The independent synthesis of the biscarbene complexes [Ir(cod)(vegi R )]PF 6 (2) (cod =1,5-cyclooctadiene, vegi R = bidentate N-heterocyclic carbene) as well as their isomerized complexes [Ir(1-κ-4,5,6-η-C 8 H 12 )(NCCH 3 )(vegi R )]-PF 6 (3) 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)(F 3 CCO 2 )(H)(vegi nPr )]PF 6 (7a) 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)]PF 6 (8) (2,2-bipy = 2,2′-bipyridine).
Attempts to generate the free bis(N-heterocyclic carbene) vegi R (R = nPr, tBu; vegi R = 2,7-dihydro-2,7dialkyldiimidazo[1,5-b:5′,1′-f ]pyridazine-1,8-diylidene) from its imidazolium salts 1 with alkali-metal bases resulted exclusively in the formation of the respective lithium, sodium, and potassium complexes 2−4 due to the strongly chelating properties of the dicarbene. DFT calculations reveal pronounced dispersion interactions in the case of N-tertbutyl substituents as the reason for the formation of the homoleptic lithium species 2b-H in solution. The dynamic behavior of the lithium complexes in equilibrium was studied by NMR techniques. Attempts to liberate the free carbenes by addition of the respective crown ethers failed for the lithium and sodium complexes. Deprotonation of the imidazolium salts 1a,b with the strong metal-free phosphazene base P4-tBu ({(Me 2 N) 3 PN} 3 PNtBu) generated successfully the free dicarbenes vegi R (5) and monocarbene 6b in solution.
A synthetic route for the selective di- and monochlorination of pyridazine annelated bis(imidazolium) salts at the formamidinium moieties with trichloroisocyanuric acid (TCCA) is presented. Due to the steric hindrance, the molecular structure of the dichlorobis(imidazolium) salt shows a pronounced torsion from planarity as well as a deviation of the C–Cl bond vectors from the ideal bisecting line of the respective NCN angles such as to avoid each other. The monochlorinated bis(imidazolium) salt is free of steric hindrance and therefore shows less deviation from the parent bis(imidazolium) salt. In the presence of acetate the chloroimidazolium salt acts as a chlorination agent for acetate leading to formation of acetyl chloride and the respective urea.
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