the first [catalytic] wave is examined; a linear plot signifies the R situation, whereas a nonlinear plot denotes the ER case. Figure 5A demonstrates that Koutccky—Levich plots for three concentrations of iodoethane are nonlinear; every experimental point oorres ponds to the average of at least nine measurements at each rotation rate. To verify that the catalytic reduction of iodoethane at a nickeiUI) salon-filmed electrode corresponds to the ER classification. two other parameters can be checked. First, the quotient hazy-“(1‘3 — til—W should be constant at all rotation rates fora given concentration. where i‘L is the limiting current for the catalytic reduction of iodoethane in the film and i; is the current for direct (uncatalyzed) reduction of iodoethane through the polymer film; results displayed in Table 1 establish that this quotient is indeed constant. Second. a Koutecky—Levich plot for the limiting current (1'2) for the second wave should be linear. as is seen in Figure SB. Therefore, these data confirm that the catalytic reduction of iodoethane at a nickelUI) selen-fllmed electrode corresponds to the ER. situation, for which the limiting current is controlled by a combination of the rate of electron diffusion and the rate of the catalytic reaction. Moreover, the catalytic process is confined to a reaction zone which is small compared with the actual thickness of the polymer film and which is adjacent to the electrode surface, suggesting that iodoethane diffuses well into the polymer film to reach the active catalytic sites. 4.5 A 4.0— fa E. as— s I so 0: :l U f/‘ 2.5— 2.0 one on: out cos ooe 0.10 o.12 RPM *112 1.5 1.mA 3 0.5 CURRENT 0.0 0.00 0.02 0.0-4 0.06 0.00 0.10 0.12 FiPM "’2 Figure 5. Koutecky—Levlch plots for (A) catalytic reduction and (E!) direct reduction of (I) 2.5. [I] 5.0. and (A) 10.0 mM solutions of lodoethane in acotonitriia containing 0.050 M TMABF. at a nloicelfll) eaten-coated electrode {5 X 10-" mol cm'zl at a scan rate oi 100 mV 8". Table 1. Hydrodynamic Data for Catalytic Reduction of Iodoothane at a Niekeltlll Salon-Filmed Electrode I'tl'i” 1(fr. - I'Ll'”2 (m0) conc roar; 100 soc i000 1500 soon {li {moi cm") rpm rpm rpm rpm rpm 2.5 5.5 0.30 0.23 0.23 0.29 0.29 5.0 5.5 0.32 0.32 0.33 033 03-1 10.0 5.5 0.37 0.38 0.39 0.42 0.43 Voltammetric Reduction of 2-Iodopropane at a Rotating Polymer-Coated Disk. A voltammogram for the catalytic reduction of 2-iodopropane at a rotating electrode filmed with polymeric nickelfli) salen (5 x 10—9 mol cm‘z) is depicted in Figure 43. Direct reduction of 2-iodopropane at more negative potentials is not observed, which probably classifies this system among the SR. S, E, and S + E regimes. Figure 6A exhibits linear Koutecky—Levich plots for three different concentrations of2-iodopropane. with slopes and intercepts that are inversely proportional to the concentration of 2-iodopr0pane. This behavior narrows the preceding four possibilities to just the S and SR cases, which can be distinguished from each other if one performs experiments With different film thicknesses. For the S situation they-intercept of a Koutecky—Levich plot will be proportional to the film thickness, whereas for the SR regime the y-interccpt will be independent ofthe film thickness. A Koutecky-Levich plot for the catalytic reduction ofa 5 mM AnaMicai Chemistry. Vol. 66, No. 1‘9. October 3'. 1994 3121 l'TIA CUHRE NT 0.00 0.02 0.04 0.06 0.00 0.10 0.12 RPM 452 3.0 i“ or CURRENT '1, mA '1 M a: 1.5 0.00 0.02 0.0-4 0.06 0.08 0,10 0.12 RPM -1f2 Figure 0. KouteckSr—Levlch plots tor the catalytic reduotlon ol 2-lodopropene at (A) a nickelt I I) selen-coateo electrode {5 X 10*” mol crn-‘t tor concentrations of (0} 2.5 [R‘ = 0.95: slope. 9.5; Intercept. 2.0). (I) 5.0 {FF = 0.98: slope. 5.1: intercept, 2.1). and (1] 10.0 mM {FF = 0.98: slope. 3.0; intercept. 1.8) and {B} tor catalytic reduction of e 5.0 mM solution ol 2-iodopropane for different thicknesses of nlckelllll-salen films: (“4.6 X 10-9 in“ = 0.93; slope. 5.5. Intercept. 2.0}. (ll 8.0 X 10"“? = 0.99: slope. 5.4: Intercept. 2.2]. and (O) 3.6 X 10'9 mol crrl'2 {Hz = 0.99: slope. 5.21 intercept. 2.3} In ecetonttrlle contalnlng 0.050 M TMABF... solution of 2-iodopropane at the rotating carbon disk coated with films of different thicknesses is shown in Figure 63. A y-intercept that is proportional to the thickness of the film is observed; thus. the catalytic reduction of 2-iodopropane at a nickellll) salon-filmed clectrodelies in theS regime. for which the limiting current is controlled by the rate of diffusion of 2-iodopropane within the polymer film. Controlled-Potential Electrolyses of Iodoethane. Prepara- tive-scale catalytic reductions ofiodoethane were performed at reticulated vitreous carbon cathodes coated with polymeric nickelUI) salon in acetonitrile containing 0.050 M TMABFt at a potential (—1.00 V} that is 100 mV more negative than the reversible redox potential for the nickel(Il)—nickel{l) couple in the film, Coulometric rt values and product distributions whicharc averages offrom four tosix electrolyses at each of two concentrations are compiled in Table 2. A coulometric in value of 1 was observed for all electrolyses. which is expected if the catalytic reduction proceeds via a radical pathway. We believe that n-butane.the major product. arises via coupling ofa pair of ethyl radicals. whereas ethane and ethylene are formed by disproportionation of ethyl radicals. [t is interesting that the product distributions in Table 2 are 3122 Analytical Chemistry. Vol. 66. No. 1.9. October 1. 1994 Table 2. Coulornetrlc Date and Product Diotxlhutlone tor Electrolytic Reduction of lodoolhane ln Acetonllrlle Containing 0.050 M TMABFt product distribution (lit) n-butanc ethane ethylene total E (V) cont: (li n —1.00“ 5 HS 32 19 "i 103 -—l .005 ii] 0.97 7'2 1? 0 95 4.00” 5 L40 90 ti 3 101 —l.65‘ 5 I.“ 32 55 6 93 “ Polymer-coated electrode (one anodic polymerization scan at 300 111V 5"), 1' With 2.00 X t0-1 M homogeneous-phase nickel(ll) salon. FBate electrode. Table 3. Coulometrlc Date and Product Dhtrlbutlono tor Electrolytic Reductlon of 2—Iodowopene In Acetonltrllo Containan 0.050 M TMABF‘ or In Dlrnetttyltonnernlde Containing 0.100 M TBABF.‘ can: product distribution (50) EN) (li n 2.3-dimcthyibutane propane propylene total 4.005 5 1.10 46 33 3G l09 —l.'00lJ it) 1.12 40 3] 26- 97 —l.00‘ S 1.31T 36 3i 23 95 ~l.-l5‘Jl 5 1.48 32 33 29 94 —l.90‘II 5 1.36 1?. 32 41 35 4,90“ 3 i156 20 52 18 10E-l ” TBABF: is tetra-n-butylammonium tetral'luoroborate {usedonly for electrolyses at —l.90 V). ” Polymer-coated electrode (one anodic polym- erization scan al 300 mV 5"]. *' With 2.00 X 10—3 M homogeneous-phase nickelfll) salen. d Bare electrode. ' With 80 told diethyi malonate. fincludes l 1% diethyl isopropylmalonate. in accord with experimental measurements in homogeneous media,“ where the rate for ethyl-radical coupling is ap- proximately 0—8 times greater than the rate of dispropor- tionation ofethyl radicals. Table 2 provides a comparison of the catalytic reduction oi" iodoethane by clectrogenerated homogeneous-phase nickelU) salen with the catalytic reduction of iodoethane at chemically modified electrodes. Product distributions from both kinds of' electrolyses are included in Table 2 and show good agreement with each other. Data from electrolyses of iodoethane at bare reticulated vitreous carbon electrodes in acetonitrile containing 0.050 M TMABF.‘ are shown in Table 2. These electrolyses were performed at a potential of —l.65 V. which is 650 mV more negative than the potential required for thecatalytic reduction ofiodoethane at a nickclUI) salen-modified electrode. When iodoethane is reduced at a bare electrode. the products are ethane, rtsbutane, and ethylene. in order of decreasing abundance. This distribution of products. along with the observed coulometrlc a value of 2. signifies that the pathway for direct reduction of iodoethane at at its re carbon surface no longer involves a one-electron radical pathway but a two— electron process to give a carbanion intermediate. Controlled-Potential Electrolyses of 2~Iodopropane. Elec— trolyses of2—iodopropane at large reticulated vitreous carbon electrodes filmed with polymeric nickelUI} salen in acetonitrile containing 0.050 M TMABH were performed at --l.00 V. Table 3 displays coulometrio n values and product distributions for electrolysis of 5 and 10 mM solutions of 2-iodopropane, Products derived from the catalytic reduction of 2-iodopropane are 2.3-dimethylbutane, propane. and propylene. and their {34] Gibian. M. J.'. Carley. R.- C. Chem. Rec. 1973. 3’3. 441—464. yields are in accord with observed rates of radical dispro- portionation and coupling. with disproportionation being favored slightly over coupling by a factor of 1.2.3'1 We compared the product distributions obtained from reduction of 2-iodopropaneat a chemically modified electrode with those derived from reductions with nickelU) salen electrogenerated homogeneously in aCetonitrile containing 0.050 M TMABFi. As the data in Table3 reveal, the yields of2,3-dimethylbutane, propane. and propylene formed in the two kinds ofelectrolyses are nearly indistinguishable. Electrolyses of 2-iodopropane at bare reticulated vitreous carbon cathodes. in acemnitrile or dimcthylformamide con- taining 0.050 M TMAB F... were performed at potentials (—1 .45 and —1.90 V) corresponding to the first and second voltam- metric waves. respectively. We expected that products from electrolyses at —l.45 V would arise via a radical pathway1 whereas products from electrolyses at —l.90 V would be generated through an anion route. Product distributions and coulometric n values for these experiments are presented in Table 3. At a potential of—l .45 V, the yields ofthe products (2.3-dimethylbutane. propane. propylene] are similar to those obtained from catalytic reductions at a filmed electrode. and it appears that these species are indeed formed through a one-electron process. In contrast to results obtained at —l.45 V. one finds that. at a potential (—1.90 V) on the second voltammetric wave. the yield of 2.3-dimethylbutane decreases. thequantityofpropylene increases, and theamount ofpropane remains essentially unchanged. It is notable in the fifth entry of Table 3 that the absolute yield of propylene is greater than that of propane. Propylene is probably produced through a base-promoted dehydrohalogenation of starting material; We believe that this process involves deprotonation of adventitious water by a 2-propyl carbanion (or an n-butyl carbanion arising from the tetra-n-butylammonium cation} which is electro- generated at —1.90 V and that the resulting hydroxide ion attacks starting material to afford the olefin. Thus, a deliberately added proton donor should react with hydroxide and should hinder the formation of propylene, and as a consequence, more starting material will undergo direct electrolytic reduction. Table 3 shows the results when diethyl malonate was added to a solution of 2-iodoprcpane electrolyzed at —1 .90 V. We found that the yield of propylene decreased from 41% to 18% and that the amount of propane increased from 32% to 52%. These findings provide strong evidence that carbanions are generated in etectrolyses of 2-iodopropane at a bare electrode held at wl.90 V. Robustness of Anodically Polymerized Films of Nickelfll) Salen on Reticulated Vitreous Carbon Electrodes. Ifa polymer~ coated electrode is to be employed successfully for practical preparative-scale electrolyses, two requirements must be met. First. the polymer film must adhere strongly to the electrode surface, and second, the polymer film must permit repetitive catalytic reductions to be performed over an extended period of time. To investigate how well a Film of anodically polymerized nickelt Ii) salen adheres to the surface of reticulated vitreous Table 4. Coulometrlc Date and Product Dlotrlbutlonl Ior Repeated Catalytic Reductlonl of 5 mM Iodooihano tn Acetonitrlle Containan 0.050 M TMABF. at a Nickel!!!) Salon-Coated Elactrodo‘ product distribution (%} elapsed time {till a u-butane ethane ethylene total 0 1.05 82 [9 i" 103 24 1.00 58 3] 7 96 48 1.40 54 4-1- ? 105 "See text for details of these cxggrimcnts. 5 Measured from the time that the reticulated vitreous car n electrode was first coated with unodically po ‘ymerized niekelUI) salen [one anodic polymerization scan at 300 mV 5— J. carbon. cyclic voltammograrns were recorded aftera polymer- coated electrode was held at —l.() V for various periods of time. No significant decreascinthe cathodic and anodic peak currents was observed after 101 20, 40, and 60 min. implying that there is no loss ofnickelllll and thus ofthe polymer film. To probe the ability of a filmed electrode to be used for repetitive electrolyses, two kinds of experiments were per- formed. First, several samples of iodoethane were injected sequentially [after catalytic reduction ofthe preceding sample was complete) into the cell, and the current—time behavior was monitored for each electrolysis. Each electrolysis produced similar current—time behavior. which suggests that the catalytic properties of a film do not diminish after at least several electrolyses. Second, another series of experiments involved removing the electroiyzed solution from the cell after a catalytic reduction, rinsing the electrode with pure aceto- nitrile and then with solvent-supporting electrolyte, adding fresh solvent-supporting electrolyte. decorating the cell with argon. and leaving the cell sealed overnight. On the following day. a fresh sample of iodoethane was injected into the cell. another electrolysis was carried out, and the product distribu- tion was determined and compared with that of the preceding day. Table 4 shows coulometric a values and product distributions for catalytic reduction of iodoethane at a polymer- coated reticulated vitreous carbon cathode on three consecutive days. Our results indicate that the yield of a-butane drops after the first day and remains eSSentially constant thereafter. that the quantity of ethane rises in absolute terms ap- proximately 12’1'6 per day, and that the amount of ethylene does not change. A nickelfli) salon-coated electrode can be used repetitively over several days for catalytic reductions of iodoethane, but we cannot yet explain why the producr distribution changes with time. ACKNOWLEDGMENT Scientific Parentoge ofAuthors. C. E. Dahrn.Ph.D. under D. G. Peters, PhD. under J. J. Lingane, PhD. under I. M. Kolthoff. Hecelvod tor rovlow March 1. 1994. Accepted June 2. 1994.9 ' Abstract published in Aduunce AC5 Abstracrs. July 15. 1994. AnaMtcatChomisrry, Vat 65, No. 15‘. October 1. 1994 3123 ...
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CatalyticReductionofIodoethaneand2Iodopropane - Anal. Chem....

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