Cyclo-tetramerization of isocyanides promoted by cyclopentadienyl chromium complexes

Yuanjin Chen, Yaqi Zhao, Xianghui Shi, Qiong Yuan, Rui Feng, Zhenfeng Xi, Junnian Wei*

Chinese Chemical Letters, 2025. DOI: 10.1016/j.cclet.2025.111130.

        The reductive cyclo-coupling of isocyanides is a pivotal reaction that facilitates the rapid construction of intricate cyclic compounds in a single-step process. In this work, treatment of simple precursor 1 (Cp^#CrLCl, L = CAAC, NHC, PCy₃, or PPh₂Et; Cp^# = Cp^* or Cp*TMS) with XylNC (2,6-dimethylphenyl isocyanide) led to the reductive coupling of isocyanides, yielding either complex 2 {(Cp^*TMSCr)2[μ-C₄(NXyl)₄]} or complex 6 {(Cp^*CrCl)2[μ-C₄(NXyl)₄]}, corresponding to the tetramerization of isocyanides. Control experiments and in-situ monitoring were carried out to understand the reaction mechanism, revealing various side reaction pathways during the isocyanide tetramerization. SQUID and DFT calculations provided insights into the electronic structures. In complex 2, the energies of nonet and broken-symmetry singlet states are close and significantly lower than other spin states, indicating two independent high-spin Cr(II) centers with weak antiferromagnetic coupling. A similar situation is observed in complex 6, where two independent high-spin Cr(III) centers are coupled antiferromagnetically. In both complexes 2 and 6, the tetrameric isocyanide rings, receiving two electrons from Cr centers, show averaged bond lengths and display moderate aromatic characteristics.