Development of Highly Active Ir–PNP Catalysts for Hydrogenation of Carbon Dioxide with Organic Bases

Abstract

Methoxy-substituted PNP–iridium(III) complexes and pyrazine-based PNP–iridium(III) complexes were developed and used to hydrogenate carbon dioxide in the presence of triethanolamine as a base. The methoxy-substituted PNP–hydridodichloridoiridium complex (C-HCl2) showed the highest turnover number, 160000; this is the highest value ever reported with an organic base in an aqueous medium. The reactivities of these complexes, derived from their ligand modification, were further studied. The results were as follows. (i) The pyrazine-based PNP–trihydridoiridium complex undergoes release of dihydrogen to afford dihydridoamido complex, possibly because of easy dearomatization of the pyrazine ring. This process was reversible, i.e., B-H2amido can readily be converted back to B-H3 on exposure to dihydrogen. (ii) The p-methoxy-substituted dihydridochlorido complex showed facile disproportionation of the chloride anion on the iridium center; this is attributed to the electron-donating nature of the methoxypyridine backbone.

Production of Liquid Solar Fuels and Their Use in Fuel Cells

“…111 The methoxy-substituted PNP-hydridodichloridoiridium complex exhibited the highest TON of 160,000 for the reduction of NaHCO 3 by H 2 with an organic base in an aqueous solution. 112 An earth-abundant metal such as an iron complex instead of precious metal complexes (see above) was also used for H 2 evolution from HCOOH by applying 0.005 mol % of Fe(BF 4 ) 2 $6H 2 O and tris[(2-diphenylphosphino)ethyl]phosphine, P(CH 2 CH 2 PPh 2 ) 3 , to a solution of formic acid in propylene carbonate, with no further additives or base to attain a TOF up to 9,425 hr À1 and TON of 92,000 at 80 C. 113 A pincer-supported iron catalyst, ( i PrPNHP)Fe(H)CO(COOH), i PrPNHP = HN {CH 2 CH 2 (P i Pr 2 )} 2 , in the presence of a Lewis acid (LiBF 4 , 10 mol %) was also reported to afford a TON of 1,000,000 for hydrogen evolution from HCOOH in dioxane at 80 C, 114 which is the largest TON reported for the dehydrogenation of HCOOH using first-row transition metal catalysts. [115][116][117][118] The Lewis acid is proposed to facilitate decarboxylation by stabilizing the negative charge that develops on the formate ligand in the transition state.…”

Efficient CO2 Hydrogenation to Formate with Immobilized Ir‐Catalysts Based on Mesoporous Silica Beads

“…It is based on a Ir III ‐pincer trihydride complex, which can achieve a turnover number (TON) of 3.5×10 6 , and a turnover frequency (TOF) of 1.5×10 5 h −1 when operating in basic aqueous solution at 120 °C and 60 MPa (Figure a, left) . The corresponding 4‐methoxy‐substituted Ir III ‐pincer monohydride complex has been also developed and achieves a TON of 1.6×10 5 when using an organic base, triethanolamine, in aqueous media …”

Metal‐ligand cooperative activation of HX (X=H, Br, OR) bond on Mn based pincer complexes

“…The well‐defined geometry and tridentate coordination mode of this class of complexes offers a stable catalytic structure. Most highly active pincers such as Ru‐MACHO complex [5,6] and Nozaki's Ir‐PNP complex [7] are based on expensive Ru and Ir metals. Catalytic systems based on such metals are not desirable for large scale ubiquitous applications due to high cost and limited availability.…”