Anion–Anion Interactions in the Crystal Packing of Functionalized Methanide Anions: An Experimental and Computational Study

Examination of the crystal structures of (Me4N)­(dcnm) (1), (Me4N)­(dcnom) (2), and (Me4N)­(nbdm) (3) [dcnm = dicyanonitrosomethanide, dcnom = dicyanonitromethanide, nbdm = nitroso-N,N-bis­(dicyanomethanide)] reveals the anions pack in an unusual columnar array, with distances between the planar species suggestive of π–π stacking. This columar packing motif is not observed in the crystal structures of (Me4N)­(ccnm) (4) and (Me4N)­(ccnom) (6) (ccnm = carbamoylcyanonitrosomethanide, ccnom = carbamoylcyanonitromethanide), in which hydrogen bonding between anions is the dominant supramolecular interaction. Ab initio calculations performed at the HF and MP2 levels of theory on ionic clusters of varying size further explored the nature and strength of anionic interactions observed in crystal structures. The first syntheses of the nbdm and ccnom anions are also reported.