Heterolytic H2 cleavage and catalytic hydrogenation by an iron metallaboratrane

Article abstract of DOI:10.1021/om400281v

Reversible, heterolytic addition of H₂ across an iron-boron bond in a ferraboratrane with formal hydride transfer to the boron gives iron-borohydrido-hydride complexes. These compounds catalyze the hydrogenation of alkenes and alkynes to the respective alkanes. Notably, the boron is capable of acting as a shuttle for hydride transfer to substrates. The results are interesting in the context of heterolytic substrate addition across metal-boron bonds in metallaboratranes and related systems, as well as metal-ligand bifunctional catalysis.

Full text of DOI:10.1021/om400281v

Article
pubs.acs.org/Organometallics
Heterolytic H2 Cleavage and Catalytic Hydrogenation by an Iron
Metallaboratrane
Henry Fong, Marc-Etienne Moret,† Yunho Lee,‡ and Jonas C. Peters*
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
S
* Supporting Information
ABSTRACT: Reversible, heterolytic addition of H2 across an iron−
boron bond in a ferraboratrane with formal hydride transfer to the
boron gives iron-borohydrido-hydride complexes. These compounds
catalyze the hydrogenation of alkenes and alkynes to the respective
alkanes. Notably, the boron is capable of acting as a shuttle for
hydride transfer to substrates. The results are interesting in the
context of heterolytic substrate addition across metal−boron bonds
in metallaboratranes and related systems, as well as metal−ligand
bifunctional catalysis.
the transfer hydrogenation of ketones catalyzed by a rhodium
diphosphine-borane complex.11
INTRODUCTION
■
Transition metal-catalyzed bond-forming reactions often
involve formal two-electron redox steps (e.g., oxidative addition
and reductive elimination). Noble metal catalysts are
commonly used in these reactions due, in part, to their
propensity to facilitate multielectron processes.1 There is
growing interest in developing catalysts for bond forming/
cleavage reactions based on earth-abundant mid to late first-row
transition metals, a goal that presents a unique set of
challenges.2 First-row transition metal catalysts that can
circumvent undesirable one-electron processes in favor of
concerted two-electron reaction steps present one plausible
design criterion.3
Herein we report on studies of heterolytic H−H bond
cleavage at a ferraboratrane complex12 of a triphosphine-borane
(TPB) ligand.13 Dihydrogen is shown to add reversibly across
the Fe−B bond of (TPB)Fe(L) complexes to form
corresponding iron-borohydrido-hydride complexes of the
type (TPB)(μ-H)Fe(L)(H). Like the nickel system reported
previously, olefin hydrogenation catalysis is accessible, albeit
much slower, thereby facilitating detailed studies. As discussed
below, other E−H bonds are also activated by the
ferraboratanes described, including the terminal C−H bonds
of arylacetylenes and the C−H bonds of formaldehyde.
Cooperative catalysis strategies that utilize ligands that
operate in tandem with a coordinated metal center to activate
substrates have shown promise in addressing this issue.4,5 First-
row metallaboratranes and related compounds that contain a
retrodative M→B σ-interaction6 are appealing as catalysts7
because of the boron center’s ability to stabilize low-valent
metals.8 Akin to frustrated Lewis pairs,9 it has also been
recently demonstrated that the metal−boron interaction can
cooperatively facilitate the activation of H2.5,7 For instance, our
lab recently reported that the diphosphine-borane nickel
RESULTS AND DISCUSSION
■
Reversible H2 Addition. Exposing the previously reported
(TPB)Fe(N2) complex12a (1) in d6-benzene to H2 (1.2 equiv)
at room temperature results in H2 addition across the Fe−B
bond to give a yellow solution of the six-coordinate
borohydride-hydride-N2 complex (TPB)(μ-H)Fe(N2)(H) (2)
(Scheme 2, Figure 1). The XRD structure of 2 shows that the
Fe−B distance is significantly elongated relative to 1 (2.604(3)
Å in 2 versus the calculated distance of 2.2 Å in 112b) (Figure
1). A terminal hydride ligand and a bridging hydride ligand
located between the B and Fe atoms can be assigned from the
electron density difference map. This structure also prevails in
(
MesDPBPh)Ni complex undergoes reversible oxidative addition
of H2 to afford a nickel-borohydrido-hydride complex (Scheme
1).10 This nickel system is an efficient catalyst for olefin
hydrogenation. Kameo and Nakazawa have also reported on
1
solution. In the H NMR spectrum (d6-benzene), the terminal
hydride ligand on iron is observed as a triplet-of-doublets at
−9.6 ppm, and the bridging hydride is observed as a broad
singlet at −30.4 ppm. Replacing H2 with D2 in the reaction
gives the corresponding isotopologue (TPB)(μ-D)Fe(N2)(D).
Scheme 1. Related H2 Activation across a Ni−B Bond
2
Along with the expected deuteride signals in the H NMR
spectrum, deuterium signals from the methine and terminal
Received: April 3, 2013
Published: May 6, 2013
© 2013 American Chemical Society
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Scheme 2. H−H and E−H Bond Activations across Fe−B
Bonds
Figure 2. XRD- (left) and DFT-optimized (right) structures of 3.
Ellipsoids shown at 50% probability. Selected bond distances (Å) and
angles (deg): XRD, Fe1−H42 = 1.56(2), B1−H42 = 1.21(2), Fe1−B1,
2.63(2), P2−Fe1−P3 = 136.54(2); DFT predicted, Fe1−H42 = 1.51,
Fe1−H43 = 1.50, Fe1−H44 = 1.62, Fe1−P45 = 1.59, B1−H42 = 1.24,
Fe1−B1 = 2.63, H44−H45 = 0.84, P2−Fe1−P3 = 140.63.
classical hydrides. We therefore turned to NMR spectroscopy
to aid in the formulation of 3.
1
The 20 °C H NMR spectrum (d8-toluene) of 3 (Figure 3)
shows a broad singlet resonance at −15.1 ppm, indicative of
1
Figure 3. H VT-NMR spectra of 3 in d8-toluene under 1 atm of H2.
hydridic protons. A broad deuteride signal is observed in the 2H
NMR spectrum when D2 is used in place of H2, and, like in 2,
deuterium signals are also observed in the methyl and methine
positions of the isopropyl groups of the TPB ligand. Cooling a
d8-toluene solution of 3 under an H2 atmosphere to −20 °C
leads to sharpening of the resonance at −15.1 ppm, which
integrates to three protons (3H). A second, broad hydridic
resonance integrating to one proton is also observed at −24.9
ppm, and in analogy with 2, this resonance is assigned to the
bridging borohydride (μ-H). Cooling to −90 °C leads to
broadening of the 3H resonance without reaching decoales-
cence, suggesting that exchange of three hydrogenic ligands is
fast on the NMR time scale. Compound 3 can be heated to 50
°C before significant sample decomposition is observed (vide
infra). At 50 °C, the 3H and μ-H signals both broaden into the
baseline, suggesting that exchange of all four hydrogens is facile
at this temperature.
To further assign the 3H unit in 3 (i.e., dihydrogen-hydride
versus trihydride), we turned to minimal longitudinal relaxation
(T1 min) measurements.14,15 The T1 min is 35 ms at −32 °C for
the 3H resonance, which suggests that the 3H unit is best
described as dihydrogen-hydride and implying the following
assignment for 3: (TPB)(μ-H)Fe(H2)(H). This interpretation
is additionally supported by DFT calculations (RB3LYP/6-
Figure 1. XRD structures of 2 (left) and 5 (right). Ellipsoids are
shown at 50% probability. Selected bond distances (Å): 2, Fe1−H1 =
1.42(2), Fe1−H2 = 1.49(2), B1−H2 = 1.17(2), Fe1−B1 = 2.604(3);
5, Fe1−H42 = 1.35(2), Fe1−H43 = 1.52(2), B1−H43 = 1.20(2),
Fe1−B1 = 2.673(2).
methyl positions of the isopropyl groups of the TPB ligand are
observed. This observation establishes that facile scrambling of
the hydridic ligands into the TPB isopropyl groups occurs,
presumably via a reversible C−H metalation process.
The dinitrogen ligand (νNN = 2070 cm−1) in 2 is labile, and it
can be substituted under excess H2 to give the dihydrogen
analogue (TPB)(μ-H)Fe(H2)(H) (3) (Scheme 2). Exposing 1
to excess H2 (1 atm) also generates 3. Its XRD structure again
confirms the presence of a bridging hydride (Figure 2).
Although electron density can be located in the difference map
between a widened P−Fe−P angle (136.54(2)°) and in the
apical position trans to the borohydride unit, the data do not
allow us to reliably distinguish between classical and non-
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31G(d)) that identify a dihydrogen-hydride structure (Figure 2
and Supporting Information) as the lowest energy isomer of 3.
A stereoisomer in which the H2 ligand occupies the equatorial
position and the hydride ligand is in the axial position trans to
boron is calculated to be 6.1 kcal/mol higher in energy. This
geometric preference parallels that of the well-characterized
iron-dihydrogen-dihydride complex mer-Fe(H2 )-
(H)2(PEt2Ph)3.16 The transition state for conversion of 3
into this higher energy stereoisomer involving H−H scission is
calculated to be 6.7 kcal/mol above the most stable isomer and
is in line with the observed exchange behavior on the NMR
time scale.
Scheme 3. Chemistry of the Related Silatrane System
Analogous (TPB)(μ-H)Fe(L)(H) complexes (L = CNtBu
(5), CO (7)) can be synthesized (Scheme 2, Figure 1). To
explore the effect of the apical ligand L on the H−H bond
activation process, (TPB)Fe(L) complexes (L = CNtBu, 4; L =
CO, 6) were prepared. The previously reported carbonyl
complex 6 displays a η3-interaction with a P−CAr−CAr unit of
the TPB ligand,12a whereas isocyanide adduct 4, whose
structure has been determined, does not: its Fe center is
rigorously five-coordinate. Complex 6 is diamagnetic, whereas 4
gives rise to a solution magnetic susceptibility (μeff = 1.7 μB) at
room temperature in C6D6. The temperature dependence of
the solution susceptibility suggests an S = 0 ground state with a
thermally accessible S = 1 state. No reaction occurs between
(TPB)Fe(CNtBu) (4, μeff = 1.7 μB) or the previously reported
(TPB)Fe(CO)12a (6) with H2 (1 atm) at room temperature
over a period of hours. Compound 4 is fully consumed by H2
(1 atm) over the course of 3 days at 40 °C to generate 5, while
compound 6 is fully consumed by H2 (1 atm) over the course
of 5 days at 80 °C to give 7. The increase in temperature and
reaction time compared to the facile room temperature reaction
between 1 and H2 is consistent with a scenario in which H2
substitution for L occurs prior to H2 addition across the Fe−B
bond. Complex 7 can be alternatively synthesized from 1 or 2
and formaldehyde (Scheme 2).
Fe−B bond distances (varying by ca. 0.5 Å)12b versus more
rigid Fe−Si bond distances (varying by ca. 0.2 Å)17−19 in
(SiPiPr3)Fe that have been observed over several formal iron
oxidation states. The rigidity of the Fe−Si interaction
presumably reflects an appreciably stronger Fe−Si bond relative
to Fe−B. One can additionally consider the relative Lewis
acidity of the Ar3B versus the Ar3Si+ subunit18 in these
respective systems and their propensity to serve as H−
acceptors, but one might then predict the Ar3Si+ to be the
better acceptor, in contrast to the experimental observations.
Indeed, this latter point may be the reason the Fe−Si
interaction is stronger than the Fe−B interaction.
Reaction with Unsaturated Substrates. The ability of
the (TPB)Fe scaffold to reversibly cleave H2 prompted us to
study if the transfer of hydrogen from (TPB)(μ-H)Fe(L)(H)
to substrates is possible. The reaction of 1 with ethylene,
styrene, and arylacetylenes was probed. A degassed d6-benzene
solution of 1 reacts with ethylene to give a light brown solution
of the paramagnetic iron-ethylene adduct (TPB)Fe(C2H4) (8;
μeff = 3.2 μB, S = 1) (Scheme 4). Brown XRD quality crystals of
Dihydrogen addition across the Fe−B bond of 1 is reversible.
Conversion of 3 to 2 and subsequently back to 1 can be
effected by exposing 3 to dynamic vacuum and then N2 or by
repeated freeze−pump−thaw−N2 cycles. Re-formation of the
Fe−B bond can also occur through hydride transfer to
unsaturated substrates (vide infra). Dihydrogen elimination
from 5 and 7 does not occur when treated similarly.
Scheme 4. Ethylene Coordination and Arylacetylene C−H
Bond Activation by 1
Worth underscoring is that cleavage of the Fe−B bond in the
present ferraboratrane system is distinct from the H2 chemistry
observed for a structurally related (SiPiPr3)Fe silatrane system
−
(SiPiPr = [Si(o-C6H4PiPr )3] ) that we have introduced
2
3
elsewhere.17−19 For instance, the reaction between (SiPiPr3)Fe-
(N2) and H2 affords (SiPiPr3)Fe(H2), and that between
[(SiPiPr3)Fe(N2)]+ and H2 affords [(SiPiPr3)Fe(H2)]+. No
disruption of the Fe−Si bond is observed in either case, even
if for instance isolated [(SiPiPr3)Fe(N2)]+ or [(SiPiPr3)Fe(H2)]+
is exposed to excess H2. Ligand substitution instead occurs.
This sharply contrasts isoelectronic (TPB)Fe(N2) 1, where H2
addition readily affords the cleavage product 2 or 3. We also
find (this report, Scheme 3) that hydrogenolysis of the iron(II)
methyl complex (SiPiPr3)Fe(Me) occurs slowly at 60 °C to give
an H2/H product, but once again without disruption of the Fe−
Si bond. Addition of exogenous donor ligands such as H2, N2,
and CO effect substitution of the coordinated H2 ligand, but
the Fe−Si bond is maintained. One factor contributing to the
difference between the two systems is likely the more flexible
Fe−B bond in the (TPB)Fe system, as reflected in the variable
8 can be grown under an atmosphere of ethylene at 0 °C. Two
molecules of 8 are found in the asymmetric unit cell. The iron
center is bound η2 to ethylene ((Fe−C)av = 2.108(1) Å) and
lies above the plane defined by the phosphine donors by an
average distance of 0.641 Å, with a corresponding elongation of
the average Fe−B distance to 2.491(2) Å (compared to 2.2 Å in
1). The average C−C bond distance ((C−C)av = 1.397(2) Å)
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of the η2-coordinated ethylene molecule is significantly
elongated from that in free ethylene (1.337 Å).20 The data
are consistent with π-back-bonding from iron to ethylene,
which confers significant ferracyclopropane character to 8.21
Storing 8 for 2 days under an atmosphere of N2 fully
regenerates 1.
Scheme 5. Reversible Arylacetylene C−H Bond Activation
Compound 1 does not afford a detectable styrene adduct but
reacts with both phenyl- and tolylacetylene with formal hydride
transfer from the terminal C(sp)−H of the arylacetylene to the
boron, forming S = 2 iron-borohydrido-arylacetylide complexes
(TPBH)Fe(C2Ar) (Ar = Ph, 9, μeff = 5.1 μB; Ar = Tol, 10, μeff =
5.2 μB) (Scheme 4). Two molecules of 10 are found in the
asymmetric unit, and the XRD structure shows the presence of
a tolylacetylide ligand coordinated to a pyramidalized iron
center (Figure 4). While the hydride on the boron cannot be
Scheme 6. Stoichiometric Hydrogenation Reactions
Figure 4. XRD structures of 8 (left) and 10 (right). Ellipsoids are
shown at 50% probability. Selected bond distances (Å) and angles
(deg): 8 (average for two molecules in the asymmetric unit cell), Fe1−
B1 = 2.491(1), Fe1−C37 = 2.103(1), Fe1−C38 = 2.113(1), C37−C38
= 1.397(2), ∑(P−Fe−P) = 338.76(3); 10 (average for two molecules
in the asymmetric unit cell), Fe1−B1 = 2.761(2), Fe1−C37 =
1.918(2) C37−C38 = 1.169(3), ∑(P−Fe−P) = 345.07(2).
reliably located by XRD, the IR spectra for both 9 and 10 show
B−H stretches at 2490 and 2500 cm−1, respectively, most
consistent with a nonbridging B−H unit. The vibrational bands
shift to 1826 cm−1 (predicted 1834 cm−1) for 9 and 1824 cm−1
(predicted 1841 cm−1) for 10 upon labeling with the
monodeuterated arylacetylene (ArCCD).
paramagnetic intermediate A and 8 can also be observed by
in situ 1H NMR spectroscopy if the same reaction is run under
ethylene (1 atm) and H2 (1 atm). The IR spectrum of the
reaction mixture shows a diagnostic terminal B−H vibration at
2470 cm−1 that is attributed to A. Akin to the iron-
borohydrido-alkynyl complexes 9 and 10, A is assigned to the
iron-borohydrido-ethyl complex (Scheme 6-i).22 Using D2 in
place of H2 in the ethylene hydrogenation reaction and
monitoring leads to the observation of both B−D and B−H
stretches in the IR spectrum, which is consistent with facile
insertion/β-hydride elimination processes prior to ethane
elimination from A. We note the similarity of A (and 9 and
10) to the well-characterized zwitterionic tris(phosphino)-
borate-iron-ethyl complex (PhBP3)Fe(Et) that was observed as
an intermediate in ethylene hydrogenation with the previously
reported iron (PhBP3)Fe system.3e
The activation of the arylacetylene C(sp)−H bond by 1 is
reversible. Mixing a d6-benzene solution of 9 with tolylacetylene
(4 equiv) and, conversely, mixing a d6-benzene solution of 10
with phenylacetylene (4 equiv) both result in a mixture of 9
and 10 (Scheme 5A). The corresponding exchange reactions
with B−D-labeled isotopologues of 9 or 10 ((TPBD)Fe-
(C2Ar)) and a different all-protio arylacetylene (Ar′CCH)
result in the exclusive formation of free ArCCD, indicating
that the arylacetylene unit is reductively eliminated from the
iron-borohydrido-arylacetylide complexes prior to activation of
an incoming acetylene substrate, presumably by reversible
hydride transfer from the boron to the arylacetylide to form
intermediate π-adducts akin to 8 (Scheme 5B).
Complexes 1, 2, and 3 hydrogenate both phenylacetylene (1
equiv) and styrene (1 equiv) to ethylbenzene (1 atm H2) with
Stoichiometric Hydrogenations. We also explored
whether the transfer of hydrogen from (TPB)(μ-H)Fe(L)(H)
species to unsaturated substrates might be possible. Exposing 8
to excess H2 (1 atm) results in complete conversion of ethylene
to ethane and 3 as the iron-containing product (Scheme 6-i) in
less than 12 h. A paramagnetic intermediate (A) and 8 can be
observed by in situ 1H NMR spectroscopy. The same
1
3 as the observable iron-containing product. The in situ H
NMR spectrum of styrene hydrogenation reactions using 1, 2,
or 3 show styrene, ethylbenzene, and 3 in solution during the
reaction course. Furthermore, for styrene hydrogenation with
2
D2 the H NMR spectrum and GC-MS data of the reaction
mixture show incorporation of deuterium onto both olefinic
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carbon atoms of free styrene, indicating that styrene
coordination to the iron center and insertion/β-hydride
elimination processes are reversible.
result offers the cautionary note that B−CAr bond cleavage to
give metal-borohydride products is a viable catalyst decom-
position pathway.
Under stoichiometric conditions, the addition of 1 equiv of
styrene to a solution of 2 in d6-benzene and under N2 (1 atm)
cleanly generates 1 equiv of ethylbenzene and 1 (Scheme 6-ii).
In contrast, running the same reaction under a static vacuum
yields a mixture of styrene, ethylbenzene, 1, and 2 (Scheme 6-
iii). These observations suggest that excess H2 or N2 is required
for the hydrogenations to proceed to completion. Moreover,
the bridging monohydride appears competent for transfer to a
substrate.
Substrates including trans-stilbene, N-benzylideneaniline,
acetone, and acetophenone were not hydrogenated under
similar conditions. Compounds 5 and 7 also do not
hydrogenate ethylene, styrene, or phenylacetylene under the
same conditions.
Catalytic Hydrogenations. Under the catalytic conditions
of 0.01 M 1, 1 atm of H2, and 30 equiv of the substrate in d6-
benzene at room temperature, ethylene, styrene, and phenyl-
acetylene are hydrogenated to ethane and ethylbenzene,
respectively (Table 1). Compounds 2 and 3 can also be used
Figure 5. Chemical line representation and XRD structure of 11.
Ellipsoids are shown at 50% probability. Selected bond distances (Å):
Fe1−H1 = 1.59(1), Fe1−H2 = 1.63(1), Fe1−B1 = 2.0900(6), Fe1−
P1 = 2.2481(2).
On the basis of the results from the stoichiometric and
catalytic experiments, we propose a plausible mechanistic
scenario to account for the observed catalytic styrene
hydrogenation by 1 (Scheme 7A) and, in doing so, underscore
a
Table 1. Catalytic Hydrogenations by 1 with H2
d
precatalyst
substrate
product
ethane
TOF (h−1
)
b
1
1
1
ethylene
15
Scheme 7. Mechanism and Alkane Elimination Pathways
b
styrene
phenylacetylene
ethylbenzene
ethylbenzene
0.27
0.16
c
a
Conditions: Room temperature, 0.01 M 1, 1 atm H2, and 0.01 M
b
ferrocene as an internal integration standard in d6-benzene. 0.3 M
substrate. 0.29 M substrate. As determined by H NMR spectros-
c
d
1
copy at >95% product.
as precatalysts. Ambient laboratory light does not affect the
reaction and the catalysis is not inhibited by elemental mercury;
it thus appears to be a homogeneous process. Norbornene is
hydrogenated to norbornane, and with an atmosphere of D2 in
place of H2 the cis-addition product exo,exo-2,3-d2norbornane23
is exclusively observed, indicating the syn-addition of hydrogen
and arguing against radical processes.
1
The hydrogenation catalysis was monitored by H NMR
spectroscopy with ferrocene as an internal integration standard.
1
As with stoichiometric ethylene hydrogenation, the in situ H
NMR spectra of the catalytic ethylene hydrogenation reaction
indicate the presence of ethylene adduct 8 and the putative
ethyl-borohydride intermediate A as the iron-containing species
during the reaction course. Complex 3 is the iron-containing
product at the completion of the reaction. For styrene
hydrogenation, styrene, ethylbenzene, and 3 are observed
during catalysis, and scrambling of deuterium into the vinylic
positions of styrene is observed under D2. In contrast, for
phenylacetylene hydrogenation complex 9 is the only observed
iron species early in the reaction when the phenylacetylene
concentration is high. As phenylacetylene is consumed, styrene
and 3 form, and ethylbenzene begins to develop slowly
thereafter.
Attempting to increase the rate of catalysis by elevating the
reaction temperature results in catalyst decomposition. The
decomposition product (11) can be synthesized independently
in near quantitative yields by heating 3 (80 °C) under H2 (1
atm) for 2 h. The XRD structure of 11 indicates that a B−CAr
bond is cleaved from the TPB ligand fragment (Figure 5). This
interesting aspects of the mechanism that remain unanswered.
Starting from precatalyst 1, addition of H2 generates 3, a species
that can be observed by 1H NMR spectroscopy during catalytic
runs (resting state). Subsequent substitution of the apical H2
ligand for styrene forms the unobserved iron-styrene-hydride-
borohydride species B, and insertion into the terminal hydride
affords the iron-alkyl intermediate A′. Intermediate A′ is
analogous to the ethyl species A (Scheme 6-i) and is also
related to the structurally characterized acetylide-borohydride
complex 10 (Scheme 4). Olefin coordination and insertion
appear to be reversible, as labeling studies show deuterium is
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exchanged into the vinylic positions of free styrene under a D2
atmosphere. Elimination of ethylbenzene in the presence of H2
regenerates the catalyst resting state.
formally abstracted by the Lewis acidic borane unit in 1 from a
C(sp)−H hydrogen.25
Dihydrogen addition across the Fe−B bond is reversible, and
the boron is also capable of shuttling the hydride equivalent
derived from H2 to unsaturated substrates under stoichiometric
hydrogenation conditions. The hydrogen chemistry of this
(TPB)Fe(L) system contrasts with the related nickel26 and
cobalt24 complexes of TPB, where the metal−boron bond
remains intact under an H2 atmosphere.
The conversion of 1 to 3 likely proceeds via the H2-adduct
intermediate C depicted in Scheme 7B by H2-for-N2 ligand
exchange. While we have not detected such a species in the
present (TPB)Fe system, its cobalt analogue (TPB)Co(H2) can
be isolated and has been thoroughly characterized,24 as has the
isoelectronic iron complex [(SiPiPr3)Fe(H2)]+.17 N2/H2 ex-
change is facile in these well-defined Co and Fe systems, and by
extension we infer it would also be facile for (TPB)Fe(N2) 1 to
afford (TPB)Fe(H2) C before additional reactions ensue. Since
H2 reacts with 1, but not with 5 and 7 at room temperature, we
think that facile H2 substitution for N2 most likely occurs prior
to H−H bond cleavage. We appreciate that while this scenario
is consistent with the data available, it is not demanded by the
available data. For instance, it is alternatively possible that
styrene substitution for the N2 ligand in 1 precedes H2 addition
to form intermediate B. No direct evidence rules out this
possibility. We prefer suggesting that the H2/dihydride species
3 precedes styrene binding because of the observation that H2
addition/activation by other (TPB)Fe(L) adducts, for example
4 and 6, is very slow and also because N2, and presumably
therefore also H2, displaces ethylene from 8 in solution
(regenerating 1 or (TPB)Fe(H2)).
The final ethylbenzene elimination step can be envisioned to
occur through two plausible routes (Scheme 7B). One such
pathway involves a reductive elimination step where hydride
transfer directly from the borohydride subunit generates the
alkane product to form 3, likely via the dihydrogen adduct
intermediate C. The other pathway proceeds through alkane
elimination by hydrogenolysis of the phenylethyl group without
hydride transfer from the borohydride subunit.
While the available data do not firmly distinguish between
the two product elimination pathways shown in Scheme 7B, the
stoichiometric hydrogenation studies described above show
that 1 equiv of styrene is completely hydrogenated to
ethylbenzene by 2 under an N2 atmosphere (Scheme 6-ii).
This observation implies that 2 can serve as the source of the
two H-atom equivalents delivered to styrene. From complex 2,
substitution of the apical N2 ligand for styrene in 2 would
generate the styrene adduct intermediate B. Subsequent
insertion of the bound styrene into the cis Fe−H would afford
intermediate A′, which in the absence of H2, at least, eliminates
ethylbenzene concomitant with N2 binding to re-form 1. The
presence of exogenous N2 (or alternatively H2) facilitates the
generation of ethylbenzene. As noted in Scheme 6-iii, the
stoichiometric reaction under static vacuum between 2 and
styrene is quite slow compared with the same reaction under
N2 (Scheme 6-ii). This observation can be explained by
presuming the conversion of intermediate A′ to 1 requires N2
association prior to elimination to generate ethylbenzene.
An understanding of the factors that govern metal−boron
bond cleavage will aid in the development of cooperative
catalytic reactions in metallaboratranes. The direct role, if any,
of the borane ligand in assisting the H2 cleavage step is an
interesting question in this context for the present iron and
recently reported diphosphine-borane-iron systems27 and also
conceptually related to the nickel system (Scheme 1).10
Determining whether there is a cooperative interaction between
the coordinated H2 ligand, the iron center, and the borane
subunit en route to H−H cleavage (and its microscopic
reverse), akin to H−H cleavage by frustrated Lewis pairs,9 calls
for detailed theoretical studies that are the subject of ongoing
research.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried out using
standard glovebox or Schlenk techniques under an N2 atmosphere.
Unless otherwise noted, solvents were deoxygenated and dried by
thoroughly sparging with N2 gas followed by passage through an
activated alumina column in the solvent purification system by SG
Water, USA LLC. Deuterated solvents and D2 gas were purchased
from Cambridge Isotope Laboratories, Inc. The deuterated solvents
were degassed and dried over activated 3 Å sieves prior to use. Unless
otherwise noted, all compounds were purchased commercially and
used without further purification. TPB,13 (SiPiPr3)Fe(Me),19 (SiPiPr3)-
Fe(N2)(H),17 and monodeuterated phenyl- and tolylacetylene
(PhC2D and TolC2D)28 were synthesized by literature procedures.
Elemental analyses were performed by Midwest Microlab, LLC,
Indianapolis, IN.
NMR spectra were recorded on Varian 300 MHz, 400 MHz, and
1
500 MHz spectrometers. H and 13C chemical shifts are reported in
ppm relative to residual solvent as internal standards. 31P and 11B
chemical shifts are reported in ppm relative to 85% aqueous H3PO4
and BF3·Et2O, respectively. Multiplicities are indicated by br (broad), s
(singlet), d (doublet), t (triplet), quart (quartet), quin (quintet),
multiplet (m), d-d (doublet-of-doublets), and t-d (triplet-of-doublets).
FT-IR measurements were obtained on samples prepared as KBr
pellets or in solution using a Bio-Rad Excalibur FTS 300 spectrometer
with Varian Resolutions Pro software at 4 cm−1 resolution. The ATR-
IR measurements were measured on a thin film of the complex
obtained from evaporating a drop of the solution on the surface of a
Bruker APLHA ATR-IR spectrometer probe (Platinum Sampling
Module, diamond, OPUS software package) at 2 cm−1 resolution. IR
intensities are indicated by s (strong), m (medium), and w (weak).
X-ray Crystallography. X-ray diffraction was measured on the
Bruker Kappa Apex II diffractometer with Mo Kα radiation. Structures
were solved using the SHELXS software and refined against F2 on all
data sets by full matrix least-squares with SHELXL. The crystals were
mounted on a glass fiber with Paratone oil.
CONCLUSIONS
■
In summary, we have demonstrated that the Fe−B bond in
ferraboratranes (TPB)Fe(L) (where L = N2, 1; CNtBu, 4; CO,
6) can facilitate heterolytic cleavage of H2 and of the C(sp)−H
and C(sp2)−H bonds of arylacetylenes and formaldehyde,
respectively, resulting in Fe−B bond rupture and formal
hydride transfer to the boron of the ligand scaffold. The formal
hydride transfer from the C(sp)−H of arylacetylenes to give
iron-acetylide complexes is distinct from traditional syntheses
of metal-acetylide complexes in that a hydride equivalent is
Computational Methods. Geometry optimizations were per-
formed using the Gaussian03 package. The B3LYP exchange−
correlation functional was employed with a 6-31G(d) basis set. The
GDIIS algorithm was used. A full frequency calculation was performed
on each structure to ensure that they were the true minima. A single
negative vibrational frequency was observed for the transition state
between 3 and its equatorial-H2 isomer, confirming that this structure
was the transition state. See Supporting Information for a full
description of the computational method.
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HD Gas Generation. D2O (1 mL) was added to an evacuated,
cooled sample (−78 °C) of solid lithium aluminum hydride (316 mg,
8.2 mmol) in a Schlenk flask. An evacuated Schlenk line was filled with
the resulting HD gas (ca. 1 atm) as the Schlenk flask was warmed to
room temperature. A J-Young NMR tube containing a freeze−pump−
thawed solution of the respective complex was exposed to the HD gas.
Synthesis of (TPB)(μ-H)Fe(N2)(H) (2). A J-Young NMR tube
containing a brown-red solution of 1 (20.3 mg, 31.1 mmol) in C6D6
(0.8 mL) was freeze−pump−thawed (3×) and, with the J-Young tube
frozen with liquid nitrogen, exposed to H2 (1.2 equiv). The solution
was thawed and mixed, giving a yellow solution. An atmosphere of N2
was subsequently introduced, and the reaction was mixed for 2 h to
yield 2 (100% yield by 1H NMR spectroscopy with a ferrocene
integration standard). Alternatively, 2 could be synthesized from 3 by
removing free H2 from a solution of 3 by freeze−pump−thaw (3×),
exposing it to an N2 atmosphere, and mixing the solution overnight
(3H), 8.6 (3H), 8.5 (9H), 6.4 (3H), 5.2 (9H), 3.7 (12H), 2.9 (9H),
−1.5 (9H), −2.3 (3H). IR (KBr, cm−1): 1972 (CN). UV−vis
(THF, nm {cm−1 M−1}): 600 {shoulder, 428}, 910 {70}. μeff (C6D6,
method of Evans, 20 °C): 1.7 μB. Anal. Calcd for C41H64BFeNP3: C,
67.50; H, 8.70; N, 1.92. Found: C, 67.20; H, 8.54; N, 1.72.
Synthesis of (TPB)(μ-H)Fe(CNtBu)(H) (5). A heavy-walled
Schlenk tube containing a yellow-brown solution of 4 (16.4 mg,
22.4 mmol) in C6H6 (10 mL) was freeze−pump−thawed (3×). Upon
warming to room temperature, the sample was exposed to H2 (1 atm)
for a few minutes. The Schlenk tube was sealed and heated under
vigorous mixing at 40 °C for 85 h. Removal of the solvent in vacuo,
extraction with C6H6, and lyophilization yielded a solid of 5 (17.3 mg,
98%). Room temperature evaporation of a solution of 5 in a diethyl
ether/pentane (2 to 1 mL) mixture yielded yellow crystals suitable for
XRD analysis. 1H NMR (C6D6, 300 MHz): δ 8.1 (1H, d, 3JH−H = 9 Hz,
Ar-H), 7.4 (3H, d, 3JH−H = 9 Hz, Ar-H), 7.2 (3H, d, 3JH−H = 6 Hz, Ar-
H), 7.1 (3H, d, 3JH−H = 9 Hz, Ar-H), 2.7 (2H, br s, PCH), 2.5 (2H, br
s, PCH), 2.2 (2H, t, 2JH−P = 9 Hz, PCH), 1.3 (16H, m, CH3), 1.1 (9H,
s, C(CH3)3), 1.0 (6H, d, 3JH−H = 6 Hz, CH3), 0.8 (6H, d-d, 3JH−P = 15
1
(100% yield by H NMR spectroscopy with a ferrocene integration
standard). Yields could not be determined by mass because 2 was
unstable to prolonged exposure to dynamic vacuum. Yellow-orange
XRD quality crystals were grown in a concentrated solution of
Hz, 3JH−H = 6 Hz, CH3), 0.7 (6H, br s, CH3), −11.7 (1H, t-d, 2JH−Pcis
=
1
2
pentane/THF (10:1) at −30 °C. H NMR (C6D6, 300 MHz): δ 7.9
87 Hz, JH−Ptrans = 27 Hz, Fe-H), −23.9 (1H, br s, Fe-(μ-H)-B). 13C
NMR (C6D6, 125 MHz): δ 176.9 (quart, 2JC−P = 8 Hz, CNtBu), 163.5
(br s, CAr), 163.0 (br s, CAr), 144.9 (m, CAr), 143.0 (d, JC−P = 20 Hz,
CAr), 131.9 (d, 2JC−P = 7.5 Hz, CAr), 130.6 (d, JC−P = 3.2 H, CAr), 130.5
(d, JC−P = 2.5 H, CAr), 128.6 (s, CAr), 127.5 (s, CAr), 124.2 (s, CAr),
123.6 (s, CAr), 55.3 (s, C(CH3)3), 31.6 (s, PCH), 30.9 (s, PCH), 29.2
(2H, d, 3JH−H = 6 Hz, Ar-H), 7.7 (1H, br s, Ar-H), 7.3 (3H, d, 3JH−H
=
3
2
6 Hz, Ar-H), 7.0 (3H, t, JH−H = 9 Hz, Ar-H), 2.7 (4H, d, JP−H = 18
3
Hz, PCH), 2.4 (2H, br s, PCH), 1.4 (6H, d, JH−H = 6 Hz, CH3), 1.3
(12H, br s, CH3), 1.1 (6H, d-d, 3JP−H = 15 Hz, 3JH−H = 6 Hz, CH3), 0.8
(6H, d, 3JP−H = 9 Hz, CH3), −9.6 (1H, d-t, 2JH−Pcis = 81 Hz, 2JH−Ptrans
=
2
2
36 Hz, Fe-H), −30.4 (1H, s, Fe-(μ-H)-B). H NMR (C6H6/C6D6, 76
MHz): δ −9.5 (1D, br s), −30.3 (1D, br s). 31P NMR (C6D6, 121
MHz): δ 73.6 (2P, s), 64.2 (1P, s). 13C NMR (C6D6, 125 MHz): δ
161.9 (s, CAr), 143.5 (s, CAr), 141.0 (s, CAr), 132.3 (s, CAr), 131.8 (s,
CAr), 131.3 (s, CAr), 130.3 (s, CAr), 124.6 (s, CAr), 124.0 (s, CAr), 32.0
(s, PCH), 29.4 (s, PCH), 28.5 (s, PCH), 22.8 (s, CH3), 20.1 (s, CH3),
19.7 (s, CH3), 18.9 (s, CH3). 11B NMR (C6D6, 128 MHz): δ 8.2 (br).
IR (KBr, cm−1): 2071 (s, NN), 1960 (w) 1934 (w). UV−vis (THF,
nm {M−1 cm−1}): 328 {shoulder, 500}, 280 {shoulder, 11250}.
Elemental analysis could not be obtained because of the instability of
the compound under dynamic vacuum.
(m, PCH), 28.5 (d, JC−P = 7.5 Hz, PCH), 23.8 (s, CH3), 20.4 (s,
CH3), 20.3 (m, CH3), 20.2 (s, CH3), 19.9 (s, CH3), 19.7 (s, CH3). 31
P
2
NMR (C6D6, 121 MHz): δ 81.3 (2P, d, JP−P = 63 Hz), 72.4 (1P, s).
IR (KBr, cm−1): 2027 (s, CN), 1942 (w, Fe−H). UV−vis (THF,
nm {cm−1 M−1}): 205 {5530}, 224 {15437}, 245 {17142}, 255
{16635}, 285 {4117}, 335 {shoulder, 2166}, 400 {1830}. Anal. Calcd
for C41H66BFeNP3: C, 67.32; H, 8.96; N, 1.91. Found: C, 66.59; H,
8.61; N, 1.30.
Synthesis of (TPB)(μ-H)Fe(CO)(H) (7) from 6 and H2. In a J-
Young NMR tube, 6 (6.0 mg, 8.9 μmol) was dissolved in C6H6 (0.7
mL) to give a brown-red solution. The solution was freeze−pump−
thawed (3×) and subsequently exposed to H2 (1 atm) for ca. 5 min.
The reaction was then heated at 80 °C for 5 days, during which time a
clear yellow solution developed. Removal of the solvent in vacuo,
extraction with C6H6, and lyophilization yielded a yellow solid of 7
(5.9 mg, 98%). Room temperature evaporation of a solution of 7 in a
diethyl ether/pentane (1 to 0.5 mL) mixture yielded yellow
Synthesis of (TPB)(μ-H)Fe(H2)(H) (3). A J-Young NMR tube
containing a brown-red solution of 1 (21 mg, 31.1 mmol) in C6H6 (0.8
mL) was freeze−pump−thawed (3×). Upon warming to room
temperature, the sample was exposed to H2 (1 atm), resulting in a
clear yellow solution. The reaction was mixed for 24 h to give 3 (100%
by 1H NMR spectroscopy with a ferrocene integration standard). The
yield could not be determined by mass because 3 was unstable to
prolonged exposure to dynamic vacuum. Yellow-orange XRD quality
crystals were grown under 1 atm of H2 in a concentrated solution of
1
3
analytically pure 7. H NMR (C6D6, 300 MHz): δ 8.1 (2H, d, JH−H
= 6 Hz, Ar-H), 7.9 (1H, d, 2JP−H = 9 Hz, Ar-H), 7.2 (4H, m, Ar-H), 7.0
(4H, m, Ar-H), 2.6 (2H, t, 2JH−P = 3 Hz, PCH), 2.4 (2H, q, 2JH−P = 6
1
2
pentane/THF (10:1) at −78 °C. H NMR (C6D6, 300 MHz): δ 8.0
Hz, PCH), 2.2 (2H, t, JH−P = 6 Hz, PCH), 1.4 (6H, d, 3JH−P = 6 Hz,
(3H, d, 3JH−H = 6 Hz, Ar-H), 7.3 (3H, t, 3JH−H = 9 Hz, Ar-H), 7.2 (3H,
t, 3JH−H = 9 Hz, Ar-H), 7.0 (3H, d, 3JH−H = 6 Hz, Ar-H), 4.47 (s, free
CH3), 1.2 (12H, m, CH3), 0.9 (6H, d, 3JH−P = 6 Hz, CH3), 0.8 (6H, d-
d, 3JH−P = 8 Hz, 3JH−H = 6 Hz, CH3), 0.6 (6H, d, 3JH−P = 6 Hz, CH3),
−11.6 (1H, t-d, 2JH−Pcis = 81 Hz, 2JH−Ptrans = 21 Hz, Fe-H), −20.0 (1H,
br s, Fe-(μ-H)-B). 2H NMR (C6H6, 76 Hz): δ −12.3 (1D, t, 2JP‑D = 10
Hz), −20.8 (1D, br s). 13C NMR (THF with 1 drop of C6D6, 125
3
3
H2), 2.3 (6H, m, PCH), 1.0 (18H, d-d, JH−P = 15 Hz, JH−H = 6 Hz,
CH3), 0.8 (18H, d-d, 3JH−P = 15 Hz, 3JH−H = 6 Hz, CH3), −15.1 (br s,
2H). T1 min (d8-toluene): 35 ms (δ −15.1, −32 °C). 2H NMR (C6H6/
C6D6, 76 MHz): δ −15.4 (1D, br s). 31P NMR (C6D6, 121 MHz): δ
90.0 (3P, s). 13C NMR (C6D6, 125 MHz): δ 163.5 (s, CAr), 144.6 (s,
CAr), 144.1 (s, CAr), 130.9 (d, JP−C = 23 Hz, CAr), 124.5 (s, CAr), 123.9
(s, CAr), 28.6 (s, PCH), 21.2 (s, CH3), 19.9 (s, CH3). 11B NMR (C6D6,
128 MHz): δ 7.5 (br). IR (KBr, cm−1): 2278 (w), 19618 (w), 1845
(w). UV−vis (THF, nm {M−1 cm−1}): 377 {shoulder, 1532}, 275
{14532}. Elemental analysis could not be obtained because of the
instability of the compound under dynamic vacuum.
MHz): δ 222.7 (br s, CO), 161.8 (br s, CAr), 142.9 (br s, JC−P = 19
1
Hz, CAr), 140.8 (br s, JC−P = 16 Hz, CAr), 131.0 (s, CAr), δ 129.8 (s,
2
CAr), 128.5 (s, CAr), 128.0 (s, CAr), 124.0 (s, CAr), 123.4 (s, CAr), 30.8
(s, PCH), 28.3 (s, PCH), 27.7 (s, PCH), 22.3 (s, CH3), 19.1 (s, CH3),
18.8 (s, CH3), 18.1 (s, CH3). 31P NMR (C6D6, 121 MHz): δ 83.4 (2P,
d, 2JP−H = 21 Hz), 72.8 (1P, s). IR (KBr, cm−1): 1898 (s, CO), 1967
(w, Fe−H). UV−vis (THF, nm {cm−1 M−1}): 270 {4333}, 280
{4111}, 390 {1400}. Anal. Calcd for C37H56BFeOP3: C, 65.70; H,
8.34. Found: C, 65.64; H, 8.08.
Synthesis of (TPB)(μ-H)Fe(CO)(H) (7) from Formaldehyde.
Compound 1 (8 mg, 11.8 μmol) or 2 (6 mg, 8.9 μmol) was mixed
with excess paraformaldehyde in C6H6 for 3 h to give a turbid, light
yellow solution. The excess paraformaldehyde was filtered away, and
the solution was pumped down to give 7 as a yellow solid (from 1, 8
mg, 100%; from 2, 7 mg, 100%). Spectroscopic data are identical to
those listed above.
Synthesis of (TPB)Fe(CNtBu) (4). tert-Butyl isocyanide (20 mg,
0.24 mmol) was added to a brown solution of (TPB)Fe(N2) 1 (40 mg,
59 μmol) in benzene (2 mL), causing an instantaneous darkening
upon gentle shaking. The volatiles were removed by lyophilization, and
the residue was extracted with tetramethylsilane (2 mL). The resulting
dark brown solution was slowly concentrated down to ca. 0.2 mL by
vapor diffusion into hexamethyldisiloxane. Removal of the mother
liquor by decantation, washing with cold tetramethylsilane (2 × 0.1
mL), and drying in vacuo afforded (TPB)Fe(CNtBu) 4 as brown
Synthesis of (TPB)Fe(C2H4) (8). A J-Young NMR tube containing
an orange solution of 3 (8.4 mg, 12.5 μmol) in C6D6 (0.8 mL) was
1
crystals (33 mg, 77%). H NMR (C6D6, 300 MHz): δ 11.2 (3H), 9.2
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freeze−pump−thawed (3×) and exposed to ethylene gas (1 atm) for
ca. 1 min. Mixing immediately gave a brown solution of 8. Removal of
solvent in vacuo yielded a brown solid (8.3 mg, 99%) of 8. Dissolution
of this solid under N2 atmosphere gave mostly 8 and small amounts of
1. Over time, 8 in solution converted to 1. Crystals suitable for XRD
were grown in a saturated, cold pentane/diethyl ether (2:1) solution
MHz): δ 41.0 (br). UV−vis (THF, nm {cm−1 M−1}): 264 {shoulder,
31650}, 520 {826}. IR (KBr, cm−1): 2088 (s, B−H), 1838 (m, B−H).
Anal. Calcd for C51H86FeP4B2Si (i.e., 11 + Me3SiH): C, 65.96; H, 9.33.
Found: C, 65.70; H, 9.16.
Synthesis of (SiPiPr3)Fe(H2)(H). In a 100 mL Schlenk tube a red
solution of (SiPiPr3)Fe(Me) (1.05 g, 1.547 mmol) in C6H6 (50 mL)
was degassed by freeze−pump−thaw (3×). H2 gas (1 atm) was
charged into the reaction mixture. The reaction was heated at 60 °C
for over a week. The reaction solution was then quickly filtered
through Celite, and volatiles were removed in vacuo to give a light
yellow powder. The solid was collected on a glass frit and washed with
pentane (3 mL × 2). The resulting product (SiPiPr3)Fe(H2)(H) (950
mg, 1.425 mmol, 92%) was obtained as a light yellow powder after
drying under vacuum. 1H NMR (C6D6, 300 MHz): δ 8.3 (3H, d, 3JH−P
= 6.8 Hz, Ar-H), 7.3 (3H, m, Ar-H), 7.2 (3H, t, 3JH−H = 7.2 Hz, Ar-H),
1
under an ethylene atmosphere. H NMR (C6D6, 300 MHz): δ 33.1
(1H), 28.8 (1H), 18.7 (4H), 13.3 (1H), 5.25 (s, free C2H4), 4.9 (3H),
4.1 (1H), 1.9 (2H), −3.0 (10H), −6.2 (11H), −9.2 (11H), −10.0
(3H). UV−vis (THF, nm {cm−1 M−1}): 309 {shoulder, 6032}, 553
{1804}, 938 {418}. μeff (C6D6, method of Evans, 20 °C): 3.2 μB (S =
1). Elemental analysis could not be obtained because of the instability
of the compound under dynamic vacuum.
Synthesis of (TPBH)Fe(C2Ph) (9). Phenylacetylene (40.8 mg, 400
μmol) was added to a C6H6 solution (5 mL) of 3 (9.0 mg, 13 μmol),
immediately giving a gray solution. Removal of the solvent in vacuo
3
7.1 (3H, t, JH−H = 7 Hz, Ar-H), 4.5 (s, free H2), 2.2 (6H, m, PCH),
1
yielded a black powder of 9 (10 mg, 100%). H NMR (C6D6, 300
1.0 (18H, m, CH3), 0.8 (18H, br s, CH3), 0.16 (s, free CH4), −10.0
2
MHz): δ 28.8 (3H), 13.1 (4H), 4.7 (18H), 2.6 (2H), 2.3 (4H), 1.2
(2H), −29.3 (1H), −30.9 (5H). μeff (C6D6, method of Evans, 20 °C):
5.1 μB (S = 2). UV−vis (THF, nm {cm−1 M−1}): 325 {20670}, 439
{1051}, 479 {971}, 522 {955}, 601 (br abs extending from 400 to 700
nm, 930}, 883 {1466}. IR (KBr, cm−1): 2040 (s, CC), 2490 (m, B−
H). Anal. Calcd for C44H60FeP3B: C, 70.60; H, 8.08. Found: C, 70.39;
H, 7.89.
Synthesis of (TPBD)Fe(C2Ph). The B−D-labeled complex of 9
was generated by the same method described for 9, except that PhC2H
was replaced with PhC2D. The 1H NMR spectrum was identical to 10.
IR (thin film, cm−1): 1826 (br m, B−D; predicted 1832).
(3H, quin, JP−H = 18.4 Hz, Fe-H). T1 min (d8-toluene): 32 ms (δ
−10.0, −30 °C). 31P NMR (C6D6, 121 MHz): δ 100 (br); (d8-toluene,
121 MHz, −80 °C): δ 117.9 (d, 3JP−P = 43.5 Hz), 94.7 (br s), 84.7 (d,
3JP−P = 43.5 Hz). 13C NMR (THF with 1 drop of C6D6, 125 MHz): δ
157.3 (d, JC−P = 22.5 Hz, CAr), 150.5 (d, JC−P = 21.3 Hz, CAr), 130.1
(d, JC−P = 9.3 Hz, CAr), 128.6 (s, CAr), 127.4 (s CAr), 126.0 (d, JC−P
=
2.5 Hz, CAr), 29.0 (br s, PCH), 20.4 (br s, CH3), 19.2 (br s, CH3).
UV−vis (THF, nm {cm−1 M−1}): 353 {3040}. IR (KBr pellet; cm1):
1941 (w, Fe−H). Elemental analysis could not be obtained because of
the instability of the compound under prolonged exposure to N2.
Isotopomers of (SiPiPr3)Fe(H2)(H): (SiPiPr3)Fe(H2)(H), (SiPiPr3)-
Fe(HD)(H), (SiPiPr3)Fe(H2)(D), (SiPiPr3)Fe(D2)(H), (SiPiPr3)Fe(HD)-
(D), and (SiPiPr3)Fe(D2)(D). HD gas was generated by the method
described above and charged into a J-young tube containing a degassed
solution of (SiPiPr3)Fe(Me) in C6D6. The solution was heated at 60 °C
Synthesis of (TPBH)Fe(C2Tol) (10). Tolylacetylene (16.1 mg, 138
μmol) was added to a C6H6 solution (5 mL) of 3 (22.9 mg, 33.0
μmol), immediately giving a gray solution. Removal of the solvent in
vacuo yielded a black powder of 10 (24.0 mg 100%). Black XRD
quality crystals of 10 were grown by layering hexamethyldisiloxane on
top of a concentrated THF solution of 10, and allowing the solution to
1
for over a week. Monitoring the progress of the reaction by H NMR
revealed the gradual disappearance of 1 and formation of diamagnetic
isotopomers (SiPiPr3)Fe(H2)(H). Spectroscopic features in the 1H
NMR spectrum were identical to (SiPiPr3)Fe(H2)(H) except for the
hydridic proton resonances, where isotopologues were observed.
1
sit overnight. H NMR (C6D6, 300 MHz): δ 44.1 (1H), 29.4 (1H),
13.3 (1H), 4.6 (2H), 3.3 (1H), 2.7 (2H), 2.3 (4H), 1.8 (2H), −32.3
(1H). μeff (C6D6, method of Evans, 20 °C): 5.2 μB (S = 2). UV−vis
(THF, nm {cm−1 M−1}): 326 {24 476}, 444 {1243}, 486 {1138}, 527
{shoulder, 975}, 624 (915}, 887 {1575}. IR (KBr, cm−1): 2039 (s,
CC), 2500 (m, B−H). Anal. Calcd for C45H62FeP3B: C, 70.88; H,
8.20. Found: C, 70.44; H, 7.80.
Synthesis of (TPBD)Fe(C2Tol). The B−D-labeled complex of 10
was generated by the same method described for 10, except that
TolC2H was replaced with TolC2D. The 1H NMR spectrum was
identical to 10. IR (thin film, cm−1): 1824 (br m, B-D; predicted
1841).
2
1H{31P} NMR (C6D6, 300 MHz): δ −10.0 (3H, quart, JP−H = 18.4
Hz, H3), −10.0 (t, 1JH‑D = 9.5 Hz, H2D), −10.2 (quin, 1JH‑D = 9.3 Hz,
HD2).
Synthesis of (SiPiPr3)Fe(CO)(H). In a 50 mL Schlenk tube a yellow
solution of (SiPiPr3)Fe(H2)(H) (330 mg, 0.495 mmol) in C6H6 (20
mL) was freeze−pump−thawed (3×). The solution was charged with
CO (1 atm). The reaction was mixed overnight at RT and then for 1 h
at 60 °C, resulting in a light yellow solution. After completion, the
solution was degassed by freeze−pump−thaw (3×). The solution was
filtered through Celite, and the volatiles were removed in vacuo to give
a light yellow powder. The solid was collected on a glass frit and
washed with pentane (5 mL × 3). Removal of the solvent in vacuo
yielded (SiPiPr3)Fe(CO)(H) (266 mg, 0.384 mmol, 78%) as a light
yellow powder. Crystals suitable for X-ray diffraction were obtained by
RT evaporation of a pentane solution of (SiPiPr3)Fe(CO)(H). 1H
NMR (d8-toluene, 300 MHz): δ 8.2 (2H, d, 3JH−H = 7.2 Hz), 8.1 (1H,
Synthesis of 11. A yellow, C6D6 solution of 1 (18.2 mg, 27 μmol)
was heated in a J-Young NMR tube under H2 (1 atm) at 80 °C for 2 h,
giving a turbid red-purple solution. The solvent was removed in vacuo,
and the dark crude material was redissolved in hexamethyldisiloxane (3
mL) and filtered through a glass frit to remove a black solid
(presumably iron metal). Removal of the solvent in vacuo gave a
purple solid that is a mixture of 11 (1 equiv) and diisopropyl-
phosphino-benzene (iPr PPh, 1 equiv). Orange XRD quality crystals of
3
2
d, JH−H = 7.2 Hz), 7.3 (3H, m), 7.2 (3H, m), 7.1 (3H, m), 2.7 (2H,
3
3
11 can be grown from slow evaporation of a concentrated
hexamethyldisiloxane solution of 11 at room temperature (5.1 mg,
18%). Dissolution of these crystals by heating in benzene or THF
results in decomposition. Therefore, spectral data are reported on the
m), 2.4 (2H, m), 2.2 (2H, m), 1.5 (6H, d-d, JH−P = 15.2 Hz, JH−P
=
6.8 Hz), 1.3 (6H, m), 1.1 (6H, d-d, 3JH−P = 12.6 Hz, 3JH−H = 6.6 Hz),
0.8 (12H, m), 0.5 (6H, s), −14.9 (1H, t-d, 3JH‑Pcis = 81.0 Hz, 3JH‑Ptrans
=
14.1 Hz). 31P NMR (C6D6, 121 MHz, RT): δ 88.0 (br), 90.0 (s); (d8-
toluene, 121 MHz, −80 °C): δ 109.9 (t, 3JP−P = 65 Hz, 3JP−H = 80 Hz),
90.0 (s), 77.9 (t, 3JP−P = 65 Hz, 3JP−H = 80 Hz). 13C NMR (THF with
1 drop of C6D6, 125 MHz): δ 223.0 (d, 2JC−P = 6.3 Hz, CO), 157.2 (d,
iPr2
mixture of 11 with PPh. Compound 11 appears to be fluxional at
1
RT. H NMR (C6D6, 300 MHz): δ 8.4 (4H, s, Ar-H), 7.6 (4H, s, Ar-
H), 7.5 (3H, d, 2JH−P = 6 Hz, Ar-H), 2.7 (1H, d, 2JH−P = 6 Hz, PCH),
3
JC−P = 8.8 Hz, CAr), 155.2 (d, JC−P = 10.6 Hz, CAr), 150.5 (d, JC−P
=
2.5 (1H, s, PCH), 1.9 (1H, quart, JH−H = 6 Hz, PCH), 1.3 (6H, d-d,
3JP−H = 4 Hz, 3JH−H = 2 Hz, CH3), 1.2 (6H, d, 3JH−H = 3 Hz, CH3), 1.1
11.3 Hz, CAr), 150.2 (d, JC−P = 8.8 Hz, CAr), 148.6 (d, JC−P = 18.1 Hz,
CAr), 132.4 (d, JC−P = 9.4 Hz, CAr), 131.9 (d, JC−P = 8.8 Hz, CAr), 130.9
(s, CAr), 128.3 (s, CAr), 128.5 (s, CAr), 127.1 (s, CAr), 126.8 (s, CAr),
126.4 (s, CAr), 125.8 (s, CAr), 124.7 (s, CAr), 32.4 (s, PCH), 30.7 (s,
PCH), 30.0 (s, PCH), 29.6 (s, PCH), 28.7 (s, PCH), 22.8 (s, CH3),
22.1 (s, CH3), 20.2 (s, CH3), 19.5 (s, CH3), 19.2 (s, CH3), 19.1 (s,
CH3), 18.9 (s, CH3), 18.0 (s, CH3). UV−vis (THF, nm {cm−1 M−1}):
340 {2050}, 400 {1500}. IR (KBr pellet; cm−1): 1882 (s, CO), 1944
3
3
(6H, quart, JP−H = 4 Hz, JH−H = 2 Hz, CH3), 0.9 (12H, m, CH3),
−17.0 (0.25H, t, 2JH−P = 36 Hz, B-H). 13C NMR (C6D6, 125 MHz): δ
157.4 (br s, CAr), 144.2 (br s, CAr), 134.9 (d, 2JC−P = 19 Hz, CAr), 130.5
2
(s, CAr), 129.3 (s, CAr), 125.7 (d, JC−P = 23 Hz, CAr), 25.5 (br s,
PCH), 24.9 (br s, PCH), 24.4 (br s, PCH), 23.7 (br s, PCH), 23.1 (br
s, PCH), 20.0 (m, CH3), 19.4 (m, CH3), 18.2 (m, CH3). 31P NMR
(C6D6, 121 MHz): δ 99.1 (4P, s), 9.5 (2P, s). 11B NMR (C6D6, 128
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Article
(m, Fe−H). Anal. Calcd for C37H57FeOP3Si: C, 64.16; H, 8.00.
Found: C, 64.15; H, 8.13.
Present Addresses
†(M.-E. Moret) Organic Chemistry and Catalysis Group,
Faculty of Science, Utrecht, Universiteitsweg 99, 3584 CG
Utrecht, The Netherlands; e-mail: m.moret@uu.nl.
Synthesis of (SiPiPr3)Fe(13CO)(H). In a J-Young NMR tube an
orange C6D6 solution of (SiPiPr3)Fe(H2)(H) was degassed by freeze−
pump−thaw (3×). Subsequently, 13CO (1 atm) was added and the
reaction was allowed to mix overnight at RT. The 1H NMR spectrum
was identical to (SiPiPr3)Fe(CO)(H). IR (KBr; cm−1): 1836 (s, 13C
O).
‡(Y. Lee) Department of Chemistry, School of Molecular
Science, Korea Advanced Institute of Science and Technology,
291 Daehak-ro Yuseong-gu, Daejeon 305-701, Republic of
Korea; e-mail: yunholee@kaist.ac.kr.
Generation of (TPBH)Fe(Et) (A). Compound A was observed as
an intermediate of catalytic ethylene hydrogenation under the reaction
conditions described in the Catalytic Hydrogenation Studies section.
We were also able to generate A starting from complex 8. The
procedure described below is more amendable to observing A
spectroscopically. A dark yellow C6D6 solution (0.5 mL) of 8 (2.8
mg, 4.2 μmol) under ethylene (1 atm) in a J-Young tube was frozen
(−196 °C), and H2 was added (1 atm). The reaction was thawed and
quickly mixed only immediately prior to measuring the 1H NMR
spectrum, revealing a mixture of 8 and A. Further mixing of the
solution for ca. 45 min yielded a purple solution of A with a small
residual amount of 8 by 1H NMR spectroscopy. Free C2H4, C2H6, and
H2 were also observed in the 1H NMR spectrum. Compound A could
not be isolated as a solid due to its instability. For example, an ATR-IR
spectrum of a thin film of the reaction mixture obtained by solvent
evaporation under an N2 atmosphere over a period less than 30 s gave
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the Gordon and Betty Moore Foundation and
the NIH (GM070757) for funding. M.-E.M. acknowledges a
Fellowship for Advanced Researchers from the Swiss National
Science Foundation. Dr. Charlene Tsay and Lawrence Henling
are acknowledged for assistance with X-ray crystallography, and
Dr. David VanderVelde is acknowledged for assistance with
NMR spectroscopy. We acknowledge the Gordon and Betty
Moore Foundation, the Beckman Institute, and the SanofiA-
ventis BRP at Caltech for their generous support of the
Molecular Observatory.
1
vibrational bands diagnostic of 1, 2, and A. H NMR (C6D6, 300
MHz): δ 17.3 (1H), 6.6 (1H), 5.26 (s, free C2H4), 4.4 (1H), 4.47 (br
s, free H2), 3.3 (2H), 0.80 (s, free C2H6), −1.9 (8H), −5.5 (6H). IR
(thin film; cm−1): 2470 (br s, B−H of A), 2069 (m, NN of
(TPB)(μ-H)Fe(N2)(H) 2), 2009 (s, NN of (TPB)Fe(N2) 1). UV−
vis, obtained 45 min after exposing A under 1 atm of ethylene to 1 atm
of H2 (THF, nm {cm−1 M−1}): 325 {20670}, 439 {1051}, 479 {971},
522 {955}, 601 (br abs extending from 400 to 600 nm, 930}, 883
{1466}. Magnetic data could not be obtained due to residual 10 in the
reaction mixture. Elemental analysis could not be obtained because of
the instability of the compound under dynamic vacuum.
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1
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ASSOCIATED CONTENT
■
S
* Supporting Information
Spectral data, catalysis data, computational details, solid-state
structures (4 and (SiPiPr3)Fe(CO)(H)), and CIF files (for 2, 3,
4, 5, 8, 10, 11, and (SiPiPr3)Fe(CO)(H)). This material is
available free of charge via the Internet at http://pubs.acs.org.
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■
Corresponding Author
*E-mail: jpeters@caltech.edu.
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