On the Catalytic Activity of [RuH₂(PPh₃)₃(CO)] (PPh₃=triphenylphosphine) in Ruthenium‐Catalysed Generation of Hydrogen from Alcohols: a Combined Experimental and DFT study

Abstract

Using density functional theory calculations (at the B97‐D2//BP86 level) and measurements of kinetic isotope effects, we explored the mechanism of [RuH2(PPh3)3(CO)] (22) in catalytic acceptor‐less dehydrogenation of methanol to formaldehyde. 22 is found to exhibit a similar activity as the previously studied [RuH2(H2)(PPh3)3] (1 b) complex. On the computed pathway, η2→η1 slippage of Ru‐bound formaldehyde prior to decoordination is indicated to be rate‐limiting, consistent with the low kH/kD KIE of 1.3 measured for this reaction. We also explored computationally the possibility of achieving complete dehydrogenation of methanol (into CO2 and H2), through subsequent decarbonylation of formaldehyde and water‐gas shift reaction of the resulting carbonyl complex. Complete pathways of this kind are traced for 22 and for [RuH2(PPh3)2(CO)2]. An alternative mechanism, involving a gem‐diol intermediate (obtained upon attack of OH− to coordinated formaldehyde), has also been investigated. All these pathways turned out to be unfavourable kinetically, in keeping with the lack of CO2 evolution experimentally observed in this system. Our calculations show that the reactions are hampered by the low electrophilicities of the CO and HCHO ligands, making OH− uptake unfavourable. Consequently, the subsequent intermediates are too high‐lying on the reaction profiles, thus leading to high kinetic barriers and preventing full dehydrogenation of methanol to occur by this kind of mechanism.

Panoramic Mechanistic Insights into Hydrogen Production via Aqueous-Phase Reforming of Methanol Catalyzed by Ruthenium Complexes of Bis-N-Heterocyclic Carbene Pincer Ligands

“…To this end, efforts have been devoted to H 2 production by catalytically dehydrogenating certain liquid organic hydrogen carrier (LOHC) molecules that are otherwise perfectly stable under ambient conditions. − Being inexpensive and readily available, methanol is one of the most attractive LOHCs; three molecules of H 2 can be generated by aqueous-phase reforming of methanol (APRM, eq ). ,,− normalC normalH 3 O H + H 2 normalO → 3 H 2 + normalC normalO 2 …”

Acceptorless Dehydrogenation of Primary and Secondary Alcohols Catalysed by Phosphine‐free C, C Chelated Ir (III) NHC Complexes

“…Moreover, mixed phosphine Ru complexes with protic NHC, [38] NHC, [39] and Schiff‐base ligands [40] are also explored for this transformation. Furthermore, transformation can also be accomplished in the presence of RuH 2 (CO)(PPh 3 ), [41] . However, most of these catalytic systems described above rely upon moisture/air‐sensitive multi‐dentate phosphine‐based ligands which are difficult to prepare in ambient conditions.…”

“…Besides homogeneous catalysts, heterogeneous catalysts based on Ag, [50] Pd, [51] Cu, [52] Ru, [53] Co, [54] Ni, [55] Pt, [56] Au, [57] Re, [57] and Ir [58] have also been employed for the same purpose. However, most of these reported catalytic systems suffer from limitations such as high catalyst loadings (up to 10 mol %), high temperature [37–45,48–49,52,54a,c,55,57] (up to 200 °C), long reaction time (48 h), [48] and limited substrate scope [43–46,54,55] . Furthermore, although dehydrogenation of secondary alcohol can easily be accomplished in the presence of a suitable catalyst, selective conversion of primary alcohol to aldehyde is a challenging task to accomplish as dehydrogenation product (aldehyde) can react with an active catalyst to furnish inactive carbonyl complex [59] .…”