Borane-protected cyanides as surrogates of h-bonded cyanides in [FeFe]-hydrogenase active site models

Article abstract of DOI:10.1021/ic500470z

Triarylborane Lewis acids bind [Fe₂(pdt)(CO)₄(CN) ₂]^2- [1]^2- (pdt^2- = 1,3-propanedithiolate) and [Fe₂(adt)(CO)₄(CN) ₂]^2- [3]^2- (adt^2- = 1,3-azadithiolate, HN(CH₂S^-)₂) to give the 2:1 adducts [Fe₂(xdt)(CO)₄(CNBAr₃) ₂]^2-. Attempts to prepare the 1:1 adducts [1(BAr ₃)]^2- (Ar = Ph, C₆F₅) were unsuccessful, but related 1:1 adducts were obtained using the bulky borane B(C₆F₄-o-C₆F₅)₃ (BAr ^F₃). By virtue of the N-protection by the borane, salts of [Fe₂(pdt)(CO)₄(CNBAr₃)₂] ^2- sustain protonation to give hydrides that are stable (in contrast to [H1]^-). The hydrides [H1(BAr₃)₂]^- are 2.5-5 pK_a units more acidic than the parent [H1]^-. The adducts [1(BAr₃)₂]^2- oxidize quasi-reversibly around -0.3 V versus Fc^0/+ in contrast to ca. -0.8 V observed for the [1]^2-/- couple. A simplified synthesis of [1] ^2-, [3]^2-, and [Fe₂(pdt)(CO) ₅(CN)]^- ([2]^-) was developed, entailing reaction of the diiron hexacarbonyl complexes with KCN in MeCN.

Full text of DOI:10.1021/ic500470z

Article
pubs.acs.org/IC
Borane-Protected Cyanides as Surrogates of H‑Bonded Cyanides in
[FeFe]-Hydrogenase Active Site Models
Brian C. Manor, Mark R. Ringenberg, and Thomas B. Rauchfuss*
School of Chemical Sciences, University of Illinois, Urbana, Illinois 61801, United States
*
S Supporting Information
ABSTRACT: Triarylborane Lewis acids bind [Fe (pdt)-
2
2
−
2−
2−
(
[
CO) (CN) ] [1] (pdt = 1,3-propanedithiolate) and
4 2
Fe (adt)(CO) (CN) ] [3]2− (adt = 1,3-azadithiolate,
2−
2−
2 4 2
−
HN(CH S ) ) to give the 2:1 adducts [Fe (xdt)-
2
2
2
2
−
(
[
CO) (CNBAr ) ] . Attempts to prepare the 1:1 adducts
1(BAr )] (Ar = Ph, C F ) were unsuccessful, but related
4 3 2
2
−
3
6 5
1
:1 adducts were obtained using the bulky borane B(C F -o-
6 4
F
C F ) (BAr * ). By virtue of the N-protection by the borane,
salts of [Fe (pdt)(CO) (CNBAr ) ] sustain protonation to
give hydrides that are stable (in contrast to [H1] ). The
6
5 3
3
2
−
2
4
3 2
−
−
hydrides [H1(BAr ) ] are 2.5−5 pK units more acidic than
3
2
a
−
2−
0/+
the parent [H1] . The adducts [1(BAr ) ] oxidize quasi-reversibly around −0.3 V versus Fc in contrast to ca. −0.8 V
3
2
2
−/−
2−
2−
−
−
observed for the [1]
couple. A simplified synthesis of [1] , [3] , and [Fe (pdt)(CO) (CN)] ([2] ) was developed,
2 5
entailing reaction of the diiron hexacarbonyl complexes with KCN in MeCN.
INTRODUCTION
Although cyanide is not compatible with many modeling
schemes owing to its ambidentate reactivity, nature prevents
such problems by encapsulating the active site in globular
■
A highlight in the history of bioorganometallic chemistry is the
discovery of cyanide ligands as cofactors in the [FeFe] and
NiFe] hydrogenases (H ases). The role of cyanide in H ases
proteins. The crystal structures of the [FeFe] H ases
1
2
[
2
2
demonstrate the significance of hydrogen bonding to cyanide
remains underdetermined since few models incorporating this
cofactor are available for testing, and those models that do exist
exhibit reactions highly complicated by the behavior of the
as observed in the 1.39 Å structure for the CpI enzyme from
13
Clostridium pasteurianum (Figure 1). The distal Fe−CN is
2
cyanide ligands. Hydrogen bonding between the protein
backbone and cyanide is known to anchor the active sites,
3
which are otherwise lightly tethered to the protein. Cyanide is
a powerful σ-donor ligand that in part serves to stabilize the
ferrous state, as illustrated by the redox inactivity of the
4
Fe(CN) (CO) center in the [NiFe] H ases. The situation is
2
2
different in the [FeFe] H ases since each Fe center carries one
2
5
cyanide and at least one of the Fe centers undergoes redox.
In synthetic analogues, cyanide interferes with the two key
attributes of functional models: acid/base and redox reactions.
2
−
2−
Thus, even though salts of [Fe (pdt)(CO) (CN) ] (pdt
1
=
2
4
2
2
−
6,7
,3-propanedithiolate), [1] , were described in 1999, little
Figure 1. Active site of [FeFe] H ase (PDB 3C8Y) depicting
hydrogen bonding environment to the cyanide ligands.
2
work has been reported since their initial discovery in part
because the products of protonation and oxidation of [1]2 are
unstable. Protonation of (Et N) [1] occurs initially at the basic
−
4
2
Fe−CN followed by formation of the μ-hydrido derivative
Et N[(μ-H)Fe (pdt)(CO) (CN) ] (Et N[H1]), which exists
strongly hydrogen-bonded to the ammonium center of Lys358
4
2
4
2
4
8
,9
14
as two major isomers. The salt Et N[H1] is labile, with a half-
(N···N distance = 2.74 Å). The lysine group is highly
4
life of minutes in solution. Both chemical and electrochemical
conserved, and this specific hydrogen-bonding interaction has
3
,15
2−
10
been shown to be critical for activity.
The proximal cyanide
studies indicate that oxidized derivatives of [1] are unstable.
As a consequence of the problems associated with cyanide-
centered reactions, most functional models feature phosphine
ligands as surrogates.
Received: February 27, 2014
Published: July 3, 2014
1
1,12
©
2014 American Chemical Society
7241
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Inorganic Chemistry
Article
+
is also weakly hydrogen bonded to Ser232 (N···O distance =
Table 1. IR Bands in the ν and ν Regions for Et N and
K Salts of [1] , [2] , and [3]
CN
CO
4
+
2−
−
2−
2.92 Å).
The present work is premised on the idea that hydrogen
compound
νCN (cm−1)
νCO (cm−1
)
bonding to Fe−CN centers can be simulated by FeCN−BR3
interactions. The ability of triarylboranes to bind to cyanide
ligands is well-established as exemplified by trans-RuH-
a
(
Et N) [1]
2075
2077
2094
2092
2075
2078
1964, 1924, 1885
1967, 1928, 1890
4
2
a
K [1]
2
b
Et N[2]
2029, 1974, 1955, 1941, 1917
2031, 1976, 1956, 1946, 1916
1969, 1925, 1892
(
(
CNBPh )(diphos) , Ni(diphos)(π-allyl)(CNBPh ), [M-
4
3
2
3
a
F
2−
F
K[2]
CNBAr ) ] (M = Ni, Pd; BAr = B(C F ) ), and
3
4
3
6 5 3
a
F
16−18
(Et N) [3]
FeCp(CNBAr )(CO) (diphos = diphosphine).
vated by interest in frustrated Lewis pairs (FLPs) and the role
of boranes in alkene-polymerization, the chemistry of boranes
Moti-
4
2
3
2
a
19
K [3]
1969, 1929, 1893
2
20
a
b
MeCN solution. THF solution, ref 10.
21
has rapidly developed, as have methods to evaluate their
Lewis acidities. Recently, BArF was used to convert the
2
2
2−
3
[Fe (pdt)(CO) (CN) ] +2BAr → [Fe (pdt)(CO) (CNB
2
4
2
3
2
4
unreactive species (CO) (CN) Fe(pdt)Ni(dxpe) (dxpe =
2−
2−
2
2
[1]
[1(BAr ) ]
3 2
dppe, dcpe) into hydride-containing model catalysts capable
23
(1)
of catalytic oxidation of H2. In parallel with these results,
F
In addition to BAr , the weaker Lewis acids BPh and B(2,4,6-
C H F ) (BAr 3) also formed related adducts. Treatment of a
CH Cl solution of (Et N) [1] with 2 equiv of these Lewis
borane Lewis acids were examined with [FeFe]-H ase active
3
3
2
F#
site models, as described below.
6
2 3 3
2
2
4
2
RESULTS
acids resulted in lightening of the solution color from red to
orange. Upon binding of the borane, IR bands in the νCO and
νCN regions shift to higher frequencies (Table 2). On the basis
■
(
−
Synthesis of [Fe (pdt)(CO) (CN)] and [Fe (xdt)-
2
5
2
2
−
CO) (CN) ]
(xdt = adt, pdt). A new synthesis was
4
2
developed for cyanide-containing [FeFe]-H ase models using
KCN as a cyanide source (Scheme 1). Quaternary ammonium
2
Table 2. IR Bands for (Et N) [1(BAr ) ] and Related
4
2
3 2
Complexes in CH Cl Solution
2
2
Scheme 1
νCN (cm−1
νCO (cm−1
)
compound
Et N) [1]
)
(
(
(
(
(
2075
2137
2136
2147
2136
1964, 1924, 1885
1984, 1946, 1911
1986, 1949, 1914
1986, 1947, 1915
1990, 1954, 1922
1979, 1942, 1889
1997, 1972, 1933
4
2
Et N) [1(BPh ) ]
4
2
3 2
Et N) [3(BPh ) ]
4
2
3
2
F#
Et N) [1(BAr ) ]
4
2
3 2
F
3 2
Et N) [1(BAr ) ]
4
2
3
1
Fe (pdt)(CO) (PMe )
3 2
2
4
t
32
Fe (pdt)(CO) (CN Bu)
2
2145
2
4
of the νCO bands for the adducts, the relative Lewis acidities of
+
F
F#
salts (R N , R = Et, Bu) salts of cyanide had previously been
the boranes were BAr > BAr
3
3
> BPh
computational methods, and other
The coordination of Lewis acids greatly increased
the solubility of the complexes. Whereas the uncapped
compound (Et N) [1] is soluble in MeCN and CH Cl , the
adducts [1(BAr are soluble in THF and, in the case of
] , even in Et O.
Rapid turnstile rotation of Fe(CO)
indicated by observation of a single BAr
3
, as established by
4
6
,7
22,27,28
used to generate the same anions. The salt K [1] formed in
good yields upon treating Fe (pdt)(CO) with an excess of
KCN in refluxing MeCN. The salt K [Fe (adt)(CO) (CN) ]
adt = 1,3-azadithiolate, HN(CH S ) ) (K [3]) was prepared
analogously, also in good yield. The potassium salts are soluble
in MeCN and even dissolve in tetrahydrofuran (THF) and
Et O in the presence of small amounts of MeCN. As we show
below, the K in these salts usefully modifies the basicity of the
Fe−CN centers, allowing the synthesis of complexes
unattainable as their Et N salts. The monocyanides K-
Fe (pdt)(CO) (CN)] (K[2]) and K[Fe (adt)(CO) (CN)]
were observed as respective intermediates in the formation of
K [1] and K [3]. Using a slight deficiency of KCN, K[2] could
Child’s methods,
29,30
2
studies.
2
6
2
2
4
2
2−
−
(
4
2
2
2
2
2
2
2−
) ]
3 2
F
2−
[1(BAr
)
3
2
2
(CNBAr ) centers was
2 3
2
+
environment in the
3
1
2−
F#
2−
H NMR spectra of [1(BPh
)
]
and [1(BAr
)
] .
and
3
2
3
2
+
19
F#
2−
Similarly, the F NMR spectra of [1(BAr ) ]
[1(BAr ) ] showed only a single averaged environment.
4
3 2
F
2−
[
2
5
2
5
3 2
11
The Fe−CNBAr linkage was confirmed by B NMR spectra.
F 2−
3
The 11B NMR spectrum of [1(BAr ) ] adducts feature a
2
2
3 2
be prepared in excellent yield from the hexacarbonyl. Like
broadened signal at δ −14. For the alternative linkage isomer
18
Et N[2], K[2] exhibits good solubility in MeCN and THF.
Fe−NC−BAr , a sharp signal is expected near δ −20.
4
3
11 2−
In the IR spectra of K [1] and K [3], the ν and νCN bands
Similarly, the B NMR spectrum of [1(BPh ) ] exhibits a
broad signal at δ −4.5, consistent with a M−CNBPh
linkage. The B NMR spectrum of [1(BAr ) ] exhibits
2
2
CO
3 2
+
−
occur at slightly higher energy than the analogous Et N
4
3
1
0,24
17
11
F#
2−
compounds (Table 1).+
This shift may be attributed to
3 2
the coordination of K to the cyanide ligands as observed
a broad signal at δ −15.
The structure of (Et N) [1(BAr ) ] was confirmed by X-ray
25
F
previously for K[CpFe(CO)(CN)2]. The salt K [1] was
2
4
2
3 2
found to undergo anion exchange with PPNCl and Et NCl in
crystallography (Figure 2). Reassuring for our modeling efforts,
no obvious structural differences exist between
4
MeCN, shifting νCN and νCO to lower energy, matching the
6
,26
F
previously reported IR spectrum.
:1 Borane Adducts of [Fe (pdt)(CO) (CN) ] . The
(Et N) [1(BAr ) ] and the previously reported structure of
4
2
3 2
2
−
10
2
(Et N) [1] (Supporting Information, Table S3). For
2
4
2
4 2
F
2−
Lewis acid BAr 3 binds to the nitrogen centers of [1] to give
example, the Fe−Fe, CN, Fe−CO, and CO distances
differ by less than 0.02 Å. Slight shortening of the Fe−CN bond
stable adducts (eq 1).
7
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Inorganic Chemistry
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F
Figure 3. IR spectra of K [1] in MeCN (top), K [1(BAr * )] in
2
2
3
F
CH Cl (middle), and K [1(BAr * ) ] in CH Cl (bottom).
2
2
2
3
2
2
2
F
2−
Figure 2. Thermal ellipsoid plot of [1(BAr ) ] at 50% probability
3
2
and with H atoms omitted for clarity. Pentafluorophenyl groups are
deemphasized for clarity.
solvent (CH Cl vs MeCN). The solvent effect is reversible. In
2
2
F
contrast, the IR spectrum for (Et N) [1(BAr * ) ] was
4
2
3 2
unchanged in these solvents (Table 3).
F
is observed upon coordination of BAr , with the Fe−CN bond
3
F
0
.06 Å shorter for (Et N) [1(BAr ) ] than for (Et N) [1].
Table 3. IR Bands for K [1] and (Et N) [1] and Their
4
2
3 2
4
2
2
4
2
2
−
F
2
:1 Borane Adducts of [Fe (adt)(CO) (CN) ] . Given
BAr * Adducts
2
4
2
3
2−
the basicity of the NH center in [3] , adducts of this complex
compound
solvent νCN (cm−1)
νCO (cm−1
)
with boranes were anticipated to be more complex than they
2
−
(Et N) [1]
MeCN
MeCN
MeCN
2075
2077
1964, 1924, 1885
1967, 1928, 1890
4
2
were for [1] . IR measurements indicated that addition of 2
F
K [1]
2
equiv of BAr 3 to (Et N) [3] resulted in an unstable complex.
4
2
F
K [1(BAr * )]
2
2096, 2081 1990, 1978, 1942, 1931,
913, 1888
3
The less Lewis-acidic BPh afforded the desired 2:1 adduct
3
1
(
Et N) [Fe (adt)(CO) (CNBPh ) ], (Et N) [3(BPh ) ]. The
F
4
2
2
4
3 2
4
2
3 2
K [1(BAr * )]
CH Cl2 2126, 2058 1985, 1950, 1905
2
2
3
IR spectrum of this salt is very similar to that for
F
K [1(BAr * ) ]
MeCN
2098
1991, 1959, 1947, 1932,
1910
1
2
3 2
(
Et N) [1(BPh ) ] (Table 2). Its H NMR spectrum displayed
4 2 3 2
F
a single BPh environment, consistent with both BPh groups
K [1(BAr * ) ]
CH Cl2 2119
1989, 1951, 1916
1991, 1932
3
3
2
3
2
2
F
being bound to Fe-CN centers and rapid rotation of the
Fe(CO) (CNBPh ) subunits.
(Et N) [1(BAr * ) ]
MeCN
2098
4
2
3 2
F
2
3
(Et N) [1(BAr * ) ]
4
2
3
2
CH Cl2 2100
2
1990, 1928
2
−
1
:1 Borane Adducts of [Fe (pdt)(CO) (CN) ] . Attempts
were made to generate the 1:1 borane adducts of [1] . As
indicated by IR spectroscopy, treatment of (Et N) [1] with 1
2 4 2
2
−
4
2
F
−
equiv of BAr 3 or BPh gave a mixture of (Et N) [1(BAr ) ],
Hydride Derivatives [(μ-H)Fe (xdt)(CO) (CNBAr ) ]
2 4 3 2
3
4
2
3 2
starting material, and insoluble solids (that do not redissolve in
(xdt = adt, pdt). In contrast to the complications reported
2−
9
the presence of boranes). In an effort to stabilize 1:1 adducts,
for the protonation of [1] , protonation of the borane
F
2−
the bulky borane B(C F -o-C F ) (BAr * ) was examined. The
adducts [1(BAr3)2] proceeded straightforwardly (eq 3).
6
4
6
5 3
3
F
reaction of (Et N) [1] with 1 equiv of BAr * precipitated
intractable solids, even when the addition was conducted slowly
4
2
3
Fe (pdt)(CO) (CNBAr ) ] + H+
2−
[
2
4
3 2
F
and on dilute solutions. When 2 equiv of BAr * were added
−
3
→
[(μ‐H)Fe (pdt)(CO) (CNBAr ) ]
(3)
F
2
4
3 2
quickly, the 2:1 adduct (Et N) [1(BAr * ) ] formed in good
4
2
3 2
yield.
Exploiting their distinctive solubility, the borane complexes
(Et N) [1(BAr ] were protonated as an ether or THF
solution with excess HCl·Et O. Protonation is accompanied by
a lightening in the solution color and precipitation of Et NCl.
The formation of 1:1 adducts required starting with K [1]
4
2
)
3 2
2
+
instead of the Et N salt. Treatment of K [1] with 1 equiv of
2
4
2
F
F
BAr * gave K [1(BAr * )] (eq 2).
4
3
2
3
Protonation causes a shift of both νCO and νCN to higher
F
K [Fe (pdt)(CO) (CN) ] + BAr *
2
2
4
2
3
energies, consistent with oxidation of the diiron centers (Table
4).
F
→
K [Fe (pdt)(CO) (CN)(CNBAr * )]
(2)
2
2
4
3
1
The hydrido complexes exhibit characteristic high-field H
1
−
The IR spectrum of this adduct shows ν bands at disparate
NMR signals. H NMR spectra of [H1(BAr ) ] in CD Cl
CN
3 2 2 2
−
1
energies (2126, 2058 cm ), consistent with the coordination
solution display two or three hydride signals, depending on the
F
of BAr * to a single cyanide ligand (see Figure 3).
borane. Ignoring the conformation of the Fe pdt center, four
3
2
F
−
The salt K [1(BAr * ) ] was produced in good yield. This
isomers are possible for [H1(BAr ) ] , three of which are
2
3 2
3 2
−
pale red solid exhibits good solubility in ether and THF, as
symmetrical with equivalent CNBAr3 ligands (Figure 4). All
F
found for (Et N) [1(BAr ) ]. The IR spectra of
K [1(BAr * )] and K [1(BAr * ) ] were found to vary with
four isomers are observed for the [(μ-H)Fe (pdt)-
4
2
3
2
2
F
F
t
+
32
(CO) (CN Bu) ] system.
2
3
2
3 2
4 2
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Inorganic Chemistry
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Table 4. IR Bands in νCN and νCO Regions for Et N[H1(BAr ) ] and Related Complexes
4
3 2
compound
solvent
νCN (cm−1)
νCO (cm−1
)
Et N[H1]
MeCN
2118, 2112
2180
2045, 2024, 1985
2064, 2043, 2009
2066, 2046, 2012
2071, 2051, 2022
2029, 1989
4
Et N[H1(BPh ) ]
4
CH Cl2
3
F#
2
2
Et N[H1(BAr ) ]
4
CH Cl2
2194
3
2
2
F
3
Et N[H1(BAr ) ]
4
CH Cl2
2185
2
2
3
1
[
[
(μ-H)Fe (pdt)(CO) (PMe ) ]BF
CH Cl2
2
4
3
2
4
2
t
32
(μ-H)Fe (pdt)(CO) (CN Bu) ]BF
CH Cl2
2187
2076, 2060, 2028
2
4
2
4
2
−
−
Figure 4. Isomers of [H1(BAr ) ] . Only for the a/b isomer are the CNBAr ligands nonequivalent.
3
2
3
F
−
F
−
Freshly generated solutions of [H1(BAr ) ] consist of
[H1(BAr ) ] was calculated to be 10.8 ± 0.2. Examination of
3 2
3
2
1
F#
2−
MeCN
three isomeric hydrides, indicated by H NMR signals at δ
mixtures of [1(BAr ) ] and pyridinium (pKa
12.53) gave a pK
[H1(BPh ) ] with pyridine did not cleanly reform
=
3
2
33
MeCN
−
13.7, −16.4, and −19.0 in a ratio of 1:11:5. Over the course of
of 13.5 ± 0.1. Deprotonation of
a
−
2
d, the isomer ratio shifted to 1:36:32. The structures of these
3
2
1
9
2−
isomers can be partially inferred from the F NMR spectra,
[1(BPh ) ] , probably because the adducts of this less-acidic
3
2
since the p-F signals are well-resolved. On the basis of these
borane are labile. The values for ν
are similar for
CO
1
F#
−
−
data, the H NMR signal at δ −16, which corresponds to the
kinetically favored species, is assigned to the unsymmetrical
apical/basal (a/b) isomer. The other two hydride signals arise
[
H1(BAr ) ] and [H1(BPh ) ] , suggesting that their pK
3 2 3 2 a
values are probably comparable. The hydridic nature of
F
−
[
H1(BAr ) ] was investigated by treatment of a CD Cl
3
2
2
2
1
9
from symmetrical complexes (single F NMR signals),
F
−
F24
1
solution of [H1(BAr ) ] with Me NHBAr
analysis failed to indicate any interaction.
The presence of adt in [FeFe]-H ase models has been
shown to greatly affect the protonation pathway. The
protonation of (Et N) [3] has been previously reported as
. H NMR
3
2
3
probably the b/b and a/a isomers.
1
F#
−
The H NMR spectrum of [H1(BAr ) ] consisted of only
2−
3
2
2
two high-field signals, at δ −16.4 and −19.2, in a 3:1 ratio. After
equilibration over the course of 3 d, this ratio shifted to 1:1.5.
4
2
In the 19F NMR spectrum of [H1(BAr ) ] , the ratio of the
F#
−
−
3
2
occurring at the amine, forming [Fe (Hadt)(CO) (CN) ] , [α-
H3] , followed by slow proton transfer over days to form the
2
4
2
o-F and p-F signals compared to the ratio of the hydride signals
are consistent with one symmetric and one asymmetric isomer
Supporting Information). The species with the H NMR
signal at δ −16.4 is assigned to the a/b isomer on the basis of
the relative intensities of H and F NMR signals.
Freshly generated solutions of [H1(BPh ) ] consisted of
−
−
unstable bridging hydride [(μ-H)Fe (adt)(CO) (CN) ] ,
1
2
4
2
(
−
34
[
[
H3] .
Unlike the straightforward protonation of
2−
2−
1(BAr ) ] , protonation of [3(BPh ) ] proved complicated
1
19
3 2
3 2
F6
and sensitive to solvent. Using H(Et O) B(Ar ) protonation
−
2
2
4
3
2
in MeCN solution resulted in N-protonation (Δν ≈ 10
1
CN
three isomers, indicated by H NMR signals at δ −13.3, −16.1,
−1
−1
cm , Δν ≈ 20 cm ), whereas protonation in a CH Cl
CO
2
2
and −19.2 in a ratio of 1.6:4.3:1. This mixture isomerizes over
1
solution produced some bridging hydrides (δ −15.3 and δ
the course of 2 d to a 1:11:11 ratio. The H NMR spectrum of
F6
−
−18.4) (Ar = C
H -3,5-(CF ) , Supporting Information).
6 3 3 2
[
H1(BPh ) ] features four signals assigned to the 2,6-protons
on BPh , the ratio of which are consistent with two symmetric
isomers and one asymmetric isomer. On the basis of the
integrations of the hydride signals and ortho-aryl signals of
H1(BPh ) ] , the high-field signal at δ −16.14 could be
assigned as the a/b isomer. The H NMR signals for the
symmetrical isomers of [H1(BAr ) ] are assigned in analogy
to the assignments for [(μ-H)Fe (pdt)(CO) (CN Bu) ] :
The most downfield hydride signal (δ −13.5) is assigned to one
of the b/b isomers, presumably trans for electrostatic reasons,
and the most upfield hydride signal (δ −19.0) is assigned to the
a/a isomer.
3
2
2−
Electrochemistry of [Fe (pdt)(CO) (CNBAr ) ] and
2
4
3 2
3
−
[
(μ-H)Fe (pdt)(CO) (CNBAr ) ] . According to its cyclic
2 4 3 2
voltammogram, a freshly prepared CH Cl2 solution of
(Et N) [1] irreversibly oxidizes at −0.79 V (Figure 5, all
potentials vs Fc ). The process is accompanied by the
appearance of a deposit on the working electrode. In contrast,
(Et N) [1(BAr ) ] was found to oxidize quasi-reversibly
without fouling the electrodes. The oxidation potentials vary
between −0.23 and −0.36 V depending on the Lewis acid
Table 5).
2
−
4
2
[
3
2
0/+
1
−
3
2
t
+
32
4
2
3 2
2
4
2
(
The redox properties of Et N[H1] have not been evaluated
because of the compound’s instability. Borane-capping allowed
for the observation of a quasi-reversible reduction for
4
The boranes noticeably affect the basicity of the diiron unit.
MeCN
−
The pKa
of [H1] is estimated to be 16 as it is fully
MeCN
33
F
0/+
deprotonated by 1 equiv of benzylamine (pK
= 16.91).
Et
[1(BAr
4
N[H1(BAr
3
)
2
] at −1.65 V vs Fc . The potentials of the
a
2−
−
2−/−
−/2−
Protonation of [1] to give [H1] occurs with one equiv 2,4,6-
3
)
2
]
and [H1(BAr
3
)
2
]
couples correlated with
MeCN
33
trimethylpyridinium (pKa
= 14.98). As observed with
the Lewis acidity of the borane (Table 5). Reduction of
−
F
−
F#
−
−
F#
−
[
H1] , both [H1(BAr ) ] and [H1(BAr ) ] can also be
[H1(BPh ) ] and [H1(BAr ) ] occurred near −1.74 V;
3 2 3 2
3
2
3 2
1
9
reversibly deprotonated. By F NMR spectroscopy using
however, reversibility was only observed at scan rates above 0.5
MeCN
33
MeCN
F#
anilinium (pKa
= 10.62) as the acid, the pKa
of
V/s. Interestingly, BPh and BAr had similar effects on the
3
3
7
244
dx.doi.org/10.1021/ic500470z | Inorg. Chem. 2014, 53, 7241−7247

Inorganic Chemistry
Article
H ase models do not engage in intramolecular hydrogen
2
3
7
II
bonding. Underscoring the nonhydridic nature of Fe (μ-
H)Fe species, we observed no interaction between
H1(BAr ) ] and Me NH , despite an electrostatic driving
II
−
+
[
3 2 3
force.
The strengths of the hydrogen bonds from the protein to the
−
two CN cofactors differ, as reflected by the distances between
distal
hydrogen-bonded heteroatoms. The Fe −CN···N (lysine ε-
proximal
ammonium) distance is 2.74 Å, whereas the Fe
(
−CN···O
13
serine hydroxyl) distance is 2.92 Å. To simulate the disparate
nature of these two hydrogen-bonding interactions, attempts
2−
were made to bind 1 equiv of a Lewis acid to [1] . The
F
F
reaction of 1 equiv of BAr 3 and BAr * with (Et N) [1] gave
3
4
2
intractable products. The targeted asymmetrically protected
dicyanide complex was obtained by combining the bulky
F
borane BAr * and K [1]. We propose that this adduct is
3
2
+
Figure 5. Cyclic voltammograms of (Et N) [1] and
stabilized by binding of K to the uncapped FeCN center. This
4
2
F
3
(
Et N) [1(BAr ) ] (1 mM solutions in CH Cl ) with 100 mM
interaction is consistent with the solvent dependence of the IR
4
2
2
2
2
F
F
Bu NPF as electrolyte recorded at 0.1 V/s.
4
6
spectra of K [1(BAr * )] and K [1(BAr * ) ]. In CH Cl
2 3 2 3 2 2 2
F
solutions, BAr * coordinates strongly to Fe−CN, whereas in
3
+
F
MeCN solutions K appears to compete with BAr * for
3
redox potentials even though their Lewis acidities are quite
different.
2
8,29,35
coordination to Fe−CN. The IR spectrum of
F
(
Et N) [1(BAr * ) ] was found not to vary with solvent.
4
2
3 2
F −
In terms of ligand properties, how does CNBAr 3 compare
DISCUSSION
■
with related ligands? The answer depends on the method of
A new synthesis of [1] and [3]2− was developed involving the
use of KCN. The method offers advantages over the traditional
2−
measurement. Using the criterion of νCO, CNBAr3− is less basic
31
than PMe . Similar ranking is indicated when comparing the
3
route: not only is KCN cheaper and safer to use than Et NCN,
it is commercially available in isotopically labeled forms. Once
isolated, K [1] undergoes anion exchange to afford the
corresponding Et N and PPN salts. In contrast to the use
MeCN
38
4
pKa
of the corresponding hydrido complexes. On the
2−/−
other hand, in terms of redox properties, the [H1(BAr ) ]
couple (ca. −1.7 V vs Fc ) is far more cathodic than the
3
2
0/+
2
+
+
+/0
39
4
couple [HFe (pdt)(CO) (PMe ) ] (−1.39 V).
2
4
3 2
of Et NCN, substitution using KCN proceeds via the readily
2−
4
Now that adt has been established as the dithiolate
cofactor, the question remains: what steps are required to
and [(μ-H)Fe (adt)-
CO) (CN) ] into functioning catalysts? Previously the
isolated intermediate K[Fe (xdt)(CO) (CN)].
40
2
5
The coordination of the boranes to [1] and [3]2−
suppresses reactions at the Fe−CN sites, which complicated
previous studies on the protonation and redox of these very
appealing models. Capping the Fe−CN sites with boranes
2−
2−
convert [Fe (adt)(CO) (CN) ]
2
4
2
2
−
(
4 2
barrier to this activation was the interfering reactivity of the
cyanide ligands, which are both bases and potentially bridging
ligands. This distracting behavior has been addressed with the
introduction of the borane protecting groups, yet the diferrous
hydride remains bridging. One obvious gap is the absence of
the 4Fe−4S cluster−ligand on the proximal Fe center. Beyond
that omission, the rotated structure may require a more
protein-like environment, especially a hydrogen-bonding
2
−
allowed for straightforward protonation of [1(BAr ) ] to give
the μ-hydrido derivatives [H1(BAr ) ] . Unlike [H1] ,
[
[
which is similar for [(μ-H)Fe (pdt)(CO) (CN Bu) ] .
Dihydrogen bonds between a metal hydrides and a protic
group is a key interaction in the evolution and oxidation of
hydrogen by H ases. “Hydridic hydrides” are expected to form
dihydrogen bonds with acids. Indeed, crystallographic study
of the [FeFe]-H ase model [HFe (Hadt)(CO) (dppv) ] ,
which contains both an N-protonated adt and terminal iron
hydride, reveals a short NH···HFe distance of 1.8 Å (dppv =
cis-C H (PPh ) ). Similarly, NMR measurements indicate
intermolecular dihydrogen bonding between [(CO)-
(
3
2
−
−
3
2
−
H1(BAr ) ] was readily isolated and handled. In solution,
H1(BAr ) ] exists as a mixture of isomers, the distribution of
3 2
−
3
2
t
+
32
2
4
2
3,41
interaction directed at a basal cyanide on the distal Fe.
2
3
6
EXPERIMENTAL SECTION
2+
■
2
2
2
2
Materials and Methods. Standard Schlenk or glovebox
techniques were used. Solvents used for syntheses were high-
performance liquid chromatography grade, further purified by using
an alumina filtration system (Glass Contour Co., Irvine, CA), and were
deoxygenated prior to use. NMR solvents were purchased from
1
2
2
2
2 2
F
−
+
23
CNBAr ) Fe(H)(pdt)Ni(dppe)] and Me NH . In con-
Cambridge Isotope Laboratories, dried with CaH , and stored under
3
2
3
2
trast, we show in this work that the bridging hydride in [FeFe]-
nitrogen over activated molecular sieves. The preparations of
2
−
−
0/+
Table 5. Redox Potentials (V) and Current Ratios of Adducts of [1(BAr ) ] and [H1(BAr ) ] in CH Cl vs Fc (Scan Rate
3
2
3 2
2
2
ν = 0.1 V/s)
2−/−
−/2−
−/0
BAr3
none
E1/2 [1(BAr ) ]
i /i
E1/2 [H1(BAr ) ]
i /i
E1/2[H1(BAr ) ]
i /i
3
−0.79
−0.36
−0.34
−0.23
2
pa pc
3
2
pc pa
3
2
pa pc
irrev.
0.16
0.69
0.82
a
BPh3
−1.72
−1.76
−1.65
irrev. (0.20)
F#
a
BAr
0.21 (0.44)
0.28 (0.51)
1.11
1.24
0.76
0.57
3
BArF
a
3
a
ν = 1.0 V/s.
7
245
dx.doi.org/10.1021/ic500470z | Inorg. Chem. 2014, 53, 7241−7247

Inorganic Chemistry
Article
2
4,42
BArF was
(
Et N) [1] and (Et N) [3] are described elsewhere.
passed through Celite and then evaporated under vacuum to yield a
tacky red solid, which was triturated with pentane (3 × 10 mL). Yield:
50 mg (68%). Anal. Calcd for C BFe S H O N F K ·0.33CH CN·
4
2
4
2
3
purchased from Boulder Scientific and twice sublimed under vacuum
at 90 °C prior to use. BPh was prepared by pyrolysis of
3
45
2
2
6
4
2
27
2
3
43
F
F#
HNMe BPh . The preparations of BAr * and BAr have been
OEt (found): C, 39.66 (40.03); H, 1.14 (0.88); N, 2.17 (2.03). IR
3
4
3
3
2
3
0,44
reported.
1
IR spectra were recorded using a PerkinElmer Spectrum
(MeCN): 2096, 2081, 1990, 1978, 1942, 1931, 1913, 1888. IR
(CH Cl ): 2100, 1990, 1927.
00 FT-IR instrument using a CaF solution cell. These data are
2
2
2
−1
F
F
reported in cm . Elemental analyses were conducted at the University
of Illinois Microanalysis Laboratory. Cyclic voltammetry was
performed under nitrogen at room temperature using a CHI 630D
potentiostat with glassy carbon working electrode, Pt wire counter
electrode, pseudoreference electrode Ag wire, and with ferrocene as an
internal standard. NMR spectra were recorded on a Varian Mercury
K [Fe (pdt)(CO) (CNBAr * ) ], K [1(BAr * ) ]. A solution of K [1]
2 2 4 3 2 2 3 2 2
(24 mg, 0.052 mmol) in MeCN (5 mL) was treated with a solution of
F
BAr *
then removed under vacuum. The residue was extracted into 5 mL of
Et O, and this extract was treated with 20 mL of pentane, precipitating
(100 mg, 0.11 mmol) in MeCN (10 mL). The solvent was
3
2
a red oil that was triturated with pentane (3 × 10 mL) to yield a red
solid. Yield: 84 mg (68%). IR (MeCN) 2098, 1991, 1959, 1947, 1932,
5
00 MHz spectrometer, and the 1H NMR chemical shifts were
1
1
referenced to the residual protons of deuterated solvents.
recorded on a Varian 400 MHz instrument) and F NMR spectra
were referenced to external standards of BF ·Et O and 1% CFCl in
B
1909. IR (CH
Et N[(μ-H)Fe
of (Et N) [1(BAr )
3 2
2
Cl
2
): 2119, 1989, 1951, 1916.
(CNBArF N[H1(BArF
], Et
] (0.282 g, 0.169 mmol) in ether (50 mL) was
19
(
4
(pdt)(CO)
2
4
3
)
2
4
) ]. A solution
3 2
F
3
2
3
4
2
CDCl , respectively.
treated with HCl in ether (0.093 mL, 2 M, 0.186 mmol). The reaction
solution color became lighter, and a colorless precipitate formed. The
reaction solution was stirred for 30 min and was filtered through a plug
3
K [Fe (pdt)(CO) (CN) ], K [1]. A Schlenk flask was charged with
Fe (pdt)(CO) (1.0 g, 2.6 mmol), KCN (0.51 g, 7.8 mmol), and
MeCN (100 mL). The mixture was heated at reflux for 28 h, with
monitoring by IR spectroscopy. The mixture was filtered through
Celite, and the filtrate was evaporated under vacuum. The resulting
solid was washed with Et O (50 mL) and hexanes (50 mL) and then
dried under vacuum. Yield: 0.91 g (76%). Anal. Calcd for
C H Fe K N O S ·0.33CH CN (found): C, 24.49 (24.35); H, 1.49
1.64); N, 6.81 (6.88). IR (MeCN): 2077, 1967, 1928, 1890.
K[Fe (pdt)(CO) (CN)], K[2]. A two-necked Schlenk flask was charged
with Fe (pdt)(CO) (0.50 g, 1.30 mmol), 0.9 equiv of KCN (0.076 g,
.17 mmol), and MeCN (50 mL). The mixture was heated at reflux for
h, with monitoring by IR spectroscopy. The mixture was filtered
through Celite, and the filtrate was dried under vacuum. The resulting
2
2
4
2
2
2
6
of Celite to remove Et NCl. The filtrate was collected, the solvent was
4
evaporated to ca. 10 mL, and pentane (20 mL) was added. The
resulting red solid was extracted into ether (10 mL), and this extract
was layered with pentane (30 mL) to yield yellow microcrystals. Yield:
2
107 mg (44%). Anal. Calcd for C53
B
Fe
S
H
O
N
F
30 (found): C,
Cl ): 2185,
): δ −13.7 (s, Fe-H), −16.4 (s,
Fe-H), −19.0 (s, Fe-H). F NMR (CD Cl ): δ −134.8 (dd, o-F),
−134.9 (m, o-F), −160.4 (t, p-F), −160.8 (t, p-F), −160.9 (t, p-F),
2
2
2
27
4
3
41.41 (41.29); H, 1.77 (1.75); N, 2.73 (2.66). IR (CH
9
6
2
2
2
4
2
3
2
2
1
(
2071, 2051, 2022. H NMR (CD Cl
2 2
19
2
5
2
2
2
6
1
1
1
6
−166.2 (m, m-F), −166.5 (td, m-F), −166.6 (td, m-F). B NMR
(CD
Et
of (Et
Cl
4
): δ −15.
N[(μ-H)Fe (pdt)(CO)
N) [1(BAr
2
2
(CNBArF#
) ] (100 mg, 0.069 mmol) in THF (1 mL) was
3 2
) ], Et ) ]. A solution
N[H1(BArF#
2
4
3 2 4 3 2
F#
oil was extracted into Et O, and product was precipitated with
4
2
2
pentane. The resulting solid was collected by filtration, washed with
pentane (50 mL), and dried under vacuum. Yield: 0.42 g (85%). Anal.
Calcd for C H Fe KNO S (found): C, 25.55 (25.62); H, 1.43 (1.71);
treated with a solution of HCl in ether (0.1 mL, 2M, 0.2 mmol),
resulting in the color of the solution lightening and the appearance of
some solid precipitate. The reaction solution was cooled to −30 °C to
9
6
2
5 2
N, 3.31 (3.17). IR (MeCN): 2092, 2031, 1976, 1956, 1946, 1916.
precipitate Et NCl and was then filtered through a plug of Celite. The
4
F
F
3 2
(
Et N) [Fe (pdt)(CO) (CNBAr ) ], (Et N) [1(BAr ) ]. A slurry of
solution was evaporated under vacuum, and the resulting oil was
triturated with pentane (3 × 10 mL) and left to dry under vacuum
overnight, yielding a red solid. Yield: 91 mg (99%). Anal. Calcd for
4
2
2
4
3 2
4
2
(
Et N) [1] (2.40 g, 3.74 mmol) in CH Cl (50 mL) was treated with
4 2 2 2
F
BAr 3 (3.88 g, 7.57 mmol) in CH Cl (10 mL). After stirring the
2
2
C H B F Fe N O S (found): C, 48.18 (48.51); H, 2.97 (3.17); N,
reaction solution for 30 min, the solvent was evaporated to yield a red
oil, which a red powder upon standing under vacuum. The product
53 39
2
18
2
3
4 2
1
3
(
(
.18 (3.16). IR (CH Cl ): 2194, 2066, 2046, 2012. H NMR
2 2
1
9
CD Cl ): δ −16.41 (s, Fe-H), −19.16 (s, Fe-H). F NMR
was triturated with pentane (3 × 10 mL) and crystallized from CH Cl2
2
2
2
CD Cl ): δ −100.6 (s, o-F), −100.8 (s, o-F), −116.1 (s, p-F),
(
10 mL) layered with Et O (10 mL) and pentane (30 mL). Yield: 5.25
2
2
2
−
116.3 (s, p-F).
pKa Determinations. Solutions of (Et N) [1(BAr ) ] and
g (99%). Anal. Calcd for C B Fe S H O N F (found): C, 43.97
61
2
2
2
46
4
4 30
F
(
1
43.50); H, 2.78(3.07); N, 3.36 (3.60). IR (CH Cl ): 2136, 1990,
4
2
3 2
2
2
F24
F#
F24
1
Ph NHBAr ·2Et O or (Et N) [1(BAr ) ] and PyHBAr
were
954, 1922. H NMR (CD Cl ): δ 3.10 (16 H, m, (CH CH ) N), 1.73
3
2
4
2
3 2
2
2
3
2 4
2
−
prepared in MeCN with approximate [1(BAr ) ] /acid ratios of 1:2,
(
4 H, t, SCH CH CH S), 1.48 (2 H, m, SCH CH CH ), 1.23 (24 H,
3 2
2
2
2
2
2
2
1
:1, and 2:1 and examined by 19F NMR spectroscopy. Solutions
1
1
19
t, (CH CH ) N). B NMR (CD Cl ): δ −14. F NMR (CD Cl ): δ
3
2
4
2
2
2
2
reached equilibrium at room temperature over 24 h. The pK was then
−
134 (d, o-F), −162 (t, p-F), −167 (t, m-F). Crystals suitable for X-
a
determined using the Henderson−Hasselbach equation.
ray diffraction were obtained by the slow diffusion of pentane into a
F
THF solution of (Et N) [1(BAr ) ] at −30 °C.
4
2
3 2
Et N) [Fe (pdt)(CO) (CNBAr ) ], (Et N) [1(BArF#3)2]. A solution of
F#
ASSOCIATED CONTENT
Supporting Information
(
4
2
2
4
3 2
4
2
■
(
Et N) [1] (0.16 g, 0.25 mmol) in CH Cl (10 mL) was treated
4
2
2
2
*
S
F#
dropwise with a solution of BAr (0.20 g, 0.50 mmol) in 10 mL of
3
Synthetic procedures, IR and NMR spectra, cyclic voltammo-
grams, and .cif files. This material is available free of charge via
the Internet at http://pubs.acs.org.
CH Cl . The solvent was evaporated to yield a red oil, which was
2
2
extracted into CH Cl (5 mL). The extract was layered with pentane
2
2
(
20 mL) and cooled at −30 °C for 2 d, producing a red oil. The filtrate
was decanted off, and the oil was triturated with pentane (3 × 10 mL)
to produce a red solid. Yield: 0.34 g (95%). Anal. Calcd for
C H B F Fe N O S (found): C, 50.51 (50.82); H, 4.03 (4.04); N,
AUTHOR INFORMATION
Corresponding Author
E-mail: rauchfuz@uiuc.edu.
■
61
58
2
18
2
4
4 2
1
3
.86 (4.15). IR (CH Cl ): 2147, 1986, 1947, 1915. H NMR
2 2
*
(
CD Cl ): δ 6.41 (12 H, t, m-ArH), 3.09 (16H, m, (CH CH ) N),
2
2
3
2
4
1
.70 (4 H, t, SCH CH CH S), 1.42 (2 H, m, SCH CH CH ), 1.22 (24
Notes
2
2
2
2
2
2
1
1
19
H, t, (CH CH ) N). B NMR (CD Cl ): δ −15. F NMR (CD Cl ):
δ −99.9 (s, o-F), −117.1 (s, p-F).
The authors declare no competing financial interest.
3
2
4
2
2
2
2
F
F
K [Fe (pdt)(CO) (CN)(CNBAr * )], K [1(BAr * )]. A solution of
2
2
4
3
2
3
ACKNOWLEDGMENTS
■
K [1] (26 mg, 0.058 mmol) in MeCN (20 mL) was treated over
the course of 2 h dropwise with a MeCN solution (10 mL) of BAr *3
2
F
This research was supported by the U.S. National Institutes of
Health and the International Institute for Carbon Neutral
Energy Research (WPI-I2CNER), sponsored by the World
(
50 mg, 0.052 mmol). The solvent was then removed under vacuum
and the residue was extracted into 5 mL of Et O. The extract was
2
7
246
dx.doi.org/10.1021/ic500470z | Inorg. Chem. 2014, 53, 7241−7247

Inorganic Chemistry
Article
Premier International Research Center Initiative (WPI),
MEXT, Japan. We thank Dr. D. Gray for assistance with the
crystallography.
(28) Nicasio, J. A.; Steinberg, S.; Ines, B.; Alcarazo, M. Chem.Eur. J.
2
013, 19, 11016.
(29) Durfey, B. L.; Gilbert, T. M. Inorg. Chem. 2011, 50, 7871.
(30) Chakraborty, D.; Rodriguez, A.; Chen, E. Y. X. Macromolecules
2
003, 36, 5470.
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