Structure and dynamics of crowns containing the phenyldinaphthylmethane subunit (a three-bladed propeller): observations of correlated rotation of the propeller blades and certain ether segments

Abstract

We describe some crowns (dinaphthopolyoxacycloalkanins) containing the phenyldinaphthylmethane subunit, regarded as a propeller, the aryl rings (blades) of which are fluxional, twisting about the aryl–methine bonds. This torsional propeller movement is fluxional at room temperature on the n.m.r. timescale for some crowns, with ΔG^‡_230–240 of ca. 10–11 kcal mol^–1, while for others (regarded as locked propellers at room temperature), ΔG^‡₃₈₀ is ca. 19 kcal mol^–1. Whether fluxional in solution or no, the crowns and their salt complexes commonly exhibit just one propeller isomer in the solids; X-ray crystal structures of one example of a crown (4), and one example of a salt complex (7), are described in this paper. Crystal data for (4): C₃₈H₄₀O₈·0.5(CH₃)₂CO, monoclinic, space group P2₁/n, a= 14.403(2), b= 13.428(1), c= 17.797(3)Å, β= 93.18(1)°, R= 0.070. Crystal data for (7): C₄₀H₄₄O₉KNCS·(CH₃)₂CO, monoclinic, space group P2₁/c, a= 12.721(1), b= 15.197(1), c= 22.038(3)Å, β= 92.55(1)°, R= 0.065. N.m.r. spectroscopic evidence in solution suggests more than one propeller isomer in solution for fluxional crowns with six oxygen donors, while for locked crowns there is only one. The single observable process (two-ring flip) for the fluxional crowns with 5 and 7 oxygen donors exchanges environments of the two differentiable naphthyl rings and phenyl edges for pairs of isomers which cannot themselves be differentiated by n.m.r. spectroscopy. A lower energy flip exchanges differentiable isomers via a flip of one naphthyl and one phenyl ring for the six-donor crowns. The flexing of the polyether segments in a locked propeller has been shown to be correlated with the twisting of its propeller, using a combination of dynamic n.m.r. studies with ¹H and ²H nuclei.