Effect of Counter Cations on Base Catalysis of [Nb₁₀O₂₈]⁶⁻

Abstract

The tetraalkylammonium salts of decaniobate ((C_nH_2n+1)₄N⁺)₆[Nb₁₀O₂₈]⁶⁻ (TAA_nNb₁₀; n = 1–4) were applied as homogeneous catalysts for the Knoevenagel condensation reaction between benzaldehyde and phenylacetonitrile. The base catalytic activity of TAA_nNb₁₀ was dependent on the alkyl chain length n of TAA_n; the initial reaction rate (r_n) increased monotonously with n and reached a plateau at n = 3. The catalytic activity of TAA₄Nb₁₀ decreased with the addition of tetraalkylammonium bromide (C_nH_2n+1)₄N⁺Br⁻ (TAA_nBr; n = 1, 2). These results indicate that TAA_n (n = 1, 2) acts as a poison for the base catalyst Nb₁₀. The adsorption–desorption equilibrium constants of TAA_n (K_n) estimated using a simple kinetic model decreased monotonously with n and became negligible at n ≥ 3. The anticorrelation between K_n and r_n led us to conclude that the catalytic poisoning by TAA_n (n = 1, 2) is caused by the blocking of the base sites of Nb₁₀ due to the electrostatic interaction. These findings suggest that a counter cation with a small charge density is suitable for the application of polyoxometalates as base catalysts.