Cooperative H2 Activation across a Metal-Metal Multiple Bond and Hydrogenation Reactions Catalyzed by a Zr/Co Heterobimetallic Complex

Article abstract of DOI:10.1021/acscatal.8b04390

In the quest for active and selective catalysts featuring nonprecious metals, bimetallic cooperativity poses a unique opportunity to promote catalytic reactions and influence selectivity. While examples of stoichiometric H₂ activation across metal-metal bonds have been reported, there have been limited advances toward the incorporation of a well-defined cooperative bimetallic H₂ activation process into a catalytic cycle for the hydrogenation of unsaturated hydrocarbons. Herein, we demonstrate that facile activation of H₂ by two nonprecious metals is facilitated by metal-metal cooperativity in the coordinatively unsaturated Zr/Co bis(phosphinoamide) complexes (THF)(I)Zr(XylNPⁱPr₂)₂CoPR₃ (3-PMe₃ and 3-PMePh₂, Xyl =3,5-dimethylphenyl), which feature highly polar Zr-Co triple bonds. Owing to the stabilizing nature of the metal-metal bond, the H₂ activation products (THF)(I)Zr(μ-H)(XylNPⁱPr₂)₂Co(H)(PR₃) (4-PMe₃ and 4-PMePh₂), which feature one terminally bound Co hydride and one hydride bridging the two metals, have been isolated and crystallographically characterized. The Zr/Co bimetallic complex 3-PMePh₂ is an active catalyst for the hydrogenation of alkenes and semihydrogenation of alkynes, and relevant intermediates including 4-PMePh₂ and alkene (5) and alkyne (6) adducts have been identified spectroscopically in situ and isolated and characterized through independent synthesis. The alkyne semihydrogenation reaction catalyzed by 3-PMePh₂ exhibits high selectivity for alkene over alkane products and generates an unselective distribution of (E)- and (Z)-alkenes via direct formation of both stereoisomers. These findings lend insight into the roles that metal-metal bonding and cooperativity play in the activation of small molecules and the promotion and selectivity of subsequent catalytic transformations.

Full text of DOI:10.1021/acscatal.8b04390

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Article
2
Cooperative H Activation Across a Metal-Metal Multiple Bond and
Hydrogenation Reactions Catalyzed by a Zr/Co Heterobimetallic Complex
Kathryn M Gramigna, Diane A. Dickie, Bruce M Foxman, and Christine M Thomas
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04390 • Publication Date (Web): 01 Mar 2019
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ACS Catalysis
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Cooperative H Activation Across a Metal-Metal
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†
†,§
†
,†,‡
Kathryn M. Gramigna, Diane A. Dickie, Bruce M. Foxman, Christine M. Thomas*
†Department of Chemistry, Brandeis University, 415 South Street, Waltham, Massachusetts 02453, United States
‡Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio
4
3210, United States
KEYWORDS: heterobimetallic, dihydrogen activation, alkyne semi-hydrogenation, alkene hydrogenation, metal-metal
bonds, cooperative reactivity
ABSTRACT: In the quest for active and selective catalysts featuring non-precious metals, bimetallic cooperativity poses
a unique opportunity to promote catalytic reactions and influence selectivity. While examples of stoichiometric H
activation across metal-metal bonds have been reported, there have been limited advances towards the incorporation of
a well-defined cooperative bimetallic H activation process into a catalytic cycle for the hydrogenation of unsaturated
hydrocarbons. Herein, we demonstrate that facile activation of H
2
2
2
by two non-precious metals is facilitated by metal-
i
metal cooperativity in coordinatively unsaturated Zr/Co bis(phosphinoamide) complexes, (THF)(I)Zr(XylNP Pr
2
)
2
CoPR
3
(
3-PMe
bond, the H
terminally bound Co-hydride and one hydride bridging the two metals, have been isolated and crystallographically
characterized. The Zr/Co bimetallic complex 3-PMePh is found to be an active catalyst for the hydrogenation of alkenes
and semi-hydrogenation of alkynes, and relevant intermediates including 4-PMePh and alkene (5) and alkyne (6)
adducts have been identified spectroscopically in situ and isolated and characterized through independent synthesis. The
alkyne semi-hydrogenation reaction catalyzed by 3-PMePh exhibits high selectivity for alkenes over alkane products,
3
and 3-PMePh
2
), that feature highly polar Zr-Co triple bonds. Owing to the stabilizing nature of the metal-metal
i
2
activation products (THF)(I)Zr(-H)(XylNP Pr
2
)
2
Co(H)(PR
3
) (4-PMe
3
and 4-PMePh ), which feature one
2
2
2
2
and generates an unselective distribution of E and Z alkenes via direct formation of both stereoisomers. These findings
lend insight into the roles that metal-metal bonding and cooperativity play in the activation of small molecules and the
promotion and selectivity of subsequent catalytic transformations.
INTRODUCTION
Bimetallic complexes have attracted interest in
polar substrates, the concept has been applied herein to
1
c, 1g
the activation of the smallest nonpolar molecule, H
Dihydrogen activation is classically depicted to
transpire via homolytic cleavage of H by a precious metal
catalyst, which is oxidized by two electrons to generate a
metal-dihydride complex (Scheme 1, top). H activation
by a bimetallic complex, however, can proceed through
2
.
the literature owing to their ability to exhibit reactivity or
selectivity divergent from that of analogous
2
1
monometallic compounds. Cooperativity between two
base metal centers can facilitate multielectron redox
events inaccessible to related monometallic compounds,
providing advantages in applications such as small
molecule activation and catalysis. In particular, tethering
a Lewis acidic early metal to an electron-rich late metal
can (1) enable more facile access to stabilized reduced
species poised for redox processes at the late metal site
and (2) induce a highly polarized metal-metal interaction,
which is advantageous for synergistic substrate activation
2
1
f, 3
one of three different pathways (Scheme 1, bottom): (1)
oxidative addition at one metal center to generate a
classical metal dihydride complex in which in the metal
center involved is oxidized by two electrons; (2)
cooperative homolytic H
centers are oxidized by one electron; or (3) cooperative
heterolytic cleavage of H across the metal-metal bond, in
2
cleavage, in which both metal
2
1
a, 1d, 1f, 1h, 2
across the metal-metal bond.
Although the
which one metal center is oxidized by two electrons. The
third pathway is likely to occur across a polarized metal-
metal bond in an early-late heterobimetallic complex,
early-late heterobimetallic strategy seems best suited to
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to activate H
Page 2 of 14
across the metal-metal bond (pathway (3))
inducing the formation of one hydridic M-H bond with
the early metal and one acidic M-H bond with the late
metal. The effective polarization of a traditionally
nonpolar substrate offers plausibility for atypical
reactivity of the resulting dihydride bimetallic complex.
The Lu group reported a transition metal-main group
2
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
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3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
to generate a complex with a bridging hydride and
terminal hydride bound to Zr (Scheme 2). Exposure of B
1
e
6a
to H in the presence of alkenes generates the expected
2
hydrogenation products; however, it was determined
that the active catalyst is likely to be a small impurity
rather than the characterized heterobimetallic complex.
metal bimetallic complex capable of binding H
2
to afford
)Ni(N(o-
2
the
NCH
nonclassical
H
2
adduct
( -H
2
Scheme 2. H
reported by Bergman.
2
activation by early-late heterobimetallics
i
(
2
P( Pr)
2
)C
6
H
4
)
3
)Ga, which is catalytically active
5a
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
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2
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5
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2
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9
0
1
2
3
4
5
6
7
8
9
0
towards alkene hydrogenation. This reaction is proposed
to proceed through a HNi(-H)Ga intermediate, but no
spectroscopic or experimental evidence for this
3
hypothesis could be obtained. The employment of a
more coordinatively unsaturated bimetallic complex
could serve to stabilize such an intermediate and
influence reactivity.
Scheme 1. Top: Classical
H
2
activation by
a
monometallic complex. Bottom: Three different
pathways for activating H
complex.
2
by a heterobimetallic
A previous example of H
heterobimetallic complex was reported by our group in
2
activation by a Zr/Co
6
d
i
2010. Exposure of (THF)Zr(MesNP Pr
2,4,6-trimethylphenyl) to 1 atm H for 12 hours results in
the addition of two equivalents of H to generate the
dihydride complex D (Scheme 3), in which one equivalent
of H cleaves and hydrogenates the P-N bond of one
phosphinoamide ligand (possibly the cause of the long
reaction time). The second equivalent of H adds across
2
)
3
CoN
2
(C, Mes =
2
2
2
2
the metal-metal bond to generate one hydride bridging
the two metals and a terminal hydride bound to the Co
center. Further reactivity with this dihydride complex
was not pursued owing to the requisite ligand
degradation. In addition to H
2
activation, the C=O bonds
of CO and diaryl ketones were shown to oxidatively add
across the Zr-Co bond of C, in both cases requiring
dissociation of one of the phosphinoamide ligands from
2
Several examples of isolable heterobimetallic H
2
activation products in which hydrogen oxidatively adds
5
to one metal center and in which H
2
is cooperatively
8
cobalt.
6
activated have been reported in the literature. Of these,
Scheme 3. P-N bond cleavage previously observed upon
only the Cp
2
Ta(-CH
2
)
2
Ir(CO)(PPh
3
) complex (A, Scheme
treatment of a tris(phosphinoamide) Zr/Co complex
2
) reported by Bergman and coworkers shows activity
6d
with H .
2
5
a
towards hydrogenation. A more recent report by
Bergman and Ess outlines a mechanism by which A
catalyzes the hydrogenation of alkenes, and it is shown
that the bimetallic complex goes through a different
mechanism than does the related monometallic complex
Ph
2
P(-CH
2
)
2
Ir(CO)(PPh
3
), emphasizing the importance
7
of the second metal site for the high catalytic activity. In
The P-N bond cleavage and requisite ligand
dissociation processes observed with the
tris(phosphinoamide) Zr/Co system prompted us to
construct more reactive compound bis(phosphinoamide)
this case, H activation occurs solely at the Ir center to
2
generate an Ir dihydride complex (pathway (1), vide
supra), and the early metal facilitates the reductive
elimination and oxidative addition of substrates through
the methylene bridge, which comparatively cannot be
complexes. Indeed,
complex
a
Ti/Co bis(phosphinoamide)
CoPMe (Xyl 3,5-
i
ClTi(XylNP Pr
2
)
2
3
=
accomplished by the phosphorus ylide analogue. A
dimethylphenyl) was found to be much more reactive
t
related Zr/Ir complex Cp
2
Zr(-N Bu)IrCp* (B) was shown
2
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oxidation state, halide identity, and coordination
towards ketones than its tris(phosphinoamide) analogue,
promoting stoichiometric reductive coupling of ketones
to alkenes (McMurry coupling).
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
geometry.
8
c, 9
Given than Zr/Co
complexes have, in our hands, generally proven to be
significantly more reactive than their Ti/Co analogues,
we now turn our attention to bis(phosphinoamide) Zr/Co
complexes and highlight their enhanced reactivity with
dihydrogen, including applications to catalytic
hydrogenation of unsaturated C-C bonds.
Scheme 4. Synthesis of Zr/Co heterobimetallic
complexes 1, 2 and 3.
8
c
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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0
RESULTS AND DISCUSSION
Synthesis of Zr-Co bis(phosphinoamide)
complexes. To generate
a
complex analogous to
, a stepwise synthetic procedure
The monometallic precursor
was prepared using a reported
was then added under reducing
to afford the
i
9
ClTi(XylNP Pr
2
)
2
CoPMe
3
was
Me
procedure. CoI
developed.
i
(
2
N)
2
Zr(XylNP Pr
)
2 2
1
0
2
conditions using one equivalent of KC
8
IV
I
bimetallic
Me
Zr /Co
complex
(Me
2
N)Zr(-
i
1
2
N)(XylNP Pr
2
)
2
CoI (1, Scheme 4). The H NMR
spectrum of 1 features five paramagnetically shifted
resonances consistent with a C -symmetric structure.
s
1
Owing to their proximity to the unpaired electrons, the H
NMR signals for both sets of dimethylamide and
isopropyl methine resonances are too broad to resolve.
Treatment of 1 with two equivalents of TMSI generates
i
the iodide-bridged dimer [IZr(-I)(XylNP Pr
2
)
2
CoI] (2).
2
The symmetry of 2 is akin to that of 1 such that a similar
pattern of five paramagnetically shifted peaks are
1
apparent in the H NMR spectrum. The effective magnetic
moments of 1 and 2 were found using Evans’ method to
be 3.52 and 4.87 
B
, consistent with S = 1 (SO = 2.83 
B
)
Reduction of 2 with four equivalents of Na/Hg in
and S = 2 (SO = 4.90 
B
) ground states, respectively.
the presence of either PMe
monomeric complexes (THF)(I)Zr(XylNP Pr
3
or PMePh
2
affords the
CoPR (3-
i
2
)
2
3
Comparisons between metal-metal distances are
3
1
1
PMe
of 3-PMe
3
and 3-PMePh
2
, Scheme 4). The P{ H} NMR spectra
display two peaks in a 2:1 ratio,
simplified by the use of Cotton’s formal shortness ratio
11
3
and 3-PMePh
2
(
FSR), defined by the ratio of the metal-metal distance to
1
2
namely a doublet corresponding to the equivalent
phosphines of the phosphinoamide ligand centered at
the sum of the single-bond atomic radii (Pauling’s R ) of
1
each metal. X-ray crystallography of single crystals of 1
and 2 (Figures 1A and S31) confirms their connectivity
and reveals that the metal-metal distance in 2 (2.6579(4)
Å; FSR = 1.02) is slightly elongated from that in the
precursor 1 (2.5832(4) Å; FSR = 0.99) owing to either the
modest increase in electron density at Zr resulting from
the coordination of an additional  donating ligand or the
difference in geometric constraints imposed by the
different bridging ligands. Both distances suggest a single
dative CoZr bond and are comparable to the metal-
4
8.9 (3-PMe
corresponding to the terminally-bound phosphine donor
centered at -24.8 (3-PMe ) and 2.8 ppm (3-PMePh ). By
3
) and 46.2 (3-PMePh ) ppm and a triplet
2
3
2
analogy to Zr/Co complex C, whose oxidation states were
assigned based on Co K-edge X-ray absorption
1
4
spectroscopy, the oxidation states of the metal centers in
IV
-
complexes 3-PMe
.
3
and 3-PMePh
2
are assigned as Zr /Co
I
This assignment was further corroborated by structural
changes that occur upon reduction of 2.
i
metal interaction in the related complex (XylNP Pr
2
)Zr(-
Single-crystal X-ray diffraction of 3-PMePh
2
i
Br)(XylNP Pr
2
)
2
CoBr, in which the Zr-Co distance is
.7602(3) Å (FSR = 1.01). All three metal-metal distances
(Figure 1B) reveals a decrease in the average Co-P
distance (2.20 Å) of about 0.1 Å from that in the precursor
2 (2.30 Å), while the Zr-N distance remains virtually
unchanged (2.13 Å). The Co-P contraction is consistent
with the two-electron reduction occurring at the Co
N
13
2
are longer than that in one-electron reduced [(-
P
i
9
Cl)Ti(XylNP Pr
2
)
2
CoI]
2
(2.2051(4) Å; FSR = 0.89). Further
N
comparison between the structures of the Ti/Co complex
and 2 is not straightforward owing to the differences in
1
4-15
center
owing to increased metal to ligand -
3
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backbonding. Notably, a marked decrease in the Zr-Co therefore, the bonding is best classified as a compilation
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
distance to 2.2733(6) Å (FSR = 0.84) indicates a significant
increase in metal-metal bond order. Corrected for
differences in radii, the short intermetallic distance in 3-
of three dative bonds. The results of a natural bond orbital
(NBO) calculation support this description by revealing
three Zr-Co NBOs that correspond to one  and two 
bonds (Figure 3), which are all comprised of significantly
more Co character (average of 88% Co) than Zr character
(average of 12% Zr). The computed Wiberg bond index
(WBI, 1.23) and Mayer bond order (MBO, 1.73) are also
indicative of metal-metal multiple bonding. While both
PMePh
2
is comparable to the Ti-Co triple bond in the
i
analogous Ti/Co complex ClTi(XylNP Pr
2
)
2
CoPMe , in
3
9
which the Ti-Co distance is 2.0236(9) Å (FSR = 0.81). The
modest increase in FSR in 3-PMePh can be attributed to
poorer 4d-3d orbital overlap. Furthermore, the slightly
shortened Zr-Co distance in 3-PMePh as compared with
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
computationally
determined
values
tend
to
that
in
the
tris(phosphinoamide)
CoN
analogue,
(2.3816 Å; FSR = 0.91), is
underestimate the extent of metal-metal bonding in early-
late heterobimetallic systems owing to the highly dative
nature of the interaction, comparisons between related
compounds remain informative. These values fall in
between those previously reported for doubly-bonded
i
13
(THF)Zr(XylNP Pr
2
)
3
2
likely a consequence of the poorer π-accepting and
stronger σ-donating properties of PMePh compared to
, amplifying donation from Co to Zr.
2
N
2
i
15b
16
(
THF)Zr(MesNP Pr
2
)
3
CoN
2
(FSR = 0.90; WBI = 0.95;
14
i
MBO = 1.49 ) and for ClTi(XylNP Pr
2
)
2
CoPMe (FSR =
3
9
0
.81; WBI = 1.59; MBO = 1.76), which features a more
covalent Ti-Co triple bond.
Figure 2. Depictions of the calculated frontier molecular
orbitals of 3-PMePh .
2
Figure 1. Displacement ellipsoid (50%) representations of 1
A) and 3-PMePh (B). Hydrogen atoms have been omitted
(
2
for clarity. In the case of 1, the phosphorus-bound isopropyl
groups are disordered over two positions, and only one
position is shown for clarity.
The Zr-Co bonding in 3-PMePh was probed
2
Figure 3. Calculated NBO surfaces depicting the Zr-Co
bonding interactions in 3-PMePh
computationally using density functional theory (DFT) to
examine the frontier molecular orbitals (MOs, Figure 2).
Appreciable overlap between the dz2 orbitals of both
metal centers gives rise to a Zr-Co  bond, and two 
bonds are evident as a result of overlap between the two
2
.
H activation. Both 3-PMe and 3-PMePh were
2
3
2
found to readily activate H across the Zr-Co bond at
2
room temperature in less than five minutes to generate
IV
I
d
xz and the two dyz orbitals. It is apparent that the electron
density is localized predominantly on the Co center;
the Zr /Co
dihydride complexes (THF)(I)Zr(-
i
H)(XylNP Pr ) Co(H)(PR ) (4-PMe3 and 4-PMePh2,
2
2
3
4
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1
Scheme 5). Two hydride signals are evident in the H
been omitted for clarity. The Zr-bound THF ligand and one
of the Ph groups of the PMePh ligand were disordered over
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
NMR spectra of both complexes (Figures S9 and S12,
respectively). For 4-PMe , one doublet of triplets centered
2
two positions, but only one position is shown for clarity.
Right: Qualitative representations of “open” and “closed” 3-
center-2-electron bonding depicting varying degrees of M-M
interaction.
3
at -7.3 ppm corresponds to the bridging hydride and
another centered at -18.0 ppm corresponds to the terminal
1
hydride. The H NMR spectrum of 4-PMePh
2
reveals the
same pattern in the hydride region shifted slightly
metal-metal bond. This is further corroborated by the
3
1
1
downfield to -7.1 and -17.4 ppm. The P{ H} NMR spectra
of both complexes show broad signals in a 2:1 ratio for the
phosphinoamide ligands at 69.1 and 66.5 ppm and for the
terminal phosphine ligands at -1.9 and 31.1 ppm of 4-
calculated WBI (0.89) and MBO (1.13) for 4-PMe
suggest a formal Zr-Co single bond but a stronger
interaction than that in singly-bonded
ClZr(MesNP Pr CoI (WBI = 0.52; MBO = 0.73 ).
3
, which
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
i
16
15a
)
2 3
PMe
3
and 4-PMePh
2
, respectively.
The frontier molecular orbital diagram (Figure
Scheme 5. H
2
activation by 3-PMe
3
and 3-PMePh .
2
5
), computed using DFT, offers insight into the bonding
interactions in 4-PMe . Upon examination of the highest
3
occupied MOs, one Zr-Co  bond and one 3-center-2-
electron Zr-H-Co bond are evident owing to overlap
between d orbitals of appropriate symmetry. The two
metal dxz orbitals interact with the hydride s orbital to
form three molecular orbitals of M-H bonding,
nonbonding, and antibonding character (Figure 5). The
LUMO, which lies over 2 eV higher in energy than the
HOMO, is M-H nonbonding in character. The HOMO is
mostly nonbonding Co dxy in character, and the electron
density in the highest occupied orbitals is predominantly
Co-based. In addition to the interaction through the
bridging hydride, the Zr and Co centers are also involved
in a 2-center-2-electron  bond. Both bonding motifs are
additionally corroborated by NBO calculations (Figure 6).
The NBO depicting the Zr-Co  bond is 89.6% Co and
The solid state structures of both 4-PMe
3
and 4-
PMePh
2
were determined via X-ray diffraction of single
crystals (Figure 4, left, and Figure S43), and the Zr-Co
distances (2.4543(3) and 2.4695(3) Å, respectively; FSR =
0
.94) reveal an increase from that in 3-PMePh
The hydrides were located on the Fourier difference map
and refined. In 4-PMePh , the bridging hydride is 2.03(3)
2
by ~0.2 Å.
2
Å and 1.45(3) Å from the Zr and Co centers, respectively,
with a Zr-H-Co angle of 89.0°. The terminal hydride is
located 1.32(4) Å from Co, in a position trans to the
bridging hydride (H-Co-H = 175(2)°). The acute Zr-H-Co
angle suggests a “closed” M-H-M interaction (Figure 4,
right) in which there is overlap between the d orbitals of
the two metal centers and thus a distinct metal-metal
1
0.4% Zr in character, supporting a dative interaction in
which electron density is donated from the electron-rich
Co center to the electron-deficient Zr center. The 3-center-
2
-electron Zr-H-Co bond
17
interaction. Although the metal centers are in the same
IV
I
Zr /Co oxidation states as in both 1 and 2, the Zr-Co
distance is shorter in 4-PMe and 4-PMePh owing to the
3
2
bridging hydride and the more electron-releasing nature
of the phosphine ligand vs. iodide or amide ligands,
allowing for increased CoZr donation. Thus, in this
case, the shortened intermetallic distance is indicative of
a stronger
Figure 4. Left: Displacement ellipsoid (50%) representation
of 4-PMePh . Hydrogen atoms (except for hydrides) have
2
5
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Page 6 of 14
Catalytic hydrogenation of unsaturated C-C
bonds. In light of hydride lability demonstrated by H/D
exchange studies, the activity of towards the
hydrogenation of alkenes was investigated. Reported
base metal catalysts that promote alkene hydrogenation
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3
from H
2
employ different types of cooperativity to
circumvent their propensity towards one-electron redox
1
c, 18
processes.
Related examples include the installation of
highly donating, strong-field ligands to Co to stabilize
1
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
two-electron redox cycles, Co complexes that utilize
metal-ligand cooperativity via redox non-innocence of
the ligand or direct participation of the ligand in substrate
2
0
+
21
coordination, a Zr /amine FLP complex, and a Ni/Ga
4
bimetallic complex. Thus, we reasoned that cooperativity
between a redox-active Co center and a Lewis acidic Zr
center could allow for novel reactivity.
As an initial probe of the catalytic alkene
hydrogenation activity of the Zr/Co system, a mixture of
3
-PMe
atm H
the styrene had been converted to ethylbenzene.
However, by simply substituting PMe for the less
donating PMePh , the reaction proceeded to 93%
3
and one equivalent of styrene was subjected to 1
. After 20 hours at room temperature, only 22% of
2
3
2
Figure 5. Depictions of the calculated frontier molecular
orbitals of 4-PMe and relevant orbitals depicting M-H-M 3-
center-2-electron bonding.
conversion in the same amount of time (20 h) at room
temperature. The origin of the increased reaction rate is
evident while monitoring
3
is represented by an NBO comprised of 18.1% Zr, 52.5%
H, and 29.4% Co character.
Figure 6. Calculated NBO surfaces depicting the Zr-H-Co
3
(left) and Zr-Co (right) bonding interactions in 4-PMe .
H/D exchange studies were carried out using
D
both dihydride 4-PMePh
2
and dideuteride 4 -PMePh
2
.
When 4-PMePh is subjected to 1 atm D
2
2
, both hydrides
are substituted for deuterides after four hours at 40°C.
1
This is evident in the H NMR spectra (Figure S36), as the
D
hydride signals disappear upon formation of 4 -PMePh
2
.
D
Similarly, exchange of the deuterides in 4 -PMePh
2
for
hydrides under complementary conditions was observed
2
by H NMR spectroscopy (Figure S37), which depicts the
loss of the two deuteride signals as 4-PMePh
2
is
Figure 7. Progression of the hydrogenation of one equivalent
generated. The relative integration of the µ-H/D and Co-
H/D signals with respect to each other remains identical
during the course of these reactions and no HD is
31
1
of styrene monitored by P{ H} NMR spectroscopy. (A)
Immediately after addition of 1 equiv styrene to 3-PMePh
under 1 atm H . (B) After 2 h. (C) After 4 h. (D) After 8 h.
Signal labeled as * denotes the small amount of free
2
2
1
2
observed in either the H or H NMR spectra, suggesting
that H/D exchange occurs via concerted loss/addition of
i
2
HXylNP Pr that is carried through synthetically.
H
2
/D .
2
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ACS Catalysis
the reaction by 31P NMR spectroscopy (Figure 7). After
addition of H to the mixture of 3-PMePh and styrene, 4-
PMePh is immediately generated (Figure 7A). As the
reaction progresses, two peaks centered at 49.9 and 39.4
ppm become evident, along with a signal at -26.6 ppm
Given that 3-PMePh demonstrates very slow
2
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2
2
activity towards the hydrogenation of internal alkenes,
we were interested in probing its propensity towards the
semi-hydrogenation of internal alkynes. Traditional
hydrogenation catalysts for the semi-hydrogenation of
2
2
2
corresponding to free PMePh
new downfield peaks correspond to asymmetric complex
, in which PMePh has dissociated and styrene is bound
in its place. 5 can also be independently synthesized by
adding one equivalent of styrene to 3-PMePh and
2
(Figure 7B-D). The two
alkynes, such as the Schrock-Osborn catalyst or
2
3
Lindlar’s catalyst, undergo syn-addition of H to the
2
5
2
alkyne to generate the cis-alkene, and rely on a two-
electron oxidation at a single precious metal center,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
typically Ru, Rh, or Pd. Owing to the nature of H
2
stirring for two hours at room temperature or for 30
minutes at 40°C. As the hydrogenation of styrene to
ethylbenzene proceeds, continued monitoring of the
reaction by NMR spectroscopy reveals accumulation of 4-
addition, elimination of the trans-alkene is less common.
Thus, the most pervasive route for achieving this
selectivity does not use H as the terminal reductant and
2
instead involves dissolving stoichiometric amounts of Li
or Na in ammonia. Several recently reported semi-
PMePh
2
in the reaction mixture along with requisite
is evident in the
P{ H} NMR spectrum upon full conversion of styrene.
Addition of further equivalents of styrene to this reaction
mixture regenerates 5 and exposure to 1 atm H resumes
hydrogenation. Catalytic turnover can be achieved using
-PMePh : at 10 mol % catalyst loading, 93% of styrene is
converted to ethylbenzene in 3 hours at 60°C under 1 atm
consumption of 5, until only 4-PMePh
2
hydrogenation catalysts that employ H
2
as the reductant
31
1
achieve E-selectivity via initial generation of the Z isomer
2
4
followed by catalytic Z- to E-isomerization. In 2015, the
2
Mankad group reported the heterobimetallic complex
(NHC)Au-RuCp(CO)
trimethylphenyl)imidazole-2-ylidene), which catalyzes
alkyne semihydrogenation under 1 atm of H in 24 hours
2
(NHC
=
N,N’-bis(2,4,6-
3
2
2
H
2
. In contrast, neither free PMe
3
nor 5 are observed in the
at 150°C and is selective for the E alkene after
isomerization from the Z to the E isomer under the
3
1
1
P{ H} NMR spectrum at any point during styrene
2
4b
hydrogenation using 3-PMe
excess styrene or exposure to elevated temperatures.
Therefore, under the same conditions, 3-PMe catalyzes
the hydrogenation of styrene in only 71% conversion in 3
hours and requires twice as long as 3-PMePh (6 h) to
3
, even after addition of
reaction conditions. Although no H
is observed upon exposure of (NHC)Au-RuCp(CO)
, the addition of alkynes at elevated temperatures
generates the semi-hydrogenation products. It was
proposed that H is activated across the metal-metal bond
of (NHC)Au-RuCp(CO) to generate short-lived
2
activation product
2
to
3
H
2
2
2
reach 93% conversion. The lability of the terminal Co-
bound ligand is an essential consideration in the
development of more active hydrogenation catalysts.
2
monometallic metal hydride complexes, followed by
alkyne 1,2-insertion and protonolysis to eliminate the
alkene, and it was suggested that H
turnover-limiting step in the catalytic cycle.
2
activation is the
Having established 3-PMePh as a competent
2
2
4b, 25
Among
catalyst for the hydrogenation of styrene, the reactivity
towards the hydrogenation of several additional olefinic
substrates was probed (Table S1). More hindered terminal
alkenes such as 3,3-dimethylbutene were hydrogenated
more slowly (Table S1, entry 2), likely owing to the
diminished ability to form the alkene adduct analogous
to 5 as a result of increased steric hindrance between the
tert-butyl group of the substrate and the isopropyl groups
of the phosphinoamide ligand. This is corroborated by the
the rare examples of alkyne semi-hydrogenation
processes that have been shown to directly form E
alkenes, Fürstner’s [Cp*Rh(COD)Cl] catalyst, which has
been shown to directly perform trans-hydrogenation via
a carbene intermediate, is the most practical and widely
2
6
applicable. Muetterties and coworkers reported the
selective formation of trans olefins by a homobimetallic
Rh complex, and Bargon and coworkers reported an
NMR spectroscopy study confirming direct trans-
hydrogenation of alkynes by a homobimetallic Ru
3
1
1
absence of a new metal species (analogous to 5) by P{ H}
1
and H NMR spectroscopy when 3-PMePh
2
is treated
27
complex. In both of the latter cases, it is proposed that
with 3,3-dimethylbutene. Similarly, internal alkenes such
as cyclohexene are hydrogenated much more slowly than
terminal alkenes (Table S1, entry 3). It was also found that
this unique selectivity is a result of the hydrogenation of
the same alkyne by two different metal centers upon
insertion into a bridging metal hydride. We reasoned that
3
-PMePh catalyzes the isomerization of terminal to
2
3
could promote similar bifunctional catalysis but might
internal olefins (Table S1, entries 3, 4 and 5). When chain
walking occurs more rapidly than hydrogenation, longer
reaction times are required to hydrogenate the resulting
internal olefin (Table S1, entries 5a and 5b).
allow for divergent reactivity owing to the facile H
activation that occurs at an intact metal-metal bond.
2
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Page 8 of 14
orbital is poised to interact with both a terminal and a
bridging hydride.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 9. (A) Displacement ellipsoid (50%) representation of
6
. Hydrogen atoms have been omitted for clarity. The THF
and iodide ligands were disordered over two positions, but
only one position is shown for clarity. (B) Calculated
representation of the highest occupied molecular orbital in 6.
Upon exposure of
diphenylacetylene and 3-PMePh
the peaks corresponding to 4-PMePh
the P{ H} NMR spectrum as the hydrogenation reaction
a
3:1 mixture of
to an atmosphere of H
begin to appear in
2
2
,
2
3
1
1
Figure 8. Progression of the hydrogenation of three
equivalents of diphenylacetylene monitored by P{ H} NMR
spectroscopy. (A) After addition of 3 equivalents of
proceeds (Figure 8B-D). The signals assigned to 6 and free
31
1
PMePh
2
simultaneously decrease until all of the
diphenylacetylene is consumed, at which point only 4-
2
diphenylacetylene to 3-PMePh . (B) Following the addition
3
1
1
PMePh remains in the P{ H} NMR spectrum. 92%
2
of 1 atm H
2
. (C) After 6 h. (D) After 16 h. Signal labeled as *
i
conversion is achieved in 18 hours at room temperature
to predominantly produce the two possible stilbene
products (Z/E = 72:28) over 1,2-diphenylethane in a ratio
of 92:8 (Table S2, entry 1b).
denotes the small amount of free HXylNP Pr
2
that is carried
through synthetically.
As in the hydrogenation of styrene, 3-PMePh
2
is
significantly more active than 3-PMe towards the
3
The semi-hydrogenation reaction proceeds
catalytically at room temperature at 10 mol % catalyst
loading, but requires 30 hours to achieve 85% conversion
catalytic hydrogenation of diphenylacetylene. Upon
addition of three equivalents of diphenylacetylene to 3-
PMePh , a new complex is generated after two hours at
2
(Table 1, entry 1). Both Z- and E-stilbene products are
31
1
room temperature. The P{ H} NMR spectrum of the
reaction mixture (Figure 8A) shows two distinct peaks:
observed in a ratio of 79:21 under these conditions, along
with a small amount of the over-hydrogenated alkane
(stilbene/1,2-diphenylacetylene = 93:7). Reaction times
can be reduced by elevating the reaction temperature.
When the hydrogenation reaction is run for three hours at
one at -26.6 ppm, confirming PMePh
another at 55.4 ppm, which corresponds to the
phosphinoamide ligands of a new C -symmetric complex.
2
dissociation, and
s
The structure of this compound was determined
crystallographically to be the diphenylacetylene adduct 6
6
0°C and 10 mol % catalyst loading under 1 atm H
of the diphenylacetylene is converted to stilbene (Z/E =
7:43) and 1,2-diphenylethane in a ratio of 98:2 (entry 2).
2
, 98%
(
Figure 9A). The Zr-Co distance in 6 is elongated from
that in 3-PMePh to 2.3716(4) Å (FSR = 0.91), in line with
the stronger  accepting nature of the diphenylacetylene
ligand as compared with PMePh . -backbonding from
the Co dyz orbital to a diphenylacetylene * orbital
significantly disrupts the second Zr-Co bond,
5
2
For comparison, after three hours at room temperature
only 31% conversion is achieved but with marginally
increased selectivity for the Z-alkene (Table S2, entry 2a).
Further elevation of the reaction temperature to 75°C
leads to nearly complete conversion in just one hour
without generation of additional overhydrogenation
products (Table 1, entry 3). On the other hand, reducing
the catalyst loading to 5 mol % and running the reaction
at 60°C still allows for full conversion with similar
selectivity, but predictably the reaction takes twice as
long (Table 1, entry 4 vs entry 2). Catalytic turnover can
2

supporting the presence of a formal Zr-Co double bond
in 6. Similar attenuation of the metal-metal bond order
has been observed in other Zr/Co heterobimetallics when
15b

-acids such as N bind to Co. Activation of the alkyne
2
C-C bond is substantiated by both the elongated C1-C2
distance (1.292(3) Å) and the bent Ph-C1-C2 angle
(135.8(2)°). DFT calculations reveal that the HOMO is
be achieved using 3-PMe ; however, at identical catalyst
3
predominantly Co dz2 in character (Figure 9B), and the
loadings the reaction is much slower and requires double
8
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ACS Catalysis
hypothesized that a Z- to E-isomerization process was
the reaction time to reach full conversion (Table 1, entry 5
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
vs entry 2), pursuant to the trend in reactivity towards the
hydrogenation of one equivalent of styrene (vide supra).
Catalytic semi-hydrogenation facilitated by isolated 6
proceeds more rapidly (Table 1, entry 6), reaching 93%
conversion in 2 h, confirming that 6 is an on-cycle
intermediate. However, an increase in selectivity for the
Z-isomer was observed in this case (Z/E = 69:31).
occurring on the same timescale as the semi-
hydrogenation reaction, and that 1,2-insertion of the cis-
alkene into the M-H bond is competitive with alkyne
insertion. The limited amount of 1,2-diphenylethane
produced would then suggest that -hydride elimination
to generate the trans-alkene is initially more facile than
reductive elimination to generate the alkane.
To
investigate this hypothesis, the reaction mixture from
entry 2 (10 mol % catalyst loading, 60°C) that had already
reached 98% conversion was allowed to continue heating
at 60°C for a total reaction time of 72 hours (Table S2,
entry 3c). The ratio of Z/E-stilbene decreases significantly
to 22:78 after 72 h, but the majority of this results from the
depletion of the Z-isomer as it is overhydrogenated to the
alkane (stilbene/1,2-diphenylethane = 72:28).
Both E- and Z-stilbene are formed under all
conditions investigated, even at early time points in the
reaction with low conversion (Table S2), and minimal
changes in the ratios of stilbene/1,2-diphenylethane and
Z/E-stilbene are observed over the course of the semi-
hydrogenation reaction (Table S2, entries 1a vs 1b, 2a vs
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
b, 3a vs 3b, 5a vs 5b, 6a vs 6b). Based on other semi-
2
4
hydrogenation catalysts in the literature, we initially
Table 1. Comparison of activity and selectivity for the catalytic semi-hydrogenation of diphenylacetylene promoted
a
by 3-PMePh
2
, 3-PMe , and several relevant previously reported catalysts.
3
cat. loading
conv
(%)
stilbene/1,2-
diphenylethaneb
Z/Eb
entry
catalyst
T (°C)
t (h)
solvent
b
(mol %)
1
2c
3
3-PMePh
3-PMePh
3-PMePh
3-PMePh
2
2
2
2
10
10
10
5
23
60
75
60
60
60
30
150
150
rt
30
3
C
6
D
6
85
98
93:7
98:2
97:3
98:2
98:2
97:3
82:18
99:1
97:3
44:56
b
79:21
57:43
51:49
56:44
61:39
69:31
0:100
4:96
6 6
C D
1
C
C
C
C
6
6
6
6
D
6
D
6
D
6
D
6
98
4
6
99
5
3-PMe
3
10
10
1
6
94
6
6
2
93
7d
8e
9e
(MesCCC)Co(N
)(PPh
)
17
24
24
3
THF
100
96
2
3
(IMes)Ag-RuCp(CO)
2
20
20
2
xylenes
xylenes
(IMes)Cu-FeCp(CO)
2
64
81:19
100:0
i
)]+
1
0f
[Cp*
2
Zr(OCH
2
CH
2
N Pr
2
C
6
D
5
Br
>99
a
1
Catalytic conditions for entries 1-5: 0.1 M diphenylacetylene in ca. 700 L C
6
D
6
2
, 1 atm H . Determined by H NMR
c
d
24c Mes
e
24b
integration. Average of three runs. Ref.
bis(2,4,6-trimethylphenyl)imidazole-2-ylidene). Ref. ; 1.5 bar H
;
CCC = bis(mesityl-benzimidazol-2-ylidenephenyl). Ref.
; IMes = N,N’-
f
21
2
.
To probe Z/E isomerization directly, Z-stilbene and 1 atm
with the observation that the Z/E ratio increases very little
throughout the course of the semi-hydrogenation
reactions under all conditions (Table S2), indicating that
Z to E isomerization is not a dominant reaction pathway
during the semi-hydrogenation of diphenylacetylene.
H
2
were added to 10 mol % 3-PMePh
2
. After one hour at
1
7
5°C, the Z/E ratio was determined to be 81:19 by H NMR
spectroscopy, confirming the modest ability of 3-PMePh
2
to catalyze Z/E isomerization. However, under the same
conditions diphenylacetylene is hydrogenated nearly
quantitatively to stilbene with a Z/E ratio of 51:49 after
one hour (Table 1, entry 3). The generation of significantly
more E-stilbene in the alkyne semi-hydrogenation
reaction suggests that a second hydrogenation pathway,
in which diphenylacetylene is directly hydrogenated to
the trans-alkene, may be operative. This is also consistent
To further explore the selectivity of the semi-
hydrogenation reaction and its origins, the substrate and
product distribution was monitored every 30 min
throughout the 3 h reaction run at 60°C and 10 mol %
loading of 3-PMePh
2
(Figure 10A). After 30 minutes
(Table S2, entry 3a), the reaction proceeds to 35%
conversion and an appreciable quantity of E-stilbene is
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already generated (Z/E = 52:48). Throughout the reaction,
hydrogenation to Z-stilbene proceeds more rapidly;
however, the isomer is present in substantial
concentrations at early reaction times and continues to
accumulate throughout the progression of the reaction.
possibility of a secondary mechanism in which much of
1
2
3
4
5
6
7
8
9
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1
1
1
1
1
1
1
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1
2
2
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2
2
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5
6
the trans-alkene is generated directly by alkyne
hydrogenation, rather than being formed exclusively via
a Z- to E- isomerization process.
E
The possibility of
a bimolecular reaction
pathway for direct trans-hydrogenation was initially
probed by varying the concentration of the catalytic
reactions. Running the reactions under the optimized
conditions (10 mol% 3-PMePh
2
, 60 °C, 1 atm H , 3 h) but
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
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0
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2
3
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8
9
0
1
2
3
4
5
6
7
8
9
0
in a Schlenk flask equipped with a stir bar rather than a J.
Young tube led to greater selectivity for the Z-isomer (Z/E
=
80:20). However, changing the concentrations of the
reactions carried out under these conditions resulted in
identical conversions and selectivities (Table S4),
suggesting that either a bimolecular pathway mediated
by two Co sites is not responsible for the generation of the
E isomers. To gain further insight into the mechanism, the
diphenylacetylene semi-hydrogenation reaction was
carried out in a J. Young tube under the optimized
reaction conditions using a 50:50 mixture of H
2
2
:D .
Generation of the mixed H/D alkene for both stilbene
isomers was confirmed by GC-MS (see Figures S38-S39),
along with di-protonated and di-deuterated stilbene. H/D
scrambling into only the E isomer would have suggested
that it was forming via Z/E isomerization; however the
observation of comparable incorporation of a single
deuterium atom into both alkene isomers does not
confirm or refute the proposed direct formation of trans-
stilbene and more detailed mechanistic studies are
required to evaluate the mechanism further.
Figure 10. (A) Progression of the hydrogenation of
diphenylacetylene at 60°C. (B) Progression of the
hydrogenation reaction at 75°C after full diphenylacetylene
conversion.
The scope of the alkyne semi-hydrogenation
catalysis facilitated by 3-PMePh
2
was probed using
In an attempt to fully convert the
diphenylacetylene to a single product, the reaction was
run at 75°C (for convenience) and 10 mol % loading of 3-
several different internal alkynes (Table S5). Asymmetric
alkynes were hydrogenated to predominantly afford the
corresponding alkene over the alkane (Table S5, entries 2
and 3). Selectivity for the alkene over the alkane in these
cases is slightly diminished compared with that observed
during the hydrogenation of diphenylacetylene;
however, a concomitant increase in Z/E selectivity is
apparent. The effect of alkyne coordination on increased
hydrogenation activity is emphasized by the prolonged
reaction times necessary for hydrogenation of alkynes
with more sterically encumbered substituents (Table S5,
entries 3b and 4b). Hydrogenation of 1,2-
bis(trimethylsilyl)acetylene is very slow and results in
predominant generation of overhydrogenated 1,2-
bis(trimethylsilyl)ethane (Table S5, entry 4). Dialkyl-
substituted 4-octyne is selectively hydrogenated to cis-4-
octene: minimal overhydrogenation is observed and no
formation of trans-4-octene is detected during any point
in the reaction (Table S5, entry 5). However, after 3 h
significant catalyst decomposition was discerned by both
PMePh
conversion for a total time of 84 h. As shown in Figure
0B, there is minimal change in the amount of E-stilbene
2
, and was monitored past the point of full
1
over time following consumption of the alkyne. However,
as the mole fraction of Z-stilbene decreases over time, the
amount of 1,2-diphenylethane increases in a similar
proportion. This observation suggests that Z-stilbene
hydrogenation occurs at a much faster rate than either
isomerization to E-stilbene or E-stilbene hydrogenation,
accounting for the lower Z/E ratio observed at later
reaction times following the consumption of
diphenylacetylene (e.g. Table S2, entries 4b and 4c).
Under the optimized catalytic conditions (60 °C, 1 atm H
2
,
1
0 mol% 3-PMePh ), hydrogenation of Z-stilbene was
2
confirmed to proceed moderately faster than
hydrogenation of E-stilbene (Table S3). The observations
(1) that both E- and Z-stilbene are generated at early
reaction times and (2) that the mole fraction of E-stilbene
changes very little at longer reaction times support the
1
31
1
H and P{ H} NMR spectroscopies; thus the observed
1
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imparted by the metal-metal multiple bond. While the
activity and selectivity in this case should be considered
with caution as it cannot be definitively attributed to 3-
PMePh or 4-PMePh
While it is not the case that all monometallic Co
1
2
3
4
5
6
7
8
9
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1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
order of the Zr-Co bond fluctuates throughout catalysis
to accommodate changes in coordination number and d-
electron count, the foundational bonding interaction
remains intact and therefore imparts advantageous
stability.
2
2
.
or monometallic Zr complexes are generally ineffective
alkyne semi-hydrogenation catalysts (Table 1, entries 7
and 10), the lack of reactivity of monometallic complexes
CONCLUSIONS
i
analogous to bimetallic 3 and 4,
I
2
Zr(XylNP Pr
2
) and
2
A highly reactive reduced Zr/Co complex was
synthesized by designing a coordinatively unsaturated
bis(phosphinoamide)-ligated complex with a formal
triple bond between the metal centers. The metal-metal
multiple bonding stabilizes the complex while
maintaining open coordination sites to facilitate substrate
binding. The polarized nature of the interaction lends
i
ICo(PPh
2
HN Pr)
3
,
towards either
H
2
activation or
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
diphenylacetylene hydrogenation highlights the requisite
nature of the bimetallic system (Table S6, entries 2 and 3).
Furthermore, the absence of reactivity by 2 (Table S6,
entry 4) emphasizes the significance of the electron-rich
Co center and open coordination sites in 3-PMePh . The
2
presence of the Lewis acidic Zr site serves to stabilize a
highly reduced Co center, allows for two-electron redox
processes at Co, and supports the binding of substrates
itself to extremely facile heterolytic cleavage of H at room
2
temperature. The reactivity of 3 and 4 are highly
dependent on the nature of the Co-bound terminal ligand,
as it needs to be donating enough to stabilize 3, but labile
enough to dissociate during catalysis.
2
across the two metal centers. The catalytic activity of 3-
PMePh is relatively similar to the best reported examples
2
of monometallic Co and Zr semi-hydrogenation catalysts
3
-PMePh
2
exhibits good catalytic activity
Mes
i
+ 21,
(
CCC)Co(N
2
)(PPh
3
) and [Cp*
2
Zr(OCH
2
CH
2
N Pr
2
)] ,
towards the hydrogenation of styrene and semi-
hydrogenation of diphenylacetylene, and the significant
improvement in bimetallic-mediated activity can be
largely attributed to the dual nature of the metal-metal
bond in helping to lower the barrier to H activation and
to accommodate structural and electronic changes to the
catalyst. The tractable, well-defined hydrogenation
2
4c
but the observed differences in selectivity
(alkene/alkane and E/Z) suggest a different mechanistic
pathway that may provide further opportunities for
optimization once fully understood.
2
The catalytic activity of 3-PMePh
2
towards
alkyne semihydrogenation is comparable to that of other
2
1, 24
recently reported base metal catalysts
(Table 1, entries
1
31
catalysis can be easily monitored by H and P NMR
spectroscopies, and relevant reactive species have been
7
, 9, and 10), and the mild reaction conditions compare
favorably. However, the poor selectivity for one stilbene
isomer over the other could likely be improved by tuning
the steric bulk of the phosphinoamide ligand and by
isolated
and
characterized
structurally
and
computationally, lending valuable insight into the metal-
metal cooperativity that is often postulated for
heterobimetallic species. To the best of our knowledge, 4-
employing insights gleaned from
a
mechanistic
investigation. Both of these research directions will be the
subject of future work. While the aforementioned
PMe
3
and 4-PMePh
2
are the first isolated and structurally
activation products of heterobimetallic
characterized H
2
(NHC)Ag-RuCp(CO)
2
catalyst demonstrates high E-
complexes that are active for further stoichiometric or
catalytic reactivity. Furthermore, 3 and 4 employ two base
metals to work in tandem to catalyze the two-electron
transformations necessary for
hydrogenation that are typically regulated by precious
metal catalysts.
selectivity (entry 8), substitution of the composite
precious metals for base metals Cu and Fe results in a
catalyst that exhibits diminished activity and selectivity,
and is more selective for the Z-isomer (entry 9). In this
H
2
activation and
system, the turnover-limiting step was proposed to be H
2
activation, and the authors suggest that in order to
improve catalytic activity (particularly with Earth-
abundant metals) and lower the reaction temperature, it
would be necessary to lower the barrier for
Although the semi-hydrogenation of internal
alkynes is not selective for one alkene isomer over the
other, experimental evidence suggests the possibility of
two simultaneous mechanistic pathways, one of which
results in direct trans-hydrogenation. At present, we
speculate that insertion of the alkyne into the terminal Co-
H or the bridging Zr-H-Co hydride could lead to direct
formation of either Z or E alkene products. This
hypothesis would be consistent with literature examples
in which dirhodium and diruthenium complexes
facilitate the direct formation of E-alkenes in semi-
hydrogenation reactions following alkyne insertion into a
heterobimetallic H
2
activation and to stabilize the
2
5
(NHC)M’-H intermediates. Thus the enhancement in
activity (including more favorable catalyst loading,
reaction time, and temperature) of the base metal-
mediated bimetallic catalysis reported herein is likely a
direct consequence of the remarkably facile H activation
2
by 3 and the stabilized dihydride complex 4. Both features
are facilitated by the simultaneous flexibility and support
1
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L. H. Highly Polar Metal-Metal Bonds in "Early-Late" Heterodimetallic
bridging hydride, further cementing the importance of
bimetallic cooperativity in tuning both catalyst activity
and selectivity. A mechanistic pathway similar to
Fürstner’s that involves a carbene intermediate is also
Complexes. Angew. Chem. Int. Ed. 2000, 39, 2658-2678; (e) Bullock, R.
M.; Casey, C. P. Heterobimetallic Compounds Linked by
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Cooper, B. G.; Napoline, J. W.; Thomas, C. M. Catalytic Applications of
Early/Late Heterobimetallic Complexes. Cat. Rev. - Sci. Eng. 2012, 54, 1-
40; (g) Wheatley, N.; Kalck, P. Structure and Reactivity of Early-Late
Heterobimetallic Complexes. Chem. Rev. 1999, 99, 3379-3420; (h)
Thomas, C. M. Metal-Metal Multiple Bonds In Early/Late Heterobimetallic
Complexes: Applications Toward Small Molecule Activation and
Catalysis. Comments Inorg. Chem. 2011, 32, 14-38.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
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4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
possible.
Nonetheless, mechanistic studies will be
required to probe the role of bimetallic cooperativity in
this transformation and determine exactly how the trans
alkene products are formed. Additional catalytic
applications of these complexes along with further
catalyst optimization and mechanistic studies are
currently underway.
2
.
Coombs, J.; Perry, D.; Kwon, D.-H.; Thomas, C. M.; Ess, D. H.
Why Two Metals Are Better Than One for Cobalt Phosphinoamide
Catalyzed Kumada Coupling. Organometallics 2018, 37, 4195-4203.
0
1
2
3
4
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0
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2
3
4
5
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7
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0
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2
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9
0
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2
3
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5
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0
1
2
3
4
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9
0
3
.
Cammarota, R. C.; Clouston, L. J.; Lu, C. C. Leveraging
2 2
Molecular Metal–Support Interactions for H and N Activation. Coord.
Chem. Rev. 2017, 334, 100-111.
AUTHOR INFORMATION
Corresponding Author
4.
Cammarota, R. C.; Lu, C. C. Tuning Nickel with Lewis Acidic
Group 13 Metalloligands for Catalytic Olefin Hydrogenation. J. Am. Chem.
Soc. 2015, 137, 12486-12489.
*thomasc@chemistry.ohio-state.edu
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(a) Hostetler, M. J.; Butts, M. D.; Bergman, R. G. Scope and
Mechanism of Alkene Hydrogenation/Isomerization Catalyzed by
Complexes of the Type R E(CH ) M(CO)(L) (R = Cp, Me, Ph; E =
Present Addresses
§Department of Chemistry, University of Virginia,
Charlottesville, VA 22904, United States
2
2 2
3
Phosphorus, Tantalum; M = Rhodium, Iridium; L = CO, PPh ). J. Am.
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The manuscript was written through contributions of all
authors. All authors have given approval to the final
version of the manuscript.
6.
(a) Baranger, A. M.; Bergman, R. G. Cooperative Reactivity in
the Interactions of X-H Bonds with a Zirconium-Iridium Bridging Imido
Complex. J. Am. Chem. Soc. 1994, 116, 3822-3835; (b) Riddlestone, I. M.;
Rajabi, N. A.; Lowe, J. P.; Mahon, M. F.; Macgregor, S. A.; Whittlesey, M.
ASSOCIATED CONTENT
Supporting Information.
The Supporting Information is available free of charge on the
ACS Publications website at DOI:
K. Activation of H over the Ru−Zn Bond in the Transition Metal−Lewis
2
+
Acid Heterobimetallic Species [Ru(IPr)
2
(CO)ZnEt] . J. Am. Chem. Soc.
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016, 138, 11081-11084; (c) Campos, J. Dihydrogen and Acetylene
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Co/Zr Multiple Bonds in Early/Late Heterobimetallic Complexes. Chem.
Commun. 2010, 46, 5790-5792.
Experimental procedures, spectroscopic data for
complexes 1-6 and representative catalytic runs,
crystallographic data collection and refinement
details for 1, 2, 3-PMePh , 4-PMe , 4-PMePh , and
2 3 2
6, additional computational details, and XYZ
coordinates of DFT-optimized geometries for 3-
7
.
Zhang, Y.; Roberts, S. P.; Bergman, R. G.; Ess, D. H.
Mechanism and Catalytic Impact of Ir–Ta Heterobimetallic and Ir–P
Transition Metal/Main Group Interactions on Alkene Hydrogenation. ACS
Catalysis 2015, 5, 1840-1849.
PMePh
Crystallographic data for 1, 2, 3-PMePh
PMePh , and 6 (CIF)
2 3
, 4-PMe , and 6 (PDF)
8
.
(a) Krogman, J. P.; Foxman, B. M.; Thomas, C. M. Activation
2
, 4-PMe
3
, 4-
of CO
2
by a Heterobimetallic Zr/Co Complex. J. Am. Chem. Soc. 2011, 133,
1
4582-14585; (b) Marquard, S. L.; Bezpalko, M. W.; Foxman, B. M.;
2
Thomas, C. M. Stoichiometric C-O Bond Oxidative Addition of
Benzophenone by a Discrete Radical Intermediate To Form a Cobalt(I)
Carbene. J. Am. Chem. Soc. 2013, 135, 6018-6021; (c) Zhang, H.; Wu, B.;
Marquard, S. L.; Litle, E. D.; Dickie, D. A.; Bezpalko, M. W.; Foxman, B.
M.; Thomas, C. M. Investigation of Ketone C═O Bond Activation
Processes by Heterobimetallic Zr/Co and Ti/Co Tris(phosphinoamide)
Complexes. Organometallics 2017, 36, 3498-3507.
ACKNOWLEDGMENTS
This material is based upon work support by the U.S.
Department of Energy, Office of Science, Office of Basic
Energy Sciences, Chemical Sciences, Geosciences, and
Biosciences Division, under Award No. DE-SC0014151. The
Ohio State University Department of Chemistry and
Biochemistry and Sustainable and Resilient Economy
program are also gratefully acknowledged for financial
support. The authors thank Dr. Benjamin R. Reiner for
insightful discussions. The authors are also grateful for
access to the Brandeis University high-performance
9
.
Wu, B.; Bezpalko, M. W.; Foxman, B. M.; Thomas, C. M. A
Heterobimetallic Complex Featuring a Ti-Co Multiple Bond and its
Application to the Reductive Coupling of Ketones to Alkenes. Chem. Sci.
2015, 6, 2044-2049.
1
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Saper, N. I.; Bezpalko, M. W.; Foxman, B. M.; Thomas, C. M.
Synthesis of Chiral Heterobimetallic Tris(phosphinoamide) Zr/Co
Complexes. Polyhedron 2016, 114, 88-95.
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