摘要:

1978 1603Solvation Effects on the Kinetics of Diamine Replacement in Penta-cya no (d i a m i ne ) fer rate ( 11) Complexes t *By NBstor E. Katz, on leave from Facultad de Bioquimica, Quimica y Farmacia, Universidad Nacional de Tucumiin,Miguel A. Blesa, CONICET and Facultad de Farmacia y Bioquimica, Universidad Nacional de Buenos Aires,Jos6 A. Olabe, Departamento de Polltica Cientifica y Tecnol6gica, Universidad Nacional de Lujiin, LujBn,Pedro J. Aymonino," CONICET and Facultad de Ciencias Exactas, Universidad Nacional de La Plata, 47 y 11 5,TucumBn. ArgentinaBuenos Aires, ArgentinaArgentina1900 La Plata, ArgentinaThe rates of release of ligand L from [FeZr(CN),LJn- complexes (L = unidentate terminal diamines NH,-[CH,],NH, (n = 3-6) unprotonated or monoprotonated) have been measured in aqueous solutions a t variouspH and temperatures. The kinetic parameters are interpreted on the basis of solvation effects.LIGAND-SUBSTITUTION processes in [FeI1(CN),LI7'- have unprotonated, or monoprotonated, terminal diamines ofbeen investigated by several workers and the mechan- general formula NH,[CH,],NH, (n = 3-6).Ourism shown to be dissociative ( I d or D).§ Recently we previous results with ethylenediamine (n = 2) werefound that for the release of aliphatic monoamines the repeated as an internal check, enabling us to attributeprocess of ligand solvation was operative, as indicated by real significance to small changes (the new results do notthe random influence of the ligand basicity on the differ greatly from the previous values).TABLE 1pH Dependence of the limiting rate constants K - L for ligand exchange in [FerI(CN),L]"- at 25 "C and I = 1mol dm-3 (NaCl) *L = pd7 /-.-A-8.00 8.159.00 8.399.25 8.359.40 8.129.60 7.609.90 7.7510.00 7.7510.20 7.2910.40 7.1910.50 6.5510.70 7.2011.00 6.4311.10 6.4011.35 6.1711.60 5.8111.80 5.7312.16 5.5312.75 5.15PH 103k-~/s-~1, = bd---l-h___.7 pH 10'k~,/~-'8.30 6.789.50 6.8310.45 6.6510.65 6.7110.75 6.5010.90 6.1711.00 6.3211.05 6.3611.30 5.7811.80 5.3313.50 4.80* See also Table 2.limiting rate. The present work is intended to givefurther support to this argument and provide additionalinformation about the effect of protonation of one of thedonor sites of bifunctional ligands on the kinetics ofsubstitution .3We report the results of a study on the kinetics ofrelease of ligand L from [FeI'(CN),L]"- complexes inaqueous solution, where the ligands L are unidentate,t A communication on this and related subjects was presenteda t the 18th International Conference on Co-ordination Chemistry,Sa6 Paulo, Brazil, 18th-23rd July, 1977.$ No reprints available.$ Recent evidence (S.ASperger, 18th International Conferenceon Co-ordination Chemistry, Sad Paulo, Brazil, 18th-23rd July,1977) confirms our previous postulations (see ref. 3) that themechanism is more likely to be In.7 en = Ethylenediamine, pd = propane-1,3-diamine, bd =butane-l,Cdiamine, ptd = pentane-1,5-diamine, hxd = hexane-1,6-diamine, and py = pyridine.L = ptdr------ -7 pH 10ak.-~/s-'9.60 6.859.80 6.0710.15 6.1910.50 6.3210.65 6.5310.70 6.25,10.90 6.5711.20 6.5011.45 6.0011.80 5.9311.90 4.87For L = en see ref.3.L7-PH9.259.8510.9011.2011.4011.6011.7011.8012.2012.7013.3513.40= hxd.-Ap31O3k-~/s-'5.396.394.814.474.694.394.454.584.254.134.244.09EXPERIMENTALThe sodium salts of the complexes [Fe(CN),LI2- (L =[Hen]+, [Hpd]+, [Hbd]+, [Hptd]+, or [Hhxd]+,T wereprepared from Na,[Fe(CN),(NO)] (Merck) by a methodpreviously described.8 They were characterized by chemi-cal analyses, and i.r. and u.v.-visible spectra.' Except forthe [Hen]+ complex, the complexes are new.H. E. Toma and J. M. Malin, Inorg.Chem., 1973, 12, 1039,2084.2. Bradid, M. Pribanid, and S. Agperger, J.C.S. Dalton, 1975,353.M. A. Blesa, J. A. Olabe, and P. J. Aymonino, J.C.S.Daltora, 1976, 1196.A. D. James, R. S. Murray, and W. C. E. Higginson, J.C.S.Dalton, 1974, 1273.N. E. Katz, P. J. Aymonino, M. A. Blesa, and J . A. Olabe,Inorg. Chem., 1978, 17, 556.8 J. A. Olabe and P. J. Aymonino, J . Inorg. Nuclear Chem.,1974,56, 1221.7 N. E. Katz, J. A. Olabe, and P. J. Aymonino, unpublishedwork1604 J.C.S. DaltonPropane- 1,3-diamine (Aldrich) was used as available.Butane-1,4-diamine was obtained by neutralization of thecorresponding dihydrochloride (Fluka) , Pentane- 1,5-diamine was prepared by a modified Gabriel synthesis.8Hexane- lt6-diamine was used as supplied (7 1 yo aqueoussolution, Ducilo).The kinetic studies were carried out in a thermostatted1-cm optical cell of a Spectronic 600 E spectrophotometer.In a typical experiment, to an aqueous solution containing asmall amount of the complex (final concentration 2 xRESULTSTables 1 and 2 present the results obtained for the limitingrate constants LL at different pH and temperature.At25 "C there are two limiting values when pH is varied; thehigher one at lower pH evidently corresponds to the releaseof the monoprotonated diamine and the value a t high pHcorresponding to the release of the unprotonated diamine .As shown in Table 3, LHL decreased regularly from [Hen]+to [Hhxd]+ and the difference between the rate con-stants K-HL and k~~ becomes progressively smaller as theTABLE 2Temperature dependence of the saturation rate constants k-L for the monoprotonated and unprotonatedforms of L at I = 1 mol dmb3 (NaC1)L = e n * L = pd L = bdr- 1 r----.--..-A 7 r-l----h 7PH 0J"C 103k-~/s-l PH ecioc i o v ~ ~ p PH7.9 14.5 2.09 8.45 11.9 1.13 8.0 20.3 3.497.9 20.0 4.94 8.45 15.0 1.99 8.0 25.0 6.977.9 25.0 9.87 8.45 20.0 4.04 8.0 29.3 13.57.9 29.0 18.1 8.45 25.0 8.17 8.0 33.8 23.77.9 33.8 31.1 8.45 29.3 14.1 13.0 20.3 2.2313.8 14.5 1.17 12.4 15.3 1.23 13.0 25.0 4.3813.8 20.0 2.52 12.4 21.5 2.90 13.0 29.3 8.8513.8 25.0 5.21 12.4 22.0 2.96 13.0 33.8 14.613.8 29.0 9.29 12.4 25.0 5.0713.8 33.8 17.8 12.4 29.1 8.59LOJ"C 103k-~/s-112.4 33.6 16.9L = hxd L = ptd10.0 15.4 1.67 9.0 16.9 2.0310.0 20.5 3.71 9.0 20.9 3.7110.0 25.0 6.77 9.0 25.0 6.3910.0 29.1 11.8 9.0 29.3 11.510.0 33.3 21.5 9.0 34.3 21.014.0 15.4 1.18 14.0 16.9 1.3314.0 20.5 2.39 14.0 20.9 2.3614.0 25.0 4.57 14.0 25.0 4.1314.0 29.1 8.61 14.0 29.3 7.8314.0 33.3 15.5 14.0 34.3 14.8* Data are improved values with respect to ref.3.mol dm-3) and an excess of free ligand (final concentration5 x mol dm-3) was added pyridine in sufficiently highconcentration (final value 0.1 mol dm-3) to ensure first-orderconditions and to obtain the limiting value hobs. = k-L forthe loss of ligand.1-3 The pH was controlled by sodiumtetraborate-phosphate or phosphate-sodium hydroxidebuffers and measured with a Sargent-Welch model LSX pHmeter. Tempera.tures were maintained to kO.1 "C.Sodium chloride was used to maintain constant ionicstrength at I = 1 mol dm-3.The rate constants weremeasured at pH 8-14 and between 10 and 35 "C. Experi-ments were monitored at 365 nm (Amax. for the py complex)and in all cases the reaction proceeded to completion asdeduced from the experimental A , values. The K-L valueswere calculated from plots of log [ ( A , - A t ) / ( A , - A,)]against t , which were linear up to a t least 3 half-lives.Duplicate runs were performed for each experimentalcondition; the rate constants were found to be reproducibleto within 1 yo. The activation parameters were determinedfrom Eyring plots of log(R~/T) against (l/T) by a least-squares method; an analysis of the variance about regres-sion was used to obtain the confidence limits with 90%probability .98 H.Raymond and H. Manske, J . Chem.,Soc., 1926,128, 2348.B H. A, Laitinen, Chemical Analysis, McGraw-Hill, NewUork, 1960.chain length of the diamine increases. In all cases, theinflection point nearly coincides with the first pK, of thecorresponding conjugate acid of the diamine.lOThis behaviour can be explained by equations (1)-(4) :[Fe(CN),(HL)12- [Fe(CN),L]3- + H+ (2)k-L [Fe(CN),LI3- [Fe(CN),I3- + LThe experimental data fit equation (5) which can easily bededuced from reactions (1)-(3) when [pyj is large. Valuesof AHT and AS1 were derived from the temperature depend-ence of K-HL and Jz-L (Table 4).nr scuss~o~The low sensitivity of the rate constants to the basicityof the ligands is shown by the small gradient of a plot oflog LHL against second pK, (= 0.10 & 0.03), in agree-ment with the behaviour of aliphatic monoamines andlo ' The Chemistry of the Amino Group,' ed.S. Patai, Inter-science, London, 1968, p. 1771978 1605the ‘ soft ’ or ‘ b ’ character of this particular metal~ e n t r e . ~ , l l Thus, changing K, by a factor of 103 merelyTABLE 3Comparison of the limiting rate constants for the release ofmonoprotonated and unprotanated forms of diaminesfrom [Fe*1(CN)5L]n- a t 25 “C and I = 1 mol dmm3(NaCl)Ligand lO3k-~&l 103k-~/s-l ~ - H L : k-Len 10.4 5.6 1.98.3 5.4 1.56.9 4.6 1.56.4 4.5 1.4 E 6.3 4.1 1.3E:doubles the rate constant. This lack of sensitivity toligand basicity seems to be a general phenomenon in the‘FABLE 4Activation parameters for ligand substitution in[FeIr(CN),LITL-Monoprotonated diaminer-----------h----- 7 AH$/ ASXILigand kJ mo1-I J K-I rno1-len 101 f 5 54 & 1754 f 2163 f 2554 rf 13101 f 6105 i 8102 f 39::Eii 97 f 3 38 f 8Unprotonated diaininer- mAH$/ AS11kj mol-1 J K-1 rno1-l102 -& 4103 & 6103 & 15104 f 5101 f 350 f 1359 f 2159 f 6059 f 1746 f 13aqueous chemistry of [E’e11(CN)5L]rL- ions.j9l2 The factthat the rate constants for the reverse reaction (i.e.formation) are also relatively independent of the liganddonor properties causes the stability constants for thecomplexes of substituted amines to be rather insensitiveto the nature of substituent groups;l with the presentligands the constancy of the p& of the ligand 011 co-ordination also emphasizes the similarity of thest ability const ants of [ Fe (CN) (HL)] 2- and [ Fe (CN) 5L] 3-.In this respect, aqueous [Fe(CN),I3- behaves ratherpeculiarly, since protonated and unprotonated ligandsgenerally show very different affinities towards othermetallic moieties.13 Thus, solvation effects are evident.Table 3 shows clearly that in all cases k-=L > k-L, aresult which is also obtained for other ligands l2 andwhich can be related to differences in the activation freeenergies of processes (1) and (3), due to the different solv-ation of the anions [Fe(CN),(HL)I2- and [Fe(CN),LI3-,the former being destabilized in water with respect to thelatter.In going to the activated complex, less desolv-ation of the 2- ion must be reflected in a higher rate ofsubstitution. The operation of such a solvation effect isfrequently observed in the form of rather large andalmost cancelling changes in AH1 and ASX.l2 In thepresent case, however, the activation parameters do notyield much information, since they are all similar, viz.:l1 T. W. Swaddle, Co-ordination Chem. Rev., 1974, 14, 242.l2 M. A. Blesa, I. A. Funai, P. J. Morando, and J. A. Olabe,la M. D. Alexander and C. A. Spillert, Inorg. Chew., 1970, 9,J.C.S. Dalton, 1977, 2092.2344.AH$ = ca, 100 kJ mol-l and A S = ca. 55 J K-l mol-l(see Table 4). There seems to be a trend in the rightdirection (except for bd) but the changes are in any casevery small.Higher AG++ values seem to be associatedwith higher AH1 and AS$ values, as expected fromprevious work.12On the other hand, in making vertical comparisons inTable 4, we can try to demonstrate the influence ofsolvation of the outgoing ligand on the rate of substitu-tion. The free energy of solvation for the substitutedammonium ions increases by a nearly constant amountfor each CH, group, while the free energy of solvation forthe free amines increases only slightly (due to an almostexact balancing of entropy and energy effects) =I4 Weobserved a similar trend in the rate constants (and con-sequently in A G ) for the release of the monoprotonatedand unprotonated diamines. Table 3 shows that therate of release of the monogrotonated ligands decreasesregularly with increasing chain length, while the un-protonated ligands are released a t nearly the same rateand the numerical value is very close to that of mono-amines.,For the longest protonated diamines, since the chargedend of the co-ordinated diamine already ‘ dips ’ into thebulk solvent, only the solvation of the co-ordinated endwhich is released upon activation will contribute to theenergetics of the process and the AGt value which resultsis again similar to that of monoamines. Also, since thepositive charge on the terminal N is more separated fromthe [Fe( CN)J3- moiety, smaller solvation differences areexpected between the complexes [Fe(CN),(HL) J2- and[Fe(CN),LI3-.This fact explains the decrease in theK-HL : k-L ratio as the chain length increases.The importance of the contribution of the solventmedium to AGI for ligand-substitution processes hasalready been established by Caldin and Bennetto l5 intheir work on exchange reactions of bivalent transition-metal cations in various solvents. Recently l6 thekinetics of substitution of pentacyano(ligand)ferrate(II)complexes was investigated in mixed aqueous solvents,and the importance of the [Fe(CN),I3- moiety wasassessed.Thus, the ratio of substitution rates in waterand in 40% aqueous t-butyl alcohol was found to bevery similar for leaving groups having great differencesin solvation. Consequently, the solvation of the remain-ing [Fe(CN),l3- moiety, rather than that of the departingligand, determined the difference in rates when the sol-vent was varied. In the present work the solvent isfixed and the structure of the ligands is varied in asystematic way. Since the solvation of the [Fe(CN),I3-moiety is expected to be the same for all our complexes(if the cationic environment is maintained constant) othereffects are shown to be operative; namely, the solvationl4 E. M. Arnett, F. M. Jones 111, M. Taagepera, W. G. Hender-son, J. L. Beauchamp, D. Holtz, and R. W. Taft, J . Amer. Chem.SOC., 1972, 94, 4724.l6 E. F. Caldin and H. P. Bennetto, J . Solution Chem., 1973, 2,277.l6 M. J . Blandamer, 3. Burgess, and R. I. Haines, J.C.S.Dulton, 1976, 12931606 J.C.S. Daltonof the leaving ligand and/or the solvation of the initial We thank CONICET for support, Dr. J. Zinczulr for thecomplex (i.e. the charge type of the reactant ion). We preparation of pentane-Wdiamine, and the Organiccan therefore conclude that solvation effects in these Chemistry Department (Facultad de Farmacia Y Bio-reactions can be observed even in water due to theconstancy of all the other relevant parameters. [7/892 Received, 23rd May, 19771quimica, U.B.A.) for a gift of various diamines

ISSN:1477-9226    

DOI:10.1039/DT9780001603

出版商:RSC    

年代:1978

数据来源: RSC