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In coordination chemistry, a transition metal NHC complex is a metal complex containing one or more N-heterocyclic carbene ligands. Such compounds are the subject of much research, in part because of prospective applications in homogeneous catalysis. One such success is the second generation Grubbs catalyst. [1] IMes is a popular NHC ligand.
NHCs as ligands are known for their strong σ donation and high structural tunability. [11] More in-depth computational analyses also highlight the role of M to NHC π back-donation, with respect to a description of an M-NHC bond. [12] Contour line diagrams of the Laplacian distribution of three (NHC)MCl complexes.
Adding "steric hindrance" to a molecule's reactive site through bulky groups is a common strategy in molecular chemistry to stabilize reactive moieties within a molecule. [3] In this case bulky ligands like terphenyls , bulky alkoxides , aryl-substituted NHCs , etc. serve as a protective wrapper on the molecule. [ 1 ]
Ever since the first crystalline carbene structure was isolated by Arduengo ins 1990, tuning different properties of NHCs has been a popular area of study in main group chemistry. [4] The first NHC boryl anion was synthesized by Segawa in 2006. [1]
This effect can be used to stabilize highly reactive main group and transition metal compounds. [6] [22] Because excessive steric hindrance can be an issue for some reactivity, NHCs and CAACs bearing substituents with multiple spatial conformations (e.g. cyclohexyl) offer "flexible steric bulk" for catalysis.
When a dimethyl group was added to C exo, the reaction no longer proceeds this way, and was able to polymerize lactide, δ-valerolactone, and ω-pentadecalactone. While this broadened the scope and speed of the polymerization, the reaction was difficult to control due to the formation of an enolate intermediate.
The transmetallated NHCs can either be isolated for subsequent reaction with palladium in a two-step method, or generated in the presence of palladium in a one-pot reaction. However, generation of Pd-NHC complexes by Ag transmetallation is cost-prohibitive and hampered by Ag complexes’ light sensitivity.
Main group organometallic chemistry concerns the preparation and properties of main-group elements directly bonded to carbon. The inventory is large. The inventory is large. The compounds exhibit a wide range of properties, including ones that are water-stable and others that are pyrophoric . [ 1 ]