CO Coupling Chemistry of a Terminal Mo Carbide: Sequential Addition of Proton, Hydride, and CO Releases Ethenone

Article abstract of DOI:10.1021/jacs.9b07743

The mechanism originally proposed by Fischer and Tropsch for carbon monoxide (CO) hydrogenative catenation involves C-C coupling from a carbide-derived surface methylidene. A single molecular system capable of capturing these complex chemical steps is hitherto unknown. Herein, we demonstrate the sequential addition of proton and hydride to a terminal Mo carbide derived from CO. The resulting anionic methylidene couples with CO (1 atm) at low temperature (-78 °C) to release ethenone. Importantly, the synchronized delivery of two reducing equivalents and an electrophile, in the form of a hydride (H^- = 2e^- + H⁺), promotes alkylidene formation from the carbyne precursor and enables coupling chemistry, under conditions milder than those previously described with strong one-electron reductants and electrophiles. Thermodynamic measurements bracket the hydricity and acidity requirements for promoting methylidene formation from carbide as energetically viable relative to the heterolytic cleavage of H₂. Methylidene formation prior to C-C coupling proves critical for organic product release, as evidenced by direct carbide carbonylation experiments. Spectroscopic studies, a monosilylated model system, and Quantum Mechanics computations provide insight into the mechanistic details of this reaction sequence, which serves as a rare model of the initial stages of the Fischer-Tropsch synthesis.

Full text of DOI:10.1021/jacs.9b07743

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Article
CO Coupling Chemistry of a Terminal Mo Carbide: Sequential
Addition of Proton, Hydride, and CO Releases Ethenone
Joshua A. Buss, Gwendolyn A. Bailey, Julius Oppenheim, David
G. VanderVelde, William A. Goddard, and Theodor Agapie
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b07743 • Publication Date (Web): 03 Sep 2019
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CO Coupling Chemistry of a Terminal Mo Carbide: Sequential Addition
of Proton, Hydride, and CO Releases Ethenone
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Joshua A. Buss, Gwendolyn A. Bailey, Julius Oppenheim, David G. VanderVelde, William A. Goddard
III, and Theodor Agapie
Division of Chemistry and Chemical Engineering, California Institute of Technology, 1200 E. California Blvd. MC 127-
72, Pasadena, CA, USA
ABSTRACT: The mechanism originally proposed byFischer and Tropsch for carbon monoxide (CO) hydrogenative catenation
involves C–C coupling from a carbide-derived surface methylidene. A single molecular system capable of capturing these
complex chemical steps is hitherto unknown. Herein, we demonstrate the sequential addition of proton and hydride to a
terminal Mo carbide derived from CO. The resulting anionic methylidene couples with CO (1 atm.) at low-temperature (-78
°C) to release ethenone. Importantly, the synchronized delivery of two reducing equivalents and an electrophile, in the form
of a hydride (H- = 2e- + H+), promotes alkylidene formation from the carbyne precursor and enables coupling chemistry, under
conditions milder than those previously described with strong one-electron reductants and electrophiles. Thermodynamic
measurements bracket the hydricity and acidity requirements for promoting methylidene formation from carbide as ener-
getically viable upon formal heterolysis of H2. Methylidene formation prior to C–C coupling proves critical for organic product
release, as evidenced by direct carbide carbonylation experiments. Spectroscopic studies, a monosilylated model system, and
Quantum Mechanics computations provide insight into the mechanistic details of this reaction sequence, which serves as a
rare model of the initial stages of the Fischer Tropsch synthesis.
Previous work in our group has focused on reductive C–O
INTRODUCTION
catenation chemistry employing one electron reductants
The Fischer-Tropsch (FT) synthesis is a well-established
(KC8) and silyl electrophiles, demonstrating the deoxygena-
industrial-scale process that uses synthesis gas, a mixture of
tion of bound CO to a terminal carbide (1, Scheme 1).25 This
COand H2, asa feedstock for the generation ofa broad range
reactivity models the early steps proposed for FT, achieving
of catenated products.1-5 While the variation in outputs can
the six-electron cleavage of CO using reducing equivalents
be versatile, it is often cost prohibitive in regards to petro-
stored in a redox non-innocent supporting ligand. C–C cou-
leum substitution.1-2 As efforts increase to synthesize bio-
pling was achieved via an isolated silyl alkylidyne complex,
diesels from renewably sourced synthesis gas,6-7 shifting FT
which, upon addition of two equiv. of strong reductant (KC8
catalysis toward specific desirable products becomes ever
or naphthalenide) followed by strong electrophiles (silyl
more important.7 Understanding the operative mecha-
chlorides), afforded disilylketene (Scheme 1, top).25-26
nism(s) of FT provides potential for rational catalyst design
In contrast to these relatively harsh reducing conditions,
and, in turn, improved selectivity.1-2, 8
several molecular FT models have employed proton and hy-
While there is little consensus as to the operative mecha-
dride equivalents as the products of heterolytic cleavage of
nism(s) of FT,9-10 all proposals share two fundamental com-
H2.27-29 Herein, we revisit CO coupling chemistry from ter-
monalities—CO reduction (via C–H bond formation and/or
minal carbide 1, the first reported example of a terminal
C–O cleavage) and C–C coupling. The elementary step of C–
group VI carbide bearing d-electrons. While C–C bond for-
C coupling between CO and surface hydrocarbons has been
mation from this species is facile, it ultimately results in del-
studied extensively in model systems.1,11-12,13-14 In contrast,
eterious ligand functionalization. Sequential proton and hy-
the investigation of carbide/CO coupling remains largely
underexplored. In seminal studies of carbonyl-bridged
dride delivery convert complex 1 to a terminal methylidene
via an isolated terminal methylidyne (Scheme 1, bottom).
metal carbide ensembles,15 more exposed carbides demon-
Importantly, thermochemical experiments with NaBHPh3
strated enhanced reactivity, including C/CO coupling in the
Fe3 cluster.16 Additional examples of C≡O cleavage and cou-
pling have been reported, likely via carbide intermediacy,
and [Ar2PhPMe]Cl (Ar = 2,4,6-trimethoxyphenyl) provide a
formal demonstration that thermodynamically H2 can pro-
mote reduction and protonation of complex 1, illustrating
with oxophilic metals.17-19 To our knowledge, CO coupling
the feasibility of using proton and hydride as H2 surrogates
has yet to be demonstrated from a terminal carbide precur-
in carbide conversion chemistry. Addition of exogenous CO
sor, likely due to the paucity of reactive yet well-character-
ized examples of such motifs.20-24
to the formed methylidene releases ketene, demonstrating
the sequential formation of two C–H bonds and C/CO cou-
pling, to yield a silicon-free, metal-free C2O1 organic.
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a Carbide 1 was synthesized cleanly in situ via deprotonation
of P2Mo(CH)(CO)(Cl) 4 with benzyl potassium. For more de-
tails, see the Supporting Information.
Scheme 1. Mechanistically Distinct Modes of C/CO Cou-
pling to Free Ketene Fragments 30
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A single-crystal X-ray diffraction (XRD) study of 2 con-
firmed the proposed assignment (Figure 1). The short C31–
C32 bond length and wide C31–C32-O1 angle (1.2885(11)
Å and 179.01°, respectively) support assignment of the η1-
C(P)=C=O bonding motif.33 Mo engages the central arene in
an η6 fashion, to stabilize the low-valent metal center result-
ing from C–C coupling. Transition metal-bound phospho-
ranylidene ketenes, relativelyrare motifs, are generally pre-
pared via reaction of a metal precursor with preformed ke-
tene, R3PCCO.34 In one example, the P–C linkage was formed
at the metal center, from cleavage of carbon suboxide.35
Complex 2 therefore represents the first example of such a
motif in which both the P–C and C–C bonds were formed at
the metal center.
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RESULTS AND DISCUSSION
Carbide/CO Coupling Reactivity
Direct carbide carbonylation represents an early bifurca-
tion in the mechanistically complicated FT process; surface
C2 formationvia carbide/COcoupling hasbeenproposedfor
syngas conversion on iron foils.31 To explore carbide/CO
coupling in a well-defined system, complex 1 was treated
with CO (Scheme 2).30,32 Admission of one atmosphere (1
atm) of 13CO to a THF solution of 1 at -80 °C showed no ef-
fect; however, uponwarming to25 °C, new spectral features
were observed. A peculiar high-field doublet of doublets (-
32.14 ppm, 1J(P,C) = 118 and 1J(C,C) = 103 Hz) was present
in the 13C{1H} NMR spectrum, in addition to two downfield
resonances. Concomitantly, two new features were ob-
served in the 31P{1H} NMR spectrum, including a doublet of
doublet of doublets (37.64 ppm, 1J(P,C) = 118.1 Hz, 2J(P,C) =
33.2 Hz, & 3J(P,P) = 4.2 Hz) displaying strong scalar coupling
to the high-field carbon. The large P–C and C–C spin-spin
coupling constants, in addition to the broken symmetry of
the terphenyl diphosphine (P2) ligand, are most consistent
with formation of a Mo-bound phosphoranylideneketene, 2.
Figure 1. Solid-state structure of 2. Thermal anisotropic dis-
placement ellipsoids are shown at the 50% probability level. H-
atoms are omitted. Selected bond distances [Å] and angles [°]:
P1–C31 1.7144(11), C31–C32 1.2885(11), C32–O1 1.1865(10),
Mo1–C31 2.2899(16), Mo1–C33 1.9563(12), C33–O2
1.1693(10), C31–C32–O1 179.01(8), P1–C31–Mo1 130.73(5).
We hypothesized that CO binding could induce C–C bond
formation36-37 in 1 via a putative dicarbonyl carbide inter-
mediate (Scheme 2). Labeling experiments were conducted
to explore this process. Addition of 12CO to 1 results in a
50:50 mixture of isotopes at the phosphoranylideneketene
α-carbon (Figure 2). Likewise, the 31P{1H} NMR spectrum
displays a broad resonance for the Mo-bound phosphine
arm comprised of an overlapping doublet of doublets and
doublet, the former split by partial (ca. 50%)13C enrichment
at the carbonyl carbon. Taken together, these data are con-
sistent with an intramolecular coupling pathway from a
proposed dicarbonyl carbide complex. This process con-
trasts the ostensibly similar cleavage and coupling of CO by
Ta(silox)3,38 which, through a series of detailed mechanistic
studies, was shown to form the C–C bond prior to the first
deoxygenation event, en route to a ditantalum dicarbide
((silox)3Ta≡CC≡Ta(silox)3).39 Related transformations start-
ing from alkylidyne carbonyl complexes, with addition of
phosphines leading to the assembly of neutral PC(R)=C=O
ligands, have also been reported.40-43 Combined, these mo-
lecular systems represent complementary precedents for
potential elementary steps in the early stages of the FT syn-
thesis.
Scheme 2. CO Coupling Chemistry of Terminal Molyb-
denum Carbidesa
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Figure 2. High-field partial 13C{1H} NMR spectra of 2 generated
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under 13CO (top) and 12CO (bottom) showing splittings for the
phosphoranylidene 13C nucleus circled in grey.
Recognizing that carbide 1 need not bind an additional
molecule of CO to form a ketenylidene, it was slowly
warmed to RT. While this results in a composite mixture of
largely intractable species,25 one of these complexes proved
highly soluble. Extraction ofthe crude residue with hexame-
thyldisiloxane afforded a deep purple solution, which upon
standing at -35 °C, precipitated 3 as a purple powder. While
single crystals of 3 have proven elusive, the structure can be
confidently inferred from multinuclear NMR spectroscopy.
A diagnostic carbidic 13C{1H} resonance is observed at
569.86 ppm, further downfield shifted from that of 1. Two
distinct features in the 31P{1H} NMR spectrum at 91.44 and
-4.08 ppm support an arm-on arm-off binding mode for the
a Carbide 1 was synthesized cleanly in situ via deprotonation
of P2Mo(CH)(CO)(Cl) 4 with benzyl potassium. For more de-
tails, see the Supporting Information.
Gratifyingly, 4 is thermally stable, facilitating both its use
for reaction chemistryand permitting growth of single crys-
tals. A single-crystal XRD study corroborates the structure
inferred from spectroscopy (Figure 3). The Mo center sits
above the central arene ring at a distance of 2.749 Å, sug-
gesting negligible metal arene interaction. The Mo≡CH dis-
tance is appropriately short (1.764(2) Å) but slightly longer
than that in a structurally characterized Mo(VI) methyli-
dyne (1.702(5) Å),44 likely a manifestation of the more-re-
duced metal center.
1
P2 ligand, and four upfield aromatic H shifts corroborate
an η6 metal-arene interaction. Addition of 1 atm of CO to so-
lutions of 3 likewise did not afford the coupled product 2,
however. Instead, an intractable mixture is formed, to-
gether with free P2, reflecting additional, deleterious path-
ways that are enabled by dissociation of the second phos-
phine arm.
Carbide Protonation, Hydride Transfer, and Methylidene/CO
Coupling
Toward modeling the proposed role of surface hydrides in
FT, and ultimately forming C–H bonds, carbide protonation
was targeted.21, 44 Addition of Et3NHCl to a -78 °C THF solu-
tion of 1 results in formation of a new compound upon
warming to room temperature. A single resonance between
3.5 and 6.5 ppm is observed in the 1H NMR spectrum, a dou-
blet of triplets centered at 5.06 ppm (1J(C,H) = 147.8 Hz,
3J(P,H) = 3.1 Hz), with a relative integration of one. This sug-
gests both a terminal methylidyne20, 24, 45-51 and a five-coor-
dinate pseudo-square pyramidal structure, as in 4, without
metal-arene interactions (vide infra; Scheme 3). The 31P{1H}
and 13C{1H} NMR data further corroborate the proposed as-
signment of 4, with a doublet of doublets at dP 40.21 ppm
(2J(C,P) = 18.6 and 10.2 Hz) and a characteristic methyli-
dyne resonance at dC 281.19 ppm, respectively.45, 51
Figure 3. Solid-state structure of 4. Thermal anisotropic dis-
placement ellipsoids are shown at the 50% probability level. H-
atoms are omitted, except for the methylidyne proton, which
was refined freely. Selected bond distances [Å] and angles [°]:
Mo1–C31 1.764(2), Mo1–C32 1.930(4), O1–C32 1.197(6).
Hydride addition to both terminal21 and bridging52 me-
thylidyne ligands has precedent, and we sought to target
this reactivity, considering proton and hydride as a crude
surrogate for H2 (vide infra). NaBEt3H was added to a C7D8
Scheme 3. Sequential Proton and Hydride Addition en
route to Ketene Formation a
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solution of 4 at -78 °C. Mixing the reagents immediately re- methylidene dicarbonyl complex. We favor the latter, given
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sulted in a color change to dark brown, and VT NMR spec-
troscopy showed signals in the 31P{1H} (s, 52.07 ppm),
13C{1H} (br s, 296.91 and br s, 237.52 ppm), and 1H (15.03,
d, 1J(C,H) = 114.90 Hz and 13.27, d, 1J(C,H) = 142.7 Hz) spec-
tra, in line with formation of methylidene 5 (Scheme 3).21
Warming the sample to 0 °C, showed quantitative conver-
sion to a new asymmetric complex with a triplet of doublets
(84.68 ppm, 2J(P,CCO) = 15.5 & 2J(P,CCH2) = 6.0 Hz) and dou-
blet of doublets (47.41 ppm, 1J(P,CCH2) = 21.9 ppm &
2J(P,CCO) = 17.0 Hz) in the 31P{1H} NMR spectrum. The
13C{1H} NMR spectrum likewise showed two signals, a
broad doublet of triplets centered at 251.88 ppm (2J(CCO,P)
that coordination to Mo facilitates C–C coupling chemistry
in the related carbide/CO chemistry described above.
Reactivity of a Silylcarbyne Model System
Expecting that similar C/CO coupling chemistry would be
achieved from silyl carbyne 9, affording a more stable or-
ganic product, a related reactivity sequence was explored
with this silyl model system (Scheme 4). Low-temperature
addition of NaBEt3H to a C7D8 solution of alkylidyne 9 and
subsequent warming to -20 °C resulted in the gradual for-
mation of a new species demarcated by a 31P{1H} resonance
at 48.53 ppm and low-field doublet in the 1H NMR spectrum
(δ = 15.79 ppm, 1J(C,H) = 121.7 Hz). Such a signal could be
consistent with either formyl generation,74-77 via CO inser-
tion into an intermediate Mo hydride (a process that is fa-
cilitated by borane Lewis acids),29, 78-80 or hydride attack at
the carbyne to give a silyl alkylidene. The 13C NMR spectrum
was more informative, displaying a broad doublet at 315.09
ppm (1J(C,H) = 121.7 Hz) and a broad singlet at 251.83 ppm
(Figure S28). C–H coupling to the more downfield reso-
nance is suggestive of a carbene/carbonyl isomer rather
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2
= 15.3 & J(CCO,CCH2) = 2.2 Hz) and an extremely high-field
resonance at -34.87 ppm (ddd, 1J(CCH2,P) = 21.8, 2J(CCH2,P) =
6.2 Hz & 2J(CCH2,CCO) = 2.2 Hz). The high-field chemical shift
in the 13C{1H} NMR spectrum49 and the C1 symmetry are
consistent with P–C bond formation, giving 6,53-54 unexpect-
edly suggesting some electrophilic carbene character in
1
electron-rich, anion 5.53-55 The small J(P,CCH2) spin-spin
coupling56 observed for 6 is consistent with phosphonium
ylide complexation to transition metal centers; ligation re-
sults in rehybridization—C(sp2) to C(sp3)—that is reflected
in the smaller one-bond scalar coupling constant.57 The
structure of 6 was confirmed in a single-crystal XRD study,
which corroborated the assignment of the h1-C-bound ylide
moiety (see the SI).
1
than the alternative carbyne/formyl complex. 2D H/13C
correlation experiments further disambiguated the struc-
ture (Figure S29), with strong HMBC cross peaks between
the H-bound carbon atom and the SiMe3 methyl protons.
These data point to hydrogen transfer to the alkylidyne car-
bon,21, 81-83 affording silyl carbene 10, in analogy to the reac-
tivity observed for parent methylidyne 4 (vide supra).
We hypothesized that similarly to the effect observed for
carbide/CO coupling, methylidene/CO coupling may be fa-
vored by addition of exogenous CO, either via direct CO at-
tack58 or coordination to Mo. Placing a -78 °C solution of in
situ generated 5 under a 13CO atmosphere resulted in a
gradual color change to bright orange, as the gas diffused
down the sample (Scheme 3). The 13C{1H} NMR spectrum at
-78 °C, showed both free 13CO and three resonances in the
metal-bound region—the CO signals of previously charac-
terized P2Mo(13CO)3.59
Scheme 4 Hydride-Initiated C–C Coupling.
Anticipating that zero-valent Mo was indicative of a C–C
coupling process, identification of ethenone by 13C{1H} NMR
spectroscopy wasattempted. The diagnostic signals for free
ketene (13C δ = 193.7 and 2.7 ppm)60 were absent, but cou-
pling doublets at 154.69 and 83.90 ppm (1J(C,C) = 76.1 Hz)
were observed, attributed to a putative ethenone-triethyl
borane adduct.61 While further efforts to detect free
ethenone (7) were unsuccessful, ketene trapping with eth-
anol has precedent, giving stable and easily detectable ethyl
acetate (EtOAc).60, 62 Forming 5 at low temperature, adding
13CO, and condensing EtOH into the reaction tube afforded
EtOAc-13C2, 8, unequivocally demonstrating the generation
and ejection of parent ketene (Scheme 3). Alkylidene car-
bonylation hasbeen demonstrated ona varietyof molecular
scaffolds,13 but often requires CO overpressures,58, 63-65 pro-
ceeds from activated Fischer carbene complexes,13, 66-68 or
occurs at bridging alkylidene ligands.69-70 Rarely is this cou-
pling achieved at monometallic complexes with carbene
fragments derived from CO.67, 71-73
While 10 was the major species observed at -20 °C, it
proved transient, slowly reacting further to give a mixture
of two Mo complexes with a doublet and singlet in the 31
P
NMR spectrum at 58.38 and 76.24 ppm, respectively. The
latter corresponds to previously characterized Mo0-N2 ad-
duct 12,59, 84 demonstrating loss of the carbon-based lig-
ands, presumably via C–C coupling. The isotopically en-
riched 13C signals at 84.76 and 191.35 ppm showed strong
scalar coupling (1J(C,C) = 69.7 Hz), the latter split into a dou-
blet of triplets (2J(C,P) = 26.3 Hz) by the trans-spanning
phosphines of the P2 ancillary ligand. This spectral signa-
ture isconsistent withC–C bond formation affording the tri-
methylsilyl ketene complex 11. Interestingly, in this case,
CO addition is not requisite for C–C coupling.
Isotopic labeling studies were conducted in which 12CO
was added to 5. Mixed isotopolog P2Mo(12/13CO)3 was
formed, consistent with either i) rapid exchange of the 13CO
ligand in 5 with free 12CO and on-metal C–C bond formation
or ii) 12CO binding and C–C coupling preceding from a
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The assignment of 11 was further supported via inde-
level. H-atoms are omitted for clarity. Selected bond dis-
tances [Å] and angles [°]: Mo1–O1 2.1858(7), Mo1–C31
2.1629(10), Mo1–Carene (ave.) 2.2599(9), O1–C31
1.2799(12), C31–C32 1.3536(14), ∠O1-C31-C32 132.32(9).
pendent synthesis.85 Anticipating that the observed mixture
of 11 and 12 resulted from a ligand substitution equilib-
rium between the isoelectronic Mo0 adducts, a C6D6 solution
of 12 was treated with trimethylsilyl ketene.86 While only
starting material was observed under an N2 atmosphere,
degassing the reaction mixture resulted in significant con-
version to 11, as evidenced by 31P{1H} NMR spectroscopy.
The N2/ketene exchange was borne out in the hydride-initi-
ated C–C coupling chemistry; in addition to the coupling
doublets of the bound ketene, a pair of doublets at 192.48
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Quantum Mechanics Computations
Akin to the sequential addition of potassium graphite and
silyl electrophile to alkylidyne 9,25-26 NaBEt3H proved profi-
cient for enacting the reductive coupling of silyl carbyne
with CO. The latter reactivity manifold, preceding through a
carbene intermediate (10) rather than a dicarbyne com-
plex, is accessible under much milder conditions and shows
equal efficacy in C–C coupling. Given the facility with which
the catenation proceeds, Quantum Mechanics computation
(using the M06-2X flavor of Density Functional Theory)93
was employed to interrogate the electronic structure re-
quirements in the critical bond-forming step. The optimized
structure following hydride addition to 9 resembles silyl al-
kylidene 10,94 in agreement with experiment; however, a
close contact between Na+ and the Cl ligand is observed in
addition to borane binding to the carbonyl (Figure S42). 95
The HOMO of this complex, with dxz parentage, is dominated
by a π-bonding interaction with CO (10•BEt3, Figure 5, in-
set). As the C–C distance contracts, the alkylidene ligand ro-
tates, directing the Mo–CSiπ-systemtowards CO (1X, Figure
5). This geometry has a dyz HOMO stabilized by M-arene
backbonding. With further C–C shortening, the ground state
crosses to a triplet, resulting from electron promotion from
the dyz (Mo– arene π) to the dxz (Mo–CSi π*) and representing
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and 60.28 ppm (1J(C,C) = 73.5 Hz) were observed in the 13
C
NMR spectrum, corresponding to the metal-free C2 frag-
ment.61
An XRD study of single crystals of 11 grown from slow
evaporation of liquid butane confirmed the inferred η2-CO
ketene assignment (Figure 4), with comparable O1-C31
(1.2799(12) Å) and C31-C32 (1.3536(14) Å) distances.87-89
η2-CC ketene adducts are known,90-92 but the O-bound form
is often thermodynamically favored.90 Contrasting the
structure of 9,25 As in the structure of 2 above, the central
arene is engaged in η6 coordination to Mo, consistent with
the more reduced Mo center formed on C–C coupling.
3
cleavage of the Mo–C π-bond (1X to X, Figure 5, right in-
sets). This alkylidene activation process, with significant
electron density localization on the carbon, is facilitated by
the proximal Na+ ion, without which a low-energy pathway
could not be identified. Bond formation is additionally as-
sisted by borane coordination, stabilizing the oxygen lone
pair aselectron density is transferred toCO.95 The direct im-
pact of the Lewis acids Na+ and BH3 on this C–C coupling
process is particularly notable. While BEt3 has no direct rel-
evance to FT, there is precedent for both Lewis acidic pro-
moters in FT catalysis10 and selectivity perturbations re-
sulting from alkali metal cation addition in electrochemical
CO96-97 reduction. Our computations highlight in a well-de-
fined molecular system the necessity of these interactions
for coupling chemistry resulting in ketene.
Figure 4. Solid-state structure of 11. Thermal anisotropic
displacement ellipsoids are shown at the 50% probability
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Figure 5. Potential Energy Landscape for C–C Bond Formation. Truncated transition state structures, computed C–C bond distances,
and energies (relative to 10•BEt3) are provided. The insets depict key molecular orbitals (0.05 eÅ-3 isosurfaces) involved in bond-
breaking and making steps.
The conjugate ylide Ar2PhP=CH2 was not detected, how-
ever (dP 7.00 ppm). Instead, a new peak in the 31P NMR at
7.4 ppm was observed, together with free P2 (dP –3.45
ppm), suggesting displacement of P2 by ylide in the formed
methylidyne complex. Competing decomposition of the car-
bide complex by other pathways, however, cannot be
strictly ruled out. Taken together, these protonation studies
suggest a lower limit of 33 for the pKa (Cl) of methylidyne 4,
roughly three orders of magnitude higher than the closest
previously reported comparator, Mo(≡CH)(NRAr)3.20 The
higher effective basicity of carbide 1 may reflect diminished
π-donation from the carbide ligand in this more electron-
rich, formally Mo(IV), complex, and/or the favored energet-
ics of chloride binding to Mo.
Thermochemistry of Proton and Hydride Transfer to Carbide
1
Thus far, a moderate acid (Et3NHCl, pKa = 14.9 in THF)98 and
a strong hydride donor (NaBHEt3, ΔG°H- = 26 kcal/mol in
MeCN99) have been described in the reaction chemistry for
the formation of ethenone from 1. These reagents are only
a crude surrogate for H2, as the free energy of hydrogen for-
mation is favorable by ca. 25 kcal/mol, considering the re-
action free energy of H2 heterolysis in MeCN.100 Given the
basicity of related Mo(VI) and W(VI) carbide complexes
(Mo(≡CH)(NRAr)3, pKa
~
30 kcal/mol in THF;20
W(≡CH)(CO)2(Tp*), pKa = 28.7 kcal/mol in THF21), we an-
ticipatedthat weaker acidscould be employedtoaccessme-
thylidyne 4.
Scheme 5. Protonation studies of carbide 1.a, b
Further protonation studies assessed the basicity of car-
bide 1, and supported a pKa (Cl) for methylidyne 4 of ≥ 33.101
Treatment of carbide 1 with the phosphazene base
P2Et•HCl (Scheme 5; pKa 26.6 in THF102) resulted in quanti-
tative conversion to methylidyne 4, with observation of the
expected conjugate base P2Et at 15.1 ppm in the 31P{1H}
NMR spectrum. Reaction with the weaker acid
[Ar2PhPMe]Cl (Ar = 2,4,6-trimethoxyphenyl; pKa 33.5 in
THF102) likewise furnished methylidyne 4, along with a new
species, 4’, whose NMR signature mirrored that of 4, sug-
gesting a closely related isomer. Most diagnostic was a 31
P
chemical shift at 41.81 ppm, shifted only ca. 1.5 ppm down-
field from 4, and a diagnostic 13C resonance at 267.8 ppm,
1
which correlated by HMQC analysis to a triplet in the H
NMR spectrum at 2.64 ppm for the methylidyne proton
(3J(H,P)= 3.8 Hz). Independent synthesis, together with a
single-crystal XRD study, confirmed 4’ to be an isomer of 4
in which the methylidyne is oriented cis with respect to the
central arene (Figure 6). As in the previously reported cis
silylcarbyne isomer,26 the cis methylidyne isomer adopts a
pseudo-octahedral geometry with a clear h2-arene interac-
tion trans to CO. The Mo–C31 distances are similar between
the two isomers (cis/trans 1.781 Å vs 1.764(2), respec-
tively), and both are consistent with a Mo-C triple bond.
apKa values of HCl salts in THF: P2Et•HCl, 26.6;98
[PhAr2PMe][Cl], 33.5.102 b Carbide 1 was synthesized cleanly in
situ via deprotonation of P2Mo(CH)(CO)(Cl) 4 with benzyl po-
tassium. For more details, see the Supporting Information.
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Figure 6. Solid-state structure of 4’. Thermal anisotropic
displacement ellipsoids are shown at the 50% probability
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level. H-atoms are omitted for clarity, except the methyli-
dyne proton. Selected bond distances [Å] and angles [°]:
Mo1–C31 1.781(2), Mo1–C32 1.972(2), O1–C32 1.155(2).
CONCLUSION
In summary, a terminal Mo carbide complex has been
demonstrated to undergo direct carbonylation, in conjunc-
tion with ligand functionalization, affording a phospho-
ranylideneketene complex. Carbide protonation provides a
thermally stable methylidyne carbonyl complex, which,
upon sequential treatment with hydride and CO, induces C–
C coupling. Experimental and Quantum Mechanics compu-
tational interrogation of reactions between the silyl alkyli-
dyne congener and hydride suggest that this C–C bond for-
mationdiffers fromthe mechanismestablished for stepwise
reduction/electrophile addition, proceeding through car-
bene carbonyl intermediates rather than dicarbynes.
To gain insight into the hydride affinity of methylidyne 4,
weaker hydride sources were employed. Hydride transfer
was complete using NaBHPh3 (DG°H– = 36 in MeCN99), as
judged bythe quantitative formation of 6 upon warming the
reaction to RT. However, in a variable temperature NMR
study, methylidene 5 was not observed on maintaining the
reaction at –78 °C. Instead, observation of 5 necessitated
warming to ca. –30 °C, consistent with a higher activation
energyof hydride transfer fromthe bulkier borohydride. Up
to equal proportions of 5 and 4 were observed at –20 °C,
suggesting that the hydride affinity of methylidyne 4 is at
least comparable to that of NaBHPh3. Competing conversion
of 5 to6 occurred at these temperatures; however, prevent-
ing extraction of the exact thermodynamic parameters.
Notably, this reaction sequence models, with high fidelity,
the proposed mechanism of FT: surrogates of H2 react with
a terminal carbide (1) to afford a reactive methylidene (5)
(cf. B – F, Figure 7). This methylidene is the precursor to C–
C bond formation, coupling with CO to give ketene. These
steps provide valuable precedent for the reactivity pro-
posed for the heterogenous FT process, mimicking the
multi-electron C–C coupling reactivity postulated following
C–O scission at a well-defined monometallic Mo carbide
complex. Critically, the use of hydride as a FT relevant two-
electron reductant provides an efficient strategy to accom-
plish C–C coupling and C–H bond formation. The reaction of
the 1 with weak acid and hydride sources (the products of
formal H2 heterolytic cleavage) to access a methylidene con-
trasts the lack of reactivity observed when treated with H2
directly, highlighting the need for pre-activation to achieve
productive carbide hydrogenation in either the present mo-
lecular model system or via cleavage on the catalyst surface
in FT.
Addition of NaBHPh3 at -78 °C, followed by reaction with
CO (1 atm), likewise furnished P2Mo(CO)3 on warming to
RT, as expected based on the thermodynamic favorability of
conversion from 5 described above. A small peak assigned
to 5 wasapparent only at intermediate temperatures (0 °C),
again consistent with the higher activation barrier using
NaBHPh3.
Reconsidering the relevance to H2 splitting, the pKa of
[Ar2PhPMe]Cl and hydricity of NaBHPh3 suggest that a reac-
tion between carbide 1 and H2 is thermodynamically down-
hill by at least 5 kcal / mol (Scheme 6).103 These experi-
ments therefore constitute a thermochemical demonstra-
tion that H2 is a suitable source of protons and hydrides in
the studied system. However, warming a THF solution of 1
under 1 atm H2 showed only slight conversion to 3. Direct
reaction of carbides with H2 therefore appears to be kinet-
ically disfavored.104 Cleavage of H2―either heterolytically or
on a surface―may be important to reaction with carbide
and subsequent enabling of C–C coupling reactivity. This is
in contrast to the reactivity of transition metal imides and
carbenes, for which direct activation of R–H bonds (R = C,
H) is kinetically allowed.105-107
Scheme 6. Thermochemical Cycle Evaluating Relevance
to Syngas Chemistry
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Catalysts and Catalytic Reactions. ACS Catal. 2019, 3026-
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Figure 7. i) Schematic representation of the early steps of
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ASSOCIATED CONTENT
Supporting Information
Detailed experimental procedures, full characterization, crys-
tallographic details (CIF), and spectroscopic data. Thismaterial
is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: agapie@caltech.edu
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
We thank Larry Henling and Mike Takase for invaluable crys-
tallographic assistance. J.A.B. isgrateful for an NSF graduate re-
search fellowship, G.A.B. for NSERC and Resnick Sustainability
Institute fellowships, and J.O. for an Ernest H. Swift Summer
Undergraduate Research Fellowship. We thank the NSF (CHE-
1800501), the Dow Next Generation Education Fund (instru-
mentation), and Caltech for funding. The computational studies
were supported by the NSF (CBET-1805022).
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1
56. For reference, the J(P,CCH2) coupling constant in
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P
chemical shift for 5 is detected slightly upfield at 49.4 ppm
(cf. 52.1 ppm in the reaction with BEt3). No such change in
chemical shift was detected for 6 in these two reactions,
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however; nor was it detected for bound ketene complex 11 103. Because of the limited availability of hydricity
1
2
3
4
5
6
7
8
generated in the presence or absence of BEt3. We conclude
that O–borane interactions are likely important for
stabilizing reaction intermediates during C–C coupling, but
that they are not ubiquitously present in the ground-state
structures.
96. Pérez-Gallent, E.; Marcandalli, G.; Figueiredo, M. C.;
Calle-Vallejo, F.; Koper, M. T. M. Structure- and Potential-
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Crystal Electrodes. J. Am. Chem. Soc. 2017, 139 (45), 16412-
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99. Heiden, Z. M.; Lathem, A. P. Establishing the
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Organometallics 2015, 34 (10), 1818-1827.
100. Curtis, C. J.; Miedaner, A.; Ellis, W. W.; DuBois, D. L.
Measurement of the Hydride Donor Abilities of
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Activation of Hydrogen. J. Am. Chem. Soc. 2002, 124 (9),
1918-1925.
101. The effective pKa of methylidyne 4 is designated as
pKa(Cl) to indicate that the observed reactivity reflects both
the free energy of proton transfer (pKa) and the free energy
of chloride binding to Mo.
values reported in THF, and the reactivity of Mo-P2
complexesinMeCN, we rely on the hydricity values inMeCN,
but use the pKa values determined in THF, to get an
approximation of the Gibbs free energy of H2 addition to 1.
To correct for these solvent effects, we used Morris’ method
to estimate a lower bound for the pKa of methylidyne 4 in
MeCN from the experimentally determined pKa in THF. Even
with this correction factor applied, the addition of H2 to 1
remains thermoneutral (pKaMeCN > 28; ΔG < 1.5 kcal / mol).
See: Morris, R. H. Estimating the Acidity of Transition Metal
Hydride and Dihydrogen Complexes by Adding Ligand
Acidity Constants. J. Am. Chem. Soc. 2014, 136 (5), 1948-
1959.
104. Soluble KCl is carried forward from the in situ
synthesis of 1, as evidenced by experiments in which 1 was
treated with acids bearing BArF4 counterions, and chloride-
bound methylidynes 4/4’ were generated as the primary
products (see the SI). Even so, no reaction was observed
between carbide 1 and H2.
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insertion. Nature 2008, 451, 417.
106. Park, Y.; Kim, Y.; Chang, S. Transition Metal-
Catalyzed C–H Amination: Scope, Mechanism, and
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107. Chu, J. C. K.; Rovis, T. Complementary Strategies for
Directed C(sp3)−H Functionalization: A Comparison of
Transition-Metal-Catalyzed Activation, Hydrogen Atom
Transfer, and Carbene/Nitrene Transfer. Angew. Chem. Int.
Ed. 2018, 57 (1), 62-101.
102. Saame, J.; Rodima, T.; Tshepelevitsh, S.; Kütt, A.;
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Experimental Basicities of Superbasic Phosphonium Ylides
and Phosphazenes. J. Org. Chem. 2016, 81 (17), 7349-7361.
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