Efficient Base-Free Aqueous Reforming of Methanol Homogeneously Catalyzed by Ruthenium Exhibiting a Remarkable Acceleration by Added Catalytic Thiol

Production of H2 by methanol reforming is of particular interest due the low cost, ready availability, and high hydrogen content of methanol. However, most current methods either require very high temperatures and pressures or strongly rely on the utilization of large amounts of base. Here we report an efficient, base-free aqueous-phase reforming of methanol homogeneously catalyzed by an acridine-based ruthenium pincer complex, the activity of which was unexpectedly improved by a catalytic amount of a thiol additive. The reactivity of this system is enhanced by nearly 2 orders of magnitude upon addition of the thiol, and it can maintain activity for over 3 weeks, achieving a total H2 turnover number of over 130 000. On the basis of both experimental and computational studies, a mechanism is proposed which involves outer-sphere dehydrogenations promoted by a unique ruthenium complex with thiolate as an assisting ligand. The current system overcomes the need for added base in homogeneous methanol reforming and also highlights the unprecedented acceleration of catalytic activity of metal complexes achieved by the addition of a catalytic amount of thiol.


Supporting Notes
Note S1. Proposed pathways and potential energy surface Figure S1. Proposed mechanism including inner-sphere dehydrogenation of water. S3 Figure S2. Potential energy surface for the pathway including inner-sphere dehydrogenation of water (in MeOH, T = 383.15 K, pressure = 1). Note that Ru-7 with formaldehyde coordinated in a κ 1 -O fashion is 6.0 kcal/mol higher in energy than Ru-7 with η 2 -coordinated C=O, which is presented in this Figure.  Figure S7. Proposed mechanism with calculated Gibbs energy difference between each intermediates and transition states based on outer-sphere dehydrogenation of methanol and methandiol involving Ru-4; The transformations of hydrogen, CO 2 and CO from the condensed phase to the gas phase are not additionally corrected in the free energy quantities provided. S9 Figure S8. Potential energy surface for the pathway including outer-sphere dehydrogenation of methanol and methandiol based on ruthenium thiolate complex (in MeOH, T = 383.15 K, pressure = 1). S10

Note S2. Control experiments with Ru-4
Control experiments were carried out using Ru-4 as the catalyst. Interestingly, under the reaction conditions with the addition of 1 equiv thiol, 240 mL gas was collected after 12 h heating at 150 o C (Supporting Figure 9). However, nearly no gas was collected without the addition of thiol.

Figure S9. Control experiments based on Ru-4.
To figure out how Ru-4 worked in the reaction system in the presence of thiol, mechanistic studies were conducted heating the biscarbonyl acridine Ru-complex Ru-4 in acetic acid under Ar flow. The utilization of acetic acid is to mimic formic acid in the methanol reforming system, which under the reaction conditions is quite easy to undergo dehydrogenation and thus makes the capture of the key intermediate, ruthenium formate, very difficult. Interestingly, both CO and H 2 were detected by GC during the reaction and the ruthenium acetate complex Ru-16 was generated as the major product ( Figure S10). 1 The result proves the lability of the second CO on the ruthenium center and thus in the case of current methanol reforming reaction, it is reasonable to propose that Ru-4 can also react with the intermediate formic acid to release one molecule of CO and H 2 to regenerate monocarbonyl complex Ru-12. The Gibbs energy difference between ruthenium formate complex Ru-12 and Ru-4 is calculated as + 10.5 kcal/mol (from Ru-12 to Ru-4, Supporting Figure 7), which can be overcome under the reaction conditions. It is noted the transformation of hydrogen and CO from the condensed phase to the gas phase is not additionally corrected in the free energy quantities. 2 Based on the above results, it is proposed that Ru-4 is the off-cycle resting state of the catalyst during most of the reaction time. Even though Ru-4 might be able to catalyze the reaction, it should be very slow as reflected from ref. 1 by the calculated S11 activation barriers for the dehydrogenation of formic acid (overall kinetic barrier of 41.9 kcal/mol with Ru-4 vs 25.1 kcal/mol with Ru-1 in benzene). Thus in the current methanol reforming system, the monocarbonyl catalysts Ru-1 -Ru-3 are likely still the actual catalysts of the reaction despite the generation of CO. Figure S10. Control experiments relevant to CO loss from Ru-4. S12 Note S3. Factors related to the amount of CO generated in the reaction 1. Reaction temperature. Control experiments were carried out heating the reaction at different temperature. While nearly no CO was detected in the collected gas after heating at 120 o C, higher concentration of CO was observed after heating at 135 o C and 150 o C (Supporting Figure S11). Thus, the reaction temperature can affect the generation of CO in the system. [3][4][5][6] One thing worthwhile to mention is the internal temperature of the system. Since we employed a closed system, the internal temperature of the reaction increased along with the accumulation of the pressure, which might also affect the generation of CO at different stage of the reaction. is supposed to lower the amount of CO in the collected gas. Note that in addition to the direct coordination by CO, Ru-4 can also be generated by the reaction of Ru-1 either with formic acid (Supporting Table S1), or with CO 2 and H 2 (Supporting Figure   S35).   In the first 72 h of the long-term reaction (in 3.6 mL MeOH and 0.4 mL H 2 O), the amount of CO increased as the reaction continued. For example, while only 0.02% CO was detected in the collected gas of the first cycle (first 12 h), higher amounts of CO were detected in the following 5 cycles (Supporting Figure S14). Considering water was consumed during the reaction, making its concentration continuously decreased, these results also supported the concentration of water in the system could affect the generation of CO.  Figure S15), however, no CO was detected in the resulting gas mixture. The same result was obtained directly starting with Ru-1 as the catalyst without the addition of a catalytic amount of thiol (See Supporting Figure S35). Figure S15. Test the possibility of reverse water-shift reaction.
2. Thermo-decomposition of formic acid. 6 The dehydrogenation of formic acid was examined under the reaction conditions with or without water (Supporting Table S1).
As expected, with Ru-1 as the catalyst, the reactions were very efficient heating at 150 o C with a TOF (H 2 ) more than 10,000 h -1 in 10 minutes. However, no CO was detected after the reaction in the collected gas (entries 1 and 2). Similar results were obtained employing Ru-2 as the catalyst with only trace amount of CO detected in the collected gas (entries 3 and 4, Ru-2 was prepared in situ). The efficiency of Ru-2 is slightly lower than that of Ru-1, which also suggests that Ru-1 is the real catalyst for the dehydrogenation of formic acid step in current methanol reforming reaction. 1 In addition, the biscarbonyl ruthenium complex Ru-4 was also used as the catalyst (entries 5 and 6). Although the reaction rate decreased, still no CO was detected in the collected gas after 20 min heating. 3. Thermo-decomposition of formaldehyde. [3][4][5] Trioxane (anhydrous formaldehyde surrogate) was tested under the reaction conditions. In order to avoid its further conversion into formic acid, MeOH was used as the only solvent without the addition of water. Moreover, in consideration of internal temperature and atmosphere in the real system, 4 bar H 2 gas was introduced before the reaction in the control experiments ( Figure S16). Interestingly, after 24 h heating, 0.28% CO was detected by GC in the collected gas (Note little amount of CO 2 was detected by GC, indicating little amount of water existed in the system). A similar result was also obtained using Ru-1 as the catalyst. These experiments support the generated CO in the developed methanol reforming system mostly comes from the thermo-decomposition of the formaldehyde intermediate. Figure S16. Decomposition of formaldehyde to CO and H 2 . S16 Supporting Experimental Procedures

General considerations
All experiments with metal complexes and phosphine ligands were carried out under an atmosphere of purified nitrogen in a Vacuum Atmosphere glovebox equipped with a MO 40-2 inert gas purifier or using standard Schlenk techniques. All solvents were reagent grade or better. All non-deuterated solvents were purified according to standard procedures under argon atmosphere. Deuterated solvents were degassed with argon and directly used. All solvents were degassed with argon and kept in the glove box over 3Å molecular sieves. Water used in this system is deionized water which is further degassed with argon and kept in glovebox. All 1 H NMR, 13 C NMR or 31 P NMR spectra were recorded on a Bruker AVANCE III 300MHz, 400MHz spectrometer and reported in ppm (δ). Chemical shifts were referenced to the residual solvent peaks ( 1 H NMR, 13 C NMR) or an external standard of phosphoric acid (85% solution in D 2 O) at 0.0 ppm ( 31 P NMR). NMR spectroscopy abbreviations: br, broad; s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet. GC analysis was performed on an HP 6890 chromatograph (TCD detector) with helium as the carrier gas.
The scale of the graduated cylinder is 10 mL and the last digit of the volume of collected gas is estimated as 5 or 0.
The turnover number (TON) of H 2 was calculated from the amount of gas collected.
GC analysis indicate the composition of gas is almost 3:1 H 2 /CO 2 (<0.1% CO is not taken into consideration). Every hydrogen molecule represents one catalyst turnover number. Thus, the TON (H 2 ) value was calculated as: TON (H 2 ) = V gas × (3/4) ÷ 24.5 ÷ n cat S17 2. Preparation of standard curves for CO 2

General experimental procedures
In an N 2 -filled glovebox, a 90 mL Fischer-Porter tube was charged with Ru-1 (1.4 mg, 2.5 μmol), HexSH (0.35 μL in 0.1 mL methanol, freshly prepared; 2.5 μmol), methanol (1.7 mL) and a stirring bar, and the mixture was stirred at room temperature for 1 min to allow for the in situ generation of Ru-2 (2.5 μmol) in methanol. 11 H 2 O (0.2 mL) was then added and the Fischer-Porter tube was sealed and taken out of the glovebox. The reaction mixture was heated to 150 °C (oil bath temperature) and stirred at this temperature for 12 h, at which point the pressure gauge of the Fischer-Porter tube reached ~8.5 bar (under heating). Subsequently, the sealed tube was cooled to room temperature and the observed pressure decreased to ~5 bar. The Fischer-Porter tube was then connected to an inverted graduated cylinder filled with silicone oil and the tube's valve was slowly opened to allow the gas to flow into the cylinder and displace the oil. After no more gas bubbles were observed, the valve was closed and the collected gas (470 mL) was analyzed by GC. Cautions: (i) The Fischer-Porter tube should be shielded with an iron net and heated behind a shielder.
(ii) Hydrogen is a flammable gas. Reactions associated with H 2 gas should be handled carefully inside proper fume hoods without any flame, spark or static electricity sources nearby.
(iii) Hexanethiol can be smelled when the Fischer-Porter head (adaptor and pressure gauge) is removed, which can also serve as an alarming reagent of leaking.  Fischer-Porter tube was cooled to room temperature and connected to an inverted graduated cylinder filled with silicone oil. The tube's valve was carefully opened and the gas was collected in the inverted cylinder. After no more gas bubbles were observed, the valve was closed and the collected gas was analyzed by GC. The Fischer-Porter tube was then reintroduced into the oil bath and the reaction mixture was stirred at 150 °C for another heating cycle. The above procedure was repeated until ~80% of the water had been consumed (usually within 3 days). At this point, the Fischer-Porter tube was transferred into the glovebox, and both methanol and water were injected into it to restore the solvent ratio to 3.6 mL MeOH/0.4 mL H 2 O, based on the amounts of solvent that were consumed, as estimated from the volume of collected gas (trace amounts of CO were not taken into consideration). The Fischer-Porter tube was then taken out of the glovebox and the above procedure was repeated. Heating of the reaction mixture continued for a total of 592 h, during which 10930 mL of the H 2 /CO 2 gas mixture were collected.     Figure S25. 31 P NMR of the resulting species in benzene-d6 after 592 h heating (from the 9:1 system); Considerable amount of Ru-4, the resting state of the catalyst, was detected without the observation of free ligand.
S25 Figure S26. Details of the short continuous reaction in 3.6 mL MeOH and 0.9 mL H 2 O.  Figure S27. 31 P NMR of the resulting species in benzene-d6 after 302 h heating (from 6:1 system), indicating the major species is still Ru-4, the resting state of the catalyst. S27 Figure S28. Details of the short continuous reaction in 7.    Figure S33. Screening of additives.

Control experiments
The Fischer-Porter tube was sealed and transfered back to the glove box after the reaction (following the general procedure). The resulting solution was directly measured by 31 P NMR, indicating that a new species was formed. Then the solvent (methanol and water) was directly removed under vacuum and the resulting solid was dissolved in THF for NMR analysis. Both the signals of 31 P NMR and 1 H NMR (hydride peak) match a biscarbonyl ruthenium acridine complex Ru-4. Note the characterization of Ru-2 was done in MeOH-d4 directly mixing Ru-1 and 1 equiv hexanethiol. 11 S32 Figure S34. 31 P NMR of resulting species after reaction.
In a N 2 glove box, Ru-1 (2.8 mg, 5 μmol), HexSH (0.7 μL, 5 μmol, by micro-syringe) and toluene-d 8 (0.6 mL) were added to a 30 mL steel autoclave fitted with a Teflon sleeve. The autoclave was taken out of the glove box and pressurized with 6 bar CO 2 and 20 bar H 2 sequentially. After heating at 150 o C for 36 h with stirring, the steel autoclave was cooled in a cold water bath for 30 min. Then the gas was vented off carefully and analyzed by GC. The resulting solution was analyzed by GCMS and NMR. However, CO and MeOH were not detected.

S34
In a N 2 glove box, Ru-4 (3.0 mg, 5 μmol), dioxane (0.5 mL) and water (0.1 mL) were added to a J. Young NMR tube. The NMR tube was taken out of the box and heated at 135 o C for 24 h, after which it was cooled to room temperature and measured by 31 P NMR and 1 H NMR. Two species were detected by NMR, one of which was Ru-4 as signed below. The NMR tube was transferred back into the box and dioxane and water was removed under vacuum. Then toluene (0.5 mL) was added and the NMR tube was taken out of the box and measured again. Both 31 P NMR and 1 H NMR indicated that no Ru-1 was regenerated, eliminating the possibility of water-gas-shift reaction in the system.     where E ωB97M-V Methanol is the single point energy; and where corr M06-L freq is the thermal correction to the Gibbs free energy from the frequency calculation (at T = 383.15K and P = 1 atm).
Free energy values (Gº ) were then corrected to account for changes in standard Standard state corrections 24 were employed such that all species are treated as 1M (using an ideal gas approximation), with the exception of H 2 , CO 2 , CO (maintained as 1 atm), water (1 atm to 5.5M) and methanol (1 atm to 22M). [25][26][27] Other than these standard state corrections, the transformation of hydrogen, CO 2 and CO from the condensed phase to the gas phase is not additionally corrected in the free energy quantities provided.

S42
Ethanethiol were studied as minimal models for hexanethiol in the system.
Directionality of ∆G and ∆G TS values are indicated by the ordering of X,Y and all energies are reported in kcal/mol.