Interplay of Halogen and π-π Charge-Transfer Bondings in Intermolecular Associates of Bromo- or Iododinitrobenzene with Tetramethyl-p-phenylenediamine

Two modes of intermolecular interactions (halogen and pi-pi charge-transfer bonding) between bromo- or iodorlinitrobenzene (XDNB) and tetramethyl-p-phenylenediarnine (TMPD) are compared. X-ray crystallography revealed that TMPD-XDNB cocrystals comprise alternating donors/acceptors stacks formed by pi-bonded (cofacial) TMPD and XDNB molecules. These structures also show two-point (C-X center dot center dot center dot O-N) halogen bonding between XDNB molecules resulting in formation of (XDNB)2 dithers. In solutions, XDNB and TMPD molecules formed 1:1 complexes showing strong absorption bands near 550 nm which followed the same Mulliken correlation as the associates of TMPD with the (halogen-free) nitro- and cyanobenzenes. In accord with the experimental data, density functional theory calculations with the M062X functional showed that TMPD-XDNB associates formed via pi-pi charge-transfer bonding are more stable (by 6-12 kcal/mol) than their halogen-bonded analogues. If XDNB is replaced with iodo- or bromoperfluorinated benzenes, or TMPD is replaced with pyridine, the energy gap between the pi-pi and halogen-bonded associates decreased. The analysis of the molecular-orbital interactions and surface electrostatic potentials of the interacting species indicated that charge-transfer contributions represent a critical component which determines variations of the strength of halogen bonding in these systems.