The reactions of pyridinyl thioesters with triiron dodecacarbonyl: Their novel diiron carbonyl complexes and mechanistic investigations

Article abstract of DOI:10.1039/c2dt30798g

Reaction of Fe₃(CO)₁₂ with pyridinyl thioester ligand PyCH₂SCOCH₃ (L₁, Py = pyridin-2-yl) produced complex, [Fe₂(κ-COCH₃)(μ-SCH₂Py)(CO) ₅] (1) (PyCH₂S = pyridin-2-ylmethanethiolate). When complex 1 reacted with PPh₃, a monosubstituted complex, [Fe ₂(κ-COCH₃)(μ-SCH₂Py)(CO) ₄PPh₃] (2), was derived. Reaction of the same precursor with analogous thioester ligand PyCH₂SCOPy (L₂) generated three novel diiron complexes, [Fe₂(κ-Py)(μ-SCH ₂Py)(CO)₅] (3), [Fe₂(κ-Py)′(μ- SCH₂Py)(CO)₅] (4), and [Fe₂(κ-Py)(μ- SCH₂Py)(CO)₆] (5). Complexes 3 and 4 are structural isomers. Complex 5 could be converted into complex 4 but the conversion from complex 5 to the isomer 3 was not observed. All the five complexes were fully characterised using FTIR, NMR, and other techniques. Their structures were determined using X-ray single crystal diffraction analysis. The oxidative formation of complexes 1, 3, 4, and 5 involved C-S and/or C-C bonds cleavages. To probe possible mechanisms for these cleavages, DFT calculations were performed. From the calculations, viable reaction pathways leading to the formation of all the isolated products were delineated. The results of the theoretic calculations also allowed rationalisation of the experimental observations.

Full text of DOI:10.1039/c2dt30798g

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PAPER
The reactions of pyridinyl thioesters with triiron dodecacarbonyl: their novel
diiron carbonyl complexes and mechanistic investigations†
Li Long,a,b Zhiyin Xiao,b Giuseppe Zampella,*c Zhenhong Wei,a Luca De Gioiac and Xiaoming Liu*a,b
Received 13th April 2012, Accepted 22nd May 2012
DOI: 10.1039/c2dt30798g
Reaction of Fe3(CO)12 with pyridinyl thioester ligand PyCH2SCOCH3 (L1, Py = pyridin-2-yl) produced
complex, [Fe2(κ-COCH3)(μ-SCH2Py)(CO)5] (1) (PyCH2S = pyridin-2-ylmethanethiolate). When complex
1 reacted with PPh3, a monosubstituted complex, [Fe2(κ-COCH3)(μ-SCH2Py)(CO)4PPh3] (2), was
derived. Reaction of the same precursor with analogous thioester ligand PyCH2SCOPy (L2) generated
three novel diiron complexes, [Fe2(κ-Py)(μ-SCH2Py)(CO)5] (3), [Fe2(κ-Py)′(μ-SCH2Py)(CO)5] (4), and
[Fe2(κ-Py)(μ-SCH2Py)(CO)6] (5). Complexes 3 and 4 are structural isomers. Complex 5 could be
converted into complex 4 but the conversion from complex 5 to the isomer 3 was not observed. All the
five complexes were fully characterised using FTIR, NMR, and other techniques. Their structures were
determined using X-ray single crystal diffraction analysis. The oxidative formation of complexes 1, 3, 4,
and 5 involved C–S and/or C–C bonds cleavages. To probe possible mechanisms for these cleavages,
DFT calculations were performed. From the calculations, viable reaction pathways leading to the
formation of all the isolated products were delineated. The results of the theoretic calculations also
allowed rationalisation of the experimental observations.
Introduction
There are three hydrogenases in nature.1,2 All the three metal-
loenzymes, [FeFe]-, [NiFe]-, and [Fe]-hydrogenases, possess
Fe(I or II)–S and Fe(I or II)–CO bonding features, as shown in
Fig. 1.3–5 Due to the relevance of these enzymes to both catalytic
hydrogen production and activation, iron–sulphur–carbonyl
chemistry has attracted much attention in recent years. [Fe]-
hydrogenase is unlike the other two enzymes in that this enzyme
performs its catalysis via a specific substrate, methenyltetra-
Fig. 1 Schematic views of the metal centres, (a) [FeFe]-, (b) [NiFe]-,
hydromethanopterin.6,7 Structural analysis revealed that the iron
and (c) [Fe]-hydrogenases.
centre of the co-factor of the enzyme adopts essentially an octa-
hedral geometry coordinated with a cysteine sulfur atom, two
cis-CO ligands, a bidentate pyridone derivative which offers two
ligating atoms, a pyridone nitrogen and an acyl carbon, and an
unidentified ligand which is either mono or diatomic .3,8–12
Spectroscopic investigations into the cofactor and its mimicking
chemistry have confirmed that the metal centre is a low spin
Fe(II).13–17
Modelling the iron centre based on the structural information of
the enzyme may not only provide insight into the co-factor, for
instance shedding light on the nature of the unknown ligand
around the metal centre, but also has relevance to the activation
of dihydrogen. In the past few years, attention has been paid on
the monoiron centre of this enzyme. Those reported mimics pro-
vided insight into the oxidation state of the iron centre and showed
the pyridinyl N–Fe(II) and acyl C–Fe(II) bonding feature found in
the cofactor.18–28 It was reported that the reaction of the thioester
with either Fe3(CO)12 or Fe2(CO)9 produced diiron hexacarbonyl
complexes with the binding feature of acyl C–Fe(I) bond.29,30
Recently, Rauchfuss and co-workers reported a type of reaction
between thioester ligands and Fe2(CO)9.19,22 The oxidative reac-
tions produced “unstable” monoiron complexes bearing the acyl-
binding feature which dimerised slowly in solution. Being inspired
by this chemistry, we were interested in the reactions of multidentate
aDepartment of Chemistry, Nanchang University, Nanchang 330031,
China
bCollege of Biological, Chemical Sciences and Engineering, Jiaxing
University, Jiaxing 314001, China. E-mail: xiaoming.liu@mail.zjxu.edu.cn,
xiaoming.liu@ncu.edu.cn; Tel: +86 (0)57383643937
cDepartment of Biotechnology and Biosciences, University of Milano-
Bicocca, 20126 Milano, Italy. E-mail: giuseppe.zampella@unimib.it;
Fax: +39 (0)264483478; Tel: +39 (0)264483416
†Electronic supplementary information (ESI) available: Spectral data of
complex 4 and the conversion from complexes 5 to 4 and schematic
details and extended descriptions of the reaction pathways generated
from DFT calculations. CCDC 860436–860440 for complexes 1–5. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2dt30798g
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Scheme 1 Synthetic route of ligands L1, L2, and complexes 1–5 (DCM = dichloromethane, DCC = dicyclohexylcarbodiimide).
ligands bearing both pyridinyl and thioester moieties with
Fe3(CO)12. This is because not only has the triiron carbonyl cluster
been a precursor widely employed in the preparation of diiron car-
bonyl complexes as models of the diiron subunit of the [FeFe]-
hydrogenase, but also its reaction often initiates cleavage/formation
C–S(N) bonds which may lead to novel complexes/clusters.29,31
Herein, we report the synthesis of two thioester ligands,
PyCH2SCOCH3 (L1) and PyCH2SCOPy (L2), their reactions
with Fe3(CO)12 and derived novel diiron complexes (please note
that throughout this work, Py denotes the pyridin-2-yl group).
Mechanistic investigations into the reactions were also per-
formed using DFT calculations. Both ligands gave novel diiron
complexes involving the cleavages of C–S and C–C bonds of
the ligands. For ligand L1, complex [Fe2(κ-COCH3)(μ-SCH2Py)-
(CO)5] (1) was isolated. Further reaction of complex 1 with PPh3
afforded substituted complex [Fe2(κ-COCH3)(μ-SCH2Py)-
(CO)4PPh3] (2) without affecting the diiron integrity. While in
the reaction of ligand L2, three complexes, [Fe2(κ-Py)-
(μ-SCH2Py)(CO)5] (3), [Fe2(κ-Py)′(μ-SCH2Py)(CO)5] (4) and
[Fe2(κ-Py)(μ-SCH2Py)(CO)6] (5), were produced, of which com-
plexes 3 and 4 are structural isomers with subtle differences in the
coordination of the pyridin-2-yl group to the diiron centre. Under
the aid of decarboxylating agent (Me3NO), complex 5 was irrever-
sibly converted into complex 4 but no complex 3 was observed. All
ligand (L2) was further derived via two-step synthesis,
Scheme 1. In the preparation of ligand L2, the conversion of
ligand L1 into pyridin-2-ylmethanethiol via hydrolysis35,36 was
nearly quantitative and its reaction with picolinic acid with the
aid of DCC (dicyclohexylcarbodiimide) to achieve ligand L2
was also in satisfactory yield (ca. 90%). The stretching fre-
quency of the carbonyl group of ligand L2 is 20 cm−1 higher
than that of ligand L1 due to the deficiency in electrons of the
adjacent pyridin-2-yl group.
Reaction of Fe3(CO)12 with ligand L1 in THF afforded a
diiron pentacarbonyl complex as the sole isolatable product
with an {FeIFeI} core, [Fe2(κ-COCH3)(μ-SCH2Py)(CO)5] (1),
Scheme 1. Unlike other analogues widely reported,37–41 one of
the bridging linkages is an acetyl group in a κ-coordinating
mode. Such a coordinating mode for acetyl group in a diiron
analogue could also be achieved by nucleophilic attack on a
bound carbon monoxide by a methyl anion, such as LiCH3, or
electrophilic reaction of methyl iodide or acetyl chloride with
metal carbonyl anion,42,43 in addition to the methodology
described by Seyferth and co-workers.29,30 Due to its asym-
metric bridging linkages which degrade the entire symmetry of
this molecule, complex 1 shows slightly more complicated infra-
red spectral profile (Fig. 2) compared to other diiron
analogues.38–41 The major difference lies in the middle of the
spectrum, as shown in Fig. 2. In complex 1, it is a main band
with a well-resolved shoulder at its right side rather than a single
band as observed in the analogues reported previously.38–41
Since ligand L2 is rather similar to ligand L1 (Scheme 1), one
could expect that its reaction with the same triiron precursor
would exhibit rather similar chemistry. But, in fact, the reaction
for this ligand showed significant differences in that (i) both hexa-
carbonyl (5) and pentacarbonyl (3 and 4) products were isolated
and, particularly, the pentacarbonyl products were structural
isomers (3 and 4), and (ii) the bridging linkage is not a picoli-
noyl group analogous to the acetyl moiety as found in complex
1
the complexes were fully characterised using FTIR, H/13C NMR,
X-ray crystallography. Plausible reaction pathways leading to those
diiron complexes were also delineated using DFT calculations.
Results and discussion
Synthesis of thioester ligands (L1 and L2) and their reactions
with Fe3(CO)12
Ligand L1 was prepared from 2-methylpyridine through bromi-
nation32,33 and then thioesterisation with thioacetate.34 The other
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Structural analysis
The five diiron carbonyl complexes are all reasonably stable in
solution. Diffusion of hexanes into solutions of these complexes
in dichloromethane afforded deep-coloured crystals of com-
plexes 1–5. The crystals were suitable for X-ray single crystal
diffraction analysis. Crystallographic details of these complexes
are tabulated in Table 1. Their structural views are shown in
Fig. 3–5. Like other diiron carbonyl complexes reported in the
literature,37–41,44,45 the iron atoms adopt essentially slightly dis-
torted octahedral geometries, as shown by the selected bonding
parameters for the five complexes summarised in Table 2. For
complexes 1–4, it is noticeable that the bond length of the Fe1–S
bond is shorter by 0.3 to 0.5 Å compared to the Fe2–S1 bond.
This shortening is mainly caused by the binding of the pyridinyl
N. The Fe1–N1 and Fe1–S1 bonds are parts of a 5-membered
ring which is nearly an isosceles pentagon. In complexes 1 and
2, the κ-bridging acetyl bond has a length of 1.26 Å, about
0.1 Å longer than the end-on binding acyl group,20 which is
probably attributed to both its κ-bridging nature and lower oxi-
dation state in the two complexes. Distinguishing crystallogra-
phically the binding modes offered by the C and N atoms of the
pyridin-2-yl anion to the diiron centre in the structural isomers 3
and 4 is a nontrivial task since the two neighbouring atoms are
electronically highly similar. Repetitively preparing single crys-
tals with high quality allowed us to determine unequivocally the
proper assignment of the two structural isomers. It is interesting to
note that the Fe1–C13 bond in complex 3 is shorter by 0.03 Å
than the Fe2–C13 bond in complex 4, and similar shortening for
the two Fe1–N2 bonds in complexes 3 and 4 is also observed. In
other words, the bond Fe–X (X = Cl3 and N2) trans to CO is
always longer than its analogous bond trans to pyridinyl group on
the other iron atom. This is likely attributed to the difference in
trans effect exerted by CO and the pyridinyl group, respectively.
Fig. 2 Infrared spectra of complexes 1, 3, 4, and 5 in dichloromethane.
Inset: stretching bands for the bound bridging acetyl group in complexes
1 and 2 (KBr pellets).
1, generated from the cleavage of S–CO(pyridin-2-yl) bond.
Instead, it is a pyridin-2-yl group, the decarboxylated species of
picolinoyl group. The pyridin-2-yl group bridged the two iron
atoms in a κ-coordinating mode in two manners to give the two
structural isomers (3 and 4), Scheme 1. Due to the same cause as
for complex 1, the three complexes isolated from the reaction of
ligand L2 with Fe3(CO)12 show also more absorption bands
compared to their analogues with symmetric bridging lin-
kages.38,39,41,44,45 It is not uncommon that in both Ru and Os
dinuclear complexes, a pyridin-2-yl anion binds isomeri-
cally.46,47 But this isomeric binding has not been observed for a
diiron complex although one of the analogous structures was
recently reported.48 The two isomers have very similar
absorption bands in their FTIR spectra, as shown in Fig. 1. In
their proton magnetic resonance, only subtle differences in
the chemical shifts of the pyridin-2-yl rings in complexes 3 and
4 were observed. It is noteworthy that complex 5 could
be cleanly converted into complex 4 under the aid of a
decarboxylating agent, as indicated spectroscopically (Fig. S1†).
This conversion is informative in deducing the reaction
mechanism of the ligands with the triiron precursor. Indeed,
theoretic calculations revealed that it is a viable pathway
(vide infra). But, interestingly, an analogous conversion into
complex 3 was not observed. This observation is particularly
peculiar as both sterically and electronically the two {Fe(CO)3}
units are generally identical and classic cis-/trans-effect
would not be sufficient to explain this discrimination in the
intramolecular substitution reaction.
Possible pathways for the cleavage of C–S (complexes 1, 3, 4,
and 5) and C–C(Py) bonds (complexes 3, 4, and 5)
Oxidative reaction of triiron dodecacarbonyl, Fe3(CO)12, with
thiols could involve cleavage/formation of C–C and/or C–X
(X = S, N) bonds to produce unexpected diiron complexes
depending on the nature of the thiols employed.29–31 In the reac-
tion of Fe2(CO)9 with thioesters,19,22 the formation of diiron
complexes involving acyl binding were achieved via unstable
monoiron intermediate. Therefore, it is feasible to speculate that
the reaction of Fe3(CO)12 with ligands L1 and L2 would adopt a
similar pathway. Upon the reaction of the ligands with the triiron
precursor, a key monoiron intermediate via the cleavage of the
C–S bond of the thioester ligands could form, Scheme 2. It is
noteworthy that, in the case of ligand L2, the picolinoyl is
further decarboxylated to generate a pyridin-2-yl group after the
C–S bond cleavage. Possibly, the initially formed picolinoyl is
unstable, probably due to the electron-deficient nature of the pyr-
idinyl group. Further reaction of the monoiron intermediate with
iron–carbonyl, either the precursor or the fragments generated in
the reaction, {Fe(CO)n}m (m = 1–3, n > 0), produced complexes
1, 3, 4, and 5, Scheme 2. More details about the cleavage mech-
anism and reaction pathways were obtained by theoretic
investigations.
Reaction of PPh3 with complex 1 produced a mono-substi-
tuted diiron pentacarbonyl complex as with other reported diiron
analogues,49 [Fe2(κ-COCH3)(μ-SCH2Py)(CO)4PPh3] (2), with
both the acetyl binding and Fe–Fe bond intact, Scheme 1. The
resultant product exhibits generally similar infrared spectral
pattern to those diiron tetracarbonyl analogues reported but again
the asymmetric bridging linkages increase the multiplicity of its
infrared spectrum in the absorption region of CO (Fig. S2†).40 In
solution, both complexes do not show any absorption band for
the bridging acetyl group but in the solid state a weak band is
evident around 1600 cm−1, Fig. 2 (inset).
9484 | Dalton Trans., 2012, 41, 9482–9492
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Table 1 Crystallographic data and processing parameters for complex 1–5a
Complex
1
2
3
4
5
Formula
Mw (g mol−1
T (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C
13H9Fe2NO6S
C30H24Fe2NO5PS
653.24
296(2)
C16H10Fe2N2O5S
454.03
296(2)
Monoclinic
P21/c
8.784(5)
12.502(7)
16.016(8)
90
92.832(6)
90
1756.6(16)
4
1.717
912.0
C16H10Fe2N2O5S
454.03
296(2)
C17H10Fe2N2O6S
482.04
296(2)
Orthorhombic
Pccn
16.518(2)
18.976(3)
12.3490(17)
90.00
90.00
90.00
3870.74
8
1.654
)
418.98
296(2)
Triclinic
Triclinic
Triclinic
ˉ
ˉ
ˉ
P1
P1
P1
8.034(2)
8.357(2)
12.834(4)
98.666(3)
93.134(3)
110.001(3)
795.2(4)
2
1.750
420.0
2.64–28.37
7478/3987
1.034
9.0878(18)
12.580(3)
13.299(3)
69.848(2)
84.859(2)
88.663(2)
1421.5(5)
2
1.526
668.0
2.25–28.47
13 543/7181
1.008
9.0510(19)
9.2243(19)
11.929(4)
101.145(3)
102.516(3)
107.424(2)
891.3(4)
2
1.692
456.0
2.41–28.31
8239/4293
0.999
γ (°)
V [Å3]
Z
Dcalcd (g cm−3
F (000)
θ (°)
)
1936.0
2.32–28.35
15 226/4391
1.030
2.32–28.30
34 515/4818
1.040
Reflns collected/ unique
GOF on F2
R1 [I > 2σ(I)]
0.0221
0.0594
0.298, −0.320
0.0299
0.0821
0.348, −0.495
0.0654
0.1545
1.402, −0.813
0.0242
0.0638
0.346, −0.312
0.0472
0.1224
1.538, −0.614
wR2 [I > 2σ(I)]
Residual electron density (e Å−3
)
1
2 2
2
2
2 2
a R1 = ∑kFo| − |Fck/∑|Fo| and wR2 = [∑(|Fo − Fc |) /∑(wFo ) ] .
Fig. 3 Crystal structures of complexes 1 (left) and 2 (right) (ellipsoids
were drawn at probability level of 30% and hydrogen atoms were
omitted for clarity).
Fig. 5 Crystal structure of complex 5 (ellipsoids were drawn at prob-
ability level of 30% and hydrogen atoms were omitted for clarity).
complexity of the reactions and impossibility in isolating any
stable intermediates. To explore possible relevant pathways to
delineate the reaction mechanisms for the formation of the diiron
complexes, a series of theoretic calculations were carried out
using DFT protocols previously validated in the study of models
of the [FeFe]-hydrogenase active site.50–57
The results of DFT calculations allowed us to outline the main
features of the reaction pathways of both ligands with Fe3(CO)12.
Due to the multiplicity of the reaction pathways and the large
number of transition states and intermediates involved, we depict
the reaction mechanisms in a highly simplified manner in
Schemes 3 and 4, respectively, and leave the very detailed
schemes in ESI† for further reference. Therein has been also
reported an extended description to complement all the
Schemes.†
We must point out that the same denotation shown in the two
schemes (Schemes 3 and 4) does not necessarily mean exactly
the same species. To envision clearly all the pathways shown in
the two schemes leading to the complexes (1, 3, 4, and 5), those
reaction pathways and related energies were also tabulated in
Tables 3 and 4, respectively. In such Tables and related Schemes,
Fig. 4 Crystal structures of complexes 3 (left) and 4 (right) (ellipsoids
were drawn at probability level of 30% and hydrogen atoms were
omitted for clarity).
DFT calculations
As mentioned above, it is difficult to sketch out more detailed
reaction pathways than those shown in Scheme 2 due to the
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Table 2 Selected bond lengths and angles for complex 1–5
Complex
1
2
3
4
5
Fe1–Fe2
Fe1–S1
Fe2–S1
Fe1–N1
2.5626(6)
2.2195(6)
2.2672(6)
2.0594(13)
2.5535(6)
2.2369(6)
2.2762(7)
2.0643(19)
2.5932(16)
2.2080(16)
2.2673(17)
2.028(4)
2.5938(6)
2.2269(5)
2.2604(7)
2.0112(13)
2.5778(7)
2.2400(10)
2.2515(10)
Fe2–N2
1.979(4)
Fe1–N2 (trans to Py)
Fe1–C13 (trans to Py)
Fe2–C13
C1–O1
C6–O6 (Acyl)
1.9649(12)
1.981(3)
1.956(4)
1.150(6)
1.9839(15)
1.138(2)
1.984(3)
1.130(6)
1.140(2)
1.2555(19)
1.141(2)
1.261(2)
Scheme 2 The formation of complexes 1, 3, 4, and 5 involving hypothesised monoiron intermediates ({Fe(CO)n}m could be either the triiron precur-
sor or a fragment generated in the reaction).
the denotation DLn refers to the ligands (L1 and L2) with an
appropriate donor atom (D) which initiates the reaction with the
triiron precursor.
on the activation barrier of key steps, such as the iron unit incor-
poration, which can be exerted by possibly existing accelerators
(such as Fe2(CO)8). DFT data indicate that the energy barrier
associated to the transfer of the mono-iron unit is actually
decreased by evident and different amounts in the different path-
ways investigated.
As for the diiron unit formation, a possible route might imply
a single-step reaction of the ligands with the triiron precursor by
simply ejecting a monoiron–carbonyl fragment. However, DFT
results indicate that the diiron core can be assembled following a
stepwise reaction pathway. The first iron atom was abstracted
when the ligand reacted with the triiron precursor and a
{Fe2(CO)8} fragment was cleaved. This cleavage was highly
thermodynamically favoured. Other cleavages have been taken
into account such as a single CO releasing from Fe3(CO)12 upon
L1/L2 addition, but they all resulted in kinetically disfavoured
pathways (associated barrier higher by 24 kcal mol−1 than those
associated to the presented reaction routes). The formed mono-
iron species underwent further reactions and the second iron was
added to the monoiron unit via its reaction with the precursor.
An attempt has been made to explore catalytic effects of labile
species (such as Fe2(CO)8), which may be possibly present in
solution as by-products of the early step of the reaction of the
ligands (L1/L2) with Fe3(CO)12. Even though it is conceivable
that such unsaturated species may react with CO molecules
which are released during some of the steps along the investi-
gated pathway, it is interesting to explore the extent of decrease
It was experimentally observed in the reactions involving both
ligands that increasing the ratio of the triiron precursor over the
ligand from 1 : 1 to 2 : 1 could almost double the overall reaction
yield. This suggests that involvement of the triiron precursor
could be dominant in assembling the second iron into the diiron
core. The outlined pathways are consistent with this observation.
As shown in Tables 3 and 4, the formation of all the products is
overall exergonic. The multidentate nature of the ligands offered
several ways to initiate the reactions. But no matter which donor
atom of the ligands attacked first at one of the three iron atoms
of the precursor, the right first step was kinetically up-hill with
an associated ΔG‡ of ca. 30 kcal mol−1. This explains well that
heating was necessary to drive the reactions to proceed. For both
ligands, their reactions with the triiron precursor involved bond
cleavage of C–S and/or C–C bonds before the final products
formed. Such a bond cleavage could occur either before or after
the diiron unit was assembled. In either way, those bond clea-
vages are highly exergonic.
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Scheme 3 Reaction pathways leading to the formation of complex 1 as derived from DFT calculations, related energies (kcal mol−1), and the calcu-
lated structures of the intermediates involved in the reactions.
The reaction pathways revealed by the theoretic calcu-
lations for ligand L2 showed that the reaction did not lead
directly to diiron pentacarbonyl complexes. Instead, the most
favourable one among the reaction pathways led to the diiron
hexacarbonyl 5, as shown in Scheme 4a and Table 4. After
the reaction between Fe3(CO)12 and L2 occurred, complexes
5, 3 and 4 were isolated in an approximate ratio of 2 : 1 : 1,
which is in line with the DFT-calculated relative stabilities
associated to the three complexes. Although the formation
energies for both complexes 3 and 4 are globally exergonic,
the energies of the conversion from complexes 5 to 3/4 are
slightly endergonic (ΔG is 0.5, 1.0 kcal mol−1, respectively;
Table 4). This is consistent with the observation that heating
a solution of complex 5 did not produce either complexes 3
or 4. As mentioned earlier, under the aid of decarboxylating
agent, complex 5 was solely converted into complex 4
without any of its isomer 3 isolated. This ought to be attribu-
ted to kinetic reason of the chemical reaction rather than to
thermodynamics matters.
acetyl (COCH3) or pyridin-2-yl were apparently generated in situ
in the reactions, that is, resulting from fragmentation of the
parent ligands involving C–S and C–C bond cleavage. The
chemical features of the obtained complexes were established
using FTIR, NMR spectroscopies, and microanalysis. Their
crystal structures were also determined using X-ray single crystal
diffraction analysis.
DFT calculations provided some insight into the reaction
mechanisms of the triiron carbonyl precursor with the
ligands, L1 and L2, and disclosed multiple plausible
pathways leading to the novel diiron products. The S atom
has a slight preference over the N atom in the reaction with
the triiron precursor according to the theoretical data. The
diiron unit of the complexes was assembled stepwise rather
than retaining a diiron core by cleaving a monoiron carbonyl
fragment in the reaction with the triiron cluster Fe3(CO)12.
During the various reaction occurrences, the cleavage of the
acetyl groups (acetyl, CH3CO and picolinoyl, PyCO)
occurring via breaking the C–S bond is thermodynamically
spontaneous. Further cleavage of one CO from the group,
2-PyCO, was also strongly favoured. These bond cleavages
were certainly driven by the coordination of different ligands
(S, N, O) to the metal atom. Since triiron dodecacarbonyl
cluster (Fe3(CO)12) is a universal precursor leading to other
iron–carbonyl complexes, understanding mechanistically its
reaction would certainly make its products more predictable
and, therefore, be beneficial to designing novel diiron–
carbonyl complexes, which are widely regarded as mimics of
the diiron subunit of the [FeFe]-hydrogenase.
Conclusions
Two pyridinyl thioesters ligands and their reactions with
Fe3(CO)12 were described and four novel diiron complexes (1, 3,
4, and 5) with κ-COCH3 or -pyridin-2-yl coordination modes
were isolated. Reaction of complex 1 with PPh3 led to complex
2 leaving the κ-COCH3 binding intact. The structural isomers,
complexes 3 and 4, exhibit a subtle difference in the manner of
bridging the two iron atoms, κ-(C, N)Py and κ-(N, C)Py. The
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Scheme 4 (a) Reaction pathways leading to the formation of complexes 3, 4, and 5 as derived from DFT calculations and related energies
(kcal mol−1). Please note that N′ in N′L2 refers to the N atom of the pyridinyl ring bearing a methylene group and the calculated structures of those
intermediates are shown in Scheme 4b except for 5CO and Y′. (b) Key intermediates involved in the reaction pathways shown in Scheme 4a.
9488 | Dalton Trans., 2012, 41, 9482–9492
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Table 3 Reaction pathways (1Pn) of the triiron precursor with ligand
Table 4 Reaction pathways (2Pn) of the triiron precursor with ligand L
L1 and related energies (kcal mol−1
)
and related energies (kcal mol−1
)
DL1
NL1
SL1
1Pn
Reaction pathway
ΔG
ΔG′ b
30.2
28.3
DL2
NL2
2Pn
Reaction pathway
ΔG
ΔG′ b
a
a
1P1
1P2
1P3
1P4
1P5
→ A → m → M → M′ → MFeFe → 1
→ A → m → M → MFeFeCO → 1
→ a → MFeFeCO → 1
→ a → m → M → MFeFeCO → 1
→ a → m → M → M′ → MFeFe → 1
−4.0
−4.0
−4.0
−4.0
−4.0
2P1
2P2
2P3
2P4
2P5
2P6
2P7
2P8
2P9
→ MFe → MFeFeCO → 5CO → 5
2P1 (5) → 3CO → 3
−7.3 32.0
−6.8
2P1 (5) → 4CO → 4
−6.3
SL2
→ SFe → SFe′ → X → Y → 5
→ SFe → SFe′ → X → YCO → 5
→ SFe → Pre-M → X → Y → 5
→ SFe → Pre-M → X → YCO → 5
→ SFe → Pre-M → MFeFe → 3CO → 3 −6.8
→ SFe → Pre-M → MFeFe′ → 3
−7.3 31.8
−7.3
−7.3
−7.3
a D refers to donor atoms, N and S. b ΔG′ refers to the energy involved
in the right first step of the ligand attacking the triiron precursor.
−6.8
−6.8
−6.8
−6.8
−6.8
−6.3
−6.3
−6.3
−6.3
−7.3 34.6
−7.3
−6.8
−6.8
−6.8
−6.8
−6.3
−6.3
1.0
2P10 2P4 (5) → 3CO → 3
2P11 2P5 (5) → 3CO → 3
2P12 2P6 (5) → 3CO → 3
2P13 2P7 (5) → 3CO → 3
2P14 2P4 (5) → 4CO → 4
2P15 2P5 (5) → 4CO → 4
2P16 2P6 (5) → 4CO → 4
2P17 2P7 (5) → 4CO → 4
2P18 → Pre-M → X → Y → 5
2P19 → Pre-M → X → YCO → 5
2P20 → Pre-M → M → MFeFe → 3CO → 3
2P21 → Pre-M → M → MFeFe′ → 3
2P22 2P18 → 3CO → 3
Experimental
General procedures
N′L2
Unless otherwise stated, all operations were carried out under Ar
atmosphere using standard Schlenk techniques. Reaction vessels
were oven-dried at 150 °C prior to use. Solvents were dried prior
to use following to standard procedures. Fe3(CO)12 were syn-
thesised following modified literature procedures.19,58 FTIR
spectra were recorded on Scimitar 2000 (Varian) using a cell
with a spacer of 0.1 mm or KBr pellets. NMR spectra were
measured on Bruker Avance (400 or 600 MHz) with tetramethyl-
silane as internal standard. Elemental analyses were performed
on a Heraeus CHN–O-Rapid.
2P23 2P19 → 3CO → 3
2P24 2P18 → 4CO → 4
2P25 2P19 → 4CO → 4
5 → 4CO → 4
5 → 3CO → 3c
0.5
a D refers to donor atoms, N, N′, and S (please refer to the caption of
Scheme 4a for the nature of N′). b ΔG′ refers to the energy involved in
the right first step of the ligand attacking the triiron precursor with a
specific donor atom (N, N′, and S). c This conversion was not
experimentally observed.
Crystal structure analyses
In the crystallographic data collection of complexes 1–5,
standard procedures were used for mounting the crystals on a
Bruker CCD area detector diffractometer at 296(2) K. Crystals
were routinely coated with paraffin oil before being mounted.
Intensity data were collected using Mo Kα radiation (λ =
0.71073 Å) by a φ- and ω-scan mode. The SMART and SAINT
programs were used for integration and absorption
correction. The structures were solved by direct method using
SHELXS program and refined on F2 with SHELXTL.1 All non-
hydrogen atoms were refined anisotropically and all hydrogen
atoms were included at calculated positions with fixed thermal
parameters.
(RC). After performing the vibrational analysis of the
constrained minimum energy structures, the negative
eigenmode associated to the RC is followed to locate the true
transition state structure, which corresponds to the maximum
energy point along the trajectory which joins two adjacent
minima.
Free energy (G) values have been obtained from the electronic
SCF energy considering three contributions to the total
partition function (Q), namely qtranslational, qrotational, qvibrational
,
under the approximation that Q may be written as the product
of such terms. Evaluation of H and S contributions has been
made by setting T and P values at 298.15 K and 1 bar,
respectively.
DFT calculations
In all steps investigated which entailed the presence of
Fe2(CO)8 and Fe(CO)4, the energy difference existing between
the S = 0 state and the S = 1 state has been taken into account at
the same level of theory that has been adopted for the present
investigation. The B-P86 functional is known to yield a small
energy split between the two different spin states, which is
slightly less than that found by wave-function based methods,
i.e. high correlated post-HF methods such as CCSD and
CCSD(T).62–64
In light of available experimental data and considering the
chemical nature of the ligands, only low-spin species of Fe com-
plexes, which have been subjected to DFT analysis, have been
considered.
Density functional theory (DFT) calculations have been carried
out by adopting the TURBOMOLE suite of programs,59 at the
B-P86/TZVP60,61 level of theory, which has been reported to be
suitable for investigating [FeFe]-hydrogenase models.50–52,54–57
All kinds of stationary points on the potential energy surface
have been determined by means of energy gradient techniques
and a full vibrational analysis has been carried out to further
characterise each point. Transition state structures have been
searched by a procedure based on a pseudo-Newton-Raphson
algorithm.
Preliminarily, the geometry optimization of a guessed tran-
sition state structure is carried out by freezing the molecular
degrees of freedom corresponding to the reaction coordinate
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Synthesis
Preparation of PyCH2SCOCH3 (L1). 2-Bromomethyl pyridine
(400 MHz, d6-DMSO): 8.95 (d, J = 7.9 Hz, 1 H, Py-H), 7.68 (t,
J = 9.40 Hz, 1 H, Py-H), 7.38 (d, J = 9.40 Hz, 1 H, Py-H), 7.23
(t, J = 9.3 Hz, 1 H, Py-H), 4.67 (d, J = 28.8 Hz, 1 H, Py-CH2),
4.32 (d, J = 28.8 Hz, 1 H, Py-CH2), 1.99 (s, 3 H, FeCO–CH3).
13C NMR (600 MHz, CDCl3): 219.2, 216.3, 213.3, 164.7,
155.6, 138.2, 138.2, 124.9, 124.4, 48.0, 45.0. Microanalysis for
Fe2C13H9NSO6 (MW = 418.97), calc. (%): C, 37.27; H, 2.17; N,
3.34. Found (%): C, 37.65; H, 1.81; N, 3.25.
hydrochloride (1.00 g, 4.8 mmol), triethylamine (1.4 mL,
9.6 mmol) and CH3COSK (0.55 g, 4.8 mmol) in methanol
(20 mL) were stirred for 2 h at room temperature. After being
concentrated under vacuum, the solution was extracted with
ethyl acetate (6 × 30 mL). The organic phases were combined
and dried with anhydrous MgSO4 before the solvent was
removed. Purification of the residue was achieved using flash
chromatography (column: silica gel; eluent: a mixture of ethyl
acetate–petroleum ether (1 : 4)). The purification gave a colour-
less oily liquid (0.67 g, 83%). IR (DCM, v/cm−1): 1695 (–CO).
1H NMR (600 MHz, CDCl3): 8.13 (s, 1H, Py-H), 7.20 (d, J =
2.8 Hz, 1 H, Py-H), 6.94 (s, 1H, Py-H), 6.75 (d, J = 3.1 Hz, 1 H,
Py-H), 3.87 (d, J = 5.5 Hz, 2 H, PyCH2), 1.93 (t, J = 1.6 Hz,
3 H, COCH3).
Preparation of [Fe2(κ-COCH3)(μ-SCH2Py)(CO)4PPh3] (2).
Complex 1 (18.70 mg, 0.045 mmol) and PPh3 (11.71 mg,
0.09 mmol) in THF (10 mL) was heated at 55 °C for 2.5 h. The
reaction mixture was evaporated to dryness under vacuum and
purified with column chromatography (eluent: ethyl acetate–
petroleum ether = 1 : 4) to produce a red solid (12 mg, 42%) which
was recrystallised from DCM/hexanes at −24 °C. IR (DCM,
1
v/cm−1): 1987, 1947, 1913, 1897 (CO). H NMR (600 MHz,
Preparation of PyCH2SH. K2CO3 (2.50 g, 18.1 mmol) in
H2O (40 mL) was added to a methanol (50 mL) solution of L1
(2.00 g, 12.0 mmol). The mixture was heated to 85 °C and
stirred for 2.5 h. The methanol was removed under reduced
pressure to give a residual which was extracted using dichloro-
methane (6 × 30 mL). The organic phases were combined and
dried with anhydrous MgSO4 before the solvent was removed.
The residue was purified using flash chromatography (eluent:
with ethyl acetate–petroleum ether = 1 : 4). A colourless oily
CDCl3): 8.951 (s, 1 H, Py-H), 7.57–6.94 (m, 21 H, Ar-H and
Py-H), 4.05 (s, 1 H, Py-CH2), 3.39 (s, 1 H, Py-CH2), 2.01 (s, 3
H, OCCH3). 31P NMR (600 MHz, CDCl3): 46.09. Microanalysis
for Fe2C30H24NSPO5 (MW = 653.24), calc. (%): C, 55.16; H,
3.70; N, 2.14. Found (%): C, 55.03; H, 2.96; N, 2.08.
Preparation of [Fe2(κ-Py)(μ-SCH2Py)(CO)5] (3), [Fe2(κ-Py)′-
(μ-SCH2Py)(CO)5] (4), and [Fe2(κ-Py)(μ-SCH2Py)(CO)6] (5).
Fe3(CO)12 (1.31 g, 2.61 mmol) and ligand L2 (0.60 g,
2.61 mmol) in THF (40 mL) was heated at 65 °C for 1.5 h. The
reaction mixture was evaporated to dryness under vacuum and
purified using column chromatography (eluent: ethyl acetate–
petroleum ether = 1 : 4) to give following three products:
1
liquid was obtained (1.4 g, 94%). H NMR (600 MHz, CDCl3):
8.53 (s, 1 H, Py-H), 7.656–7.627 (m, 1 H, Py-H), 7.33 (d, J =
7.8 Hz, 1 H, Py-H), 7.162–7.140 (m, 1 H, Py-H), 3.84 (d, J =
7.2 Hz, 2 H, PyCH2), 2.027 (t, J = 7.8 Hz, 1 H, SH).
Complex 3 as a red solid (49.3 mg, 17%) which was recrystal-
lised from DCM/hexanes at −24 °C. IR (DCM, v/cm−1): 2034,
1975, 1957, 1907 (CO). 1H NMR (600 MHz, CDCl3): 9.02 (s, 1
H, Py-H), 7.42 (d, J = 24.0 Hz, 2 H, Py-H), 7.26 (s, 1 H, Py-H),
7.15 (s, 1 H, Py-H), 6.98 (s, 1 H, Py-H), 6.78 (s, 2 H, Py-H),
6.41 (s, 1 H, Py-H), 4.47 (d, J = 15.6 Hz, 1 H, Py-CH2), 4.05 (d,
J = 15.6 Hz, 1 H, Py-CH2). 13C NMR (600 MHz, CDCl3):
220.3, 216.9, 213.1, 197.0, 164.2, 154.3, 152.0, 137.7, 135.6,
131.2, 122.9, 122.1, 118.3, 44.0. Microanalysis for
Fe2C16H10N2SO5 (MW = 454.02), calc.: C, 42.33; H, 2.22;
N, 6.17. Found: C, 41.88; H, 1.82; N, 6.16.
Complex 4 as a red solid (57 mg, 19%) which was recrystal-
lised from DCM/hexanes at −24 °C. IR (DCM, v/cm−1): 2029,
1981, 1947, 1917 (CO). 1H NMR (600 MHz, CDCl3): 8.51 (s, 1
H, Py-H), 7.29 (s, 1.4 H, Py-H), 7.12 (s, 1 H, Py-H), 6.99 (s, 1
H, Py-H), 6.80 (s, 2.6 H, Py-H), 6.36 (s, 1H, Py-H), 4.44 (s, 1
Preparation of PyCH2SCOPy (L2). To a stirred solution of
pyridin-2-ylmethanethiol (1.02 g, 8.1 mmol) in CH2Cl2 (DCM)
(20 mL) was successively added picolinic acid (1.01 g,
8.1 mmol) and dicyclohexylcarbodiimide (DCC) (1.68 g,
8.1 mmol). The solution was stirred for 3 h at ambient tempera-
ture and a white suspended solid was formed. The solid was
removed by filtration and the filtrate was concentrated under
reduced pressure and purified using column chromatography
(eluent: ethyl acetate–petroleum ether = 1 : 1). A white solid
1
(1.65 g, 88%) was obtained. H NMR (600 MHz, CDCl3): 8.68
(d, J = 4.20 Hz, 1 H, Py-H), 8.55 (d, J = 4.8 Hz, 1 H, Py-H),
7.97 (d, J = 7.8 Hz, 1 H, Py-H), 7.85 (t, J = 7.5 Hz, 1 H, Py-H),
7.63 (t, J = 7.5 Hz, 1 H, Py-H), 7.521–7.500 (m, 1 H, Py-H),
7.44 (d, J = 7.8 Hz, 1 H, Py-H),7.16 (t, J = 6.6 Hz, 1 H, Py-H),
4.43 (s, 2 H, Py-CH2). 13C NMR (600 MHz, CDCl3): 192.9,
157.6, 151.7, 149.5, 149.2, 137.2, 136.7, 127.9, 123.3, 122.1,
120.5, 35.1. Microanalysis for C12H10N2SO (MW = 230.29),
calc. (%): C, 62.59; H, 4.38; N, 12.16. Found (%): C, 62.40; H,
5.54; N, 11.92.
H, Py-CH2), 3.93 (s,
1 H, Py-CH2). Anal. Calc. for
Fe2C16H10N2SO5 (MW = 454.02): C, 42.33; H, 2.22; N, 6.17;
S, 7.06. Found: C, 41.99; H, 2.31; N, 6.12; S, 7.01.
Complex 5 as a orange-red solid (106 mg, 18%) which was
crystallised from ether/hexanes at −24 °C. IR (DCM, v/cm−1):
2067, 2026, 1990, 1960 (CO). 1H NMR (600 MHz, CDCl3):
8.63 (s, 1 H, Py-H), 7.69 (s, 1 H, Py-H), 7.50 (s, 1 H, Py-H),
7.33 (d, J = 6.6 Hz, 1 H, Py-H), 7.22 (s, 1 H, Py-H), 7.04 (s, 2
H, Py-H), 6.64 (s, 1 H, Py-H), 3.926–3.849 (m, 2 H, Py-CH2).
13C NMR (600 MHz, CDCl3): 213.6, 210.2, 191.6, 159.1,
153.6, 149.9, 137.6, 136.9, 133.3, 122.8, 122.4, 120.4, 45.5.
Microanalysis for Fe2C17H10N2SO6 (MW = 482.03), calc. (%):
Preparation
of
[Fe2(κ-COCH3)(μ-SCH2Py)(CO)5]
(1).
Fe3(CO)12 (1.09 g, 2.2 mmol) and ligand L1 (0.361 g,
2.2 mmol) in THF (20 mL) was heated at 70 °C for 2.5 h. The
resulting reddish brown mixture was concentrated under reduced
pressure and purified with column chromatography (eluent: ethyl
acetate–petroleum ether = 1 : 4) to give as a red solid (0.29 g,
32%) which was crystallised from DCM/hexanes at −24 °C. IR
(DCM, v/cm−1): 2046, 1986, 1969, 1919 (CO). 1H NMR
9490 | Dalton Trans., 2012, 41, 9482–9492
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Acknowledgements
We thank the Natural Science Foundation of China (Grant No.
20871064, 21171073), Ministry of Science and Technology of
China (973 program, 2009CB220009), and the provincial gov-
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