1,4-Alkyl migration associated with simultaneous S–C bond cleavage and N–C bond formation in platinum complexes of 2-aminothioethers. Characterization of intramolecular interligand charge transfer phenomenon

Abstract

Chemical reactions of Pt(pap)Cl₂ [pap = 2-(phenylazo)pyridine] with the N,S-donor atom ligands, 2-alkylthioanilines (HL, (H₂N⁁SR), where R = Me, -CH₂Ph, -CH₂-CHCH₂) in acetonitrile solvent under alkaline conditions yielded mixed chelate donor–acceptor complexes, [Pt(pap)(HN⁁SR)]⁺ (1⁺, R = Me), [Pt(pap)(HN⁁S)] (2, HN⁁S = 2-amidothiophenolate, R = Me, -CH₂Ph, -CH₂-CHCH₂) and [Pt(pap)(RN⁁S)] (RN⁁S = 2-(N-alkyl)amidothiophenolate 3 and 4, R = -CH₂Ph, -CH₂-CHCH₂). Unusual types of 1,4-alkyl group migration associated with simultaneous S–C bond cleavage and N–C bond formation were observed for R = -CH₂Ph, -CH₂-CHCH₂. However, for R = Me, only S–C bond cleavage occurred producing compound 2 at a higher temperature. Under identical experimental conditions the reaction of Pt(bpy)Cl₂ [bpy = 2,2′-bipyridine] with 2-methylthioaniline afforded [Pt(bpy)(HN⁁SR)]⁺ (5⁺, R = Me) as the only product with no S–C bond activation. The complexes have been characterized by ¹H NMR, UV-vis-NIR, ESI-MS, EPR and cyclic voltammetry studies. Single-crystal X-ray structures of the complexes, [1][OTf] (OTf = trifluoromethanesulfonate), 2, 3, 4 and [5][OTf] are reported. The Pt-pap complexes (1⁺ and 2–4) showed intense interligand charge transfer (ILCT) transition in the NIR-region (>800 nm). This band in the Pt-bpy analogue, [5]⁺ shifted to a much higher energy, at 545 nm. The cationic complex, [1][OTf] displayed two reversible responses at −0.21 and −0.94 V along with two irreversible anodic responses at 0.48 and 0.90 V. The molecular compounds, 2, 3 and 4 showed two reversible waves near −0.60 and −1.30 V and an irreversible anodic response near 0.5 V. The response at the anodic potential is presumably due to oxidation of 2-amidothioether/2-amidothiophenolate ligand while the reversible responses at cathodic potentials are due to successive reductions of the azo chromophore of the coordinated pap ligand. The redox processes are characterized by EPR and spectroelectrochemistry. Density-functional theory calculations were employed to confirm the structural features and to support the spectral and redox properties of the complexes.