Synthesis and Characterization of Tetrakis(carbene)ruthenium(II) Complexes Featuring an [Ru(NHC) 4 ] 2+ Core

The synthesis of different ruthenium(II) complexes with an [Ru(NHC) 4 ] 2+ (NHC = N‐heterocyclic carbene) core is reported. The reaction of [(η 6 ‐C 6 H 4 Me i Pr)RuCl(μ‐Cl)] 2 with 1 equiv. of the NHC i Pr 2 Im in thf leads to the clean formation of [(η 6 ‐C 6 H 4 Me i Pr)Ru( i Pr 2 Im)Cl 2 ] ( 1 ), but use of an excess of the carbene at higher temperatures affords side products such as [Ru( i Pr 2 Im) 4 H]Cl ( 9 ). Complexes of the type [Ru(R 2 Im) 4 Cl 2 ] [R = Me ( 2 ), n Pr ( 3 ), Me i Pr ( 6 )] were synthesized by the reaction of [Ru(PPh 3 ) 3 Cl 2 ] with Me 2 Im, n Pr 2 Im, and Me i PrIm. Compound 6 was isolated as a mixture of isomers that differ in the relative orientation of the asymmetrically substituted NHC ligand with respect to the Ru–Cl vector. Replacement of the chlorido ligands in 2 and 3 with acetonitrile leads to the formation of [Ru(R 2 Im) 4 (CH 3 CN)]Cl 2 [R = Me ( 4 ), n Pr ( 5 )]. At higher temperatures, complex 6 eliminates HCl under C–H activation of one of the NHC ligand i Pr groups to give the cyclometallation product 7 . The reaction of [Ru(PPh 3 ) 3 Cl 2 ] with i Pr 2 Im afforded the cyclometallation product [Ru( i Pr 2 Im) 3 { i Pr(C 3 H 6 )Im}Cl] ( 8 ) or the hydrido complex [Ru( i Pr 2 Im) 4 (H)]Cl ( 9 ) as the main product depending on the reaction conditions. The complexes 1 , 3 , 4 , and 7 as well as two isomers of 6 were structurally characterized.