Reactivity of a Ruthenium–Carbonyl Complex in the Methanol Dehydrogenation Reaction

Abstract Finding new catalysts for the release of molecular hydrogen from methanol is of high relevance in the context of the development of sustainable energy carriers. Herein, we report that the ruthenium complex Ru(salbinapht)(CO)(Pi‐Pr3) {salbinapht=2‐[({2′‐[(2‐hydroxybenzyl)amino]‐[1,1′‐binaphthalen]‐2‐yl}imino)methyl]phenolato} (2) catalyzes the methanol dehydrogenation reaction in the presence of base and water to yield H2, formate, and carbonate. Dihydrogen is the only gas detected and a turnover frequency up to 55 h−1 at 82 °C is reached. Complex 2 bears a carbonyl ligand that is derived from methanol, as is demonstrated by labeling experiments. The carbonyl ligand can be treated with base to form formate (HCOO−) and hydrogen. The nature of the active species is further shown not to contain a CO ligand but likely still possesses a salen‐derived ligand. During catalysis, formation of Ru(CO)2(H)2(P‐iPr3)2 is occasionally observed, which is also an active methanol dehydrogenation catalyst.


1) General methods
All reactions were carried out under an inert (argon/nitrogen) atmosphere using standard Schlenk techniques. THF was distilled from sodium benzophenone ketyl; acetonitrile, methanol and dichloromethane were distilled from CaH 2 and dioxane was distilled from sodium all under nitrogen. NMR spectra ( 1 H, 13 C and 31 P, H-H COSY and HSQC) were measured on a Bruker AMX 400, a Varian Mercury 300, a Bruker DRX 500, or a Bruker DRX 300 spectrometer. Infrared spectra were recorded on a Thermo Nicolet NEXUS 670 FT-IR. The high resolution mass spectrum were recorded on a JEOL AccuTOF LC, JMS-T100LP mass spectrometer using electron spray ionization (ESI) on a JEOL AccuTOF GC v 4g, JMS-T100GCV mass spectrometer using field desorption (FD).

Ru(1-H 2 )(CO)P(i-Pr 3 )MeCN (2)
: 100 mg (0.21 mmol) of Ru(Cl) 2 (DMSO) 4 was transferred to a flame dried schlenk flask equipped with a reflux condenser and the vessel was purged three times with argon-vacuum cycles. Then 10 mL of methanol was added, followed by the addition of 80 L (0.42 mmol) of P(i-Pr) 3 . The colourless solution with suspended yellow solid was heated under reflux until the solid had dissolved and the solution turned orange-red (approximately 5 minutes). The reaction mixture was cooled to room temperature after which 0.42 mL of a LiOMe solution (0.42 mmol, 1M in methanol) was added and stirred for 5 minutes. Then the orange solution was brought to reflux for 30 minutes. In the meantime 102 mg (0.21 mmol) of (R)-2,2'-bis(salicylideneamino)-1,1'-binaphthyl (1) was transferred to a different flame dried Schlenk flask, which was purged three times with argon-vacuum cycles. To this yellow solid 20 mL of methanol and 0.42 mL (2.55 mmol) of LiOMesolution (1M in methanol) where added. The yellow suspension was added via a syringe to the stirring solution containing the ruthenium phosphine complex. The reaction mixture was heated for 45 min which resulted in an orange solution. The reaction mixture was concentrated in vacuo to approximately 2 mL, leading to the formation of an orange-yellow precipitate. The filtrate was removed using a syringe and the orange-yellow solid was dried and subsequently recrystallized from acetonitrile giving the orange-yellow crystalline complex in pure form. The acetonitrile filtrate was concentrated to ~1 mL giving a second crop of 2. Overall yield: 148 mg, 90 %.
Only one diastereoisomer is formed during the synthesis: the N-atom of the amine group ( Figure S3, N-H) has S configuration, the ruthenium centre is coordinated in a OC-6-56-A configuration 3 , and binapthyl remains in R configuration.     4 was transferred to a flame dried Schlenk flask equipped with a reflux condenser and the vessel was purged three times with argon-vacuum cycles. Then 3 mL of 13 C-labelled methanol was added, followed by the addition of 0.11 mL (0.56 mmol) of P(i-Pr) 3 . The colourless solution with suspended yellow solids was heated under reflux until the solid had dissolved and the solution turned orange-red (approximately 5 minutes). In the meantime 81.1 mg (0.16 mmol) of the (R)-2,2'-bis(salicylideneamino)-1,1'binaphthyl (1) was transferred to a different flame dried Schlenk flask, which was purged three times with argon-vacuum cycles. To this yellow solid, 3 mL of 13 C-labelled methanol and 8 mg (0.35 mmol) of solid sodium where added. The resulting yellow suspension was added via a syringe to the stirring solution containing the ruthenium phosphine complex. The reaction mixture was heated for 55 min which resulted in a dark red solution that gave an orange-yellow precipitate upon concentration of the solvent to ±1 mL. After standing one day at 5 ˚C, the filtrate was taken off and the orange-yellow solid was dried and subsequently recrystallized from 2 mL hot acetonitrile, giving the orange-yellow crystalline complex in pure form.

Ru(1)(DMSO) 2 (3):
103.4 mg (0.21 mmol) of (R)-2,2'-bis(salicylideneamino)-1,1'-binaphthyl (1) and 95.4 mg (3.98 mmol) of NaH were transferred to a Schlenk flask and purged 3x with vacuum argon cycles. Then 4.5 mL of THF was added and this suspension was stirred until gas evolution had ceased. The yellow solution was filtered off, the white solid was washed with 3x 2mL THF and the combined filtrates were evaporated to dryness. To this yellow solid was added 64.9 mg (0.11 mmol) of [Ru(Cl) 2 p-cymene] 2 which was purged 3x with vacuum argon cycles. Then 10 mL of MeCN was added and the solution was stirred overnight. The resulting orange suspension was filtered and the solids were washed with DCM. The filtrates were combined and evaporated to dryness resulting in an orange red solid. This solid was dissolved in 3 mL DMSO and stirred overnight. After evaporation of the DMSO and pcymene, the product was obtained in pure form. Yield: 159.3 mg, 99%

Synthesis of Ru(CO) 2 (H) 2 (P-iPr 3 ) 2 (4)
102.5 mg (0.214 mmol) of Ru(Cl) 2 (DMSO) 4 was added to a flame dried schlenk and purged 3x with vacuum argon cycles. Then 10 mL of methanol and 83 L (0.427 mmol) of P-iPr 3 were added and this was heated to reflux for 1h turning into a red solution. The solution was cooled to room temperature, 0.85 mL of LiOMe (1M, 0.85 mmol) was added and stirred for 5 minutes after which the solution was heated to reflux and stirred overnight yielding an orange/brown solution. The solution was filtered and evaporated to dryness in vacuo. The solid was extracted with dcm and the yellow product was crashed out adding pentane to this solution giving a yellow solid that matched literature data 4 . The solid was used without further purification for catalysis.

3) Hydrogen evolution experiments
Typical gas evolution experiment The specified amount of base was transferred to a flame dried setup as depicted in Figure 21 and purged 3x with vacuum argon cycles until point "A". Then 29 mL of solvent (20.3 mL of methanol, 2.2 mL of water and 6.5 mL of dioxane) was added and the mixture was heated to reflux, while flushing the tube and the cylinder until point "B" with argon. When reflux temperature was reached the flushing was ended and heating was continued for 0.5-1h. Meanwhile, ~12-13 mol of the specified ruthenium complex was transferred to another flame dried schlenk flask, purged 3x with vacuum argon cycles and subsequently dissolved in a total amount of 1.5 mL dioxane. This catalyst solution was added to the refluxing mixture and the measurement of gas evolution was started. The displacement of water in the cylinder (see S21 for the set up) was measured in time. All catalysis runs were measured in duplo and the gas formed was injected in the gas GC after every measurement. Molecular hydrogen was always the only gas present.  A typical gas evolution experiment was set up using 13.50 mg of 2, in a 8M KOH solution of 8 mL dioxane, 9 mL CD 3 OD, 11 mL 13 CD 3 OD and 2.2 mL water. Catalysis was ran for 2h producing molecular hydrogen in a linear way (Figure 14). The reaction mixture was cooled to room temperature and extracted with 1 x 10 mL and 1 x 5 mL of dichloromethane. The organic solution was evaporated to dryness and dissolved in MeCN-d 3 : no enriched 13 CO-signal could be detected in 13 C-NMR. 13 C-NMR of the methanol/water phase showed presence of the 13 C-enriched H/DCOOindicating that the 13 C-labelled methanol was dehydrogenated. Thus, the carbonyl complex is not regenerated after the attack of base on the carbonyl.