Hydrogen Activation with Ru-PN3P Pincer Complexes for the Conversion of C1 Feedstocks

The hydrogenation of C1 feedstocks (CO and CO2) has been investigated using ruthenium complexes [RuHCl(CO)(PN3P)] as the catalyst. PN3P pincer ligands containing amines in the linker between the central pyridine donor and the phosphorus donors with bulky substituents (tert-butyl (1) or TMPhos (2)) are required to obtain mononuclear single-site catalysts that can be activated by the addition of KOtBu to generate stable five-coordinate complexes [RuH(CO)(PN3P–H)], whereby the pincer ligand has been deprotonated. Activation of hydrogen takes place via heterolytic cleavage to generate [RuH2(CO)(PN3P)], but in the presence of CO, coordination of CO occurs preferentially to give [RuH(CO)2(PN3P–H)]. This complex can be protonated to give the cationic complex [RuH(CO)2(PN3P)]+, but it is unable to activate H2 heterolytically. In the case of the less coordinating CO2, both ruthenium complexes 1 and 2 are highly efficient as CO2 hydrogenation catalysts in the presence of a base (DBU), which in the case of the TMPhos ligand results in a TON of 30,000 for the formation of formate.


General
All moisture and oxygen sensitive compounds were prepared using standard vacuum line, Schlenk and cannula techniques.A standard N2 or Ar-filled glove box was used for any subsequent manipulation and storage of these compounds.

NMR spectroscopy
Standard 1 H, 31 P, 19 F, and 13 C NMR spectra were recorded using a Bruker 400 MHz or 500 MHz spectrometer at room temperature (RT) unless otherwise stated. 1 H NMR and 13 C NMR chemical shifts (δ) were referenced to the residual non-deuterated solvent signal and the 13 C signal of the deuterated solvent respectively. 19F and 31 P NMR chemical shifts were referenced externally to CFCl3 and 85% H3PO4 in H2O respectively.NMR spectra for air sensitive compounds were recorded under a nitrogen or argon atmosphere.NMR spectra for samples under gas greater than atmospheric pressure were recorded using a thick walled, high-pressure tube.NMR signal multiplicities are described by the following abbreviations: s (singlet), d (doublet), t (triplet), q (quartet), m (unresolved multiplet), br (broad).

Further characterisation
Mass spectra were recorded using a Waters LCT Premier ES-ToF (ESI) or a Micromass Autospec Premier (LSIMS) spectrometer.Standard FTIR spectra were measured using a Perkin Elmer Spectrum GX spectrometer.Elemental analyses were carried out by the Elemental Analysis Service at London Metropolitan University or at the Institute of Sustainability for Chemicals, Energy and Environment (ISCE 2 , A*STAR).UV-Vis spectra were measured using a Perkin-Elmer Lambda 20 spectrometer.Gas chromatography analysis was carried out using an Agilent 6890 Series system with an Agilent HP-5, 30m x 0.25 mm I.D., (5% Phenylmethylpolysiloxane) column.

Solvents and reagents
Solvents and reagents were used as supplied unless otherwise specified.Solvents used in the reactions of oxygen and moisture sensitive compounds were dried and degassed according to standard techniques.Et2O and THF were dried and freshly distilled from sodium benzophenone ketyl, under a nitrogen atmosphere.Toluene, pentane and hexane were dried by passage through a column containing 3Ǻ molecular sieves and Q5 reagent (activated Cu(II)O).C6D5Cl was dried by stirring over CaH2 followed by trap to trap distillation, and was stored in a glass ampoule inside a N2 or Ar filled glovebox.RuCl3.3H2O was donated by Johnson Matthey and used without further purification.t Bu2PCl was donated by Solvay (Rhodia) and used without further purification.All other chemicals were obtained commercially and used without further purification.
[ 4 were prepared according to literature procedures.N,N′-bis(di-tert-butylphosphino)-2,6diaminopyridine was prepared by a modification of the literature procedure. 3TMPhos was prepared according to a previously reported procedure. 5
n BuLi (12 mL of 1.6 M solution in hexanes, 19.2 mmol) was slowly added to this solution by syringe.A solution of t Bu2PCl (3.49 g, 19.3 mmol) in THF (10 mL) was added dropwise by cannula at −78 °C.The mixture was allowed to warm to RT and heated at reflux temperature overnight.After cooling to room temperature, the THF was removed under reduced pressure and DCM (30 mL) added to the light yellow residue.The suspension was filtered through celite and then silica and the DCM removed under reduced pressure to give an off-white powder (2.72 g, 75 %).[RuHCl(CO)(PPh3)3] (1.00 g, 1.05 mmol) and PN 3 P-t Bu (0.50 g, 1.26 mmol) were added to a Schlenk flask under a nitrogen atmosphere and dry THF (15 mL) was added by cannula.The suspension was heated at reflux for 3 hours, resulting in an orange solution and a yellow precipitate.The THF was reduced to about 5 mL to complete the precipitation and the mixture was filtered to leave a yellow solid.This was dissolved in minimal DCM, and pentane added to precipitate a yellow powder that was washed with pentane and dried in vacuo (500 mg, 85 %).The complex is soluble in polar organic solvents and partially soluble in aromatic solvents.

[RuH(CO)(PN 3 P-t Bu-H)] (3)
[RuHCl(CO)(PN 3 P-t Bu)] (120 mg, 0.213 mmol) and KO t Bu (30 mg, 0.268 mmol) were added to a Schlenk flask in a nitrogen filled glovebox and dry THF (10 ml) was added.The suspension was stirred at RT until a red/orange solution was obtained (30 minutes) and the THF was removed under reduced pressure.The red residue was extracted with DCM (10 mL), filtered and the DCM removed under reduced pressure.The red powder was washed with pentane at −78 °C and dried in vacuo (70 mg, 51 %).Noteworthy, the colour of freshly prepared [RuH(CO)(PN 3 P-t Bu-H)] is red, but was reported by Huang and co-workers as a brown-red solid. 6We observed that this complex slowly decomposes at RT under an inert atmosphere, turning from red to brown on a timescale of several months, though when stored in a freezer this decomposition is not observed.[RuH(CO)(PN 3 P-TMPhos-H)] (4) [RuHCl(CO)(PN 3 P-TMPhos)] (2) (25 mg, 0.04 mmol) and KO t Bu (6.0 mg, 0.051 mmol) were dissolved in MeCN (2 mL) and stirred at RT for 2 h.The solution was passed through a glass filter and the solvent removed in vacuo to give an off-white solid.Yield = 20 mg (80%).The complex also appears to decompose in solution gradually darkening over a few days with the formation of precipitate.

[RuH(CO)2(PN 3 P-t Bu-H)] (5)
[Ru3(CO)12] (100 mg, 0.16 mmol) and PN 3 P( t Bu) (187 mg, 0.47 mmol) were added to a Schlenk flask and toluene (5 mL) was then added.The orange/yellow suspension was heated at reflux for 17 h and cooled to RT. Pentane (10 mL) was added to the resulting suspension to complete precipitation of the pink/orange powder that was isolated by filtration, washed with pentane and dried in vacuo.(172 mg, 66 %).(99 mg, 78 %).Crystals suitable for Single Crystal X-ray diffraction were obtained by diffusion of hexane into a benzene solution of the complex.

General procedure for the hydrogenation of CO2 to formate
A 300 mL stainless steel PARR reactor was evacuated at 140 °C for 3 h.After this time is was allowed to cool to RT and cycled 3 times with argon/vacuum.Under a stream of argon, DMF (30 mL), catalyst solution (5 mL) and DBU (10 mL) were introduced to the reactor.Mechanical stirring was started (1000 rpm) and the vessel was heated to 90 °C.At this temperature CO2/H2 (7.5:7.5 bar) was introduced into the reactor from a gas reservoir.The pressure in the reaction vessel was maintained throughout the reaction.After 1 h the reaction was cooled to RT and the reaction vessel vented.An accurate amount of internal standard (mesitylene) was added and the liquid fraction analysed by 1 H NMR spectroscopy.