Molecular Mimics of Heterogeneous Metal Phosphides: Thermochemistry, Hydride-Proton Isomerism, and HER Reactivity

Article abstract of DOI:10.1002/anie.201808307

A new series of low-valent dinuclear molybdenum complexes bearing phosphido or phosphinidene bridging ligands was synthesized as a structural model of heterogeneous metal phosphide catalysts. Addition of acid to a monocationic Mo₂-μ-P complex results in phosphide protonation, affording a dicationic Mo₂-μ-PH species. Alternatively, reaction of an isoelectronic Mo₂-μ-P precursor with LiBEt₃H gives a Mo₂H-μ-P complex. Mixing these species, one bearing a Mo?H and the other a P?H bond, results in facile H₂ production at room temperature.

Full text of DOI:10.1002/anie.201808307

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Accepted Article
Title: Molecular Mimics of Heterogeneous Metal Phosphides:
Thermochemistry, Hydride-Proton Isomerism, and HER
Reactivity
Authors: Joshua A. Buss, Masanari Hirahara, Yohei Ueda, and
Theodor Agapie
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201808307
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Molecular Mimics of Heterogeneous Metal Phosphides:
Thermochemistry, Hydride-Proton Isomerism, and HER Reactivity
Joshua A. Buss, Masanari Hirahara, Yohei Ueda, and Theodor Agapie*
Abstract:
A new series of low-valent dinuclear molybdenum
complexes bearing phosphido or phosphinidene bridging ligands was
synthesized as a structural model of heterogeneous metal phosphide
catalysts. Addition of acid to a monocationic Mo2-μ-P complex results
in phosphide protonation, affording a dicationic Mo2-μ-PH species.
Alternatively, reaction of an isoelectronic Mo2-μ-P precursor with
LiBEt3H gives a Mo2H-μ-P complex. Mixing these species, one
bearing a Mo–H and the other a P–H bond results in facile H2
production at room temperature.
Heterogeneous metal pnictogenides represent a diverse class
of solid-state materials with applications ranging from
optoelectronics[1] to reductive catalysis.[2] With respect to the latter,
transition metal phosphides have demonstrated high activities
and stabilities in solar-fuels relevant catalyses[3] such as HER
(2H+ + 2e-  H2)[2b,4] and ORR (O2 + 4e- + 4H+  2 H2O),[2a,4a] with
overpotentials and lifetimes that rival state of the art platinum
catalysts.[2b,4c] Despite this remarkable reactivity, the operative
mechanism of these catalysts is not known.[2b] Computation
suggests that under HER conditions, both M- and P-protonation
are viable pathways for generation of surface hydrogen adducts
that, upon application of a reducing potential, can lead to
formation of H2 (Figure 1, top).[5] While intense research has been
devoted to understanding the nucleation and growth of metal
pnictogenide nanocrystals, affording atomically precise pictures
of these materials,[6] homogenous models of bulk transition metal
phosphides are scarce.
Figure 1. A representation of heterogeneous metal-phosphide catalysed HER
(top) and classes of known molecular μ-phosphido complexes (bottom).
maintains a short (ca. 2.15 Å) M–P distance, consistent with
preservation of a formal triple bond.[8a,8c] These complexes are not
ideal models for HER chemistry by heterogeneous metal
phosphide catalysts because of the propensity for protonolysis of
supporting amide or alkoxide ligands. Furthermore, they are
electronically distinct, as the oxidation states of these molecular
compounds do not reflect the low oxidation states found in
heterogeneous binary transition metal phosphides.
We recently reported the first examples of transition metal
complexes with both terminal phosphide ligands and d-electrons
(2, Scheme 1).[9] Herein, we extend this molecular Mo
pnictogenide chemistry, reporting the preparation of a series of
low-valent μ-phosphido complexes. These compounds model
important steps invoked in the proposed mechanism of
Transition metal phosphides are structurally diverse, with the
stoichiometric or metal-rich phases displaying bridging phosphide
building blocks.[2b] This μ-P motif is likewise known for molecular
metal phosphides; however, M–P–M linkages reported to date
either feature high/mid oxidation state metals[7] or largely
asymmetric phosphide coordination[8] (Figure 1, bottom).
Dinuclear symmetric bridging phosphides have been
demonstrated as intermediates in intermetal P-atom transfer
reactions[7c] and have been prepared by the activation of white
phosphorous,[7a] phosphine,[7d,7e] and phosphides.[7b] Capping
reactive terminal phosphides with transition metal-based Lewis
acidic fragments has been employed to stabilize the M≡P moiety,
resulting in asymmetric μ-P complexes.[8c-e] The high-valent metal
[*]
J. A. Buss, M. Hirahara, Y. Ueda, and Prof. T. Agapie
Division of Chemistry and Chemical Engineering
California Institute of Technology, Pasadena, CA 91125 (USA)
E-mail: agapie@caltech.edu
Scheme 1. Synthesis of a low-valent Mo–P–Mo core.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.xxxxxxxxx.
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heterogeneous HER and provide an experimental benchmark for
3
the thermochemistry of a bridging phosphinidene P–H bond.
Addition of Mo(0) complex 1 to a solution of terminal metal
phosphide 2 affords a new species with five distinct 31P{1H} NMR
resonances, at 1161.0, 82.9, 73.7, 70.3, and -5.3 ppm. This
spectral signature, consistent with a C1 symmetric Mo–P–Mo
structure (3, Scheme 1), was corroborated by the 1H NMR
spectrum, which showed eight discrete resonances for the central
arene protons between 3.5 and 5.5 ppm. Though the low-field 31
P
signal in 3 resonates several hundred ppm downfield of
diamagnetic symmetric or asymmetric bridging phosphide
complexes,[7a,8c] the μ-P assignment was conclusively established
by XRD. The structure of 3 is dinuclear with a bridging phosphide
ligand, one trans-spanning para-terphenyl diphosphine (para-P2)
ligand, and one arm-on arm-off para-P2 unit (Figure 2). The Mo–
P3 distances are similar for both metal centers (2.2831(3) (Mo1)
and 2.2409(3) (Mo2) Å), most consistent with double bonds
between both Mo ions and the μ-P.[7c] The chloride remains bound
(2.5157(3) Å), prompting dissociation of one of the phosphine
arms of the para-P2 ligand. Both arenes bind in an η6 fashion with
the Mo-arene contacts averaging 2.281(1) and 2.309(1) Å for Mo1
and Mo2, respectively.
4
A higher symmetry bridging phosphide was targeted via
exchanging the chloride for a non-coordinating counterion
(Scheme 2). Halide abstraction from 3 with one equiv. of
[Na][BArF24] results in simpler NMR characteristics; the 31P{1H}
NMR spectrum shows only two resonances at 1123.3 and 68.6
ppm in a 1:4 ratio, in accord with formation of 4. Complex 4
crystalizes with C2 symmetry (Figure 2). The μ-P ligand lies on a
crystallographic special position, imposing equivalent M–P
distances of 2.2747(2) Å, which agree well with related symmetric
μ-P complexes bearing metal phosphide double bonds.[7a,7c] The
Mo η6-arene contacts in 4 are slightly elongated (2.301(2) Å,)
when compared to the analogous fragment in 3, potentially a
consequence of steric congestion resulting from phosphine arm
recoordination.
5H
Stoichiometric reactions of
[(Et2O)2H][BArF (BArF
4
with the strong acid
]
24
=
tetrakis[3,5-bis(trifluoromethyl)
24
phenyl]borate)[10] result in formation of a new species with four
signals in the central arene region of the 1H NMR spectrum—6.08,
5.74, 5.24, and 4.92 ppm—suggesting a Cs symmetric dinuclear
solution structure. The 31P{1H} chemical shift for the terphenyl
phosphines corroborate this assignment, resonating at 86.4 and
74.6 ppm. A third, low-field 31P resonance is observed; a broad
peak (1046.1 ppm, ΔfFWHM = 112 Hz) shifted 77 ppm upfield of the
μ–P signal of 4 (Figure S21). Addition of [(Et2O)2D][BArF24] to 4
forms the same species with a slightly shifted (1047.1 ppm) low-
field resonance (Figures S21 and S24). Neither a new proton
(HBArF24) nor deuteron (DBArF24) resonance was observed for
this new complex; every observed 1H NMR feature can be
assigned and corresponds to the terphenyl diphosphine ligands.
To our knowledge, only diamagnetic bridging phosphinidene (μ-
PH) complexes of the actinides have been prepared previously;[11]
the 31P resonances are observed at 24.5 and 74.0 ppm, for the
Th2-μ-PH and U2-μ-PH compounds, respectively.[12] Terminal
phosphinidenes supported by U and Zr and sterically protected by
Figure 2. Solid-state structures of 3, 4, and 5H. Protons and counterions (4) are
omitted for clarity. Anisotropic thermal displacement ellipsoids are shown at a
50% probability level. Select bond distances [Å] and angles (°): 3 – Mo1–P3:
2.2831(3), Mo2–P3: 2.2409(3), ∠ Mo1–P3–Mo2 154.23(1); 4 – Mo1–P3
2.2747(2); 5H – Mo1–P3 2.3250(4), Mo2–P3 2.2180(5), ∠ Mo1–P3–Mo2
174.75(2).
bulky TREN variants were reported recently.[13] The 31P{1H}
chemical shifts for these PH moieties vary significantly,
resonating at 2460 ppm for the U complex[13b] and at 247 ppm for
the Zr compound,[13a] complicating phosphinidene assignment
strictly on the basis of 31P chemical shift. The chemical shift of the
P-bound proton is likewise highly variable,[13a,14] and in some of
1
these complexes is (as in complex 6H) not observed in the H
NMR.[12,13b] IR spectra were collected for both 6H and 6D and no
clear P–H stretch was observed.[15] Despite this, the data,
particularly the isotopic sensitivity of the μ-P 31P{1H} NMR
resonance and the lack of a Mo hydride signal in the 1H NMR, are
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to the Mo(II)Mo(II)/Mo(II)Mo(III) couple, and one quasi-reversible
reduction (-2.19 V) (Figure S25). With the standard potential for
oxidation of 4 in hand, a pKa for phosphide protonation was
required to complete a square-scheme for HAT from 6H
(Scheme 2, blue). Addition of pyridinium triflate to the THF
solution of 4 displayed partial protonation of the μ-P motif (Figure
S30). Acid titration afforded a pKa value for the bridging
phosphide—3.6 ± 0.1 in THF—the first observed value for the
acidity of bridging phosphinidene. This starkly contrasts a
reported Mn2-μ-PH complex, in which the P-bound H-atom reacts
[19]
as a hydride.[7e] Calculating the BDE(P–H)
,
including a THF
solvent correction,[20] gives a value of 59 kcal/mol; this P–H
bond is significantly weaker than those of phosphine (BDE
H2P–H = 83.9 ± 0.5),[21] methyl phosphine (BDE MeH2P–H = 77 ±
3),[22] and comparable to that of diphosphine (BDE H2PHP–H =
61.2).[23] The P–H bond of 6H is only slightly stronger than the
S–H bond of (CpMo)2(S2CH2)(S)(SH) (55 kcal/mol), a molecular
catalyst for electrochemical HER.[24]
It is notable that the most Brönsted-basic site of complex 4 is
the μ-P. In HER catalysis, the basicity of P atoms is invoked in
forming surface bonded H-atoms; however, H adsorption at P is
proposed to occur only after metal sites have been occupied.[5,25]
For comparison, a Mo hydride μ-phosphido was targeted by
treating asymmetric 3 with 1 equiv. of LiBEt3H. A new diamagnet
with an apparent triplet at -1.55 ppm (2J(P,H) = 17.0 Hz)[26] in the
1H NMR spectrum was observed, in line with generation of 5
(Scheme 2). The 31P{1H} NMR spectrum shows four resonances
(1057.4, 103.9, 69.5, and -4.4 ppm), indicating maintenance of
the C1 structure. The solid-state structure corroborates the
solution spectroscopy, demonstrating an asymmetric dinuclear
complex in which one diphosphine ligand is trans-spanning and
the other adopts an arm-on arm-off binding motif (Figure 2). The
Mo–μ-P distances remain relatively similar at 2.3250(4) and
2.2180(5) Å for the diphosphine and monophosphine coordinated
Mo centers, respectively. These metrics suggest maintenance of
the two Mo=P double bonds[7a,7c] following hydride installation.
Scheme 2. Reactivity and thermochemical parameters of P- and PH-bridged
Mo2 complexes.
most consistent with protonation of 4 at the phosphide, giving μ-
phosphinidene 6H.[16] Efforts to crystallize 6H uniformly resulted
in formation of single crystals of a complex formally missing a
hydrogen atom—dinuclear Mo(II)–μ-P–Mo(III), 7 (Figure S38).
Dication 7 can be prepared independently by the one electron
oxidation of 4 with [Fc][BArF24]. The μ-phosphide in the solid-state
structure of 7 (grown from a reaction of 4 with acid) lies on a
symmetry special position, giving two equivalent Mo–P distances
of 2.276(1) Å. The independently synthesized sample (from single
electron oxidation) crystallizes as an isoform of 7; the unit cells
differ but the Mo–Pphosphide distances are consistent with those
observed previously at 2.294 and 2.260 Å, corroborating the
absence of a hydrogen atom and in close agreement with the Mo–
Pphosphide distances observed for 4. The X-band CW EPR
spectrum of 7 shows a broad rhombic signal centred at g =
2.027 (Figure S28), consistent with an S = ½ species.[17] The
formation of 7 from protonation of 4 (via 6H) is suggestive of
an intermolecular H2 evolution pathway.
Hydride 5H represents a two-electron reduced isomer of
phosphinidene 6H. Interested in the potential to undergo a Mo-to-
P H-shift, 5H was oxidized with 2 equiv. of [Fc][BArF24] (Scheme
2). Multinuclear NMR spectroscopy shows moderate conversion
to 6H (39%), demonstrating the feasibility of this rearrangement.
Hydride-proton isomerism has been discussed in the context of
ligand non-innocence for transition metal catalysed HER.[27] Our
results indicate that both phosphide and metal moieties can serve
as sites for hydrogen binding. This process provides experimental
precedent for the proposed “active role” of P-sites in HER[28] and
mimics the fluxionality proposed for surface adsorbed H-
atoms.[4c,29] Low-energy pathways have been determined
computationally in which H-atoms migrate on the surface to
promote H2 evolution,[30] a mobility mimicked in the conversion of
5H to 6H, albeit in an oxidative process. The inverse reaction,
reduction of 6H with 2 equiv. of CoCp2, however, formed 4,
presumably due to H2 production upon reduction (a significantly
weaker P–H bond is anticipated for a reduced Mo(I)Mo(II) bridging
The conversion of 6H to 7 represents the formal loss of a
hydrogen atom, a mechanism proposed be relevant to HER
catalysis—the Volmer-Tafel pathway.[4c,5] There is little
experimental thermochemical data in the literature relevant to
metal phosphide catalyzed HER,[18] a shortcoming we sought
to rectify. The cyclic voltammogram of 4 shows two reversible
oxidation features (-0.52 V and 0.16 V), the former corresponding
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number CHE-1151918 (T.A.), the Dow Next Generation Fund,
and Caltech.
Keywords: μ-Phosphide • μ-Phosphinidene • Thermochemistry
• Proton Reduction • Molybdenum
[1]
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Scheme 3. H2 evolution from well-defined Mo–H and P–H precursors.
phosphinidene). It is also possible that terphenyl diphosphine
chelation prohibits reversion of 6H to 4.
With access to complexes with both Mo–H and P–H bonds,
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formation of monocationic complex of 4 was observed (Figure
S33). Monitoring the reaction over time showed good material
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remained relatively constant (Figure S33), though the previously
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binuclear hydrogen production pathway between 5H and 6H. The
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[3]
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complex, [Rh(tBuxanPOP)Cl]
(
tBuxanPOP
=
4,5-bis(di-tert-
[7]
[8]
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acids (Figure S52), falling between 35.7 and 40.8 kcal/mol. The
relatively strong hydricity of 5H is in agreement with the observed
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In summary, we have synthesized a new series dinuclear
metal phosphides that model elementary steps proposed for
heterogeneous metal phosphide HER catalysts. With a conserved
para-terphenyl diphosphine ancillary ligand set, both Mo2 μ-PH
and Mo2 monohydride μ-P complexes were characterized. From
the former, the bond dissociation enthalpy of the P–H was
determined, giving a weak bond strength consistent with the
observation of slow intermolecular H2 evolution. From the latter,
oxidation results in a Mo–H to P–H isomerization, mimicking the
fluxionality proposed for surface adsorbed H-atoms.[4c] Mixing
these species, 5H and 6H, results in facile H2 formation under
ambient conditions in a demonstration of stoichiometric HER with
a molecular metal phosphide.
J. A. Buss, P. H. Oyala, T. Agapie, Angew. Chem. Int. Ed. 2017, 56,
14502.
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We thank Lawrence M. Henling and Dr. Mike Takase for
crystallographic assistance and Dr. David VanderVelde for NMR
expertise. T.A. is grateful for a Dreyfus fellowship, J.A.B. for an
NSF graduate research fellowship, M.H. for funding from the
Grant-in-Aid for Young Scientists (B) (17K18433), and Y.U. for
financial support from the JSPS Research Fellowships for Young
Scientists. This research was supported by the NSF, grant
[15] Relevant IR data for P–H/D bonds can be found in references 12-14.
[16] We acknowledge that a Mo–H structure for 5H cannot be explicitly ruled
out; however, the preponderance of data supports a P–H assignment. A
detailed discussion is provided in the Supplemental Information.
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[26] The scalar coupling constants to both the bound phosphine and bridging
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10.1002/anie.201808307
Angewandte Chemie International Edition
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Molecularly Precise: A series
of reduced dinuclear μ-
phosphido complexes have
been prepared as models for
Joshua A. Buss, Masanari Hirahara,
Yohei Ueda and Theodor Agapie*
Page No. – Page No.
heterogeneous
proton
Molecular Mimics of Heterogeneous
Metal Phosphides: Thermochemistry,
Hydride-Proton Isomerism, and HER
Reactivity
reduction catalysts. These
molecular complexes provide
insight into elementary reaction
steps and thermochemical
parameters relevant to their
solid-state counterparts. μ-P
protonation,
Mo-to-P
H-
migration, and H-atom transfer
have all been demonstrated, en
route to H2 formation.
This article is protected by copyright. All rights reserved.