Expanded ring N-heterocyclic carbene adducts of group 15 element trichlorides: synthesis and reduction studies

Abstract

Reactions of the expanded ring N-heterocyclic carbene, 6-Dip (:C{N(Dip)CH₂}₂CH₂, Dip = 2,6-diisopropylphenyl), with group 15 element trichlorides have yielded the monomeric complexes, [(6-Dip)ECl₃] (E = P, As or Sb), two examples of which (E = P and Sb) have been crystallographically characterised. Reduction of [(6-Dip)PCl₃] with KC₈ yielded the unusual tetraphosphorus dicationic complex, [(6-Dip)₂(μ-P₄)]Cl₂, the X-ray crystal structure of which shows it to be an ion-separate salt. The compound can also be prepared from the direct reaction of excess 6-Dip with PCl₃. Treatment of the cyclic amidinium salt, [6-MesH]Br (6-MesH = [HC{N(Mes)CH₂}₂CH₂]⁺, Mes = mesityl) with KC₈, leads to reductive coupling of the heterocycle and formation of the hindered bis(hexahydropyrimidine), (6-MesH)₂. An X-ray crystallographic analysis of (6-MesH)₂ shows the compound to have a long central C–C bond, while an electrochemical analysis reveals it to undergo an irreversible two-electron oxidation in dichloromethane solutions.