Synthesis, Molecular Structures and Electrochemical Investigations of [FeFe]‐Hydrogenase Biomimics [Fe₂(CO)_6‐n(EPh₃)_n(µ‐edt)] (E = P, As, Sb; n = 1, 2)

Abstract

A series of ethane‐dithiolate (edt = S(CH2)2S) complexes [Fe2(CO)5(EPh3)(µ‐edt)] and [Fe2(CO)4(EPh3)2(µ‐edt)] (E = P, As, Sb), biomimics of the core of [FeFe]‐hydrogenases, have been prepared and structurally characterised. The introduced ligand(s) occupies apical sites lying trans to the iron‐iron bond. NMR studies reveal that while in the mono‐substituted complexes the Fe(CO)3 moiety undergoes facile trigonal rotation, the Fe(CO)2(PPh3) centres do not rotate on the NMR timescale. The reductive chemistry has been examined by cyclic voltammetry both in the presence and absence of CO and the observed behavior is found to be dependent upon the nature of the substituents. With L = CO or SbPh3 potential inversion is seen leading to a two‐electron reduction, while for others (L = PPh3, AsPh3) a quasi‐reversible one‐electron reduction is observed. Protonation studies reveal that [Fe2(CO)5(PPh3)(µ‐edt)] is only partially protonated by excess HBF4·Et2O, thus ruling complexes [Fe2(CO)5(EPh3)(µ‐edt)(µ‐H)]+ out as a catalytic intermediates, but [Fe2(CO)4(PPh3)2(µ‐edt)] reacts readily with HBF4·Et2O to produce [Fe2(CO)4(PPh3)2(µ‐edt)(µ‐H)]+. While all new complexes are catalysts for the reduction of protons in MeCN, their poor stability and relatively high reduction potentials does not make them attractive in this respect.

Amine‐containing tertiary phosphine‐substituted diiron ethanedithioate (edt) complexes Fe2(μ‐edt)(CO)6‐nLn (n= 1, 2): Synthesis, protonation, and electrochemical properties

“…‐2.2 V (Table 2), which are respectively ascribed to the Fe I Fe I to Fe I Fe 0 and Fe I Fe 0 to Fe 0 Fe 0 reduction processes. [ 27,32,45,47 ] In contrast, the disubstituted complexes 5 and 6 show only an irreversible initial reduction peak at −2.18 V and −2.19 V (Table 2) assigned to the Fe I Fe I to Fe I Fe 0 reduction process. [ 27,45 ] Of importance to note is that the first reductive peaks of 1 – 4 show a more positive reduction potential ( E pc1 ) in comparison to those of 5 and 6 , indicating that the former (monosubstituted) are more easily reduced in contrast to the latter (disubstituted).…”

“…[ 27,32,45,47 ] In contrast, the disubstituted complexes 5 and 6 show only an irreversible initial reduction peak at −2.18 V and −2.19 V (Table 2) assigned to the Fe I Fe I to Fe I Fe 0 reduction process. [ 27,45 ] Of importance to note is that the first reductive peaks of 1 – 4 show a more positive reduction potential ( E pc1 ) in comparison to those of 5 and 6 , indicating that the former (monosubstituted) are more easily reduced in contrast to the latter (disubstituted). Additionally, the electrochemical behaviors of the amine‐containing homologues 1 – 3 and their N‐protonated products [1H N ] + , [2H N ] + , [3H N ] + isolated above are studied and compared in the absence of acid through CV (Figures S4(a)‐(c)).…”

“…The IR spectra of the monosubstituted complexes 1–4 show three to five strong absorption bands in the range of 2044–1925 cm −1 whereas those of the disubstituted complexes 5 and 6 display three strong absorption bands in the region of 1997–1924 cm −1 , which are all assigned to their terminal carbonyls. [ 38–45 ]…”

Substituent effects of tertiary phosphines on the structures and electrochemical performances of azadithiolato‐bridged diiron model complexes of [FeFe]‐hydrogenases

“…[21] Meanwhile, the P atom of tertiary phosphines in 1 and 3 lies in an apical position of the square-pyramidal coordination sphere around the Fe2 atom, as observed in most of known diiron model complexes of the type [Fe 2 (μ-dithiolate) (CO) 5 (ĸ 1 -monophosphine)]. [30][31][32][33] Further, the Fe1-Fe2 distance of 2.5053(5) Å in 1 is slightly shorter than that of 2.5287(8) Å in 3 (Table 3), which are much shorter than those (2.55-2.62 Å) found in native [FeFe]-hydrogenases. [4,5] Interestingly, the Fe2-P1 distance of 2.2350 (7) Å is a bit shorter but the Fe1-C1 distance of 1.812(3) Å is a little longer in complex 1 as compared with those of 2.2477 (11) and 1.791(5) Å in its analog 3 (Table 3).…”

“…As shown in Figure 3a and Table 4, complexes 1-3 all show an irreversible reduction peak at E pc = À1.71, À1.80, and À1.81 V, respectively, attributed to their one-electron reduction processes of Fe I Fe I to Fe I Fe 0. [32][33][34] Such an assignment is further evidenced by the comparison of the initial reduction peak currents of 1-3 with that of a reference diiron complex Fe 2 (μ-bdt)(CO) 6 (bdt = benzene-1,2-dithiolate) that is known to undergo a reversible two-electron reduction, [35,36] wherein the peak current height of the former is almost half of that of the latter under the same CV condition (see Figures S13-S15). Meanwhile, the CV curve of 1 shows that a more negative reduction peak at À2.10 V might resulted from the one-electron reduction process of its CH 3 CN-substituted species.…”