Co(III) complexes of tetradentate X3L type ligands: Synthesis, electronic structure, and reactivity

Co(III) complexes of DPP (DPP = (Z)-2-(2-((5-(2-hydroxyphenyl)-1H-pyrrol-2-yl)(phenyl)methylene)-2H-pyrrol-5-yl)phenol) and two fluorinated DPP variants are prepared, structurally characterized, and their reactivity explored in the catalytic ring-opening of 1,2- epoxyhexane with alcohols. While the related Co(III) salen and porphyrin complexes are highly active for this catalytic transformation, the Co(III) DPP complexes are shown to be catalytically inert under the conditions studied. Computational characterization of the electronic structure of all the complexes shows that the Co(III) complexes ligated by the DPP ligands (tetradentate X3L type) do not have sufficient Lewis acidity to activate the epoxide, whereas the Co(III) porphyrin and salen (tetradentate X2L2 type) incorporate a counter-ion that draws electron density away from the metal center, creating a more Lewis acidic Co species that is catalytically active for alcohol ring-opening of epoxides. The work thus gives insight into the requirements for effective Lewis acidic epoxide ring-opening catalysts based on cobalt. (C) 2015 Elsevier B.V. All rights reserved.