A Convenient Approach for the Synthesis of Multidentate N-Heterocyclic Carbene Ligand Precursors

Multidentate bis-NHCs ligands with various spacers are expected to combine the advantages of coordination chemistry and catalysis. These offer opportunities to construct various organometallic frameworks involving transition metals and main group elements. Therefore, developing a general procedure for synthesizing a variety of carbene salt precursors using a convenient technique is key in this context. The extended protocol of a solvent-free approach for synthesizing various bridged bis-imidazolium carbene salts, including tris and tetrakis-imidazolium precursors, is reported here. This method can be performed in the laboratory, leading to high yields (80-95 %) and isolated as analytically pure, multigram, bench-stable compounds.

Preparation of Mixed Bis-N-Heterocyclic Carbene Rhodium(I) Complexes

A series of mixed bis-NHC rhodium(I) complexes of type RhCl(η2-olefin)(NHC)(NHC') have been synthesized by a stepwise reaction of [Rh(μ-Cl)(η2-olefin)2]2 with two different NHCs (NHC = N-heterocyclic carbene), in which the steric hindrance of both NHC ligands and the η2-olefin is critical. Similarly, new mixed coumarin-functionalized bis-NHC rhodium complexes have been prepared by a reaction of mono NHC complexes of type RhCl(NHC-coumarin)(η2,η2-cod) with the corresponding azolium salt in the presence of an external base. Both synthetic procedures proceed selectively and allow the preparation of mixed bis-NHC rhodium complexes in good yields.

Air and Moisture Tolerant Synthesis of a Chelated bis(NHC) Methylpalladium(II) Complex Relevant to Alkyl Migration Processes in Catalysis

An air- and moisture-tolerant alternate synthetic pathway to the preparation of a cationic chelated bis(NHC) methylpalladium(ii) complex, [{(MesIm)2CH2}Pd(Me)(NCMe)][PF6], is described. The pathway involves the isolation of a bis(NHC) AgI complex, [{(MesIm)2CH2}2Ag2][PF6]2, via metallation of the corresponding diimidazolium salt with Ag2O followed by carbene transfer to [(COD)PdBrMe]. This new method avoids a previously reported unstable intermediate that displayed rapid decomposition at room temperature, attaining the targeted cationic methylpalladium(ii) complex in high yield. CO/ethylene copolymerisation catalysis trials are reported showing solvent dependent catalyst lifetime and copolymer yields. Preliminary ethylene insertion studies are also outlined revealing possible pathways leading towards catalyst deactivation.

Synthesis and structural characterization of 20-membered macrocyclic rings bearing trans-chelating bis(N-heterocyclic carbene) ligands and the catalytic activity of their palladium(ii) complexes

Macrocycles consisting of a 20-membered ring containing two imidazolium salt functionalities and of the formula [PhCH2N(CH2CH2CH2)Im(CH2CH2CH2)2][Br]2 (Im = imidazole = 3a, benzimidazole = 3b) were synthesized in 70-75% yields. These salts serve as precursors to macrocycles containing two N-heterocyclic carbene (NHC) moieties. Reaction of the macrocyclic salts 3a and 3b with silver oxide afforded macrocyclic-bis(NHC)silver(i) complexes 4a and 4b. Single-crystal X-ray diffraction studies of macrocyclic-bis(NHC)silver(i) complex 4a revealed a tetranuclear silver core with a short Ag-Ag distance (2.9328 Å). Complexes 4a and 4b serve as carbene transfer reagents to Pd. The treatment of macrocyclic-bis(NHC)silver(i) complexes 4a and 4b with one equivalent of PdCl2(MeCN)2 in methylene chloride afforded square-planar trans-macrocyclic-bis(NHC)Pd(ii)X2 complexes 5a and 5b. Preliminary screening of these palladium complexes showed they are competent precatalysts for Heck and Suzuki coupling reactions.

Synthesis of water-soluble palladium(ii) complexes with N-heterocyclic carbene chelate ligands and their use in the aerobic oxidation of 1-phenylethanol

Water-soluble palladium(ii) complexes with NHC chelate ligands have been prepared and studied as catalysts in the aerobic oxidation of alcohols in water.

Chelated bis(NHC) complexes of saturated (imidazolin-2-ylidene) NHC ligands: structural authentication and facile ligand fragmentation

The first structurally characterised chelated, saturated bis(NHC) complexes are reported along with facile ligand fragmentations during their synthesis.

Ruthenium(ii) and iridium(iii) complexes featuring NHC–sulfonate chelate

Three new complexes bearing a chelating (κ²C,O) NHC-SO₃ ligand have been prepared.

The free-energy barrier to hydride transfer across a dipalladium complex

We use density-functional theory molecular dynamics (DFT-MD) simulations to determine the hydride transfer coordinate between palladium centres of the crystallographically observed terminal hydride locations, Pd–Pd–H, originally postulated for the solution dynamics of the complex bis-NHC dipalladium hydride [{(MesIm)₂CH₂}₂Pd₂H][PF₆], and then calculate the free-energy along this coordinate. We estimate the transfer barrier-height to be about 20 kcal mol⁻¹ with a hydride transfer rate in the order of seconds at room temperature. We validate our DFT-MD modelling using inelastic neutron scattering which reveals anharmonicity of the hydride environment that is so pronounced that there is complete failure of the harmonic model for the hydride ligand. The simulations are extended to high temperature to bring the H-transfer to a rate that is accessible to the simulation technique.