New reactions of terminal hydrides on a diiron dithiolate

Article abstract of DOI:10.1021/ja501366j

Mechanisms for biological and bioinspired dihydrogen activation and production often invoke the intermediacy of diiron dithiolato dihydrides. The first example of such a Fe₂(SR)₂H₂ species is provided by the complex [(term-H)(μ-H)Fe₂(pdt)(CO)(dppv) ₂] ([H1H]⁰). Spectroscopic and computational studies indicate that [H1H]⁰ contains both a bridging hydride and a terminal hydride, which, notably, occupies a basal site. The synthesis begins with [(μ-H)Fe₂(pdt)(CO)₂(dppv)₂]⁺ ([H1(CO)]⁺), which undergoes substitution to afford [(μ-H)Fe ₂(pdt)(CO)(NCMe)(dppv)₂]⁺ ([H1(NCMe)] ⁺). Upon treatment of [H1(NCMe)]⁺ with borohydride salts, the MeCN ligand is displaced to afford [H1H]⁰. DNMR (EXSY, SST) experiments on this complex show that the terminal and bridging hydride ligands interchange intramolecularly at a rate of 1 s^-1 at -40 °C. The compound reacts with D₂ to afford [D1D]⁰, but not mixed isotopomers such as [H1D]⁰. The dihydride undergoes oxidation with Fc⁺ under CO to give [1(CO)]⁺ and H₂. Protonation in MeCN solution gives [H1(NCMe)]⁺ and H₂. Carbonylation converts [H1H]⁰ into [1(CO)]⁰.

Full text of DOI:10.1021/ja501366j

Article
pubs.acs.org/JACS
New Reactions of Terminal Hydrides on a Diiron Dithiolate
Wenguang Wang, Thomas B. Rauchfuss,* and Lingyang Zhu
School of Chemical Sciences University of Illinois - Urbana, Urbana, Illinois 61801, United States
Giuseppe Zampella*
Department of Biotechnology and Biosciences, University of Milano-Bicocca 20126 Milan, Italy
S
* Supporting Information
ABSTRACT: Mechanisms for biological and bioinspired
dihydrogen activation and production often invoke the
intermediacy of diiron dithiolato dihydrides. The first example
of such a Fe2(SR)2H2 species is provided by the complex
[(term-H)(μ-H)Fe2(pdt)(CO)(dppv)2] ([H1H]0). Spectro-
scopic and computational studies indicate that [H1H]0
contains both a bridging hydride and a terminal hydride,
which, notably, occupies a basal site. The synthesis begins with
[(μ-H)Fe2(pdt)(CO)2(dppv)2]+ ([H1(CO)]+), which under-
goes substitution to afford [(μ-H)Fe2(pdt)(CO)(NCMe)-
(dppv)2]+ ([H1(NCMe)]+). Upon treatment of
[H1(NCMe)]+ with borohydride salts, the MeCN ligand is
displaced to afford [H1H]0. DNMR (EXSY, SST) experiments
on this complex show that the terminal and bridging hydride
ligands interchange intramolecularly at a rate of 1 s−1 at −40
°C. The compound reacts with D2 to afford [D1D]0, but not
mixed isotopomers such as [H1D]0. The dihydride undergoes
oxidation with Fc+ under CO to give [1(CO)]+ and H2.
Protonation in MeCN solution gives [H1(NCMe)]+ and H2.
Carbonylation converts [H1H]0 into [1(CO)]0.
INTRODUCTION
■
For several years, diiron hydrides have been heavily studied
because they are intermediates in the enzymatic production and
oxidation of H2 by enzymes.1 Since these enzymes are so
efficient and so fast, many model complexes have been
synthesized to mimic aspects of this catalysis. Focusing on
the hydride intermediates, the greatest challenge in the
modeling area is the tendency of the terminal hydrides, with
the connectivity Fe−Fe−H, to isomerize to bridging hydrides,
with the connectivity Fe−H−Fe (Figure 1). Indeed the Fe2(μ-
H) entity is predicted to be unusually stable.2 In the [FeFe]-
hydrogenase, it appears that supramolecular interactions
prevent this isomerization, but in models, the isomerization
can only be slowed (but not stopped) by sterically inhibiting
the rotation at the FeH center.3,4 This paper describes a new
approach to the “terminal hydride problem”incorporate both
terminal and bridging hydrides into such diiron complexes.
Several diiron di- and polyhydrides have been investigated,5
and this situation is poised to accelerate in step with the interest
in catalysis by first row metals.6 Examples include diiron(III) di-
and polyhydrides, e.g., Cp*2Fe2(μ-H)4,7 and diiron(II)
dihydrides,8 e.g., H2Fe2(NacNac)2.9 Polyiron di- and poly-
hydrides are implicated in other areas of homogeneous
Figure 1. (Left) “Bridging hydride structure” proposed for active site
of [NiFe]-hydrogenase. (Right) “Terminal hydride structure” proposed
for active site of [FeFe]-hydrogenase in Hred state.
catalysis5 including nitrogenase.10 With few exceptions,11 the
hydride ligands bridge the metal centers. Although diiron
dithiolato dihydrides have not been described,12 they are
implicated in proton reduction catalysis (hydrogen evolution
reaction, or HER) by bioinspired catalysts.13 Di- and even
trihydrides of diiron dithiolates are also implicated in the
Received: February 8, 2014
Published: March 22, 2014
© 2014 American Chemical Society
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Scheme 1. Synthesis and Reactions of [(t-H)(μ-H)Fe2(pdt)(CO)(dppv)2] ([H1H]0])
exchange between H2 and D2 photocatalyzed by [HFe2(pdt)-
(CO)4(PMe3)2]+.14
RESULTS AND DISCUSSION
■
Dihydride [(t-H)(μ-H)Fe2(pdt)(CO)(dppv)2]. The syn-
thesis and reactions of the new diiron dihydride, [(t-H)(μ-
H)Fe2(pdt)(CO)(dppv)2] ([H1H]0]) are outlined in Scheme
1.
The synthetic approach exploits the lability of CO ligands in
the cationic diferrous hydride [(μ-H)Fe2(pdt)(CO)2(dppv)2]+
([H1(CO)]+). Photolysis of MeCN solutions of this dicarbonyl
resulted in clean monosubstitution, affording [(μ-H)Fe2(pdt)-
(CO)(MeCN)(dppv)2]+ ([H1(NCMe)]+). For this cation νCO
= 1942 cm−1, which can be compared with 1952 (s) and 1972
(sh) cm−1 in [H1(CO)]+. As in [H1(CO)]+, the 31P NMR
spectrum of [H1(NCMe)]+ indicates two isomers in a 90:10
ratio. In the major isomer, labeled ab−bb, one dppv occupies
apical and basal sites (ab) and the other dppv occupies the two
basal sites (bb) on the other Fe center. The MeCN ligand
occupies only one of two nonequivalent sites. In the minor
Figure 2. Structure of major ab−bb isomer of [H1(NCMe)]+.
(13%) isomer of [H1(NCMe)]+, both dppv ligands are
disposed in the ab mode and the MeCN binds to one of two
equivalent basal sites in this ab−ab isomer. This isomer displays
four 31P NMR signals.
−
Counterion (BF4 ) and solvent of crystallization are not shown.
Thermal ellipsoids set at the 50% probability level. For clarity, phenyl
groups are drawn as lines and hydrogen atoms (except the hydride
ligand) are omitted. Selected distances (Å): Fe(1)−Fe(2), 2.6549(4);
Fe(1)−H(1), 1.72(2); Fe(2)−H(1), 1.63(2); Fe(1)−Savg, 2.2555(5);
Fe(2)−Savg, 2.2674(5); Fe(1)−P(1), 2.2502(5); Fe(1)−P(2),
2.2195(6); Fe(2)−P(3), 2.2093(5); Fe(2)−P(4), 2.2334(5); Fe(1)−
N(1), 1.9328(16); N(1)−C(27), 1.134(2). Angles (deg): S(1)−
Fe(1)−S(2), 84.013(18); S(1)−Fe(2)−S(2), 83.471(19); Fe(1)−
Fe(2)−H(1), 38.9(7); Fe(2)−Fe(1)−H(1), 36.4(7).
The structure of a major isomer of [H1(NCMe)]BF4 was
confirmed crystallographically (Figure 2). The MeCN ligand
occupies a basal position. The bridging hydrido ligand, the
location of which was identified in the difference map, refined
to a position slightly closer to the Fe(NCMe) center (1.63(2)
Å) than to the Fe(CO) site (1.72(2) Å). In contrast, the two
Fe−H distances in [H1(CO)]+ are almost identical with values
7
́
of 1.65 (3) and 1.68 (3) Å.
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The MeCN ligand in [H1(NCMe)]+ is displaced by a
hydride donated by NBu4BH4 to afford the targeted complex
[H1H]0, isolated as a green toluene-soluble solid. The νCO
band for this complex is found at 1915 cm−1 (Figure 3). This
complex is stable at room temperature but is susceptible to a
variety of thermally and chemically induced reactions as
described below.
Figure 4. 1H−31P HMBC spectrum of [H1H]0 in THF-d8 at −40 °C.
The multibond JH−P was set to 60 Hz.
is coupled to two 31P NMR signals (δ 108 and 105). These
1
results are consistent with the assignment of the H NMR
signal at δ −12.2 to the bridging hydride, and the signal at δ
−18.9 to the terminal hydride. The relative chemical shifts of
the hydride signals are reversed from the usual pattern of
terminal hydrides being lower field than bridging hydrides in
diiron dithiolates compounds,3 but the usual trend may only
apply to terminal, apical hydrides. In addition, based on the
Figure 3. IR spectra in νCO region for (a) [H1(NCMe)]BF4 in THF
solution after treatment with one equiv of solid [NBu4]BH4 for (b) 1
min, and (c) 10 min.
1
JP−H coupling value of ∼62 Hz obtained in the H NMR
spectrum, this terminal hydride is cis to the two phosphorus
centers (δ 108 and 105). As repeatedly observed in related
complexes3 and in simpler iron phosphine hydride complexes,
Attempted preparation of the related dihydride [H2Fe2(pdt)-
(dppv)(CO)3] failed because the precursor [(μ-H)Fe2(pdt)-
(CO)3(NCMe)(dppv)]+ oxidizes borohydride. This result can
be rationalized qualitatively on the basis of redox potentials: the
1
two-bond H−31P couplings are characteristically large when
these nuclei are mutually cis.17 Overall, the pattern in HMBC
spectrum is consistent with a terminal, basal hydride (δ −18.9)
and a hydride (δ −12.2) that bridges a,b- and b,b-Fe(dppv)
centers. Coupling between the two hydride centers is not
observed. In mononuclear complexes this coupling constant
can be small.18 These assignments lead to the structure shown
in Figure 5.
(irreversible) oxidation BH4 is −1.64 V (−1.24 V vs SHE15)
−
and the couple [(μ-H)Fe2(pdt)(CO)3(NCMe)(dppv)]+/0 is
the relatively mild −1.38 V vs Fc+/0 (Supporting Information
[SI]). In contrast, the [H1(NCMe)]+/0 is −1.83 V. Indeed,
treatment of the tricarbonyl cation with NBu4BH4 gave ∼50%
the reduced species Fe2(pdt)(CO)4(dppv) as well as other
unidentified species.
Structure of [H1H]0. Since [H1H]0 did not crystallize
suitably, its structure was examined by DFT calculations. As the
DFT scheme, we adopted B-P86/TZVP, one of the best
performers (even better than the very popular B3LYP) in
reproducing the crystallographic parameters as well as IR
spectra (νCO/CN) of active-site mimics of the [FeFe] and
The structure of [H1H]0 was initially assigned on the basis of
its NMR properties. The room temperature 1H NMR spectrum
features broad peaks around δ −12 and −19. These signals
sharpen at lower temperatures (T < 0 °C). At −40 °C, well-
resolved peaks are observed at δ −12.2 and −18.9. The δ −12.2
signal appears as a quartet with JP−H ≈ 24 Hz. The upper field
signal δ −18.90 appears as a triplet, with JP−H ≈ 62 Hz. Four
signals are observed in the 31P{1H} NMR spectrum: doublets at
δ 108 (d, JP−P = 56.5 Hz) and 105 (d, JP−P = 56.5 Hz) and
singlets at δ 80.6 and 77.9. Typically apical−basal dppv ligands
exhibit 31P NMR signals above δ 90, whereas dibasal dppv
ligands absorb below δ 90.16 In the two-dimensional (2D)
31P−31P TOCSY experiment, cross-peaks are only observed
between the apical−basal dppv signals at δ 108 and 105. No
31P−31P interaction was observed within the dibasal dppv (SI).
The structure of [H1H]0 was further examined by 2D
1H−31P HMBC (heteronuclear multiple-bond correlation)
spectroscopy. In the HMBC spectrum (Figure 4), with the
use of the long-range coupling of 60 Hz, three cross-peaks are
observed: between the hydride at δ −12.2 and the phosphorus
(δ 105, 80.6, and 77.9), and two cross-peaks between the
hydride at δ −18.9 and the phosphorus (δ 108 and 105). These
results indicate that the 1H signal at δ −12.2 is coupled to three
31P signals (δ 105, 80.6, and 77.9), and the 1H signal at δ −18.9
Figure 5. Connectivity for three possible isomers of the
H2Fe2S2(CO)(P2C2H2)2 core of [H1H]0 with nonequivalent
phosphorus centers. The proposed isomer is shown with NMR
assignments.
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[NiFe] hydrogenase,19,20 even if this success may be partially
attributable to the effect of error cancellation.21 Our
calculations indicate that the isomer with a basal terminal
hydride is 2 kcal/mol lower in energy than the isomer with the
more precedented apical terminal hydride. Despite this small
calculated energy difference, no evidence experimentally was
observed for a second isomer. Previous DFT calculations had
predicted that apical and basal hydrides may be quite close in
energy in other diiron dithiolates.22 Qualitatively, the proposed
structure is consistent with the general trend that μ-hydride
ligands prefer weak donors or strong acceptors at the two trans
(apical) sites. Although the complex’s stereochemistry is
noteworthy, the calculated bond distances and angles are
unremarkable (Figure 6).
Figure 7. DFT-calculated HOMO for [H1H]0 with isosurface value of
0.02 showing that the basal hydride ligand is embedded in this orbital.
The NBO partial charge calculations indicate qbridging‑H = −0.14 and
qbasal‑H = −0.16.
terminal basal hydride is noteworthy because previous examples
of terminal (apical) hydrides absorb near δ −4.3 For the
isomeric structure (Figure 5) with the terminal hydride, the
hydrides absorb at δ −12.3 and −12.4. The 31P NMR spectra
observed for [H1H]0 and calculated for the analogous
(Me2PCHCHPMe2)2 complex do not agree, but such shifts
are known to be very sensitive to alkyl vs aryl substituents on P.
Nonetheless, calculated 31P chemical shifts are qualitatively
more consistent on the basal hydride assignment (SI).
Figure 6. DFT-Calculated structure of [H1H]0. Distances in Å: Fe1−
H1, 1.673; Fe1−H2, 1.527; Fe1−P1, 2.175; Fe1−P2, 2.200; Fe1−S1,
2.270; Fe1−S2, 2.255; Fe2−C, 1.748; Fe2−H1, 1.750; Fe2−P3, 2.248;
Fe2−P4, 2.251; Fe2−S1, 2.321; Fe2−S2, 2.286.
Hydride Interchange in [H1H]0. Insight into the exchange
between the hydride sites was obtained by a combination of
1
1H−1H 2D EXSY and 1D H spin saturation transfer (SST)
experiments. At −40 °C, the 2D EXSY spectrum (mixing time
of 300 ms, see SI) shows an intense cross-peak linking the
signals at δ −12.2 and −18.9. This cross-peak has the same sign
as the diagonal peaks, indicating it arises from chemical
exchange of the two hydrides (cross-peaks from nuclear
Overhauser enhancement (NOE) take the opposite sign of
the diagonal peaks). In spin saturation transfer experiments,
presaturation (continuous low-power irradiation) of the
hydride resonance at δ −18.9 leads to selective loss of intensity
of the signal at δ −12.2 and vice versa. Other peaks were
unaffected. As shown in Figure 8, the rate of exchange between
the hydride sites is indicated by the time dependence of the
intensity of the bridging hydride resonance (at δ −12.2) as a
function of the duration (τ) of presaturation at δ −18.9. The
rate constant for H/H exchange, k = 1/τ, was calculated to be
1.14 s−1 by fitting these data. This reverse rate constant
(saturated at δ −18.9, observed δ −12.2) was fit to be 0.91 s−1,
which is close to 1.14 s−1 for the forward rate.
The isosurface of the HOMO (Figure 7) shows qualitatively
that bridging and terminal basal hydrides should feature
different chemical properties. This mapping indicates that the
basal-H should be more nucleophilic and more basic than the
bridging hydride.23
DFT was also used to calculate IR and NMR spectra for all
structures reported in Figure 5. The DFT-calculated value of
νCO is 1923 cm−1, vs 1915 cm−1 observed. The deviation is
attributed to the harmonic approximation employed in all
quantum chemistry program codes.24 The DFT-calculated
value of νCO for the apical hydride isomer of [H1H]0 is 1930
cm−1. According to the calculations, νC−O is weakly coupled
with the Fe−Hba and Fe−H−Fe vibrations. Consistent with
weak coupling between CO and Fe-H/D modes, for [D1D]0,
νCO = 1919 cm−1, 4 cm−1 shift. Calculations predict a 1 cm−1
shift. Moreover, in the DFT-calculated IR spectrum of [D1D]0,
the vibronic coupling, observed in [H1H]0 for νC−O and Fe−H
modes, disappears.
The rate of H−H exchange was found to be independent of
concentration of the diiron complex. For the sample
concentration from 24 to 8 mM, the rate constants for hydride
exchange were almost the same (0.91 and 0.88 s−1) by
presaturation of resonances at δ −18.9 and −12.2, respectively.
These rate constants are within 20% of the values found for the
For the DFT-calculated NMR data, the model was simplified
by replacing Ph with methyl groups. The calculations reproduce
the chemical shifts for the hydride ligands, predicting δ −19.5
and −12.9 for the terminal and bridging hydrides, respectively.
The agreement between the chemical shifts calculated for the
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1
with the H DNMR data that indicate that interchange of the
hydrides occurs at ∼1 s−1 (−40 °C), the 31P−31P 2D EXSY
result suggests that H−H exchange occurs without symmetriz-
ing the complex. A pathway consistent with this finding is
shown in eq 1.
The D/D exchange rate was evaluated for [D1D]0, which
was prepared by the reaction of D2 with [H1H]0 (see below).
Although the 31P NMR chemical shifts are almost identical for
[H1H]0 and [D1D]0, the coupling patterns are fairly
distinctive. Four signals were observed: broad doublet−
multiplets (dm, 44.9 Hz) at δ 80.0 and at 77.9 as well as
broad singlets at δ 108 and 105. The D/D exchange rate at −40
°C was determined to be 7.89 s−1 vs 1.14 s−1 for H/H exchange
in [H1H]0 (2H-SST was used to determine the exchange rate
since the signals are weak in the EXSY experiment). As
examined by DFT methods, the low-energy pathway for H−H
exchange is proposed to proceed via a transition state with an
H−H-bonding interaction (rH−H = 0.996 A, eq 1). The barrier
is 10.3 kcal/mol, which nicely reproduces the experimental
value. Aside from the Fe−H and H−H distances, no other
bonds change by >0.02 Å relative to the ground-state geometry.
Attempts were made to obtain the monodeuteride [(t-H)(μ-
D)Fe2(pdt)(CO)(dppv)2] by treatment of [D1(NCMe)]+ with
Figure 8. Spin saturation transfer spectra of [H1H]0 at −40 °C in
THF-d8 (sample concentration: 24 mM), showing the decay of the
hydride signal at δ −12.2 upon irradiating the hydride resonance at δ
−18.9 (top), and the plot of decay vs presaturation time at δ −12.2
(bottom). The presaturation time was increased by 0.05 s for each
successive measurement. Fitting results: At/A0= 1/(1 + τ/0.748) ×
exp[−t × (1/0.748 + 1/τ)] + 1/(1 + 0.748/τ), τ = 0.874 s, where τ is
lifetime (1/k) for the exchanging H at δ −12.2.
more concentrated solution, indicative of an intramolecular
mechanism for exchange.
The spin saturation transfer measurements at various
temperatures between −60 to 0 °C were acquired (Figure 9).
−
BH4 . The product proved to be a mixture of not only
[D1H]0/[H1D]0 but also [H1H]0 (and we assume [D1D]0), as
indicated by 2-D EXSY measurements that showed cross peaks
1
for exchange between the H sites.
The interchange of terminal and bridging hydrides has been
demonstrated in the case of Cp*2Fe2(term-H)(μ-H)(PPh2)2
and (term-H)(μ-H)Os3(CO)11.7,25 In the latter case, exchange
is proposed to occur via an intermediate where both hydrides
are terminal.25 This mechanism does not appear to apply to
[H1H]0, as explained above.
Reductive Elimination of H2. The reductive elimination
from dihydrido complexes is an established route to
unsaturated complexes.26 The reductive elimination of H2
from [H1H]0 in principle should afford “Fe2(pdt)(CO)-
(dppv)2”, a 32e− diiron(I) dithiolate. Such a highly unsaturated
species is expected to be reactive. Upon heating in THF,
[H1H]0 yields H2 and a complex mixture of organometallic
products, as indicated by 31P NMR analysis. Alternatively, when
[H1H]0 was allowed to decompose in the presence of ∼0.5 atm
CO, the organometallic product is the well-known species
[1CO]0,3 as indicated by NMR and IR analyses (SI).
Figure 9. Eyring plot of H/H exchange for [H1H]0 taken from SST
data. Results: ΔH⧧ = +41.2 ( 0.8) kJ/mol, ΔS⧧ = −60 ( 3.4) J/K·
mol.
The rate constant k increases from 0.387 s−1 (−60 °C) to 79.0
s−1 at 0 °C. The activation energy for the exchange process was
determined to be 43.2 kJ/mol, and the temperature depend-
ence of k gives ΔH⧧ = +41.2 ( 0.8) kJ/mol, ΔS⧧ = −60 ( 3.4)
J/K·mol (Figure 9).
Solutions of [H1H]0 also react with D2 and produce [D1D]0.
When the reaction was monitored in a sealed NMR tube, H2
was observed (δ 4.56, s) together with only ∼10% HD (see SI).
The 2H NMR spectrum shows signals at δ −12.2 and −18.9 in
THF at −40 °C (Figure 10). The νCO band slightly shifts from
1915 to 1919 cm−1 upon conversion of [H1H]0 to [D1D]0.
The 31P−31P 2D EXSY spectrum recorded at various mixing
times in the range of 50−500 ms showed no cross-peaks,
indicating the slowness of site exchange at −40 °C. Together
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Figure 11. FT-IR spectra in the νCO region and ESI-MS spectra (([M-
BArF4]+) for [H1H]0 in THF solution after treatment with 1 equiv of
H(OEt2)2BArF4 under CO (a and c), and 13CO (b and d) atmosphere.
NMR tube, a difficult experiment, afforded HD and H2 in a 2:1
molar ratio and almost exclusively [D1CO]+. Protonolysis of
[H1H]0 with H(OEt2)2BArF4 under an N2 atmosphere gave
only a complex mixture of products, suggesting that [H1(N2)]+,
if it forms at all, is unstable.
2
Figure 10. (Top) H NMR spectrum of [D1D]0 generated from the
reaction of [H1H]0 and D2 in THF at −40 °C. (Bottom) Plot of decay
vs time at δ −12.24 from SST experiments. Fitting results: At/A0 = 1/
(1 + τ/0.1005) × exp[−t × (1/0.1005 + 1/τ)] + 1/(1 + 0.1005/τ), τ =
0.1267s.
Oxidation of [H1H]0. According to its cyclic voltammo-
gram (CV), a THF solution of [H1H]0 undergoes a quasi-
reversible oxidation at −78 mV (ipc/ipa = 0.52) vs Fc+/0 at the
scan rate of 100 mV/s. For comparison, the couple [Fe2(pdt)-
(CO)2(dppv)2]0/+ occurs at −360 mV vs Ag/AgCl (−850 mV
vs Fc+/0 in CH2Cl2).28 The CV of [H1H]0 shows no reductive
events out to −2.00 V (vs Fc+/0).
Protonolysis. In contrast to known hydrides (terminal and
bridging) of other diiron dithiolates, [H1H]0 reacts readily with
acids with evolution of H2. With addition of one equivalent of
H(OEt2)2BArF4, the color of a THF solution of [H1H]0
immediately changed from dark green to brown. Yields of
H2, quantified by GC analysis, approached 95.7% (four
experiments, standard deviation of 9.2%). Weak acids such as
PhCO2H (pKaMeCN = 21.5127) also induce hydrogen evolution,
although these reactions require a few minutes at room
temperature.
The oxidation of [H1H]0 could also be effected by treatment
with ferrocinium salts. Addition of one equivalent of FcBArF4 to
[H1H]0 resulted in nearly quantitative release of H2 (97.6%,
four experiments, standard deviation of 1.3%).
Judging from the FT-IR and ESI-MS spectrum analysis,
oxidation in the presence of CO afforded [Fe2(pdt)-
(CO)2(dppv)2]+, [1CO]+. The Fc+-induced reaction of
[H1H]0 with CO is noticeably faster than the direct
carbonylation (see above). In the absence of CO, oxidation
produces a complex mixture of organometallic products. The
FT-IR spectrum of [1CO]+ exhibits strong bands at 1885 and
1955 cm−1, assigned to νμ‑CO and νt‑CO, respectively (Figure
12). The product, [1CO]BF4, was characterized crystallo-
graphically (Figure 13). The cation adopts a highly unsym-
metrical, “rotated” structure reminiscent of the Hox state of the
[FeFe]-hydrogenase.28,29 The terminal CO is coordinated to
the Fe center with an apical−basal dppv, and the semibridging
CO is more strongly bonded to the rotated Fe site. The two Fe-
μ-CO distances are very different, being 2.618 (‘proximal’ Fe1)
The protonolysis of [H1H]0 by H(OEt2)2BArF4 was
explored under a variety of conditions. When conducted
under an atmosphere of CO, [H1(CO)]+ was regenerated in
87% yield as confirmed by IR as well as 1H and 31P{1H} NMR
analyses (eq 2 and Figure 11). Protonation under a 13CO
atmosphere afforded [H1(13CO)]+ wherein one of the two CO
ligands is labeled. For the singly labeled species, the νCO band at
1953 is unaffected, but the second band shifts from 1971 to
1925 cm−1. The 13C NMR spectrum displays a triplet at δ 218
with JP−C = 15 Hz and a small singlet at δ 212.6 for two CO
groups. The 31P NMR spectrum exhibits doublets (JC−P = 18
Hz) at δ 87.4 and 83.8 as well as singlets at δ 82.2 and 76.2.
The results show that 13CO ligand is coupled to the ab dppv.
Overall, it is clear that CO is installed stereoselectively (eq 2).
The ESI-MS spectrum of [H113CO]+ also confirmed the single
label. Protonolysis of [H1H]0 in the presence of MeCN gave
[H1(NCMe)]+. Protonolysis of [D1D]0 under CO in a sealed
and 1.786 Å (‘distal’, rotated Fe1). The bridging CO group is
́
bent with <Fe(2)−C(57)−O(2) = 169.9°.
Oxidation of [H1H]0 under an atmosphere of 13CO afforded
[Fe2(pdt)(13CO)2(dppv)2]+ as indicated by ESI-MS. Relative to
the spectrum for [1CO]+ both νμ‑CO and νt‑CO are shifted by
about 40 cm−1 to lower energy. The double labeling arises
because such 33e dimers are known to exchange with
exogenous CO.30 For example, a solution of [1CO]+ under
an atmosphere of 13CO atmosphere converts to the doubly
labeled derivative in ∼30 min. (SI).
Hydride and Hydrogen Transfer from [H1H]0. The
hydridic character of [H1H]0 was also demonstrated by its
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Solutions of [H1H]0 were also found to react with
dimethylacetylenedicarboxylate (DMAD) to give dimethyl
maleate (eq 4). The cis stereochemistry of the product is
Figure 12. IR spectra of THF solutions of [H1H]0 after addition of
consistent with a hydrogenation reaction. When this process is
conducted under an atmosphere of CO, [1CO]0 was produced.
The DMAD-induced reaction proceeds in minutes at room
temperature, whereas the reaction of CO + [H1H]0 to give
[1CO]0 requires hours. Treatment of [H1H]0 with DMAD
under an atmosphere of D2 gave 2,3-dideutero-dimethyl
maleate, which implies that this reaction is catalytic.
FcBArF under (a) ∼0.5 atm CO and (b) ∼0.5 atm 13CO. Spectrum
4
(c) is for [Fe2(pdt)(CO)2(dppv)2]+ obtained from treating Fe2(pdt)-
(CO)2(dppv)2 with FcBArF in THF.
4
CONCLUSIONS
■
The complex [H1H]0 is a unique example of an isolable
dihydride derivative of the [FeFe]-hydrogenase mimics. Several
mechanistic studies invoke di- or even trihydride intermedi-
ates,1,32 but such species have been unavailable to directly
probe their reactivity. Once the targeted dihydride [H1H]0 had
been generated, the first question addressed, both computa-
tionally and spectroscopically, was its structure. The occurrence
of a basal terminal hydride is unprecedented in the chemistry of
diiron dithiolates, although basal hydride intermediates are
implicated in the conversion of terminal, apical hydrides to μ-
hydrides (Scheme 2).22
Scheme 2
Figure 13. Structure of the cation [Fe2(pdt)(CO)2(dppv)2]+
−
([1(CO)]+). Counterion (BF4 ) and solvent of crystallization are
not shown. Thermal ellipsoids are set at the 50% probability level. For
clarity, phenyl groups are drawn as lines and hydrogen atoms are
́
omitted. Selected distances Å: Fe(1)−Fe(2), 2.5942(5); Fe(1)−
In addition to the calculations and 2D NMR analyses, the
basal stereochemistry of the terminal hydride is consistent with
the fast intramolecular H/H exchange, a process subject to a
substantial inverse isotope effect (kH/kD ≈ 0.14 at −40 °C).
This result is consistent with calculations showing the H−H
(D−D) bond forms at the transition state for exchange of the
hydride sites.33 Typically, normal kinetic isotope effects (kH/kD
> 1) are observed for the reductive elimination of dihydrides.33
The hydridic character of [H1H]0 contrasts starkly with
previous examples of diiron hydrides, which are unaffected by
mild oxidants and strong acids. For example, [term-HFe2(pdt)-
(CO)2(dppv)2]+ requires reduction to undergo protonolysis.3
In contrast, even weak acids induce H2 evolution from [H1H]0.
The enhanced reactivity of [H1H]0 is attributable to its neutral
charge and by the donor strength of the μ-hydride. The
complex [H1H]0 is sufficiently hydridic to behave like the
“third hydrogenase” (Hmd). In Hmd, a single Fe center
C(56), 1.750(3); Fe(2)−C(57), 1.786(3). Selected angles (deg):
Fe(1)−Fe(2)−C(57), 70.67(8); Fe(2)−Fe(1)−C(56), 109.66(9);
Fe(2)−C(57)−O(2), 169.9(2); Fe(1)−C(56)−O(1), 179.2(3).
reactivity toward 1,3-dimethylbenzimidazolium salts. Treatment
of this weak hydride acceptor31 with [H1H]0 produced 1,3-
dimethylbenzimidazoline. When the reaction was conducted in
the presence of MeCN, [H1H]0 converted to [H1(NCMe)]+.
[H1H]0 was, however, found to be unreactive toward PhCHO
and CO2.
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(83%). 1H NMR (500 MHz, THF-d8, −40 °C): δ 8.04−6.75 (m, 42H,
40 × ArH, Ph2PCHCHPPh2), 6.30 (m, 1H, Ph2PCHCHPPh2),
5.76 (m, 1H, Ph2PCHCHPPh2), 2.13−1.25 (m, 6H,
SCH2CH2SCH2), −12.2 (q, 1H, JP−H = 24 Hz, μ-H) and −18.9 (t,
1H, JP−H = 62 Hz, t-H). 31P{1H} NMR (202 MHz, CD2Cl2, −40 °C):
δ108, 105, 80.6, and 77.9. FT-IR (THF): 1915. Anal. Calcd (found)
for C56H52Fe2OP4S2: C, 64.63 (64.42); H, 5.04 (5.49).
catalyzes the transfers of hydride to a dihydroimidazolium
substrate, one step in the methanogenesis cycle.34 The
emergence of hydride transfer reactivity is manifested also in
the hydrogenation of an electrophilic alkyne by [H1H]0. These
reactions may foreshadow the development of bioinspired
catalysts for reactions other than H2/H+ redox.
Oxidants induce reductive elimination of H2 from [H1H]0,
resulting in net reduction. With the HOMO strongly localized
on the terminal hydride, the oxidatively induced reductive
elimination can be envisioned as follows (eqs 5 and 6):
Reaction of [H1H]0 with D2. A THF-d8 solution of [H1H]0 was
1
sealed in a J-young tube under 6 psi of D2. H NMR spectra were
recorded at 2 h intervals for several hours. The H2 produced by the
reaction was observed at δ 4.57 (s) together with small amount of HD
(∼10%). In the 2H NMR spectrum of the product, signals were found
at δ −12.2 and −18.9 in ratio of 1:1. No hydrides signals were
H−FeII−H−FeII → H−FeIII−H−FeII + e−
(5)
1
observed in the high field range of the H NMR spectrum. Although
H−FeIII−H−FeII + CO → FeI−CO−FeII + H2
the 31P NMR chemical shifts are almost identical to those for [H1H]0,
the coupling patterns of the signals differ from those of [H1H]0. Broad
double multiplets (dm, 44.9 Hz) at δ 80.0 and 77.9, as well as broad
signals at δ 108 and 105. FT-IR (THF): 1919.
(6)
The finding that the FeI−CO−FeII species does not bind H2 is
consistent with our proposal that H2 activation requires an
auxiliary oxidant.35
Protonolysis of [H1H]0 in the presence of CO. A J. Young
NMR tube containing [H1H]0 (15 mg, 0.0144 mmol) and
H(OEt2)2BArF4 (15 mg, 0.0148 mmol) was cooled in liquid nitrogen
and evacuated. About 0.5 mL of THF was transferred on the top of the
solids, and the frozen sample was purged with CO (6 psi) or 13CO
(∼6 psi). Then, the NMR tube was vigorously shaken, and the sample
was warmed up to room temperature. During this period, the color of
the solution changed from dark green to brown. After reaction, the
solvent was removed under vacuum. The residue was redissolved in
CD2Cl2, and the yields of [H1(CO)]+ were quantified using
The new compound [H1H]0 is a precursor to the following
highly unsaturated species: the 31e- and 32e-cations [Fe2(pdt)-
(CO)(dppv)2]+ and [HFe2(pdt)(CO)(dppv)2]+ and the 32e-
neutral species [Fe2(pdt)(CO)(dppv)2]0. Oxidation is known
to greatly enhance the acidity of hydrides and induce reductive
elimination of H2.36 Proposed intermediates are trapped as
their CO derivatives. It is interesting that these hydrogenase
mimics, like the hydrogenases, show no affinity for N2 despite
their high sensitivity toward CO.37
[HFe2(edt)(CO)4(PMe3)2]BArF as an internal integration standard
4
(SI). NMR, ESI-MS, and IR analyses indicated the formation of
[H1(CO)]+ when the protonolysis was conducted under CO, and
production of [H1(CNMe)]+ in the presence of MeCN.
EXPERIMENTAL SECTION
■
Material and Methods. Experimental methods have been recently
described.13 NBu4BH4, 13CO and D2 were purchased from Sigma-
Aldrich and used as received. FT-IR measurements (in cm−1) are
reported for the νCO region only.
Oxidation of [H1H]0 in the Presence of CO. The oxidation
experiment was conducted similarly to the protonolysis, using FcBArF
4
or FcBF4. After reaction, solvent was removed under vacuum. The
residue was washed with hexane (5 mL × 3) and redissolved in
CH2Cl2. Crystals of [1(CO)]BF4 were grown by allowing a hexane
layer to diffuse into a CH2Cl2 solution at −20 °C. The compound was
isolated in an estimated yield of 80%. The product was identified as
[1(CO)]BF4 by IR, ESI-MS, and X-ray crystallography.
[HFe2(pdt)(CO)(NCMe)(dppv)2]BF4 ([H1(NCMe)]BF4). A solu-
tion of [H1]BF4 (345 mg, 0.3 mmol)13 in 100 mL of CH2Cl2/MeCN
(v/v = 90/10) in a Pyrex Schlenk tube was photolyzed (>400 nm
cutoff filter) with a 200 W medium-pressure mercury lamp under
argon flow. The reaction was monitored by FT-IR spectroscopy. The
color of the reaction mixture changed from brown to black gradually
over the course of the reaction. After ∼6 h, the solvent was removed
under vacuum. The residue was extracted into 10 mL of CH2Cl2. The
solution was filtered through a plug of Celite, and the filtrate was
diluted by 30 mL of diethyl ether, resulting in precipitation of black
H2 Elimination from [H1H]0 in the Presence of CO. A THF
solution of [H1H]0 in a J. Young NMR tube was frozen, and the tube
was evacuated. The frozen sample was purged with CO (6 psi) or
13CO. The sample was warmed to room temperature, the sample was
shaken, and the 13P{1H} NMR spectra were recorded at 2 h intervals.
A new signal appeared at δ 90.7 concomitant with the disappearance of
signals at δ 108, 105, 80.6, and 79.7 for [H1H]0. This new species
generated features of νCO bands at 1896(sh), 1884(s), 1849(sh), and
1838(s) cm−1. According to mass spectrum data (ESI-MS: m/z
1067.4), [Fe2(pdt)(CO)(13CO)(dppv)2] was generated.
1
solid. Yield: 310 mg (88%). H NMR (500 MHz, CD2Cl2, 20 °C): δ
8.02−6.05 (m, 44H, 40 × ArH, 2 × Ph2PCHCHPPh2), 2.55−1.15
(m, 6H, SCH2CH2SCH2), 0.93 (s, 3H, CH3CN), −12.68 (m, 1H, μ-
H). 31P{1H} NMR (202 MHz, CD2Cl2, 20 °C): ab−bb isomer, δ 96.6
(s), 85 (s), 84.9 (s) and 78 (s); ab−ab isomer, δ 87.3 (s), 85.4 (s),
84.8 (s) and 82.6 (s). The ab−bb/ab−ab ratio is 87:13. FT-IR (THF):
1942. ESI-MS, m/z: 1080.5 ([M-BF4]+). Anal. Calcd (found) for
C58H54NBF4Fe2OP4S2·CH2Cl2: C, 56.58 (56.72); H, 4.51 (4.58); N,
1.12 (1.33). Single crystals were obtained by allowing a hexane layer to
diffuse into a CH2Cl2 solution of [H1(NCMe)]BF4 at −20 °C.
The compound [D1(NCMe)]BF4 was prepared analogously from
[D1(CO)]BF4, according to the same procedure of preparing
[H1(NCMe)]BF4. 31P{1H} NMR (202 MHz, CD2Cl2, 20 °C): ab−
ab isomer, δ 94.5 (s), 83 (s) and 76 (s), and with the ratio of 1:2:1;
ab−ab isomer, δ 85.4 (s), 83.4 (s), 82.7 (s) and 80.6 (s). The ab−bb/
ab−ab ratio is 85:15. FT-IR (THF): 1942.
Hydride Transfer to N,N-Dimethylbenzimidazolium from
[H1H]0. A solution of 15 mg (0.0144 mmol) [H1H]0 in 2 mL of
THF/MeCN (v/v = 1/1) was treated by N,N′-dimethylbenzimidazo-
lium hexafluorophosphate38 (5.0 mg, 0.0171 mmol). After 20 min, the
solvent was removed under vacuum. The residue was dissolved in
CD2Cl2 for NMR analysis. In the 1H NMR spectrum, two singlets at δ
4. 30 (2H, −CH3 NCH2 NCH3 −) and
δ 2. 72 (6H,
−CH3NCH2NCH3−) for 1,3-dimethylbenzimidazoline were observed;
and a hydride signal was shown at −12.65 (m). 31P NMR and IR
spectra matched that for the two isomers of [H1(NCMe)]+.
Hydrogenation of Dimethyl Acetylenedicarboxylate with
[H1H]0. A solution of [H1H]0 (10 mg, 0.01 mmol) in 5 mL of THF
was frozen with liquid nitrogen. The atmosphere over the frozen
solution was then filled with CO (6 psi). A sample of DMAD (∼1.5
μL, 0.02 mmol) was injected. The mixture was allowed to warm to
room temperature and stirred for a further 30 min. Solvent was
removed under gentle vacuum, and hexane (5 mL × 3) was added to
the green residue to extract organic compounds. The extracts were
combined and dried under vacuum, and the oily residue was dissolved
[H2Fe2(pdt)(CO)(dppv)2] ([H1H]0). To
a solution of
[H1(NCMe)]BF4 (233 mg, 0.2 mmol) in 10 mL of THF was
added solution of NBu4BH4 (52 mg, 0.2 mmol) in 5 mL of THF over
the course of 5 min. The reaction mixture was then stirred at room
temperature for 30 min, during which time the solution color changed
from virtually black to green. The solvent was removed under vacuum.
The residue was extracted into toluene (5 mL × 3). The extracts was
filtered through Celite, concentrated, and diluted with 30 mL of
pentane, resulting in precipitation of a deep-green solid. Yield: 173 mg
1
1
in CDCl3 for H NMR analysis. The H NMR spectrum displays a
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1
singlet at δ 6.27 for cis-MeO2CCHCHCO2Me (note: H NMR for
trans-MeO2CCHCHCO2Me is δ 6.86). IR and 31P NMR analysis of
the residue indicate that [1(CO)] was produced. When the same
procedure was conducted using [H1H]0 and under an atmosphere of
method46 was used for approximating expensive four-center integrals
(describing the classical electron−electron repulsive contribution to
the total energy) through a combination of two three-center integrals.
This procedure is made possible by expanding the density ρ in terms
of an atom-centered and very large auxiliary basis set. IR simulations
were performed with the same full-electron TZVP basis set that was
employed to investigate energies and structures of isomeric forms of
interest.
As for the DFT-calculated NMR data, the model was simplified by
replacing Ph by methyl groups. The TZVP and TZVPP basis sets have
been checked to provide approximately the same chemical shifts.
Nuclear magnetic shielding tensors were computed using B-P86/
TZVP geometries and the gauge including atomic orbital (GIAO)
method.47
2
D2, H NMR analysis shows a singlet at δ 6.27 for cis-MeO2CCD
CDCO2Me.
DNMR Experiments. DNMR experiments were performed on a
Varian Inova 600 MHz spectrometer equipped with a 5 mm AutoX
dual broadband probe with z-gradient capability. The exchange rates of
hydrides were measured by spin saturation transfer experiments.39,40 A
series of 50 spectra were collected with the saturation (selective
irradiation) on either the bridging (δ −12.2) or the terminal hydride
(δ −18.9) at different saturation times. The saturation times ranged
from 0.002 to 2.5 s. The saturation power at 12 dB was chosen, which
gave about 80% decrease of the signal with 1 s of saturation compared
with a normal proton spectrum. The relaxation delay between pulses
was 4.10 s. The intensity (integral) of the other hydride (the one not
irradiated) was measured and was fitted to:
ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR and 31P{1H}NMR spectra for new compounds. X-ray
crystallographic data in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
⎛
⎜
⎞
⎟
I(t)
I(0)
τ*
τ
−t
τ*
τ*
1
=
exp
+
⎝
⎠
T
AUTHOR INFORMATION
Corresponding Authors
rauchfuz@illinois.edu (T.B.M.)
giuseppe.zampella@unimib.it (G.Z.)
where T1 is the spin−lattice relaxation time of the hydride, τ is the
■
−1
lifetime of the exchanging hydride center, and τ* = (T1 + τ−1)−1.
The spin−lattice relaxation times T1 for the hydride signals at δ
−12.2 and δ −18.9 were measured to be 0.748 and 0.644 s,
respectively for [H1H]0 at −40 °C in THF-d8 solution (sample
concentration was 24 mM). For [D1D]0, the T1 value for the signal at
δ −12.2 was found to be 0.101 s.
Notes
The authors declare no competing financial interest.
The two-dimensional (2D) 1H−31P HMBC41 spectra were collected
at different multibond J values; the spectrum with J = 60 Hz is shown
in Figure 4. In the 2D 1H−31P HMBC experiments, the spectral
windows of 1H and 31P were set to 26991 and 14567 Hz, respectively.
ACKNOWLEDGMENTS
■
This research was conducted under contracts DEFG02-
90ER14146 and DE-SC0001059 with the U.S. Department of
Energy by its Division of Chemical Sciences, Office of Basic
Energy Sciences.
1
A total of 32 transients were acquired in the H dimension and 100
transients were acquired in the 31P dimension, using a relaxation delay
of 1 s and acquisition time of 0.15 s. The spectra were processed with a
90°-shifted sine-bell square window function in Mnova 8.8.1
(Mestralab Research S.L.).
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