Abstract
Protic NHC (PNHC) complexes with N1H, N2-alkyl/aryl imidazolylidene ligands are relatively rare, and routes for their synthesis differ from what is used to make non-protic analogs. Prior work from our group and others showed that in the presence of a tethering ligand (phosphine or in one case, pyridine), CpM and Cp*M (M = Ir, Ru) PNHC complexes could be made by heating. Here, we find that the use of ionizing agents to activate [Cp*MIIICl(μ-Cl)]2 (M = Ir, Rh) allows for what we believe is unprecedented ambient temperature formation of PNHC complexes from neutral imidazoles; the product complexes are able to perform transfer hydrogenation.
Highlights
The last 25 years have seen the establishment of N-heterocyclic carbenes (NHCs) as essential tools for creation of powerful metal-containing catalysts and organocatalysts
NH derivatives may be described as protic NHCs (PNHC) [1,2], Whereas thousands of literature references describe NHCs, less than one hundred describe PNHCs, yet for PNHCs, there is the additional possibility of bifunctional chemistry and catalysis enabled by the protic functionality
In some syntheses of NHC complexes, free NHCs are made by deprotonating imidazolium salts; this route is precluded in making PNHCs, because the most acidic proton on the imidazolium salt precursor would be that on one of the nitrogens
Summary
The last 25 years have seen the establishment of N-heterocyclic carbenes (NHCs) as essential tools for creation of powerful metal-containing catalysts and organocatalysts. Synthetic routes to PNHC complexes must differ from those used to make NHC complexes. In some syntheses of NHC complexes, free NHCs are made by deprotonating imidazolium salts; this route is precluded in making PNHCs, because the most acidic proton on the imidazolium salt precursor would be that on one of the nitrogens. A suitable N-protecting group has been used [9] All three of these routes involve three synthetic steps. Another route starts with a heterocycle functionalized at C2 with a halide; oxidative addition, followed by protonation afford PHNC complexes [10,11,12]
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