Reactivity comparison of five-and six-membered cyclometalated platinum(II) complexes in oxidative addition reactions

Abstract

The compound [PtMe(bzpy)(DMSO)] (1; bzpy = 2-benzylpyridinate) was synthesized by reaction of cis-[PtMe₂(DMSO)₂] with 1 equiv. of bzpyH under reflux conditions in toluene through C–H activation of the carbon–hydrogen bond in 2-benzylpyridine. Then, the complex [PtMe(bzpy)(PPh₃)], 2, was prepared by addition of PPh₃ to complex 1. Complex 2 undergoes oxidative addition with methyl iodide to give [PtMe₂I(bzpy)(PPh₃)], 3. NMR spectroscopy (¹H and ³¹P) and X-ray crystallography (supported by DFT calculations) clearly showed that the thermodynamic isomer product 3, with iodide trans to C of bzpy rather than the related kinetic isomer, 3, in which iodide is trans to methyl, is obtained. Mechanistic studies using UV-vis spectroscopy and DFT calculations indicate that the reaction occurs via a S_N2 mechanism. The kinetic study of the oxidative addition of methyl iodide to the non-planar, six-membered cyclometalated complex with that of the five-membered cyclometalated [PtMe(ppy)(PPh₃)], in which ppy = 2-phenylpyridinate, shows that the ring size of the chelating unit has a significant impact on the rate of the reaction.

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1. Introduction

Recently, there has been intense interest in transition metal-catalyzed carbon–hydrogen bond activation. In particular, C–H bond activation by organoplatinum complexes is currently an active topic of study in organometallic and organic chemistry,¹ as part of a search for useful catalysts and in order to gain a deeper understanding of the reactivity and mechanisms of such reactions.² Although the most well-studied examples of these compounds are five-membered ring complexes containing a N–Pt–C(phenyl) moiety,^3–5 six-membered cyclometalated complexes are relatively rare.^6–11 One candidate for obtaining a six-membered cyclometalated complex utilizes 2-benzylpyridine (bzpyH). This substrate can react with a transition metal complex to give an ortho-metalated compound with the help of nitrogen coordination to the metal (Fig. 1).

Fig. 1 Ligands giving rise to 5- and 6-membered cyclometalated platinum(II) complexes.

It has been well established that cyclometalated platinum(II) complexes that possess nitrogen and phosphorus donor ligands are highly reactive toward oxidative addition reactions.^12–21 For example, the trans or cis oxidative additions of an alkyl halide to cyclometalated platinum(II) complexes give the corresponding alkyl-cyclometalated platinum(IV) complex. In this regard, it is possible to incorporate many functional groups into the alkyl group for preparation of supramolecular materials and photoactive complexes.^22–24 This article describes the synthesis of a new, six-membered, cyclometalated platinum(II) complex [PtMe(bzpy)(PPh₃)], in which bzpy = 2-benzylpyridinate, and the study of the reactivity of this complex toward methyl iodide, which is a typical electrophile in oxidative addition reactions. In continuation of our interest in oxidative addition reactions of cyclometalated platinum(II) complexes, we test the rate of oxidative addition by methyl iodide towards [PtMe(bzpy)(PPh₃)] and compare these data to those reported previously for the corresponding reaction involving the five-membered analog, [PtMe(ppy)(PPh₃)] (ppy = 2-phenylpyridinate).

2. Experimental

The ¹H NMR spectra were recorded on a Bruker Avance DPX 250 MHz spectrometer and the ³¹P NMR spectra were recorded on a Bruker Avance DRX 400 MHz spectrometer. Chloroform-d was used as solvent. The chemical shifts and coupling constants are in ppm and Hz, respectively. The microanalyses were performed using a Thermofinigan Flash EA-1112 CHNSO rapid elemental analyzer and melting points were recorded on a Buchi 530 apparatus. Kinetic studies were carried out by using a Perkin-Elmer Lambda 25 spectrophotometer with temperature control using an EYELA NCB-3100 constant-temperature bath. Samples of 2-benzylpyridine (bzpyH), dimethylsulfoxide (DMSO), and triphenylphosphine (PPh₃) were purchased from commercial sources. The precursor complex [PtMe₂(μ-SMe₂)]₂ was prepared by the literature method.²⁵ Chloroform and dichloromethane were dried over CaCl₂. Sodium was added to toluene for a minimum of 24 hours to remove any water.

2.1. Preparation of complexes

2.1.1. [PtMe₂(DMSO)₂]. This compound had been previously synthesized by the reaction of [PtCl₂(DMSO)₂] with SnMe₄ at 80 °C for 24 h with yield of 55%.²⁶ We used an alternative method. Pure dimethylsulfoxide (70 μL, 1 mmol) was added to a solution of [PtMe₂(μ-SMe₂)]₂ (140 mg, 0.24 mmol) in dichloromethane at room temperature. The solution was stirred for 2 h and then the solvent was evaporated. The white powder was washed with diethyl ether and dried under vacuum to give the final product. Yield: 141 mg, 76%. mp = 131 °C (decomp.). Anal. calcd for C₆H₁₈O₂S₂Pt; C, 18.9; H, 4.7; S, 16.8. Found: C, 19.1; H, 4.8; S, 17.1. ¹H NMR in chloroform-d: δ 0.69 (s, ³J_PtH = 80.0 Hz, 6H, MePt), 3.14 (s, ³J_PtH = 12.5 Hz, 12H, Me of DMSO).

2.1.2. [PtMe(bzpy)(DMSO)], 1. A 63 μL portion (0.39 mmol) of 2-benzylpyridine was added to a solution of cis-[PtMe₂(DMSO)₂] (148 mg, 0.39 mmol) in toluene. The reaction mixture was heated under reflux for 1 h, and then cooled. Then the mixture was filtered and evaporated to dryness. The oily residue was extracted with diethyl ether and the solvent was removed under reduced pressure. The residue was dissolved in a minimum amount of dichloromethane and a cream colored powder was obtained after dropwise addition of n-hexane after 2 days at 0 °C. Yield: 44 mg, 25%. mp = 121 °C (decomp.). Anal. calcd for C₁₅H₁₉NOSPt; C, 39.5; H, 4.2; N, 3.1; S, 7.0. Found: C, 40.0; H, 4.0; N, 3.4; S, 7.4. ¹H NMR in chloroform-d: δ 0.67 (s, ²J_PtH = 85.0 Hz, 3H, MePt), 3.08 (s, ²J_PtH = 17.5 Hz, 3H, Me of DMSO ligand), 3.20 (s, ²J_PtH = 17.5 Hz, 3H, Me of DMSO ligand), 3.81 (d, ²J_HH = 12.5 Hz, 1H, CH₂ group in the six-membered ring of bzpy ligand), 4.52 (d, ²J_HH = 12.5 Hz, 1H, CH₂ group in the six-membered ring of bzpy ligand), 6.77–9.0 (br, 8H, hydrogens of bzpy ligand).

2.1.3. [PtMe(bzpy)(PPh₃)], 2. To the in situ yellowish brown solution of [PtMe(bzpy)(DMSO)], 1, [prepared by addition of 2-benzylpyridine (104 μL, 0.65 mmol) in dry toluene to a solution of [PtMe₂(DMSO)₂] (82 mg, 0.22 mmol) in the same solvent at 90–95 °C for 3 h under an inert atmosphere of dry argon] triphenylphosphine (56.4 mg, 0.22 mmol) was added. The reaction was stirred for 30 min to give a dirty-white solution. The reaction mixture was then evaporated and washed with cold diethyl ether and dried under vacuum. Yield: 56 mg, 41%. mp = 231 °C (decomp.). Anal. calcd for C₃₁H₂₈NPPt; C, 58.1; H, 4.4; N, 2.2. Found: C, 58.2; H, 4.5; N, 2.6. NMR in chloroform-d: δ(¹H) = 0.76 (d, ²J_PtH = 83.0 Hz, ³J_PH = 7.1 Hz, 3H, MePt), 3.80 (d, ²J_HH = 12.5 Hz, 1H, CH₂ group in the six-membered ring of bzpy ligand), 4.63 (d, ²J_HH = 12.5 Hz, J_PtH = 16.3 Hz, 1H, CH₂ group in the six-membered ring of bzpy ligand), 6.30 (t, ³J_H⁵H⁶ = ³J_H⁵H⁴ = 6.6 Hz, 1H, H⁵ of bzpy ligand), 6.9–7.9 (br, 23H, hydrogens of aromatic region), 7.78 (d, ³J_PtH^6′ = 20 Hz, ³J_H^6′H^5′ = 6 Hz, 1H, H^6′ of bzpy ligand); δ(³¹P) = 29.8 (s, ¹J_PtP = 2562 Hz, 1P).

2.1.4. [PtMe₂I(bzpy) (PPh₃)], 3. An excess of methyl iodide (58.5 μL, 0.94 mmol) was added to a solution of complex [PtMe(bzpy)(PPh₃)], 2, (30 mg, 0.05 mmol) in 20 mL of acetone. The mixture was stirred at room temperature for 2 h, and then the solvent was removed under reduced pressure. The residue was washed with very cold acetone, and the product was dried under vacuum. Yield: 20 mg, 52%. mp = 219 °C (decomp.). Anal. calcd for C₃₂H₃₁NPIPt; C, 49.1; H, 4.0; N, 1.8. Found: C, 49.5; H, 4.1; N, 2.1. NMR in chloroform-d: δ(¹H) = 1.63 (d, ²J_PtH = 72.5 Hz, ³J_PH = 10 Hz, 3H, Me ligand trans to N), 1.71 (d, ²J_PtH = 60.0 Hz, ³J_PH = 7.5 Hz, 3H, Me ligand trans to P), 3.28 (d, ²J_HH = 12.5 Hz, 1H, CH₂ group in the six-membered ring of bzpy ligand), 3.44 (d, ²J_HH = 12.5 Hz, 1H, CH₂ group in the six-membered ring of bzpy ligand), 6.50 (d, ³J_PtH = 17.5 Hz, ³J_H⁵H⁶ = 7.5 Hz, 1H, H⁶ of bzpy ligand), 6.91 (t, ³J_H⁵H⁶ = ³J_H⁵H⁴ = 7.5 Hz, 1H, H⁵ of bzpy ligand) 7.0–7.7 (br, 23H, other aromatic hydrogens), 10.2 (d, ³J_PtH^6′ = 15.0 Hz, ³J_H^6′H^5′ = 5.1 Hz, 1H, H^6′ of bzpy ligand); δ(³¹P) = −6.8 (s, ¹J_PtP = 1103 Hz, 1P).

2.2. Kinetic study of reaction of [PtMe(bzpy)(PPh₃)], 2, with methyl iodide in CHCl₃

In a typical experiment, a solution of complex 2 in CHCl₃ (50 mL, 1.25 × 10⁻⁴ M) in a cuvette was thermostated at 25 °C and methyl iodide was added using a micro syringe. After rapid stirring, the absorbance at corresponding wavelength was monitored with time.

2.3. Kinetic studies using ¹H NMR monitoring

The platinum(II)/platinum(IV) conversion process in chloroform-d was monitored by ¹H NMR spectroscopy. A sample of [PtMe(bzpy)(PPh₃)], 2, (10 mg) was dissolved in 0.75 mL of chloroform-d in a sealed NMR tube, and an equimolar amount of methyl iodide was added. The NMR spectra were recorded several times at room temperature over about 1 day until the reaction was gradually completed.

2.4. Crystallography

Single crystals of complex 3 were grown from a concentrated dichloromethane solution by slow diffusion of n-pentane. There are two molecules of 3 in the asymmetric unit. Each displays a small disorder where one of the two methyl groups bonded to platinum is interchanged with an iodide. For Pt(1), the disorder C(1)/I(2) is about 5% I and for Pt(2), the disorder C(33)/I(4) is about 8%. Appropriate restraints were applied during refinement to model this disorder. The formula given in Table 1 omits the disorder. Further details are given in the ESI.† The crystal was treated as a two-component twin.

Table 1 Crystal data and structure refinement for complex 3

Empirical formula	C32H31NIPPt·CH2Cl2
Formula weight	836.51
Temperature, K	90
Wavelength, Å	0.71073
Crystal system	Monoclinic
Space group	P21/c
Unit cell dimensions	a = 9.2180(13) Å
	b = 16.848(3) Å
	c = 40.285 Å
	β = 93.339(3) °
Volume	6246.0(16) Å^3
Z	8
Density (calculated)	1.859 g cm^−3
Absorption coefficient	5.788 mm^−1
F(000)	3224
Theta range for data collection	1.58–27.48°
Reflections collected	76 587
Independent reflections	27 213
Observed reflections	21 121
Completeness to theta = 25.24°	0.916
Absorption correction	Multi-scan
Refinement method	F^2
Data/restraints/parameters	27 213/16/714
Goodness-of-fit on F^2	1.029
Final R indices [I > 2σ(I)]	0.0487
R indices (all data)	0.1180
Largest diff. peak and hole	1.574/−2.156 e Å^−3

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2.5. Computational details

Density functional calculations were performed with the program suite Gaussian03 (ref. 27) using the B3LYP level of theory. The LANL2DZ basis set^28,29 was chosen to describe Pt and I. The 6-31G(d) basis set was used for other atoms. The geometries of the complexes were fully optimized by employing the density functional theory without imposing any symmetry constraints. To evaluate and ensure the optimized structures of the molecules, frequency calculations were carried out using analytical second derivatives.

3. Results and discussion

3.1. Synthesis and characterization of the complexes

The synthetic procedure is summarized in Scheme 1.

Scheme 1 Reactions studied in the present work.

The reaction of the dimeric organoplatinum(II) complex [Pt₂Me₄(μ-SMe₂)₂] with dimethylsulfoxide at room temperature in dichloromethane gave the starting platinum(II) complex cis-[PtMe₂(DMSO)₂]. The new cyclometalated platinum(II) complex [PtMe(bzpy)(DMSO)], 1, was prepared by the reaction of a solution of [PtMe₂(DMSO)₂] with 1 equiv. of 2-benzylpyridine in refluxing toluene and isolated as an air-stable product. The structure of complex 1 was clearly deduced from its ¹H NMR spectrum in chloroform-d. The protons of the methyl ligand appeared at δ = 0.67 with ¹⁹⁵Pt satellites with ²J_PtH = 85.0 Hz. Two singlets with platinum satellites at δ = 3.08 and 3.20 were attributed to the protons of the Me groups of DMSO ligand with ³J_PtH value of 17.5 Hz. Hydrogen atoms of the CH₂ group in the chelating six-membered ring of 2-benzylpyridinate appeared at δ = 3.81 and δ = 4.52 as two doublets in a 1 : 1 ratio with ²J_HH = 12.5 Hz. The presence of two signals for the methyl groups of the DMSO ligand and also for hydrogen atoms of the CH₂ group in the six-membered ring indicates that 2-benzylpyridinate ligand adopts a rigid-boat conformation and a non-planar six-membered ring.⁷

The reaction of a yellow solution of [PtMe(bzpy)(DMSO)], 1, [prepared in situ by reaction of [PtMe₂(DMSO)₂] with an excess of 2-benzylpyridine in dry and distilled toluene under an argon atmosphere and refluxing for 3 h], with PPh₃ at room temperature gave the cyclometalated complex [PtMe(bzpy) (PPh₃)], 2, in good yield by replacement of DMSO ligand with PPh₃. The complex 2 was characterized by its ¹H and ³¹P spectra. In the ¹H NMR spectrum of complex 2 (see Fig. 2A), the resonance of the platinium-bound methyl group appeared at δ = 0.76 as a doublet (³J_PH = 7.1 Hz), with ²J_PtH value equal to 83.0 Hz. Hydrogen atoms of the CH₂ group in the chelating ring of 2-benzylpyridinate were observed at δ = 3.80 and 4.63 ppm as two doublets in a 1 : 1 ratio with ²J_HH = 12.5 Hz. The nonequivalence of these hydrogen atoms of CH₂ group indicates that the bowing of the six-membered chelate ring is not very easily inverted and the bzpy ligand adopts a rigid-boat conformation (which is also supported by DFT calculations, see theoretical section). The methylene proton resonance at δ = 4.63 shows a small coupling to ¹⁹⁵Pt (J_PtH = 16.3 Hz, the value being very close to that obtained for similar complex [PtCl(bzpy)(PPh₃)]⁷ with J_PtH = 16 Hz), but no corresponding coupling is observed for the other methylene proton at δ = 3.80. In the ³¹P NMR spectrum of the complex 2 (see Fig. 2B), as expected, a singlet at δ = 29.8 having ¹⁹⁵Pt satellites with ¹J_PtP = 2562 Hz was observed.

Fig. 2 ¹H (A and C) and ³¹P{¹H}NMR (B and D) spectra of complexes 2 (A and B) and 3 (C and D), respectively. The peak labeled # in ¹H NMR is due to water of chloroform-d solvent.

The platinum(IV) complex [PtMe₂I(bzpy)(PPh₃)], 3, was obtained as a very pale yellow, air-stable solid from the reaction of the methylplatinum(II) complex [PtMe(bzpy)(PPh₃)], 2, with methyl iodide in acetone at room temperature. The cyclometalated platinum(IV) product of reaction contained the thermodynamic isomer 3 in which the incoming iodide group is trans to C atom of bzpy ligand (see Scheme 1). This indicates that the thermodynamic isomer 3 is more stable than the kinetic isomer, with the incoming Me and iodide ligands in trans disposition to each other. The greater stability of 3 compared to kinetic isomer was further confirmed by DFT calculations (see theoretical section). The synthesized platinum(IV) complex was fully characterized by using ¹H and ³¹P NMR spectroscopies and elemental analysis. In the ¹H NMR spectrum of [PtMe₂I(bzpy)(PPh₃)], 3, (shown in Fig. 2C), the two doublets at δ = 1.63 ppm (with ³J_PH = 10.0 Hz and ²J_PtH = 72.5 Hz) and 1.71 ppm (with ³J_PH = 7.5 Hz and ²J_PtH = 60.0 Hz) were assigned to the Me ligands trans to N and P, respectively. This indicates that PPh₃ has greater trans influence than metalated N atom of the 2-benzylpyridinate ligand. The hydrogen atoms related to the CH₂ group in the chelating ring of 2-benzylpyridinate were observed at δ = 3.28 and δ = 3.44 ppm with ²J_HH = 12.5 Hz. The hydrogen atom of the CH group adjacent to N ligating atom of the 2-benzylpyridinate ligand appeared as a doublet at δ = 10.2 ppm with ³J_HH = 5 Hz and ³J_HPt = 15 Hz. In the ³¹P{¹H} NMR spectrum of the platinum(IV) complex 3 (see Fig. 2D), a singlet at δ = −6.8 appeared with a ¹J_PtP value of 1103 Hz, which, as expected, is much lower than the corresponding value of 2562 Hz found for the starting platinum(II) complex 2.

Complex 3 was further characterized by single crystal X-ray diffraction analysis as shown in Fig. 3. The complex has octahedral coordination environment that consists of two Me groups, the nitrogen atom and the ortho C of the bzpy ligand, one iodide and the phorphorus atom of PPh₃. The bzpy ligand adopts a boat conformation with the bridging CH₂ group directed toward PPh₃ and away from I.

Fig. 3 A view of one of the two complexes in the asymmetric unit in the crystal structure of 3. Ring hydrogen atoms were omitted for clarity.

The tetrahedral geometry at the methylene group of the bzpy ligand forces the phenyl and pyridyl rings to be twisted with respect to the Pt/N/C/I/CH₃ plane, forming a “V” or “boat” shape. Fig. 4 (1 plane) illustrates the ring puckering and twisting of the cyclometalated ligand and gives the displacements of the atoms (Å) from the least-squares plane of Pt1/I1/C1/N1/C14. The distance between the platinum(IV) centre and the methylene carbon of the bzpy group is 3.247(9) Å and 3.262(9) Å for molecules 1 and 2, respectively. Additional geometric features are collected in Table 3.

Fig. 4 A projection of the structure of [PtMe₂I(bzpy)(PPh₃)], 3, down the square plane containing the bzpy group and showing displacements (Å) from the Pt1/I1/C1/N1/C14 least-squares plane.

3.2. Kinetic and mechanism of the reaction of complex 2 with methyl iodide

On the basis of the NMR and UV-vis spectroscopic studies, described below, a mechanism for reaction of complex 2 with methyl iodide is suggested as shown in Scheme 2.

Scheme 2 Suggested mechanism for oxidation of 2.

3.2.1. Kinetic study using UV-vis spectroscopy. An excess of methyl iodide was added to the six-membered cyclometalated platinum complex 2 in CHCl₃ solution and the disappearance of the MLCT band of platinum(II) complex at λ_max = 340 nm was used to monitor the reaction. The pseudo-first-order rate constants (k_obs) were evaluated by nonlinear least-squares fitting of the absorbance–time profiles to the first-order equation, A_t = A_∞ + (A₀ − A_∞)exp(−k_obst). Graphs of these first-order rate constants, k_obs, against the concentration of methyl iodide gave good straight-line plots passing through the origin with no intercepts (see Fig. 5), showing a first-order dependence of the rate on the concentration of methyl iodide. The slope in each case gave the second-order rate constant. Therefore, the reaction followed a simple second-order rate law, first order in both the complex 2 and methyl iodide. The kinetic data for this reaction suggest the operation of a classical S_N2 oxidative addition mechanism. The same method was used at other temperatures and activation parameters were obtained using Eyring equation (see Fig. 5). The data are collected in Table 2. The large negative value of ΔS^‡ obtained for the reaction of complex 2 with methyl iodide is typical of oxidative addition by a common S_N2 mechanism, which involves nucleophilic attack of the cycloplatinated centre at the methyl group of methyl iodide and confirms the associative nature of the reaction. Therefore, on the basis of the gathered data from the above kinetic investigations, a mechanism depicted in Scheme 2 is suggested.

Fig. 5 Left, plots of first-order rate constants (k_obs/s⁻¹) for the reaction of complex [PtMe(bzpy)(PPh₃)], 2, with methyl iodide in CHCl₃ at different temperatures ((a) 10 °C; (b) 20 °C; (c) 25 °C; (d) 30 °C; (e) 40 °C) versus concentration of methyl iodide in CHCl₃; right, Eyring plot for the reaction of complex 2 with methyl iodide in CHCl₃.

Table 2 Second-order rate constants^a and activation parameters for the reaction of the six- and five-membered cyclometalated platinum(II) complexes with methyl iodide in CHCl₃

Complex	λ/nm	10^2k2/L mol^−1 s^−1 at different temperatures (°C)						ΔH^‡/kJ mol^−1	ΔS^‡/J K^−1mol^−1
		10	15	20	25	30	40
a Estimated errors in k2 values are ±5%.b From ref. 30.
[PtMe(ppy) (PPh3)]^b	359	0.28	0.40	0.66	0.85	1.10	—	47.9 ± 0.4	−124 ± 2
[PtMe(bzpy) (PPh3)]	340	0.56	—	1.00	1.40	1.89	3.39	41.9 ± 0.1	−140 ± 1

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As was mentioned before, the thermodynamically preferred isomer 3 (in which the incoming Me and I are in a cis disposition) is obtained, when the ancillary ligand PPh₃ forces steric encumbrance on the kinetically favored trans isomer. The suggested mechanism for the isomerization of kinetic product to 3 is shown in Scheme 2. The oxidation of 2 with methyl iodide occurs through a polar transition state to give the five-coordinate cationic platinum(IV) intermediate A. The reversible trapping of A by iodide can then give the complex 3. On the other hand, it is possible for A to give B by a pseudo-rotation. The cationic intermediate B can also be trapped by iodide to give 3. The results have also been confirmed by DFT calculations (see next section), suggesting that the greater steric bulk of the phosphine group favors the product 3.

It is interesting to note that the obtained rate constants for the reaction of six-membered cyclometalated platinum(II) complex, [PtMe(bzpy)(PPh₃)], with methyl iodide are higher than the related rate constants reported for five-membered cyclometalated platinum(II) complex [PtMe(ppy)(PPh₃)]³⁰ (see Table 2). For example, methyl iodide in chloroform at 25 °C reacted nearly 1.6 times faster with [PtMe(bzpy)(PPh₃)] (k₂ = 1.40 × 10⁻² L mol⁻¹ s⁻¹) than with [PtMe(ppy)(PPh₃)] (k₂ = 0.85 × 10⁻² L mol⁻¹ s⁻¹). It is suggested that the twisting of the phenyl and pyridyl groups out of the square plane of the platinum(II) centre containing 2-benzylpyridinate, but not for 2-phenylpyridinate, is an important factor in determining reactivity.³¹ This trend could be explained by the lower π-acceptor ability of bzpy compared to ppy in platinum(II) complexes. The ppy ligand is located in the square plane of the platinum centre, resulting in the more metal-to-ligand π-back bonding. This decrease in the electron density of platinum(II) in the ppy analogue compared to the bzpy decreases the reaction rate of [PtMe(ppy)(PPh₃)] compared to [PtMe(bzpy)(PPh₃)]. It should be noted that the increased rate for 6-membered ring compared to the 5-membered ring might also be partly a result of the inductive effect of the CH₂ group of bzpy ligand.

3.2.2. Monitoring the reaction by ¹H and ³¹P NMR spectroscopies. The reaction of complex [PtMe(bzpy)(PPh₃)], 2, with methyl iodide was also monitored by using ¹H NMR spectroscopy. Attempts were made to detect reaction intermediates by monitoring the reactions by ¹H NMR spectroscopy, but no intermediates could be detected under these conditions. The ¹H NMR spectra of a 1 : 1 reaction mixture of 2 with methyl iodide in chloroform-d at 27 °C are shown in Fig. 6. Comparison of the spectra in Fig. 6 shows that after addition of methyl iodide to the platinum(II) complex 2, the signals due to starting complex 2 gradually disappeared and those for the complex 3 appeared. A study of the reaction at low temperatures, followed by warming to room temperature, gave similar NMR spectra, showing the rapid conversion of any intermediates to the final product 3. Since only one type of platinum(IV) complex is recognized, it can be concluded that the isomerization of trans- to cis-addition product (see Scheme 2) occurs very fast.

Fig. 6 Monitoring the platinum(II)/platinum(IV) transformation process by ¹H and ³¹P NMR spectroscopies in chloroform-d at 27 °C. The first spectrum (t = 0 min) is obtained from the starting complex [PtMe(bzpy)(PPh₃)], 2, and the next spectra are obtained after addition of methyl iodide to the starting complex 2.

The ¹H NMR data under 1 : 1 stoichiometric condition ([methyl iodide]₀ = [Pt complex]), could be used to measure the reaction rate. The disappearance of the signal at δ = 0.76 (due to Me group coordinated to Pt centre in complex 2) was used to monitor the reaction. The rate of the disappearance of complex 2 was found to be 1.96 (0.12) × 10⁻² L mol⁻¹ s⁻¹. As can be seen, the values resulted by this method is comparable with that obtained by UV-vis spectroscopy (see Table 2). Using ¹H NMR spectroscopy, we found that the rate of disappearance of the starting platinum(II) complex 2 is almost equal to the rate of appearance of the platinum(IV) complex 3 (by following the appearance of a peak at δ = 10.18 due to H^6′ of bzpy ligand), showing that any intermediate was converted rapidly to final product 3.

3.3. Computational studies

The computed structure of complex 2 is shown in Fig. 7. It confirms that the chelate ring is in the boat conformation, with one CH₂ group oriented below the square plane of the platinum atom, with Pt⋯CH₂ = 3.179 Å. The angles about the platinum atom and carbon atom of CH₂ group are close to the ideal values for square-planar and tetrahedral centers, respectively, and so indicate that there is little strain in the six-membered chelate ring. The pyridyl and phenyl groups of bzpy are twisted by an average of 57° out of the square plane of the platinum centre.

Fig. 7 Optimized structures of platinum complexes 2 and 3.

The ligand 2-benzylpyridinate forms platinum(IV) complex 3 in which the PtNC4 chelate ring is in the boat conformation with the CH₂ group anti to the incoming Me group (Fig. 7). The calculated structures and energies of the isomers of [PtMe₂I(bzpy)(PPh₃)], 3, in different conformations, are illustrated in Fig. 8. The experimentally observed conformation 3-cis-anti was predicted to be more stable. The next in energy is the 3-trans-anti isomer, which is calculated to be 4.3 kJ mol⁻¹ higher in energy than the 3-cis-anti. The predictions of the DFT calculations are consistent with the experimental observations and the latter is the only isomer detected in formation of complex 3. The theoretically observed conformation 3-cis-anti was also calculated to be more stable than 3-cis-synand 3-trans-syn by 17.8 and 12.8 kJ mol⁻¹, respectively (see Fig. 8).

Fig. 8 Calculated structures and relative energies of isomers of complex 3 in different conformations.

In the most stable isomer, 3-cis-anti, the phenyl and pyridine groups of bzpy ligand are twisted out of the MePtICN plane by an average value of 46°, compared to 57° in complex 2 (see Fig. 6). The lower twist, and therefore flatter boat structure, in the platinum(IV) complex 3-cis-anti arise from steric repulsion between the PPh₃ group and the CH₂ group of the bzpy ligand, aided by longer Pt–N distance than that in complex 2 (the calculated Pt–N bond lengths are 2.257 and 2.307 Å in complexes 2 and 3-cis-anti, respectively). Selected calculated bond lengths of optimized geometries of complexes at B3LYP/6-31G(d) level (LANL2DZ potential for Pt and I) and the corresponding experimental crystallographic data for complex 3-cis-anti are given in Table 3. The computed structural details are in reasonable agreement with the experimental parameters and error in some cases is due to comparison of the geometrical parameters calculated in solution phase with those obtained experimentally in solid state.

Table 3 Selected calculated bond distances (Å) and angles (°) for complexes 2 and 3-cis-anti compared to the experimental data of complex 3

	2	3-cis-anti	3-exp
Pt–C (trans to N)	2.068	2.080	2.066(9), 2.057(10)
Pt–C (bzpy)	2.052	2.065	2.043(9), 2.048(10)
Pt–N	2.257	2.307	2.204(7), 2.184(8)
Pt–P	2.431	2.652	2.470(2), 2.471(2)
Pt–C (cis to N)	—	2.102	2.093(8), 2.072(8)
Pt–I	—	2.968	2.7855(8), 2.8007(8)
C(bzpy)–Pt–N	84.6	87.7	88.1(3), 86.9(3)
C(bzpy)–Pt–P	179.2	100.2	102.1(2), 100.3(2)
C(bzpy)–Pt–I	—	172.0	169.8(2), 170.9(2)
C(bzpy)–Pt–C (trans to N)	89.7	91.4	90.3(4), 91.0(4)
C(bzpy)–Pt–C (cis to N)	—	83.8	83.1(4), 83.7(3)
I–Pt–P	—	87.7	88.03(6), 88.77(6)

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The oxidative addition of methyl iodide to complex 2 gave [PtMe₂I(bzpy)(PPh₃)], 3, as the only product, although the two faces of platinum(II) complex 2 are not equivalent and methyl iodide can approach from either side (see optimized structure of complex 2 shown in Fig. 7). We considered both approach possibilities in terms of the S_N2 mechanism shown in Scheme 3.

Scheme 3 Suggested S_N2 mechanism for the reaction of complex 2 with methyl iodide.

The oxidative addition of methyl iodide to the six-membered cyclometalated platinum(II) complex 2 can be considered to occur through an S_N2 mechanism. As shown in Scheme 3, the platinum centre of complex 2 (using its 5d_z²) as a nucleophile attacks the carbon atom of methyl iodide to give either transition state TS1 or TS2, on approach from either side of the square plane of platinum(II). The transition state TS1 structure includes the I–C_Me–Pt and Pt–C_Me–H arrangements with bond angles of 177.7 and 88.4°, respectively, which shows a linear I⋯C_Me⋯Pt arrangement in this transition state. The hydrogen atoms of the incoming CH₃ group in TS1 are located in the equatorial plane of the five-coordinated carbon atom with a trigonal bipyramidal arrangement at C centre. The most significant changes in bond lengths of TS1 are calculated for the I–Me and Pt–Me bonds. The calculated bond distances of 2.195 Å for methyl iodide increases to 2.659 Å in transition structure TS1, while the Pt⋯Me distance decreases from far apart in the reactants to 2.552 Å. The accuracy of transition state TS1 is confirmed by observation of the imaginary frequency (−347 cm⁻¹). The DFT calculations (see Fig. 9) suggest that the energy barriers for both paths (44.5 and 44.7 kJ mol⁻¹ for transition states TS1 and TS2, respectively) are the same, showing that there is no difference in steric hindrance of both sides. These values are in excellent agreement with the experimental value of 41.9 kJ mol⁻¹ (see Table 2). To show that the steric effect is important in the addition of the alkyl halide to square planar platinum(II) complexes,¹² we substituted the hydrogen atoms of CH₂ group by Me and found that the energy barriers changed from 44.5 and 44.7 kJ mol⁻¹ to 43.5 and 73.6 kJ mol⁻¹, respectively, for TS1 and TS2. Therefore methyl iodide approach to the less hindered side of complex 2 (when we have CMe₂ linker instead of CH₂) to give intermediate TS1 is preferred over formation of TS2 by approach to the more hindered side. In the next step, the formation of the transition state TS1 or TS2 is followed by completely breaking and forming of the I–Me and Pt–Me bonds, respectively, giving the cationic five-coordinate IM1 or IM2, respectively. These intermediates have a square pyramidal geometry, with the incoming Me group and the iodide ion located in the apical position and in the outer sphere of metal intermediate, respectively. Each of these intermediates can abstract iodide to form the methyl iodide trans addition products, i.e. 3-trans-anti and 3-trans-syn. The 5-coordinate intermediates can also undergo pseudorotation to give IM1′ and IM2′, and iodide coordination can then give 3-cis-anti and 3-cis-syn-platinum(IV) complexes. Iodide dissociation from any of the octahedral isomers can reform the 5-coordinate precursor complexes and then further isomerization can occur. As shown in Scheme 3, it is also clear that pseudorotation of the 5-coordinate intermediates is competitive with iodide coordination. Finally, the free iodide ion coordinates to the platinum(IV) centre of each intermediate to give final platinum(IV) products with an octahedral geometry (see Fig. 9). In most stable product 3-cis-anti, the Pt–I bond length is 2.968 Å. As expected, the bond lengths of the starting platinum(II) complex are shorter than those of the corresponding platinum(IV) product. For example the Pt–C(bzpy) and Pt–P bonds in 2 are shorter (2.052 and 2.431 Å, respectively) than those in 3-cis-anti (2.065 and 2.652 Å, respectively). According to DFT calculations, which is in agreement with experimental finding, complex 3-cis-syn is thermodynamically more stable than other conformers (see Fig. 7) because the larger PPh₃ group is located in the axial position as compared with the equatorial position in trans addition products. The conformer of 3-cis-anti is also thermodynamically more stable than 3-cis-syn because in the former the phosphine group (with Pt–P bond distance of 2.652 Å) is syn to the hydrogens of CH₂ group and the methyl group (with Pt–C bond length of 2.102 Å, which is significantly shorter than Pt–P bond) is anti to the CH₂ group. It is worthy to note that the calculated entropy of activation (ΔS^‡) for the oxidative addition reaction of methyl iodide with complex 2 has large negative value consistent with an S_N2-type mechanism, in agreement with the experimental work.¹²

Fig. 9 Calculated structures and relative energies (kJ mol⁻¹) for products, intermediates and transition state, arising from the addition of methyl iodide through anti (top) or syn (bottom) side respect to CH₂ group of bzpy ligand in CHCl₃ solvent.

Conclusions

The kinetics of reaction of excess methyl iodide with [PtMe(bzpy)(PPh₃)] were studied. According to the kinetic results, the oxidative addition reaction of methyl iodide with [PtMe(bzpy)(PPh₃)] follows second order kinetics, first order with respect to both reactants. The entropy of activation, ΔS^‡, has a large negative value consistent with an S_N2-type mechanism. By comparing the above kinetic results with those reported previously to proceed by similar mechanism, for the reaction of methyl iodide with the analogous complex [PtMe(ppy)(PPh₃)] (see Table 2),³⁰ we now find that the ring size of cyclometal has significant impact on rate of the reaction of methyl iodide with the studied complexes containing PPh₃ as a monodentate ligand. In reactions of the cyclometalated platinum(II) complexes with methyl iodide at different temperatures, the complex having the chelated six-membered ring reacted faster than the corresponding five-membered analogues. For example, methyl iodide at 10 °C reacted 2 times faster with [PtMe(bzpy)(PPh₃)] (k₂ = 0.56 × 10⁻² L mol⁻¹ s⁻¹) than with [PtMe(ppy)(PPh₃)] (k₂ = 0.28 × 10⁻² L mol⁻¹ s⁻¹), therefore increasing the twist angle between the aromatic rings and probably the inductive effect of the CH₂ group of bzpy ligand should lead to increased reactivity. In conclusion, the chelate ring size of the supporting cyclometalated ligands is an important factor in oxidative addition reactions and should be considered in the design of more active catalysts.

As a result of its non-planarity, 2-benzylpyridinate has the same σ-donor ability and lower π-acceptor ability than the 2-phenylpyridinate ligand. Therefore, it is expected that the platinum centre in the six-membered cyclometalated platinum(II) complex 2 is more electron rich than the platinum centre in five-membered cyclometalated platinum(II) complexes toward nucleophilic attack. This higher electron density at the platinum centre, complies with the higher rate of oxidative addition with methyl iodide for the six-membered cyclometalated platinum(II) complex as compared with that of the five-membered cyclometalated platinum(II) complexes. This behavior is also consistent with the trends found for the ¹J_PtP values in their ³¹P NMR spectra in [PtMe(ppy)(PPh₃)] (2105 Hz)³² and [PtMe(bzpy)(PPh₃)] (2562 Hz). These results also suggest that the trans influence of the metalated C atom of 2-benzylpyridinate ligand should be lower than that of 2-phenylpyridinate ligand.

Acknowledgements

We thank the Shiraz University Research Council and the Iran National Science Foundation (Grant No. 93038832) for financial support. We also thank Prof. M. Rashidi, Dr M. Golbon Haghighi, and Dr F. Niroomand Hosseini for helpful comments.

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Footnote

† Electronic supplementary information (ESI) available: The computed Cartesian coordinates of all of the molecules reported in this study, and thermal ellipsoid plots of the structure of [PtMe₂I(bzpy)(PPh₃)], 3. CCDC 1420815. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra17421j

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