摘要:
J. CHEM. SOC. DALTON TRANS. 1982 1285Configurational Effects in the Reaction of Cobalt(i) Schiff-base Com-plexes with Alkyl BromidesBy Anna Puxeddu and Giacomo Costa,' lnstituto di Chimica, UniversitA di Trieste, 341 27 Trieste, ItalyElectrogenerated diasteroisomeric cobalt(1) chelate complexes of quadridentate Schiff bases derived from sub-stituted ethylenediamines and salicylaldehyde react with PrnBr, Bu'Br, and ButBr at different rates. The dif-ference in the second-order rate constant can be attributed to the various distortions of the cobalt(1) complex indifferent configurations. The rate-limiting step is independent of the formation of the cobalt-carbon bond and itis proposed that the reaction occurs via an inner-sphere alkyl-bridged electron transfer, with a [CoJ(chelate)]-R+ X- transition state.The reaction is sensitive to purely stereochemical perturbations of the co-ordinationsphere of the metal atom.THE use of model compounds is a technique which hasbeen extensively employed in order to gain informationabout the mechanism of reaction at co-ordination sites oncobalt and iron atoms in coenzyme B,, chemistry and inoxygen transport and activation chemistry respective1y.lPreviously, most work has been devoted to the rationaliz-ation of the behaviour of model and naturally occurringcompounds as regards the electronic effects of ligands,manifest in redox potentials and the thermodynamics andkinetics of chemical reactivity. However, steric factorsare an essential element in the control of biochemicalfunctions, e.g.through conformational changes due toligand binding. Hence, the ' intrinsic ' kinetic barriersdeserve as much attention as the thermodynamic aspectsof reactivity. Knowledge of the mechanism which leadsto selectivity is relevant not only to the understandingof enzymatic reactions, but also in the development ofnew catalysts.Special steric features were introduced in model com-pounds for oxygen transport and activation with the aimof reproducing the obstacle to axial ligation which pre-vents dimerization of the chelate, as in the naturallyoccurring oxygen carrier.2. tA series of cobalt chelates which can be obtained indifferent configurations involving only minor changes inthe electronic structure was prepared by Ugo and co-worker~.~ Enantiomers, and diastereoisomeric, optic-ally active, and meso forms, were synthesized fromquadridentate Schiff-base ligands formed by the con-densation of salicylaldehyde with mono- and di-sub-stituted ethylenediamine. The equilibrium and kineticparameters of the reversible formation of oxygen adductsof these chelates were discussed in terms of the structiualdistortions, as induced by different conformations.4J~We have found significant differences in redox poten-tials between diastereoisomers 6*7 which can be related todifferences in chemical reactivity.Electrochemicaltechniques can be used to investigate the mechanisms in-volving heterogeneous electron transfer@) and homo-geneous chemical reaction undergone by cobalt chelatesThe axial ligation constants of the ' capped ' metal por-phyrins differ from those of the flat porphyrins, although smallmolecules, e.g.CO and 0,, can co-ordinate in the axial positioninside the cap (P. E. Ellis, jun., J. E. Linard, T. Szymanski, R. D.Jones, J. R. Budge, and F. Basolo, J . Am. Chem. Soc., 1980,102, 1889; J. E. Baldwin, T. Klase, and M. Peters, J . Chem. Soc.,Chem. Commun., 1976, 881).and their organometallic derivatives.8* $ We have alsoreported an approximately linear relationship betweenthe polarographic half-wave potential (E,) of the revers-ible, heterogeneous one-electron reduction (1) and the[CoII(chelate)] +", [CoI(chelate)]-[CoJ(chelate)]- + RX 2 [CoR(chelate)] + X- (2)log of the rate constant of the reaction (2) where thechelate is a quadridentate Schiff base formed from salicyl-aldehyde or o-hydroxyacet ophenone and ethylenediamineor o-phenylenediamine, and R = CH,.The methyl-donor compound, RX, can be a methyl halide, methyl-sulphonium, or methylammonium ion. This reaction ofnucleophilic low-valent metal complexes is one of themore general routes to the preparation of cobalt(i1~)-alkyl comp1exes.l. 8The present work attempts to evaluate the influence ofsteric factors on the rate of reaction (2) and to separatethe kinetic components from the driving force of thereaction.7 The second-order rate constants of the reac-tions of PmBr, BunBr, and RutBr with a range of optic-ally active and diastereoisomeric model cobalt (I)chelates obtained in situ by electrochemical reduction ofthe corresponding cobalt (11) chelates have thus beendetermined.The Ei value of reaction (1) is related tothe driving force of reaction (2) to show that the changesin the free energies of activation and of reaction are in-dependent of the formation of the organocobalt deriv-ative. The Marcus model as extended to group transferreactions lo is applied in order to obtain informationabout the nature of the transition state and the reactionmechanism.EXPERIMENTALMaterials.-The cobalt complexes, [CoII(chelate) 1, werekindly donated by Professor R. Ugo. The parent ligand,3 A ' quantitative ' comparison of vitamin B,, model corn-pounds was carried out recently with electrochemical techniquesby R.G . Finke, B. L. Smith, M. W. Droege, C. M. Elliott, and E.Hershenhart, J . Organornet. Chem., 1980, 202, C26.5 The synthesis of organocobalt derivatives by chemicalreduction of [CoL1(salen)] has been reported (G. Costa and G.Mestroni. J . Organomet. Chem., 1968, 11, 333).T In the study of charge transfer between alkylmetals andoxidants it was proposed that the separation of polar from stericeffects is possible by meta, para, and ortho substitution in theorganic moiety (C. L. Wong and J. K. Kochi, J . Am. Chem. Soc.,1980, 108, 2928)1286 J. CHEM. SOC. DALTON TRANS. 1982[Coll(chelate )]Chelate R RNN'-Ethylenebis(salicy1ideneiminate) : salen H HH CH8H C,H,CH, CH,-(CH34-NN'-Propane-l,2-diylbis(salicylideneiminate) : sal( + )pn or sal( j- )pnN W - l-Phenylethylenebis(salicy1ideneiminate) : sal( - )penNN'-Butane-2,3-diylbis(salicylideneiminate) : sal( +)bn or sal(m)bnN W - 1,2-Diphenylethylenebis(salicylideneiminate) : sal( + )dpen or sal(m)dpen C6H5 C6H& NN'-1,2-Cyclohexylenebis(salicylideneiminate) : sal( + )chxn, sal(m)chxn, or sal( -)chxnNN'-ethylenebis(salicy1ideneiminate) (salen) , was obtainedfrom ethylenediamine while from o-phenylenediamine, thecorresponding ligand, NN'-o-phenylenebis( salicylideneimin-ate) (salphen), was obtained.The synthesis and charac-terization of the complexes have been described in detailel~ewhere.~Dimethylformamide (dmf) (Erba) was dried over 4 Amolecular sieves before use. Sodium perchlorate (Erba)was dried at 50 "C in 'uacuo.t-Butyl bromide was purifiedby successive distillations.Afiparutus.-The apparatus used for cyclic voltammetricmeasurements has already been described.*Kinetic Measurements .-Reactions between the electro-generated [CoI(chelate)]- complexes and alkyl bromides werefollowed by cyclic voltammetry as previously reported .aThe kinetic measurements were performed under pseudo-first-order conditions in the presence of an excess of sub-strate RX and the second-order rate constants were cal-culated as before l1 from the rate of disappearance of[CoI(chelate)]- by Nicholson's method.11*12 The reactionwith ButBr is electrocatalytic and was investigated as pre-viously described.13Catalysis of alkyl halide reduction by vitamin B,, wasreported by Hill et aZ.14 and was subsequently investigatedby Lexa et aZ.lb in their studies on electrochemistry andelectrocatalysis of vitamin B,, derivatives and porphyrins.We independently reported the electrocatalytic reduction ofButBr by [Co1(salen)]-.13Half-wave potentials of irreversible two-electron reduc-tions of saturated aliphatic monobromides in dmf-[NBu,]-[CIO,] (0.01 mol dm-3) are likely to be much more negativethan the standard potential (although not known pre-cisely).16-18 On the other hand, cobalt(r) and nickel(0)complexes occasionally show a one-electron reductionpotential which is less cathodic than the actual irreversiblereduction potential of the alkyl halides but more cathodicthan their standard potential for a hypothetical one-electronreduction of the latter.1s-22RESULTS AND DISCUSSIONThe differences in the E, data for reaction (1) are sig-nificant not only between compounds with chelatingagents bearing different substituents in the ethylenedi-amine ring, but also between diastereoisomers differingonly in their configuration.In the presence of PrnBr and BunBr in dmf solution,the electrogenerated [CoI(chelate)]- yields the cor-responding organometals according to reaction (2) ,&while reaction with ButBr yields isobutene and hydrogentogether with [CoTI(chelate)] according to reactions (3)and (4).13k3 [CoI(chelate)]- + ButBr kl[CoBut (Br) (chelate)] - (3)(4)[CoBut(Br) (chelate)]- - k,[CoIr(chelate)] + C,H, + &H, + Br-The second-order rate constants of reactions (2) and (3)The data span a range of ca. 1.7 are given in the Table.Polarographic half-wave potentials for reaction (1) and kinetic data for reactions (2) and (3) in dmf-NaC10, (0.1 moldm-3) a t 0 "Ck , */dm3 mol-1 s-l k , */dm8 mol-l s-'[ CoII(che1ate)l CO'I CO' R = R" R = Bu" R = But[ Co(salen)] - 1.119 1041 (3.02) 1436 (3.16) 1019 (3.01)-1.124 731 (2.86) 946 (2.98)- 1.084 464 (2.67) 531 (2.73)118 (2.07) -1.126 191 (2.28) 210 (2.32)[Co{sal(m) bn)] - 1.146 945 (2.98) 1250 (3.10) 1 176 (3.07)-1.044 40 (1.60) 76 (1.88) 146 (2.16)213 (2.33) 190 (2.28) 176 (2.24)- 1.171 2 096 (3.32) 3 174 (3.60) 3 140 (3.50) [Co(sal( - )chxn}][ Co (salphen)] - 1.043 506 (2.70) 762 (1.79)(Ei),/V for[Coisal( *)pn)l[COW( - )pen11[Co{sal(+ )bn)l[ C O W + )dpen11CCo{sal(m) d P 4 l[Co{sal(m)chxn)] -1.164 1542 (3.19) 2 100 (3.32) 2 200 (3.34)- 1.067* log k Values are given in parenthesesJ.CHEM. SOC. DALTON TRANS. 1982 1287logarithmic units but allow a meaningful discussion ofdifferences and trends. A plot of log k against E+ isshown in Figure 1.4 Q00@88 9910-1-05 -1.10 -1.15E, / V vs. s.c.e.2FIGURE 1 Relationship between the rates (log k) of reactions(2) and (3) and the half-wave potentials, Eh. Data from theTable: (a) PrnBr, (0) BunBr, (0) ButBr; salphen (l),sal( -)pen (2). salen (3), sal( f)pn (4). sal(m)bn (5), sal(m)chxn(6), sal( -)chxn (7), sal(+)dpen (a), sal(m)dpen (Q), sal(+)bn(10)It can be seen that log k increases with E, for the com-plexes [Co(salphen)], [Co(salen)], the monosubstitutedderivatives [Co(sal( j-)pn}], [Co(sal( -)pen}], and thecyclohexyl derivatives.With the exception of [Co-{sal( +)dpen}] , all the chelates examined show similarreaction rate constants with PrnBr, BunBr, and ButBr,within 0.2 logarithmic units. Thus, the log k valuesappear to be influenced by the structure of the metalchelate to the same extent for all substrates.Moreover, an approximately linear relationship be-tween log k and Et is found for [Co(salphen)], [Co-(salen)], and the two diastereoisomers [Co(sal( -)chxn}]and [Co(sal(m)chxn)]. Deviations from linearity areobserved for the monosubstituted complexes, [Co-(sal(&)pn}J and [Co(sal(-)pen}], as well as for the mesoform [Co(sal(m)bn}], stronger deviations being found forboth the diastereoisomers [Co(sal( +)dpen)] and [Co(sal-(m)dpen}] and for [Co(sal(+)bn}].Significant differences are found between the reactionrate constants of diastereoisomers: the meso forms arereduced at more cathodic potentials and with higher rateconstants than the optically active forms of the butane-diamine and the diphenylethylenediamine derivatives.An opposite trend is observed for the cyclohexyldiaminederivatives, whose meso form is reduced at Less cathodicpotentials and with smaller rate constants than theoptically active forms.These results are in agreement with the variations inthe circular dichroism (c.d.) spectra in the 350-nm region.%A complete inversion of the sign of the c.d. spectrum wasobserved for [Co{sal( -)chxn}] with respect to all opticallyactive species.This is attributed to the fact that themost stable conformation of [Co{sal( +)bn}j and [Co{sal-(+)dpen}] has the substituents on the ethylenediaminecarbon atoms in axial positions with respect to theCo-N-C-C-N ring which is in the half-chair conformationdue to steric repulsion between the substituents and thehydrogen atoms of the azomethine group. In the optic-ally active species, [Co(sal(+)chxn}], the only possible con-formation is bis-equatorial due to the steric requirementsof the cyclohexyl ring fused with the Co-N-C-C-N ring.As regards the difference in log k values between twodiastereoisomers, it is observed that the [Co(sal( +)bn}]-[Co(sal(m)bn}] couple show the greatest difference whilethat for the [Co(sal( +)chxn}]-[Co{sal(m)chxn}] coupleis :he least.The position of the various chelates relative to theapproximately linear plot of Figure 1 can be rationalizedin terms of the distortion caused by substitution and interms of conformation of the Co-N-C-C-N ring in thechelate macrocycle.The strongest deviations, observedfor [Co(sal( +)bn)] and [Co(sal( +)dpen}], can beattributed to the bis-axial conformation which presum-ably causes a strong distortion from the planar geometryof the equatorial co-ordination bonds, unlike the mesoforms with axial-equatorial conformations, which arelikely to be less distorted. The smaller deviations fromlinearity shown by [Co(sal( -)pen}] and [Co(sal( &)pn>]can be explained by the less constrained geometry of themonosubstituted Co-N-C-C-N ring.Conversely, inthe cyclohexylenediamine derivatives the less distortedgeometry is likely to be that of the optically active di-astereoisomer which has the bis-equatorial conformation.Moreover, the cyclohexyl derivatives are probably morerigid than the other chelates, preventing major distortionof the co-ordination bonds around the cobalt atom.Apart from the evidence for conformational effects ofthe ligands on the rate constant of reaction (2), the aboveresults can also be examined with reference to the reac-tion mechanism. The formation of alkylcobalt com-pounds from cobalt ( I) chelates of bis(dimethylg1yoxim-ato)-dianions, quadridentate Schiff bases, and vitaminB,, itself is generally classified as a nucleophilic substitu-tion at the or-carbon atom of the alkyl group, occurring witha classical SN2 mechanism.lfv= This mechanism was alsoproposed for the alkylat ion of tet ra-aza-macroc ycles .MHowever, the high rate constant found for the react-ion of Me1 with [COIL] (L = Fi,7,7,12,14,14-hexamethyl-1,4,8,1l-tetra-azacyclodeca-4,ll-diene) led Tait et aLZ5and Endicott and co-workers26 to suggest that thisspecies and other Curtis-type macrocycles which fail togive easily isolable cobalt (III)-alkyl compounds do notfollow the classical S N 2 mechanism but rather react viaan electron-transfer nxchanism generating CoII, CH,',and I-.Further information concerning the detailedmechanism of this process was obtained from analysis o128810J.CHEM. SOC. DALTON TRANS. 1982t \.:4 0the relationship between the free energy of activation,AGJ, and the driving force of the reaction, AGe, assumingthe validity of the Marcus model extended to the methyl,and more generally, to the saturated primary alkyl grouptransfer reactions.10s27, * The linear relationship pre-viously found between reaction rate constants of a seriesof [CoI(chelate)]- with EtBr, PhBr, and (Me,SC,H,Me)+and the reversible redox potentials of the CoII-CoIcouple 8~ suggests that when bulky substituents are notc !- E"4bV x\(3 a" t a9a9a 1024 25 26 27F€1/ kcal mol-'2FIGURE 2 Relationship between the free energy of activationfor reaction (2) and the half-wave potentials (1 cal = 4.184 J) ;RX = PrnBr.AGS is calculated from the equation: k = 2 exp-(-AGt/RT) where 2 w 10" dm3 mol-l s-1; key as in Figure 1present on the quadridentate equatorial ligand the Eivalue for the CoII-CoI process is related to the thermo-dynamics of the reaction. The driving force of the reac-tion may thus be assumed to be linearly proportional tothe difference between the standard potential of the one-electron oxidation of the cobalt@) species and that of theone-electron reduction of the substrate RX. Unfor-tunately, the electrochemical reduction of alkyl halidesoccurs with electron transfer before and after bondcleavage and the standard potential of the first reversibleone-electron transfer cannot be measured.Nevertheless, for a series of reactions of [CoI(chelate)]-with the same organic halide, the diference in the drivingforce, AG, - AG2, of two such reactions can be assumedto be linearly related to the diference in the hetero-geneous one-electron transfer potentials, (IT&), - (EJ2.The slope of the relationship between the free energies,AGI and AG*, can be found from a plot of FE, againstAGX, as calculated from the second-order rate constant(Figure 2).The main result obtained from such a plot* Recent work of S. Fukuzumi, C. L. Wong, and J . K. Kochi,J . Am. Chem. Soc., 1980, 102, 2928, points to the potential of theMarcus model together with the charge-transfer theory of Mullikenin evaluating steric effects as quantitative probes for outer- andinner-sphere electron transfer between organometals and variousoxidizing agents.The application of the Marcus model to atomand group transfer reactions was recently reviewed by Albery 108and was adopted by Endicott et ~ 1 . ~ 0 in studies of energetics anddynamics of methyl-bridged electron transfer for the methyl-cobalamin-cobalamin(I1) couple and model chelates.is the value of the symmetry factor, a, which is related tothe location of the transition state along the reaction co-ordinate in the electron, atom, or group transfer. Inequation (5) AG,,,: and AG,,,,,~ are the free energies ofactivation of the symmetrical transfer reaction betweenthe same donor and acceptor system.A linear dependence of AGI on AGe is only found, inprinciple, when changes in AGe along the reaction seriesare limited.In the present case, the range of AGeis relatively small; one of the AGX terms for the sym-metrical reaction (RBr + Br-) remains constant whilethe other changes only slightly when the less distorted[Col(chelate)] - nucleophiles are considered. An ap-proximately linear relationship between log k and Eior between AGI and AGe is thus expected in these cases.For the reactions of [CoI(salen)]-, [CoI(salphen)]-,[Co*(sal(m)chxn}]-, and [CoI{sal( -)chxn}]- with PrnBra linear relationship between AGI and FE, is found witha slope of a = 0.27. This value is below the lowestfound lo for the reaction of C,H,SO,Me with conventionalnucleophiles such as OH- ( a = 0.33) and CN- (a = 0.34)and, hence, is in accord with the ' downhill ' process dueto the strong nucleophilicity of [CoI(chelate)]-. More-over, this value of a suggests a reactant-like character ofthe transition state.The similar kinetic behaviour of primary and tertiaryalkyl substrates, irrespective of the formation of organo-cobalt derivatives, is also relevant to the understandingof the reaction mechanism.This feature suggests thatthe rate-determining step and its transition state are notqualitatively different when either primary or tertiarycarbon atoms are involved. This confirms that thetransition state precedes the formation of the cobalt-carbon bond, which is stable only with primary andsecondary carbon atoms.Furthermore, an ion-pairintermediate ensuing from the rate-limiting electrontransfer can be envisaged.The nature of the transition state of the rate-determin-ing step of an S N ~ reaction has been the subject of exten-sive discussion.10**28 If the transition state is morereactant- than product-like, the possibility of chargeseparation within the substrate and the formation of anion pair is suggested. A pictorial representation of theS N ~ ion-pair mechanism was proposed by Bordwell andco-workers; 29 in the present case it can be representedby equations (6) and (7). The transition state leads to[CoI(chelate)]- + RBr - {[CoI(chelate)]- R+ Br-} (6)transition staterate {[CoI(chelate)]- - R+ - Br-} determining ___._.__t{[CoII(chelate)] - R * Br-1 (7)intermediatethe rate-determining step which involves electron trans-fer from the nucleophile to the carbon atom.Only if J. CHEM. SOC. DALTON TRANS. 1982 1289is a primary or secondary alkyl group is the organocobaltproduct formed from the intermediate electron-transferproduct. In the case of ButBr, the bulkiness of theradical prevents the formation of a stable Co-C bond andthe reaction proceeds with elimination of hydrogen.The intermediate formation of a cobalt hydride cannotbe ruled out .*ConcZtcsiorc.-Our results obviously have a bearing oncurrent studies of methyl transfer between [CoII-(chelate)] 2+,o and [CoIIIMe (chelate)] 2 + ~ o (chelate = non-charged tetra-aza-macrocyclic ligands, corrin, or di-methylglyoximato-dianion).Endicott et d 3 O recentlysuggested a transition state involving a three-centre-three-electron bond, [CoI*-(R)-CoI*], relating the activ-ation barrier to the transition-state binding energy forthese reactions. Within the context of a three-centre-four-electron, {[CoI(chelate)]--(R+)-X-}, transition-statebinding model as in the present study, the critical para-meters may be the same as those examined by Endicott,i.e. the transition-state bond energy and the criticaldistance of separation in the transition state.A significant increase in the Co-C bond distance hasbeen attributed to the geminal methyl groups in themacrocycle 5,7,7,12,14,14-hexamet hyl-, 1,4,8,11 -tetra-azacyclotetradeca-4,l l-diene relative to the less en-cumbered tetra-aza-macrocycle, corrin, or dimethyl-glyoximato-dianion.The lengthening of the Co-Cbond contributes in turn to an increase in the intrinsicbarrier to the methyl-mediated inner-sphere electrontransfer.Present results show that even in the case of [CoI-(chelate)]- with alkyl bromides, the separation betweenthe donor and acceptor centres in the transition state isincreased by the presence of methyl substituents on theCo-N-C-C-N ring. This increase may be sufficient toprevent the formation of a stable bond in the final pro-ducts, as in the case of the reaction with ButBr.* A rate-limiting electron transfer leading to a common ion-pair intermediate Ni*RX*- has been proposed in the reaction ofnickel@) complexes with aromatic halides leading to mixtures ofnickel(1) and organometallic nickel(rr) products (T.T. Tsou andJ. R. Kochi, J . Am. Chem. Soc., 1979,101, 6319).[1/1489 Received, 24th September, 19811REFERENCES1 (a) J. M . Pratt, ‘ Inorganic Chemistry of Vitamin BIZ,’Academic Press, London, 1972; (b) G. Costa, Pure Appl. Chem.,1972, 80. 336; (c) G. Costa, Coord. Chem. Rev., 1972, 8, 63;(d) H. A. 0. Hill, in ‘ Inorganic Biochemistry,’ ed. G. L. Eichorn,Elsevier, Amsterdam, 1973, p. 1076; (e) J. Halpern, Ann. N.Y.Acad. Sci., 1974, 289, 1; (f) G. N. Schrauzer, Angew. Chem., Int.Ed. Engl., 1976, 16, 417; (g) M. W. Witman and J. H. Weber,Inorg. Chim. Acta, 1977, 28, 263.* J. P. Collman, Am. Chem. Res., 1977, 10, 205; J. 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ISSN:1477-9226
DOI:10.1039/DT9820001285
出版商:RSC
年代:1982
数据来源: RSC