Nickel(II) and Palladium(II) Thiaethyneporphyrins. Intramolecular Metal(II)−η2-CC Interaction inside a Porphyrinoid Frame

3,18-Diphenyl-8,13-di-p-tolyl-20-thiaethyneporphyrin ([18]­thiatriphyrin­(4.1.1)), which formally contains an C1–C2 ethyne moiety instead of pyrrole embedded in the macrocyclic framework of 21-thiaporphyrin, was obtained in a modification of the “3 + 1” approach using the ethyne analogue of tripyrrane (1,4-diphenyl-1,4-di(pyrrol-2-yl)but-2-yne) and 2,5-bis­(p-tolylhydroxymethyl)­thiophene. The spectroscopic and structural properties of 20-thiaethyneporphyrin reflect its macrocyclic aromaticity, revealing a combination of the acetylene (≥C–CC–C≤) and cumulene (>CCCC1H NMR spectroscopy and were confirmed by density functional theory calculations involving chemical shifts and nucleus-independent chemical shift analysis. Protonation of 20-thiaethyneporphyrin yielded a nonaromatic tautomer of iso-20-thiaethyneporphyrin, locating the saturated meso carbon adjacent to thiophene. Insertion of palladium­(II) and nickel­(II) into 20-thiaethyneporphyrin afforded planar palladium­(II) thiaethyneporphyrin and low-spin diamagnetic nickel­(II) 20-thiaethyneporphyrin as determined by X-ray crystallography. 20-Thiaethyneporphyrin acts as a dianionic ligand that coordinates through the two nitrogen and one sulfur donors. Metal­(II) ions are uniquely exposed to form an intramolecular metal­(II)−η2-CC bond, whereas the organometallic fragment is coplanar with the whole macrocycle. Coordination of pyridine converts diamagnetic nickel­(II) thiaethyneporphyrin into its paramagnetic counterpart as determined by 1H NMR.