Fig. 2

Crystallographic structures and packing of 2a and 2b. a, c Crystallographic structures of 2a and 2b with thermal ellipsoids at the 50% probability level. The bowl depths of 2a and 2b are indicated. b, d Packing structures of 2a in the a–c plane and 2b in the b–c plane. C atom is shown in gold and N atom in navy blue. All hydrogen atoms are omitted for clarity