Studies on molecular complexes of macrocyclic polyethers

Abstract

The investigations report the ability of several donor macrocyclic polyethers, 15?Crown?5, 18?Crown?6, benzo?15?Crown?5, dibenzo?18?Crown?6, dicyclohexyl?18?Crown?6 and dibenzo?24? Crown?8, to form molecular complexes with diverse organic ??acceptors, dichlorodicyanobenzoquinone (DDQ), trinitro, tetranitro, 9?dicyanomethylene fluorenones (TEF, TEHP, CNTF), cyanomethylene indanedione (CNIND), tetracyano pyrazine (TCNPY) and hexafluorobenzene (HFB). Optical absorption, ¹H, ¹?F and ¹³C NMR and IR spectral methods were employed to study binding constants (K), thermodynamic parameters (?H and ?S) and charge transfer transition energies (h?_CT). It is found that the stability of the complexes increases with the number of oxygen atoms in the donor ethers. The ground state stabilization in these electron?donor?acceptor complexes is ascribed to both ?–? and n–? interactions. The n–? interaction forms 10% of the total contribution to the stabilisation. The electron affinity (EA) of the ??acceptors correlates well with the observed binding constants of the complexes and also with the h?_CT values indicating that charge transfer forces are dominant. In all the cases, the ionization energy of donor ethers (I_D) is evaluated. The appearance of multiple charge transfer absorptions in these complexes is explained on the basis of I_D values (energies of the highest occupied molecular orbitals) and available acceptor molecular orbitals (energies of the lowest unoccupied molecular orbitals). The ¹H and NMR spectra of the complexes provided information as to the geometry of the complexes in solution. That there exists no specific site binding in these complexes is revealed by the IR spectra of the complexes. The thermodynamic parameters (?H and ?S) are discussed in terms of energetics of packing and orientation effects present in these molecular complexing systems. These values are low compared to systems where ion?dipole interactions dominate. Based on these studies it is proposed that crown ethers form weak complexes with ??acceptors wherein both ?–? and n–? interactions form the major source of ground state stabilization.