A Persistent Ni(0)–Pentalene Complex with High Fluxionality: Does Stronger Antiaromaticity Promote Metal–Ligand Interactions?

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Abstract
Despite the long-standing use of transition metal complexation for stabilizing antiaromatic hydrocarbons, the underlying factors that govern their coordination behavior remain unclear. A Ni(0) complex, [Ni(cod)(1)], featuring a benzothiophene-S,S-dioxide-fused pentalene ligand, was synthesized and characterized, and compared to a benzothiophene-fused analog, [Ni(cod)(2)], which bears a more antiaromatic yet less electron-accepting pentalene ligand. Single-crystal X-ray diffraction, X-ray absorption spectroscopy, and variable-temperature NMR, combined with computational analyses, reveal that the enhanced stability of [Ni(cod)(1)] arises primarily from substantial π-back donation facilitated by strongly electron-withdrawing SO2 groups that lower the LUMO energy of 1. Comparisons to [Ni(cod)(2)] further demonstrate that a more antiaromatic core does not necessarily lead to stronger coordination to the Ni center. Surprisingly, the robust [Ni(cod)(1)] complex is fluxional in solution; the Ni center migrates reversibly between the two five-membered rings of the pentalene ligand via a bond shift mechanism.
Funding Information
  • Japan Society for the Promotion of Science (20H05864)
  • Japan Society for the Promotion of Science (21H01916)
  • Japan Society for the Promotion of Science (24H00458)
  • Japan Society for the Promotion of Science (22K05095)
  • Japan Society for the Promotion of Science (24H01851)
  • National Institute of General Medical Sciences (R35GM133548)

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