Phosphine-substituted diiron complexes Fe2(μ-Rodt)(CO)6?n(PPh3)n(R = Ph, Me, H andn= 1, 2) featuring desymmetrized oxadithiolate bridges: structures, protonation, and electrocatalysis

Article abstract of DOI:10.1039/d1nj03398k

Two new series of phosphine-substituted diiron complexes Fe₂(μ-Rodt)(CO)_6?n(PPh₃)_n(n= 1 for4-6andn= 2 for7-9) bearing desymmetrized oxadithiolate bridges (i.e., Rodt bridges), which can be considered as diiron subsite models of [FeFe]-hydrogenases, were prepared through the Me₃NO-induced replacements of all-CO diiron precursors Fe₂(μ-Rodt)(CO)₆(Rodt = SCH(R)OCH₂S; R = Ph (1), Me (2), and H (3)) with one equivalent or excess PPh₃. All the as-obtained complexes have been fully characterized by elemental analysis, various spectroscopy methods, and especially for4,7-9by X-ray crystallography. Further protonation and electrochemistry of4-6and7-9with Rodt bridges are studied and compared without and with CF₃CO₂H (TFA) and CH₃CO₂H (HOAc) as strong and weak acidic proton sources byin situIR plus NMR spectroscopies and cyclic voltammetry. On the one hand, under excess TFA, the monosubstituted complexes4-6are partially protonated to produce a mixture of main precursors4-6and minor Fe-protonated species[4(μH)]⁺,[5(μH)]⁺, and[6(μH)]⁺, whereas the disubstituted counterparts7-9are completely protonated to form an isomeric mixture of their hydride species[7(μH)]⁺,[8(μH)]⁺, and[9(μH)]⁺intransoid-dibasal and apical-basal fashions. On the other hand, complexes7-9show better electrocatalytic proton reduction activities (i.e., higher turnover numbers/TONs) under TFA or HOAc relative to counterparts4-6, in which the TONs of4,5and7,8with Phodt or Meodt bridges are a little higher than those of6and9with odt bridges, respectively. These findings reveal that the redox and electrocatalytic behaviors of4-6and7-9with Rodt bridges are affected not only by phosphine coordination modes (PPh₃, mono-vs.di-substitution) but also by desymmetrized dithiolate bridges (Rodt, R = Ph, Mevs.H).

Full text of DOI:10.1039/d1nj03398k

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Phosphine-substituted diiron complexes Fe2(μ-Rodt)(CO)6-n(PPh3)n
(R = Ph, Me, H and n = 1, 2) featuring desymmetrized oxadithiolate
bridges: Structures, protonation, and electrocatalysis
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Xiao-Li Gu,# Jian-Rong Li,# Bo Jin, Yang Guo, Xing-Bin Jing and Pei-Hua Zhao*
School of Materials Science and Engineering, North University of China, Taiyuan
030051, P. R. China
*Corresponding author.
Email: zph2004@nuc.edu.cn (P.-H. Zhao).
#The two authors equally contributed to this work.
1

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Abstract
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Two new series of phosphine-substituted diiron complexes Fe2(μ-Rodt)(CO)6-n(PPh3)n
(n = 1 for 4-6 and n = 2 for 7-9) bearing desymmetrized oxadithiolate bridges (i.e.,
Rodt bridges), which can be considered as diiron subsite models of
[FeFe]-hydrogenases, were prepared through the Me3NO-induced replacements of
all-CO diiron precursors Fe2(μ-Rodt)(CO)6 (Rodt = SCH(R)OCH2S; R = Ph (1), Me
(2), and H (3)) with one equivalent or excess PPh3. All the as-obtained complexes
have been fully characterized by elemental analysis, various spectroscopies, and
especially for 4, 7-9 by X-ray crystallography. Further protonation and
electrochemistry of 4-6 and 7-9 with Rodt bridges are studied and compared without
and with CF3CO2H (TFA) and CH3CO2H (HOAc) as strong and weak acidic proton
sources by in situ IR plus NMR spectroscopies and cyclic voltammetry. On the one
hand, under excess TFA, the monosubstituted complexes 4-6 are partially protonated
to produce the mixture of main precursors 4-6 and minor Fe-protonated species
[4(μH)]+, [5(μH)]+, [6(μH)]+, whereas the disubstituted counterparts 7-9 are
completely protonated to form the isomeric mixture of their hydride species [7(μH)]+,
[8(μH)]+, [9(μH)]+ in transoid-dibasal and apical-basal fashions. On the other hand,
complexes 7-9 show better electrocatalytic proton reduction activities (i.e., higher
turnover numbers/TONs) under TFA or HOAc relative to counterparts 4-6, in which
the TONs of 4, 5 and 7, 8 with Phodt or Meodt bridges are a little larger than those of
6 and 9 with odt bridge, respectively. These findings reveal the redox and
electrocatalytic behaviors of 4-6 and 7-9 with Rodt bridges are affected by not only
phosphine coordination modes (PPh3, mono- vs. di-substitution) but also
desymmetrized dithiolate bridges (Rodt, R = Ph, Me vs. H).
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Keywords: [FeFe]-hydrogenases, diiron model complexes, desymmetrized
oxadithiolate bridge, protonation, electrochemistry.
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1. Introduction
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At present, molecular hydrogen (H2) has been considered as a promising alternative
energy carrier due to its high combustion heat and eco-friendliness.1 Many efforts are
made to develop an inexpensive and durable proton-reduction catalyst as a substitute
for noble-metal platinum that has efficient eletrocatalytic ability of water splitting into
H2.2,3 A family of metalloenzyme named as [FeFe]-hydrogenases can reversibly
transfer protons into H2 with remarkable catalytic efficiency in nature.4,5 The
crystallographic, spectroscopic, and theoretical studies have shown that the active site
of [FeFe]-hydrogenases (so-called H-cluster) as shown in Fig. 1, contains a butterfly
[Fe2S2] subcluster (denoted as diiron subsite) and a cubic [Fe4S4] subcluster which are
connected by cysteine S atom.6,7 In the H-cluster, two Fe atoms of diiron subsite are
coordinated by several diatomic CO/CN- ligands and a dithiolate cofactor that is
definitively determined to be an azadithiolate bridge (adtNH = SCH2N(H)CH2S).8-12
Further, this native adtNH bridge has been extensively replaced with the
chemically-synthesized dithiolate bridges such as pdt (SCH2CH2CH2S), adtNR
(SCH2N(R)CH2S), odt (SCH2OCH2S), and sdt (SCH2SCH2S), etc.13-16 Meanwhile, it
is proposed that the highly-efficient catalysis of proton reduction to H2 by
[FeFe]-hydrogenases is resulted from the presence of the “rotated” geometry at one Fe
core of diiron subsite bearing a vacant coordination site and a bridging carbonyl
ligand (Fig. 1).17,18
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NH
vacant
Cys
site
S
S
[S4Fe4]
S
Fe
NC
Fe
CO
OC
CN
C
O
diiron subsite
Fig. 1 Schematic structure of the active site of [FeFe]-hydrogenases
In this context, a large number of diiron dithiolate complexes, which are
regarded as diiron subsite models of [FeFe]-hydrogenases, have been obtained in
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order to better mimic the above structural features of diiron subsite and to deeply
explore the effective catalytic factors of proton reduction into H2 in enzyme.19,20
Specifically, density functional theory (DFT) calculation has predicted that the
asymmetry of diiron cores in diiron dithiolate complexes helps to form the desired
“rotated” geometry in diiron subsite of [FeFe]-hydrogenases for boosted H2
evolution.21 To this end, many examples have been focused on the design and
synthesis of diphosphine-chelated diiron ditholate complexes through the
Me3NO-induced or UV-irradiated substitutions of diiron hexacarbonyl complexes by
some special diphosphines with rigid backbone or small bite-angle to favorably
produce the asymmetry of diiron cores.22-29 However, no attention is so far paid to the
installing of desymmetrized dithiolate bridge into phosphine-substituted diiron model
complexes as a straightforward strategy to construct the asymmetry of diiron cores,
which maybe result in the highly-efficient catalytic proton reduction to H2.
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On the basis of the aforementioned considerations and as a continuation of our
concern on the biomimetic chemistry of [FeFe]-hydrogenases,27-32 we recently carried
out the Me3NO-induced decarbonylating of two new all-CO diiron asymmetric
oxadithiolate complexes Fe2(μ-Rodt)(CO)6 (Rodt = SCH(R)OCH2S; R = Ph (1) and
Me (2)) and a known reference analogue 3 (R = H) with one or two equivalents of
phosphine (PPh3) to afford two new series of PPh3-substituted diiron model
complexes Fe2(μ-Rodt)(CO)6-n(PPh3)n (n = 1 for 4-6 and n = 2 for 7-9) featuring
desymmetrized oxadithiolate (i.e., Rodt) bridges. Herein, we report the synthesis,
spectroscopic characterization, and X-ray crystallography of three all-CO diiron
precursors 1-3 and their PPh3-substituted complexes 4-9 with Rodt bridges. Further
protonation and electrochemistry of the mono- and disubstituted complexes 4-6 and
7-9 with Rodt bridges are investigated and compared in the absence and presence of
strong acid CF3CO2H (TFA) and weak acid CH3CO2H (HOAc) as different proton
sources by using in situ spectroscopic technique (IR, NMR) and cyclic voltammetry
(CV), respectively.
2. Results and discussion
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2.1 Preparation and spectroscopic characterization of all-CO diiron precursors
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1-3 and their PPh3-substituted diiron complexes 4-9 with Rodt bridges
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As displayed in Scheme 1, two new all-CO diiron asymmetric oxadithiolate
precursors Fe2(μ-Rodt)(CO)6 [Rodt = SCH(R)OCH2S; R = Ph (1) and Me (2)] were
firstly prepared in low yields by using the similar synthetic method to the previously
reported complex 3 (R = H),33 i.e., the condensations of the in situ formed
intermediate A34 with different aldehydes RCHO (R = Ph and Me) followed by the
dehydration with concentrated H2SO4 in dry CH2Cl2. Further, Me3NO-induced
substitution reactions of the above-obtained precursors 1-3 with one or two
equivalents of ligand PPh3 in dry MeCN at room temperature resulted in the
formations of the mono- and disubstituted complexes Fe2(μ-Rodt)(CO)6-n(PPh3)n with
the yields of 66-79% for 4-6 (n =1) and 28-79% for 7-9 (n = 2), respectively.
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O
R
OC
1.2 equiv. PPh3
Me3NO, MeCN
S
S
CO
Fe
Fe
O
OC
Ph3P
CO
R
CO
H
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H
S
4-6
S
S
i) RCHO, THF, r.t.
4
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[R = Ph ( ), Me ( ), H ( )]
OC
CO
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CO
ii) con. H2SO4, DCM
O
Fe
Fe
Fe
Fe
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OC
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OC
OC
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CO
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S
2.2 equiv. PPh3
Me3NO, MeCN
A
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CO
Ph3P
OC
1
2
3
[R = Ph ( ), Me ( ), H ( )]
Fe
Fe
CO
OC
7-9
7
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9
[R = Ph ( ), Me ( ), H ( )]
Scheme 1 Synthesis of complexes 1-3, 4-6, and 7-9 with Rodt bridges, respectively
All the new complexes 1, 2, 4, 5, and 7-9 together with two known complexes 3, 6
are stable in air and have been fully characterized by elemental analysis, FT-IR as
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well as NMR (1H, P) spectroscopies. The FT-IR spectra of all-CO precursors 1-3
show four absorption bands in the range of 2078‒1981 cm-1 assigned to terminal
carbonyls.15,33 The PPh3-monosubstituted complexes 4-6 display four IR carbonyl
absorption bands in the range of 2046‒1933 cm-1 whereas the PPh3-disubstituted
complexes 7-9 exhibit four IR carbonyl absorption bands in the region of 2000‒1907
cm-1. It is found that relative to all-CO precursors 1-3 (ca. 2080 cm-1), the first
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(CO) bands of the PPh3-substituted complexes 4-6 (ca. 2045 cm ) and 7-9 (ca.
2000 cm-1) are respectively moved by approximately 35 and 80 cm-1 towards lower
energy due to the stronger electron-donating ability of PPh3 in contrast to CO.
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The H NMR spectra of homologues 1, 4, and 7 with Phodt bridges respectively
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show a set of proton signals at δH 4.92, 4.93, 4.03 ppm assigned to their methines and
another two sets of proton signals at δH 4.82/4.15, 4,14/3.95, 4.03/2.90 ppm ascribed
to their methenes. In contrast, those of homologues 2, 5, and 8 with Meodt bridges
display two groups of the higher-field NMR proton signals for their methines
observed at δH 4.69, 3.80, 2.91 ppm and for their methenes observed at δH 3.91,
3.80/3.68, 2.65/2.39 ppm, respectively. This is consistent with the following NMR
observation that the electron-withdrawing phenyl proton signals of the Phodt bridges
in 1, 4, and 7 are found in the region of δH 7.56-6.87 ppm whereas the
electron-donating methyl proton signals of the Meodt bridges in 2, 5, and 8 appear at
δH 1.45, 1.16, and 1.34 ppm. Furthermore, the 31P{1H} NMR spectra of the
PPh3-monosubstituted complexes 4-6 show only a sharp singlet at δp ca. 64 ppm for
their P atoms linked to one Fe core, whereas those of the PPh3-disubstituted
counterparts 7-9 exhibit one or two singlet(s) at δp ca. 61 ppm for their P atoms
attached to two Fe cores, as observed by the following X-ray crystallography.
2.2 Crystal structures of diiron model complexes 4 and 7-9
The molecular structures of complexes 4 and 7-9 are unambiguously confirmed by
single-crystal X-ray diffraction analysis as shown in Fig. 2. The selected bond lengths
and angles for 4 and 7-9 are listed in Table 1 for comparison.
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Fig. 2 Molecular structures of PPh3-substituted diiron complexes 4 (a), 7 (b), 8 (c), and 9 (d)
drawn at 35% probability level ellipsoids. All hydrogen atoms and solvents are omitted for clarity.
Table 1 Selected bond distances (Å) and angles (°) for complexes 4 and 7-9.
Complex
4
7
8
9
Fe(1)-Fe(2)
Fe(1)-P(1)a
Fe(2)-P(2)a
P(1)-Fe(1)-Fe(2)a
P(2)-Fe(2)-Fe(1)
C(1)-Fe(1)-Fe(2)
2.5214(8)
2.2366(11)
2.2428(11)
153.89(4)
154.81(4)
102.25(14)
2.5174(11)
2.2380(16)
2.2391(17)
155.87(6)
153.86(6)
100.01(19)
2.5176(10)
2.2332(14)
2.2417(15)
154.47(5)
2.5910(6)
-
2.2494(9)
108.90(3)
-
154.89(5)
101.7(2)
141.16(11)
aFe(1)-C(3), Fe(2)-P(1), and P(1)-Fe(2)-Fe(1) are labeled in complex 4 (Fig. 2a).
As illustrated in Fig. 2, complexes 4 and 7-9 are composed of a butterfly-shaped
[Fe2S2] cluster core that is bridged by a desymmetrized oxadithiolate moiety (i.e.,
Rodt = SCH(R)OCH2S) to form two fused six-membered FeS2C2O rings in a chair
and a boat conformations. Clearly, the C-substituents (R = Ph or Me) of the Rodt
bridges in 4, 7, and 8 occupy the sterically-favored equatorial bond in the
aforementioned two six-membered FeS2C2O ferracycle as shown in Figs. 2a-c.
Moreover, one PPh3 ligand in 4 resides in a basal position whereas two PPh3 ligands
in 7-9 are all located in an apical-apical manner of the square-pyramidal geometries of
one or two Fe core(s). Meanwhile, the Fe1-Fe2 distance of 4 (2.5910(6) Å) is longer
than those of 7-9 (ca. 2.52 Å) with an order of 7 (2.5176(10) Å) ≈ 8 (2.5174(10) Å) <
9 (2.5214(8) Å). These differences in 4 versus 7-9 are obviously caused by the steric
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effect of dithiolate bridges (Rodt) and the electronic effect of phosphine ligands
(PPh3). Noticeably, the distances of Fe2-P2 close to the bridgehead of the Rodt bridge
(i.e., Rodt bridgehead) are longer by 0.0085, 0.0062, and 0.0011 Å than those of
Fe1-P1 away from the Rodt bridgehead in 7-9 (Table 1), respectively.
Correspondingly, the P2-Fe2-Fe1 angles are larger by 0.42o and 0.92o for 7 (R = Ph)
and 8 (R = Me) but much smaller by 2.01o for 9 (R = H) in contrast to their
P1-Fe1-Fe2 angles (Table 1). These findings indicate that the desymmetrization of the
Rodt bridges could influence the geometric symmetries of two PPh3 ligands in
homologues 7-9.
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2.3 Protonation studies of diiron model complexes 4-6 and 7-9
Considering the interplay between proton source and diiron subsite in native
[FeFe]-hydrogenases is one of the key steps in the catalytic cycle of proton reduction,
the protonations of the mono- and disubstituted complexes 4-6 and 7-9 were treated
with CF3CO2H (TFA) and CH3CO2H (HOAc) as strong and weak acids at room
temperature, respectively. Their protonation results are monitored by using in situ
FT-IR (CO) and NMR (31P, 1H) techniques as follows.
Protonations of the monosubstituted complexes 4-6 with 0 and 10 equiv. strong
acid TFA are studied as depicted in the Fig. 3. The additions of excess TFA into the
respective dry CH2Cl2 solution of homologues 4-6 all give rise to a new but very
weak CO absorption band at about 2100 cm-1 in company with several original
strong CO absorption bands in the region of 2049-1942 cm-1 as shown Figs. 3a-c.
For the first CO bands of 4-6, there is a larger IR shift by about 50 cm-1 toward
higher energy upon addition of excess TFA (Figs. 3a-c), indicating that a small
amount of the Fe-protonated species [(μ-H)Fe2(μ-Rodt)(CO)5(PPh3)]+ (R = Ph,
[4(μH)]+; Me, [5(μH)]+; and H, [6(μH)]+) are produced.29,35-38 Further, the FT-IR
spectra of 4-6 in the range of 2400-2600 cm-1 show no specific differences in the
absence and presence of 10 equiv. TFA (Fig. S1), thus suggesting the no occurrence
of sulfur protonation.19c This is line with the corresponding 31P{1H} NMR
observations (Figs. 3d-f) that in addition to one major original phosphorus signal at δP
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ca. 64 ppm ascribed to neutral precursors 4-6, the other minor new phosphorus signals
at δP 24.0, 48.8, and 50.4 ppm assigned to their Fe-protonated products 4(μH)]+,
[5(μH)]+, [6(μH)]+ are respectively found when excess TFA were added into the
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respective CDCl3 solution of 4-6 (Figs. S2-S4). Indeed, in the high-field region H
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NMR spectra of 4-6 with excess TFA (Figs. S5 and S6), a hydride signal is noticed as
a well-resolved doublet at δH -15.01 ppm (2JPH = 15.6 Hz) attributed to
[5(μH)]+29,35,37-39 in the case of 5 although no hydride signals appear for 4 and 6. This
result is probably due to the following fact that the molar contents of the
Fe-protonated species formed via the in situ treatments of 4-6 with excess TFA show
an order of [5(μH)]+ (38%) > [4(μH)]+ (21%) ≈ [6(μH)]+ (25%) (Figs. 3d-f), which
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are measured by integration of phosphorus peaks in their P{1H} NMR spectra with
excess TFA (Figs. S2-S4). It is worthy to note that the higher content of species
[5(μH)]+ than those of species [4(μH)]+ and [6(μH)]+ is probably owing to the
stronger electron-donating ability of the Meodt bridge in the former than those of the
Phodt and odt bridges in the latter. Therefore, these results indicate that the
monosubstituted complexes 4-6 with excess TFA are partially protonated to form the
mixture of main neutral precursors 4-6 and minor Fe-protonated species [4(μH)]+,
[5(μH)]+, [6(μH)]+ (the middle red box of Fig. 3). In addition, several known diiron
monophosphine complexes Fe2{μ-(SCH2)2X}(CO)5(L) (X = null, L = PPh3, AsPh3,
SbPh3 and X = SnMe2, L = PPh3, P(OMe)3) required a certain amount of strong acid
HBF4·Et2O to partially protonate the Fe-Fe bond.19c,39 In contrast, the as-prepared
complexes 4-6 with Rodt bridges can be more readily protonated to form the partial
Fe(μ-H)Fe product in the presence of weaker acid TFA (pKaMeCN = 12.7)40 relative to
MeCN
HBF4·Et2O (pKa
= 0.2),40 further suggesting that the introduction of the R
substituent and the presence of the O atom in dithiolate bridges are more favorable to
protonate the Fe-Fe bond in diiron model complexes.
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Fig. 3 FT-IR spectra (a-c) and 31P{1H} NMR spectra (d-f) recorded for the in situ protonations of
the monosubstituted complexes 4-6 (0.015 mmol) with 0 (black line), 10 (red line) equivalents of
strong acid TFA at 298 K for 4 h, respectively. Middle: Protonation results of 4-6 with excess
TFA is shown in red box.
For comparison, protonations of the disubstituted complexes 7-9 with 0 and 10
equiv. TFA are investigated as displayed in the Fig. 4. The additions of excess TFA
into the respective dry CH2Cl2 solution of 7-9 all develop three or four new strong
CO absorption bands in the range of 2046-1930 cm-1 and lead to a large IR shift by
approximately 50 cm-1 for the first CO band toward higher energy than those
(1997-1911 cm-1) of neutral precursors 7-9 (Figs. 4a-c), suggesting the formations of
their Fe-protonated species [(μ-H)Fe2(μ-Rodt)(CO)4(PPh3)2]+ (R = Ph, [7(μH)]+; Me,
[8(μH)]+; and H, [9(μH)]+).29,35-39 Further analysis of the FT-IR spectral region from
2400-2600 cm-1 displays no absorption peaks for 7-9 without and with 10 equiv. TFA
(Fig. S7), indicating the sulfur protonation didn’t take place.19c Indeed, the high-field
1
region H NMR spectra of 7-9 with excess TFA (Fig. 4d) all exhibit two sets of the
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bridging hydride signals due to the isomeric presence of the Fe-protonated species
[7(μH)]+, [8(μH)]+, [9(μH)]+,29,35,37,38 which are observed as a triplet at δH ca. -14
ppm (2JPH = ca. 16 Hz) and a doublet at δH ca. -15 ppm (2JPH = ca. 19 Hz) assigned
respectively to their transoid-dibasal and apical-basal isomers with the corresponding
molar ratios of 1:1, 4:1, and 5:2 (Figs. S8-S10).38 It is worth noting that the different
ratios of the transoid-dibasal versus apical-basal isomers in species [7(μH)]+,
[8(μH)]+, [9(μH)]+ are obviously owing to the substituent (R) effect of the Rodt
bridges in them, in which species [8(μH)]+ show a higher isomeric ratio.
Correspondingly, the 31P{1H} NMR spectra show that the original phosphorus signals
of precursors 7-9 at δp ca. 61 ppm have been completely changed into two new main
phosphorus signals at δp 65.8/5.6, 67.1/5.5, and 53.6/5.5 ppm upon additions of excess
TFA into 7-9 (Figs. S11-S13). Thus, these findings suggest that the disubstituted
complexes 7-9 are completely protonated to produce the isomeric mixture of the
hydride species [7(μH)]+, [8(μH)]+, [9(μH)]+ with different molar ratios of the
transoid-dibasal versus apical-basal isomers (the bottom red box of Fig. 4).
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Fig. 4 FT-IR spectra with 0 (black line), 10 (red line) equivalents of strong acid TFA (a-c) and
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high-field region H NMR spectra with 10 equivalents of strong acid TFA (d) recorded for in situ
protonations of the disubstituted complexes 7-9 (0.015 mmol) at 298 K for 3 h, respectively.
Bottom: Protonation results of 7-9 with excess TFA is illustrated in red box.
9
In addition, the protonation reactions of both 4-6 and 7-9 with 0, 10 equiv. weak
acid HOAc are monitored by in situ FT-IR monitor. As shown in Fig. S14, the
additions of excess HOAc didn’t cause any shift of the CO absorption bands of 4-6
and 7-9, indicating that the basicity of diiron cores in these neutral precursors is not
enough to accept protons of HOAc and thus no protonation with HOAc occurs.
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2.4 Electrochemical and electrocatalytic properties of diiron model complexes
4-6 and 7-9
To explore the influences of phosphine coordination modes (PPh3) and
desymmetrized oxadithiolate bridges (Rodt) on the redox and electrocatalytic
properties of asymmetric diiron subsite models of [FeFe]-hydrogenases for proton
reduction to H2, the electrochemical behaviors of PPh3-substituted diiron model
complexes 4-6 and 7-9 with Rodt bridges are studied in 0.1 M n-Bu4NPF6/MeCN
solution at a scan rate of 0.05 V s-1 in the absence and presence of TFA and HOAc as
different proton sources using CV technique, as displayed in Figs. 5-7. The relevant
electrochemical data of 4-6 and 7-9 without and with the added acids (TFA or HOAc)
are listed in Table 2.
Table 2 Relevant electrochemical data of 4-6 and 7-9 without and with TFA or HOAc
Complex aEpa1 (V)
aEpc1 (V)
-1.76
aEpc2 (V)
-2.17
bEpc3 (V)
-1.65
cEpc4 (V)
4
5
6
7
8
9
+0.49
+0.43
+0.43
+0.14
+0.13
+0.13
-
-1.81
-1.65
-
-2.20
-1.76
-1.64
-
-2.18
-1.98
-1.55
-1.70
-1.82
-1.70
-2.19
-2.05
-1.65
-2.25
-2.02
-1.48
-2.21
a The original reduction and oxidative potentials (V) are observed at 0 mM TFA or HOAc.
b The new reduction potential (V) is observed at 2 mM TFA.
c The new reduction potential (V) is observed at 4 mM TFA.
The redox behaviors of 4-6 and 7-9 have been investigated by using CV without the
added acid as depicted in Fig. 5. The monosubstituted complexes 4-6 show two
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irreversible reductive peaks at ca. -1.80 V and ca. -2.18 V as well as one irreversible
oxidative peak at ca. +0.43 V (Fig. 5a). Similarly, the disubstituted complexes 7-9
also display two reductive and one oxidative peaks at ca. -2.00 V, -2.21 V, and ca.
+0.13 V (Fig. 5b). Their two reductive and one oxidative peaks are respectively
attributed to two one-electron reduction processes of FeIFeI to FeIFe0 and FeIFe0 to
Fe0Fe0 as well as one two-electron oxidative process of FeIFeI to FeIIFeII for 4-6 and
7-9.41-43 We noted that the first reductive peak potentials (Epc1) of complexes 4-6 are
positively shifted by about 220 mV whereas their initial oxidative peak potentials
(Epa1) are negatively shifted by approximately 300 mV in comparison to counterparts
7-9 (Table 2), reflecting that the former are more easily reduced but are more
difficultly oxidized in contrast to the latter owing to the electron-donating ability of
PPh3. Meanwhile, it is also noticeable that complexes 5 and 8 with Meodt bridges
exhibit two more negative reduction potentials (Epc1 and Epc2) relative to their
respective homologues 4, 6 and 7, 9 with Phodt or odt bridges (Table 2), resulting
from the stronger electron-donating property of the Me moiety with respect to the Ph
and H groups in the Rodt bridges.
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Fig. 5 CV curves of (a) 1.0 mM monosubstituted complexes 4 (black line), 5 (red line) and 6 (blue
line) and (b) 1.0 mM disubstituted complexes 7 (black line), 8 (red line) and 9 (blue line) in 0.1 M
n-Bu4NPF6/MeCN at a scan rate of 0.05 V s-1. All potentials are versus the ferrocene/ferrocenium
(Fc0/+) couple.
Further electrocatalytic responses of 4-6 and 7-9 to the additions of 0–10 mM
MeCN
strong acid TFA (pKa
= 12.7)40 are studied in 0.1 M n-Bu4NPF6/MeCN solution
by means of CV (Figs. 6 and 7). The CV curves of the monosubstituted complexes
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4-6 with 0-10 mM TFA exhibit very similar electrocatalytic behaviors for proton
reduction to H2 as displayed in Fig. 6. The initial reductive peak currents of 4-6 at Epc1
= ca. -1.80 V increase obviously with the consecutive additions of TFA (2-10 mM),
wherein the initial peak currents (icat, labeled I of Figs. 6a-c) display a linear
dependence on the added acid concentrations (black lines, inserts of Figs. 6a-c). This
indicates that complexes 4-6 are active for the electrocatalytic proton reduction to
H2.29,44-46 Meanwhile, a new reduction peak at more positive potential Epc3 = ca. -1.65
V appears upon the addition of 2 mM TFA and this peak current (icat, labeled II of
Figs. 6a-c) shows a linear increasement with the added concentrations of TFA (red
lines, inserts of Figs. 6a-c), demonstrating the formations of the protonated species of
4-6 (i.e., [4(μH)]+, [5(μH)]+, and [6(μH)]+ as proposed in the above Fig. 3) that are
electrocatally active for proton reduction to H2.29,44-46 These observations are probably
due to the fact that the incomplete protonations of 4-6 with excess TFA as revealed in
their aforementioned chemical protonations (Fig. 3) lead to two different
electrocatalytic processes (waves I and II of Figs. 6a-c) of proton reduction to H2 by
precursors 4-6 at Epc1 = ca. -1.80 V through a suggested ECCE (E = electrochemical
and C = chemical) mechanism29,41,43 and by protonated species [4(μH)]+, [5(μH)]+,
[6(μH)]+ at Epc3 = ca. -1.65 V through a proposed ECEC mechanism (Scheme
S1),29,35,47 respectively.
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Fig. 6 CV curves of 1.0 mM monosubstituted complexes 4 (a), 5 (b) and 6 (c) with strong acid
TFA (0–10 mM) in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 0.05 V s-1. Inserts in (a)-(c): Plots of
catalytic currents (icat, μA) of two catalytic waves I (black lines) and II (red lines) for 4-6 upon
increasing concentrations of acid ([TFA], mM). All potentials are versus the
ferrocene/ferrocenium (Fc0/+) couple.
In contrast to complexes 4-6, the CV curves of the disubstituted counterparts 7-9
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with 0-10 mM TFA display the distinct electrocatalytic proton reduction features as
shown in Fig. 7. The addition of 2 mM TFA into the MeCN solution of 7-9 each gives
rise to two new reduction peaks at two more positive potentials as Epc3 = -1.55, -1.65,
-1.48 V and Epc4 = -1.70, -1.82, -1.70, respectively. Further addition of 4 mM TFA
leads to the complete disappearance of two initial reduction peaks at Epc1 = ca. -2.00
V and Epc2 = ca. -2.21 V for 7-9. Interestingly, two new reduction peak currents of 7-9
(icat, labeled I and II of Figs. 7a-c) at Epc3 = ca. -1.50 V (-1.65 V for 8) and Epc4 = ca.
-1.70 V (-1.82 for 8) all show a concomitant and linear relationship with the
increasing concentrations of TFA (2-10 mM [TFA], black and red lines in inserts of
Figs. 7a-c), suggesting the formations of the electrocatally-active protonated species
of 7-9 (i.e., [7(μH)]+, [8(μH)]+, and [9(μH)]+ as proposed in the above Fig. 4) for
proton reduction to H2.29,44-46 These findings indicate that the complete protonation of
7-9 with excess TFA as observed in their above-mentioned chemical protonations
(Fig. 4) lead to two distinct electrocatalytic processes (waves I and II of Figs. 7a-c) of
proton reduction to H2 by protonated species [7(μH)]+, [8(μH)]+, and [9(μH)]+ at Epc3
= ca. -1.50 V via a proposed ECEC mechanism and at Epc4 = ca. -1.70 V via a
suggested CEEC mechanism (Scheme S2),29,35,47 which are very similar to those
reported previously for several diphosphine-chelate diiron dithiolate analogues
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Fe2(μ-odt)(CO)4(κ2-PNP),
Fe2(μ-edt)(CO)4(κ2-dppen)
and
[(μ-H)Fe2(μ-pdt)(CO)4(κ2-dppe)]+ with strong acids like HOTs, HBF4, TFA under CV
condition.29,35,47
Fig. 7 CV curves of 1.0 mM disubstituted complexes 7 (a), 8 (b) and 9 (b) with strong acid TFA
(0–10 mM) in 0.1 M n-Bu4NPF6/MeCN at a scan rate of 0.05 V s-1. Inserts in (a)-(c): Plots of
catalytic currents (icat, μA) of two catalytic waves I (black lines) and II (red lines) for 7-9 upon the
increasing concentrations of acid ([TFA], mM). All potentials are versus the
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ferrocene/ferrocenium (Fc0/+) couple.
5
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7
8
Meanwhile, the electrocatalytic responses of 4-6 and 7-9 to the additions of 0-10
mM weak acid HOAc (pKaMeCN = 22.3)48 are also studied through CV technique. As
illustrated in Fig. S15, the CV curves of 4-6 and 7-9 display very similar
electrocatalytic behaviors for proton reduction to H2 as follows. The initial reductive
peak currents at Epc1 = ca. -1.80 V for 4-6 and ca. -2.00 V for 7-9 hardly grow.
However, the second reductive peak currents at Epc2 = ca. -2.18 V for 4-6 and ca.
-2.21 V for 7-9 increase dramatically and linearly with the increasing concentrations
of HOAc (2-10 mM [HOAc], blue lines in inserts of Figs. S15a-f). These observations
suggest that a typical and similar electrocatalytic process of proton reduction to H2
can happen via a proposed EECC mechanism in the presence of 4-6 and 7-9 as active
electrocatalysts as well as HOAc as proton source (Scheme S3).29,41,43
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2.5 Electrocatalytic HER activity of diiron model complexes 4-6 and 7-9
Notably, the electrochemical stability and electrocatalytic activity of complexes 4-6
and 7-9 for hydrogen evolution reaction (HER) are investigated and compared
through the controlled-potential electrolysis (CPE) experiments in the presence of
TFA or HOAc as illustrated in Fig. 8, respectively.
Fig. 8 Controlled-potential electrolysis (CPE) experiments and the maximum TON values of 0.5
mM monosubstituted complexes 4 (black), 5 (red), 6 (blue) and 0.5 mM disubstituted complexes 7
(black), 8 (red), 9 (blue) in 5 mM TFA/MeCN (a-c) and HOAc/MeCN (d-f) solutions for 5 h at
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-2.30 V vs. Ag/AgNO3, respectively.
5
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7
8
First of all, the CPE experiments of complexes 4-6 (0.5 mM) and 7-9 (0.5 mM)
in 5 mM TFA/MeCN or HOAc/MeCN solutions at -2.30 V vs. Ag/AgNO3 all show a
steady and linear consumption of charge within 5 hours (Fig. 8), that is, a constant
catalytic current is shown during their electrolysis (Figs. S16 and S17). Thus, two
types of diiron model complexes 4-6 and 7-9 with Rodt bridges are all found to
possess a good stability under long-term electrochemical conditions with TFA or
HOAc as different proton sources.32,49,50
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Further, the turnover numbers (TONs) of 4-6 and 7-9 during electrolysis are
evaluated as a measure of their electrocatalytic HER activity.32,49,50 As displayed in
Fig. 8a, the 5 h electrolysis of the monosubstituted complexes 4-6 (0.5 mM) with
excess strong acid TFA (5 mM) passed the total charges of 3.70, 3.32, and 2.61 C,
respectively, leading to the H2 evolutions of 19.2, 17.2, and 13.5 μmol (Figs. S18a-c).
Thus, the maximum TONs of homologues 4-6 are separately estimated to be 3.84,
3.44, and 2.70 (Figs. 8c and S18a-c), suggesting the somewhat greater electrocatalytic
HER efficiency of 4 and 5 with Phodt or Meodt bridges relative to reference 6 with
odt bridge in the presence of TFA. Similarly, it is deduced from Fig. 8b that the total
charges consumed by the disubstituted complexes 7-9 (0.5 mM) are found to 5.31,
4.01, and 3.69 C in the presence of excess TFA (5 mM) during 5 hours, respectively,
giving rise to the H2 amounts of 27.5, 20.8, and 19.1 μmol (Figs. S18d-f). Hence, the
theoretical TONs of homologues 7-9 show the following order of 7 (5.50) > 8 (4.16) >
9 (3.82) (Figs. 8c and S18d-f), which are larger than those of their respective
counterparts 4-6 (Fig. 8a). This finding indicates the better electrocatalytic HER
activities of the disubstituted complexes 7-9 in contrast to their monosubstituted
counterparts 4-6 under TFA as a proton source.
Additionally, as depicted in Figs. 8d and 8e, the electrocatalysis of 4-6 and 7-9
(0.5 mM) with excess weak acid HOAc (5 mM) consumed the total charges of 4.53,
4.46, 4.20 C and 5.60, 4.88, 4.48 C within 5 hours, respectively, resulting in the H2
formations of 23.5, 23.1, 21.8 and 29.0, 25.3, 23.2 μmol (Figs. S19a-f).
Correspondingly, their theoretical TONs display an order of 4 (4.70) > 5 (4.62) > 6
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(4.36) and 7 (5.80) > 8 (5.06) > 9 (4.64) (Figs. 8f and S19a-f) in the presence of
HOAc respectively, which is very similar to the results found above for them in the
presence of TFA (Fig. 8c).
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3. Experimental section
3.1 Materials and methods
All reactions were performed using standard Schlenk and vacuum-line techniques
under N2 atmosphere. Solvents MeCN and CH2Cl2 were distilled from CaH2 whereas
THF was treated with sodium/benzophene ketyl under N2. Starting materials LiHBEt3
(1.0 M in THF), 37% aqueous HCHO, CH3CHO, PhCHO, CF3CO2H (TFA),
concentrated H2SO4, PPh3, and Me3NO·2H2O were obtained commercially and used
as received. Complex Fe2(µ-S)2(CO)6 was prepared according to literature
procedures.51 Preparative thin layer chromatography (TLC) was carried out on glass
plates (25 cm x 20 cm x 0.25 cm) coated with silica gel G (10-40 mm). Flash column
chromatography was performed on silica gel (45-75 µm). Elemental analyses were
performed on a PerkinElmer 240C analyzer. FT-IR spectra were recorded on a
Nicolet iS 10 FTIR spectrophotometer. NMR (1H and 31P) spectra were obtained on a
Bruker Avance spectrometer.
3.2 Preparation of all-CO diiron complexes Fe2(μ-Rodt)(CO)6 (Rodt =
SCH(R)OCH2S, 1-3)
A dry THF (40 mL) solution of Fe2(µ-S)2(CO)6 (2.0 mmol) was cooled to −78 °C and
was treated dropwise with LiHBEt3 (4.0 mmol) to get a green solution. After stirring
for 20 min, CF3CO2H (5.0 mmol) was added at −78 °C to cause an immediate color
change from green to red. The reaction mixture was stirred for 15 min at −78 °C and
then was treated with RCHO (R = Ph, 6.0 mmol; R = CH3, 10.0 mmol; R = H, 4.2
mmol 37% aqueous solution). The reaction mixture was warmed to room temperature
and stirred overnight at room temperature. After solvent THF and unreacted RCHO
were removed under vacuum, the resulting viscous liquid was dissolved in 40 mL dry
CH2Cl2, which was treated with concentrated H2SO4 (44.2 mmol). This reaction
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mixture was constituted to be stirred for 5-6 h at room temperature. After solvent
CH2Cl2 were removed under vacuum, the resulting residue was chromatographed on
silica gel column eluting with petroleum ether/CH2Cl2 (10/1, v/v). Collecting the main
orange band afforded the brown-red viscous liquid or solid as all-CO diiron Rodt
complexes.
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Complex 1 (R = Ph). Yield: 0.101 g (11% based on Fe2S2(CO)6). Anal. calcd. for
C14H8Fe2O7S2: C, 36.24; H, 1.74 %. Found: C, 36.09; H, 1.91%. FT-IR (KBr disk):
CO 2076 (vs), 1938 (vs), 2000 (vs), 1984 (w) cm1. H NMR (600 MHz, CDCl3,
1
TMS): 7.40-7.35 (m, 5H, PhH), 4.92 (d, J = 10.8 Hz, 1H, SCHPh), 4.82 (s, 1H,
SCHeHa), 4.15 (d, J = 10.8 Hz, 1H, SCHeHa) ppm.
Complex 2 (R = Me). Yield: 0.079 g (10% based on Fe2S2(CO)6). Anal. calcd. for
C9H6Fe2O7S2: C, 26.89; H, 1.50 %. Found: C, 27.01; H, 1.77%. FT-IR (KBr disk):
CO 2077 (vs), 2038 (vs), 1999 (vs), 1983 (w) cm1. H NMR (600 MHz, CDCl3,
1
TMS): 4.69 (s, 1H, SCHCH3), 3.91 (s, 2H, SCH2), 1.45 (s, 3H, CH3) ppm.
Complex 3 (R = H). Yield: 0.359 g (46% based on Fe2S2(CO)6). Anal. calcd. for
C8H4Fe2O7S2 : C, 24.77; H, 1.04 %. Found: C, 24.56; H, 1.32%. FT-IR (KBr disk):

CO 2078 (vs), 2037 (vs), 2001 (vs), 1981 (w) cm1.
3.3
Preparation
of
PPh3-monosubstituted
diiron
complexes
Fe2(μ-Rodt)(CO)5(PPh3) (Rodt = SCH(R)OCH2S, 4-6)
A mixture of the above-prepared precursors 1, 2, or 3 (0.20 mmol), PPh3 (0.24 mmol)
and Me3NO·2H2O (0.24 mmol) was dissolved in 20 mL dry MeCN. The reaction
mixture was stirred at room temperature for 1 h until TLC monitor indicated the
respective starting precursors 1-3 were completely consumed. After solvent was
removed in vacuo, the resulting residue was chromatographed by preparative TLC
separation eluting with petroleum ether/CH2Cl2 (v/v, 3:1). From the main red band (Rf
= 0.5), the PPh3-monosubstituted complexes were obtained as red solids.
Complex 4 (R = Ph). Yield: 0.096 g (69%). Anal. calcd. for C31H23Fe2O6PS2: C,
53.32; H, 3.32%. Found: C, 53.45; H, 3.52%. FT-IR (KBr disk): CO 2045 (vs), 1987
(vs), 1961 (w), 1935 (s) cm1. 1H NMR (600 MHz, CDCl3, TMS): 7.64 (t, J = 9.0 Hz,
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6H, 3xPPhH-o), 7.56 (s, 1H, CHPhH-p), 7.44 (t, J = 7.2 Hz, 4H, CHPhH-o, m),
7.36-7.33 (m, 9H, 3xPPhH-m, p), 4.93 (s, 1H, SCHPh), 4.14 (s, 1H, SCHeHa), 3.95 (d,
J = 10.2 Hz, 1H, SCHeHa) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): 64.5
(s) ppm.
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Complex 5 (R = Me). Yield: 0.100 g (79%). Anal. calcd. for C26H21Fe2O6PS2: C,
49.08; H, 3.33%. Found: C, 49.25; H, 3.13%. FT-IR (KBr disk): CO 2046 (vs), 1985
1
(vs), 1958 (w), 1933 (s) cm1. H NMR (600 MHz, CDCl3, TMS): 7.73 (s, 6H,
3xPPhH-o), 7.42 (s, 9H, 3xPPhH-m, p), 3.80-3.68 (m, 3H, SCHCH3 and SCH2), 1.16
(s, 3H, SCH3) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): 64.3 (s) ppm.
Complex 6 (R = H). Yield: 0.082 g (66%). Anal. calcd. for C25H19Fe2O6PS2: C,
48.26; H, 3.08%. Found: C, 48.56; H, 3.26%. FT-IR (KBr disk): CO 2038 (vs), 1974
1
(vs), 1955 (s), 1934 (s) cm1. H NMR (600 MHz, CDCl3, TMS): 7.72 (s, 6H,
3xPPhH-o), 7.43 (s, 9H, 3xPPhH-m, p), 3.76 (s, 2H, 2xSCHeHa), 3.40 (s, 2H,
2xSCHeHa) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): 63.7(s) ppm.
3.4 Preparation of PPh3-disubstituted diiron complexes Fe2(μ-Rodt)(CO)4(PPh3)2
(Rodt = SCH(R)OCH2S, 7-9)
A mixture of the above-prepared precursors 1, 2, or 3 (0.20 mmol), PPh3 (0.44 mmol)
and Me3NO·2H2O (0.44 mmol) was dissolved in 20 mL of MeCN. The reaction
mixture was stirred at room temperature for 1 h until TLC monitor indicated the
respective starting precursors 1-3 were completely consumed. After solvent was
removed in vacuo, the residue was chromatographed by preparative TLC separation
eluting with petroleum ether/CH2Cl2 (v/v, 3:1). From the brown-red band (Rf = 0.3),
the PPh3-disubstituted complexes were obtained as brown-red solids.
Complex 7 (R = Ph). Yield: 0.053 g (28%). Anal. Calcd. For C48H38Fe2O5P2S2: C,
61.82; H, 4.11%. Found: C, 61.63; H, 4.34%. FT-IR (KBr disk): CO 2000 (vs), 1958
1
(vs), 1936 (vs), 1912 (w) cm1. H NMR (600 MHz, CDCl3, TMS): 7.80, 763 (s, s,
12H, 6xPPhH-o), 7.41 (s, 12H, 6xPPhH-m), 7.23 (s, 6H, 6xPPhH-p), 7.14-7.09 (m,
3H, CHPhH-o, p), 6.87 (s, 2H, CHPhH-m), 4.03 (s, 2H, SCHeHa, SCHPh), 2.90 (s,
1H, SCHeHa) ppm. 31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): 61.0 (s) ppm.
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Complex 8 (R = Me). Yield: 0.100 g (79%). Anal. Calcd. For C43H36Fe2O5P2S2:
5
6
C, 59.33; H, 4.17%. Found: C, 59.54; H, 4.44%. FT-IR (KBr disk): CO 1998 (vs),
7
8
1
1956 (vs), 1934 (vs), 1907 (w) cm1. H NMR (600 MHz, CDCl3, TMS): 7.79, 7.71
9
(s, s, 12H, 6xPPhH-o), 7.52-7.45 (m, 6H, 6xPPhH-p), 7.37 (s, 12H, 6xPPhH-m), 2.91,
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2.65, 2.39 (s, s, s, 3H, SCHPh, SCH2), 1.34 (s, 3H, SCH3) ppm. P{1H} NMR (243
MHz, CDCl3, 85% H3PO4): 61.5 and 61.3 (s, s, basal-apical and basal-basal isomers)
ppm.
Complex 9 (R = H). Yield: 0.128 g (75%). Anal. Calcd. For C42H34Fe2O5P2S2: C,
58.90; H, 4.00%. Found: C, 58.69; H, 3.82%. FT-IR (KBr disk): CO 1998 (vs), 1955
(vs), 1935 (vs), 1913 (w) cm1. H NMR (600 MHz, CDCl3, TMS): 7.81 (s, 12H,
1
6xPPhH-o), 7.46 (s, 18H, 6xPPhH-m,p), 2.83 (s, 4H, 2xSCHeHa) ppm. 31P{1H} NMR
(243 MHz, CDCl3, 85% H3PO4): 61.0 (s) ppm.
3.5 Protonations of 4-6 and 7-9 with excess strong acid TFA
A dry CH2Cl2 (5 mL) solution of complexes 4-6 and 7-9 (0.015 mmol) was treated
with 10 equivalents of TFA (11.1 μL, 0.15 mmol), respectively. The reaction mixture
was stirred and monitored at room temperature for 4 h for 4-6 and 3 h for 7-9 until the
carbonyl absorption bands didn’t change by using in situ IR monitor. Similarly, a
CDCl3 (0.5 mL) solution containing 4-9 (0.015 mmol) and 10 equivalents of TFA
(11.1 μL, 0.15 mmol) was placed in a J-Young NMR tube at room temperature for 4 h
for 4-6 and 3 h for 7-9, which was monitored by NMR (1H, 31P) spectroscopies.
For 4 with excess TFA: FT-IR (KBr disk): CO 2098 (w), 2048 (vs), 1992 (vs),
1967 (w), 1944 (s) cm1. 1H NMR (600 MHz, CDCl3, r.t.): no hydride signal. 31P{1H}
NMR (243 MHz, CDCl3, 85% H3PO4): 64.0 (s, 79%) ppm for 4 and 24.0 (s, 21%)
ppm for [4(μH)](CF3CO2).
For 5 with excess TFA: FT-IR (KBr disk): CO 2111 (w), 2049 (vs), 1993 (vs),
1966 (w), 1942 (s) cm1. H NMR (600 MHz, CDCl3, r.t.): -15.01 (d, JPH = 15.6 Hz,
1
μH). P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): 64.6 (s, 62%) ppm for 5 and
31
48.8 (s, 38%) ppm for [5(μH)](CF3CO2).
For 6 with excess TFA: FT-IR (KBr disk): CO 2100 (w), 2049 (vs), 1989 (vs),
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1
1
31
1
1967 (w), 1944 (s) cm . H NMR (600 MHz, CDCl3, r.t.): no hydride signal. P{ H}
NMR (243 MHz, CDCl3, 85% H3PO4): 63.7 (s, 75%) ppm for 6 and 50.4 (s, 25%)
ppm for [6(μH)](CF3CO2).
9
For 7 with excess TFA: FT-IR (KBr disk): CO 2044 (vs), 1992 (vs), 1964 (s),
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1
1930 (w) cm1. H NMR (600 MHz, CDCl3, r.t.): -14.15 (t, JPH = 15.6 Hz, 1xμH,
transoid-dibasal isomer) and -14.75 (d, JPH = 21.0 Hz, 1xμH, apical-basal isomer).
31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): 65.8 (s) and 5.6 (s) ppm for
[7(μH)](CF3CO2).
For 8 with excess TFA: FT-IR (KBr disk): CO 2046 (vs), 1992 (vs), 1943 (s)
cm1. 1H NMR (600 MHz, CDCl3, r.t.): -14.45 (t, JPH = 16.2 Hz, 4xμH,
transoid-dibasal isomer) and -15.17 (d, JPH = 19.2 Hz, 1xμH, apical-basal isomer).
31P{1H} NMR (243 MHz, CDCl3, 85% H3PO4): 67.1 (s) and 5.5 (s) ppm for
[8(μH)](CF3CO2).
For 9 with excess TFA: FT-IR (KBr disk): CO 2042 (vs), 2003 (vs), 1943 (s)
cm1. 1H NMR (600 MHz, CDCl3, r.t.): -14.35 (t, JPH = 16.2 Hz, 2.5xμH, apical-basal
isomer) and -15.01 (d, JPH = 18.6 Hz, 1xμH, apical-basal isomer). 31P{1H} NMR (243
MHz, CDCl3, 85% H3PO4): 53.6 (s) and 5.5 (s) ppm for [9(μH)](CF3CO2).
3.6 X-ray structure determination
Single crystals of complexes 4 and 7-9 suitable for X-ray diffraction analysis were
grown by slow evaporation of the CH2Cl2/n-hexane solution at 5oC. The crystals were
mounted on a Bruker-CCD diffractometer. Data were collected at 150(2) K using a
graphite monochromator with Mo-Kα radiation (λ = 0.71073 Å) in the ω-φ scanning
mode. The structure was solved by direct methods using the SHELXS-97 program
and refined by full-matrix least-squares techniques (SHELXL-97) on F2.52 Hydrogen
atoms were located using the geometric method. The crystallographic parameters,
data collection, and structure refinement details of 3, 4, 6 and 8 are summarized in
Table S1 in the supporting information.
3.7 Electrochemistry
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Electrochemical measurements were performed with a CHI 660E electrochemical
analyzer (CH Instruments, Inc., Shanghai). The electrochemical experiments of
complexes 4-9 were studied in a conventional three-electrode system using a glassy
carbon electrode (GCE, Ø = 3 mm) as the working electrode, a Pt wire as the counter
electrode, and an Ag/AgNO3 (0.01 M in MeCN) electrode as the reference electrode.
Prior to the experiments, the GCE was polished using a polishing cloth using 0.05 μm
alumina paste to obtain a mirror-like surface, followed by ultrasonic cleaning in water
and acetone. All electrochemical experiments were conducted under a nitrogen
atmosphere. The potential scale was calibrated against the ferrocene/ferrocenium
(Fc/Fc+) couple and was reported versus this reference system.
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4. Conclusions
In summary, we have prepared and structurally characterized two types of
phosphine-substituted diiron complexes 4-6 and 7-9 with Rodt bridges related to
[FeFe]-hydrogenases. Most significantly, a comparative study on the influence of
desymmetrized dithiolate bridges on the structural, protophilic, and electrochemical
behaviors of 4-6 and 7-9 are performed by using X-ray crystallography, in situ IR plus
NMR spectroscopies, and CV technique.
In contrast to reference homologues 6 and 9 with symmetric odt bridges, complexes
4, 5 and 7, 8 with desymmetrized Phodt or Meodt bridges display several meaningful
results as follows. Firstly, the differences among two Fe-P distances are more obvious
while the distinctions among two P-Fe-Fe angles are smaller in their solid-state
structures of 4, 5 and 7, 8, revealing that the desymmetrization of the Rodt bridges
could tune geometric symmetries of two PPh3 ligands and further lead to the
asymmetry of diiron cores. Secondly, under excess TFA, the partial protonation of 5
with Meodt bridge produced more of protonated species [5(μH)]+, while the complete
protonation of 8 with Meodt bridge formed the higher molar ratios of the
transoid-dibasal versus apical-basal isomers of hydride species [8(μH)]+. Thirdly,
complexes 5 and 8 with Meodt bridges show the more negative initial reduction
potential (Epc1), whereas complexes 4, 5 and 7, 8 exhibit the relatively higher
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electrocatalytic HER activity with slightly larger TONs under excess TFA or HOAc.
These outcomes suggest that the desymmetrization of the ditholate bridges could play
a certain role in the redox behaviors of diiron cores and the electrocatalytic HER
activity of asymmetric diiron model complexes.
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Author contributions
Xiao-Li Gu: Methodology, Investigation, Data Curation, Writing - original draft.
Jian-Rong Li: Methodology, Investigation, Data Curation.
Bo Jin, Yang Guo, and Xing-Bin Jing: Validation, Investigation, Formal analysis.
Pei-Hua Zhao: Conceptualization, Supervision, Writing -review & editing.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We are grateful to Research Project Supported by Shanxi Scholarship Council of
China (No. 2021-119), National Natural Science Foundation of China (No.
21301160), and Science Foundation of North University of China (in 2013) for the
financial support of this work.
Appendix A. Supplementary data
CCDC numbers 2083414-2083417 for 4, 7-9 (cif files), relevant protonation data of
4-6 and 7-9 (Figs. S1-S14), electrolysis experiments and TON calculations of 4-6 and
7-9 (Schemes S1-S3 and Figs. S15-S19), and spectroscopic data of 1-3, 4-6, and 7-9
(Figs. S20-S42) may be available in the Electronic Supporting Information (ESI).
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