Homogeneously catalysed conversion of aqueous formaldehyde to H2 and carbonate

Small organic molecules provide a promising solution for the requirement to store large amounts of hydrogen in a future hydrogen-based energy system. Herein, we report that diolefin–ruthenium complexes containing the chemically and redox non-innocent ligand trop2dad catalyse the production of H2 from formaldehyde and water in the presence of a base. The process involves the catalytic conversion to carbonate salt using aqueous solutions and is the fastest reported for acceptorless formalin dehydrogenation to date. A mechanism supported by density functional theory calculations postulates protonation of a ruthenium hydride to form a low-valent active species, the reversible uptake of dihydrogen by the ligand and active participation of both the ligand and the metal in substrate activation and dihydrogen bond formation.

the gas phase in the experiment of formaldehyde. Elemental analyses were performed by the microanalytical laboratory of the ETH Zürich. Melting points were determined with a Büchi melting point apparatus and are not corrected. X-Ray diffraction was measured on a Bruker SMART Apex II diffractometer with CCD area detector; Mo-Kα radiation (0.71073 Å) at T = 100 K. The refinement against full matrix (versus F 2 ) was performed with SHELXTL (ver. 6.12) and SHELXL-97. For all complexes all nonhydrogen atoms were refined anisotropically.
Supplementary Note 1. Data

Synthesis of [(dodecyl)Me 3 N][Ru(trop 2 dad)] (1Ab)
To a solution of complex 1K (50 mg, 0.066 mmol, 1.0 equiv) in THF (6 mL) [(dodecyl)Me 3 N]Br (22.4 mg, 0.072 mmol, 1.1 equiv) was added as solid. The suspension was stirred for 3 hours at room temperature and filtered through a pad of Celite. All volatiles of the filtrate were removed under reduced pressure and the crude dark purple product was washed carefully with Et 2 O (3 x 1 mL) and 5 mL of n-hexane. Drying the residue under high vacuum gave the product as air sensitive dark red powder. Yield: 41.5 mg, 82 %.
Crystals suitable for single crystal X-ray diffraction analysis were grown from an n-hexane layered solution of the product in a DME / THF (1:1) mixture.

Synthesis of [Ru(trop 2 dad)(CO)Ru(trop 2 dad)].thf (5)
Complex 2 (prepared as described above) (0.5 mmol, 1.0 equiv) was dissolved in 20 mL THF. The solution was placed in a Schlenk flask with a J. Young valve. Argon was purged through the solution by the freezepump-thaw method (three times), and the flask was subsequently filled with CO(g) (1.0 bar). The colour changed from dark brown to orange within seconds , accompanied by the precipitation of a microcrystalline orange solid, identified as complex 5. The reaction mixture was filtered and the filtrate was layered with n-hexane (6 mL) and stored at -32°C. After 2 days, a second crop of orange crystals of 5, suitable for x-ray diffraction analysis were isolated by filtration and dried in a stream of argon.

Synthesis of [Ru(trop 2 dad)Ru(trop-dad-trop H )(CO)] (5H 2 )
The previous solution was extracted under argon with 0.6 mL of degassed D 2 O and the aqueous phase was analysed by 1 H and 13 C NMR, revealing the presence of HCO 2 M and M 2 CO 3 (M = K, Bu 4 N). The organic phase was concentrated to dryness under vacuum. The obtained dark orange was dissolved in 2 mL of THF / DME (1:1) and layered with n-hexane (1 mL). After 5 days at -32 °C the compound 5H 2 was obtained as orange crystals. The mother liquor was decanted and the obtained air sensitive crystals were washed with n-hexane prior drying in a stream of argon. Yield: 3.1 mg, 10%.
Complex 3a is obtained as main product along with O=PPh 3 (ca. 20%) and two minor additional P containing species. 1 H NMR analysis of a dried aliquot which was dissolved in d 8 -THF indicated the formation of complex 1K and other hydride containing complexes . The deep orange solution was extracted under an argon atmosphere with D 2 O (2 x 0.5 mL) and analysed. The orange organic phase was evaporated to dryness and the residue was washed with diethyl ether. The obtained orange solid was dissolved in DME (0.5 mL), filtered and layered with n-hexane (1 mL). After 1 day at room temperature air sensitive orange reddish single crystals of 7 were isolated by filtration, washed with n-hexane and dried in a stream of argon. Yield: 11.6 mg, 12% yield.   3. Catalytic dehydrogenation of formaldehyde or paraformaldehyde / water mixtures.

General method
In a typical experiment, a 25 mL two-neck round-bottom flask was connected to a reflux condenser with argon inlet/outlet which is coupled to a water filled gas burette (see Supplementary Figure1)

Calculations
All DFT geometry optimizations were carried out with the Turbomole program 4 coupled to the PQS Baker optimizer 5 via the BOpt package. 6 Geometries were fully optimized as minima or transition states using the BP86 functional 7,8 and the resolution-of-identity (ri) method 9 using the Turbomole def2-TZVP basis 10 for all atoms. Grimme's dispersion corrections (D3 version, implemented with the keyword disp3 in Turbomole) were applied in all geometry optimizations. 11 All minima (no imaginary frequencies) and transition states (one imaginary frequency) were characterized by calculating the Hessian matrix. ZPE and gas-phase thermal corrections (entropy and enthalpy, 298 K, 1 bar) from these analyses were calculated.
The relative (free) energies obtained from these calculations are reported in the main text of this paper.
The nature of the transition states was confirmed by following the intrinsic reaction coordinate (IRC).
By calculation of the partition function of the molecules in the gas phase, the entropy of dissociation or coordination for reactions in solution is overestimated (overestimated translational entropy terms in the gas phase compared to solutions). For reactions in solution we therefore corrected the Gibbs free energies for all steps involving a change in the number of solute species (we did not apply any corrections for loss of gaseous H 2 or CO 2 ). The applied correction term is a correction for the condensed phase (CP) reference volume (1 L mol -1 ) compared to the gas phase (GP) reference volume (24.5 L mol -1 ). This leads to an entropy correction term (S CP = S GP + Rln{1/24.5} for all species, which combined with neglecting the RT term, corrects the relative free energies (298 K) of all associative (2.5 kcal mol -1 ) and dissociative steps (+2.5 kcal mol -1 ), 12 except those involving H 2 and CO 2 gas molecules.
In order to make computations more time efficient and less expensive, a reduced model for catalyst was employed. Li and Hall, 13 used this approach on same system and it has been established that MERP is not affected by this simplification in any significant way. We first explored the conversion of species 4 m to 4' m , which is exergonic and proceeds over a rather lowbarrier transition state (Supplementary Figure 22). This is essentially a proton transfer reaction from the ligand to the metal. 15  The computed pathway from 4' m involves initial association of methanediol via a double hydrogen-bond interaction between the ligand and the substrate producing 4-A, which is exergonic by 10 kcal mol -1 .

Supplementary
Proton-transfer to the amido moiety and coordination of the alcoholate moiety to Ru is downhill by another 2-3 kcal mol -1 , producing complex 4-B. Subsequent hydride transfer from methanediol to Ru over 4-TS1 has a remarkably low barrier (2.4 kcal mol -1 ). This is not a common beta-hydride elimination step, as was computed for complex 2 m (see main text), but is better described as an ion-pair polarized, double hydrogen-bond stabilized transition state without a significant RuO interaction (RuO distance: 3.52 Å).
This step directly produces intermediate 4-C', which is best described as a formic acid adduct stabilized by two hydrogen-bonds involving the amine moieties of the ligand, as well as by a weak interaction between the hydride and the carbonyl moiety (C carbonyl -H hydride distance: 2.08 Å). Complex 4-C' then rearranges to form complex 4-C, which has a distinct dihydrogen bond between the RuH moiety and the acidic proton of the formic acid moiety (H hydride H acid distance: 1.20 Å) and still contains a double hydrogen bond interaction between the two amines and the carbonyl group of the formic acid moiety.

OPTIMIZED GEOMETRIES
All energy values reported below (combined with the xyz-coordinates) are SCF energies in atomic units.