Influence of Dithiolate Bridges on the Structures and Electrocatalytic Performance of Small Bite-Angle PNP-Chelated Diiron Complexes Fe₂(μ-xdt)(CO)₄{κ²-(Ph₂P)₂NR} Related to [FeFe]-Hydrogenases

Abstract

As a further exploration of the asymmetrically substituted diiron models for the active site of [FeFe]hydrogenases, two new types of small bite-angle aminodiphosphine [(Ph 2 P) 2 NR; denoted as PNP in this study]chelated diiron N-phenyl-aza-and ethanedithioate complexes Fe 2 (μ-xdt)(CO) 4 {κ 2 -(Ph 2 P) 2 NR} (1a−1e) and (2a−2e), respectively, were successfully synthesized by the carbonyl substitution reactions of all-carbonyl diiron complexes Fe 2 (μxdt)(CO) 6 (xdt = SCH 2 N(Ph)CH 2 S (adt NPh ) and SCH 2 CH 2 S (edt)) with PNP (PNP = (Ph 2 P) 2 NR, R = CMe 3 , CH 2 CHMe 2 , (CH 2 ) 3 Me, (CH 2 ) 3 Si(OEt) 3 , and (CH 2 ) 3 NMe 2 ) in the presence of Me 3 NO•2H 2 O or UV irradiation.All the new complexes obtained above have been well characterized by elemental analysis, FT-IR, NMR spectroscopy, and particularly for 1a, 1b, 2a, and 2d by single-crystal X-ray diffraction analysis. By comparison, 31 P{ 1 H} NMR and X-ray crystallographic studies have clearly revealed that the change of the dithiolate bridge from adt NPh to edt has a significant influence on the coordination geometry of the chelating PNP ligands in Fe 2 S 2 complexes, in which the basal−basal configuration in the adt NPh complexes 1a−1e is favorable whereas the apical−basal conformation in the edt complexes 2a−2e is main. In addition, the electrochemical properties of complexes 1b and 2b as a pair of representative counterparts are evaluated and compared by cyclic voltammetry in the absence and presence of HOAc as a proton source, indicating that they are found to be electrocatalytically active.

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

“…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.…”

“…The (Ph 2 P) 2 NBu n ligand of 3b situates in a apical‐basal conformation and is coordinated asymmetrically to one of two Fe centers in the solid state, thus resulting in the formation of the two chemically unequivalent P atoms that correspond to a major phosphorus signal at δ P 110.6 ppm for its apical‐basal isomer in the aforementioned solution 31 P{ 1 H} NMR spectrum of 3b (Table ). Moreover, the apical‐basal coordination mode of the PNP ligand in 3b is comparable to those of the reported diiron chelate complexes with edt or adt NR bridges, but is distinct from the basal‐basal configuration of the related analogues with pdt or odt bridges . This result implies that the coordination geometry (apical‐basal or basal‐basal) of the chelating diphosphines probably depends on the steric hindrance interaction between the diphosphine ligands and the dithiolate bridges.…”

“…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 …”