Zn-Co Double Metal Cyanides as Heterogeneous Catalysts for Hydroamination: A Structure-Activity Relationship

Zn-Co double metal cyanide (DMC) materials are effective heterogeneous catalysts for intermolecular hydroaminations. Using the reaction of 4-isopropylaniline with phenylacetylene as a test, the effect of different catalyst synthesis procedures on the catalytic performance is examined. The best activities are observed for double metal cyanides with a cubic structure and prepared with a Zn2+ excess, and for nanosized particles prepared via a reverse emulsion synthesis. Detailed study of the active Zn2+ sites in the cubic material by EXAFS gives evidence for coordinative vacancies around the Zn, with four cyanide ligands in close proximity of the Zn. The substrate scope of the hydroaminations was successfully expanded to both aromatic and aliphatic alkynes and other aromatic and aliphatic amines. Even with styrenes the reaction proceeded with aromatic amines. The DMC catalysts are truly heterogeneous, possess a high thermal stability and are perfectly reusable.