Silver(I) and Nickel(II) Complexes with Oxygen‐ or Nitrogen‐Functionalized NHC Ditopic Ligands and Catalytic Ethylene Oligomerization

Recent Advances in Catalysis Involving Bidentate N-Heterocyclic Carbene Ligands

Since the discovery of persistent carbenes by the isolation of 1,3-di-l-adamantylimidazol-2-ylidene by Arduengo and coworkers, we witnessed a fast growth in the design and applications of this class of ligands and their metal complexes. Modular synthesis and ease of electronic and steric adjustability made this class of sigma donors highly popular among chemists. While the nature of the metal-carbon bond in transition metal complexes bearing N-heterocyclic carbenes (NHCs) is predominantly considered to be neutral sigma or dative bonds, the strength of the bond is highly dependent on the energy match between the highest occupied molecular orbital (HOMO) of the NHC ligand and that of the metal ion. Because of their versatility, the coordination chemistry of NHC ligands with was explored with almost all transition metal ions. Other than the transition metals, NHCs are also capable of establishing a chemical bond with the main group elements. The advances in the catalytic applications of the NHC ligands linked with a second tether are discussed. For clarity, more frequently targeted catalytic reactions are considered first. Carbon-carbon coupling reactions, transfer hydrogenation of alkenes and carbonyl compounds, ketone hydrosilylation, and chiral catalysis are among highly popular reactions. Areas where the efficacy of the NHC based catalytic systems were explored to a lesser extent include CO2 reduction, C-H borylation, alkyl amination, and hydroamination reactions. Furthermore, the synthesis and applications of transition metal complexes are covered.

Imidazolium salts and [Pt(cod)2]: from NHC hydrido complexes to the unprecedented olefinic tetrahedral cluster [Pt4(μ-H)(cod)4]BF4

Whereas the bis(imine)imidazolium salt 1·Cl is a potential precursor to a N^imineC^NHCN^imine pincer-type ligand, it reacted with [Pt(cod)₂] to give the Pt^II NHC hydrido complex 3, in which chloride coordination and C^NHCN^imine chelation is preferred over pincer formation. Unexpectedly, reaction of 1·BF₄ with [Pt(cod)₂] afforded the unprecedented, 56 CVE tetrahedral cluster [Pt₄(μ-H)(cod)₄]BF₄ (7), which was fully characterized. Imidazolium salts are ubiquitous sources of the much studied NHC ligands and not expected to lead to the formation of metal clusters.

Ni-Based Complexes in Selective Ethylene Oligomerization Processes

Linear alpha-olefins are widely used in the petrochemical industry and the world demand for these compounds increases annually. At present, the main method for producing linear alpha-olefins is the homogeneous catalytic ethylene oligomerization. This review presents modern nickel catalysts for this process, mainly systems for obtaining of one of the most demanded oligomer—1-butene—which is used for the production of linear low density polyethylene (LLDPE) and high density polyethylene (HDPE). The dependence of the catalytic performance on the composition and the structure of the used activated complexes, the electronic and coordination states of the nickel center was considered.