Mechanism of Molybdenum-Mediated Carbon Monoxide Deoxygenation and Coupling: Mono- and Dicarbyne Complexes Precede C-O Bond Cleavage and C-C Bond Formation

Article abstract of DOI:10.1021/jacs.6b10535

Deoxygenative coupling of CO to value-added C_≥2 products is challenging and mechanistically poorly understood. Herein, we report a mechanistic investigation into the reductive coupling of CO, which provides new fundamental insights into a multielectron bond-breaking and bond-making transformation. In our studies, the formation of a bis(siloxycarbyne) complex precedes C-O bond cleavage. At -78 °C, over days, C-C coupling occurs without C-O cleavage. However, upon warming to 0 °C, C-O cleavage is observed from this bis(siloxycarbyne) complex. A siloxycarbyne/CO species undergoes C-O bond cleavage at lower temperatures, indicating that monosilylation, and a more electron-rich Mo center, favors deoxygenative pathways. From the bis(siloxycarbyne), isotopic labeling experiments and kinetics are consistent with a mechanism involving unimolecular silyl loss or C-O cleavage as rate-determining steps toward carbide formation. Reduction of Mo(IV) CO adducts of carbide and silylcarbyne species allowed for the spectroscopic detection of reduced silylcarbyne/CO and mixed silylcarbyne/siloxycarbyne complexes, respectively. Upon warming, both of these silylcarbynes undergo C-C bond formation, releasing silylated C₂O₁ fragments and demonstrating that the multiple bonded terminal Mo≡C moiety is an intermediate on the path to deoxygenated, C-C coupled products. The electronic structures of Mo carbide and carbyne species were investigated quantum mechanically. Overall, the present studies establish the elementary reactions steps by which CO is cleaved and coupled at a single metal site.

Full text of DOI:10.1021/jacs.6b10535

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Article
Mechanism of Molybdenum Mediated Carbon Monoxide
Deoxygenation and Coupling: Mono- and Dicarbyne Complexes
Precede C–O Bond Cleavage and C–C Bond Formation
Joshua A. Buss, and Theodor Agapie
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b10535 • Publication Date (Web): 21 Nov 2016
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Mechanism of Molybdenum Mediated Carbon Monoxide De-
oxygenation and Coupling: Mono- and Dicarbyne Complexes
Precede C–O Bond Cleavage and C–C Bond Formation
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Joshua A. Buss and Theodor Agapie*
Division of Chemistry and Chemical Engineering, California Institute of Technology, 1200 E. California Blvd. MC 127-72, Pasadena,
CA, USA
ABSTRACT: Deoxygenative coupling of CO to value-added C≥2 products is challenging and mechanistically poorly understood. Herein we
report a mechanistic investigation into the reductive coupling of CO, which provides new fundamental insights into a multi-electron bond
breaking and bond making transformation. In our studies, the formation of a bis(siloxycarbyne) complex precedes C–O bond cleavage. At -78
°C, over days, C–C coupling occurs without C–O cleavage. However, upon warming to 0 °C, C–O cleavage is observed from this
bis(siloxycarbyne) complex. A siloxycarbyne/CO species undergoes C–O bond cleavage at lower temperatures, indicating that monosilyla-
tion, and a more electron rich Mo center, favors deoxygenative pathways. From the bis(siloxycarbyne), isotopic labeling experiments and
kinetics are consistent with a mechanism involving unimolecular silyl loss or C–O cleavage as rate determining steps toward carbide for-
mation. Reduction of Mo(IV) CO adducts of carbide and silylcarbyne species allowed for the spectroscopic detection of reduced silyl-
carbyne/CO and mixed silylcarbyne/siloxycarbyne complexes, respectively. Upon warming, both of these silylcarbynes undergo C–C bond
formation, releasing silylated C2O1 fragments and demonstrating that the multiple bonded terminal Mo≡C moiety is an intermediate on the
path to deoxygenated, C–C coupled products. The electronic structures of Mo carbide and carbyne species were investigated quantum me-
chanically. Overall, the present studies establish the elementary reactions steps by which CO is cleaved and coupled at a single metal site.
electron reduced CO coupling product. This reaction proceeds
through a silylated dicarbyne complex,33,34 and, following hydro-
genation, can yield metal-free siloxy-substituted olefins.32-34 Alt-
hough these alkenes represent a four-electron reduced CO-derived
product, both C–O bonds of the starting COs remain intact. It is
rare that both C–O bond cleavage and C–C bond formation are
simultaneously achieved in homogeneous model systems.15,24,25
This is in stark contrast to the product distributions found for
CO coupling catalysis in F–T,4,5 by oxide-derived Cu electrodes,35
and by nitrogenase enzymes.36,37 The hydrocarbons and alcohols
formed in these systems require at least one C–O bond breaking
step. Despite experimental38-40 and computational41,42 studies, in-
sight into the operative mechanisms of the electrocatalytic and
enzymatic systems is sparse. Most mechanistic proposals for the
more thoroughly studied F–T process invoke C–O bond cleavage
to a surface carbide prior to C–C coupling.3 Only a handful of ho-
mogeneous complexes bearing bridging17,43 and terminal44-46 car-
bides have been synthesized from CO, overcoming the strong C≡O
triple bond (257 kcal/mol).47 Functionalization of these carbides
with electrophiles provides alkylidynes,44,45 motifs that have been
shown to undergo C–C bond formation with CO ligands,48-52 but
only recently has a terminal transition metal carbide been clearly
demonstrated as an intermediate in a homogeneous CO coupling
reaction.46
1. INTRODUCTION
With the rise of atmospheric carbon dioxide (CO2) levels and geo-
political constraints on the availability of reduced carbon reserves,
there is significant interest in the generation of liquid fuels from
oxygenated C1 feedstocks.1,2 Synthesis gas (syngas), a mixture of
carbon monoxide (CO) and dihydrogen (H2), is one such feed-
stock obtained from biomass, coal, and natural gas.3,4 Using syngas
as an alternative to crude petroleum for the generation of fuels and
chemical precursors is demonstrated in the Fischer-Tropsch (F–T)
process; F–T converts syngas to a complex mixture of hydrocar-
bons at high temperatures on heterogeneous surfaces.3-5 Increasing
the selectivity of F–T is of significant interest and a recent report
demonstrates that decoupling CO activation and C–C bond for-
mation is a viable strategy.6 Deeper mechanistic insight into the
elementary transformations involved in F–T holds promise for
developing more efficient and selective catalysts. However, detec-
tion of reaction intermediates in these systems is challenging.3-5
Molecular systems have been studied extensively to gain insight
into the steps of reductive CO coupling chemistry.3 Leveraging
strong M–O interactions,7-17 insertion into early transition-metal
hydride bonds,18-25 inducing radical character at carbon,26,27 and
electrophilic functionalization28-33 have all proven viable strategies
for CO reduction, C–O bond cleavage, and C–C bond formation.
The addition of silyl electrophiles to electron-rich dicarbonyl com-
plexes forms bound bis(siloxy)acetylene units,28-30,32,34 a two-
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Scheme 1: CO activation, cleavage, and coupling at a single Mo center.46
Taking advantage of the coordinative flexibility and redox activi-
ty of noninnocent para-terphenyldiphosphine ligands,53-59 we have
recently described the role of complexes 1-8 in C–O bond cleavage
and C–C bond formation between two highly activated CO ligands
(Scheme 1). Terminal Mo carbide 7 was proposed as the product
of C–O bond cleavage (Scheme 1, II) and Mo silylcarbyne 8 was
demonstrated as a precursor to C–C coupling (Scheme 1, III),
resulting in a four-electron reduced C2O1 fragment (Scheme 1, 6c).
In the present study, we provide a more detailed and complete
mechanistic picture of the pivotal C–O bond cleaving and C–C
bond forming steps in this complex reaction sequence, including
the characterization of several reaction intermediates.
2. RESULTS AND DISCUSSION
2.1. Previously Reported Molybdenum Dicarbonyl
Complexes. We recently described the synthesis and characteri-
zation of polyanionic dicarbonyl complexes 3 and 4.46 A full de-
scription of their structural details follows. Dianion 3 crystallizes as
a potassium bridged dinuclear cluster—two Mo(CO)2 units are
bridged by four potassium ions. The cations display interactions
with the carbonyl ligands, the dangling phosphine arm, the central
arenes of the terphenyl backbone, and solvating THF molecules.
Significant distortion of the central arene from planarity is ob-
served, with an angle of 46.2° between the C6–C1–C2–C3 and C6–
C5–C4–C3 planes. This deplanarization, as well as localization of
short C–C contacts between C1 and C2 and C4 and C5 (Figure 1), is
consistent with partial cyclohexadienyldianionic character.60,61 No-
tably, the dianion shows Mo–CO and C–O bond metrics that are
significantly contracted and elongated, respectively, from neutral
species 2 (Figure 1).
Trianion 4 likewise adopts a polynuclear structure—a Mo4K12
tetramer—in the solid state. Two disparate K3Mo(CO)2 subunits
are observed. One is quite similar in structure to dianion 3, with an
η4 metal-arene interaction, a deplanarization of the central arene
ring of 43.7°, and short Mo–CO and long C–O contacts (Figure 1,
4-η4). The second subunit has antifacial phosphines, one coordi-
nating Mo and the other a potassium cation, on opposite faces of
Figure 1: Line representations (left) and bond metrics (right) as
determined by single-crystal XRD for dicarbonyl complexes 2-4.
For anions 3 and 4, representative mononuclear KnMo(CO)2
fragments of the polynuclear complexes are shown. All bond dis-
tances are reported in Å.
the
central
ring
(Figure
1,
4-η3).
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This arene ring, which displays η3 Mo-allyl character,56 has two
3-13C, which serves as a sacrificial two-electron reductant. This
cation-π interactions with the potassium counter ions62 and is more
planar (∠C3–C2–C1)(C3–C4–C6–C1) = 34.8°). The Mo–C and C–
O distances are comparable in both the η3 and η4-bound compo-
nents, and along with the arene bond metrics, support significant
delocalization of the stored reducing equivalents between the cen-
tral ring, Mo center, and CO ligands. Formal oxidation state as-
signments of Mo in 3 and 4 are complicated by this high degree of
charge delocalization.
two-electron oxidation of 3-13C leads to Mo(0) dicarbonyl com-
plex 2-13C. Reduction of 7-13C by two electrons coupled with
silylation forms the C2O1 species, 6a-13C, and Mo(0)-N2 complex
5. The four electrons required for the conversion of two molecules
of CO to 6a-13C are provided by two equivalents of dianion 3-13C,
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forming equimolar amounts of the Mo(0) complexes 5 and 2-13
(depicted in blue in Scheme 2).
C
The product ratio of 2 and 5 suggests additional unproductive
redox chemistry from dianion 3-13C, likely involving reduction of
electrophile (depicted in red in Scheme 2). Though the same
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2.2. Variable Temperature NMR Studies of Electro-
philic Quenching of Anion 3. Variable temperature (VT)
NMR studies monitoring addition of both small and large silyl
electrophiles to complex 3 were conducted in an attempt to spec-
troscopically characterize reaction intermediates.
products are observed when either dianion 3-13C or trianion 4-13
C
i
is treated with Pr3SiCl at low temperature, the latter reacts quanti-
tatively (in reducing equivalents) toward CO deoxygenative cou-
pling chemistry to form the desired products 5 and 6a (75% con-
31
version, by P{1H} NMR).46 The quantitative reactivity of trianion
4-13C suggests that fully pre-loading the complex with electrons
promotes C–O reduction chemistry and disfavors unproductive
outer-sphere redox, an important feature for designing highly selec-
tive systems.
i
Scheme 2: Pr3SiCl addition to dianion 3. Product distribution,
as determined by 31P{1H} NMR spectroscopy, is shown in brackets.
i
2.2.1. VT NMR Studies of Reaction With Pr3SiCl. A solution of
13CO enriched dianion 3 (3-13C) was treated with four equiv. of
iPr3SiCl at -78 °C (Scheme 2). No reaction was observed at this
temperature by P{1H} or C{1H} NMR spectroscopy. Warming
the sample to -30 °C resulted in the appearance of carbide 7-13C, as
a minor species, as indicated by the characteristic low-field carbidic
resonance at 547.2 ppm in the 13C{1H} NMR spectrum (Figure
S1). Concomitant formation of oxyacetylene 6a, dinitrogen adduct
5, and dicarbonyl 2-13C were observed by coupled doublets at
108.3 and 25.2 ppm (1J(C,C) = 167.7 Hz) in the 13C{1H} NMR
spectrum, a 31P{1H} resonance at 76.4 ppm, and by 13C{1H} (229.9
31
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Scheme 3: Me3SiCl mediated C–O cleavage and C–C coupling
transformations from dianion 3.
2
2
ppm, d, J(P,C) = 11.1 Hz) and 31P{1H} (95.9 ppm, t, J(P,C) =
11.2 Hz) NMR spectroscopy, respectively. Further warming of the
solution resulted in complete conversion to the Mo-N2 complex 5,
oxidized dicarbonyl 2-13C—in a ca. 1:2 ratio (by 31P{1H}
NMR)—and the C2O1 organic fragment, 6a. These data support
the intermediacy of carbide 7 in the reaction sequence. No other
intermediates were detected.
2.2.2. VT NMR Studies of Reaction With Me3SiCl. The addition of
the smaller silyl electrophile, Me3SiCl, to dianion 3-13C was like-
wise investigated at low temperature. Thawing a frozen THF solu-
tion of 3-13C and Me3SiCl to -80 °C resulted in the observation of a
single triplet resonance at 63.8 ppm (2J(C,P) = 17.32 Hz) in the
31P{1H} NMR spectrum, indicating that the free phosphine arm in
3-13C rebinds the Mo center. Two broad resonances at 285.8 and
275.8 ppm were present in the 13C{1H} NMR spectrum (Figure
S2). These downfield 13C signals, suggest the formation of di-
carbyne 9-13C (Scheme 3).63 Similar 13C chemical shifts are report-
ed for siloxycarbyne complexes of Fe (230-260 ppm)33,64 and Ta
The observed reactivity is consistent with multiple reaction
pathways from dianion 3-13C (Scheme 2). Productive CO reduc-
tion chemistry involves carbonyl silylation and C–O bond cleavage,
providing carbide 7-13C and (iPr3Si)2O. The intermediate Mo(IV)
complex 7-13C is envisioned to react with an equimolar amount of
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(240 ppm)65. To the best of our knowledge, bis(siloxycarbyne)
bond cleavage was observed, 10 was treated with excess Me3SiCl;
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8
complexes have not been reported for Mo, though analogous
no reaction was observed. In low temperature reactions of 3 with
silyl electrophiles, the dianion itself acts as a reductant (vide supra).
To probe if 10 reacts under reductive conditions in the presence of
Me3SiCl, in situ reduction of the above mixture was conducted.
Addition of two equiv. of [Na][C10H8] at -78 °C resulted in the
formation of an intractable mixture as evidenced by 13C{1H} and
31P{1H} NMR (Figure S4). Even upon warming to room tempera-
ture, no 13C resonances consistent with C–O cleavage chemistry
from 10-13C were observed, consistent with C–C coupled 10 be-
ing off-path to C–O cleavage and the observed four-electron reduc-
tive coupling chemistry.
13
bis(aminocarbyne) motifs are known for both Mo and W, with C
resonances in the range of 250-280 ppm.66-69
A pseudo-square pyramidal coordination geometry of dicarbyne
9-13C is proposed with two 13COSiMe3 moieties in distinct chemi-
cal environments, axial and equatorial (Figure S2, right), account-
13
ing for the two unique resonances observed in the C{1H} NMR
spectrum at -80 °C. Warming 9-13C to -20 °C results in coalescence
of these resonances to a broad signal at 281.9 ppm, consistent with
two Mo≡13COSiMe3 fragments involved in a fast exchange process
on the NMR time scale. This fluxionality is attributed to ring-
slipping of the η2-arene interaction on opposite edges of the arene,
while also interconverting the axial and equatorial alkylidynes.
Cooling the sample to -100 °C returns the disparate signals, and
resolves each as a broad triplet (284.9 and 276.0 ppm). These re-
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1
solved resonances show no J(C,C), demonstrating a lack of a C–C
bond and further supporting the dicarbyne assignment. The
2J(P,C) data from 31P{1H} NMR spectroscopy also support this
exchange process (Figure S2).
The spectrum collected at -100 °C, where the carbyne moieties
are under the slow exchange limit, was simulated; modeling the
exchange dynamics up to -20 °C provided rates for the carbyne
exchange (Figure S2). The Eyring equation was used to calculate
the activation parameters of this process giving ∆G‡ = 9.4 ± 0.4
kcal/mol at -40 °C (Table S1), which is in reasonable agreement
with the estimated value from the approximate coalescence tem-
perature (∆G‡ = 9.9 kcal/mol, Tc = -40 °C). The calculated activa-
tion parameters are ∆H‡ = 6.6 ± 0.4 kcal/mol and ∆S‡ = -12 ± 2
e.u. (Figure S3). The large and negative ∆S‡ is consistent with a
non-dissociative process with a highly ordered transition state,
likely required by the steric constraints of shifting of the two car-
byne ligands in the terphenyl diphosphine cleft without phosphine
arm dissociation.
Crystallization of bis(siloxycarbyne) 9 was attempted; slow
evaporation of liquid butane over days furnished maroon single
crystals of a new species corresponding to two electron CO cou-
pling chemistry—bis(siloxy)acetylene complex 10 (Scheme 3).
The solid-state structure of 10 (Figure 2) is similar to that of other
bis(siloxy)acetylene complexes of group V metals, with a short C31–
C32 distance (1.332(2) Å), an acute C31–Mo–C32 angle (38.0°), and
long C31/32–O1/2 distances (1.367(2) Å).30,32 The Mo center adopts
an η6 metal-arene interaction, with coordination of a single phos-
phine donor.
31
The P{1H} NMR data for the bis(siloxy)acetylene adduct 10-
Figure 2: Solid-state structures of 10 and 11 with anisotropic
displacement ellipsoids shown at the 50% probability level. Hydro-
gen atoms are omitted for clarity. Selected bond lengths [Å] and
angles [°]: 10 Mo1–Carene(ave.) 2.280(2), Mo1–C31 2.054(1), Mo1–
C32 2.032(1), C1–C2 1.424(2), C2–C3 1.427(2), C3–C4 1.430(2),
C4–C5 1.430(2), C5–C6 1.426(2), C6–C1 1.428(2), C31–O1
1.362(2), C32–O2 1.368(2), C31–C32 1.332(2), ∠ C31-Mo1-C32
38.02(6); 11 Mo1–C5 2.524(1), Mo1–C6 2.520(1), Mo1–C31
1.976(1), Mo1–C32 1.796(1), Mo1–Cl1 2.588(1), C1–C2 1.383(1),
C2–C3 1.407(1), C3–C4 1.382(1), C4–C5 1.421 (1), C5–C6 1.395
(1), C6–C1 1.424(1), C31–O1 1.158(1), C32–Si1 1.882(1).
13C corroborate the conservation of a monophosphine coordina-
tion environment in solution, with resonances at 86.7 and -6.8 ppm
13
for the Mo-bound and free phosphines, respectively. The C{1H}
NMR spectrum shows a single doublet for the acetylene carbons at
207.9 ppm (2J(C,P) = 19.9 Hz), despite their asymmetry in the
solid-state. Holding dicarbyne complex 9-13C at -78 °C shows slow
13
conversion to C–C coupled 10-13C by C{1H} NMR spectrosco-
py, providing a route to a metal-bound two-electron reduced CO
coupling product (Scheme 3). Importantly, characterization of 10
by crystallography provides indirect structural evidence for pro-
posed dicarbyne 9, as dicarbyne complexes are demonstrated in-
termediates in the formation of C2O2 fragments from CO.28,33,34
Studies at higher temperatures (-78 °C to room temperature)
indicate that acetylene adduct 10 does not undergo C–O bond
cleavage chemistry. To mimic reaction conditions in which C–O
C–O bond cleavage was observed, however, when a THF solu-
tion of dicarbyne 9-13C was warmed from -78 to 0 °C (Scheme 3).
A broad resonance in the 31P{1H} NMR spectrum at 41.9 ppm
supported the formation of a more oxidized Mo center. Signals in
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the 13C{1H} NMR spectrum at 344.9 and 241.9 ppm are consistent terphenyl framework makes this mechanism unlikely.75,76 It is pro-
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5
6
7
8
with silyl carbyne and carbonyl ligands, respectively. Mo(IV) trial-
kyl- and triaryl silyl carbyne complexes have been prepared previ-
ously and show diagnostic low-field 13C NMR resonances in the
range of 320-360 ppm.70-72 Low temperature crystallization (-35
°C) provided single-crystals suitable for solid-state analysis, con-
firming this intermediate 11 as a constitutional isomer of the pre-
viously reported silyl carbyne 8 (Figure 2).46 Contrasting the pseu-
do-square pyramidal geometry of carbyne 8, isomer 11 adopts a
pseudo-octahedral geometry with a clear η2 metal-arene interac-
tion; the Mo–Carene contacts average 2.52 Å in 11 compared to 2.82
Å in 8. The Mo-C32 distances are similar between the alkylidyne
isomers (1.796(1) Å for 11, 1.767(2) Å for 8), with both being
consistent with Mo–C triple bonds.70,71,73,74
posed that dissociation of a ligand (chloride or phosphine) facili-
tates rearrangement to the thermodynamic product 8, where the
alkylidyne is now located trans to the arene ring, a sterically pre-
ferred coordination geometry (Table S2).
2.3. Mechanistic Investigation of C–O Bond Cleavage.
From dicarbyne 9, there are several envisioned pathways by which
C–O bond cleavage could provide carbide 7 and hexamethyl-
disiloxane (Scheme 4, Paths 1-5). This reaction proceeds to com-
pletion in the course of ca. 2 h at 0 °C, to provide a mixture of 11
and 8, without the spectroscopic observation of additional inter-
mediates. If C–O bond cleavage proceeds from 9, upon its for-
mation in an irreversible process, three pathways are envisioned
(Scheme 4, I). Intramolecular silyl ether formation—nucleophilic
attack of a siloxide oxygen on the silyl group of the other si-
loxycarbyne ligand—would provide carbide 7 directly (Scheme 4,
Path 1). Alternatively, external silyl electrophile attack on the siloxy
carbyne oxygen could induce C–O bond cleavage to form a pre-
sumably highly unstable carbide/carbene complex, B (Scheme 4,
Path 2). Silyl chloride elimination from this species would yield
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Warming samples of silyl alkylidyne 11 resulted in isomerization
31
within one hour to 8 at 25 °C as judged by P{1H} NMR spectros-
copy (Figure S5). These data suggest that 11 is the kinetic product
of carbide silylation. The cis geometry of the silyl carbyne moiety
versus the central arene supports the proposed pseudo-square py-
ramidal geometry for carbide 7, which forms immediately following
C–O bond cleavage (Scheme 3). From 7, electrophilic attack by
silyl electrophile on the carbide followed by chloride coordination
on the back-side of the molecule (as drawn) provides 11. Although
isomerization form 11 to 8 could occur in the six-coordinate com-
plex by twists of trigonal faces of the octahedron, the rigidity of the
c a r b i d e
7
;
s u b s e q u e n t
Scheme 4: Mechanistic possibilities (Paths 1-5, in blue) for the elementary reaction step of C–O bond cleavage from dicarbyne 9.
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silyl migration and chloride association would provide silyl alkyli- C, which is expected to be a strong silylating agent. The formation
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7
8
dyne 11. C–O bond cleavage could also proceed from dicarbyne 9
via siloxide anion (Me3SiO-) dissociation, generating a cationic
siloxycarbyne/carbide intermediate, C (Scheme 4, Path 3). This
species could react with the released Me3SiO- to generate
(Me3Si)2O, or lose Me3Si+ (or Me3SiCl upon attack by Cl–—under
the reaction conditions, chloride anions are necessarily present in
solution) which would undergo metal-free reaction with Me3SiO-
to generate the observed (Me3Si)2O.
of HMDSO-d0, albeit in a small amount, is more convoluted as it
requires a silyl group exchange. A control reaction demonstrates
that Me3SiO- undergoes substitution chemistry with disiloxane,
even in the presence of excess silyl electrophile (Figure S15). Such
a side reaction accounts for the formation of the observed
HMDSO-d0 from the deutero dicarbyne. Assuming dicarbyne for-
mation is irreversible, this pathway is the most consistent with both
the reaction kinetics and labeling studies and cannot be ruled out
based on the experimental data.
Both kinetics and isotopic labeling were employed to aid in ex-
perimentally evaluating these mechanistic possibilities. Kinetic
analysis of the consumption of dicarbyne 9-13C and formation of
silyl carbynes 11-13C and 8-13C at 0 °C (as at this temperature,
carbide 7-13C is silylated rapidly (vide infra)) shows that the rate of
C–O bond cleavage is independent of electrophile concentration
(Figures 3 and S8). A zeroth order reaction in Me3SiCl is consistent
with two of these mechanistic scenarios (Eq. S1-S3); Path 2 is ex-
pected to show first order rate dependence on silyl electrophile
concentration. Isotopic labeling studies invalidate both Paths 1 and
2. Condensing two equiv. of Me3SiCl-d9 onto a frozen THF solu-
tion of dianion 3-13C and allowing the mixture to thaw to -78 °C
generated deuterated 9-13C-d18. This dicarbyne complex was
warmed to room temperature in the presence of an excess of
(CH3)3SiCl, facilitating C–O bond cleavage, and the end products
were analyzed by both NMR spectroscopy and GC/MS. The major
hexamethyldisiloxane (HMDSO) isotopolog observed was
HMDSO-d18 (81%, Scheme 5). Incorporation of at least one
(CH3)3Si group (as was observed in 19% of the product disilox-
anes) excludes an intramolecular cleavage event (Scheme 4, I, Path
1). Electrophilic attack by external silyl electrophile, as in Path 2,
can likewise be ruled out; if operative, this pathway would provide
exclusively HMDSO-d9. These results are further supported by the
complementary experiment of warming 9-13C in an excess of
(CD3)3SiCl (Scheme 5).
9
However, formation of dicarbyne 9 need not be irreversible
(Scheme 4, II). Either a fast pre-equilibrium or rate limiting trime-
thylsilyl dissociation may precede C–O bond cleavage, via carbyne
anion A. Anionic carbyne A can be envisioned to act as a precursor
to C–O bond cleavage; electrophilic attack by external silyl chloride
(Scheme 4, II, Path 4) or siloxide dissociation (Scheme 4, II, Path
5) would both result in direct generation of carbide 7.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
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58
59
60
Scheme 5: End-product distributions in isotopic labeling studies of
the C–O bond cleavage step. The para-terphenyldiphosphine ligand is
omitted for brevity. Endproduct isotopolog distributions were
determined by GC/MS.
Although a fast pre-equilibrium followed by reaction of A with
external Me3SiCl (Scheme 4, II, Path 5) would be overall zeroth
order in electrophile (Eq. S4a), such a process can be ruled out by
the isotopic labeling studies. Any pathway involving a fast pre-
equilibrium would scramble the silyl electrophile label statistically,
which is not observed either spectroscopically during the course of
the reaction (2H NMR, Figure S13) or in the end product
isotopolog distributions (Scheme 5). Rate-limiting formation of A
is likewise consistent with the observed reaction kinetics (Eqs. S6
and S7a). Further information was sought through direct studies of
species A. Species A can be generated at low temperatures as part
of a mixture, when using limiting amounts of electrophile. Careful
addition of a single equiv. of Me3SiCl to dianion 3-13C at -78 °C
provides a mixture of 3-13C, dicarbyne 9-13C, and a third species.
13
This new species displays C{1H} NMR resonances at 263.6 and
232.8 ppm, chemical shifts consistent with siloxycarbyne and car-
bonyl ligands, respectively. The 31P{1H} NMR spectrum shows
signals at 97.7 and –2.8 ppm, supporting the tentative assignment
of this new species as anionic carbyne A (Figure S16). Addition of
a second equiv. of silyl electrophile to this mixture yields dicarbyne
9-13C almost quantitatively, consistent with A being a partially
silylated species. Warming a mixture of 3-13C, A, and 9-13C to -50
Figure 3: Kinetic data supporting C–O bond cleavage is zeroth
order in silyl electrophile.
A siloxide dissociation pathway (Scheme 4, I, Path 3) is both
zeroth order in silyl electrophile (Eq. S3) and accounts for the gen-
eration of all three HMDSO isotopologs. From 9-13C-d18, the
liberated siloxide anion could react with the proximal silyl group of
cation C (giving HMDSO-d18) or it could react off-metal with the
excess Me3SiCl (giving HMDSO-d9). The preference for HMDSO-
d18 is potentially a consequence of the high local concentration of
13
°C demonstrates complete consumption of complex A by C{1H}
31
and P{1H} NMR spectroscopies, the characteristic resonances of
3-13C and 9-13C persist at these temperatures. Anionic car-
byne/CO adduct A forms a mixture of C–O cleaved products
(Figure S17), as indicated by the characteristic low-field 13C{1H}
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NMR signals for silyl carbynes 8 and 11, as well as mixed dicarbyne
15 (vide infra), observed upon exhaustively silylating this reaction
mixture at -78 °C. These data are consistent with C–O bond cleav-
age occurring more readily from anionic carbyne A than neutral
dicarbyne 9, though C–O scission occurring from both complexes
at 0 °C cannot be ruled out conclusively.
1
2
3
4
5
6
7
8
9
Though the end-product isotopolog distribution in a rate limit-
ing silyl dissociation mechanism is convoluted by a dependence on
the relative rates of C–O bond cleavage from A and resilylation of
2
A to provide isotopologs of 9, in situ H NMR spectroscopy is
10
11
12
13
14
15
16
17
18
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21
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60
more consistent with C–O scission by siloxide dissociation (Path
5) than external electrophile attack (Path 4). From 9, no H incor-
2
poration is observed in the metal complex during the course of the
reaction with (CD3)3SiCl (Figure S13), consistent with C–O bond
cleavage being faster than resilylation at 0 °C. If formation of car-
bide 7 proceeded from A via external silyl electrophile attack (Path
4), HMDSO-d9 would be formed starting from either 9 or 9-d18,
inconsistent with the experimental data (Scheme 5). Moreover,
this reactivity pathway fails to account for the HMDSO-d0 and
HMDSO-d18 observed from 9-d18 and 9, respectively. A siloxide
dissociation pathway (Scheme 4, Path 5) is most consistent with
the observed end-products, with off-metal substitution chemistry
facilitating the formation of all three HMDSO isotopologs.
We therefore favor Path 5, a reaction mechanism in which silyl
dissociation from dicarbyne 9 is rate limiting, providing anionic
carbyne A, the precursor to C–O cleavage. At 0 °C, siloxide dissoci-
ates from this electron-rich species more rapidly than resilylation
can occur—consistent with both spectroscopic studies and the
observed reaction kinetics—providing carbide 7 and siloxide ani-
on. As indicated above, direct siloxide dissociation from 9 (Path 3)
is also a viable mechanism for carbide formation. The observation
of C–O bond cleavage at lower temperature when starting from A
is notable, and offers a method to facilitate selective deoxygenation
chemistry, avoiding non-deoxygenative C–C coupling, by limiting
electrophile concentration. These studies provide detailed mecha-
nistic insight for an unprecedented observation of the elementary
reaction step(s) of C–O bond cleavage to form a terminal transi-
tion metal carbide.
Scheme 6: In situ generation and silylation of terminal carbide 7.
complex 12-13C (Scheme 6), is observed. Consistent with for-
mation of this silyl carbyne are the upfield shifted resonance at 34.4
31
ppm in the P{1H} NMR spectrum, and resonances at 360.8 and
250.8 ppm in the 13C{1H} NMR spectrum. The stability of the
carbide up to -10 °C in the presence of this bulky silyl electrophile is
consistent with its spectroscopic detection in reactions of dianion
3-13C with iPr3SiCl (Scheme 2). However, formation of organic 6a
proceeds below this temperature when treating either anions 3 or 4
i
with Pr3SiCl, ruling out the intermediacy of isopropyl analogs of 8
and 11 at low temperatures. Based on this reactivity, coupling may
occur from carbide 7-13C directly, which prompted investigation of
reduction prior to silylation.
2.5. Investigation of C–C Bond Formation. Reduction of
carbide 7 in the presence of silyl electrophiles was targeted. One
i
equiv. of Pr3SiCl was added to in situ generated carbide 7-13C at -
78 °C, resulting in no change to the 13C{1H} or 31P{1H} NMR spec-
tra (Figure S19). Two equiv. of [Na][C10H8], as a solution in THF,
were then added at low temperature. The 13C{1H} and 31P{1H}
NMR spectra showed new broad resonances at 327.8 and 58.2
ppm, respectively. Expecting two resonances for the isotopically
enriched nuclei in the 13C NMR spectrum (given the chemically
inequivalent C–Si and C–O motifs), the sample was cooled to -100
°C, resulting in further broadening, but maintenance of a single
2.4. Reactions from Terminal Carbide 7. 7-13C is a spec-
i
troscopically observed intermediate in the addition of Pr3SiCl to
dianion 3-13C en route to 5 and 6a (Scheme 2). It is a proposed
intermediate in the formation of trimethylsilyl alkylidyne 11 from
dicarbyne 9 (Scheme 3). The addition of both electrophiles to
independently prepared 7-13C was investigated at low tempera-
ture.
13
signal. Warming the sample to -60 °C resolved two broad C reso-
nances at 327.9 and 327.0 ppm, again in the chemical shift range of
Mo alkylidynes. The product of carbide reduction in the presence
of iPr3SiCl was tentatively assigned as silyl carbyne/oxycarbyne 13-
13C (Scheme 7).
Treating a frozen THF solution of carbide 7-13C with one equiv.
of Me3SiCl resulted, upon warming to -78 °C, in complete disap-
pearance of the carbidic resonance at 546.3 ppm and the growth in
of the upfield shifted silylcarbyne resonances of 11-13C (ca. 75%
by 31P{1H} NMR integration) and 8-13C (ca. 25% by 31P{1H}
NMR integration) at 344.9 and 355.9 ppm, respectively (Scheme
6). These data support the hypothesis that 7-13C is a precursor to
silyl alkylidynes 11 and 8 (vide supra). In contrast, addition of
iPr3SiCl to a frozen THF solution of carbide 7-13C did not show
any conversion of starting material upon thawing to -78 °C. The
Independent synthesis of the trimethylsilyl variant (14) was
targeted via reduction of silyl carbyne 8-13C (Scheme 7). Treat-
ment with two equiv. of [Na][C10H8] at -78 °C demonstrated for-
mation of a new species with broad resonances at 330.8 and 58.5
ppm in the 13C{1H} and 31P{1H} NMR spectra, respectively. The
chemical shift differences between these resonances and those
assigned to 13-13C are small, supporting assignment as the mixed
carbyne 14-13C (Scheme 7). Cooling the sample to -100 °C re-
13
solved two broad triplets in the C{1H} NMR—that with the larg-
2
er J(C,P) (19.3 Hz) being shifted further downfield. Warming the
13
spectroscopic features of 7-13C persist in the C{1H} NMR spec-
sample to -60 °C likewise showed two resonances, now with the
peak displaying the larger 2J(C,P) (21.4 Hz) moving further upfield
(Figure S20). The lack of C–C coupling in the 13C{1H} NMR spec-
trum supports the dicarbyne motif, and rules out silylated oxyacety-
trum up to -10 °C, at which temperature slow conversion to the
triisopropylsilyl
alkylidyne
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13
lide or ketenylidene structures. Additional analysis of the tempera-
ture dependent fluxionality was inhibited by the narrow tempera-
ture range at which the complex is stable.
and the growth in of a pair of doublets in the C{1H} NMR spec-
1
1
2
3
4
5
6
trum—131.3 and 5.9 ppm, J(C,C) = 139.6 Hz—consistent with
formation of sodium silyl ethynolate, 6b (Scheme 6).77 As reported
previously,78,79 addition of a small silyl electrophile to the trime-
thylsilyl ethynolate yielded disilyl ketene, 6c (13C{1H} δ = 167.5
and 1.1 ppm, 1J(C,C) = 82.3 Hz), the thermodynamically preferred
isomer of the disilylated C2O1 product.
7
8
9
To gain further support for the proposed structures of anions
13-13C and 14-13C, silylation of the oxycarbyne fragment of 14-
13C was attempted at low temperature, targeting a mixed si-
lyl/siloxy dicarbyne, 15-13C (Scheme 7). Treating a solution of
14-13C with Me3SiCl at -78 °C resulted in two new resonances in
10
11
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14
15
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59
60
13
the C{1H} NMR spectrum at 378.9 and 283.7 ppm, assigned to
the silyl- and siloxycarbyne resonances of 15-13C, respectively. The
31P{1H} NMR spectrum of 15-13C displays a broad apparent tri-
plet at 56.8 ppm at -80 °C, lacking resolution necessary to assign
two 2J(P,C) coupling constants. Warming the sample to -20 °C
resulted in resolution of this 31P resonance to a doublet of dou-
blets—2J(P,C) = 18.3 and 13.4 Hz—consistent with the proposed
mixed dicarbyne structure of 15-13C. The 13C{1H} NMR signals
likewise were resolved as triplets, coupling the trans-spanning 31P
nuclei. Upon warming further, growth of dinitrogen complex 5 was
31
observed as a singlet at 76.4 ppm in P NMR spectrum. Concur-
13
rently, resonances in the C{1H} NMR showed formation of met-
al-free bis(trimethylsilyl)ketene 6c.
Crossover experiments were performed to determine if the C–C
bond forming step is unimolecular, both starting from silyl carbyne
8 and from a one-pot reduction and silylation of dicarbonyl dica-
t i o n 1 ( S c h e m e 8 ) . A 1 : 1 m i x t u r e o f 8 a n d 8 -
Scheme 7: Synthesis of mixed dicarbyne complexes and subse-
quent C–C bond formation.
Warming either 13-13C or 14-13C above -60 °C under N2 re-
sulted in conversion of the Mo species to dinitrogen complex 5,
Scheme 8: Crossover-type experiments from 8/8-13C and 1/1-13C.
13C was exposed to the conditions leading to C–C bond formation
of two cofacial carbyne ligands. Similarly, in the one-pot reduction
and silylation of a mixture of 1 and 1-13C, only 6a-13C is observed.
13
(Scheme 8). The organic fragment was interrogated by C NMR
spectroscopy for the presence of 6b with a single 13C enriched posi-
tion. Only 6b-13C (displaying two 13C enriched positions) is ob-
served, consistent with intramolecular C–C bond formation. For-
mation of silyl ethynolates directly from mixed dicarbynes 13 and
14 indicates that the C–C bond is formed via reductive elimination
Though coupling of carbyne and CO ligands to form substituted
oxyacetylenes is known,50,80 these reactions involve electrophile
addition at oxygen after C–C bond formation with ketenyl com-
plexes as established reaction intermediates.49,80,81 In contrast, com-
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ure S48). These results, in conjunction with natural population
plexes 13-15 demonstrate that in the present system, C–C bond
formation occurs following functionalization at oxygen.
1
2
analysis, support a strong formal Mo≡C triple bond and agree well
with computations for neutral Ru and Fe carbide complexes.96,97
The electronic structure of transition metal dicarbyne complexes
has been studied extensively in the context of CO coupling.98-100
The optimized structure of dicarbyne 9 shows short Mo–C dis-
tances of 1.850 Å, consistent with the proposed dicarbyne assign-
ment. The Mo–C distance to the nearest arene carbons averages
2.593 Å, in agreement with the weak M-arene interaction inferred
3
4
5
6
7
8
9
2.6. Quantum Mechanics Studies. With myriad complexes
bearing Mo–C multiple bonds in a variety of coordination envi-
ronments, calculations were performed to investigate the electronic
structures of these molecules. Revised TPSS exchange and correla-
tion functionals82,83 were employed at the LANL2DZ84-87 level of
theory. The diisopropylphosphine and trimethylsilyl moieties were
modeled as dimethylphosphine and silyl groups, respectively, un-
less noted otherwise. The optimized geometries of the computed
structures agree well with experimentally determined solid-state
parameters established by single-crystal XRD (for 2, 8, 10, and 11,
Table S4). To further test the validity of the calculated structures,
CO stretching frequencies and 13C NMR shifts (GIAO)88,89 were
computed. The trends observed experimentally are matched well
computationally (Table S3).
1
in solution from H and 13C{1H} NMR spectroscopies. The four
10
11
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highest energy occupied molecular orbitals of 9 are composed of
Mo d and C–O π* orbitals (Figure 4, b). Four orbitals have π con-
tributions to the Mo–C bonds, in two orthogonal pairs. The
HOMO-1 and HOMO-2, with metal orbital contributions of dxz
and dyz parentage (Figure S51), show considerable in-phase orbital
overlap between the carbyne carbons, akin to calculations per-
formed for adjacent cylindrical π-interacting ligands on the unique
face of a capped trigonal prism.98
The frontier molecular orbitals of mixed dicarbyne 15 (Figure 4,
c) are quite similar to 9 (Figure 4, b), again demonstrating signifi-
cant in-phase orbital overlap between the carbyne C atoms. Con-
traction of the C–Mo–C angle, as would be expected in the reac-
tion coordinate for formation of a C–C bond from either 9 or 15,
would stabilize these orbitals (for instance, they become the C–C
π-bonds of the alkyne fragment in 10, Figure 4, d), an electronic
justification for the observed C–C bond formation from these in-
termediates.99,101 All of these orbitals are comparable to those calcu-
lated for model systems for carbyne-carbyne coupling—a hypo-
thetical tungsten dicarbyne99,100 and an isolated iron diphosphine
dicarbyne.33
As a rare example of a terminal transition metal carbide,44,45,90-95
the electronic structure of 7 is of particular interest. The highest
occupied molecular orbital (HOMO) is non-bonding with respect
to the carbide ligand and has stabilizing contributions from the π-
acidic antibonding orbitals of both the central arene and the CO
(Figure 4, a). The η2 metal-arene interaction is supported experi-
mentally; the arene H and 13C resonances of the metal-bound
1
arene fragment show a distinct upfield shift (6.5 and 86.2 ppm,
respectively), evidencing donation of electron density from Mo
into an arene π* orbital. The Mo-carbide π-bonds show contribu-
tions from the metal dxz and dyz orbitals and C px and py orbitals
(Figure 4, a, HOMO-2 and HOMO-3). The Mo-carbide σ-bond is
slightly higher in energy (HOMO-1) with primarily C pz character
and contribution from a metal-based orbital of dz2 parentage (Fig-
Figure 4: Calculated valence molecular orbitals of models of 7 (a), 9 (b), 15 (c), and 10 (d). Orbital energies (relative to the HOMO) in eV are
given in parentheses. Isosurfaces are displayed at the 0.04 e/Å3 level.
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Conclusions
Crystallography, isotopic labeling, kinetics, spectroscopy, and
computation have provided insight into the mechanism by which
Mo-bound CO molecules can be cleaved and coupled to a C2O1
product at a single metal center supported by a terphenyl diphos-
phine ligand. When dianion 3 is treated with Me3SiCl, a
bis(siloxycarbyne) complex is formed, the first example of Mo sup-
porting such a motif. This species can either undergo C–C cou-
pling—forming a bis(siloxy)acetylene adduct of Mo, a two electron
CO coupling product—or it can undergo C–O cleavage. Kinetics
and isotopic labeling studies are consistent with two mechanisms,
one involving rate determining siloxide dissociation, the second
silyl electrophile dissociation followed by fast siloxide loss. An ani-
onic Mo-siloxycarbyne/CO species indeed undergoes C–O cleav-
age at low temperatures and demonstrates that scission of this bond
is more facile from a mono- rather than bis-silylated species.
The carbide resulting from C–O bond cleavage is trapped rapid-
ly in the presence of excess electrophile at the temperatures re-
quired to break the C–O bond, giving a pseudo-octahedral silyl-
alkylidyne complex. Further warming results in rearrangement to
the thermodynamic product, a sterically preferred five-coordinate
isomer lacking a metal-arene interaction. Reduction of this species
at low temperature provides evidence for a mixed dicarbyne, which
undergoes C–C bond formation at temperatures as low as -60 °C,
releasing the silylethynolate fragment and binding dinitrogen to
give the final product 5.
Combined, the findings above provide valuable insight into the
use of coordinatively flexible, redox noninnocent ligand scaffolds
for multi-electron small molecule transformations. The adaptive
binding of the arene plays a central role in the stabilization of
unique molecular motifs throughout the reaction scheme—ranging
from an extremely reducing trianion to high-valent molybdenum
complexes bearing Mo–C multiple bonds: dicarbyne, carbide, and
alkylidyne complexes. Phosphine arm hemilability is likewise criti-
cal for accessing the highly reduced species. Work to tune the selec-
tivity and expand the scope of this reductive functionalization
chemistry to other electrophiles and small molecule substrates is
ongoing.
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ASSOCIATED CONTENT
Supporting Information.
Detailed experimental procedures, full characterization, crystallograph-
ic details (CIF), and spectroscopic data. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: agapie@caltech.edu
ACKNOWLEDGMENT
We thank Larry Henling and Mike Takase for invaluable crystallo-
graphic assistance and David VanderVelde for NMR expertise. We are
grateful to Jay Labinger for insightful suggestions regarding the synthe-
sis of mixed molybdenum dicarbyne complexes. Sibo Lin is thanked for
advice and discussions regarding the computations. T.A. is grateful for
Sloan and Dreyfus fellowships and J.A.B. for a NSF graduate research
fellowship. This research was funded by the NSF (CHE-1151918) and
Caltech.
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