摘要:

1976 2341Ring-size Effects of Macrocyclic Ligands on Complex Formation : theCopper(i1) Complex of the 15-Membered Cyclic Tetra-amine 1,4,8,12-Tetra-azacyclopentadecaneBy Mutsuo Kodama, Department of Chemistry, College of General Education, Hirosaki University, Bunkyo,Eiichi Kimura," Institute of Pharmaceutical Sciences, Hiroshima University School of Medicine, Kasumi,Hirosaki, JapanHiroshima, JapanThe thermodynamics and kinetics of complex formation between CuII and 1.4.8.1 2-tetra-azacyclopentadecane( L4) have been studied polarographically. The stability constant of I 024.4+0-2 (/ = 0.20 mol dm-3, 25 "C) for the1 :I complex containing 5,6,6,6-membered chelates i s 10'- and 103-fold greater than those for complexes ofthe corresponding open-chain tetra-amines 4.8-diazaundecane-I ,I 1 -diamine ( L5) containing 6,6,6-chelates and4.7-diazadecane-I ,I O-diamine ( L6) containing 5.6.6-chelates, respectively. Depending on the choice of thereference complexes, either the AH term (cf. the L5 complex) or the AS term (cf.the L6 complex) makes a majorcontribution to the macrocyclic effects. A significant effectof the macrocyclic ring expansion i s a facile protonationof the complexed ligand in acetate buffer solution, KCu(HL)H = [CuL2+] [H+]/[CU(HL)~*] = (3.0 * 0.2) x 1 0-6mol dm-3. The rate law for the formation of the complex of L4 in acetate buffer i s of the form d [Cu( H L)3+]/dt = k5-[Cu(O,CMe)+][HL+], where k5 = (4.0 f 0.5) x lo8 dm3 m0I-l s-1 a t 25 "C and the activation parameters areAH: = 13.7 f 0.3 kcal mol-l and AS: = 26.7 f 1 .O cal K - l mol-l.The larger formation and dissociation rateconstants suggest that the 1 5-membered macrocycle is more flexible relative to the smaller analogues.IN the preceding thermodynamic and kinetic studies of and 13-membered ligands (1,4,7,10-tetra-azacyclotri-the complex formation between CuII and macrocyclic decane, L 2 ) . 3 p 4 Both complexes showed a ca. 105-foldstability enhancement relative to the correspondingn 0 n complexes of the linear tetra-amines. However, theirstability increases do not seem to stem from the samefavourable ligand configuration, outweighs a smaller-AH value (probably due to the unfavourable stericL' L.2 L3 L4 constraint), and for Lz the increase in AS is smaller, butf iu u Utetra-amine ligands we have disclosed the cyclic ring- is compensated for by a considerable AH contribution.size effects of 12 - (1,4,7,l0-tetra-azacyclododec~ne, L1) i 23 M.KodaInaandE. Kimura, J.C.S. Chem. Comm., 1975, 891.M. Kodama and E. Kimura, T.C.S. Chenz. Comm.. 1975. 326. M. Kodama and E. Kimura, T.C.S. Dalton. 1976. in the - "M. Kodama and E. Kimura,"J.C.S. Dalton, 1976, 116.- press2342 J.C.S. DaltonAn unusually large AH value was first reported for thestability of the nickel(r1) complex of 1,4,8,11-tetra-azacyclotetradecane(L3) .5-7 The effects of the ligandcyclization on the kinetics are most notable in thedissociation rates which are retarded to a much greaterextent than the formation rates.1.2 As the ring sizeincreases, however, the steric rigidity diminishes and thedissociation rates tend to in~rease.~.~In order to cast more light on the thermodynaniic andkinetic macrocyclic ring-size effects, we have nowstudied the reaction of a &membered cyclic tetra-amine, 1,4,8,12-tetra-azacyclopentadecane (L4) withCuII.A similar study of 12- to 18-membered cyclictetrathiaethers was recently reported.8EXPERIMENTALReage.mzts.-l,4,8,12-Tetra-azacyclopentadecane (L4) wasprepared by the method of Martin et aLB Its tetrahydro-chloride was purified by two recrystallizations fromaqueous 6~ HC1, m.p. 240-245 "C (decomp.) (Found: N,Effectsdefined single wave of the diffusion-controlled nature at thedropping mercury electrode (d.m.e.) . The polarographicbehaviour of the complex was independent both of acetateand valinate concentrations from 0.05 to 0.2 mol dm-3,showing that no mixed-ligand complex involving acetate orvalinate anions was formed under the present conditions.Plots of log[i/(id - i)] against d.c. potential, E, were straightlines with a reciprocal gradient of ca.-32 mV, a valuecorresponding to a reversible two-electron reduction at thed.m.e. The half-wave potential of the complex [CUIIL~],(E~)c~I,, shifted to more negative potentials on increasingthe logarithm of the concentration of free L4, [LIf, in eitherbasic or acidic conditions [relation (l)]. Moreover, inbasic solutions (EB)CuL shifted linearly to more negativepotentials with increasing pH, according to relation (2). *A(Ei)cuL/A log [L]f = -30 mVA(E$C~L/A log ( ~ H ) L = 30 mV(1)(2)A typical result obtained in valinate buffer is given inTable 1.On the other hand, in acidic solution, relation (2)TABLE 1of ligand, CuII, pH, and valine in alkaline media on the half-wave potentials at I = 0.20 mol dme3 and 25 "CA (EdcuLlmV(Ef)CuL -- 103[L4] 103[CuII] [valine]rnol dm-3 PH log ( a H ) L V vevsus s.c.e. calc . obs .Ligand concentration effect3.88 0.101.94 0.100.97 0.100.97 0.100.97 0.100.97 0.100.97 0.100.97 0.100.97 0.10pH EffectValine concentration effect0.100.100.100.100.100.100.050.100.208.708.708.708.709.309.608.708.708.70-0.524 0 " 0-0.151 8.9 9 - 0.507 17.8 a 173.918 -0.507 O b 02.763 - 0.542 -34.2 ' - 352.210 -0.556 - 50.5 - 49- 0.506-0.507- 0.506Copper (11) concentration effect1.94 0.10 0.10 8.70 -0.5151.94 0.20 0.10 8.70 -0.614Using equation (I).Using equation (2).12.9. Calc. for L4*4HCl*4H20 : N, 13.0%), D,L-Valinewas recrystallized twice from aqueous solution by addinghydrochloric acid and ethanol. The preparation of othersolutions was described earlier .2*Apparatus and Procedures.-All the apparatus and theprocedures employed in this study were the same as thosedescribed p r e v i ~ u s l y . ~ ~ ~ As in the cases of L1,2 L2,4 andL3,4 mixed-mode protonation constants of L4 were deter-mined by potentiometric acid-base titration : pK, 1 1.2 f0.1, pK, 10.1 &- 0.1, and pK, and pK4 ca. 2.0 a t I = 0.20mol dm-3 and 25 "C.In the present study, acetate(3.4 < pH < 5.4) and valinate (8.7 < pH < 9.8) bufferswere used.RESULTSEquilibrium Studies.-The complex of CuII with L4 inboth acetate and valinate buffer solutions gave a well* All the symbols used were, unless otherwise noted, definedel~ewhere.~F. P. Hinz and D. W. Margerum, J . Amer. Chem. SOC., 1974.96, 4993.F. P. Hinz and D. W. Margerum, Inorg. Chem., 1974, 13,2941.did not hold, but a plot of antilog{[(E~)C,,~+ - (E$)C,,L]/0.0296 + 1og(Kcu2+/KCuL) - log[[L]f/(a~)~] 1 against [H+]was linear with an intercept (Figure).These results, and a finding that variation of [Cu2+] hadno effect on (Eq)cu~ (see Table l), indicate that Cu2+ formsa complex [CuLI2+ in alkaline solution and an additionalcomplex [Cu(HL)]3+ in acidic solution. The electrodereaction mechanism for the reduction of the complexes isthen expressed by (3).The shift in the half-wave potentialdue to complex formation, A(Eg), is given by the generalG. F. Smith and D. W. Margerum, J.C.S. Chem. Comw.,1975, 807.8 T. E. Jones, L. L. Zimmer, L. L. Diaddario, D. B. Rora-bacher, and L. A. Ochrymowycz, J . Amer. Chem. Soc., 1975, 97,7163.L. Martin, L. J. DeHayes, L. J. Zompa, and D. H. Busch,J . Amer. Chem. SOC., 1974, 96, 40461976 2343equation (4). Here, kcu~/kcu~+ = 0.80 (as experimentallyestablished) and Ka(HLIH = [CUL~+][H+]/[CU(HL)~+].NEt) = ( - W C , P + - (Ef)CuL[Llf I log KcU~(1 + [H+]/KC~~(HL>~) . - +( ~ H ) Llog "'-> (4)kCua+Under the alkaline conditions, the term (1 + [H+]/KCu(HLIH)in equation (4) can be approximated to 1, thus affordingthe relations (1) and (2).TABLE 2Comparison of the stability constant, enthalpy, and entropyof formation of tetra-amine complexes a t 25 "CConiplex log KML kcal mol-l cal K-l mol-1 mol dm-3 Ref.-AH A S I[CUL4]2[CUL2]2+[CU L'] 2+[CULS] 2+[ c u L6] 2+[CU L71 z+[CU La] 2+[NiL3] 2+[NiL7] 2+[NiL8] 2+24.4 & 0.2 26.5 & 0.329.1 29.224.8 18.317.3 19.521.8 25.923.9 27.720.2 21.622.2 31.015.8 19.413.8 14.022.7 &- 2 0.2 *33.7 0.2 3, 451.4 0.2 1,212.8 0.1 10, 1113.1 0.5 1216.5 0.5 1319.5 0.1 11-2 0.1 5, 67.2 0.1 1316.0 0.1 13* This work.Uncertainties are standard deviations.Determination of K c U ~ and KCu(~~)IT.Plot of equation (4) atmol dm-3, [Cu2+] = 0.20 x lop3 mol dm-3, [L4J = 5.96 x[MeCO,-] = 0.05 mol dm-3, I = 0.20 mol dm-3, and 25 "CThe stability constant, KcuL, determined from A(E4) inalkaline media, was 1024.450.2 a t 25 OC, which, togetherwith the values of l O Z 5 e 1 and dm3 mol-l at 15 and35 OC, respectively, gives thermodynamic parameters forthe complex formation (Table 2). The values of pK, useda t 15 and 35 "C were 11.4 and 11 .O, respectively, and pK, was10.3 and 9.9, obtained experimentally. The valueof K ~ L =1024.6 dn13 mol-l determined independently from theintercept in the Figure is in very good agreement with theabove value.mol dm-3 was also estimated from the intercept/gradient ofthe Figure.*Kinetic Studies.-The formation of the complex of CuIIwith L4 proceeded at a measurable rate in acetate buffersolutions (pH <4.0).As in the case of L1,, the observedformation rate constant, k p , was determined from the* The IYc,,(HL)H values were virtually temperature inde-pendent (15-35 "C) .The value of KC^(^^)^ = (3.0 & 0.2) xinitial gradient. In aiialysiiig the kinetic data, the follow-ing results were found (Table 3); (2) a t constant pH and[MeCO,-1, K p was independent of the initial concentrationsof copper and L4; (ii) at given pH and concentrations ofcopper and L4, the values of h~ multiplied by P0,cMe/K c ~ ( o , ~ ~ : >[MeCO,-) were independent of [MeCO,-] ; (iii) atTABLE 3Rate data for reactions of CuII and L4 at I = 0.20in01 dm-3 and 10 "C(a) Dependence on initial concentrations of CuII and L 1 (60.05 0.20 29.80.10 0.20 29.60.10 0.40 29.50.20 0.4.0 30.00.10 0.80 29.6(b) Dependence on acetate concentration0.05 0.577 29.6 51.40.10 0.582 30.2 52.00.20 0.475 25.2 53.0(c) Dependence on pH3.97 2.14 44.7 9.573.78 2.31 29.6 9.803.54 5.75 16.8 9.663.39 8.14 11.8 9.55a At [MeCO,-j = 0.05 mol dm-3 and pH 3.78.6 At[CuII], app.=0.10 x rnol ~lm-~, and pH3.78. At [ C U I I ] ~ , ~ ~ ~ . = 0.10 >: [L], = 0.20 x andmol dm-3, [L], = 0.20 x[MeCO,-] = 0.05 mol dm-3.constant concentrations of copper, L4, and acetate ion, thevalues of k~ multiplied by (ccH)L/[H+] were independentof [H+].The kinetic results in conjunction with the equilibriumstudy establish that the reaction occurs predominantlybetween a monoprotonated ligand, [HL]+, and a 1 : 1copper(I1)-acetate species, [Cu(O,CMe)]+, to give a mono-protonated complex, [Cu (HL) J3+, as displayed in ( 5 ) , whereCU2+rapidk6k-6 slow[Cu(HL)IS+ + MeC0,- ( 5 )[Cu(O,CMe)]+ + [HL]+ \ me)h'w 02cxydCu(O,CMe),the second-order rate constant in the rate law d[Cu(HL)3+]/dt = k,[Cu(O,CMe)+][HL+] is derived from equation (6).The rate constants ( K J of 1.1 X lo8, 2.2 X los, and 4.0 x1 0 8 dm3 mol-1 s-l at 10, 17, and 25 "C, respectively, gavethe activation parameters in Table 4 which are comparedwith values for other inacrocyclic systems2344 J.C.S.DaltonTABLE 4Summary of the rate constants and associated activationparameters for the reaction of [Cu(O,CMe)]+ withinacrocyclic tetra-amine ligands a t 25 "C and I = 0.20mol dm-3Reactingform of the Forward rate constant AH: A Stligand dm3 mol-1 s-1 kcal mol-' cal I<-' mol[HL1]+ a (1.8 f 0.2) x lo6 16.9 f 0.3 27.3 f 1[HL2]+ (5.6 f 0.5) x lo6 16.1 f 0.3 36.9 f 1[HL4]+c (4.0 f 0.5) x lo8 13.7 i 0.3 26.7 $ 1Refs.2 and 4. b Ref. 4. This work.DISCUSSIONThe 15-membered cyclic tetra-aniine in valinate buffersolutions forms a 1 : 1 complex with CuII composed of5,6,6,6-membered chelate rings. Its stability constantis cn. lo7, lo3 times greater than those for complexes ofthe related linear tetra-amines 4,8-diazaundecane-1, 11-diamine (L5) and 4,7-diazadecane-l,lO-diamine (LG),respectively (see Table 1) Compared with otherA n n Q, r N N, /N 1 (: ,") /NJL9 LSmacrocyclic systems the complex is as stable as thatwith L1,1s2 but four orders of magnitude less stable thanthose with L2 (refs.3 and 4) and L3.6914 It is of interestthat, despite having similar protonation constants pK,and pK, (11.1 and 10.1 for L2),4 the ligands L2 and L4form copper(I1) complexes with widely differing stabilityconstants. The diminished stability of the 15-memberedcomplex results from less favourable enthalpy as well asentropy changes. Apparently, a square-planar con-formation of the 15-membered ring9 should be moresensitive to internal steric and entropic factors than aprotonated conformation.A comparison of the thermodynamic parameters forthe reactions of CuII with L4 and L5 (ref.10) may revealthat effect of the 15-membered macrocycle is primarilydue to an enthalpy contribution [A(AH) -7 kcal mol-l]together with a minor entropy effect [A(AS) 10 cal K-lmol-l].* However, we prefer to define the effect byconsidering similarities or differences between thegeometries of the macrocyclic and linear ligands in thecomplexes. A large enthalpy contribution to themacrocyclic effect [A(AH) -12 kcal mol-l], however,was previously reported for NiII reacting with L3, wherea decreased ligand solvation of the macrocycle was* 1 cal = 4.184 J.i ,4 similar protonation was reported for the related linear' 0 R. Barbucci, L. Fabbrizzi, and P. Paoletti, J.C.S. Dalton,l1 R. Barbucci, L. Fabbrizzi, and P.Paoletti, Co-oydimtioTzl2 P. Paoletti, L. Fabbrizzi, and R. Barbucci, Inorg. Chem.,l3 P. Paoletti, L. Fabbrizzi, and R. Barbucci, J.C.S. Dalton,copper(I1) complex of L6, ~Kc,,(HL)~ 3.6.121972, 745.Chew. Rev., 1972, 8.1973, 12, 1861.1973, 1763.i n ~ o k e d . ~ - ~ When the comparison is made with theL6 complex,12 the enthalpy contribution becomes small[A(AH) -1 kcal mol-l] with the entropy term [A(AS) 10cal K-l mol-l] emerging as the major term. Accordingto this choice of reference complexes, the macrocycliceffect for L4 is interpreted in terms of the entropy term,as for L1.1y2 Obviously, compared with the 12-memberedring, the more flexible 15-membered ring suffers morerestriction of freedom on complex formation, leading tothe observed smaller entropy contribution [A( AS) 10compared with 32 cal K-l mol-l].The similar A Hvalues for L4 and L6 systems may reflect the analogouscomplex geometries and strengths of the Cu-N bonds.lj?l6The most striking manifestation of the expansion ofthe macrocyclic ring is the easy access of a proton to oneof the nitrogen atoms in the complex, pKCum~>H being5.5.7 Mechanical constraint and conformational straincaused by the longer methylene chains results in aweaker metal-donor atom interaction, thus makingavailable a nitrogen lone pair for the protonation. Bycontrast, no such protonation was detected for tetra-amine complexes having smaller ring size9 under the same(acetate-buffered) conditions.The kinetic studies in acetate buffer solutions showthat complex formation occurs between [Cu(O,CMe):-and the monoprotonated ligand.Although there was asignificant contribution of the diprotonated ligand to therate of the reactions1-* of L1 and L2, this term isnegligible in the present instance due to an increasedrate constant (A5) of the inonoprotonated species. Offurther significance is the fact that the rate of reactionof [HL4]+ is close to those of linear p o l y a m i n e ~ , ~ ~ . ~ ~suggesting the blocking effects of a nearby proton, andof steric constraint, on the donor sites are also small withthe 15-membered ring.Regarding the activation parameters for the macro-cyclic systems Table 4 shows that for the 12-, 13-, and15-membered series the AS: term is cn.27 cal K-l mol-lwhile AH2 gradually decreases as the ring expands.The transition state should involve loss of watermolecules bound to the metal ion, thus resulting in thesame positive A S value regardless of the macrocyclicring size. The more facile ligand-metal outer-sphereinteraction or easier water dissociation due to less sterichindrance may be responsible for the inore favourableAH1 term for the larger macrocycle. These results areconsistent with the usual dissociative reaction mechan-ism as proposed earlier.4 A comparison with the re-actions of linear polyamines 18 supports the notion thatgenerally negative AS: terms for linear systems areindicative of unfavourable ligand configurations whichl4 D. K. Cabbiness and D. W. Margerum, J .Amev. Chem. SOC.,l5 P. Paoletti, L. Fabbrizzi, and R. Barbucci, Inovg. Chem.,l6 C. Bianchini, L. Fabbrizzi, P. Paoletti, and A. B. P. Lever,l7 D. B. Moss, C. Lin, and D. B. Rorabacher, J . A m w . Chem.la T. S. Roche and R. G. Wilkins, J . Amev. Chew. SOC., 1974,1969,91,6540.1973, 12, 1961.Inorg. Chem., 1976, 14, 107.SOC., 1973, 95, 5179.96, 508211976 2345outweigh the positive AS; of the water loss, and thatgenerally higher AH$ values for the macrocyclic systemsrepresent larger energies required for the loss of waterattached to Cu2+.The dissociation rate constant (k-5) is estimated to be3.8 x dm3 mol-1 s-l from equation (7). Whenreaction (8) is considered as was the case for L1 (refs. 1arid 2) and L2,3,4 the dissociation rate constant, k-5f, is[Cu(O,Chk)]+ +- iHL]+ =G= [CuLI2* + MeC0,H (8)calculated to be 10-7.65 dm3 mol-l s-l from k,/k-,’ =I ~ C , ~ L I ~ ~ ~ / ~ - ( ’ ~ ( O , C I N ~ ) K ~ ~ ~ C O , ~ . A comparison with thek-5‘L1 and L2 systems shows that the dissociation rate isdefinitely faster for the present L4 system, indicatingagain the relief of the steric rigidity due to the cyclicgeometry.In the macrocyclic series, the correlation between theforward reaction rate and equilibrium constants ofcomplex formation (including their parameters) is verypoor. Provided that all the macrocycles follow thesame reaction path, this leads to the conclusion that thediffering thermodynamic functions arise from processesoccurring after the formation of the transition state.For example, the difference, if any, in the macrocycliceffects of the 12-15-membered systems should occurafter the formation of the first bond.[6/817 Received, 27th April, 1976

ISSN:1477-9226    

DOI:10.1039/DT9760002341

出版商:RSC    

年代:1976

数据来源: RSC