the stoichiometry, both [(IMes)Ir(COE)(N 2)Cl] and [(IMes) 2 Ir(COE)Cl] (198 in Scheme 75) were successfully isolated. 44 Finally, as Scheme 20 in Section 4.4 shows, a series of stable (NHC) 2 Ir(COE)Cl complexes were isolated either by the free carbene or the silver transmetallation method. Scheme 7. The synthesis of half-sandwich NHC-Ir(III) 24. The most commonly used precursor for the synthesis of NHC-Ir(III) complexes is [Cp*IrCl 2 ] 2. In 2000, Herrmann showed that treatment of [Cp*IrCl 2 ] 2 with 1 equivalent of ICy per iridium yielded 24 in an excellent 90% yield (Scheme 7). 45 Termaten et al. employed the same procedure for the synthesis of (IiPr Me)Ir(Cp*)Cl 2 (26a in Scheme 8). 15 An alternative synthetic route to access free NHCs was described in 1993 by Kuhn and Kratz, who prepared free carbenes via the reduction of thiones 25 by metallic potassium in THF (Scheme 8). 46 Using this method, Yamaguchi and coworkers prepared 26a-c by cannulating the solution of the in situ generated NHCs to a solution of [Cp*IrCl 2 ] 2 in THF (Scheme 8). 47 Scheme 8. Synthesis of NHC-Ir(III) from free carbenes. 4.2. Alkoxy metal precursors and base-assisted one-pot methods It was Köcher and Herrmann who first developed a one pot procedure in which [Ir(COD)(µ-OEt)] 2 was prepared in situ by reacting [Ir(COD)Cl] 2 with four equivalents of NaOEt. Subsequently, four equivalents of 1,3-bis(diphenylmethyl)imidazolium bromide were added and the air and moisture stable 27 was isolated two days later (Scheme 9). 48 Interestingly, despite the fact that a large excess of the NHC salt was employed, formation of a cationic biscarbene complex was not observed. Scheme 9. NHC-Ir complex synthesis from an in situ generated metal-alkoxide. The procedure starting with the alkoxy metal precursor gave bis-NHC substituted complexes (28) when sterically less demanding NHC salts were applied (Scheme 10). Notably, even a 2.2:1 ratio of NHC:Ir was sufficient to isolate 28a-c in good yields (67-86%). Iridium and rhodium bis-NHC complexes with chelating N-N bridged NHCs were prepared in the same way. 39 Scheme 10. Cationic biscarbene complexes. Recently, Savka and Plenio described a one-step synthesis to access (NHC)M(COD)Cl (M = Rh, Ir) as well as (NHC)M(CO) 2 Cl complexes by employing a relatively weak base (K 2 CO 3) in acetone under air (Scheme 11). 49 Scheme 11. One pot synthesis of (NHC)Ir(COD)Cl and (NHC)Ir(CO) 2 Cl complexes. 4.3. The silver transmetallation method Scheme 12. The silver transmetallation method. The silver transmetallation method for the synthesis of NHC-iridium complexes was first published by Crabtree and coworkers. 50 The NHC-silver adducts (29) were generated in excellent yields by modifying the original procedure of Wang and Lin. 51 Subsequent treatment of [Ir(COD)Cl] 2 at room temperature gave 30a and 30b in 81 and 68% yield, respectively (Scheme 12). X-ray analysis of the yellow crystals of 30a revealed square planar geometry around the iridium center. Complexes 30 smoothly reacted with CO to form bis-carbonyl compound...
