“Pincer” pyridine–dicarbene–iridium and -ruthenium complexes and derivatives thereof

The cationic pincer-type complexes [Ir^I(CN^MeC)L]X {CN^MeC = [2,6-bis(2,6-diisopropylphenyl)imidazol-2-ylidene]-3,5-dimethylpyridine, L = CO, X = PF₆ 4; L = CH₃CN, X = PF₆ 5; L = pyridine, X = BAr^F₄, Ar^F = 3,5-bis-trifluoromethyl-phenyl 6}, that were obtained from [Ir^I(CN^MeC)Cl] (1) by displacement of the chloride ligand were structurally characterized. Complexes 4 and 5 adopt square planar, in-plane distorted geometries, and in 6 the metal environment shows substantial pyramidalization. Theoretical calculations of the cations in 4 and 6 reproduce the experimental structures and rationalize their features. 1 undergoes oxidative transformations with CH₂Cl₂ to cis-[Ir^III(CN^MeC)(CH₂Cl)Cl₂] (7) and with PhICl₂ to mer-[Ir^III(CN^MeC)Cl₃] (8). The ruthenium derivatives trans-[Ru^II(CNC)Cl₂L] {CNC = [2,6-bis(2,6-diisopropylphenyl)imidazol-2-ylidene]-pyridine, L = pyridine, 10} and [Ru^II(CNC)(η²,η²-nbd)](X)₂. 2L (nbd = 2,5-norbornadiene, L = CH₃CN, X = BF₄ 11), were prepared by the reaction of cis-trans-[RuCl₂(nbd)(py)₂] and trans-cis-[RuCl₂(nbd)(pip)₂] (pip = piperidine) with the ligand CNC, respectively; both adopt distorted octahedral structures. The back-bonding in 11 is comparable to that in its precursor complex, indicating minimal contribution of the NHC donors to this effect. Substitution of both chlorides in the known cis-[Ru^II(CNC)Cl₂L] (L = PPh₃) by azido ligands gave cis-[Ru^II(CNC)(N₃)₂L] (L = PPh₃, 12), which by photolytic cleavage of the coordinated N₃ failed to produce well-defined complexes.