Ruthenium(ii) complexes of N-heterocyclic carbenes derived from imidazolium-linked cyclophanes

Abstract

The present work seeks to characterize, in the light of electronic structure calculations, an unusual metal–[(η¹-NHC)₂:(η⁶-arene)] bonding situation in a set of ruthenium(II) complexes containing the ortho-xylylene-linked-bis(NHC)cyclophane ligand (NHC-cyclophane) (1), which binds to the ruthenium center through two carbene carbons and one of the arene rings. The nature of ruthenium(II)–[(η¹-NHC)₂:(η⁶-arene)] bonding was investigated in the light of EDA-NOCV, NBO and QTAIM analyses by adopting 1 as a model compound. The interplay between the ortho-cyclophane scaffold with different families of five-membered carbenes, such as imidazole, 1, triazole-based NHCs (Enders’ carbenes), 2, and P-heterocyclic carbenes (PHCs), 3, was investigated. The metal–[(η¹-NHC)₂:(η⁶-arene)] bonding situation was also extended to heavier analogues, such as N-heterocyclic silylenes (NHSi) and N-heterocyclic germylenes (NHGe), in order to address how the basicity of NHC, NHSi and NHGe is affected by the cyclophane framework. The results reveal that ruthenium(II)–[(η¹-NHC)₂:(η⁶-arene)] is more covalently than electrostatically bonded and that the degree of covalence is larger in PHCs than in NHCs or Enders’ carbenes. It is also revealed that the covalent character in the ruthenium(II)–[(η¹-NHGe)₂:(η⁶-arene)] and ruthenium(II)–[(η¹-NHSi)₂:(η⁶-arene)] bonds is larger than in ruthenium(II)–[(η¹-NHC)₂:(η⁶-arene)].