Structure and bonding of tetracyanopyrrolides

Salts bearing the tetracyanopyrrolide anion (TCP) were synthesized from tetracyano-1,4-dithiin and sodium azide according to a modified synthetic protocol. The solid-state structures of HTCP, MeTCP, Me₄NTCP, NaTCP, and the ionic liquid EMIMTCP (EMIM = 1-ethyl-3-methylimidazolium) are discussed for the first time. According to experimental and computational data, the TCP anion can be regarded as a pseudohalide with π bonds highly delocalized over the entire anion including the four cyano groups. The planar anion is C _2v symmetric and represents a multifunctional binary CN ligand, which is capable of forming classic coordination bonds through the lone pairs of the CN groups or the ring nitrogen atom. In addition, noncovalent interactions such as π-π interactions are found and can be used to strengthen structural and electronic communication between organic ligands.