Intermetallic Nanocatalysts from Heterobimetallic Group 10-14 Pyridine-2-thiolate Precursors

Article abstract of DOI:10.1021/acs.organomet.9b00803

Intermetallic compounds are atomically ordered inorganic materials containing two or more transition metals and main-group elements in unique crystal structures. Intermetallics based on group 10 and group 14 metals have shown enhanced activity, selectivity, and durability in comparison to simple metals and alloys in many catalytic reactions. While high-temperature solid-state methods to prepare intermetallic compounds exist, softer synthetic methods can provide key advantages, such as enabling the preparation of metastable phases or of smaller particles with increased surface areas for catalysis. Here, we study a generalized family of heterobimetallic precursors to binary intermetallics, each containing a group 10 metal and a group 14 tetrel bonded together and supported by pincer-like pyridine-2-thiolate ligands. Upon thermal decomposition, these heterobimetallic complexes form 10-14 binary intermetallic nanocrystals. Experiments and density functional theory (DFT) computations help in better understanding the reactivity of these precursors toward the synthesis of specific intermetallic binary phases. Using Pd₂Sn as an example, we demonstrate that nanoparticles made in this way can act as uniquely selective catalysts for the reduction of nitroarenes to azoxyarenes, which highlights the utility of the intermetallics made by our method. Employing heterobimetallic pincer complexes as precursors toward binary nanocrystals and other metal-rich intermetallics provides opportunities to explore the fundamental chemistry and applications of these materials.

Full text of DOI:10.1021/acs.organomet.9b00803

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Article
Intermetallic Nanocatalysts from Heterobimetallic Group 10−14
Pyridine-2-thiolate Precursors
Carena L. Daniels, Megan Knobeloch, Philip Yox, Marquix A. S. Adamson, Yunhua Chen, Rick W. Dorn,
Hao Wu, Guoquan Zhou, Huajun Fan, Aaron J. Rossini, and Javier Vela*
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ABSTRACT: Intermetallic compounds are atomically ordered
inorganic materials containing two or more transition metals and
main-group elements in unique crystal structures. Intermetallics
based on group 10 and group 14 metals have shown enhanced
activity, selectivity, and durability in comparison to simple metals
and alloys in many catalytic reactions. While high-temperature
solid-state methods to prepare intermetallic compounds exist,
softer synthetic methods can provide key advantages, such as
enabling the preparation of metastable phases or of smaller
particles with increased surface areas for catalysis. Here, we study a
generalized family of heterobimetallic precursors to binary
intermetallics, each containing a group 10 metal and a group 14 tetrel bonded together and supported by pincer-like pyridine-2-
thiolate ligands. Upon thermal decomposition, these heterobimetallic complexes form 10−14 binary intermetallic nanocrystals.
Experiments and density functional theory (DFT) computations help in better understanding the reactivity of these precursors
toward the synthesis of specific intermetallic binary phases. Using Pd2Sn as an example, we demonstrate that nanoparticles made in
this way can act as uniquely selective catalysts for the reduction of nitroarenes to azoxyarenes, which highlights the utility of the
intermetallics made by our method. Employing heterobimetallic pincer complexes as precursors toward binary nanocrystals and
other metal-rich intermetallics provides opportunities to explore the fundamental chemistry and applications of these materials.
hydrogenation,36 and Ni3Sn catalyzes the decomposition of
INTRODUCTION
■
methanol for the production of hydrogen.37
Intermetallic compounds are composition-specific materials
where two or more metals crystallize in ordered structures that
Other binary and multinary group 10−14 intermetallics
promise to be just as interesting. Germanides Ni19Ge12, Pt3Ge,
are distinct from those adopted by the constituent elements.
and PtGe2 catalyze the hydrogen evolution reaction (HER)
Along with adopting unique structures, intermetallic com-
under acidic conditions.38,39 Pd2Ge nanoparticles are active in
pounds are known to possess many interesting properties
ethanol oxidation for fuel cell applications,40 where they are
including magnetism, superconductivity, shape memory,
hydrogen sorption, and catalytic activity.1−4 For example, in
stable for up to 250 cycles. Nickel silicides have also received
attention: for example, Ni31Si12, Ni2Si, NiSi2, and NiSi in
comparison to simple, single-element metals, binary group 10−
hydrogenation reactions.41−45 Ni2Si and Pd2Si were inves-
14 intermetallics show enhanced catalytic activity, selectivity,
tigated for the HER46 and for selective phenylacetylene
and durability.5−7 Among these, Pt−Sn nanocatalysts such as
Pt3Sn and PtSn have been studied for a variety of reactions,
hydrogenation.47 Recently, ternary silicide intermetallics such
including alkane dehydrogenation,8−20 electro-oxidation of
CO, formic acid, and methanol fuel cells,21−26 methane
conversion to ethylene, benzene, and napththalene,27 dehy-
droisomerization of butane to isobutene,28 and highly selective
hydrogenation of nitrobenzene to aniline,29 of 3-nitrostyrene
to 3-aminostyrene,30 and of furfural to furfuryl alcohol.31 Less
studied are Pd−Sn and Ni−Sn binaries. Nonetheless, PdSn is
known to catalyze the electrooxidation of formic acid for fuel
cells32,33 and Pd2Sn the ethanol oxidation reaction (EOR) in
direct alcohol fuel cells,34 as well as the denitration of water.35
In addition, Ni3Sn, Ni3Sn2, and Ni3Sn4 catalyze acetylene
as LaScSi, LaCoSi, and LnNiSi (Ln = La−Nb) have attracted
attention due to their ability to act as electridescompounds
which have anionic-like electrons localized in interstitial sites
which renders them useful in hydrogen storage as well as in
Special Issue: Organometallic Chemistry at Various
Length Scales
Received: November 27, 2019
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catalytic ammonia synthesis at moderate temperature (400 °C)
and pressure (1 atm).48−52
While many high-temperature solid-state methods to
synthesize intermetallic compounds exist, low-temperature
solution-phase methods provide some advantages: for example,
in enabling the preparation of metastable phases or of
intermetallics with smaller particle sizes with increased surface
areas for catalytic applications.2−4 Foremost among such “soft”
chemistry routes is polyol coreduction, where soluble metal
precursors are heated in ethylene glycol or another mildly
reducing solvent, usually to produce the desired intermetallic
compound.53−60 While the use of this method is widespread, it
may result in phase-segregated products due to the different
reduction potentials of the metals. Another synthetic method,
which is a modification of the polyol approach, is coined “ship
in a bottle”.31 In this method, a mesoporous silica shell is
grown around a metal nanoparticle, followed by the addition of
a second metal precursor to form the intermetallic product
inside the mesoporous shell. The intermetallic nanoparticles
made in this way are more stable against aggregation and can
stand higher temperatures during catalysis. However, this is a
longer, multistep process, and the resulting products may also
suffer from partial phase segregation.
Recently, we prepared binary group 10−15 and 11−15
intermetallics through the use of heterobimetallic precursors.61
Rather than starting with two separate metallic precursors and
reducing them together, this approach uses premade
complexes where two metals are already present and are
bonded together.62−68 In this work, we study the solid-state
decomposition of a family of group 10−14 heterobimetallic
complexes as precursors to the preparation of group 10−14
intermetallic compounds. Our approach is related to the use of
single-source precursors (SSPs), which are commonly
employed in chemical vapor deposition in order to lower
decomposition temperatures and prevent phase segrega-
tion.69,70 However, our approach differs from SSPs in that
the stoichiometry of the final inorganic phase that is produced
may or may not be the same as the initial ratio of the two
metals in the heterobimetallic precursor, as dictated by the
reactivity of the specific complex and reaction conditions used
(see below). Here, we use experiments and density functional
theory (DFT) to better understand the effect of the
heteronucleating pyridine-2-thiolate and ancillary phospho-
rus-based ligands on the formation of different binary
intermetallic phases. Finally, we test Pd2Sn nanoparticles
made in this way as catalysts for the selective hydrogenation of
nitroarenes, which demonstrates the utility of the intermetallics
synthesized by our method.
Figure 1. Families of group 10−14 heterobimetallic complexes found
in the CSD.
by similar ligands. We then narrowed our search to include
only complexes with easily substitutedand thus potentially
tunableelements and ligands. Our search yielded four
distinct families of complexes, as shown in Figure 1.72−75
Because of the wide variety of elemental and ligand
substitutions that it afforded, as well as because of its general
ease of synthesis, we decided to focus our efforts on the
bis(pyridine-2-thiolate)-based family of heterobimetallic com-
plexes shown in Figure 1a.
In this family of complexes, a group 10 transition metal (Ni,
Pd, Pt) and a group 14 tetrel (Ge, Sn) are held together by two
bridging pyridine-2-thiolate ligands. The heterobimetallic unit
is further capped by two halides (normally chlorides or
pseudohalides, see below) on the tetrel side and a phosphorus-
based ligand (alkyl or aryl phosphine or phosphite) on the
transition-metal side (Chart 1). Previous studies examined the
appropriate resonance assignment for the pyridine-2-thiolate
ligands in these complexes (see below).72 On the basis of
crystallographic and computational (density functional theory)
data, it was determined that the tetrel (Tt) in these complexes
normally acts as an X-type ligand but can become more Z- or
L-type depending on ligand substitution (see further discussion
below).72
Synthesis of Heterobimetallic Precursors. The group
10−14 heterobimetallic complexes have been made by reacting
Cl2Tt(pyridine-2-thiolate)2 (Tt = Ge, Sn) and M(PPh3)4 (M =
Pd, Pt) (for M = Ni, Ni(cod)2 and an excess of PPh3 are first
mixed in situ) (Scheme 1a).72 We find that this route is
reproducible and often resorted to it in order to prepare ca.
half of the previously reported complexes in Chart 1. Another
route used to make M−Sn complexes (M = Pd, Pt) involves
reacting Sn(Py)2 with MCl2(PPh3)2 (not shown).72 For the
sake of synthetic ease, we have developed a slightly simpler,
more flexible route that consists of reacting together
stoichiometric amounts of Cl2Tt(pyridine-2-thiolate)2 (Tt =
Ge, Sn), a soluble zerovalent metal precursor M(L)2 (M = Pd,
Pt), and 1 equiv of PR3 (phosphine, R = Ph, Cy, nBu;
phosphite, R = OPh) (Scheme 1b). By avoiding the use of
excess phosphine, we find that this route enables an easier
preparation and purification of a wider range of hetero-
bimetallic complexes.
RESULTS AND DISCUSSION
■
Precursor Design and Screening. A family of pyridine-2-
thiolate-based heterobimetallic complexes containing the tetrel
elements Ge and Sn bonded to the transition metals Ni, Pd,
and Pt, pictured in Figure 1a, was synthesized and thermally
decomposed to form intermetallic nanoparticles. In order to
find suitable heterobimetallic complexes to screen as possible
single-source precursors to group 10−14 intermetallics, we
searched the Cambridge Structural Database (CSD).71 First,
we looked for crystallographically characterized complexes that
contained both group 10 and group 14 metals, ideally already
bonded to each other. We did not place any restrictions on the
metal−metal bond distance or the order of the bond. Next, we
grouped the initial results into families of complexes supported
Using this approach, we were able to expand the range of
pyridine-2-thiolate heterobimetallic complexes to Ge−Ni and
B
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a
Chart 1. Family of Pyridine-2-thiolate-Based Heterobimetallic Complexes Studied as Precursors in This Work
a
Oak Ridge Thermal Ellipsoid Plot (ORTEP) diagrams at 50% probability are shown for compounds 1, 3, 8, and 9, which were crystallographically
characterized in this work (see the Experimental Section). Compounds 272 and 4−772,81 were reported previously.
“scrambled” PnBu3 complex 9 obtained by single-crystal X-ray
diffraction analysis are shown in Chart 1 (also Table 1 and
Table S1).
Scheme 1. Synthetic Routes to Bis(pyridine-2-thiolate)
Heterobimetallic Complexes
The heterobimetallic complexes 1, 3, and 8 display a
pseudo-trigonal-bipyramidal geometry around the tetrel (Tt),
with the pyridine nitrogens (Ns) in pseudoaxial positions (the
N−Tt−N angles are between 163 and 170°; see Table 1). In
turn, the two chlorides and the group 10 metal (M) occupy
pseudo-equatorial positions around Tt (the X−Tt−X and X−
Tt−M angles are between 100 and 110° and ca. 126°,
respectively). Interestingly, the scrambled compound 9
containing the tripodal tris(pyridine-2-thiolate) (Tt(PyS)3)
ligand is a special case. Specifically, the N on the “free” PyS
arm points toward the heterobimetallic Sn−Pd core. Curious
about whether this was due to any N−Sn or N−Pd
interactions, we measured both of these distances and
compared them each to the corresponding sum of their van
der Waals radii (Table 1). Both of these distances were under
this sum (2.6463(58) Å vs 3.85 Å for N−Sn and 3.5643(54) Å
vs 3.65 Å for N−Pd),76 suggesting that there is, indeed, N−Sn
and N−Pd bonding interactions in the solid state (thus
increasing the apparent coordination number around Sn to 6
and that around Pd to 5 in this complex). Interestingly, 9
shows only one set of PyS resonances by 1H and 13C NMR in
dmso-d6, suggesting fast exchange of the bound and “free” PyS
arms in solution. To best describe the geometry around the
group 10 transition metals (M) in these complexes, we used a τ
parameter: τ = (∑(angles around M) − 360°)/(438° −
360°).77,78 In this way, a perfect square-planar geometry has τ
= 0, whereas a perfect tetrahedral geometry has a τ = 1. In all of
the heterobimetallic complexes, the sum of the angles of M is
Ge−Pt analogues for the first time (Table 1 and Table S1).
The molecular structures of two of these new complexes, 1 and
3, respectively, were obtained by single-crystal X-ray diffraction
and are shown in Chart 1. In addition, we used this approach
in altering the nature of the M-capping phosphine ligand. In
the case of the Sn−Pd complexes, PPh3 was previously
exchanged in solution by exposure to an excess of the better σ-
donor PCy3.72 Here, we were able to use the new approach
outlined in Scheme 1b to explore a wider range of
heterobimetallic Sn−Pd complexes capped with PCy3, P-
(OPh)3, and PnBu3. Interestingly, during the isolation of the
PnBu3 complex, a byproduct where one of the tin-bound
chlorides (Cl−) was replaced by a pyridine-2-thiolate group
formally a pseudohalidecrystallized preferentially. The
molecular structures of the P(OPh)3 complex 8 and of the
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Table 1. Selected Bond Lengths (Å) and Angles (deg) for New Group 10−14 Heterobimetallic Complexes
a
param
Cl2Ge(μ-PyS)2NiPPh3 (1)
Cl2Ge(μ-PyS)2PtPPh3 (3)
Cl2Sn(μ-PyS)2PdP(OPh)3 (8)
Cl(PyS)Sn(μ-PyS)2PdPnBu3 (9)
M−Tt
M−S1
M−S2
M−P
Tt−N1
Tt−N2
Tt−Cl1
Tt−X (X = Cl2, S)
Pd- - -N3
2.2766(16)
2.148(2)
2.143(2)
2.2245(18)
2.157(4)
2.161(4)
2.2084(19)
2.2283(19)
n.a.
2.379(3)
2.2990(7)
2.2809(8)
2.3262(6)
2.195(2)
2.137(2)
2.2157(6)
2.2001(7)
n.a.
2.5067(3)
2.2939(7)
2.3001(7)
2.3017(7)
2.296(2)
2.311(2)
2.3823(7)
2.3608(6)
n.a.
2.5087(5)
2.3016(13)
2.2885(13)
2.3243(14)
2.362(4)
2.304(4)
2.4237(13)
2.4744(16)
b
c
3.5643(54)
c
Sn- - -N3
n.a.
n.a.
n.a.
2.6463(58)
P−M−Tt
P−M−S1
P−M−S2
S1−M−S2
S1−M−Tt
S2−M−Tt
M−Tt−N1
M−Tt−N2
N1−Tt−N2
178.41(5)
89.63(6)
87.04(6)
173.79(6)
91.41(6)
91.89(6)
98.62(10)
98.257(11)
163.12(13)
109.80(6)
125.98(6)
124.22(6)
174.895(16)
89.26(2)
89.10(2)
177.26(2)
90.20(2)
91.633(19)
96.40(6)
98.43(6)
165.14(9)
108.25(3)
126.28(2)
125.39(2)
176.121(19)
92.10(2)
89.07(2)
177.67(3)
89.337(17)
89.368(19)
95.17(5)
94.90(6)
169.60(8)
100.56(2)
125.573(18)
133.815(19)
177.83(4)
91.18(5)
88.62(5)
174.38(5)
90.94(4)
89.23(4)
94.11(11)
96.05(10)
169.61(15)
95.08(6)
118.85(4)
145.78(4)
b
Cl1−Tt−X
Cl1−Tt−M
X−Tt−M
M = Ni, Pd, Pt (group 10); Tt = Ge, Sn (group 14). X = Cl , S. ∑(van der Waals radii): Pd + N = 3.65 Å, Sn + N = 3.85 Å.76
a
b
c
approximately 360°, and thus τ is very close to ≃0 (fairly
square planar).
Insights from Computations. In order to gain a better
understanding of the Ge−M and Sn−M heterobimetallic
complexes (M = Ni, Pd, Pt), as well as their hypothetical Si−M
analogues and a few Sn−Pd complexes with different
phosphorus ligands, we studied them computationally using
density functional theory (DFT). For all complexes, the
popular B3LYP with hybrid exchange functional79 was used to
optimize their molecular geometries, while an additional
functional, TPSS,80 was used on Cl2Sn(μ-PyS)PdPPh3 (5)
(see the Experimental Section). The optimized geometries
show a relatively flat Tt−(μ-pyS)2−M frame, with the
phosphorus center adopting a pseudo-tetrahedral geometry.
A careful look at some of the key computationally optimized
Tt−M, Tt−N, M−S, M−P (Tt = Si, Ge, Sn; M = Ni, Pd, Pt)
bond distances show that they overestimate those determined
experimentally from the crystal structures by ca. 2.6%;
however, both follow similar trends (Figure 2 and Table 2).
Generally, Tt−N bond distances stay relatively the same for
compounds with the same group 14 element (Tt). Tt−M, M−
S, and M−P bond distances increase as the group 10 metal
(M) becomes larger. When M is kept constant, increasing the
size of the Tt leads to longer Tt−M and Tt−N bond distances,
while the M−S and M−P distances stay the same.
Figure 2. Selected bond distances for (a) Cl2Tt(μ-PyS)NiPPh3, (b)
Cl2Tt(μ-PyS)PdPPh3, and (c) Cl2Tt(μ-PyS)PtPPh3 with Tt = Si, Ge,
Sn and (d) Cl2Sn(μ-PyS)PdPR3 with R = Cy, Ph, OPh, nBu.
group 10−14 heterobimetallic complexes. Interestingly, the
cost of breaking one of the Tt−N(pyridine) bonds is relatively
small, ranging from 1.38 kcal/mol for the Cl2Si(μ-PyS)NiPPh3
complex to 8.99 kcal/mol for the Cl2Ge(μ-pyS)2PtPPh3
complex and becoming prohibitively high for the Cl2Sn(μ-
pyS)2PtPPh3 complex. We were also able to estimate overall
metal (M) binding energies according to the general Scheme 2.
In this way, the metal binding energies calculated for Cl2Sn(μ-
In the case of the Sn−Pd complexes, the C−S bond distance
increases in the order R = OPh < Cy ≈ Ph < nBu. We also
calculated the Wiberg bond orders for these complexes to
measure the electron density between the atoms.82 The Wiberg
bond order at S reveals that this is accompanied by a shifting of
the electron density. The bond order around S increases in the
order R = Cy ≈ nBu < Ph < OPh. Natural bond orbital (NBO)
analysis suggests that the electron density increases at the S
center in the order R = OPh < Ph < nBu < Cy.
pyS)2Pd(PR3) increase in the order PnBu3 < PCy3
<
P(NMe2)3 < P(OPh)3 < P(C6F5)3 < PPh3 (Table 2).
Importantly, using computations we were able to assess the
relative strengths of different interactions within a series of
Thermolysis of Heterobimetallic Precursors. To assess
their potential as precursors to intermetallic compounds, we
D
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a
Table 2. Calculated Structural and Thermodynamic Data for Group 10−14 Heterobimetallic Complexes
a
b
M−Tt distance M−P distance M−S distance Tt−N distance
M−ligand binding energy
M−P binding energy
compound
(Å)
(Å)
(Å)
(Å)
(kcal/mol)
(kcal/mol)
Cl2Ge(μ-PyS)2NiPPh3
(1)
2.322
2.346
2.234
2.208
−794.4
−8.87
Cl2Ge(μ-PyS)2PdPPh3
(2)
2.416
2.453
2.370
2.216
−815.4
−8.40
Cl2Ge(μ-PyS)2PtPPh3 (3)
Cl2Sn(μ-PyS)2NiPPh3 (4)
Cl2Sn(μ-PyS)2PdPPh3 (5)
Cl2Sn(μ-PyS)2PtPPh3 (6)
Cl2Sn(μ-PyS)2PdPCy3
(7)
2.416
2.463
2.552
2.547
2.551
2.416
2.339
2.416
2.416
2.507
2.376
2.238
2.381
2.381
2.380
2.212
2.313
2.321
2.321
2.321
−847.2
−786.8
−809.0
−839.4
−798.8
−17.3
−9.24
−6.91
−16.7
−20.8
Cl2Sn(μ-
2.542
2.547
2.397
2.442
2.297
2.324
2.313
2.327
−841.7
−842.3
−14.7
−19.9
PyS)2PdP(OPh)3 (8)
Cl2Sn(μ-
PyS)2PdPnBu3(9)
a
b
Using density functional theory (DFT): B3LYP with hybrid exchange functional or TPSS (see the Experimental Section). Average between the
c
two M−S bonds. Average between the two Tt−N bonds.
Scheme 2. Calculation of Overall Metal Binding Energies
from the Dissociation of Cl2Sn(μ-pyS)2Pd(PR3) into
Cl2Sn(μ-PyS)2 (L), Pd, and PR3 (R = Cy, Ph, OPh, nBu)
studied the solid-state thermal decomposition of all the Ge-
and Sn-based heterobimetallic complexes shown in Chart 1.
Differential scanning calorimetry (DSC) and thermogravimet-
ric analysis (TGA) were simultaneously monitored from near
21 °C (room temperature or RT) to 600 °Cin all cases,
under an inert dry nitrogen (N2) atmosphere (Figure 3 and
Table 3). In general, these heterobimetallic compounds
undergo thermally induced decomposition between 240 and
325 °C, before mass loss eases and a plateau is reached around
400−500 °C (Figure 3). After it was cooled to RT, the leftover
solid residue was further analyzed by powder X-ray diffraction
(XRD).
XRD analysis of the solid products left behind after
decomposition shows that both Ni complexes, specifically
Ge−Ni (1) and Sn−Ni (4) complexes, produced oxides and
sulfides, respectively, rather than one of the originally intended
or more desirable intermetallic compounds (see the Support-
ing Information). We suspect that this may be due to the
relative electronegativity of Ni in comparison to the tetrel in
each case (see below), as well as to the higher oxophilicity of
Ni, especially in comparison to Pd and Pt. Also, the Ge−Pt
complex 3 did not form an intermetallic product, which could
be due to a high crystallization temperature because the TGA
trace does not plateau by 600 °C, like the others do.
Nonetheless, all of the other heterobimetallic complexes form
intermetallics when they are heated to at least at 400 °C. In
this way, Pd2Ge, Pd2Sn, and PtSn intermetallic compounds are
produced from the thermolysis of complexes Cl2Ge(μ-
PyS)PdPPh3 (2), Cl2Sn(μ-PyS)PdPPh3 (5), and Cl2Sn(μ-
PyS)PtPPh3 (6), respectively (Figure 4). Notably, there are no
crystalline sulfide impurities detected by XRD in any of these
intermetallic products, suggesting that the sulfur from the
pyridinethiolate ligandand up to half of the tetrel, in some
cases (see below)may be able to come off as a volatile
byproduct during thermolysis. Further, when it is heated to
600 °C, complex 5 produces Pd3Sn2, while complexes 2 and 6
still make the same intermetallic product that they did at 400
°C. Interestingly, many of the remaining intermetallic phases,
Figure 3. TGA and DSC for (a) Cl2Ge(μ-PyS)2PdPPh3 (2), (b)
Cl2Sn(μ-PyS)2PdPPh3 (5), and (c) Cl2Sn(μ-PyS)2PtPPh3 (6) upon
heating from 40 to 600 °C.
with the exception of PtSn, do not contain the same 1:1 metal
to tetrel ratio that is present in the heterobimetallic precursors.
Critically, elemental analysis of binary nanoparticle products
such as Pd2Sn by energy-dispersive X-ray (EDX) diffraction
agrees with the new observed stoichiometry (2:1, see also XRD
below). Further, the 119Sn solid-state (ss) NMR spectrum of
Pd2Sn nanocrystals exhibits a broad NMR signal with the
E
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Table 3. Thermolysis of Group 10−14 Heterobimetallic Complexes
a
b
precursor
amt (g/mol)
Tdec (°C)
XRD (temp (°C))
Ni2GeO4 (600)
Pd2Ge (400, 600)
res mass Δ (%)
Cl2Ge(μ-PyS)2NiPPh3 (1)
Cl2Ge(μ-PyS)2PdPPh3 (2)
Cl2Ge(μ-PyS)2PtPPh3 (3)
Cl2Sn(μ-PyS)2NiPPh3 (4)
Cl2Sn(μ-PyS)2PdPPh3 (5)
Cl2Sn(μ-PyS)2PtPPh3 (6)
Cl2Sn(μ-PyS)2PdPCy3 (7)
Cl2Sn(μ-PyS)2PdP(OPh)3 (8)
Cl(PyS)Sn(μ-PyS)2PdPnBu3 (9)
684.82
732.55
821.21
730.90
778.63
867.29
796.77
826.63
233
244
291
266
294
322
250
260
300
32
6.9
30
3.1
10
15
14
12
amorph (400, 600), Pt (800)
NiS + Ni3Sn2S2 + SnS + Ni (400, 600)
Pd2Sn (400), Pd3Sn2 (600)
c
PtSn + Pt3Sn (400, 600)
PdSn (400, 600)
Pd2Sn (400), Pd20Sn13 (600)
Pd2Sn (400, 600)
793.36
−10
a
b
c
Thermal decomposition onset from TGA/DSC. Difference between mass left after TGA and mass of M + Tt: Δ = TGAmf − theominor. Minor
impurity.
Figure 5. Different resonance structures for Cl2Tt(pyridine-2-
thiolate)2 showing that the Tt can act as a Z-, L-, or X-type ligand
toward M (the phosphorus-based ligand always acts as an L-type
ligand).72,84 The chart shows the difference in Pauling electro-
negativities between the group 10 metal (M) and tetrel (Tt) in each
case, calculated as ΔλP = λM − λTt.
Figure 4. Selected powder XRD patterns and main product unit cells
for solids obtained after the decomposition of (a) Cl2Ge(μ-
PyS)2PdPPh3 (2) (600 °C), (b) Cl2Sn(μ-PyS)2PdPPh3 (5) (600
°C), (c) Cl2Sn(μ-PyS)2PdPPh3 (5) (400 °C), and (d) Cl2Sn(μ-
PyS)2PtPPh3 (6) (600 °C, * = Pt3Sn). Standard patterns are shown in
red (Pd2Ge 52052, Pd3Sn2 197134, Pd2Sn 158364, PtSn 42593).87
maximum intensity observed near the same 119Sn chemical
shift as that observed for Pt3Sn and PtSn intermetallics and is
free of unreacted heterobimetallic precursor (Figure S15).83
Therefore, we conclude that some of the tetrel content in the
original heterobimetallic complexes is lost as a volatile material
during thermal decomposition, along with the organic ligands.
To better understand why in some cases we obtain sulfides
and oxides and in other cases we obtain intermetallics, we
examined the different resonance structures of the tetrel ligand
and compared them to the electronegativity difference between
the metal and tetrel elements in each case (Figure 5). When
the two pyridine nitrogens are negatively charged and both
(thione) sulfurs are neutral, the tetrel or group 14 (IV)
element (Tt) acts as a Lewis acid or Z-type ligand toward the
group 10 metal (M). When the two pyridine nitrogens are
neutral and both (thiolate) sulfurs are negatively charged, the
Tt acts a Lewis base or L-type ligand toward M. In the
intermediate case, where one each of either of the two
nitrogens and either of the two sulfurs is negative and the other
one is neutral, Tt may be considered as an X-type ligand
toward M.72,84
than the metal (M) and thus Tt will behave more like a Z-type
ligand. In this case, the sulfur atoms more strongly donate their
electron density toward the metal, strengthening the M−S
interactions. In all other cases, this electronegativity difference
is positive, meaning that M is more electronegative than Tt,
and thus Tt will behave more like an L-type ligand. In this case,
it is the more volatile pyridine nitrogens that interact more
strongly with the tetrel, rather than the sulfur atoms interacting
with the metal. This provides a possible framework from which
we may be able to rationalizeor at least predictwhy it is
more favorable to form intermetallics, with no sulfur
incorporation, in the latter case.
Interestingly, there are well-defined trends in the decom-
position temperatures (Tdec) across many of the precursors. All
precursors decompose between 244 and 322 °C, with Tdec
increasing on going down the group 10 transition metals
(Figure 6a and Table 3). Also, Tdec increases in going from Ge
to Sn complexes. A comparison between the remaining mass
measured by TGA and the theoretical weight of inorganic Tt +
M metals in each precursor reveals the presence of excess mass
left over after thermolysis, with this excess increasing from Ni
to Pt (Figure 6b). Using Raman spectroscopy, we have
previously shown that this is due to the formation of graphitic
carbon on the surface of the intermetallic nanoparticles.61
Conveniently, this graphitic impurity can be very easily
In the cases where sulfides/oxides are obtained, specifically
for with the Ge−Ni (1) and Sn−Ni (4) complexes, the
electronegativity difference between Ni and Sn or Ge is
negative, meaning that the tetrel (Tt) is more electronegative
F
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complexes were decomposed at both 400 and 600 °C under
inert conditions. At 400 °C, all of the complexes give Pd2Sn as
the intermetallic product, except that with PCy3 the product is
the PdSn phase. At 600 °C, PCy3 still produces PdSn. The
PPh3 complex gives Pd3Sn2, the P(OPh)3 complex gives
Pd20Sn13, and the PnBu3 complex forms Pd2Sn (Figure 8 and
Figure 6. (a) Decomposition temperatures and (b) difference
between percent mass left after TGA and theoretical mass of M +
Tt for different pyridine-2-thiolate heterobimetallic complexes (Tt =
Ge, Sn; M = Ni, Pd, Pt).
removed, if necessary, by plasma cleaning.85,86 Even more
importantly, the presence of such an impurity does not
preclude nor impede catalytic activityor other possible
applicationsof these intermetallic compounds (see below).
On the basis of these data together, our method of choice to
routinely prepare phase-pure Pd2Sn nanocrystalsfor exam-
pleis the solid-state thermolysis of Cl2Sn(μ-PyS)PdPPh3 at
400 °C under N2. As shown in Figure 7, this typically produces
Figure 8. Formation of Pd−Sn binary phases as a function of
thermolysis temperature (600 °C) and homoleptic phosphorus ligand.
Figure S12). Interestingly, the level of incorporation of Sn in
the intermetallic product correlates well with our calculated
metal binding energies (see above). These predict that Sn and
Pd are held together increasingly more strongly in the
following order: PnBu3 < P(OPh)3 < PPh3 < PCy3. Thus,
when PCy3 is used, Pd and Sn are bound most strongly,
resulting in the retention of the 1:1 Pd:Sn stoichiometry from
the precursor through thermal decomposition and into the
product. As the binding energy decreases, Sn is not held as
strongly, and some of it is lost upon thermolysis, resulting in
more Sn deficient intermetallic products (Pd3Sn2, Pd20Sn13,
and Pd2Sn).
Intermetallic Catalysts and Selectivity. Because catal-
ysis is one of the most remarkable and sought-after
applications of many intermetallic compounds, we tested
whether the binary intermetallic nanocrystals prepared by our
heterobimetallic precursor approach could be catalytically
active. Specifically, we tested Pd2Sn against the reduction of
nitroarenes by NaBH4 (Table 4). As a control, we used
recently reported Au/TiO2 heterostructures, which are known
to be active catalysts for this transformation.88 Indeed, our
studies show that Pd2Sn is also a very active catalyst in the
hydrogenation of nitroarenes. However, unlike Au/TiO2,
which is known to covert nitroarenes all the way to the
corresponding anilines, Pd2Sn instead converts nitroarenes
preferentially to just the corresponding azoxyarenes (Figure 9).
Normally, azoxyarenes are intermediates in this nitro- to
Figure 7. Representative TEM of Pd2Sn produced by thermolysis of
Cl2Sn(μ-PyS)PdPPh3 (5) at 400 °C.
Pd2Sn nanocrystals with a 41 16 nm diameter. d spacings
measured for several lattice fringes and multiple EDX area
scans as observed by TEM are consistent with this Pd2Sn
phase, with an overall elemental composition of 67 2 atom %
Pd and 33 2 atom % Sn.
Phosphine Tuning. Heterobimetallic Sn−Pd pyridine-2-
thiolate complexes were synthesized with four different
phosphine ligands: triphenylphosphine, tricyclohexylphos-
phine, triphenyl phosphite, and tri-n-butylphosphine. All four
G
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the monodentate phosphorus ligand and that the Sn:Pd ratio
in the binary products correlates to how well the Sn ligand
binds to Pd. To demonstrate the utility of the intermetallic
binaries made in this way, we used Pd2Sn as a catalyst for the
hydrogenation of nitroarenes. In comparison to typical metallic
catalysts, which usually produce the corresponding anilines,
intermetallic Pd2Sn nanoparticles made through our hetero-
bimetallic precursor approach are selective to the intermediate
azoxyarenes. Using this group 10−14 family of heterobimetal-
lic complexes, we were able to show a versatile approach
toward the synthesis of intermetallic nanocatalysts with unique
selectivity, underscoring how molecular organometallic com-
plexes can be utilized and developed toward the synthesis of
materials with interesting applications.
Table 4. Catalytic Reduction of Nitroarenes
a
b
c
d
cat.
substrate
conv. (%)
product(s)
selec. (%)
Pd2Sn
nitrobenzene
91
B
54
23
15
70
11
15
100
4
C
D
Pd2Sn
p-nitrotoluene
95
B
C
D
Au/TiO2
Au/TiO2
nitrobenzene
p-nitrotoluene
100
97
C
B
C
93
n.d.
n.d.
none
none
nitrobenzene
p-nitrotoluene
9.7
0.9
n.d.
n.d.
a
b
20 mg of the catalyst. 0.1 mmol of the substrate in 2 mL of ethanol.
c
d
After 16 h at 25 °C. Abbreviations: B = azoxybenzene (R = H) or
4,4′-azoxytoluene (R = Me), C = aniline (R = H) or p-toluidine (R =
Me), and D = azobenzene (R = H) or 4,4′-azotoluene (R = Me) (see
Figure 9).
EXPERIMENTAL SECTION
■
Materials. Bis(1,5-cyclooctadiene)Ni (Ni(COD)2, 98+%),
tetrakis(triphenylphosphine)platinum (Pt(PPh3)4, 98%), bis-
(dibenzylideneacetone)palladium (Pd(dba)2), triphenylphosphine
(PPh3, 99%), triphenyl phosphite (P(OPh)3, 97%), tri-n-butylphos-
phine (PnBu3, 99%), tricyclohexylphosphine (PCy3, 97%), and gold
nanoparticles on titania (1% Au/TiO2) were purchased from Strem.
Tetrahydrofuran (THF, 99.9%, extra dry, anhydrous), sodium
borohydride powder (NaBH4, 98+%), and 4-nitrotoluene (99%)
were purchased from Acros Organics. Nitrobenzene (99%) was
purchased from Oakwood Chemical. Diethyl ether (Et2O, 99.7+%,
anhydrous) was purchased from MilliporeSigma. CDCl3, DMSO-d6,
and CD2Cl2 were purchased from Cambridge Isotope Laboratories.
The tetrel-based ligands Cl2Sn(PyS)2 and Cl2Ge(PyS)2 and the
heterobimetallic complexes Cl2Ge(μ-PyS)2PdPPh3, Cl2Sn(μ-
PyS)2NiPPh3, Cl2Sn(μ-PyS)2PdPPh3, Cl2Sn(μ-PyS)2PtPPh3, and
Cl2Sn(μ-PyS)2PdCy3 were synthesized according to literature
procedures.72,81,99
aminoarene (aniline) transformation. They are important and
useful organic products themselves, as they can be widely used
in industry as dyes, polymerization inhibitors, reducing agents,
chemical stabilizers, and important building blocks in many
natural compounds and functional materials.89−93 Azoxyarenes
have been made through the selective reduction of nitroarenes
or the selective oxidation of anilines.94−98 Yet it is relatively
uncommon for nitroarene reduction to stop at the azoxyarene
intermediate step, making the Pd2Sn nanocrystals made here a
much more interesting catalyst than we could have first
envisioned, due to their catalytic activity and unique selectivity.
Precursor Synthesis. The new heterobimetallic complexes
Cl2Ge(μ-PyS)2NiPPh3, Cl2Ge(μ-PyS)2PtPPh3, Cl2Sn(μ-PyS)2PdP-
(OPh)3, and Cl2Sn(μ-PyS)2PdPnBu3 were synthesized under a
nitrogen atmosphere as follows.
Cl2Ge(μ-PyS)2NiPPh3 (1). Cl2Ge(PyS)2 (100 mg, 0.275 mmol) and
PPh3 (72.1 mg, 0.275 mmol) were added to a solution of
Ni(COD)2(75.6 mg, 0.275 mmol) in THF (5 mL). After it was
stirred for 2 h at RT, the solution was filtered and slowly layered with
5 mL of Et2O. Brownish-yellow X-ray-quality crystals were formed
within 1 day, isolated by decantation, washed with diethyl ether, and
dried under vacuum. Yield: 29 mg (15%). Tdec = 233 °C.
CONCLUSIONS
■
In summary, we have synthesized an expanded family of
bis(pyridine-2-thiolate)-based heterobimetallic complexes that
contain a group 10 transition metal (Ni, Pd, or Pt) bonded to a
tetrel element (Ge or Sn). Upon thermal decomposition, these
heterobimetallic complexes form a variety of group 10−14
binary intermetallics. To better understand these precursors,
we performed DFT calculations to identify geometry trends
and to understand the behavior and binding of the Tt-based
pyridine-2-thiolate ligand. We also discovered that different
Sn−Pd intermetallic phases could be produced by changing
Cl2Ge(μ-PyS)2PtPPh3 (3). Cl2Ge(PyS)2 (60 mg, 0.165 mmol) was
added to a solution of Pt(PPh3)4 (200 mg, 0.161 mmol) in THF (4
Figure 9. Reduction of nitroarenes (R = H, Me) with NaBH4 in ethanol catalyzed by Pd2Sn nanocrystals and Au/TiO2.
H
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mL). After 1 h of stirring at 50 °C, the solution was filtered. Orange
single crystals suitable for X-ray diffraction were obtained by slow
vapor diffusion of Et2O for 3 days at RT. Yield: 35 mg (26%). H
inserted into the furnace of the TGA-DSC under an Ar atmosphere.
After an initial 5 min isothermal step near RT, the temperature was
increased to 600 °C at a ramp rate of 10 °C/min. After it was cooled
to room temperature, the remaining solid was removed and analyzed
by powder X-ray diffraction (XRD).
Characterization. Structural. Single crystal X-ray diffraction
(XRD) measurements were performed at 173(2) K using a Bruker
APEX-II CCD area detector diffractometer (Cu Kα radiation, λ =
1.54178 Å) for Cl2Ge(μ-PyS)2NiPPh3 (1) and Cl2Ge(μ-PyS)2PtPPh3
(3). For Cl2Sn(μ-PyS)2PdP(OPh)3 (8) and Cl2Sn(μ-PyS)2PdPnBu3
(9), a Bruker Venture D8 diffractometer with a Photon CMOS
detector was used at 173(2) K (Mo Kα radiation, λ = 0.71073 Å).
Powder XRD. Powder XRD was collected on a Rigaku Ultima IV
diffractometer using a Cu Kα radiation source (40 kV, 44 mA). Each
sample was prepared by smearing 2−5 mg of powder onto a
backgroundless quartz slide.
1
NMR (CDCl3, 21 °C): δ 6.99 (m, 2H, C5H PyS), 7.55 (m, 2H, C4H
PyS), 7.40−7.50 (mm, 9H, Phmeta,para), 7.69 (m, 2H, C3H PyS), 7.85
(mm, 6H, Phortho), 8.31 (m, 2H, C6H PyS). 13C{1H} NMR (CDCl3,
21 °C): δ 126.0 (C5 PyS), 128.4 (d, OPhortho), 131.3 (C3 PyS), 134.4
(d, OPhmeta), 135.0 (br, OPhipso), 138.6 (br, OPhpara), 143.8 (C4
PyS), 150.7 (C6 PyS), 160.3 ppm (C2 PyS). 31P{1H} NMR (CDCl3,
21 °C): δ 10.7 (J(195Pt−31P) = 3171 Hz). 195Pt{1H} NMR (CDCl3,
21 °C): δ −4961 (J(31P−195Pt) = 3173 Hz). Anal. Found (calcd): C,
40.95 (41.26); H, 2.82 (2.69); N, 3.41 (3.66). Tdec = 291 °C.
Cl2Sn(μ-PyS)2PdP(OPh)3 (8). Cl2Sn(PyS)2 (100 mg, 0.244 mmol)
and P(OPh)3 (0.1 mL, 0.381 mmol) were added to a solution of
Pd(dba)2 (140 mg, 0.243 mmol) in THF (5 mL). The solution was
stirred for 1 h at RT and filtered. Red crystals suitable for X-ray
diffraction were obtained by slow vapor diffusion of Et2O for 2 days at
RT. Yield: 159 mg (79%). 1H NMR (CDCl3, 21 °C): δ 7.00 (m, 2H,
C5H PyS), 7.10−7.21 (mm, 15H, OPhortho,meta,para), 7.39 (m, 2H,
C4H PyS), 7.65 (m, 2H, C3H PyS), 8.47 (m, 2H, C6H PyS).
13C{1H} NMR (CDCl3, 21 °C): δ 118.6 (C5 PyS), 120.9 (d,
OPhmeta), 124.8 (br, OPhipso), 125.1 (C3 PyS), 129.9 (d, OPhortho),
138.1 (C6 PyS), 145.6 (C4 PyS), 151.3 (br, OPhpara), 157.6 ppm (C2
PyS). 31P{1H} NMR (CDCl3, 21 °C): δ 127.8 (br). 31P{1H} ssNMR
(21 °C): δ 127.8 (br). Tdec = 260 °C.
Spectroscopy. NMR spectroscopy was recorded on a Bruker
Avance III 600 spectrometer at room temperature at 600.39 MHz for
1H, 150.97 MHz for 13C, 243.04 MHz for 31P, 223.89 MHz for 119Sn,
1
and 129.06 MHz for 195Pt. H and 13C NMR shifts are given in ppm
and referenced to residual protonated solvent signals. 31P, 119Sn, and
195Pt NMR shifts are given in ppm and referenced to the 1H spectrum
residual solvent peaks using indirect referencing.112
Intermetallic Catalysis. For each reaction, the nitroarene
substrate (nitrobenzene or 4-nitrotoluene, 0.1 mmol) was placed in
a 5 mL vial, followed by the addition of ethanol (2 mL), catalyst (Au/
TiO2 or Pd2Sn, 10−20 mg), and NaBH4 (0.6 mmol). The reaction
mixture was stirred for 16 h. During the reaction, small aliquots (∼0.2
mL) were taken at specific intervals, filtered, and diluted for analysis
by gas chromatography−mass spectroscopy (GCMS).
Cl2Sn(μ-PyS)2PdPnBu3 (9). Cl2Sn(PyS)2 (100 mg, 0.244 mmol)
and PnBu3 (0.1 mL, 0.405 mmol) were added to a solution of
Pd(dba)2 (140 mg, 0.243 mmol) in THF (5 mL). The solution was
stirred for 2 h at RT and filtered. Orange X-ray-quality crystals were
1
obtained by slow solvent evaporation. Yield: 90 mg (47%). H NMR
(DMSO-d6, 21 °C): δ 7.37 (m, 3H, C5H PyS), 7.47 (m, 3H, C4H
PyS), 7.80 (m, 3H, C3H PyS), 8.41 (m, 3H, C6H PyS), 1.10−1.60
(18H, C3CH2CH2CH2, PnBu3), 0.87 (m, 9H, C4CH3 PnBu3).
13C{H} NMR (DMSO-d6, 21 °C): δ 125.7 (C5 PyS), 128.8 (C3
PyS), 130.5 (C6 PyS), 134.7 (C4 PyS), 142.8 (C2 PyS), 25.8
(C1CH2 PnBu3), 23.4 (C2CH2 PnBu3), 13.6 (C3CH2 PnBu3), 13.2
(C4CH3 PnBu3). 31P{1H} NMR (DMSO-d6, 21 °C): δ 36.4 (s). Tdec
= 300 °C.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.9b00803.
Computational data (frontier orbitals), crystallographic
data, full thermal analysis and powder XRD measure-
ments, particle size histogram for Pd2Sn, and solid-state
(ss) NMR measurements for Cl2Sn(μ-PyS)2PdP(OPh)3
(5) and Pd2Sn (PDF)
Calculations. All molecules were constructed using GaussView
and optimized with the Gaussian 09 package.100 The popular B3LYP
with hybrid exchange functional79 was used for all geometry
optimization and frequency calculations. Additional calculations
were performed on the Cl2Sn(μ-PyS)2PdPPh3 complex using the
TPSS functional,80 in order to compare the results to the B3LYP DFT
functional. One of the biggest deficiencies in DFT methods is the
long-range dispersion correction in B3LYP, which is 1.05 and rather
large in comparison with other DFT functionals.101 In comparison to
other density functionals, TPSS generally gives excellent results for a
wide range of systems and properties, correcting overestimated PKZB
(Perdew−Kurth−Zupan−Blaha) bond distances in molecules, hydro-
gen-bonded complexes, and ionic solids. The LANL2DZ basis set
with effective core potentials (ECP) was used for the metal (Ni, Pd,
Pt) and tetrel (Ge and Sn) elements102,103 and the 6-31G(d,p) basis
set for the rest of elements,104,105 for the initial geometry
optimization. Other comparisons were made with the Stuttgart ESC
1997 ECP basis set for Pd,106 Stuttgart ELC ECP basis set for Sn,107
cc-pvDZ-PP basis set for Pd and Sn,108−110 and aug-cc-pvTZ-PP
diffuse basis set for Pd and Sn.106−108 The basis sets and inputs were
adapted from the “Basis Set Exchange” Web site maintained by The
Molecular Sciences Software Institute.111 All structures were fully
optimized, and frequency analyses were performed to ensure a
minimum was achieved, which had zero imaginary vibrational
frequencies as derived from a vibrational frequency analysis. The
thermodynamic parameters of the reaction such as zero-point
corrected energy (ΔEZPE), enthalpies (ΔH°), and Gibbs free energies
(ΔG°) were calculated at 298.15 K and 1 atm.
Accession Codes
CCDC 1968037−1968040 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
Javier Vela − Department of Chemistry, Iowa State University,
Ames, Iowa 50011, United States; Ames Laboratory, Ames,
Iowa 50011, United States; orcid.org/0000-0001-5124-
6893; Email: vela@iastate.edu
Authors
Carena L. Daniels − Department of Chemistry, Iowa State
University, Ames, Iowa 50011, United States
Megan Knobeloch − Department of Chemistry, Iowa State
University, Ames, Iowa 50011, United States
Philip Yox − Department of Chemistry, Iowa State University,
Ames, Iowa 50011, United States
Precursor Thermolysis. Thermogravimetric analysis and differ-
ential scanning calorimetry (TGA-DSC) were measured on a Netzsch
DSC/TGA instrument (STA449 F1). For each experiment, 5−10 mg
of precursor was placed into an aluminum crucible, which was then
Marquix A. S. Adamson − Department of Chemistry, Iowa
State University, Ames, Iowa 50011, United States
I
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Yunhua Chen − Department of Chemistry, Iowa State
University, Ames, Iowa 50011, United States
Rick W. Dorn − Department of Chemistry, Iowa State
University, Ames, Iowa 50011, United States; Ames Laboratory,
Ames, Iowa 50011, United States
Hao Wu − School of Materials and Chemical Engineering,
Ningbo University of Technology, Ningbo, Zhejiang, People’s
Republic of China
Guoquan Zhou − School of Materials and Chemical
Engineering, Ningbo University of Technology, Ningbo,
Zhejiang, People’s Republic of China
Huajun Fan − College of Chemical Engineering, Sichuan
University of Science and Engineering, Zigong, Sichuan, People’s
Republic of China
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Aaron J. Rossini − Department of Chemistry, Iowa State
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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■
J.V. thanks the U.S. National Science Foundation for a grant
from the Division of Chemistry, Macromolecular, Supra-
molecular, and Nanochemistry Program (1905066). Y.C.,
R.W.D. and A.J.R. were supported by the U.S. Department of
Energy (DOE), Office of Science, Basic Energy Sciences,
Materials Science and Engineering Division. The Ames
Laboratory is operated for the U.S. DOE by Iowa State
University under contract #DE-AC02-07CH11358. We thank
Arkady Ellern and Amrit Venkatesh for assistance with single-
crystal X-ray diffraction and solid-state NMR, respectively.
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https://dx.doi.org/10.1021/acs.organomet.9b00803
Organometallics XXXX, XXX, XXX−XXX