Influences on the rotated structure of diiron dithiolate complexes: Electronic asymmetry vs. secondary coordination sphere interaction

Article abstract of DOI:10.1039/c0dt01332c

In the pursuit of a "rotated" structure, and exploration of the influence of the aza nitrogen lone pair, the Fe^IFe^I model complexes wherein two Fe(CO)_3-xP_x moieties are significantly twisted from the ideal confi

Full text of DOI:10.1039/c0dt01332c

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PAPER
Influences on the rotated structure of diiron dithiolate complexes: electronic
asymmetry vs. secondary coordination sphere interaction†
Yu-Chiao Liu,a Ling-Kuang Tu,a Tao-Hung Yen,a Gene-Hsiang Leeb and Ming-Hsi Chiang*a
Received 5th October 2010, Accepted 8th December 2010
DOI: 10.1039/c0dt01332c
In the pursuit of a “rotated” structure, and exploration of the influence of the aza nitrogen lone pair, the
FeIFeI model complexes wherein◦two Fe(CO)3-xPx moieties are significantly twisted from the ideal
2
2+
configuration (torsion angle >30 ) are reported. [Fe2(m-S(CH2)2NiPr(X)(CH2)2S)(CO)4(k -dppe)]2
(X = H, 4; Me, 5) prepared from protonation and methylation, ◦respectively, of [Fe2(m-S(CH2)2NiPr-
2
(CH2)2S)(CO)4(k -dppe)]2, 1, possess U angles of 34.1 and 35.4 (av.), respectively. Such dramatic twist
is attributed to asymmetric substitution within the Fe2 unit in which a dppe ligand is coordinated to one
2
Fe site in the k -mode. In the presence of the N ◊ ◊ ◊ C(COap) interaction, the torsion angle is decreased to
10.8◦, suggesting availability of lone pairs of the aza nitrogen sites within 1 is in control of the twist.
Backbones of the bridging diphosphine ligands also affect distortion. For a shorter ligand, the more
compact structure of [Fe2(m-S(CH2)2NiPr(CH2)2S)(m-dppm)(CO)4]2, 7, is formed. Dppm in a bridging
manner allows achievement of the nearly eclipsed configuration. In contrast, dppe in
[Fe2(m-S(CH2)2NiPr(CH2)2S)(m-dppe)(CO)4]2, 6, could twist the Fe(CO)3-xLx fragment to adopt the
least strained structure. In addition, the N ◊ ◊ ◊ C(COap) interaction would direct the twist towards a
˚
specific direction for the closer contact. In return, the shorter N ◊ ◊ ◊ C(COap) di◦stance of 3.721(7) A and
◦
˚
larger U angle of 26.5 are obtained in 6. For comparison, 3.987(7) A and 3.9 of the corresponding
parameters are observed in 7. Conversion of (m-dppe)[Fe2(m-S(CH2)2NiPr(CH2)2S)(CO)5]2, 2, to
complex 1 via an associative mechanism is studied.
the enzymatic active center adapts an unusual inverted square
Introduction
pyramid structure.6,7 It creates an accessible site for proton
reduction/hydrogen oxidation as well as a possible electron-
transfer route through a semi-bridging carbonyl group that could
mediate electron flux between the Fed site and the protein via the
cysteinyl bridged [4Fe4S] ferredoxin cluster.8
The research of [Fe2(SR)2(CO)6] and its derivatives has been
rejuvenated in recent years mainly due to discovery of the
active site of Fe-only hydrogenase that consists of a similar
{Fe2S2} organometallic unit.1 Numerous synthetic routes have
been developed to better model the [2Fe2S] unit within the H-
cluster in order to gain understanding of the electrocatalytic
mechanism.2 In the known synthetic models,3 the phosphine
substitution is widely approached to mimic the CN- ligation on
the Fe sites in reference to that phosphines are an appropriate
s-donating group which does not lead to polymerization of the
diiron units.4 A few examples are reported to consist of different s-
or p-type substituents.5 Most of the synthetic complexes, however,
fail to replicate a key structural feature of the active site of Fe-
only hydrogenase, which is proposed as being essential for its
high catalytic turnover frequency. One of the Fe moieties of
From their theoretical analysis, Hall and Darensbourg recently
have suggested the presence of a sterically demanding S–S linker
and an asymmetric electronic structure of the Fe sites are effective
synthetic targets to achieve the expected inverted geometry of the
Fe site.9 In the examples of diiron(I) complexes, they have success-
fully shown the torsion angle (U, vide infra) between the FeI(CO)3
and FeI(CO)2L units increases from 0.0◦ of (m-pdt)[Fe(CO)3]2
(pdt = 1,3-propanedithiolate) with the smallest steric bulk to 15.8◦
of the bulky dithiolate species, (m-depdt)[Fe(CO)3]2 (depdt = 2,2-
diethyl-1,3-propanedithiolate). In the presence of larger steric lig-
ands, the torsion angle of 4.26◦ in (m-pdt)[Fe(CO)3][Fe(CO)2IMes]
(IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) is dra-
matically increased to 40.7◦ in (m-dmpdt)[Fe(CO)3][Fe(CO)2IMes]
(dmpdt = 2,2-dimethyl-1,3-propanedithiolate).10
aInstitute of Chemistry, Academia Sinica, Nankang, Taipei 115, Taiwan
bInstrumentation Center, National Taiwan University, Taipei 106, Taiwan.
E-mail: mhchiang@chem.sinica.edu.tw
† Electronic supplementary information (ESI) available: A summary of
the crystallographic data for complexes 4 and 5 after the SQUEEZE
refinement (Table S1). CCDC 793181 (1), 793182 (2), 793183 (3), 793184
(4), 793185 (5), 793186 (6) and 793187 (7). For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c0dt01332c
Asymmetry of the electronic structure about the FeIFeI centers
can be achieved by inequivalent coordination of ligands. In De
Gioia’s theoretical investigation, the transition state to form
the terminal hydride in (m-edt)[Fe(PH3)3][Fe(PH3)(CO)2] (edt =
1,2-ethanedithiolate) has a lower energy by 11 kcal mol-1 than
2528 | Dalton Trans., 2011, 40, 2528–2541
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that in (m-edt)[Fe(PH3)2(CO)]2.11 The resultant t-hydride species
for the former is stabilized by 5 kcal mol-1. Several complexes
with unequal arrangement of the ligands have been reported.12
Most of them are prepared by use of phosphine chelates.13
Few have the torsion angle near 30◦. Two Fe(CO)3-xLx moieties
to release the ring strain. On the other hand, too long backbones
would inhibit formation of the chelated species due to the high
flexibility of the ligand. For dppe (dppe = Ph2P(CH2)2PPh2), three
out of four kinds of species have been observed. In contrast to
formation of (m-dppe)[Fe2(SS)(CO)5]2 in the presence of limited
amount of dppe, [Fe2(SS)(CO)4(m-dppe)] is generally afforded
in the presence of an excess amount of dppe and under reflux
are twisted by 27.7◦ in [Fe2(m-pdt)(CO)4(k -dppm)] (dppm =
2
bis(diphenylphosphino)methane) where both phosphorus atoms
2
2
are situated at the basal position.14 In [Fe2(m-edt)(CO)4(k -dppv)]
conditions.24–26 The k -dppe substituted species was obtained in
(dppv = cis-1,2-bis(diphenylphosphino)ethylene), the torsion an-
gle is 30.2◦ and the diphosphine unit is arranged at the apical/basal
configuration.15 If NO+ is introduced to the same Fe moiety at
which the pho◦sphine is located, the twist becomes more significant,
reaching 36.1 .16 The second approach involves a mixed-valent
FeIIFeI system. Since it is assumed as the Hox state of the active site
of Fe-only hydrogenase,17 many efforts have been paid to explore
this type of chemistry. Ligands with better donating ability are used
to stabilize the FeII center. Besides, bulkier substituents are em-
ployed to possibly preserve the open site. Pickett,18 Darensbourg19
and Rauchfuss20 have reported the formation of thermal sensitive
FeIIFeI species containing a bridging CO group, which have been
characterized by crystallography and spectroscopy.
the presence of Me3NO at 70 ◦C.27
Previously, we have shown that the distance between the aza
nitrogen and the apical carbonyl carbon can be treated as a
dynamic measure of electron density about the Fe centers.28
Herein, we would like to explore the influence of the aza
nitrogen site on the electronic structure of the Fe2 subunits. Two
diphosphine-substituted FeIFeI species in which two Fe moieties
are greatly distorted (torsion angle >30◦) are reported while lone
pairs are not available. Factors such as asymmetric substitution,
the presence of the aza nitrogen lone pair and backbones of the
bridging phosphines in controlling the deviation from the ideal
eclipse configuration are discussed.
Substitution reactions of [Fe2(SS)(CO)6] (SS = dithiolate
bridges) in the presence of diphosphine ligands have been studied
since the 1970s21 and recently they received high attention.
Coordination configuration of the diphosphines within the Fe2S2
moiety depends on various factors: substituent sterics and rigidity
of the ligand, and reaction conditions.22 In general, four kinds of
species are generated, which are depicted in Scheme 1.
Results and discussion
Synthesis and characterization of 1–3
When a toluene solution of [Fe2(m-S(CH2)2NiPr(CH2)2S)(CO)6]2
was treated with dppe at elevated temperature, two products
were isolated (Scheme 2). [Fe2(m-S(CH2)2NiPr(CH2)2S)(CO)4(k -
dppe)]2, 1, as an olive green solid in toluene was separated
from the solution. (m-dppe)[Fe2(m-S(CH2)2NiPr(CH2)2S)(CO)5]2,
2, with higher solubility was purified via column chromatography.
It is reported that the refluxed reactions of the pdt and adt
analogues in the presence of dppe gave [Fe2(m-xdt)(CO)4(m-
dppe)].24,25 In contrast, the identifiable product for [Fe2(m-
S(CH2)2NiPr(CH2)2S)(CO)6]2 in the similar condition is com-
plex 1. It is tentatively suggested that its poor solubility in-
2
2
hibits rearrangement of the k species to the m-isomer in the
Scheme 1 The coordination mode of a diphosphine ligand.
For example, Rauchfuss et al. reported the reaction of [Fe2(m-
pdt)(CO)6] in the presence of dppv at ambient temperature
2
afforded [Fe2(m-pdt)(CO)4(k -dppv)].15 The driving force for for-
2
mation of the product in the k -manner is mainly attributed to the
rigid cis-mode of dppv and thermodynamically-preferred config-
uration of the five-membered ring. When [Fe2(m-odt)(CO)6] (odt =
oxadithiolate) reacts with (Ph2PCH2CH2OCH2)2 in a ratio of 2 : 1,
Song et al. isolated (Ph2PCH2CH2OCH2)2[Fe2(m-odt)(CO)5]2 as
a sole product where the two ends of the phosphine coordinate
to two {Fe2S2} units.23 The length of the ligand backbone also
2
determines the fate: symmetric m-form vs. asymmetric k -form. For
smaller backbones, the acute bite angles would prefer the former
Scheme 2
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Table 1 A list of nCO bands of complexes 1–8 and selected related diphosphine complexes in CH2Cl2
CO/cm-1
Complex
n
DnCOa/cm-1
2
[Fe2(m-S(CH2)2NiPr(CH2)2S)(CO)4(k -dppe)]2, 1
2016, 1944, 1895
2041, 1985, 1971, 1960, 1929
2017, 1943, 1894
121
112
123
120
119
91
83
85
108
111
106
88
(m-dppe)[Fe2(m-S(CH2)2NiPr(CH2)2S)(CO)5]2, 2
2
[Fe2(m-S(CH2)5S)(CO)4(k -dppe)]2, 3
[Fe2(m-S(CH2)2NiPr(H)(CH2)2S)(CO)4(k -dppe)]22+, 4
2024, 1958, 1945, 1904
2026, 1961, 1940, 1907
1982, 1954, 1914, 1891
1982, 1958, 1916, 1899
1983, 1955, 1917, 1898
2023, 1953, 1915
2
[Fe2(m-S(CH2)2NiPr(Me)(CH2)2S)(CO)4(k -dppe)]22+, 5
2
[Fe2(m-S(CH2)2NiPr(CH2)2S)(m-dppe)(CO)4]2, 6
[Fe2(m-S(CH2)2NiPr(CH2)2S)(m-dppm)(CO)4]2, 7
[Fe2(m-S(CH2)5S)(m-dppe)(CO)4]2, 8
2
[Fe2(m-edt)(CO)4(k -dppv)]15
2
[Fe2(m-pdt)(CO)4(k -PNP)]22
2019, 1939, 1908b
2
[Fe2(m-pdt)(CO)4(k -dppm)]14
2023, 1952, 1917c
[Fe2(m-pdt)(CO)4(m-dppe)]24
[Fe2(m-pdt)(CO)4(m-dcpm)]14
1990, 1953, 1920, 1902
1975, 1942, 1906, 1889d
86
a The energy difference between the highest energy and the lowest energy nCO bands. b In CH3CN solution. PNP = (EtO)2PN(Me)P(OEt)2. c In hexane
solution. d dcpm = (C6H11)2PCH2P(C6H11)2.
2
solution. [Fe2(m-S(CH2)5S)(CO)4(k -dppe)]2, 3, was isolated from
the similar reaction with [Fe2(m-S(CH2)5S)(CO)6]2. No evidence of
(m-dppe)[Fe2(m-S(CH2)5S)(CO)5]2 and [Fe2(m-S(CH2)5S)(CO)4(m-
dppe)]2 was observed. Different reactivity towards dppm has been
reported for the pdt and adt analogues.29 Combined with the
current results, this may suggest the bridgehead of the dithiolate
linkers being a non-innocent role in the substitution reactions.
The IR spectra of complexes 1 and 3 feature three nCO bands
each, consistent with the {Fe2S2} core wherein dppe is chelated
to one Fe center. Energies of the nCO bands for both complexes
are lower than their pdt analogues. The IR results are tabulated
in Table 1 together with those of the related diphosphine chelated
complexes for comparison. DFT calculations have suggested that
energies of the highest and lowest nCO bands are an approximate
2
Fig. 1 Molecular structure of [Fe2(m-S(CH2)2NiPr(CH2)2S)(CO)4(k -
gauge of electron richness about the Fe(CO)3 and Fe(CO)P2
dppe)]2, 1, thermal ellipsoids drawn at 30% probability level. All hydrog◦en
moieties.15 The difference of these two bands could be a measure of
electron asymmetry between the P-substituted and unsubstituted
fragments. The values of DnCO for 1 and 3 are greater than
120 cm-1, which is larger than the values of the xdt analogues,
110 cm-1.14,24,27,30 Interestingly, such large difference is usually
observed in the mono-substituted derivatives.31 For the sake of
comparison, DnCO is 70–90 cm-1 for the symmetrically diphosphine
species.32
˚
atoms are omitted for clarity. Selected bond lengths (A) and angles ( ):
Fe–Fe, 2.5406(5); Fe–S, 2.2736(8); Fe–P, 2.2024(8); Fe–CCO,ap, 1.805(3);
Fe–CCO,ba
67.93(2).
, 1.758(3); S–Fe–S, 79.64(3); S–Fe–Fe, 56.04(2); Fe–S–Fe,
additional resonance from its isomer at 58.4 ppm appeared in
a quantitative intensity. Distances of the metal–metal bond for
complexes 1 and 3 are similar (2.54 vs. 2.55 A). For 2, the distance is
˚
X-Ray crystallographic analyses of 1–3 were carried out at
150 K. Their molecular structures are displayed in Fig. 1–3 and
selected metric parameters are included in the figure captions. All
three complexes have a dimer-of-dimer structure, wherein each
dimer composes of a {Fe2S2} unit that resembles the active site of
Fe-only hydrogenase. Each Fe center within the {Fe2S2} moiety is
coordinated by five ligands and the metal–metal bond completes
the distorted octahedral coordination geometry. For complexes
1 and 3, the dppe ligand is coordinated to one of the Fe sites
via the chelated mode, leaving the other {Fe(CO)3} fragment
intact. Two P atoms arrange at the apical/basal configuration,
which has been confirmed by 31P NMR spectroscopy (83.2 and
85.5, and 84.4 and 85.3 ppm for 1 and 3, respectively). For
complex 2, the dppe ligand bridges two dimeric units within the
molecule. Two ends of the diphosphine coordinate to the apical
sites, which pulls two P substituted sides close to each other. Only
one 31P signal at 54.1 ppm was recorded at the NMR spectrum
of 243 K. When it was increased to ambient temperature, an
˚
shorter by about 0.03 A. Sums of the angles about the aza nitrogen
site are 339.7 and 341.9 on average for 1 and 2, respectively. These
indicate the presence of sp3 N site and in turn availability of its
lone pair. The distance between the aza nitrogen and the closest
˚
apical carbonyl carbon is 3.796(4) A in 1, which is shorter than
˚
the averaged value of 4.011(3) A in its parent molecule, [Fe2(m-
S(CH2)2NiPr(CH2)2S)(CO)6]2.28 In 2, the dppe bridge between two
separated {Fe2S2} units constructs a basket-like structure. No
˚
similar N ◊ ◊ ◊ C(COap) distance less than 4 A is observed.
Conversion of 2 to 1
When complex 2 was reacted with dppe, 1 was generated. The
reaction occurs at room temperature at a slow rate. Prolonged
reaction time causes decomposition of the complexes. Fig. 4
displays the IR spectra recorded for the reaction of 2 in the
presence of 1 equiv. of dppe at 55 ◦C in a THF solution.
Intensities of the IR signatures of 2 at 2042, 1986, 1977, 1962,
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Fig.
4
IR spectra for the reaction of (m-dppe)[Fe2(m-S(CH2)2-
NiPr(CH2)2S)(CO)5]2 with 1 equiv. of dppe in a THF solution at 55 ◦C. The
IR spectra recorded at 0, 1, 3, 5 and 22 h are shown.
Fig. 2 Molecular structure of (m-dppe)[Fe2(m-S(CH2)2NiPr(CH2)2S)-
(CO)5]2, 2, thermal ellipsoids drawn at 50% probability level. All hydrog◦en
˚
lived species since no apparent evidence is observed for its presence
in the IR and 31P NMR spectra or the following steps: Fe–P bond
breaking, CO migration and P-chelation occur simultaneously.
The proposed conversion mechanism is displayed as route 1 in
Scheme 3.
atoms are omitted for clarity. Selected bond lengths (A) and angles ( ):
Fe–Fe, 2.5076(7); Fe–S, 2.2722(10); Fe–P, 2.2252(10); Fe–CCO,ap, 1.810(5);
Fe–CCO,ba
67.98(3).
,
1.777(4); S–Fe–S, 79.67(3); S–Fe–Fe, 56.51(3); Fe–S–Fe,
A different reaction route can be proposed if 2 is treated as
a complex of two {Fe(CO)5P} subunits. It is well known that
reactions of [Fe2(m-SS)(CO)5P1] and P2 (P1, P2 = monodentate
phosphines) lead to formation of disubstituted products, (m-
SS)[Fe(CO)2P1][Fe(CO)2P2].33 In the conversion of 2 to 1, the
exogenous dppe ligand could substitute one of the CO groups
via monodentate coordination to the Fe center, which is followed
by coordination of the second P end and CO migration (route 2
in Scheme 3). Since identification of the pendant species, complex
B, is not feasible, dppe is replaced by PPh2Me to confirm whether
formation of the similar species of B, complex C occurs. Instead,
complex 9 is generated, shown in Fig. 5. The results suggest that
the CO-bridge mechanism of route 1 which opens up a site for the
P coordination is favored over the CO-substitution mechanism
(route 2) in the initial step.
2
Fig. 3 Molecular structure of [Fe2(m-S(CH2)5S)(CO)4(k -dppe)]2, 3, ther-
mal ellipsoids drawn at 50% probability level. All hydrogen atoms are
omitted for clarity. Selected bond lengths (A) and angles ( ): Fe–Fe,
◦
˚
2.5488(4); Fe–S, 2.2648(6); Fe–P, 2.1991(6); Fe–CCO,ap, 1.810(2); Fe–CCO,ba
1.763(3); S–Fe–S, 80.21(2); S–Fe–Fe, 55.759(16); Fe–S–Fe, 68.483(18).
,
1936, 1928 cm-1 decrease while the IR bands of 1 (2017, 1947,
1897 cm-1) increase with time. Their intensities only reach a
maximum and then the product is precipitated due to limited
solubility of complex 1. Decomposition occurred without for-
mation of complex 1 while the ligand was absent. This suggests
complex 2 is one of the intermediates in the reaction of [Fe2(m-
S(CH2)2NiPr(CH2)2S)(CO)6]2 and dppe to the formation of 1.
On the basis of the larger U angle (vide infra), it is tentatively
assumed that energy barrier to the rotation of the Fe(CO)3 moiety
is lowered in 2. The phosphine coordination and the turnstile
rotation of the Fe(CO)3 moiety occur in a concerted manner,
concomitant with formation of a bridging or semi-bridging
CO group. The subsequent CO migration results in formation
Distortion of the ideal eclipse configuration
Electronic asymmetry vs. the aza nitrogen bridgehead. The
idealized structure in [Fe2(SR)2(CO)6] possesses a C2v symmetry.
Theoretical calculations show that two Fe centers have equal
Mulliken charges. If one of the Fe(CO)3 moieties is rotated by 60◦,
subsequent breaking of the metal–metal bonding and asymmetric
distribution of Mulliken charges occur.34 It would be interesting to
examine our complexes by the W angle, which is the angle between
1
of the pendant species, [Fe2(m-S(CH2)2NiPr(CH2)2S)(CO)5(k -
dppe)]2. Decarbonylation and chelation of the dppe ligand lead to
complex 1. It is assumed either this pendant complex A is a short-
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Scheme 3
Scheme 4
as the carbonyl spins about the Fe–Fe axis to form a bent CO
bridge. From the DnCO value, it is expected dissimilarity of electron
density between the Fe(CO)3 and Fe(CO)P2 moieties in complex 1
is significant but a W value of 97.9◦ is observed (Table 2). The main
reason for this result could be attributed to the aza nitrogen factor.
Since the IR spectra were recorded at ambient temperature, the
thermal energy has overcome the weak N ◊ ◊ ◊ C(COap) interaction
and the nCO bands are the averaged result of fluxional behavior
of the CO groups, which directly reflects dissimilarity of electron
density. On the other hand, the crystallographic data was collected
at 150 K. The fluxional mechanism of the CO groups has been shut
down at this temperature. In view of its pointing towards the apical
carbonyl and the shorter N ◊ ◊ ◊ C(COap) distance, the presence of
the N lone pair is possibly in control of rotation of the Fe(CO)3
subunit.
Fig. 5 31P NMR spectra recorded over the course of 10 h for the reaction
of 2 and PPh2Me in toluene at 45 ◦C. The signal at -26.47 ppm is from
PPh2Me.
the Fe–Fe vector and the CO group underneath it defined by
Crabtree,35 displayed in Scheme 4. A typical value around 100◦ is
observed for the terminal carbonyl, whereas the W angle decreases
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Table 2 A list of complexes 1–7 and the related diphosphine complexes with U > 25◦ or W < 95◦
Complex
Ua/◦
Wb/◦
2
[Fe2(m-S(CH2)2NiPr(CH2)2S)(CO)4(k -dppe)]2, 1
10.8
97.9
98.5
96.8
98.9
(m-dppe)[Fe2(m-S(CH2)2NiPr(CH2)2S)(CO)5]2, 2
20.7
13.2c
2
[Fe2(m-S(CH2)5S)(CO)4(k -dppe)]2, 3
6.8
0.8c
96.6
[Fe2(m-S(CH2)2NiPr(H)(CH2)2S)(CO)4(k -dppe)]22+, 4
34.1 (33.7)e
36.3 (36.0)e
34.5c (35.8)e
26.5
91.4 (91.4)e
85.9 (85.9)e
99.6 (99.3)e
94.4
2
[Fe2(m-S(CH2)2NiPr(Me)(CH2)2S)(CO)4(k -dppe)]22+, 5
2
[Fe2(m-S(CH2)2NiPr(CH2)2S)(m-dppe)(CO)4]2, 6
[Fe2(m-S(CH2)2NiPr(CH2)2S)(m-dppm)(CO)4]2, 7
4.1
99.0
99.8
105.1
99.7
99.4
3.6c
2
[Fe2(m-edt)(CO)4(k -dppv)]15
30.2
12.3d
27.7
2
[Fe2(m-pdt)(CO)4(k -dppm)]14
[Fe2(m-(SCH2)2NnPr)(CO)4(m-dppm)]29
31
94.9
a The angle, ∠Lap-Fe–Fe-Lap, is defined by Darensbourg to measure the distortion between two apical ligands. b The angle, ∠Fe–Fe–C(CO), is defined
by Crabtree as an indicator for the bent semi-bridging, bridging and terminal carbonyls. c Values are taken from two asymmetric Fe2 units within
one molecule. d This complex crystallizes with two asymmetric molecules in its unit cell. e The values in parentheses are obtained with the SQUEEZE
treatment.
2
[Fe2(m-S(CH2)2NiPr(H)(CH2)2S)(CO)4(k -dppe)]2(CF3COO)2,
2
[4](CF3COO)2, and [Fe2(m-S(CH2)2NiPr(Me)(CH2)2S)(CO)4(k -
dppe)]2(OTf)2, [5](OTf)2, respectively were afforded when complex
1 was reacted with TFA (trifluoroacetic acid) and MeOTf (methyl
trifluoromethanesulfonate). The molecular structures of both
complexes are displayed in Fig. 6 and 7. Their selected metric
Fig. 7 Molecular structures of [Fe2(m-S(CH2)2NiPr(Me)(CH2)2S)(CO)4-
2
(k -dppe)]22+, 5, thermal ellipsoids drawn at 30% probability level. All
hydrogen atoms are omitted for clarity. The side view of partial molecular
˚
structure is shown to highlight the distortion. Selected bond lengths (A)
and angles (◦): Fe–Fe, 2.5444(19); Fe–S, 2.272(3); Fe–P, 2.215(3); Fe–CCO,ap
,
1.795(12); Fe–CCO,ba, 1.773(13); S–Fe–S, 80.26(10); S–Fe–Fe, 55.95(8);
Fe–S–Fe, 68.11(9).
parameters are listed in the corresponding figure captions. The
presence of the protonated and methylated aza N sites provides
an opportunity to examine the possible involvement of the N lone
pair on the W angle. In complex 4, it decreases to 91.4◦ and a
smaller W value, 85.9◦, is obtained for complex 5.
Fig. 6 Molecular structures of [Fe2(m-S(CH2)2NiPr(H)(CH2)2S)(CO)4-
2
(k -dppe)]22+, 4, thermal ellipsoids drawn at 30% probability level. All
hydrogen atoms are omitted for clarity. The side view of partial molecular
structure is shown to highlight the distortion. Selected bond lengths
◦
˚
(A) and angles ( ): Fe–Fe, 2.5946(17); Fe–S, 2.273(3); Fe–P, 2.215(3);
The second parameter can be used to evaluate the extent of
electron asymmetry. The U angle is defined as the torsion angle
Fe–CCO,ap, 1.811(10); Fe–CCO,ba, 1.764(10); S–Fe–S, 81.45(9); S–Fe–Fe,
55.19(7); Fe–S–Fe, 69.62(8).
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of Lap–Fe–Fe–Lap, introduced by Darensbourg, Scheme 4.10 This
value is practically zero in [Fe2(m-pdt)(CO)6] and increases to
15.8◦ for the bulky bridgehead derivative, (m-depdt)[Fe(CO)3]2.
In combination of inequivalent substitution, the U angle reaches
40.7◦ in (m-dmpdt)[Fe(CO)3][Fe(CO)2IMes]. For the current work
if the steric influence is diminished to the minimum such as in
the protonated and methylated species the N sites are swung away
from the Fe(CO)3 subunits, the U angle would practically reflect
the electronic dissimilarity in complexes 4 and 5. Up to date,
2
including complexes 4 and 5, only four k -diphosphine FeIFeI
carbonyl dithiolate complexes that possess the U angle close to
or larger than 30◦ are characterized (Table 2). Compared with
◦
2
2
[Fe2(m-edt)(CO)4(k -dppv)] (U = 30.2 )15 and [Fe2(m-pdt)(CO)4(k -
dppm)] (U = 27.7◦),14 complexes 4 (U = 34.1◦) and 5 (U = 36.3◦)
have the most distorted Fe(CO)3 centers.
The bridging phosphine vs. the aza nitrogen bridgehead. Dppe
is a diphosphine ligand that consists of a two-carbon backbone.
This gives the ligand a higher degree of freedom to adopt the
least strained structure when dppe bridges two Fe centers. For
comparison, the distance between two phosphorus atoms within
the Fe2 subunits of [Fe2(m-S(CH2)2NiPr(CH2)2S)(m-dppe)(CO)4]2,
˚
6, is 3.787 A while the distances of the two basal carbonyl carbons
˚
and Fe–Fe bond are 3.137 and 2.549 A, respectively. A twist away
from the eclipsed configuration between two Fe(CO)2P moieties is
observed in 6, as displayed in Fig. 8a. The U angle is measured to
26.5◦ (Scheme 5). The N ◊ ◊ ◊ C(COap) interaction could facilitate the
˚
rotation as well. Compared with 4.011(3) A of the parent molecule,
[Fe2(m-S(CH2)2NiPr(CH2)2S)(CO)6]2,28 a shorter N ◊ ◊ ◊ C(COap)
distance of 3.721(7) A is observed. The N ◊ ◊ ◊ C(COap) interaction
Fig. 8 Molecular structures of [Fe2(m-S(CH2)2NiPr(CH2)2S)(CO)4(m-L)]2
((a) L = dppe, 6; (b) L = dppm, 7), thermal ellipsoids drawn at 30%
probability level. All hydrogen atoms are omitted for clarity. Selected bond
˚
is expected as an additive factor that would conduct the twist
towards a specific direction for the closer contact between the
apical carbonyl and the aza nitrogen. This phenomenon would
tentatively explain the small U angles in the other dppe related
species: 8 and 6.5◦ for [Fe2(m-SCH2N(iPr)CH2S)(m-dppe)(CO)4]30
and [Fe2(m-pdt)(m-dppe)(CO)4],26 respectively. Repulsion due to
◦
˚
lengths (A) and angles ( ) for 6: Fe–Fe, 2.5485(9); Fe–S, 2.2604(12); Fe–P,
2.2111(13); Fe–CCO,ap, 1.785(5); Fe–CCO,ba, 1.761(5); S–Fe–S, 80.52(4);
S–Fe–Fe, 55.69(3); Fe–S–Fe, 68.63(4); for 7: Fe–Fe, 2.5222(10); Fe–S,
2.2622(15); Fe–P, 2.2141(15); Fe–CCO,ap, 1.772(6); Fe–CCO,ba, 1.751(6);
S–Fe–S, 80.10(5); S–Fe–Fe, 56.12(4); Fe–S–Fe, 67.77(4).
i
steric bulk of the Pr substituent overcomes stabilization from
the anomeric effect.36 This enforces the alkyl group to the
equatorial position, leaving the lone pair of the aza nitrogen
at the axial position close to the apical carbonyl. The possible
N ◊ ◊ ◊ C(COap) and H(CH2) ◊ ◊ ◊ O(COap) interactions could dictate
the almost eclipsed configuration within these two species. The
“Newman projections” in Scheme 5 provide a clear representation
how the N ◊ ◊ ◊ C(COap) and H(CH2) ◊ ◊ ◊ O(COap) interactions could
influence stereochemistry of the species.
On the other hand, the Fe2(CO)4P2 unit with a dppm bridge
˚
has the more compact structure: 2.522 and 2.549 A for the Fe–Fe
bond distances in [Fe2(m-S(CH2)2NiPr(CH2)2S)(m-dppm)(CO)4]2,
7, (Fig. 8b) and complex 6, respectively. Dppm is a relatively
rigid bridging ligand that allows the eclipsed configuration
to be achieved. In other words, the comparable U angle and
N ◊ ◊ ◊ C(COap) distance are expected between the dppm-substituted
derivatives and the all-CO molecules: 3.9 (av.) vs. 6.0◦ and 3.987(7)
i
˚
vs. 4.011(3) A for 7 and [Fe2(m-S(CH2)2N Pr(CH2)2S)(CO)6]2,
respectively (Scheme 6). For comparison, the Fe(CO)2P moieties in
[Fe2(m-pdt)(m-dppm)(CO)4] are twisted by 5.2◦.29 Interestingly, one
exception is found in [Fe2(m-SCH2N(nPr)CH2S)(m-dppm)(CO)4]
in which the twist angle is as large as 31◦.29 In contrast to other
adt analogues, the alkyl substituent in this species is located at the
Scheme 5
2534 | Dalton Trans., 2011, 40, 2528–2541
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Table 3 Electrochemical properties of complexes 1–8 (1 mM in THF, 0.1 M nBu4PF6, v = 100 mV s-1)
Complex
Oxidation potential/V (vs. Fc/Fc+)
Reduction potential/V (vs. Fc/Fc+)
2
[Fe2(m-S(CH2)2NiPr(CH2)2S)(CO)4(k -dppe)]2, 1
0.38,a 0.28,a -0.08, -0.29
0.58, 0.46
-2.44, -2.64,a -2.80b
-2.15, -2.28, -2.59b
-2.36, -2.60,a -2.74b
-2.09, -2.46,c -2.61,a -2.78b
-1.89, -2.15, -2.40, -2.61, -2.78b
-2.59, -2.73b
(m-dppe)[Fe2(m-S(CH2)2NiPr(CH2)2S)(CO)5]2, 2
2
[Fe2(m-S(CH2)5S)(CO)4(k -dppe)]2, 3
0.57,b 0.35,a -0.07, -0.2
0.44,a 0.04,a -0.04
0.54,b 0.19
[Fe2(m-S(CH2)2NiPr(H)(CH2)2S)(CO)4(k -dppe)]22+, 4
2
[Fe2(m-S(CH2)2NiPr(Me)(CH2)2S)(CO)4(k -dppe)]22+, 5
2
[Fe2(m-S(CH2)2NiPr(CH2)2S)(m-dppe)(CO)4]2, 6
[Fe2(m-S(CH2)2NiPr(CH2)2S)(m-dppm)(CO)4]2, 7
[Fe2(m-S(CH2)5S)(m-dppe)(CO)4]2, 8
0.4, 0.29, 0.09
0.42, 0.14
0.36, 0.15
-2.52, -2.65b
-2.56, -2.72b
2
a Responses from the m-species. b Quasi-reversible, inhibited under CO atmosphere. c Responses from the k -species.
Scheme 6
axial position. It exerts a large steric influence that repels
the nearby apical carbonyl. A similar observation has been
reported in the [Fe2(m-SS)(CO)6] series (SS = pdt, depdt).10 No
example with an equatorial alkyl substituent is available for
comparison.
Electrochemistry
2
Cyclic voltammograms of [Fe2(m-S(CH2)2X(CH2)2S)(CO)4(k -
dppe)]2 (X = NiPr, 1; CH2, 3) at room temperature in the accessible
Fig. 9 (a) Cyclic voltammograms of 1 (1 mM) in THF under N2 (v =
100 mV s-1, 0.1 M nBu4NPF6, vitreous carbon electrode). The inset shows
the voltammogram of 1 recorded in CH2Cl2 under the same conditions.
(b) Differential pulse voltammograms of 1 in THF under N2 and CO
(amplitude = 50 mV, sample width = 16.7 ms, pulse width = 50 ms, pulse
period = 200 ms). The redox wave marked as an asterisk is attributed to the
reduction event of the pendant species. The inset shows DPVs of 6 under
the same experimental conditions.
electrochemical window of THF under N2 exhibit two irreversible
reduction processes and one quasi-reversible reduction event at
the most negative potential in the absence of any acid (Table 3
and Fig. 9). The first reduction waves recorded at -2.44 and
-2.36 V are assigned to the reduction of 1 and 3, respectively
(all potentials in this paper are vs. Fc/Fc+). Recently, it is reported
2
that electrocatalytic isomerization of the k -species occurs to form
the m-species.30 On the basis of the electrochemical behavior of
complexes 1 and 3, the same electron transfer-catalyzed (ETC) pro-
cess is expected. [Fe2(m-S(CH2)2X(CH2)2S)(m-dppe)(CO)4]2 (X =
NiPr, 6; CH2, 8) in quantitative yields were obtained from
the selected metric data are included in the figure caption. Results
of electrochemical studies of 6 and 8 confirm the two reduction
2
responsesofthe k -complexes inthemorenegativepotentialregion
2
controlled-potential electrolysis of the corresponding k -species.
(-2.64 and -2.80 V for 1; -2.60 and -2.74 V for 3) are originated
from their m-isomers. Their oxidation waves are also observed in
The molecular structure of complex 6 is shown in Fig. 8a and
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Scheme 7
2
the voltammograms of the k -species. The responses at 0.28 and
0.38 V are ascribed to oxidations of 6 and the one at 0.35 V to 8.
The voltammograms recorded under CO provide evidences to
further differentiate the events at -2.64 and -2.60 V corresponding
to the reduction of 6 and 8, respectively, and at -2.80 and -2.74 V
to the products due to decomposition of the reduced m-species.
Fig. 9b shows voltammetric changes of 1 and 6 recorded by
differential pulse technique. The reduction responses at the most
negative potential are inhibited by CO purge, referring to that CO
dissociation occurs upon reduction of the m-species. To bear in
mind, other chemical reactions are possibly involved such as Fe–S
bond cleavage. Since the redox process of the decomposed species
is quasi-reversible at least at electrochemical timescale, electron
transfer to the m-complex triggers formation of one stable product
that is yet unknown at this moment. It is postulated any species
analogous to the reduction products of [Fe2(m-xdt)(CO)6] (xdt =
pdt, adt) are generated.37
2
Transformation of the k -complex to the corresponding m-
isomer is suppressed upon CO exposure along with appearance of
a new species marked as an asterisk in Fig. 9b. Various experimen-
tal and theoretical studies have shown that the rotated geometry
is favored in the asymmetrically disubstituted [Fe2(m-SS)(CO)4L2]
complexes in which one apical CO group lies underneath the Fe–
Fe vector to form the CO bridge or semi-bridge. This assists the
CO migration and one coordination site is opened up in such
transient species, which is available for exogenous CO-binding. On
the other hand, CO dissociation occurs in the reduced state.38 It
is not known whether these two CO events proceed consecutively
2
or concomitantly; either regenerates the parent k -species. This
mechanism can explain why isomerization is hindered in the CO
atmosphere (Scheme 7). If CO migration occurs instead of CO
liberation, the pendant species is formed. Reduction of [Fe2(m-
S(CH2)2X(CH2)2S)(CO)5L]2 has been recorded at ca. -2.10 V.28
For instance, (m-dppe)[Fe2(m-S(CH2)2NiPr(CH2)2S)(CO)5]2, 2, is
reduced at -2.15 V. Consequently, the minor response at -2.09
and -2.12 V during reduction of 1 and 3, respectively, under CO
Fig. 10 (a) Cyclic voltammogram of 3 (1 mM) in CH2Cl2 (v = 100 mV s-1,
0.1 M nBu4NPF6, vitreous carbon electrode) under N2, showing two suc-
cessive reversible two-electron transfer events. (b) Cyclic voltammograms
n
of 7 (1 mM) in THF (v = 100 mV s-1, 0.1 M Bu4NPF6, vitreous carbon
electrode) under N2 and CO. The inset shows the changes of differential
pulse voltammograms of 7 (amplitude = 50 mV, sample width = 16.7 ms,
pulse width = 50 ms, pulse period = 200 ms).
1
is assigned to [Fe2(m-S(CH2)2X(CH2)2S)(CO)5(k -dppe)]2.
In contrast to its m-isomer, reversibility of the two oxidation
2
waves is conserved for the k -complex in CH2Cl2. For 3, they are
c
a
fully reversible (ip /ip = 1), shown in Fig. 10a. These oxidation
events are assigned to the FeIFeI/FeIIFeI and FeIIFeI/FeIIFeII
redox pairs. The DE1/2 difference for the first and second oxidation
processes of 170 mV indicates weak electronic communication be-
tween the two Fe centers. A comproportionation constant (Kcomp
)
of 752 was calculated according to DE1/2 (V) = 0.0591(log Kcomp).
This suggests that removal of electrons takes place from almost
electronically discrete redox levels. The metal–metal distances of
2536 | Dalton Trans., 2011, 40, 2528–2541
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˚
complexes 1 and 3 (2.541 and 2.549 A) are comparable to those of
optimal stereoisomer for catalysis. The bridging diphosphines with
an appropriate length within the [Fe2(m-xdt)(CO)4P2] unit well
resemble a combination of CN- ligands and the peptide chain as
the donor ends and the chain backbone are already present. For
the donor ability this can be tuned by different substituents on the
phosphines. However, modulation of electron richness about the
Fe centers via hydrogen bonding is a dynamic process in proteins.
This will be a challenge for synthetic chemists to overcome.
˚
the known species (2.53–2.57 A), in which mixed-valence states
are isolated.18–20 Theoretical calculations have shown that the
HOMO is predominately from Fe contributions. Thus factors
other than d-d overlap are possible origins for the existence
of mixed-valence FeIIFeI complexes.39 Likewise, two reductions
of complex 7 are better resolved than for its dppe counterpart
under the same experimental conditions, as displayed in Fig. 10b.
Contributions of ligand field due to structural distortion should
not be negligible. Management of electronic structure and electron
transfer within the Fe centers via tuning the ligand field is essential
to understanding of catalytic property of the model complexes.
Further studies are currently under investigation.
Concluding remarks
Factors that are in control of the distortion within the [Fe2] units
are discussed in this report. In addition to electronic asymmetry,
the results indicate the intramolecular N ◊ ◊ ◊ C(COap) interaction
plays a crucial role on rotation of the Fe(CO)3 moiety. Fig. 11
displays an almost eclipsed configuration within complex 1 is
observed in the presence of the interaction. In contrast, a
significant twist is formed upon removal of the interaction from
availability of the aza nitrogen lone pairs. The distortion angles
of 34.1 and 35.4◦ (av.) for complexes 4 and 5, respectively, are the
largest among their class.
Experimental section
General methods
All reactions were carried out by using standard Schlenk and
vacuum-line techniques under an atmosphere of purified nitro-
gen. All commercial available chemicals were of ACS grade
and used without further purification. Solvents were of HPLC
grade and purified as follows: diethyl ether and THF were
distilled from sodium/benzophenone under N2. Hexane was
distilled from sodium under N2. Dichloromethane was distilled
from CaH2 under N2. Acetonitrile was distilled first over CaH2
and then from P2O5 under N2. Deuterated solvents obtained
˚
from Merck were distilled over 4 A molecular sieves under N2
prior to use. Preparation of [Fe2(m-S(CH2)2NiPr(CH2)2S)(CO)6]2,
i
[Fe2(m-S(CH2)5S)(CO)6]2 and PrN(CH2CH2SH)2 was previously
described.41
Infrared spectra were recorded on a Perkin Elmer Spectrum One
using a 0.05-mm CaF2 cell. 1H, 13C{ H} and 31P{ H} NMR spectra
were recorded on a Bruker AV-500 or DRX-500 spectrometer
operating at 500, 125.7 and 202.49 MHz, respectively. Spectra are
1
1
Fig. 11 Presentation of the distortion along the Fe–Fe axis upon removal
of the N ◊ ◊ ◊ C(COap) interaction within 1 via protonation/methylation of
the aza nitrogen sites. Only the partial molecules are highlighted for a
clearer view.
1
1
referenced to tetramethylsilane (TMS) for H and 13C{ H}, and
1
85% H3PO4 for 31P{ H} NMR spectra. Mass spectral analyses
were done on a Waters LCT Premier XE at Mass Spectrometry
Center in the Institute of Chemistry, Academia Sinica. Elemental
analyses were performed on an Elementar vario EL III elemental
analyzer.
2
The k -mode coordination of dppe in complex 1 generates the
electron richer Fe(CO)P2 and the more electron deficient Fe(CO)3
moieties. Electronic inequivalence is attested by the substantially
shorter distance between the aza nitrogen and the apical carbonyl
˚
carbon of the Fe(CO)3 moiety, 3.796(4) A, compared to that of
Electrochemistry
˚
4.011(3) A in the all-CO molecule.
Different degrees of distortion are also caused by the bridging
diphosphines. A larger U angle of 26.5◦ for 6 is probably resulted
from the least strained structure with the bridge of dppe to two
Fe centers. On the contrary, the short backbone of dppm does not
allow any substantial twist but a compact structure of 7 with U =
3.9◦ (av.).
In reality, the active site of Fe-only hydrogenase is encapsulated
inside the protein pocket wherein hydrogen bonds between the
bound CN- ligands and the peptide chain are present.7,40 The
peptide backbone that is in control of the rotated geometry of
the active site via hydrogen bonding would slightly modulate
the twist as morphology of the protein changes to adopt the
Electrochemical measurements were recorded on a CH Instru-
ments 630 C electrochemical potentiostat using a gastight three-
electrode cell under N2 at room temperature or at the specific
temperature mentioned. A glassy carbon electrode and a platinum
wire were used as working and auxiliary electrodes, respectively.
Reference electrode was a non-aqueous Ag/Ag+ electrode (0.01 M
AgNO3/0.1 M nBu4NPF6). All potentials are measured in 0.1 M
nBu4NPF6 solution in CH2Cl2 or THF. They are reported against
ferrocene/ferrocenium (Fc/Fc+). Controlled-potential electrolysis
of complexes 1 and 3 was performed at the working potential 100
mV more negative than the corresponding first reduction event in
order not to exceed the reduction potential of the m-isomer.
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Table 4 X-Ray crystallographic data
1·2CH2Cl2
[5](OTf)2·1.5CH2Cl2·
[4](TFA)2·4CH2Cl2· O(CH2CH3)2·
2·2CH2Cl2
3
2O(CH2CH3)2
2CH3CN
6·THF
7·4THF
Empirical formula
C76H82Cl4Fe4- C52H58Cl4Fe4- C70H68Fe4O8- C90H108Cl8F6Fe4-
C87.5H103Cl3F6-
Fe4N4O15P4S6
2210.73
C78H86Fe4-
N2O9P4S4
1671.01
150(2)
Tetragonal
P41212
15.5258(4)
15.5258(4)
33.5050(8)
90
90
90
8076.4(4)
4
1.374
C88H106Fe4-
N2O12P4S4
1859.27
150(2) K
Monoclinic
Cc
37.7214(7)
13.5805(3)
21.1039(4)
90
123.2606(12)
90
9040.0(3)
4
1.366
0.851
N2O8P4S4
1768.76
150(2)
Monoclinic
P21/n
N2O10P2S4
1426.38
150(2)
Monoclinic
P21/n
P4S4
N2O14P4S4
2314.90
150(2)
Monoclinic
P21/n
Mr
T/K
Crystal system
1512.76
150(2)
Triclinic
¯
P1
150(2)
Triclinic
¯
Space group
P1
˚
a/A
15.7127(2)
13.9213(2)
18.6454(3)
90
94.2687(7)
90
13.2880(6)
21.5592(9)
21.8668(9)
90
102.760(1)
90
10.63200(10)
17.8511(2)
20.8444(2)
110.0752(6)
96.6827(6)
106.1587(6)
3469.11(6)
2
17.133(3)
13.281(2)
23.904(4)
90
107.252(8)
90
16.4839(5)
17.0452(5)
21.2654(4)
94.2320(2)
108.7318(14)
113.5410(11)
5045.1(2)
2
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
4067.20(10)
2
6109.7(5)
4
5194.5(15)
2
Z
Dc/Mg m-3
m/mm-1
1.444
1.065
1.551
1.350
1.448
1.086
1.480
0.965
1.455
0.903
2282
63604
0.942
3472
27175
F(000)
1824
23077
2920
46682
1560
42631
2384
20998
3888
30121
Reflections collected
Independent reflections 9311
14029
0.0489
1.068
15859
0.0338
1.017
9021
0.0997
1.031
17651
0.0917
1.008
0.1006 (0.1789)
0.2871 (0.3409)
9224
0.0442
1.068
18044
0.0508
1.030
Rint
0.0452
Goodness-of-fit on F2 1.001
R1 [I > 2s(I)] (all data)a 0.0445 (0.0772) 0.0557 (0.0722) 0.0322 (0.0491) 0.0920 (0.1806)
wR2 [I > 2s(I)] (all
0.0487 (0.0706) 0.0511 (0.0862)
0.1344 (0.1466) 0.1119 (0.1279)
0.1119 (0.1254) 0.1287 (0.1380) 0.0749 (0.0825) 0.2289 (0.2928)
data)b
ꢀ
ꢀ
ꢀ
ꢀ
2
a R1 = ( ꢀFo| - |Fcꢀ)/( |Fo|). b wR2 = [ w(Fo - Fc2)2/ w(Fo2)2]1/2
.
2
Molecular structure determinations
solution is almost clear. [Fe2(m-S(CH2)2NiPr(CH2)2S)(CO)4(k -
dppe)]2, 1, was obtained as a dark green solid in 60%
The X-ray single crystal crystallographic data collections for 1–
7 were carried out at 150 K on a Bruker SMART APEX CCD
four-circle diffractometer with graphite-monochromated Mo-Ka
2
yield (524 mg). Crystals of [Fe2(m-S(CH2)2NiPr(CH2)2S)(CO)4(k -
dppe)]2·2CH2Cl2 suitable for X-ray crystallographic analysis were
grown from the CH2Cl2–MeOH solution at -20 ◦C. The red
extract was dried under vacuum and purified by chromatography
on silica gel with CH2Cl2 as the eluent. The red band, 2,
was obtained as a red solid in 21% yield (143 mg). Crystals
of (m-dppe)[Fe2(m-S(CH2)2NiPr(CH2)2S)(CO)5]2·2CH2Cl2 suitable
for X-ray crystallograph◦ic analysis were grown from the CH2Cl2–
MeOH solution at -20 C. 1: IR (CH2Cl2, cm-1): nCO 2016 (vs),
˚
radiation (l = 0.71073 A) outfitted with a low-temperature,
nitrogen-stream aperture. The structures were solved using di-
rect methods, in conjunction with standard difference Fourier
techniques and refined by full-matrix least-squares procedures.
A summary of the crystallographic data for complexes 1–7 is
shown in Table 4. 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 by the
riding model. All software used for diffraction data processing
and crystal structure solution and refinement are contained in the
SHELXL-97 program suites.42 The X-ray data refinement R1 =
0.092 of complex 4 was improved to 0.0759 after removal of co-
crystallized solvents by the SQUEEZE function in the SHELXL
program. A similar refinement was operated for complex 5. The
X-ray data refinement R1 = 0.1006 was improved to 0.0791. A
summary of the crystallographic data for complexes 4 and 5 after
the SQUEEZE refinement is shown in Table S1 (ESI†).
1
1944 (vs), 1895 (w). H NMR (500 MHz, CD2Cl2, 253 K): 0.69
3
3
(d, JHH = 6 Hz, 6H, 2 CH3), 0.75 (d, JHH = 6 Hz, 6H, 2 CH3),
1.74 (t, 2H, 1 CH2), 2.09 (m, 6H, 3 CH2), 2.35 (m, 4H, 1 CH2 + 2
NCH), 2.46 (m, 3H, 2 CH2), 2.64 (m, 7H, 4 CH2), 2.80 (m, 4H, 2
1
CH2), 7.05–7.88 (m, 40H, 8 Ph) ppm. 13C{ H} NMR (125.7 MHz,
CD2Cl2, 253 K): 19.44 (2 CH3), 20.12 (1 CH2), 20.57 (2 CH3),
29.14 (2 CH2), 36.44 (2 CH2), 51.23 (2 CH2), ~54 (CH2 + 2
NCH, overlapped with d-solvent), 127.69, 128.80, 129.20, 129.84,
130.56, 131.80, 132.58, 133.79, 134.99, 137.43 (8 Ph), 213.71 (CO)
1
ppm. 31P{ H} NMR (202.48 MHz, CD2Cl2, 253 K): 83.23 (d,
JPP = 12.8 Hz, 2P), 85.47 (d, JPP = 12.8 Hz, 2P) ppm. ESI-MS:
+
m/z 1599.95 {1 + H+} . Anal. Calc. for C74H78Fe4N2O8P4S4: C,
55.59; H, 4.92; N, 1.75. Found: C, 55.37; H, 4.93; N, 1.74. 2: IR
(CH2Cl2, cm-1): nCO 2041 (vs), 1985 (vs), 1971 (sh), 1960 (sh), 1929
Synthesis of [Fe2(l-S(CH2)2NiPr(CH2)2S)(CO)4(j2-dppe)]2, 1, and
(l-dppe)[Fe2(l-S(CH2)2NiPr(CH2)2S)(CO)5]2, 2
1
3
(w). H NMR (500 MHz, CD2Cl2, 243 K): 0.71 (d, JHH = 6 Hz,
6H, 2 CH3), 1.09 (d, 3JHH = 6 Hz, 6H, 2 CH3), 1.41 (b, 2H, 1 CH2),
1.66 (m, 3H, 2 CH2), 2.08 (m, 4H, 2 CH2), 2.50 (m, 2H, 1 CH2),
2.58 (m, 4H, 2 CH2 + 1 NCH), 2.87 (m, 4H, 2 CH2), 3.10 (m, 3H,
A solution of [Fe2(m-S(CH2)2NiPr(CH2)2S)(CO)6]2 (500 mg, 0.55
mmol) and dppe (800 mg, 2 mmol) in 17 mL of toluene was
heated at 90 ◦C for 6 h. The precipitate was settled and the
red solution was transferred to a different flask. The dark green
powder was then washed with toluene a few times until the upper
1
1 CH2 + 1 NCH), 7.41–7.67 (m, 20H, 4 Ph) ppm. 13C{ H} NMR
(125.7 MHz, CD2Cl2, 243 K): 19.09 (4 CH3), 26.55 (2 CH2), 28.11
(2 CH2), 40.51 (2 CH2), ~54 (2 CH2, overlapped with d-solvent),
2538 | Dalton Trans., 2011, 40, 2528–2541
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55.4 (1 NCH), 56.15 (1 NCH), 56.27 (2 CH2), 129.23, 129.40,
129.55, 129.65, 129.95, 130.17, 130.42, 130.93, 131.57, 132.45,
133.90, 136.11 (4 Ph), 211.10, 213.61, 213.89, 215.40 (CO) ppm.
2.41–3.35 (m, 17H, 2 NCH + 8 CH2), 7.82, 7.87 (br, 2H, 2NH),
1
6.93–7.99 (m, 40H, 8 Ph) ppm. 31P{ H} NMR (202.48 MHz,
CD2Cl2, 225 K): 81.89 (2P), 84.33 (2P) ppm. ESI-MS: m/z 1713.06
1
+
+
+
31P{ H} NMR (202.48 MHz, CD2Cl2, 243 K): 54.14 ppm; (298
{[4](TFA)} , 1598.99 {4 - H+} , 1487.17 {4 - 4CO - H+} . Anal.
Calc. for C78H80F6Fe4N2O12P4S4: C, 51.28; H, 4.41; N, 1.53. Found:
C, 50.87; H, 4.43; N, 1.53%.
+
K): 54.91, 58.36 ppm. ESI-MS: m/z 1256.92 {2 + H+} . Anal.
Calc. for C50H54Fe4N2O10P2S4: C, 47.79; H, 4.33; N, 2.23. Found:
C, 47.74; H, 4.35; N, 2.23%.
Synthesis of [Fe2(l-S(CH2)2NiPr(Me)(CH2)2S)(CO)4(j2-
dppe)]2(OTf)2, [5](OTf)2
Synthesis of [Fe2(l-S(CH2)5S)(CO)4(j2-dppe)]2, 3
A solution of 174 mg (0.11 mmol) of 1 in 8 mL of CH2Cl2 was
treated with 120 mL (1.1 mmol) MeOTf. After it was stirred for
1 h at room temperature, the solution was dried under vacuum.
The brick-red residue was washed by 20 mL of ether twice. The
brick-red powder was precipitated from the CH2Cl2 solution upon
the addition of excess hexane. The product was collected and dried
under vacuum in 96% yield (203 mg) after the upper solution was
To
a flask containing 185 mg (0.22 mmol) of [Fe2(m-
S(CH2)5S)(CO)6]2 and 312 mg (0.78 mmol) of dppe was added
6 mL of toluene. The flask was transferred to an oil-bath and
heated at 90 ◦C for 8 h. The upper solution was removed via
cannula. The green solid was washed by at least three portions
of 4 mL of toluene until the upper solution was almost clear.
The solid was dried under vacuum and re-dissolved in CH2Cl2.
The green powder was precipitated upon the addition of hexane
to the CH2Cl2 solution. The green product was dried and its
yield was calculated to 59% (199.5 mg). Crystals suitable for X-
2
removed. Crystals of [Fe2(m-S(CH2)2NiPr(Me)(CH2)2S)(CO)4(k -
dppe)]2(OTf)2·1.5CH2Cl2·O(CH2CH3)2·2CH3CN suitable for X-
ray crystallographic analysis were grown from the CH3CN–
CH2Cl2 mixed solution layered with ether at -20 ◦C. IR
2
ray crystallographic analysis of [Fe2(m-S(CH2)5S)(CO)4(k -dppe)]2
1
(CH2Cl2, cm-1): nCO 2026 (vs), 1961 (vs), 1940 (vs), 1907 (w). H
were◦grown from the CH2Cl2 solution layered with methanol at
-20 C. IR (CH2Cl2, cm-1): nCO 2017 (vs), 1943 (vs), 1894 (w).
1H NMR (500 MHz, CD2Cl2, 263 K): 0.10 (t, J = 12 Hz, 2H, 1
CH2), 0.85 (m, 4H, 2 CH2), 0.92 (m, 2H, 1 CH2), 1.55 (m, 2H,
1 SCH2), 1.76 (m, 2H, 1 SCH2), 1.91 (m, 2H, 1 CH2), 2.02 (m,
2H, 1 CH2), 2.15 (m, 2H, 1 SCH2), 2.31 (m, 2H, 1 SCH2), 2.70
(m, 6H, 3 PCH2), 2.87 (m, 2H, 1 PCH2), 7.03–7.92 (m, 40H, 8
NMR (500 MHz, CD2Cl2, 253 K): 0.65–1.47 (m, 20H, 4 CH3 + 4
CH2), 1.71–3.9 (m, 24H, 8 CH2 + 2 NCH3 + 2 NCH), 6.78–7.94
1
(m, 40H, 8 Ph) ppm. 31P{ H} NMR (202.48 MHz, CD2Cl2, 225
+
K): 83.39 (2P), 80.44 (2P) ppm. ESI-MS: m/z 1777.1 {[5](OTf)} ,
2+
814.1 {[5]} . Anal. Calc. for C79H86F6Fe4N2O14P4S6Cl2: C, 47.16;
H, 4.31; N, 1.39. Found: C, 47.03; H, 4.54; N, 1.48%.
Ph) ppm. 13C{ H} NMR (125.7 MHz, CD2Cl2, 263 K): 22.87 (2
1
Electrosynthesis of [Fe2(l-S(CH2)2NiPr(CH2)2S)(l-dppe)-
(CO)4]2, 6
CH2), 25.68 (1 CH2), 29.13 (d, JPC = 11.3 Hz, 1 PCH2), 29.35 (d,
JPC = 11.3 Hz, 1 PCH2), 30.02 (2 CH2), 31.02 (1 CH2), 31.36 (d,
JPC = 15.6 Hz, 1 PCH2), 31.58 (d, JPC = 15.6 Hz, 1 PCH2), 32.10 (2
SCH2), 38.10 (2 SCH2), 127.79, 127.73, 128.75, 128.82, 129.23,
129.71, 129.91, 130.61, 131.91, 131.99, 132.08, 132.91, 132.96,
133.92, 133.99, 134.95, 135.05, 135.18, 135.37, 138.15, 138.36,
30 mL of THF was added to an electrochemical cell containing
70 mg (0.044 mmol) of 1 and 1.16 g (3 mmol) of nBu4NPF6.
Graphite rods (6.15 mm in diameter) were used as working and
auxiliary electrodes. The reference electrode was a non-aqueous
Ag/Ag+ electrode. The working potential was set to -2.54 V. The
solution was monitored by in situ FTIR spectroscopy (Mettler
Toledo ReactIR iC10 equipped with a MCT detector and a
0.625-inch SiComp probe) throughout bulk electrolysis. The IR
signatures of 1 decreased as those of 6 increased. The electrolysis
experiment was stopped when the IR bands of 1 disappeared. The
charge passage of 1.57 C (0.37 F mol-1 of 1) was recorded. The
solution was then transferred to a Schlenk flask via cannula and
dried under vacuum to obtain a red–brown solid. Three portions
of 30 mL of ether were used to extract the product from the
mixture containing the electrolyte. The extract was purified by
column chromatography on silica gel with CH2Cl2 as the eluent
to remove free dppe ligand and then with CH2Cl2–ethyl acetate–
hexane (v/v/v 1/1/2) to obtain a red–brown band. The yield
of 6 is 70% (49 mg). Crystals of [Fe2(m-S(CH2)2NiPr(CH2)2S)(m-
dppe)(CO)4]2·THF suitable for X-ray crystallographic analysis
were grown from a THF solution layered with hexane at-20 ◦C. IR
138.50, 138.80 (8 Ph), 213.95 (6 CO), 218.07 (CO), 218.28 (CO)
ppm. 31P{ H} NMR (202.48 MHz, CD2Cl2, 263 K): 84.43 (d, JPP
=
1
14.6 Hz, 2P), 85.34 (d, JPP = 14.6 Hz, 2P) ppm. FAB+-MS: m/z
1514.03 {3 + H+} . Anal. Calc. for C70H68Fe4O8P4S4: C, 55.57; H,
+
4.53. Found: C, 55.14; H, 4.52%.
Synthesis of [Fe2(l-S(CH2)2NiPr(H)(CH2)2S)(CO)4(j2-
dppe)]2(TFA)2, [4](TFA)2
A solution of 100 mg (0.063 mmol) of 1 in 5 mL of CH2Cl2
was treated with 14 mL (0.19 mmol) of TFA. After it was
stirred for 20 min, the solution was dried under vacuum. The
dark brown residue was washed by 30 mL of ether three times
and was re-dissolved in 3 mL of CH2Cl2. The product was
precipitated as a dark green powder upon the addition of
30 mL of hexane. The product was collected and dried under
vacuum in 80% yield (92 mg) after the upper solution was
2
1
removed. Crystals of [Fe2(m-S(CH2)2NiPr(H)(CH2)2S)(CO)4(k -
(CH2Cl2, cm-1): nCO 1982 (m), 1954 (vs), 1914 (s), 1891 (w). H
dppe)]2(CF3CO2)2·4CH2Cl2·2O(CH2CH3)2 suitable for X-ray crys-
tallographic analysis were grown from the CH2Cl2 solution layered
with ether at -20 ◦C. IR (CH2Cl2, cm-1): nCO 2024 (vs), 1958 (vs),
1945 (vs), 1904 (w); nCOO 1684 (w), 1671 (w). 1H NMR (500 MHz,
NMR (500 MHz, CD2Cl2, 253 K): 0.95 (m, 4H, 2 CH2), 1.01 (d,
3JHH = 6 Hz, 12H, 4 CH3), 1.80 (m, 2H, 1 CH2), 2.24 (b, 4H, 2 CH2),
2.62 (t, 3JHH = 8 Hz, 4H, 2 CH2), 2.78 (b, 6H, 2 CH2 + 2 NCH), 2.91
(b, 4H, 2 CH2), 3.66 (m, 2H, 1 CH2), 7.31–7.75 (m, 40H, 8 Ph) ppm.
1
CD2Cl2, 298 K): 0.75 (d, 3JHH = 5 Hz, 6H, 2 CH3), 0.83 (d, 3JHH
=
13C{ H} NMR (125.7 MHz, CD2Cl2, 253 K): 18.46 (4 CH3 + 2
5 Hz, 6H, 2 CH3), 0.87–1.28 (m, 8H, 4 CH2), 1.94 (t, 1H, 1 CH2),
CH2), 26.04 (1 CH2), 27.85 (2 CH2), 39.02 (2 CH2), 51.17 (2 CH2),
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51.98 (2 NCH), 53.47 (2 CH2), 68.26 (1 CH2), 128.43, 128.51,
The solution was gently heated at 45 ◦C and was monitored
by FTIR and 31P NMR spectroscopy frequently. The solution
was dried under reduced pressure once the spectra indicated
PPh2Me was consumed. The solid was re-dissolved in CH2Cl2
and was washed by CH2Cl2–MeOH several times to obtain a
128.98, 129.05, 129.91, 130.58, 130.90, 130.94, 134.20, 134.26 (8
1
Ph), 215.35, 215.40, 217.42, 217.60 (CO) ppm. 31P{ H} NMR
(202.48 MHz, CD2Cl2, 233 K): 55.57, 66.48 ppm. ESI-MS: m/z
+
+
1599.04 {6 + H+} , 1487.16 {6 - 4CO + H+} . Anal. Calc, for
C74H78Fe4N2O8P4S4: C, 55.59; H, 4.92; N, 1.75. Found: C, 55.89;
H, 5.07; N, 1.79%.
2
brown solid. [Fe2(m-S(CH2)2NiPr(CH2)2S)(CO)4(k -dppe)][Fe2(m-
S(CH2)2NiPr(CH2)2S)(CO)5(PPh2Me)], 9, was obtained in 75%
(51 mg) yield. IR (CH2Cl2, cm-1): nCO 2039 (s), 2015 (m), 1979
(vs), 1949 (vs), 1934 (sh), 1895 (w). 1H NMR (500 MHz, CD2Cl2):
0.71 (m, 6H, 2 CH3), 0.84 (m, 6H, 2 CH3), 2.16 (m, 3H, PCH3),
1.76–2.74 (m, 22H, 10 CH2 + 2 NCH), 7.06–7.97 (m, 30H, 6 Ph)
Synthesis of [Fe2(l-S(CH2)2NiPr(CH2)2S)(l-dppm)(CO)4]2, 7
To a flask containing 300 mg (0.33 mmol) of 1 and 290 mg
(0.75 mmol) of dppm was added 15 mL of toluene. The flask
was transferred to an oil-bath and heated at 105 ◦C for 19 h.
The solution was then refluxed for 5 h to complete the reaction.
The solution was dried under vacuum to afford a red solid. It was
purified by chromatography on silica gel with CH2Cl2 as the eluent
to remove free dppm and then CH2Cl2–diethyl ether (v/v 6/1) to
obtain the product, 7, as a red band. It was dried under reduced
pressure to afford a red solid in 88% yield (454 mg). Crystals of
[Fe2(m-S(CH2)2NiPr(CH2)2S)(m-dppm)(CO)4]2·4THF suitable for
X-ray crystallographic analysis were grown from THF–hexane
1
ppm. 31P{ H} NMR (202.48 MHz, CD2Cl2, 296.5 K): 42.89, 86.90
(d, JPP = 11 Hz, 1P), 88.97 (d, JPP = 11 Hz, 1P); 43.70, 87.57 (d,
JPP = 11 Hz, 1P), 89.80 (d, JPP = 11 Hz, 1P); 43.55, 85.72 (d, JPP
=
11 Hz, 1P), 86.63 (d, JPP = 11 Hz, 1P) ppm in a ratio of 6 : 2 : 1.
+
ESI-MS: m/z 1429.03 {9 + H+} .
Acknowledgements
We are grateful to financial support from National Science Council
of Taiwan and Academia Sinica. We also thank Drs. Mei-Chun
Tseng and Su-Ching Lin for help with MS analysis and 2D NMR
experiments, respectively.
◦
solution at -20 C. IR (CH2Cl2, cm-1): nCO 1982 (m), 1958 (vs),
1916 (s), 1899 (sh). 1H NMR (500 MHz, CD2Cl2, 253 K): 0.96 (d,
3
3JHH = 6.5 Hz, 12H, 4 CH3), 2.44 (t, JHH = 7.5 Hz, 4H, 2 CH2),
2.50 (b, 4H, 2 CH2), 2.66 (t, 3JHH = 8.0 Hz, 4H, 2 CH2), 2.80 (m,
4H, 2 CH2 + 2 NCH), 3.41 (dt, J = 14, 11 Hz, 2H, PCH2P), 4.03
(dt, J = 14, 11 Hz, 2H, PCH2P), 7.29–7.61 (m, 40H, 8 Ph) ppm.
References
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31P{ H} NMR (202.48 MHz, CD2Cl2, 253 K): 51.18 (s) ppm. ESI-
+
MS: m/z 1570.98 {7 + H+} . Anal. Calc. for C76H82Fe4N2O9P4S4:
C, 55.56; H, 5.03; N, 1.71. Found: C, 55.58; H, 5.32; N, 1.66%.
Electrosynthesis of [Fe2(l-S(CH2)5S)(l-dppe)(CO)4]2, 8
The similar procedure to synthesis of complex 6 was applied
to preparation of complex 8. The working potential was set to
-2.46 V. The charge passage of 0.96 F mol-1 of 3 was recorded.
The yield of 8 as a red solid is 90%. IR (CH2Cl2, cm-1): nCO 1983
1
(m), 1955 (vs), 1917 (s), 1898 (w). H NMR (500 MHz, CD2Cl2,
298 K): 1.54 (m, 2H, CH2), 1.71 (m, 8H, 4 CH2), 2.11 (m, 2H,
CH2), 2.36 (m, 4H, 2 CH2), 2.45–2.53 (m, 12H, 6 CH2), 7.35–7.79
1
(m, 40H, 8 Ph) ppm. 13C{ H} NMR (125.7 MHz, CD2Cl2, 298
K): 24.24 (4 CH2), 25.44 (1 CH2), 26.08 (2 CH2), 27.41 (1 CH2),
29.84 (1 CH2), 30.20 (1 CH2), 31.24 (1 CH2), 32.74 (1 CH2), 36.11
(1 CH2), 37.99 (1 CH2), 128.36, 128.43, 128.86, 128.93, 129.84,
130.31, 131.08, 131.15, 133.84, 133.91, 133.96, 136.31, 136.61,
139.43, 139.77 (8 Ph), 215.22, 215.30, 217.36, 217.55 (CO) ppm.
1
31P{ H} NMR (202.48 MHz, CD2Cl2, 298 K): 60.48 ppm. Anal.
Calc. for C70H68Fe4O8P4S4: C, 55.57; H, 4.53; S, 8.48. Found: C,
55.24; H, 4.52; S, 8.47%.
Reaction of (l-dppe)[Fe2(l-S(CH2)2NiPr(CH2)2S)(CO)5]2 and
PPh2Me
To a red solution of 60 mg (0.048 mmol) of complex 2 in
10 mL of toluene was added 9 mL (0.048 mmol) PPh2Me.
2540 | Dalton Trans., 2011, 40, 2528–2541
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The Royal Society of Chemistry 2011
©

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