Identifying and Rationalizing the Conditions for the Isomerization of 1,5-Cyclooctadiene in Iridium Complexes by Experimental and Theoretical Mechanistic Studies

The independent synthesis of the biscarbene complexes [Ir­(cod)­(vegi^R)]­PF₆ (<b>2</b>) (cod =1,5-cyclooctadiene, vegi^R = bidentate N-heterocyclic carbene) as well as their isomerized complexes [Ir­(1-κ-4,5,6-η-C₈H₁₂)­(NCCH₃)­(vegi^R)]­PF₆ (<b>3</b>) is described. We elucidated acetic acid as the catalyst and coordinated acetonitrile as the thermodynamic driving force for this cod-isomerization. By using the stronger trifluoroacetic acid, we isolated complex [Ir­(cod)­(F₃CCO₂)­(H)­(vegi^<i>n</i>Pr)]­PF₆ (<b>7a</b>) as an intermediate of the isomerization. From H/D exchange experiments as well as DFT calculations, we conclude that after formation of the Ir–H complex, an olefin insertion, followed by a concerted metalation-deprotonation step and a coordination of acetonitrile, is the mechanistic pathway. On the basis of our findings, we were able to carry out the cod-isomerization for the first time also for the less-electron-rich complex [Ir­(2,2′-bipy)­(cod)]­PF₆ (<b>8</b>) (2,2-bipy = 2,2′-bipyridine)

Identifying and Rationalizing the Conditions for the Isomerization of 1,5-Cyclooctadiene in Iridium Complexes by Experimental and Theoretical Mechanistic Studies

The independent synthesis of the biscarbene complexes [Ir­(cod)­(vegi^R)]­PF₆ (<b>2</b>) (cod =1,5-cyclooctadiene, vegi^R = bidentate N-heterocyclic carbene) as well as their isomerized complexes [Ir­(1-κ-4,5,6-η-C₈H₁₂)­(NCCH₃)­(vegi^R)]­PF₆ (<b>3</b>) is described. We elucidated acetic acid as the catalyst and coordinated acetonitrile as the thermodynamic driving force for this cod-isomerization. By using the stronger trifluoroacetic acid, we isolated complex [Ir­(cod)­(F₃CCO₂)­(H)­(vegi^<i>n</i>Pr)]­PF₆ (<b>7a</b>) as an intermediate of the isomerization. From H/D exchange experiments as well as DFT calculations, we conclude that after formation of the Ir–H complex, an olefin insertion, followed by a concerted metalation-deprotonation step and a coordination of acetonitrile, is the mechanistic pathway. On the basis of our findings, we were able to carry out the cod-isomerization for the first time also for the less-electron-rich complex [Ir­(2,2′-bipy)­(cod)]­PF₆ (<b>8</b>) (2,2-bipy = 2,2′-bipyridine)

1,10-Phenanthroline Analogue Pyridazine-Based N-Heterocyclic Carbene Ligands

Synthesis of a planar, π-conjugated pyridazine-based biscarbene is reported. Starting from 3,6-dimethylpyridazine, the bisimidazolium salt <b>1</b>*<b>2HPF</b>_<b>6</b> was prepared in a four-step synthesis by chlorination, amination, formylation, and cyclization. The free carbene <b>1</b> can be generated in situ by addition of base. Despite the rigid annelated tricycle, the carbene ligand turns out to be highly flexible upon coordination of transition-metal complexes. With silver­(I) oxide or copper­(I) oxide binuclear carbene complexes with a bridging coordination mode of the ligand are obtained. Transmetalation of both complexes to the respective gold complex is described. The bridging coordination mode of the carbene ligand is similar to that of 2,2′-bipyridine. Reaction of the bisimidazolium salt <b>1*2HPF</b>_<b>6</b> with potassium acetate and [RhCl­(COD)]₂ leads to a mononuclear rhodium complex with the chelating binding mode of <b>1</b>, resembling strongly the coordination properties of 1,10-phenanthroline

Rhodium Complexes Bearing 1,10-Phenanthroline Analogue Bis-NHC Ligands Are Active Catalysts for Transfer Hydrogenation of Ketones

Synthesis of a new example of the pyridazine annelated bis­(N-heterocyclic carbene) ligand <b>3b</b> (vegi) bearing benzyl substituents is reported, as well as the synthesis of its cationic Rh­(cod) complex <b>5b</b>. The mechanism of formation of the [Rh­(cod)­(vegi)]⁺ complexes was investigated, showing a stepwise deprotonation of the imidazolium moieties via the mono-carbene imidazolium Rh­(cod)Cl species <b>4</b>. The [Rh­(cod)­(vegi)]⁺ complexes <b>5a</b>,<b>b</b> show catalytic activity in the transfer hydrogenation of even sterically hindered ketones

Dinuclear Coinage-Metal Complexes of Bis(NHC) Ligands: Structural Features and Dynamic Behavior of a Cu–Cu Complex

Binuclear complexes of copper, silver, and gold bearing a 2,2′-bipyridine analogue, the pyridazine annelated bis­(N-heterocyclic carbene) ligand (vegi) <b>1</b>, were prepared and structurally characterized. They all feature the shortest metal–metal distances that have been measured so far in complexes with this structural motif bearing neutral bidentate ligands, indicative of d¹⁰–d¹⁰ interactions. While in the silver complex the linear coordination of each silver atom with two carbene ligands results in a planar complex, the ligand planes are twisted by 70° in the Cu complex <b>4</b> and by 31° in the gold complex <b>3</b>. The copper complex shows a solvent-dependent equilibrium between the [Cu₂L₂]²⁺ complex and a [Cu₂L₃]²⁺ complex along with solvated CuPF₆

Dinuclear Coinage-Metal Complexes of Bis(NHC) Ligands: Structural Features and Dynamic Behavior of a Cu–Cu Complex

Binuclear complexes of copper, silver, and gold bearing a 2,2′-bipyridine analogue, the pyridazine annelated bis­(N-heterocyclic carbene) ligand (vegi) <b>1</b>, were prepared and structurally characterized. They all feature the shortest metal–metal distances that have been measured so far in complexes with this structural motif bearing neutral bidentate ligands, indicative of d¹⁰–d¹⁰ interactions. While in the silver complex the linear coordination of each silver atom with two carbene ligands results in a planar complex, the ligand planes are twisted by 70° in the Cu complex <b>4</b> and by 31° in the gold complex <b>3</b>. The copper complex shows a solvent-dependent equilibrium between the [Cu₂L₂]²⁺ complex and a [Cu₂L₃]²⁺ complex along with solvated CuPF₆