Biomimetic model featuring the NH proton and bridging hydride related to a proposed intermediate in enzymatic H2 production by fe-only hydrogenase

Article abstract of DOI:10.1021/ic900564q

Iron azadithiolate phosphine-substituted complex and its protonated species featuring the NH proton and/or bridging Fe hydride, [Fe₂(μ- S(CH₂)₂NⁿPr(H)_m(CH₂) ₂S)(M-H)_n(

Full text of DOI:10.1021/ic900564q

7604 Inorg. Chem. 2009, 48, 7604–7612
DOI: 10.1021/ic900564q
Biomimetic Model Featuring the NH Proton and Bridging Hydride Related to a
Proposed Intermediate in Enzymatic H2 Production by Fe-Only Hydrogenase
Ming-Hsi Chiang,*,† Yu-Chiao Liu,† Shu-Ting Yang,† and Gene-Hsiang Lee‡
†Institute of Chemistry, Academia Sinica, Nankang, Taipei 115, Taiwan, and ‡Instrumentation Center, National
Taiwan University, Taipei 106, Taiwan
Received January 21, 2009
Iron azadithiolate phosphine-substituted complex and its protonated species featuring the NH proton and/or bridging
(2m+2n)+
Fe hydride, [Fe2(μ-S(CH2)2NnPr(H)m(CH2)2S)(μ-H)n(CO)4(PMe3)2]2
(1, m = n = 0; [1-2HN]2+, m = 1,
n = 0; [1-2HN2HFe]4+, m = n = 1), are prepared to mimic the active site of Fe-only hydrogenase. X-ray crystallo-
graphic analyses of these three complexes reveal that two diiron subunits are linked by two azadiethylenethiolate
bridges to construct a dimer-of-dimer structure. 31P NMR spectroscopy confirms two trimethylphosphine ligands within
the diiron moiety are arranged in the apical/basal configuration, which is consistent with the solid-state structural
characterization. Deprotonation of the NH proton in [1-2HN]2+ and [1-2HN2HFe]4+ occurs in the presence of
triethanolamine (TEOA), which generates 1 and [1-2HFe]2+, respectively. Deprotonation of the Fe hydride is
accomplished by addition of bistriphenylphosphineimminium chloride ([PPN]Cl). It is observed that the Fe hy-
dride species, [1-2HFe]2+, is a kinetic product which converts to its thermodynamically stable tautomer, [1-2HN]2+, in
solution, as evidenced by IR and NMR spectroscopy. The pKa values of the aza nitrogen and the diiron sites
are estimated to be 8.9-15.9 and <8.9, respectively. [1-2HN2HFe]4+ has been observed to evolve H2 electro-
catalytically at a mild potential (-1.42 V vs Fc/Fc+) in CH3CN solution. Catalysis of [1-2HN2HFe]4+ is found to be as
efficient as that of the related diiron azadithiolate complexes. In the absence of a proton source, [1-2HN2HFe]4+
undergoes four irreversible reduction processes at -1.26, -1.42, -1.82, and -2.43 V, which are attributed to the
reduction events from [1-2HN2HFe]4+, [1-2HFe]2+, [1-2HN]2+, and 1, respectively, according to bulk electrolysis and
voltammetry in combination of titration experiments with acids.
Introduction
via a cysteinyl thiolate on the peptide chain.5 The primary
coordination sphere of the diiron unit in addition to the Scys
bridge is occupied by cyanide and carbon monoxide ligands
as well as an abiological dithiolate bridge (-SCH2XCH2S-).
The putative central atom of the dithiolate linker is presumed
to be NH6 albeit the possibility of CH2 or O can not be
excluded.2,7 The presence of the NH bridgehead is important
because of its potential function to act as a base to either
convey protons from the proteinsurface to the catalytic metal
site in the process of enzymatic H2 evolution or polarize
hydrogen molecules in the heterolytic cleavage process of H2
uptake.
Recent efforts have been paid to the structural and func-
tional design of biomimetic models of Fe-only hydrogenases
in the hope of finding a substitute for expensive platinum
working electrodes used for industrial hydrogen production.1
Fe-only hydrogenases isolated from Desulfovibrio desulfur-
icans2 and Clostridium pasterianum3 perform both meta-
bolism and production of H2 (2H+ + 2e- T H2) in mild
reduction potentials (-100 to -500 mV) and in high effi-
ciency (6ꢀ103 s-1).4 The active site, the H-cluster, is com-
posed of a [2Fe2S] unit linked to a [4Fe4S] ferredoxin cluster
The proposed mechanism of enzymatic H2 production
involves two protonated species: the N-protonated species
and the N-protonated, Fe-hydride species where the hydride
is presumably in a terminal manner, shown in Scheme 1.8
*To whom correspondence should be addressed. E-mail: mhchiang@
chem.sinica.edu.tw.
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r
pubs.acs.org/IC
Published on Web 07/14/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 16, 2009 7605
Protonation of the iron propanedithiolate complexes and
their derivatives to form the Fe(μ-H)Fe products has been
well studied.9-15 Recently the transient terminal hydride
intermediates have been spectroscopically characterized.16
On the other hand, isolation of the protonated species of the
azadithiolate analogues encounters difficulties. The presence
of the protonatedderivatives inthe reaction solution has been
evidenced by NMR and FTIR spectroscopy.17-19 The first
structural report available is on [{(μ-SCH2)2N(H)-
(CH2C6H4-o-Br)}Fe2(CO)6]+ by Sun et al. In this molecule
the NH proton is stabilized because of the intramolecular
interaction initiated by Br of the o-bromobenzyl group.20
One can speculate that hydrogen bonding within the mole-
cule might prohibit proton relay to the catalytic Fe center
from theaza nitrogen site viathe agostic interaction. The only
example close to the NH-FeH intermediate is the pdt deri-
vative featuring the bridging hydride and N-protonated
diphosphine chelate, which is recently published by the same
group.12 Unfortunately, no structural evidence of the iron
thiolate phosphine derivatives possessing the azadithiolate
NH proton and bridging iron hydride has been reported yet.
To elucidate the importance of the aza nitrogen site and
its role in the mechanism of enzymatic H2 production,
we designed a series of iron complexes bearing azadiethyle-
nethiolate ligands. Herein, we report the isolation and
characterization of three unprecedented iron azadithiolate
complexes, [Fe2(μ-S(CH2)2N(H)nPr(CH2)2S)(CO)4(PMe3)2]2-
(BF4)2, [1-2HN]2+, featuring non-supported NH protons,
Scheme 1. Possible Intermediates in the Mechanism of Enzymatic H2
Production by Fe-Only Hydrogenases
[Fe2(μ-H)(μ-S(CH2)2N(H)nPr(CH2)2S)(CO)4(PMe3)2]2(BF4)4,
[1-2HN2HFe]4+, featuring both non-supported NH protons
and bridging iron hydrides, and their parent molecule,
[Fe2(μ-S(CH2)2NnPr(CH2)2S)(CO)4(PMe3)2]2, 1. Electroca-
talytic study of the NH-FeH complex shows it catalyzes
reduction of protons at a mild potential in the presence of an
acid.
Results and Discussion
Synthesis and Characterization of [Fe2(μ-S(CH2)2NnPr-
(CH2)2S)(CO)6]2, 1, [1-2HN]2+, and [1-2HN2HFe]4+
.
Reaction of nPrN(CH2CH2SH)2 with triiron dodecacar-
bonyl in tetrahydrofuran (THF) at refluxing temperature
afforded a brown red solution that contained Fe(CO)5
and an orange-red product with the general formula
Fe2(μ-SR)2(CO)6 on the basis of carbonyl vibrational
frequency.21 Fe(CO)5 was removed under reduced pres-
sure, and the resultant orange-red semi-oil was subject to
column chromatography. [Fe2(μ-S(CH2)2NnPr(CH2)2S)-
(CO)6]2, which possesses three IR bands at 2069 (m), 2037
(vs), and 1990 (s) cm-1, was obtained as a red solid.
Broadness of the lowest-energy band reveals a combina-
tion of more than one peak close in energy, which is well
resolved to two bands at 1999 (s) and 1988 (s) cm-1 in a
less polar solvent such as hexane.
(9) Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Mejia-Rodriguez, R.;
Chiang, C.-Y.; Darensbourg, M. Y. Inorg. Chem. 2002, 41, 3917–3928.
(10) Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Yarbrough, J. C.;
Darensbourg, M. Y. J. Am. Chem. Soc. 2001, 123, 9710–9711.
(11) (a) Gloaguen, F.; Lawrence, J. D.; Rauchfuss, T. B.; Benard, M.;
Rohmer, M.-M. Inorg. Chem. 2002, 41, 6573–6582. (b) Morvan, D.; Capon, J.-
F.; Gloaguen, F.; Le Goff, A.; Marchivie, M.; Michaud, F.; Schollhammer, P.;
Talarmin, J.; Yaouanc, J.-J.; Pichon, R.; Kervarec, N. Organometallics 2007, 26,
2042–2052. (c) Dong, W.; Wang, M.; Liu, X.; Jin, K.; Li, G.; Wang, F.; Sun, L.
::
Reaction of [Fe2(μ-S(CH2)2NnPr(CH2)2S)(CO)6]2 with
PMe3 readily affords its trimethylphosphine derivative at
room temperature, [Fe2(μ-S(CH2)2NnPr(CH2)2S)(CO)4-
(PMe3)2]2, 1. Four CO bands are identified for 1 at 1975,
1939, 1905, and 1887 cm-1, which is a typical IR char-
acteristic of the disubstituted Fe2(μ-dithiolate)(CO)4L2
complexes. The phosphine ligands are fluxional in solu-
tion as evidenced by 31P NMR spectroscopy. One broad
peak at 20.3 ppm is observed. This fluxional behavior is
suppressed at 218 K in which two sharp bands at 13.3 and
32.6 ppm are recorded. Appearance of two 31P bands
indicates that these phosphine groups reside at the apical
and basal positions in two different iron centers at low
temperature, which is consistent with X-ray crystallo-
graphic analysis. The molecular structure of 1, which is
displayed in Figure 1, reveals that two identical diiron
units are linked by two azadithiolate bridges to construct
a dimer-of-dimer structure.
Chem. Commun. 2006, 305–307. (d) Thomas, C. M.; Rudiger, O.; Liu, T.;
Carson, C. E.; Hall, M. B.; Darensbourg, M. Y. Organometallics 2007, 26, 3976–
3984. (e) Dong, W.; Wang, M.; Liu, T.; Liu, X.; Jin, K.; Sun, L. J. Inorg. Biochem.
2007, 101, 506–513. (f) Zhao, X.; Chiang, C.-Y.; Miller, M. L.; Rampersad, M. V.;
Darensbourg, M. Y. J. Am. Chem. Soc. 2003, 125, 518–524. (g) Xu, F.; Tard, C.;
Wang, X.; Ibrahim, S. K.; Hughes, D. L.; Zhong, W.; Zeng, X.; Luo, Q.; Liu, X.;
Pickett, C. J. Chem. Commun. 2008, 606–608. (h) Heiden, Z. M.; Zampella, G.;
De Gioia, L.; Rauchfuss, T. B. Angew. Chem., Int. Ed. 2008, 47, 9756–9759.
(i) Mejia-Rodriguez, R.; Chong, D.; Reibenspies, J. H.; Soriaga, M. P.; Dar-
ensbourg, M. Y. J. Am. Chem. Soc. 2004, 126, 12004–12014. (j) Gloaguen, F.;
Lawrence, J. D.; Rauchfuss, T. B. J. Am. Chem. Soc. 2001, 123, 9476–9477.
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Commun. 2008, 5800–5802.
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(13) Ezzaher, S.; Capon, J.-F.; Gloaguen, F.; Petillon, F. Y.; Schollham-
mer, P.; Talarmin, J.; Kervarec, N. Inorg. Chem. 2009, 48, 2–4.
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(14) Orain, P.-Y.; Capon, J.-F.; Kervarec, N.; Gloaguen, F.; Petillon, F.;
Pichon, R.; Schollhammer, P.; Talarmin, J. Dalton Trans. 2007, 3754–3756.
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2009, 1919–1926.
(16) (a) Barton, B. E.; Rauchfuss, T. B. Inorg. Chem. 2008, 47, 2261–2263.
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(b) Ezzaher, S.; Capon, J.-F.; Gloaguen, F.; Petillon, F. Y.; Schollhammer, P.;
Talarmin, J.; Pichon, R.; Kervarec, N. Inorg. Chem. 2007, 46, 3426–3428. (c) van
der Vlugt, J. I.; Rauchfuss, T. B.; Whaley, C. M.; Wilson, S. R. J. Am. Chem. Soc.
2005, 127, 16012–16013. (d) Barton, B. E.; Olsen, M. T.; Rauchfuss, T. B. J. Am.
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S.; Lomoth, R. Chem.;Eur. J. 2007, 13, 7075–7084.
Formation of [1-2HN]2+ from reaction of 1 with 2
equiv of HBF4 is confirmed by the IR spectra in CH3CN.
The IR spectral pattern, displayed in Figure 2, remains
unchanged but ν(CO) shows a shift of 10 cm-1 toward
higher energy from 1 to [1-2HN]2+, which is character-
istic of the presence of the protonated aza nitrogen.18,20
The NH proton is characterized as a broadband at
(18) Schwartz, L.; Eilers, G.; Eriksson, L.; Gogoll, A.; Lomoth, R.; Ott, S.
Chem. Commun. 2006, 520–522.
˚
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Trans. 2007, 896–902. (b) Wang, F.; Wang, M.; Liu, X.; Jin, K.; Dong, W.; Sun, L.
Dalton Trans. 2007, 3812–3819.
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(20) Wang, F.; Wang, M.; Liu, X.; Jin, K.; Dong, W.; Li, G.; Akermark,
B.; Sun, L. Chem. Commun. 2005, 3221–3223.
(21) King, R. B. J. Am. Chem. Soc. 1962, 84, 2460.

7606 Inorganic Chemistry, Vol. 48, No. 16, 2009
Chiang et al.
Figure 1. Molecular structure of [Fe2(μ-S(CH2)2NnPr(CH2)2S)(CO)4-
(PMe3)2]2, 1, thermal ellipsoids drawn at 50% probability level. All
hydrogen atoms and methyl groups of PMe3 are omitted for clarity.
Figure 3. 31P NMR spectra of (a) [1-2HN]2+ in CD2Cl2, (b) [1-
2HN2HFe 4+
]
in CD2Cl2, and (c) High-field 1H NMR spectrum of [1-
in CD2Cl2.
2HN2HFe 4+
]
2
hydride resonance of [1-2HN2HFe]4+, featuring a JHP
constant of 21.5 Hz, indicates that two PMe3 groups in
each Fe2 subunit, which have the 31P NMR resonances
located at 19.58 and 23.43 ppm, are arranged in the
apical/basal configuration. The 31P NMR assignments
for both [1-2HN]2+ and [1-2HN2HFe]4+ are consistent
with their solid-state structures analyzed by single-crystal
X-ray crystallography. Figure 4 shows that the PMe3
ligands coordinate to apical and basal positions per iron
of a dimer, and the Fe Fe distance of each dimer is 2.55
3 3 3
and 2.58 A for [1-2HN]2+ and [1-2HN2HFe]4+, respec-
˚
tively. The Fe Fe distance of [1-2HN2HFe]4+ is com-
parable with the metric parameters previously reported
3 3 3
in the Fe hydride complexes, [Fe2(μ-H)(μ-SS)(CO)4-
,10
(P)2]+ (SS = dithiolate linkers; P = phosphines).9
Deprotonation and Tautomerization. Triethanolamine
(TEOA, pKaTEOA = 15.9 in CH3CN)22 is an effective
agent to remove protons of the quaternary ammonium
cations.15,18 It was found that 1 was quantitatively recov-
ered from reaction of [1-2HN]2+ with TEOA. The same
deprotonation process can be observed for [1-
2HN2HFe]4+ in the presence of 2 equiv TEOA. A new
IR spectrum with the similar peak pattern with that of [1-
2HN2HFe]4+ emerged in the lower energy region. This
approximate 10 cm-1 shift is comparable with the results
observed for the adt diiron analogues with respect to their
adt diiron NH species.17,18 The new species is assigned
to [1-2HFe]2+. It is discovered that [1-2HFe]2+ is not a
Figure 2. FTIR spectra of 1 in CH2Cl2 (in short dash), [1-2HN]2+ in
CH3CN (in red), and [1-2HN2HFe 4+
in CH3CN (in blue).
]
7.42 ppm at room temperature. Two proton signals for
the trimethylphosphine ligands are observed at 1.50 and
1.64 ppm with the doublet pattern because of their
coupling to the phosphorus atom. Two 31P NMR reso-
nances are recorded at 10.71 and 29.43 ppm, shown in
Figure 3, and are assigned to two PMe3 groups located at
the apical and basal positions, respectively.9
[1-2HN2HFe]4+ is generated from the CH3CN solu-
tion of [1-2HN]2+ in the presence of excess HBF4. The
formation of the Fe(μ-H)Fe species because of insertion
of a proton into the Fe-Fe vector is evidenced by shifts of
the IR peaks to higher energy by about 90 cm-1. In
contrast to a minor difference corresponding to proton-
ation of the aza nitrogen, which causes insignificant
influence on the electron density about the Fe centers,
such dramatic change on the IR energy, shown in
Figure 2, reflects that the oxidation state of both Fe
centers has altered from the original +1 state to +2 upon
formation of the bridging iron hydride unit.9,10,14 The
bridging hydride is identified at -15.46 ppm in 1H NMR
spectroscopy, displayed in Figure 3. 2JHP values for the cis
position of the hydride and phosphine are in the range
of 19-24 Hz. Small J values of 3-5 Hz are characterized
for the trans orientation.9 The doublet pattern of the
stable species which converts to its tautomer, [1-2HN]2+
,
in solution over time, as indicated by the infrared spectra
(Figure 5). This observation is in contrast to the results of
the adt diiron complexes in which the FeH species is
presumably more stable than its NH counterpart.17
Rauchfuss et al. has discovered that [Fe2((μ-
SCHMe)2NH2)(CO)4(PMe3)2]+ slowly tautomerizes to
its bridging hydride isomer, [Fe2((μ-SCHMe)2NH)(μ-H)-
(CO)4(PMe3)2]+, in CH3CN solution.23 Proton transfer
(22) Izutsu, K. Acid-Base Dissociation Constants in Dipolar Aprotic
Solvents; Blackwell: Oxford, U.K., 1990.
(23) Stanley, J. L.; Heiden, Z. M.; Rauchfuss, T. B.; Wilson, S. R.; De
Gioia, L.; Zampella, G. Organometallics 2008, 27, 119–125.

Article
Inorganic Chemistry, Vol. 48, No. 16, 2009 7607
Figure 5. FTIR spectral changes for [1-2HN2HFe 4+
tion by TEOA in CH3CN.
]
upon deprotona-
processes of 1 and its protonated species is depicted in
Scheme 2.
Electrochemistry of [1-2HN2HFe]4+ in the Presence of
HBF4. Electrocatalytic activity of [1-2HN2HFe]4+ was
monitored in CH3CN solution under N2 with stepwise
addition of HBF4, shown in Figure 6. In the absence of a
proton source, [1-2HN2HFe]4+ undergoes four irrever-
sible reduction processes at -1.26, -1.42, -1.82, and
-2.43 V (vs Fc/Fc+). The redox wave at the potential of
-1.26 V is attributed to reduction of [1-2HN2HFe]4+
from FeIIFeII to FeIFeII. According to bulk electrolysis
at -1.34 V (vide infra), the subsequent reduction event at
-1.42 V is assigned to the reduction pair (FeIIFeII/
Figure 4. Molecular structures of (a) [Fe2(μ-S(CH2)2NnPr(H)(CH2)2S)-
(CO)4(PMe3)2]22+, [1-2HN]2+, thermal ellipsoids drawn at 50% prob-
FeIFeII) of [1-2HFe]2+ 12,17
The second reduction peak
.
ability
level.
(b)
[Fe2(μ-H)(μ-S(CH2)2NnPr(H)(CH2)2S)(CO)4-
shows a tremendous increase of its current intensity, but
the remainder of the cyclic voltammograms does not
appear to show any significant influence when the acid
is added into the electrochemical cell. Such enhancement
of the peak current for the reduction process at -1.42 V,
which shows the linear relationship with sequential incre-
ments of acid added, is consistent with electrocatalytic H2
formation at this potential.25
A new reduction wave at -1.73 V is observed only when
HBF4 is present. Its current intensity also increases linearly
with the acid concentration. When its peak current was
plotted against equivalents of acid added, a linear depen-
dence (slope = 7.7 ꢀ 10-5) was obtained. This steeperslope
compared with that (slope = 6.1ꢀ10-5) for the reduction
wave at -1.42 V, shown in Figure 7, and the appearance of
the second catalytic event specific in the presence of more
acids indicate that protonation of the reduced species in
acidic solution occurs, and the rate of this second electro-
catalytic process of proton reduction is faster than the
initial one. Analysis of gas evolved from both electroche-
mical processes by gaschromatography confirms hydrogen
to be the sole content in the gaseous product.
(PMe3)2]24+, [1-2HN2HFe 4+
]
, thermal ellipsoids drawn at 20% prob-
ability level. All hydrogen except NH hydrogen and bridging hydride
atoms are omitted for clarity. All methyl groups of PMe3 are also omitted
for clarity.
from the pendant nitrogen site in [Fe2(μ-pdt)-
(κ2-(Ph2PCH2)2N(H)CH3)(CO)4]+ to the Fe center
was recently observed by Talarmin et al.13 To confirm
[1-2HN]2+ being the thermodynamically stable pro-
duct, prolonged stirring was applied to the solution
containing [1-2HN]2+. No spectral change for forma-
tion of [1-2HFe]2+ was witnessed. When excess
TEOA was added to the solution of [1-2HN2HFe]4+
,
both of [1-2HFe]2+ and 1 were formed immediately and
eventually the latter is the final product present in the
solution. It is rationalized that 1 is formed owing to
instant deprotonation of [1-2HN]2+ which is generated
via the tautomerization process from [1-2HFe]2+
.
Deprotonation of the iron hydride is accomplished by
HCl
addition of Cl- (pKa
≈ 8.9 in CH3CN),22 as previously
reported.18,24 Addition of 2 equiv of [PPN]Cl promotes the
dehydride reaction of [1-2HN2HFe]4+ and [1-2HFe]2+ to
generate [1-2HN]2+ and 1, respectively. The pKa values
of the aza nitrogen (8.9<pKaN<15.9) and the diiron sites
(pKaFeFe < 8.9) are then estimated from studies of pro-
tonation and deprotonation. The reaction scheme invol-
ving the protonation, deprotonation, and tautomerization
ꢀ
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3982–3983. (b) Felton, G. A. N.; Vannucci, A. K.; Chen, J.; Lockett, L. T.;
Okumura, N.; Petro, B. J.; Zakai, U. I.; Evans, D. H.; Glass, R. S.; Lichtenberger,
D. L. J. Am. Chem. Soc. 2007, 129, 12521–12530. (c) Capon, J.-F.; Ezzaher, S.;
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7608 Inorganic Chemistry, Vol. 48, No. 16, 2009
Chiang et al.
Scheme 2. Reactions Scheme for Protonation of 1, Deprotonation of [1-2HN]2+, [1-2HFe 2+
]
, and [1-2HN2HFe 4+
]
, and Tautomerization of
[1-2H
]
Fe 2+ to [1-2HN]2+
Controlled-potential electrolysis of [1-2HN2HFe]4+
performed in CH3CN solution at -1.34 V in the absence
of acid was monitored by in situ infrared spectroscopy in
a custom-made airtight cell under N2. H2 evolution is
visible on the surface of the working electrode during
electrolysis, which causes erratic spikes in the current-
potential plot. The intensity of the initial IR peaks
from [1-2HN2HFe]4+ decreases, and the ν(CO) bands of
[1-2HFe]2+ emerge at the lower energy (2049, 2032, and
1993 cm-1). Figure 8 displays the IR changes from
[1-2HN2HFe]4+ to [1-2HFe]2+ in which the isosbestic
behavior is observed, indicative of the absence of
intermediates.26 [1-2HFe]2+ subsequently transforms
to [1-2HN]2+ in solution as evidenced by growth of its
The result is compared with an independent voltammo-
gram of [1-2HN]2+ recorded in CH3CN solution, con-
firming the observation of two irreversible processes at
the identical potentials. With increasing acid concentra-
tion, the redox behavior of the solution reveals the
similarity to that of [1-2HN2HFe]4+ in the presence of
acid. The FTIR spectra indicate that [1-2HN]2+ is
generated initially and then its amount decreases with
increase of [1-2HN2HFe]4+ which eventually is the sole
product in the acidic solution.
Our results point to two electrocatalytic routes to
.
H2 production from the complex [1-2HN2HFe]4+
Current intensity of process I reaches a plateau when
acid concentration is about 4 equiv. This suggests that
reduction of the catalyst at the electrode has the same
rate as its oxidation by the acid under such condition.27
Catalytic activity of [1-2HN2HFe]4+ can be compared
with that of the related adt diiron complexes if its
proton reduction mechanism involved with process I
is presumably similar with that supported by the adt
diphosphine species since a close catalytic environment
including an aza nitrogen and two phosphine sites is
present within each diiron unit. The peak current
of process I is found to level off at about 340 μA, which
refers to 170 μA for each Fe2 subset in 1 mM of
IR signature bands at 1982, 1945, 1914, and 1892 cm-1
.
Assignment of the two reduction waves at the most
negative potential is made possible by titration experi-
ments of 1 with HOTf and HBF4, respectively, monitored
by FTIR spectroscopy and cyclic voltammetry. Figure 9
shows results from the HOTf reaction. A new irreversible
peak appearing at -1.82 V in addition to the initial
reduction wave at -2.43 V corresponding to the 12-/1
pair is confirmed by IR to be the reduction of the
protonated species, [1-2HN]2+, when 2 equiv of HOTf
is added to the CH3CN solution of 1. This 600 mV shift
toward more positive potential is consistent with the
introduction of protons to the aza nitrogen sites of 1.
(27) (a) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Funda-
mentals and Applications; John Wiley & Sons: New York, 1980. (b) Andrieux,
0
ꢀ
(26) Olsen, M. T.; Bruschi, M.; De Gioia, L.; Rauchfuss, T. B.; Wilson, S.
C. P.; Blocman, C.; Dumas-Bouchiat, J. M.; M Halla, F.; Saveant, J. M.
R. J. Am. Chem. Soc. 2008, 130, 12021–12030.
J. Electroanal. Chem. 1980, 113, 19–40.

Article
Inorganic Chemistry, Vol. 48, No. 16, 2009 7609
Figure 6. Cyclic voltammograms of [1-2HN2HFe 4+
(1 mM) with
]
HBF4 in CH3CN (ν = 100 mV s-1, potentials are versus Fc/Fc+).
Figure 9. Cyclic voltammograms of titration of 1 (1 mM in CH3CN) by
HOTf and their corresponding FTIR spectra.
Figure 7. Dependence of current (icred) vs equivalents of HBF4 added
The catalytic rate of both model complexes is thus esti-
mated to be the same. In other words, structural discre-
pancy of [1-2HN2HFe]4+ from [Fe2(μ-adt)(CO)4(P)2] (P =
phosphine ligand) does not affect its ability to electrocata-
lyze the formation of H2.
for [1-2HN2HFe 4+
(at RT, 1 mM in CH3CN), where ic is peak
]
red
catalytic current.
Conclusions
This report presents the characterization of the biomimetic
iron thiolate complexes of the Fe-only hydrogenase active
site, [1-2HN2HFe]4+, that possesses both the NH proton and
bridging Fe-hydride, and its precursor [1-2HN]2+ that
possesses the NH proton. The protons coordinate to the
aza nitrogen sites of 1 generate [1-2HN]2+ which further
leads to [1-2HN2HFe]4+ with hydrides embedded within the
Fe-Fe vectors while more protons are available. Such
introduction of protons to the aza nitrogen site and the
Fe-Fe bond, respectively, moves the reduction potential
for about 1.2 V toward a more positive region. This result
suggests that the ability to accept protons for the aza nitrogen
and the Fe sites is essential for the enzymatic H2 production
at the mild potential.
Figure 8. Spectral changes monitored by in-situ IR spectroscopy over
time for [1-2HN2HFe 4+
upon electrolysis at -1.35 V in CH3CN.
]
Site-specific deprotonation of [1-2HN2HFe]4+ is facili-
tated by usage of different reagents. TEOA removes the
NH protons of [1-2HN2HFe]4+ to generate the kinetic
product, [1-2HFe]2+, which converts to its thermodynami-
cally stable tautomer, [1-2HN]2+. Deprotonation of the Fe
hydride proceeds in the presence of chloride. The pKa values
[1-2HN2HFe]4+. This value is comparable to 150 μA
observed for [Fe2(μ-SCH2N(iPr)CH2S)(CO)4(κ2-dppe)].28
ꢀ
(28) Ezzaher, S.; Orain, P.-Y.; Capon, J.-F.; Gloaguen, F.; Petillon, F. Y.;
Roisnel, T.; Schollhammer, P.; Talarmin, J. Chem. Commun. 2008, 2547–2549.

7610 Inorganic Chemistry, Vol. 48, No. 16, 2009
Chiang et al.
Table 1. X-ray Crystallographic Data of 1, [1-2HN]2+, and [1-2HN2HFe 4+
]
1
[1-2HN](BF4)2 2CH2Cl2
[1-2HN2HFe](BF4)4 4CH3CN
3
3
empirical formula
formula weight
T, K
C
34H68Fe4N2O8P4S4
C36H72B2Cl4F8Fe4N2O8P4S4
1451.90
150(2)
C42H82B4F16Fe4N6O8P4S4
1621.90
150(2)
1108.42
150(2)
crystal system
space group
triclinic
P1
orthorhombic
Pna21
26.2091(17)
11.9446(7)
20.3932(13)
90
90
90
6384.2(7)
4
1.511
1.355
2976
47855
14451
0.0495
1.154
0.0467 (0.0511)
0.1116 (0.1159)
triclinic
P1
˚
a, A
11.7164(9)
14.0018(15)
16.7238(17)
112.986(9)
90.204(7)
101.705(8)
2462.7(4)
2
1.495
1.499
1156
18111
18111
0.0000
1.060
0.0435 (0.0669)
0.1040 (0.1181)
12.0713(5)
12.7630(5)
14.1186(5)
107.990(2)
94.040(2)
113.709(2)
1846.32(12)
1
1.459
1.056
832
17715
6428
0.0326
1.018
0.1031 (0.1225)
0.2985 (0.3227)
˚
b, A
˚
c, A
R, deg
β, deg
γ, deg
3
˚
V, A
Z
F
calcd, Mg m-3
μ, mm-1
F (000)
reflections collected
independent reflections
Rint
Goodness-of-fit on F2
R1 [I > 2σ(I)] (all data)a
wR2 [I > 2σ(I)] (all data)b
P
P
P
P
a R1= ||Fo| - |Fc||/ |Fo|. b wR2 = [ w(Fo2 - Fc2)2/ w(Fo2)2]1/2
.
of the aza nitrogen (8.9 < pKaN < 15.9) and the diiron sites
(pKaFeFe < 8.9) are estimated from studies of protonation
and deprotonation.
Molecular Structure Determinations. The X-ray single crystal
crystallographic data collections for 1, [1-2HN]2+, and
[1-2HN2HFe]4+ were carried out at 150 K on a Bruker SMART
APEX CCD four-circle diffractometer with graphite-mono-
Four irreversible reduction events are recorded in the
absence of a proton source for [1-2HN2HFe]4+. According
to bulk electrolysis and voltammetry in combination of
titration experiments with acids, they are assigned to reduc-
tion of [1-2HN2HFe]4+, [1-2HFe]2+, [1-2HN]2+, and 1
from the ease of the working potential. Their presence in
solution tentatively suggests that these four species probably
are involved in the mechanism of electrocatalytic H2 forma-
tion. Further studies to probe their roles in the catalytic
mechanism are currently under investigation.
˚
chromated Mo KR radiation (λ = 0.71073 A) outfitted with a
low-temperature, nitrogen-stream aperture. The structures were
solved using direct methods, in conjunction with standard
difference Fourier techniques, and refined by full-matrix
least-squares procedures. A summary of the crystallographic
data for complexes 1, [1-2HN]2+, and [1-2HN2HFe]4+ is
shown in Table 1. Selected metric data of 1, [1-2HN]2+, and
[1-2HN2HFe]4+ are listed in Table 2. An empirical absorption
correction (multiscan) was applied to the diffraction data for all
structures. All non-hydrogen atoms were refined anisotropi-
cally, and all hydrogen atoms were placed in geometrically
calculated positions by the riding model. All software used for
diffraction data processing and crystal structure solution and
refinement are contained in the SHELXTL97 program suites.30
The crystals of 1 suitable for X-ray analysis are in twin form. The
crystallographic data are solved by twin refinement, and two
molecules are presents in the unit cell. For [1-2HN]2+, the
methyl groups from one of the PMe3 ligands, C(26), C(27), and
C(28), are disordered. For [1-2HN2HFe]4+, the fluorine atoms
from one of the BF4 anions, F(5), F(6), F(7), and F(8), exhibit
disorder.
Experimental Section
General Methods. All reactions were carried out by using
standard Schlenk and vacuum-line techniques under an atmo-
sphere of purified nitrogen. All commercial available chemicals
from Aldrich were of ACS grade and used without further
purification. Solvents were of HPLC grade and purified as
follows: Hexane, diethyl ether, and tetrahydrofuran (THF) were
distilled from sodium/benzophenone under N2. Dichloro-
methane was distilled from CaH2 under N2. Acetonitrile was
distilled first over CaH2 and then from P2O5 under N2. Deut-
Electrochemistry. Electrochemical measurements were re-
corded on a CH Instruments 630C electrochemical potentiostat
using a gastight three-electrode cell under N2 at room tempera-
ture. A glassy carbon electrode (3 mm in diameter) and a
platinum wire were used as working and auxiliary electrode,
respectively. Reference electrode was a non-aqueous Ag/Ag+
electrode (0.01 M AgNO3/0.1 M n-Bu4NPF6). All potentials are
measured in 0.1 M n-Bu4NPF6 solution in CH3CN and reported
against ferrocene/ferrocenium (Fc/Fc+). Increments of acids
were added by microsyringe.
Spectroelectrochemistry was performed by a Mettler Toledo
ReactIR iC10 in situ FTIR system equipped with a MCT
detector and a 0.62500 SiComp probe. Graphite rods (6.15 mm
in diameter) were used as working and auxiliary electrodes.
Reference electrode was a non-aqueous Ag/Ag+ electrode
˚
erated solvents obtained from Merck were distilled over 4 A
molecular sieves under N2 prior to use. nPrN(CH2CH2SH)2
were prepared according to related literature procedures.29
Infrared spectra were recorded on a Perkin-Elmer Spectrum
One using a 0.05 mm CaF2 cell. 1H, 13C{1H}, and 31P{1H} NMR
spectra were recorded on a Bruker AV-500 or DRX-500 spectro-
meter operating at 500, 125.7, and 202.49 MHz, respectively.
Mass spectral analyses were done on a Waters LCT Premier XE
at Mass Spectrometry Center in Institute of Chemistry, Acade-
mia Sinica. Elemental analyses were performed on an Elementar
vario EL III elemental analyzer. Analysis of H2 gas from bulk
electrolysis was performed on an Agilent 6890 gas chromato-
graph with a TCD detector and a Supelco Carboxen 1000
column. Argon was used as the carrier gas.
(29) (a) Karlin, K. D.; Lippard, S. J. J. Am. Chem. Soc. 1976, 98, 6951–
6957. (b) Harley-Mason, J. J. Chem. Soc. 1947, 320–322. (c) Snyder, H. R.;
Stewart, J. M.; Ziegler, J. B. J. Am. Chem. Soc. 1947, 69, 2672–2674.
(30) SHELXTL, ver. 6.10; Bruker Analytical X-Ray Systems: Madison, WI,
2000.

Article
Inorganic Chemistry, Vol. 48, No. 16, 2009 7611
Table 2. Selected Bond Lengths (A) and Angles (deg) for 1, [1-2HN]2+, and
[1-2HN2HFe 4+
before it was dried in vacuo. The solid was washed three times
with 10 mL portions of hexane. A red powder was obtained in
85% yield. Crystals of 1 suitable for X-ray crystallographic
analysis were grown from a CH2Cl2 solution layered with
hexane. IR (CH2Cl2, cm-1): νCO 1975 (s), 1939 (vs), 1905 (s),
1887 (m). 1H NMR (500 MHz, CD2Cl2): 0.83 (t, 6H, CH3), 1.39
(hex, 4H, CH3CH2CH2), 1.53 (d, 2JHP = 9 Hz, 36H, PMe3), 2.10
(br, 4H, SCH2), 2.33 (t, 8H, SCH2, CH3CH2CH2), 2.49 (t, 4H,
NCH2), 2.68 (t, 4H, NCH2) ppm. 31P NMR (202.49 MHz,
CD2Cl2): 20.34 (br) ppm. ESI-MS: m/z 1106.99 {1 + H+}+,
554.00 {1 + 2H+}2+. Anal. Calcd for C34H66Fe4N2O8P4S4: C,
36.91; H, 6.01; N, 2.53, S, 11.59. Found: C, 36.84; H, 5.97; N,
2.51, S, 11.67. CV (THF, V vs Fc/Fc+): -2.43 (Epc).
˚
]
1
Fe(1)-Fe(2)
Fe(1)-S(1)
Fe(1)-S(2)
Fe(2)-S(1)
Fe(2)-S(2)
Fe(1)-P(1)
Fe(2)-P(2)
Fe-CCO,ap
Fe-CCO,ba
2.5433(6)
2.2737(8)
2.2528(8)
2.2724(8)
2.2597(8)
2.2000(9)
2.2110(9)
1.763(3)
1.757b
S(1)-Fe(1)-S(2)
S(1)-Fe(1)-Fe(2)
S(2)-Fe(1)-Fe(2)
S(1)-Fe(2)-S(2)
S(1)-Fe(2)-Fe(1)
S(2)-Fe(2)-Fe(1)
Fe(2)-S(1)-Fe(1)
Fe(2)-S(2)-Fe(1)
81.18(3)
55.96(2)
55.82(2)
81.06(3)
56.01(2)
55.57(2)
68.03(3)
68.61(3)
Synthesis of [Fe2(μ-S(CH2)2NnPr(H)(CH2)2S)(CO)4(PMe3)2]2-
(BF4)2, [1-2HN](BF4)2. To a solution of 1 (300 mg, 0.27 mmol)
in CH2Cl2 (10 mL) was added HBF4 (68.4 μL, 0.54 mmol,
50 wt % in H2O) via an airtight microsyringe. The reaction
mixture was stirred at room temperature for 30 min and
monitored by FTIR spectroscopy. After the reaction was fini-
shed, it was dried in vacuo. The solid residue was washed twice
with 10 mL portions of ether. A red powder was obtained in
[1-2HN]2+
S(1)-Fe(1)-S(3)
Fe(1)-Fe(2)
Fe(3)-Fe(4)
Fe(1)-S(1)
Fe(1)-S(3)
Fe(1)-P(1)
Fe(2)-S(1)
Fe(2)-S(3)
Fe(2)-P(2)
Fe(3)-S(2)
Fe(3)-S(4)
Fe(3)-P(3)
Fe(4)-S(2)
Fe(4)-S(4)
Fe(4)-P(4)
Fe-CCO,ap
Fe-CCO,ba
2.5641(8)
2.5348(8)
2.2517(11)
2.2485(12)
2.2277(13)
2.2567(11)
2.2496(11)
2.2127(13)
2.2619(11)
2.2707(11)
2.2088(12)
2.2836(11)
2.2763(12)
2.2208(13)
1.768a
80.97(4)
55.43(3)
55.27(3)
80.84(4)
55.24(3)
55.23(3)
81.21(4)
56.22(3)
56.51(3)
80.63(4)
56.01(3)
55.70(3)
69.33(3)
67.79(3)
69.51(4)
67.76(4)
S(1)-Fe(1)-Fe(2)
S(3)-Fe(1)-Fe(2)
S(1)-Fe(2)-S(3)
S(1)-Fe(2)-Fe(1)
S(3)-Fe(2)-Fe(1)
S(4)-Fe(3)-S(2)
S(4)-Fe(3)-Fe(4)
S(2)-Fe(3)-Fe(4)
S(4)-Fe(4)-S(2)
S(4)-Fe(4)-Fe(3)
S(2)-Fe(4)-Fe(3)
Fe(1)-S(1)-Fe(2)
Fe(4)-S(2)-Fe(3)
Fe(2)-S(3)-Fe(1)
Fe(3)-S(4)-Fe(4)
almost quantitative yield. Crystals of [1-2HN](BF4)2 2CH2Cl2
3
suitable for X-ray crystallographic analysis were grown from a
CH2Cl2 solution layered with ether. IR (CH2Cl2, cm-1): νCO
1
1987 (s), 1948 (vs), 1920 (s), 1891 (m). H NMR (500 MHz,
CD2Cl2): 1.03 (t, 6H, CH3), 1.50 (d, 2JHP = 9.5 Hz, 18H, PMe3),
1.64 (d, 2JHP = 9 Hz, 18H, PMe3), 1.80 (m, 4H, CH3CH2CH2),
2.11-2.64 (m, 8H, SCH2), 2.94-3.78 (m, 12H, NCH2), 7.42
(br, 2H, NH) ppm. 31P NMR (202.49 MHz, CD2Cl2): 10.71 (d,
JPP = 9.9 Hz), 29.43 (d, JPP = 9.5 Hz) ppm. ESI-MS:
m/z 1195.04 {[1-2HN](BF4)}+, 1107.03 {[1-2HN] - H+}+,
554.01 {[1-2HN]}2+. Anal. Calcd for C36H72B2Cl4F8Fe4N2O8P4S4:
C, 31.85; H, 5.35; N, 2.19. Found: C, 31.33; H, 5.46; N, 2.18. CV
(CH3CN, V vs Fc/Fc+): -1.82 and -2.43 (Epc), +0.01 (Epa).
Synthesis of [Fe2(μ-H)(μ-S(CH2)2NnPr(H)(CH2)2S)(CO)4-
(PMe3)2]2(BF4)4, [1-2HN2HFe](BF4)4. Method A. To a solu-
tion of 1 (300 mg, 0.27 mmol) in CH3CN (7 mL) was added
HBF4 (854.4 μL, 6.75 mmol, 50 wt % in H2O) via an airtight
microsyringe. The reaction mixture was stirred at room tem-
perature for at least 1.5 h until FTIR indicated no more of the
starting material was left. After the reaction was finished, it was
dried in vacuo. The semi-solid residue was washed twice with
15 mL portions of ether first and then CH3CN/ether solution
until the powder was obtained. The yield was 85%.
1.762b
[1-2HN2HFe 4+
]
Fe(1)-Fe(2)
Fe(1)-S(1)
Fe(1)-S(2)
Fe(1)-P(1)
Fe(2)-S(1)
Fe(2)-S(2)
Fe(2)-P(2)
Fe-CCO,ap
Fe-CCO,ba
2.5770(15)
2.281(2)
2.273(2)
2.236(2)
2.282(2)
2.268(3)
2.248(3)
1.786(9)
1.788b
S(2)-Fe(1)-S(1)
S(2)-Fe(1)-Fe(2)
S(1)-Fe(1)-Fe(2)
S(2)-Fe(2)-S(1)
S(2)-Fe(2)-Fe(1)
S(1)-Fe(2)-Fe(1)
Fe(2)-S(1)-Fe(1)
Fe(2)-S(2)-Fe(1)
81.26(8)
55.32(7)
55.64(6)
81.35(8)
55.52(6)
55.59(6)
68.77(7)
69.16(7)
a Averaged values of d(Fe-COap). b Averaged values of d(Fe-COba).
(0.01 M AgNO3/0.1 M n-Bu4NPF6), which was placed in a
separated compartment with a fine porosity glass frit. The
solution was stirred under N2 throughout bulk electrolysis.
Synthesis of [Fe2(μ-S(CH2)2NnPr(CH2)2S)(CO)6]2. A solu-
tion of Fe3(CO)12 (1.0 g, 1.98 mmol) in THF (20 mL) was treated
with nPrN(CH2CH2SH)2 (0.394 g, 2.2 mmol). The reaction
mixture was then stirred at a refluxing temperature for 30 min.
The resulting brown-red mixture was evaporated to dryness in
vacuo, and the crude product was purified by chromatography
on silica gel with CH2Cl2/hexane (v/v 1/1) as the eluent. From
the red band, [Fe2(μ-S(CH2)2NnPr(CH2)2S)(CO)6]2 (0.145 g,
16%) was obtained as a red solid. IR (hexane, cm-1): νCO
2069 (m), 2037 (vs), 1999 (s), 1988 (s), 1973 (w), 1955 (vw).
Method B. To a solution of [1-2HN](BF4)2 (221 mg, 0.2 mmol)
in CH3CN (7 mL) was added HBF4 (630 μL, 5 mmol, 50 wt % in
H2O) via an airtight microsyringe. The solution was stirred at
room temperature for 1 h before it was dried under vacuum. The
semi-oil residue was washed three times with 20 mL portions
of ether. The powder product was obtained after the second
wash with CH3CN/ether. The yield was 87%. Crystals of [1-
2HN2HFe](BF4)4 4CH3CN suitable for X-ray crystallographic
3
analysis were grown from a mixed CH3CN/CH2Cl2 solution
layered with ether. IR (CH3CN, cm-1): νCO 2058 (s), 2039 (vs),
2006 (s), 1993 (s, sh). 1H NMR (500 MHz, CD2Cl2): -15.46 (d,
3
1H NMR (500 MHz, CDCl3): 0.88 (t, JCH = 7.3 Hz, 6H,
2JHP = 21.5 Hz, 2H, FeHFe), 0.96 (t, 6H, CH3), 1.74 (d, 2JHP
=
3
CH2CH2CH3), 1.50 (sext, JCH = 7.5 Hz, 4H, CH2CH2CH3),
2
10.5 Hz, 18H, PMe3), 1.97 (d, JHP = 11 Hz, 18H, PMe3),
1.70-2.01 (m, 4H, CH3CH2CH2), 2.24-2.92 (m, 8H, SCH2),
3.23-3.74 (m, 12H, NCH2), 9.08 (br, 2H, NH) ppm. 31P NMR
2.34 (t, 4H, CH2CH2CH3), 2.35 (t, 4H, SCH2), 2.43 (t, 4H,
NCH2), 2.46 (t, 4H, SCH2), 2.69 (t, 4H, NCH2) ppm. 13C{1H}
NMR (125.7 MHz, CDCl3): 11.68 (CH2CH2CH3), 20.36
(CH2CH2CH3), 23.65 (SCH2), 38.13 (SCH2), 55.08 (NCH2),
56.02 (NCH2), 57.03 (CH2CH2CH3), 208.87 (CO) ppm.
ESI-MS: m/z 914.8 {M+H+}+. Anal. Calcd for C26H30Fe4-
N2O12S4: C, 34.16; H, 3.31; N, 3.06; S, 14.03. Found: C, 34.16;
H, 3.34; N, 3.00; S, 14.05.
(202.49 MHz, CD2Cl2):19.58 (d, JPP = 7.89 Hz), 23.43 (dd, JPP
=
7.69 Hz) ppm. ESI-MS: m/z 1371.06 {[1-2HN2HFe](BF4)3}+,
1283.04 {[1-2HN2HFe](BF4)2 - H+}+, 1195.02 {[1-2HN2HFe]-
(BF4) - 2H+}+, 1106.8 {[1-2HN2HFe] - 3H+}+, 598.02 {[1-
2HN2HFe](BF4) - H+}2+, 554.1 {[1-2HN2HFe] - 2H+}2+
.
Synthesis of [Fe2(μ-S(CH2)2NnPr(CH2)2S)(CO)4(PMe3)2]2,
Anal. Calcd for C42H82B4F16Fe4N6O8P4S4: C, 31.10; H, 5.10; N,
5.18. Found: C, 31.32; H, 5.12; N, 5.15. CV (CH3CN, V vs
Fc/Fc+): -1.26, -1.42, -1.82, and -2.43 (Epc).
1. To
a
solution of [Fe2(μ-S(CH2)2NnPr(CH2)2S)(CO)6]2
(500 mg, 0.55 mmol) in THF (10 mL) was added PMe3
(2.74 μL, 2.75 mmol, 1 M in THF) via an airtight microsyringe.
The reaction solution was stirred at room temperature for 2 h
Reaction of [1-2HN2HFe](BF4)4 with Triethanolamine
(TEOA). To a 5 mL CH3CN solution of [1-2HN2HFe](BF4)4

7612 Inorganic Chemistry, Vol. 48, No. 16, 2009
Chiang et al.
(97 mg, 0.067 mmol) was added 2 equiv of TEOA (17.7 μL, 0.133
mmol) by microsyringe. The reaction was monitored by IR
spectroscopy, indicating that the IR bands of [1-2HN2HFe]-
(BF4)4 were red-shifted about 10 cm-1 within 10 min. The
product was assigned to [1-2HFe](BF4)2. Attempt to isolate
the pure sample of [1-2HFe]2+ failed because of its instability in
solution. [1-2HFe]2+ converted to [1-2HN]2+ in solution,
evidenced by IR spectroscopy. [1-2HN]2+ was the final product
if [1-2HFe]2+ was left in solution overnight, confirmed by
NMR and FTIR analyses. When excess TEOA (5 equiv) was
initially added to the solution instead, both of [1-2HFe]2+ and 1
were observed immediately in the IR spectrum. IR changes of
the reaction revealed that intensity of the IR bands from 1
increased as that of [1-2HFe]2+ decreased. Complex 1 even-
tually was the sole product according to IR and NMR char-
acterization. [1-2HFe]2+: IR (CH3CN, cm-1): νCO 2049 (m),
2032 (vs), 1993 (s).
was added 8 equiv of [PPN]Cl (317 mg, 0.552 mmol). The
reaction was monitored by IR spectroscopy. A new set of
ν(CO) bands at 1982, 1945, 1914, and 1892 cm-1 was observed
within 4 min, indicating formation of [1-2HN](BF4)2. The
solution was dried under vacuum and washed by CH2Cl2/
hexane. Characterization of the solid to be [1-2HN]2+ was
confirmed by NMR and MS analyses. [1-2HN]2+: IR
(CH3CN, cm-1): νCO 1982 (s), 1945 (vs), 1914 (s), 1892 (m).
Acknowledgment. We are grateful to financial support from
National Science Council of Taiwan and Academia Sinica. We
thank Prof. Wen-Feng Liaw (National Tsing Hua University)
for insightful discussions. We also thank Drs. Mei-Chun Tseng
and Su-Ching Lin for help with MS analysis and 2D NMR
experiments, respectively.
Supporting Information Available: Crystallographic data of 1,
[1-2HN]2+, and [1-2HN2HFe]4+ in CIF format. This material
is available free of charge via the Internet at http://pubs.acs.org.
Reaction of [1-2HN2HFe](BF4)4 with [PPN]Cl. To a 10 mL
CH3CN solution of [1-2HN2HFe](BF4)4 (100 mg, 0.069 mmol)