Metall-?-Komplexe von Benzolderivaten. XXXV. Bis(η6-bimesityl)chrom. Magnetische Abschirmung und konformative Beweglichkeit

Article abstract of DOI:10.1016/0022-328X(90)80174-X

Metal-atom ligand-vapor cocondensation affords the complexes bis(η⁶-bimesityl)chromium (2), (η⁶-bimesityl)(η⁶-benzene)chromium (5) and bis(η⁶-2,4,8,10-tetramethylbiphenyl)chromium (4), and conventional carbonyl substitution yields the compounds (η⁶-bimesityl)(tricarbonyl)chromium (7) and μ(ν⁶ : η⁶-bimesityl)bis<(tricarbonyl)chromium> (8). 1H NMR data for 2 suggest that in the rotamer of lowest energy the two bimesityl ligands with regard to the sandwich axis have a twist angle, α, of about 90 of 270 deg, respectively.These two rotamers undergo torsional interconversion with an activation barrier of ΔG_c^<*> 75 kJ/mol.The 1H NMR spectra of 2, 5, 7 and 8 are assigned and the large chemical shift differences are traced to the unique positions the protons adopt in the periphery of the bis(η⁶-arene)metal core.The NOE difference spectrum of 2 is temperature dependent; rapid exchange on the T₁ scale is achieved at 53 deg C, a temperature at which, on the chemical shift time scale, slow exchange still prevails.In 4, the ortho-methyl groups of the 8,10-dimethyl substituent relative to the central metal, exclusively occupy exo-positions.Correspondingly, the activation barrier for the conformational interconversion is decreased in 4 compared to 2.Owing to the lower symmetry of the ligand o,o',p,p'-tetramethylbiphenyl (3), the binary complex (3)2Cr is formed as a mixture of the forms meso-4 and rac-4.Although 4 is already in the fast exchange region at room temperature, the 1H NMR spectrum still displays a marked temperature dependence.The latter is caused by the fact that for meso-4 the two equilibrating rotamers are enantiomers, whereas for rac-4 they are diastereomers.Thus, in contrast to meso-4, for rac-4 the population ratio of the two interconverting rotamers is temperature dependent.