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

Article abstract of DOI:10.1002/aoc.5523

To further extend diiron subsite models of [FeFe]-hydrogenases, the various substitutions of all-carbonyl diiron complex Fe₂(μ-Me₂pdt)(CO)₆ (A, Me₂pdt = (SCH₂)₂CMe₂) with monophosphines or small bite-angle diphosphines are studied as follows. Firstly, the monodentate complexes Fe₂(μ-Me₂pdt)(CO)₅{κ¹-P(C₆H₄R-p)₃} [R = Me (1a) and Cl (1b)] and Fe₂(μ-Me₂pdt)(CO)₅{κ¹-Ph₂PX'} [X' = NHPh (2a) and CH₂PPh₂ (2b)] are readily afforded through the Me₃NO-assisted reactions of A with monophosphines P(C₆H₄R-p)₃ (R = Me, Cl) and diphosphines (Ph₂P)₂X (X = NPh, CH₂ (dppm)) in MeCN at room temperature, respectively. Secondly, the chelate complexes Fe₂(μ-Me₂pdt)(CO)₄(κ²-(Ph₂P)₂X) [X = NPh (3a) and NBuⁿ (3b)] can be efficiently prepared by the UV-irradiated reactions of A with small bite-angle diphosphines (Ph₂P)₂X (X = NPh, NBuⁿ) in toluene. Thirdly, the bridge complexes Fe₂(μ-Me₂pdt)(CO)₄(μ-(Ph₂P)₂X) [X = NPh (4a) and CH₂ (4b)] are well obtained from the refluxing solutions of A and diphosphines (Ph₂P)₂X (X = NPh, CH₂) in xylene. Rarely, the diphosphine-bridge complex 4b may be produced in low yield via the UV-irradiated solutions of A and the dppm ligand in toluene emitting at 365 nm. Eight new complexes obtained above have been well characterized by using element analysis, FT-IR, NMR (¹H, ³¹P) spectroscopies, and particularly for 1a, 1b, 2a, 3b, 4a, 4b by X-ray crystallography. Meanwhile, the electrochemical and electrocatalytic properties of three representative complexes 2a, 3a, and 4a with pendant N-phenyl groups are investigated and compared by using cyclic voltammetry (CV) in the absence and presence of trifluoroacetic acid (TFA) as a proton source, indicating that they are all found to be active for electrocatalytic proton reduction to hydrogen (H₂).

Full text of DOI:10.1002/aoc.5523

Received: 13 November 2019
DOI: 10.1002/aoc.5523
Revised: 15 December 2019
Accepted: 20 December 2019
F U L L P A P E R
Synthesis, structures, and electrocatalytic properties of
phosphine-monodentate, −chelate, and -bridge diiron
2,2-dimethylpropanedithiolate complexes related to [FeFe]-
hydrogenases
1
1
1
1
Meng-Yuan Hu | Pei-Hua Zhao
| Jian-Rong Li | Xiao-Li Gu
|
1
2
Xing-Bin Jing | Xu-Feng Liu
1
School of Materials Science and
To further extend diiron subsite models of [FeFe]-hydrogenases, the various
Engineering, North University of China,
Taiyuan, 030051, P. R. China
substitutions of all-carbonyl diiron complex Fe (μ-Me pdt)(CO) (A,
2
2
6
2
School of Materials and Chemical
Me pdt
= (SCH ) CMe ) with monophosphines or small bite-angle
2 2 2
2
Engineering, Ningbo University of
diphosphines are studied as follows. Firstly, the monodentate complexes Fe (μ-
2
Technology, Ningbo, 315211, P. R. China
1
Me pdt)(CO) {κ -P(C H R-p) } [R = Me (1a) and Cl (1b)] and Fe (μ-Me pdt)
2
5
6
4
3
2
2
Correspondence
1
5 2 2 2
(
CO) {κ -Ph PX'} [X' = NHPh (2a) and CH PPh (2b)] are readily afforded
P.-H. Zhao, School of Materials Science
and Engineering, North University of
China, Taiyuan 030051, P. R. China.
Email: zph2004@nuc.edu.cn
X.-F. Liu, School of Materials and
Chemical Engineering, Ningbo University
of Technology, Ningbo 315211, P. R.
China.
through the Me NO-assisted reactions of A with monophosphines P(C H R-p)
3
3
6
4
(R = Me, Cl) and diphosphines (Ph P) X (X = NPh, CH (dppm)) in MeCN at
2
2
2
room temperature, respectively. Secondly, the chelate complexes Fe (μ-
2
2
n
Me pdt)(CO) (κ -(Ph P) X) [X = NPh (3a) and NBu (3b)] can be efficiently
2
4
2
2
prepared by the UV-irradiated reactions of A with small bite-angle
n
diphosphines (Ph P) X (X = NPh, NBu ) in toluene. Thirdly, the bridge com-
2
2
Email: nkxfliu@126.com
plexes Fe (μ-Me pdt)(CO) (μ-(Ph P) X) [X = NPh (4a) and CH (4b)] are well
2
2
4
2
2
2
Funding information
obtained from the refluxing solutions of A and diphosphines (Ph P) X
2
2
Natural Science Foundation of Zhejiang
Province, Grant/Award Number:
LY19B02002; Scientific Research
Foundation for the Returned Overseas
Chinese Scholars of Shanxi Province;
Natural Science Foundation of Shanxi
Province, Grant/Award Number:
(
X = NPh, CH ) in xylene. Rarely, the diphosphine-bridge complex 4b may be
2
produced in low yield via the UV-irradiated solutions of A and the dppm
ligand in toluene emitting at 365 nm. Eight new complexes obtained above
1
31
have been well characterized by using element analysis, FT-IR, NMR ( H, P)
spectroscopies, and particularly for 1a, 1b, 2a, 3b, 4a, 4b by X-ray crystallogra-
phy. Meanwhile, the electrochemical and electrocatalytic properties of three
representative complexes 2a, 3a, and 4a with pendant N-phenyl groups are
investigated and compared by using cyclic voltammetry (CV) in the absence
and presence of trifluoroacetic acid (TFA) as a proton source, indicating that
they are all found to be active for electrocatalytic proton reduction to hydrogen
201701D121035; National Natural Science
Foundation of China, Grant/Award
Number: 21301160
(H2).
K E Y W O R D S
diiron 2,2-dimethylpropanedithiolate complex, electrocatalytic property, facile synthesis,
phosphine ligands, structural characterization
Appl Organometal Chem. 2020;e5523.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
1 of 12
https://doi.org/10.1002/aoc.5523

2
of 12
HU ET AL.
1
| INTRODUCTION
observe the potential influence of the diverse coordina-
tion modes (monodentate, chelate, bridge) of phosphine
Hydrogen (H ) as a green energy carrier may be gener-
ligands in diiron complexes with bulky Me pdt bridge
2
2
ated from the electrolysis of water through noble-metal
platinum catalyst, limiting the wide use of hydrogen
on their formation, structures, and electrocatalytic
behaviors, and to ii) further develop the biomimetic
chemistry of [FeFe]-hydrogenases for proton reduction.
As a result, we herein report the facile synthesis and
structural comparison of three types of diiron phos-
phine complexes, i.e., the monodentate complexes
[1]
fuel. Thus, it is very necessary for hydrogen produc-
tion to seek a low-cost and environmentally-friendly
catalyst instead of platinum. In nature, a subclass of
metalloenzymes named [FeFe]-hydrogenases can revers-
ibly and efficiently catalyze the reduction of proton to
1
Fe (μ-Me pdt)(CO) {κ -P(C H R-p) } [R = Me (1a) and
2
2
5
6
4
3
1
H . At the end of the last century, [FeFe]-hydrogenases
2
Cl (1b)] and Fe (μ-Me pdt)(CO) {κ -Ph PX'} [X' = NHPh
2 2 5 2
isolated from Clostridium pasteurianum and Des-
ulfovibrio desulfuricans have begun to be characterized
using the high-resolution single-crystal X-ray diffraction
analysis. Further spectroscopic and theoretical studies
have revealed that i) the active site of [FeFe]-
hydrogenases is made up of a butterfly [Fe S ] cluster
(2a) and CH PPh (2b)], the chelate complexes Fe (μ-
2 2 2
2
n
Me pdt)(CO) (κ -(Ph P)X) [X = NPh (3a) and NBu
2
4
2
(3b)], and the bridge complexes Fe (μ-Me pdt)(CO) (μ-
2
2
4
(Ph P)X) [X = NPh (4a) and CH (4b)]. Further, the
2
2
electrochemical and electrocatalytic properties of three
representative complexes 2a, 3a, 4a with pendant N-
phenyl groups are investigated and compared through
cyclic voltammetry (CV).
2
2
as a catalytic active center and a cubic [Fe S ] sub-
4
4
cluster as an electron transfer channel, which are
[2,3]
linked through a sulfur atom of cysteine;
ii) the two
Fe cores in the [Fe S ] cluster (i.e., diiron subsite) are
2
2
coordinated by a dithiolate bridge and several diatomic
2 | EXPERIMENTAL
CO/CN− ligands;
[4,5]
and iii) the dithiolate cofactor has
been definitively determined to be an azadithiolate unit
2.1 | Materials and methods
[6–8]
(
adt = (SCH ) NH) in natural enzymes
and may be
2
2
extensively replaced with other groups or atoms such
All reactions and operations were carried out under a
dry, oxygen-free nitrogen atmosphere with standard
Schlenk and vacuum-line techniques. MeCN was distilled
from CaH under N , whereas toluene and xylene were
[
9]
CH , CR , SiR , O, S, Se, etc in synthetic models.
2
2
2
In this context, in order to explore a class of cheap
and efficient artificial catalysts for electrocatalytic proton
2
2
reduction to H , a great number of diiron dithiolate
distilled under N2 from sodium/benzophenone ketyl.
2
complexes that mimic the diiron subsite of [FeFe]-
hydrogenases were obtained over the decades.
Me NOÁ2H O, P(C H R-p) (R = Me, Cl), (Ph P) CH
3
2
6
4
3
2
2
2
[
10–15]
(dppm) and CF CO H (TFA) were commercially avail-
3 2
Among the preparation of diiron subsite models, the
interesting substitutions of all-carbonyl diiron complexes
Fe (μ-xdt)(CO) (xdt = pdt, (SCH ) CH ; edt, (SCH ) ;
able and used as received. Compounds Fe (μ-Me pdt)
2 2
(CO) (A) and aminodiphosphine (Ph P) NR (R = Ph,
6
2
2
n
Bu ) were prepared according to the literature proce-
dures.
2
NR
6
2 2
2
2 2
[32,36]
adt , (SCH ) NR) by small bite-angle diphosphines
Preparative TLC was performed on glass
2
2
(
Ph P) X (X = CR named PCP and X = NR’ labeled PNP
plates (25 cm x 20 cm x 0.25 cm) coated with silica gel G
(10–40 mm). Elemental analyses were obtained on a
Perkin-Elmer 240C analyzer. FT-IR spectra were
recorded on a Nicolet iS 10 FT-IR spectrometer. NMR
( H, P{ H}) spectra were obtained on a Bruker Avance
600 MHz spectrometer.
2
2
2
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 H2.
Inspired by these findings mentioned above and in
comparison with the previous studies on the substitu-
[16–28]
1
31
1
tions of diiron hexacarbonyl complexes bearing some
NR [18–31]
dithiolate bridges such as pdt, edt and adt
,
little
2.2 | Preparation of Fe (μ-Me pdt)
2
2
1
attention has been however paid to the reactions of all-
carbonyl diiron complexes with bulky dithiolate bridges
such as Me pdt [(SCH ) CMe ], Et pdt [(SCH ) CEt ],
(CO) {κ -P(C H Me-p) } (1a) and
5 6 4 3
1
Fe (μ-Me pdt)(CO) {κ -P(C H Cl-p) }
2
2
5
6
4
3
.
(1b) using Me NO 2H O in MeCN
2
2 2
2
2
2 2
2
3
2
[32–35]
etc.
In this work, we carried out the treatments of
parent complex Fe (μ-Me pdt)(CO) (A) with common
A
dry MeCN solution (20 ml) of
A (0.104 g,
2
2
6
monophosphines P(C H R-p) (R = Me, Cl) or small
0.250 mmol), Me NOÁ2H O (0.042 g, 0.375 mmol) and
6
4
3
3
2
bite-angle diphosphines (Ph P) X (X = CH (dppm),
P(C H Me-p) (0.114 g, 0.375 mmol) was stirred at
6 4 3
2
2
2
n
NPh, NBu ) in different conditions, mainly aiming to i)
room temperature for 50 min to give a solution color

SYNTHESIS, STRUCTURES, AND ELECTROCATALYTIC PROPERTIES
3 of 12
3
1
1
change from red to brown-red. Then, the solvent was
removed under reduced pressure and the residue was
subjected to preparative TLC separation using petro-
leum ether/dichloromethane (4:1, v/v) as eluent. Col-
lecting the main red band afforded 0.134 g (78%) of
P{ H} NMR (243 MHz, CDCl , 85% H PO ): δ 91.5
3 3 4
(s) ppm. Red single crystals of 2a were grown from by
slow evaporation of CH Cl /n-hexane solutions at −5 C.
Complex 2b was afforded as a red solid (0.129 g,
67%) by a procedure similar to that of 2a except for
dppm (0.144 g, 0.375 mmol). Anal. Calcd. for
C H Fe O P S : C, 54.57; H, 4.19%. Found: C,
ꢀ
2
2
complex 1a as
a
red solid. Anal. Calcd. for
C H Fe O PS : C, 53.93; H, 4.53%. Found: C,
31
31
2
5
2
35 32
2 5 2 2
5
1
3.75; H, 4.74%. FT-IR (KBr disk): νC ≡
979 (vs), 1955 (w), 1928 (m) cm .
2041 (vs),
54.32; H, 4.42%. FT-IR (KBr disk): νC ≡ O 2040 (vs),
O
−1
1
−1
1
H
NMR
1979 (vs), 1954 (w), 1920 (m) cm .
H NMR
(
(
(
(
600 MHz, CDCl , TMS): δ 7.48 (s, 6H, 3xC H -o), 7.14
(600 MHz, CDCl , TMS): δ 7.47–7.44 (m, 4H, 2xPC H -
3 6 4
3
6
5
s, 6H, 3xC H -m), 2.30 (s, 9H, 3xC H CH -p), 1.72
o), 7.17–7.07 (m, 16H, 2xPC H -o and 4xPC H -m,p),
6
4
6
4
3
6
5
6
5
s, 2H, 2xSCH H ), 1.17 (s, 2H, 2xSCH H ), 0.86, 0.37
3.14 (d, JPH = 7.8 Hz, 2H, PCH ), 1.81 (d, J = 13.2 Hz,
e
a
e
a
2 HH
31
1
s, s, 6H, CH C(CH ) ) ppm. P{ H} NMR (243 MHz,
2H, 2xSCH H ), 1.68 (d, J = 13.2 Hz, 2H, 2xSCH H ),
e a HH e a
2
3 2
31
1
CDCl , 85% H PO ): δ 62.6 (s) ppm. Red single crystals
0.83, 0.75 (s, s, 6H, CH C(CH ) ) ppm. P{ H} NMR
2 3 2
3
3
4
of 1a were grown from by slow evaporation of CH Cl /
n-hexane solutions at −5 C.
(243 MHz, CDCl , 85% H PO ): δ 57.9 (d, J = 78 Hz,
3 3 4 PP
FeP), −26.3 (d, JPP = 78 Hz, free P) ppm.
2
2
ꢀ
Complex 1b was afforded as a red solid (0.133 g,
7
1%) by a procedure similar to that of 1a except for
P(C H Cl-p) (0.137 g, 0.375 mmol). Anal. Calcd. for
2.4 | Preparation of Fe (μ-Me pdt)
2 2
6
4
3
2
C H Cl Fe O PS : C, 44.74; H, 2.95%. Found: C,
(CO) {κ -(Ph P) NPh} (3a), Fe (μ-Me pdt)
4 2 2 2 2
2 n
28
22
3
2
5
2
4
1
4.87; H, 3.17%. FT-IR (KBr disk): νC ≡
983 (vs), 1960 (w), 1932 (m) cm .
2046 (vs),
(CO) {κ -(Ph P) NBu } (3b), and
4 2 2
O
−1
1
H
NMR
Fe (μ-Me pdt)(CO) (μ-dppm) (4b) using UV
2 2 4
(
600 MHz, CDCl , TMS): δ 7.56 (s, 6H, 3xC H -o), 7.42
irradiation in toluene
3
6
4
(
s, 6H, 3xC H -m), 1.89 (s, 2H, 2xSCH H ), 1.39 (s, 2H,
6 4 e a
2
xSCH H ), 0.94, 0.55 (s, s, 6H, CH C(CH ) ) ppm.
A dry toluene solution (100 ml) of A (0.104 g, 0.250 mmol)
e
a
2
3 2
3
1
1
P{ H} NMR (243 MHz, CDCl , 85% H PO ): δ 64.6
and (Ph P) NPh (0.173 g, 0.375 mmol) was irradiated in a
3
3
4
2
2
(
s) ppm. Red single crystals of 1b were grown from by
Pyrex Schlenk tube with LED lamp at 365 nm for 1 hr to
give a red solution. After this reaction, the solvent was
removed under reduced pressure and the residue was
subjected to flash column chromatography on neutral
alumina using petroleum ether/dichloromethane (2:1, v/
v) as eluent. Collecting the main red band afforded
0.085 g (42%) of complex 3a as a red solid. Anal. Calcd.
for C H Fe NO P S : C, 57.16; H, 4.30; N, 1.71%.
slow evaporation of CH Cl /n-hexane solutions at
−
2
2
ꢀ
5 C.
2
(
.3 | Preparation of Fe (μ-Me pdt)
2 2
1
CO) {κ -Ph P(NHPh)} (2a) and
5 2
1
Fe (μ-Me pdt)(CO) (κ -dppm)
(
2
2
5
39 35
2
4 2 2
.
2b) using Me NO 2H O in MeCN
Found: C, 57.49; H, 4.71; N, 1.98%. FT-IR (KBr disk):
3
2
−
1 1
νC ≡ O 2016 (vs), 1949 (vs), 1941 (vs), 1912 (w) cm . H
NMR (600 MHz, CDCl , TMS): δ 7.75 (s, 8H, 4xPC H -o),
A dry MeCN solution (20 ml) of A (0.104 g, 0.250 mmol),
Me NOÁ2H O (0.042 g, 0.375 mmol) and (Ph P) NPh
3
6
5
7.45 (s, 12H, 4xPC H -m,p), 6.93 (s, 5H, NC H ), 2.09
3
2
2
2
6
5
6
5
(
0.173 g, 0.375 mmol) was stirred at room temperature for
(d, JHH = 12.6 Hz, 2H, 2xSCH H ), 1.76 (d, J = 12.6 Hz,
e a HH
5
0 min to give a solution color change from red to brown-
2H, 2xSCH H ), 1.40, 1.26 (s, s, 6H, CH C(CH ) ) ppm.
e a 2 3 2
P{ H} NMR (243 MHz, CDCl , 85% H PO ): δ 116.5 (s,
3 3 4
apical-basal isomer, 62%), 98.2 (s, basal-basal isomer,
38%) ppm.
31
1
red. Then, the solvent was removed under reduced pres-
sure and the residue was subjected to preparative TLC
separation using petroleum ether/dichloromethane (2:1,
v/v) as eluent. Collecting the main red band afforded
Complex 3b was afforded as a red solid (0.095 g,
47%) by a procedure similar to that of 3a except for
0
.072 g (43%) of complex 2a as a red solid. Anal. Calcd.
n
for C H Fe NO PS : C, 50.70; H, 3.95; N, 2.11%.
(Ph P) NBu (0.165 g, 0.375 mmol). Anal. Calcd. for
28
26
2
5
2
2
2
Found: C, 50.48; H, 4.12; N, 2.32%. FT-IR (KBr disk): ν
C H Fe NO P S : C, 55.59; H, 4.92; N, 1.75%.
37 39 2 4 2 2
C ≡ O
−
1 1
2
043 (vs), 1983 (vs), 1957 (w), 1930 (m) cm . H NMR
Found: C, 55.39; H, 5.18; N, 1.96%. FT-IR (KBr disk):
−
1 1
(
(
600 MHz, CDCl , TMS): δ 7.78 (s, 4H, 2xPC H -o), 7.42
νC ≡ O 2014 (vs), 1946 (vs), 1935(vs), 1911 (w) cm . H
NMR (600 MHz, CDCl , TMS): δ 7.68 (s, 8H, 4xPC H -
3
6
4
s, 6H, 2xPC H -m,p), 6.99, 6.78, 6.52 (s, s, s, 5H,
6
4
3
6
5
NHC H ), 5.49 (d, J
= 18 Hz, 1H, PNH), 1.94
o), 7.44 (m, 12H, 4xPC H -m,p), 2.96–2.89 (m, 2H,
6
5
PH
6
5
(
d, JHH = 13.2 Hz, 2H, 2xSCH H ), 1.61 (d, J = 13.2 Hz,
NCH ), 2.14–2.00 (m, 2H, 2xSCH H ), 1.71–1.57 (m, 2H,
e
a
HH
2
e
a
2
H, 2xSCH H ), 0.97, 0.73 (s, s, 6H, CH C(CH ) ) ppm.
2xSCH H ), 1.26 (s, 4H, NCH CH CH ), 0.99 (s, 3H,
e a 2 2 2
e
a
2
3 2

4
of 12
HU ET AL.
CH CH ), 0.88, 0.64 (s, s, 6H, CH C(CH ) ) ppm.
2.6 | X-ray crystal structure
determination
2
1
3
2
3 2
31
P{ H} NMR (243 MHz, CDCl , 85% H PO ): δ 110.6
3
3
4
(
s, apical-basal isomer, 78%), 98.7 (s, basal-basal isomer,
2%) ppm. Red single crystals of 3b were grown from
by slow evaporation of CH Cl /pentane solutions at
2
Single crystals of complexes 1a, 1b, 2a, 3b, 4a, and 4b
suitable for X-ray diffraction analysis were mounted on a
Bruker-CCD diffractometer. Data were collected at
150(2) K using a graphite monochromator with Mo-
Kαradiation (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-
2
2
ꢀ
−
5 C.
Complex 4b was prepared as an orange-red solid
0.054 g, 29%) by a procedure similar to that of 3a except
(
for dppm (0.144 g, 0.375 mmol). Anal. Calcd. for
C H Fe O P S : C, 55.01; H, 4.34%. Found: C, 55.18; H,
34
32
2 4 2 2
2
[37]
4
.56%. FT-IR (KBr disk): ν
1984 (vs), 1949 (vs), 1916
squares techniques (SHELXL-97) on F .
Hydrogen
C ≡ O
−
1
(vs), 1900 (m) cm . Red single crystals of 4b were grown
atoms were located using the geometric method. Details
of crystallographic data and structure refinement for
complexes 1a, 1b, 2a, 3b, 4a, and 4b are summarized in
Table S1 in the supporting information.
from by slow evaporation of CH Cl /n-hexane solutions
2
2
ꢀ
at −5 C.
2
.5 | Preparation of Fe (μ-Me pdt)
2 2
(CO) (μ-(Ph P) NPh) (4a) and
2.7 | Electrochemistry
4
2
2
Fe (μ-Me pdt)(CO) (μ-dppm) (4b) using
refluxing in xylene
2
2
4
Electrochemical and electrocatalytic properties of com-
plexes 2a, 3a, and 4a were studied by cyclic voltammetry
(CV) in MeCN solution. As the electrolyte, n-Bu NPF
A dry xylene solution (20 ml) of A (0.104 g, 0.250 mmol)
4
6
and (Ph P) NPh (0.173 g, 0.375 mmol) was refluxed at
was recrystallized multiple times from a CH Cl solution
2 2
2
2
ꢀ
1
38 C for 4 hr to give a solution color change from
by the addition of n-hexane. CV scans were obtained in a
three-electrode cell with a glassy carbon electrode (3 mm
diameter) as the working electrode, a platinum wire as
red to brown-red. After this reaction, the solvent was
removed under reduced pressure and the residue was
subjected to preparative TLC separation using petro-
leum ether/dichloromethane (3:2, v/v) as eluent. Col-
lecting the main orange red band afforded 0.179 g
+
the counter electrode, and a nonaqueous Ag/Ag elec-
trode as the reference electrode. The potential scale was
+
calibrated against the Fc/Fc couple and reported versus
(88%) of complex 4a as an orange red solid. Anal.
this reference system.
Calcd. for C H Fe NO P S : C, 57.16; H, 4.30; N,
39
35
2
4 2 2
1
.71%. Found: C, 57.03; H, 4.09; N, 1.98%. FT-IR (KBr
disk): νC 1997 (vs), 1961 (vs), 1925 (vs), 1912
3 | RESULTS AND DISCUSSION
≡
1
O
−1
(
(
(
vs) cm . H NMR (600 MHz, CDCl , TMS): δ 7.87
3
s, 4H, 2xPC H -o), 7.42 (s, 12H, 4xPC H -m,p), 7.03
3.1 | Facile synthesis of complexes 1a/1b,
2a/2b, 3a/3b, and 4a/4b
6
5
6
5
s, 4H, 2xPC H -o), 6.77 (s, 1H, NC H -p), 6.58 (s, 2H,
6
5
6
5
NC H -o), 6.09 (s, 2H, NC H -m), 2.39 (s, 2H,
6
5
6
5
2
xSCH H ), 2.21 (s, 2H, 2xSCH H ), 1.20 (s, 6H,
To obtain the diverse diiron phosphine complexes as
diiron subsite models of [FeFe]-hydrogenases, the CO-
replacement reactions of diiron hexacarbonyl complex
Fe (μ-Me pdt)(CO) (A) bearing bulky dithiolate bridge
e
a
e
a
31
1
CH C(CH ) ) ppm. P{ H} NMR (243 MHz, CDCl ,
2
3 2
3
8
5% H PO ): δ 124.5 (s) ppm. Red single crystals of 4a
3 4
were grown from by slow evaporation of CH Cl /n-
hexane solutions at −5 C.
2
2
2
2
6
ꢀ
with two types of phosphine ligands like PR3 and
(Ph P) X are studied under different reaction conditions
Complex 4b was prepared as an orange-red solid
2
2
(0.142 g, 77%) by a procedure similar to that of 4a
as shown in Scheme 1.
except for dppm (0.144 g, 0.375 mmol). Anal. Calcd. for
C H Fe O P S : C, 55.01; H, 4.34%. Found: C,
As shown in route (i) of Scheme 1, the substitution
reactions of precursor A with 1.5 equiv. common mon-
ophosphines P(C H R-p) (R = Me and Cl) in the pres-
34
32
2 4 2 2
5
1
4.78; H, 4.53%. FT-IR (KBr disk): νC ≡
953 (vs), 1917 (vs), 1904 (m) cm .
1988 (vs),
O
6
4
3
−
1
1
H
NMR
ence of Me NOÁ2H O as CO-removing agent at room
3
2
(
600 MHz, CDCl , TMS): δ 7.59 (s, 4H, 2xPC H -o),
temperature readily afforded the monodentate complexes
3
6
5
1
7
.32 (s, 16H, 2xPC H -o and 4xPC H -m,p), 3.71, 3.19
Fe (μ-Me pdt)(CO) {κ -P(C H R-p) } [R = Me (1a) and
6
5
6
5
2
2
5
6
4
3
(
s, s, 2H, PCH ), 2.11 (s, 4H, 2xSCH ), 1.08 (s, 6H,
Cl (1b)] in good yields. In contrast, the similar Me NO-
assisted decarbonylations of A by 1.5 equiv. small bite-
2
2
3
31
1
CH C(CH ) ) ppm. P{ H} NMR (243 MHz, CDCl ,
2
3 2
3
8
5% H PO ): δ 54.5 (s) ppm.
angle diphosphines (Ph P) X (X = NPh and CH )
3
4
2
2
2

SYNTHESIS, STRUCTURES, AND ELECTROCATALYTIC PROPERTIES
5 of 12
SCHEME 1 Preparation of
complexes 1a/1b, 2a/2b, 3a/3b, and 4a/
4
b
resulted in the unexpected formation of the monodentate
and 3a/3b as confirmed by the following spectroscopic
and crystallographic techniques.
1
complexes Fe (μ-Me pdt)(CO) {κ -Ph PX'} [X' = NHPh
2
2
5
2
(
2a) and CH PPh (2b)] in moderate yields (see route
2 2
(
ii) in Scheme 1), as reported previously in several known
NPh
[24–26]
diiiron pdt or adt
analogues.
Furthermore, the
3.2 | Spectroscopic characterization of
complexes 1a/1b, 2a/2b, 3a/3b, and 4a/4b
UV-irradiated toluene solutions of A and small bite-angle
n
PNP diphosphines (Ph P) NPh or (Ph P) NBu resulted
2
2
2
2
in the respective formation of the PNP-chelate complexes
All the new complexes 1a-4a and 2a-2b are air-stable red
solids, which have been well characterized by elemental
analysis and various spectroscopic techniques. Their
FT-IR and P{ H} NMR data were summarized in
Table 1 for comparison.
2
Fe (μ-Me pdt)(CO) (κ -(Ph P)X) [X = NPh (3a) and
2
2
4
2
n
NBu (3b)] in moderate yields, whereas the similar reac-
tion of A with one PCP diphosphine (Ph P) CH (dppm)
31
1
2
2
2
rarely produced a diphosphine-bridge diiron complex
Fe (μ-Me pdt)(CO) (μ-dppm) (4b) in low yield (see route
For the phosphine-monodentate complexes 1a/1b
and 2a/2b, their FT-IR spectra show four absorption
bands in the range of 2046–1920 cm− for their terminal
carbonyls (Table 1), the first band of which is each
observed at about 2045 cm− as reported in diiron mono-
2
2
4
28]
[
(
iii) in Scheme 1).
solutions of
Ph P) NPh and dppm gave rise to the similar bridge
By comparison, the refluxing xylene
and two different diphosphines
1
A
(
2
2
1
complexes Fe (μ-Me pdt)(CO) (μ-(Ph P)X) [X = NPh
2
2
4
2
(
(
4a) and CH (4b)] in high yields, as displayed in route
iv) of Scheme 1.
phosphine analogues of the type Fe (μ-dithiolate)
2
2
1
[24–26,38–41]
(CO) (k -monophosphine).
The average νC ≡ O
5
Thus, we found that i) at mild condition such as
values of 1a/1b and 2a/2b exhibit the order of 1b > 2a >
1a > 2b (Table 1), apparently due to the fact that
the electron-donating abilities of these phosphines
with the following order of P(C H Cl-p) < Ph P(NHPh)
Me NO assistant, the oxidative decarbonylations of
3
A with common monophosphines and small bite-angle
diphosphines formed the monodentate complexes 1a/1b
and 2a/2b, respectively. And ii) at forcing conditions
such as UV irradiation and refluxing, the photolytic and
pyrolytic decarbonylations of A with the PNP or PCP
ligands produced the chelate complexes 3a/3b and the
bridge counterparts 4a/4b respectively, wherein the PCP-
diphosphine ligand (dppm) is only bridged to two iron
6
4
3
2
< P(C H Me-p) < dppm. This finding implies that the
6
4
3
small bite-angle diphosphine like dppm has stronger
coordination effect relative to the common mono-
phosphine, which might well tune the redox properties of
the iron core in diiron phosphine complexes. In the
31
1
meanwhile, the
P{ H} NMR spectra of the
cores in 4b whereas the PNP-diphosphine ligands
monophosphine-monodentate complexes 1a, 1b, and 2a
display only one strong singlet at δP 62.6, 64.6, and
91.5 ppm for their P atoms attached to one iron core
n
(
(Ph P) NR, R = Ph and Bu ) are not only bridged to two
2
2
iron centers but also chelated to one iron center in 4a

6
of 12
HU ET AL.
3
1
1
TABLE 1 IR (νC ≡ O) and P{ H} NMR data of complexes 1a/1b, 2a/2b, 3a/3b, and 4a/4b
1
31
1
Complex
Fe (μ-Me
IR for νC ≡ O (average νC ≡ O, cm−
)
P{ H} NMR (δ/ppm)
2
2
1
pdt)
2041/1979/1955/1928 (1976)
62.6
64.6
91.5
(
CO)
(μ-Me
1b)
5
{k -P(C
6
H
4
3
Me-p) }(1a)
1
Fe
2
2
2
pdt)(CO)
5
{k -P(C
6
H
4
Cl-p)
3
}
2046/1983/1960/1932 (1980)
2043/1983/1957/1930 (1978)
(
1
Fe
2
(μ-Me
2a)
pdt)(CO)
5
{κ -Ph
2
P(NHPh)}
(
1
Fe
Fe
2
(μ-Me
2
2
pdt)(CO)
pdt)(CO)
5
4
(κ -dppm) (2b)
2040/1979/1954/1920 (1973)
2016/1949/1941/1912 (1955)
57.9, −26.3
2
2
(μ-Me
{κ -(Ph
2
P)
2
NPh}
116.5, 98.2
(
3a)
2
n
Fe
(
2
(μ-Me
3b)
2
2
2
pdt)(CO)
pdt)(CO)
pdt)(CO)
4
4
4
{κ -(Ph
2
P)
2
NBu }
2014/1946/1935/1911 (1952)
1997/1961/1925/1912 (1949)
1988/1953/1917/1904 (1941)
110.6, 98.7
124.5
Fe
(
2
(μ-Me
4a)
(μ-(Ph
2 2
P) NPh)
Fe
2
(μ-Me
(μ-dppm) (4b)
54.5
(
Table 1), respectively. By contrast, two well-resolved
98.7 ppm (Table 1) ascribed to the apical-basal and basal-
2
doublets with the JPP value of 78 Hz are observed at δP
7.9 and − 26.3 ppm for the coordinated and free P atoms
basal isomers with the isomeric ratio from 1.6:1 in 3a to
31
5
3.5:1 in 3b, which matches well with the P NMR signal
31
1
in the P{ H} NMR spectrum of the diphosphine-
monodentate complex 2b (Table 1), being consistent with
the known dppm-monosubstituted diiron complexes of
the type Fe (μ-dithiolate)(CO) (k -dppm) with pdt, edt,
odt or adt
patterns observed in the known PNP-chelate diiron com-
2
plexes Fe (μ-xdt)(CO) {k -(Ph P) NR} with pdt, edt, odt
2
4
2
2
NR
[22,23,25–28,42]
or adt bridges.
The outcome demonstrates
1
that the PNP-diphosphine ligands chelated to one iron
core exit in the more favorable apical-basal configuration.
2
NR
5
[19–21,28]
1
bridges.
In addition, the H NMR
1
spectra of 1a, 1b, 2a, and 2b all display two sets of aryl
Furthermore, the H NMR spectra of 3a and 3b exhibit
proton signals in the downfield region of δ 7.8–7.1 ppm
two sets of aryl proton signals in the range of δH
H
for their phenyls of the phosphine ligands and two
7.8–7.4 ppm assigned to their PC H moieties of the PNP
6
5
groups of alkyl proton signals in the upfield region of δH
ligands and two sets of alkyl proton signals within the
scope of δH 2.1–1.5 ppm plus two singlet signals at δH
1.40/1.26 (for 3a) or 0.88/0.64 (for 3b) ppm attributed to
their (SCH ) C(CH ) units of the dithiolate bridges. In
2
.0–1.1 ppm together with two singlet signals in the more
upfield region of δ 1.0–0.4 ppm for their methenes and
H
1
methyls of the Me pdt bridges. In the H NMR spectra of
2
2 2
3 3
1
2
a and 2b, one additional doublet formed due to the
addition to these H NMR signals described above,
phosphorus-hydrogen coupling can be observed at δ
another a set of aryl proton signal appears at δ 6.93 ppm
H
H
2
5
(
.49 ppm ( J = 18 Hz for 2a) and at δH 3.14 ppm
JPH = 7.8 Hz for 2b), respectively, which are assigned to
for the NC H group in 3a whereas there are three sets of
PH
6
5
2
the residual alkyl proton signals at δH 2.90, 1.26,
0.99 ppm with the integral ratio of 2:4:3 for the
NCH (CH ) CH unit in 3b.
their respective NH and PCH moieties.
2
For the diphosphine-chelate complexes 3a and 3b,
2
2 2
3
their IR spectra show four absorption bands in the
For the diphosphine-bridge complexes 4a and 4b,
−
1
region of 2016–1911 cm
for their iron carbonyls
value of approximately
their IR spectra display the similar carbonyl absorption
−
1
(
2
Table 1), wherein the first ν
patterns in the region of 1997–1904 cm while the
C ≡ O
015 cm− is well accordant with those of the previously-
1
31
P{ H} NMR spectra give only a sharp singlet at δ 124.5
1
P
2
reported diiron chelate complexes Fe (μ-xdt)(CO) (k -
diphosphine) (xdt = pdt, edt, odt, adt ).
or 54.5 ppm (Table 1), being in well agreement with the
observation in the reported diiron bridge complexes
Fe (μ-xdt)(CO) {μ-(Ph P) X} (X = CH , NR) with pdt, edt,
2
4
NR [16,17,22,23,25–28,42]
−
1
The average νC ≡ O value (1955 cm ) of 3a is slightly big-
2
4
2
2
2
−
1
NR
[18–20,22,23,26,28]
ger than that (1952 cm ) of 3b (Table 1), clearly resulting
odt, adt
bridges.
It is also seen from
from the electron-donating effect of the N-substituent
Table 1 that the average νC ≡ O value of the PNP-bridge
complex 4a is bigger than those of the PCP-bridge ana-
logue 4b, possibly owing to the fact that the weaker
electron-donating abilities of the PNP ligands such as
(Ph P) NPh relative to the PCP ligand like dppm lead to
n
(
NR) with the following order of NPh (3a) < NBu (3b) in
31
1
their (Ph P) NR ligands. At the same time, the P{ H}
2
2
NMR spectra of 3a and 3b both display a major singlet at
δ 116.5 or 110.6 ppm and a minor singlet at δ 98.2 or
P
P
2
2

SYNTHESIS, STRUCTURES, AND ELECTROCATALYTIC PROPERTIES
7 of 12
the lower back-donating π-bonding electron transfer from
1
Fe to CO. Meanwhile, in the H NMR spectra of 4a and
4
b, two or three sets of aryl proton signals are observed
in the downfield region of δ 7.9–7.0 ppm for their PPh2
H
units of the diphosphine ligands, while three sets of alkyl
proton signals are found in the upfield region of δH
2
.4–1.0 ppm for their Me pdt groups of the dithiolate
2
1
bridges. Additionally, the H NMR spectrum of 4a gives
three redundant sets of the phenyl proton signals at δH
6
.77, 6.58, and 6.09 ppm with the integral ratio of 1:2:2
for its NPh group of the PNP ligand, whereas that of 4b
shows two broad singlets at δH 3.71 and 3.19 ppm with
the integral ratio of 1:1 for its PCH moiety of the dppm
2
ligand.
3
1
.3 | Molecular structures of complexes
a, 1b, 2a, 3b, 4a, and 4b
FIGURE 2 Molecular structure of the diphosphine-chelate
complex 3b with thermal ellipsoids at 50% probability. Hydrogen
atoms are omitted for clarity. Selected bond lengths (Å) and bond
angles (deg): Fe(1)-Fe(2) 2.5048(6); P(1)-Fe(2)-P(2) 72.09(3); P(1)-
N(1)-P(2) 99.17(13)
The solid-state structures of 1a, 1b, 2a, 3b, 4a, and 4b
have been unambiguously determined by X-ray crystal-
lography, as illustrated in Figures 1–3 with the selected
bond lengths and bonds angles.
As shown in Figure 1, the phosphine-monodentate
complexes 1a, 1b, and 2a are composed of a butterfly-
shaped [Fe S ] skeleton with a monodentate phosphine,
to that of 1b (2.5065(13) Å) but is shorter than that of 2a
(2.4853(5) Å), which all are much shorter in comparison
to those found in natural [FeFe]-hydrogenases
2
2
[
2,3]
a dimethylpropanedithiolate bridge, and five terminal
carbonyls, being in accordance with the known
phosphine-monosubstituted diiron complexes of the type
(2.55–2.62 Å).
This finding suggests that the
P(C H R) monophosphines in 1a and 1b have the dis-
6
4
3
tinct electron-donating effect relative to the dppm
diphosphine in 2a, as observed in their IR spectra
1
Fe (μ-dithiolate)(CO) (k -
monophosphine).
2
5
[
24–26,28,29,38–41]
Clearly, the P atoms of
described above. Furthermore, the
P
(1)-Fe(2)-
apical
o
o
the monodentate ligands such as P(C H Me-p) in 1a,
Fe(1) bond angles of 1a, 1b, and 2a are 0.8 , 2.13 , and
6
4
3
o
P(C H Cl-p) in 1b, and Ph P(NHPh) in 2a are all bound
4.84 smaller than their Capical(1)-Fe(1)-Fe(2) ones respec-
6
4
3
2
in the apical position of the distorted-square-pyramidal
coordination geometry of the Fe(2) atom. Notably, the
Fe(1)-Fe(2) bond length of 1a (2.5022(5) Å) is very closer
tively, obviously due to the observation that the
bridgehead-C substituents (CMe ) of the dithiolate brid-
2
ges in them are all pushed away from the apical position
FIGURE 1 Molecular structures of the phosphine-monodentate complexes 1a (left), 1b (middle), and 2a (right) with thermal ellipsoids
at 50% probability. Hydrogen atoms and solvent molecules are omitted for clarity. Selected bond lengths (Å) and bond angles (deg): Fe(1)-
Fe(2) 2.5022(5) for 1a, 2.5065(13) for 1b, 2.483(5) for 2a; P(1)-Fe(2)-Fe(1) 157.95(2) for 1a, 155.60(5) for 1b, 154.83(3) for 2a; C(1)-Fe(1)-
Fe(2) 158.75(10) for 1a, 157.73(16) for 1b, 159.67(10) for 2a

8
of 12
HU ET AL.
FIGURE 3 Molecular structures of the diphosphine-bridge complexes 4a (left) and 4b (right) with thermal ellipsoids at 50% probability.
Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg): Fe(1)-Fe(2) 2.4656(13) for 4a, 2.4958(6) for 4b;
P(1)-N(1)-P(2) 118.8(3) for 4a; P(1)-C(10)-P(2) 115.85(16) for 4b
o
o
of the monodentate ligands and towards the Fe (CO) site
(72.09(3) ) and P-N-P (99.17(13) ) in 3b are characteristic
3
(
Figure 1).
As illustrated in Figure 2, the diphosphine-chelate
complex 3b contains a typical butterfly [Fe S ] cluster
of the reported diiron dithiolate complexes with chelating
[
22–28,42–44]
small bite-angle diphosphines like (Ph P) NR.
2
2
As depicted in Figure 3, the diphosphine-bridge com-
2
2
with a chelating PNP ligand, a Me pdt bridge, and four
plexes 4a and 4b consist of a similar [Fe S ] framework to
2
2 2
n
terminal carbonyls. The (Ph P) NBu ligand of 3b situ-
3a except for a bridging diphosphine instead of a chelat-
ing one. The (Ph P) NPh or dppm ligands of 4a and 4b
2
2
ates 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 phos-
2
2
lie in a cisoid basal-basal fashion and are bridged symmet-
rically to two Fe centers in the crystalline state, being
31
1
accordant with the above solution P{ H} NMR observa-
tion that only one phosphorus signal at δP 124.5 or
54.5 ppm is attributed to the two chemically equivalent P
atoms in 4a and 4b (Table 1). Furthermore, the cisoid
dibasal coordination manners of the (Ph P) NPh or dppm
phorus signal at δ 110.6 ppm for its apical-basal isomer
P
31
1
in the aforementioned solution P{ H} NMR spectrum of
b (Table 1). Moreover, the apical-basal coordination
3
mode of the PNP ligand in 3b is comparable to those of
2
2
NR
the reported diiron chelate complexes with edt or adt
bridges,
ligands in 4a and 4b match well with those of the known
[27,43,44]
but is distinct from the basal-basal config-
diiron dithiolate complexes with bridging small bite-angle
[
18,20,22,23,26,28]
uration of the related analogues with pdt or odt
diphosphines such as (Ph P) NR and dppm.
2
2
[22–26,28,42]
bridges.
This result implies that the coordina-
Meanwhile, the Fe(1)-Fe(2) bond length of the PNP-
tion geometry (apical-basal or basal-basal) of the chelat-
ing diphosphines probably depends on the steric
hindrance interaction between the diphosphine ligands
and the dithiolate bridges. In the meanwhile, the Fe(1)-
bridge complex 4a (2.4656(13) Å) with Me pdt bridge is
2
akin to those of the known PNP-bridge analogues (ca.
[
22,23,26,28]
2.47 Å) with pdt bridge,
but is much shorter
than those of the dppm-bridge analogue 4b (2.4958(6) Å)
and other related dppm-bridge complexes (ca.
Fe(2) bond length of 3b (2.5048(6) Å) with Me pdt bridge
2
[27]
NR
[18,20,28]
is closer to its chelate analogues with edt bridge,
but is
2.48–2.52 Å) with edt, pdt, odt or adt bridges.
o
apparently shorter in contrast to the known diiron che-
Notably, the P-N-P bite angle of 4a (118.8(3) ) is similar
NR
late complexes with pdt, odt, adt bridges as well as the
to the P-C-P one of its dppm-bridge analogue 4b
[2,3,22–26,28,42–44]
o
o
natural [FeFe]-hydrogenase enzymes.
It is
(115.85(16) ) but is significantly larger up to 19.62 than
o
worth noting that the small bite angles of P-Fe-P
that of its PNP-chelate counterpart 3b (99.18(12) )

SYNTHESIS, STRUCTURES, AND ELECTROCATALYTIC PROPERTIES
9 of 12
I
I
I
0
mentioned above. It is therefore noted that the opening-
up of the P-N-P bite angle from the bridge 4a to the che-
late 3b is apparently due to the following observation that
a less steric five-membered Fe P N ferracycle is produced
process of Fe Fe to Fe Fe based on the reported similar
cases.
[
22,23,25–28,41,45–47]
It can be noticed from Table 2
that the monodentate complex 2a exhibits a more posi-
tive reduction potential (E ) of about 360 mV in contrast
2
2
pc
through one bridging PNP ligand and two Fe atoms in
the former (Figure 3), whereas a more steric four-
to the bidentate complexes 3a and 4a, thus indicating
that the electron density of the iron core of 2a is lower
than those of 3a and 4a, as consistent with the above IR
results (Table 1). In their anodic regions of CVs, the first
oxidation potential (Epa1) shows the following order of 3a
membered FeP N ferracycle is formed by one chelating
2
PNP ligand and one Fe atom in the latter (Figure 2).
(−0.14 V) < 2a (+0.34 V) < 4a (+0.41 V) (Table 2),
3
.4 | Electrochemical and
suggesting that the chelate complex 3a seems to be easily
oxidized so as to favorably accept proton in the
electrocatalytic process.
electrocatalytic studies of complexes 2a, 3a,
and 4a
To investigate the electocatalytic ability of proton
For comparison with the effect of three coordination
modes (monodentate, chelate, bridge) of phosphine
ligands on the electrochemical and electrocatalytic prop-
erties of diiron model complexes, the cyclic vol-
tammetries (CVs) of three representative complexes 2a,
reduction to H by complexes 2a, 3a, and 4a, their elec-
trochemical behaviors were observed in the presence of
0–10 mM TFA through CV, as illustrated in Figure 5. The
2
initial reduction peak currents (i ) of 2a, 3a, and 4a at
cat
Epc = ca. −1.76, −2.12, and − 2.05 V increase rapidly
upon sequential additions of TFA (0–10 mM) and have a
linear correlation with TFA concentration (insert in
3
a, and 4a with pendant N-phenyl groups were done in
the absence or presence of trifluoroacetic acid (TFA) as a
proton source in the [n-Bu N][PF ]/MeCN solution at
Figure 5). This result suggests that
a
typical
4
6
−
1
the scan rate of 0.1 V s , as displayed in Figures 4 and 5.
Their relevant electrochemical and electrocatalytic data
are listed in Table 2.
electrocatalytic process for proton reduction to H takes
2
[
48–51]
place,
that is to say, complexes 2a, 3a, and 4a are
all found to be electrocatalytically active under TFA as a
proton source. Additionally, relative to the monodentate
2a (Figure 5-I), the CV curves of the bidentate 3a and 4a
(Figures 5-II and 5-III) display that a new reduction peak
is found at Epc = ca. -1.66 V possibly attributed to the for-
mation of their protonated species upon the addition of
The redox properties of 2a, 3a, and 4a are studied by
using CV techniques in the [n-Bu N][PF ]/MeCN solu-
4
6
tion without TFA, as shown in Figure 4. In their cathodic
regions of CVs, the first irreversible reduction peak (Epc)
is found at −1.76 V for 2a (blank line), −2.12 V for 3a
[
28,48,49]
(red line), and − 2.05 V (blue line) for 4a (Table 2),
2 mM TFA,
gradually with increasing of TFA concentration
2–10 mM) but are always lower than those of the initial
the catalytic currents of which raise
respectively, which are all ascribed to the reduction
(
reduction peaks at Epc = ca. -2.12 or − 2.05 V mentioned
above. This observation indicates that an additional and
minor electrocatalytic process at around −1.66 V happens
and is accompanied by a main electrocatalysis at about
−2.12 or − 2.05 V in the presence of 3a or 4a as catalysts
and TFA as proton source.
More importantly, the overpotential and turnover fre-
quency (TOF) are investigated in order to give a further
estimation of the catalytic proton reduction activities of
2a, 3a, and 4a as the active electrocatalysts. First of all,
the overpotentials of proton reduction to H catalyzed by
2
them can be calculated by the difference between theo-
retical half-wave reduction potential for TFA reduction
T
(
E1/2 ) and the observed potential at half-maximum of
the catalytic current (E
) for TFA reduction in the
cat/2
52,53]
[
presence of 2a, 3a, and 4a,
wherein the equation is|
T
E1/2 – Ecat/2|. It is seen from Table 2 that the over-
potentials of 2a, 3a, and 4a in 10 mM TFA-MeCN solu-
tion are calculated to be 0.89, 0.71, and 0.86 V,
respectively, based on the following fact that (i) the value
FIGURE 4 CVs of 1.0 mM 2a (blank line), 3a (red line), and
4
4 6
a (blue line) in 0.1 M [n-Bu N][PF ]/MeCN solution at a scan rate
−1
0/+
of 0.1 V s . All potentials are versus ferrocene/ferrocenium (Fc
couple
)

10 of 12
HU ET AL.
FIGURE 5 CVs of 1.0 mM 2a (I), 3a (II), and 4a (III) with TFA (0–10 mM) in 0.1 M [n-Bu
4
N][PF
6
]/MeCN solution at a scan rate of
−1
0
.1 V s . Insert in (I)-(III): Plots of catalytic currents (icat, μA) at around −1.76, −2.12, −2.05 V with increasing concentrations of acid
0
/+
(
[CF
3
CO
2
H], mM) for 2a, 3a, and 4a, respectively. All potentials are versus ferrocene/ferrocenium (Fc ) couple
a
TABLE 2 Relevant electrochemical and electrocatalytic data for complexes 2a, 3a, and 4a
2
3
K )
obs (TOF, s−1
Complex
E
pc (V)
E
pa1 (V)
i
p
(μA)
i
cat (μA)
(icat/i
6.20
p
)
max
E
cat/2 (V)
Overpotential (V)
2a
3a
4a
-1.76
−2.12
−2.05
+0.34
−0.14
+0.41
47.62
295.10
252.32
300.70
−1.69
−1.51
−1.66
0.89
0.71
0.86
7
42.90
41.32
+
5.88
7
7.28
10
a
All the data for 2a-4a (1 mM) are versus the Fc/Fc couple in 0.1 M [n-Bu
4
N][PF
6
]/MeCN.
b
p
i is defined as the reduction peak current at the initial reduction potential (Epc) without acid.
c
i
cat is defined as the largest catalytic peak current at the highest concentration of TFA (10 mM).
T
of E1/2 for 10 mM TFA in MeCN is calculated to be
causing a decrease up to 180 mV of overpotential in com-
parison to 2a and 4a.
[52,53]
−
0.80 V (see the supporting information),
ii) the catalytic half-wave potentials (Ecat/2) of 2a, 3a,
and 4a determined for 10 mM TFA are found to be
1.69, −1.51 and − 1.66 V respectively (Figures S1-S3).
This outcome suggests that the chelate complex 3a may
more efficiently catalyze proton reduction to H2 by
and
(
In the meanwhile, the turnover frequencies (TOF) for
H production catalyzed by 2a, 3a, and 4a are estimated
2
−
through the kinetic rate constant (kobs) of hydrogen evo-
lution reaction with them as electrocatalysts, which are
determined by plotting the concentration of added acid
(
[TFA]) against the ratio of catalytic current (i ) versus
cat
[
53–55]
reduction current (ipc).
On the basis of the above
information, complexes 2a, 3a, and 4a all display a linear
relationship between [TFA] and the i /i ratio as the
cat
p
TFA concentration is changed from 2 to 10 mM
Figure 6), indicating that they all do not decompose in
(
this range. Moreover, at this linear level, the kobs (TOF)
value of 2a, 3a, and 4a can be calculated by using the fol-
2
[53–55]
lowing equation: kobs = 1.94ν(i /i ) .
It is therefore
cat
p
seen from Table 2 that the i /i values of 2a, 3a, and 4a
cat
p
are found to be 6.20, 5.88, and 7.28 at the highest concen-
tration of TFA (10 mM); correspondingly, the k (TOF)
obs
−
1
value of 4a is calculated to be 10 s , which is slightly
−
1
larger than those of 2a and 3a (ca. 7 s ), suggesting that
these complexes have a similar electrocatalytic efficiency
for proton reduction to H2.
In addition, we further investigated the CV perfor-
mance of the chelate complex 3b in the presence of
0–10 mM TFA (see Table S2 and Figures S4, S5) and
FIGURE 6 Plots of icat/i
p
versus [TFA] (mM) for 1.0 mM 2a
(
[
blank line), 3a (red line), and 4a (blue line) in 0.1 M [n-Bu N]
4
]/MeCN solution at a scan rate of 0.1 V s−
1
found that it has
a
similar CV feature and
PF
6

SYNTHESIS, STRUCTURES, AND ELECTROCATALYTIC PROPERTIES
11 of 12
electrocatalytic property to its chelate analogue 3a men-
ORCID
tioned above.
Pei-Hua Zhao https://orcid.org/0000-0002-5480-6128
Xu-Feng Liu https://orcid.org/0000-0002-9717-5645
4
| CONCLUSIONS
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We are grateful to the National Natural Science Founda-
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[
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