Oxidative Addition of Ethyl Iodide to a Dimethylplatinum(II) Complex

Organometallics 2010, 29, 6359–6368 DOI: 10.1021/om100810t

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Oxidative Addition of Ethyl Iodide to a Dimethylplatinum(II) Complex: Unusually Large Kinetic Isotope Effects and Their Transition-State Implications S. Masoud Nabavizadeh,† Sepideh Habibzadeh,† Mehdi Rashidi,†,‡,§ and Richard J. Puddephatt*,‡ †

Department of Chemistry, Faculty of Sciences, Shiraz University, Shiraz 71454, Iran, and ‡ Department of Chemistry, University of Western Ontario, London, Canada N6A 5B7. § On sabbatical leave from Shiraz University, Iran Received August 18, 2010

The mechanism of oxidative addition of ethyl iodide to [PtMe2(2,20 -bipyridine)], 1, has been investigated by product analysis and by study of secondary deuterium kinetic isotope effects (KIEs), using the reagents C2H5I, C2D5I, CH3CD2I, and CD3CH2I. The reactions in acetone and benzene give [PtIMe2Et(bipy)], mostly as the product of trans oxidative addition, but with some of the isomeric product of cis oxidative addition and some [PtIMe3(bipy)], resulting from methyl group transfer. The reaction in benzene is light-sensitive, giving additional major products [PtI2Me2(bipy)] and [PtIMe2(OOEt)(bipy)], as well as several minor products indicative of a photochemically initiated free-radical reaction. No H/D exchange within the ethyl group was observed in any of the products. The dark reactions in acetone and benzene follow second-order kinetics, with large negative values of the entropy of activation, indicating the SN2 mechanism of oxidative addition of ethyl iodide to 1. However, for reaction with C2H5I vs C2D5I, values of the KIE kH/kD range from 1.32 to 1.72 in acetone and from 1.44 to 1.90 in benzene solution, and studies with CH3CD2I and CD3CH2I show that R- and β-deuterium KIEs make about equal contributions to the overall KIE. These are the first reports of isotope effects on the rate of oxidative addition reactions of ethyl halides, and the high values of the secondary deuterium KIE were unexpected for the SN2 mechanism. Possible reasons for these observations are discussed.

Introduction

Scheme 1. Mechanism of Oxidative Addition by the SN2 Mechanism and Isomerization at Platinum(IV)a

The oxidative addition of alkyl halides to square-planar transition metal complexes is recognized as a fundamental step in many catalytic processes, so these reactions have been studied in detail.1 The square-planar platinum(II) complexes of formula [PtR2(NN)], where R is an alkyl or aryl group and NN is a diimine ligand, are particularly suited to mechanistic studies of the oxidative addition of alkyl halides because they have high reactivity and because there is typically a change in color on oxidation of platinum(II) to platinum(IV), which allows easy monitoring of reaction rates by UV-visible spectroscopy.2-5 For example, the oxidative addition of CD3I to *Corresponding author. E-mail: [email protected]. (1) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. J. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987. (2) Rendina, L. M.; Puddephatt, R. J. Chem. Rev. 1997, 97, 1735. (3) (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. (4) Nabavizadeh, S. M.; Hoseini, S. J.; Momeni, B. Z.; Shahabadi, N.; Rashidi, M.; Pakiari, A. H.; Eskandari, K. Dalton Trans. 2008, 2414. (5) (a) Rashidi, M.; Nabavizadeh, S. M.; Akbari, A.; Habibzadeh, S. Organometallics 2005, 24, 2528. (b) Habibzadeh, S.; Rashidi, M.; Nabavizadeh, S. M.; Mahmoodi, L.; Hosseini, F. M.; Puddephatt, R. J. Organometallics 2010, 29, 82.

[PtMe2(NN)], with NN = 2,20 -bipyridine or 1,10-phenanthroline, has been shown to occur by the SN2 mechanism according to Scheme 1 (R = CD3).7 The oxidation of A occurs in the first step to give either the five-coordinate, 16-electron platinum(IV) complex B or the octahedral solvent complex C. If monitored by NMR spectroscopy at low temperature in a polar solvent, the product that is first observed is complex C

r 2010 American Chemical Society

Published on Web 11/08/2010

a

S = solvent.

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(S = Me2CO or MeCN). Complex C is thus the product of kinetic control, but it can dissociate solvent to give back complex B, and reversible trapping by iodide can then give the complex D. Alternatively, B can isomerize by pseudorotation to give E, which can also be trapped by either solvent to give F or by iodide to give G. At equilibrium, the product is a statistical 1:2 mixture of the platinum(IV) complexes D and G (R = CD3). These oxidative addition reactions of methyl iodide to platinum(II) complexes follow second-order kinetics, occur more rapidly in polar solvents, and are characterized by large negative values of the entropy of activation, as expected for a bimolecular reaction with an ionic intermediate.1-8 Comparison of the rates for reactions of CH3I and CD3I shows a small positive or inverse secondary deuterium kinetic isotope effect (KIE),5 which is considered to be a characteristic feature of the SN2 mechanism.5,9,10 Most primary alkyl halides, including ethyl iodide, are thought to react by a similar SN2 mechanism, but the oxidative addition step of the reactions often occurs too slowly with respect to subsequent steps (Scheme 1, R = Et) to allow detection of intermediates.3,11,12 Secondary alkyl halides, such as isopropyl iodide, can react either by the SN2 mechanism of Scheme 1 or by a free-radical chain reaction shown in Scheme 2.11,13 Initiation gives an alkyl radical and a platinum(III) radical H, which can react with more alkyl halide to give the diiodoplatinum(IV) complex I and a second alkyl radical (R = i-Pr). Under air-free conditions, the propagation occurs by reaction of the alkyl radical with [PtMe2(NN)] to give the platinum(III) complex J, which reacts with alkyl iodide to give the product K and an alkyl radical to continue the chain. Alternatively, if oxygen is present, the alkyl radical can react to give an alkylperoxy radical, which can give the (6) (a) Jawad, J. K.; Puddephatt, R. J. J. Chem. Soc., Dalton Trans. 1977, 1466. (b) Monaghan, P. K.; Puddephatt, R. J. J. Chem. Soc., Dalton Trans. 1988, 595. (c) Anderson, C. M.; Crespo, M.; Tanski, J. M. Organometallics 2010, 29, 2676. (d) Chaudhury, N.; Puddephatt, R. J. J. Organomet. Chem. 1975, 84, 105. (e) Jawad, J. K.; Puddephatt, R. J. J. Organomet. Chem. 1976, 117, 297. (7) (a) Crespo, M.; Puddephatt, R. J. Organometallics 1987, 6, 2548. (b) Byers, P. K.; Canty, A. J.; Crespo, M.; Puddephatt, R. J.; Scott, J. D. Organometallics 1988, 7, 1363. (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. (9) (a) Stang, P. J.; Schiavelli, M. D.; Chenault, H. K.; Breidegam, 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. (10) (a) Westaway, K. C. Adv. Phys. Org. Chem. 2006, 41, 217. (b) Westaway, K. C. J. Label. Compd. Radiopharm. 2007, 50, 989. (c) Shiner, V. J., Jr.; Rapp, M. W.; Pinnick, H. R., Jr. J. Am. Chem. Soc. 1970, 92, 232. (11) (a) Monaghan, P. K.; Puddephatt, R. J. J. Chem. Soc., Dalton Trans. 1988, 595. (b) Monaghan, P. K.; Puddephatt, R. J. Inorg. Chim. Acta 1983, 76, L237. (c) Hill, R. H.; Puddephatt, R. J. J. Am. Chem. Soc. 1985, 107, 1218. (d) Ferguson, G.; Monaghan, P. K.; Parvez, M.; Puddephatt, R. J. Organometallics 1985, 4, 1669. (12) (a) Werner, M.; Bruhn, C.; Steinborn, D. Transition Met. Chem. 2009, 34, 61. (b) Sobanov, A. A.; Vedernikov, A. N.; Dyker, G.; Solomonov, B. N. Mendeleev Commun. 2002, 12, 14. (c) Bennett, M. A.; Canty, A. J.; Felixberger, J. K.; Rendina, L. M.; Sunderland, C.; Willis, A. C. Inorg. Chem. 1993, 32, 1951. (13) Free-radical chain and nonchain mechanisms of oxidative addition have been proposed in early studies, and more recently, free-radical mechanisms continue to be reported for important steps in catalytic reactions. (a) Kramer, A. V.; Labinger, J. A.; Bradley, J. S.; Osborn, J. A. J. Am. Chem. Soc. 1974, 96, 7145. (b) Hall, T. L.; Lappert, M. F.; Lednor, P. W. J. Chem. Soc. Dalton 1980, 1448. (c) Tejel, C.; Ciriano, M. A.; Edwards, A. J.; Lahoz, F. J.; Oro, L. A. Organometallics 2000, 19, 4968. (d) Astruc, D. Chem. Rev. 1988, 88, 1189. (e) Manolikakes, G.; Knochel, P. Angew. Chem., Int. Ed. 2009, 48, 205. (f) Boisvert, L.; Denney, M. C.; Hanson, S. K.; Goldberg, K. I. J. Am. Chem. Soc. 2009, 131, 15802.

Nabavizadeh et al. Scheme 2. aFree-Radical Chain Mechanism of Oxidative Addition with Possible Formation of Alkylperoxo Complexes of Platinum(IV)

a

R = i-Pr.

platinum(III) complex L and then the alkylperoxoplatinum(IV) complex M. Complex K can also be formed by the SN2 mechanism, but the observation of complexes I and M can be considered as indicators that at least part of the reaction occurs by a free-radical mechanism.11,13 This paper describes a detailed study of the oxidative addition reactions of ethyl iodide with [PtMe2(2,20 -bipyridine)], 1. It includes the identification of several minor products and, in particular, a study of product analysis with the isotopically labeled CD3CD2I, CD3CH2I, and CH3CD2I and of the secondary deuterium kinetic isotope effect on the rates of reaction. The reactions are expected to occur primarily by the mechanism of Scheme 1, and there is the potential for the ethyl groups to undergo reversible β-elimination or to form β-agostic CH 3 3 3 Pt interactions within the cationic 16-electron platinum(IV) intermediate E (Scheme 1, R = Et). Ethylplatinum(IV) complexes often decompose by β-elimination,14 but there are few precedents for either R- or β-agostic interactions in alkylplatinum(IV) complexes,15 though they are well known for platinum(II).16 It is already known that reactions involving β-elimination or in which coordinatively unsaturated platinum(II) complex intermediates are stabilized by β-agostic interactions can lead to the observation of large deuterium kinetic isotope effects.17,18 However, there seem to be no previous studies of kinetic isotope effects for oxidative addition of ethyl iodide, in which any β-agostic interactions would involve platinum(IV) intermediates (Scheme 1). There are many studies of secondary deuterium kinetic isotope effects on the rates of SN2 reactions with classical nucleophiles, and (14) (a) Brown, M. P.; Puddephatt, R. J.; Upton, C. E. E.; Lavington, S. W. J. Chem. Soc., Dalton Trans. 1974, 1613. (b) Brown, M. P.; Hollings, A.; Houston, K. J.; Puddephatt, R. J.; Rashidi, M. J. Chem. Soc., Dalton Trans. 1976, 786. (c) Zamashchikov, W.; Mitchenko, S. A.; Shubin, A. A. Kinet. Catal. 1992, 33, 400. (15) (a) Baber, A.; Fan, C.; Norman, D. W.; Orpen, A. G.; Pringle, P. G.; Wingad, R. L. Organometallics 2008, 27, 5906. (b) Puddephatt, R. J. Angew. Chem., Int. Ed. 2002, 41, 261. (c) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2001, 123, 6423. (d) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L. J. Am. Chem. Soc. 2001, 123, 6425. (e) Hill, G. S.; Puddephatt, R. J. Organometallics 1998, 17, 1478. (f) Prokopchuk, E. M.; Jenkins, H. A.; Puddephatt, R. J. Organometallics 1999, 18, 2861. (16) Carr, N.; Mole, L.; Orpen, A. G.; Spencer, J. L. J. Chem. Soc., Dalton Trans. 1992, 2653. (17) (a) Lloyd-Jones, G. C.; Slatford, P. A. J. Am. Chem. Soc. 2004, 126, 2690. (b) Takacs, J. M.; Lawson, E. C.; Clement, F. J. Am. Chem. Soc. 1997, 119, 5956. (18) Romeo, R.; D’Amico, G.; Sicilia, E.; Russo, N.; Rizzato, S. J. Am. Chem. Soc. 2007, 129, 5744.

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Chart 1. Isotopically Labeled Products

small values of the secondary deuterium KIE (both R- and β-effects), which are considered typical for the SN2 reaction, have been observed.10,19 Larger values are observed with the platinum(II) nucleophile studied here.

Results Oxidative Addition and Product Analysis. The reaction of complex 1 with ethyl iodide, followed by recrystallization, is known to give complex 2a (Chart 1), the product of trans oxidative addition.3b This has been confirmed in the present work, and the structure of complex 2a has been determined crystallographically (Figure 1). The complex crystallizes solvent-free from dichloromethane solution (Figure 1a), but with two independent molecules in the unit cell, and it crystallizes as the solvate 2a 3 0.5C6H6 from benzene solution (Figure 1b). In each case, the ethyl group is oriented above one of the pyridyl rings, and there is disorder of the ethyl group between two positions in the benzene solvate. The angle C-C-Pt is greater than the tetrahedral angle, probably as a result of steric effects (Figure 1), but otherwise there are no unusual features.3-5 The NMR parameters for complex 2a have been reported earlier,3b but it was important in the context of the present work to establish if reversible β-elimination can occur in the proposed five-coordinate 16-electron platinum(IV) intermediates (Scheme 1, R = Et). Therefore, the NMR spectra of the complexes 2a* and 2a**, prepared from isotopically labeled CH3CD2I and CD3CH2I, respectively, were also recorded. Figure 2 shows the methyl resonance of the ethylplatinum group in complexes 2a and 2a* (Chart 1). The resonance for 2a (Figure 2A) appears as a triplet due to coupling with the CH2 protons [3J(HH) = 8 Hz], with satellites arising (19) (a) Leffek, K. T.; Llewellyn, J. A.; Robertson, R. E. Can. J. Chem. 1960, 38, 1505. (b) Leffek, K. T.; Llewellyn, J. A.; Robertson, R. E. Can. J. Chem. 1960, 38, 2171. (c) Westaway, K. C.; Fang, Y.-R.; MacMillar, S.; Matsson, O.; Poirier, R. A.; Islam, S. M. J. Phys. Chem. A 2008, 112, 10264. (d) Garver, J. M.; Fang, Y.-R.; Eyet, N.; Villano, S. M.; Bierbaum, V. M.; Westaway, K. C. J. Am. Chem. Soc. 2010, 132, 3808.

Figure 1. Structures of complex 2a in (a) one of two independent molecules of 2a and (b) 2a 3 1/2C6H6. Selected bond parameters (A˚, deg): 2a, Pt(1)-C(11) 2.047(3); Pt(1)-C(12) 2.051(3); Pt(1)-C(13) 2.079(3); Pt(1)-N(1) 2.151(2); Pt(1)-N(2) 2.153(3); Pt(1)-I(1) 2.8101(3); C(13)-C(14) 1.469(5); C(14)C(13)-Pt(1) 116.6(3); 2a 3 0.5C6H6, Pt(1)-C(11) 2.058(6); Pt(1)C(12) 2.056(5); Pt(1)-C(13) 2.112(10); Pt(1)-N(1) 2.159(4); Pt(1)-N(2) 2.135(4); Pt(1)-I(1) 2.7831(5).

Figure 2. 1H NMR spectra in CD2Cl2 of (A) trans-[PtIMe2Et(bipy)], 2a, and (B) trans-[PtIMe2(CD2CH3)(bipy)], 2a*, in the region of the CH3 group of the ethyl ligand.

from the coupling 3J(PtH) = 66 Hz, but the corresponding resonance for 2a* (Figure 2B) appears as a broad singlet, clearly showing the presence of a CH3CD2Pt group, with no scrambling of H/D labels. The PtCH2 resonance for 2a [δ = 1.48, q, 3J(HH) = 8 Hz, 2J(PtH) = 70 Hz] was absent in 2a*. The presence of the CD3CH2Pt group in 2a**, with no H/D scrambing, was established in an analogous way. Confirmation of the absence of H/D scrambling was obtained from the 13C{1H} NMR spectra of complexes 2a, 2a*, and 2a**, shown in Figure 3A, B, and C, respectively. The methylplatinum resonance [δ = 0.0, 1J(PtC) = 698 Hz] was observed in all cases, but the CH3C resonance [δ = 19.8, 2 J(PtC) = 37 Hz] was not observed for 2a** and the CH2Pt resonance [δ = 25.5, 1J(PtC) = 661 Hz] was not observed for

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Figure 4. Structure of complex 3. Selected bond distances (A˚): Pt(1)-C(1) 2.024(10); Pt(1)-C(7) 2.054(7); Pt(1)-N(1) 2.153(6); Pt(1)-I(1) 2.7731(8). Symmetry for equivalent atoms: x, -y, z. Scheme 3. Oxidative Addition of EtI by the SN2 Mechanism, with Alkyl Exchange to Give 3 and 4a

Figure 3. 13C NMR spectra (CD2Cl2 solution) in the region of the methyl- and ethylplatinum groups of (A) trans-[PtIMe2Et(bipy)], 2a; (B) trans-[PtIMe2(CD2CH3)(bipy)], 2a*; and (C) trans[PtIMe2(CH2CD3)(bipy)], 2a**.

2a* because the intensities were too low. The oxidative addition reactions were carried out in both acetone and benzene solution, and no H/D scrambling was observed in either case over a period of several days in solution. The NMR spectra were recorded for the total product, isolated by evaporation of the solvent after reaction of complex 1 with ethyl iodide in acetone or benzene solution, and several minor products were identified. From the acetone solution, the complexes cis-[PtIMe2Et(bipy)], 2b (Chart 1), and [PtIMe3(bipy)], 3, were identified by their NMR spectra. Complex 3 is a known compound, and an authentic sample was prepared to confirm the assignment.6a,7,20 Its structure has not been reported previously, so it was determined (Figure 4) in order to compare to the structure of 2a. The molecule of 3 is located on a crystallographic mirror plane (Figure 4). The Pt-I distance is slightly longer in 2a than in 3 [2.8101(3) vs 2.7731(8) A˚] and the axial Pt-C distance is slightly longer for the ethylplatinum complex 2a compared to the methylplatinum complex 3 [2.079(3) vs 2.024(10) A˚]. The bond parameters for the equatorial PtMe2(bipy) groups are very similar in 2a and 3 (Figures 1 and 4). The 1H NMR spectrum of 2b in CD2Cl2 solution contained two equal-intensity methylplatinum resonances [δ = 0.66, 2J(PtH) = 71 Hz, MePt trans to I; δ = 1.55, 2J(PtH) = 72 Hz, MePt trans to N] and a triplet for the CH3 protons of the ethyl group [δ = 1.23, 3J(HH) = 8 Hz, 3J(PtH) = 53 Hz]. Due to the low symmetry of the molecule 2b, the PtCH2 protons are diastereotopic and appeared as two multiplets [δ = 2.11, 2J(HH) = 11 Hz, 3J(HH) = 8 Hz, 2J(PtH) = 69 Hz; δ = 2.48, 2J(HH) = 11 Hz, 3J(HH) = 8 Hz, 2J(PtH) = 92 Hz]. The assignments were confirmed by comparison with the spectra of 2b* and 2b** (Chart 1). Thus, the 1H NMR spectrum of 2b* contained no PtCH2 resonances, and the 1H NMR (20) Clegg, D. E.; Hall, J. R.; Swile, G. A. J. Organomet. Chem. 1972, 38, 403. According to the stoichiometry of Scheme 3, the total yield of 4a þ 4b should equal the yield of 3 but was lower in the dark benzene reaction. We considered the possibility that some 3 might be formed from trace impurity of methyl iodide in the ethyl iodide, but none could be detected. Probably some of the intermediate [PtMeEt(bipy)] decomposes before oxidative addition to give 4a/4b.

a

N-N = 2,20 -bipyridine.

spectrum of 2b** contained no CH3C resonance. The presence of the complex 2b was expected (Scheme 1, R = Et), but complex 3 was not expected. Two other complexes were detected in very low abundance and were tentatively identified as 4a and 4b by their NMR spectra. Only the triplet resonance for the CH3C protons of the axial ethylplatinum groups [δ = 0.17, 3J(HH) = 7 Hz, 3J(PtH) = 63 Hz] and the methylplatinum resonance [δ = 1.25, 2J(PtH) = 72 Hz, MePt trans to N] were resolved for complex 4a, and only the axial methylplatinum resonance for 4b [δ = 0.43, 2J(PtH) = 75 Hz, MePt trans to I], with other peaks obscured by those of more abundant products. A likely mechanism for the formation of both major and minor products is shown in Scheme 3. The unexpected product 3 is probably formed by intermolecular transfer of a methyl group between the cationic intermediate and the dimethylplatinum(II) complex 1, and the initial coproduct [PtMeEt(bipy)] is expected to react with ethyl iodide to give 4a (Scheme 3) and, with an extra isomerization step, its isomer 4b. Methyl transfer reactions between platinum centers have been observed previously,21 but the six-coordinate platinum(IV) complexes are inert to alkyl group (21) (a) Aye, K.-T.; Canty, A. J.; Crespo, M.; Puddephatt, R. J.; Scott, J. D.; Watson, A. A. Organometallics 1989, 8, 1518. (b) Scott, J. D.; Puddephatt, R. J. Organometallics 1986, 5, 2522. (c) Puddephatt, R. J.; Thompson, P. J. J. Chem. Soc., Dalton Trans. 1975, 1810. (d) Puddephatt, R. J.; Thompson, P. J. J. Organomet. Chem. 1979, 166, 251.

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Figure 5. Plots of first-order rate constants (kobs/s-1) for the reaction of [PtMe2bipy], 1, with EtI in benzene at different temperatures vs the concentration of EtI.

transfer, as shown by the observation that no reaction occurred between complex 1 and an equilibrium mixture of complexes 2a and 2b, as monitored by NMR spectroscopy over a period of several days. No significant differences in product ratios were observed for the reactions in acetone carried out either in the dark or in diffuse light. The similar reaction of complex 1 with ethyl iodide in benzene solution in the dark occurred more slowly but gave a similar mixture of products, with the ratio 2a:2b:3 = 13:1:3, also with evidence for formation of 4a. However, if the reaction was carried out in diffuse laboratory light, the NMR spectrum of the product mixture contained additional resonances assigned to trans-[PtI(OOEt)Me2(bipy)], 5, and trans-[PtI2Me 2(bipy)], 6. The ratio of these products was 2a:2b:3:5:6 = 10:1:2.5:1:1. Complex 6 was prepared independently by reaction of complex 1 with iodine and so was positively identified. Complex 5 was characterized by its 1 H NMR spectrum, in particular by observation of peaks characteristic of an ethoxy group [δ(1H) = 0.53, t, 3 J(HH) = 7 Hz, CH3; δ = 3.18, q, 3J(HH) = 7 Hz, CH2], with the expected variations in the labeled complexes 5* and 5** (Chart 1). If the reaction in benzene was carried out with exposure to sunlight, it occurred more rapidly and 6 and 5 were then the major products. A new complex, trans-[PtI2MeEt(bipy)], 7, was tentatively identified by its downfield methylplatinum resonance [δ = 2.07, 2J(PtH) = 73 Hz, MePt trans to N]. It is the iodine adduct of the proposed intermediate platinum(II) complex [PtMeEt(bipy)]. The formation of 5-7 probably occurs in an analogous way as proposed for oxidative addition with isopropyl iodide (Scheme 2 with R = Et for 5 and 6) and is indicative of a free-radical component to the mechanism of reaction when carried out in the presence of light.11,13 Kinetics and Secondary Deuterium Kinetic Isotope Effects. The kinetics of the oxidative addition reactions of ethyl iodide and isotopically substituted derivatives with the red complex 1 in both benzene and acetone solutions were studied using UV-visible spectroscopy. In each case, excess ethyl iodide was used and the disappearance of the MLCT band for the complex 1 (λmax = 474 nm in acetone; 508 nm in benzene)6d was used to monitor the reaction. For minimization of errors in determining the KIE values, the reaction rates of ethyl


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Figure 6. Eyring plots for the reaction of [PtMe2bipy], 1, with (a) EtI in acetone; (b) C2D5I in acetone; (c) EtI in benzene; and (d) C2D5I in benzene.

iodide and its deuterium-substituted derivatives were measured in parallel using the same stock solution of the platinum(II) complex 1. Initially, the reactions with C2H5I and C2D5I were studied over a range of temperatures. The reactions followed good first-order kinetics, and the pseudo-firstorder rate constants (kobs) were evaluated by nonlinear least-squares fitting of the absorbance-time profiles to the first-order equation. Although there was evidence for a minor free-radical component to the reaction mechanism when carried out in benzene solution (see above), the rates were not retarded by radical scavengers such as DPPH or galvinoxyl, and there was no induction period, so any freeradical chain component is minor under the conditions of the kinetic studies. The second-order rate constants were then obtained from the first-order relationship of the values of kobs with the concentration of ethyl iodide (Figure 5). The activation parameters and associated error limits were obtained from the Eyring equation (Figure 6),22 and the resulting data are collected in Table 1. The rates of the reactions in benzene at different temperatures were slower than the rates of similar reactions in acetone by a factor of about 4, and the values of the entropy of activation were large and negative. These are strong indicators of the SN2 mechanism of oxidative addition.2-7 In particular, the increase of rate in more polar solvents indicates that a polar transition state is involved, while the large, negative values of ΔSq are similar to those observed in the Menschutkin reaction and arise from greater solvent ordering in a polar transition state.1-10 For comparison, the reaction of 1 with MeI and EtI in acetone solution gives activation parameters ΔHq = 22.3 and 44.6 kJ mol-1 and ΔSq = -138 and -117 J K-1 mol-1, respectively.5b The higher activation enthalpy for ethyl iodide is expected as a result of the increased steric hindrance in the transition state, as well as from the electronic effects of the methyl substituent, while the lower value of the entropy of activation can be interpreted in terms of a somewhat less polar transition state compared to the reaction with methyl iodide. (22) Morse, P. M.; Spencer, M. D.; Wilson, S. R.; Girolami, G. S. Organometallics 1994, 13, 1646.

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Table 1. Second-Order Rate Constants and Activation Parameters for the Reaction of [PtMe2(bipy)], 1, with C2H5I and C2D5I in Acetone or Benzene Solution solvent

reagent

T (°C)

102k2 (L mol-1 s-1)

(kH/kD)

ΔHq (kJ mol-1)

ΔSq (J K-1 mol-1)

acetone

C2H5I

10 15 20 25 30 35 10 15 20 25 30 35 10 15 20 25 30 35 10 15 20 25 30 35

2.81 ( 0.08 3.87 ( 0.14 5.26 ( 0.03 7.55 ( 0.17 10.40 ( 0.19 14.00 ( 0.11 1.63 ( 0.04 2.35 ( 0.02 3.41 ( 0.03 5.27 ( 0.05 7.50 ( 0.09 10.60 ( 0.09 0.59 ( 0.01 0.92 ( 0.02 1.28 ( 0.02 1.83 ( 0.03 2.39 ( 0.04 3.48 ( 0.02 0.31 ( 0.02 0.51 ( 0.02 0.74 ( 0.01 1.16 ( 0.02 1.66 ( 0.01 2.41 ( 0.01

1.72 ( 0.06 1.65 ( 0.06 1.54 ( 0.02 1.43 ( 0.03 1.39 ( 0.03 1.32 ( 0.02

44.6 ( 3.0

-117 ( 3

52.6 ( 3.1

-93 ( 3

1.90 ( 0.13 1.80 ( 0.08 1.73 ( 0.04 1.58 ( 0.04 1.44 ( 0.03 1.44 ( 0.01

47.8 ( 1.3

-118 ( 4

56.6 ( 3.8

-92 ( 3

C2D5I

benzene

C2H5I

C2D5I

The surprising feature in the data of Table 1 is that large values of the secondary isotope effect kH/kD are observed, ranging from 1.32 to 1.72 in acetone and from 1.44 to 1.90 in benzene solution. The large values arise as a result of a higher enthalpy of activation for the reactions of 1 with C2D5I compared to C2H5I, and the values of kH/kD are therefore temperature dependent, with higher values of kH/kD at lower temperatures (Table 1). These differences in activation parameters are statistically significant and give values of ΔΔHq = ΔHq(C2H5I) - ΔHq(C2D5I) = -8.0 ( 4.3 or -8.8 ( 4.0 kJ mol-1 and ΔΔSq = ΔSq(C2H5I) - ΔSq(C2D5I) = -24 ( 5 or -26 ( 4 J K-1 mol-1 in acetone or benzene, respectively, indicating an earlier, less polar transition state in the reaction with C2D5I. On the basis of the activation parameters, the value of kH/kD = 1 is expected to be observed at about 60 °C in acetone and 65 °C in benzene solution. The values of kH/kD in Table 1 arise from a combination of R- and β- secondary deuterium KIEs on the rates of reaction. Because it has previously been shown that the R-effect is small and either positive or inverse in oxidative addition of CH3I/CD3I to complex 1, giving values of kH/kD at 25 °C of 1.02(2) in acetone and 0.93(4) in benzene,5b it seemed likely at this stage that a large β-effect was responsible. Nevertheless, the high values of kH/kD in Table 1 are surprising for a classical SN2 mechanism of reaction.10,19,23 For example, values of kH/kD of 0.96 and 1.03 have been reported for water solvolysis at 80 °C of C2H5I/CH3CD2I and C2H5I/CD3CH2I, respectively, showing that both the R- and β-secondary deuterium KIEs are small and can be either inverse or positive.19a,b These are typical small values of both the R- and β-secondary deuterium KIEs on the rates of most known SN2 reactions.10,19,23 Larger KIEs are usually observed for SN1 mechanisms, as illustrated by values of kH/kD of 1.05 and 1.31 for water solvolysis at 60 °C of Me2CHI/Me2CDI and (CH3)2CHI/(CD3)2CHI, respectively.19a,b Clearly, the oxidative addition reactions of ethyl iodide studied in this work cannot occur by an SN1 mechanism, so either an explanation (23) Smith, M. B.; March, J. March’s Advanced Organic Chemistry, 5th ed.; Wiley: New York, 2001.

Table 2. Summary of Secondary Deuterium Kinetic Isotope Effects for the Reaction of Complex [PtMe2(bipy)], 1, with Ethyl Iodide in Acetone or Benzene solvent

kH/kD kH/kD kH/kD kH/kD reagent 102k2 L mol-1 s-1 kinetica per Da,c NMRb per Db,c

acetone C2H5I C2D5I acetone C2H5I CH3CD2I CD3CH2I benzene C2H5I C2D5I CH3CD2I CD3CH2I

7.55 ( 0.17 5.27 ( 0.05 7.01 ( 0.09 5.63 ( 0.05 5.78 ( 0.09 1.83 ( 0.03 1.16 ( 0.02 1.43 ( 0.07 1.47 ( 0.06

1.43

1.07

1.4d

1.07

1.24 1.21

1.11 1.07

1.23 1.18

1.11 1.06

1.58 1.28 1.24

1.09 1.13 1.07

1.5d 1.24 1.19

1.08 1.11 1.06

a KIE from kinetics at 25 °C. b KIE from NMR product analysis at 22 °C; note that the error limits will be higher for reagent C2D5I. c KIE per deuterium atom = (KIE)1/n, where n = number of D atoms; note that for C2D5I this is an average for the two R- and three β-deuterium atoms. d Error estimated as 10%; see text.

of the unusually high values of the KIEs for the SN2 mechanism or an alternative mechanism that is consistent with the large KIE values is needed. Next, experiments were carried out to determine the relative importance of the R- and β-secondary deuterium KIEs by using the reagents CH3CD2I and CD3CH2I, respectively. Results are listed in Table 2. The KIE values were determined by kinetic measurements in each case at 25 °C in acetone or benzene, as for the values in Table 1, and independently by using a competition experiment protocol in which complex 1 was allowed to react with an excess of a 1:1 mixture of ethyl iodide and CH3CD2I, CD3CH2I, or CD3CD2I, then measuring the ratio of products 2a:2a*, 2a:2a**, or 2a:2a***, respectively, by integration of the 1H NMR spectra. The unexpected result is that the overall contributions from the R- and β-secondary deuterium KIEs are about equal and that, on a per-deuterium basis, the secondary R-deuterium KIE is greater than the secondary β-deuterium KIE (Table 2). The values of the secondary deuterium KIEs are similar in the two solvents, with slightly higher values in benzene solution (Table 2).

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Scheme 4. Possible Second-Order Reaction Mechanisms: (i) Free-Radical Nonchain; (ii) Concerted

Discussion The oxidative addition of alkyl halides to transition metal complexes is an important reaction, so it is to be expected that the factors affecting reactivity and mechanism have been studied intensively.1,2 It is notable that the study of secondary deuterium kinetic isotope effects as a mechanistic probe has previously been applied only in the oxidative addition of methyl iodide and has supported the SN2 mechanism of reaction.9 This work has revealed unusually high values of the secondary deuterium isotope effect in the oxidative addition of ethyl iodide to complex 1, which were unexpected for an SN2 mechanism depicted in Schemes 1 and 3 and which contrasted with the small values previously observed for oxidative addition of methyl iodide.5,9 Simply on the basis of precedents from physical-organic chemistry, the high values of the secondary KIEs (Table 2) could be considered as evidence for a concerted or free-radical mechanism of reaction.9,23,24 The kinetics rule out a free-radical chain mechanism as a major pathway, at least when the reactions are carried out in the absence of light.13 However, it should be recognized that product analysis does support a major radical component when the solvent is benzene and the reaction is photochemically initiated. The activation parameters and solvent effect on the rate provide support for the SN2 mechanism, rather than a less polar radical nonchain or concerted mechanism (Scheme 4).1 The mainly trans stereochemistry of the oxidative addition to give 2a is consistent with a concerted mechanism if isomerization of initially formed cis oxidative addition product 2b to 2a occurs more rapidly than the oxidative addition. One important factor that could affect the mechanism is the large steric bulk of the square-planar platinum(II) nucleophile. Thus, with most nucleophiles, the relative rates for substitution of MeI:EtI are about 30:1,25 but for oxidative addition at platinum(II) the relative rates are ca. 1000:1, and this difference has been attributed to steric effects in the transition state.11 Because this steric effect disfavors the SN2 mechanism, the other mechanisms shown in Scheme 4 may become competitive. Thus, an analysis of the overall evidence on the mechanism was required. Consider first that the expected SN2 mechanism, which is certainly consistent with the solvent effect on rate and with the activation parameters, is correct. One possible explanation of a large β-deuterium KIE is that there is a β-hydrogen interaction with platinum in the transition state.17,18 To gain some insight into this effect, DFT calculations were carried (24) (a) Takada, T.; Koizumi, H.; Ichikawa, T. J. Phys. Chem. 2000, 104, 703. (b) Griller, D.; Ingold, K. U. J. Am. Chem. Soc. 1975, 97, 1813. (25) Streitweiser, A. Jr. Solvolytic Displacement Reactions; McGraw-Hill: New York, 1962. (26) (a) Te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; van Gisbergen, S.; Guerra, C. F.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001, 22, 931. (b) A. Becke, A. Phys. Rev. A 1988, 38, 3098.

Figure 7. Calculated structures and relative energies of starting materials and products of oxidative addition. Only the H atoms of the ethyl group are shown for 2a and 2b. Scheme 5. Possible Cationic Intermediates

out.26 The method used was tested by carrying out calculations on the isomeric products 2a and 2b and potential intermediates (Figure 7). Complex 2a was calculated to be 7.5 kJ mol-1 more stable than 2b, which, allowing for statistical factors, would give an equilibrium constant for 2a:2b of ca. 10, compared to the observed value of 11.5 and indicating good agreement. The agreement between calculated and observed distances is not as good. For example, for the ethylplatinum group in 2a calculated (found) parameters are Pt-C = 2.18 (2.08); C-C = 1.54 (1.47) A˚; Pt-C-C = 116 (117)o. Possible intermediates [PtMe2Et(bipy)]þ and [PtHMe2(C2H4)(bipy)]þ are shown in Scheme 6. The 16-electron cations [PtMe2Et(bipy)]þ, N and O, could become less coordinatively unsaturated by forming a β-agostic interaction in P or Q or achieve an 18-electron configuration in the ethylene hydride complexes [PtHMe2(C2H4)(bipy)]þ, R or S. The coordinatively unsaturated cations O and N were calculated to have essentially equal energy (O more stable by only 0.5 kJ mol-1), and they were only slightly more stable than the β-agostic complex Q (Scheme 5, Figure 8). The distance Pt 3 3 3 H is 2.80 A˚ in O but 2.35 A˚ in Q, and the angle Pt-C-C is 104o in O but 95o in Q. The β-agostic interaction is clearly weak for platinum(IV) in Q, and further distortion to the hydrido(ethylene) complex S is unfavorable, at least in part because the stereochemistry of S, with hydride trans to methyl, is unfavorable.27 Complex N cannot form a β-agostic interaction unless a methyl group migrates to be trans to ethyl (27) Platinum(IV) compounds with three strong σ-donor groups (such as alkyl or hydride) always adopt the fac rather than mer stereochemistry, because the mer configuration requires two strong σ-donor groups to be mutually trans. When two strong σ-donor groups are forced to be mutually trans, such as in tetramethylplatinum(IV) complexes, one of these groups is easily cleaved. (a) Shahsavari, H. R.; Rashidi, M.; Nabavizadeh, S. M.; Habibzadeh, S.; Heinemann, F. W. Eur. J. Inorg. Chem. 2009, 3814. (b) Wik, B. J.; Tilset, M. J. Organomet. Chem. 2007, 692, 3223. (c) Crumpton-Bregel, D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2003, 125, 9442. (d) Hill, G. S.; Yap, G. P. A.; Puddephatt, R. J. Organometallics 1999, 18, 1408.

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Nabavizadeh et al. Scheme 6. Possible Resonance Forms Involving β- or r-Hyperconjugation, for N* and N**

Chart 2. Possible Transition States for Oxidative Addition

Figure 8. Possible intermediates [PtMe2Et(bipy)]þ and [PtHMe2(C 2H4)(bipy)]þ. Only the H atoms from the ethyl group are shown, for clarity.

in P, but this stereochemistry has two strong trans-influence ligands in mutually trans positions, which is unfavorable.27 If P could form, the calculations indicate that it would spontaneously β-eliminate to form R, which is only 13 kJ mol-1 higher in energy than N. However, because there is no easy pathway from N to R (Scheme 5, Figure 8), the β-elimination cannot occur easily. The platinum-ethylene bonding in the platinum(IV) cation R is evidently weak (Pt-C = 2.57, 2.54 A˚; C-C 1.37 A˚), which accounts for it being at higher energy than N, even though it has a more favorable 18-electron configuration. Together, these calculations explain why no H-D scrambling occurs within the cations [PtMe2Et(bipy)]þ, which might be formed during the oxidative addition of ethyl iodide by an SN2 mechanism or afterward by iodide dissociation from 2a or 2b,28 and they also indicate that any β-agostic interaction in the transition state for oxidative addition by the SN2 mechanism is very improbable. If the β-agostic interaction is not responsible for the high value of the β-deuterium KIE, then other factors must be considered. The stabilization of incipient radicals or cations by hyperconjugation has been used to explain the significant positive β-deuterium KIEs in processes involving radical or carbonium ion formation in the transition state, but the effect is generally small in reactions occurring by the SN2 mechanism.10,19,23,24,29 It could be argued that the hyperconjugation might be enhanced for the platinum(II) nucleophile because it would lead to some alkene-platinum character in the incipient cation (N* in Scheme 6). The calculations do predict a slight shortening of the C-C distance (1.54 and 1.53 A˚ in 2a and N) and lengthening of the β-anti C-H distance (1.107 and 1.110 A˚ in 2a and N), but the effect appears to be modest even in the fully cationic intermediate. The R-deuterium KIE usually arises through changes in the out-of-plane CH bending vibration between the ground and transition states.10,19,23 With few exceptions, 10 this R-deuterium KIE is close to unity for SN2 reactions, including reactions of ethyl iodide with several nucleophiles.10,19,23 The small positive or inverse values for SN2 reactions are usually discussed in terms of steric crowding at the carbon atom, with tight transition states (short distances between entering and leaving groups) often giving small positive values (28) Goldberg, K. I.; Yan, J. Y.; Breitung, E. M. J. Am. Chem. Soc. 1995, 117, 6889. (29) (a) Angelis, Y. S.; Hatzakis, N. S.; Smonou, I.; Orfanopoulos, M. Tetrahedron Lett. 2001, 42, 3753. (b) Koenig, T.; Wolf, R. J. Am. Chem. Soc. 1967, 89, 2948. (c) Seltzer, S; Hamilton, E. J., Jr. J. Am. Chem. Soc. 1966, 88, 3775.

and loose transition states (long distances between entering and leaving groups) giving small inverse values of the KIE.10,23 Of the exceptional cases in which SN2 reactions exhibit larger positive values of the R-deuterium KIE, the case in which the transition state contains a highly nonlinear arrangement of the entering and leaving groups and the reacting carbon center in the transition state appears most relevant to the present reaction.10 The very bulky square-planar platinum nucleophile will be forced to approach from a direction remote from the methyl substituent of the ethyl group, and so a nonlinear Pt 3 3 3 C 3 3 3 I unit will be present in the transition state.10 There is a prediction from theory that the SN2 mechanism involving frontside, instead of the usual backside, attack should give high values of the R-deuterium KIE, but there are no well-established examples of this mechanism, and it would lead to steric hindrance between the platinum complex nucleophile and the iodine substituent.9 We note that R-hyperconjugation could stabilize the cationic intermediate through carbene character, but the calculations do not indicate that this is significant in N** (Scheme 6). Dissociative mechanisms, as in free-radical or SN1 mechanisms, in which the carbon center becomes three-coordinate (sp2) in the reaction intermediate can give high positive values of the secondary R-deuterium KIE.10,23,29 The values observed in this work fall into the known ranges for freeradical or SN1 mechanisms and are much higher than those observed for oxidative addition of methyl iodide.5,9 Because ethyl iodide does not undergo SN1 reactions under the mild conditions used in this work,19 the observation of high values of the R-deuterium KIEs could be taken as evidence for a free-radical mechanism of oxidative addition (Scheme 3).

Summary and Conclusions Of the possible mechanisms of oxidative addition, the observation of good second-order kinetics rules out the SN1 and free-radical chain mechanisms. The likely transition states for the remaining mechanisms are shown in Chart 2. The SN2 mechanisms with back- or frontside attack would give transition states T and U, respectively. These appear most consistent with the observed kinetic activation parameters and with the solvent effect on the rate, both of which indicate a polar transition state. The concerted mechanism and freeradical nonchain mechanism would give less polar transition states V and W, respectively. We note that transition states between U and V or between V and W could also be possible.

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Any free-radical mechanism through transition state W would need to occur in large part within a caged radical pair, to be consistent with the second-order kinetics, the lack of inhibition by radical scavengers, and the absence of products typical of free-radical mechanisms under the conditions of the kinetic studies. The high values of both the R- and β-deuterium secondary KIE are most easily interpreted in terms of a transition state with some ethyl radical character,10,23,24 since the present work indicates that potential factors such as agostic interactions do not play an important role. If the expected transition state T is correct, the results could be rationalized in terms of a bent Pt 3 3 3 C 3 3 3 I geometry (Chart 2), with only weak overlap of the carbon 2p orbital with the much larger platinum and iodine σ orbitals, or having some character of an electron transfer intermediate [PtMe2(bipy)]•þ[EtI]•-. This is perhaps the most likely scenario, but the KIE results, considered alone, are more naturally accommodated if the transition state lies somewhere on the U, V, W continuum. We suggest that further insights into alkyl halide oxidative additions are still possible and that further experimental and theoretical studies on the secondary deuterium KIEs on the rates of these reactions are needed.

Experimental Section The alkyl halides, CH3CH2I (99%), CD3CH2I, and CH3CD2I (98 atom % D), and CD3CD2I (99.5 atom % D) were purchased from Aldrich Chemical. The 1H and 13C NMR spectra were recorded by using a Varian Mercury 400 spectrometer (in CD2Cl2), with TMS as reference. Kinetic studies were carried out by using either a Perkin-Elmer Lambda 25 or a Cary 300 spectrophotometer. Temperature was controlled by a EYELA NCB3100 low-temperature thermo regulation bath and checked by using a second thermocouple. DFT calculations were carried out by using the Amsterdam density functional program based on the Becke-Perdew functional, with first-order scalar relativistic corrections.26 The starting complex [PtMe2(bipy)], 1, was prepared as a pure red crystalline complex by reaction of [Pt2Me4(μ-SMe2)2] with 2,20 -bipyridine, bipy, in diethyl ether, and authentic samples of [PtIMe3(bipy)], 3, and [PtI2Me2(bipy)], 6, were prepared by reaction of complex 1 with MeI and I2, respectively.6,30 The NMR spectra were identical with those reported below. The 1H and 13C NMR spectra of the products were recorded in benzene-d6, acetone-d6, and CD2Cl2 solvents, in order to resolve partially overlapping peaks in some spectra, but only data for CD2Cl2 are reported. For complexes prepared using deuterium-substituted EtI, only the changes observed in the corresponding NMR data compared to the complex prepared from C2H5I are given. Reaction of [PtMe2(bipy)], 1, with EtI in Acetone. To a solution of [PtMe2(bipy)], 1 (40 mg), in acetone (10 mL) was added an excess amount of EtI (0.5 mL). The mixture was stirred for 1 h, and then the solvent was removed to give an off-white solid, which was dried under vacuum. The solid was identified using 1 H and 13C NMR spectroscopy as a mixture of trans-[PtMe2EtI(bipy)], 2a, cis-[PtMe2EtI(bipy)], 2b, and [PtMe3I(bipy)], 3, as well as small amounts of a complex tentatively assigned as [PtMeEt2I(bipy)], 4. Product ratio for 2a:2b:3 is 11.5:1:2. trans-[PtIMe2Et(bipy)], 2a: δ(1H) = 0.10 [t, 3H, 3JHH = 8 Hz, 3 JPtH = 66 Hz, MeCH2]; 1.48 [q, 2H, 3JHH = 8 Hz, 2JPtH = 70 Hz, PtCH2]; 1.50 [s, 6H, 2JPtH = 71 Hz, PtMe]; δ(13C) = 0.0 [s, 1J(PtC) = 698 Hz, PtMe]; 19.8 [s, 2J(PtC) = 37 Hz, CH3CH2]; 25.5 [s, 1J(PtC) = 661 Hz, PtCH2]; 128.8 [J(PtC) = 8 Hz], 132.0 [J(PtC) = 15 Hz], 144.1, 152.5 [J(PtC) = 15 Hz], (30) (a) Scott, J. D.; Puddephatt, R. J. Organometallics 1983, 2, 1643. (b) Monaghan, P. K.; Puddephatt, R. J. Organometallics 1984, 3, 444.

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160.2 [bipy]. trans-[PtIMe2(CD2CH3)(bipy)], 2a*: δ(1H) = 0.08 [br s, 3H, 3JPtH = 68 Hz, MeCD2]; no signal in the 1H NMR for the PtCH2 group, and the multiplet for PtCD2 in the 13C NMR was too weak to resolve. trans-[PtMe2(CH2CD3)I(bipy)], 2a**: δ(1H) = 1.45 [br s, 2H, 2JPtH = 70 Hz, PtCH2]; no signal in the 1 H NMR for the MeCH2 group, and the multiplet for the CD3CH2 carbon in the 13C NMR was too weak to resolve. trans[PtMe2(CD2CD3)I(bipy)], 2a***: no signals for the ethyl group. cis-[PtIMe2Et(bipy)], 2b: δ(1H) = 0.66 [s, 3H, 2JPtH = 71 Hz, MePt trans to I]; 1.23 [t, 3H, 3JHH = 7 Hz, 3JPtH = 53 Hz, MeCH2]; 1.55 [s, 3H, 2JPtH = 72 Hz, MePt trans to N]; 2.11 [dd, 1H, 2JHH = 11 Hz, 3JHH = 8 Hz, 2JPtH = 69 Hz, PtCHaHb]; 2.48 [dd, 2JHH = 11 Hz, 3JHH = 8 Hz, 2JPtH = 92 Hz, PtCHaHb]. cis-[PtIMe2(CD2CH3)(bipy)], 2b*: δ(1H) = 1.22 [br s., 3H, 3JPtH = 53 Hz, MeCD2]; no signal for PtCH2. cis-[PtIMe2(CH2CD3)I(bipy)], 2b**: δ(1H) = 2.08 [br d, 1H, 2 JHH ≈ 10 Hz, 2JPtH not resolved, PtCHaHb]; 2.46 [d, 1H, 2JHH ≈ 10 Hz, 2JPtH not resolved, PtCHaHb]; no signal for MeCH2. cis-[PtIMe2(CD2CD3)(bipy)], 2b***: no signals for the ethyl group. [PtIMe3(bipy)], 3: δ(1H) = 0.70 [s, 3H, 2JPtH = 71 Hz, MePt trans to I]; 1.57 [s, 6H, 2JPtH = 71 Hz, MePt trans to N]. cis-[PtIMeEt2(bipy)], 4a: δ(1H) = 0.17 [t, 3H, 3JHH = 7 Hz, 3 JPtH = 63 Hz, MeCH2 trans to I]; 1.25 [s, 3H, 2JPtH = 72 Hz, MePt trans to N]. trans-[PtIMeEt2(bipy)], 4b: δ(1H) = 0.43 [s, 2JPtH = 75 Hz, MePt trans to N]. Reaction of [PtMe2(bipy)], 1, with EtI in Benzene. The reaction was performed similarly to that for the acetone solution except that the reaction time was 10 h. On the basis of 1H NMR data, the product was shown to contain not only the above-mentioned complexes 2a, 2b, 3, and 4 but also trans-[PtIMe2(OOEt)(bipy)], 5, and trans-[PtI2Me2(bipy)], 6. trans-[PtIMe2(OOEt)(bipy)], 5: δ(1H) = 0.49 [t, 3H, 3JHH = 7 Hz, MeCH2]; 1.86 [s, 6H, 2JPtH = 72 Hz, MePt]; 3.18 [q, 2H, 3 JHH = 7 Hz, MeCH2]. trans-[PtIMe2(OOCD2CH3)I(bipy)], 5*: δ(1H) = 0.47 [br s., 3H, MeCD2]; no signal for MeCH2. trans-[PtMe2(OOCH2CD3)I(bipy)], 5b**: δ(1H) = 3.18 [br s., 2H, MeCH2]; no signal for MeCH2. trans-[PtIMe2(OOCD2CD3)(bipy)]: no signals for the ethyl group. trans-[PtI2Me2(bipy)], 6: δ(1H) = 2.47 [s, 6H, 2JPtH = 73 Hz, MePt]. trans-[PtI2MeEt(bipy)], 7: δ(1H) = 2.07 [s, 6H, 2JPtH = 73 Hz, MePt]. Approximate yields (%) of products whose abundance could be determined by NMR analysis. Dark reaction: 2a, 74; 2b, 7; 3, 15; 4a, 2; 4b, 2. Diffuse light: 2a, 75; 2b, 8; 3, 6; 4a 2; 5, 2; 6, 4. Sunlight: 2a, 11; 2b, 1; 3, 9; 4a, 4; 4b, 4; 5, 19; 6, 47; 7, 4. KIEs by Competition Experiments. To a solution of [PtMe2(bipy)] (10 mg) in acetone (40 mL) was added a solution containing a mixture of excess CH3CH2I (100 μL) and CH3CD2I (100 μL) in acetone (5 mL). The solution was stirred at 25 °C for 1 h, and then the solvent was removed and the residue was dried under vacuum. The ratio of products 2a:2a* was obtained from the 1H NMR spectrum in benzene-d6 by integration of the CH3CH2 resonance of trans-[PtMe2EtI(bipy)], 2a, against the CH3CD2 resonance of trans-[PtIMe2(CH3CD2)(bipy)], 2a*, to obtain the secondary R-deuterium isotope effect. The reaction in benzene solvent was performed similarly with a reaction time of 2 h. The isotope effect on the chemical shift allowed the resolution of the resonances. A similar procedure using a 1:1 mixture of CH3CH 2I/ CD3CH2I was used to obtain the secondary β-deuterium isotope effect by integration of the resonances for the MeCH2 protons of 2a versus the CD3CH2 protons of 2a**. In the case of CH3CH2I/CD3CD2I competition, the product ratio was determined by integration of the ethyl resonances of 2a versus the MePt resonances of both isotopomers 2a and 2a***. The calculated intensity of the MePt resonance for 2a was subtracted from the total integration of the MePt resonance to give the relative concentrations of 2a and 2a***. This necessary subtraction step will lead to higher error limits on the KIE values.

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Table 3. Crystal and Refinement Data for the Complexes

formula fw T/K λ/A˚ cryst syst space gp a/A˚ b/A˚ c/A˚ R/deg β/deg γ/deg V/A˚3 Z d(calc)/Mg m-3 μ/mm-1 data/restr/params R1 [I > 2σ(I)] wR2 (all data)

2a

2a 3 0.5C6H6

3

C14H19IN2Pt 537.30 150(2) 0.71073 triclinic P1 9.9002(3) 11.7696(4) 13.4227(4) 88.490(2) 88.747(2) 85.119(2) 1557.52(8) 4 2.291 10.977 8756/0/331 0.0199 0.0374

C17H22IN2Pt 576.36 150(2) 0.71073 triclinic P1 9.3910(4) 10.5204(5) 10.7511(6) 67.924(2) 70.377(2) 70.536(2) 900.69(8) 2 2.125 9.499 5054/7/204 0.0346 0.0641

C13H17IN2Pt 523.28 150(2) 0.71073 monoclinic C2/m 16.2374(16) 13.2518(12) 7.8193(8) 90 117.454(3) 90 1493.0(3) 4 2.328 11.448 1603/0/84 0.0270 0.0662

Kinetic Studies. In a typical experiment, a solution of [PtMe2(bipy)], 1, in benzene (3 mL, 3 10-4 M) in a cuvette was thermostated at 25 °C, and a known excess of C2D5I was added using a syringe. After rapid stirring, the absorbance at λ = 508 nm was collected with time. The absorbance-time curves were analyzed by pseudo-first-order method. The pseudo-first-order rate constants (kobs) were evaluated by nonlinear least-squares fitting of

the absorbance-time profiles to a first-order equation. A plot of kobs versus [C2D5I] was linear, and the slope gave the secondorder rate constant. The same method was used at other temperatures, and activation parameters were obtained from the Eyring equation. The same reaction in acetone was similarly investigated at λ = 480 nm. The resulting data are collected in Table 1. The kinetics of reactions involving partially deuterated reagents, CH3CD2I or CD3CH2I, at 25 °C were studied similarly. Note that the assumption is made that equal volumes of EtI and its deuterium-substituted derivatives give equal concentrations. X-ray Structure Determinations. Data were collected using a Bruker diffractometer. The frames were integrated with the Bruker SAINT software package using a narrow-frame algorithm. Data were corrected for absorption effects using the multiscan method (SADABS). The structures were solved by direct methods and refined using the Bruker SHELXTL Software Package. Details are given in Table 3.

Acknowledgment. We thank the NSERC (Canada) and the Iran National Science Foundation (Grant No. 88001391) for financial support. M.R. thanks Shiraz University for granting sabbatical leave. We thank Dr. G. Popov for expert assistance with the X-ray structure determinations, and the referees for their insight. Supporting Information Available: Tables of X-ray data for the complexes in cif format are available free of charge via the Internet at http://pubs.acs.org.

Oxidative Addition of Ethyl Iodide to a Dimethylplatinum(II) Complex

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