Rh- and Ir-NHC-based complexes

Research in the chemistry of rhodium and iridium Af-heterocyclic carbene (NHC) complexes has extraordinarily evolved since 2000. A quick search for rhodiimi-NHC and iridiimi-NHC complexes in the SCl-expanded database, with a 2005-2013 timespan, results in more than 360 hits for rhodium, and more than 340 for iridiiun, which gives a good idea on the interest that rhodium and iridium NHC-based chemistry have achieved in the last few years. It is important to note that a nimiber of reviews and book chapters specifically concerning the chemistry of NHC-based compounds of rhodium and iridiiun have recently appeared [1]. This chapter will deal with all new aspects of the NHC-M (M = Rh, Ir) chemistry not reviewed before, and therefore is mainly restricted to the last 4-5 years. The chapter is classified into two main sections, the first of which deals with relevant structural and electronic features of Rh-NHC and Ir-NHC complexes, and the second with the catalytic applications of these compounds. While not pretending to be completely comprehensive, we have tried to describe the most relevant examples assigned to each section. Some other relevant applications of these complexes have not been considered, such as the emerging biochemical applications, mostly referred to Rh-NHC complexes [2], and the luminescent properties of some Ir-NHC complexes, mostly used for the fabrication of electro-optical devices [3]. 

Numerous C H activations of the NHC ligand have also been observed in ruthenium clusters, where extensive decomposition could be observed. As observed with Rh and Ir complexes, mesityl substituents were more stable than phenyl substituents, but can be sensitive to C-H activation at the methyl group. In 2003, Leitner reported the facile exchange of hydrogen atoms on the methyl groups of an IMes-based Ru complex with deuterated solvent. This phenomenon was used in the design of a catalyst for the deuteration of aromatic molecules. ... 

The metal catalysed hydroboration and diboration of alkenes and alkynes (addition of H-B and B-B bonds, respectively) gives rise to alkyl- or alkenyl-boronate or diboronate esters, which are important intermediates for further catalytic transformations, or can be converted to useful organic compounds by established stoichiometric methodologies. The iyn-diboration of alkynes catalysed by Pt phosphine complexes is well-established [58]. However, in alkene diborations, challenging problems of chemo- and stereo-selectivity control stiU need to be solved, with the most successful current systems being based on Pt, Rh and An complexes [59-61]. There have been some recent advances in the area by using NHC complexes of Ir, Pd, Pt, Cu, Ag and Au as catalysts under mild conditions, which present important advantages in terms of activity and selectivity over the established catalysts. 

The MCR toward 2//-2-imidazolines (65) has found apphcation in the construction of A(-heterocyclic carbene (NHC) complexes (74). Alkylation of the sp Af-atom with an alkyl halide followed by abstraction of the proton at C2 with a strong base (NaH, KOtBu) resulted in the formation of the free carbene species, which could be trapped and isolated as the corresponding metal complexes (Ir or Rh) [160]. The corresponding Ru-complexes were shown to be active and selective catalysts for the transfer hydrogenaticm of furfural to furfurol using iPrOH as hydrogen source [161]. 

Major achievements in cyclometallation processes involving NHC ligands have been achieved by Nolan and coworkers. It was observed that the solvent could play a crucial role in the formation of rhodium- and iridium-based complexes. As shown in Scheme 39, the reaction performed in pentane between [M(COE)2Cl]2 (M = Rh or Ir) and four equivalents of FBu led only to COE substitution by the NHC ligand affording rhodium-based product (243) (the iridium complexing proving difficult to isolate). On the other hand, the same reaction carried out in hexanes gave, via C H activation, the hydride complexes (244) and (245). Finally, in benzene, a unique double cyclometallation process occurred to yield the coordinatively unsaturated 16-electron... 

At the end of this section, a special mention should be made of the intensive work performed by Peris on the development of poly-NHC-based hetero-bimetallic complexes. In these complexes, the two metal centers are capable of catalyzing two mechanistically distinct reactions on the same substrates, thereupon enabfing overall tandem synthetic process. Several examples have been provided, including Ir/Rh, Ir/Pd, Ir/Pt, and Ru/Pd systems (see also Section 2.3), and several combinations of tandem processes with these metal centers as catalysts. This subject has been thoroughly reviewed in 2014 by... 

The introduction of fluorine substituents instead of hydrogen may be a useful strategy for the synthesis of metalation-resistant NHCs. For example, vV-pentafluorophenylmethylene substituted NHC complexes of Ir and Rh showed no evidence of activation of C-H, C-C or C-F bonds. Grubbs and co-workers also prepared a NHC ligand designed to be resistant to orthome-talation by virtue of the presence of ortho fluorine substituents, and the metathesis catalysts prepared from these ligands displayed increased activity when compared to traditional IMes-based catalysts. ... 

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