Hydrophilic Quaternary Ammonium‐Group‐Containing [FeFe]‐Hydrogenase Models: Synthesis, Structures, and Electrocatalytic Hydrogen Production

Abstract

The first quaternary ammonium-group-containing [FeFe]-hydrogenase models [(μ-PDT)Fe (CO) {κ -(Ph P) N(CH ) NMe BzBr}] (2; PDT=propanedithiolate) and [(μ-PDT)Fe (CO) {μ-(Ph P) N(CH ) NMe BzBr}] (4) have been prepared by the quaternization of their precursors [(μ-PDT)Fe (CO) {κ -(Ph P) N(CH ) NMe }] (1) and [(μ-PDT)Fe (CO) {μ-(Ph P) N(CH ) NMe }] (3) with benzyl bromide in high yields. Although new complexes 1-4 have been fully characterized by spectroscopic and X-ray crystallographic studies, the chelated complexes 1 and 2 converted into their bridged isomers 3 and 4 at higher temperatures, thus demonstrating that these bridged isomers are thermodynamically favorable. An electrochemical study on hydrophilic models 2 and 4 in MeCN and MeCN/H O as solvents indicates that the reduction potentials are shifted to less-negative potentials as the water content increases. This outcome implies that both 2 and 4 are more easily reduced in the mixed MeCN/H O solvent than in MeCN. In addition, hydrophilic models 2 and 4 act as electrocatalysts and achieve higher i /i values and turnover numbers (TONs) in MeCN/H O as a solvent than in MeCN for the production of hydrogen from the weak acid HOAc.

Synthesis, structures, and electrocatalytic properties of phosphine‐monodentate, −chelate, and ‐bridge diiron 2,2‐dimethylpropanedithiolate complexes related to [FeFe]‐hydrogenases

“…For the diphosphine‐bridge complexes 4a and 4b , their IR spectra display the similar carbonyl absorption patterns in the region of 1997–1904 cm −1 while the 31 P{ 1 H} NMR spectra give only a sharp singlet at δ P 124.5 or 54.5 ppm (Table ), being in well agreement with the observation in the reported diiron bridge complexes Fe 2 ( μ ‐xdt)(CO) 4 { μ ‐(Ph 2 P) 2 X} (X = CH 2 , NR) with pdt, edt, odt, adt NR bridges . It is also seen from Table that the average ν C ≡ O value of the PNP‐bridge complex 4a is bigger than those of the PCP‐bridge analogue 4b , possibly owing to the fact that the weaker electron‐donating abilities of the PNP ligands such as (Ph 2 P) 2 NPh relative to the PCP ligand like dppm lead to the lower back‐donating π‐bonding electron transfer from Fe to CO.…”

“…The average ν C ≡ O value (1955 cm −1 ) of 3a is slightly bigger than that (1952 cm −1 ) of 3b (Table ), clearly resulting from the electron‐donating effect of the N‐substituent (NR) with the following order of NPh ( 3a ) ˂ NBu n ( 3b ) in their (Ph 2 P) 2 NR ligands. At the same time, the 31 P{ 1 H} NMR spectra of 3a and 3b both display a major singlet at δ P 116.5 or 110.6 ppm and a minor singlet at δ P 98.2 or 98.7 ppm (Table ) ascribed to the apical‐basal and basal‐basal isomers with the isomeric ratio from 1.6:1 in 3a to 3.5:1 in 3b , which matches well with the 31 P NMR signal patterns observed in the known PNP‐chelate diiron complexes Fe 2 ( μ ‐xdt)(CO) 4 { k 2 ‐(Ph 2 P) 2 NR} with pdt, edt, odt or adt NR bridges . The outcome demonstrates that the PNP‐diphosphine ligands chelated to one iron core exit in the more favorable apical‐basal configuration.…”

“…For the diphosphine‐chelate complexes 3a and 3b , their IR spectra show four absorption bands in the region of 2016–1911 cm −1 for their iron carbonyls (Table ), wherein the first ν C ≡ O value of approximately 2015 cm −1 is well accordant with those of the previously‐reported diiron chelate complexes Fe 2 ( μ ‐xdt)(CO) 4 ( k 2 ‐diphosphine) (xdt = pdt, edt, odt, adt NR ) . The average ν C ≡ O value (1955 cm −1 ) of 3a is slightly bigger than that (1952 cm −1 ) of 3b (Table ), clearly resulting from the electron‐donating effect of the N‐substituent (NR) with the following order of NPh ( 3a ) ˂ NBu n ( 3b ) in their (Ph 2 P) 2 NR ligands.…”

“…Inspired by these findings mentioned above and in comparison with the previous studies on the substitutions of diiron hexacarbonyl complexes bearing some dithiolate bridges such as pdt, edt and adt NR , little attention has been however paid to the reactions of all‐carbonyl diiron complexes with bulky dithiolate bridges such as Me 2 pdt [(SCH 2 ) 2 CMe 2 ], Et 2 pdt [(SCH 2 ) 2 CEt 2 ], etc . In this work, we carried out the treatments of parent complex Fe 2 ( μ ‐Me 2 pdt)(CO) 6 ( A ) with common monophosphines P(C 6 H 4 R‐ p ) 3 (R = Me, Cl) or small bite‐angle diphosphines (Ph 2 P) 2 X (X = CH 2 (dppm), NPh, NBu n ) in different conditions, mainly aiming to i) observe the potential influence of the diverse coordination modes (monodentate, chelate, bridge) of phosphine ligands in diiron complexes with bulky Me 2 pdt bridge on their formation, structures, and electrocatalytic behaviors, and to ii) further develop the biomimetic chemistry of [FeFe]‐hydrogenases for proton reduction.…”

“…In this context, in order to explore a class of cheap and efficient artificial catalysts for electrocatalytic proton reduction to H 2 , a great number of diiron dithiolate complexes that mimic the diiron subsite of [FeFe]‐hydrogenases were obtained over the decades . Among the preparation of diiron subsite models, the interesting substitutions of all‐carbonyl diiron complexes Fe 2 ( μ ‐xdt)(CO) 6 (xdt = pdt, (SCH 2 ) 2 CH 2 ; edt, (SCH 2 ) 2 ; adt NR , (SCH 2 ) 2 NR) by small bite‐angle diphosphines (Ph 2 P) 2 X (X = CR 2 named PCP and X = NR' labeled PNP in this study) have attracted much attentions since diverse diiron phosphine complexes can be afforded and have also shown different electrocatalytic properties of proton reduction to H 2 …”