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

Abstract

The known rollover cycloplatinated(II) complex [Pt 2 Me 2 (PPh 3 ) 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 [Pt 2 Me 4 I 2 (PPh 3 ) 2 (µ-bpy-2H)], 2. Stereochemistry of 2 was fully identified by NMR spectroscopy ( 1 H and 31 P) 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 kinetic of its reaction with MeI; a double MeI oxidative addition was observed and classical S N 2 mechanism was suggested for the both steps of reaction. The large negative entropy of activation (∆S ‡ ), 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)(PPh 3 )] and found to be higher (in step 1) and usually lower (in step 2). Theoretical computations of geometry of the possible reaction transition 2 sates and intermediates revealed that each step of the reaction take places via a transition state with a nearly linear arrangement of the I-CH 3 -Pt moiety. The computational results are in a good agreement with the experimental findings, confirming the proposed mechanism.16 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 kJmol -1 ) and TS2 (+48.2 kJmol -1 ) are in excellent agreement with the observed values of ∆H ≠ = 40.9 and 51.7 kJmol -1 for the first and second steps, respectively. Fig. 6. The B3LYP/(LANL2DZ, 6-31G(d))/chloroform optimized structures involved during oxidative addition reaction of [Pt 2 Me 2 (PPh 3 ) 2 (µ-bpy-2H)], 1, with CH 3 I. The H atoms are removed for clarity (except for the methyl group of the CH 3 I). Selected bond angles (°) and bond lengths (Å) are also shown.

Phenylpyrazolate cycloplatinated(II) complexes: Kinetics of oxidation to Pt(IV) complexes

“…This is consistent with our DFT calculations which will be discussed in the next section. MeI, to give the transition state TSa (calculated energy +43.9 kJmol −1 ); the result being in excellent agreement with the observed value of ΔH ‡ = 37.6 kJmol −1 (see Table 2 [3,9,26,39,40], and then is coordinated to iodide to give the corresponding thermodynamic observed product 3a. We believe that the thermodynamic isomer 3a is favored over the kinetic isomer 3a-t as in the former more sterically demanding phosphine ligand L is situated in the "axial" position as compared with that in the latter in which L is located in the S N 2 mechanism and the rate of reaction involving 2b (k 2 = 1.31×10 -2 Lmol -1 s -1 ) is significantly faster than that involving 2a (with k 2 = 0.35×10 -2 Lmol -1 s -1 ).…”

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

“…2 Although the most well-studied examples of these compounds are five-membered ring complexes containing N−Pt−C(phenyl) moiety, [3][4][5] six-membered cyclometalated complexes are relatively rare. [12][13][14][15][16][17][18][19][20][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. [12][13][14][15][16][17][18][19][20][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.…”

Stable trans isomer as the kinetic and theromodynamic product for the oxidative addition of MeI to cycloplatinated(II) complexes comprising isocyanide ligands

“…In this way, the structures of 1a and 1b together with the TS (transition state), IMt ( trans intermediate) and IMc ( cis intermediate) were optimized without imposing any symmetry constraints (Figure , the xyz coordinates for all the optimized structures are shown in Tables S4‐S8). Additionally, the dichloromethane solvation effects were modelled by Conductor‐like Polarizable Continuum Model (CPCM) . The calculated energy profile for the oxidative addition reaction is also depicted in Figure .…”

“…As can be seen, TS is generated through the nucleophilic attack of 5d Z 2 atomic orbital of Pt(II) center on the σ* of Me‐I bond. In the structure of TS , the methyl group of the MeI is planar and the Pt‐C(Me)‐I angle is almost linear with a value close to 180° . In the optimized transition ( TS ) structure, the negative imaginary frequency belongs to the vibration of planar Me group between Pt and I atoms which means Me is going far from I and closing to platinum center.…”

“…Interestingly, there are many reports in the literature that theoretical calculations support the experimental kinetic data . In the previous investigations, particularly for the reported cycloplatinated(II) complexes, it has been proven that the roles of cyclometalated and ancillary ligands are very important in determining the reaction rate . For example, the ancillary monophosphanes, containing a wide range of substituents, are the appropriate choices for tuning the oxidative addition reaction rate of cycloplatinated(II) complexes due to their various electronic and steric effects …”