Structural characterization of 2,3,5,6-tetramethyl-p-phenylenediamine radical cation and its dimer in molecular crystals

Abstract Reaction between 2,3,5,6-tetramethyl- p -phenylenediamine (DDA) and bromanil (BA) gave two different crystalline products, needle-like and block-shaped crystals. The former was an alternate-stack 1:1 charge-transfer complex between DDA and BA with an electron-transfer ratio of about 0.7. The latter was a bromide salt of DDA, which was crystallized simultaneously with the decomposition of BA anions. The DDA molecules in the salt were dimerized even at room temperature. The similar products were obtained by a reaction between DDA and chloranil. The molecular and electronic structures are discussed with the calculation results of the density functional theory. This report has two additional meanings on the synthesis of charge-transfer complexes with using haloanils, which are commonly used as electron acceptors: (1) special attention must be paid for contaminating halide ions in characterizing reaction products and in discussing their properties, and (2) a new moderate synthetic root of halide complexes becomes available when using haloanils.