Trifluoromethanesulfonate (triflate) as a moderately coordinating anion: Studies from chemistry of the cationic coordinatively unsaturated mono- and diruthenium amidinates

Triflate complexes of mono- and diruthenium amidinates, (η⁶-C₆R₆)Ru(κ¹-OTf){η²-R′NC(R′′)NR′} (1: R = Me; 2: R = H) and (η⁵-C₅Me₅)Ru(μ-η²-ⁱPrNC(Me)NⁱPr)Ru(κ¹-OTf)(η⁵-C₅R₅) (3: R = Me; 4: R = H), are synthesized, and coordination behavior of the triflate anion to the coordinatively unsaturated ruthenium species is investigated by crystallography and variable temperature (VT) NMR spectroscopy (¹⁹F, ¹H). The monoruthenium amidinate complexes have three-legged piano-stool structures in single crystals, which include a κ¹-OTf ligand with the Ru–O bond of 2.15–2.20 Å. In contrast, reversible dissociation of OTf is observed in variable temperature ¹H NMR spectroscopy in liquid states; the activation energy for the dissociation and recombination of the OTf ligand is varied with the substituents on the arene and amidinate ligand in the corresponding ruthenium cation and the solvent used. A typical example of moderately coordinating ability of the OTf ligand is seen in ¹⁹F NMR spectra of (η⁶-C₆Me₆)Ru(κ¹-OTf){η²-ⁱPrNC(Me)NⁱPr} (1a) and (η⁶-C₆H₆)Ru(κ¹-OTf){η²-ⁱPrNC(Me)NⁱPr} (2a) in CD₂Cl₂ at the temperature range from −90 to 20 °C, in which the OTf anion is dissociated in 1a, whereas 2a has a relatively robust Ru–OTf bond. Combination of crystallography and VT NMR contributes to understanding the difference in coordination behavior of the OTf ligand between two diruthenium amidinates, (η⁵-C₅Me₅)Ru(μ-η²-ⁱPrNC(Me)NⁱPr)Ru(κ¹-OTf)(η⁵-C₅Me₅) (3) and (η⁵-C₅Me₅)Ru(μ-η²-ⁱPrNC(Me)NⁱPr)Ru(κ¹-OTf)(η⁵-C₅H₅) (4); the results suggest that the electron-donating and sterically demanding η⁵-C₅Me₅ helps for dissociation of the triflate ligand. Moderate coordinating ability of the triflate anion sometimes provides characteristic reactions of mono- and diruthenium amidinates which differ from the corresponding neutral halogeno-compounds or cationic coordinatively unsaturated homologues bearing fluorinated tetraarylborates; a typical example is given by inhibition of coordination of ethylene to the (η⁶-C₆H₆)Ru{η²-^tBuNC(Ph)N^tBu}]⁺ species by the OTf ligand.