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Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Carbon−Oxygen Bond Forming Reductive Elimination from Cycloplatinated(IV) Complexes Marzieh Dadkhah Aseman,†,‡ S. Masoud Nabavizadeh,*,†,§ Fatemeh Niroomand Hosseini,§,∥ Guang Wu,§ and Mahdi M. Abu-Omar*,§ †

Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71467-13565, Iran Faculty of Chemistry, Kharazmi University, Tehran, Iran § Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, California 93106, United States ∥ Department of Chemistry, Shiraz Branch, Islamic Azad University, Shiraz 71993-37635, Iran ‡

S Supporting Information *

ABSTRACT: The synthesis of the two cycloplatinated(IV) complexes [PtMe(OAc)2(C∧N)(H2O)] (C∧N = 2-phenylpyridinate (2a), benzo[h]quinolate, 2b) by reaction of [PtMe(C∧N)(SMe2)] with PhI(OAc)2 is described. Complexes 2 undergo carbon− oxygen bond forming reductive elimination instead of C−C reductive elimination to produce MeOAc as protected methanol. The kinetics and mechanism of both Pt−O bond formation and C−O reductive elimination have been experimentally and theoretically investigated. The results suggest that formation of methyl acetate proceeds via nucleophilic attack of the dissociated acetate ligand at the methyl group carbon in a cationic five-coordinate intermediate cycloplatinated(IV) complex.

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INTRODUCTION Oxidative addition reactions of transition-metal complexes are some of the most fundamental transformations in organometallic chemistry and are often key steps in catalytic reactions. In contrast to oxidative addition, its microscopic reverse, reductive elimination, has received considerably less attention. Among the C−X reductive elimination processes, examples of reductive eliminations that form carbon−heteroatom (C−O, C−N, or C−S) bonds have been less investigated in comparison to reductive elimination reactions, which form C−H and C−C bonds.1 Reductive elimination reactions to form C−O bonds can result in the production of valuable chemicals such as alcohols, ethers, and esters.2−4 Most examples of C−O bond forming reactions are encountered for low-valent d8 metal centers which involve aryl or acyl carbon groups.2,5,6 C−O reductive eliminations of O−Calkyl bonds have also been reported.7−13 Recently reductive elimination of C−O bonds from Pd(IV) intermediates has been studied by several groups (Scheme 1 for some examples), forming valuable organic products.14−18 In particular, C−OAc bond formation via reductive elimination is the most studied case, providing various acetates for further synthetic transformation.11,19−24 C−O bond reductive elimination from Pt(IV) complexes, including kinetics and mechanisms of the C−O coupling reaction, has also been investigated, some examples of which including the C−OAc group from Pt(IV) complexes are shown in Scheme 2.7−9,12,25 © XXXX American Chemical Society

In this study we describe the synthesis and characterization of isolable Pt(IV)−diacetate complexes using PhI(OAc)2 as the acetate source. These cycloplatinated(IV) diacetate complexes undergo smooth C(sp3)−O reductive elimination. Competing C(sp2)−C(sp3) coupling at Pt(IV) center is not observed during thermolysis. A kinetic study of oxidative addition to the Pt(II) center by PhI(OAc)2 and reductive elimination of Me− OAc from the resulting Pt(IV) center is discussed.

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EXPERIMENTAL SECTION

General Remarks. 1H and 13C NMR spectra were recorded on a Bruker Avance DPX 400 or Varian 500 or 600 MHz spectrometer and referenced to external TMS (0.00 ppm). All chemical shifts and coupling constants are given in ppm and Hz, respectively. UV−vis spectra and kinetics were recorded on a Cary-60 spectrophotometer equipped with a temperature controller capable of holding four cells or a PerkinElmer Lambda 25 spectrophotometer with temperature control using an EYELA NCB-3100 constant-temperature bath. [PtMe(ppy)(SMe2)] (1a) and [PtMe(bhq)(SMe2)] (1b) were prepared as reported previously.26 Synthesis of Cycloplatinated(IV) Complexes. The NMR labeling for the ligands is shown in Scheme 3 to clarify chemical shift assignments. [PtMe(ppy)(OAc)2(H2O)] (2a). [PtMe(ppy)(SMe2)] (40 mg, 0.09 mmol), was dissolved in CH2Cl2 (15 mL), and PhI(OAc)2 (30 mg, 0.09 mmol) was added to the solution. The mixture was stirred at room temperature over 8 h. The solvent was evaporated from the Received: October 6, 2017

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DOI: 10.1021/acs.organomet.7b00745 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

resulting pale yellow solution, and the residue was washed with ether and hexane. Yield: 88%. Mp: 244 °C dec. Anal. Calcd for C16H19O5NPt: C, 38.4; H, 3.8; N, 2.8. Found: C, 38.2; H, 3.7; N, 3.0. 1H NMR in CDCl3: δ 1.75 (s, 6H, CH3 of OAc ligand), 2.20 (s, 2 J(PtH) = 68.6 Hz, 3H, MePt), 7.20−7.30 (m, 3H), 7.49 (t, 3J(HH) = 7.2 Hz, 1H, H5′), 7.70 (dd, 3J(H5H6) = 5.2 Hz, 3J(H5H4) = 8.2, 1H, H5), 7.97 (t, 3J(HH) = 8.2 Hz, 1H, H4), 8.05 (d, 1H, 3J(H3H4) = 8.3 Hz, H3), 9.08 (d, 3J(H6H5) = 5.6 Hz, 1H, H6). 13C NMR: δ 1.77 (s, 1 J(PtC) = 609.8 Hz, MePt), 23.38 (s, 2C, 3J(PtC) = 46.6, acetate ligand), 119.78, 123.44, 124.85, 126.30, 127.82, 130.45, 130.58, 139.62, 141.12, 147.52, 159.72 (s, 11C, aromatic carbons), 182.30 (s, 2J(PtC) = 43.1, 2C, OAc ligand). [PtMe(bhq)(OAc)2(H2O)] (2b). This compound was made similarly using [PtMe(bhq)(SMe2)] (45 mg, 1 mmol) and PhI(OAc)2 (32 mg, 1 mmol). Yield: 80%. Mp: 244 °C dec. Anal. Calcd for C18H19O5NPt: C, 41.2; H, 3.6; N, 2.6. Found: C, 41.1; H, 3.8; N, 2.5. 1H NMR data in CDCl3: δ 1.68 (s, 6H, CH3 of acetate ligand), 2.41 (s, 2J(PtH) = 68.1 Hz, 3H, MePt), 7.54 (td, 3J(HH) = 7.56 Hz, 4J(HH) = 2.00 Hz, 1H, H10), 7.57 (m, 3J(HH) = 7.53 Hz, 1H, H9), 7.71 (dd, 3J(H11H10) = 7.60 Hz, 4J(H11H9) = 1.11 Hz, 1H, H11), 7.75 (d, 3J(H6H7) = 8.7 Hz, 1H, H6), 7.79 (dd, 3J(H3H4) = 7.9 Hz, 3J(H3H2) = 5.38 Hz, 1H, H3), 7.86 (d, 1H, 3J(H7H6) = 8.8 Hz, H7), 8.34 (dd, 1H, 3J(H4H3) = 8.28 Hz, 4J(H4H2) = 1.30 Hz, H4), 9.29 (dd, 1H, 3J(H2H3) = 5.14 Hz, J(H2H4) = 1.23 Hz, H2). 13C NMR: δ 1.87 (s, 1C, 1J(PtC) = 578.09 Hz, MePt), 23.37 (s, 2C, 3J(PtC) = 43.80, Me of acetate ligand), 122.43, 124.01, 124.98, 127.49, 127.95, 129.13, 129.24, 130.27, 134.20, 137.05, 138.08, 146.94, 149.71 (s, 13C, aromatic carbons), 182.24 (s, 2 J(PtC) = 42.77, 2C, acetate ligands). General Procedure for Investigation of the C−O Reductive Elimination Process. Reductive elimination reactions were carried out in different solvents such as CDCl3, CD3CN, and C6D6 at 60 °C. The formation of methyl acetate (MeOAc) was monitored by 1H NMR spectroscopy. Crystals suitable for X-ray diffraction could not be obtained for Pt(II) complexes 3 because they decompose and convert to platinum black under vacuum. Complexes 3 were trapped by SMe2 to form the complexes [Pt(C∧N)(OAc)(SMe2)], which were easily characterized by NMR spectroscopy. [Pt(ppy)(OAc)(H2O)] (3a). 1H NMR in C6D6: δ 2.29 (s, 3H, CH3 of acetate ligand), 5.81 (dd, 1H, 2J(HH) = 7.89 Hz, 2J(HH) = 12.0 Hz, 3 J(PtH) = 24.0 Hz), 5.90−8.20 (br, aromatic protons), 8.25 (d, 2 J(HH) = 5.80 Hz, 3J(PtH) = 41.2 Hz, 1H). [Pt(bhq)(OAc)(H2O)] (3b). 1H NMR in C6D6: δ 2.39 (s, 3H, CH3 of acetate ligand), 5.83 (dd, 1H, 2J(HH) = 5.3 Hz, 2J(HH) = 7.78 Hz, 3 J(PtH) = 18.8 Hz), 5.90−8.20 (br, aromatic protons), 8.25 (d, 2 J(HH) = 5.6 Hz, 3J(PtH) = 45.20 Hz, 1H). [Pt(ppy)(OAc)(SMe2)] (4a). 1H NMR in CDCl3: δ 1.98 (s, methyl of OAc group, 3H), 2.75 (s, 3J(PtH) = 52.7 Hz, SMe2 trans to N, 6H), 9.51 (d, J(HH) = 5.9 Hz, 3J(PtH) = 36.1 Hz, the C−H proton adjacent to N of ppy, 1H), other aromatic protons of ppy ligand 7.51− 8.42. [Pt(bhq)(OAc)(SMe2)] (4b). 1H NMR in CDCl3: δ 2.10 (s, methyl of OAc group, 3H), 2.86 (s, 3J(PtH) = 53.8 Hz, SMe2 trans to N, 6H), 9.86 (d, J(HH) = 7.0 Hz, 3J(PtH) = 38.7 Hz, the C−H proton adjacent to N of bhq, 1H), other aromatic protons of bhq ligand 7.51− 8.42. Kinetic Study. For oxidative addition reactions, a solution of complex 1 in CH2Cl2 (3 mL, 3.0 × 10−4 M) in a cuvette was thermostated at 25 °C and PhI(OAc)2 was added using a microsyringe under second-order 1:1 stoichiometric conditions ([PhI(OAc)2]0 = [Pt]0). After rapid stirring, the absorbance at the corresponding wavelength (360 nm for complex 1a and 400 nm for complex 1b) was monitored over time. The absorbance−time profiles were analyzed using the second-order equation (eq 1). For each temperature, at least three kinetic experiments were run and the mean value was taken as the second-order rate constant. The data at other temperatures were obtained similarly, and activation parameters were obtained from the Eyring equation (eq 2).

Scheme 1. Examples of C−O Bond Formation via Reductive Elimination from Palladium(IV) Complexesa

a

Reprinted (adapted or reprinted in part) with permission from the corresponding references. Copyright 2009 and 2015 American Chemical Society and 2014 Royal Chemical Society.

Scheme 2. Examples of C−O Bond Formation via Reductive Elimination from Platinum(IV) Complexesa

a

Reprinted (adapted or reprinted in part) with permission from the corresponding references. Copyright 1999, 2004, and 2007 American Chemical Society.

Scheme 3. NMR Labeling of Complexes 2

B


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Organometallics Scheme 4. Reactions Studied in This Work

Figure 1. 1H NMR of complex 2b in CDCl3. The inset shows HH-COSY NMR in the aromatic region.

Abst = Abs∞ +

(Abs0 − Abs∞) 1 + [Pt]0 × k 2 × t

⎛ k ⎞ ΔS ⧧ ⎛k ⎞ ΔH ⧧ ln⎜ 2 ⎟ = ln⎜ B ⎟ + − ⎝T ⎠ ⎝h⎠ R RT

Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC 1568574. Computational Details. Gaussian 09 was used29 to fully optimize all the structures at the B3LYP level of density functional theory. The effective core potential of Hay and Wadt with a double-ξ valence basis set (LANL2DZ) was chosen to describe Pt and I.30 The 6-31G(d) basis set was used for all other atoms. A polarization function (ξf = 0.993 for Pt and ξd = 0.289 for I) was also added. Single-point energy calculations were performed at the B3LYP level using the def2-QZVP basis set on Pt and I along with the corresponding ECP and the 6311+G(2d,p) basis set on other atoms. Frequency calculations were carried out at the same level of theory to identify whether the calculated stationary point is a minimum (zero imaginary frequency) or a transition-state structure (one imaginary frequency). All data were calculated at standard temperature and pressure (298.15 K and 1.0 atm). The solvation energies were calculated by the CPCM model in CHCl3 and CH2Cl2 solvent for reductive elimination and oxidative addition, respectively, which reflect the operating experimental conditions. The ionic species were considered as an ion pair.

(1)

(2)

The C−O reductive elimination process was monitored by 1H NMR spectroscopy as follows: a small sample (10 mg) of complex 2 was dissolved in 0.75 mL of deuterated solvent in a sealed NMR tube and heated to the desired temperature. Triphenylmethane (Ph3CH) was used as an integral reference. NMR spectra were recorded several times over about 24 h until the reaction was gradually completed. A kinetic study of the reductive elimination reaction was also monitored by UV−vis spectroscopy. In this case, a solution of complex 2b in solvent (3 mL, 3.0 × 10−4 M) in a cuvette was thermostated at 60 °C and the absorbance changes at the corresponding wavelength were monitored over time. These UV−vis experiments for complex 2b were repeated in the presence of excess water and N(n-Bu)4OAc to study the effect of these species. The concentrations used for these experiments were 0.018−0.18, (3.3−8.3) × 10−3 and 3.0 × 10−4 M for water, N(n-Bu)4OAc, and 2b, respectively. Crystallographic Data. Single-crystal X-ray diffraction data of complex 2b were collected on a Bruker KAPPA APEX II diffractometer equipped with an APEX II CCD detector using a TRIUMPH monochromator with a Mo Kα X-ray source (λ = 0.71073 Å). The crystal was mounted on a cryoloop under Paratone-N oil and kept under nitrogen. Absorption correction of the data was carried out using the multiscan method SADABS.27 Subsequent calculations were carried out using SHELXTL.28 Structure determination was done using intrinsic methods. Structure solution, refinement, and creation of publication data were performed using SHELXTL. Crystallographic information is presented in Table S1 in the Supporting Information.

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RESULTS AND DISCUSSION The reaction studied in this work is divided into two processes as illustrated in Scheme 4; (a) oxidative addition involving Pt− O bond formation and (b) C−O bond formation via reductive elimination from platinum(IV) to give MeOAc. Oxidative Addition Process. Synthesis and Characterization of Cycloplatinated(IV) Acetate. As depicted in Scheme 4, the reaction of complexes [PtMe(SMe2)(C∧N)] (1) with PhI(OAc)2 in CH2Cl2 cleanly yields the air-stable complexes [PtMe(OAc)2(C∧N)(H2O)] (2) by oxidative addition of two acetate groups onto the Pt(II) center in a trans manner. The C


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carboxylate π bond is rather delocalized. The ESI mass spectrum of 2b (positive ion mode in Figure 3) shows a base peak at m/z 509, assignable to the species [Pt(OAc)2(bhq)(H2O)]+, which confirms the formation of cyclometalated complex 2b containing two acetate groups.

resulting Pt(IV) center probably absorbs H2O (from moisture) to replace the SMe2 ligand. The reaction in dry solvent under an inert atmosphere forms [PtMe(OAc)2(C∧N)(SMe2)] complexes, which are not stable over prolonged times. When the NMR tube samples of these complexes are opened in air or a solid sample is dissolved in an NMR solvent that is not dried, the SMe2 complexes are converted to aqua complexes 2, as confirmed by NMR spectroscopy. The Pt(IV) complexes 2 are pale yellow solids and are stable in common organic solvents such as chloroform, benzene, and CH2Cl2 for several days at room temperature. This stability enabled their full characterization by multinuclear (1H and 13C) NMR spectroscopy, mass spectrometry (ESI-MS), and X-ray crystal structure determination. As an example, the 1H NMR spectrum of 2b (Figure 1) indicated methylplatinum groups at δ 2.45, which was coupled with Pt to give satellites with 2J(PtH) = 68.1 Hz, confirming that the methyl is directly bonded to platinum(IV). The two methyls of acetate appeared as a singlet at δ 1.75. The equivalency of the two acetate CH3 groups suggests that the acetate ligands must be trans to each other. The spectroscopic data are consistent with two acetate ligands bonded to 1b, as the integration ratio of Me and OAc groups is 1:2. The inset of Figure 1 shows the HH-COSY NMR of 2b, which was used to assign the aromatic hydrogens. The solid-state structure of complex 2b was further confirmed crystallographically and is shown in Figure 2. The

Figure 3. Expanded (left) and simulated (right) isotopic pattern ESI mass spectrum of the cation [Pt(bhq)(OAc)2(H2O)]+ fragment of complex 2b at m/z 509, showing the expected intensity due to isotopic distribution.

NMR Monitoring of Formation of Cycloplatinated(IV) Complexes. The reaction of [PtMe(bhq)(SMe2)] (1b) with PhI(OAc)2 was also monitored by 1H NMR spectroscopy. Attempts were made to detect reaction intermediates, but no intermediates could be detected under our reaction conditions. The 1H NMR spectra (aromatic region) of a 1:1 reaction mixture of 1b with PhI(OAc)2 in CD2Cl2 at 23 °C is shown in Figure 4. Comparison of the spectra in Figure 4 shows that, after addition of PhI(OAc)2 to Pt(II) complex 1b, the signals due to starting complex 1b gradually disappeared and those for the complex 2b and PhI emerged. A study of the reaction at low temperature (from −40 °C) followed by warming to room temperature gave similar NMR spectra, showing no detectable intermediates. Kinetic Study of Oxidative Addition Process. Complexes 1 contain an MLCT band in the visible region that could be used to monitor their reactions with PhI(OAc)2 by using UV−vis spectroscopy. The kinetics of oxidative addition reactions were studied in dichloromethane solution. The reactions were conveniently followed under second-order conditions by using equimolar concentrations of 1a or 1b and PhI(OAc)2. A typical set of UV−visible spectra taken during a kinetic run is shown in Figure 5. Under these conditions, the reactions followed second-order kinetics, first order in each reactant, the corresponding Pt complex, and PhI(OAc)2. The resulting rate constants are given in Table 1. Eyring plots for the addition of PhI(OAc)2 to Pt(II) complexes are shown in Figure 6, from which the activation parameters were calculated (Table 1). The negative entropies of activation are typical values for an associative mechanism.31 On the basis of kinetic data, a possible mechanism is shown in Scheme 5. The reaction is initiated by nucleophilic attack of the Pt center onto the iodide atom of PhI(OAc)2, which can be considered as a ligand exchange process at I. The next step involves elimination of PhI to give a five-coordinate intermediate (see the next paragraph for DFT calculations). The supposed intermediate can absorb an acetate ion to give

Figure 2. Structure of [PtMe(bhq)(OAc)2(H2O)] (2b). Selected geometrical parameters (Å, deg): Pt1−O1 2.022(2); Pt1−O3 2.020(2); Pt1−O5 2.174(8); Pt1−C11 2.015(3); Pt1−C18 2.052(17); Pt1(6)−N1 2.161(15); O1−Pt1−O3 173.4(4); O3− Pt1−O5 90.4(2); O1−Pt1−O5 95.3(8); N1−Pt1−C11 82.0(1); C11−Pt1−C18 84.2(1); O1−Pt1−N1 88.31(9).

crystal was grown by slow diffusion of hexane into a CH2Cl2 solution of 2b in ambient air and contains a coordinated water molecule. It confirms the characterization of 2b as an octahedral platinum(IV) complex as a result of trans oxidative addition of acetate groups of PhI(OAc)2. The Pt(IV)−C and Pt−O bond lengths are comparable to those in other reported structures with similar trans ligands.8 The two Pt−O distances of acetate ligands are approximately equal (2.022(2) and 2.020(2) Å). The formal carbon−oxygen single and double bonds in the acetate fragment have small differences in bond lengths (1.298(4) and 1.237(4) Å), suggesting that the D


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Figure 4. Monitoring the platinum(II)/platinum(IV) transformation process by 1H NMR spectroscopy (aromatic region) in CD2Cl2 at 23 °C. The first spectrum is obtained from the starting complex [PtMe(bhq)(SMe2)] (1b), and the next spectra are obtained after addition of PhI(OAc)2 to the starting complex 1b. The peaks of oxidant (PhI(OAc)2, #) and consumed oxidant (PhI, *) are assigned.

Figure 6. Eyring plot for the reaction of [Pt(C∧N)(Me)(SMe2)] (1) with PhI(OAc)2 in CH2Cl2.

To shed some light on the suggested mechanism (depicted in Scheme 5), DFT calculations were carried out on the precursor complex 1b, final product 2b, the potential transition state, and intermediates. The conductor-like polarizable continuum model (CPCM) was used to model general solvation effects by CH2Cl2. The reaction is initiated by nucleophilic attack by the 5dz2 HOMO of the Pt(II) center of 1b onto the iodide atom of PhI(OAc)2. Substitution of an acetate group by the Pt center, followed by simultaneous partial removal of an acetate ion, results in formation of the transition state TS (Figure 7). The free energy barrier for this step was calculated to be 16.1 kcal mol−1 in CH2Cl2 at 298.15 K, which is in excellent agreement with the experimental value of 16.4 kcal mol−1. In TS, the bond angle Pt−I−Ph (96.7°) is close to 90°, showing a vertical arrangement with the iodide atom. During the formation of TS, the most significant changes in

Figure 5. Changes in the UV−visible spectrum during the reaction of [PtMe(ppy)(SMe2)] (1a; 3 mL of 3 × 10−4 M solution) with PhI(OAc)2, under second-order 1:1 stoichiometric conditions, in CH2Cl2 at 25 °C. The inset shows the variation of absorbance at 360 nm (the MLCT band) over time.

the final cyclometalated Pt(IV) complex.32 A similar mechanism was suggested for the reaction of PhI(OAc)2 with nucleophiles.33 As reported in Table 1, the rate constant for oxidative addition process is almost 2 times faster for complex 1b (having bhq ligand) than for complex 1a (with ppy as chelating ligand), which could be attributed to the more extended π conjugation system in the bhq complex in comparison to that for ppy. It should be noted that the cyclometalated Pt(II) complexes are among the most active of the noble-metal complexes in oxidative addition reactions.31,34

Table 1. Rate Constantsa and Activation Parameters for the Reaction of Complexes [PtMe(C∧N)(SMe2)] (1) with PhI(OAc)2 in Dichloromethane k2 (L mol−1 s−1)

a

complex

15 °C

20 °C

25 °C

30 °C

35 °C

ΔH‡ (kcal mol−1)

ΔS⧧ (cal mol−1 K−1)

ΔG⧧ (kcal mol−1)

1a 1b

1.23 2.97

1.84 4.03

2.51 5.12

3.88 6.83

5.14 8.68

12.3 ± 0.3 8.8 ± 0.1

−15 ± 1 −26 ± 5

16.8 ± 0.3 16.4 ± 0.3

Estimated errors are ±6%. E


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Organometallics Scheme 5. Proposed Mechanism for Reaction of Pt(II) Complexes with PhI(OAc)2

an acetate ion to the platinum center gives intermediate IM2. The latter intermediate can absorb another acetate ion released in an earlier step to form IM3, which then proceeds to cyclometalated Pt(IV) complex 2b by replacement of SMe2 by H2O. Product 2b has an octahedral geometry around the Pt(IV) center (see Scheme 5 and Figure 7) with Pt−OAc bond lengths of 2.062 Å, in excellent agreement with the experimental X-ray value of 2.022 Å. Reductive Elimination Process. Carbon−Oxygen Bond Formation. Complexes 2 are prone to lose methyl acetate by heating in different organic solvents through a carbon−oxygen reductive elimination process (see Scheme 6). The Pt(II) products obtained from reductive elimination were characterized by NMR and trapped by addition of SMe2. Monitoring of MeOAc Formation. The thermolyses of complexes 2 at 60 °C were studied in different organic solvents such as C6D6, acetone-d6, CD3CN, and CDCl3. During heating C(sp3)−O bond forming reductive elimination was observed, forming methyl acetate along with [Pt(C∧N)(OAc)(H2O)] (3). No competitive reductive elimination of the carbon− carbon bond was observed. To investigate the detail of the mechanism of C−O bond reductive elimination, thermolyses of complexes 2 were monitored in CD3CN in a sealed NMR tube at 60 °C (see Figure 8). As the time was passing at this temperature, the signals due to starting complex 2b at 1.57 and 2.36 ppm gradually disappeared and those for methyl acetate at 2.01 and 3.63 ppm appeared. The product 3b is not stable at high temperature for a long time and decomposes. Kinetic Study of C−O Bond Formation. The rate constants for the C−O reductive elimination product were obtained using 1 H NMR spectroscopy by following the signal for MeOAc formation in different solvents. Triphenylmethane (Ph3CH) was used as an integral reference. The data are collected in Table 2. A first-order kinetic plot of thermolysis of 2b, as an

Figure 7. Free energy diagram and calculated structures of starting material, transition state, intermediates, and product of oxidative addition (2b) in CH2Cl2 solvent. The H atoms are omitted for clarity.

bond distances are observed for I−Ac and Pt−I distances. The I−OAc bond increases from 2.182 Å in PhI(OAc)2 to 2.444 Å in TS, whereas the Pt−I distance decreases from being far apart in the reactant to 2.975 Å in TS. This confirms that oxidative addition of PhI(OAc)2 to 1b involves simultaneous cleavage of the I−OAc bond and formation of a Pt−I bond, which is followed by formation of the cationic intermediate IM1 by completely breaking the I−OAc bond and forming Pt−I. The intermediate IM1 has a square-pyramidal geometry around Pt, with the incoming PhI(OAc) group occupying the apical position and the acetate ion being in the outer sphere of IM1. The elimination of PhI from IM1 followed by coordination of

Scheme 6. Carbon−Oxygen Bond Formation by Reductive Elimination from Pt(IV) Complexes

F


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Figure 8. 1H NMR spectra (aliphatic region) of thermolysis of complex 2b at 60 °C in CD3CN. The time interval is 30 min. The residual peak of CD3CN appeared at 1.94 ppm. Small traces of impurities around 2.10−2.15 ppm are due to the water of the deuterated solvent and Pt complex after decomposition. Asterisks show Pt satellites.

Table 2. Rate Constantsa and Yieldsb for Thermolysis of Complexes 2a,b in Different Organic Solvents at 60 °C 2a

was used to determine the rate of the reaction as a function of Pt(IV) concentration over a concentration range of (1.5−9.0) × 10−4 M (see Figure 9C). Under our reaction conditions, the C−O reductive elimination from cycloplatinated(IV) complex 2b showed a first-order dependence on Pt complex (1.06 ± 0.06). The rate constants for carbon−oxygen bond reductive elimination were obtained and are reported in Table 2. The obtained rate constants from UV−vis studies are in good agreement with those obtained from NMR experiments. The C−O reductive elimination reaction of complex 2b was also investigated at different excess amounts of N(n-Bu)4OAc (over a concentration range of (3.3−8.3) × 10−3 M) in comparison to Pt(IV) complex 2b under pseudo-first-order conditions. Under our reaction conditions, the C−O reductive elimination from cycloplatinated(IV) complex 2b showed a zero-order dependence on N(n-Bu)4OAc (see Figure 10A). As is clear from Figures 9D and 10A, the results indicate that the reaction obeys the simple first-order rate law rate = k[Pt(IV)]. The same experiment was also done to investigate the effect of added water on the rate of C−O reductive elimination. It was found that water has no significant effect on the rate and the order of the reaction in water is zero (see Figure 10B). Theoretical Mechanistic Study of C−O Coupling Reactions. Because of the carbon−heteroatom bond forming reductive elimination from Pt(IV), commonly observed to proceed through a dissociative mechanism,9,35 and the key fivecoordinate intermediates accessed through ligand loss from the ground-state octahedral structure, three different mechanisms for C−O reductive elimination can be proposed (Scheme 7). The first possible mechanism (mechanism A) is thermally induced dissociation of an acetate ligand to form a cationic fivecoordinate cycloplatinated(IV) complex, followed by nucleophilic attack of dissociated acetate to the Me group carbon to give methyl acetate and Pt(II) product. Alternatively, the remaining coordinated acetate can concertedly form a bond to the Me group and produce methyl acetate. The second mechanism (mechanism B), or the concerted bond formation accompanied by chelate dissociation, involves direct C−O reductive elimination bond formation from a five-coordinate platinum(IV) complex. The third possible mechanism (mech-

2b

solvent

dielectric constant

105k (s−1)

yield (%)

C6D6 CDCl3 acetone-d6 CD3CN

2.27 4.81 20.7 37.5

1.8 3.1 2.1 3.2

68 77 16 87

105k (s−1) 0.9 1.6 1.2 1.7

(1.3) (1.1) (2.0) (3.2)

yield (%) 79 74 20 88

a Estimated error is ±10%, using NMR (or UV−vis) spectroscopy. bBy NMR on the basis of MeOAc formation.

example, is shown in Figure 9A. The rate constants of MeOAc formation from 2a are generally higher than those for 2b, showing that complex 2a is more reactive toward C−O reductive elimination than 2b. Dielectric constants of solvents used for reductive elimination process are also included in Table 2. The solvent polarity has no effect on the rate of methyl acetate formation. For example, the rate of reaction in the more polar solvent acetone is almost identical with that obtained in the less polar solvent benzene. The yield of MeOAc, reported in Table 2, shows a range of about 16% in acetone to 88% in acetonitrile. Although the only organic product obtained during the reductive elimination process is MeOAc (see the NMR experiment shown in Figure 8), Pt complexes did decompose in solution when they were heated for a long time (black Pt metal was usually observed). This may be a reason for the low yield in acetone and the less than quantitative yield of MeOAc in the other solvents. The carbon−oxygen bond reductive elimination from complex 2b at 60 °C has also been monitored by UV−vis spectroscopy. During this process, the cycloplatinated(IV) complex 2b is converted to complex 3b by losing methyl acetate. Using UV−vis spectroscopy a known concentration of 2b is used and the appearance of the MLCT band at the corresponding λmax (400 nm for CH3CN, CHCl3, and benzene and 380 nm for acetone) is used to monitor the C−O reductive elimination reaction. The change in the spectrum during a typical run is shown in Figure 9B. The method of initial rates G


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Organometallics

Figure 9. (A) First-order kinetic plot of thermolysis of 2b, in CD3CN at 60 °C using 1H NMR spectroscopy. (B) Changes in the UV−visible spectrum during thermolysis of [Pt(bhq)(Me)(OAc)2(H2O)] (2b; 3 mL, 3 × 10−4 M solution) in CH3CN at 60 °C. (C) First-order kinetic plot of the thermolysis of 2b showing first-order dependence of rate on [2b]. (D) Plot of initial rate versus concentration of 2b fit to y = m1*m0∧m2 where y = initial rate, m0 = [2b], m1 = rate constant (k), and m2 = order in Pt(IV).

Figure 10. Plot of initial rate versus concentration of N(n-Bu)4OAc (A) and H2O (B) showing zero-order dependence of rate on free ion acetate and H2O.

anism C) initiates with predissociation of H2O ligand followed by internal unimolecular C−O coupling bond formation. To find the most suitable mechanism for C−O bond forming reductive elimination from the cycloplatinated(IV) acetate complex, the proposed mechanisms were theoretically investigated using density functional theory calculations to gain more insight into the mechanism, as well as to determine the geometries of the transition states, intermediates, and energy barriers of each proposed mechanism (see Figure 11). The optimized geometry for complex 2b is in excellent agreement with X-ray crystallographic data, and our calculation successfully reproduces the bond angles and bond distances within precisions of 1.2° and 0.05 Å, respectively. This indicates the

reliability of the selected theoretical method for this study, and therefore we employ it for our mechanistic investigation. Mechanism A. According to this mechanism, the cycloplatinated(IV) complex 2b with coordination number 6 first loses an acetate ion to give a cationic five-coordinate (IMa) as the reaction intermediate. The free energy barrier for this step is calculated to be 23.6 kcal mol−1 in CHCl3 solution. It should be noted that no significant structural geometry change was found in intermediate IMa. The corresponding intermediate in which coordinated acetate ion acts as a bidentate ligand is found to be less stable in comparison to the fivecoordinate intermediate IMa due to ring strain present in the four-membered ring of this structure with an O−Pt−O angle of H


Article

Organometallics Scheme 7. Proposed Mechanisms for Carbon−Oxygen Reductive Elimination from Cycloplatinated(IV) Complex 2ba

a

The main mechanism is pathway 2b → IMa → IMa1 → TSa2 → 3b.

Figure 11. Calculated geometries of the species involved in the C−O reductive elimination process from complex 2b and the energy barriers of each proposed mechanisms (A in black, B in red, and C in blue). Anionic components of ionic species are omitted, and only DFT-optimized structures of Pt complexes are shown for more clarity.

58.6°. Although the intermediate IMa could undergo a carbon−oxygen bond formation to give transition state TSa (not shown in Figure 11; see Scheme 7), this transition state cannot be located and instead TSa1 is found. The intermediate

IMa then isomerizes to IMa1, in which the methyl ligand occupies an axial position. Formation of such intermediates with the axial methyl group is well-established in a number of C−O coupling reactions at Pt(IV).10,25 There are two I


Article

Organometallics possibilities from this intermediate. The first is an intramolecular C−O reductive elimination of the remaining coordinated acetate ion and Me group (transition state TSa1) to give methyl acetate. The energy barrier for this step is calculated as 69.4 kcal mol−1, which is higher than that found for nucleophilic attack of dissociated acetate onto the Me group. Therefore, this path (IMa1 → TSa1 → 3b + MeOAc) is ruled out. The second possible pathway for IMa1 is nucleophilic attack of dissociated acetate to the Me group carbon to give methyl acetate and Pt(II) product through TSa2, which includes a Pt−CMe−O bond angle of 157.0°. During the formation of TSa2, the most significant changes in bond distances are observed for Pt−CMe and C−O bonds. The Pt−CMe length increases from 2.054 Å in IMa1 to 2.272 Å in transition structure TSa2, whereas the O···CMe distance decreases from being far apart to 2.185 Å. The accuracy of transition state TSa2 is confirmed by the observation of an imaginary frequency (−347 cm−1), which corresponds to a Pt− C−O stretching vibrational mode in which the Pt−O bond ruptures and the C−O bond forms. TSa2 has a free energy of +24.1 kcal mol−1 in CHCl3 in comparison to starting complex 2b. The formation of the transition state TSa2 is followed by complete breaking of the Pt−CMe bond and formation of Me− OAc. The overall free energy change of this C−O bond forming reductive elimination process is −21.9 kcal mol−1. Mechanism B. In this mechanism, the starting cyclometalated Pt(IV) complex 2b, as the starting material, undergoes a reductive elimination process. Here, the reactant concurrently releases a pyridyl arm of the bhq ligand to form the five-coordinate Pt(IV) transition sate TSb. In TSb, the O− Pt−CMe bond angle decreases from 90.4° (in 2b) to 49.2°, while the Pt−N bond increases from 2.237 Å (in 2b) to 2.765 Å. The Pt−C and Pt−O bond distances also increase from 2.067 to 2.549 Å and from 2.062 to 2.086 Å, respectively. The free energy barrier for this step is +35.7 kcal mol−1 in CHCl3 solution. The direct product of this mechanism is the squareplanar cycloplatinated(II) complex 3b with an overall free energy of −21.9 kcal mol−1 in comparison to reactant 2b. This mechanism is ruled out due to the high energy barrier calculated for this pathway in comparison to those calculated for Path A. Mechanism C. The third possible C−O coupling mechanism, C, involves preliminary water dissociation to produce the five-coordinate neutral intermediate IMc. Direct C−O coupling reductive elimination from this intermediate is ruled out due to the high energy barrier of 63.9 kcal mol−1 calculated for TSc (see Scheme 7 and Figure 11). Isomerization of IMc to IMc3 is also not possible because the latter is located at higher energy in comparison to IMc. Therefore, the formation of the intermediate IMc is followed by isomerization to intermediate IMc1 and then intermediate IMc2, in which the Me group is located in an axial position of Pt(IV) complex. Although transition state TSc1 (shown in Scheme 7) from IMc1 could not be located, the reaction may proceed through IMc1 and then transition state TSc2, where Me ligand in an axial position can make a bond with the OAc group trans to C of the bhq ligand. Complete Pt−O and Pt−C bond breaking and C−O bond forming results in the neutral four-coordinate [Pt(bhq)(OAc)(H2O)] product 3b, having a square-planar geometry, with the water group located in a position trans to the carbon atom of the cyclometalated bhq ligand. The energy barrier for this mechanism is calculated as +36.9 kcal mol−1 in CHCl3.

In all of these proposed mechanisms studied above using DFT calculations, the rate-determining step is C−O bond forming reductive elimination. The free energy barriers for mechanisms A−C (through transition states TSa2, TSb, and TSc2) are +24.1, +35.7, and +36.9 kcal mol−1, respectively. These values suggest that the favored mechanism should be mechanism A with the lowest energy barrier. On the other hand, using transition state theory with the transmission coefficient equal to 1, the first-order rate constants at a temperature of 60 °C for mechanisms A−C are calculated as 1.2 × 10−5, 3.7 × 10−14, and 5.1 × 10−15 s−1, respectively. If we compare these calculated rate constants with our measured experimental value of 1.6 × 10−5 or 1.1 × 10−5 s−1 (from NMR and UV−vis measurements, respectively) in CHCl3, the best theoretical value in agreement with experimental finding is 1.2 × 10−5 s−1, which provides further support for the pathway 2b → IMa → IMa1 → TSa2 → 3b.

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CONCLUSION In the presented work, first the synthesis of two stable cycloplatinated(IV) complexes, [PtMe(C∧N)(OAc)2(H2O)], is described. PhI(OAc)2 is used as the acetate source to make Pt− O bonds through an oxidative addition process. Kinetic studies show that the bhq complex reacts more rapidly with PhI(OAc)2 than its ppy analogue. These cyclometalated Pt(IV) complexes are used to study the kinetics and mechanism of C−O bond forming reductive elimination. On the basis of kinetics and DFT investigations, we propose that C−O reductive elimination proceeds via acetate dissociation to generate a cationic five-coordinate intermediate, followed by nucleophilic attack of acetate to give methyl acetate and Pt(II) product. On the basis of this mechanism, simplified in eqs 3−5, the rate law can be derived as eq 6. k1

2b HooI IMa + OAc−

(3)

k −1 fast

(4)

IMa ⎯→ ⎯ IMa1 k2

IMa1 + OAc− → 3b + MeOAc

rate =

(5)

k1k 2[OAc−][2b] k k [2b] = 1 2 k −1[OAc−] + k 2[OAc−] k −1 + k 2

(6)

The rate equation shown in eq 6 is consistent with the experimental data, as it predicts first order in [2b] (see Figure 9) and zero order in [OAc−] (see Figure 10A): i.e., rate = k[2b] where k = k1k2/(k−1 + k2). The same mechanism has been previously suggested for C−O bond reductive elimination from Pt(IV) complexes. For example, Williams et al.12 suggested the same mechanism in thermolysis of platinum(IV) hydroxide complexes which occurs from dissociation of hydroxide ion followed by nucleophilic attack on the Pt(IV) methyl group. Bercaw and co-workers have also shown that a C−O reductive elimination from their Pt(IV) system proceeds via an SN2 reaction.3 Similarly, the same mechanism for other organometallic systems including Pd(IV) (see Scheme 1) and Rh(III) had been proposed.36 The calculated rate constant using DFT for the proposed mechanism (mechanism A through transition state TSa2) is in agreement with the experimentally determined rate constant. It is noteworthy that the acetate leaving group attacks the Me group of IMa1 in which the Me ligand is located at an axial position. As a result, a cationic five-coordinate Pt J


Organometallics

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intermediate that is susceptible to C−O reductive elimination emerges. One more piece of evidence for this mechanism is the lack of solvent effect. As shown in Table 2, the rate of C−O reductive elimination from complex 2a in the more polar solvent acetonitrile (ε = 37.5) is almost identical with that obtained in the less polar solvent chloroform (ε = 3.1); therefore, the rate of C−O coupling is almost independent of solvent polarity, which is consistent with the suggested mechanism through acetate ion dissociation.8 To confirm that the same product is formed in the presence of excess N(n-Bu)4OAc, complex 2b was heated at 60 °C in CD3CN using [2b] = 0.02 M and [N(n-Bu)4OAc] = 0.2 M and the formation of MeOAc was confirmed by 1H NMR with lower yield in comparison to that observed in the absence of acetate ion. On the basis of the first step of the suggested mechanism, which is an equilibrium (eq 3), it can be expected that the formation of IMa could be suppressed by the addition of OAc ion to the thermolyses of 2b. Although, on the basis of eq 5, the addition of acetate ion would be expected to increase the amount of product, an overall lower amount of product formation in the C−O reductive elimination could be explained on the basis of a larger influence of the free acetate ion on the equilibrium step.8

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00745. Crystallographic data, UV−vis spectra, and DFT data (PDF) Cartesian coordinates for the calculated structures (TXT) Accession Codes

CCDC 1568574 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for S.M.N.: [email protected]. *E-mail for M.M.A.-O.: [email protected]. ORCID

S. Masoud Nabavizadeh: 0000-0003-3976-7869 Fatemeh Niroomand Hosseini: 0000-0002-5856-8104 Mahdi M. Abu-Omar: 0000-0002-4412-1985 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.M.N. and F.N.H. wish to acknowledge Shiraz University and the Islamic Azad University, Shiraz Branch, for their sabbatical leave at University of California, Santa Barbara (UCSB). We acknowledge support from the Department of Chemistry and Biochemistry at UCSB and Shiraz University. The Center for Scientific Computing from the CNSI, MRL: NSF MRSEC (DMR-1121053) and NSF CNS-0960316 is also acknowledged. K


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