Diorganoplatinum(ii) complexes with chelating PN ligand 2-(diphenylphosphinoamino)pyridine; synthesis and kinetics of the reaction with MeI

Abstract

New organoplatinum(II) complexes [PtR₂(PN)] (PN = 2-(diphenylphosphinoamino)pyridine, R = Me, 1a, or p-MeC₆H₄, 1b) were synthesized by the reaction of [Pt(p-MeC₆H₄)₂(SMe₂)₂] or [Me₂Pt(μ-SMe₂)₂PtMe₂] with 1 and 2 equiv. of PN, respectively. The reaction of Pt(II) complexes 1 with MeI gave the Pt(IV) complexes [PtR₂(PN)MeI] (R = Me; 2a, and p-MeC₆H₄; 2b). All the complexes were fully characterized using multinuclear (¹H, ³¹P, ¹³C, and ¹⁹⁵Pt) NMR spectroscopy. Density functional theory calculations have been performed to find approximate structures for all described complexes. The platinum(II) complexes have a 5d_π(Pt)-π*(PN) metal-to-ligand charge-transfer band, which was used to easily follow the kinetics of their reactions with MeI. The classical S_N2 mechanism was suggested. The rates of the reactions at different temperatures were measured and were consistent with the proposed mechanism, large negative ΔS^‡ values were found in each reaction. The PN chelating complexes [PtR₂(PN)], 1, reacted almost 100 or 300 times slower with MeI as compared to that of the NN chelating complex [PtR₂(bpy)] (bpy = 2,2′-bipyridine) in acetone or benzene, respectively. This was attributed to the π-acceptance through the P ligating atom of PN ligand, which decreases the electron density of Pt(II) in PN chelating complexes.