Osmium(II)–bis(dihydrogen) complexes containing caryl,CNHC–chelate ligands: Preparation, bonding situation, and acidity

Abstract

The hexahydride complex OsH6(PiPr3)2 (1) reacts with the BF4-salts of 1-phenyl-3-methyl-1-H-benzimidazolium, 1-phenyl-3-methyl-1-H-5,6-dimethyl-benzimidazolium, and 1-phenyl-3-methyl-1-H-imidazolium to give the respective trihydride-osmium(IV) derivatives OsH3(κ2-Caryl,CNHC)(PiPr3)2 (2–4). The protonation of these compounds with HBF4·OEt2 produces the reduction of the metal center and the formation of the bis(dihydrogen)-osmium(II) complexes [Os(κ2-Caryl,CNHC)(η2-H2)2(PiPr3)2]BF4 (5–7). DFT calculations using AIM and NBO methods reveal that the Os–NHC bond of the Os-chelate link tolerates a significant π-backdonation from a doubly occupied dπ(Os) atomic orbital to the pz atomic orbital of the carbene carbon atom. The π-accepting capacity of the NHC unit of the Caryl,CNHC-chelate ligand, which is higher than those of the coordinated aryl group and phosphine ligands, enhances the electrophilicity of the metal center activating one of the coordinated hydrogen molecules of 5–7 toward the heterolysis. As a result, these compounds are strong Brønsted acids with pKawater values between 2.5 and 2.8. In acetonitrile the hydrogen molecules of 5 and 6 are displaced by the solvent, the resulting bis(solvento) compounds [Os(κ2-Caryl,CNHC)(CH3CN)2(PiPr3)2]BF4 (8, 9) react with acetylacetonate (acac) and cis-1,2-bis(diphenylsphosphino)ethylene (bdppe) to give Os(κ2-Caryl,CNHC)(acac)(bdppe) (10, 11) as a mixture of the two possible isomers, namely with P trans to the aryl group or to the NHC moiety.