Thermodynamic Analysis of Metal–Ligand Cooperativity of PNP Ru Complexes: Implications for CO₂ Hydrogenation to Methanol and Catalyst Inhibition

Abstract

The hydrogenation of CO 2 in the presence of amines to formate, formamides, and methanol (MeOH) is a promising approach to streamlining carbon capture and recycling. To achieve this, understanding how catalyst design impacts selectivity and performance is critical. Herein we describe a thorough thermochemical analysis of the (de)hydrogenation catalyst, (PNP)Ru−Cl (PNP = 2,6-bis(di-tertbutylphosphinomethyl)pyridine; Ru = Ru(CO)(H)) and correlate our findings to catalyst performance. Although this catalyst is known to hydrogenate CO 2 to formate with a mild base, we show that MeOH is produced when using a strong base. Consistent with pK a measurements, the requirement for a strong base suggests that the deprotonation of a sixcoordinate Ru species is integral to the catalytic cycle that produces MeOH. Our studies also indicate that the concentration of MeOH produced is independent of catalyst concentration, consistent with a deactivation pathway that is dependent on methanol concentration, not equivalency. Our temperature-dependent equilibrium studies of the dearomatized congener, (*PNP)Ru, with various H−X species (to give (PNP)Ru−X; X = H, OH, OMe, OCHO, OC(O)NMe 2 ) reveal that formic acid equilibrium is approximately temperature-independent; relative to H 2 , it is more favored at elevated temperatures. We also measure the hydricity of (PNP)Ru−H in THF and show how subsequent coordination of the substrate can impact the apparent hydricity. The implications of this work are broadly applicable to hydrogenation and dehydrogenation catalysis and, in particular, to those that can undergo metal−ligand cooperativity (MLC) at the catalyst. These results serve to benchmark future studies by allowing comparisons to be made among catalysts and will positively impact rational catalyst design.

Phosphines and N‐Heterocycles Joining Forces: an Emerging Structural Motif in PNP‐Pincer Chemistry

“…Instead, 87 seems to be a resting state of the catalyst, which releases the active pincer complex on demand . Similar C–C bond formations were also discovered upon exposing de‐aromatized pincer complexes of the general type of complex 86 to CO 2 , which commonly results the formation of compounds of type 88 , , , , , , . Nitriles or other unsaturated molecules were found to react in a similar and generally fully reversible manner , , , , …”

Temperature and Solvent Effects on H2 Splitting and Hydricity: Ramifications on CO2 Hydrogenation by a Rhenium Pincer Catalyst

“…Metal-ligand cooperative examples include 1,2-addition of H2 across Fe-N or Fe-B bonds (ΔΗ˚1 ≈ -9 kcal•mol -1 in toluene 11 or benzene 9 ) and 1,3-addition of H2 in (PNP * )RuH(CO) (ΔΗ˚1 = -17.4 kcal•mol -1 in THF; PNP* = 2-( t Bu2PCH2)-6-( t Bu2PCH)-C5H3N). 10 The entropy of H2 splitting by 2 is far more unfavorable than any comparable systems reported in the literature. The ∆S˚1 of 2 varies from -63.3 cal•mol -1 •K -1 in MeCN to -76.0 cal•mol -1 •K -1 in toluene.…”

“…These solvents not only span a range of polarity and donor ability, but THF and toluene are commonly employed in catalysis and were the solvents used in thermodynamic studies of H2 splitting by metal-ligand cooperative systems. 10,11 While MeCN is not often employed in hydrogenation catalysis, it is a classic solvent for thermochemical studies. 12,34,35 Solutions of dicarbonyl 2 and various organic bases were prepared in each of the three solvents and placed under 1 atm H2 to probe for hydrogen cleavage reactivity.…”

“…Hydricities in THF have not been reported until recently. 10,13 Comparing the hydricity of hydrides in THF (see Table S1 in the SI), complex 3 is more hydridic than neutral (PNP)Ru(H)2(CO) (∆G°H-in THF = 44.6 kcal•mol -1 ; PNP = 2,6-bis( t Bu2PCH2)-C5H3N), and falls in the range of anionic bimetallic cobalt hydrides. 10,13 On the basis of an estimated hydricity of formate in THF and the known catalytic activity of anionic bimetallic cobalt hydrides, 10,13,41 complex 3 should also be sufficiently hydridic to produce formate from hydride transfer to CO2 in THF.…”

“…5,7,8 Enthalpy and entropy of reaction parameters have been determined for a few catalysts that split H2 via the metal-ligand cooperative mechanism. [9][10][11] However, to our knowledge no analogous temperature-dependent thermochemical data is available for catalysts that follow an outer sphere H2 splitting mechanism, precluding comparisons between mechanisms.…”