Coordination Singularities of a Bis(p-xylyl)bis(benzimidazolylidene) Ligand and the Bis-iridium and -rhodium-Related Complexes

The reaction of bis(alpha,alpha'-p-xylyl)bis(benzimidazolium) dichloride with [IrCp*Cl-2](2) or [RhCl(COD)](2) affords the corresponding dimetallic bis-N-heterocyclic carbene complexes of Ir and Rh. The reaction with the iridium complex occurs by the transmetalation method, in the presence of Ag2O, while the reaction with the rhodium complex is carried out in the presence of NaOtBu. The two complexes display an anti configuration of the bis-NHC ligand, with the two metal atoms pointing at different faces of the bis-carbene ligand. In both complexes, the two metal fragments disclose different coordination environments (in-out, with respect to the inner and outer part of the cyclophane-bis-NHC), as a consequence of noncovalent interactions. DFT calculations have been used to rationalize this "less intuitive" coordination singularity. The reaction of the bis(alpha,alpha'-p-xylyl)bis(benzimidazolium) dichloride with [RhCl(CO)(2)](2) in the presence of Ag2O affords a dirhodium complex in which the two metals are on the same side of the ligand, which adopts a syn conformation. In the latter case, the two metals are bridged by a chloride and hydroxyl ligands, therefore facilitating the syn disposition of the ligand.