Reactivity of coordinated nitriles

Abstract

The base hydrolysis of coordinated acrylonitrile in [ (NH₃)₅CoN=CCH=CH₂]³⁺ to the acrylamide complex in carbonate buffers obeys the rate law k(obsd) = k(OH)OH⁻] + k(c)[C0₃²⁻] (k(OH) = 35 M⁻¹ s⁻¹; k(c) = 1 M⁻¹ s⁻¹; 25 °C, μ = l.O), and ¹⁸O tracer studies indicate the mechanism of hydrolsis by carbonate ion to involve direct nucleophilic attack at the nitrile group by C0₃²⁻ with subsequent elimination of C0₂. Coordination of malononitrile, cyanoacetic acid, and ethyl cyanoacetate to the (NH₃)₅CO(III) moiety leads to greatly increased acidity of the methylene protons (pK(a) of [(NH₃)₅CoN=CCH₂C=N]³⁺ is 5.7 ie 0.1 ( μ = 1.0,25 °C) compared with 11.3 for uncoordinated malononitrile). In basic aqueous solutions, these complexes equilibrate rapidly between the original complexes and the complexes containing the deprotonated activated methylene group. The protonated complexes then hydrolyze to the coordinated amides with both hydroxide ion and water acting as nucleophiles while the coordinated carbanions undergo an intramolecular electron transfer of a single electron to yield Co(II) and the ligand radical. The C-deprotonated complexes also function as nucleophiles for appropriate substrates (such as methyl iodide, methyl pyruvate, or bromine) to form the corresponding substituted species. In nonaqueous media, generation of the ligand radical initiates polymerizations of monomers methacrylate, styrene, acrylonitrile, and methacrylonitrile by a radical (rather than an ionic) mechanism.