FULL PAPER DOI: 10.1002/ejic.200801161 CO 2 , Magnesium, Aluminum, and Zinc Adducts of N-Heterocyclic Carbenes as (Latent) Catalysts for Polyurethane Synthesis Bhasker Bantu, [a] Gajanan Manohar Pawar, [a] Klaus Wurst, [b] Ulrich Decker, [a] Axel M. Schmidt, [c] and Michael R. Buchmeiser* [a,d] Keywords: Carbenes / Magnesium / Aluminum / Zinc / Polyurethane A series of magnesium, aluminum, and zinc complexes of imidazol-2-ylidene-, imidazolin-2-ylidene-, and tetrahydro- pyrimidin-2-ylidene-derived N-heterocyclic carbenes (NHCs) was prepared. In addition, both symmetrical and un- symmetrical CO 2 -protected imidazol-2-ylidenes and imid- azolin-2-ylidenes were prepared. Selected single-crystal X- ray 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 rel- evant catalyst dibutyltin dilaurate in terms of catalytic ac- Introduction N-Heterocyclic carbenes (NHCs) have become an impor- tant class of ligands for both main-group and transition metals. [1–7] The corresponding NHC–metal complexes have been introduced into organometallic catalysis some 15 years ago and now hold a strong position in that area of research. More recently,NHCs have established themselves as highly valuable catalysts in organic synthesis, thereby entering and further developing the field of organocatalysis. [8,9] During the last years, however, an increasing number of reports on the use of NHCs in polymer synthesis has emerged. In- spired by some excellent reports, [10–16] we were interested whether protected NHCs could be of some use as catalysts in polyurethane (PUR) synthesis and, if this was true, whether latent systems could be created. Such latent sys- tems are of significant interest in industry, as they allow the premixing of all components including the precatalyst and, after triggering the catalyst, formation of the desired poly- [a] Leibniz-Institut für Oberflächenmodifizierung (IOM), Permoserstr. 15, 04318 Leipzig, Germany Fax: +49-341-235-2584 E-mail: michael.buchmeiser@iom-leipzig.de [b] Institut für Allgemeine Anorganische und Theoretische Chemie, Universität Innsbruck, Innrain 52a, 6020 Innsbruck, Austria [c] BAYER MaterialScience AG, 51368 Leverkusen [d] Institut für Technische Chemie, Universität Leipzig, Linnéstr. 3, 04103 Leipzig, Germany Supporting information for this article is available on the WWW under http://www.eurjic.org or from the author. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2009, 1970–1976 1970 tivity. Even more important, the Mg- and Zn-derived NHC complexes as well as the CO 2 -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 sys- tems with significant pot lives. Pseudo-second-order kinetics were found for all Mg- and Zn-based NHC complexes as well as for all CO 2 -protected NHCs indicating fast precatalyst de- composition prior to (poly)urethane formation. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) mer or polymeric network. Here, we report on some se- lected results we have obtained so far. Results and Discussion NHC–metal complexes 1 and 7–9 (Figure 1) were pre- pared by reaction of the free NHC with the corresponding water-free metal salt in 83–93% yield. Compounds 5 and 6 were accessible in high yields (89 and 90%) by reaction of the corresponding CO 2 -protected NHCs [17–19] 1,3-dimes- itylimidazolium-2-carboxylate (3) [20] and 1,3-dimesityl- 3,4,5,6-tetrahydropyridinium-2-carboxylate, [21] respectively, with anhydrous ZnCl 2 . Compounds 2 and 4 were prepared in 93 and 70% yield by reaction of the corresponding imid- azolium salt with potassium tert-butoxide and KH, respec- tively, followed by reaction with dry CO 2 . Compound 1 crystallizes in the monoclinic space group P2 1 /n, a = 1089.28(3) pm, b = 1236.14(4) pm, c = 2018.18(6) pm, β = 101.559(2)°, Z = 4 (Figure 2). Its structure and selected bond lengths and angles are summarized in Figure 2. The Mg–carbene distance in 1 [220.0(2) pm] is slightly shorter than the one in Mg(1,3-dimesitylimidazol-2-ylidene)- Et 2 (227.9 pm), [22] which is most probably a result of the stronger salt character of 1. The ligand arrangement around the central Mg II ion results in a distorted tetrahedral com- plex as supported by the angles C(1)–Mg(1)–Cl(1) [109.27(7)°], O(1)–Mg(1)–Cl(2) [101.09(6)°], C(1)–Mg(1)– Cl(2) [111.08(7)°], and Cl(1)–Mg(1)–Cl(2) [122.99(5)°].