Rapid, reversible heterolytic cleavage of bound H2

Article abstract of DOI:10.1021/ja405755j

Heterolytic cleavage of dihydrogen into a proton and a hydride ion is a fundamentally important step in many reactions, including the oxidation of hydrogen by hydrogenase enzymes and ionic hydrogenation of organic compounds. We report the facile, reversible heterolytic cleavage of H₂ in a manganese complex bearing a pendant amine, leading to the formation of a manganese hydride and a protonated amine that undergo H⁺/H ^- exchange at an estimated rate of >10⁷ s^-1 at 25°C.

Full text of DOI:10.1021/ja405755j

Communication
pubs.acs.org/JACS
Rapid, Reversible Heterolytic Cleavage of Bound H2
Elliott B. Hulley, Kevin D. Welch, Aaron M. Appel, Daniel L. DuBois, and R. Morris Bullock*
Center for Molecular Electrocatalysis, Physical Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, K2-57,
Richland, Washington 99352, United States
S
* Supporting Information
environments and oxidation states allows for multiple means of
ABSTRACT: Heterolytic cleavage of dihydrogen into a
proton and a hydride ion is a fundamentally important step
in many reactions, including the oxidation of hydrogen by
hydrogenase enzymes and ionic hydrogenation of organic
compounds. We report the facile, reversible heterolytic
cleavage of H2 in a manganese complex bearing a pendant
amine, leading to the formation of a manganese hydride
and a protonated amine that undergo H+/H− exchange at
an estimated rate of >107 s−1 at 25 °C.
controlling the catalytic rate and overpotential. We previously
studied cationic Mn(I) complexes containing pendant amines
and found that the stable Mn(η2-H2)+ adducts showed no
evidence for heterolytic cleavage of H2.9 Reasoning that the
acidity of the H2 ligand in our previous complexes was
insufficient to lead to heterolysis, we found in the present work
that incorporation of an electron-deficient diphosphine ligand
into a new Mn complex (Figure 2) increased the electro-
philicity at Mn, facilitating rapid, reversible heterolytic cleavage
of H2.
The cationic complex 1 (Figure 2) was prepared by halide
he H−H bond of dihydrogen is the simplest chemical
abstraction from (P2PhN2Bn)Mn(Br)(CO)(bppm) [bppm =
T
bond and has served as the prototypical bond for
fundamental experimental and theoretical studies. The
formation and cleavage of H2 can occur homolytically (H·/
H·) or heterolytically (H+/H−) and is an essential step in many
reactions, including the production of H2,1 the oxidation of H2,2
and catalytic hydrogenation of a diverse range of unsaturated
bonds.3 In polar environments, typical of enzymatic or
electrochemical reactions, the heterolytic bond strength for
H−H cleavage4 is lower than the homolytic bond strength,
favoring heterolytic scission. Bifunctional interactions that
facilitate the heterolysis of H2 (Figure 1) are important to
Figure 1. Intramolecular deprotonation of bound H2.
the function of many types of hydrogenation catalysts and the
family of hydrogenase enzymes.5,6 A detailed understanding of
the heterolysis of a nonpolar bond would greatly inform the
design of systems based on heterolytic activation. Here we
report an extremely facile, reversible heterolytic cleavage of the
H−H bond in a Mn complex, wherein the proton and hydride
ion undergo exchange through an energetically accessible
Mn(η2-H2)+ intermediate at an estimated rate of >107 s−1 at 25
°C.
To mimic a salient feature in the active sites of hydrogenase
enzymes,7 pendant amine bases have been incorporated into
families of Ni, Co, and Fe complexes, and their electrochemical
behavior under acidic and basic conditions has been studied.
This has led to the development of catalysts for H2 production
and oxidation using inexpensive, abundant transition metals
rather than precious metals such as platinum.8 Exploiting a
large variety of metal complexes with varying coordination
Figure 2. Syntheses of [(κ3-P2PhN2Bn)Mn(CO)(bppm)][BArF ] (1) and
4
[(PP2hNB2n H)MnH(CO)(bppm)][BArF ] (2) and the truncated solid-
4
state structure of the cation of 2 (−123 °C, 50% probability ellipsoids).
Received: June 8, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja405755j | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Figure 3. Selected regions of (A) 1H (with deconvolution) and (B) 1H{31P} NMR spectra of 2; (C) 1H{31P}, (D, top) 15N{1H}, and (D, bottom)
2
1
15N NMR spectra of 2[15N]; and (E) H and (F) H NMR spectra of 2-(H/D) from −20 to −70 °C.
(PArF )2CH2; ArF = 3,5-bis(trifluoromethyl)phenyl] through
consistent with rapid proton−hydride exchange that results in
the observation of an average coupling constant for a “normal”
1JNH (∼70 Hz)12 from the protonated amine and negligible 15
coupling (JNH ≈ 0 Hz) to the hydride ligand. Similarly, the
magnitude of JPH is approximately half of the value expected for
manganese hydrides in this structural family (2JPH = 15 Hz
observed vs 30−40 Hz expected).11
2
treatment with excess NaBArF4 in fluorobenzene. While similar
Mn cations previously prepared by Kubas and co-workers are
intensely blue,10 1 is red-orange as a result of coordination of
one of the pendant amines to the Mn center [Figure S1 in the
Supporting Information (SI). The 31P NMR resonance for the
P2N2 ligand of 1 was broad at room temperature (Figure S2B),
consistent with dynamic behavior related to reversible
dissociation of the coordinated pendant amine.
N
Addition of HD to a CH2Cl2 solution of 1 at −20 °C resulted
in the observation of an 1H NMR spectrum containing
resonances identical to those of 2 except for the absence of a
peak near 2.5 ppm. Instead, a signal integrating to one proton
was observed at −2.78 ppm that appeared as a pseudopentet
(JPH = 28 Hz) and collapsed to a singlet upon broadband 31P
decoupling. The 2H NMR spectrum of the same sample
revealed only a resonance at +7.8 ppm. In the variable-
temperature 2H and 1H NMR spectra of 2-(H/D) from −20 to
Addition of H2 (1 atm) to 1 in PhF or CD2Cl2 at −20 °C
resulted in a color change from red to yellow and the formation
of a single new product (see the 31P NMR spectrum in Figure
S3). Yellow-orange crystals of 2 were grown from CH2Cl2
solutions under H2, and the solid-state molecular structure
determined by X-ray diffraction (Figure 2) is consistent with a
static structure containing Mn−H and N−H bonds resulting
from heterolytic cleavage of H2.11
1
2
−70 °C (Figure 3E,F), both the H and H chemical shifts
showed a strong temperature dependence, linearly shifting by
>1 ppm in opposite directions (upfield and downfield,
respectively, with decreasing temperature) over a 50 °C
Spectral studies of 2 in solution indicated dynamic behavior.
1
Besides the expected resonances from ligand protons, the H
NMR spectrum of 2 in CD2Cl2 exhibited a pseudopentet (JPH
≈ 15 Hz, 2H) at 2.55 ppm that collapsed to a broad singlet
upon broadband 31P decoupling (Figure 3A, B). The modest
31P coupling [not observed in similar Mn(η2-H2)+ com-
plexes9,10] and the unusual chemical shift are inconsistent
with those of Mn(η2-H2)+ (typically −2 to −5 ppm)9 or a static
structure containing a Mn−H bond (typically −4.5 to −6
ppm)11 and a N−H bond in the protonated P2N2 ligand (14 to
7 ppm). When D2 was used to produce 2-(D/D), the 1H NMR
spectrum was identical to that of 2 except for the absence of the
1
2
range. The average of these chemical shifts in the H and H
NMR spectra was 2.5 ppm, consistent with a dynamic process
involving rapid exchange of H and D between the amine and
metal sites, with a preference for D at the amine and the H at
the metal.13 Although the H signal became too broad below
2
−70 °C to allow the chemical shift to be quantified reliably, the
1H signal continued to shift consistently upfield, reaching −4.32
ppm at a temperature of −94 °C (Figure S5).
1
2
The assignment of the resonance at 2.55 ppm in the H
peak at 2.55 ppm, and the H NMR spectrum contained a
1
NMR spectrum of 2 to rapidly exchanging proton and hydride
singlet at 2.5 ppm (Figure S4). The existence of a single H
resonances is thus consistent with the observed average
NMR resonance attributable to the Mn−H and N−H positions
indicated that those moieties are in rapid exchange, and further
isotopic labeling studies supported this conclusion.
1
2
chemical shift, the magnitudes of JNH and JPH, and the
coupling of two hydrogen atoms to the corresponding 15N
nucleus in the 15N-labeled complex. Furthermore, the variable-
temperature behavior of 2-(H/D) is expected in the rapid
exchange limit. The large opposing changes in the chemical
In the 1H NMR spectrum of the 15N-labeled analogue
2[15N] prepared from 1[15N] and H2, the Mn−H/N−H
resonance appeared as a doublet of pentets that collapsed to a
doublet upon broadband 31P decoupling (JNH = 37 Hz; Figure
3C). The 15N nucleus coupled to the Mn−H/N−H resonance
(assigned by 1H,15N-HSQCAD) appeared as a triplet of triplets
in the 15N NMR spectrum (JNP = 7.4 Hz, JNH = 37 Hz; Figure
3D), indicating coupling between this 15N nucleus and two
dynamically equivalent protons. The magnitude of JNH is
1
2
shifts of the H and H signals of 2-(H/D) as the temperature
was varied are indicative of an equilibrium isotope effect (EIE)
wherein D preferentially resides at the amine while H is bound
to Mn.14
The magnitude of the preference for N−D over N−H
increased at lower temperatures, consistent with the expected
B
dx.doi.org/10.1021/ja405755j | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
The Mn−H and N−H sites are proposed to exchange
through an energetically accessible Mn(η2-H2)+ complex.
Exchange between dissolved H2 and 2 was shown by exchange
spectroscopy (EXSY) experiments, which revealed that the
Mn−H/N−H resonance was in chemical exchange with free H2
at an approximate rate of 0.1−10 s−1 at 25 °C (Figure S8).
Moreover, addition of H2 to 1 was only slightly favorable at 1
atm [ΔG = −2.1(3) kcal mol−1],11 and removal of the H2
atmosphere at 20 °C resulted in complete conversion back to
red-orange 1 as rapidly as the solution could be degassed. Since
no significant HD coupling was observed in 2-(H/D)
(expected JHD ∼ 30 Hz for an η2-HD complex),15,21 the
complex 2-(η2-HD) likely lies at least 1 kcal mol−1 above 2-(H/
D). On the basis of this result and the negligible temperature
dependence of the 1H NMR chemical shifts of 2, the
intermediate 2-(η2-H2) likely also lies at least 1 kcal mol−1
above 2. Because of the higher energy of 2(η2-H2) relative to 2,
the barrier for heterolytic cleavage of H2 in this intermediate is
correspondingly lower than that for the overall exchange
process between the proton and hydride sites in 2 (i.e., ΔG⧧ ≤
5.8 kcal mol−1 for H2 cleavage). Moreover, the temperature
Figure 4. Qualitative energy diagram for exchange of the proton and
hydride sites of 2 via 2-(η2-H2) (red) and a depiction of the effect of
isotopic substitution.
influence of the zero-point energy (ZPE) differences in the
Mn−(H/D) and N−(H/D) bonds (Figure 4). ZPE differences
are expected to dominate this equilibrium (Figure S6), in
contrast to other systems such as [M(H)(H2)] studied by
Heinekey and others,15,16 for which the EIEs may have
additional contributions.14 Those systems can also facilitate
rapid H2 activation, but the nature of the activation is quite
different from that in the present system.16b
1
dependence of the H NMR signal of 2-(H/D), indicative of
the EIE and thus still rapid H+/H− exchange, did not show a
deviation that would indicate a kinetic isotope effect (KIE).
Since a significant primary KIE would be expected for an event
involving substantial H−H bond breaking/making, this
suggests that the minimum H+/H− exchange rate of 1.5 ×
104 s−1 at −95 °C may be a considerable underestimate.
Observation of the extremely rapid, reversible heterolytic
cleavage of H2 into a proton and a hydride ion in this
bifunctional manganese complex may give insight into synthetic
catalysts and enzymes that heterolytically cleave H2. Energy
matching between the heterolytic cleavage product and the
Mn(η2-H2)+ complex was obtained, such that exchange
between the Mn−H and N−H sites was facile. The rate of
the overall exchange has been estimated to be >107 s−1 at 25
°C, which is orders of magnitude higher than the rates of
catalysis for H2 oxidation or production in [FeFe] hydro-
genase5 and reported molecular catalysts that mimic the
function of this enzyme.22 The facility of H2 activation shown
here suggests that other suitably energy-matched systems may
be able to perform facile heterolysis of nonpolar substrates, an
as-yet underutilized strategy in the activation of, for example,
alkane C−H bonds.
The temperature dependence of the chemical shifts in 2-(H/
D) is entirely due to thermodynamics: for the chemical shifts to
reflect weighted averages of the different isotopomers, exchange
must be fast on the NMR time scale. Assuming the chemical
shift of the static hydride in 2-(H/D) to be between −4.5 and
−5.5 ppm, we estimate that the EIE for (2-(MnH/ND) ⇄ 2-
(MnD/NH)) is in the range 0.13−0.23 at −20 °C.11 An
intramolecular EIE of similar magnitude (K = 0.41 at 56 °C)
was observed by Toomey et al.17 in the amido hydride (η5-
C5(CH3)4H)2Zr(H/D)(N(D/H)tBu), wherein slow intramo-
lecular H/D exchange facilitated an equilibrium favoring the
[Zr(H)][N(D)tBu] isotopomer.
The rate of the H+/H− interconversion in 2 is related to the
difference in the frequencies of the two resonances that are
being averaged, but the isolated chemical shifts could not be
1
established by “freezing out” the dynamism. In the H NMR
spectra of 2 from −25 to −95 °C, the proton−hydride
resonance remained coalesced over the entire temperature
range (Figure S7A), and similarly, the 2H NMR spectrum of 2-
(D/D) remained coalesced down to −90 °C (Figure S7B). The
1H NMR spectra of 2 did show appreciable broadening of the
Mn−H/N−H resonance as the temperature was lowered, but
concomitant broadening of ligand and solvent resonances
suggested that this was due to ion-pair formation and/or an
increase in solution viscosity. On the basis of the lowest
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, syntheses, and X-ray crystallographic
data (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
1
measured H chemical shift of 2-(H/D) (−4.32 ppm) and the
AUTHOR INFORMATION
Corresponding Author
morris.bullock@pnnl.gov
■
known averaged chemical shift of 2 (2.55 ppm), we estimate a
minimum N−H/Mn−H peak separation of 13.7 ppm,
corresponding to a reversible heterolytic H2 cleavage rate
greater than 1.5 × 104 s−1 at −95 °C. This minimum rate is
consistent with a ΔG⧧ of less than 6.8 kcal/mol for the overall
exchange process. If a similar barrier at room temperature is
assumed, H+/H− exchange occurs at a rate of at least 107 s−1 at
25 °C.11,18 There are several previous examples of directly
observed H+/H− exchange,19 although they occur at substan-
tially lower rates, and others where exchange is implied as part
of an overall process.20
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to Dr. Monte Helm for assistance in revising
this manuscript. We thank the U.S. Department of Energy,
Office of Basic Energy Sciences, Division of Chemical Sciences,
Geosciences and Biosciences, for support of the initial parts of
C
dx.doi.org/10.1021/ja405755j | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
this work. The current work was supported by the Center for
Molecular Electrocatalysis, an Energy Frontier Research Center
funded by the U.S. Department of Energy, Office of Science,
Office of Basic Energy Sciences. Pacific Northwest National
Laboratory is operated by Battelle for the U.S. Department of
Energy.
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