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J. CHEM. SOC. DALTON TRANS. 1994 1805Reactivity of Neutral Nitrogen Donors in Planar d8 MetalComplexes. Part I . The System [I .2-Bis(phenylsulfanyI)-et haneldichloroplat inum( 11) with Pyridi nes in Methanol.Effect of Basicity and Steric HindranceMarcello Bellicini, Lucio Cattalini, Giampaolo Marangoni and Bruno Pitteri *Dipartimento di Chimica, Universita di Venezia, Calle Larga S. Marta 2737, 30723 Venezia, ItalyThe kinetics of the forward and reverse steps of the process [Pt(PhSCH,CH,SPh)CI,] + am---- [Pt(PhSCH,CH,SPh)(am)CI]+ + CI- (am = one of a number of pyridines and other heterocyclicnitrogen bases covering a wide range of basicity) has been studied in methanol at 25 "C. Both forward andreverse reactions obey the usual two-term rate law observed in square-planar substitution.The second-order rate constants for the forward reactions, k:, show only a slight dependence upon the nature ofthe entering pyridine, and steric hindrance due to the presence of one or two methyl groups in aposition to the nitrogen markedly decreases the reactivity. The first- and second-order rate constantsfor the reverse reaction are very sensitive to the basicity of the leaving group and a plot of log k,'against the pKa of the conjugate acids of unhindered pyridines is linear with a slope of -0.56. Stericretardation for monosubstituted a-methylpyridines is relatively small. The equilibrium constants forthese reactions have been determined from the ratio of the rate constants and a plot of log K againstthe pKa of the unhindered pyridines is linear with a slope of 0.58.The results are compared with datafrom the literature and discussed in terms of the reaction profile.Neutral nitrogen donors (am) have seldom been investigatedas nucleophiles towards planar four-co-ordinate d8 metalcomplexes. As far as platinum(r1) substrates are concernedthere are indications that proton basicity, as measured by thepK, values of the donors, plays a minor role in determiningthe relative nucleophilicity, the major differences among re-action rates arising from steric retardation effects. However,there are cases2*, in which the reactivity increases linearlywith the basicity of isosteric entering groups and others4*'where aromatic nitrogen donors, like pyridines, behavedifferently from aliphatic and/or alicyclic donors.Less attention has been paid to the study of the relationshipbetween the basicity and the substitution lability of co-ordinated nitrogen donors mainly because of the generalinertness of the Pt-N bond toward substitution.6 No systematicstudies have been carried out, to our knowledge, to correlatethis information to the charge of the substrate or to the natureof the non-participating ligands.As studies on the role of basicity and steric effects in this typeof reaction can provide a useful tool to understand the intimateasynchronous mechanism of substitution, we decided to make asystematic study of this aspect of ligand-substitution reactions.In this paper we report the kinetics of the forward and reversesteps of the process (1) in methanol at 25 "C and discuss the[Pt(PhSCH,CH,SPh)Cl,] + am[Pt(PhSCH,CH,SPh)(am)Cl] + + C1- (1)results in terms of both electronic and steric effects.ExperimentalMaterials.-Platinum(II) chloride was obtained from JanssenChimica. Pure reagent-grade ~Bun4][C104], ~Bu",]Cl andAgNO, (Fluka and Aldrich) were dried over P,05 in a vacuumdesiccator and used without further purification. The pyridinesand the other nitrogen donors were recrystallized or distilledbefore use when necessary.Instruments.-Infrared spectra (4000-400 cm-' , KBr discs;400-200 cm-', polyethylene pellets) were recorded on aPerkin-Elmer 683 spectrophotometer. Electronic spectra wereobtained and kinetic measurements made on a personal-computer-controlled Perkin-Elmer Lambda 1 6 spectrophoto-meter.Proton NMR spectra were recorded on a Bruker AC 200F spectrometer and referred to tetramethylsilane. Conductivitymeasurements were carried out with a CDM 83 RadiometerCopenhagen conductivity meter and a CDC 334 immersioncell. Elemental analyses were performed by the MicroanalyticalLaboratory of the University of Padua.Preparation of 1 ,2-Bis(phenylsulfanyl)ethane.-This com-pound was prepared according to a published method7 andits purity confirmed by elemental analysis, IR, UV/VIS and'H NMR spectra.Preparation of Complexes.-Bis(benzonitri1e)dichloroplati-num(rr), [Pt(PhCN),Cl,], and [Pt(PhSCH,CH,SPh)Cl,] wereprepared as reported.'.'[ 1,2- Bis(phenylsulfany1)ethanel chloro(pyridine)platinum(rI)nitrate, [Pt(PhSCH,CH,SPh)(py)Cl]NO,.Silver nitrate(0.170 g, 1 mmol) was added to a warm solution (60 "C) of[Pt(PhSCH,CH,SPh)CIJ (0.5 12 g, 1 mmol) in dimethyl-formamide (dmf) (25 cm3) and the mixture stirred in the darkfor 30 min. The AgCl formed was filtered off and the solutiontreated with pyridine (0.079 g, 1 mmol) and stirred at 80 "C for10 min. After cooling at room temperature, the crude productprecipitated with diethyl ether was filtered off, washed twicewith diethyl ether and dried under reduced pressure. Yield0.42 g (70%).All the other complexes of the type [Pt(PhSCH,CH,SPh)-(R-py)CI]NO, (R = 4-chloro, 2-methyl, 4-methyl, 2,4-dimethylor 4-cyano) were prepared similarly.Analytical and some physicochemical data for the complexesare collected in Table 1 .Kinetics.-The reactions were followed (a) spectrophoto-metrically and/or (b) conductometrically 1806 J.CHEM. SOC. DALTON TRANS. 1994Table 1 Analytical and physical data for the complexesAnalysis (%)Complex[Pt(PhSCH2CH2SPh)C12] '[Pt(PhSCH2CH2SPh)(NC,H4R)Cl]NO,R = H4-C12-Me4-Me2,4-Me24-CNC32.8(32.8)35.6(36.9)33.0(35.0)36.4(38.0)37.1(38.0)38.5(39.0)36.8(37.3)H2.75(2.75)3.20(3.10)2.80(2.80)3.40(3.35)3.45(3.35)3.60(3.60)3.00(2.80)N-4.75(4.55)4.15(4.30)4.45(4.45)4.70(4.45)4.20(4.35)7.00(6.55)S12.1(12.5)--9.60(10.1)9.45(10.1)10.0(9.95)(9.95)9.30ColourYellowPale yellowDeep yellowPale yellowCreamCreamPale yellowM.p.("C)> 230> 230> 230> 230> 230> 230> 230AM blJZ-' cm2 mol-'-686970716675IRP(Pt-Cl)/cm-'335,320335335335335335330Calculated values in parentheses.In methanol at 25 "C. ' 'H NMR [(CD,),SO, reference SiMe,]: 6 3.14 (s, 4 H) and 7.30-8.09 (m, 10 H).' C1 13.9 (13.8)%. Cl 10.5 (10.9)%.(a) Spectrophotometric changes. Reactions were initiated byadding a 0.05 mol dmP3 dmf solution (5-20 pl) of the substratecomplex, [Pt(PhSCH,CH,SPh)Cl,] or [Pt(PhSCH,CH,SPh)-(am)Cl]+, to a methanolic solution (3 an3) of the appropriatereagent, the nitrogen donor or chloride ion respectively,previously brought to the reaction temperature (25 "C) in athermostatted cell in the spectrophotometer.The concentrationof the entering group was always large enough to providepseudo-first-order conditions. After preliminary repetitive scanexperiments in the range 280-360 nm to search for isosbesticpoints and spectral changes, the kinetics was studied bymeasuring the changing absorbance at suitable wavelengths as afunction of time. Pseudo-first-order rate constants (kobs/S-')were obtained either from the gradients of plots of log@, -A , ) us. time or from a non-linear least-squares fit ofexperimental data by A, = A, + (A, - A,)exp( -kobst) withA , , A , and kobs as the parameters to be optimized (A, =absorbance after mixing of reactants, A , = absorbance atcompletion of reaction).(6) Conductivity changes.Reactions were initiated by addinga 0.05 mol dmP3 dmf solution (60 p1) of the substrate complex[Pt(PhSCH,CH,SPh)Cl,] to a methanolic solution (10 cm3)of the appropriate nitrogen-donor reagent in the thermostattedcell of the conductivity meter (25 "C), under the pseudo-first-order conditions. The conductivity, initially nearly zero, in-creased with time, and the change followed a first-order ratelaw. Rate constants (kobs/SP1) were obtained either from thegradients of plots of log(A, - A,) vs. time or from a non-linearleast-squares fit of experimental data to A, = A, + (Ao -A,)exp( -kobst) with A,, A, and kobs as the parameters to beoptimized (A, = conductivity after mixing of reactants, A, =conductivity at completion of reaction).ResultsKinetics of Displacement of Chloride by Nitrogen Donors from[Pt(PhSCH,CH,SPh)Cl,] .-The spectrophotometric changesobserved in repetitive scanning of the spectrum of the reactionmixture are characteristic of a single chemical stage, with wellmaintained isosbestic points.Careful examination of the spec-tral changes (subsequent changes have never been observed)which occur after the reagents are mixed and the close similarityof the spectra at the end of the reaction with those of authenticsamples of the expected reaction product demonstrate that allthe reactions that have been studied kinetically involve thedisplacement of co-ordinated chloride by the nitrogen base. Therate constants were determined in the usual way from thechange in absorbance as a function of time at a convenientwavelength within the region 310-330 nm.The products ofsome substitution reactions were not independently synthesisedand characterized, but the general similarity of the spectralchanges indicated that the same type of reaction was beingobserved. When am = 4-aminopyridine, morpholine, quino-line, 2-methylquinoline, 2,6-dimethylpyridine or 2,4,6-trimethyl-pyridine, the reactions were studied kinetically from the changein conductivity as a function of time and in any case theconductivity values at the end of reaction (A,) were typical of1 : 1 electrolytes in methanol. The reactions with 4-methyl-pyridine were followed with both methods and the measuredrate constants are in close agreement. All the reactions werestudied in the presence of a sufficient excess of nucleophile overthe substrate to provide pseudo-first-order conditions and theobserved rate constants, kobs, collected in Table 2, obey thegeneral relationship kobs = k,' + k,'[am], which is usual fornucleophilic substitution at planar four-co-ordinate d8 metalcomplexes.lo The spectral intervals used, the isosbestic pointscharacteristic of the stage, the conditions of measurements aswell as the values of k,' and k2f are summarized in Table 4.Kinetics of Displacement of Nitrogen Donors by Chloride from[Pt(PhSCH,CH,SPh)(am)Cl] + .-In all the reactions with anexcess of chloride ion the spectrophotometric changes arecharacteristic of a single chemical stage, with the same isosbesticpoints as those observed for the reverse reactions.The spectra atthe end of the reaction coincide with those of authentic samplesof the expected products [Pt(PhSCH,CH,SPh)Cl,] + Ham + at the same concentration, so that the displacement of co-ordinated nitrogen donor by chloride is assured. This confirmsthat the presence of the S-bonded thioether chelate ligandprovides sufficient trans-labilizing power for it to be possible toobserve the displacement of the trans N-donor by a rather poornucleophile like chloride. The reactions were studied in thepresence of 0.01 mol dmP3 HClO, at constant ionic strength( I = 0.1 mol dm-3, [NBu",][ClO,]). Preliminary experimentshad shown that, at constant chloride concentration, the rate ofreaction was independent of the concentration of acid over therange 0.005-0.020 mol dm-3.The acid serves simply toprotonate the released am and to prevent the reverse reaction.The observed rate constants, kobs, collected in Table 3, obey thegeneral relationship kobs = kl' + k,'[Cl], where k,' and k2rhave the usual meaning and their values at I = 0.1 mol dm-3 arereported in Table 4 together with the corresponding values k02rextrapolated to zero ionic strength. The extrapolation to zeroionic strength uses the Debye-Huckel relationship, log ko2' J. CHEM. soc. DALTON TRANS. 1994 1807Table 2 First-order rate constants, kobs, for the reaction W(PhSCH2CH,SPh)Cl2] + am - [Pt(PhSCH,CH,SPh)(am)Cl]+ + C1- inmethanol at 25 "Cam [am J/mol dm-3 103k,,/~-' am [am]/mol dm-3 103k0,/s-1CAminopyridine * 0.00350.00500.00750.01000.01250.01 500.02000.0250CCyanopyridinePyridineMorpholine *Quinoline2-Meth ylpyridine0.00350.00500.00650.00800.00950.0 1200.01500.02000.0030.0050.0070.0100.0 120.0150.0180.0200.00350.00500.00750.01000.01 250.01500.02000.02500.0250.0500.1000.1500.2000.2500.3000.0550.1030.1530.2560.3580.5110.6522,4-Dimethylpyridine 0.0120.0500.0800.1000.1250.1500.200* Conductivity measurements.2.48 k 0.033.25 f 0.054.32 f 0.065.40 f 0.076.53 f 0.087.78 f 0.0610.04 f 0.0512.25 f 0.073.10 f 0.093.57 f 0.064.02 f 0.074.2 f 0.14.70 f 0.075.32 f 0.056.1 f 0.27.4 f 0.11.68 k 0.022.24 f 0.032.82 f 0.023.60 f 0.024.18 f 0.035.12 f 0.025.95 * 0.046.44 f 0.053.66 f 0.064.46 f 0.095.32 f 0.026.16 f 0.057.2 f 0.38.15 f 0.0410.1 f 0.112.3 f 0.11.25 f 0.021.61 f 0.012.22 f 0.012.97 f 0.033.53 f 0.044.21 f 0.054.87 f 0.041.40 k 0.031.70 f 0.042.22 f 0.033.0 f 0.13.89 f 0.045.1 f 0.16.4 f 0.21.06 f 0.021.50 ? 0.041.78 f 0.042.0 f 0.12.23 f 0.052.52 f 0.43.06 f 0.2log k,' - 2daZbI+/(1 + PIf) with 2, = + 1, z b = - 1, a =1.90 and p = 1.55 (estimated from the relative permittivity ofmethanol at 25 "C).l 1 The rate of processes between the neutralplatinum(1r) substrates and the nitrogen donor is not influencedby primary salt effects, whereas that of the reverse reactionsbetween the cationic species and chloride ion depends uponionic strength.As a consequence, also the equilibriumconstants, K, depend upon ionic strength and in Table 4 arereported the values K =kZf/kzr and KO = kz'/kozr determined3-Methylpyridine 0.001 50.00300.0045O.Oo600.0075O.Oo900.01050.0 120CMethylpyridine * 0.0030.0040.0070.0100.01 50.0200.025CMeth ylpyridine 0.0030.0040.0060.0070.0090.0100.0150.0200.0252-Methylquinoline * 0.050.200.500.751 .001.251 S O2.003.002,6-Dimethylpyridine * 0.050.100.200.400.600.801 .002,4,6-Trimethylpyridine * 0.050.100.300.501.301.802.403 .001.52 f 0.052.04 f 0.022.62 f 0.033.26 f 0.023.80 f 0.034.42 f 0.034.91 f 0.025.45 f 0.042.02 f 0.072.28 f 0.053.32 f 0.054.43 f 0.046.12 f 0.057.8 f 0.19.45 f 0.041.99 f 0.042.24 f 0.022.96 _+ 0.033.21 f 0.023.99 f 0.014.40 f 0.035.99 k 0.047.68 2 0.059.38 f 0.031.06 f 0.031.1 f 0.11.2 f 0.11.25 f 0.071.31 f 0.021.39 f 0.041.48 f 0.041.6 f 0.11.88 2 0.051.03 f 0.031.06 f 0.041.1 f 0.11.25 f 0.021.35 f 0.041.47 f 0.021.57 4 0.021.38 f 0.031.41 f 0.021.51 f 0.011.67 f 0.022.18 f 0.042.59 f 0.052.93 k 0.073.22 f 0.08from the ratios of the second-order rate constants for theforward and reverse reactions.DiscussionThe complex ~(PhSCH,CH,SPh)Cl,] can exist in twoisomeric forms, depending on the projection of the SPh groupson the Same (meso isomer) or different sides (DL isomer) of thechelate ring.In the present case, as well as in the previous wor1808 J. CHEM. soc. DALTON TRANS. 1994Table 3 First-order rate constants, kobs, for the reaction [Pt(PhSCH,CH,SPh)(am)Cl]+ + C1- - [R(PhSCH,CH,SPh)CI,] + am inmethanol at 25 "C (I = 0.1 mol dm-3, [NBu",][CIO,]; 0.01 mol dm-3 HCIO,)am [Cl-]/mol dm-34-C yanop yridine 0.0030.0040.0060.0080.0100.0150.0200.0254-Chlorop yridinePyridine0.0050.0100.01 50.0200.0300.0400.0500.0 100.0100.0200.0250.0300.0300.0500.070I am4.26 i- 0.04 4-Methylpyridine5.01 rl: 0.056.92 f 0.058.56 k 0.0810.50 rl: 0.0214.8 i- 0.119.3 k 0.223.9 rl: 0.10.95 k 0.031.50 k 0.022.08 k 0.022.61 k 0.023.84 +_ 0.034.95 rl: 0.046.10 k 0.060.17 k 0.020.19 2 0.030.28 rl: 0.020.35 2 0.010.40 k 0.010.41 k 0.010.66 2 0.030.91 f 0.012-Meth ylpyridine[CI-]/mol dm-30.0050.0100.0150.0200.0300.0400.0500.020.040.050.070.090.102,CDimethylpyridine 0.020.040.060.080.101 03kobs/S-10.047 k 0.0030.077 _+ 0.0010.101 k 0.0030.120 f 0.0010.173 k 0.0010.220 _+ 0.0020.273 i- 0.0010.023 f 0.0010.043 f 0.0010.056 k 0.0060.073 f 0.0050.095 f 0.0070.105 rl: 0.0010.0068 2 0.00050.0181 f 0.00030.0244 k 0.00060.0348 f 0.00040.0372 k 0.0002in which all the mutual interconversions of the [Pt(PhSCH,-CH,SPh)X(Y)] species (X, Y = C1, Br or I) were examined,' thesolid-state structure of crystalline [Pt(PhSCH,CH,SPh)Cl,],obtained on slow cooling at room temperature of a hot dmf-water solution of the crude product shows it to be the DLisomer. 'All the reaction mixtures used for the present kinetic studywere prepared by dissolving the crystalline product in dmf atroom temperature and, considering the rather high temperaturefor sulfur inversion of platinum(I1) complexes of this type,any isomerization process appears unlikely.Accordingly, the'H NMR spectrum of crystalline [Pt(PhSCH,CH,SPh)Cl,]in (CD,),SO solution indicates the presence of the DL isomeronly and the kinetics of chloride substitution obeys a singlemonoexponential rate law.The only reaction occurring between nitrogen donors and[Pt(PhSCH,CH,SPh)Cl,] in methanol at 25 "C is thedisplacement of a chloride ligand.It is known' that, underthe same experimental conditions, the substrate undergoeschloride displacement upon reaction with anionic nucleophileslike Br- and I-. However, with a number of pyridines in1,2-dimethoxyethane at the same temperature, ring openingfollowed by substitution of the chelating sulfur ligand occurs14instead of chloride substitution, showing that the nature ofthe solvent plays an important role in determiming the courseof the reaction.The chloride displacement reactions (2) obey the two-term[Pt(PhSCH,CH,SPh)Cl,] + am -~t(PhSCH,CH,SPh)(am)Cl] + + C1- (2)rate equation, kobs = k,' + k,'[am], usually found in substitu-tions at platinum(r1) complexes. The k,' term, referring to thepathway in which the rate-determining step is nucleophilicattack of the solvent followed by rapid entry of am into thesolvento-complex,' is obviously independent of the nature ofthe amine so that k,' is the same C(0.97 2 0.06) x lo-' s-'J inall the reactions within the limit of experimental errors.Thisvalue is also comparable to that determined for replacement ofchloride by bromide and iodide ions, under the same experi-mental conditions.' The only exception amongst pyridines is' 0 2 4 6 8 1 0PKaFig. 1 Plots of log k2' for the reaction [Pt(PhSCH,CH,SPh)Cl,] +am - [Pt(PhSCH,CH,SPh)(am)Cl]+ + C1- against pK, of Ham': (e), morpholine, pyridine and para-substituted pyridines; (m), quino-line, 2-methyl- and 2,4-dimethyl-pyridine; ( +), 2-rnethylquinoline7 2,6-dimethyl- and 2,4,6-trimethyl-pyridinerepresented by the reaction with 4-cyanopyridine, having arelatively higher k,' value C(2.24 k 0.05) x lop3 s-'1.This canprobably be related to the low basicity of 4-cyanopyridineso that its reactivity towards the solvento-intermediate isdecreased, as found in other cases with entering groups of lownucleophilicity .The k,' values depend both on the electronic and stericfeatures of the nucleophile, as expected for an associativeprocess. A plot of log k,' us. the basicity of am, as usuallymeasured by the pKa values of the conjugate acids Ham+ inwater at 25 OC," allows the nucleophiles to be separated intothree groups (Fig. 1). The first group is formed by the fiveisosteric para-substituted pyridines, covering a pKa range from1.9 to 9.1 1.The entry of these nucleophiles obeys a linear-free-energy relationship of the type (3) with a = 0.035 2 0.009log kzf = a(pKa) + constant (3)which measures the ability of the substrate to discriminateamong entering nitrogen donors of different basicity havingthe same form of hindrance6 Also morpholine, the stericrequirements of which are not very different from that oTable 4 First- and second-orderin methanol at 25 "Crate constants" and equilibrium constants for the reaction [Pt(PhSCH2CH2SPh)C12] k,' + k2'[am]k,' + k,"CI-] +am. [Pt(PhSCH,amh/nmIsosbestic point Used for calculations s-l dm3 mol-' s-l s-' dm3 mol-' s-' dm3103kl'/ 103k2'/ 1 03k1 * b/ 103kzrb/ 14-C yanopyridine 3224-Chlorop yridine 310Pyridine 3093-Methylpyridine 3094-Methylpyridine'CMethylpyridine 3084Aminopyridine 'Morpholine'Quinoline '2-Meth ylpyridine 31 12,4-Dimethylpyridine 31 I2-Methylquinoline '2,6-Dimethylpyridine '2,4,6-TrimethylpyridineC 3083103203303203203203203202.24 f 0.05 257 f 40.82 f 0.03 283 f 30.94 f 0.03 379 f 40.97 f 0.03 341 f 20.93 f 0.03 338 f 30.91 f 0.04 454 f 32.32 f 0.08 393 f 60.94 f 0.03 13.1 f 0.10.90 f 0.04 8.4 f 0.10.95 f 0.02 10.4 k 0.21.05 f 0.01 0.278 f 0.0041.00 k 0.01 0.58 f 0.011.35 f 0.02 0.65 f 0.021.55 f 0.07 887 f 7 56800.36 k 0.020.047 k 0.008 12.2 f 0.20.024 f 0.002 4.94 f 0.070.003 k 0.001 1.02 f 0.020.001 k 0.003 0.39 f 0.04115 f 1" Determined by weighted linear regression of kobs values vs, nucleophile concentration.I = 0.1 mol dm-3, ~Bu",][ClO,]; 0.01 mol dm-' HC104. Conductivit1810 J. CHEM. SOC. DALTON TRANS. 1994pyridine, can be considered to belong to this group, at least to afirst approximation, and it seems that in these processes n-back donation from the filled d orbitals of the metal toantibonding orbitals of the aromatic ring of pyridine, even iftheoretically possible, does not contribute significantly tobond formation.As found in other planar four-co-ordinate systems,6 a secondgroup is formed by pyridines having a methyl group ortho to thenitrogen (2-methyl- and 2,4-dimethylpyridine in the presentcase), to which quinoline may be also added, at least to a firstapproximation.The steric retardation, typical for associativeprocesses, can be measured by the value of A x 1.6 (Fig. 1).Pyridines with two o-methyl substituents (2,6-dimethyl- and2,4,6-trimethylpyridine and possibly 2-methylquinoline) formthe third group with steric retardation A z 2.8 with respectto unhindered 4-substituted pyridines (Fig. 1).The parameters a and A, together with the rate of entry ofpyridine [k,'(py) = 0.283 dm3 mol-' s-'1, which can beassumed as the typical nucleophile for this class of reagents,characterize the system of reactions considered.The entry of pyridine (and solvent) can be compared with thesubstitution of chloride by Br- and I- (k, = 7.22 x and0.304 dm3 mol-' s-' respectively) measured under the sameexperimental condition^.^ Whereas the nopt nucleophilicityscale ' would lead to a reactivity sequence I- > Br- B pyridine> MeOH, the results show that the k2f value for pyridine iscomparable to that of iodide (0.283 and 0.304 dm3 mol-' s-'respectively) and also the solvent, MeOH, is more reactive asexpected (klf = 9.7 x lo4 s-').Such an anomaly, alreadyreported for substrates containing ligands of high tram effect, ''seems to be related to the presence of a thioether sulfur atomtrans to the leaving group, and it has been observed also withthe parent compound [Pt(MeSCH,CH,SMe)Cl,], ' whereas itis not present with substrates like [Pt(en)(Me,S)Cl] + (en =ethane- 1,2-diamine) ' and [2,6- bis(methylsulfanylmethy1)-pyridine]chloroplatinum(r~) cations,,' having one or twothioether sulfur atoms cis to the leaving group.However, itwould be necessary to extend this type of study over a muchwider range of nucleophiles before any firm conclusions couldbe drawn.The substrate [Pt(bipy)Cl,] (bipy = 2,2'-bipyridine) reactswith amines with k,(py) = 0.01 15 dm3 mol-' s-l, a = 0.06 andA = 1.0., The relatively larger reactivity of the presentcompound containing the chelating thioether can be probablyattributed to a combination of a (trans + cis)-labilizing effectof the sulfur donors, expected to be somewhat larger than thatof nitrogen donors, as found for the reactions of the samesubstrates with ionic n~cleophiles.~ The discrimination abilityof [Pt(PhSCH,CH,SPh)Cl,] is less than that of [Pt(bipy)Cl,],suggesting a lower electrophilicity of the former reaction centreas compared to the latter.Both values (0.035 and 0.06) howeverare relatively low as compared to data for other reactioncentres [e.g. for cationic gold(rn) substrates values of a up to0.89 have been r e p ~ r t e d ] , ~ ~ indicating again that protonbasicity plays a minor role in determining the reactivity ofthese processes.Another substrate which has been studied under the sameexperimental conditions is the neutral N,N' chelate complex[Pt { Ph( Me)NN=C( Me)C( Me)=NN(Me)Ph}C12],3 for whichk,(py) = 0.73 dm3 mol-' s-' and a = 0.05. The relatively largereactivity was attributed to hydrogen-bonding assistance to theleaving chloride in the transition state, and the ability todiscriminate among different amines is very small indeed.A different behaviour has been reported for the anionicsubstrate [Pt(dmso)Cl,]- reacting with nitrogen donors (am) toform the neutral complexes trans-[Pt(dmso)(am)Cl,] (dmso =dimethyl s~lfoxide).~ In this case the reactivity is relatively high,as expected for the moderate trans-labilizing effect of dmso, andsteric retardation effects have not been studied, but a dis-crimination factor of about 0.13 can be evaluated for primaryand secondary amines, whereas the reactivity of pyridines seemsto decrease slightly on increasing their proton basicity.Similartrends have been observed with the anionic substrate [Pt-(Me,S)Cl,]-.' This anomalous behaviour may perhaps bedue to some n interaction between the filled d metal orbitalsand the antibonding orbitals of the aromatic ligands, as aconsequence of the negative charge of the trigonal-bipyramidaltransition state and/or the simultaneous presence of n-bondedsulfur in the trigonal plane.The role of the total charge in thisclass of processes is known to be important in gold(1Ir) substi-tution reactions,,l but not enough data are available forpla tinum(r1) complexes.It has been already mentioned that when the solvent is 1,2-dimethoxyethane the substrate [Pt(PhSCH,CH,SPh)Cl,], onreacting with pyridines, undergoes ring opening instead ofchloride substitution and the corresponding parameters area = 0.14 and A = 1.6.14 In both cases in the trigonal-bipyramidal transition state an S, Cl pair of ligands will liein the trigonal plane, together with the entering nitrogen donor,am.The different course of the reaction seems then to becontrolled by the solvation of the leaving group in the transitionstate, methanol thus leading to chloride substitution anddimethoxyethane to ring opening.The steric retardation due to the interaction between the o-methyl group(s) in the pyridine ring and the axial non-participating ligands in the trigonal-bipyramidal transitionstate is much the same in the two processes. On the contrary,the reactivity is nearly two orders of magnitude smaller for ringopening than for chloride substitution and the discriminationamong the entering amines somewhat larger. This seems tosuggest that, when the reagents are allowed to react in methanol,a relatively small degree of bond formation between the metaland the incoming nitrogen is required to form the transitionstate and to promote the release of solvated chloride ion inmethanol. In I ,2-dimethoxyethane, however, the interactionwith the solvent is less efficient in assisting a leaving chloride,the Pt-N bond is more substantially formed and ring openingoccurs.Bearing in mind that the forward and reverse reactions mustproceed along the same reaction profile, a kinetic study of thereverse processes can provide further information on thetransition state.The data reported in Table 4 for the kinetics ofthe processes (4) show that the rate of substitution decreases on[Pt(PhSCH,CH,SPh)(am)Cl] + + C1-[Pt(PhSCH,CH,SPh)Cl,] + am (4)increasing basicity of the displaceable pyridine.This is clearlyevident for the second-order rate constants k,', but alsoobservable for the first-order rate constants k,' relative to thesolvolytic processes.A plot of log k,' us. pK, is linear (Fig. 2) for unhinderedpyridines, with a slope of -0.56 f 0.04. Steric retardationfor monosubstituted 2-methylpyridines is relatively small(A x 0.6), whereas the reactivity of substrates containingdisubstituted 2,6-dimethylpyridines is too small to allow aproper measure. It may be pointed out that, if steric retardationdue to hindrance of the entering nucleophiles is obvious andexpected in associative processes, the same is not necessarilytrue when the hindrance lies with the displaceable ligand.Thepresence of sterically hindered groups in the substrate can shieldto some extent the approach of the nucleophile and, in a limitingcase, prevent the associative second-order process ('pseudo-octahedral complexes'). ,, On the contrary, in dissociativeprocesses, steric ameleration can be expected. l oHowever, since steric destabilization can affect the energylevels of both the ground and transition states, a certain amountof mutual compensation can be expected, so that the stericeffects in the displacement of pyridines should be less than intheir entry. A limiting case of the complete absence of stericeffects has been reported for the displacement of pyridines (L)from the gold(m) substrates [AuLC~,].~J.CHEM. SOC. DALTON TRANS. 1994 181 1'1-1 O I \-341Fig. 2 Plots of log k,' for the reaction [Pt(PhSCH,CH,SPh)(am)Cl]+ + C1-- [Pt(PhSCH,CH,SPh)CI,] + am against pK, of Ham': (a), pyridine and para-substituted pyridines; (m), 2-methyl- and 2,4-dimeth yl-pyridineIn the present case the relatively large dependence ofsubstitution lability upon ligand basicity suggests that theformation of the transition state requires a considerable changein the Pt-N bond, confirming therefore the observationreported above that the transition state can be described as astructure containing a well formed Pt-C1 and a weak Pt-Nbond.In the reactions4 (5) the reactivity is smaller than that fortrans-[Pt(dmso)(am)Cl,] + C1- -~t(dmso)Cl,]- + am (5)the systems reported here (second-order rate constants for thedisplacement of pyridine being 1.96 x lo4 and 1.22 x lo-,dm3 mol-' s-' respectively) thus indicating that the trans-labilizing effect of dmso combined with the cis effect of chlorideis not as efficient as the (trans + cis)-labilizing effect of the twosulfur atoms (bearing in mind that the different charge of thesubstrates can also play a role). The dependence of the reactivityupon the basicity of the leaving group is however surprisinglysimilar ( - 0.55 us.- 0.56). Data for the displacement of differentpyridines from trans-[Pt(Me,S)(am)Cl,] by chloride areunfortunately not available.It is finally to be noted that in the case of 4-cyanopyridinethe equilibrium constant at I = 0.1 mol dm-, is less than 1which may also be related to the anomaly in the k , valuereported above.The linear-free-energy relationship between log K for theequilibria (1) and the pK, of the unhindered pyridines, log K =O.58(pKa) + constant, provides a clear example of the influenceof the a-donor ability of the heterocyclic bases upon the stabilityof these platinum(r1) complexes.AcknowledgementsWe thank the Italian Ministry of University and ConsiglioNazionale delle Ricerche (Rome) for financial support andMiss Tatiana Bobbo for technical assistance.References1234567891011121314151617181920212223M. L.Tobe, Comprehensive Coordination Chemistry, eds G.Wilkinson, R. D. Gillard and J. A. McCleverty, Pergamon, Oxford,1987, vol.1, p. 282.L. Cattalini, A. Orio and A. Doni, Inorg. Chem., 1966,5, 1517.G. Annibale, L. Cattalini, L. Maresca, G. Michelon and G. Natile,Inorg. Chim. Acta, 1974, 10,211.R. Romeo and M. L. Tobe, Inorg. Chem., 1974,13,1991.B. P. Kennedy, R. Gosling and M. L. Tobe, Inorg. Chem., 1977,16,1744.L. Cattalini, MTP Int. Rev. Sci., Inorg. Chem., Ser. I , 1973,9.F. R. Hartley, S. G. Murray, W. Levason, H. E. Soutter andC. A. McAuliffe, Inorg. Chim. Acta, 1979,35, 265.M. S. Kharasch, R. C. Seyler and F. R. Mayo, J. Am. Chem. SOC.,1938,60,882.B. Pitteri, G. Marangoni, L. Cattalini and L. Canovese, J. Chem.SOC., Dalton Trans., 1994, 169.F. Basolo and R. G. Pearson, Mechanism of Inorganic Reactions,2nd edn., Wiley, New York, 1967.R. A. Robinson and R. H. Stokes, Electrolyte Solutions, 2nd edn.,Butterworths, London, 1959, p. 23 1.G. Marangoni, B. Pitteri, V. Bertolasi, V. Ferretti and G. Gilli,personal communication.E. W. Abel, R. P. Bush, F. J. Hopton and C. R. Jenkins, Chem.Commun., 1966, 58; R. J. Cross, I. G. Dalgleish, G. J. Smith andR. Wardle, J. Chem. Soc., Dalton Trans., 1972,992.M. Martelli, G. Marangoni and L. Cattalini, Gazz. Chim. Ital.,1968,98, 1031.H. K. Hall, J. Phys. Chem., 1956,60, 63.R. G. Pearson, H. Sobel and J. Songstad, J. Am. Chem. SOC., 1968,90,319.M. L. Tobe, A. T. Treadgold and L. Cattalini, J. Chem. Soc.,Dalton Trans., 1988,2347.B. Pitteri, personal communication.M. Bonivento, L. Canovese, L. Cattalini, G. Marangoni,G. Michelon and M. L. Tobe, Inorg. Chem., 1981,20,3728.B. Pitteri, L. Canovese, G. Chessa, G. Marangoni and P. Uguagliati,Polyhedron, 1992,11,2363.L. Cattalini, M. Nicolini and A. Orio, Inorg. Chem., 1966, 5, 1674;L. Cattalini, A. Doni and A. Orio, Inorg. Chem., 1967,6,280.W. H. Baddley and F. Basolo, J. Am. Chem. SOC., 1966,88,2944.L. Cattalini and M. L. Tobe, Inorg. Chem., 1966,5,1145; L. Cattalini,A. Orio and M. L. Tobe, Inorg. Chem., 1967,6,75.Received 30th December 1993; Paper 310759450 Copyright 1994 by the Royal Society of Chemistr

ISSN:1477-9226    

DOI:10.1039/DT9940001805

出版商:RSC    

年代:1994

数据来源: RSC