Steric and Solvent Effects on the Secondary Kinetic α-Deuterium

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Organometallics 2010, 29, 82–88 DOI: 10.1021/om900778u

Steric and Solvent Effects on the Secondary Kinetic r-Deuterium Isotope Effects in the Reaction of Methyl Iodide with Organoplatinum(II) Complexes: Application of a Second-Order Technique in Measuring the Rates of Rapid Processes Sepideh Habibzadeh,† Mehdi Rashidi,*,† S. Masoud Nabavizadeh,*,† Leila Mahmoodi,† Fatemeh Niroomand Hosseini,‡ and Richard J. Puddephatt§ †

Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran, ‡Department of Chemistry, Islamic Azad University, Shiraz Branch, Shiraz 71993-37635, Iran, and §Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 5B7 Received September 7, 2009

The kinetic isotope effects (KIEs), kH/kD, have been determined for reaction of CH3I/CD3I with several organoplatinum(II) complexes [PtR2(NN)], in which the bidentate NN ligand is bpy=2,20 -bipyridine, t Bu2bpy=4,40 -bis(tert-butyl)-2,20 -bipyridine, phen=1,10-phenanthroline, or Me2phen=2,9-dimethyl1,10-phenanthroline, at different temperatures and in solvents having different polarities. The values obtained for the secondary R-deuterium KIEs are close to 1 and are dependent on the solvent; values of up to 7-10% larger are obtained for the reactions in the polar solvent acetone as compared to those obtained in the nonpolar solvent benzene. The data also indicate that the steric crowding around the squareplanar coordination sphere of the platinum(II) complexes with the ligand Me2phen results in higher KIEs. The reactions involving dimethylplatinum(II) complexes, [PtMe2(NN)], were fast, and a 1:1 molar ratio (of complex and reagent) technique was successfully used to measure the rate constants accurately by conventional UV-visible spectroscopy. It is shown that there are significant advantages to measuring the reaction rates under second-order condition, as compared to the usual pseudo-first-order method. Introduction The kinetic isotope effect (KIE) is considered an important tool in obtaining information about reaction mechanism and structure of the transition state.1 In this respect, investigation of the secondary kinetic R-deuterium isotope effects in the oxidative addition reaction of alkyl halides with d8 squareplanar complexes2 can be very useful, as these reactions are of fundamental importance in both chemistry and applied chemistry, with particular relevance to the petrochemical industry.3,4 *Corresponding authors. E-mail: [email protected] (M.R.); [email protected] (S.M.N.). (1) (a) Westaway, K. C. J. Labelled Compd. Radiopharm. 2007, 50, 989. (b) Parkin, G. Acc. Chem. Res. 2009, 42, 315. (2) (a) Stang, P. J.; Schlavelll, M. D.; Chenault, H. K.; Breldegam, J. L. Organometallics 1984, 3, 1133. (b) Griffin, T. R.; Cook, D. B.; Haynes, A.; Pearson, J. M.; Monti, D.; Morris, G. E. J. Am. Chem. Soc. 1996, 118, 3029. (3) (a) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Fink, R. J. Principles and Application of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA 1987. (b) Crabtree, R. H. Organometallic Chemistry of the Transition Metals, 3rd ed.; John Wiley & Sons: New York, 2001. (c) Atwood, J. D. Inorganic and Organometallic Reaction Mechanism, 2nd ed.; Wiley-VCH: New York, 1997. (4) (a) Manna, J.; Kuehl, C. J.; Whiteford, J. A.; Stang, P. J. Organometallics 1997, 16, 1897. (b) Fraser, C. S. A.; Eisler, D. J.; Jennings, M. C.; Puddephatt, R. J. J. Chem. Soc., Chem. Commun. 2002, 1224. (c) Fraser, C. S. A.; Jennings, M. C.; Puddephatt, R. J. J. Chem. Soc., Chem. Commun. 2001, 1310. (d) Achar, S.; Puddephatt, R. J. J. Chem. Soc., Chem. Commun. 1994, 1895. (e) Barrios-Landeros, F.; Hartwig, J. F. J. Am. Chem. Soc. 2005, 127, 6944. (f) Alcazar-Roman, L. M.; Hartwig, J. F. Organometallics 2002, 21, 491. pubs.acs.org/Organometallics

Published on Web 12/08/2009

Bimolecular oxidative addition reactions of SN2 type involving addition of a wide variety of alkyl halides to dimethyl or diaryl organoplatinum(II) complexes have been investigated extensively,5 including the diimine platinacyclopentane complex [Pt(CH2CH2CH2CH2)(NN)], in which the bidentate NN ligand is either 2,20 -bipyridine or 1,10-phenanthroline.6 Although the reactions are well documented and established, we have recently presented some new insights by a closer look at the kinetic behavior.7 The kinetics of reaction of MeI with some binuclear organoplatinum(II) complexes, held together by a robust bridging biphosphine ligand, have recently been studied, and a stepwise oxidative addition of MeI to the platinum(II) centers of each complex was described.8 In all these reactions involving the oxidative addition of alkyl halides to organoplatinum(II) complexes, it is (5) (a) Canty, A. J., Ed. Comprehensive Organometallic Chemistry III, Vol. 8; Elsevier: Amsterdam, 2007. (b) Capape, A.; Crespo, M.; Granell, J.; Font-Bardia, M.; Solans, X. Dalton Trans. 2007, 2030. (c) Hager, E.; Clayton, A. H. S.; Mogorosi, M. M.; Moss, J. R. Coord. Chem. Rev. 2008, 252, 1668. (d) Canty, A. J.; Watson, R. P.; Karpiniec, S. S.; Rodemann, T.; Gardiner, M. G.; Jones, R. C. Organometallics 2008, 27, 3203. (e) Rostovtsev, V. V.; Labinger, J. A.; Bercaw, J. E.; Lasseter, I. L.; Goldberg, K. I. Organometallics 1998, 17, 4530. (f) Rendina, L. M.; Puddephatt, R. J. Chem. Rev. 1997, 97, 1735. (6) (a) Rashidi, M.; Momeni, B. Z. J. Organomet. Chem. 1999, 574, 286. (b) Rashidi, M.; Esmaeilbeig, A. R.; Shahabadi, N.; Tangestaninejad, S.; Puddephatt, R. J. J. Organomet. Chem. 1998, 568, 53. (7) Nabavizadeh, S. M.; Hoseini, S. J.; Momeni, B. Z.; Shahabadi, N.; Rashidi, M.; Pakiari, A. H.; Eskandari, K. Dalton Trans. 2008, 2414. (8) (a) Jamali, S.; Nabavizadeh, S. M.; Rashidi, M. Inorg. Chem. 2005, 44, 8594. (b) Jamali, S.; Nabavizadeh, S. M.; Rashidi, M. Inorg. Chem. 2008, 47, 5441. r 2009 American Chemical Society

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well established that the reactions normally proceed by an SN2-type mechanism involving a pentacoordinated or a solvent six-coordinated cationic intermediate.5 A secondary R-deuterium KIE study involving the reaction of MeI with some organoplatinum(II) complexes has been reported in which, as is consistent with an SN2-type mechanism, very small inverse to very small positive secondary kinetic R-deuterium isotope effect, kH/kD, values of 0.94-1.14 were observed.9 The factors influencing KIE, in reactions involving both organic and inorganic nucleophiles, have been thoroughly discussed,2,10 but further theoretical and experimental investigations are needed to give a more complete understanding of the subject. In continuation of our interest in the oxidative addition reactions of alkyl halides with organoplatinum(II) complexes and the related KIE, in the present work, two interesting findings are reported. First we have investigated the effects of solvent and of steric hindrance (implemented by ligands on the axial directions of the square-planar geometry) on the secondary R-deuterium KIEs in reaction of MeI with some organoplatinum(II) complexes and confirmed that the effects are significant. Second, many oxidative addition reactions, especially those involving diimine-dimethylplatinum(II) complexes, are very rapid and so either stopped-flow or fastscanning techniques have usually been necessary in order to follow the kinetics of the reactions with UV-visible spectroscopy.11 In the present study, we have measured the rates of such rapid reactions under second-order conditions, and we have shown that the reactions can be followed easily by conventional UV-visible spectroscopy under these conditions. In particular, there are significant savings of time and material, with no loss of accuracy in the results, when compared to kinetic data obtained under pseudo-first-order conditions. We anticipate that the technique could easily be used for investigating other fast oxidative addition reactions involving platinum or other transition metal complexes.

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Figure 1. Changes in the UV-visible spectrum during the reaction of [Pt(p-MeC6H4)2(Me2phen)], 1b (3  10-4 M), with MeI (133.3 mM) in acetone at T = 20 C: (a) initial spectrum (before adding MeI); (b) spectrum at t = 0; successive spectra recorded at intervals of 35 s. Scheme 1

Results The reactions involved in the present study are described in Scheme 1. The reddish diorganoplatinum(II) complexes 1 reacted with excess MeI in either acetone or benzene to form pale yellow solutions, from which solid products 2 were obtained with the geometry shown in Scheme 1. The starting organoplatinum(II) complexes 1c,12 1d,13 1e,12 and 1f14 and the product organoplatinum(IV) complexes 2c,15 2d,9 2e,16 and 2f17 have been synthesized and characterized as reported elsewhere. The other complexes were characterized by elemental analysis and 1H NMR spectroscopy, and the details are collected in the Experimental Section. (9) Rashidi, M.; Nabavizadeh, S. M.; Akbari, A.; Habibzadeh, S. Organometallics 2005, 24, 2528. (10) Holm, T. J. Am. Chem. Soc. 1999, 121, 515. (11) Monaghan, P. K.; Puddephatt, R. J. J. Chem. Soc., Dalton Trans. 1988, 595. (12) Monaghan, P. K.; Puddephatt, R. J. Organometallics 1984, 3, 444. (13) Achar, S.; Scott, J. D.; Vittal, J. J.; Puddephatt, R. J. Organometallics 1993, 12, 4592. (14) Klein, A.; McInnes, E. J. L.; Kaim, W. J. Chem. Soc., Dalton Trans. 2002, 2371. (15) Jawad, J. K.; Puddephatt, R. J. J. Chem. Soc., Dalton Trans. 1977, 1466. (16) Aye, K. T.; Canty, A. J.; Crespo, M.; Puddephatt, R. J.; Scott, J. D.; Watson, A. A. Organometallics 1989, 8, 1518. (17) De Felice, V.; Giovannitti, B.; De Renzi, A.; Tesauro, D.; Panunzi, A. J. Organomet. Chem. 2000, 593, 445.

The Kinetic Investigations. Reactions Involving Ditolyl Complexes. The oxidative addition reactions of CH3I/CD3I with the reddish ditolyl complexes 1a and 1b in both benzene and acetone were studied using UV-visible spectroscopy. In each case, excess CH3I or CD3I was used and the disappearance of the MLCT band for the complex 1a (λmax=445 nm in acetone or λmax= 480 nm in benzene) or 1b (λmax=435 nm in acetone or λmax= 470 nm in benzene) was used to monitor the reaction. For minimization of errors, the addition reaction rates of both CH3I and CD3I were measured in parallel with the identical stock solution of the related platinum(II) complex. The reactions followed good first-order kinetics (see Figures 1 and 2 for typical cases). The pseudo-first-order rate constants (kobs) were evaluated by nonlinear least-squares fitting of the absorbance-time profiles to a first-order equation (eq 1):

Abst ¼ Abs¥ þ ðAbs0 -Abs¥ Þ expð -kobs tÞ

ð1Þ

Graphs of these first-order rate constants against the concentration of the halide gave good straight-line plots

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Figure 2. Absorbance-time curves for the reaction of [Pt(pMeC6H4)2(Me2phen)], 1b, with CD3I (26.7-133.3 mM with [CD3I] increases reading downward) in acetone at 25 C.

Figure 4. Eyring plots for the reactions (a) [Pt(p-MeC6H4)2(phen)], 1a, with CD3I in benzene; (b) [Pt(p-MeC6H4)2(Me2phen)], 1b, with CD3I in benzene; and (c) [Pt(p-MeC6H4)2(phen)], 1a, with MeI in acetone.

under the pseudo-first-order conditions ([MeI]0 > 10 [diarylPt complex]) and uses eq 1 to fit the data, would not be appropriate, and either stopped-flow or fast-scanning techniques are usually needed to measure the rates.11 For this type of fast reaction we have found that the method that uses 1:1 stoichiometry, in reactions that follow second-order kinetics and consume one mole of each of the two reactants, could be used to measure the reaction rates by conventional UV-visible spectroscopy. Thus, the reactions of MeI with the methyl-diimine complexes 1c-1f were monitored by the same procedure described above for the corresponding reaction involving the diaryl complexes 1a and 1b, except that the data were obtained under second-order 1:1 stoichiometric conditions ([MeI]0 = [Pt complex]) and the data were fit to eq 3 with the programs that are based on the leastsquares method.18

Abst ¼ Abs¥ þ ðAbs0 -Abs¥ Þ=ð1 þ ½Pt complex0  k2  tÞ ð3Þ Figure 3. Plots of first-order rate constants (kobs/s-1) for the reaction of [Pt(p-MeC6H4)2(phen)], 1a, with MeI in benzene at different temperatures vs concentration of MeI.

passing through the origin, showing a first-order dependence of the rate on the concentration of the halide (Figure 3), and the slope gave the overall second-order rate constant. The same method was used at other temperatures, and activation parameters were obtained from the Eyring equation (eq 2 and Figure 4). The resulting data are collected in Tables S1 and S2.

    k2 kB ΔSq ΔS q ln ¼ ln þ T h R RT

ð2Þ

Reactions Involving Dimethyl Complexes. The reaction of MeI with the dimethylplatinum(II) complexes containing chelating diimine ligands, such as complexes 1c-1e, is known to be very fast, and therefore measurement of the rate of the reaction by the above technique, which works

The changes in the UV-visible spectrum during a typical reaction are shown in Figure 5, with the absorbance/time curves shown in Figure S1. The k2 values were evaluated by nonlinear least-squares fitting of the absorbance-time profiles to eq 3. This method was used at different temperatures, and activation parameters were obtained from the Eyring equation (eq 2 and Figure S2). The results are collected in Tables S3-S6.

Discussion As has been reported previously, the reaction of dimethylplatinum(II) complexes containing diimine ligands with MeI is very fast and so cannot be measured accurately by the more conventional UV-visible spectrophotometric techniques.11 In this study we have succeeded in measuring the rate of these fast reactions by using a 1:1 molar ratio of (18) Espenson, J. H. Chemical Kinetics and Reaction Mechanisms, 2nd ed.; McGraw-Hill: New York, 1995.

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Figure 6. CR-H(D) out-of-plane bending vibrations in the substrate and in the SN2 transition state.

Figure 5. Changes in the UV-visible spectrum during the reaction of [PtMe2(bpy)], 1c (3 mL of 3  10-4 M solution), with MeI, under second-order 1:1 stoichiometric conditions, in benzene at T=30 C: (a) initial spectrum (before adding MeI); (b) spectrum at t = 0; successive spectra recorded at intervals of 30 s; (c) final spectrum.

complex and reagent (second-order conditions). This makes the reaction slow enough for measurement by the conventional UV-visible spectroscopy. For reactions operating by the SN2 mechanism, both time measurements and concentrations of all the reagents are important, and so the accuracy of measurements under second-order conditions is probably expected to be less than that of those under pseudo-first-order conditions. However, the measurements by the second-order technique used in the present work have confirmed that this is not always true, especially for fast reactions, and there are several more advantages: (i) The time needed to measure the rate constants was very significantly shortened. (ii) The amount of the materials needed was much less, and this is in particular important when the precious metal complexes, and especially expensive deuterated compounds, such as CD3I reagent used in the present study, are consumed during the process. (iii) The measurements at comparatively higher temperatures were easily feasible. (iv) The slower reaction allows a greater fraction of the reaction to be monitored, under a wider temperature range, and gives improved accuracy of the results and a more complete kinetic study. The above advantages are clear in the kinetic results obtained in the present study. The second-order rate constants, k2, reported in the literature for the reaction of [PtMe2(bpy)], 1c, with MeI in acetone at 20, 3.6, and -7.5 C are 40 ( 1, 22 ( 1, and 14 ( 1 dm3 mol-1 s-1, respectively;19 by extrapolation, the value of k2 at 30 C is calculated to be 56 ( 1 dm3 mol-1 s-1. The data have been obtained under pseudo-first-order conditions, and the ΔSq value is found to be -129 ( 1 J K-1 mol-1. These values are (19) Byers, P. K.; Canty, A. J.; Crespo, M.; Puddephatt, R. J.; Scott, J. D. Organometallics 1989, 8, 1363.

in agreement with the rate constant values obtained in the present study for the same reaction, using 1:1 stoichiometric techniques, in acetone at 20 and 30 C, which are 38.2 ( 0.4 and 55.5 ( 0.9 dm3 mol-1 s-1, respectively, with the ΔSq value being -138 ( 4 J K-1 mol-1. It is interesting to note that comparatively higher temperatures were used in the latter method to obtain the ΔSq value (see Table S3). Note also that in a separate report for the same reaction in acetone at 30 C,15 the value of k2 has been found to be 46.8 dm3 mol-1 s-1, a value that is some 20% lower than those obtained by the two independent procedures just mentioned. This may support the conclusion that the 1:1 stoichiometric technique has probably provided more reliable data as compared to the pseudo-first-order technique, especially at mild or elevated temperatures, where the rates are increased and yet the measurements are much easier to perform. Investigation of the data obtained from the reaction of diorganoplatinum(II) complexes 1 with CH3I/CD3I revealed some interesting secondary R-deuterium KIE trends. It is believed that the vibration contribution has a major role in the magnitude of total R-deuterium KIEs in SN2 reactions. This has been factored into stretching and bending components, and it has been shown that both are important in determining KIEs. Westaway has described that, in an SN2 reaction, the tetrahedral substrate is converted into a pentavalent transition state (Figure 6), with corresponding change in the sp3 character of the carbon atom, and this increases the energy of the CR-H(D) out-of-plane bending vibrations and leads to the values of the normal or small inverse KIEs that are observed for SN2 reactions.1a Thus, the trends observed in the present work have been explained as follows: (a) Solvent effect. The results in Tables S1-S6 (summarized and typified in Table 1) clearly reveal that the values of kH/kD for each reaction at the same temperatures are some 5-10% lower in the nonpolar solvent benzene than in the polar solvent acetone. As mentioned above, the bending contribution, (kH/kD)bend, to the total KIE, kH/kD, is significant, and therefore a looser (the looseness is defined as the nucleophile-leaving group distance) SN2 transition state results in a larger kH/kD value, while a tighter one results in a smaller or inverse kH/kD value. In a polar solvent such as acetone, the polar [Pt] 3 3 3 CH3 3 3 3 I system in the transition state is more loosened than that in a nonpolar solvent such as benzene, and this makes the bending contribution, (kH/kD)bend, to the total KIE higher in acetone than in benzene. This “looseness” suggestion can also be modified by the “crowdedness” concept suggested by Davico and Bierbaum.20 As such, the polar acetone solvent molecules surround the positively charged CH3 section of the (20) Davico and Bierbaum believe that crowdedness rather than looseness is a better way of understanding the secondary R-deuterium KIE magnitude in polyatomic nucleophiles; see: Davico, G. E.; Bierbaum, V. M. J. Am. Chem. Soc. 2000, 122, 1740.

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Table 1. Rate Constants (10  kH or 10  kD / dm3 mol-1 s-1)a,b, Activation Parameters,b and Kinetic r-Deuterium Isotope Effectsa for the Reaction of Complexes [PtR2(NN)], 1, with CH3I/CD3I in Acetone or Benzene at 25 C complex

solvent

10  kHb (dm3 mol-1 s-1)

kH/kD

ΔH # b,c (kJ mol-1)

ΔS# b,c (J K-1 mol-1)

1a

acetone

8.7 ( 0.3 (8.3 ( 0.1) 1.02 ( 0.02 (1.04 ( 0.02) 22.5 ( 0.2 (20.3 ( 0.1) 4.89 ( 0.01 (4.57 ( 0.01) 450.5 ( 3.5 (440.4 ( 6.7) 109.3 ( 2.9 (117.6 ( 3.7) 1120.1 ( 9.4 (1061.4 ( 7.1) 221.6 ( 9.0 (219.7 ( 6.0) 724.0 ( 3.9 (700.3 ( 8.1) 140.3 ( 2.9 (154.4 ( 4.1) 251.1 ( 3.2 (250.1 ( 1.1)

1.05 ( 0.04

28.9 ( 2.0 (28.0 ( 2.0) 32.2 ( 2.2 (30.0 ( 2.0) 29.7 ( 1.8 (28.5 ( 1.7) 26.4 ( 1.6 (26.0 ( 1.5) 22.3 ( 1.2 (22.0 ( 1.2) 21.6 ( 1.4 (20.3 ( 1.7) 30.1 ( 2.1 (28.3 ( 2.0) 34.4 ( 2.2 (33.7 ( 2.2) 29.1 ( 1.5 (28.7 ( 1.5) 34.2 ( 2.2 (32.9 ( 2.1) 30.2 ( 1.9 (29.7 ( 1.8)

-149 ( 7 (-152 ( 7) -156 ( 7 (-163 ( 6) -138 ( 6 (-143 ( 6) -162 ( 5 (-164 ( 5) -138 ( 4 (-139 ( 4) -153 ( 5 (-157 ( 6) -105 ( 7 (-111 ( 7) -104 ( 8 (-106 ( 7) -112 ( 5 (-113 ( 5) -108 ( 7 (-112 ( 7) -117 ( 7 (-119 ( 6)

benzene 1b

acetone benzene acetone

1c

benzene 1d

acetone benzene acetone

1e

benzene 1f

d

benzene

0.98 ( 0.03 1.11 ( 0.01 1.07 ( 0.01 1.02 ( 0.02 0.93 ( 0.04 1.05 ( 0.01 1.01 ( 0.03 1.03 ( 0.01 0.91 ( 0.03 1.00 ( 0.01

a Estimated errors were calculated from least-squares regression analysis. b Values in parentheses are for reactions with CD3I. c Obtained from the Eyring equation. The errors were computed using the published formula.22 d The complex was not stable enough in acetone for any kinetic measurements.

[Pt] 3 3 3 CH3 3 3 3 I transition state, and this leads to an increase in energy of the CR-H(D) out-of-plane bending vibrations compared to the case when the solvent is benzene. Note also that when comparing the values of kH/kD for reactions involving different Pt nucleophilic complexes, the difference is more pronounced in benzene than in acetone. (b) Steric effect. As mentioned above, the magnitude of R-deuterium KIEs in an SN2 reaction is influenced by the amount of steric crowding at the CR-H(D) out-of-plane bending vibrations,1 and a smaller KIE would be found for a reaction with a tighter transition state. In the reaction of diimineplatinum(II) complexes 1 with MeI studied in the present work, the Me group attacks the platinum center in a direction perpendicular to the square-planar coordination plane of the complex, interacting with the filled Pt dz2 orbital. The rate of the reaction is, among other factors, dependent on the steric bulk of the ligands, and for example, the orthosubstituted phenyl ligands inhibit the reaction.15

Figure 7. Natural populations analysis (NPA) charge of nitrogen atoms in bidentate N-donor ligands at B3LYP/6-311þG*.

The X-ray structural analyses of a number of Pd(II) and Pt(II) complexes containing 2,9-dimethyl-1,10-phenanthroline, Me2phen, e.g., [PtPh2(Me2phen)],14 [PdMeCl(Me2phen)],21a and [PtCl2(Me2phen)],21b have revealed that the plane of the Me2phen ligand in each complex has a remarkable rotation around the N-N direction with respect to the coordination plane, and this creates crowding around the coordination sphere. We believe that this crowding is probably responsible for the observation that the magnitude of R-deuterium KIEs for reactions involving the complex [PtMe2(Me2phen)], 1f, in benzene and at different temperatures is some 7-9% greater than that of [PtMe2(phen)], 1e, at the same conditions. A similar trend was also observed for

the ditolyl analogous complexes [Pt(p-MeC6H4)2(Me2phen)], 1b, and [Pt(p-MeC6H4)2(phen)], 1a (see Table 1). Theoretical studies using density functional calculations were performed to calculate the charge on nitrogen atoms of the NN ligands.23 The data were used in connection with the kinetic data to confirm the steric effect of the Me2phen ligand on the kinetic behaviors. Thus the charges were measured and the results are indicated in Figure 7. The graph of log(k2) for the reaction of complexes [PtMe2(NN)], 1c-1f, with MeI at 25 C in benzene versus charges on the NN ligands, Figure 8a and Table 2, is linear for the series of NN ligands bpy, phen, and tBu2bpy, and this confirms that the rate of reactions correlates linearly with the charges on the NN

(21) (a) De Felice, V.; Albano, V. G.; Castellari, C.; Cucciolito, M. E.; De Renzi, A. J. Organomet. Chem. 1991, 403, 269. (b) Fanizzi, F. P.; Intini, F. P.; Maresca, L.; Natile, G.; Lanfranchi, M.; Tiripicchio, A. J. Chem. Soc., Dalton Trans. 1991, 1007. (22) Morse, P. M.; Spencer, M. D.; Wilson, S. R.; Girolami, G. S. Organometallics 1994, 13, 1646.

(23) Density functional calculations were performed with the program suite Gaussian03 using the B3LYP method.24 The 6-311þG* basis set was used with B3LYP. All geometries were optimized. To evaluate and ensure the optimized structures of the molecules, frequency calculations were carried out using analytical second derivatives. In all cases only real frequencies were obtained for the optimized structures.

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Figure 8. Plot showing a correlation between the rate constants on a logarithmic scale {log(k2)} for (a) the reaction of complexes [PtMe2(NN)], 1c-1f, with MeI at 25 C in benzene and (b) the reaction of complexes [Pt(p-MeC6H4)2(NN)] with MeI at 25 C in acetone and the charge of the nitrogen atom on the NN ligands. The values for the Me2phen ligand were omitted from the linear fit. Table 2. Charges of Nitrogen Atoms on the NN Ligandsa and the k2 Values (experimental and calculated)b for the Reaction of Complexes [PtR2(NN)] (R = Me or p-MeC6H4), 1, with MeI at 25 C, in Benzene or Acetone NN ligand

bpy

phen

N charge experimental (or calculated)b k2 values for reaction of [PtMe2(NN)] þ MeI in benzene at 25 C experimental (or calculated)b k2 values for reaction of [Pt(p-MeC6H4)2(NN)] þ MeI in acetone at 25 C

-0.418 10.9 (10.5) 0.62c (0.60)

a

b

t

Bu2bpy

Me2phen

-0.429 14.0 (15.3)

-0.438 22.2 (21.1)

-0.478 25.1 (83.9)

0.84 (0.94)

1.43c (1.37)

2.03 (7.06)

a N charge applies to the natural populations analysis (NPA) charge of nitrogen atoms in bidentate NN ligands at B3LYP/6-311þG*; see Figure 7. Values of k2 in parentheses were calculated according to log(k2) = m(N charge) þ b equations shown in Figure 8. c From ref 9.

ligands. However, when NN is Me2phen, the related point is quite off the linear correlation, and if the line is extrapolated, then the rate of reaction of complex [PtMe2(Me2phen)], 1f, at the corresponding conditions of temperature 25 C in benzene would be estimated to be nearly 84 dm3 mol-1 s-1. This theoretical rate obtained for the reaction based on the electronic effect of the NN ligand is considerably higher than that obtained experimentally at the same conditions, i.e., nearly 25 dm3 mol-1 s-1. A similar trend was observed for the reaction of the analogous tolyl complexes [Pt(p-MeC6H4)2(phen)], 1a, [Pt(p-MeC6H4)2(Me2phen)], 1b, [Pt(p-MeC6H4)2(bpy)], and [Pt(p-MeC6H4)2(tBu2bpy)] with MeI at 25 C (see Table 2 and Figure 8b). We attribute this (24) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.;. Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;. Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,. K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;. Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.;. Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03; Revision B03; Gaussian, Inc.: Pittsburgh, PA, 2003.

significant rate reduction to the steric effect imposed by Me groups of the Me2phen ligand on the coordination sphere of the square-planar complexes [PtR2(NN)].

Conclusions The reaction of dimethyl-diimine platinum(II) complexes with methyl iodide is fast, and therefore either stopped-flow or fast-scanning techniques have usually been needed in order to follow the kinetics of the reactions under pseudofirst-order conditions with UV-visible spectroscopy.11 However, in the present work we have shown that, by using a 1:1 stoichiometric method (under second-order conditions), the rate of the reaction of MeI with several dimethylplatinum(II) complexes [PtMe2(NN)], in which the bidentate NN ligand is 2,20 -bipyridine, 1,10-phenanthroline, or their alkyl-substituted derivatives, can be determined accurately, even at higher temperatures, by using more conventional UV-visible spectroscopy. The other advantages of this 1:1 stoichiometric method are significant time and material savings. We anticipate that the technique can successfully be used to determine the rate of other fast reactions involving other organometallic complexes. We have determined the rate of reactions of a series of organoplatinum(II) complexes with CH3I/CD3I, in acetone or benzene at different temperatures, and indicated that in each case a secondary R-deuterium KIE close to 1 (either 1.00, small inverse up to 0.91, or small positive up to 1.11) is obtained. The results show that in cases where steric

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crowding is created around the coordination sphere the bending contribution to the total KIE is affected and leads to a considerable increase in the magnitude of the secondary R-deuterium KIE. We also have clearly indicated that when the nonpolar solvent benzene is used, the values of secondary R-deuterium KIE are 7-10% lower than when the more polar solvent acetone is used. This trend is proposed to arise because the transition state in benzene is tighter than that in acetone. It is also clearly shown that, in the same reactions involving different nucleophilic platinum complexes, the difference is more pronounced in benzene than in acetone. We therefore suggest that, in this kind of study, using the reaction rates obtained in a less polar solvent can provide a more reliable comparison.

Experimental Section The 1H NMR spectra of the complexes were recorded as CDCl3 solutions on a Bruker Avance DPX 250 MHz spectrometer, and TMS (0.00) was used as an external reference. All the chemical shifts and coupling constants are given in units of ppm and Hz, respectively. Kinetic studies were carried out by using a Perkin-Elmer Lambda 25 spectrophotometer. Temperature was carefully controlled by a EYELA NCB-3100 low-temperature thermoregulation bath. The starting organoplatinum(II) complexes 1c,12 1d,13 1e,12 and 1f14 and the product organoplatinum(IV) complexes 2c,15 2d,9 2e,16 and 2f17 have been synthesized and characterized as reported elsewhere. The starting complexes 1, used for kinetic measurements, nicely precipitated in ether solutions, and the separated crystals were carefully dried under vacuum. The 1H NMR data and the microanalysis results of the complexes indicate that the complexes are of very high purity. [Pt(p-MeC6H4)2(phen)], 1a. [Pt(p-MeC6H4)2(SMe2)2] (504 mg, 1 mmol) was dissolved in diethyl ether (20 mL), and 1,10-phenanthroline (180 mg, 1 mmol) was added to it. The mixture was stirred at room temperature over a period of 2 h, after which the solution turned yellow. The solvent was evaporated and the yellow residue was washed with n-hexane and dried under vacuum. Yield: 95%; mp 288 C (dec). Anal. Found: C, 55.57; H, 3.91; N, 4.85. C26H22N2Pt requires: C, 56.01; H, 3.98; N, 5.02. 1H NMR: δ 2.30 (6H, s, ArCH3), 6.95 (4H, d, 3J(HoHm)=7.8 Hz, Hm of Ar), 7.48 (d, 4H, 3J(HmHo)=7.8 Hz, 3J(PtHo)=68.7 Hz, Ho of Ar), 9.02 (d, 2H, 3J(H2H3) = 5.0 Hz, 3J(PtH2) = 18.6 Hz, H2 and H9 of phen), 8.55 (d, 2H, 3J(H3H4)=8.2 Hz, H4 and H7 of phen), 7.97 (s, 2H, H5 and H6 of phen), 7.77 (dd, 2H, 3J(H4H3) = 8.2 Hz, 3 J(H2H3) = 5.0 Hz, H3 and H8 of phen). [Pt(p-MeC6H4)2(Me2phen)], 1b, was prepared by the same method using [Pt(pMeC6H4)2(SMe2)2) and 2,9-dimethyl-1,10-phenanthroline. Yield: 97%; mp 276 C (dec). Anal. Found: C, 57.71; H, 4.57; N, 4.38.

Habibzadeh et al. C28H26N2Pt requires: C, 57.43; H, 4.48; N, 4.78. 1H NMR: δ 2.19 (s, 6H, Me of Me2phen), 2.23 (s, 6H, ArCH3), 6.75 (d, 4H, 3 J(HoHm)=7.7 Hz, Hm of Ar), 7.32 (d, 4H, 3J(HmHo)=7.7 Hz, 3 J(PtHo)=73.5 Hz, Ho of Ar), 7.49 (d, 2H, 3J(H3H4) = 8.3 Hz, H3 and H8 of phen), 7.83 (s, 2H, H5 and H6 of phen), 8.31 (d, 2H, 3 J(H3H4) = 8.3 Hz, H4 and H7 of phen). [PtIMe(p-MeC6H4)2(phen)], 2a. Excess MeI (1 mL) was added to a solution of [Pt(p-MeC6H4)2(phen)] (100 mg) in acetone (20 mL). The mixture was stirred at room temperature for 2 h. The solvent was evaporated from the resulting solution, and the residue was washed with ether and dried under vacuum. Yield: 91%; mp 234 C (dec). Anal. Found: C, 45.94; H, 3.61; N, 3.86. C27H25IN2Pt requires: C, 46.36; H, 3.60; N, 4.00. 1H NMR: δ 2.32 (s, 3H, 2J(PtH) = 72.0 Hz, Me-Pt), 2.36 (s, 6H, ArCH3), 6.65 (d, 4H, 3J(HoHm)=7.6 Hz, Hm of Ar), 7.43 (d, 4H, 3 J(HmHo)=7.6 Hz, 3J(PtHo)=34.9 Hz, Ho of Ar), 9.12 (d, 2H, 3 J(H2H3) = 5.0 Hz, 3J(PtH2) =11.6 Hz, H2 and H9 of phen), 8.56 (d, 2H, 3J(H3H4) = 8.2 Hz, H4 and H7 of phen), 8.07 (s, 2H, H5 and H6 of phen), 7.82 (dd, 2H, 3J(H4H3) = 8.2 Hz, 3 J(H2H3) = 5.0 Hz, H3 and H8 of phen). The following complexes were made similarly using the appropriate platinum(II) complex and CH3I or CD3I. [PtIMe(p-MeC6H4)2(Me2phen)], 2b. Yield: 82%; mp 236 C (dec). Anal. Found: C, 47.40; H, 3.60; N, 3.51. C29H29IN2Pt requires: C, 47.88; H, 4.02; N, 3.85. 1H NMR: δ 1.21 (s, 3H, 2 J(PtH)=70.3 Hz, Me-Pt), 2.22 (s, 6H, methyl of Me2phen), 2.43 (s, 6H, ArCH3), 6.77 (4H, d, 3J(HoHm) = 8.2 Hz, Hm of Ar), 7.52 (d, 4H, 3J(HmHo)=8.2 Hz, Ho of Ar), 8.55 (d, 2H, 3J(H3H4) =8.5 Hz, H4 and H7 of phen), 7.86 (s, 2H, H5 and H6 of phen), 7.71 (d, 2H, 3J(H4H3) = 8.5 Hz, H3 and H8 of phen). [PtI(CD3)(p-MeC6H4)2(phen)]. This compound had 1H NMR data similar to data obtained for 2a, but without the MePt peak. [PtI(CD3)(p-MeC6H4)2(Me2phen)]. This compound had 1H NMR data similar to data obtained for 2b, but without the MePt peak. Kinetic Study. In a typical experiment, a solution of complex 1 in acetone or benzene (3 mL, 3.0  10-4 M) in a cuvette was thermostated at 25 C, and MeI with a known concentration was added using a microsyringe. After rapid stirring, the absorbance was monitored with time.

Acknowledgment. We thank the Iran National Science Foundation and the Shiraz University Research Councils (Grant Nos. 88-GR-SC-12 and 88-GR-SC-18) for financial support. Supporting Information Available: Figures S1-S2 (kinetic data and Eyring plots), Tables S1-S6 (primary kinetic data). These materials are available free of charge via the Internet at http://pubs.acs.org.