An in-depth investigation on the C–I bond activation by rollover cycloplatinated(ii) complexes bearing monodentate phosphane ligands: kinetic and kinetic isotope effect

Abstract

In this study, a series of platinum(II) complexes with the general formula of [Pt(O-bpy)(Me)(L)], 1, O-bpy = cyclometalated rollover 2,2′-bipyridine N-oxide and L = PPh₃ (1a), PPh₂Me (1b) and PPhMe₂ (1c) was prepared. The oxidative addition reaction of MeI reagent to the Pt(II) center was performed and kinetically investigated. In the Pt(II) → trans-Pt(IV) reaction (kinetic product), the electronic effects of the phosphanes determine the rates of the reactions and the following trend was obtained: PPhMe₂ > PPh₂Me > PPh₃. However, in the trans-Pt(IV) → cis-Pt(IV) isomerization (thermodynamic product), the steric effects induced by phosphane ligands controlled the conversion rates (PPh₃ > PPh₂Me > PPhMe₂). Also, a secondary α-deuterium kinetic isotope effect (KIE) study was performed for some of the reactions using the CD₃I reagent. The obtained small KIE values are indicative of a classic S_N2 mechanism with a tight transition state. Moreover, the reactions of 1a with MeI and CD₃I were monitored by (¹H and ³¹P{¹H}) NMR spectroscopy in order to gain an appropriate proposed mechanism. The NMR monitoring data for CD₃I exhibited the presence of scrambling twins in which the incoming CD₃ ligand occupies the CH₃ ligand position in the parent Pt(II) complex and vice versa. 1a, containing PPh₃ ligand, indicated the mentioned scrambling for both trans and cis products. However, the trans to cis isomerization for 1b (PPh₂Me) and 1c (PPhMe₂) was very slow, so NMR data showed the scrambling only for trans geometry.