An insight into the protonation property of a diiron azadithiolate complex pertinent to the active site of Fe-only hydrogenases

Article abstract of DOI:10.1039/b513270c

Protonation of [{(μ-SCH₂)₂N(C₆H ₄-p-NO₂)}{Fe(CO)₂(PMe₃)} ₂] in the presence of 4 equiv. of HOTf afforded two species, a μ-hydride diiron complex, the molecular structu

Full text of DOI:10.1039/b513270c

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An insight into the protonation property of a diiron azadithiolate
complex pertinent to the active site of Fe-only hydrogenases{
Weibing Dong,a Mei Wang,*a Xiaoyang Liu,ab Kun Jin,a Guanghua Li,b Fujun Wanga and Licheng Sun*ac
Received (in Cambridge, UK) 19th September 2005, Accepted 31st October 2005
First published as an Advance Article on the web 22nd November 2005
DOI: 10.1039/b513270c
property of 2 on the iron and the m-S atoms, and the molecular
structures of 2 and its m-hydride species [2(FeHFe)]+[PF6]2.
The all-carbonyl diiron azadithiolate complex [{(m-SCH2)2N
(C6H4-p-NO2)}Fe2(CO)6] (1) was prepared according to the
procedure of Rauchfuss et al.12 The PMe3-disubstituted complex
2 was obtained in a good yield by double CO-displacement of 1
with 4 equiv. of PMe3 in toluene. The n(CO) values of 2 at 1985,
1949 and 1906 cm21 (Fig. 1, left (a)) were substantially lowered as
Protonation of [{(m-SCH2)2N(C6H4-p-NO2)}{Fe(CO)2(PMe3)}2]
in the presence of 4 equiv. of HOTf afforded two species, a
m-hydride diiron complex, the molecular structure of which
was crystallographically characterized, and a m-S-protonated
species, which was readily deprotonated in the presence of
pyridine.
In recent years, special attention has been paid to the molecular
models of the Fe-only hydrogenase active site due to its
remarkable efficiency in H2 production and its structural
resemblance to well-known complexes [(m-SR)2{Fe(CO)2L}2].1
One of the challenging issues is to elucidate the mechanism of
enzymatic hydrogen production and uptake by figuring out which
heteroatom of the diiron substrate might act as a favorable internal
basic site in the heterolytic cleavage of H2, or as a preferred acidic
site in a protonated form in the enzymatic H2-evolution. The DFT
calculations relevant to the Fe-only hydrogenase active site and its
molecular models suggested three controversial pathways for the
H2-heterolytic splitting with the formation of the iron hydride
species, that is, protonation on the N atom, either in the CN2
ligand or in the 2-azapropane bridge,2 or protonation on one of
the m-S atoms.3 The experimental proof for the formation of the
m-S protonated species of the 2Fe2S model complexes is as yet
undisclosed.
compared with those of 1 at 2078, 2040 and 2001 cm21 11c
The
.
1
selected region (d = 2.5–9.0) of the H NMR spectrum of 2 is
shown in Fig. 2 (a).
Although the protonation of model complexes bearing a pdt-
bridge (pdt = 1,3-propanedithiolato) and their catalytic properties
for electrochemical proton reduction are well studied,4–10 the
diiron azadithiolate model complexes and their reactivity towards
proton acids have been less described so far. The rare reports on
the protonation properties and electrochemical proton reduction
of the adt-bridged (adt = 2-azapropanedithiolato) diiron com-
plexes are confined to the all-carbonyl complexes.11 To have
an insight into the protophilic property of the PMe3-
disubstituted diiron azadithiolate complex, [{(m-SCH2)2N(C6H4-
p-NO2)}{Fe(CO)2(PMe3)}2] (2) was prepared and the protonation
process of 2 was explored by IR, 31P{1H} and 1H NMR
spectroscopy in the presence of different amounts of HOTf.
Here we describe the spectroscopic evidence for the protonation
Fig. 1 FT-IR (left) and 31P{1H} NMR (right) spectra of (a) 2 in
CH3CN, (b) + 4 equiv. HOTf, (c) [2(FeHFe)]+[PF6]2 in CH3CN, (d)
[2(SH)]+[OTf]2 in CH3CN, containing
[2(FeHFe)]+[OTf]2.
a
small amount of
aState Key Laboratory of Fine Chemicals, Dalian University of
Technology, Zhongshan Road 158-46, 116012 Dalian, P. R. China.
E-mail: symbueno@dlut.edu.cn; Fax: +86 411 83702185;
Tel: +86 411 88993886
bState Key Laboratory of Inorganic Synthesis and Preparative
Chemistry, Jilin University, 130012 Changchun, P. R. China
cKTH Chemistry, Organic Chemistry, Royal Institute of Technology,
10044 Stockholm, Sweden
{ Electronic supplementary information (ESI) available: Experimental
details and selected bond lengths and angles of 2 and [2(FeHFe)]+. See
DOI: 10.1039/b513270c
Fig. 2 1H NMR spectra of the selected region: (a) 2 in CDCl3, (b)
[2(FeHFe)]+[PF6]2 in CD3CN, (c) [2(SH)]+[OTf]2 in CD3CN, containing a
small amount of [2(FeHFe)]+[OTf]2.
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1
To get a preliminary perception of the protonation process of 2,
the in situ IR and 31P{1H} NMR spectra of 2 in different amounts
of HOTf were recorded in CH3CN. The three n(CO) bands in the
IR and the signal in the 31P{1H} NMR spectra of 2 (Fig. 1 (a)) did
not display any noticeable shift as an equiv. of HOTf was added to
the CH3CN solution of 2. With an increase of the molar ratio of
HOTf up to 1 : 4 (2 : HOTf, mol/mol), the n(CO) bands of 2
completely disappeared and four blue-shifted bands at 2111, 2071,
2035 and 1995 cm21 were observed (Fig. 1, left (b)). At the same
time two new signals at d = 22.52 and 21.09 appeared in the
31P{1H} NMR spectrum (Fig. 1, right (b)), shifting high field by
4.67 and 28.28 ppm, respectively, as compared with that of the
non-protonated complex 2. It is noteworthy that the signal at
d = 21.09 vanished when an excess of pyridine was added,
accompanied by regeneration of the signal of 2 at d = 27.19, while
the intensity of the signal at d = 22.52 did not show observable
change. The IR and 31P{1H} NMR spectra imply that two
different protonated species of 2 are formed in a 1 : 4 molar ratio
of 2 : HOTf in CH3CN. The one with the 31P NMR signal at
d = 22.52 is relatively stable in the presence of excess pyridine, and
the other protonated species with the signal at d = 21.09 is readily
deprotonated by pyridine.
Both 31P{1H} and H NMR spectra suggest that [2(SH)]+ is the
dominant component in the orange paste, which is contaminated
with a small amount of [2(FeHFe)]+. The protonation of one of the
m-S atoms leads to a decrease in the electron-donating capability of
the S atom, resulting in the decrease of the electron density of the
iron atoms. Consequently the abated feedback bonds cause the
great blue-shifts of n(CO) frequencies of [2(SH)]+. The splitting and
the very large down-field shifts of the signals of the CH2 groups
can be rationally explained by the proposed m-S-protonated
species, which is suggested by theoretical calculations on the
reaction mechanism at the diiron subsite of Fe-only hydrogenases
but has not yet been identified by experiment.3 Although the
attempt to obtain a single crystal of [2(SH)]+ for an X-ray
diffraction measurement was thwarted by the instability of
[2(SH)]+
in
solution,
the
structurally
characterized
[(m-SCH2CH2SEt){Fe(CO)2(PMe3)}2]+[OTf]2, obtained from the
m-S-alkylation of [(m-SCH2CH2S){Fe(CO)2(PMe3)}2] with EtOTf
in CH2Cl2,13 gives an indirect support for the argument of the m-S
protonation of 2 in the presence of 4 equiv. of HOTf.
The molecular structures of 2 and its m-hydride diiron complex
[2(FeHFe)]+[PF6]2 were determined by X-ray analyses of single
crystals (Fig. 3).{ The PMe3-disubstituted complex 2 exists in the
crystalline state as two configurational isomers (Fig. 3 (top)),
transoid basal–basal (ba–ba) and apical–basal (ap–ba). The two
The pure m-hydride diiron complex [2(FeHFe)]+ was isolated as
2
a PF6 salt by addition of a few drops of saturated aqueous
2
NH4PF6 solution to the CH2Cl2–EtOH (2 : 15, v/v) solution of 2
and HCl (48 equiv., 12 M). Complex [2(FeHFe)]+[PF6]2 is quite
stable in the solid state and in O2-free neutral solution. The n(CO)
frequencies of [2(FeHFe)]+ at 2035 and 1995 cm21 (Fig. 1, left (c))
and the signal of the PMe3 ligands at d = 22.52 (Fig. 1, right (c))
indicate that [2(FeHFe)]+ is one of the two protonated species
detected in the CH3CN solution of 2 and HOTf. The high-field
triplet at d = 215.02 with JPH = 21.7 Hz in the 1H NMR spectrum
provides an unambiguous proof for the formation of the m-hydride
isomers pair up in a crystal cell. In contrast, the PF6 salt of
1
diiron complex [2(FeHFe)]+. The H NMR signals of the CH2
groups and the ortho-protons of the C6H4-p-NO2 group shift 0.43
and 0.37 ppm (Fig. 2 (b)), respectively, to lower field relative to the
corresponding signals of 2 (Fig. 2 (a)).
The results of further experiments incline us to assume that the
other protonated form in Fig. 1 (b) is the species with the proton
on a bridging thiolate. The addition of 4 equiv. of HOTf into the
CHCl3 solution of 2 afforded an orange paste, which was quickly
washed with O2-free CHCl3 and dried in vacuo. The high-
resolution mass spectrum (m/z 604.9496) indicates that the orange
paste is mainly composed of mono-protonated species of 2. The
orange paste displays two strong bands at the relatively high
wavenumbers of 2112 and 2075 cm21 in the IR spectrum (Fig. 1,
left (d)) and a signal at d = 2 1.09 in its 31P{1H} NMR spectrum
(Fig. 1, right (d)) with a small signal from the m-hydride
[2(FeHFe)]+ at d = 22.52. In the 1H NMR spectrum of the
orange paste the signal of the CH2 groups of 2 at d = 4.20 (Fig. 2
(a)) splits into two signals with marked down-field shifts to d =
6.66 and 5.39 (Fig. 2 (c)), and the signals of aromatic protons of
the C6H4-p-NO2 group at d = 8.14 and 6.68 for 2 also move to
down field by 0.17 and 0.93 ppm, respectively. According to the
spectroscopic evidence the P- and the bridging-N-protonated
species can be ruled out for the orange paste, which is proposed to
be the complex with one of the m-S atoms protonated, [2(SH)]+.
The additional signal at d = 3.05 in the 1H NMR spectrum of the
orange paste is tentatively assigned to the proton on the m-S atom.
Fig. 3 Molecular structures of two isomers of 2 (top) and the structure
of [2(FeHFe)]+ (bottom) with thermal ellipsoids set at 30% probability.
˚
Selected distances (A) and angles (u) for the ba–ba isomer of 2 (top, left):
Fe1–Fe2 2.5671(10), Fe–P(av.) 2.2275(6), Fe–CCO(av.) 1.761(9), Fe–S(av.)
2.264(1), Fe1–S1–Fe2 69.15(4), S1–Fe1–S2 85.32(5), sum of angles at N1
357.7(9); for the ap–ba isomer of 2 (top, right): Fe3–Fe4 2.5280(14), Fe–
P(av.) 2.223(1), Fe–CCO(av.) 1.763(5), Fe–S(av.) 2.275(6), Fe3–S3–Fe4
67.66(5), S3–Fe3–S4 85.04(5), sum of angles at N3 358.2(1); for
+
…
[2(FeHFe)] : Fe1 Fe2 2.5879(8), Fe–H(av.) 1.75, Fe–P(av.) 2.2489(6),
Fe–CCO(av.) 1.789(7), Fe–S(av.) 2.2713(4), Fe1–Fe2–H 41.5(16), Fe2–
Fe1–H 42.9(15), Fe1–S1–Fe2 69.38(4), S1–Fe1–S2 84.46(4), sum of angles
at N1 360.0(2).
306 | Chem. Commun., 2006, 305–307
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3
c = 82.83(3)u, V = 2616.4(9) A ; rcalcd = 1.534; m = 1.425 mm21
;
[2(FeHFe)]+ possesses the sole transoid ba–ba geometry (Fig. 3
(bottom)), which is identical with the coordination orientation of
other reported m-hydride diiron analogues.4,8,9 The crystallo-
graphic evidence indicates that a rotation of the Fe(CO)2PMe3
unit in the ap–ba isomer of 2 occurs during the protonation
process of the iron atoms. In solution, the coordination
configuration of 2 might be mobile structural forms.1a,5 This kind
of ligand-rotation phenomenon has been observed in the
protonation processes of diiron and diruthenium propanedithio-
lates.4,8,14 The general characteristics of the molecular structures,
e.g. the butterfly framework of the 2Fe2S center, the pseudo-
pyramidal coordination geometry of each iron atom in 2 and the
distorted octahedral coordination sphere of the iron atoms in
[2(FeHFe)]+, are in agreement with previously reported 2Fe2S
˚
T = 293(2) K; Z = 4; R1 = 0.0485 and wR2 = 0.1174 for 11823 reflections
with I > 2s(I). CCDC 246474. Crystal data for [2(FeHFe)]+[PF6]2:
C18H27F6Fe2N2O6P3S2, M = 750.15; monoclinic; P2(1)/n; a = 11.3070(6),
˚
b = 14.1755(5), c = 18.9123(9) A, a = 90.00, b = 104.846(3), c = 90.00u,
V = 2930.1(2) A ; rcalcd = 1.700; m = 1.371 mm21; T = 293(2) K; Z = 4;
3
˚
R1 = 0.0531 and wR2 = 0.1382 for 7212 reflections with I > 2s(I). CCDC
279852. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b513270c
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˚
shorter than that in its ba–ba counterpart because of the smaller
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tinguishable in the two configurational isomers of 2, while the
˚
mean Fe–S bond in the ap–ba isomer is 0.011(5) A longer than
…
˚
that in the ba–ba geometry. The Fe Fe distance (2.5879(8) A) of
[2(FeHFe)]+ shows
slight increase as compared to the
a
˚
corresponding ba–ba isomer (2.5671(10) A) of 2, in which there
is an Fe–Fe bond. The mean Fe–CCO and Fe–P coordination
bonds of the m-hydride complex are also lengthened by 0.027(8)
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In this work, two protonated species of the diiron azadithiolate
2 were obtained and characterized. One is a m-hydride diiron
complex [2(FeHFe)]+ and the other is assumed to be a m-S-
protonated cation [2(SH)]+, which is readily deprotonated in the
presence of pyridine. The S atom is preferred over the N atom in
the protonation process of 2 due to the weak basicity of the
bridging-N, resulting from the strong electron-withdrawing NO2
group on the para position of the N,N-dialkylaniline. The results of
this work provide indirect experimental evidence for the protona-
tion capability of the m-S atoms in the diiron subsite of Fe-only
hydrogenases. Further studies on protonation of diiron azadithio-
late model complexes are under way.
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We are grateful to the Natural Science Foundation of China
(Grants 20471013 and 20128005), the Swedish Energy Agency and
the Swedish Research Council for financial support of this work.
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Notes and references
¯
{ Crystal data for 2: C18H26Fe2N2O6P2S2, M = 604.17; monoclinic; P1;
˚
a = 13.248(3), b = 14.676(3), c = 15.028(3) A, a = 64.96(3), b = 82.40(3),
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 305–307 | 307