Cyclometallated Pt(ii) and Pd(ii) complexes with a trithiacrown ligand

We report the synthesis and full characterization for a series of cyclometallated complexes of Pt(ii) and Pd(ii) incorporating the fluxional trithiacrown ligand 1,4,7-trithiacyclononane ([9]aneS₃). Reaction of [M(C^N)(µ-Cl)]₂ (M = Pt(ii), Pd(ii); C⁁N = 2-phenylpyridinate (ppy) or 7,8-benzoquinolinate (bzq)) with [9]aneS₃ followed by metathesis with NH₄PF₆ yields [M(C⁁N)([9]aneS₃)](PF₆). The complexes [M(C⁁P)([9]aneS₃)](PF₆) (M = Pt(ii), Pd(ii); C⁁P = (CH₂C₆H₄P(o-tolyl)₂-C, P)⁻) were synthesized from their respective [Pt(C⁁P)(µ-Cl)]₂ or [Pd(C⁁P)(µ-O₂CCH₃)]₂ (C⁁P) starting materials. All five new complexes have been fully characterized by multinuclear NMR, IR and UV-Vis spectroscopies in addition to elemental analysis, cyclic voltammetry, and single-crystal structural determinations. As expected, the coordinated [9]aneS₃ ligand shows fluxional behavior in its NMR spectra, resulting in a single¹³C NMR resonance despite the asymmetric coordination environment of the cyclometallating ligand. Electrochemical studies reveal irreversible one-electron metal-centered oxidations for all Pt(ii) complexes, but unusual two-electron reversible oxidations for the Pd(ii) complexes of ppy and bzq. The X-ray crystal structures of each complex indicate an axial M–S interaction formed by the endodentate conformation of the [9]aneS₃ ligand. The structure of [Pd(bzq)([9]aneS₃)](PF₆) exhibits disorder in the [9]aneS₃ conformation indicating a rare exodentate conformation as the major contributor in the solid-state structure. DFT calculations on [Pt([9]aneS₃)(ppy)](PF₆) and [Pd([9]aneS₃)(ppy)](PF₆) indicate the HOMO for both complexes is primarily d_z² in character with a significant contribution from the phenyl ring of the ppy ligand and p orbital of the axial sulfur donor. In contrast, the calculated LUMO is primarily ppy π* in character for [Pt([9]aneS₃)(ppy)](PF₆), but d_x²−y² in character for [Pd([9]aneS₃)(ppy)](PF₆).