Phosphane and phosphite unsymmetrically disubstituted diiron complexes related to the Fe-only hydrogenase active site

Article abstract of DOI:10.1002/ejic.200601184

A series of unsymmetrically disubstituted diiron complexes [(μ-pdt){Fe(CO)₂L¹}{Fe(CO)₂L²}] [pdt = 1,3-propanedithiolato; L¹ = PMe₃, L² = PMe₂Ph, 4; PPh₃, 5; PCy₃, 6; P(OEt) ₃, 7; L¹ = PMe₂Ph, L² = PPh ₃, 8; P(OEt)₃, 9; L¹ = P(OEt)₃, L² = PPh3, 10; PCy₃, 11] and [(μ-edt){Fe(CO) ₂PMe₃}{Fe(CO)₂PPh₃}] (edt = 1,2-ethanedithiolato, 12) were prepared by means of stepwise CO displacements of [(μ-pdt)Fe₂(CO)₆] and [(μ-edt)-Fe ₂(CO)₆] by different tertiary phosphane and phosphite ligands. The interconversion of the irondithiacyclohexane ring and the rotation of the [Fe(CO)₂PR₃] subunit were studied using by variable-temperature ³¹P{¹H} NMR spectroscopy of 4, 6 and 12 in solution. The molecular structures of 4-6, 8-10 and 12 show that complexes 4-6, 8, 9 and 12 possess an apical/basal coordination mode and complex 10 has an apical/apical conformation. The X-ray analyses indicate that the PMe ₂Ph ligand in the apical position of the starting complex [(μ-pdt){Fe(CO)₃}{FeCO₂(PMe₂Ph)}] rotates to the basal position on conversion to the products 8 and 9. Cyclic voltammograms of 4-11 were studied both under argon and CO. The influences of the phosphane and phosphite ligands on the redox properties of the unsymmetrically disubstituted diiron complexes are discussed. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200601184

FULL PAPER
DOI: 10.1002/ejic.200601184
Phosphane and Phosphite Unsymmetrically Disubstituted Diiron Complexes
Related to the Fe-Only Hydrogenase Active Site
Ping Li,[a] Mei Wang,*[a] Chengjiang He,[a] Xiaoyang Liu,[a,b] Kun Jin,[a] and
Licheng Sun*[a,c]
Keywords: Bioinorganic chemistry / Carbonyl displacement / Diiron complexes / Fe-only hydrogenase / P ligands
A series of unsymmetrically disubstituted diiron complexes
[(µ-pdt){Fe(CO)2L1}{Fe(CO)2L2}] [pdt = 1,3-propanedithiol-
ato; L1 = PMe3, L2 = PMe2Ph, 4; PPh3, 5; PCy3, 6; P(OEt)3, 7;
L1 = PMe2Ph, L2 = PPh3, 8; P(OEt)3, 9; L1 = P(OEt)3, L2 = PPh3,
10; PCy3, 11] and [(µ-edt){Fe(CO)2PMe3}{Fe(CO)2PPh3}] (edt
= 1,2-ethanedithiolato, 12) were prepared by means of step-
wise CO displacements of [(µ-pdt)Fe2(CO)6] and [(µ-edt)-
Fe2(CO)6] by different tertiary phosphane and phosphite li-
gands. The interconversion of the irondithiacyclohexane ring
and the rotation of the [Fe(CO)2PR3] subunit were studied
using by variable-temperature 31P{1H} NMR spectroscopy of
4, 6 and 12 in solution. The molecular structures of 4–6, 8–10
and 12 show that complexes 4–6, 8, 9 and 12 possess an api-
cal/basal coordination mode and complex 10 has an apical/
apical conformation. The X-ray analyses indicate that the
PMe2Ph ligand in the apical position of the starting complex
[(µ-pdt){Fe(CO)3}{FeCO2(PMe2Ph)}] rotates to the basal posi-
tion on conversion to the products 8 and 9. Cyclic voltammo-
grams of 4–11 were studied both under argon and CO. The
influences of the phosphane and phosphite ligands on the
redox properties of the unsymmetrically disubstituted diiron
complexes are discussed.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
gate the electrochemistry and protonation properties of the
derivatives. Tertiary phosphane ligands, which can avoid the
intricacy of the protonation process on the cyanide nitrogen
atom, are widely used in many model complexes of the Fe-
only hydrogenase, especially for hydridodiiron dithiolate
complexes.[22–24] Although the CO displacement of [(µ-SR)2-
Fe2(CO)6] (R = Me, Et, Ph), [(µ-edt)Fe2(CO)6] (edt = 1,2-
ethanedithiolato), [(µ-pdt)Fe2(CO)6] and [(µ-adt)Fe2(CO)6]
(adt = 2-azapropane-1,3-dithiolato) by tertiary phosphanes
has been extensively studied,[10–17] to the best of our knowl-
edge the model complexes in the form of [(µ-pdt){Fe(CO)2-
L1}{Fe(CO)2L2}] with two different π ligands have been
scarcely reported.[10] To continue a systematic study of the
influence of phosphane and phosphite ligands on the coor-
dination structures, the reactivity and the redox properties
of the iron centres,[15] a series of unsymmetrically disubsti-
tuted model complexes [(µ-pdt){Fe(CO)2L1}{Fe(CO)2L2}]
[L1 = PMe3, L2 = PMe2Ph, 4; PPh3, 5; PCy3, 6; P(OEt)3, 7;
Introduction
Iron-only hydrogenases with popular metals and simple
diatom ligands in their active sites can catalyse the proton
reduction to molecular hydrogen more efficiently than other
types of hydrogenases. In recent years, synthesis of struc-
tural and functional models of the Fe-only hydrogenase
active site has been a hot topic in bioinorganic and organo-
metallic chemistry. The well-studied organometallic diiron
complex [(µ-pdt)Fe2(CO)6] (pdt = 1,3-propanedithiolato)
serves as a synthetic model of the Fe-only hydrogenase
active site in terms of the structure revealed by X-ray crys-
tallographic studies.[1,2] Various good donor ligands, such
as CN–,[3–9] PR3,[8–17] CNR[18,19] and NHC (N-heterocyclic
carbene)[20,21] have been successfully introduced to the
model complexes by CO displacement in order to investi-
[a] State Key Laboratory of Fine Chemicals, DUT-KTH Joint
Education and Research Centre on Molecular Devices, Dalian L1 = PMe2Ph, L2 = PPh3, 8; P(OEt)3, 9; L1 = P(OEt)3, L2
University of Technology (DUT),
= PPh3, 10; PCy3, 11] and [(µ-edt){Fe(CO)2PMe3}{Fe(CO)2-
Zhongshan Road 158-46, Dalian 116012, China
PPh3}] (12) with two different tertiary phosphane and
phosphite ligands were prepared and among them 4–6, 8–
10 and 12 were crystallographically characterised. Here we
describe the preparation, variable-temperature 31P{1H}
NMR spectra, molecular structures and cyclic voltammog-
rams of diiron complexes unsymmetrically disubstituted by
phosphane and phosphite ligands.
E-mail: symbueno@dlut.edu.cn
[b] State Key Laboratory of Inorganic Synthesis and Preparative
Chemistry, Jilin University,
Changchun 130012, China
[c] Department of Chemistry, Royal Institute of Technology
(KTH),
10044 Stockholm, Sweden
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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Phosphane and Phosphite Diiron Complexes
FULL PAPER
Results and Discussion
Preparation and Spectroscopic Characterisation of
Complexes 4–12
The monosubstituted complexes [(µ-pdt)Fe2(CO)5L1] [L1
= PMe3, 1; PMe2Ph, 2; P(OEt)3, 3] were prepared in moder-
ate yields according to the previous report.[15] Treatment of
1 with PMe2Ph, PPh3 and P(OEt)3 in toluene at reflux af-
forded products 4, 5 and 7, respectively, in good yields
(Scheme 1). Complex 6 could be obtained from 1, in a mod-
erate yield, in the presence of 1 equiv. of PCy3 using
Me3NO as a CO-removing reagent. The disubstituted com-
plexes 4, 8 and 9 were obtained in yields of 40–70% by the
replacement of CO in 2 with 1 equiv. of the corresponding
tertiary phosphane or phosphite ligand in toluene at reflux
(Scheme 2). Complexes 7 and 9 could also be prepared by
treatment of 3 with a slight excess of PMe3 and PMe2Ph
under the same conditions (Scheme 3). The reaction of
complex 3 with 1 equiv. or an excess of PPh3 in toluene at
reflux did not afford complex 10, presumably due to the
high activation energy caused by the large bulk and weak
donating capability of the PPh3 ligand. Consequently,
Me3NO was used as CO removing reagent to prepare com-
plex 10 (Scheme 3). Although the volume of PCy3 is larger
than that of PPh3, complex 11 could be obtained in a good
yield from a solution of 3 and 1 equiv. of PCy3 in toluene
at reflux. The stronger donating capability of PCy3 benefits
the PCy3/CO displacement. Complex 12 was prepared ac-
cording to a similar procedure used for 5. The unsymmetri-
cally disubstituted diiron dithiolate complexes 4–12 are rel-
atively thermally and air-stable in the solid state.
Scheme 2.
Scheme 3.
The products 4–12 were characterised by ESI- or APCI-
MS, IR, 1H and 31P{1H} NMR spectroscopy as well as ele-
mental analysis. The results of the elemental analyses for all
products are in good agreement with the proposed compo-
sitions. The [M + H]+ peaks of all the diiron dithiolates
were detected in the mass spectra in ESI- or APCI-positive
mode. The disubstituted complexes 4–11 each display three
ν(CO) bands in the region of 1900–2000 cm–1. The IR data
of ν(CO) for 4–11 are listed in Table 1 together with the
Scheme 1.
Table 1. A comparison of ν(CO) bands of 1–11 and the all-CO parent complex in CH3CN.
Complex
L1, L2
ν(CO)/cm–1
∆ν(CO)av/cm–1[a]
All-CO complex
–
2074 (m), 2036 (s), 1995 (s)
2037 (s), 1980 (s), 1919 (m)
2040 (s), 1980 (s), 1921 (m)
2046 (s), 1989 (s), 1936 (m)
1981 (m), 1943 (s), 1908 (m)
1985 (s), 1946 (s), 1905 (s)
1976 (s), 1937 (s), 1901 (s)
1991 (s), 1949 (s), 1907 (s)
1982 (s), 1946 (s), 1913 (s)
1991 (s), 1952 (s), 1922 (s)
1997 (s), 1955 (s), 1934 (s)
1993 (s), 1943 (s), 1933 (s)
–
1
PMe3, –
PMe2Ph, –
P(OEt)3, –
56
55
45
91
90
97
86
88
80
73
79
2
3
4
PMe3, PMe2Ph
PMe3, PPh3
PMe3, PCy3
PMe3, P(OEt)3
PMe2Ph, PPh3
PMe2Ph, P(OEt)3
P(OEt)3, PPh3
P(OEt)3, PCy3
5
6
7
8
9
10
11
[a] ∆v(CO)av = [∑v(CO)all-CO – ∑v(CO)]/3.
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P. Li, M. Wang, C. He, X. Liu, K. Jin, L. Sun
FULL PAPER
ν(CO) data of complexes 1–3 and the all-CO complex.[15,25]
A comparison of the ν(CO) bands for the three subsets (1
vs. 4–7, 2 vs. 4, 8 and 9, and 3 vs. 7 and 9–11) of complexes
4–11 shows that the introduction of the second phosphane
and phosphite ligand has a considerable effect on the ν(CO)
bands of the diiron complexes. The variation of the ν(CO)
bands implies the influence of different tertiary phosphane
and phosphite ligands on the electron density of the Fe
centres. Compared with the all-carbonyl complex, the
average values of the three strong ν(CO) bands are red-
shifted by 73–97 cm–1 for 4–11. The shift values [∆ν-
(CO)av] are consistent with the electron-donating capabili-
ties of different phosphane and phosphite ligands.
Variable-Temperature 31P{1H} NMR Spectra of Complexes
4, 6 and 12
The variable-temperature 31P{1H} NMR spectra of 4, 6
and the edt-bridged analogue 12 are shown in Figures 1 and
Figure 1. Variable-temperature 31P{1H} NMR spectra of 4 and 6
2. Complexes 4 and 6 each display two signals at δ = 25.5 in CDCl3.
and 29.6 ppm for 4, and δ = 23.3 and 68.1 ppm for 6, in the
31P{1H} NMR spectra at room temperature [Figure 1(a)
and (b)]. On lowering the temperature, each 31P{1H} NMR
signal of complex 4 splits into two resonances (δ = 26.5,
26.8 for PMe3 and δ = 32.5, 33.3 ppm for PMe2Ph) at
–80 °C [Figure 1(aЈ) and the inset]. Two resonances for the
PMe3 ligand and two broad signals for the PCy3 ligand can
be observed for complex 6 at –80 °C [Figure 1(bЈ)]. The
broad resonance of the PCy3 ligand may result from the
slow rotation of the bulky PCy3 ligand. The splitting of the
31P{1H} NMR signals at low temperature can be attributed
to the slowing down of the interconversion of the FeS2C3
six-membered rings in complexes 4 and 6. The resonance
difference for the PMe3 ligand of 6 (δ = 2.4 ppm) is appar-
ently larger than that for the splitting of the PMe3 ligand
of 4 (δ = 0.3 ppm), presumably due to the large interaction
between the propanedithiolato group and the bulky PCy3
ligands. A similar interconversion of the irondithiacyclohex-
ane rings in the symmetrically PR3-disubstituted diiron
complexes has been previously reported by Darensbourg
and co-workers.[13,26]
To explore the rotation flexibility of phosphane ligands
Figure 2. 31P{1H} NMR spectra of 12 at variable temperature in
CDCl3.
of the [Fe(CO)2PR3] subunit and to avoid the interconver-
sion of the S-to-S bridge, an edt-bridged analogue [(µ-
edt){Fe(CO)2PMe3}{Fe(CO)2PPh3}] (12) was prepared for
study by variable-temperature 31P{1H} NMR spectroscopy.
Complex 12 displays two 31P resonances at δ = 22.7 ppm
for PMe3 and δ = 60.3 ppm for PPh3 at room temperature the obtained variable-temperature 31P{1H} NMR spectra
[Figure 2(top)]. When the measuring temperature was low- of 12 is that the splitting of the 31P resonances at low tem-
ered to –80 °C, four 31P signals were observed for the edt- perature is caused by the slowing down of the rotation of
bridged diiron complex 12 [Figure 2(bottom)], two strong the subunit [Fe(CO)2PMe3],[13] and the subunit [Fe(CO)2-
signals at δ = 27.3 ppm for PMe3, δ = 60.7 ppm for PPh3 PPh3] does not rotate in solution within the range between
and two weak signals at δ = 17.0 ppm for PMe3 and δ = –80 and 25 °C but maintains its apical position as in the
59.9 ppm for PPh3. It is noticeable that the large resonance crystalline state. Accordingly, the two strong signals can be
difference (10.3 ppm) between the two 31P signals for the attributed to the major apical (PPh3)/basal (PMe3) isomer
PMe3 ligand of 12 can be observed, while the difference for of 12 and the two weak signals to the ap/ap isomer. The
the PPh3 ligand is only 0.8 ppm. A rational explanation for 31P{1H} NMR spectra of 12 suggest that the coordination
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Phosphane and Phosphite Diiron Complexes
FULL PAPER
orientation of the PMe3 ligand in the [Fe(CO)2PMe3] sub-
The X-ray diffraction analyses clearly show that the con-
unit does not exhibit an apparent effect on the chemical formations of the two coordinating phosphane and phos-
shift of the PPh3 ligand attached to the vicinal iron atom.
phite ligands in complexes 4–6, 8, 9 and 12 are predomi-
nantly determined by the relative sizes of the ligands. Com-
plexes 4–6, 8, 9 and 12 feature an ap/ba conformation in
the crystalline state which is a common conformation for
double-CO displacement by two non-carbonyl ligands on
each iron atom of [(µ-pdt)Fe2(CO)6].[4,9,20] The size of the
second phosphane or phosphite ligand determines the ori-
entation of the PMe2Ph group in complexes 4, 8 and 9,
which is in the apical position of 4 with PMe3 as its coun-
terpart and in the basal positions of 8 and 9 with PPh3 or
P(OEt)3 on the other iron atom. Complex 10 containing
two bulky ligands possesses an ap/ap conformation which,
to the best of our knowledge, is the first structurally charac-
terised unsymmetrically disubstituted diiron dithiolate com-
plex of the ap/ap conformation except for the symmetrically
PMe2Ph-, PPh2(BrC6H4)- and CN-t-Bu-disubstituted ana-
logues.[15,19,27] Our previous crystallographic studies clari-
fied that the ligand PMe2Ph in the starting complex 2 is in
an apical orientation.[15] The basal position of the PMe2Ph
ligand in 8 and 9 clearly verifies the conformational mo-
bility of the [Fe(CO)2PMe2Ph] subunit of the diiron dithio-
late model complex in solution.
Molecular Structures of Complexes 4–6, 8–10 and 12
Single-crystal X-ray diffraction studies were carried out
on complexes 4–6, 8–10 and 12 to determine the coordina-
tion conformations of the phosphane and phosphite ligands
in the diiron complexes. The molecular structures of 4–6,
8–10 and 12 are presented in Figure 3 and selected bond
lengths and angles are listed in Table 2. The central 2Fe2S
cores of complexes 4–6, 8–10 and 12 are all in butterfly
conformation as for previously reported model com-
plexes[10–15] and each iron atom is coordinated with a dis-
torted square-pyramidal geometry. The vertex carbon
atoms of the FeS2C3 six-membered rings all orient to the
phenyl-containing phosphane ligands, either in apical (4, 5
and 10) or in basal (9) positions. In contrast, the ellipsoid
of the vertex carbon atom C(14) is disordered in the crystal-
line state of 8 with two phenyl-containing phosphane li-
gands attached to two iron atoms. The distances of the Fe–
Fe bonds in 4–6, 8 and 9 are quite similar (2.534–2.544 Å),
while the Fe–Fe bond of 10 [2.5077(5) Å] with an ap/ap
conformation is apparently shorter than that of its analo-
gous ap/ba isomers 4–6, 8 and 9. The Fe–Fe bond
Cyclic Voltammograms of Complexes 4–11
The cyclic voltammograms (CVs) of complexes 4–11
[2.5553(4) Å] of the edt-bridged diiron complex 12 is longer were studied to evaluate the effects of different phosphane
than those of the pdt-bridged complexes. The dihedral an- ligands on the redox properties of the iron atoms. Except
gle, defined by the S(1)Fe(1)Fe(2) and S(2)Fe(1)Fe(2) pla- for 6 and 11, the CV measurements were carried out in
nes, increases to 73.30(2)° for 10 featuring two bulky ligands CH3CN for all complexes. The CVs of complexes 6 and 11
as compared to 70.89(3)° for 4 bearing two small ligands were recorded in toluene/CH3CN (1:3, v/v) because of the
and it increases to 77.72(2)° when the pdt-bridge is replaced poor solubility of 6 and 11 in CH3CN.
by the edt-bridge. The dihedral angle is affected by the ste-
Under argon, complexes 5, 8 and 10 containing a PPh3
ric interactions between the phosphane or phosphite ligand ligand each display two reduction peaks in the range of
and the S-to-S linker.
–1.98 to –2.26 V (vs Ag/Ag+, 0.01  AgNO3) in their CVs
Table 2. Selected bond lengths [Å] and angles [°] for 4–6, 8–10 and 12.
Complex
Bond lengths
4
5
6
8
9
10
12
Fe–Fe
Fe–S[a]
Fe–Pap
Fe–Pba
Fe–Cap
Fe–Cba
S···S
2.5430(5)
2.2643(8)
2.2299(7)
2.2335(6)
1.755(2)
1.777(2)
3.053
2.5383(6)
2.2636(7)
2.2518(7)
2.2312(8)
1.767(2)
1.756(2)
3.039
2.5375(5)
2.2741(7)
2.2822(7)
2.2293(8)
1.762(3)
1.758(3)
3.039
2.5437(10)
2.2658(11)
2.2534(10)
2.2352(11)
1.762(4)
1.768(4)
3.044
2.5341(9)
2.2625(14)
2.1589(13)
2.2278(13)
1.768(5)
1.764(4)
3.039
2.5077(5)
2.2745(7)
2.2117(7)[b]
–
2.5553(4)
2.2494(6)
2.2444(5)
2.2198(6)
1.764(2)
1.759(2)
2.877
–
1.771(2)[d]
3.045
[c]
Bond angles
S–Fe–S[e]
Fe–S–Fe[f]
Dihedral[g]
84.77(2)
68.33(2)
70.89(3)
84.35(2)
68.20(2)
71.65(2)
83.86(3)
67.82(2)
72.75(2)
84.41(4)
68.30(4)
71.47(4)
84.38(5)
68.12(4)
71.68(5)
84.04(2)
66.91(2)
73.30(2)
79.65(2)
69.34(2)
77.72(2)
[a] Average over four Fe–S bonds. [b] Average over two Fe–Pap bonds. [c] Average over three Fe–Cba bonds. [d] Average over four Fe–
Cba bonds. [e] Average over two S–Fe–S angles. [f] Average over two Fe–S–Fe angles. [g] Defined by the intersection of the two SFe2
planes.
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P. Li, M. Wang, C. He, X. Liu, K. Jin, L. Sun
FULL PAPER
Figure 3. Molecular structures of 4, 5, 6, 8, 9, 10 and 12 with thermal ellipsoids drawn at the 30% probability level. Hydrogen atoms
have been omitted for clarity.
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Phosphane and Phosphite Diiron Complexes
FULL PAPER
[Figure 4(a) for 5] and the other complexes 4, 6, 7, 9 and Furthermore, it was found that the current height ratio [i(I)/
11 without PPh3 ligands each show a broad peak including i(II)] of the two reduction peaks under argon varies with
two overlapping reduction events from –2.09 to –2.28 V the scan rate. The i(I)/i(II) ratio increases when the CV is
[Figure 4(b) for 9]. Considering the small potential differ- recorded at a higher scan rate. As shown in Figure 5, the
ences of the two reduction peaks for each complex, the sec- values of the i(I)/i(II) ratios are ca. 1.0, 2.0 and 3.3 at scan
ond reduction peak cannot be attributed to the further re- rates of 0.1, 0.4 and 1.0 Vs–1, respectively. The CVs of mul-
duction process of FeIFe0 to Fe0Fe0. In order to assign the tiple scans for complex 5 at a scan rate of 4.0 Vs–1 show
reduction peaks, CVs of 4–11 were performed under CO in that the current height of the second reduction peak appar-
a CO-saturated CH3CN solution. As the two sub-groups ently increases in the second scan cycle accompanied by a
display similar CVs under CO (complexes with a PPh3 li- decrease in the height of the first reduction peak (see Sup-
gand and those without PPh3 ligand), we chose complexes porting Information). After three scan cycles, the current
5 and 9 as examples for comparison of the CVs recorded height ratio of the two reduction peaks does not exhibit any
under argon and under CO (Figure 4). The CVs of other observable change. In view of the results of the CO control
complexes (4, 6–8, 10 and 11) under argon and under CO experiments, the variable scan rate measurements and the
are given in the Supporting Information.
CVs of multiple scans, the second reduction peak detected
under argon, either as a separated peak for the complex
with a PPh3 ligand or as an overlapping peak for the com-
plex without PPh3, is most likely due to the reduction of a
new complex, presumably generated by a quick chemical
reaction following the first reduction of the diiron(I,I) com-
plex. In comparison, the reported symmetrically disubsti-
tuted diiron analogues [(µ-pdt){Fe(CO)2L}2] [L = PMe3,
PMe2Ph, P(OEt)3, pta] each exhibit only one reduction
peak in the FeIFeI/FeIFe0 region of the CVs.[14,15,27] It seems
that the reductive [FeIFe0] intermediates formed by the un-
symmetrically disubstituted diiron complexes, especially the
complexes containing a PPh3 ligand, are less stable than
those derived from the symmetrically disubstituted diiron
analogues.
Figure 4. Cyclic voltammograms of complexes 5 (a) and 9 (b) under
argon and under CO; 1.0 m diiron complex, 0.05  nBu4NPF6 in
CH3CN, scan rate 0.1 Vs–1.
The current of the first reduction peak at –2.01 V in the
CV of 5, which has been assigned to the FeIFeI/FeIFe0 pro-
cess, increases under CO relative to that detected under ar-
gon. In the meantime the current height of the second re-
duction peak at –2.23 V apparently decreases when com-
pared with that detected under argon. Similarly, the broad
reduction peak in the range of –2.0 to –2.5 V for complex
9 under argon changes to a sharp reduction peak at –2.09 V
Figure 5. Cyclic voltammograms of complex 5 (1.0 m) under ar-
gon; scan rate 0.1 [i(I)/i(II) = 1.0], 0.4 [i(I)/i(II) = 2.0] and 1.0 Vs–1
[i(I)/i(II) = 3.3].
The redox potentials under CO for complexes 4–11 are
under CO and the more cathodic overlapping peak disap- listed in Table 3. The anodic peaks at –0.02 to +0.21 V have
pears. The peak at –2.09 V can be attributed to the FeIFeI/ been assigned to the first oxidation process of FeIFeI to
FeIFe0 process. The different CVs recorded under argon FeIIFeI as described in the literature.[28] Complexes 6, 7 and
and under CO reminded us of similar changes in the CVs 9–11 exhibit quasi-reversible oxidation peaks, while 4, 5 and
reported for PMe2Ph- and pta- (1,3,5-triaza-7-phosphaada- 8 display irreversible peaks for the first oxidation process.
mantane) -monosubstituted complexes[14,15] in which the It is interesting to notice that the P(OEt)3 and PCy3 ligands
second peaks detected under argon or N2 may be attributed show a certain capability to stabilise the FeIIFeI species
to a solvent-substituted species resulting from the 19-elec- formed in the first oxidation process. This phenomenon
tron radical anion formed at the first reduction process. was also observed in the CVs of the previously prepared
Eur. J. Inorg. Chem. 2007, 3718–3727
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3723

P. Li, M. Wang, C. He, X. Liu, K. Jin, L. Sun
FULL PAPER
P(OEt)3 and PMe2Ph symmetrically disubstituted com-
plexes.[15] The FeIFeI to FeIIFeI oxidative peak was quasi-
reversible for [(µ-pdt){Fe(CO)2P(OEt)3}2] and totally
irreversible for [(µ-pdt){Fe(CO)2PMe2Ph}2]. The redox po-
tentials of 4–11 are consistent with the electron-donating
capabilities of the phosphane and phosphite ligands. Com-
plex 6 containing PMe3 and PCy3 displays the most nega-
tive redox potentials, –0.02 V for the oxidation process of
FeIFeI to FeIIFeI and –2.15 V for the FeIFeI to FeIFe0 pro-
cess. In contrast, complex 10 featuring P(OEt)3 and PPh3
shows the most positive redox potentials, +0.21 V for the
oxidation process of FeIFeI to FeIIFeI and –1.98 V for the
FeIFeI to FeIFe0 process. In general, the variations in the
redox potentials by changing phosphane and phosphite li-
gands are ca. 230 mV for the first oxidation process and ca.
170 mV for the first reduction process.
Experimental Section
General Procedures: All reactions and operations related to organo-
metallic complexes were carried out under dry oxygen-free dinitro-
gen using standard Schlenk techniques. Solvents were dried and
distilled prior to use according to the standard methods. Commer-
cially available chemicals, 1,3-propanedithol, Fe(CO)5 and all terti-
ary phosphanes and phosphite compounds were reagent grade and
used as received. Compounds [(µ-pdt)Fe2(CO)5L] [L = PMe3, 1;
PMe2Ph, 2 and P(OEt)3, 3] and [(µ-edt)Fe2(CO)5PMe3] were pre-
pared according to previously reported procedures.[15,25] Infrared
spectra were recorded in CH3CN solutions with a JASCO FT/IR
430 spectrophotometer. 1H and 31P{1H} NMR spectra were re-
corded with a Varian INOVA 400 NMR spectrometer. Elemental
analyses were performed with a Thermoquest-Flash EA 1112 ele-
mental analyser and the mass spectra of the iron complexes were
recorded with an HP1100 MSD instrument.
Synthesis of [(µ-pdt){Fe(CO)2PMe3}{Fe(CO)2L}] [L = PMe2Ph (4),
PPh3 (5), P(OEt)3 (7)]: Complex 1 (0.43 g, 1.0 mmol) was treated
with PMe2Ph (0.14 g, 1.0 mmol) in toluene (40 mL) at reflux for
5 h. Complex 4 was obtained by column chromatography on neu-
tral alumina with hexane and CH2Cl2 as gradient eluents. Yield:
0.11 g (40%). C18H26Fe2O4P2S2 (544.2): calcd. C 39.73, H 4.82;
found C 39.77, H 4.87. 1H NMR (CDCl3): δ = 7.68, 7.42 (2 s, 5
H, Ph), 1.87 (s, 6 H, the CH3 of PMe2Ph), 1.67–1.56 (br. m, 6 H,
CH2CH2CH2), 1.45 (s, 9 H, PMe3) ppm. 31P{1H} NMR (CDCl3):
Table 3. Redox potentials of complexes 4–11.[a]
Complex L1, L2
Epa/V
Epc/V
FeIFeI/FeIIFeI FeIFeI/FeIFe0
4
PMe3, PMe2Ph
PMe3, PPh3
PMe3, PCy3
PMe3, P(OEt)3
PMe2Ph, PPh3
PMe2Ph, P(OEt)3
P(OEt)3, PPh3
P(OEt)3, PCy3
0.0
–2.12
–2.04
–2.15
–2.08
–2.01
–2.09
–1.98
–2.14
5
+0.10
–0.02
+0.11
+0.11
+0.09
+0.21
+0.14
6[b]
7
8
9
δ = 30.99 (PMe Ph), 25.89 (PMe ) ppm. IR (CH CN): νCO = 1981,
˜
2
3
3
1943, 1907 cm–1. APCI-MS: m/z = 544.8 [M + H]+. Complex 5 was
prepared according to a procedure similar to that of 4, with PPh3
(0.26 g, 1.0 mmol) as the ligand. Yield: 0.40 g (60%).
C28H30Fe2O4P2S2 (668.3): calcd. C 50.32, H 4.52; found C 50.24,
10
11[b]
[a] nBu4NPF6 (0.05 ) in a CO-saturated CH3CN solution under
CO; scan rate: 0.1 Vs–1; working electrode: glassy carbon electrode
of diameter 3 mm; reference electrode: non-aqueous Ag/Ag+ elec-
trode (0.01  AgNO3 in CH3CN); counter electrode: platinum wire.
[b] Data collected in CO-saturated toluene/CH3CN (1:3, v/v) mix-
ture.
1
H 4.63. H NMR (CDCl3): δ = 7.75, 7.33 (2 s, 15 H, Ph), 1.47 (d,
9 H, PMe3), 1.66–1.12 (br. m, 6 H, CH2CH2CH2) ppm. 31P{1H}
NMR (CDCl3): δ = 62.81 (PPh3), 25.66 (PMe3) ppm. IR (CH3CN):
νCO = 1985, 1946, 1905 cm–1. APCI-MS: m/z = 669.0 [M + H]+.
˜
Complex 7 was prepared according to a similar procedure used for
4 but with P(OEt)3 (0.17 g, 1.0 mmol) as the ligand. Yield: 0.36 g
(62%). C16H30Fe2O7P2S2 (572.2): calcd. C 33.59, H 5.28; found C
33.71, H 5.18. 1H NMR (CDCl3): δ = 4.16 (s, 6 H, OCH2), 1.46
(d, 9 H, PMe3), 1.37 [s, 9 H, the CH3 of P(OEt)3], 1.61–1.21 (br.
m, 6 H, CH2CH2CH2) ppm. 31P{1H} NMR (CDCl3): δ = 175.38
Conclusions
Nine unsymmetrically disubstituted diiron dithiolate
complexes 4–12 with various phosphane and phosphite li-
gands were prepared and well characterised. The rotation of
the [Fe(CO)2PMe3] subunit can be detected by the variable-
temperature 31P{1H} NMR spectra of 12, which also sug-
gest that the [Fe(CO)2PPh3] subunit does not rotate in solu-
tion in the range between –80 and 25 °C. In the substitution
reactions of [(µ-pdt){Fe(CO)3}{Fe(CO)2PMe2Ph}] (2) by
PPh3 and P(OEt)3, the PMe2Ph ligand in the apical position
of 2 moves to the basal position on formation of the prod-
ucts 8 and 9 and this provides clear evidence for the confor-
mational flexibility of the [Fe(CO)2PMe2Ph] subunit in
solution. The CVs of complexes 4–11 under argon and un-
der CO show that the reductive [FeIFe0] intermediates
formed by the unsymmetrically disubstituted diiron com-
plexes are not stable under argon. A considerable part of
the [FeIFe0] intermediate is converted to a new complex by
a quick chemical reaction which can be suppressed when
the CV measurement is carried out under CO. The redox
potentials of PR3-disubstituted diiron dithiolate complexes
can be adjusted in a range of ca. 200 mV.
[P(OEt) ], 27.10 (PMe ) ppm. IR (CH CN): ν = CO = 1991, 1949,
˜
3
3
3
1907 cm–1. APCI-MS: m/z = 572.8 [M + H]+.
Synthesis of [(µ-pdt){Fe(CO)2PMe3}{Fe(CO)2PCy3}] (6): Complex
6 can be obtained by using Me3NO as a CO-removing reagent. A
solution of complex 1 (0.43 g, 1.0 mmol) in CH3CN (10 mL) was
treated with Me3NO·2H2O (0.11 g, 1.0 mmol) at 50 °C for 30 min
and PCy3 (0.28 g, 1.0 mmol) was then added to the solution. The
mixture was stirred for 1 h and the resultant solution concentrated
to dryness under vacuum. The crude product was purified by flash
column chromatography on neutral alumina with hexane/CH2Cl2
(1:1) as eluent. Yield: 0.15 g (22%). C28H48Fe2O4P2S2 (686.42):
1
calcd. C 48.99, H 7.05; found C 48.93, H 6.94. H NMR (CDCl3):
δ = 1.47 (s, 9 H, PMe3), 2.12–1.29 (br. m, 39 H, CH2CH2CH2 and
PCy3) ppm. 31P{1H} NMR (CDCl3): δ = 68.54 (PCy3), 23.49
(PMe ) ppm. IR (CH CN): νCO = 1976, 1937, 1901 cm–1. ESI-
˜
3
3
MS: m/z = 687.0 [M + H]+.
Synthesis of [(µ-pdt){Fe(CO)2(PMe2Ph)}{Fe(CO)2L}] [L = PPh3
(8), P(OEt)3 (9)]: Triphenylphosphane (0.26 g, 1.0 mmol) was
added to a solution of complex [(µ-pdt)Fe2(CO)5(PMe2Ph)] (2)
(0.50 g, 1.0 mmol) in toluene (40 mL). The solution was heated to
reflux with stirring for 5 h, and the colour turned deep red. After
the solution was concentrated by solvent evaporation under vac-
3724
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 3718–3727

Phosphane and Phosphite Diiron Complexes
FULL PAPER
uum, the crude product was purified by column chromatography
on neutral alumina with hexane/CH2Cl2 (2:1) as eluent. Product 8
was obtained by cooling the concentrated hexane/CH2Cl2 solution
to –20 °C. Yield: 0.51 g (69%). C33H32Fe2O4P2S2 (730.4): calcd. C
54.27, H 4.42; found C 54.20, H 4.45. 1H NMR (CDCl3): δ = 7.72,
7.39 (2 br. s, 20 H, Ph), 1.79 (d, 6 H, the CH3 of PMe2Ph), 1.48–
1.0 mmol) in toluene (40 mL) at reflux for 6 h. Complex 11 was
obtained by column chromatography on neutral alumina with hex-
ane and CH2Cl2 as gradient eluents. Yield: 0.51 g (65%).
C31H54Fe2O7P2S2 (776.5): calcd. C 47.95, H 7.01; found C 47.68,
1
H 6.94. H NMR (CDCl3): δ = 4.18 (s, 6 H, OCH2), 1.38 [s, 9 H,
CH3 of P(OEt)3], 2.12–1.29 (br. m, 39 H, CH2CH2CH2 and PCy3)
1.26 (br. m, 6 H, CH2CH2CH2) ppm. 31P{1H} NMR (CDCl3): δ = ppm. 31P{1H} NMR (CDCl3): δ = 174.77 [P(OEt)3], 69.97 (PCy3)
62.95 (PPh ), 32.98 (PMe Ph) ppm. IR (CH CN): νCO = 1982,
ppm. IR (CH CN): νCO = 1993, 1943, 1933 cm–1. ESI-MS: m/z =
˜
˜
3
2
3
3
1946, 1913 cm–1. APCI-MS: m/z = 731.0 [M + H]+. Complex 9 was
prepared according to a similar procedure used for 8 or by using
complex 2 as a starting compound. The reaction of 2 (0.50 g,
1.0 mmol) with P(OEt)3 (0.17 g, 1.0 mmol) was carried out in tolu-
ene. Yield: 0.40 g (64%). C21H32Fe2O7P2S2 (634.2): calcd. C 39.77,
777.2 [M + H]+.
Synthesis of [(µ-edt){Fe(CO)2PMe3}{Fe(CO)2PPh3}] (12): Complex
[(µ-edt){Fe(CO)2PMe3}{Fe(CO)3}] (0.42 g, 1.0 mmol) was treated
with PPh3 (0.26 g, 1.0 mmol) in toluene (40 mL) at reflux for 5 h.
The workup of complex 12 was similar to that for 5. Yield: 0.42 g
(64%). C27H28Fe2O4P2S2 (654.3): calcd. C 49.56, H 4.31; found C
49.78, H 4.54. 1H NMR (CD3COCD3): δ = 7.65 (m, 9 H, Ph), 7.50
(s, 6 H, Ph), 1.81 (d, 2 H, CH2), 1.53 (d, 9 H, PMe3), 1.06 (d, 2 H,
CH2) ppm. 31P{1H} NMR (CDCl3): δ = 60.31 (PPh3), 22.68
1
H 5.09; found C 39.77, H 5.15. H NMR (CDCl3): δ = 7.62, 7.38
(2 s, 5 H, Ph), 4.10 (s, 6 H, OCH2), 1.77 (s, 6 H, the CH3 of
PMe2Ph), 1.32 [s, 9 H, the CH3 of P(OEt)3], 1.52–1.20 (br. m, 6 H,
CH2CH2CH2) ppm. 31P{1H} NMR (CDCl3): δ = 175.15 [P-
(OEt) ], 34.38 (PMe Ph) ppm. IR (CH CN): νCO = 1991, 1952,
(PMe ) ppm. IR (CH CN): νCO = 1982, 1945, 1907 cm–1. APCI-
˜
3
2
3
˜
3
3
1922 cm–1. ESI-MS: m/z = 634.8 [M + H]+.
MS: m/z = 654.9 [M + H]+.
Synthesis of [(µ-pdt){Fe(CO)2P(OEt)3}{Fe(CO)2L}] [L = PPh3 (10),
X-ray Structure Determination of Complexes 4–6, 8–10 and 12: The
PCy3 (11)]: A solution of [(µ-pdt)Fe2(CO)5P(OEt)3] (3) (0.52 g, single-crystal X-ray diffraction data were collected with an AFE5R
1.0 mmol) in CH3CN (15 mL) was treated with Me3NO·2H2O
(0.11 g, 1.0 mmol) at room temperature. The red solution was
stirred for 15 min and turned dark red. Triphenylphosphane
(0.26 g, 1.0 mmol) was added to the mixture in one portion with
stirring. The reaction mixture was stirred for 20 min, and the result-
ant solution was concentrated to dryness under vacuum. The crude
product was purified by flash column chromatography on neutral
alumina with hexane/CH2Cl2 (1:1) as eluent. Product 10 was ob-
tained by cooling the concentrated hexane/CH2Cl2 solution to
–20 °C. Yield: 0.54 g (72%). C31H36Fe2O7P2S2 (758.4): calcd. C
49.10, H 4.78; found C 49.12, H 4.89. 1H NMR (CDCl3): δ = 7.74,
Rigaku diffractometer for 4, 8 and 9 and a Siemens SMART CCD
diffractometer for 5, 6, 10 and 12, with graphite-monochromated
Mo-Kα radiation (λ = 0.071073 Å) at 293 K using the ω-2θ scan
mode. Data processing was accomplished with the SAINT pro-
cessing program.[29] Intensity data for 4–6 and 8–10 were corrected
for absorption using the SADABS program.[30] All structures were
solved by direct methods and refined on F2 against full-matrix le-
ast-squares methods by using the SHELXTL97 program pack-
age.[31] All non-hydrogen atoms were refined anisotropically. Hy-
drogen atoms were located by geometrical calculation. Details of
crystal data, data collections and structure refinements are summa-
7.41 (2 s, 15 H, Ph), 4.15 (s, 6 H, OCH2), 1.55 (br. s, 6 H, rised in Tables 4 and 5. CCDC-627499 (4), -627500 (5), -627501
CH2CH2CH2), 1.35 [s, 9 H, CH3 of P(OEt)3] ppm. 31P{1H} NMR (6), -627502 (8), -627503 (9), -627504 (10) and -630623 (12) contain
(CDCl3): δ = 175.00 [P(OEt)3], 64.19 (PPh3) ppm. IR (CH3CN):
˜
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
νCO = 1997, 1955, 1934 cm–1. ESI-MS: m/z = 759.1 [M + H]+.
Complex 3 (0.52 g, 1.0 mmol) was treated with PCy3 (0.14 g, graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Table 4. Crystallographic data and processing parameters for complexes 4–6.
Complex
4
5
6
Empirical formula
Formula mass
Crystal system
Space group
a [Å]
b [Å]
c [Å]
C18H26Fe2O4P2S2
544.15
monoclinic
P21/n
14.802(3)
10.337(2)
16.090(3)
90
C28H30Fe2O4P2S2
668.28
monoclinic
P21/n
8.431(2)
22.350(5)
16.403(4)
90
C28H48Fe2O4P2S2
686.42
monoclinic
P21/c
11.992(1)
12.077(1)
22.658(3)
90
α [°]
β [°]
105.53(3)
90
2372.0(8)
4
93.598(3)
90
3085.0(12)
4
98.146(1)
90
3248.3(6)
4
γ [°]
V [Å3]
Z
Dcalcd. [gcm–3]
Crystal size [mm]
θmin/max [°]
1.524
1.439
1.404
0.15ϫ0.28ϫ0.30
3.29–27.44
5395
4694, 0.0242
253
1.074
0.0278
0.08ϫ0.16ϫ0.22
2.21–27.00
6727
5325, 0.0262
343
1.067
0.0322
0.20ϫ0.26ϫ0.32
1.92–27.00
7064
5223, 0.0403
343
1.009
0.0369
Reflections collected
Unique data, Rint
Parameters
GOF on F2
R1 [I Ͼ 2σ(I)]
wR2 [I Ͼ 2σ(I)]
Residual electron density [eÅ–3]
0.0676
1.067,–0.501
0.0767
0.382, –0.259
0.0793
0.325, –0.301
Eur. J. Inorg. Chem. 2007, 3718–3727
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3725

P. Li, M. Wang, C. He, X. Liu, K. Jin, L. Sun
FULL PAPER
Table 5. Crystallographic data and processing parameters for complexes 8–10 and 12.
Complex
8·0.5C6H14
9
10
12
Empirical formula
Formula mass
Crystal system
Space group
a [Å]
b [Å]
c [Å]
C33H32Fe2O4P2S2·0.5C6H14
C21H32Fe2O7P2S2
634.23
C31H36Fe2O7P2S2
758.36
orthorhombic
Pbca
20.154(3)
15.815(2)
21.852(3)
90
C27H28Fe2O4P2S2
654.25
773.43
triclinic
triclinic
orthorhombic
P212121
12.2677(2)
15.1411(3)
16.1701(3)
90
¯
¯
P1
P1
10.780(5)
12.459(5)
14.972(7)
79.304(14)
84.660(14)
69.358(10)
1848.2(15)
2
8.600(2)
11.586(2)
15.036(3)
96.25(3)
92.93(3)
105.13(3)
1432.7(5)
2
α [°]
β [°]
90
90
90
90
γ [°]
V [Å ]
3
6965.2(18)
8
1.446
0.12ϫ0.28ϫ0.30
1.86–26.00
6852
5741, 0.0298
398
3003.54(10)
4
1.447
0.30ϫ0.40ϫ0.52
1.84–30.00
8482
7146, 0.0322
353
Z
Dcalcd. [gcm–3]
Crystal size [mm]
θmin/max [°]
1.390
1.470
0.12ϫ0.13ϫ0.25
2.06–28.28
8963
7411, 0.0174
425
0.10ϫ0.24ϫ0.30
2.99–27.45
6369
4515, 0.0381
307
Reflections collected
Unique data, Rint
Parameters
GOF on F2
1.055
1.058
1.057
1.006
R1 [I Ͼ 2σ(I)]
wR2 [I Ͼ 2σ(I)]
Residual electron density [eÅ–3]
0.0464
0.1222
0.940, –0.539
0.0501
0.1378
0.857, –0.695
0.0315
0.0810
0.732, –0.551
0.0304
0.0657
0.241, –0.299
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were obtained in a three-electrode cell under argon using a BAS
100B electrochemical workstation. The working electrode was a
glassy carbon disk (0.071 cm2) polished with 3 and 1 µm diamond
pastes and sonicated for 10 min prior to use. The reference elec-
trode was a non-aqueous Ag/Ag+ (0.01  AgNO3 in CH3CN) elec-
trode and the counter electrode was a platinum wire. A solution of
0.05  nBu4NPF6 (Fluka, electrochemical grade) in CH3CN was
used as the supporting electrolyte for complexes 4, 5 and 7–10. A
mixed solution of 0.05  nBu4NPF6 (Fluka, electrochemical grade)
in toluene and acetonitrile (1:3, v/v) was used as supporting electro-
lyte for complexes 6 and 11 due to its poor solubility in pure
CH3CN. Acetonitrile (Aldrich, spectroscopic grade) used for elec-
trochemical measurements was freshly distilled from CaH2 under
N2.
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Supporting Information (see footnote on the first page of this arti-
cle): Cyclic voltammograms of complexes 4, 6–8, 10 and 11 under
argon and CO, as well as the multiple scans for complex 5 at a
scan rate of 4.0 Vs–1 are included.
[15]
[16]
[17]
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Acknowledgments
We are grateful to the Chinese National Natural Science Founda-
tion (Grant nos. 20471013 and 20633020), the Swedish Energy Ag-
ency, the Swedish Research Council and the K & A Wallenberg
Foundation for financial support of this work.
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