X-ray structure of 2d. (a) ORTEP view of 2d (50% probability displacement ellipsoids). Hydrogen atoms are drawn as circles of arbitrary radii. (b) Difference-Fourier map plotted in the mean plane of labelled atoms. Dotted contour lines indicate negative, dashed lines zero, and full lines positive residual electron density. (c) Packing of molecules of 2d as viewed along a axis.