Hydrogen activation by biomimetic [NiFe]-hydrogenase model containing protected cyanide cofactors

Article abstract of DOI:10.1021/ja404580r

Described are experiments demonstrating incorporation of cyanide cofactors and hydride substrate into [NiFe]-hydrogenase (H₂ase) active site models. Complexes of the type (CO)₂(CN)₂Fe(pdt)Ni(dxpe) (dxpe = dppe, 1; dxpe = dcpe, 2) bind the Lewis acid B(C₆F ₅)₃ (BAr^F₃) to give the adducts (CO)₂(CNBAr^F₃)₂Fe(pdt)Ni(dxpe), (1(BAr^F₃)₂, 2(BAr^F₃) ₂). Upon decarbonylation using amine oxides, these adducts react with H₂ to give hydrido derivatives [(CO)(CNBAr^F ₃)₂Fe(H)(pdt)Ni(dxpe)]^- (dxpe = dppe, [H3(BAr^F₃)₂]^-; dxpe = dcpe, [H4(BAr^F₃)₂]^-). Crystallographic analysis shows that Et₄N[H3(BAr^F₃)₂] generally resembles the active site of the enzyme in the reduced, hydride-containing states (Ni-C/R). The Fe-H...Ni center is unsymmetrical with r_Fe-H = 1.51(3) A and r_Ni-H = 1.71(3) A. Both crystallographic and ¹⁹F NMR analyses show that the CNBAr ^F₃^- ligands occupy basal and apical sites. Unlike cationic Ni-Fe hydrides, [H3(BAr^F₃) ₂]^- and [H4(BAr^F₃)₂] ^- oxidize at mild potentials, near the Fc^+/0 couple. Electrochemical measurements indicate that in the presence of base, [H3(BAr ^F₃)₂]^- catalyzes the oxidation of H₂. NMR evidence indicates dihydrogen bonding between these anionic hydrides and R₃NH⁺ salts, which is relevant to the mechanism of hydrogenogenesis. In the case of Et₄N[H3(BAr ^F₃)₂], strong acids such as HCl induce H ₂ release to give the chloride Et₄N[(CO)(CNBAr ^F₃)₂Fe(Cl)(pdt)Ni(dppe)].

Full text of DOI:10.1021/ja404580r

Article
pubs.acs.org/JACS
Hydrogen Activation by Biomimetic [NiFe]-Hydrogenase Model
Containing Protected Cyanide Cofactors
Brian C. Manor and Thomas B. Rauchfuss*
School of Chemical Sciences, University of Illinois, Urbana, Illinois 61801, United States
S
* Supporting Information
ABSTRACT: Described are experiments demonstrating incorporation of
cyanide cofactors and hydride substrate into [NiFe]-hydrogenase (H2ase)
active site models. Complexes of the type (CO)2(CN)2Fe(pdt)Ni(dxpe)
(dxpe = dppe, 1; dxpe = dcpe, 2) bind the Lewis acid B(C6F5)3 (BArF ) to
3
give the adducts (CO)2(CNBArF )2Fe(pdt)Ni(dxpe), (1(BArF )2,
3
3
2(BArF )2). Upon decarbonylation using amine oxides, these adducts react
3
with H2 to give hydrido derivatives [(CO)(CNBArF )2Fe(H)(pdt)Ni-
3
(dxpe)]− (dxpe = dppe, [H3(BArF )2]−; dxpe = dcpe, [H4(BArF )2]−).
3
3
Crystallographic analysis shows that Et4N[H3(BArF )2] generally resembles
3
the active site of the enzyme in the reduced, hydride-containing states (Ni−
C/R). The Fe−H···Ni center is unsymmetrical with rFe−H = 1.51(3) Å and
rNi−H = 1.71(3) Å. Both crystallographic and 19F NMR analyses show that
the CNBArF − ligands occupy basal and apical sites. Unlike cationic Ni−Fe
3
hydrides, [H3(BArF )2]− and [H4(BArF )2]− oxidize at mild potentials, near the Fc+/0 couple. Electrochemical measurements
3
3
indicate that in the presence of base, [H3(BArF )2]− catalyzes the oxidation of H2. NMR evidence indicates dihydrogen bonding
3
between these anionic hydrides and R3NH+ salts, which is relevant to the mechanism of hydrogenogenesis. In the case of
Et4N[H3(BArF )2], strong acids such as HCl induce H2 release to give the chloride Et4N[(CO)(CNBArF )2Fe(Cl)(pdt)Ni-
3
3
(dppe)].
contacts between otherwise nonbonded heteroatoms, the Fe−
CN centers engage in hydrogen bonding. For example, in the
[NiFe]-H2ase from D. fructosovorans, one cyanide ligand
hydrogen bonds with serine 499 (N···N, N···O = 2.98, 2.88 Å,
respectively). The second cyanide interacts with arginine 476,
with N···N distances of 2.87 and 3.25 Å.6 Some of these amino
acids can be mutated with retention of catalytic activity,7 which
suggests that the N-terminus of the cyanide cofactors could be
modified in models as well.
Motivated both by the novelty of the active site as well as by
possible practical applications, much effort has been invested into
the structural and functional models of the [NiFe]-H2ase active
site.1,8 Since 2009, several nickel−iron hydrides have been
reported, but all of these models feature Fe(CO)3−x(PR3)x
centers (x = 0, 1, 2).9−12 In contrast to these cationic models,
the active site is expected to be anionic because six ligands, two
cyanide cofactors and four thiolate groups, are anionic.13
Incorporation of cyanide cofactors in models is avoided because
of the reactivity of the basic nitrogen site of cyanide.14 Structural
modes for a CO-inhibited state of the [NiFe]-H2ases with
cyanide cofactors have been reported, (CN)2(CO)2Fe(SR)2Ni-
(dppe) (dppe = (C2H4(PPh2)2)15 and [(CN)2(CO)2Fe-
(SR)2Ni(S2CNEt2)]−.16 Both complexes feature Fe-
(CO)2(CN)2 centers bound to Ni(SR)2L2 sites. These
INTRODUCTION
■
The hydrogenases (H2ases) are represented by three main
families of enzymes defined by the metals at their active sites, the
[FeFe]-, [Fe]-, and [NiFe]-H2ases.1 Other enzymes consume or
produce hydrogen,2 but the [FeFe]- and [NiFe]-H2ases are
renowned for their ability to catalyze the oxidation of H2 and
reduction of protons at high rates and low overpotentials.3
The active sites of the H2ases are characterized by the presence
of several remarkable cofactors (Figure 1). Crystallographic and
spectroscopic analyses consistently point to the presence of
Fe(CO)x(CN)y sites in both types of redox active H2ases.4 The
diatomic cofactors are unusual in biology.5 As indicated by close
Figure 1. Left: Active site of [NiFe]-H2ase active site (Ni−B state) of D.
fructosovorans with hydrogen-bonding interactions between Fe−CN and
nearby residues (PDB 1YRQ). Right: Drawing of the active site of
[NiFe]-H2ase showing various bridging ligands.
Received: May 7, 2013
Published: July 11, 2013
© 2013 American Chemical Society
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Article
Scheme 1
complexes exhibit no biomimetic reactivity because of the
coordinatively saturated nature of the Fe(CN)2(CO)2(SR)2
sites.
This Article describes a method for modifying Fe−CN centers
to suppress their high basicity14 but retain the charge provided by
the anionic cofactors. The approach entails attaching organo-
boranes to the Fe−CN groups. Although novel for metal-
lobiochemistry, the interaction of boranes with metal cyanides is
well-known17 and is exploited in commercial hydrocyanation
catalysis.18
In view of the inertness of cyanide-containing models for the
[NiFe]-H2ases, attachment of boranes to the Fe−CN centers was
expected to both labilize one CO ligand and suppress reactions of
the Fe−CN centers, giving rise to reactive NiII−FeII centers
capable of capturing a hydride substrate from dihydrogen.
RESULTS AND DISCUSSION
■
N-Protected Active Site Models. Modeling efforts focused
on the dicyano Ni−Fe dithiolate 1, which was prepared by
combining NiCl2(dppe) and [Fe(CN)2(pdt)(CO)2]2−. Initial
efforts to induce reactivity reminiscent of the H2ases focused on
the decarbonylation of this species. The FT-IR spectrum of 1
exhibits a relatively high frequency νCO band at 2053 cm−1,
suggesting that CO would be labile. UV-irradiation of a THF
solution of 1, however, afforded only insoluble solids, which are
tentatively attributed to the formation of Fe−CN−M (M = Fe,
Ni) linkages. Solutions of 1 were also found to be inert toward
the decarbonylating agent Me3NO.
Figure 2. Structure of [(CO)2(CNBArF )2Fe(pdt)Ni(dppe)],
3
1(BArF )2, with ellipsoids shown at 50% probability and with H atoms
3
omitted for clarity. Phenyl and pentafluorophenyl groups are de-
emphasized for clarity.
weak Ni···CN π-interaction (2.421 Å) apparent in 115 is absent in
1(BArF )2.
3
To examine the effects of the basicity of nickel, (CO)2(CN)2-
Fe(pdt)Ni(dcpe) (2), featuring the highly basic ligand dcpe
(C2H4(P(C6H11)2)2), was also investigated. IR and 31P NMR
spectroscopy shows that 2 exists as a mixture of isomers,
consisting of both the cis-CO/trans-CN arrangement (as seen in
1) and the cis-CO/cis-CN arrangement. The decreased Lewis
acidity of the Ni(dcpe) center, associated with the strong donor
properties of dcpe,20 is proposed to weaken the Ni−NCFe
interaction, giving rise to the second isomer. Much like 1, 2 was
found to be unreactive toward Me3NO and unstable in ambient
Next examined was the use of boranes to protect the Fe−CN
centers while also electrophilically activating the iron center. A
variety of triarylboranes were found to form adducts with 1, but
this work focuses on B(C6F5)3 (BArF ). Adducts of this strong
3
Lewis acid are often highly soluble in nonpolar solvents
compatible with H2 activation experiments.19 Treatment of
CH2Cl2 solutions of 1 with 2 equiv of BArF quantitatively
3
afforded the 2:1 adduct (CO)2(CNBArF )2Fe(pdt)Ni(dppe),
3
1(BArF )2 (Scheme 1). The attachment of BArF3 strongly affects
light. Complexation of 2 with BArF gave a pair of isomeric
3
3
adducts of the formula (CO)2(CNBArF )2Fe(pdt)Ni(dcpe),
the νCN and νCO bands, resulting in shifts of about 100 and 50
cm−1, respectively (Table S1). The pattern of the νCO/CN bands is
similar for the precursor and the adduct, consistent with
retention of the cis-CO/trans-CN arrangement. 31P and 19F
NMR spectra are also consistent with the proposed stereo-
chemistry.
3
2(BArF )2. Over the course of several hours, this mixture
3
converted to the cis-CO/trans-CN arrangement seen for
1(BArF )2.
3
The attachment of BArF to the cyanide ligands stabilizes
3
isomers not observed in 1 and 2. As determined by 19F and 31
P
The structure of 1(BArF )2 was verified by X-ray crystallog-
NMR spectroscopy, the cis-CO/trans-CN isomer of 1(BArF )2
3
3
raphy (Figure 2). Because of the steric bulk of the BArF3 groups,
the Ni−Fe distance increased from 2.809 in 1 to 3.218 Å in the
adduct.15 Similarly, the Ni−S−Fe angles opened from 75.98° to
converted to the cis-CO/cis-CN isomer upon heating at 60 °C.
Upon UV-irradiation (365 nm), 1(BArF )2 converted to the
3
trans-CO/cis-CN isomer as well as a small amount of the cis-CO/
cis-CN isomer. Upon standing, trans-CO/cis-CN reverted to one
of the two cis/cis isomers. Irradiation of the latter regenerated
the trans-CO/cis-CN isomer. In no case was the cis-CO/trans-
86.60°. The BArF substituents only subtly affect the diatomic
3
ligands: the Fe−CN bonds are slightly elongated (Δavg = 0.03 Å),
and the Fe−CO bond is slightly shortened (Δavg = −0.03 Å). The
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CN isomer of 1(BArF )2 observed to reform, thus, it is apparently
Table 1. IR Bands for the CN-Protected Species
[H3(BArF )2]− and [H4(BArF ) ]− as well as [NiFe]-H2ase
3
a kinetically stabilized isomer.
3
3 2
Enzymes in the Ni−R State,21 and Other Ni−R Models10
Irradiation of the cis-CO/trans-CN isomer of 2(BArF )2 gave
3
the trans-CO/cis-CN isomer. Unlike in 1(BArF )2, the cis-CO/
3
compound
νCN (cm−1
)
νCO (cm−1
)
cis-CN isomer of 2(BArF )2 was not found to reform. Iron
3
Ni−R state, A. vinosum
Ni−R state, D. gigas
2072, 2059
2073, 2060
2074, 2060
2074, 2061
2162, 2137
1936
1940
1938
1948
1963
carbonyl complexes with high frequency νCO bands are typically
photolabile. Indeed, photolysis of cis-CO/trans-CN 1(BArF )2
3
under an atmosphere of 13CO gave complete exchange of CO
Ni−R state, D. fructosovorans
Ni−R state, D. vulgaris
while forming the trans-CO/cis-CN isomer. Furthermore, the
[(CO)(CNBArF )2Fe(H)(pdt)Ni(dppe)]−
photoisomerization of 1(BArF )2 was found to be inhibited by
3
([H3(BArF )2]−)
3
3
CO. These results are consistent with a dissociative pathway for
[(CO)(CNBArF )2Fe(H)(pdt)Ni(dcpe)]−
2158, 2131
1952
3
the photoisomerization of 1(BArF )2 and 2(BArF )2.
([H4(BArF )2]−)
3
3
3
[(CO)2(PPh3)Fe(H)(pdt)Ni(dppe)]+
2016, 1964
Hydride Derivatives. Being coordinatively saturated, the
adducts 1(BArF )2 and 2(BArF )2 are unreactive toward H2.
3
3
Addition of a decarbonylation agent, however, was found to
induce rapid uptake of H2 (eq 1).
Thus, treatment of a CH2Cl2 solution of 1(BArF )2 with 1.2
3
equiv of an amine oxide under an atmosphere of H2 afforded the
hydride [(CO)(CNBArF )2Fe(H)(pdt)Ni(dppe)]−, which was
3
isolated as the salt Et4N[H3(BArF )2]. The use of D2 in place of
3
1
2
H2 gave Et4N[D3(BArF )2], as verified by H and H NMR
3
analyses. The decarbonylation agent Me3NO gave an initial
mixture of isomers with the CNBArF − ligands in dibasal (b/b)
Figure 3. Structure of the anion in Et4N[(CO)(CNBArF )2Fe(H)-
3
3
(pdt)Ni(dppe)], Et4N[H3(BArF )2]), with ellipsoids shown at 50%
and apical-basal (a/b) arrangements in a 1:9.5 ratio. Upon
standing in solution, the b/b isomer converted solely to the a/b
3
probability and H atoms except for the bridging hydride omitted. Phenyl
and pentafluorophenyl groups are de-emphasized for clarity.
isomer. Decarbonylation of 1(BArF )2 with N-methylmorpho-
3
line-N-oxide (NMO) produced the b/b isomer as the major
species (2.5:1 b/b:a/b ratio). Similarly, decarbonylation of
1.51(3) Å is shorter than the Ni−H bond (1.71(3) Å). In several
2(BArF )2 under H2 produced the a/b isomer of Et4N[(CO)-
respects, the structure of [H3(BArF )2]− resembles the high-
3
3
(CNBArF )2Fe(H)(pdt)Ni(dcpe)] (Et4N[H4(BArF )2]) when
resolution (1.4, 1.5 Å) structures of the hydride-containing states
(Ni−C/R) of D. vulgaris (Table 2).22 The Fe···Ni distance of
2.5497(5) Å reasonably matches the values 2.59 and 2.57 Å
found in these hydride-containing states. Reminiscent of the
active site, the coordination geometry of the NiS2P2 center is
distorted from square planar, with a twist angle between the NiS2
and NiP2 planes of 34.53°. Like the enzyme, iron is bound to one
CO and two CN-derived ligands, although the locations of the
3
3
Me3NO was used as a decarbonylating agent with no b/b isomer
observed.
The hydrides [H3(BArF )2]− and [H4(BArF )2]− were also
3
3
produced using conventional hydride sources instead of H2.
Thus, treatment of 1(BArF )2 with amine oxides followed by
3
Et4NBH4 gave Et4N[H3(BArF )2]. However, attempted syn-
3
thesis of Et4N[H4(BArF )2] by treatment of 2(BArF )2 with
3
3
amine oxides and Et4NBH4 produced more complicated results,
yielding both mono- and dicarbonyl hydrides. Overall, H2 is a
milder, more selective reagent for generating hydrido complexes.
CO and the one CNBArF − are interchanged relative to the
3
enzyme.
Reactivity of the Hydrido Complexes. The redox
As observed in the enzymes, the IR spectra of [H3(BArF )2]−
properties of [H3(BArF )2]− and [H4(BArF )2]− were assessed
3
3
3
and [H4(BArF )2]− display two νCN and one νCO bands. The
by cyclic voltammetry on CH2Cl2 solutions. These results
3
energy for νCO of [H4(BArF )2]− is 11 cm−1 lower than that of
underscore the influence of anionic ligands on the Fe site. Both
3
[H3(BArF )2]−, indicating that changes at the Ni center (dppe vs
hydrides oxidize at mild potentials: −0.08 V vs Fc0/+ (ipc/ipa
=
3
dcpe) influence the Fe site. Values of νCO for [H3(BArF )2]− and
0.43, scan rate = 0.1 V/s) for Et4N[H3(BArF )2] and −0.10 V
3
3
[H4(BArF )2]− were found to be within the range observed for
(ipc/ipa = 0.69, scan rate = 0.1 V/s) for Et4N[H4(BArF )2]
3
3
the Ni−R state of [NiFe]-H2ase (Table 1).
(Figures S29, S30). The insensitivity of the [H3(BArF )2]−/0
3
Structure of Et4N[H3(BArF ) ] and Comparisons with
versus [H4(BArF )2]−/0 couples to the diphosphine suggests that
3 2
3
Enzyme Active Site. The structure of [H3(BArF )2]− was
oxidation is localized on Fe. In the enzyme, oxidation of the Ni−
R state occurs at Ni.
Reflecting their anionic character, both complexes reduce at
more negative potentials than previously reported NiFe hydrides.
3
confirmed by single-crystal X-ray diffraction (Figure 3). The
hydride present in [H3(BArF )2]− was located in the difference
3
Fourier map and refined isotropically. The Fe−H distance of
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Table 2. Key Distances (Å) and Angles (deg) of
[H3(BArF ) ]− and the Active Site of D. vulgaris Miyazaki F in
3 2
the Reduced State(s)22
D. vulgaris
D. vulgaris
[(CO)
Miyazaki F Ni− Miyazaki F Ni−
(CNBArF )2Fe(H)
(pdt)Ni(3dppe)]−
C/R (PDB:
1H2R)
C/R (PDB:
1WUL)
Fe−Ni
Fe−H
2.5497(5)
1.51(3)
2.59
N/A
2.57
N/A
Ni−H
1.71(3)
N/A
N/A
Fe−Capical
1.889(3)
1.84 (CO)
1.80 (CO)
(CNBArF −
)
3
Fe−Cbasal
1.758(3) (CO),
1.866(2)
1.86 (CO),
2.21 (SO)
1.99 (CN−),
2.03 (CO)
(CNBArF −
)
3
Fe−S
2.3246(6),
2.2977(8)
2.29, 2.36
2.43, 2.33
2.26, 2.32
2.23, 2.52
Ni−S
2.1952(8),
2.3046(8)
Ni−term−L
2.1382(8),
2.1591(8)
(L = PR3)
2.32, 2.24
(L = SR−)
2.13, 2.21
(L = SR−)
Figure 4. 1H NMR chemical shift for Et4N[H3(BArF )2] upon titration
3
with HNMe3[BArF24] in CD2Cl2 (20 °C). Inset: 1H NMR (500 MHz)
Cbasal−Fe−Cbasal
Cbasal−Fe−Capical
94.0(1)
89
93
spectra of the titration of Et4N[H3(BArF )2] with HNMe3[BArF24] in
90.6(1), 96.0(1)
90, 98
90, 95
3
CD2Cl2 (20 °C): (a) 0, (b) 1.0, (c) 2.0, (d) 3.0, and (e) 4.0 equiv.
A reversible reduction was observed for Et4N[H3(BArF )2] at
3
1
the H NMR spectra when [H4(BArF )2]− was titrated with
−1.99 V (Figure S28), but no reduction was observed for
3
Et4N[H4(BArF )2] within the potential window of CH2Cl2 (i.e.,
3
[pyrrolidinium]BArF24 (pKaMeCN = 19.56).24 Although compet-
itive with degradation of the complex, [H3(BArF )2]− was found
less reducing than −2.5 V). Cationic Ni−Fe hydrides, based on
Fe(CO)3−x(PR3)x centers reduce at milder potentials (−1.33 to
−1.56 V vs Fc0/+).10,11 Reduction potentials for those cationic
NiII−FeII hydrides are sensitive to substitution at Ni (dppe vs
dcpe) and Fe (CO vs PR3).10,11
3
to undergo H/D scrambling with D2, forming HD. This process
+
occurs only in the presence of HNMe3 .
Although weak acids form dihydrogen bonds with
[H3(BArF )2]−, stronger acids (e.g., PhNH3+ pKaMeCN = 10.62)
3
The acid-base properties of [H3(BArF3)2]− and
induce release ∼1 equiv of H2. Using HCl, the unsaturated
[H4(BArF )2]− were examined, with limited success. Unlike
3
product is trapped as the chloride adduct [Cl3(BArF )2]−. The
3
cationic Ni−Fe hydrides, which exhibit pKaMeCN near 14, CH2Cl2
complex [Cl3(BArF )2]− was independently prepared by the
3
solution of [H3(BArF )2]− did not undergo deprotonation upon
3
reaction of 1(BArF )2 with Me3NO and Et4NCl.
3
treatment with the strong base DBU (pKaMeCN = 24.34). Solution
of [H3(BArF )2]− exhibits no exchange with D2O.
CONCLUSIONS
3
■
Et4N[H3(BArF )2] was found to be an electrocatalyst for the
3
Borane protecting groups enable the preparation of the first
models of the [NiFe]-H2ases with biomimetic ligation at Fe. The
protecting groups suppress reactions at the Fe−CN centers,
while retaining the anionic charge that is characteristic of these
active sites.
oxidation of H2. When a solution was examined by cyclic
voltammetry under 1 atm of H2, the current at −0.08 V,
corresponding to the [H3(BArF )2]−/0 couple, was found to
3
increase upon addition of the base DBU (see the Supporting
Information).
Similar to work with the enzyme,21 models in this work were
obtained in an inhibited state. Unlike the enzyme,25 the CO-
inhibited NiFe models contain an extra CO ligand on Fe.26 Upon
decarbonylation, the borane-protected Ni−Fe derivatives
abstract hydride from H2. The generation of hydrides from H2
is unusual in H2ase models12,27 but is a key reaction of the
enzyme. On the basis of structural and νCO data, these models
compare well with the Ni−R state.28
Dihydrogen Bonding. The solution properties of
[H3(BArF )2]− are indicative of its hydridic character, consistent
3
with its resemblance to the hydrogen-evolving state Ni−R.
Specifically, the high field 1H NMR signal of Et4N[H3(BArF )2]
3
is sensitive to the presence of protic reagents that engage in
dihydrogen bonding. For example, the addition of
HNMe3[BArF24] (BArF24 = B(C6H3-3,5-(CF3)2)4; pKaMeCN
=
17.61)23 shifts the hydride signal (δH) by about 2 ppm (Figure 4,
The redox properties of the hydrides [H3(BArF )2]− and
3
+
[H4(BArF )2]− provide insights into the design of improved
eq 2). The dependence of δH on the concentration of HNMe3 is
3
active site models. Because the basicities of dppe and dcpe differ
greatly20 and the oxidations potentials differ by only 20 mV, these
oxidations are assigned to the FeII/III couple. In contrast,
oxidation of the Ni−R state to the Ni−C state is localized on
nickel, that is, a NiII−FeII/NiIII−FeII couple.29 These findings
point to the limitations of diphosphines as mimics for the
terminal ligands on the Ni site. As suggested with a recent
model,30 the terminal thiolate ligands in the enzyme1 change
their protonation state upon redox of the Ni center. Future
models could benefit by including terminal ligands that have
ionizable protons such as RSH.
consistent with an equilibrium constant of (9.0 1.0) × 103 L/
mol in CD2Cl2 (20 °C). Dihydrogen bonding is also evident in
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As compared to cationic nickel−iron hydrides,10−12 the
anionic models exhibit more biomimetic electrochemical and
acid−base properties. Most importantly, anionic nickel−iron
hydrides have hydridic character31 as indicated by NMR studies
showing dihydrogen bonding, which leads to hydrogen
evolution. On the basis of these results, a similar dihydrogen
bond in the Ni−R state is proposed to precede H2 evolution
(Figure 5).
(CO)2(CN)2Fe(pdt)Ni(dcpe), 2. A solution of Et4N[FeBr-
(CO)3(CN)2] (0.500 g, 1.23 mmol) in MeOH (20 mL) was treated
with a solution of K2pdt (0.228 g, 1.23 mmol) in MeOH (20 mL). The
mixture was let to stir for 3 h at which time the solvent was removed
under vacuum. In the absence of light, the residue was extracted into 20
mL of MeCN, and the mixture was treated with a suspension of
NiCl2(dcpe) (0.682 g, 1.23 mmol) in MeCN (20 mL). After being
stirred for 15 min, the mixture was filtered, and the solvent was stripped
from the filtrate. The crude product was extracted into 20 mL of THF,
the mixture was filtered, and the product was precipitated from the
filtrate upon the addition of pentane. Unlike 1, 2 did not require further
purification by column chromatography. Yield: 0.423 g (45%). Anal.
Calcd for C33H54FeN2NiO2P2S2: C, 52.75; H, 7.24; N, 3.73. Found: C,
52.34; H, 7.59; N, 3.99.
Data for cis-CO/trans-CN Isomer. IR(CH2Cl2, cm−1): νCN = 2113,
2090, νCO = 2045, 1995. 31P NMR (CD2Cl2): δ 66.3 (s).
Dats for cis-CO/cis-CN Isomer. IR (CH2Cl2, cm−1): νCN = 2120, νCO
= 2033, 1982. 31P NMR (CD2Cl2): δ 67.8, 66.5 (AB quartet, JPP = 33
Hz).
(CO)2(CNBArF ) Fe(pdt)Ni(dppe), 1(BArF ) . A solution of
Figure 5. Dihydrogen bonding in [H3(BArF )2]− and the analogous
3 2
3 2
3
(CO)2(CN)2Fe(pdt)Ni(dppe) (290 mg, 0.4 mmol) in 10 mL of
CH2Cl2 was treated with a solution of BArF3 (410 mg, 0.8 mmol) in 30
mL of CH2Cl2 dropwise. The reaction caused a change in color from a
red to an orange solution. The reaction solution was concentrated to 20
mL and layered with 100 mL of pentane followed by cooling to −30 °C.
Yield: 570 mg (82%). Crystals suitable for X-ray diffraction were grown
interaction proposed for Ni−R.
Bridging hydrides in related systems have protic character and
do not engage in dihydrogen bonding.32 Dihydrogen bonding
involving terminal iron hydrides is observed in models for the
active site of the [FeFe]-H2ases.33 Thus, it appears that the
hydrides of [FeFe]- and [NiFe]-H2ases (in the Ni−R state) are
similar in being hydridic.
from a concentrated THF solution of 1(BArF )2 layered with pentane
3
and cooled at −30 °C. IR (CH2Cl2, cm−1): νCN = 2194, 2166, νCO
=
2088, 2050. 31P NMR (CD2Cl2): δ 56.4 (s). 19F NMR (CD2Cl2): δ
−133.7 (d, o-F) −135.1 (d, o-F), −159.2 (t, p-F) −159.4 (t, p-F), −165.0
( d t , m - F ) , − 1 6 5 . 6 ( d t , m - F ) . A n a l . C a l c d f o r
C69H30B2F30FeN2NiO2P2S2: C, 47.32; H, 1.73; N, 1.60. Found: C,
47.24; H, 1.64; N, 1.52.
EXPERIMENTAL SECTION
■
All procedures were carried out in a nitrogen-filled MBraun glovebox or
using standard Schlenk techniques. All solvents were degassed by
sparging with nitrogen and dried by passage through activated alumina.
IR spectra were recorded on a Perkin-Elmer Spectrum 100. Cyclic
voltammetry was performed under argon at room temperature using a
CHI 630D potentiostat with glassy carbon working electrode, Pt wire
counter electrode and the pseudoreference electrode Ag wire, and with
Fc internal standard. Production of H2 was quantified by gas
chromatography on a column packed with 5 Å molecular sieves (carrier
gas: Ar), using an Agilent 7820A instrument equipped with a thermal
conductivity detector. The response factor for H2/CH4 was 3.8 under
experimental conditions as established by calibration with standard H2−
CH4 mixtures. NMR spectra were recorded on a Varian Unity 500 MHz
NMR spectrometer. Signals for 19F and 31P NMR spectra were
referenced using external standards of 1% CFCl3 in CDCl3 and 85%
H3PO4, respectively. 1H NMR spectra were referenced to internal
solvent signals. CD2Cl2 (Cambridge Isotope Laboratories) was dried
over CaH2 before being vacuum distilled onto activated 4 Å molecular
sieves. B(C6F5)3 (Boulder Scientific) was sublimed under vacuum at 90
°C. Literature routes were followed for NiCl2(dppe),9 NiCl2(dcpe),11
Et4N[Fe(CO)4(CN)], K2pdt,16 and Et4N[FeBr(CO)3(CN)2].16
Revised Synthesis of (CO)2(CN)2Fe(pdt)Ni(dppe), 1. A solution
of Et4N[FeBr(CO)3(CN)2] (0.900 g, 2.22 mmol) in MeOH (100 mL)
was treated with a solution of K2pdt (0.410 g, 2.22 mmol) in MeOH (50
mL). The mixture was allowed to stir for 3 h at which time the solvent
was removed under vacuum. NiCl2(dppe) (1.17 g, 2.22 mmol) was
added along with 100 mL of MeCN, and the mixture was stirred for 15
min. The mixture was filtered, and the solvent was removed from the
filtrate under vacuum. The crude product was extracted into THF, and
the crude product was precipitated from the filtered extract upon the
addition of pentane. The crude product was dissolved in MeCN and
loaded onto a neutral alumina column (Brockmann Level I). A faint red
band was washed off the column with MeCN. The product eluted with
MeOH as a red band. The solution was evaporated under vacuum, and
the residue was extracted into THF. This solution was filtered through
Celite, and product was precipitated from the filtrate with pentane. The
product16 was collected by filtration and dried under vacuum. Yield:
0.958 g (59%).
(CO)2(CNBArF ) Fe(pdt)Ni(dcpe), 2(BArF ) . Analogously to the
3 2
3 2
preparation of 1(BArF )2, a solution of 2 (0.144 g, 0.0192 mmol) in
3
CH2Cl2 (5 mL) was treated with a solution of BArF3 (0.196 g, 0.0383
mmol), resulting in a color change from red or orange. The solvent was
removed under vacuum, redissolved in THF, and crystallized by the
addition of pentane followed by cooling to −30 °C. Yield: 0.253 g
(74%).
Data for cis-CO/trans-CN Isomer. IR (CH2Cl2, cm−1): νCN = 2190,
2149, νCO = 2085, 2046. 31P NMR (CD2Cl2): δ 69.8 (s). 19F NMR
(CD2Cl2): −133.8 (d, o-F), −134.2 (d, o-F), −159.4 (t, p-F), −160.6 (t,
p-F), −165.3 (t, m-F), −166.4 (t, m-F).
Data for cis-CO/cis-CN Isomer. IR (CH2Cl2, cm−1): νCN = 2210, νCO
= 2068, 2030. 31P NMR (CD2Cl2): δ 70.2, 69.5 (AB quartet, JPP = 30
Hz). 19F NMR (CD2Cl2): −133.1 (d, o-F), −134.9 (o-F), −158.7 (t, p-
F), −159.3 (t, p-F), −165.0 (t, m-F), −165.5 (t, m-F).
Et4N[(CO)(CNBArF ) Fe(H)(pdt)Ni(dppe)], Et4N[H3(BArF ) ]. In
3 2
3 2
a thick-walled glass pressure tube, a solution of 1(BArF )2 (150 mg,
3
0.0857 mmol) in CH2Cl2 (10 mL) was frozen by inserting the tube into
liquid nitrogen. Under a flow of Ar, a buffer layer of CH2Cl2 (5 mL) was
frozen on top of the frozen solution of 1(BArF )2. A solution of Me3NO
3
(7.7 mg, 0.103 mmol) in CH2Cl2 (2 mL) was frozen on top of the buffer
layer. The head space was evacuated and refilled with 40 psig of H2
followed by thawing of the frozen solution. After the mixture was stirred
for 2 h, the solvent was removed under vacuum, and the residue was
extracted into Et2O. The extract was filtered through Celite and analyzed
by 19F NMR spectroscopy, which showed 60% of signals corresponded
to product. A solution of this sample in 10 mL of CH2Cl2 was stirred
with excess Et4NCl (142 mg, 0.857 mmol). The solvent was removed
under vacuum, and the product was extracted into 10 mL of THF, which
was then filtered through Celite and evaporated. The solid was dissolved
in a minimal amount of THF and loaded onto a column of neutral
alumina (Brockmann Level IV) packed with THF. A red band eluted
with THF and was discarded. The remaining brown band was eluted off
the column using CH2Cl2. The solution was concentrated to 5 mL,
layered with 20 mL of pentane, and cooled to −30 °C yielding red
crystals (44 mg, 28%). IR (CH2Cl2, cm−1): νCN = 2162, 2137, νCO
=
1
1963. H NMR (CD2Cl2): δ −7.05 (t, JHP = 5.2 Hz). 31P{1H} NMR
11899
dx.doi.org/10.1021/ja404580r | J. Am. Chem. Soc. 2013, 135, 11895−11900

Journal of the American Chemical Society
Article
(CD2Cl2): δ −66.6 (s). 19F NMR (CD2Cl2): δ −134.4 (o-F), −162.5,
−162.9 (t, p-F), −167.4, −167.6 (t, m-F). Anal. Calcd for
C76H51B2F30FeN3NiOP2S2·0.3CH2Cl2: C, 48.83; H, 2.77; N, 2.30.
Found: C, 48.81; H, 2.43; N, 2.24. Use of N-methylmorpholine-N-oxide
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in place of Me3NO favored the b/b isomer. Et4N[H3(BArF )2] was
3
prepared from 1(BArF )2 and Me3NO using Et4NBH4 as the hydride
3
source (see the Supporting Information).
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Et4N[(CO)(CNBArF ) Fe(H)(pdt)Ni(dcpe)], Et4N[H4(BArF ) ].
3 2
3 2
This compound was prepared in a manner and yield similar to the
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preparation of Et4N[H3(BArF )2]. IR (CH2Cl2, cm−1): νCN = 2158,
Physiol. 2006, 51, 1. Peters, J. W.; Broderick, J. B. Annu. Rev. Biochem.
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3
2132, νCO = 1952. 1H NMR (CD2Cl2): δ −6.56 (t, JHP = 3 Hz). 31P{1H}
NMR (CD2Cl2): δ −85.1 (s). 19F NMR (CD2Cl2): δ −134.3 (d, o-F),
−162.5, −163.0 (t, p-F), −167.3, −167.8 (t, m-F). Anal. Calcd for
C76H75B2F30FeN3NiOP2S2: C, 48.58; H, 4.02; N, 2.24. Found: C, 48.83;
H, 4.00; N, 2.35.
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Et4N[(CO)(CNBArF ) Fe(Cl)(pdt)Ni(dppe)], Et4N[Cl3(BArF ) ]. A
3 2
3 2
solution of Et4N[H3(BArF )2] (10 mg, 0.0054 mmol) in CD2Cl2 (0.7
3
mL) was treated with HCl (2 M in Et2O, 6 μL, 2.2 equiv, 0.012 mmol),
resulting in that the solution turning slightly lighter in color. 1H NMR
confirmed the formation of H2. 31P NMR displayed a major species
(∼60%) as an AB quartet at δ −51.5 and −48.6, and 19F NMR displayed
signals for two inequivalent BArF3 environments, both consistent with
an apical/basal arrangement of CNBArF − ligands. Assignment of this
3
major species as Et4N[Cl3(BArF )2] was confirmed by independent
3
(12) Ogo, S.; Ichikawa, K.; Kishima, T.; Matsumoto, T.; Nakai, H.;
Kusaka, K.; Ohhara, T. Science 2013, 339, 682.
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synthesis of Et4N[Cl3(BArF )2] from 1(BArF )2 and Et4NCl. Thus, a
3
3
solution of 1(BArF )2 (0.10 g, 0.057 mmol) in CH2Cl2 (10 mL) was
3
treated with a solution of Me3NO (5.1 mg, 0.069 mmol) in CH2Cl2 (1
mL) followed by Et4NCl (9.5 mg, 0.057 mmol) in CH2Cl2 (5 mL). After
the solution was allowed to stir for 15 min, solvent was removed under
vacuum. The crude solid was dissolved in a minimal amount of THF and
loaded onto a column of neutral alumina (Brockmann Level IV) eluting
THF. The first red band was discarded, and the remaining brown band
was eluted with CH2Cl2. The solution was concentrated to 5 mL, layered
with 20 mL of pentane, and cooled to −30 °C precipitating an oil. Upon
storing under vacuum, the oil was converted to a solid (40 mg, 37%). IR
(CH2Cl2, cm−1): νCN = 2181, 2149, νCO = 1996. 31P{1H} NMR
(CD2Cl2): δ −51.5, −48.6 (AB quartet, JPP = 29 Hz). 19F NMR
(CD2Cl2): δ −134.1, −134.6 (d, o-F), −161.9, −162.4 (t, p-F), −167.0,
−167.2 (t, m-F). Anal. Calcd for C76H50B2ClF30FeN3NiOP2S2: C, 48.33;
H, 2.67; N, 2.22. Found: C, 48.30; H, 2.43; N, 2.43.
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ASSOCIATED CONTENT
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■
S
* Supporting Information
1H NMR, 19F NMR, 31P NMR, and IR spectra for new
compounds. CIF files giving X-ray crystallographic data. This
material is available free of charge via the Internet at http://pubs.
acs.org.
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Soc. 2002, 124, 11628.
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AUTHOR INFORMATION
■
Corresponding Author
rauchfuz@illinois.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by NIH and the International
Institute for Carbon Neutral Energy Research (WPI-I2CNER),
sponsored by the World Premier International Research Center
Initiative (WPI), MEXT, Japan. We thank Dr. Mark Ringenberg
for advice, Dr. Danielle Gray for assistance with the
crystallography, and Prof. Steven Zimmerman and Ying Li for
assistance with the calculations of the binding constants.
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van Gastel, M.; Neese, F.; Lubitz, W. J. Am. Chem. Soc. 2012, 134, 20745.
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