CRYSTAL-STRUCTURE AND SOLID-STATE BEHAVIOR OF A DIAZONIUM IODIDE - 3-CARBOXY-2-NAPHTHALENEDIAZONIUM IODIDE HYDRATE

The photochemically and thermally unstable orthorhombic crystal structure of 3-carboxy-2-naphthalenediazonium iodide hydrate (1a, C11H7N202I•H2O; a = 15.42, b = 16.96, c = 9.59 Å; space group Pbca, Z = 8) has been examined at -50°C (R = 0.12 for 303 observed reflections) and its solid-state behavior compared with that of the corresponding triclinic diazonium bromide hydrate, 1b. The immediate environment of the essentially planar diazonium cation is similar in both structures: three halide ions, the carbonyl oxygen of a neighboring cation, and a water molecule are arranged about the C+N≡N group with the shortest halide-nitrogen distance involving the outer nitrogen (3.4 Å in 1a, 3.27 Å in 1b). Each halide is coordinated to two diazonium groups and hydrogen bonded to two waters. Each water is hydrogen bonded to two halides and the carboxyl group. Despite these similarities, 1a and 1b exhibit markedly different solid-state stabilities. TGA and DSC studies indicate that la rapidly decomposes in the interval 30-60°C with simultaneous loss of N2 and H2O, and is transformed to the corresponding iodo acid (83% isolated yield). By contrast, the more stable 1b decomposes stepwise: loss of H2O (65-100°C), loss of N2 (100-135 °C) (heating rate 2.5 °C/min). Solid-state decomposition mechanisms in 1a involving either a displacement of -N2+ by iodide ion or a unimolecular ionization to give aryl cations seem unlikely in view of the above comparisons. Instead, an electron-transfer process in which iodide ion is oxidized to atomic iodine (possibly through a radical-chain mechanism) is proposed. © 1979, American Chemical Society. All rights reserved.