coiicludv that only one out of ten encounters betwecan I ) ' a n d 0,leads to reaction. These calculatetl lifetiiiies delilolistrate that oxygen reacts with the longlived iiietastahle species 11' rather tlmi with the first cxcitctl siiiglet species I)*. Although oxygen quenched tliis fluorescence of chlorophyll,3'1 this step d o e s not appear to be of importance for clilorophyllin-sensitized ~~Iii~tocisitlatioii. The data for the chlorophyllir~deprii(leiicts sliows that the induced ticcoriiposition. stel, I:$, i s 275 times iiiore rapid than the oxidation of the sulistrate, step 1.i. I ! i i ~ ( I i i i i ( (it' chloroph~-lliiiti to PI-P corisiderahly ciiliaiices its ~ ~ l ~ o t o r e d u c tas i o ~shown i i n the photosensitizrd retluction of fast red S just as is the case for For sensitized photooxidation the cffcct oi liititliiig is less prcinouiiced because diffusion ut' cisygen to t h e excited inolecule is retardetl by the cii~-elopiiig residues of the high polynier.li
[ ~ O \ l ~ R l B L l T I O FSK l l ' r l TIIE
1) E P A K I ' I I [ i \
I ( I F C I I E \ I I S l R Y , S I A N F O R I ) L-SIVEKSIT'Y, S T A S F O K I > , CALIF., A N D I ) I V I S I O S O F M A T E K I A L S
SCILChCL'5, S I A Y F O K I ) f < E s E A K C l l I S S l I l r T E ,
hIESI.0
P A R K , CALIF.]
Electron Transfer through Organic Structural Units. Aromatic and Heterocyclic Carboxylates as Bridging Groups in Oxidation--Reduction Reactions
exaniined. Moreover, scant attention has been paid to Although the effects of ring ~ ~ i l ~ s t i t i i ton i o nthe reducl)eii~cintcil)erit~ialrlnliiiiiiicc~~I~~i!t i 111) c o i i i p l e ~ ~ ~ slieteroc>-clic carboxylato groups as "electron-transfer bridges" in such reactions. In the oxidation-reduction have I)ceii itivesti,qated i l l soiiie detail.' a ~iurnbcrof reactions comprising the present study, only one repotentiiilly instructive substitueiits ha\-e not yet tleen ~ . ~ , , ~ S~e n~i u ri po,t,i,,ctc,ral ~ t i ~ ~ ~ V ~ I I , , ~ , ,q t J n r < l r c i CiuciIlg agent, Cr'+, has been used, b u t v~ have introi lditl,,nai I ' n i \ c r ~ i t v Nfj2 I duced considerable variation in the carboxylato group t i o n oi
-\j)ril 3 . lR(i-1
CARBOXYLATES IS OXIDATION-REDUCTION REACTIONS
plexes derived from furan-, thiophene-, pyrrole-, pyrThe most inrolidine-, and pyridinecarboxylic acids teresting ligands in this series appear to be those having a carbonyl group or a pyridine ring, and those which may form a chelated chromium(II1) product
Experimental Aquopentaamminecobalt(II1) Perchlorate, [Co(NH, iiHI.O!coniples was prepared from carbonatopentaaniiiiineciibalt( 111 1 nitrate, which was, in turn, prepared by air oxitlaticin of a solution of Co(SO3)2, SH?, arid ISH,)?COJ.,' I n our hands, preparation of the nitrate was capricious, for the product I\ as {Iften heavily contaminated \%-it11the carbonatotetraarninine cornpie\, coloring it maroon rather than the correct brick red. The best preparations were obtained when the air oxidation was very slow. The carbonato nitrate was converted to the aquo perchlorate by dissolving in hot water, then adding cilnci-ntrated HC10, until effervescence ceased and further addition caused no further precipitatioii. An additional treatment with HCIO, was needed to remove the last traces of nitrate from the brick red complex. The pure aquopentaarrimine complex, when heated with concentrated HCI, gives the purple precipitate, [Co(S H dljCI]Clr, and an a l m ( ~ s tcolorless solution; a sample contaminated with the diaquotetraammine coniplex will yield a blue solution (CijC142-). Additional Inorganic Materials.-Chloropentaamrninecobaltf I1 I ) perchlorate was prepared from the corresponding chloride, [Co(SH31iC1]C1?,by dissolving the latter in hot 0.1 .\I HCIO,, adding one-third the volume of 6 0 5 HCIOd, then cooling. The desired perchlorate slowly crystallized and was filtered off, washed with ice water, and dried in air. Chromous solutions were p r e p a r d by reducing a master solutioii o f cliromiutn( 111i perchlorate, made up by adding the hesahydrate to WC, HCIOc, then diluting tenfold with boiled distilled water. T h e reduction was carried out under nitrogen with zinc arnalgani, and the solution was stored over zinc amalgam i n a serun-capped vessel. Samples were withdrawn with a volumetric syringe. The concentration of the solution was determined by adding a known volume to an excess of chloropentaarnniinecobalt(111) perchlorate solution under nitrogen, diluting tenfold with concentrated HCI, and reading the absorbance a t 692 n i p ( t 560) of the cobalt(I1) formed. Sodium perchlorate solutions for kinetic experiments were prepared by neutralizing reagent grade €107~ HCIO, with primary standard S a ? C 0 3 ,boiling off the CO? formed, and then diluting to the required volume with boiled distilled water, Organic Acids.-Commercially available acids were used as received, o r , if badly discolored, after crystallization from methanol or water. Monometh\-l terephthalate was obtained as a research sample from Hercules Powder Co.? 2,2'-Thiodibenzoic acid was prepared from the sodium salts of thiosalicylic acid and o-chlorobenzoic acid.j o-Phenoxybenzoic acid resulted from metalation and carbonation of diphenyl ether by the procedure described by Gilman and Oita for preparation of 2,2'-oxydibenzoic acid,6 m . p . 112" ( l i t . 6 114O). 2-Methyl-2'-carboxydiphenyl ether was prepared from ochlurobenzoic acid, o-cresol, a n d sodium methoxide.' The reaction mixture was kept a t 210-215' under nitrogen for 20 min., cooled, and then dissolved in 5Tc S a O H . T h e acid and excess cresol were precipitated with HCI, redissolved in N a H C 0 3 , and the cresol extracted with ether (five times the volume of aqueous solution). The acid was reprecipitated with HCI, then recrystallized from E t O H . X 2.3-g. portion of the acid was then dissolved in 20 ml. of 2yc S a O H , heated to 95", and carefully oxidized with 1.6 g. of K M n 0 4 . The mixture was allowed to stand overnight, the permanganate color discharged b>-addition ( ClO, ;,j.--This
( 3 ) F. Basolo and R K Xlurmann, Inorg S y n , 2 , 1 7 1 ( 1 R 4 6 ) 1 4 ) a ' e t h a n k Dr H G T e n n e n t of Hercules Research C e n t e r , Wilmingt o n . Del , for this sample ( 5 ) H. Gilman and D L E s m a y , J A m . C'hem Soc , 76, 2 8 1 (1953). Reasonable yields of this acid were obtained only if t h e reaction mixture was melted a n d kept a t 270' for 10 m i n . , r a t h e r t h a n t h e recommended :min a t 250'. Success in converting this acid t o t h e p e n t a a m m i n e c o b a l t ( l I 1 ) complex was achieved only when t h e acid was free of yellow contaminating matter. (6) H. Gilman a n d K. Oita, ibid., 79, 339 (1957) 17) F. Ullmann a n d M. Zlokasoff. Ber., 38, 2111 (1905)
1319
of ethanol, the M n 0 2 filtered off, and the resulting 2,2'-oxpdibenzoic acid precipitated with HCI, after which it was recrystallized from glacial HO.1c. -l-~~ethyl--l'-carboxydiplienvl ether was prepared, with some difliculty, from p-cresol and p-chlorobenzoic acid, using an analogous procedure. However, after removal of solvent a t 130", the reaction mixture was pulverized, then kept under nitrogen a t 310-320° for 15 inin. The acid obtained by a work-up similar to that described for the 2,2'-product was oxidized with KMnO,, yielding roughly equal amounts of the desired 4,1'-oxydibenzoic acid and p-chlorobenzoic acid. The two acids were separated by dissolving in boiling methanol and then cooling, whereupon the dibasic acid crystallized. o-nletliyltIiercaptobenzoic acid was prepared by methylating the sodium salt of thiosalic)-lic acid with dimethyl sulfate. 4,l'-L)ithiodibenzc,ic acid was prepared from p-aminobenzoic acid b)- diazotization and coupling with Sa&, using the procedure described for the 2,2'-isomer,b then reduced to p-niercaptobenzoic acid using sodium hyrosulfiteY (hood!). After refluxing for 1 hr. the reaction mixture was cooled and acidified. The precipitated acid was filtered off, pressed dry, arid separated from elemental sulfur by dissolving in hot ethanol and filtering. Sitrogen was bubbled through the ethanolic solution for 30 inin. to remove HzS, the solution dried with anhydrous K ? S O I , and the ethanol removed in mcuo. hiudibenzoic acid was prepared from the salts of pchlorobenzoic acid and p-mercaptobenzoic acid, using a procedure analogous to t h a t for the %,2'-isomer.j The reaction mixture was kept a t 300-320" for 10 min., cooled, and extracted with lo?& NaOH . Acidification precipitated a crude green acid, which was crystallized from methanol, giving a white acid, n1.p. 295', in 155; yield. Anal. Calcd. for Cl4H,~O4S: C, 61.2; H , 3.7; S, 11.7. Found: C, 61.3; H , 3.8; S, 11.9. Furan-3,4-dicarboxylic acid was prepared by saponification of its diethyl ester.'" Thiophene-~,~-dicarboxjIicacid diethyl ester was prepared from P2Ss and diethyl l-fornijI-2-diethoxymethylsuccinate10 by refluxing for 4 hr. in toluene (hood!)," then saponifying to its parent acid. N-Metliyl-2-carboxyp~ridiniurn perchlorate and the corresponding 3- and 4-carboxy isomers were prepared in solution by methylation of the appropriate pyridinecarboxylic acid with methyl iodide.'* The unreacted methyl iodide \vas removed in zlacuo, the iodide precipitated with a slight excess of AgCIOl, and the excess silver ion precipitated by slow addition of saturated SaCI. The preparation was filtered, concentrated to about 2 N by rotary evaporation, then acidified to pH 3-1. The resulting solution was used for preparation of the cobalt complex. An analogous procedure with pyridine-2,j-dicarbosylic acid led to S-tnethyl-2,5-dicarbox~-pyridiniu111 perchlorate solution, but pyridine-2.6-dicarboxylic acid resisted methylation, Preparation of the Complexes.-The general procedure, applicable t o well over half of the complexes studied, involved treating 20 Inmoles of the parent acid with 10 ml. of 1.75 NaOH, heating a t 80" for 10 min., arid then filtering. T o the filtrate, which should have a pH value less than 6.5, was added a solution of 1.0 g. of aquopentaamminecobalt(II1) perchlorate in 2 ml. of hot water. The mixture was kept a t 75-80" for 2 hr. ( a bath of refluxing ethanol was convenient here), then cooled, and added to a large separatory funnel containing 5 ml. of water, 1.9 mi.of 9 F HCIOI, and 150 ml.of ether. The unchanged parent acid precipitated, but dissolved in the ether layer o n shaking. After a 15-inin. wait, the layers were separated, and the aqueous layer was re-extracted with another 150 ml. of ether. Ether was then removed from the aqueous layer by drawing air through for 10 niin., 10 rnl. of 9 F HC104 was added, and the preparation was allowed to stand a t -10" for 2 hr. The precipitated complex was filtered off, dissolved in a minimum quantity of hot water (XO"), and the solution cooled to 0". The recrystallized complex was then filtered off and dried in Vacuo a t 35' for 10 hr. Ik'ashing with ethanol or methanol*cwas not advisable in most cases since many of the complexes dissolved readily in the alcohols when ( 8 ) C . F. 411en a n d D D M c K a y , "Organic Syntheses,' Call Vu1 11, John Wiley a n d Sons, Inc , S e w E'ork, X J' , 19.13, p 580 ('3) C . Hansch and H. G Lindwall, J Org C h r m , 10, H8:3 (1'345) (10) We are indebted to Dr Reuben G. Jones of Eli Lilly a n d Co fur generous gift samples of furan-3,4-dicarboxylic ester and diethyl l - f o r m y l - 2 diethoxymethylsuccinate. (11) E C. Kornfeld a n d R . G. Jones, J . Oug. Chenz., 19, 1674 (1954) (12) H. M e y e r , Monalsh., 2 4 , 199 (1903).
1320
EDWIK S . COULD A N D HENRY TAUBE
slightly damp. The most troublesome impurity was the parent carboxylic acid, which often persisted, even after several recrystallizations but, fortunately, was shown not t o affect our kinetic measurements. T o remove the last traces of parent acid, the complex was dissolved in hot water, saturated NaHC08 solution added dtopwise t o p H 8, and the solution quickly cooled; this procedure was not applicable t o complexes of dibasic acids. Yields tend t o be small (often less than 20y0), with the greatest losses occurring during the final recrystallization(s); somewhat higher yields, sometimes as great as 50y0, were obtained from complexes derived from furan-, thiophene-, and pyridinecarboxylic acids. T h e general procedure was modified in a number of instances. For acids having basic nitrogen atoms (an amino group or a pyridine ring), the complex was precipitated with saturated NaClOt, rather than HC104, unless the hydroperchlorate was desired. If the acid or its salt was easily oxidized by air or by tripositive cobalt (aniline and phenol derivatives), reaction conditions were milder (60' heating for 3 hr., keeping p H below 5 ) . The anions of a number of the acids (0-iodobenzoic, 2,2'-diphenic, and 2 2'-thiodibenzoic acids) formed insoluble salts with the Co(NH3),H203+cation; in such cases, the mixture was heated a t 100" for 3 hr., then worked u p in the usual fashion. With very insoluble acids ( e . g . , anthraquinone-2-carboxylic, p-bromobenzoic, and 4,4'-thiodibenzoic acids), treatment with NaOH yielded soluticins having p H above 7, and heating such solutions with the aquocobalt complex resulted in precipitation of brown cobaltic oxide and release of ammonia; in these instances, reaction was carried o u t a t 55" f o r 16 hr., the mixture filtered t o remove the cobalt oxides which inevitably formed, and the filtrate acidified and repeatedly extracted with ether before precipitating the complex. With a number of the dicarboxylic acids, 16 ml., rather than 10 tnl., of 1.75 N NaOH was used, converting the acid t o a mixture of its mono- and disodium salts; the complexes formed from such solutions often precipitate from the hot reaction mixture as sparingly soluble carboxylate salts, b u t may be converted to perchlorates b y heating with 2 M HCI04. In preparing complexes from 1,3,5-benzenetricarboxyIicand 1,2,4,5benzenetetracarboxylic acids, enough NaOH was added to neutralize all but one of the carboxyl groups, and after treatment with the aquocobalt complex, the solution was passed through an anion-exchange column (BioRad AGI-X4) t o remove the unreacted carboxylate anion. Removal was incomplete, however for subsequent acidification precipitated significant quantities of the carboxylic acid (anion-exchange resins did not appear t o be effective in removing anions of dicarboxylic acids). For very soluble complexes ( e . g . , those derived from levulinic acid, anthraquinone-2-carboxylic acid, pyrrolecarboxylic acid, and proline), treatment of the reaction mixture with HC104 and ether was omitted; instead, the mixture was evaporated ( a t 60-65") t o a volume of 3-5 ml., a n equal volume of saturated NaCIOd solution added, and the solution cooled t o -5". In a few cases (complexes from sebacic acid and the benzoylbenzoic acids) a second liquid phase, purple and viscous, separated from the reaction mixture. This contained the desired complex and much unchanged carboxylic acid, and was dissolved in hot 90% methanol, quickly filtered, then added to the water-etherHCIOI mixture in the usual way. Special care should be used in working up benzoato complexes having an aldehyde or keto group meta or para t o the carboxylate, for these undergo hydrolysis surprisingly rapidly. After the excess of parent acid was removed, the preparations were cooled rapidly, and the subsequent recrystallizations were carried out quickly so t h a t the solutions did not remain above 30" for more than a few minutes; the solid complexes, however, are quite stable when d r y . T h e 2,5-dihydroxybenzoato complex is less stable, t h e crystalline solid becoming black after standing in air a few minutes at room temperature; for this complex, drying and weighing steps were omitted, and kinetic measurements were carried o u t o n solutions immediately after preparation. Table I lists 38 complexes giving satisfactory analyses. In only one case, t h e complex of 1,2,4,5-benzenetetracarboxylic acid, did t h e analysis indicate t h a t a dinuclear complex had been isolated. In addition, mononuclear complexes from the following 12 acids were prepared, b u t were contaminated with from 5 t o l5Yo of the parent acid: p-aminobenzoic, o-thiomethylbenzoic, 1,3,5-benzenetricarboxyIic, 4,4'-oxydibenzoic, 2-o-cresoxybenzoic, an thraquinone-2-carboxylic,3,4-furandicarboxylic, 3,4-thiophenedicarboxylic, 2-pyrrolecarboxylic, glycine, proline, and sebacic. Kinetic studies of the reductions of these impure acids were carried
VOl. 86
out, but final purifications were not, either because the quantity remaining was too small or because the complexes exhibited uninteresting kinetic behavior. Several attempts were made to prepare complexes from each of the following acids but without success: anthranilic, N-acetylanthranilic, terephthalic, 4,4'diphenic, 3,4-dihydroxybenzoic, oxaloacetic (this decarboxylated), 2,6-dihydroxypyridine-4-carboxylic, r-pyrone-Z,B-tiicarboxylic, 3-acetyl-2,4ditnethylpyrrole-5-carboxylic, 3-indoleacetic, and ferrocene-1,l '-dicarboxylic.
TABLE I ANALYSES OF CARBOXYLATOPEKTAAMMI~ECOBALT( 111) PERCHLORATES, R e o ( NHl)a(C104)z ~
R
--
(/c
Calcd
o-Fluorobenzoato 0-C hioro benzoato o-Bromobenzoa to o-Iodobenzoato Salicyla to
12 11 10 10 12
m-Hydroxybenzoato p-Hydroxybenzoato o-Methox ybenzoato m-M ethoxybenzoato p-Methoxybenzoato
2 8
9
Cu I'ouod
12 3 11 4 10 8
0
9 5
3
12 1
12 12 11 11 11
3 3 9 9 9
11 11 12 11 11
8 9 0 5 7
2,4 Dihydroxybenzoato 2,6-Dihydrox~-henzoato o-Phenoxybenzoato p-Xitrobenzoato p-Carbomethoxybenzoato
11 11 10 11 11
9 9 7 6 3
11 11 11 11 11
8 5 1 4 3
n i - Aminobenzoato o- Acetyl benzoato o- Benzoylbmzoato p-Benzoylbenzoato p-Formylbenzoato
12 11 10 10 12
3
12 11 10 10 12
0 5 3 5 1
2-Hydroxy-5-formylbenzoato 2,2 '-Oxydibenzoato
11 7 9 9
11 4
2,2'-Thiodibenzoato 2,2'-Dicarboxydiphenyl 2-Furoato
9 6 10 1 13 0
9 5 9 6 12 9
2-Thiophenccarhoxylato 3-Thiophenecar boxylato 2-Pyridinecarboxylnto 3-Pyridinecarboxylato 4-Pyridinecarboxylato 2,6-Dicarboxypyridine 2,5-Dicarboxypyridine (He104 adduct)
12 12 12 12 12 11 9 53
(ClOa) N-Methyl-2-carboxypyridinea N-Methyl-3-carbox ypyridine" N-Methyl-4-carboxypyridine' N-Methyl-2,5-dicarboxypyridine"
(ClOa) Levulinato 4,4'-Thiodibenzoato ( H )
(C) 1,2,4,5-Renzenetetracarboxylato(binuclear) (H) (C) a
7 4 4 0
9 5
6 6 7 7 7 7 6 0
12 12 12 12 12 11 9 53
5 5 0 6 6 7 1
10 1 10 1 10 1
48 8
10 10 10 9 48
0 1 0 7 2
12 9 3 9 27 4
12 8 4 1 26 6
3 6 12 8
3 1 12 9
9 5
4
Triperchlorates.
Spectra.-Each pure complex t h a t was examined showed an absorption maximum a t 502-503 mM. The e-value for this peak lay very close t o 80, irrespective of the nature of the substituents (phthalato, 79.5; p-methoxybenzoato, 78.0; p-phenoxybenzoato, 80.5; p-nitrobenzoato, 78.8; 4-pyridinecarboxylato, i7.9), the two exceptions being the p-hydroxybenzoato complex ( e 90.0) and the 2,2'-dicarboxybiphenyl complex ( e 55.0). A
April 5 , 1964
1321
CARBOXYLATES IN OXIDATION-REDUCTION REACTIONS
second peak, near 350 mp, present in the spectra of the phthalato and isophthalato complexes, is absent from the spectra of most benzenemonocarboxylato complexes, b u t is observed in the pyridinecarboxylato complexes ( e 64). A striking and reversible change occurs when a complex having a hydroxy group ortho or para t o the coordinated carboxy group is treated with base. The color changes from pink t o bright orange, and two of the absorption maxima (lying, for example, a t 503 ( e 76.0) and 235 m p ( e 22,000) in the o-hydroxybenzoato complex) disappear. The extinction coefficient for a third peak, near 298 mp, is little changed on addition of base ( e 4400 in acid, 3900 in base), but the peak is much broader in base. S o such effect is observed with m-hydroxy compounds. Analyses.-Cobalt analyses were carried o u t by adding 2.5 ml. of 0.1 S S a O H to a 10-30 mg. sample of the complex. The mixture was boiled, allowed t o cool, then brought t o a boil once more. I t was then diluted to 25.00 ml. with concentreted HC1, any residue removed by pressure (not suction) filtration, the solution transferred t o a 1-cm. cell, and the absorbance a t 692 nip read. The estinction coefficient for Co(1I) in this medium is about 560, but is a rather sensitive function of the exact concentration of HC1 used. T h e procedure was therefore standardized internally for each set of runs, using a pure sample of the aquo comples. This method was exceedingly rapid and, with internal standardization, gave values reproducible t o better than 1yG. For perchlorate analysis, a 15-mg. sample of complex was dissolved in 15 ml. of a solution 2.5 .%fin S a C l and 0.1 Jf in HCI. T!ie mixture was heated t o go", and a twofold excess of tetraphenylarsonium chloride in water was added slowly with vigorous stirring. The mixture was cooled, allowed t o stand 2 hr., and the precipitated tetraphenylarsonium perchlorate filtered off, washed with 10 ml. of ice water in small portions, dried 2 hr. a t l l O " , and weighed. Kinetic Experiments.-Rates were evaluated by following the decrease of absorbance a t 502 mp. Samples were dissolved in 5.0 ml. of warm water or hot methanol, diluted t o 25.0 ml. with 3.2 Jf SaCIO?, with 3.2 .If HC104,or with mixtures of the two, and transferred t o a 10.0-cm. cylindrical cell. Xitrogen was bubbled through for 10 min., and a known excess of Cr(I1) was added. Generally, good pseudo-first-order plots were obtained, and in those cases where successive runs with different Cr(I1) concentrations were made, first-order dependence on Cr(I1) was confirmed. R a t e constants from several points in a single run were reproducible t o better than 5%, whereas those obtained from different runs checked to within about 87c. LVhen an acid dependence was suspected, runs were carried o u t a t a t least three acidities lying between 0.01 and 3.0 M . R a t e experiments were generally carried out with p = 3.0, but a number of complexes (particularly those difficultly soluble in 3 M perchlorate) were examined a t p = 1.0. For those complexes studied a t both ionic strengths, increasing p from 1.0 t o 3.0 was found t o increase the specific rate of reduction by 15 f 2Yc. The presence of 5 ml. of methanol in 27 ml. of solution lowered the rate only slightly (about 47,). The reduction of the p-benzoylbenzoato complex a t concentrations greater than 8 X 10-4 AI resulted in precipitation of a white crystalline solid, which was filtered off and dried. This melted a t 193" and did not depress the melting point of the pure parent acid. Stoichiometry Experiments.-A sample (15 to 30 mg.) of the complex was dissolved in 2.5 m l . of boiled distilled water or 1 M HClO,, the solution purged with nitrogen, and 0.50 ml. of a solution 0.9 21 in HClO4 and 0.046 ,M in Cr(I1) was added. After 20 min., the solution was diluted t o 25.0 ml. with concentrated HCI, the absorbance read a t 692 mp, and the amount of Co(II1) reduced calculated from the amount of Co(I1) released. Reduction of Acids.-Samples (8 m g . ) of organic acids were dissolved in 2 ml. of boiled water ori n methanol; nitrogen was bubbled through the solutions t o remove 02, and 0.50 ml. of 0.046 .if Cr(I1) added as above. The solutions were allowed t o stand 40 min., after which a 0.50 Af solution of chloropentaamminecobalt( 111) perchlorate was added through the cap (syringe) until the maroon color persisted. The solutions were then uncapped and diluted to 25.00 ml. with concentrated HCI; the absorbance a t 692 mp was read and the quantity of Cr(I1) not consumed by the organic acid calculated from the Co(I1) produced. Some air oxidation of Cr(I1) always occurred, for even when no organic acid was used, the Co(I1) measured corresponded to only 85-90% of the Cr(I1) added. T h e figures summarized in Table 1-11 are corrected for this loss.
Results Spectra of the Products.-Normally, the products formed from reduction of a carboxylatopentamminecobalt(II1) complex with Cr(I1) in acid are Co2+, NH4+, and the carboxylato complex of Cr(II1). When there was reason to suspect t h a t this was not so, a spectrum of the resulting solution was taken, and the spectrum of the chromium(II1) product obtained by subtraction of the known absorbencies of C o 2 + and (excess) Cr2+. Table I1 compares the spectra of a number of these products with the spectra of known Cr(II1) complexes. Data for two of the maxima, one TABLE I1 ABSORBANCIES OF Cr(II1) PRODUCTS OBTAINED FROM CHROREDUCTION OF CARBOXYLATOPEFTAAMMINECOBAL~ (111) COMPLEXES
MOUS
Amax,
Amax,
Llgand
mr
tl
Aquo" Acetato' Lactato' Malonato'
406 406 409 416
15 6 20 7 31 2 27 2
57 1 568 563 559
13 21 25 32
Salicylato m-Hydroxybenzoato p-H ydroxybenzoato 2,4-Dihydroxybenzoato
415 417 411 420
36 43 8 40 5
38 5
555 569 572 560
35 0 37 4 38 7 36 5
o-hlethoxybenzoato Phthalato Isophthala to 2,2 '-Thiodibenzoato
410 417 414
31 2 21 7 30 4
568 573 56i 580
26 20 30 25
4 5 8 2
2,2'-Biphenyldicarboxylato o- Acetylhenzoato o-Benzoylbenzoato 2-Hydroxy-5-formylbenzoato
417 412 412
13 0 25 2 30 5
573 5i5 575 555
20 22 21 54
6 6 4 1
Pyridine-4-carboxylato Pyridine-3-carboxylato XMethylpyridine-4-carboxylato Pyridine-2-carboxylato
410 410 408 406
23 22 28 50
6 6
579 577
5
600
8
553
19 21 20 32
Pyridine-2,6-dicarboxylato Pyridine-2,5-dicarboxylato
428 403
44 40 9
577 562 543
102 29 2 21 6
Pyrrole-2-carboxylato
mr
(2
4
6 7 4
5 2
0 8
a R . I. Colemar and F. W. Schwartz, J . Am. Chem. Soc., 54, 3204 (1932). R. E. Hamm, R . L . Johnson, R . H . Perkins, and R . E. Davis, ibid.,80, 4469 (1958).
near 410 mp) the other near 575 mp, are included, b u t the second of these peaks is often very broad and considerable difficulty is encountered in estimating the position of A,,. The extinction coefficients, however, allow a clear choice between the aquo- and a carboxylatochromium(II1) complex and appear to indicate, for the pyridine derivatives, whether the product is chelated. In cases most favorable for coordination through nitrogen, one or both of the €-values exceed 40. For substituted benzoato complexes, the picture is less clear. Products thought, on the basis of other evidence, to be chelated (e.g., the salicylato and the 2,4dihydroxybenzoato Cr(II1) derivatives) have higher extinction coefficients than some of the nonchelated products, b u t absorb less intensely than other complexes (e.g.> the m- and p-hydroxybenzoato compounds) which clearly cannot be chelated. In general, no serious
EDWINs. GOULDAND HENRYTAUBE
1322
inconsistency is introduced by assuming nonchelation in products for which both €-values are less than 35, but reserving judgment when one or both values exceed this. Range of Rates.-The specific rates for reduction of pentaamminecobalt(II1) complexes with C r 2 + (adjusted to 0% methanol and P = 3.0) are summarized in Tables 111-V. Over two-thirds of the values lie TABLE 111 REDUCTIONO F CARBOSYLATOPEXTAAMMINECOBALT(III) Ioss DERIVED FROM M o v o ~ ~ s r c
SPECIFIC RATES FOR CHROMOUS
hCIDS“
Ligand
k
TARLE I\-
SPECIFIC RATESFOR CHROMOUS REDUCTIOSOF CARBOXYLAI oP E S T A A M M ~ X E C O B A L T ( 111) IOSS DERIVED FROM UIDASIC XCIDS” kHa (acid P K H . ~ path)
I.igand
Sebacnto Salicylato m-Hydroxybenzoato p-H~drosT\~heiizoato* 2,4- Dili?dros!-benzoaco
2,5~I>ih)-dro~ybenzoato ( 7 . 7 j r “,A-Dihydrox).berizoato 7 72 2-H)-drox)--5-forrn)-lbenzoato ( 9 1)‘ 2,2’-Oxydibenzoato 2 28 2,2 ’-Thiodibctizoato 2.15
0 0 0 0 0
2-Furoato 2-Thiophenecarboxylato 3-Thiophenecarboxylato p-Carbomethoxybenzoato o- Acetylbenzoato
0.43 0.30 0.23 0 20
1,3,5-Benzenetricarbox?.iato 1,2,1,~~-Benzenetetracarboxylato (binuclearj
0.80
* Our data for these complexes are not
o-Benzoylbenzoato 2-.%nthraquinonecarbxylato p-Benzol-lbenzoato p-Formylbenzoato ( 0 ”j
5 .1 5 0
In 1. mole-’ set.-', a t 2 5 ” ;
I/ 11 38
3
x
106
11
0.19 0 22 ( 2 .8)c 0 16
yiatoh
a
0.022:K~~ ( = 10)
2 94
0.075
9
2 97
0 078
21
+
3 . 0 ; ( H + ) = 0.16-2.90 F .
reported by Fraser2b (0.083 at p = 1.0) However, his value for the o-chloro compound (0.074) is less than t h a t for the iodo compound, whereas we observe the reverse trend in the o-halogenobenzoato series. The authors are indebted t o Mr Robert Jordan for a sample of this complex.
between 0.08 and 0.30. Thus, the rates of reactions in this series are strikingly insensitive to the usual inductive, conjugative, and steric effects which so often influence reactivity in organic compounds. In the extreme case, replacement of the three a-hydrogens in the acetate group with fluorines retards the reduction of the pentaamminecobalt(II1) derivatives only by a factor of 3 , whereas such a replacement in the parent acid increases its dissociation constant by over five powers of 10. Although one hesitates to rationalize small rate differences in the present series on a structural basis, i t is probably safe to assume t h a t a rate constant greater than 0 . j indicates the operation of one or more special effects. Remote Attack through p-Carbonyl Substituents.The “special effect” which rests, a t present, on firmest experimental ground is remote attack, i.e., reduction of Co(II1) by attack by the reducing agent a t a noncoordinated carboxyl or carbonyl, which is separated from the coordinated carboxyl by a system of conjugated double bonds.2c In the present series, this type of attack is exemplified by the reduction of the p -
0 38 p
= 3.0; ( H + j = 0.06-2.90 F
in agreement with those of Fraser,2bwho reports a value of 0.13 for the p-hydroxy complex, and, for the biphenyl derivative, an acid independent term of 0.10, a first-order acid term having a specific rate of 3.3, but n o inverse acid term. ‘ Estimated value.
TABLE V
+ 0 0081 [H ’1
’ Our value for the p-iodo complex is in good agreement with that
0 23
In I. mole-’ set.-' a t 2 5 ’ ;
0 0A2 0 35 p =
106
Biplienyl-2,2’-dicarbox-
0.32 1 5[H+] 29 5 . 4 X 102[H-1
+
x
0 2 7 l K ~ a(=IOY)
0 55 0.20 0 19
m-Methoxybenzoato p-Methoxybenzoa to o- Phenoxybenzoa to 2-o-Cresoxybenzoato o-Thiomethylbenzoato
22 19
2
0 29 0 0 1 6 / K ~(. =~ l o 6 ) 0.096 5 x 106
0.15 0.11 0 096 0 095 0.28
4,4’-Oxydibenzoato 4,4’-Thiodibenzoato I;uran-3,3-dicarboxylato Thiophene-3.i-dicarboxylato
k 4(basic path1
0 28 0.15 0 20 0 23 0 16
9 77 8 80 9.91 7 75
o-Fluorobenzoato o-Chlorobenz~ato~ o-Bromobenzoato o-Iodobenzoa toh o-h!Ietho\ybenzoato
Trifluoroacetato“ Levulinato a
Vol. 8Ci
SPECIFIC RATESFOR REDUCTIOS OF CARBOXYLATOPE\TAAM111) I O N S DERIVEDFROM NITROGES-CONTAINING ACIDS“
MI\ECOBALT(
k m + Ligand
p--%ininobenzoato m -Aminobenzoato I’rolinato
(acid ~ K H Bp a~t h )
kn (basic p a t h )
0 14 0 12 0 027
2
x
Pyridine-2-carboxylato Pyridine-3-carboxylato Pyridine-4-carboxylato Pyridine-2,5-dicarboxylato p: rid1ne-2,6-dicarboxy-lato ’
4.49 4 . 7 3 0 11 4 79 1 3 150
Pyrrole-2-carbosylato
2 . 7 0 0 28
2 6
,
kMc INmethyl deriv 1
105
0 8; 0 13 1 1 8
I n 1. mole-’ set.-' a t 2 5 ’ ; p = 3.0; ( H - j = 0.03-2.90 F limit refers t o t h e S-protonated (tripositive) form of the complexes Second acid dissociation constants (for the dipositive forms) have been found t o be 3 45 (for the 2,6-cotnpIex j and 3.80 (for the 2,5).
* This upper
benzoyl and p-formyl (p-aldehydo) derivatives, the latter being reduced “immeasurably rapidly” a t room temperature.13 In addition to the high rates, there are two other indications of remote attack: The rate laws for these complexes include large terms which are first order in hydrogen ion, a characteristic which has (13)
K T hI Fraser, J A m C h e w S O L ,83, 4920
(1961)
April 5 , 1964
CARBOXYLATES I N OXID;\TI~N~-REUVCTION REACTIOSS
1323
been found in the pentaamniine complexes only in cases where remote attack greatly predominates over adjacent attack.2hs’4 Moreover, we have observed kA-(CrTS)(Ko-X-) (1) t h a t reduction of the p-benzoyl complex yields acjuochromium(II1) and the free parent acid, and Fraser’j where k ~ . kand k ~ -the , specific rates for the acidic and has made the corresponcling observation with the p basic fornis of the complex, ma>-be evaluated from the formyl derivative. Release of the free acid which has dependence of reaction rate on acidity and from Kr3x, never been reported for cases involving the more usual the acidity constant of Ko-.I-H. The second term, adjacent attack, occurs because the “remote” carbonyl representing the basic path. does not always appear, group, although it must come into contact with Cr’+ to particularly when the acidic centers in the complex are accept the reducing electron, is too weakly basic to far apart. i n which case the specific rate is independerit remain bonded to it in an aqueous environment. of acidity. I t is evident that the observed rate conChelation and Possible Electron Transfer Through stants for the basic paths fall into two distinct groups. o-Carbonyl Groups.--~ -The rates for the o-benzoyl and Those associated with the complexes of dicarboxylic %-anthraquinone derivatives, and, to a lesser extent, acids cluster near 10, whereas those from the o-hydroxythe o-acetyl derivative, indicate t h a t a carbonyl group benzoic acids lie well above 10,i. lying oriho to the coordinated carboxylate group may Reductions of Dicarboxylato Complexes.- -The niagalso accelerate reduction. Remote attack might be exnitude of the rate constants associated with the basic pected here as well! for, classically speaking, the ortho paths for reduction of our dicarboxylato coniplexes, and para positions are corijugatively equivalent. and, more particularly, comparison of niolecular models .I-o term Nevertheless, important differences appear. first ordcr in h y d r o g m ion has brcvz o b s m e d iii the rat(> of complexes which exhibit this path with those of comlaws, and the reduction >tieid$ tiw carbos~~latorhromic~~n-plexes that do not, suggests t h a t the basic path proceeds through a chelated transition state, although the low ( I I I ) prodzict, rather tiinn t i i r Jrc’c, parcnt crcid (Table 11). absorbancies of the resulting solutions in such cases T h e identity of the product tells us t h a t chromium is (Table 11) indicate that chelation does not persist in bound to the carboxyl group in the activated complex the products. Strainless models of chelated transifor reduction, and we may infer t h a t the ortho carboxyl tion states have been constructed for each dicarboxylato group serves to stabilize the transition state through complex displaying an inverse acid term in its reduction chelation (with the rings for the biphenylL2,2’-dicarboxylatoand 2,%’-thiodibenzoato complexes having eight and nine atonis, respectively), whereas those complexes for which such models cannot be made (c.g., the isophthalato. the 4,4’-thiodibenzoato, and the l,:~,j-benzenetricarboxylato complexes) are reduced a t rates independent of (H T). T h e alternative interpretation, t h a t the k.1 values for R the dicarboxylato complexes in this series exceed the k ~ . values k b y a factor of about I O 2 because of the extra unit of negative charge associated with the bridging although, once again, the weakly basic carbonyl oxygen group, seems less satisfactory, for there is other evicannot remain bound to chromium, and a chelated dence t h a t reactions in this series are strikingly insenproduct would not be expected (nor is it observed). sitive to the charge type of the electron-transfer bridge. However, this cannot be the entire story, for the phthaThus, it may be noted that the protonated and methylato complex, which may form a chelate in just the manlated forms of the complexes in Table V (in which the ner shown, is reduced with a specific rate of 0.073 at ligand bears a net charge of zero) are reduced about as 2j3,’lC Z . P . , more slowly than virtually all other benzoato rapidly as many of the substituted benzoato coniplexes complexes in the series, including the y11- and p-SHR’ in Table 111 (in which the ligand bears a t net charge of substituted complexes (Table I-) having an additional - 1). unit of positive charge. I t is suggested, therefore, Ii-hen formation of a six-membered chelate is possithat conductivity associated with the carbonyl l i d ble. k . 4 - increases significantly, being near 100 for the age is important here (as is the case with the p-benzoyl dimethylmalonato complex16and 2300 for the rnalonnto and p-formyl compounds), and t h a t electron transport complex. l 4 I t nevertheless remains well below the takes place, a t least in part, into the keto group, K,%values noted here for the hydroxybenzoate c o n through the ring. and thence into the carboxyl group. plexes. Since both carboxyl oxygens in the transition state are already attached to positive centers (one to Co(II1). Chelation of Reduction of o-Hydroxybenzoate Dethe other to C r ( I I ) ) , hydrogen ion catalysis is not obof a hydroxy group ortho to rivatives.-Incorporation served the carboxylato group in this series should greatly favor chelation in the transition state, for here the chelate Duality of Paths for Complexes of Dibasic Acids.-would be an almost planar, conjugated, six-nieinbrred Reductions of the complexes of dibasic acids (Table IV) ring. may be expressed as the sum of two terms, pertaining. .ilthough the spectra of the resulting solutions (Table respectively, to the acid form of the complex (designated 11) do not easily distinguish a chelated Cr(II1) product Ro-;\-H), and to its conjugate base (Ro-;\~-) from a nonchelated one and, in any event, say nothing ( 1 4 ) G Svatos and H T a u b e , J A m C h r m .So< , 83, 4172 llU6l (1:) I< T hi Fraser. P h D. Thesis. U n i v e r s i t y of Chicaeo
I
!I81 I< Butler a n d H. T a u h e , unpublished r e ~ u l t . .
EDWINS.GOULD AND HENRY TAUBE
1324
co’
0
I
yr\
/
about occurrence of chelation in the activated complex, comparison of the specific rates for the acidic and basic paths is instructive. The KHAvalues for reduction of the isomeric hydroxybenzoato complexes are not greatly different from each other and fall well within the range characteristic of the “ordinary” benzoato complexes in Table 111, indicating that the activated complexes for reduction of the acid forms of the o-hydroxybenzoato complexes are not stabilized significantly b y chelation. For the basic path, i t is quite another matter. The very high k A - values in Table IVappear only when one or two hydroxyls lie ortho to the coordinated carboxyl, strongly suggesting chelation in tfie basic paths. The effect (corresponding to a rate increase of three to six powers of 10 over the reduction of the malonato complex) is striking, particularly since i t is substantially a reflection merely of the difference in stability between a conjugated planar ring system and one that is nonconjugated and puckered. Although the magnitude is not out of line with observed differences in stabilities between malonato and salicylato complexes,” i t is not unreasonable to ask whether an additional mode of reduction, i.e., electron transfer through phenolic oxygen, has come into play in the basic path. The data a t hand give scant support to such an alternate mechanism, although they do not rule it out. For it may be argued that the para and ortho positions are conjugatively equivalent and t h a t any mode of electron transfer through an 0-0-substituent should occur also through a p - 0 - group, although the effect might be attenuated. Lye find no evidence for a basic path in reduction of the @-hydroxycomplex, but such a path might easily pass undetected if it constituted less than 5% of the entire reduction. Because of the very high ~ K Hvalue A for the para compound, this “upper limit” corresponds to a k A - value of about 106 in our range of acidity. The question thus remains open, and extension of these measurements to lower acidities is highly desirable. Nontransmission through Sulfur.-Values in Table I V for the sulfur-containing dicarboxylato complexes derived from 4,4’-thiodibenzoic (11), 2,2’-thiodibenzoic ( I I I ) , and thiophene-3,4-dicarboxylic(IV) acids are
Vol. 86
of some interest, not because they are large, but rather because they are very similar to their respective oxygen analogs. In these complexes, the coordinated carboxyl is separated from the noncoordinated carboxyl by conjugated carbon systems which are, in turn, separated from each other by sulfur atoms. T o the extent t h a t these sulfur atoms can transmit electronic effects from one conjugated carbon system to the other by “d-orbital resonance,”18 the two carboxyl groups may be considered to be conjugated with each other, although there is considerable variation, both in steric relationships and detailed electronic distribution within this triad. Since there is no evidence that the reduction of these complexes occurs by remote attack, as has been observed for the fumarato and terephthalato coiiiplexes,2c we may infer that the sulfur atom, ho similar it may be to a pair of doubly-bonded carbons in other respects,Ig does riot easily transmit a reducing electron from one conjugated system to another.1o Electron Transfer through Nitrogen. Pyridinecarboxylato Complexes. *l-Each of the ligands listed in Table V has a basic nitrogen atom, although the basicity of the pyridinedicarboxylato complexes is too weak to be detected in dilute aqueous solution. A%gain, rates of reduction may be expressed as the sum of two terms, involving specific rates for the acidic ( k n +)~ aiid ) of the complex. In this group, rebasic ( k ~ forms ductions of the pyridinecarboxylato complexes provide what appears to be the first strong evidence for remote attack involving electron transfer through nitrogen. As with other conjugation-related effects exhibited by pyridine derivatives, this path has been observed for a- and y-, b u t not for @-substituents. In reduction of the pyridine-2-carboxylato complex, only the basic path was detected, even in 3 -12 HClOA. Both paths were observed with the &carboxylate coni(18) D P Craig, A. iifaccoll, R S. I i y h o l m , 1,. E Orgel, and 1.. E Surton. J . Chenz SOL.,332 (19%). (19) For a recent s u m m a r y of t h e evidence t h a t suliur, like C: = C , but unlike oxygen, m a y stabilize adjacent free-radical or carbanion centers, see C C . Price and S Oae, “Sulfur Bonding,” Ronald Press, Yew I’ork, N I’ , 1962, p p 26-60 (20) Remote a t t a c k does not appear t o be involved in reduction of t h e hinuclear complex of 1,2,4,5-benzenetetracarboxylicacid. I t is found t o react substantially more rapidly t h a n t h e phthalato complex (ref 2c) and t h e binuclear terephthalato complex ( D . K. Sebera, P h . D Thesis, University of How Chicago), both of which have structural features in common with it ever, no first-order acid t e r m was observed, even in 2 5 JI HClOi This observation does not tell us whether t h e complex has structure \’, \‘I,
0
Ro- Cn -0
0
0 n C O O H
HOOC
Ro- CI! - O ~ O - C - iiR O
C-0-RO
v
HOOC
&
COOH
VI 0
HOOCn o - C II -Ro HOOC
0 I1
HooC
w c-
’
I11 O -Ro
‘S/
IV (Ro
=
“roseo” = Co(NHs)j)
(17) For example, t h e p K values (2.5”) for t h e monomalonato a n d monosalicylato complexes of C u ( 1 I ) are reported as 5.60 (R. W M o n e y and C . W. D a v i e s . T u u n s . Fuuaduy Soc., 28, 609 (1932)) and 10.60 ( A . K . Babko, Z h . Obsch Kbiwa., 17. 413 ( 1 9 4 7 ) ) , respectively.
HOOC
0-C-RO
VI1
6
or VI1 (where Ro = “roseo” = Co(NHa)s), although VI1 would be considered unlikely on a steric basis. Remote a t t a c k would not be expected in t h e reduction of v, since no -COOH group lies pura t o a -COORo group. I t would, as well, be unlikely in reductions of \TI a n d V I I , for t h e -COOKo groups are pushed o u t of t h e plane of t h e benzene ring b y “ouiho interference.” (21) A brief report, summarizing t h e results in this and t h e following section, has appeared ( E S Gould a n d H. Taube, J . A m . Chem Soc., 8 6 , 3706 (1963)).
April 5 , 1964
CARBOXYLATES IN
OXIDATION-REDUCTION REACTIONS
plex, whereas only the acidic path appears with the 3isomer, which reacts a t a rate comparable not only to its N-methylated derivative, but also to the many “normal” carboxylato complexes in Tables 111-V. Rates corresponding to these basic paths are surprisingly high, considering particularly t h a t the basic forms of these complexes have one more positive charge than the basic forms of the complexes in Table IV. The marked structural similarity between the pyridine4-carboxylato (VIII) and the fumarato (IX) complexes
1325
The fact t h a t no spectral irregularities attributable to an N-coordinated Cr(II1) product could be detected, even when an excess of Co(II1) complex was used in the reaction, suggests t h a t the specific rate of such a “follow-up reaction” compares with that for the initial reduction and may be greater. A similar exchange, leading t o a carboxylatochromium(II1) product, may be imagined for a -CEO-Cr(II1) intermediate resulting from reduction of a p-carbonyl complex, but it is not observed. Thus, it may be presumed t h a t such a step, in this case, does not compete favorably with hydrolysis to Cr(Hz0)83+.
The high absorbance of the product derived from reduction of the pyridine-2-carboxylato complex indicates that a chelated product, having a Cr(II1)-N bond, has been formed. Moreover, the extreme rapidity of the basic path points to chelation in the activated complex, but the Cr atom in the product is bound both to oxygen and nitrogen, and the direction of electron flow, as in the case of reduction of the salicylato complex, has not been determined. However, to the extent t h a t the data on reduction of the 4-carboxylato compound suggest remote attack through nitrogen, electron transfer through nitrogen must be considered also in reduction of the 2-carboxylato isomer. In view of the long recognized similarities in conjugative relationships between the 2- and 4-positions of pyridine derivative^,^^ i t is difficult to see why electron transfer would be permitted in one case but prohibited in the other. The reduction of the pyrrole-2-carboxylato complex contrasts with t h a t of its counterpart in the pyridine series. The low value of emiLxa t 543 mp for Cr(II1) species in the resulting solution indicates that the predominant product is not chelated. The basic path for reduction, although kinetically detectable, is slower than that for the pyridine-2-carboxylato complex by a factor of lo5,but still significantly faster than reduction of the corresponding furan and thiophene derivatives. A chelated transition state is not ruled out for the small portion of the reduction of the pyrrole derivative proceeding through the basic path, but because of the tetrahedral configuration about the pyrrole nitrogen, the chelate ring would not approach planarity and any CrN bond should, in contrast to the case of the pyridine complex, lie well out of the plane of the heterocyclic ring. Interestingly, the specific rate for the basic path for the pyrrole complex approximates those for the dicarboxylato complexes in Table 1V which, we believe, are reduced through nonplanar chelated activated coniplexes. Reductions of Pyridinedicarboxylato Complexes.Reductions of the complexes of pyridine-2,5-dicarboxylic and -2,,6-dicarboxylicacids, even in 3 M HC104,are immeasurably fast under our conditions and may be presumed to proceed predominantly by the basic path. Our lower limits for the rate constants fall so much below the values for the 2- and 4-pyridinemonocarboxylato complexes because the dicarboxylato complexes are much the weaker bases. The Cr(II1) products formed in these reductions are chelates, as indicated by their spectra (Table 11), the color of the 2,B-product being unusually intense for this series. It is likely t h a t the 2,6-derivative is reduced in the same manner as the 2carboxylato complex; the ambiguity with respect to direction of electron flow is here also. Discussion of the mode of reduction of the 2,j-derivative requires t h a t we know whether, in the preparation, cobalt(II1) has become attached to the 2- or the 5carboxyl group. Although we cannot be sure, comparing its base strength with that of the 2,B-derivative allows a resonable choice. The complexes themselves are immeasurably weak bases in water, but the basicities of their nonprotonated forms (X and XI) are measurable. As indicated, the monopositive ion derived from the 2,5-complex is about twice as strong a base
(22) See, for example, R. E Hamm. R L Johnson, R H Perkins, and R. E. Davis, J A m Chem S O L ,80, 4469 (1958)
(23) See, for example, G . W. Wheland, “Resonance in Organic Chemistry,” John Wiley and Sons, Inc., New York, N. Y . , 1955, pp. 484-486
suggests to us that the basic path for reduction of the 4carboxylato complex is associated with remote attack, in this case a t nitrogen. However, the spectrum of the product is very similar to that obtained from the isomeric 3-carboxylato complex, and similar also to t h a t of the acetato complex (Table 11). Apparently, then, the N-Clr bond does not survive, b u t the N-coordinated Cr(II1) complex is rapidly re-equilibrated to the more stable carboxylato-coordinated complex. If this picture is true, reduction of the pyridine-4carboxylato complex is similar to reduction of the pbenzoyl- and p-formylbenzoato complexes in t h a t chromium does not remain bound to the site of attack in the Co(II1) complex, but the two types of reduction differ with respect to the ultimate fate of the Cr(II1) product. The aquochromium(II1) complex, resulting from reductions of p-carbonylbenzoato derivatives, is not observed as a product from reduction of the pyridine derivative, nor is it a t all likely that the observed carboxylato-coordinated product is formed from the aquo, for such conversions a t room temperature are known to be extremely slow.22 We propose instead that the N-coordinated product is converted directly to the carboxylato-coordinated product by action of the unreacted C r ( I I ) , again by electron transfer through the pyridinecarboxylato system.
1326;
EDWIN
r
s
G O U L D AND
0
J
X P K B -10.55
r
0
Vol. Hfi
N-methyl compound was exposed to air for 3 days, the color faded, and a maximum appeared a t 406 m p , the expected wave length for an N-methylated carboxylatochromium(II1) derivative (Table I I ) , indicating partial re-oxidation to the pyridine ring Competition for Cr(I1).-Experiments in which a deficiency of Cr(I1) was treated with (a) a number of carboxylato complexes and (b) the corresponding parent acids are summarized in Tables 17 anti [?I1
l+
L
HENRY TAUBE
1'
TABLE \-I T I E L D S OF
co(II) FROM
R E I I U C T I O S O F CARROXTLATOPES1 A A M -
MISECOBALT(
111) COMPLEXES
-(Hi)
(25", p = 0.0075) as its 2,6-isomer, indicating that in the 2,5-isomer the base-weaking Co(II1) center is coordinated to the 5 - , rather than the 2-, carboxyl group Since the coordinated and noncoordinated carboxy groups lie "para" to each other, remote attack, as thought to occur in reduction of the terephthalato complex, may occur here also. If, as we suspect, the cobalt is bound to the 5- rather than the 2-carboxyl group, formation of a chelated product suggests that chelation may facilitate remote attack as well as adjacent attack.
- 0-CO(I1I)
0,IZYield
-(H*) < d I I11
Cr(lJ)
CofII)
CriII:
3.0 3.0 3.0 1.5
0 99
1 25 1 25 1,5 3.0 15
1 00 0.89 0,90 0 87 0,99
1.25 3.0 1.5 Pyridine-2,5-dicarboxy1atoa 1. 5 N-R/Iethylpyridine-?carboxylato 1 5 ?;-Meth~-lpvridine-2,5dicarboxylato I'y-role-2-car boxy lato
Ligand
hquo p-Sitrobenzoato 2-Furoato p-Forrnylbenzoato
C
S
N-
The position of Co(II1) in the N-methylated 2,sdicarboxylato complex is also undetermined, although we suggest t h a t formation of the complex is more likely at the 5- rather than a t the more hindered 2-position. In any event, remote attack, b u t not chelation, is possible in the reduction of this complex, and the observed specific rate is significantly greater than for reductions of the other N-methylated complexes in the series. It is, however, only one-tenth as rapid as the reduction of the terephthalato complex,2c probably because the acarboxyl group (be i t "adjacent" or "remote" with respect t.o cobalt) is kept out of the plane of the pyridine ring by interference with the N-methyl group. U-hen the 2,5-dicarboxylato complex or its N-methyl derivative was treated with an excess of Cr2+, more deeply colored species were slowly formed. A bluepurple product (+,,,, 63.0 (-104 mp), -17.0 (540 mp)) was obtained from the nonrnethylated complex and a deep green product (emax 71.5 (570 m p ) , 12,OOO (284 mp)) from the methylated complex. These are almost certainly Cr(II1) complexes also, possibly with the rings reduced to dihydropyridine or related structures, for the 2,S-dicarboxypyridine system (both methylated a n d nonlnethylated) has been found to be readily reduced by Cr(I1) in the absence of Co(II1) (Table 1'11). Moreover, when the product obtained from the
-
I O \TiC,(]
C'qll
0.0 0.91
3.0
I)
100
3 0
l.!)O
3.0 3.0
98 0 91
1.00 0 82 1.00 0.95
3.0 3,0
0.92 0.97
0.96
3.0
0 88
0.99 1 00
3 0 3 0
095 0.78
!94
p-Benzoylbenzoato a-Benzoylbenzoato a-hcetylbenzoato 2-hnthraquinonecarboxylato Levulina t o 2-€I?.dros).-5-form\.lbenzoato Pyridine-~-carbosVIato Pyridine-3-carboxylate Pyridine-4-carboxylato
OH
0
o-!*h
-
CO(1II) _
1.5 3.0
a A similar determination with the complex of pyridine-P,6dicarboxylic acid could not be carried out sitice the color of the Cr(II1) product interfered with that of Co(1I) in HCI
A Co(I1) yield substantially below unity in Table 1'1 signifies that some entity is conipeting with coordinated Co(I11) for Cr(I1) Behavioral extremes i n this series are exemplified, on one hand by the p nitrobenzoato complex and, on the other, by the { I formylbenzoato complex The nitro group quickly takes up all reducing electrons from C r 2 L ,is itself reduced, and transmits none through the ring to Co(II1) whereas the p-CHO group accepts reducing electrons but transmits virtually all of them through the ring to Co(II1) Many of the intermediate values may be rationalized by assuming that Cr(I1) reduces the organic portion of the Cr(II1) product formed in the main reaction This supposition is consistent with the decreased yields of Co(I1) (insofar as these changes lie outside of experimental error), as (H+) is raised from 0 15 to 1 0, the competing reduction, to a more nearly saturated organic product, almost certainly requires protons Since the p-benzoyl derivative yields the free parent acid, rather than the Cr(II1) complex, on reduction, the observed discrepancy here between Cr(I1) com sunied and Co(I1) produced cannot be attributed to partial reduction of a Cr(II1) complex In this case,
April 5, 1964
CARBOXYLATESI N
OXIDATION-REDUCTION REACTIONS
TABLE VI1 REDUCTION OF
CARBOXYLJC .%CIDS
Acid
( H ’)
WITH
CHROMICM(II 1”
Cr(I1) consumed
Color
R
produced
2-Furoic p-Forrn? Ibenzoic p-Benzoyl benzoic o- Benzoylbenzoic o-.-lcetyl benzoic
0 0 0 0 0 0
40 15 15 40 15 40
10 0 5 10 0 20
;\nthraquinone-~-rarboxylic Levulinic .?-Pyridinecarboxylic 3-~’~ridint.carbozylic 4-Pyridinecarboxylic
0 15 0 15 0 15 0 15 0 15 0 40
5 24
0 2,j-Pyridinedicarboxylic 0 ~,A-I’yridinedicarboxylic S - ~ r e t h y l - ~ - p S . r i d i n e c a r b o x y l i c0 N-Methyl-3-pyridinecarboxylic 0 0 S-?rlethyl-4-p~-ridinecarbox~-lic S-?rleth?l-%,5-pyridinedicarboxylic 2-Pyrrolecarboxylic
15 15 15 15 15
0 15 0 40
b
Dark brown Red-brown
20
>40
Purple
20 80
> 85 5 0 23
> 64
Purple Dark blue
Purple Green
0
a -40 min., 2 5 ” . concentration of acid, 4 0 g./l color prevented estimation of Cr( 11) consumed
Intense
’;YG
about of the Cr(I1) is oxidized by the free parent acid, by a radical-like intermediate, or by a combination of both. As shown in Table VII, p-benzoylbenzoic acid does indeed oxidize C r ( I I ) , but the reaction is slow; after 40 min. (twice the reaction period for the competition experiments), and in the absence of Co(III), only 5% of the Cr(I1) is oxidized b y the free acid. Intervention of a radical-like intermediate, which may either form the predominant products or react with a second C r ( I I ) , is thus a distinct possibility here, although the differences are too small to be anything but suggestive. Likewise, analogous intermediates cannot be ruled out for reduction of other complexes in Table VI for which yields of Co(I1) lie well below unity; rate data on the reduction of carboxylatochromium(II1j complexes with Cr(I1) would be pertinent here. Relevance of Reducibility of Ligands to Reduction of Complexes. Evidence for Radical-Ion Intermediates. -The values in Table VI1 may be taken as rough indications of the relative rates a t which the various parent carboxylic acids are reduced by C r 2 + ,since concentrations of Cr2+ and carboxylic acid, temperature, and reaction period are kept nearly constant throughout the series. These figures emphasize an important distinction between the manner in which the carbonylbenzoato groups, on one hand, and the pyridinecarboxylato groups, on the other, facilitate reduction of coordinated Co(II1) by Cr2+. As Conant has shown,24ordinary aldehydes and ketones are reduced extremely slowly by C r 2 + a t room temperature, and our figures reveal little, if any, difference in reducibilities among the various carbonylbenzoic acids. This suggests that the ligand in such 1241 J B C o n a n t and H . B C u t t e r , J .4m. Chem. Soi , 48, 102.5 (1926)
1327
cases serves largely to transmit the electron deficiency associated with Co(II1) to another (possibly more accessible) site, for without Co(III), no transfer would occur. In this respect, it is interesting that the levulinato complex reacts a t a “normal” rate, although its parent acid (y-ketovaleric acid) is more easily reduced than any other carbonyl acid in our series. The keto group apparently has some capacity for accepting a reducing electron, but this electron cannot find its way through the -CHa-CH2- chain to the coordinated carboxyl. The same sort of “conduction” through the ligand almost certainly occurs in reduction of certain of the pyridinecarboxylato complexes (particularly the basic forms of the 2- and 4-carboxylato derivatives) ; but for the series as a whole, reducibility of the ring system appears to have assumed increased importance, for it is to be noted that those acids which are reduced most easily with C r 2 + form complexes which are, in turn, reduced easily. We suggest that certain of the pyridinecarboxylato ligands may facilitate electron transfer by accommodation of the reducing electron on the pyridine ring itself, forming a radical-ion intermediate. T h e electron may enter the system through a noncoordinated carboxyl group, or through the ring nitrogen, if it is neither protonated nor alkylated. If neither route is open, the more usual mode of reduction, in which the incoming electron attacks the coordinated carboxyl, is to be e ~ p e c t e d , ? ~ b u t even here, in favorable cases, the pyridine ring may facilitate reduction by conjugative stabilization of the intermediate radical ion, e.g., XI1 5+
Me
XI1
-
The latter description would seem to apply to the reduction of the N-methylated and the protonated 4-pyridinecarboxylato derivatives, for the rates lie well above those for the corresponding 2- and 3carboxylato species,26being, a t the same time, far below those for other reductions in the series in which a noncoordinated carboxyl, or a basic nitrogen (or both), is available. Moreover, as shown in Table VII, the 4acid and its N-methyl derivative are reduced more easily by Cr(1I) than are the corresponding h u b s t i tuted analogs and the methylated 2-analog. Since the initial step in reduction of the pyridinecarboxylic acids with C r 2 + is almost certainly the transfer of an electron to the ring, forming a radical, we may infer that the same electronic factors which tend to stabilize radicals derived from the uncoordinated 4-carboxy ( 2 8 ) Attack b y C r 2 + on t h e *-orbital system of t h e ring is conceivable T h e possibility is ruled o u t here, however, by invariable f o r m a t i o n of a caTboxylato- rather t h a n a n a q u o c h r o m i u m ( I I 1 ) p r o d u c t , unless strict, and highly unusual, orientational requirements are associated with s u c h a n attack. 126) Dr. K. T. M. Fraser ( p r i v a t e communication) reports specific r a t e s for reduction (0 2 M HCIOP, p = 1) as 1 2 (methylated] a n d 0 9.5 (nonmethylated). His values, if increased b y IS$$ t o ciirrect for difierencei in ionic s t r e n g t h , are in reasonable agreement with ours
1328
CARLOS h D R A D E A N D
acids (in comparison with those derived from the 2and %systems) also stabilize radical-ions which intervene in reduction of the respective ( N H 3 ) 5 C ~ ( I I I ) complexes. *' Stabilization of a radical-ion intermediate, facilitating electron transfer by adjacent attack, comes to light here, largely because other effects which have been invoked to account for acceleration of electrontransfer reactions in carboxylatocobalt(II1) complexes (,i.e., remote attack, chelation, and increased negative charge on the bridging group) are ruled out in the reduction of the methylated and protonated +pyridinecarboxylato complexes. The lesson to be learned is that specific rate of reduction is a reliable guide neither to the site of attack nor the direction of electron flow. In particular, the "fast" reduction of such complexes as the o-benzoylbenzoato (Table 111) and the pyruvatoI6 derivatives could conceivably be rationalized b y considering stabilimtion of a radical-ion intermediate in attack on the carboxylato group ( e . g . , X I I I ) , rather than b y assuming transient chelation and electron transfer through the carbony1 group. However, the difficulty (271 N-F,thyl-.l-carbethoxypyridinyl, a stable free radical closely related t o o u r N-substituted 2-carboxypyridine complexes, has been isolated recently ( E 11. Kosower and E J. Poziomek, J . A m Cliern S O C . , 88, 2035 (1963)).
HENRYT c l U B E
Vol. 86
-b XI11
J
in reducing the carbonylbenzoato acids in our series, in contrast to the ease in reducing many of the pyridinecarboxylic acids, leads us to favor the radical-ion intermediate in rapid reductions of pyridinecarboxylato complexes but not in reductions of the carbonyl complexes. With the carbonyl complexes, a low-lying vacant orbital, while not low enough to accept an electron from Cr2+, may nevertheless be involved in the resonance transfer of an electron from the reducing agent to the Co(II1) center. Acknowledgment.-The authors wish to thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. Funds to purchase the spectrophotometer were made available by the National Science Foundation under Grant No. 22611.
GEORGEHERBERT JONES LABORATORY O F THE UNIVERSITY O F CHICAGO, DEPARTMENT OF CHEMISTRY, STAXFORD UNIVERSITY, STANFORD, CALIF.]
[COSTRIBUTION FROM THE
CHICAGO, I L L . , AND THE
Tracer Experiments on Alkaline Hydrolysis of Some Oxalate Ammonia Cobalt (111) Complexes BY CARLOS a N D R A D E
ASD
HENRY TAUBE
RECEIVED KOVEMBER 12, 1963 LVhen C O ( S H ~ ) ~ releases C ~ O ~ C~ Z O ~in~ -alkaline solution a t 25', C-0 bond breaking takes place in - i n k , S Y , 1961; D H. B u s c h , K S w a m i n a t h a n , a n d TI. W C m k e , i u o r q C'hrrn , 1, 200 (1962)
in water. Analysis showed 20.2070 oxalate (calcd., 20.37Yo) and 46.17% C10,- (calcd., 46.04%). [ C o ( e n ) ~ C ~ O ~ ](not Cl0~ necessarily anhydrous) was prepared from the chloride3by double decomposition. The perchlorate salt was not analyzed, but a report of analysis of [ C o ( e n ) ~ C ~ 0 ~ 3H20 ] C l ~showed 16.48Yc Co (calcd., 16 .52yc).). Procedure.-For all experiments, salts of normal isotopic composition were used, added to aqueous alkaline solutions enriched in 0 ' 8 . WTith the oxalato complexes as reactants, reaction is so slow that the product complex undergoes serious deterioration ( a s in the case of the pentaammine complex), or it undergoes exchange (as in the case of the bisethylenediamine complex) during the course of the reaction, and isotopic analysis of only the product oxalate is feasible. The solutions after the reaction were filtered t o remove solid material, then acidified to a faint acid reaction, and a solution of i\gNO3 was added to precipitate Ag2CzOi. The .4gtCZ04 was collected by filtration, washed with water of normal isotopic composition and then with CHaOH, and dried a t 40" for 12 hr. and then in a vacuum desiccator for 12 hr. The COZreleased by heating the solid t o complete decomposition was collected and analyzed b y mass spectrometer (.4tlas hdodel M86). The isotopic composition of the solvent was determined by the Anbar-Guttmann, method. ( 2 ) P S a s r and H T a u h e . J A m Chem S o c . , 88, 1 3 (1960) (3) A Werner a n d A Wilmos, Z o n o r g . Chcm , 2 1 , 145 (1899) (4) M . Antmr and S . G u t t m a n n . Iiilprn J . A b p l R a i l i d i o n l ~ o l o P r ~ 6, s, 233 f15159)
