Complexes of acridine and 9-chloroacridine with I-2: Formation of unusual I-6 chains through charge-transfer interactions involving amphoteric I-2

Acridine and 9-chloroacridine form charge-transfer complexes with iodine in which the nitrogen-bound I-2 molecule is amphoteric; one end serves as a L ewis acid to the heterocyclic donor, while the other end acts as a Lewis ba se to a second I-2 molecule that bridges two acridine I-2 units. In the acr idine derivative [(acridine .I-2)(2).I-2, I], the dimer has a "zigzag" conf ormation, while in the 9-chloroacridine derivative [(9-Cl-acridine .I-2)(2) .I-2, 2], the dimer is "C-shaped". The thermal decomposition of the two com plexes is very different. Compound 1 loses one molecule of I-2 to form an a cridine .I-2, intermediate, which has not been isolated. Further decomposit ion gives acridine as the form II polymorph, exclusively. Decomposition of 2 involves the loss of two molecules of I-2 to form a relatively stable int ermediate [(9-Cl-acridine)(2). I-2, 3]. Compound 3 consists of two 9-Cl-acr idine molecules bridged through N . . .I charge-transfer interactions by a single I-2 molecule. This compound represents the first known example, in w hich both ends of an I, molecule form interactions in a complex that is not stabilized by the extended interactions of an infinite chain structure. Th e ability of the terminal iodine of an N-bound I-2 to act either as an elec tron donor (complexes 1 and 2) or as an electron acceptor (complex 3) can b e understood through a quantum mechanical analysis of the systems. Both ele ctrostatic interactions and the overlap of frontier molecular orbitals cont ribute to the observed behavior.