Determining the Catalyst Properties That Lead to High Activity and Selectivity for Catalytic Hydrodeoxygenation with Ruthenium Pincer Complexes

Abstract

Ten ruthenium pincer complexes were evaluated as catalysts for the hydrodeoxygenation (HDO) reaction on a lignin monomer surrogate, vanillyl alcohol. Four of these complexes are reported herein with the synthesis and full characterization data for all and single-crystal X-ray diffraction data for three complexes bearing OH/O − , NMe 2 , and Me substituents on the pincer. A systematic study of these CNC pincer complexes revealed that the π-donor substituent on the pyridine ring plays a key role in enhancing the yield of the desired deoxygenated product. While OMe, OH, and NMe 2 are all effective as π-donor substituents on the central pyridine ring in the pincer, the highest conversion to products and the best selectivity was observed with OH substituents and added sodium carbonate as a base. Base serves to deprotonate the OH group and form 1 Oas observed spectroscopically. Furthermore, efforts to use other catalysts have revealed that free or labile sites are needed on the ruthenium center and an electronically rich and nonbulky CNC pincer is optimal. At low catalyst loadings (0.01 mol %), the OH-substituted catalyst 1 OH in the presence of base serves as a homogeneous catalyst and is able to achieve quantitative and selective conversion of vanillyl alcohol to desired the HDO product, creosol, with up to 10000 turnovers. With this knowledge in hand, we can design the next generation of homogeneous catalysts with increased reactivity toward all of the oxygenated sites on lignin-derived monomers.

Transition Metal Complexes Supported by N‐Heterocyclic Carbene‐Based Pincer Platforms: Synthesis, Reactivity and Applications

“…In a more recent work, the same group employed a series of pincer analogs of 19 to catalyze the hydrodeoxygenation of alcohols (Scheme 9). [20] It was revealed that 19 and analogs that bear pyridine π‐donor substituents (OMe, OH, NMe 2 ) are superior catalytic performers. This was rationalized by the proposal that a π‐donor substituent makes the Ru center more electron‐rich and thus weakens the ligated H−H bond in the transient intermediate B .…”

Structure Function Relationships in Ruthenium Carbon Dioxide Reduction Catalysts with CNC Pincers Containing Donor Groups

“…The syntheses of the ruthenium complexes, Ru OMe , Ru H , Ru OH , Ru NMe2 , and Ru Me , complexes, were reported recently by us. [4a], These complexes were studied as alcohol hydrodeoxygenation (HDO) catalysts. It was demonstrated by both spectroscopic and crystallographic methods that the oxygen and nitrogen substituents donate electron density towards the metal center and thereby enhance catalytic rates .…”

“…It was demonstrated by both spectroscopic and crystallographic methods that the oxygen and nitrogen substituents donate electron density towards the metal center and thereby enhance catalytic rates. [7] Furthermore, Ru OH is readily deprotonated by mild bases, which results in an even stronger π donor group and the most active HDO catalyst. However, the complexes Ru OH , Ru NMe2 , and Ru Me have not been studied for photocatalysis.…”

“…[4a,7] Similarly, for Ru NPh2 and Ru 3OMe , the ligand synthesis starts from 2,6-dihalopyridine (1) with the remote group, R, already installed on the pyridine ring either from a selective nucleophilic aromatic substitution reaction with R = NPh 2 , or via a C-H activation-borylation/oxidation with R = 3-OMe (Scheme 1, see the Experimental Section for Schemes 2 and 3). [7,8] Molecule 1 is treated with deprotonated imidazole to give 2,6-diimidazolepyridine 2 in yields ranging from 52-63 %, which is then methylated to yield the pre-ligand bis(imidazolium) salts 3 in 78-88 % yield. This pincer precursor is then treated with triethylamine and dichloro(p-cymene)ruthenium(II)dimer in acetonitrile for an in situ metalation with yields ranging from 35-47 %, similar to our previously reported procedures.…”

“…2,6-Difluoro-3-methoxypyridine (8): Note: In treating 2,6-difluoropyridine with reagents for C-H activation/borylation/oxidation, [7,8] 2,6-difluoropyridin-3-ol is formed as a minor product in a mixture of products (with 2,6-difluoropyridin-4-ol as the major product). Though the major product could be isolated in pure form by column chromatography (silica gel), [7] the minor product could not be isolated. Hence, a mixture of compounds was used for the synthesis of 2,6-difluoro-3-methoxypyridine.…”