Allylic C–H Deprotonation of Olefins with PtII(OH) to Form η3-Allyl PtII

ARTICLE pubs.acs.org/Organometallics

Allylic C
H Deprotonation of Olefins with PtII(OH) to Form η3-Allyl PtII Complexes in Water and Aprotic Organic Solvents Julia R. Khusnutdinova,† Peter Y. Zavalij, and Andrei N. Vedernikov* Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States

bS Supporting Information ABSTRACT: Hydroxo olefin platinum(II) complex (dpms)PtII(OH)(olefin) (olefin = cis-cyclooctene) reacts in water, in methanol, and, much faster, in aprotic solvents, DMF, acetone, or CH2Cl2 at 20 °C to produce the corresponding η3-allylic complex. Allylic C
H bond deprotonation in (dpms)PtII(OH)(cis-cyclooctene) is reversible, leading to the selective H/ D exchange of the olefin in D2O solutions. Attempted olefin-for-olefin ligand exchange in (dpms)PtII(OH)(C2H4) with olefin = cycloheptene or cyclopentene aimed at the preparation of other (dpms)PtII(OH)(olefin) complexes in water
organic solvent mixtures leads to corresponding η3-allylic derivatives via the intermediacy of (dpms)PtII(OH)(olefin) species. Consistent with mechanistic tests and results of DFT calculations, the PtII(OH) group is responsible for deprotonation of the allylic C
H bond of the coordinated olefin.

’ INTRODUCTION The cleavage of C
H bonds in hydrocarbons with late transition metal aqua,1 hydroxo, alkoxo, and aryloxo complexes2
7 attracts special attention since it may be important for applications in the long sought after selective transition metal mediated aerobic C
H functionalization in aqueous media.8 A plausible catalytic cycle for such reactions is shown in Scheme 1.9 Transition metal hydroxo complexes suitable for hydrocarbon C
H cleavage are scarce.2,4,7 Little is known about mechanisms of these reactions. C
H cleavage reactions involving hydroxo platinum(II) complexes and hydrocarbons less acidic than phenylacetylene10 were never documented. Deprotonation of allylic C
H bonds of 1-butene and 1-hexene in the coordination sphere of cationic chloro N,N,N0 ,N0 -tetramethylethylenediamine platinum(II) complexes in dichloromethane solutions was reported to require the presence of a triethylamine external base.11 Activation of the allylic C
H bond in cyclohexene and indene with μ-hydroxo-bridged dinuclear platinum(II) complexes, [(diimine)2PtII2(μ-OH)2]2þ, in benzene, 1,2-dichloroethane, CH2Cl2, or trifluoroethanol solutions reported recently12 requires the presence of stoichiometric amounts of a strong acid and was proposed to involve formation of aqua species [(diimine)PtII(OH2)2]2þ, but the mechanism of the C
H cleavage step in these systems was not discussed. In the case of palladium analogues indene could be converted to corresponding η3-indenyl complexes in the absence of acids in CH2Cl2 solutions.6 It was suggested that [(diimine)PdII(OH)(solv)]þ (solv = TFE or MeOH additive) might be involved in this transformation, but no [(diimine)PdII(OH)(olefin)]þ intermediates were detected. The role of the hydroxo ligand at the C
H bond cleavage step in that system is yet to be determined. Herein we report the intra- and intermolecular C
H deprotonation of propene, cyclopentene, cycloheptene, and cis-cyclooctene r 2011 American Chemical Society

Scheme 1

Chart 1

olefinic substrates, with neutral hydroxo platinum(II) complexes supported by a di(2-pyridyl)methane sulfonate ligand Received: March 28, 2011 Published: June 01, 2011 3392

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Organometallics (dpms) to produce their corresponding platinum(II) η3-allyl derivatives. Some of these reactions involve monomeric, isolable hydroxo complexes such as 1 or 12 (Chart 1) and can be carried out in aprotic solvents such as acetone, DMF, and dichloromethane, as well as in pure water and water
organic solvent mixtures (Schemes 2
4). On the basis of available experimental observations and our density functional theory (DFT) modeling, the intramolecular C
H bond cleavage of complex 1 involves reversible deprotonation of the coordinated olefin with the PtII(OH) fragment or solvent and the subsequent virtually irreversible loss of the resulting aqua ligand to form the corresponding η3-allyl derivative 8 (Scheme 2). In the case of intermolecular C
H bond cleavage involving complexes 12 (Scheme 2) and 2 (Scheme 3), the olefin coordination is often rate limiting and the nature of the solvent dramatically affects the rate of the C
H bond cleavage step.

’ RESULTS AND DISCUSSION Previously we reported that cis-cyclooctene hydroxo platinum(II) complex 1 undergoes facile aerobic oxidation in alkaline aqueous solutions to cleanly produce PtIV oxetanes.13 Now we found that in the absence of oxidants complex 1 dissolved in CH2Cl2, acetone, DMF, methanol, water, or aqueous methanol is Scheme 2

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also reactive and produces η3-cyclooctenyl complex 8 in high yield (eq 1): ðdpmsÞPtII ðOHÞðη2 -C8 H14 Þ f ðdpmsÞPtII ðη3 -C8 H13 Þ þ H2 O ð1Þ Formation of one equivalent of H2O could be detected by 1H NMR spectroscopy in dry DMF-d7 as a solvent. Depending on the medium, formation of 8 may be accompanied by some decomposition of 1, leading to cis-cyclooctene and 10
15% of complex 13, (dpms)2Pt2(μ-OH)2.14 Analytically pure 8 could be isolated when the reaction is performed in acetone. In solutions complex 8 exists as a mixture of two isomers with the dpms ligand sulfonate tail and the (CH2)5 fragment of the allyl ligand being on the same (cis) or the opposite (trans) sides with respect to the metal coordination plane, as established by NOE (nuclear Overhauser effect) NMR experiments (Scheme 5). The trans-8 complex was also characterized by single-crystal X-ray diffraction (Figure 1; crystal and refinement data are given in Table 1). The rate of the reaction given by eq 1 is strongly solventdependent (Scheme 2, top). The reaction half-life at 20 °C, τ1/2, is 17 min in acetone (ΔG293q = 21.4 kcal/mol), 12 min in dichloromethane (ΔG293q = 21.2 kcal/mol), and 10 min in DMF (ΔG293q = 21.1 kcal/mol). The reaction rate is much slower in a 8:1 (v/v) methanol
water mixture with a half-life of 12 min at 70 °C. In this mixture, the reaction is complete after 3 h, and 8 can be isolated in 85
90% yield. Finally, in pure water the reaction in eq 1 is the slowest, τ1/2 = 70 min at 70 °C or 40 days at 20 °C (ΔG293q = 26.1 kcal/mol), and the contribution of the side reaction leading to (dpms)2Pt2(μ-OH)2 is most pronounced. Interestingly, slow H/D exchange was observed in a D2O Scheme 4

Scheme 3

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Scheme 5

Table 1. Crystal Structure and Refinement Data for Complexes 8 and 5 8

Figure 1. ORTEP plots (50% probability ellipsoids) for trans-8. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Pt1
C1, 2.143(5); Pt1
C3, 2.132(5); Pt1
C2, 2.086(5); Pt1
N1, 2.102(4); Pt1
N2, 2.089(4); C1
C2, 1.440(7); C2
C3, 1.428(7); C1
C2
C3, 118.8(4).

solution of 1 in allylic C
H positions selectively, indicative of reversible allylic C
H deprotonation in this solvent (vide infra). Complex 8 could also be obtained as a result of an intermolecular C
H bond cleavage in cis-cyclooctene, by reacting complex (dpms)PtII(OH)(DMSO), 12, with excess olefin (Scheme 2, bottom) in either neutral or weakly alkaline methanol
water mixtures. When 12 was heated with 70 equivalents of the olefin in a 50:1 MeOH
water mixture at 60 °C, formation of complex 8 was observed with a half-life τ1/2 = 5 h. The allylic product forms in low yield (20
30%) due to competing formation of insoluble μ-hydroxo-bridged dinuclear complex (dpms)2Pt2(μ-OH)2, 13. Similar reaction rates and yields were observed in the absence or in the presence of NaOH additives. We propose that complex 8 results from the cyclooctene-for-DMSO substitution in 12, eq 2, with subsequent allylic C
H bond cleavage in the resulting intermediate 1, eq 1. ðdpmsÞPtII ðOHÞðDMSOÞ þ cyclo-C8 H14 f ðdpmsÞPtII ðOHÞðη2 -C8 H14 Þ þ DMSO

ð2Þ

Since the hydroxo complex 1 was not detected by means of 1H NMR spectroscopy in the reaction mixtures above, the cyclooctene-for-DMSO substitution is the rate-limiting step. To explore the substrate scope of the allylic C
H bond cleavage with the PtII(OH) fragment, preparation of some other (dpms)PtII(OH)(olefin) complexes was attempted using the hydroxo ethylene complex 215 and cycloolefin cyclopentene, cyclohexene, or cycloheptene taken in excess. The reagents were allowed to react in a 1:1 water
acetone or 1:1 water
1,4dioxane mixture at 20 °C for 2
5 days. In the reaction involving cyclopentene, no (dpms)PtII(OH)(cyclo-C 5 H10 ) complex expected from the cycloolefin-forethylene substitution was observed during the reaction period. Instead, the product of intermolecular C
H bond cleavage, η3-cyclopentenyl derivative 9 (60
70% NMR yield; complete conversion of 2), formed in 5 days (Scheme 3, top) along with some unidentified species. In the case of cyclohexene, the

5

formula

C19H22N2O3PtS 3

2(C19H23N2SO3PtCl) 3

fw

H2O 571.55

CH3OH 1212.03

cryst syst

triclinic

monoclinic

space group

P1

C2/c

a (Å)

9.3702(9)

32.1173(15)

b (Å)

9.5020(9)

7.4449(3)

c (Å)

11.3905(11)

17.5732(8)

R (deg)

72.504(2)

90

β (deg) γ (deg)

88.877(2) 81.837(2)

104.0610(10) 90

Z

2

4

density, calc (g/cm3)

1.983

1.975

abs coeff (mm
1)

7.467

7.145

F(000)

556e

2360e

cryst size (mm3)

0.45  0.21  0.07

0.28  0.24  0.12

cryst habit

colorless prism

yellow prism

θ range (deg) index ranges

2.85 to 27.50
12 e h e 12

2.62 to 27.50
41 e h e 41


12 e k e 12


9 e k e 9


14 e l e 14


22 e l e 22

no. of reflns collected

11 602

25 159

no. of indep reflns

4324

4683

no. of obs reflns

3979

4382

final R indices

0.0295

0.0145

R1, I > 2σ(I) wR2, all data

0.0771

0.0351

Rint

0.0235

0.0282

Rsig

0.0254

0.0186

reagents remained intact after 5 days in the water
1,4-dioxane mixture. In contrast, the reaction with cycloheptene led to the slow formation of the η3-cycloheptenyl complex 10 (15% NMR yield; 40% conversion of 2) when a 1:1 acetone
water mixture was used as a solvent or, in the case of a purely aqueous solution, to a mixture of the olefin hydroxo complex 4 and the η3-allyl complex 10 in 1:1 molar ratio in a 55% combined yield after 2 days. On the basis of these observations, we suggest that the formation of η3-allylic complexes 9 and 10 in the intermolecular C
H bond cleavage reactions in aqueous organic solvents involves rate-limiting cycloolefin-for-ethylene displacement in 2 (Scheme 3) to produce corresponding (dpms)PtII(OH)(olefin) complexes. The absence of reaction between 2 and cyclohexene could be due to the lower nucleophilicity of cyclohexene compared to other, strained cycloalkenes used in this work. In the case of aqueous organic solvents, the corresponding olefin complexes (dpms)PtII(OH)(olefin) undergo fast intramolecular allylic C
H bond cleavage of the coordinated olefin and transformation into the corresponding η3-allylic complex, 9 or 10. In pure water, the rate of the allylic C
H bond cleavage in the cycloheptene hydroxo complex 4 is not as fast, similar to the reaction of cis-cyclooctene complex 1 (Scheme 2), where the rate of the C
H bond cleavage was shown to be strongly solvent-dependent. The C
H bond cleavage in 3394

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free cis-cyclooctene and cationic diaqua complex (dpms)PtII(OH2)2þ (eq 3) was observed: ðdpmsÞPtII ðOHÞðolefinÞ þ H3 Oþ f þ

ðdpmsÞPtII ðOH2 Þ2 þ olefin

Figure 2. ORTEP plots (50% probability ellipsoids) for 5. Selected bond lengths (Å): Pt1
Cl1, 2.3034(6); Pt1
N1, 2.0305(18); Pt1
N2, 2.0707(18); Pt1
C31, 2.192(2);Pt1
C32, 2.183(2); C31
C21, 1.389(3).

(dpms)PtII(OH)(olefin) complexes is noticeably faster than the ligand exchange in aqueous organic solvents and slower in pure water. To probe the effect of an anionic ligand X in (dpms)PtII(X)(olefin) complexes on their ability to undergo allylic C
H bond cleavage, the corresponding chloro complexes (dpms)PtIICl(olefin) (5, olefin = cis-cyclooctene, and 6, olefin = propene) were prepared and isolated in analytically pure form. Complex 5 was characterized by X-ray diffraction (Figure 2; crystal and refinement data are given in Table 1). Both complexes 5 and 6 were found to be stable in neutral CD3OD or D2O solutions for 1
2 days at 20 °C. In contrast, when two diastereomeric chloro propene complexes 6 were allowed to react with 1 equiv of NaOH or 1 equiv of Ag2O (Scheme 4) in aqueous solution at 20 °C, the formation of the η3allylic complex 11 was observed. The formation of 11 proceeds via the intermediate diastereomeric hydroxo complex (dpms)PtII(η3-propene)(OH), 3, which was detected in the reaction mixture by means of 1H NMR spectroscopy and electrospray ionization mass spectrometry (ESI-MS). The intermediates were consumed to produce 11 in 85
90% NMR yield when the temperature was raised to 50 °C. Complex 11 could be isolated from the reaction mixture in 42
47% yield in analytically pure form. On the basis of these observations, the allylic C
H deprotonation in these complexes may involve formation of hydroxo olefin species. The attempted reaction of 5 with NaOH or Ag2O led to liberation of free cis-cyclooctene along with formation of intractable mixtures of unidentified products. To shed light on the mechanism of the allylic C
H deprotonation in coordinated cis-cyclooctene, eq 1, several mechanistic tests were performed with complex 1. Previously an allylic C
H bond cleavage in cyclohexene and indene with μ-hydroxobridged dinuclear PtII and PdII diimine complexes in CH2Cl2 solutions containing aqueous HBF4 was reported, and the corresponding mononuclear olefin aqua complexes were proposed as the reaction intermediates.12 No C
H bond cleavage was observed in those systems in the absence of the acid. These results prompted us to test the effect of acid additives in reaction 1. The reaction of 1 in the presence of one equivalent of HBF4 in a 1:1 MeOH
H2O mixture was monitored using 1H NMR spectroscopy at 50 °C. Under these conditions formation of poorly soluble reaction product 8 was observed after 0.5
1 reaction half-life; hence initial reaction rates were determined as an average of 2 or 3 runs. Besides complex 8 (eq 1) formation of

ð3Þ

The rate of disappearance of 1 and the rates of accumulation of (dpms)PtII(OH2)2þ and 8, both of which are stable in acidic media, were determined using 1H NMR integration of signals of these complexes. The rate of disappearance of 1 was found to be equal to the rate of formation of 8 plus the rate of formation of (dpms)PtII(OH2)2þ:


d½1 d½8 d½ðdpmsÞPtðOH2 Þ2 þ  ¼ þ dt dt dt d½8 ¼ kallyl ½1 dt d½ðdpmsÞPtðOH2 Þþ 2 ¼ ksolv ½1 dt

We found that the rate of formation of 8 is slightly suppressed by the acid additive with the first-order rate constant kallyl changing from (5.9 ( 0.2)  10
5 s
1 (no acid) to (5.0 ( 0.6)  10
5 s
1 (one equivalent of acid) at 50 °C. In contrast, the overall rate of disappearance of 1 has increased significantly in the presence of the acid due to a fast water-for-olefin ligand substitution leading to free cis-cyclooctene and (dpms)PtII(OH2)2þ, eq 3. The olefin loss is accelerated more than 10-fold under these conditions. The observed pseudo-first-order rate constant for the olefin substitution, ksolv, increases from (1.2 ( 0.2)  10
5 s
1 in neutral solutions to (1.32 ( 0.06)  10
4 s
1 in the presence of one equivalent of HBF4 at 50 °C. We propose an involvement of the cationic aqua complex (dpms)PtII(OH2)(cis-cyclooctene)þ, 7, eq 4, in this olefin displacement reaction: ðdpmsÞPtII ðOHÞðolefinÞ þ H3 Oþ h ðdpmsÞPtII ðOH2 ÞðolefinÞþ

ð4Þ Cationic complex 7 is expected to be more electrophilic than 1 and more reactive toward nucleophilic attacks of the solvent, leading to the displacement of a weakly bound olefin.16 In turn, the low sensitivity of the rate of accumulation of 8 to the presence of the acid additive suggests that the hydroxo complex 1 and not its protonated form, the aqua complex 7, is involved in this transformation. The basicity of (dpms)PtII(olefin)(OH) complexes is low enough (pKa = 3 for ethylene derivative 215 in pure water) to preserve most of the complex 1 from protonation in the presence of one equivalent of HBF4 (solutions used in our experiments were 1.40
2.80 mM in HBF4). The conclusion that neutral 1 and not cationic 7 is a kinetically viable intermediate leading to the allylic C
H bond cleavage is also consistent with the fact that similar yields of 8 were obtained in reaction of 12 with cis-cyclooctene (eq 2) in the absence and in the presence of NaOH additives. Additional information about the mechanism of the transformation of cis-cyclooctene complex 1 into η3-cyclooctenyl complex 8, eq 1, can be obtained from the results of an H/D exchange between D2O solvent and allylic protons of the coordinated olefin in 1 (Scheme 6). The C
H bond cleavage product, 3395

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Scheme 6

Figure 3. DFT-calculated Gibbs energy reaction profiles for C
H bond cleavage in complex 1 in aqueous solution (first number) and in gas phase (the number after slash): (i) path a, C
H oxidative addition to PtII; (ii) path b, allylic C
H deprotonation with the PtII(OH) fragment.

η3-allylic complex 8, was found to be inert toward H/D exchange in neutral or basic solutions in D2O
CD3OD mixtures at 20 °C. The H/D exchange in 1 was observed at 20 °C with a half-life τ1/2 = 6.6 days (ΔG293q = 25.1 kcal/mol), which is faster than the disappearance of 1 (τ1/2 = 40 days; ΔG293q = 26.1 kcal/mol). No noticeable deuterium incorporation into the vinylic positions of cis-cyclooctene ligand was seen. These observations imply the presence of a reversible C
H activation step leading to a presumed η1-cyclooctenyl aqua intermediate A (Figure 3) that produces the η3-cyclooctenyl complex 8 virtually irreversibly as a result of an intramolecular substitution of the aqua ligand with the η3-cyclooctenyl. Interestingly, the rate of the H/D exchange in 1 is virtually the same for the allylic hydrogens facing the Pt(OH) fragment (Scheme 6, top) and those pointing in the opposite direction (Scheme 5, bottom; see Table S2).

We presume that the basicity of the Pt(OH) fragment in 1 is significantly diminished in pure water due to hydrogen bonding to the OH ligand, so that the intramolecular deprotonation of the allylic C
H bonds with the Pt(OH) fragment (Scheme 6, top) can occur at about the same rate as the intermolecular, solventmediated deprotonation of the allylic C
H bonds inaccessible for an intramolecular attack by the Pt(OH) group (Scheme 6, bottom). This second mechanism of the allylic C
H bond cleavage must be absent in aprotic solvents. Four plausible mechanisms for the transformation of 1 into 8 that might be operational in aprotic solvents as well as aqueous neutral or basic solutions were analyzed using DFT calculations. Paths a and b are presented in Figure 3. Standard Gibbs energies were calculated for both aqueous solutions (first number in Scheme 3) and, in most cases, for gas phase (second number after 3396

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Organometallics

Figure 4. DFT-calculated Gibbs energy reaction profiles for C
H bond cleavage in complex 1 in aqueous solution (first number) and in gas phase (the number after slash): (i) path c, 1,2 C
H bond addition across the Pt
OH bond; (ii) path d, allylic C
H deprotonation with the sulfonate group.

a slash). Path a corresponds to a C
H bond oxidative addition/ reductive elimination mechanism including σ-complex intermediate B, whereas the path b involves deprotonation of the allylic C
H bond of the coordinated olefin with the PtII(OH) fragment. Both reactions lead to a η1-allylic intermediate A. Path a involves a η1-allylic PtIV hydride C and a high-energy transition state TSoa, whereas path b involves a CH-to-OH proton transfer transition state TSd connected to both 1 and the intermediate A via a single minimal-energy reaction path. Since the energy of TSd is much lower than that of TSoa for both aqueous and gas phase, we presume that formation of A is not likely to occur via the C
H bond oxidative addition mechanism a. Two alternatives to the allylic C
H deprotonation mechanism b leading to the common intermediate A were also considered: (i) path c, 1,2-addition of an allylic C
H bond across the Pt
OH bond in the intermediate B, and (ii) path d, the allylic C
H deprotonation of the olefin with one of the oxygen atoms of the sulfonate group leading to a sulfonic acid-derived intermediate D (Figure 4). In the case of path c the corresponding transition state TSa connecting it to the product A is higher in energy than both TSoa and TSd, thus making this path even less competitive than path a. In the case of path d the transition state TSds is higher in energy than TSd, suggesting that this path is also not operational in the reaction in eq 1. On the basis of the results presented in Figures 3 and 4, we can conclude that the most likely mechanism of C
H bond cleavage in complex 1 involves allylic C
H deprotonation with the PtII(OH) fragment (path b, Figure 3). The intramolecular ligand substitution leading from intermediate A to the reaction product 8 is common for all four mechanisms of C
H bond cleavage considered. The transition state TSls corresponding to the aqua ligand substitution in water is higher in energy than the TSd corresponding to the allylic C
H deprotonation step. Hence, the aqua ligand loss is rate

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limiting in the case of the most viable mechanism, b, which allows one to account for the observed multiple H/D exchange at the allylic positions of the coordinated olefin prior to the formation of 8. By contrast, the C
H activation step is rate limiting in mechanisms a and c, which is inconsistent with the observed H/D exchange in the coordinated olefin in D2O solutions. This result provides an additional support for mechanism b for aqueous solutions. According to the DFT calculations a transition from water to gas phase as a reaction medium is expected to decrease the activation barrier for the C
H deprotonation in 1 by 1.4 kcal/ mol and the overall reaction barrier by 4.5 kcal/mol. On the basis of this trend caused by the solvation effects, we presume that formation of 8 from 1 should be faster in solvents that are less polar than water, which is consistent with the reactivity trend observed experimentally in protic versus aprotic solvents (Scheme 2, top). In the same vein, due to the presence of the PtII(OH) fragment, complex 1 can be viewed as a more basic species compared to the transition states TSd and TSls in Figure 3 so that preferential stabilization of 1 via hydrogen bonding between H2O and the hydroxo ligand in 1 becomes possible, which must lead to even slower rates of reaction given by eq 1 in water. Although this hydrogen bonding was not modeled explicitly in this DFT study, this type of interaction can be the major factor contributing to slower allylic C
H deprotonation of 1 in protic media. Finally, assuming that the allylic C
H deprotonation of cyclopentene complex (dpms)Pt(OH)(cyclopentene) follows the same mechanism as for its cis-cyclooctene analogue 1, path b, we calculated the Gibbs activation energies for the transition states TSd(cyclopentene) and TSls(cyclopentene) in aqueous solutions. The value found for TSd(cyclopentene) is 16.2 kcal/mol versus 15.2 kcal/mol for the cyclooctene analogue (Figure 3), and for TSls(cyclopentene) it is 18.9 kcal/mol versus 25.1 kcal/mol for the cyclooctene derivative (Figure 3). These results predict much faster conversion of (dpms)Pt(OH)(cyclopentene) to its η3-allylic derivative 9 as compared to the cyclooctene analogue, conversion of 1 into 8. This result is consistent with the absence of (dpms)Pt(OH)(cyclopentene) intermediate in reactions presented in Scheme 3. The higher reactivity of the cyclopentene derivatives may be due to the more rigid allylic ligand structure compared to cyclooctene derivatives, allowing for more facile accommodation of the conformation preferred at the η3-allyl-foraqua ligand substitution step.

’ SUMMARY In summary, the reactivity of (dpms)PtII(X)(olefin) complexes toward deprotonation of the allylic C
H bond of the coordinated olefins was found to be highly dependent on the nature of the solvent and the anionic ligand X. While neutral chloro olefin complexes (X = Cl) are essentially unreactive toward allylic C
H bond cleavage, facile formation of allylic complexes is observed in solutions of hydroxo complexes (dpms)PtII(OH)(olefin) in aprotic as well as in protic solvents. The nature of the solvent is an important factor determining the reactivity of (dpms)PtII(OH)(olefin) complexes in the allylic C
H bond deprotonation: aprotic solvents promote facile formation of allylic complexes, whereas slower reaction is observed in water and water
organic solvent mixtures. Combined experimental and DFT studies of these systems suggest that allylic C
H bond cleavage can involve an intramolecular 3397

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Organometallics direct deprotonation of the allylic C
H bonds of PtII-coordinated olefins with the Pt-bound hydroxo group.

’ EXPERIMENTAL SECTION General Procedures. All manipulations were carried out under purified argon using standard Schlenk and glovebox techniques. All reagents for which synthesis is not given are commercially available from Aldrich, Acros, Alfa Aesar, or Pressure Chemicals and were used as received without further purification. Potassium di(2-pyridyl)methanesulfonate,17 (dpms)Pt(cis-cyclooctene)(OH),13 (dpms)PtII(OH)(DMSO),13 and LPt(CH2dCH2)(OH)15 were synthesized as described previously. Deuterated solvents, DMSO-d6, DMF-d7, CD2Cl2, and CD3OD, were purchased from Cambridge Isotope Laboratories and were dried over CaH2, vacuum-transferred or distilled, and stored in Teflon-sealed flasks in an argon-filled glovebox. Water was deaerated by repeating freezing
pumping cycles and stored under argon in a Teflon-sealed Schlenk flask in a glovebox. 1H (400 or 500 MHz) and 13C NMR (100 or 125 MHz) spectra were recorded on Bruker Avance 400 and Bruker DRX-500 spectrometers. Chemical shifts are reported in ppm and referenced to residual solvent resonance peaks. The assignment of proton and carbon resonances was made on the basis of NOE, DEPT, 2D 1H
1H COSY, and 2D 1H
13C HSQC NMR experiments. IR spectra were recorded on a JASCO FT/IR 4100 spectrometer. ESI-MS experiments were performed using a JEOL AccuTOF-CS instrument. Elemental analyses were carried out by Chemisar Laboratories Inc., Guelph, Canada. Computational Details. Theoretical calculations in this work have been performed using the density functional theory method, specifically functional PBE, implemented in the Jaguar program package with the LACVP relativistic basis set with two polarization functions. Full geometry optimization has been performed without constraints on symmetry. For all species under investigation frequency analysis has been carried out. All minima have been checked for the absence of imaginary frequencies. All transition states possessed just one imaginary frequency. Using the method of intrinsic reaction coordinates, reactants, products, and the corresponding transition states were proven to be connected by a single minimal-energy reaction path. The solvation of all complexes in water was modeled using a Poisson
Boltzmann continuum solvation model implemented in the Jaguar program package. Preparation of (dpms)PtII(η3-C8H13), 8. A sample of a complex (dpms)PtII(cis-cyclooctene)(OH),1 1 (79.8 mg, 140 μmol), was placed into a 50 mL round-bottom flask, equipped with a stirring bar and a reflux condenser; 20 mL of dry acetone was added. Because of the poor solubility of 8 in acetone, heating was necessary to complete the reaction: the stirred suspension of 1 was heated at 40 °C for 8 h. A yellowish-white, powdery precipitate formed. The precipitate was filtered off, washed with water (1 mL), acetone (5 mL), and pentane (5 mL), and dried under vacuum. White solid: yield 52 mg (84 μmol), 67%. The product is soluble in DMSO, DMF, and methanol, slightly soluble in acetone and CH2Cl2, and almost insoluble in water. According to NMR, the allylic complex (dpms)Pt(η3-C8H13) also forms upon dissolving (dpms)Pt(C8H14)(OH) in aprotic solvents such as DMF-d7 and CD2Cl2 at room temperature with a half-life of 10
20 min with the release of one equivalent of water. The ratio of isomers cis8:trans-8 was 3.3:1 in DMF-d7, 3:1 in dmso-d6, and 1:1 in CD3OD. cis-8. 1H NMR (dmso-d6, 22 °C), δ: 0.74
1.67 (m, 10H, (CH2)5 of cyclooctenyl), 3.87 (m, 2J195PtH = 37 Hz, 2H, Hsyn), 5.22 (t, J = 7.3 Hz, 2 J195PtH = 90 Hz, 1H, Hmeso), 5.58 (s, 1H, CHSO3), 7.01 (ddd, J = 7.8, 5.6, 1.3 Hz, 2H, Hm2), 7.50 (d, J = 7.8 Hz, 2H, Hm1), 7.72 (td, J = 7.8, 1.3 Hz, 2H, Hp), 8.54 (d, J = 5.6, 3J195PtH =31 Hz, 2H, Ho). 1H NMR (DMF-d7, 22 °C), δ: 1.20
2.21 (m, 10H, (CH2)5 of cyclooctenyl), 4.40 (m, 2J195PtH = 48 Hz, 2H, Hsyn), 5.75 (t, J = 7.3 Hz, 2J195PtH = 93 Hz, Hmeso), 6.07 (s, 1H, CHSO3), 7.52 (ddd, J = 7.9, 5.8, 1.5 Hz, 2H, Hm2),

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8.06 (d, J = 7.9 Hz, 2H, Hm1), 8.21 (td, J = 7.9, 1.5 Hz, 2H, Hp), 9.12 (dd, J = 5.8, 3J195PtH = 40 Hz, Ho) trans-8. 1H NMR (dmso-d6, 22 °C), δ: 0.7
2.24 (m, 10H, (CH2)5 of cyclooctenyl), 3.81 (m, 2J195PtH = 40 Hz, 2H, Hsyn’), 4.83 (t, J = 7.6 Hz, 2 J195PtH = 74 Hz, 1H, Hmeso’), 5.51 (s, 1H, CHSO3), 7.10 (ddd, J = 7.9, 5.8, 1.3 Hz, 2H, Hm2’), 7.47 (d, J = 7.9 Hz, 2H, Hm1’), 7.74 (td, J = 7.9, 1.3 Hz, 2H, Hp’), 8.65 (d, J = 5.8 Hz, 3J195PtH = 27 Hz, 2H, Ho0 ). 1H NMR (DMF-d7, 22 °C), δ: 1.3
2.2 (m, 8H, (CH2)4 of cyclooctenyl), 2.65 (m, 2H, CH2 of cyclooctenyl), 4.36 (m, 2J195PtH = 44 Hz, 2H, Hsyn0 ), 5.39 (t, J = 7.2 Hz, 2J195PtH = 79 Hz, Hmeso’), 6.00 (s, 1H, CHSO3), 7.62 (ddd, J = 7.7, 5.6, 1.4 Hz, 2H, Hm20 ), 8.05 (d, J = 7.7 Hz, 2H, Hm10 ), 8.24 (td, J = 7.7, 1.4 Hz, 2H, Hp0 ), 9.23 (dd, J = 5.6, 3J195PtH = 34 Hz, 2H, Ho0 ). XRD quality crystals of 8 were obtained by slow crystallization from aqueous solutions of 1 at room temperature. IR (KBr), ν: 3076 (w), 2924 (w), 2870 (w), 1607 (w), 1484 (m), 1249 (m), 1228 (s), 1184 (w), 1171 (w), 1038 (s), 809 (m), 767 (m), 685 (m) cm
1. ESI-MS of the solution in trifluoroethanol acidified with HBF4, m/z 554.1; calculated for (dpms)Pt(η3-C8H13)*Hþ, C19H23N2O3195PtS, 554.1. Anal. Found: C, 40.95; H, 4.23; N, 5.07. Calculated for allyl complex (dpms)Pt(η3C8H13), C19H22N2O3PtS: C, 41.23; H, 4.01; N, 5.06. NOE/EXSY Experiments. All experiments were performed using a mixing time of 0.5 s and delay time of 4 s if not indicated otherwise.

Geometry of the isomers was deduced from 1D-difference NOE experiments and 2D NOESY/EXSY and is shown above. cis-8 (major isomers in DMF-d7). Irradiation of a resonance at 9.12 ppm (ortho-H of pyridyl, Ho) gives intensity enhancement (positive NOE) of a multiplet at 4.40 ppm (Hsyn, 5%) and a multiplet at 7.52 ppm (C(5)-H of pyridyl Hm2, 3%). An exchange peak that has the same phase as the irradiated Ho signal is observed in the 1D-NOE spectrum as a doublet at 9.23 ppm (ortho-H of pyridyl of exo-8, Ho0 ). trans-8 (minor isomers in DMF-d7). Irradiation of a resonance at 9.23 ppm (ortho-H of pyridyl, Ho0 ) showed a positive NOE with a multiplet at 2.65 ppm (Hc0 , 3%) and a multiplet at 7.62 ppm (C(5)-H of pyridyl Hm20 , 3%). The assignment of the Hc’ resonance at 2.65 ppm was confirmed by a positive NOE between Hc’ and an allylic syn-proton at 4.36 ppm (Hsyn’, 9%). Irradiation of Hc’ gave other positive NOEs with a multiplet at 2.05
2.19 ppm (Hd’, 10%) and Ho’ (3%); an exchange peak (with the same phase as the irradiated Hc’) was observed at 1.86
2.06 ppm (multiplet, Hc of cis-8). 2D NOESY/EXSY spectrum (mixing time 0.3 s) confirmed the signal assignments above. In the EXSY spectrum exchange correlation peaks were observed between meso-protons (Hmeso and Hmeso’), ortho-protons (Ho and Ho’), and Hc/Hc’ protons of cis-8/trans-8. Formation of 8 in Protic Solvents. Identification of Pt-Containing Products of Decomposition of 1 in Water. 1. Reaction in Water at Room Temperature. A solution of 50 mg (87 μmol) of (dpms)PtII(ciscyclooctene)(OH) (1) in 5 mL of H2O was stored for 2 weeks at rt under argon. Conversion of the starting material was 78%. A mixture of a white microcrystalline solid and larger colorless crystals precipitated from the solution. The larger crystals were analyzed by XRD and were identified as complex 8. The precipitate was filtered off, washed with water, and dried; yield 4.5 mg (9%). Solution of this product in trifluoroethanol acidified with HBF4 showed the presence of two mass envelopes, corresponding to protonated (dpms)PtII(η3-C8H13) (8) and (dpms)2PtII2(μ-OH)2 (13), described previously.14 ESI-MS: m/z 554.1 and 923.0; calculated for (dpms)PtII(η3-C8H13)*Hþ, C19H23N2O3195PtS, 554.1; calculated for (dpms)2PtII2(μ-OH)2*Hþ, C22H21N4O8195Pt2S2, 923.0. 3398

dx.doi.org/10.1021/om2002766 |Organometallics 2011, 30, 3392–3399

Organometallics 2. Reaction in Water at 70
80 °C. A solution of complex 1 (3.3 mg) in a D2O
H2O mixture (1:1 v/v) was placed into an NMR Young tube; dioxane (2 μL) was added as an internal standard. The solution was heated at 60
80 °C; NMR spectra were recorded periodically. Complex 1 reacts with a half-life of 2 h at 70 °C to produce insoluble products; the reaction is complete after 15 h at 80 °C. A white precipitate was collected and dissolved in dmso-d6 in the presence of internal standard (dioxane, 1 μL). The yield of allyl complex 8 was estimated to be ∼30%, based on integration of doublets of ortho-protons of cis- and trans-8 at 9.00 and 9.18 ppm, respectively. ESI analysis showed the presence of peaks corresponding to protonated (dpms)PtII(η3-C8H13) (8) and (dpms)2PtII2(μ-OH)2 (13), m/z 554.1 and 923.0, respectively. 3. Reaction in Methanol and Water
Methanol Mixtures. A solution of 3.3 mg of 1 in a mixture of D2O (75 μL) and CD3OD (615 μL) reacts at a much faster rate (half-life 12 min at 70 °C), compared to reaction in neat water, to produce allyl complex 8 in 85
90% yield after 3 h, as determined by 1H NMR. Complex 1 is poorly soluble in neat methanol and produces allyl complex 8 with a half-life of ∼1.5 h at 20 °C; the reaction was complete in CD3OD after heating at 50 °C for 14 h to give 6 (80% yield), free cyclooctene (20%), as determined by 1H NMR, and a white precipitate (presumably (dpms)2Pt2(μ-OR)2, R = H, Me).

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(12) Williams, T. J.; Caffyn, A. J. M.; Hazari, N.; Oblad, P. F.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2008, 130, 2418–2419. (13) Khusnutdinova, J. R.; Newman, L. L.; Zavalij, P. Y.; Lam, Y. F.; Vedernikov, A. N. J. Am. Chem. Soc. 2008, 130, 2174–2175. (14) Vedernikov, A. N.; Binfield, S. A.; Zavalij, P. Y.; Khusnutdinova, J. R. J. Am. Chem. Soc. 2006, 128, 82–83. (15) Khusnutdinova, J. R.; Zavalij, P. Y.; Vedernikov, A. N. Organometallics 2007, 26, 2402–2413. (16) Hahn, C. Chem.—Eur. J. 2004, 10, 5888–5899. (17) Vedernikov, A. N.; Fettinger, J. C.; Mohr, F. J. Am. Chem. Soc. 2004, 126, 11160–11161.

’ ASSOCIATED CONTENT

bS

Supporting Information. Experimental and computational details and CIF files for 5 and trans-8 are available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Present Addresses †

Department of Chemistry, Washington University, One Brookings Drive, St. Louis, Missouri 63130-4899.

’ ACKNOWLEDGMENT We thank the University of Maryland, the NSF (CHE-0614798), and the US-Israel Binational Science Foundation for the financial support of this work. ’ REFERENCES (1) Goldshleger, N. F.; Shteinman, A. A.; Shilov, A. E.; Eskova, V. V. Russ. J. Phys. Chem. 1972, 46, 785–786. (2) Feng, Y.; Lail, M.; Barakat, K. A.; Cundari, T. R.; Gunnoe, T. B.; Petersen, J. L. J. Am. Chem. Soc. 2005, 127, 14174–14175. (3) Hanson, S. K.; Heinekey, D. M.; Goldberg, K. I. Organometallics 2008, 27, 1454–1463. (4) Kloek, S. M.; Heinekey, D. M.; Goldberg, K. I. Angew. Chem., Int. Ed. 2007, 46, 4736–4738. (5) Tenn, W. J.; Young, K. J. H.; Bhalla, G.; Oxgaard, J.; Goddard, W. A.; Periana, R. A. J. Am. Chem. Soc. 2005, 127, 14172–14173. (6) Bercaw, J. E.; Hazari, N.; Labinger, J. A.; Oblad, P. E. Angew. Chem., Int. Ed. 2008, 47, 9941–9943. (7) Bercaw, J. E.; Hazari, N.; Labinger, J. A. Organometallics 2009, 28, 5489–5492. (8) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507–514. (9) Vedernikov, A. N. Chem. Commun. 2009, 4781–4790. (10) Klein, A.; Klinkhammer, K. W.; Scheiring, T. J. Organomet. Chem. 1999, 592, 128–135. (11) Bandoli, G.; Dolmella, A.; Di Masi, N. G.; Fanizzi, F. P.; Maresca, L.; Natile, G. Organometallics 2002, 21, 4595–4603. 3399

dx.doi.org/10.1021/om2002766 |Organometallics 2011, 30, 3392–3399