Tricarbonylchromium(0) complexes of 1,3,5-triphenylbenzene: an X-ray crystallographic and high field NMR spectroscopic study

The reaction of 1,3,5-triphenylbenzene (TPB) with Cr(CO)6 yields products in which a Cr(CO)3 moiety is bonded to the central ring or to one, two or all three peripheral phenyl rings. The structure of the TPB · 3Cr(CO)3 complex has been determined by single crystal X-ray diffraction techniques. The complex crystallizes in the space group P21/n with unit cell parameters a 8.316(2) Å, b 15.498(5) Å, c 22.254(4) Å, β 92.18(2)°, V 2865.8(11) Å3 and Z = 4. The structure has been solved by direct and Fourier methods and refined by block diagonal matrix least squares to R1 and R2 values of 0.053 and 0.053, respectively, for 2830 observed reflections. The structure reveals that the chromium tricarbonyl groups are bonded to the three peripheral phenyls and these aryl rings adopt dihedral angles of 35° −26° and 14° with respect to the central ring; these dihedral angles are remarkably similar to those in the free ligand itself. All three Cr(CO)3 tripods are oriented to within a few degrees of being perfectly staggered with respect to the aryl carbon atoms. The 500 MHz 1H and 125 MHz 13C NMR spectra of these complexes reveal that the peripheral rings do not exhibit restricted rotation even at −90°C. This contrasts with the situation for (hexaphenylbenzene)Cr(CO)3 the 1H NMR spectrum of which may indicate slowed rotation of the phenyl rings even at room temperature.