Oxidation of a rollover cycloplatinated(II) dimer by MeI: a kinetic study

Abstract

The known rollover cycloplatinated(II) complex [Pt₂Me₂(PPh₃)₂(μ-bpy-2H)], 1, in which bpy-2H acts as a bridging rollover ligand, was reacted with MeI to give a new binuclear rollover cycloplatinated(IV) complex [Pt₂Me₄I₂(PPh₃)₂(μ-bpy-2H)], 2. The stereochemistry of 2 was fully identified by NMR spectroscopy (¹H and ³¹P) and confirmed by DFT calculations. To the best of our knowledge, 2 is the first example of a diplatinum(IV) complex having a bridging rollover bipyridine ligand. 1 has a 5d_π(Pt) → π*(bpy) MLCT band in the visible region which was used to easily follow the kinetics of its reaction with MeI; a double MeI oxidative addition was observed and classical S_N2 mechanism was suggested for both steps of the reaction. The large negative entropy of activation (ΔS^‡), found in each step, complies with an associative process. The rates are almost 3–5 times slower in the second step as compared to the first step, due the electronic effects transmitted through the rollover bpy ligand. The rates were also compared with that reported for the corresponding monomeric cyclometalated complex [PtMe(bpy-H)(PPh₃)] and found to be higher (in step 1) and usually lower (in step 2). Theoretical computations of the geometry of the possible reaction transition states and intermediates revealed that each step of the reaction takes place via a transition state with a nearly linear arrangement of the I–CH₃–Pt moiety. The computational results are in good agreement with the experimental findings, confirming the proposed mechanism.

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1. Introduction

The chemistry of cyclometalated compounds is currently one of the most active topics in organic synthesis and homogeneous catalysis. The C–H bond activation using a directing group such as 2-phenylpyridine or 2,2′-bipyridine is a suitable method for achieving easy C–H bond functionalization.^1–3 2-Phenylpyridine is a typical compound, and its reaction with a transition metal complex forms an orthometalated intermediate with the help of nitrogen coordination to the metal. A new class of cyclometalation is C–H bond metalation to form rollover complexes and the most studied ligand in this area is 2,2′-bipyridine (bpy).⁴ In this type of cyclometalation, the two nitrogen atoms of bpy at first coordinate to metal and then the coordination of one nitrogen to the metal is switched to carbon–metal bond by rotation of the pyridine ring (Scheme 1).^5,6 Although rollover cyclometalated complexes are currently reported to be very easy to prepare, their reactivity have not been studied in-depth.

Scheme 1 Classical and rollover cyclometalation.

Oxidative addition of MeI to platinum(II) centers is a fundamental process in organometallic chemistry with significant implications in catalysis. A classic example of such a system is the oxidative addition of MeI to [PtX₄]²⁻ complex in the Shilov process of activation of C–H bonds.⁷ It is well-known that electron-rich Pt(II) complexes (e.g. [PtMe₂(NN)], in which NN are various diimine ligands such as 2,2′-bipyridine and 1,10-phenanthroline) undergo faster oxidative addition than electron-poor Pt(II) complexes.^8–10 Electron density on the Pt(II) center of a [PtR₂(LL)] complex can be manipulated by selecting R ligand, which has either electron-withdrawing or electron-donating ability.^11–14

Our previous study has described in detail the effect of usual cyclometalated groups on rate of oxidative addition of MeI to the complexes of type [PtMe(C^N)L], in which C^N = 2-phenylpyridinate, 2-(p-tolyl)pyridinate or benzo[h]quinolate and L = PPh₃ or PPh₂Me.¹⁵ However, report on oxidative addition of alkyl halides on rollover cyclometalated platinum(II) complexes are very rare,^9,16 with only one report existing on kinetic study of this type of complexes with MeI in which effect of rollover cyclometalated group on the rate of oxidative addition is investigated.⁹ There, a study of reaction between MeI and the monomeric Pt(II) complexes [PtMe(bpy-H)(L)] (bpy-H = κ²N,C-2,2′-bipyridine; L = PPh₃ or PPh₂Me)] led to formation of the Pt(IV) rollover complexes [PtMe₂I(bpy-H)(L)]. Despite of this, any such reactions involving the related binuclear complexes are not reported. We report here, for the first time, on the reactivity toward oxidative addition of MeI with dimeric rollover cyclometalated Pt(II) complex, [Pt₂Me₂(PPh₃)₂(μ-bpy-2H)], 1, to investigate the cooperative steric or electronic effects between the two adjacent metal centers. Finally, using density functional theory for structure optimizations on the related complexes, geometries of transition states and intermediates in the complete reaction sequence are proposed.

2. Experimental section

The ¹H and ³¹P NMR spectra were recorded on a Bruker Avance DRX 400 MHz spectrometer in CDCl₃ as solvent. The operating frequencies and references, respectively, are shown in parentheses as follows: ¹H (400 MHz, TMS) and ³¹P (162 MHz, 85% H₃PO₄). The chemical shifts and coupling constants are in ppm and Hz, respectively. Kinetic studies were carried out by using a Perkin-Elmer Lambda 25 spectrophotometer with temperature control using an EYELA NCB-3100 constant temperature bath. The precursor [Pt₂Me₂(PPh₃)₂(μ-bpy-2H)], 1, was prepared according to reported procedure.¹⁷

2.1. Synthesis of [Pt₂Me₄I₂(PPh₃)₂(μ-bpy-2H)], 2

To a solution of [Pt₂Me₂(PPh₃)₂(μ-bpy-2H)], 1, (50 mg in 25 mL toluene) was added an excess of MeI (550 μL) at room temperature and the mixture was stirred for 5 days. The solvent was removed under reduced pressure, and the residue was washed with diethyl ether and acetone. The product was dried under vacuum. Yield 40 mg; 65%, mp 203 °C (decomp.). Anal. calcd for C₅₀H₄₈I₂N₂P₂Pt₂: C, 43.4; H, 3.5; N, 2.0; found: C, 43.0; H, 3.2; N, 1.8. NMR data: δ(¹H) 1.15 (d, ³J_PH = 12.0 Hz, ²J_PtH = 60.0 Hz, 6H, Me group trans to P), 1.64 (d, ³J_PH = 12.0 Hz, ²J_PtH = 75.0 Hz, 3H, Me groups trans to N), (aromatic protons): 7.0–7.9 (overlapping multiplets of phenyl groups of PPh₃), 8.80 (d, ³J_HH = 7.9 Hz, 2H, CH groups adjacent to coordinated N atoms), 6.84 (d, ³J_HH = 12.0 Hz, ³J_PtH = 64 Hz, 2H, CH groups adjacent to coordinated C atoms), 6.52 (t, ³J_HH = 20.0 Hz, 2H); δ(³¹P) 10.4 (s, ¹J_PtP = 1004 Hz, 2P).

2.2. Kinetic study

The oxidative addition reaction was monitored using UV-vis (by monitoring the change in absorbance at λ = 375 nm) spectrophotometer. All kinetic measurements were monitored under pseudo-first-order conditions in CHCl₃ solution. Concentration of [Pt₂Me₂(PPh₃)₂(μ-bpy-2H)], 1, was 0.0003 M for UV-vis measurements. Concentration range used for MeI was 0.1–1.5 M. Kinetic measurements, under pseudo-first-order conditions for different concentrations of 1 (ranges from 2 × 10⁻⁴ to 4 × 10⁻⁴ M) at a constant [MeI], confirmed that the Pt complex concentration had no influence on value of the observed rate constant. The observed first-order rate constants were obtained from least-square fits of absorbance versus time data according to eqn (1).

2.3. Computational details

Gaussian 03 was used¹⁸ to fully optimize all the structures at the B3LYP level of density functional theory. The solvation energies were calculated by CPCM model in CHCl₃. The effective core potential of Hay and Wadt with a double-ξ valence basis set (LANL2DZ) was chosen to describe Pt and I.^19,20 The 6-31G(d) basis set was used for 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 structure (one imaginary frequency).

3. Results and discussion

3.1. Synthesis and characterization of the rollover dimeric Pt(IV)–Pt(IV) complex 2

The studied reaction is depicted in Scheme 2.

Scheme 2 Reaction studied in the present work.

The known starting rollover methylplatinum(II) [Pt₂Me₂(PPh₃)₂(μ-bpy-2H)], 1,¹⁷ was reacted cleanly with an excess of MeI in toluene at room temperature to give the air-stable solid product [Pt₂Me₄I₂(PPh₃)₂(μ-bpy-2H)], 2. The synthesized Pt(IV) complex was fully characterized using ¹H and ³¹P NMR spectroscopy and elemental analysis (full data are collected in the Experimental section). Thus, structure of product of the oxidative addition, 2, is proposed to adopt an octahedral geometry in which the rollover bpy ligand, iodide and Me groups are located in an equatorial plane with the methyl and PPh₃ ligands in the axial positions. In ¹H NMR spectrum of 2, the two equivalent Me groups trans to P were observed at δ 1.15 as a doublet with ³J_PH = 12.0 and ²J_PtH = 60.0 Hz, while the two equivalent Me groups locating trans to N ligating atoms were appeared at δ 1.64, with a considerably higher ²J_PtH value of 75.0 Hz, due to lower trans influence of N atom as compared with that of P atom; these data are in agreement with those reported for the related monomeric complexes, such as [PtMe₂I(bpy-H)(PPh₃)] with chemical shift values of 1.12 (with ²J_PtH = 61 Hz) and 1.64 ppm (with ²J_PtH = 71 Hz) for Me groups trans to P and N atoms, respectively.²¹ Hydrogens related to CH groups adjacent to ligating N and C atoms of the rollover bpy-2H ligand each appeared as a doublet at δ = 8.80 (with ³J_HH = 7.9 Hz) and 6.84 (with ³J_HH = 12.0 Hz and ³J_PtH = 64.0 Hz), respectively. In the ³¹P NMR spectrum of [Pt₂Me₄I₂(PPh₃)₂(μ-bpy-2H)], 2, a singlet at δ 10.4 (with ¹J_PtP = 1004 Hz), assigned to the two equivalent P ligating atoms, was observed with its ¹J_PtP value, as expected, being much lower than the corresponding value of ¹J_PtP = 2347 Hz found for the starting Pt(II) complex 1.¹⁷ Although the data comply well with relative disposition of different ligands on each Pt center of the complex 2 (with chirality at each Pt center), it is not possible to use the present data to actually propose any “frozen” conformer(s) for complex 2 resulting from rotation around one or two of the Pt–P bonds. As twice the “expected” numbers of signals were observed in NMR spectra of 2, the formation of a statistical 1 : 1 mixture of two stereoisomers may be a reasonable explanation.

3.2. Kinetic studies

Kinetics of oxidative addition of MeI in CHCl₃ to the dimeric rollover cycloplatinated(II) complex 1 was studied using UV-vis spectroscopy. An excess of MeI was used and disappearance of the MLCT band was followed to monitor the reaction. Changes in the spectrum during a typical run are shown in Fig. 1. Absorbance–time curves monitored at 375 nm for reaction of 1 with MeI in CHCl₃ solution were obtained in the presence of a large excess of MeI, such that its concentration remained effectively constant in each experiment.

Fig. 1 Changes in the UV-vis spectrum during reaction of 1 with MeI in CHCl₃; successive spectra were recorded at intervals of 150 s.

Time-dependence curves of spectra of the reaction at this condition are shown in Fig. 2. The data failed to fit properly in equation Abs_t = Abs_∞ + (Abs₀ − Abs_∞)[exp(−k_obst)], which corresponds to a monophasic kinetic behavior. However, the data were successfully fitted in eqn (1) with two exponentials, showing the occurrence of two sequential first-order reactions. Thus, pseudo-first-order rate constants, k_obs(1) and k_obs(2), for the two steps of the reaction were calculated by nonlinear least-square fitting kinetic data to the biphasic first order equation (eqn (1) and Fig. 2).

Abs_t = α[exp(−k_obs(1)t)] + β[exp(−k_obs(2)t)] + Abs_∞ (1)

Fig. 2 Absorbance–time curves for reaction of 1 with MeI (0.27–1.33 M with [MeI] increases reading downward) in CHCl₃ at 15 °C. The inset shows the biphasic fit to the experimental curve when [MeI] = 0.27 M.

The experimentally determined pseudo-first-order rate constants, k_obs(1) and k_obs(2), were converted to second order rate constants (k₂ for the first step and k′₂ for the second step) by determining slope of the linear plots of k_obs(1) and k_obs(2) against the concentration of MeI reagent (Fig. 3). Non-zero intercepts implied that k_obs(1) = k₂[MeI] + k₁ for the first step and k_obs(2) = k′₂[MeI] + k′₁ for the second step and that the k₁ or k′₁ step in the proposed reaction mechanism should exist. Thus, in addition to second-order terms k₂ and k′₂, which are suggested for the simple associative S_N2-type mechanism, a much smaller terms, k₁ and k′₁, are also observed for associative substitutions by solvent. The results are given in Table 1. The findings are consistent with the two-step sequence shown in Scheme 3 for reaction of 1 with MeI. It should be noted that k₂ > k′₂. The same method was used at other temperatures and activation parameters were obtained from the Eyring equation (eqn (2)):

Fig. 3 Plots of first-order rate constants for the reaction of 1 with MeI at 25 °C versus [MeI] in CHCl₃.

Table 1 Rate constants^a and activation parameters^b for reaction of 1 with MeI in CHCl₃

	Rate constants at different temperatures
	15 °C	20 °C	25 °C	30 °C	40 °C	ΔH^‡/kJ mol^−1	ΔS^‡/J K^−1 mol^−1
a Estimated error in k values are ±5%.b Obtained from the Eyring equation.c From ref. 9.
10^3k2/(L mol^−1 s^−1)	2.83	3.72	4.83	7.02	11.74	40.9 ± 1.5	−152 ± 5
10^3k1/s^−1	0.74	0.81	1.12	—	—	26.6 ± 9.9	−210 ± 33
10^3k′2/(L mol^−1 s^−1)	0.59	0.78	0.99	2.22	3.16	51.7 ± 7.5	−127 ± 25
10^3k′1/s^−1	0.20	0.25	0.39	—	—	44.9 ± 9.5	−160 ± 33
10^3k2/(L mol^−1 s^−1) for [PtMe(bpy-H)(PPh3)]^c	0.70	0.90	1.50	2.00	4.60	55.1 ± 3.2	−114 ± 10

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Scheme 3 Suggested mechanism for the two-step oxidative addition reaction of 1 with MeI (S = solvent).

In this equation ΔH^‡ = activation enthalpy, ΔS^‡ = activation entropy, k = rate constant, k_B = Boltzmann's constant, T = temperature, h = Planck's constant, R = universal gas constant (see Fig. 4). The data are shown in Table 1.

Fig. 4 Eyring plots for the reaction of 1 with MeI in CHCl₃.

ΔS^‡ values are large and negative for the both steps, complying with the classical S_N2 type mechanism. Oxidative addition of organoplatinum(II) complexes with alkyl halides have been extensively investigated^8,22 and we have also studied a secondary α-deuterium KIE study involving the reaction of MeI/CD₃I to confirm operation of S_N2 mechanism in the oxidative addition of MeI to some organoplatinum(II) complexes.^23–25 So it is well established that these reactions proceed by an S_N2 mechanism. Although a redox pathway is also a possibility, it has never been demonstrated by experimental evidence. On the basis of these kinetic data and the related DFT calculations (described in the next section) we propose that (see Scheme 3) in the first step, one of the electron rich Pt(II) centers of [Pt₂Me₂(PPh₃)₂(μ-bpy-2H)], 1 (as a nucleophile), attacks on carbon atom of MeI through an S_N2 type mechanism (with rate constant k₂) and the mix valence Pt(II)–Pt(IV) kinetic binuclear intermediate IM-A is formed, via the transition state TS1, which is then equilibrated with the analogous intermediate IM-B with Me and I being in cis disposition at Pt(IV) center. The latter intermediate, IM-B, is then converted to the Pt(II)–Pt(IV) compound [PtMe(PPh₃)(μ-bpy-2H)PtMe₂I(PPh₃)], A. The complex A is then reacted with MeI in the second step, again by an S_N2 type mechanism, with the rate constant k′₂ being nearly 3–5 times smaller than value for rate constant of the first step (i.e. k₂, see Table 1) and the Pt(IV)–Pt(IV) binuclear intermediate IM-C in equilibrium with the analogous intermediate IM-D is formed via the transition state TS2. The latter intermediate, IM-D, gives the final product [Pt₂Me₄I₂(PPh₃)₂(μ-bpy-2H)], 2.

On the basis of the UV-vis data for the two-step oxidative addition reaction studied in this work (see Table 1) and using the Espenson's approach, in which the biphasic equation was used in evaluating the molar absorptivities of intermediates and the two rate constants,^26–28 the second step seems to be slower than the first step by a factor of nearly 3 to 5. This is suggested to be due to the fact that in the first step one of the Pt(II) centers of Pt(II)–Pt(II) complex 1 attacks on methyl group of MeI, while in the second step, MeI reacts with the Pt(II) center of the Pt(II)–Pt(IV) complex A, in which the adjacent Pt(IV) reduces nucleophilicity of the Pt(II) center (through the bridging rollover bpy-2H ligand) when compared with situation in the first step during which the attack is performed on a Pt(II)–Pt(II) species.

For the purpose of comparison, the kinetic data for reaction of the related monomeric complex [PtMe(bpy-H)(PPh₃)] with MeI⁹ are also included in Table 1. Thus, on reaction with MeI, 1 in the first step reacts nearly 3 time faster than the monomer [PtMe(bpy-H)(PPh₃)]; we attribute this to less electron withdrawing ability of the neighboring Pt(II) center in 1 to increase the electron density of the attacking Pt(II) center in Pt(II)–Pt(II) complex 1 when compared with the case of monomer. As such, 1 in the second step is reacted with MeI with usually lower rate constant.

3.3. DFT calculations

In order to shed some light on the suggested mechanism of double oxidative addition reaction involving [Pt₂Me₂(PPh₃)₂(μ-bpy-2H)], 1, with MeI (depicted on Scheme 3), DFT calculations were carried out on the precursor complex 1, final product 2, potential transition states and intermediates. The Conductor-like Polarizable Continuum Model (CPCM) was used to model general solvation effects by chloroform. The reaction is initiated by nucleophilic attack by 5d_z² HOMO−1 of one of the Pt(II) centers of 1 on the σ* LUMO of MeI (Fig. 5). Substitution of iodide by the Pt center, followed by simultaneous partial removal of an iodide ion, results in formation of the transition state TS1 (see Fig. 6). In TS1, the bond angles I–C_Me–Pt (177.6°) and Pt–C_Me–H (89.7°) are close to 180° and 90°, respectively, showing a linear I⋯C_Me⋯Pt arrangement with the hydrogen atoms of CH₃ group forming a trigonal bipyramidal arrangement around the C center of methyl group. During the formation of TS1, the most significant changes in bond distances are observed for I⋯Me and Pt⋯Me (Me is from MeI) distances increasing from 2.227 Å in MeI to 2.698 Å in TS1, whereas the Pt⋯Me distance decreases from far apart (randomly chosen) in the reactant to 2.517 Å in the TS1. This confirms that oxidative addition reaction of MeI to the rollover dimeric complex 1 involves simultaneous cleavage of Me–I bond and formation of the Pt–Me bond, which is then followed by formation of the cationic intermediate IM-A by completely breaking and forming of Me–I and Pt–Me bonds, respectively. The intermediate IM-A has a square pyramidal geometry, with the incoming Me group locating in the apical position and the iodide ion in the outer sphere of dimer complex. As was mentioned before, oxidative addition of MeI to 1 yielded almost quantitatively the cis addition isomer 2, which is the stable thermodynamic product. Therefore the intermediate IM-A is quickly performing a facile trans to cis isomerization of Pt(IV) center to form the intermediate IM-B having Me and I in cis disposition. The free iodide ion coordinates to the platinum(IV) center of IM-B forming the mixed Pt(II)–Pt(IV) complex A having an octahedral geometry around the Pt(IV) center (see Scheme 3 and Fig. 6) with Pt–I and Pt–C_Me bond lengths being 2.907 and 2.087 Å, respectively; notice that although the intermediate IM-A is theoretically shown to be slightly more stable (see Fig. 7) than the intermediate IM-B, from sterical point of view the latter is more favorable in forming A. As expected (see Fig. 6), in A, bond lengths of the Pt(II) center are shorter than those of the Pt(IV) center; for example Pt–C_Me and Pt–P bonds (2.065 and 2.409 Å, respectively) in the Pt(II) center of A are shorter than those in the Pt(IV) center (2.087 and 2.608 Å, respectively).

Fig. 5 (a) HOMO−1 of rollover dimer 1, (b) LUMO of MeI, (c) interactions of d_z² orbital of electron-rich Pt(II) center of dimer 1 with σ* LUMO of MeI and (d) HOMO of TS1.

Fig. 6 The B3LYP/(LANL2DZ, 6-31G(d))/chloroform optimized structures involved during oxidative addition reaction of [Pt₂Me₂(PPh₃)₂(μ-bpy-2H)], 1, with CH₃I. The H atoms are removed for clarity (except for the methyl group of the CH₃I). Selected bond angles (°) and bond lengths (Å) are also shown.

Fig. 7 Energy profile for oxidative addition of MeI to [Pt₂Me₂(PPh₃)₂(μ-bpy-2H)], 1, in CHCl₃.

A is then reacted with MeI in the second step, again by an S_N2 type mechanism, upon which iodide separates from carbon of the MeI reagent and the transition state TS2 is formed (see Scheme 3 and Fig. 6 and 7), having an approximately collinear arrangement of the I–CH₃–Pt moiety (the angle being 176.8°) with the Pt–C_Me–H angle (91.8°) close to 90°, corresponding to an S_N2 type of reaction. A similar trend is observed in the second step giving the transition state TS2 following by formation of the intermediate IM-C which is then equilibrate with the intermediate isomer IM-D,¹⁵ to finally give the thermodynamic isomer 2. Formation of the thermodynamic isomer 2 over the kinetic isomer (in which iodide groups are located trans to the incoming Me groups) is favored as in 2 the more sterically demanding PPh₃ ligand is situated in “axial” position when compared with that in the kinetic isomer in which it is located in the “equatorial” position.

Although formation of 2 through a concerted pathway is a possibility, the theoretical suggested S_N2 mechanism is consistent with the experimental finding.

Calculated structures and energies for the compounds, shown in Scheme 3, in chloroform solution (Fig. 6 and 7) indicate that, in consistent with the experimental observations, 1 is easily reacted with MeI to give A which is followed by a slower reaction to give 2. Calculated energies of the transition states TS1 (+45.5 kJ mol⁻¹) and TS2 (+48.2 kJ mol⁻¹) are in excellent agreement with the observed values of ΔH^‡ = 40.9 and 51.7 kJ mol⁻¹ for the first and second steps, respectively.

A summary of calculated atomic charges (based on Atomic Polar Tensors population (APT) analysis) for the selected compounds of Scheme 3 is given in Table 2. Charges on Pt(II) centers of 1 are significantly lower than those on the other compounds, supporting that 1 has the most potential to act as nucleophile in the reaction with MeI (Fig. 5). Atomic charges on Pt(IV) and Pt(II) centers of the mixed valence Pt(IV)–Pt(II) complex A are +0.553 and −0.201, respectively. As one moves from 1 to 2, the electron population on the iodide atom increases. These results confirm the fact that the oxidative addition reaction in such systems happens through transfer of electron density from Pt center to Me group and then to the iodide atom.

Table 2 APT (Atomic Polar Tensors) atomic charges of the selected compounds

Compounds	qPt(1)/qPt(2)	qI(1)^a/qI(2)^b	qC(1)^a/qC(2)^b
a First incoming MeI.b Second incoming MeI.
1	−0.204/−0.204	−0.236/−0.236	+0.274/+0.274
IM-B	+0.440/−0.189	−1/−0.236	+0.203/+0.274
A	+0.553/−0.201	−0.833/−0.236	+0.306/+0.274
IM-D	+0.540/+0.469	−0.788/−1	+0.290/+0.211
2	+0.544/+0.554	−0.809/−0.809	+0.298/+0.298

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A further rationale of oxidative addition process in terms of nature of frontier molecular orbitals of the Pt compounds (Fig. 8) may be helpful. Composition and energy of HOMO and LUMO of 1 and A are also reported in Fig. 8. The highest occupied molecular orbitals 1 consist mainly of the Pt orbitals (42% and 84% for HOMO and HOMO−1, respectively) with some contribution from orbitals of the bridged bpy-2H ligand (53 and 8% for HOMO and HOMO−1, respectively). The LUMO in 1 is predominately localized on the bpy-2H unit orbitals. The HOMO and HOMO−1 of A consist of the iodide ligand and the LUMO of this complex is mainly localized on the bpy-2H ligand (see Fig. 8). Value of energy separations between the HOMO and LUMO of 1 is greater than that of A and are equal to 4.157 and 3.588 eV, respectively, meaning that the kinetic stability of A is lower than 1. Main contribution to Pt⋯C_MeI in the transition state TS1 comes from overlap of d_z² HOMO−1 of the platinum center with the LUMO of MeI (which is the C–I σ* antibonding molecular orbital, see Fig. 5). So it is reasonable to view the oxidation process formally as removal of electrons from HOMO−1 of 1 into the LUMO of MeI. The second step including the reaction of A with a second incoming MeI shows similar frontier orbitals (see Fig. 8).

Fig. 8 Qualitative frontier molecular orbital scheme for 1 and A.

4. Conclusions

The rollover cyclometalated Pt(II)–Pt(II) dimeric complex [Pt₂Me₂(PPh₃)₂(μ-bpy-2H)], 1,¹⁷ is an interesting case of the less common complexes of this kind containing a bridging double deprotonated bipyridine ligand, in which the two Pt fragments are identical. As supported by experimental and theoretical investigations, a two-step oxidative addition reaction of MeI with 1 is suggested to occur via S_N2 mechanism, during a number of stages (see Scheme 3), to finally give the thermodynamic Pt(IV)–Pt(IV) product [Pt₂Me₄I₂(PPh₃)₂(μ-bpy-2H)], 2 (in which the added Me and I ligands on each of the two Pt centers are situated in cis position to each other); high steric demand of PPh₃ ligands are probably responsible for isomerization of the kinetic product in each of the two steps (having the added Me and I ligands in trans position) to the thermodynamic analogous. Kinetics of the reaction, as investigated by UV-vis spectrophotometry and confirming by DFT studies, shows the following results:

(1) In the first step, one of the electron rich Pt centers of Pt(II)–Pt(II) starting complex 1 attacks the carbon of MeI to form A (suggested to be a Pt(II)–Pt(IV) complex) via the transition state TS1. The Pt(II) center in A is then reacted with MeI with a rate considerably slower (by a factor of nearly 3–5) than that in the first step involving the starting complex 1, which is in consistent with a significant electronic effect transmitted through the rollover bpy bridging ligand. Less importantly, the fact that coordination around A seems to be more sterically hindered than that of 1 is probably responsible for the rate differences.

(2) MeI in the first step reacts with 1 in CHCl₃ at 25 °C with a rate constant k₂ = 4.83 × 10⁻³ L mol⁻¹ s⁻¹, which is some 3 times faster than that with the related mononuclear cycloplatinated(II) complex [PtMe(bpy-H)(PPh₃)] at the same condition (with k₂ = 1.50 × 10⁻³ L mol⁻¹ s⁻¹). This is attributed to less electron withdrawing ability of neighboring Pt(II) center in 1 to increase the electron density of the under attacking Pt(II) center in the Pt(II)–Pt(II) complex 1 as compared to that in monomer [PtMe(bpy-H)(PPh₃)].

(3) Rate of MeI oxidative addition in the second step involving 1 (see Table 1) is found to be usually lower than that involving the mononuclear complex [PtMe(bpy-H)(PPh₃)].

Acknowledgements

This work has been supported by Shiraz University and the Iran National Science Foundation (Grant No. 93035108).

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