Lithium binding in fluorinated phosphazene trimers

Density functional theory is used to model a series of cyclic phosphazenes, with and without coordinated Li+. Two pendant groups are used, ethoxy and 2,2,2-trifluoroethoxy, in varying combinations to generate phosphazenes with 0, 1, 2, 3, or 6 trifluoro groups. In all cases, Li+ sits in a pocket on the ring, always bonding to a ring nitrogen, and can be three- or four-coordinate, depending on the local environment. Three-coordinate occurs when no fluorines are close enough to interact, with the lithium bonding to a ring nitrogen and the two adjacent oxygens. When Li+ is four-coordinate, the bonding varies between the ring nitrogen, two adjacent oxygens, and one fluorine, or the ring nitrogen, one adjacent oxygen, and two fluorines. All of the possible symmetry unique structures have been calculated. The binding strength of Li+ steadily decreases, from 74.93 kcal/mol with no trifluoro groups, to 58.01 kcal/mol when 6 trifluoro groups are present. The decrease is attributed to the electron withdrawing effect of the trifluoro groups, and also to distortions in the geometry to accommodate Li–F interactions.