Activation of Carbon Dioxide by Bicyclic Amidines Eduardo R. Pe ´rez,* ,† Regina H. A. Santos, † Maria T. P. Gambardella, † Luiz G. M. de Macedo, † Ubirajara P. Rodrigues-Filho, † Jean-Claude Launay, ‡ and Douglas W. Franco* ,† Instituto de Quı ´mica de Sa ˜ o Carlos, Universidade de Sa ˜ o Paulo, C. P. 780, CEP 13560-970, Sa ˜ o Carlos-SP, Brasil, and Institut de Chimie de la Matie ` re Condense ´ e de Bordeaux, CNRS UPR 9048, F-33608 Pessac, France ed5bras@mailcity.com; douglas@iqsc.sc.usp.br Received May 4, 2004 Activation of the carbon dioxide molecule was achieved using bicyclic amidines (DBU, PMDBD, and DBN). The solution reaction of CO 2 with amidines yielded the corresponding zwitterionic complexes through the formation of a N-CO 2 bond. 13 C NMR data confirmed the carbamic nature of the carbamic zwitterions, DBU-CO 2 and PMDBD-CO 2 . However, when these adducts were crystallized, the X-ray analyses of the single crystals were in agreement with bisamidinium bicarbonate salt structures, indicating that structural changes occurred in the crystallization process. The elemental and thermogravimetric analysis data for the carbamic zwitterions, DBU-CO 2 and PMDBD-CO 2 , initially obtained by the direct reaction of amidines with CO 2 , suggest that these molecules are probably associated with one molecule of water by hydrogen-bond formation (amidinium + -COO - ‚‚‚H 2 O). A correlation was observed between the thermal stability and the transcarboxylating activity for the amidine-CO 2 complexes. Theoretical calculations of hardness were performed at the B3LYP/cc-pVTZ level of theory and showed concordance with the experimental reactivity of DBU and PMDBD toward CO 2 . Introduction Carbon dioxide is considered to be a weak Lewis acid, 1 and a preliminary activation of the CO 2 molecule 2 is commonly required to insert it into organic molecules. The activation of carbon dioxide has been performed by electrochemical reduction in both aqueous 3 and nonaque- ous 4 media. Inorganic and organometallic compounds have also been used for this effect. 5 Hindered amidine and guanidine bases have been used as catalysts in reactions involving the use of carbon dioxide. 2a,c-e,6 The catalytic activity of these bases is, in many cases, associated with their proton-transfer activity. 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Chem. 2004, 69, 8005-8011 8005 Published on Web 10/09/2004