Influence of the introduction of cyanido and phosphane ligands in multifunctionalized (mercaptomethyl)silane [FeFe] hydrogenase model systems

Article abstract of DOI:10.1002/ejic.201000918

Cyanido- and phosphane-substituted [FeFe] hydrogenase models are known to show increased basicity of the [2Fe2S] cluster. Following our continued interest in [2Fe2S(Si)] compounds, substitution reactions of such complexes with tetraethylammonium cyanide as well as triphenylphosphane were investigated to yield the respective monosubstituted triphenylphosphane complexes 2a-c and cyanido complexes 3a-c. Cyclic voltammetry was used to investigate the electrocatalytic H₂ formation with acetic acid.

Full text of DOI:10.1002/ejic.201000918

FULL PAPER
DOI: 10.1002/ejic.201000918
Influence of the Introduction of Cyanido and Phosphane Ligands in
Multifunctionalized (Mercaptomethyl)silane [FeFe] Hydrogenase Model
Systems
Ulf-Peter Apfel,[a] Yvonne Halpin,[b] Helmar Görls,[a] Johannes G. Vos,*[b] and
Wolfgang Weigand*[a]
Dedicated to Professor Grzegorz Mloston on the occasion of his 60th birthday
Keywords: Silicon / Cyanides / Phosphanes / Iron / Sulfur / Hydrogenase
Cyanido- and phosphane-substituted [FeFe] hydrogenase
models are known to show increased basicity of the [2Fe2S]
cluster. Following our continued interest in [2Fe2S(Si)] com-
pounds, substitution reactions of such complexes with tetra-
ethylammonium cyanide as well as triphenylphosphane
were investigated to yield the respective monosubstituted tri-
phenylphosphane complexes 2a–c and cyanido complexes
3a–c. Cyclic voltammetry was used to investigate the electro-
catalytic H2 formation with acetic acid.
Introduction
Since the structure of the active site of the [FeFe] hydro-
genase was determined,[1,2] much attention has been paid to
the synthesis of structural and functional model complexes
that contain different S–to–S linkers.[3–7] Recently,
[2Fe2S(Si)] complexes that contain functionalized (mercap-
tomethyl)silanes were reported, and the influence of the sili-
con moiety on the catalytic cycle during electrocatalysis as
well as their unique structural features were investigated.[8]
Thereby a sulfur···proton interaction was observed, which
can be explained by an increased nucleophilicity of the co-
ordinated sulfur atoms,[8] which was also observed for
[Fe2(CO)6{(SCH2)2Sn(CH3)2}].[9]
It has been reported that replacement of a carbonyl li-
gand by either cyanide[10] or phosphane[11] leads to in-
creased basicity of the iron atoms in the [2Fe2S] subsite.
This enables formation of terminal or bridging hydrides.[12]
However, since CN– ligands in model systems are not pro-
tected by the peptide environment,[1] isocyanic acid com-
plexes can be formed, which has been reported in detail for
several LnM–CNH complexes (Scheme 1).[13,14]
Scheme 1. Isocyanic acid complexes.
In contrast to CN–, triphenylphosphane is an abiological
ligand and cannot be protonated in its coordinated form.
Nevertheless, trimethylphosphane reveals a similar donor
character and can serve as an abiological surrogate for
CN–.[15]
Inspired by the properties of [2Fe2S(Si)] complexes and
the natural abundance of the strong donor ligand CN–,[16]
[FeFe] hydrogenase model complexes 1a–c (Scheme 2) were
treated with triphenylphosphane and tetraethylammonium
cyanide to afford monosubstituted, asymmetric [2Fe2S(Si)]
cluster compounds. The resulting products were investi-
gated according to their chemical and structural properties
by cyclic voltammetry and crystal structure analysis.
[a] Institut für Anorganische und Analytische Chemie, Friedrich-
Schiller Universität Jena,
August-Bebel-Strasse 2, 07743 Jena, Germany
Fax: +49-3641-948102
E-mail: wolfgang.weigand@uni-jena.de
[b] Solar Energy Conversion SRC, School of Chemical Sciences,
Dublin City University, Dublin 9, Ireland
E-mail: han.vos@dcu.ie
Scheme 2. [2Fe2S(Si)] model complexes 1a–c.[8]
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The Fe2–P1 distances and the bonding angles (Fe1–Fe2–
P1) of 2a–c are the same within the standard deviation and
are in good agreement with the recent reported complex
[Fe2(CO)5(PPh3){(SCH2)2CHOH}].[21] Short Fe–Fe bonds
are detected within all three molecules and reveal bond
lengths of 250.81(6) (2a), 251.16(5) (2b), and 249.68(5) pm
(2c) (cf. Table 1). The silicon atoms exhibit a tetrahedral
geometry. However, the angles S1–C1–Si1 and S2–C2–Si1
are about 120° and therefore much larger than the expected
109.5° for sp3 hybridization (cf. Table 1).
Results and Discussion
Synthesis and Characterization of PPh3-Substituted Diiron
Complexes 2a–c
Initial experiments to obtain the PPh3-substituted com-
plexes 2a–c by means of treatment of the hexacarbonyl
complexes 1a–c with triphenylphosphane at room tempera-
ture[17] or in toluene heated to reflux[18] required long reac-
tion times and showed only partial conversions. A fast and
selective exchange of CO was established by oxidative ab-
straction of CO by using trimethylamine N-oxide, followed
by addition of triphenylphosphane (Scheme 3).[19]
Table 1. Bond lengths [pm] and angles [°] for compounds 2a–c.
Initial reaction of trimethylamine N-oxide with com-
plexes 1a–c in acetonitrile yields the respective acetonitrile
complexes through decarboxylation.[20] Attempts to isolate
the unstable nitrile complexes failed, and the synthesis was
therefore performed as a one-pot reaction. Further treat-
ment of the in situ formed acetonitrile complexes with tri-
phenylphosphane resulted in monosubstituted compounds
2a
2b
2c
Fe1–Fe2
Fe2–P1
Fe1–S1
Fe2–S2
S1–C1
S2–C2
C1–Si1
250.81(6)
224.30(9)
226.69(9)
229.26(9)
183.0(3)
182.3(3)
188.1(4)
186.6(4)
186.3(4)
186.2(4)
251.16(5)
225.60(8)
227.71(7)
226.21(8)
182.1(3)
182.9(3)
187.5(3)
186.3(3)
188.5(3)
188.6(3)
249.68(5)
223.71(7)
226.91(7)
227.44(7)
181.21(3)
183.5(3)
186.0(3)
186.1(3)
186.1(3)
186.7(3)
2a–c. The identities of 2a–c were verified by elemental C2–Si1
analysis, NMR spectroscopy, MS, and IR studies. Thus,
Si1–C3
Si1–C4/C6/C7
mass spectrometry afforded the [M]+ peaks at m/z = 664
(2a), 690 (2b), and 704 (2c). IR spectroscopy provided ad-
ditional confirmation for the structures of 2a–c: νCO bands
are visible between 2041 and 1930 cm–1. Single-crystal X-
ray analysis revealed the proposed structures as depicted in
Figure 1. The iron atoms in 2a–c are coordinated in a dis-
torted octahedral fashion by the S–to–S linker and by CO
groups and triphenylphosphane in the equatorial position.
S1–Fe1–S2
S1–Fe2–S2
Fe1–Fe2–P1
S1–C1–Si1
86.18(3)
86.30(3)
85.80(3)
86.08(3)
86.66(2)
86.45(2)
156.74(3)
122.05(19)
118.37(18)
108.19(15)
112.85(18)
108.58(18)
156.66(3)
120.08(15)
117.65(15)
108.29(13)
115.41(14)
95.22(15)
156.26(3)
121.76(15)
119.30(13)
108.09(12)
113.82(13)
103.63(13)
S2–C2–Si1
C1–Si1–C2
C1–Si1–C3
C3–Si1–C4/C6/C7
Scheme 3. Reaction pathway towards monosubstituted compounds 2a–c containing the PPh3 ligand.
Figure 1. Molecular structures of compounds 2a–c (left to right; ellipsoids at the 50% level). The molecular structure of 2c shows one of
two independent molecules.
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Cyanido and Phosphane Ligands in [2Fe2S(Si)] Hydrogenase Model Systems
Synthesis and Characterization of the CN–-Substituted
Diiron Complexes 3a–c
C9–N1 are in good agreement with those reported for
(Et4N)[Fe2(S2C3H6)(CN)(CO)5].[3]
The replacement of a CO ligand by CN– follows similar Table 2. Bond lengths [pm] and angles [°] for compound 3b.
procedures[22] as described for the tiphenylphosphane com-
Fe1–Fe2
Fe1–S1
Fe2–S2
S1–C1
S2–C2
C1–Si1
252.32(8)
224.82(11)
227.36(10)
182.8(4)
183.0(4)
187.3(4)
C2–Si1
Si1–C3
Si1–C6
Fe1–C9
C9–N1
186.9(4)
187.5(4)
187.5(84)
193.5(4)
115.4(5)
plexes 2a–c and leads to the negatively charged complexes
3a–c. Again, the labile acetonitrile complex is formed first
and is easily substituted by the addition of tetraethylammo-
nium cyanide. Further purification affords compounds 3a–
c (Scheme 4) as dark red substances.
S1–Fe1–S2
S1–Fe2–S2
S2–C2–Si1
S1–C1–Si1
88.49(4)
87.22(4)
119.8(2)
118.4(2)
C1–Si1–C2
C1–Si1–C3
C3–Si1–C6
Fe2–Fe1–C9
106.57(18)
112.32(19)
95.64(19)
107.50(13)
Experiments that used two or more equivalents of CN–
or PPh3 afforded only monosubstituted complexes. An in-
creased electron density at the coordinated CO ligand can
be observed due to the σ-donor ability of CN– or PPh3
ligands.[23] Further reactions of 1a–c with 1,10-phen-
anthroline were carried out. By using the same reaction
conditions as for 2a–3c, the desired [2Fe2S(phenanthrol-
ine)] complex was not obtained;[24] in contrast, decomposi-
tion to [Fe(phenanthroline)3]2+ was observed.
Scheme 4. Synthesis of monosubstituted complexes 3a–c contain-
ing the CN– ligand.
The identities of 3a–c were established by NMR spec-
troscopy, MS, and IR studies. Consequently, mass spec-
trometry reveals the [M]– peaks at m/z = 428 (3a), 454 (3b),
and 468 (3c). IR spectroscopy shows the νCO bands between
2030 and 1930 cm–1 and the νCN band at around 2088 cm–1.
Slow diffusion of hexane into a solution of 3b, dissolved
in thf, at 0 °C affords single crystals suitable for structure
determination (Figure 2).
IR Spectroscopy
As pointed out, the basicity of the iron centers can be
increased by using the strong-field ligands CN– and PPh3,
whereby CN– is more relevant in the hydrogenase sys-
tem.[1,2] The basicity (electron density) of the iron atoms
depends mainly on two different aspects: (i) the oxidation
state,[18,25] and (ii) the σ/π-bonding properties of the li-
gands.[10,11] A characteristic feature that can be used to de-
termine the basicity of the iron centers is the νCO. The
smaller the νCO value, the larger the basicity of the iron
centers.[26] The infrared spectroscopic data for complexes
1a, 2a, and 3a are listed in Table 3 and are representative
for 1b–c, 2b–c, as well as 3b–c. By evaluating the IR data,
a decrease of νCO can be observed from 1a to 2a and 3a,
which show an increased electron density at the iron atoms.
Thus, CN– is the strongest donor ligand in this series. Sim-
ilar electronic effects have been previously reported for
cyanide and phosphane derivatives of [Fe2(CO)4(L)2(SR)2]-
type complexes.[3,27]
Figure 2. Molecular structure of compound 3b (ellipsoids at the
50% level). Hydrogen atoms are omitted for clarity.
Table 3. νCO infrared bands of complexes 1a–3a (KBr pellets).
Compound
νCO
νCN
1a[8]
2a
2074 (vs), 2031 (vs), 1989 (vs)
2041 (vs), 1981 (vs, b)
2027 (vs), 1973 (vs)
–
–
2088
The molecular structure of 3b shows the complex anion
[2Fe2S(Si)(CN)]–, in which the counterion is a tetraethyl-
ammonium cation. The iron atom Fe1 is coordinated by a
cyanido ligand in a basal position, two CO ligands, the Fe2
atom, and the dithiolato bridge. Both iron atoms are coor-
dinated in a distorted octahedral way. In contrast to the
phosphane complex 2b, the iron–iron distance is signifi-
cantly longer (cf. Table 2). The silicon atom is tetrahedrally
3a
Electrochemistry
Cyclic and differential pulse voltammetry were carried
coordinated, and unusually large bond angles S1–C1–Si1 out to investigate the electrocatalytic properties of the tri-
and S2–C2–Si1 of 118.4(2) and 119.8(2)°, respectively, are phenylphosphane complexes 2a–c and cyanide complex 3c
observed (see Table 2). The bond lengths for Fe1–C9 and towards H2 development. The reduction and oxidation po-
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FULL PAPER
tentials of the complexes are listed in Table 4. We were not
able to investigate the electrochemical properties of com-
pound 3a and 3b, because of their low solubility in any
solvent.
Table 4. Electrochemical data of the hexa- and pentacarbonyldiiron
hydrogenase model compounds 1a–c, 2a–c, and 3c. All potentials
are corrected for the Ag/AgNO3 (0.1 m nBu4NPF6, 0.01 m AgNO3
in acetonitrile) reference electrode with internal Fc/Fc+ reference.
Compound
FeIFeI/FeIIFeII [V] FeIFeI/Fe0Fe0 [V]
1a (acetonitrile)[8]
+0.79 (Epa
+0.81 (Epa
+0.81 (Epa
+0.57 (Epa
+0.44 (Epc)
+0.62 (Epa
+0.49 (Epc)
+0.61 (Epa
+0.46 (Epc)
+0.86 (Epa
)
)
)
)
–1.40 (Epa
–1.48 (Epc)
–1.40 (Epa
–1.48 (Epc)
–1.39 (Epa
–1.49 (Epc)
–1.83 (Epc)
)
1b (acetonitrile)[8]
1c (acetonitrile)[8]
)
)
2a (dichloromethane)
2b (dichloromethane)
2c (dichloromethane)
3c (acetonitrile)
Figure 3. Cyclic voltammetry (black) and differential pulse voltam-
metry (gray) of compound 2a (representative also for 2b and 2c)
[1 mm] on a glassy carbon working electrode, in dichloromethane,
by using 0.05 m nBu4NPF6 as the supporting electrolyte and Ag/
AgCl as the reference electrode.
)
–1.80 (Epc)
–1.85 (Epc)
–2.04 (Epc)
)
)
Triphenylphosphane ligands in 2a–c led to an increased
electron density around the diiron center. This is clearly
shown by the oxidation and reduction potentials of the
three hydrogenase models (cf. Table 4). When comparing
the redox potentials of these complexes with their hexacar-
bonyl analogues 1a–c, and allowing for effects of the dif-
ferent solvent (dichloromethane results in positive shifts
compared with the corresponding process in acetonitrile),
it is evident that the former are both easier to oxidize and
harder to reduce. Upon inspection of the cathodic waves in
each of the cyclic voltammograms, a second reduction pro-
cess is observed that occurs at potentials very close to the
first (least negative) reduction process, although not at all
clear in the differential pulse. This type of two-electron re-
duction has been observed for a similar monosubstituted
complex, [Fe2(bdt)(CO)5{P(OMe)3}] (bdt = 1,2-benzenedi-
thiolate),[28] and may be related to the coordination asym-
metry of the two Fe centers in the molecules. The two-elec-
tron reduction step coincides well with the hexacarbonyl
complexes 1a–c, for which the first reduction signal has
been assigned to an electron transfer–chemical reaction–
electron transfer (ECE) mechanism.[8] From the cyclic vol-
tammogram in Figure 3, it can be seen that the reversibility
of the oxidative process is greatly improved with the PPh3-
substituted complexes. This is, because the σ-donor charac-
ter of this ligand stabilizes the FeIIFeII redox state, whereas
the reduced state is destabilized by this ligand, as indicated
by the irreversibility of the cathodic waves in each cyclic
voltammogram.[29] The PPh3-functionalized complexes are
oxidized at potentials approximately 200–250 mV less posi-
tive than the hexacarbonyl analogues. However, it must be
noted that this is not a direct comparison as the electro-
chemical solvent used for the PPh3 complexes is dichloro-
methane (these complexes were insoluble in acetonitrile),
Upon addition of 1 mmol of HOAc to the complexes,
the separation of this first cathodic wave into two peaks
was observed with a minor positive shift in potential as for
the first wave. The second, now more prominent peak expe-
rienced a shift of approximately 200 mV towards more
negative potentials. An increase in the current of this sec-
ond cathodic wave was observed, and the process shifted to
more negative potentials, with these shifts continuing with
each increment of acid added thereafter. It is tentatively
proposed that any hydrogen evolved occurs from the Fe0Fe0
redox state. This type of behavior is observed for all three of
the monosubstituted complexes containing PPh3 (Figure 4).
Figure 4. Cyclic voltammetry of 2a [1 mm] with HOAc (0–10 mmol)
in dichloromethane (electrolyte: 0.05 m nBu4NPF6), representative
also for compound 2b and 2c.
The pentacarbonyldiiron complex 3c was electrochemi-
whereas acetonitrile was the solvent for the hexacarbonyl cally investigated by using acetonitrile as solvent. The oxi-
derivatives. dation wave observed for this complex occurs at a potential
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Cyanido and Phosphane Ligands in [2Fe2S(Si)] Hydrogenase Model Systems
(+0.86 V vs. Ag/Ag+) that is approximately 50 mV more this moment to clearly assign this process as hydrogen evol-
positive than the corresponding process in the hexacarbonyl ution catalyzed by the FeFe complex. In contrast to the
analogue 1c that is oxidized at +0.81 V vs. Ag/Ag+. This is literature-known complex [Fe2{μ-S2(CH2)3}(CN)(CO)5]–, a
counterintuitive if one considers the presence of the CN– protonation at the N atom of the cyanide was not ob-
ligand, which should increase the electron density around served.[23,31]
the diiron active site. The complex appears to pacify the
electrode with each successive cycle as indicated by the
gradual decrease in current.
Conclusion
The increased electron density on the metal atom is re-
Over the course of this study, a suitable synthesis route to
flected in the reduction potential recorded for 3c (Figure 5).
The (FeIFeI + 2 e– Ǟ Fe0Fe0) is observed at a potential
(–2.04 V vs. Ag/Ag+) that is approximately 500 mV more
negative than that of the hexacarbonyl analogue 1c. A sim-
ilar negative shift of the reduction potential was already
observed for the negatively charged monocyanide [Fe2{μ-
S2(CH2)3}(CN)(CO)5]–.[3] However, the reduction potential
obtained for compound 3c is about 300 mV more negative
than for the respective propanedithiolato species.
monosubstituted asymmetric [2Fe2S(Si)] model complexes
that contain triphenylphosphane (2a–c) and cyanide (3a–c)
was established. Single-crystal X-ray determination revealed
a basal configuration mode of the introduced cyanide and
an apical for the triphenylphosphane in the solid state. As
already shown for 1a–c, the substitution pattern on the sili-
con atom reveals no or negligible influence on the electroca-
talytic cycle.[7] During the investigation of the electrochemi-
cal properties, unexpected trends arose in the evaluation of
the redox potentials in certain complexes of this series. Ad-
dition of σ-donor ligands such as CN– or PPh3 led to an
increase in electron density around the metal centers, which
should result in an increase in basicity of the active site.
This theory is not completely consistent with all oxidation
potential values obtained. In contrast to results of Poilblanc
et al.,[11,32] who showed that complexes [Fe2(SR)2(CO)4L2]
(L = donor ligand) are both easier to oxidize and harder to
reduce, monosubstituted cyanido complex 3c experiences
an oxidation potential that is more positive than that of
the corresponding hexacarbonyl analogues. Therefore, the
effect of these ligands on the nature of the HOMO and
LUMO must be considered. The addition of a negatively
charged ligand may change the location of these orbitals,
thus resulting in a complicated interpretation of results.
Both the oxidation and reduction potentials of complexes
2a–c experience shifts in the negative direction. Two closely
spaced reduction steps were observed for these compounds.
Catalytic behavior was observed for all complexes. How-
ever, direct comparison of all six complexes was not pos-
sible as a result of solubility issues in different solvents.
Figure 5. Cyclic voltammetry of compound 3c [1 mm] with HOAc
(0–10 mmol) on a glassy carbon working electrode, in acetonitrile,
by using 0.05 m nBu4NPF6 as the supporting electrolyte and Ag/
Ag+ as the reference electrode (0.1 m nBu4NPF6 and 0.01 m AgNO3
in CH3CN).
The expected basicity around the iron–iron bond or the
nitrile ligand itself in 3c is not reflected in its electrocatalytic
behavior in the presence of acetic acid. The presence of
these basic sites led to an increased susceptibility to proton-
ation prior to reduction of the complex, which is indicated
by a positive shift in the reduction potential of the least
negative cathodic process.[30] Upon addition of 1 mmol of
Experimental Section
General Procedures: All syntheses were carried out under dry nitro-
gen or argon. The organic solvents used were dried and purified
according to standard procedures and stored under dry argon.
Chemicals were used as received from Fluka and Acros without
HOAc, the reduction potential of this process shifted by further purification. Compounds 1a–c were prepared according to
literature procedures.[8] Thin layer chromatography (TLC): Merck
approximately 200 mV in the negative direction, and an in-
silica gel 60 F254 plates; detection under UV light at 254 nm. Flash
crease in the height of the peak was observed. The intensity
chromatography (FC): Fluka silica gel 60. 1H, 13C{1H}, and
of the current continued to increase with each increment of
31P{1H} NMR spectra were recorded with a Bruker Avance 200 or
acid added thereafter, which is indicative of an electrocata-
400 MHz spectrometer, whereby the multiplicities of proton reso-
lytic current. These results suggest that the evolution of hy-
nances are defined as s (singlet), d (doublet), t (triplet), and m
drogen is mediated from the Fe0Fe0 redox state. However,
the observed catalytic wave is within the region of that ob-
served when acetic acid itself is reduced in the absence of a
catalyst (ca. –2.4 V vs. Ag/Ag+), which was also observed
(multiplet). CDCl3 was used as the solvent. Chemical shifts (δ
[ppm]) were determined relative to internal CHCl3 [1H (CDCl3): δ
= 7.24 ppm], CDCl3 [13C (CDCl3): δ = 77.0 ppm], or external TMS
[1H (CDCl3): δ = 0.00 ppm]. Analysis and assignments of the 1H
by Darensbourg et al.,[29] and as such it is not possible at NMR spectroscopic data were supported by H,1H COSY, 13C,1H
1
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heteronuclear multiple quantum coherence (HMQC), and 13C,1H
heteronuclear multiple bond correlation (HMBC) experiments. As-
signment of the 13C NMR spectroscopic data was supported by
DEPT 135, 13C,1H heteronuclear single quantum coherence
(HSQC), and 13C,1H HMBC experiments. IR spectra were re-
corded with a Perkin–Elmer 2000 FTIR spectrometer. Electron im-
pact and atom bombardment mass spectrometry was carried out
at 70 eV with a Finnigan SSQ710 by using desorption electron ion-
ization (DEI) or fast atom bombardment (FAB) mode. Elemental
analyses (C, H, N, S) were carried out with a Leco CHNS-931
instrument.
(m), 1436 (m) cm–1. C29H27Fe2O5PS2Si (689.9): calcd. C 50.45, H
3.94, S 9.29; found C 50.56, H 3.69, S 8.99.
Pentacarbonyl(triphenylphosphane)diiron Complex 2c: Complex 1c
(50 mg, 0.106 mmol) was treated with trimethylamine N-oxide di-
hydrate (12 mg, 0.106 mmol) and triphenylphosphane (28 mg,
0.106 mmol) according to Method A. After purification, 2c (19 mg,
1
0.03 mmol, 25%) was obtained. H NMR (CDCl3, 200 MHz): δ =
7.63 (m, 9 H, H-c/d), 7.38 (m, 6 H, H-b), 1.53–1.26 (m, 10 H,
CH2SiCH2CH2CH2), 0.70 (m, 2 H, SiCH2) ppm. 13C{1H} NMR
(CDCl3, 50 MHz): δ = 213.8/209.4 (CO), 135.8 (C-a), 133.5 (C-b),
130.1 (C-d), 128.4 (C-c), 29.3 (SiCH2CH2CH2C), 23.8
(SiCH2CH2C), 13.5 (SiCH2C), 2.9 (SCH2Si) ppm. 31P{1H} NMR
(CDCl3, 81 MHz): δ = 68.6 ppm. MS (DEI): m/z = 704 [M]+, 648
[M – 2 CO]+, 620 [M – 3 CO]+, 564 [M – 5 CO]+, 262 [PPh3]+. IR
(KBr): ν = 3054 (w), 2919 (m), 2852 (m), 2041 (vs), 1981 (vs), 1930
˜
(s), 1637 (m), 1435 (m) cm–1. C30H29Fe2O5PS2Si (703.9): calcd. C
51.15, H 4.15, S 9.10; found C 51.49, H 4.10, S 9.11.
Pentacarbonyl(cyanido)diiron Complex 3a: Compound 1a (55 mg,
0.128 mmol) was treated with trimethylamine N-oxide (14 mg,
Synthetic Methods
0.128 mmol)
and
tetraethylammonium
cyanide
(20 mg,
Method A: Replacement of CO by PPh3 was performed as follows:
The hexacarbonyl complex (1a–c) was dissolved in acetonitrile
(20 mL). Me3NO·2H2O (1 equiv.) was added to give the respective
nitrile complex within 30 min, visible by darkening of the red solu-
tion. Subsequently, PPh3 (1 equiv.) was added, and the reaction
mixture was stirred at room temperature for 7 h. Concentration to
dryness afforded the crude product, which was purified by means
of FC (thf/hexane, 1:3).
0.128 mmol) according to Method B to yield compound 3a (41 mg,
0.07 mmol, 57%). 1H NMR (CD3CN, 200 MHz): δ = 3.19 (m, 8
H, NCH2C), 1.39 (s, 2 H, SCH2Si), 1.21 (m, 14 H, SCH2Si and
CH3), 0.10/–0.05 (2s, 6 H, SiCH3) ppm. 13C{1H} NMR (CD3CN,
50 MHz): δ = 216.9/213.3 (CO), 53.6 (NCH2C), 8.3 (CH3), 7.3
(SCH2Si), –1.0 (SiCH3) ppm. MS (FAB, neg.): m/z = 428 [M –
Et N]–. IR (KBr): ν = 2924 (m), 2088 (w, CN), 2027 (vs, CO), 1973
˜
4
(vs, CO), 1630 (m), 1484 (m) cm–1. Elemental analysis data was not
obtained.
Method B: A solution of the complex (1a–c) dissolved in acetoni-
trile (20 mL) was treated with Me3NO·2H2O (1 equiv.) and was
stirred at room temperature for 30 min. Following that, tetraethyl-
ammonium cyanide (1 equiv.) was added, and the solution was
stirred for 20 h. Removal of the solvent under reduced pressure
afforded the crude product. The residue was dissolved in thf, and
the remaining solid was filtered off. Repeated evaporation and
washing with hexane afforded the respective cyanide complexes.
Pentacarbonyl(cyanido)diiron Complex 3b: Compound 1b (53 mg,
0.116 mmol) was treated with trimethyalamine N-oxide dihydrate
(13 mg, 0.116 mmol) and tetraethylammonium cyanide (18 mg,
0.116 mmol) according to Method B to afford 3b (37 mg,
0.06 mmol, 54%) as a red oily substance. 1H NMR (CD3CN,
400 MHz): δ = 3.14 (m, 8 H, NCH2C), 1.45–1.19 (m, 18 H,
SCH2Si, SiCH2CH2C and CH3), 0.65/0.49 (2 m, 4 H, SiCH2C)
ppm. 13C{1H} NMR (CD3CN, 50 MHz): δ = 216.8/212.9 (CO),
53.2 (NCH2C), 27.3 (SiCH2CH2C), 13.9 (SiCH2C), 7.8 (CH3), 6.9
(SCH2Si) ppm. MS(FAB, neg.): m/z = 454 [M – Et4N]–. IR (KBr):
Pentacarbonyl(triphenylphosphane)diiron Complex 2a: Compound
1a (148 mg, 0.344 mmol) was treated with trimethylamine N-oxide
dihydrate (39 mg, 0.348 mmol) and triphenylphosphane (93 mg,
0.48 mmol) according to Method A to yield 2a (66 mg, 0.1 mmol,
29%). 1H NMR (CDCl3, 200 MHz): δ = 7.67 (m, 9 H, H-c/d), 7.43
(m, 6 H, H-b), 0.09 (s, 6 H, SiCH3), –0.31 (s, 4 H, SCH2Si) ppm.
13C{1H} NMR (CDCl3, 50 MHz): δ = 213.8/209.4 (CO), 135.8 (C-
a), 133.5 (C-b), 130.1 (C-d), 128.5 (C-c), 4.9 (SCH2Si), 0.5/–0.2
(SiCH3) ppm. 31P{1H} NMR (CDCl3, 81 MHz): δ = 68.5 ppm. MS
(DEI): m/z = 664 [M]+, 608 [M – 2 CO]+, 580 [M – 3 CO]+, 524
[M – 5 CO]+, 262 [PPh3]+. IR (KBr): 3060 (w), 2041 (vs), 1981 (vs,
br.), 1628 (m), 1435 (m) cm–1. C27H25Fe2O5PS2Si (663.9): calcd. C
48.81, H 3.79, S 9.65; found C 48.65, H 3.83, S 9.49.
ν = 2982 (s), 2925 (s), 2853 (m), 2087 (w, CN), 2027 (vs, CO), 1974
˜
(vs, CO), 1630 (s), 1484 (m) cm–1. Elemental analysis data was not
obtained.
Pentacarbonyl(cyanido)diiron Complex 3c: Compound 1c (57 mg,
0.12 mmol) was treated with trimethyalamine N-oxide dihydrate
(13 mg, 0.12 mmol) and tetraethylammonium cyanide (19 mg,
0.12 mmol) according to Method B. Yield: 32 mg (0.05 mmol,
1
44%). H NMR (CD3CN, 400 MHz): δ = 3.14 (m, 8 H, NCH2C),
1.57 (m,
4 H, SCH2Si), 1.19 (m, 16 H, SiCH2CH2C,
SiCH2CH2CH2C and CH3), 0.88 (m, 2 H, SiCH2CH2C), 0.74/0.54
(2 m, 4 H, SiCH2C) ppm. 13C{1H} NMR (CD3CN, 50 MHz): δ =
212.4 (CO), 54.1 (NCH2C), 30.3 (SiCH2CH2CH2C), 26.2
(SiCH2CH2C), 24.6 (SiCH2C), 8.7 (CH3), 5.0 (SCH2Si) ppm. MS
Pentacarbonyl(triphenylphosphane)diiron Complex 2b: Complex 1b
(147 mg, 0.322 mmol) was treated with trimethylamine N-oxide di-
hydrate (36 mg, 0.322 mmol) and triphenylphosphane (84 mg,
0.322 mmol) according to Method A to yield the desired product as
a brownish powder (48 mg, 0.07 mmol, 21.6%). 1H NMR (CDCl3,
200 MHz): δ = 7.69 (m, 9 H, H-c/d), 7.41 (m, 6 H, H-b), 1.55–1.24
(m, 8 H, SiCH2CH2C and SCH2Si), 0.65 (t, 3J = 7 Hz, 4 H,
SiCH2C) ppm. 13C{1H} NMR (CDCl3, 50 MHz): δ = 213.2/209.2
(FAB, neg.): m/z = 468 [M – Et N]–. IR (KBr): ν = 2922 (s), 2089
˜
4
(w, CN), 2027 (vs, CO), 1973 (vs, CO), 1637 (m) cm–1. Elemental
analysis data was not obtained.
Structure Determinations: The intensity data for the compounds
were collected with a Nonius KappaCCD diffractometer by using
(CO), 136.1 (C-a), 133.4 (C-b), 130.1 (C-d), 128.6 (C-c), 26.9/26.2 graphite-monochromated Mo-Kα radiation. Data were corrected
(SiCH2CH2C), 13.8 (SiCH2C), 3.8 (SCH2Si) ppm. 31P{1H} NMR for Lorentz and polarization effects but not for absorption ef-
(CDCl3, 81 MHz): δ = 68.7 ppm. MS (DEI): m/z = 690 [M]+, 634
[M – 2 CO]+, 606 [M – 3 CO]+, 550 [M – 5 CO]+, 262 [PPh3]+. IR
(KBr): 3056 (w), 2925 (s), 2854 (m), 2041 (vs), 1982 (vs, br.), 1630
fects.[33,34] Crystallographic data as well as structure solution and
refinement details are summarized in Table 5. The structures were
solved by direct methods (SHELXS[34]) and refined by full-matrix
586
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 581–588

Cyanido and Phosphane Ligands in [2Fe2S(Si)] Hydrogenase Model Systems
Table 5. Crystal data and refinement details for the X-ray structure determinations of the compounds 2a, 2b, 2c, and 3b.
Compound
2a
2b
2c
3b
Empirical formula
C27H25Fe2O5PS2Si·0.5C2H8O2 C29H27Fe2O5PS2Si C30H29Fe2O5PS2Si C12H12Fe2NO5S2Si·C8H20N
Mr [gmol–1]
696.39
–90(2)
triclinic
690.39
–90(2)
triclinic
704.41
–90(2)
monoclinic
P21/n
584.39
–90(2)
monoclinic
P21/c
T [°C]
Crystal system
Space group
¯
P1
¯
P1
a [Å]
b [Å]
c [Å]
α [°]
10.0550(4)
12.8059(4)
13.8444(6)
75.360(3)
77.349(3)
68.226(3)
1586.08(11)
2
10.4099(5)
12.6449(5)
12.8477(3)
105.563(2)
94.001(3)
110.728(2)
1498.03(10)
2
16.6325(3)
20.7724(3)
18.3780(5)
90
99.982(1)
90
6253.4(2)
8
1.496
17.1886(9)
12.5652(6)
12.2768(3)
90
100.704(3)
90
2605.4(2)
4
1.490
β [°]
γ [°]
V [Å3]
Z
ρ [gcm–3]
1.458
11.73
1.531
12.39
μ [cm–1]
11.88
13.52
Measured data
Data with IϾ2σ(I)
Unique data/Rint
wR2 (all data, on F2)[a]
R1 [IϾ2σ(I)][a]
s[b]
11013
5500
7136/0.0293
0.1312
0.0465
1.022
1.844/–0.576
10386
5132
6813/0.0327
0.0829
0.0384
1.025
0.403/–0.459
43489
8918
14280/0.0627
0.0832
0.0382
0.962
18053
3858
5955/0.0752
0.1324
0.0500
1.019
Max/min residual density [eÅ–3]
0.673/–0.362
1.170/–0.506
2 2
[a] Definition of the R indices: R1 = (Σ||Fo| – |Fc||)/Σ|Fo|. wR2 = {Σ[w(Fo2 – Fc ) ]/Σ[w(Fo2)2]}1/2 with w–1 = σ2(Fo2) + (aP)2. [b] s =
2
2 2
{Σ[w(Fo – Fc ) ]/(No – Np)}1/2
.
least-squares techniques against Fo (SHELXL-97[35]). All hydro-
gen atom positions were included at calculated positions with fixed
thermal parameters. All non-hydrogen atoms were refined aniso-
tropically.[36] XP (SIEMENS Analytical X-ray Instruments, Inc.)
was used for structure representations. CCDC-753495 (2a), -753496
(2b), -753497 (2c), and -753498 (3b) 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.
2
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Acknowledgments
Financial support for this work was provided by the Studienstif-
tung des Deutschen Volkes (U.-P. A.). Y. H. and J. G. V. thank the
Science Foundation Ireland for financial support (grant no. 06/
CHPRFP/029).
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Received: August 27, 2010
Published Online: December 12, 2010
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Eur. J. Inorg. Chem. 2011, 581–588