Rare-Earth Supported Nickel Catalysts for Alkyne Semihydrogenation: Chemo- And Regioselectivity Impacted by the Lewis Acidity and Size of the Support

Article abstract of DOI:10.1021/jacs.0c00905

Bimetallic catalysts of nickel(0) with a trivalent rare-earth ion or Ga(III), NiML₃ (where L is [ⁱPr₂PCH₂NPh]^-, and M is Sc, Y, La, Lu, or Ga), were investigated for the selective hydrogenation of diphenylacetylene (DPA) to (E)-stilbene. Each bimetallic complex features a relatively short Ni-M bond length, ranging from 2.3395(8) ? (Ni-Ga) to 2.5732(4) ? (Ni-La). The anodic peak potentials of the NiML₃ complexes vary from -0.48 V to -1.23 V, where the potentials are negatively correlated with the Lewis acidity of the M(III) ion. Three catalysts, Ni-Y, Ni-Lu, and Ni-Ga, showed nearly quantitative conversions in the semihydrogenation of DPA, with NiYL₃ giving the highest selectivity for (E)-stilbene. Initial rate studies were performed on the two tandem catalytic reactions: DPA hydrogenation and (Z)-stilbene isomerization. The catalytic activity in DPA hydrogenation follows the order Ni-Ga > Ni-La > Ni-Y > Ni-Lu > Ni-Sc. The ranking of catalysts by (Z)-stilbene isomerization initial rates is Ni-Ga ? Ni-Sc > Ni-Lu > Ni-Y > Ni-La. In operando ³¹P and ¹H NMR studies revealed that in the presence of DPA, the Ni bimetallic complexes supported by Y, Lu, and La form the Ni(η²-alkyne) intermediate, (η²-PhCCPh)Ni(ⁱPr₂PCH₂NPh)₂M(κ²-ⁱPr₂PCH₂NPh). In contrast, the Ni-Ga resting state is the Ni(η²-H₂) species, and Ni-Sc showed no detectable binding of either substrate. Hence, the mechanism of Ni-catalyzed diphenylacetylene semihydrogenation adheres to two different kinetics: an autotandem pathway (Ni-Ga, Ni-Sc) versus temporally separated tandem reactions (Ni-Y, Ni-Lu, Ni-La). Collectively, the experimental results demonstrate that modulating a base-metal center via a covalently appended Lewis acidic support is viable for promoting selective alkyne semihydrogenation.

Full text of DOI:10.1021/jacs.0c00905

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Article
Rare-Earth Supported Nickel Catalysts for Alkyne Semihydrogenation: Chemo-
and Regioselectivity Impacted by the Lewis Acidity and Size of the Support
Bianca L. Ramirez, and Connie C Lu
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 24 Feb 2020
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Rare-Earth Supported Nickel Catalysts for Alkyne Semihydrogenation: Chemo- and
Regioselectivity Impacted by the Lewis Acidity and Size of the Support
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Bianca L. Ramirez† and Connie C. Lu*,†
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†
Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455-
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0431, United States
Abstract: Bimetallic catalysts of nickel(0) with a trivalent rare-earth ion or Ga(III), NiML3 (where L is
[iPr2PCH2NPh]−, and M is Sc, Y, La, Lu, or Ga), were investigated for the selective hydrogenation of
diphenylacetylene (DPA) to (E)–stilbene. Each bimetallic complex features a relatively short Ni–M bond
lengths, ranging from 2.3395(12) Å (Ni–Ga) to 2.5732(4) Å (Ni–La). The anodic peak potentials of the
NiML3 complexes vary from –0.48 V to –1.23 V, where the potentials are negatively correlated with the
Lewis acidity of the M(III) ion. Three catalysts, Ni–Y, Ni–Lu, and Ni–Ga, showed nearly quantitative
conversions in the semihydrogenation of DPA, with NiYL3 giving the highest selectivity for (E)-stilbene.
Initial rate studies were performed on the two tandem catalytic reactions: DPA hydrogenation and (Z)-
stilbene isomerization. The catalytic activity in DPA hydrogenation follows the order: Ni–Ga > Ni–La >
Ni–Y > Ni–Lu > Ni–Sc. The ranking of catalysts by (Z)-stilbene isomerization initial rates is: Ni–Ga >>
31
1
Ni–Sc > Ni–Lu > Ni–Y > Ni–La. In operando P and H NMR studies revealed that in the presence of
DPA, the Ni bimetallic complexes supported by Y, Lu, and La form the Ni(η2-alkyne) intermediate, (η2-
i
2 i
2
≡
PhC CPh)Ni( Pr2PCH2NPh)2M(κ - Pr2PCH2NPh). In contrast, the Ni–Ga resting state is the Ni(η -H2)
species, and Ni–Sc showed no detectable binding of either substrate. Hence, the mechanism of Ni-catalyzed
diphenylacetylene semihydrogenation adheres to two different kinetics: an auto-tandem pathway (Ni–Ga,
Ni–Sc) versus temporally separated tandem reactions (Ni–Y, Ni–Lu, Ni–La). Collectively, the experimental
results demonstrate that modulating a base-metal center via a covalently appended Lewis acidic support is
viable for promoting selective alkyne semihydrogenation.
Introduction
The recent emergence of coordination compounds containing metal–metal bonds between transition metals
(TMs) and rare earth (RE) elements, that is, the group 3 metals and the lanthanides (Ln), has inspired their
use in diverse applications.1-14 For example, RE–TM bonded complexes are currently being explored for
single-molecule magnetism.15-16 In the realm of catalysis, multimetallic cooperativity between rare earth
ions and transition metals can elicit beneficial activity in both heterogeneous17-18 and cluster19-22 systems by
affecting the catalyst stability, electronics, and/or substrate binding affinity. Even so, direct RE–TM bonds,
especially those involving lanthanides, is rarely seen in homogeneous catalysis. Relevant precedents are
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the rare-earth complexes supported by ferrocene diamide ligands that catalyze ring-opening polymerization,
wherein a weak metal-metal interaction was proposed to influence catalytic activity.23-25 Fueled by the rise
of bifunctional transition-metal catalysts with an appended Lewis acidic main group or Group 4 metal ion,26-
40 the extension to rare earth metals seemed ripe for exploration.
Our interest in using rare earth ions as a Lewis acidic support for tuning base metal reactivity was
realized with the discovery of the catalytically active Ni−Lu complex, NiLu(iPr2PCH2NPh)3 (1). We
demonstrated that the Lu(III) promotes Ni(0) activity in olefin hydrogenation. We also showed that the least
coordinately saturated (4–coordinate) Lu center, which gave the shortest Ni−Lu bond, also gave the fastest
catalytic rates.41 Because this work was the first example of a directly bound lanthanide ion being used as
promoter for molecular base-metal catalysis, we became interested in exploring the catalytic reactivity of
other nickel rare-earth bimetallic complexes, and also turned our attention to the semihydrogenation of
alkynes.
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The semihydrogenation of alkynes to alkenes is an industrially important transformation in bulk and
fine chemical manufacturing.42-43 However, due to the competitive formation of both (E)- and (Z)-isomers,
as well as the overhydrogenated byproduct, the main challenge is to fully control both the chemo- and
stereoselectivity of this reaction.44 Classic alkyne semihydrogenation catalysts, such as Lindlar’s catalyst45
or the Schrock–Osborn catalyst,46 can produce (Z)-alkenes efficiently, though careful control of reaction
conditions is sometimes necessary to avoid overhydrogenation. The stereocomplementary formation of (E)-
alkenes using H2 as a reductant is significantly more challenging. As such, well-defined semihydrogenation
catalysts that operate under an atmosphere of H2 are surprisingly limited in number.46-51
With rare exceptions,52-56 (E)-alkenes are most commonly formed via an initial hydrogenation of the
alkyne to the (Z)-alkene, followed by cis-to-trans-alkene isomerization.57-64 Within a broader catalytic
context, the employment of non-precious metal catalysts has attracted intense interest, not only for their
affordability and sustainability, but also for their potential to provide unprecedented reactivities and
selectivities.65-66 Several (E)-selective semihydrogenation catalysts based on Co and Fe have been reported,
but common drawbacks include limited substrate scope, the need for a high operating temperature and/or
lack of overall selectivity.57-59,63 Recently, the first example of Ni-mediated semihydrogenation using H2 as
a reductant was reported using a mixture of Ni(NO3).6H2O and triphos.60 However, forcing reaction
conditions (120°C, 30 bar) were needed to achieve high yields and selectivity.
In this work, we have synthesized and structurally isolated an isostructural series of Ni(0)–RE(III)
complexes, NiRE(iPr2PCH2NPh)3 (where RE is Sc, Y, and La). In conjunction with the previously reported
Ni−Lu complex (1), and NiGa(iPr2PCH2NPh)3 for comparison with a non-rare earth support, the catalytic
utility of the Ni bimetallic complexes toward the (E)-selective semihydrogenation of internal alkynes using
H2 at moderate pressure and temperature is presented.
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Results
Synthesis and Characterization of ML3 and NiML3. Scheme 1 illustrates the two synthetic routes
used to prepare the NiML3 series, where L is [iPr2PCH2NPh]−. In the first route, the rare-earth
metalloligands were isolated prior to metalation with Ni(0). The metalloligands, ScL3 (2) and YL3 (3), were
obtained as white powders in good yields (85% and 88% yields, respectively) from the reaction of the
corresponding MCl3 (M = Sc or Y) and 3 equiv of the deprotonated ligand, Li[iPr2PCH2NPh]. The La(III)
metalloligand, LaL3 (4), was obtained from the reaction of La[N(TMS)2]3 and 3 equiv of iPr2PCH2NHPh in
70% yield. In the second synthetic approach, the metalation sequence was flipped by installing the Ni(0)
center first. From the reaction of 3 equiv of Li[iPr2PCH2NPh] with Ni(COD)2, where COD = 1,5-
cyclooctadiene, the Ni(0) complex, Ni(Li∙L)3 (5), was isolated as a purple solid (90% yield). For complexes
2, 4, and 5, the corresponding 31P{1H} NMR spectra each display a singlet at −18.8, –12.6, and 20.1 ppm,
respectively (Figure S1). The 31P NMR spectrum of 3 shows a doublet at −14.1 ppm with a coupling
constant (JP-Y) of 36.5 Hz to the I = ½ 89Y nucleus. The 1H{31P} NMR spectrum of 2 has 10 peaks, which
is consistent with C3 symmetry: 3 aryl protons, 2 diastereotopic methylene protons, 2 unique methine
protons, and 3 methyl peaks that integrate to 18H, 9H, and 9H. The 2:1:1 ratio supports the assignment of
P(CHMe2)(CHMe′Me″), where one of the two iPr substituents displays hindered P−C bond rotation (Figure
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1
S4). On the other hand, the H{31P} NMR spectra of 3, 4, and 5 each display a singlet for the methylene
protons. These complexes are presumably more fluxional, such that the averaged solution structure exhibits
C3v symmetry (Figures S6, S8, and S10).
PiPr2
PiPr2
iPr2 P
N
RE
Ni(COD)2
N
N
70°C
toluene
PiPr2
PiPr2
iPr2 P Ni
RE = Sc, Y, La
M
2
3
4
ScL3
YL3
LaL3
N
N
N
PiPr2
PiPr2
iPr2 P Ni
M = Sc, Y, La, Ga
Li
GaCl3
N
N
Li
6
7
8
9
NiScL3
NiYL3
NiLaL3
NiGaL3
N
Li
Et2O, RT
5
Scheme 1. Synthetic pathways of heterobimetallic NiML3 complexes 6–9, where M = Sc, Y, La, and Ga.
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The heterobimetallic complexes NiScL3 (6), NiYL3 (7), and NiLaL3 (8) were isolated as bright red to
orange crystals from the reaction of Ni(COD)2 with 2, 3, and 4, respectively (crystalline yields 75 to 80%).
Further, addition of GaCl3 to 5 afforded NiGaL3 (9) after workup as a brown solid (60% yield). The NMR
spectra of all these NiML3 compounds display a single 31P{1H} resonance and a single 1H{31P} resonance
for the methylene protons, consistent with C3v symmetry in solution (Figures S12−S15, S18, S19, and S21).
Of note, the 31P{1H} NMR shifts in aromatic solvents for the Ni–rare earth series vary subtly (range: –0.8
ppm for 1 to 3.1 ppm for 6, Figure S12), whereas the resonance of Ni–Ga 9 appears significantly more
downfield at 31.8 ppm.
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Previously, we noted that adding THF to a toluene-d8 solution of NiLuL3 (1) generated an equilibrium
mixture of 1 and 1−THF, in which a single molecule of THF binds to the Lu center.41 Complete THF
binding resulted in a significant 11-ppm downfield shift of the 31P NMR resonance. Herein, a similar
behavior was observed for 7 (Δ(31P δ) = +10 ppm) and to a smaller extent for 8 (Δ(31P δ) = +2.5 ppm at rt,
+ 6.6 ppm at –83 ºC), suggesting that 7−THF and 8−THF are accessible (Figure S13 and S100). By
comparison, the change in the 31P δ for 6 or 9 is negligible when dissolved in THF-d8 versus that in toluene-
d8 (Δ(31P δ) < 0.3 ppm). These findings indicate that THF binding to a NiML3 complex can be controlled
by the size of M, where a sufficiently large M center (Lu, Y, and La) allows the solvent to bind.
To probe the impact of the M(III) supports on the electronics at Ni, a cyclic voltammetry (CV) study
of the NiML3 series was performed. The cyclic voltammograms were collected at 100 mV/s in 0.1 M
[nPr4N][BArF ]/1,2-difluorobenzene electrolyte solutions (where ArF is 3,5-bis(trifluoromethyl)phenyl) and
4
internally referenced to the [FeCp2]+/0 potential. Each NiML3 complex showed an irreversible Ni(I/0)
oxidation process, with the anodic peak potential (Epa) shifting overall by 0.75 V (Figure S27). The Epa
values increase in the order: −1.23 V (for 7) < –1.17 V (8) < −1.00 V (1)41 < −0.57 V (6) < −0.48 V (9).
The Ni(I/0) peak potentials in this NiML3 series correlate well with the Lewis acidity of the M(III) ion,
푅2
( 푎푑푗= 0.89, Table S3, Figure
which is parameterized by the pKa values of the aquo species [M(OH2)6]3+
S29).67-70 This is in accord with the rationale that more Lewis acidic M(III) ions will withdraw electron
density from Ni to a greater extent, which leads to more positive Ni(I/0) peak potentials. Of interest to us,
the peak potentials showed a worse correlation with size of the M(III) ion using the Shannon M(III) ionic
푅2
radii ( 푎푑푗 = 0.77, Table S3),39,71which suggests that the flexible nature of L does not accentuate size effects
of the M(III) support on the Ni electronics.72
X-ray Crystallography. Single-crystal X-ray diffraction studies were conducted on all monometallic and
bimetallic compounds. The solid-state structures and geometrical parameters for monometallic 2 – 5 are
provided in the SI (Figure S30–S33, Table S4). The solid-state structures of the bimetallic Ni–M complexes
6 – 9 are shown in Figure 2, with selected bond distances and other geometrical parameters in Table 1. The
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NiML3 complexes are all isostructural and possess approximate C3 symmetry. By examining the structures
of 1 and 6 – 9, the impact of M on various bond lengths was assessed. Of note, the Shannon ionic radii of
the M(III) ions vary by 0.41 Å, with Ga(III) possessing the smallest ionic radius (0.620 Å) and La(III) the
largest (1.032 Å).73 Both the M–N and Ni–M bonds in 1 and 6 – 9 adhere to the same trend: these bonds
elongate according to M = Ga < Sc < Lu < Y < La. This accords well with the expected trend based on
M(III) radii (Figures S36–S38, Table S7). In comparing Ni–Ga 9 and Ni–La 8, which represent the two
M(III) size extremes, the M–N and Ni–M bond distances differ by 0.51 Å and 0.23 Å, respectively. On the
other hand, the Ni–P bond lengths do not substantially change (Δ = 0.03 Å).
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Ni
Ni
Y
P
Sc
N
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6
2.24
2.43
Ni
Ga
Ni
La
8
9
Figure 2. Molecular structures of 6 – 9 shown at 50% thermal ellipsoid probability. Hydrogen atoms and
lattice solvent molecules have been omitted for clarity. The average bond lengths (Å) are shown. Atom
colors: Sc, yellow; Y, light blue; La, purple; Ga, red; Ni, pink; P, orange; N, blue; C, gray.
Table 1. Geometrical Parameters, Including Bond Lengths (Å)
and Angles (deg), for 6–9
1
6
7
8
9a
1.032 (La) 0.620 (Ga)
M(III) ionic radius (Å)b 0.861 (Lu) 0.745 (Sc)
0.900 (Y)
8.43
pKa of [M(OH2)6]3+
7.90
5.11
9.06
2.60
Ni–M
2.4644(2)
2.3569(7)
2.4823(3)
2.5732(4)
2.3395(8)
rc
0.91
0.91
0.91
2.2268(6)
2.258(1)
0.89
2.2352(5)
2.429(1)
1.00
Ni–Pd
M–Nd
Σ P–Ni–P
2.2188(7)
2.213(2)
359.40(1)
2.2184(2)
2.102(1)
359.53(1)
2.2027(3)
1.9181(5)
357.57(3)
354.81(10) 359.74(1)
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Σ N–M–N
352.39(8)
0.3559(7)
0.0995(3)
357.68(10) 359.75(4)
0.4427(15) 0.2994(10) –0.06(6)
359.83(1)
345.74(6)
0.4243(5)
0.1991(2)
M to N3 planee
Ni to P3 planef
0.0881(6)
0.0645(4)
–0.07(2)
aTwo molecules in the asymmetric unit. Values are averages. Shannon ionic radius of six-coordinate
M(III).73 cFormal shortness ratio.74 See main text. dAverage value. ePositive value indicates M is positioned
closer to Ni. fPositive value indicates Ni is positioned away from M.
b
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The intermetal distances in 1 and 6 – 9 are all shorter than the sum of the two metals’ covalent radii,
signifying a significant Ni−M bonding interaction. To better compare the metal-metal interactions, we
calculated the formal shortness ratio (r), or the ratio of the Ni−M bond distance to the sum of their respective
covalent radii using Pyykkö covalent radii values.74 The r values across the rare-earth supported bimetallic
complexes are all similar, 0.89 to 0.91. This suggests that the bonding interaction between Ni and the rare
earth ion is independent of the rare earth identity. We further note that the similar r values are not simply a
consequence of the bridging ligand L since Ni−Ga 9 has a significantly larger r value of 1.00.
Catalyst Screening for Diphenylacetylene Semihydrogenation. The effect of varying the Lewis acid
support on both catalytic activity and selectivity was investigated using the semihydrogenation of
diphenylacetylene (DPA) as a model reaction (Table 2, Figures S39−S45). In a standard catalytic run, a J
Young NMR tube loaded with the catalyst (2.5 mol%) and substrate (0.45 M DPA) in toluene–d8 was
charged with 4 atm of H2 and heated to 70 °C for 24 h (4.6 atm H2 at 70°C). A mixture of Ni(COD)2 and 3
equiv of the protioligand was tested as a control for a monometallic nickel catalyst (entry 1). This catalyst
system gave poor DPA conversion (<40%). In addition, alkyne semihydrogenation proceeded
nonselectively to a ~1:1 mixture of (E)- and (Z)-stilbene with a small amount (6%) of the double alkyne
insertion byproduct, 1,2,3,4-tetraphenyl-1,3-butadiene, which will simply be referred to as the (Z,Z)-diene.
Mononickel complex 5 performed similarly poorly (entry 2). The catalytic performance of bimetallic 1 and
6 – 9 are shown in entries 3 to 7. With the single exception of Ni−Sc 6, all the Ni−M catalysts showed
nearly complete DPA conversion (≥98%) under the standard catalytic conditions. The best performer was
Ni−Y 7, which quantitatively converted DPA to (E)-stilbene (entry 4). Both Ni−Lu 1 and Ni−Ga 9
generated (E)-stilbene in high yields (91%); however, both catalysts also generated a significant amount
(7-9% yield) of the undesired byproducts: 1,2-diphenylacetylene, which results from double hydrogenation,
and (Z,Z)-diene.
Table 2. Catalyst Screening for Semihydrogenation of Diphenylacetylene (DPA)a, b
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DPA conversion
stilbene
(%)
E:Z stilbene diphenylethane Z,Z-dienec
entry
catalyst
(%)
38
29
>99
74
>99
98
>99
>99
>99
ratio
52:48
7:21
>99:1
40:60
>99:1
20:80
>99:1
40:60
>99:1
(%)
0
<1
7
1
<1
2
1
21
<1
(%)
6
1
2
3
4
5
6
7
8d
9e
Ni(COD)2 + 3 HL
5 Ni(Li∙L)3
1 (Lu)
32
28
91
58
99
91
92
78
99
<1
2
15
<1
5
7
<1
<1
6 (Sc)
7 (Y)
8 (La)
9 (Ga)
7–THF-d8
7 + Hg0
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a Catalytic conditions: 2.5 mol % catalyst, 0.45 M DPA, ca. 500 L of toluene–d8, 4.6 atm H2 at 70 C, 24
b
1
h. Values (%) are averages of triplicate runs and were determined by H NMR integration using
trimethoxybenzene as an internal standard(error = ± 0.5%). c Z,Z-diene = 1,2,3,4-tetraphenyl-1,3-butadiene.
d THF–d8 solvent. 300 equiv Hg metal.
e
Unexpectedly, Ni−La 8 showed an inverted preference for (Z)-stilbene over the (E)-isomer by a factor
of 4. We note, however, that during the catalysis time course of 8, we observed the gradual growth of
different decomposition species, including iPr2PH. Hence, a more reasonable explanation is that the cis-to-
trans stilbene isomerization was impeded by catalyst death. In support, all NiML3 complexes, including 8,
catalyze the isomerization of (Z)-stilbene to (E)-stilbene under the standard catalytic conditions (SI Table
S10). Among the bimetallic catalysts, Ni−Sc 6 performed the worst, as it reacted the most sluggishly and
produced the most of the (Z,Z)-diene byproduct (15%).
The choice of solvent was found to be critical for the high catalytic selectivity of 7. Upon changing the
solvent to THF–d8, the performance of catalyst 7, as the THF adduct 7-THF, suffered greatly, as evidenced
by marked decline in both regioselectivity (E:Z stilbene ratio of 2:3) and chemoselectivity (alkyne versus
alkene hydrogenation), as indicated by >20% yield of 1,2-diphenylethane (entry 8). Lastly, catalysis by 7
was uninhibited in the presence of excess Hg, which supports a homogeneous active species (entry 9).
Initial Rate Studies. The (E)-selective hydrogenation of disubstituted alkynes proceeds by an auto-
tandem catalytic process, in which a single catalyst executes both the syn-hydrogenation of the alkyne and
the cis-to-trans isomerization of the resulting alkene. To disentangle the two catalytic cycles, we separately
conducted initial rate studies of DPA hydrogenation and (Z)-stilbene isomerization. While H2 is not
consumed in the latter reaction, H2 was found to be necessary because alkene isomerization either does not
proceed, or is very sluggish, in the absence of H2 (Table S11). The initial rates were determined by
monitoring the consumption of the starting material (DPA or (Z)-stilbene) under the standard catalytic
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conditions until either 10% conversion or up to 1 h. The kinetic plots are shown in Figure 3, and the initial
rates are listed in Table 3. Of note, catalyst 8 did not decompose during this short time period.
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Figure 3. Kinetic plots of substrate concentration (M) versus initial time (s) for: A) the hydrogenation of
diphenylacetylene; and B) isomerization of (Z)–stilbene, as catalyzed by 1 and 6−9 under the standard
catalytic conditions. For more details, see SI Figures S46−S56.
Table 3. Initial Rate Data for the Syn-Hydrogenation of DPA and (Z)-to-(E) Stilbene
Isomerization by NiML3 Catalystsa
Initial Rate (×10−5 M/s)
entry catalyst
−d[DPA]/dtb
−d[(Z)-
stilbene]/dt
1
2
3
4
5
1 (Lu)
6 (Sc)
7 (Y)
1.71(8)
0.67(4)c
1.9(4)
1.4(1)
3.0(2)
1.1(1)
8 (La)
9 (Ga)
2.13(1)
2.5(4)
0.45(3)
40.4(7)
aCatalytic conditions: 2.5 mol % catalyst, 0.45 M DPA or (Z)-stilbene, ca. 500 L of toluene–d8, 4.6 atm
H2 at 70 C, ≤ 1 h with an internal standard of 1,3,5-trimethoxybenzene (0.02 mmol). bIn all cases except
6, (Z)-stilbene was formed cleanly in the syn-hydrogenation, and the rate of the reaction is −d[DPA]/dt.
cFor 6, formation of both (Z,Z)-diene and (E)-stilbene were observed, and the corrected rate of syn-
hydrogenation = −d[DPA]/dt − d[Z,Z-diene]/dt. See SI for details.
In both reactions, Ni−Ga 9 was the fastest of the NiML3 catalysts. Changing the metal support impacted
the catalytic activity, especially in the (Z)-stilbene isomerization reaction, where the initial rates varied by
nearly 90-fold. On the other hand, the initial rates for DPA hydrogenation only varied by a factor of 3. With
the exception of Ni−Ga 9, the two reactions showed opposite trends in catalytic activity for the different
metal supports. For example, catalytic activity in DPA hydrogenation follows the order: 9 (M = Ga) > 8
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(La) > 7 (Y) > 1 (Lu) > 6 (Sc). By comparison, the ranking of catalysts by (Z)-stilbene isomerization
activity is: 9 (M = Ga) >> 6 (Sc) > 1 (Lu) > 7 (Y) > 8 (La). Upon excluding 9, a positive correlation was
found between the rate of DPA hydrogenation and the pKa value of the corresponding aquo species
푅2
3+
−5
[M(OH2)6] (Figure 4, slope = +0.37(1) x 10 M/sper pKa unit; 푎푑푗=0.99). This finding suggests that less
Lewis acidic supports result in greater Ni-based activity in alkyne syn-hydrogenation (Figure S57). As an
aside, a reasonable correlation also exists between the rate of DPA hydrogenation and Epa for the Ni-rare
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푅2
earth catalysts ( 푎푑푗=0.94, Table S8). Also excluding 9, the plot of the (Z)-stilbene isomerization rate versus
the pKa value of [M(OH2)6]3+ showed a negative correlation (Figure 4, slope = −0.62(5) x 10−5 M/s per pKa
푅2
unit; 푎푑푗=0.99), indicating that more Lewis acidic supports lead to higher activity in cis-to-trans alkene
isomerization (Figure S58). However, for both reactions, the initial rates for 9 are significantly faster than
the correlative values based on Lewis acidity (Figures S59−S60), suggesting the GaIII supporting ion, which
is by far the most Lewis acidic, results in a significant change in the nature of the reactive species and/or
the mechanism.
푅2
Figure 4. Plot of initial rate of syn-hydrogenation of diphenylacetylene (black circles, 푎푑푗 = 0.99) and
cis-to-trans isomerization of (Z)–stilbene (blue squares,
catalysts 1 and 6–8. The slope, m, is provided in the plots.
푅2
= 0.99) versus pKa of M(OH2)63+ for
푎푑푗
In operando catalytic studies. To further investigate the impact of the supporting metal, the full
reaction time course from DPA to (E)-stilbene was monitored by 1H NMR spectroscopy under the standard
catalytic conditions. Three catalysts, Ni–Ga 9, Ni–Sc 6, and Ni–Y 7, were selected because they exhibit
markedly different rates and/or selectivity. In the reaction catalyzed by Ni–Ga 9 (Figure 5A), (Z)-stilbene
is the first intermediate to accumulate, reaching maximum concentration at ~4.5 h, before fully decaying as
it converts to (E)-stilbene within 8 h. By comparison, the Ni–Sc 6 catalyst required a significantly longer
time period (70 h) for the reaction to fully consume (Z)-stilbene (Figure 5B). In addition to the (E)-stilbene
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product, the (Z,Z)-diene byproduct was formed in significant amounts, and primarily at early reaction times
when DPA is present. This suggests that after DPA insertion into a Ni−H bond to form a Ni−vinyl
intermediate, an off-pathway insertion of a second DPA is competitive with the on-pathway reductive
elimination of (Z)-stilbene.
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Figure 5. Reaction time course for the semihydrogenation of diphenylacetylene to (E)–stilbene under
standard conditions for catalysts: A) Ni–Ga 9, B) Ni–Sc 6, and C) Ni–Y 7.
The reaction time course for Ni–Y 7 was remarkably distinct in that a temporal separation of the two
catalytic reactions was clearly observed. The cis-to-trans isomerization of stilbene proceeded only after the
DPA was almost entirely consumed in the syn-hydrogenation reaction (Figure 5C). Temporal separation in
auto-tandem catalysis is rare, and to our knowledge, the single well-studied precedent is
[Ru(Cp)(bpy)NCCH3]+, which converts terminal alkynes into the corresponding alcohols through an
acetaldehyde intermediate.75 The temporal separation of the two catalytic cycles also indicates a greater
chemoselectivity for DPA relative to (Z)-stilbene by 7. Also of relevance, strong alkyne adsorption and its
impact on kinetics had been observed for Cu/SiO2, a heterogeneous catalyst for (Z)-selective
semihydrogenation.76 Lastly, we note that during the reaction time course shown in Figure 5C, the rate of
stilbene isomerization exceeded that of DPA hydrogenation. This was unexpected because the initial rate
studies showed the opposite order, where DPA hydrogenation was 1.7 times faster than that of stilbene
isomerization. We do not currently understand the origin of the rate enhancement, but we speculate that
small amounts of DPA might alter the catalyst speciation to a more reactive species for stilbene
isomerization (Figure S75).
Catalyst Resting State. The reaction time course experiments were also monitored by 31P{1H} NMR
spectroscopy to elucidate the catalyst resting state. Throughout the full reaction time course, the catalytic
reaction mixture of Ni–Sc 6 only showed the presence of 6 (Figure S65). Hence, no observable binding of
either H2 or DPA was found for the Sc(III)-supported Ni site. During the in operando catalysis by Ni–Ga
9, a single complex with a corresponding 31P{1H} singlet at 48.5 ppm was observed throughout the full
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time course (Figures S71 and S77). This new species also exhibited a broad H resonance at –2.03 ppm,
which is typical of a Ni(η2-H2) adduct (Figure S84).77-78 Also, consistent with H2 binding, the reaction
solution color bleached from a dark red-orange (9) to a light yellow-orange (9−H2).71 Hence, the strongly
Lewis acidic Ga(III) ion promotes H2 binding at the supported Ni site. Further characterization of the
independently synthesized 9−H2 includes a T1(min) value of 28(1) ms (500 Hz), which confirms an intact
H−H bond (Figure S85).77 An H−H distance of 0.86 Å (c.f. 0.74 Å in free H2) was determined from the JH−D
value of 35 Hz for the corresponding HD adduct (Figure S86).78 H2 binding was reversible as evacuating a
solution of 9−H2 regenerates 9. Of note, the H2 binding behavior of 9 contrasts that of Ni−Lu 1, where H2
binding is so weak that it was only perceptible at low temperatures of –80 °C and under 4 atm H2. The other
rare-earth supported Ni complexes herein also show similarly weak H2 binding akin to 1 (Figures S87–
S101).
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The in operando catalysis of 7 also showed complete formation of a new species that persisted until t
~3.5 h. After t ~3.5 h, the only species observed was 7. The 31P{1H} spectrum of this new complex consists
of a singlet resonance at 30.4 ppm and doublet at –11.7 ppm (J = 48.5 Hz), with an integration ratio of 2:1,
respectively (Figure S67). The observed coupling makes evident that one phosphine donor is coordinated
to 89Y (>99% abundance, I = ½). We further infer that this ligand hemilability allows for strong alkyne
binding at Ni. In support, identical spectroscopic features were observed when 5 equiv DPA was added to
7 (Figure S26). Structural elucidation of this species by single-crystal X-ray diffraction confirmed its
2
i
2 i
≡
assignment as the DPA adduct, (η -PhC CPh)Ni( Pr2PCH2NPh)2Y(κ - Pr2PCH2NPh), or 7–DPA (Figure
6). The favorable formation of 7–DPA also rationalizes the greater chemoselectivity of 7 for DPA over
stilbene, as well as the temporal separation of the two tandem reactions.
N
C2
P
C1
Y
Ni
P
N
N
P
Figure 6. Molecular structure of 7–DPA shown at 50% thermal ellipsoid probability; hydrogen atoms and
non-coordinating solvent molecules are omitted for clarity. Only one of the two complexes from the
asymmetric unit is shown for clarity. Selected interatomic distances (Å) and angles (deg.) as an average of
the two crystallized molecules in the asymmetric unit: Ni–Y, 2.7031(2); Y–P, 2.9898(9); C1–C2, 1.281(5);
Ph–C1–C2, 142.48. See Table S6 for more details.
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In the solid-state structure of 7–DPA, two molecules crystallized in the asymmetric unit with an average
Ni–Y bond length of 2.7031(2) Å, which is longer than that in 7 by 0.22 Å. The Ni–Y bonding interaction
remains intact as suggested by the corresponding r value of 0.99. The average Y–P distance of 2.9898(9)
Å is similar to those in the monometallic YL3, 3. Significant π-back-bonding into the alkyne is evidenced
by the elongated C1–C2 distance (avg 1.281(5) Å) and the bent Ph–C1–C2 angle (avg 142.48°). The
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2
i
≡
geometric changes in the DPA ligand are similar to those in (η -PhC CPh)Co( Pr2PNXyl)2ZrI(THF),
59
which is also a relevant intermediate in DPA semihydrogenation. However, the regioselectivity of the
Co-Zr catalyst is poor as stilbene was formed as a mixture of both the cis and trans isomers.59 Lastly, the
rearrangement of the [iPr2PCH2NPh]− ligand in 7–DPA is reminiscent of the hemilability behavior observed
in related heterobimetallic Co-Zr complexes bearing three bridging phosphinoamide ligands.79-80
The combination of ligand hemilability and DPA binding was also observed for Ni−Lu 1 and Ni−La 8
(Figure S61). Also of interest, catalysts 1 and 8 showed a clear temporal separation during catalysis, akin
to 7. These findings suggest that the strong alkyne chemoselectivity is linked to the M(III) rare-earth
supports with sufficiently large ionic radii. Hence, we hypothesize that larger supporting metal ions enhance
ligand hemilability, which then favors DPA binding. In support, neither Ni−Ga 9 nor Ni−Sc 6 showed any
detectable binding of DPA even when DPA was present in large excess (40 equiv) or cooled to –60 °C
(Figures S76–S77). To test whether a vacant coordination site at Y(III) in 7 is vital to its catalytic
performance, we monitored the catalysis when the Y(III) center is saturated by a coordinating solvent
molecule, such as THF. During the catalytic time course in THF, 7–DPA was never observed (40 equiv
DPA). Instead, the only observed species was the THF–adduct, 7–THF, which was independently
synthesized and crystallographically characterized (Figures S34 and S73, Table S6). The deterioration of
the catalyst performance for 7–THF suggests that having an open coordination site on Y to promote ligand
hemilability may be critical to the enhanced performance of 7.
Substrate Scope and Isotope-Labeling Studies. Encouraged by preliminary catalytic studies with
DPA, we sought to investigate the utility of 7 toward the semihydrogenation of an assortment of substrates
(Table 4, also see Figures S102−S110), using the standard catalytic conditions, unless otherwise noted.
Para-substituted diarylacetylene featuring a boronate ester proceeded cleanly to the trans-diarylethene 10b
with excellent regioselectivity (>99:1 E:Z) and in 99% yield. 3-Hexyne, a dialkyl-substituted acetylene,
gave nearly exclusively cis-3-hexene (10c) and some trace hexane. The clean formation of the cis-isomer
suggests that dialkyl-substituted alkenes may not undergo the critical olefin insertion into the Ni−H species
during the alkene-isomerization cycle (vide infra). Of interest, 1-phenylpropyne yielded the trans-alkene
10d fairly cleanly. Terminal alkynes, on the other hand, are nonviable substrates. An attempted catalytic
run with phenylacetylene resulted in catalyst decomposition and no hydrogenation product(s). If instead,
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the trimethylsilyl protecting group is employed, such as 1-phenyl-2-trimethylsilylacetylene, then the trans-
alkene 10e is formed nearly exclusively. Several 1-aryl-2-trimethylsilylacetylene substrates were also
investigated, including methoxy or methyl ortho-substituted or para-substituted fluorine derivatives. For
these substrates, the corresponding trans products (10f - 10h) were obtained in good yields with excellent
regioselectivity, though either higher catalyst loadings (up to 5 mol%) or longer reaction times (up to 60 h)
were necessary. Lastly, a substrate bearing two internal alkynes reacted cleanly in a double
semihydrogenation with excellent (E,E)-selectivity (10i).
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Table 4. Substrate Scope
aCatalytic Conditions: 2.5 mol % 7, 0.45 M alkyne in ca. 500 L of toluene–d8, 4.6 atm H2, heated to 70 C, unless
otherwise noted. Conversion is reported as an average of triplicate runs (±0.5 error bars) at 24 h (unless otherwise
noted), as determined by 1H NMR integration against an internal trimethoxybenzene standard (0.02 mmol). bAfter
60h. c3.5 mol % 7. d5 mol % 7, full catalyst decomposition after 24 h, remainder was alkyne. eafter 48h.
As previously discussed, the alkene cis-to-trans isomerization does not consume H2, but H2 is required
for the reaction to be kinetically relevant. An isotopic labeling experiment was performed using catalysts 7
and 9. Using D2 and otherwise standard catalytic conditions, (Z)-stilbene was converted to (E)-stilbene with
deuterium incorporation. Also, formation of HD, and a minor amount of H2, was observed for catalyst 7
(Figure S79–S80). A control experiment showed that 7 does not scramble HD to produce H2 and D2 under
the same conditions. Hence, the origin of the protons in HD and H2 is most likely the (Z)-stilbene substrate.
This supports the alkene isomerization proceeding via alkene insertion into a Ni−H bond and subsequent
C−C bond rotation and β-hydride elimination.
Discussion
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The mechanistic and kinetic studies provide a comprehensive understanding of how the choice of the
M(III) support impacts the Ni–M semihydrogenation catalysis. Both the size and the Lewis acidity of the
M(III) ion play significant roles. The larger M(III) ions with an ionic radius ≥ 0.86 Å all favor a catalyst
resting state of the alkyne adduct, Ni(η2-PhC≡CPh), during the first catalytic cycle. This subgroup includes
the Ni–Ln catalysts, where M is Lu, Y, and La. During the second catalytic cycle when DPA is depleted,
these catalysts change their resting state to the “naked” species, NiML3, which suggests that both alkene
and H2 binding is weak. Of note, the observation of the Ni(η2-PhC≡CPh) catalyst resting state is directly
coupled to the rare observation of a temporally separated auto-tandem catalysis, which to our knowledge
has not been reported for alkyne semihydrogenation. Hence, the temporal separation is a direct consequence
of the nearly perfect chemoselectivity of the Ni–Ln catalysts for reacting with alkynes over alkenes when
both substrates are present.
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The Lewis acidity of the M(III) support can influence both the catalyst resting state and the catalytic
rates. The H2 adduct resting state, Ni(η2-H2), is only observed for Ni−Ga 9. Previously, increasingly
electron-deficient Ni(0) centers were shown to bind H2 more strongly.39,71 The observation of 9-H2 as the
resting state in both catalytic cycles follows a similar logic in that Ga(III), which is by far the most Lewis
acidic ion in this series, would withdraw the most Ni electron density. Catalyst 9 further stands out for its
fastest initial rates in (Z)-stilbene isomerization in the series, by nearly two orders of magnitude. Even
among the remaining Ni−RE catalysts, the more Lewis acidic RE ions promote faster (Z)-stilbene
isomerization (Figure 4). By contrast, the more Lewis acidic RE ions yield slower initial rates in the syn-
hydrogenation of DPA. We posit that alkyne binding is favored by a more electron-rich Ni center, which
can better π-back-bond to the alkyne. Furthermore, we propose that alkyne binding is more critical to the
first catalytic cycle, whereas H2 binding and activation plays a more important role during the second
catalytic cycle.
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Figure 7. Proposed catalytic cycles for the nickel–catalyzed (E)-selective semihydrogenation of
diphenylacetylene.
Figure 7 shows the proposed mechanism for the selective hydrogenation of DPA to (E)-stilbene. Since
Ni–Ga 9 and the Ni–Ln (where Ln = Lu, Y, and La) complexes feature different catalyst resting states, their
mechanisms are shown separately. The Ni–Ga catalyst first binds H2 to generate 9–H2, the observed resting
state. Activation of H2, followed by phosphine dissociation35 and then DPA binding81-83 leads to the
formation of intermediate I. The bridging and terminal hydrides, HNi(μ−H)Ga, in I is based on the
analogous HNi(μ−H)B species reported by Peters,28,84 as well as Thomas’ HCo(μ−H)Zr intermediate79. We
note, however, that the mechanistic data does not discern whether alkyne binding or H–H cleavage proceeds
first in the cycle. Next, migratory insertion of the alkyne into the Ni−H bond forms the Ni vinyl
intermediate, II. The first catalytic cycle ends with the reductive elimination of (Z)-stilbene and binding of
H2 to regenerate 9–H2. In the second catalytic cycle, H2 activation, followed by phosphine dissociation to
allow for (Z)-stilbene binding, would provide intermediate III, which is the alkene counterpart to I.
Migratory insertion of the alkene into the Ni−H generates the Ni diphenylethyl intermediate IV. Rotation
of the ethyl C−C bond followed by β-hydride elimination would generate (E)-stilbene, as supported by the
isotope labeling studies described above. Alternatively, from IV, reductive elimination may proceed to
form the 1,2-diphenylethane byproduct.
The Ni–Ln catalysts are proposed to operate through similar intermediates, I′–IV′, though their
mechanism diverges in a couple key ways. First, by favoring DPA binding, the Ni–Ln catalysts operate via
the Ni(η2-PhC≡CPh) resting state, wherein the dissociated phosphine binds to the Ln ion. Second, owing to
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the favorability of the alkyne adduct, (Z)-stilbene cannot enter the cis-to-trans isomerization cycle until
nearly all the DPA is depleted, and Ni(η2-PhC≡CPh) is no longer the default resting state. The isomerization
cycle is similar to that described above for Ni–Ga 9, except that the “naked” NiLnL3 species is the new
resting state. Likewise, H2 binding is proposed to precede alkene binding since the NiLnL3 complexes all
exhibit weak binding of H2, whereas alkene binding has not yet been detected for any of these catalysts.
For Ni–Sc 6, the catalyst resting state throughout the full catalysis is NiScL3. Presumably, the Sc(III)
support is neither sufficiently large (0.75 Å) to accommodate a stable Ni(η2-PhC≡CPh) adduct, nor
particularly Lewis acidic (pKa 5.1) to promote strong H2 binding at Ni. The Ni–Sc 6 showed the slowest
initial rate in the syn-hydrogenation of DPA, which hinders its performance in the overall catalysis.
Moreover, Ni–Sc generates the highest percentage of the (Z,Z)-diene byproduct (15%) in the series. The
formation of the Z,Z-diene points to an alternative path whereby migratory insertion of DPA into the Ni
vinyl bond in intermediate II occurs prior to reductive elimination.
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Conclusion
The Ni–rare earth complexes add to the few first-row transition metal–rare earth complexes with direct
metal-metal bonding interactions. More significantly, this study demonstrates the utility of rare earth metal
ions as promoters in base-metal catalysis. Both the ionic size and Lewis acidity of the metal support are
important tunable parameters for the catalytic activity and selectivity. First, the Ni electronics are
significantly perturbed by variation of the M(III) support: the Ni(I/0) peak potentials were tuned overall by
0.75 V and showed a strong correlation with M(III) Lewis acidity. Second, the M(III) support can remotely
tune the substrate-binding preference of the active Ni center, and consequently, determine the catalyst
resting state. For example, the weak Lewis acidity and the large ionic size of the rare-earth supports are
posited to be responsible for the strong Ni-alkyne binding in Ni–Y 7, Ni–Lu 1, and Ni–La 8. Presumably,
these more electron-rich Ni centers can better π-back-bond into the alkyne unit, while the large rare-earth
supports provide an intramolecular, open coordination site to favor hemilability of the phosphine ligand.
On the other hand, strong Lewis acidity appears to be the dominant parameter in priming Ni for strong H2
binding as only the highly Lewis acidic Ga(III) formed a stable Ni(η2-H2) adduct under ambient conditions.
The delineation of size and Lewis acidity in determining substrate binding is corroborated by the fact that
Ni−Sc 6, which falls in the middle in terms of both Lewis acidity and size, showed neither alkyne nor H2
binding under ambient conditions.
The stark differences in substrate binding among the Ni−M catalysts have important mechanistic and
kinetic ramifications. When the Ni–M complex binds alkyne strongly, the ensuing auto-tandem catalysis
shows a clear temporal separation between the two catalytic cycles. This temporal separation also indicates
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a stronger chemoselectivity for alkyne over alkene reactivity. Similarly, for the Ni-rare earth catalysts, the
rate of DPA syn-hydrogenation appears to be directly influenced by its ability to bind alkyne, such that less
Lewis acidic RE ions yield faster initial rates in the syn-hydrogenation of DPA. The opposite trend was
found for the rate of alkene isomerization, where more Lewis acidic RE ions yield faster initial rates. This
may suggest that H2 binding and activation is dependent on the Lewis acidity of the support and is the
critical step in alkene isomerization for these catalysts.
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Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI:
10.1021/.
X-ray crystallographic data for complexes 2–9, 7–THF, 7–DPA, and 8’ (CIF)
Experimental procedures, characterization and spectroscopic data (PDF)
Author Information
Corresponding Author
*clu@umn.edu
ORCID
Bianca L. Ramirez: 0000-0001-7907-6312
Connie C. Lu: 0000-0002-5162-9250
Notes
The authors declare no competing financial interest.
These cif files for complexes 2–9, 7–THF, 7–DPA, and 8’ have been deposited in the Cambridge CCDC
as nos. 1979220–1979230.
Acknowledgements
B.L.R is grateful to 3M for a research fellowship. The experimental work was supported by the American
Chemical Society Petroleum Research Fund (57192-ND3). C.C.L. acknowledges the National Science
Foundation (CHE-1665010 and CHE-1954751) for support. X-ray diffraction experiments were
performed using a crystal diffractometer acquired through an NSF-MRI award (CHE-1229400) in the X-
ray laboratory supervised by Dr Victor G. Young, Jr. B.L.R. thanks Dr Victor G. Young, Jr. for assistance
with X-ray crystallography and Dr. Letitia J. Yao for assistance with NMR spectroscopy.
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References
5
6
(1) Liddle, S. T.; Mills, D. P. Metal-metal bonds in f-element chemistry. Dalton Trans. 2009, 5592-5605.
7
8
9
(2) Butovskii, M. V.; Kempe, R. Rare earth–metal bonding in molecular compounds: recent advances,
challenges, and perspectives. New Journal of Chemistry 2015, 39, 7544-7558.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(3) Oelkers, B.; Kempe, R., Group 3, Lanthanide, and Actinide Metal–Metal Bonds. In Molecular Metal-
Metal Bonds, 1st ed.; Liddle, S. T., Ed. Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2015; pp
47-71.
(4) Spannenberg, A.; Oberthür, M.; Noss, H.; Tillack, A.; Arndt, P.; Kempe, R. Metal—Metal
“Communication” of Rh or Pd with Nd in Novel Heterobinuclear Complexes. Angew. Chem. Int. Ed.
1998, 37, 2079-2082.
(5) Butovskii, M. V.; Tok, O. L.; Wagner, F. R.; Kempe, R. Bismetallocenes: Lanthanoid–
Transition‐Metal Bonds through Alkane Elimination. Angew. Chem. Int. Ed. 2008, 47, 6469-6472.
(6) Takanori, S.; Zhaomin, H. Activation and Dehydrogenative Silylation of the C–H Bonds of
Phosphine-coordinated Ruthenium in Lu/Ru Heteromultimetallic Hydride Complexes. Chem. Lett. 2008,
37, 298-299.
(7) Döring, C.; Dietel, A. M.; Butovskii, M. V.; Bezugly, V.; Wagner, F. R.; Kempe, R. Molecular
[Yb(TM)2] Intermetalloids (TM=Ru, Re). Chem. Eur. J. 2010, 16, 10679-10683.
(8) Nakajima, Y.; Hou, Z. Rare-Earth-Metal/Platinum Heterobinuclear Complexes Containing Reactive
Ln-alkyl groups (Ln = Y, Lu): Synthesis, Structural Characterization, and Reactivity. Organometallics
2009, 28, 6861-6870.
(9) Arnold, P. L.; McMaster, J.; Liddle, S. T. An unsupported transition metal-lanthanide bond; synthesis
and crystal structure of an Nd-Fe amido N-heterocyclic carbene complex. Chem. Commun. 2009, 818-
820.
(10) O, W. W. N.; Kang, X.; Luo, Y.; Hou, Z. PNP-Ligated Heterometallic Rare-Earth/Ruthenium
Hydride Complexes Bearing Phosphinophenyl and Phosphinomethyl Bridging Ligands. Organometallics
2014, 33, 1030-1043.
(11) Volcker, F.; Muck, F. M.; Vogiatzis, K. D.; Fink, K.; Roesky, P. W. Bi- and trimetallic rare-earth-
palladium complexes ligated by phosphinoamides. Chem. Commun. 2015, 51, 11761-11764.
(12) Volcker, F.; Roesky, P. W. Bimetallic rare-earth/platinum complexes ligated by phosphinoamides.
Dalton Trans. 2016, 45, 9429-9435.
(13) Cui, P.; Xiong, C.; Du, J.; Huang, Z.; Xie, S.; Wang, H.; Zhou, S.; Fang, H.; Wang, S.
Heterobimetallic scandium–group 10 metal complexes with LM → Sc (LM = Ni, Pd, Pt) dative bonds.
Dalton Trans. 2020, 49, 124-130.
(14) Du, J.; Huang, Z.; Zhang, Y.; Wang, S.; Zhou, S.; Fang, H.; Cui, P. A Scandium Metalloligand-
Based Heterobimetallic Pd−Sc Complex: Electronic Tuning Through a Very Short Pd→Sc Dative Bond.
Chem. Eur. J. 2019, 25, 10149-10155.
ACS Paragon Plus Environment

Page 19 of 25
Journal of the American Chemical Society
1
2
3
4
5
6
(15) Burns, C. P.; Yang, X.; Sung, S.; Wofford, J. D.; Bhuvanesh, N. S.; Hall, M. B.; Nippe, M. Towards
understanding of lanthanide–transition metal bonding: investigations of the first Ce–Fe bonded complex.
Chem. Commun. 2018, 54, 10893-10896.
7
8
9
(16) Burns, C. P.; Yang, X.; Wofford, J. D.; Bhuvanesh, N. S.; Hall, M. B.; Nippe, M. Structure and
Magnetization Dynamics of Dy−Fe and Dy−Ru Bonded Complexes. Angew. Chem. Int. Ed. 2018, 57,
8144-8148.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(17) (a) Schwob, T.; Kunnas, P.; de Jonge, N.; Papp, C.; Steinrück, H.-P.; Kempe, R. General and
selective deoxygenation by hydrogen using a reusable earth-abundant metal catalyst. Sci. Adv. 2019, 5,
eaav3680. (b) Bruix, A.; Rodriguez, J. A.; Ramírez, P. J.; Senanayake, S. D.; Evans, J.; Park, J. B.;
Stacchiola, D.; Liu, P.; Hrbek, J.; Illas, F. A New Type of Strong Metal–Support Interaction and the
Production of H2 through the Transformation of Water on Pt/CeO2(111) and Pt/CeOx/TiO2(110) Catalysts.
J. Am. Chem. Soc. 2012, 134, 8968-8974.
(18) Ahmadi, M.; Mistry, H.; Roldan Cuenya, B. Tailoring the Catalytic Properties of Metal
Nanoparticles via Support Interactions. J. Phys. Chem. Lett. 2016, 7, 3519-3533.
(19) Evangelisti, F.; Moré, R.; Hodel, F.; Luber, S.; Patzke, G. R. 3d–4f {CoII Ln(OR)4} Cubanes as Bio-
3
Inspired Water Oxidation Catalysts. J. Am. Chem. Soc. 2015, 137, 11076-11084.
(20) Hodel, F. H.; Luber, S. Redox-Inert Cations Enhancing Water Oxidation Activity: The Crucial Role
of Flexibility. ACS Catal. 2016, 6, 6750-6761.
(21) Kumar, P.; Griffiths, K.; Anson, C. E.; Powell, A. K.; Kostakis, G. E. A tetranuclear CuII DyIII
2
2
coordination cluster as a Suzuki (C–C) coupling reaction promoter. Dalton Trans. 2018, 47, 17202-
17205.
(22) SaiꢀManojꢀGorantla, N. V. T.; GuruprasadꢀReddy, P.; AbdulꢀShakoor, S. M.; Mandal, R.; Roy, S.;
Mondal, K. C. Tetranuclear 3d/4f Coordination Complexes as Homogeneous Catalysts for
Bis(indolyl)methane Syntheses. ChemistrySelect 2019, 4, 7722-7727.
(23) Broderick, E. M.; Diaconescu, P. L. Cerium(IV) Catalysts for the Ring-Opening Polymerization of
Lactide. Inorg. Chem. 2009, 48, 4701-4706.
(24) Broderick, E. M.; Guo, N.; Wu, T.; Vogel, C. S.; Xu, C.; Sutter, J.; Miller, J. T.; Meyer, K.; Cantat,
T.; Diaconescu, P. L. Redox control of a polymerization catalyst by changing the oxidation state of the
metal center. Chem. Commun. 2011, 47, 9897-9899.
(25) Wang, X.; Brosmer, J. L.; Thevenon, A.; Diaconescu, P. L. Highly Active Yttrium Catalysts for the
Ring-Opening Polymerization of ε-Caprolactone and δ-Valerolactone. Organometallics 2015, 34, 4700-
4706.
(26) Amgoune, A.; Bourissou, D. σ-Acceptor, Z-type ligands for transition metals. Chem. Commun. 2011,
47, 859-871.
(27) Buchwalter, P.; Rosé, J.; Braunstein, P. Multimetallic Catalysis Based on Heterometallic Complexes
and Clusters. Chem. Rev. 2015, 115, 28-126.
(28) Harman, W. H.; Lin, T.-P.; Peters, J. C. A d10 Ni–(H2) Adduct as an Intermediate in H−H Oxidative
Addition across a Ni−B Bond. Angew. Chem. Int. Ed. 2014, 53, 1081-1086.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 20 of 25
1
2
3
4
(29) Mankad, N. P. Selectivity Effects in Bimetallic Catalysis. Chem. Eur. J. 2016, 22, 5822-5829.
5
6
7
(30) Bouhadir, G.; Bourissou, D. Complexes of ambiphilic ligands: reactivity and catalytic applications.
Chem. Soc. Rev. 2016, 45, 1065-1079.
8
9
(31) Powers, I. G.; Uyeda, C. Metal–Metal Bonds in Catalysis. ACS Catal. 2017, 7, 936-958.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(32) Pye, D. R.; Mankad, N. P. Bimetallic catalysis for C–C and C–X coupling reactions. Chem. Sci.
2017, 8, 1705-1718.
(33) Thomas, C. M. Metal-Metal Multiple Bonds in Early/Late Heterobimetallic Complexes:
Applications Toward Small Molecule Activation and Catalysis. Comments on Inorganic Chemistry 2011,
32, 14-38.
(34) You, D.; Gabbaï, F. P. Unmasking the Catalytic Activity of a Platinum Complex with a Lewis
Acidic, Non-innocent Antimony Ligand. J. Am. Chem. Soc. 2017, 139, 6843-6846.
(35) Cammarota, R. C.; Clouston, L. J.; Lu, C. C. Leveraging molecular metal–support interactions for H2
and N2 activation. Coord. Chem. Rev. 2017, 334, 100-111.
(36) Sircoglou, M.; Bontemps, S.; Bouhadir, G.; Saffon, N.; Miqueu, K.; Gu, W.; Mercy, M.; Chen, C.-
H.; Foxman, B. M.; Maron, L.; Ozerov, O. V.; Bourissou, D. Group 10 and 11 Metal Boratranes (Ni, Pd,
Pt, CuCl, AgCl, AuCl, and Au+) Derived from a Triphosphine−Borane. J. Am. Chem. Soc. 2008, 130,
16729-16738.
(37) Takaya, J.; Iwasawa, N. Synthesis, Structure, and Catalysis of Palladium Complexes Bearing a
Group 13 Metalloligand: Remarkable Effect of an Aluminum-Metalloligand in Hydrosilylation of CO2. J.
Am. Chem. Soc. 2017, 139, 6074-6077.
(38) You, D.; Gabbaï, F. P. Tunable σ-Accepting, Z-Type Ligands for Organometallic Catalysis. Trends
in Chemistry 2019, 1, 485-496.
(39) 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.
(40) Cammarota, R. C.; Vollmer, M. V.; Xie, J.; Ye, J.; Linehan, J. C.; Burgess, S. A.; Appel, A. M.;
Gagliardi, L.; Lu, C. C. A Bimetallic Nickel–Gallium Complex Catalyzes CO2 Hydrogenation via the
Intermediacy of an Anionic d10 Nickel Hydride. J. Am. Chem. Soc. 2017, 139, 14244-14250.
(41) Ramirez, B. L.; Sharma, P.; Eisenhart, R. J.; Gagliardi, L.; Lu, C. C. Bimetallic nickel-lutetium
complexes: tuning the properties and catalytic hydrogenation activity of the Ni site by varying the Lu
coordination environment. Chem. Sci. 2019, 10, 3375-3384.
(42) Crespo-Quesada, M.; Cárdenas-Lizana, F.; Dessimoz, A.-L.; Kiwi-Minsker, L. Modern Trends in
Catalyst and Process Design for Alkyne Hydrogenations. ACS Catal. 2012, 2, 1773-1786.
(43) de Vries, J. G.; Elsevier, C. J., eds., The Handbook of Homogeneous Hydrogenation. Wiley-VCH:
Weinheim, 2007.
(44) Swamy, K. C. K.; Reddy, A. S.; Sandeep, K.; Kalyani, A. Advances in chemoselective and/or
stereoselective semihydrogenation of alkynes. Tet. Lett. 2018, 59, 419-429.
ACS Paragon Plus Environment

Page 21 of 25
Journal of the American Chemical Society
1
2
3
4
5
(45) Lindlar, H., Dubuis, R. Palladium Catalyst for the Partial Reduction of Acetylenes. Organic
Syntheses 1966, 46, 89.
6
7
8
(46) Schrock, R. R.; Osborn, J. A. Catalytic hydrogenation using cationic rhodium complexes. II. The
selective hydrogenation of alkynes to cis olefins. J. Am. Chem. Soc. 1976, 98, 2143-2147.
9
(47) van Laren, M. W.; Elsevier, C. J. Selective Homogeneous Palladium(0)-Catalyzed Hydrogenation of
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Alkynes to (Z)-Alkenes. Angew. Chem. Int. Ed. 1999, 38, 3715-3717.
(48) Gianetti, T. L.; Tomson, N. C.; Arnold, J.; Bergman, R. G. Z-Selective, Catalytic Internal Alkyne
Semihydrogenation under H2/CO Mixtures by a Niobium(III) Imido Complex. J. Am. Chem. Soc. 2011,
133, 14904-14907.
(49) LaꢀPierre, H. S.; Arnold, J.; Toste, F. D. Z-Selective Semihydrogenation of Alkynes Catalyzed by a
Cationic Vanadium Bisimido Complex. Angew. Chem. Int. Ed. 2011, 50, 3900-3903.
(50) Tseng, K.-N. T.; Kampf, J. W.; Szymczak, N. K. Modular Attachment of Appended Boron Lewis
Acids to a Ruthenium Pincer Catalyst: Metal–Ligand Cooperativity Enables Selective Alkyne
Hydrogenation. J. Am. Chem. Soc. 2016, 138, 10378-10381.
(51) Gorgas, N.; Brünig, J.; Stöger, B.; Vanicek, S.; Tilset, M.; Veiros, L. F.; Kirchner, K. Efficient Z-
Selective Semihydrogenation of Internal Alkynes Catalyzed by Cationic Iron(II) Hydride Complexes. J.
Am. Chem. Soc. 2019, 141, 17452-17458.
(52) Fürstner, A. trans-Hydrogenation, gem-Hydrogenation, and trans-Hydrometalation of Alkynes: An
Interim Report on an Unorthodox Reactivity Paradigm. J. Am. Chem. Soc. 2019, 141, 11-24.
(53) Leutzsch, M.; Wolf, L. M.; Gupta, P.; Fuchs, M.; Thiel, W.; Farès, C.; Fürstner, A. Formation of
Ruthenium Carbenes by gem-Hydrogen Transfer to Internal Alkynes: Implications for Alkyne trans-
Hydrogenation. Angew. Chem. Int. Ed. 2015, 54, 12431-12436.
(54) Guthertz, A.; Leutzsch, M.; Wolf, L. M.; Gupta, P.; Rummelt, S. M.; Goddard, R.; Farès, C.; Thiel,
W.; Fürstner, A. Half-Sandwich Ruthenium Carbene Complexes Link trans-Hydrogenation and gem-
Hydrogenation of Internal Alkynes. J. Am. Chem. Soc. 2018, 140, 3156-3169.
(55) Radkowski, K.; Sundararaju, B.; Fürstner, A. A Functional‐Group‐Tolerant Catalytic transꢀ
Hydrogenation of Alkynes. Angew. Chem. Int. Ed. 2013, 52, 355-360.
(56) Schleyer, D.; Niessen, H. G.; Bargon, J. In situ 1H-PHIP-NMR studies of the stereoselective
hydrogenation of alkynes to (E)-alkenes catalyzed by a homogeneous [Cp*Ru]+ catalyst. New Journal of
Chemistry 2001, 25, 423-426.
(57) Srimani, D.; Diskin-Posner, Y.; Ben-David, Y.; Milstein, D. Iron Pincer Complex Catalyzed,
Environmentally Benign, E-Selective Semi-Hydrogenation of Alkynes. Angew. Chem. Int. Ed. 2013, 52,
14131-14134.
(58) Tokmic, K.; Fout, A. R. Alkyne Semihydrogenation with a Well-Defined Nonclassical Co–H2
Catalyst: A H2 Spin on Isomerization and E-Selectivity. J. Am. Chem. Soc. 2016, 138, 13700-13705.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 22 of 25
1
2
3
4
5
6
(59) Gramigna, K. M.; Dickie, D. A.; Foxman, B. M.; Thomas, C. M. Cooperative H2 Activation across a
Metal–Metal Multiple Bond and Hydrogenation Reactions Catalyzed by a Zr/Co Heterobimetallic
Complex. ACS Catal. 2019, 9, 3153-3164.
7
8
9
(60) Murugesan, K.; Bheeter, C. B.; Linnebank, P. R.; Spannenberg, A.; Reek, J. N. H.; Jagadeesh, R. V.;
Beller, M. Nickel-Catalyzed Stereodivergent Synthesis of E- and Z-Alkenes by Hydrogenation of
Alkynes. ChemSusChem 2019, 12, 3363-3369.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(61) Karunananda, M. K.; Mankad, N. P. E-Selective Semi-Hydrogenation of Alkynes by
Heterobimetallic Catalysis. J. Am. Chem. Soc. 2015, 137, 14598-14601.
(62) Karunananda, M. K.; Mankad, N. P. Heterobimetallic H2 Addition and Alkene/Alkane Elimination
Reactions Related to the Mechanism of E-Selective Alkyne Semihydrogenation. Organometallics 2017,
36, 220-227.
(63) Muhammad, S. R.; Nugent, J. W.; Tokmic, K.; Zhu, L.; Mahmoud, J.; Fout, A. R. Electronic Ligand
Modifications on Cobalt Complexes and Their Application toward the Semi-Hydrogenation of Alkynes
and Para-Hydrogenation of Alkenes. Organometallics 2019, 38, 3132-3138.
(64) Higashida, K.; Mashima, K. E-Selective Semi-hydrogenation of Alkynes with Dinuclear Iridium
Complexes under Atmospheric Pressure of Hydrogen. Chem. Lett. 2016, 45, 866-868.
(65) Chirik, P.; Morris, R. Getting Down to Earth: The Renaissance of Catalysis with Abundant Metals.
Acc. Chem. Res. 2015, 48, 2495-2495.
(66) White, M. C. Base‐Metal Catalysis: Embrace the Wild Side. Advanced Synthesis & Catalysis 2016,
358, 2364-2365.
(67) Herbert, D. E.; Lionetti, D.; Rittle, J.; Agapie, T. Heterometallic Triiron-Oxo/Hydroxo Clusters:
Effect of Redox-Inactive Metals. J. Am. Chem. Soc. 2013, 135, 19075-19078.
(68) Lin, P.-H.; Takase, M. K.; Agapie, T. Investigations of the Effect of the Non-Manganese Metal in
Heterometallic-Oxido Cluster Models of the Oxygen Evolving Complex of Photosystem II: Lanthanides
as Substitutes for Calcium. Inorg. Chem. 2015, 54, 59-64.
(69) Tsui, E. Y.; Agapie, T. Reduction potentials of heterometallic manganese–oxido cubane complexes
modulated by redox-inactive metals. Proc. Natl. Acad. Sci. 2013, 110, 10084-10088.
(70) Tsui, E. Y.; Tran, R.; Yano, J.; Agapie, T. Redox-inactive metals modulate the reduction potential in
heterometallic manganese–oxido clusters. Nat. Chem. 2013, 5, 293.
(71) Cammarota, R. C.; Xie, J.; Burgess, S. A.; Vollmer, M. V.; Vogiatzis, K. D.; Ye, J.; Linehan, J. C.;
Appel, A. M.; Hoffmann, C.; Wang, X.; Young, V. G.; Lu, C. C. Thermodynamic and kinetic studies of
H2 and N2 binding to bimetallic nickel-group 13 complexes and neutron structure of a Ni(η2-H2) adduct.
Chem. Sci. 2019, 10, 7029-7042.
(72) You, D.; Gabbaï, F. P. Tunable σ-Accepting, Z-Type Ligands for Organometallic Catalysis. Trends
in Chemistry 2019, 1, 485-496.
(73) Shannon, R. Revised effective ionic radii and systematic studies of interatomic distances in halides
and chalcogenides. Acta Cryst. Sec. A 1976, 32, 751-767.
ACS Paragon Plus Environment

Page 23 of 25
Journal of the American Chemical Society
1
2
3
4
5
(74) Pyykkö, P. Additive Covalent Radii for Single-, Double-, and Triple-Bonded Molecules and
Tetrahedrally Bonded Crystals: A Summary. J. Phys. Chem. A 2015, 119, 2326-2337.
6
7
8
(75) Li, L.; Herzon, S. B. Temporal separation of catalytic activities allows anti-Markovnikov reductive
functionalization of terminal alkynes. Nat. Chem. 2014, 6, 22-27.
9
(76) Kaeffer, N.; Larmier, K.; Fedorov, A.; Copéret, C. Origin of ligand-driven selectivity in alkyne
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
semihydrogenation over silica-supported copper nanoparticles. J. Catal. 2018, 364, 437-445.
(77) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J. Assessment of the "T1 criterion" for
distinguishing between classical and nonclassical transition-metal hydrides: hydride relaxation rates in
tris(triarylphosphine)osmium tetrahydrides and related polyhydrides. J. Am. Chem. Soc. 1991, 113, 4173-
4184.
(78) Luther, T. A.; Heinekey, D. M. Synthesis, Characterization, and Reactivity of Dicationic Dihydrogen
Complexes of Osmium and Ruthenium. Inorg. Chem. 1998, 37, 127-132.
(79) Thomas, C. M.; Napoline, J. W.; Rowe, G. T.; Foxman, B. M. Oxidative addition across Zr/Co
multiple bonds in early/late heterobimetallic complexes. Chem. Commun. 2010, 46, 5790-5792.
(80) 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.
(81) Bianchini, C.; Meli, A.; Peruzzini, M.; Frediani, P.; Bohanna, C.; Esteruelas, M. A.; Oro, L. A.
Selective hydrogenation of 1-alkynes to alkenes catalyzed by an iron(II) cis-hydride η2-dihydrogen
complex. A case of intramolecular reaction between η2-H2 and sigma-vinyl ligands. Organometallics
1992, 11, 138-145.
(82) Bianchini, C.; Meli, A.; Peruzzini, M.; Vizza, F.; Zanobini, F.; Frediani, P. A homogeneous iron(II)
system capable of selectivity catalyzing the reduction of terminal alkynes to alkenes and buta-1,3-dienes.
Organometallics 1989, 8, 2080-2082.
(83) Kubas, G. J. Fundamentals of H2 Binding and Reactivity on Transition Metals Underlying
Hydrogenase Function and H2 Production and Storage. Chem. Rev. 2007, 107, 4152-4205.
(84) Harman, W. H.; Peters, J. C. Reversible H2 Addition across a Nickel–Borane Unit as a Promising
Strategy for Catalysis. J. Am. Chem. Soc. 2012, 134, 5080-5082.
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