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 CO 2 -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(CH 3 COO) 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 ac-

Catalytic rivals: Both CO(2)-protected tetrahydropyrimidin-2-ylidene-based N-heterocyclic carbenes (NHCs) and Sn(II)-1,3-dimesitylimidazol-2-ylidene, as well as Sn(II)-1,3-dimesitylimidazolin-2-ylidene complexes (example displayed), have been identified as truly latent catalysts for polyurethane (PUR) synthesis rivaling all existing systems both in activity and latency.A series of CO(2)-protected pyrimidin-2-ylidenes as well as 1,3-dimesitylimidazol-2-ylidene and dimesitylimidazolin-2-ylidene complexes of Sn(II) have been prepared. Selected single-crystal X-ray structures are reported. The new compounds were investigated for their catalytic behavior in polyurethane (PUR) synthesis. All compounds investigated showed excellent catalytic activity, rivaling the industrially most relevant catalyst dibutyltin dilaurate. Even more important, all compounds displayed pronounced latent behavior, in selected cases rivaling and exceeding the industrially relevant latent catalyst phenylmercury neodecanoate both in terms of latency and catalytic activity. This allows for creating one-component PUR systems with improved pot lifetimes. Pseudo-second-order kinetics were found for both CO(2)-protected tetrahyropyrimidin-2-ylidenes and for [SnCl(2)(1,3-dimesityldihydroimidazol-2-ylidene)], indicating a fast pre-catalyst decomposition prior to polyurethane formation. 1,3-Di(2-propyl)tetrahydropyrimidin-2-ylidene was additionally found to be active in the cyclotrimerization of various isocyanates, offering access to a broad variability in polymer structure, that is, creating both urethane and isocyanurate moieties within the same polymer.

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