Nickel(II) and Palladium(II)
Thiaethyneporphyrins.
Intramolecular Metal(II)−η<sup>2</sup>-CC Interaction
inside a Porphyrinoid Frame
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Abstract
3,18-Diphenyl-8,13-di-<i>p</i>-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(<i>p</i>-tolylhydroxymethyl)thiophene. The spectroscopic and structural
properties of 20-thiaethyneporphyrin reflect its macrocyclic aromaticity,
revealing a combination of the acetylene (≥C–CC–C≤)
and cumulene (>CCCC<) character of the
C18–C1–C2–C3 linker. The magnetic manifestations
of aromaticity and antiaromaticity of thiaethyneporphyrin and its
two-electron-oxidized derivative were observed using <sup>1</sup>H
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 <i>iso</i>-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)−η<sup>2</sup>-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 <sup>1</sup>H NMR
