ReviewPreparation and application of N-heterocyclic carbene complexes of Ag(I)
Introduction
In the past few years, N-heterocyclic carbenes (NHCs) have emerged as versatile ligational building blocks for a large variety of coordination compounds [1]. Earlier work considered NHCs as phosphine analogues, but the recent observations emphasized some differences in steric and electronic properties and thus their chemical behavior [2]. Since these ligands act as excellent strong σ-donors, they can produce stable metal–NHCs with strong metal–carbon bonds. For this reason, the metal–NHCs have been widely used as highly reactive and rather selective catalysts for numerous chemical transformations [3]. The utilization of metal–NHCs has also been useful in medicinal science applications [4]. Many research groups around the world are currently focusing on metal–NHC complexes.
Because of the usefulness of the metal–NHC complexes, many synthetic methods have been explored. According to the reports published on metal–NHCs, the most widely used preparation methods can be divided broadly into five types: (1) reaction of free NHCs with metal precursors [5], (2) reaction of electron-rich olefin dimers with organometallic fragments [6], (3) reaction of imidazolium salts with suitable basic transition metal salts [7], (4) reaction of azolium salts with metal precursors under basic phase transfer catalysis (PTC) conditions [8], and (5) transmetallation with Ag(I)–NHCs [9a]. The last strategy is now well-established for the preparation of various transition metal–NHCs including Au(I) [9], [10], [11], [12], [13], [14], [15], [16], [17], Pd(II) [9], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], Rh(I) [25], [30], [35], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], Rh(III) [41], [57], Ir(I) [40], [41], [47], [49], [51], [58], [59], Ir(III) [41], [60], [61], Cu(I) [62], [63], [64], Cu(II) [65], [66], [67], Ru(II) [56], [68], [69], [70], [71], [72], Ru(III) [69], Ru(IV) [66], [73], Ni(II) [74], [75], Pt(II) [26], [31], [76], and proved to be a convenient method over other methods under certain conditions.
Section snippets
Aims and objectives
Metal–NHC chemistry has been reviewed by pioneers in the field. When Herrmann and Bertrand published excellent reviews in 2000 concerning the synthesis of metal–NHCs, few Ag(I)–NHCs were known [1], [3]. Indeed interest in the synthesis of Ag(I)–NHCs and their application in transmetallation reactions has only greatly increased in recent years. In this regard, we reviewed Ag(I)–NHCs in 2004 [77] providing general trends in Ag(I)–NHC chemistry. Arnold also had a mini review on Ag(I)–NHCs in 2002
Discussion
As depicted in Scheme 2, different approaches such as (1) reaction of azolium salts with silver base, (2) reaction of free NHC with silver salts, and (3) reaction of azolium salts with silver salts under basic PTC conditions, have been used to prepare Ag(I)–NHCs. As early as 1993, Arduengo reported the first Ag(I)–NHC by the reaction of Ag(I) salt with a free NHC [80]. Later, Bertrand in 1997, reported that the reaction between triazolium triflate salt and a silver base such as Ag(OAc) (OAc =
Conclusion
The present work revealed that the Ag2O technique has been the most popular method in the synthesis of Ag(I)–NHCs and in their subsequent uses as transmetallating agents. The Ag2O technique is tolerant of activated hydrogen atoms adjacent to functional groups. The use of normal azolium halide led to the formation of ionic or neutral Ag(I)–NHCs in the solid-state, depending on factors such as counter-ions, carbene cores, N-substituents and the crystallization conditions. A fluxional behavior
Acknowledgments
We thank National Science Council (NSC) of Taiwan for providing financial assistance. The authors also thank H.M.J. Wang, C.K. Lee, K.M. Lee, J.C. Chen, T.W. Huang, R.Y. Yang for their contribution to the work of Ag(I)–NHCs.
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