Synthesis, characterisation and photochemistry of PtIVpyridylazido acetato complexes

Abstract

Pt^II azido complexes [Pt(bpy)(N₃)₂] (1), [Pt(phen)(N₃)₂] (2) and trans-[Pt(N₃)₂(py)₂] (3) incorporating the bidentate diimine ligands 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen) or the monodentate pyridine (py) respectively, have been synthesised from their chlorido precursors and characterised by X-ray crystallography; complex 3 shows significant deviation from square-planar geometry (N₃–Pt–N₃ angle 146.7°) as a result of steric congestion at the Pt centre. The novel Pt^IV complexes trans, cis-[Pt(bpy)(OAc)₂(N₃)₂] (4), trans, cis-[Pt(phen)(OAc)₂(N₃)₂] (5), trans, trans, trans-[Pt(OAc)₂(N₃)₂(py)₂] (6), were obtained from 1–3viaoxidation with H₂O₂ in acetic acid followed by reaction of the intermediate with acetic anhydride. Complexes 4–6 exhibit interesting structural and photochemical properties that were studied by X-ray, NMR and UV-vis spectroscopy and TD-DFT (time-dependent density functional theory). These Pt^IV complexes exhibit greater absorption at longer wavelengths (ε = 9756 M⁻¹ cm⁻¹ at 315 nm for 4; ε = 796 M⁻¹ cm⁻¹ at 352 nm for 5; ε = 16900 M⁻¹ cm⁻¹ at 307 nm for 6, in aqueous solution) than previously reported Pt^IV azide complexes, due to the presence of aromatic amines, and 4–6 undergo photoactivation with both UVA (365 nm) and visible green light (514 nm). The UV-vis spectra of complexes 4–6 were calculated using TD-DFT; the nature of the transitions contributing to the UV-vis bands provide insight into the mechanism of production of the observed photoproducts. The UV-vis spectra of 1–3 were also simulated by computational methods and comparison between Pt^II and Pt^IV electronic and structural properties allowed further elucidation of the photochemistry of 4–6.