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 (DN) and Lewis acidities and polarity (ET). By use of this approach both DN and ET 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 [kHg 1.16 × dm3 mol-1 s-1, kAg 2.85 × dm3 mol-1 s-1, kCd 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 Kobs = Ks + Km[n+] except for Ag+, where the rate law takes the form Kobs = (ks + kK[Ag+])/(1 + K[Mn+]) with kAg = 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 RhIII and IrIII complexes also exhibit catalysis by Ag+ and Hg2+ in aqueous solution.