Boosting homogeneous chemoselective hydrogenation of olefins mediated by a bis(silylenyl)terphenyl-nickel(0) pre-catalyst†

The isolable chelating bis(N-heterocyclic silylenyl)-substituted terphenyl ligand [SiII(Terp)SiII] as well as its bis(phosphine) analogue [PIII(Terp)PIII] have been synthesised and fully characterised. Their reaction with Ni(cod)2 (cod = cycloocta-1,5-diene) affords the corresponding 16 VE nickel(0) complexes with an intramolecular η2-arene coordination of Ni, [E(Terp)E]Ni(η2-arene) (E = PIII, SiII; arene = phenylene spacer). Due to a strong cooperativity of the Si and Ni sites in H2 activation and H atom transfer, [SiII(Terp)SiII]Ni(η2-arene) mediates very effectively and chemoselectively the homogeneously catalysed hydrogenation of olefins bearing functional groups at 1 bar H2 pressure and room temperature; in contrast, the bis(phosphine) analogous complex shows only poor activity. Catalytic and stoichiometric experiments revealed the important role of the η2-coordination of the Ni(0) site by the intramolecular phenylene with respect to the hydrogenation activity of [SiII(Terp)SiII]Ni(η2-arene). The mechanism has been established by kinetic measurements, including kinetic isotope effect (KIE) and Hammet-plot correlation. With this system, the currently highest performance of a homogeneous nickel-based hydrogenation catalyst of olefins (TON = 9800, TOF = 6800 h−1) could be realised.


Electronic Supplementary Information
Boosting chemoselective hydrogention of olefins with molecular hydrogen mediated by a bis(silylenyl)terphenyl-nickel(0) pre-catalyst Marcel Cyclic Voltammetry Measurement: Cyclic Voltammetry (CV) measurements were carried out at 295 K by using a Biologic SP-150 potentiostat and a three electrode setup. Pt-wire was used as an auxiliary electrode. A freshly polished glassy carbon disc (3 mm diameter) as a working electrode and a pseudo reference electrode Ag/Ag + was used.
All cyclic voltammograms were referenced against the Cp2Fe/Cp2Fe + redox couple which was used as an internal standard. As an electrolyte, 0.3 M solutions of TBAPF6 in THF was used. The iR-drop was determined and compensated by using the impedance measurement technique implemented in the EC-Lab Software V10.

Synthesis of [E(Terp)E]Ni(CO)2 (E = P,Si). A J. Young NMR tube was charged with
4 mg of the Ni(0) complex and 0.45 mL THF was added into the NMR tube. After three freeze-pump-thaw cycles, the solution was subjected to 1 bar CO gas at room temperature with an immediate color change from dark red to pale yellow. After 2 minutes at room temperature, volatiles were removed under reduced pressure and the obtained pale yellow solid was subjected to an FT-IR spectroscopic analysis revealing the formation of the corresponding [Ni]CO2 complexes 4-CO and 5-CO as indicated by two very strong IR-absorption bands. The other minor absorption band, which is observed for both complexes (4-CO and 5-CO) possibly corresponds to the related 18VE Tricarbonylcomplex which is formed in small quantities in the presence of an excess CO.

8) Reversible H2-activation by bis(silylene)Ni(η 2 -arene) 5
A J. Young NMR tube was charged with 5 mg of the Ni(0) complex and 0.45 mL d8-THF was added into the NMR tube. After three freeze-pump-thaw cycles, the solution was subjected to 1 bar H2 gas at room temperature upon which no color change did occur. 1 Hproton NMR analysis revealed the partial formation of 5-H2 with a characteristic singlet resonance at δ = -1.50 ppm (2 H) and δ = + 5.77 ppm for the central arene protons (4 H).
A mixture of initial Ni(0) complex 5 and 5-H2 exist in an equilibrium under these conditions.
This process was repeated and identical results were obtained with no indication of decomposition (S26    The equilibrium constant k1/k-1 was determined using ferrocene as internal standard and was found to be k1/k-1 = 0.66. Addition of D2 at -80 °C to a solution of 5/5-H2 inside a J. Young NMR tube resulted in fast H-D scrambling (δ = 4.50 ppm, 1 JH,D = 43 Hz) and the appearance of an additional triplet resonance signal at δ = -1.24 ppm, 2 JH,D = 4.88 Hz) corresponding to 5-HD (S31). H,H-EXSY NMR additionally shows the expected cross-signal for the hydride-signal and dissolved H2 gas in d8-THF (S33).

9) Hydrogenation of Olefins
General Procedure for Olefin Hydrogenation. A stock solution was used containing ferrocene (0.022 mmol/0.45 mL) and catalyst 5 (1.1 µmol/0.45 mL, 2 mol%). From the stock solution, 0.45 mL were taken and transferred into a sealed J. Young NMR tube and olefin (0.054 mmol) was added. Prior to H2 addition, a 1 H NMR was recorded. After three freeze-pump-thaw cycles, the solution was subjected to 1 bar H2 at room temperature.
The reaction progress was further monitored by 1 H NMR analysis.

Mercury Drop Test.
In a 3 mL vial, norbornene (21.0 mg, 0.223 mmol), ferrocene (17 mg, 0.091 mmol) and catalyst 5 were dissolved in 2 mL C6D6. The red solution was transferred into a 25 mL Schlenk tube containing a stir bar. The solution was frozen in liquid nitrogen and mercury (250 mg, 1.15 mmol) was added under a N2 atmosphere.
After three freeze-pump-thaw cycles, the solution was subjected to 1 bar H2 at room temperature and stirred for 14 h. Conversion was determined by 1 H NMR analysis. A full conversion (>99 %) was achieved in the presence of Hg, indicating a homogeneous reaction. However, decomposition was observed resulting in decoloration and formation of a black solid. When Ni(0) complex 5 is exposed to mercury (exc.) inside a sealed J.
Young NMR tube. Slow decomposition was observed in the absence and presence of 1 bar H2 gas in d8-THF resulting in decoloration and the formation of a black solid. In the absence of mercury, 5 and 5-H2 are stable in solution and show no decomposition within 7 days at room temperature.

Catalyst comparison in nbe hydrogenation.
Two 3 mL vials were charged with Ni(0) catalyst (1.1 µmol, 2 mol%), ferrocene (0.022 mmol) and norbornene (0.054 mmol). The mixture was dissolved in 0.45 mL C6D6 and transferred into a sealed J. Young NMR tube. Prior to H2 addition, a 1 H NMR was recorded. After three freeze-pump-thaw cycles, the solution was subjected to 1 bar H2 at room temperature. The reaction progress was further monitored by 1 H NMR analysis.
Data points were fitted with a linear function (R 2 = 0.97). Bis(silylene) complex 5 catalyzes the hydrogenation of norbornene 12 times faster compared to bis(phosphine) complex 4 under identical reaction conditions. Thus, 120 h were needed to reach full conversion using 2 mol% of 4.
A J. Young NMR tube was charged with 5.6 mg of the Ni(0) complex, 3.3 mg of norbornene (Nbe) and 0.45 mL d8-THF was added into the NMR tube resulting in a homogeneous dark red solution. 1 H-proton NMR analysis revealed the partial formation of 5-nbe with a characteristic pattern in the hydride region at δ = -0.32 ppm and δ = -0.52 ppm similar to those of literature known Ni(nbe) adducts. [S2] H 2 (1bar) Then, 20 equiv. of Nbe was added and the experiment was repeated. After full conversion, the same results were obtained with no observable catalyst degradation.

Kinetic isotope effect:
Two NMR-samples were prepared, each containing 2 mol% of 5 (0.8 mg, 1.1 µmol) from a stock solution in C6D6 containing ferrocene as internal standard (0.022 mmol, 4 mg) and norbornene (0.054 mmol, 5 mg). 1 H-NMR spectra were obtained at regular intervals approx. 1 h and the NMR tubes were shaken after each measurement. The concentration of norbornene was plotted against time and the data points were fitted with a linear function (R 2 = 0.98/0.99).

TOF / TON measurement
A mixture of norbornene (9f, 1.07 mmol), ferrocene (0.43 mmol, internal standard) and 5 (1.07 µmol) was prepared inside a 3 mL vial and dissolved in 3 mL C6D6. The obtained red solution was transferred to a 25 mL Schlenk tube containing a stir bar. After three freeze-pump-thaw cycles, the solution was subjected to 1 bar H2 at room temperature.

Determination of Reaction order in [olefine], [Ni]
Two NMR-samples were prepared, each containing 2 mol% of 5 (0.8 mg, 1.1 µmol) from a stock solution in C6D6 containing ferrocene as internal standard (0.022 mmol, 4 mg) and 5 / 2.5 mg of norbornene. Prior to H2 addition, a 1 H NMR was recorded. After addition of H2, 1 H-NMR spectra were obtained at intervals of approx. 1 h. The concentration of norbornene was plotted against time (8 hours) and the data points were fitted with a linear function (R 2 > 0.98).

mM 27 51 mM 25
For both concentrations of nbe, a very similar TON was observed within 8 hours. We conclude, that the reaction is 0. order in [Olefin]. The TOF value after >99% conversion was the same (3.2 h -1 ). We conclude, that the reaction is 1. order in [5].

Determination of Reaction order in [H2]
Three solutions were prepared, each containing 0.5 mol% of 5 (1.1 µmol) from a stock solution in C6D6 (1.5 mL) containing ferrocene as internal standard (0.088 mmol, 16 mg) and 20 mg of norbornene. The solutions were subjected to different H2 pressures (1.0, 2.0, 3.0 bar) inside a Schlenk tube containing a stir bar after three freeze-pump-thaw cycles. During hydrogenation, the H2-cylinder was directly attached to the Schlenk tube to ensure a constant H2 pressure. After t = 10 min, 0.1 mL of the reaction mixture were removed and analyzed by 1 H NMR spectroscopy. The conversion of norbornene was plotted against the H2-pressure and the data points were fitted with a linear function. A linear correlation was obtained (R 2 = 0.98). We conclude, that the reaction is 1. order in [H2].

Calculation of the experimental date law:
The overall reaction rate can be expressed by the following equation based on our experimental findings discussed above: The

Hammet-Plot analysis
Four solutions were prepared in a vial, each containing 0.5 mol% of 5 (1.1 µmol) from a stock solution in C6D6 (1.5 mL) containing ferrocene as internal standard (0.088 mmol, 16 mg) and para-substituted styrene derivatives (0.216 mmol). The solutions were transferred into a 10 mL Schlenk tube containing a stir bar. After three freeze-pump-thaw cycles, the solution was subjected to 1 bar H2 at room temperature. The reaction progress was monitored by 1 H NMR spectroscopy by removal of 0.1 mL from the reaction mixture within the first 3 hours. Data points were fitted with a linear function (R 2 = 0.99).