Single‐Site Ruthenium Pincer Complex Knitted into Porous Organic Polymers for Dehydrogenation of Formic Acid

Abstract

Owing to its capacity for reversible hydrogen storage, formic acid (FA) holds great promise as an alternative energy carrier to conventional fossil fuel systems. Whereas the decomposition of FA to hydrogen (H2) and carbon dioxide (CO2) through homogeneous catalysis is well established, the selective and efficient dehydrogenation of FA by a robust heterogeneous catalyst remains a challenge. A new heterogeneous ruthenium pincer framework with single‐atomic sites was prepared in one step by the direct knitting of a phosphorus–nitrogen PN3P‐pincer ruthenium complex in a porous organic polymer. The heterogeneous ruthenium complex efficiently dehydrogenates formic acid in both organic and aqueous media with remarkably enhanced stability. Notably, no detectable CO was generated and a turnover number (TON) of 145 300 was attained in a continuous experiment with no significant decline in catalytic activity (in sharp contrast, a total TON of only 5600 was obtained with the homogeneous analog under the same conditions). The single‐atomic sites in the porous framework combined the desirable attributes of high reactivity and selectivity of a homogeneous catalyst with the significantly enhanced catalyst stability and reusability benefits of heterogeneous catalysis.

An Update on Formic Acid Dehydrogenation by Homogeneous Catalysis

“…As the hydrogen transfer of ketones was demonstrated using the Ru−PN 3 (P) pincer complexes, it was not a surprise that the FA dehydrogenation could also proceed . This well‐defined PN 3 ‐Pincer Ru catalytic system ( 14 ) can efficiently and selectively liberate H 2 from FA under mild conditions without CO and reached a TOF of 7,000 h −1 and a TON more than 1.1 million during a long lifetime over 150 hours .…”

“…In 2011, Nozaki introduced an Ir−PNP complex to demonstrate the reversible FA dehydrogenation providing TOF of 120,000 h −1 in tert ‐butanol during the first minute at 80 °C . A great number of catalysts bearing pincer type ligands were also investigated in the selective FA decomposition under different conditions . These studies are described in section 4.…”

Visible‐Light Photocatalytic Reduction of CO2 to Formic Acid with a Ru Catalyst Supported by N,N′‐Bis(diphenylphosphino)‐2,6‐diaminopyridine Ligands

“…Furthermore, formic acid has been identified and explored as ap otential carrier of dihydrogen. [24][25][26] Furthermore, the potentialo fp incerc omplexes as efficient electrocatalysts for CO 2 reduction under mild conditions has recently been demonstrated for Mn, [27] Ru, [28] and Ir [29,30] complexes. [3][4][5][6] Photocatalysis is ap articularly appealing approacha s it relies on an essentially limitless and clean solar energy source.P hotocatalytic systemsc onsisto fi ntegrated components that include ap hotosensitizer (PS)f or harvesting the energy of the light, an electron donor (ED)t hat provides electrons for the reduction, and ac atalyst (CAT)t hat is as ite for the transformation of CO 2 .T oo ur knowledge,s ince their discovery in 1985, [7] all of the reported molecular photocatalysts based on Ru II have used a-diimine supporting ligandsa nd these speciesh ave fallen into two broad groups.O ne class are bis(a-diimine) speciesr epresentedb ycis-[Ru(N^N) 2 (CO) 2 ] 2 + [8][9][10][11] and the other are mono(a-diimine) catalysts represented by cis,trans-[Ru(N^N)(CO) 2 Cl 2 ].…”

“…[1,2] Homogeneous catalysts have been developed for CO 2 reduction under both electrochemical and photochemical conditions. [24][25][26] Furthermore, the potentialo fp incerc omplexes as efficient electrocatalysts for CO 2 reduction under mild conditions has recently been demonstrated for Mn, [27] Ru, [28] and Ir [29,30] complexes. [12][13][14] Althoughavariety of substituents on the a-diimine ligands have been productively explored to improvec atalystp erformance, given the maturity of this field, ab roader variationo fm olecular architecture is required to provide new insights and stimulate new concepts in this field.…”

“…[25] This provides as hortcut in the catalytic cyclef or formic acid production. Alternative accesst oH 2 could come from the combination of two of these metal hydrides or perhaps throughd ehydration of formica cid.…”