Protonation/Reduction of Carbonyl-Rich Diiron Complexes and the Direct Observation of Triprotonated Species: Insights into the Electrocatalytic Mechanism of Hydrogen Formation

Article abstract of DOI:10.1021/acscatal.5b02646

Both the reduced and the protonated states of diiron dithiolate complexes, which are key intermediate species for the electrocatalytic production of hydrogen, have been spectroscopically and theoretically investigated in this study. Five important states

Full text of DOI:10.1021/acscatal.5b02646

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Article
Protonation/Reduction of Carbonyl-Rich Diiron Complexes
and the Direct Observation of Triprotonated Species: Insights
into the Electrocatalytic Mechanism of Hydrogen Formation
Yu-Chiao Liu, Kai-Ti Chu, Yi-Lan Huang, Cheng-Huey Hsu,
Gene-Hsiang Lee, Mei-Chun Tseng, and Ming-Hsi Chiang
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 03 Mar 2016
Downloaded from http://pubs.acs.org on March 3, 2016
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Protonation/Reduction of Carbonyl-Rich Diiron Complexes and the Direct
Observation of Triprotonated Species: Insights into the Electrocatalytic
Mechanism of Hydrogen Formation
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†
†,‡,§
†
†
║
Yu-Chiao Liu, Kai-Ti Chu,
Yi-Lan Huang, Cheng-Huey Hsu, Gene-Hsiang Lee, Mei-
†
,†,‡
Chun Tseng, Ming-Hsi Chiang*
†
Institute of Chemistry, Academia Sinica, Nankang, Taipei 115, Taiwan.
Molecular Science and Technology Program, TIGP, Institute of Chemistry, Academia Sinica,
Nankang, Taipei 115, Taiwan
Department of Chemistry, National Tsing Hua University, Hsinchu 300, Taiwan
‡
§
║
Instrumentation Center, National Taiwan University, Taipei 106, Taiwan
*To whom correspondence should be addressed. E-mail: mhchiang@chem.sinica.edu.tw.
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Abstract
Both the reduced and the protonated states of diiron dithiolate complexes, which are key
intermediate species for the electrocatalytic production of hydrogen, are spectroscopically and
theoretically investigated in this study. Five important states in the process of H
characterized. In the presence of a super acid, protonation occurs onto the Fe-Fe vector of [(μ-
xdt)Fe (CO) ] (xdt = pdt, 1,3-propanedithiolate; edt, 1,2-ethanedithiolate; bdt, 1,2-
2
evolution are
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benzenedithiolate) to yield the cationic μ-H species (the C state). A single reduction at 193 K
leads to the neutral species (the CE state), with similar structures for the pdt and edt bridgeheads.
The CE species of the bdt analog is unstable under the same condition. An open structure
resulting from the rupture of one Fe-S bond is suggested by DFT calculations. Subsequently, a
second reduction induces a dramatic structural rearrangement in which the CEE state possesses
an open structure exhibiting a μ-H and a μ-CO group. Protonation onto the terminal sulfur site of
the CEE state affords the CEEC state, which readily converts to the parent hexacarbonyl
complex accompanied by the liberation of H
2
at higher temperatures. In the presence of excess
acid, the CEECC state is achieved and the third proton is coordinated to the Fe center. The S-
1
2
proton and Fe-hydride are characterized by H- and D-NMR spectroscopy. Electrocatalytic
hydrogen production involving the CEEC and CEECC states is investigated by DFT calculation.
In combination with the spectroscopic results, this information allows us to construct the possible
catalytic routes and study the plausible role of the triply protonated species least explored in
biomimetic catalysis.
Keywords: hydrogen evolution, electrocatalysis, hydrogenases, hydrides, protonation, iron
complexes.
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Introduction
Iron hydride species have been implicated as key intermediates for energy transfer and
storage in biocatalysis. For instance, in hydrogenases, the Fe hydride state is generated when the
reductive production or heterolytic cleavage of molecular hydrogen takes place.1-3 In biological
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state,4-6
nitrogen fixation, the FeMo cofactor stores reducing equivalents as Fe hydrides in the E
4
7
which are used for the subsequent activation of coordinated N
2
substrates. The Fe hydride states
are transient and difficult to characterize because the enzymatic reactions involve rapid and
8
efficient turnovers. The catalytic hydride intermediates have thus remained relatively unknown
despite the substantial effort expended on their study.
Synthetic iron hydride complexes, in contrast, have been characterized and widely
employed in organic syntheses since Fe(H)
2
(CO) , the first transition-metal hydride complex,
4
was prepared by Hieber in the 1930s.9,10 These complexes have proven to be attractive
successors to homogeneous catalysts containing precious metals, as they display superior
reactivity and chemo-/regio-selectivity compared to heterogeneous catalysts in a wide variety of
catalytic reactions. Iron hydrido complexes play important roles in the hydrogenation,11-15
hydrosilylation16-18 and hydroboration19-21 of unsaturated hydrocarbons, ketones and aldehydes,
the hydrogenation of bicarbonates and carbon dioxide,22-24 and C-C/C-E (E = heteroatom) cross-
coupling reactions.25-28
In the reversible biocatalytic cycle of proton reduction/hydrogen oxidation, three states
have been characterized for [FeFe]hydrogenases with respect to the oxidation state of the Fe
centers.29 The most oxidized (Hox) state possesses a mixed-valence Fe Fe subset in the H-
I
II
cluster.30,31 The reduced (Hred) state is achieved upon a one-electron reduction, in which an EPR
I
I
32,33
inactive Fe Fe configuration is realized.
H-cluster remains oxidized. A subsequent reduction event at a more negative potential converts
the Hred state to the super-reduced (Hsred) state. In the transition, the Fe subset maintains an
In both states, the attached [4Fe4S] subcluster in the
2
invariant oxidation state while the [4Fe4S] cluster is singly reduced to +1.34,35 The Hsred state has
been clearly identified only in eukaryotic [FeFe]hydrogenase (Chlamydomonas reinhardtii,
CrHydA1) but not in its prokaryotic homologs from Desulfovibrio desulfuricans (DdH) and
Clostridium pasteurianum (CpI). It has been conjectured that the determining factor for the
observation of the Hsred state is the inherent protein environment in distinct organisms, namely,
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whether auxiliary iron-sulfur clusters (the F-cluster) are situated near the H-cluster. In bacterial
FeFe]hydrogenases, the oxidized [4Fe4S] cluster is quickly reinstated by the neighboring F-
cluster, affording the Hred state and making the Hsred state the transient species.
FeFe]hydrogenases in algae, which lack such auxiliary iron-sulfur clusters, instead terminate
electron shuttling in and out of the H-cluster, making detection of the Hsred state possible. The
sred state has been characterized as consisting of a {Fe } subunit bearing a bridging hydride
[
[
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H
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based on recent infrared and site-selective X-ray absorption/emission spectroscopic
measurements.35-37 The results of protein film voltammetry and chronoamperometry indicate that
the catalytic current does not reveal a substantial decrease, and high turnover rates of hydrogen
production are recorded at reduction potentials that are negative enough to reach the Hsred state.38
These findings suggest the participation of the Hsred state in catalysis. Interestingly, this
assignment is in sharp contrast to the common understanding that the related {Fe
is energetically stable and less reactive for catalysis.39-43 Instead, the Fe
structure with a rotated
geometry and a terminally bound hydride is responsible for H
production.44
2
(μ-H)} moiety
2
2
This finding regarding the Hsred state has rejuvenated the chemistry of synthetic bridging
hydrido diiron complexes, which can be prepared by the protonation of the corresponding [(μ-
xdt)Fe
2
(CO)6-x(L)
]n (xdt = dithiolate linkers; L = ligands; n = number of the charge).45-52 The
x
II
II
I
II
I
{
Fe (μ-H)Fe } synthetic mimics have been proposed to achieve the {Fe (μ-H)Fe } and {Fe (μ-
I
H)Fe } states under reducing conditions, which have recently been characterized by
spectroelectrochemical techniques.53 These species are presumably capable of undergoing
additional protonation steps to form doubly protonated54-58 and triply protonated products,59,60
I
I
and the subsequent liberation of H from these species restores the parent Fe Fe state. These
2
proposed intermediates and mechanistic insights have only been established on the basis of
electrochemical and DFT calculation results. None of them has been characterized by
spectroscopy.
In this work, we describe the syntheses and characterization of a series of hydrido
intermediate species possessing the {Fe (μ-H)} moiety. The corresponding stepwise reduced
2
I
I
products, including the most reduced species with the {Fe (μ-H)Fe } core and its subsequent
double- and triple-protonation products, were investigated by in-situ low-temperature NMR, UV-
vis and FTIR spectroscopy combined with DFT calculations. The confirmation of H
2
release
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from these intermediates and recovery of the starting complexes demonstrated that all of them
are active catalysts in electrocatalytic processes for hydrogen production.
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Experimental Section
General Methods. All reactions were conducted using standard Schlenk and vacuum-line
techniques under an atmosphere of purified nitrogen. All commercially available chemicals from
Aldrich were of ACS grade and used without further purification. Solvents were of HPLC grade
and were purified as follows: hexane was distilled from sodium; and dichloromethane was
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distilled from CaH
molecular sieves under N
propanedithiolate, 1; edt, 1,2-ethanedithiolate, 2; bdt, 1,2-benzenedithiolate, 3) were prepared
2
under N
2
. Deuterated solvents obtained from Merck were distilled over 4 Å
2
prior to use. The complexes [(μ-xdt)Fe (CO) ] (xdt = pdt, 1,3-
2
6
according to related procedures in the literature.61,62 [(Et
H][B(C
F
5
3
Si)
2
6
4
) ] was prepared as
63
described in the literature. Infrared spectra were recorded on a Perkin Elmer Spectrum One
using a 0.05-mm CaF
2
cell. In-situ FTIR measurements were performed using a Mettler Toledo
ReactIR iC10 system equipped with a HgCdTe (MCT) detector and a 0.625-inch SiComp probe.
1
13
1
19
1
2
11
1
H, C{ H}, F{ H}, D and B{ H} NMR spectra were recorded on a Bruker AV-500 or
DRX-500 spectrometer operating at 500, 125.7, 470.59, 76.8 and 160.46 MHz, respectively. UV-
vis-NIR spectra were recorded on a Varian Cary 5000 spectrophotometer using a 1-cm quartz
cell. In-situ UV-vis spectra were recorded on a Varian Cary 5000 spectrophotometer equipped
with a Hellma All-Quartz Immersion Probe (661.302) at a low temperature. Mass spectral
analyses were performed on a Waters LCT Premier XE at the Mass Spectrometry Center in the
Institute of Chemistry, Academia Sinica. Elemental analyses were performed on a Thermo Fisher
EA1112 elemental analyzer. Continuous wave EPR measurements were performed at the X band
using a Bruker E580 spectrometer equipped with a Bruker ELEXSYS super-high-sensitivity
cavity. EPR spectra of 1μH–3μH in frozen CH
2
Cl solution were obtained in a 4 mm JY-type
2
EPR tube at 77 K. The following experimental conditions were used: for 1μH, a microwave
power of 0.1917 mW, microwave frequency of 9.397 GHz, conversion time of 20.48 ms,
receiver gain of 1002.374 and modulation amplitude of 10 G/100 kHz were used; for 2μH, a
microwave power of 0.1922 mW, microwave frequency of 9.439 GHz, conversion time of 20.48
ms, receiver gain of 10023.74 and modulation amplitude of 10 G/100 kHz were used; and for
3
μH, a microwave power of 19.14 mW, microwave frequency of 9.394 GHz, conversion time of
20.48 ms, receiver gain of 3990.525 and modulation amplitude of 10 G/100 kHz were used. EPR
spectra were simulated using the WINEPR SimFonia program.
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Molecular Structure Determinations. The X-ray single crystal crystallographic data collections
for [1μH][B(C
6
F
5
)
4
] (CCDC1424168), [2μH][B(C
6
F
5
)
4
] (CCDC1424167), [3μH][B(C
6
F
5
)
4
]
(CCDC1424056) and [2μH][CF
3
SO ] (CCDC1424169) were performed at 200 (for the first three)
3
or 150 K (for the last) on a Bruker SMART APEX CCD four-circle diffractometer with graphite-
monochromated Mo Kα radiation (λ = 0.71073 Å) and outfitted with a low-temperature nitrogen-
stream aperture. The structures were solved using direct methods in conjunction with standard
difference Fourier techniques and refined by full-matrix least-squares procedures. A summary of
the crystallographic data for all complexes is shown in Table S1. Selected metric data of each
structure are listed in the figure captions. The experimental data and the computed results are
summarized in Table S2 for comparison. An empirical absorption correction (multi-scan) was
applied to the diffraction data for all structures. All non-hydrogen atoms were refined
anisotropically, and all hydrogen atoms were placed in geometrically calculated positions using
the riding model. All software used for diffraction data processing and crystal structure solution
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and refinement are contained in the SHELXTL97 program suite. There are two asymmetric
+
]– anions and one CH
]. Only one of the complexes is discussed in the study. The C8 and C17 of the
propane substituent on the bridging propanedithiolate group were disordered. There was one
CH Cl solvent molecule co-existing in the unit cell of [2μH][B(C ]. The Cl2 of the co-
solvated CH Cl molecule is disordered. There were two CF SO H molecules co-existing in the
]. The F4–F6 and O8–O9 atoms of the co-solvated CF SO H molecule
1
μH cations, two [B(C
6
F
5
)
4
2
Cl
2
molecule in the unit cell of
[1μH][B(C
6
F )
5 4
2
2
6
F )
5 4
2
2
3
3
unit cell of [2μH][CF
3
SO
3
3
3
were disordered.
Electrochemistry. Electrochemical measurements were recorded on a CH Instruments 630C
electrochemical potentiostat using a gastight three-electrode cell under N at 298 K unless
2
otherwise stated. A vitreous carbon electrode (1 or 3 mm in diameter) and a platinum wire were
used as the working and auxiliary electrodes, respectively. The reference electrode was a
+
n
nonaqueous Ag /Ag electrode (0.01 M AgNO
3
/0.1 M [ Bu
4
N][B(C
6
F
5
4
) ]). All potentials were
n
measured in CH Cl
2
2
solution containing 0.1 M [ Bu
4
N][B(C
6
F )
5 4
]. The potentials are reported
+
against the ferrocenium/ferrocene (Fc /Fc) couple.
Computational Studies. All Kohn-Sham DFT calculations were performed using the
6
5
66,67
Gaussian09 suite of the ab initio program with a hybrid functional B3LYP
to obtain the
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geometry optimization and free energy. The Fe atom used the effective core potential and the
6
8
associated basis set of LANL2DZ, and the 6•31G(d,p) basis set was used for the O, S, C and H
atoms. The computational solvent effect was applied using the polarizable continuum model
(
PCM)69,70 for CH
2
Cl (ε = 8.93) in all calculated results. All geometries were fully optimized
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and applied for harmonic vibrational frequency calculations under 298 K. Transition states were
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explored using a quasi-Newton algorithm to obtain a single imaginary frequency. The orbital
contributions of atoms were analyzed by the QMForge program.72 Time-dependent DFT
calculation73,74 was employed to investigate vertical electronic transitions using the
functional/basis set combination. The first 50 excitations were calculated for the species. The
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computed transitions were analyzed by the SWizard program. The half-bandwidths, D1/2,I, were
–
1
taken to be 3000 cm . Validation of the proposed intermediates related to complex 2 and its
derivatives based on the comparison of computed vibrational frequency with the experimental
results was performed using both B3LYP and a GGA functional BP8676,77 with the same basis
set. The results using BP86 were in agreement with the results using B3LYP. The computed IR
spectra for the species, their optimized structures and the predicted vertical electronic transitions
are displayed in either the main text or the supporting section.
Synthesis of [(μ-pdt)(μ-H)Fe
2
(CO)
6
][B(C
6
5
F )
4
] ([1μH][B(C
6
F
5
)
4
]). First, [(μ-pdt)Fe
2
(CO)
6
]
(
(
100 mg, 0.25 mmol) and [(Et
3
Si) H][B(C
2
6
F
5
)
4
] (470 mg, 0.51 mmol) were dissolved in CH
2
Cl
2
4 mL) solution at room temperature. The resulting red-orange solution was stirred for 1 hr, and
the reaction was constantly monitored by FTIR spectroscopy until completion. The solution was
filtered and layered with hexane (15 mL). Orange-red crystals of [(μ-pdt)(μ-
H)Fe
2
(CO)
6
][B(C
6
F
5
)
4
]∙0.5CH
2
Cl
2
suitable for X-ray crystallographic analysis were grown at
–1
2
2
53 K. The yield was 210 mg (76 %). IR (CH
2
Cl
2
, cm ): νCO 2132 (m), 2111 (vs), 2082 (s),
1
069 (s), 2039 (sh); νBC 1645 (m). H NMR (500 MHz, CD
CH CH
S), 2.77 (t, 4H, 3JHH = 6.0 Hz, SCH
C{ H} NMR (125.77 MHz, CD Cl , 298 K): 21.73 (s, 2C, SCH CH CH S), 29.04 (s, 1C,
SCH CH CH S), 123.23 (br, 4C, ipso-B(C ),
), 135.84 (d, 8C, 1JCF = 242.5 Hz, m-B(C
37.77 (d, 4C, 1JCF = 243.1 Hz, p-B(C ), 147.67 (d, 8C, 1JCF = 241.0 Hz, o-B(C
), 195.88
(s, 4C, COba), 199.98 (s, 2C, COap) ppm. F{ H} NMR (470.52 MHz, CD , 298 K): –133.27
2
Cl
2
, 298 K): –15.52 (s, 1H, FeHFe),
2.32 (q, 2H, 3JHH = 6.0 Hz, SCH
CH CH S) ppm.
2
2
2
2
2
2
1
3
1
2
2
2
2
2
2
2
2
6
F
5
)
4
6
F )
5 4
1
6
F
5
)
4
6
F )
5 4
1
9
1
2
Cl
2
(
br s, o-B(C
6
F
5
)
4
), –164.37 (t, 3JFF = 19 Hz, p-B(C
6
F
5
)
4
), –167.58 (br s, m-B(C
6
F ) ) ppm.
5
4
1
1
1
+
+
B{ H} NMR (160.46 MHz, CD
2
Cl
2
, 298 K): –16.69 ppm. FAB -MS: m/z 386.8 {M} , 358.8
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+
+
+
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+
{
M – 1CO} , 330.8 {M – 2CO} , 302.8 {M – 3CO} , 274.8 {M – 4CO} , 246.8 {M – 5CO} ,
+
2
18.9 {M – 6CO} . Anal. Calcd for C67
H
16
B
2
Cl
2
F
40Fe
4
O
12
S : C, 36.30; H, 0.73. Found: C, 36.27;
4
H, 0.69.
Synthesis of [(μ-edt)(μ-H)Fe
2
(CO)
6
][B(C
6
5
F )
4
] ([2μH][B(C
6
F
5
)
4
]). First, [(μ-edt)Fe
2
(CO)
6
]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(
(
100 mg, 0.26 mmol) and [(Et
3
Si) H][B(C
2
6
F
5
)
4
] (490 mg, 0.53 mmol) were dissolved in CH
2
Cl
2
4 mL) solution at room temperature. The resulting red-orange solution was stirred for 2 hr, and
the reaction was constantly monitored by FTIR spectroscopy until completion. The solution was
filtered and layered with hexane (18 mL). Orange crystals of [(μ-edt)(μ-
H)Fe
2
(CO)
6
][B(C
6
F
5
)
4
]∙CH
2
Cl
2
suitable for X-ray crystallographic analysis were grown at 253 K.
–1
The yield was 250 mg (89 %). IR (CH
2
Cl
2
, cm ): νCO 2134 (m), 2113 (vs), 2084 (s), 2074 (s),
1
2036 (sh); νBC 1644 (m). H NMR (500 MHz, CD
2
Cl
2
, 298 K): –17.93 (s, 1H, FeHFe), 3.25 (s,
Cl , 298 K): 39.08 (s, 2C, S(CH S),
), 136.72 (d, 8C, 1JCF = 242.6 Hz, m-B(C ), 138.65 (d, 4C, 1JCF
), 148.55 (d, 8C, 1JCF = 240.5 Hz, o-B(C
), 197.59 (s, 2C, COba),
97.65 (s, 2C, COba), 198.85 (s, 2C, COap) ppm. F{ H} NMR (470.52 MHz, CD Cl , 298 K): –
133.95 (br s, o-B(C ), –168.41 (br s, m-B(C ) ppm.
), –164.56 (t, 3JFF = 19 Hz, p-B(C
B{ H} NMR (160.46 MHz, CD Cl , 298 K): –16.69 ppm. FAB -MS: m/z 372.8 {M} , 344.8
1
3
1
4
1
H, S(CH
24.52 (br, 4C, ipso-B(C
243.6 Hz, p-B(C
2
)
2
S) ppm. C{ H} NMR (125.77 MHz, CD
2
2
)
2 2
6
F
5
)
4
6
F )
5 4
=
6
F
5
)
4
6
F )
5 4
1
9
1
1
2
2
6
F
5
)
4
6
F
5
)
4
6 5 4
F )
1
1
1
+
+
2
2
+
+
+
+
+
{
M – 1CO} , 316.8 {M – 2CO} , 288.8 {M – 3CO} , 260.8 {M – 4CO} , 232.8 {M – 5CO} ,
+
2
04.9 {M – 6CO} . Anal. Calcd for C33
H
5
BF20Fe
2
O
6
2
S : C, 36.54; H, 0.48. Found: C, 36.69; H,
0.86.
Synthesis of [(μ-bdt)(μ-H)Fe
2
(CO)
6
][B(C
6
F
5
)
4
] ([3μH][B(C
6
F
5
)
4
]). First, [(μ-bdt)Fe
2
(CO)
6
]
(
(
100 mg, 0.24 mmol) and [(Et
3
Si) H][B(C
2
6
F
5
)
4
] (450 mg, 0.49 mmol) were dissolved in CH
2
Cl
2
5 mL) solution at room temperature. The resulting red-orange solution was stirred for 4 hr, and
the reaction was constantly monitored by FTIR spectroscopy until completion. The solution was
filtered and layered with hexane (20 mL). Dark-red crystals of [(μ-bdt)(μ-H)Fe (CO) ][B(C
suitable for X-ray crystallographic analysis were grown at 253 K. The yield was 230 mg (88 %).
2
6
6
F
5
4
) ]
–1
1
IR (CH
2
Cl
Cl
C{ H} NMR (125.77 MHz, CD
2
, cm ): νCO 2135 (m), 2116 (vs), 2084 (s), 2042 (sh); νBC 1645 (m). H NMR (500
, 298 K): –13.44 (s, 1H, FeHFe), 7.06 (m, 2H, S ), 7.55 (m, 2H, S ) ppm.
Cl , 298 K): 123.49 (br, 4C, ipso-B(C
MHz, CD
2
2
2
C
6
H
4
2
C
6
H
4
1
3
1
2
2
6
F
5
)
4
), 130.07 (s, 2C,
), 137.76 (d, 4C,
S
2
C
6
H
4
), 130.95 (s, 2C, S
2
C
6
H
4
), 135.83 (d, 8C, 1JCF = 241.6 Hz, m-B(C
6
F
5
)
4
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1JCF = 243.7 Hz, p-B(C
B(C ), 195.39 (s, 4C, COba), 198.88 (s, 2C, COap) ppm. F{ H} NMR (470.52 MHz, CD
298 K): –133.86 (br s, o-B(C ), –168.33 (br s, m-
), –164.57 (t, 3JFF = 20 Hz, p-B(C
F
5
)
), 143.10 (s, 2C, ipso-S
C
6
H
), 147.68 (d, 8C, 1JCF = 243.5 Hz, o-
6
4
2
4
1
9
1
6
F
5
)
4
2
Cl
2
,
6
F
5
)
4
6
F )
5 4
1
1
1
+
B(C
6
F
5
)
4
) ppm. B{ H} NMR (160.46 MHz, CD
2
Cl
2
, 298 K): –16.66 ppm. FAB -MS: m/z
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
+
+
+
+
+
4
2
20.8 {M} , 392.8 {M – 1CO} , 364.8 {M – 2CO} , 336.8 {M – 3CO} , 308.8 {M – 4CO} ,
+
+
80.8 {M – 5CO} , 252.8 {M – 6CO} . Anal. Calcd for C36
H
5
BF20Fe
2
O
6
S
2
: C, 39.31; H, 0.46.
Found: C, 39.41; H, 0.80.
Reduction of [2μH][B(C
6
F
5
)
4
] by cobaltocene (Cp
2
Co). To a CH
Cl
atmosphere at 193 K. The resulting light green solution was stirred
for 10 min and monitored by in situ FTIR spectroscopy. The formation of 2μH was confirmed
2
Cl
2
solution (7 mL) of
[2μH][B(C F
6 5
)
4
] (80 mg, 0.076 mmol) was added a CH
2
2
solution (0.5 mL) of Cp
2
Co (14.5
mg, 0.076 mmol) under an N
2
–
1
according to the FTIR spectrum of the resultant solution. IR (CH
2
Cl
2
, cm , 193 K): νCO 2074
Co (19
(
m), 2045 (vs), 1995 (s). To this solution was added a CH Cl solution (0.5 mL) of Cp
2
2
2
mg, 0.1 mmol) under an N
2
atmosphere at 193 K. The solution species was converted to the
–
Cl
, cm–1,
doubly reduced species 2μH* according to the in-situ FTIR spectral results. IR (CH
193 K): νCO 2040 (m), 1991 (vs), 1960 (s), 1934 (sh), 1755 (w, br).
2
2
Reduction of [1μH][B(C
6
F
5
)
4
] or [3μH][B(C
6
F
5
)
4
] by cobaltcene (Cp
] and [3μH][B(C
, cm , 193 K): νCO 2072 (m), 2043 (vs), 1993 (s). For the species from one-
2
Co). Similar synthetic
procedures were applied for the reduction reactions of [1μH][B(C
6
5
F )
4
6
F ) ].
5 4
–
1
For 1μH: IR (CH
electron reduction of 3μH : IR (CH
species from two-electron reduction of 3μH : IR (CH
2
Cl
2
+
–1
2
Cl , cm , 193 K): νCO 2077 (m), 2041 (vs), 1999 (s). For the
2
+
–1
2
Cl
2
, cm , 193 K): νCO 2045 (w), 1995 (m),
–
1
967 (s), 1917 (vs), 1866 (vs), 1850 (sh), 1751 (w, br), 1665 (w, br). For 1μH* : IR (CH
2
Cl ,
2
–
1
–
cm , 193 K): νCO 2042 (m), 1992 (vs), 1972 (s), 1953 (s), 1934 (m), 1752 (w, br). For 3μH* : IR
–
1
(CH
2
Cl
2
, cm , 193 K): νCO 2045 (m), 1996 (vs), 1979 (s), 1963 (s), 1935 (m), 1750 (w, br).
–
Cl
solution of the doubly reduced species 2μH*–
Protonation of 2μH* with HOTf. To the CH
2
2
was added 1.1 equiv. of HOTf at 193 K. The resulting orange-brown solution was stirred for 10
min. The sulfur-protonated species 2μHSH* was generated according to the in-situ FTIR
–
1
spectral results. IR (CH
2
Cl , cm , 193 K): νCO 2067 (m), 2020 (vs), 1990 (s), 1969 (sh), 1777 (w,
2
br). Further protonation of 2μHSH* occurred when an additional 6 equiv. of HOTf was added by
+
microsyringe. The triply protonated species, 2μHtHSH* , was generated after 20 min according
1
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–1
to the in-situ FTIR spectral results. IR (CH
2
Cl
2
, cm , 193 K): νCO 2130 (s), 2097 (vs), 2082 (s),
2
075 (s), 2046 (m), 2036 (m).
–
–
Protonation of 1μH* or 3μH* with HOTf. Similar synthetic procedures were applied for the
–
–
protonation reactions of 1μH* and 3μH* . The species 1μHSH* was extremely unstable at 193
10
11
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16
17
18
19
20
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22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
–1
K and its IR bands could not be clearly identified. For 3μHSH*: IR (CH
2
Cl
2
, cm , 193 K): νCO
+
–1
2
070 (m), 2024 (vs), 2005 (s), 1973 (sh), 1780 (w, br). For 3μHtHSH* : IR (CH
2
Cl
2
, cm , 193
K): νCO 2132 (s), 2097 (vs), 2078 (s, br), 2041 (m, br).
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1
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2
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2
2
2
2
2
2
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5
5
5
6
Results and discussion
Synthesis and characterization of [(μ-xdt)(μ-H)Fe
+
+
+
2
(CO) ] (1μH –3μH )
6
Diiron dithiolato hexacarbonyl complexes are generally inert upon contact with
conventional acids, and protonation across the Fe-Fe vector of the Fe core typically occurs when
the basicity of the metal center has been enriched. It has been reported that the substitution of
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
7
8
two CO groups by PMe
3
increases the rate constant of protonation by 2 orders. Recent methods
developed by Heinekey et al., which allow for the preparation of protonated species of the diiron
7
9
hexacarbonyl complexes, require the use of either HCl gas or arenium ions in the presence of
SiEt ][B(C ] to generate super strong Brønsted acids that are used to activate the Fe-Fe bond.
We adopted a modified procedure based on this methodology to synthesize hexacarbonyl
bridging hydride complexes [(μ-xdt)(μ-H)Fe (CO) ][B(C ] (C state, xdt = pdt, 1,3-
propanedithiolate, [1μH][B(C ]; edt, 1,2-ethanedithiolate, [2μH][B(C ]; bdt, 1,2-
benzenedithiolate, [3μH][B(C ]) from their corresponding parent molecules.
[
3
6
F )
5 4
2
6
6
F )
5 4
6
F
5
)
4
6
F
)
5 4
6
F )
5 4
Scheme 1. Reaction scheme for the protonation-reduction processes involving the diiron
hexacarbonyl complexes with different bridgeheads.
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The CH
2
Cl
2
solution of [(μ-xdt)Fe
2
(CO)
6
] (xdt = pdt, 1; edt, 2; bdt, 3) was reacted with
[(Et
3
Si) H)[B(C
2
6
F
5 4
)
] to afford the protonated species in high yields (Scheme 1). The following
synthetic discussion on the C state focuses on the edt and bdt analogs because the complex 1μH+
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13
14
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16
17
18
19
20
21
22
23
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33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
has been reported by Heinekey et al. The resultant FTIR spectra exhibited carbonyl signature
–
1
+
–1
bands at 2134 m, 2113 vs, 2084 s, and 2074 s cm for 2μH and 2135 m, 2116 vs, 2084 s cm
for 3μH (Figures 1 and 6, Table 1). The shift of the ν(CO) bands to higher energy by ca. 78 cm–
+
1
relative to the parent complexes suggested the formation of the {Fe(μ-H)Fe} core with two FeII
1
centers. The high-field H-NMR signals at –17.93 and –13.44 ppm were assigned to the bridging
+
+
13
hydrides in 2μH and 3μH , respectively (Figure S2). Two sets of C-NMR resonances were
+
recorded for 3μH : 195.39 and 198.88 ppm, which were assigned to the basal and apical
1
3
+
carbonyls, respectively. Three distinct C-NMR singlets were observed in 2μH , indicating two
+
+
types of basal carbonyl groups. The protons in 1μH –3μH are extremely labile and susceptible
to instantaneous removal by THF or diethyl ether solvent residues to restore complexes 1–3.
Attempts to prepare adt (adt = azadithiolate) derivatives failed. Similar synthetic procedures
n
were applied to the reactions of [(μ-NR(CH
2
S)
2
)Fe
2
(CO)
6
] (R = H, Pr), yielding the N-
protonated products. Further protonation of the Fe
did not occur in the presence of excess super acid.
2
core for the formation of bridging hydride
1
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2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
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3
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5
5
5
6
0.16
.14
0.12
2113
2
020
0.08
0
1990
0
.06
0
0
.10
.08
2084
2
074
2067
1
969
0.04
0.02
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
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5
6
7
8
9
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1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
0.06
.04
0.02
.00
0.14
.12
0.10
2134
0
1
777
0
.00
*
*
0
0
0
.12
.10
2045
2097
1
995
0
0.08
0.06
0.04
0.02
2082
2
075
2046
2
0
0
0
0
0
.08
.06
.04
.02
.00
2130
036
2074
0
.00
-0.02
-0.04
.14
0
1
991
2033
1
0
.08
0.12
0
0
0
0
0
0
.10
.08
.06
.04
.02
.00
996
1
0.06
0.04
1960
989
2040
1934
2073
0
0
.02
.00
1
755
2
200
2100
2000
1900
1800
1700 2200
2100
2000
1900
1800
1700
-
1
-1
Wavenumber (cm
)
Wavenumber (cm )
+
–
Figure 1. In-situ FTIR spectra of 2μH (black), 2μH (red), 2μH* (blue), 2μHSH* (olive green),
+
2
μHtHSH* (brick red), and 2 (pink) in CH
2
Cl solution at 193 K. Asterisks represent 2 in-situ
2
+
generated from deprotonation of 2μH by moisture residue. The spectrum of 2 was collected
from the solution after the warm-up experiment of 2μHSH*.
+
+
The molecular structures of 1μH –3μH were characterized by X-ray crystallography and
are displayed in Figures 2, S4 and S5, with selected corresponding metric parameters included in
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the figure captions. All three complexes possessed a {Fe
2
S
2
} unit that resembled the active site
of [FeFe]hydrogenase. Each Fe center within the {Fe
S
2 2
} core was coordinated by 3 carbonyls
and 2 thiolates, and the “bent” metal-metal bond was replaced by the bridging hydride to
complete the distorted octahedral geometry. The Fe-Fe distances in three species were slightly
elongated in comparison with the parent complexes. The hydride was asymmetrically
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35
36
37
38
39
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41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
+
coordinated to two Fe centers. The hydride of 2μH was located close to one Fe site (1.59(5) vs.
+
1.81(4) Å). A similar result (1.54(6) vs. 1.70(6) Å) was observed in 1μH , and the Fe-H
+
distances of 3μH (1.63(4) vs. 1.66(4) Å) were relatively close. The Fe-H distance determined
using X-ray single crystal diffraction is generally considered to be less accurate, but the values
are still reliable references for comparison. The significant difference between the two Fe-H
+
13
distances in 2μH may have been the origin of two sets of chemically non-equivalent CO
resonances.
+
Figure 2. Molecular structure of 2μH . All hydrogen atoms except for the hydride are omitted
for clarity. Selected bond length (Å) and angles (deg): Fe−Fe, 2.5477(7); Fe−S, ave. 2.2461(10);
Fe−CCO,ap, ave. 1.819(4); Fe−CCO,ba, ave. 1.832(4); Fe1−H1, 1.81(4); Fe2−H1, 1.59(5); S−Fe−S,
ave. 80.35(4); S−Fe−Fe, ave. 55.45(3); Fe−S−Fe, ave. 69.11(3).
+
+
Electrochemistry and chemical reductions of 1μH –3μH
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2
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2
2
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5
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5
5
5
5
5
5
5
6
+
+
The electrochemical properties of 1μH –3μH were investigated in CH
2
Cl solution.
2
+
Cyclic voltammograms of 1μH showed a quasi-reversible one-electron reduction event (ipa/ipc
=
red
0.9) at E1/2 = –0.69 V (Epc = –0.74 V, Epa = –0.64 V) at 200 K and a fully irreversible reduction
process at 298 K, as shown in Figures 3 and S6. Similar electrochemical behaviors were
recorded for the analogs, for which the reductions occurred at –0.68 V (E1/2red; Epc = –0.72 V, Epa
0
1
2
3
4
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
+
+
=
–0.64 V) at 210 K for 2μH and –0.67 V (Epc) at 213 K for 3μH . The redox process was less
reversible for the latter (ipa/ipc = 0.61) because 3μH was relatively unstable and readily
regenerated 3 during hydrogen production (vide infra). There is a significant concept to be
addressed here: in contrast to the corresponding parent complexes, the protonated complexes
displayed insignificant variations in reduction potentials despite their different bridgeheads.
Given the electron-withdrawing ability of the bdt unit, the potential for electron addition to 3 was
decreased by at least 300 mV relative to the pdt and edt analogs. The current electrochemical
results, however, suggested that the FeII
core in 1μH –3μH stemming from the formation of
+
+
2
the hydride group played a greater role in the reduction than the bridgehead.
1.0
0.8
0.6
+
/0
E
E
E
(1H ) = -0.69 V
1/2
1/2
pc
+
/0
(2H ) = -0.68 V
+/0
(3H ) = -0.67 V
0.4
0.2
0.0
0.2
0.4
-
-
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
+
Potential vs. Fc /Fc (V)
1
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Figure 3. Cyclic voltammograms of 1μH (blue line, 200 K), 2μH (red line, 210 K) and 3μH
–
1
(black line, 213 K) in CH
2
Cl
2
solution under N
2
atmosphere ([complex] 1 mM, υ = 100 mV s , 1
mm vitreous carbon electrode, 0.1 M [n-Bu
4
N][B(C ]).
6
F
5 4
)
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60
+
The chemical reduction of 2μH by 1 equiv. of Cp
2
Co (cobaltocene) in CH
2
Cl
2
solution
at 193 K afforded [(μ-edt)(μ-H)Fe
2
(CO)
6
] (2μH, the CE state). The IR profile of 2μH exhibited
–
1
three carbonyl bands at 2074 m, 2045 vs and 1995 s cm (Figure 1), which was closely similar
–
1
to the IR profile of complex 2 (2073 m, 2033 vs, 1996 s, 1989 s cm at 193 K) with a slight
energy shift. Similar FTIR spectral results were observed regardless of the bridgehead identity.
–
1
At 193 K, a set of ν(CO) peaks for 1μH was recorded at 2072 m, 2043 vs and 1993 s cm . For
+
the one-electron reduction of 3μH , these peaks were observed at 2077 m, 2041 vs and 1999 s
–
1
cm . These carbonyl vibrational bands were comparable to the ones for the corresponding parent
–1
molecules at 193 K: 2075 m, 2034 vs, 1999 s, 1993 s cm for 1; and 2079 m, 2041 vs, 2002 s
–
1
cm for 3. The affirmative evidence for the formation of 2μH and 1μH was a color change from
+
orange-yellow to green. The electronic spectrum of 2μH at 193 K revealed one visible band at
–
1
–1
4
58 (ε = 500 M cm ) nm, which was readily replaced by two new transitions at 451 (ε = 1,700
–
1
–1
–1
–1
M cm ) and 625 (ε = 740 M cm ) nm upon contact with the reducing agent (Figure 4).
–1
–1
Similarly, 1μH exhibited two visible transitions at 457 (ε = 2,700 M cm ) and 666 (ε = 1,300
–
1
–1
M cm ) nm. Figure 5 displays the 77 K EPR spectra of 1μH and 2μH in frozen CH
2
Cl
2
solution. A rhombic signal with hyperfine interactions with hydride was recorded, in which gxyz
=
=
2.0083, 1.9776, 1.9705, Axyz = 2.1, 2.1, 3.9 G for 1μH and gxyz = 1.9924, 1.9696, 1.9637, Axyz
5, 5, 4.1 G for 2μH.
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1H
H
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0
4
57
4
51
666
625
400
500
600
700
800

(nm)
Figure 4. Electronic spectra of 1μH (blue line) and 2μH (red line) in CH
under N atmosphere.
2
Cl
2
solution at 193 K
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exp
exp
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gxyz = 1.9924, 1.9696,
.9637.
Axyz = 5, 5, 4.1.
gxyz = 2.0083, 1.9776,
1
1.9705.
Axyz = 2.1, 2.1, 3.9.
sim
sim
3200
3300
3400
3500
3600
3200
3300
3400
3500
3600
Magnetic Field (Gauss)
Magnetic Field (Gauss)
Figure 5. CW X-band EPR spectra (black line) of 1μH (left panel) and 2μH (right panel)
recorded in frozen CH Cl solution at 77 K. The red line represents the simulation result.
2
2
Lomoth et al. recently derived a conclusion from the photolysis results of [(μ-
bdt)Fe
2
(CO)
6
] in the presence of sensitizers and acids with pK
a
≤ 12.7 that [Fe
2
(bdt)(CO) ] and
6
80
[
Fe (bdt)(CO)
2
6
H] exhibit similar IR and UV-vis spectra. [Fe
2
(bdt)(CO) H] possesses a similar
6
structure to 1μH and 2μH. From our initial in-situ FTIR results, it is tempting to state that the
+
one-electron reduced species of 3μH could be 3μH, though the DFT calculations suggested that
μH* was thermodynamically more favorable. The species 3μH* was formulated as [(μ,κ2-
3
bdt)(μ-H)Fe (CO) ] with the rupture of one Fe-S bond, and the optimized structure exhibited the
2
6
bridging hydride. The details of the calculation results are discussed in the following paragraph.
The species 3μH* was not as stable as 1μH or 2μH, which exhibited no detectable
decomposition in CH
2
Cl solution at 193 K. Under the same conditions, 3μH* partially
2
converted to 3 and produced gas bubbles (Figure 6); such evolution did not occur until higher
–
temperatures for the pdt and edt analogs. In addition, trace amount of 3μH* possibly generated
via disproportionation was detected. The formation of molecular hydrogen was confirmed by GC
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analysis, and the yield of H
2
was estimated to be approximately 60 % after the resulting solution
was kept at 193 K for 5 minutes. The yield was increased to approximately 98 % in 30 minutes,
indicating that the temperature was not low enough to prevent 3μH* from decomposing. The
conversion could be obscured in the in-situ FTIR spectra. A prominent difference was observed
0
1
2
3
4
5
6
7
8
9
0
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0
+
in the electronic and EPR spectra for the one-electron reduction of 3μH relative to 1μH and
+
2
μH. The solution from the singly reduced reaction of 3μH exhibited only one detectable
–
1
–1
visible band at 459 (ε = 1,200 M cm ) nm in addition to one UV absorption at 333 (ε = 11,400
–
1
–1
M cm ) nm (Figure S8). These two transitions were close to the energy transitions of 3: 333 (ε
–
1
–1
–1
–1
=
20,600 M cm ) and 457 (ε = 1,000 M cm ) nm. The absence of the band at ~ 650 nm in
the observed electronic spectra may have suggested that 3 was the dominant species in solution
+
even at 193 K. The single reduction of 3μH displayed a weak EPR signal at 77 K relative to the
strong EPR signals for 1μH and 2μH (Figure S9), which supports the suggestion based on the
UV-vis results. Figure S10 compares the IR and UV-vis spectra of the optimized structures of 3,
+
3
μH and 3μH*. Both species, 3 and 3μH*, possessed similar vibrational patterns with similar
energies. This result suggests that it was difficult to characterize 3μH*, especially when the
major species in solution was 3. Time-dependent DFT (TD-DFT) calculations of the geometry of
the ground state predicted electronic excitations between 550 and 800 nm for 3μH*. By contrast,
3
did not possess noticeable transitions in the same region. No distinct assignment upon the
+
single reduction of 3μH could be made based on the experimental observations. Failure to
observe the excitation signature of 3μH* at approximately 650 nm in the experiments can be
rationalized by the small extinction coefficient and the short lifetime of 3μH* in solution at 193
K.
2
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116
0
0
0
0
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.10
.08
.06
.04
.02
.00
2084
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2
135
1
999
0.10
0.08
2
041
0.06
0.04
0.02
0.00
2077
1917
*
0
.15
1866
*
1967
0.10
0.05
*
1
*
850
1995
1665
*
2
045
1751
0
.00
.08
0
1
996
0
0
0
.06
.04
.02
1
979
1963
2045
1
935
1750
0.00
2
200
2100
2000
1900
1800
1700
1600
-
1
Wavenumber (cm
)
2
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+
+
Figure 6. In-situ FTIR spectra of 3μH (black), the single reduction of 3μH (red), the double
+
–
reduction of 3μH (blue), and 3μH* (olive green) in CH
2
Cl
2
solution at 193 K. The species
+
–
2
]2–. The
from the double reduction of 3μH contain 3μH* and [(μ,κ -bdt)(μ-CO)Fe
2
(CO)
5
2
2–
–1
–
asterisks denote [(μ,κ -bdt)(μ-CO)Fe
2
(CO)
5
] . The signal of 1645 cm is from [B(C
6
F
5
)
4
] .
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The 35-electron complex 2μH was susceptible to additional reduction at 193 K for the
formation of a new species (the CEE state) that possessed an IR profile of 2040 m, 1991 vs, 1960
–
1
s, 1934 sh and 1755 w cm (Figure 1). This new species could also be yielded by the reaction of
+
2
μH with 2 equiv. of Cp
2
Co. The product was deduced to be a doubly reduced complex. The
magnitude of the red shift of the carbonyl bands due to the single reduction of 2μH was smaller
+
–1
than the magnitude of the shift from the conversion of 2μH to 2μH (Δν ≈ 34 vs. 58 cm ). In
–
1
association with the presence of the bridging CO peak at 1755 cm , the molecular structure of
+
–
the doubly reduced 2μH , which was designated as 2μH* , was assigned to a {Fe
2
2
S } core in
which one thiolate, one hydride and one carbonyl spanned between two metal sites and one
2
thiolate was terminally coordinated to one Fe site. The {(μ,κ -edt)(μ-CO)(μ-H)Fe
2
} skeleton of
–
2
2
μH* resembled the structural feature in [(μ,κ -bdt)(μ-PPh
2
)(μ-H)Fe
2
(CO)
5
] with a PPh
2
8
1
bridging group instead of the bridging CO. Both species revealed similar IR signatures in the
–
terminal CO region, though the IR signature was blue shifted for 2μH* . The formulation of the
–
2
(CO)
]–
complex 2μH* was supported by computational studies as [(μ,κ -edt)(μ-H)(μ-CO)Fe
2
5
(vide infra). Similar IR profiles were recorded for the singly reduced 1μH and 3μH. The
–
–
–
resulting species, 1μH* and 3μH* , had identical structures to 2μH* . The electronic spectra of
–
–
–
–
1
μH* –3μH* were featureless in the region of 550–800 nm. The species 1μH* –3μH* were
degraded to 1–3, respectively, and molecular hydrogen when the temperature was raised to room
temperature.
–
+
It was not possible to quantitatively generate 3μH* from 3μH under reducing
conditions at low temperature. The in-situ FTIR spectra for the two consecutive one-electron
+
–
reduction reactions of 3μH at 193 K revealed the presence of 3μH* , which was masked by
2
(CO)
]2–. The latter was the major species (~80 %) in solution. We used an
[(μ,κ -bdt)(μ-CO)Fe
2
5
–
2
alternative procedure to obtain the distinct IR profile of 3μH* . The complex [(μ,κ -bdt)(μ-
CO)Fe (CO) Cl solution at 193 K and reacted with HOTf
]2– was synthesized first in CH
2
5
2
2
2
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–
(
trifluoromethanesulfonic acid) to afford 3μH* . The resulting carbonyl bands at 2045 m, 1996
–
1
–
–
vs, 1979 s, 1963 s, 1935 m and 1750 w cm were similar to the bands of 1μH* and 2μH* ,
–
suggesting that 3μH* had enhanced stability relative to 3μH* in CH
2
Cl
2
solution at 193 K.
–
–
Protonation of 1μH* –3μH*
10
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60
t
–
–
The presence of the terminally coordinated thiolate (the S site) in 1μH* –3μH*
t
provided an opportunity to examine the capability of the S site for proton attachment as it
2
].81,82 The reaction of 1μH* –3μH* with one
–
–
occurred in [(μ,κ -bdt)(μ-PPh
2
)(μ-H)Fe
2
(CO)
5
2
equiv. of HOTf at 193 K afforded an S-protonated product [(μ,κ -xdtH)(μ-H)(μ-CO)Fe
2
(CO) ]
5
(
1μHSH*–3μHSH*, the CEEC state), which was characterized by in-situ FTIR spectroscopy.
Species 1μHSH* was extremely unstable even at this temperature. Complexes 2μHSH*–
–
–
3μHSH* exhibited similar IR patterns to 2μH* –3μH* but were shifted by an average of 28
cm–1 to a higher energy. These results were consistent with previous observations on the
t
protonation of the S site.
The consecutive protonation of 2μHSH* occurred in the presence of extra amounts of
acid at 193 K, leading to a new species (the “3H” species, the CEECC state) possessing ν(CO)
–
1
bands at 2130 s, 2097 vs, 2082 s, 2075 s, 2046 m, 2036 m cm . The disappearance of the CO
band in the lower energy region indicated a structural rearrangement upon the addition of the
third proton. All carbonyl groups were thus assigned to be terminally ligated. Compared to
–
1
2μHSH*, the shift of the IR profile to higher energy by ca. 63 cm suggested that two Fe
II
II
centers were converted to the Fe Fe state due to the formation of an additional Fe-hydride. The
S-proton and the 2 Fe-hydrides were characterized by NMR spectroscopy. The SD resonance of
2
μHSD* was observed at 2.3 ppm at 193 K in CH
2
Cl solution (Figure S11). This signal retained
2
the same chemical shift when additional DOTf was injected into a CH
2
Cl solution of 2μHSD*
2
2
at 193 K, while an up-fielded D-NMR signal located at –12.98 ppm replaced the μH of 2μHSD*
1
at –12.38 ppm. Additionally, a H-NMR signal of –13.0 ppm was identified for the same species.
1
2
The H/ D-NMR resonances at ~ –13 ppm for the multi-hydride species, which are displayed in
8
3
Figure 7, may have originated from the hydride in either bridging or terminal fashion. We
2
(CO)
]+ (2μHtHSH* ) with the
+
assigned the “3H” species to [(μ,κ -edtH)(μ-H)(t-H)Fe
2
6
assistance of computational studies (vide infra). These results suggested that the exchange
between the terminal and bridging hydrides/deuterides was fast on the NMR time scale at this
2
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1
1
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2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
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3
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+
temperature. The species 2μHtDSD* was also characterized by electrospray ionization mass
spectrometry (ESI-MS) analysis. High-resolution ESI-MS (positive mode) revealed a prominent
2
peak at m/z = 1056.8368 (Figure S12), which corresponded to [(μ,κ -edtD)(μ-H)(t-
2
D)Fe
2
(CO)
6
]+•HB(C
]+•HB(C
6
F
5
)
4
(calcd = 1056.8389). The observed isotopic pattern of [(μ,κ -edtD)(μ-
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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3
4
5
6
7
8
9
0
H)(t-D)Fe
2
(CO)
6
6
F
5
)
4
was in excellent agreement with the simulated one.
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2
1
Figure 7. Variable-temperature (A) D and (B) H NMR spectra from the in-situ reaction of
2
μHSD* with DOTf in CH
2
Cl
2
solution. The solvent signal is indicated as an asterisk.
+
It was observed that 2μHtHSH* had higher stability in solution at low temperature
+
compared to 2μHSH*, which slowly transformed at 203 K. In contrast, for 2μHtHSH* , no
detectable change was observed for days under the same conditions. We attempted to crystallize
+
2μHtHSH* and successfully grew orange-red crystals at 193 K over the course of 2 months.
These crystals were structurally characterized as [(μ-edt)(μ-H)Fe
2
(CO) ][OTf] ([2μH][OTf]), as
6
+
shown in Figure S13. The species 2μH in OTf salt was extremely unstable and was readily
deprotonated to 2 when dissolved in CH
2
Cl solution at 205 K. This result was consistent with
2
2
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the understanding that the Fe
2
core in a hexacarbonyl complex is electronically deficient and
cannot accept protons unless a super acid is employed.
Computational studies
0
1
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0
Theoretical studies were undertaken to make better assignments, although the
spectroscopic evidence provided a data point for conjecturing the structures of the intermediates.
The calculation work mainly focused on the protonation/reduction intermediates of complex 2,
as similar intermediates were generated for its analogous complexes. A summary of the solvent-
corrected free energy for all computed species and transition states is provided in the supporting
materials.
DFT calculations were employed to obtain the optimized structures and free energies of
+
all possible intermediates from the C to the CEECC species. Herein, we defined 2µH as the C
species resulting from protonation (the chemical step) of the parent complex 2, 2µH as the CE
species resulting from the initial protonation and subsequent reduction (referred to as the related
electrochemical step in cyclic voltammetry) of complex 2, and so forth. We initially compared
+
the experimental structural parameters of 2 and 2µH with the corresponding calculated values
because the structures of both species have been characterized by X-ray single crystallography.
Consistent results were obtained, suggesting that the applied functional/basis set was valid for
this purpose. The energetically lowest-lying species were used as the reference ground to make
an initial guess for the following state. In addition, a validation check was performed between
the experimental and computational carbonyl stretching frequencies, resulting in a linear
relationship84 that indicated that the calculated models made reliable claims regarding the
corresponding intermediate species. Finally, mechanisms of two hydrogen-evolution processes
are discussed in detail.
Computed structures and relative stabilities of potential intermediates
+
The CE state: For the one-electron reduced species of 2µH , several isomers were proposed.
Only two of them, 2µH and 2µH*, were investigated, as shown in Figure 8. The plausible third
species with a terminal hydride and a µ-CO group was eliminated on the basis of the
+
experimental FTIR data. Complex 2µH retained the structure of 2µH , though the Fe-Fe bond
was elongated by 0.17 Å. The μ-H was slightly displaced from the symmetric position to one Fe
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center. Complex 2µH* was a species featuring an open structure with one thiolate terminally
coordinated to one Fe center, which resulted from the rupture of one Fe-S bond. The assumption
of 2µH* is still under consideration because the Fe-S bond breakup or elongation has been
experimentally observed or theoretically predicted, respectively, in the reduced products of [(μ-
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xdt)Fe
2
(CO)
6
] (xdt = thiolate bridges).85-93 A comparison of the free energy of the optimized
structures indicated that the species 2µH was thermodynamically more stable than 2µH* by
−1
+10.28 kcal mol . We therefore assigned the CE state as 2µH, which was consistent with the
–
57,94
previous conclusion concerning the protonated species of [(μ-xdt)Fe
on the DFT calculations, De Gioia et al. and Hall et al. later proposed the best candidate for the
EC product in proton binding of the mono-reduced complex to be [(μ-xdt)(μ-H)Fe (CO) ], which
preserves the typical {(μ-xdt)Fe } core observed in 2µH. For the pdt analog, a similar result was
2
(CO)
6
] .
Earlier, based
2
6
2
−1
obtained in favor of 1µH by +10.09 kcal mol (Figure S14). In contrast, the open structure of
2
3
µH*, designated [(μ,κ -bdt)(μ-H)Fe
2
(CO) ], was more stable than the structure of 3µH by
6
−1
+
9.48 kcal mol .
Figure 8. Computed structures of the CE species for the edt and bdt analogs. The free energy
−1
difference is reported in kcal mol . All hydrogen atoms except for the hydride are omitted for
clarity. Color scheme: Fe, dark green; S, yellow; O, red; C, grey and H, white.
The CEE state: The subsequent reduction of the CE state led to a Fe(I)Fe(I) species, for which
we considered two electronic states: the diamagnetic state (the singlet state) and the
paramagnetic state (the triplet). Throughout the discussion, the multiplicity of this species was
one if not otherwise specified. The geometry of the resulting species was distorted to a structure
featuring a lengthened Fe-Fe distance of 3.352 Å when the reduced 2μH was optimized. The
2
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hydride was converted to the terminal mode from the bridging mode and remained in the
location underneath the Fe-Fe vector, as displayed in Figure 9. The new species was thus
–
–
designated as 2tH . For the electronic isomer 2μH (M = 3), the optimized structure was similar
to 2μH. Next, two electronic isomers of 2μH* were also investigated. A Fe core bearing
2
0
1
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0
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0
–
asymmetric bridging hydride and carbonyl groups was obtained for 2μH* . A one-electron
addition to 2μH* increased the Fe-S distances by 0.04 Å while decreasing the Fe-Fe distance by
–
S
} skeleton of 2μH–
0.12 Å. Optimization of 2μH* (M = 3) resulted in the related {(μ-H)Fe
2
2
(M = 3) due to restoring the Fe-S bond, which had an almost identical energy. Among these four
–
–
possible isomeric products, 2μH* was the most favorable species. The species 2μH (M = 3)
–
–1
and 2tH were less stable by +7.97 and +15.74 kcal mol , respectively.
We also considered two less plausible scenarios. First, 2μH(CO) –
, a high-energy, mono-
5
CO-eliminated species with ΔGCEE = +12.23 kcal mol , was produced. For the triplet state, an
–1
open structure resulting from splitting the Fe-S bond, which was designated as 2μH(CO)
5
*– (M
=
3), was found by convergence. This structure was more thermodynamically favorable than its
electronic congener but was still destabilized by +2.34 kcal mol–1 compared to 2μH* . The
–
attractive characteristic of both species was the presence of an apical vacant site, which could
facilitate succeeding protonation steps. The plausible presence of 2μH(CO) –
M = 3) in solution was excluded because these species did not exhibit the μ-CO signature
observed in the experiments.
and 2μH(CO)
*–
5
5
(
2
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Figure 9. Computed structures of the CEE species for the edt analog. The free energy difference
−1
is reported in kcal mol . All hydrogen atoms except for the hydride and formyl hydrogen are
omitted for clarity. Color scheme: Fe, dark green; S, yellow; O, red; C, grey and H, white.
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–
(C(O)H)] (2HCO–) proved fruitless.
Exploration of the formyl species [(μ-edt)Fe
2
(CO)
5
First, to produce the formyl species, an intramolecular rearrangement from its anionic hydride
analogs was attempted. De Gioia et al. previously reported that the conversion from [(μ-pdt)(μ-
–1 57
H)Fe
2
(CO)
]– to its formyl tautomer requires overcoming an energy barrier of +11.4 kcal mol ,
6
suggesting a slow conversion. It could be reasonably assumed that the kinetic barrier of the
possible related transformation for the edt analog was as high as in the pdt species, although the
calculated free energy of 2HCO– was only +3.24 kcal mol higher than 2μH* . Second, it has
–1
–
been reported that the formyl complex of the pdt analog is solely synthesized from the reaction
54
between the hexacarbonyl complex and metal hydrides. It was thus reasonable to propose that
HCO– could be prepared via a similar synthetic procedure. From the experiments, it was found
2
–
that 2μH* did not react by hydrogen evolution when exposed to excess HOTf in solution. This
result revealed the acidic character of the bridging hydride in the species. Consequently, the
hydride attack of the carbonyl group to form the formyl species was not favored. The last piece
of evidence is that the FTIR data lacked the corresponding formyl vibrational band. In the
current study, it was concluded in light of theoretical and experimental evidence that the
intramolecular isomerization of 2μH* to 2HCO– was not feasible.
–
The CEEC state: Figure S15 displays the highest occupied molecular orbital (HOMO) of 2μH*–,
t
with significant contributions from the Fe
2
(44.2 %), terminal thiolate (S , 18.9 %) and bridging
b
CO (CO , 17.6 %). These electron-rich sites are prone to protonation. The initial models bearing
two hydrides attached to the Fe
2
core ultimately converged to structures consisting of one μ-H
s
a
and one terminal hydride (t-H) in 2μHtH * and 2μHtH * (Figure 10). The species with the t-H
t
μ
μ
and S groups on the same side relative to the plane of the FeS FeH rhomb was designated
s
a
2
μHtH * and the opposite side was designated 2μHtH *. The t-H group, along with 2 CO and
the μ-S groups, was coordinated in the equatorial position about the distal Fe with respect to the
t
t
in
S site. One another isomer composed with a t-H and a S -proton was designated 2tHSH *.
Protonation onto the μ-CO group afforded the formyl species 2μHHCO and restored the compact
2
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