CO2, Magnesium, Aluminum, and Zinc Adducts of N-Heterocyclic Carbenes as (Latent) Catalysts for Polyurethane Synthesis

A series of magnesium, aluminum, and zinc complexes of imidazol-2-ylidene-, imidazolin-2-ylidene-, and tetrahydropyrimidin-2-ylidene-derived N-heterocyclic carbenes (NHCs) was prepared. In addition, both symmetrical and unsymmetrical CO2-protected imidazol-2-ylidenes and imidazolin-2-ylidenes were prepared. Selected single-crystal Xray structures are reported. All compounds were investigated for their catalytic behavior in (poly)urethane (PUR) synthesis. Out of nine different compounds, the dimeric complex [Zn(CH3COO)(2)(1,3-dimesitylimidazol-2-ylidene)](2) (7) proved to be the most active one, rivaling the industrially most relevant catalyst dibutyltin dilaurate in terms of catalytic activity. Even more important, the Mg- and Zn-derived NHC complexes as well as the CO2-protected imidazol-2-ylidenes displayed pronounced latent behavior combined with high catalytic activity in selected cases, rivaling and exceeding the industrially relevant latent catalyst phenylmercury neo-decanoate. This allows for creating one-component PUR systems with significant pot lives. Pseudo-second-order kinetics were found for all Mg- and Zn-based NHC complexes as well as for all CO2-protected NHCs indicating fast precatalyst decomposition prior to (poly)urethane formation. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)