Solvent dependence, metal ion catalysis and base catalysis of nitrito to nitro linkage isomerization of pentaamminenitritocobalt(III)

Abstract

The spontaneous rate of nitrito to nitro isomerization of [(NH₃),Co(ONO)]²⁺ spans two orders of magnitude for 16 solvents studied. The volume of activation is small and negative (from - 3.5 to -7 cm3 mol^-1) for water, dimethyl sulfoxide, N-methylformamide and sulfolane. Combined with the observation of negligible competitive solvolysis, and of an isokinetic plot of ΔH^‡ against ΔS^‡, this indicates that isomerization is intramolecular in all solvents. The solvent dependence is interpreted in terms of a dual-parameter equation involving terms for the Lewis basicities (D_N) and Lewis acidities and polarity (E_T). By use of this approach both D_N and E_T of the solvents are shown to be major contributing factors to reactivity. Further studies on base-catalysed nitrito to nitro isomerizations reveal mild catalysis by F^-, AcO^- and Et₃N in Me₂SO. The metal ions HgZ⁺, Ag⁺ and Cd²⁺ also catalyse the isomerization in water [k_Hg 1.16 × dm³ mol^-1 s^-1, k_Ag 2.85 × dm³ mol^-1 s^-1, k_Cd 4.4 × 10^-5 dm³ mol^-1 s^-1; ,μ 1.0 M (CIO₄^-), 25°C]. The lack of competition by AcO^-, NO₃^- and H₂O in the Hg²⁺-catalysed isomerization reaction also implicates an intramolecular process for this path. In acetone, the ions Hg²⁺, Ag⁺, Cd²⁺ and Zn²⁺ catalyse the isomerization reaction with a rate law of the form

K_obs = K_s + K_m[ⁿ⁺]

except for Ag⁺, where the rate law takes the form

K_obs = (k_s + kK[Ag⁺])/(1 + K[Mⁿ⁺])

with k_Ag = kK = 2.11 × dm³ mol^-1 s^-1, k = 1.74 × 10^-4 s^-1 and K = 12 ± 1 dm³ mol^-1. The limiting rate is ascribed to a high stability constant for the silver adduct intermediate. The analogous Rh^III and Ir^III complexes also exhibit catalysis by Ag⁺ and Hg²⁺ in aqueous solution.