Structures, protonation, and electrochemical properties of diiron dithiolate complexes containing pyridyl-phosphine ligands

Article abstract of DOI:10.1039/b814336f

Diiron complexes containing pyridyl-phosphine ligands, that is, (μ-pdt)[Fe₂(CO)₅L] (pdt = S(CH₂) ₃S, L = Ph₂PCH₂Py, 3a; Ph₂PPy, 3b) and (μ-pdt)[Fe(CO)₂(PMe₃)][Fe(CO)₂L] (L = Ph₂PCH₂Py, 4a; Ph₂PPy, 4b) were prepared as model complexes of the [FeFe]-hydrogenase active site. Protonation of 3a and 3b by HOTf afforded the pyridyl-nitrogen protonated products [3aH_N][OTf] and [3bH_N][OTf], respectively. The molecular structures of 3a, 3b, 4a, 4b, as well as [3aH_N][OTf] and [3bH_N][OTf] were confirmed by X-ray diffraction studies, which show that the Ph ₂PCH₂Py ligand occupies the basal position both in 3a and its protonated species [3aH_N][OTf], while the Ph₂PPy ligand prefers the apical position in 3b and [3bH_N][OTf]. The double protonation process of complex 4b was monitored by in situ IR, ¹H and ³¹P NMR spectroscopy at low temperature. The spectroscopic evidence indicates that the protonation of 4b occurs first at the Fe-Fe bond and then at the pyridyl-nitrogen atom. Cyclic voltammograms reveal that protonation of 3a and 3b results in a considerable decrease in the overpotential for electrocatalytic proton reduction in the presence of HOTf, while the efficiency is not influenced by protonation. The electrocatalytic efficiency of 4a for proton reduction in the presence of HOAc in CH₃CN-H₂O (50:1, v/v) is 5 times higher than that in pure CH₃CN. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b814336f

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PAPER
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Structures, protonation, and electrochemical properties of diiron dithiolate
complexes containing pyridyl-phosphine ligands†
Ping Li,a Mei Wang,*a Lin Chen,a Jihong Liu,a Zhenbo Zhaoa and Licheng Sun*a,b
Received 18th August 2008, Accepted 8th December 2008
First published as an Advance Article on the web 28th January 2009
DOI: 10.1039/b814336f
Diiron complexes containing pyridyl-phosphine ligands, that is, (m-pdt)[Fe2(CO)5L] (pdt =
S(CH2)3S, L = Ph2PCH2Py, 3a; Ph2PPy, 3b) and (m-pdt)[Fe(CO)2(PMe3)][Fe(CO)2L] (L = Ph2PCH2Py,
4a; Ph2PPy, 4b) were prepared as model complexes of the [FeFe]-hydrogenase active site. Protonation of
3a and 3b by HOTf afforded the pyridyl-nitrogen protonated products [3aHN][OTf] and [3bHN][OTf],
respectively. The molecular structures of 3a, 3b, 4a, 4b, as well as [3aHN][OTf] and [3bHN][OTf] were
confirmed by X-ray diffraction studies, which show that the Ph2PCH2Py ligand occupies the basal
position both in 3a and its protonated species [3aHN][OTf], while the Ph2PPy ligand prefers the apical
position in 3b and [3bHN][OTf]. The double protonation process of complex 4b was monitored by in situ
IR, 1H and 31P NMR spectroscopy at low temperature. The spectroscopic evidence indicates that the
protonation of 4b occurs first at the Fe–Fe bond and then at the pyridyl-nitrogen atom. Cyclic
voltammograms reveal that protonation of 3a and 3b results in a considerable decrease in the
overpotential for electrocatalytic proton reduction in the presence of HOTf, while the efficiency is not
influenced by protonation. The electrocatalytic efficiency of 4a for proton reduction in the presence of
HOAc in CH3CN–H2O (50 : 1, v/v) is 5 times higher than that in pure CH3CN.
bridging S atom may act as a proton relay to form a proton–
Introduction
hydride diiron dithiolate intermediate,3–6 which is an inevitable
The crystallographic and spectroscopic studies of [FeFe]-
state in the heterolytic H–H forming/breaking processes at
hydrogenases ([FeFe]-Hases) have revealed that the active site of
the [FeFe]-Hase active site (H-cluster). There are two possible
[FeFe]-Hases has a simple diiron structure, closely resembling
basic sites in the H-cluster, that is, the nitrogen atoms in the
SCH2NHCH2S bridge and the CN- ligand.7,8 The potential
function of these pendant bases of the H-cluster in the process of
enzymatic dihydrogen generation and uptake is still a problematic
issue.
the structure of a well-known diiron dithiolate complex (m-
pdt)[Fe2(CO)6] (pdt = S(CH2)3S).1,2 This finding has inspired
organometallic chemists to devote more attention to the chemistry
of diiron dithiolate complexes. Studies on the diiron dithiolate
complexes containing a pendant internal base are of interest
for better understanding the mechanism of enzymatic proton
reduction and dihydrogen oxidation, and for designing efficient
iron-based homogeneous catalysts. A pendant internal base or the
Spectroscopic evidence has proved that the protonation of
the bridging nitrogen atom of diiron azadithiolate complexes
occurred rapidly,9,10 and two nitrogen-protonated products were
crystallographically characterized.11,12 Recently, two examples of
the proton–hydride species of diiron azadithiolate complexes
have been reported. The one, formed in situ by double pro-
tonation of [(m-SCH2)2NH][Fe(CO)(dppv)]2 (dppv = cis-1,2-
bis(diphenylphosphino)ethene) at -78 ◦C, contains a terminal
hydride and a bridging-ammonium centre.5 The other proton–
hydride species, generated in situ by double protonation of [(m-
SCH2)2NBn][Fe(CO)2(PMe3)]2 at 25 ◦C, bears a m-hydride and
an ammonium centre.3,4 The proton carried by the bridging
nitrogen atom is adjacent to the apical terminal hydride in the
proposed structure of the former example (Fig. 1(a)), but it
aState Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and
Research Centre on Molecular Devices, Dalian University of Technology
(DUT), Dalian, 116012, China
bKTH School of Chemical Science and Engineering, Organic Chemistry,
10044, Stockholm, Sweden
† Electronic supplementary information (ESI) available: React-IR spectra
for the protonation of 4b in the presence of HOTf; packing diagrams and
geometric parameters of [3aHN][OTf] and [3bHN][OTf]; cyclic voltammo-
grams of complex 3b in the presence of HOAc and HOTf. CCDC reference
numbers 643385–643390. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b814336f
Fig. 1 Possible structures for doubly protonated species of [(m-SCH2)2NH][Fe(CO)(dppv)]2, [(m-SCH2)2NBn][Fe(CO)2(PMe3)]2, 4a, and 4b.
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Table 1 IR data of the v(CO) bands for 3a, [3aHN][OTf], 3b, [3bHN][OTf],
4a, and 4b
is distant from the m-hydride in the latter example (Fig. 1(b)).
Many experiments have proved that at ambient temperature the
protonation of the diiron complexes containing strong electron
donating ligands, such as PR3,13,14 CN-,15 CNR,16 N-heterocyclic
carbene (NHC),17 and 1,10-phenanthroline,18 generates only the
m-hydride diiron complexes. Although the [FeII(m-H)FeII] species
exhibit no inherent reactivity toward proton reduction, some of
them show H/D exchange activity with D2 and catalytic activity
for H/D scrambling between D2O and H2 in the presence of
light.14 The results of density functional studies on heterolytic
H–H forming/breaking at the diiron sub-cluster suggest that
[FeII(m-H)FeII] species may act as an intermediate state in the
catalytic cycle of the H-cluster.19,20 Supposing that a m-hydride
diiron species is involved in the H–H forming/breaking process,
the diiron model with a pendant basic site in a s-donating ligand
might be catalytically preferred over that with a secondary amine
in the 2-azapropane-1,3-dithiolato (adt) bridge, considering the
distance between the m-hydride and the proton carried by the
pendant base.
Dn1st(CO)a
Complex
n(CO) KBr/cm-1
KBr/cm-1
3a
2033(s), 1982(vs), 1956(vs), 1926(m)
[3aHN][OTf] 2041(s), 1990(s), 1971(s), 1963(s), 1935(m)
8
3b
2039(vs), 2003(s), 1986(s), 1969(s), 1940(sh)
[3bHN][OTf] 2049(vs), 1989(vs), 1972(sh), 1957(m), 1944(m) 10
4a
4b
1976(s), 1945(s), 1907(s), 1884(m)
1982(s), 1947(vs), 1918(s), 1896(s)
a Dn1st(CO) = n1st(CO)protonated - n1st(CO)non-protonated
.
Results and discussion
Preparation and spectroscopic characterization of 3a, 3b, 4a, 4b,
[3aHN][OTf], and [3bHN][OTf]
Diiron dithiolate complexes 3a, 3b, 4a, and 4b containing
pyridyl-phosphine ligands were prepared by CO displacement
of (m-pdt)[Fe2(CO)6] (1) and (m-pdt)[Fe2(CO)5(PMe3)] (2), respec-
tively, in the presence of CO-removing reagent Me3NO·2H2O in
CH3CN. Addition of 5 equiv. of HOTf to the Et2O solution of 3a
and 3b afforded pyridyl-nitrogen protonated species [3aHN][OTf]
and [3bHN][OTf], respectively. All complexes were characterized by
The pyridyl group has been introduced to the diiron model
complexes, either as part of the chelating ligand or as a pendant
arm tethered to the central carbon of the pdt bridge,21–24 which
gives an example for the protonation competition of the pyridyl
group and the Fe–Fe bond. We designed diiron complexes
containing pyridyl-phosphine ligands, Ph2PCH2Py and Ph2PPy,
in which the pyridyl nitrogen atom may act as a proton relay
similar to the bridging nitrogen atom of the adt bridge.3–5,9–12 In
terms of the turnstile rotation of the ligands and the rotation
around the Fe–P and P–C (Py) single bonds, the distance
between the pyridyl-nitrogen and the m-hydride is changeable for
the model complexes 4a and 4b (Fig. 1). The flexible position
of the pendant base towards the m-hydride of the protonated
diiron complex could benefit the intramolecular proton–hydride
combination in the catalytic dihydrogen generation process. Here
we report the preparation and characterization of the pyridyl-
phosphine mono-substituted complexes (m-pdt)[Fe2(CO)5L] (L =
Ph2PCH2Py, 3a; Ph2PPy, 3b) and the disubstituted complexes (m-
pdt)[Fe(CO)2(PMe3)][Fe(CO)2L] (L = Ph2PCH2Py, 4a; Ph2PPy,
4b, Chart 1), and the molecular structures of 3a, 3b, 4a, 4b,
as well as the pyridyl-nitrogen protonated species [3aHN][OTf]
and [3bHN][OTf]. The doubly protonated species [4bHmHN]2+
containing a m-hydride on the iron atoms and a proton on
1
IR, MS, H and 31P NMR spectroscopy, and elemental analysis.
The IR data of the v(CO) bands of 3a, 3b, 4a, 4b, [3aHN][OTf],
and [3bHN][OTf] are summarized in Table 1. The frequencies of
the first v(CO) bands of 3a and 4a are 6 cm-1 lower than their
corresponding complexes 3b and 4b, implying a slightly stronger
donating ability of Ph2PCH2Py than Ph2PPy. Protonation of 3a
and 3b on the pyridyl-nitrogen atom causes a blue-shift of 8–
10 cm-1 for the first v(CO) bands. The blue-shifts of the v(CO)
bands for 3a and 3b are somewhat smaller than that (15–20 cm-1)
caused by protonation of the bridging nitrogen atom of analogous
diiron complexes.3–5,9–12
Protonation of 4b
The red color of the CH3CN solution of 4a or 4b quickly faded
upon addition of HOTf at room temperature and all v(CO) bands
disappeared in the IR spectrum, indicating the decomposition
of the protonated diiron complexes. Due to the poor stability of
the protonated species of 4a and 4b at room temperature, the
1
the pyridyl-nitrogen atom were detected by in situ IR, H and
1
1
31P NMR spectroscopy at low temperature. The spectroscopic
evidence shows that the protonation of the Fe–Fe bond in 4b is
preferred over the pyridyl-nitrogen atom in the presence of HOTf.
In addition, the electrochemical properties of 3a, 3b, and 4a for
proton reduction in the presence of different acids are also reported
in this paper.
protonation process of 4b was◦studied by 31P{ H} and H NMR
spectroscopy in CD2Cl2 at -55 C.25 Fig. 2 shows the variations of
the 31P (left) and 1H (right) resonances in the selected regions with
addition of 0–4 equiv. of HOTf to the solution of 4b.
Complex 4b displays two 31P resonances at 64.2 ppm for Ph2PPy
and 26.2 ppm for PMe3 (Fig. 2(a)). The relatively broad signal
for PMe3 at -55 ◦C indicates the slowdown of the rotation of
the Fe(CO)2(PMe3) subunit. The similar case has been reported
for (m-pdt)[Fe(CO)2(PMe3)]2 by Darensbourg and coworkers.26
When 1 equiv. of HOTf is added, a broad hydride signal at
-14.5 ppm appears in the 1H NMR spectrum (Fig. 2(b¢)),
indicating the formation of the m-hydride diiron complex
[4bHm]+.26–28 Concomitantly, two additional sharp signals at
62.8 ppm for Ph2PPy and 21.3 ppm for PMe3 appear in the
31P{H} NMR spectrum (Fig. 2(b)). The protonation of the
Fe–Fe bond results in high-field shifts of 4.9 and 1.4 ppm
Chart 1
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Fig. 2 Selected regions of 31P{ H} (left) and 1H NMR (right) spectra for protonation of 4b in CD2Cl2 at -55 ◦C.
1
for the 31P signals of PMe3 and Ph2PPy, respectively. Upon
addition of 2 equiv. of HOTf, the appearance of two hydride
signals at -14.5 and -15.2 ppm (Fig. 2(c¢)) gives unambiguous
evidence for the existence of two types of m-hydride species, that
is, the m-hydride complex [4bHm]+ and the doubly protonated
species [4bHmHN]2+ (see Scheme 1). An additional proton signal
at 16.1 ppm is observed, which is ascribed to the proton on
Ph2PPy ligand upon protonation of the pyridyl-nitrogen atom
of [4bHm]+ are not clear up to now. The tentative explanation
is that the protonation of the pyridyl-nitrogen atom results in a
movement of the m-hydride to the Fe(CO)2(PMe3) subunit with a
strong donating ligand, which leads to the high-field shift of the
m-hydride resonance. Accordingly, the effect of the movement of
m-hydride to the Fe(CO)2(PMe3) subunit could partially offset the
counter effect of the protonation at the pyridyl-nitrogen atom on
the electron density of the Fe(CO)2(Ph2PPy) subunit, resulting in
a small shift of the 31P signal for the Ph2PPy ligand of [4bHmHN]2+
relative to the corresponding signal for [4bHm]+.
The protonation process of 4b in the presence of 0–4 equiv. of
HOTf was also monitored by react-IR spectroscopy in CH2Cl2 at
-55 ◦C (Fig. S1 in ESI†). First, one equiv. of HOTf was added
to the CH2Cl2 solution of 4b, and when no change was observed
from the IR spectra, one more equiv. of HOTf was immediately
added. The selected IR spectra evincing the protonation process
of 4b are shown in Fig. 3.
Complex 4b displays four v(CO) bands at 1985, 1951, 1916, and
1900 cm-1 (Fig. 3(a)). A large blue shift (ca. 77 wavenumbers) of
the average value of the n(CO) bands is found upon addition of 1
equiv. of HOTf (Fig. 3(b)), which is indicative of the protonation
of the Fe–Fe bond of 4b.3,4,26–28 In contrast, the further addition
of HOTf up to 4 equiv. results only in a slight blue shift (ca. 9
wavenumbers) of the n(CO) bands (Fig. 3(d)), suggesting that the
singly protonated complex [4bHm]+ is transformed to the doubly
protonated species [4bHmHN]2+. The result from IR spectroscopic
study further proves that in the presence of HOTf complex 4b is
protonated first at the Fe–Fe bond and then at the pyridyl-nitrogen
atom in CH2Cl2 at low temperature. In contrast, protonation of 3a
and 3b takes place at the pyridine-N atom and does not occur at
the Fe–Fe bond. Compared with 3a and 3b, complex 4b contains
an additional good electron donating ligand PMe3, the electron
the pyridyl-nitrogen atom of [4bHmHN]2+. Simultaneously, the 31
P
signals of 4b completely disappears, accompanied with appearance
of a new 31P signal at 19.6 ppm in addition to the signal at
21.3 ppm for the PMe3 ligand of [4bHm]+ (Fig. 2(c)). The 31P signal
at 19.6 ppm is attributed to the PMe3 ligand of [4bHmHN]2+. The
31P signal at 62.8 ppm for Ph2PPy in Fig. 2(c) does not show
an observable shift, but the intensity of this signal relative to
that of the 31P NMR signal at 21.3 ppm in Fig. 2(c) apparently
increases as compared to the intensity ratio of these two 31P
NMR signals in Fig. 2(b), suggesting an overlap of the 31P NMR
signals of the Ph2PPy ligands in [4bHm]+ and [4bHmHN]2+. When the
accumulated quantity of HOTf is up to 4 equiv., the initial complex
4b is quantitatively transformed to the doubly protonated species
[4bHmHN]2+, suggested by the evidence that the signal at -14.5 ppm
in Fig. 2(c¢) and the 31P signal at 21.3 ppm in Fig. 2(c) completely
disappears in Fig. 2(d¢) and (d). The broad signal for the proton
on the pyridyl-nitrogen atom shifts from 16.1 to 15.4 ppm as the
acidity of the solution is increased (Fig. 2(d¢)), and the 31P signal
for the Ph2PPy ligand of [4bHmHN]2+ is slightly shifted to 63.1 ppm.
The protonation of the pyridyl-nitrogen atom of [4bHm]+ causes a
high-field shift of the m-hydride signal from -14.5 to -15.2 ppm. In
contrast, the m-hydride resonance in the doubly protonated species
(m-adtH)(m-H)[Fe(CO)2(PMe3)]2 (adt = N-benzyl-2-azapropane-
1,3-dithiolato) is not affected by the protonation at the bridging
nitrogen atom.3,4 The reason for the high-field shift of the m-hydride
resonance and for the relatively small shift of the 31P signal of the
Scheme 1
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Ph2PCH2Py and Ph2PPy, in 3a and 3b as well as in [3aHN][OTf]
and [3bHN][OTf] is that the insertion of a CH2 group to the bond
between the phosphorus atom and the pyridyl ring decreases the
steric interaction of the basal phosphine and the carbonyl ligands.
Disubstituted complexes 4a and 4b both feature a basal/apical
geometry with PMe3 in a basal position and Ph2PCH2Py or
Ph2PPy in an apical position.
Fig. 4(b and d) and the metric data for Fe ◊ ◊ ◊ N distances listed
in Table 2 show that the orientation of the pyridyl-nitrogen atom in
3a and 3b is changed after protonation. The N–H ◊ ◊ ◊ O hydrogen
bond among the pyridyl NH unit and the [HOTf]- anion accounts
for the change of the pyridyl orientation. The Fe ◊ ◊ ◊ N distances in
˚
thesolidstateof3aand3bare3.653(3)and3.919(3)A, respectively,
while these distances are apparently enlarged by the rotation of the
pyridyl group in the solid state of [3aHN]+ and [3bHN]+. Crystal
packing diagrams and geometric data showing the hydrogen bonds
in the crystalline state of [3aHN][OTf] and [3bHN][OTf] are given
in Fig. S2 and S3 and Tables S1 and S2 in ESI.†
Fig. 3 Selected region of IR spectra of 4b in the presence of 0–4 equiv. of
HOTf in CH2Cl2 at -55 ◦C.
density of the Fe–Fe bond in 4b is much larger than that in 3a
and 3b. This leads to the easy protonation of 4b at the Fe–Fe
bond in the presence of HOTf. The protonation order of 4b is
different from most of other reported analogous diiron complexes
containing internal bases,3,5,28 in which protonation of the internal
base is prior to the Fe–Fe bond. The only example which showed
a similar protonation order as in our case was reported by Ott and
co-workers for protonation of (m-adt)[Fe(CO)2(PMe3)]2 (adt = N-
benzyl-2-azapropane-1,3-dithiolato) in the presence of HCl.4 The
proposed protonation process of 4b is shown in Scheme 1.
Electrochemical properties of 3a, 3b, and 4a
The cyclic voltammograms (CVs) of 3a and 3b in the presence of
different acids were measured to explore the role of the pyridyl-
nitrogen atom in the electrocatalytic proton reduction process. The
CVs of 3a in the presence of HOAc and HOTf, and the plots of
the current heights of electrocatalytic events vs. the concentration
of acids in CH3CN are given in Fig. 5 (For the CVs of 3b, see Fig.
S4 and S5). Complexes 3a and 3b display the first reductive events
(FeIFeI/FeIFe0) at -1.86 and -1.83 V vs. Fc/Fc+, respectively,
which are catalytically active for proton reduction in the presence
of HOAc, while the electrocatalytic efficiencies of 3a and 3b are
relatively low. The overpotentials for the electrocatalytic reduction
of protons from HOAc are 400 mV for 3a and 370 mV for 3b.30
Upon addition of 1 equiv. of HOTf, new reductive events are
observed at -1.50 V for 3a and -1.34 V for 3b (Fig. S5 in ESI†).
The positive shifts of 360–490 mV, resulting from the protonation
of the pyridyl-nitrogen atoms of 3a and 3b in the presence of HOTf,
are comparable to that (400–600 mV) caused by the protonation
of the bridging nitrogen atom in the diiron 2-azapropane-1,3-
dithiolate complexes.4,10,28 The current height of the new event
at -1.50 V is enhanced in the same rate as that for the original
event at -1.86 V with increase in the concentration of HOTf from
1 to 5 mM (Fig. 5(c)). The calculated value of DIav is 16.3 mA per
Molecular structures of 3a, 3b, 4a, 4b, [3aHN][OTf], and
[3bHN][OTf]
The expected square-based-pyramidal coordination geometry is
found for 3a, 3b, 4a, 4b, and the protonated species [3aHN][OTf]
and [3bHN][OTf] according to the calculated structural factors (t)
for the iron atoms.29 ORTEP diagrams for the crystal structures
are shown in Fig. 4. Selected bond lengths, angles, and t values
are listed in Table 2. The X-ray analysis shows that the Ph2PPy
ligand in 3b and [3bHN][OTf] prefers an apical position to minimize
the special crowd in the molecule, which is coincident with
the reported structure for PPh3-monosubstituted complex [(m-
pdt)Fe2(CO)5(PPh3)].19 In contrast, the Ph2PCH2Py ligand in 3a
and [3aHN][OTf] occupies a basal position with the CH2Py group
close to the apical CO ligand. The possible reason for the difference
in the coordination position of the pyridyl-phosphine ligands,
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for 3a, [3aHN][OTf], 3b, [3bHN][OTf], 4a, and 4b
Complex
3a
[3aHN][OTf]
3b
[3bHN][OTf]
4a
2.5519(3)
4b
2.5437(6)
Fe–Fe
Fe–PPy
Fe–PMe
Fe ◊ ◊ ◊ N
P ◊ ◊ ◊ N
S ◊ ◊ ◊ S
2.5650(6)
2.2365(6)
—
2.5654(6)
2.2366(7)
—
2.5266(12)
2.2438(13)
—
2.5247(8)
2.2248(11)
—
2.2227(6)
2.2342(7)
4.060(2)
3.650(3)
3.028(1)
84.41(2)
84.02(2)
68.89(2)
68.78(2)
71.25(2)
0.089
2.2392(8)
2.2385(9)
3.681(3)
2.692(3)
3.042(1)
84.86(3)
83.64(2)
68.40(2)
68.10(2)
71.75(3)
0.058
3.919(3)
3.629(3)
3.038(1)
84.78(3)
84.78(2)
69.29(2)
69.49(2)
69.83(2)
0.005
4.894(2)
3.548(2)
3.033(1)
84.47(3)
85.20(3)
69.50(3)
69.66(3)
69.56(3)
0.080
3.653(3)
2.716(3)
3.048(2)
84.67(4)
84.40(3)
67.82(3)
67.72(3)
71.76(4)
0.342
4.202(3)
2.809(3)
3.035(1)
84.78(4)
84.18(4)
67.84(4)
68.14(3)
71.65(4)
0.296
S–Fe–S
Fe–S–Fe
S-2Fe–S
t Fe(1)
t Fe(2)
0.029
0.083
0.209
0.121
0.050
0.132
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Fig. 4 ORTEP diagrams for 3a (a), [3aHN][OTf] (b), 3b (c), [3bHN][OTf] (d), 4a (e), and 4b (f). Hydrogen atoms are omitted for clarity except the proton
on pyridyl-nitrogen atom in [3aHN][OTf] and [3bHN][OTf].
millimole of HOTf for both events, suggesting that the protonation
of the internal base can effectively reduce the overpotential for
proton reduction but not influence the electrocatalytic efficiency.
The pyridyl-nitrogen atom in the phosphine ligand of 3a and 3b
plays a role of the proton relay, similar to the function of the
bridging nitrogen atom in the diiron azadithiolate complexes.4,10,28
The effect of water on the electrocatalytic efficiency of 4a
was studied by the cyclic voltammetry in CH3CN and in
CH3CN–H2O (50 : 1, v/v) mixed solution. Good CVs for 4a and
4b in the presence of HOTf are not obtained due to the poor
stabilities of the doubly protonated species at room temperature.
The electrochemical property of 4a was studied in the presence of
HOAc. Electrocatalytic proton reduction by 4a occurs at -2.20 V
in pure CH3CN (Fig. 6(a)). Under this condition, the DIav is
10.4 mA per millimole of HOAc according to slope 1 in Fig. 6(c).
The second reductive event is tentatively ascribed to the reduction
of an unknown product decomposed from the one-electron
reduced species according to the previous report.31 The current
height of the electrocatalytic event is considerably enhanced in
CH3CN–H2O (50 : 1, v/v, Fig. 6(b)). The electrocatalytic efficiency
for proton reduction by 4a in CH3CN–H2O (50 : 1, v/v) increases
ca. 5 times (DIav = 51.6 mA per millimole of HOAc, slope 2
in Fig. 6(c)) as compared to that in CH3CN. This result is
different from the previous report for the m-hydride diphosphine-
substituted diiron complex [(m-pdt)(m-H){Fe(CO)2PMe3}2][BF4]
and the PTA-substituted diiron complex (m-pdt) [Fe(CO)2(PTA)]2
(PTA = 1,3,5-triaza-7-phosphaadamantane).32 The former dis-
plays no difference in the electrocatalytic efficiency for reduction of
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Fig. 5 Cyclic voltammograms of 3a in the presence of (a) HOAc, (b) HOTf in CH3CN, and (c) the plots of the current heights of electrocatalytic events
vs. the concentration of acids.
Fig. 6 Cyclic voltammograms of 4a in the presence of HOAc, (a) in pure CH3CN, (b) in CH3CN–H2O (50 : 1, v/v), and (c) the plots of the current
heights of electrocatalytic events vs. the concentration of acids.
protons from HOAc in pure CH3CN or in CH3CN–H2O mixture,
and the electrocatalytic efficiency for the latter in CH3CN–H2O
(10 : 1 to 1 : 3, v/v) is enhanced less than twice as compared to that
found in pure CH3CN. To estimate the direct proton reduction
of HOAc at the electrode, a control experiment was carried out
in the absence of diiron complex, which shows that the DIav is
17.4 mA in CH3CN–H2O (50 : 1, v/v, slope 3 in Fig. 6(c)). It is
not currently clear how a small amount of added water could
apparently enhance the electrocatalytic efficiency of 4b in the
presence of HOAc.
leading to the decrease of the overpotential for electrocatalytic
proton reduction by 360–490 mV, and the hydrophilicity of the
pyridyl group in the diiron complex renders the electrocatalytic
efficiency more susceptible to water.
Experimental
Reagents and instruments
All manipulations related to pyridyl-phosphine ligands and
organometallic complexes were performed using standard
Schlenk technique under N2. Commercially available chemi-
cals, 1,3-propanedithiol, Fe(CO)5, Me3NO·2H2O, picoline, 2-
bromopyridine, and Ph2PCl were reagent grade and used
as received. The pyridyl-phosphine ligands, Ph2PCH2Py34 and
Ph2PPy,35 diiron dithiolate complexes [(m-pdt){Fe(CO)3}2] (1)36
and [(m-pdt){Fe2(CO)5(PMe3)}] (2)19 were prepared according to
literature methods. All organic solvents were distilled by standard
Conclusions
In this work, a pyridyl group was introduced to the phosphine
ligand of the diiron complex as a pendant basic site, which could
approach to or stand apart from the m-hydride of a protonated
diiron complex by ligand and single bond rotation. Four diiron
dithiolate complexes containing pyridyl-phosphine ligands and
two corresponding protonated species were structurally character-
ized. The 1H and 31P NMR evidence at low temperature suggests
that protonation of (m-pdt)[Fe(CO)2(PMe3)][Fe(CO)2(Ph2PPy)]
(4b) by HOTf occurs first at the Fe–Fe bond and then at the
pyridyl-nitrogen atom. This result shows that the protonation
process of diiron models can be readily alternated either by
increasing the electron density of the iron centres,33 or by adjusting
the basicity of the internal base. Therefore, both the basicity and
the relative position of the internal base to the hydride in a designed
catalyst are important factors for hydrogen generation and uptake.
The pyridyl group in the phosphine ligand of diiron complexes
can act as a proton relay in the presence of strong acid just like
the secondary amine in the diiron azadithiolate complexes does,
1
methods and degassed prior to use. The H and 31P{H} NMR
spectra were collected with a Bruker AVANCE II/400 spectrom-
eter. Infrared spectra were recorded with a JASCO FT/IR 430
spectrophotometer. Elemental analyses were performed with an
Elementar Vario EL III elemental analyzer. Mass spectra of iron
complexes were recorded on an HP1100 MSD instrument.
Preparation of 3a, [3aH][OTf], 3b, [3b][OTf], 4a, and 4b
(l-pdt)[Fe2(CO)5(Ph2PCH2Py)] (3a). One portion of Me3NO·
2H2O (0.11 g, 1 mmol) was added to a solution of (m-
pdt)[Fe(CO)3]2 (1) (0.39 g, 1 mmol) in acetonitrile (30 mL) under
N2. The mixture was stirred for 10 min, followed by the addition
of the pyridyl-phosphine ligand Ph2PCH2Py (0.28 g, 1 mmol) to
1924 | Dalton Trans., 2009, 1919–1926
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the solution. After 20 min, the volatiles were removed in vacuo.
The resulting solid was purified by column chromatography on
neutral alumina with hexane and CH2Cl2 as gradient eluents. The
analytically pure solid of 3a (0.44 g, 69%) was obtained as dark
red powder. Crystals suitable for X-ray analysis were grown from a
mixed solution of 3a in hexane and CH2Cl2 at room temperature.
Found: C, 48.93; H, 3.63; N, 2.17. C26H22Fe2NO5PS2 requires C,
49.16; H, 3.49; N, 2.20%; vmax(KBr)/cm-1 2033, 1982, 1956 and
1926 (CO); dH (400 MHz; CDCl3) 8.43 (1 H, s, Py), 7.72–7.43 (10
H, 2 s, Ph), 7.33–6.42 (3 H, 3 s, Py), 4.10 (2 H, d, PCH2), 1.85–
1.50 (6 H, br m, SCH2CH2CH2S); dF (400 MHz; CDCl3) 64.24 (s,
Ph2PCH2Py); m/z(APCI+) 636.0 (M + H+).
v
max(KBr)/cm-1 2039, 2003, 1986, 1969, 1940 and 1923 (CO); dH
(400 MHz; CDCl3) 8.79 (1 H, s, Py), 7.77 (5 H, d, Ph and Py),
7.52 (7 H, d, Ph and Py), 7.41 (1 H, s, Py), 1.77–1.55 (6 H, br m,
SCH2CH2CH2S). dF (400 MHz; CDCl3) 66.70 (s, Ph2PPy); m/z
(APCI+) 621.8 (M + H+).
[(l-pdt)Fe2(CO)5(Ph2PPyH)](OTf)
([3bHN][OTf]). Proto-
nated complex [3bHN][OTf] (0.05 g, 62%) was obtained by
the same procedure as for [3aHN][OTf] but with 3b as starting
complex. Found: C, 40.27; H, 2.82; N, 1.88. C26H21F3Fe2NO8PS3
requires C, 40.49; H, 2.74; N, 1.82%; vmax(KBr)/cm-1 2049, 1989,
1972, 1957 and 1944 (CO); dH (400 MHz; CD2Cl2) 14.68 (1 H, s,
NH), 9.30–8.04 (4 H, 4 s, Py), 7.81–7.64 (10 H, 2 s, Ph), 1.94–1.37
(6 H, 3 s, SCH2CH2CH2S); dF (400 MHz; CD2Cl2) 74.15 (s,
Ph2PPy).
[(l-pdt)Fe2(CO)5(Ph2PCH2PyH)](OTf) ([3aHN][OTf]). Anhy-
drous HOTf (12 mL, 0.13 mmol) was added to a stirring solution
of 3a (0.06 g, 0.1 mmol) in Et2O (20 mL). The red precipitate
immediately appeared. The mixture was stirred for 3 min, and
then stood for 10 min. The upper layer of solvent was decanted.
The deep red powder was collected and washed with Et2O (5 ¥
5 mL). The solid was dried in vacuo to give the protonated product
[3aHN][OTf] (0.04 g, 50%). Crystals suitable for X-ray diffraction
were obtained by the direct diffusion of hexane to the concentrated
CH2Cl2 solution of [3aHN][OTf] over night at room temperature.
Found: C, 41.55; H, 3.01; N, 2.03. C27H23F3Fe2NO8PS3 requires C,
41.29; H, 2.95; N, 1.78%; vmax(KBr)/cm-1 2041, 1990, 1971, 1963
and 1935 (CO); dH (400 MHz; CDCl3) 14.88 (1 H, s, NH), 8.54–
7.76 (3 H, 3 s, Py), 7.70 (4 H, t, Ph), 7.55 (6 H, m, Ph), 7.33 (1 H,
d, Py), 4.44 (2 H, d, PCH2), 1.99–1.45 (6 H, m, SCH2CH2CH2S);
dF (400 MHz; CDCl3) 64.31 (s, Ph2PCH2Py).
(l-pdt)[Fe(CO)2(PMe3)][Fe(CO)2(Ph2PCH2Py)]
(4a). One
portion of Me3NO·2H2O (0.11 g, 1 mmol) was added to a solution
of (m-pdt)[Fe2(CO)5(PMe3)] (2) (0.43 g, 1 mmol) in acetonitrile
◦
(30 mL) under N2 at 35 C. The mixture was stirred for 10 min,
followed by addition of Ph2PCH2Py (0.28 g, 1 mmol) to the
solution. After 40 min, the volatiles were removed in vacuo.
The resulting solid was purified by column chromatography on
neutral alumina with hexane and CH2Cl2 as gradient eluents.
The analytically pure solid of 4a was collected as dark red
powder (0.29 g, 43%). Crystals suitable for X-ray analysis were
obtained by cooling the solution of 4a in hexane and CH2Cl2 to
-20 ◦C. Found: C, 49.31; H, 4.63; N, 2.03. C28H31Fe2NO4P2S2
requires C, 49.22; H, 4.57; N, 2.05%; vmax(KBr)/cm-1 1976, 1945,
1907 and 1884 (CO). dH (400 MHz; CDCl3) 8.37 (1 H, s, Py),
7.81(4 H, s, Ph), 7.42 (6 H, s, Ph), 7.34–6.65 (3 H, 3 s, Py), 4.18
(2 H, s, PCH2), 1.74–1.54 (6 H, br m, SCH2CH2CH2S), 1.45 (9
H, s, PMe3); dF (400 MHz; CDCl3) 59.15 (s, Ph2PCH2Py), 24.78
(s, PMe3); m/z (APCI+) 684.0 (M + H+).
(l-pdt)[Fe2(CO)5(Ph2PPy)] (3b). Complex 3b was prepared by
using Ph2PPy ligand with a similar procedure as that used for
3a. The yield of 3b is 77% (0.48 g). Found: C, 48.21; H, 3.33;
N, 2.22. C25H20Fe2NO5PS2 requires C, 48.33; H, 3.25; N, 2.25%;
Table 3 Crystallographic data and processing parameters for 3a, [3aHN][OTf], 3b, [3bHN][OTf], 4a, and 4b
Complex
Formula
3a
[3aHN][OTf]
3b
[3bHN][OTf]
4a
4b·0.5(C6H14)
C26H22Fe2NO5PS2 C27H23F3Fe2NO8- C25H20Fe2NO5-
C26H21F3Fe2NO8- C28H31Fe2NO4-
PS2
C30H36Fe2NO4-
P2S2
PS2
PS2
P2S2
Mw
635.24
Triclinic
¯
P1
785.31
Triclinic
P1
621.21
Monoclinic
P21/c
9.285(6)
17.400(11)
16.844(11)
90.00
103.196(9)
90.00
2650(3)
4
771.29
Monoclinic
P21/c
683.30
712.36
Triclinic
¯
P1
Crystal system
Space group
Monoclinic
c
¯
P21
˚
a/A
9.0353(13)
12.3801(18)
12.7353(19)
97.723(2)
108.443(2)
91.284(2)
1335.9(3)
2
10.8040(6)
10.9749(8)
14.5372(9)
97.749(4)
109.389(3)
94.117(4)
1598.53(18)
2
19.1778(6)
8.7364(3)
18.9530(6)
90.00
93.071(2)
90.00
3170.92(18)
4
1.228
9.6520(2)
14.0097(3)
11.7256(3)
90.00
103.012(1)
90.00
1544.84(6)
2
1.211
9.165(2)
12.253(2)
15.845(3)
82.194(2)
77.273(2)
73.081(2)
1655.6(6)
2
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
m/mm-1
1.340
1.219
1.349
1.133
Crystal size/mm
0.02 ¥ 0.07 ¥ 0.52 0.20 ¥ 0.22 ¥ 0.30 0.13 ¥ 0.40 ¥ 0.50 0.12 ¥ 0.25 ¥ 0.48 0.30 ¥ 0.30 ¥ 0.40 0.04 ¥ 0.07 ¥ 0.30
2.20–26.97
7773
5618/334
0.0109
1.026
0.0305, 0.0389
0.0750, 0.0804
0.586/–0.248
q range/◦
1.89–27.63
15173
7369 /410
0.0239
2.25–29.01
16207
6432/322
0.0374
2.13–27.50
14629
7058/401
0.0363
1.78–30.00
18855
7618/362
0.0213
1.74–26.00
9005
6349/370
0.0166
Reflns collected
Unique data/Params
Rint
GOF on F2
1.027
1.018
1.024
1.031
1.045
R1 aI > 2s(I), all data
0.0403, 0.0656
0.0935, 0.1065
0.474/–0.350
0.0398, 0.0702
0.0812, 0.0930
0.591/–0.417
0.0502, 0.1008
0.1096, 0.1317
0.613/–0.336
0.0291, 0.0344
0.0691, 0.0717
0.426/–0.236
0.0342, 0.0460
0.0805, 0.0860
0.292/–0.320
wR2 bI > 2s(I), all data
-3
˚
Residual electron density/e A
ꢀ
ꢀ
ꢀ
ꢀ
2
a R1 = ꢀFo| - |Fcꢀ/ |Fo|. b wR2 = [ (w|Fo - Fc2|2)/ w(Fo2)2]1/2
.
c The Flack parameter for 4a in P21 is 0.016(10).
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(l-pdt)[Fe(CO)2(PMe3)][Fe(CO)2(Ph2PPy)] (4b). Complex 4b
was prepared using Ph2PPy ligand with a similar procedure as that
used for 4a. The yield of 4b is 52% (0.35 g). Found: C, 49.28; H,
4.79; N, 1.88. C27H29Fe2NO4P2S2·0.25C6H14 requires C, 49.55; H,
4.74; N, 2.03%; v(KBr)/cm-1 1982, 1947, 1918 and 1896 (CO). dH
(400 MHz; CDCl3) 8.74 (1 H, s, Py), 7.86(4 H, s, Ph), 7.62 (2 H, s,
Py), 7.41 (6 H, s, Ph), 7.24 (1 H, s, Py), 1.65–1.53 (6 H, br m,
SCH2CH2CH2S), 1.45 (9 H, s, PMe3); dF (400 MHz; CDCl3) 63.13
(s, Ph2PPy), 24.84 (s, PMe3); m/z (APCI+) 669.9 (M + H+).
4 G. Eilers, L. Schwartz, M. Stein, G. Zampella, L. Gioia, S. Ott and R.
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6 W. Dong, M. Wang, X. Liu, K. Jin, G. Li, F. Wang and L. Sun, Chem.
Commun., 2006, 305–307.
7 H.-J. Fan and M. B. Hall, J. Am. Chem. Soc., 2001, 123, 3828–3829.
8 Z. Cao and M. B. Hall, J. Am. Chem. Soc., 2001, 123, 3734–3742.
9 J. D. Lawrence, H. Li, T. B. Rauchfuss, M. Be´nard and M.-M. Rohmer,
Angew. Chem., Int. Ed., 2001, 40, 1768–1771.
˚
10 S. Ott, M. Kritikos, B. Akermark, L. Sun and R. Lomoth, Angew.
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11 P. Das, J.-F. Capon, F. Gloaguen, F. Y. Pe´tillon, P. Schollhammer and
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Crystal structure determination of 3a, [3aHN][OTf], 3b,
[3bHN][OTf], 4a, and 4b
˚
12 F. Wang, M. Wang, X. Liu, K. Jin, W. Dong, G. Li, B. Akermark and
L. Sun, Chem. Commun., 2005, 3221–3223.
13 E. J. Lyon, I. P. Georgakaki, J. H. Reibenspies and M. Y. Darensbourg,
Angew. Chem., Int. Ed., 1999, 38, 3178–3180.
14 A. L. Cloirec, S. P. Best, S. Borg, S. C. Davies, D. J. Evans, D. L. Hughes
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15 M. Schmidt, S. M. Contakes and T. B. Rauchfuss, J. Am. Chem. Soc.,
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The X-ray diffraction data were collected on a Bruker SMART
Apex II CCD diffractometer at room temperature. Structures
were solved by direct methods using SHELXTL-97 software
package.37 All non-hydrogen atoms were refined anisotropically.
The hydrogen atoms on pyridyl-N in [3aHN][OTf] and [3bHN][OTf]
were located by difference Fourier maps while other H atoms were
placed at calculated positions and refined with fixed isotropic
displacement parameters. Crystallographic data and processing
parameters are summarized in Table 3.
17 J. L. Nehring and D. M. Heinekey, Inorg. Chem., 2003, 42, 4288–4292.
18 J.-F. Capon, S. E. Hassnaoui, F. Gloaguen, P. Schollhammer and J.
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19 P. Li, M. Wang, C. He, G. Li, X. Liu, C. Chen, B. Akermark and L.
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20 L. Schwartz, J. Ekstro¨m, R. Lomoth and S. Ott, Chem. Commun., 2006,
4206–4208.
Electrochemistry
21 L. Duan, M. Wang, P. Li, Y. Na, N. Wang and L. Sun, Dalton Trans.,
Cyclic voltammograms were recorded on a BAS-100B electro-
chemical potentiostat using a three electrode cell at a scan rate of
100 mV s-1 under argon atmosphere. The working electrode was a
glass carbon disk (0.071 cm2) polished with 3 and 1 mm diamond
pastes and sonicated in ion-free water for 10 min prior to use. The
reference electrode was a non-aqueous Ag/Ag+ (0.01 M AgNO3)
in CH3CN and the auxiliary electrode was a platinum wire. The
acetonitrile (Aldrich, spectroscopy grade) used for electrochemical
measurements was freshly distilled from CaH2 under nitrogen. A
solution of 0.05 M nBu4NPF6 (Fluka, electrochemical grade) in
CH3CN was used as electrolyte. All potentials reported are versus
Fc/Fc+ in CH3CN.
2007, 1277–1283.
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25 Upon addition of HOTf to the CD2Cl2 solution of 4a at -55 ◦C,
precipitates appeared and the red color of the solution faded. The
in situ IR and NMR measurements did not give any useful information
for the protonation process of 4a.
26 X. Zhao, I. P. Georgakaki, M. L. Miller, R. Mejia-Rodriguez, C.-Y.
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27 X. Zhao, I. P. Georgakaki, M. L. Miller, J. C. Yarbrough and M. Y.
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28 F. Wang, M. Wang, X. Liu, K. Jin, W. Dong and L. Sun, Dalton Trans.,
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Acknowledgements
We are grateful to the Chinese National Natural Science Founda-
tion (Grant no. 20633020), the National Basic Research Program
of China (Grant No. 2009CB220009), the Program for Changjiang
Scholars and Innovative Research Team in University (IRT0711),
the Swedish Energy Agency, the Swedish Research Council, and K
and A Wallenberg Foundation for financial supports of this work.
29 F. I. Adam, G. Hogarth, I. Richards and B. E. Sanchez, Dalton Trans.,
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30 G. A. N. Felton, R. S. Glass, D. L. Lichtenberger and D. H. Evans,
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©