摘要:
J. CHEM. SOC. DALTON TRANS. 1983 2415Solvent Dependence of the Stereochemistry of Base-catalyzed Solvolysisof t r a n ~ - [ C o ( N H , ) , ( ~ ~ N k i , ) X ] ~ + ' ~ + (X = Me2S0 or CI) IonsSijbe Balt,' Hendrikus J. Gamelkoorn, and Willem E. RenkemaDepartment of Inorganic Chemistry, Free University, De Boelelaan 1083, 1081 HV Amsterdam,The NetherlandsThe cis-trans product ratio for the base-catalyzed solvolysis of the 15NH3 labelled~ ~ ~ ~ S - [ C O ( N H , ) , ( ~ ~ N H ~ ) C I ] ~ + ion has been determined as a function of the solvent. The solvents usedwere H20-CH30H and H20-dmso (dimethyl sulphoxide) mixtures, as well as anhydrous CH30H andCH3NH2. The trans-[C0(NH,),(~5NH~)(drnso)]3+ ion was included also in the study. The basehydrolysis results do not show any solvent or leaving-group dependence, within experimental error(2%). All trans systems give (44 f 1)% rearrangement.Small, significant differences are shown by thepenta-amminechloro-complex in CH3NH2 (50% rearrangement) and by the dmso complex (48%rearrangement in H20 and 46% in CH30H). The results are interpreted as indicating the adequacy of amechanistic model involving a five-co-ordinate intermediate.Stereochemical change in base hydrolysis is still to a largeextent an unsolved problem in the conjugate base (c.b.)mechanism. This mechanism generally consists of a fast pro-ton transfer pre-equilibrium, followed by the rate-determiningdissociative elimination of the leaving g~oup.~J Early stereo-chemical theories assumed the five-co-ordinate intermediate,inherent in the dissociative activation, to have a fixed geo-metry.This idea was combined with that of a stereospecificentry of the incoming ligand, directed by the amido-group.4VsRecent results on the ' red ' s and p (secondary and primary)isomers of the [CoL(NH3)XI3 + I 2 + ion [L = 2,2'-diamino-2"-methylaminotriethylamine, X = C1, Br, N3, or NH3], bearingon the lifetime of the intermediate,'jn7 have been interpretedas indicating that the five-co-ordinate species do not relaxtowards a common trigonal-bipyramidal structure, althoughthis process would require only a small change in bond angle.The resulting more dynamic picture is the immediate cap-ture of a solvent molecule, before bond angles can equilibrate.If indeed the process of capture of the entering group by thefive-co-ordinate species competes in rate with angle equilibr-ation and solvent structure rea~rangement,~.~ relative rates andconsequently product stereochemistry should be affected bythe nature of the solvent.This conclusion does not seem con-sistent with the fact that experiments in our laboratory re-vealed a close agreement in product stereochemistry for thebase-catalyzed reactions of [c~(en)~XY]~+'+ (en = ethylene-diamine; X = C1, Br, or dmso; Y = Cl, NCS, NO2, N3, orNH3) complexes in liquid ammonia and in aqueous solu-t i ~ n . ~ * ~ In this report we investigate the solvent dependence inthe base-catalyzed solvolysis reaction of tran~-[Co(NH~)~-(15NH3)X]3+/2+ (X = dmso or C1) ions, a reaction for whicha fixed five-co-ordinate intermediate seems to explainaqueous stereochemical results well.ExperimentalMateriaZs.-[Co(NH3)5(S03)]Cl was prepared accordingto Siebert and Wittke." The purity of the product proved tobe sufficient for the synthesis.tran~-[Co(NH~)~(OH)(S0~)1 was prepared as described inthe literature," with some modifications to improve thepurity of the product.[CO(NH~)~(SO~)]CI ( 5 g) was dissolved ina 3% aqueous solution of lithium hydroxide (125 cm9. After5 min stirring, Amberlite MB-3 cation-anion exchange resin(25 cm3) was added and after another 2 min stirring, the solu-tion was filtered. The hydroxosulphito-complex was pre-cipitated from the filtrate by adding 96% ethanol (150 cm3).Subsequent centrifuge recovery, washing with ethanol anddiethyl ether, and vacuum drying yielded tetra-ammine-(hydroxo)sulphitocobalt(~rr). This compound was used as astarting material for the synthesis of the trans-substitutedcomplex.For tran~-[Co(NH~)~('~NH~)C1][Cl0~]~, the method ofBuckingham et al." was followed, using a more strict pH con-trol than indicated by these workers.A solution of lSNH4C1(1 g) (VEB Berlin-Chemie) in water (10cm3) was brought to pH9.7 with solid lithium hydroxide monohydrate. Freshly pre-pared tetra-ammine(hydroxo)sulphitocobalt(iII) (1.25 g) wasthen dissolved in this solution; after 5 min stirring the re-sulting precipitate was filtered off. After readjusting the pHto 9.7, another 1.25 g of complex were added and this last pro-cedure repeated once.The brown-yellow product was washedwith ethanol and dried in vacuo. This trans-[C~(NH~)~(l~NH,)-(S03)]Cl was converted into the chloro-complex by boiling inconcentrated hydrochloric acid saturated with lithiumchloride, and converted into the perchlorate salt by treatmentwith silver perchlorate in solution. The anhydrous complexwas obtained by heating for 1 h in U ~ C U O at 100 "C, yield 70%.The purity of the compound was checked by n.m.r. spec-troscopy.'O[Co(NH3)4('"~,)(dm~o)l[C10413 (dmso = dimethylsulphoxide) was prepared from trans-[C~(NH~)~(l~NH~)-(H20)][C104]3 according to Piriz Mac-Coll and Beyer,12 andrecrystallized from acidified aqueous solution. The aqua-complex was obtained from the mercury(1r)-catalyzedaquation of trans-[Co(NH3)4('sNH3)Cl][C104]2.10 This pro-cedure yielded a mixture of cis and trans dmso complex, thecomposition of which (determined by 'H n.m.r.spectroscopy)depended on the reaction temperature. A reaction at roomtemperature during 10 d gave a 90% pure trans complex.The other reagents and solvents used were purchased asanalytical grade and purified further, and when necessarydried by routine methods.I3Base-catalyzed Solvolysis of the 15NH3 Labelled Penta-ammines.-Penta-amminechlorocobalt(I1~) perchlorate wassubjected to base-catalyzed solvolysis at 25 "C in water, water-methanol mixtures, and water-dmso mixtures (up to 40% v/vdmso ; higher percentages of dmso decomposed the complex)by dissolving the complex (200 g) in the (mixed) solvent(10 cm7, followed by the addition of sodium hydroxide(1 cm3, 2 mol dm-9.After 5 min, an excess of concentratedperchloric acid was added to precipitate the penta-ammine-aqua-complex. This complex was filtered off, washed witJ. CHEM. SOC. DALTON TRANS. 1983ethanol and diethyl ether, and dried in uacuo. The dmso com-plex in water was treated analogously. The reactions of[CO(NH,)~(~~NH~)X]~+"+ (X = dmso or C1) in anhydrousmethanol were performed in a closed system under dry nitro-gen. The perchlorate salt (100 mg) was dissolved in driedmethanol (25 cm3) in which metallic sodium (0.25 g) had beendissolved previously. After 30 min, the slightly turbid solutionwas filtered and an excess of concentrated perchloric acidwas slowly added with cooling in ice.The pink complex,[CO(NH,)~( 15NH3)( CH,OH)] [ C104l3, crystallizing from thesolution, was filtered off, washed with ethanol and diethylether, dried, and kept in uacuo.The penta-amminechloro-complex was dissolved in methyl-amine by condensing methylamine at -70 "C on to the per-chlorate salt (200 mg) in a closed system. On warming to-10 "C the reaction started and was complete (yellowsolution) in 30 min. The reaction product was isolated byevaporating the solvent. The resulting solid [CO(NH,)~-(15NH3)(CH3NH2)][C104]2Cl was converted into the perchlor-ate by treatment with AgC104, washed with cold ethanol anddiethyl ether, and dried in uacuo.In none of the cases studied was a subsequent rearrange-ment of the products observed.N.M.R.Spectra.-Hydrogen-1 n.m.r. spectra of the 15NH3labelled compounds were recorded on a Bruker WM 250 or 500Fourier spectrometer. Solutions were made up in 5-mmdiameter tubes, using acidified (trace of D2S04) ['H6]dmso assolvent. Chemical shifts are in p.p.m. relative to sodium 3-trimethylsilylpropane-1 -sulphonate (tsp) as internal reference,The chemical shifts of cobalt(rr1) amine complexes aresusceptible to medium effect~.'~ We found these effects to beminimal for the perchlorates ( <0.02 p.p.m.). The chloridesalts consistently show a downfield shift of the ammineresonances compared to the perchlorates and are more proneto medium effects. Integrals of resonance areas were repro-ducible within 2%.ResultsHydrolysis in Water and Aqueous Mixtures.-For aqueoussolutions and the solvent mixtures entered in the Table themain products of the base-catalyzed reactions after acidific-ation are cis- and ~~u~~-[CO(NH~)~('~NH~)(H~O)][CIO~]~.*The products were identified from the 'H n.m.r. spectrum.The two geometric isomers have clearly separated 15NH3resonances: a characteristic doublet [J(15N-H) = 70 Hz] ofcomparatively narrow lines, its centre coinciding with thebroad 14NH3 resonance at 2.81 p.p.m.(ISNH3 trans to H20)or at 3.83 p.p.m. (cis). The resonance of co-ordinated HzOwas found at 5.72 p.p.m. These positions agree with publishedspectra.'O The cis-trans product distribution was calculatedfrom the 14NH3 peak areas and the 15NH3 peak areas or peakheights.The results are given in the Table.Reaction in Methanol.1n the product of base-catalyzedsolvolysis in anhydrous methanol the aqua-resonances werecompletely absent. The identification of the final product asthe [CO(NH~)~(~'NH~)(CH,OH)]~ + ion is evident from themethanol exchange of this compound in D20 l6 and the 'Hn.m.r. spectrum. Resonances were found at 2.81 p.p.m. [trans-14NH3 and the t~ans-*~NH~ doublet, J(15N-H) = 70 Hz], 4.01p.p.m. (cis-NH3), 2.86 p.p.m. [co-ordinated CH30H triplet,* Due to incomplete precipitation the relative amount of methanolcomplex in the precipitates was smaller than expected l5 in theH20-CH30H solutions. No co-ordinated dmso was observed.We assume isotopic effects on the solubility to be negligible.Table.Product distribution of base-catalyzed solvolysis of trans-[CO(NH,),(~~NH~)X]~ + I 2 + complexes in water-co-solvent mixturesand pure solvents at 25 "CXCIc1 c1c1c1c1c1c1Clc1dmsodmsoC*solven t-CHjOHCHjOHCHaOHCHSOHCHjOHdmsodmsoCH3NH2CHjOH-Percentageco-solven t(v/v)-104050701002040100100-Percentagecis504445454543444444504846* If no co-solvent is indicated, the reaction medium is pure water.Estimated error in percentage of cis product is 2%. The entriesare averaged over at least two independent measurements. Fromref. 10, error 5%. At - 10 "C.J(H-H) = 7 Hz], and 6.74 p.p.m. (co-ordinated CH30H).Again the cis-trans product distribution followed from theresonance areas and peak heights.Reaction in Methylamine.-The solvolysis reaction was donewithout the addition of base, as reactions in liquid ammonia l7and amines Is have been shown to follow the c.b.mechanism,even when no base is added; no spontaneous reaction routeis detected. In addition, the presence of amide ions must beavoided, as it induces polymerization and condensation~eacti0ns.I~ The product of the reaction could be identified as[CO(NH~)~('~NH~)(CH~NH~)][C~O~]~C~ by 'H n.m.r., i.r., andu.v.-visible spectra." In addition, a minor amount ( < 1%) ofthe unreacted substrate and some 15NH3-containing hexa-ammine (4%) was invariably detected. Because of the smalldifference in ligand field strength between ammonia andmethylamine, ammine hydrogens cis and trans with respect toco-ordinated methylamine in the complex are expected toshow a very small difference in chemical shift.The resonancescould only be clearly resolved in the 500-MHz spectrum ofthe triperchlorate, where they appeared as two narrowdoublets [J(ISN-H) = 70 Hz] of equal intensity, thecentre of each coinciding with the broad cis-14NH3 (3.36p.p.m.) or trans-14NH3 (3.38 p.p.m.) signals. The assignmentof the lower field resonances to trans-NH3 follows from therelative intensities. It does not agree with the lower effectiveligand field of methylamine compared to ammonia in co-balt(ri1) comple~es.~~-~~ Additional broader bands were foundat 4.17 p.p.m. (CH3NHz, unresolved quartet) and 1.91 p.p.m.[CH3NH2, triplet, J(H-H) = 7 Hz].The spectrum is displayedin the Figure.Base Hydrolysis of [C0(NH~)~(~~NH~)(drnso)][C10~ J3.-Because no pure trans-15NH3 compound could be synthesized,the percentage rearrangement on base hydrolysis was calcul-ated from the cis-trans product distribution (determined by250-MHz lH n.m.r.) in the initial mixture and the product,using the formula for the experimental fraction x of cis isomerin the penta-ammineaqua-product, x = ab + (1 - b)(l -0.25a), with a as the fractional rearrangement in the basehydrolysis of the original trans compound and b as the initialfraction of trans in the starting compound. Experimentally wefound for b = 0.65 that x = 0.60 and for b = 0.82 that x =0.55, giving a = 0.44 and 0.48 respectively. The isomericallJ.CHEM. SOC. DALTON TRANS. 1983 2417Y '0085.CIS- NH3 dmso4.0 35 3.0 2.5 206 /p.p,m.Figure. 'H N.m.r. (500 MHz, in [*&]dmso, reference tsp) spectrumof the cis- trans- [ Co( NH3),( :"H3)(CH3NH2)] [ CIO& reactionproduct. The upper lines show the resonances on an enlarged scale.For the NH2 and the CH3 signals, the resolution-enhanced spectrum(using Gaussian deconvolution) is displayed alsomost pure compound contained 92% of the trans isomer (b =0.92) and yielded 51% of the cis aqua-isomer; the calculatedcis-trans ratio for this most accurate case is given in the Table(a = 0.48). Agreement with the results from the less iso-metrically pure samples is satisfactory and illustrates the cor-rectness of the calculation.Reaction of the 92% trans compound in anhydrous methanolgave 49% of the ~~~-[CO(NH~)~('~NH~)(CH~OH)J~ + isomer(a = 0.46).DiscussionThe product distribution of base-catalyzed solvolysis of the~~~~S-[CO(NH~)~('~NH~)C~]~+ ion in various solvents (with theexception of methylamine), as displayed in the Table, isremarkably independent of the nature of the solvent.In factthe percentage rearrangement (percentage of cis, Table) has anaverage of 44 f 1. The reaction product in methylamineshows a small but significant deviation from this average. Thecause may be the difference in reaction temperature (35 "C).It has been convincingly argued7 that the lifetime of thefive-co-ordinate intermediate is so short that this situationis on the border of the possibility of a thermodynamicdistinction 25 between transition state and intermediate. Inthis picture, which has been obtained from studies of poly-chelate complexes, the stereochemistry of base hydrolysis willbe strongly influenced by the fact that the five-co-ordinatespecies tends to react with the incoming group even before aneasily accessible and expectedly stable configuration isreached.Then the position of entry of the incoming solventmolecule will be dependent on the moment of entry, as theprocesses of bond angle adjustment and solvent entry arethought to be comparable in ~ate.6'~If this situation is also accepted for the penta-ammine-chlorocobalt(n1) ion, the considerable steric rearrangement,observed on base hydrolysis, will in all probability depend onthe solvent.This will then certainly be shown in the range ofsolvents used here, as these have widely different structure anddifferent nucleophilicity (HzO, CH30H, CH3NH2) and, prob-ably even more important for the fast reactions involved,different stereochemical properties. The absence of a solventeffect on the product distribution is a strong indication thatthe position of entry of the neutral solvent molecule occurs atan approximately fixed position on the reaction co-ordinate.The mechanistic possibility of other positions of entry (andconsequently other cis-trans product ratios) is confirmed bythe results of anion competition studies,'O and the presentresults on the solvolysis of the dmso-penta-ammine complex.Our conclusion may be expressed more specifically as indi-cating the presence of a definite intermediate (or intermediates),in which the directive effects are not much affected by thesolvation sphere.This interpretation is along the lines of thatpostulated in the now classic model of Nordmeyer.s However,this model needs to be refined, as the cis-trans product ratitdiffers significantly from the predicted 50-50 ratio.The results on the stereochemical course of base hydrolysisof the [CO(NH,),(~~NH~)(~~SO)]~+ ion fit this picture. Thesmall but solvent-independent - (H20,CH30H) leaving-groupeffect, which is on the border of significance, can be explainedby the presence of the leaving dmso in the solvation shell of thetransition state and the intermediate.ls It is then reasonable toassume that this bulky group will influence the relative ease ofentering of the solvent.In addition, the difference in cationiccharge between the chloro- and the dmso-penta-amminecomplexes will influence the composition of the solvent sheetof the substrate. If it is accepted that this solvent sheet isinherited in the five-co-ordinate intermediate, it may thusaffect the product distribution. The latter explanation followsthe exposition of charge dependence of competition results forthe [CO(NH~)~X]"+ series by Sargeson and co-worker~.~~These authors have rejected the assumption of an interchangemechanism to explain the exceptional position of especiallydmso as leaving group in anion competition results.26The results obtained in this study can be interpreted toimply that for base hydrolysis of cobalt(1u) amine complexesthe existence of one or more discrete intermediate(s) of re-duced co-ordination number is still a reasonable assumption,at least for the simple monodentate amine complexes.Theanalogous stereochemical behaviour of bis(ethy1enediamine)-cobalt(II1) complexes in water and liquid ammonia * i 9 makesthe extension of this conclusion to these compounds reason-able. However, this contradicts the results obtained with somepolychelate complexes that are now being studied extensively.Of course the simplicity of the complexes of the present reportmakes it difficult to reach more detailed conclusions.Finally, it must be emphasised again that the independenceof the solvent and consequently that of the entering group onthe steric course, demonstrated by the results presented here,is restricted to neutral entering groups.An anionic ligand suchas the azide ion, used in competition experiments, doeschange the steric course of the reaction.'' Another restrictionon the present conclusion is the choice of the leaving group.In view of the competition results it would be interesting toextend this study to more reactive and to less reactive penta-amminecobalt(Ir1) complexes than those reported here.AcknowledgementsThe authors wish to thank Mr. P. C. M. van Zijl for recordingthe 500-MHz n.m.r. spectra and the Netherlands Foundationfor Chemical Research (S.O.N.) for access to the National500j200 HF-NMR facilities at Nijmegen.References1 See, for example, D.Fenemor and D. A. House, J. Inorg. Nucl.Chem., 1976, 38, 1559; D. A. Buckingham, C. R, Clark, andT. W. Lewis, Inorg. Chem., 1979, 18, 1985 and refs. therein.2 F. Basolo and R. Pearson, ' Mechanisms of Inorganic Reactions,'2nd edn., Wiley, New York, 1967, p. 177.3 M. L. Tobe, Acc. Chem. Res., 1970, 3, 377.4 Ref. 2, p. 261.5 F. R. Nordmeyer, Znorg. Chem., 1969, 8, 2780.6 D. A. Buckingham, J. D. Edwards, T. W. Lewis, and G. M.7 D. A. Buckingham, J. D. Edwards, and G. M. Faughlin, Inorg.McLaughlin, J. Chem. SOC., Chem. Commun., 1978, 892.Chem., 1982,21, 27702418 J. CHEW SOC. DALTON TRANS. 19838 S. Balt, J. Breman, and W. de Kieviet, J. Inorg. Nucl. Chem.,9 S . Balt, H. J. Gamelkoorn, H. J. A. M. Kuipers, and W. E.10 D. A. Buckingham, I. I. Olsen, and A. M. Sargeson, J. Am.1 1 H. Siebert and G. Wittke, 2. Anorg. Allg. Chem., 1973, 399,12 C. R. Piriz Mac-Coll and L. Beyer, Inorg. Chem., 1973, 12,13 J. A. Riddick and W. B. Bunger, ‘Techniques of Chemistry,14 R. Bramley, I. I. Creaser, D. J. Mackey, and A. M. Sargeson,15 N. E. Dixon, W. G. Jackson, W. Marty, and A. M. Sargeson,16 R. B. Jordan, A. M. Sargeson, and H. Taube, Inorg. Chem.,17 S . Balt and A. Jelsma, Inorg. Chem., 1981, 20, 733 and refs.1979, 41, 331.Renkema, Inorg. Chem., in the press.Chem. SOC., 1967, 89, 5129; 1968,90, 6539.43.9.Organic Solvents,’ 3rd edn., 1970, vol. 11.Inorg. Chem., 1978, 17, 244.Inorg. Chem., 1982, 21, 688.1966, 5, 1091.therein.18 S . Balt, M. W. G. de Bolster, and H. J. Gamelkoorn, Inorg.19 0. Schmitz-Du Mont, Rec. Chem. Prog., 1968, 29, 13.20 M. W. G. de Bolster and J. W. M. Wegener, J . Inorg. Nucl.21 H. Yoneda and Y. Nakashima, Bull. Chem. Soc. Jpn., 1974,22 R. Mitzner, W. Depkat, and P. Blankenburg, 2. Chem., 1970,23 L. F. Book, K. Y. Hui, and W.-K. Li, 2. Anorg. Allg. Chem.,24 M. W. G. de Bolster and J. W. M. Wegener, unpublished25 W. P. Jencks, Ace. Chem. Res., 1980, 13, 161.26 W. L. Reynolds and S. Hafezi, Inorg. Chem., 1978, 17, 1819.Chim. Acta, 1979, 35, L329.Chem., 1977,39, 1459.47, 669.10, 34.1976,426, 215.work.Received 7th February 1983 ; Paper 311 8
ISSN:1477-9226
DOI:10.1039/DT9830002415
出版商:RSC
年代:1983
数据来源: RSC