An Anionic Dinuclear Ruthenium Dihydrogen Complex of Relevance for Alkyne gem‐Hydrogenation

Abstract During an investigation into the fate of ruthenium precatalysts used for light‐driven alkyne gem‐hydrogenation reactions with formation of Grubbs‐type ruthenium catalysts, it was found that the reaction of [(IPr)(η6‐cymene)RuCl2] with H2 under UV‐irradiation affords an anionic dinuclear σ‐dihydrogen complex, which is thermally surprisingly robust. Not only are anionic σ‐complexes in general exceedingly rare, but the newly formed species seems to be the first example lacking any structural attributes able to counterbalance the negative charge and, in so doing, prevent oxidative insertion of the metal centers into the ligated H2 from occurring.


INTENSITY STATISTICS FOR DATASET
The investigated crystal was small and the structure contains several disordered regions. One isopropyl group of the anion exhibits a 60:40 % disorder over two positions. A second isopropyl group of the cation shows a disorder over three positions with 33.3 % occupancy for each component. All three CH2Cl2 solute molecules are disordered over two positions with varying occupancies (50:50 %, 75:25 % and 70:30 %).
One CH2Cl2 molecule could only be satisfactorily described by isotropic displacement parameters.

General
All reactions were carried out under argon in flame-dried glassware, ensuring rigorously inert conditions. The solvents were purified by distillation over the indicated drying agents and were stored and handled under argon: CH2Cl2 (CaH2), MeCN (CaH2), pentane (Na/K alloy), THF (Na/K alloy).
Hydrogen and HD were handled with standard balloon techniques.
NMR spectra were recorded on Bruker AVIII 400, AVIII 500 or AVneo 600 MHz NMR spectrometers at 298 K unless otherwise indicated; chemical shift (δ) given in ppm relative to TMS, coupling constants (J) in Hz.
As can be seen in Figure  was dissolved in toluene (15 mL). The Schlenk tube was closed with a septum and a hydrogen-filled balloon was connected to a needle, which was pierced through the septum. The Schlenk tube was flushed with hydrogen for 2 min through an outlet cannula (the cannula did not reach into the solution to ensure that only the head space of the tube was flushed). The exit cannula was removed but the hydrogen-filled balloon remained attached to the Schlenk tube to ensure constant hydrogen pressure throughout the reaction. The mixture was heated to 90 °C for 1 h under a hydrogen atmosphere in the dark. After this time, the red solution was evaporated in vacuo. The remaining material was suspended in pentane and filtered with a filter canula into another Schlenk flask. The filtrate was concentrated in vacuum to provide the title compound as a red crystalline solid (34 mg, 42%). Red Kinetic Profiling. The catalytic performance of complexes 8 and 9 was evaluated by means of the standard hydrogenative metathesis reaction of enyne 6. In case of complex 8, presence of cyclopentene 7 in the resulting complex crude reaction mixture could not be confirmed.
A flame-dried quartz Schlenk tube was charged with 9 (77.6 mg, 0.05 mmol, 20 mol%) [or 1b (69.6 mg, 0.1 mmol, 20 mol%)], enyne 6 (110 mg, 0.5 mmol) and toluene (5 mL). The Schlenk tube was closed with a septum and then transferred into the PhotoRedOxBox TC, cooled to approx. 23 °C. A hydrogen-filled balloon was connected to a needle, which was pierced through the septum. The Schlenk tube was flushed with hydrogen for 2 min through an outlet cannula (the cannula did not reach into the solution to ensure that only the head space of the tube was flushed). The exit cannula was removed but the hydrogen-filled balloon remained attached to the Schlenk tube to ensure constant hydrogen pressure throughout the reaction. The light source was switched on and samples were taken manually with oven-dried syringes every 2 mins; the samples were filtered through a short pad of silica and the filtrates analyzed by GC-MS.  The spin-lattice relaxation times (T1) were determined with the inversion recovery sequence (Bruker pulse program: t1ir) using 32 non-linear spaced inversion times ranging from 0.001 ms to approx. 5 * T1, which were adjusted individually to the different field strengths. The pulse offset during the measurement was set on-resonance on the hydride signal ( 11 ppm). The obtained 2D datasets were analyzed with the Bruker TOPSPIN T1T2 module by integrating the hydride signal and fitting it to the following equation: I0 describes the magnetization at thermal equilibrium,  is the individual inversion time at each step in the 2D sequence, P is the polarization at t = 0.
Morris et al. developed equations that can be used to calculate an upper and lower limit of dHH, depending on whether the H2 ligand is spinning slower or faster than the spectrometer frequency: