A Ketimide-Stabilized Palladium Nanocluster with a Hexagonal Aromatic Pd7 Core

Article abstract of DOI:10.1021/acs.inorgchem.9b03276

Herein, we report the synthesis and characterization of the mixed-valent, ketimide-stabilized Pd₇ nanosheet, [Pd₇(N=C^tBu₂)₆] (1), via reaction of PdCl₂(PhCN)₂ and Li(N=C^t

Full text of DOI:10.1021/acs.inorgchem.9b03276

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
A Ketimide-Stabilized Palladium Nanocluster with a Hexagonal
Aromatic Pd7 Core
,‡
Andrew W. Cook,† Peter Hrobarik,* Peter L. Damon,† Guang Wu,† and Trevor W. Hayton*,†
́
†Department of Chemistry and Biochemistry, University of California Santa Barbara, Santa Barbara, California 93106, United States
‡Department of Inorganic Chemistry, Faculty of Natural Sciences, Comenius University, SK-84215 Bratislava, Slovakia
S
* Supporting Information
ABSTRACT: Herein, we report the synthesis and character-
ization of the mixed-valent, ketimide-stabilized Pd7 nanosheet,
[Pd7(NCtBu2)6] (1), via reaction of PdCl2(PhCN)2 and
t
Li(NCtBu2). BuCN, isobutylene, and isobutane are also
formed in the reaction. The presence of these products
suggests that Li(NCtBu2) acts as a reducing agent in the
transformation, converting the Pd(II) starting material into
the mixed-valent Pd(I)/Pd(0) product. Complex 1 features a
hexagonal planar [Pd7]6+ core stabilized by six ketimide
ligands, which surround the [Pd7]6+ center in an alternating
up/down fashion. In situ NMR spectroscopic studies, as well
as density functional theory (DFT) calculations, suggest that 1 is formed via the intermediacy of the bimetallic Pd(II) ketimide
complex, [(tBu2CN)Pd(μ-N,C−NC(tBu)C(Me)2CH2)Pd(NCtBu2)] (2). DFT calculations also reveal that 1 is a rare
example of an all-metal aromatic nanocluster with hexagonal symmetry, sustaining a net diatropic ring-current of 10.6 nA/T,
which is similar to that of benzene (11.8 nA/T) or other well-established transition-metal aromatic systems. Finally, we have
found that 1 reacts with Ph3P, cleanly forming the tris-ligated 16-electron Pd(0) phosphine complex, Pd(PPh3)3 (3), suggesting
that 1 could be a useful precatalyst for a variety of cross-coupling reactions.
In an effort to address the synthetic challenge of preparing
Pd nanoclusters, we have been searching for new ligands that
can stabilize low-valent Pd. In this regard, the ketimide ligand
(R2CN−) appears to be a suitable choice. The ketimide
ligand is known to be a good π-acceptor30,31 and thus should
be able to stabilize low-valent metal clusters. Moreover, it is
known to promote metal−metal bonding, as evidenced by the
isolation of the bimetallic complexes [Li(12-crown-4)2]-
[M2(NCtBu2)5] (M = Mn, Fe, Co),32 each of which
contains a strong M−M interaction. Ketimide ligands have also
found use in the stabilization of high-oxidation states as well, as
shown by the isolation of [M(NCtBu2)4] (M = V, Nb, Ta,
Cr, Mo, W, Mn, Fe, Co).30,31,33−36 Moreover, ketimides are
useful co-ligands for olefin polymerization catalysts,37−41 and
they can serve as “masked” nitrenes,42 which can mediate C−H
bond activation chemistry when unmasked. They have also
appeared as intermediates in C−N bond-forming reac-
tions.43,44 Yet, despite these diverse roles, ketimides have not
been previously used to stabilize APNCs.
INTRODUCTION
■
Atomically precise nanoclusters (APNCs) are attracting
attention for their use in a variety of applications, including
quantum computing,1−3 catalysis,4−6 and imaging.7,8 While
most attention has been paid to Ag- and Au-containing
nanoclusters in the past few years,9−11 the synthesis of Pd- and
Pt-containing nanoclusters has been historically important to
the APNC field.12−14 These clusters typically feature a mixture
of CO and PR3 as stabilizing ligands and can achieve large
nuclearities.15 Notable examples include [Pd4(CO)4(OAc)4],16
[Pd10(CO)12(PnBu3)6],17 [Pd23(CO)20(PEt3)10],18 and [(μ12-
Pt)Pd164−xPtx(CO)72(PPh3)20] (x ≈ 7).19 In these cases, the
electron-rich, low-valent M(0) core is likely stabilized by the
presence of the π-accepting CO and PR3 ligands. More
recently, several low-valent Pd nanoclusters that do not contain
CO or phosphine co-ligands have also been reported,20 such as
[Pd3(μ3-C7H7)2]2+,21 [Pd13(μ4-C7H7)6]2+,20,22 and [Pd6(μ-
GePh2)2(CN-2,6-Me2C6H3)8(μ-CN-2,6-Me2C6H3)2].23 Other
noteworthy Pd nanoclusters include the Pd11 nanosheet,
[Pd11(SiiPr)2(SiiPr2)4(CNtBu)10],24,25 the Pd5 nanosheet,
Herein, we report the synthesis and characterization of the
ketimide-stabilized Pd7 nanosheet, [Pd7(NCtBu2)6], dem-
onstrating for the first time that ketimides can promote the
formation of low-valent palladium APNCs.
21
[Pd5(C18H12)2]2−
,
and the Pd6 “bow-tie” cluster,
[Pd6Cl2(SiPh2)2(CNtBu)8].26 While these complexes do not
contain CO or PR3, in many cases they still contain π-
accepting ligands, such as isocyanides,27 tropylium cation
22
(C7H7 ), and silylenes.28,29 That said, Pd nanoclusters are
still relatively rare, and only handful of ligands are known to
stabilize this class of materials.
+
Received: November 7, 2019
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of the Palladium Nanosheet Pd7(NCtBu2)6 (1)
Figure 1. ORTEP diagram of 1. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Pd1−N1 = 1.990(6), Pd1−
N2 = 1.995(6), Pd2−N3 = 1.986(8), Pd2−N1 = 2.015(8), Pd4−N2 = 2.003(4), Pd4−N3 = 2.005(4), Pd1−Pd3 = 2.7024(5), Pd1−Pd4 =
2.7128(8), Pd1−Pd2 = 2.7137(11), Pd2−Pd4 = 2.7018(6), Pd2−Pd3 = 2.7159(6), Pd3−Pd4 = 2.7079(8), avg. N−Pd−N = 172.
equiv of Li(NCtBu2) results in formation of a mixture of the
RESULTS AND DISCUSSION
■
linear, two-coordinate Pt(II) complex, [Pt(NCtBu2)2], and
the bimetallic Pt(II) complex, [(tBu2CN)Pt(μ-N,C-N
C(tBu)C(Me)2CH2)-Pt(NCtBu2)].45 No evidence is ob-
served for the formation of the analogous Pt7 nanosheet,
[Pt7(NCtBu2)6], in that reaction (see further discussion
below).
Reaction of PdCl2(PhCN)2 with 2 equiv of Li(NCtBu2) in
THF results in the formation of a dark green solution. Workup
of the reaction mixture results in the isolation of [Pd7(N
CtBu2)6] (1), in 40% yield as deep green blocks (Scheme 1).
Complex 1 is soluble in pentane, hexanes, Et2O, benzene,
toluene, and THF and somewhat soluble in MeCN, but
quickly decomposes in the presence of CH2Cl2. It is stable as a
solid under inert atmosphere at −25 °C for at least several
months. For comparison, reaction of PtCl2(1,5-COD) with 2
Complex 1 crystallizes in the triclinic space group P-1. Its
planar [Pd7]6+ core lies upon a center of inversion and consists
of a hexagonal arrangement of six Pd atoms (Figure 1). A
B
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Figure 2. ESI-MS of Pd7(NCtBu2)6 (1) in THF. The peak at m/z 1586.2362 is assignable to the [M]+ ion, whereas the peak at m/z 740.1635 is
assignable to the [Pd3(NCtBu2)3]+ fragment. Inset: Experimental (bottom) and calculated (top) peaks assignable to the [M]+ ion.
seventh Pd atom resides at the center of the hexagon and
features a hexagonal planar coordination geometry. All seven
Pd atoms are co-planar. Each ketimide ligand bridges two
outer, adjacent Pd atoms in an alternating up/down fashion. As
a result, the complex has overall D3d symmetry. The Pd−Pd
bond distances fall within a narrow range (2.7024(5) to
2.7159(6) Å) and are within the range of those reported for
Pd−Pd single bonds.46 The average Pd−N bond length is 2.00
Å, which is comparable to the Pd−N bond lengths reported for
the terminal Pd(II) ketimides, [Pd(PCy3)2(O2CC6F5)(N
CRR′)] (1.956(3) Å for RR′ = C10H10; and 1.999(3) Å for
RR′ = Me{CHPh2}),44,47 as well as terminal Pd(II)
amides.48,49 Finally, the average N−Pd−N angle is 172°,
highlighting the fact that the coordination geometry about the
outer Pd ions is nearly linear if the Pd−Pd bonds are ignored.
The hexagonal planar coordination geometry is relatively
rare. Tanase and co-workers recently reported [Pt{Pd-
(dmpe)}3(μ3-SiPh2)3], wherein the Pt atom is ligated by a
hexagonal array of three Pd atoms and three Si atoms,50 while
Crimmin and co-workers recently reported the hexagonal
planar Pd complex, [PdH3(Mg LMe)3] (LMe = {(2,6-iPr2C6H3)-
NC(Me)}2CH).51 Also of note is [Ni(cyclo-PtBu)6], which
cores,56−58 but the central metal ions in these structures are
also bridged by μ3-hydride ligands.
The 1H NMR spectrum of 1 in C6D6 features a sharp singlet
at 1.65 ppm (Figure S1), consistent with the high-symmetry
observed in the solid-state. The 13C{1H} NMR spectrum
displays a diagnostic resonance at 181.0 ppm (Figure S2),
assignable to the ketimide NC carbon. A similar chemical
shift (188.5 ppm) was observed for the copper ketimide
cluster, [Cu(NCtBu2)]4.59 The ESI mass spectrum of 1
features a signal at m/z 1586.2362 (Figure 2), corresponding
to the parent [M]+ ion (calcd m/z 1586.1953). A second
prominent peak at m/z 740.1635 is assigned to the
fragmentation product, [Pd3(NCtBu2)3]+ (calcd m/z
740.1459). The presence (or absence) of hydride ligands in
transition metal nanoclusters is sometimes difficult to ascertain,
as shown by the recent recharacterization of [Ag40(S-2,4-
Me2C6H3)24(PPh3)8H12]2+.60,61 However, in this case, the
excellent agreement between the observed and calculated
isotope distributions for both signals (Figures S8 and S9)
supports the absence of bridging hydride ligands in this
complex.
To better understand the formation of 1 and identify the
c o n t a i n s
a
h e x a g o n a l p l a n a r N i c e n t e r , 5 2
overall stoichiometry of the transformation, we recorded a H
1
[Mn7(THF)6(CO)12]−, which contains a hexagonal planar
Mn center,53 and [M5Fe4(CO)16]3− (M = Cu, Ag), which
contains a hexagonal planar group 11 ion.54,55 The clusters
[Co7H6{N(SiMe3)2}6], Pd[Re2(CO)8(μ-SbPh2)(μ-H)]2, and
[Rh7H18(PiPr3)6]2+ contain comparable hexagonal planar [M7]
NMR spectrum of the reaction mixture in THF-d8, in the
presence of an internal standard (Figures S3 and S4). This
spectrum reveals the presence of resonances assignable to 1,
t
HNCtBu2, BuCN, isobutylene, and isobutane. Resonances
at 2.38, 1.66, 1.60, 1.54, 1.30, 1.24, and 1.18 ppm are also
C
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Figure 3. Comparison of DFT (PBE0-D3(BJ)/ECP/def2-TZVP) optimized structures and pertinent bond lengths in analogous Pd and Pt
ketimide complexes, M(NCtBu2)2, [(tBu2CN)M(μ-N,CNC(tBu)C(Me)2CH2)M(NCtBu2)], and M7(NCtBu2)6 (M = Pd, Pt).
Experimentally determined bond lengths are indicated in blue.
present, which we have assigned to [(tBu2CN)Pd(μ-N,C-
NC(tBu)C(Me)2CH2)Pd(NCtBu2)] (2) (Scheme 1 and
Figure 3 for comparison of structures of analogous Pd and Pt
complexes). This complex is the Pd analogue of the recently
reported bimetallic Pt ketimide, [(tBu2CN)Pt(μ-N,C−N
C(tBu)C(Me)2CH2)Pt(NCtBu2)].45 These resonances are
observed in a 2:9:9:9:6:9:9 ratio. Both the chemical shifts and
relative intensities are essentially identical to those observed
for the Pt analogue, supporting our assignment.
In situ NMR spectroscopic monitoring appears to confirm this
hypothesis (Figure S5). Thermochemical calculations at the
density functional theory (DFT) level are also consistent with
this proposal, as the formation of Pd7(NCtBu2)6 (1) from
[(tBu2CN)Pd(μ-N,C-NC(tBu)C(Me)2CH2)Pd(N
CtBu2)] (2) was found to be highly exergonic (Scheme 2).
Moreover, we have previously demonstrated that thermolysis
of M(NCtBu2)4 (M = Mn, Fe) results in ketimide oxidation
and formation of Mn3(NCtBu2)6 and Fe2(NCtBu2)5,
lene.34,35 Incidentally, our calculations also explain why the
Pt analogue of 1, [Pt7(NCtBu2)6], is not experimentally
observed during the reaction of PtCl2(1,5-COD) with Li(N
CtBu2).45 Its formation from the [(tBu2CN)Pt(μ-N,C-N
C(tBu)C(Me)2CH2)Pt(NCtBu2)] intermediate is actually
slightly endergonic due to large positive reaction enthalpy
t
Given the distribution of products in the reaction mixture,
we hypothesize that the formation of 1 proceeds via the
intermediacy of 2. Specifically, we propose that reaction of
PdCl2(PhCN)2 with Li(NCtBu2) results in initial formation
of 2 and HNCtBu2. Complex 2 then converts to low-valent 1
via oxidation of its ketimide ligands, concomitant with
respectively, along with BuCN, isobutane, and isobuty-
t
formation of BuCN, isobutylene, and isobutane (Scheme 1).
D
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Scheme 2. Computed Reaction Enthalpies (ΔHr0), Entropies (ΔSr0), and Free Gibbs Energies (ΔGr0) for Selected
a
Transformations of Analogous Pd and Pt Ketimide Complexes
a
PBE-D3(BJ)/def2-TZVPP results in gas-phase.
associated with higher bond-breaking costs of the more
covalent Pt−N/Pt−C bonds in comparison to their Pd−L
counterparts.45
aromaticity was confirmed by the gauge-including magnetically
induced current (GIMIC) method,62,63 which was used to
evaluate ring currents and visualize current densities in the
[PdnL6] (n = 6, 7) clusters in comparison with other widely
recognized aromatics (cf. Table 1 as well as Table S4 and
Figure S14 together with a short discussion in Supporting
Information). Hence, complex 1 sustains a net diatropic ring-
current of 10.6 nA/T (see also Figure 5 for the current
density), which is comparable with that of benzene (11.8 nA/
T) and other well-established 4d transition-metal aromatics,
such as [Mo3O9]2− (11.1 nA/T) and [(Ar3P)3Pd3(SR)3]+
(14.8 nA/T), the latter being noble-metal analogues of the
cyclopropenium ion.64−66 In addition, the computed vertical
electron detachment energy (VDE) for 1 is relatively high
(5.30 eV) and comparable with that of [Mo3O9]− (4.0 eV).65
We also recorded the cyclic voltammogram of 1 in THF at a
variety of scan rates. Most notably, complex 1 features a
reversible reduction feature at E1/2 = −2.32 V (vs Fc/Fc+)
(Figures 6, S12, and S13 and Table S2). The reversibility of
this feature suggests that [1]− could be isolable. Additionally,
we observe the presence of an irreversible oxidation feature at
In an effort to understand the unique structural features of
complex 1, we performed a computational bonding analysis
using quasi-relativistic DFT calculations at the PBE0-D3(BJ)/
ECP/def2-TZVP level of theory. Calculations were performed
on 1 as well as the truncated model complexes, Pd7(N
CMe2)6 and Pd7(NCH2)6. These calculations reveal a rich
manifold of Pd−Pd bonding interactions (see Figures 4 and
S16 for canonical MOs and Figure S17 for ELF analysis). For
example, the HOMO and HOMO−1 for Pd7(NCH2)6 are
highly delocalized and predominately Pd-based (Figure 4).
Similar results have been found for other Pd nanoclusters.20,23
The calculated NPA atomic charges for the outer and inner
Pd atoms of 1 are +0.41 and +0.08, respectively, consistent
with their formal oxidations states of +1 and 0. Neither the
ligand-free [Pd6]6+ nor [Pd7]6+ cores are stable alone (no
stationary points on the potential energy surface were found),
and both require anionic bridging ligands to hold the outer,
positively charged Pd atoms together. We note further that
dispersion forces play only a minor role in the overall stability
of the neutral Pd7(NCtBu2)6 cluster (the Pd−Pd bonds are
elongated only by 0.005 Å when optimizing the structure
without Grimme’s D3(BJ) dispersion corrections; cf. Table
S3), and the central Pd(0) atom features a strong bonding
energy (375 kJ/mol) to the outer hexagonal [Pd6] ring, as
revealed from energy decomposition analysis (EDA). Interest-
ingly, the structures of both the [Pd7] and [Pd6] cores remain
planar, and Pd−Pd distances are almost unaffected upon
replacing the bridging ketimide ligands with other mono-
anionic π-donors, such as halides, thiocyanates, or alkyl-
thiolates (Table 1), hinting at the extra stabilization energy
(e.g., aromaticity) within the hexagonal ring. The presence of
E
p,a = −0.18 V (vs Fc/Fc+, 100 mV/s scan rate). This feature is
irreversible at all scan rates studied. Interestingly, after
scanning past −0.18 V, a new feature appears in the cyclic
voltammogram, suggesting that the oxidation of 1 results in
formation of a new species via an ECE-type mechanism. This
new feature appears at −1.35 V (vs Fc/Fc+, 100 mV/s scan
rate) and is irreversible at all scan rates studied. The identity of
this new species is unknown. For comparison, the Pd13
nanocluster [Pd13(μ4-C7H7)6]2+ features a reversible reduction
feature at −1.08 V and reversible oxidation features at +0.05 V
and +0.73 V (vs Fc/Fc+).20 The Pd6 nanocluster, [Pd6(μ-
GePh2)2-(CN-2,6-Me2C6H3)8(μ-CN-2,6-Me2C6H3)2], also ex-
hibits rich electrochemistry, including reversible redox features
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Table 1. Computed Pd−Pd Distances and Ring-Current
Strengths in Selected [Pdn] (n = 7, 6, 3) Clusters,
a
[Mo3O9]2−, and Benzene
b
ring-current strength [nA/T]
d(Pd−Pd) [Å] diatropic paratropic
net
Pd7(N=CtBu2)6 (1)
Pd7(N=CMe2)6
Pd7(N=CH2)6
Pd7(SCN)6
Pd7(SMe)6
Pd7F6
Pd7Cl6
Pd7Br6
Pd7I6
Pd6(N=CMe2)6
[(H3P)3Pd3(SH)3]+
[Mo3O9]2− (D3h)
C6H6
2.745
2.718
2.721
2.724
2.713
2.720
2.720
2.714
2.717
2.681
2.859
21.8
16.3
15.9
16.9
16.3
13.6
16.9
17.8
22.8
15.6
15.2
11.5
16.8
−11.2
−4.4
−3.9
−8.4
−8.2
−4.9
−7.7
−7.1
−8.4
−6.5
−0.4
−0.4
−5.0
10.6
11.9
12.0
8.5
8.1
8.7
9.2
10.7
14.4
9.1
14.8
11.1
11.8
−
−
a
Structures optimized at the PBE0-D3(BJ)/ECP/def2-TZVP level.
The integrated current passing outer ring (B3LYP-D3(BJ)/TZVPP
b
results; cf. computational details). An integration plane is placed into
the geometrical ring center or into the center of Pd3 triangles in the
case of [Pd7] clusters, so that the integration plane is perpendicular to
the planar ring and it passes through the center of M−L bonds
(ligating atoms are indicated in bold). See also Table S4 for more
details and calculated nuclear independent chemical shift (NICS)
values.
Figure 4. Frontier MOs (isosurface plots 0.03 au) in Pd7(N
CH2)6 (a truncated model complex of 1).
at ca. −1.35 V and −1.03 V (vs Fc/Fc+).23 While quantitative
comparisons are a challenge because of their different
nuclearities and ligands sets, it is clear that reversible addition
or removal of electrons to the Pd bonding framework is a
common trait among Pd nanoclusters.
Finally, we undertook a preliminary reactivity study of
complex 1. Addition of 21 equiv of PPh3 to 1 in THF resulted
is a slow color change from deep forest green to pale green
over the course of 48 h at room temperature (Scheme 3).
Workup of the reaction mixture resulted in the isolation of
Figure 5. Signed modulus of the current density in Pd7(NCMe2)6
(a truncated model of 1). Cut-plane plot within the [Pd7] plane.
Diatropic currents are illustrated in blue, and paratropic currents are
illustrated in red. For more details, see Supporting Information.
1
Pd(PPh3)3 (3) as yellow blocks in 82% yield. The H and
31P{1H} NMR spectra of 3 in C6D6 match closely to those
previously reported for this material in toluene-d8 (Figures S6
and S7).67,68 To account for the reduction of the Pd centers in
1 upon formation of 3, we propose the ketimide ligands are
CONCLUSIONS
■
The isolation of the Pd7 nanosheet, Pd7(NCtBu2)6 (1), via
reaction of PdCl2(PhCN)2 and Li(NCtBu2), demonstrates
that ketimides are effective at stabilizing low-valent group 10
nanoclusters, thereby broadening the scope of ligands that are
known to promote the formation of this class of materials.
Complex 1 is built around a [Pd7]6+ core, and its central Pd
atom features the rare hexagonal planar coordination
geometry. According to DFT calculations, 1 features a similar
level of aromaticity as other previously reported 4d transition-
metal aromatic clusters, such as [Mo3O9]2− or
[(Ar3P)3Pd3(SR)3]+, but 1 is unique because it is the first
t
oxidized, forming BuCN, isobutane, and isobutylene as
byproducts. A similar process is occurring during the initial
formation of 1 and further demonstrates the redox activity of
the ketimide ligand. Significantly, the facile formation of Pd(0)
upon decomposition of 1 suggests that it could be a useful
catalyst precursor for a variety of cross coupling reactions.69−71
Indeed, Pd nanoclusters and nanoparticles have previously
been shown to act as reservoirs for catalytically competent
monometallic Pd(0) species.72
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13C nuclei were referenced by using the residual solvent peaks (1H
NMR experiments) or the characteristic resonances of the solvent
nuclei as internal standards (13C{1H} NMR experiments).
IR spectra were recorded on a Nicolet 6700 FT-IR spectrometer.
UV−vis/NIR spectra were recorded on a Shimadzu UV3600 UV-NIR
spectrometer. Mass spectra were collected by the Mass Spectrometry
Facility at the University of California, Santa Barbara, using an
electrospray ion (ESI) source on positive ion mode with a Waters
Micromass QTOF2 Quadrupole/Time-of-Flight Tandem mass
spectrometer. Elemental analyses were performed by the Micro-
analytical Laboratory at University of California (Berkeley, CA).
Cyclic Voltammetry (CV) Measurements. CV experiments
were performed with a CH Instruments 600c Potentiostat, and the
data were processed using CHI software (version 6.29). All
experiments were performed in a glovebox using a 20 mL glass vial
as the cell. The working electrode consisted of a platinum disk
embedded in glass (2 mm diameter), the counter electrode was a Pt
wire, and the reference electrode was a Pt wire. Solutions employed
during CV studies were typically 1 mM in complex 1 and 0.1 M in
[Bu4N][PF6]. All potentials are reported versus the [Cp2Fe]0/+
couple. For all trials, ip,a/ip,c = 1 for the [Cp2Fe]0/+ couple, while ip,c
increased linearly with the square root of the scan rate (i.e., √v).
Redox couples which exhibited behavior similar to the [Cp2Fe]0/+
couple were considered reversible.
Figure 6. Cyclic voltammogram of complex 1 (200 mV/s, vs Fc/Fc+),
measured in THF with 0.1 M [NBu4][PF6] as supporting electrolyte.
The electrochemical feature indicated by (#) is assigned to the [1]0/−
couple. The electrochemical feature indicated by (∧) is due to the
irreversible oxidation of 1 via an ECE-type mechanism, which gives
rise to the reduction at −1.4 V. The electrochemical features indicated
by (*) are likely due to a decomposition product, as they grow in
intensity during the course of the experiment. The electrochemical
feature indicated by (†) is ascribed to a trace amount of 2.
Synthesis of Pd7(NCtBu2)6 (1). In a 20 mL scintillation vial
equipped with a magnetic stir bar, PdCl2(PhCN)2 (107 mg, 0.28
mmol) was dissolved in THF (2 mL) to give an orange solution,
which was subsequently cooled to −25 °C. Concurrently, 2 equiv of
Li(NCtBu2) (82 mg, 0.56 mmol) was dissolved in THF (2 mL) to
give a colorless solution, which was also cooled to −25 °C. Over the
course of 1 min, the Li(NCtBu2) solution was added dropwise to a
stirring solution of PdCl2(PhCN)2. The reaction mixture was then
allowed to warm to room temeprature with sitrring. The resulting
mixture slowly turned to a dark forest green color. After 5 h, the
volatiles were removed in vacuo to give a dark green oily solid. This
solid was dissolved in pentane (2 mL) and filtered through a Celite
column supported on glass wool (0.5 cm × 2 cm) to give a clear dark
green filtrate, while leaving a white precipitate on the Celite. The filter
pad was washed with pentane (2 × 1 mL) and added to the filtrate.
The filtrate (in a 5 mL vial) was placed inside of a 20 mL scintillation
vial containing 4 mL of isooctane. Storage of this two-vial system at
−25 °C for 7 d resulted in the deposition of dark green blocks of 1,
which were isolated by decanting the supernatant (26 mg, 40% yield).
The 1H NMR data for this sample also reveal the presence of a small
amount of [(tBu2CN)Pd(μ-N,C-NC(tBu)C(Me)2CH2)Pd(N
CtBu2)] (2). Complexes 1 and 2 were present in a 6:1 ratio. All
attempts to isolate analytically pure 2 have thus far been unsuccessful.
1H NMR (C6D6, 25 °C, 400 MHz): δ 1.15 (2, s, 9H, C(CH3)3), 1.27
(2, s, 9H, C(CH3)), 1.28 (2, s, 9H, C(CH3)3), 1.47 (2, s, 9H,
C(CH3)3), 1.49 (2, s, 6H, C(CH3)2(CH2)), 1.53 (2, s, 9H,
C(CH3)3), 1.65 (1, s, 108H, C(CH3)3), 3.08 (2, s, 2H,
C(CH3)2(CH2)). 13C{1H} NMR (C6D6, 25 °C, 126 MHz): δ 32.97
(1, s, C(CH3)3), 42.83 (1, s, C(CH3)3), 180.97 (1, s, NC). Anal.
calcd for C54H108N6Pd7: C, 40.88; H, 6.86; N, 5.30. Found: C, 40.75;
all-metal system with hexagonal (i.e., benzene-like) symmetry.
Preliminary reactivity studies suggest that 1 can act as a source
of Pd(0), a consequence of the facile oxidative decomposition
of the ketimide ligands. Going forward, we plan to explore the
small molecule reactivity of Pd7(NCtBu2)6, which, given the
unique coordination environment of the central Pd(0) atom,
could result in new modes of reactivity. In addition, we will
explore the ability of the ketimide ligand to stabilize higher
nuclearity Pd clusters, which could potentially be achieved by
changing the alkyl substituents on the ketimide ligand.
EXPERIMENTAL SECTION
■
General Procedures. All reactions and subsequent manipulations
were performed under anaerobic and anhydrous conditions in the
glovebox under an atmosphere of dinitrogen. Hexanes, diethyl ether
(Et2O), and tetrahydrofuran (THF) were dried by passage over
activated molecular sieves using a Vacuum Atmospheres DRI-SOLV
solvent purification system. Pentane, dichloromethane, and benzene
were dried on an MBraun solvent purification system. Isooctane and
acetonitrile were degassed and stored over activated 3 Å molecular
sieves for 72 h prior to use. C6D6, THF-d8, and hexamethyldisiloxane
(HMDSO) were dried over activated 3 Å molecular sieves for 24 h
prior to use. Li(NCtBu2)73 and PdCl2(PhCN)274 were prepared by
literature procedures. All other reagents were purchased from
commercial suppliers and used as received.
1H and 13C{1H} NMR spectra were recorded on an Agilent
Technologies 400-MR DD2 400 MHz spectrometer or Varian Unity
Inova 500 MHz spectrometer at 25 °C. The chemical shifts of 1H and
Scheme 3. Synthesis of Pd(PPh3)3 (3) from Pd Nanocluster 1 and PPh3
G
DOI: 10.1021/acs.inorgchem.9b03276
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
H, 6.66; N, 5.04. IR (KBr pellet, cm−1): 3003 (m), 2946 (s), 2922 (s),
1579 (ν(CN), s), 1566 (ν(CN), s), 1477 (s), 1443 (s), 1383
(m), 1360 (s), 1260 (vw), 1214 (s), 1103 (w), 1042 (w), 973 (w),
925 (w), 840 (w), 688 (w), 668 (w). UV−vis/NIR (pentane, 18.9
μM, 25 °C, L·mol−1·cm−1) 251 nm (ε = 99,900), 290 nm (sh, ε =
69,500), 369 nm (ε = 56,200), 420 nm (sh, ε = 33,400), 604 nm (ε =
17,100), 771 nm (ε = 16,300). ESI-MS: m/z 1586.2362 [M]+ (calcd
m/z 1586.1953), 740.1635 [Pd3(NCtBu2)3]+ (calcd m/z
740.1459).
Initiative in Sustainability at UCSB for a summer fellowship.
Calculations were performed using the supercomputing
infrastructure acquired in projects ITMS 26230120002 and
26210120002 supported by the Research and Development
Operational Program funded by the ERDF. P.H. also
acknowledges financial support from the European Union’s
Horizon 2020 research and innovation program under the
Marie Skłodowska-Curie grant no. 752285.
Reaction of 1 with PPh3. To a stirring, deep green solution of 1
(24 mg, 0.016 mmol) in THF (2 mL) was added PPh3 (87 mg, 0.33
mmol) as a clear colorless THF solution (2 mL). The reaction
mixture was stirred at room temperature for 48 h, whereupon the
mixture lightened to pale green. The volatiles were removed in vacuo
to give a pale green solid. The solid was rinsed with hexanes (2 mL)
to afford a yellow solid. The solid was then dissolved in toluene (5
mL), and the resulting solution was filtered through a Celite column
supported on glass wool (0.5 cm × 2 cm) to give a clear yellow
filtrate. The filtrate was concentrated to ca. 2 mL and layered with
hexanes (5 mL). Storage of this solution at −25 °C for 24 h resulted
in the deposition of yellow blocks of Pd(PPh3)3 (3), which were
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b03276.
■
S
Experimental and computational details together with
additional data/analysis for complexes 1 and 2. (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: hayton@chem.ucsb.edu.
*E-mail: peter.hrobarik@uniba.sk.
ORCID
■
Andrew W. Cook: 0000-0002-8850-8946
́
Peter Hrobarik: 0000-0002-6444-8555
Trevor W. Hayton: 0000-0003-4370-1424
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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This work has been supported by the National Science
Foundation (CHE 1764345) and the Slovak grant agencies
VEGA (grant nos. 1/0507/17 and 1/0712/18) and APVV
(grant no. APVV-17-0324). NMR spectra were collected on
instruments supported by an NIH Shared Instrumentation
Grant (SIG, 1S10OD012077-01A1). ESI mass spectra were
acquired at the MRL Shared Experimental Facilities, supported
by the MRSEC Program of the NSF under award no. DMR
1720256 and a member of the NSF-funded Materials Research
Facilities Network. A.W.C. thanks the Mellichamp Academic
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DOI: 10.1021/acs.inorgchem.9b03276
Inorg. Chem. XXXX, XXX, XXX−XXX