Thermodynamics of Molecular Metal Formation: Metallophthalocyanine and Tetrathiafulvalene Iodides

Solid-state electrochemical cells are employed to investigate the formation reaction of several iodinated materials, DI_x, where D is a metallophthalocyanine (M(Pc), M = H₂, Ni, Co, Cu) or tetrathiafulvalene (TTF). I₂ and [(CH₃)₄N]I₃ cells were also studied as controls. The M(Pc)I constitute a set of isostructural and isoionic molecular conductors; in contrast, the TTFI_x (x = 0.7, 2, 3) differ in structure, level of oxidation, and the state of the iodine. For none of the DI_x studied does the formation reaction of the parent D and molecular iodine have a free energy more favorable than −5.2 kcal/mol. This is so despite the highly favorable lattice stabilization energies, L, that are calculated from appropriate Born-Haber cycles: L ranges from −103 to −117 kcal/mol in the nonintegrally (partially) oxidized materials while L = −164 ± 5 kcal/mol in the integrally oxidized TTFI₃. Individual contributions to L, namely the nonbonded, ionic, and mixed-valence, or carrier-delocalization, contributions, are also discussed. The first two of these are large and contribute in different proportions for the M(Pc)I and (TTF)I_x. The third, though small, can be of the same magnitude as or greater than the total free energy of formation.