Chromium tricarbonyl complexes with biphenylene as η6 ligand: synthesis, structure, dynamic behaviour in solid state and thermal η6,η6-haptotropic rearrangements. Experimental (NMR) and theoretical (DFT) studies

The reaction of biphenylene (1) with Cr(CO)3Py3/BF3·OEt2 at 25 °C gives a mixture of two known isomeric complexes: tricarbonyl(1-4,4a,8b-η6-biphenylene)chromium (2) (yield 62%) and hexacarbonyl(μ-η6:η6-biphenylene)bis-chromium (3) (yield 4%). Complexes 2 and 3 were separated by thin layer chromatography on silica and characterised by 1H and 13C, as well as 13C CP MAS-NMR. X-ray structure analysis for 3 proved trans-orientation of the chromium tricarbonyl groups. A novel regioselective method for the synthesis of (η6-biphenylene)chromium tricarbonyl complexes bearing a substituent R (D (4), CH3 (5)) in position 1 of the coordinated ring was proposed. The kinetics of η6,η6-inter-ring haptotropic rearrangement (IRHR) for 4 in inert, noncoordinating C6F6 was investigated by 2H–{1H}-NMR spectroscopy. Density function theory (DFT) calculations for ground, intermediate and transition states were done and fitted well with experiment.