Ag4(NHC)4(NO3)4: a Tetrameric Silver (I) Complex with Bridging N-Heterocyclic Carbene Ligands

In recent years the coordination chemistry of N-heterocyclic carbene showed a boosting increment, since these new types of ligands have many valuable properties in terms of stability, versatility, possibility to introduce new functionalities.1 Silver complexes are among the most widely studied since they have been used for several applications, including pharmaceutical, photophysical and catalytic.2 In such a large number of characterized compounds, only two examples of bridging coordination of the NHC groups have been reported, one for copper complex,3 and the other (with silver) assisted through symmetric pendant donor groups.4 Looking for new neutral silver complexes, we explored the reaction between Ag(cod)NO3 and [Ag(IPrIm)Cl]2, in CH2Cl2. The product was crystallized by diffusion of heptane, and the final yield was about 50 %. IR , elemental analyses and X-ray diffraction concurred to establish the actual formula of the compound as (HNC-Ag-NO3)4 In the solid state, the compound is composed by tetrameric unit with two short and one long Ag-Ag contacts. The short ones are bridged by the N-heterocyclic carbene, and the long one by monodendate nitrate anions. According to DFT calculations, the tetramer can exist in two isomers and the one with terminal-only carbene ligands should be slightly more stable. Conversely, with other anions such as halide, only one isomer can be formed.  Dedicated to the memery of Mario Manassero 1. Special issues on metal-carbene complexes: a) Dalton. Trans., 2013, 42, 7245-7432; b) Dalton. Trans., 2009, 6873-7316; c) Coord. Chem. Rev., 2007, 251, 595-896 d) Chem. Rev., 2009, 109, 3209-3884; e) Jones, W.D.; J. Am. Chem. Soc., 2009, 131, 15075-15077 f) J. Organomet. Chem.., 2005, 690, 5397-6252 2. Garrison, J.C.; Youngs, W.J.; Chem. Rev.,2005, 105, 3978-4008. 3. Han, X.; Koh, L.-L.; Liu, Z.-P.; Weng, Z.; Hor, T.S.A.; Organometallics, 2010, 29, 2403-2405 4. Garrison, J. C.; Simons, R. S.; Kofron, W. G.; Tessier, C. A.;Youngs, W. J. Chem. Commun. 2001, 1780–1781.