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Kinetics and mechanism of the acid-catalysed decarboxylation of thecis-carbonato(5,12-dimethyl-1,4,8,11-tetra-azacyclotetradeca-4,11-diene)cobalt(III) ion |
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Dalton Transactions,
Volume 1,
Issue 9,
1979,
Page 1343-1346
Robert W. Hay,
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摘要:
0Kinetics and Mechanism of the Acid-catalysed Decarboxylation of thecis-Carbonat0(5,12-dimethyl-l,4,8,1 I -tetra-azacyclotetradeca-4,11-diene)cobalt(iii) IonBy Robert W. Hay and Bakir Jeragh, Department of Chemistry, University of Stirling, Stirling FK9 4LA,ScotlandThe acid-catalysed decarboxylation of the title compound has been studied over the range of perchloric acid con-centrations 0.1-0.5 mol dm-s at / = 0.5 mol dm-, (Na[CIO,]), and 25,34.8, and 45.4 "C. Over this acidity rangekobs. = ko + k,[H+] ; however, the ko term does not make a significant contribution. The values of the activationparameters (fork,) are AH1 = 82.9 kJ mol-'and ASTzSs = -0.4 J K- lmokl. The reaction exhibits a solvent deuter-ium-isotope effect k,,ro/ka,o = 2.6 at 25 "C; the solvent deuterium-isotope effect is consistent with a rapidpre-equilibrium protonation followed by rate-determining ring opening, and excludes a mechanism involving con-certed attack by [H,O]+.The value of AS3 close to zero, and the magnitude of the solvent isotope effect,suggests an A-1 type reaction to give a five-co-ordinate intermediate.THE kinetics of the acid-catalysed aquation (or decarb-oxylation) oi a number of carbonatotetramineco-balt (111) complexes have been investigated and dis-cussed over recent years.,-7 For such reactions therate law normally observed is kobs. = k, + k,[H+],where kobs. is the observed first-order rate constant atconstant hydrogen-ion concentration, and k, and k, arethe rate constants for the parallel rate-determiningcarbonato ring-opening processes (1) and (2) below[ (N4) is a tetra-amine]. The decarboxylation steps[(N4)Co(C03)]+ + H30+ -%cis-[(N,)Co(OH,)(CO,H)]2+ -%cis-[ (N4)Co( OH,) (CO,H)]'+ (2)C~~-[(N~)C~(OH)(OH~)]~+ + CO, (3)cis-[ (N,)Co(OH) ( CO,H)] + Lc~s-[(N~)CO(OH),]+ + CO, (4)represented by equations (3) and (4) are rapid, and theabove reaction scheme is consistent with the observedkinetics if k, > kl[H30+] and k, > k,.The decarboxylation of a number of carbonatocomplexes of the type [CoL(C0,)l2+, where L representsthe macrocyclic ligands C-rac-5,7,7,12,14,14-hexa-methyl-1,4,8,1 l-tetra-azacyclotetradecane (L1 = tet-b) ,l 5,7,7,12,14,14-hexamethyl-l,4,8,1 l-tetra-azacyclo-tetradeca-4,ll-diene (L2 = trans-Me6[l4]diene) ,I and1,4,8,11-tetra-azacyclotetradecane (L3 = cyclam) havebeen studied.The present paper is concerned with the acid-catalyseddecarboxylation of [Co(L4) (CO3)l2+. The work wasundertaken to investigate the influence of steric effectsand ligand unsaturation on decarboxylations of thistype.Ligand unsaturation may lead to stabilisation offive-co-ordinate intermediates by d,,-P,, bonding. Inaddition, it was hoped that the determination of activ-ation parameters and the solvent deuterium-isotopeeffect would clarify a number of problems connectedwith the intimate mechanism of the acid catalysis whichrequire consideration.6p8\N HN') U M eMe h et' L2nI"" "">'NH HN J HN U Me IUL3 LLEXPERIMENTALcis-Carbonat0(5,12-dimethyl-l,4,8,1 l-tetra-azacyclotetra-deca-4,ll-diene)cobalt(rIr) perchlorate sesquihydrate, cis-[Co(L4) (CO,)] [C104]*1.5H20, was prepared as previouslydescribed 9 (Found: C, 33.15; H, 5.6; N, 11.9.Calc. forCI3H2,C1CoN4O,.,: C, 33.2; H, 5.8; N, 11.9:h).tetra-azacyclotetradeca-4,ll-diene)cobalt (HI) cation wasprepared by two routes; (a) by acid-catalysed ring openingof cis-[Co(L4)(C03)]+, and ( b ) by direct synthesis, and iso-lated as the perchlorate salt.cis-[Co(L4)(C0,)J[C10,]*1.5H,0 (0.1 g) wasdissolved in the minimum volume of perchloric acid (0.5mol dm-3) and the solution heated on a water-bath for ca. 5min. The solution was then set aside a t room temperaturefor ca. 1 week, during which time reddish green crystalsbegan to appear. The product was filtered off, washed withcold ethanol and then ether, and dried in vucuo (Found: C,22.6; H, 4.9; N, 8.4.C,2H2,C1,CoN,0,4*1.5H20 requiresC, 22.35; H, 4.85; N, 8.7%).Synthesis.-The trans-diaqua(5,12-dimethyl-1,4,8,11-Method (a)1344 J.C.S. DaltonMethod (b).-A solution of L4-2HC10, (4.25 g, 0.1 mol)in ethanol-water (50 cm3, 1 : 1 v/v) was added t o a solutionof Co[C104J2*6H20 (3.66 g, 0.01 mol) in ethanol-water (50cm3, 1 : 1 v/v) and the mixture heated on a steam-bath forca. 1 h. During heating, air was passed through the mixtureand this was continued for a further 3 h. The mixture wasfiltered and the filtrate made up to ca. 30% in HCIO, andheated for a further 0.5 h. The volume of the solution wasreduced on a rotary evaporator to ca.30 cm3 and the solu-tion set aside a t room temperature. After several hoursgreen crystals of the product appeared, which were filteredoff, washed with cold ethanol and then ether, and dried invacuo (Found: C, 21.9; H, 4.7; N, 8.6. C12H2,Cl,CoN4-01,*2H20 requires C, 22.05; H, 4.9; N, 8.6%).Kinetics.-The reactions were monitored spectrophoto-metrically by following the decrease in absorbance a t 280nm. Linear plots of log ( A , - A,) vs. time t were observedin all cases. The reactions were carried out at I = 0.5mol dm-3, the ionic strength being adjusted by the additionof sodium perchlorate. The concentration of the per-chloric acid solutions was determined by titration with stan-dard sodium hydroxide solution. Kinetic measurementswere carried out with a Gilford 2400s instrument, the celltemperature being maintained to within 0.05 "C by cir-culating water through a metal cell-block holder.Thetemperature was monitored throughout the reaction, andeach kinetic measurement was carried out in triplicate.Routine u.v.-visible spectral measurements, includinginterval scan spectra, were made with a Perkin-Elmer 402instrument.RESULTS AND DISCUSSIONThe complex cis-[CoL4(C0,)]+ has Amax. 504 (E = 133dm3 mol-l cm-l) and 360 nm (167 dm3 mol-l cm-l). Inacidic solution (0.1-0.5 mol dm-3 H[C10,]) the absorb-ance decreases with time at both wavelengths with theband at 504 nm moving to ca. 490 nm. The resultingspectrum is consistent with the formation of a cis-diaqua-complex.Thus ~is-[CoL~(0H,),]~+ has A,,,. 506( E = 110) and 367 nm (99).l0 This relatively rapidreaction is followed by a much slower reaction, in whichthe absorbance decreases with time, suggesting iso-merisation to the trans-diaqua-complex. The finalproduct has A,, 362 ( E = 79), 424 (49), and 564 nm (27).This spectrum is identical to that of an authentic sampleof trans-[CoL4(0H2),l3+, which has A,,,. 362 ( E = 77),424 (48), and 564 nm (26).The spectral changes are thus consistent with thereaction scheme ;H+ cis- [C0L4( COJ] + + cis- [C0L4( OH,) ,I3+cis- [C0L4( OH,),]3+ trans- [C0L4( OH,) 213+The kinetics of the acid-catalysed ring-opening step weremonitored spectrophotometrically using the decrease inabsorbance at 280 nm. As the isomerisation reaction isvery slow, it was possible to obtain values of A , withlittle difficulty.The acid-catalysed aquation wasstudied over the range of perchloric acid concentrations0.1-0.5 mol dmb3, at I = 0.5 mol dm-3, adjusted withsodium perchlorate. Values of kobs. (the observed first-order rate constant at constant hydrogen-ion concen-tration) were obtained from plots of log ( A t - A,)which were linear in every case. The rate constantskobs. at 25, 34.8, and 45.4 "C are listed in Table 1.TABLE 1Kinetic data for the acid-catalysed aquation ofcis-[CoL4(C03)]+ at I = 0.5 mol dm-3 (Na[ClO,]) t25 "C[HC10,1/mol dm-3 1 03kOb,. / ~ - 10.099 1.670.196 2.900.320 4.780.400 5.930.500 7.33k, = 1.46 x 10-2 dm3 mol-l s-l; k , = 7.5 x loW6 s-l34.8 "C[HC10,1/mol dm-3 1 0 3 k ~ ~ ~ S - l0.050 2.550.099 4.730.151 7.490.196 9.380.250 10.980.320 12.760.400 18.65k, = 4.25 x dm3 mol-'s-l; k, = 6.34 x lO-'s-l45.4 "C[HC10,1/rnol dm-3 103kobs./s-10.050 7.070.099 15.360.151 20.330.196 25.400.250 35.040.320 42.84k, = 12.89 x dma mol-l s-l; k, = 1.59 x s-lt Values of k , and k , were obtained by linear regressionanalysis; the k, constants are subject to considerable error,and must be regarded as order-of-magnitude values only.Values of kobs.are almost directly proportional to[H+] over the acidity range studied, indicating that k,does not make a significant contribution to the overallreaction. Linear regression analysis gives k, = 1.46 xdm3 mol-l s-l and k, = 7.5 x loW5 s-l at 25 "C.Atthe acidities used in the present study it is difficult todetermine k, with any degree of precision. Attempts todetermine k, at lower acidities were frustrated by thecis+trans isomerisation reaction of the diaqua-species.Dasgupta 5 has reported a small contribution from k, inthe ring-opening of [CoL3(C03)]+ where k, = 2.5 xs-1 and k, = 7.1 x dm3 mol-l s-l at 40 "C. Therequisite constants k, at 34.8 and 45.4 "C are 4.25 xdm3 mol-l s-1 and 12.89 x lo-, dm3 mol-l s-l. Theappropriate activation parameters are AH1 = 82.9 kJmol-l and AS,, = -0.4 J K-l mol-l. At 25 "C, therate of the acid-catalysed aquation of the L4 complex isca. twice that for L2 and ca. 11.5 times faster than forL3 (see Table 2). The activation parameters for theacid-catalysed decarboxylations of the macrocycliccomplexes (Table 2) are quite similar with AH1 ca.85k J mol-1 and close to zero. Stenc effects appearto be unimportant ; however, complexes containingunsaturated ligands do decarboxylate rather faster thanthe cyclam derivative1979 1345r0( N, )CO/ ‘c=O\ / i 0r l+++ t i + f H20/O-0OH2\C //O‘OH-2 +kA-2 - fastH++2 ++2 +Mechanism ( A 1KfastH+ -k B - lH2° TiErk B - 2 - f a s tH+k A - lsl owc-(N4 ICOOH2\Mechanism ( 6 )(Nb denotes the macrocyclic ligandSCHEME Possible mechanisms for acid-catalysed decarboxylationThe intimate mechanism of the acid catalysis requiresconsideration. Early work by Sastri and Harrisfavoured a mechanism in which there is rate-determiningproton transfer from hydronium ion to the carbonato-moiety in the transition state, leading to ring-openingby metal-oxygen bond cleavage [Mechanism (A),Scheme].An alternative mechanism involves rapidpre-equilibrium protonation of the complex followed byrate-determining ring opening [Mechanism (B) , Scheme].It is possible to distinguish between these two mechan-isms by using solvent deuterium-isotope effects. Forthe value of kD,O/KHxO = 2.6 (Table 3), fully consistentwith a rapid pre-equilibrium protonation. More recentwork by Harris and his co-workers has established thatkD,o/ka,o for the acid-catalysed decarboxylation of thecarbonatotetrakis( pyridine)cobalt ( 1x1) ion falls in therange 1.7-1.9, while a re-investigation * of the solventdeuterium-isotope effect for the acid-catalysed decarb-TABLE 2Rate constants and activation parameters for the acid-catalysed decarboxylation of macrocyclic [CoL(CO,)]+mechanism (A) a rate-determining protbn-transfer stepis involved and kD,o/ka,o < 1.In mechanism (B), Ligand dm3 mo1-l s-1 k J mol-l J K-’ mol-l Ref.kD,O/kH,O > 1 since D20 is less basic than water, and MeZ[l4]diene ( ~ 4 ) 1.5 x 10-2 82.9 -0.4 Thishence the substrate will be able to compete with the worksolvent for the deuteron in D20 more effectively than for ~ ~ $ ~ ~ ~ ~ ~ (L2) 1.3 x 86.2the proton in H,O.complexes aA l l AHXI A S /88.6 +3.8b 1- 10.9 6values of k, a t 25 “C. bcalculated from Kernohan andFor solutions ca.0.5 mol dm-3 in hydrogen ion at 25 “C, Endicott (ref. 1)1346 J.C.S. DaltonTABLE 3Solvent deuterium-isotope effect at 25 "C a[HCl]/mol dm-s 1 0' kob. IS-' dm9 mo1-I s-l0.493 8.3 1.681O2kob,. [H+I-'/[DCl]/mol dm-8 23.7 4.35kHlo/kDlo = 2.6a The small contribution of k, to koa. was ignored in the cal-culation of the isotope effect. b Mean of three kinetic runs.oxylation of [Co(en),(C03)]+ gave a value of 2.3. There isnow considerable evidence to support the view that acid-catalysed decarboxylations of carbonato-complexesoccur by a rapid pre-equilibrium protonation, followedby a slow ratedetermining ring-opening.The reaction could occur by either an A-1 or an A-2type mechanism. For an A-1 reaction;K1 [(N4)CO(C03)I+ -I- H+ * [(N4)Co(C03H)12+[ (N4)Co( C0;H)l2+ kl slow [I ( N4)Co( OCO,H)I2+and the rate constant k, = K,k,; while for the A-2situation,K1 I: (N4) CO( CO3)I + H+ * [ (N4) CO (CO3H)I 2+AsC(N4)C0(COP)12' 4- H2Ocis- [ ( N4)C0( OH,) (CO,H)I2+and K, = K,k3[OHJ.The A-1 mechanism wouldinvolve decomposition of the protonated species to givea five-co-ordinate intermediate, while the A-2 mechanismwould involve nucleophilic attack by water on theprotonated species. For the A-1 reaction the entropyof activation is close to zero since the reaction is uni-molecular, while A-2 reactions have substantial negativeentropies of activation since they are bimolecular. ForA-1 reactions the rate of the reaction at very low aciditiesis proportional to the hydrogen-ion concentration and athigher acidities to h, (k a plot of log kobs. vs. - H , islinear with a slope of unity, where H, = -log h,).The acid-catalysed hydrolysis of cis-[CoL4(C0,)] + dis-plays a solvent deuterium-isotope effect kDIO/kH,O of2.6. This ratio is in the range (1.9-2.6) consideredtypicallo for A-1 type hydrolysis, and is much largerthan the values of 1.3-1.4 which have been observed lofor A-2 reactions. The present evidence thus favours anA-1 mechanism.[8/1077 Received, 10th June, 19781REFERENCESJ. A. Kernohan and J. F. Endicott, J . Amer. Chem. SOC..1969, 91, 6977; J . F. Endicott, N. A. P. Kane-Maguire, D. P.Rillema, and T. S. Roche, Inorg. Chem., 1973, 12, 1818.T. P. Dasgupta and G. M. Harris, J . Amer. Chem. SOC., 1971,93, 91.D. J. Francis and R. B. Jordan, Inorg. Chem., 1972, 11, 461.D. J. Francis and G. H. Searle, Austral. J . Chem., 1974, 27,269.T. P. Dasgupta, Inorg. Chim. Acta, 1976, 20, 33.13 K. E. Hyde, G. H. Fairchild, and G. M. Harris, Inorg. Chem.,V. S . Sastri and G. M. Harris, J . Amer. Chem. SOC., 1970, 92,* G. M. Harris and K. E. Hyde, Inorg. Chem., 1978, 17, 1892.@ R. W. Hay and G. A. Lawrance, J.C.S. Dalton, 1975, 1466.lo J. L. Kice and J. M. Anderson, J . Amer. Chem. SOC., 1966,88,1976,15, 2631.2943.5242
ISSN:1477-9226
DOI:10.1039/DT9790001343
出版商:RSC
年代:1979
数据来源: RSC
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