Redox reactions of [FeFe]-hydrogenase models containing an internal amine and a pendant phosphine

Article abstract of DOI:10.1021/ic4025519

A diiron dithiolate complex with a pendant phosphine coordinated to one of the iron centers, [(μ-SCH₂)₂N(CH₂C ₆H₄-o-PPh₂){Fe₂(CO)₅}] (1), was prepared and structurally characterized. The pendant phosphine is dissociated together with a CO ligand in the presence of excess PMe₃, to afford [(μ-SCH₂)₂N(CH₂C ₆H₄-o-PPh₂){Fe(CO)₂(PMe ₃)}₂] (2). Redox reactions of 2 and related complexes were studied in detail by in situ IR spectroscopy. A series of new Fe ^IIFe^I ([3]⁺ and [6]⁺), Fe ^IIFe^II ([4]²⁺), and Fe^IFe ^I (5) complexes relevant to H_ox, H_ox ^CO, and H_red states of the [FeFe]-hydrogenase active site were detected. Among these complexes, the molecular structures of the diferrous complex [4]²⁺ with the internal amine and the pendant phosphine co-coordinated to the same iron center and the triphosphine diiron complex 5 were determined by X-ray crystallography. To make a comparison, the redox reactions of an analogous complex, [(μ-SCH₂)₂N(CH ₂C₆H₅){Fe(CO)₂(PMe ₃)}₂] (7), were also investigated by in situ IR spectroscopy in the absence or presence of extrinsic PPh₃, which has no influence on the oxidation reaction of 7. The pendant phosphine in the second coordination sphere makes the redox reaction of 2 different from that of its analogue 7.

Full text of DOI:10.1021/ic4025519

Article
pubs.acs.org/IC
Redox Reactions of [FeFe]-Hydrogenase Models Containing an
Internal Amine and a Pendant Phosphine
Dehua Zheng,† Mei Wang,*,† Lin Chen,† Ning Wang,†,‡ and Licheng Sun†,§
†State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular Devices, Dalian University
of Technology (DUT), Linggong Road 2, Dalian 116024, China
‡School of Chemistry and Chemical Engineering, Henan University of Technology, 100 Lianhua Street, Zhengzhou 450001, China
§Department of Chemistry, KTH Royal Institute of Technology, Teknikringen 42, Stockholm 10044, Sweden
S
* Supporting Information
ABSTRACT: A diiron dithiolate complex with a pendant phosphine coordinated
to one of the iron centers, [(μ-SCH2)2N(CH2C6H4-o-PPh2){Fe2(CO)5}] (1), was
prepared and structurally characterized. The pendant phosphine is dissociated
together with a CO ligand in the presence of excess PMe3, to afford [(μ-
SCH2)2N(CH2C6H4-o-PPh2){Fe(CO)2(PMe3)}2] (2). Redox reactions of 2 and
related complexes were studied in detail by in situ IR spectroscopy. A series of
new FeIIFeI ([3]+ and [6]+), FeIIFeII ([4]2+), and FeIFeI (5) complexes relevant to
Hox, HoxCO, and Hred states of the [FeFe]-hydrogenase active site were detected.
Among these complexes, the molecular structures of the diferrous complex [4]2+
with the internal amine and the pendant phosphine co-coordinated to the same
iron center and the triphosphine diiron complex 5 were determined by X-ray
crystallography. To make a comparison, the redox reactions of an analogous
complex, [(μ-SCH2)2N(CH2C6H5){Fe(CO)2(PMe3)}2] (7), were also inves-
tigated by in situ IR spectroscopy in the absence or presence of extrinsic PPh3, which has no influence on the oxidation reaction
of 7. The pendant phosphine in the second coordination sphere makes the redox reaction of 2 different from that of its analogue
7.
can give some insights into the mechanism for H2 evolution
and uptake at the [FeFe]-H2ase active site. The better
understanding of enzymatic mechanism will help chemists in
designing highly efficient bioinspired catalysts for hydrogen
production and oxidation. Over the past decade, numerous
synthetic models of Hred were reported, and their chemical and
electrochemical properties were widely studied.9−14 Very
recently, some FeIFeI complexes with a rotated geometry
related to the active site of [FeFe]-H2ase were reported.15,16 In
contrast, reports on the reactivity of [FeFe]-H2ase models in
oxidized states are scarce. To date, only a few structurally
characterized mixed-valence diiron models of Hox were
reported.17−22 These models either bear special σ ligands,
IMes [1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene],15
dppv (cis-1,2-bis(diphenylphosphino)ethane),18,19 and dppn
(1.8-bis(diphenylphosphino)naphthalene),20 or feature a
bulky bridge, dmpdt (2,2-dimethyl-1,3-propanedithiolate).21
Recently, a functional model of Hox containing both an internal
amine and a redox active unit, FcP* (FcP* = Cp*Fe-
(C5Me4CH2PEt2), Cp* = C5Me5), was reported.23 Under CO
atmosphere, the HoCxOmodels readily convert to the correspond-
ing mimics of Hox with extrinsic CO coordinating to the
vacant apical site at the rotated iron center of Hox models.24−26
INTRODUCTION
■
The structural and functional mimicking of the [FeFe]-H2ase
active site and investigation of the enzymatic mechanism for
dihydrogen oxidation and proton reduction have been the focus
in the fields of organometallic and bioinorganic chemistry since
the crystal structure of [FeFe]-hydrogenases ([FeFe]-H2ases)
was unveiled in the end of the most recent century.1,2 The key
structural features of [FeFe]-H2ase active site are a diiron
dithiolate core, an amine cofactor in the S−to−S bridge, and a
4Fe4S cluster tethered to the diiron core through a cysteine
residue.3,4 On the basis of theoretical and experimental studies,
different redox states of [FeFe]-H2ases, namely the super-
reduced state (Hsred),5 the reduced state (Hred), the oxidized
active state (Hox), the CO-inhibited state (HoxCO), and the
oxidized inactive state (Hoxair), have been proposed for the
active site of [FeFe]-H2ases (Figure 1).6−8 Studies on the redox
properties and reactions of [FeFe]-H2ase models of these states
Figure 1. Hsred, Hred, Hox, HoxCO, and Hoxair states of the [FeFe]-H2ase
active site.
Received: October 10, 2013
Published: January 14, 2014
© 2014 American Chemical Society
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Scheme 1. Redox Reactions of 2 and [3]+ in CH2Cl2 at −70 °C
In addition, in situ spectroscopic investigations showed that the
one-electron oxidation processes of 2Fe3S models are
accompanied by coordination of the pendant thioether group
to the rotated iron center.27,28 Some diferrous dithiolate
and 2 were also characterized by 1H and 13C NMR
spectroscopy (Supporting Information).
Electrochemistry of 2. The oxidation of 2 was first probed
by cyclic voltammetry in a CH2Cl2 solution containing 0.1 M
Bu4NPF6 at −40 °C. The cyclic voltammogram (CV) of 2
displays a quasireversible oxidation event at Epa1 = −0.24 V
versus Fc+/0 for the FeIFeI/FeIIFeI couple and an irreversible
oxidation peak at Epa2 = −0.02 V versus Fc+/0 for the FeIIFeI/
FeIIFeII process (Figure 2). When the anodic scan returned at
−0.16 V to avoid the second oxidation process, the reversibility
of the first oxidation event was slightly improved.
air
complexes were also reported as Hox models of [FeFe]-
H2ases.24,29−32
Herein we present a series of redox reactions starting from an
[FeFe]-H2ase model containing an internal amine and a
pendant phosphine, [(μ-adtP){Fe(CO)2(PMe3)}2] (2, adtP =
(μ-SCH2)2NCH2C6H4-o-PPh2). The two-step one-electron
oxidation process of 2 accompanied by intramolecular P- and
N-coordination, the reaction of the in situ generated mixed-
valence species under CO atmosphere, and the reductions of
oxidized FeIIFeI and FeIIFeII species were studied by in situ IR
spectroscopy. The structures of the diferrous complex [4]2+ and
its reduced product 5 were determined by crystallographic
analyses. These [FeFe]-H2ase models are interesting because,
in [4]2+, the internal amine-N atom and the pendant phosphine
cocoordinate to the same iron center, and in 5, there are three
donor ligands and three CO ligands like the [FeFe]-H2ase
active site. A comparative study on the redox reaction of an
analogous complex [(μ-SCH2)2N(CH2C6H4){Fe(CO)2-
(PMe3)}2] (7) shows that the pendant phosphine in the
second coordination sphere makes the redox property of 2
different from its analogue 7.
Figure 2. Cyclic voltammograms of 2 (1.0 mM) in 0.1 M Bu4NPF6/
CH2Cl2 at −40 °C with a scan rate of 100 mV s−1.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of 1 and 2. The starting
compound, [(μ-adtP)Fe2(CO)5] (1), was prepared as a purple
powder in 38% yield from the reaction of formaldehyde and 2-
Ph2PC6H4CH2NH2 with a freshly prepared complex [(μ-
SH)2Fe2(CO)6] in THF.33 Treatment of 1 with 4 equiv of
PMe3 in refluxing toluene for 6 h afforded a diphosphine
complex (2) in 48% yield (Scheme 1), together with a
byproduct of PMe3-monosubstituted diiron complex [(μ-
SCH2)2NCH2C6H4-o-PPh2){Fe(CO)2}{Fe(CO)2(PMe3)}]
(8) in ∼28% yield (see the Supporting Information). The
crystallographic study (Supporting Information Figure S1,
Tables S1−S4), together with ESI-MS (1, m/z = 633.8; 8,
681.9784 for [M + H]+), 31P{1H} NMR (1, δ = 52.9; 8, 24.6
and 49.9), and IR (1, νCO = 1928, 1975, 2041 cm−1; 8, 1909,
1944, 1981 cm−1) spectroscopic data, confirms that the
pendant phosphine is coordinated to one of the iron centers
in 1 and 8. In contrast, the 31P{1H} NMR spectrum of 2
displays a signal at δ −13.5 for the pendant phosphine in
addition to a signal at δ 26.3 for two PMe3 ligands. This
31P{1H} NMR evidence together with MS (m/z = 758.0225 for
[M + H]+) and IR (νCO = 1896, 1941, 1980 cm−1)
spectroscopic data of 2 clearly indicates that the CO-
displacement of 1 by two PMe3 ligands leads to dissociations
of the pendant phosphine and a CO from the iron centers. A
similar association/dissociation of an internal sulfur atom has
been previously described for a 2Fe3S complex.34 Complexes 1
Chemical Oxidations of 2 and [3]+. Chemical oxidation
of 2 was monitored in CH2Cl2 by in situ IR spectroscopy at
−70 °C (Supporting Information Figure S2a). Upon addition
of an equivalent of FcBArF [ArF = 3,5-(CF3)2C6H3], the CO
4
absorptions of 2 shift by 50−80 cm−1 to higher energy for its
oxidized species [3]+ (νCO = 1977, 1992, 2036 cm−1),
accompanied by appearance of a typical band at 1772 cm−1
for a bridging or semibridging CO (Figure 3). Generally, the μ-
CO absorptions of FeIIFeI complexes featuring a vacant apical
site at the rotated FeI center appear in the region of 1830−1940
cm−1,17−21 while the μ-CO bands are observed in the lower
energy region of 1770−1800 cm−1, which is typical for FeIIFeI
complexes with the vacant apical site occupied by CO or a
pendant donor ligand.24−26 Noticeably, the IR absorption
pattern of [3]+ in the CO absorption region has apparently
changed as compared to that of its parent complex 2,
implicating a considerable change in the symmetry of the
molecule. Taking into account this IR evidence, we assume that
the one-electron oxidation of 2 concurs with coordination of
the pendant phosphine (Scheme 1), as observed for the one-
electron oxidation of diiron complexes bearing a pendant
thioether group.27,28 The mixed-valence complex [3]+ is only
stable in CH2Cl2 below −50 °C. The characteristic CO
absorptions of 2 were quantitatively recovered when an
equivalent of Cp2Co was added to the CH2Cl2 solution of
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PMe3 in refluxing toluene, or by direct treatment of the toluene
solution of 2 in the presence of Me3NO or under irradiation of
light (Scheme 2).
Scheme 2. Substitution and Redox Reactions of 1, 2, and
[4]2+
Figure 3. IR spectra of 2, [3]+, [4]2+, and 5 in CH2Cl2 at −70 °C.
[3]+ at −70 °C, indicative of a good chemical reversibility of
redox reactions between 2 and [3]+ at low temperature
(Supporting Information Figure S2b).
Molecular Structures of [4]2+ and 5. X-ray crystallo-
graphic analysis of [4]2+ demonstrates that the pendant
phosphine and internal amine are co-coordinated to the same
iron center with the S−Fe−N angles of 73.95(10)° and the N−
Fe(1)−P(1) angle of 90.67(11)° (Figure 4, Supporting
Information Tables S1 and S2). This iron center features a
six-coordinate distorted octahedral geometry, in which a PMe3
and pendant phosphine together with two thiolates are cis-
coordinated to the iron center in a square plane with the
coordinated internal N atom and a CO ligand in the two apexes
of octahedron. The other iron center is coordinated in a nearly
standard octahedral geometry by two thiolates, a PMe3 and
three CO ligands in a meridional configuration. The long Fe···
Fe distance of 3.5174(12) Å indicates that the two FeII ions are
nonbonding but still linked by two thiolates. The spectroscopic
and crystallographic results show that the oxidation of [3]+ is
accompanied by coordination of the internal amine to the
rotated iron center and by transfer of the semibridging CO
from the rotated iron center to the nonrotated one. The
structure of [4]2+ is uncommon. To our knowledge, only one
example was reported to date on the [FeFe]-H2ase model with
an internal amine-N atom of the S-to-S bridge coordinated to
an iron center.32
The irreversibility of the second oxidation event of 2 in CV
implies that some irreversible chemical reaction occurs rapidly
following the oxidation reaction. The chemical oxidation of
[3]+, in situ generated by oxidation of 2 with 1 equiv of
FcBArF in CH2Cl2 at −70 °C, was monitored by in situ IR
4
spectroscopy (Supporting Information Figure 2Sa). Upon
addition of an extra equivalent of FcBArF , the absorptions of
4
terminal CO ligands of [3]+ immediately shift by ∼80 cm−1 to
higher energy, and no band is observed in the bridging CO
absorption region 1770−1940 cm−1 (Figure 3). The 31P{1H}
NMR spectrum of the diferrous complex ([4]2+) displays three
31P{1H} NMR signals at δ 11.21 (d, Jpp = 38.15 Hz, PMe3),
15.79 (s, PMe3), and 54.20 (d, Jpp = 38.15 Hz, PPh2Ar, Ar =
C6H4-o-CH2N(CH2S-μ)2), indicating an unsymmetrical struc-
ture of [4]2+. Accordingly, the hydrogen atoms of three CH2
groups in [4]2+ exhibit two doublets at δ 6.11, 6.32 (2H,
CH2Ar) and two doublets of doublets at δ 4.71, 5.48 [4H,
N(CH2S)2], while two singlets are observed for these CH2
groups in 1 (δ 3.07 (4H), 4.22 (2H)) and 2 (δ 3.19 (4H), 4.19
(2H)). The NMR spectroscopic evidence suggests the
coordination of the internal amine-N atom to one of the iron
centers of [4]2+.
Chemical Reduction of [4]2+. Treatment of [4]2+ with 2
equiv of Cp2Co in CH2Cl2 at −70 °C leads to dramatic shifts of
CO absorptions to 1945 and 1882 cm−1 for the reduced FeIFeI
complex (5) with an apparent change in IR CO absorption
pattern (Figure 3 and Supporting Information Figure S3).
Complex 5 displays three 31P{1H} NMR singlets at δ 17.42,
21.18, and 60.90, indicating that the pendant phosphine is still
coordinated to the iron center. The specialty of 5 is that it has
three donor ligands and three CO ligands like the [FeFe]-H2ase
active site. Although a number of diphosphine-substituted
diiron dithiolate complexes have been reported, only a few
trisphosphine diiron complexes were found in the literature
because of difficulty in the further CO-displacement of
diphosphine complexes.19,23,35,36 For example, complex 5
cannot be obtained either by treatment of 1 with 4 equiv of
Single crystal X-ray analysis of 5 (Figure 4, Supporting
Information Tables S1 and S2) shows that one of the CO
ligands is lost and the internal amine is dissociated from the
iron center, resulting in the reformation of an Fe−Fe bond with
́
a bond length (2.5861(4) Å) close to that (2.6 Å) for the
[FeFe]-H2ase active site.37 Complex 5 has a similar butterfly
conformation as previously reported diiron dithiolato com-
plexes.38,39 The angle of P(2)−Fe(1)−Fe(2) (102.30(2)°) is
apparently smaller than that of P(3)−Fe(2)−Fe(1)
(109.19(2)°) because of the bulkiness of pendant phosphine
ligand.
Comparative Study on the Redox Reaction of 7.
Complex 7, [(μ-SCH2)2NCH2C6H4){Fe(CO)2(PMe3)}2], is a
well-known diiron complex, and it was prepared as a reference
compound according to the literature procedure.40 To
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Figure 4. Molecular structures of [4]2+ (left) and 5 (right) with thermal ellipsoids set at 30% level. Hydrogen atoms were omitted for clarity.
Selected bond lengths (Å) and angles (deg) for [4]2+: Fe(1)−Fe(2), 3.5174(12); Fe(1)−N(1), 2.037(4); Fe(1)−P(1), 2.250(13); Fe(1)−P(2),
2.2891(15); Fe(2)−P(3), 2.2725(16); Fe(1)−Sav, 2.3660; Fe(2)−Sav, 2.3165; Fe(1)−C(1), 1.749(5); Fe(2)−Cav, 1.826; N(1)−Fe(1)−P(1),
90.67(11); C(1)−Fe(1)−S(2), 101.32(17); N(1)−Fe(1)−S(2), 73.95(10); P(1)−Fe(1)−S(1), 92.16(5); S−Fe−Sav, 79.94; C(2)−Fe(2)−C(4),
175.6(2). For 5: Fe(1)−Fe(2), 2.5862(4); Fe−Sav, 2.2637; Fe(1)−P(1), 2.2505(6); Fe−P(PMe3)av, 2.2353; Fe(1)−C(1), 1.731(2); Fe(2)−Cav,
1.7635; P(1)−Fe(1)−S(2), 106.45(2); P(2)−Fe(1)−Fe(2), 102.30(2); P(1)−Fe(1)−Fe(2), 149.945(19); P(2)−Fe(1)−P(1), 101.32(2).
Scheme 3. Redox Reactions of 7 at −70 °C
Figure 5. IR spectra monitoring (a) the reaction of [3]+ with CO over a period of 90 s and (b) the process of the back reaction of [6]+ to [3]+ over a
period of 18 min in CH2Cl2 at −70 °C under Ar.
understand the influence of a pendant phosphine in the second
coordination sphere, the oxidation reactions of 7, an analogue
of 2, were also studied in the absence and presence of extrinsic
PPh3. Similar to 2, complex 7 also displays a quasireversible
oxidation event for the FeIFeI/FeIIFeI couple (Epa1 = −0.16 V vs
Fc+/0) and an irreversible oxidation peak for the FeIIFeI/FeIIFeII
process (Epa2 = +0.08 V vs Fc+/0, Supporting Information
Figure S4) at potentials 80−100 mV more positive than those
of 2 in 0.1 M Bu4NPF6/CH2Cl2.
2007, and 1866 cm−1 (Supporting Information Figure S5a)
regardless of absence or presence of PPh3. The last band with
lowest energy is attributed to the absorption of a μ-CO ligand
in the one-electron oxidized species [7]+. Upon addition of
PPh3 to the CH2Cl2 solution of [7]+, no change was observed
in the in situ IR spectra, implicating that the extrinsic PPh3
cannot coordinate to the iron center of the rotated
Fe(CO)2(PMe3) unit (Scheme 3). The oxidative coordination
reactivity of 7 with two identical units, Fe(CO)2(PMe3), linked
by a dithiolate bridge is different from diiron dithiolate
complexes containing an unsymmetrical chelating ligand. It is
reported that (μ-pdt){Fe(CO)(L∧L)}{Fe(CO)3} (L∧L = 1,2-
bis(diphenylphosphino)ethane (dppe), IMe-CH2−IMe, IMe = 1-
The in situ IR monitoring shows that, upon addition of an
equivalent of FcBArF , the CO absorptions of 7 at 1985, 1948,
4
and 1908 cm−1 disappeared within 2 min at −70 °C in CH2Cl2,
accompanied with appearance of three new CO bands at 2045,
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methylimidazol-2-ylidene) complexes are readily coordinated
by an extrinsic phosphite ligand at the rotated Fe(CO)3 unit
after one-electron oxidation,41,42 most possibly due to less
electron density and steric hindrance of the Fe center in the
rotated Fe(CO)3 unit of (μ-pdt){Fe(CO)(L∧L)}{Fe(CO)3} as
compared to the Fe(CO)2(PMe3) moiety of [7]+. The large
difference in the μ-CO bands, νμ‑CO = 1772 cm−1 for [3]+ and
1866 cm−1 for [7]+, gives further support for the coordination
of the pendant phosphine during the oxidation process of 2 to
[3]+, while an extrinsic PPh3 is not involved in the one-electron
oxidation reaction of 7. The oxidized species [7]+ returns to 7
immediately upon addition of an equivalent of Cp2Co
(Supporting Information Figure S5b). In situ IR monitoring
(Ar) was bubbled into the solution of [6]+ at −70 °C, the four
CO absorptions of [3]+ was completely recovered in about 20
min (Figure 5b), indicating that the extra CO in [6]+ can be
readily replaced by the pendant phosphine under Ar to
regenerate [3]+.
Moreover, the purple solution of [6]+ immediately changed
to reddish yellow upon addition of an equivalent of FcBArF .
4
The resulting solution gives an exactly identical IR spectrum to
that of [4]2+, indicating that during the further one-electron
oxidation process of [6]+ the coordination of both the internal
amine and the pendant phosphine to the same iron center
occurs, accompanied by loss of a CO and transformation of the
semibridging CO to a terminal CO (Supporting Information
Figure S7).
also shows that further oxidation of [7]+ by FcBArF , even in an
4
excess (up to 4 equiv), is difficult due to the small driving force
considering the potentials for the second oxidation of 7 and the
reduction of Fc+ to Fc.
CONCLUSIONS
■
Redox reactions of 2, [3]+, [4]2+, and [6]+ are all accompanied
by association/dissociation of the pendant phosphine ligand
and/or internal amine. The coordinated pendant phosphine is
readily replaced by extrinsic CO and PMe3. The facile
association/dissociation of the internal amine implicates that
the central N atom of S-to-S bridge can function not only as a
proton transfer relay in the process of proton reduction to
hydrogen, but also as an electron donor to stabilize the
coordination-unsaturated FeII center in the oxidation reaction
of [FeFe]-H2ase models. The flexibility of 2Fe2S framework is
evinced by reversible conversion of 2Fe2S core between a
butterfly and quasisquare configuration in the oxidation process
of [3]+ to [4]2+ and the reduction process of [4]2+ to 5.
Successive oxidative/reductive CO-displacement of diphos-
phine complex 2 in one pot affords a triphosphine FeIFeI
complex 5 in good yield, which cannot be prepared by direct
CO-displacement of 1 and 2. Comparative studies on the
oxidation of 7 in the absence and presence of PPh3 show that
the effect of the pendant phosphine in the second coordination
sphere on the stability of oxidized species of [FeFe]-H2ase
models cannot be surrogated by an extrinsic phosphine ligand.
Reaction of [3]+ with CO and Chemical Oxidation of
[6]+. It is reported that the mixed-valence diiron models readily
take up extrinsic CO to the open site of the rotated FeI
center.24−26 Although there is no vacant coordination site in
[3]+, upon bubbling of CO into the CH2Cl2 solution of [3]+ at
−70 °C, the CO absorptions of [3]+ disappeared within 2 min
(Figure 5a). The most noticeable change is the semibridging
CO absorptions, shifting from 1770 to 1800 cm−1 (Supporting
Information Figure S6). The terminal CO bands at 1990 and
1975 cm−1 for [3]+ merge into one band, which shifts to 2011
cm−1 for the resulting complex ([6]+), while the position of the
highest energy CO stretch of [3]+ at 2037 cm−1 remains
unchanged but the intensity of this band appears to increase
relative to the other CO bands. On the basis of the changes in
IR spectra, we assume that the coordinated pendant phosphine
at the open site of the rotated FeI center is replaced by extrinsic
CO to form a product of symmetric structure containing two
Fe(CO)2PMe3 units linked by a pair of thiolatos and a μ-CO
ligand (Scheme 4). The shift of the CO bands to higher
wavenumbers is due to the better π-acceptor ability of CO
ligand as compared to PPh3. The easy displacement of the
coordinated pendant phosphine by CO in the formation of [6]+
from [3]+ and by PMe3 in the preparation of 2 from 1 shows
that the coordination of the pendant phosphine in the mixed-
valence diiron complex [3]+ is quite labile. When the inert gas
EXPERIMENTAL SECTION
■
Reagents and Instruments. All reactions and operations related
to organometallic compounds were carried out under dry, oxygen-free
dinitrogen atmosphere with standard Schlenk techniques. Solvents
were dried and distilled prior to use according to the standard
methods. The reagents LiEt3BH and 3,5-bis(trifluoromethyl)-
bromobenzene were purchased from Aldrich. Other commercially
available chemicals such as Fe(CO)5, Cp2Fe, Cp2Co, trifluoroacetic
acid, and 36% formalin were purchased from local suppliers and used
Scheme 4. Redox Reactions of [3]+, [4]2+, and [6]+ at −70
°C
as received. Compounds [(μ-S2)Fe2(CO)6],43 7,40 (2-
(diphenylphosphino)phenyl)methylamine,44 and FcBArF
prepared according to literature procedures.
were
45
4
Infrared spectra were measured with a JASCO FT/IR 430
spectrophotometer. In situ IR spectra were recorded using a
Mettler-Toledo ReactIR 15 System equipped with an MCT detector
and a Dsub AgX SiComp in situ probe. 1H and 31P{1H} NMR spectra
were collected with a Varian INOVA 400 NMR spectrometer. Mass
spectra were recorded either on a TOF mass spectrometer
(Micromass) or an Agilent 6224 Accurate-Mass TOF mass
spectrometer.
Preparation of 1. To a red solution of [(μ-S2)Fe2(CO)6] (2.0 g,
5.81 mmol) in THF (30 mL) at −78 °C was added 12 mL of a 1.0 M
LiEt3BH (12.0 mmol). The resulting green solution was allowed to stir
for another 2 h, followed by dropwise addition of trifluoroacetic acid
(1.0 mL). After the solution was allowed to warm to room
temperature, a portion of a 36% formalin solution (1.0 mL, 12.0
mmol) was added, and the solution was stirred for 1 h. Compound [2-
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(diphenylphosphino)phenyl]methylamine (1.69 g, 5.83 mmol) was
then added, and the mixture was stirred at 60 °C overnight. The
solvent was then removed on a rotary evaporator. The crude product
was purified by chromatography on silica gel first with CH2Cl2 and
then with petroleum ether as eluents. Product 1 was isolated as a
purple powder in 38% yield (1.4 g) after the solvent was removed
under reduced pressure. Crystals of 1 were obtained from CH2Cl2/
hexane at room temperature. IR (CH2Cl2): νCO = 1928, 1975, 2041
cm−1. 1H NMR (CDCl3): δ = 3.07 (s, 4H, N(CH2S)2), 4.22 (s br, 2H,
NCH2Ar), 7.06, 7.30, 7.41, 7.61 (4t, 4H), and 7.49 (s br, 10H) for Ph
and Ar. 13C NMR (CDCl3): δ 209.1, 207.9, 141.4, 138.8, 133.4, 132.2,
130.7, 128.8, 127.9, 55.3, 51.7. 31P{1H} NMR (CDCl3): δ = 52.9 (s,
PPh2Ar). ESI-MS: found for [M + H]+, m/z 633.8; for [M + Na]+,
655.8; calcd for [1]+, m/z 632.9. Anal. Calcd for C26H20NO5S2PFe2: C,
49.31; H, 3.18; N, 2.21%. Found: C, 49.18; H, 3.17; N, 2.19%.
Preparation of 2. To a purple solution of 1 (0.16 g, 0.25 mmol) in
toluene (50 mL) was added a PMe3 hexane solution (0.11 mL, 1.0
mmol, 9.7 M). After the resulting solution was refluxed for 6 h, the
solvent was removed on a rotary evaporator. The crude product was
purified by chromatography on silica gel with hexane/CH2Cl2 (2:1, v/
v) as eluent. The first red band afforded 2 in 48% yield (91 mg). The
second band contained mainly the PMe3-monosubstituted diiron
complex 8 (yield: ∼28%, 50 mg), which was always contaminated with
2 as these mono- and disubstituted diiron complexes have very similar
polarity and solubility.
1.37 [2s, 18H, P(CH3)3], 3.40 [s br, 4H, N(CH2S)2], 4.11 (s, 2H,
NCH2Ar), 6.90−7.95 (br, 14H, Ph and Ar). 31P{1H} NMR (CDCl3):
δ = 17.42 (s, PMe3), 21.18 (s, PMe3), 60.90 (s, PPh2Ar). HR-ESI-MS
(m/z): found, 730.0266 for [M + H]+; calcd for 5, 729.0205. Anal.
Calcd for C30H38NO3S2P3Fe2: C, 49.40; H, 5.25; N, 1.92%. Found: C,
49.05; H, 5.22; N, 1.89%.
X-ray Structure Determination of 1, [4](BArF ) , and 5. The
4 2
single-crystal X-ray diffraction data were collected with an Bruker
Smart Apex II CCD diffractometer with graphite-monochromated Mo
Kα radiation (λ = 0.0710 73 Å) using the ω−2θ scan mode. Data
processing was accomplished with the SAINT processing program.46
Intensity data were corrected for absorption by the SADABS
program.47 All structures were solved by direct methods and refined
on F2 against full-matrix least-squares methods by using the SHELXTL
97 program package.48 All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were located by geometrical
calculation. Details of crystal data, data collections, and structure
refinements are summarized in Supporting Information Table S1, and
selected bond lengths and bond angles are listed in Supporting
Information Table S2. CCDC-953860 (1), -953859 ([4](BArF )2),
4
and -953861 (5) contain the supplementary crystallographic data for
this Article. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Electrochemistry Measurements of 2 and 7. Cyclic voltammo-
grams were recorded in a three-electrode cell under Ar using CHI
630D electrochemical workstation. The working electrode was a glassy
carbon disc (diameter 3 mm) polished with 3 and 1 μm diamond
pastes and sonicated in ion-free water for 15 min prior to use. The
reference electrode was a nonaqueous Ag+/Ag (0.01 M AgNO3 in
CH3CN) electrode, and the counter electrode was platinum wire. A
solution of 0.1 M nBu4NPF6 (Fluka, electrochemical grade) in CH2Cl2
was used as supporting electrolyte, which was degassed by bubbling
with dry argon for 15 min before measurement. The ferricinium/
ferrocene (Fc+/Fc) couple was used as an internal reference, and all
potentials given in this work are referred to the Fc+/0 potential.
Characterization data for 2 follow. IR (CH2Cl2): νCO = 1896, 1941,
1980 cm−1. 1H NMR (CDCl3): δ = 1.46 and 1.48 (2s, 18H, P(CH3)3),
3.19 (s, 4H, N(CH2S)2), 4.19 (s, 2H, NCH2Ar), 6.85, 7.14, 7.30, and
7.45 (4s br, 14H, Ph and Ar). 13C NMR (THF-d8): 207.9, 141.1,
138.0, 133.6, 131.3, 129.8, 128.2, 127.3, 58.4, 51.7, 19.2, 18.9. 31P{1H}
NMR (CDCl3): δ = 26.3 (s, 2PMe3), −13.5 (s, PPh2Ar). HR-ESI-MS:
found for [M + H]+, m/z 758.0225; calcd for 2, 757.0154. Anal. Calcd
for C31H38NO4S2P3Fe2: C, 49.16; H, 5.06; N, 1.85%. Found: C, 48.92;
H, 5.09; N, 1.84%.
Characterization data for 8 follow. IR (CH2Cl2): νCO = 1909, 1944,
1
1981 cm−1. H NMR (CDCl3): δ = 1.47 (s, 9H, P(CH3)3), 3.25 (s,
4H, NCH2S), 3.40 (s, 2H, NCH2Ar), 6.99, 7.22, 7.30, 7.50 (4s br,
14H, Ph and Ar). 31P{1H} NMR (CDCl3): δ = 24.6 (s, 1P, PMe3),
49.9 (s, 1P, PPh2Ar). HR-ESI-MS: calcd for C28H29NO4S2P2Fe2, m/z
680.9712; found for [M + H]+, m/z 681.9784. Anal. Calcd for
C28H29NO4S2P2Fe2: C, 49.36; H, 4.29; N, 2.06%. Found: C, 48.98; H,
4.26; N, 2.05%.
ASSOCIATED CONTENT
■
S
* Supporting Information
Cyclic voltammogram of 7; in situ IR spectra for the redox
reactions of 2, [4]2+, [6]+, and 7, and for the reaction of [3]+
Preparation of [4]2+. Complex 2 (0.03 g, 0.04 mmol) was
dissolved in CH2Cl2 (3 mL), and the solution was cooled to −70 °C
under Ar. The precooled solution of FcBArF4 (83.2 mg, 0.08 mmol) in
CH2Cl2 (3 mL) was transferred to the above solution. After the
mixture was stirred at −70 °C for 0.5 h, the precooled hexane (10 mL)
was added to precipitate the product, which was collected by filtration.
The reddish brown crude product was washed with 2 × 2.5 mL
pentane to further remove impurities. Complex [4]2+ was obtained in
92% yield (92 mg). Crystals suitable for single crystal analysis was
obtained by recrystallization of [4]2+ from CH2Cl2/hexane at 4 °C. IR
(CH2Cl2): νCO = 2115, 2074, 2051, 1963 cm−1. 1H NMR (CDCl3): δ
= 1.80 and 1.83 [2s, 18H, P(CH3)3], 4.67 and 4.76 [2d, JHH = 16.8 Hz,
2H, N(CH2S)2], 5.44 and 5.52 [2d, JHH = 13.2 Hz, 2H, N(CH2S)2],
6.11 and 6.32 (2d, JHH = 12.6 Hz, 2H, NCH2Ar), 7.15, 7.22, 7.61, and
7.85 (4s br, 14H, Ph and Ar). 31P{1H} NMR (CDCl3): δ = 11.21 (d,
Jpp = 38.15 Hz, PMe3), 15.79 (s, PMe3), 54.20 (d, Jpp = 38.15 Hz,
PPh2Ar). ESI-MS (m/z): found, 378.0 for [M]2+; calcd for [4]2+,
378.5.
with CO; crystallographic data of 1, [4](BArF )2, 5, and 8;
4
crystal structures of 1 and 8. Crystallographic data in CIF
format. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: symbueno@dlut.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the Natural Science Foundation of China
(Grants 21373040, 21120102036, and 21101057), the Basic
Research Program of China (Grant 2014CB239402), the
Swedish Energy Agency, and the K & A Wallenberg
Foundation for financial support of this work.
Preparation of 5. Complex [4](BArF )2 (198.8 mg, 0.08 mmol)
4
was dissolved in CH2Cl2 (5 mL), and the solution was cooled to −70
°C under Ar. The precooled solution of Cp2Co (30.4 mg, 0.16 mmol)
in CH2Cl2 (5 mL) was transferred to the above solution. After the
mixture was stirred at −70 °C for 0.5 h, the solvent was removed
under reduced pressure, and the residue was extracted with CH2Cl2/
hexane (1:3, v/v). Complex 5 was obtained in 52% yield (30 mg).
Crystals suitable for single crystal analysis was obtained by
recrystallization of 5 from CH2Cl2/hexane at room temperature. IR
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(CH2Cl2): νCO = 1945, 1883 cm−1. H NMR (CDCl3): δ = 1.25 and
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