Synthetic models for the active site of the [FeFe]-hydrogenase: Catalytic proton reduction and the structure of the doubly protonated intermediate

Article abstract of DOI:10.1021/ja309216v

This report compares biomimetic hydrogen evolution reaction catalysts with and without the amine cofactor (adt^NH): Fe₂(adt ^NH)(CO)₂(dppv)₂ (1^NH) and Fe ₂(pdt)(CO)₂(dppv)₂ (2) [(adt^NH) ^2- = HN(CH₂S)₂^2-, pdt^2- = 1,3-(CH₂)₃S₂^2-, and dppv = cis-C₂H₂(PPh₂)₂]. These compounds are spectroscopically, structurally, and stereodynamically very similar but exhibit very different catalytic properties. Protonation of 1^NH and 2 gives three isomeric hydrides each, beginning with the kinetically favored terminal hydride, which converts sequentially to sym and unsym isomers of the bridging hydrides. In the case of 1^NH, the corresponding ammonium hydrides are also observed. In the case of the terminal amine hydride [t-H1 ^NH]BF₄, the ammonium/amine hydride equilibrium is sensitive to counteranions and solvent. The species [t-H1^NH ₂](BF₄)₂ represents the first example of a crystallographically characterized terminal hydride produced by protonation. The NH - -HFe distance of 1.88(7) - indicates dihydrogen-bonding. The bridging hydrides [-H1^NH]⁺ and [-H2]⁺ reduce near 1.8 V, about 150 mV more negative than the reductions of the terminal hydride [t-H1^NH]⁺ and [t-H2]⁺ at 1.65 V. Reductions of the amine hydrides [t-H1^NH]⁺ and [t-H1^NH ₂]²⁺ are irreversible. For the pdt analogue, the [t-H2]^+/0 couple is unaffected by weak acids (pK_a ^MeCN = 15.3) but exhibits catalysis with HBF₄-Et ₂O, albeit with a turnover frequency (TOF) around 4 s^-1 and an overpotential greater than 1 V. The voltammetry of [t-H1 ^NH]⁺ is strongly affected by relatively weak acids and proceeds at 5000 s^-1 with an overpotential of 0.7 V. The ammonium hydride [t-H1^NH₂]²⁺ is a faster catalyst, with an estimated TOF of 58 000 s^-1 and an overpotential of 0.5 V.

Full text of DOI:10.1021/ja309216v

Article
pubs.acs.org/JACS
Synthetic Models for the Active Site of the [FeFe]-Hydrogenase:
Catalytic Proton Reduction and the Structure of the Doubly
Protonated Intermediate
Maria E. Carroll,† Bryan E. Barton,† Thomas B. Rauchfuss,*,† and Patrick J. Carroll‡
†School of Chemical Sciences, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States
‡Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
S
* Supporting Information
ABSTRACT: This report compares biomimetic hydrogen evolution reaction catalysts
with and without the amine cofactor (adtNH): Fe2(adtNH)(CO)2(dppv)2 (1NH) and
2−
Fe2(pdt)(CO)2(dppv)2 (2) [(adtNH
)
= HN(CH2S)22−, pdt2− = 1,3-(CH2)3S22−, and
dppv = cis-C2H2(PPh2)2]. These compounds are spectroscopically, structurally, and
stereodynamically very similar but exhibit very different catalytic properties. Protonation
of 1NH and 2 gives three isomeric hydrides each, beginning with the kinetically favored
terminal hydride, which converts sequentially to sym and unsym isomers of the bridging
hydrides. In the case of 1NH, the corresponding ammonium hydrides are also observed. In
the case of the terminal amine hydride [t-H1NH]BF4, the ammonium/amine hydride
equilibrium is sensitive to counteranions and solvent. The species [t-H1NH ](BF4)2
2
represents the first example of a crystallographically characterized terminal hydride
produced by protonation. The NH---HFe distance of 1.88(7) Å indicates dihydrogen-
bonding. The bridging hydrides [μ-H1NH]+ and [μ-H2]+ reduce near −1.8 V, about 150
mV more negative than the reductions of the terminal hydride [t-H1NH]+ and [t-H2]+ at
2+
−1.65 V. Reductions of the amine hydrides [t-H1NH]+ and [t-H1NH
]
are irreversible. For the pdt analogue, the [t-H2]+/0
2
couple is unaffected by weak acids (pKaMeCN = 15.3) but exhibits catalysis with HBF4·Et2O, albeit with a turnover frequency
(TOF) around 4 s−1 and an overpotential greater than 1 V. The voltammetry of [t-H1NH]+ is strongly affected by relatively weak
acids and proceeds at 5000 s−1 with an overpotential of 0.7 V. The ammonium hydride [t-H1NH
]
is a faster catalyst, with an
2+
2
estimated TOF of 58 000 s−1 and an overpotential of 0.5 V.
a
5
,
Table 1. Activities of [FeFe]- and [NiFe]-Hydrogenases
INTRODUCTION
■
In nature, hydrogen is primarily produced and oxidized by the
hydrogenase (H2ase) enzymes.1 For example, under fermenta-
tive conditions, organisms release accumulated reducing
equivalents as H2. This H2 can be captured by other organisms,
where it is utilized, via hydrogenases, to reduce oxides such as
sulfate.2 These enzymes have attracted attention as proven
motifs for the processing of hydrogen.3,4 A key goal in this area
is the elucidation of catalytic mechanisms.
reaction
FeFe
28000
6000−9000
NiFe
H2 oxidation
700
700
H2 production
a
Rates quoted in moles of H2 per mole of enzyme per second
measured at 30 °C.
The first step in characterizing a catalytic mechanism is
understanding the structure of the active sites. Two genetically
unrelated hydrogenases have been identified: they contain Ni
and/or Fe thiolate centers, the latter bound to CN− and CO.
These enzymes are generally rich in Fe−S clusters, emphasizing
the central role of electron transfer.5,6 [FeFe]-H2ases are more
active than [NiFe]-H2ases, and they more commonly function
as catalysts for H2 production (Table 1).7 In the case of [FeFe]-
H2ases, one 4Fe-4S cluster is directly tethered to the diiron
active site; the 6Fe ensemble is called the H-cluster. The
[FeFe]-H2ases also feature an amino-dithiolate cofactor that
bridges the two organoiron centers (Figure 1).8
Figure 1. Structure proposed for the active site of [FeFe]-H2ase in the
Hred state.5
compounds that are structurally similar to the active site.3,9
This approach benefits from many decades of research on
organoFe-S clusters.10 Indeed, soon after the structures of the
One widely embraced method for mechanistic analysis of the
Received: September 17, 2012
enzymes entails studies on synthetic diiron dithiolato
© XXXX American Chemical Society
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Article
Figure 2. Structure of Fe2(adtNH)(CO)2(dppv)2. Left: with thermal ellipsoids drawn at the 50% and those for phenyl carbon atoms omitted. Selected
distances (Å): Fe1−Fe2, 2.6027(6); Fe1−C27, 1.7381(30); Fe1−P1, 2.1884(8); Fe1−P2, 2.2108(8); Fe1−S1, 2.2108(8); Fe1−S2, 2.2668(8); Fe2−
C30, 1.7508(30); Fe2−P3, 2.1937(9); Fe2−P4, 2.2296(8); Fe2−S1, 2.2785(8); Fe2−S2, 2.2668(8). Right: space-filling model showing that the Ph
groups block the Fe−Fe bond (red = O, blue = N, yellow = S, orange = Fe, violet = P).
enzymes from Clostridium pasteurianum and Desulfovibrio
desulfuricans were reported, the diiron dithiolato dicyanides
[Fe2(pdt)(CN)2(CO)4]2− (pdt = 1,3-propanedithiolate) were
described.11 Unfortunately, these species proved to be not very
useful in developing functional models. Since that time, two
simplifications have been particularly enabling. First, tertiary
phosphine (and other) ligands are used in place of cyanide (and
some CO) ligands found in the natural catalysts.12 Second, in
place of the appended 4Fe-4S cluster, models usually rely on
electrodes to supply and accept electrons. No compromise is
required for the amine-dithiolate cofactor (adt), which is
incorporated into our models without modification.13
reduce at a potential ca. 100 mV less negative than that
required for the isomeric bridging hydride complex.19
Herein we summarize an extensive investigation of the
structural and protonation chemistry of Fe2(adtNH)-
(CO)2(dppv)2 (1NH) [adtNH 2− = [(SCH2)2NH]2−], with
parallel studies on Fe2(pdt)(CO)2(dppv)2 (2), which lacks
the amine cofactor. Overall, the results indicate that the
combination of a terminal hydride and the azadithiolate
cofactor greatly facilitates reduction of protons to form H2 by
diiron complexes. We also describe a rare crystal structure of a
terminal hydride of this series of model diiron dithiolato
complexes.
Consensus from biophysical2 and organometallic3,12 studies
points to the intermediacy of iron hydrides in the catalytic
function of [FeFe]-H2ase. Although much is known about iron
hydrides,14 our understanding of diiron dithiolato hydride
frameworks is still underdeveloped. CO-rich compounds such
as Fe2(SR)2(CO)6 are only protonated by very strong acids,15
hence the requirement that some CO ligands be replaced by
more basic donors. In almost all hydride derivatives of diiron
dithiolates, the hydride ligand bridges the two metals. This
geometry resembles that found in the [NiFe]-H2ases, but
biophysical2 and computational studies16 on the [FeFe]-H2ases
strongly indicate that the hydride is located at the apical
position of a single organoFe center. A number of diiron
complexes with terminal hydride ligands have been charac-
terized, although usually only by NMR spectroscopy. The
terminal hydride complexes [HFe2(pdt)(CO)4(chel)]+ (chel =
chelating ligand) are often observable by NMR spectroscopy,
but above ca. −30 °C they isomerize to bridging hydride
complexes.17,18 The stability of these terminal hydride
complexes is enhanced for sterically crowded or very
electron-rich diiron dithiolato carbonyls.19,20 Thus, Fe2(pdt)-
(CO)2(dppv)2 (2) [dppv = cis-C2H2(PPh2)2] undergoes
protonation, albeit only with strong acids, to give a terminal
hydride with a half-life of several minutes at room temperature.
Additionally, this terminal hydride derivative was shown to
RESULTS
■
Characterization of Fe2(adtNH)(CO)2(dppv)2. The main
catalyst of interest is Fe2(adtNH)(CO)2(dppv)2 (1NH), a
greenish-brown, air-sensitive solid that is highly soluble in
dichloromethane and toluene. The IR spectra for 1NH (νCO
=
1888, 1868 cm−1) and related derivatives Fe2(pdt)-
(CO)2(dppv)2 (2), Fe2(edt)(CO)2(dppv)2, and Fe2(odt)-
(CO)2(dppv)2 [edt2− = 1,2-C2H4S22−; odt2− = O(C2H4S)2
]
2−
are similar, which indicates that they adopt similar structures
and that the donor properties of the dithiolates are similar.21
Compounds 1NH and 2 are stereochemically nonrigid in two
ways: (i) “turnstile rotation” of the Fe(dppv)(CO) subunits
and (ii) “flipping” of the dithiolate bridge. With respect to the
latter process, the X(CH2S)2Fe rings (X = CH2, NH, O, etc.)
are subject to a chair−chair equilibration,22 as seen in
cyclohexane and piperidine derivatives.23
At −80 °C, the 31P NMR spectrum of 1NH displays four
equally intense signals, indicating that the two dppv ligands are
chemically inequivalent. In contrast, spectra for the related pdt
complexes show only a pair of signals at low temperatures, an
observation that suggests that pdt2− is a more flexible dithiolate
than adt2−. In the pdt derivative, turnstile rotation at the
Fe(dppv)(CO) unit is halted on the NMR time scale at −80
°C, but flipping of the dithiolate remains fast. In the adt
B
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derivative, both processes are either slow or cease to occur at
low temperatures, which may reflect a higher barrier for the
flipping of the adt compared to the structurally related pdt
derivative. [Note: The barriers for ring flipping for the
Fe2(xdt)(CO)4(dppv) [xdt2− = pdt, odt, and adt] from
DNMR studies (at coalescence temperatures, K) are as follow:
pdt, 42 (225); odt, 43.3 (235); and adt, 55.9 kJ/mol (293). The
variation reflects increased stiffness of the two C−N bonds in
the adt backbone, as a consequence of the interaction of the
amine lone pair and the C−S σ* orbitals. M. T. Olsen,
unpublished results.] At −10 °C, this conformational
equilibrium becomes rapid on the NMR time scale, and the
31P NMR spectrum simplifies to a broad singlet. The dynamics
of the adt and the turnstile rotation of the Fe(CO)(dppv)
centers appear to be coupled since both processes change from
slow to fast exchange regimes at the same temperature range
(see Supporting Information (SI)).
The structure of 1NH was confirmed crystallographically
(Figure 2), the details being consistent with the NMR results.
Space-filling models show that the phenyl groups protect the
Fe−Fe bond, which is relevant to the regiochemistry of the
protonation of 1NH and 2.
Protonated Derivatives of Fe2(adtNH)(CO)2(dppv)2.
Protonation of 1NH gives three isomeric hydrides as well as
ammonium derivatives or combinations of both. Addition of
[H(OEt2)2]BArF to a CH2Cl2 solution of 1NH at −80 °C
4
initially afforded the terminal hydride [t-HFe2(adtNH)-
−
(CO)2 (dppv)2 ]+ ([t-H1N H ]+ ) (BArF
= B(3,5-
4
−
1
C6H3(CF3)2)4 , Scheme 1). Its high-field H NMR spectrum,
Scheme 1. Protonation of 1NH and Isomerization Reactions
of the Resulting Hydrides
Figure 3. (Top) 1H and (bottom) 31P NMR spectra of the
protonation of 1NH with [H(OEt2)2]BArF in CD2Cl2 at various
4
times and temperatures. Spectra a: −80 °C ([t-H1NH]+). Spectra b:
sample warmed to −10 °C. Spectra c: sample warmed to 20 °C (sym-
[μ-H1NH]+; this spectrum does not change upon recooling to 0 °C).
Spectra d: previous sample after standing at 20 °C for 24 h (sym- and
unsym-[μ-H1NH]+, the latter indicated by asterisks). Spectra e: same
sample after addition of 1 equiv of [H(OEt2)2]BArF (mainly sym-[μ-
4
2+
H1NH
]
and a small amount of the unsym isomer).
2
+
The NMR spectrum of [t-H1NH
]
changes at higher
1
temperatures. Above −20 °C, the triplet at δ −4.2 in the H
NMR spectrum broadens, as do the 31P NMR signals,
consistent with exchange between the hydride and ammonium
tautomers (Scheme 2). Near room temperature, [t-H1NH +
]
converts sequentially to two isomers that feature bridging
hydride ligands (“μ-hydrides”, Scheme 1). The first μ-hydride
isomer, sym-[μ-H1NH]+, has C2-symmetry (ignoring the adt
ligand, which is rapidly flexing and hence effectively planar). In
sym-[μ-H1NH]+, each dppv ligand spans one apical and one
basal site. Upon recooling a solution of sym-[μ-H1NH]+ to 0 °C,
the terminal hydride species does not re-form. Within minutes
at room temperature, sym-[μ-H1NH]+ converts to an unsym-
metrical isomer labeled unsym-[μ-H1NH]+. In unsym-[μ-
H1NH]+, one diphosphine remains apical-basal and the other
is dibasal as indicated by the 31P NMR spectrum, which consists
of four singlets. After the solution was allowed to stand for 12 h
at room temperature, a 1:5 equilibrium distribution was
reached, favoring unsym-[μ-H1NH]+. The isomerization of ([t-
H1NH]+ is similar to that observed for the hydrides of 2.18
a triplet near δ −4.2, indicates a single isomer (Table 2, Figure
3). Such relatively low field chemical shifts are often associated
with terminal hydrides of diiron dithiolates.17−20 The 31P NMR
spectrum, which features four equally intense singlets (Table
2), uniquely defines the stereochemistry at the diiron center:
the diphosphine on the FeH center is dibasal and the
diphosphine on the other (“proximal”) Fe center spans apical
and basal sites (31P−31P coupling is often weak in such
complexes).24
Addition of [H(OEt2)2]BArF to a CH2Cl2 solution of [μ-
4
H1NH]+ shifts the νCO pattern about 15 cm−1 to higher energy.
C
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Table 2. Spectroscopic Properties for 1NH, 2, and Their Protonated Derivatives in CD2Cl2 Solution
complex
IR: νCO (cm−1
)
1H NMR: δ hydride (ppm)
31P NMR (ppm)
Fe2(adtNH)(CO)2(dppv)2, 1NH
1888, 1868
1965, 1915
1986, 1925
not obsvd
1948, 1969
1967, 1985
not obsvd
1888, 1868
1965, 1905
not obsvd
1951, 1968
91.2; at −70 °C: 102.6, 93.4, 92.2, 88.6
103.2, 94.5, 84.0, 75.1
98, 89, 76, 74
+
[t-HFe2(adtNH)(CO)2(dppv)2]+, [t-H1NH
]
−4.2 (t), J = 73 Hz
−4.95 (t), J = 72 Hz
−14.8 (tt) J = 25, 5 Hz
−13.7 (dtd)
−15.6 (tt), J = 20, 5 Hz
−14.5 (dtd)
[t-HFe2(adtNH )(CO)2(dppv)2] , [t-H1
]
2+
NH2 2+
2
[Fe2(adtNH)(μ-H)(CO)2(dppv)2]+, sym-[μ-H1NH
]
91.1, 90.9
+
[Fe2(adtNH)(μ-H)(CO)2(dppv)2]+, unsym-[μ-H1NH
]
90.1, 87.6, 84.6, 79.2
91.3, 88.7
+
[Fe2(adtNH )(μ-H)(CO)2(dppv)2] , sym-[μ-H1
]
2+
NH2 2+
2
[Fe2(adtNH )(μ-H)(CO)2(dppv)2] , unsym-[μ-H1
]
92.3, 86.0, 82.4, 79.0
90.8; at −80 °C: 97, 88.5
99, 91, 86, 68
2+
NH2 2+
2
Fe2(pdt)(CO)2(dppv)2, 2
[HFe2(pdt)(CO)2(dppv)2]+, [t-H2]+
[Fe2(pdt)(μ-H)(CO)2(dppv)2]+, sym-[μ-H2]+
[Fe2(pdt)(μ-H)(CO)2(dppv)2]+, unsym-[μ-H2]+
−3.5 (t), J = 78 Hz
−15.6 (tt), J = 24, 6 Hz
−14.5 (dddd), J = 24, 19, 19, 10 Hz
89.6, 89.4
76.7, 82.7, 84.2, 87.9
Scheme 2. Proposed Hydrogen-Bonding Interaction
+
−
between [1NH ] and BF4 Resulting in Stabilization of the
2
Ammonium Tautomer
This change is indicative of N-protonation to form the
ammonium hydride [μ-H1NH
]
2+ (Scheme 1).25 N-Protonation
2
Figure 4. FT-IR spectra in the νCO region of 1NH (bottom) and
1
is also indicated by the H NMR signal for the hydride, which
products from its protonation in CH2Cl2. A mixture of the ammonium
2+
shifts to higher field. The symmetrical isomer, sym-[μ-H1NH ] ,
2
+
+
cation [1NH
]
and [t-H1NH
]
generated by addition of excess
2
[Bu4N]BF4 to a solution of [t-H1NH]+. The terminal hydride, [t-
is the major species in solution, in contrast to the situation for
[μ-H2]+ and [μ-H1NH]+.
H1NH]+, formed by protonation of 1 with 1 equiv of [H(OEt2)2]BArF
4
at −40 °C. The terminal hydride ammonium dication [t-H1NH
]
2+
2
IR spectroscopic measurements on the protonation reactions
are consistent with the sequence of reactions proposed in
Scheme 1. The main feature of interest is the ca. 70 cm−1 shift
formed by protonation of 1NH with 2 equiv of HBF4·Et2O at −40 °C.
in νCO,avg upon protonation of 1NH by [H(OEt2)2]BArF . IR
Diprotonation of Fe2(adtNH)(CO)2(dppv)2. Relevant to
the electrocatalysis discussed below, 1NH undergoes double
4
spectra in the νCO region also distinguish terminal and bridging
hydrides. The terminal hydrides shows two well-resolved bands,
νCO = 1965 and 1915 cm−1, with the band at lower frequency
assigned to the semibridging CO (Figure 4). The IR spectra of
μ-hydrides show two bands, νCO ≈ 1948 and 1969 cm−1.
Protonation of Fe2(adtNH)(CO)2(dppv)2 with
HBF4·Et2O. The identity of the acid affects the course of
protonation of 1NH. Thus, treatment of CH2Cl2 solutions with
protonation. Addition of excess of [H(OEt2)2]BArF ,
4
]
HBF4·Et2O, or CF3CO2H to a CH2Cl2 solution of [t-H1NH +
gave a new species assigned as the ammonium hydride [t-
2+
1
H1NH
]
(Figures 3 and 4). According to H and 31P NMR
2
spectra, [t-H1NH
]
is structurally similar to [t-H1NH]+, i.e., a
2+
2
single isomer with both dibasal and apical-basal diphosphine
+
ligands (Table 2, Scheme 1). As seen for 1NH vs [1NH ] , N-
2
HBF4·Et2O (vs [H(OEt2)2]BArF discussed above) afforded
protonation shifts νCO bands to higher energy by 20 cm−1,
compared to that for [t-H1NH]+.
4
not only [t-H1NH]+ but also a second species. The pattern for
the νCO bands for the other species is identical to that for 1NH
,
After attempts over the course of several years, single crystals
of an ammonium hydride were obtained in the form of the salt
but the bands are shifted approximately 20 cm−1 to higher
energy, characteristic of N-protonation (Figure 4).25,26 The
[t-H1NH ](BF4)2 (Figure 5). In the dication, two Fe(dppv)-
2
new species is thus assigned as the ammonium tautomer
(CO) centers are linked in the usual way by a dithiolate. One
CO ligand is semibridging with Fe−Fe−C and Fe−C−O angles
of 66.68(14) and 166.1(4)°, respectively.29 The dispositions of
the dppv groups, being dibasal and apical-basal, conform with
the NMR measurements. The crystals are of sufficient quality
that the H centers attached to nitrogen and iron were located
and refined. Two phenyl groups on the Fe(1)(dppv) center,
equivalent to the distal Fe in the active site, form a pocket
around the Fe−H and N−H centers. The Fe−H distance of
1.44(4) Å is normal for a ferrous hydride.30 The ammonium
+
([1NH ] , Schemes 1 and 2). The formation of the tautomer is
2
−
attributed to the ability of BF4 to participate in hydrogen-
bonding (Scheme 2),27 which has been observed previously in
complexes of protonated adt ligands.28 The ratio [t-H1NH]+/
+
[1NH ] increased from 2:1 to approximately 1:1 in the presence
2
−
of 10 equiv of [Bu4N]BF4. Although BF4 stabilizes the
ammonium tautomer, the mixture isomerizes in the usual way,
forming the μ-hydride species over the course of minutes at
room temperature.
D
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behavior of 1NH, the pdt derivative, 2, is unaffected by weak
acids.
Redox Properties of the Protonated Derivatives of
Fe2(xdt)(CO)2(dppv)2 [xdt = adtNH, pdt]. In CH2Cl2
solutions, the μ-hydrides [μ-H1NH]+ and [μ-H2]+ reversibly
reduce near −1.8 V. Reduction of the doubly protonated
2+
species [μ-H1NH
]
occurs about 100 mV positive of the [μ-
2
H1NH +/0
couple, as observed for other adt-hydrido com-
]
plexes.33 The terminal hydrides [t-H1NH + and [t-H2]+ are
]
redox-active at milder potentials, the pdt derivative [t-H2]+
reducing quasi-reversibly (ipa/ipc = 1.57) at −1.67 V. Reduction
of [t-H2]+ is a 1e− process, as indicated by the similarity of the
19
dependence of ip vs ν1/2 for the couples [t-H2]+/0 and [2]0/+
.
We have independently established the stoichiometry of the
[2]0/+ couple.34 The amino hydride [t-H1NH]+ reduces at nearly
the same potential (−1.64 V), but the couple is irreversible, in
contrast to the [t-2H]+/0 couple. The ammonium hydride [t-
2+
H1NH
]
also reduces irreversibly but at the mild potential of
2
Figure 5. Structure of [t-HFe2(adtNH )(CO)2(dppv)2](BF4)2 with
2
−1.3 V (Table 3). Although the reduction potential is milder,
thermal ellipsoids drawn at 50% probability and those for phenyl
carbon atoms omitted for clarity. Counterions and solvent of
crystallization are not shown. Selected distances (Å): H1---H2,
1.88(7); Fe1−C1, 1.794(5); Fe1−P2, 2.2141(13); Fe1−P1,
2.2196(12); Fe1−S2, 2.2610(12); Fe1−S1, 2.3009(13); Fe1−Fe2,
2.6155(9); Fe1−H1, 1.44(4); Fe2−C2, 1.769(5); Fe2−P3,
2.2122(14); Fe2−S2, 2.2348(14); Fe2−S1, 2.2564(12); Fe2−P4,
2.2686(14). Angles (°): C1−Fe1−Fe2, 66.68(14); Fe2−Fe1−H1,
130.3(18); S2−Fe1−S1, 83.50(4); S2−Fe2−S1, 85.12(4); O1−C1−
Fe1, 166.1(4).
stronger acids are required to generate this doubly protonated
species. Finally, both the [t-H1 ]
NH +/0 and the [t-H2]+/0 couples
proved insensitive to the electrolyte, i.e., [Bu4N]BArF and
4
[Bu4N]PF6.
Proton Reduction Catalysis by Fe2(pdt)(CO)2(dppv)2.
For comparison with the catalyst that contains the amine
cofactor, the catalytic properties of [t-H2]+ and [μ-H2]+ were
evaluated. Strong acids such HBF4·Et2O (pKaMeCN = −3)35 are
required to protonate 2, readily giving the terminal hydride [t-
H2]+, which is stable for many minutes at 0 °C. The [t-H2]+/0
couple at −1.67 V is partially reversible even at scan rates as
slow as 25 mV/s. Addition of ClCH2CO2H (pKaMeCN = 15.3)35
has no effect on the electrochemical properties of [t-H2]+.
Addition of HBF4·Et2O results in an increase in current for the
couple at −1.67 V, indicative of catalysis. The ratio of the
catalytic current to the peak current in the absence of excess
acid, ic/ip, increases linearly with [H+] up to 8 equiv of acid,
indicating a catalytic pathway that is second order with respect
to [H+]. Above this level, ic/ip reaches a plateau near 5. The
turnover frequency (TOF) is calculated to be ∼5 s−1 (see SI).
The overpotential is estimated to be 1.3 V. Overpotentials
represent the difference between the catalytic potential, Ecat,
and the standard reduction potential of the acid, E°HA/H2 (see
Table 4).
center of the dithiolate cofactor adtH+ is adjacent to the Fe-H
center. The NH---HFe distance of 1.88(7) Å is shorter than 2.4
Å distance that is twice the van der Waals radius of hydrogen,31
indicative of dihydrogen-bonding.
Protonation of Fe2(xdt)(CO)2(dppv)2 [xdt = adtNH, pdt]
with Weak Acids. In this section we discuss protonations
using acids with pKaMeCN between 12 and 19. Weak acids that
do not convert 1NH into detectable levels of [t-H1NH]+ at low
temperatures quantitatively give [μ-H1NH]+ near room temper-
ature. Thus, [HNMe3]BArF (pKaMeCN = 17.6)32 rapidly
4
converted 1 to [μ-H1NH]+, whereas the same reaction using
[HNEt3]BF4 (pKaMeCN = 18.6)32 required days for 50%
conversion.
The catalytic properties of [μ-2H]+ were also examined.
Addition of ClCH2CO2H to a solution of [μ-2H]BF4 results in
an increase in the current for the −1.8 V couple (see SI). Plots
of ic/ip vs [H+] are linear up to 10 equiv of acid, and the rate of
catalysis by this derivative is calculated to be ∼3 s−1. The
overpotential is estimated to be 0.95 V for this very slow
process.35
Addition of >2 equiv of CF3CO2H (pKaMeCN = 12.7)32 to
2+
1NH at −40 °C gave [t-H1NH ] . The IR spectrum (νCO = 1986
2
and 1950 cm−1) of the trifluoroacetate differs from that (νCO
=
1986 and 1925 cm−1) obtained by protonation with [H-
(OEt2)2]BArF and HBF4·Et2O, a difference tentatively
4
attributed to hydrogen-bonding between the trifluoroacetate
and the protonated adt ligand. In contrast to the acid−base
a
Table 3. Selected Electrochemical Properties of 1 and 2 and Their Protonated Derivatives
complex
E1/2 in CH2Cl2 (V vs Fc+/0
)
assignment
ipc/ipa
+
+/0
[t-HFe2(adtNH)(CO)2(dppv)2]+, [t-H1NH
]
−1.64
−1.4
−1.86
−1.77
−1.67
−1.8
[t-H1NH
[t-H1NH
[μ-H1NH
[μ-H1NH
]
2
b
2+
NH2 2+
2+/+
[t-HFe2(adtNH )(CO)2(dppv)2] , [t-H1
]
]
b
2
[Fe2(adtNH)(μ-H)(CO)2(dppv)2]+, [μ-H1NH
]
]
1.19
1.00
1.57
1.07
+
+/0
[Fe2(adtNH )(μ-H)(CO)2(dppv)2] , [μ-H1
]
]
2+
NH2 2+
2+/+
2
2
[HFe2(pdt)(CO)2(dppv)2]+, [t-H2]+
[Fe2(pdt)(μ-H)(CO)2(dppv)2]+, [μ-H2]+
[t-H2]+/0
[μ-H2]+/0
a
b
Electrolytes for measurements: [Bu4N]BArF (for 1) and [Bu4N]PF6 (for 2). Irreversible.
4
E
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Table 4. Selected Electrocatalytic Properties of Protonated Derivatives of 1 and 2
equiv of H+
a
b
c
catalyst
acid
linear region plateau region (ic/ip)max
k (s−1
)
Ecat (V vs Fc+/0
)
overpotential (V)
+
[HFe2(adtNH)(CO)2(dppv)2]+, [t-H1NH
]
ClCH2CO2H
CF3CO2H
50−400
40−560
2−50
1−8
2−10
600−1200
740−1300
60−90
10−20
14−40
69
167
10
5000
−1.49
−1.11
−1.72
−1.49
−1.78
0.71
0.51
0.90
1.32
0.95
[HFe2(adtNH H)(CO)2(dppv)2]2+, [t-H1NH
[Fe2(adtNH)(μ-H)(CO)2(dppv)2 , [μ-H1NH
]
58000
2+
2
+
+
]
ClCH2CO2H
HBF4·Et2O
ClCH2CO2H
20
5
[HFe2(pdt)(CO)2(dppv)2]+, [t-H2]+
5
[Fe2(pdt)(μ-H)(CO)2(dppv)2]+, [μ-H2]+
3.5
3
a
b
ic is the average catalytic current in the plateau region Ecat was calculated at each acid concentration in the linear region by the method described
by Fourmond et al.,37 considering the effects of homoconjugation for the two carboxylic acids. The values listed are averages from each acid
concentration in the linear region. Overpotential = E°HA/H2 − Ecat, where E°HA/H2 is the standard reduction potential of the acid.37
c
Figure 6. Cyclic voltammograms of a 1.00 mM solution of 1NH (0 °C,
0.125 M [Bu4N]BArF , CH2Cl2, scan rate = 0.5 V/s, glassy carbon
Figure 8. Cyclic voltammograms of a 0.5 mM solution of 1NH (0 °C,
4
0.125 M [Bu4N]BArF , CH2Cl2, scan rate = 1.0 V/s, GC working
working electrode, Pt counter electrode, Ag wire pseudo reference
electrode, Fc internal standard) recorded with increasing equivalents
of ClCH2CO2H (shown to right of voltammograms).
4
electrode, Pt counter electrode, Ag wire pseudo reference electrode, Fc
internal standard) recorded with increasing equivalents of CF3CO2H
(shown to right of voltammograms).
Figure 9. Dependence of ic/ip on equivalents of CF3CO2H added to a
0.5 mM solution of 1NH with [Bu4N]BArF as supporting electrolyte.
4
Figure 7. Graph of ic/ip vs equivalents of acid for the addition of
ClCH2CO2H to a 1.00 mM solution of 1NH with [Bu4N]BArF as
H1NH +/0 couple closely matched the current for the [1NH 0/+
]
]
4
supporting electrolyte (blue) and a 1.0 M solution of [μ-H1NH]+ with
[Bu4N]PF6 as supporting electrolyte (red). Inset: magnified data for
[μ-H1NH]+.
couple, indicating that both events correspond to one-electron
couples. Whereas the couple [2H]+/0 is partially reversible, the
+/0
[t-H1NH
]
couple is completely irreversible (SI). Upon
addition of ClCH2CO2H to solutions of 1NH, the reductive
current at −1.64 V sharply increased and ipc shifts to more
positive potential (Ecat = −1.43 V), indicative of catalytic
proton reduction (Figure 6).35 A plot of ic/ip vs [H+] is linear
over the range 50−400 equiv. The maximum ic/ip ≈ 69
corresponds to a rate of 5000 s−1 (Figure 7). Gas chromato-
graphic analysis showed that a 90% yield of hydrogen was
produced by bulk electrolysis.
Proton Reduction Catalysis by Fe2(adtNH)(CO)2(dppv)2.
In contrast to the behavior of the pdt complex, the voltammetry
of [t-H1NH +
] is strongly affected by relatively weak acids.
Experiments employed [Bu4N]BArF as the electrolyte to
4
maximize the equilibrium concentration of [t-H1NH]+ vs its
+
tautomer [1NH ] . When [t-H1NH]+ was generated in situ from
2
1NH and one equiv of [H(OEt2)2]BArF , the current for the [t-
4
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By the standards of synthetic catalysts, [t-H1NH]+ is a fast
(MeCN scale). Being a stronger acid, [t-H1NH]+ is more easily
reduced than is [μ-H1NH]+.
catalyst, but [t-H1NH
]
2+ appears to be even faster. Thus, in the
2
presence of strong acids such as HBF4·Et2O or trifluoroacetic
acid (TFA), a catalytic wave is observed at −1.22 V. Monitoring
the current at −1.2 V, the variation of ic/ip vs [TFA] was found
to be linear over 40−560 equiv. The maximum ic/ip ≈ 167
corresponds to an estimated TOF of 58 000 s−1 (Figures 8 and
9).
For both the pdt and adt derivatives, the terminal hydrides
reduce at potentials that are 100−200 mV more positive than
the corresponding bridging hydrides. The difference in
reduction potentials between the isomeric hydrides is proposed
to reflect differences in the localization of the reduction. In the
case of the μ-hydride isomer, reduction is likely more
delocalized between the two equivalent Fe centers.38 When
the hydride is bound to a single Fe atom, reduction likely
occurs at the Fe center with no hydride ligand.39 The
crystallographic results presented above indicate how this
reduction could induce the formation of a mixed valence
dihydrogen complex (eq 1).
Hydrogen evolution by electrolysis is typically subject to
substantial isotope effects,36 and this aspect was observed with
1
NH. The dependence of the ic/ip vs [ClCH2CO2D] is linear up
to 60 equiv of acid and reaches a plateau at ic/ip = 18, which
corresponds to a TOF of 420 s−1. The rate decreased 10-fold
compared to that exhibited in experiments that utilize
ClCH2CO2H (maximum ic/ip = 66, k = 4500 s−1). Similarly,
when CF3CO2D is substituted for CF3CO2H, k decreases by
about 7-fold, with the maximum ic/ip = 106 and k = 6600 s−1.
Electrocatalysis experiments were conducted with the
bridging hydride [μ-H1NH +
] also at 0 °C. At −1.8 V, the
current increases with the addition of ClCH2CO2H. A plot of
ic/ip vs [H+] is linear over the range 2−50 equiv of
ClCH2CO2H but begins to plateau at 60 equiv, corresponding
to ic/ip ≈ 10 (Figure 7). The catalytic rate is estimated as 20
s−1. The μ-hydride operates at an overpotential of 0.9 V, which
is 0.2−0.4 V higher than the overpotentials for the terminal
species.
This scenario underscores the role of electron transfer,
perhaps proton-coupled electron transfer (PCET),40 in the
hydrogen evolution reaction (HER, Scheme 3). Upon
reduction, hydrogen evolution would produce the mixed
valence derivative, analogous to the Hox state of the enzyme.
In HER catalyzed by [μ-H1NH]+, it is apparent that 1NH is
not regenerated. Otherwise, we would have observed increased
rates during the electrolysis since [t-H1NH]+, which is such an
CONCLUSIONS
■
Hydrogen evolution by biomimetic diiron dithiolates is
accelerated by the amine cofactor, to which the hydride ligand
must be adjacent. The protonated derivatives of 1NH and 2
exhibit very different catalytic properties, although the neutral
complexes are almost indistinguishable spectroscopically. Thus,
the adt and pdt ligands have comparable inductive effects,
resulting in complexes of very similar thermodynamic proper-
ties.
Scheme 3. Proposed Mechanism for Proton Reduction
Catalyzed by [t-H1NH]+: Two Subcycles Are Shown for
Strong and Weak Acids
2+
The crystallographic analysis of [t-H1NH
]
provides a
2
unique insight into a probable intermediate in both the
evolution and oxidation of hydrogen. The specific point of
interest is the short distance between the hydride and the
proton on the ammonium cofactor. The implied dihydrogen-
bonding provides a snapshot of a step in the formation and
scission of a dihydrogen molecule at the active site of the
[FeFe]-H2ase. An attractive NHδ+---Hδ− interaction may be
relevant to the small separation between the first and second
protonation constants, estimated at only 102 in CH2Cl2
solution. In comparison with [t-H1NH]+, [t-H1NH
]
2+ isomerizes
2
to the μ-hydride more slowly, perhaps because this interaction
stabilizes the terminal hydride.
The basicities of two sitesthe terminal Fe center and the
amineare closely matched. In CH2Cl2 solution, Fe is more
basic, but the equilibrium shifts in the presence of hydrogen-
bond acceptors as demonstrated by the influence of [Bu4N]BF4
on this equilibrium. The lower basicity of a terminal Fe site vs
+
the Fe−Fe bond is indicated by the conversion of [1NH ] into
2
[μ-H1NH]+ under conditions where [t-H1NH]+ is undetectable.
A sequence of three processes is invoked to explain the
formation the μ-hydride under these conditions: (i) N-
protonation, albeit unfavorable; (ii) tautomerization of
ammonium species to the terminal hydride [t-H1NH]+; and
(iii) isomerization of [t-H1NH]+ to [μ-H1NH]+. The pKa of [t-
H1NH]+ is estimated at 16, and the pKa of [μ-H1NH]+ is >18.6
G
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δ 91.0 and 90.8 appear in the 31P NMR spectrum, which are assigned
active catalyst, would be formed by protonation of 1NH. We
conclude therefore that the μ-hydride ligand in [μ-H1NH]+ is a
spectator. A related mechanism was invoked by Talarmin for
catalysis by [Fe2(pdt)(μ-H)(CO)4(dppe)]+.41
Although the bioinspired complexes described in this report
are highly active proton reduction catalysts,42 challenges
remain. These biomimetic catalysts suffer from relatively
negative reduction potentials leading to high overpotentials.5
One possible solution involves incorporation of redox-active
center to mediate the PCET that is implicated for the breaking
and making of the H−H bond.40
to sym-[μ-H1NH]BArF . Upon warming of the sample to 20 °C, signals
4
-
corresponding to [t-H1NH]BArF disappear, and only sym-[μ-H1NH
]
4
BArF is observed. After ∼10 min at 20 °C, an additional H NMR
1
4
multiplet at δ −13.7 appears, and four 31P NMR singlets appear at δ
88.2, 85.7, 82.7, and 77.3, which are assigned to unsym-[μ-
H1NH]BArF . After ∼12 h at room temperature, the ratio of the
4
sym:unsym isomers is 1:5.
Upon addition of a second equivalent of [H(OEt2)2]BArF (5 mg,
4
1
0.005 mmol) to the solution, the H NMR spectrum showed a triplet
of triplets at δ −15.5 (Fe-μH, JPH = 25, 5 Hz) assigned to sym-[μ-
F
H1NH ](BAr )2 and a multiplet at δ −14.3 assigned to unsym-[μ-
2
4
F
4
H1NH ](BAr )2. Additionally, in the 31P NMR spectrum, signals are
2
observed at δ 91.3 and 88.7 (sym-[μ-H1NH ](BAr )2) and at δ 92.3,
F
4
2
86.0, 82.4, and 79.0 (unsym-[μ-H1NH ](BAr )2). The two isomers are
F
EXPERIMENTAL SECTION
2
■
4
present in a ratio of 5:1 sym:unsym. An IR spectrum of the CD2Cl2
Reactions were typically conducted using Schlenk techniques at room
temperature. Most reagents were purchased from Aldrich and Strem.
Solvents were HPLC-grade and dried by filtration through activated
alumina or distilled under nitrogen over an appropriate drying agent.
HBF4·Et2O (Sigma-Aldrich) was supplied as 51−57% HBF4 in Et2O
solution features bands at 1967 and 1985 cm−1.
F
NH2
F
[t-HFe2(adtNH )(CO)2(dppv)2](BAr ) ([t-H1 ](BAr ) ). Into a
2
4 2
4 2
liquid nitrogen-cooled J. Young NMR tube, ∼0.5 mL of CD2Cl2 was
distilled onto a mixture of 1NH (5 mg, 0.005 mmol) and
[H(OEt2)2]BArF (10 mg, 0.010 mmol). The sample was then gently
(6.91−7.71 M). [H(OEt2)2]BArF was prepared by literature
4
4
methods.43 [Bu4N]PF6 was purchased from GFS Chemicals and was
recrystallized multiple times by extraction into acetone followed by
warmed to near −78 °C before analysis by low-temperature NMR
1
spectroscopy. High-field H NMR (600 MHz, CD2Cl2, −40 °C): δ
2
−4.95 (t, Fe−H, JPH = 72 Hz). 31P{1H} NMR (242 MHz, CD2Cl2,
1
precipitation by ethanol. H NMR spectra (500 and 400 MHz) are
referenced to residual solvent referenced to TMS. 31P{1H} NMR
spectra (202 and 161 MHz) are referenced to external 85% H3PO4.
FT-IR spectra were recorded on a Perkin-Elmer 100 FT-IR
spectrometer. CF3CO2D was purchased from Sigma-Aldrich.
CCl2HCO2D was prepared by treating the acid in D2O.
−40 °C): δ 98 (s), 89 (s), 76 (s), 74 (s). Decoupling experiments
verified that the high-field 1H NMR signal is coupled to the 31P NMR
signals at δ 89 and 74.
Crystallization of [t-HFe2(adtNH )(CO)2(dppv)2](BF4)2. Crystals
2
were obtained by treating a CH2Cl2 solution of 1NH, at 0 °C, with 2
equiv of HBF4·Et2O and layering with cold pentane. Storage at −20 °C
gave dark brown crystals.
44
Fe2(adtNH)(CO)4(dppv). A solution of Fe2(adtNH)(CO)6 (175
mg, 0.45 mmol) and dppv (179 mg, 0.45 mmol) in 15 mL of toluene
was treated with a solution of anhydrous Me3NO (34 mg, 0.45 mmol)
in ca. 5 mL of MeCN. Bubbles appeared, and the reaction mixture
darkened. After heating of the mixture at reflux for 5 h, the FT-IR
spectrum indicated complete conversion to product. Solvent was
removed under vacuum, and the product was extracted into 5 mL of
CH2Cl2. Addition of 50 mL of hexane precipitated the product. Yield:
0.260 g (80%). FT-IR, 1H NMR, and 31P NMR results matched
reported values.24
[Fe2(adtNH)(μ-H)(CO)2(dppv)2]BArF ([μ-H1NH]BArF ). A 10 mL
4
4
solution of 1NH (35 mg, 0.03 mmol) in CH2Cl2 was treated with a 5
mL solution of [H(OEt2)2]BArF (33 mg, 0.03 mmol) in CH2Cl2.
4
Within 5 min, the solution color changed from green to brown. After
being stirred at room temperature for 22 h, the solution was
concentrated to ∼5 mL, and the brown product was precipitated with
addition of 20 mL of Et2O. IR (CH2Cl2): νCO = 1948, 1969 (sh) cm−1.
1H and 31P NMR data match those described above. Anal. Calcd for
C9H62BF24Fe2NO2P4S2·2CH2Cl2·C4H10O (found): C, 51.89 (52.19);
H, 3.52 (3.13); N 0.64 (0.76).
Fe2(adtNH)(CO)2(dppv)2 (1NH). A solution of Fe2(adtNH)-
(CO)4(dppv) (400 mg, 0.55 mmol) in 40 mL of toluene was treated
with dppv (872 mg, 2.2 mmol) in 10 mL of toluene. The mixture was
irradiated at 365 nm until the conversion was complete (∼24 h) as
indicated by IR spectroscopy. The solvent was removed in vacuum,
and the product was extracted into ∼3 mL of toluene. The green
product precipitated upon addition of methanol to the toluene extract.
The product was then re-extracted into ∼2 mL of CH2Cl2, and the
product precipitated as an olive-green powder upon the addition of
100 mL of hexanes. Yield: 150 mg (26%). 1H NMR (500 MHz,
CD2Cl2): δ 8.1−7.0 (m, 40H, P(C6H5)2), 2.3 (s, 4H, (SCH2)2NH).
31P NMR (CD2Cl2, 20 °C): δ 92. 31P NMR (CD2Cl2, −70 °C): δ
102.6, 93.4, 92.2, 88.6. FT-IR (CH2Cl2): νCO = 1888, 1868 cm−1. Anal.
Calcd for C56H49Fe2O2NP4S2·CH2Cl2·CH3OH (found): C, 58.8
(58.15); H, 4.68 (4.63); N 1.18 (1.10). Diffraction-quality crystals
were grown at −20 °C from a CH2Cl2 solution of 1 layered with
pentane.
Protonation of Fe2(adtNH)(CO)2(dppv)2 with 1.5 equiv of
[H(OEt2)2]BArF . Into a liquid nitrogen-cooled J. Young NMR tube,
4
CD2Cl2 was distilled onto a mixture of 1NH (5 mg, 0.005 mmol) and
[H(OEt2)2]BArF4 (7.5 mg, 0.0075 mmol). The sample was then gently
warmed to near −78 °C before analysis by low-temperature NMR
1
spectroscopy. High-field H NMR (600 MHz, CD2Cl2, −40 °C): δ
2
x2
−4.95 (Fe-H, JPH = 72 Hz), −4.2 (Fe−H,
J
= 73 Hz).
PH
Effect of [Bu4N]BF4 on Equilibrium between Tautomers [t-
H1NH]+ and [1NH ] . A 7 mM solution of [t-H1NH]BArF4 was prepared
+
2
by dissolving 1NH (15 mg, 0.014 mmol) and [H(Et2O)2]BArF4 (15 mg,
0.014 mmol) in 2 mL of CH2Cl2, which had been precooled to −78
°C. Aliquots of a 0.6 M [Bu4N]BF4 solution were added to the
solution. After each addition, 0.1 mL aliquots were removed and
immediately analyzed by IR spectroscopy.
[t-HFe2(pdt)(μ-CO)(CO)(dppv)2]BF4 ([t-H2]BF4). A dark green
solution of 2 (205 mg, 0.192 mmol) in 5 mL of CH2Cl2 was treated at
−40 °C with HBF4·Et2O (5 mL 0.04 M, 0.2 mmol). The resulting
darker green solution was then diluted with 100 mL of cold (−78 °C)
Et2O, producing a green precipitate. The precipitate was collected by
filtration at −78 °C, washed with 2 × 10 mL of Et2O, and dried under
[HFe2(adtNH)(CO)2(dppv)2]BArF ([t-H1NH]BArF ), [Fe2(adtNH
)
4
4
(CO)2(dppv)2]BArF ([μ-H1NH]BArF ), and [Fe2(adtNH )(μ-H)-
2
4
4
F
(CO)2(dppv)2](BArF ) ([μ-H1NH ](BAr ) ). Into a liquid nitrogen-
2
4 2
4 2
cooled J. Young NMR tube, ∼0.5 mL of CD2Cl2 was distilled and
frozen onto a mixture of 1NH (5 mg, 0.005 mmol) and [H(OEt2)2]-
1
BArF (5 mg, 0.005 mmol). The sample was then gently warmed to
vacuum prior to storage at −30 °C. Yield: 140 mg (63%). H NMR
4
(500 MHz, CD2Cl2): δ −3.5 (t, Fe−H, 2JPH = 76 Hz). 31P{1H} NMR
1
near −78 °C before analysis by NMR spectroscopy. H NMR (600
MHz, CD2Cl2, −80 °C): δ −4.2 (t, Fe−H, JPH = 73 Hz). 31P{1H}
(202 MHz, CD2Cl2): δ 99 (s), 91 (s), 86 (s), 68 (s). Selective 31P-
2
1
NMR (242 MHz, CD2Cl2, −40 °C): δ 103.2 (s), 94.5 (s), 84.0 (s),
decoupled H NMR verified that the signals at δ 91 and 86 were
75.1 (s). Selective 31P decoupling of the H NMR verified that only
coupled to the hydride signal. FT-IR (CH2Cl2, cm−1): νCO = 1965,
1905, 1988.
1
the signals at δ 94 and 84 were coupled to the hydride, and presumably
these correspond to dibasal phosphines attached to the FeH center. At
the sample temperature of −10 °C, a triplet of triplets at δ −14.8 (Fe-
μH, JPH = 25, 5 Hz) appears in the 1H NMR spectrum, and singlets at
[Fe2(pdt)(μ-H)(CO)2(dppv)2]PF6 ([μ-H2]PF6). In a 250 mL
Schlenk flask, a dark green solution of 1 (200 mg, 0.187 mmol) in 5
mL of CH2Cl2 was treated at room temperature with a solution of 2.0
H
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Journal of the American Chemical Society
Article
M HCl in Et2O (0.2 mL, 0.2 mmol). Solvent was removed under
vacuum, and the solid was redissolved in MeOH. The product was
precipitated as its PF6 salt by addition of ca. 5 mL of a saturated
(2) Fontecilla-Camps, J. C.; Amara, P.; Cavazza, C.; Nicolet, Y.;
Volbeda, A. Nature 2009, 460, 814.
(3) Tard, C.; Pickett, C. J. Chem. Rev. 2009, 109, 2245.
(4) Rakowski DuBois, M.; DuBois, D. L. Acc. Chem. Res. 2009, 42,
1974. Catalysis without Precious Metals; Bullock, R. M., Ed.; Wiley-
VCH: Weinheim, 2010.
(5) Fontecilla-Camps, J. C.; Volbeda, A.; Cavazza, C.; Nicolet, Y.
Chem. Rev. 2007, 107, 4273.
(6) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C.
Science 1998, 282, 1853.
(7) Frey, M. ChemBioChem 2002, 3, 153.
−
aqueous solution of NH4PF6. The brownish precipitate was then
transferred anaerobically onto a Celite plug in a chromatography
column, where it was washed with 6 × 30 mL of H2O and 6 × 30 mL
of Et2O. The product was then extracted off the Celite with CH2Cl2,
and solvent was removed under vacuum. The product was dissolved
with 5 mL of CH2Cl2, which was diluted with 5 mL of MeOH.
Addition of 50 mL of hexane to this solution yielded a brown powder,
which was collected by filtration and dried. Yield: 170 mg (75%).
High-field 1H NMR (500 MHz, CD2Cl2): δ −14.5 (dddd, μ-H, JPH1
=
(8) Silakov, A.; Wenk, B.; Reijerse, E.; Lubitz, W. Phys. Chem. Chem.
Phys. 2009, 11, 6592.
24, JPH2 = 19, JPH3 = 10 Hz), −15.6 (tt, μ-H, JPH1 = 24, JPH2 = 6 Hz).
31P{1H} NMR (202 MHz, CD2Cl2): δ 89.6 (s), 89.6 (s), δ 76.7 (s),
82.8 (s), 84.2 (s), 87.8 (s). FT-IR (CH2Cl2, cm−1): νCO = 1963, 1951.
Anal. Calcd for C57H51F6Fe2O2P5S2 (found): C, 56.45 (56.28); H, 4.24
(4.28); Fe 9.21 (8.79).
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Electrochemistry. Electrochemical experiments were carried out
on CH Instruments model 600D series electrochemical analyzer or a
BAS-100 electrochemical analyzer. Cyclic voltammetry experiments
were conducted using a 10-mL one-compartment glass cell with a
tight-fitting Teflon top. The working electrode was a glassy carbon
(GC) disk (diameter = 3.00 mm). A Ag wire was used as a quasi-
reference electrode, and the counter electrode was a Pt wire. Ferrocene
was added as an internal reference, and each cyclic voltammogram
(CV) was referenced to this Fc0/+ couple = 0.00 V. iR compensation
was applied to all measurements using the CH Instruments or BAS
software. Cell resistance was determined prior to each scan, and the
correction applied to the subsequently collected CV. During
prolonged experiments, additional solvent was added to compensate
for evaporative loss. Between scans, the solution was purged briefly
with N2, and the working GC electrode was removed and polished.
The duration of typical electrochemical titrations was 30 min. For all
experiments, the electrolyte solution was prepared and sparged in the
cell, which was fitted with electrodes. A CV of the electrolyte was
collected prior to the addition of Fe2 compound, in order to check the
purity of the electrolyte. The diiron compound was then dissolved in
the electrolyte solution and transferred to the cell.
(18) Barton, B. E.; Zampella, G.; Justice, A. K.; De Gioia, L.;
Rauchfuss, T. B.; Wilson, S. R. Dalton Trans. 2010, 39, 3011.
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Due to their tendency to isomerize above 0 °C, [t-H1NH]+, [t-
2+
+
H1NH ] , and [t-H2] were generated in situ. The corresponding
2
−
bridging hydrides were isolated as their BF4 salts prior to
electrochemical experiments. All electrochemical experiments were
conducted at 0 °C.
(22) Winter, A.; Zsolnai, L.; Huttner, G. Z. Naturforsch. 1982, 37b,
1430.
ASSOCIATED CONTENT
* Supporting Information
Results from NMR, IR spectra, X-ray crystallographic, and
electrochemical analyses. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
S
(23) Lambert, J. B.; Featherman, S. I. Chem. Rev. 1975, 75, 611.
(24) Justice, A. K.; Zampella, G.; De Gioia, L.; Rauchfuss, T. B.; van
der Vlugt, J. I.; Wilson, S. R. Inorg. Chem. 2007, 46, 1655.
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AUTHOR INFORMATION
Corresponding Author
rauchfuz@illinois.edu
Notes
■
(26) Lawrence, J. D.; Li, H.; Rauchfuss, T. B.; Ben
M.-M. Angew Chem., Int. Ed. 2001, 40, 1768.
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M. J.; Lledos, A.; Maseras, F.; Manez, M. A. J. Am. Chem. Soc. 2007,
129, 6608.
(28) Das, P.; Capon, J.-F.; Gloaguen, F.; Pet
́
ard, M.; Rohmer,
́
dez-Trujillo,
́
́
̃
The authors declare no competing financial interest.
́
illon, F. Y.;
ACKNOWLEDGMENTS
Schollhammer, P.; Talarmin, J.; Muir, K. W. Inorg. Chem. 2004, 43,
■
8203.
This work was supported by the National Institutes of Health
(Grant GM61153). This work was also supported in part by the
ANSER Center, an Energy Frontier Research Center funded by
the U.S. Department of Energy, Office of Science, Office of
Basic Energy Sciences, under Award Number DE-SC0001059.
We thank Danielle Gray for collecting crystallographic data.
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Wilson, S. R. J. Am. Chem. Soc. 2008, 130, 12021.
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