Reversible Hydride Transfer to N,N′-Diarylimidazolinium Cations from Hydrogen Catalyzed by Transition Metal Complexes Mimicking the Reaction of [Fe]-Hydrogenase

Abstract

[Fe]-hydrogenase is a key enzyme involved in methanogenesis and facilitates reversible hydride transfer from H to N,N-methenyltetrahydromethanopterin (CH-HMPT). In this study, a reaction system was developed to model the enzymatic function of [Fe]-hydrogenase by using N,N'-diphenylimidazolinium cation (1) as a structurally related alternative to CH-HMPT. In connection with the enzymatic mechanism via heterolytic cleavage of H at the single metal active site, several transition metal complex catalysts capable of such activation were utilized in the model system. Reduction of 1[BF] to N,N'-diphenylimidazolidine (2) was achieved under 1 atm H at ambient temperature in the presence of an equimolar amount of NEt as a proton acceptor. The proposed catalytic pathways involved the generation of active hydride complexes and subsequent intermolecular hydride transfer to 1. The reverse reaction was accomplished by treatment of 2 with HNMePh as the proton source, where [(η-CMe)Ir{(p-MeCHSO)NCHPhCHPhNH}] was found to catalyze the formation of 1 and H with high efficiency. These results are consistent with the fact that use of 2,6-lutidine in the forward reaction or 2,6-lutidinium in the reverse reaction resulted in incomplete conversion. By combining these reactions using the above Ir amido catalyst, the reversible hydride transfer interconverting 1/H and 2/H was performed successfully. This system demonstrated the hydride-accepting and hydride-donating modes of biologically relevant N-heterocycles coupled with proton concentration. The influence of substituents on the forward and reverse reactivities was examined for the derivatives of 1 and 2 bearing one para-substituted N-phenyl group.

Biomimetic Hydrogenation Catalyzed by a Manganese Model of [Fe]‐Hydrogenase

“…The 2,6‐difluoro substituents in 9 a was not essential for a substrate to be hydrogenated. Other imidazolinium salts bearing similar N ‐aryl groups, such as 9 c – e , were also good substrates of hydrogenation, with yields of over 90 %. Both electron‐withdrawing Cl and electron‐donating OMe groups were tolerated.…”

Biomimetics of [NiFe]-Hydrogenase: Nickel- or Iron-Centered Proton Reduction Catalysis?

“…Hydrogenases (H2ase) have attracted increasing attention because of their efficient production and oxidation of molecular hydrogen, a carbon-neutral energy carrier. In contrast to the more common biomimetics for [Fe]-H2ases , and [FeFe]-H2ases, , biomimetics for [NiFe]-H2ases have proven challenging. , The active site of the [NiFe]-H2ases features a nickel tetrathiolate (four cysteines) with two S bridges to an Fe­(CN) 2 (CO) center . Four key states in the catalytic cycle are Ni–SI a (Ni II Fe II ), Ni–L (Ni I Fe II ), Ni–C (Ni III μ­(H)­Fe II ), and Ni–R (Ni II μ­(H)­Fe II ). − Despite intensive design efforts experimentally and theoretically, the metal-hydrides in nearly all of the synthetic NiFe models feature a short Fe–H bond and a long Ni–H bond, including the only structurally characterized ones by the Ogo and Rauchfuss ,,, groups.…”

Structural and functional biomimetics of [Fe]-hydrogenase featuring a mono-, di- or tetrasubstituted pyridine ligand with a fac-C, N, and S ligation

“…Trifluoroacetic acid, potassium tert -butoxide, sodium hydride (60% dispersion in paraffin liquid), p -MeC 6 H 4 SH, iodine, thionyl chloride, p -MeC 6 H 4 SO 2 Cl, 2-ClCH 2 C 5 H 4 N·HCl, AgBF 4 , H 2 and other reagents were available commercially and used as received. Disodium thiosalicylate, 40 1,3-diphenylimidazolium tetrafluoroborate ImBF 4 , 41 Na 2 Fe(CO) 4 ·(1,4-dioxane) 1.5 , 42 2-HOCH 2 -6-TsOCH 2 C 5 H 3 N 43 and 2-TsOCH 2 -3,5-Me 2 -4-MeOC 5 HN 44 were prepared according to the published methods. IR spectra were recorded on a Bruker Vector 22 or Bruker Tensor 27 FT-IR infrared spectrophotometer, while 1 H and 13 C{ 1 H} NMR spectra were obtained on a Bruker Avance 400 NMR spectrometer.…”