Syntheses, Structures and Conformational Dynamics of 1,3,5-Tris(3″-ethynylbi­phen­yl-2′-yl)benzene Derivatives.

Following a route described by Müllen et al. we improved the preparation of 1,2,3-tris(3″-iodobiphenyl-2′-yl)benzene ( 4). During the required iododesilylation with iodine monochloride the unexpected formation of a mono-chlorinated compound 6 as major product was observed, when an excess of the iodination reagent was employed. The reaction mechanism for the formation of this compound involving an oxidizing process is discussed. X-ray crystallography of the subsequent alkynylated product 7 proves the location of the chlorine atom at the central benzene ring. The related compound 5 was likewise investigated by X-ray analysis revealing a preferred solid-state conformation ('2-up-1-down') different from that of the chlorinated compound ('propeller' conformation). Compound 7 showed remarkable conformational dynamics in solution as observed by 1H NMR spectroscopy. Temperature-dependent measurements allowed the calculation of an energy difference of two major conformers of ca. 2 kJ/mol and a rotational barrier Δ Grot of ca. 77 kJ/mol. These values were convincingly confirmed by DFT calculations. A subsequent Sonogashira reaction of 4 with a suitable bipyridine derivative provided a unique trivalent bipyridine derivative. [ABSTRACT FROM AUTHOR]