Enhanced activity of sterically crowded N,N'-ethylenebis(salicylideneaminato)cobalt(II) derivatives as catalysts for oxygenation of 3-methylindole.

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Abstract
Six derivatives of N, N'-ethylenebis (3-tert-butylsalicylideneaminato) cobalt (II), abbreviated as Co (Busalen), were prepared by varying the diamine moiety with ethylenediamine, (R)-1, 2-propanediamine, (2S, 3S)-2, 3-butanediamine, meso-2, 3-butanediamine, (1R, 2R)-1, 2-cyclohexanediamine, and cis-1, 2-cyclohexanediamine. The spectral and electrochemical properties of the complexes are reported. These complexes and four derivatives of N, N'-ethylenebis (salicylideneaminato) cobalt (II), Co (salen), were used as catalysts for the oxygenation of 3-methylindole. With increasing steric hindrance of the metal complex catalyst, the rate of the steady-state consumption of the substrate increased, and the induction period increased with increasing steric hindrance at the apical region of Co (Busalen) derivatives. A linear relationship was found between the logarithm of the rate constants and the half-wave potentials for CoII/III couples : log (102k/mol-1dm-3s-1)=a (E1/2/V)+b, where a and b are 5.2 and -0.56 for the Co (Busalen) series, and 5.5 and -1.25 for the Co (salen) series, respectively. Based on these observations, it is concluded that the catalysis is controlled by the redox potential and tert-butyl groups of the Co (Busalen) derivatives, and the activation of the substrate, not dioxygen, is the critical step of this reaction.

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