Computational and experimental structural studies of selected chromium(0) monocarbene complexes

• Synthesis of chromium Fischer carbene complexes, with different heteroatom and heteroaromatic substituents.
• Density functional theory calculations on different conformations of the carbene substituents.
• Comparison with X-ray crystal structures.
• NBO calculations of carbonyl-heteroarene interactions.

A set of chromium Fischer carbene complexes, with different heteroatom and heteroaromatic substituents on the carbene carbon atom, were studied. Density functional theory as well as single crystal diffraction techniques were employed. The complexes studied, [Cr(CO)5{C(X)Z}], had their substituents varied systematically to give six complexes, with X = ethoxy (–OCH2CH3) or amino (–NH2,) substituents as the heteroatom substituents and Z = 2-furyl (–C4H3O), 2-thienyl (–C4H3S), or 2-(N-methyl)pyrrolyl (–C4H3NCH3) as the heteroaromatic substituents. These complexes were studied in terms of the conformation of the complexes, in particular the dihedral angle between the heteroatom of the heteroatom substituent and the heteroatom of the heteroaromatic substituents. Five of the six complexes preferred the syn conformation, which seems to be the trend for this type of complex. Only one complex, [Cr(CO)5{C(OEt)2-furyl}], (1), has a distinct preference for the anti conformation according to DFT calculations. Calculated and experimentally measured infrared vibrational spectra of the complexes were compared. X-ray structural studies in the solid state were performed for all six of the complexes. It was found that these structures corresponded to the calculated structures. Using NBO calculations, an explanation for the unexpected anti conformation of [Cr(CO)5{C(OEt)2-furyl}], (1) was investigated. It was concluded that both steric and electronic factors influence the conformations of the carbene complexes, with the extent of contribution of these two factors varying for each of the different carbene substituents.

A DFT and X-ray structural study was undertaken to explain the preference for the anti conformation of the carbene complex [Cr(CO)5C(OEt)(F)], (F = 2-furyl) 1, while the trend for analogous heteroarene-containing carbene complexes with general formula [Cr(CO)5C(X)(Z)], (X = OEt, NH2; Z = 2-furyl, 2-thienyl) is to exhibit the syn conformation.Figure optionsDownload as PowerPoint slide