Synthesis and characterisation of mixed ligand Pt(ii) and Pt(iv) oxadiazoline complexes

Abstract

The nitrile ligands in trans-[PtX₂(PhCN)₂] (X = Cl, Br, I) undergo sequential 1,3 dipolar cycloadditions with nitrones R¹R²CN⁺(Me)–O⁻ (R¹ = H, R² = Ph; R¹ = CO₂Et, R² = CH₂CO₂Et) to selectively form the Δ⁴-1,2,4-oxadiazoline complexes trans-[PtX₂(PhCN) {NC(Ph)–O–N(Me)–CR¹R²}] or trans-[PtX₂{NC(Ph)–O–N(Me)–CR¹R²}₂] in high yields. The reactivity of the mixed ligand complexes trans-[PtX₂(PhCN){NC(Ph)–O–N(Me)–CR¹R²}] towards oxidation and ligand substitution was studied in more detail. Oxidation with Cl₂ or Br₂ provides the Pt(IV) species trans-[PtX₂Y₂(PhCN){NC(Ph)–O–N(Me)–CH(Ph)}] (X, Y = Cl, Br). The mixed halide complex (X = Cl, Y = Br) undergoes halide scrambling in solution to form trans-[PtX_(4−n)Y_n(PhCN){NC(Ph)–O–N(Me)–CH(Ph)}] as a statistical mixture. Ligand substitution in trans-[PtCl₂(PhCN){NC(Ph)–O–N(Me)–CR¹R²}] allows for selective replacement of the coordinated nitrile by nitrogen heterocycles such as pyridine, DMAP or 1-benzyl-2-methylimidazole to produce mixed ligand Pt(II) complexes of the type trans- [PtX₂(heterocycle){NC(Ph)–O–N(Me)–CR¹R²}]. All compounds were characterised by elemental analysis, mass spectrometry, IR and ¹H, ¹³C and ¹⁹⁵Pt NMR spectroscopy. Single-crystal X-ray structural analysis of (R,S)-trans-[PtBr₂{NC(Ph)–O–N(Me)–CH(Ph)}₂] and trans-[PtCl₂(C₅H₅N){NC(Ph)–O–N(Me)–CH(Ph)}] confirms the molecular structure and the trans configuration of the heterocycles relative to each other.