Comparative study of electronic and steric properties of bulky, electron-rich bisphosphinoethane, bis-NHC and phosphino-NHC chelating ligands in analogous rhodium(I) and iridium(I) COD and carbonyl complexes

• Characteristic distortions of the square planar geometries were found.
• Larger ligands in trans-position to the phosphino-NHC ligand favour its abnormal binding.
• The Bis-NHC and normally bound NHCP ligands possess similar electron donating abilities .
• The abnormally bound aNHCP ligand shows exceptionally high donating character.

The synthesis and characterization of rhodium(I) and iridium(I) COD (1,5-cyclooctadiene) and carbonyl complexes containing small-bite angle and bulky bisphosphine (bis(di-tert-butylphosphino)ethane = dtbpe), bis-NHC (1,1′-di-tert-butyl-3,3′-methylenediimidazolin-2,2′-ylidene = BisNHC) and phosphino-functionalized NHC (3-tert-butyl-1-(di-tert-butylphosphinomethyl)-imidazolin-2-ylidene = NHCP) ligands is described. X-ray diffraction analysis of most compounds made and spectroscopic data permitted a valuable comparison of the characteristic steric and electronic properties of these related ligand classes and also of the “abnormally” bound form of the NHCP ligand (aNHCP). While the dicarbonyl complexes indicated similar overall donating abilities for the BisNHC and NHCP ligands, dtbpe and the aNHCP ligand were shown to be significantly less or more electron donating, respectively. These complexes also allowed the evaluation of the different σ-donating influences of the NHC moiety in comparison to the phosphine moiety within the hybrid NHC–P systems.

The NHC-phosphine hybrid system studied in this work – despite containing only one NHC unit – shows similar or even greater donor capabilities than a related bulky bis-NHC ligand. The bulky bisphosphine ligand dtbpe was characterized clearly as the weakest donor ligand of the series.Figure optionsDownload as PowerPoint slide