Influence of tertiary phosphanes on the coordination configurations and electrochemical properties of iron hydrogenase model complexes: Crystal structures of [(μ-S2C3H6)Fe 2(CO)6-nLn] (L = PMe2Ph, n = 1, 2; PPh3, P(OEt)3, n = 1)

Article abstract of DOI:10.1002/ejic.200400947

A series of mono- and disubstituted diiron complexes [(μ-pdt)-Fe ₂(CO)₅L] [pdt = 1,3-propanedithiolato; L = PMe₃ (2), PMe₂Ph (3), PPh₃ (4), P(OEt)₃ (5)] and [(μ-pdt)Fe₂(CO)₄L₂] [L = PMe₂Ph (6), PPh₃ (7), P(OEt)₃ (8)] were prepared as Fe-only hydrogenase-active-site models by controllable CO displacement of [(μ-pdt)Fe₂(CO)₆] by tertiary phosphanes. The coordination configurations of 3-6 were characterized by X-ray crystallography. Disubstituted diiron complex 6 features an apical/apical coordination mode, instead of the typical transoid basal/basal configuration. The electrochemistry of 2-6 and 8 was studied by cyclic voltammetry to evaluate the effects of different tertiary phosphane ligands on the redox properties of the iron atoms of model complexes. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.

Full text of DOI:10.1002/ejic.200400947

FULL PAPER
Influence of Tertiary Phosphanes on the Coordination Configurations and
Electrochemical Properties of Iron Hydrogenase Model Complexes: Crystal
Structures of [(μ-S2C3H6)Fe2(CO)6–nLn] (L = PMe2Ph, n = 1, 2; PPh3,
P(OEt)3, n = 1)
Ping Li,[a] Mei Wang,*[a] Chengjiang He,[a] Guanghua Li,[b] Xiaoyang Liu,[a,b]
Changneng Chen,[c] Björn Åkermark,[d] and Licheng Sun*[a,e]
Keywords: Binuclear iron complexes / Bioinorganic chemistry / Carbonyl displacement / Fe-only hydrogenase / P ligands
A series of mono- and disubstituted diiron complexes [(μ-pdt)-
Fe2(CO)5L] [pdt = 1,3-propanedithiolato; L = PMe3 (2),
PMe2Ph (3), PPh3 (4), P(OEt)3 (5)] and [(μ-pdt)Fe2(CO)4L2] [L
= PMe2Ph (6), PPh3 (7), P(OEt)3 (8)] were prepared as Fe-only
hydrogenase-active-site models by controllable CO displace-
ment of [(μ-pdt)Fe2(CO)6] by tertiary phosphanes. The coor-
dination configurations of 3–6 were characterized by X-ray
crystallography. Disubstituted diiron complex 6 features an
apical/apical coordination mode, instead of the typical
transoid basal/basal configuration. The electrochemistry of
2–6 and 8 was studied by cyclic voltammetry to evaluate the
effects of different tertiary phosphane ligands on the redox
properties of the iron atoms of model complexes.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
with the active site of Fe-only hydrogenase. The introduc-
tion of good donor ligands, such as cyanide, tertiary phos-
phanes and isonitriles, in the [(μ-pdt)Fe2(CO)6] complex
renders the iron atoms more electron-rich and more proto-
philic. In recent years, there have been numerous reports
Introduction
The Fe-only hydrogenases, which can effectively catalyze
the reduction of protons to dihydrogen in numerous micro-
organisms,[1,2] are of significance to the economical pro-
duction of H2. The composition and the structure of the
active site of Fe-only hydrogenases have been established
by high-resolution X-ray crystallographic and spectroscopic
studies of Fe-only hydrogenases isolated from Desulfovibrio
desulfuricans and Clostridium pasteurianum.[3,4] These
achievements in molecular biology and protein crystallogra-
phy provide an opportunity for chemists to chemically
mimic the structure and function of the Fe-only hydro-
genase-active site and to explore cheap and efficient catalyst
candidates for proton reduction.
regarding the CO displacement of [(μ-pdt)Fe2(CO)6] by
[10–13]
CN–,[5–9] PR3
and CNR,[14,15], as well as the proton-
ation and electrochemistry of the CN–- and PR3-substituted
derivatives.[16–19] Tertiary phosphane ligands are preferably
used in many model complexes of the Fe-only hydrogenase-
active site for the following reasons: 1) The electronic char-
acteristics of tertiary phosphane ligands towards the Fe
atom are similar to those of CN– ligands. 2) The steric and
electronic properties of PR3 are tunable by changing the R
groups. 3) The replacement of CN– by PR3 can avoid the
complications of protonation on the cyanide nitrogen atom.
Although CO-displacement reactions of [(μ-SR)2Fe2(CO)6]
with tertiary phosphanes have been extensively studied
since the 1970s,[19–23] to the best of our knowledge, only a
limited number of structurally characterized PR3-monosub-
stituted derivatives of diiron dithiolate complexes have been
The propanedithiolato-bridged dinuclear complex [(μ-
pdt)Fe2(CO)6] (1) bears remarkable structural similarities
[a] State Key Laboratory of Fine Chemicals, Dalian University of
Technology,
Zhongshan Road 158–46, Dalian 116012, China
E-mail: symbueno@vip.sina.com
[b] State Key Laboratory of Inorganic Synthesis and Preparative reported in the literature.[11–13] In addition to the crystal
Chemistry, Jilin University,
structures of PMe3-disubstituted complexes [(μ-SR)2-
Changchun 130012, China
Fe2(CO)4(PMe3)2] (R = Me, Et)[17,24] and [(μ-SRS)Fe2-
[c] State Key Laboratory of Structural Chemistry, Fujian Institute
(CO)4(PMe3)2] (R = CH2CH2, CH2CH2CH2),[10,17] the co-
of Research on the Structure of Matter,
Fuzhou 350002, China
ordination configurations of other PR3-disubstituted com-
[d] Department of Organic Chemistry, Arrhenius Laboratory,
plexes, [(μ-pdt)Fe2(CO)4L2] (L = PMe2Ph, PPh3), are still
not known. Albeit complex [(μ-pdt)Fe2(CO)4(PMe2Ph)2]
was reported recently[17] and the PPh3-disubstituted com-
plexes [(μ-SR)Fe2(CO)4(PPh3)2] (R = Me, Ph) were pre-
Stockholm University,
10691 Stockholm, Sweden
[e] KTH Chemistry, Organic Chemistry, Royal Institute of Tech-
nology,
10044 Stockholm, Sweden
2506
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejic.200400947
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Influence of Tertiary Phosphanes on Iron Hydrogenase Model Complexes
FULL PAPER
pared in the early 1970s,[25–27] the coordination positions of the PPh3 ligand in 4 and 7 was indicated by 31P NMR spec-
PMe2Ph and PPh3 in these diiron complexes are hitherto tral studies.
unknown. A series of PR3-substituted diiron complexes,
[(μ-pdt)Fe2(CO)5L] [L = PMe3 (2), PMe2Ph (3), PPh3 (4),
P(OEt)3 (5)] and [(μ-pdt)Fe2(CO)4L2] [L = PMe2Ph (6),
PPh3 (7), P(OEt)3 (8)], were prepared for a systematic study
of the influence of phosphane ligands on the coordination
structures and the redox potentials. Here we describe the
preparation, spectroscopic characterization and the coordi-
nation configurations of complexes 2–8, as well as the elec-
trochemical properties of complexes 2–6 and 8. The X-ray
crystallographic study shows that the PMe2Ph ligands on
the Fe atoms of 6 are in an apical/apical coordination mode,
which is in contrast with the transoid basal/basal configura-
tion of [(μ-pdt)Fe2(CO)4(PMe3)2][10] and the recently re-
ported complex [(μ-pdt)Fe2(CO)4(PTA)2] (PTA = 1,3,5-tri-
aza-7-phosphaadamantane).[12]
Scheme 1. Reagents and conditions: (i) for 2, L = PMe3 (1 equiv.),
hexane, room temp., 5 h; for 3, L = PMe2Ph (1 equiv.), benzene,
room temp., 10 h; for 4, L = PPh3 (2 equiv.), toluene, reflux, 6 h;
for 5, L = P(OEt)3 (1 equiv.), toluene, reflux, 3 h; (ii) L = PPh3,
P(OEt)3 (4 equiv.), toluene, reflux, 6 h.
Results and Discussion
Preparation and Spectroscopic Characterization of
Complexes 2–8
Monosubstituted complexes [(μ-pdt)Fe2(CO)5L] (2–5)
were readily prepared in moderate to good yields by treat-
ing 1 with PR3, according to literature procedures (see
Scheme 1).[20,26] Different solvents, namely, hexane, benzene
and toluene, were used for the individual reactions in the
light of the solubility of the tertiary phosphanes. The reac-
tion of 1 with 1 equiv. of a less bulky tertiary phosphane (L
Scheme 2.
The products obtained were characterized by IR, 1H and
= PMe3, PMe2Ph) can be controlled at the monosubstituted 31P NMR spectroscopy and elemental analysis. The results
stage at room temp. Complex 5 was obtained by treating 1 of the elemental analyses for all products are in good agree-
with 1 equiv. of P(OEt)3 in refluxing toluene. PPh3 (2 ment with the supposed compositions. Complexes 2–8 each
equiv.) is required to obtain 4 in good yield for the displace- show three or four infrared bands in the ν(CO) stretching
ment of CO in 1 by PPh3 in refluxing toluene. The PMe2Ph- region (1900–2050 cm–1). The IR data of ν(CO) for 2–8 are
disubstituted complex 6 was obtained in satisfactory yield listed in Table 1 for a comparison, together with the ν(CO)
either by treatment of the monosubstituted complex 3 with data of [(μ-pdt)Fe2(CO)6] and [(μ-pdt)Fe2(CO)4L2] [L =
an additional equiv. of PMe2Ph in refluxing toluene (see PMe3 (9)].[10,28,29] The ν(CO) bands are considered as a use-
Scheme 2) or by a straightforward reaction of the all-CO ful indicator for detecting the variation in the electron den-
complex 1 with 4 equiv. of PMe2Ph. The formation of PPh3- sity of the Fe atoms in the model complexes of the Fe-only
disubstituted complex 7 is more difficult than that of the hydrogenase-active site. A comparison of the ν(CO) bands
PMe3-[10] and PMe2Ph-disubstituted derivatives,[17] presum- for the four subsets of mono- and disubstituted homologues
ably due to the large bulk of the PPh3 ligand. The reaction in Table 1 (2 vs. 9, 3 vs. 6, 4 vs. 7 and 5 vs. 8) indicates that
of 1 with PPh3 in a molar ratio of 1:4 (1/PPh3) in refluxing the introduction of a phosphane ligand has a considerable
toluene for 5 h afforded the PPh3-monosubstituted complex effect on the ν(CO) bands, while the difference in phos-
4 as a major product together with a low yield of the PPh3- phane ligands appears to have a smaller influence on the
disubstituted complex 7. With the PPh3-monosubstituted shift values of the ν(CO) bands of 2–8. The red shifts of
complex 4 as a starting reactant, the reaction also gave 7 in 44–58 cm–1 for the first ν(CO) bands and of 29–38 cm–1 for
low yield after 10 h of reflux in toluene but the major part the second ones are observed (see Table 1) as the monosub-
of 4 did not react, even in excess PPh3. A similar behaviour stituted complexes 2–5 are converted into their disubsti-
to PPh3 was found for the double CO-displacement of 1 by tuted homologues 6–9. Compared with the all-carbonyl
P(OEt)3. The reaction of 1 with 4 equiv. of P(OEt)3 in re- complex 1, the average values of the three strong ν(CO)
fluxing toluene gave 5 as the major product and the P(OEt)3- bands for the PR3-disubstituted complexes are lowered by
disubstituted complex 8 in low yield. All complexes (2–8) about 95, 89, 75, and 69 cm–1 for 9, 6, 7, and 8, respectively.
are relatively thermo- and air-stable in the solid state. The The order of the red-shift values of the ν(CO) bands for
PPh3 derivatives, 4 and 7, are less stable in solution than 6–9 is consistent with the electron-donating capabilities of
their analogous PMe3-, PMe2Ph- and P(OEt)3-substituted different phosphane ligands, which exhibit a clear ranking
diiron complexes. In toluene at room temp., dissociation of of PMe3 Ͼ PMe2Ph Ͼ PPh3 Ͼ P(OEt)3.
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P. Li, M. Wang, C. He, G. Li, X. Liu, C. Chen, B. Åkermark, L. Sun
Table 1. Comparison of ν(CO) bands in PR3-mono- and -disubstituted complexes of 1.[a]
FULL PAPER
Δν[b] [cm–1]
Complex
ν(CO) [cm–1]
Note
Δν of 1st ν(CO)
Δν of 2nd ν(CO)
1
2074(m), 2036(s), 1995(s)
2037(s), 1980(s), 1919(m)
2040(s), 1980(s), 1921(m)
2044(s), 1984(s), 1931(m)
2046(s), 1989(s), 1936(m)
1982(s), 1946(s), 1910(s)
1997(s), 1951(s), 1933(s)
2002(s), 1960(s), 1935(s)
1979(s), 1942(s), 1898(s)
–
–
ref.[29]
2
–37
–34
–30
–28
–58
–47
–44
–58
–56
–56
–52
–47
–34
–33
–29
–38
this work
this work
this work
this work
this work
this work
this work
ref.[10]
3
4
5
6
7[c]
8
9
[a] The spectra were measured in CH3CN unless stated otherwise. [b] For the PR3-monosubstituted complexes 2–5: Δν = ν(CO)mono
ν(CO)allCO and for the PR3-disubstituted complexes 6–9: Δν = ν(CO)di – ν(CO)mono. [c] The spectrum was measured in CHCl3.
–
Molecular Structures of Complexes 3–6
plexes are all in the butterfly conformation as in previously
reported models,[9–17] and each iron atom is coordinated
with a pseudo-square-pyramidal geometry. The CO dis-
placement by one or two molecules of tertiary phosphane
ligands has only a small effect on the Fe–Fe distances
[2.5048(10) Å in 3, 2.5247(6) Å in 4, 2.5142(9) Å in 5 and
2.5198(13) Å in 6] as compared to that of [(μ-pdt)Fe2-
(CO)6] [2.5103(11) Å].[28]
Complexes 4 and 6 were previously prepared according
to the same reactions,[11,17] but until now their coordination
configurations had not been structurally characterized. As
the coordination positions of the phosphane ligands in the
diiron complexes cannot be determined solely by the spec-
troscopic data, X-ray diffraction studies were undertaken
for 3–6. The molecular structures of 3–6 are displayed in
Figure 1 and selected bond lengths and angles are listed in
The coordination configurations of the monosubstituted
Tables 2 and 3. The central 2Fe2S structures of four com- complexes 3, 4 and 5 are essentially identical with that of
Figure 1. Molecular structures of 3 (a), 4 (b), 5 (c) and 6 (d).
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Eur. J. Inorg. Chem. 2005, 2506–2513

Influence of Tertiary Phosphanes on Iron Hydrogenase Model Complexes
FULL PAPER
Table 2. Selected bond lengths [Å] and angles [°] for complexes 3– approximately facing the six-membered ring of the pro-
5.
panedithiolate. The angles of C(14)S(1)Fe(1) [114.9(2)°] and
C(16)S(2)Fe(1) [114.04(18)°] in 3 are a bit wider than the
corresponding angles of C(14)S(1)Fe(2) [110.7(2)°] and
C(16)S(2)Fe(2) [111.93(19)°]. Compared with 3, even larger
differences between the counterpart angles C(24)S(1)Fe(1)
[116.29(15)°] vs. C(24)S(1)Fe(2) [108.84(15)°] and C(26)S(2)-
Fe(1) [116.62(15)°] vs. C(26)S(2)Fe(2) [109.71(15)°] are ob-
served for the structure of 4. It shows that the six-mem-
bered ring of the propanedithiolate in 3 and 4 is pushed
away from the site occupied by an apical PMe2Ph or PPh3
ligand, leading to the tilt of the propanedithiolate ring
towards the Fe(CO)3 site. The P(1)Fe(1)Fe(2) angle is
10.17(17)° larger than the C(4)Fe(2)Fe(1) angle in 4, while
the difference between two angles is only 1.99(34) and
0.57(21)° for 3 and 5, respectively. The PR3 ligands used in
the present work display the steric effects on the coordina-
tion structures in the order PPh3 ϾϾ PMe2Ph Ͼ P(OEt)3.
The Fe(1)–P(1) bond lengths of 2.2222(15) Å in 3,
2.2566(9) Å in 4 and 2.1696(12) Å in 5 are similar to the
values of Fe–P bond lengths reported for PR3-coordinating
diiron complexes.[9–13,17–19] The average Fe(2)–C(CO)
bonds, 1.785–1.787 Å for 3–5, are slightly shorter than in
[(μ-pdt)Fe2(CO)6] [av. 1.800(3) Å],[28] and the average Fe(1)–
C(CO) distance [1.757(9) Å] of 4 is considerably shortened
by coordination of PPh3 to Fe(1).
Theoretically, there are four possible coordination config-
urations for double-CO displacement by two identical non-
carbonyl ligands on each iron atom of [(μ-pdt)Fe2(CO)6],
that is, ap/ap, ap/ba, transoid and cisoid ba/ba configura-
tions (see Scheme 3).[17] The first three possible configura-
tions can each be related to one or two well-characterized
examples, reported in recent years as biomimetic models of
the Fe-only hydrogenase-active site.[5,10,12,15] The structur-
ally characterized diiron propanedithiolate with an Fe–Fe
bond for the cisoid ba/ba configuration model has not yet
been reported. Only a protonated diiron complex with
double-CO displacement by PMe3 and CN– on two iron
atoms was found in a cisoid ba/ba configuration.[18] The X-
ray single-crystal analysis shows that the PMe2Ph-disubsti-
tuted complex 6 possesses an ap/ap coordination configura-
tion, which is identical to the coordination mode of the
large isocyanide ligand tBuNC,[15] but in contrast with the
transoid ba/ba configuration of [(μ-pdt)Fe2(CO)4(PMe3)2]
and [(μ-pdt)Fe2(CO)4(PTA)2],[10,12] presumably due to the
steric requirement of PMe2Ph. It could be predicted that
for tertiary phosphane ligands of large volume an ap/ap
configuration should be sterically less crowded than a ba/
Complex
3
4
5
Bond lengths
Fe(1)–Fe(2)
Fe(1)–S(2)
Fe(1)–S(1)
Fe(2)–S(1)
Fe(2)–S(2)
Fe(1)–P(1)
Fe(1)–C(1)
Fe(1)–C(2)
Fe(2)–C(3)
Fe(2)–C(4)
Fe(2)–C(5)
Bond angles
P(1)–Fe(1)–S(1)
P(1)–Fe(1)–S(2)
P(1)–Fe(1)–C(1)
P(1)–Fe(1)–C(2)
S(1)–Fe(1)–C(1)
S(1)–Fe(1)–C(2)
S(2)–Fe(1)–C(1)
S(2)–Fe(1)–C(2)
S(1)–Fe(1)–Fe(2)
S(2)–Fe(1)–Fe(2)
S(1)–Fe(2)–C(3)
S(1)–Fe(2)–C(4)
S(1)–Fe(2)–C(5)
S(2)–Fe(2)–C(3)
S(2)–Fe(2)–C(4)
S(2)–Fe(2)–C(5)
2.5048(10)
2.2653(14)
2.2617(13)
2.2653(14)
2.2707(13)
2.2222(15)
1.791(5)
1.772(5)
1.776(6)
1.788(6)
1.797(6)
2.5247(6)
2.2639(10)
2.2700(9)
2.2598(9)
2.2629(10)
2.2566(9)
1.759(4)
1.756(4)
1.786(4)
1.797(4)
1.771(5)
2.5142(9)
2.253(4)
2.267(4)
2.276(4)
2.259(4)
2.1696(12)
1.771(14)
1.779(15)
1.785(12)
1.800(7)
1.770(19)
107.04(6)
105.45(5)
93.1(2)
107.13(3)
111.79(4)
97.15(12)
94.94(13)
89.37(12)
157.80(13)
150.99(12)
85.34(13)
55.94(3)
104.86(15)
105.42(14)
98.0(5)
94.9(4)
87.7(5)
160.2(4)
156.5(5)
88.5(4)
56.56(11)
56.24(11)
83.0(5)
103.7(5)
155.8(6)
154.2(5)
106.3(5)
89.4(5)
97.61(19)
87.72(18)
155.34(19)
161.3(2)
88.31(18)
56.47(4)
56.59(4)
88.2(2)
105.37(16)
155.3(2)
154.8(2)
104.59(17)
85.4(2)
56.08(3)
86.70(13)
101.93(12)
157.88(14)
158.12(16)
101.73(14)
87.53(14)
Table 3. Selected bond lengths [Å] and angles [°] for complex 6.
Complex
6
Bond lengths
Fe(1)–Fe(1A)
Fe(1)–S(1)
Fe(1)–S(1A)
Fe(1)–P(1)
Fe(1)–C(11)
Fe(1)–C(12)
Bond angles
P(1)–Fe(1)–S(1)
P(1)–Fe(1)–S(1A)
P(1)–Fe(1)–C(11)
P(1)–Fe(1)–C(12)
S(1)–Fe(1)–C(11)
S(1)–Fe(1)–C(12)
S(1A)–Fe(1)–C(11)
S(1A)–Fe(1)–C(12)
S(1)–Fe(1)–Fe(1A)
S(1A)–Fe(1)–Fe(1A)
2.5198(13)
2.2750(14)
2.2691(14)
2.2286(13)
1.757(5)
1.724(5)
103.81(5)
110.50(5)
96.54(18)
93.24(17)
159.64(17)
88.53(18)
87.96(16)
156.21(17)
56.43(4)
56.21(4)
their analogues [{μ-S(C6H3-m-CH3)S}Fe2(CO)5(PPh3)],[11] ba and an ap/ba configuration. It is noteworthy that the
[(μ-pdt)Fe2(CO)5(PTA)][12] and [{(μ-SCH2)2N(C6H4-4-Br)}- complex [(μ-pdt)Fe2(CO)4(CN)2]2– with two small CN– li-
Fe2(CO)5(PPh3)].[13] Tertiary phosphane ligands PMe2Ph, gands on each iron atom features an ap/ba configuration,[5]
PPh3 and P(OEt)3 are coordinated to an apical site on Fe(1) implying that not only the steric requirement but also the
and roughly trans to the Fe–Fe bond. Both 31P NMR and electronic effects of a ligand can influence the coordination
X-ray crystallographic analyses of 3–5 suggest that one CO configurations of diiron propanedithiolates. Another no-
displacement by tertiary phosphane in [(μ-pdt)Fe2(CO)6] af- ticeable fact is that although the protonated species [(μ-
fords only an apical isomer, as shown in Figure 1(a–c). One H)(μ-pdt)Fe2(CO)4(PMe2Ph)2] is in a transoid ba/ba config-
of the phenyl groups of the apical PPh3 ligand in 4 and uration in the crystalline state,[17] the coordination structure
the sole phenyl group of the apical PMe2Ph ligand in 3 are of its parent complex [(μ-pdt)Fe2(CO)4(PMe2Ph)2] is in an
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P. Li, M. Wang, C. He, G. Li, X. Liu, C. Chen, B. Åkermark, L. Sun
FULL PAPER
Scheme 3. Four possible coordination configurations resulting from the L orientation.
ap/ap geometry in the crystalline state. In solution, the co-
ordination configurations of the PR3-substituted diiron
complexes might be mobile structural forms.[12] It is unam-
biguous that there exists a rotation of CO and PMe2Ph li-
gands during the protonation process of iron atoms in 6,
just as in the very recently reported isomerization in the
protonation process of diruthenium propanedithiolate.[30]
The phenyl groups of the apical PMe2Ph ligands in 6
both face the inner top side of the molecule, framing a
glider-shape conformation with a C2 symmetry. The dihe-
dral angle between the S(1)C(9)C(9A)S(1A) and S(1A)-
Fe(1)S(1) planes and the counterpart angle between the
planes of S(1A)C(9)C(9A)S(1) and S(1A)Fe(1A)S(1) are
found to have the same value (131.5°) for 6. The average
Fe–C(CO) bond of 1.741 Å in 6 is shortened, and mean-
while the average C–O bond [1.163(1) Å in 6] is elongated
by coordination of the PMe2Ph ligands to the iron atoms,
as compared with [(μ-pdt)Fe2(CO)6] [Fe–C(CO)av.
1.800(3) Å and C–Oav. 1.136(1) Å].[28] The CO displacement
by the better donating tertiary phosphane ligands enhances
electron accumulation on the iron centers of a diiron dithi-
olate model, resulting in stronger back-bonding from the
metal atoms to the CO ligands and at the same time weak-
ening the CO triple bonds.
Figure 2. Cyclic voltammograms of complexes 2–5.
Cyclic Voltammograms of Complexes 2–6 and 8
The redox properties of model complexes are one of the
interesting features in the structural and functional mimics
of the Fe-only hydrogenase-active site. The cyclic voltam-
mograms (CV) of complexes 2–6 and 8 were studied to eval-
uate the effects of different phosphane ligands on the redox
properties of the central iron atoms. The CV measurements
were carried out in CH3CN and scanned in the cathodic
Figure 3. Cyclic voltammograms of 3 (1×10–3 m) under Ar and CO.
direction as indicated in Figures 2, 3 and 4. The redox po- FeIIFeII. Complexes 2–5 exhibit the first reductive peaks
tentials of 2–6 and 8, as well as their parent complex [(μ- at –1.86, –1.82, –1.76 and –1.73 V, respectively, which are
pdt)Fe2(CO)6], are given in Table 4. A good CV of the attributed to the reduction process of FeIFeI to Fe0FeI (E1).
PPh3-disubstituted complex 7 was not obtained because of The second reductive peaks for 2–5 in the range of –2.06
the poor solubility of 7 in CH3CN.
to –2.19 V, detected under Ar, were not observed when the
Each phosphane-monosubstituted diiron complex dis- CV measurement was carried out under CO (Figure 3). The
plays four irreversible peaks, two oxidative and two re- CV control studies of 2–5 suggest that the more negative
ductive peaks. Compared with the electrochemical data of peaks at –2.06 to –2.19 V might be raised by the one-elec-
the all-carbonyl diiron complex
1
and other ana- tron reduction process of FeIFeI/FeIFe0 for a CH3CN-
logues,[9,29,31,32] the peaks at 0.33–0.52 V are assigned to the substituted
species,
[(μ-pdt)[Fe(CO)2(PR3)][Fe(CO)2-
one-electron oxidation process of FeIFeI to FeIIFeI, and the (NCCH3)].[12] The assignment of the third irreversible re-
more anodic peaks at 0.70–0.74 V are proposed to be the ductive peak for complex 4 at ca. –2.29 V is not clear. The
response of the further oxidation process of FeIIFeI to first reduction potential is shifted in a cathodic direction
2510
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Eur. J. Inorg. Chem. 2005, 2506–2513

Influence of Tertiary Phosphanes on Iron Hydrogenase Model Complexes
FULL PAPER
+0.33 to –0.06 V for the conversion of 3 to 6 and from
+0.52 to +0.08 V for 5 to 8. Another oxidative peak at
+0.28 V for 6, with a cathodic shift of 460 mV compared
with the corresponding peak of 3, is tentatively ascribed to
the oxidation process of FeIIFeI to FeIIFeII.
Experimental Section
General Procedures: All reactions and operations related to organo-
metallic complexes were carried out under dry, oxygen-free dinitro-
gen with standard Schlenk techniques. Solvents were dried and dis-
tilled prior to use according to the standard methods. Commer-
cially available chemicals, 1,3-propanedithiol, [Fe(CO)5] and all ter-
tiary phosphanes and phosphite, were reagent grade and used as
received. The starting compound [(μ-pdt)Fe2(CO)6] (1) was pre-
pared according to the literature.[28,33] Infrared spectra were re-
corded from KBr pellets with a JASCO FT/IR 430 spectrophotom-
eter. 1H and 31P NMR spectra were collected with a Varian INOVA
400 NMR spectrometer. Elemental analyses were performed with
a Thermoquest-Flash EA 1112 elemental analyzer.
Figure 4. Cyclic voltammograms of complexes 6 and 8.
Table 4. Redox potentials of complexes 1–6 and 8.[a]
Epa [V]
FeIIFeI/FeII-
FeII
Epa [V]
FeIFeI/FeII-
FeI
Epc [V]
FeIFeI/
Fe0FeI
Complex
Synthesis of [(μ-pdt)Fe2(CO)5L] [L = PMe3 (2), PMe2Ph (3)]: Tri-
methylphosphane (46 mg, 0.6 mmol) was added to a solution of
hexacarbonyldiiron dithiolate 1 (240 mg, 0.6 mmol) in hexane
(30 mL). The red solution was stirred for 5 h and the color turned
dark red. After the solution was concentrated by solvent evapora-
tion in vacuo, the crude product was purified by flash column
chromatography on silica gel, first with hexane and then with hex-
ane/CH2Cl2 (6:1) as eluents. Product 2 was obtained by cooling
the concentrated hexane solution to –20 °C. Yield: 0.16 g (61%).
C11H15O5PS2Fe2 (434.0): calcd. C 30.44, H 3.48; found C 31.28, H
1
2
3
4
5
6
8
+0.84
+0.73
+0.74
+0.70
+0.71
+0.28
–
–
–1.57
–1.86
–1.82
–1.76
–1.73
–2.22
–2.19
+0.39
+0.33
+0.34
+0.52
–0.06
+0.08
[a] nBu4NPF6 (0.1 m⁾ in CH3CN; scan rate: 0.1 Vs–1; working elec-
trode: glassy carbon electrode of diameter 3 mm; reference elec-
trode: non-aqueous Ag/Ag+ electrode (0.01 m AgNO3 in CH3CN);
counter electrode: platinum wire.
3.57. IR (KBr, νCO): ν = 2037, 1980, 1919 cm–1. 1H NMR
˜
(CDCl3): δ = 1.95 (m, 4 H, SCH2), 1.71 (m, 2 H, CCH2C), 1.48
(d, JP–H = 9.19 Hz, 9 H, CH3) ppm. 31P NMR (CDCl3): δ = 24.96
(PMe3) ppm. Complex 3 was prepared by a procedure similar to
that of 2. The reaction of 1 (3.0 g, 7.8 mmol) with PMe2Ph (1.07 g,
7.8 mmol) was carried out in benzene (100 mL). Analytically pure
complex 3 was obtained by column chromatography on silica gel,
with hexane and then hexane/CH2Cl2 (3:1) as eluents. Yield: 2.8 g
(72%). C16H17Fe2O5PS2 (496.1): calcd. C 38.74, H 3.45; found C
by ca. 290, 250, 190 and 160 mV for 2–5, respectively, as
compared to that of the all-carbonyl parent complex.[31]
The better donor character of PMe3 and PMe2Ph renders
the reduction of the iron core more difficult. Complexes 4
and 5, with either a PPh3 or a P(OEt)3 ligand, display
smaller cathodic shifts in CV relative to 2 and 3. The re-
duction potential (E1) for 5 is obviously less cathodic than
that for complex 2 by 130 mV, indicative of a considerable
influence of different phosphane ligands on the redox prop-
erties of the iron atoms of Fe-only hydrogenase-active-site
model complexes.
38.90, H 3.51. IR (KBr, νCO): ν = 2040, 1980, 1921 cm–1. 1H NMR
˜
(CDCl3): δ = 7.57, 7.41 (2 s, br., 5 H, Ph), 1.94 (m, 4 H, SCH2),
1.80 (s, br., 6 H, CH3), 1.66 (m, 2 H, CCH2C) ppm. 31P NMR
(CDCl3): δ = 32.66 (PMe2Ph) ppm.
Synthesis of [(μ-pdt)Fe2(CO)5L] [L = PPh3 (4), P(OEt)3 (5)]: Tri-
phenylphosphane (2.0 g, 6.5 mmol) was added to a solution of 1
(1.3 g, 3.2 mmol) in toluene (100 mL). The solution was refluxed
for 6 h and the color turned purple-red. After the solution was
concentrated by solvent evaporation in vacuo, the crude product
was purified by flash column chromatography on silica gel with
As for previously reported CVs of PMe3-disubstituted
derivatives,[29] 6 and 8 each show only one reductive peak
at ca. –2.2 V, which is attributed to the reduction process of
FeIFeI to Fe0FeI. The second reductive peaks of 6 and 8 are CH2Cl2 as eluent. Purple crystals of 4 were obtained by recrystalli-
zation in hexane/CH2Cl2. Yield: 1.2 g (64%). C26H21O5PS2Fe2
(620.2): calcd. C 50.35, H 3.41; found C 50.29, H 3.45. IR (KBr,
not accessible within the solvent window in Figure 4. The
first reductive peaks of 6 and 8 apparently move to more
negative potentials by 400 and 460 mV, respectively, as com-
pared with that of the corresponding monosubstituted
homologues 3 and 5. It suggests that the introduction of
the second tertiary phosphane ligand exerts a stronger in-
fluence on the redox potentials of diiron complexes than
the introduction of the first one. With the conversion of
mono- to disubstituted derivatives, the anodic peak, as-
νCO): ν = 2044, 1984, 1931 cm–1. 1H NMR (CDCl3): δ = 7.69,
˜
7.43 (2 s, 15 H, Ph), 1.73 (m, 4 H, SCH2), 1.54 (m, 2 H, CCH2C)
ppm. 31P NMR (CDCl3): δ = 65.74 (PPh3) ppm. Complex 5 was
prepared by a procedure similar to that for 4. The reaction of 1
(386 mg, 1.0 mmol) with P(OEt)3 (0.17 mL, 1.0 mmol) was carried
out in toluene (20 mL) for 3 h. Analytically pure complex 5 was
obtained by recrystallization of the product in hexane/CH2Cl2 at
–20 °C. Yield: 0.158 g (33%). C14H21Fe2O8PS2 (524.1): calcd. C
signed to the oxidation of FeIFeI to FeIIFeI, shifts from 32.08, H 4.04; found C 32.04, H 4.04. IR (KBr, νCO): ν = 2046,
˜
Eur. J. Inorg. Chem. 2005, 2506–2513
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P. Li, M. Wang, C. He, G. Li, X. Liu, C. Chen, B. Åkermark, L. Sun
Table 5. Crystallographic data and processing parameters for complexes 3–6.
FULL PAPER
Complex
3
4
5
6
Empirical formula
Mw
C16H17Fe2O5PS2
496.09
C26H21Fe2O5PS2
620.22
C14H21Fe2O8PS2
524.10
C23H28Fe2O4P2S2
606.21
T [K]
Crystal system
Space group
293(2)
293(2)
monoclinic
P2(1)/c
293(2)
monoclinic
C2
293(2)
orthorhombic
Fdd2
triclinic
¯
P1
a [Å]
b [Å]
c [Å]
α [°]
9.061(2)
9.155
9.2533(4)
17.5257(6)
16.8340(6)
90.00
102.7620(10)
90.00
2662.54(17)
4
1.547
20.5038(8)
13.8441(7)
8.4154(5)
90.00
103.953(4)
90.00
2318.3(2)
4
1.502
25.633(5)
27.503(6)
7.7193(15)
90.00
90.00
90.00
5441.8(19)
8
1.480
14.067(4)
106.5720(10)
96.578(2)
108.048(2)
1036.81(4)
2
β [°]
γ [°]
V [Å3]
Z
ρcalcd. [gcm–3]
F (000)
1.589
504
1264
1072
2496
Crystal size [mm]
θmin/max [°]
0.09×0.10×0.28
1.55/25.04
5375/3607
235
1.019
0.0461
0.07×0.15×0.20
1.70/25.06
7906/4204
325
1.036
0.0386
0.05×0.08×0.13
1.79/28.30
8384/ 5091
244
0.920
0.0447
0.18×0.25×0.34
3.18/27.48
11919/2893
154
1.050
0.0500
Reflections collected/unique
Parameters refined
Goodness-of-fit on F2
Final R1 [I Ͼ σ(I)]
Final wR2
0.1268
0.524, –0.416
0.0931
0.621, –0.344
0.0981
0.581, –0.356
0.1038
0.481, –0.514
Residual electron density [eÅ–3]
1989, 1936 cm–1. 1H NMR (CDCl3): δ = 4.13 (s, 6 H, OCH2), 2.05 the SADABS program.[35] The structure was solved by direct meth-
(m, 4 H, SCH2), 1.76 (m, 2 H, CCH2C), 1.37 (s, 9 H, CH3) ppm. ods and refined on F2 against full-matrix least-squares methods
31P NMR (CDCl3): δ = 171.09 [P(OEt)3] ppm.
by using the SHELXTL97 program package.[36] All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were located
by geometrical calculation, but their positions and thermal param-
eters were fixed during the structure refinement. Crystal data and
parameters for data collections and refinements of complexes 3–6
are listed in Table 5. CCDC-218501 (3), -228422 (4), -255204 (5)
and -253139 (6) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Synthesis of [(μ-pdt)Fe2(CO)4(PMe2Ph)2] (6): Product 6 can be pre-
pared directly from [(μ-pdt)Fe2(CO)6] in one pot according to the
reported protocol[17] or by a stepwise CO displacement with 3 as
a starting compound. The crystal suitable for X-ray analysis was
obtained from a solution of hexane at –20 °C.
Synthesis of [(μ-pdt)Fe2(CO)4L2] [L = PPh3 (7), P(OEt)3 (8)]: Com-
pound 1 (2.5 g, 6.5 mmol) was dissolved in toluene (150 mL) and
then an excess of PPh3 (6.8 g, 26.0 mmol) was added. The red solu-
tion was refluxed for 6 h and the color turned dark red. The solu-
tion was concentrated by solvent evaporation in vacuo. Mono- and
disubstituted complexes 4 and 6 were separated by column
chromatography on silica gel with hexane/CH2Cl2 (2:1) and then
CH2Cl2 as eluents. Product 4 was obtained from the first band
Electrochemistry Studies of Complexes 2–6 and 8: Acetonitrile (Ald-
rich, spectroscopy grade) used for electrochemical measurements
was dried with molecular sieves and then freshly distilled from
CaH2 under N2. A solution of 0.1 m nBu4NPF6 (Fluka, electro-
chemical grade) in CH3CN was used as electrolyte, which was de-
gassed by bubbling with dry argon for 10 min before measurement.
Electrochemical measurements were recorded using a BAS-100W
electrochemical potentiostat at a scan rate of 200 mV/s. Cyclic vol-
tammograms were obtained in a three-electrode cell under argon.
The working electrode was a glassy carbon disc (diameter 3 mm)
successively polished with 3- and 1-μm diamond pastes and soni-
cated in ion-free water for 10 min. The reference electrode was a
non-aqueous Ag/Ag+ electrode (0.01 m AgNO3 in CH3CN) and the
auxiliary electrode was a platinum wire.
(1.5 g, 37%) and
7 from the second band (1.1 g, 20%).
C43H36Fe2O4P2S2 (854.5): calcd. C 60.44, H 4.25; found C 60.27,
H 4.28. IR (KBr, νCO): ν = 1997, 1951, 1933 cm–1. 1H NMR
˜
(CDCl3): δ = 7.69, 7.34 (2s, 30 H, Ph), 1.85 (m, 4 H, SCH2), 1.56
(m, 2 H, CCH2C) ppm. 31P NMR (CDCl3): δ = 62.79 (PPh3) ppm.
Essentially the same procedure as for 7 was applied for the prepara-
tion of 8, by refluxing a toluene solution (35 mL) of 1 (1.3 g,
3.4 mmol) and P(OEt)3 (2.0 mL, 11.6 mmol) for 6 h. Product 5 was
obtained from the first band (1.0 g, 56%) and 8 from the second
band (0.57 g, 26%). C19H36Fe2O10P2S2 (662.3): calcd. C 34.46, H
5.48; found C 34.64, H, 5.63. IR (KBr, νCO): ν = 2002, 1960,
˜
1
1935 cm–1. H NMR (CDCl3): δ = 4.15 (s, 12 H, OCH2), 1.95 (m,
4 H, SCH2), 1.70 (m, 2 H, CCH2C), 1.36 (s, 18 H, CH3) ppm. 31P
Acknowledgments
NMR (CDCl3): δ = 175.43 [P(OEt)3] ppm.
We are grateful to the Chinese National Natural Science Founda-
tion (Grant nos. 20471013 and 20128005), the Swedish Energy Ag-
ency and the Swedish Research Council for financial supports of
this work.
Crystal Structure Determination of Complexes 3, 4 and 6: The sin-
gle-crystal X-ray diffraction data were collected with a Siemens
SMART CCD diffractometer for 3 and 4 and with an AFE5R
Rigaku diffractometer for 5 and 6 with graphite-monochromated
Mo-Kα radiation (λ = 0.71073 Å) at 293 K using the ω-2θ scan
mode. Data processing was accomplished with the SAINT pro-
cessing program.[34] Intensity data were corrected for absorption by
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Influence of Tertiary Phosphanes on Iron Hydrogenase Model Complexes
FULL PAPER
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Received November 10, 2004
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